Organosilicon Chemistry V

Edited by N. Auner and J. Weis

Further Reading from Wiley-VCH

Jutzi, P., Schubert, U. (Eds.) Silicon Chemistry 2003.3-527-30647- 1

Auner, N., Weis, J. (Eds.) Organosilicon Chemistry IV 2000.3-527-29854-1

Meyer, G., Naumann, D., Wesemann, L. (Eds.) Inorganic Chemistry Highlights 2002.3 -5 27- 30265 -4

Lehmann, V. Electrochemistry of Silicon 2002. 3-527-29321-3

Organosilicon Chemistry V From Molecules to Materials

Edited by Norbert Auner and Johann Weis

WILEY- VCH

WILEY-VCH GmbH & Co. KGaA

Prof. Dr. N. Auner Prof. Dr. J. Weis Department of Inorganic Chemistry Consortium of Electrochemical Industry GmbH University of Frankfurt Zielstattstrae 20 Marie-Curie-StraBe 11 81379 Munich 60439 Frankfurt am Main Germany Germany

This book was carefully produced. Nevertheless, editors, authors and publisher do not warrant the information contained therein to be free of errors. Readers are advised to keep in mind that statements, data, illustrations, procedural details or other items may inadvertently be inaccurate.

Library of Congress Card No.: applied for

A catalogue record for this book is available from the British Library.

Bibliographic information published by Die Deutsche Bibliothek Die Deutsche Bibliothek lists this publication in the Deutsche Nationalbibliografie; detailed bibliographic data is available in the Internet at http://dnb.ddb.de

ISBN: 3-527-30670-6

0 2003 WILEY-VCH Verlag GmbH & Co. KgaA, Weinheim Printed on acid-free paper. All rights reserved (including those of translation in other languages). No part of this book may be reproduced in any form - by photoprinting, microfilm, or any other means - nor transmitted or translated into machine language without written permission from the publishers. Registered names, trademarks, etc. used in this book, even when not specifically marked as such, are not to be considered unprotected by law. Printing: betz-druck gmbh, Darmstadt. Bookbinding: Litges & Dopf Buchbinderei GmbH, Heppenheim. Printed in the Federal Republic of Germany.

In 2000 we published Organosilicon Chemistry - From Molecules to Materials, Vol. IV, which contained the lectures and poster contributions that were presented at the N. Munchner Silicontage, held in April 1998. However, this Volume V of the same series collects all scientific contributions that were presented at the 1st European Silicon Days, Munich, 2001. Between 1998 and 2001 the Munich Silicon Days had been transformed into European Silicon Days. This important reorganization happened because it was requested by the international community of scientists in both academia and industry. Furthermore, the Munich Silicon Days, jointly organized by the Gesellschaft Deutscher Chemiker and by Wacker-Chemie GmbH, which started in 1992 and continued every second year, were most successful in bringing together an exceptionally large number of participants from industry and academia.

Especially the impressive number of participating students and young scientists was always convincing evidence of the high interest in these meetings. The transformation of Munich Silicon Days into an European meeting includes the participation of the most important european companies that produce silicones, such as Wacker-Chemie GmbH, Dow Coming Ltd., GE Bayer Silicones, Degussa including Goldschmidt, OSi, and Rhodia Silicones with the intention to organize future conferences at different places in different European countries.

The continuing favourable response of organosilicon researchers from all over the world to the Conference Proceedings “Organosilicon Chemistry - From Molecules to Materials” encouraged us to take the workload with the present edition of Volume V thus representing the scientific contributions of the 1st European Silicon Days. We already mentioned in the previous four volumes, that the Conference Proceedings are not to be considered as a classical textbook in the sense that it would describe the basic knowledge of a discipline, but provides accounts and summaries of the latest results of organosilicon research, written by experts in a fascinating field of rapidly growing main-group chemistry. Furthermore, in editing the present volume, we used this occasion to update the authors’ contributions to the different topics reviewed at the symposium with references of papers that were published during the past two or three years to ensure utmost actuality.

In earlier volumes, the contributions to each of the different chapters were mostly accompanied by a short overview and a summary of current research directions and developments in organosilicon chemistry. In order to avoid repetition and in consideration of the fact that research emphasis changes only slightly over periods of only two to three years, we have omitted a similarly detailed work-up of the material. Being at the beginning of a new century, we have tried to analyze where future organosilicon chemistry will be going, and to understand market requirements and research needs as was reflected by the motto “Possibilities for the 21st Century“ of “The 12th International Symposium on Organosilicon Chemistry”, Sendai, Japan, 1999. The requirements towards (i) the generation of silicon-based novel materials and the structural understanding of their properties, (ii) the increasing importance of the physics of the silicon atom and its compounds as well as the fact that (iii) biochemistry of silicon and silicones is a fascinating, rapidly growing new facet in organosilicon research and (iv) the understanding of the natural formation of different silicate structures, as well as the environmental aspects of silicodsilicones, etc., will be briefly touched in the “Introduction” of this Volume “Organosilicon Chemistry - Facts and Perspectives ”.

VI Preface

During the I. Munchner Silicontage (1992), the two pioneers of organosilicon chemistry, Prof. Dr. Richard Muller and Prof. Dr. Eugene Rochow, were awarded the Wacker-Silicon-Preis on the occasion of the 50th anniversary of the Direct Synthesis.

By that they were integrated into an impressive assembly of award recipients, which is listed below:

200 1

1998

1996

1994

1992

1999 1989 1988 1987

Prof. Dr. M. Weidenbruch (I. European Silicon Days, September 2001) Prof. Dr. R. Corriu (IV. Munich Silicon Days, April 1998) Prof. Dr. H. Schmidbaur (111. Munich Silicon Days, April 1996) Prof. Dr. E. Hengge (11. Munich Silicon Days, August 1994) Prof. Dr. R. Muller and Prof. Dr. E.G. Rochow (I. Munich Silicon Days, August 1992) Prof. Dr. H. Sakurai Prof. Dr. R. West Prof. Dr. N. Wiberg, Prof. Dr. R. Tacke (junior award) Prof. Dr. P. Jutzi, Prof. Dr. N. Auner (junior award)

At the I. European Silicon Days Prof. Dr. M. Weidenbruch (Carl von Ossietzky University of Oldenburg) was honored with the Wacker-Silicon-Preis 2001 for this outstanding research on the widespread field of the chemistry of low-coordinated silicon, including silylenes, disilenes, silabutadienes, ring systems, and related higher homologues of the group 14 elements. Prof. Weidenbruch’s work is acknowledged and emphasized in this book by his contribution entitled: From Silylenes and Disilenes to a Tetrasilabutadiene and Related Compounds.

The collection and the publication of papers in this volume should reflect the diversity of silicon chemistry as well as the fascination dealing with this element. As much as we know today, the chemistry - but in particular also the physics and the biology of silicon and its compounds, have not at all been exhaustively treated yet. The future will certainly bring many beautiful and also surprising results. This Volume V shall continue to essentially stimulate young researchers to focus on basic silicon science and its transfer into costumer-oriented applications expressed by the design of materials with new and extraordinary chemical, physical, and biological properties. There are still many challenging problems, which should be identified, discussed, and finally solved. The editors hope that future European Organosilicon meetings will also provide a solid platform to establish silicon as the key element of the new century.

July 2003 Profi Dr, NorbertAuner, Pro5 Dr. Johann Weis

Acknowledgments

First of all we thank the authors for their contributions and intense cooperation, which made this overview of current organosilicon chemistry possible. The tremendous work to achieve the attractive layout of this volume was performed by Dr. Yu Yang, and Mrs. Hannelore Bovermann helped to organize the editorial work. Furthermore we are very grateful to Christian Bauch, Martin Bleuel, Jens Elsner, Andreas Frost, Tatiana Hennegriff, Andreas Hess, Dr. Sven Holl, Fariba Maysamy-Tmar, Dr. Ajax Mohamed, Dr. Thomas Muller, Dr. Bahman Solouki, Natalie Spomer, and Dr. Duanchao Yan for their very active assistance to read, compare and correct.

We thank all of them for their admirable engagement!

Prof. Dr. Norbert Auner Johann Wolfgang Goethe-Universitat Frankfurt

Prof. Dr. Johann Weis Wacker-Chemie GmbH Munchen

Contents

Introduction 1 Norbert Auner, Johann Weis

Reactions of Silicon Atoms — an Access to Unusual Molecules 5 Gunther Maier, Hans Peter Reisenauer, Heiko Egenolf, Jorg Glatthaar

Reactions of Silicon Atoms with Methane and Silane in Solid Argon: A Matrix Spectroscopic Study 11 Hans Peter Reisenauer, Jorg Glatthaar, Gunther Maier

Cryogenic Trapping Reactions of Silicon Atoms: New Insights into the Photochemistry of Complexes of Silicon Atoms with Donor Molecules 15 Jorg Glatthaar, Hans Peter Reisenauer, Heiko Egenolf, Gunther Maier

New Reactions of Stable Silylenes 19 Robert West, Daniel F. Moser, Michael Haaf, Thomas A. Schmedake, Ilia Guzei

Insertion Reactions of the Stable Silylene Si[(NCH2/Bu)2C6H4-l,2] 27

Floria Antolini, Xiaoping Cai, Barbara Gehrhus, Peter B. Hitchcock, Michael F. Lappert, Massimo Parrucci, J. Chris Slootweg

A Model System for the Generation of Silyl Cationic Species of Different Reactivity and Stability 34 Thomas Miiller

Synthesis and Chemistry of Some Bridged Silicocations 45 PaulD. Lickiss, Phindile C. Masangane, Wazir Sohal, Guilaine L. Veneziani

New Donor-Stabilized Organosilicon Cations: Synthesis, Structure and Reactivity 50 Andreas Bockholt, Thomas Braun, Peter Jutzi, Beate Neumann, Anja Stammler, Hans-Georg Stammler

Novel Pentacoordinate Siliconium Complexes Stabilized by Oxygen and Nitrogen Donors: Highly Sensitive and Unusual Equilibrium between Ionic Penta- and Neutral Hexacoordinate Compounds 55 Daniel Kost, Vijeyakumar Kingston, Inna Kalikhman

Binuclear Ethylene-Bridged Silicon Chelates: Equilibrium between Neutral Hexacoordinate and Ionic Pentacoordinate Siliconium Complexes 61 Inna Kalikhman, Vijeyakumar Kingston, Daniel Kost, Dietmar Stalke, Bernhard Walfort

Bonding in Silicon Compounds — Long-Range Si/N Interactions in Organosilicon Molecules and Molecular Cations 66 Hans Bock

Thermolytic Formation and Trapping of Silenes Strongly Influenced by Reversed Polarization 78 Henrik Ottosson, Tamaz Guliashvili, Ibrahim El-Sayed

X Contents

Synthesis, Structure and Reactivity of Intramolecularly Donor-Stabilized Silenes 82 Martin Mickoleit, Matthias Potter, Ute Baumer, Kathleen Schmohl, Hartmut Oehme, Rhett Kempe

Cyclotrimetallenes Consisting of Heavier Group 14 Elements: A New Unsaturated Small Ring System 92 Akira Sekiguchi, Vladimir Ya. Lee

On the Way to a Disilyne RSi^SiR 101 Nils Wiberg

From Silylenes and Disilenes to a Tetrasilabuta-l,3-diene and Related Compounds 114 Manfred Weidenbruch

The Formation of a Solid from the Reaction SiCU(g) + 02(g) = Si02(s) + 2 Cl2(g) 126 Michael Binnewies, Andreas Kornick, Marion Jerzembeck, Andreas Wilkening, Heike Quellhorst

Stepwise Formation of Si-0 Networks by Means of Hydrolysis/Condensation 130 Nicola Soger, Heike Quellhorst, Michael Binnewies

The Conformational Preference of the Methyl Group in 1-Methyl-1-silacyclohexane 135 Ingvar Arnason, Agust Kvaran, Sigridur Jonsdottir, Palmar L Gudnason, Heinz Oberhammer

Origin of Photoluminescence in Organosilicon Compounds Containing Styrene Subunits 139 Duanchao Yan, Thomas Milller, Michael Bolte, Norbert Auner

Photoluminescence Characteristics of Linear Methyl- and Phenyl-Substituted Siloxanes 145 Michael Backer, Udo Pernisz

Syntheses of Silyllithium Reagents Starting from Tetraorganosilanes 150 Jan Hornig, DominikAuer, Carsten Strohmann

Selective Transformations Starting from a Diastereomerically Enriched Lithiated Benzylsilane.. 155 Carsten Strohmann, Daniel H M. Buchold, Kerstin Wild, Daniel Schildbach

Synthesis of a Highly Enantiomerically Enriched Silyllithium Compound 167 DominikAuer, Jan Hornig, Carsten Strohmann

Alkynylsilyl Anions — Versatile Building Blocks for Silicon-Containing Polymers 171 Christian Mechtler, Judith Baumgartner, Christoph Marschner

Reactions of Trimethylsiloxychlorosilanes (Me3SiO)RPhSiCl (R = H, Me, Et, 'Pr, 'Bu, Ph, Mes) with Lithium — Formation and Reactivity of Trimethylsiloxysilyllithiums 175 Jorg Harloff, Eckhard Popowski

Silacyclobutanes: Head-to-Head Dimerization Versus Anionic Polymerization — a-Silyl Substituted Carbanions as Reactive Intermediates 180 Hans-Uwe Steinberger, Duanchao Yan, Norbert Auner

Contents XI

Polysilylanions — Easily Available Building Blocks for the Synthesis of Oligosilyl Transition Metal Compounds 186 Roland Fischer, Dieter Frank, Christian Kayser, Judith Baumgartner, Christoph Marschner

Experimental Determination of the Inversion Barriers of Oligosilyl Anions 190 Roland Fischer, Christoph Marschner

Regiospecific and Enantioselective Polymerization to Poly[(dibutylamino)(trimethyldisilene)] by the Masked Disilene Method 195 Hidehi Sakurai

The Cationic Rearrangement of (3-Hydroxy-l-propenyl)tris(trimethylsilyl)silanes into (l-Trimethylsilyl-2-propenyl)-bis(trimethylsilyl)silanols 202 K. Schmohl, H. Reinke, H. Oehme

Chiral (3-Silyl Aldehydes as Precursors of Chiral p-Hydroxy Acids and Chiral 1,3-Diols 207 Joachim Sommer, Hubertus Ahlbrecht

Revisiting the S12CI6 Cleavage of Group 14 Element Phosphanes: Phosphane-Catalyzed Rearrangements 210 W.-W. du Mont, E. Seppdld, T. Gust, L. Miiller

Some New Nucleophile-Induced Reactions Involving SiC^, GeCl2 and GeMe2 Transfer 213 Emma Seppdla, Wolf- Walther du Mont, Thorsten Gust, Jens Mahnke, Lars Miiller

Synthesis, Structure and Reactivity of Novel Oligosilyl Anions 217 H. Reinke, C. Krempner

Synthesis of SiJfc-Containing Polymers Using Silyl Triflate Intermediates 222 Wolfram Uhlig

Silicon Compounds with Geminal Donor Centers 226 Norbert W. Mitzel, Krunoslav Vojinovic, Udo Losehand

Cyclic Silylhydrazines — Synthesis, Isomerizations, and Quantum Chemical Calculations 233 Uwe Klingebiel, Stefan Schmatz

Silylhydroxylamines — Synthesis, Isomerisation, and Quantum Chemical Calculations 246 Christina Ebker, Friedhelm Diedrich, Stefan Schmatz, Uwe Klingebiel

SiO and SiOSiN Chains, Rings and Cages 254 Susanne Kliem, Clemens Reiche, Uwe Klingebiel

Isomeric Cyclosilazanes and their Application as Precursors for Silicon-Based Ceramics 261 Nina Helmold, Verena Liebau, Uwe Klingebiel, Stefan Schmatz

Silicon and Germanium Compounds with Amidinate Ligands 270 Hans H. Karsch, Thomas Segmuller

XII Contents

Development of Force Field Parameters for Amino-Substituted Organodisilanes 277 Uwe Bdhme, Birgit Schluttig, Robert K. Szildgyi

Novel Cyclic and Polycyclic Chalcogenides of Silicon 282 Uwe Herzog, Uwe Bdhme, Gerd Rheinwald

Hypersilylchalcogenolate Derivates of Group 14 Elements 288 Heike Lange, Uwe Herzog, Gerhard Roewer

Syntheses and Properties of Novel Cage-Shaped Molecules Containing an Extended Silicon Backbone 294 Roland Fischer, Judith Baumgartner, Karl Hassler, Guido Kickelbick

Synthesis and Reactivity of Novel Tin-Modified Oligosilanes 299 Thorsten Schollmeier, Markus Schurmann, Frank Uhlig

Mutual Effects between the Trialkylsilyl Substituents and the MmP„ Cages of Phosphanediides (M = Mg, Ca, Sr, Ba, and Sn) 303 Matthias Westerhausen, Stefan Schneiderbauer, Sabine Weinrich

Differing Affinities of the Triorganylsilyl and -stannyl Substituents for Oxygen or Nitrogen, and Phosphorus, Respectively 307 G. Becker, G. Ditten, S. Horner, A. H Maulitz, E,-U Wurthwein

Mono- and Oligosilanes with Pyrazole Ligands 312 Klaus Hiibler, Jan Uwe Berner, Steffen E. F. Merz

A New Type of Silicon Complex with Salen-Type Ligands 317 J. Wagler, U Bdhme, G. Roewer

Gas-Phase Reactions of Free Methyl Cations with Amines and Their Organosilicon Analogues: A Radiochemical Study 321 T A. Kochina, D. V. Vrazhnov, E. N. Sinotova, B. F. Shchogolev

Calculation of 29Si Chemical Shifts Using a Density-Functional Based Tight-Binding Scheme ...324 Marc Milbradt, Heinrich Marsmann, Thomas Heine, Gotthard Seifert, Thomas Frauenheim

29Si NMR Chemical Shifts of Four- and Five-Membered Organosilacycles: Experimental and Theoretical Studies 329 Katja Strohfeldt, Katrin Andres, Rudiger Bertermann, Eric Wack, Martin Kaupp, Carsten Strohmann

29Si NMR Chemical Shifts of Siloxanes: Ab Initio and Density Functional Study 334 Georgios Tsantes, Norbert Auner, Thomas Muller

Domain Size Determination of Poly(phthalamide)/Poly(dimethylsiloxane) Block Copolymers by *H Solid-State Spin Diffusion NMR Spectroscopy 339 Axel Kretschmer, Robert A. Drake, Simeon J. Bones, Michael Neidhoefer, Manfred Wilhelm, Hans Wolfgang Spiess

Contents XIII

Esterification Study of Acetoxysilane by Alcohols and Phenols 344 Victor Kopylov, Vladimir Ivanov, Marina Zheneva, Vyacheslav Kireev, Valerii Djakov

Organosilicon Compounds in Medicine and Cosmetics 348 Valerii D 'yakov

Synthesis and Biological Activity of Silocanyl- and Silatranylmethyl Ethers of Acrylic Acids ....352 V.M.D 'yakov, S. V Loginov

Biodegradability and Silatrane Effect Mechanism 356 Evgenii Ofitserov, Valerii D 'yakov, Maksud Rasulov

Intensification of Unsaturated Organomagnesium Chloride Production Reaction 3 60 Vladimir Zhun, Alia Zhun, Evgenii Chernyshev

Silylative Coupling and Cross-Metathesis of Alkenes and Dienes with Vinyl-Silicon Derivatives — New Catalytic Routes to Synthesis of Organosilicon Compounds 363 Bogdan Marciniec

Thermolytic Formation and Trapping of Silenes Strongly Influenced by Reversed Polarization... 375 Henrik Ottosson, Tamaz Guliashvili, Ibrahim El-Sayed

Tailoring Properties of Silicon-Containing Oxide Catalysts via the Thermolytic Molecular Precursor Route 379 Kyle L. Fujdala, T. Don Tilley

Organosilicon Chemistry and Nanosciences 389 Bruno Boury, Robert Corriu

Catalytic Activity of Rhodium-Siloxide Complexes in Hydrosilylation of Allyl Ethers and Allyl Esters 415 Bogdan Marciniec, Edyta Walczuk, Paulina Blazejewska-Chadyniak, Dariusz Chadyniak, Malgorzata Kujawa-Welten, Stanislaw Krompiec

Synthesis and Complex Chemistry of Novel Di- and Trihydroxyoligosilanes 420 D. Hoffmann, H Reinke, C. Krempner

Thioether Functionalized Octasilsesquioxanes 425 H J, Konig, H C. Marsmann, M. C Letzel

Synthesis of Cyclopentadienyl-Substituted Polyhedral Zirconasiloxanes 429 Hans Martin Lindemann, Beate Neumann, Hans-Georg Stammler, Anja Stammler, Peter Jutzi

Preparation of Highly Porous Silicates by Fast Gelation of H-Silsesquioxane 435 Duan Li Ou, Pierre M Chevalier

Metal Complexes Containing Extended-Reach Siloxypyridine and Related Ligands 447 DavidM. L. Goodgame, Paul D. Lickiss, Stephanie J. Rooke, AndewJ. P. White, David J. Williams

XIV Contents

Agostic versus Hypervalent Si-H Interactions in Half-Sandwich Complexes of Nb and Ta 451 Alexei A. Merkulov, Georgii I. Nikonov, Philip Mountford

The Reactivity of Platinum Complexes Containing Hemilabile Ligands towards Silanes and Stannanes 456 Frank Stohr, Susan Thompson, Dietmar Sturmayr, Jiirgen Pfeiffer, Ulrich Schubert

57Fe-M6ssbauer Spectra and X-ray Structures of Dipolar Ferrocenylhexasilanes 462 Harold Stiiger, Hermann Rautz, Guido Kickelbick, Claus Pietzsch

Dipolar 1,2-Af,Af-Dimethylaminomethylferrocenyl Complexes for Nonlinear Optics? 467 Christian Beyer, Uwe Bohme, Gerhard Roewer, Claus Pietzsch

Metallo-silanols — Precursors for the Generation of Novel Metallo-siloxanes and Metallo-heterosiloxanes 473 Marco Hofinann, Matthias Vogler, Dirk Schumacher, Wolfgang Malisch

Half-Sandwich Complexes of Iron and Tungsten with Silanol-Functionalized Cyclopentadienyl Ligand 486 Andreas Sohns, Holger Bera, Dirk Schumacher, Wolfgang Malisch

Synthesis and Electrochemical Properties of Silanes with Iron-Containing Donors 490 Helmut Fallmann, Gottfried Furpafi, Harald Stiiger, Christa Grogger

Sustainable Silicon Production 495 Gunnar Halvorsen, Gunnar Schussler

Reactivity of Doped Silicon in the Direct Synthesis of Methylchlorosilanes 509 L. Lorey, G. Roewer

Solvent Role in the Triethoxysilane Direct Process 514 Alexander Gorshkov, Victor Kopylov, Anna Markacheva, Alexander Polivanov

Methylsilane Production by Means of Methyldiethoxysilane Catalytic Disproportionation 518 Evgenii Belov, Galina Dubrovskaya, Nikolay Efimov, Salomonida Kleshcevnikova, Evgenii Korobkov, Evgenii Lebedev

Investigations of the Reactivity of Methylchloro- and Methylaminodisilanes toward Alkenes 522 Claudia Knopf Gerhard Roewer, Gerd Rheinwald, Heinrich Lang

New Organofunctional Silanes for Adhesives, Sealants and Spray Foams 527 A. Bauer, T. Kammel, B. Pachaly, O. Schafer, W. Schindler, V. Stanjek, J. Weis

Isocyanatopropyltrimethoxysilane — Key Intermediate of New Silane Coupling Agents 536 Hieronim Maciejewski, Bogdan Marciniec, Agnieszka Wyszpolska

Development of Adhesion Promoters on the Basis of Secondary Reactions of Carbofunctional Organosilicon Monomers 541 V. A. Kovyazin, V. M. Kopylov, A. V. Nikitin

Contents

Synthesis of Chiral Amino-Substituted Organosilanes 545 Uwe Bohme, Betty Gilnther, Ben Rittmeister

Water-Borne Fluoroalkylsilanes: a New Family of Products for Surface Modification 551 K. Weifienbach, B. Standke, P. Jenkner

Mineral-Filled Thermoplastics: How Silanes Make the Difference 557 Helmut Mack

The Role of Silanes in Filled and Crosslinked Polymers 562 Peter Kraxner, Louis Boogh, Alain Lejeune

Hybrid Coatings Based on Silanes: Precursor Methods to Make Hybrid Organic-Inorganic Coatings 573 B. Borup, R. Edelmann, J. Monkiewicz

Marketable Products Based on Secondary Raw Materials from Organosilicon Production Facilities 581 Anatolii Shapatin

Correlation of the Viscosity and the Molecular Weight of Silicone Oils with the Ti NMR Relaxation Times 584 Joachim Gotz, Horst Weisser, Stefan Altmann

Oligoethylsiloxane Modification 595 Aleksei Gureev, Vladimir Zverev, Tafyana Koroleva, Mikhail Lotarev, Sergei Natsjuk

Comblike Oligosiloxanes with Higher Af-Alkyl Substituents — A Basis for Lubricants of the New Century 600 Mikhail Sobolevskii, Vladimir Zverev, Igor Lavygin, Victor Kovalenko

Permeability of Silicone-Water Interfaces in Water-in-Oil Emulsions 606 Michael A. Brook, Paul Zelisko, Meaghan Walsh

New Textile Softener, Rhodorsil® Hydrosoft 612 Gilles Lorentz, Josette Chardon, Martial Deruelle, Caroll Vergelati

Nature Meets Silicones — Synthesis and Properties of Modern Organomodified Silicones 622 Philipp C. Tomuschat

Organo-Modified Hydropolysiloxanes for Release Control in Silicone Paper Coatings 632 Christine Strissel, Oskar Nuyken, Jochen Dauth, Christian Herzig, Hans Lautenschldger

Catalytic Hydrosilylation of Fatty Compounds 638 Arno Behr, Franz Naendrup, Dietmar Obst

Polycarbosilanes as Precursors of Novel Membrane Materials 641 Hieronim Maciejewski, Piotr Pawluc, Bogdan Marciniec, Ireneusz Kownacki, Wioletta Maciejewska, Mariusz Majchrzak

XVI Contents

Innovative Hybrid Coatings for Facades 645 U. Posset, K Rose

Adhesion of Silicone Coatings to Plastic Films 650 Lesley-Ann O 'Hare, Stuart R. Leadley, Bhukan Parbhoo, John G Francis

Thermoplastic Silicone Elastomers 659 Andreas Bauer, O. Schdfer, J. Weis

LC Silicones Improving the Temperature-Dependent Optical Performance of STN Displays 664 Eckhard Hanelt, Thomas Kammel, Masato Kuwabara

Self-Adhesive Liquid Silicone Rubbers (LSRs) for the Injection Molding of Rigid Flexible Combinations 671 Stephan Bofihammer

Oil-Bleeding Properties of Self-Lubricating Liquid Silicone Rubbers 678 Klaus Pohmer

PDMS-6-PEO Block Copolymers as Surfactants in the Synthesis of Mesostructured Silica: A Theoretical and Practical Approach 689 Dietmar Sturmayr, Josef Bauer, Beatrice Launay, Guido Kickelbick, Nicola Hiising, Anthony P. Malanoski, DhavalA. Doshi, Frank van Swol

Preparation and Properties of Porous Hybrids Silicone Resin for Interlayer Dielectronic Application 696 P. M Chevalier, D. L. Ou, L MacKinnon, K. Eguchi, R. Boisvert, K Su

Control of the Dispersion of Metal Oxide Phases in Silica Gels via Organically Modified Alkoxysilanes 700 Wolfgang Rupp, Gregor Trimmel, Nicola Hiising, Ulrich Schubert

Interaction of Silica Particles in a Model Rubber System: The Role of Silane Surface Treatments 705 Antoine Guillet, Jacques Persello, Jean-Claude Morawski

The Structure of a PDMS Layer Grafted onto a Silica Surface Studied by Means of DSC and Solid-State NMR 715 V. M. Litvinov, H. Barthel, J. Weis

Novel Routes for the Preparation of Nanoporous Silica Particles 736 P. M. Chevalier, D. L. Ou

Particle Size Distribution of Fumed Silica Agglomerates at Low Shear Stress 741 Michael Stintz, Herbert Barthel, Mario Heinemann, Johann Weis

Hydroxylation of Amorphous Fumed Silicas Demonstrated by IGC, Solid-State NMR and IR Spectroscopies 747 H Barthel, H Balard, B. Bresson, A, Burneau, C Carteret, A. P. Legrand

Contents XVII

Fumed Silica — Rheological Additive for Adhesives, Resins, and Paints 752 Herbert Barthel, Michael Dreyer, Torsten Gottschalk-Gaudig, Victor Litvinov, Ekaterina Nikitina

Morphology of Toner-Silica Interfaces 767 Sabine Hild, Herbert Barthel, Mario Heinemann, Ute Voelkel, Johann Weis

Selective Surface Deposition of Colloidal Particles 772 Christian Kruger, Esther Barrena, Ulrich Jonas

Synthesis and Functionalization of Monodisperse Nanoparticles with High Optical Density Based on Inorganic Networks 785 Carsten Blum, Heinrich Marsmann, Klaus Huber, Siegmund Greulich-Weber, Holger Winkler

Oxidation States of Si and Ge Sheet Polymers 789 Gunther Vogg, Martin S. Brandt, Martin Stutzmann

Light-Emitting Properties of Size-Selected Silicon Nanoparticles 797 F. Huisken, G. Ledoux, O. Guillois, C. Reynaud

Spinel-SiAlONs — A New Group of Silicon-Based Hard Materials 808 Marcus Schwarz, Rama S. Komaragiri, Andreas Zerr, Edwin Kroke, RalfRiedel, Gerhard Miehe, James E. Lowther

Aluminosiloxanes as Molecular Models for Aluminosilicates 814 Roisin Reilly

Investigation of Silicone-Modified Photocatalytic Ti02 Formation by Solid-Liquid Reaction and Its Structural Changes under Irradiation 819 Akira Nakabayashi

Author Index 827

Subject Index 833

Introduction: Organosilicon Chemistry - Facts and Perspectives

In Volume IV of “Organosilicon Chemistry: From Molecules to Materials” the editors provided a comprehensive summary on the basics of organosilicon chemistry, dealing in depth with “how to make silicon-containing compounds and how to transfer them into new materials”. Entering a new century expressed by the motto of the 12th International Symposium on Organosilicon Chemistry, Sendai 1999 “Possibilities for the 21st Century” and enhanced by stimulating discussions about ‘Yuture trends in organosilicon chemistry” during the 13th International Symposium in Guanajuato, Mexico (August 25-28, 2002), the editors decided to deal now with some special topics of new developments in organosilicon chemistry: biology and physics. We simply want to encourage the discussion of which directions (0rgano)silicon chemistry will be taking in the future in academia as well as in industry. This ongoing discussion, which might be controversial and which is - for sure - biased, is even more important reflecting the global challenge of the chemical industry and a decreasing public acceptance of chemistry itself, impressively shown by a dramatic reduction of students in chemistry, especially in Germany at the end of the last century.

Fig. I : The Universe of Silicon Technology

Organosilicon chemistry - how did it develop and which way will it go? Organosilicon-based chemistry began in the late 19th century when silicon tetrachloride was transferred into tetraorganosilanes and organochlorosilanes using zinc, mercury or magnesium organyles as reaction partners. This initiating work of chemists such as Frankland, Friedel, Crafts, Pape, and Ladenburg was further improved by the “pioneer” of organosilicon chemistry, Sir Stanley Kipping, who formed silicon-carbon bonds using mixtures of alkali metals and chlorocarbohydrates. Figure 1, “The Universe of Silicon Technology” demonstrates that first silicon-based products entered the market in 1930, and since then an impressive scientific and economic development started, strongly supported by the large-scale production of methylchlorosilanes, especially dimethyldichlorosilane using the Direct Process technology explored by Miiller and Rochow. The triumphant progress of the silicone industry began around 1950. The Miiller-Rochow Process was a fundamental step,

Ovganosilicon Chemisty V Edited by N. Auner and J. Weis

Copyright 0 2003 WILEY-VCH Verlag GmbH & Co. KgaA, Weinheim

2 N. Auner, J. Weis

because up to now no silicon-carbon compounds could be found or identified, which are formed by nature.

According to Figure 1, the overall development is best described by the exponential increase in numbers of patents, publications, commercial products, and sales, which cannot be discussed in this Introduction. This astonishing development is still ongoing. New rapidly developing markets arise in Asia, especially in China and products with new chemical and/or physical properties expand the fields of application. New requirements have to be identified, options have to be discussed, and solutions have to be found: Where are the needs and opportunities of silicon chemistry in the new century? In principle academic basic silicon research and industrial market-oriented research cannot be separated; they profit from and influence each other. Today’s activities in both fields, academia and industry, may be described by the following triangle.

on silicon and of silicon compounds

Analysis of the broad field of academic research activities worldwide leads to a special main focus on the following topics:

The evaluation of the syntheses, the synthetic potential, and the structural features of low- valent silicon compounds, such as silylenes and silicon unsaturated species Si=E, is still under investigation. Exciting results in the past years are now pointing the to the existence of stable silicon triple-bonded compounds SkE; the first pieces of evidence were reported recently. Trivalent cationic silicon ( S f ) and donor- or n-stabilized derivatives are currently under intensive exploration: Reports about their syntheses, structures, and reactivities are an important part of organosilicon conferences worldwide, and more exciting results can be expected in the near future, especially about their catalytic activities. Due to economic requests and environmental needs the direct formation of silicon-carbon from silicon-oxygen bonds (Si-0 activation) either by metal-complex-based catalysis or by chemical synthesis using “unconventional” reaction conditions (e.g. electrochemistry, ultrasonic, or plasma technology), or even using a biological (enzymatic) approach is becoming more and more attractive. In this field academic and industrial research are coming closer together. The synthesis of polysilanes and the subsequent study of their physical properties have led to promising results, thus stimulating researchers to expand the field. The design and synthesis of new tailor-made organosilanes, especially a-organofunctional silanes and their use as building blocks for hybridpolymers with very specific chemical and physical properties will remain an ongoing challenge for academia and industry in the future.

Introduction 3

Theoretical calculations to predict reaction routes as well as the existence and stability of, for example, highly reactive intermediates or compounds is convincingly influencing the preparative work. This is demonstrated by quite a number of exciting papers during the past years. The prediction and interpretation of experimental results by computational methods will become more and more important for future research. Separately, rather complex phenomena and processes such as the understanding of biological activity or of physical material properties are theoretically modeled and described with increasing accuracy, allowing the design of drugs and materials with specific properties. The editors expect an increasing importance of the interaction between theory and practice in these specific fields in the near future.

While in the past silicon-based bulk materials with unique properties were produced using a chemical approach (e.g. new synthetic routes to silicones, variation of functional groups and of crosslinking processes, variation of organic substituents at silicon for the production of different silsesquioxane-based resin materials), today the physical properties of silicon and its compounds with unique structural features are widely used for the design of new materials and their application. Reflecting to increasing energy costs, the conventional energy-consuming carbothermal silicon production process has to be optimized, for example a more efficient heat recovery is required. With respect to energy aspects in general, the use of renewable energy, especially the combination of solar light and photovoltaic or solarthermal technologies, is of increasing importance, and efforts to improve the technical efficiency are requested for future work.

Other areas of increasing interest are listed below: Silicon-oxygen-based cage compounds for electronic applications, catalysis, storage systems, e.g. for hydrogen, nanostructured silicates (“nanotubes”, microreactors). Silicon-based materials with unique (opto)electronic properties; photoluminescent materials for flat panel technology, displays, light-emitting diodes, sensors; electroluminescence, nonmetallic conductors, e.g. siloles, polysilanes, 2,3-diphenyl-l-silacyclobutene chemistry; design and application of liquid crystals. Design and synthesis of hybrid polymers and of dendrimers for special applications and as building blocks for carbosilanes, polysilanes; catalysis and highly crosslinked coatings. Inorganic magnets, e.g. metalla-silsesquioxanes or sol-gel processes in the presence of metal oxides. Zeolites for catalysis and as storage systems. Silica and SiOz-based ceramics, silicon carbide and silicon nitride: improvement of ceramic properties. Communication technology; glass fibers and fabrication of quartz glass goods.

This list cannot claim to be complete and might be continued by other fields of interesting research areas and applications not mentioned. Thus, we would like to encourage all researchers to identify more specific topics of interest.

4 N. Auner, J. Weis

During all ongoing discussions about future silicon technology, the field of bioorganic silicon chemistry or of “Silicon and Life Science” is addressed as one of the most challenging directions for academic and industrial research. What do these areas deal with?

Silicone-based delivery systems for bioactive compounds with controlled release Based on the fact that fine-tuning of the physical and chemical properties of silicones is possible and that silicones are considered to be physiologically inert, the development of biocompatible silicone-based delivery systems for bioactive compounds seems a challenging future task. Biodegradation of silicones Based on the knowledge that biodegradation of organosilicon compounds has been demon- strated in various biological systems and referring to the fact that accumulation of silicones in the environment is currently seriously discussed as an ecological issue, the search for microorganisms that break down silicones is strongly required, especially for applications using silicone emulsions with respect to waste-water purification plants. Biologically active organosilicon compounds In some cases, low-molecular-weight organosilicon compounds exhibit specific biological effects; the biological profile depends on the molecular structure - this might be of interest for practical application. Biological activities might be improved by 0-, N-, S- and C-silylation or by carbodsilicon exchange in existing drugs. Furthermore, new silicon-based drugs can be designed and synthesized, the carbon analogues of which are unknown. Applications in pharmacology, as diagnostics in medicine, as plant-protective agents in agriculture, etc. might be worth studying. Biotransformations of organosilicon compounds Pioneering work during the past years demonstrated that enzymes accept organosilanes as substrates for enzymatic conversions. These conversions are characterized by high chemo-, regio-, diastereo-, and enantioselectivity and mild reaction conditions. In future investigations, the high potential of biocatalysis might be used for the successful preparation of polyfunctional organosilicon compounds with well-defined stereochemistry. Nanostructured silicone- and silica-based materials For exciting and pioneering work in this field, the editors refer to the “Introduction” of Volume IV of this series. Enzymatic silicon-carbon bond formation Nature forms various element-carbon bonds E-C (E = P, As, S, Hg, Co, and even Ge), but surprisingly no silicon-carbon bonds could be identified. Although the Si-C bond is thermodynamically less stable than the Si-0 bond, it is kinetically more stable in aqueous biological systems. The challenge in this extraordinary and important field is the development of a biotechnological process for enzymatic Si-C bond formation for the large-scale production of organosilicon compounds: The “bio-route” to silicones!

Let’s move with enthusiasm to new frontiers of organosilicon chemistry!

July 2003 Pro$ Dr. Norbert Auner, Pro$ Dr. Johann Weis

Reactions of Silicon Atoms - an Access to Unusual Molecules

Giinther Maier, Hans Peter Reisenauer, Heiko Egenolf, Jorg Glatthaar

Institut fur Organische Chemie, Justus-Liebig-Universitat GieBen Heinrich-Buff-Ring 58,35392 GieBen, Germany

Fax: +49 641 9934309 E-mail: Guenther.Maier@org.Chemie.uni-giessen.de

Keywords: matrix isolation, photoisomerizations, co-condensation

Summary: Based on a screening study with a purposeful selection of substrate molecules, the versatile reactivity of silicon atoms under matrix conditions (argon, 10 K) has been studied. The observations can be turned into a general scheme unraveling the characteristic features which govern the wide-ranging potential behavior of silicon atoms.

Introduction

Since the late 1990s, we have been studying the reactions of thermally generated silicon atoms with low molecular weight reactants in an argon matrix. The reaction products were identified by means of IR and UVNis spectroscopy, aided by comparison with calculated spectra. The method turned out to be very versatile and successful [I]. The experimental procedure has been described before [ lh] and is based on the earlier work of Skell [2], Weltner [3], and Margrave [4], the pioneers in this field. The reactions carried out by us so far cover a wide range of substrate molecules (Scheme 1). To assess the potential of silicon atoms, we selected examples which belong to four different groups, namely (n) systems, (n + n) systems, (0 + n) systems, and pure (0) systems.

(n) Systems

Of the partner molecules indicated in Scheme 1 only a few representatives (boxed) are discussed. As far as compounds with carbon-carbon double or triple bonds are concerned, one can differentiate between three subgroups: those with isolated, conjugated, or aromatic (n) systems.

Isolated ( x ) Systems

The reactions of acetylene and ethylene, the parent compounds with isolated n bonds, with silicon

Ovganosilicon Chemisty V Edited by N. Auner and J. Weis

Copyright 0 2003 WILEY-VCH Verlag GmbH & Co. KgaA, Weinheim

6 G. Maier, H. P. Reisenauer, H. EgenolJ; J. Glatthaar

atoms lead to the corresponding n adducts [ 1 b,d]. These primarily formed cyclic silylenes rearrange upon matrix irradiation to the ring-opened isomers, ethynyl and vinyl silylene, respectively (e.g. 1 + 2 + 3 in Scheme 2).

G SYSTEMS f 7c SYSTEMS

(o+n) SYSTEMS

Scheme 1. Substrate molecules treated with silicon atoms in argon at 10 K.

.. /H

Si - S i * hv ..

c'=\, H - C s - s i H .. H - C C C - H "0

2 3 x ADDUCT INSERTION 1

PRODUCT

Scheme 2. Reaction of acetylene with silicon atoms in argon at 10 K.

Reactions of Silicon Atoms - an Access to Unusual Molecules 7

Conjugated (IC) Systems

The addition behavior of a conjugated x system is illustrated by cyclopentadiene 4 (Scheme 3). Co-condensation with silicon atoms yields the [1,4] x adduct 5. Remarkably, no [1,2] addition product is detected. According to calculations the silylene bridge in 5 is strongly coordinated with the double bond. Upon irradiation 5 rearranges to a new isomer, which again has an unusual structure 6. The compound contains a SiH group, but it is not to be regarded as a simple silylene (formal insertion product). Theory demands distinct electron delocalization in the five-membered ring and at the same time a threefold coordination of the silicon atom. In accordance with this structural prerequisite it is possible to establish a photoequilibrium between two exo/endo isomers 6 and 7.

‘“Si hv

S!NH -si - hv # I 4 - -

H 2 C 3 - H*C a \- - hv

INSERTION INSERTION PRODUCT PRODUCT

5 6 7 4

[1,4] x ADDUCT

Scheme 3. Co-condensation of cyclopentadiene (4) with silicon atoms in argon at 10 K.

Aromatic (IC) Systems

Even benzene (8) reacts very cleanly with silicon atoms, again in the specific [l, 41 manner (Scheme 4). Once more, in x adduct 9 the silicon atom is additionally coordinated to one of the two double bonds. The formal C,C insertion product 10, silacycloheptatrienylidene (a planar compound with six delocalized x-electrons) is generated upon photoexcitation of 9.

..

a 9 10

[1,4] 7c ADDUCT INSERTION PRODUCT

Scheme 4. Co-condensation of benzene (8) with silicon atoms in argon at 10 K.

8 G. Maier, H. P. Reisenauer, H. EgenolJ; J. Glatthaar

(x + n) Systems

If the substrate molecule offers a x bond and a free electron pair it is always the lone pair which wins. This has already been shown in the reactions of silicon atoms with hydrogen cyanide (11) [lb,c] (Scheme 5), formaldehyde [lf,g], and nitrogen [lh]. In all these cases the first thermal step is the formation of the n adduct. The corresponding cyclic x adducts were only found on subsequent photochemically induced isomerization of the initial n adducts (e.g. 12 + 13).

.. Si ..

.Si. hv / \

H/C=N H-CEN - 'C-N=Si: -

11 12 13

n ADDUCT n ADDUCT

Scheme 5. Addition of HCN (11) to silicon atoms in argon at 10 K.

(a + n) Systems

Substrates belonging to this group, such as water, methanol, dimethyl ether etc., have been studied intensively [5 ] . Another typical candidate is ammonia (14). The primary n adduct 15 (triplet ground state) is stable enough to be detected by IR spectroscopy and does not spontaneously isomerize into the insertion product 16. However, this rearrangement with formation of aminosilylene 16 occurs upon photoexcitation (h = 436 nm) of 15. Secondary irradiation of 16 (h >310 nm) leads - as is already known [6] - to iminosilylene 17.

16 17 14 15

n ADDUCT INSERTION PRODUCT

Scheme. 6 Addition of ammonia (14) to silicon atoms in argon at 10 K.

(a) Systems

The classical case is the reaction between silicon atoms and molecular hydrogen [la, 4c], which takes place quite rapidly, in spite of the fact that a (r bond has to be broken. However, strong resistance against insertion is found for methane (18). In our hands 18 turned out to be inert under

Reactions of Silicon Atoms - an Access to Unusual Molecules 9

our standard conditions, but the reaction 18 + 19 (Scheme 7) can be enforced by irradiating (h = 185 or 254 nm) the matrix [7]. The photoequilibrium 19 t 20 has been studied previously

PI.

H

18

-s i . , A r , l O K * .. - S i m , Ar, I O K

hv ! 185nm; 254nm

H \

hv 436 nm

Hi& - H ‘H hv 19

INSERTION 254 nm

PRODUCT

20

Scheme 7. Insertion of silicon atoms in methane (18) in argon at 10 K.

Conclusion

Our studies illustrate the great potential of silicon atoms to react with all kinds of substrate molecules. If we take into account all our observations we come to the conclusions summarized in Scheme 8.

A, ISOLATED INSERTION PRODUCTS - xADDUCTS

x SYSTEMS

INSERTION PRODUCTS

‘ONJUGATED - [1,4] x ADDUCTS- x SYSTEMS

INSERTION PRODUCTS - [1,4] x ADDUCTS-

A3 AROMATIC n SYSTEMS

c (o+n) SYSTEMS - n ADDUCTS - INSERTION PRODUCTS

D oSYSTEMS - INSERTION PRODUCTS

Scheme 8. Principal reactions of silicon atoms with different kinds of substrate molecules.

10 G. Maier, H. P. Reisenauer, H. Egenog J. Glatthaar

References a) G. Maier, H. P. Reisenauer, A. Meudt, H. Egenolf, Chem. Ber./Recueil 1997, 130, 1043-1046; b) G. Maier, H. P. Reisenauer, H. Egenolf, in: Organosilicon Chemistry III: From Molecules to Materials (Eds.: N. Auner, J. Weis), VCH, Weinheim, 1998, p. 31-35; c) G. Maier, H. P. Reisenauer, H. Egenolf, J. Glatthaar, Eur. J. Org. Chem. 1998, 1307-131 1; d) G. Maier, H. P. Reisenauer, H. Egenolf, Eur. J. Org. Chem. 1998, 1313-1317; e) G. Maier, H P. Reisenauer, H. Egenolf, Monatsheftefur Chemie 1999, 130, 227-235; f) G. Maier, H. P. Reisenauer, H. Egenolf, Organometallics 1999, 18, 2155-2161; g) G. Maier, H. P. Reisenauer, H. Egenolf, in: Organosilicon Chemistry IV: From Molecules to Materials (Eds.: N. Auner, J. Weis), Wiley-VCH, Weinheim, 2000, p. 64-69; h) G. Maier, H. P. Reisenauer, J. Glatthaar, Organometallics 2000, 19, 47754783; i) Summary: G. Maier, A. Meudt, J. Jung, H. Pacl, in: The Chemistry of Organic Silicon Compounds, Vol. 2 (Eds.: Z. Rappoport, Y. Apeloig), Wiley, New York, 1998, Chapter 19, p.1143-1185. a) P. S. Skell, P. W. Owen, J. Am. Chem. SOC. 1967, 89, 3933-3934; b) P. S. Skell, P. W. Owen, J. Am. Chem. SOC. 1972,94,5434-5438. R. R. Lemke, R. F. Ferrante, W. Weltner, Jr., J. Am. Chem. SOC. 1977,99,416-423. a) J. W. Kauffman, R. H. Hauge, J. L. Margrave, ACS Symp. Ser. 1982, 179, 355-362; b) Z. K. Ismail, R. H. Hauge, L. Fredin, J. W. Kauffman, J. L.Margrave, J. Chem. Phys. 1982, 77, 1617-1625; c) Z. K. Ismail, L. Fredin, R. H. Hauge, J. L.Margrave, J. Chem. Phys. 1982, 77, 1626-1631 d) L. Fredin, R. H. Hauge, Z. K. Ismail, J. L.Margrave, J. Chem. Phys. 1985, 82,

More details about the reactions of silicon atoms with water, methanol, dimethyl ether, and methylchloride are discussed separately (J. Glatthaar, H. P. Reisenauer, H. Egenolf, G. Maier, p. 11 and p. 15 ). a) G. Maier, J. Glatthaar, H. P. Reisenauer, Chem. Ber. 1989,122,2403-2405; b) G. Maier, J. Glatthaar, Angew. Chem. 1994,106,486488; Angew. Chem. Int. Ed. 1994 33,473475. The reactions of silicon atoms with S i b and CHq are presented separately (H. P. Reisenauer, J. Glatthaar, G. Maier, p. 1 1 and p. 15). G. Maier, G. Mihm, H. P. Reisenauer, D. Littmann, Chem. Ber. 1984,117,2369-2381.

3542-3545.

Reactions of Silicon Atoms with Methane and Mane in Solid Argon:

A Matrix Spectroscopic Study

Hans Peter Reisenauer, Jorg Glatthaar, Giinther Maier

Institut fur Organische Chemie, Justus-Liebig-Universitat GieSen Heinrich-Buff-Ring 58, D-35392 GieSen, Germany

Fax: +49 641 9934309 E-mail: hans.p.reisenauer@ org.chemie.uni-giessen.de

Keywords: matrix isolation, photoisomerizations, silicon atoms

Summary: Under matrix conditions (Ar, 10 K) silicon atoms do not react spontaneously with methane. An additional activation by irradiation is necessary to produce silene and methylsilylene. In contrast to the behavior of methane, silane is attacked by Si atoms with formation of disilene and silylsilylene (7-S), even at 10 K. Both compounds have been detected for the first time and can be interconverted photochemically. Photoexcitation of disilene also leads to elimination of HZ yielding disilyne and formation to small quantities of a new isomer, which is tentatively assigned to a doubly bridged structure.

In continuation of our studies on the reactions of silicon atoms with low molecular weight reactants [ 13 we investigated the behavior of methane (1) and silane (5). Silicon atoms were vaporized from a resistively heated silicon rod at ca. 1380 "C and co-condensed with silane or methane and a large excess of argon at 10 to 15 K on a spectroscopic window of a diplex closed-cycle cryostat. The reaction products, which were formed during the condensation andor after subsequent irradiations, were investigated by UVNis and IR spectroscopy. To support the identification of the species formed and to model the reactions, DFT calculations (B3LYP/6-31 l+G**) were performed.

Calculations of the changes of the potential energy during the approach of a Si atom in its triplet ground state to a methane molecule showed a steep increase of energy at distances lower than ca. 3 A. No stable complex (2) between the Si atom and methane could be found in our calculations. On the other hand, the approach to silane (5 ) leads to a successive drop in energy leading to the stable triplet complex 6. This different behavior reflects the electrophilicity of the Si atom and higher hydridic character of the H atoms of silane (5) compared to methane (1).

Indeed, the experiments show that methane (1) does not react spontaneously with Si atoms. But after irradiation of the matrices (h = 185, 254 nm) the well-known IR bands of methylsilylene (4), and after a secondary photolysis with h > 400 nm, of silaethylene (3) could be recorded (Scheme 1)

Ovganosilicon Chemisty V Edited by N. Auner and J. Weis

Copyright 0 2003 WILEY-VCH Verlag GmbH & Co. KgaA, Weinheim

12 H. P. Reisenauer, J. Glatthaar, G. Maier

3Si +

AT

H

1 Ar, 10 K L

h= 185, 254 nrn

-I 2

hv ,

H H h=254nrn .. /H

H H h > 400 nrn

Si-C

H H

\ / Si=C / \ hv , I \''H

3 4

Scheme 1. Reactions of silicon atoms with methane in argon at 10 K.

/<lo Ar, 10 K K

H hv, h = 334 nm - '. / H H \ / ?=Si C-- Si-Si

\ H H H H hv,h>570nm I F H

lHt

'H'

Si-Si + H,

9 10

Scheme 2. Reactions of silicon atoms with silane in argon at 10 K.

Reactions of Silicon Atoms with Methane and Silane in Solid Argon 13

0.6

J! 5 0.4

Fig. 1. UVNis spectra of D4-disilene and D4-silylsilylene. Difference spectrum of the photoisomerization of the

silylsilylene to disilene in argon at 10 K. Positive bands increase, negative decrease during the irradiation

with h > 570 nm.

2000 1500 1000

Wavenumber cm-' 500

Fig. 2. IR spectra of D4-disilene and D4-silylsilylene. Top and bottom: Calculated with B3LYP/6-3 1 l+G**. Middle:

Difference spectrum of the photoisomerization of the disilene to the silylsilylene in argon at 10 K. Positive

bands increase, negative decrease during the irradiation with h = 334 nm.

14 H. P. Reisenauer, J. Glatthaar, G. Maier

On the contrary silane (5) reacts without any additional activation (Scheme 2). The reaction probably proceeds via the initial complex 6 and triplet silylsilylene (7-T), which could not be observed spectroscopically. IR and UVNis spectra, taken directly after the co-condensation, show bands of both silylsilylene (7-S) and disilene (8). Both isomers can be photochemically interconverted using different excitation wavelengths according to their respective UVNis absorption bands (e.g. Drdisilene: I,,,,,, = 329 nm (strong), D4-silylsilylene: I,,,,,, = 582 nm (very weak), Fig. 1). The IR spectra of the parent compounds and the tetradeutero derivatives (Fig. 2) are in good accordance with the calculated spectra. On irradiation of disilene, a side reaction leads to the elimination of hydrogen and the formation of disilyne (9)[la]. An additional IR band at 1355.5 cm-' (1000.0 cm-' for tetradeuteration) is tentatively assigned to the doubly bridged b S i 2 isomer 10 [3].

Acknowledgments: This work was supported by the Fonds der Chemischen Industrie and the Deutsche Forschungsgemeinschaft.

References [ l ] a) G. Maier, H. P. Reisenauer, A. Meudt, H. Egenolf, Chem. Ber./Recueil 1997, 130, 1043-

1046; b) G. Maier, H. P. Reisenauer, H. Egenolf, in: Organosilicon Chemistry III: From Molecules to Materials (Eds.: N. Auner, J. Weis), Wiley-VCH, Weinheim, 1998, pp. 31-35; c) G. Maier, H. P. Reisenauer, H. Egenolf, J. Glatthaar, Eur. J. Org. Chem. 1998, 1307-131 1; d) G. Maier, H. P. Reisenauer, H. Egenolf, Eur. J. Org. Chem. 1998, 1313-1317; e) G. Maier, H. P. Reisenauer, H. Egenolf, Monatsh. Chem. 1999, 130, 227-235; f) G. Maier, H. P. Reisenauer, H. Egenolf, Organometallics 1999, 18, 2155-2161; g) G. Maier, H. P. Reisenauer, H. Egenolf, in: Organosilicon Chemistry IV: From Molecules to Materials (Eds.: N. Auner, J. Weis), Wiley-VCH, Weinheim, 2000, pp. 64-69; h) G. Maier, H. P. Reisenauer, J. Glatthaar, Organometallics 2000, 19,47754783; i) summary: G. Maier, A. Meudt, J. Jung, H. Pacl, in: The Chemistry of Organic Silicon Compounds, Vol. 2 ( Eds.: Z. Rappoport, Y. Apeloig), Wiley, New York, 1998, Chapter 19, pp. 1143-1185. a) G. Maier, H. P. Reisenauer, G. Mihm, Angew. Chem. Int. Ed. 1981, 20, 579-589; b) H. P. Reisenauer, G. Mihm, G. Maier, Angew. Chem. Int. Ed. 1982,2I, 854-855; c) G. Maier, H. P. Reisenauer, D. Littmann, Chem. Ber. 1984, I 17,2369-2381. M. T. Swihart, R. W. Carr, J. Phys. Chem. A 1998,102,785-792.

[2]

[3]

Cryogenic Trapping Reactions of Silicon Atoms: New Insights into the Photochemistry of Complexes of Silicon Atoms with Donor

Molecules

Jorg Glatthaar, Hans Peter Reisenauer, Heiko Egenolf, Giinther Maier

Institute of Organic Chemistry, Justus-Liebig-Universitat Giessen Heinrich-Buff-Ring 58, D-35392 Giessen, Germany

Fax: +49 641 9934309 E-mail: Guenther.Maier@org.chemie.uni-giessen.de

Keywords: matrix isolation, photoisomerizations, co-condensation, direct process

Summary: The reactions between silicon atoms and a series of oxygen-containing donor molecules like water, methanol and dimethyl ether were studied by means of IR and UVNis spectroscopy. General trends in reactivity as well as substituent effects - especially the effect of isotopic substitution - can be derived from the observed photochemical reactions. As a tribute to the famous “direct process” the reaction of atomic silicon and methyl halides was also investigated.

Introduction

In one of the first experiments in this field, the co-deposition of thermally generated silicon atoms with water in an argon matrix at 12 K was investigated by Margrave [l] in 1982. In an earlier exploration by our group [2] of the reaction of atomic silicon with methanol we could only detect the formation of s,cis- and s, trans-methoxysilylene (CH30SiH). Recently Khabashesku [3] has studied the co-deposition of thermally generated silicon atoms and methanol and dimethyl ether.

Silicon Atoms with Water

Quantum mechanical calculations clearly show that s,truns-hydroxysilylene 2b is the key molecule for the formation of silanone. It was missing in the earlier spectra [ l ] and this fact was explained later by quantum mechanical tunneling [4].

Repeating the co-condensation experiment, we observed the formation of the triplet Si/HzO complex 1 (Scheme 1). In the dark - or much faster upon irradiation with h = 534 nm - 1 was transformed into s,cis-2a and into the previously unobserved s, trans-hydroxysilylene 2b by

Ovganosilicon Chemisty V Edited by N. Auner and J. Weis

Copyright 0 2003 WILEY-VCH Verlag GmbH & Co. KgaA, Weinheim

16 J. Glatthaar, H. P. Reisenauer, H. EgenolJ; G. Maier

insertion of the silicon atom into the 0-H bond. The thermal isomerization process is very slow. At a slightly shorter wavelength (h = 366 nm) the isomerization of 2a into 2b was stimulated and besides the known photoproduct, silicon monoxide 3, silanone 4 was generated for the first time. The results were confirmed by isotopic labeling experiments with H180, HDO and D20. All reactions showed strong kinetic isotope effects.

Ar, 10K / 3Si + 0, -

H

1 h = 5 3 4 n m

or in the 'daW

h ~ 4 0 0 n m 0-Sj Si=O + ti, 4 d H

3

I h=254nm

2a

It h = 3 6 6 n m

,H Si=O * 0-Si

2b

Scheme 1. Reaction of silicon atoms with water.

Silicon Atoms with Methanol and with Dimethyl Ether

Methanol

We have re-investigated this reaction in a more detailed study including deuteration experiments. We obtained surprising results on using the DI- or the D4-isotopomer (Scheme 2) . In the f i s t step a stable silicon methanol complex 5 is formed. Adduct 5 shows a strong UV maximum at about 280 nm and a weak absorption around 550 nm. Irradiation at the short-wavelength maximum leads to the formation of methylsilanone 7. Irradiation with h = 580 nm gives hydroxymethylsilylene 6 by insertion of the silicon atom into the 0-C bond instead of insertion into the 0-D bond, which is the only reaction path in the reaction of silicon and CH30H. This compound can be photochemically isomerized further to 1 -hydroxysilene 8. No isomerization to methylsilanone 7 occurs.

Dimethyl ether

By comparison of the calculated and experimental IR spectra it can be shown that the stable dimethyl ether complex of silicon 9, which was formed primarily, has a triplet ground state. The

Cryogenic Trapping Reactions of Silicon Atoms 17

adduct 9 exhibits four UV bands at 280, 330, 380 and 580 nm. Specific transformation into methylmethoxysilylene 10, the main product at 580 nm, or dimethylsilanone 11, the main product at 280 nm, are triggered by selective irradiations using wavelength ranges according to one of the absorption maxima mentioned. Again, methylmethoxysilylene 10 was easily isomerized into silene 12 [5] . The conversion of 10 into silanone 11 was very slow.

-,-. '3, 599

6, lO

7,11 0,12

Scheme 2. General reaction pathways of silicon atoms with HJCOD and H3COCH3.

Silicon Atoms with Methyl Halides

IR:

7 H'

3si + "-c-x

X = CI, Br 13

X = CI: VcI = 679 cm'

X = Br: S,,, = 1418 c m l

H, /x h = 405 nm

H-?\ * c=sj h = 2 M n m H' H

Si

H H

14 15 UVNIS: UVNIS:

X = CI: h,, = 410 nm

X = Br: h, = 425 nm

X = CI: h,= 253 nm

X = Br: h,, = 256 nm

Scheme 3. General reaction pathways of silicon atoms with methyl halides.

18 J. Glatthaar, H. P. Reisenauer, H. EgenoK G. Maier

Faced with the milestone of silicon chemistry, the “Rochow process”, we have also investigated the “direct process” of silicon atoms with methyl halides under matrix isolation conditions. As illustrated in Scheme 3, the formation of the halomethylsilylene 14 [6] was observed, which can be photochemically isomerized to the 1-halosilene 15 [6]. The primary reaction product, the triplet CH3-WSi adduct 13, can be observed directly.

Conclusions

The silicon complexes with oxygen donor molecules show strong kinetic isotope effects. Only the deuterated complexes are stable under matrix isolation conditions. With methanol the insertion reaction is so fast that the complex could not be observed. The reason might be that the insertion reaction in the protonated species is effected by quantum mechanical tunneling. In the case of HDO, only HSiOD is formed.

The photochemical C-0 insertion 5 ---f 6 of the CH30D complex of silicon 5 is remarkable, especially if one bears in mind that, in the case of methanol itself, only 0-H insertion is observed. This is true even under irradiation during the deposition process.

Isolated silicon atoms show a similar behavior compared to the bulk contact mass of the Rochow synthesis. This result may support Okamoto’s observation [7], who was able to show by scavenger reactions that the contact mass must contain highly reactive silicon areas which produce chloromethylsilylene.

References [l]

[2] [3]

[4] [5]

[6]

Z. K. Ismail, R. H. Hauge, L. Fredin, J. W. Kauffman, J. L. Margrave, J. Chem. Phys. 1982,

G. Maier, H. P. Reisenauer, H. Egenolf, Monatsh. Chem. 1999,130,227-235. V. N. Khabashesku, K. N. Kudin, J. L. Margrave, L. Fredin, J. Organomet. Chem. 2000, 595,

T. Taketsugu, N. Watanabe, K. Hirao, J. Chem. Phys. 1999,8,3410-3419. G. Maier, H. P. Reisenauer, K. Schottler, U. Wessolek-Kraus, J. Organomet. Chem. 1989,

a) H. P. Reisenauer, G. Mihm, G. Maier, Angew. Chem. 1982, 94, 864-865; Angew. Chem. Int. Ed. 1981, 21, 854-855; Angew. Chem. Suppl. 1982, 1785-1791; Angew. Chem. Suppl. lnt. Ed. 1982, 21, 1785-1791; b) G. Maier, G. Mihm, H. P. Reisenauer, Chem. Ber. 1984,

M. Okamoto, S. Onodera, T. Okano, E. Suzuki, Y. Ono, J. Organomet. Chem. 1997, 531,

77,1617-1625.

248-260.

366,277-235.

1 17,235 1-2368.

[7] 67-7 1.

New Reactions of Stable Silylenes

Robert West," Daniel F. Moser, Michael Haaf, Thomas A. Schmedake, Ilia Guzei

Organosilicon Research Center, University of Wisconsin-Madison 1101 University Ave., Madison, WI 53706-1396, USA

Fax: +1608 262 6143 E-mail: west @chem.wisc.edu

Keywords: silicon-metal complexes, halocarbons, polymerization

Summary: Some recent chemistry of the stable silylenes, (CHNtBuhSi: (1) and (CH2NtBu)zSi: (2) is reported [l]. The X-ray crystal structure of 1 has finally been determined. Both silylenes react with transition metal carbonyls to displace CO and form silylene complexes. Complexes of 1 and 2 with Cr, Mo, W, Fe, Ru, and Ni have been prepared and studied structurally. Reactions of 1 and 2 with halocarbons yield either simple addition of the C-X bond to the silylene, or 2:l silylene : halocarbon products containing a Si-Si bond, depending on the halocarbon. Silylene 1 catalyzes the polymerization of nearly all compounds containing carbon-carbon double or triple bonds. Possible mechanisms are proposed for several of the reactions described above.

Crystal Structure of the Unsaturated Silylene

When the first stable silylene, 1 (Fig. l), was synthesized in 1994 we were unable to determine its structure by X-ray diffraction, in spite of many attempts. Proof of the structure was obtained from electron diffraction measurements, carried out by Arne Haaland and his group at the University of Oslo [2]. For 2, synthesized shortly afterward, there was no such difficulty and the X-ray structure was promptly reported [3].

Fig. 1. Stable silylenes 1 and 2.

Recently, the crystal structure of 1 has at last been determined [4]. A thermal ellipsoid diagram is

Ovganosilicon Chemisty V Edited by N. Auner and J. Weis

Copyright 0 2003 WILEY-VCH Verlag GmbH & Co. KgaA, Weinheim

20

shown in Fig. 2. The molecular structure provides no great surprises; the bond lengths are very close to those originally reported from electron diffraction. The molecule is fully planar in the solid state, and slightly nonplanar in the vapor phase, due to thermally induced vibrational motion. The packing diagram for 1, displayed in Fig. 3, is striking, with the silylene molecules lined up in perfect columns, head to tail.

R. West, D. F. Moser, M. H Q Q ~ T. A. Schmedake, I. Guzei

b d Fig. 2. Molecular structure of 1.

Fig. 3. Packing diagram of 1.

Silylene-Transition Metal Complexes

Stable silylenes 1 and 2 are isolobal with phosphines, and isoelectronic with the stable carbenes

New Reactions of Stable Silylenes 21

synthesized by Arduengo and co-workers [ 5 ] , which form many transition metal complexes [6] . The silylenes can also act as Lewis bases toward transition metals, and produce various silylene-metal complexes.

Earlier we reported the reaction of 1 with Ni(C0)4, in which two carbon monoxide molecules are displaced to yield the tetrahedral complex, (1)2Ni(C0)2 [7]. When bis(cyc1ooctadiene)nickel is treated with 1 or 2, both cyclooctadiene molecules are lost and the trigonal planar complexes (IhNi and (2)3Ni are obtained [8]. The structure of (1)3Ni is shown in Fig. 4. This trigonal structural arrangement for nickel complexes is unusual, but not unprecedented.

Fig. 4. Crystal structure of (1)3Ni.

Silylene 1 reacts with Fe(C0)S or Fe2(CO)9 to give the complex (I)Fe(CO)4, also reported earlier [2]. With ruthenium pentacarbonyl, however, 1 reacts with removal of two CO groups, yielding (1)2Ru(C0)3 [9]. This complex has an unusual unit cell containing two different structures. As shown in Fig. 5, both kinds of molecules have essentially a trigonal bipyramidal geometry, but in half of the molecules two CO groups occupy the (near) axial positions, and in the other half the silylenes take the (near) axial sites.

With chromium, molybdenum, and tungsten carbonyls, 1 and 2 displace two CO molecules and form the trans complexes, (1)2M(C0)4 and (2)2M(C0)4 (M = Mo, Cr and W). As an example, the structure of (1)2Cr(CO)4 is shown in Fig. 6. To provide a measure of the strength of these ligands as electron donors, the C=O stretching frequencies for these complexes were determined by IR spectroscopy [9]. The frequencies for the molybdenum complexes, and for a number of isostructural Mo compounds, are shown in Table 1. The data indicate that toward Mo(CO)4 as a reference acid, 1 is about equal to triphenylphosphine in donor ability, while 2 is slightly weaker, more resembling a trialkoxyphosphine. The stable carbene isostructural with 1 is an extremely powerful Lewis base toward molybdenum, however, surpassing even trialkylphosphines.

22 R. West, D. F. Moser, M. H m J T. A. Schmedake, I. Guzei

Fig. 5. Crystal structure of (l)zRu(C0)3.

Fig. 6. Molecular structure of (1)2Cr(CO)4.

Table 1. Infrared C=O stretching frequencies for molybdenum complexes, irans-kMo(C0)4.

Compound IR (cm-')

[a] 7 is the stable carbene isostructural with 1.

New Reactions of Stable Silylenes 23

Silylene complexes with more complex structures have not yet been investigated in any detail. One reaction of this type which has however been studied is that between 1 and the dinuclear complex, [C~ZP(CH,)~PC~~]RU-C~~-RUC~~P(CHZ)~PC~Z] [lo]. The surprising product 3 is obtained, in which a new Ru-C bond has been formed. A possible pathway to 3 is illustrated in Scheme 1.

Scheme 1.

Reaction of Stable Silylenes with Halocarbons

The reactions of stable silylenes with halocarbons have given quite unexpected results. With chloroform, 1 reacts even at -70 "C, to give a single product, the disilane 4 (Scheme 2). No 1:l adduct is produced, even in the presence of a 100-fold excess of chloroform! Analogous reactions were observed for 1 with CH2C12 and CC4, and for 2 with CHC13.

R I

c>si:

I R

Scheme 2.

24 R. West, D. F. Moser, M. Huu. T. A. Schmeduke, I. Guzei

Equally surprising was the reaction of 1 with t-butyl chloride, also shown in the scheme. Again only a single product was rapidly obtained, in quantitative yield, but this time it was the 1:l adduct of simple insertion into the C-Cl bond.

Chlorobenzene did not react with 1, even when the mixture was heated. However with bromobenzene as the substrate the reaction proceeded slowly, yielding both types of products. In the presence of excess bromobenzene only the 1: 1 monosilane compound 5 was obtained, but with excess silylene the disilane 6 was the major product (Scheme 3).

Scheme 3.

I

c>i: - Ph-Br

I tBu

1

Ratio, Ph-Br : 1

4 :1

1:l

1 :2

9

tBu leu 5 8

% 5 % 6

99

67 33

32 68

/I ,3 shift

R R

Scheme 4.

New Reactions of Stable Silylenes 25

How can these reactions be explained? The reaction with t-butyl chloride cannot be a simple nucleophilic displacement, which should have yielded isobutene. Nor can it be a single-electron transfer, free radical reaction, which should have produced many side products. A clue may come from the reactions of the carbene analogous to 1, which forms weak complexes with some halogen compounds, in which the carbene acts as a Lewis base and the halogen as a Lewis acid [12].

Imagine that the silylene similarly forms a weak complex with the halogen in these halocarbon compounds, as shown in Scheme 4. Removal of charge from silicon to halogen would make the silicon atom slightly Lewis acidic [13]. Another silylene molecule could approach and, acting as a Lewis base, weakly bond to the first silicon. Now, a 1,3-shift could generate the disilane product. This process is illustrated for CCl4 in the scheme. To account for the 1:l insertion products, as with t-butyl chloride, we suppose that there is no participation of a second silylene, for steric or electronic reasons or both. Instead a 1,Zshift takes place from the 1:l acid-base complex, yielding the insertion product.

Obviously, this mechanism is speculative, and must be tested through further experiments and by theoretical calculations.

Silylenes as Polymerization Catalysts

Earlier we reported that 2 reacts as a typical silylene with 2,3-dimethylbutadiene, to give the spirocyclic adduct 5 (Eq. 1) [14].

tBU tBu I

Eq. 1.

For 1, the expected cyclic product was not obtained, instead the product appeared to be polymeric. This led us to try the reaction of 1 with simple olefins. To our surprise, the reaction of 1 with 1-hexene also yielded a polymer, poly( 1-hexene). Moreover, experiments showed that the process is a catalytic one, in which only a small amount of silylene is needed.

Silylene 1 is an unusually versatile catalyst for alkene and alkyne polymerization. The list of compounds polymerized by 1 includes ethene, propene, 1-hexene, styrene, dimethylbutadiene, vinylidene chloride, vinyl ethyl ether, methyl methacrylate, and phenylacetylene. The polymerization does not seem to take place by any of the usual mechanisms, anionic, cationic or free-radical. Instead it somewhat resembles coordination polymerization, as observed for Ziegler- Natta type catalysts. Silylene 2 also catalyzes the polymerization of 1-hexene, but the polymerization is 10 to 100 times slower than with 1.

26 R. West, D. F. Moser, M. Haaf; T. A. Schmedake, I. Guzei

The polymers produced so far, after workup in air, are insoluble, which has made complete characterization difficult. In all our initial experiments we have used 4 mol% of silylene, much of which can be recovered unchanged after polymerization is complete.

However, some of the silylene is apparently introduced into the growing polymer chain. Upon workup the Si-N bonds may hydrolyze to Si-OH groups, which lead to crosslinking and consequent insolubility. We are now taking steps to prevent such crosslinking, and to further characterize the polymers.

Acknowledgment: This work was supported by grants from the National Science Foundation, and by the sponsors of the Organosilicon Research Center at the University of Wisconsin.

References [l] For reviews on stable silylenes, see: M. Haaf, T. A. Schmedake, R. West, Acc. Chem. Res.

2000,33,704; B. Gehrhus, M. F. Lappert, J. Organomet. Chem. 2001,617,269. [2] M. Denk, R. Lennon, R. Hayashi, R. West, A. V. Belyakov, H. R. Verne, M. Wagner, N.

Metzler, J. Am. Chem. SOC. 1994,116,2691. [3] R. West, M. Denk, PureAppl. Chem. 1996,68,785. [4] D. F. Moser, I Guzei, R. West, Main Group Metal Chem. 2001,24, 891. [5] A. J. Arduengo 111, Acc. Chem. Res. 1999,32,913. [6] W. A. Henmann, T. Weskamp, V. P. W. Bohm, Advan. Organomet. Chem. 2002,48,1. [7] M. Denk, R. Hayashi, R. West, J. Am. Chem. SOC. 1994,116, 10813. [8] T. A. Schmedake, M. Haaf, B. J. Paradise, D. R. Powell, R. West, Organometallics 2000, 19,

3263. [9] M. Haaf, T. A. Schmedake, B. J. Paradise, A. J. Millevolte, D. R. Powell, R. West, J.

Organomet. Chem. 2001,17,636. [lo] D. Amoroso, M. Haaf, G. P. A. Yap, R. West, D. E. Fogg, Organometallics, 2001,21,534. [ l l ] B. Gehrhus, P. B. Hitchcock, M. F. Lappert, H. Maciejewski, Organometallics 1998, 17,

5599. [ 121 A. J. Arduengo 111, Paper presented at 2000 International Chemical Congress of Pacijic Basin

Societies, Honolulu, HI, Dec. 14-19,2000, INOR 430. [13] B. Gehrhus, P. B. Hitchcock, M. F. Lappert, J. Chem. SOC. Dalton Trans. 2000,3094. [14] M. Haaf, T. A. Schmedake, B. J. Paradise, R. West, Can. J. Chem. 2000, 78,1526.

Insertion Reactions of the Stable Silylene Si[ (NCHiBu)&H4- 1,2]

Floriu Antolini, Xiuoping Cai, Barbara Gehrhus, * Peter B. Hitchcock, Michael F. Lappert, * Massimo Parrucci, J. Chris Slootweg

The Chemistry Laboratory, University of Sussex, Brighton, BN1 9QJ, UK Tel.: +44 01273 606755 -Fax: +44 01273 677196

Email: B.Gehrhus @ sussex.ac.uk

Keywords: insertion, silylene, amino-functionalised silyl anions

Summary: The silylene S ~ [ ( N C H ~ ~ B U ) Z C ~ ~ - ~ , ~ ] 1 inserts readily into the Li-C, Li-Si or M-N (M = Li, Na or K) bond of the lithium alkyl LiR [R = Me, ‘Bu or CH(SiMe3)2], the lithium sisyl Li[Si(SiMe&], or the alkali metal amide MNR’R” (M = Li, Na or K, R’ = R” = SiMe3; M = Li, R’ = SiMe3 and R” = 2,6-Me2C&, or R’ = R’ = Me or ‘Pr) to yield the corresponding amino-functionalized lithium silyl for LiR, Li[Si(SiMes)s] and LiNR’R” (R‘ = R” = Me or ‘Pr), but alkali amide for MNR’R” (R’ = R“ = SiMe3). Insertion of 1 into the Li-N bond of the lithium enamide Li[N(SiMezOMe)(AdC=CHSiMe3)] (Ad = 1-adamantyl) affords a new azatrisilacyclobutane. The mechanism of the reactions will be discussed.

Introduction

In recent years the chemistry of silylenes has experienced major developments following the isolation of thermally stable bis(amino)silylenes [ 1, 21. The bis(amino)silyleneSi[(NCH2fBu)2C6H~- -1,2] 1 was shown to undergo typical silylene reactions [3] such as insertions or additions. Additionally, the silylene 1 behaves as a ligand in transition metal complexes and can react as a reducing reagent. A summary of the diversity of its reactions is published in a recent review [2].

In this article we focus solely on new insertion reactions of the silylene Si[(NCH2fBu)zC6H4-1,2] 1 [abbreviated as Si(NN)]. Reactions previously reported include insertions of 1 into 0-H, C-I, Gel1<, Ge*’-N, Ge’I-0, Sn”-C, Sd-N, Sn”-0, Pbn-N, Pd-Cl and Pt-C1 bonds (see refs. in Ref

[21). Now we report the extension of silylene insertion reactions into the domain of lithium alkyls LiR

[R = Me, ‘Bu or CH(SiMe&] and alkali metal amides MNR’R’’ (M = Li, Na or K, R’ = R” = SiMe3; M = Li, R’ = SiMe3 and R” = 2,6-Me2C&, or R’ = R” = Me or ‘Pr). Further we present the reaction of 1 with the lithium sisyl compound Li[Si(SiMe&] and the lithium enamide Li[N(SiMezOMe)(AdC=CHSiMe3)] (Ad = 1-adamantyl).

Ovganosilicon Chemisty V Edited by N. Auner and J. Weis

Copyright 0 2003 WILEY-VCH Verlag GmbH & Co. KgaA, Weinheim

28 F. Antolini, X . Cai, B. Gehrhus, P. B. Hitchcock, M. F. Lappert, M. Parrucci

Insertion into a Compound Containing an LCC Bond

The silylene 1 inserted under very mild conditions into the Li-C bond of the appropriate lithium alkyl LiR [R = Me, 'Bu or CH(SiMe3)zI to afford the colourless, crystalline, X-ray-characterized, new silyllithium compounds 2-4 in good yield (ca. 80 %) (Scheme 1) [4]. Each was thermally stable at ambient temperature, with 2 (mp 2 41 "C) the most labile. This robustness is in contrast with previously reported amino-functionalized silyllithium compounds, which are only stable below 0 "C [cf. Li( Si(NEtz),Ph3,)1 [5,61.

Np = CHiBu

2 R = M e EtzO; n = 2

3 R='Bu thVEt20; n = 1:l and 3:O

4 R = CH(SiMe3)z EtzO; n = 2

Scheme 1.

The 29Si NMR chemical shifts for 2-4 were at unusually high frequency [6 53.8 (2), 47.5 (3), 48.0 (4)] compared with values for covalent SiN compounds [cf. (NN)Si(I)Me, 6 -8.91 and highest for 2, consistent with the weaker electron-releasing effect of Me than 'Bu or CH(SiMe&. These data are also available for comparison with those for LiSiPhz(NEt2) (6 19.3), LiSiPh(NEt2)Z (6 27.9), ClSiPhz(NEt2) (6 -6.8) and ClSiPh(NEt2)z (6 -18.8) [5, 61. It has been suggested that amino substituents at the silicon atom have a deshielding effect on 6[29Si], a view supported by computational data on appropriate silyllithium compounds [7].

Insertion into a Compound Containing an Li-Si Bond

The reaction of the silylene 1 with the lithium sisyl [Li{Si(SiMe3)3}(thf)3] proceeded under very mild conditions to yield the yellow, crystalline, X-ray-characterized compound 5 in good yield (80 %) (Scheme 2) [4,8].

Variable-temperature 29Si NMR spectra of the sisylsilyllithium compound 5 revealed that it readily dissociated into its factors in solution. Thus at 298 K in tolueneltoluene-dg the 29Si NMR chemical shifts were at 6 91.1, -5.8 and -180.75, which are close to the values of 1 (6 96.9) and [Li(Si(SiMe3)3)(thf)3] (6 -5.3 and -185.4) [9]. Upon cooling the solution of 5 to 198 K, the 29Si signals were observed as a quartet at 6 59.9 [1J(29Si-7Li) = 63 Hz] and singlets at 6 -9.9 and -154.5, assigned to the silicon nuclei a-, 'y- and p- to the lithium atom of 5, respectively.

Insertion Reactions of the Stable Silylene Si[(NCH2'Bu)2C&-l,2] 29

in solution - - (NN)Si + [Li( Si(SiMe3)3}(thf)2I /si(siMe3)3

\ (NN)Si

5 Li(thf)2

Scheme 2.

Insertion into a Compound Containing an M-N Bond (M = Li, Na or K)

The reaction of 1 with M[N(SiMe&] (M = Li, Na or K) under mild conditions did not lead to the expected silyllithium compound (A) but instead to the colorless, crystalline, X-ray- characterized, new alkali metal amides 6-8 in good yield (60-90 %) (Scheme 3) [8]. The 29Si NMR data for 6-8 confirmed the proposed structures. The chemical shifts for the silicon atom a- to the alkali metal M are at 6 -14.8 (6 and 7) and -18.9 (8) and are consistent with a covalent Siw compound. Additionally, the SiMeJ groups appeared at 6 -16.7 and -25.5 (6), -19.4 and -25.0 (7) and at -22.8 and -26.1 (8).

a $ s i N

1 Np

M-N(SiMe,),

thf, -30 T

6 M=Li, x = 2 , n = 1 7 M = N a , x = 3 , n = l S a M = K , x = O , n = o o S b M = K , x = 3 , n = 1

n

6 - 8

Scheme 3.

30 F. Antolini, X . Cai, B. Gehrhus, P. B. Hitchcock, M. F. Lappert, M. Parrucci

In the reaction of 1 with LiN[(Ar)SiMe3] (Ar = 2,6-MezCsH3) it was possible to isolate the silyllithium compound 9 (29Si NMR at 6 15.2 and -4.5) (Scheme 4), which is related to the intermediate A in Scheme 3. Compound 9 was not thermally stable. Heating a C6D6 solution of 9 for several hours, converted the silyllithium compound into its corresponding lithium amide 10, as shown by 'H NMR spectra.

Ar = 2,6-M%C6H3 I.,. Scheme 4.

The reaction of 1 with LiNR2 (R = Me or 'Pr), however, led to the isolation of the colorless, crystalline, X-ray-characterized, new silyllithium compounds 11 and 12 in 60 and 80 % yield, respectively (Scheme 5). The 29Si NMR signals were typically (cf. the second section above) shifted to higher frequency at 6 29.6 (11) and 15.6 (12).

11 R = Me, 12 R = 'Pr

Scheme 5.

The silylene 1 inserted also into the Li-N bond of the enamide 13, which led surprisingly to the isolation of the colorless, crystalline, X-ray-characterized azatrisilacyclobutane 14 in 50 % yield (Scheme 6) [lo]. Compound 13 was formed by reaction of the a-hydrogen-free nitrile, 1-adamantyl cyanide, with the chiral bis(sily1)methyl compound Li[CH(SiMezOMe)(SiMe3)] [ 11, 121.

lnsertion Reactions of the Stable Silylene Si[(NCH;Bu)2Cd-€4-1,2] 31

MeO-SiMe,

MeOMe Si AdCN '*i

Et20. 25 "C Me3Si

13

(-LiOMe)

N NP

[E Si(NN)]

1

Scheme 6.

Proposed pathways

The pathways to compounds 2-12 probably require that in the first step the silylene 1 behaves towards MX as a nucleophile yielding an intermolecular donor-acceptor adduct B, followed by subsequent intramolecular insertion of either 1 into the M-X bond or X into the Si-M bond (Scheme 7), resulting in C.

a 5 s i + M X - B w, N

NP MX = alkali alkyl, sisyl or amide

Scheme 7.

32 F. Antolini, X. Cai, B. Gehrhus, P. B. Hitchcock; M. F. Lappert, M. Parmcci

For X = N(SiMe&, however, C is only the intermediate to D (cf. intermediate A in Scheme 3) and undergoes a further Me& shift from N to Si.

In one case, namely for the reaction of 1 with LiN[(Ar)SiMe3] (Ar = 2,6-Me2C~H3), the migration of the SiMe3 was slow at ambient temperature and compound 9, an analogue of C bearing SiMe3 at N, was isolated. The reason for this is evident by comparing the X-ray structures of 9 and, e.g., 12 (Fig. 1).

Fig. 1. X-ray structure of 9 (left) and 12 (right).

Although there is a planar environment at N(3) in both complexes, the angles that encompass N(3) are significantly different. In 9 the Si-N(3)Si and Si-N(3)-Cipso angles are much wider and narrower, respectively, compared to analogous angles in 12. The aryl substituent in 9 is therefore in closer proximity to the Li atom (Li Cv, 3.28/3.35 A) and this may be the reason for the stability of 9.

Fig. 2. Intermediates E and F.

Insertion Reactions of the Stable Silylene Si[(NCH~Bu)~C&-l,2] 33

We anticipated a similar outcome for the reaction of 1 with the lithium enamide 13; this would have led to the formation of the insertion product E. We suggest that E was formed in the first instance but readily extruded LiOMe, generating the azadisilacyclopropane F (Fig. 2), which in turn underwent insertion of a further molecule of 1 to yield the azatrisilacyclobutane 14.

Acknowledgments: We thank the EPSRC for an Advanced Fellowship for B.G., the Royal Society for a K. S. Wong fellowship for X.C., the EU and later the University of Bologna for provision of a studentship for F.A., the Toso-Montanari Foundation of the University of Bologna for support for M.P. and the Free University of Amsterdam for support for J.C.S.

References [l] M. Haaf, T. A. Schmedake, R. West, Acc. Chem. Research 2000,33,704. [2] B. Gehrhus, M. F. Lappert, J. Organomet. Chem. 2001,617,209. [3] P. P. Gaspar, R. West, in The Chemistry of Organosilicon Compounds, Eds. Z. Rappoport, Y.

Apeloig, Wiley, New York, 1998. [4] X. Cai, B. Gehrhus, P. B. Hitchcock, M. F. Lappert, J. C. Slootweg, J. Organomet. Chem.

2002,643,272; erratum, J. Organomet. Chem. 2002,651,150. [5] A. Kawachi, K. Tamao, Bull. Chem. SOC. Jpn. 1997, 70,945. [6] A. Kawachi, K. Tamao, J. Am. Chem. SOC. 2000,122,1919. [7] C. Strohmann, 0. Ulbrich, D. Auer, Eur. J. lnorg. Chem. 2001, 1013. [8 ] B. Gehrhus, P. B. Hitchcock, M. F. Lappert, J. C . Slootweg, Chem. Commun. 2000, 1427. [9] G. Gutekunst, A. G. Brook, J. Organomet. Chem 1982,225, 1. [lo] F. Antolini, B. Gehrhus, P. B. Hitchcock, M. F. Lappert, Angew. Chem. Znt. Ed. 2002, 41,

2568. [ 111 T. F. Bates, S. A. Dandekar, J. L. Longlet, K. A. Wood, R. D. Thomas, J. Organomet. Chem.

2000,595, 87. [12] F. Antolini, P. B. Hitchcock, M. F. Lappert, X.-H. Wei, Organometallics 2003, in press.

A Model System for the Generation of Silyl Cationic Species of Different

Reactivity and Stability

Thomas Miiller

Institut fur Anorganische Chemie, Johann Wolfgang Goethe-Universitat Frankfurt Marie Curie-Str. 11, D-60439 Frankfurt am Main, Germany

Tel.: +49 69 79829166 - Fax: +49 69 79829188 E-mail: dr.thomas.mueller@chemie.uni-frankfurt.de

Keywords: carbocations, computational chemistry, NMR spectroscopy, reactive intermediates, silyl cations, P-silyleffect, quantum mechanical calculations

Summary: On the basis of the results of quantum-mechanical investigations a model system, 1, is suggested which allows the synthesis and the investigation of highly electrophilic silyl cationic species. The cations 1 are stabilized by intramolecular interaction with different electron-rich donor substituents X in a constant molecular environment. This model provides the possibility to compare the different stabilizing ability of the ligands X, which is the first step on the way to silyl cations with controlled and tunable reactivity. The applicability of this model system is demonstrated for three different stabilizing groups X (X = aryl, alkynyl and hydrogen).

Introduction

The recent report on the solid-state structure of trimesitylsilylium by Reed, Lambert and coworkers [lo] has finally resolved the so-called “silylium ion problem” [l], the quest for a chemistry of cationic silicon, however, remains. Reactions with trivalent silylium ions as intermediates are extremely rare in organosilicon chemistry and, furthermore, silylium ions, once generated, are highly reactive, leading to irreversible formation of products in not always clearly defined reaction sequences [ 2 ] . The major challenge is therefore to reduce the high electrophilicity of silylium ions in order to control and to exploit their reactivity, for example in Lewis acid-catalyzed transformations in organic synthesis. The concept we follow here utilizes the reversible intramolecular reactions of transient silylium ions R3Si+ with electron-donating groups X to synthesize stable silyl cationic species with different and, as a future and desirable aspect, controlled electrophilicity and reactivity.

For a systematic study a model system is required which should meet several prerequisites:

Ovganosilicon Chemisty V Edited by N. Auner and J. Weis

Copyright 0 2003 WILEY-VCH Verlag GmbH & Co. KgaA, Weinheim

A Model System for the Generation of Silyl Cationic Species 35

the intramolecular stabilized cation should have a constant molecular framework, which allows a straight forward comparison of various donor substituents X; the steric demand for the formation of the intramolecular stabilized species should be small; from a synthetic point of view it is advantageous for the stabilizing group X to be easily exchangeable.

Results and Discussion

The Model

Our model system which combines all these requirements is the cyclic cation 1. In cation 1 (Fig. 1) the positive charge is divided between the two silicon atoms and the intramolecular stabilizing ligand X. The nature of X determines the extent of the charge transfer from one silicon atom via the X-group to the second silicon atom.

Me,Si, ,SiMe, X

1

Fig. 1. The cyclic cation 1.

Cations 1, with different substituents X, can be synthesized by hydride transfer reactions [3] from 2,5-disilaheptanes 2. The reaction of 2 with one equivalent trityl tetrakis(pentafluoropheny1) borate (TPFPB) in benzene at room temperature yields the trivalent silylium ions 3 and triphenylmethane (Scheme 1). The silylium ions 3 are only transient species and undergo intramolecular cyclization which yield the cations 1.

Me,Si-SiMe, Me,Si, ,SiMe,

X

Me,Si-SiMe, TPFPB

I X

I I H X

2

Scheme 1. Synthesis of cations 1.

3 1

The 2,5-disilahexane framework can be synthesized in a few steps from basic materials (Scheme 2). Key compounds in this synthetic scheme are the chlorides 4 and 5. Starting from these compounds nearly every X-group can be introduced into the 2,5-disilaheptane framework.

36 T. Miiller

Me2Si -SiMe2

I I Me,Si -a

5 C’ H

4 I H

2. Me2SiCIX L Me,Sf-SiMe2 I I li X

2

Scheme 2. Synthetic routes to 2,6-disilaheptanes 2.

nl+ Me2Si, ,SiMe,

X

1 ---

Me,Si, n ,SiMe, O1+--. X 7

Fig. 2. Energetic relation between the cations 1,3,6 and 7.

A comparative computational study [4] for different substituted cations 1 provided important insights in the stability and the existence of these species in solutions prior to any experimental investigation. The four energy terms EA-ED determine whether cations 1 can exist in solution (Fig. 2). The most important term is the intramolecular stabilization energy, EA, which is the energy difference between the trivalent silylium ion 3 and the cyclic cation 1. EA provides a direct measure

A Model System for the Generation of Silyl Cationic Species 37

for the relative stabilization ability of different substituents X. The terms EB and EC describe the interaction between the cations 3 and 1, respectively, and the solvent. In this particular case, with benzene as solvent, the terms EB and Ec are calculated using the structure and energies of the benzenium ion 6 and aggregate 7. Finally, the energy difference ED determines which species, 6 or 7, prevails in solution.

Table 1. Calculated energies EA - ED for differently substituted cations 1

(B3LYP/6-31G(d), in kJ mol-'; see also Fig. 2.

Entry X

1 H

2 Ph

3 vinyl

4 =Me

5 s P b

6 F

I c1

8 OMe

9 SMe

10 NMez

11 PMez

12 SiMe,

EA EB EC ED

-106.7 -85.8 -17.6 -38.5

-109.6 -95.8 -17.2 -3 1 .O

-101.0 -101.0 -22.3 -22.3

-143.9 -96.7 -18.0 -65.3

-182.8 -96.2 -15.1 -101.7

-145.2

-116.3

-200.4

-182.0

-219.7 -90.4 -18.8 -148.1

-238.1

+18.4

From the computed results for various cations 1 with different substituents X the following conclusions can be drawn (see Table 1).

The enormous electrophilicity of the open silylium ions 3 results in a strongly negative intramolecular stabilization energy EA for nearly all substituents X, i.e. the cyclic cations 1 are markedly more stable than their trivalent isomers 3. An interesting exception is the trimethylsilyl group (Table 1, entry 12), for which the computations predict a significant destabilization of the cyclic cation 1 (X = SiMe3) relative to the open ion 3 (X = SiMe3). Although the energy of formation for the benzenium ions 6, EB, is strongly negative, the reaction is less exothermic than the intramolecular reaction for nearly all substituents X. For all these cases the formation of the cyclic ion 1 is favored in benzene solution. The small association energy Ec predicted for the cations 1 suggests that at ambient conditions no significant cation-solvent interactions are present. This is supported by calculations which take entropy and temperature effects into account. These calculations show, for example, that the formation of 7 (X = Ph) is endergonic at 298 K (AG298 (7, X = Ph) = +14.6 kJ mol-', at B3LYP/6-31G(d)).

38 T. Miiller

The size of EB as well as of Ec is nearly independent of the substituent X for all cations investigated (see Table 1). Consequently, knowledge of the intramolecular stabilization energy EA already allows the prediction of which cation, the cyclic 1 or the benzenium ion 6, prevails in benzene solution. The formation of stable Si-X bonds and the stability of the resulting onium ions of the group 15 and 16 elements contribute to the high negative values for EA of cyclic cations 1 with n- donors as stabilizing substituents X, as for example X = NMe2, PMe2, OMe, SMe. Remarkably high negative values for EA are also calculated for substituted arenes and alkynes, and even for a weak o-donor like the Si-H bond the formation of a stable cyclic cation 1, X = H is predicted by the calculations.

Aryl Groups as Intramolecular Stabilizing Substituents: Formation of Bissilylated Arenium Ions [5]

Treatment of 2-aryl-2,6-disilaheptanes 8 with one equivalent of TPFPB in benzene results in the exclusive formation of the bissilylated arenium ions 9 (Scheme 3).

8a-g

n

9a-g

R' R' R3 R4 R5

H H H H H

CH3 H H H H

CH3 CH3 H H H

H CH3 H H CH3 H CH3 CH3 H H H H H CH, CHB

CH3 CH3 CH3 H H

Scheme 3. Synthesis of bissilylated arenium ions 9.

The arenium ions 9 are identified by their characteristic NMR spectra. One single line in the 29Si Nh4R spectra (629Si = 19.1-25.6) indicates in each case the formation of a symmetric species. The downfield shift of the 29Si NMR signal, A6"Si, which occurs upon ionization of 8, is significant (ASZ9Si = 22-29 for the Si-aryl group) and indicates an accumulation of positive charge at silicon. A629Si is clearly dependent on the electron-donating ability of the aryl group, that is, A629Si is largest for the benzenium ion 9a (A629Si = 29.7) and smallest for the mesitylenium ion 9g (A629Si = 23.7). For the benzenium ion 9a, nearly the same 29Si NMR chemical shift is measured in [Dgltoluene as in [Dslbenzene (629Si = 26.9 and 25.6, respectively), indicating negligible solvent-cation interactions. The typical I3C NMR chemical shift pattern for arenium ions was found for each of the cations 9a-g. The ortho and para carbons of the aryl substituents in cations 9a-g (613C0"'h0 = 160.9-182.0, 613Cpara = 150.2-169.9) are strongly deshielded relative to the starting silanes (A6I3Coflho = 39.6-28.9, A6'3Cp"ru = 35.1-21.6), while the metu carbons are only slightly affected by the ionization (613Cmra = 132.5-146.9, A613C"'" = 9.9-6.3). The strong highfield

A Model System for the Generation of Silyl Cationic Species 39

shifts of Cipso signals relative to the precursor silanes (S'3Cip" = 89.0-102.9, d613Cip" = -36.5 to 49 .4 ) are consistent with the rehybridization of Cip" from sp2 to sp3, which occurs upon addition of the silyl cation to the aryl ring. These characteristic changes in the I3C NMR chemical shifts of the arene substituents indicate the delocalization of positive charge into the aryl substituent as rationalized by the dienyl cation-like representations 9A,B (Scheme 4). Comparison of the I3C NMR data for the bissilylated arenium ions 9 with reported data for their non-silylated counterparts clearly reveal the importance of the "no-bond'' resonance structure 9C to the ground-state structure of the cations 9 (Scheme 4) [5 ] .

- R R R

9A 9B 9c

Scheme 4. Schematic representation of the delocalization of positive charge in arenium ions 9.

The P-stabilizing effect of two silyl substituents confers to the arenium ions 9 a high thermodynamic stability, i.e. the results of density functional calculations suggest that the bissilylated benzenium ion 9A is more stable than the parent benzenium ion (C6H7') by 157.2 kJ mol-' (at B3LYP/6-31 lG(d,p)//B3LYP/6-31G(d) + AZPVE).

Intramolecular Addition to C=C Triple Bonds: Formation of Unusually Stable Vinyl Cations [61

Vinyl cations were established as short-living reaction intermediates in the 1970s [7] and only in the 1990s were some specific examples of this highly intriguing class of carbocations synthesized in superacidic media and characterized by NMR spectroscopy at temperatures below -100°C. At higher temperatures rapid decomposition of the vinyl cations was observed [7b,c]. In contrast, reaction of the alkynylsilanes 10 with one equivalent of TPFFB in benzene solution at r.t. results in the clean formation of the vinyl cations 11 and 12 (Scheme 5). In the absence of air and moisture 12 proves to be stable for weeks [6].

As a typical example, the 29Si and 13C NMR spectra of vinyl cation 12 are shown in Fig. 3. The divalent coordinated C" atom accounts for the signal at S= 185.8 and at S= 84.1 the resonance for the formal sp2-hybridized Cp atom is detected. Both NMR chemical shifts are characteristic of the [Ca=Cp]' group and point to the particular bonding situation in vinyl cations [7c]. The formal sp hybridization of the linear Ca atom is responsible for its relatively highfield shifted resonance compared to trivalent carbenium ions (i.e. 613C(C') = 254 for the cumyl cation Me2C+-Ph) and its closeness to the CB atom results in a significant shielding for Cp compared to unsaturated carbon

40 T. Miiller

atoms in neutral alkenes. The 29Si NMR signal of 12 (a2'% =22.8) is markedly deshielded compared to the precursor silanes (A.629Si = 39.2), indicating delocalization of positive charge from the Ca atom to the p-silyl substituents by hyperconjugative interaction. As in the case of the arenium ions, the combined stabilizing effect of two p-silyl groups results in high thermodynamic stabilization of the vinyl cations 11 and 12. The vinyl cations are, however, highly reactive. A few typical reactions of vinyl cation 12 are summarized in Scheme 6. Addition of acetonitrile results in desilylation of the vinyl cation 12 and formation of the nitrilium ion 13 in which the 2-alkynyl-2,6 -disilaheptane moiety is recovered. The reaction with methylene chloride leads to a fragmentation process with chloronium ion 14 as the only detectable product. Treatment of a solution of 12 in benzene with allylstannane gives the product of C-C bond formation, the diene 15, in 65 % isolated yield.

Me2Si MSiMe2 TPFw Me2Si ,CB,SiMe2

6 - 111 CsHs, RT cat II 11,R=CH, I H

I 12, R = C6H5 10 R 5: R

Scheme 5. Synthesis of vinyl cations.

H&CN Me2SiMSiMe2

t I R T - H~CCN'

Me2Si n ,C,SiMe2

I1 C + I Ph

12

I Ph

RT

Scheme 6. Reactions of vinyl cation 12.

A Model System for the Generation of Silyl Cationic Species 41

a.

100 50 0 +- 6 -50

b.

C"?. I '.I C'"

SiMe,

L

200 150 100 - 6 50 0

Fig. 3. a) 49.7 MHz 29Si( 'H) Inept NMR spectrum of 12 in [D8]toluene; b) 62.9 MHz I3C( 'H) NMR spectrum of 12 in [D8]toluene.

Intramolecular Stabilization of Silylium Ions By o-Bonds: the First Example of a Si-H-Si Three Center Bond [8]

Our theoretical investigation suggests that the cyclic silyl cation 16 with a symmetrical Si-H-Si bridge is significantly more stable than its acyclic isomer 17, and that 16 should be stable in benzene solution at room temperature (see Table 1, entry 1). In agreement with this theoretical prediction, only one product is formed in the hydride transfer reaction between disilane 18 and trityl cation (Scheme 7). The NMR spectra of the product (see Fig. 3) are in accordance with the cyclic structure 16.

18 17 16

Scheme 7. Synthesis of silyl cation 16.

42 T. Miiller

Highly diagnostic for the cyclic structure of the silyl cation 16 is the shielding of the bridging hydrogen atom (6'H(br) = 1.47) compared to the precursor silane 18, by A6'H = -2.43 and the single, strongly deshielded 29Si resonance (aZ9Si = 76.7, ASz9% = 91) with an unusual small coupling constant of 'J(SiH) =39 Hz (see Fig. 4a). Low-temperature NMR studies and the determination of deuterium isotope effects on the chemical shifts provide conclusive arguments for the static structure of 16 and against a conceivable fast equilibrium of two trivalent silylium ions 17 (Fig. 5):

No kinetic line broading of NMR signals is observed in the available temperature range

The hexadeuterated cation [DG] 16 exhibits only a small temperature-independent intrinsic deuterium effect on the 29Si NMR chemical shift (Agintr = -0.23), which is only consistent with the static structure 16 [9]. For 1:1 mixtures of cations 16 and [Dl116 a negative primary deuterium isotope effect on the hydrogen chemical shift (A6('H,'H) = -0.30) is detected. Negative (A6 ('H,2H) values indicate a single minimum potential [ 101 and are found for symmetrical bridged carbocations [ 1 11.

(-40°C - 30°C).

Fig. 4. a) Lower trace: 49.7 MHz 29Si( 'H) Inept NMR spectrum of 16; upper trace: part of the 49.7 MHz 29Si NMR

spectrum. b) Calculated structure (at B3LYP/6-3 1 lG(d,p) and MP2/6-3 1 lG(d,p) (underlined) levels) and

charge distribution (italics: NBO analysis at B3LYP/6-3 1 lG(d,p)//B3LYP/6-3 1 lG(d,p) of 16.

A Model System for the Generation of Silyl Cationic Species 43

The experimental characterization of 16 is strongly corroborated by quantum-mechanical calculations of geometries and of 29Si NMR chemical shifts. For 16, 829Si = 87 is predicted (GIAO/MP2/6-311G(2df,p)) with a coupling constant between silicon and hydrogen I 'J(SiH)I = 43.8 Hz (SOS/IGLO/DFPT/basis III), both values in close agreement with the experiment. The calculated structure of 16 reveals a regular chair conformation for the six-membered ring with long SiH bonds (162.3 pm (MP2/6-31 lG(d,p)), as expected for a three-center two-electron Si-H-Si bond (Fig. 4b).

Me,Si,H +SiMe, Me,Si ,SiMe,

syn, syn-17

+ H

Fig. 5. Fast equilibrating silylium ions 17 (left) an- ..-: static bridged ions [Dl116 and [Ds]16.

Conclusion

The three examples provided in this short overview demonstrate the possibilities of our model system 1. The high electrophilicity of the silylium ions 3 is strongly reduced by the intramolecular reaction with the donor substituent; however, its reactivity is also preserved in the cyclic isomers 1. In all cases described here, the reaction of 1 with weak nucleophiles like acetonitrile results in breakdown of the intermolecular interaction and formation of nitrilium ions 19 (Scheme 8) [5,6, 81.

1 19 CH3

Scheme 8. Formation of nitrilium ions 19 (X = aryl, alkynyl, H).

Our ultimate goal, the control of the reactivity of silylium ions is very ambitious, and we are still far away from a chemistry of cationic silicon in solution, in particular from a controlled chemistry. The benefits of our model system 1, however, are already obvious. The two silicon substituents confer to the cyclic cations 1 a high thermodynamic stability. An example is shown for the arenium ions 9 and the vinyl cations 11 and 12 and can be easily extended to every substituent X acting as a n-donor, such as alkenes, nitriles, allenes or aides. In addition the very weakly nucleophilic conditions applied in our reactions allow the generation of only weakly stabilized cations with very uncommon structural features, as for example the hydrogen bridged ion 16.

44 T. Miiller

Acknowledgments: This work was supported by the German-Israeli Foundation (GIF) and by the Deutsche Forschungsgemeinschaft (DFG). Chemetall and Wacker GmbH supported this research with generous gifts of alkyllithium compounds and silanes.

References [ l ] a) K.-C. Kim, C. A. Reed, D. W. Elliott, L. J. Mueller, F. Tham, L. Lin, J. B. Lambert,

Science, 2002, 297, 825. b) J. B. Lambert, Y. Zhao, Angew. Chem. Int. Ed. 1997, 36, 400. c) T. Miiller, Y. Zhao, J. B. Lambert, Organometallics 1998, 17,278. d) J. B. Lambert, Y. Zhao, H. Wu, W. C. Tse, B. Kuhlmann, J. Am. Chem. SOC. 1999,121,5001. Recent reviews : a) C. Maerker, P. v. R. Schleyer, in The Chemistry of Organic Silicon Compounds, Vol. 2 (Eds.: Z. Rappoport, Y. Apeloig), Wiley, Chichester, 1998, p. 513. b) P. D. Lickiss, in The Chemistry of Organic Silicon Compounds, Vol. 2 (Eds.: Z. Rappoport, Y. Apeloig), Wiley, Chichester, 1998, p. 557. c) S. Fornarini, in The Chemistry of Organic Silicon Compounds, Vol. 3 (Eds.: Z. Rappoport, Y. Apeloig), Wiley, Chichester, 2001, p. 1027. a) J. Y. Corey, J. Am. Chem. Soc. 1975, 97, 3237-3238. b) H. Mayr, N. Basso, J. Am. Chem. Soc. 1992, 114, 3060. c) Y. Apeloig, 0. Merin-Aharoni, D. Danovich, A. Ioffe, S . Shaik, Isr. J. Chem. 1993,33,387-402. All calculations have been performed with Gaussian 98. Gaussian 98 Revisions A3-A9, Gaussian Inc., Pittsburgh PA, 1999. R. Meyer, K. Werner, T. Muller, Chem. Eur. J . 2002,8, 1163. T. Miiller, R. Meyer, D. Lennartz, H.-U. Siehl, Angew. Chem. 2000, 112, 3203; Angew. Chem. Int. Ed. 2000,39, 3074. Reviews for vinyl cations: a) P. J. Stang, Z. Rappoport, M. Hanack, L. R. Subramanian, Vinyl Cations, Plenum Press, New York, 1979. b) P. J. Stang, Z. Rappoport (Eds.), Dicoordinated Carbocations, Wiley, Chichester, 1997. c) H.-U. Siehl, in Dicoordinated Carbocations (Eds: P. J. Stang, Z. Rappoport.), Wiley, Chichester, 1997, chapter 5.

[8] T. Miiller, Angew. Chem. 2001,113,3123; Angew. Chem. Int. Ed. 2001,40,3033. [9] For a review on isotope effects on the NMR chemical shifts, see H.-U. Siehl, Adv. Phys. Org.

Chem. 1987,23,63. [lo] a) The primary isotope effect on the ‘H NMR chemical shift is defined as: A6(’H,’H) =

6lH-~5~H. b) L. J. Altman, D. Laungani, G. Gunnarsson, H. Wennerstrom, S . ForsCn, J. Am. Chem. Soc. 1978,100,8264.

[ l l ] J. E. McMurry, T. Lectka, C. N. Hodge, J. Am. Chem. SOC. 1989,111,8867.

[2]

[3]

[4]

[5] [6]

[7]

Synthesis and Chemistry of Some Bridged Silicocations

Paul D. Lickiss, Phindile C. Masangane, Wazir Sohal, Guilaine L. Veneziani

Department of Chemistry, Imperial College of Science Technology and Medicine, London SW7 2AY, UK Tel.: +44 207 5945761 -Fax: +44 207 5945804

E-mail: p.lickiss@ic.ac.uk

Keywords: silicocations, synthesis, structure, equilibria, trityl salts

Summary: Treatment of the sterically hindered silanes TsiSiRR'H (where RR' = Hz or PhMe and Tsi = (MesSi)3C) with the hydride abstraction agent Ph&B(C&)4 affords cationic species TsiSiRR* which are in equilibrium with bridged cationic species. Bridged cations in which Ph or H bridge between 1,3-silicon atoms have been isolated and characterized by X-ray crystallography.

Introduction

In 1979 Eaborn et al. published work on the chemistry of bulky silyl iodides TsiSiRR'I and silanes TsiSiRR'H (Tsi = (Me&)3C) with electrophilic reagents that invoked the intermediacy of 1,3-Si-to-Si bridged cations [l]. Since that time, more than 100 publications detailing the chemistry of Tsi-silicon derivatives, particularly chemistry involving silicocationic intermediates, have been published and many novel compounds and unusual chemistry have been described. This work has been reviewed recently [2] and usually involved the use of reagents such as silver or mercury salts, and solvents such as alcohols or ethers. Any silicocationic species formed in the presence of reactive solvents or strongly coordinating anions such as F or OAc- is rapidly consumed and mechanistic arguments were mainly based on product analysis. The strongly electrophilic nature of silicocations has thus frustrated these and many other attempts by other workers to isolate stable compounds containing a three-coordinate R3Si' ion (for a review, see Ref. [3]). A more successful approach to isolating R3Si' ions has been to abstract H- from a silane R3SiH using a trityl salt such as Ph3CB(C&)4, where the anion has a very weakly coordinating nature [3]. The work described here combines the very bulky nature of the Tsi group, which should sterically protect a reactive R3Si' ion, together with the hydride abstraction synthetic method, involving poorly coordinating anions and solvents.

Ovganosilicon Chemisty V Edited by N. Auner and J. Weis

Copyright 0 2003 WILEY-VCH Verlag GmbH & Co. KgaA, Weinheim

46 P. D. Lickiss, P. C. Masangane, W. Sohal, G. L. Veneziani

Preparation of the Reactants

The synthetic route to the bulky silanes is shown in Scheme 1 and uses TsiLi (prepared by metallation of TsiH by MeLi) as a Tsi transfer reagent. The silyl chloride intermediates are all air- stable, which gives a strong indication of the degree to which the Tsi group protects the functional Si centers. TsiSiH3 is prepared in a similar way by reduction of TsiSiCl3.

TsiLi + RRSiC12 - TsiSiRRCl

RR' = Me, or PhMe LiAlH, 1 Scheme 1. Synthesis of bulky silanes. TsiSiRRH

Synthesis of Ph3CB(C6F& was carried out in a one-pot modification of the literature method [4]. Treatment of the silanes with Ph3CB(C&)4 was done in a PhMePhCl solvent mixture, under an inert atmosphere, at room temperature, which gave rise to orange reaction solutions.

Reaction of TsiSiPhMeH with Ph3CB(C6F5)4

The 'H NMR spectrum of the reaction solution after 24 h showed two singlets in the SiMe region, signals due to Ph3CH and no TsiSiPhMeH. The SiMe signals were at first attributed to the expected formation of TsiSiMe2' as shown in Scheme 2, but it was then found that the smaller of the two SiMe signals actually corresponded to (Me3Si)&. This thus means that the larger signal must be due to a series of equilibria, as shown in Scheme 2, that exchange all the Me groups rapidly so as to give one signal only. The Ph groups are difficult to monitor due to the presence of other aryl signals from the solvents, the Ph3C' and Ph3CH.

Scheme 2. Some of the possible equilibria in the TsiSiPhMe' system.

Synthesis and Chemistry of Some Bridged Silicocations 47

Although the ‘H NMR spectrum showed only a single signal for the equilibria in Scheme 2, the 29Si spectrum showed many signals between zero and ca. 60 ppm, some of which were broad and some sharp. On warming the sample to 60°C these signals coalesced and became a single sharp signal, again indicative of a system in equilibrium. Addition of pentane to the reaction solution gave crystals that were examined by X-ray crystallography and found to be the B(C&)4- salt of the bridged cation shown in Fig.1. This is the first example of such a bridged cation to be isolated and structurally characterized and is precisely the type of cation proposed by Eaborn et al. [2] to account for rearrangements seen in earlier work.

I- +

Fig. 1. The cation isolated from the reaction between TsiSiPhMeH and Ph3CB(C&5)4.

The bridge formed by the Ph group between the two Si atoms is nearly symmetrical, S i x distances are 2.104(8) and 2.021(7) A, and these distances are longer than would be expected for a “normal” Si-Ph bond. The Si to central carbon distances are shorter [1.867(5) and 1.872(6) A] for the SiMez groups when compared to the Me3Si-C distances [1.921(5) and 1.915(5) A]. The four-membered ring is puckered about the Si.-.Si vector with a fold angle of 13.5”, similar to the 14” found in the AlMezPh dimer. The presence of a phenyl rather than a methyl bridge in the cation might also be expected by comparison with aluminum chemistry.

Reaction of TsiSiH3 with Ph3CB(C6F&

The reaction between TsiSiH3 with Ph3CB(C6F5)4 was carried out in a similar fashion to that with TsiSiPhMeH and gave a solution with only a single sharp signal in the SiMe region of the ‘H NMR spectrum and a broad signal in the SiH region. Again, the ”Si NMR spectrum is verj complicated, as shown in Fig. 2, and includes signals from about -20 to 90 ppm, also indicative of a system containing several equilibrating species. Equilibria of the type shown in Scheme 2, but having H bridges instead of Ph bridges, would account for the spectra.

Crystals obtained by addition of pentane to this reaction solution unexpectedly gave the B(C&)4- salt of the cation shown in Fig. 3.

The formation of the cation shown in Fig. 3 is surprising as it contains ten Me groups, whereas the starting material TsiSiH3 contains only nine. There has thus been an intermolecular exchange of Me, presumably with Si-H, as well as Me groups migrating between silicon atoms in an intramolecular fashion. It has not been possible so far to isolate other compounds that must be formed as byproducts in such reactions. Rapid intermolecular methyl group exchange would also

48 P. D. Lickiss, P. C. Masangane, W. Sohal, G. L. Veneziani

explain the formation of (MesSi)& in the reaction of TsiSiPhMeH with Ph3CB(C6F5)4. The Si-H-Si bridge is symmetrical, having Si-H distances of 1.657 and 1.652 8, and an Si-H-Si angle of 97.6'. The four-membered ring is again puckered but this time about the H . .C vector with a fold-angle of 12.7'. The presence of H bridging rather than Me bridging is again expected by comparison with aluminum chemistry.

-.... , , , , , , , , , , , ,- , . , . ,.".?" I (-------

200 150 100 so 0 -50 -100 -150 ..l,.ll

Fig. 2. 29Si NMR spectrum of reaction solution of TsiSiH3 with Ph3CB(C&)4.

Fig. 3. The cation isolated from the reaction between TsiSiH3 and Ph3CB(C&)4.

The isolation of 1,3-Si-to-Si bridged cations confirms their presence in reactions of Tsi-Si derivatives with electrophilic reagents. Unfortunately the formation of complicated equilibrium solutions has, so far, hindered investigation of their chemistry. The use of better bridging groups such as OMe or OAc might allow more stable bridged species to be formed and this should allow for easier investigation of their chemistry.

Synthesis and Chemistry of Some Bridged Silicocations 49

Acknowledgments: P.D.L. thanks Professor Colin Eaborn for many stimulating discussions about silicocations over the years, and Professor M. McPartlin and N. Choi of the University of North London for the X-ray crystallographic structure determinations. Financial support is gratefully acknowledged from the EPRSC and the Dow Coming Corporation (to W.S.), the British Council for a Commonwealth Scholarship (to P.C.M.) and the European Community for a Fellowship (to G.L.V.) .

References [ 13 [2] [3]

[4]

C. Eaborn, D. A. R. Happer, S. P. Hopper, S. D. Safa, J. Organomet. Chem. 1979,170, C9. C. Eaborn, J. Chem. SOC., Dalton Trans. 2001,3397. P. D. Lickiss Silicenium Ions - Experimental Aspects in: The Chemistry of Organic Silicon Compounds, Vol. 2 , Part 1, (Eds.: Z. Rappoport Y. Apeloig), Wiley, 1998, p. 557. J. B. Lambert, S. Zhang, S. M. Ciro, Organometallics 1994,13,2430.

New Donor-S tabilized Organosilicon Cations: Synthesis, Structure and Reactivity

Andreas Bockholt, Thomas Braun, Peter Jut$ Beate Neumann, Anja Stummler, Hans-Georg Stummler

Fakultat fur Chemie der Universitat Bielefeld UniversitatsstraBe 25, D-33615 Bielefeld, Germany

Tel.: +49 521 1066181 - Fax: +49 521 1066026 E-mail: peter.jutzi@uni-bielefeld.de

Keywords: silyl cation, pincer ligands, 29Si NMR, solid-state structure

Summary: Silyl triflates containing organosilicon cations stabilized by the 2,6-bis(methoxymethyl)phenyl or by the 2,6-bis(methylthiomethyl)phenyl ligand have been prepared starting from the corresponding chlorosilanes. Ionic structures are found in solution as well as in the solid state. The cations all show a trigonal bipyramidal geometry at silicon in the solid state. Oxygen- and sulfur-stabilized organosilyl cations differ remarkably in their reactivity towards nucleophiles.

Introduction

Highly reactive silyl cations (silicenium ions) can be stabilized by the coordination of “hard” o-donor ligands and according to recent findings also by the coordination of “soft” o- or n-donor ligands [ 11. The so-called “pincer” ligand I with “hard” nitrogen donors was introduced into silicon chemistry by Corriu’s group and has proven to be an excellent tool for the stabilization of silyl cations [2]. We now report on results concerning the synthesis and characterization of compounds containing silyl cations stabilized by the “pincer” ligands I1 and I11 with oxygen and sulfur donors (Fig. 1).

Fig. 1. Pincer ligands.

Ovganosilicon Chemisty V Edited by N. Auner and J. Weis

Copyright 0 2003 WILEY-VCH Verlag GmbH & Co. KgaA, Weinheim

New Donor-Stabilized Organosilicon Cations 51

Results and Discussion

The general procedure for the synthesis of 9-15 is shown in Scheme 1. Bromobenzene 1 was prepared according to Bickelhaupt et al. [3], and the novel bromobenzene 2 was synthesized by the reaction of two equivalents of sodium methane thiolate with one equivalent of 2,6-bis(bromomethyl)bromobenzene. Compounds 1 and 2 were then converted with butyllithium to the corresponding lithiated species which reacted with one equivalent of either methyldichlorosilane, phenyldichlorosilane or dimethyldichlorosilane to afford the chlorosilanes 3-8. The chlorosilanes were then treated with one equivalent of trimethylsilyl triflate to yield the silyl triflates 9-14. Silyl triflate 10 could also be prepared by the reaction of chlorosilane 4 with one equivalent of triflic acid. The silyl tetrakis(pentafluorpheny1)borate 15 was synthesized by the reaction of chlorosilane 4 with one equivalent of lithium tetrakis(pentafluorpheny1)borate. Compounds 9-15 are dissociated in dichloromethane solution.

1: D=OMe 2 D=SMe

CF,SO,H

-HCI 4 - 10

3 4 9-14

3,9 D=OMe, RkMe, R2=H 4,lO D=SMe, Rl=Me, R2=H 5.11: D=OMe, RI=Ph, RZ=H 6.12 M M e , RkPh. R2=H 7,13 M M e , RI=Me, ==Me 8.14 D=SMe, RI=Me, R k M e

\-D

15: DTSMe, RI=Me, R2=H

Scheme 1. Synthesis of the silyl triflates and tetrakis(pentafluorpheny1)borates.

The 29Si NMR shifts of the sulfur-stabilized triflates 10, 12 and 14 are found to be at considerably higher field than the shifts of their oxygen-stabilized analogues 9, 11 and 13 (Table 1). The 'J(Si-H) coupling constants prove sp2 hybridization at silicon.

Surprisingly, the tetrakis(pentafluorpheny1)borate 15 shows a downfield shift compared to the triflate 10 in spite of identical cations. This indicates a weak triflate-cation interaction in solution.

The molecular structures of 11 and 12 (Fig. 2 ) show a slightly distorted trigonal bipyrarnidal geometry at the silicon centers with the donor atoms in the axial positions. The ions are separated in the solid state.

52 A. Bockholt, T. Braun, P. Jutzi, B. Neumann, A. Stammler, H.-G. Stammler

Table 1. 29Si NMR data (r.t.) of 9-15.

9 10 11 12 13 14 15

Si [ppml -20.6 -33.0 -29.1 4 0 . 0 6.9 -6.5 -19.9 29

'JSiH [Hzl 276 277 280 290 - - 280

Fig. 2. Structures of 11 (left) and 12 (right); selected bond lengths [pm]: 11: Si(l)-O(l): 195.40(14), Si(lW(2):

197.23(14); 12: Si(l)-S(l): 243.88(14), Si(l)-S(2): 246.93(14); selected angles ["I: 11: O(l)-Si(1)-0(2):

161.90(6); 12: S(l)-Si(l)-S(2): 172.25(5).

In 10, 12, 14 and 15 the coordinated sulfur atoms are chiral centers. The triflates 10, 12 and 14 crystallize as racemates of A and A' whereas 15 crystallizes as meso form B (Fig. 3). Diastereomer C was not found in any of the solid-state structures. In Figure 2 only one enantiomer of 12 is depicted. In solutions of 10, 12 and 15, interconversion of A, A' and B is observed giving rise to broad signals in the r.t. 'H and 29Si NMR spectra.

S- -S -S S-

A A. B C

10 R l =Me, R2 =H / \ Si,.LJI1 12: R1 =Ph, RZ =H

14 R1 = R 2 = Me qi \R2 15: R1 =Me, RZ = H

S \

Fig. 3. Stereochemical properties of 10,12,14 and 15 (Fischer-type projections of the cationic units).

New Donor-Stabilized Organosilicon Cations 53

The oxygen- and sulfur-stabilized salts show remarkable differences in reactions with nucleophiles. Compound 11 reacts with water to give 16 containing an oxygen-bridged dication. On reaction of 12 with water quantitative decomposition into the protonated pincer ligand and a polysiloxane is observed (Scheme 2).

2 OSO,CF,o

16

Scheme 2. Hydrolysis of 11 and 12.

Like organosilyl cations stabilized by two 2-(methoxymethy1)phenyl ligands [4], 11 reacts with pyridine with transfer of a methyl group and formation of a cyclic silyl ether 17. In contrast to organosilyl cations chelated by two 2-(methylthiomethy1)phenyl ligands, a displacement of the intramolecular donors by pyridine is not possible in 12 (Scheme 3).

11

Scheme 3. Reactions of 11 and 12 with pyridine.

Further investigations are under way concerning the properties of the cations as Lewis acids in stoichiometric and catalytic reactions.

References [l] For example: M. Kira, T. Hino, H. Sakurai, J. Am. Chem. SOC. 1992, 114, 6697; H.-U.

Steinberger, T. Miiller, N. Auner, C. Maerker, P. v. R. Schleyer, Angew. Chem. 1997, 109,

54 A. Bockholt, T. Braun, P. Jutzi, B. Neumann, A. Stammler, H.-G. Stammler

667; U. H. Berlekamp, P. Jutzi, A. Mix, B. Neumann, H.-G. Stammler, W. W. Schoeller, Angew. Chem. 1999,38,2071. C. Chiut, R. J. P. Corriu, A. Mehdi, C. Reye, Angew. Chem. 1993,105,1372. P. R. Markies, R. M. Altink, A. Villena, 0. S. Akkermann, F. Bickelhaupt, J. Organomet. Chem. 1991,402,289. U. H. Berlekamp, Dissertation, Universitat Bielefeld, 1999.

[2] [3]

[4]

Novel Pentacoordinate Siliconium Complexes Stabilized by Oxygen and Nitrogen Donors: Highly Sensitive and Unusual Equilibrium

between Ionic Penta- and Neutral Hexacoordinate Compounds'

Daniel Kost, Vgeyakumar Kingston, Inna Kalikhman

Department of Chemistry, Ben-Gurion University of the Negev Beer-Sheva 84105, Israel

Tel.: +972 8 646 1192 - Fax: +972 8 647 2943 E-mail: Kostd@bgumail.bgu.ac.il

Keywords: ionic dissociation, equilibrium reaction, siliconium compounds, neutral hexacoordinate complexes, negative entropy

Summary: The first ionic dissociation of the Si-Cl bond in neutral hexacoordinate silicon complexes is reported. An equilibrium reaction between the ionic siliconium chloride and its neutral precursor (dissociation-recombination) is observed. The population ratio can be controlled by temperature or by replacement of the chloro ligand by a triflate group. The reaction enthalpy and entropy of the dissociation are both negative, suggesting that solvent organization facilitates dissociation at low temperature.

A convenient method for the preparation of neutral bis(N+Si) hexacoordinate silicon complexes has been developed and reported recently, consisting of ligand exchange between a polychlorosilane (1) and 0-trimethylsilyl derivatives of hydrazides (2, Eq. 1) [2]. An attempt to utilize this synthetic route for the preparation of isomeric 043 coordinated chelates did not lead to the expected hexacoordinate complexes, but to ionic siliconium chloride salts stabilized by two ( 0 4 3 ) dative bonds (5, Eq. 2) [3].

This unexpected difference between two apparently similar reactions raises the question of whether the N+Si coordinated octahedral 3 can also, under certain conditions, dissociate to form pentacoordinate siliconium chloride salts. To answer this question we have re-investigated the chemistry of 3 in some detail. We now report on the reversible ionic dissociation of 3 to siliconium salts, and the resulting control of the coordination number of silicon by various means: temperature,

'See Ref. [ l ] .

Ovganosilicon Chemisty V Edited by N. Auner and J. Weis

Copyright 0 2003 WILEY-VCH Verlag GmbH & Co. KgaA, Weinheim

56 D. Kost, V. Kingston, I. Kalikhman

counterion, and various ligands and substituents. This is the first report of an ionic dissociation of a hexacoordinate silicon complex.

3

Eq. 1.

I - X S ~ C I ~ + 2 Me3SiWWCC-Ph ++ I Me

MB 1 4 Ph Ph

5

Eq. 2.

The 29Si NMR spectra of compounds of series 3 in CD2C12 solution change dramatically

downfield upon cooling, from a predominantly hexacoordinate resonance (ranging between -120 to -150 ppm) to a weighted average signal of increasingly pentacoordinate chemical shift, and finally to a predominantly pentacoordinate resonance at -60 to -75 ppm (Fig. 1) [4]. The latter was assigned to the dissociated siliconium chloride complex 6, based on an almost exact chemical shift analogy with the corresponding siliconium triflate 7 (Table l), which was characterized by an X-ray crystal structure. At low temperatures (5 200 K) the single broad signal split into two separate signals (Fig. l), one corresponding to 3 (assigned from the [D8]toluene NMR, in which no dissociation took place, Table 1) and the other to the pentacoordinate siliconium chloride (6, Eq. 3).

3 a , R = M e 6

b, R = 1-Bu c , R = C W h d, R = Ph

Eq. 3.

Novel Pentacoordinate Siliconium Complexes 5'7

Fig. 1. 29Si NMR spectra of 3a f 6a in CDzClz solution at various low temperatures.

The spectral changes are completely reversible with temperature changes, and thus represent an equilibrium dissociation-recombination process, the first reported dissociation of a neutral hexacoordinate silicon complex [5 ] . The 29Si chemical shifts of the equilibrium mixtures at various temperatures are listed in Table 1.

Table 1. 29Si chemical shifts for the equilibrium mixtures of 3 and the corresponding siliconium

chlorides 6 at two temperatures, and for siliconium triflates (7).

3 z 6 3 7 T9 WI CD~CIZ [Dsltoluene CDCl3 Compound R

300 -131.3 -135.6 -73.2

165 -73.0 -135.6 321 Me

300 -132.9

165 -73.6 -135.9 3b t-Bu

-73.6

300 -133.7 -73.4

165 -73.4 -135.9

300 -133.7 -136.9 -72.6

165 -73.2 -136.9

3c PhCH2

3d Ph

58 D. Kost, V. Kingston, I. Kalikhman

The equilibrium position is strongly affected by a variety of factors, and can be shifted to either extreme under suitable conditions. It is thus possible to effectively control the coordination number of silicon to be either 5 or 6, in a reversible manner, and to modify the complex's properties accordingly. This may prove useful in the future design of molecular switches.

Table 2 shows the effect of temperature upon the population ratio K = [6]/[3]. It is evident that K changes with temperature, and given the right substitution on the chelate rings (R), the equilibrium can be shifted almost completely to the ionic side at low temperature, and to the hexacoordinate side at room temperature.

Table 2. Equilibrium constants at selected temperatures, enthalpies and entropies for the dissociation-recombination

reaction, and approximate coalescence temperatures and activation free energies for the dissociation.

AHo Aso Tc ''I AG* P I K = [6]/[3] [kcal mol-'I [cal mol-'K-'l [K] [kcal mol-'1 300 K 180 K

Compound R

3a Me -2.84 -15.0 215 8.4 0.08 I .38

3b 'Bu -3.94 -21.8 220 8.6 0.05 0.85

3c CH2Ph -1.94 -9.4 230 9.0 0.14 0.69

3d Ph -0.82 -8.6 220 8.6 0.05 0.13

[a] +5. [b] k5.

y = -0.000x3 +0.065x2 - 18.403x+ 1608.593 CDCI,

y = -0.052~ - 115.603 CD2C12

I

-1 34 I I

250 260 270 280 290 300 -134 I I

250 260 270 280 290 300

Temperature [1(1

Fig. 2. Temperature dependence of the 29Si chemical shift for 3a in two solvents.

Interestingly, the extent of dissociation increases with decreasing temperature, contrary to what might be expected from a dissociative process, generating two particles from each molecule. Correlations of In K vs. reciprocal temperature for the four complexes 3a-d yielded reaction enthalpies and entropies (Table 2) which clearly confirm the counter-intuitive process: the

Novel Pentacoordinate Siliconium Complexes 59

dissociation is associated with negative entropies and enthalpies. A plausible explanation for this result may be solvent aggregation about the ions, which serves to stabilize the ions at the expense of greater system organization, and hence the negative entropy.

Support for this rationale comes from comparison of the temperature dependence of the 29Si chemical shift for one compound (3a) in two solvents: CD2C12 and CDC13 (Fig. 2). It is

immediately evident from the figure that the temperature dependence of the chemical shift is greater in chloroform than it is in the less polar and less hydrogen-bonding solvent dichloromethane. This indicates that solvent aggregation, associated with negative reaction entropy, is stronger in chloroform.

Table 2 also lists the kinetic data for 3a-d: because of the very large 29Si chemical shift differences between 3 and 6, the exchange broadening and coalescence phenomena extend over a large temperature range. As a result the activation barriers cannot be determined accurately, and are believed to be correct within * 0.5 kcal mol-l.

The hexa-pentacoordinate equilibrium can be further controlled by manipulating the monodentate ligand: the reaction of 3a-d with trimethylsilyl triflate (TMSOTf) led to quantitative substitution of the chloro ligand by the less nucleophilic triflate group (Eq. 4). The 29Si NMR spectra indicated a complete shift of the equilibrium toward the ionic side (Table 1). The overwhelming predominance of the triflate salt at room temperature facilitated the growth of a single crystal of 7a, suitable for a crystallographic analysis (Fig. 3). The figure clearly confirms the ionic nature of 7a, with well separated ions and a distorted trigonal bipyramidal geometry. The N- Si distances are shorter than in previously reported N+Si coordinated siliconium ions [6].

Fig. 3. Crystal structure of 7a at 50 % probability. Hydrogen atoms have been omitted for clarity. Selected bond

lengths [A]: Si-N1, 1.9645(13); Si-N4, 1.9747(13); Si-01, 1.691 l(11); Si-02, 1.6833(11); Si-C9,

1.8453(15). Selected bond angles ["I: N2-Si-N4, 157.24(6); 02-Si-01, 137.05(6).

60 D. Kost, V. Kingston, I. Kalikhman

a , R = M e 6 b, R = t-Bu c , R = C W ~ d, R = Ph

Eq. 4.

7

Acknowledgments: Financial support from the Israel Science Foundation and from the German Israeli Fund for Scientific Research (GIF) is gratefully acknowledged.

References For a preliminary communication see: V. Kingston, B. Gostevskii, I. Kalikhman, D. Kost, Chem. Commun. 2001,1272. a) D. Kost, I. Kalikhman, M. Raban J. Am. Chem. SOC. 1995,117, 11512. b) I. Kalikhman, S. Krivonos, D. Stalke, T. Kottke, D. Kost Organometallics, 1997, 16, 3255. c) D. Kost, I. Kalikhman, S. Krivonos, D. Stalke, T. Kottke J. Am. Chem. SOC. 1998, 120, 4209. d) A. 0. Mozzhukhin, M. Yu. Antipin, Yu. T. Struchkov, B. A. Gostevskii, I. D. Kalikhman, V. A. Pestunovich, M. G. Voronkov Metalloorg. Khim. 1992, 5, 658; Chem. Abstr. 1992, 117, 234095 w . I. Kalikhman, S. Krivonos. L. Lameyer, D. Stalke, D. Kost Organometallics 2001,20, 1053. The huge 29Si chemical shift difference between exchanging species (ca 6000 Hz!) enabled observation of the exchange and resolution of the individual species at low temperature. This resolution was not achieved in the 1H and 13C NMR spectra. The only previously reported comparable case of temperature-dependent dissociation is an internal nucleophilic substitution from a covalent pentacoordinate to anionic pentacoordinate complex: D. Schiir, J. Belzner, in Organosilicon Chemistry ZZZ, ed. N. Auner, J. Weis, VCH, Weinheim, 1997, p. 429. a) C. Brelibre, F. C a d , R. J. P. Corriu, M. W. C. Man J. Chem. Soc., Chem. Commun. 1994, 2333. b) J. Belzner, D. Schiir, B. 0. Kneisel, R. Herbst-her Organometallics 1995, 14, 1840. c) K. Hensen, T. Zengerley, T. Muller, P Pickel Z. Anorg. Allg. Chem. 1988,558, 21.

Binuclear Ethylene-Bridged Silicon Chelates: Equilibrium between Neutral Hexacoordinate

and Ionic Pentacoordinate Siliconium Complexes

Inna Kalikhman, Vijeyakumar Kingston, Daniel Kost

Department of Chemistry, Ben-Gurion University of the Negev Beer-Sheva 84105, Israel

Tel.: +972 8 646 1192 - Fax: +972 8 647 2943 E-mail: Kostd@bgumail.bgu.ac.il

Dietmar Stalke, Bernhard Walfort

Institut fur Anorganische Chemie, Universitat Wurzburg, D-97074 Wurzburg, Germany Tel.: +49 931 888 4783 -Fax: +49 931 888 4619

E-mail: Dstalke@chemie.uni-wuerzburg.de

Keywords: siliconium cations, Hexacoordinate silicon, binuclear silicon complexes

Summary: Binuclear hexacoordinate silicon complexes with two N+Si dative bonds (per Si atom) have been prepared. The crystal structure conforms to a distorted octahedron, with the N+Si bonds trans to each other. The binuclear complex undergoes Si-Cl dissociation of one S i x 1 bond in CD2C12 and CDC13 solution, to form a

binuclear, monosiliconium chloride salt, in reversible equilibrium with its precursor. The dissociation and equilibrium reactions are observed by variable-temperature 29Si NMR spectra. The extent of ionic dissociation increases as the temperature is decreased. The equilibrium population ratio is shifted completely to the di-ionic side at room temperature, by replacement of the chloride by the less nucleophilic triflate anion. The crystal structure of a disiliconium ditriflate shows a) well separated ions, b) that the geometry about silicon is almost an exact square pyramid, and c) that the N+Si bonds are among the shortest coordination bonds of this kind.

In search for new oligomers and polymers containing hypervalent silicon chelates, the reactions of bis(trichlorosily1)ethane (1) with 0- and N-trimethylsilylated hydrazides (2 and 3, respectively) have been studied. The reaction of 1 with 3 was reported to lead to ionic binuclear disiliconium dichloride salts (4, Eq. 1) [ 11. It was not obvious how the reaction of 2 with 1 might proceed: would it form a neutral binuclear hexacoordinate complex, like the mononuclear analogues [2], or would it form ionic complexes, in analogy to 4?

Ovganosilicon Chemisty V Edited by N. Auner and J. Weis

Copyright 0 2003 WILEY-VCH Verlag GmbH & Co. KgaA, Weinheim

62 I . Kalikhman, V. Kingston, D . Kost, D . Stalke, B . Walfort

Eq. 1.

The reaction of 1 with 2 (Eq. 2) proceeded like that of XSiC13 with 2 [2], forming an

ethylene-bridged binuclear hexacoordinate complex (5). This result was confirmed by the single- crystal X-ray analysis shown in Fig. 1 for 5b. The crystal features two equivalent essentially octahedral silicon atoms, with the donor-nitrogens in trans positions relative to each other.

a

c4

c5

Fig. 1. Crystal structures of bis-hexacoordinate 5b (left) and bis-siliconium M a t e 8b (right).

The room-temperature "Si NMR spectra of 5a, b in CDC13 solutions show resonances which are

at significantly lower field than expected for similar hexacoordinate complexes (Sa, -106.3 and Sb, -117.1 ppm, respectively). The CD2C12 solution 29Si resonance of 5b shifts strongly downfield

upon cooling, and splits below 200 K into two resonances (Fig. 2), characteristic of penta- (-61.8

Binuclear Ethylene-Bridged Silicon Chelates 63

ppm) and hexacoordinate (-1 30.8 ppm) complexes, suggesting that the compound undergoes the equilibrium reaction shown in Eq. 3. These changes are fully reversible.

Cl3SiCH2CH2SiCl3 + 4Me,NN=C(R)OSiMe,+

1 2

R = Me (a) R = Ph (b)

Sa, b

Eq. 2.

! 165 K

Fig. 2. Variable-temperature 29Si NMR spectra of 5b in CDzClz solution.

Ionization and possible dissociation of 5 may proceed stepwise and result either in mixed penta- hexacoordinate monocations (6), or in dications (7, Eq. 3). The *'Si NMR spectral changes with temperature provide evidence to answer this question. At temperatures below the coalescence temperature the two signals for the penta and hexacoordinate species reach equal intensities, which no longer change upon further cooling. This proves that the ionization produces the monoionic 6, and stops at this stage without further ionization to the dicationic 7.

The equilibrium population ratio, which at room temperature leans primarily towards 5b, can be shifted completely to the ionic side by replacement of the counter-anion [4]. Treatment of a solution of 5b in chloroform or dichloromethane with trimethylsilyl triflate (Me3SiOS02CF3, TMSOTf)

64

resulted in a downfield shift of the 29Si resonance completely to the pentacoordinate side, forming the first binuclear siliconium triflate salt (8b) stable at room temperature. The presence of two chiral silicon centers in 8b causes the appearance of two 29Si chemical shifts, adjacent to each other, due to two diastereoisomers (-63.2 and -63.7 ppm) at a 6:l population ratio.

I . Kalikhman, V. Kingston, D. Kost, D. Stalke, B. Walfort

6a,b 7a,b

Eq. 3.

The crystal structure obtained for a single crystal of 8b is depicted in Fig. 1. A striking difference is found between the molecular geometries of 8b and its mononuclear analogue 9 [3] (Table 1): while in 9 the geometry about silicon is a TBP distorted toward a square pyramid (SP) to about 65-70 % [5], in the present compound (8b), the geometry about the silicons is almost exactly SP, with the 0-Si-0 and N-Si-N angles being equal [146.01(9) and 146.93(10)", respectively]. Furthermore, previously published mononuclear siliconium salts with the SiNzCzH ligand framework had almost pure TBP geometries, as opposed to the present results, and their N+Si dative bonds were significantly longer (2.052-2.080 8) [4c, 61 than in 8b (1.931-1.936 A).

Table 1. Selected bond lengths and angles from crystal structures of neutral binuclear hexacoordinate compex (5b),

binuclear (8b) and mononuclear (9) ionic siliconium triflates.

Bond lengths [A] Angles [deg]

Bond 5b 8b 9 [a1 Angle 5b 8b

Sil-01 1.7892 (15) 1.7134 (19) 1.6833 (11) 01-Sil-C19 173.70 (8) 107.13 (11) 110.16 (10)

Sil-02 1.7768 (15) 1.7216 (19) 1.6911 (11) 02-Sil-CII 171.01 (5)

Sil-NI 2.0578 (17) 1.931 (2) 1.9645 (13) N4-Sil-N2 168.20 (7) 144.14 (11) 157.24 (6)

Sil-N2 2.0441 (17) 1.931 (2) 1.9747 (19) 01-Sil-Cl 107.13 (11) 112.09 (6)

Sil-ClI 2.2410 (7) 01-Sil-02 149.62 (10) 137.07 (6)

Sil-C 1.925 (2) 1.864 (3) 1.8453 (15)

9 [a1

[a] Ref. [3].

Acknowledgment: Financial support from the Israel Science Foundation and the German Israeli

Binuclear Ethylene-Bridged Silicon Chelates 65

Foundation (GIF) is gratefully acknowledged.

References I. Kalikhman, S. Krivonos, L. Lameyer, D. Stalke, D. Kost, Organometallics 2001,20, 1053. a) D. Kost, I. Kalikhman, M. Raban, J . Am. Chem. SOC. 1995, 117, 11512. b) D. Kost, I. Kalikhman, S. Krivonos, D. Stalke, T. Kottke, J . Am. Chem. SOC. 1998,120,4209. V. Kingston, B. Gostevskii, I. Kalikhman, D. Kost, Chem. Commun. 2001, 1272. a) M. Chauhan, C. Chuit, R. J. P. Corriu, C. Reyt, Tetrahedron Lett. 1996, 37, 845. b) M. Chauhan, C. Chuit, R. J. P. Corriu, A. Mehdi, C. Reyt, Organornetallics, 1996,15,4326. c) J. Belzner, D. Schtir, B. 0. Kneisel, R. Herbst-Irmer, Organometallics, 1995,14, 1840. d) U.-H. Berlekamp, P. Jutzi, A. Mix, B. Neumann, H.-G. Stammler, W. W. Schoeller, Angew. Chem. lnt. Ed. 1999,38,2048. a) W. S. Sheldrick, in The Chemistry of Organic Silicon Compounds, S. Patai, Z . Rappoport,. Eds., Wiely, Chichester, U.K., 1989, I , p. 227. b) R. R. Holmes, J. A. Deiters, J . Am. Chem. SOC. 1977,99,3318. C. Brelibre, F. C a d , R. J. P. Corriu, M. Wong Chi Man, J . Chem. Soc., Chem. Commun. 1994,2333.

Bonding in Silicon Compounds - Long-Range Si/N Interactions in Organosilicon Molecules and

Molecular Cations

Hans Bock

Institut fiir Anorganische Chemie, Johann Wolfgang Goethe-Universitat Frankfurt Marie-Curie-Str. 11, D-60439 Frankfurt, Germany

Tel.: + 49 69 798 29180 -Fax: + 49 69 798 29188

Keywords: hypercoordinate organosilicon compounds, Si-N bonds, long-range Coulomb interactions, DFT calculations

Summary: The importance of rather low effective nuclear charges at Si centers in silicon compounds is emphasized on the basis of photoelectron spectroscopic comparison of the electronically isovalent molecules H3Si-SiH3 and H3C-CH3. For long-range N&+Si” interactions in two selected six- and seven-coordinate organosilicon Si-N complexes of known structure, density functional theory calculations (DFT) including intramolecular van der Waals contribution corrections predict distance-dependent N-Si bond enthalpies between 2 and 14 kJ mol-’ for bond lengths between 312 and 287 pm. An additional MP4-correlated calculation for the model compound H3N+Sib provides a potential curve with a minimum at 300 pm and the resulting NBO (natural bond orbital) charges of qsi = +1.6 and N = -0.5 correspond to an electrostatic attraction of about 60 kJ mol-’. The “through space” Coulomb-dominated donor-acceptor “bonding” N&+Si”, which is expected to vanish in organosilicon molecular cations, (N-Si’), presumably plays an essential role also in intermolecular interactions X&-...Si” between numerous heteroorganosilicon

compounds.

Introduction

The tremendous development of silicon chemistry together with adequate methods to measure and to calculate the properties of individual compounds allowed, in addition, various energy values to be specified for many molecules of known structure. As outlined in detail in the Wacker Silicon Days Lecture in 1998 [l], the resulting qualitative molecular state model for structure w energy relations [ 1-41 incorporates aspects of molecular dynamics, covers phenomena such as redox reactions or photochemical excitation and helps to tackle molecular self-recognition and self-organization in crystals. It also is of advantage to rationalize Coulomb-dominated long-range

Ovganosilicon Chemisty V Edited by N. Auner and J. Weis

Copyright 0 2003 WILEY-VCH Verlag GmbH & Co. KgaA, Weinheim

Bonding in Silicon Compounds 67

intramolecular interactions, the novel extension of bonding in silicon compounds discussed here.

Effective Nuclear Potential of Silicon Centers: Radical Cation State Comparison of the Isovalent Molecules H3SiSiH3 and H3CCH3

The qualitative molecular state model [ 1-41, the application of which is strongly recommended, can usually be transparently parametrized by physical measurement data such as vertical ionization energies [l, 2, 5, 61. Therefore, the potentials at the individual Si nuclear centers no longer have to be circumscribed by ill-defined quantities such as “electronegativity” or “hard and soft”, but are advantageously characterized by their “effective nuclear charges” introduced by Slater [7], nowadays inherent in many quantum-chemical calculation procedures.

For an eye-catching demonstration of the rather large difference between the effective nuclear charges of Si and C centers, the photoelectron spectroscopic ionization patterns of the “smallest chain polymers” disilane and ethane [ 1,2,5] are presented (Fig. 1).

,c- c

T

Fig. 1. a) Photoelectron spectroscopic radical cation state comparison of the electronically isovalent molecules

disilane and ethane including Jahn-Teller splitting JT; b) an approximate sequence of effective nuclear

potentials for some main group element centers in heteroorganic molecules [2].

68 H. Bock

The comparison of the electronically isovalent molecules reveals surprisingly large differences, especially for the two radical cation states with dominant Si contributions (Fig. 1): The energy of the H3SiSiH3.’ state 3ssi with the largest Si component at IE; = 16.5 eV [5] is lowered

tremendously by AIE: = 3.6 eV relative to the 2sc one of ethane! In close analogy, the oCc(a,)

ionization band of ethane with its vertical ionization maximum IE: - 13 eV is shifted by 3.0 eV to

become the osisi(a,) ground state of the disilane radical cation at 10.6 eV (Fig. l), demonstrating that the rather low effective nuclear charge of the silicon centers may even cause changes in the M’ state sequence relative to that of electronically isovalent carbon molecules.

The low effective nuclear charge of Si centers in silicon organic molecules relative to those of other main group elements such as S , P, C, 0 or N (Fig. l b ) has numerous essential consequences not only for the significantly different properties of silanes and alkanes (Fig. 1) [ 1 4 ] including even the “band structure” of (doped) polysilanes [6], but also for instance for the breath-taking donor effect of P-trimethylsilyl groups, which allow many (H3C)3SiCHz-substituted organic n-systens to be oxidized in aprotic AlCl3/H*CC12 solution to organosilicon radical cations [n(CH2Si(CH3)3),]*’, stabilized by [AlC147 counteranions [ 1-41.

Long-Range Si...N Interactions in Organosilicon Compounds with Hepta- and Octacoordinate Silicon Centers

Si centers in organosilicon compounds exhibit coordination numbers between 1 and 10, [ 13 and the resulting distances vary over wide ranges, as exemplified by the histogram for Si-N interactions registered in the Cambridge Structural Database [7] (Fig. 2 with N hits within 10 pm ranges), which contains a total of 574 entries for d ~ i . . . ~ distances greater than 200 pm. The shorter interactions are generally assigned to so-called “Si-N single bonds” such as in the 1-amino-8-silylnaphthalene derivative sitrol (Fig. 2b). Compounds with Si-N distances between 220 and 260 pm have repeatedly been defined as intramolecular donor-acceptor complexes. What type of interactions is, however, represented by the numerous Si...N distances exceeding 275 pm?

For density functional theory calculations the known structures of his[ (2-dimethyl- aminomethyl)phenyl]silane with a heptacoordinate Si Center (Fig. 3) and of bis[2,6-bi(dimethyl- aminomethyl)phenyl]silane with an octacoordinate Si center (Fig. 4), which complement each other, were selected [7].

For both relatively large molecules, containing 59 or even 68 centers respectively, an NEC SX4 supercomputer had to be used (for structural and computational details, see Ref. [7]).

Tris[(2-dimethylaminoethyl)phenyl]silane (Fig. 3a) contains a formally sevenfold coordinated silicon center with averaged intramolecular Si-N distances of 299 and 302 pm. The energy of the Si-N interactions has been approximated as follows [7]: the vector of each nitrogen electron pair, which deviates by only 23” from its idealized Si-N bond axis, was twisted perpendicular to this axis under continuous structure control on a computer screen to avoid additional severe overlap of the substituents within the molecule (Fig. 3b). The enthalpy difference due to the inevitably varied van

Bonding in Silicon Compounds 69

der Waals contacts in the periphery of the molecule is calculated after removal of the central SiH group accompanied by H saturation of the ruptured bond of each phenyl substituent, and estimated to amount to about 60 kJ mo1-I (Figs. 3c and d). The sum of all DFT calculated energy contributions for a rotation of the three N vectors out of their Si-N axial positions between the initially seven- and finally four-coordinate Si centers amounts to AAHdSi-N) = 42 W mol-’ or to about 14 kJ mol-’ for a cooperation-free Si-N interaction over a distance of about 300 pm (Fig. 3).

Fig. 2. Differences in various Si-N bond lengths: a) histogram for the range betwen 200 and 350 pm, as revealed by

a search in the Cambridge Structural Database, in which N is the number of hits within 10 pm ranges; b), c) selected examples of Si-N interactions in 1-amino-8-silylnaphtalene derivatives, in which d) N...Si

distances longer than 250 pm are also observed [7].

For bis[2,6-bis(dirnethylaminoethyl)phenylsilane, the second selected “hypercoordinate” organosilicon compound, its structure determination proves a formally eight-coordinate silicon center with averaged intramolecular distances of %and 310 pm for the two different N+Si interactions in the differently overcrowded donor-acceptor complex halves (Fig. 4a). They vary by 21 pm in length and their bond enthalpies have to be each separately approximated, analogously to the ones in the seven-coordinate compound (Figs. 3 and 4). Altogether four rotations around the

70 H. Bock

H3C6-CH2 and H~C-N(CH~)Z bonds (Figs. 4b and c) turn the nitrogen electron pairs of the (H~CZ)~N substituents perpendicular to the structurally characterized SiN connecting axes. The resulting van der Wads perturbations in the molecular periphery are approximated by two DFT calculations after removal of the central SiHz group and H saturation of the phenyl substituents (Figs. 3d and e). The enthalpy contributions calculated for the interactions over the shorter and the longer Si-N distances of 10 and 14 kJ mol-' (limit of deviation k 2 kJ mol-') [7] differ predominantly because of the individual van der Waals wrapping of the two molecular halves.

3 -SiH

Etota, = -1 504.8741 95 a.u.

ERot = -1504.834800 a.u. 1103 kJ mol-' /I

@ EslH = -1216.500253 a.u. @ ED: -1216.477034 a.u. 1 X(AEsl ,,) 61 kJ mo'-l 1 42 kJ mol-'

Fig. 3. Density functional theory calculated total energies for tris[2-dimethylamino)methylphenylsilane [7] based on

its experimental structural parameters; b) after twisting its three €L,C6-CH2 bonds by o1 = 90" as well as the

H,C-N bond by 02 = 30"; c) and d) after removal of the central SiH, subunit from the structures a) and b),

respectively, accompanied by H saturation of the ruptured bonds of the phenyl substituents (see text).

The calculations for a second selected aminosilane complex with an octahedral Si center (Fig. 4), again (Fig. 3) twisting the nitrogen lone pairs nN perpendicular to the Si...N interaction axes while accounting for the changed intramolecular van der Waals interactions, were well worth all the computational effort: the resulting approximate energy differences M s i . . . ~ for the additional long bonds in the individual fragments (Fig. 4b and c) - amounting to 10 and 4 kJ mol-' for bond lengths of about 289 and 311 pm - strongly indicated a distance-dependent potential minimum around the value AEsi . . .N of 14 kJ, which has been estimated for the three almost coincident bond lengths of 300 pm in the seven-coordinate R3HSi(...NR'RF) complex (Fig. 3). This eye-opening conclusion stimulated a highly correlated calculation of the potential curve for H3N.a - S i b , the simplest model adduct imaginable [7].

Bonding in Silicon Compounds 71

Fig. 4. Density functional theory calculated total energies for bis[2,6-bis(dimethylamino)methylphenyl]silane: a)

based on its structural parameters; b) after twisting of the 289 pm-distant or c) the 310 pm-distant (H3C)2N

substituent groups by angles o(H3CGCH2) = 120" as well as o(H2C-N(CH3)2) = 30"; d,e) after removal of

the central SiH2 unit accompanied by H saturation of the phenyl substituents starting from the structures in b)

and c), respectively (see text).

Potential Curve the Model Adduct &Si..-NH3

The energy differences h E s i . . . ~ estimated by the DFT calculations (Figs. 3 and 4) obviously cannot be correlated (directly) with the structurally determined bond lengths dsi-N. To test a nonlinear correlation with potential curves, calculations for the simplest model adduct H4Si.e.NH3 imaginable with large double-zeta basis sets at MP2 as well as MP4 levels have been performed [7]. The resulting geometry-optimized potential curve shows a minimum hEtod = 11 kJ mol-' at 300 pm (Fig. 5), which corresponds to the DFT energy difference M s i . . . ~ -- -14 kJ mol-' estimated for three

experimental bond lengths of about 300 pm (Fig. 3). Remarkably, both the higher ( h E s i . . . ~ - 5 kJ mol-' for dsi-N = 289 pm) as well as the lower value

( h E s i - ~ - 2 kJ m0l-I for dSi-N = 31 1 pm) confirm the predictions of the potential curve (Fig. 5): the higher energy value is shifted by 11 pm from the minimum towards the steeper slope for repulsive interaction and the lower one 11 pm towards the flatter part of the curve calculated for dissociative interaction [7]. The rather small DFT energy differences (estimated deviation limit 2 kJ mol-' [7]) are, nevertheless, convincingly supported by the nonlinear correlation (Fig. 5).

72 H. Bock

H4Si-,NH3 T (M P4/aug-cc-pVDZ) &al

[U mol“)

a)

: I : I

: I : I : I

j I

11

-14

Fig. 5. a) Total potential energy curve for the model adduct H4Si...NH3 calculated with basis sets (aug-cc-pVDZ)

and using MP2 correlation for the geometry optimization or MP4 correlation for all single point calculations [7]; b) Si-N interaction energies AEsi. N, DFT-estimated based on the selected crystal structures of the amino

organosilyl compounds with seven- and eight-coordinate Si centers (Figs. 3 and 4).

Si.-.N Charge Densities

For the three long-range Si. .N interactions of different lengths in the two hypercoordinate

organosilicon molecules investigated (Figs. 3,4 and 5b), in addition the DFT “natural bond orbital” charge densities q pN have been calculated - complemented by those for 1-dimethylamino-8

-trichlorosilylnaphtalene (sitrol, Fig. 2b) and -trifluoro-silylnaphthalene (Fig. 2c). Data inspection (Fig. 6) reveals that the calculated DFT/NBO charge densities do not correlate

with the experimental bond distances dsi. . .~ The charges at the N centers seem to be constant for each specific intramolecular environment and the increase in positive charge at the Si center, especially on Cl-F exchange, can be rationalized by the sequence of effective nuclear charges: Si < C1 < N < F (Fig. lb). In the hypercoordinate aminosilane complexes elaborated upon, the charge density at the Si centers is lowered slightly with increasing number of Si-N interactions, whereas - as pointed out already - the charge at the N centers seems to remain constant.

Bonding in Silicon Compounds 73

Fig. 6. DFT/NBO charge densities for Si..*N interactions at distances between 176 and 300 pm in amino organosilyl

compounds with Si coordination numbers between 5 and 8 [7].

The complexity of the data presented (Fig. 6) - experimental Si...N bond lengths stretching

over a range of 300-176 = 124 pm (!) combined with DFTNBO calculated charge densities for four individual Si-N compounds with Si centers exhibiting coordination numbers between 5 and 8 - deserve some additional spotlight comments to indicate the multifaceted discussion actually required.

The shortest bond length of 176 pm in the trichlorosilyl compound sitrol (Figs. l b and 6) might be energetically favored for steric reasons by the undulated perhydropyridine ring. The longest bond length of 300 pm (Figs. 3 and 6) can be rationalized by the minimum on the distance-dependent potential curve for the model compound &Si...NH3 (Fig. 5). General attention has to be drawn to the spatial overcrowding in almost all hypercoordinate compounds and especially the organoaminosilanes investigated (Figs. 3 and 4), containing numerous van der Waals contacts in the molecular periphery that are presumably bonding [2, 81 because of the considerable positive charge at their Si centers (cf. Fig. 6).

Altogether, geometry-optimized and MP4-correlated calculations with large basis sets for the simplest analogous complex imaginable, &Si. * ”H3, not only yielded the expected minimum at about 300 pm Si...N distance, but predicted a potential curve with a steeper

repulsive gradient at shorter distances and a shallower dissociative one at longer distances (Fig. 5).

For the possible origin of the long-range Si...N interactions at distances above 260 pm (Fig. l),

the DFTNBO analysis of the charge distribution [7] calculated for the aminosilane with the seven-coordinate Si center provides as a simple tentative suggestion a more or less overlap-free, through-space Coulombic attraction. Inserting, for instance, the resulting charges qsi of +1.6 and q N

of -0.5 at 300 pm distance (Fig. 6) into the Coulombic formula predicts a pure electrostatic attraction between the two bond centers of about 60 kT mol-’. Accordingly, a Coulomb-dominated donor-acceptor bonding over long distances is proposed for the hypercoordinate Si...N compounds discussed here.

The concept of the Coulombic attraction through space between differently charged centers

74 H. Bock

within or between molecules exemplified for hypercoordinate Si(N), compounds should be widely generalizable and quite useful to rationalize intra- or intermolecular phenomena. The inherent principle of effective nuclear charges (Fig. 1) might help in qualitative assessment of aspects concerning self-recognition and self-organization [9] of other hetero-substituted organosilicon molecules, especially those containing oxygen centers.

Enhanced Long-Range Si-. -N Interactions in Organosilicon Cations

For an extension of the DFT calculation approach developed, organosilicon cations also containing Si...N interactions have been selected to study the effect of a tremendous charge increase. The project started with the simplest imaginable model organosilicon cation H3N. .SiH3+, for which

potential curves have been calculated by density functional theory at the B3LYP level by applying both MP2 and MP4 perturbation procedures and, in addition, by a Coupled Cluster approach [lo]. Relative to the analogous potential curve for the uncharged adduct, H3N**.Si&, a considerable increase of the long-range Si.-.N interaction energy accompanied by bond shortening is predicted. To test the reliability of the quantum chemical procedure, an analogous hypercoordinate, structurally characterized dication salt, [(3-picoline)4SiH2I2+ [Cl-... (HCCl3)2]2, which contains a hexacoordinated Si center with four equivalent Si.. .N interactions, has been selected from the literature to reproduce its structure by DFT geometry optimization. The results substantiate the increase of the Si...N interactions as predicted by the potential curves for the model adduct H3N...SiH3+.

Correlated Potential Curves for a Model Cation H3N..-SiH3+

The interaction on adduct formation between H3N and the SiH3' cation is investigated quantum- chemically at several levels of sophistication (Fig. 7) and the accuracy of the procedure further tested by a calculation with a basis set of (aug-cc-pVTZ) quality [lo]. For the potential curves (Fig. 7), the distance N...Si+ was changed between 200 and 500 pm in steps of 10 pm and all the remaining degrees of freedom were optimized.

The most striking feature on structural comparison of the neutral and charged adducts, H3N-..Sib versus H3N-.-SiH3+ containing five- and four-coordinate Si centers, is the shortening of the 310 pm N..-Si distance to the cationic N-Si' bond of only 194 pm (Fig. 8), with considerably polarized NS--Si" due to the large difference in the effective nuclear charges. The cationic charge is predominantly localized in the SiH3 group ( C q s i ~ ~ = +2/3), although the H3N fragment (ZqNH3 = +1/3) indicates some ammonium character. In contrast, the neutral and only weakly bonded complex H3N+Si& (Figs. 5 and 6 ) exhibits a small o-electron transfer N+Si as indicated also by the slightly elongated axial Si-H bond. Altogether as demonstrated by the differing charges of the N and SiH centers, the interaction causes a considerable N%Si" charge transfer.

Bonding in Silicon Compounds 75

Fig. 7. Distance-dependent potential energy curves for the ammonia adduct to the silyl cation, H3N...SiH3+ at

different levels of correlated wavefunctions: density functional theory (DlT-B3LYP) with a basis set of

triple-zeta quality (aug-cc-pVTZ), MP2 as well as MP4 perturbation, and coupled cluster CCSD(T)

calculations with double-zeta basis sets (aug-cc-pVDZ).

(-1.051

Fig. 8, CCSD(T) optimized structures of the ammonia adducts to silane H3N...SiH4 and to silyl cation H3N-..SiH3+

together with DFTiNBO charges at the individual centers.

76 H. Bock

DFT Structural Reproduction of Dihydridotetrakis(3-pico1ine)silicon Dication Containing a Hexacoordinated Si Center

The partly surprising information from the highly correlated calculations for the neutral or positively charged ammonia adducts to silane or to the respective silyl cation stimulated a literature search concerning "experimental" structural evidence for N-substituted organosilicon cations or dications. A single hit was provided by a structurally characterized dication with a fourfold nitrogen-substituted silicon center of total coordination number six: the dihydridotetrakis(3- -picoline)silicon dichloride, [(H3C-&C5)4SiH2]2+[Cl- (HCC13)2]2, in which the chloride anions are each solvated by two chloroform molecules [ 111. The salt with two hydrogen-bonded Cl--.*HCCb

chloride counteranions contain an N-tetrasubstituted, hexacoordinate Si dication of slightly distorted C2h symmetry. The DFT fully optimized structure of a slightly simplified model compound with pyridine substituents instead of picoline ones reproduced the Si dication structure almost perfectly (Fig. 9).

Fig. 9. Fully DFT-optimized structure of the dihydridotetrakis(pyridine)silicon dication with selected calculated

versus experimental (in parentheses) bond lengths and charges (in brackets) from a DFTPJBO analysis.

Both the structurally characterized Si(tetrapico1ine) and the DFT geometry-optimzed Si(tetrapyridine) dications are free from any steric strain and, therefore, four equivalent strong Si-N bonds about 200 pm long formed (cf. Fig. 2) with an approximate interaction energy of 310 kJ mol-'.

Conclusions

The Si.. .N interaction energy increases considerably in organosilicon cations and, correspondingly,

the (Si-N)"' bonds exhibit a tremendous shortening relative to the long-range distances in the

Bonding in Silicon Compounds 77

uncharged aminosilane adducts. In the absence of steric overcrowding, bond lengths of about 200 pm are observed with estimated interaction energies of about 300 kJmol-’. This type of interaction, for which the Coulombic origin suggests sensitivity to polarization and induction effects, can be predicted even in detail by DFT (B3LYP) calculations with polarized double-zeta quality basis sets.

In the hope that both the method developed and the computational results for the selected Si-N compounds are of more general interest, their application to rationalizing other long-range interaction phenomena in main group element chemistry is recommended.

Acknowledgment: All calculations were performed by Dr. Zdenek Havlas from the Academy of Science in Prague and part of the graphics were provided by Dip1.-Chem. Volker Krenzel at the University of Frankfurt. The project has been supported by the Deutsche Forschungsgemeinschaft, the Fonds der Chemischen Industrie and the Supercomputer Centers in Stuttgart as well as in Prague.

References H. Bock, “Fascinating Silicon Chemistry - Retrospection and Perspectives”, in: Organosilicon Chemistry IV: From Molecules to Materials (Eds. N. Auner, J. Weis) Wiley-VCH, Weinheim, 2000, p. 37-58. Shortened version of plenary lecture at the Wacker Silicon Days, Munich, April 8th 1998 and 34th essay on “Molecular Properties and Models”. H. Bock,Angew. Chem. 1989,101,1659-1682; Angew. Chem. Int. Ed. 1989,28, 1627-1650. H. Bock, Coll. Czech. Chem. Commun. 1997,62, 1-41 and references cited therein. H. Bock, Angew. Chem. 1977, 89, 631-655; Angew. Chem. Int. Ed. 1977, 16, 613-637, and the numerous references cited therein. H. Bock, B. Solouki, “Photoelectron Spectra of Silicon Compounds” in The Chemistry of Organosilicon Compounds (Eds. S. Patai, Z. Rappoport), Wiley & Sons, Chichester, 1989, p. 555-653 (252 references). H. Bock, B. Solouki “(Helium I) Photoelectron Spectra of Silicon Compounds: History and Achievements Concerning Their Molecular States” in The Chemistry of Organic Silicon Compounds (Eds. Z . Rappoport, Y. Apeloig), Wiley & Sons, 2001, p. 165-222 (100 references). H. Bock, Z. Havlas, V. Krenzel, Angew. Chem. 1998,110,3305; Angew. Chem. Int. Ed. 1998, 37, 3165. M. Weidenbruch, B. Blintjer, K. Peters, H.G. von Schnering, Angew. Chem. 1986, 98, 1090; Angew. Chem. Int. Ed. 1986,25,1129. J.-M. Lehn, Supramolecular Chemistry; Concepts and Perspectives, VCH-Verlag, Weinheim, 1995.

[lo] Z. Havlas, H. Bock, Coll. Czech. Chem. Commun. 2001,66,473. [ 113 H. Fleischer, K. Hensen, T. Stumpf, Chem. Ber. 1996,129,765.

Thermolytic Formation and Trapping of Silenes Strongly Influenced by Reversed Polarization

Henrik Ottosson, * Tamaz Guliushvili, Ibrahim El-Sayed

Department of Organic Chemistry Box 599, Uppsala University, 751 24 Uppsala, Sweden

Tel.: +46 18 471 3809 -Fax: +46 18 471 3818 E-mail: Henrik.Ottosson@kemi.uu.se

Keywords: silenes, reversed polarization, DFT, rearrangements

Summary: The thermal rearrangement of silylamides into 1,l -bis(trimethylsilyl)-2- amino-2-trimethylsilyloxysilenes and their subsequent trapping by 2,3-dimethyl-1,3 -butadiene were investigated both computationally and experimentally, with focus on the geometric and electronic structure of these silenes.

Reversed Si=C bond polarization, which is manifested by delocalization of negative charge from n- electron donating C substituents to Si (resonance structures I1 and III, Scheme l), has been considered as the single most important electronic factor that influences the reactivity of silenes [ 11.

Scheme 1.

The first solid, stable silene (Me$i)2Si=C(OSiMe3)Ad of Brook and co-workers [2], and the 4-silatriafulvenes of Kira's group [3], are influenced by reversed polarization, as manifested by elongation of their Si=C bonds when compared to that of H2Si=CH2 [4]. However, when resonance structures I1 and I11 completely dominate the electronic structure, the Si=C bond should turn into an Si-C single bond and the Si atom should be strongly pyramidalized since it will resemble the Si of a silyl anion. These silenes therefore have nonclassical bent structures similar to many other heavy alkenes, species whose structures are explained by the theory of Carter, Goddard, Trinquier and Malrieu (CGTM) [S]. One may thus anticipate a connection between the reversed polarization effect and the CGTM theory. So far, no silene that is strongly affected by reversed polarization has been generated that is stable at ambient temperatures, even though such species extrapolate to silylene-carbene complexes, such as that of Lappert and co-workers [6].

Ovganosilicon Chemisty V Edited by N. Auner and J. Weis

Copyright 0 2003 WILEY-VCH Verlag GmbH & Co. KgaA, Weinheim

Thermolytic Formation and Trapping of Silenes Strongly InJluenced by Reversed Polarization 79

We reasoned that a silene with two n-electron donating groups at the C atom would be dominated by resonance structures I1 and 111. Quantum chemical calculations at the B3LYP/6-31G(d) level [7] reveal that this is the case since the Si-C bond elongates gradually as more strongly donating groups are attached to C (Fig. 1). For (H3Si)zSi=C(OSiH3)NMez (4) the Si-C bond has turned into an Si-C single bond, whereas for (H3Si)zSi=CH(OSiH3) (2) and (HsSi)2Si=CHNMez (3), this bond length is between that of Si=C and Si-C bonds.

Fig. 1. B3LYP/6-3 1G(d) geometries of (H3Si)2Si=CH2 (l), (H3Si)2Si=CH(OSiH3) (2), (H3Si)2Si=CH(NMe2) (3) and

(H3Si)2Si=C(NMe2)(0SiH3) (4). (Distances in A and angles in deg. XSi refers to the sum of valence angles at

Si.)

Our idea was to generate reverse-polarized silenes through thermolysis or photolysis of silylamides (5) in a manner similar to that used by Brook and co-workers [2] when forming (Me3Si)zSi=C(OSiMe3)Ad. This would lead to silenes that resemble 4. However, we soon found the discouraging fact that photolysis of silylamides had been attempted before, and only starting material was recovered after long irradiation [8]. In despair of this finding we resorted to computations to investigate the properties of silenes formed upon thermolysis of silylamides [9] Since these silenes are Si-C single-bonded, a facile back-transfer of the TMS group from 0 to Si could occur once they are formed. Indeed, B3LYP/6-31G(d) computations indicate that the barrier that separates (Me3Si)2Si=C(OSiMe3)NMez (6a) from the corresponding silylamide (Sa) is merely 9.1 kcdmol, and 5a is 17.5 kcdmol lower in energy than 6a (Fig. 2). The chances to isolate silenes 6 at thermolytic or photolytic conditions should therefore be very small.

0.0

Fig. 2. Energy surface connecting tris(trimethylsily1)silyl-N,N-dimethylarnide (5a) with l,l-bis(trimethylsilyl)-2 -N,N-dimethylamino-2-trimethylsiloxysilene (6a). Calculations at the B3LYP/6-3 1G(d) level. (Energies in

kcaYmol .)

80 H. Ottosson, T. Guliashvili, I. El-Sayed

To probe whether 6 is formed upon heating of 5 we used 2,3-dimethyl-l,3-butadiene as a trapping reagent. With this reagent added we observed that a new product was formed while monitoring the reaction by NMR. To our surprise the product was not the anticipated Diels-Alder adduct 7 between 6 and 2,3-dimethyl-l,3-butadiene (Scheme 2), but that in which the OTMS and TMS groups had changed positions (8). Even more surprisingly, the product was formed in nearly quantitative yields. And finally, the required temperature and reaction times varied widely, depending on the amino substituent. Whereas the thermolysis of 5a required two days at 180 "C for completion, only 2 h at 100 "C was needed for 5c. The latter reaction could therefore be carried out under standard reflux conditions.

' x 5 6

a: R = R' = Me, T = 180 OC, 2 days, toluene, 88 % yield

c: R = Me, R' = Ph, T = 100 O C , 3 h, benzene, 95 % yield b: R = R' = Ph, T = 100 OC, 2 h, benzene, 97 % yield TMS NRR'

Scheme 2.

B P

= 62.2 = -104.2

Fig. 3. Geometries of TMS,Si=C(OTMS)NMe2 (6a) and TMS2Si=C(OTMS)NMePh (6c) at the B3LYPI6-3 1G(d)

level. (Distances in 8, and angles in deg. CSi refers to the sum of valence angles at Si.)

These experiments provide us with ample data for further computational investigations. We feel confident that silenes 6 are formed, but how does the subsequent trapping reaction and the formation of 8 proceed? Moreover, one may ask whether there is a direct electronic influence of the N-phenyl groups on the reactivity of the silene or whether these groups have an indirect influence caused solely by their steric bulk? B3LYP/6-31G(d) computations show that they force the amino

Thermolytic Formation and Trapping of Silenes Strongly Influenced by Reversed Polarization 81

group out of conjugation with the Si=C bond (Fig. 3), making the silenes less influenced by reversed polarization. However, further computational as well as experimental studies are required to determine the exact reason why the N-phenyl groups affect the reactivity of the silene. We hope to provide a more detailed report on this and other subtleties in the reactions of silenes 6 in the near future.

Acknowledgments: Financial supports from the Wenner-Gren Foundations and from the Swedish Research Council (Vetenskapsridet), as well as a generous allotment of computer time from the National Supercomputer Center (NSC) in Linkoping, Sweden, are gratefully appreciated.

References Y. Apeloig, M. Karni, J. Am. Chem. SOC. 1984,106,6676. A. G. Brook, F. Abdesaken, B. Gutekunst, G. Gutekunst, R. K. Kallury, J. Chem. SOC., Chem. Commun. 1981, 191. a) K. Sakamoto, J. Ogasawara, H. Sakurai, M. Kira, J. Am. Chem. SOC. 1997,119,3405; b) T. Veszprimi, M. Takahashi, J. Ogasawara, K. Sakamoto, M. Kira, J. Am. Chem. SOC. 1998, 120, 2408; c) T. Veszprimi, M. Takahashi, B. Hajgat6, J. Ogasawara, K. Sakamoto, M. Kira, J. Phys. Chem. A. 1998, 102, 10530; d) M. Takahashi, K. Sakamoto, M. Kira, Znt. J. Quant. Chem. 2001, 84, 198; e) K. Sakamoto, J. Ogasawara, Y. Kon, T. Sunagawa, C. Kabuto, M. Kira, Angew. Chem. Int. Ed. 2002,41, 1402. S . Bailleux, M. Bogey, J. Demaison, H. Burger, M. Senzlober, J. Breidung, W. Thiel, R. Fajgar, J. Pola, J. Chem. Phys. 1997,24, 10016. a) E. A. Carter, W. A. Goddard 111, J. Phys. Chem. 1986, 90, 998; b) J.-P. Malrieu, G. Trinquier, J. Am. Chem. SOC. 1989,111,5916. W. M. Boesveld, B. Gehrhus, P. B. Hitchcock, M. F. Lappert, P. v. R. Schleyer, J. Chem. SOC., Chem. Commun. 1999,155. M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria, M. A. Robb, J. R. Cheeseman, V. G. Zakrzewski, J. A. Montgomery, R. E. Jr. Stratmann, J. C. Burant, S. Dapprich, J. M. Millam, A. D. Daniels, K. N. Kudin, M. C. Strain, 0. Farkas, J. Tomasi, V. Barone, M. Cossi, R. Cammi, B. Mennucci, C. Pomelli, C. Adamo, S . Clifford, J. Ochterski, G. A. Petersson, P. Y. Ayala, Q. Cui, K. Morokuma, D. K. Malick, A. D. Rabuck, K. Raghavachari, J. B. Foresman, J. Cioslowski, J. V. Ortiz, A. G. Baboul, B. B. Stefanov, G. Liu, A. Liashenko, P. Piskorz, I. Komaromi, R. Gomperts, R. L. Martin, D. J. Fox, T. Keith, M. A. Al-Laham, C. Y. Peng, A. Nanayakkara, C. Gonzalez, M. Challacombe, P. M. W. Gill, B. G. Johnson, W. Chen, M. W. Wong, J. L. Andres, M. Head-Gordon, E. S . Replogle, and J. Pople, Gaussian 98 (Revision A.9), Gaussian, Inc., Pittsburgh PA, 1998. S. S. Al-Juiad, Y. Derouiche, P. B. Hitchcock, P. D. Lickiss, A. G. Brook, J. Organomet. Chem. 1991,403,293. I. El-Sayed, T. Guliashvili, R. Hazell, A. Gogoll, H. Ottosson, Org. Lett. 2002,4, 1915.

Synthesis, Structure and Reactivity of Intramolecularly Donor-Stabilized Silenes

Martin Mickoleit, Matthias Potter, Ute Baumer, Kathleen Schmohl, Hartmut Oehme*

Fachbereich Chemie der Universitat Rostock, D- 18051 Rostock, Germany Tel.: +49 381 498 1765 -Fax: +49 381 498 1763

E-mail: hartmut.oehme@chemie.uni-rostock.de

Rhett Kempe

Institut fur Organische Katalyseforschung an der Universitat Rostock, Germany

Keywords: silanes, silenes, donor-acceptor systems, structure elucidation

Summary: Using a new synthetic pathway, four intramolecularly donor-stabilized silenes, R(Me$i)Si=C(SiMe& (10: R = 8-dimethylamino-1-naphthyl; 11: R = 2 -(dimethylaminomethyl)phenyl; 14: R = 2,6-bis(dimethylaminomethyl)phenyl; 15: R = 2,6-bis(diethylaminomethyl)phenyl)), were prepared by the reaction of (dichloromethyl)tris(trimethylsilyl)silane (lc) with the respective dialkylamino substituted aryllithium compounds (molar ratio 1:2). X-ray structural analyses of the four silenes revealed strong donor-acceptor interactions between the dialkylamino groups and the electrophilic silene silicon atoms, leading to pyramidalization at the silicon centers. The configuration at the silene carbon atoms was found to be planar. The chemical behavior of the new silenes, particularly reactions with water and methanol, treatment with methyl iodide or methyl triflate and conversions with benzaldehyde, are discussed.

Introduction

Donor-acceptor interactions between the electrophilic Si atoms of three-coordinate silicon derivatives and suitable bases lead to an effective stabilization of these usually extremely reactive systems. Thus, for example, silenes and silanimines have been shown to be stabilized by tertiary amines [ 11. Extraordinary stabilizations have been achieved by intramolecular Si-N coordinations, making possible the isolation of stable silanethiones [2] and silylium salts [3]. Intramolecularly donor-stabilized silenes have been unknown till now, possibly because of the lack of suitable synthetic methods.

Ovganosilicon Chemisty V Edited by N. Auner and J. Weis

Copyright 0 2003 WILEY-VCH Verlag GmbH & Co. KgaA, Weinheim

Synthesis, Structure and Reactivity of Intramolecularly Donor-Stabilized Silenes 83

Reaction of (Dichloromethy1)oligosilanes with Organolithium Reagents

Recently we found (dichloromethy1)oligosilanes (Me3Si)2R1Si-CHC12 (la-c) (a: R' = Me; b: R' = Ph; c: R' = SiMe3) to react with organolithium compounds R2Li (R2 = Me, nBu, Ph) producing (after hydrolysis) [bis(trimethylsilyl)methyl]silanes R'R22Si-CH(SiMe3)2 (4a-c). As described previously [4], in the course of this reaction, which is outlined in Scheme 1, highly reactive silenes 2a-c and 3a-c occur as intermediates, which in the case of the reaction of the starting dichloromethylsilane with MeLi, nBuLi or PhLi, respectively, (molar ratio 1 :3), are immediately trapped by excess R2Li to give, after protonation during the aqueous workup, the silanes 4a-c. Attempts to isolate dimers of the silenes 2a-c or 3a-c by reducing the molar ratio of the reactants to 1:2 or 1:l were unsuccessful. Under these conditions 4a-c were obtained simply in lower yields. We assume the deprotonation of la-c to be a slow process compared with the consecutive steps. Thus, the silenes formed always meet an effective excess of R2Li and the reaction proceeds through all the intermediates given in Scheme 1 to the final products 4a-c.

Me3Si, + R2Li [ Me3:; ] Me3Si-SiCHClp - Me3Si-Si-C\-CI

- R ~ H R"

1 a-c / 1- LiCI

1,2-Li,SiMes-exchange

silylcarbene-silene-rearrangement LiCl 1 /

2a-c

a: R' =Me

c: R' =SiMe3 b: R' = Ph

3a-c

-1. i. + R'Li ii. + H20, - LiOH

R2 \ H

R1Si-C:-SiMe3 R' SiMe3

R2 =Me, nBu, Ph 4a-c

Scheme 1. The reaction of the (dich1oromethyl)oligosilanes la-c with organolithium reagents R2Li (Rz = Me, nBu, Phl.

But if, in this reaction, organolithium derivatives are applied with groups R2, which, when introduced to the silene silicon atom of 3a-c, provide sufficient stabilization to the Si=C system by

84

its steric bulk or by any other means, so that further addition of R2Li is prevented, the reaction may stop at this stage and the resulting silenes may be expected to be stable. Following this concept, l c was treated with mesityllithium (molar ratio 1:2). But, unexpectedly, we obtained no silene or its dimer, but the sterically crowded silane 5 (Scheme 2). Obviously, the steric protection of the silene 3c (R2 = Mes) by one mesityl group and three trimethylsilyl substituents is insufficient to prevent further addition of mesityllithium.

M. Mickoleit, M. Potter, U. Baumer, K. Schmohl, H. Oehme, R. Kempe

1. Deprotonation, carbenoid formation 2. Elimination of LiCI, formation of 3c (R2 = Mes) 3. Addition of Mes-Li to the Si=C bond 4. Repeated loss of LiCl and renewed

rearrangement and silene formation

MesSi,

/ Me3Si

Me3Si-Si-CHClp + 3 Li

l c

Mes, SiMe3 MesLi Mes\ ,SiMe3 [ Me3si/Si=$iMe3 ] y Me3Si/ Mes-si-c~ SiMe3

5

Scheme 2. The reaction of (dichloromethyl)tris(trimethylsilyl)silane (lc) with mesityllithium (molar ratio 1:3).

Me3Si, Me3Si-Si-CHClp + 2 TipLi

R”

1 a-c -1Pr3CeH3

Tp\ /SiMe3 Tp\ /SiMe3

P i SiMe3 Me3SI/ SiMe3 Si=C, Si=C,

6 7 8

1 TipLi I H20

Tip! ,SiMe3

Me/ SiMe3 Tip-Si-C-H

9

Scheme 3. The reaction of the (dichloromethy1)oligosilanes la-c with 2,4,6-triisopropylphenyllithium (TipLi).

Synthesis, Structure and Reactivity of Intramolecularly Donor-Stabilized Silenes 85

To improve the kinetic stabilization of the silenes 3, 2,4,6-triisopropylphenyllithium (TipLi) was used instead of MesLi in the reaction with la-c, and, indeed, two indefinitely stable silenes, 7 and 8, were formed (Scheme 3). Unfortunately, we did not succeed in crystallizing these kinetically stabilized silenes and in isolating the two compounds in pure form. This is mainly due to problems in a complete separation of 1,3,5-triisopropylbenzene, always formed as a byproduct. But the oily silenes were characterized by their conversion with water into the respective silanols, by their reaction with methanol to give methoxysilanes as well as by [2+2] cycloaddition reactions with benzaldehyde to afford stable 1,2-oxasiletanes [4b]. Interestingly, the reaction of l a with TipLi gave no stable silene, but the sterically extremely congested silane 9 was obtained. That means, in case of the small methyl substituent in 6, that the kinetic stabilization of the silene with respect to further additon of the organolithium reagent is insufficient and a second triisopropylphenyl substituent is introduced to the central silicon atom.

Synthesis and Structure of Intramolecularly Donor-Stabilized Silenes

The efficiency of the new synthetic method of generating stable silenes by the reaction of (dichloromethy1)oligosilanes with organolithium reagents is best demonstrated by the reaction of l c with dimethylamino-functionalized aryllithium compounds to give intramolecularly donor- stabilized Si=C systems. Actually, in a straightforward reaction, l c with two equivalents of 8-dimethylamino-1-naphthyllithium afforded the crystalline, yellow silene 10 in a yield of 79 % (Scheme 4). Similarly, l c and 2-(dimethylaminomethyl)phenyllithium (1 :2) gave the silene 11 (74 %). The compound is almost colorless. Both silenes proved to be indefinitely stable at room temperature and, obviously, are resistant with respect to further addition of excess of the organolithium compound to the Si=C double bond.

for RLi = &yMeSiMe3 / si=c I 1

Li NMe2/ Me3Si/ SiMe3

10 Me3Si,

Me3Si’ - 2 LiCl Me3Si-Si-CHC12 + 2 RLi - RH

lc

11

Scheme 4. Synthesis of the intramolecularly donor-stabilized silenes 10 and 11 by the reaction of l c with

8-dimethylamino- 1 -naphthyllithium or 2-(dimethylaminomethyl)phenyllithium, respectively.

86 M. Mickoleit, M. Potter, U. Baumer, K. Schmohl, H. Oehme, R. Kempe

The X-ray structural analysis of 10 offers the expected picture of an intramolecularly amine -coordinated silene (Fig. 1). The electron pair donation of the amino group causes pyramidalization at the silene silicon atom. The sum of angles at the central Si atom is 345.5'. The geometry at the silene carbon atom is still trigonal planar. The Si-N distance of 2.069(2) 8, is longer than a usual silicon-nitrogen single bond, which amounts to approx. 1.75 A. The silicon-carbon bond length is 1.751(3) A. The structural data obtained for 13 are comparable (see Table 1).

Fig. 1. Molecular structure of 10 in the crystal (ORTEP, 30 % probability, H atoms omitted for clarity); selected

bond lengths [A] and angles ["I: Cl-Sil 1.751(3), Cl-Si3 1.830(3), C1-Si4 1.832(3), Sil-C2 1.889(3),

Si 1Si2 2.3827( 12), Sil-NI 2.069(2); Sil-Cl-Si3 123.03( 1 3 , Sil -C1 -Si4 120.17( 1 3 , Si3-Cl -Si4

115.66(14), C1-Sil-C2 119.98(12), C1-Sil-Si2 125.63(10), Cl-Sil-Nl 113.93(11), C2-Sil-Si2 99.90(8),

C2-Sil-Nl 86.30(10), Si2-Sil-N1 103.28(7).

The silicon-carbon double bond in intramolecularly donor-stabilized silenes is also extremely reactive. This is demonstrated by the reaction of (dichloromethyl)tris(trimethylsilyl)silane with 8-dimethylaminomethyl- 1 -naphthyllithium, which deviates from the general reaction pattern. We expected to get another stable silene, but we obtained in a yield of 51 % the 1-sila-acenaphthene 13. Obviously, the silene 12 is unstable, and the silicon-carbon bond is inserted into one carbon-hydrogen bond of the benzylic methylene group (Scheme 5).

Of particular interest is the reaction of the same starting (dichloromethy1)silane l c with 2,6-bis(dimethylaminomethyl)phenyllithium, since after formation of the silene along the reaction path discussed, one or two dimethylamino groups may coordinate to the silene silicon atom, i.e. the silicon can be tetra- or pentacoordinate (Scheme 6). The X-ray analysis of the colorless

Synthesis, Structure and Reactivity of Intramolecularly Donor-Stabilized Silenes 87

compound 14, which is obtained in a very straightforward reaction in a yield of 57 %, unambiguously revealed the coordination of only one amino group. The Si-N distance between the silene silicon atom and the coordinated dimethylamino group is 2.035(3) A, which is in agreement with the other derivatives already discussed; the Si-N distance to the other amino group (3.441(3) A) is considerably longer. The sum of angles at the silene silicon atom amounts to 348.5'; the sum of angles at the silene carbon atom is 359.9'. Similarly, 1-[2,6- bis(diethylaminomethyl)phenyl]- 1,2,2-tris(trimethylsilyl)silene (15) was obtained from l c and 2,6- bis(diethylaminomethy1)phenyllithium (1 :2) (49 %).

Me3Si, Me3Si-/Si-CHClz +2 RLi -

Me3Si - RH - 2 LiCl

l c

RLi = 93 NMez

{ SiMe3 g T l i M e 3 Si=C,

Me3Si

12

SiMe3

1 MezN-CH-Si-CH(SiMe3)z

65 13

Scheme 5. Generation of the transient silene 12 and its conversion into the 1-silaacenaphthene 13.

L - L - - - Si=C - 1 ,3-(Me2NCH2)C6H4 Me3S1/ SiMe3

Me2N Me3Si,

M e 3 d Me3Si-Si-CHC12 + 2

- 2 LiCl Me2N

lc 14

Scheme 6. Reaction of l c with 2,6-bis(dimethylaminomethyl)phenyllithium (molar ratio 1:2).

In Table 1 the structural data of the synthesized intramolecularly donor-stabilized silenes 10, 11, 14 and 15 are summarized and additionally, for comparison, some data taken from the literature are given. The silicon-carbon distances, the silicon-nitrogen distances and the sums of angles at the silene silicon atoms as well as at the silene carbon atoms of the unsaturated silicon compounds

88 M. Mickoleit, M. Potter, U. Baumer, K. Schmohl, H. Oehme, R. Kempe

described in this paper are quite comparable. All these data are in reasonable agreement with values obtained by Wiberg for an acyclic silene-amine adduct [lb]. In this compound also, the geometry at the silicon atom is pyramidal; the configuration at the carbon atom is planar. A silicon-nitrogen distance of approx. 2.0 8, appears to be a standard value for silene-amine adducts. Coordination of an amine to the silene silicon atom obviously leads to a weakening and elongation of the silicon- carbon double bond. The Si=C distance of 1.702(5) 8, in the uncomplexed kinetically substituted silene, made by Wiberg, is significantly shorter [5 ] . The Si=C bond lengths in the silene made in the Brook's group [6] as well as in Apeloig's silene [7] are longer and almost equal to those of the donor-stabilized derivatives. But this - as theoretical work by Apeloig indicated - is due to particular substituent effects [7, 81.

Table 1. Selected structure parameters of the intramolecularly donor-stabilized silenes 10, 11, 14 and 15, and, for

comparison, some structural data of an acyclic silene-amine adduct and some uncomplexed kinetically

stabilized silenes.

Sum of angles [deg]

Sil c 1 Compound Si=C [A] Bl-Nl[A]

10 Me3Si' SiMe2

& r 2 , S i M e 2 Si=C, 11 Me3Si' SiMe2

Me2"

15 Gr p=C\ ,SiMe3

,) Me& 'SiMe3

EBN

MezEtN.MezSi=C( SiMe2Ph),["

Me2Si=C(SiMe3)( SiMetBuz)'bl

(Me3Si)2Si=C(OSiMe3)Ad[c'

(Me3Si)zSi=CAd"d1

1.75 l(3)

1.749(3)

1.759(3)

1.764(2)

1.761(4)

1.702(5)

1.764

1.741(2)

2.069(2)

2.004(2)

2.035(3)

2.077(2)

1.988(4)

-

-

-

345.5 358.9

343.6 359.3

348.5 359.9

345.7 359.5

341.8 359.2

360.0 360.0

359.9 359.8

359.8 359.9

Ad = 1-adamantyl; Ad'= 2-adamantylidene

[a1 Ref. [Ih]. [bl Ref. [51. [c] Ref. [61. [dl Ref. [71.

Synthesis, Structure and Reactivity of Intramolecularly Donor-Stabilized Silenes 89

The donation of the nitrogen lone pairs to the silene silicon atoms of 10, 11, 14 and 15 produces systems which might better be described as ylides (Scheme 7). The structural data agree with this picture and also the reactivity is consistent with a high negative charge, i.e. a nucleophilic center at the silene carbon atom.

c NMe2,SiMe3 c k 2 , S i M e 3 Si=C Si-cd

Me3Si/ \SiMe3 Me3Si/ SiMe3

C N M e 2 - SiMe3 c :Me2 SiMen

Scheme 7. Ylide character of donor-Stablized silenes 10, 11,14 and 15.

The chemical reactivity of these silenes - compared with uncomplexed Si=C systems - is dramatically reduced. The compounds are thermally stable up to temperatures higher than 100 "C. By no means were we able to obtain dimers. Dimethylbutadiene, regularly used for the characterization of silenes, did not react with 10, 11, 14 and 15; the silenes were recovered unchanged. As expected, water and methanol are added to the Si=C bonds to afford silanols or methoxysilanes, respectively.

@lMe2,SiMe3 \ Si=C

Me3S/ SiMe3

- + Me1

\

10 1- Me1

Me Si-C,-SiMe3

/

Me3!$ SiMe3

16

Scheme 8. The reaction of the intramolecularly donor-stabilized silene 10 with methyl iodide.

The reaction of 10, 11, 14 and 15 with methyl iodide or methyl triflate is expected to produce

90 M. Mickoleit, M. Potter, U. Baumer, K. Schmohl, H. Oehme, R. Kempe

C"" Si=O /

Me3Si

intramolecularly donor-stabilized silylium salts, which - in some cases - are known to be stable compounds [3]. As the result of respective studies we obtained complex mixtures of products, insoluble in nonpolar solvents, but we did not succeed in isolating a pure silylium salt. The reaction of 10 with methyl iodide afforded the cyclic aminosilane 16 (Scheme 8). This is understood to be the result of the methylation of the silene carbon atom by a nucleophilic reaction. But the silylium iodide formed is obviously not stable. Attack of the iodide ion at the methyl group of the amino substituent produces the final product 16.

The silenes 10 and 14 were chosen for studies of the behavior of the new compounds towards benzaldehyde. As products of a standard Peterson mechanism, mixtures of the stereoisomers of the silanone dimers 18 and 19, respectively, were obtained, besides 2,2-bis(trimethylsilyl)styrene (17) (Scheme 9).

Me3SiC,SiMe3

H/"Ph

+ I1

CHMe2,SiMe3 Si=C + Ph-C\ I/ 0 - [ cPMe2,SiMe3 ./gi-y'SiMe3 ] Mess! 'SiMe3 H Me3S' O-CHPh

10,14 I 17

2x I

18,19

18: c NMe2 = 8-dimethylamino-1 -naphthyl

19: <!Me2 = 2,6-bis(dimethylaminomethyl)phenyl

Scheme 9. Conversion of the intramolecularly donor-stabilized silenes 10 and 14 with benzaldehyde into 2,2-

bis(trimethylsily1)styrene (17) and the silanone dimers 18 and 19, respectively.

Synthesis, Structure and Reactivity of lntramolecularly Donor-Stabilized Silenes 91

Acknowledgments: This work was supported by the State of Mecklenburg-Vorpommem, by the Deutsche Forschungsgemeinschaft, and by the Fonds der Chemischen Industrie. We thank Prof. M. Michalik, Dr. W. Baumann, and Prof. N. Stoll for recording NMR and mass spectra.

References a) N. Wiberg, H. Kopf, J. Organomet. Chem. 1986, 315,9; N. Wiberg, G. Wagner, G. Reber, J. Riede, G. Miiller, Organometallics 1987, 6, 35; b) N. Wiberg, K.-S. Joo, K. Polborn, Chem. Ber. 1993,126,67; c) N. Wiberg, K. Schurz, J. Organomet. Chem. 1988,341, 145; G. Reber, J. Riede, N. Wiberg, K. Schurz, G. Miiller, 2. Naturjorsch. Teil B 1989,44, 86; S. Walter, U. Klingebiel, D. Schmidt-Base, J. Organomet. Chem. 1991,412,319. P. Arya, J. Boyer, F. CarrC, P. Corriu, G. Lanneau, J. Lapasset, M. Perrot, C. Priou, Angew. Chem. 1989, 101, 1069; Angew. Chem. Znt. Ed. 1989, 28, 1016; J. Belzner, H. Ihmels, B. 0. Kneisel, R. Herbst-her, Chem. Ber. 1996,129, 125. C. Chuit, R. Comu, A. Mehdi, C. ReyC, Angew. Chem. 1993, 105, 1372; Angew. Chem. Int. Ed. 1993,32, 131 1; C. Brelikre, F.CarrC, R. Comu, M. Wong Chi Man, J. Chem. Soc., Chem. Commun. 1994, 2333; M. Chauhan, C. Chuit, R. J. P. Corriu, A. Mehdi, C. ReyC, Organometallics 1996, 15, 4326; J. Belzner, D. Schk, B. 0. Kneisel, R. Herbst-her, Organometallics 1995,14, 1840. a) K. Schmohl, T. Gross, H. Reinke, H. Oehme, Z. Anorg. Allg. Chem. 2000,626, 1100; b) K. Schmohl, H. Reinke, H. Oehme, Eur. J. lnorg. Chem. 2001,481. N. Wiberg, G. Wagner, J. Riede, G. Miiller, Organometallics 1987,6,32. A. G. Brook, S. C. Nyburg, F. Abdesaken, B. Gutekunst, G. Gutekunst, R. K. M. R. Kallury, Y. C. Poon, Y.-M. Chang, W. Wong-Ng, J. Am. Chem. SOC. 1982,104,5667. Y. Apeloig, M. Bendikov, M. Yuzefovich, M. Nakash, D. Bravo-Zhivotovskii, D. Blaser, R. Boese, J. Am. Chem. SOC. 1996,118,12228. Y. Apeloig, M. Karni, J. Am. Chem. SOC. 1984,106, 6676.

Cyclotrimetallenes Consisting of Heavier Group 14 Elements:

A New Unsaturated Small Ring System

Akira Sekiguchi, * W i m i r Ya. Lee

Department of Chemistry, University of Tsukuba, Tsukuba, Ibaraki 305-8571, Japan E-mail: sekiguch @ staff.chem.tsukuba.ac.jp

Keywords: cyclotrimetallene, doubly bonded silicon, small ring

Summary: The synthesis, structural characteristics, and physical properties of unsaturated three-membered rings containing heavier group 14 elements with endocyclic metal-metal double bonds, including cyclotrigermenes, cyclotrisilenes, cyclotristannene, and disilagermirenes, is summarized.

Introduction

The chemistry of doubly bonded silicon, germanium, tin, and lead compounds, as well as the chemistry of highly strained three-membered rings, is one of the most exciting fields in the organometallic chemistry of heavier group 14 elements [ l , 21. The unsaturated analogues of cyclotrimetallanes, that is, cyclotrimetallenes of heavier group 14 elements with endocyclic metal-metal double bonds, have been prepared quite recently. Undoubtedly, cyclotrimetallenes are very unusual molecules, since they possess the properties of both a highly strained three-membered skeleton and a highly reactive endocyclic metal-metal double bond. Therefore, one can reasonably expect unusual attributes in such compounds, including structural characteristics, photochemical behavior, and enhanced reactivity. In this overview, the chemistry of cyclotrimetallenes consisting of heavier group 14 elements will be considered, covering the literature up to 2001. As well as the cyclotrimetallenes, there are some other examples of cyclic unsaturated compounds composed of heavier group 14 elements. These include two cyclotetrasilenes, hexakis(tert-butyldimethylsily1)- cyclotetrasilene [3] and 3,4-diiodo- 1,2,3,4-tetrakis(tri-tert-butylsilyl)cyclotetrasilene [4], and three five-membered cyclic compounds, thia-, selena- and telluratetrasilacyclopentenes [5]. However, since these compounds do not represent examples of three-membered rings, they will not be discussed in this article.

Ovganosilicon Chemisty V Edited by N. Auner and J. Weis

Copyright 0 2003 WILEY-VCH Verlag GmbH & Co. KgaA, Weinheim

Cyclotrimetallenes Consisting of Heavier Group 14 Elements 93

C yclotrigermenes

The first record of three-membered unsaturated ring systems consisting of heavier group 14 elements appeared in 1995 [6], when Sekiguchi et al. reported the unexpected formation of tetrakis(tri-tert-butylsily1)- and tetrakis(tri-tert-butylgermy1)cyclotrigermenes (la,b) by the reaction of GeCl2.dioxane with two equivalents of 'Bu3SiNa or 'Bu3GeLi in THF at -70 "C (Scheme 1). Compounds la,b were isolated in 20 and 13 % yield respectively, as dark red air-sensitive crystals. They appeared to be thermally labile and photosensitive: heating at 100 "C in toluene or irradiation with light of wavelength longer than 300 nm caused their decomposition due to cleavage of the exocyclic Ge-Si or Ge-Ge bonds. The mechanism of the formation of cyclotrigermenes was elucidated later by the same authors [7]. It is interesting that under different reaction conditions (room temperature, excess 'BusSiNa) Wiberg et al. obtained a completely different product, tetrakis(tri-tert-butylsily1)tetragermatetrahedrane [8].

THF 2 'Bu3MM' + :GeCI2.dioxane - M=Si, M '=Na M=Ge, M'=Li

'Bu3M /Ge=Ge \ MfBu3

la: M = S i lb: M = G e

-70 "C -+ r.t.

Scheme 1. Synthesis of cyclotrigermenes la and lb.

Shortly after the discovery of the first cyclotrigermenes la,b, a cyclotrigermenium cation ('Bu3Si)3Ge3' (2') was prepared by the oxidation of l a with Ph3C+-BAr4- as the first free germyl cation with an aromatic two n-electron system [9]. Such a stable cation was found to be a very convenient source for the synthesis of new, unsymmetrically substituted, cyclotrigermenes. Thus, treatment of 2'sTFPB- (TFPB- = [3,5-(CF3)2C&]J-) with an equimolar amount of the appropriate nucleophile ('BusSiNa, 'Bu3GeLi, (Me-,Si)sSiLi, (Me3Si)3GeLi, 'BuzMeSiNa, MesLi, MeLi) in Et2O at -78 "C results in the formation of the corresponding 3-substituted cyclotrigermenes la, 3-8 in good yields (Scheme 2) [lo].

The crystal structures of cyclotrigermenes have been determined for la, 4, 6, and 7. It was found that the geometry around the Ge=Ge double bond strongly depends on the substituents (Fig. 1). Thus, for example, tri-tert-butylsilyl- or tri-tert-butylgermyl-substituted cyclotrigermenes la,b showed a completely planar geometry with a Ge=Ge double bond length of 2.239(4) A, which was supported by theoretical calculations of the model H3Si- and H3Ge-substituted cyclotrigermenes (Ge=Ge double bond length of 2.228 and 2.231 A, respectively) [6]. In contrast, the mesityl-substituted cyclotrigermene 7 showed a trans-bent structure, with a bend angle of 11.1" and Ge=Ge double bond length of 2.2680(4) A. Such trans geometry was characteristic of all previously

94 A. Sekiguchi, V. Ya. Lee

isolated and characterized digermenes [ 11. Changing the substituent from mesityl to the tris(trimethylsily1)silyl group caused a dramatic change in the geometry. Thus, cyclotrigermene 4 has a cis-bent structure with bend angles of 12.5" and 4.4" for sp2 Ge atoms and a Ge=Ge bond length of 2.264(2) 8, [lo]. Such geometry was well reproduced by the theoretical calculations, which gave values for the cis-bending angles of 8.8" and 5.8", respectively.

Ge

TFPB@ R-M/THF or EQO

-100 "C + r.t. 'Bu3Si Si'Bu3 'Bu3Si

2+*T FPB - R-M = 'Bu3SiNa,

'B u3GeNa

(Me3Si)3SiLi TFPB- = [3,5-(CF3)&H3]4B-

(Me3Si)3GeLi

'BuzMeSiNa

MesLi

MeLi

Scheme 2. Synthesis of cyclotrigermenes la, 3,4,5,6,7, and 8.

la: R = 'Bu3Si

3: R = 'Bu3Ge

4 R = (Me3Si)3Si

5: R = (Me3Si)3Ge

6: R = SiMefBu2

7: R = Mes

8:R=Me

Si'Bu3 'Bu3si\ Ge '*""'

'Bu3Si /

la: planar 4: cis-bent 7: trans-bent

Fig. 1. Schematic representation of cyclotrigermenes la, 4, and 7.

Cyclotrisilenes

Only three examples of cyclotrisilenes have been reported to date. The first two isolable cyclotrisilenes were reported independently by Era's and Sekiguchi's groups in 1999 [ l l , 121. The reduction of ('BuMe2Si)3SiSiBr2CI with potassium graphite (Kc8) in THF at -78 "C produced cyclotrisilene 9 as air-sensitive dark red crystals in 11 % yield (Scheme 3) [ l l ] . The existence of the doubly bonded silicon atoms was shown by the 29Si NMR spectrum: the two downfield signals at +81.9 and +99.8 ppm, which are shifted significantly upfield relative to those for the acyclic tetrasilyldisilenes (+142-154 ppm) [ 131. The major product of the reductive condensation of

Cyclotrimetallenes Consisting of Heavier Group 14 Elements 95

('BuMe2Si)3SiSiBr*Cl depends on the reaction conditions. Thus, treatment of ('BuMe2Si)3SiSiBr~Cl with sodium at room temperature gave the previously prepared hexakis(tert-butyldimethylsily1)- cyclotetrasilene 10 in 64 % yield rather than 9.

-78 "C + r.t. / KCflHF

9

PiR3 R3Si,

S e S i %.. 1 Ti\

Si=Si R3Si / 'SiR3

R3SiSiBrzCI -( 12

R = SiMez'Bu Ndtoluene

r.t. Si=Si, R' R

10

Scheme 3. Synthesis of cyclotrisilene 9, cyclotetrasilene 10, and spiropentasiladiene 12.

A symmetrically substituted cyclotrisilene and the first crystal structure of cyclotrisilene derivatives were reported by Sekiguchi et al. [12]. Cyclotrisilene 11 was prepared in 9 % yield by the reductive coupling of ('BuzMeSi)2SiBrz and 'Bu~MeSiSiBr3 with sodium in toluene at room temperature (Scheme 4). Cyclotrisilene 11 was isolated as air- and moisture-sensitive red-orange crystals, which showed a downfield signal at +97.7 ppm in the 29Si NMR spectrum attributable to the unsaturated silicon atoms.

'Bu2MeSi+< fMe'Bu2

Ndtoluene

r.t., 3 h * A 2 'BuzMeSiSiBr3 + ( ' B U ~ M ~ S ~ ) ~ S ~ B ~ ~

'BunMeSi /si-si 'SiMe'BuZ

11

Scheme 4. Synthesis of cyclotrisilene 11

X-ray crystallographic analysis of 11 (Fig. 2) showed a trans-bent configuration of the Si=Si double bond with a bend angle of 31.9" and an Si=Si double bond length of 2.138(2) A, which is one of the shortest Si=Si bond lengths reported thus far (2.138-2.261 A) [2]. One of the possible reasons for such bending may be the eclipsed conformation of the two SiMe'Bu2 groups attached to the unsaturated silicon atoms.

96 A. Sekiguchi, V. Ya. Lee

Fig. 2. X-ray crystal structure of cyclotrisilene 11.

The last example of a cyclotrisilene has an unusual spiro structure: tetrakis[tris(tert- butyldimethylsilyl)silyl]spiropentasiladiene 12, which was reported quite recently by Kira's group as a side product of the above-mentioned reaction of ('BuMe2Si)3SiSiBr2Cl with KC8 (see Scheme 3) [14]. Such a compound, which has no carbon analogue in organic chemistry, exhibited a significant through-space interaction between the two perpendicular double bonds, which was demonstrated by the spectroscopic data and theoretical calculations.

Cyclotristannene

The only example of a stable cyclotristannene was reported in 1999 by Wiberg et al. [15]. The synthesis of this compound was performed starting from the stable stannylenes :Sn[N(SiMe3)2]2 or :Sn(O'Bu)z by reaction with 'Bu3SiNa in pentane at low temperature (Scheme 5). The very interesting point about this synthesis is that the primary product in the reaction of : S I I [ N ( S ~ M ~ ~ ) ~ ] ~ with 'Bu3SiNa is tristannaallene 13, which isomerizes at room temperature to the cyclotristannene 14. Tristannaallene 13, which was isolated in 20 % yield as air- and moisture-sensitive dark blue crystals, has a half-life of 9.8 hours at 25 "C. Cyclotristannene 14, isolated in 27 % yield as dark red-brown crystals, is an air-sensitive but moisture-stable compound, which decomposes at 166 "C in the solid state. NMR data of cyclotristannene 14 correspond well with the structure. The very informative "'Sn NMR spectrum showed two resonances, of which the downfield signal of +412 ppm is characteristic of unsaturated Sn atoms in the distannene unit, and the upfield signal of -694 ppm can be assigned to a saturated endocyclic Sn atom. X-ray crystallographic analysis showed the almost planar configuration of the Sn=Sn double bond. The most striking feature of the molecule is the short Sn=Sn double bond length of 2.59 A, which is the shortest Sn=Sn distance of

Cyclotrimetallenes Consisting of Heavier Group 14 Elements 97

all the distannenes reported thus far [16].

w ( ' B u ~ S ~ ) ~ S ~ ~ ~ " ~ S ~ ( S ~ ' B U ~ ~ ~ 'Bu3SiNdpentane - C6D6 : S ~ I [ N ( S ~ M ~ ~ ) ~ ] ~

13 -1 96 "C + -25 "C

half-life period 9.8 h at 25 O C P

'Bu3Sg ,Si'Bu3

'Bu3Si /Sn-sn 'Si'Bus 14

'Bu3SiNa/pentane w

: Sn(O'Bu)z -78 "C + r.t. z\ Scheme 5. Synthesis of tristannaallene 13 and cyclotristannene 14.

"Mixed" Cyclotrimetallenes

It was expected that "mixed" cyclotrimetallenes, that is, three-membered ring compounds consisting of different group 14 elements, would possess specific and unexpected properties, which may distinguish them from their homonuclear analogues. Until now, only two examples of such molecules have been synthesized. In 2000, Sekiguchi et al. reported the first representatives of "mixed" cyclotrimetallenes: 1- and 2-disilagermirenes 15 and 16 [ 171. 1-Disilagennirene 15 was prepared by the Wurtz-type reductive coupling reaction of R2GeC12 and RSiBr3 (R = SiMe'Bu2) with sodium in toluene at room temperature (Scheme 6).

'BuzMeSb SiMe'Bu2 .. 1

Ndtoluene

r.t., 6 h 2 'BuzMeSiSiBr3 + ( ' B U ~ M ~ S ~ ) ~ G ~ C I ~ + L

'Bu2MeSi 'SiMe'Bu2

15

Scheme 6. Synthesis of 1-disilagermirene 15.

Compound 15 was isolated as ruby crystals in 40 % yield and appeared to be highly air- and moisture-sensitive. The 29Si NMR spectrum displayed a downfield resonance at +107.8 ppm, which is characteristic of the sp2 Si atom. The molecular structure of 1-disilagermirene 15 was determined

98 A. Sekiguchi, V. Yu. Lee

by X-ray crystallography, which showed a trans-bent Si=Si double bond (bend angle 37") with a bond length of 2.146( 1) 8, (Fig. 3).

Fig. 3. X-ray crystal sructure of 1-disilagermirene 15.

Under photolysis (h > 300 nm, 4 h) or thermolysis (mesitylene, 120 "C, 1 day or without solvent, 215 "C, 20 minutes) of 1-disilagermirene, a migration of the silyl substituent with the clean and quantitative formation of the isomeric 2-disilagermirene 16 takes place (Scheme 7).

SiMe'Bup

I hv ( > 300 nm), 4 h, r.t. *

or A (120 "C, 1 day 'Bu,MeSi 'SiMe'Bu, or 215 "C, 20 min) 'Bu2MeSi

15

Scheme 7. Isornerization of 1-disilagermirene 15 to 2-disilagermirene 16.

Compound 16 represents the first example of a stable germasilene reported to date. Baines previously prepared a tetramesitylgermasilene, but it is unstable and can survive only at low temperature [ 181. The 2-disilagermirene 16 was isolated as highly air- and moisture-sensitive scarlet crystals. The 29Si NMR spectrum showed a downfield resonance of the doubly bonded Si atom at +100.7 ppm. The crystal structure determination exhibited the trans-bent configuration of the Si=Ge double bond with a bend angle of 40".

Cyclotrimetallenes Consisting of Heavier Group 14 Elements 99

Conclusion

The chemistry of cyclotrimetallenes of heavier group 14 elements is a quite new and very promising field, which started only in the mid-1990s. Despite the great progress that has been made subsequently in the synthesis and characterization of these compounds, there are still many questions to be solved. Firstly, they concern the development of effective new methods for the synthesis of the title compounds, particularly for the preparation of cyclotriplumbene derivatives, which are still unknown. Another great synthetic challenge is the reactivity of such compounds, which needs to be widely investigated, since preliminary studies showed very exciting and unusual properties of cyclotrimetallenes. The high research activity in this field permits us to hope that these problems will be solved in the near future.

Acknowledgments: We thank the Ministry of Education, Science and Culture of Japan and the TARA (Tsukuba Advanced Research Alliance) fund for financial support. The authors are grateful to Dr. M. Ichinohe, Dr. N. Fukaya, T. Matsuno, Y. Ishida, and H. Sekiyama for their experimental contributions.

References [1] a) M. Weidenbruch, Eur. J. Inorg. Chem. 1999, 373. b) P. P. Power, Chem. Rev. 1999, 99,

3463. c) J. Escudit, H. Ranaivonjatovo, Adv. Organomet. Chem. 1999,44, 113. [2] M. Kaftory, M. Kapon, M. Botoshansky, The structural chemistry of organosilicon

compounds, Vol. 2, Part 1 , in The Chemistry of Organic Silicon Compounds, (Eds: Z . Rappoport, Y. Apeloig), Wiley, Chichester, 1998, Chapter 5, p. 181. M. Kira, T. Iwamoto, C . Kabuto, J. Am. Chem. SOC. 1996,118, 10303. N. Wiberg, H. Auer, H. Noth, J. Knizek, K. Polborn, Angew. Chem., Znt. Ed. 1998,37,2869. A. Grybat, S. Boomgaarden, W. Saak, H. Marsmann, M. Weidenbruch, Angew. Chem., Znt. Ed. 1999,38, 2010. A. Sekiguchi, H. Yamazaki, C. Kabuto, H. Sakurai, S . Nagase, J. Am. Chem. SOC. 1995, 117, 8025. M. Ichinohe, H. Sekiyama, N. Fukaya, A. Sekiguchi, J. Am. Chem. SOC. 2000,122,6781. N. Wiberg, W. Hochmuth, H. Noth, A. Appel, M. Schmidt-Amelunxen, Angew. Chem., Znt. Ed. 1996,35, 1333. a) A. Sekiguchi, M. Tsukamoto, M. Ichinohe, Science 1997, 275, 60. b) M. Ichinohe, N. Fukaya, A. Sekiguchi, Chem. Lett. 1998, 1045. c) A. Sekiguchi, N. Fukaya, M. Ichinohe, Y. Ishida, Eur. J. Inorg. Chem. 2000, 1155.

[lo] A. Sekiguchi, N. Fukaya, M. Ichinohe, N. Takagi, S. Nagase, J. Am. Chem. SOC. 1999, 121, 11587.

[ l l ] T. Iwamoto, C. Kabuto, M. Kira, J. Am. Chem. SOC. 1999,121,886. [12] M. Ichinohe, T. Matsuno, A. Sekiguchi, Angew. Chem., Znt. Ed. 1999,38,2194.

[3] [4] [5]

[6]

[7] [8]

[9]

100 A. Sekiguchi, V. Ya. Lee

M. Kira, T. Maruyama, C. Kabuto, K. Ebata, H. Sakurai, Angew. Chem., Int. Ed. 1994, 33, 1489. T. Iwamoto, M. Tamura, C. Kabuto, M. Kira, Science 2000,290,504. N. Wiberg, H.-W. Lerner, S.-K. Vasisht, S. Wagner, K. Karaghiosoff, H. Noth, W. Ponikwar, Eur. J. Inorg. Chem. 1999, 1211. a) S. Masamune, L. R. Sita, J. Am. Chem. SOC. 1985,107,6390. b) M. A. Della Bona, M. C. Cassani, J. M. Keates, G. A. Lawless, M. F. Lappert, M. Sturmann, M. Weidenbruch, J. Chem. SOC., Dalton Trans. 1998, 1187. c) D. E. Goldberg, D. H. Harris, M. F. Lappert, K. M. Thomas, J. Chem. SOC., Chem. Commun. 1976, 261. d) D. E. Goldberg, P. B. Hitchcock, M. F. Lappert, K. M. Thomas, A. J. Thorne, T. Fjeldberg, A. Haaland, B. E. R. Schilling, J. Chem. SOC., Dalton Trans. 1986,2387. e) K. W. Klinkhammer, W. Schwarz, Angew. Chem., Int. Ed. 1995, 34, 1334. f) K. W. Klinkhammer, T. F. Fassler, H. Griitzmacher, Angew. Chem., Int. Ed. 1998,37, 124. V. Ya. Lee, M. Ichinohe, A. Sekiguchi, N. Takagi, S . Nagase, J. Am. Chem. SOC. 2000, 122, 9034. K. M. Baines, J. A. Cooke, Organometallics 1992,II, 3487.

On the Way to a Disilyne RSiSiR

Nils Wiberg

Department of Chemistry, Ludwig-Maximilians-Universitiit Miinchen, Butenandtstr. 5-13 (Haus D), 80377 Munich, Germany

Tel.: +49 89 2180 7458 -Fax: +49 89 2180 7865 E-mail: niw @cup.uni-muenchen.de

Keywords: silicon, disilene, disilyne, silylene, X-ray structure analysis

Summary: Supersilyltrihalosilanes R*SiX3 (X = C1, Br, I), with the bulky, chemically inert supersilyl group R* = Si'Bu3 (cone angle 130"; dependence of C-Si-C angle and Si-C bond lengths in R*X on X is discussed) as well as disupersilyltetrahalodisilanes R*XzSi-SiXzR* react with NaR* in THF to form tetrasupersilyl-tetrahedro-tetrasilane R*4Si4 in quantitative yields. The tetrahedrane is obtained via disilenes R*XSi=SiXR* (trans-configuration) as reactive intermediates (trapped by DMB with formation of [2+2] and [2+4] cycloadducts as well as ene reaction products). R*XSi=SiXR* themselves are formed from R*SiX3/NaR* via silanides R*SiXzNa and silylenes R*SiX which - after insertion into the Si-Na bond of R*SiXNa and NaX elimination - gives R*&; in addition they are formed from R*XzSi-SiX2R* via the disilanides R*XZSi- SiNaXR", which eliminate NaX. Obviously, R*XSi=SiXR* reacts further with NaR* with formation of the disilenides R*XSi=SiNaR*, which may eliminate NaX to form an intermediate disilyne R*SiaSiR*. The latter then reacts with (among others) its precursor R*X2Si-SiNaXR* with formation of R*4Si4 via cyclotetrasilenes R*&Xz. R*4Si4 is very thermostable. The action of 12 or 0, on R*4Si4 gives the cyclotetrasilene R*4Si& (convertible with BI3 into R*4Si$, or with HzO into R*4S40) and the tetrahedrane oxides R*4Si402 and R*4S404, whereas NaCloH8 reacts with R*& to the tetrasilanediide NazSi& (convertible with H' or Me' into R*4S&Hz or R*4Si&lez). Reaction of R'HBrSi-SiBrHR' (R' = R*zHSi) with NaR* leads to yellow R'HSi=SiHR' (first isolated disilene with H at unsaturated Si; adds MeOH; decomposes at -20 "C) and the cyclotetrasilene R*4Si4Hz, formed possibly via the thermolabile disilyne R'SkSiR'. Reaction of R**ClSi=SiClR** (R** = R*zMeSi) with NaR* leads to orange- red high-melting R* *ClSi=SiClR** (first isolated disilene with C1 atoms at unsaturated Si; planar; Si-Si distance 2.16 A; reacts neither with 02, nor with H20, HF or NaR*). Reduction of the disilene with LiCloH8 in THF occurs with formation of a product which is probably the disilyne R**Si=SiR** (6(29Si) = 91.5 ppm; chemical ionization leads to a mass peak for the disilyne plus two oxygen atoms; crystals suitable for X-ray analysis are awaited).

Ovganosilicon Chemisty V Edited by N. Auner and J. Weis

Copyright 0 2003 WILEY-VCH Verlag GmbH & Co. KgaA, Weinheim

102 N. Wiberg

Introduction

In a review on the cluster chemistry of the heavier main group IV elements, Tsumuraya et al. stated in their concluding remarks that the syntheses of a tetrasilatetrahedrane R&, a disilyne RzSiz and a l,l,l-pentasilapropellane R6Sb (Fig. 1) were currently the greatest challenges for silicon chemists

R 1 2

for R = SitBu3 = R* for R = R*,MeSi= R** 3

for R to be found

Fig. 1. Tetrasilane R.,Si4, disilyne R2Si2 and 1,1,1-pentasilapropellane &Si4.

We believed at first that the generation at least of a tetrahedro-tetrasilane and a disilyne should be easy. We wondered, whether it could well be possible to dehalogenate trihalosilanes RSiHal3 to form disilynes 2 which, with very bulky substituents R, should be isolable or otherwise could dimerize with the formation of a tetrasilatetrahedrane 1. However, the facts were not so simple, and the path to 2 was difficult to find and full of stones.

First we have to look at the nature of the group R. If it is as small as hydrogen, a disilyne no longer exists. According to ab-initio calculations [2], a molecule of SbH2 (2, R = H) takes the configuration of a butterfly with a Si-Si edge twice bridged by H atoms. On the other hand, bulky R group may only effectively stabilize the disilyne configuration if they do not react with the Si-Si triple bond to form secondary species.

First of all, I propose to introduce the group R which we are using as a bulky substituent. Then I will describe a molecular tetruhedro-tetrasilane 1 [3] which we were able to synthesize as the first and hitherto single species of this class of compounds. In this connection, new insights into the mechanism of its formation as well as new results concerning its properties are presented. Finally, our efforts to synthesize a disilyne 2 will be mentioned.

The Supersilyl Group R*

As substituent R in 1 we used the tri-tert-butylsilyl group SitBu3, which we call “supersilyl” and symbolize as R*. And indeed, this moiety is an overcrowded one: three bulky tertiary butyl groups fully shield a silicon atom on one side and leave only a small place for a fourth substituent on the

On the Way to a Disilyne RSiSiR 103

other side. In addition, R* has much merit as a bulky substituent: for example, it is easy to prepare - provided the co-worker gets assistance from Above. Then it is exceptionally inert chemically. Further, it leads to compounds which are comparatively soluble. And finally, it may exist as a radical, as an anion, or next to a cation.

The supersilyl anions [4], (Fig. 2) work as excellent reaction partners. The tetrahydrofuran adduct NaR*.2THF (4) of supersilyl sodium is easily prepared from supersilyl bromide and sodium in refluxing THF. It crystallizes in beautiful yellow needles which - according to the X-ray structure analysis - contain contact pairs of the molecule. On the other hand, the Ph4DTA adduct (5) of NaR* and the benzene adduct 6 of KR* form monomeric molecules in the crystals. Donor- free NaR* (7) is dimeric, whereby silicon gains the coordination number 5 , which is very unusual for supersilyl silicon.

6

Fig. 2. Structures of supersilyl alkali metals.

\ 2.91 8, /

N

tBu

tBu-Si-Na / tBu

- 5

7

By transfer of supersilyl anions, radicals or cations, supersilyl derivatives of almost all the main group elements have been obtained and studied structurally. As is demonstrated in Fig. 3, the cone angle of the supersilyl amounts to about 130". The value of the C-Si-C angle a depends subtly on steric and electronic effects of the fourth substituent X. Normally, a ranges from 110" to 112". If the substituent is as small as hydrogen or withdraws electrons like tribromostannate(n), a is larger than 112". If, on the other hand, it is as bulky as supersilyl or delivers electrons like alkali metals, a is smaller than 110". Furthermore, it should be pointed out that the S i x bond lengths r in supersilyl decrease as the Si-C-Si bond angles a increase.

104 N. Wiberg

C M e 3

tBu3S@ I I I t B u 3 S i e I

steric I ! I

\ 110" I I

I f' I '

I effects I normal I I

I I I I I I I I

I I DCi I I 115" I '\

I \

I I I ? I I I I I electronic] effects

I I

I tBu3Si-SnBr3 \ I

< \ tBgSi-Na I region

a 106.6" 107.3"; 110-112" ; 113.7" 115.3" d 2.00 8, 1.99 8,; 1.95 -1.94 ; 1.93 8, 1.92 8,

Fig. 3. The geometry of supersilyl.

Syntheses of R*&i4

One of the many applications of the supersilyl sodium just introduced concerns its use as an excellent dehalogenation agent. It transforms, for example, the dibromodisilane R*BrZSi-SiBrzR* in THF quantitatively into tetrasupersilyl-tetruhedro-tetrasilane R*dSh (1) [3]. The compound forms orange crystals, which are stable towards water and heat, but are reducible and oxidizable. Naturally, the question arises about the tetrahedrane formation via a disilyne intermediate 2 (R = Si'Bu3). This question stimulated detailed studies on the mechanism of the dehalogenation reactions.

We observed that supersilyl sodium dehalogenates all tetrahalodisilanes R*X2Si-SiX2R* (X = C1, Br, I) just like trihalosilanes R*SiX3 to form tetrahedrane R*4S4 [5, 61. As the reductions take place in high yields, one expects a straightforward mechanism for the tetrahedrane formation, which was accomplished by cleverly planned trapping experiments.

Obviously, the first step of the reaction of R*SiX3 with NaR* in THF consists - according to Scheme 1 and Ref. [7] - in an exchange of halogen against sodium. The monoanions formed are metastable at -78 "C, but eliminate NaX at about -50 "C with the formation of silylenes R*SiX (8), which may then insert into the Si-Na bond of their precursors several times to build a silicon chain. The di-, tri-, or tetrasilanides obtained may transform, by elimination of NaX, into dihalodisilenes R*XSi=SiXR* (9), cyclotrisilanes (R*XSi)3 (10) and cyclotetrasilanes (R*XSi)4 (11).

On the Way to a Disilyne RSizYiR 105

X

X R*-sX

I R* = SitBu3; X = C1, Br. I I 1 + NaR*; - R*X

Na x x Na + 8 X X X N a + 8 X

R*-Si Na +8, R*-Si-Si-R* R*-Si-Si-Si-R* 4 R*-Si-Si-Si-Si-R* X x x x x x x x x x

J-NaX J-NaX

R*Si X R* R*

/ \ R* R* Si-Si x\xx\x

Si-Si

X

R*

/

Si- Si X R* R*

8 9 10 11

R* R*2 R*

Si Si R* R*

1 10a 1

Scheme 1. Reaction of R*SiX, with Na R*.

Certainly, the product yields depend on the relative rates of the insertion and elimination processes. For X = Br or I, only trans-configurated disilenes are formed which may further react with NaR* into 1. For X = C1, in addition to a disilene 9, a cyclotri- and -tetrasilane 10 and 11 may be isolated. Obviously, they react with NaR* to give a cyclotrisilene 10a or the silatetrahedrane 1.

To demonstrate our efforts to clarify the existence of intermediates, I will discuss only the above-mentioned dihalodisilenes, which - according to Scheme 2 - may be trapped for example with 2,3-dimethylbutadiene with formation of [2+2] cycloadducts 12 and [2+4] cycloadducts 13 as well as ene reaction products 14. Obviously, the yields of 12 and 13 decrease for the disilenes in order X = C1> Br > I, and - as a consequence - the yields of 14 increase in the same order.

Next, the question arises as to how the disilenes 9 are transformed in the presence of NaR* into the tetrahedrane 1. According to Scheme 3, the first step of the reaction of disilanes R*RXSi-SiXRR* consists in an exchange of halogen for sodium. The anions formed then eliminate NaX at about -20 "C to form disilenes R*RSi=SiRR*. For R = halogen, the latter exist only as short-lived intermediates which may be trapped as shown in Scheme 2 [5 ] , but for R equal to more

106 N. Wiberg

bulky substituents such as phenyl the disilenes become isolable [8].

X R*\ /

X H S i ' S j R* 9

X R* X R* X R* R*Si-SiX % + R*Si-SiX +yY

12 13 14 X=C1 25% 46% 29%

Br 23% 38% 39% I 0% 27% 73%

H

Scheme 2. Trapping of disilenes 9 with DMB.

x x X Na I 1 + NaR* I t

I t - R*X I I - NaX R R R R /si=s\ R*

R* -Si-Si-R* R* -Si-Si-R* d

R

A B C

4 - N . X 4 -NaX

X R* R*Si-SiX

I 1 Si-Si

X = C1, Br, I R* R*

R* Si

R* R*

15 l l a R* 1

Scheme 3. Reactions of disilanes R*RXSi-SiXRR* (X = Hal; X = Hal, Organyl) with NaR*.

The above-mentioned dihalodisilenes 9 - now called A - transform in the presence of NaR*

On the Way to a Disilyne RSiSiR 107

quantitatively into 1. This fact may easily be explained by supposing - according to Scheme 3 - that A reacts via B to the disilyne C which dimerizes. Indeed, our studies speak for the intermediacy of B, but all experiments on trapping C have so far gone wrong. Certainly, the latter fact does not really speak against C as an intermediate (traps may react with A before A is transformed into C). Maybe B works as a trap for C, but B might as well react with A with formation of a tetrasilabutadiene 15 or a cyclotetrasilene l la , which then in the presence of NaR* transforms into 1. Most probable seems a reaction of C with its precursors R*XzSi-SiXNaR* (cf. Scheme 3, first line) with formation of a tetrasilenide which - after NaX elimination - transforms in the presence of NaR* by way of l l a into 1.

Reactions of R*4Si4

To obtain further insights into the formation of 1, we studied its reactivity. The already mentioned high themostability of 1 points towards special stability associated with the tetrahedral S 4 framework. Thus, the following question may be formulated with Fig. 4. Is the existence of an isolable modification of silicon a possibility, in which S4 tetrahedranes take the positions of carbon atoms in diamond? I wonder if the generation of such a material would be a challenge to solid-state chemists and silicon manufacturers. Such a material would certainly show unexpected properties.

Fig. 4. A new modification of silicon?

I also mentioned the reactivity of 1 against oxidation and reduction. According to Scheme 4, the oxidation of 1 with equimolar amounts of I, in benzene at r.t. leads quantitatively to the red

108 N. Wiberg

cyclotetrasilene lla (X = I) [9]. It can be converted back into 1 by its reaction with NaR*. The unsaturated character of the Si-Si double bond, observed in disilenes, is suppressed in this cyclotetrasilene owing to shielding by the bulky groups. Water does not attack the disilene moiety, and instead substitutes the iodo groups to form a light-yellow tetrahedrane oxide 16 [9]. The fact that the conversion of 1la (x = I) into 16 occurs very fast at r.t. argues against an s N 2 mechanism, the rate of which slowed down drastically with increasing steric overcrowding at the substitution center. On the other hand, the latter disposition facilitates s N 1 processes. An observation in favor of the latter mechanism is the formation of a cation lla' in the presence of BI3 [9].

R*

11;

R* Me " - s i H ~ MeS,i- SX

R*

18 lla

+ 2 0 2 + 2 Me+ - 2 Na t R*

R*Si-SiI I R* + I ~ + 2 N a R * S j l N a

Si=Si I l f - 1 - - I2 - 2 N a R*Si\\ //Na R* R*

I - Si

lla (X = I) 19 R*

1 + H20, - 2 HI + 2 H+ 1- 2 Na

17 20 16

Scheme 4. Reactions of 1.

Dry air reacts slowly with solid 1 at r.t. The first detectable product is the tetrahedrane dioxide 17, which oxidizes subsequently into tetrahedrane tetraoxide 18 [5] . However, oxygen uptake in solution proceeds faster. Mass spectra of solutions of 1 show the mass of 17 besides the mass of 1.

On the Way to a Disilyne RSiSiR 109

The reduction of 1 with sodium naphthalenide in THF at -78 “C leads to a dianion R*4Sb2- (19) [ 5 ] , which may be structured as shown in Scheme 4. It could not be transformed into crystals suitable for X-ray structure analysis. However, the existence of 19 is doubtless as shown by reactions with strong acids and with dimethyl sulfate, leading to the bicyclo-tetrasilane 20 and the cyclotetrasilene l l a (X = Me), respectively.

Now, returning to the question of the mechanism of formation of 1 from dihalodisilenes 9 and NaR*, Scheme 4 points at the dihalocyclotetrasilenes l l a (cf. Scheme 3) as possible reaction intermediates. In fact, the reaction of R*I2Si-SiIzR* with an insufficient amount of NaR* for a complete formation of 1 yields -besides 1 - the cyclotetrasilene l l a (X = I).

On the Way to a Disilyne RSi=SiR

Altogether, one can be sure that the overcrowding of R* substituents would not suffice to stabilize a disilyne against its dimenzation. One needs substituents which are markedly more overcrowded than the supersilyl groups. Pietschnig et al. made use of the sterically erloaded 2,6- disupersilylphenyl group known as dmp. In fact, they reduced RSiF3 (R = dmp) with Na and obtained a product which might have been formed via intermediate dmp-SiiSi-dmp, wherein the substituents dmp were not able to stabilize the Si-Si triple bond and hence change the Si-Si triple into a single bond by intramolecular migration. On the other hand, we used the sterically overloaded trimesitylsilyl groups and prepared a disilane Mes3Si-SiHz-SiH2-SiMes3. Unfortunately, the extreme insolubility of the compound hindered further experiments for the present.

We then turned over to supersilyl-substituted silyl groups as candidates for disilyne substituents. Certainly, an R*$3 group does not exist for steric reasons because of a cone angle of 130” for R* (see above). On the other hand, R*2RSi groups with R = H, Me, Ph are preparable. At first, we planned to synthesize tetrahalodisilanes R’XzSi-SiX2R’ (R’ = R*ZHSi), the dehalogenation of which might produce R’SiESiR’. Unfortunately, the disilane R’H2SiSiH2R’ formed from R’SiH2Cl and Na, could only be dihalogenated. More overcrowded R**SiH2Cl (R** = R*zMeSi) is even more unreactive than R’SiH2C1. In fact, we were not able to dehalogenate it into a disilane R**HZSi- SiH2R**. However, the silane R**SiHzCl may be halogenated into trihalosilanes R**SiX3 which may also play the role of a precursor for the disilyne R**Si=SiR** (cf. dehalogenation of R*SiX,, above).

Dehalogenation of R*zHSiSiHBrSiHBrSiHR*z

As the main product of the dehalogenation of R*2HSi-SiHBr-SiHBr-SiHR*2 with NaR* (R’ = R*zHSi), we obtained - according to Scheme 5 - the disilene 21 as yellow crystals [ l l ] . However, these were not suitable for an X-ray structure analysis, but the constitution of the compound doubtless follows from its properties. Indeed, the disilene shows a low-field NMR signal (doublet of doublets) for the unsaturated Si atoms at 145 ppm. Further, the reaction of 21 with methanol consists in the addition of MeOH at the double bond. Finally, the disilene already

110 N. Wiberg

decomposes slowly at -20°C into the colorless product 22, the structure of which could be studied by X-ray analysis. By the way, 21 is the first disilene with H atoms bound to unsaturated Si atoms which could be isolated as a pure substance. It can be positioned halfway in the direction of the parent disilene, Si2b.

Br Br I I

R*2HSi, / H ,Si=Si, (NaR*)

H SiHR"2

21

/L\ R* R* H H2C SitBu2

\R*- Si-Si

I I I R* H H H H

I \ I R*2HSi- Si-Si- Si- R*

22 l l a (X = H)

Scheme 5. Dehalogenation of R*2HSi-SiHBr-SiHBr-SiHR*2.

25

Besides yellow crystals of 21, red crystals were also obtained from R*zHSi-SiHBr-SiHBr- SMR*2 and NaR* which - after an X-ray structure analysis - were found to contain cyclotetrasilene molecules l l a (X = H). In fact, the latter product may be formed from the reactants after elimination of two molecules of HBr set off by NaR* via the disilyne 24 as an intermediate. The 24 should then transform with simultaneous migration of two supersilyl groups into a tetrasilabutadiene 25, which thereafter transforms by an electrocyclic conrotatory process into the cyclotetrasilene l l a (X = H). Certainly, the formation of the latter product does not prove the intermediacy of a disilyne but nevertheless it is an indication of that.

Dehalogenation of R*NeSi-SiBrCIz

Obviously, the overcrowding of R*2HSi groups used as substituents in a disilyne again does not

On the Way to a Disilyne RSiSiR 111

suffice to stabilize the latter against further reactions. Therefore, we turned to the more overcrowded R*2MeSi group, which we jokingly call megasilyl and symbolize it as R**. At first we prepared the trihalosilane R**SiBrC12, which is obtained as a 2:l mixture with R**SiBrzCl by reaction of R**SiH2Cl with Br2 in CC4 at 0 “C. The action of NaR* on R**SiBrC12 gives - according to Scheme 6 - the orange disilene 26 [12]. The intermediates in the reaction of R**SiBrC12 with NaR* are possibly an anion R*SiC12Na (dark red) and then a silylene R*SiCl which dimerizes slowly. Compound 26 is the first disilene to be isolated as a pure substance with halogen substituents on unsaturated Si atoms. It forms orange-red crystals, which are almost insoluble in organic solvents and behave comparatively inertly against oxygen, water, methanol, hydrogen fluoride, or supersilyl sodium. The disilene 26 is much more thermostable than 25 and does not decompose until 228°C. Obviously, it is only a small step from R*2HSi to R*zMeSi, but the effect is evident.

c1 R* * - NaCl \SFs( R* x 2 Cl’

26

Scheme 6. Dehalogenation of R*2MeSi-SiBeC12.

According to an X-ray structure analysis [12], the central framework of 26 is planar with both the C1 atoms in trans-positions. In fact, only a few disilenes are analogously structured, whereas most of the studied disilenes show trans-bent and/or twist angles around the double bond [13]. Obviously, the planar conformation is a consequence of special electronic and steric effects of the substituents at the Si-Si double bond. Certainly, the approach of two singlet silylenes with formation of a planar disilene requires some activation energy, slowing down the rate of silylene dimerization. The Si-Si double bond length is - in spite of the overcrowding of the “megasilyl” groups - as short as 2.16 A, speaking for a strong bond. In the Raman spectrum the Si-Si double -bond vibration occurs at 589 cm-’.

Now, to dehalogenate the disilene 26, we combined it with lithium naphthalenide in THF at -78 “C. After heating the reaction mixture to r.t., evaporation of volatile products (including

112 N. Wiberg

naphthalene) and treating the residue with benzene, we obtained an orange-red solution. As demonstrated in Scheme 6, the following question arises: Does the very oxygen-sensitive solution contain the disilyne 2? To date, we have been unable to get crystals from the solution suitable for X-ray structure analysis. The solution shows an 29Si NMR signal at about 90 ppm. Certainly, the signal appears in a region which makes sense for unsaturated disilyne Si atoms. In addition, according to mass spectra obtained by chemical ionization of the solution, the latter is chlorine-free and shows the mass of 2 plus two oxygen atoms. Thus, it appears that the dehalogenation of 26 leads to a disilyne which takes up molecular oxygen before entering the mass spectrometer. In this connection, it is worth mentioning that - according to ab-initio calculations [14] - R**Si=SiR** is predicted to be stable enough to be isolable with a short Si-Si bond of 2.072 A that deserves to be a triple bond.

Conclusion

We are waiting for suitable crystals of 26.

Acknowledgment: The author’s own work on disilenes and disilynes was undertaken with Dr. H. Auer, Dr. G. Fischer, Dr. H.-W. Lerner, Dr. W. Niedermayer, Prof. S.-K. Vasisht and Dr. S . Wagner, and in addition, has been achieved by cooperation with specialists in X-ray structure analyses (Prof. P. Kliifers, Prof. H. Noth, Dr. K. Polborn and their groups). It has received financial support from the Deutsche Forschungsgemeinschaft (Schwerpunkt “Spezifische Phiinomene in der Siliciumchemie”) and the Fonds der Chemischen Industrie. All of the above are gratefully acknowledged.

References [ l ]

[2] [3]

[4]

[5]

[6] [7] [8] [9]

T. Tsumuraya, S . A. Batcheller, S . Masamune, Angew. Chem. 1991,103,916; Angew. Chem. Int. Ed. 1991,30,902. K. Kobayashi, N. Takagi, S . Nagase, Organometallics 2001,20,237 and refs. cited therein. N. Wiberg, Ch. M. M. Finger, K. Polborn, Angew. Chem. 1993,105, 1140; Angew. Chem. Int. Ed. 1993,32, 1054. N. Wiberg, K. Amelunxen, H.-W. Lerner, H. Schuster, H. Noth, I. Krossing, M. Schmidt -Amelunxen, T. Seifert, J. Organomet. Chem. 1997,542, 1. N. Wiberg, H. Auer, S . Wagner, K. Polborn, G. Kramer, J. Organomet. Chem. 2001,619, 110 and refs. cited therein. N. Wiberg, W. Niedermayer, 2. Naturjorsch., Teil B, 2000,55,406 and refs. cited therein. N. Wiberg, W. Niedermayer, J. Organomet. Chem. 2001,628,57. N. Wiberg, W. Niedermayer, K. Polborn, P. Mayer, Chem. Eur. J. 2002,8,2730. N. Wiberg, H. Auer, H. Noth, J. Knizek, K. Polborn, Angew. Chem. 1998,110,3030; Angew.

On the Way to a Disilyne RSiSiR 113

Chem. Int. Ed. 1998,37,2869. [lo] R. Pietschnig, R. West, D. R. Powell, Organometallics 2000,19,2724. [Ill N. Wiberg, W. Niedermayer, H. Noth, M. Warchhold, 2. Anorg. Allg. Chem. 2001, 627,

1717. [I21 N. Wiberg, W. Niedermayer, G. Fischer, H. Noth, M. Suter, Eur. J. Znorg. Chem. 2002, 1066. [I31 M. Weidenbruch, Recent Advances in the Chemistry of Silicon-Silicon Multiple Bonds, in:

The Chemistry of Organic Silicon Compounds (Eds.: Z . Rappoport, Y. Apeloig), Vol. 3, Wiley, Chichester, 2001, p. 391.

[14] N. Takagi, S. Nagase, Eur. J. Inorg. Chem. 2002,2775.

From Silylenes and Disilenes to a Tetrasilabuta-l,3=diene and Related Compounds

Manfred Weidenbruch

Fachbereich Chemie, Universitat Oldenburg Carl-von-Ossietzky-StraBe 9-1 1, D-26111 Oldenburg, Germany

Tel.: +49 441 798 3655 -Fax: +49 441 798 3352 E-mail: manfred.weidenbruch@uni-oldenburg.de

Keywords: silylenes, disilenes, tetrasilabutadiene, multiple bonds, addition reactions, cycloadditions, germanium compounds

Summary: Di-tert-butylsilylene and tetra-tert-butyldisilene, generated by photolysis of hexa-tert-butylcyclotrisilane, react with the C-C triple bonds of di- and oligoynes to furnish C-C-linked bis(disilacyc1obutenes) and di- and oligosilirenes, which, upon prolonged irradiation, rearrange to disilabicyclohexadiene derivatives. The reactions of 1,3-diynes with a diarylgermylene proceed differently to provide conjugated acetylene- linked bis(germaethenes). The first, and as yet only, tetrasilabutadiene reacts with small molecules by 1,2-and 1,4-additions. Recently, the first cycloaddition reactions could be realized. In addition, a tetragermabutadiene was synthesized by two independent routes. The structures of all the new compounds were confirmed by analytical and spectral data and by X-ray crystallography.

Silylene and Disilene Reactions with Oligoynes

Silylenes and disilenes are well-known compounds whose properties have been documented in several review articles [ l , 21. We first came into contact with these classes of compounds in 1984 when we achieved a successful synthesis of the long sought-after hexa-tert-butylcyclotrisilane 1, one of the most strained silacycles [3]. Thermolysis of this molecule leads - depending on the reactions partner present - to acyclic trisilanes or ring-expansion products. However, under photolytic conditions 1 decomposes with cleavage of two silicon-silicon bonds to furnish the corresponding, sterically encumbered di-tert-butylsilylene 2 and the marginally stable tetra-tert- butyldisilene 3 (Eq. 1). Both compounds can undergo addition or cycloaddition reactions to a variety of multiple bonds to afford small silicon-containing ring systems [ 1,2]

Irradiation of the 1,4-di-tert-butyl-substituted butadiyne in the presence of excess 1 furnished the carbon-carbon-linked bis(si1irene) 4; this is in fact a trans-butadiene with each of the double bonds being bridged by silylene molecules. The compound is thermally stable and, for example, is

Ovganosilicon Chemisty V Edited by N. Auner and J. Weis

Copyright 0 2003 WILEY-VCH Verlag GmbH & Co. KgaA, Weinheim

From Silylenes and Disilenes to a Tetrasilabuta-1,3-diene 115

recovered unchanged after heating for 30 days to 120 "C. Its behavior under photochemical conditions is completely different. After only 6 h at room temperature it has rearranged to the disilabicyclohexadiene derivative 5 (Scheme 1)[4].

/ \ = tBuzSi: + tBu2Si=SitBu2 tBuzSi-SitBuz

1 2 3

Eq. 1.

tBu2

tBu - hv tBu7SlrfBu SitBq /

?\ (~Bu-CEC-)~ + 2 2 + tBu,C=C,

4 cvc tBu 5 Si tBu2

Scheme 1.

The corresponding reaction of 1 with a tert-butyl-substituted tetrayne yielded three products. The cycloaddition of 2 to each of the four carbon-carbon triple bonds of the tetrayne afforded the C-C-linked quatersilirene 6 whose carbon skeleton has an s-cis-trans-cis configuration, apparently in order to minimize the steric interactions between the bulky tert-butyl group (Scheme 2). Disilene 3 reacts with the tetrayne by a two-fold [2+2] cycloaddition giving the carbon-carbon linked disilacylobutenes with retention of the terminal alkynyl groups 7. An interesting feature of the structure of 7 is the presence of an s-cis butadiene framework bridged by two disilene moieties [5 ] . In contrast, the s-trans-form dominates in butadiene itself and many of its derivatives.

Upon prolonged irradiation the quatersilirene 6 rearranges to the disilabicyclohexadiene derivative with two terminal silirene groupds 8 (Eq. 2) [5 ] . This behavior is no longer unusual since, for example, the bis(si1irene) 4 is also smoothly converted to the analogous bicyclic species 5.

hv 6 -

Eq. 2.

116 M. Weidenbruch

tBu tBu

Scheme 2.

We then examined the question of whether the partial replacement of the conjugated carbon-carbon triple bonds in the oligoynes by double bonds would lead to molecules containing both silirene and silirane increments next to each other or whether the cycloaddition of silylene 2 to only one of these types of multiple bonds would be preferred. In the case of an acetylene linked by a carbon-carbon double bond, the triple bonds appear to be more reactive than the double bond that is also present, because only compound 9 is isolated (Eq.3).

Eq. 3.

The low reactivity of 2 towards C-C double bonds is also seen in the reaction with a diacetylene linked by a C-C double bond. Again it can be assumed that the reaction sequence is initiated by a four-fold cycloaddition of four molecules of 2 followed by a rearrangement to furnish the two disilabicyclohexadiene units linked by a C-C double bond 10 (Eq. 4). In both molecules the C-C double bonds adopt an all-trans arrangement [6] . These compounds behave like polyacetylenes, where the all-trans form is more stable then the all-cis configuration [7].

From Silylenes and Disilenes to a Tetrasilabuta-1,3-diene 117

hv SitBu,

SitBu, 10

Eq. 4.

In addition to the acetylenes linked by olefinic double bonds, we have also examined the reactivity of oligoacetylenes separated by one or two aromatic multiple bonds. With a 1,3,5- triethynylbenzene the tris(silireny1)benzene 11 is formed (Eq. 5). However, an analogous reaction of the silylene 2 with a 1,2-diethynylbenzene followed an unexpected course, to furnish the tricyclic compound 14 with an exocyclic di-tert-butyl group (Scheme 3). Again it can be assumed that the reaction sequence is initiated by a two-fold cycloaddition of the silylene to the C-C triple bonds to furnish compound 12. Silirenes bearing hydrogen atoms at the C-C double bond are known to be unstable and can rearrange to the vinylidene derivative 13. This reactive species can undergo an insertion reaction into the Si-C bond of the second silirene to give the tricyclic molecule 14 as final product [6].

KH \\ H + 2 2

tBu2!$H

-- tBu,.CjiH 5 SitBu,

13 H

14

Scheme 3.

118 M. Weidenbruch

Eq. 5.

The somewhat unexpected reactions of 1 with oligoynes prompted us to repeat the cycloadditions with a gennylene instead of a silylene. Some years ago we isolated the tetraaryldigermene 15, which was found to have a planar environment of its substituents. This digermene is stable in the solid state but, in solution, it dissociates almost completely in the germylene molecules 16 (Eq. 6) [8]. Treatment of several diynes with 16 leads to intensely colored solutions, from which dark red crystals of the methyl- or n-butyl-substituted compounds can be isolated. In the case of the reaction with 1,4-diphenylbutadiyne a violet solution was formed, from which blue-black crystals of 19 were obtained (Eq. 7). Compounds 17-19 are the first known molecules to possess conjugated gennanium-carbon double bonds. The structures of these molecules do not provide any information about a possible conjugation between the multiple bonds. The electronic spectra are more informative. The dark-blue solution of 19 in n-hexane, for example, displays a longest wavelength absorption at 595 nm which is bathochromically shifted by nearly 200 nm in comparison to the yellow or orange solutions of the simple germaethenes [9].

/R R \ ,Ge:

16

\ R

R \ - 2 /Ge=Ge 15 R

Eq. 6.

//G”R2 17 R’= cH3 (R’-C--(:-)* + 216 - 18 R’=nC&

R’ 19 R’=C6H,

Eq. 7.

Compound 19, which is obtained in almost quantitative yield, reacts differently from the simple germaethenes. Although these molecules undergo smooth cycloaddition reactions with phosphaalkynes or with dimethylbutadiene [lo], the conjugated Ge-C double bonds do not react

From Silylenes and Disilenes to a Tetrasilabuta-1,3-diene 119

with these partners even under harsher conditions. In contrast, they participate in mostly rapid reactions with electron-poor multiple bond systems, such as 1 ,2-quinones or 1 ,2-dicyano compounds [ 1 11.

The successful synthesis of the acetylene-linked bis(germaethenes) prompted us to address the question of whether analogous compounds with conjugated silicon-carbon double bonds could be isolated or, at least, be detected by indirect means, through the use of suitable substituents on the 1,3-diynes. Irradiation of a cyclohexenyl-substituted 1,3-diyne in the presence of 1 furnished the acetylene-linked bis(silacyc1obutene) derivative 22 (Scheme 4). Like all other additions of silylenes to oligoynes, the reaction sequence begins with the formation of the carbon-carbon-linked bis(si1irene) 20. Subsequent opening of the silicon-carbon bonds would then lead to the acetylene- coupled bis(si1aethene) intermediate 21, which in this case does not afford a bicyclic compound but rather undergoes two-fold [2+2] cycloadditions with the double bonds of the cyclohexenyl rings to furnish the isolated product 22 [12].

Scheme 4.

Hexaaryltetrasilabut-ld-diene

We recently obtained the first, and as yet only, tetrasilabutadiene 24 from the tetraaryldisilene 23 by a sequence of metallation, halogenation, and coupling reactions (Scheme 5).

Scheme 5.

The X-ray structure analysis of the reddish-brown crystals of this diene revealed that the

120 M. Weidenbruch

compound approaches the s-gauche form with a dihedral angle of 51" for the Si4 skeleton. Evidence for the conjugation of both silicon-silicon double bonds is provided by the electronic spectrum, in which the absorption at longest wavelength at 518 nm is bathochromically shifted by about 100 nm in comparison to the signals of disilenes with a similar substitution pattern [13].

Cycloaddition Reactions

In view of the preference of 24 for the s-gauche form, it seemed worthwhile to examine its behaviour in [4+2] cycloadditions of the Diels-Alder type. However, until very recently all reactions with olefins, acetylenes, and the C=O bond of ketones have remained unsuccessful. The effective shielding of the two double bonds of 24 by the bulky aryl groups and, above all, the large 1,Cseparation between the terminal silicon atoms appear to be responsible for the failures.

Since genuine [4+2] cycloaddition products had previously not been prepared, it was surprising to find that the action of sulfur on 24 resulted in a formal [4+1] cycloaddition to furnish the first five-membered ring with an endocyclic silicon-silicon double bond 25. The heavier chalcogens selenium and tellurium did not react with 24. However, in the presence of small amounts of triethylphosphane smooth reactions with these elements did occur to furnish two further five- membered ring compounds 26, 27, each with an endocyclic silicon-silicon double bond (Eq. 8)

~ 4 1 .

R R R R

R,P-X /+=Y R2si\ /SiR2

24 + X

25 26 X=Se;27 X = T e

Eq. 8.

Although the mechanism of formation of these ring compounds cannot be proven experimentally, the following proposal seems to be reasonable. In analogy to the reactions of disilenes [15], the process could be initiated by a [2+1] cycloaddition of a chalcogen atom to one of the double bonds, followed by a rearrangement of these intermediates into the presumably less strained five-membered rings.

In contrast to the attempted cycloadditions of 24 to several multiple bonds, the reaction with maleic anhydride (28) furnished the tetracyclic compound 29 in high yield (Eq. 9). It can be assumed that the reaction sequence is initiated by the [2+2] cycloaddition of one of the silicon- silicon double bonds to the highly reactive carbonyl group, followed by a second cycloadditon of the remaining silicon-silicon double bond across the C-C double bond of the anhydride to complete the formation of 29 [ 161.

From Silylenes and Disilenes to a Tetrasilabuta-1,3-diene 121

0

Eq. 9.

Since the anhydride 28 apparently exhibits a high reactivity towards silicon-silicon multiple bonds, we have also examined its reaction with disilene 23, from which the bicyclic compound 32 was obtained in high yield (Scheme 6). Compound 32 can formally be considered as a [2+3] cycloadduct of the starting materials after a spontaneous 1,Zhydrogen shift. The driving force for the formation of 32 is assumed to be the oxophilicity of silicon. This leads to the dipolar addition product 30 that, in turn, affords the intermediate 31 through 1P-addition. The last step of the sequence would then be a 1,Zproton shift to furnish the isolated compound [16].

23 + 28

Y 0 / SiR2

R*si\o+o

30

1 4 addition L

1,2-proton shift t

Scheme 6.

1,2- and 1,4-Additions

In contrast to the limited number of cycloadditions, reactions of the diene 24 with small molecules were more successful. It reacts readily with water [17], ammonia [18], hydrazine [19], or chlorine [ 181 by two-fold 1 ,Zadditions (Scheme 7).

With small amounts of water the five-membered ring 33, analogous to tetrahydrofuran, is formed in high yield. However, when the amount of water available is sufficient large, a tetrasilanediol is obtained. We believe that the reactions of 24 with water are probably initiated by

122 M. Weidenbruch

the 1,2-addition of a molecule of water to one of the double bonds. When the amount of available water is small, hydrogen shift and ring closure to the five-membered ring 33 occurs. However, when the amount of water is sufficiently large, a further 1,a-addition on the remaining double bond occurs to provide the diol as final product [17].

The reactions of 24 with ammonia or hydrazine are most unusual. Although the correspondingly substituted disilene does not react, the diene participates in spontaneous 1,2-additions even at room temperature to furnish the 1 ,Cdiamino- (34) or 1,4-dihydrazinotetrasilane (35) in almost quantitative yields. On account of the presence of two stereogenic centers in these molecules the existence of a diastereomeric meso form in addition to the enantiomeric R,R and S,S form is possible. The X-ray crystallographic analyses of both compounds revealed the existence of conglomerates of enantiomerically pure substances [ 18, 191. Although the individual molecules of the tetrachlorotetrasilane 36 are also chiral, this compound crystallizes as a racemate [ 181.

H \ R R/H

7- \

R2Si'O/SiR2 'Cl c1

24

34

Scheme 7.

H&. . HNH

35

It is well known from organic chemistry that butadienes undergo not only 1,2-additions but also 1,Caddition reactions. Very recently we have found that treatment of 24 with hydrogen halides, which were slowly generated from trichlorosilane or lithium bromide and trifluoroacetic acid, respectively, furnished unsymmetrically substituted disilenes as formal 1 ,Caddition products (Eq. 10) [20].

From Silylenes and Disilenes to a Tetrasilabuta-1,3-diene 123

R R

/si=s\ 37 x=c1 ,SiR2 38 X=Br \

24 + [HX] + R2Si

H X

Eq. 10.

In analogy to the reaction of 24 with small amounts of water, it can be assumed that these reactions are also initiated by a 1 ,Zaddition of the respective hydrogen halide to one of the silicon- silicon double bonds, followed by a 1,3-hydrogen shift to furnish the disilenes 37 and 38. The decisive factor is apparently the slow liberation of the respective hydrogen halide so that the 1,3- hydrogen shift presumably proceeds faster than the competing addition of a second hydrogen halide molecule to the remaining double bond of the diene 24.

The successful synthesis of the diene 24 posed the question as to whether an analogous compound with conjugated Ge-Ge double bonds would be accessible following a similar route.

As a starting compound we chose the digermene 39, whose structural integrity, according to previous investigations, is retained in solution [21]. Treatment of 39 with lithium did not afford the lithium compound 41 but instead we obtained dark red crystals of the ionic compound 40, containing an allyl-like unit of three germanium atoms as part of the four-membered ring (Scheme 8). Shortening the reaction time to such an extent that most of the germanium had reacted before the formation of 40 became the main reaction, did indeed lead to the lithium compound 41, which by subsequent reaction with mesityl bromide presumably resulted in the bromine compound 42. Intermolecular lithium bromide elimination then furnished the tetragermabutadiene 43 in a very low yield.

R R

o""\ /Br +41 ~

R2Ge=Ge, / R MesBr ~

Li -LiMes R2Ge=Ge,

R -LiBr R2Ge GeR2 41 42 43

Scheme 8.

124 M. Weidenbruch

In the solid state 43 approaches the s-cis form with a dihedral angle for the Ge4 skeleton of 22". The germanium-germanium double bonds are considerably elongated, although they are still in the range typical for digermenes. The electronic spectrum of the dark blue solution of 43 in n-hexane confirms the conjugation of the two double bonds. It shows a longest wavelength absorption at 560 nm which, compared to the yellow or orange digermenes, corresponds to a bathochromic shift of about 140 nm [22].

Very recently we have found that 43 is easily accessible by a simple one-pot sythesis (Eq. 11). Treatment of the germanium dichloride dioxan complex with the aryl Grignard reagent in the presence of magnesium furnishes 43 in one step and in an acceptable yield of about 30 % [23].

4 GeC12. dioxan + 6 RMgBr + 3 Mg - 43 + 4 MgC12 + 3 MgBq

Eq. 11.

Acknowledgment: Financial support of our work by the Deutsche Forschungsgemeinschaft and the Fonds der Chemischen Industrie is gratefully acknowledged.

References [ 13 Recent reviews on silylenes: a) M. Weidenbruch, Coord. Chem. Rev. 1994, 130, 275; b) P. P.

Gaspar, R. West, in: The Chemistry of Organic Silicon Compounds (Eds.: Z . Rappoport, Y. Apeloig), Vol. 2, Part 3, Wiley, Chichester, 1998, p. 2463; c) N. Tokitoh, R. Okazaki, Coord. Chem. Rev. 2000, 210, 251; d) M. Haaf, T. A. Schmedake, R. West, Acc. Chem. Res. 2001, 33,704; e) B. Gehrhus, M. F. Lappert, J. Organomet. Chem. 2001,617-61 8,209. Recent reviews on disilenes: a) R. Okazaki, R. West, Adv. Organomet. Chem. 1996, 39, 231; b) M. Weidenbruch, in: The Chemistry of Organic Silicon Compounds (Eds.: Z . Rappoport, Y. Apeloig), Vol. 3, Wiley, Chichester, 2001, p. 391.

[3] A. Schafer, M. Weidenbruch, K. Peters, H. G. von Schnering, Angew. Chem. 1984, 96, 311; Angew. Chem. Int. Ed. Engl. 1984,23, 302.

[4] D. Ostendorf, L. Kirmaier W. Saak, H. Marsmann, M. Weidenbruch, Eur. J. Inorg. Chem. 1999,2301.

[5] D. Ostendorf, W. Saak, H. Marsmann, M. Weidenbruch, Organometallics 2000,19,4938. [6] D. Ostendorf, W. Saak, H. Marsmann, M. Weidenbruch, Organometallics 2002,21,636. [7] H. Shirakawa, Angew. Chem. 2001,113,2642; Angew. Chem. Int. Ed. 2001,40,2574. [8] M. Weidenbruch, M. Sturmann, H. Kilian, S . Pohl, W. Saak, Chem. Ber. 1997,130,735. [9] F. Meiners, W. Saak, M. Weidenbruch, Organometallics 2000,19,2835. [ 101 Review: K. M. Baines, W. G. Stibbs, Adv. Organomet. Chem. 1996,39,275. [ 111 F. Meiners, D. Haase, R. Koch, W. Saak, M. Weidenbruch, Organometallics, 2002,21, 3990. [12] D. Ostendorf, W. Saak, D. Haase, M. Weidenbruch, J. Organomet. Chem. 2001,636,7. [13] M. Weidenbruch, S. Willms, W. Saak, G. Henkel, Angew. Chem. 1997, 106, 2612; Angew.

[2]

From Silylenes and Disilenes to a Tetrasilabuta-I,3-diene 125

Chem. Int. Ed. Engl. 1997,36,2503. A. Grybat, S . Boomgaarden, W. Saak, H. Marsmann, M. Weidenbruch, Angew. Chem. 1999, 111,2161;Angew. Chem. Int. Ed. 1999,38,2010. a) R. West, D. J. DeYoung, K. J . Haller, J. Am. Chem. SOC. 1985,107,4942; b) R. P. Tan, G. R. Gillette, D. R. Powell, R. West, Organometallics 1991, 10, 546. S . Boomgaarden, W. Saak, M. Weidenbruch, H . Marsmann, Organometallics 2001,20,2451. S. Willms, A. Grybat, W. Saak, M. Weidenbruch, H. Marsmann, Z. Anorg. Allg. Chem. 2000, 626, 1148. S . Boomgaarden, W. Saak, H. Marsmann, M. Weidenbruch, Z. Anorg. Allg. Chem. 2001,627, 349. S . Boomgaarden, W. Saak, H. Marsmann, M. Weidenbruch, Z. Anorg. Allg. Chem. 2001,627, 805. S . Boomgaarden, W. Saak, H. Marsmann, M. Weidenbruch, Organometallics 2003,22,1302. a) J . Park, S. A. Batcheller, S. Masamune, J. Organomet. Chem. 1989, 367, 39; b) W. Ando, H. Itoh, T. Tsumuraya, Organometallics 1989, 8, 2759; c) H. Schiifer, W. Saak, M. Weidenbruch, Organometallics 1999,18,3 159. H . Schafer, W. Saak, M. Weidenbruch, Angew. Chem. 2000, 112, 3847; Angew. Chem. Znt. Ed. 2000,39,3703. A. Schafer, G. Ramaker, M. Weidenbruch, unpublished results.

The Formation of a Solid from the Reaction SiC14(g) + 02(g) == SiOZ(s) + 2 ClZ(g)

Michael Binnewies, Andreas Kornick, Marion Jerzembeck, Andreas Wilkening, Heike Quellhorst

Institut fur Anorganische Chemie, Universitat Hannover Callinstr. 9, D-30167 Hannover, Germany

Tel.: +49 51 1 7622254 - Fax: +49 51 1 76219032 E-mail: binn@ aca.uni-hannover.de

Keywords: growth pattern, CVD process, chlorosiloxanes, silicon dioxide, aerosil

Summary: The reaction of silicon(rv) chloride with oxygen forming silicon dioxide is strongly exothermic. The equilibrium is completely on the product’s side, but activation of the reaction needs considerably high temperatures. The target of our investigations was to understand the formation of a solid starting from small gaseous molecules as precursors. It was shown that during combustion of silicon(rv) chloride in oxygen a great variety of intermediate chlorosiloxanes have been detected. On the basis of their compositions, structures and stabilities, growth mechanisms of Si-0 networks can be derived. The stepwise formation of solid SiOz can be described in terms of three basic reactions: i) formation of highly reactive O=SiClZ, ii) insertion of O=SiC12 into an S i x 1 bond and iii) thermally induced elimination of S ic4 from the resulting chlorosiloxanes.

Introduction

During several CVD processes solid compounds are formed from gaseous starting materials. Over the past 30 years solid-state chemists have become increasingly interested in their formation and in the intermediates during the formation process. The role of clusters and colloids as links between molecules and solids is of actual interest. From the present point of view, it can be stated that the traditional separation between molecules and solids is neither up to date nor scientifically substantiated. Our investigations on the title reaction show that this seemingly simple reaction is highly complicated. Many intermediates are formed; some of them can be isolated and structurally characterized.

Ovganosilicon Chemisty V Edited by N. Auner and J. Weis

Copyright 0 2003 WILEY-VCH Verlag GmbH & Co. KgaA, Weinheim

The Formation of a Solid from the Reaction SiCl4(g) + Oz(g) =SiOz(s) + 2 ch(g) 127

Results

It could be shown that during the title reaction in the temperature range from 800 "C to 1000 "C it is not SiOz that would be expected from a thermodynamic point of view. Instead, a multitude of chlorosiloxanes Si,O,Cl, (x = 2.a.60, molar weights up to 7000 D) are formed [l]. In these

compounds the chlorine atoms of the starting Sic14 are partially replaced by oxygen atoms. These compounds can be considered as intermediates that are formed step by step in the course of the reaction. Figure 1 gives an overview of the X-ray-structures of some chlorosiloxanes.

Fig. 1. Structures of chlorosiloxanes determined by X-ray diffraction methods.

128 M. Binnewies, A. Kornick, M. Jerzembeck, A. Wilkening, H. Quellhorst

By means of matrix-IR investigations [4] it could be shown that at first the highly reactive silaphosgene (SiOC12) is formed in the gas phase. By means of this intermediate the build-up of catena-chlorosiloxanes can be understood (see Scheme 1).

Scheme 1. Formation of Si-0 chains by means of 0=SiCl2.

This mechanism allows some understanding of the growth of the chains. However, it does not provide an explanation for the formation of mono- and oligocyclic siloxanes (Fig. 1). In a growth reaction, O=SiCl2 will invariably introduce Si and 0 atoms into the newly formed siloxanes in a 1: 1 ratio. Since oxygen-richer compounds are undoubtedly formed the insertion reaction of O=SiCl2 cannot be the only reaction responsible for the build-up of the Si-0 frameworks of oxygen-rich siloxanes and SiOz. Experimental results and thermodynamic considerations show that chlorosiloxanes are thermodynamically unstable. They must decompose for entropy reasons, preferably to the following decomposition products [5] :

Scheme 2 shows the formation of the cyclic Si303Ck as an example.

Scheme 2. Decomposition reaction leading to cyclic products.

According to Scheme 2 the stoichiometry of decomposition of the chlorosiloxanes that are built-up stepwise differs from that of their formation. The chlorosiloxane molecules become smaller but oxygen-richer in each decomposition step. Figure 2 shows the build-up and decomposition

The Formation of a Solid from the Reaction Sic&) + O2(g) = S o d s ) + 2 c12(g) 129

reactions as a graph.

2.0

. . . . I

I - I

.. --.

1.5 -

40) n(Si) m- - - . *----

- . - - - - m - - - - . - - - - a - - - - .- - - - - 1.0 -

--'1

0.5 -

0.0 I I I I I

6 8 n (si) 2

Fig. 2. Growth and decomposition of chlorosiloxanes leading to the formation of SiOz.

I

In summary we can say that O=SiClZ is formed in a first step of the reaction between Sic14 and 0 2 . The formation of Si-0 frameworks in the chlorsiloxanes and finally in SiOz can be regarded as an interplay between the build-up reaction by means of O=SiClZ and the entropy-driven decomposition reaction of the chlorosiloxanes.

Using the knowledge about this mechanism for the title reaction, we are now able to understand the influence of temperature and reaction time on the growth of small Si02 particles.

Acknowledgment: The authors are grateful to the Deutsche Forschungsgemeinschaft and the Funds der Chemischen Industrie for financial support.

References [l] [2] [3] [4] [5]

M. Binnewies, M. Jerzembeck, A. Wilkening, Z. Anorg. Allg. Chem. 1997,623, 1875. U. Wannagat et al., 2. NaturJorsch. Teil B 1991,46, 931. K. W. Tomroos, G. Calzafem, R. Imhof, Acta Crystallogr., Sect. C 1995,51, 1732. M. Junker, A. Wilkening, M. Binnewies, Hg. Schnockel, Eur. J. Inorg. Chem. 1999, 1531. M. Binnewies, K. Jug, Eur. J. Znorg. Chem. 2000, 1127.

Stepwise Formation of Si-0 Networks by Means of Hydrolysis/Condensation

Nicola Soger, Heike Quellhorst, Michael Binnewies

Institut fur Anorganische Chemie, Universitat Hannover Callinstr. 9, D-30167 Hannover, Germany

Tel.: +49 511 7625187 -Fax: +49 511 76219032 E-mail: soeger@ web.de

Keywords: chlorosiloxanes, ethoxysiloxanes, siloxanoles, GC-MS, hydrolysis

Summary: The hydrolysis of Six4 (X = C1, OEt), leading to Si02 as the final product, gives a great multitude of different siloxanes and siloxanoles as intermediates. The compositions of the reaction mixtures were observed by GC-MS methods. Using certain perchlorosiloxanes as starting materials instead of Sic14 the number of different products can be reduced. However, hydrolysis of ethoxysiloxanes leads to the cleavage of Si-0-Si bonds.

Introduction

The hydrolysis of different silanes Six4 - followed by condensation processes - is a commonly used method for the preparation of Si-0 networks with varying properties. Thus, the "continuous flame hydrolysis" of silicon tetrachloride, SiC4, leads to highly disperse, pyrogenic Si02 (Aerosil, Fa. Degussa). The sol-gel process is frequently introduced by the hydrolysis of tetraethoxysilane, Si(OEt)4. During these procedures oligo- and polymeric siloxanes with the general formula SinOn+mXfn-Zm are built as intermediates. Depending on the chosen conditions, the compositions of the reaction mixtures vary considerably. Observing the oligomers developed in the early stage of the hydrolysis systematically contributes significantly to the understanding of the reaction course.

Hydrolysis of Perchlorosilanes and -Siloxanes [ 11

In a first step the silanol SiC130H is built via nucleophilic attack of the H20 molecule. Using a lower-molecular chlorosiloxane, such as Si2OC16, as precursor instead of the monomeric S ic4 the corresponding siloxanol is obtained according to Scheme 1.

Isolation of these reactive molecules could not be achieved because of their strong tendency to perform self-condensation. Their existence was proved by carrying out the reaction in the vapor phase [2]. However, their condensation products could be observed by GC-MS methods. Mass-

Ovganosilicon Chemisty V Edited by N. Auner and J. Weis

Copyright 0 2003 WILEY-VCH Verlag GmbH & Co. KgaA, Weinheim

Stepwise Formation of Si-0 Networks by Means of HydrolysisKondensation 131

spectra of several perchlorosiloxanes could therefore be obtained after separating the species by gas chromatography [3].

\ \ -Si-Cl -Si-OH / - / + H,O I - HCI -

Scheme 1. First reaction steps during the hydrolysis of Sic& / Si20Cl6.

I / %

0

2 4 6 8 10 12 tR,, / min

Fig. 1. Chromatogram of a reaction mixture obtained after partial hydrolysis of Si20Cl6 (ethox = ethoxysiloxanes as

byproducts).

During the hydrolysis of Si2OC16 there was no evidence for the formation of oligomers with an odd number of Si atoms, as Fig.1 illustrates. Consequently, the reverse reaction to form SiC14, requiring cleavage of the Si-0 bond, does not seem to occur.

Hydrolysis of Ethoxysilanes and -Siloxanes [4]

Compared to the previously described chlorosiloxanols, the OH-substituted intermediates in this system have a lower tendency to perform self-condensation. Therefore, the appearance of siloxanols in solution leads to an even greater variety of products [5 ] . In addition, a noticeable conversion requires acid or base catalysis because of very low reaction rates.

Hydrolysis Si(0Et)d

The composition of the product mixture strongly depends on the initial concentration of the

132 N. Soger, H. Quellhorst, M. Binnewies

precursors and on the water-to-silane ratio (R = n(HzO)/n(Si(OEt>4). The concentrations of some relevant trisiloxanes are plotted versus the reaction time in Fig. 2. The example compares a reaction with low water content (R = 1) to one with a significantly higher one ( R = 4).

4 a 5 R=4: cyclic tisilolane

0 -1.00 -0.50 0.00 0.3 1.00 1.50 2.00

log I

Fig. 2. Formation and further reaction of some trisiloxane species: amounts of components Are,. vs. log t . C ( R = 1)

exceeds the extent of the diagram.

A higher water-to-silane ratio (see Fig. 2) basically causes: a) higher reaction rates (the products appear more quickly and disappear earlier); b) lower ratios of siloxanes to the corresponding siloxanols; c) higher ratios of cyclic to chain-like products.

Hydrolysis of SizO(OEt)6

In order to improve the possibility of following the reaction course, Si20(0Et)6 is hydrolyzed under comparable conditions. Figure 3 illustrates the concentrations of important components of the reaction mixture as a function of time.

xdisilomne

cyclic tetrasiloxane

0 chamlike tetrasilolrane

-2.5 -1.5 -0.5 0.5 1.5 2.5

f

Fig. 3. Amounts of the main components in reaction mixture vs. log t (water content: R = 1.5).

u.. . ... . . . .......... . ............ . . ... . . . . . . ........ . . ..... . . . . . . . . ...... . .I... .... ....... 1 ‘(130kS, ,!Si(OlE)

......... . ......... . ...

134 N. Soger, H. Quellhorst, M. Binnewies

1 4 1 EtOH "", Si(OEt),

(EtO),Si-0' HO

0

(EtO),

/EtOH

0-Si(OEt),

HO;s{ ' 0 I ,OEt EtO I

0 \ . (EtO),SI-O

Scheme 2. Cyclization reaction, possible reaction pathways: A) one-step condensation building the eight-

membered ring directly; B) two-step condensation with chain prolongation and subsequent cyclization.

Acknowledgment: The authors are grateful to the Deutsche Forschungsgemeinschaft for financial support.

References [l]

[2] [3]

H. Quellhorst, A. Wilkening, N. Soger, M. Binnewies, Z. Natugorsch. Ted B 1999, 54,

H. Quellhorst, A. Wilkening, M. Binnewies, Z. Anorg. AlZg. Chern., 1997,623, 1871-1874. N. Soger, M. Binnewies, Z. Anorg. Allg. Chem., in press.

577-582.

The Conformational Preference of the Methyl Group in 1-Methyl-1-silacyclohexane

Ingvar Arnason, * Agust Kvaran, Sigridur Jonsdotlir, Palmar I. Gudnason

Science Institute, University of Iceland, Dunhaga 3, IS- 107 Reykjavik, Iceland Tel.: +354 5254800 - Fax: +354 552891 1

E-mail: ingvara @ raunvis . hi. is

Heinz Oberhammer

Institut fur Physikalische und Theoretische Chemie, Universitat Tiibingen Auf der Morgenstelle 8,72076 Tiibingen, Germany

Keywords: electron diffraction, NMR, silacyclohexane, conformational analyses

Summary: The conformational equilibrium of the title compound has been investigated experimentally in the gas phase by gas electron diffraction and in solution at low temperatures by 13C NMR, and theoretically by quantum chemical calculations. Both experimental methods result in a preference of the equatorial position of the methyl group, 68(7) % in the gas phase at 298 K and 74(1) % in solution at 110 K. The calculations predict 66 % equatorial conformer at room temperature.

Introduction

The conformational behavior of monosubstituted saturated six-membered ring systems (Fig. 1) plays an important role in organic stereochemistry. According to a low-temperature 13C NMR experiment methylcyclohexane 1 exists as a mixture of 95 % equatorial and 5 % axial conformers, corresponding to an A value of 1.80(2) kcal mol-' [l]. Similarly, in silylcyclohexane 2 the equatorial conformer is strongly preferred [2, 31. On the other hand, force field calculations for the title compound 3 predict a preference for the axial form with slightly negative A values [4]. This is in agreement with the interpretation of room-temperature 'H NMR spectra, resulting in A = -0.34 kcal mol-' [5]. These conformational properties are unusual for a monosubstituted cyclohexane-like ring and prompted us to perform further experimental investigations, using gas electron diffraction (GED), low-temperature 13C NMR and theoretical calculations (HF, MP2 and DFT).

Ovganosilicon Chemisty V Edited by N. Auner and J. Weis

Copyright 0 2003 WILEY-VCH Verlag GmbH & Co. KgaA, Weinheim

136 I. Amason, A, Kvaran, S. Jonsdottir, P. I. Gudnason, H. Oberhammer

1 2 3

7

& - - - E = x

A = -A@ = Rnn(K)

Fig. 1. Conformational behavior of monosubstituted saturated six-membered ring systems.

Gas Electron Diffraction

The electron diffraction intensities were recorded at room temperature in the s-range 2 to 35 A-' (s = (4dh) sin8/2, h = electron wavelength, 8 = scattering angle). The experimental radial distribution of the molecular scattering intensities is shown in Fig. 2 together with calculated curves for the axial and equatorial conformers. The geometric parameters of 3 (Table 1) and the fraction of the equatorial conformer (see Table 3) were obtained by least squares fitting of the molecular intensities.

Table 1. Skeletal experimental and calculated geometric parameters for the equatorial conformer of 3.["'

GED [bl HF/6-31G* MP2/6-31G* B3LYP/6-31G*

S i x 2 1.867(4) 1.893 1.887 1.897

Si-C7 1.862(4) 1.890 1.885 1.892

C2-C3 1.534(3) 1.542 1.539 1.546

c3-c4 1.528(3) 1.536 1.533 1.540

C2-Si-C6 102.8(20) 104.1 103.8 104.2

c3-c4-c5 116.7(34) 114.5 114.2 114.5

Si-C2-C3 110.5(16) 111.1 110.3 111.1

C2-C3-C4 112.4(27) 113.8 113.5 113.9

C2-Si-C7 112.5(24) 112.9 113.0 113.0

Flap(Si) [" 46.0(3 1) 40.5 42.8 40.6

n a p ( ~ 4 ) ['I 55.9(20) 56.7 58.4 56.6

#Si-C2-C3-C4) 56.6( 10) 55.0 56.5 55.0

#C2-C3-C4C5) -62.9(9) 4 5 . 0 -65.6 -64.9

#CZ-Si-CS-CS) 49.8(28) 43.7 46.0 43.8

[a] Values in 8, and degrees. [b] Uncertainties are 30 values. [c] Flap angle from the plane of C2, C3, C5, and C6.

The Conformational Preference of the Methyl Group in 1 -Methyl-I -silacyclohexane 137

-.--

0 1 2 3 4 5 6 RIA

Fig. 2. Radial distribution functions of 3 (left) and molecular model of the equatorial conformer (right).

NMR Spectra

13C NMR spectra of 3 were recorded with a Bruker AC 250 instrument at room temperature and at low temperatures (Fig. 3). The spectrum at the lowest temperature demonstrates the presence of two conformers. Coalescence occurs around 124 K. The assignment of the signals to the various carbon atoms of 3 and to equatorial and axial conformers is based on calculated chemical shift values (Table 2) . This assignment is also in agreement with data for 1. The ratio of the two conformers was obtained by analysis of the spectrum at 110 K.

A T = 1 1 8 K

A ~ k A AA h A T = I l O K

I I I I I I ppm 30 20 10 0 -1 0 -20

Fig. 3. Low-temperature "C NMR spectra of 3.

138 I. Arnason, A. Kvaran, S. Jonsdottir, P. I. Gudnason, H. Oberhammer

Table 2. "C NMR chemical shifts for 3 in CDC13 solution at 293 K and in 1: 1:3 solution of CD2C12, CHFC12 and

CHFzCl at 110 K. Calculated values (RI-DlT) are given in parentheses.

T[Kl (36) C3(5) c 4 c 7

3 293 11.73 24.82 29.93 -5.62

3 (eq) 110 11.92 (13.02) 25.79 (26.17) 29.81 (29.68) -4.32 (-2.99)

3 (ax) 110 10.71 (11.99) 24.04 (24.84) 29.81 (29.56) -7.31 (-5.45)

Results

The conformational properties of the monosubstituted six-membered rings 1, 2 and 3 are summarized in Table 3. In contrast to previous interpretation of NMR data and MM calculations, our experimental and theoretical investigations result in a preference for the equatorial conformer in the title compound 3. Thus, the conformational properties of this compound are similar to those of 1 and 2. The A value of 3 is considerably smaller but still positive.

Table 3. Conformational properties for 1 ,2 and 3.

Ring system Method Equatorial [%I A [kcal mol-'1 Ref.

NMR (157 K) 95 1.80(2) 1

ab initio (298 K) 97 2.05 I

NMR (188 K) 92 1 . 4 3 1) 2

ab initio (298 K) 96 1.90 3

NMR (298 K) 36 -0.34 5

MM2 (298 K) 45 -0.13 4

1

2

3 NMR(110K) 74(1) 0.23(2) this work

GED (298 K) 6 W ) 0.45(14) this work

ab initio (298 K) 66 0.40 this work

References [l]

[2] [3] [4] [5 ]

K. B. Wiberg, J. D. Hammer, H. Castejon, W. F. Bailey, E. L. DeLeon, R. M. Jarret, J. Org. Chem. 1999,64,2085. K. G. Penman, W. Kitching, W. Adcock, J. Org. Chem. 1989,54,5390. S. G. Cho, 0. K. Rim, Y.-S. Kim, J. Mol. Struct. (Theochem) 1996,364,59. M. R. Frierson, M. R. Iman, V. B. Zalkow, N. L. Allinger, J. Org. Chem. 1988,53,5248. R. Carleer, M. J. 0. Anteunis, Org. Magn. Reson. 1979,12,673.

Origin of Photoluminescence in Organosilicon Compounds Containing Styrene Subunits

Duanchao Yan, Thomas Miiller, Michael Bolte, Norbert Auner"

hstitut fur Anorganische Chemie der Universitat Frankfurt, Marie Curie-Str. 11, D-60439 Frankfurt am Main, Germany

Tel.: +49 69 79829591 -Fax: +49 69 79829188 E-mail: Auner@chemie.uni-frankfurt.de

Keywords: silanes, fluorescence, stilbene

Summary: The observation of an intense blue photoluminescence in 1,l -diorgano-2,3- diphenyl-4-neopentyl- 1 -silacyclobut-2-enes upon irradiation with UV light led to the investigation of the origin of the phenomenon in this class of compounds. In this report we describe the synthesis of seven silicon-based model compounds which contain stilbene or styrene subunits, respectively. The compounds were characterized spectroscopically and by single-crystal X-ray crystallography. Their photoluminescence spectra were recorded in the solid state as well as in dilute THF solution. The interpretation of the spectra revealed that the photoluminescence in this series of compounds originates from the presence of the stilbene or its vinylogue subunits. The different linkages of these groups to the silicon centers (cyclic or open structures, four- or five-membered cycles) strongly affect both the excitation as well as the emission spectra.

Introduction

Several different types of phenyl groups containing silicon-based compounds show an intense blue photoluminescence (PL) upon excitation with UV light [ 11. For silacyclobutenes 1, the exocyclic substituents R at silicon affect both the strength and wavelength of the PL (Fig. 1).

In extension of these studies, a broad range of differently substituted cyclic and acyclic silicon compounds were synthesized and their PL spectra were investigated to identify the origin of the PL and to determine the influence of substituents and molecular structure on the phenomena. All the compounds contain stilbene, stilbene vinylogue or styrene subunits.

Ovganosilicon Chemisty V Edited by N. Auner and J. Weis

Copyright 0 2003 WILEY-VCH Verlag GmbH & Co. KgaA, Weinheim

140 D. Yan, T. Miiller, M. Bolte, N. Auner

1000-

a**\ ' \

800 -

R = M e

: I 400

200

7.3 320 350 400 450 500 560

nm

Ph R\H+ 1 Ph

Fig. 1. PL of silacyclobutenes 1 (solid state).

Syntheses of Model Compounds

The air-stable compounds I, 11, VI, and VII were synthesized by treating the corresponding dichlorosilanes [2] with 2 equiv. of PhCCMgBr at room temperature (Scheme 1). The final product formation from the reaction of 1 (R = Cl) with PhCCMgBr (reaction (e)) is strongly controlled by the reaction conditions: (i) addition of the Grignard reagent to a solution of 1 gave the expected substitution product VI, while (ii) the reverse addition resulted in the isolation of ring-opened compound VII. Reaction of silole I1 with lithium naphthalenide and ClSiHMe2 yielded a complicated mixture, from which V could be isolated in small amounts. Silacyclobutenes I11 and IV were synthesized according to literature procedure [3]. All compounds were characterized by NMR spectra. The structures of compounds I, 11, V, and VII were determined by X-ray single-crystal diffraction analysis (Fig. 2) .

Crystal Structures

The phenyl groups of the stilbene subunit in compound I adopt a cis-configuration. In compound 11, the four phenyl rings at the d o l e ring exhibit a propeller-like arrangement. In silacyclopentene V, the two Me2SiH- groups are in the cis-position. In alkene VII, the carbon-carbon bond lengths are 150 (Cl-C2) and 132 pm (C2-C3), confirming the double bond shift and the disintegration of the former stilbene subunit.

Origin of Photoluminescence in Organosilicon Compounds 141

(b)

Ct, ,ct

EtZO FilRph THF

Sic14 PhC-CMgBr PhCECPh + Li -

II 59%

IV: R=Ph 67%

v 8%

I PhCtCMgBr ~ ‘Ys/

reverse + 1N addition HCI +$/+ VII 31%

Scheme 1.

142 D. Yan, T. Miiller, M. Bolte, N. Auner

UV Spectra and Photoluminescence

The acyclic compounds I and VII exhibit very similar UV-vis absorptions (Fig.3), and their maximum absorptions are closer to styrene than to stilbene. Silole I1 has a broad absorption band around 380 nm, which is ascribed to the n -+ n* transition of the silole n-conjugated moiety [4]. Its absorption in the short wavelength region is quite similar to that of silacyclopentene V. The UV-vis spectrum of silacyclobutene VI is close to that of silacyclopentene V, but has a different fine structure. The silacyclobutenes I11 and IV have very similar UV-vis spectra in the region >250 nm, with the absorption maximum around 345 nm. These bands probably arise from the n 4 n* transition of 1,4-diphenyl-trans,truns-buta- 1,3-diene subunits. All compounds show the phenyl group absorption at 212-214 nm. It is obvious that the extended conjugation (in I11 and IV) causes the long-wave length shift (red shift) of I,,,,,,.

CI

C

I I1

I

c12

V

Fig. 2. Molecular structures of I, 11, V, and VII.

C1241

C114l

VII

Origin of Photoluminescence in Organosilicon Compounds 143

0.8- .

0.84 , I : , . . .’ .. . . ,. . .

200 300 400 500

Wavelength [nm] Wavelength [nm]

Fig. 3. UV-vis absorption spectra of I-VII in THF (lo” M).

The PL emission spectra (Figs. 4,5) give evidence that the origin of PL in the presented series of compounds is not caused by the styrene subunit, because the styrene-containing compound VII shows no PL emission. The other samples generally exhibit strong PL in the solid state despite their different molecular structures. Therefore the PL should originate from stilbene or its vinylogue subunits. The reason for the weak PL of silole I1 in solution is not obvious. The size of the conjugated system determines the red shifts of the emission bands (compare 11, 111, IV with I, V, VI), while the effect of cyclization is only small, as shown by the comparison between VI and I.

.“ : :. ,-\

350 400 450 500 550 nm

Fig. 4. PL emission spectra of I-VII (solid state).

144 D. Yan, T. Miiller, M. Bolte, N. Auner

400

z 3w 2w

1W

1 0

1 I ’,I \ 1 I \

320 340 360 380 4w 420 440 460 480 5W nm

Fig. 5. PL emission spectra of I-VII in THF solution.

300 369 I 250 4

nm

Conclusion

The PL of this series of compounds originates from stilbene or its vinylogue subunits. Extended conjugation causes a red shift of the maximum of both the absorption and the emission. Cyclization has only minor effects on the PL emission; however, it significantly influences the excitation spectra.

Acknowledgement: We thank DFG and Dow Corning Corporation for financial support.

References U. Pernisz, N. Auner, “Photoluminescence of Organically Modified Cyclosiloxanes”, in: Organosilicon Chemistry IV - From Molecules to Materials (Eds.: Auner, N.; Weis, J.), VCH, Weinheim 2000, pp. 505-520. a) M. Green, J. L. Spencer, F. G. A. Stone, C. A. Tsipis, J. Chem. Soc., Dalton Trans. 1977, 1525; b) N. Auner, C. Seidenschwarz, E. Herdtweck, Angew. Chem. 1991,102, 1172; Angew. Chem., Int. Ed. Engl. 1991, 30, 1151; c) W.-C. Joo, J.-H. Hong, S.-B. Choi, H.-E. Son, C. H. Kim, J. Organomet. Chem. 1990,391,21. Z. Xi, R. Fischer, R. Hara, W.-H. Sun, Y. Obora, N. Suzuki, K. Nakajima, T. Takahashi, J. Am. Chem. Soc. 1997,119, 12842. S . Yamaguchi, T. Endo, M. Uchida, T. Izumizawa, K. Furukawa, K. Tamao, Chem. Eur. J . 2000,6, 1683.

Photoluminescence Characteristics of Linear Methyl- and Phenyl- Substituted Siloxanes

Michael Backer," Udo Pernisz

Dow Coming Limited, New Ventures Business, Science and Technology Cardiff Road, Barry, Vale of Glamorgan, CF 63 2YL, UK

Tel.: +44 1446 723712 - Fax: +44 1446 730495 E-mail: michae1.w .backer @ dowcorning.com

Keywords: phenylsiloxanes, silanediols, photoluminescence, intensity contour plots

Summary: Short linear methyl- and phenyl- substituted siloxanediols were investigated as solids in powder form at room temperature, and complete photoluminescence excitation-emission maps in the ultraviolethisible spectral range were obtained. The position of the emission maxima and Stokes shifts were analyzed from intensity contour plots. Different types of luminescence behavior could be identified that correlate with molecular structure and chemical substitution patterns. The iterative replacement of methyl groups in siloxanes with phenyl substituents and the increase in the chain lengths both lead to a bathochromic shift of the emission maxima into the near-UV region and generally give rise to an increase in emission intensity.

Short siloxanediols are important precursors in a wide variety of industrial products. They can be considered also as the monomeric representatives of a series of polymeric compounds which exhibit unique and highly desirable features such as specific rheological behaviour or thermal and chemical resistance. However, despite the broad knowledge about their physical and material properties, only limited information is available on the photoluminescence behaviour of these classes of compounds [ 1-61. In order to obtain a basic understanding of the influence of methyl and phenyl substituents on the photoluminescence of siloxanes, several siloxanediols have been prepared and investigated.

Tetramethyldisiloxane- 1,3-diol 1 was produced by hydrolysis of the dichloro-substituted analogue in diethyl ether in the presence of trialkylamine, and subsequent crystallization from pentane [8, 91. Hydrolysis of a stereo isomeric mixture of 1,3-dichloro-l,3-dimethyl-l,3-diphenyl- disiloxane in ether in the presence of sodium bicarbonate led to the formation of the corresponding disiloxane- 1,3-diols. Fractional crystallization of the mixture from tetrachloromethane yielded pure ruc- 1,3-dimethyl- 1,3-diphenyldisiloxanediol 2 [ 101, while subsequent recrystallization of the residual material from hexamethyldisiloxane/petroleum ether failed to result in the formation of the pure meso compound 3. Therefore, compound 3 was not investigated further. Diphenylsilanediol4 [ 1 11 and tetraphenyldisiloxane- 1,3-diol 5 [ 121 have also been prepared by hydrolysis of their

Ovganosilicon Chemisty V Edited by N. Auner and J. Weis

Copyright 0 2003 WILEY-VCH Verlag GmbH & Co. KgaA, Weinheim

146 M. Backer, U. Pernisz

analogous chlorosilanes [5 ] and subsequent crystallization from 2-butanone/chloroform or hot toluene/petroleum ether. Hexaphenyltrisiloxane- 1,5-diol 6 [ 121 can be generated via ring opening reaction of hexaphenylcyclotrisiloxane in tetrahydrofuran in the presence of aqueous n-hexylamine followed by crystallization from toluenehexane [ 131.

The photoluminescence spectra were obtained at room temperature with a SPEX Fluorolog 2 spectrophotometer (JobidYvon) using a 150 W high-pressure Xe arc lamp. The single grating excitation and emission monochromators were equipped with 1200 lines/nm gratings blazed at 330 nm and 500 nm, respectively. With a focal length o f f= 0.22 m, the system had a dispersion of about 3 nrdmm [ 11. The spectra of the compounds were taken at room temperature.

Tetramethyldisiloxane- 1,3-diol 1 exhibits only low-intensity photoluminescence when irradiated with light of wavelengths 270, 300, or 330 nm. As seen in the spectra shown in Fig. 1, the intensity is decreasing continuously towards longer excitation wavelengths. The analysis of the vibrational structure seen in the emission spectra indicates different group frequencies spaced from Av = 1330 cm-' to 1720 cm-' which are assigned to the Si-Me2 vibration and combinations of this mode with the Si-OH and the symmetric Si-0-Si stretch vibration [ 141.

- .E 6.E+07 a

- 4.E+07

g 3.E+07

.- ! 2.E+07

.- i.E+07

p i.E+06

c 5.E+07

.- 1:

D

-270 nm

-300 nm

z 290 310 330 350 370 390 410 430 450 470 490

Wavelength [nm]

Fig. 1. Emission spectra of tetramethyldisiloxane-1,3-diol 1 at excitation wavelengths of A= 270,300, and 330 nm.

-270 nm

-300 nm

330nm 1 6.E+07 - 2 '5 5.E+07 -

4.E+07 ~

c 3.E+07 ~

'3 2.E+07 -

E l.E+07 - 1 .E+06 --

C

.- 0

.- w

290 310 330 350 370 390 410 430 450

Wavelength [nm]

Fig. 2. Emission spectra of ruc-1,3-dimethyl-1,3-diphenyldisiloxanediol2 excited at 1 = 270,300, and 330 nm.

Photoluminescence Characteristics of Linear Siloxanes 147

The exchange of one methyl substituent per silicon unit in 1 by a phenyl group to form rac- 1,3-dimethyl- 1,3-diphenyldisiloxanediol 2 results in a slightly higher overall intensity, but also in the loss of the vibrational structure and of the emission maximum at 317 nm for the spectrum taken at & = 270 nm, (Fig. 2). At longer excitation wavelengths, the local emission maxima at 348, 367, and 385 nm are, however, still present, thus confirming the group vibrational structure concept.

, c) 350;

i

E m s o n Wmk@h [MI] I n v r r l ~ m W M h bml

Fig. 3. Photoexcitation-emission intensity maps (contour plots) of a) rac-1,3-dimethyl-l,3-diphenyldisiloxanediol 2,

b) diphenylsilanediol 4, c) tetraphenyldisiloxane-l,3-diol 5, and d) hexaphenyltrisiloxane-1,3-diol 6,

measured at room temperature with spectral bandwidth of 1 nm.

148 M. Backer, U. Pemisz

This surprising substitution effect is more clearly seen in Fig. 3a, which shows an excitation-emission intensity contour plot for compound 2, where a saddle point separates the emission maximum at 360 nm from the short-wavelength features. This explains the two qualitatively different emission characteristics observed for 2 in Fig. 2 for high and low excitation energy.

Diphenylsiloxanes exhibit a drastically enhanced photoluminescence compared to the dimethyl- or mixed methyl-phenyl-substituted siloxanes due to the unique interaction between their 7c-

electron systems in the substituents and the Si atom. The disiloxane group in 5, (Fig. 3c), especially, apparently constitutes a unit with particularly high photoluminescence efficiency [2].

As presented in Figures 3b-d, the emission intensities in the diphenylsil(ox)anediols 4-6 have very different distributions that depend on the number of diphenylsiloxy subunits, although all three display the same vibrational bands except for 5, which apparently lacks the distinct feature in the high-energy region at kS = 320 nm. The bathochromic shift of the emission maxima in the longer siloxanes 5 and 6, from A,,,s = 320 nm to 356 nm, gives rise to the assumption of a conjugation between the chromophores via the Si-0-Si bonds. Surprisingly, trisiloxanediol 6 exhibits the luminescence features of silanediol4 and the dimer 5 so that, in photoluminescence terms, it can be considered possibly as a (2+1) combination of silicone units. Investigation of compounds like the tetramer or pentamer could elucidate this model which then, if confirmed, could serve as a guide for the preparation of molecules with desired luminescence or perhaps electroluminescence properties.

Acknowledgments: The authors thank Prof. N. Auner, Johann Wolfgang Goethe Universitat, Frankfurt, for many stimulating discussions. Helpful comments on the data analysis by G. Zank and computational support by T. Lauer are gratefully acknowledged. Some of the photoluminescence spectra were taken by A. Hart with great skill and expert handling of the equipment. We also thank Brian Harkness for the donation of pure trisiloxanediol 6. (The last four are all at Dow Coming Corporation.)

References [l] U. C. Pernisz, M. W. Backer, Photoexcitation and Emission Spectra of Phenyl Substituted

Cyclosiloxanes, in Synthesis and Properties of Silicones and Silicone-Modijied Materials, ACS Symposium Series 838 (Eds.: S. J. Clarson, M. E. Van Dyke, J. J. Fitzgerald, M. J. Owen, S. D. Smith), Am. Chem. SOC., Washington DC, 2003, chapter 10, p. 105. U. C. Pernisz, M. W. Backer, Polym. Prepr. 2001,42(1), 122. U. C. Pernisz, N. Auner, M. W. Backer, Photoluminescence of Phenyl- and Methyl- Substituted Cyclosiloxanes, in Silicones and Silicone-Modijied Materials, ACS Symposium Series 729 (Eds.: S. J. Clarson, J. J. Fitzgerald, M. J. Owen, S. D. Smith), Am. Chem. SOC., Washington DC, 2000, p. 115. U. C. Pernisz, N. Auner, Photoluminescence of Organically Modified Cyclosiloxanes, in Organosilicon Chemistry - From Molecules to Materials (Eds.: N. Auner, J. Weis),

[2] [3]

[4]

Photoluminescence Characteristics of Linear Siloxanes 149

VCH-Wiley, Weinheim, 2000, p. 505. [5] M. W. Backer, Silacyclobutene - Synthese, Struktur, Reaktivitat und Materialien, Mensch

und Buch Verlag, Berlin, 1999 (Ph.D. Thesis, Humboldt Universitat zu Berlin, 1999). [6] U. C. Pernisz, N. Auner, M. W. Backer, Polym. Prepr. 1998,39(1), 450. [7] J. A. Cella, J. C . Carpenter, J. Organomet. Chem. 1994,480, 23. [8] P. D. Lickiss, A. D. Redhouse, R. J. Thompson, W. A Staczyk, K. Rozga, J. Organomet.

Chem. 1994,453,13. [9] J. Hickton, A. Holt, J. Homer, A. W. Jarvie, J. Chem. SOC. 1966, 149. [ 101 J. K. Fawcett, N. Camerman, A. Camerman, Can. J. Chem. 1977,55,363 1. [ l l ] V. E. Shklover, Y. T. Struchkov, I. V. Karpova, V. A. Odinets, A. A. Zhdanov, Zh. Strukt.

Khim. 1985,16, 125. [12] H. Behbehani, B. J. Bridson, M. F. Mahon, K. C. Molloy, J. Organomet. Chem. 1993, 463,

41. [13] M. Tachikawa, K. Takeuchi, B. R. Harkness, Chem. Muter. 1998,10,4154. [14] E. D. Lipp, A. L. Smith, Infrared, Raman, Near-Infrared, and Ultraviolet Spectroscopy, in:

The Analytical Chemistry of Silicones, Chemical Analysis Series, Vol. 112 (Ed.: A. L. Smith), John Wiley, New York, 1991, p. 305.

Syntheses of Silyllithium Reagents Starting from Tetraorganosilanes

Jan Hornig, Dominik Auer, Carsten Strohmann*

Institut fur Anorganische Chemie, Universitat Wurzburg Am Hubland, D-97074 Wurzburg, Germany

Tel.: +49 931 888 4613 -Fax: +49 931 888 4605 E-mail: c.strohmann@mail.uni-wuerzburg.de

Keywords: lithium, Si-C cleavage, silyl anions, silyllithium compounds, tetraorganosilanes

Summary: Lithiated silanes were generated by Si-C bond cleavage of tetraorganosilanes. Reaction of 9-methylfluorenyl tetraorganosilanes with lithium metal results in the formation of silyllithium and organometallic compounds. A more convenient method is the selective Si-C bond cleavage of diphenylmethyl-substituted tetraorganosilanes. The latter method is a route to functionalized silyllithium compounds, which can be used as building blocks in organic and organometallic synthesis and correspond to silyl anion and silyl dianion synthons.

Introduction

Silyllithium compounds are useful and important reagents for silyl group transfer reactions to organic molecules or organometallic systems [l]. Lithiated silanes can be prepared by reaction of lithium metal with chlorosilanes or disilanes. The latter method is limited to systems bearing at least one aryl group [2].

As part of our studies we are investigating the Si-C bond cleavage of tetraorganosilanes in order to find more stable starting materials for the preparation of silyllithium compounds. Review articles and reports do not describe the cleavage of Si-C bonds with lithium or other alkaline metals as the most commonly used method so far [24 ] . Only a few examples can be found, most of them under drastic conditions requiring the use of strongly coordinating additives such as HMPA [24] . One of the first reported successful Si-C cleavage reactions without such additives is the reaction of Ph,SiCMe,Ph (1) with NdK in EtzO leading to the metallosilane Ph,SiK (2) (Eq. 1) [2, 51. Another example is the reaction of tetraphenylsilane (4) with lithium metal in THF, which results in the formation of triphenylsilyllithium (5) and phenyllithium (6) (Eq. 2) [3,6].

Ovganosilicon Chemisty V Edited by N. Auner and J. Weis

Copyright 0 2003 WILEY-VCH Verlag GmbH & Co. KgaA, Weinheim

Syntheses of Silyllithium Reagents Starting from Tetraorganosilanes 151

Nan< 2 Li p h 3 s i ~ p h - !%$iK + phMe&K SiPh4 THF- Ph3SiLi + PhLi

Et20 1 2 3 4 5 6

Eq. 1. Eq. 2.

Unfortunately this reaction is rather special and does not allow further functionalization at the silicon center. Therefore we decided to search for alternative starting materials with activated Si-C bonds, which can be cleaved under “attractive” conditions without any additives like HMPA. These precursors should furthermore allow a variety of functional groups at the silicon center and thus be useful building blocks for the field of synthetic chemistry. In general these tetraorganosilane precursors correspond to silyl anion and silyl dianion synthons of type B and D in Eqs. 3 and 4.

R3Si-CR3 & R3Si@ n ‘sio R/ 0

R

A

Eq. 3.

0 D

Si-C Bond Cleavage of 9-Methylfluorenyl-Substituted Organosilanes

Our first approach for such a stable precursor were 9-methylfluorenyl-substituted silanes such as 7a and 7b, synthesized starting from lithiated 9-methylfluorene and various chlorosilanes. Cleavage of the activated Si-C bonds with lithium metal led to the silyllithium systems 8a and 8b (Scheme 1). However, the resulting compounds 8a and 8b partially react with the starting material 7a (or 7b respectively) to form disilanes 9a and 9b. Under the selected conditions only the aryl-substituted disilane 9b was cleaved in a further reaction with lithium metal to give 8b. This is in good agreement with observations that only disilanes bearing at least one aryl group can be cleaved with lithium metal [ 2 ] . Therefore this access to the lithiated silanes 8a and 8b via the cleavage of 9-methylfluorenyl-substituted organosilanes 7a (or 7b respectively) is limited. The observed side reactions reduce the selectivity of this method and thus its usefulness in synthesis. After trapping reactions with chlorosilanes the products formed could be isolated in poor yields only (yield for trapping product of 8a with PhMezSiCl: 20 %; cleavage reaction for 9 h at -80 “C and 3 h at r.t.) (yield trapping product of 8b with MesSiCl: 13 %; cleavage reaction for 30 min at -50 “C and 45 min at r.t.).

152 J. Hornig, D. Auer, C. Strohmann

Me, /Li 7a Me, /SiMe3 2Li Me, /Li Si - Si + 2 Si

9a 8a 1- -LiFI Me’ ‘Me Me’ \Me

Me, /FI Si

Me/- 7a, 7b 1 2Li ~ Me, ,Li 7b Me, ,SiMepPh 2Li Me, , ~ i - 2 Si Si - Si Me’ \ph Me’ \Ph -LiFl Me/ xPh - LiFl

8b 9b 8b

Scheme 1. Si-C bond cleavage of 9-methylfluorenyl-substituted organosilanes.

Si-C Bond Cleavage of Diphenylmethyl-Substituted Organosilanes

Two other starting compounds investigated were the diphenylmethyl-substituted organosilanes 10a and lob, which were prepared from lithiated diphenylmethane and the corresponding chlorosilanes. By Si-C bond cleavage with lithium metal it is possible to generate silyllithium species of types lla and llb (Scheme 2). Trapping reactions with chlorotrimethylsilane gave products 13a, 13b (yield of 13a: 70 %, yield of 13b: 83 %) and 14.

In contrast to the route via the 9-methylfluorenyl-substituted silanes 7a and 7b, no side reactions with the starting material were observed and therefore the products were formed with higher selectivity and yields. However, it was not possible to use (diphenylmethy1)trimethylsilane (14) as a precursor for the synthesis of lithiotrimethylsilane @a), because the corresponding Si-C bond could not be cleaved under these conditions.

10a, 10b l l a , l l b 12 13a, 13b 14

S i x bond cleavage of diphenylmethyl-substituted organosilanes. Scheme 2.

Our approach for starting material suitable as a silyl dianion synthon (D) was via the bis(diphenylmethy1)-substituted silane 15. It was selectively cleaved to give the silyllithium compound 16, which resulted, after a trapping reaction with chlorotrimethylsilane, in disilane 17 (Scheme 3; yield of 17: 80 %). The isolated and purified disilane 17 then was cleaved at the Si-C bond with lithium metal, resulting in the lithiated silane 18. Another trapping reaction with chlorotrimethylsilane gave the trisilane 19 (yield of 19: 78 %).

Syntheses of Silyllithium Reagents Starting from Tetraorganosilanes 153

Ph, ,CHPh2 2Li Ph, ,Li 2Me3SiCl Ph, ,SiMe3

Ph’ ‘CHPh2 THF Ph’ ‘CHPh2 Si - TI+ Ph2CHLi -2Licl - Si,

15 16 12 17

Ph’ CHPh2 - PhZHCSiMea

2Me3SiCl Ph, ,SiMe3

Ph’ ‘SiMe3 Si

18 12 19 - PhzHCSiMe3

Scheme 3. Successive twofold Si-C bond cleavage of bis(diphenylmethy1)-substituted organosilanes.

This method for the synthesis of trisilane systems like 19 is very versatile, since the substituents are introduced sequentially and thus allow the construction of unsymmetrical trisilanes.

Furthermore functionalized and chiral diphenylmethyl-substituted compounds, like rac-20, are potential precursors for enantiomerically enriched silyllithium compounds [7, 81. They can be transformed selectively and in good yields to systems of type rac-21. A trapping reaction with chlorotrimethylsilane resulted in the products rac-22 (yield of rac-22: 89 %) and 14 (Scheme 4).

zMe,SiCI Ph, /SiMe3 Ph, ,SiMe3

tPh2CHLi Mejsi-N2; ph/‘‘H

rac-20 rac-21 12 rac-22 14

Scheme 4. S i x bond cleavage of chiral diphenylmethyl-substituted organosilanes.

Conclusion

With substituents like 9-methylfluorene and diphenylmethane, Si-C bonds can be activated for a cleavage under mild conditions. In contrast to the 9-methylfluorenyl-substituted silanes 7 a and 7b, diphenylmethyl-substituted tetraorganosilanes of types 10a, 10b and rac-20 have proven to be valuable precursors for the synthesis of silyllithium reagents like l l a , l l b and rac-21 (Eq. 5). Therefore they correspond well to the silyl anion synthons B. Furthermore the bis(diphenylmethy1)- substituted silane 15 allows a sequential synthesis of unsymmetrical trisilanes and thus is a valuable silyl dianion synthon D (Eq. 6) .

154 J. Homig, D. Auer, C. Strohmann

- 10a, lob, rac-20

Eq. 5.

15

Eq. 6.

Acknowledgment: We are grateful to the Deutsche Forschungsgemeinschaft (DFG), the Graduiertenkolleg 690, the Fonds der Chemischen Industrie (FCI) and the Institut fur Anorganische Chemie der Universitat Wurzburg for financial support. Furthermore we acknowledge Wacker- Chemie GmbH for providing us with special chemicals.

References [ l ] C. Strohmann, 0. Ulbrich, D. Auer, Eur. J. Znorg. Chem. 2001, 1013. [2] K. Tamao, A. Kawachi, Adv. Organomet. Chem. 1995,38, 1. [3] P. D. Lickiss, C. M. Smith, Coord. Chem. Rev. 1995,145,75. [4] A. Sekiguchi, V. Ya. Lee, M. Nanjo, Coord. Chem. Rev. 2000,210, 1 1 . [ 5 ] R. A. Benkeser, R. G. Severson, J. Am. Chem. Soc. 1951,73,1424. [6] M. Porchia, N. Brianese, U. Casellato, F. Ossola, G. Rossetto, P. Zanella, R. Graziani,

J. Chem. SOC., Dalton Trans. 1989,677. [7] C. Strohmann, J. Homig, D. Auer, Chem. Commun. 2002,766. [8] D. Auer, J. Hornig, C. Strohmann, Synthesis of a Highly Enantiomerically Enriched

Silyllithium Compound, in: Organosilicon Chemistry V: From Molecules to Materials (Eds. N. Auner, J. Weis), Wiley-VCH, Weinheim, 2003, p. 167

Selective Transformations Starting from a Diastereomerically Enriched Lithiated

Benzylsilane

Carsten Strohmann, * Daniel H. M. Buchold, Kerstin Wild, Daniel Schildbach

Institut fur Anorganische Chemie, Universitat Wiirzburg Am Hubland, D-97074 Wiirzburg, Germany

Tel.: +49 931 888 4613 - Fax: +49 931 888 4605 E-mail: c.strohmann@mail.uni-wuerzburg.de

Keywords: organosilanes, lithium, Si-C cleavage, quantum chemical calculations, stereochemistry

Summary: The solid-state structure of the lithiated (aminomethy1)benzylsilane Me&( [R]-[CHLiPh] }(CH$MP) [(R,S)-2] [CHzSMP = (S)-2-(methoxymethyl)pyrroli- dinomethyl] and the absolute configuration at the stereogenic metalated carbon center of this alkyllithium compound, which is highly diastereomerically enriched in solution, were determined by single-crystal X-ray diffraction methods. By computational methods, the mechanism of the deprotonation of the starting compound, (aminomethy1)benzylsilane 1, as well as the stability of configuration of lithiated (R,S)- 2, were examined. The stereochemical course of an integral sequence of further transformations, starting from 1, was experimentally clarified. For the selective reaction of (R,S)-2 with MeI, inversion of the configuration at the metalated carbon center was observed.

Introduction

Lithium alkyls are some of the most important building blocks in synthetic organic and inorganic chemistry. They are widely used as strong bases or nucleophilic reagents. Using a-metalated organosilanes instead, such as substituted (lithiomethyl)silanes, implies some important advantages: a) avoidance of P-H elimination (a major problem with polar metal alkyls); b) the availability of special methods of functionalization (e. g. Peterson olefination or Si-C bond cleavage); c) stabilization effects on a-negative charges by vicinal silicon centers; d) the possibility of influencing the reactivity and solubility of these compounds by judicious choice of substituents at the silicon center; and e) a great variety of possible synthetic routes towards carbosilane precursors. For the generation of diastereomerically enriched [ 13 a-metalated organosilanes, we have been using (aminomethyl)(lithiomethyl)silanes as starting materials. The merits of this class of

Ovganosilicon Chemisty V Edited by N. Auner and J. Weis

Copyright 0 2003 WILEY-VCH Verlag GmbH & Co. KgaA, Weinheim

156 C. Strohmann, D. H. M. Buchold, K. Wild, D. Schildbach

compounds are summarized in Fig. 1.

Si center prevents intramolecular coordination fl-elimination R' (stabilization) of reactive centers

Si center stabilizes chiral amine can introduce a-metalated C atom stereochemical information

Stabilization effects in (aminomethyl)(metallomethyl)silanes on their metalated centers. Fig. 1.

The synthesis of enantiomerically or diastereomerically enriched alkyllithium compounds, in which the metalated carbon center is the stereogenic center, has been studied intensively since the early 1980s [2]. Among these compounds are several enantiomerically enriched benzyllithium reagents [2-51. Stereogenic metalated carbon centers are usually generated by deprotonation using alkyllithium bases, like tert-butyllithium. The stereochemical information can be introduced prior to the deprotonation reaction by using enantiomerically enriched C-H acidic precursors bearing only one proton at the stereogenic carbon center. In contrast, methods for the introduction of the stereochemical information during the deprotonation reaction are either intermolecularly by chiral auxiliaries, such as (-)-sparteine, or intramolecularly by the side-arm donation of chiral substituents, such as the (S)-2-(methoxymethyl)pyrrolidinomethyl substituent (CHzSMP). This latter method is applied by our workgroup.

Two different reaction conditions for the synthesis of the diastereomerically enriched benzyllithium compounds have been used in the study: a kinetically controlled route, diastereotopos-differentiating deprotonation at low temperatures, mostly observed in nonpolar solvents (e.g. in toluene, n-pentane); or a thermodynamically controlled route, epimerization between two diastereomeric lithium alkyls at higher temperatures, mostly observed in polar solvents (e.g. in THF) (Scheme 1).

OR2 Ph Ph 4 YLi -0Me Me, Deprotonation 7 Me

Me /Si L.,& t (R,S)-S.ORz

Epimerization 2

OR2

Ph

Epimerization 1

Ph,, 4 Me;&-Y

* Me L N , ~

1

(S,S)-2.OR2

Scheme 1. Relevant deprotonation reactions and epimerization equilibria starting from 1.

Selective Transformations Starting from a Lithiated Benzylsilane 157

The kinetically controlled reactions Deprotonation 1 and Deprotonation 2 are discussed in the next section, and the thermodynamically controlled reactions Epimerization 1 and Epimerization 2 in the following section. The third section deals with selective reactions, starting with title compound. The focus will be particularly on this lithiated (aminomethy1)benzylsilane (R,S)-2, whose synthesis, stability of configuration and further transformations (stereochemical course) have been investigated thoroughly.

Synthesis of the Diastereomerically Enriched Lithiated Benzylsilane

The lithiation of 1 -[(benzyldimethylsilyl)methyl]-(S)-2-(methoxymethyl)py~olidine (1) with sec-butyllithium in THF and selective substitution reactions with alkyl halides, which showed very high diastereomeric ratios (d.r.), were first reported by Chan and co-workers [4a]. A significant solvent effect on the diastereomeric ratios of the transformations with iodomethane was observed when the reaction was carried out in THF (d.r. = 75 : 25) and diethyl ether (d.r. >_ 98:2). Due to the absence of knowledge of the solid-state structure for 1 -{ [(R)-(lithiomethylphenyl)dimethylsilyl]- methyl) -(S)-2-(methoxymethyl)pyrrolidine [(R,S)-2], the authors had to deduce the absolute configuration at the metalated stereogenic carbon center from the absolute configurations of the trapping product, which is formed “presumably with retention of stereochemistry” [4a], and by comparison of the product of the Si-C cleavage reaction, (S)-1-phenylethanol [(S)-4], with a purchased authentic sample [4c]. Therefore it was of great interest to determine the structure (and the absolute configuration) of lithiated organosilane (R,S)-2 in the crystal, especially since only two enantiomerically pure lithium alkyls have been investigated in the solid state until now [2a].

Scheme 2.

The lithiation of (aminomethy1)benzylsilane 1 was carried out with tert-butyllithium at -90 “C in tolueneln-pentane (Scheme 2). At -30 “C, yellow colored needles of the metalated product (R,S)-2 could be isolated as crystals in 80 % yield. The result of the single-crystal X-ray diffraction study is shown in Fig. 2.

The crystal structure reveals a coordination polymer in the solid state with (R) configuration at the metalated carbon center. The chains of this polymer are formed by n-interactions between a lithium center and the phenyl group of an adjoining silane molecule. The lithium center Li(1) of the metalated a-carbon center is coordinated by the two donor groups N(l) and 0(1) of the

158 C. Strohmann, D. H. M. Buchold, K. Wild, D. Schildbach

aminomethyl ligand, transferring the stereochemical information onto the metal fragment. The coordination sphere of Li(1) is completed by contacts to the three aromatic carbon centers C(6’), C(7’) and C(8’) of the adjoining silane molecule. The benzylic carbon atom C(3) is almost planar, but is nevertheless a stereogenic center due to the C(3)-Li(l) contact. The sum of the angles in the “carbanionic” moiety around C(3) is 360(3)”. In contrast to the expectations of Chan and Fraenkel [4a, 51, the phenyl substituent and the methoxymethyl substituent are pointing in the same direction, showing no steric interactions. In agreement with the well-known fact that silicon centers have a stabilizing effect on a-carbanionic centers, the C(3)-Si(l) bond of (R,S)-2 is shortened by this polarization effect, compared to the value in the unmetalated compound (S,S)-3.HI [7]: 1.797(7) A versus 1.9 14(9) A. In a related lithiated benzyl(piperidinomethy1)silane whose synthesis and crystal structure were reported recently [8], the corresponding bond length of Si(l)-C(3) (1.821(3) is of a comparable value, indicating an analogous stabilizing effect by the silicon center of this molecule.

C-

Fig. 2. Molecular structure and numbering scheme of(R,S)-2 in the crystal (Schakal plot) [6]. Selected bond

distances (A) and angles ( O ) : Si(l)-C(3) 1.797(7), Li(l)-C(3) 2.269(14), C(3)-C(4) 1.419(9), N(l)-Li(l)

2.183(12), C(3)-Si(l)-C(lO) 107.9(3).

An insight into the deprotonation reaction of 1 with tert-butyllithium can be gained by computational methods [9]. In the experiment, this reaction is effected at very low temperatures (-70 to -90 “C) under kinetically controlled conditions. Under these conditions, the reaction will proceed via the energetically more stable transition state. Two reaction paths are plausible, resulting in the formation of the two possible epimers (R,S)-2 and (S,S)-2. Each of these reaction paths will be considered separately. With the implication of the complex induced proximity effect (CZPE} [ 103, the reaction is believed to start with a pre-coordinated tert-butyllithium molecule, whose lithium

Selective Transfornations Starting from a Lithiated Benzylsilane 159

center is coordinated by the oxygen and nitrogen atoms of the SMP ligand. By the specific conformation of the Si-C bond in MIN-1, the pro-R hydrogen (HR) atom is being abstracted in the deprotonation reaction, proceding via the transition state TS-1, in which the carbon-lithium contact is formed simultaneously with the carbon-hydrogen bond cleavage. The reaction products (metalated silane and tert-butane) form the stationary point MIN-2 (Deprotonation 1; Scheme 1, Fig. 3, Table 1). The results of this calculation together with the analogous deprotonation reaction, where the abstraction of the pro-S hydrogen (Hs) atom gives the other epimer MIN-4 (Deprotonation 2; Scheme 1, Table I), are summarized in Table 1.

The energies of the two conformers MIN-1 and MIN-3 are equal within the margin of errors of the DFT method used [ 1 I]. On the reasonable assumption that there is an equilibrium between MIN-1 and MIN-3, the stereoselectivity of the reaction would be influenced by the difference between the two activation barriers of the potential energy surface of deprotonation reactions 1 and 2. This activation barrier for Deprotonation 1 is 6 kJ/mol smaller than that for the abstraction of the pro-S hydrogen atom. This is reflecting the trend of the kinetically controlled experiment, where the epimer with the (R)-configured metalated center is highly enriched [ 121. After the deprotonation reaction, two equilibria (Epimerizations 1 and 2 in Scheme 1) can influence the distribution of the stereoisomers. This is examined in the following section.

Fig. 3. Relative-energy profile of stationary points MIN-1, TS-1 and MIN-2 for the deprotonation reaction towards

the (R,S)-epimer, Deprotonation 1, B3LYP/6-31+G(d).

160 C. Strohmann, D. H. M. Buchold, K. Wild, D. Schildbach

Table 1. Calculated energies for the stationary points MIN-1-MIN-4, TS-1 and TS-2 of the deprotonation reactions

towards the (R,S)-/(S,S)-epimer, B3LYP/6-3 1+G(d) (ZPVE = zero-point vibrational energy).

Electronic energy relative to MIN-1

corrected by ZPVE uncorrected Deprotonation Reaction towards (R,S)-/(S,S)-Epimer

Deprotonation 1 ,CMe3 MIN-1 0 0

Ph fi Ee3 Ph H

yH iiJOMe )-Li+OMe TS-1 55 65 Me Me:Si L d , & --..--Me, Me /Si ~ ~ $ 6 t

-100 -102 MIN-2

MIN-2 MIN-1

Deprotonation 2 ,CMe3 MIN-3 0 0

Ph, H H Ph t e 3

FH LiJOMe TS-2 61 71

MIN-3 MIN-4 MIN-4 -85 -88

Me, P L i - O M e Me>Si Me Ld,h -Me 3 L,,,,G t

Stability of Configuration

In general, “carbanions” have a low barrier of inversion (since they are isoelectronical to tertiary amines). Therefore, unsubstituted systems of this type lose their stereochemical information even at low temperatures. For the racemization, the following process is assumed (Scheme 3), where the two enantiomeric lithium alkyls A and ent-A are interconverting by separatiodfixation of the lithium center (steps a and d) and the inversion of the configuration via the planar transition state C (steps b and c):

A B C enf-B ent-A

Scheme 3. Supposed racemization process of enantiomerically enriched lithium alkyls: a) separation of the Li

fragment; b)+c) inversion of the “carbanion” via planar transition state; d) fixation of the Li center.

The two equilibria (Epimerizations 1 and 2 in Scheme 1) can influence the distribution of the stereoisomers after the deprotonation reaction. The situations in nonpolar and polar solutions are of course different. While it is reasonable to imply the existence of a vacant coordination site at the

Selective Transformations Starting from a Lithiated Benzylsilane 161

lithium center for nonpolar solvents (Fig. 4, see also the crystal structure in Fig. 2) , a polar solvent molecule like ether will complete the coordination sphere of the metal center. Thus, the energies of the stationary points of the epimerization were calculated with a three-coordinate lithium center (Epimerization 1; Scheme 1, Fig. 4, Table 2 ) and a four-coordinate lithium center with an additional dimethyl ether molecule (Epimerization 2; Scheme 1, Table 2 ) to take this into account.

The two reaction products (R,S)-2 and (S,S)-2 (equivalent to MINd and MIN-6, in Epimerization 1; Scheme 1, Fig. 4, Table 2 ) might be involved in an equilibrium process of interconversion. This process, which is undesirable for the kinetically controlled synthesis, is required for the thermodynamically controlled route, where a polar solvent is typically added.

By preceding calculations and experimental NMR studies [ 5 ] , it could be shown that the energies for removing the intramolecularly coordinating ligands from the lithium center, and for removing the lithium center from the metalated carbon center, are much higher than the energies discussed in Tables 1 and 2. Therefore it is reasonable to exclude a racemization proceeding via dimeric structures or carbanions.

The energy differences of the stationary points MIN-5, TS-3 and MIN-6 (Fig. 4, Table 2 ) indicate that an epimerization process at room temperature is unlikely to occur in a nonpolar solvent. In this model system for (R,S)-2 in that type of solvent with a vacant coordination site at the lithium center, the activation barrier on the potential energy surface of Epimerization 1 is 93 kJ/mol.

r

reaction coordinate

Fig. 4. Relative-energy profile of stationary points MIN-5, TS-3 and MIN-6 for the epimerization reaction (vacant

coordination site) between the (R.s)-/(s,s)-epimers, Epimerization 1, B3LYP/6-3 l+G(d).

162 C. Strohmann, D. H. M. Buchold, K. Wild, D. Schildbach

In the model system for (R,S)-2 in a polar solvent with a coordinating dimethyl ether molecule at the lithium center (Epimerization 2 in Scheme I), the energy difference between MIN-7 and TS-4 is smaller by 25 kJ/mol. Furthermore, in contrast to the nonpolar case described, there is an energy difference of 9 kJ/mol between the two diastereomeric minimum structures MIN-7 and MIN-I. The conclusion from these studies is that from ether solution, a highly diastereomerically enriched (R,S)- 2 should be obtained by epimerization at room temperature, while a diastereotopos-differentiating deprotonation without noticeable epimerization should be possible in nonpolar solution (under the reaction conditions described).

Table 2. Calculated energies for the stationary points MINd-MIN-8, TS-3 and TS-4 of the epimerization reactions

between the (R,S)-/(S,S)-epimers, B3LYP/6-3 l+G(d) (ZPVE = zero-point vibrational energy).

Epimerization reaction in nonpolar/polar solvent Electronic energy relative to MIN-5MIN-7

corrected by ZPVE uncorrected

MIN-5 0 0

MIN-7 0 0

68 71 OMe2 OMe2 Epirnerization 2

Me, PLi-OMe TS-4 Ph I

YLi-OMe 3 t Me,

Me?-,!,,& MIN-7 --+ Me MIN-8 L~,& MIN-I 9 9

0 -<

0

71

9

All our calculations are in agreement with the results of dynamic 13C NMR studies by Fraenkel and co-workers. That group was able to show that the benzyllithium compound (R,S)-2 exists as a monomeric single diastereomer in THF solution between -93 and +27 "C [ 5 ] . This again fits with the results of the trapping reactions at variable temperatures performed by Chan and our research group which prove a stability of configuration on at least the time scale of the reaction.

Stereochemical Course of Further Transformations

The stereochemical course of further transformations with the lithiated silane (R,S)-2 is not affected by crystallization and isolation of the product; reactions starting with the product as prepared and maintained in solution give the same d.r. values. Beginning with a known absolute configuration at the metalated carbon center, we were able to examine and explain the stereochemical course of an integral sequence of transformations by experimental and computational methods, starting from the

Selective Transfoimations Starting from a Lithiated Benzylsilane 163

unmetalated silane 1 [7]. After the solid (R,S)-2 had been dissolved in toluene, the trapping reaction with iodomethane was carried out at -90 "C and the reaction mixture was subsequently warmed to room temperature. Methylated silane (S,S)-3/(R,S)-3 was formed in 98 % yield with a diastereomeric ratio of d.r. [(S,S)-3:(R,S)-3] = 96:4.

- Me1 Me %Me H HI

toluene Me 'p zE - Lil

- ;si -90 "C--r.t.

OMe OMe

( R S M (S09-3 80 %, only one 98 %

d.r. = 96:4 diastereomer detected [7]

1) KOHIHpO qMe EtpO, - KI

2) KFiHpOp H THFMeOH Ho

(S, 9-3.H I ( 9 - 4 94 Yo 98 Yo

d.r. t 98:2 e.r. 2 96:4

Scheme 4.

For the determination of the absolute configuration of the major diastereomer of silane (S,S)-3, the hydrogen iodide adduct (S,S)-3.HI was formed by reaction of the compound with anhydrous hydrogen iodide (solution in diethyl ether) at 0 "C. Colorless needles of (S,S)-3.HI were obtained as single crystals from CHzClZln-pentane.

The Si-C cleavage reaction with K F / H 2 0 2 was effected using the method of Chan [4c] and Itami [13] and co-workers in a mixture of THF and methanol at 50 "C to give benzyl alcohol (S)-4. The absolute configuration of this alcohol was determined by 13C NMR methods. The NMR shift reagent erbium tris[3-(trifluoromethylhydroxymethylene)-(+)-camphorate] [Er(tfc)s] was used to determine the selectivity {ex. [(S)-4:(R)-4] 196:4} and the absolute configuration of (S)-4 by comparison with an authentic sample.

In the literature, both the (R,S) and the (S,S) diastereomers of lithium alkyl2 have been proposed as the major diastereomer [4a, 5a]. Based on these predictions, both retention and inversion of the configuration at C(3) have been postulated for the reactions of (R,S)-2 with alkyl halides. Our X-ray structural analysis of (R,S)-2 shows that the reaction of the (2-3)-configured stereoisomer and inversion of configuration [for the reaction of (R,S)-2 with iodomethane in toluene] do in fact take place.

The literature reveals that both retention and inversion of the configuration have been observed for the reactions of lithiated benzylic and related systems with various electrophiles.[l4] In most cases no solid-state structure of the corresponding lithium alkyl could be determined to confirm the absolute configuration of the stereogenic metalated carbon center. The stereochemical course of the reaction of our lithium alkyl with the electrophile iodomethane in toluene can be understood on the basis of the solid-state structure of compound (R,S)-2, which was crystallized from the same solvent as was used in the substitution reaction with the alkyl iodide, in combination with computational studies.

164 C. Strohmann, D. H. M. Buchold, K. Wild, D. Schildbach

The modeling of monomeric (R,S)-2 [B3LYP/6-3 l+G(d)] indicates that the highest occupied molecular orbital (HOMO) is chiefly located at the metalated carbon center and the aromatic ring system (Fig. 5). It can be deduced from the calculated orbital coefficients that both inversion and retention of configuration are almost equally likely to result from electrophilic attack. Only the fact that the site opposite the lithium center is sterically accessible to attack by electrophiles (the coordination polymer of the solid-state structure should be broken up in solution) makes it possible for (R,S)-2 to react selectively with inversion of configuration at C(3) under kinetic control in nonpolar solvents.

Fig. 5. B3LYP/6-3 l+G(d)-optimized structure of monomeric (R,S)-2 and visualization of the highest occupied

molecular orbital (HOMO) (Molekel plot [15]; numbering scheme adopted from Fig. 2).

Conclusions and Outlook

The stereochemical pathway of an integral sequence of transformations, starting with the deprotonation of (aminomethy1)benzylsilane 1, has been clarified experimentally. This deprotonation reaction, leading to highly diastereomerically enriched (R,S)-2, and the stability of configuration of this compound were additionally examined by computational methods. In the first studies, high diastereoselectivities were observed for metathesis reactions of highly diastereomerically enriched (aminomethyl)(lithiomethyl)silane (R,S)-2 with mercury and palladium compounds [16]. It is now of interest to determine whether the results of our investigations may be of use in understanding the general stereochemical pathways of benzyllithium systems in reactions, or whether studies of systems like (R,S)-2 are too specialized to set up common rules for the stereochemistry of this class of compounds.

Currently, our work is concentrating mainly on the stereochemical pathways of aggregated

Selective Transformations Starting from a Lithiated Benzylsilane 165

lithium alkyls (e.g. dimeric molecular structures of highly diastereomerically enriched lithium alkyls). In a recent review article [2a], the following comment on the main challenges in this research field can be found: “We believe that the first of these challenges is the development of a systematic structure-function relationship for the stereochemical behavior of organolithium compounds. There have only been a few instances in which structural information about organolithium aggregates have been used as the starting point for the design of stereoselective lithiatiodsubstitution sequences.”

Acknowledgement: We are grateful to the Institut fur Anorganische Chemie der Universitat Wurzburg, the Deutsche Forschungsgemeinschaft (DFG), the Sonderforschungsbereich 347, the Graduiertenkolleg 690, and the Fonds der Chemischen Industrie (FCI) for financial support. D.S. and K.W. thank the FCI for the grant of two scholarships. Furthermore we acknowledge Wacker- Chemie GmbH for providing us with special chemicals.

References In general, we speak of enantiomerically enriched metal alkyls when we focus our interest on the stereogenic metalated carbon center. In the real case, these molecules are almost always diastereomerically enriched metal alkyls, due to the presence of stereogenic centers other than the metalated one alone. a) A. Basu, S . Thayumanavan, Angew. Chem. lnt. Ed. 2002, 41, 716, and literature cited therein; b) D. Hoppe, T. Hense, Angew. Chem. lnt. Ed. Engl. 1997, 36, 2282, and literature cited therein. For examples see: a) 0. Stratmann, B. Kaiser, R. Frohlich, 0. Meyer, D. Hoppe, Chem. Eur. J. 2001, 7 , 423; b) P. Beak, D. R. Anderson, M. D. Curtis, J. M. Laumer, D. J. Pippel, G. A. Weisenburger, Acc. Chem. Res. 2000, 33, 715; c) G. A. Weisenburger, N. C. Faibish, D. J. Pippel, P. Beak, J. Am. Chem. SOC. 1999, 121, 9522; d) D. J. Pippel, G. A. Weisenburger, S. R. Wilson, P. Beak, Angew. Chem. lnt. Ed. 1998, 37, 2522; e) D. Hoppe, B. Kaiser, 0. Stratmann, R. Froehlich, Angew. Chem. Znt. Ed. Engl. 1997, 36, 2784; f) H. Ahlbrecht, J. Harbach, R. W. Hoffmann, T. Ruhland, Liebigs Ann. 1995, 211; g) G. Boche, M. Marsch, J. Harbach, K. Harms, B. Ledig, F. Schubert, J. C. W. Lohrenz, H. Ahlbrecht, Chem. Ber. 1993, 126, 1887; h) M. Marsch, K. Harms, 0. Zschage, D. Hoppe, G. Boche, Angew. Chem. lnt. Ed. Engl. 1991,30,321. a) T. H. Chan, P. Pellon, J. Am. Chem. Soc. 1989, 111, 8737; b) T. H. Chan, S . Lamothe, Tetrahedron Lett. 1991,32, 1847; c) T. H. Chan, K. T. Nwe, J. Org. Chem. 1992,57,6107. a) G. Fraenkel, J. H. Duncan, K. Martin, J. Wang, J. Am. Chem. SOC. 1999,121, 10538; b) G. Fraenkel, K. Martin, J. Am. Chem. SOC. 1995,117, 10336. E. Keller, Schakal99, University of Freiburg (Germany): Freiburg 1999. C. Strohmann, K. Lehmen, K. Wild, D. Schildbach, Organometallics 2002,21, 3079. C. Strohmann, K. Lehmen, A. Ludwig, D. Schildbach, Organometallics 2001,20, 4138.

166 C. Strohmann, D. H. M. Buchold, K. Wild, D. Schildbach

[9] Gaussian 98, Revision A.9, Gaussian, Inc., Pittsburgh PA, 1998. [lo] F. Haeffner, P. Brandt, R. E. Gawley, Org. Lett. 2002,4, 2101. [ 111 All corrected and uncorrected electronic energies are given relative to MIN-1. [12] At this point, the influence of the indispensable correction of all optimized structures by the

zero-point vibrational energy (ZPVE) becomes obvious, since it decreases the absolute energy of the transition states more than that of the minimum structures. This, and the computational method itself, significantly affect the electronic part of the activation energy. Thus, conclusions derived from over-accurate comparisons of calculations with experimental data have to be drawn very carefully, but can be found in the literature time and time again.

[13] K. Itami, T. Kamei, K. Mitsudo, T. Nokami, J. Yoshida, J. Org. Chem. 2001,66, 3970. [14] For examples see: a) F. Marr, R. Frohlich, D. Hoppe, Org. Lett. 1999, I , 2081; b) see Ref.

[3a]; c) see Ref. [3e]; d) N. C. Faibish, Y. S. Park, S. Lee, P. Beak, J. Am. Chem. SOC. 1997, 119, 11561; e) M. D. Curtis, P. Beak, J. Org. Chem. 1999, 64, 2996; f) F. Hammerschmidt, A. Hanninger, B. C. Simov, H. Vollenkle, A. Werner, Eur. J. Org. Chem. 1999,351 1.

[ 151 S . Portmann, Molekel, Version 4.l.win-32, ETH Zurich (Switzerland), Zurich 2001. [16] C. Strohmann, B. C. Abele, K. Lehmen, F. Villafaiie, L. Sierra, S. Martin-Banios,

D. Schildbach, J. Organomet. Chem. 2002,661, 149.

Synthesis of a Highly Enantiomerically Enriched Silyllithium Compound

Dominik Auer, Jan Hornig, Carsten Strohmann * Institut fiir Anorganische Chemie, Universitat Wiirzburg

Am Hubland, D-97074 Wurzburg, Germany Tel.: +49 931 888 4613 -Fax: +49 931 888 4605

E-mail: c.strohmann@mail.uni-wuerzburg.de

Keywords: lithiosilanes, chirality, enantiomerically enriched, metathesis, ab-initio calculations

Summary: The highly enantiomerically enriched silyllithium compound lithiomethylphenyl(1-piperidinylmethy1)silane (2) reacts stereospecifically with chlorosilanes, but over a period of several hours slow racemization in solution at room temperature occurs, which can be supressed by a metathesis reaction with [Mg(thf)d]Brz. Quantum chemical calculations of solvated model systems allow an assessment of possible intermediates during the racemization process.

Introduction

Silyllithium compounds are useful reagents for the introduction of silyl groups in organic and organometallic systems [ l ] and for the synthesis of complex polysilanes [2]. The preparation of lithiosilanes starting from chlorosilanes or disilanes is a well established method [3]. Sommer and co-workers described the first synthesis of an optically active silyllithium compound by the cleavage of an enantiomerically enriched disilane with lithium metal [4, 51. Via a cobalt-lithium exchange reaction, Corriu and co-workers also succeeded in the preparation of an optically active silyllithium system [6]. The stability of configuration of the metalated silicon center was estimated by NMR experiments to be at least 100 kJ/mol [7]. On the basis of these studies the configuration was expected to be stable at room temperature and the mechanism of racemization has been described in review articles [3a, 8, 91 and textbooks [ 10, 111 as the inversion of a free silyl anion. A recent study by Kawakami and co-workers reports on the synthesis of an enantiomerically enriched silyllithium system starting from enantiomerically enriched stannosilanes [ 121. The authors report the first racemization of a silyllithium compound, but key intermediates and products have not been characterized sufficiently.

Ovganosilicon Chemisty V Edited by N. Auner and J. Weis

Copyright 0 2003 WILEY-VCH Verlag GmbH & Co. KgaA, Weinheim

168 D. Auer, J. Homig, C. Strohmann

time c [mol/l]

0 h 0.36

2.0 h 0.36

2.0 h 0.36

0 h 0.36

Results and Discussion

T[K] electrophile produd

- Me3SiCl 3

293 Me3SiCI 3

293 Me3SiCI 3

- Ph,MeSiCI (R)-1

The starting material for our studies was enantiomerically pure (R)- 1,2-dimethyl-l,2,2-Criphenyl-l- -( 1-piperidinylmethy1)disilane (R)-(l), which can be obtained by a three-step synthesis followed by a separation of enantiomers with (R)-mandelic acid [13]. All reported ee values of the disilane products (R)-1 and 3 were determined by 'H NMR spectroscopy by addition of 3 equiv. of (R)- mandelic acid resulting in the separation of the resonance signals of the methyl groups and the SiCHzN group (AB system). Determination of the absolute configuration at the silicon center of (R)-1 was performed by single-crystal structure analysis [ 131.

In the experiments described the enantiomerically pure disilane (R)-1 was cleaved with lithium metal in THF at -70 "C (Scheme 1). The completeness of the cleavage reaction was proven by GC-MS and 'H NMR spectroscopy after a trapping reaction with MesSiCl. The resulting disilane 3 could be isolated with ee > 98 %. In solution lithiomethylphenyl( 1-piperidinylmethy1)silane (2) racemizes within a few hours at temperatures between 0 and 20 "C, but a very slow decomposition of 2 was also observed. Due to this decomposition no exact determination of the reaction rate law was possible.

ee > 98

53

> 981a1

> 98 -

Metathesis reaction with [Mg(thfk]Br, at -70 "C.

+ Me3SiCl Ph, * .SiMe3

-80 "C Me'siLN2 - LiCl 3

SiMePhp + Ph,MeSiCl -80 "C M$e';si~N3 - LiCl

(R-1 . .

1 .) [Mg(thf)4]Br2[a1

2.) MqSiCl Ph,* ,SiMe3

-80 "C * M e / s i L N 3 - LiCl 3

Scheme 1. Synthesis and reactions of the enantiomerically enriched silyllithium compound 2.

Solutions of 2 left at room temperature for 2 h show reduced ee values of 53 % after trapping with Me3SiC1. In order to determine stabilizing effects on the configuration at the silicon center, we performed a metathesis reaction with [Mg(thf)d]Brz at -70 "C. After 2 h at room temperature no significant racemization of the resulting metalated silane could be observed (ee > 98 %).

This increase in stability caused by the change of the metal is in contrast to the proposed mechanism of racemization for metalated silanes. Since the rate-determining step of the racemization process is discussed in the literature [3a, 8-1 13 as the inversion of the free silyl anion, no drastic effect of transmetalation should be expected.

The results of cleavage of (R)-1 to give 2, followed by the trapping reaction with PhzMeSiCl to give (R)-1 again (ee > 98 %), show that overall the configuration of the stereogenic silicon center is

Synthesis of a Highly Enantiomerically Enriched Silyllithium Compound 169

retained (see Scheme l), although it is not clear whether this is due to retention of the configuration at each step or to a twofold inversion during the course of the reaction. Retention of configuration can also be observed if a metathesis step with [Mg(thf)4]Br2 is involved during the reaction.

Recent experiments revealed dependence between the rate of the racemization process and the concentration of the silyllithium compound 2 in solution. Due to decomposition the reaction rate cannot be determined exactly, but it is not simply first or second order. We believe that a solvated lithium cation plays an important role in the inversion process of 2. Thus, this process can be described by the interaction of the solvated lithium cation with compound 2 (model system 4).

Quantum chemical calculations allow an assessment of possible intermediates during the racemization process, since the results can be correlated with experimental observables. Starting from model system 4, it is possible to locate transition state TS-4 for the inversion at the silicon center (Fig. 1). The calculated barrier (159 kJ/mol for the inversion) is decreased drastically if the methyl groups at the silicon center are exchanged by phenyl groups, because these substituents can stabilize the transition state. These results prove once more the importance of the presence of solvated molecules in calculations in order to obtain the sufficient description of inversion processes and barriers, which can be compared with experimental results (inversion barrier for Me&-: 199 kJ/mol). Nevertheless, when calculations are considered in the present literature, free silyl anions and unsolvated silyllithium compounds are still discussed as appropriate model systems

~ 4 1 .

Fig. 1. Inversion of 4 [barrier calculated with B3LYP/6-31+G(d), all energies were corrected by zero point

vibrational energies].

170 D. Auer, J. Hornig, C. Strohmann

Conclusion

We were able to prove that it is possible to synthesize the highly enantiomerically enriched silyllithium compound 2 (ee > 98 %) in large amounts and to perform stereospecific reactions with chlorosilanes. Due to the observed racemization of 2 in solution, we believe that a thorough reassessment of previous studies concerning optically active silyllithium species is in order.

Acknowledgment: We gratefully acknowledge the Deutsche Forschungsgemeinschaft DFG, the Graduiertenkolleg 690, the Fonds der Chemischen Industrie (FCI) and the Institut fiir Anorganische Chemie der Universitat Wurzburg for financial support. Furthermore we acknowledge Wacker- Chemie GmbH for providing us with special chemicals.

References [ l ] a) I. Fleming, R. S. Roberts, S. C. Smith, J. Chem. Soc., Perkin Trans. 1 1998, 1215; b) U.

Schubert, A. Schenkel, Transition Met. Chem. 1985,210. [2] A. Sekiguchi, V. Ya. Lee, M. Nanjo, Coord. Chem. Rev. 2000,210, 11 . [3] a) K. Tamao, A. Kawachi, Adv. Organomet. Chem. 1995,38, 1; b) P. D. Lickiss, C. M. Smith,

Coord. Chem. Rev. 1995,145,75. [4] L. H. Sommer, J. E. Lyons, H. Fujimoto, J. Am. Chem. SOC. 1969,91,7051. [5] L. H. Sommer, R. Mason, J. Am. Chem. Soc. 1965,87,1619. [6] a) E. Colomer, R. J. P. Corriu, J. Chem. Soc., Chem. Commun. 1976, 176; b) E. Colomer, R.

J. P. Corriu, J. Organomet. Chem. 1977,133, 159. [7] J. Lambert, M. Urdaneta-Pkrez, J. Am. Chem. SOC. 1978,100, 157. [8] R. J. P. Corriu, C. Guerin, J. J. E. Moreau in The Chemistry of Organic Silicon Compounds,

Part 1 (Eds. S . Patai, Z. Rappoport), Wiley, Chichester, 1989, p. 305. [9] J. B. Lambert, W. J. Schulz, in The Chemistry of Organic Silicon Compounds, Part II (Eds. S .

Patai, Z. Rappoport), Wiley, Chichester, 1989, p. 1007. [lo] A. F. Holleman, E. Wiberg, Lehrbuch der Anorganischen Chemie, lOlst edn., Walter de

Gruyter, Berlin, 1995, p. 899. [ 111 Ch. Elschenbroich, A. Salzer, Organometallics, 3rd edn., VCH, Weinheim, 1992, p. 112. [12] M. Omote, T. Tokita, Y. Shimizu, I. Imae, E. Shirakawa, Y. Kawakami, J. Organomet. Chem.

2000,611,20. [ 131 C. Strohmann, J. Homig, D. Auer, Chem. Commun. 2002,766. [14] M. Flock, C. Marschner, Chem. Eur. J . 2002,8,1024.

Alkynylsilyl Anions - Versatile Building Blocks for Silicon-Containing Polymers

Christian Mechtler, Judith Baumgartner, Christoph Marschner

Institut fur Anorganische Chemie, Technische Universitat Graz Stremayrgasse 16, A-8010 Graz, Austria

Tel.: +43 316 873 8216 -Fax: +43 316 873 8701 E-mail: marschner@ anorg.tu-graz.ac.at

Keywords: silyl anions, oligosilanes, alkynes

Summary: Alkynyl-substituted oligosilanes can be easily converted to potassium silanides. The reaction proceeds very fast, exceeded only by that of the phenylethynyl -substituted silanes. Linking two or three hypersilyl moieties via an ethynylene or ethynylphenylene bridge leads to multifunctional silyl anions.

Oligosilyl anions can easily be prepared by cleavage of a Si-SiMe3 bond with tBuOK in THF, if the central silicon is stabilized by at least two -SiR3 or phenyl substituents [l]. Conversion of the alkynyl oligosilanes to the corresponding potassium silanides at room temperature (Scheme 1) proceeds up to 50 times faster than that of alkylated or silylated hypersilyl silanes [2], because the alkynyl substituent seems to stabilize the negative charge at the silicon atom extremely well. We varied the substitution pattern of the alkyne, using -H, methyl, decyl and phenyl groups, and found that the reaction times (seconds to few minutes) as well as the 29Si NMR shifts of the potassium silyl(-155M ppm) were in quite a small range.

SiMe3 SiMe3

BuOK, THF I I

I R = Si-SiMe3 -+ R = Si-K

I I SiMe3

-BuOSiMe3

Scheme 1. Cleavage of the Si-SiMe3bond with tBuOK; R = H (l), Me (2), CloHzl (3), Ph (4).

I SiMe3

The fact that different substitutions at the ethyne did not affect cleavage of the Si-Si bond, at least not significantly, made us feel quite positive about obtaining di- or oligoanions in the same easy way.

The branched oligosilanes 5 to 7 were synthesized by coupling of hypersilyl ethyne 1 with the respective iodobenzenes using a modified Sonogashira coupling reaction [3]. 1,3,5Triiodobenzene

Ovganosilicon Chemisty V Edited by N. Auner and J. Weis

Copyright 0 2003 WILEY-VCH Verlag GmbH & Co. KgaA, Weinheim

172 C. Mechtler, .I. Baumgartner, C. Marschner

is no longer commercially available and had to be prepared from tribromobenzene (Scheme 2 ) [4]. Compound 8 was prepared from lithiated 1 and hypersilyl chloride [5 ] . The structures of 4,5, 6 and 8 are shown in Fig. 1.

HYP-O-Hyp Hyp-Hyp

HYP 6 8

HYP A 5

Hyp = Si(SiMe3)3

6

C l l l l

4

5 8

Fig. 1. Structures of compounds 4, 5, 6 and 8 determined by single-crystal X-ray diffraction. The drawings are

thermal ellipsoids with 30 % probability. Hydrogen atoms are omitted for clarity.

Reacting 2 to 5 with one equivalent tBuOK did not cleanly yield the monoanion, but stoichiometric (2 or 3 eq., respectively) amounts formed the di- or trianion quantitatively within seconds. This is rather astonishing, considering that in all other attempts to form dianions from “linked hypersilyls” the second step evidently takes much longer or needs harsher reaction conditions [6]. Furthermore the tripotassium derivative of 7 is the first observed silicon trianion.

Further derivatization leads to the magnesium [7] and zirconium [8] compounds (Scheme 3). It has to be noted that addition of MgBr2 to the oligo-potassium compounds of 5, 6 and 7 first produced a gel-like suspension which turned to a solution again on subsequent addition of excess

Alkynylsilyl Anions - Versatile Building Blocksfor Silicon-Containing Polymers 173

MgBr2. This indicates a crosslinking via Si-Mg-Si bonds. Further reaction with Cp2ZrC12 yielded, besides the expected silyl zirconium compounds, also considerable amounts of insoluble material which is likely to be polymeric.

Br ,TMS /'

'I \ TMS

\ Br

?Me3 Me3Si-Si- ( ~ 1 &Me3 1

Synthesis of 1,3,5-trishypersilylethynylbenzene (7). (i) TMSCI, nBuLi, THF/pentane, -78 "C; (ii) ICI,

CC14, 0 "C; (iii) 5 mol % Pd(PPh3k, CuI, EQN, THF, 60 "C.

(iii) HYP =

YP 7

Scheme 2.

SiMe, SiMe3 I

-KBr I -MgBrCI I

SiMe3 I MgBr2.Etp0 I CppZrClp

SiMe3

- R Si-K - R Si-MgBr- R - Si-ZrCp2CI

SiMe3 SiMea I

Scheme 3. Derivatization as Mg and Zr compounds.

Derivatization of compound 8 following Scheme 3, however, proceeded without any troubles. The structure of its dizirconium derivative is shown in Fig. 2.

Fig. 2. Structures of the di-zirconium derivative of 8 determined by single-crystal X-ray diffraction. The drawings

are thermal ellipsoids with 30 % probability. Hydrogen atoms are omitted for clarity.

174 C. Mechtler, J. Baumgartner, C. Marschner

The easy accessibility of mono-, di- and trifunctional monomers should make them excellent precursors for tailor-made, nanostructured compounds or polymers. The reactivity at the central silicon can be tuned by metal-metal exchange reactions as well.

Acknowledgments: Financial support by the Fonds zur Forderung der wissenschaftlichen Forschung in Osterreich (FSP Siliciumchemie, S7902) and the Austrian Ministry for Education, Science and Culture (START 120-Y) is gratefully acknowledged. Wacker Chemie GmbH, Burghausen, kindly provided various organosilanes as starting materials.

References a) Ch. Marschner, Eur. J. Inorg. Chem. 1998,221. b) Ch. Kayser, R. Fischer, J. Baumgartner, Ch. Marschner, Organometallics 2002, 21, 1023. The term “hypersilyl” represents the tris(trimethylsily1)silyl group as proposed by N. Wiberg at the Xth International Symposium on Organosilicon Chemistry, Posnan, Poland, 1993. Y. Yao, J. M. Tour, J. Org. Chem. 1999,64,1968. P.-J. Prest, Ph.D. Thesis, Univ. Illinois, Urbana-Champaign, 1999. H. Bock, J. Meuret, H. Schoedel, Chem. Ber. 1993,126,2227. Ch. Kayser, G. Kickelbick, Ch Marschner, Angew. Chem. 2002,114, 1031. J. D. Farwell, M. F. Lappert, Ch. Marschner, Ch. Strissel, T. D. Tilley, J. Organomet. Chem. 2000,603,185 a) B. K. Campion, J. Falk, T. D. Tilley, J. Am. Chem. Soc. 1987, 109, 2049. b) Ch. Kayser, Ch. Marschner, Monatsh. Chem. 1999,130,203.

Reactions of Trimethylsiloxychlorosilanes (Me3SiO)RPhSiCl (R = H, Me, Et, iPr, 'Bu, Ph,

Mes) with Lithium - Formation and Reactivity of Trimethylsiloxysilyllithiums

Jorg Harlofi, Eckhard Popowski*

Fachbereich Chemie, Universitat Rostock Albert-Einstein-StraBe 3a, D- 18059 Rostock, Germany

Keywords: trimethylsiloxysilyllithiums, trimethylsiloxydisilanes, trimethylsiloxy- trisilanes, self-condensation, silylenoids

Summary: Reaction of chlorosilanes (Me3SiO)RPhSiCl (la: R = H, lb: R = Me, lc: R = Et, Id: R = 'Pr, le: R = 'Bu, I f R = Ph, lg: R = Mes) with lithium metal in THF at -78 "C and in a mixture of THF/diethyl etherln-pentane in volume ratio 4:l:l (Trapp mixture) at -1 10 "C gives the silyllithium derivatives (Me3SiO)RPhSiLi (2b-g), (Me3SiO)RPhSiRPhSiLi (3a-g), Me3SiRPhSiLi (4a-g) and the trisiloxanes (Me3SiO)zSiRPh (5a-g). The siloxydisilanyllithiums 3a-g are formed by self- condensation of the corresponding siloxysilyllithiums 2a-g. Reaction of (Me3SiO)RPhSiLi (2f R = Ph, 2g: R = Mes) with n-butyllithium leads to the silyllithiums "BuRPhSiLi (14f, 14g). Silacyclopentene 16 is obtained in the reaction of 2g with 2,3-dimethylbutadiene.

Introduction

Organolithium compounds which bear a lithium atom as well as a leaving group such as a halogen atom or an alkoxy group on the same carbon atom - lithium carbenoids - are a well-characterized class of compounds [l-51. They react as nucleophiles or electrophiles depending on the chosen conditions; the electrophilic reactivity is typical of carbenoids.

For analogous silicon compounds, reports of only a few investigations are available [6, 71. Known stable silicon-functionalized silyllithiums with an a-heteroatom are (EtzN),Ph3-,SiLi (n = 1, 2) [7], (Et2N)MePhSiLi [7] , [(Me3Si)zN]Mez-,,PhflSiLi (n = 1, 2) [8], (RO)Ph2SiLi (R = Me&, MeZCH, Me) [7] and (Me3Si0)Me~_~Ph,SiLi (n = 1, 2) [9]. Electrophilicity and nucleophilicity were only proven for the compounds (RO)PhzSiLi, [(Me&)zN]MePhSiLi and (Me3SiO)Me2-flPh,SiLi (n = 1,2).

Herein we report on the results of the preparation of substituted siloxysilyllithiums

Ovganosilicon Chemisty V Edited by N. Auner and J. Weis

Copyright 0 2003 WILEY-VCH Verlag GmbH & Co. KgaA, Weinheim

176 J. Harlofi E. Popowski

J. R I

Me~SiO-Si-SiMe2R I

Ph 6b-g: R'= Me 7b-g: R'= H

(Me3SiO)RPhSiLi by reaction of siloxychlorosilanes (Me3SiO)RPhSiCl with lithium metal, and on their reactivity.

Results and Discussion

R I I

Ph

Me3SiO-Si-Cl la: R= H, lb: R= Me, lc: R= Et, Id R= iPr, le: R= 'Bu, If: R= Ph, Ig: R= Mes

+Li (4 eq)

or B: THFIEt201 n-pentane

A: THF, -78 "C, 0.5-3 h

(4:1:1), -1 10°C, l-6 h

J 2b-g

R R I. I I 1

Ph Ph 3a-g

Me3SiO-Si-Si--Li

-LiC 1 -LiOSMe3

1 + LiOSMe3

R I

I Ph 4a-g

Me3Si-Si-Li

5a-g (1-18 %)

Scheme 1.

Reactions of Trimethylsiloxychlorosilanes with Lithium 177

The reactions of chlorosilanes (Me3SiO)RPhSiCl (la: R = H, lb: R = Me, lc: R = Et, Id: R = 'Pr, le: R = 'Bu, If R = Ph, lg: R = Mes) with lithium chips in THF at -78 "C (A) and in a Trapp mixture [ 11 at -1 10 "C (B) produced the trisiloxanes (Me3SiO)zSiRPh (5a-g) and the silyllithium derivatives (Me3SiO)RPhSiLi (2b-g), Me$%O(RPhSi)zLi (3a-g) and Me3SiRPhSiLi (4a-g), which were trapped with Me3SiC1 and HMeZSiC1 (Scheme 1).

In addition to 2 b 5 b a small amounts Me3SiO(MePhSi)3Li was formed in the reactions of lb with lithium [9]. The siloxysilyllithium 2e partially reacted with lithium at -78°C as well as at -1 10 "C with Si-0 bond splitting to give the silanolate (Li0)'BuPhSiLi (12), which was trapped as the siloxydisilane (13) (Scheme 2).

'BU 'BU 'Bu I

HMe2SiO-Si-SiMezH Me3SiO-Si-Li -IiSiMe; Liasli-Li -21icI w I

Ph

I +2 Ii +2 HMe,SiCI

I Ph

I Ph

2e 12 13 ( A 34 %; B: 17 %)

Scheme 2.

The Si-0 bond splitting was the cause of the unexpectedly small yield of 2e in the reaction of le with lithium at -78 "C (Table 1). The data of Table 1 show that the stability of the siloxysilyllithiums 2a-g depends on the substituents R and on the temperature. The ranking of stability is R = Mes > Ph > 'Bu > 'Pr > Et - Me > H and stability is greater at 6 = -1 10 "C than at -78 "C.

Table 1. Proportions of the siloxysilyllithiums (Me3SiO)RF'hSiLi (2a-g) [%I in the mixture of

silanides determined by trapping products (Me&O)RPhSiSiMezR' (6b-g, 7b-g).

Temperature (Me$iO)RF'hSiLi (2a-g) [%]

r o c 1 R = H Me Et 'Pr 'Bu Ph Mes

-1 7 9 4 19 51

17 16 32 38 41 66

- la1

- [a1

-7 8

-1 10

[a] No trapping products (Me3SiO)HPhSiSiMe2R' for (Me3SiO)HPhSiLi (2a).

The siloxysilyllithiums 2b-g partially underwent self-condensation at both temperatures, and the corresponding siloxydisilanyllithiums Me$iO(RPhSi)zLi (3b-g) were formed (Scheme 3), which were trapped as the siloxytrisilanes Me3SiO(RPhSi)2SiMe2R (8b-g, 9b-g) (Scheme 1 ; yields, Table

2). Disilanes 6a and 7a, the trapping products of (Me3SiO)HPhSiLi (2a) were not obtained.

Compound 2a is most likely to be generated in the first step and immediately to undergo

178 J. Harlofi E. Popowski

self-condensation. In the self-condensation one molecule of 2a-g behaves as a nucleophile and the other one as an electrophile. This behavior points to the ambiphilic reactivity (silylenoid character) of the siloxysilyllithiums (Me3SiO)RPhSiLi (2a-g). The electrophilicity of the Si(Li) atom in these compounds is also demonstrated by the nucleophilic alkylation of 2f and 2g. Both compounds reacted with n-butyllithium to give the n-butyl-substituted silyllithiums "BuRPhSiLi (14f, 14g), which were trapped with Me3SiC1 as the corresponding disilanes "BuRPhSi-SiMe3 (15f, 15g) (Scheme 4).

R R R R I. I I I

Ph Ph I

Ph

2b-g 2b-g 3b-g

Me3SiO-Si-Si-Li MgSiO-di-Li + Me3SiO-Si-Li -LiOSiMe3* I I

Ph

Scheme 3.

Table 2. Yields of the trapping products Me3SiO(RPhSi)2SiMe2R' (8a-g: R' = Me; 9a-g: R' = H)

Temperature Me3SiO(RPhSi)2SiMe2 R' 8a-g [%]/9a-g [%I ["CI R=H Me Et 'Pr 'Bu Ph MI3

-1 8 1017 71/78 67/75 56/63 514 27/29 24/28

-1 10 47/45 751- 70168 36/46 411 281- 20118

R I I

Ph

R I I

Ph

THFIEt~OIn-pentane, [. 1 ] +Me3SiC1 ~ "Bu-Si-SiMe3 Bu-Si-Li -Licl Me3SiO-Si-Li + "BuLi -LiOSMe3

2f: R = P h 2g: R = Mes

14f R = Ph 14g: R = Mes

15f R=Ph(16%) 15g: R = Mes (26 %)

Scheme 4.

A trapping reaction of (Me3SiO)MesPhSiLi (2g) with 2,3-dimethylbutadiene affords the silacyclopentene (16) (Scheme 5).

1 h,-11OoC+-78"C) I THF/Et;?Oln-pentane Me3SiO-Si-Li +

3 h, -78 "C Ph I

Ph -LOSMe3

2g 16 (16 %)

Scheme 5.

Reactions of Trimethylsiloxychlorosilanes with Lithium 179

Acknowledgment: We thank the Fonds der Chemischen Industrie for financial support

References [I] [2] [3] [4] [5] [6] [7] [8] [9]

G. Kobrich, Angew. Chem. 1972,84,557; Angew. Chem. Int. Ed. 1972,II, 473. H. Siegel, Top. Curr. Chem. 1982,106, 55 . A. Maercker, Angew. Chem. 1993,105, 1072; Angew. Chem. Int. Ed. 1993,32, 1023. G. Boche, F. Bosold, J. C. W. Lohrenz, A. Opel, R. Zulauf, Chem. Ber. 1993,126, 1873. M. Braun, Angew. Chem. 1998,110,444; Angew. Chem. Int. Ed. 1998,37,430. K. Tamao, A. Kawachi, Adv. Organomet. Chem. 1995,38, 1. A. Kawachi, K. Tamao, Bull. Chem. SOC. Jpn. 1997, 70, 945. I. Rietz, E. Popowski, H. Reinke, M. Michalik, J. Organomet. Chem. 1998,556,67. J. Harloff, E. Popowski, H. Fuhrmann, J. Organomet. Chem. 1999,592, 136.

Silacyclobutanes: Head-to-Head Dimerization Versus Anionic

Polymerization - a-Silyl Substituted Carbanions as Reactive Intermediates

Hans-Uwe Steinberger

Polymerics GmbH, Landsberger Allee 378, D-12681 Berlin, Germany

Duanchao Yan, Norbert Auner"h

Johann-Wolfgang-Goethe Universitat Marie Curie-Str. 11, D-60439 Frankfurt am Main, Germany

Tel.: +49 69 79829591 -Fax: +49 69 79829188 E-mail: Auner@chemie.uni-frankfurt.de

Keywords: carbanions, sil yl-stabilization, hydrogen migration, silacyclobutanes

Summary: Reacting 2-neopentyl substituted silacyclobutanes la,b with MeLi/HMPA (hexamethylphosphoric triamide) anionic polymerization to give polymers 3a,b plays only a minor role for product formation. Instead, the head-to-head dimers 2a,b are isolated as main products. Their formation is explained by a complex reaction mechanism, in which various carbanionic, highly reactive intermediates are discussed. Obviously, the bis-a-silyl substituted carbanions 10a,b are remarkably stable, as can be concluded from 29Si NMR spectroscopic investigations at low temperature and from the products formed by trapping reactions with alcohols.

Reaction of Silacyclobutanes with MeLi/HMPA

Dichlorosubstituted silacyclobutanes can easily be synthesized from the reaction of trichlorovinylsilane and tert-butyllithium using the corresponding conjugated dienes as trapping agents [l]. After methylation with methyl Grignard reagent the starting compounds la,b are isolated in high yield.

During the reaction of the silicon dimethylsubstituted silacyclobutanes la,b with MeLi/HMPA and subsequent methanolysis (Scheme l), the head-to-head dimers 2a,b, which are separated from simultaneously formed oligomers 3a by distillation, are isolated as main products. The linear dimers 2a,b are fully characterized by the usual analytical methods ('H, 13C, 29Si NMR, GPC, and MS). Obviously, the formation of oligomers 3a results from anionic ring opening polymerization of la,

Ovganosilicon Chemisty V Edited by N. Auner and J. Weis

Copyright 0 2003 WILEY-VCH Verlag GmbH & Co. KgaA, Weinheim

Silacyclobutanes: Head-to-Head Dimerization Versus Anionic Polymerization 181

while silacyclobutane lb does not yield any oligomeric material 3b at all. Performing the reaction of l a without HMPA under comparable conditions no products are formed and the educt l a is recovered quantitatively. While the purity of dimer 2a is checked by GPC investigations, the oligomer 3a is characterizied to be a mixture of compounds ranging from dimers to hexamers (Figs. 1 and 2 ) by the same method.

2a,b a: R=H 1 .MeLi/HMPA b: R=Me

‘ d la,b a: R=H b: R=Me

Scheme 1.

A Plausible Reaction Mechanism

Reacting la,b with MeLi in HMPA as active solvent and in the presence of MeOH as trapping agent, the attack of a methyl anion at the silicon atom of the silacyclobutane is the fiist reaction step and gives a pentacoordinated silicon anion (Scheme 2). Such five-coordinated species are discussed as intermediates during the ring opening polymerization of silacyclobutenes, -butanes, and -pentenes [ 2 ] . Furthermore, five-coordinated silicon species are well described to be stable compounds [3].

Me& I MeLiIHMPA,

la,b a: R=H b: R=Me

Scheme 2.

A 4a,b

- 4 .

6a,b

For five-coordinated anionic intermediates 4a,b two options for ring opening reactions are possible, subsequently forming the carbanions 5a,b and 6a,b (Scheme 2). Due to their sterically distinctive features the primary anions 5a,b are preformed to attack another silacyclobutane moiety, while the secondary anions 6a,b are sterically hindered by their large neighbouring groups.

182 H . 4 . Steinberger, D . Yan, N . Auner

However, no ring opening products resulting from anions 5a,b are detected. Based on the higher basicity of the secondary anions obviously, a 1,5-H-shift giving the more stable a-silyl-substituted carbanions 7a,b (a-effect) takes place (Scheme 3).

6a,b a: R=H b: R=Me

Scheme 3.

The less hindered “needle-like’’ anions 7a,b again are allowed to attack silacyclobutane la,b. In accordance to the following reaction scheme the formation of two different anions, the primary anions 8a,b and the secondary anions 9a,b is plausible (Scheme 4). As discussed for the formation of 7a,b, the secondary anions 9a,b rearrange to the doubly a-silyl-stabilized anions 10a,b by subsequent 1,5-H-shift (Scheme 5). The carbanions 10a,b are stable at -78 OC and, as an example, 10b is detected by NMR spectroscopic methods. Due to steric reasons, no indication of a further attack of the anions 10a,b to silacyclobutane la,b is observed.

Scheme 4.

Finally, the anionic attack at the silacyclobutane la,b is finished by a methanol addition to carbanion 10a,b, and after purification the products 2a,b are isolated as main products (Scheme 5).

Silacyclobutanes: Head-to-Head Dimerization Versus Anionic Polymerization 183

13'- Me !h$-~i-~&:,+ y" Me - I

Me Me R

lOa,b a: R=H b: R=Me

Scheme 5.

M% 2a,b a: R=H b: R=Me

Stability of the Bis-a-silyl-substituted Carbanions 10a,b

Generally, in organic chemistry the stabilizing effects of silyl- and phosphino-substituents on carbanions are well-known. From electron affinity and gas-phase measurements it is concluded that a-silyl-substituents stabilize carbanionic species by about 20 kcal/mol [4, 51 compared to simple carbanions. Furthermore, the electron affinity of doubly a-silyl-substituted carbanions is additionally increased by about 16 kcal/mol relative to the singly substituted compound [6]. In conclusion, the stabilizing effect of two a-silyl substituents to a carbanionic center is about 36 kcal/mol as compared with simple carbanions.

Si NMR Investigations 29

The NMR spectroscopic investigations were examplified studying the reaction of l b with MeLi/HMPA at -78 "C giving an orange reaction solution - that is a typical colour for lithiated compounds. The 29Si NMR spectrum at -78 "C in a solvent mixture of THF/ds-toluene = 75:25 shows a 6 29Si NMR shift at -3.25 ppm (referenced to external TMS). This is in good agreement with chemical shifts detected for tris-silyl- (4.2-10.6 ppm) and bis-silyl-substituted carbanions (0,O ppm: MM'(OS02R)[CH(SiMe3)2]; M = Li, K; M' = Ca, Sr, Ba [ S ] ) . At -20 "C the signal of 10b slowly changes, accompanied by decolouring of the solution. At 0 "C the signal collapses, and the solution is now completely decoloured. In accordance to trapping experiments of carbanion lob, the 29Si NMR data give great evidence for the formation and the remarkabe stability of the bis-a-silyl- stabilized carbanion 10b [7].

Experiments to Trap the Carbanion 10b with MeOH and MeOD

Performing the reactions of silacyclobutane la,b with MeLi/HMPA in the presence of MeOH, the doubly a-silyl-substituted carbanions 10a,b are protonated and transformed into the corresponding

184 H . 4 . Steinberger, D . Yan, N . Auner

compounds 2a,b, which are completely characterized. The proposed reaction mechanism resulting in the formation of carbanions lOa,b is additionally

confirmed by an isotope marking experiment starting from silacyclobutane lb. Using MeOD as “quenching reagent” the deuterium isotope is unequivocally localized between the two silicon atoms of the bridging CHD-group of product 11. In the I3C NMR spectrum two 1:l:l triplets (6 (I3C) = 5.15 and 4.61 ppm) are detected for the two diastereomers. The deuterium marking is furthermore impressively demonstrated by mass spectroscopy. Neither in compound 2a, nor in 11, the molecule ion M+ can be registered. Under EI (electron impact) conditions fragmentation is strongly favored. However, compared to compound 2a the fragments 422 (2a: 421), 284 (2a: 283), and 226 (2a: 225) clearly verify the deuterated species 11.

Table 1. Summarizes the chemical shifts 6 29Si of compounds 1,2, lob, and 11.

No. 6 (29Si) [ppm] No. 6 (29si) [ P P ~ I

l a 11.63 2a 4 . 2 5

9.76 2b -0.62

lb 10.17 10b -3.25 [bl

10.70 11 4 . 2 9

[a] CDC13 as solvent, 293 K, referenced to external TMS. [b] NMR signal detected at -78 “C in THF/d8-toluene = 75/25 solution.

Formation and Characterization of Oligomer 3a

While from the reaction of l a with MeLi/HMPA only a small amount of oligomer 3a can be isolated, from lb no oligomeric material (3b) is obtained. The products 2a and 3a are analyzed by GPC analysis. From Fig. 1 it can be concluded that 2a is only contaminated by small amounts of trimeric and tetrameric products. The GPC of the distillation residue (Fig. 2) proofs the existence of an oligomeric product mixture 3a (dimers to hexamers).

1.80% 100% 0,59% 2 100%

0% , , , , , , , 0% 3.3 3.2 3.1 3.0 2,s 2,8 2 ,7LogMW 3,8 3,6 3.4 3.2 3.0 2,s Log MW

Fig. 1. GPC of dimer 2a. Fig. 2. GPC of oligomer 3a.

Silacyclobutanes: Head-to-Head Dimerization Versus Anionic Polymerization 185

It is known from literature that the anionic polymerization of silaheterocycles [2] usually occurs very fast. From that it can be concluded that the formation of any primary carbanions, such as 5a,b and 8a,b, and its subsequent attack at a silacyclobutane should lead to chain propagation. In contrast, the formation of a secondary carbanion stops polymerization. From the poor yield of oligomers 3a we suppose that the formation of primary carbanions is disfavoured. The reason for that might result from the bulky neopentyl-substituent, which shields the silicon from the attack giving primary carbanions. In the case of educt l b the steric overcrowding at the silicon center is even more enlarged by the 1 -methylvinyl-substitent. This obviously completely suppresses the formation of the primary carbanion 5b and consequently, no oligomeric and polymeric materials are formed.

Acknowledgement: We gratefully thank Wacker-Chemie GmbH for gifts of chlorosilanes and Dow Coming Corporation for financial support.

References N. Auner “Neopentylsilenes: Laboratory Curiosities or Useful Building Blocks for Synthesis of Silaheterocycles?” in Organosilicon Chemistry - From Molecules to Materials, N. Auner, J. Weis (eds.), VCH, Weinheim, 1993, 103. X. Zhang, Q. S. Zhou, W. P. Weber, R. F. Horvath, T. H. Chan, G. Manuel, Macromolecules 1988, 21 1563; Y. T. Park, W. P. Weber, Polymer Bulletin 1989, 22, 349; Y. T. Park, G. Manuel, W. P. Weber Macromolecules 1990, 23, 349; Q. S. Zhou, W. P. Weber Macromolecules 1990,23, 1915. C. Chiut, R. J. P. Comu, C. Reye, J. C . Young Chem. Rev. 1993,93, 1371; M. A. Barrow, E. A. V. Ebsworth, M. M. Harding J . Chem. SOC., Dalton Trans. 1980, 1838; A. A. Macharshvili, V. E. Shklover, Yu. T. Struchkov, G. I. Oleneva, E. P. Kramarova, A. G. Shipov, Yu. I. Baukov J . Chem. SOC., Chem. Comm. 1988, 683; V. F. Sidorkin, V. V. Vladimirov, M. G. Voronkov, V. A. Pestunovich J . Mol. Struct. (Theochem.) 1991, 228, 1; Yu. E. Ovchinnikov, A. A. Macharshvili, Yu. T. Struchkov, A. G. Shipov, Yu. I. Baukov J . Struct. Chem. 1994, 35, 91; D. Kost, I. Kalikhman, in The Chemistry of Organosilicon Compounds, Z. Rappaport, Y. Apeloig (eds.), Wiley, New York, 1998, vol. 2, 1339-1446; R. R. Holmes, Chem. Rev. 1996,96,927. R. Damrauner, S. R. Kass, C. H. DePuy, Organometallics 1988,7,637. D. M. Wetzel, J. I. Braumann J . Am. Chem. SOC. 1988,110,8333. E. A. Brinkmann, S. Berger, J. I. Braumann J . Am. Chem. SOC. 1988,110, 8304. A. G. Avent, D. Bonafoux, C. Eaborn, M. S. Hill, P. B. Hitchcock, J. D. Smih, J . Chem. SOC., Dalton Trans. 2000,2183. A. D. Frankland, M. F. Lappert J . Chem. SOC., Dalton Trans. 1996,4151.

Polysilylanions - Easily Available Building Blocks for the Synthesis of

Oligosilyl Transition Metal Compounds

Roland Fischer, Dieter Frank, Christian Kayser, Judith Baumgartner, Christoph Marschner

Institut f i r Anorganische Chemie, Technische Universifat Graz Stremayrgasse 16,8010 Graz, Austria

Tel.: +43 316 873 8209 -Fax.: +43 316 873 8701 E-mail: marschner@anorg.tu-graz.ac.at

Keywords: silyl anions, transition metal silyl compounds, potassium

Summary: A number of differently substituted mono- and dianionic oligosilyl compounds have been reacted with zirconocene and hafnocene dichloride to give mono and bissilylated metallocenes. The use of a TMEDA adduct of tris(trimethylsily1)silyl potassium enables the formation of a “Cp-free” hafnium silyl compound.

Recently we have shown that the reaction of oligosilanes with potassium alkoxides is a useful method to obtain a whole array of oligosilyl mono- and dianions [ 1,2]. As part of our investigations of the reactivity of these compounds we studied their reactions with group 4 halides [3]. As was shown initially by Tilley et al., it is easily possible to introduce the tris(trimethylsily1)silyl group to zirconocene and hafnocene chlorides by reaction with the respective lithium reagent [4]. Therefore it was expected that similar reactions can be done with our oligosilylpotassium compounds. This expectation was fully met for a number of differently substituted tris(sily1)silyl groups (Scheme 1; Fig. 1) [ 5 ] .

SiMe, SiMe,

I I I I

R-Si-K CpzMC1z, Cp,M(CI)-Si-R

SiMe3 SiMe3

R = SiMe3, SiMe21Bu, SiMe2(CMezCHMe2), SiMe2Si(SiMe3),, NEQ, H M = Zr, Hf

Scheme 1. Synthesis of oligosilyl zircono- and hafnocene chlorides.

If, however, one of the trimethylsilyl groups in the tris(trimethylsily1)silylanion reagent is replaced by methyl, the reaction with the metallocene dichlorides does not work cleanly any more.

Ovganosilicon Chemisty V Edited by N. Auner and J. Weis

Copyright 0 2003 WILEY-VCH Verlag GmbH & Co. KgaA, Weinheim

Synthesis of Oligosilyl Transition Metal Compounds 187

The reason for this seems to be the enhanced reduction properties of the silyl anion, which are caused by the removal of a charge-stabilizing group. This called for a reactivity moderation, which can easily be accomplished via the transmetallation of potassium against magnesium [6]. The reagent obtained then reacts smoothly with the metallocene dichloride (Scheme 2).

SiMe3 5ime3 I 1. fert-BuOK / THF or DME Me ci 1 Zr(C,)Cp,

SiMe, I 3’ Cp,ZrC12

Me-Si-SiMe3 1 2. MgBr2

51me3

Scheme 2. Synthesis of methylbis(trimethylsilyl)silylzirconocene chloride.

In order to introduce groups into the silyl anion, which can be used for further functionalization, we also investigated reactions of hydride and amino-substituted bis(trimethylsily1)silylpotassium compounds with the metallocenes. Both types of compounds underwent the transformation smoothly (Scheme 1).

Bissilylated metallocenes can be obtained via the reaction of two equivalents of tris(trimethylsily1)silyl potassium with the respective metallocene dichloride in pentane (Scheme 3; Fig. 2).

CP, ,C’

+ 2 Me3Si

SiMe3

I I

-Si-K

SiMe3

pentane

M = Zr, Hf

me351 \ /SiMe3

”\ / S i l S i M e 3

cp/M\Si,SiMe3

me351 / ‘SiMe,

Scheme 3. Bissilylated zircono- and hafnocenes

Fig. 1. Structure of Cp2Zr(C1)Si(SiMe&SiMe2(CMe2CHMe2). Fig. 2. X-ray structure of CpzZr[Si(SiMe3)3]2.

Dianions which possess enough conformational flexibility for both ends of the chain to be able to react with the same atom are excellently suited to the syntheses of homo- and especially

188 R. Fischer, D. Frank, C. Kayser, J. Baumgartner, C. Marschner

heterocyclosilanes. This has been demonstrated for the reactions of the 1,3- and the 1,4-dianions with either group 4 metallocene or group 14 dihalides (Scheme 4; Fig. 3) [2].

Me3Si \Si,SiMe3 SiMe3 SiMe3 I I CP2MC12 CP~M, / \ /(SiMe2)n

K-Si-(SiMep),Si-K

Si ’ ‘SiMe, n = 1 , 2

M = Zr, Hf

I I SiMe3 SiMe3

Scheme 4. Zirconocena- and hafnocenacyclosilanes.

Besides the metallocene group 4 metal silyl compounds already discussed, substances which do not contain the cyclopentadienyl ligand are also of interest [7]. Remarkably, the seemingly most straightforward synthetic route, namely the reaction of silyl anions with group 4 tetrahalides, has not been reported so far. The reason for this seems to be connected to the highly Lewis acidic character of the metal halides, which causes ether cleavage and other side reactions.

In a recent attempt we have reacted a TMEDA-adduct of tris(trimethylsily1)silyl potassium with hafnium tetrachloride [8]. This represents the first example of a reaction of a silyl anion with a group 4 tetrahalide. The product obtained is tris(trimethylsily1)silylhafnium trichloride, with one TMEDA molecule coordinated. With three potential leaving groups still present in the molecule, we hope that it might serve as a useful precursor for a number of differently substituted silyl hafnium compounds.

Scheme 5. TMEDA-trichloro[tris(trimethylsilyl)silyl]hafnium compound.

Fig. 3. Structure of Cp,Zr[{Si(SiMe~)z}zSiMe~]. Fig. 4. Structure of C13HfSi(SiMe&TMEDA.

Synthesis of Oligosilyl Transition Metal Compounds 189

In conclusion, we have been able to demonstrate the high potential of the easily accessible oligosilyl anions as precursors for the synthesis of silyl transition metal compounds.

Acknowledgments: Financial support by the Fonds zur Forderung der wissenschaftlichen Forschung in Osterreich (FSP Silicumchemie, S7902) and the Austrian Ministry for Education, Science and Culture (START 120-Y) is gratefully acknowledged. The Wacker Chemie GmbH, Burghausen, kindly provided various organosilanes as starting materials.

References a) Ch. Marschner, Eur. J. Inorg. Chem. 1998, 221; b) Ch. Kayser, R. Fischer, J. Baumgartner, Ch. Marschner, Organometallics 2002, 21, 1023. Ch. Kayser, G. Kickelbick, Ch. Marschner, Angew. Chem. 2002, 114, 1031; Angew. Chem Int. Ed. 2002,41,989. a) T. D. Tilley, Transition-Metal Silyl Derivatives, in The Chemistry of Organic Silicon Compounds; S. Patai, Z. Rappoport, (eds.); John Wiley: Chichester, 1989, Chapter 24; p. 1415; b) T. D. Tilley, Appendix to Transition-Metal Silyl Derivatives, in The Silicon- Heteroatom Bond S . Patai, Z. Rappoport, (eds.); John Wiley: Chichester, 1991, Chapter 10; p. 309; c) M. S. Eisen, Transition-Metal Silyl Derivatives in The Chemistry of Organic Silicon Compounds; Z. Rappoport, Y. Apeloig, (eds.); John Wiley: Chichester, 1998; Vol. 2, Chapter 35; p. 2037. B. K. Campion, J. Falk, T. D. Tilley, J. Am. Chem. SOC. 1987,109,2049. Ch. Kayser, Ch. Marschner, Monatsh. Chem. 1999,130,203. J. D. Farwell, M. F. Lappert, Ch. Marschner, Ch. Strissel, T. D. Tilley, J. Organomet. Chem. 2000,603,185. a) I. Castillo, T. D. Tilley, J. Organomet. Chem. 2002, 643-644, 431; b) X. Liu, Z. Wu, Z. Peng, Y.-D. Wu, Z. Xue, J. Am. Chem. SOC. 1999, 121, 5350 and references therein; c) R. H. Heyn, T. D. Tilley, Inorg. Chem. 1989,28, 1768. D. Frank, J. Baumgartner, Ch. Marschner, J. Chem. SOC., Chem. Commun. 2002, 1190.

Experimental Determination of the Inversion Barriers of Oligosilyl Anions

Roland Fischer, Christoph Marschner

Institut fur Anorganische Chemie, Technische Universitat Graz Stremayrgasse 16,8010 Graz, Austria

Tel.: +43 316 873 8209 -Fax: +43 316 873 8701 E-mail: marschner@anorg.tu-graz.ac.at

Keywords: silyl anions, dynamic NMR spectroscopy, inversion barrier, activation parameters

Summary: Employing temperature-dependent dynamic NMR spectroscopy we have been able to measure both the activation enthalpy and entropy of inversion processes. Moreover we have performed a systematic study of the influence of the substituent groups, the nature of the cation, and solvent effects on the height of the inversion barrier.

Introduction

As silyl groups find widespread use as protective groups and for directing purposes in both organic and inorganic chemistry, it would be highly appreciated to have access to chiral silyl substituents. The introduction of configurationally stable, optically active silyl groups might allow insight to reaction mechanisms, for example stereocontrolled polymerization reactions of silanes, and could provide new synthetic tools, not only in organometallic chemistry. For these purposes it is important to understand how the substituent pattern, the solvent effects or the nature of the cation affects the configurational stability and reactivity of silyl anions.

Although the introduction of silyl moieties is usually accomplished by electrophilic silyl reagents, recently their anionic congeners began to enjoy more and more popularity, as they are readily available by metal-halogen exchange or by cleavage reactions.

The configurational stability of silyl anions has been a matter of investigation for some time. Although theoretical studies [I] predict a higher inversion barrier for silyl anions than for their carbon analogues, experimental data are scarce. The pioneering synthetic studies on chiral silyl anions by Sommer [2] and Corriu [3] and the first NMR studies of inversion processes by Lambert [4] have shown that chemistry can be done with optically active silanes but provides only a rough estimate for the lower limit of the inversion barrier, as does the recently published work by Tamao and co-workers [5] on nitrogen-substituted silyl anions. Tilley’s group investigated the stability of siloles under the influence of inversion [6].

Ovganosilicon Chemisty V Edited by N. Auner and J. Weis

Copyright 0 2003 WILEY-VCH Verlag GmbH & Co. KgaA, Weinheim

Experimental Determination of the Inversion Barriers of Oligosilyl Anions 191

Results and Discussion

As was recently shown by our group, the cleavage of silicon-silicon bonds with potassium tert-butoxide [7] is completely selective if silyl groups with different stenc demands are present [8]. As Scheme 1 shows, this approach was used to create oligosilanes with enantiotopic trimethylsilyl

Me3Si\ %Me3 Me 3 Si \si./siMe3 Me\ /SiMe2Ph Si., Si: 4 % 4 % 4 % 5

PhMqSi SiMe2Ph

- tert-BuOSiMe2Ph + tert-BuOK

- 5a

Me\ /SiMe2Ph I

1 J

PhLi

S i, Si.. Me3%\ / %Me3

4 % .

SiMe3

Me3Si SiMe3

+ tert-BuOK - tert-BuOSiMe,

Rx 4 %

I Me3%\ SiMe3

Si, 4 % Me3Si K Me$i R K SIMe;?Ph

1 , 2 , 3 , 4 , 5

iso-Pr3SiC1 + tefl-BuOK - tert-BuOSiMe,

Me, /SiMqPh S i, 4 %

J Me3+ SiMe, Me3%\ / SiMe,

Si, tert-BuMe2SiC1 si> 4 % 4 4 % tert-BuMe2Si R K R iso-Pr3Si SiMqPh 6, 7, 8, 9, 10 10 la, 2a, 3a, 4a, 5a \

R3 Si \si./M- M \si/siR33

4 % 4 % R2Me2Si 5' R2Me2Si 5'

6a-e; 7a; 8a; 9a,b; 1Oa-c

7a: M = K, R' = Et, R2 = tert-Bu, R3 = Me: i

8a: M = K, R' = i-Pr, R' = tert-Bu, R3 = Me: i

9a: M = K ; R' = Ph, R2 = tert-Bu, R3 = Me: i

9b: M = Li; R' = Ph, R2 = tert-Bu, R3 = Me: iii

10a: M = K; R' = Me, R2 = Ph, R3 = iso-Pr: i

lob: M = Na; R' = Me, R2 = Ph, R3 = iso-Pr: ii

1Oc: M = Li; R' = Me, R2 = Ph, R3 = iso-Pr: iii

iv = 1) tert-BuOK, THF 2) MgBriEt20

v = tert-BuOK, 18-cr-6, toluene

vi = tert-BuOK, THF-d8

6a: M = K, R', R3 = Me, R2 = tert-Bu: i

6b: M = Na , R', R3 = Me, R2 = tert-Bu: ii

6c: M = Li, R', R3 = Me, R2 = tert-Bu: iii

6d: M = MgBr, R', R3 = Me, R2 = tert-Bu: iv

6e: M = K, R', R3 = Me, R2 = tert-Bu: v

6f M = K, R', R3 = Me, R2 = tert-Bu: vi

Reaction conditions:

i = tert-BuOK, THF

ii = tert-BuONa, 15-cr-5, toluene

iii = 1) tert-BuOK, THF 2) LiCl

Scheme 1. Synthetic routes for the preparation of the chiral silyl anions.

192 R. Fischer, C. Marschner

-24 -

-26 -

-28

or dimethylphenylsilyl groups. Cleavage of one of these groups using potassium tert-butoxide in THF or in a solution of crown ethers and alkali metal alkoxides in aromatic solvents affords the racemic chiral silyl anions in quantitative yield.

Transmetallation of silyl potassiums 6a, 9a and 10a using LiCl or MgBr2.EtzO yields the corresponding Li and Mg compounds 6c, 6d, 9b and 1Oc. NMR samples were prepared by removing the solvent in vacuo when THF was used for the cleavage and adding 0.7mL of benzene-d6 or tOlUene-dg. Samples produced by this procedure usually contain one equivalent of complexed THF. The other samples were directly prepared in deuterated aromatic solvents.

We relied on temperature-dependent 'H NMR spectroscopy of diastereotopic methyl groups as these singlets proved to exhibit a shift difference at the low-temperature limit large enough to follow the inversion over a temperature range large enough to obtain rate constants for an Eyring evaluation, typically from -80 to +120 "C. Rate constants were extracted from the NMR spectra by evaluation of the line broadening due to inversion using the appropriate expression for the low- and high-temperature limit or by simulating the corresponding NMR spectra with the g-NMR simulation program 191.

Figure 1 shows the temperature-dependent NMR spectra of 9b. The diastereotopic methyl groups exhibit coalescence and finally merge into a single line, whereas the residual trimethylsilyl group remains unaffected by the line broadening due to inversion. The rate constants given in Fig. 1 were obtained by complete lineshape simulation. In Fig. 2 the Eyring-plot for the inversion of 9b is given.

? \

'IT [K-'

90" I

tert-Butyldimethylsil ylpheny 1 in(krmbT) trimethylsilylsilyllithium

-22

" T " r " r T m l m 7 m 0,68 0,60 0,52

0,68 0,60 0,52

Fig. 1. Temperature-dependent NMR spectra of 9b. Fig. 2. Eyring plot of 9b.

As Tamao [ 101 stated, silyl lithiums do exist as monomers in solution. This is in contrast to alkyl lithium compounds, which usually form larger aggregates. Therefore, Lambert and Tamao suggested a first-order rate law for the inversion process. We could verify this assumption by investigating the influence of the concentration of 6a ranging from 2 to 100 mg/mL. The inversion does indeed follow a first-order rate law as the evaluation of the rate constant with expressions for first-order kinetics afforded activation parameters which were independent of concentration.

Experimental Determination of the Inversion Barriers of Oligosilyl Anions 193

Yet the rate-determining step remains unrevealed. Either the dissociation of the ion pair or the inversion at the central silicon center might be crucial. The assumption that the formation of a solvent-separated ion pair (SSIP) is the rate-determining step is favored by the fact that elements such as lithium and magnesium, that are less electropositive than potassium and sodium, increase the barrier quite dramatically.

The destabilizing effect is more pronounced with silyl groups than with phenyl rings. A comparison with the isoelectronic phosphines shows that silyl-substituted phosphines exhibit a smaller inversion barrier than their phenyl-substituted analogues do [ 111. As our investigations concerning the cleavage of (Me3Si)3SiSi(SiMe&Ph with one equivalent of potassium tert-butoxide revealed, only K(SiMe3)2SiSi(SiMe&Ph is formed, which indicates that - as silylpotassiums react with oligosilanes in order to form a thermodynamic mixture of silane/silyl potassiums, as was shown in cross-metallation reactions - metallation at a silicon center with more silyl substituents is strongly favored over the metallation at the silicon center bearing phenyl groups [S].

When comparing the inversion barriers of 6a, 6e and 6f (Table l), the influence of the solvent systems becomes obvious. The solution of 6a in toluene-dg contains one equivalent of THF complexing the cation. The inversion barrier is considerably higher for this system than for 6e, for which dissociation is easier as the crown ether is complexing the cation, and 6f, which was investigated in an ethereal solvent system. Again this hints at a mechanism involving SSIPs.

Table 1. Inversion barriers of oligosilyl anions.

AH' [k.J/mol] AS* [J/mol K]

TBDMS TMS Me Si K 6a

TBDMS TMS Me Si Na, 15-crown-5 6b

TBDMS TMS Me Si Li 6c

TBDMS TMS Me Si Mg Br 6d

TBDMS TMS Me Si K, 18-crown-6 6e

TBDMS TMS Me Si K, THF-d8 6f

TBDMS TMS Et Si K 7a

TBDMS TMS iPr Si K 8a

TBDMS TMS Ph Si K 9a

TBDMS TMS Ph Si Li 9b

DMPS TIPS Me Si K 10a

DMPS TIPS Me Si Na, 15-crown-5 10b

DMPS TIPS Me Si Li 1Oc

80.3 -2 1

57.8 -6.7

102 +46

coalescence temperature z 110 "C

68.9 +1.9

71.4 -23

62.6 -7.3

39.5 -36

32.8 -67

59.4 -25

72.4 -44

65.2 -12

109 4 5

From these findings we interpret the configurational stability as a function of the Si-cation bonding interaction and the planarity of the anion, which is in good agreement with theoretical results obtained in our group [ 11.

194 R. Fischer, C. Marschner

Acknowledgments: Financial support by the Fonds zur Forderung der wissenschaftlichen Forschung in Osterreich and the Austrian Ministry for Education, Science and Culture (START 120-Y) is gratefully acknowledged. Wacker Chemie GmbH, Burghausen, kindly provided various organosilanes as starting materials.

References M. Flock, Ch. Marschner, Chem. Eur. J. 2002,8, 1024, and references therein. L. H. Sommer, R. Mason, J. Am. Chem. SOC. 1965,87,1619. R. Corriu, E. Colomer, J. Chem. Soc., Chem. Commun. 1976,176. J. B. Lambert, M. Urdaneta-PCrez, J. Am. Chem. SOC. 1978,100, 157. K. Tamao, A. Kawachi, H. Maeda, Organometallics 2002,21, 1319. W. P. Freeman, T. D. Tilley, L. M. Liable-Sands, A. L. Rheingold, J. Am. Chem. SOC. 1996, 118,10457. Ch. Marschner, Eur. J. Inorg. Chem. 1998,221-226. Ch. Kayser, R. Fischer, J. Baumgartner, Ch. Marschner, Organornetullics 2002,21, 1023. gNMR version 4.1, Cherwell Scientific Limited, The Magdalen Centre Oxford OX3 4GA (UK). The method is based on G. Binsch, J. Am. Chem. SOC. 1969, 91, 1304; D. S. Stephenson, G. Binsch, J. Magn. Res. 1978,30,625. K. Tamao, A. Kawachi, Adv. Organomet. Chem. 1995,38, 1, and references therein. J. B. Lambert, Topics Stereochem. 1971,6, 19.

Regiospecific and Enantioselective Polymerization to

Poly [ (dibutylamino)( trimethyldisilene)] by the Masked Disilene Method

Hideki Sakurai

Department of Pure and Applied Chemistry, Faculty of Science and Technology Tokyo University of Science, Noda, Chiba 278-85 10, Japan

Tel.: +8 1 22 277 2615 - Fax: +8 1 22 279 0973 E-mail: sakuraiq-h5 16@gamma.ocn.ne.jp

Keyword: polysilanes, regiospecific polymerization, stereoselectivity, masked disilene

Introduction

Polysilanes [ 11 have attracted considerable interest in recent decades because of their interesting electronic and photophysical properties and potential applications in the field of materials such as ceramic precursors [2], electrical conductors [3] and photo-conductors [4], and for microlithographic [5] and nonlinear optical devices [6]. The unusual properties of polysilanes are extremely sensitive to the polymer conformation and the substituents attached to the polymer backbone. However, the most common synthetic method employed for the preparation of polysilanes, the Wurtz coupling of dichlorosilanes with alkali metals, limits the potential routes for introducing functional organic groups into the polysilanes. It is also extremely difficult to control regio- and stereochemical features of polysilanes by the Wurtz coupling method. Beyond all question, controlling the structure is of prime importance in polymer chemistry.

Regiospecific Polymerization of Masked (Dibutylamino)(trimethyl)disilene and Functional Transformation

Anionic polymerization of masked disilenes presents exciting opportunities for the synthesis of polysilanes of well-defined structure and functionality [7, 81. Indeed, we have found that amino-substituted masked disilenes could be prepared and polymerized successfully to unprecedented amino-substituted polysilanes with a completely head-to-tail structure, poly[ 1,1,2-trimethy1-2-(dibutylamino)disilene]s [9].

Chlorination of the Si-N bond of the amino-substituted polysilanes affords chloro-substituted polysilanes, which could be substituted in a subsequent step by nucleophiles to give a new class of

Ovganosilicon Chemisty V Edited by N. Auner and J. Weis

Copyright 0 2003 WILEY-VCH Verlag GmbH & Co. KgaA, Weinheim

196 H. Sakurai

polysilanes. We report first on regioselective substitution on amino-substituted polysilanes as a synthetic route to a new class of polysilanes.

Amino-substituted polysilanes were synthesized in high yield by the anionic polymerization of masked disilenes in THF with a catalytic amount of n-butyllithium (Scheme 1).

P i 1 2

3

Scheme 1.

4

The 'H, 13C, and 29Si NMR spectra showed that the structure of the polymer, poly(1- dibutylamino- 1,2,2-trimethyldisilene) (2), is completely head-to-tail. The reaction of the amino- substituted polysilane and acetyl chloride at room temperature gave poly( 1 -chloro- 1,2,2- trimethyldisilene) (3) in high yield as a slightly soluble white powder. Although the exact structure of the polymer could not be determined because of very low solubility, the elemental analyses showed at least that the polysilane did not contain any nitrogen atoms, and the structure was confirmed by solid-state MAS NMR.

Next, substitutions of the Si-C1 bonds of the polymer were carried out with a variety of nucleophiles (Table 1). For example, alkylation of the chloro-substituted polysilane with butylmagnesium bromide gave poly( 1-butyl- 1,2,2-trimethyldisilene) in high yield. The SEC measurement demonstrated that the molecular weight of the polymer agreed very closely with calculated values on the basis of that of the amino-substituted polysilane, indicating no degradation occurred in this reaction process. The structure of the polymer was estimated by NMR analyses. In the 'H and 13C NMR spectra, the peaks of butyl groups on the polymer were observed, while the peaks of dibutylamino groups on the amino-substituted polysilanes completely disappeared. Figure 1 shows the dimethylsilylene region of the 13C NMR spectrum for the polymer and a polysilane prepared by anionic polymerization of a 6:4 mixture of 1-butyl- 1,2,2-trimethyl-substituted masked disilene isomers. Two sharp peaks assignable to the head-to-tail structural units of -SiMeZ- and -BuSiMe- were observed for the polysilane prepared from the chloro-substituted polysilane, but four peaks assigned for the methyl groups of the head-to-tail and head-to-head structures were observed for the polysilane prepared from the butyl-substituted masked disilenes. The 29Si NMR spectrum is also consistent with those for the polymer chain with highly head-to-tail regulated

RegiospeciJic and Enantioselective Polymerization 197

structure. These results suggest that the regioselective substitution reaction occurred.

I I I I I -2 -3 -4 -5 -6

ppm

Fig. 1. Dimethylsilylene region of the I3C NMR of polysilane prepared a) by anionic polymerization, and b) by

Wurtz coupling of of 6:4 mixture of masked disilene.

Table 1. Substitution of amino groups on side-chain of the polysilanes.

Mn Run Reagent Yield [%] MwlMn

Obs. ['I Calc. [bl

1 BuMgBr 91 22 000 24 000 2.4

2 PhLi 78 15 000 22 000 2.2

3 AllMgCl "I 45 8 800 7 900 2.4

4 BuOH/Et3N 52 33 000 21 000 1.9

[a] Determined by GPC with polystyrene standards. [b] Based on amino-substituted polysilanes. [c] All = allyl(2-propenyl).

This synthetic strategy has allowed us to synthesize a real head-to-tail polysilastyrene. In 1981 West et al. reported the first synthesis of a soluble polysilane copolymer from methylphenyldichlorosilane and dimethyldichlorosilane [ 101, The copolymer was called polysilastyrene, based on its structural similarity to carbon-based polymer, i.e., polystyrene.

198 H. Sakurai

However, the structure of the copolymer was blocky with runs of the respective monomer units, and hence the polymer was not the real polysilastyrene [l l] . Phenylation of the chloro-substituted polysilane 3 with phenyllithium gave poly( lfl,2-trimethyl-2-pheny1disilene) in good yield. The structure of the polymer was confirmed with the SEC and NMR spectra. We have now succeeded in the preparation of the real head-to-tail polysilastyrene (4, R = Ph) about 20 years after the first reported synthesis of soluble polysilastyrene. This procedure is a simple method for preparing polysilanes of special structure with functional groups which are otherwise very difficult to prepare.

Enantioselective Polymerization of the Masked Disilene (1)

Enantiotopic polymerization of a prochiral vinyl monomer leads to either isotactic or syndiotactic polymer. Stereocontrol of the polymerization is a main issue of the polymer chemistry. Contrary to the situation for the vinyl monomers, a masked disilene such as 1 is chiral. Monomers are obtained as a racemic mixture, which can be separated into each enatiomer by using a chiral column on liquid chromatography. Herein we report the first highly enatioselective polymerization of the racemic masked monomer. The stereochemical course of the propagation step in the anionic polymerization of 1 should be extremely interesting.

Very recently, we have disclosed the stereoselective anionic polymerization of dibutylamino- substituted masked disilenes 1 [12]. The stereochemistry of the polymers is analyzed based on diad and triad sequences. Under the appropriate conditions, the polymerization produced a polymer rich in syndiotacticity up to 89 % in diad. Typical examples of the analysis are shown in Table 2. A high r content in the diad tacticity as well as a high rr fraction in the triad tacticity indicates clearly that the polysilane is rich in syndiotacticity.

Table 2. Determination of tacticity for dibutylamino-substituted polysilanes.

Run Initiator Polymn. Conditions Diad tacticity ['I Triad tacticity la]

r m rr rm mm

1 PhzMeSiLi r.t., 40 min, TI-F 0.77 0.23 0.68 0.23 0.09

2 PhzMeSiK r.t., 15 min, THF 0.73 0.27 0.57 0.32 0.11

3 PhzMeSiK + r.t., 3min, benzene 0.69 0.31 0.49 0.38 0.13 cryptand[2.2.2]

[a] Determined by 'H NMR (600 MHz, C6D6),

The syndiotacticity also depends on the nature of the initiator used. The initiators were changed from Ph2MeSiLi to PhzMeSiK and the effect of an added cryptand was also examined. The interactions between the anionic ends and cations became freer on progressing down Table 1 and the syndiotacticity of the polysilanes decreased as the interaction became freer. The result suggests that the syndiotacticity should increase at lower temperature since the ion pair interaction should increase at lower temperature. This was indeed the case, as shown in Table 3 and Fig. 2.

Regiospec$c and Enantioselective Polymerization 199

I I \

r -Si(NBu2)Me- -

rr - mr(rm) - m

mm A

m \ 4 I

L

L I

0.9 0.8 0.7 0.6

PPm

-

i 0.5

Fig. 2. I3C NMR spectra (600 MHz) of dibutylamino-substituted polysilanes in C,& (Si-Me region): a) 23 "C,

b) -19 "C, C) 4 0 "C.

200 H. Sakurai

Table 3. Anionic polymerization of dibutylamino-substituted masked disilene 1 at various temperatures.

Diad tacticity [b] Triad tacticity [b] Mn [a1 (PD)

Polymn. Yield conditions [%I r m rr rm mm

Run

1 23 "C, lh, THF 66 24000 (1.46) 0.76 0.24 0.67 0.23 0.10

2 O T , 12h,THF 40 21 200(1.65) 0.78 0.22 0.69 0.20 1011

3 -1O"C, 12h,THF 89 15 400 (1.40) 0.80 0.20 0.72 0.19 0.09

4 -3O"C, 12h,THF 89 25 500 (1.67) 0.82 0.18 0.75 0.18 0.07

5 4 0 ° C . 12h,THF 66 20500 (1.47) 0.89 0.11 0.78 0.16 0.06

[a] Determined by GPC (polystyrene standards, eluent: toluene). [b] Determined by 'H NMR (600 MHz, C6D6).

The syndiotacticity as measured by the diad tacticity recorded a value of 0.89 at -60 "C. In this polymerization, the selective attack of the propagating ends to each enatiomer of the monomers resulted in high stereoselectivity. It is well documented that the configurational stability of silyl anions is high enough to prevent pyramidal inversion of the growing ends during the propagation steps. The formation of the syndiotactic polysilanes indicates that propagating silyl anion ends with D configuration selectively attack the L monomer, and vice versa. This polymerization demonstrates the first example of the control of the stereochemistry of polysilanes.

References [ l ] For reviews of polysilanes, see: a) R. West, J. Organomet. Chem. 1986, 300, 327; b) R. D.

Miller, J. Michel, Chem. Rev. 1989, 89, 1359; c) H. Sakurai, ed., Advanced Technology of Organosilicon Polymers, CMC, Tokyo, 1996. S. Yajima, K. Okamura, J. Hayashi, M. Omori, J. Am. Ceram. SOC. 1976, 59, 324; b) S. Yajima, J. Hayashi, M. Omori, Chem. Lett. 1975,931. R. West, L. D. David, P. I. Djurovich, K. L. Stearley, K. S . V. Srinivasan, H. Yu, J. Am. Chem. SOC. 1981,103,7352. F. Kajzar, J. Messier, C. Rosilio, J. Appl. Phys. 1986, 60, 3040. R. D. Miller, D. Hofer, J. N. Fickes, C. G Willson, E. E. Marinero, P. T. Trefonas III, R. West, Polym. Eng. Sci. 1986,26, 1129. Y. Moritomo, Y. Tokura, H. Tachibana, Y. Kawabata, R. D. Miller, Phys. Rev. B 1991, 43, 14746. a) K. Sakamoto, K. Obata, H. Hirata, M. Nakajima, H. Sakurai, J. Am. Chem. SOC. 1989, 111, 7641; b) K. Sakamoto, M. Yoshida, H. Sakurai, Macromolecules 1990,23,4494. For review, see: H. Sakurai, Macromolecular Design of Polymeric Materials, K. Hatada, T. Kitayama, 0. Vogl, eds., Marcel Dekker, New York, 1997, Chapt. 27. For recent application of the masked disilene method to the preparation of supramolecular assemblies, see: a) T. Sanji, F. Kitayama, H. Sakurai, Macromolecules, 1999, 32, 5718; b) T.

[2]

[3]

[4] [5]

[6]

[7]

[8]

Regiospecific and Enantioselective Polymerization 201

Sanji, Y. Nakatsuka, F. Kitayama H. Sakurai, Chem. Commun. 1999, 2201; c) T. Sanji, Y. Nakatsuka, S. Ohnishi, F. Kitayama, H. Sakurai, Macromolecules 2000,33, 8524. H. Sakurai, K. Sakamoto, Y. Funada, M. Yoshida, Inorganic and Organometallic Polymers II, Advanced Materials and Intermediates, ACS Symposium Series, Vol. 572, Wisian, P.; Allcock, H. R.; Wynne, K. J. eds. 1994, Chapt. 2.

[lo] R. West, L. D. David, P. I. Djurovich, K. L. Stearley, K. S. Srinivasan, H. Yu, J. Am. Chem. SOC. 1981,103,7352.

[ l l ] A. R. Wolff, I. Nozue, L. Maxka, R. West, J. Polym. Sci., Polym. Chem. Ed. 1988,26, 701. [ 121 T. Sanji, R. Honbori, H. Sakurai, submitted for publication.

[9]

The Cationic Rearrangement of (3-Hydroxy- 1- propenyl) tris( trimethylsil y1)silanes

into (1 - Trime thylsilyl-2-propenyl) - bis( trimethylsily1)silanols

K. Schmohl, H. Reinke, H. Oehme

Fachbereich Chemie der Universitat Rostock D- 1805 1 Rostock, Germany

Tel.: +49 381 498 1765 -Fax: +49 381 498 1763 E-mail: hartmut.oehme @ chemie.uni-rostock.de

Keywords: silanes, silylcarbenium ions, rearrangements, 1,2-Si, C-trimethylsilyl migration, hypersilyl alcohols

Summary: (3-Hydroxy-l-propenyl)tris(trimethylsilyl)silanes (Me3Si)3Si-CH=CHC- (OH)R2 4a-c (a: R = H; b: R = Me; c: R = Ph), made by addition of tris(trimethylsily1)silane (3) to propargylic alcohol, 2-methyl-3-butyn-2-01 and 1,l -diphenyl-2-propyn- 1-01, respectively, were treated with HCl and HzS04. Whereas 4a proved to be stable under these conditions, 4b and 4c underwent a rapid isomerization to give the ( 1 -trimethylsilyl-2-propenyl)bis(trimethylsilyl)silanols (Me3Si)2Si(OH)CH(SiMe3)CH=CRz 8b and 8c. A possible mechanism of the rearrangement reaction is discussed. Following a similar reaction path, 4b and 4c were converted by boron trifluoride to give the fluorosilanes (Me3Si)2Si(F)CH(SiMe3)CH=CR2 (10b,c).

Introduction

In the presence of acid, l-hydroxyalkyltris(trimethylsily1)silanes (Me3Si)3Si-C(OH)R1R2 (1) undergo a rapid isomerization into 1 -trimethylsilylalkylbis(trimethylsilyl)silanols (Me$ji)zSi(OH)- C(SiMe3)R'R2 (2) [l]. The reaction involves the migration of one trimethylsilyl group from the central Si atom to a neighboring carbenium carbon atom, formed by the acid-induced elimination of water from the alcohols 1. Attack of X, the conjugate base of the acid used as the catalyst, at this Si atom and subsequent hydrolysis affords 2. The crucial step of the reaction is believed to be the isomerization of a silyl-substituted carbenium ion into a silylium ion, c o n f i i n g expectations drawn from theoretical calculations that a-silyl carbenium ions should generally be less stable than the isomeric silylium ions [2].

Ovganosilicon Chemisty V Edited by N. Auner and J. Weis

Copyright 0 2003 WILEY-VCH Verlag GmbH & Co. KgaA, Weinheim

The Cationic Rearrangement of silanes into silanols 203

Results and Discussion

In the present paper we pursue the question of whether tris(trimethylsily1)silylcarbenium ions with a delocalized positive charge similarly undergo the above-described isomerization. The delocalization is expected to decrease the energy difference between the carbenium ion and the silylium ion and the rearrangement may fail.

As model compounds we have chosen (3-hydroxy- 1 -propenyl)tris(trimethylsilyl)silanes (4a-c), which were obtained by AIBN-initiated additions of tris(trimethylsi1yl)silane (3) to propargylic alcohol, 2-methyl-3-butyn-2-01 and l,l-diphenyl-2-propyn-l-01, respectively (Eq. 1). In agreement with literature data [3], in the case of the reaction of 3 with propargylic alcohol the Z-olefin 4a was obtained. NMR studies of 4b and 4c revealed an E-configuration for the two olefins.

Me3Si

Me3Si

3

\ I

I Me3Si-Si-H + HCX-C. (AIBN) 4a \ R

R

\ SI 2 R \

Me3Si/ I SiMe3

4b,c

b: R=Me: c: R=Ph

Eq. 1. Synthesis of the 3-hydroxy-1 -propenyltris(trimethylsilyl)silanes (4a-c) by AIBN-induced addition of

tris(trimethylsily1)silane (3) to the triple bond of different propynols.

The alcohols 4a-q dissolved in ether, were treated with ethereal HC1 or, alternatively, solutions of the alcohols in pentane were stirred with a few drops of sulfuric acid. Under both conditions 4a was always recovered unchanged, but 4b and 4c, independently of the catalyst applied, underwent a rapid rearrangement and aqueous workup of the mixtures afforded the silanols 8b and 8c.

The proposed mechanism of the reaction is outlined in Schemel. Comparably with the described conversion 1+2, after protonation of the alcohols 4b and 4c water is eliminated to form the carbenium ions 5b,c. As mentioned, 4a is reluctant to undergo the rearrangement. Obviously, under the conditions applied the elimination of water from the primary alcohol 4a fails and therefore the whole process cannot occur. Despite the delocalization of the positive charge in 5b,c and the enhanced stability of the carbenium ion, one trimethylsilyl group migrates from the central silicon atom to the neighboring carbon atom, generating the transient silylium ions 6b,c. Attack of the

204 K. Schmohl, H. Reinke, H. Oehme

OH R

Me3Si, 4~ +HX Me3Si, dR x~ Me3SI/ I ' R - - H20 1 Me3S/riMe3 1 SiMe3

L

4b,c 5b,c

6b,c

J Me3 Si

SiMe3

7b,c

(H20)1/ \(MeoH)

8b,c 9b,c

b: R=Me: c: R=Ph

Scheme 1. The acid-induced conversion of the 3-hydroxy-l-propenyltris(trimethylsilyl)silanes 4h,c into the

silanols 8b,c or the methoxysilanes 9b,c, respectively.

The Cationic Rearrangement of silanes into silanols 205

counterions C1- or HS04- at the positive silicon centers of 6b,c leads to 7b,c and the hydrolysis of the chlorosilanes or silylsulfates, respectively, during the aqueous workup affords the silanols 8b,c. As mentioned, a-silyl carbenium ions are generally less stable than the isomeric silylium ions, and this obviously also applies to the delocalized silylcarbenium ions 5b,c and the isomeric silylium ions 6b,c. This was confirmed by ab-initio calculations, which indicated a stabilization of 6b relative to 5b by approx. 23 kcal mol-' [4]. For the reaction of 4c with ethereal HC1 the intermediate chlorosilane 7c (R = C1) could be isolated. As expected, with water 7c is rapidly converted into the silanol 8c. The reaction of 4b,c with sulfuric acid in methanol afforded the methoxysilanes 9b,c (Scheme 1).

According to a related mechanism, treatment of 4b,c with boron trifluoride produced the fluorosilanes (Me3Si)zSi(F)-CH(SiMe3)CH=CR2 (lob: R = Me; 1Oc: R = Ph). IR, NMR and MS studies confirm the proposed structures of 4b,c, 7c, 8b,c, 9b,c and 10b,c. For

the methoxysilane 9c an X-ray structural analysis was performed (Fig. 1). Bond lengths and angles were found to meet standard values

c24

Fig. 1. Molecular structure of 9c in the crystal (H atoms omitted, except C1H and C2H); selected bond lengths [A] and angles ["I: Sil-Cl 1.906(4), Sil-Si2 2.3621(19), Sil-Si3 2.3630(18), Sil-01 1.645(4), Cl-Si4

1.898(4), Cl-C2 1.496(5), C 2 4 3 1.338(5); Si2-Sil-Si3 111.27(7), Sil-C1-Si4 114.60(19), Sil-01-C19

131.6(5), Si3-Sil-Cl 112.53(13), Si2-Sil-CI 109.54(13), 01-Sil-C1 105.74(19).

Acknowledgment: We gratefully acknowledge the support of our work by the Deutsche Forschungsgemeinschaft and the Fonds der Chemischen Industrie. We thank Prof. M. Michalik, Dr. W. Baumann and Prof. N. Stoll for recording the NMR and mass spectra.

206 K, Schmohl, H. Reinke, H. Oehme

References [ 11

[2]

[3] [4]

K. Sternberg, M. Michalik, H. Oehme, J. Organomet. Chem. 1997,533,265; K. Sternberg, H. Oehme, Eur. J. Inorg. Chem. 1998,177. Y. Apeloig, A. Stanger, J. Am. Chem. SOC. 1985, 107, 2806; Y. Apeloig, A. Stanger, J. Am. Chem. Soc. 1987,109,212. K. Miura, K. Oshima, K. Utimoto, Bull, Chem. SOC. Jpn. 1993,66,2356. K. Schmohl, D. Wandschneider, H. Reinke, A. Heintz, H. Oehme, Eur. J. Inorg. Chem. 2002, 597.

Chiral p-Silyl Aldehydes as Precursors of Chiral P-Hydroxy Acids and Chiral1,3-Diols

Joachim Sommer, Hubertus Ahlbrecht

Institut fur Organische Chemie, Justus-Liebig-Universitat GieSen Heinrich-Buff-fing 58, D-35392 Giessen, Germany Tel.: +49 641.99 34340 - Fax: +49 641 99 34309

E-mail: hubertus.ahlbrecht @org.chemie.uni-giessen.de

Keyword: homoenolate, chiral, formal hydroxylation, 1 -aminoallyl anions, silylation, alkylation

We have developed a completely new synthesis of chiral aldols 12 by formal enantioselelective connection of a C-0 and a C-C bonding at the C3-position of a homoenolate dianion synthon 11 (Scheme 1). Because the natural polarity of the reactants is reversed twice the direct approach is not possible. Therefore we used the ally1 amine 1 and enamine 2 as excellent synthesis equivalents of homoenolates, which can be silylated and alkylated, respectively (Scheme 2).

Scheme 1.

Allyl-SMP 1 is easily to deprotonate with the Lochmann-Schlosser base at -78 "C in TBME. Subsequent silylation with DMPSCl provides quantitatively the P-silylated enamine 2, which can be metallated directly again in the same pot.

SMP serves as a chiral auxiliary and can be obtained on a large scale from the amino acid proline. Hydrolysis of enamine 5 under very mild conditions releases SMP without any loss of chirality.

The introduction of a silyl group has several decisive advantages: it reacts with 1 quantitatively in the 3-position, it is stable under the reaction conditions, it facilitates the second metallation by its a-effect, it improves the diastereoselectivity of alkylation of 4 by its strong steric influence, and lastly it can be converted into a hydroxy group with complete retention of configuration.

The one-pot synthesis sequence of metallation, silylation, metallation, and alkylation of allyl-SMP 1 generates almost enantiopure (R)-P-silyl aldehydes 6. These aldehydes 6 are oxidized

Ovganosilicon Chemisty V Edited by N. Auner and J. Weis

Copyright 0 2003 WILEY-VCH Verlag GmbH & Co. KgaA, Weinheim

208 J. Sommer, H. Ahlbrecht

and subsequently oxidatively desilylated to give P-hydroxy acids 8, or reduced and subsequently oxidatively desilylated to give chiral 1,3-diols 10. Following this concept of silylation and alkylation of the simple allyl-SMP 1 and subsequent oxidative cleavage of the C-Si bonding, it is possible to prepare a complete library of highly enantiopure chiral aldol compounds (Tables 1 and

2) .

1. tBuLilKOT 2. DMPSCl

I

Allyl-Br : I

I

H

/ O d

SMP

4

I tBuLilKOT

3 I R-X

I

J"'" c.,......,..,.........

R-0 '*

" W C O O H 8 H

1. HBFd 2. H202

R-OH '* 8H 10

Scheme 2.

Chiral PS i l y l Aldehydes as Precursors of Chiral PHydroxy Acids and Chiral I,3-Diols 209

Table 1. Silylation and alkylation of 1.

fi mR '3 Yieldof6 Inductionof 6 Si SMP Si SMP si SMP [%I [ % eel

Electrophile

2 [moll 5 [%I 4 [%I Methyl iodide

Ethyl iodide

Ethyl bromide

n-Propyl iodide

n-Propyl bromide

n-Butyl iodide

n-Butyl bromide

nButyl chloride

n-Undecyl bromide

n-Pentadecyl bromide

n-Pentadecyl iodide

Isopropyl bromide

TMSCl

Dimethyl disulfide

Benzyl bromide

Ally1 bromide

Benzaldehyde

0

3

3

5

1

0

3

2

0

5

5

1

2

-

3

1

1

0

27

19

19

19

22

19

10

13

15

16

70

38

-

51

47

99

100

70

78

76

80

78

78

88

87

80

77

29

60 - [cl

46

52

0

83

55

53

56

60

64

53

65

62

63

50

46

48

37

21

29

0

[a] Yield is related to ally1 amine 5. [b] Cannot be determined by "C NMR. [c] Cannot be determined by GC.

[d] Change of priority. [el 8 h at -78 "C, subsequently warmed up. [ f l3 days at -78 "C, subsequently warmed up.

[g] 6 days at -78"C, subsequently warmed up.

Table 2. Reduction and oxidation with subsequent oxidative desilylation.

p-Silyl aldehyde 6 p-Silyl alcohol 9 1,3-DiollO P-Silyl acid 7 P-Hydroxy acid 25 [ % eel [%I [%I [%I [%I

R

Methyl 98 (R) 98 71 65 -

Ethyl >97 (R) 88 - 74 90

n-Propyl 98 (R) 91 58 73 -

n-Butyl >98 (R) 91 79 69 49

n-Undecyl >98 ( R ) 91 96 73 73

Phenyl Ibl 95 (R) 96 79 76 82

Revisiting the SiZC16 Cleavage of Group 14 Element Phosphanes: Phosphane-Catalyzed

Rearrangements

W.- W. du Mont, * E. Seppalii, T. Gust, L Muller

Institut fur Anorganische und Analytische Chemie Technische Universitat Braunschweig

Hagenring 30, D-38 106 Braunschweig, Germany

Keywords: alkylidenediphosphane, diphosphene, nucleophile-catalyzed, 31P NMR

Summary: Me3GeSiCl3 is a useful new source for the nucleophile-catalyzed generation of Sic12 under very mild conditions.

Introduction

The intended (formal) Sic12 insertion into the P-Cl bond of the P-chloroalkylidenephosphane (Me3Si)2C=PCl using Si2C16 [ l ] did not lead to the desired novel P- (trichlorosily1)alkylidenephosphane (Me3Si)&=PSiCls, but an unexpected diphosphene R*-P=P-R* [R* = (Me$i)2(C13Si)C-] was isolated [2]. 31P NMR spectra from the reaction mixture showed a (d, d) pattern (due to J 31P, 31P) suggesting the presence of an alkylidenediphosphane (Me3Si)*C=P-P(SiC13)R* that may be the diphosphene precursor. To learn more about this reaction, we were looking for alternative sources of SiC13 or Sic12 groups and their reactivity towards alkylidenediphosphanes. The recent observation that syntheses of compounds R3MSiCl3 (M = Ge, Sn) - contrary to earlier expectations [2] - are affected by their nucleophile-catalyzed disproportionations [3-51 led us to consider the stable compound Me3GeSiCl3 as a potential source of SiC12.

Formation and Decomposition of Trimethyl(trichlorosily1)germane (1)

Me3GeSiCl3 (1) is obtained from Me3GeCl/€€SiCl?/NEt3 or from R#GeMe3/Si2Cl6 as a distillable liquid. The formation of 1 is, however, followed by incomplete base-catalyzed decomposition reactions leading to novel solid (Me3Ge)~Si(SiCl3)2 2 [3,5].

Distilled 1 is thermally stable, but, upon addition of small amounts of Et3N or iPr3P, it decomposes incompletely into 2, Me3GeC1, and SiC4. A careful study by NMR [5] reveals that a sequence of several reaction steps involving transmetallatiodtranssilylation reactions leads to 2

Ovganosilicon Chemisty V Edited by N. Auner and J. Weis

Copyright 0 2003 WILEY-VCH Verlag GmbH & Co. KgaA, Weinheim

Revisiting the Si2C16 Cleavage of Group 14 Element Phosphanes 211

(Scheme 1).

R2PGeMe3 + Si2CI6 R2PSiC13 + Me3GeSiCb

i i i) I ’ R2PSi2CI, + Me3GeCI (Me3Ge)*Si(SiC& + SiCI4 + Me3GeCl

2

Scheme 1. i) Me3Ge/Si2C15 exchange at phosphorus leading from R‘R”PGeMe3 (R’ = tBu, R“ = iPr)/SizC16 to

RRPSi2C15 and Me3GeC1; ii) SiClz (or SiC13- ) transfer from R’R”PSi2C15 to germanium yielding

Me,GeSiC13 (1) and R‘RPSiC13; iii) dismutation of Me3GeSiC13 into (Me3Ge)2Si(SiC13)2 (2), Me3GeC1,

and SiC14 Ref. 151.

The last step iii) implies the nucleophile-induced transfer of one Me3Ge group and two Sic13 groups to the Si atom of a Me3GeSiCl3 molecule. We assume, that nucleophilic attack at the silicon atom of the Me3GeSiCl3 molecule can give the MesGe moiety the character of an anionic leaving group that attacks another Me3GeSiCl3 molecule with nucleophilic substitution of a C1 atom, leading to a (Me3Ge)2SiC12 intermediate that does not allow the attack of another Me3Ge group but is susceptible to subsequent trichlorosilylation by latent SiCl3- nucleophiles (CVSiCl3 exchange at Si) or by nucleophile-coordinated dichlorosilylene [i.e. by Sic12 insertions into Si-Cl bonds of (Me3Ge)2SiC12], providing 2.

Formation and Decomposition of Trichlorosilylstannanes

Chlorotrialkylstannanes R3SnC1 (R = CH3, C2H5, nC4H9) react with HSiCl3/NEt3 providing only traces of silylstannanes R3SnSiC13 (3a-c) but substantial amounts of the branched silylstannanes (R3Sn)2Si(SiCl3)2 (4a-c) [3]. Neopentane-like branched compounds 4 are also formed when dialkyl(trialkylstanny1)phosphanes R’R”PSnR3 are cleaved by hexachlorodisilane [5 ] .

In the series of silylstannanes 3a-c, formation and decomposition reactions were both faster than with germylsilane 1. When stannylphosphanehexachlorodisilane or trialkylchloro- stannane/HSiCl3/NEt3 reactions were carried out using the nBu3Sn substituent, formation of nBu3SnSi(SiC13)3 (5c) from intermediate nBu3SnSiCl3 (3c) was preferred to that of (nBu3Sn)2Si(SiCl3)2 (4c) [3-51. This suggests, that steric reasons might limit the ability of R3M groups to attack R3MSiCl3 (M = Ge, Sn) molecules in such a way that they have to compete with the trichlorosilylation of R3MSiC13 leading to R3MSi2C15 intermediates that are rapidly silylated further.

212 W.-W. du Mont, E. Seppala, T. Gust, L. Miiller

The SiClz Trapping Reaction of Me3GeSiC13 (1) with Alkylidenediphosphane (Me3Si),C=P-P(tBu)(iPr) (6)

The reaction of Sic12 precursor 1 with phosphaalkene 6 [6] at room temperature furnished a new compound 7 that exihibits a 31P NMR (d, d) pattern involving one 31P nucleus with a large upfield shift whereas the 31P NMR resonance of the iPr(tBu)P group (PA) appears at 45.6 ppm (“normal” for a crowded PR2 group). The upfield shift of PM (-112.3 ppm) indicates the presence of a ring system, which was apparently formed by cycloaddition of Sic12 with the P=C double bond of 6 [7]. Analytical data suggest that compound 7 contains two equivalents of SiC12. Among several 29Si resonances, only one exihibits resolvable couplings to the two phosphorus nuclei (J = 36 Hz and 8.1 HZ), indicating the connectivity c12si-PM-PA. TWO further pairs of 2 9 ~ i NMR lines can be assigned preliminarily to Me& groups (line distance 4.1 Hz) and to another Sic12 moiety (line distance 8.3 Hz). These patterns would be consistent with a novel P-phosphanyl-substituted phosphadisiletane ring system, i.e. a cyclic alkylidenetetrachlorodisilanylphosphane moiety bonded to a P(tBu)(iPr) group. To explain the presence of only one set of 31P and 29Si NMR patterns for 7 requires either high diastereoselectivity of the reaction leading to 7, orland fluxional behavior (inversion of PMIring inversion) of 7. These questions are under study.

The straightforwardness of the formation of this highly functionalised double Sic12 trapping product 7 supports the concept of regarding trichlorosilylgermanes, like trimethyl(trichlorosily1)- germane (l), as valuable new Sic12 precursors.

Acknowledgment: We thank the Deutsche Forschungsgemeinschaft and the Fonds der Chemischen Industrie for long-time financial support.

References A. Zanin, M. Karnop, J. Jeske, P. G. Jones, W.-W. du Mont, J. Organomet. Chem. 1994 ,475, 95. R. Martens, W.-W. du Mont, Chem. Ber. 1993, 126, 1115; R. Martens, W.-W. du Mont, in Organosilicon Chemistry - From Molecules to Materials, N. Auner, J . Weis, Eds., VCH, Weinheim 1994, p. 35. L. Miiller, W.-W. du Mont, F. Ruthe, P. G. Jones, H. C. Marsmann, J. Organomet. Chem. 1999,579,156. W.-W. du Mont, L. Miiller, F. Ruthe, Phosphorus, Sulfur and Silicon 1999,15&151, 149. W.-W. du Mont, L. Miiller, R. Martens, P. M. Papathomas, B. A. Smart, H. E. Robertson, D. W. H. Rankin, Eur. J. Znorg. Chem. 1999, 1381. J. Mahnke, A. Zanin, W.-W. du Mont, F. Ruthe, P. G. Jones, Z. Anorg. Allg. Chem. 1998, 624, 1447. W.-W. du Mont, E. Seppala. T. Gust, J. Mahnke, L. Muller, Main Group Chem. 2001, 24, 609.

Some New Nucleophile-Induced Reactions Involving SiC12, GeC12 and GeMe2 Transfer

Emma Seppalii, Wolf- Wulther du Mont, Thorsten Gust, Jens Muhnke, Lars Miiller

Institut fur Anorganische und Analytische Chemie der Technischen Universitat Postfach 3329, 38023 Braunschweig, Germany

Fax: +49 5313915387 E-mail: e.seppaelae@tu-bs.de

Keywords: silylenes, silylgermanes, phosphaalkenes, diphosphenes, silylphosphanes

Summary: Considering the stable compound Me3GeSiCl3 (1) as a potential nucleophile-induced source of Sic12 moieties and to evaluate the properties of MezGe(SiC13)z as nucleophile-induced source of Sic12 and GeMez moieties that could be generated by a kind of a domino-a-elimination, alkylidenediphosphane 5 (Me,Si),C=P-PRR‘ (R = Qu, R’ = iPr) was chosen as a new multifunctional trapping

reagent for six-electron species Sic12 and GeMez. Reaction of 1 with 5 leads to the formation of a stable 2-phospha- 1,3-disiletane-ring system. This observation indicates that trichlorosilylgermanes will be of importance as sources of generation of nucleophile-stabilized dichlorosilylene under very mild conditions.

Introduction

Trihalogenosilyl compounds are of general importance as trifunctional precursors for the synthesis of highly functionalized silicon compounds such as branched silicones and silsesquioxanes. Trihalogenosilylstannanes and related germanes, being a kind of a-halogeno(metal)silane, would be most desirable precursors for further transformation.

We have recently observed that the syntheses of R,MSiCl, (M = Ge, Sn) from “Benkeser” reaction are accompanied by their amine-catalyzed disproportionation into Me,MCl, SiC1, and neopentane-like branched products (R3M)zSi(SiC13)2 [ 11 (Scheme 1). This observation led us to consider the stable compounds Me,GeSiCl, (1) and Me,Ge(SiCl,), (2) as potential nucleophile- induced sources of SiCl, moieties (1 and 2) and of GeMe, moieties (2) that could be generated by a kind of a domino-a-elimination. As a new multifunctional trapping agent for six-electron species, we chose P-phosphanylphosphaalkene (Me,Si),C=P-PRR‘ (5) (R = QU, R’ = iPr) [2, 31. Experiments with GeC1, led to novel intermediates of the type R*-P=P-C(SiMe3)2GeC1,PRR’ [R* = (Me,Si),(Cl,Ge)C-] that decomposedrearranged by liberation of chlorophosphanes, providing bicyclic [PC(SiMe,),GeCl,],.

Ovganosilicon Chemisty V Edited by N. Auner and J. Weis

Copyright 0 2003 WILEY-VCH Verlag GmbH & Co. KgaA, Weinheim

214 E. Seppala, W. - W. du Mont, T. Gust, J. Mahnke, L. Miiller

Me,GeCI Me2GeC12

+ HSiCI, / Et3N

-Et3NHCI I + 2 HSiCI3 / Et3N - 2EtsNHCI

Me2Ge(SiC13), (2) Me3GeSiC13 (1)

- Me3GeCI

- Sic14 1 (Me3Ge)2Si(SiC13)2 (3)

Scheme 1. Syntheses of the trichlorosilylgermanes by the trichlorosilane method.

Disproportionation of the MezGe(SiC1S)z

Bis(trich1oro)silylgermane 2 undergoes a totally different kind of disproportionation than

Nu + { Me2Ge, 2' SiCI3 51c13

(2) 1

SiCI3 51c13

(4)

Scheme 2. Disproportionation of the dimethylbis(trichlorosily1)germane.

Some New Nucleophile-Induced Reactions 215

trimethyltrichlorosilylgermane. Compound 2 is stable in the presence of Et3N but undergoes a

disproportionation in the presence of other nucleophiles, for instance iPr3P. A new digermane, 1,2- bis(trichlorosily1)digermane Cl,Si(Me),Ge-Ge(Me),SiC13 (4), is formed (Scheme 2) instead of the

branched neopentane-like silicon product that was observed for the other silylgermanes and -stannanes. The postulated mechanism might involve a kind of a domino-a-elimination: the silicon atom would undergo a (slow) a-elimination, after which a nucleophile-stabilized dichlorosilylene species and an a-silylchlorogermane, Me,Ge(Cl)SiCl,, would be formed. This a-silylchlorogermane would then spontaneously eliminate SiCl,, which could be considered as the a-elimination at the germanium. The second half of the a-silylchlorogermane, the Me,Ge species,

would then undergo a fast insertion into the G e S i bond of the starting material 2 to form the digermane 4.

The formation of this product leads us to suggest that 2 may serve as a source of SiCl, and of Me,Ge as well. At present, we are subjecting this hypothesis to experimental evidence.

P-Phosphanylphosphaalkene Reaction

The reaction of SiC1, precursors 1 and 2 with P-phosphanylphosphaalkene 5 furnished a new

compound 6 that exhibits a 31P NMR (d,d) resonance involving one 31P nucleus with a large upfield shift whereas the resonance of the 'Pr('Bu)P group appears in the usual range for a crowded R,P

group (6 = -113.5 (d, J(P,P) = 196.1 Hz, ,'Si satellites J(P,Si) = 36.2 Hz); 44.4 (d, J(P,P) = 196.1 Hz, P'Bu'Pr). This upfield shift indicates a presence of a ring system that was apparently formed by cycloaddition of SiCl, with the P=C double bond of 5. Compound 6 contains two equivalents of SiC1,. The NMR and analytical data suggest a novel P-phosphanyl-substituted

2-phospha-l,3-disiletane ring system, i.e. a cyclic alkylidenetetrachlorodisilanylphosphane moiety bonded to a 'Pr('Bu)P group [4] (Scheme 3). Only when SiCl,/GeMe, precursor 2 is used, is the

above (d,d) pattern accompanied by another one (6= -17.9L22.7, J(P,P) = 297.5 Hz), suggesting that GeMe, as well has been trapped by 5.

Me3Si,

Me3Si ;C=P\ptBuipr

5

+ 2 x 1 - - 2 Me3GeCI

Me3Si

Me3Si

6

Scheme 3. Formation of 2-phospha-l,3-disiletane ring system 6 from 1 and 5.

The straightforwardness of the formation of the highly functionalized double SiCl, trapping

216

product 6 supports the concept of trichlorosilylgermanes as valuable new SiC1, precursors.

E. Seppala, W.-W. du Mont, T. Gust, J. Mahnke, L. Muller

Discussion

Me,GeSiCl, (1) and Me,Ge(SiCl,), (2) are obtained as colorless liquids from the “Benkeser” type of reaction of chloromethylgermanes with Et,N/HSiCl,. The formation of 1 is followed by the

incomplete base-catalyzed decomposition reactions leading to the novel crystalline solid (Me,Ge),Si(SiCl,), (3). Pure 1 is totally stable but nucleophiles catalyze the incomplete

disproportionation to 3. We suggest that this disproportionation undergoes different nucleophile-catalyzed steps that are very much like the ones in the known disilane disproportionation [5]. A new digermane is observed in the nucleophile-catalyzed reaction of the bis(trichlorosily1)germane. We suggest a kind of a domino-a-elimination as a possible mechanism. Reaction of alkylidenediphosphanes with 1 or 2 lead to the formation of a stable 2-phospha-1,3 -disiletane (6). This observation indicates that trichlorosilylgermanes will be of importance as sources for the generation of nucleophile-stabilized dichlorosilylene under very mild conditions.

References [ l ]

[2]

[3]

[4]

[5]

L. Muller, W.-W. du Mont, F. Ruthe, P. G. Jones, H. C. Marsmann, J. Organomet. Chem. 1999,579, 156-163. J. Mahnke, A, Zanin, W.-W. du Mont, F. Ruthe, P. G. Jones, Z. Anorg. Allg. Chem. 1998,

W.-W. du Mont, E. Seppala, T. Gust, J. Mahnke, L. Muller, Main Group Metal Chemistry 2001,24,609-612. W.-W. du Mont, T. Gust, E. Seppala, C. Wismach, P. G. Jones, L. Emst, J. Grunenberg, H. C. Marsmann, Angew. Chem. 2002,114,3977; Angew. Chem. lnt. Ed. 2002,41,3829. G. Urry, Acc. Chem. Res. 1970,3, 306.

624, 1447-1454.

Synthesis, Structure and Reactivity of Novel Oligosilyl Anions

H. Reinke, C. Krempner*

Fachbereich Chemie, Abteilung Anorganische Chemie, Universitat Rostock Einsteinstr. 3a, D-18055 Rostock, Germany

Tel.: +49 381 4986406 -Fax: +49 381 4986382 E-mail: clemens.krempner@chemie.uni-rostock.de

Keywords: oligosilanes, oligosilyl anions, chlorosilanes, fluorosilanes

Summary: The synthesis of the sterically overcrowded oligosilyl anions tris[methylbis(trimethylsilyl)sily1]silylpotassium [TBTS-K], -lithium [TBTS-Li] (2) and heptakis(trimethylsilyl)tetrasilacyclobutylpotassium [HTSB-K] (9) is reported. The new oligosilanes, TBTS-SiF3 (9, TBTS-SiF2Ph (6) and TBTS-SiH20Ph (7) were readily prepared by treatment of 2 with SiF4, PhSiF3 and (PhO)sSiH, respectively. The molecular structure of 6 has been derived from X-ray diffraction data. The reaction of 9 with SiF4, PhSiF3 and PhSiHzCl led to the formation of the cyclic oligosilanes HTSB-SiF3 (lo), HTSB-SiF2Ph (11) and HTSB-SiH2Ph (12), respectively.

Introduction

In recent years, space-demanding silyl anions have become of great interest, not only from the structural point of view, but also due to the fact that these highly reactive reagents could be used as anionic ligands for the kinetic stabilization of low-valent intermediates [ 11. These developments prompted us to design greatly overcrowded anionic ligands with electron-releasing properties that might be of interest for the stabilization of reaction intermediates or highly reactive compounds both kinetically and electronically. Herein we report the f is t synthesis of the oligosilyl anions tris[methylbis(trimethylsilyl)silyl]silylpotassium [TBTS-K], -lithium [TBTS-Li] and heptakis(trimethylsilyl)tetrasilacyclobutylpotassium [HTSB-K] and we describe their reaction behavior towards several chloro- and fluorosilanes.

Results and Discussion

Recently, we were successful in synthesizing TBTS-Br (l), a compound in which three Me(Me3Si)zSi groups and the central silicon atom form an extended hemispherical shield providing a center, to which the TBTS group is fixed, with exceptional steric protection [2]. These remarkable

Ovganosilicon Chemisty V Edited by N. Auner and J. Weis

Copyright 0 2003 WILEY-VCH Verlag GmbH & Co. KgaA, Weinheim

218 H. Reinke, C. Krempner

requirements make 1 a useful starting material for the synthesis of sterically overcrowded oligosilyl anions (Scheme 1).

TI

M = Li, K Me

Me3Si, I ,SiMe3 Me3Si,

Me-;Si-Si- = -TBTS Me3Si/ &,

Me3SI/I SiMe3 Me

k TBTS-SiF3 5 (87%)

lBTS-SiF2Ph 6 (93 %)

rn TBTS-SiH20Ph 7 (45 %) + HSi(OPh),

Scheme 1. Synthesis and reaction behavior of TBTS-Li (2).

In fact, treatment of 1 with an excess of Li powder in THF at r.t. leads to the formation of TBTS- Li (2) which can be isolated after crystallization from hot pentane as a red-orange crystalline material containing four molecules of THF (yield 76 %). The donor-free silyl potassium compound TBTS-K (2) was obtained nearly quantitatively by heating a solution of 1 in heptane in the presence of an excess of Na-K alloy. Although attempts to obtain single crystals of 2 suitable for an X-ray structure analysis failed, the structure proposed was in full agreement with the MS and NMR data, and especially the 29Si NMR data.

In view of the extreme bulkiness of 2, the fixation of one TBTS group at a silicon atom by a salt elimination reaction using simple chlorosilanes appears to be a difficult undertaking. Thus, we obtained no coupling products after treatment of 2 with S ic4 or HSiC13. Instead, in the case of Sic14 metal halogen exchange was observed yielding the chlorosilane 4, and the relatively acidic HSiC13 was deprotonated by the strong base 2 giving the hydridosilane 3 quantitatively. Even the reaction of less acidic chlorosilanes such as MeSiHC12, MezSiHCl, PhSiHC12, and PhSiH2C1 with 2 at -78 "C yields only mixtures of 3 and the desired coupling products, which could not be separated. However, when SiF4 or PhSiF3 reacted with TBTS-Li (2), the corresponding fluorosilanes 5 and 6, respectively, could be isolated in nearly quantitative yields as crystalline materials, which are stable in air and moisture.

The 29Si NMR chemical shift data for the TBTS compounds 1-7 are summarized in Table 1. As can be seen, the replacement of the bromine in TBTS-Br (1) by the silyl groups in 5-7 and finally by lithium in TBTS-Li (2) leads to a strong high-field shift of the central silicon atom ($Si3), which can be attributed mainly to the electronegativity difference between these groups.

In addition, the molecular structure of 6 (Fig. 1) has been derived from X-ray diffraction data. The results confirm the expected extensive shielding of the SiFzPh group by the hemispherical TBTS substituent which forces a remarkable widening of the CCSiS-Sil angle [123.21"] of the PhF2Si-Si tetrahedron. The geometry around the central silicon atom Sil is described best as

Synthesis, Structure and Reactivity of Novel Oligosilyl Anions 219

distorted tetrahedral, with a Si2-Sil-Si3 angle of 116.09" and a Si5-Sil-Si2 angle of 101.85'. As expected, most of the Si-Si bonds are slightly elongated within the range of 237-241 pm.

Table 1. 29Si-Nh4R chemical shifts [ppm] for TBTS substituted compounds 1,2 and 5-7.

Compound 6 @Si3) 6 (SiMe) 6 @Me3) 6 (Six3)

TBTS-Br (1) 12.3 4 8 . 7 -9.8 -

TBTS-Li (2) -170.2 -75.6 -12.3 -

TBTS-SiF3 (5) -135.2 -75.4 -9.7 4 4 . 5 (SiF3)

TBTS-SiF2Ph (6) -1 19.0 -74.6 -9.6 5.3 (SiF2Ph)

TBTS-SiH20Ph (7) -117.1 -75.2 -9.8 -14.3 (SiH20Ph)

C7 C16

Fig. 1. Structure of TBTS-SiF2Ph (6) [hydrogen omitted for clarity]; selected bond lengths [A] and angles ["I: Fl-Si5, 1.595(2); F2-Si5, 1.596(2); Sil-Si(S), 2.3535(13); Sil-Si2, 2.4000(13); Sil-Si4, 2.4099(13);

Sil-Si3, 2.4143(13); Si5-C4, 1.859(4); Si5-Sil-Si2, 101.85(5); Si5-Sil-Si4, 108.64(5); Si2-Sil-Si4,

114.98(5); Si5-Sil-Si3, 104.03(5); Si2-Sil-Si3, 116.09(5); Si4-Sil-Si3, 110.06(5); C4-SiS-Si1, 123.21(12);

Fl-Si5-F2, 103.27(15).

A second useful precursor for the synthesis of sterically overcrowded oligosilyl anions is octakis(trimethylsily1)tetrasilacyclobutane (8) [3], a cyclic oligosilane which can easily be converted into HTSB-K (9) by selective cleavage of the Si-SiMe3 bond with tBuOK in THF [4]. After removal of the solvent, a highly moisture- and air-sensitive yellow-orange powder was obtained, which could be identified by NMR spectroscopy as the THF adduct of 9 (Scheme 2).

In comparison with TBTS-Li (2), the interaction of the sterically less demanding HTSB-K (9) with several halosilanes proceeded as smoothly as expected. For example, treatment of 9 with SiF4,

220 H. Reinke, C. Krempner

PhSiF3 and PhSiH2Cl at -78°C in pentane gave the oligosilanes 10-12, respectively, as crystalline materials in excellent yields (Scheme 2) . However, attempts to connect two HTSB groups with a disilane fragment by reaction of 2 equiv. of 9 with ClMezSi-SiMezCl failed. After crystallization of the reaction mixture from acetone only the monosilylated product 13 could be obtained.

HTSB-SiF3 10(89%) + BuOK + SiF4 HTSB-SiMe3 8 - - BuOSiMea

HTSB-SiF2Ph 11 (92 %)

HTSB-SiH2Ph 12 (93 %)

HTSB-SiMe2SiMe$.21 13 (57 %)

Me3Si SiMe3 ' ! SiMe3 Me$% ,'\ Me3SySi\,fSi, = -HTSB

Me3S/ &Me3

Scheme 2. Synthesis and reaction behavior of HTSB-K (9).

The 29Si NMR spectra of the HTSB compounds (Table 2) are more complex, but they show roughly the same features as observed for the corresponding TBTS derivatives. For example, the signal for the anionic silicon atom in 9 (6 = -156.6 ppm) is shifted significantly to higher field in comparison with that of the silyl-substituted HTSB derivatives 10-13.

Table 2. 29Si NMR chemical shifts [ppm] for the ring silicon atoms and the Six3 group of the compounds 9-13.

Compound 6 (ring silicon atoms) 6 (Six31

HTSB-K (9) -156.6, -99.0, -94.5 -

HTSB-SiF3 (10) -133.9, -96.0, -87.9 -44.7 (SiF3)

HTSB-SiFZPh (11) -123.8, -93.0, -87.4 5.4 (SiFzPh)

HTSB-SiH2Ph (12) -106.7, -92.3, -90.3 49 .3 (SiHQh)

HTSB-SiMezSiMezC1 (13) -92.0, -88.9, -83.2 -37.5 (SiMez), 17.1 (SiMezC1)

Conclusions

We have prepared the sterically overcrowded silyl anions TBTS-K, TBTS-Li (2) and HTSB-K (9) which can react with PhSiF3 and SiF4 to give the oligosilanes 5, 6 and 10, 11, respectively, in excellent yields. The X-ray analysis revealed TBTS-SiFZPh (6) to be a space-filling molecule in which the SiFzPh group is strongly shielded by the TBTS substituent. TBTS-SiF3 (5) and HTSB- SiF3 (ll), especially, might be attractive as precursors for low-valent silicon species [5]. Further

Synthesis, Structure and Reactivity of Novel Oligosilyl Anions 221

investigations concerning the synthesis, isolation and structural characterization of such species are in progress.

Acknowledgment: We gratefully acknowledge the support of our work by the Fonds der Chemischen Industrie and we thank Prof. H. Oehme for his generous support.

References [ l ] a) Y. Apeloig, M. Yuzefovich, M. Bendikov, D. Bravo-Zhivotovskii, D. Blaser, R. Boese,

Angew. Chem. 2001,113, 3106; Angew. Chem. Znt. Ed 2001,40, 3016; b) K. Klinkhammer, Chem. Eur. J. 1997,3, 1418; c) N. Wiberg, W. Niedermayer, K. Polborn, Chem. Eur. J. 2002, 12, 2130; d) N. Wiberg, W. Niedermayer, G. Fischer, Eur. J. Inorg. Chem. 2002,5, 1066. S. Chtchian, R. Kempe, C. Krempner, J. Organomet. Chem. 2000,613,208. Y . 3 . Chen, P. P. Gaspar, Organometallics 1982,1, 1410. C. Marschner, Eur. J. Znorg. Chem. 1998,221. R. Pietschnig, R. West, D. R. Powell, Organometallics 2000,19,2724.

[2] [3] [4] [5]

Synthesis of SiH2-Containing Polymers Using Silyl Triflate Intermediates

Wolfram Uhlig

Laboratorium fur Anorganische Chemie Eidgenossische Technische Hochschule Ziirich

ETH-Honggerberg, CH-8093 Ziirich, Switzerland Tel.: 4 1 16333405 - Fax: +41 16321 149

E-mail: uhlig@inorg.chem.ethz.ch

Keywords: polysilylene-phenylenes, pol ysilylene-ethynylenes, silyl triflates

Summary: New synthetic routes to organosilicon polymers containing SiH2 groups and organic x-electron units in the polymer main chain are described. The polymer backbone is formed by condensation of a,mbis(trifluoromethylsulfonyloxy)-substituted organosilicon compounds containing SiH2 groups with the organometallic dinucleophiles Li2C2, Li2C4, and 1,4-BrMg-C6&-MgBr. We could confirm the formation at low temperatures, in short reaction times, and with high yields.

Introduction

Organosilicon polymers have been studied intensively because of the possibility of application in the field of ceramics [l-31. The final ceramic properties are governed to a large extent by the polymer design in the preceramic polymer routes. Criteria for useful preceramic polymers are high ceramic yields, processability, latent reactivity and controlled composition [ 11. Crosslinking is required during the pyrolysis to obtain high ceramic yields and to avoid large amounts of degradation products. Incorporation of aromatic groups can be an alternative route because of the higher thermal resistance. However, aromatic groups in preceramic polymers yield high free carbon contents that are not suitable for high-strength materials. The design of polymers with smaller carbon contents will serve the purpose. The introduction of Si-H groups can lower the free carbon content in the pyrolysis residue, which is essential to prepare silicon-containing polymers with good mechanical properties. The Si-H groups can also be used for further modification of the polymers. Crosslinked polymers can be obtained by platinum-catalyzed hydrosilylation reactions. However, only a few articles on organosilicon polymers containing Si-H functional groups have been reported (reviews: Refs. [4, 51). The pyrolysis of organosilicon polymers having Si-H groups leads to higher ceramic yields than the pyrolysis of those bearing no Si-H bonds. Therefore, we investigated new synthetic routes to organosilicon polymers containing Si-H groups and

Ovganosilicon Chemisty V Edited by N. Auner and J. Weis

Copyright 0 2003 WILEY-VCH Verlag GmbH & Co. KgaA, Weinheim

Synthesis of SiHz-Containing Polymers Using Silyl Trijlate Intermediates 223

unsaturated groups in the polymer main chain. The syntheses based on silyl triflate derivatives [6] are characterized by high regioselectivity and excellent yields.

Results and Discussion

The highly reactive silyl triflates are valuable reagents in organosilicon chemistry [7]. In principle two synthetic routes to new organosilicon polymers based on triflate derivatives are realizable. Firstly, derivatizations can be carried out on finished polymers. Recent papers by Matyjaszewski [8] and by our group [9] have shown the feasibility of this route. We describe here examples for the second synthetic method, which consists of formation of the polymer chains by condensation of a,mbis(trifluoromethylsulfonyloxy)-substituted organosilicon compounds with dinucleophiles [ 101. We prepared the hydrogen-containing a,wbis(silyl triflates) la-lc by relatively simple methods. Normally, the required silyl triflates are obtained from the corresponding phenyl derivatives. Schmidbaur observed that the experiments with p-tolyl and p-anisyl instead of phenyl groups always led to superior yields of pure products with high selectivity under less stringent conditions [ l l , 121. Therefore, the described experiments are based mainly on the p-tolyl- and p-anisyl-substituted precursors. The crude products of high purity are necessary, because the triflate derivatives often cannot be distilled without decomposition. Therefore, they should be used for consecutive reactions without purification. The synthesis of the hydrogen derivatives la-lc [ 13, 141 is shown in Scheme 1.

H H H H H

I 2Li I I 2 TmH I I I I I -2LCI

2 Tol-Si-CI - Tol-Si-Si-To1 - TfO-Si-Si-OTf l a - 2 C6H5-CH3

H H I I

H H H

c c H H H

I u2c2 I 1 PTfOH

H H H H I -2LOTf

2 Tol-Si-OTf - Tol-Si-CEC-Si-To1 - Tfo-Si-C=C-Si-OTf 1 b

I H

I I -2GH&H3 I

Si-OTf Ic

H 1. B N g -@@r - TfO-Si

H

I I

2 Anis-Si-OTf 1. 2TfOH

H

To1 = ptolyl; Anis = panisyl

Scheme 1. Synthesis of compounds la-lc.

We prepared numerous organosilicon polymers containing Si-H groups using la-lc as electrophilic starting materials. The dinucleophilic reactants were mainly organometallic compounds. The reactions of la-lc with 1,4-BrMg-C&-MgBr, Li2C2, Li2C4, and

224 W. Uhlig

PhzSi(OH)Z/NEt3 illustrate the potential of this method. Co-condensations of the three electrophiles with the four dinucleophiles gave 12 different structured polymers 2a-2d, 3a-3d, 4 a 4 . We could confirm their formation at low temperatures, in short reaction times, and with high yields. The preparation of the polymers is summarized in Scheme 2 [ 131.

H

H I n

2a-4a 2b-4b L i C f C L i

H H

TfO-Si-Y-Si-OTf 1 a-1 c I I

I I H H

PhpSi(0H)Z / 2 NEt3

H

H H Ph

2c4c 2d4d

Scheme 2. Synthesis of the polymers 2a-2d (without Y), 3a-3d (Y: C S ) , 4 a 4 d (Y: p-C61-LJ

The structural characterization of 2a4d was mainly based on NMR spectroscopy. *'Si NMR chemical shifts are particularly useful (Table 1). The 'H, I3C, and 29Si NMR spectra of all the compounds are consistent with the proposed structures of the polymer chain. As expected, one observes relatively broad signals, which are typical for organosilicon polymers. However, the half-bandwidths of the 29Si NMR signals, 1.5-3.0 ppm, are much narrower than those in the case of polysilanes and polycarbosilanes prepared by Wurtz reactions. The narrower signals of 2a4d indicate the regular alternating arrangement of the building blocks in the polymer backbone resulting from the fact that the condensation reactions are not accompanied by exchange processes analogous to metal-halogen exchange. Weight-average molecular weights in the range of M, = 10 000-20 000, relative to polystyrene standards, were found by GPC. They correspond to polymerization degrees of n = 100-150. The polydispersities (Mw/Mn) were found in the range 2.3-3.1. It must be emphasized that the molecular weights are determined by the reaction conditions. Higher values of M , were obtained using more concentrated solutions of the reactants. The exact compliance with the stoichiometric ratio of 1:l is another important requirement. It is therefore necessary to determine the content of the organometallic compounds quantitatively before use. However, the molecular weight can also fall below 5000 when diluted solutions are used. Other changes, such as the use of different solvents and reaction temperatures, are currently being investigated. Thus, the molecular weights reported in Table 1 are those found under the conditions specified in the experimental section of Ref. [ 131.

Synthesis of SiH2-Containing Polymers Using Silyl Trijlate Intermediates 225

Table 1. 29Si NMR data and molecular weights of the polymers 2 a 4 .

H H I

H H n

H

X

-EC- 28: S "Si: -76.8 ppm &: 11500; PD: 2.6

+=c+ 2b: S "Si: -74.5 ppm &: 14600; PD: 2.7

+ 2c: Sz8Si: -61.5ppm

&: 14300; PD: 2.8

Phz - 0 - S r V 2d: S "Si: -53.9;43.4 ppm

A&: 19300; PD: 2.6

a: 6%: -80.1 ppm &: 12400; PD: 2.3

3b: 6 29Si:-78.0 ppm &: 16100; PD: 2.9

3 ~ : s 29~i:-~7.8 ppm A&: 14100; PD: 2.5

3d: s 2g~i:-49.9; -44.0 pprn &: 1MW: PD: 2.6

48: 6 "si: 4 8 . 0 ppm M,,: 17800; PD: 2.9

4b: s zg~i: -59.2 ppm M,,: 18800; PD: 2.6

4 ~ : s 29si:-41 .a ppm 4: 22500; PD: 3.1

4d: s "si: -35.0; 4 . 8 ppm M,,: 21300; PD: 3.0

Acknowledgment: This work was supported by the ETH Zurich, by Schweizer Nationalfonds zur Forderung der Wissenschaften, by Wacker-Chemie GmbH, Burghausen, and by Siemens (Schweiz) AG. Furthermore the author thanks Prof. R. Nesper for support of this investigation.

References [l] M. Birot, J. P. Pillot, J. Dunogubs, J. Chem. Rev. 1995,95, 1443. [2] M. Jansen, H. P. Baldus, Angew. Chem. 1997,109,338. [3] R. Riedel, A. Kienzle, W. Dressler, L. Ruwisch, J. Bill, F. Aldinger, Nature 1996,382,796. [4] L. V. Interrante, Q. Liu, I. R. Rushkin, Q. J. Shen, J. Organomet. Chem. 1996,521, 1. [ 5 ] W. Uhlig, Progr. Polym. Sci. 2002,27,255. [6] W. Uhlig, J. Prakt. Chem. 2000,342, 132. [7] W. Uhlig, Chem. Ber. 1996,129,733. [8] K. Matyjaszewski, H. K. Kim, Y. L. Chen,ACS Symp. Ser. 1988,360,78. [9] W. Uhlig, Syntheses, Functionalization, and Crosslinking Reactions of Organosilicon

Polymers in: Solid State Organometallic Chemistry (Eds. M. Gielen M, R. Willem, B. Wrackmeyer), John Wiley, Chichester, 1999, p. 397.

[ 101 W. Uhlig, Organometallics 1994,13,2843. [ l l ] M. Soldner, A. Schier, H. Schmidbaur, J. Organomet. Chem. 1996,521,295. [12] C. Rudinger, H. Beruda, H. Schmidbaur, Chem. Ber. 1992,125,1401. [13] W. Uhlig, Silicon Chemistry, 2002,1, 129. [ 141 W. Uhlig, Z. Naturj-orsch. Teil B 1999,54,270.

Silicon Compounds with Geminal Donor Centers

Norbert K Mitzel, Krunoslav Vojinovic, Udo Losehand

Anorganisch-chemisches Institut, Technische Universitat Miinchen Lichtenbergstr. 4, 85747 Garching, Germany

(present address: Institut f i r Anorganische und Analytische Chemie, Westfalische Wilhelms-Universit Miinster, Wilhelm-Klemm-Str. 8,48 149 Miinster, Germany)

Tel.: +49 251 83 36006 - Fax: +49 251 83 36007 E-mail: MitzelBuni-muenster.de

Keywords: hypercoordination, three-membered rings, hydroxylamine, hydrazine, phase-dependent structures

Summary: Small model compounds with a nitrogen donor h c t i o n in geminal position to a silicon atom have been prepared, including F3SiONMe2, F3SiCH2NMe2, F3SiNMeNMe2, CCl3SiNMeNMe2, U!1H2SiNMeNMez, tF3SiN(SiMe3)NMe2, Bnd F3SiN(SnMe3)NMe2. Strong phase-dependent interactions between Si and N atoms in F3SiONMe2 (three-membered SiON ring) have been determined by single-crystal X-ray crystallography, combined NMWtheoretical studies and gas-phase electron diffraction. Crystallographic examination of the hydrazine compounds showed the interactions to become stronger in the series Cl3SiNMeNMe2 < ClHzSiNMeNMe2 < F3SiNMeNMe2, F3SiN(SnMe3)NMe2 < F,SiN(SiMes)NMez.

Silicon compounds with geminal donor functions show a range of interesting reactivities such as the silylhydroxylamine-based nitrene generators R3SiN(R)OSiR3 [ 13 or the a-fluoromethylsilanes, which form fluorosilanes with extrusion of carbenes [2]. Other, more applied examples from silicon chemistry, where intriguing reactivities could be rationalized with the presence of geminal donor functions, are the highly active silicone cold-curing catalysts based on hydroxylaminosilanes [3] and the alcoholysis reaction of Si-H functional polysilanes catalyzed by N,N- dialkylhydroxylamines [4].

In the light of these facts it seems surprising that so little is known about the structural chemistry of simple but representative systems containing silicon and a geminal donor function. A few years ago we started investigating three classes of compounds in more detail, hydroxylaminosilanes with Si-0-N units, hydrazinosilanes with Si-N-N units and aminomethylsilanes with S ix -N units. During these investigations it turned out that attractive interactions between the Si and N atoms are present and structure-determining. The strengths of these interactions are dependent on various parameters, including the atom linking the Si and N functions, the electronic requirements of the substituents at Si and N, the orientation of these substituents relative to the Si-X-N plane

Ovganosilicon Chemisty V Edited by N. Auner and J. Weis

Copyright 0 2003 WILEY-VCH Verlag GmbH & Co. KgaA, Weinheim

Silicon Compounds with Geminal Donor Centers 227

(conformations) and the medium and its dielectric constant which surrounds the compounds. Besides the SiON compounds [ 5 ] , we have also investigated those with GeON and SnON units [6]. The strongest interactions leading to three-membered SiON rings with hypercoordinate silicon have been found in ClH2SiONMe2 [7] (prepared from H2SiClz and LiONMe2) and F3SiONMez [8] (prepared from SiF4 and LiONMez).

0 m

Fig. 1. Molecular structure of F3SiONMe2 in the solid state.

The molecular geometry of F3SiONMe2 is markedly dependent on the polarity of the surrounding medium. The SiON angle is smallest in the crystal (77.1"), as was shown by low-temperature X-ray crystallography of single-crystal grown in situ. This angle is larger in the gas-phase (94.1"), as determined by electron diffraction. The respective Si...N distances are 1.963(1) and 2.273(17) A. This structural difference between the phases finds its parallels in many donor-acceptor adducts such as the amine-borane adduct, which also have shorter B-N distances in the solid state than in the gas phase [9].

We have also studied the geometry of F3SiONMe2 in CsD6 solution, which is a medium having an intermediate dielectric constant between the vacuum (gas-phase structure) and the highly polar solid state. This was done by the ab-initio/IGLO/NMR method and the theoretical part was carried out by D. Cremer and A. Wu (University of Goteborg, Sweden). The result of this investigation was a valence angle at oxygen for F3SiONMe2 of 87.1", which is also between the solid-state and the gas-phase value, demonstrating the extreme dependence of the molecular structure of F3SiONMeZ on the medium surrounding it.

In order to investigate the possibility of Si.0.N interactions between such geminal atoms in

compounds containing the SiCN linkage, we carried out model calculations on FHzSiCH2NMez up to the MP2/6-311G(d,p) level of theory. The results are depicted in Fig. 2. These calculations predict the angle SiCN (and distance Si...N) to be markedly dependent on the conformation of the molecule, while the three conformations investigated differ only marginally in energy. This would make it possible that the intermolecular forces in a crystal lattice could override this energy barrier and favor the Cs transition state due to its higher dipole moment leading to stronger forces in the crystal. As was shown above for F3SiONMe2, the interaction with other highly polar molecules could strengthen the Si...N interaction even more.

Experimentally we came closest to the model compound FHzSiCHzNMe2 with the triply fluorinated F3SiCH2NMez. CLSiCH2NMe2 was prepared from [HzC=NMe2]Cl under the conditions

228 N. W. Mitzel, K. Vojinovic, U. Losehand

of a Benkeser reaction with Cl3SiH and NEt3 (Eq. l), then fluorination carefully with SbF3 (Eq. 2). It should be noted that our sample of Cl3SiCH2NMe2 had different spectroscopic properties from those described earlier in the literature [lo]. We could confirm our data with a crystal structure determination of Cl3SiCH2NMe2.

Symmetry C, Cl C, (transition state) F-Position anti gauche anti

LSiCN 101.0" 112.8' 94.4" zlp,SiCN 2 1 .O' 38.5' 0.0" E [kJ/mol] 0.0 0.3 0.8 P[D] 2.74 2.24 3.16

Fig. 2. Calculations on different conformers of FH2SiCH2NMez up to the MP2/6-3 1 lG(d,p) level of theory.

[H2C=NMez]Cl + HSiCl3 + NEB - [HNEbICl + C13SiCH2NMe2

Eq. 1.

Cl3SiCH2NMe2 + SbF3 - SbC13 + F3SiCH2NMe2

Eq. 2.

The fluorination also produces SiF4 in the course of the reaction, which is very difficult to separate from the desired product, as it forms a 1:2 adduct, which could be obtained in crystalline form and therefore structurally elucidated by X-ray diffraction. The result is shown in Fig. 3.

Expectedly the wide angle Si-C-N proves the absence of any attractive force between the F3Si group and the N atoms, because these N-functions are already coordinated to the SiF4 unit. Crystallization experiments with the SiF4 free compound have not been successful so far, but a determination of the gas-phase structure of F3SiCH2NMez by electron diffraction is in progress.

Our search for Si...N attractive interactions in Si-N-N units led us earlier to investigate the simple systems H3SiMeNNMez and (H3Si)2NNNMez, where we found very weak attractive interactions between the geminal Si and N atoms [ 111. We intended to study other systems with electronegative substituents bound to the Si atom, namely C1 and F substituents.

Using the reactions represented by Eqs. 3 and 4, we prepared ClH$3NMeNMez and F3SiNMeNMez.

Silicon Compounds with Geminal Donor Centers 229

Fig. 3.

Eq. 3.

Eq. 4.

F

7 Molecular structure of [F3SiCH2NMe2 I2SiF4 as obtained by low-temperature X-ray crystallography.

Important bond lengths and angles are: SiCN6-F 1.648(1E1.655(1), Sim4-F 1.572(1~1.589(1),

SiCN6-N 2.000(1), SicN4-c 1.855(1), c-N 1.501(1) A; sic~4-C-N 121.1(1)0.

C12H2Si + LiNMeNMez - LiCl + ClH2SiNMeNMez

F4Si + LiNMeNMez - LiF + F3SiNMeNMe2

The latter reaction is not completely selective and also yields FzSi(NMeNMe2)2; both can be separated by fractional condensation through a series of cold traps.

The compounds were identified by multinuclear NMR spectroscopy ('H, I3C, "N, I9F and 29Si), by mass spectrometry and by gas-phase IR spectroscopy. Particularly informative were the triplet of triplets splitting of the 29Si signals at -33.5 ppm for ClHzSiNMeNMez ('J= 261.7, 3 J = 3.1 Hz) and the quartet in the 29Si NMR spectrum of F3SiNMeNMez ('J = 193.8 Hz) centered at -98.4 pprn (Fig. 4) and the occurrence of two signals in the "N NMR spectra of both compounds, at -300.8 and -3 18.9 pprn for ClHzSiNMeNMez and -3 11.5 and -326.4 ppm for F3SiNMeNMez.

Single crystals of both low-melting compounds (m.p. ClHzSiNMeNMe:! -73 OC, m.p. FsSiNMeNMez -52 "C) could be grown in situ on the diffractometer in sealed Duran@ capillaries and thus made it possible to determine the structures of these compounds in the solid state which are displayed in Fig. 5.

In both structures the Si-N-N angles are much smaller than the expected 120" for a planar-coordinate N atom. However, the Si-N-N angles of 104.1(1>0 for F3SiNMeNMez and 109.9(1)" for ClHzSiNMeNMez are still much larger than the corresponding valence angles at oxygen in ClH2SiONMe2 (79.7(1)") and F3SiONMe2 (77.1(1)").

230 N. W. Mitzel, K. Vojinovic, U. Losehand

I 7 I

-75 -100

Fig. 4. "Si NMR spectra of F2Si(NMeNMe2)* (left) and F3SiNMeNMe2 (right).

F121

Fig. 5. Crystal structures of one of the two independent molecules of CIHzSiNMeNMez (left) and of F3SiNMeNMe2

(right). Important bond lengths and angles are: C1H2SiNMeNMe2 Si-Cl 2.078(1), Si-N 1.686(2), N-N

1.434(2) A; Si-N-N 109.9(1), Si-N-C 127.3(1), N-Si-CI 113.5(1)0; and F3SiNh4eNMe2Si-F

1.566(2t1.571(1), Si-N 1.644(1),N-N 1.441(1) A; Si-N-N 104.1(1), C-N-N 120.7(1)0.

In contrast to the hydroxylamines, the hydrazines open up the possibility of a modification of the substituent at the a-nitrogen atom and thus allow a variation of the electronic situation at this center. Our intention was to attach an electropositive group to this N atom instead of the methyl group. We succeeded in doing so with the preparation of F3SiN(SiMe3)NMe2 and F3SiN(SnMe3)NMe*, which were obtained from the reactions shown in Eqs. 5 and 6.

F4Si + LiN(SiMe3)NMez - LiF + F3SiN(SiMe3)NMe2

Eq. 5.

0 Silicon Compounds with Geminal Donor Centers 231

F4Si + (Me3Sn)zNMez - Me3SnF + &N(SnMe3)NMez

Eq. 6.

The latter reaction, starting with the doubly stannylated (Me3Sn)~NMez [12], leads only to a single substitution if conducted at low temperatures. However, some side products occur, as indicated by unidentified peaks in the spectra. Moreover, the compound is not stable at ambient temperature and slowly decomposes giving less volatile products.

Both compounds were identified by means of multinuclear NMR spectroscopy ('H, 13C, I9F) and gas-phase IR spectroscopy. F3SiN(SiMe3)NMez was hrther characterized by "N and 29Si NMR and by mass spectrometry.

The 15N{'H} NMR spectrum of F3SiN(SiMe3)NMe2 is shown in Fig. 6 and contains two well resolved quartets, one caused by a 'JNF coupling, the other by a 3 J ~ ~ coupling.

-90 2 -280 . -2- 6 -3,- a -3,n 50 -3LB 7 5 , -

Fig. 6. Two sections kom the '% NMR spectrum of F3SiNMeNMe2. The left quartet corresponds to the doubly

silylated N atom, the right to the N of the. NMe2 group.

Both compounds, FsSiN(SiMe3)NMez and F3SiN(SnMe3)NMez, could be turned into single crystals by in-situ crystallization techniques. The results of the structure determinations by X-ray diffraction are presented in Fig. 7.

The structural data show that substitution of the a-methyl group in F&N(Me)NMez by the more electropositive substituents SiMe3 and SnMe3 leads to a dramatic strengthening of the attractive interaction between the Si and geminal N atoms. The SiMe3 group is in this respect the most effective substituent of the two, and leads to a valence angle of the a-N atom of only 83.6(1)", while with the SnMe3 group a larger angle of 89.6( 1)" can be achieved. This corresponds to geminal distances Si-N of 2.102(1), and 2.204(2) A. Although these values have to interpreted as a weaker interaction between the geminal donor and acceptor centers than in F3SiONMe2, this is an unequivocal proof of the existence of pronounced interactions of this type in silylhydrazines.

232 N. W. Mitzel, K, Vojinovic, U. Losehand

F F

Fig. 7. Molecular structures in the solid state of F3SiN(SiMe3)NMe2 and F3SiN(SnMe3)NMe2. Important bond lengths and angles are: F3SiN(SiMe3)NMe2 Si-F 1.577(1)-1.591(1), SiFN 1.662(1), SicN 1.758(1), Si...N

2.102(1), N-N 1.487(1) A; SiF-N-N 83.6(1), Sic-N-N 130.0(1), Si-N-Si 145.9(1)”; and F3SiN(SnMe3)NMe2

Si-F 1.574(2k1.582(2), Si-N 1.651(2), Sn-N 2.078(2), Si...N 2.204(2), N-N 1.472(3) A; Si-N-N 89.6(1),

Si-N-Sn 141.2(1), Sn-N-N 128.4(1)0.

Acknowledgments: We are grateful for support through Deutsche Forschungsgemeinschafi, Fonds der Chemischen Industrie and the Leonhard-Lorenz-Stiftung.

References

[l] [2]

[3]

Y. H. Chang, F.-T. Chiu, G. Zon, J. Org. Chem. 1981,46,352. a) R. N. Haszeldine, J. C. Young, J . Chem. SOC. 1959, 394; b) H. Beckers, H. Biirger, J. Organomet. Chem. 1990,385,207. A collection of patent literature references can be found in: M. G. Voronkov, E. A. Maletina, V. K. Roman, Heterosiloxanes, Vol. 2: Derivates of Nitrogen and Phosphorus, Hanvood Academic Publishers, Chur, Switzerland, 1991.

[4] Y. Hamada, S. Mori, Proceedings of the 29th Organosilicon Symposium, March 1996, Evanston, USA, 1996.

[5] N. W. Mitzel, U. Losehand, Angew. Chem. Znt. Ed. 1997,36,2807. [6] CN. W. Mitzel, U. Losehand, A. Richardson, Organometallics 1999,18,2610. [7] N. W. Mitzel, U. Losehand, J. Am. Chem. SOC. 1998,120,7320. [8] N. W. Mitzel, U. Losehand, A. Wu, D. Cremer, D. W. H. Rankin, J. Am. Chem. SOC. 2000,

122,4471. [9] K. R. Leopold, M. Canagaratna, J. A. Phillips, Acc. Chem. Res. 1997, 30, 57. [ 101 A. Tzschach, W. Uhlig, K. Kellner, J. Organomet. Chem. 1984,266, 17. [l 13 N. W. Mitzel, Chem. Eur. J. 1998,4,692. [12] N. Wiberg, M. Veith, Chem. Ber. 1971,104,3191.

Cyclic Silylhydrazines - Synthesis, Isomerizations, and Quantum

Chemical Calculations

Uwe Klingebiel

Institut fur Anorganische Chemie, Georg-August-Universitat TammannstraBe 4, D-37077 Gottingen, Germany Tel.:+49 551 393052 - Fax:+49 551 393373

E-mail: uklinge@gwdg.de

Stefan Schmatz

Institut fur Physikalische Chemie, Georg-August-Universitat Tammannstral3e 6, D-37077 Gottingen, Germany

E-mail: sschmat@gwdg.de

Keywords: silylhydrazines, rings, isomerizations, expansions, transition states, density functional theory

Summary: The synthesis and isomerization reactions of acyclic and cyclic silylhydrazines and silylparazolones are described. Topics of the discussion are: 1) the formation of the Si-form of silylpyrazolones; 2) the "side on" and "end on" coordination of lithium in silylhydrazides; 3) the expansion of the three-membered Si(SiN2) ring to the four-membered (SiN)2 ring by lithiation of the (SiN)2 ring and by thermal silyl group insertion into the N-N bond; 4) the expansion of a three-membered (SiN2) ring to a five-membered (CSi2N2) ring by insertion of SiCH2 into the Si-N bond; 5) the formation of isomeric four- and six-membered silylhydrazine rings; 6) the expansion of a five-membered (N2Si2N)N ring to the isomeric six-membered (SiNN)2 ring. The mechanisms of the isomerization are elucidated by quantum chemical calculations, and the molecular structures are verified by crystal structure determinations.

Introduction

The syntheses of the first acyclic and cyclic silylhydrazines were reported by Aylett and Wannagat in 1956-1958. Two main methods of preparing them were developed (Scheme 1). The first (route a) is the treatment of a hydrazine with a halosilane and the second (route b) the treatment of a lithiated hydrazine with a halosilane.

Ovganosilicon Chemisty V Edited by N. Auner and J. Weis

Copyright 0 2003 WILEY-VCH Verlag GmbH & Co. KgaA, Weinheim

234 U. Klingebiel, S. Schmatz

Me$/ H + BuLi + ClSiR, . BuH

Silylhydrazines condense very easily. The tendency to condense depends on the bulkiness of the substituent. For example: the condensation product of tert-butyldimethylsilylhydrazine can be isolated as the mono-, the trimethylsilylhydrazine as the bis-, and the dimethylsilylhydrazine as the tris(silyl)hydrazine, and the methylsilylhydrazine condenses with formation of the tetrakis(sily1)hydrazine ( M ~ ~ C S ~ M ~ ~ N H - N H Z , Me3SiNH-NHSiMe3, (Me2HSi)zNNHSiHMe2, (MeH2Si)zN-N(SiHzMe)2, respectively).

. LiCl

Hal H H Hal H H I I I \ I I I

a) -Si- + IN-NI - I I I H H

SMe3 / + H\

+ N,H, - N2H4. HHal 1

- LiF I I

b) -Si-F + LNH-NH2

Scheme 1.

In the absence of strong steric or electronic constraints, the bis(organosily1)hydrazines give essentially equal amounts of the N,N and N,N' isomers at equilibrium (Scheme 2).

Me3SiC1 *I Scheme 2.

Tris- and tetrakis(sily1)hydrazines were obtained by the reaction of lithium derivatives with halosilanes. Starting with either isomer, the expected and the rearranged products were isolated. Bailey and West discovered in 1964 that organosilicon groups migrate from one nitrogen to the other in anions of silicon-substituted hydrazines. In order to understand the silyl group migration we

Cyclic Silylhydrazines - Synthesis, Isomerizations, and Quantum Chemical Calculations 235

isolated anions of silylhydrazines and found in lithium derivatives “side on” and “end on” coordination of the lithium cation to the N-N bond. For example, the lithium derivative of di-fert- butylmethylhydrazine crystallizes as a hexamer with two tautomeric silylhydrazide units I and I1 (Scheme 3).

Scheme 3.

-Si-N-NHf + - [ 1 H ,I’ -Si-N--

4 +

This phenomenon accounts for the isomerizations during secondary substitutions. Monosilylhydrazines react with aldehydes and ketones to hydrazones. Even formaldehydehydrazone can be isolated (Scheme 4).

/ \

R-NH-NH2 + O=C

1 R

R\ / R

/ \ //”

H ‘R M-N=C C H r C N-N=C

R\

‘OR“

R = Silyl, R , R” = H, Alkyl, Aryl

Scheme 4.

Five-membered aromatic heterocycles can be prepared from silylhydrazines and ethyl acetate (Scheme 5). Condensation of ethanol leads to pyrazolones. Starting with organohydrazines, for example, with phenylhydrazine three isomeric rings are known. The CH form is isolated in nonpolar solvents. In water 90 % of the NH form with 10 % of the OH form exist in an equilibrium. The hitherto unknown Si form could be isolated by starting with silylhydrazines.

Dr. Stefan Schmatz from Gottingen carried out quantum chemical calculations in order to understand the rearrangement process. The ring closure occurs with formation of the CH form. Migration of a hydrogen from the CH2 to the C=O group leads to the OH form via the saddle point TS1 (Fig. 1). Rotation of the C=O group to the SiMe3 group via saddle point TS2 leads to compound 3. Hydrogen migration from carbon to carbon via TS3 to 4 and from carbon to nitrogen

236 U. Klingebiel, S. Schmatz

A

via TS4 gives the most stable isomer, the Si form, compound 5. The NH form would be the result of silyl group migration from oxygen to nitrogen.

-EtOH

R \

M-N=C C H r C

CH Form OH Form NH Form Si Form

Scheme 5.

100 -

80 -

60 -

- f 40- 8

20 - Y

e 0-

- L

@

OH form CH form NH form -60

reaction coordinate

Fig. 1. Isomerization of a silylpyrazolone.

Back to the lithium silylhydrazides: lithium can be coordinated “side on” and “end on” to the N-N bond. The coordination depends on the silyl substituent. For example: Monolithium

Cyclic Silylhydrazines - Synthesis, Isomerizations, and Quantum Chemical Calculations 237

derivatives of bis(sily1)hydrazines crystallize as N,N-bis(sily1)hydrazines or as N , N - bis(silyl)hydrazines, for example as shown in Fig. 2.

Ph Ph I I

Li SiMe2

MeZSi ’ ‘Li/ H ‘SiMez

H\ / \ /

N-N N,-N

I Ph

I Ph

Fig. 2. Crystallization products of bis(sily1)hydrazine monolithium derivatives.

The lithium derivative of bis(dimethylphenylsily1)hydrazine crystallizes as N,N-bis(sily1) -hydrazide dimer. In this compound only end-on coordination is found for the lithium ions.

Dilithium salts of bis(silyl)hydrazines, which could be isolated as monomers, dimers, trimers, and tetramers in the solid state, are excellent precursors of fluorofunctional rings. These molecules have a twist conformation (Fig. 3). The endocyclic Si-N bonds are much shorter than the exocyclic ones (170 pm, 179 pm).

I I y-”\

I t

\ / + SIP N-N

L a L l I I -2L.1; - F+f /s”2 N-N

Fig. 3. Cyclization of bis(sily1)hydrazine dilithium salts.

238 U. Klingebiel, S. Schmatz

Salt elimination from lithiated N-silyl-N'-fluorosilylhydrazines leads to the formation of three-, four-, five- or six-membered rings, depending on the bulkiness of the substituents (Scheme 6).

bulky substituents

Scheme 6.

small sustituents

We obtained a crystal structure of a three-membered BN2 ring with the longest N-N bond (168 pm) measured so far. This explains the instability of three-membered SiNz rings. Above room temperature ring cleavage occurs and a diaminosilane is formed (Scheme 7). In this case the hydrogen comes from the butyl substituent. We characterized the diaminosilane as its dilithium salt (Fig. 4).

Li Li

\ /

H I I , R-N N-R R H NR RN

+2RLi Si \ / Si

/ \

N-N R \

-2RH- / \ Me2HC CqH7

\ / - Me2HC CqH7 Si

/ \ Me2HC C g 9

Scheme 7.

A u Fig. 4. Structure of the diaminosilane formed in Scheme 7.

Si(1)-N(1): 169.9 pm Li-N: 195.5 - 198.3

An excess of BuLi in the preparation of the three-membered ring also leads to the cleavage of the

In reactions with difluorosilanes, e.g. MezSiF2, cyclodisilazanes are formed. If there is no other N-N bond (Scheme 8).

Cyclic Silylhydrazines - Synthesis, Isomerizations, and Quantum Chemical Calculations 239

compound to react with the three-membered ring an unknown ring expansion occurs. For example, the SiFz-containing three-membered ring forms a cyclodisilazane by the mechanism of Scheme 9.

R L i L i Me Me R R N-N I 1 R\

Scheme 8.

Me$ Ph Me$, ,Ph Ph, ,CMe3

R ' / Si.

N-SiFz : 170.2pm N-SiPhz : 174.4 pm

Scheme 9.

The reductive insertion of a silyl group into the N-N bond and migration of a phenyl group from the silicon to a nitrogen atom which occurred are both unprecedented so far.

In order to understand the formation of the four-membered ring from the three-membered ring in a unimolecular isomerzation process, quantum chemical calculations were carried out for the N,N- bis(trimethylsily1)hydrazine derivative (Scheme 10). The three-membered ring is planar. The SiMe3 groups are tilted out of the SiN2 plane with a dihedral angle of 115.8'. The nitrogen atoms show sp3 hybridization. The most important structural parameters are calculated as follows: r(N-N) = 173.6 pm, a(N-Si-N) = 62.1, r(FzSi-N) = 168.4 pm, r(Me3Si-N) = 177.9 pm, a(N-N-SiMe3) = 114.5'.

Scheme 10.

F2 Si

F2

In summary, the unimolecular rearrangement process can be described as follows. The reaction starts with the stretching and eventually breaking of the strained N-N single bond. Measured from

240 U. Klingebiel, S. Schmatz

the reactant side, the barrier height amounts to 34.0 kcal mol-'. Thus, the energy needed for the N-N bond cleavage is relatively small. The process is compensated by the energy gain through the delocalization of one of the two N-N bond electrons along the MesSi-N-SiFz structure on the nonreactive side. This explanation is supported by the small deviation from linearity for this structure and the fact that both Si-N bonds are shortened by more than 8 pm. At the transition state a triangle is formed. Thus, the rate-determining simultaneous motion is the fission of the S i x bond and the formation of the C-N bond - in summary, a methyl group transfer. After formation of the C-N bond, the SiMe2 moiety swings inward and recombines with the unsaturated nitrogen atom. As usual for radical pair recombinations, no activation energy is required. Within the error bars of both theory and experiment the calculated geometry of the four-membered ring agrees nicely with the structure obtained by X-ray diffraction. The energetic difference, AE, between the rings is calculated to be 74.7 kcal mol-'.

From a 1,2-Diaza-3-silacyclopropane to a 1,2-Diaza-3,5-disilacyclopentane

Ring Expansion with Insertion of a SiCHz Unit into the Si-N Bond

Another unexpected reaction has been observed in the case of the LiF elimination of lithiated N,N'- bis(di-tert-butylmethylsilyl)-N-difluorosilylhy~azines. The 1,2-diaza-3-silacyclopropane is transformd into a 1,2-diaza-3,5-disilacyclopentane via silylmethylene group insertion and subsequent protonation of one of the nitrogen atoms (Scheme 11).

Me$, ,CMq Si R H

\ / N-N

Si, ,Si(CMe2)2

R', ' \

\ /

Me$, /CMe3

R, ,Si, N-N CH3 - R, I I N-N C H ~ +BuLi

/ \ -xi? S', F/ - LiF

R F H2 ,SF2 H

R

a, b R = Ph (a), N(SiMe,), (h): R = Si(CMe,)*Me

Scheme 11.

Quantum Chemical Study of SiCHz Insertion into the Si -N Bond

The formation of the diazadisilacyclopentane from a three-membered SiN2 ring (Scheme 12) was studied by means of quantum chemical calculations for the system (Me3SiN)2SiH2 [l]. A bimolecular reaction mechanism is possible, but experimental results give clear evidence that the reaction proceeds in a unimolecular way since no cross over products could be detected. It was calculated that the N-N bond length in the three-membered ring is 162.4 pm. The geometrical parameters from the calculation and the crystal structure show also good agreement. The calculated reaction enthalpy A~H"(298 K) amounts to -23.1 kcal mol-'.

Cyclic Silylhydrazines - Synthesis, Isomerizations, and Quantum Chemical Calculations 241

Me3Si, /SMe3

N;;N Sir N-N: 162.4pm

H' H

TS 1

TS 2

Scheme 12.

The reaction pathway from the three-membered ring to the five-membered ring (Fig. 5) is more complicated. Two transition states (TS1 and TS2) are involved.

TS2 TSI

1 diazasilacyclopropane -20

diazadisilacyclopentane -40

reaction coordinate

Fig. 5. Reaction pathway from diazasilacyclopropane to diazasilocyclopentane.

242 U. Klingebiel, S. Schmatz

In summary, the isomerization process can be described as follows: First, one of the trimethylsilyl groups swings inward. A bond is formed between one of the carbon atoms and the silicon atom of the ring, resulting in a bicyclic structure. The silicon-methyl bond breaks with subsequent methyl group transfer and a stable dimethylsilahydrazone is formed. The transferred methyl group now attacks the silicon atom of the iminosilyl group and a transition state containing a five-membered ring is formed. Finally, hydrogen migration occurs from the methyl carbon atom via the silicon atom to the nitrogen atom and the diazasilacyclopentane is produced.

From Lithium Triorganylsilyl Fluoro Diorganylsilylhydrazides to Four- and Six- Membered Silylhydrazine Ring Systems

In some cases sterically encumbered lithium derivatives exhibit similar chemical behavior to that of some lithium fluorosilylamides. Treatment with Me3SiC1 does not afford the expected substitution product, but leads to a fluorine<hlorine exchange at the silicon atom and lithium N-silyl-N- chlorosilylhydrazides are formed (Scheme 13). These react with LiCl elimination to produce dimeric silahydrazones. Isomers of cyclic silylhydrazines can be isolated.

ii.’ F

A -LiF

R

I H \ / N-N

/ \

\ / N-N

\ R’ H

R#i SRZ

Chair-Conformation

N-N: 148pm X HN(N)Si : 349.9

R = CMq, R’ = SiMe2CMe3

A -LiCl I R2

Si H

Si

R\ / \ ,

H’ \ / R N-N N-N,

R2

N-N: 144.4pm X Si2N-N : 350.1 O

R = CHh4e2, R = SiMe(CMe&

Scheme 13.

Cyclic Silylhydrazines - Synthesis, Isomerizations, and Quantum Chemical Calculations 243

Expansion of 1,2,4-Triaza-3,5-disilacyclopentane to 1,2,4,5-Tetraaza-3,6-disila- cyclohexane - Insertion of an NH Group into an Si-N Group

Five- or six-membered ring systems are obtained from lithiated monofluorosilylhydrazines. For instance, bis(diisopropy1amino)fluorosilylhydrazine yields the five-membered ring system according to Scheme 14. When the five-membered ring (Fig. 6a) is either heated to 180" C for a short period or dissolved in boiling n-hexane, the isomeric six-membered ring (Fig. 6b) is formed at ambient temperature.

I I -Si-NH-NH + -Si-N-NHz

I '\ / I '\ / F 'Li F L i

A -LiF

?-I

Scheme 14.

C

Fig. 6. Structures of a) 1,2,4-triaza-3,5-disilacyclopentane; b) 1,2,4,5-tetraaza-3,6-disilacyclohexane.

244 U. Klingebiel, S. Schmatz

Quantum Chemical Study of The Isomerization from a Triazadisilacyclopentane to a Tetraazadisilacyclohexane

The asymmetric five-membered ring is not planar. The exocyclic amino group and the ring form angles of 118.7' and 127.9", respectively. The H-N bonds are almost equal in length at 101.6 pm (ring) and 102.1 pm (amino group), respectively.

The most stable conformation of the six-membered ring is the twist form, in agreement with the experimental result. It is more stable by 3.7 kcal mol-' than the five-membered ring (Fig. 7). A comparison of the calculated geometry with the X-ray structure data shows good agreement.

TS 8o 1

reaction coordinate

Fig. 7. Reaction pathway from triazadisilacyclopentane to tetraazadisilacyclohexane (twist form).

The mechanism of the isomerization can be described as follows. First, the amino group turns towards the N and the Si atoms, yielding a three-center structure (Scheme 15). The attack on the silicon atom is nucleophilic. The Si-N bond of the five-membered ring breaks heterolytically.

H H \ I

N \ ' 3 Megi' SMe2

\ I / N-N,

H H

H H / '\ /

N-N

N-N. I I

H H

7-"\ Me@ \ ?Me2

N-N H H

H H TS

Scheme 15.

Cyclic Silylhydrazines - Synthesis, Isomerizations, and Quantum Chemical Calculations 245

Finally, a proton migrates from the NH2 group to the lone electron pair of the N atom. The corresponding transition state TS is definitely the highest point of this isomerization reaction.

Acknowledgments: We gratefully acknowledge the Fonds der Chemischen Industrie and the Deutschen Forschungsgemeinschaft for finacial support during this work. Most of the calculations were carried out on workstations of the Gesellschaft fiir Wissenschaftliche Datenverarbeitung Gottingen (GWDG).

Reference [ 11 S . Schmatz, Organornetallics 2002,21, 864.

Silylhydroxylamines - Synthesis, Isomerisation, and

Quantum Chemical Calculations

Christina Ebker, Friedhelm Diedrich, Stefan Schmatz, Uwe Klingebiel

Institute of Inorganic Chemistry, Georg August-University Gottingen Tammannstr. 4, D-37073 Gottingen, Germany

Tel.: +49 551 39 3052 -Fax: +49 551 39 3313 E-Mail: uklinge@gwdg.de

Keywords: silylhydroxylamines, rearrangement, calculations

Summary: Silylhydroxylamines were able to undergo anionic, neutral and thermal rearrangements. Lithium derivatives of silylhydroxylamines were formed in the reaction of N,O-bis(sily1)hydroxylamines with butyllithium and crystallized as 0-lithium-N,N-bis(sily1)hydroxylamides under silyl group migration from the 0-atom to N-atom. Stable 0,O’-bis(hydroxy1amino)silanes have been isolated and characterized in a two-phase reaction. In the N,O-bis(tert-butyl-dmethylsilyl)-N-trimethyl-stannyl- hydroxylamine a 1,2-anionic stannyl group migration from oxygen to nitrogen and a silyl group migration from nitrogen to the oxygen via a dyotropic transition state was proved for the first time. Reactions, crystal structures, rearrangements and quantum chemical calculations are presented.

Introduction and Explanation

Pioneering work in the synthesis and in studies of rearrangements in tri(organy1)silyl-hydroxylamines was done in the groups of Wannagat and West [ 11.

We report here new results of preparation, reactions and rearrangement in silylhydroxylamine chemistry (Scheme 1).

Ovganosilicon Chemisty V Edited by N. Auner and J. Weis

Copyright 0 2003 WILEY-VCH Verlag GmbH & Co. KgaA, Weinheim

Silylhydroxylamines 247

R3Si- ONH2

I ;:L3 + R3SiCl 1 - l,ZSiR,

R3SiO- N-SiR’3 B R3SiO- N- SR3 I I

H

+ BuLi

- 1.2 anionic rearrangemnt kfH [ W O - N(SR3)(SR*3)] n C

IF R2S++N-SR3

I sa; I F

I F

‘2 / \

\ / R,Si-N -0

Si R3

- FSWZ - s i 3

dyotropic them1 - siR3 irreversible rearrangemnt

t F

t R ’

D E

Scheme 1.

A: In contrast to organic substituted hydroxylamines, monosilylhydroxylamines are only known with the silyl group bonded to the oxygen atom. B: Depending on the reaction process isomeric bis(sily1)hydroxylamines can be prepared. C: In the reaction of bis(sily1)hydroxylamines with lithium, organic lithium always attacks the oxygen atom. This includes an anionic 1,2-silyl group migration from oxygen to the nitrogen atom. Isomeric bis(sily1)hydroxylamines form the same lithium compound which reacts with

248 C. Ebker, F. Diedrich, S. Schmatz, U. Klingebiel

halosilanes to give tris(si1yl)hydroxylamines. D: In case of 0-fluorosilyl substituted tris(sily1)hydroxylamines a neutral irreversible rearrangement involving positional exchange between the fluorosilyl group at the oxygen and one of the organic silyl group at the nitrogen takes place via a dyotropic transition state [2]. E: In a thermal rearrangement at temperatures up to 200 "C, tris(organosily1)hydroxylamines undergo an intramolecular isomerisation involving the insertion of a silicon moiety into the bond between nitrogen and oxygen and the transfer of an organic group from silicon to nitrogen, to form silylaminodisiloxanes [3]. This conversion occurs in high yields. We studied rearrangements of N-fluorosilyl-N,O-bis(organosily1)hydroxylamines and found that the insertion of a silicon moiety into the N-0 bond occurs exclusively with a silicon which was bonded to the nitrogen. The insertion of a SiF2 unit into the N-O bond has not been observed.

For a better understanding of our results we report quantum chemical investigations of the isomerizations of tris(sily1)hydroxylamines for several model compounds and the thermal rearrangements of two N,N,O-tris(sily1)hydroxylamines with formation of the isomeric silylaminodisiloxanes.

Lithium Derivatives of Bis(sily1)hydroxylamines

Lithium derivatives of silylhydroxylamines have been used for more than thirty years. Now we are able to present the first crystal structures [4, 51. Lithium is bonded side-on and end-on in these silylhydroxylamides (Fig. 1).

side on

Fig. 1.

end on

\ /N-o-Li

Depending on the reaction conditions and the bulkiness of the substituents, dimeric, trimeric and tetrameric oligomers were found (Fig. 2):

Silylhydroxylamines 249

Bond lengths [pm] and angles ["I Bond lengths [pm] and angles ["I

Li(l)-O(l) 182.6 Z N( 1) 359.9 O(1)-Li(1) 176.3 Z N(l) 352.1

O(l)-N(l) 147.4 Z Li( 1) 360.0 N(1)-Li(l) 209.7 X Li(1) 356.5

O(1)-N(l) 150.0

Fig. 2. Crystal structure of a dimeric and a tetrameric silylhydroxylamide.

Bis(hydroxy1amino)silanes

We are able to present the first bis(hydroxy1amino)silanes [6], obtained in the following reaction (Eq. 1):

Eq. 1.

The bis(hydroxy1amino)silanes can be used for the synthesis of cyclic and acyclic 0,O-bis(silylhydroxy1amino)silanes. One of the examples is illustrated in Eq. 2. The crystal structure of a bis(silylhydroxy1amino)silane is shown in Fig. 3.

250 C. Ebker, F. Diedrich, S. Schmatz, U. Klingebiel

+ 2 NEt, H3C\ ,CH,

H S ~ ~ C H , Me3C, ,0-N

+ 2 TMSCl

H,CNSi'O--NH, 0-N

I'

Me& ,0-NH, - Hii'CH,

H,C CH3

Eq. 2.

c,,3, Si-N 174.9-175.8 pm

O(1)-N(l) 147.9pm

..o 0(2)-N(2) 146.8 pm .a

Fig. 3.

Cyclic Silylhydroxylamines

Fluorofunctional bis(sily1)hydroxylamines were excellent precursors for rings. These molecules underwent an 1,Zanionic rearrangement after lithiation and two of these lithium compounds reacted with one another to form six-membered ring systems [4] (Eq. 3).

SiR,

* R2Si, SiR2

/

/

+ 2 n-BuLi FN\ /

- 2 n-BuH N-0 2 FR2SiONH(SiR3)

R3Si -2L iF

Eq. 3.

Crystal structure of a cyclic silylhydroxylamine, (Me&MeSi-O-N-SiMezCMe3)2, is shown in Fig. 4.

Silylhydroxylamines 251

Fig. 4.

2,4-Bis(tert-butyldimethylsilylhydroxyla~no)-1-dimethylalano-3-lithio-2,4 -dioxoc y clobutane

The first aluminium containing silylhydroxylamine was isolated in the reaction of O-lithium-N,N-bis(tert-butyldimethylsilyl)hy~oxyl~de with chlorodimethylalan [4] (Eq. 4).

2

Eq. 4.

Me&

Me2Si I \

I Me2Si IN-o-L Me3C

CMe3 Me37 Me Me I /SMe2 Me2Si 1 1

\

I

+ 3CIAIMe2 - 3 ./"-" \

SMe2

CMe3

LJ -3Licl MezSi

Me3C I

3

Figure 5 illustrates a solid-state structure of this (Al+-Li)-ring system. The crystal is monoclinic, space group P21In with Z = 4. This compound forms an (Al-0-Li-0) four-membered planar ring. Neither the lithium nor the aluminium atom is bonded side-on to the N-0 unit.

252 C. Ebker, F. Diedrich, S. Schmatz, U. Klingebiel

Cllll

a331 Bond lengths [pm] and angles ["I

Si-N 173.8-174.6

O(I)-N(l) 147.7

0(2)-N(2) 148.4

C N(l) = 375.75

I: N(2) = 355.19'

Fig. 5.

N,O-Bis(tert-butyldimethylsily1)-N-( trimethyl-stanny1)hydroxylamine

In the reaction of 0-lithium-N,N-bis(sily1)hydroxylamide with trimethylchlorostannan N,O-bis(tert-butyldimethylsilyl)-N-(~methylst~nyl)hydroxylamine was formed (Eq. 5) . In contrast to the expected N,N-bis(sily1)-0-stannylhydroxylamine the reaction product was N,O-bis(sily1)-N-stannylhydroxylamine. The migration of the stannyl group from the oxygen to the nitrogen atom can be explained with the Pearson concept. A solid-state structure was elucidated by X-ray diffraction method (Fig. 6).

Eq. 5.

U71 C181

Cll41

Bond lengths [pm] and angles ["I Sn(1)-N(1) 208.8 O(1)-N(1)-Si(1) 105.36

N( l )S i ( l ) 174.9 Si(1)-N(1)-Sn(1) 126.57

N(1)-O(1) 149.7 O(1)-N(1)-Sn(1) 108.59

0(1)-Si(2) 166.6 N(l)-O(l)-Si(2) 115.70

N = 340.52"

Fig. 6.

Silylhydroxylamines 253

-100

-I50

-200

-250

Quantum chemical calculations of the stannyl group rearrangement were carried out for a model compound by Dr. S. Schmatz [7] (Scheme 2).

Me Me

Me3Si-O-Si -N-SnMe3 C I I I

Me I -243.5

reaction coordinate

Scheme 2.

The difference between the AHf of the model compounds A and B is very small whereas in the

Further reactions and quantum chemical calculations are in preparation. thermal rearrangement of B to C it is more than 240 kJ/mol.

Acknowledgments: This work has been supported by the Deutsche Forschungsgemeinschaft and the Fonds der Chemischen Industrie.

References [ 11 [2] [3] [4]

[5] [6] [7]

P. Boudjouk, R. West, Zntra-Sci. Chem. Rep. 1973, 7,65-82. R. Wolfgramm, T. Muller, U. Klingebiel, Organometullics 1998, 17,3222-3226. S . Schmatz, F. Diedrich, C. Ebker, U. Klingebiel, Eur. J. Znorg. Chem. 2003, in preparation. F. Diedrich, U. Klingebiel, F. Dall’Antonia, C. Lehmann, M. Noltemeyer, T. R. Schneider, Organometallics 2000,19,5376-5383. F. Diedrich, U. Klingebiel, M. Schiifer, J. Organomet. Chem. 1999,588,242-246. F. Diedrich, C. Ebker, U. Klingebiel, Phosphorus, Sulfur and Silicon 2001,169,253-256. C . Ebker, S. Schmatz, U. Klingebiel, in preparation.

SiO and SiOSiN Chains, Rings and Cages

Susanne Kliem, Clemens Reiche, Uwe Klingebiel

Institute of Inorganic Chemistry, Georg-August-University Gottingen Tammannstr. 4, D-37077 Gottingen, Grmany

Tel.: +49 551 39 3052 - Fax: +49 551 39 3373 E-mail: uklinge@gwdg.de

Keyword: siloxanes, heteroatomic siloxanes, acyclic, rings, cages

Summary: Alkaline derivatives of silanols and aminosilanols are isolated as rings, cubanes and prisms. They react stepwise with element halides to give chains or four-, six-, and eight-membered SiO and SiOSiN ring molecules. In these reactions, for example, the smallest four-membered rings containing two silicon atoms and the longest Si-0 chains were formed. Syntheses and structures are presented.

Introduction

The alkaline derivatives of silanols and aminosilanols shown in Fig. 1 have been the starting materials for preparation of SiO and SiOSiN chains, rings and cages.

0

0 X = F, NH2, OH, -< X = NH2, OH

M = Na,K

Fig. 1. General formulas of the starting materials.

Silanols and Siloxanes

In contrast to carbon chemistry, in silicon chemistry diols and silanol halides can be stabilized by bulky groups. Stepwise synthesis of cyclic and acyclic siloxanes is possible with fluorosilanes and alkaline derivatives (Scheme 1).

The two hydroxy groups of the trisiloxane look like a pair of tongs. By forming intramolecular

Ovganosilicon Chemisty V Edited by N. Auner and J. Weis

Copyright 0 2003 WILEY-VCH Verlag GmbH & Co. KgaA, Weinheim

SiO- and SiOSiN- Chains, Rings and Cages 255

hydrogen bonds both pairs of tongs yield an eight-membered Si304H ring. The O(1)-0(2) distance amounts to about 276 pm, and the O( 1)-O( 1A) distance to about 277 pm.

7Me3 7HMe2 CMe3 I Me3C\ ,Ofi + ( cHM~~)~sz~

Me3C OH OH CHMe2 OH

w Me$-Si-0-Si-0-Si-CMe3 / \ - 2LiF I I I

Si 2

Scheme 1.

Because of this structure (Fig. 2), H2O can be cleaved off by heating. The resulting six- membered ring is planar (Fig. 3).

Fig. 2. Crystal structure of the trisiloxane product in

Scheme 1.

Fig. 3. Crystal structure of the planar six-membered

ring compound after cleavage of HzO

By alternating lithiation and substitution involving LiF elimination, siloxane units up to heptasiloxanes (Fig. 4) were obtained.

F CMe3 Me CMe3 Me CMe3 F

Fig. 4. Heptasiloxanes formed by alternating lithiation and substitution involving LiF elimination.

256 S. Kliem, C. Reiche, U. Klingebiel

A tetrafluoro-functional eight-membered SiO ring is formed in the reaction of the dilithiated silandiol with tetrafluorosilane (e.g. as in Scheme 2) . The structure of the product is represented by Fig. 5.

Scheme 2.

ClBl

c121

Fig. 5. Crystal structure of eight-membered ring product in Scheme 2.

Aminosilanols and Aminosiloxanes

Amino-di-tert-butylsilanol (Fig. 6 ) can be used as a good precursor for the synthesis of heteroatomic siloxanes because of its two different functional groups.

Me3C, ,OH

Me$’ “ I 3 2

Si

Fig. 6. Amino-di-tert-butylsilanol.

SiO- and SiOSiN- Chains, Rings and Cages 257

In the crystal structure (Fig. 6) no intramolecular H bonds were found, but intermolecular H bonds connect two molecules, yielding a dimer (Fig. 7). The hydrogen atom of the hydroxy group forms a bond to the electron lone pair of the nitrogen atom of the neighboring molecule. This is in agreement with the expectation that the nitrogen atom will give up its electron lone pair to form the H bond because its electronegativity is lower than that of the oxygen atom.

O(1 )-N(l) 216 pm

O(1)-N(1A) 285 pm

Fig. 7. Dimerization of amino-di-tert-butylsilanol.

The lithium derivative of the aminosilanol crystallizes as LiO cubane, the sodium and potassium derivatives as hexagonal prisms (Fig. 8).

Si-0 : 159.7 pm Si-N : 176.4 pm Na(1)-O(1) : 241.6pm Na(ltO(2) : 229.0 pm Na( 1)-N(l) : 249.5 pm Na(2)-N(2) : 252.9 pm

Fig. 8. Alkali metal derivative of amino-di-?el?-butylsilaneol.

258 S. Kliem, C. Reiche, U. Klingebiel

Usually the attacking silyl group is bonded to the 0 atom, which means a 1,3-(O-N)-silyl group migration occurs (Scheme 3, Fig. 9). This rearrangement can be prevented by bulky substituents.

I

/ 51111 Me3C -Si-O-Si-N,

I I 0111 H-,N,yF CHMez

I / ' ,

X X .S!

SiFR2 I o\

+ n-BuLi Li

x x - n-BuH

Si N: 'OLi

RzFSi' H

SiFRz

+HalSiFR2 x x Si

H2N 'OLi -LiHal HZN' 'o\

Scheme 3.

Scheme 4.

SiO- and SiOSiN- Chains, Rings and Cages 259

Using other element halides, e.g. (SiOBN)2, eight-membered rings are obtained, e.g. as in

The Si-0-B angle in the product (Fig. 10) is found to be larger than the Si-N-B angle. Scheme 4.

Si-0

Si-N

B-F

B-N

B-O

Si-0-B

Si-N-B

I: (B) = 360.0"

162.8 pm

173.4 pm

135.5 pm

139.7 pm

130.9 pm

161.0"

131.7'

Fig. 10. Crystal structure of the eight-member ring product of Scheme 4.

Four membered rings (e.g. Fig. 11) are isolated by using bulky substituents. The C( 1) atom has a nonplanar environment. (C" C(l) = 355.7"). The Si-0 bonds are longer than the Si-N bonds.

N H

Si( 1)-O( 1) 167.2 pm

Si(2)-O( 1) 170.2 pm

Si( 1)-N( 1) 166.6 pm

Si(2)-N( 1) 169.1 pm

Fig. 11. Four-membered aminosiloxane with bulky substituents

The lithium derivative (Fig. 12) crystallizes from n-hexane as a dimer. The crystal structure shows some irregularities. The dimer is formed via a four-membered (Li-N)2 ring, which is not entirely planar but folded by 10.7" across the Li-Li line. The (Si-0-Si-N)-rings are at an angle of 86" to the central (LiN)2 ring and at 5.8" to each other. As far as we know this is the smallest four- membered ring containing two Si atoms. The transannular Si...Si distance is found to be 237.2 pm.

260 S. Kliern, C. Reiche, U. Klingebiel

Both lithium atoms of the dimer can be substituted in the reaction with a difluorosilane. The product is shown in Fig. 13.

Fig. 12. Dimeric lithium derivative of the compound in Fig. 1 1.

Fig. 13. Difluorosilyl derivative of lithiated dimer in Fig. 12.

Isomeric Cyclosilazanes and their Application as Precursors for Silicon-Based Ceramics

Nina Helmold, Verena Liebau, Uwe KlingebieP

Institut fur Anorganische Chemie, Georg-August-Universitat Gottingen Tammannstr. 4, D-37077 Gottingen, Germany

E-mail: uklinge@gwdg.de

Stefan Schmatz*

Institut fur Physikalische Chemie, Georg-August-Universitiit Gottingen Tammannstr. 6, D-37077 Gottingen, Germany

E-mail: sschmat@gwdg.de

Keywords: cyclosilazanes, isomerization, quantum chemical calculations

Summary: Lithiated octamethylcyclotetrasilazane (OMCTS) and hexamethylcyclotrisilazane (HMCTS) react with pyrazoles, fluorosilanes and fluoroboranes in different ways. Ring contraction with formation of the isomeric six- membered or four-membered ring occurs. Quantum Chemical Calculations improve the understanding of the isomerization mechanism. Cyclosilazanes have been intensively studied with regard to their application for ceramics in the Si-C-N and Si-B-C-N systems.

Introduction

The transformation of organoelement compounds into inorganic solids by pyrolysis of these molecules is a new way to obtain ceramic materials. The advantage of this method is that pure compounds can be used. Suitable molecules are boron-silicon-amines, silylcarbodiimides and cyclosilazanes.

Here we present new results of quantum chemical calculations and basic research about the isomerization of cyclosilazanes and their use as precursor molecules.

Results

Quantum Chemical Calculations

Cyclosilazanes frequently tend to isomerize. The equilibrium between the eight-, six- and

Ovganosilicon Chemisty V Edited by N. Auner and J. Weis

Copyright 0 2003 WILEY-VCH Verlag GmbH & Co. KgaA, Weinheim

262 N. Helmold, V. Liebau, S. Schmatz, U. Klingebiel

four-membered (Si-N) rings depends on thermal and kinetic effects, the size of the substituents and the properties of the attacking ligand.

To investigate the ring contraction of octamethylcyclotetrasilazane (OMCTS) quantum chemical studies have been carried out (Gaussian98, B3LYP, 6-3 1G*).

lsomerization of OMCTS

The eight-membered ring compound OMCTS can exist in three conformations: the twist form (A), the boat conformation (B) and the chair form (C). Further isomeric structures are a six-membered (D), a symmetrical (E) and a nonsymmetrical (F) four-membered ring with exocyclic SiMeZNH2 groups (Fig. 2). D has an energy difference of +0.4 kcal mol-' compared to C. The four-membered rings have obviously higher energy differences (Fig. 1).

l 8 1

D C I

I

reaction coordinate

F I

Fig. 1. Energy diagram of the neutral isomers A-F. Eo refers to the most stable isomer A and includes zero-point

energy contributions.

Isomeric Cyclosilazanes and their Application as Precursors for Silicon-Based Ceramics 263

264 N. Helmold, V. Liebau, S. Schmatz, U. Klingebiel

r- -Ti- Me -7 H -Ti- Me

JI

F

Fig. 2. The isomeric forms of OMCTS

Isomerization of Anionic OMCTS

For the OMCTS anion only the twist form (A') exists. The six- (D') and the symmetrical four-membered ring (E') form a bicyclic isomer (Fig. 3). Compared to the neutral molecule the energy difference of the anion E' is obviously higher (Fig. 4).

H

A'

Isomeric Cyclosilazanes and their Application as Precursors for Silicon-Based Ceramics 265

D’

E’

Me2

Me2 /si\ \ /N-H

Si-N

Si Me2

Si’ Me2 \ N N H

I H

H I

H

I N

Si, ’ ‘SiMe2 Me2 / ‘\@/

Si Me2

/Si-N

H

F’

Fig. 3. The isomeric forms of the OMCTS anion.

266 N. Helmold, V. Liebau, S. Schmatz, U. Klingebiel

18

15

I2 - r

Lo

Y E a 9 - 0

' 6

3

0 A'

reaction coordinate

Fig. 4. Energy diagram of the anionic isomers A'-F'. Eo refers to the most stable isomer A' and includes zero-point

energy contributions.

Reactions of Cyclosilazanes as Precursors for Ceramic Materials

Reactions of OMCTS

In the reactions of pyrazole and 3,5-dimethylpyrazole with octamethylcyclotetrasilazane different isomers are obtained (Scheme 1). Due to the high reactivity of the NH2 group the eight-membered rings contract via elimination of NH3.

r-

b-H +

R

L R = H, CH,

Scheme 1.

R 2 S1:N=4:5

Isomeric Cyclosilazanes and their Application as Precursors for Silicon-Based Ceramics 267

Dilithiated OMCTS reacts with F2BN(CHMe2)2 to 4 (Si:B:N = 4:2:6; Scheme 2).

I 12

Si(1)-N(l) = 173.9 (10) pm Si(1)-N(2) = 172.4 (10) pm B(1)-N(l) = 145.4 (14) pm B(l)-N(12) = 140.1 (14)pm EN( 1) = 358.4" CN( 12) = 359.4"

FlA

Scheme 2.

In the reaction of lithiated OMCTS with Me2SiF2 5 has been obtained (Si:N = 7:4; Scheme 3).

H Me, Me2 Me

I Si Me

Scheme 3.

268 N. Helmold, V. Liebau, S. Schmatz, U. Klingebiel

Reactions of Hexamethylcyclotrisilazane

Reactions of lithiated HMCTS with FzBN(i-C3H7)2 lead to mono-, di- and trisubstituted products by retention of the ring size, e.g. 6 was obtained (Si:B:N = 3:3:6) according to Scheme 4.

M<HC\N/CH,Me

I H

N N ’ ‘SiMe,

N N Me2Si I + 3 n-BuLi + 3 C~H~.$JBFZ - 3 n-BuH - 3 LiF

I ’ ‘SiMe,

I H’ \si/ ‘H

Me2 BF\ N/CHl Me

- 6 I CH

I CH

Me’ ‘Me Me’ ‘Me

Scheme 4.

Fluorosilyl substituents stabilize the cyclosilazane unit due to the -I effect. Lithiated (fluorodimethylsilyl) hexamethylcyclotrisilazane reacted with n-butyllithium with retention of the six-membered ring 2 (Si:N = 4:3; Scheme 5).

H I I

N ’ ‘SiMe2

Me Me2 i I \si/N\si/N\H

F’\ Me2

Si( 1)-N( 1)

Si( 1)-N(2)

Si(2)-N(3)

rn + n-BuLi

- n-BuH - Y2

169.7 (10) pm

177.1 (11) pm

172.7 (1l)pm

2

Scheme 5.

Isomeric Cyclosilazanes and their Application as Precursors for Silicon-Based Ceramics 269

In reactions of the mono- and dilithium salts with fluoroboranes we succeeded in the coupling of SiN ring systems and FBN compounds in variable molar ratios, e.g. 1:2 yields S (Si:B:N = 4:2:5; Scheme 6).

H I

+ 2 n-BuLi

- 2 n-BuH - 2 LiF

ii * ’ \SiMe, + 2 C ~ H L ~ N B F ~

I Me N

BF\ Me

I I Me

C k CH

Me’ \Me - 8 Me’ \Me

Si-N(endo) 172.1-174.2 pm

Si-N(exo) 170.5 pm

Scheme 6.

Silicon and Germanium Compounds with Amidinate Ligands

Hans H. Karsch,” Thomas Segmiiller

Anorganisch-chemisches Institut, Technische Universitat Miinchen, Lichtenbergstral3e 4, D-85747 Garching, Germany Tel.: +49 89 289 13132 -Fax: +49 89 289 14421

E-mail: Hans.H.Karsch@lrz.tum.de

Keywords: silicon amidinates, germanium amidinates, hypervalency, low valency

Summary: A series of penta- and hexacoordinate silicon and germanium compounds with amidinate ligands have been synthesized and investigated spectroscopically and structurally. In contrast to silicon, germanium also forms stable compounds with amidinate ligands in the +2 valence state.

Introduction

Amidinate ligands play a prominent role not only in transition metal and lanthanoid coordination chemistry, but also in main group metal derivatives. However, examples of silicon and germanium compounds with amidinate ligands beyond our own studies are rather scarce [ 1-31. This perhaps is due to the fact that amidinate complexes have been synthesized mainly for catalytic purposes, but silicon and germanium compounds are not generally regarded relevant for this application. In foregoing studies, we have introduced amidinate ligands in silicon and germanium compounds in the context of a new and different concept: the formation of strong element-to-nitrogen bonds together with the introduction of two nitrogen atoms with one anionic ligand enables the generation of silicon and germanium centers with high coordination numbers, where the central elements are imbedded in a nitrogen-rich environment.

In this way, amidinate complexes of silicon and germanium might provide valuable precursors for CVD purposes (Si/(C)/N). In the form of dianionic ligands, i.e. [N-C(R)-NRf]*-, they may even provide easily tunable structural units in polymeric arrangements with pseudo-chalcogenide bridges, resembling silicate structures with 0’- bridges, which may further be converted to Si/C/N/X high temperature resistant materials.

Beyond any application purposes, silicon compounds with high coordination numbers (“hypervalent” compounds) are of general interest and examples with four-membered rings, as are formed by chelating amidinate ligands, are almost unknown. Finally, with the aid of these ligands, low-valent complexes of these elements (Sin, Gen’) seem accessible.

Ovganosilicon Chemisty V Edited by N. Auner and J. Weis

Copyright 0 2003 WILEY-VCH Verlag GmbH & Co. KgaA, Weinheim

Silicon and Germanium Compounds with Amidinate Ligands 271

Results

By variation of the substitution pattern of the amidinate ligands and of the silicon or germanium coordination centers, we obtained a number of compounds with a variable degree of WGe-N hypervalent interaction, i.e. compounds with essentially no bones and ones with very strong element-nitrogen bonds ("double bonds") are formed, providing tetra-, penta- or hexa-coordinated species [4]. New or newly structurally characterized examples of silicon amidinate complexes with pentacoordinate silicon are obtained (Eq. 1; Table 1).

1 a d

Eq. 1.

Table 1. NMR data for compounds la-d.

R R' 6 (29Si NMR)

l a CY 'Bu -89.16

l b CY Mes -98.57

l c CY CH2PMe2 -98.40 a'

Id 'Pr 'Bu -99.08

[a] 6 (3'P NMR) = -37.60.

The NMR spectra of the compounds l a 4 clearly indicate a chelating coordination mode of the amidinate ligands. Both R groups show only one set of signals in the temperature range from +30 to -90 "C ('H, 13C NMR), indicative for a fast pseudo-rotation of the molecular skeleton in each case, whereas the *'Si NMR resonances confirm the presence of a pentacoordinate silicon centers. The 31P NMR signal of l c indicates that the phosphine functionality is not involved in the coordination to silicon, as was observed in pentacoordinate silicon phosphinomethanide complexes studied earlier by us [S]. Obviously, nitrogen coordination is preferred over phosphorus coordination despite the fact that in the latter case a five-membered ring may be formed instead of four- membered rings of the amidinate chelates.

The molecular structures of l a and lb, as determined by X-ray diffraction, are shown in Figs. l a and Ib, and relevant structural parameters are given in Table 2.

The structures of la and l b confirm the spectroscopic findings. The silicon atoms are each surrounded by three chlorine and two nitrogen atoms in a distorted trigonal bipyramidal fashion. As expected, the axial bonds to nitrogen and chlorine are longer than the respective equatorial ones, but

272 H. H. Karsch, T. Segrniiller

the overall structural features closely resemble those for related compounds previously described [4]. Thus it may be concluded, that the substituents R and R’ in compounds of type 1 have only a minor influence on the basic structural features. This is at variance with earlier findings on pentacoordinate silicon amidinates with fluoroalkyl substituents at nitrogen, which differ considerably in the bonding parameters from the complexes described here [6] . It is also at variance with the findings on germanium($ complexes with amidinate ligands, Ge[(RN)2CR’]2, where very small changes in the substitution pattern of the amidinate ligands R and R’ cause a complete change in the overall structure, from yr-trigonal bipyramidal to yr-tetrahedral[3,7].

C

Fig. la. Molecular structure of la. Fig. lb. Molecular structure of lb.

Table 2. Selected distances [A] and angles [“I for l a and l b

la l b

Si-N(1)

Si-N(2)

Si-Cl( 1)

Si-Cl(2)

Si-Cl(3)

“1) -C(1)

N(2- (1)

N(2)-Si-N( 1)

N( l)-Si-C1(3)

N( l)-Si-C1(2)

N( 1)-Si-Cl( 1)

N(2)-C( 1)-N( 1)

1.9209 (9)

1.7810 (9)

2.0738 (10)

2.0800 (4)

2.1507 (4)

1.3152 (11)

1.3814 (12)

70.09 (4)

167.70 (3)

93.51 (3)

93.30 (3)

104.23 (8)

1.9578(13)

1.7992(12)

2.0841(5)

2.0641(5)

2.1414(5)

1.2971( 18)

1.3624(16)

69.07(5)

163.33(4)

94.92(4)

91.92(4)

106.69( 12)

Silicon and Germanium Compounds with Amidinate Ligands 273

Amidinate complexes [ (RN)2CR']2SiC12 2 with hexacoordinate silicon obtained by a related reaction to Eq. 1, but with a 2: 1 stoichiometry, have been described previously by us [4,7].

The structure of the newly synthesized [(CyN)2CMe]2SiC12 2b (6 29Si NMR = -168.61) (Fig. 2; Table 3) shows the same trend as observed in the pentacoordinate cases discussed above: all comparable structural parameters are virtually identical to those of the related complex [MeC(NiPr)2]zSiClz 2a [7]. In particular, the Si-N and Si-Cl distances are equal within experimental error and the C1-Si-Cl angle is close to 90°, thus confirming the octahedral structure principle.

Fig. 2. Molecular structure of 2b.

Table 3. Selected distances [A] and angles ["I for 2b.

Si-N( 1) 1.839( 18) CI( I)-Si-C1(2) 90.06(3)

Si-N(2) 1.917(2) N( l)-Si-N(2) 68.90(8)

Si-N(3) 1.907(2) N(3)-Si-N(4) 69.03(8)

Si-N(4) 1.833 18) N( 1)-Si-N(4) 163.6(9)

Si-Cl(1) 2.193(9) N(2)-Si-N(4) 99.12(8)

Si-Cl(2) 2.200( 8) N( I)-C( 1)-N(2) 106.6(2)

N( 1 )-C( 1) 1.337(3) N(3)-C(3)-N(4) 106.2( 19)

N(2)-C( 1) 1.3 15(3) Cl( l)-Si-N(2) 90.79(7)

N(3)-C(3) 1.320(3) Cl( l)-Si-N(3) 164.0(6)

N(4)-C(3) 1.333(3) C1(2)-Si-N(2) 164.8(6)

Attempts to reduce these hexacoordinate compounds to silicon(II) derivatives were unsuccessful so far, however. The Si-Cl functionality in these compounds turned out to be rather untractable: even simple metathesis reactions in most cases failed. Therefore we turned to the related

274 H. H. Karsch, T. Segmuller

compounds [(RN)2CR']*SiHCl, 3a,b, which were synthesized according to Eq. 2.

HSiCI, + 2Li

Eq. 2.

3a, b

Table 4. NMR data for compounds 3a, b.

R R' 6 ( 2 9 ~ i ~ ~ ) JSI-H [Hzl

3a CY Me -176.29 199.55

3b 'Pr Me -117.42 276.91

From the NMR spectroscopic data (Table 4), it is concluded that these compounds are fluctional in solution and contain hexacoordinate silicon centers. Attempts to use these compounds as precursors for silicon(n) derivatives by HCl abstraction in the presence of a strong base likewise have failed so far, however.

Unsymmetrical amidinate ligands with an N-H functionality have likewise been shown to be suitable ligands for silicon coordination centers [4, 71. In the case of germanium($, a tetrameric compound 4 has been obtained according to Eq. 3 [4].

8 Li[N=C(Ph)(NH'Bu)] + 4 GeC12.Dioxan TH F * (Ge[N=C(Ph)N'Bu)]}, - 4 HN=C(Ph)(NH'BU)

4

Eq. 3.

We have now succeeded in obtaining crystals of compound 4, suitable for an X-ray structure determination (Fig. 3; Table 5) . Originally having assigned a cubane-like structure, it is now obvious that the tetrameric unit adopts an eight-membered ring [Gen-N]4. The Ge" atoms are

additionally chelated by the remainder of the dianionic amidinate ligands (double-deprotonated amidines), which thus function as both bridging and chelating ligands to Gen Each germanium atom

Silicon and Germanium Compounds with Amidinate Ligands 275

is surrounded by three planar (sp2) nitrogen atoms in a y-tetrahedral environment with a rather acute chelate angle N-Ge-N (-65.4'). This resembles the coordination mode in the monomeric bis- amidinate germanium(n) compounds Ge[(RN)2CR']2 for R = Cy; R' = Me or 'Bu [3], but is at variance to the case with R = 'Pr; R' = Me [7].

Table 5. Selected distances [A] and angles ["I for 4. k Ge( I)-N( 1) 2.O425( 13)

Ge( 1)-N(2) 1.9897( 13)

Ge(2)-N(21) 1.9935(12)

Ge( 2)-N(22) 2.0402( 13)

Ge(2)-N(2) 1.9009( 13)

Ge( 1)-N(21A) 1.9020( 13)

N(2)-Ge( 1)-N( 1) 65.42(5) I +

Fig. 3. Molecular structure of 4.

N(21)-Ge(2)-N(22) 65.41(5)

Ge[N($u)C( Ph)N]}4 (NW4

I I

I

I I ,

Fig. 4. Comparison of the ring puckering in 4 and (NSF)d.

276 H. H. Karsch, T. Segmuller

But the most interesting feature in the structure of 4 is the puckering of the eight-membered ring [Ge-N]4. Far from being random, the ring adopts a geometry which closely mimics that of the [ S-N]4 eight-membered ring of tetrameric thiacyl fluoride molecule (NSF)4 (Fig. 4). At first glance it is rather surprising that there is any resemblance of these grossly different species, but it emerges at closer sight that indeed compound 4 and (NSF)4 are closely related by electronic and geometric necessities. Thus germanium(n) is electronically related to sulfur(rv); their environment may be denoted as N[(N,N)Ge] and F[(N,N)S+] respectively and thus is virtually identical within the eight- membered rings. Also, the endocyclic nitrogen atoms adopt a planar sp2 configuration in both cases, either as formally neutral X3N (4) or as formally anionic X2N- ((NSF)4). This relationship between the two molecules considered here is a unique case where a comparison can be made between Gen and S", which donates similarly even in the folding of larger rings, and in turn reveals that the ring

puckering is by no means determined by serendipity.

Conclusion

This study demonstrates the utility of amidinate ligands for the synthesis of hypervalent silicon and germanium compounds, and also for low-valent germanium species. In addition, a surprising relationship between iso(valence)electronic Gen and S" is documented.

Acknowledgments: We thank Dr. N. Mitzel for performing the X-ray determinations and the Fonds der Chemischen Industrie for financial support.

References [l] 0. J. Scherer, 0. Hornig, Chem. Ber. 1968,101,2533. [2] H. W. Roesky, B. Meller, M. Noltemeyer, H. G. Schmidt, U. Scholz, G. M. Sheldrick, Chem.

[3] S . R. Foley, C. Bensimon, D. S . Richeson, J. Am. Chem. SOC. 1997,119,10359. [4] H. H. Karsch, P. A. Schluter in: Organosilicon Compounds ZV - From Molecules to Materials

[ 5 ] H. H. Karsch, R. Richter, E. Witt, J. Organomet. Chem. 1996,521, 185. [6] a) H. H. Karsch, F. Bienlein in: Organosilicon Compounds ZZ - From Molecules to Materials

(Eds.: N. Auner, J. Weis), Wiley-VCH, Weinheim 1996, p. 133 and literature cited therein. b) H. H. Karsch, P. A. Schluter in: Organosilicon Compounds I l l - From Molecules to Materials (Eds.: N. Auner, J. Weis), Wiley-VCH, Weinheim 1998, p. 53.

[7] H. H. Karsch, P. A. Schluter, M. Reisky, Eur. J. Znorg. Chem. 1998,433.

Ber. 1988,121,1403.

(Eds.: N. Auner, J. Weis), Wiley-VCH, Weinheim 2000,287.

Development of Force Field Parameters for Amino-Substituted Organodisilanes

Uwe Bohme,* Birgit Schluttig

Institut fur Anorganische Chemie, Technische Universitit Bergakademie Freiberg, Leipziger Stral3e 29, D-09596 Freiberg, Germany

E-mail: Uwe.Boehme@chemie.tu-freiberg.de

Robert K . Szikigyi

Department of Chemistry, Stanford University 333 Campus Dr, Stanford, CA 94305, USA

Keywords: aminosilane, ab initio, DFT, force field, molecular modeling

Summary: We have constructed a force field for amino-substituted organodisilanes. Since there are not enough crystal structures of amino-substituted oligosilanes available, we chose to develop new parameters from quantum chemical calculations. Ab initio, DFT and MP2 calculations on a set of model compounds provided structural parameters necessary to develop an initial set of force field parameters. Simultaneously we have synthesized a number of aminoorganodisilanes. The data from X-ray structure analyses of crystalline derivatives are used to prove the quality of the parameter set.

Introduction

Molecular mechanics (MM) force fields are commonly used today for a wide variety of applications [l]. The MM approach is the optimal choice for studying the conformational space and isomerization processes of oligomeric and polymeric organosilanes, which have been synthesized and characterized by our group and others [2-71. Amino-substituted oligosilanes have captured our interest especially, because they are expected to be new precursors for silicon-containing materials and for the deposition of silicon nitride [8-101. Force fields have already been developed for monomeric organosilanes [ll-131. However, the accuracy of these parameter sets for oligomeric organosilanes is not superior to those of advanced molecular builders.

The aim of our work is to develop an MM2-type force field capable of handling a wide range of oligomeric organosilanes. The MMX force field was chosen as a suitable environment for the implementation of new parameters [ 113.

Ovganosilicon Chemisty V Edited by N. Auner and J. Weis

Copyright 0 2003 WILEY-VCH Verlag GmbH & Co. KgaA, Weinheim

278 U . Bohme, B. Schluttig, R. K . Sziligyi

Methodology

Force fields can be parameterized by reference to experimental data or by getting constants from high-level ab-initio calculations. For our force field for amino-substituted organodisilanes the chemical environment of interest is R(N)CSi-SiC(N)R. We found only very few suitable structures (1-4, Fig. 1) in the Cambridge Crystallographic Database.

R

\ yE,? NE t2 /NEt'ph

1 R = f-but$ [14] 2 R = thexyl [14] 3 R=phenyl [16]

Fig. 1. Amino-substituted organodisilanes R(N)CSCSiC(N)R.

4 [151

Some structures with the structural motif of interest are from cyclic compounds [ 17-19]. These are not appropriate for our purposes, since the disilacyclus causes additional ring strain effects. In addition to these existing structures we have synthesized the new amino(chloro)methyldisilanes 5-7 (Fig. 2). Nevertheless, there are not sufficient experimental data to obtain reliable parameters. Accordingly, theoretical methods provide the only practical means of deriving force field constants. The X-ray structural data of the available compounds 1-7 will be used to prove the quality of the new developed molecular modeling parameters.

S i \

Fig. 2. New amino(ch1oro)methyldisilanes 5-7.

5 R, = N(Me)Ph R2 = N(Me)Ph [20]

6 R1 =N(Me)Ph R2 = CI POI 7 R1 = CI R2 = CI PI

A set of model compounds was designee. which contains t.2 geometrical parameters of interest. Table 1 shows our set with changing numbers of dimethylamino groups and a list of all the possible configurations and conformations. The amino groups are kept simple to save computing time. The trans isomers should generally have the lower total energy compared to the gauche isomers. Therefore we restricted the model compounds to the trans isomers (C-Si-Si-C torsion angle) I-VII. Three isomers, RR-trans (111), SS-trans (IV), and RS-trans (V), were calculated of the

Development of Force Field Parameters 279

compound Cl(Me2N)MeSi-SiMe(NMez)Cl to evaluate the possible influence of different stereoisomers. The preferred conformation of the dimethylamino group was investigated with a potential energy surface calculation of the torsion angle Si-Si-N-C of compound VI. The geometries of the model compounds were fully optimized with RHF, B3LYP and MP2 methods. All calculations were made by using the 6-31G* basis set and the global minima were verified by calculating the Hessian Matrix [21].

Table 1. Set of model compounds.

Molecule ConfiguratiodConformation (Me2N)2MeSi-SiMe(NMe2)2 trans I

gauche

(MezN)zMeSi-SiMe(NMez)C1 R-trans I1 S-trans

R-gauche S-gauche

Cl(MezN)MeSi-SiMe(NMe2)Cl RR-trans 111 RS-trans V

RR-gauche RS-gauche

SS-trans IV

SS-gauche

ClzMeSi-SiMe(NMe2)C1 R-trans VI S-trans

ClzMeSi-SiMeClz

R-gauche S-gauche

trans VII

gauche

Force Field Parameterization

Force field parameters can be derived from structural parameters, such as bond lengths and angles, obtained from quantum chemical calculations. A detailed statistical analysis of the calculated model compounds was performed. Table 2 shows an example of the comparison between calculated and experimental structures. Please notice that the calculated model compound contains dimethylamino groups whereas the molecule from the X-ray structure has N-methylanilino groups. Therefore deviations in bond angles and the conformation of the molecules were expected.

The B3LYP/6-31G* method represents the closest match to the experimental values of the X-ray structures. The bond lengths agree well. Small deviations are found due to different configurations and conformations. We chose to use this method to develop force field constants [22]. To prove the quality of our new parameter set we optimized the structure of 1 with the extended MMX force field [23] and compared the structural data with the already known X-ray structure. Table 3 reveals good agreement of bond lengths. A large difference is found for the bond angles Si-Six.

280 U . Bohme, B. Schluttig, R . K. Szilhgyi

Table 2. Comparison of the crystal structure of 7 with the optimized geometry of 111

(bond lengths in A, angles in degrees).

Si-Si Si-N Si-C Si-Si-C Si-Si-N

RHF/6-31G* 2.372 1.718 1.876 113.61 111.64

B3LYPJ6-3 lG* 2.374 1.733 1.880 113.78 110.68

MP2/6-3 lG* 2.349 1.731 1.872 114,28 110.49

X-rav structure 2.421 1.744 1.898 112.53 110.26

Table 3. Comparison of bond lengths [A] and angles [deg] for ‘Bu(EtzN)2SiSi(NJ3z)~Bu (1).

Bond Lengths

Si-Si Si-N Si-C

Bond Angles Torsion

Si-Si-C Si-Si-N C-Si-Si-C

X-ray 2.476 1.743 1.938

structure 1.743 1.938

1.749

1.749

Optimized 2.475 1.773 1.987

structure 1.776 1.985

1.779

1.784

110.46 108.4 -180.0

110.46 108.4

114.92

114.92

115.0 108.8 -178.8

114.9 110.2

114.2

116.0

Conclusions

We have extended the MM2 force field with parameters for amino-substituted organooligosilanes. Applied to existing compounds and compared to their X-ray structures, we found that the parameters reproduce bond lengths very well. The parameter optimization is still in progress. The parameters for bond angles need especially some improvement.

Acknowledgment: Freiberg for supplying disk space and computing time.

Special thanks are given to the Computing Center of the TU Bergakademie

References [l] J. R. Maple, Force Fields: A General Discussion, in Encyclopedia of Computational

Chemistry (Editor in Chief P. v. RaguC Schleyer), John Wiley & Sons, Chichester, 1998, 1017. U. Bohme, B. Giinther, B. Rittmeister, Inorganic Chem. Commun. 2000,3,428. [2]

Development of Force Field Parameters 281

[3] K. Trommer, U. Herzog, G. Roewer, J . Prakt. Chem./Chem.-Ztg. 1997,339, 637; U. Herzog, K. Trommer, G. Roewer, J. Organomet. Chem. 1998,552,99.

[4] A. Kawachi, K. Tamao, J. Organomet. Chem. 2000,601,259. [5] J. Heinicke, S. Mantey, A. Oprea, M. K. Kindermann, P. G. Jones, Heteroatom. Chem. 1999,

10,605; J. Heinicke, S. Mantey, Heteroatom. Chem. 1998,9,311. [6] G. Huber, H. Schmidbaur, Monatsh. Chem. 1999,130, 133. [7] I. Rietz, E. Popowski, H. Reinke, M. Michalik, J. Organomet. Chem. 1998,556,67, [8] C. Ackerhans, B. Rtike, P. Miiller, H. W. Roesky, I. Uson, Eur. J. Znorg. Chem. 2000,827. [9] Th. Schlosser, A. Sladek, W. Hiller, H. Schmidbaur, Z . Natugorsch., Teil B 1994,49, 1247. [lo] G. Huber, A. Schier, H. Schmidbaur, Chem. Ber. 1997,130, 1167. [ 111 MMX: J. J. Gajewski, K. E. Gilbert, J. McKelvey in Advances in Molecular Modeling, Vol. 2

(Ed.: D. Liotta), JAI Press, Greenwich, CT, 1990, p. 65. [12] TRIPOS: M. Clark, R. D. Cramer, 111, N. Van Opdenbosch, J. Comp. Chem. 1989,10,982. [I31 UFF: A. K. Rappe, C. J. Casewit, K. S. Colwell, W. A. Goddard, W. M. Skiff, J. Am. Chem.

SOC. 1992,114,10024. [14] M. Unno, M. Saito, H. Matsumoto, J. Organomet. Chem. 1995,499,221. [I51 F. Huppmann, M. Noltmeyer, A. Meller, J. Organomet. Chem. 1994,483,217. [16] K. Tamao, A. Kawachi, Y. Nakagawa, Y. Ito, J. Organomet. Chem. 1994,473,29. [ 171 U. Wannagat, S. Klemke, D. Mootz, H. D. Reski, J. Organomet. Chem. 1979,178,83. [ 181 A. Sakakibara, Y. Kabe, T. Shimizu, W. Ando, J. Chem. Soc., Chem. Commun. 1991,43. [ 191 M. Weidenbruch, E. Lesch, K. Peters, J. Organomet. Chem. 1991,407,3 1. [20] U. Bohme, B. Gunther, B. Rittmeister, Eur. J. Znorg. Chem. 2003,751. [21] All quantum chemical calculations were performed using the GAUSSIAN 98 series of

programs: GAUSSIAN 98, Revision A.6, Gaussian, Inc., Pittsburgh PA, 1998. [22] The parameterization was performed by Robert Sziliigyi. [23] The modeling calculations were performed with PCMODEL Version 6.0 from Serena

Software, Bloomington, IN 47402-3076.

Novel Cyclic and Polycyclic Chalcogenides of Silicon

Uwe Herzoge, Uwe Bohme

Institut fur Anorganische Chemie, TU Bergakademie Freiberg D-09596 Freiberg, Germany

Tel.: +49 313 1 394343 - Fax: +49 373 1 394058 E-mail: Uwe.Herzog @chemie.tu-freiberg.de

Gerd Rheinwald

Institut fur Chemie, TU Chemnitz, Stral3e der Nationen 62, D-09111 Chemnitz, Germany

Keywords: silthianes, selenium, tellurium, polycycles, adamantanes

Summary: Starting from methylchlorooligosilanes, a great variety of new cyclic and polycyclic silicon chalcogenides is accessible by reaction with either HzS/NEt3 or LizE (E = S, Se, Te). The results show that products with Si3Ez or S4E five-membered rings are formed preferentially, or otherwise Si4E2 or Si3E3 six-membered rings are observed. Molecular structures of bicyclo[2.2.l]heptanes, bicyclo[3.3.0]octanes, bicyclo[2.2.2]octanes, adamantanes, noradamantanes, [3.3.3]propellanes and further ring systems are reported. In some cases also the heavier group 14 elements (Ge, Sn, Pb) have been introduced into these ring systems

The chemistry of organo-group 14 chalcogenides is dominated by the formation of cyclic and polycyclic compounds such as (RzME)~,~ and adamantane-like ( M E & They are formed by reaction of the corresponding chloro derivatives either with HzS in the presence of a base or with alkali chalcogenides.

Starting from methylchlorooligosilanes, a great variety of new cyclic and polycyclic silicon chalcogenides is accessible. Scheme 1 gives a summary of the products obtained from chloro- substituted disilanes and mixtures with chloro-substituted monosilanes, -germanes, -stannanes and -plumbanes and Fig. 1 shows some examples. DFT calculations on the conversion reaction represented by Eq. 1 have shown that the formation of the five-membered ring compound is accompanied by a decrease of 20 kJ/mol in the total energy which is in accordance with the observed favored formation of this ring size [ 11.

Besides its deduction from crystal structure analyses the formation of five-membered rings is also evident from the Si NMR spectra. The incorporation into a five-membered ring causes a strong

Ovganosilicon Chemisty V Edited by N. Auner and J. Weis

Copyright 0 2003 WILEY-VCH Verlag GmbH & Co. KgaA, Weinheim

Novel Cyclic and Polycyclic Chalcogenides of Silicon 283

downfield shift of the Si (as well as Sn or Pb) NMR signals in comparison with acyclic compounds with the same first coordination sphere at the silicon atom (e.g. Me2Si(ER)z or MeSi(ER)3 [ 2 4 ] ) while a smaller effect in the opposite direction can be seen in six-membered rings [5]. On the other hand, the "Se and '25Te NMR data show no special effect due to changes in ring size.

Me Me Me Me Me

Me,p,&/Me ,Si Si, 3 I I + 2 M e I I M e - 6 I si:

Me I S I Me s, Si / s Me I S Me Me / \ Me

Me Me

\ A,/ Me l S i A s \ I Me

,Si, / Me ,Si, ,Si,

Eq. 1

Me

Ti.€\ ,Me M

Me, I

;-, ,Si. Me

Me Me Me I Me, I ,E, I ,Me

M M I 1 I t

Si ,Si E, /E

Me,MNMe 2\ Me\ / c l / Me'&E hiMe

Me Me Me I ,E,I ,Me

'Si Si

, I "I

M I

~ Ph CI \ /

Me- Si -Si-Me / \

CI , CI

Me

I

I Me Me

E . Qi Me

'E,Si,

3~11 p P h E Si

Ph-.,.&/)'.E/ YMe Me > Me

Me

Me '"' "'Me /I (E = Se)

Me

Me? ,si. Me /,Me I si

/Si,se,Si, Me-7' Me Me Me

I "I , I E I: H,S / NE1, or Li,E (E = Se, Te)

II: a) Me,SiCI / Li / THF, b) AcCl/ AICI,

Me 111: PhMgBr

Scheme 1. Cyclic and polycyclic chalcogenides with disilane units (M = Si, Ge, Sn, Pb; E = S, Se, Te).

Starting from Z(SiMe2Cl)s (Z = SiMe, CH) the reactions with RMC13 (R = Me, Ph; M = Si, Ge, Sn) and Li2E (E = S, Se, Te) yielded bicyclo[2.2.2]octanes Z(SiMe2E)3MR in moderate to high yields [ 10, 111 (Fig. 2).

284 U. Herzog, U. Bohme, G. Rheinwald

Novel Cyclic and Polycyclic Chalcogenides of Silicon 285

Me6Si6S6 [ I ] (MeSi)& [91

Fig. 1. Molecular structures of some examples of the title compounds.

i

Fig. 2. Crystal structures of three examples of bicyclo[2.2.2]octanes Z(SiMe2E)3MMe (Z = CH, SiMe; M = Si, Ge,

Sn).

A new class of silicon chalcogenides with adamantane structures, (MeSi)4(CH&E4, resulted from the reaction of the carbosilane C12MeSi-CH2-SiMeC12 with LizE (E = S, Se, Te) [9] (Fig. 3).

Fig. 3. Crystal structures of the adamantanes (MeSi)4(CH2)2E4 (E = S, Se, Te).

286 U. Herzog, U. Bohme, G. Rheinwald

The tellurium compound was the first structurally characterized organosilicon telluride with an adamantane-like structure. Due to the larger Si-E bond lengths and smaller bond angles at the heavier chalcogen atoms, the adamantane structures become more and more distorted from the sulfur to the tellurium compound. Corresponding noradamantanes (MeSi)4(CHz)E4 resulted from reactions of a mixture of the carbosilane and tetrachlorodimethyldisilane with LizE [9].

Tricyclic silicon chalcogenides with a [3.3.3]propellane skeleton have been synthesized by reactions of hexakis(chlorodimethylsily1)disilane (Fig. 4) with LizE (E = S , Se, Te) [ 121 according to Scheme 2.

Me CI Me,l Me Me Me

Me Me SiO ye/,, Me Me

Me \Si'Si'Me 6 AcCl I AICI,

\ I . Si I Me.

Me.Si,Si\ ,Me / I I Si,

Me Me /Si, MeMe Me I Me Me Me

Me CI

Scheme 2. Formation of 3,7,10-trichalcogenaoctasila[3.3.3]propellanes.

0

Siz(SiMezC1)6 [ 121 Si2(SiMe2)6S3 [121

Fig. 4. Crystal structures of hexakis(chlorodimethylsily1)disilane and its reaction product with Li2S.

References [l]

[ 2 ]

U. Herzog, U. Bohme, G. Roewer, G. Rheinwald, H. Lang, J. Organomet. Chem. 2000, 602, 193. U. Herzog, G. Roewer, Main Group Metal Chem. 1999,22,579.

Novel Cyclic and Polycyclic Chalcogenides of Silicon 287

[3] U. Herzog, J. Prakt. Chem. 2000,342,379. [4] U. Herzog, Main Group Metal Chem. 2001,24,3 1. [ 5 ] U. Herzog, G. Rheinwald, J. Organomet. Chem. 2001,627, 23. [6] U. Herzog, G. Rheinwald, J. Organomet. Chem. 2002,648, 220. [7] U. Herzog, U. Bohme, G. Rheinwald, J. Organomet. Chem. 2001,627, 144. [8] U. Herzog, U. Bohme, E. Brendler, G. Rheinwald, J. Organomet. Chem. 2001,630, 139. [9] U. Herzog, G. Rheinwald, J. Organomet. Chem. 2001,628, 133. [ 101 U. Herzog, G. Rheinwald, Organometallics 2001,245369. [ 111 U. Herzog, G. Rheinwald, H. Borrmann, J. Organomet. Chem. 2002,660,27. [12] U. Herzog, G. Rheinwald, Eur. J. Znorg. Chem. 2001,3107.

Hypersilylchalcogenolate Derivates of Group 14 Elements

Heike Lunge, Uwe Herzog, Gerhard Roewer

Institute of Inorganic Chemistry, TU BA Freiberg Leipziger Str. 29, D-09596 Freiberg, Germany

Tel.: +49 3731 393583 -Fax: +49 3731 394058 E-mail: Heike.Lange@chemie.tu-freiberg.de

Keywords: hypersilyl, sulfur, selenium, tellurium, tin

Summary: The reaction of potassium hypersilylide KSi(SiMe3)3, prepared from Si(SiMe3)4 with KO'BU, with elemental chalcogens led to hypersilyl chalcogenolates KESi(SiMe3)s. Salt elimination reactions with organ0 group 14 halides yielded the acyclic hypersilylchalcogenolate derivates R,M[ESi(SiMe3)3]kx, (R = Ph, Me; M = Si, Ge, Sn, E = S, Se).

Introduction

Beside a great variety of cyclic silchalcogenides (R2SiE)2,3, polycyclic silsesquithianes and -selenanes with adamantane and doubledecker-like structures [ 1-31, a broad spectrum of acyclic silicon-sulfur and silicon-selenium counterparts is still unknown because of the high tendency to form cyclic compounds. So far most of these compounds are simple disilylchalcogenides (R3Si)2E. Furthermore there are some reports on organochalcogenolate derivatives (RE),SiR&, (ER = SMe [4], SBu [5], SeBu [6], TeBu [7]). We have been interested especially in gaining NMR data of acyclic silicon chalcogen compounds in order to examine trends in chemical shifts and to compare the data with those of known cyclic and polycyclic organosilicon chalcogenides. Because of the facile synthetic availability and stability we have chosen the hypersilyl chalcogeno unit to build acyclic chalcogenides containing R&i(ESi), (x = 1 4 ) units.

Results and Discussion

Hypersilylchalcogeno-Substituted Silanes

The hypersilyl anion has been prepared by several routes starting from tetrakis(hypersily1)silane with either MeLi [8] or KO'BU [9].

Elemental chalcogens react with silyl anions with formation of silylchalcogenolates (Eq. 1) [ 101.

Ovganosilicon Chemisty V Edited by N. Auner and J. Weis

Copyright 0 2003 WILEY-VCH Verlag GmbH & Co. KgaA, Weinheim

Hypersilylchalcogenolate Derivates of Group 14 Elements 289

30 -

20 -

- 10- k 5 0- n v

-10-

-20 -

Me Me Me, I ,Me Me, (.,Me

\ I

Me Si

""\ 9' / I M = Li, K Me / I si

Me si Me-Si-Si- M+ + E Me-Si-Si-E M'

Me/ I \Me Me E = S, Se, Te

Me' I \Me Me

Eq. 1. Formation of hypersilylchalcogenolates.

The potassium hypersilylchalcogenolates (E = S, Se) were reacted with organochlorosilanes under formation of hypersilylchalcogeno substituted silanes (Eq. 2). The NMR data are summarized in Table 1.

Me

RcXSiCIx - - x KCI

Me

Eq. 2. Hypersilylchalcogeno substituted silanes.

1

R4.xSi

Me

Ti ,Me

I 'Me Me/r\Me

Me

Me\I ,Me

-E-Si -Si -Me

X

-0- SSi(SiMe,), -A- SeBu -v- SeSi(SiMe,J,

v I I I I I

0 1 2 3 4

X

Fig. 1. 29Si NMR chemical shifts of Me6&i[(ESi(SiMe&1, and Me&(EBu),.

290 H. Lange, U. Herzog, G. Roewer

-200- -250-

-300- & -350: n a -400

% -450: -500- -550

v

Table 1. 29Si and 77Se NMR chemical shifts and coupling constants of R&3[(ESi(SiMe3)3]x, (E = S, Se).

___i_j/___. I

-

-

-

-

-

-

-

-

-

-502

-502

-508

4 1 2

4 1 7

4 2 7

-310

-302

-

-

-

-

-

-

-

-

-

123.4

132.2

162.3

180.1

188.3

193.4

196.5

-

-

-

-

-

-

-

-

-

117.6

122.0

124.4

114.6

114.7

117.0

117.1

15.6

8.1

2.0

-4.0

30.4

20.0

9.9

25.3

14.7

11.1

4.9

-0.2

25.9

16.2

8.0

8.6

-0.3

-58.0 60.6

-58.4 60.3

-57.7 59.3

-57.1 57.8

-54.4 58.8

-53.0 61.2

-51.9 57.4

-51.6 61.2

-50.2 58.8

-69.0 58.8

-68.0 58.3

-67.4 57.8

-61.8

-60.6 55.9

-59.3 55.8

-58.0 55.4

-56.6

-11.2

-11.1

-10.8

-10.5

-10.4

-10.2

-10.0

-10.0

-9.9

-11.6

-11.6

-11.5

-10.8

-10.6

-10.5

-10.5

-10.3 . . . ..

A comparison of the "Si NMR data of Me&3[(ESi(SiMe3)3]x with the corresponding Me4,Si(EBu), revealed significant differences in the chemical shifts for x > 1. An especially strong high-field shift occurs for the tetrakis(hypersilylcha1cogeno)silanes (Fig. 1). This may be interpreted as a result of steric overcrowding of these compounds containing four hypersilylchalcogeno units.

Fig. 2. 77Se NMR data as a function of the number of

SeSi(SiMe3)3 units (x) in Me4_xSi[SeSi(SiMe3)3]x.

-70 3 4

* x 1

Fig. 3. 6(29Si) of Me&3[Ea(SiMe3)3], as a

function of the number of ESi(SiMe3)3

units (x).

Hypersilylchalcogenolate Derivates of Group 14 Elements 291

An increasing number of SeSi(SiMe3)s groups in the molecules (x) causes a downfield shift in the 77Se NMR spectra (Fig. 2). Also, the central silicon atoms of the hypersilyl units show a downfield shift with an increasing number of hypersilyl units (Fig. 3).

Tetrakis(hypersilylcha1cogeno)-Substituted Silanes, Germanes and Stannanes

Reactions of potassium hypersilylchalcogenolates with SiCl4, GeC4 and SnC14 yielded the tetrasubstituted silanes, germanes and stannanes despite four sterically demanding substituents (Eq. 3). The NMR data are summarized in Table 2.

CI I Me

\ . 1 . . 4 Me-Si-St-E K+ + CI-M-CI -

I - 4 KCI Me' Si I CI

Me'LiMe E = S, Se, Te M = Si, Ge, Sn

Me MeMe Me \A< Me

Me,\Si I J.-Me

Me Me/ 'Si' '\Me Me \ /Me I

I Me 'si

Me I

Me E

\ /

Eq. 3. Formation of [(Me3Si)3E]4M.

Table 2. NMR data of M[ESi(SiMe,)& (M = Si, Ge, Sn, E = S, Se, Te).

[ (Me3Si)3SiS]4Si

[(Me3Si)3SiSe]4Si

[(Me&)3SiTe]4Si

[ (Me3Si)3SiS]4Ge

[ (Me3Si)3SiSe]4Ge

[(Me3Si)3SiTeI4Ge

[(Me3Si)3SiS]4Sn

[(Me3Si),SiSeI4Sn

5.9

-21.2

-1 12.0

-

-

25

-356

- -

235 -213

-468

- -

- -28

- -88

- -

1955 -208

-

115.0

269.7

-

123.9

272.0

-

132.5

-46.8 55.2 - -9.5

-51.9 54.4 - -9.8

-79.6 52.0 - -9.9

-47.1 57.8 - -9.6

-51.8 54.4 - -10.0

-76.2 51.0 - -9.0

4 . 6 56.9 64.1 -9.4

-52.5 55.2 55.8 -10.2

However, instead of the expected [(Me3Si)Si3Te]&, the reaction of SnC4 and potassium hypersilyltelluride yielded bis(hypersily1)ditelluride by reduction of the stannane (Eq. 4).

"Si NMR data of the middle silicon atoms of the hypersilyl units show that 6si depends on the nature of the chalcogen E but is almost independent to the nature of the central atom M. As observed earlier for other group 14 chalcogenides the 77Se and Iz5Te NMR chemical shifts of the

292 H. Lange, U. Herzog, G. Roewer

germanium compounds are downfield from the values found for the corresponding silicon and tin derivatives.

Me Me Me

\ I -2KCI Me

- SnClp / I

CI I I

2 Me -Si -Si -Te-K+ + CI -Sn -CI - Me -Si -Si -Te -Te-Si-Si -Me

Qi 'Me Me Si CI / I

Me Si Me' I 'Me Me' I 'Me Me' I 'Me

Me Me Me

Eq. 4. Reaction of potassium hypersilyltellurolate with tin tetrachloride.

Bis- and Tris(Hypersilylcha1cogeno)-Substituted Organostannanes

As for organochlorosilanes, the reactions of potassium hypersilylchalcogenolates with organochlorostannanes RzSnClz and RSnCl3 yielded R,,-xSn[ESi(SiMe3)3].. In the case of Ph2Sn[ESi(SiMe3)3]2 we obtained (PhzSnE)3 [ 111 and (Me3Si)sSiCl as by-products in 10-20 % yield. The molecular structures of MeSn[ESi(SiMe3)3]3 have been determined [12] (Figs. 4 and 5), and the NMR data of the new stannanes are given in Table 3.

Fig. 4. Molecular structure of MeSn[SSi(SiMe&],.

(Av.: Sn-S 2.396 A; Si-S 2.180 A; Sn-S-Si

117.1").

Fig. 5. Molecular structure of M ~ S I I [ S ~ S ~ ( S ~ M ~ ~ ) ~ ] ~ .

(Av.: Sn-Se 2.522 A; Si-Se 2.321 A; Sn-Se-Si

114.3").

Hypersilylchalcogenolate Derivates of Group 14 Elements 293

Table 3. 29Si, "Se and 'I9Sn NMR chemical shifts and coupling constants of R&3n[(ESi(SiMe3)3lx, (n = 1,2;

E = S, Se; R =Me, Ph).

Compound 8% 'JSS, hSn 8si Si&i 'Jsi+~se 'Jsisi 2Jssn &i&iMes

[(Me3Si)3SiS]3SnMe - - 115 -49.6 - 53.8 49.6 -10.5

[(Me3Si)3SiS]3SnPh - - 61 -47.6 - 58.8 51.4 -10.2

[(Me3Si)3SiSe]3SnMe -364 1686 -110 -58.3 127 56.9 45.7 -10.8

[(Me3Si),SiSeI3SnPh -384 1779 -140 -56.1 128 55.9 46.2 -10.5

[ (Me3SiC)3SiAS]2SnPh2 - - 11 -50.6 - 58.1 45.4 -10.6

[(Me3SiC)3SiASe]2SnPh2 -504 1595 -84 -59.5 125 56.9 42.4 -10.9

Acknowledgments: The authors thank Dr. H. Bomann, MPI f. Chemische Physik fester Stoffe (Dresden, Germany) for performing the crystal structure analyses and the Deutsche Forschungsgesellschaft for financial support.

References [ l ] U. Herzog, G. Rheinwald, J. Organomet. Chem. 2001, 627, 23. [2] U. Herzog, G. Rheinwald, J. Organomet. Chem. 2001,628, 133. [3] M. Unno, Y. Kawai, H. Shioyama, H. Matsumoto, Organometallics 1997, 16,4428. [4] E. V. Van den Berghe, G. P. van der Kelen, J. Organomet. Chem. 1976,122, 3290. [5] U. Herzog, G. Roewer, Main Group Metal Chemistry 1999, 22, 579. [6] U. Herzog, J. Prakt. Chem. 2000,342,379. [7] U. Herzog, Main Group Metal Chemistry 2001,24, 31. [8] H. Gilman, J. M. Holmes, C. L. Smith, Chem. Znd. (London) 1965,848. [9] C. Marschner, Eur. J. Znorg. Chem. 1998,221. [lo] B. 0. Dabbousi, P. J. Bonasia, J. Arnold, J. Am. Chem. Soc. 1991,113, 3186. [ 111 H. Lange, U. Herzog, U. Bohme, G. Rheinwald, J. Organornet. Chem. 2002,660,43. [12] MeSn[SSi(SiMe3)3]3: P21/n, a = 13.5590(5) A, b = 17.7502(8) A, c = 24.3564(9) A,

p = 91.58(1)"; and MeSn[SeSi(SiMe3)3]3: P 2 h a = 13.7468(4)A, b = 17.7159(4)& c = 24.5401(7) A, p = 91.22(1)".

Syntheses and Properties of Novel Cage-Shaped Molecules Containing an Extended Silicon

Backbone

Roland Fischer, Judith Baumgartner, Karl Hassler

Institut fiir Anorganische Chemie, Technische Universitat Graz Stremayrgasse 16,8010 Graz, Austria

Tel.: +43 316 873 8209 -Fax: +43 316 873 8701 E-mail: hassler@ anorg.tu-graz.ac.at

Guido Kickelbick

Institut fiir Materialchemie, Technische Universitat Wien Getreidemark 9, 1060 Wien, Austria

Tel.: +43 1 58801 15321 -Fax: 4 3 1 58801 15399

Keywords: silicon, cages, NMR spectroscopy, X-ray structures, NMR shift calculations

Summary: When branched hexabromoheptasilanes are reacted with divalent nucleophiles such as H20, H2S, NH3 or MeNH2, cage structures analogous to that of adamantane are obtained in almost quantitative yields. The reduction of hexabromoheptasilanes with bulkier terminal tert-butyl groups affords heptasilanortricyclenes, which have a molecular geometry often met in group 15 chemistry. The structures of these compounds were determined by NMR spectroscopy and X-ray crystallography. Structural features were further investigated by means of ab- initio calculations.

Introduction

A large number of compounds with tricyclo[2.2.1 .02*6]heptane and tricycl0[3.3.1. 13v71decane structures (nortricyclenes and adamantanes, respectively) are known in both organic and inorganic chemistry. In addition to the parent hydrocarbons, a large number of cages consisting of heavier group 15 and 16 elements are known. Therefore it is surprising that no such structures containing silicon-silicon bonds have been described so far. All adarnantane-analogue compounds containing silicon are characterized by alternating Si-E bonds. Their general formula is either (RzSi)4Xs with X = 0, S , Se, NH, PPh, or (RSi)& with X = P, As, where the silicon atoms form either the corners

Ovganosilicon Chemisty V Edited by N. Auner and J. Weis

Copyright 0 2003 WILEY-VCH Verlag GmbH & Co. KgaA, Weinheim

Syntheses and Properties of Novel Cage-Shaped Molecules 295

or the edges of the cages [I]. Only two nortricyclene analogue compounds E&(SiMe2)3 (E = P [2], As [3]) have been fully characterized by X-ray diffraction so far. These compounds do not contain Si-Si bonds as the SiMe2 groups are located between an apical E atom and a three-membered ring at the base of the molecule.

As nortricyclenes and adamantanes are structurally closely related we were able to obtain members of both classes from hexa-functional heptasilanes by reactions with nucleophiles and reductive elimination respectively. Products were characterized by means of X-ray diffraction and a combination of ab-initio calculations and NMR spectroscopy [4].

Results and Discussion

Starting from commercially available halosilanes, the hexaphenylheptasilanes 1 and 2 were prepared according to Scheme 1 [5 ] . Proto-dearylation with neat, liquid HBr afforded branched hexabromoheptasilanes l a and 2a.

Me3SiC1 + MeSiC13

1"" Me

rBuPh,SiCl Me3Si/,si,SiMe, MePh2SiC1

Ndxylenes I SiMe, I Ndtoluene + 1Me3siCI/A1C13

t tBuPh2SiSiPh2tBu MePh2SiSiPh2Me

Me Me Me2SiqS1-'SiMe2 H B ~ (1) M e 2 S i ~ s 1 ~ S i M e 2

RPh,Si 1 SiRPh, Br2RSi I SiRBrz I SiMe, I * ISiMe2 I SiRPh, SiRBr,

1 R=tBu 2 R=Me

Scheme 1. Syntheses of branched hexabromoheptasilanes l a and 2a.

la R=tBu 2a R=Me

Crystals of la suitable for X-ray diffraction were obtained upon recrystallization from n-pentane (Fig. 1). They were mounted onto the tip of a glass fiber, and the data collection was performed

296 R. Fischer, J. Baumgartner, G. Kickelbick, K. Hassler

with a Bruker-AXS Smart CCD diffractometer. The structures were solved by direct methods and refined by full-matrix least-squares method (SHELXL97) [6] .

Upon reaction of 2a with nucleophiles such as H20 or H2S, or amines like NH3 and MeNH2 (Scheme 2), desired heptasilaadamantanes were obtained in high to excellent yields. The products were characterized by means of NMR spectroscopy and X-ray diffraction (Fig. 2) . Comparison of selected experimental and calculated bond lengths and angles is given in Table 1. The theoretical values were obtained with Gaussian94 using RHF methods employing 6-3 1G* basis sets.

Me Me

Me,Sil/ ISiMe, Si\SiMe2 I MeSi.1. /SiMe

O'Me

Me2Si /Si\SiMeZ

I Me 7 tBu)iKTM>sitBU SitBu / si.Oo/

3 M e 2 S i T S ' l S i M e 2

Br2RSi I SiRBrz

la, 2a

I SiMe, I 7

SiRBr, Me

ISiMe, I \ iii); iv) ~ e , ~ i l / ~ ~ ~ S i M e ,

Et,O/n-heptane MeSi-1- N R + 3 M e /,SilNR

RN Me ii) H2S/imidazole, Et20/n-heptane iii) NH,, n-heptane

4 iv) MeNHz, n-heptane 5 6 v) Li-naphthalenide, THF

Scheme 2. Syntheses of adamantane- and nortricyclene-analogue compounds 3-7.

C1341

Fig. 1. X-ray structure of la. ORTEP drawing of 30 %

ellipsoids; hydrogen atoms are omitted for

Fig. 2. X-ray structure of 4. ORTEP drawing of

30 % ellipsoids; hydrogens are omitted for

clarity. clarity.

Syntheses and Properties of Novel Cage-Shaped Molecules 297

Table 1. Selected experimental and calculated bond lengths and angles for 3,4,5 and 6.

Si7Melo03 3 Si7MeloS3 4

Calc. Exp. Calc. Exp. Calc. Exp. Exp.

Si( 1)-Si(2)

Si(2)-Si(3)

Si(3)-X

Si(2)'-Si( 1)-Si(2)

Si( l)-Si(2)-Si(3)

Si(2)-Si(3)-X

Si(3)-X-Si(3)

X-Si(3)-X

2.394

2.364

1.660

105.8

101.3

106.4

128.7

105.5

2.354

2.343

1.656

104.9

102.1

108.7

126.9

106.4

2.380

2.369

2.169

107.0

107.5

111.0

108.5

111.6

2.339

2.342

2.151

105.5

108.5

112.2

105.0

112.2

2.396

2.372

1.746

106.6

102.1

108.0

126.2

105.5

2.343

2.342

1.728

105.7

103.9

109.3

123.6

106.7

2.351

2.357

1.720

106.7

101.6

107.3

124.2

107.5

By reduction of l a with lithium naphthalenide at -70 "C in THF the nortricyclene-analogue cage 7 is formed. Its structure was determined by a combination of ab-initio methods and NMR spectroscopy as no crystals suitable for X-ray diffraction could be obtained. Calculated NMR shifts were obtained with the GIAO method at RHF level using 6-31G* basis sets and are reported relative to the calculated shift of tetramethylsilane.

Table 2 gives a comparison of calculated NMR shifts and experimental values for 3, 4, 5 and 7. The good overall correlation is also represented in Fig. 3.

Table 2. Comparison of calculated and experimental 29Si Nh4R shifts.

Si7Me7tBu3 calc. -88.1 -74.3 -19.8

7 exp. -87.4 -80.5 -14.6

Si7Melo03 calc. -69.5 -39.2 0.2

3 exp. -80.0 4 0 . 3 1.0

Si7MeloS3 calc. -69.9 -31.7 23.4

4 exp. -78.4 -31.6 13.4

Si7Mel,,N3H3 calc. -68.6 -35.6 -4.8

5 exp. -80.0 -38.9 -0.7

*'Si INEPT-INADEQUATE NMR spectroscopy afforded further evidence for the nortricyclene structure of 7 as the coupling constants (Fig. 4) are consistent with the proposed atom arrangement.

298 R. Fischer, J. Baumgartner, G. Kickelbick, K. Hassler

-100

Exp. G[ppml NMR shift ;- -20 -

0 -

2 0 7

\ m 4 : + i

, I . I I

52.7 Hz

48.4 Hz

9.5 Hz

8.6 Hz

Fig. 3. Experimental vs. calculated 29Si NMR shifts. Fig. 4. ‘J and *J coupling constants for 7.

Acknowledgments: Financial support by the Fonds zur Forderung der wissenschaftlichen Forschung in Osterreich via the project P11878-CHE is gratefully acknowledged. Wacker Chemie GmbH, Burghausen, kindly provided various organosilanes as starting materials.

References [ 11 I. Haiduc, The Chemistry of Inorganic Ring Systems, Parts 1 and 2, Wiley, London, 1970. [2] G. Fritz, R. Uhlmann, Z. Anorg. Allgem. Chem. 1978,440, 168. [3] K. Hassler, Silicon-Phosphorus, -Arsenic, -Antimony, and -Bismuth Cages, in:

Organosilicon Chemisty 11: From Molecules to Materials (Eds.; N. Auner, J. Weis), VCH, Weinheim, 1996, p. 203. R. Fischer, J. Baumgartner, K. Hassler, Ch. Marschner, G. Kickelbick, Adamantanes, Nortricyclenes and Dendnmers with Extended Silicon Backbones, manuscript in prep. R. Fischer, J. Baumgartner, K. Hassler, Ch. Marschner, G. Kickelbick, Syntheses of Branched Functionalised Oligosilanes, manuscript in prep. G. M. Sheldrick, SHELX97 Programs for Crystal Structure Analysis (97-2), Universitat Gottingen, Gottingen (Germany), 1998.

[4]

[5]

[6]

Synthesis and Reactivity of Novel Tin-Modified Oligosilanes

Thorsten Schollmeier, Markus Schiirmann, Frank Uhlig*

Anorganische Chemie 11, Universitat Dortmund Otto-Hahn-Stral3e 6, D-44221 Dortmund, Germany

Keywords: tin-substituted oligosilane, cyclic silane, silicon-tin bond

Summary: The synthesis of four-, five-, and six-membered tin-substituted oligosilanes via the reactions of dilithio- or dichloro-substituted V,T-bis(diorganylstanny1)oligo- silanes is reported. Especially, the reactions with the dilithio derivatives allow an easy access towards Si-Sn compounds with additional elements in the ring skeleton. The reaction of a branched dihydrotin oligosilane with lithium diisopropylamide (LDA) and CC14 yields a five-membered ring with an exocyclic chlorosilyl substituent. The X-ray structure of a bis(hydridodiorganylstanny1)oligosilanes is also shown.

The chemistry of mono- and polycyclic oligosilanes has been well investigated, but only little is known about cyclic silanes containing the higher elements of group 14. In recent years our studies have been focused on tin-modified oligosilanes, compounds which contain at least one silicon-tin bond [ 1-51.

One synthetic route towards such derivatives is the reaction of lithium hydridodiorganylstannides (RR'Sn(H)Li) with a,o-dichlorodiorganosilanes (Cl-(SiMe&-Cl, n = 1-6) yielding the bis(hydridodiorganylstanny1)oligosilanes 1 and 2 nearly quantitatively. Besides the recently published di(t-buty1)tin derivatives 1 [ 13 compounds with chiral tin centers (2) were also available on this reaction pathway (Eq. 1).

CI-(SiMe,),-CI + 2 RR'Sn(H)Li - RR'(H)Sn-(SiMe,),-Sn(H)RR' - 2 LiCl

1,2

R,R-='Bu la , n = l R=Ph, I?'='&

lb , n = 2 2b, n = 2

lc , n = 3 2c, n = 3

Id, n = 4 2d, n = 4

le, n = 6

Eq. 1.

Ovganosilicon Chemisty V Edited by N. Auner and J. Weis

Copyright 0 2003 WILEY-VCH Verlag GmbH & Co. KgaA, Weinheim

300 T. Schollmeier, M. Schurtnunn, F. Uhlig

Figure 1 shows the molecular structure of la. Compound lb, the first in this series of derivatives, crystallizes in the monoclinic space group Cc. Selected bond lengths and bond angles are given in Table 1.

Fig. 1. Molecular structure of la.

Table 1. Selected bond lengths and angles of la.

Bond length [A] Bond angle [“I Sn( 1)Si ( I ) 2.594(4) C(21)-Si( 1)-C(22) 107.8(9)

Sn(2)-Si( 1) 2.561(6) Sn( 1)-Si( 1)-Sn(2) 112.5(2)

Si( 1)-C(21) 1.915(15) C(5)-Sn( 1)-C( 1) 112.3(7)

Sn(l)-C(I) 2.207( 16) C(5)-Sn( I)-Si( 1) 110.2(4)

C(1)-Sn( I)-Si( 1) 113.2(4)

V,T-Bis(diorganylhydridostanny1)oligosilanes of types 1 and 2 are suitable precursors for the synthesis of cyclic tin-modified oligosilanes. They can easily be converted into V,T-dihalogen -substituted stannylsilanes of type 3 by reacting with two equivalents of haloforms (Scheme 1). The dihalogenated chains 3 give access to the cyclic compounds 5-7 containing a distanna bridge in the ring skeletons [2]. For example, reaction of 3 in the presence of magnesium leads to the four-, five- and six-membered rings 5-7 by elimination of magnesium chloride or bromide.

The dilithio species 4 are available by the reaction of 1 or 2 with two equivalents of lithium diisopropylamide (LDA). Surprisingly, these syntheses are accompanied by a number of side reactions for the di- and tetrasilanes yielding six- ([-‘BuzSn-SiMe2-SiMe2-12 for n = 2) and five-membered rings ([-‘Bu2Sn-(SiMe2)5-] for n = 4) as byproducts [4]. Nevertheless, compounds of type 4 can be used as starting materials for the synthesis of a large variety of tin- and silicon-containing rings (9, 10a) as well as for novel heteroatom-containing ring systems (8, lob). It should be noted that the phosphorus-containing heterocycle 8 has been unique until now. Reactions of 4 with phenyl- or ethyldichlorophosphine always result in a Si-Sn bond cleavage and

Synthesis and Reactivity of Novel Tin-Mod@ed Oligosilanes 301

transmetallation products for the phosphorus part only are observed.

Me, 3,

‘ SnRR‘ MepSi ’ SntBu, Me2Si SntBu,

Me,Si Sn’Bu, \ SnRR‘ MepSi ,SntBu,

Me2Si

M%SI,

Me$ Si Me2 6a, R, R’= ‘Bu

4 5 4 6b, R = Ph, R’= ‘Bu 4 7

1 + M g n = 4 ~ + M g - MXZ - mx, + Mg n = 3

- MgX, n = 2

RR’(X)Sn-(SiMe,),Sn(X)RR’ 3

+ 2 CHX ~ - 2CH,X, x = CI, Br 3~

1; R, R’=‘Bu, Ph

2; R, R’=‘Bu RR’(H)Sn-(SiMe,),-Sn(H)RR’ 1,2 n = 2 - 6

+ 2 LiNfPr), - 2 HN(’Prh

1 RR’(Li)Sn-(SiMe,),-Sn(Li)RR‘ 4

~

I

+ R”$Cl, n = 2 ! I - 2 LiCl

1 + R“$CI, 1 -2LiCl n = 2

1 t f tBu , tBu2 R=Me,Ph tBu, Sn Sn Sn

M e,Si MepSi \ Me,SJ ,

Me$i .. Me,Si ’ ‘Sll ‘Sn ‘Sn

PtBu Si Ri’ I ER“, M ~ S I 1

‘b 2 ‘Bu, tB up

a 9a, R” =Me

%, R” = Ph

Scheme 1. Synthesis and reaction behavior of the tin-modified oligosilanes 1-10.

lOa, R”2E = ’BupSn

1 Ob, R‘;E = Me,Ge

The branched trihydridotin compound 11 reacts with LDA in a complex bond cleavage-bond closure reaction to the monocyclic ring 13 with an exocyclic dimethylsilyl unit [3]. Tetra(t-buty1)distannane [5] is formed as the second product. The remaining hydrido function at the

302 T. Schollmeier, M. Schurmann, F. Uhlig

exocyclic silicon atom can be converted into a chloro substituent by treating compound 12 with CCld (Scheme 2).

A much simpler access to the chlorosilyl derivative 13 and further investigations into the reactivity of 13 are in progress and will be discussed later.

Me,

SiMe,- Sn(H)'Bu, + 3 LiN 'Pr2 Me\ /Si'Sn'Bu,

MeSi-SiMe,-Sn(H)'Bu,

SiMe,- Sn(H)'Bu,

si I Si

/

\ Me,(H)Si/ \ ,Sn'Bu, - I / , 'Su,Sn(H)-Sn(H)'Su,

Me,

+ CC14 - CHCI, 1 l2 11

13

Scheme 2. Reaction of the branched oligosilanes 11 with LDA and CCL.

Acknowledgment: The authors thank the Deutschen Forschungsgemeinschaft and the federal state Nordrhein-Westfalen for financial support; the ASV-innovative Chemie GmbH (Bitterfeld) is gratefully acknowledged for the generous gift of silanes. We are grateful to Prof. Dr. K. Jurkschat for support.

References [l] F. Uhlig, C. Kayser, R. Klassen, U. Hermann, L. Brecker, M. Schiirmann, K. Ruhlandt-Senge,

U. Englich, Z. Naturforsch. Teil B 1999,54,278-287. [2] U. Hermann, M. Schiirmann, F. Uhlig, J. Organomet. Chem. 1999,585,211-214. [3] B. Costisella, U. Englich, I. Prass, M. Schiirmann, K. Ruhlandt-Senge, F. Uhlig,

Organometallics 2000,19,2546-2550. [4] U. Hermann, G. Reeske, M. Schiirmann, F. Uhlig, Z. Anorg. Allg. Chem. 2001,627,453-457. [5 ] U. Englich, U. Hermann, I. Prass, T. Schollmeier, K. Ruhlandt-Senge, F. Uhlig, J.

Organomet. Chem. 2002,646,271-276.

Mutual Effects between the Trialkylsilyl Substituents and the M,P, Cages of

Phosphanediides (M = Mg, Ca, Sr, Ba, and Sn)

Matthias Westerhausen, Stefan Schneiderbauer, Sabine Weinrich

Department Chemie, Ludwig-Maximilians-Universitat Munchen Butenandtstr. 9 (House D), D-81377 Munich, Germany

Tel.: 4 9 89 2180 7481 -Fax: +49 89 2180 7867 E-mail: maw @cup.uni-muenchen.de

Keywords: alkaline earth metals, cages, phosphanediides, tin, trialkylsilyl substituents

Summary: The phosphanediides of the alkali metals, magnesium, and tin($ oligomerize and form spherical cages. The degree of aggregation depends strongly on the steric demand of the trialkylsilyl substituents. The partial substitution of tin atoms in [SnPSitBus]4 preserves the heterocubane structure, the general formula for these heterocubane structures being [MnSwn(PR)4] with n = 0, 1, 2, and 3. Homometallic cages of the heavier alkaline earth metals show the trigonal M2P3 bipyramid as the structure-dominating motif. These bipyramids can be combined via common faces thus leading to M3P4 cages. For the less reactive arsanediides an aggregation via common corners is also observed. Homoleptic phosphanediides of the heavy alkaline earth metals are unknown as yet.

Introduction

The phosphanides of the alkali metals regained high interest in the last decade for several reasons. Due to the commercial availability of butyl- and methyllithium, primary and secondary phosphanes are easily lithiated. The diagonal relationship between P and CH could imply a chemical similarity between carbanions and phosphanides [ 11. Therefore, the structure-reactivity relationship is one of the main interests in this area. Furthermore, the lithium phosphanides are widely used reagents in organometallic synthesis. The most common examples are phosphanide transfers to other metals (metathesis reactions), Lewis acid-base interactions (addition reactions), and transformations of the phosphanide ligands (such as for example for the phosphaalkene and -alkyne synthesis). Regarding the importance of these lithium phosphanides it is not surprising that numerous review articles deal with this subject [2].

Dimeric molecules of the type [MX2]2 of the heavier alkaline earth metals M tend to form polycyclic molecules (cages). Kaupp and Schleyer [3] calculated the structures of the hydrides of

Ovganosilicon Chemisty V Edited by N. Auner and J. Weis

Copyright 0 2003 WILEY-VCH Verlag GmbH & Co. KgaA, Weinheim

304 M. Westerhausen, S. Schneiderbauer, S. Weinrich

magnesium, calcium, strontium, and barium. Whereas monocyclic molecules of the type HM(p-H)zMH are the minimum structures for Mg and Ca, bicyclic structures with C3” symmetry of the type HM(p-H)3M are expected for Sr and Ba. The reason for these somewhat unexpected structures is a combination of the polarization of the heavy, soft metal dications by the hard anions (“reverse” polarization) and a small but significant d-orbital participation at the metal atoms [3]. Neutral coligands such as THF should not alter the coordination geometries, but influence the bond lengths and angles.

Results and Discussion

The phosphanediides of lithium tend to form oxygen-centered cages consisting of lithium and phosphorus atoms. If oxygen and moisture were excluded carefully during the preparation procedures, the phosphanediides of lithium showed a metal deficiency and compounds of the type [(LizPR),(PR),] with Li2nP(n+m) cages were isolated. The formation of spherical polyhedra seems to

be the structural motif for the lithium

n Sn[N(SiMe,),], + n H,E-SiR,

I

phosphanediides. Investigations of phosphanides of the heavier alkali metals are far less common. However, the tetraanion of the heterobimetallic solvent-separated ion

pair (Li+)2 [ (THF)6Na+] 2 [Naz0(PSiiPr3) I ~ “ 1 consists of a phosphorus icosahedron with the sodium atoms above each face.

Scheme 1 shows the synthesis of tin@) trialkylsilylphosphanediides 1 and 2 as well as the arsanediide 3. The steric demand of the trialkylsilyl substituent determines the degree of aggregation. Similar observations were made for the magnesium complexes

(Scheme 2) . The magnesium tri(tert-buty1)silylphosphanediide (4 to 7) and asanediide (8) crystallized as a tetrameric structure with a central Mg4E3 heterocubane moiety. Decreasing steric strain by omitting the neutral coligands such as THF or other ethers led to the formation of hexamers with an inner hexagonal Mg6E6 prism, as found for 9 (E = P) and 10 (E = As). The addition of Lewis bases gave the compounds 11,12, and 13, but no formation of 7 was observed.

Mixed phosphanideslphosphanediides were available by choosing a different stoichiometry. In 14 a dimeric magnesium trialkylsilylphosphanediide is capped by two magnesium trialkylsilylphosphanide molecules, whereas in 15 two opposite square planes of a hexagonal Mg6P6 prism are capped by Mg[P(H)SiR& molecules. In the latter complex, these molecules can be substituted by THF to give 16, which is similar to 11 but with smaller trialkylsilyl groups.

For the chemistry of the heavier alkaline earth metals characteristic differences are observed. The motif of a hexagonal M6E6 prism is as yet unknown for the phosphanediides of calcium,

- 2n HN(SiMe,), n=4 F i n = .

7 n - p Sn-

Sn Sn

fgn pJqJ Sn

0 = PSitBu, (1)

Scheme 1.

0 = PSiiPr, (2), AsSiiPr, (3)

Synthesis of the cage compounds 1,2, and 3.

Mutual Effects between the Trialkylsilyl Substituents and the Phosphanediides 305

strontium, and barium. The trigonal M2E3 bipyramid with the metal atoms in apical positions seems to play a dominant role. The bis(ph0sphanides) of calcium and strontium favor the bicyclic

according to ab initio calculations; this was found also for dimeric bis[bis(trialkylsilyl)phosphanide]s of calcium, strontium, and barium [4]. The incorporation of phosphanediide substituents according to Scheme 3 leads to M3P4 cages which can be regarded as two trigonal bipyramids with a common face. This cage can be capped by [MPSiR3]2 moieties (20) or dimerize (21 to 25). The lower reactivity of the trialkylsilylarsane led to a higher arsanidehrsanediide ratio and to trigonal MzAs~ bipyramids with a common corner (17 to 19). Homoleptic phosphanediides or arsanediides of the heavier alkaline earth metals are as yet unknown. However, such molecules can be prepared by incorporation of tin(1I) atoms into the cage compounds, as shown in the bottom part of Scheme 3. The heterobimetallic phosphanide of barium and tin($ in a ratio of 1:2 (26) gives bicyclic structures, whereas a ratio of 2: 1 leads to a cage compound of two trigonal bipyramids with a common MP2 face, thus giving a mixed phosphanidelphosphanediide compound (27 to 29). Homoleptic

structures H~P-M(/.I-PH~)~M

L

L L

0 = PSiiPr,, L = THF (16)

- 2 Mg[P(H)SiiPr,], 1 + 4 THF

0 = P(H)SifBu,, 0 = PSitBu, (14) 0 = P(H)SiiR,, 0 = PSiiPr, (15)

- (x + 2y) BuH

= 4+n 112 x + y = n = I

n MgBu, + (x + y) H,E-SiR,

I x + y = n

n = 4 - 2n BuH n = 8 xly = 412 xly = 416

L\ @CL Mg @g Mg

0 = PSifBu, (9). AsSifBu, (10) /Mg

L

J 0 = PSiiPr,, L = THF (4),

0 = PSitBu,, L = THF (7)

0 = AsSiiPr,, L = THF (8)

DME (5). EhO (6) L\

L

SC

0 = PSitBu,, L = THF (11). PhCN (12), Ph-NCO (13)

L.

?me 2. Synthesis of homo- and heteroleptic magnesium

complexes with phosphanediide ligands (4 to 16).

phosphanediides of tin($ and calcium (31) or barium (30, 32, 33) form heterocubane structures which can be derived from the homometallic tin@) compound 1.

306 M. Westerhausen, S. Schneiderbauer, S. Weinrich

Acknowledgments: We thank the Deutsche Forschungsgemeinschaft and the Fonds der Chemischen Industrie for generous financial support.

References [ l ] K. B. Dillon, F. Mathey, J. F.

Nixon: Phosphorus: The Carbon Copy, Wiley, Chichester, 1998. a) G. Fritz, P. Scheer, Chem. Rev. 2000, 100, 3341-3401; b) K. hod, Adv. Inorg. Chem.

Driess, Adv. Inorg. Chem.

M. Kaupp, P. v. R. Schleyer, J. Am. Chem. SOC. 1993,115,

[4] a) M. Westerhausen, Trends Organomet. Chem. 1997, 2, 89-105; b) M. Westerhausen, Coord. Chem. Rev. 1998,

M. Westerhausen, C. Birg, H. Piotrowski, T. Habereder, M. Suter, H. Noth, Z. Anorg. Allg. Chem. 2001, 627,

[2]

2000, 50, 33-107; C) M.

2000,50,235-284. [3]

11202-1 1208.

176,157-210.

[5]

882-890.

M = Ca (17). Sr (18),

E = A s R = i h

Ba (19)

0 =P(H)SiR, 0 =PSiR,

M & M = Ca (%I) E = A s R = tBu

E = P, R = iPr: M = Sr (21); E = P, R = tBu: M =Ca (22).

E = As, R = tBu: M = Ba (25) Sr (23). Ba (24);

I I I

n l i= 1/21 nli = 1/2,2/1/ n/i = 1/3,2/2,3/1{ an Sn M = Ca (27),

Ba (28)

26

&I Ca

0 =PRR 0 =PR R = SiMe,, SitBu, R = H, SiMe,

29

a n Sn

30 a n Sn

M = Ca (31). Ba (32) a n Ba

33

Scheme 3. Synthesis of homo- (above) as well as heterometallic

complexes (below) of the heavy alkaline earth metals

and tin(n).

Differing Affinities of the Triorganylsilyl and -stannyl Substituents for Oxygen or Nitrogen, and Phosphorus, Respectively

G. Becker, G. Ditten, S. Horner

Institut fur Anorganosche Chemie der Universitat Stuttgart Pfaffenwaldring 55, D-70569 Stuttgart, Germany

Tel. + 49 711 685 4172 - Fax. + 49 711 685 4201 E-mail: becker@iac.uni-stuttgart.de

A. H. Maul&, E.-U. Wiirthwein

Institut fiir Organosche Chemie der Westfiilischen Wilhelms-Universit Corrensstr. 40, D-48149 Munster, Germany

Keywords: acylbis(trimethylstannyl)phosphanes, amino-h3-phosphaalkyes, imino- methylidenephosphanide anion, lh3,3h3-diphosphetanes

Summary: The differing affinities of triorganylsilyl and -stannyl substituents for oxygen and phosphorus, respectively, are illustrated by the structures of acylbis(trimethylsilyl/trimethylstannyl)phosphanes, and similarly for nitrogen and phosphorus by the reaction products of the ['Pr-N=C=P]- anion.

Acylbis(trimethylstanny1)phosphanes

Acylbis(trimethylsilyl)phosphanes, R-CO-P(SiMe&, easily accessible from either the tris(trimethylsily1) compound or lithium bis(trimethylsily1)phosphanide and the corresponding acyl chloride, have been known since the mid-1970s to be thermally unstable. They rearrange via migration of a trimethylsilyl group from phosphorus to oxygen (Scheme 1). The h3-phosphaalkenes thus formed are valuable starting materials in the synthesis of h3-phosphaalkynes [ 11.

Whereas the reaction of acyl chlorides R-CO-Cl (R = Me, MeO, 'Bu, 1-adamantyl, mesityl) with tris(trimethylstanny1)phosphane leads successfully to the analogous bis(trimethylstanny1) derivatives, R-CO-P(SnMe&, the corresponding trimethylstannyl migration does not take place. As evidenced by NMR and IR spectroscopic studies, both trimethylstannyl groups remain bound to phosphorus [2]. Indeed, quantum mechanical calculations on H-CO-P(SnH3)2 show this isomer to be 1.24 kcal mol-' more stable than the h3-phosphaalkene, whereas for silicon, the h3-phosphaalkene is favored by 7.98 kcal mol-'. Activation energies AH# of the corresponding

Ovganosilicon Chemisty V Edited by N. Auner and J. Weis

Copyright 0 2003 WILEY-VCH Verlag GmbH & Co. KgaA, Weinheim

308 G. Becker, G. Ditten, S. Homer, A. H. Maulitz, E.-U. Wiirthwein

migration were calculated to be 11.46 and 20.85 kcal mol-', respectively.

E = Si, Sn y\ y e 3 Me,Si-0, n C=P+SiMe,

8 - Me,E-CI R EMe, E = Si only d - F-P\ RdC,C, + P(EMeJ,

Scheme 1.

An X-ray structural analysis of the 1-adamantoyl compound (m.p. 57 "C) reveals a short intramolecular Sn...O contact of 299.7 pm and an eclipsing of the C=O and one of the P-Sn bonds (Fig. l), suggesting that the stannyl group stabilizes the acyl isomer. The affinity of tin for oxygen is, in contrast to silicon, not large enough to effect a migration [3]. The colorless crystalline compounds become yellow in the melt and in solution, indicating the existence of different conformers. Characteristic NMR data for the 1 -adamantoy1 compound (benzene-&) are as follows:

P -120.4, Il9Sn +11.1 ppm; 'Jsnp 617.2 Hz. 31

C38 P21Ic; R1

c1=01 c1-P P-Snl P-Sn2

0.0557; 173(2) K

121.7 pm 187.7 pm 251.7pm 251.1 pm

C1-P-Snl C1-P-Sn2 Ol-Cl-P-Sn2

98.3" 93.1° -1.1O

Fig. 1. Molecular structure of 1-adamantoylbis(trimethy1stannyl)phosphane.

The ['Pr-N=C=P] - Anion

The h3-phosphaalkyne 'Pr(MesSi)N-C=P, prepared from tris(trimethylsily1)phosphane and isopropyl isocyanate via addition and NaOH-catalyzed elimination of hexamethyldisiloxane [4] and characterized by a low-temperature X-ray structural analysis (Fig. 2a), reacts with potassium tert-butoxide to yield the corresponding salt K+['Pr-N=C=P]- (Scheme 2) [5].

The potassium salt was structurally characterized as the [ 18]crownd ether complex (Fig. 2b). A rather long C-P and relatively short C-N distance with respect to the initial h3-phosphaalkyne (Fig. 2a) classify the anion as an iminomethylidenephosphanide rather than an anionic lh3-phosphaalkyne. The potassium cation lies 58.9 pm above the plane formed by the oxygen atoms of the crown ether [6].

Differing AfJinities of the Triorganylsilyl and -stannyl Substituents 309

Me,Si Tetraglyme : e N a O H L + 20°C; . (Me,Si),O

MeCH 0-SiMe, N - C I P

\ / Me,CH N=C\

'N=C=O ,P-SiMe, Me,CH

+

Me$

<THF> : -50°C

E b 0 ; 3 d

r.t.

P(SiMe,), Me,CH \ P N-C,

Me3Si' ,P-SiMe, Me,CH 7- Me,Si ["-.I 'N=C=P

Scheme 2.

a)

9 C11' 173(2) K

PS 155.8pm

C-N 131.5pm

N U 149.9pm

N-Si 177.4pm

P S - N 178.7'

C-N-C1 117.0"

C-N-Si 123.5'

173(2) K

P=C 160.3pm

C=N 124.8pm

N-CI 149.0pm

P=C=N 174.8'

C=N-C1 119.3'

K...N 320.8 pm

K...C 304.1 pm

K...P 340.3 pm

Fig. 2. Structures of a) 'Pr(Me3Si)N-C=P (Ref. [6]), and b) [K([18]crown-6)1'['Pr-N=C=P]-.

Reactivity of the ['Pr-N=C=P]- Anion

The potassium salt reacts differently with chlorotriorganylsilanes and -stannanes (Scheme 3) [7]. The stronger affinity of the silicon reagents for nitrogen over phosphorus leads again to h3-phosphaalkynes, whereas the softer tin compounds attack at phosphorus, thereby forming 1h3-phosphaketene-imines, 'Pr-N=C=P-SnR3. Compounds of this type are known to dimerize [ 81 at the highly reactive C=P bond to give 2,4-bis(isopropylimino)- 1,3-triorganylstannyl- 1 h3, 3h3-diphosphetanes as a mixture of Ci and C, isomers. Characteristic NMR data are compiled in Table 1 and the structure of the Ci isomer is shown in Fig. 3. The analogous germanium and lead compounds are currently being investigated.

310 G. Becker, G. Ditten, S. Homer, A. H. Maulitz, E.-U. Wurthwein

Isomers

\ N=C=P Me,CH HCMe,

Scheme 3.

Table 1. Characteristic NMR data [ppm, Hz] for the h3-phosphaalkyne 'Pr(Me3Si)N4kP and its derivatives.

Silicon compounds 3lP l3C 'JCP 2 9 ~ i 3Jsi~

Kf [ 'Pr-N=C=P]- -226.1 +178.8 45.7 - -

'Pr(Me3Si)N-C=P -137.6 +153.9 18.2 +13.2 3.5

'Pr(Ph3Si)N<=P -129.5 +154.1 21.4 -9.3 3.1

Tin compounds 31P lI9sn ' J s e 3 J s e 2JPP

['Pr-N- -P-SnMe312 Ci +55.0 +4.6 679.4 10.5 -

C, +36.21+76.8 +13.41-3.2 679.01687.5 4.711 1.4 34.3 TI

['Pr-N=C-P-SnPh3lz Ci +53.1 -1 19.6 818.1 7.6 -

C, +30.71+75.8 -1- ['I 828.11844.2 -1- 34.0 I I

[a] Signals of C, isomer too weak to be observed in Il9Sn NMR spectrum.

- P1; R10.0464; 173(2) K

Nl=Cl 125.7 pm

c1-P1 187.6 pm

PI-Snl 252.5 pm

Cl=Nl-C2 118.5'

Nl=Cl-Pl 132.5'

c 1-PI-c 1 ' C2-Nl=Cl-P1 -3.1"

8 1.1

Fig. 3. Structure of 2,4-bis(isopropylimino)-l ,3-bis(triphenylstannyl)-1h3,3h3-diphosphetane, Ci isomer.

Direring AfJinities of the Triorganylsilyl and -stannyl Substituents 311

Calculations on the ['Pr-N=C=P]- Anion

Quantum chemical DFT calculations (B3LYP/6-31+G**) were canied out on the ['Pr-N=C=P]- anion and the results are shown in Fig. 4a. The largest negative charge was established from an NPA (Natural Population Analysis) to be on nitrogen, which is therefore the site of preferred kinetic attack of hard electrophiles (i.e., silicon). In contrast, Fig. 4b shows that the largest HOMO coefficient is on phosphorus, which is thus the preferred site of kinetic attack of soft electrophiles (i.e., tin).

Fig. 4. a) Molecular parameters [A, deg] and NPA charges shown in brackets of the [ 'Pr-N=C=P]- anion. b) HOMO

of the ['Pr-N=C=P]- anion (RHF/6-31G*).

References [ I ]

[2]

[3]

[4] [5]

[6]

[7] [8]

R. Appel in: Multiple Bonds and Low Coordination in Phosphorus Chemistry (Eds.: M. Re- gitz, 0. J. Scherer), Thieme Verlag, Stuttgart 1990, p. 157ff; M. Regitz, ibid., p. 58ff. For earlier studies on similar Sn-P compounds, see e.g.: Yu. A. Veits, E. G. Neganova, A. A. Borisenko, V. L. Foss, J. Gen. Chem. USSR (Engl. Transl.) 1990,60, 1822. G. Becker, G. Ditten, A. H. Maulitz, W. Schwarz, E.-U. Wiirthwein, Z. Anorg. Allg. Chem., to be published. R. Appel, M. Poppe, Angew. Chem. Znt. Ed. Engl. 1989,28,53. G. Heckmann, G. Becker, S. Homer, H. Richard, H. Kraft, P. Dvortsak, 2. Natu~orsch. Teil B 2001,56, 146. G. Becker, M. Bohringer, S. Homer, W. Schwarz, R. Streubel, E.-U. Wiirthwein, to be published. G. Becker, S. Homer, E.-U. Wurthwein, unpublished results. G. Becker, H. Riffel, W. Uhl, H.-J. Wessely, Z. Anorg. Allg. Chem. 1986,534,31.

Mono- and Oligosilanes with Pyrazole Ligands

Klaus Hiibler, Jan Uwe Berner, Steffen E. F. M e n

Institut fur Anorganische Chemie, Universitat Stuttgart Pfaffenwaldring 55, D-70569 Stuttgart, Germany

Tel.: +49 711 685 4205 -Fax: 4 9 1212511 676047 E-mail: k.huebler@rus.uni-stuttgart.de

Keywords: silanes, germanes, pyrazoles, X-ray structures, ab initio calculations

Introduction

Halogenosilanes SiH,&, (n = 1 4 ) react with nitrogen-containing organic bases to give

different Lewis acid-base adducts with molar compositions of 1:1, 1:2 and 1:4 as reported by Ebsworth et al. in 1966 [l]. Under certain circumstances the amine can act as a catalyst to the dismutation of the halogenosilane. This effect becomes more dominant as the reaction temperature rises. Hensen et al. [2] found the silicon atoms, in the products of the reactions of halogenosilanes with aromatic N-heterocycles such as substituted pyridines, to be hexacoordinated, and the halogenosilanes often to react with dismutation according to Eq. 1.

2 SiHnXkn # SiHn-lXkn+l + SiHn+lWn-l n = 1-3

Eq. 1.

In 1998 Boudjouk et al. published similar results [3] for the reaction of trichlorosilane with tetraethylethylenediamine (teeda) or tetramethylethylenediiamine (tmeda), showing that a 1 : 1 adduct is formed with or without previous dismutation, yielding SiHzClz4eeda or SiHC134meda, respectively.

In addition to these base-catalyzed dismutations, another common reaction of hydrohalogenosilanes is the base-induced elimination of hydrogen chloride (Eq. 2) [4].

EHC13 # "EC12" + HC1 E = Si, Ge

Eq. 2.

We report here the reactions of trichlorosilane and trichlorogermane with various substituted pyrazoles such as 3,Sdimethyl- (HDmpa), 3,5-bis(trifluoromethyl)- (HFpa) and 3-phenylpyrazole (HPpa) carried out in dry tetrahydrofuran or 1 ,2-dimethoxyethane with the added base triethylamine (NEt3) to precipitate triethylammonium chloride. Apart from a simple substitution of ligands, all mechanisms discussed in this paper involve the two basic reactions of EHC13 (E = Si, Ge), namely

Ovganosilicon Chemisty V Edited by N. Auner and J. Weis

Copyright 0 2003 WILEY-VCH Verlag GmbH & Co. KgaA, Weinheim

Mono- and Oligosilanes with Pyrazole Ligands 313

dismutation (Eq. 1) and HC1 elimination (Eq. 2) .

Results

To support the mechanisms mentioned above for the reactions of trichlorosilane with pyrazoles, one has to characterize both hydrogen-rich and hydrogen-poor products (for the dismutation) as well as compounds of silicon or germanium in a lower oxidation state (for the elimination). This goal could be achieved by reacting 3,5-dimethyl- (HDmpa) or 3,5-bis(trifluoromethyl)pyrazole (HFpa) in tetrahydrofuran in the presence of triethylamine. Immediately after trichlorosilane was added, triethylammonium chloride was formed with both pyrazoles, indicating a ligand exchange of chloride by pyrazolyl as a first step. In the case of HDmpa we were able to isolate Si& as the hydrogen-rich and disilane 1 as the hydrogen-poor product, as characterized by IR spectrum and solid-state structure, respectively. This clearly shows that the mechanism includes the dismutation of trichlorosilane or a similar, substituted intermediate according to Eq. 1. The base-induced elimination of HCI (Eq. 2) also seems to play an important role since product 1 contains two silicon atoms of the formal oxidation state +nr (Eq. 3). For this reaction we were not able to isolate any species containing two or three hydrogen atoms on silicon but only gaseous Si& as the final product of a series of dismutations.

<NEt >

- H,SiCl, 2 HSiC1,Dmpa SiCl,Dmpa,

N-N

<NEt >

- H,SiCl, 2 HSiC1,Dmpa SiCl,Dmpa,

N-N Dmpa = 3,5-Me2N,C,H

Eq. 3.

On the other hand when 3,5-bis(trifluoromethyl)- was used instead of 3,5-dimethylpyrazole, compound 2 (Fig. l), a derivative of the hydrogen-rich SiH2Cl2, could be isolated and characterized by X-ray analysis. The silicon atom is additionally stabilized by two tetrahydrofuran molecules to achieve an overall octahedral coordination, which is most probably due to the nature of the electron-withdrawing pyrazolyl ligands.

A remarkable feature of the structure of compound 1 (Fig. 1) is the exceptionally short Si-Si bond (226.7 pm), which can be attributed to either electronic or steric reasons [5]. A similarly short Si-Si distance of 227.2 pm is also found for the analogous disilane 3a which crystallizes, together with the ion pair 3b/3c (Fig. 3), if the reaction mixture leading to compound 1 contains traces of water. The mechanism resulting in 3b/3c features a complex series of reactions, again involving dismutations and elimination of hydrogen chloride, and has to be discussed in detail elsewhere. The

314 K. Hiibler, J. U. Berner, S. E. F. Merz

Fig. 1. Structure models of compounds 1 and 2. Selected bond lengths [pm] and angles ["I: 1; Si-Si' 226.7(2), Si-CI

204.8(1), Si-N1 190.3(3), Si-N2' 190.9(3), Nl-N2 137.7(4), N1-Si-N2' 153.0(1), CI-Si-Si' 126.52(4),

Cl-Si-CI"' 106.96(7), 2; Sil-H1 141(2), Sil-H2 144(2), Sil-N1 193.1(2), Sil-N2 193.3(2), Sil-01 189.8(2),

Sil-02 190.9(2), Nnl-Nn2 135.6(2), E-Sil-E 178.0(11)-179.52(7), E-Sil-E 88.2(8)-91.6(8).

Fig. 2.

W

Structure model of compound 3b. Selected bond lengths

[pm] and angles ["I: Si6-Si6 227.2(4), SitLC16 203.8(3),

Si6-N91 189.4(5), Si6-N92' 193.6(6), N91-N92

138.2(8); N91-Si6-N92' 153.0(3), C16-Si6-Si6

122.15(14), C16-Si6-N91 98.5(2).

anion 3c, are well reproduced by the calculations

final product was characterized by X- ray crystallography. To confirm that the anion 3c contains a bridging hydrogen atom, we performed ab initio calculations on two model compounds [6]. Although all three hydrogen atoms were able to be located at the appropriate positions in the solid-state structure, it is preferable to compare distances and angles of only the heavier elements derived from X-ray structure analysis and theoretical calculations. In this case, the Si-Si bond length of 299.4pm and the distance of the octahedral coordinated silicon atoms from their C12N2 plane of 17.6 and

18.9 pm, found experimentally in for the anion itself (Si-Si: 307.5;

Si-.plane(C12Nz): 17.7 pm). In contrast, the geometry optimization for a theoretical, neutral compound without the bridging hydrogen atom results in a much longer Si-Si bond of 357 pm and a greater distance of 46.1 pm between Si and the C12N2 plane.

Cation 3b shows a trigonal planar coordinated oxygen atom surrounded by three

Mono- and Oligosilanes with Pyrazole Ligands 315

hexacoordinated silicon atoms which are bridged by six p2-pyrazole ligands.

Fig.

Fig.

I 3. Structure model of the ion pair 3bk Selected bond lengths [pm] and angles ["I: 3b; 0-Si 172.3(4)-174.0(4),

Si-N 187.0(5)-191.2(5), Si-H 138(6)-155(6), Si-0-Si 119.3(2)-120.4(2), 0-Si-H 176(2)-177(2), 3c; Si-C1

216.7(2)-218.5(2), Si-N 187.0(5)-188.9(5), Sin-Hn 137(6)/151(6), Si-H45 155(6)/170(6), Si4-H45-Si5

134(3).

4. Structure models of compounds 4 and 5. Selected bond lengths [pm] and angles ["I: 4; Gel-CI

226.9(1)/227.6(1), Gel-N41 196.7(3), Gel-N(bridge) 199.8(3)-200.7(3), Ge2-N 199.5(3)-201.3(3),

Ge2-0 282.3(4), 5; 0 4 e 1 / 2 187.5(4)/187.6(4), 0-Ge3 192.0(4), Ge-N5n/N6n 201.1(6)-203.7(6), Gel-

N1 IN21 and Ge2-N31/N41 195.0(6)-196.0(6), Ge3-N 230.0(5)-239.5(5), Ge-0-Ge 116.0(2)-122.2(2), 0- Ge-C1 175.0( 1)/176.1(2).

316 K. Hiibler, J. U. Berner, S. E. F. Merz

Very similar products could be isolated from the reaction of 3-phenylpyrazole with trichlorogermane. As discussed for the analogous reaction with trichlorosilane, a combination of substitution and base-induced dismutation as well as HC1 elimination leads to a compound containing two group 14 elements in the formal oxidation states of +m (see Eq. 3). In this case, the more favorable alternative seems to be the formation of the mixed Ge(n)/Ge(rv) species 4 (Fig. 4). This arrangement is even more preferred because Ge2 can be base-stabilized by an additional molecule, 1,2-dimethoxyethane. Even in the partial hydrolysis of 4, we find some analogy to the formation of cation 3b. In the presence of traces of water in the reaction mixture, crystals of the neutral compound 5 were isolated. Compared to 3b, 5 also shows a central, trigonal planar oxygen atom, linked to three germanium atoms, that are again bridged by six p2-pyrazole ligands (Fig. 3).

The only significant difference is that Ge3 has no additional ligand trans to the central oxygen atom and therefore has to be assigned a formal oxidation state of +II.

References

[ l ] [2] [3]

[4]

[ 5 ] [6]

H. J. Campbell-Ferguson, E. A. V. Ebsworth, J. Chem. SOC. A 1966, 1508. K. Hensen, M. Kettner, T. Stumpf, M. Bolte, Z. Natugorsch. Teil B 2000,55,901. P. Boudjouk, S. D. Kloos, B.-K. Kim, M. Page, D. Thweatt, J. Chem. SOC., Dalton Trans. 1998,877. N. Auner in Synthetic Methods of Organometallic and Inorganic Chemistry (ed. W. A. Herrmann), Vol. 2, Thieme, New York, 1996, p. 247. K. Hubler, S. Merz, submitted for publication. Gaussian98 (B3LYP/LANL2DZ), Rev. A.7, Gaussian, Inc., Pittsburgh PA, 1998.

A New Type of Silicon Complex with Salen-Type Ligands

J. Wagler," U. Bohme, G. Roewer

Institut fur Anorganische Chemie, Technische Universitat Bergakademie Freiberg Leipziger Str. 29, D-09596 Freiberg, Germany

Tel.: +49 3731 39 3174 -Fax: +49 3731 39 4058 E-mail: gerhardxoewer @chemie.tu-freiberg.de

Keywords: chelates, enamines, hypervalent compounds, schiff bases, silicon

Summary: Our investigations on silicon compounds of ethylene-NJV"'bis(2 -hydroxyacetophenoneiminate) led to the synthesis and X-ray structure analysis of a new kind of salen complex - hypervalent silicon compounds with a threefold deprotonated salen ligand and an enamine structure. This structural unit provides access to new routes for synthesizing hypercoordinate silicon complexes. Addition reactions between various Br#nsted acids and these new pentacoordinate silicon compounds were carried out to precipitate complexes bearing hexacoordinate silicon atoms.

In transition metal complexes the tetradentate salen ligand usually contributes two M-0 bonds plus two M-N dative bonds to complex formation. Recently salen-silicon complexes obtained by reaction of an easily available silicon source, e.g. SiC4, with the salen ligand acid, as well as preparations of such complexes by salt elimination routes, have been described [ 1-31. However, only a few preparations have been published yet, starting from salen-silicon compounds which are then modified by nucleophilic substitution reactions at the silicon atom. 29Si NMR data of these salen-silicon complexes as well as single-crystal X-ray structure analyses of two fluorinated silicon compounds with salen ligands prove the hexacoordination at the silicon atom provided by the OANANAO unit plus two fluorine atoms [ l , 21. Besides this, the ability of ethylene-N,N- bissalicylideneimine to realize only one Si-0 bond plus two dative Si-N bonds by complex synthesis via the ligand monosodium salt has been already discussed, but this hypothesis was not confirmed by single-crystal X-ray structure analysis [3].

Our recent research resulted in successful synthesis of a completely new type of salen-silicon complexes relying on a novel synthesis route yielding hexacoordinate silicon compounds. Generally, the possibility of an imine-enamine tautomerism within a salen ligand arises from the C-H function in the a-position to the imine group. Thus the ligand Salen*Hz (ethylene-N,N'-bis(2- hydroxyacetophenoneimine)) 1 bearing two imine a-methyl groups should be able to engender two tautomeric molecules, but up to now only silicon compounds having imine-type Salen* ligands have

Ovganosilicon Chemisty V Edited by N. Auner and J. Weis

Copyright 0 2003 WILEY-VCH Verlag GmbH & Co. KgaA, Weinheim

318 J. Wagler, U. Bohme, G. Roewer

been found. We have now succeeded in eliminating HCl from 2 by an excess of amine. On the other hand, it was also possible to realize 1,Caddition of 1 equiv. of hydrogen chloride at 3 (Scheme 1).

4 R=MeS03

5 R = picrate 6 R=CeHs-COO

7

Scheme 1. Formation of penta- and hexacoordinate salen-silicon complexes by elimination and addition of acids.

In a comfortable one-pot synthesis 3 was obtained from 1, diethylamine and PhSiCls in THF. Separation of the clear solution from the precipitated diethylamine hydrochloride and crystallization from the solution gave monoclinic crystals of 3 in high yield and purity. The crystals of 3 (space

A New Type of Silicon Complex with Salen-Type Ligands 319

group P21) contain only one enantiomer each. Its molecular structure is presented in Fig. 1. The coordination geometry at the silicon atom is distorted trigonal bipyramidal with the dative Si-N bond in the axial position. Selected bond lengths and angles are given in Table 1.

h

Fig. 1. Molecular structure of 3.

Table 1. Selected bond lengths [A] and angles [deg] of 3.

Atoms Distance [A] Atoms Angle [deg]

Si( 1)-O( 1) 1.684(2) O( 1)-Si( 1)-N(2) 13 1.8( 1)

Si( 1)-0(2) 1.720(2) O( 1)-Si( 1)-C(19) 111.3(1)

Si( 1)-N( 1) 2.010(3) N(2)-Si( 1)-C(19) 116.0(2)

Si( 1)-N(2) 1.756(3) O( 1)-Si( l)-N(l) 87.1(1)

Si( I)<( 19) 1.871(3) 0(2)-Si(l)-N( 1) 167.9( 1)

C( 16)<(17) 1.340(6) N(2)-Si( 1)-N( 1) 82.6(1)

C( 16)-N(2) 1.4 1 l(4) C( 19)-Si( 1)-N( 1) 92.3(1)

The covalent Si-N bond (1.756(3) A) is significantly shorter than the dative one (2.010(3) A). On the other hand the enamine C-N bond (1.457(4) A) is longer than the imine C=N bond (1.289(4) A). The C=C bond length of the enamine group is representing a typical C-C double bond (1.340(6) A). Both the chemical shift and high anisotropy of the 29Si CPMAS-NMR signal of 3 indicate typical pentacoordinate silicon atoms. There is no significant influence of the solvent used for NMR spectroscopy since 29Si chemical shifts of 3 in the solid state (6 = -116.1 ppm) and in CDCl3 (6 = -115.8 ppm) differ only little. Even dimethyl sulfoxide has no important influence on the silicon coordination sphere of 3 ( ~ ( L ~ I D M S O ) = -1 17.1 ppm).

320 J. Wagler, U. Bohme, G. Roewer

At the methylidene group carbon atom of 3 a relatively high electron density (13C NMR(CDCl3) 6 = 85.5 ppm) is located which could be caused by the mesomeric electron-releasing effect of the silicon-substituted enamine nitrogen atom. Therefore this methylidene group carbon atom is able to pick up a proton from some acids. Even weak acids react with 3 if a stable Si-0 bond can be formed with the corresponding anion. 1 ,CAddition of methanesulfonic acid, picric acid, benzoic acid and hydroquinone to 3 (solution in THF) led to hexacoordinate salen-silicon complexes by precipitation of compounds 4, 5, 6 and 7 respectively (Scheme 1). The 29Si chemical shifts of the solids obtained (CPMAS-NMR) are presented in Table 2.

Table 2. 29Si CPMAS-NMR chemical shifts of hypercoordinate silicon complexes 27.

Compound 6 [ppm] Compound 6 [ppm]

2 -168.4 5 -173.5

3 -116.1 6 -186.9

4 -174.3 7 -180.7, -182.9

References [ l ] [2] [3]

F. Mucha, U. Bohme, G. Roewer, Chem. Commun. 1998, 1289-1290. F. Mucha, J. Haberecht, U. Bohme, G. Roewer, Monatsh. Chem. 1999,130(1), 117-132. M. S. Singh, P. K. Singh, Main Group Met. Chem. 2000,23(3), 183-188.

Gas-Phase Reactions of Free Methyl Cations with Amines and Their Organosilicon Analogues: A

Radiochemical Study

T. A. Kochina, D. V. Vrazhnov, E. N. Sinotova, B. F. Shchogolev

Institute for Silicate Chemistry of Russian Academy of Sciences ul. Odoevskogo 2412, Saint Petersburg 199155, Russia

Tel.: +7812 3284802 -Fax: +7812 3285401 E-mail: kom@isc.nw.ru

Keywords: methyl cations, amines, ion-molecule reactions

Summary: Reactions of free methyl cations with diisobutylamine (l), isobutylaminotrimethylsilane (2) and hexamethyldisilazane (3) were studied by the radiochemical method. It was shown that in all cases studied, the proton transfer is a predominant channel. Probabilities for the reaction to enter this channel are: 0.87 for 1, 0.67 for 2 and 0.93 for 3. The last value contradicts the well-known dependence of the lowering of the proton affinity of amines upon the CH3/SiH3 ratio. However, quantum chemical calculations have shown that the interaction of the silazane HOMO with the CH3' LUMO is symmetry forbidden. This fact may result in the lowering of the methyl cation affinity for silazanes and preference for the proton transfer channel.

Nitrogen-containing compounds form an important class of N-bases due to their role in life processes. This is one of the reasons for our interest in reactions of free carbenium ions with amines [ 1-31. In the course of these studies we found that, in contrast to the other N-bases, the interaction of carbocations with amines occurs via two competing channels, i.e. the formation of the condensation complex as a result of the overlap of the vacant p-orbital of the cation with the lone pair orbital of nitrogen and the proton transfer from the carbenium cation to the amine. The latter channel is very effective, due to the high proton affinities of amines.

This study deals with the effect of Si/C substitution on the mechanism of ion-molecule reactions for series of compounds containing C-N-C, C-N-Si, and Si-N-Si linkages. Reactions of nucleogenic free methyl cations with diisobutylamine, isobutylaminotrimethylsilane, and hexamethyldisilazane were studied for this purpose.

Nuclear-chemical generation of CT3' cations by P-decay of tritium in CTq [4] and subsequent radiochromatographic identification of the neutral products of their reaction with the amines under investigation were employed in this study.

Assuming similar mechanisms for the reactions of methyl cations with amines and with their

Ovganosilicon Chemisty V Edited by N. Auner and J. Weis

Copyright 0 2003 WILEY-VCH Verlag GmbH & Co. KgaA, Weinheim

322 T. A. Kochina, D. V. Vrazhnov, E. N. Sinotova, B. F. Shchogolev

silicon analogues, we may give Scheme 1 for the latter reactions by analogy with the schemes proposed by us earlier for amines [ 1-31.

Scheme 1. The mechanism of the reaction of free methyl cations with amines RlNHR2 (R, = R2 = iBu; R, = iBu,

Rz = SiMe,; RI = R2 = SiMe,).

The radiochromatographic analysis of the products of reactions of methyl cations with diisobutylamine, isobutylaminotrimethylsilane and hexamethyldisilazane has shown that the reaction mechanism which includes two competitive channels is also operative in the case of organosilicon amines.

The relative probabilities of these two channels (condensation and proton transfer), determined by the relative yields of the labeled products observed for the ion-molecule reactions of diisobutylamine, isobutylaminotrimethylsilane and hexamethyldisilazane with methyl cations, are shown in Table 1.

The analysis of these results reveals that for all three amines the dominant channel remains that of proton transfer. This may be rationalized by assuming the higher proton affinity of amines compared to methylene [5] which is a product of such a transfer.

The substitution of the alkyl groups in amines by silyl groups decreases their proton affinity [6] and, as a result, the proton transfer reactions from the methyl cation are less competitive with condensation. Indeed, the probability of proton transfer decreases in going from diisobutylamine to isobutylaminotrimethylsilane. However, in the case of hexamethyldisilazane this rule is broken.

Gas-Phase Reactions of Free Methyl Cations with Amines and Their Organosilicon Analogues 323

Table 1. Reaction channel probabilities calculated from the product yields.

Substrate Condensation Proton transfer

Diisobutylamine 0.13 0.87

Isobutylaminotrimethylsilane 0.33 0.67

Hexamethyldisilazane 0.07 0.93

In order to explain this fact we use the frontier orbital theory [7]. First of all we examine the probability of interaction between the methyl cation and hexamethyldisilazane in the condensation channel.

The interaction of the methyl cation with disilazanes was modeled by quantum chemical methods. The equilibrium geometry and electronic structure of the methyl cation-disilazane adduct were obtained by the Hartree-Fock and MP2/6-31G* methods. It was shown that the interaction between the highest occupied molecular orbital (HOMO) of the H3SiNHSiH3 molecule with the lowest unoccupied molecular orbital (LUMO) of the methyl cation is prohibited by symmetry, but the interaction between the next to the highest occupied molecular orbital of the silazane molecule (with the lower energy) and the methyl cation LUMO is possible. So, the gap between these molecular orbitals increases and the overlap probability decreases. Thus, the condensation channel for the H3SiNHSiH3 molecule becomes less realizable and as a consequence the probability of the proton transfer channel increases. In the case of the aminosilane molecule the same situation is not true because of the absence of symmetry prohibition.

Consequently, the seeming discrepancy between the proton affinity in the (iC4Hg)ZNH, iC4HgNHSi(CH3)3, [(CH3)3Si]2NH and reaction probabilities in the proton transfer channel becomes understandable.

References [l]

[2]

[3] [4] [5] [6] [7]

I. Ignatyev, T. Kochina, V. Nefedov, E. Sinotova, E. Kalinin, Zh. Obshch. Khim. 1995, 65, 297. I. Ignatyev, T. Kochina, V. Nefedov, E. Sinotova, D. Vrazhnov, Zh. Obshch. Khim. 1995, 65, 304. T. Kochina, D. Vrazhnov, I. Ignatyev, J. Organomet. Chem. 1997,549,45. M. Speranza, Chem. Rev. 1993,93,2933. E. Uggerud, J. Am. Chem. Soc. 1994,116,6873. M. Hendewerk, R. Frey, D. Dixon, J. Phys. Chem. 1983,87, 2026. G. Klopman, J. Am. Chem. Soc. 1968,90,223.

Calculation of 29Si Chemical Shifts Using a Density-Functional Based Tight-Binding Scheme

Marc Milbradt," Heinrich Marsmann

Anorganische und Analytische Chemie, Universitat Paderborn Warburger Str. 100,33098 Paderborn, Germany

E-mail: marc .milbradt @ gmx.de

Thomas Heine

Department of Physical Chemistry, University of Geneva, Switzerland

Gotthard Seqert

Institut fur Physikalische Chemie und Elektrochemie Technische Universitat Dresden, Germany

Thomas Frauenheim

Theoretische Physik, Universitat Paderborn, Germany

Keywords: DFTB, 29Si, chemical shift calculation

Summary: The shielding constants o f o r 29Si, and from it the chemical shifts 6 are calculated for a series of silicon compounds using the IGLO-DFTB method (Individual Gauge for Localized Orbitals, Density-Functional based Zight-Binding). The calculated values of silanes SinH2,+2 (n = 1-.5), methylsilanes H,Si(CH&,, and phenylsilanes H,Si(G,HS)k, (n = 1-3), are compared with DFT calculations and experimental values. Geometries have been optimized using the DFTB method. Calculated geometries are in good agreement with experiment. Calculated chemical shifts correlate quite well with experimental values. After an empirical correction, the chemical shifts for silanes and methylsilanes obtained by DFTB are equivalent to those of DFT calculations. The rms errors with respect to experiment are 4.3 ppm for silanes, 6 pprn for methylsilanes and 6.8 ppm for phenylsilanes.

Theory

The DFTB method is based on an LCAO Ansatz for the Kohn-Sham molecular orbitals g of basis functions @, (Eq. 1).

Ovganosilicon Chemisty V Edited by N. Auner and J. Weis

Copyright 0 2003 WILEY-VCH Verlag GmbH & Co. KgaA, Weinheim

Calculation of 29Si Chemical Shifts 325

The basis functions are represented as linear combinations of Slater-type orbitals (STO) or here Gaussian-type orbitals (GTO). Expansion coefficients are found by solving the secular problem:

Eq. 2.

With the Kohn-Sham matrix FPv = (@p If +Veff 14") and the overlap matrix SPv = ( @ P I@"). The

effective potential V,, is approximated as a superposition of atomic contributions, each determined by an LDA-DFT calculation. Only two-center elements of the Kohn-Sham matrix are considered (Eq. 3).

Eq. 3.

Vj,k are the effective potentials of atomsj, k that carry functions &, 4". The total energy can be

written as sum of occupied Kohn-Sham states and a repulsive, short-range, two-particle interaction U (Eq. 4).

ccc

i j # k

Eq. 4.

The repulsive potential U is fitted by the difference between the Kohn-Sham energies and LDA-DFT energies of a reference system.

Results and Discussion

The DFTB method combined with the IGLO concept gives good results in calculating 13C-chemical shifts [I]. The extension to silicon systems has been done by testing the method on simple silicon compounds, e.g. silanes, methylsilanes and phenylsilanes. The chemical shift values of these compounds covers a range between -170 and 0 ppm.

DFTB-optimized geometries are in good agreement with experimental values. The largest

326 M. Milbradt, T, Heine, H. Marsmann, G. Seifert, T. Frauenheirn

deviations between calculated and experimental values are found for disilane (Si-Si bond length 0.02 A). The Si-H bonds generally agree within 0.02 A, Si-Si within 0.01 A, and bond angles within 0.7".

In a scatter plot of DFTB vs. experiment, one can see a too-small slope with respect to experiment. This is known from 13C NMR DFTB calculations and originates from the limited basis used for the sum-over-states calculation of the paramagnetic part of the shielding constants. Further, the silicon values in silanes are affected by a systematic shift of each site, proportional to the number of silicon neighbors of the site, which is also observed in full DFT calculations [2].

Two schemes were combined for empirical correction: 1) Forsyth and Sebag proposed to fix the slope and intercept at 1.0 and 0.0 [3]; 2) for the systematic deviations of silicon atoms in silanes, the correction in Eq. 5 is used [2], where nsi = number of silicon neighbors; a = difference between intercepts of nsi = 1 and nsi = 2.

ams ( X ) = O(TMS) - O( x ) + %i (X )a

Eq. 5.

For silanes both correction schemes are used, but for methyl- and phenylsilanes only 2 ) is applied. The results are shown in Tables 1-3 and Figs. 1-3 (all values are given in ppm).

Table 1. Chemical shifts of silane molecules with respect to TMS. Asterisks mark empirical corrected values.

DFTB DFTB* DFT*

SiH4

Si2H6

-46.2

-62.6

Si& (1) -61.9

(2) -80.8

n-Si4Hlo (1) -63.2

n-Si4Hlo (2) -79.3

i-Si4H10 (1) -60.5

i-Si4Hlo (2) -102.8

n-Si5H12 (1) -64.2

n-Si5HIz (2) -79.8

n-Si5Hlz (3) -77.7

i-Si~Hlz (1) -61.1

i-SiSH12 (2) -101.9

i-Si~H12 (3) -78.6

i-Si5HI2 (4) -63.3

neo-Si~Hlz (1) -58.7

neo-SiSHlz (2) -128.5

-98.0 -95.6

-101.4 -103.1

-98.9 -98.0

-111.2 -115.7

-103.5 -97.9

-105.8 -111.1

-93.9 -93.6

-134.6 -136.7

-107.1 -98.4

-107.6 -1 11.5

-100.1 -107.3

-96.0 -94.1

-131.4 -131.5

-103.3 -106.9

-103.9 -99.0

-87.4 -89.6

-171.3 -165.9

Exp.

-95.6

-103.1

-98.0

-115.7

-97.8

-111.1

-93.6

-136.3

-98.4

-111.5

-107.3

-94.1

-131.5

-106.9

-99.0

-89.6

-165.9

Calculation of 29Si Chemical Shifts 327

Table 2. Chemical shifts of methylsilane molecules; Me = CH,.

Conventions as in Table 1.

DFTB DFTB* DFT Exp.

SiH4 4 6 . 2 -97.0 -120.3 -95.6

H3SiMe -34.4 -72.2 -77.8 -65.2

HzSiMez -22.3 4 8 . 8 -46.3 -37.7

HSiMe3 -10.6 -22.3 -18.6 -15.5

TMS 0.0 0.0 0.0 0.0

Table 3. Chemical shifts of phenylsilane molecules; Ph = C~HS.

Conventions as in Table 1.

DFTB DFTB* Exp.

SiH4 -46.2 -92.1 -95.6

H3SiPh -38.5 -63.6 -61.5

H2SiPh2 -32.6 4 1 . 7 -34.5

HSiPh, -28.6 -26.9 -21.1

SiPh, -22.0 -2.5 -14.0

-170 -150 -130 -110 -90

%* Fig. 1. Scatter plot of calculated and experimental chemical shifts of silanes. The uncorrected DFT values are taken

from Ref. [2]. The correlation with experiment is 0.975 and the rms error 4.3 ppm.

328 M. Milbradt, T. Heine, H. Marsmann, G. Seifert, T. Frauenheim

0

-40

1 3

(0

-80

-120 -1 20 -80 -40 0

s,, Fig. 2. Scatter plot of calculated and experimental chemical shifts of methylsilanes. DFT values (uncorrected) are

taken from Ref. [2]. The correlation with experiment is 0.994 and the rms error 6.0 ppm.

-10

-30

% -50 a

-70

-90

'exp

Fig. 3. Scatter plot of calculated and experimental chemical shifts of phenylsilanes. The correlation with experiment

is 0.975 and the rms error 6.8 ppm.

References [l] [2] [3]

T. Heine, G. Seifert, P. W. Fowler, F. Zerbetto, J. Phys. Chem. A 1999,103, 8783. T. Heine, A. Goursot, G. Seifert, J. Weber, J. Phys. Chem. A 2001,105,620. D. A. Forsyth, A. B. Sebag, J. Am. Chem. SOC. 1997,119,9483.

29Si NMR Chemical Shifts of Four- and Five-Membered Organosilacycles:

Experimental and Theoretical Studies

Katja Strohfeldt, Katrin Andres, Riidiger Bertermann, Eric Wack, Martin Kaupp, Carsten Strohmann"

Institut fur Anorganische Chemie, Universitat Wurzburg Am Hubland, D-97074 Wurzburg, Germany

Tel.: +49 931 888 4613 -Fax: +49 931 888 4605 E-mail: c.strohmann@mail.uni-wuerzburg.de

Keywords: silacycles, crystal structure, 29Si NMR, quantum chemical calculations, chemical shift

Summary: Experimental and theoretical 29Si NMR studies on cyclic and analogous acyclic organosilanes show that variation of the CRz-Si-CR2 bond angle results in opposite effects on the "Si NMR shift for cyclic and acyclic systems. Therefore structural predictions for strained organosilanes, based on 29Si NMR data, remain a challenge. A direct correlation of the 29Si NMR shift and the CRz-Si-CR2 bond angle is only possible for related systems. Explanations for this reverse dependence of the 29Si chemical shifts on CRZ-Si-CR2 bond angles in cyclic and acyclic systems can be obtained by detailed quantum chemical analyses of the shielding tensors.

As part of our studies on four- and five-membered silacycles [l, 21, we are interested in 29Si NMR chemical shifts as a structural probe for these ring systems. A general survey of cyclic organosilanes shows characteristic downfield shifts in comparison with corresponding acyclic systems [3]. These characteristic experimental downfield shifts of the "Si NMR resonance signals for cyclic molecules with decreasing bond angles at the silicon center were confirmed by quantum chemical calculations (Fig. 1) [4].

Fig. 1. "Si NMR shifts of organosilacycles and corresponding acyclic systems [3]. Bond angles and the 6 *'Si value

in brackets pertain to computational results [HF/6-3 lG(d)//HF/6-3 1 1+G(2d,p), (GIAO method)].

Ovganosilicon Chemisty V Edited by N. Auner and J. Weis

Copyright 0 2003 WILEY-VCH Verlag GmbH & Co. KgaA, Weinheim

330 K. Strohfeldt, K. Andres, R. Bertennann, E. Wack, M. Kaupp, C. Strohmann

The four-membered organosilacycles, especially, show unusually small CR2-Si-CR2 bond angles (4: 78.6") resulting in large downfield shifts (4: dZ9Si = 18.9 ppm). Obtaining an experimental variation of this bond angle is possible by arranging silicon and heteroatoms in the 1,3-positions of cyclobutanes [l]. For oxygen as a heteroatom, a decrease in the CRz-Si-CRZ bond angle to 75.1" (5)/74.5' (7) is observed. The heteroatom sulfur causes an opening of this angle to 89.0" (6)/89.5' (8). If the silicon center of the four-membered silacycles is part of an additional ring system (7, 8), in our case a five-membered ring, an enhancement of the 29Si NMR downfield shift is observed. Molecular structures of four-membered silacycles in the crystal in combination with 29Si NMR data in solution are presented in Fig. 2 [l]. A decrease in the CRz-Si-CR2 angle results in a downfield shift of the 29Si NMR resonance signal in comparable systems.

8 Ph' 'Ph

5 6 7 a CR2-Si-CR2 bond angles 75.1" 89.0" 74.5"/97.0" 89.5"/ 95.6"

"Si NMR shift 32.0 ppm 25.7 ppm 52.6 ppm 45.8 ppm

Si NMR shift calculations 35.0 ppm 26.9 pprn - 29 -

Fig. 2. Molecular structures of 3-silaoxetanes and 3-silathietanes in the crystal in combination with 29Si NMR data

in solution (CDQ) and calculated 29Si NMR shifts [B3LYP/6-3 1G(d)//HF/6-3 1 1+G(2d,p), (GIAO method)].

29Si NMR Chemical Shifts of Four- and Five-Membered Organosilacycles 331

Quantum chemical studies with acyclic systems might clarify the situation, if the characteristic downfield shift can be correlated to the CR2-Si-CR2 bond angle. Therefore, a relaxed scan minimization of 1 (variation of the CH2-Si-CH2 bond angle with reoptimization of all other degrees of freedom) and additional 29Si NMR calculations were performed (Fig. 3).

Fig. 3. Quantum chemical 29Si NMR shift calculations [HF/6-3 1 G(d)//HF/6-3 ll+G(2d,p), (GIAO method)] for

acyclic compounds with a fixed CH2-Si-CH2 bond angle.

These quantum chemical results indicate that a variation of the CH2-Si-CH2 bond angle causes opposite effects for cyclic and acyclic compounds. A decrease in the CH2-Si-CH2 bond angle in acyclic organosilanes results in a highfield shift of the 29Si NMR resonance signal. The decrease in this angle causes an opening of the other angles around the silicon center. But also, no correlation could be found between the sum of all bond angles at the silicon center and the 29Si NMR chemical shift.

Explanations for the reverse dependence of 629Si on the CHz-Si-CH2 bond angle in cyclic and acyclic systems may be obtained by detailed quantum chemical analyses of the shielding tensors. Recall that nuclear shielding is usually divided into diamagnetic and paramagnetic parts, cd and cp, respectively. For non-hydrogen nuclei, u p ~ is assumed to dominate the relative shifts between different chemical environments. Ramsey's sum-over-states expression for c p ~ may be given as Eq. 1 [5 ] . In this formula, the external magnetic field B (represented by the angular momentum operator Lo) and the nuclear magnetic moment ,UN (represented by the term L, / ri3 ) couple ground

and excited states of the molecule in double perturbation theory. In a molecular-orbital framework (e.g. in density functional calculations), the energy difference E 0 - ' E n between ground and singlet excited states Yo and Y, may be translated into energy differences between occupied and

unoccupied molecular orbitals (MOs), which are coupled by the two perturbations. Obviously, smaller energy differences favor larger (negative) paramagnetic contributions cp and therefore shifts to lower field. In the literature one frequently sees attempts to correlate the HOMO-LUMO energy difference with the chemical shifts. However, Eq. 1 shows clearly that:

Eq. 1. Sum-over-states expression for bp

332 K. Strohfeldt, K. Andres, R. Bertermann, E. Wack, M. Kaupp, C. Strohmann

more than just one coupling between ground and excited states may contribute to 8; not only the energy denominators but also the products of the perturbation matrix elements in the numerators are important in determining oP.

Our recent quantum chemical analyses of 29Si shieldings for both cyclic and acyclic model systems [6] lead to the following consequences: In acyclic systems, a distortion from the equilibrium structure to smaller angles as in Fig. 3 reduces the energy denominators of the most important transitions in Eq. 1. Nevertheless, d' becomes less negative (and thus a shift to higher field results), as the smaller energy denominators are overcompensated by reduced numerators (perturbation matrix elements). The latter point may be understood from the fact that distortion polarizes the relevant Si-C bonds more towards the carbon end, and therefore coefficients at silicon decrease. In cyclic systems with small rings (in particular for four-membered rings), the trend is just the opposite. Here lower angles lead to lower coefficients at Si for Si-C bonds within the ring. This leads to reduced d' (via reduced numerators in Eq. 1). Further analyses are currently being carried out to establish the detailed relation between chemical shifts and bond angles in cyclic and acyclic compounds [6].

Conclusion

Experimental and theoretical 29Si NMR studies on cyclic and analogous acyclic organosilanes show that variation of the CR2-Si-CR2 bond angle results in opposite effects on the 29Si NMR shift for corresponding cyclic and acyclic systems. Therefore, predicting structures for strained organosilanes out of 29Si NMR data is still a challenge. Explanations for the 29Si NMR shifts by quantum chemical analyses of the shielding tensors show that they depend on:

the energy differences of all coupling orbitals; polarization of the relevant Si-C bonds, leading to changes in the perturbation matrix elements.

In the compounds studied, it appears that the latter point dominates the dependence of 29Si NMR chemical shifts on bond angles in both cyclic and acyclic compounds.

Acknowledgment: We are grateful to the Institut f i r Anorganische Chemie der Universitat Wiirzburg, the Deutsche Forschungsgemeinschaft (DFG), the Sonderforschungsbereich 347, the Graduiertenkolleg 690 and the Fonds der Chemischen Industrie (FCI) for financial support. We acknowledge Wacker-Chemie GmbH for providing us with special chemicals.

29Si NMR Chemical Shifts of Four- and Five-Membered Organosilacycles 333

References a) C. Strohmann, Chem. Ber. 1995, 128, 167; b) C. Strohmann, E. Wack, in: Organosilicon Chemistry: From Molecules to Materials III (Eds.: N. Auner, J. Weis), VCH, Weinheim, 1997, p. 217; c) N. P. Toltl, M. Stradiotto, T. L. Morkin, W. J. Leigh, Organometallics 1999, 18, 5643; d) C. Strohmann, 0. Ulbrich, in: Organosilicon Chemistry: From Molecules to Materials IV (Eds.: N. Auner, J. Weis), VCH, Weinheim, 2000, p. 220. C. Strohmann, S. Ludtke, 0. Ulbrich, Organometallics 2000,19,4223. F. Uhlig, U. Hermann, H. Marsmann, 29Si NMR Database, Version 1 . 1 , University of Dortmund, 1999. G. Magyarfalvi, P. Pulay, Chem. Phys. Lett. 1995,241,393. For examples see: Multinuclear NMR (Ed.: J. Mason): Plenum Press, New York, 1987, and references cited therein. K. Strohfeldt, M. Kaupp, C. Strohmann, unpublished results.

29Si NMR Chemical Shifts of Siloxanes: Ab Initio and Density Functional Study

Georgios Tsantes, Norbert Auner," Thomas Miillefi

Institut fur Anorganische Chemie, Johann Wolfgang Goethe-Universitat Frankfurt a. M. D-60439 Frankfurt a. M., Germany

Tel.: +49 69 798 29166 -Fax: +49 69 798 29188 E-mail: tsantes@chemie.uni-frankfurt.de

Keywords: 29Si NMR, siloxanes, Computational Chemistry, NMR spectroscopy

Since 1990 the ab initio calculation of NMR chemical shift parameters has been established as a reliable tool for structure elucidation of various types of compounds [l]. Based on a set of model disiloxanes, the predictive powers of various computational levels (HF, DFT, MP2) [2] for 29Si NMR chemical shift calculations for linear, cyclic and cage-like polysiloxanes are compared. The dependence of the calculated NMR parameters on structural changes is discussed. Finally, the performance of our theoretical approach is tested using a series of 25 siloxanes and silanes.

A survey of structural data for siloxanes [3] reveals that the scattering of the 0-Si-0 bond angle is comparatively small (105-1 14"), but the spread of the Si-0-Si bond angle and of the Si-0 bond length is rather large (129-177' and 1.550-1.701 A). Therefore, we investigated the effect of the flexibility of these structural parameters on the 29Si NMR chemical shift.

Disiloxane was choosen as a model compound for two reasons:

it is the smallest molecule which has an Si-OSi linkage; it is the major building block for siloxanes.

The optimized molecular structure [4] of disiloxane at the B3LYP/6-31G(d) level of theory has an Si-0-Si angle of 153.3' and an Si-0 bond length of 1.648 A. The energy for linearization is as low as 1 kcal mol-I. This is in good qualitative agreement [5] with calculations at higher levels of theory which predict linearization barriers between 0.50 (MP2/6-311+G(3df)) and 1.36 kcal mol-' (CCSD(T)/6-31 lG(2d)). Figure 1 shows the relative potential energy surface of disiloxane as calculated at the B3LYP/6-31G(d) level of theory. It is interesting to note the flat bottom of the surface, marked by light grey, which represents a potential energy well of only 2 kcal mol-I. A change of the Si-0-Si angle from 120 to 180" is covered by this energy range. Within the same energy band the 29Si NMR shift decreases by 8-10 ppm (Fig. 2).

The Si-0-Si bond angle and the Si-0 bond length were changed in 10' and in 0.03 A steps, and after a partial optimization of the disiloxane structure at the B3LYP/6-31G(d) level of theory a subsequent NMR calculation (GJAO-MP2/A) [6, 71 was performed. The combined effect of both

Ovganosilicon Chemisty V Edited by N. Auner and J. Weis

Copyright 0 2003 WILEY-VCH Verlag GmbH & Co. KgaA, Weinheim

29Si NMR Chemical Shifts of Siloxanes: Ab Initio and Density Functional Study 335

structural parameters on the calculated 29Si Nh4R shift is shown in Fig. 2. Associated with the increase of the Si-0-Si bond angle is a significant reduction of the Si-0 bond length. Both structural changes result in a high-field shift of 829Si; 629Si decreases by approximately 25 pprn in the range from 120 to 180" and 1.750 to 1.570 A. This corresponds roughly to A 8 4 ppm per 10" and -4 ppm per 0.05 A. A similar picture results when the 29Si NMR shift is calculated at different levels such as GIAO-HF/A, GIAO-B3LYP/A, or GIAO-MPWlPW91/A [6, 71, the only difference being in the absolute values.

Fig. 1. H3Si-O-SiH3 potential energy surface calculated at the B3LYP/6-31G(d) level of theory.

Fig. 2. "Si NMR chemical shift from H3Si-O-SiH3 calculated at the GIAO-MP2/A // B3LYP/6-31G(d) level of

theory [6,7].

336 G. Tsantes, N . Auner, T . Miiller

The prediction of NMR chemical shifts depends crucially on the quality of the optimized structure and on the precision of the subsequent NMR calculation. Therefore various computational methods in combination with different basis sets were tested for their ability to predict the important geometrical parameters like the Si-0-Si angle and the Si-0 bond length. After a full geometry optimization, four theoretical levels were used to calculate the 29Si NMR shift. Disiloxane, hexafluorodisiloxane, hexachlorodisiloxane and hexamethyldisiloxane were used as model compounds (Fig. 3).

31 0

32 0

330 340 35 0 36 0

39.0 I -40.0 ! -41.0 I -42.0 j -4w I -44.0 I -45.0 : -4 .0 ~ . -47 0

. *

I . . * . :

.a,o j

-500 j -51.0 I -52.0 j -53.0 ~

-49.0 ; "

- L . O I

NMR chemical shift calculations on disiloxane [8].

When using HF theory for the geometry optimization the use of two sets of polarization functions is necessary to reproduce the Si-0-Si angle. Methods which include electron correlation (i.e. MP2- or DFT-based methods) do not require such extended basis sets. Based on the same geometry, GIAO-HF/A [6, 71 and GIAO-B3LYP/A [6, 71 methods result in very similar shifts, usually with a difference of only 1-2 ppm from each other. In the case of the 29Si NMR shift of hexachlorodisiloxane, all combinations of theoretical levels used result in significantly greater errors in comparison to the other three disiloxanes. The exact prediction of the Si-0-Si angle is not sufficient for an accurate 8 29Si NMR prediction. In the case of hexamethyldisiloxane the HF/6-3 1G(d) geometry optimization results in an Si-0-Si angle of approximately 180", in contrast to the experimentally derived

29Si NMR Chemical Shifts of Siloxanes: Ab Initio and Density Functional Study 337

value of 148". Nevertheless the calculated GIAO-B3LYP/A [6, 71 29Si NMR chemical shift is in good agreement with the experimental shift due to the calculated Si-0 bond length of 1.636 A (exp. 1.63 A). This is in contrast to the MP2/6-31G(d) geometry, where the Si-0-Si bond angle is in agreement with the experimental value but the Si-0 bond length differs by 0.04 A. As a consequence the subsequent GIAO-B3LYP/A [6, 71 29Si NMR chemical shift calculation results in a greater difference from the experimental value than the GIAO-B3LYP/A//HF/6-3 1G(d) level of theory.

0 -

. . E -20:

-40-

ri -60- ' -60: -100-

-120-

Finally, the predictive power of our theoretical approach was tested using a series of 25 siloxanes and silanes, including a silsesquioxane and a cyclotrisilthiane (Fig. 4). All the structures were fully optimized and characterized as minima on the potential energy surface and the 29Si NMR chemical shifts were calculated. The GIAO-HF/A [6, 71 calculations agree slightly better with the experimental shifts than the GIAO-B3LYP/A [6, 71 level of theory. Both theoretical levels have difficulties in predicting correct 6 29Si NMR chemical shifts of compounds containing Si-Cl bonds, regardless of whether chlorodisiloxanes or chlorosilanes are involved.

0 - E . / ./ ' 3

58' 8 .20- . .

f,';" $ 40- Y ) ' ,Y - m . / & -60-

" -80-

9, /. ,#' -100-

/ , -120- ,/ /'. . 1 4 0 - , . , . , , , . , , , , , , , , , I

338 G . Tsantes, N . Auner, T . Muller

Acknowledgments: This work was supported by Dow Coming Corporation and SBC GmbH.

References [ l ]

[2 ]

a) Helgaker, M. Jaszunski, K. Ruud, Chem. Rev. 1999, 99, 293; b) J. R. Cheeseman, M. J. Frisch, G. W. Trucks, T. A. Keith, J . Chem. Phys. 1996,104,5497. For an introduction in the applied methods and basis sets, see: a) W. J. Hehre, L. Radom, P. v. R. Schleyer, J. A. Pople, Ab initio Molecular Orbital Theory, Wiley: New York, 1986; b) J. B. Foresman, E. Frisch, Exploring Chemistry with Electronic Structure Methods, 2nd edn., Gaussian Inc.: Pittsburgh, 1996. M. Kaftory et al., The Structural Chemistry of Organosilicon Compounds, in Chemistry of Organic Silicon Compounds, Z . Rappoport, Y. Apeloig, Eds., John Wiley: New York., 1998, VOl. 2, Part 1. All calculations were performed with Gaussian 98 (Revision A.9), M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria, M. A. Robb, J. R. Cheeseman, V. G. Zakrzewski, J. A. Montgomery, R. E. Stratmann, J. C. Burant, S. Dapprich, J. M. Millam, A. D. Daniels, K. N. Kudin, M. C. Strain, 0. Farkas, J. Tomasi, V. Barone, M. Cossi, R. Cammi, B. Mennucci, C. Pomelli, C. Adamo, S. Clifford, J. Ochterski, G. A. Petersson, P. Y . Ayala, Q. Cui, K. Morokuma, D. K. Malick, A. D. Rabuck, K. Raghavachari, J. B. Foresman, J. Cioslowski, J. V. Ortiz, B. B. Stefanov, G. Liu, A. Liashenko, P. Piskorz, I. Komaromi, R. Gomperts, R. L. Martin, D. J. Fox, T. Keith, M. A. Al-Laham, C. Y. Peng, A. Nanayakkara, C. Gonzalez, M. Challacombe, P. M. W. Gill, B. G. Johnson, W. Chen, M. W. Wong, J. L. Andres, M. Head- Gordon, E. s. Replogle, J. A. Pople, Gaussian, Inc., Pittsburgh PA, 1998. G. I. Csonka, J. Reffy, Chem. Phys. Lett. 1994,229, 191. A: Si: 6-311+G(2df), all other elements 6-31G(d). K. Wolinski, J. F. Hilton, P. Pulay, J . Am. Chem. Soc. 1990,112,8251. a) Identical calculations were done also for hexafluorodisiloxane, hexachlorodisiloxane, and hexamethyldisiloxane; b) For experimental data about siloxanes: W.S. Sheldrick, Structural Chemistry of Organic Silicon Compounds, in The Chemistry of Organic Silicon Compounds, S . Patai, Z . Rappoport, Eds., Wiley: New York, 1989, Vol. 1, Chap. 3, p. 242.

[3]

[4]

[5] [6] [7] [8]

Domain Size Determination of Pol y ( p h t halamide)/Pol y (dime t h y lsiloxane) Block Copolymers by 'H Solid-state Spin

Diffusion NMR Spectroscopy

Axel Kretschmer

Dow Corning SA, Parc Industriel, Zone C, B-7180 Seneffe, Belgium Tel. +32 64 888986 - Fax: +32 64 888420 E-mail: axel.kretschmer@dowcorning.com

Robert A. Drake, Simeon J. Bones

Dow Coming Ltd., Cardiff Road Barry, Vale of Glamorgan, CF63 2YL, UK

Michael Neidhoefer, Manfred Wilhelm, Hans Wolfsang Spiess

Max Planck Institute for Polymer Research Ackermannweg 10, D-55128 Mainz, Germany

Keywords: block copolymers, domain size, 'H solid-state NMR, spin diffusion

1 Summary: H solid-state spin diffusion NMR spectroscopy was applied to calculate domain sizes of separated phases in poly(phthalamide)/poly(dimethylsiloxane) block copolymers. To initiate spin diffusion, the dipolar filter technique was used. The correlation between longer PDMS chains in the copolymer and bigger domains of the PDMS phase could be shown.

Introduction

Poly(phthalamide)/poly(dimethylsiloxane) copolymers (PAPDMS) can be used as thermopolastic elastomers at high temperatures. Modern NMR spectroscopy supplies several tools for characterization and has the potential to answer the questions about composition, mobility and domain size of the separated phases.

For domain size determination the 'H solid-state spin diffusion technique under static conditions was used. This method has been applied so far on many different copolymer systems [l-91. The term spin diffusion was introduced by Bloembergen in 1949 [lo] and means transfer of

Ovganosilicon Chemisty V Edited by N. Auner and J. Weis

Copyright 0 2003 WILEY-VCH Verlag GmbH & Co. KgaA, Weinheim

340 A. Kretschmer, R. A. Drake, S. J. Bones, M. Neidhoefer, M. Wilhelm, H. W. Spiess

magnetization between nuclei via dipolar coupling. The first step of the spin diffusion experiment is the selective magnetization of one part of the sample. The selection can be made on chemical shift differences or on TZ spin-spin relaxation time differences [l I]. In our case TZ selection was applied due to the mobility difference (and therefore TZ difference) between the rigid organic part (PA) and the mobile siloxane part (PDMS). The dipolar filter sequence was used to obtain selective magnetization of the PDMS phase, and in the following mixing time tm within the pulse sequence magnetization transfer between the PDMS and the PA phase takes place.

Using a series of experiments varying the mixing time tm, one can follow the decrease in the PDMS signal. This curve (signal intensity vs. tml") contains the information about the domains of the phase-separated PDMS. As described in Ref. [ 1 I], during the mixing time tm TI spin lattice relaxation also reduces the intensity of the PDMS signal and this has to be taken into account. Therefore, the spin diffusion curve must be divided by the TI relaxation curve, which is measured separately for all mixing times.

The initial part of this TI-corrected spin diffusion curve is linear, as described elsewhere [9, 1 I]. Extrapolation of this linear decay for intensity = 0 leads to the value (tmsso)l/z that is needed to determine the domain size according to Eq. 1.

Eq. 1.

In this equation, E is the number of orthogonal directions relevant for the spin diffusion process. Its value depends on morphology and is 1 for lamellar block copolymers, 2 for phases with a cylinder-like morphology in a matrix, and 3 for discrete phases (for example spheres in a matrix). The remaining parameter in Eq. 1 is D,*, the effective spin diffusion coefficient. It can be calculated according to Eq. 2.

Eq. 2.

For Drigidthe value of 0.8 nm/msz was determined in a previous study [9] and is generally applied for rigid components [ l l ] . To determine Dmobile Tz measurements were carried out to use the correlation between D and T2 which was measured by Mellinger et al. [ 1 I].

Experimental Section

The 'H spin diffusion NMR experiments were performed on a 300 MHz Bruker DSX spectrometer,

Domain Size Determination by 'H Solid-state Spin DlfSusion NMR Spectroscopy 341

operating at a 'H frequency of 300.22 MHz under static conditions. In the spin diffusion experiment the dipolar filter was applied with successive mixing time tm [ll], using the pulse train of 12 90' pulses four times. The 90" pulse length was 4 ps. The distance between the 90" pulses during the dipolar filter was set to 10 ps. The mixing time tm was varied between 100 ps and 8 s in 53 steps. The relaxation delay was 3 s, and the number of scans was 16.

For the TI compensation, measurements with the same mixing time but without the dipolar filter were recorded. The T2 values were obtained by using a Can-Purcell-Meiboom-Gill (CPMG) sequence. The curves were analyzed by a biexponential decay representing the T2 for the rigid and the mobile component.

Results and Discussion

For three different PAPDMS copolymers spin diffusion decays were measured: PAloPDMS11,

The curves showed different decay behavior; the example for PAloPDMSll is shown in Fig. 1. The smaller the PDMS chain, the steeper is the extrapolated line of the initial linear decay of the curves. The extrapolation leads to very different (tms30)1'2 values. For the short-chain PDMS sample (PAloPDMSII), this value is 8 mslR, increasing to 43 msl" for the sample PAloPDMSso and up to 90 ms'" for sample PAloPDMSlm (see Table 1). Due to Eq. 1, shorter tmS9O values lead to smaller domain sizes.

PAloPDMSso and PAloPDMSloo.

PDMSIPPAM 15744/016A sddfakn2, T1 corrected

Fig. 1. 'H NMR spin diffusion curve for sample PAl0PDMS~, (Tl-corrected).

The T2 values needed for the domain size determination are also listed in Table 1. The longer the PDMS chain, the longer is the T2 for the PDMS phase, reflecting the increasing mobility of the siloxane phase. The difference between the T2 of PAloPDMS11 and the T2 of the two other samples

342 A. Kretschmer, R. A. Drake, S. J. Bones, M. Neidhoefer, M. Wilhelm, H. W. Spiess

is one order of magnitude. Therefore, the mobility for the short-chain PDMS sample is much lower than in the other, longer, PDMS chain samples.

From the T2 values the diffusion coefficient of the mobile phase could be determined. The corresponding D values are listed in Table 1 as well. Using Eq. 2, the effective spin diffusion coefficient Deff can be calculated (see Table 1).

Looking back to Eq. 1, the only parameter we still need for the domain size determination is E

which reflects the expected morphology. It is suggested that for P M D M S copolymers E =3 be used.

Taking the (tms30)1'2 values from the corrected spin diffusion curves, the D,ff values and E =3 for the expected morphology domain size values can be determined for the PDMS phase in the copolymers. These values are also listed in Table 1. The results show the longer the PDMS chain (with equal poly(phtha1amide) chain length), the greater is the size of the phase-separated PDMS.

Table 1. Data from spin diffusion static solid-state NMR measurements on three different PAmDMS copolymers.

Composition (tms!o)'n T2 (mobile part) DeE" Domain size

PAloPDMSl I 8 2.5 0.41 10-12

PAIoPDMSSo 43 24 0.17 25-27

PAloPDMSlw 90 40 0.16 49-5 1

[m.9vz1 [ml [(nm'/ms)"] (mobile part) [nm]

Conclusions

Spin diffusion measurements of poly(phthalamide)/poly(dimethylsiloxane) block copolymers having separated phases (rigid poly(phthalamide) part, mobile PDMS part) showed different domain sizes depending on the length of the PDMS chain. The shorter the chain, the smaller is the domain size of the separated PDMS phase. To determine the spin diffusion coefficient of the mobile phase spin-spin, relaxation time T2 was measured. It showed the big mobility difference between the sample with the short chain (PAloPDMS11) and the other two samples (PA~oPDMSSO and PAloPDMSlm) - one order of magnitude. Therefore, the PDMS chain with 11 links is fairly restricted in its mobility compared with the other two chains with 50 and 100 links.

Acknowledgment: We thank Ian Moss, Anne Dupont and Iain MacKinnon from Dow Corning and Robert Graf from the Max Planck Institute for Polymer Research for their support and helpful discussions.

References [ 11 D. L. VanderHart, G.B. McFadden, Solid State Nucl. Magn. Reson. 1996, 7,45.

Domain Size Determination by ' H Solid-state Spin DifSusion NMR Spectroscopy 343

[2] D. L. VanderHart, Makromol. Chem. Makromol. Symp. 1990,34, 125. [3] P. Caravatti, P. Neuenschwander, R. R. Ernst, Macromolecules 1985,18, 119. [4] D. T. Nzudie, L. Delmotte, G. Riess, Makromol. Chem. Phys. 1994, 195,2723. [ 5 ] K. Landfester, H. W. Spiess, Acta Polym. 1998,49,451. [6] F. Mellinger, M. Wilhelm, K. Landfester, H. W. Spiess, A. Haunschild, J. Packusch, J. Acta

Polym. 1998,49, 108. [7] K. Landfester, C. Boeffel, M. Lambla, H. W. Spiess, Macromolecules 1996,29,5972. [8] S . Spiegel, K. Landfester, G. Lieser, C. Boeffel, H. W. Spiess, N. Eidam, Macromol. Chem.

Phys. 1995,196,985. [9] J . Clauss, K. Schmidt-Rohr, H. W. Spiess,Acta Polym. 1993,44, 1. [ 101 N. Bloembergen, Physica, 1949, XV, 386. [ 113 F. Mellinger, M. Wilhelm, H. W. Spiess, Macromolecules 1999,32,4686.

Esterification Study of Acetoxysilane by Alcohols and Phenols

Victor Kopylov, W i m i r Zvanov, Marina Zheneva

Federal State Unitary Enterprise State Research Institute for Chemistry and Technology of Organoelement Compounds, 38 Shosse Entuziastov, Moscow, 1 1 1123 Russia

Fax: +7 095 273 72 06 E-mail: vmkopylov @ aport2000.m

Vyachesluv Kireev, Valerii Djakov

The Russian Chemical Technological University of D. I. Mendeleev 9 Meusscaja Place, Moscow, Russia 125047

Keywords: esterification, acetoxysilane, alcohols, phenols

Summary: The interaction of organoacetoxysilanes with menthol, eugenol, vanillin, citronellol, phenol, cyclohexanol and hexanol was investigated. Products of full and partial esterification were obtained. The hydrolysis of alkoxy(aroxy)silanes and acetoxyalkoxy(aroxy)silanes in a solution of methyl ethyl ketone or THF and on a cellulose surface was investigated. Rates of acetoxyalkoxy(aroxy)silane hydrolysis on the cellulose surface were by 1-2 orders lower than in a solution, but the dependence on the nature of the substituents remained.

The interaction of tetraacetoxysilane, methyltriacetoxysilane and cyclohexantriacetoxysilane with alcohols and phenols was investigated. Menthol, eugenol, vanillin, citronellol, phenol, cyclohexanol, and hexanol were used as alcohols and phenols. Products of full and partial esterification (Table 1) were obtained.

Si(OOCCH3)d + nROH + Si(OOCCH&,(OR), + nCH3COOH (n = 1-4)

Eq. 1.

The reaction (Eq. 1) was not accompanied by parallel formation of the acetates of the alcohols or phenols. The products of partial esterification were shown by the 'H and 29Si NMR methods to be a mixture of acetoxyalkoxy(aroxy)silanes of which the composition depended on the initial molar ratio of acetoxysilanes, alcohols or phenols. The hydrolysis of alkoxy(aroxy)silanes and acetoxyalkoxy(aroxy)silanes in a solution of methyl ethyl ketone or THF and on a cellulose surface

Ovganosilicon Chemisty V Edited by N. Auner and J. Weis

Copyright 0 2003 WILEY-VCH Verlag GmbH & Co. KgaA, Weinheim

Esteri$cation Study of Acetoxysilane by Alcohols and Phenols 345

was investigated (Table 2 and Fig. 1).

Table 1. Properties of the esterification products.

Compound d,M n,” Appearance

S~[OEU~]~[OAC]~

Si[OEug13[0Ac]

Si[OEug14

Si[OCitr],

Si[OEugl[OCitr13

Si[OEug]z[OCitr]z

Si[OEug]3[OCitr]

MeSi[0A~]~[OMent]

MeSi[OCitr]z[OMent]

MeSi[OAcIz [OEug]

MeSi[OAc] [OEuglz

Si[OCitr]z[OAc]z

c-HexSi [OEug]

c-HexSi[OMent],

c-HexSi[OVanilI3

c-HexSi[OCitr],

Si[OCitrlz[OMentlz

Si[OCitr][OMent],

Si[OCitr],[OMent]

Si[OMentI4

Si[OMent]z[OAc]z

Si[0Vanill4

Si[OVanil][OCitr],

Si[OVanil]z[OCitr],

MeSi[OEug],

1.1712

1.1541

1.1297

0.9117

0.9497

0.9915

1.0605

1.0066

0.9092

0.9475

1.0338

0.9778

0.9184

-

0.9318

0.9241

0.9146

-

1.0159

-

1.1317

1.5205

1.5475

1.5550

1.4605

1.4790

1.4995

1.5275

1.4405

1.4585

1.4351

1.4470

1.4521

-

-

1.4666

1.4410

-

1.4422

-

1.4495

-

light yellow liquid

light yellow liquid

light yellow liquid

light yellow liquid

light yellow liquid

light yellow liquid

light yellow liquid

light yellow liquid

light yellow liquid

light yellow liquid

light yellow liquid

light yellow liquid

light yellow liquid

solid product

resin

light yellow liquid

light yellow liquid

wax

viscous liquid

white crystals

light yellow liquid

resin

wax

wax

light yellow liquid

[a] OEug = 4-allyl-2-methoxyphenoxy; OCitr = 3,7-dimethoxy-6-octenoxy; Ovanil= 4-phormyl-2- methoxyphenoxy; OAc = acetoxy; c-Hex = cyclohexyl; OMent = 5-methyl-2-( 1- methylethy1)cyclohexanoxy.

The hydrolysis of alkoxy(aroxy)silanes and acetoxyalkoxy(aroxy)silanes in solution showed that the hydrolysis rate of alkoxy(aroxy)groups was slowed down with an increase in the number of alkoxy(aroxy)groups in a molecule, in the steric factors of these groups and in the organic substituents at silicon. Aroxy groups were split faster than alkoxy groups. When hydrolysis occurred on a cellulose surface, acetoxyalkoxy(aroxy)silanes were grafted to it. Rates of

346 V. Kopylov, V. Ivanov, M. Zheneva, V. Kireev, V. Djakov

acetoxyalkoxy(aroxy)silane hydrolysis on the cellulose surface were lower by 1-2 orders than in solution, but the dependence on the nature of the substituents remained. The hydrolysis of alkoxysilanes on a cellulose surface proceeds within 3-5 months, and aroxysilanes within 1-5 weeks. The research carried out shows that alkoxy(aroxy)silanes and acetoxyalkoxy(aroxy)silanes can be used in quality fragrance and biologically active substances with prolonged action, due to slow release of hydrolytic products into the environment.

Table 2 . The production of alcohols and phenols by hydrolysis of a solution of esterification (0.2 mom)

in methyl ethyl ketone (1.5 rnol H20/ROSi group at 20 "C for 200 h.

Alcohols and phenols produced by hydrolysis for 200 h [%fbl

Citronellol Eugenol Menthol Vanillin"'

Si[OA~]~[0Ci t r ]~

Si[Oeug] [OCitr]

Si[Oe~g]~[OCitr]~

Si[OEug][OCitr],

Si[OVanil]z[OCitr]z

Si[OVanil] [OCitr],

Si[OCitr],

Si[OAc] [OEug13

Si [OEug14

Si[OAc][OVanilI3

Si[OVanilI4

S i [OA~]~[0Ment]~

Si[OMent]z[OCitr]z

Si[OMent],

ChexSi[OEug13

ChexSi[OVanilI3

MeSi[OAcI2 [OEug]

MeSi[OAc] [OEuglZ

MeSi[OEug13

MeSi[OA~]~[Oment

MeSi[OAc][OMent] 2

MeSi[OMent]

MeSi[OA~l[OVanil]~

100

75

25

10

85

34

1

81

42

10

88

67

-

100

100

92

99

94

1

1

61 -

67

100

57

44

89

34

2 -

62

[a] OEug = 4-allyl-2-methoxyphenoxy; OCitr = 3,7-dimethoxy-6-octenoxy; Ovanil= 4-phormyl-2- rnethoxyphenoxy;

[b] The emission of alcohols and phenols was determined by method of GLC.

[c] Hydrolysis time 0.5 h.

OAc = acetoxy; c-Hex = cyclohexyl; OMent = 5-methyl-2-( 1- rnethy1ethyl)cyclohexanoxy.

Esterifcation Study of Acetoxysilane by Alcohols and Phenols 347

90-

80-

7 0 -

60-

50-

40-

30-

20-

10 -

0 -

I

I

I 1 I 1 I I I

Days 0 10 20 30 40 50

Fig. 1. The producation of alcohols and phenols by hydrolysis of acetoxyalkoxy(aroxy)silane on a cellulose surface

in air. 1, MeSi[OAclz[OVanill; 2, MeSi[OA~l[OVanil]~; 3, Si[OAc][OEug]3; 4, S ~ [ O A C ] ~ [ O E U ~ ] ~ ;

5 , Si[OAc]z[OMent]z; 6, Si[0A~]~[OCitr]~; 7, Si[OAc][0Citrl3.

Acknowledgment: The work was funded by Proctor 8z Gamble.

Organosilicon Compounds in Medicine and Cosmetics

Valerii D’yakov

Federal State Unitary Enterprise State Research Institute for Chemistry and Technology of Organoelement Compounds, 38 Shosse Entusiastov, Moscow, 11 1123 Russia

Tel.: +7 095 273 6323 -Fax: +7 095 913 2538 E-mail: mivalin@online.ru

Keywords: silatranes, silocanes, bioactive silicon, herbs, cosmetics, medicine

Summary: The biological activity and pharmacological effects of silatrane, silocanes and their silicon analogs of heteroauxine, choline, acethylcholine, colamine, and phosphorus oxyacids have been studied for the first time. New drugs for “alopecia areata” (silocast), chronic periodontitis (sident), and osteoporosis (silymine) have been developed. Silatranes and herbs (silicon-philes) are being used for development of new medicinal cosmetics (sila-cosmeceutics).

We were the first to synthesize, patent and study the biological and pharmaceutical effects of new compounds relating to the silatrane class and their monocyclic analogs, silocines and silocanes.

All these compounds are organosilicon ethers of biogenic amines (triethanolamine, diethylamine) or diethylene glycol. But we failed to obtain similar atrane derivatives of glycerin. At the same time we obtained and described for the first time silatrane analogs of indolylacetic acid (heteroauxin) and of synthetic phitohormones from phenoxyacetic acids.

At our disposal we have sila-analogs of choline, acetylcholine, colamine, biogenic amines, and oxygen-containing acids of phosphorus a.0. But we failed to isolate aspirin and ascorbic acid sila- analogs.

“Sila-cosmetics” a new trend in cosmetology, is being developed on the basis of the recent scientific achievements in the field of physiological function regulation of connective tissue, including our skin. Present-day investigations are carried out at the level of cell membrane processes, using surprising and unique formulas containing plants that are silicon-philes or comprise bioactive organic silicon. The latter, in contrast to inorganic silicon (as its oxide) is bio- available. Thus, organism tissue is found to interact with cosmetic preparations at the level of biological membranes. As a result of this interaction the following functions are provided:

respiratory function of skin;

Ovganosilicon Chemisty V Edited by N. Auner and J. Weis

Copyright 0 2003 WILEY-VCH Verlag GmbH & Co. KgaA, Weinheim

Organosilicon Compounds in Medicine and Cosmetics 349

redox processes in skin and tissues; absorption function of skin relating to aqueous-saline solution and fat-like substances; secretion function of skin promoting salt and slag removal.

These cosmetic compositions stimulate all types of metabolism (hydrocarbon, protein, fat). Methodical and competent use of cosmetic preparations is favorable for the normal physiological processes in organisms. What is the difference between “sila-cosmetics” and other cosmetic preparations?

Primarily, the distinction lies in the use of bioactive silicon (silatrane and silocane substances) as the active component (AC), and in natural Russian cosmetics “Baikal collection” medicinal plants - silicon-philes (nettle, horse-tail, fern, greater celandine, maral root etc.) as well as such truly Russian Siberian components as cedar, buckthorn, fir needles, birch oils; fly-agaric, birch black mushroom, asp rind, bemes and leaves of mountain ash; bergenia, aconite, Rhodiola and maral roots are used. The composition of sila-cosmetics may be represented by:

Base - emulsions of vegetable oils. Active components (AC) - silatranes, silocanes, Zn, Cu, Ca, Fe, Se, ethereal oils. Additives - extracts of herbs, roots, mushrooms, berries and fragrances.

We are paying great attention to the development of “Sila-cosmetics”, and “Sila-pharmaca” preparations are waiting their “hour of triumph”. Active investigations in this field are being carried out in Germany, in Professor R. Take’s laboratory.

The original collection of “Sila-stimulators” against wrinkles attracts particular attention and makes women tremble with joy. This preparation is made in accordance to the ying-yang principle and contains the mask-balsam “Silimin” (masculine origin, heat) and lifting-cream “Trecresan” (female origin, coolness). The Korean company “Bumil” has purchased this unique collection under contract and it is licensed in Germany. Requests for “Sila-stimulators” have been received from various companies from Switzerland, the Netherlands, the USA and Sweden.

An idea for development of an elite “Valery” collection of cosmetics in ampoules arose in 1996 in France during the XI International Symposium on Organosilicon Chemistry, and was realized in 1998 in Moscow.

The compositions and properties of the “Valery” ampoules are described in annotations. We succeeded in combining a glycine amino acid, widely used vitamin and antioxidant (ascorbic acid) with a biostimulator, antidote and strong “Cresazin” antioxidant in “Trecresan-C” gel. Our Cresazin is superior as an antidote and antioxidant to the classical standards - succinic, ascorbic and folic acids, and vitamin E (a-tocopherol) (Tables 1,2).

The idea for a series of medicinal balsams-cosmeceutics for prophylaxis and therapeutic osteochondrosis, radiculitis, rheumatism, and cellulite treatment arose in the Khamar Daban mountains, so it was named “Baikal collection”. Scientific sources were found in medieval German alchemy texts. They have something in common with a recipe of the Tibetan canon “Zhud-Shi”. The components include camphor (or camphor oil), eucalyptus extract (or oil), mint (menthol),

350 V. D’yakov

derivatives of orthosalicylic acid (they are contained in cranberries, red bilberries, willow and asp rust). In Old Russian medicinal books we found delightful descriptions of red fly-agaric. Official medicine recognizes it as poisonous. Popular medicine, in contrast often uses it in practice. Thus in formulas of Tibetan lama healers each component plays its own role. Some of the components are “tsar and tsarevitch”, “councilor and serf’, “horses and guides”. In “Amanit” balsam tsar is red fly- agaric (Am. Anita muskaria), and its nearest relation Caesar fly-agaric (Am. caesarea) is an edible mushroom. Tsar fly-agaric contains choline, acetylcholine, alkaloids muscarine, mescaline and ascorbic acid. These substance are effective as anesthetics and a nervous system stimulants, and improve the flow of nerve impulses. Fly-agaric toxin (bufothenine) has only a hallucinogenic effect. A fatal dose of muscarine may be isolated only from 125 kg of mushrooms.

Table 1. Effect of trecrezan on free-radical oxidative processes under the action of chlorophos.

Student’s t-test criteria Test reliability P Light sum chemoluminescence IMP/3 [min] Variant

Control (standard) 10 078f245 - -

Chlorophos 29 936k1 828 15.64 <0.05

Ascorbic acid 7 521f539 10.88 <0.05

Trecrezan 2 15of478 2.0 >0.05

[a] Registration of chemoluminescence in bream blood serum for the system luminol-H202, pH 6.8. Detector Q3Y-37, nuclear analyzer JIC-428B.

Table 2 Effect of antidote in the case of hydrobiont [‘I poisoning by chlorophos (0.01 mg/L).

Initial no. of Live individuals within 6 days

individuals No. [Initial value] Species Antidote (anti-poison)

Ascorbic acid (0.0005) daphnia 40 30 75

Succinic acid (0.0005) daphnia 40 15 37.5

Cresazin (0.0005) daphnia 40 78 195

Control 1 (without antidote) daphnia 80 30 37.5

Ascorbic acid (0.0005) Protozoacb1 52 30 57.7

Cresazin (0.0005) Protozoacb1 45 70 155.6

Control 2 (without antidote) Protozoacb’ 60 16 26.7

[a] Hydrobionts = inhabitants of aqueous media. [b] spatidium.

In the Middle Ages healers in many countries used an infusion of spruce fir and copper vitrol against podagra and for bone fracture. In the 20th century scientists demonstrated that silicon content increases 50-fold at the fracture site.

Until now people in Europe and Asia have used an alcohol infusion of nettle in cases of confusion, inflammation and wounds. Nettle is a strong silicon-phile, containing oligosilicic acid.

Organosilicon Compounds in Medicine and Cosmetics 351

Previously, many people used an infusion of red fly-agaric in vodka in the rheumatism cases. Clinical trials of “Asp Solegon” balsam astonished many cosmeticians and medical specialists.

Thus after external application of this balsam in, for example, cases of pain in the lumbar-saclar bond of the back section, knee and upper arm, ankylosis along with severe rheumatic pains and crunch control inflammation processes in blood and lymph disappear. This is confirmed quantitatively by blood analysis. We used “Asp Solegon” in combination with magneto-laser therapy (Milt-apparatus, 5 Hz-5 mA). For 85 % of patients in the 29-68 age group (18 women and 12 men) the ESR was normalized from 30-55 mm/Hz to 6-12 mm/Hz during five to seven treatments.

Asp contains tannin matter, benzoic acid, glycoside, populin and salicin. By the way, in addition to bornyl acetate, caffeic and ascorbic acids, pinosylvin and pinoabin spruce fir needles contain iron, manganese, copper and chromium.

In conclusion, the primary peculiarities of “Sila-cosmetics” are:

unique biocompatibility with human skin; mild stimulation of the natural biochemical mechanisms of skin functional states; dermatological compatibility of natural components and classic synthetic substances; absence of a “habituation effect”.

Now investigations in the “Sila-cosmetics” field have been awarded by gold medals by the All- Russian Exhibition Center (1998-2000) and other numerous diplomas at Russian Exhibitions. In Russia these developments have obtained favorable conditions for practical application and are welcomed by scientific community; we believe they will find wide commercial use.

In Europe, particularly in Germany (Munich, Cologne, Berlin, Braunschweig etc.) preliminary comments, reports and reviews have been obtained. Women in Germany have paid much attention to the preparations of “Sila-cosmetics” that Russian women appraised five years ago, namely, “Sila- stimulators”, “Valery”, “Fluffy eyelashes” and the “Baikal collection”.

Synthesis and Biological Activity of Silocanyl- and Silatranylmethyl Ethers of Acrylic Acids

V. M. D’yakov, S. V. Loginov

Federal State Unitary Enterprise State Research Institute for Chemistry and Technology of Organoelement Compounds, 38 Shosse Entuziastov, Moscow, 11 1 123 Russia

E-mail: eos 0eos.incotrade.ru

Keyword: silatranes, silocanes, acrylic acids, wounds, burns, granulation-fibroid tissue

Summary: Synthesis of previously unknown silatranes and silocanes methyl ethers of acrylic- and metacrylic acids has been performed. It is shown that silocanyl derivatives similar to silatrane posses wound cicatrizing effects and stimulate bums healing.

We have synthesized previously unknown silocyne, silocane and silatrane derivatives of acrylic acids according to Scheme 1, reaction I in the presence of polymerization inhibitors. Acrylates [ 13 are also obtained by concurrent synthesis on the basis of previously described halomethylsilocanes via the scheme 1, reaction 11.

M = O X = C1, Br; R = H, Me

Scheme 1.

Silatrane acrylates are obtained in accordance with Scheme 2, reaction 111. In contrast to silocanes, their silatrane analogues comprise the crystalline substances that are not susceptible to easy polymerization. Their toxicity and wound cicatrizing effects have been studied.

Ovganosilicon Chemisty V Edited by N. Auner and J. Weis

Copyright 0 2003 WILEY-VCH Verlag GmbH & Co. KgaA, Weinheim

Synthesis and Biological Activity of Silocanyl- and Silatranylmethyl Ethers 353

M = 0, NR"; R" = H, Me, Ph R = H, Me; R'= Me, Et

Scheme 2.

All the silocane acrylates, even with R = Ph, are viscous high-boiling liquids with characteristic odors. They are not toxic (LD50 2 2.0 gkg).

Their effect on regeneration of connective tissues at wound defects and deep thermal bums has been studied. In experiments carried out and designed to reveal the biological activity the compounds obtained (A and B, Fig. 1) it has been found that they considerably accelerate wound cicatrizing (see Table 1).

CHz=C(Me)COO H2 7 (B)

CH2=C(Me)COO H2

(A) MeSi(OCH2CH2)20

7 Si(Me)(OEt)z

Fig. 1. Alkoxysilane (A) and silocane (B) acrylates.

Table 1. Effect of alkoxysilane (A) and silocane (B) acrylates on period of cicatrizing rat's skin wounds.

Times of primary (I) and secondary (II) scabs falling off [day]

Control Placebo B A

I I1 I I1 I I1 I n

9 18

10 19

10 20

11 21

11 21

12 21

12 22

12 23

13 23

13 23

11.3 21.1

f0.84 f1.05

10

10

10

11

11

12

12

13

14

14

11.7

f0.84

19

20

20

21

22

22

23

23

24

27

22.1

f1.68

8

8

8

9

9

9

9

10

10

10

9.1

H.42

14

18

18

19

19

19

20

21

21

-

18.8

f1.41

8

8

9

9

9

9

9

9

10

10

9.0

f0.21

16

16

17

17

18

18

18

18

19

20

17.7

f0.84

354 V. M. D’yakov, S. V. Loginov

As shown in Table 1, both silocanes activate the cicatrizing uncomplicated skin wounds in experimental animals. It should be noted that for B this activation is practically completed in the period before the primary scab falls off; the range between the primary and the secondary scabs remains unchanged. The wound-cicatrizing effect of B is determined by its influence on granulation-fibroid tissue formation, i.e. the connective tissue component of the cicatrizing process. Silocane A, along with the same effect on the f i s t process stage, shortens (on the average by one day) the second one - a stage of wound defect epithelization; as a result, the total activation effect of A is found to be more marked.

Silocanes are no less effective than silatranes in activation of the burns cicatrizing period. Therefore silocanes improve and accelerate the scab falling off period (see Table 2).

From the present results it follows that both silocanes B and A provide reliable activation of cicatrizing of the deep uncomplicated skin thermal bum. The silocane effect on average terms of primary scab falling off, upon completion of granulation-fibroid tissue development at the site of bum necrosis, was more pronounced; this acceleration stage approaches to 15 % relative the control. Somewhat less pronounced (about 6-7 %) was activation of secondary scab falling off but it is also of practical value too, taking into account the fact of uncomplicated bum cicatrization, in comparison with solkoseril, which is a highly efficient preparation in clinical practice but which turned out to be ineffective for an uncomplicated burning process in this experiment.

Table 2. Effect of some silocanes (in comparison with solkoseril) on cicatrizing of experimental burns of rats.

Time of the primary (I) and the secondary (11) scabs falling off [day]

Control B A Solkoseril

I I1 I I1 I I1 I II 15 22 13 25 12 24 18 26

15 25 14 25 14 24 19 21

17 25 15 26 14 25 20 29

19 30 16 26 17 26 22 30

21 31 18 28 18 29 23 31

11.4 21.2 15.2 26.0 15.0 25.6 20.4 28.6

k3.06 f4.59 k2.55 k1.53 f3.06 f2.55 f2.55 f2.55

It is of great importance that bioactive silicon in molecules of both silatranes and silocanes intensifies biosynthetic processes (see Table 3).

Attention is drawn to the decrease in average mass value of the granulation-fibroid tissue formed under the influence of both silocanes. Under the effect of B a decrease in the ribonucleic acids concentration has occurred that demonstrates the rise in the rate of biosynthetic processes in the tissue. Collagen concentrations, determined via hydroxyproline, remain at nearly the same level as the control series (under A, there are even decreased), but meanwhile a considerable decrease takes place in the concentration of hydroxylysine - an amino acid that is specific for collagens and takes

Synthesis and Biological Activity of Silocanyl- and Silatranylmethyl Ethers 355

direct part in cross-bonding of collagenic molecules. This means that in this case collagens differ in their lower structure stability (maturity) and this situation may be the basis of favorable conditions for morphogenesis of wound scrap, particularly for mesenchymal+pithelial interaction. In addition, a shift in the fractional distribution of glycosaminoglycans (GAG), determined as their cetylpyridine derivatives, may provide these conditions. A decrease in the fraction dissolved in a 0.4 M solution of sodium chloride comprising mainly hyaluronate is noted; accumulation of the latter is characteristic of immature granulation-fibroid tissue. At the same time fractions of “mature” GAG (predominantly chondroitin-sulfates and heparin) dissolved in 1.2 and 2.1 M solutions of sodium chloride are increased.

Table 3. Silocane effect on some biochemical characteristics of granulation-fibroid tissue.

Characteristics Control Placebo B A

Tissue mass [g]

Ribonucleic acid la]

Hydroxyproline la’

Hydroxylysine la’

Hexauronic acids

Fractions GAG Ib1

0.4

1.2 1.

L. 1

1.51kO.12

2.15M.05

2.57f0.07

0.21M.03

0.76M.03

45.9

38.9

15.2

1.51M.22

2.26M.05

2.71M.04

0.23M.01

0.76M.06

55.2

27.7

17.1

1.1039.18

2.41M.01

2.21M.07

0.22M.01

0.75M.02

51.1

35.9

13.0

1.13M.11

1.56M.12

1.96M.03

1.14M.03

0.86M.02

50.5

30.8

18.7

[a] g/100 g dried degreased tissue. [b] percentage of total glycosaminoglycan quantity.

Thus the experimental tests carried out show that two representatives of the silocane class: B (2-methyl-2-metacryloxymethyl-1,3,6,2-thioxysilocane) and A (methacryloxymethylmethyldi- ethoxysilane) are capable of activating uncomplicated experimentally cut wounds and deep thermal bums of skin in single applications of these preparations as high-concentration (5 %) liniments to fresh wound surfaces. As shown, the results of biochemical investigations of granulation-fibroid tissue activation are determined by changes in the dynamics of metabolic processes in developing wound scrap. Thus it may be concluded that the silocanes under study are able to directly influence the functional state of connective tissue cells participating in the cicatrizing process by activation of their morphogenetic potentials.

Reference [ 11 V. M. Dyakov, A. N. Kir’yanova, Zh. Obshch. Khirnii 1988,58(3), 539-547.

Biodegradability and Silatrane Effect Mechanism

Evgenii Ofitserov, Valerii D'yakov, Maksud Rasulov

Federal State Unitary Enterprise State Research Institute for Chemistry and Technology of Organoelement Compounds, 38 Shosse Entuziastov, Moscow, 11 1123 Russia

Tel.: +7095 273 6323 -Fax: +7095 913 2538 E-mail: eos @eos.incotrade.ru

Keywords: silatranes, pharmacokinetics, hepatoprotection effect

Summary: The degradation rate of biologically active hypervalent silicon and germanium compounds (atranes) was investigated under various pH conditions. Stimulating effects on proliferation and reparation processes in experimental animals were demonstrated.

The silatranes have been found to posses higher hydrolytic stability than similar di- and triethoxysilanes XSi(OCzH5)3 and tris(2-aminoethoxy)silanes XS~(OCH~CH~NHZ)~. The relationship between the XSi(OCH2CH2)3N neutral hydrolytic reaction rate and the nature of the substituent X (X = H, CHz=CH, CH3, CzH5, C3H7, C6H5, CH30, CzH50, C6H5O etc.) was studied. The hydrolysis kinetics in 0.01 mol/L aqueous solutions at 20 "C was analyzed. The process of hydrolysis, being a combination of parallel-serial reactions, corresponds to a first-order reaction. The reaction order is established graphically and the hydrolysis rate constant is calculated by Eq. 1, where t = time [h] from the beginning of the reaction till the sampling moment, vt = volume [mL] of HCl used for sample titration, v, = volume [mL] of HCl used for titration of the completely hydrolyzed sample.

k = 2.31t log (v,/v,-v~)

Eq. 1.

The comparison of the rate constants obtained shows that the titration of the silatrane hydrolysis rate drops in the following series according to the X substituents:

Thus, the silatranes studied are subdivided into two reaction series: I) where X = R (alkyl); and 11) where X = RO (alkoxyl), where good linear correlation between log k and ox* is observed. In

Ovganosilicon Chemisty V Edited by N. Auner and J. Weis

Copyright 0 2003 WILEY-VCH Verlag GmbH & Co. KgaA, Weinheim

Biodegradability and Silatrane Effect Mechanism 357

series I the growth of the R-radical induction constant facilitates hydrolysis (p* > 0). This testifies to the fact that the total rate of alkylsilatrane multi-stage hydrolytic reaction is determined by nucleophilic attack of the silatrane molecule reaction center by a water molecule (or aquated HO-ion), which is the slowest stage of the process.

Spatial influence of substituents on the series I, alkylsilatrane, hydrolysis rate evidently does not produce a significant effect. 1-Hydridosilatrane (X = H) is also referred to reaction series I. Some deviation of log k from the correlation straight line can be explained by the competitive co-hydrolysis of the Si-H bond besides the Si-0-C group cleavage. The log k values of l-vinyl- and 1-phenylsilatranes significantly deviate from the correlation line I and their co-hydrolysis rate turned out to be much lower than the anticipated one.

The hydrolysis rate of 1-alkoxysilatranes (series 11) is considerably lower than one could anticipate on the basis of the correlation straight line obtained I. The high hydrolytic stability of 1- alkoxysilatranes can be explained by steric barriers and prelogic interactions typical of intermediate cycles. In contrast to 1 -alkoxysilatranes, the reaction rate of para-substituted 1-aroxysilatrane hydrolysis drops with an increase of electron-accepting substituent in the aromatic nucleus; log k values of these compounds correlate linearly with op or op+ values of the aromatics. The reaction constant in this series has a negative sign, which sets one speaking of an electrophilic mechanism for 1-aroxysilatrane synthesis. Its small value (p* = -0.40) testifies to the small substituent influence in the aromatic nucleus on the reaction rate.

In contrast to hydrolytic cleavage in a neutral medium, acid hydrolysis of 1-organoxysilatranes is described by a kinetic equation of the second order. The hydrolysis was studied in diluted aqueous solutions at 25 "C in the presence of HCl and KC1. No salting effect was manifest at KCl concentrations from 0.05 to 0.25 mom. The hydrolytic rate constants (k ) obtained, their mean square deviations, o * o ~ values and average lethal doses (LD50) of 1-aroxysilatranes are presented in Table 1. Quantitative evaluation of k as a function of the nature of the substituent was carried out in a similar way by applying the Taft equation.

All the compounds analyzed are also subdivided into two reaction series. The first series involves 1 dkoxysilatranes, the other 1-aroxysilatranes and 1-benzyloxysilatrane. In series I, where p* = -1.26 (R = C2H50, C3H70, (CH3)2CHO, (CH&CO, (CH3)3CCH20), and series I1 (R =

where p*= 0.23, the hydrolytic reaction rate drops as the induction constant o*o~ of substituent is growing. Negative values of the reaction constant p* testify to the positive charge forming at the reaction center in a transition state. The log k value for 1-tert-butoxysilatrane sharply deviates from the correlation straight line (its hydrolytic rate proves to be much slower than expected) resulting from the steric effect of the substituent.

Linear dependence between hydrolysis rate and medium pH value demonstrated that for both reaction series an electrophilic attack of the reaction center by hydroxonium ion was a limiting stage. The following stages of endocyclic Si-0 bond breakdown proceed very quickly with triethanolamine hydrochloride as the hydrolytic reaction end product. Potentiometric titration curves testify to this fact.

CrjH50, 2-CH3C6&0, 3-CH3C6&0, 4-CH3C6&0, 4-CH30C&O, 4-c1c&40, CsH&H20),

358 E. Ofitserov, V. D'yakov, M. Rasulov

Table 1. Hydrolysis rate constants for silatranes ROSi(OCH2CH2)3N.

R k2= [mom s] 0.011 LDSO bg/kg*I [" 0.37M.01 1.37 -

i-C3H7 0.51M.01 1.26 -

i-C4H9 0.12M.02 1.15 -

CIH5 0.33M.01 2.38 200

2-CH,C& 0.38M.01 2.19 1200

3-CH&& 0.35M.01 2.31 708

4-CH3C6H4 0.36M.01 2.23 710

4-CH30C& 0.37M.01 2.22 345

4-C1C& 0.39M.02 2.11 565

C6H5CH2 0.57&.02 1.37 2250

CIH5

[a] For white mice of 18-20 g weight. The silatranes were administered intrapentoneally in water-based emulsions.

In one of our first studies we demonstrated that silatranes raised lymphocyte membrane resistance to damaging agents (mechanical, chemical, or thermal). Hemocoagulation indices and histology results led to this conclusion about the adaptogenic properties of the silatranes. For the example of CMS (chlormethylsilatrane) it was shown that they produced a high anti-hemolytic effect in response to ultrasound and circulatory damages of erythrocyte membranes.

Diuron mom), as a photosynthetic inhibitor, as well as dinitrophenol (DNP mom), as an oxidative phosphorylation uncoupler, facilitated the adverse effect of high

temperature. This can testify to the influence of CMS on stabilization of electron transport chain carriers in the lipid membrane.

Hence the causes of the strengthening of the protein-lipid bonds and the structural integrity of membranes under silatrane action can be understood. Protein-lipid bond strengthening can be of functional importance for energy-storing membranes.

In the regenerating liver after two-thirds tissue allation the bile-duct epithelium is never produced from the hepatocytes. However, if 2-acetylaminofluorene or dipinum is introduced simultaneously with the liver resection, the regeneration proceeds in an unusual way: after 24 hours m-RNA is accumulated in lipocytes under HGF (hepatocyte generation factor) action. At the same time m-RNA of lc-met receptor appears in bile-duct epithelium. Theses events can be interpreted as oval curve proliferation start-up by means of HGF mitogen supplied by active lipocytes [l].

From the experimental point of view it was shown that, in mice to which aqueous solutions of trecresan and mival(5 mg/L) were administered for 20 days, DNA and RNA content in live animals rose to 110.5 and 134.2 respectively against 98.6 in the test group. Use of trecresan in the partial hepatoectomy experiments in rats results in intensification of hepatocyte regeneration, a rise in macroergons and acceleration of individual phases of the mitotic cycle. These processes occur simultaneously with suppression of lipid peroxide oxidation in the hepatocytes and a decrease in oxygen transfer rate through mitochondrial membranes. Similar tests for the analysis of the

Biodegradability and Silatrane Effect Mechanism 359

liver-protecting effect of 1 -ethoxysilatrane and isopropoxygermatrane, conducted in our laboratory by N. K. Nurbekov, demonstrated that use of atranes and trecresan at liver resection results in acceleration of reparative process that is observable by liver sample microscopy. Biochemical processes determining this phenomenon were therefore studied. On the grounds of the idea that initial contact of the atrane molecule with cytoplasmic membrane is followed by “signal” transfer through messenger relay - a race to intracellular organelles [2, 31 - the problem of identification of possible “relay race” participants was set. Taking into consideration that we are talking about prior protein synthesis with the purpose of compensation for the lost part of the organ, we investigated the dynamics of metal-competent enzymes (aminoacyl-tRNA synthetases, or ARSes) activity under liver regeneration conditions. In the total ARS fraction, employing analysis of their acylating activity tagged amino acid (tryptophan, phenylalanine and lysine) tRNA acylation kinetics on lys yl-ARSes pairs the tRNA(Lys) concentration was found to have a direct linear dependence on aminoacylation reaction time. A similar picture was observed when tRNA(Phe) acylation kinetics was analyzed, i.e. a definite increase in protein-synthesizing system components, in particular in the enzyme complex catalyzing the main stage of protein biosynthesis in a cell-, and tRNA-specific aminoacylation were evident. Precise correlation between the stimulating effects of atranes on reparative processes and the rise in the protein-synthesizing cell mechanism was found for the example of the increase in total ARSes preparation activity increase. It is possible that the increase in activity of the protein-synthesizing liver cell mechanism reflects a rise in the total protein-synthesizing activity; it may not always be related to the proliferation but can reflect an increase in secretory activity, which means that the atrane effect is complicated, diverse and very complex.

Thus, the silatranes improve plant resistance to the unfavorable environment that may result from membrane stabilization. This phenomenon and fast biodegradability of the silatranes in soil and water prove that organosilicon plant growth regulators should be looked upon as a new generation of environmentally safe plant protection agents.

References

[ 11 [2] [3]

V. Repin, G. Sykhikh, Medical Cell Biology, BEBiM, Moscow, 1998, p.200. Yu. Pisarskii, V. Kazimirovskaya, M. Voronkov, Doklady ANSSSR 1987,293(3), 724. M. Rasulov, I. Kuznetsov, M. Voronkov, Doklady AN SSSR 1989,307(3), 762.

Intensification of Unsaturated Organomagnesium Chloride Production Reaction

Vladimir Zhun, Alla Zhun, Evgenii Chernyshev

Federal State Unitary Enterprise State Research Institute for Chemistry and Technology of Organoelement Compounds, 38 Shosse Entuziastov, Moscow 1 11 123, Russia

Tel.: +7 95 2736346, - Fax: +7 95 1877058 E-mail: postmaster@chimex.msk.m.

Keywords: organomagnesiumchlorides, intensification, ultrasound

Summary: The efficiency of the effect of ultrasound irradiation on the reaction mixture for vinyl- and phenylchlorosilane synthesis is determined by ultrasound irradiation (USI) frequency as well as by the exposure time and the origin of starting organohalide. In the case of vinylmagnesium chloride, the formation period of the major reaction product under continuous US1 exposure shortened 2.3-fold. When US1 affected the synthesis during half the reaction period, the latter duration shortened by 1.4 times. In the case of phenylmagnesium chloride the process period also shortened by 2 and 1.2 times respectively for irradiation times that were 100 and 50 % of the reaction time.

First attempts to obtain organic magnesium derivatives were made in 1859, but after Victor Grignard discovered in 1900 [l] that when diethyl ether is used as the solvent for organic magnesium derivatives production, stable products can be obtained, these reagents were employed in a diversity of chemical conversions. But synthesis of organomagnesium compounds with unsaturated radicals with multiple bonds in the P-position to magnesium was implemented only in the middle of the 20th century. Henry Norman [2] found that when tetrahydrofuran is employed as a solvent, unsaturated organomagnesium compounds are produced with high yields. Researchers from our Institute have been engaged in commercial application of vinyl- and phenylmagnesium chlorides and their application in organoelement product synthesis since the mid- 1970s. The critical point of this reaction is a process initiation, a so-called “provocation”. The most careful raw material and equipment preparation will never guarantee fast process initiation. At the same time the presence of cutting steel (Crl8NilOT brand) in the reaction mass as well as asbestos, graphite, and polyvinyl chloride-based gasket material does not prevent process initiation and then its full progress (up to 90-95 % magnesium conversion). Reaction mass heating did not promote reaction initiation either, or its further successful. In the case of vinyl chloride this may be explained by its solubility decrease in THF on heating, and the resulting concentration in the reaction mixture diminishing. Growth in the intensity of blending did not facilitate process initiation either - only

Ovganosilicon Chemisty V Edited by N. Auner and J. Weis

Copyright 0 2003 WILEY-VCH Verlag GmbH & Co. KgaA, Weinheim

Intensification of Unsaturated Organomagnesium Chloride Production Reaction 361

an intensive grinding of magnesium chips was observed but no reaction "provocation" occurred.

shown in Scheme 1.

The conventional radical chain reaction of organomagnesium compounds formation [3, 41 is

R B r + M g - RMg' + Br '

RMg'+ :s* - RMg' : s

R' + Mg - RMg':S + RBr - RMgBr + R' Wg'

MgBr'+ : S* - MgBf:S MgBf:S + RBr - RMgBr+ Br '

Br' + Mg + MgBr'

Scheme 1.

It was interesting to analyse the US1 effect on initiation and progress of the formation of unsaturated organomagnesium compounds.

The US1 effect on vinyl- and phenylmagnesium chloride synthesis in THF was studied. The following parameters were modified in the process of analysis: 1) ultrasound irradiation frequency 20 and 44 kHz; 2) the period during which the ultrasound irradiation took-effect on organomagnesium production. The data are presented in the Table 1.

Table 1. Effect of US1 on organomagnesium chloride production.

Organo halide Process parameters RMgCl yield

Frequency Treatment intervals Synthesis Temperature (by reaction with [kHz] (treatment/processing) time [h] ["CI MeJSiCl) [ %]

22

22

22

44

22

22

22

44

without treatment

continuous

5/10

5/15

Continuous

without treatment

continuous

5/10

15/15

continuous

6

3

5

5

5

14

6

12

10

10

60-70

6@70

60-70

60-70

60-70

6 M 5

60-65

60-65

60-65

60-65

89

90

88

85

90

85

87

80

83

85

The US1 effect was found to intensify vinyl- and phenylmagnesium chloride formation. US1 of 22 kHz frequency is most efficient; US1 of 44 kHz frequency is less efficient in terms of decrease in reaction time. US1 efficiency for the reaction mixture was found to be determined by both the exposure time and the origin of the starting organohalide. In the case of vinylmagnesium chloride

362 V. Zhun, A. Zhun, E. Chemyshev

synthesis, the formation time of the major reaction product under continuous exposure to 22 kHz US1 was shortened 2.5 times, and when the exposure period amounts to 50 % of the reaction time the process time was observed to shorten only by 1.4 times. In the case of phenylmagnesium chloride synthesis, the reaction time also decreased by 2 and 1.2 times (the irradiation periods were 50 and 30 % of the reaction time). It should be emphasized that for vinylmagnesium chloride synthesis, the ultrasound influence on the first reaction stage (induction period) was not efficient: magnesium cuttings were abraded to powder but the reaction did not start. This is presumed to be the result of a string dispersive effect of ultrasound on the solid phase (metal) and equilibrium displacement in the diffused layer at the magnesium surface due to a decrease in the vinyl chloride concentration. The influence of US1 on vinylmagnesium chloride is efficient only after “reaction provocation”. No byproducts are actually formed by the Wurtz reaction when the process in conducted under USI.

References

[I] [2] [3]

[4]

V. Grinard, Compt. Rend. 1900,130,1322. H. Normant, Compt. Rend. 1954,239,1510. P. M. Hirak, V. A. Pal’m, U. I. Soogenbits, Reactsionnai Sposobnost Organicheskih Soedinenii 1975, 11(4), 705. H. R. Rogers, C. L. Hill, Y. Fujiwara, R. J. Rogers, H. Lee, G. M. Whitesides, J. Am. Chem. SOC. 1980,102(1), 217-231.

Silylative Coupling and Cross-Metathesis of Alkenes and Dienes with Vinyl-Silicon

Derivatives - New Catalytic Routes to Synthesis of Organosilicon Compounds'

Bogdan Marciniec

Adam Mickiewicz University, Faculty of Chemistry Grunwaldzka 6,60-780 Poznari, Poland

Tel.: +48 618291366 - Fax: +48 618291508 E-mail: marcinb @amu.edu.pl

Keywords: silylative coupling, silylation, cross-metathesis, polymerization, vinylsilanes, catalysis

Summary: Recent achievements in two catalytic reactions, i.e., silylative coupling and cross-metathesis of alkenes and dienes with vinyl-silicon compounds, which resulted in new synthetic routes to organosilicon molecular and macromolecular compounds are presented. The silylative coupling, also called dehydrogenative or trans-silylation and silyl group transfer, is catalyzed by metal complexes which either contain or initiate the formation of M-H and M-Si bonds, where M = Ru, Rh, Co and Ir. Cross-metathesis, which was developed very recently, proceeds in the presence of metallacarbenes, mainly those of ruthenium (e.g., Grubbs catalyst).

Introduction

Substituted vinylsilanes, RCH=CHSiR3 and R(SiR3)C=CH2, have become a commonly used class of organosilicon reagents in organic synthesis [ 11. There have been usually prepared by traditional methods, such as hydroboration of silylalkyne derivatives followed by alcoholysis of the B-C bond [2], silylation of terminal alkenes with halogenotrimethylsilane in the presence of organolithium [3] or organomagnesium [4] compounds, or hydrogenation of silylalkynes catalyzed by palladium complexes [ 5 ] .

In the past decade two new catalytic reactions occurring between the same parent substances have been developed, namely silylative coupling (trans-silylation) (Eq. 1) and cross-metathesis of alkenes (Eq. 2) with vinylsilicon compounds.

In 1984 we reported the first very effective example of disproportionation (called "metathesis")

' Delicated to Professor Florian Domka on the occasion of his retirement.

Ovganosilicon Chemisty V Edited by N. Auner and J. Weis

Copyright 0 2003 WILEY-VCH Verlag GmbH & Co. KgaA, Weinheim

364 B. Marciniec

of vinyl-substituted silicon compounds which could be catalyzed by Ru and Rh complexes [6]. Evidence for the migratory insertion of ethylene [7] and vinylsilane [8] into the Ru-Si bond (yielding vinylsilane and two regioisomers (1,2- and 1,l -bis(silyl)ethene), respectively, showed that in the reaction referred to as the "metathesis" of vinylsilanes and their "co-metathesis" with alkenes, instead of the C=C bond cleavage (formally characterizing alkene metathesis, Eq. 2), a new type of olefin conversion was revealed - silylative coupling of alkenes with vinylsilanes, called also dehydrogenative silylation trans-silylation or silyl group transfer (Eq. 1).

Eq. 1.

Subsequent extensive synthetic and catalytic studies have shown that silylative coupling of alkenes with vinyl-substituted silicon compounds proceeds (similarly to the hydrosilylation and dehydrogenative silylation reactions) via active intermediates containing M-Si (silicometallics) and M-H bonds (where M = Ru, Rh, Ir, Co, Fe). The insertion of alkene into M-Si bonds and vinylsilanes into M-H bonds, followed by elimination of vinylsilane and ethene, respectively, are the key steps in this new process [9].

On the other hand, although well-defined or in-situ initiated metallacarbenes are inactive for self- metathesis of vinyl-substituted silanes and siloxanes, we revealed recently a high catalytic activity of Grubbs catalyst in cross-metathesis of vinyltrialkoxysilanes and vinyltrisiloxanes with styrene, 1-alkenes and selected ally1 ethers and other derivatives [ 10-121.

The reaction occurs under mild conditions according to Eq. 2.

Eq. 2.

This review is aimed at presenting recent achievements in the search for new synthetic routes to unsaturated molecular and macromolecular organosilicon compounds using the two above- mentioned catalytic reactions.

Silylative Coupling and Cross-Metathesis of Alkenes and Dienes 365

Synthesis of (E)-Styrylsilanes and -Siloxanes

In our recent study [13, 141 on stereo- and regioselective coupling reactions of styrene with vinylsilanes, catalyzed by ruthenium complexes, we used p-substituted styrenes as well as new, more efficient rhodium and ruthenium catalysts. In the presence of Grubbs catalyst, a reaction of two initial substances, proceeding via a metallacarbene mechanism and yielding the same products, was recently revealed [ l l , 121. The two reactions often proceed quantitatively according to Eq. 3.

xw SiR1R2R3 [Ru], [Rh] x w S i R 1 R2R3 + - - /

Eq. 3.

Silylative Coupling

Catalysts = [(cod)Rh(,~-OSiMe)3]z [ 151, RuHCl(CO)(PPh3)3 [ 161, RuHCl(CO)(PCy3)3 [ 161; X = H, C1, Br, Me, OMe; R'R2R3 = H, C1, Me, Ph, OMe, OEt, OSiMe3; [vinylsilane]:[styrene] = 15-10; T = 40-90 "C; t = 1-24 h; catalyst loading = 0.01-0.5 mol %.

Cross-metathesis

Catalyst = (PCy3)2C12Ru=CHPh [ l l ] ; X = H, C1, OMe, Me; [vinylsilane]:[styrene] = 5:l; T = 40 OC; t = 3 h; catalyst loading = 0.1-1 mol %.

Synthesis of (E+Z)-1-Silyl-ZN(0 or S)-substituted -ethenes and -propenes

On the basis of all the experiments with vinyl-substituted silanes and siloxanes with heteroatom- functionalized alkenes, catalyzed by ruthenium complexes, we were able to propose general synthetic routes. The reaction of vinyl-substituted silanes with vinyl-substituted heteroorganic (N,O,S) compounds proceeds effectively and yields, under optimum conditions (usually a five-fold excess of alkene; 80-1 10 "C) and in the presence of ruthenium complexes containing or generating Ru-H and/or Ru-Si bonds, l-silyl-2-N(O or S)-substituted ethenes with a high preference for the E-isomer, according to Eq. 4, where R3 = Me3, MepPh, or (OEt)3.

On the other hand, the reactions of vinylsilanes with allyl-substituted heteroorganic compounds in the presence of Grubbs catalyst are examples of successful cross-metatheses and yield, under optimum conditions (10-15 mol % excess of vinylsilane, 40 "C), l-silyl-2-N(O or S)-substituted propenes according to Eq. 5.

366 B. Marciniec

==, RuH, Ru-Si R3Si JX -SiR3 + X

X = OR' [17], where R' = Et, Pr, Bu, fert-Bu, Hex, Me3Si or X = COOMe [7], Catalysts =

RuHCI(CO)(PP~~)~, RUCI(S~M~~)(CO)(PP~,)~, R U C I ~ ( P P ~ ~ ) ~ ; yield = 18-73 %.

)( = N 5 , N b , N-g-H [19], yield = 1S97 %.

0

X = S-C-Me,, catalyst = RUHCI(CO)(PC~,)~ [19].

Eq. 4.

R = Me, Et, SiMe,, Y = OR' where R' = Et, mBu, Cy, Ph, PhCH2, Gly, Me,Si [12], yield = 50-95 %;

R = Me, Et, Pr, Y = OCOR [la], yield = 87-89 %;

R = Et, Y = NMePh, N(C0)MePh 1191, yield = 5743%;

R = Et, Y = SCMe3 [19], yield = 57 %.

Eq. 5.

Synthesis of Bis(sily1)decadienes

Our recent study on the activity for metathesis revealed a high reactivity of 1,9-decadiene and cyclooctene in their reactions with triethoxy- and trisiloxy-substituted vinylsilanes [20]. When the mixture containing vinylsilane and 1 ,Pdecadiene was heated in the presence of Grubbs catalyst, the formation of mono- and bis(silyl)dienes, accompanied by a polymeric product, was detected. The replacement of 1,9-diene with cyclooctene in the reaction mixture has resulted in the same products. The reactions can be illustrated by Eqs. 6 and 7. 1,lO-bis(sily1)-substituted dienes were isolated with 50-70 % yields and characterized spectroscopically. In the case of disubstituted products separated by distillation, no double bond migration was observed. The only process observed in that case was E n isomerization, so only ZZ-disubstituted products could be isolated.

The two reactions of vinylsilanes with cycloalkenes and alkadienes discussed above compete with ROMP and ADMET polymerization of the organic parent substances. Scheme 1 has been proposed to combine all the competitive/consecutive reactions. The successful formation of silyldiene and bis(sily1)diene occurs exclusively when n = 4 (see Scheme 1).

Silylative Coupling and Cross-Metathesis of Alkenes and Dienes 367

+ polymer

Eq. 6.

[Ru]=CHPh

/ SiR3 R3Si% + / SiR3 -H2C=CH2 * r351-

Eq. 7.

polym. ADMET / 3 \ROMP

R3Si*SiR3

2

Scheme 1.

Functionalization of Polyvinyl-Substituted Organosilicon Compounds

The reaction of trans-silylation can be used for effective functionalization of 1,3,5- trivinylsilylbenzene, vinylcyclosiloxanes, vinylcyclosilazanes and vinyloctasilsesquioxane.

368 B. Marciniec

Products of the reactions with styrene were isolated and identified [21-231 (examples are shown in Fig. 1).

7 -Si-

Ph

-Si- /I

Fig. 1. Some products of the trans-silylation of polyvinyl-substituted organosilicon compounds.

Polycondensation vs. Intermolecular Cyclization of Divinyl-Substituted Silicon Compounds

As has already been mentioned, divinyl silicon derivatives, similarly to monovinyl-substituted silicon compounds, are also completely inert to productive homometathesis, particularly as far as acyclic diene metathesis (ADMET) polymerization is concerned. However, we have shown in earlier reports that in the presence of ruthenium, rhodium and cobalt complexes containing or generating M-H andor M-Si bonds, divinyl-substituted silicon compounds undergo de-ethenated (po1y)condensation to yield a mixture of oligomers and cyclic unsaturated siloxanes, silazanes and carbosilanes, as shown in Scheme 2 [21-291.

Silylative Coupling and Cross-Metathesis of Alkenes and Dienes 369

Trans-tactic polymers have been synthesized and identified in the presence of [RuC12(C0)3]2. Examples are:

polysilylene-vinylenes [30]: poly(l,l-dimethyl-l-sila-2-propene), Mw = 8250, PDI = 1.32, poly( l,l-diphenyl-l-sila-2-propene), Mw = 3800, PDI = 1.50;

poly( 1,1,3,3-tetraethoxy-l,3-disila-2-oxo-4-pentene), Mw = 4900, PDI = 1.58;

poly( 1,1,3,3-tetraethoxy-l,3-disila-2-aze-4-pentene), Mw = 813 [28];

poly(l,l,3,3-tetraethoxy-1,3-disila-4-pentene), M , = 9500, PDI = 1.35.

polysiloxylene-vinylenes [30]:

polyvinylene-silazanylene-vinylenes [28]:

polysil ylene-alkylene-silylene-vinylenes [ 3 11 :

In the presence of rhodium complexes as catalysts, the initial divinyltetramethylsiloxane, divinyldimethylsilane and divinyltetramethyldisilazane undergo condensation predominantly to dimeric and trimeric general products, ring closure of which then yields the respective cyclocarbosiloxane [32], cyclocarbosilane [2 11 and cyclocarbosilazane [2 11 with exocyclic methylenes (see Fig. 2 for the formulas).

CH3 CH3

H3C CH3 \ /

X=O,NH

Fig. 2. Intermolecular cyclization products from reactions catalyzed by rhodium complexes.

Wakatsuki et al. [33] synthesized cyclocarbosilane via ruthenium-catalyzed intermolecular ring closure of bis(vinyldimethylsily1)ethane and bis(vinyldimethy1)silylbutane (60-80 "C, 24 h) [33] with one exocyclic methylene.

Silylene-arylene-vinylene Polymers

Poly-p-phenylene-vinylene-based polymers containing a silicon atom in the main chain are of great interest because of their efficient photoluminescence and potentially useful electroluminescence properties [ 341. Poly(pheny1ene-vinylene4lylene)s are usually synthesized by polyhydrosilylation

370 B. Marciniec

of diethynylbenzene with bis(hydrosily1)benzene [35, 361. The silylative coupling (SC) polycondensation procedure has been used effectively to synthesize various poly(ary1ene-silylene- viny1ene)s according to Eqs. 8-10 [37-391.

Eq. 8.

\--I

Mw = 710042000, PDI

Ru(OAc)(CO)(PPh& [39].

Eq. 9.

w \ = /

= 1.5-3.2; catalysts = [RuCI~(CO)~]~ [37], RuHCI(CO)(PCy& [38],

Eq. 10.

Siloxylene-vinylene-alkenylene Polymers via ADMET Copolymerization and Tandem ROWCM Polymerization

High efficiency of the cross-metathesis of 1,9-decadiene and ROM/CM of cyclooctene with vinylsilanes points to a possibility of effective runs of the ADMET copolymerization of 1,9- decadiene [40] and tandem ROM/CM polymerization of cyclooctadiene [20], in both cases with divinylsilicon compounds. The reactions have proceeded according to Eq. 11, yielding polymeric material isolated and analyzed by GPC and NMR methods.

OEt OEt

W Mw = 18000, PDI = 2.9 (ADMET copolymerization); A& = 4500, PDI = 3.2 (ROM/CM polymerization).

Eq. 11.

Silylative Coupling and Cross-Metathesis of Alkenes and Dienes 371

Functionalization vs. Degradation of Polybutadiene in the Reaction with Vinyltrialkoxysilanes Catalyzed by Ruthenium Complexes

Organosilicon unsaturated compounds, except vinylsilanes (esters, ethers, imides), were used successfully to degrade 1,4-polybutadiene with the formation of difunctional polybutadiene oligomers [41, 421. When polybutadiene [Mw = 400 0001 underwent a treatment with vinyltriethoxysilane in the presence of Grubbs or Noels catalysts, a partial degradation was observed with the formation of silyl-terminated unsaturated oligomers (Mw = 5 1 000 and 113 000, respectively [43]). However, if this procedure was performed at a higher temperature (120-130 "C) in the presence of complexes containing or generating Ru-H bonds, exclusive functionalization of =C-H bonds of a polymer (Mw - 500 000) [44] was observed to proceed according to the silylative coupling reaction. A general illustration of the differences is given in Scheme 3.

Si R3'

FUNCTIONALWTION k + l = n p + r + s = n

DEGRADATION

Scheme 3.

Conclusions

Two new catalytic reactions occumng between the same parent substances have been developed since 1984, i.e. the silylative coupling (dehydrogenative or trans-silylation, silyl group transfer) and cross-metathesis of alkenes with vinylsilicon compounds. They make it possible to synthesize molecular and macromolecular compounds with vinylsilicon functionality. Stereo-, regio- and chemoselective syntheses of p-substituted styrylsilanes and siloxanes occur in the presence of ruthenium, rhodium and iridium complexes containing or generating M-H and/or M-Si bonds according to the non-metallacarbene mechanism and, if catalyzed by Grubbs complexes (in the case of trialkoxy and trisiloxy substituents at silicon), according to the metallacarbene mechanism. The reactions of polyvinyl-substituted organosilicon compounds (e.g. 1,3,5-trivinylbenezene, vinylcyclosiloxane and vinylcyclosilazane) with styrene lead to synthesis of the respective vinyl-substituted derivatives according to the non-metallacarbene process.

372 B. Marciniec

While vinylsilanes undergo productive cross-metathesis (Mo and Ru carbenes) with allyl-substituted functionalized alkenes, their effective transformation with derivatives containing a functionalized group attached directly to a carbonkarbon double bond can be achieved only via silylative coupling catalyzed by metal complexes containing (or generating) M-H and/or M-Si bonds (M = Ru, Rh, Ir). In view of the inactivity of metallacarbenes in ADMET polymerization of divinylsilicon compounds, the latter can undergo effective de-ethenated (po1y)condensation to yield, in the presence of appropriate catalysts (e.g. [RuCl,(C0)3]), either trans-tactic polymers (e.g. silylene- (siloxylene,silazane)-vinylene-silylene-alkylene-vinylene, silylene-arylene-vinylene) or the respective cyclic compounds containing exocyclic methylidene, e.g. cyclocarbosiloxanes, cyclocarbosilanes, cyclocarbosilaxanes (rhodium catalysts). Metathesis of 1,9-decadiene and cyclooctene with trialkoxy- and trisiloxy-substituted vinylsilanes in the presence of Grubbs catalyst, carried out in appropriate conditions, leads to the formation of bis(sily1)diene with a high yield. Similar processes performed with divinyl-substituted siloxane lead to the formation of silicon-containing polymers (via ADMET copolymerization and tandem ROWCD polymerization), thus opening a new convenient route to synthesis of unsaturated organosilicon copolymers. Organic polyenes, e.g. 1 ,Cpolybutadiene, undergo partial degradation while reacting with vinyltrialkoxysilanes in the presence of ruthenium carbenes and functionalization, if the reaction proceeds in the presence of ruthenium complexes containing (or generating) Ru-H and Ru-Si bonds.

Acknowledgments: My warmest thanks are due to the co-workers whose names appear in the references. Our recent research was partly supported by the State Committee for Scientific Research (Poland), Project No. K026/T09/2001.

References T. H. Chan, I. Fleming, Synthesis 1979,761. C. V. Ponomarev, M. V. Ericova, S . N. Nikolaeva, R. Zel, A. C. Kostyuk, Zh. Obshch. Khim. 1987,57, 1741. I. V. Efimova, M. A. Kazankova, I. F. Lutsenko, Zh. Obshch. Khim. 1985,55,1646. F. Barbot, P. Miginiac, Bull. Chem. SOC. Fr. 1983, 1-2,II-41. The Chemistry of Organosilicon Compounds, S . Patai, Z. Rappoport, Eds.; Wiley: Chichester, 1998. B. Marciniec, J. Gulinski, J. Organomet. Chem. 1984,266, C19. Y. Wakatsuki, H. Yamazaki, M. Nakano, Y. Yamamoto, J. Chem. SOC., Chem. Cornmun. 1991,703. B. Marciniec, C. Pietraszuk, J. Chem. SOC., Chem. Commun. 1995,2003. For recent reviews on the silylative coupling of olefins with vinylsilanes see: (a) B.

Silylative Coupling and Cross-Metathesis of Alkenes and Dienes 373

Marciniec, New J. Chem. 1997,21, 815; (b) B. Marciniec, in Applied Homogeneous Catalysis with Organometallic Compounds, 2nd edn., B. Cornils, W. A. Herrmann Eds.; VCH: Weinheim, 2002; Chapter 2.6; (c) J. A. Reichl, D. H. Berry, Adv. Organomet. Chem. 1998, 43, 203; (d) B. Marciniec, Appl. Organomet. Chem. 2000, 14, 527; B. Marciniec, in Ring Opening Metathesis Polymerization and Related Chemistry, E. Koshravi, T. Szymaiiska- Buzar, Eds.; Kluwer Academic Publishers: Dordrecht, 2002, p. 391.

[lo] C. Pietraszuk, B. Marciniec, H. Fischer, Organometallics 2000,29, 913. [ 111 C. Pietraszuk, H. Fischer, M. Kujawa, B. Marciniec, Tetrahedron Lett. 2001,42, 1175. [ 121 M. Kujawa-Welten, C. Pietraszuk, B. Marciniec, Organometallics 2002, 21, 840. [13] B. Marciniec, C. Pietraszuk, Organometallics 1997,16,4320. [14] Ch. S. Yi, Z. He, D. W. Lee, Organometallics 2000, 19,2036. [ 151 B. Marciniec, E. Walczuk-GuSciora, C. Pietraszuk, Organometallics 2001,20,3423. [ 161 C. Pietraszuk, B. Marciniec, M. Jankowska (unpublished results). [I71 B. Marciniec, M. Kujawa, C. Pietraszuk, Organometallics 2000, 19, 1677. [18] M. Kujawa-Welten, B. Marciniec, J. Mol. Catal. 2002, 190, 79. [I91 D. Chadyniak, S. Krompiec, B. Marciniec (unpublished results). [20] C. Pietraszuk, B. Marciniec, M. Jankowska, Adv. Synth. Catal. 2002,344,789. [21] B. Marciniec, E. Malecka, M. Majchrzak, Y. Itami, Macromol. Symposia 2001, 174, 137. [22] Y. Itami, B. Marciniec (unpublished results). [23] Y. Itami, B. Marciniec, M. Kubicki (unpublished results). [24] F. J. Feher, D. Soulivong, A. G. Eklund, K. D. Wyndhan, Chem. Cornmun. 1997,1185. [25] B. Marciniec in Ring Opening Metathesis Polymerization and Related Chemistry, E.

Koshravi, T. Szymanska-Buzar, Eds.; Kluwer Academic Publishers: Dordrecht, 2002, p. 33 1. [26] B. Marciniec, M. Lewandowski, J. Polym. Sci., Part A - Polym. Chem. 1996,34, 1443. [27] B. Marciniec, M. Lewandowski, J. Inorg. Organomet. Polym. 1995,15, 115. [28] B. Marciniec, E. Malecka, Macromol. Rapid Cornmun. 1999,20,475. [29] B. Marciniec, Mol. Cryst. Liquid Cryst. 2000,353-354, 173. [30] E. Malecka, B. Marciniec (unpublished results). [3 I] E. Malecka, B. Marciniec, Macromolecules (accepted for publication). [32] B. Marciniec, M. Lewandowski, E. Bijpost, E. Malecka, M. Kubicki, E. Walczuk-GuSciora,

Organometallics 1999,18, 3968. [33] T. Mise, Y. Takaguchi, T. Umemiya, S. Shimizu, Y. Wakatsuki, Chem. Commun. 1998,690. [34] D. J. Sandman, Trends. Polym. Sci. 1994, 2, 44; D. J. Sandman, Trends. Polym. Sci. 1997, 5,

7 1 , and references therein. [35] D. S. Kim, S. C. Shim, J. Polym. Sci., PartA-Polym. Chem. 1999,37,2933. [36] D. Y. Son, D. Bucce, T. M. Keller, Tetrahedron Lett. 1996,37, 1576. [37] M. Majchrzak, Y. Itami, B. Marciniec, P. PawluC, Macromol. Rapid Cornmun. 2001,22,202. [38] M. Majchrzak, B. Marciniec, Y. Itami (unpublished results). [39] M. Majchrzak, Y. Itami, B. Marciniec, P. PawluC, Tetrahedron Lett. 2000,41, 10303. [40] E. Malecka, B. Marciniec, C. Pietraszuk, A. C. Church, K. B. Wagener, J. Mol. Catal. 2002,

190, 27.

374 B. Marciniec

[41] J. C. Marmo, K. B. Wagener, Macromolecules 1995,28, 2602. [42] K. B. Wagener, J. C. Marmo, Macromol. Rapid. Commun. 1995,16,557. [43] B. Marciniec, M. Lewandowski, J. Gulinski, A. F. Noels, A. Demonceau, E. Malecka, D. Jan,

Polymer 2000,41, 821. [a] B. Marciniec, E. Malecka, J. Gulinski, M. Grundwald-Wyspiahska, M. Lewandowski, Can. J.

Chem. 2002,79,775.

Thermolytic Formation and Trapping of Silenes Strongly Influenced by Reversed Polarization

Henrik Ottosson, %. Tamaz Guliashvili, Ibrahim El-Sayed

Department of Organic Chemistry Box 599, Uppsala University, 75 1 24 Uppsala, Sweden

Tel.: +46 18 471 3809 -Fax: +46 18 471 3818 E-mail: Henrik.Ottosson@kemi.uu.se

Keywords: silenes, reversed polarization, DFT, rearrangements

Summary: The thermal rearrangement of silylamides into 1,l -bis(trimethylsilyl)-2- amino-2-trimethylsilyloxysilenes and their subsequent trapping by 2,3-dimethyl- 1,3 -butadiene were investigated both computationally and experimentally, with focus on the geometric and electronic structure of these silenes.

Reversed Si=C bond polarization, which is manifested by delocalization of negative charge from K -

electron donating C substituents to Si (resonance structures I1 and 111, Scheme l), has been considered as the single most important electronic factor that influences the reactivity of silenes [ 11.

Scheme 1.

The first solid, stable silene (Me3Si)&=C(OSiMe3)Ad of Brook and co-workers [2], and the 4-silatriafulvenes of Kira's group [3], are influenced by reversed polarization, as manifested by elongation of their Si=C bonds when compared to that of HZSi=CH2 [4]. However, when resonance structures I1 and I11 completely dominate the electronic structure, the Si=C bond should turn into an Si-C single bond and the Si atom should be strongly pyramidalized since it will resemble the Si of a silyl anion. These silenes therefore have nonclassical bent structures similar to many other heavy alkenes, species whose structures are explained by the theory of Carter, Goddard, Trinquier and Malrieu (CGTM) [5]. One may thus anticipate a connection between the reversed polarization effect and the CGTM theory. So far, no silene that is strongly affected by reversed polarization has been generated that is stable at ambient temperatures, even though such species extrapolate to silylenexarbene complexes, such as that of Lappert and co-workers [6].

Ovganosilicon Chemisty V Edited by N. Auner and J. Weis

Copyright 0 2003 WILEY-VCH Verlag GmbH & Co. KgaA, Weinheim

376 H. Ottosson, T. Guliashvili, I. El-Sayed

We reasoned that a silene with two x-electron donating groups at the C atom would be dominated by resonance structures I1 and 111. Quantum chemical calculations at the B3LYP/6-31G(d) level [7] reveal that this is the case since the Si-C bond elongates gradually as more strongly donating groups are attached to C (Fig. 1). For (H3Si)2Si=C(OSiH3)NMez (4) the Si-C bond has turned into an Si-C single bond, whereas for (H3Si)zSi=CH(OSiH3) (2) and (H3Si)nSi=CHNMe2 (3), this bond length is between that of Si=C and Si-C bonds.

= 360 0 SI = 359 0

Fig. 1. B3LYP/6-3lG(d) geometries of (H3Si)&=CH2 (l), (H3Si)zSi=CH(OSiH3) (2), (H3Si)2Si=CH(NMez) (3) and

(H3Si),Si=C(NMez)(OSiH3) (4). (Distances in 8, and angles in deg. ZSi refers to the sum of valence angles at

Si.)

Our idea was to generate reverse-polarized silenes through thermolysis or photolysis of silylamides (5 ) in a manner similar to that used by Brook and co-workers [2] when forming (Me3Si)zSi=C(OSiMe3)Ad. This would lead to silenes that resemble 4. However, we soon found the discouraging fact that photolysis of silylamides had been attempted before, and only starting material was recovered after long irradiation [8]. In despair of this finding we resorted to computations to investigate the properties of silenes formed upon thermolysis of silylamides [9] Since these silenes are Si-C single-bonded, a facile back-transfer of the TMS group from 0 to Si could occur once they are formed. Indeed, B3LYP/6-31G(d) computations indicate that the barrier that separates (Me3Si)2Si=C(OSiMe3)NMe2 (6a) from the corresponding silylamide (5a) is merely 9.1 kcdmol, and 5a is 17.5 kcaVmol lower in energy than 6a (Fig. 2). The chances to isolate silenes 6 at thermolytic or photolytic conditions should therefore be very small.

0.0

Fig. 2. Energy surface connecting tris(trimethylsily1)silyl-N,N-dimethylamide (5a) with l,l-bis(trimethylsilyl)-2

-N,N-dimethylamino-2-trimethylsiloxysilene (6a). Calculations at the B3LYP/6-3 1G(d) level. (Energies in

kcal/mol.)

Thermolytic Formation and Trapping of Silenes Strongly Influenced by Reversed Polarization 377

To probe whether 6 is formed upon heating of 5 we used 2,3-dimethyl-l,3-butadiene as a trapping reagent. With this reagent added we observed that a new product was formed while monitoring the reaction by NMR. To our surprise the product was not the anticipated Diels-Alder adduct 7 between 6 and 2,3-dimethyl-l,3-butadiene (Scheme 2), but that in which the OTMS and TMS groups had changed positions (8). Even more surprisingly, the product was formed in nearly quantitative yields. And finally, the required temperature and reaction times varied widely, depending on the amino substituent. Whereas the thermolysis of 5a required two days at 180 "C for completion, only 2 h at 100 "C was needed for 5c. The latter reaction could therefore be carried out under standard reflux conditions.

5 6

a: R = R' = Me, T = 180 OC, 2 days, toluene, 88 % yield b R = R' = Ph, T = 100 "C, 2 h, benzene, 97 % yield TMS NRR

c: R = Me, R' = Ph, T = 100 OC, 3 h, benzene, 95 %yield

Scheme 2.

= 62.2 I = -104.2

Fig. 3. Geometries of TMS2Si=C(OTMS)NMe2 (6a) and TMS2Si=C(OTMS)NMePh (6c) at the B3LYP/6-31G(d)

level. (Distances in 8, and angles in deg. ZSi refers to the sum of valence angles at Si.)

These experiments provide us with ample data for further computational investigations. We feel confident that silenes 6 are formed, but how does the subsequent trapping reaction and the formation of 8 proceed? Moreover, one may ask whether there is a direct electronic influence of the N-phenyl groups on the reactivity of the silene or whether these groups have an indirect influence caused solely by their steric bulk? B3LYP/6-31G(d) computations show that they force the amino

378 H. Ottosson, T. Guliashvili, I. El-Sayed

group out of conjugation with the Si=C bond (Fig. 3), making the silenes less influenced by reversed polarization. However, further computational as well as experimental studies are required to determine the exact reason why the N-phenyl groups affect the reactivity of the silene. We hope to provide a more detailed report on this and other subtleties in the reactions of silenes 6 in the near future.

Acknowledgments: Financial supports from the Wenner-Gren Foundations and from the Swedish Research Council (Vetenskapsridet), as well as a generous allotment of computer time from the National Supercomputer Center (NSC) in Linkoping, Sweden, are gratefully appreciated.

References [ l ] [2]

[3]

Y. Apeloig, M. Karni, J. Am. Chem. SOC. 1984,106,6676. A. G. Brook, F. Abdesaken, B. Gutekunst, G. Gutekunst, R. K. Kallury, J. Chem. SOC., Chem. Cornmun. 1981, 191. a) K. Sakamoto, J. Ogasawara, H. Sakurai, M. Kira, J. Am. Chem. SOC. 1997,119, 3405; b) T. Veszprhi, M. Takahashi, J. Ogasawara, K. Sakamoto, M. Kira, J. Am. Chem. SOC. 1998, 120, 2408; c) T. Veszprkmi, M. Takahashi, B. Hajgat6, J. Ogasawara, K. Sakamoto, M. Kira, J. Phys. Chem. A. 1998, 102, 10530; d) M. Takahashi, K. Sakamoto, M. Kira, Int. J. Quant. Chem. 2001, 84, 198; e) K. Sakamoto, J. Ogasawara, Y. Kon, T. Sunagawa, C. Kabuto, M. Kira, Angew. Chem. Int. Ed. 2002,41, 1402. S . Bailleux, M. Bogey, J. Demaison, H. Burger, M. Senzlober, J. Breidung, W. Thiel, R. Fajgar, J. Pola, J. Chem. Phys. 1997,24, 10016. a) E. A. Carter, W. A. Goddard 111, J. Phys. Chem. 1986, 90, 998; b) J.-P. Malrieu, G. Trinquier, J. Am. Chem. SOC. 1989,111,5916. W. M. Boesveld, B. Gehrhus, P. B. Hitchcock, M. F. Lappert, P. v. R. Schleyer, J. Chem. SOC., Chem. Commun. 1999,755. M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria, M. A. Robb, J. R. Cheeseman, V. G. Zakrzewski, J. A. Montgomery, R. E. Jr. Stratmann, J. C. Burant, S . Dapprich, J. M. Millam, A. D. Daniels, K. N. Kudin, M. C. Strain, 0. Farkas, J. Tomasi, V. Barone, M. Cossi, R. Cammi, B. Mennucci, C. Pomelli, C. Adamo, S . Clifford, J. Ochterski, G. A. Petersson, P. Y. Ayala, Q. Cui, K. Morokuma, D. K. Malick, A. D. Rabuck, K. Raghavachari, J. B. Foresman, J. Cioslowski, J. V. Ortiz, A. G. Baboul, B. B. Stefanov, G. Liu, A. Liashenko, P. Piskorz, I. Komaromi, R. Gomperts, R. L. Martin, D. J. Fox, T. Keith, M. A. Al-Laham, C. Y. Peng, A. Nanayakkara, C. Gonzalez, M. Challacombe, P. M. W. Gill, B. G. Johnson, W. Chen, M. W. Wong, J. L. Andres, M. Head-Gordon, E. S . Replogle, and J. Pople, Gaussian 98 (Revision A.9), Gaussian, Inc., Pittsburgh PA, 1998. S. S. Al-Juiad, Y. Derouiche, P. B. Hitchcock, P. D. Lickiss, A. G. Brook, J. Organomet. Chem. 1991,403,293. I. El-Sayed, T. Guliashvili, R. Hazell, A. Gogoll, H. Ottosson, Org. Lett. 2002,4, 1915.

[4]

[5]

[6]

[7]

[8]

[9]

Tailoring Properties of Silicon- Containing Oxide Catalysts via the Thermolytic Molecular

Precursor Route

Kyle L. Fujdala, T. Don Tilley*

Department of Chemistry, University of California Berkeley Berkeley, California 94720-1460, USA

Chemical Sciences Division, Lawrence Berkeley National Laboratory 1 Cyclotron Road, Berkeley, California 94720- 1460, USA

Tel.: +1 510 6428939 -Fax: +1510 6428940 E-mail: tdtilley @ socrates.berkeley.edu

Keywords: molecular precursors, heterogeneous catalysis, tailored materials

Summary: A non-aqueous thermolytic method for the synthesis of highly homogeneous mixed-element oxides using well-defined oxygen-rich complexes based upon the OSi(O'Bu), and 02P(01Bu)2 ligands is described. This method offers molecular-level control over the nanostructure of the resulting materials and allows for the tailoring of the properties of such materials. This thermolytic molecular precursor approach has been used to obtain high surface area and well-dispersed M/Si/O and M/P/O materials with a variety of transition and main group elements. This route often provides heterogeneous catalysts with properties that are superior to those of catalysts with the same composition, but prepared by traditional aqueous methods. The molecular precursors may be used to produce mesoporous materials with complex stoichiometries and a homogeneous distribution of metal atoms within the walls via the use of templating agents. It has also been found that these molecular precursors may be used for the introduction of surface-bound and isolated catalytic species via simple grafting techniques.

Introduction

The preparation of advanced materials with tailored properties is a technical challenge of great significance [l-81. An important goal in materials chemistry targets the development of new catalysts, catalyst supports, and processes, especially since advances in catalysis are important for the global economy and will continue to benefit the environment through more efficient use of resources [9-1 I]. The properties of materials are known to be highly sensitive to features on both atomic and nanostructural scales and the spatial arrangement of atoms on the surface of a

Ovganosilicon Chemisty V Edited by N. Auner and J. Weis

Copyright 0 2003 WILEY-VCH Verlag GmbH & Co. KgaA, Weinheim

380 K. L. Fujdala, T. D. Tilley

heterogeneous catalyst is critical for the activation of substrates. Synthetic control over structures at these levels should therefore drive progress in the synthesis of tailored materials and heterogeneous catalysts and will lead to a greater understanding of the structure-property relationships for materials, and ultimately to more efficient catalysts and better catalyst supports. For example, the M-0-M' linkages in bulk heteroelement oxides, or at catalyst-support interfaces, are associated with novel chemistry and in some cases high Bronsted acidities [1240].

Our program in the synthesis of multi-component oxides features molecular design and synthesis, and low-temperature construction of solid-state networks. This strategy focuses on the generation of oxygen-rich, "single-source" molecular precursors containing OSi(O'Bu)3 and 02P(O'Bu)2 ligands and transition or main group metals with preformed M-O-E heterolinkages [41-621. These complexes cleanly convert to M/E/O materials via the elimination of isobutene and water under mild thermolytic conditions (< 200 "C) [41-621. The precursors act effectively as individual building blocks for the construction of inorganic materials that may possess properties derived from their molecular origin. The high solubility of such complexes allows for thermolytic transformations in nonpolar solvents, giving rise to xerogels upon conventional drying. We have employed this thermolytic molecular precursor method for the low-temperature synthesis of a variety of M/E/O materials with tailored properties [41-62]. This method offers several advantages over conventional sol-gel routes to multi-component oxides. For example, the preexistence of M-0-E linkages and simultaneous incorporation of heteroelements leads to highly homogeneous materials at the atomic level. In addition, the use of nonpolar solvents minimizes the hydrolytic cleavage of M-0-E linkages, and reduces pore collapse upon drying to provide xerogels with high surface areas. Finally, the preparation of novel inorganic/organic hybrid materials and the use of organic templating agents are facilitated through the use of nonpolar media. This contribution describes use of the thermolytic molecular precursor method for the synthesis of Si-containing, multi-component oxides.

Thermolytic Molecular Precursor Route to Oxide Materials

Early work in this area included use of complexes of the form M[OSi(O'Bu)3]4 (M = Ti, Zr, Hf) that convert quantitatively upon gentle heating (100-200 "C) to homogeneous carbon-free M02.4Si02 materials in the solid-state or in solution (Eq. 1) [41, 451. For example, the decomposition of Zr[OSi(OrBu)3]4 in toluene gives a transparent gel, which upon drying in the air forms a high surface area xerogel(520 m2 g-I).

A M[OSi(O$u)& - M02.4Si02 + 12 CH2=CMe2 + 6 H20

M = Ti, Zr, Hf

Eq. 1.

Tailoring Properties of Silicon-Containing Oxide Catalysts 381

A general goal of this work is to establish correlations between the chemical properties of molecular precursor building blocks and the properties (porosity, catalytic efficiency, surface acidity, etc.) of the materials derived therefrom. Such correlations should be valuable for the design and synthesis of new heterogeneous catalysts and support materials. For example, it was shown that this approach may be used to provide homogeneously dispersed metal sites in an amorphous SiOz support matrix [46, 49, 54, 59, 611. This is relevant in that certain highly homogeneous systems (Zr02-SiO2, TiOz-SiOz, V205-Si02, Cr203-Si02, etc.) exhibit superior catalytic efficiencies

Recent investigations have shown that the properties of amorphous aluminosilicate materials derived from compounds containing the OSi(O'Bu)3 ligand are greatly dependent upon the nature of the starting precursor [60]. It was observed that A1203.2Si02 materials generated from Al[OSi(O'Bu)~]~(HOiPr)~1/4[A1(OiPr)~]~ or [(OiPr)2Al0Si(O'Bu)3]2 (both containing A1 and Si in a 1 : 1 ratio) possess very different microstructures and surface Lewis and Bransted acidities. The former precursor gave rise to a xerogel with a low Bransted site concentration, a higher Lewis/Br@nsted acid site ratio, and a microstructure that more closely resembled that of mullite. The latter precursor gave rise to a xerogel that had a greater concentration of Bransted sites and a lower LewidBransted acid site ratio, presumably from the preexistence of only A1-O-Si linkages [60]. These materials are being explored as catalysts and catalyst support materials.

Other investigations have provided new Cr and V alkoxysiloxy complexes that are efficient precursors to homogeneous Cr/Si/O and V/Si/O materials, respectively (Scheme 1) [54, 611. High Cr content materials derived from ('BuO)3CrOSi(O'Bu)3 via solid-state and solution-derived thermolyses have similar surface areas (300 m2 g-') and are homogeneous; however, the solution- derived Cr/Si/O material (1 : 1 Cr/Si) exhibited phase separation and formation of Cr2O3 at an earlier stage in the calcination process. This suggests that this material is somewhat less homogeneous [54]. Interestingly, this solution-derived xerogel displayed up to three times the catalytic activity and a significant increase in selectivity (although low) for the oxidative dehydrogenation of propane relative to the solid-state derived material [54]. This significant increase in catalytic efficiency was realized despite the fact that each material had the same composition and a similar surface area [54]. It is evident that seemingly small differences in the thermolytic method employed for the generation of materials can dramatically affect their properties and catalytic performance.

The generation of single-source precursors for V/Si/O materials with varying oxidation states and silicon contents afforded the opportunity to study how these factors influence the properties of the final material [61]. For example, V/Si/O xerogels synthesized by solution (n-octane) thermolyses of a V(v) precursor, OV[OSi(O'Bu)3]3 [46], exhibited phase separation and formation of V2O5 crystallites at low temperatures (300 "C, in 0 2 ) . By comparison, xerogels derived from the V(rv) precursors ('Bu0)3VOSi(OfBu)3 and ('BuO)zV[OSi(O'Bu)3]2 produced V205 much more slowly (400 "C, in Oz), indicating that the initial materials derived from the V(w) precursors are more homogeneous (by PXRD, and TEM). In addition, the size of the V2O5 nanocrystals formed after calcination at 500 "C increased with increasing silicon content, regardless of the vanadium oxidation state. The surface areas of these V/Si/O xerogels increased with increasing silicon content and ranged from 30 to 300 m2 g-'.

[ 19-24,631.

382 K. L. Fujdala, T. D. Tilley

M(~'Bu),

/..OH -2'euO\

HOSi(O'Bu)3 2 HOSi(O'Bu)3

M = Cr: 82 % isolated M = V: 45 % isolated

M = Cr: 0-68 % isolated (variable) M = V: 61 % isolated

Scheme 1.

The construction of nanostructured materials with specific properties is a challenging goal in catalysis. Recent work with poly(alky1ene oxide) block copolymers as structure-directing agents has shown that the thermolytic molecular precursor method represents an exciting new approach to materials with complex stoichiometries and mesoporosity [56,57]. Based upon the use of molecular precursors in the presence of block copolymer templates, mesoporous two-component oxides have been prepared with the compositions ZrO2.SiO2, Ta205-6Si02, Fez03.6Si02, and Alp04 [56, 571. The new materials are mesoporous mixed-element oxides with high surface areas, homogenous dispersions of elements, and thick framework walls. The templating mechanism by which these materials form is of interest as it occurs under nonaqueous conditions and in nonpolar solvents. In contrast, previous reports on template-assisted mesoporous metal oxide formation describe polar (usually aqueous) reaction solvents.

The recent synthesis of a single-source molecular precursor containing aluminum with OSi(O'Bu)3 and 02P(OfBu)2 ligands provided an important step toward general use of molecular precursor routes to materials with more complicated stoichiometries. Indeed, it was shown that this new complex, [('BuO)~SiO]~A1[p-0~P(O'Bu)~]~A1(Me)OSi(O'Bu)~, was readily converted to homogeneous Si/AI/P/O materials under mild thermolytic conditions via loss of isobutene, H20, and CHq (Eq. 2) [%I.

O\\ /" Quo p,o 'Bu

-CH 4 -H20

Eq. 2.

Tailoring Properties of Silicon-Containing Oxide Catalysts 383

New mesoporous silicoaluminophosphates (SAPOs) were generated via solution thermolyses of this precursor in the presence of a structure-directing triblock copolymer [%I. These new SAP0 materials should prove useful in catalysis, especially for large organic substrates. It is anticipated that, in general, the design and synthesis of single-source molecular precursors containing several heteroelements will provide access to highly homogeneous materials with novel properties.

Efficient Vanadium-Based Catalysts for the Oxidative Dehydrogenation (ODH) of Propane

An important goal in materials chemistry is the development of new catalysts for selective transformations of hydrocarbons [25-271. The dehydrogenation of light alkanes proceeds only at high temperatures, where cracking and the deposition of carbon present serious problems. Alternatively, oxidative dehydrogenation (ODH) is thermodynamically favored at lower temperatures and does not suffer from coking, which decreases catalyst performance [25]. Given the high demand for propene in the production of polypropylene, acrylonitrile, and propene oxide [28], ODH (Eq. 3) has generated considerable attention as an alternative source of this valuable molecule. However, this selective oxidation is particularly challenging, given the tendency of propene to become further oxidized under the reaction conditions [25,29].

Eq. 3.

In the development of a molecular precursor route to ODH catalysts, an important discovery was that the composition of materials derived therefrom may be manipulated by co-thermolyses in solution [46, 521. Also, it is often observed that such thermolyses give well-dispersed materials. This method is inherently versatile, allows control over elemental composition, and should be applicable to many catalyst formulations. Initially, this approach was investigated in the synthesis of catalysts for the ODH of propane.

Utilizing OV[OSi(OrBu)3]3 and Zr[OCMezEt]4, a series of catalysts with varying vanadium content (2-33 wt%) were prepared (Eq. 4). These new catalysts were compared to catalysts of similar stoichiometry that were prepared by conventional (impregnation) methods. The surface areas of the catalysts prepared via the thermolytic route were high (up to 465 m2 g-' after calcination at 773 K) and the intrinsic selectivities for propene were as high as 95.5 % at 673 K. The presence of oligomeric tetrahedral vanadium sites appears to be a key component of the more active compositions (10-18 % vanadia). The V/Si/Zr/O catalysts with 18 and 23 % vanadia were as efficient as the most selective and active catalysts reported for propane ODH, and exhibited superior performance when compared to other vanadium-based catalysts. Most significantly, the novel features and impressive catalytic results for these materials suggest that molecular-level

384 K. L. Fujdala, T. D. Tilley

control over structure can provide new generations of catalysts with enhanced performance [46].

n-octane. A

Eq. 4.

Molecular Precursor Routes to Single-Site Catalysts

Many of the molecular precursors under investigation in our laboratories also serve as useful models for isolated sites on the surface of a support and dispersed within a silica matrix. For example, multiple spectroscopic techniques have shown that the vanadium in OV[OSi(OfBu)3]3 is in an environment that is very similar to that exhibited by well-dispersed vanadium on a silica support [52]. The 29Si NMR chemical shift of this model complex is quite similar to those observed for porous vanadium silicates, and Raman and infrared assignments for this species have led to insights into the structural characterization of single-site vanadium catalysts [52]. The environments of the metal centers within the molecular precursors mimic those expected for isolated metal centers in heteroelement oxides (surrounded exclusively by 0 with heteroelement nearest neighbors); hence characterization by single-crystal X-ray crystallography and various spectroscopic techniques provides invaluable information about possible bonding in single-site, heterogeneous catalysts.

The thermolytic molecular precursor route may also be used to introduce catalpc sites onto the surface of an oxide support via simple chemical reactions. Advantages to this approach stem from the potential for molecular-level control over the structure of the catalytic site. This general method begins with the chemical bonding of the precursor to the support (e.g., SiO2) via a protonolysis reaction, through loss of either HO'BU or HOSi(OfBu)3 for M[OSi(O'Bu)31n precursors, as shown in Scheme 2. Such grafting reactions can be easily monitored by use of solution NMR spectroscopy, thus providing insight into the nature of the bonding to the surface. Either M-0-(surface) or Si-0-(surface) linkages are possible using the above reaction pathway; hence quantification of the species that are released upon grafting can provide information about the types of sites that are present and the approximate quantities of each. Heat treatment of the grafted precursor then leads to elimination of isobutene from the remaining OSi(OfBu)3 ligands, to introduce "supported" metal centers onto the oxide surface in the form of MO,.nSiOz or MO,.(n-l)SiOz species.

The method outlined above was initially investigated for the introduction of isolated titanium sites onto silica support materials, as titania-silica materials have been widely studied for their superior, performance as selective oxidation catalysts. For example, in the 1970s Shell developed a silica-supported titania epoxidation catalyst for the production of propylene oxide [3O-32]. A wide range of other titania-substituted silica materials have been studied in this context, including the molecular sieves TS1 and TS2 [33-361, the zeolite Ti-P [37], and Ti-MCM41 (with Ti in the walls of the mesoporous silica [38, 391 or grafted onto its surface [40]). Our first study of the TiO2-SiOz system focused on use of the precursor Ti[OSi(OfBu)3]4 which is known to be an efficient source of

Tailoring Properties of Silicon-Containing Oxide Catalysts 385

homogeneous Ti02.4Si02 materials [5 11.

SiO,

+ M[OSi(O'Bu)J,

-HOfBu

A -HOSi((YBu),

Scheme 2.

Treatment of aerosil silica samples with the titanium precursor provided as-synthesized catalysts that were highly active and selective for the oxidation of cyclohexene to cyclohexene oxide, using cumene hydroperoxide as the oxidant. Calcination of this material prior to the catalytic reaction led to reduced activity, but the catalysts were still more active than silica treated with Ti(0'Pr)d (the Shell catalyst). Thus, the OSi(O'Bu)3 ligands appear to provide a beneficial effect on the structure of the supported titanium site, which probably involve tetrahedral Ti centers of the form OTi(0Si)s

Given the promising results observed for the above titanium system, we explored the synthesis of other single-site catalysts prepared from alternative oxygen-rich titanium precursors, based on the OSi(O'Bu)3 ligand, with varying Ti/Si ratios [62]. These studies showed that fewer siloxide ligands typically give a higher titanium loading. This is presumably due to reduction in the efficiency of the grafting reaction due to the steric bulk of the OSi(O'Bu)3 ligand. However, the presence of alkoxysiloxy ligands improves the catalytic activity and selectivity [62]. These two competing factors mean that an optimal precursor is the tris(si1oxide) ('PrO)Ti[OSi(O'Bu)3]3 [64]. The high surface area silica support materials investigated were aerosil, MCM-41 [65], and SBA-15 [66], with the best catalytic performance resulting from the mesoporous supports (MCM-41 and SBA- 15).

~ 1 1 .

Conclusions

The thermolytic molecular precursor method for the generation of multi-component oxides affords high surface area and homogeneous materials under mild conditions. In addition, there appear to be

386 K. L. Fujdala, T. D. Tilley

benefits that arise from the molecular chemistry involved in network formation. This method has produced materials with catalytic properties that are superior to those of materials with similar compositions obtained by conventional aqueous methods. We attribute these results to the formation of initial materials with a high dispersion of elements, which can result in a more efficient conversion to catalytically active species [ 19-24, 631. The synthesis of single-source molecular precursors with more than two heteroelements allows for the generation of materials with more complex stoichiometries. Such advanced materials have potential as new catalysts and catalyst support materials. The molecular precursor method can also be used for the generation of single-site catalysts on the surface of a support material with a degree of synthetic control over the nature of the active site.

Acknowledgments: The authors are extremely grateful for the support of this work by the Office of Basic Energy Sciences, Chemical Sciences Division, of the US Department of Energy under Contract No. DE-AC03-76SF00098, and for the enthusiastic efforts of the many co-workers who have contributed to this research. The contributions of our collaborator, Professor Alexis T. Bell of the Chemical Engineering Department at UC Berkeley, are also appreciated [49,52,59].

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K. Tanabe, Solid Acids and Bases; their Catalytic Properties, Academic Press: New York, 1970 K. Tanabe, T. Sumiyoshi, K. Shibata, T. Kiyoura, J. Kitagawa, Bull. Chem. SOC. Jpn. 1974, 47, 1064. J. B. Miller, S. T. Johnston, E. I . KO, J. Catal. 1994, 150,311. E. A. Mamedov, C. Corberhn, Appl. Catal. A 1995,127, 1. D. C. M. Dutoit, M. Schneider, A. Baiker, J. Catal. 1995,153, 165. R. Hutter, T. Mallat, D. Dutoit, A. Baiker, Top. Catal. 1996,3,421. X . Gao, I. E. Wachs, Catal. Today 1999,51,233. C. J . Brinker, G. W. Scherer, Sol-Gel Science, Academic Press: Boston, 1990. R. J. P. Comu, D. Leclercq, Angew. Chem., Znt. Ed. 1996,35, 1421. U. Schubert, N. Hiising, A. Lorenz, Chem. Muter. 1995,7,2010. U . Schubert, J. Chem. SOC., Dalton Trans. 1996,3343. A. H . Cowley, R. A. Jones, Angew. Chem. lnt. Ed. 1989,28, 1208. A. W. Apblett, A. C. Warren, A. R. Barron, Chem. Muter. 1992,4,167. H . H . Kung, Adv. Catal. 1994,40, 1. B. K. Warren, S. T. Oyama, Heterogeneous Hydrocarbon Oxidation, ACS. Symp. Ser. Vol. 638, ACS: Washington, 1996. S . Albonetti, F. Cavani, F. Trifir4 Key Aspects of Catalyst Design for the Selective Oxidation of Parafns, Marcel Dekker: Nwe York 1996. A. M. Thayer, Chem. Eng. News 1997, (March 24), 20. M. M. Bettahar, G. Costentin, L. Savary, J. C. Lavalley, Appl. Catal. A 1996,145, 1. F . Wattimena, H. P. Wulff, UK Patent (Shell Oil) 1 249 079; Chem Abstr. 1971, 74, 12981m. H. P. Wulff US Patent (Shell Oil) 3 923 843; Chem. Abstr. 1976,84, 89977d. F . Wattimena, H. P. Wulff, US Patent (Shell Oil) 4 021 454; Chem. Abstr. 1977, 87, 22393d. M. Taramasso, G. Perego, B. Notari, US. Patent, (Enichem) 4 410 501; Chem Abstr. 1981,95, 206272k. I . W. C. E. Arends, R. A. Sheldon, M. Wallan, U. Schuchardt, Angew. Chem., Znt. Ed. 1997, 36, 1145. C. B. Khouw, C. B. Dartt, J. A. Labinger, M. E. Davis, J. Catal. 1994,149, 195. R. Murugavel, H. W. Roesky, Angew. Chem. lnt. Ed. 1997,36,477. A. Corma, P. Esteve, A. Martinez, S . Valencia, J. Catal. 1995,152, 18. A. Corma, M. T. Navarro, J. PCrez-Pariente, J. Chem SOC., Chem. Commun. 1994, 147. P.T. Tanev, M. Chibwe, T.J. Pinnavaia, Nature 1994,368, 321. T. Maschmeyer, F. Rey, G. Sanker, J. M. Thomas, Nature 1995,378, 159. K. W. Terry, T. D. Tilley, Chem. Muter. 1991,3, 1001. K. W. Terry, P. K. Gantzel, T. D. Tilley, Chem. Muter. 1992,4, 1290. K. W. Terry, C. G. Lugmair, P. K. Gantzel, T. D. Tilley, Chem. Muter. 1996,8,274. K . Su, T. D. Tilley, Chem. Muter. 1997,9,588. K. W. Terry, C. G. Lugmair, T. D. Tilley, J. Am. Chem. SOC. 1997,119,9745. R. Rulkens, T. D. Tilley, J. Am. Chem. SOC. 1998,120,9959.

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Organosilicon Chemistry and Nanosciences

Bruno Boury, Robert Corriu * UMR 5637 - Laboratoire de chimie molCculaire et Organisation du solide

Cc007 - UniversitC Montpellier I1 Place E. Bataillon, 34 090 Montpellier, France

Tel.: +33 467 143 801 - Fax: +33 467 143 852 E-mail: comu@crit.univ-montp2.fr

Nanomaterials and Nanosciences

Introduction

Nanosciences certainly represent the most promising development in the science of matter [ 1-31. In this field one of the major aims will be to control and totally master physical and chemical properties by means that imply working on the elementary atomic or molecular scale. The object of this paper is to show a glimpse of the widespread possibilities offered to physicists and chemists working together. The emphasis is placed upon the contribution of molecular chemistry which in the field of synthesis opens up great possibilities of intervention in all the other fields, for matter either living or not. The place of organosilicon chemistry will be highlighted.

Definition of Nanosciences

A definition broad enough for the nanosciences would be to consider them as the body of research focused on the synthesis and the study of nanoobjects, enhanced by their properties (physical, chemical or biological) as well as the discovery of assembly methods allowing access to nunomaterials and also of organizational methods which make it possible to attain smart materials.

The fundamental basis of nanosciences is the creation of nanoobjects as well as the study of their properties. Superimposed, moreover, is knowledge allowing the transformation of nanoobjects into material. This corresponds to the discovery and the development of specific assembly and organization methods. It is also necessary that these methods be able to allow the production of devices in the form of films, fibers, matrixes, composites or even porosity-controlled solids. The materials thus created must present precise and useable physical, mechanical or chemical properties. In the case of smart materials these properties must be organized each in relation to the others and coupled between them in an interactively controlled manner.

Defined in this way, nanosciences correspond to the opening of a vast field of multidisciplinary research involving at least physicists and chemists; it is appropriate to illustrate the essential elements at stake.

Ovganosilicon Chemisty V Edited by N. Auner and J. Weis

Copyright 0 2003 WILEY-VCH Verlag GmbH & Co. KgaA, Weinheim

390 B. Boury, R. Corriu

Nanoobjects

These make up the elementary bricks of the materials of the future. They can be composed of an assembly of atoms such as a cluster, metallic nanoparticles or the elementary stage of a mineral combination. It can often be a question of a molecule specifically synthesized with the objective of obtaining a particular property, when may be optical, magnetic, electric, chemical (catalysis, separation), mechanical etc. In all cases the property must be precise, able to be measured, and controllable: the synthesis of the nanoobject must be focused on the property with which it is wished to enhance it.

The degree and the precision reached by the chemical synthesis permit the preparation of any chemical object conceived around a property. Wide perspectives are thus open in this field. It is important to point out that some very simple molecules are susceptible to inclusion in nanoobjects. This is the case, for example, for the lanthanide complexes which represent specific photoluminescent properties (in the red for Eu3+ and in the infra red for Er3') [4, 51 or for the paramagnetic complexes obtained by coordination of Cu2+ in a 1,4,8,1l-tetraazacyclotetradecane (cyclam) [6] (described schematically in Fig. 1).

= CU2+ = Eu3+

Fig. 1.

-1e- - XlDATlON

Fig. 2. Exemple of an redox-driven molecular motion with a catenane system.

Orgunosilicon Chemistry and Nunosciences 391

In other cases, more sophisticated systems have been designed, for example molecular engines which aim to mimic precise mechanical movements on a molecular scale (Fig. 2) [7 ] . The principle of movement is based on the variations of stability caused by oxidation or reduction of coordination complexes. In the reduced state the tetracoordination is the most favorable, whereas in the oxide state it is the pentacoordination which is the result of the liberation of a new site of coordination due to the loss of the electron. The intramolecular movement allows the molecule to adopt the most stable configuration by passing from [2+2] to [2+3] coordination by oxidation and conversely from [3+2] to [2+2] coordination by reduction. Thus chemical stability is responsible for the rotation.

Nanomaterials Obtained by Assembling Nanoobjects

The synthesis and the study of nanoobjects enhanced with properties (physical or chemical) constitutes the basis is of any research related to nanosciences. It mustbe pointed out, however, that it is only a part of the target to reach the nanomaterials, and in the long term the smart materials. The isolated nanoobject is not a material: in the absence of an assembly and structure, it will remain a laboratory curiosity.

Chemists have already discovered a large number of assembly methods and found effective organization methods for independent molecules. This chemistry has to be adapted, at least in part, in order to succeed in the assembly of nanoobjects. Thus these materials can be prepared using chemical methods, present or future, which allow the assembly of nanoobjects and at the same time access to a device in the form of a coating, a film, a fiber, a matrix or a porous solid. The preparation of smart materials moreover implies the control of the organization of several properties at the core of the material. This aspect constitutes an essential field of research for the future of the nanosciences.

General Methods and Access to Nanomaterials

These correspond to the chemical methods which allow the nanoobject to be turned into a material linking the different units to each other by chemical means, using different types of links.

Organic Polymerization

Fig. 3. Assembling a nanoobject by polyaddition.

Polymers can be used as the matrix where nanoobjects can be embedded leading to composites, multiphase materials at the nanoscale level. But polymers and polymerization can also be used for

392 B. Boury, R. Corriu

the preparation of monophasic and organized materials, assembling nanoobjects with a polymerizable group. The general assembly design of a nanoobject is represented simply here as an example, in the case of linear assembly (Fig. 3). The nanoobject must be a carrier of chemical functions allowing polymerization. The example chosen represents assembly by the formation of polyurethanes. The scope of the polymerization possibilities is extremely wide and has a good repertory: polyesters, polyamides, polyolefins etc.

Mineral Polycondensation

This corresponds to a procedure of obtaining oxides by hydrolytic polycondensation carried out on molecular organomineral precursors. It is better known as the sol-gel procedure whose discovery dating back to 1848 was due to Ebelman.

It is important to emphasize that this mineral synthesis is the sister of organic polymerization. This reaction in fact corresponds to a nucleophilic substitution of the oxygen atom on the metal with formation of M-0-M bonds, which as they propagate leads to the formation of the kinetically controlled oxide [8]. The best example known is the silica obtained by the hydrolysis of molecular precursors. Figures 4 and 5 represent both the polycondensation, in the case of a metal pentaalkoxide (for exempla V(0R)s) [9], and also the detail of the different stages leading from the precursor Si(OR)4 to silica [lo].

1 H20 I Catalyst I Solvent

Fig. 4. Chimie douce: inorganic polymerization of metal oxide precursors.

This research field will be developed further in the following section since it represents an efficient and powerful tool for the chemist, making compatible different aspects of the chemistry that initially were separated: organic chemistry, inorganic and organometallic chemistry can now be unified with solid-state chemistry as well as macromolecular chemistry. Livage and Rouxel have introduced the word “chimie douce” by contrast to the usual method that required high-temperature processes [ l l , 121. In addition, this “chimie douce” allows a control of the shape of the solid (film, fiber, matrix, monolith) that is important in the perspective of devices.

Organosilicon Chemistry and Nanosciences 393

MOLECULE H,O I Catalyst (PRECURSOR) ' OLIGOMERS

0 O b , b

fiber, matrix

SOL-

1 TRANSITION

GEL b AgedGel SOLID Ageing

J 4 IDwing MATERIALS

Fig. 5. Chimie douce: inorganic polymerization of silica precursors.

Nanomaterials Prepared by the Sol-Gel Process

Nanocomposites [Organic Host + (SiOz Matrix)]

The preparation of a material from a nanoobject can be performed by the sol-gel route, by which it is included it as a host compound in a polymerizing mixture of water, solvent and a silica source (TMOS (tetramethoxysilane), TEOS (tetraethoxysilane)) (Fig. 6). The resulting solid is a biphasic system at the molecular scale but also sometimes at the macrometer scale. As a consequence the two components can be separated by physical means.

Form this point of view, it is very convenient to have the silica as matrix, as it can be formed at room temperature by hydrolytic sol-gel polycondensation and therefore this makes it possible to combine it with an infinity of different host nanoobjects. In addition, compared to other matrixes, the properties of the silica matrix is highly suitable and flexible: its chemical and thermal stability, transparency, porosity, surface polarity, mold ability etc. are all adaptable [ 131. Nanocomposites with organic, organometallic or bioorganic molecules as well as polymers, cells or inorganic salts can be trapped in a silica matrix. Although most of these systems appear homogeneous at the macro (> 1 pm) scale, their structure around or below the nanometer domain (< 100 nm) may be more complex [14-161.

Applications, especially those in analytical chemistry (e.g. bio-, photo- or electrochemical sensors) [17, 181 have currently been developed, as has the preparation of photochromic, thermochromic, luminescent or electrooptical materials [ 19-24].

The main investigations are now directed toward a) the modification of micro-polarity by co-condensation of silica precursors with organosilanes (CH3Si(OR)3 [25-281 or C6H&(OR)3 [29]

394 B. Boury, R. Corriu

or polysiloxanes that can modify important properties like dielectric constant (cf. microelectronic applications) [30]; b) the probing of the matrix’s evolution with environmentally dependent luminescent compounds (ruthenium complexes [3 I], 1,12-bis( 1-pyreny1)dodecane [32], hemicyanine molecules [33, 341); and c) the association of the silica matrix with other oxides (A1203 [35], Zr02 [36], B203 [37,38], Ti02 [39]).

Fig. 6. Overall process for the hydrolysis-polycondensation of precursors of silica-based hybrid organic-inorganic

materials.

This approach to the synthesis of hybrid materials is very convenient and easy to work out; it is particularly efficient for the development of materials having specific physical properties. Therefore, many devices are currently developed for different applications and their presence on the marketplace is now more related to economic issues than to scientific limitations.

However, more fundamental research on this subject can be done by studying monophasic materials in which a homogeneous distribution of the organic and inorganic parts are associated at the molecular level. We will now consider such types of material prepared from molecular precursors that include in their architecture both organic and mineral parts.

Nanostructured Hybrid Materials

Silicon, the “Ideal” Element for Hybrid Materials

Compared to other elements, silicon appears as the most convenient and fruitful element for the preparation of monomers required for the design and preparation of new materials. In fact, the

Organosilicon Chemistry and Nanosciences 395

number of elements which allow the conservation of covalent metal-carbon bonds is limited. For instance, besides Si, the elements P, As, Ge and Sn open possibilities. In contrast, transition metal and group I11 elements cannot be used. There are possibilities with elements of groups IV and V but, taking into account other requirements, it is obvious that only P and Si present reactivity and stability compatible with the preparation of hybrid materials. Chemistry of organogermanes is difficult and expensive, derivatives of arsenic are toxic, and the stability of organotin and a fortiori organolead compounds is limited.

Many reasons can be mentioned to illustrate the unique character of silicon: a) organosilicon chemistry has been developed widely and its fundamental rules are now well understood; b) the characterization of solids at the molecular level is also well established by several techniques; c) the conditions required for the formation of the inorganic network are mild and soft, and almost all the types of Si-C bonds survive them: Si-Csp3 is very stable, Si-Csp2 can be cleaved in severe

conditions, Si-C,, is also stable but it can be easily cleaved in very specific conditions; d) many different leaving groups can be used for the formation of the Si-0-Si inorganic network (OR, H, C1, etc.) [40].

Above all, silicon can be introduced into almost all types of organic or inorganic structure and, especially, organosilicon chemistry offers several reactions that the chemist can use to bind an organic synthon with a reactive organosilicon moiety like those required for the sol-gel chemistry, Si(OR)3 for instance [40, 411. Hydrosilylation reaction is one of the most convenient methods that allows introduction of -Si(OR)3 or -Sic13 groups by a catalytic process. The Calas-Dunogues reaction and the Grignard reaction between aryl halogenide and chloroalkoxysilane are other examples that allow a direct grafting of -Si(OR)3 groups onto aromatic units [42]. An alternative is to form the lithiated compound, followed by silylation with chlorotrialkoxysilane. Particular Grignard reagents like C1Mg-CH2-Si(OiPr)3 or a BrMg-C&-Si(OiPr)3 are interesting synthetic tools that present the general reactivity of an RMgX function [43, 441. Another general route is the Benkeser reaction, which allows efficient preparation of precursors with ally1 or benzyl groups [45]. As a last example, the Heck reaction (or some modified procedure) is certainly one of the most powerful tools used in the preparation of precursors since this reaction can be performed with synthons bearing -Si(OR)3 substituents [46]. These examples illustrate the very rich potential of organosilicon chemistry in the perspective of inclusion of nanoobjects in silica matrixes.

Organosilsesquioxanes R’(Si01.5) The development of organosilicon chemistry makes it possible to synthesize a very large family of monosilylated R’Si(OR)3 precursors. The R‘ group can be chosen with very different architectures and functions and with a high stability of the Si-C link. This chemistry has been widely developed and several applications and fundamental results have already been published in specific reviews [47, 481. We just briefly mention below some aspects of these materials in relation to the field of nanosciences.

Co-condensation of organosilicon R’Si(OR)3 with silica precursors (TMOS, TEOS) is one of the most frequent uses of these compounds that allow fictionalization of the surface of the nanocomposite matrix or of mesoporous silica (see above) [21]. With the same idea, these

396 B. Boury, R. Corriu

organosilanes are also used for monomolecular coating of oxide or metal surface, especially silicon wafers [49].

Another very important use of these organosilanes is their hydrolysis-polycondensation to lead to compounds with nice and well-known cage architectures of nanometer size (Fig. 7).

Fig. 7. Cage-like structure of organopolysilsesquioxanes.

They are well-defined building units for hybrid materials [50-521. Those with mesogenic R groups can exhibit liquid crystal properties [53, 541, and those with polymerizable R groups (acrylate, epoxide, vinylic) can be copolymerized with organic polymer precursors. As a last example of the wide interest in this compounds, those with an incompletely condensed structure (open cage) or incorporating other metals can be models for zeolites and are used themselves as catalysts [55-57].

Finally, hydrolysis-polycondensation of organotrialkoxysilanes can lead to covalent-extended solids R'(SiOl,5), usually known as ormosils (organically modified silanes) or ormocers (organically modified ceramics), which are also a part of the nanostructured family. The formation of these glassy solids instead of the cage-like molecular compounds mentioned above is mainly dependent on both the nature of the R group and the hydrolysis-polycondensation conditions [47, 481. In relation to the nature of the R group, many applications have currently been developed, some of which are already affordable in the marketplace for insulation, sealing and coating [40, 581. Examples taken from the recent literature described them for potential applications as metal silicon oxycarbide precursors to ceramics [59, 601, catalyst supports [61-641 photochromic [65, 661, photoluminescent [67] and thin-film materials with mechanical [68-701, thermo-mechanical [33, 71,721, photo-patternable [73, 741 and low dielectric properties [75,76].

However, there are some limitations to this approach due to possible relaxation processes because here anchoring of molecule is limited to only one Si-C bond. As a consequence, in the nonlinear optics (NLO) studies, the order that is initially induced in the material by an electrical field poling effect is very often lost upon ageing [77-801.

Nanostructured Organopolysilsesquioxanes R[SiOl.5],, (n > I )

This type of solid results generally from the hydrolysis-polycondensation of precursors with general formula R'(SiX3), (n 2 2) . Different possibilities exist, depending on: the nature of the organic group; the number n of Six3 groups; their location in the molecule and the presence of a spacer between the -Six3 group and the central part of the molecules [81-871.

Organosilicon Chemistry and Nanosciences 397

This method, based on the reactions of mineral polycondensation, constitutes a very important extension of the classical preparation of oxides using hydrolytic polycondensation [84, 86, 881. One of the most interesting perspectives is its total compatibility with organic chemistry. The firm stability of the Si-C bond also allows this methodology to be extended to the elaboration of hybrid materials where organic and inorganic parts are combined and covalently bonded at the molecular level (Fig. 8).

Fig. 8. Hybrid organic-inorganic nanostructured materials. The structure shows the possibility to have remaining

uncondensed Si-OH or Si-OR functions.

In the literature devoted to this type of materials, it appears that there are only limited examples of such precursors being unable to achieve the formation of a solid. In almost all cases it is possible to obtain a nanostructured material from the corresponding precursor. Additionally, this approach respects the integrity of the organic group that is preserved (no rearrangement of the structure). As a consequence, a high homogeneous distribution of the organic fragments in the solid results.

However, things may be different when looking at the physical properties of the material that are based on the nature of the organic group (luminescence, for example). Although the properties of the organic group are basically preserved, the incorporation at the molecular level of the organic group in the solid may afford some modifications due to the matrix effect or self-association (vide infra). For example, this is the case for material 1 (Fig. 9) obtained after hydrolysis -polycondensation, which exhibits luminescent maxima at different wavelengths [ 891.

The thermal stability of the material is generally high; it depends on the organic moiety. It is frequently higher than that of the organic group alone. Such behavior is related a priori to the confinement of the organic group in the inorganic matrix, which limits the recombination and radical abstraction processes. In the case of rigid molecular geometry of the organic spacer, the stability is higher than 400 "C and sometimes up to 500 "C. In the case of a nonrigid geometry, for examples in a tetraaza macrocycle (cyclam) (Fig. lo), a thermal degradation has been observed to start from 220 "C. These nonrigid units are interesting since they are organized in the solid and in contrast to the rigid spacers that are also organized (vide infra). The difference in behavior upon thermal treatment under argon originates not from organization but from rigidity. A possible explanation is that, in the case of flexible cyclam units, as soon as a Si-C or C-C bond is broken, the free radical center formed is attached in the solid but with a possible mobility due to the

398 B. Boury, R. Corriu

flexibility of the other links. It can react with another unit located in the proximity, or on the same unit itself. Thus little by little the elimination of the organic part occurs. In contrast, in the case of a rigid spacer, the free radical centers formed by cleavage of covalent bonds cannot move enough to react with one another and especially not with themselves. The degradation starts only when the temperature is high enough to provide radical units that are completely free to move - in other words, only when two covalent bonds are broken in the same spacer.

Fig. 9. Material 1.

2 3

Fig. 10. Nanostructured hybrid organic-inorganic materials 2 and 3.

As far as solids are concerned, the texture (the specific surface area, the porosity, the pore volume and the granulometry) has to be considered. Many reports have shown that it is highly dependent on the experimental conditions (temperature, solvent, catalyst, concentration of the reagents and possibly pressure) and the nature of the organic group R. This suggests a strong kinetic control of the solid, although the precise relation between the chemical processes and the texture so far remains difficult to describe in detail [ 10,901.

Organization of Nanostructured Organopolysilsesquioxanes R[S~OIS], (n >I)

One of the most interesting aspects of these solids is their aptitude for organization, on the nano-, meso- and macroscopic scale. This observation is of great importance since the chemical and physical behavior of the resulting material can be dramatically modified by this characteristic. Silicas prepared by the sol-gel process are amorphous, crystallization being obtained only after thermal curing [8, 101. Due to the shape of the starting precursors, a different situation is observed for the xerogels containing an organic unit like 01.5SiRSi01.5. As illustrated by Fig. 11, either isotropic or anisotropic organization could be obtained following the interactions between organic units. This point is particularly important since a control of the organization of the solids prepared by sol-gel chemistry may be achieved in connection with the weak forces between organic spacers.

Organosilicon Chemistry and Nanosciences 399

- - Anisotropic material Anisotropic material short to long-range order crystallized Isotropic materials

Fig. 11. Possible anisotropic and isotropic organization in silica-based hybrid organic-inorganic materials with a rigid

rod-like organic group.

Chemical evidences: Such evidence of the organization of the solids was initially obtained by checking the reactivity of some of these hybrid materials. The very easy polymerization between the thiophene units leading to solid 4 [91, 921, or the thermal polymerization of the butadiyne units [93] at low temperature (200 "C) leading to solid 5, illustrate such behavior (Fig. 12). Similar phenomena have also been observed for bisethynyl-l,4-phenyl [94]. The reactivities of these systems illustrate the possibility of forming interpenetrating nanosize networks, but more interestingly they also suggest a close and specific positioning of the organic group for the formation of either the polythiophene chain [95] or the polybutadiyne [96-981. Since motion in this highly crosslinked system is very limited, it can be assumed that such favorable positioning of the monomer units is already present in the solid and is resulting from a self-associating process occurring during synthesis of the material.

Oxidative treetme

5

Fig. 12. Chemical transformation in the solid state of materials 4 and 5.

400 B. Boury, R. Corriu

The complexation reaction is another useful way to perform such an investigation. Aromatic organosilanes are known to form tricarbonyl(q6-organosilyarene)chromium(o) complexes easily upon reaction with Cr(C0)6 or (MeCN)&r(C0)3 [99-1011. The complexation of aryl groups of hybrid xerogels of precursors 6 and 7 has been used for comparing their reactivity, as depicted in Fig. 13. In similar experimental sol-gel conditions, these precursors with close structures lead to completely different solids. Xerogel 6 is a hydrophilic solid with a high specific surface area (550 m2 8 ' ) for which the SIMS analysis exhibits only SiOH fragments [loll. Interestingly, this xerogel does not react with Cr(C0)6, showing that the organic groups are not accessible in the solid.

In contrast, the xerogel of 7 is a hydrophobic resin with a very low specific surface area. The mass fragments corresponding to the phenyl and benzyl units are detected by TOF-SIMS and the Si-OH peak is not detectable [loll. The reaction of this xerogel with Cr(C0)6 takes place in good yield leading to a new material in which most of the aromatic groups are complexed by Cr(C0)3. This easy complexation confirms the aromatic units' accessibility.

Hydrophylic xerogel SIMS : SiOH (45)

> 500 m2 g"

Hydrophobic xerogel

<10m2g" SIMS : CeH, (77)

6 0, . 5 s i e s i o l - .5 0 1 . 5 s i ~ s i o 1 . 5 7

Fr(C0)3 U Complexation with

a stoichiometric amount U of Cr(CO)6

o ~ . ~ s ~ + s ~ o ~ . ~ - o1 .5si- SiO1.5

NO REACTION QUANTITATIVE REACTION

Fig. 13. Difference of reactivity of 6 and 7 toward a complexation reaction with chromium carbonyl.

The case of 7 can be explained by formation of almost any kind of crosslinked solid by polycondensation at silicon and from a bis-silylated precursor. The behavior of 6 can be accounted for if the organization of the organic units is obeying some requirements. In addition to the steric limitation, the relative arrangement of the phenyl group must prevent the reactivity with the bulky Cr(C0)6. Moreover, hydrophilic groups have to be present at the surface to explain both the hydrophilic character and the Si-OH fragments detected by SIMS analysis (TOF-SIMS permits detection of the functionalities which are located at the surface, for instance in the case of 01.5Si-CH2-CH2-CH2-I, the main fragments observed are I and CHzI) [102]. Finally, any cleavage allowing formation of aromatic fragments must be eliminated since the SIMS analysis does not exhibit their presence. A possible representation of such an arrangement is shown in Fig 14.

Finally, reactivity of the organic group in these solids can be different from that exhibited in solution; this is illustrated by the specific properties of chelating units such as the cyclams. Hybrid materials containing cyclams have been prepared [103, 1041. It was observed that direct coordination into these solids was very easy. By treating the solid with a solution of CuC12 in EtOH

Organosilicon Chemistry and Nanosciences 401

it is possible to coordinate quantitatively all the coordinating sites. Thus the inclusion of cyclams in the solid does not prevent the coordination. The coordination yield is always quantitative, whatever the texture of the solid (high or low specific surface area). The texture can be modified by the kinetic parameters which control the polycondensation at silicon. However, all the cyclams can always be complexed by Cu2+ or Co2+cations.

Fig.14. A schematic representation of nanostructured hybrid material 7.

Moreover, it was observed that the coordination properties in the solid are not always similar to those observed in solution. When solids containing Cu2+ have been studied, they exhibit a paramagnetism properties which are different from those observed in solution, where Cu2+ exhibits a single R. P. E. signal at 3200 G corresponding to the paramagnetism. In solids this signal is weaker and, as well, a resonance due to classical antiferomagnetic Cu-Cu interaction is observed at 1550 G. Interestingly these kinds of interaction have been observed in solution in the core of tricyclic molecules where the Cu...Cu interaction is controlled by the size and the molecular

rigidity [ 1031.

Fig. 15. Nanostructured material 8 incorporating cyclam units after complexation with copper(n).

402 B. Boury, R. Corriu

These results strongly suggest that only the presence of organization of the cyclam units in the solid permits the possible Cu...Cu interaction. This fact was evidenced by the ability of these

materials to complex Eu3+ (or Gd3+) cations in the ratio of one Eu3+ for two cyclam units. This behavior is in contrast to the observations of solutions, where the lanthanide (Eu3+ or Gd3+) salts cannot be complexed by the same chelating agents. Both facts, Cu...Cu interaction and lanthanide complexation in the solid, require a special organization of the complexing units in the solid (a proposed structure is depicted in Fig. 15).

Interestingly these experimental facts also suggest that the solid exhibits a very interesting flexibility for coordination chemistry and shows that the aptitude of the solid for coordination is not the same as that in solution. Physical evidences: The organization of these hybrid materials was evidenced from different observations. The X-ray diffraction of a nanostructured solid of the general formula R[Si01,5In (n 2 2) does not exhibit any sharp Bragg signal. However, broad signals, whose position and intensity depend on the nature of the precursors, are frequently observed along with the signal attributed to the Si-0-Si contribution (q = 1.7 A-1 corresponding to 3.5-3.7 A). These peaks are too large to be interpreted in terms of crystalline periodicity over a long-range order; however, a short-range order can be considered [105]. Indeed, when R is a rigid organic unit as in 2, the distance associated with the first peak at q1 = 0.55 A-' corresponds to a distance d = 11.4 8, (assuming in the first approximation a Bragg law), close to the Si...Si distance estimated in the

precursor (d(Si-Si): 11.45 A) (Fig. 16) [105]. A similar close correspondence is observed in other cases where R is a rigid rod-like molecule [81].

0 0,5 1 1,5 2

q (A-1)

Fig. 16. X-ray powder diffraction patterns of nanostructured hybrid organic-inorganic materials.

X-ray analysis of materials prepared by co-polymerization of a rigid precursor with various amounts of TMOS also agrees with the short-range order organization. In this case the fluctuations in the position and intensity of the peaks observed in X-ray patterns indicate the formation of a lamellar structure partially swelled one-dimensionally by the silica phase that results from the hydrolysis of the TMOS (Fig. 17) [106].

The swelling law is a linear one with a slope of 0.48 which corresponds to a one-dimensional swelling, in contrast with the law expected for curves for a two- or three-dimensional swelling,

Organosilicon Chemistry and Nanosciences 403

respectively depending on the square and cube root of molar fraction. Moreover, the slope observed is 0.48 instead of the 1.0 expected for a pure swelling, as observed in solution for example. The deviation from the ideal situation can be explained by taking into account the kinetics of polycondensation. Swelling has been established in the case of solutions, but here the swelling is being studied in solids. We have to consider the respective kinetics of polycondensation of TMOS and precursor [ 1071.

Fig. 17. Descriptive representation of the swelling effect of a silica phase by co-polycondensation of hybrid and silica

precursor.

At the meso- and microscopic level, a clear demonstration of the anisotropic organization of these hybrid xerogels is their birefringence observed in polarized light, as exemplified by Fig. 18 [105].

It has been demonstrated that this birefringence is highly dependent on two phenomena. The first is the nature of the organic group; a rigid rod-like group appears to be a requirement in order to achieve such high birefringence values. In contrast, when a flexible alkyl organic group is used a nonbirefringent solid is obtained [ 1081. The second parameter that governs the birefringence phenomenon is the experimental conditions. Thus it appears that the organization in these systems is also under the control of the kinetics of the chemical process [109]. For example, different birefringence values can be obtained, depending on the solvent. The catalyst effect is even greater, since solids can be either isotropic if prepared with NaOH as catalyst, or birefringent if prepared with HCl as catalyst. Such a difference may be due to the catalytic cleavage of the Si-0-Si bond catalyzed by NaOH [40], and it is also certainly due to the difference between the relative kinetics of the hydrolysis and the polycondensation reaction, depending on the catalyst - HCl or NaOH, for

404 B. Boury, R. Corriu

example. For F catalysis an intermediate situation is observed, the material being birefringent but less so than in the case of H' catalysis.

Fig. 18. Material 2 observed by microscopy in polarized light (100pm x 1OOpm).

Other evidence for the self-organization of the solids is their morphological aspect, revealed by TEM observations. In some cases, material with an helical fiber-like structure with 0.3-1 pm widths or with a layered structure can be observed for solids made from precursors exhibiting strong interactions via H bonding (Fig. 19) [llO, 11 11.

Fig. 19. Possible representation of the H bonding interaction between molecular units of hybrid materials.

Finally, highly anisotropic solids with an organization in long-range order were obtained with precursors like 9 (Fig. 20) where R is a mesogenic group which exhibits good flexibility [ 1121. Sol-gel processing of this compound leads to materials whose high birefringence, X-ray powder diffraction data and formation of a crack-free gel strongly support an auto-organization process resulting from the self-association of the hydrophobic part of the monomeric units.

Macroscopic effects on the organization of the solids are related to the ageing process that takes place in the gel once it is formed. As mentioned above, at the ageing step, contraction of the Si-0-Si network and expulsion + evaporation of the solvent generate stress that is released by a

Organosilicon Chemistry and Nanosciences 405

cracking phenomenon. It is a step of reorganization and reorientation of the solid. This process occurs from a few minutes to several hours after gelation, depending on the experimental parameters. The propagation of cracks through the gels seems connected with the birefringence in most of the case. Only recently, birefringent and crack-free gels have been obtained by two different routes: using precursor 9 with a mesogen-like organic moiety, or working in conditions such that elimination of solvent is strongly limited [ 1131.

9

Fig. 20.

The induction effect of an electric field on the auto-organization of organic moieties during polycondensation at silicon has been studied in the case of an organometallic monomer with a strong dipole moment (p = 5.29 D) resulting from the presence of a Cr(C0)3 unit (Fig. 21) [ 1141.

Hybrid material 10 does not exhibit any birefringence, in contrast to the situation when the preparation of the solid by hydrolytic polycondensation is performed in a cell under an electric field, and a faster gelation occurs leading to a material 11 which exhibits the same birefringence as the hybrid 12 obtained from the polycondensation of the corresponding precursor.

Precursor B Precursor A

Wlthout in THF solution J electrk Held electric tbld

O 1.5s1yJ-Lslol., ol.kly)-Lslol, c

I O 1 . P ~ s l O , . r

Wlthwt electric field HydrolysisIpolymndenMlon

o d j o O C Q 2 0 Birefringence solid

12 No.birel rlngence solid Birefringence solid 10 11

Fig. 21. Comparison of material prepared in the absence or presence of an electric field.

The same experiment was performed in a cell which contained two parts that could freely communicate: one (coated with ITO) where an electric field was present, the other where an electric field could not applied. Interestingly, the two parts of the cell led to the same birefringent material. In other words, the possibility of communication between these two parts permits the self-organization of the organic units in the part where an electric field is not applied, although the precursor does not exhibit any birefringence when studied in a cell without an electric field. This communication between the two parts is of importance during the time when the polycondensation is carried out in solution.

406 B. Boury, R. Corriu

During the polymerization and the formation of colloids (sol), the two parts of the cell are in communication. This is the step which controls the formation of the short-range order as proposed previously. Thus, the presence of the birefringence in the part of the cell which was not under an electric field strongly supports our hypothesis that the observed organization starts over a short range in solution and is extended to the macroscopic scale in the solid during ageing. In other words, the formation of a nanometric anisotropic domain appears to be a sufficient condition for the formation of a micrometric anisotropic domain.

To Understand the structure of these solids and the processes by which it is formed requires consideration of the fact that the sol-gel process goes from a homogeneous mixture (precursor/water/solvent) to a solvent-free solid. As shown in Fig. 22, the turning point is the sol-gel transition. Following this idea, the organization of a hybrid material appears strongly related to the process involved before the formation of the gel.

Fig. 22. Representation of the possible self-association process occumng during hydrolytic polycondensation.

At this point auto-association of the molecular units can occur after a long evolution process occurring at first in the liquid state and then followed by a re-organization in the solid. Let us consider the steps in solution: a) the formation of the Si-0-Si bonds; b) the interaction of the organic group with weak forces (electrostatic and dispersion forces, H bonding); c) solvent/solid hydrophobic/hydrophilic interactions. All these effects are producing the short-range order evidenced by X-ray diffraction. Then, after the gelation point and during the ageing, a reorganization of the solid occurs and leads to densification of the Si-0-Si framework, evaporation of solvent and byproducts and chemical reorganization of the Si-0-Si network, which generate stress and finally cracks. Their propagation leads to the birefringence that may result from an anisotropic stretching of the material (Fig. 23). This favors extension of the short-range order over large domains (several micrometers). However, it is important to point out that it has been possible to observe the micrometric-scale organization (birefringence) without cracks.

Organosilicon Chemistry and Nanosciences 407

Fig. 23. Representation of anisotropic stretching of the wet gel.

Organization of Mesoporous Nano Organopolysilsesquioxanes R[SiOl,5],, (n >1) The discovery of mesoporous materials (MCM-41) in 1992 opened very wide perspectives for chemistry, particularly in the field of catalysis and materials [ 115-1 171. These solids also provide a very good opportunity for organosilicon chemistry, since until now silica has been the main matrix in which these materials have been studied and developed. A few other oxides have been studied (Ti02 [118], ZrO2 [119], SnO2 [120], A1203 [121]). However, these materials are more difficult to obtain and they do not exhibit the same potentialities and flexibility as Si02, which provides very stable matrixes, easily tunable, highly compatible with almost all chemicals. Moreover it permits a very wide reactivity with organosilicon compounds.

The mesoporous materials are obtained by sol-gel type polycondensation performed in the presence of a surfactant (Fig. 24). This procedure leads to the formation of a solid in which channels have been designed by the surfactant working as a template. After elimination of the surfactant, in most of the cases the materials obtained exhibit a very regular hexagonal packing distribution of the pores in the silica matrix.

The great interest of chemists in these new materials stems from the fact that, for the first time, they are faced in a single material with all three aspects of chemistry: chemistry in the solid (in the bulk of the mesoporous material), chemistry in solution (channels), and chemistry at the surface (functionalization of channels). Thus the next goal to reach is to find chemical methods permitting the control of chemistry in the bulk but also both in the channels and on the surface. This last goal is a very interesting one since the surface area of the mesoporous material is very high, making it possible to increase the quantities of the matter linked at the surface and permitting easier characterization by directly using solid-state NMR, IR, etc. There is a very interesting enhancement of the possibilities by comparison with what can be done on a planar surface.

These materials provide an important opportunity for the development of the possibilities of organosilicon chemistry for materials science. Insofar as silica is the major matrix, organosilicon

408 B. Boury, R. Corriu

compounds are the most convenient reagents for introducing chemical functionalities either in the channels, or in the bulk, or in both. The inclusion of functionalities precisely located in the pores can be achieved either by grafting, or by direct synthesis [122-1251. In both cases, chemical functionalities will be included by using an organosilicon compound, which can be represented by the general formula 10 (Fig. 25).

1 Micellkation micellar rods Hexagonal or cubic packing

Hydrolysis and polycondensation followed by

elimination of the surfactant and drying

Fig. 24. General overview of the synthesis of mesoporous materials.

Ro\

Rd RO-Si--------x

Linker Spacer Function Fig. 25. General formula of 10, a grafting agent.

The grafting is performed by direct reaction of 10 on the solid obtained after elimination of the template. The direct synthesis is a procedure which implies the presence of 10 as a reagent during the preparation of the Si02 matrix. In this case 10 has a spacer with lipophilic properties, compatible with the core of the micelle. The C-SiOl.5 bond is attached to the matrix during the hydrolytic polycondensation, which permits the formation of Si02.

Let us consider now the great potential made available by the possible functionalization of the pores using a compound like 10. This permits introduction of much more than one simple chemical function; in fact it open up three development routes: 1) introduction of physical or chemical properties by means of metal, particles, or molecules in the pores, by playing on the nature of Z;

Organosilicon Chemistry and Nanosciences 409

2) introduction of a physical or chemical property by means of the spacer itself; 3) through possible interaction between the physical properties of both the matrix and of the pores.

Finally we have to mention a very important recent development in this field. It concerns the preparation of mesoporous materials obtained from nanostructured precursors having two Si(OMe)3 groups (Fig. 26) [126-1281. Great progress has been made over the last two years. Inagaki and Ozin and Stein succeeded in obtaining these materials, which contain organic units in the bulk with the possibility of also having another organic group in the channels. These new materials exhibit hexagonal organization of the pores and are called Periodically Organized Mesoporous Silsesquioxanes (POMS). Moreover, for one of these materials very good organization has been demonstrated.

By extending this methodology to the case of chelating units, we have obtained materials permitting inclusion of tetraaza macrocyles (3) in both the framework and the channels. Investigation of the coordination chemistry of these solids is in progress.

3 and 4 Bifunctional materials

Cyclame (N\Si(OEt)&TEOS

Phonic P123/0H2:EtOH/Decane F 0 (CH,),SNOEQ,

A

: silica 0: C Y ~ W

O - - C L , P(O)(OEt), and Cyclam

Fig. 26. Schematic structure of a POMS with grafting of a functional organic group into the pore surface.

Conclusions

The development of the nanosciences is opening wide perspectives to the chemist, since the elaboration of almost all nanoobjects will be performed by purely chemical routes. Moreover, the preparation of nanomaterials will also imply that method of assembly are mainly. Finally, access to smart materials needs the organization of two (or more) nanoobjects with different physical (or chemical) properties in a way which permits the interactive and controlled connection of these properties. The nanosciences provide a great opportunity for chemistry since they are opening up a future in which all the chemical subdisciplines will have close interconnections and in which the contact between chemists and physicists will be very fruitful for both. The physicist will have the opportunity to work on the nanometric scale with new materials able to have interconnected physical properties. For the chemist a wonderful goal will be to access of the control of matter in terms of physical and chemical properties, playing on its patterns of organization.

Of course this exciting program implies some changes in the targets, in the fields of work, and in the chemical synthetic approach. For instance, nanoobjects are not single molecules; they are molecules designed from two perspectives: their property (physical or chemical in the case of

410 B. Boury, R. Corriu

catalysis or separation), and their assembly. Thus the synthesis must take into account the property and the most appropriate way of assembling the nanoobject. Moreover, the nanomaterial will be a solid in the great majority of cases. That means that molecular chemists have also to integrate the design of the solids in their targets. In other words, the separation between the interdisciplinary fields has to be permeable.

Finally, a new, purely chemical research field will be opened up, for instance with the development of new ways of reaching oxides by hydrolytic polycondensation routes, routes permitting one to obtain new kinds of matrixes affording particular physical properties (new glasses or new resins), and chemical methods permitting control of the organization of nanoobjects; the development of chemical connections with switching properties; access to selective chemical sensors or biosensors, an so on. These wide fields of investigation are providing a very promising future for silicon chemistry, since silicon is the most convenient element which will make success possible thanks to its very great flexibility.

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Organosilicon Chemistry and Nanosciences 411

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Catalytic Activity of Rhodium-Siloxide Complexes in Hydrosilylation of Allyl Ethers and

Allyl Esters ' Bogdan Marciniec, * Edyta Walczuk, Paulina Btaiejewska-Chadyniuk, Dariusz Chadyniuk, Matgorzata Kujawa- Welten, Stanislaw Krompiec

Department of Organometallic Chemistry, Faculty of Chemistry Adam Mickiewicz University

Grunwaldzka 6,60-780 Poznd, Poland Tel.: +48618291366 - Fax: +48618291508

E-mail: marcinb @amu.edu.pl

Keywords: hydrosilylation, rhodium(1) siloxide complex, allyl derivatives

Summary: Rhodium-siloxide dimer [ [ (diene)Rh(p-OSiMe3)}2] (I) appeared to be an active catalyst (even at room tem erature) of the hydrosilylation of allyl ethers,

triethoxysilane and methylbis(trimethylsi1oxy)silane as well as of allyl esters of selected carboxylic acids, i.e. allyl acetate and allyl butyrate, to yield the usual hydrosilylation products accompanied (in the case of ethers) by traces of dehydrogenative silylation products.

CH2=CHCH2OR (R = CH2 d HCH2 , C4H9, Ph, CHzPh, (CHzCH20)7H) by

Introduction

Hydrosilylation of olefins is one of the most thoroughly investigated reactions of its type. It can be catalyzed with complexes of various transition metals [ 1-31.

Most research and industrial syntheses are carried out in the presence of platinum complexes, although two types of rhodium complexes, [RhX(R3P)3] (X = C1, R = Ph; Wilkinson's catalyst) and [RhX(CO)(R3P)2], are often employed also. In addition, dinuclear rhodium complexes containing n-acceptor ligands not involving phosphines have been used, i.e.: [Rh2X2Y2] (X = C1, R, OSiMe3; Y = C2H4, CgH14 and other olefins, CO, COD, P(OR)3, Cp and Cp*) [l , 21. Siloxy-rhodium(1) complexes of the general formula [{ (diene)Rh(p-OSiMe3)}2] (diene = COD, NBD) showed much higher catalytic activity in the hydrosilylation of 1 -hexene by triethoxysilane than respective chloro-rhodium(1) complexes, [ [ (diene)Rh(p-C1)}2] [4]. These results prompted us to investigate the scope of its catalytic activity in the hydrosilylation of allyl ethers and allyl esters which are

Part XXXV in the series: Catalysis of Hydrosilylation. For Part X X X I V see Ref. [15].

Ovganosilicon Chemisty V Edited by N. Auner and J. Weis

Copyright 0 2003 WILEY-VCH Verlag GmbH & Co. KgaA, Weinheim

416 B. Murciniec*, E. Walczuk, P. Btaiejewska-Chadyniak, D. Chadyniuk

commonly used for synthesis of silane coupling agents [5].

Hydrosilylation of Ally1 Alkyl Ethers

The addition of trisubstituted silanes, particularly triethoxysilane, to allyl alkyl (aryl) ethers catalyzed predominantly by a chloroplatinic acid precursor leads to y-functional triethoxysilanes. The process was reported to occur usually at an elevated temperature, e.g. 120-166 "C, 1.4 h, 50-65 % [6], 80 "C, 3 h, 88 % [7] and 150 "C, 1 h, 97 % [8].

Rhodium-siloxide complex [ { (diene)Rh(p-OSiMe3)}2] (I) appeared to be a very effective catalyst of the hydrosilylation of various allyl ethers; for example, hydrosilylation of allyl glycidyl ether, allyl butyl ether, allyl phenyl ether and allyl benzyl ether [9] proceeds almost quantitatively even at room temperature (Table 1). The hydrosilylation of allyl glycidyl ether by triethoxysilane leads to glycidoxypropyltriethoxysilane, which is a commercially important silane coupling agent. The reaction of allyl ethers with hydrosiloxanes catalyzed by I also occurs with very high yield (Table 2) [ 101. The hydrosilylation products have applications in the cosmetic industry [ 111.

All the reactions catalyzed by I give the hydrosilylation products (A) in very high yield accompanied by products of dehydrogenative silylation (B + C) according to Eq. 1.

Table 1. Hydrosilylation of allyl ethers by triethoxysilane.

R Yield [%]

A B + C [Rhl:[HSi(OEt)~]:[ether] Temp. ["C] Time [h]

5 x 104:1:1.5 RT 0.25 98 2

lo4: 1 : 1.5 40 2 41 1 v

24 99 1

- C4H9 5 x 104:1:1.5 RT 0.25 98 2

104:1: 1.5 40 2 68 2

24 98 2

5 x 104:1:1.5 RT 0.25 98 2

lo4: 1 : 1.5 40 2 70 1

5 x 104:1:1.5 RT 0.25 79 2

24 98 2

4 lo4: 1: 1.5 40 2 14 1

24 98 2

[a] Reaction conditions: argon, glass ampoules.

Catalytic Activity of Rhodium-Siloxide Complexes in Hydrosilylation 417

Table 2. Hydrosilylation of allyl ethers by methylbis(trimethylsi1oxy)silane. Ial

R [Rh]:[H-siloxane]: [ether] Temp. ["C] Time [h] A

Yield [%]

B + C

T 1 : 1.5 RT 2

24

10A:1:1.5[b1 60 2

- (CHzCH*O),H 10-3:1:1.25 RT 2

10A:1:1.25[b1 60 2

24

24

24

84

86

56

82

13

92

67

92

[a] Reaction conditions: argon, glass ampoules. [b] benzene.

~ O - R + HSiR3 [Rhl ~ R3Si-0,R + R'3Si-0.R + ~ 0 . ~

A B C

R = - 0 , -C4H9, -Ph, -CHZPh

R 3 = (OEt)3, (Me)(OSiMe,),

Eq. 1.

Hydrosilylation of Ally1 Esters of Carboxylic Acids

Esters of various carbocyclic acids are readily hydrosilylated to give products that are used as silane coupling agents according Eq. 2. [l, 21.

p,,OCOR + (RO)$iH [Ptl D (R0)3Si,-,-,-,0COR

Eq. 2.

Hydroxy-functional propylsilanes may be obtained by hydrosilylation of allyl esters followed by alcoholysis of the product to recover the hydroxypropylsilanes (Eq. 3).

(R'0)3Si,-,-,-,0COR (R'~)~S~-.--.JO~ + RCOOR H+

Eq. 3.

The direct hydrosilylation of unsaturated alcohols is accompanied by dehydrocondensation

418 B. Marciniec*, E. Walczuk, P. Btaiejewska-Chadyniak, D. Chadyniak

according to Eq. 4 [ 11.

Eq. 4.

Catalysis of the hydrosilylation of allyl esters has been based on the platinum complexes used in the hydrosilylation of allyl acetate (160 "C, 6.75 h, 56 % [12]), allyl acrylate (106-114 "C, 3 h, 97 % [13]) and allyl methacrylate (40-50 "C, 3 h, 87 % [14]). We studied the catalytic activity of I in the hydrosilylation of allyl acetate and allyl butyrate to obtain products according to Eq. 5. The results are described in Table 3.

KR -"fR + HSi(OEt)3 [Rhl - (EtO),Si-O

0 0

R = -CH3, C3H7

Eq. 5.

Table 3. Hydrosilylation of allyl esters by triethoxysilane.

R [IUI]:[HS~(OE~)~]: [ester] Time [h] Yield [%I -CH3 5x104:1: 1.5 1 87

-C& 5 ~ 1 0 ~ : 1: 1.5 1 97

[a] Reaction conditions: argon, glass ampoules.

These reactions occur even at room temperature and have been confirmed by analysis of the products using GC-MS, IR and 1H,'3C,29Si NMR methods.

Conclusions

Catalytic measurements of the hydrosilylation of the exemplary allyl ethers and allyl esters have shown a much higher activity of the siloxide-rhodium complexes than that of chloro-rhodium analogues. All the reactions examined occur under much milder conditions (even at room temperature) than those with previously reported platinum complexes.

Acknowledgments: This work was supported by funds from The State Committee for Scientific Research, Poland, Project No. K026/T09/2001.

Catalytic Activity of Rhodium-Siloxide Complexes in Hydrosilylation 419

References [ 11 B. Marciniec (Ed.), Comprehensive Handbook on Hydrosilylation, Pergamon Press, Oxford,

1992. [2] I. Ojima, Z. Li, J. Zhu, in The Chemistry of Organic Silicon Compounds (Eds. Z . Rappaport,

Y. Apeloig), John Wiley & Sons, Chichester, UK, 1998, Chapter 29. [3] B. Marciniec in Applied Homogenous Catalysis with Organometallic Compounds, 2nd edn.,

(Eds. B. Cornils, W. A. Hermann), Verlag Chemie, Weinheim, 2002, Chapter 2.6. [4] B. Marciniec, P. Krzyzanowski, E. Walczuk-Gukiora, W. Duczmal, J. Mol. Cut. A 1999,

144,263. [5] J. I. Kroschwitz, M. Mowe-Grant (Eds.), Kirk-Other Encyclopedia of Chemical

Technology, Vol. 22, John Wiley & Sons, New York, 1997, p. 1. [6] E. German Patent 144 413. [7] R. A. Sultanov, M. B. Kadyrova, I. A. Khudayarov, S . I. Sadylch-Zade, Uch. Zap. Azerb.

Univ., Ser. Khim. Nauk, 1971,37; Refi Zh. Khim., 24Zh730,1971. [8] German Patent 1 259 888 (1967). [9] Polish Patent P-351 449 (2001). [lo] Polish Patent P-351 451 (2001). [ l l ] R. Wagner, Appl. Organomet. Chem. 1999,13,611. [12] Polish Patent 117 627 (1978). [13] USA Patent 3 258 477 (1966). [14] British Patent 1 158 510 (1969). [15] H. Maciejewski, B. Marciniec, I. Kownacki, J. Organomet. Chem. 2000,597, 175.

Synthesis and Complex Chemistry of Novel Di- and Trihydroxyoligosilanes

D. Hoffmann," H. Reinke, C. Krempner"

Fachbereich Chemie, Abteilung Anorganische Chemie, Universitat Rostock Einsteinstr. 3a, D-18055 Rostock, Germany

Tel.: +49 381 4986406 -Fax: 4 9 381 4986382 E-mail: clemens.krempner@chemie.uni-rostock.de

Keywords: titanium, zirconium, oligosilanes, silanols, metal siloxides

Summary: The synthesis of the novel bidentate ligands (H0RMeSi)z [R = Si(SiMe&] (2) and (HOR2Si)zSiMez [R = SiMes] (7) as well as the tridentate ligands (HORMeSi)3SiMe (10a) [R = Si(SiMe3)3] and (lob) [R = SiMe(SiMe&] is reported. New cyclic metal siloxide complexes were readily prepared from the reaction of 2 and 7 with Ti(OEt)4, Ti(NEt2)4 and Zr(NEtz)4, respectively. The solid-state structures of the novel complexes TiL2 (3) and LTi(OEt)2 (4) [L = (2) - 2H] were determined by X-ray crystallography. Treatment of 10b with CpTiCl3 resulted in the formation of the novel tridentate complex CpTi-(OSiMeR)3SiMe (11).

Introduction

The chemistry of transition metal siloxide complexes has continued to attract considerable attention in the field of material science [ l ] and catalysis [2], particularly since 1980. The synthesis, reactivity, and bonding of such complexes in a wide variety of supporting ligand environments continues to be explored. In this regard numerous silanediols, disilanols and silanetriols have been described in the literature [3,4], which could be used as building blocks for the preparation of novel titanium-containing heterocubanes and titanasiloxanes [4-61.

Our interest lies mainly in the development of metal siloxide complexes using novel bidentate and tridentate ligands based on hydroxyoligosilanes in which the hydroxy groups are fixed at different silicon atoms along a oligosilane chain. We have previously shown that 1,2-dihydroxydisilanes surrounded by two bulky (Me3Si)3Si groups are stable anionic siloxide ligands for the preparation of titanium-containing five-membered ring compounds [7]. Herein we describe the synthesis of bidentate and tridentate silanol ligands as well as the synthesis of novel Ti and Zr siloxide complexes of different ring sizes.

Ovganosilicon Chemisty V Edited by N. Auner and J. Weis

Copyright 0 2003 WILEY-VCH Verlag GmbH & Co. KgaA, Weinheim

Synthesis and Complex Chemistry of Novel Di- and Trihydronyoligosilanes 421

Results and Discussion

The synthetic route to five-membered siloxide complexes of Ti and Zr is outlined in Scheme 1. The diastereomerically pure (meso)- 1,2-dihydroxydisilane 2 can be obtained by acidic hydrolysis of the dichlorodisilane 1 in short reaction times. Hydrolysis of 1 in the presence of Nfi(NH2COO) at high temperatures gave a 2: 1 mixture of (rac)-2 and (meso)-2. After column chromatographic separation of the mixture, the (rac)-diol 2 could be isolated in an overall yield of 65 %. The structures proposed are in full agreement with the straightforward 'H, 13C and 29Si NMR spectra as well as the MS data.

A suitable entry into titanium and zirconium complex chemistry is the use of group IV amides and alkoxides. For example, when Ti(OEt)4 reacted with 2 equiv. of (rac)-2 in heptane, the titanium bis(disi1oxide) 3 could be isolated in 91 % yield as a yellow microcrystalline material. The results of the X-ray analysis (Fig. 1) of 3 confirm the expected extensive shielding of the titanium atom by the two sterically demanding disiloxide ligands. The geometry around the titanium atom is described best as distorted tetrahedral, with an 02-Til-02 chelate angle of 99" and an 02-Til-01 angle of 115.5'.

R I R OH OH

I 1 l i (0Etk Me-Si-'. /'-Si I IMe

I I I 1 Me(' Silo- O,Si-Me Me Me Me Me

9' CI HZO

R-Si-Si-R - R-Si-Si-R - I 3, I

R R 1 3

R

EtO, /O-Ji ,,Me R = Si(SiMe& E t ~ 8 ' T \ o , ~ i I IMe

R 4

Scheme 1. Synthesis of the ligand 2 and the complexes 3-5.

Fig. 1. Structure of 3 (hydrogen omitted for clarity). Fig. 2. Structure of 4 (hydrogen omitted for clarity).

422 D. HofSmann, H. Reinke, C. Krempner

When Ti(OEt), reacted with 1 equiv. of (rneso)-2 in heptane for 3 h, a yellow microcrystalline material of what was thought to be the titanium complex 4 could be isolated in a yield of 76 %. However, the results of diffraction study (Fig. 2) reveal a dinuclear structure in which two titanium diethoxide fragments are linked by two ethoxide groups. The coordination geometry around the titanium centers can be regarded as distorted square pyramidal, with an 0-Ti-0 chelate angle of 91”. The bridging Ti-04 distance at 198 pm is significantly longer than the T i 4 1 and Ti-02 distances of the ligand at 182 and 183 pm. In contrast, the reaction of (rneso)-2 with Zr(NEt2)4 in heptane led to the quantitative formation of the zirconium bis(disi1oxide) 5.

As shown in Scheme 2, the pure diol7 was obtained by selective cleavage of the Si-Ph bond of diphenyltrisilane 6 with CF3S03H and subsequent hydrolysis in the presence of W(NH2COO) in quantitative yields. Reaction of 7 with Ti(NEtz)4 and ZT(NEt2)4 leads to the formation of the six-membered ring complexes 8 (M = Ti or Zr) as colorless and moisture-sensitive crystals in a yield of approximately 80-90 %.

Me3Si, TiMe3 \ I

Si-Ph Me, MeyS\

Si-Ph / I

Me3S1 SiMe3

TfOWHfl . SiMe3

Si-OH

Si-OH

Me3Si, I Me, MeyS(

./ I Me3S1 SiMe3

6 7 8

Scheme 2. Synthesis of 8 (M = Ti, Zr).

Furthermore, hydrolysis of the trichlorosilanes 9 in the presence of Nb(NH2COO) proceeds selectively, giving racemic mixtures of the triols 10 in excellent yields (Scheme 3). As determined by NMR spectroscopy, the (Me3Si)3Si-substituted compound (Zl)-9a was completely converted into the triol (1u)-lOa. In the case of the less sterically demanding triol lob, mixtures of both diastereomers 11 and lu were observed which could be separated by column chromatography [8].

Me

t + 3BuLi / CpTiCI,

R Me R yR Meis’,\ MeA, S y M e -* H20 bledSi,\ Me_Si l S i A . 4 e

OH I OH s i I I SI-R I - 3LiCI

CI OH CI I CI

9a R = Si(SiMe3)3 9b R = SiMe(SiMe3)2

10a R = Si(SiMe3)3 10b R = SiMe(SiMe& 11 R = SiMe(SiMe3)2

Scheme 3. Synthesis of 11.

Synthesis and Complex Chemistry of Novel Di- and Trihydroxyoligosilanes 423

Additionally, the solid-state structure of (1u)-lOa was determined by X-ray crystallography. Although a configurational disorder was observed for the methyl as well as the OH groups and the positions of these groups could not be refined exactly, the X-ray analysis revealed that the OH groups are fully enclosed by the bulky (Me3Si)3Si substituents and that no intra- or intermolecular hydrogen bonds exist in the solid state.

Unfortunately, attempts to synthesize tripodal complexes by treatment of the triol (1u)-lOa with Ti(OEt), or Ti(NEt& resulted in the formation of complex mixtures, presumably of bipodal and coordination polymers. It is suggested that the space demand and the stereochemical influence of the (MesSi)$i groups in (1u)-1Oa prevents a complete linkage of the three OH groups with the metal center. In contrast, the sterically less demanding triol (1u)-lob reacts selectively in the presence of 3 equiv. of BuLi with CpTiCl3 at -78 "C to produce the tripodal complex (1u)-11. Despite its high solubility in organic solvents, which prevents crystallization, the selectivity of this reaction allows the isolation of the compound as a yellow powder after filtration and evaporation of the solvent. The structure proposed was in full agreement with the MS and NMR data, especially the 29Si NMR data.

Conclusions

We have prepared di- and trihydroxyoligosilanes in which the Si-OH groups are surrounded by sterically demanding substituents. It was shown that the silanols 2 and 7 especially are useful dianionic siloxide ligands which react cleanly with T a r amides and alkoxides giving the five- and six-membered ring complexes 3-5 and 8 in high yields. The results of the X-ray analyses of 3 and 4 confirm the expected extensive shielding of the titanium atom by the disiloxide ligands. Further investigations concerning the synthesis and reactivity of novel transition metal siloxide complexes by using these silanols as bipodal and tripodal ligands are under way.

Acknowledgment: We gratefully acknowledge the support of our work by the Fonds der Chemischen Industrie and we thank Prof. H. Oehme for his generous support.

References

[ l ] a) G. Perego, G. Ballussi, C. Corno, M. Tamarasso, F. Buonomo, A. Esposito, New Developments in Zeolite Science and Technology, in Studies in Surface Science and Catalysis (Eds.: Y. Murakami, A. Iijima, J. W. Word), Elsevier, Amsterdam, 1986; b) A. Bhaumik, R. J. Kumar, J. Chem. SOC., Chem. Commun. 1995, 869; c) B. Notari, Stud. S u ~ Sci. Catal. 1988,37,413. a) H. C. L. Abbenhuis, S . Krijnen, R. A. van Santen, Chem. Commun. 1997, 331; b) I. E. Buys, T. W. Hambley, D. J. Houlten, T. Maschmeyer, F. A. Masters, A. K. Smith, J. Mol. Catal. 1994, 86, 309; c) M. Crocker, R. H. M. Herold, A. G. Orpen, Chem. Commun. 1997,

[2]

424 D. HofJiann, H. Reinke, C. Krempner

2411; d) T. Maschmeyer, M. C. Klunduk, C. M. Martin, D. S. Shepard, J. M. Thomas, B. F. G. Johnson, Chem. Commun. 1997, 1847. P. D. Lickiss, Adv. Inorg. Chem. 1995,42, 147. R. Murugavel, A. Voigt, M. G. Walawalkar, H. W. Roesky, Silanetriols: Preparation and Their Reactions, in Organosilicon Chemistry 111: From Molecules to Materials (N. Auner, J. Weis), VCH, Weinheim, 1998, p. 376. R. Muragavel, A. Voigt, M. G. Walawalkar, H. W. Roesky, Chem. Rev. 1996,96,2205. A. Voigt, R. Murugavel, V. Chandrasekhar, N. Winkhofer, H. W. Roesky, I. Uson, Organometallics 1996,15, 1610. D. Hoffmann, H. Reinke, C. Krempner, J. Organomet. Chem. 2002,662, 1 . For the Prelog-Seebach notation of 1 and u, see: D. Seebach, V. F’relog, Angew. Chem. 1982, 94,696;Angew. Chem. Int. Ed. Engl. 1982,21,654.

Thioether Functionalized Octasilsesquioxanes

H. J. Konig," H. C. Marsmann

Anorganische und Analytische Chemie, Universitat Paderborn Warburger Str. 100,33098 Paderborn, Germany

E-mail: koenig.heinrich @ freenet.de

M. C. Letzel

Organische Chemie I, Universitat Bielefeld, Germany

Keywords: octasilsesquioxane, thioether, 29Si NMR spectroscopy, MALDI-TOF-MS

Summary: Octasilsesquioxanes are cage molecules with eight functional groups. It is possible to modify them by radical addition of thiols to double bonds. Octavinylsilsesquioxane reacts without decomposition of the silsesquioxane core regioselectively to give the anti-Markovnikov product. It is also possible to modify monovinyYally1-propyl-functionalized silsesquioxanes in this manner. The reaction pathway was transferred to thiosilsesquioxanes. On reacting these HS-functionalized compounds with vinylsilsesquioxanes, products with very complex structures and high molecular masses were obtained. Interestingly they are soluble in all common organic solvents (e.g. benzene, CHCl3, THF etc.). Consequently they were characterized by 'H, I3C and 29Si NMR spectroscopy. MALDI-TOF mass examination and elemental analysis were also performed on these molecules. Based on the spectroscopic data obtained the structures of the novel compounds are proposed.

Introduction

Octasilsesquioxanes with general formula R&301,& are cube-shaped molecules with eight functional groups [ l , 21. Various functionalities can be used to synthesize different molecular structures of high symmetry. One method to prepare those structures is the radical addition of thiols to double bonds initiated by AIBN [3,4].

Results

Octathioether-functionalized silsesquioxanes can be prepared according to Scheme 1. The structures of the novel compounds were confirmed by various spectroscopic methods ('H, 13C, 29Si NMR; MALDI-TOF-MS, IR) and elemental analysis. As an example the MALDI-TOF mass spectrum

Ovganosilicon Chemisty V Edited by N. Auner and J. Weis

Copyright 0 2003 WILEY-VCH Verlag GmbH & Co. KgaA, Weinheim

426 H. J. Konig, H. C. Marsmann, M. C. Letzel

E -

1wO-

(measured and calculated) of octathiopyridylethylsilsesquioxane is depicted in Fig. 1.

RI= (f&SH

im

m 90

70

,xm

i40

350

30 20

-

. AIBN

Scheme 1. Radical addition to octavinylsilsesquioxane.

m . 0 6 1523.44

Fig. 1. Measured (left) and calculated (right) MALDI-TOF mass spectrum of octathiopyridylethyl-silsesquioxane (H species).

Scheme 2. Radical addition to monoolefins.

Thioether Functionalized Octasilsesquioxanes 427

The modification of monovinyYal1 yl-hepta-n-propylsilsesquioxanes gives completely analogous

A typical 29Si NMR spectrum of one of the above-mentioned products is shown in Fig. 2. monofunctionalized products (Scheme 2) .

2.4.5 3,6, 8

The reaction pathway was transferred to thiosilsesquioxanes, giving silsesquioxane cage block oligomers (Scheme 3). A typical 29Si NMR spectum of an oligomer is shown in Fig. 3.

Finally, the above-mentioned concept was applied to octavinylsilsesquioxane, giving molecules with high molecular masses. For example, the product of octavinylsilsesquioxane with 8 equiv. of monothiopropylhepta-n-propylsilsesquioxane has a molecular mass of 6980.66. Its MALDI-TOF mass spectrum is shown in Fig. 4.

Scheme 3. Radical addition to silsesquioxanes.

428 H. J. Konig, H. C. Marsmann, M. C. Letzel

3.6 .8 , 11, 14, 16

W V J

Fig. 3. 29Si NMR spectrum of diblock oligomer.

25001 6307.76 7102.27

1 2000 -

g I5O0- c E 1000- -

500 -

60006100620063006400650066006700M)M)690070007tM)7200730074007500

mass [m/z]

MALDI-TOF mass spectrum of a nonablock silsesquioxane. Fig. 4.

References [l]

[2] [3] [4]

R. Corriu, P. Jutzi, Tailor Made Silicon Oxygen Compounds: from Molecules to Materials, Vieweg, Braunschweig, Wiesbaden, 1996, p. 149. B. J. Hendan, Ph.D. Thesis, Universitat Paderborn, 1995. R. Weidner, N. Zeller, B. Deubzer, V. Frey (Wacker Chemie), DE 3837397, 1990. H. J. Haupt, T. Seshadri, Anal. Chem. 1988,60,47.

Synthesis of Cyclopentadienyl-Substituted Polyhedral Zirconasiloxanes

Hans Martin Lindemann, Beate Neumann, Hans-Georg Stammler, Anja Stammler, Peter JutzP

Fakultat fur Chemie, Universitat Bielefeld UniversitatsstraBe 25, 33615 Bielefeld, Germany

Tel.: +49 521 106-6181 -Fax: +49 521 106-6026 E-mail: Peter.Jutzi @Uni-Bielefeld.de

Keywords: cyclopentadienyl-silanetriols, polyhedral zirconasiloxanes, Si-0-Zr frameworks

Summary: Condensation reactions of cyclopentadienyl silanetriols (CpRSi(OH)3) with zirconium tetra-t-butoxide yield oligomeric zirconasiloxanes. Various types of polyhedral structures can be obtained by small structural modifications of the reactants or by variation of reaction parameters. These compounds can be considered as valuable model systems for zirconasilicates.

Introduction

Heterogeneous silica-supported group 4 transition metal compounds play an important role as catalysts in the petrochemical industry [ 11. The commercial importance of such catalysts has stimulated an intense interest in the chemical processes which occur on the surface of heterogeneous catalysts [2]. Mechanistic studies are difficult due to the complicated molecular structure of the surface [2]. This problem has evoked efforts to synthesize suitable model systems such as transition-metal-containing polyhedral oligometallasilsesquioxanes (POMSS) that are formally derived from the incorporation of transition-metal atoms into the silicon-oxygen frameworks of polyhedral oligosilsesquioxanes (POSS) [3]. While there are many examples of polyhedral titanasiloxanes in the literature [3], there are only a few examples of polyhedral zirconasiloxanes [4]. Our interest in silsesquioxane chemistry is aimed at developing polyhedral, structurally well-characterized group 4 metal siloxanes, possessing potential leaving groups at the core atoms. Recently, we reported the synthesis and chemistry of CpR-substituted, polyhedral titanasiloxanes [5], which have been prepared in good yields by co-condensation reactions of silanetriols (CpRSi(OH)3) and titanium alkoxides (Ti(OR)4). In this paper we present studies concerning analogeous co-condensation reactions with Zr(OfBu)4 as transition metal component.

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430 H. M. Lindemann, B. Neumann, H.-G. Stammler, A. Stammler, P. Jutzi

Results and Discussion

Condensation reactions have been performed with the silanetriols 1,2 and 3 (Fig. 1).

Me,SiFISi(OH), (1) MeFISi(OH), (2) Cp'Si(OH), (3)

Fig. 1. Silanetriols used as precursors for polyhedral zirconasiloxanes.

Interestingly, this procedure does not lead to the expected cubic Si4012Zrd frameworks. Instead, a variety of oligomeric systems have been obtained by even small structural changes in the reactants, or by variation of solvent (including donors like aniline or THF) and temperature. Scheme 1 depicts the products obtained by the reaction of equimolar amounts of 1 with Zr(OfBu)4 under different reaction conditions.

Me,SiFISi(OH),(l) + Zr(O'Bu), -78"c_ [Me,SiFlSi]dMe&FISiOH), hexane, [Zr~[ZrOBuaniline~[~O],4[p30]2 (5) aniline

Scheme 1.

Fig. 2.

Condensation reactions of Me3SiF1Si(OH)3 (1) with Zr(O'Bu)+

Ball and stick representation of 4 [6].

Compound 4 was isolated by fractional crystallization as the main product of the co-condensation reaction at r.t. and was analyzed by X-ray crystallography (Fig. 2) [6] . The core structure of 4 can be described as a Zr4012Si4 cube with a missing zirconium corner. As a consequence, Si3 and Si4 each possess a silanol group, and Si2 is connected by an p2-0x0 bond to 2 3 . The oxygen atom of the Zr3-0-Sil moiety is linked with Zr2 thus forming a

Synthesis of Cyclopentadienyl-Substituted Polyhedral Zirconasiloxanes 431

p3-0 unit. Zrl possesses two aniline ligands in addition to a t-butoxy group, Zr2 has no additional donor ligands and Zr3 is coordinated by only two aniline molecules. Zrl and Zr3 adopt distorted octahedral geometries, and Zr2 is square-pyramidally coordinated.

After co-condensation of 1 and Zr(O'Bu)4 at -78 "C, compound 5 was isolated as the main product by fractional crystallization and was analyzed by X-ray crystallography (Fig. 3). The structure of 5 resembles two fused cubic cages, each lacking a Zr comer, connected by a common face formed by Zr2, Zr3, Si2 and Si4. The Sil and Si3 atoms of one cage fragment and the Si5 and Si6 atoms of the other possess silanol groups. The Zr atoms Zrl, Zr2, and Zr3 and the atoms Zr4, Zr3 and Zr2 are connected by p3-O units. The Zrl and Zr4 atoms are each bound to a t-butoxy group and to an aniline ligand. All Zr atoms adopt distorted octahedral geometries.

Fig. 3. Ball and stick representation of 5 [6] . Fig. 4.

When the co-condensation of 1 and Zr(O'Bu)4 was carried out at -78 "C in DME as solvent, compound 6 was isolated as the main product by fractional crystallization and was analyzed by X- ray crystallography (Fig. 4). The atoms Zrl, Sil, Si2, Si3, Zr3 and Zr4 form the comers of a cube- structure with one missing Si comer. The missing comer is replaced by a more open structure formed by Zr5 and three pendant, bidentate (CpRSiOH)02) units formed by Si4, Si5 and Si6. This moiety is linked to the cage fragment by p3-O (Zr-0-Zr) and p2 (Si-0-Zr) units. The p3-O units bridge the faces of a tetrahedron formed by Zr2, Zr3, Zr4 and Zr5. Three of the tetrahedron edges are bridged by (CpRSiOH)02 units. Zr5 is coordinated by one ql-DME ligand and Zrl by one q'- and one q2-DME ligand. In this polyhedron no Zr atom still bears t-butoxy groups. The Zr atoms Zr 1-Zr4 adopt distorted octahedral geometries, and Zr5 adopts an overall sevenfold coordination.

Scheme 2 depicts the products obtained by the reaction of equimolar amounts of 2 and of 3 with Zr(O'Bu)4.

Ball and stick representation of 6 [6] .

aniline/ THF [MeFISib[ZrOlBuanilineTHF]2 * [ZrOlBuTHF][ZrO~ul~~Ol, , [~O~ (7)

4 MeFISi(OH),(2) + 4 Zr(OfBu), -12 'BuOH

- [(Cp*Si),O~[ZrO~ul~~OfBul 4 Cp*Si(OH),(3) + 4 Zr(O'Bu), -1 0 'BuOH I ~ ~ ~ ~ I ~ ~ I ~ C ~ O L I C 1 4 O l (8)

Scheme 2. Condensation reactions of 9-Mefl~orenylSi(OH)~ (2) and Cp*Si(OH)3 (3) with Zr(O'Bu),.

432 H. M. Lindemann, B. Neumann, H.-G. Stammler, A. Stammler, P. Jutzi

Single crystals of 7 suitable for an X-ray crystal structure determination were grown from a concentrated solution in hexane (Fig. 5). The core of the cage structure can be defined by four silicon and four zirconium atoms occupying alternate comers of a cube. Ten of the twelve Si-Zr edges are bridged by oxygen atoms in a p~ fashion. There are five SizTiz04 eight-membered rings which define faces of the cube. The upper cube face is strongly distorted; p3-O units are formed by coordination of the oxygen atoms of two opposing Zr-0-Si edges to the zirconium atoms Zrl and Zr2.

Fig. 5. Ball and stick representation of 7 [6]. Fig. 6. Ball and stick representation of 8 [6] .

Zr4 and Zr3 are coordinated by THF and aniline ligands in addition to t-butoxy groups, thus adopting a distorted octahedral geometry. Zr2 is coordinated by THF and a t-butoxy group, adopting an octahedral geometry. Zr2 possesses a t-butoxy group with no additional ligands; this arrangement leads to a distorted square-pyramidal geometry.

The zirconium atoms in 8 (Fig. 6) form a tetrahedron possessing a central c~q-0 unit. Two faces of the tetrahedron are bridged by [02CpRSiOSiCpROz] units forming pz-0 and p3-O links to Zr. Each of the four Zr atoms bears one t-butoxy group; in addition, Zr2,Zr3 and Zr4 are connected by a p3-O'Bu unit, and Zr3 and Zr4 are connected by a pz-O'Bu link. Zrl, Zr4 and Zr3 adopt distorted octahedral geometries, and Zr2 adopts an overall sevenfold coordination.

Compounds 6 8 can be distinguished by the coordination number of Zr, the number of donor molecules coordinated to Zr, the type of pn-0 (n = 2-4) and pn-OR (n = 2-3) linkages of the Zr atoms, and the ratio of Si to Zr.

In conclusion, three aspects of the synthesis and structure of zirconasiloxanes are important:

Even small variations of the reaction conditions lead to great structural effects. Structure elements like pn-0 (n = 2-4) and pn-OR (n = 2-3) linkages show the analogy to Zr-oxo-alkoxide chemistry. In most cases donor molecules are necessary to complete the coordination sphere.

Synthesis of Cyclopentadienyl-Substituted Polyhedral Zirconasiloxanes 433

The difficulty of obtaining cubic zirconasiloxanes can be explained by the significantly higher electrophilic character of zirconium compared to titanium [4]. Zirconasiloxanes show a stronger tendency to form oligomeric structures and Lewis base (e.g. THF) adducts. Six-coordination at the Zr centers is highly preferred, whereas four-coordination is preserved at the Si centers.

The zirconasiloxane oligomers synthesized represent valuable model compounds for understanding of

the sol-gel process to form Zr-Si mixed oxides (structure-function relationship), mechanisms and active sites at zirconasilicates in catalytic processes.

Outlook

The Cp moiety has proven to be an excellent leaving group [7]. Also, the thermal elimination of isobutene from zirconium t-butoxides has been explored [8]. A nonhydrolytic cleavage of the organic periphery should be possible and thus offer a way to new extended Si-0-Zr structures.

Acknowledgments: We gratefully acknowledge support of this work by the Deutsche Forschungsgemeinschaft (Schwerpunktprogramm “Silicium-Chemie”) and the University of Bielefeld.

References [ l ] Y. I. Yermakow, B. N. Kuznetsow, V. A. Zakhaxov, Catalysis by Supported Complexes,

Elsevier: New York, 1981; J. M. Thomas, G. Sankar, Acc. Chem. Res. 2001,34,571-581. M. Taramasso, G. Perego, B. Notari, US Patent 4 410 501, 1983; T. Maschmayer, F. Ray, G. Sankar, J. M. Thomas, Nature 1995, 378, 159; R. Hutter, D. C. M. Dutoit, T. Mallat, M. Schneider, A.Baiker, J. Chem. Soc., Chem. Commun. 1995,163. A. 0. Bouh, G. L. Rice, S. L. Scott, J. Am. Chem. Soc. 1999,121,7201; B. Notari, Stud. Surf: Sci Catal. 1988, 37, 413; D. R. C. Huybrechts, L. Bruyker, P. A. Jacobs, Nature 1990, 345, 240; G. Bellusi, A. Carati, M. G. Clerici, M. G, Maddinelli, R. Millini, J. Catal. 1992, 133, 220. F. J. Feher, T. A. Budzichowski, Polyhedron 1995, 14, 3239; F. J. Feher, K. Rahimian, T. A. Budzichowski, J. Ziller, Organometallics 1995, 14, 3920; H. C. L. Abbenhuis, S. Krijnen, R. A. van Santen, Chem. Commun. 1997, 331; M. Crocker, R. H. M. Herold, A. G. Orpen, Chem. Commun. 1997, 241 1; A. Voigt, R. Murugavel, V. Chandrasekhar, N. Winkhofer, H. W. Roesky, H. -G. Schmid, I. U s h , Organometallics 1996, 15, 1610; N. Winkhofer, A. Voigt, H. W. Roesky, Angew. Chem. Int. Ed. 1994, 33, 1352-1354; R. Murugavel, A. Voigt, M. G. Walawalkar, H. W. Roesky, Chem. Rev. 1996, 96, 2205; R. Murugavel, M. Bhattacharjee, H. W. Roesky, Appl. Organometal. Chem. 1999,13,227-243.

[2]

[3]

434 H. M. Lindemann, B. Neumann, H.-G. Stammler, A. Stammler, P. Jutzi

F. J. Feher, J. Am. Chem. SOC. 1986,108,3850; R. Duchateau, H. C. L. Abbenhuis, R. A. van Santen, A. Meetsma, S. K. H. Thiele, M. F. H. van Tol, Organometallics 1998, 17,5663. M. Schneider, B. Neumann, H. G. Stammler, P. Jutzi, Organosilicon Chemistry - From Molecules to Materials IV (Eds.: N. Auner, J. Weis), VCH, Weinheim, 2000; H. M. Lindemann, M. Schneider, B. Neumann, H.-G. Stammler, A. Stammler, P. Jutzi, Organometallics 2002, 21,3009. For clarity, the organic substituents are represented only by their ipso-C atoms: orange: zirconium; red: silicon; blue: oxygen; yellow: nitrogen. P. Jutzi, Comments Inorg. Chem. 1987, 6, 123; P. Jutzi, J. Organomet. Chem. 1990, 400; P. Jutzi, G. Reumann, J. Chem. SOC., Dalton Trans. 2000,2237-2244. K. W. Terry, C. G. Lugmair, T. D. Tilley, J. Am. Chem. SOC. 1997,119,9745.

Preparation of Highly Porous Silicates by Fast Gelation of H-Silsesquioxane

Duan Li Ou, Pierre M. Chevalier*

New Venture R&D, Dow Coming Ltd., Barry CF63 2YL, UK Tel: +44 1446 723 504 - Fax: +44 1446 730 495

E-mail: p.chevalier@dowcorning.com

Keywords: fast gelation, hydrogensilsesquioxanes, aerogel, ambient pressure drying, highly porous hybrid

Summary: Aerogels exhibiting over 75 % porosity have been prepared traditionally by supercritical drying of hydrolyzed alkoxysilanes. Recent work demonstrated the possibility of preparing, under ambient pressure, aerogel-type highly porous materials. However, a multiple-step process was required to reduce the capillary pressure (PJ, which caused shrinkage during the drying step and led to pore collapse. Surface modification of pore walls was also described, to increase the wetting angles between remaining solvent and the pore surface in order to reduce the capillary pressure. We report a fast, single-step route for the preparation of ultrahighly porous materials starting from hydrogensilsesquioxane and thus without any further surface modification. This novel approach relies on increasing the pore radius (rp) to reduce the capillary pressure upon drying. The increment of rp was achieved by the evolution of hydrogen gas during the gelation step, catalyzed by the use of activating agents. Variations of experimental conditions enable production, by a fast gelation process, of ultrahighly mesoporous monolithic materials (30 to 50 A) with a very narrow pore size distribution.

Introduction

Supercritical drying of hydrolyzed alkoxysilanes is a typical method for making aerogels that exhibit over 75 % porosity, leading to unique physical properties for many important applications. Supercritical drying is used to eliminate the capillary force from the pore fluid generated by fluid extraction during the drying process, in order to retain the original pore structure in the wet gel stage. High economic cost and safety concerns have limited the use of areogels despite the fact that they were invented around 1940 [l]. Since 1990, aerogel types of materials have also been prepared using an ambient-pressure drying process in order to address the above issues. Strategies to minimize drying shrinkage caused by capillary forces mainly included increasing the network modulus and decreasing the surface tension.

Ovganosilicon Chemisty V Edited by N. Auner and J. Weis

Copyright 0 2003 WILEY-VCH Verlag GmbH & Co. KgaA, Weinheim

436 D. L. Ou, P. M. Chevalier

Brinker, Smith, and others developed a “spring-back” process to address the issue of shrinkage, in order to maintain the pore structure in the wet gel stage [2-51. In principle, this method involved a series of solvent-exchange processes and modification of the pore inner surface silanol groups by silylation (reaction with trimethylchlorosilane), in order to reduce the contact angle between solvent and pore walls. The gel shrank strongly during evaporation of solvent from the pores, as expected. However, no irreversible narrowing of the pores with formation of Si-0-Si bonds was possible because of the silylation. The gel, therefore, was able to “spring back” to nearly its original size after reaching the critical point. The key to this approach also relied on the strengthening of the modulus of the wet gel, which was achieved by control of the aging conditions and solvent -exchange processes, thus allowing reversible shrinkage in the spring-back process.

Einarsrud and co-workers obtained similarly good results [6, 71. They managed to increase the strength and stiffness of the network drastically by aging the wet gels in solutions of tetraalkoxysilanes in aqueous alcohols, and were thus able to avoid shrinkage during the drying step.

However, the drawbacks of these approaches were that they were labor-intensive, involved complicated procedures and consumption of large quantities of chemical solvents, and required lengthy preparation times. Therefore, the cost-effectiveness of these alternative ambient-pressure processes was not so attractive. In the present studies, our main aim was to address the process issues. Thus a novel single-step process was developed, leading to highly porous aerogel-like THxQy hybrids [(HSi03/2),(Si04/2),], with total pore volumes of up to 2.24 cm3/g and prepared by fast gelation of commercially available hydrogensilsesquioxane resin [(HSi03/&] within minutes. No special precautions were needed for the drying process under ambient pressure. The whole procedure typically lasted 4-5 days and was not labor-intensive.

Pc = 2y~,cos O/rp

Eq. 1.

The capillary pressure Pc developed in the liquid during drying is given by Eq. 1, where nV is the liquid-vapor surface tension, 0 is the wetting angle and rp is the pore radius. The capillary pressure developed during drying impacts the solid network, causing it to shrink. Shrinkage stops when the capillary pressure exerted by the liquid is balanced by the network modulus. In a supercritical fluid, the liquid vapor surface tension, kv, becomes zero; thus no capillary pressure Pc occurs during supercritical drying. The key of Einarsrud’s process is to increase the network modulus to counter the capillary pressure P,. The principle of Brinker’s process is to reduce the P, by reducing the wetting angle 0, and also by increasing the network modulus to counter the capillary pressure P, at the same time. In the present study, we addressed the third parameter, the pore size rp in the wet gel stage, by increasing it in order to reduce the capillary pressure Pc during the drying process.

Preparation of Highly Porous Silicates by Fast Gelation 437

Results and Discussion

Fast Gelation Process

A one step-fast gelation process was employed for the preparation of homogenous THxQy hybrids. Systematic studies were carried out using three selected activating agents: formamide, polyethylene glycol, and ammonium hydroxide. The foam-like wet gels were formed typically within minutes at room temperature under ambient pressure, upon addition of activating agent to hydrogensilsesquioxane resin (FOx@22) solution; the conditions are summarized in Table 1.

Table 1. Formation and percentage of TH to Q conversions in the TH,Qy hybrids obtained.

Entry Activating agent (content [wt%]) Gelation time [min] TA to Q conversion [%I A1

A2

A3

A4

F1

F2

F3

F4

F5

El

E2

E3

E4

E5

c 1

c 2

F4

M1

M2

NH40H (3.58)

NH4OH (0.358)

NHIOH (0.179)

NH40H (0.0358)

formamide (4)

formamide (2)

formamide (0.8)

formamide (0.25)

formamide (0.05)

polyethylene glycol (20)

polyethylene glycol (10)

polyethylene glycol (4)

polyethylene glycol (2)

polyethylene glycol (0.2)

formamide (0.005); NH40H (0.016)

formamide (0.005); NH40H (0,0064)

formamide (0.25), y-butyrolactone (0)

formamide (0.25), y-butyrolactone (10)

formamide (0.25). y-butyrolactone (4)

< 0.5

< 0.5

5

240

< 0.5

1

2

2

4

< 0.5

< 0.5

< 0.5

1

5

1

1

2

< 0.5

< 0.5

100

100

100

100

69.1

35.9

18.7

23.1

-

86.1

79.0

57.1

46.2

18.7

-

57.6

23.1

74.2

56.7

M3 formamide (0.25), y-butyrolactone (2) < 0.5 35.1

Substantial amounts of HZ gas, generated by the hydrolysis of SiH to S O H and further condensation, were released simultaneously to the fast gelation. The hydrolysis-condensation of SiH and gas release were carefully controlled using various amounts of activating agents. Numerous micro-bubbles were formed during the fast formation of the wet gel. The ultrashort gelation times (typically within minutes) allowed a substantial proportion of these bubbles to be

438 D. L. Ou, P. M. Chevalier

trapped inside the wet gel matrix. Therefore the volume of the wet gel was greater than that of its original hydrogen silsesquioxane sol (as shown in Figs. 1 and 2), indicating that the pore size in this wet gel stage was substantially greater than the size of the pores in the wet gels formed by the conventional sol-gel process.

Fig. 1. Hydrogensilsesquioxane solution of FA before gelation.

Fig. 2. TH,Q, hybrid E4 in wet gel stage.

The capillary pressure P, during drying was therefore reduced according to Eq. 1, by increasing in the wet gel pore size rp, and drying shrinkages were decreased during pore fluid evaporation. Highly porous aerogel like THxQy hybrids could then be prepared by this simple procedure; examples of E4 in dry gel stages are shown in Figs. 3 and 4.

Preparation of Highly Porous Silicates by Fast Gelation 439

Fig. 3. T"x@ hybrid E4 in dry gel stage.

Fig. 4. SEM of THxQy hybrid EM in dry gel stage.

A further aim of this work was to gain control of the porosity of the resulting THxQ, hybrid materials through the fast gelation approach. The porosity in this class of materials was derived from the porosity in its wet gel, which was generated by H2 release during the fast gelation. Therefore controlling the amount and release rate of H2 gas should enable us to control the porosity of the resulting THxQy hybrids. This was achieved by the use of carefully selected activating agents such as formamide and polyethylene glycol. It was difficult to monitor the rate of H2 evolution since solvent evaporation also occurred during the fast gelation step. However, the amount of HZ could be evaluated by the amount of SiH lost during the process, determined by 29Si single-pulse MAS NMR.

440 D. L. Ou, P. M. Chevalier

NMR Characterization

The amount of SiH reacted during the fast gelation of hydrogensilsesquioxane was determined by 29Si MAS NMR, comparing the Q species formed with the remaining

SiH is hydrolyzed readily in aqueous base solution such as ammonium hydroxide; 29Si MAS NMR results indicated that a trace amount of NH40H (0.0358 %) was sufficient to hydrolyze 100 % of SiH into SiOH in this hydrogensilsesquioxane dioxane solution. With such a low concentration of activating agent, hydrogensilsesquoxane resin gelled in a relatively long time (4 h), leading to a microporous material. When the concentration of N h O H was higher than 0.18 %, the H-silsesquoxane resin gelled within seconds.

As a nucleophile, formamide appeared to lead to a better control of the compositions of the resulting hybrid gel. The TH to Q conversion varied from 18 to 70 % in this series, depending uponthe amount of formamide used. The proposed mechanism is shown in Scheme 1 [8].

species (Table 1).

Nu

H \ /"

/ Nu 1) H Si-

\ \

\ / / \

2) HOSi-

\ - SiOSi- + / \ 2) H,O

Nu: Forrnarnide

Scheme 1. Proposed mechanism of fast gelation of H-silsesquioxane resin using formamide as activating agent.

This reaction, catalyzed by formamide, involves nucleophilic attack of water on a pentacoordinated silicon atom to release HZ gas with the formation of SiOH groups that can further condense with each other or by reacting with SiH groups to form fully condensed Q4. This

Preparation of Highly Porous Silicates by Fast Gelation 441

activation appeared to be under better control since the amount of pentacoordinated silicon was limited by the amount of nucleophilic formamide used; thus the percentage of TH converted to Q depended upon the amount of formamide in the system.

Polyethylene glycol can also serve as an activating agent for the fast gelation of hydrogensilsesquioxane. The proposed gelation mechanism is shown in Scheme 2.

Scheme 2. Proposed mechanism of fast gelation of H-silsesquioxane resin using polyethylene glycol as activating

agent.

TH

I '

0.2 % polyethylene 11 1 ' Q

c ̂ _ _ _ - _ ..-----. ; ' __ __ _--'

40 60 -80 100 420 140 160 -40 60 -80 -100 -120 -140 -360

a b

10 % polyethylene 4 % polyethylene

140 %a -'WJ -120 :wc -' is0 ~

C d

Fig. 5. 29Si MAS NMR of E series hybrid gels and its precursor hydrogensilsesquioxae resin (a: H resin; b: E5; c:

E3; d: E2).

442 D. L. Ou, P. M. Chevalier

Trace amounts of residual sodium ion (Na') remaining in the commercially supplied polyethylene glycol (272 ppm, determined by ICP) formed a complex with the polyethlylene glycol, and this then acted as activation species. It reacted with moisture to release OH- and H+ species. OH- reacted with SiH to form SOH, thus generating H2 gas and displacing the equilibrium. SiOH then further condensed either with SiH or SiOH, generating further H2 gas or water, and so forming fully condensed Q4 species. This activation also appeared to be controllable since the amount of activation species, polyethylene-sodium complex, was limited. The percentage of TH converted to Q again depended upon the amount of OH- formed in the first step; thus, it depended on the amount of polyethylene glycol-sodium complex, and ultimately on the polyethylene glycol content in the system, as illustrated in the 29Si NMR spectrum in Fig. 5.

Porosity Measurement

Porosity measurements by the NZ sorption method were carried out on THxQy hybrids prepared by the fast gelation of hydrogensilesquioxane resin (Table 2).

Table 2. Porosity data of THQ hybrids prepared by fast gelation of H-silsesquioxane.

Entry BET surface area [m*/g] Total pore volume [cm3/g] Pore characteristics

A1

A2

A3

A4

FI

F2

F3

F4

F5

El

E2

E3

E4

E5

c1

c 2

F4

M1

M2

M3

408

616

766

772

444

788

824

358

387

127

207

277

568

63 1

798

668

358

82 1

928

97 1

2.237

0.879

1.185

0.536

0.446

1.079

0.843

0.360

0.227

0.151

1.430

1.464

1.144

0.520

0.855

0.431

0.360

1.697

1.576

1.704

cylindrical

cylindrical

cylindrical

microporous

cylindrical

cylindrical

cylindrical

ink bottle

microporous

cylindrical

cylindrical

cylindrical

cylindrical

ink bottle

cylindrical

ink bottle

ink bottle

cylindrical

cylindrical

cylindrical

Preparation of Highly Porous Silicates by Fast Gelation 443

Hybrid gels in the A series were formed by using various concentrations of ammonium hydroxide as the activating agent for fast gelation. BJH [9] pore size distributions of this series of hybrid gels are shown in Fig. 6. Hybrid gels in this series consist of both micropores and mesopores. The lowest concentration in the series (0.0358 % m 0 H ) led to a relatively slow gelation (4 h), having mainly micropores with only a small amount of mesopores in the region between 30 to 40 A, whereas increasing the concentration of the activating agent N h O H to 0.179 through 0.358 and up to 3.58 % led to a higher percentage of mesopores in the broad range of 30 to 300 A, until the micropores almost disappeared. The total pore volume increased and the BET [lo] surface area decreased with an increase in of the amount of NHAOH used.

0.0253

- 0.0227 tn \

U

I< 0.0202

u 0.0177 Y

.-. 0.0152 a > 0.0126 w

n # 0.0101 .rl g 0.0076

0.0051

0.0025

0.0000

VI

BJH Desorption Dv(dl

i I I

20 50 ; 200 500 I 2000 10 100 1000

I

Pore Diameter [A1

Fig. 6. BJH pore size distributions of A1 to A4 (o:Al; w: A2; A : A3; +: A4).

Hybrid gels in the F series were formed by using different concentrations of formamide as activating agent for fast gelation. BJH [9] pore size distributions of this series of hybrid gels are shown in Fig. 7. Hybrid gels in this series consist of both micropores and mesopores. The lowest concentration of formamide in this series (0.05 %) led to a mainly microporous material. A concentration of 0.4 % led to a very narrow pore size distribution in the mesopore region between 30 and 55 A, with mainly ink-bottle shape pores. A higher content of formamide led to a reduction in smaller pores and the formation of bigger pores, e.g. 0.8 % formamide led to a bimodal distribution in the mesopore region with the maximum distributions at 40 and 60 A. On increasing the formamide content, the narrow distribution band centered at 40 A decreased dramatically and an almost equal amount of pores centered at 60A were created. When the formamide content

444 D. L. Ou, P. M. Chevalier

increased to fivefold, to 0.4 %, only a trace of the 40 8, pore regime remained and a very broad distribution between 50 and 2008, was observed in the mesopore region. When the formamide content increased to tenfold, to 0.4 %, only trace amounts of mesopores were observed in the 40 8, band with the majority of mesopores collapsing during the fast gelation; the pore size distributions were somewhat similar to those of the lowest concentration in this series (0.05 %). The total pore volumes mainly increased when the formamide content increased from 0.4 to 2 %, and reduced dramatically at a higher content (4 %). This was probably due to the collapsing effect caused by the ultrafast gelation when an excess of activating agent was used. BET [ 101 surface areas were reduced on increasing the content of formamide.

BJH Desorption Dv[d) 0.0461

0.0415

0.0369

0.0323

- 0.0277 a b 0.0231 n r: 0.0184

0.0138

\

U

Y

0

PI

ul ; 0.0092

0.0046

0.0000 I I

I I 20 50 ; 200 500 I 2000 10 100 1000

Pore Diameter [i] Fig. 7. BJH pore size distributions of F1 to F5 ( 0 : F1; w: F2; A: F3; +: F4; 0 : F5).

Hybrid gels in the E series were formed by using various concentrations of polyethylene glycol (EG) as the activating agent for the fast gelation. BJH [9] pore size distributions of this series of hybrid gels are shown in Fig. 8. Hybrid gels in this series contained both micro- and mesopores. The lowest EG concentration in this series (0.2 %) led to a very narrow pore size distribution in the mesopore region between 30 and 55 A, centered at 40 A, with typical ink-bottle characteristics. Higher polyethylene glycol contents led to a reduction of pore size at the low end of the mesoporous region and formation of bigger pores, e.g. 2 % EG led to a great reduction of this size band and the formation of a broad size distribution range from 50 to 300 A, centered at 150 A; 4 % EG led to a very broad size distribution range from 40 to 200 A, centered at 180 A; 10 % EG led to a similar range of broad size distribution from 40 to 2000 A with the bigger pore median at 300 A.

Preparation of Highly Porous Silicates by Fast Gelation 445 ~

Yet the highest percentage in the series (20 %) caused a collapsing effect in the mesopores with a great reduction in the total pore volume. The total pore volume in this series increased when the EG concentration was below 4 % and reached a plateau at up to 10 %, and was reduced dramatically using a 20 % EG concentration. A medium concentration of activating agent led to a medium level of H2 gas formation caused by TH to Q conversion. The medium level of TH to Q conversion gave the highest porosity in the series, as shown in the correlation diagram between the TH to Q conversion and porosity in Fig. 9. This finding was in line with formamide activation (F series). Excess of activating agent led to a much higher amount of HZ gas released during the limited time of the fast gelation, which caused the pores in the wet gel to collapse. The BET [lo] surface area reduced upon an increase of EG content across the whole series.

0.0437

0.0393

‘t 0.0349 0.0306

* 0.0262 d

> 0.0218 n E 0.0175

\

U

.-.

0 .rl e, 0.0131 a

UI 0.0087

0.0044

0.0000

BJH Desorption Dv(d]

I I

I 20 50 ; 200 500 I 2000 5000 10 100 1000

Pore Diameter [ll

Fig. 8. BJH pore size distributions of El to E5 (0 : El; m: E2; A: E3; +: E4; 0: E5).

Conclusion

A novel fast gelation approach was developed for the preparation of highly porous THXQ, [(HSi03/2)x(Si04/2)y] hybrid gel using commercially available hydrogen silsesquioxane. Gelation occurred within minutes. Aerogel-like materials with a total pore volume up to 2.24 cm3/g were obtained after drying at ambient pressure. This simple approach does not require any solvent exchange and surface modifications, as opposed to previous work.

NHdOH, formamide and polyethylene glycol were used as activating agents for the fast gelation,

446 D. L. Ou, P. M. Chevalier

releasing numerous H2 gas bubbles simultaneously to the gelation. The ultrashort gelation time allowed a substantial proportion of these micro-bubbles to be trapped inside the wet gel, forming significantly bigger pores in the wet gel stage compared with the conventional sol-gel process. Capillary pressure during the drying stage was thus reduced, leading to a reduction in the pore collapsing effect. Systematic studies were carried out using these three activating agents. Indeed, the amount of HZ gas released appeared to be controlled by the amount of activating agent, e.g. formamide and polyethylene glycol, according to the solid-state NMR studies. Similar trends in the effect of the level of both activating agents on total pore volume in the resulting THxQy hybrids were observed through the porosity evaluations, with the mid-range concentration leading to the highest total pore volume. Mesopores with a narrow size distribution between 30 to 50 8, were observed in THxQy hybrids generated by the lower concentrations in both series. The simplicity and controllability of this fast gelation process are attractive. Much higher porosity would be achieved if the highly porous wet gel were surface-modified to allow the “spring back” to occur after the drying step.

Acknowledgment: The authors thank Prof. A. C. Sullivan, Prof. R. J. P. Corriu and Dr. I. A. MacKinnon for their valuable advice.

References [ l ] S. S. Kistler, US Patent 2093454, 1937. [2] D. M. Smith, R. Deshpande, C. J. Brinker, Muter. Res. SOC. Symp. Proc. 1992, 271 (Better

Ceramics through Chemistry V), 567-572. [3] S. S. Prakash, C. J. Brinker, A. J. Hurd, M. S . Rao, Nature 1995,431, 375. [4] D. M. Smith, D. Stein, J. M. Anderson, W. Ackerman, J. Non-Cryst. Solids 1995, 186,

104-1 12. [5 ] S . S . Prakash, C. J. Brinker, US Patent 5948482, 1999. [6] M. A. Einarsrud, S. Haereid, WO-B 92/20623, 1992. [7] M. A. Einarsrud, E. Nilsen, J. Sol-Gel Sci. Technol. 1998,13(1/2/3), 317-322. [8] V. Belot, R. Corriu, C, Guerin, B. Henner, D. Leclercq, H. Mutin, A. Vioux, Q. Wang, Muter.

Res. SOC. Symp. Proc. 1990,180, 3-14. [9] E. P. Barett, L. G. Joyner, P. H. Haleuda, J. Am. Chem. SOC. 1951, 73, 373. [lo] S. Brunauer, P. H. Emmett, E. J. Teller, J. Am. Chem. SOC. 1938,60, 309.

Metal Complexes Containing Extended-Reach Siloxypyridine and Related Ligands

David M. L. Goodgame, Paul D. Lickiss, Stephanie J. Rooke, Andew J. P. White, David J. Williams

Department of Chemistry, Imperial College of Science, Technology and Medicine LondonSW72AY,UK

Tel.: +44 207 5945761 - Fax: +44 207 5945804 E-mail: p.lickiss@ic.ac.uk

Keywords: siloxanes, siloxypyridines, metal complexes, polymeric complexes, crystal structures

Summary: Novel, extended-reach ligands, [R2SiO(CH2),py]20 (where n = 0, 1 or 3 and R = Me, Ph or iPr), containing both siloxane and alkoxysilane groups have been prepared and their reactions with a range of transition metal complexes investigated. The reactions of [iPrzSiOpy]20 with CuBr2, CuI and CoC12 give complexes forming infinite one-dimensional ribbons with the ligand bridging two adjacent metals so as to form 24- or 26-membered rings.

The use of extended-reach ligands containing two ligation points that are separated by different spacer units allows a wide variety of polymeric framework materials to be prepared, depending on the size and shape of the ligand and the coordination geometry of the metal center. Bis-pyridyl ligands have found significant use as extended-reach nitrogen donor ligands and have been shown to form complexes comprising large rings, sheets or three-dimensional networks [l]. The work described below was aimed at the preparation and use of extended-reach ligands containing a flexible siloxane group as a spacer unit. Such ligands were anticipated to give new types of structure as they do not have the rigidity of the majority of extended-reach ligands, that are usually based on bipyridine, biphenyl or acetylenic units.

The synthesis of the compounds for use as ligands was achieved via reactions commonly used in alcohol protection chemistry [2], and involved treatment of a chlorosilane with an alcohol in the presence of an organic base as shown in Scheme 1.

Unfortunately, the Si-O-C linkage is susceptible to hydrolysis and so most of the compounds formed in Scheme 1 are not suitable for use in protic solvents. However, the bulky isopropyl groups in [iPr2SiOpy]20 protect the reactive linkage and allow the compound to be used in solvents such as MeOH. The chemistry described below, therefore, relates only to the reactions of [iPr2SiOpy]~O, which is much more convenient to work with.

Ovganosilicon Chemisty V Edited by N. Auner and J. Weis

Copyright 0 2003 WILEY-VCH Verlag GmbH & Co. KgaA, Weinheim

448 D. M. L. Goodgame, P. D. Lickiss, S. J. Rooke, A. J. P. White, D. J. Williams

R 2 S i H o ~ S i R 2

OR' I I

R2SiHo\SiR2 Et,N, toluene I + ROH- CI OR'

I CI

R = i-Pr, Ph or Me R' = 3-pyridyl or 3-CH2py

Scheme 1. The synthesis of siloxanes for use as extended-reach ligands.

The reactions were carried out by adding a solution of the ligand in acetonitrile to a solution of the metal salt, usually in MeOH, and then leaving the solution to stand at room temperature. Crystalline products formed over a period of several days and these were characterized by IR spectroscopy (an increase of between 2 and 15 cm-' for v(Si-0-Py) is found on coordination of the ligand) and elemental analysis, which gives the metal:ligand stoichiometry. X-ray quality crystals were obtained by the layering technique. A wide range of metal salts, MX2 (where M = Cu, X = Br or N03; M = Co, X = CI, Br, NO3 or SCN; M = Ni, X = C1, Br, NO3 or SCN; M = Mn, X = C1, Br, or SCN; M = Zn, X = SCN; M = Cd, X = CI, Br or N03) were treated with solutions of the ligand in a 1:2 ratio to give insoluble complexes. The complexes formed generally have the formula [M(L)2X2] (where L = [iPr2SiOpy]20) as determined by elemental analysis, but the details of the structures could only be determined by X-ray crystallography, and this was done for the complexes [Cu(L)2Br2] and [Co(L)2C12].

The complex [Co(L)2C12] was obtained as violet crystals in 74 % yield from the reaction between the ligand [iPr2SiOpy]20 and CoC12.6H20 in a 2:l ratio. The structure comprises 24-membered macrocycles joined at the metal so as to form a one-dimensional ribbon as shown schematically in Fig. 1.

RPSi ,O,S-iRP

Fig. 1. Schematic drawing to show the linked 24-membered rings in [CO(L)~C~~] , R = iPr.

Metal Complexes Containing Extended-Reach Siloxypyridine and Related Ligands 449

The geometry about the cobalt atom in the structure shown in Fig. 1 is distorted octahedral with four nitrogens from the bridging ligands occupying equatorial positions and two trans chlorine atoms occupying axial positions. The siloxane angle of 168.8' is well within the normal range expected for Si-0-Si linkages. The cobalt atoms form a linear arrangement with a Co .*Co

separation of 10.92 A. The reaction between the ligand [iPrzSiOpy]20 and CuBrz.3HzO in a 2: 1 ratio gave the complex

[Cu(L)2Brz] as blue-green dichroic crystals in 48 % yield. The overall structure is similar to that found for [Co(L)2C12], i.e. 24-membered macrocycles comprising two metals and two ligands linking up to form a one-dimensional ribbon. However, in this case the geometry of the copper atoms is square-based pyramidal with equatorial positions occupied by four nitrogen atoms from the ligands and the apical position occupied by a bromine atom (Cu-Br distance 2.91 A). A second bromine atom approaches at a distance of 3.43 A to fill the vacant axial position. There are two significantly different Si-0-Si angles, 163.6 and 157.6', in the structure which demonstrates the flexible nature of the ligand. Again, the metal atoms are in a linear arrangement and the Cu...Cu

separation is 10.17 A. Treatment of CuI with [iPr2SiOpy]zO gave colorless crystals of [Cu4(L&] in 18 % yield. The

CU(I) iodide complex contains clusters of Cuqh (rather than the single metal atoms for the complexes described above), which link the bridging ligands as shown in Fig. 2.

Fig. 2. Schematic diagram showing the 26-membered ring structure of [Cuq(LJ14], R = iPr.

The Cu& clusters in [Cuq(L2)h] comprise a tetrahedral copper arrangement with each face capped by an iodide ion, a type of cage well documented in the literature [3]. The Cu-Cu distances in the cluster are short and range from 2.62 to 2.83 A, whilst the Cu-I distances range from 2.65 to 2.77 A and are in accordance with other Cu-I distances in similar structures [4]. The copper atoms have essentially tetrahedral coordination geometry, being connected to one nitrogen atom in the

450 D. M. L. Goodgame, P. D. Lickiss, S. J. Rooke, A. J. P. White, D. J. Williams

ligand and three iodide ions (ignoring the Cu-Cu interactions). The Si-0-Si angle is 168.4' and is similar to the CoC12 structure above. The ligand is in a cis conformation and two ligands link two Cuqb clusters together to form 26-membered macrocycles comprising four metals and two ligands. The macrocycles again connect to form a one-dimensional ribbon. The cage-centroid to cage- centroid distance within the ribbon is 9.78 8, and the angle subtended at the centroid (159.3') creates a zigzag arrangement.

This work, together with other X-ray crystallographic work showing the diversity of structures adopted by complexes containing flexible, extended-reach siloxane ligands, has recently been published [5] .

Acknowledgments: We thank the EPSRC for a Research Studentship (to S.J.R.) and for equipment.

References [ l ]

[2] [3]

[4] [5]

M. Kondo, M. Shimura, S. Noro, S. Minakoshi, A. Asami, K. Seki, S. Kiyagawa, Chem. Muter. 2000,12, 1288, and references therein. H. Kaye, S. Chang, Tetrahedron 1970,26, 1369. C . L. Raston, A. H. White, J. Chem. Soc., Dalton Trans. 1976, 2153; W. P. Schaefer, Inorg. Chem. 1986,25,2665; G. Hu, G. J. Mains, E. M. Holt, Inorg. Chim. Acta 1995,240,559. V. Schramm, Inorg. Chem. 1978,17,714. D. M. L. Goodgame, P. D. Lickiss, S. J. Rooke, A. J. P. White, D. J. Williams, Inorg. Chim. Acta 2001, 324, 218; D. M. L. Goodgame, P. D. Lickiss, S. J. Rooke, A. J. P. White, D. J. Williams, Inorg. Chim. Acta 2003,343, 61; A. Fereday, D. M. L. Goodgame, P. D. Lickiss, S . J. Rooke, A. J. P. White, D. J. Williams, Inorg. Chem. Commun. 2002, 5, 805.

Agostic versus Hypervalent Si-H Interactions in Half-Sandwich Complexes of Nb and Ta

Alexei A. Merkulov, Georgii I . Nikonov*

Chemistry Department, Moscow State University Vorob'evy Gory, Moscow, 119899 Russia

Tel: +7 95 939 19 76 - Fax: +7 95 932 88 46 E-mail: nikonov@org.chem.msu.su

Philip Mountford

Inorganic Chemistry Laboratory, University of Oxford South Parks Road, Oxford OX1 3QR, UK

Keywords: nonclassical interactions, agostic bond, hypervalent bond, silicon, hydride

Summary: A series of chlorosilyl hydride complexes of tantalum, C~(AI-N)T~(PM~~)(H)(S~CI,R~-~) (n = 0-3), was prepared and studied by 29Si NMR, X-ray diffraction and DFT calculations. An unprecedented increase of the J(Si-H) coupling constant between the hydride and silyl ligands from 14 Hz for n = 0 to 50 Hz (n = 3) was observed. Reactions of the compounds CpNb(RN)(PR'Me2)2 (R=Ar, Ar'; R= Me, Ph) with chlorosilanes HSiMe2Cl give agostic complexes Cp(RN(Me2Si- H...))Nb(PR'Me2)(Cl) studied by 29Si NMR and X-ray diffraction, whereas reaction of CpNb(ArN)(PMe& with HSiCl3 gives the silylhydrido complex Cp(ArN)Nb(PMe3)(H)(SiCl3).

Introduction

Nikonov et al. have recently reported that niobocene complexes Cp2Nb(SiMe~C1)2(H) (1) and Cp2Nb(SiMe2Cl)(H)2 (2) (Scheme 1) have non-classical interligand hypervalent interactions (IHI) of the type M-H...Si-Cl [l]. Wishing to extend these studies to other ligand systems, we set up an investigation of the isolobal complexes of the type Cp(ArN)M(SiRzCl)(H>(PMe3) (3) [2, 31. Complexes 3 can, in principle, be obtained by the reaction of the precursor Cp(RN)M(PMe3)2 (M = Nb, Ta) with silanes. Studying this reaction resulted in the rich and unexpected chemistry reported here.

Ovganosilicon Chemisty V Edited by N. Auner and J. Weis

Copyright 0 2003 WILEY-VCH Verlag GmbH & Co. KgaA, Weinheim

452 A. A, Merkulov, G. I Nikonov, P. Mounford

3, M = Nb, Ta X = H (l), X = SiMqCl(2)

Scheme 1.

Results and Discussion

Reactions of the tantalum compound Cp(ArN)Ta(PMe& (Ar = 2,6-diisopropylpenyl) with silanes HSiMe3,Cln (n = 1-3) give only compounds of the type Cp(ArN>Ta(SiMe3,Cln)(H)(PMe3) (4) with M I of the type Ta-H...Si-Cl . Important X-ray and 29Si data are given in Table 1. As found for the niobium compounds 1 and 2, complexes 4 exhibit elongated Si-C1 bonds and shortened Ta-Si bonds, signifying the presence of MI. It can be seen that the coupling constant J('H-''Si) increases with rising n, which contradicts the views that the compounds are silane o-complexes, i.e. the interligand bonding in 4 is different from that in silane o-complexes [4]. Optimization of the hydride position in complex Cp(ArN)Ta(SiMe2Cl)(H)(PMe3) by DFT calculations was in accord with the X-ray data [2 , 31. Moreover, DFT calculations showed that the strongest interligand hypervalent interaction in complexes 4 is in Cp(ArN)Ta(SiMezCl)(H>(PMe3) and decreases as the number of chlorine group increases, contrary to the expectations based on the rise of coupling constant J('H-29Si) [5].

Table 1. Selected bond lengths [A] and H-Si coupling constants [Hz].

Compound TaSi Si-Cl(1) Si-CIQ) J(1H-29Si) ~~

Cp(ArN)Ta(SiMeHPh)(H)(PMe3) Cp(ArN)Ta(SiMe2Cl)(H)(PMe3) 2.574(1) 2.174(1)

14

33

Cp(ArN)Ta(SiMeClz)(H)(PMe3) 2.569(2) 2.1 17(2) 2.064(2) 41

Cp(ArN)Ta(SiC13)(H)(PMe3) 50

In contrast, reactions of the niobium complexes Cp(ArN)Nb(PMes)z and Cp(Ar'N)Nb(PMe3)2 (Ar' = 2,6-dimethylpenyl) with the silane HSiMeZC1 result in the formally d2 complexes Cp(q3-RNSiMez-H-. .)Nb(Cl)(PMe3) which are the first examples of compounds with stretched P-agostic Si-H...Nb bonds [2 , 31. For R = Ar a mixture of two isomers, with C1 trans to the Si-H bond (I, major) or cis to it (11, minor), is formed (Figs. 1 and 2), whereas for R = Ar' only the second type of isomer is observed. Isomers I and I1 have very different coupling constants

Agostic versus Hypervalent Si-H Interactions in Complexes of Nb and Ta 453

J('H-29Si): 97 Hz for I and 116 Hz for I1 (Table 2), which means that the Si-H bond in I is in a greater degree of oxidative addition to the metal, i.e. more stretched. The analogous reaction of the complex Cp(Ar'N)Nb(PPhMe2)2 with HSiMe2C1 also exclusively results in the type I1 structure (Fig. 3). Formation of I versus I1 is determined completely by the nature of group R at nitrogen.

I I1

Fig. 1. Isomers of Cp(q3-RNSiMez-H...)Nb(CI)(PMe3) .

Table 2. Selected bond lengths [A] and H-Si coupling constants [Hz].

Compound NbSi Nb-H Nb-Cl J('H-29Si)

Cp(q3-ArNSiMez-H...)Nb(CI)(PMe3) 2.646(2) 1.91(5) 2.497(1) 97

Cp(q3-Ar'NSiMez-H.. .)Nb(CI)(PMe3) 2.795 2.414 116

Cp(q3-Ar'NSiMez-H...)Nb(C1)(PPhzMe) 2.679(2) 2.486(2) 132

Cl51

C W

CH61

Fig. 2. Molecular structure of Cp(q3-ArNSiMez-H...)Nb(C1)(PMe3) .

454 A. A. Merkulov, G. I Nikonov, P. Mountford

Surprisingly, reaction of Cp(ArN)Nb(PMe& with the much more acidic silane HSiCl3 gives the tantalum-like structure of the niobium do silylhydride complex Cp(ArN)Nb(PMe3)(H)(SiCl3). The Nb-Si bond is very short (2.541(4) A) due to the presence of three electron-withdrawing chlorine groups on the silicon atom. The S i x 1 bond lengths are the same within the experimental error (2.088(5), 2.094(7) and 2.098(5) 8).

C

Fig. 3. Molecular structure of Cp(q3-ArNSiMe2-H...)Nb(C1)(PPhMez) .

Conclusion

Reactions of Cp(RN)M(PR'& (M = Nb, Ta) with chlorosilanes give two very different types of complexes, formally d2 agostic silylamido and do hypervalent silylhydrido, depending mainly on the nature of the nature of the silyl group and the substituent R at nitrogen.

Acknowledgment: We are indebted to Dr. N. H. Rees for the help with NMR and Drs. S. R. Dubberley, M. Leech, P. A. Cooke and A. J. Blake, and Prof. J. A. K. Howard for the X-ray determinations. This work was supported by the Royal Society (London) through a joint research grant.

References [ l ] a) G. I. Nikonov, L. G. Kuzmina, D. A. Lemenovskii, V. V. Kotov, J. Am. Chem. SOC. 1995,

117,10133; b) G. I. Nikonov, L. G. Kuzmina, D. A. Lemenovskii, V. V. Kotov, J. Am. Chem. SOC. 1996, 118, 6333 (con-.); c) G. I. Nikonov, L. G. Kuzmina, S . F. Vyboishchikov, D. A.

Agostic versus Hypewalent Si-H Interactions in Complexes of Nb and Ta 455

Lemenovskii, J. A. K. Howard, Chem.-Eur. J. 1999, 5, 2497; d) V. I. Bakhmutov, J. A. K. Howard, D. A. Keen, L G. Kuzmina, M. A. Leech, G. I. Nikonov, E.V. Vorontsov, C. C . Wilson, Dalton Trans. 2000, 1631; e) S. B. Duckett, L. G. Kuzmina, G. I. Nikonov, Znorg. Chem. Commun. 2000,3(3), 126. G. I. Nikonov, P. Mountford, J. C. Green, P. A. Cooke, M. A. Leech, A. J. Blake, J. A. K. Howard, D. A. Lemenovskii, Eur. J. Inorg. Chem. 2000, 1917. G. I. Nikonov, P. Mountford, J. C. Green, P. A. Cooke, M. A. Leech, A. J. Blake, J. A. K. Howard, L.G. Kuzmina, D. A. Lemenovskii, Dalton Trans. 2001,2903. U. Schubert, Adv. Organomet. Chem. 1990,30,151. S . R. Dubberley, S . K. Ignatov, N. H. Rees, A. G. Razuvaev, P. Mountford, G. I. Nikonov, J. Am. Chem. SOC. 2003,125,642.

[2]

[3]

[4] [5 ]

The Reactivity of Platinum Complexes Containing Hemilabile Ligands Towards Silanes and Stannanes

Frank Stohr, Susan Thompson, Dietmar Sturmayr, Jurgen Pfegfer, Ulrich Schubert"

Institute of Materials Chemistry, Vienna University of Technology Getreidemarkt 9, A- 1060 Wien, Austria

E-mail: uschuber@mail.zserv.tuwien.ac.at

Keywords: platinum complexes, silyl complexes, catalysis, exchange reactions

Summary: The reactivity of Pt(n) complexes towards organosilanes is greatly enhanced by hemilabile chelating ligands RZN-R'-PPhz (PnN). (PnN)PtMez reacts with HSiR3 to yield the complexes (PnN)Pt(SiRs)Me and (PnN)Pt(SiR& along with MeSiR3. An excess of HSi(OMe)3 is catalytically converted into Si(OMe)4. PnN-substituted Pt(I1) complexes also catalyze the formation of disiloxanes from HSiR3 and C-CYSi-H exchange reactions. The latter reaction yields dechlorinated hydrocarbons and chlorosilanes. Reaction of (PnN)PtMez with ClSiPhMez results in the stoichiometric formation of Ph2Me4Si2, along with the Pt(w) complex (PnN)Pt(Cl)Me3 and the Pt(n) complex ((PnN)Pt(Cl)Me. The corresponding bisphosphine complex (dppe)PtMez undergoes none of these reactions.

Introduction

Hemilabile ligands are chelating ligands with two different donor centers and are used in coordination chemistry to enhance the reactivity of metal complexes. Their activating effect results from the different metal-ligand bond strengths: while one atom remains coordinated to the metal, the more weakly bonded second center de-coordinates more easily and thus creates a vacant coordination site at the metal. After coordination of a substrate molecule and its reaction at the metal center, the dangling center may re-coordinate with concomitant elimination of a product species. Hemilabile ligands thus promote both oxidative addition and reductive elimination reactions [l]. Another effect associated with this kind of chelate ligand is that an unsymmetrical electronic situation is thus created at the metal, i.e., the sites trans to the two donor atoms have different electronic properties.

In the chemistry of metal-silicon compounds hemilabile auxiliary ligands are hardly used, although oxidative additiodreductive elimination reactions play a much bigger role than in the organometallic chemistry of carbon compounds. We will show that the use of such ligands has

Ovganosilicon Chemisty V Edited by N. Auner and J. Weis

Copyright 0 2003 WILEY-VCH Verlag GmbH & Co. KgaA, Weinheim

The Reactivity of Platinum Complexes Containing Hemilabile Ligands 457

indeed a remarkable effect both on the outcome and the rate of these reactions. All the results discussed for PnN-substituted Pt(I1) complexes are not obtained for the corresponding bis(dipheny1phosphino)methane-substituted complexes under the same conditions.

Reaction of Dimethylplatinum Complexes with Hydrogenosilanes

A series of Pt(I1) dimethyl complexes with hemilabile P,N-chelating ligands, (PnN)PtMez (l), were prepared by reaction of (q4-2,5-norbornadiene)dimethylp1atinum(n), (nbd)PtMez, with PnN [ 2 ] . The strength (or weakness) of the Pt-N interaction can be modified by varying the size and flexibility of the chelate ring, the bulkiness of the substituents at nitrogen and the basicity of the nitrogen donor center (Fig. 1).

PC,N R=Me: PC,N PC3N PC6N

R=Et: PC2NEt

R='Pr: PC,NPr

PCC6N

Fig. 1. Hemilabile P,N-chelating ligands PnN.

According to X-ray structure analyses, the Pt-N distances of selected complexes 1 are in the range of the Pt-P distances - or even longer - despite the smaller bond radius of nitrogen compared to phosphorus. The weak Pt-N interaction results in a strengthening of the Pt-Me bond trans to N; the Pt-C bond lengths trans to N are distinctly shorter than that trans to P. The complexes 1 can thus be considered as T-shaped complexes (R3P)PtMe2, in which the fourth coordination site is screened by a more or less weakly bonded amino group.

Reaction of (PnN)PtMez (PnN = PClN, PC2N, PC6N) with 1,2-bis(dimethylsilyI)benzene yielded cyclic bis(sily1) complexes (Eq. 1) [ 2 ] . This reaction is remarkable, because two equivalents of 1,2-bis(dimethylsilyl)benzene were necessary for the complete reaction of the dimethyl complexes, and one equivalent of 1 -dimethylsilyl-2-trimethylsilylbenzene was formed as a by- product along with methane.

+ 2 Me2HSi MezHsiD Me2HsiD Me3Si

+ CHI

Eq. 1.

Formation of both methane and a methylsilane, with the methyl groups originating from the

458 F. Stohr, S. Thompson, D. Sturmayr, J. Pfeirer, U. Schubert

Pt-Me ligands, was also observed when other hydrogenosilanes were reacted with (PnN)PtMez (again, (dppe)PtMez did not react) [3, 41. The reactivity of the complexes depends on the PnN ligand employed and increases in the order PCzNPr >> PC3N >> PCzNEt > PC2N.

The formation of methysilanes implies that the formation of the bis-silyl complex from the hydrogenosilane is not a simple methyYsily1 exchange but instead a more complex process. The experimental findings suggest the mechanism depicted in Scheme 1 (the products obtained are indicated in bold).

- CH4 + HSiR, -

Scheme 1.

In the reactions of (PnN)PtMez with hydrogenosilanes, other organosilicon products were identified in addition to the methylsilane. For example, (PC2N)PtMez reacted with HSi(0Me)s to yield the Pt(n) complexes (PCzN)Pt[Si(OMe)3]Me and (PCzN)Pt[Si(OMe)3]2 along with methyltrimethoxysilane, tetramethoxysilane, and small amounts of pentamethoxydisiloxane and hexamethoxydisiloxane. The formation of Si(OMe)4 is catalytic, i.e. when HSi(OMe)3 is added to the reaction mixture once the reaction is completed, formation of Si(OMe)4 is resumed [3]. The redistribution of methyl and phenyl groups was also observed [4].

The mechanism of the Si(OMe)4 formation is still unclear. Scrambling reactions of silicon substituents in metal-silyl complexes are not uncommon and are usually explained by intermediate silylene complexes [5]. Formation of an LPt[=Si(OMe)2][Si(OMe)3](OMe) intermediate by migration of one methoxy group from silicon to the metal is possibly facilitated by the hemilabile PnN ligand. Si(OMe)4 is then formed by reductive elimination of the methoxy and the Si(0Me)s ligands.

Catalytic Oxidation of Hydrogenosilanes

When (PC6N)PtMez was reacted with 1,2-bis(dimethylsilyl)benzene, small amounts of 1,1,3,3-tetramethyl-4,5-benzo-2-oxa-1,3-disilacyclopent-4-ene were formed as a byproduct due to the presence of traces of oxygen. A closer investigation of this reaction revealed that the primarily formed bis(sily1) complex ( P C ~ N > P ~ [ O - ( M ~ Z ~ ~ ) ~ C ~ ~ ] reacted with oxygen to give the corresponding bis(si1oxy) complex (PCfi)Pt[o-(OMezSi)zC&], which was isolated and structurally characterized. In the presence of air and catalytic amounts of (PnN)PtMez,

The Reactivity of Platinum Complexes Containing Hemilabile Ligands 459

1,2-bis(dimethylsilyl)benzene was catalytically converted to the cyclic siloxane (Scheme 2 ) [6]. This is the first example of siloxy complexes being observed as intermediates in a catalytic silane oxidation reaction.

+02 i Me2HsiD Me,HSi

Scheme 2.

Catalytic Dehydrocoupling of Hydrogenostannanes

The reactivity of (PC3N)PMe2 towards hydrogenostannanes was completely different. Catalytic dehydrogenative dimerization occurred when HSnBu3 or HSnPh3 was reacted (Eq. 2) . MethyYstannyl exchange appears to be the initial step of the reaction. Redistribution reactions of organostannanes were also observed to a minor extent. The dehydrogenative stannane coupling is also catalyzed by (dppe)PtMe2, although the (PnN)PtMe2 complex is more active [7].

cat. (PnN)PtMez 2HSnR3 - Sn2% + H2

Eq. 2.

Activation of Si-Cl and C-Cl Bonds

The complexes (PnN)PtMe2 activate not only Si-H bonds but also Si-Cl bonds. For example, [(I?-P,N)-P~~PCH~CH~NM~Z]P~M~~ reacted stoichiometrically with PhMeZSiCl to give the disilane Ph2MedSi2, the Pt(IV) complex [(I?-P,N)-P~zPCH~CH~NR#~(CI)M~~ and the Pt(n) complex [ (? -P ,N)-P~~PCH~CHZNR~]P~(C~)M~ [6]. A possible reaction mechanism is shown in Scheme 3 (the observed products are indicated in bold).

The complexes (PnN)PtMe2 activate not only Si-Cl bonds but also C-Cl bonds, and a very interesting spectrum of products, i.e. the complexes (PnN)Pt(CI)Me, (PnN)Pt(C1)Me3 and (PnN)Pt(Cl)C(CI)=CH2, was found in their reaction with CCl4 [8]. The activation of both Si-H and

460 F. Stohr, S. Thompson, D. Sturmayr, J. Pfeifier, U. Schubert

C-Cl bonds in the same reaction led to the development of a highly efficient catalytic Si-WC-C1 exchange reaction (Eq. 3) [9].

Scheme 3.

cat. (PnN)PtMe2 PhMe2SiH + HCCl3 P PhMezSiCl + H2CC12/CH3CYC&

Eq. 3.

The chloro(methy1) complexes [ (I?-P,N)-P~~PCHZCHZNRZ]P~(C~)M~ are even more active as catalysts than the dimethyl complexes. The catalytic activity increases in the series R = Me < Et < Pr, i.e. parallel to the weakening of the Pt-N interaction. The reaction was tested for the reaction of ally1 chloride, Ph3CC1, CHC13, 1,1,2,2-tetrachloroethane or H2CCl2 with PhMeZSiH; the reactivity of the chlorides decreased in the given order.

Theoretical calculations have shown that the driving force for the overall reaction is the formation of the very stable Si-Cl bond. New systems to convert critical halogen-containing organic compounds into more innocent compounds based on this reaction can be envisaged.

i

Conclusions

The above results clearly demonstrate that metal-mediated stoichiometric or catalytic reactions of organosilanes can be promoted by employing hemilabile chelating ligands. The reactivity of the complexes can be varied somewhat by modifying the properties of the P,N-chelating ligand (type and length of spacer group between P and N; substituents at N).

The redistribution reactions mentioned above indicate that the opening of a coordination site not only promotes oxidative addition and possibly o-bond metathesis reactions, but - differently from carbon compounds - also induces migrations of silicon substituents with the concomitant formation of silylene complex intermediates. It was pointed out by Tilley and co-workers that the transfer of a silyl substituent from silicon to platinum is easier in three- than in four-coordinated

The Reactivity of Platinum Complexes Containing Hemilabile Ligands 461

complexes [lo]. The formation of the three-coordinate complex is of course easier with a PnN ligand than with two PR3 or a bisphosphine ligand.

Acknowledgment: We thank the Fonds zur Forderung der wissenschaftlichen Forschung for the support of our work.

References [ l ] a) C. S. Slone, D. A. Weinberger, C. A. Mirkin, Progr. Inorg. Chem. 1999, 48, 233; b) P.

Braunstein, F. Naud, Angew. Chem. 2001, 113, 702; Angew. Chem. Znt. Ed. 2001,40, 680. A. Bader, E. Lindner, Coord. Chem. Rev. 1991,108,27. J. Pfeiffer, G. Kickelbick, U. Schubert, Organometallics 2000,19, 62. J. Pfeiffer, U. Schubert, Organometallics 1999, 18,3245. S. Thompson, F. Stohr, G. Kickelbick, U. Schubert J. Organomet. Chem., submitted. L. S. Chang, M. P. Johnson, M. Fink, Organometallics 1991, 10, 1219. D. C. Pestana, T. S. Koloski, D. H. Berry, Organometallics 1994,13,4173. L. K. Figge, P. J. Carroll, D. H. Berry, Organometallics 1996, 15, 209. H. Yamashita, M. Tanaka, M. Goto, Organometallics 1992, 11, 3227. M. D. Curtis, P. S. Epstein, Adv. Organomet. Chem. 1981,19,213.

[6] J. Pfeiffer, U. Schubert, Organometallics 2000,19,957. [7] S. Thompson, U. Schubert, Inorg. Chim. Acta 2003, in press. [8] F. Stohr, D. Sturmayr, G. Kickelbick, U. Schubert, Eur. J. Inorg. Chem. 2002,2305. [9] F. Stohr, D. Sturmayr, U. Schubert, Chem. Commun. 2002,2222. [lo] J. D. Feldman, G. P. Mitchell, J.-0. Nolte, T. D. Tilley, J. Am. Chem. SOC. 1998, 120, 11184.

G. P. Mitchell, T. D. Tilley, Angew. Chem., Int. Ed. 1998,37,2524.

[2] [3] [4] [5]

57Fe-Mossbauer Spectra and X-ray Structures of Dipolar Ferrocenylhexasilanes

Harald Stiiger, * Hemtann Rautz, Guido Kickelbick, Claus Pietzsch

Institut fur Anorganische Chemie, Technische Universitat Graz, Stremayrgasse 16, A-8010 Graz, Austria

Tel.: +43 316 873 8708 -Fax: +43 316 873 8701 E-mail: Stueger@anorg.tu-graz.ac.at

Keywords: polysilanes, ferrocene, crystal structure, Mossbauer spectroscopy

Summary: Linear and cyclic ferrocenylhexasilanes 1,6-Fc-Si&fel2-X [X = Me, -C6H4CH=C(CN)2], 1,3- and 1,4-Fc-Si6Melo-X [ x = Me, <&CH=C(CN)2], have been synthesized and their Mossbauer spectra have been measured. All compounds possess localized electronic structures on the Mossbauer timescale and partial Fern character of the iron atom due to Cp+(Si,) electron transfer. The Mossbauer data obtained, furthermore, indicate considerable aryl-ferrocenyl interaction via the hexasilane moiety. The single-crystal X-ray structures of 1,3- and 1 ,~-F~-S~~M~IG--C&CH=C(CN)~ exhibit the cyclohexasilane ring in chair conformation with the bulky Fc and aryl substituents in equatorial positions, while the open-chained analogue exhibits a perfect all-trans geometry.

Introduction

Organic and organometallic materials with second-order nonlinear optical properties have been the subject of intensive investigation that led to the development of certain structure/NLO property relationships [ 11. Recently several reports have appeared in the literature concerning the synthesis and the characterization of dipolar silicon compounds showing an extended transparency range combined with nonlinear optical activities [2]. For disilanes containing ferrocene as a donor and various organic acceptor groups, only moderate optical nonlinearities have been found independently by Pannell and co-workers [3] and in our laboratories [4]. For the corresponding cyclohexasilane derivatives, however, we observed significantly increased values of the quadratic hyperpolarizabilities p which are mainly ascribed to the increased ability of the cyclohexasilane ring to act as a conjugated bridge [5]. Now we present the results of X-ray crystallography and 57Fe-Mossbauer spectroscopy studies performed on l-ferrocenyl-6-[(2,2-dicyanoethenyl)phenyl]- dodecamethylhexasilane (l), l-ferrocenyl-4-[(2,2-dicyanoethenyl)phenyl]decamethylcyclohexa- silane (2) and l-ferrocenyl-3-[(2,2-dicyanoethenyl)phenyl]decamethylcyclohexasilane (3) in order

Ovganosilicon Chemisty V Edited by N. Auner and J. Weis

Copyright 0 2003 WILEY-VCH Verlag GmbH & Co. KgaA, Weinheim

57Fe-Mossbauer Spectra and X-ray Structures of Dipolar Ferrocenylhexasilanes 463

to further assess intramolecular donor/acceptor interactions.

Synthesis

Compounds 1-3 were synthesized using slightly modified published procedures (Scheme 1) [2a]. Recrystallization of the crude reaction products of 2 and 3 from heptane at -80 "C exclusively affords orange red crystals of the pure e,e isomer (trans in the case of 2 and cis in the case of 3). The proposed structures are consistent with MS and NMR data and with the results of C, H, N elemental analyses.

~~ w/

* Cl-@-C,H,Br 1) CF,SO,H

Cl-@-Cl BrC,H4++C6H4Br 2) Licl

Li Fe

a *C6H4Br l)BuLi/DMF, - Fe 2) CH,CN,

G=a

1: @ = -(SiMe,),- I \ 0 -0

2:@= -0, ,- 0 - 0

0-0 , \

3: @ = -0, ,. 0 - 0

qZ9Si) = -16.60(1) -17.69(1)

-43.24( 1) -41.81(1)

aZ9Si) trans-e,e isomer = -39.94(1) 429Si) cis-e,e isomer= -38.93(1) -38.61(1) -39.01(1) -42.65(1) -40.68(2) -41.00(2) -41.64(1) -40.99(1) -41.08(2) -41.19(1)

Scheme 1. Synthesis and "Si NMR chemical shifts 6 (ppm vs. ext. TMS, hydrocarbon solution) of 1-3 (number of

Si atoms in brackets; = SiMe,, SiMe).

X-ray Crystallography

Drawings of the molecular structures of 1, 2 and 3 with atom labeling together with selected bond lengths and angles are depicted in Figs. 1-3. The hexasilane chain in 1 exhibits a nearly perfect all- trans geometry, while the six-membered rings in 2 and 3 adopt slightly distorted chair conformations. The bulky substituents in the 1,3 or 1,4 positions, respectively, occupy equatorial sites in order to minimize nonbonding interactions. The aromatic rings are arranged roughly perpendicular to one of the adjacent Si-Si bonds as shown by the respective torsional angles, thus providing a perfect basis for effective o(Si-Si)-n(aryl) hyperconjugation. The average Si-Si distances and the average Si-Si-Si bond angles found for 1-3 exhibit quite unexceptional values. Compounds 1,2 and 3 crystallize in centrosymmetric point groups.

464 H. Stiiger, H. Rautz, G. Kickelbick, C. Pietzsch

1 N2

C51

C24

Fig. 1. Molecular structure of 1. dsisi (mean) = 234.3 pm; dsi(l)c(ll) = 186.9 pm; dsi(6)C(41) = 186.2 pm;

ds~(,, ,~) (mean) = 187.4 pm; Lsisisi (mean) = 112.2'; Lcsic (mean) = 109.3"; LSi(I)Si(Z)C(II) = 105.3";

&(6)Si(5)C(41) = 104.5"; LN(I)C(61)C(52) = 177.5"; Lsisisisi (mean) = 178.7"; Lsi(z)si(i)C(II)c(iz) = -90.0";

LSi(5)S1(6)C(41)C(42) = -94.7'; LC(U)C(5l)C(52)C(61) = 0.4"; LC(45)C(U)C(51)C(52) = -10.4".

2

C32 U6

Nt

57Fe-Mossbauer Spectra and X-ray Structures of Dipolar Ferrocenylhexasilanes 465

Mossbauer Spectra

57Fe Mossbauer spectra of the compounds 1-3 were recorded at -80 "C together with the spectra of the reference substances l-ferrocenyl-2-[(2,2-dicyanoethenyl)phenyl]te~amethyldisilane (4), 1- ferrocenyltridecamethylhexasilane (5) and ferrocenylundecamethylcyclohexasilane (6) in order to probe the nature of the iron sites. Mossbauer effect spectral parameters are presented in Table 1. Figure 4 shows the experimentally obtained spectrum of 1.

Table 1. Mossbauer data for compounds 1 4 (6: isomeric shift relative to GFe; E: quadrupole splitting; I : intensity).

Doublet Singlet

S[mm/s] E [mds] I [ % ] s [mmlsl I [%I

0 H

1 0 k z 0.53(4) z--+.+ 2 wk 0.544(6)

GzD

H

0.532(4)

3 - C t K z e

4 WL: 0.528( 1) 0

0

5- 0.526( 1)

0.529(3)

2.313(8)

2.36( 1)

2.332(9)

2.323(3)

2.320(2)

2.335(2)

Each spectrum displays a doublet and a broad singlet with isomer shift (6) and quadrupole splitting (E) values typical for Fe" and Fe"' sites, which implies that all compounds have localized electronic structures on the Mossbauer timescale (lo-' s) [6, 71. The appearance of the Fern signals is consistent with Cp+(Si,) electron transfer resulting in partial oxidation Fen+Fem. In general, ferrocenyl groups (Fe") give spectra with large quadrupole splitting in the range 2.0-2.2 mm/s, while the spectra of ferricinium cations (FeIn) are characterized by small or vanishing quadrupole splitting [8]. The enhanced intensity of the Fe"' signal in the spectra of 1 ,2 and 3 as compared to 4 clearly indicates the increased electron-accepting character of the larger hexasilane frameworks.

466 H. Stuger, H. Rautz, G. Kickelbick, C. Pietzsch

IM)5 ~

I M W -

0

a - 4 0995 ii 5

- I e

e, b 0990 ~

0985 -

electron transfer is more pronounced, which is a clear indication of the transmission of the electron-accepting properties of the -Ph-CH=C(CN)* group via the hexasilane framework. In conclusion, the Mossbauer analysis performed in this study provides further evidence for the increased ability of larger polysilane frameworks to act as mediators for electronic effects, which is consistent with the results of recent cyclovoltammetric studies and NLO measurements [5].

* .. . . .

09801 , , , , , I , , , I , I , , , , -6 -5 -4 -3 -2 - 1 0 I 2 3 4 5 6

velocitv in mm/s Acknowledgment: We thank the Fonds zur

Fig. 4. ”Fe Mossbauer spectrum of 1 (T = -80 “C). Forderung der Wissenschaftlichen Forschung (Wien, Austria) for financial

support within the Spezialforschungsbereich “Elektroaktive Stoffe” and the Wacker Chemie GmbH (Burghausen, Germany) for the donation of silane precursors.

References a) N. J. Long, Angew. Chem. 1995, 107, 37; b) Materials for Nonlinear Optics (Eds.: S. R. Marder, J. E. Sohn, G. D. Stucky), ACS Symposium Series Vol. 455, ACS, Washington DC, 1991; c) Nonlinear Optical Properties of Organic Molecules and Crystals (Eds.: D. S. Chemla, J. Zyss), Academic Press, New York, 1987. a) G. Mignani, M. Barzoukas, J. Zyss, G. Soula, F. Balegroune, D. Grandjean, D. Josse, Organometallics 1991, 10, 3660; b) D. Hissink, P. F. van Hutten, G. Hadziioannou, J. Organomet. Chem. 1993,454,25. H. K. Sharma, K. H. Pannell, I. Ledoux, J. Zyss, A. Ceccanti, P. Zanello, Organometallics 2000,19,770. Ch. Grogger, H. Siegl, H. Rautz, H. Stuger in Organosilicon Chemistry IV: From Molecules to Materials (Eds.: N. Auner, J. Weis), Wiley-VCH, Weinheim, 2000, p. 384. Ch. Grogger, H. Rautz, H. Stuger, Monatsh. Chem. 2001,132,453. W. Palitzsch, C. Pietzsch, K. Jacob, F. T. Edelmann, T. Gelbrich, V. Lorenz, M. ht tnat G. Roewer, J. Organomet. Chem. 1998,554,139. C. Pietzsch, A. Kirsten, K. Jacob, F. T. Edelmann, 2. Phys. Chem. 1998,205,271. T. Y. Dong, D. N. Hendrickson, K. Iwai, M. J. Cohn, S. J. Geib, A. L. Rheingold, H. Sano, I. Motoyama, S. Nakashima, J. Am. Chem. SOC. 1985,107, 7996.

Dipolar 1,2-N,N-Dimethylaminomethylferrocenyl Complexes for Nonlinear Optics?

Christian Beyer,* Uwe Bohme, Gerhard Roewer

Institut fur Anorganische Chemie, Technische Universitat Bergakademie Freiberg Leipziger Stral3e 29, D-09596 Freiberg, Germany

Tel.: +49 3731 39 4346 -Fax: +49 3731 39 4058 E-mail: christian.beyer@chemie.tu-freiberg.de

Cluus Pietzsch

Institut fur Angewandte Physik, Technische Universitat Bergakademie Freiberg Bernhard-von-Cotta-Stral3e 4, D-09596 Freiberg Germany

Keywords: 1,2-N,N-dimethylaminomethylferrocenyl, ferrocene, charge transfer, silicon, Mossbauer spectroscopy

Summary: A series of substituted 1,2-N,N-(dimethylaminomethyl)ferrocenyl compounds with organic acceptor groups were synthesized and characterized by 'H NMR, I3C NMR, 29Si NMR, ES-MS, R, UVNis and 57Fe Mossbauer spectroscopy. The final products are stable in air. X-ray structure analyses of compounds with two or one phenyl groups are not so easily accessible, therefore crystalline derivatives were obtained by using hydrogen chloride or picric acid. "Fe Mossbauer spectroscopy gives strong evidence of a significant electronic coupling between the ferrocenyl unit and the organic acceptor moiety of the molecules in the ground state. Such compounds should have some special optical properties.

Introduction

There is a growing interest in the synthesis of materials for quadratic nonlinear optics (NLO) because of their potential use in optoelectronics, telecommunications, optical computing and optical storage devices [l]. Recent investigations in our group have shown that the reaction of (R,S)-l,2- (N,N-dimethylaminomethy1)ferrocenyllithium [2] with halogenosilanes leads to interesting (R,S)- 1,2-(N,N-dirnethylaminomethyl)ferrocenylsilanes [ 3-51. The advantage of the 1,2-(N, N -dimethylaminomethyl)ferrocenyl group is the possibility of derivatization to get crystalline compounds. In principle the crystalline state is the best medium for generating NLO phenomena because of the high packing density of the active components. In an effort to investigate systematically the molecular properties of dipolar (R,S)-1,2-(N,N-dimethylaminomethyl)-

Ovganosilicon Chemisty V Edited by N. Auner and J. Weis

Copyright 0 2003 WILEY-VCH Verlag GmbH & Co. KgaA, Weinheim

468 C. Beyer, U. Bohme, C. Pietzsch, G. Roewer

ferrocenylsilanes, we have started the synthesis and characterization of such compounds. Here we report the syntheses, spectroscopic properties and crystallographic characterization of new 1,2-(N,N-dimethylaminornethyl)ferrocenylsilanes with organic acceptor groups.

Synthesis of Crystalline Derivatives

The new dipolar (R,S)-2-(N,N-dimethylaminomethyl)fe~ocenyl(aryl)silanes (R,S)-FcNSiMe, -(C&4X)m { n = 2-0, m = 1, X = p-F (5); m = 2, X = p-F (6); m = 3, X = p-F (7); and m = 1, X = p-Br (14)} were formed by reaction of 2-dimethylaminomethylferrocenyllithium FcNLi with chloroarylsilanes ClSi(Me),(C&X), { n = 2-0, m = 1, X = p-F (1); m = 2, X = p-F (2); m = 3, X = p-F (3); and m = 1, X = p-Br (11)). The treatment of 5, 6, 14 and 15 with gaseous hydrogen chloride or picric acid resulted in the formation of the hydrochloride complexes 4, 10, 13 and the picrates 8,9,16 and 18 ( see Schemes 1 and 2).

FcNLi =Li%

,N\

CI

F

\ \ 9 /s\'-+&-F @--F CI F +L CI

FcNLi - LlCl 1 FcNLi - LlCl

,N\

FcNLi - LiCl

The treatment of 14 with LiR or Mg and DMF (see Scheme 2) resulted in the formation of (R,S)-2-(N,N-dimethylaminomethyl)ferrocenyl(4-fo~ylphenyl)~methylsilane 15. The crystal structures of 7,9, 13, 16 and 18 were determined by single-crystal X-ray analyses (see Figs. 1-4). Furthermore, we are extending our strategy of derivatization of the

Dipolar 1,2-N,N-Dirnethylaminomethylferrocenyl Complexes for Nonlinear Optics? 469

N,N-(dimethylaminomethy1)ferrocenylsilanes with chiral acids, with the aim of separating the diastereomers.

Characterization

Fi31

Fi2J

Crystal system: Triclinic

Space group: P i

Independent reflections: 5643

R indices [ I > 2 o(l)]: R1 = 0.0605

R indices (all data): R2=0.1143

Selected bond lengths [A]: Si-C(2,14,20,26) 1.86-1.87

F( 1,2,3)-C( 17,23,29) 1.35-1.37

Fig. 1. Molecular structure of (R,S)-FcNSi(C6H4F)3 (7).

470 C. Beyer, U. Bohme, C. Pietzsch, G. Roewer

Fig. 2. Molecular structure of (R,S)-FCNS~M~~(C~H~CHO) picrate (18).

Crystal system: Triclinic

Space group: P i

Independent reflections: 6010

R indices [I> 2 o(l)]:

R indices (all data):

Selected bond lengths [A] and angles ["I:

R1= 0.0652

R2 = 0.1272

Si-C(2,14,15,16) 1.85-1.88

O(1 )-C(22) 1.18

N( 1 )-H( 1) 1.05

H(1)-.0(2) 2.66

Fig. 3. Molecular structure of (R,S)-FcNSiMez(C&HBr)CL0.5 H20 (13).

Crystal system: Monoclinic

Space group: p21c

Independent reflections: 4666

R indices [I > 2 o(Z)]:

R indices (all data):

Selected bond lengths [A] and angles ["I: Si-C(2,14,15,16) 1.86-1.89

Br-C( 19) 1.89

N(l)-H(l) 1.03

N( 1)-H( 1)- 161.1

R1 = 0.0724

R2 = 0.1273

Crystal system: Monoclinic

Space group: p2,1a

Independent reflections: 5919

R indices [I> 2 o(I)]:

R indices (all data):

Selected bond lengths [A] and angles ["I: Si-C(2,14,15,16) 1.85-1.88

Br-C(l9) 1.90

N( 1)-H( 1) 0.91

N( 1)-H( 1)-O( 1) 165.6

R1 = 0.0925

R2 = 0.1456

Fig. 4. Molecular structure of (R,S)-FcNSiMe2(C6&Br) picrate (16).

Dipolar 1,2-N,N-Dimethylaminomethylferrocenyl Complexes for Nonlinear Optics? 471

'"1

4 4 4 3 .* 4 I I 1 1 4 I c Dism . . . . . . . . . . . _ I

veloei0. i" d s

t=15h T = 8 0 K Fe 2+: 90% Fe 3 + : 10%

"1

. . . . . . . . . . . . < d 4 J I - l I I 1 1 . $ 6

vebdly i n d s

t = 1 4 4 h T = 1 7 3 K Fe 2+: 55 % Fe 3+: 45 %

'= 1

-1

4 d 4 J .2 -1 0 I I 1 4 5 6 vebdly in d s

t = 2 8 h T=298K Fe 2+: 45 % Fe 3+: 55 %

la,

am . . . . . . . . . . . . 4 d . 4 J a . l o 3 2 % . 5 6

veloeiy in d s

t = 4 6 h T = 7 8 K Fe 2+: 92 % Fe 3+: 9 %

. . . . . . . . . . . . 6 d 4 J .2 4 I I * 3 4 3 6

velodty in m d s

a%

t = 4 6 h T = 1 7 3 K Fe 2+: 83 % Fe 3+: 17 %

. . . . . . . . . . * J 1 . 3 4 . , 0 I 1 f 1 5 6

rcbdty lonmk

uoc. 4

t = 4 6 h T=298K Fe 2+: 91 % Fe 3+: 24 %

Fig. 5. 57Fe Mossbauer spectra of (R,S)-FcNSiMe2(C6H&r) picrate (16) (top) and (R,S)-FcNSiMe(C6&F)z

picrate (9) (bottom) at different temperatures.

Table 1. 57Fe Miissbauer spectroscopic data. The Mossbauer spectra recorded at different ~ ~~

Compound T [K] DT ['I [%]

16 80 10

175 45

295 55

9 80 9

175 17

295 24

10 80 13

175 23

295 not resolved

4 80 5

7 80 11

temperatures of compounds (16) and (9) are shown in Fig. 5. Table 1 contains the results of these measurements. The observation of two doublets points to the presence of Fe2+ and Fe3+. The doublet with large quadrupole splitting was created by the iron in the ferrocene skeleton (Fe2'), and the one with small quadrupole splitting indicates fenicinium (Fe3'). The increasing amount of Fe3' with rising temperature is an indication of significant electronic coupling between the ferrocenyl unit and the organic acceptor in the ground state.

[a] Degree of transformation Fez+ - Fe3'

472 C. Beyer, U. Bohme, C. Pietzsch, G. Roewer

Acknowledgments: This work was supported by the Deutsche Forschungsgemeinschaft and the Fonds der Chemischen Industrie.

References a) G. Mignani, A. Kramer, G. Puccetti, I. Ledoux, G. Soula, J. Zyss, R. Meyrueix, Organometallics 1990, 9, 2640; b) H. K. Sharma, K. H. Pannel, I. Ledoux, J. Zyss, A. Ceccanti, P. Zanello, Organometallics 2000, 19, 770; c) C. Grogger, H. Rautz, H. Stuger, Monatsh. Chem. 2001, 132, 453; d) H. Rautz, H. Stiiger, G. Kickelbick, C. Pietzsch, J. Organomet. Chem. 2001, 627, 167; e) C. Grogger, H. Siegl, H. Rautz, H. Stuger, in Organosilicon Chemistry IV, From Molecules to Materials (Eds.: N. Auner, J. Weis), Wiley- VCH, Weinheim, 2000, p. 384-388. G. Marr, J. Organomet. Chem. 1967,9, 147. W. Palitzsch, C. Pietzsch, M. Puttnat, C. Jacob, K. Merzweiler, P. Zanello, A. Cinquantini, M. Fontani, G. Roewer, J. Organomet. Chem. 1999,587, 9. W. Palitzsch, C. Pietzsch, K. Jacob, F. Edelmann, T. Gelbrich, V. Lorenz, M. Puttnat, G. Roewer, J. Organomet. Chem. 1998,554, 139. C. Pietzsch, A. Kirsten, K. Jacob, F.T. Edelmann, Z. Physik. Chem. 1998,205, 271.

Metallo-silanols - Precursors for the Generation of Novel Metallo-siloxanes and Metallo-

heterosiloxanes’

Marc0 Hofmann, Matthias Vogler, Dirk Schumacher, Wolfgang Malisch * Institut fur Anorganische Chemie der Universitat Wurzburg

Am Hubland, D-97074 Wurzburg, Germany Tel.: +49 931 888 5277 -Fax: +49 931 888 4618 E-mail: Wolfgang.Malisch@mail.uni-wuerzburg.de

Keywords: iron, oxygenation, silanols, condensation, siloxanes

Summary: Transition metal fragment-substituted silanols, silanediols and silanetriols represent valuable precursors for the controlled generation of novel metallo-siloxanes and -heterosiloxanes due to their reduced tendency for self-condensation compared to ordinary organosilanols. Here we report a) the synthesis of a novel class of bis(metal1o)-silanols of iron and tungsten as well as condensation reactions with dimethylchlorosilane resulting in the formation of bis(metal1o)-siloxanes; b) the synthesis of group 13 and group 4 heterosiloxanes from the reaction of iron-silanols, -silanediols and -silanetriols with triorganoalanes, -gallanes, -indanes and metallocene dichlorides of titanium and zirconium, respectively; c) the synthesis of ferriomethyl-siloxanes and group 4 heterosiloxanes derived from femomethyl-silanols.

Introduction

Metallo-silanols of the general type LnM-Si(R)3-x(OH)x (x = 1-3) are characterized by a reduced acidity of the Si-OH proton due to the electron-releasing effect of the metal fragment. This property leads to a remarkably high stability towards self-condensation and allows the isolation of some of these “special” silanols [l-81, mainly of the chromium and iron group, including examples with stereogenic metal and silicon atoms [3] and even metallo-silanediols [4] and -silanetriols [5]. The synthesis of these species has mainly been accomplished by using two complementary routes: the hydrolysis of metallo-chlorosilanes in the presence of an auxiliary base and the oxofunctionalization of metallo-hydridosilanes with dimethyldioxirane [9], as outlined in Scheme 1 for the example of CsR5(OC)zFe-SiMezOH (R = H, Me) [6].

Part 27 of the series “Metallo-Silanols and Metallo-Siloxanes”. In addition, Part 55 of the series “Synthesis and Reactivity of Silicon Transition Metal Complexes”. For Part 26/54, see Ref. [I].

Ovganosilicon Chemisty V Edited by N. Auner and J. Weis

Copyright 0 2003 WILEY-VCH Verlag GmbH & Co. KgaA, Weinheim

Metallo-silanols - Precursors for Novel Metallo-siloxanes and Metallo-heterosiloxanes 477

10

Fig. 5. Bis(rnetal1o)-siloxanes.

9a 11

Eq. 2. Synthesis of the bis(femo)-disiloxanol 11 via oxygenation of 9a.

Group 13 Heterosiloxanes Derived from Ferrio-silanols

The ferrio-silanols 12a-c, although characterized by a lowered acidity compared to organosilanols, react with the triorganoalanes, -gallanes and -indanes 13a-c to yield the transition metal fragment-substituted heterosiloxanes 14a-i via elimination of alkane (Eq. 3). Molecular weight determination of 14b,c,h,i shows the presence of dimers in solution, derived from an intermolecular interaction of the Lewis acidic group 13 element and the Lewis basic oxygen atom of the SiO unit.

Me h e 121 a b c 1 3 1 8 b c 141. b c d e f g h i

E Al Ga In Al Ga In Al Ga In

R Me Ph CPr E Al Ga In R Me Me Me Ph Ph Ph i-Pr CPr i-Pr

Eq. 3. Synthesis of the heterosiloxanes 14a-i.

478 M. Hofmann, M. Vogler, D. Schumacher, W. Malisch

Using the chiral ferrio-silanols Cp(0C)zFe-SiMe(R)OH [R = Ph (12d), R = p-To1 (12e)], the aggregation to dimers of the resulting heterosiloxanes [Cp(OC)2Fe-Si(Me)(R)O-EMez]z [R = Ph, E = Ga (15a); R = p-Tol, E = In (15b)l can easily be determined due to the formation of a 1:l mixture of diastereomers (RRISS, meso), leading to three E-CH3 signals in the 'H and 13C NMR-spectra, respectively, two for the unlike-product and one for the like-product.

These results clearly indicate that an increased 0-Fi-n interaction (E = In, Ga), induced by the Cp(0C)ZFe-fragment at the silicon, is not sufficient to prevent dimerization.

In addition, the dimeric structure is confirmed by X-ray analysis of 14e,i for the solid state (Fig. 6). The structure-determining element is the rhombic, four-membered EzOz ring, a common feature of hetero atom-substituted group 13 dialkyl compounds [ 141. Heterosiloxanes 14e,i show a butterfly geometry with torsion angles of 11.24(4)' (14e) and 17.49(5)' (14i), respectively. The group 13 element-oxygen bond lengths [O-Ga 1.9815(11)/1.9754(11) 8, (14e); @In 2.1794(13)/2.1671(13)

(14i)l correspond to those found in the literature for alkoxy-substituted dialkylgallanes [15] and lie in the range between a o-bond and a coordinative bond.

a> b)

Fig. 6. Molecular structure of a) 14e and b) 14i. Hydrogen atoms are omitted for clarity.

s /OH I ,,,

\ '''.XYi R

+ EMe, + - CH,

1 2 -

oc '.a,,,, oc--Fe,

Si - /: R OH

Me Me \ 5:

? \ Me Me

- si + \ /co

Fe I,,,,,,,

a b c 1 3 / b c l 7 i a b c d e

R Me Ph Me Ph pTol

Me Ph pTol E Ga In E Ga Ga In In In

Eq. 4. Synthesis of the heterosiloxanols 17a-e.

Metallo-silanols - Precursors for Novel Metallo-siloxanes and Metallo-heterosiloxanes 479

The reaction of the ferrio-silanediols 16a-c with 13b,c, respectively, results in the formation of the heterosiloxanols 17a-e (Eq. 4). Only one OH function of 16a-c is transformed, independently of the amount of trimethylgallane or -indane used.

The same finding is valid for the reaction of the ferrio-silanetriol Cp(OC)2Fe-Si(OH)3 (18) with GaMe3 (13b). In n-hexane at room temperature the ferrio-heterosiloxanediol Cp(OC)2Fe-Si(OH)2 -0GaMe3 is initially formed. However, in the presence of two equivalents of THF and more severe reaction conditions (65 'C) the drum-shaped heterosiloxane 19 with iron-substituted silicon atoms is produced (Eq. 5). In this reaction, THF acts as a donor to stabilize the intermediates and to guarantee a controlled condensation procedure.

2 Fp-Si(OH), 4 GaMe, (13b) Et,O

b

- 6 CH,, - 4 Et,O

Fp = Cp(OC),Fe

18 CiOl c102

19

Eq. 5. Synthesis of the drum-shaped heterosiloxane 19.

Heterosiloxane 19 crystallizes with Ci symmetry and is isostructural with Roesky's compound [(2,6-iPr2C6H3)N(SiMe3)Si(OGaMe2)(0GaMe)O]2 [ 161. The structure is composed of two six- membered Ga203Si rings, which are linked by two Ga-0 and two Si-0 bonds. While the six- membered rings adopt a boat conformation, the four-membered rings are almost planar (Sil-01- Ga2-02 4.92'). The Ga-0 bonds containing p2-oxygen atoms are found to be significantly shorter than those containing p3-oxygen atoms [Ga2-03 1.8212( IS), Ga2-01 1.9585( 13), Ga2-02 1.9615(18) A].

Group 4 Heterosiloxanes Derived from Ferrio-silanols

Metallo-silanols also proved to be synthetically valuable in the context of the generation of SiO-bridged polynuclear transition metal complexes [ 171. Especially the Et3N-assisted condensation reactions with cyclopentadienyl-titanium and -zirconium chlorides result in nearly quantitative conversions due to the oxophilic nature of the group 4 metals.

Et3N-assisted condensation of the ferrio-silanol Cp(OC)(Ph3P)Fe-Si(Me)(Ph)OH (20), employed as the (RWSS)-diasteromer, and CpTiCl3 produces the diastereomerically pure heterosiloxane 21 as a brownish red microcrystalline powder in a yield of 76 % (Fig. 7). The heterosiloxanes 23a-d are prepared analogously by the condensation of the ferrio-silanediols Cp(OC)zFe-Si(R)(OH)2 [R = Me

480 M. Hofmann, M. Vogler, D. Schumacher, W. Malisch

(16a), p-Tol(16c)l with two equivalents of CpzMCl2 [M = Ti (22a), Zr (22b)l in diethyl ether (Fig. 7). The RSiO2-bridged trinuclear compounds are isolated as yellow-orange (23a,c) or beige (23b,d) microcrystalline powders, respectively, in yields of 49-80 %. Compounds 23a-d are insoluble in n- pentane and are only slightly soluble in benzene or THF.

21 q-sxe R Me Me p-To1 p-To1

Fig. 7. Group 4 heterosiloxanes 21 and 23a-d.

Using a 1 : 1 stoichiometry, the reaction of 16a,c with Cp2ZrC12 (22b) proceeds in a different way and results in the formation of the eight-membered, cyclic heterosiloxanes 24a,b (Eq. 6), which are isolated as light beige powders in yields of 53-56 % and can be stored under an atmosphere of nitrogen for several weeks without decomposition.

In the case of Ma, a cisltrans isomer ratio of 1:l is detected, whereas in the case of Mb, the formation of the cis isomer is preferred with the p-tolyl substituents on the same side of the ring plane (cisltrans 10: 1).

+

?-= R Me p-To1 22b

+ 4 NEt3

- 4 [HNEt,]CI

D

241 a b

R 1 Me p-To1

Eq. 6. Synthesis of the cyclic heterosiloxanes 24a,b.

Metallo-silanols - Precursors for Novel Metallo-siloxanes and Metallo-heterosiloxanes 481

The analogous reactions of 16a,c with CpzTiC12 (22a) according to Eq. 6 lead to a complex product mixture. However, using Cp(OC)zFe-Si(Ph)(OH)2 (16b) and toluene as a solvent, the SiOH-functionalized heterosiloxanol 25 can be isolated as orange, microcrystalline powder in a good yield of 89 % after a reaction period of 24 h (Eq. 7).

16b 22a 25

Eq. 7. Synthesis of the heterosiloxanol25.

The condensation of all OH functions with formation of the heterosiloxane 26a containing four metal centers is observed in the case of the reaction of the ferrio-silanetriol Cp(OC)zFe-Si(OH)3 (18) with an excess of Cp2TiClz (22a). Employing a 1:2 stoichiometry of 18 and 22a, the trinuclear heterosiloxanol 26b is obtained, whereas in the case of the phosphine-substituted ferrio-silanetriol Cp(OC)(Ph3P)Fe-Si(OH)3 condensation with an excess of 22a stops at the dinuclear species 27 (Fig. 8). The SiOH-functional heterosiloxanes 26b and 27 show no tendency for controlled condensation to give cage-like metallo-siloxanes.

Compounds 23-27 represent the first examples of a novel class of heterodi-, tri- and tetranuclear metal complexes involving an RSi02 or a Si03 bridging unit.

q-&+ Fig. 8. Trinuclear (26a,b) and dinuclear (27) heterosiloxanols.

Ferriomethyl-silanols, -siloxanes and -heterosiloxanes

27

In more recent studies [18], we were looking for metallo-silanols in which the stabilizing effect of

482 M. Hofmann, M. Vogler, D. Schumacher, W. Malisch

the transition metal group is decreased in order to get access to metal-substituted oligo- and polysiloxanes via controlled self-condensation reactions. In this context, we have synthesized the metallo-silanols 28-30 in which the metal and the silanol group are separated by a methylene spacer group (Fig. 9). Compounds 28-30 have been generated either by EtsN-assisted hydrolysis of SiC1-functionalized precursors or by oxofunctionalization of the corresponding SiH-functional metallomethyl-silanes with dimethyldioxirane.

291 a b

L 1 CO Me,P

301 a b

R I Me Ph

R / M e Ph Me R I Ph Me

Fig. 9. Femomethyl-silanols 2%30

The same synthetic procedures have been used for the generation of the ferriomethyl-silanediols 31a,b and the silanetriol32 (Fig. 10).

OH

3 1 1 a b

HO

32

L I CO PPh,

Fig. 10. Femomethyl-silanols 31a,b and silanetriol32.

At room temperature, the Cp(OC)zFe-CH2-substituted ferriomethyl-silanols 28a, 31a and 32 exhibit an enhanced reactivity towards self-condensation compared to the analogous Fe-Si systems; as a consequence, 28a has already exhibited conversion to the 1,3-(bisferriornethyl)-disiloxane 33 (Fig. 11) [ 181. In the case of 31a and 32 complex product mixtures are formed at room temperature after storage for several days (32) or weeks (31a), presumably due to self-condensation. Controlled condensation reactions of 28a, 31a,b and 32 with organochlorosilanes such as Me2Si(H)Cl are

Metallo-silanols - Precursors for Novel Metallo-siloxanes and Metallo-heterosiloxanes 483

L

possible, leading to the ferriomethyl-siloxanes 34a-d (Fig. 11).

CO CO PPh, CO

n 1 2 2 3

I

33

Fig. 11. 1,3-(Bisfeniomethyl)-disiloxane 33 and ferriomethyl-siloxanes 34a-d.

The SiOH function of ferriomethyl-diorganosilanols is also suitable for condensation reactions with group 4 metal chlorides, as proved for Cp(OC)zFe-CHz-SiMezOH (28a), which on treatment with an excess of Cp2MC12 [M = Ti (22a), M = Zr (22b)l produces the ferriomethyl-heterosiloxanes 35a,b (Fig. 12) [19].

Further reaction of 35b with one equivalent of 28a leads to the nearly quantitative formation of the trinuclear heterosiloxane 36 (Fig. 12) [19]. Compounds 35a,b and 36 are isolated as orange (35a) and light- yellow (35b, 36) microcrystalline powders, respectively and show good solubility in toluene and diethyl ether.

+ Fig. 12. Femomethyl-heterosiloxanes 35a,b and trinuclear heterosiloxane 36.

36

The structures of 35b and 36 reveal substantial d,(Zr)-px(0) bonding interaction indicated by Zr-0 bond distances of 1.9621(10) (35b) and 1.9545(13)-1.9724(14) (36) as well as Zr-0-Si bond angles of 160.61(7) (35b) and 161.69(9)-167.84(9)O (36), respectively. In the case of 36 two crystallographically independent molecules are found in the asymmetric unit (Fig. 13) which show a significant difference in the conformation of the Zr(0SiCFe)z backbone. In the one case a chair-like arrangement is found with a mirror plane including Zr2 and the bisector of the 010-Zr2-09 angle. In the other case a twist conformation can be observed with proximately C2

484 M. Hofmann, M. Vogler, D. Schumacher, W. Malisch

symmetry of the Zr(OSiCFe)* unit.

c7

12

3 C

08

Fig. 13. Molecular structure of 36 showing the two crystallographically independent molecules.

Acknowledgment: This work has been generously supported by the Deutsche Forschungsgemeinschaft (Schwerpunktprogramm: “Spezifische Phanomene in der Siliciumchemie”).

References [ l ] W. Malisch, M. Hofmann, M. Vogler, D. Schumacher, A. Sohns, H. Bera, H. Jehle, in:

Silicon Chemistry - From Small Molecules to Extended Systems (Eds.: P. Jutzi, U. Schubert), Wiley-VCH, Weinheim, in press. a) C. S. Cundy, M. F. Lappert, C. K. Yuen, J. Chem. SOC., Dalton Trans. 1978, 427. b) L. S. Chang, M. P. Johnson, M. J. Fink, Organometallics 1991, 10, 1219. c) W. Adam, U. Azzena, F. Prechtl, K. Hindahl, W. Malisch, Chem. Ber. 1992,125, 1409. d) H. Handwerker, C. Leis, R. Probst, P. Bissinger, A. Grohmann, P. Kiprof, E. Herdtweck, J. Bluemel, N. Auner, C. Zybill, Organometallics 1993, 12, 2162. e) W. Malisch, S. Schmitzer, G. Kaupp, K. Hindahl, H. Kab, U. Wachtler, in: Organosilicon Chemistry: From Molecules to Materials, Vol. 1 (Eds.: N. Auner, J. Weis), Wiley-VCH, Weinheim, 1994, p. 185. f) W. Malisch, S. Schmitzer, R. Lankat, M. Neumayer, F. Prechtl, W. Adam, Chem. Ber. 1995, 128, 1251. g) W. Malisch, K. Hindahl, H. Kab, J. Reising, W. Adam, F. Prechtl, Chem. Ber. 1995, 128, 963. h) R. Goikhman, M. Aizenberg, H.-B. Kraatz, D. Milstein, J. Am. Chem. SOC. 1995,117, 5865. i) S. Moller, 0. Fey, W. Malisch, W. Seelbach, J. Organomet. Chem. 1996, 507, 239. j) W. Malisch, S. Moller, R. Lankat, J. Reising, S. Schmitzer, 0. Fey, in: Organosilicon Chemistry II: From Molecules to Materials (Eds.: N. Auner, J. Weis), Wiley-VCH, Weinheim, 1996, p. 575. k) S. H. A. Petri, D. Eikenberg, B. Neumann, H.-G. Stammler, P. Jutzi, Organometallics 1999,18,2615. W. Malisch, M. Neumayer, 0. Fey, W. Adam, R. Schuhmann, Chem. Ber. 1995,128,1257.

[2]

[3]

Metallo-silanols - Precursors for Novel Metallo-siloxanes and Metallo-heterosiloxanes 485

[4]

[5]

[6]

[7] [8]

W. Malisch, R. Lankat, 0. Fey, J. Reising, S. Schmitzer, J. Chem. SOC., Chem. Commun. 1995,1917. a) C. E. F. Rickard, W. R. Roper, D. M. Salter, L. J. Wright, J. Am. Chem. SOC. 1992, 114, 9682. b) W. Malisch, R. Lankat, S. Schmitzer, J. Reising, Inorg. Chem. 1995,34, 5701. a) W. Adam, U. Azzena, F. Prechtl, K. Hindahl, W. Malisch, Chem. Ber. 1992,125, 1409. b) W. Malisch, M. Hofmann, G. Kaupp, H. Kab, J. Reising, Eur. J. Inorg. Chem. 2002, 3235. W. Malisch, H. Jehle, C. Mitchel, W. Adam, J. Organomet. Chem. 1998,566,259. a) W. Malisch, M. Vogler, in: Organosilicon Chemistry N: From Molecules to Materials (Eds.: N. Auner, J. Weis), Wiley-VCH, Weinheim, 2000, p. 442. b) W. Malisch, H.-U. Wekel, I. Grob, F.H. Kohler, Z. Natu~orsch. Teil B 1982, 37, 601. c) H.-U. Wekel, W. Malisch, J. Organomet. Chem. 1984,264, CIO. a) W. Adam, A. K. Smerz, Bull. SOC. Chim. Belg. 1996, 105, 581. b) W. Adam, J. Bialas, L. Hadajiarapolou, Chem. Ber. 1991,124, 2377. c) W. Adam, R. Mello, R. Curci, Angew. Chem. 1990,102,916;Angew. Chem. Int. Ed. 1990,29, 890.

[lo] a) W. A. Herrmann, J. G. Kuchler, J. K. Felixberger, E. Herdtweck, W. Wagner, Angew. Chem. 1988,100,420; Angew. Chem. Int. Ed. 1988,27,394. b) W. A Herrmann, F. E. Kiihn, Acc. Chem. Res. 1997,30, 169. c) W. A. Herrmann, R. W. Fischer, W. Scherer, M. U. Rauch, Angew. Chem. 1993, 105, 1209; Angew. Chem. Int. Ed. 1993,32, 1157. d) W. A. Herrmann, R. W. Fischer, D. Marz, Angew. Chem. 1991, 103, 1706; Angew. Chem. Int. Ed. 1991, 30, 1638.

[ 111 a) D. Schmidt-Base, U. Klingebiel, Chem. Ber. 1990, 123, 449. b) K. Dippel, 0. Graalmann, U. Klingebiel, Z. Anorg. Allg. Chem. 1987, 552, 195. c) 0. Graalmann, U. Klingebiel, J. Organomet. Chem. 1984,275, C1.

[12] W. Malisch, M. Vogler, D. Schumacher, M. Nieger, Organometallics 2002,21,2891. [13] W. Malisch, M. Vogler, H. Kab, H.-U. Wekel, Organometallics 2002,21,2830. [ 141 a) F. Schindler, H. Schmidbaur, Angew. Chem. 1965, 77, 206; Angew. Chem. Int. Ed. 1965,4,

201. b) H. Schmidbaur, F. Schindler, Chem. Ber. 1966, 99, 2178. c) F. Schindler, H. Schmidbaur, Angew. Chem. 1967, 79,697; Angew. Chem. Int. Ed. 1967,6,683.

[15] a) M. B. Power, W. M. Cleaver, A. W. Apblett, A.R. Barron, J.W. Ziller, Polyhedron 1992, 11,477. b) D. C. Bradley, D. M. Frigo, M. B. Hursthouse, B. Hussain, Organometallics 1988, 7, 1 1 12.

[16] a) R. Murugavel, A. Voigt, M. G. Walawalkar, H. W. Roesky, Chem. Rev. 1996,96,2205. b) R. Murugavel, V. Chandrasekhar, H. W. Roesky, Acc. Chem. Res. 1996,29,183.

[17] W. Malisch, J. Reising, M. Schneider, in: Organosilicon Chemistry III: From Molecules to Materials (Eds.: N. Auner, J. Weis), Wiley-VCH, Weinheim, 1998, p. 415.

[ 181 a) W. Malisch, M. Hofmann, M. Nieger, in: Organosilicon Chemistry IV: From Molecules to Materials (Eds.: N. Auner, J. Weis), Wiley-VCH, Weinheim, 2000, p. 446. b) W. Malisch, M. Hofmann, M. Nieger, W. W. Scholler, A. Sundermann, Eur. J. Inorg. Chem., accepted for publication.

[19] M. Hofmann, W. Malisch, D. Schumacher, M. Lager, M. Nieger, Organometallics 2002, 21, 3485.

[9]

Half-Sandwich Complexes of Iron and Tungsten with Silanol-Functionalized

Cyclopentadienyl Ligand’

Andreas Sohns, Holger Bera, Dirk Schumacher, Wolfgang Malischa

Institut fur Anorganische Chemie der Universitat Wiirzburg Am Hubland, D-97074 Wurzburg, Germany

Tel.: +49 931 888 5277 -Fax: +49 931 888 4618 E-mail: Wolfgang.Malisch@mail.uni-wuerzburg.de

Keywords: iron, tungsten, half-sandwich complexes, metallo-silanols

Summary: The silylcyclopentadienyl iron complexes [XMe2si(CsH4)](0C)~Fe-Me (X = H, OMe) (4a,b) are synthesized from the corresponding carbonyl metallates M [ F ~ ( C O ) ~ ( C S H ~ - S ~ M ~ ~ X ) (M = Li, Na) (3a-c) with methyl iodide. The novel iron fragment substituted silanol [ (HO)M~~S~(CSH~) ] (OC)~F~-M~ (5) is obtained either by hydrolysis of [(MeO)Me2Si(C&)](OC)2Fe-Me (4a) or by reaction of [HMe2Si(Csb)](OC)2Fe-Me (4b) with C02(CO)g, followed by treatment with HzO. Tungsten complexes of the type [(MeO)Me2Si(CH2),(CsH4)1(0C)3WH [n = 1 @a), 3 (8b)l are generated by reaction of (C5h)(CH2)Si(OMe)Mez (6) with NaH and W(CO),,

followed by protonation with HOAc, or treatment of the cyclopentadiene (C5H4)(CH2)3Si(OMe)Me2 (6b) With (MeCN)3W(CO)3, respectively. Hydrolysis of 8a,b leads directly to the disiloxanes (HW(C0)3[C5H4(CH2)nMe2Si]}2-0 [(n = 1 (lOa), 3 (lob)] via the intermediate silanols [(HO)Me2Si(CH2),C&](OC)3WH [n = 1 (9a), 3

( 9 ~ 1 .

Introduction

The preparation of catalysts by immobilizing active transition metal fragments on solid supports such as inorganic surfaces, organic polymers and related materials attracts widespread attention [2-71. Most studies are concentrated on phosphine ligands to fix catalysts on solid supports [8]. Surprisingly cyclopentadienyls have only marginally been studied as anchoring ligands, although a great number of organometallic compounds containing cyclopentadienyl ligands are known [4, 9-11]. In this context we have established simple synthetic routes for the generation of functionalized q5-silylcyclopentadienyl complexes of iron and tungsten, especially derivatives with

Part 28 of the series “Metallo-Silanols and Metallo-Siloxanes”. In addition, Part 56 of the series “Synthesis and Reactivity of Silicon Metal Complexes”. For Part 27/25, see Ref. [l].

Ovganosilicon Chemisty V Edited by N. Auner and J. Weis

Copyright 0 2003 WILEY-VCH Verlag GmbH & Co. KgaA, Weinheim

Half-Sandwich Complexes of Iron and Tungsten with Silanol-Functionalized Ligand 487

a silanol group, realized before, to the best of our knowledge, only in the case of [(HO)M~ZS~(C~H~)](OC)~MOR (R = H, Cl) [12].

Results

Crucial reagents in the case of the iron derivatives are the cyclopentadienyl-functionalized alkali metallates 3a-c which are obtained by the reaction of the methoxysilyl-cyclopentadiene C5H5- SiMezOMe with Fe(C0)s to give the dimeric iron complex 1, followed by cleavage with sodium amalgam. An alternative approach starts with the ferrio-silanes 2a,b which on treatment with lithium diisopropylamide yield the metallates 3b,c. This base-induced reaction involves deprotonation of the cyclopentadienyl ligand followed by an anionic silyl migration from the iron to the cyclopentadienyl ring [13]. The metallates 3a,b readily react with methyl iodide to yield the methyl iron complexes 4a,b (Scheme 1).

1 t+ e S i M e , X

+

/ 5 M- oc co

MF X OMe OMe H

+ Me1

- MI - @-- SiMe,X

y R Me Me

+t&+ Scheme 1. q5-Silylcyclopentadienyl iron complexes 3a-c and 4a,b.

The transformation of the SiOMe or SiH unit of 4a,b into a SiOH group can be realized by hydrolysis of 4a or by reaction of 4b with Coz(C0)8 in the presence of triethylamine, followed by treatment with water. The silanol 5 is obtained as a yellow crystalline solid in 62-85 % yield (Scheme 2).

+ 1/2 Co,(CO)$NE~. YO FSiMezH *,d Fe

T S i M e z O M e Fe + H,O T S i M e 2 O H - - MeOH -- HNEtJCo(C0)J oc"/ \ M ~

0 0 4a 5

Generation of the silanol5. Scheme 2.

4b

A single-crystal X-ray diffraction analysis of 5 (Fig. la) reveals a pseudo-octahedral conformation of the iron atom and hydrogen bonding resulting in the formation of dimeric units

488 Andreas Sohns, Holger Bera, Dirk Schumacher, Wolfgang Malisch

indicated by the oxygen-xygen distance of 2.703 8, (Fig. lb).

a)

Fig. 1. Molecular structure of [(HO)Me2Si(C5H4)](OC)PFe-Me (5).

The synthesis of the silanol-functionalized tungsten complexes 8a,b starts with the deprotonation of the cyclopentadiene 6a, followed by the reaction with W(CO), to give the metallate 7. Final

protonation with acetic acid yields 8a. A second method to generate this type of tungsten complex is realized in the case of 8b by treatment of the cyclopentadiene 6b with (MeCN),W(CO), in

refluxing THF (Scheme 3).

+ NaH, W(CO)@ ~ s i ( o M e ) M z

Na+ w 111,,,. -3 CO. i /Z H,

p S i ( O M e ) M e ,

I \ co 68 oc' -co

7

+HOAc -LiOAc I 6b

Scheme 3. q5-Silylalkylcyclopentadienyltungsten complexes 8a,b.

Hydrolysis of 8a,b leads directly to the disiloxanes 10a,b. Due to fast self-condensation the primarily formed silanols 9a,b cannot be isolated (Scheme 4).

Further experiments have dealt with the generation of more stable tungsten silanols via ligand exchange at the metal and the reactivity of the iron-substituted silanol5 in controlled condensation reactions.

Half-Sandwich Complexes of Iron and Tungsten with Silanol-Functionalized Ligand 489

+z Scheme 4. Hydrolysis of 8a,b.

References [I] M. Hofmann, M. Vogler, D. Schumacher, W. Malisch, in Organosilicon Chemistry V: From

Molecules to Materials (Eds.: N. Auner, J. Weis), Wiley-VCH, Weinheim, 2003.p. 473. [2] M. Capka, Collect. Czech. Chem. Commun. 1990,55,2803. [3] D. C. Bailey, S . H. Langer, Chem. Rev. 1981,81, 109. [4] B. L. Booth, G. C. Ofunne, C. Stacey, P. J. T. Tait, J. Organomet. Chem. 1986,315, 143-156. [5] U. Deschler, P. Kleinschmit, P. Panster, Angew. Chem. 1986,98,237. [6] A. Reissovi, M. Capka, Synth. React. Inorg. Met.-Org. Chem. 1986,16(5), 707. [7] J. Cermfik, M. Kvicalovi, V. Blechte, M. Capka, Z. Bastl, J. Organomet. Chem. 1996, 509,

77. [8] L. L. Murrell, in Advanced Materials in Catalysis, (Eds: J. J. Burton, R. L. Garten),

Academic Press, New York, 1977,236. [9] A. Reissovk, Z. Bastl, M. Capka, Collect. Czech. Chem. Commun. 1986,51, 1430. [lo] M. Capka, A. Reissovi, Z. Bastl, M. Capka, Collect. Czech. Chem. Commun. 1989,54, 1760. [ 111 G. C. Ofunne, B. L. Booth, P. J. T. Tait, Indian J. Chem. 1988,27A, 1040. [12] F. J. De la Mata, P. Giner, P. Royo, J. Organomet. Chem. 1999,572, 155. [13] G. "hum, W. Ries, D. Greissinger, W. Malisch, J. Organomet. Chem. 1983, 252, C67; S. R.

Berryhill, G. L. Clevenger, F. Y. Burdurlu, Organometallics 1985,4, 1509.

Synthesis and Electrochemical Properties of Silanes with Iron-Containing Donors

Helmut Fallmann, Gottfrid Fi ipaj , HaraM Stiiger, Christa Grogger*

Institut fur Anorganische Chemie, Technische Universitat Graz Stremayrgasse 16, A-8010 Graz, Austria

Tel.: +43 316 873 8217 -Fax: +43 316 873 8701 E-mail: grogger @ anorg.tu-graz.ac.at

Keywords: electrochemistry, cyclic voltammetry, chlorosilanes

Summary: In order to investigate the influence of iron-containing substituents on the redox behavior of oligosilanes a series of dicarbonylcyclopentadienyliron (Fp)- and ferrocene (Fc)-substituted silanes was subjected to cyclovoltammetric studies together with several reference compounds. UV-Vis and CV data indicate considerable donor-acceptor interaction in FpSi2Me4-C&CH=C(CN)z (1). Furthermore, electrochemical synthesis of Fp-substituted silanes is reported.

Introduction

Recently, several reports have appeared in the literature concerning the synthesis and characterization of dipolar silicon compounds showing an extended transparency range combined with nonlinear optical activities [ 11, materials of considerable interest for future technological applications. Although it is well established that the 0-electrons in polysilanes are delocalized rather effectively along the silicon backbone, resulting in unusual electronic properties like low ionization potentials of these polysilane chains [2], silicon was found to be only a weak charge transmitter when donor and acceptor moieties are connected by permethylated Si-Si chains and the resulting optical nonlinearities are just moderate [3, 41. In order to compare the donor properties of dicarbonylcyclopentadienyliron (Fp = q5-C5H5Fe(C0)2) and ferrocene (Fc), the compounds shown in Fig. 1 have been investigated by UV absorption spectroscopy and cyclic voltammetry, together with compounds 2-7, 9, 10 and 12, which represent reasonable model systems to estimate the properties of the donor and the acceptor in the absence of interaction (see Table 1).

Electrochemical Synthesis of Fp-Substituted Silanes

Based on a work by Ruiz et al., who showed that it is possible to generate silicon-iron bonds electrochemically when using dicarbonylcyclopentadienyliron dimer (Fp2) and Me3SiC1 as

Ovganosilicon Chemisty V Edited by N. Auner and J. Weis

Copyright 0 2003 WILEY-VCH Verlag GmbH & Co. KgaA, Weinheim

Synthesis and Electrochemical Properties of Silanes with Iron-Containing Donors 491

substrates (Eq. 1) [ 5 ] , we extended this method to different mono- and oligosilanes, thus yielding a variety of iron substituted silyl compounds.

D=Fp, @ 1 = Si,Me, (1)

D=Fc, @= Si,Me, (11)

D=Fc, Si = Si,Me,,, (8) 0 Fig. 1. Compounds 1,s and 11.

Fp2 +2R3SiCl Mg’e- >2FpSiR3 +MgC12

Eq. 1.

As shown in Table 1, electrochemical activation of Mg anodes is very suitable for the synthesis of silanes containing the Fp substituent and high yields of the appropriate target molecules were obtained using a simple undivided cell with a stainless steel cathode, a Mg anode and THFVBu4NBr as electrolyte. Compared to the multi-step conventional chemical route, preparation of 2, for instance, is simplified to a one-pot reaction with an overall yield of 62 % based on ClSiMe2SiMeZCl.

Table 1. Electrochemical synthesis of Fp-substituted silanes.

Starting silane Product [ % yield] No. Starting silane Product [ % yield] No.

PhMezSiSiMezCl Fp-SiMe2SiMe2Ph (62) 2 Ph2SiC12 Fp-SiPhzC1 (49) ‘a’ 15

Me3SiSiMezCl Fp-SiMe2SiMe3 (56) 3 CIMezSiSiMezC1 Fp-SiMezSiMe2-Fp (62) 16

Me3SiC1 Fp-SiMe3 (66) 4 ClMezSiSiMe2Cl Fp-SiMe2SiMe2C1 (60) 17

MezSiClz Fp-SiMezCl(57) 13 (Me3Si)3SiC1 F ~ - s i ( S i M e ~ ) ~ (65) 18

MePhSiClz Fp-SiMePhCI (54) 14

[a] Products not isolated, based on GCMS and NMR data.

But there are limits for the electrochemical pathway. The difunctional derivative FpSi2Me&&I&H=C(CN)2 (1) could not be prepared electrochemically. When Fpz is electrolyzed together with ClSiMe2SiMe2<&-CH=C(CN)2, the activated double bond of the dicyanovinyl group reacts preferentially and a complex mixture of products is obtained. Therefore, 1 was made by starting from 1 -chloro-2-(p-bromophenyl)tetramethyldisilane and using a reaction sequence similar to the procedure already applied successfully for the preparation of the ferrocenyldisilane Fc-SiMe2SiMe2-Ph-CH=C(CN)z [3]. Preparation of the Fc-substituted compounds was described previously [3,4].

492 H. Fallmann, G. FiirpaJ, H. Stiiger, C. Grogger

UV Absorption Spectra

UV-Vis data of compounds 1-12 are summarized in Table 2; Figure 2 exhibits the absorption spectra of the donor-acceptor-substituted disilane derivatives 1 and 11 together with appropriate model compounds containing the donor and acceptor moieties in the absence of interaction. The absorption spectrum of the Fc derivative 11, exhibiting the acceptor band near 335 nm and the weak ferrocene band near 450 nm, apparently fulfills the classical expectation for nonconjugatively connected chromophores, being identical to the sum spectrum of the isolated donor and acceptor models (Fig. 2A). Other compounds containing Fc as a donor behave similarly [4].

P M e S f M e M e S i M e

A

Q

250 300 350 400 450 250 300 350 400 450

wavelength [nm] wavelength [nml

Fig. 2. UV absorption spectra of 1 and 11 with those of model

compounds (3.5 x 10” mol L-’ in cyclohexane).

In contrast to this, the absorption spectrum of the Fp analogue 1 indicates marked electronic interactions between the organometallic donor and the organic acceptor. In addition to the 310 nm band assigned to the local n+x* transition within the acceptor group [lc], a second maximum, near 360 nm, appears which is present in the spectrum of neither of the model compounds 3 and 5 (compare Fig. 2B). Solvatochromism, which is generally considered as indicative of intramolecular charge transfer upon electronic excitation and as a basic requirement for high NLO responses, is not observed, however.

Cyclic Voltammetry

To compare the donor properties of ferrocene and Fp substituents, a series of compounds have been investigated by cyclic voltammetry. The pertinent data are shown in Table 2. Electrochemical measurements were performed according to standard procedures [la] and all signals referenced to the Fc/Fc+ couple [6].

The irreversible wave at high cathodic potential (1, 5, 8, 9, 11) can be attributed to reduction processes of the dicyanovinyl group [7]. For the Fp-substituted compounds 1-4 all the oxidation steps display complete electrochemical irreversibility and can be related to the Fe-Si skeleton. Introduction of the Fp donor results in distinct lowering of the oxidation potentials ( 2 4 compared

Synthesis and Electrochemical Properties of Silanes with Iron-Containing Donors 493

to 6). Going from 4 to 3 to 2, the cathodic shift increases with extension of the conjugated system. On the other hand, introduction of the 2,2-dicyanoethenyl acceptor group leads to an increase of Epa. Thus, the first oxidation potential of 1 is shifted 120 mV anodically compared to 2.

Table 2. CV and UV data for several Fp- and Fc-substituted compounds.

Compound E,' [Vl E,' [Vl EP3 [Vl EF [Vl h,. [nm] (E x lo-')

Fp-SiMe, (4) 0.64 1.24

Fp-SiMezSiMe3 (3) 0.58 0.99 284 (47), 330sh (18)

Fp-SiMezSiMezPh (2) 0.50 0.84 1.20 330sh (20)

Fp-(SiMez)zPhCHC(CN)z (1) 0.62 0.90 1.22 -1.64 309 (20), 360sh (120)

Me3SiSiMezPhCHC(CN)z (5) 1.30 -1.66 333 (264)

Me3SiSiMe3 (6) 1.46 >200

E"' [V] Epa [Vl Epc [Vl h, [nm] (E x lo-*)

Fc-Si&lell (7)

FC-S~~M~~J'~CHC(CN)~ (8)

0.02 0.93

0.04 0.97 -1.67

225 (203), 260 (85), 328 (1.5), 456 (2.2)

230 (ZOO), 279 (101), 368 (175)

Si6Mel IPhCHC(CN)z (9) 1.10 -1.69 289 (123), 366 (230)

Si6Melz (10) 1.06 230 (60), 252 (12)

Fc-(SiMeZ)zPhCHC(CN)z (11) 0.00 1.11 -1.67 335 (217), 409 (lo), 435sh (9)

PhCHC(CN)z (12) -1.72 225 (loo), 302 (240)

Regarding the Fc-substituted compounds 7, 8 and 11, the first anodic wave E"'= (Epa + Ep,)/2 displays electrochemical reversibility and can be assigned to the Fc moiety, whereas the second oxidation step displays electrochemical irreversibility. Compared with the unsubstituted analogues 6 and 10, it seems clear that this irreversible oxidation occurs in the silanyl bridge of the molecule. As there is no shift for E"' of the Fc moiety in 11, the data underline that there is a noticeable transmission effect only via the cyclohexasilanyl group in 8. However, it is rather small compared to a vinyl bridge, e.g. cyclic voltammetric studies of 2-[4-(2-ferrocenylvinyl) -benzylidene]malonitrile showed an anodic shift of 70 mV for the Fc group [3], which is about twice as much compared to 8.

Conclusions

In accordance with results obtained from UV-Vis and Mossbauer spectroscopy, the pertinent CV data underline the fact that there is a noticeable transmission effect via the disilanyl bridge in 1. They show a distinct decrease in the HOMO-LUMO gap upon introduction of the Fp group

494 H. Fullmunn, G. Furpup, H. Stiiger, C. Grogger

(- Epc + Epal = 2.96 V in 5 and 2.26 V in 1). In contrast to this, no shift in the Fc-substituted analogue 11 of either the ferrocene oxidation potential or of the dicyanovinyl reduction potential can be observed. Donor-acceptor interaction seems unlikely via the disilanyl bridge in 11, which again is in accordance with UV-Vis data. As there is some interaction via the cyclohexasilanyl bridge in 8, an enhanced communication could be expected for the Fp analogue, a topic which currently is under study.

References [ l ] a) G. Mignani, M. Barzoukas, J. Zyss, G. Soula, F. Balegroune, D. Grandjean, D. Josse,

Orgunometullics 1991, 10, 3660. b) G. Mignani, A. W i e r , G. Puccetti, I. Ledoux, G. Soula, J. Zyss, R. Meyrueix, Organometullics 1990, 9, 2640. c) G. Mignani, A. W i e r , G. Puccetti, I. Ledoux, J. Zyss, G. Soula, Organometullics 1991, 10, 3656. d) D. Hissink, P. F. van Hutten, G. Hadziioannou, J. Organomet. Chem. 1993, 454, 25. e) D. Hissink, H. J. Bolink, J. W. Eshuis, G. G. Malliaras, G. Hadziioannou, Polymer Prepr. 1993,34,721. a) R. D. Miller, J. Michl, Chem. Rev. 1989, 89, 1359. b) R. West, in G. Wilkinson, F. G. A. Stone, E. W. Abel (Eds.): Comprehensive Organometallic Chemistry II, Vol. 2, Pergamon Press, Oxford, 1995, p. 77. H. K. Sharma, K. H. Pannell, I. Ledoux, J. Zyss, A. Ceccanti, P. Zanello, Organometullics 2000,19,770. C. Grogger, H. Rautz, H. Stiiger, Monutsh. Chem. 2001,132,453. J. Ruiz, F. Serein-Spirau, P. Atkins, D. Astruc, C. R. Acud. Sci., Ser. Ilb: Mec., Phys., Chim., Astron. 1996,323, 85 1. R. R. GagnC, C. A. Koval, G. C. Lisensky, Inorg. Chem. 1980,19,2855. K. R. J. Thomas, J. T. Lin, Y. S. Wen, J. Orgunomet. Chem. 1999,575, 301.

[2]

[3]

[4] [5]

[6] [7]

Sustainable Silicon Production

Gunnar Halvorsen, * Gunnar Schiissler

Elkem ASA, Silicon Division, P.O. Box 8040,4675 Kristiansand Norway Tel.: +47 380 17000 -Fax: +47 380 17494

Keyword: silicone-based product, high energy consumption, carbothermic process, sustainable development, metallurgical-grade silicon, waste, silicon solar cell, energy carrier

Summary: The market for silicon has grown from 500 000 tons to 1 000 000 tons since 1980. The growth has accelerated during the last few years and the expected growth rate is now 5-8 % for the foreseeable future. The industrial process for production of metallurgical-grade silicon has improved its efficiency and is currently operating close to the technical limits. The world community expects from rising industries that no aspects of human life should be threatened by industrial activity and that growth should be sustainable. The silicon industry is in a good position to become a winner. The raw materials to produce silicon are quite abundant on Earth and the products made out of silicon are needed in our daily lives and can even be used to produce energy. Our challenges are the high energy consumption in the carbothermic process and the waste created in the process itself and in the following conversion steps

Introduction

Production of silicon is historically a young industry. Most products made out of this element are linked to modern society and the needs that have developed since 1950s. Aluminum-silicon alloys are today the most important automotive alloys; computers would not have reached today’s level of capacity without silicon chips, and our personal welfare in daily life is heavily influenced by silicone-based products.

From a resource utilization viewpoint this seems to be an ecologically excellent development, as silicon oxide is the most available mineral on Earth. Industrial processes needed to process minerals into useable products do, however, create wastes and consume energy. Sustainable production means that no aspects of human life should be threatened by this processing and that its products should improve living conditions on Earth. The Brundtland Commission states that sustainable development must be based on availability of metals and materials for use in building, construction, transportation, tools, and necessary equipment.

Here we aim to show that sustainable silicon production is possible, considering the improvements that have been achieved since the early 1980s.

Ovganosilicon Chemisty V Edited by N. Auner and J. Weis

Copyright 0 2003 WILEY-VCH Verlag GmbH & Co. KgaA, Weinheim

496 G. Halvorsen, G. Schiissler

The Market

The market for silicon has grown from 500000 Mt in 1980 and exceeded for the first time 1 000 000 Mt in 2000. Its growth has accelerated recently, reaching rates of 5-8 % per year. In the same time span the industrial process for production of metallurgical grade silicon has improved its efficiency by almost 25 % and is currently operating close to the technical limits. Supply has been secured by improved productivity, by erection of new furnace capacity, and by conversion of ferrosilicon furnace capacity to silicon production.

The aluminum market is still the largest market for metallurgical silicon. In 2000, consumption reached 573 000 Mt, whereas the chemical market accounted for 401 000 Mt and the electronics market for 42 000 Mt (Fig.1). The largest geographical markets are the EU (355 000 Mt), Japan (187 000 Mt) and the USA (290 000 Mt). The supply side is characterized by a few big suppliers (Elkem, Invensil) and many small ones (China, US, Brazil etc.). Norway, China, Brazil, and the USA are the four countries with the highest silicon production.

The demand side of the market is somewhat concentrated, particularly in the chemical segment, where three industrial players (Dow Coming, Wacker and GE-Bayer) account for 80 % of total consumption.

Fig. 1. Global silicon market - 1 016 000 Mt (2000).

Demand for silicon is expected to grow at an average of 4 % in the medium- to long-term perspective. The current situation, however, is reflecting a weaker world economy and the fact that some product substitution by oil-based products has taken place in the silicone sealant market. The lastest figures show that silicon consumption slumped by 5 % in 2001.

The price of silicon, in real terms, has been falling at an annual rate of 2 % since 1980 (Fig 2). A continuation of this trend has to be anticipated. The pressure on prices has mainly hit the “Western” world producers, whereas new market players from China and the CIS (Commonwealth of Independent States) do not seem to be affected. The exports from these countries will be a key

Sustainable Silicon Production 497

factor in determining the supply/demand balance going forward. The projected price development and continued price convergence between market segments underline the fact that the silicon market has many of the characteristics of a commodity market.

5 n

Fig. 2. Real price development of metallurgical grade silicon, based on the European spot price of standard silicon.

The cost of producing silicon (Fig. 3) is strongly dependent on energy and raw material. A high efficiency with regard to the use of resources is therefore a necessity for competitive production.

Fig. 3. Approximate cost distribution for silicon production. Sourec: CRU.

The Carbothermic Reduction Process

Metallurgical-grade silicon (96-99 %) is now produced in electric arc furnaces. Elkem was the leading supplier of such furnaces until the early 1980s, when the company also became the world’s largest producer of metallurgical grade silicon. The carbothermic reduction process and the basic equipment have been more or less unchanged since large-scale commercial production started in the

498 G. Halvorsen, G. Schussler

1930s. A modem silicon furnace (Fig. 4) does, however, contain process control and technology improvements that make sustainable production possible.

Fig. 4. Metallurgical production of silicon.

Raw Materials

The charge in a silicon furnace consists of lumpy quartz and reduction materials, mainly coal, coke, charcoal, and wood chips (Fig. 5).

Quartz, an eruptive modification of silica, is mined in large quarries. Quartzite is a sedimentary modification and cannot normally be used because of its iron content. There are many different qualities of quartz, but for the silicon process, three parameters are mainly predominant:

metallurgical or thermal strength, to avoid disintegration of quartz; purity: the contents of Fe, Al, Ca and trace elements (B, P); size: the normal size of furnace quartz varies from 10 to 150 mm.

The quality of the reduction materials is considered to be important in order to achieve a high silicon yield (from SiOz) in the furnace. Both the size and especially the reactivity of the carbon used will affect the performance. It has been shown [2, 31 that special grades of charcoal are the best reduction material, but the cost and impurity levels do not always allow their usage. Charcoal and wood chips (added to make the charge porous) are considered as biomaterials and do not add to the COz problem. Good reduction materials are not as available and abundant as quartz, and the industry is continuously striving to find and develop new sources.

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Fig. 5. Raw material selection for silicon production.

Electrodes

One of the most costly “raw materials” in silicon production is the furnace electrode (Table 1). Unlike ferrosilicon production, the original Soderberg electrode with steel casing and electrode self-baking paste cannot be used because of the iron contamination. For many years a pre-baked graphite electrode has been the only usable electrode for a silicon furnace. These have two major disadvantages - one being the price and the other the availability of very large diameters, thereby restricting the silicon furnace size to around 25-30 MW. In 1979 Elkem patented and developed the Bruff electrode (a modified Soderberg electrode) for silicon production [4]. Since then, Ferroatlantica developed the ELSA electrode, a self-baking paste electrode with a graphite core. With these developments electrode costs are improving, and the development of electrodes for larger silicon furnaces can be expected.

Table 1. Comparison of furnace electrodes

Electrode Engineering details cost Comment

Sederberg steel casing low high Fe in product

Pre-baked time-consuming baking high few producers of large diameters

ELSA compound electrode medium large diameter potential

Bruff self-baking Soderberg, steel m e d i u d o w large diameter potential casing removed

Reduction

The overall chemical reaction in the furnace can be written as Eq. 1, where x is the silicon recovery.

500 G. Halvorsen, G. Schiissler

SiO and CO gases bum off above the furnace and finely dispersed SiOz fume is produced. The single reactions taking place in the different furnace zones are far more complicated and the silicon yield, which is a measure of process efficiency, may range from 60 to 90 %. A highly efficient process with above 90 % silicon yield corresponds to an electric power consumption of approx. 10 500 kwhl Mt silicon produced.

SiO2 (s) + (l+x) C (s) = x Si (1) + (1-x) SiO (g) + (l+x) CO (g)

Eq. 1.

The reactions in the furnace may be represented by Scheme 1.

The overall reaction in the furnace is : siO?(S)+(l+X) C(S)=XS (I)+(l-X)SiO(q)+(l+x)co(~

Other ip r tan t furnace reactions are : 1) 2 so, (s, I) + sic (s) = sio (g) + co (g) 2) so7 (s, I) + si (I) = 2sio @1 3) sic (s) + SO(g) = 2s (I) + Cqg)

4) so, (s) +xsic= (2x-1) Si + (2-4 so+ m or corrbined:

Scheme 1. Reactions in the furnace [l] ( x = silicon recovery).

Refining

Silicon used in the chemical and electronics industry has a specification that makes refining of elements like aluminum and calcium necessary. Silicon used in high-quality aluminum alloys also needs refining. In the refining process aluminum and calcium are oxidized by oxygen (air, 02-N2

mixtures) blowing through a porous plug placed in the bottom of the ladle. Slag-forming components such as sand and calcium oxide are added to obtain a floating or sinking slag, allowing a slag-free casting of the silicon product.

Refining technology and processes have been improved since 1980s and the metal losses in the refining process are reduced from a level of 5 % to below 2 %. Whereas the ability to produce in-spec product in the refining process was formerly characterized by a 5 M O % hit rate, today it is 90-95 %.

Casting

The cooling rate of solidifying silicon has a direct influence on the resulting macro- and microstructure. Since 1985 it has been shown [5-71 that the composition, size, and distribution of intermetallic compounds can influence reactivity and selectivity in the dimethyldichlorosilane (MCS) and trichlorosilane (TCS) processes. This has led to the development of various casting

Sustainable Silicon Production 501

methods and patents (Table 2). It is well known that fast cooling gives a more homogeneous product than slow cooling. Casting in large isolated pans has therefore been changed to casting of thin layers in iron molds, with casting on a water-cooled copper plate and water granulation. Even atomization of liquid silicon has been tested on pilot-plant scale [8].

Table 2. Casting of silicon.

Casting method Engineering cost Comment

In fines pans none very low inhomogeneous qualities

In iron casings common practice low thin casting due to quality requirements

On copper plate; proprietary technology medium? low fines generation, fine metal water-cooled structure

Water granulation patents medium explosion danger, reduced crushing, less fines

Atomization patented process high promising quality for MCS and TCS

Products

Most metallic impurities contained in the raw materials will be reduced to their metallic form and remain in the product. Except for volatile elements and elements with very low liquid solubility in silicon, actually most elements that enter the furnace will be found in the metallic product with a very high recovery. Depending on the choice of raw material, the major impurities found in metallurgical silicon are iron, aluminum and calcium. Whereas aluminum and calcium can be oxidized in liquid silicon in the refining process following the tapping, the iron (and all transition metals) remain in the product. Refining processes add costs and reduce total silicon yield, as some silicon is lost in the slag created in the refining process.

Most elements can easily be alloyed to liquid silicon, through silicothermic reduction of oxides or by adding pure metals. In this way a variety of special requirements can be fulfilled. Metallurgical silicon is now supplied in a wide range of qualities. Almost every MCS or TCS process operator has his own specification, whereas the aluminum market is supplied with a range of standard specifications.

Among the metallurgical silicon qualities, the Elkem product SilgrainTM plays a very special role. SilgrainTM is produced from 92 % ferrosilicon in a leaching process. Depending on the ferrosilicon composition and on the use of different leaching agents, a wide range of silicon qualities can be produced. SilgrainTM is known as an excellent feedstock for the TCS process and for production of high-quality aluminum alloys. SilgrainTM HQ (High Quality), with as much as 99.9 % Si, is used for special products in the ceramics and electronics industry.

502 G. Halvorsen, G. Schiissler

Environmental Challenges

Production of metallurgical-grade silicon in an electric arc furnace is faced with three major environmental challenges:

high energy consumption; COZ emission from the reduction process;

0 SiOz fumes from the furnace.

Other environmental and health issues connected to the production process have found accepted solutions and are not on the short- to medium-term agenda.

Energy Consumption and Recovery

By “energy consumption” is normally meant the amount of electric energy needed to produce silicon. The energy content of the raw materials used in the reduction process is not counted. Production of 1 Mt silicon requires theoretically around 8 600 kwh. Losses in the electrical supply systems, in the electrodes and in the process itself set the technical limitation at around 10 000 kWh. Furnaces run with optimal raw materials and high operating times can produce close to this limit, and several companies have reported production at around 10 500 kWh/Mt on a yearly basis. New technology, especially with regard to furnace and electrode control systems, together with improved metallurgical process insight, have reduced electric energy consumption by close to 20 % since 1980.

The recovery of energy from silicon furnace fumes has only become possible since gas-cleaning equipment became available in the 1970s. Gases and fumes released from a furnace carry substantial amounts of energy. In general, a maximum energy recovery for production of electricity or heat and cost-effective silica powder recovery are obtained with a semi-closed furnace with reduced gas flow and high gas temperature.

The main components in an optimal energy recovery concept for power generation are:

a steam producing exhaust hood and ducting; 0 a waste heat water tube steam boiler; 0 a steam turbine with generator and condenser; 0 a membrane filter, fans and packing system.

The energy flow in silicon metal production with energy recovery for maximal electric power production has been discussed by Delbeck [ 101.

For electrical power generation based on an integrated concept, the efficiency can be as high as 25-30 %, comparing the power supply to the furnace with the power generated in the steam turbine. In addition to electrical power generation, a possible efficiency of around 100 % has been attained for production of process steam or district heating. Several ferrosilicon plants in Norway (six) and

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Sweden (one) have long experience with energy recovery systems, using shot-cleaned water tube boilers for electrical power production or self-cleaning gas tube boilers for steam and hot water production. One plant, Lilleby Metall, has installed a district heating system.

One of the most modem silicon furnaces in the world, at Elkem Thamshavn, has just recently installed a new and improved energy recovery system. The furnace, a 22 M W semi-closed arc furnace, has a steam-producing hood and ductwork. After one year of operation, last quarter figures showed a direct process energy consumption of 10 800 kWh/Mt. The electric energy recovery was reported to 2400 kWh/Mt of silicon produced; that gives a net energy consumption of 8 400 kWh/Mt. It has to be noted that auxiliary plant energy consumption is around 1 000 kWh/Mt

v11. In addition to increasing the energy recovery efficiency with the steam-producing hood, the aim

is to develop a hood that improves the silica fume quality and is environmentally preferable, having good operational qualities and lower maintenance costs than existing hood designs. Good and stable combustion conditions reduce the emissions of pollutants both in the silicon powder and in the cleaned off-gas.

Installation of the steam-producing hood and ductwork is a major step toward the future optimal concept for an energy and microsilica recovery system in Elkem’s ferroalloy and silicon metal

In areas where there is a market for low-temperature energy, an integrated process steam and district heating system will be a preferable energy recovery concept. Elkem is currently considering such a system at its Fiskaa plant in Kristiansand. The energy recovery plant will include one or two gas tube boilers integrated in the off-gas system between the furnace and the filter unit. The closeness of a city and other energy-consuming process industry may make this a viable economic solution.

plants.

coz Carbon dioxide is a relatively new threat to most metal-producing industries. Production of silicon is - and will be for many years to come - dependent on the carbothermic reduction process. Bioreduction materials such as charcoal, paper, saw dusts and other carbon-containing biomass wastes are now considered as reduction alternatives to coal and coke. Since the 1990 the West European silicon industry has reduced the COz releasemt silicon produced by nearly 20 %. The reduction is mainly a result of more efficient processes, higher silicon yields and use of charcoal and woodchips in the furnace charge. Further improvements will depend on the development of biomass reduction materials and a higher use of these materials (Table 3).

The Climate Challenge has, in a more powerful way than earlier, has set the focus on the effective use of energy in the ferroalloy and silicon industry. Through the Kyoto Protocol of November 19, 1997, a large number of countries have agreed to reduce emissions of greenhouse gases considerably from current levels. The Kyoto Protocol has initiated extensive work in the EU, defining the Best Available Technology (BAT) for energy and waste recovery from all the major industries, including the ferroalloy and silicon industry.

504 G. Halvorsen, G. Schiissler

Table 3. Environmental COz from silicon production.

Present-day standard silicon process Future sustainable Si process

Total 6100 kg C02/1000 kg Si Reduction of volatiles

Fossil COz

Biological COz

4300 kg C02/1000 kg Si

1800 kg C02/1000 kg Si

Increased amount of biological COz

Increased yield of silicon

Increased yield of energy equivalent to increase in amount of COZ

The EU’s requirements are put forward in EU Directive 96/61/EF (IPPC Directive, Integrated Pollution Prevention and Control). The IPPC Directive with BREF (BAT REFerence document) will regulate all environmental aspects in one concession including energy and silica powder recovery.

Most of the industry today has emission limits and concessions that are subject to change. The new limitations will obviously be stricter regarding emissions to air and water. Demands for higher operating time and lower emission levels will increase.

SiO2 Fumes (Microsilica)

After the Second World War environmental limits were set on emissions to air, earth and water. In particular, the visible smoke from the furnace hood and stacks were attacked not only by the general population, but also by the environmental authorities. Smoke emission from industrial plant was considerable. Although work with upgrading of the microsilica process was carried out in the early 1950s, the breakthrough was not achieved until the mid-1970s.

Since the late 1950s Elkem has invested more than US$ 100 million in filter installations. However, just filtering the silica fume led to substantial disposal problems. The fume from just one smelting plant could amount to as much as 20000 Mtlyear. More than US$ 40 million were invested in R&D and business development to utilize microsilica.

The bag-house filter cleaning method involves reverse air or reverse gas cleaning. The Elkem system features a filter bag made of PTFE membrane, which is bonded to the backing material. All the dust particles adhere to the surface of the membrane, which prevents particles from penetrating the fabric of the bag.

Sustainable Products

Sustainable products should make a positive contribution to the overall ecological resource problem. Many products made out of silicon obviously fall into this category, such as solar silicon and aluminum-silicon alloys, while others contribute to sustainability more indirectly and maybe in a long-term perspective.

Sustainable Silicon Production 505

Solar Silicon

Silicon solar cells are one of the most potentially sustainable products on Earth. The Sun is our ultimate energy source and the solar energy received can be far better utilized than it is today. A comparison of different forms of conversion of solar light to electric energy shows the tremendous potential of solar cells (Table 4).

Table 4. Conversion of solar energy to electricity.

Energy source Energy efficiency [%]

Forests 0.1

Biomass (selected crops) 1

Silicon solar cells 10-17

Today the energy payback time, defined as the time necessary for a silicon solar panel to generate the energy equivalent to that used to produce it, is 3 4 years. Panel lifetime is normally more than 20 years.

The cost of solar energy, however, is still in the range of 6 O.S/kWh, which is more than 3 4 times the current European energy price. Since 1990 the price gaps between solar and grid power has narrowed and it is expected that this trend will continue. For many applications, such as off-grid domestic power supply, mobile power supply and backup systems for electronics, solar power is already competitive. Solar panels integrated into roof construction are another very promising application.

Since the early 1980s Elkem has worked with processes aimed at a metallurgical silicon quality that can be suitable for solar cells (Fig. 6). This work has extended from selection of pure raw materials and smelting in special reduction furnaces through metallurgical refining and leaching processes [12, 131. Most projects have been performed in cooperation with solar cell producers - Exxon (1980-85), Texas Instruments (1990-94), Eurosolare (1991-93) and lately with the American company Astro Power (started 2000). The current situation is that the design of a pilot plant is under evaluation and that pilot production hopefully will start within 2002.

1 Available unit ooeratlons: I

-1 t- Slag treatment (liquid-liquid extraction) * Leaching (8olvent refining) * Crystallization

* Vacuum - Plasma

Oxidative gas blowlng

Low coat (Equipment, Chemical.) High capacity

Fig. 6. Metallurgical route to solar grade silicon.

506 G. Halvorsen, G. Schiissler

Aluminum-Silicon Alloys

In 1980 a standard middle-class automobile contained approx. 60 kg of aluminum. Since then, environmental forces, energy prices and technology developments have raised the use of aluminum to approx. 100 kg per vehicle and the automobile production has almost doubled. Aluminum-silicon alloys make up more than 70 % of the aluminum alloys in a car and contain on average 7 % silicon. Silicon contributes to alloy castability and strength and to weight reduction. In addition, aluminum-silicon alloys are fully recyclable. Without the development of silicon-containing aluminum alloys, recycling of aluminum from automobiles would have been far more complicated, since these alloys are able to consume impurity elements better than all other aluminum alloys and are still valuable in new products.

The growth in the automotive industry and in the use of aluminum in cars will make a strong impact on the consumption of silicon in the future also.

Silicon as Energy Carrier

Metallurgically produced silicon is, like aluminum, magnesium and some other light metals, a potential energy carrier. By oxidation of these elements large amounts of energy are set free. The use of aluminum powder as a rocket fuel is well known.

Auner [ 141 has recently suggested the use of silicon as an energy carrier and has shown different routes for possible conversion and usage. When a more energy-efficient method of producing silicon from its oxide can be found, some of these ideas may indeed make the concept of silicon as a future energy carrier viable in economical and environmental terms also.

Recycling of Process Waste from Chemicals and Electronics Production

Table 5. Recycling of process by-products and wastes. N, none; 0, low; X, medium; XX, high.

FeSi SiMn Burning Copper Remelt Refractory ,si Silico Productnndustry Foundry Cement -thermic

TCS-Fines X 0 X X X 0 X X 0

0 0 N 0 0 0 X 0 0 MCS-spent, upgraded

MCS-spent,

Cvclone, reactor

Fines from Milling 0 0 X 0 xx 0 X xx N

upgraded N N N N N X 0 N N

The process waste from metallurgical silicon has been discussed previously. In the following chemical processes also, additional byproducts and wastes are generated. This was more recently discussed by Freeburne [ 151 and Rich [16]. Currently such materials are treated and recovered in

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various processes, none of which has the obvious potential of long-term sustainability. The silicon and ferrosilicon industry takes some of these materials back into the processes, thereby not extracting the full value by far. Table 5 shows an evaluation of the recycling and recovery processes available today.

Considering the future growth expected in the chemicals and electronics industry, there is a need to address this area and to involve producers and suppliers in a common R&D program.

Challenges to Sustainable Production

The conclusions of this paper are:

Raw materials for the metallurgical silicon production are abundantly available. The reduction process has been substantially improved since 1980, with silicon yield rising from 60 to 90 % from modem furnaces and at the same time energy consumption improving from 12 500 to 10 500 kWh/Mt. The energy recovery from furnace fumes and gases is around 20 % of electrical energy input in modem furnaces. It can be enhanced to close to 100% if low-temperature energy can be utilized. 99.9 % of the fume can be recycled as microsilica, a byproduct that has high value. The CO:! gas emitted from the process has been reduced in modern furnaces by up to 20 %, based on the tonnage produced since 1980. Biomass-based reduction materials can reduce this emission even further. Silicon solar cells are the most potentially sustainable product made out of silicon. New process and product development will lower the price of solar cells and make solar energy even more competitive. Aluminum-silicon alloys are important for energy saving and recycling of aluminum alloys. Recycling of wastes from the chemicals and electronics industry should be a common R&D task.

Sustainable production requires a will to change, technological resources to perform the changes, and a commitment from the whole industry chain, including the consumer, to consider ecological solutions rather than short-term commercial gains.

References [ l ]

[2]

A. Schei, J . Kr. Tuset, H. Tveit (1998): High Silicon Alloys, Tapir Forlag, Trondheim, ISBN

0. Raaness, R. Gray, (1995): “Coal in the production of silicon rich alloys,” in ZNFACON 7, ed. J. Kr. Tuset, H. Tveit, I. G. Page, FFF, (The Norwegian Ferroalloy Research Organization, SINTEF, Trondheim).

82-5 19-1327-9.

508 G. Halvorsen, G. Schiissler

T. Videm (1995): “Reaction rate of reduction materials for the (ferro)silicon furnace” in ZNFACON 7, ed. J. Kr. Tuset, H. Tveit, I. G. Page, FFF (The Norwegian Ferroalloy Research Organization, SINTEF, Trondheim), p. 221-230. A. Vatland, W. Bruff (1995): “Development of an in situ prebaking electrode system for silicon metal production” in INFACON 7, ed. J. Kr. Tuset, H. Tveit, I. G. Page, FFF (the Norwegian Ferroalloy Research Organisation, SINTEF, Trondheim), p. 43 1440 . G. Halvorsen, G. Schiissler (1986): “Silicon metal qualities for the aluminum industry”, paper presented at The Institute of Metals, Aluminum Technology 1986, Royal Lancaster Hotel, London, March 11-13,1986. G. Halvorsen, G. Schiissler (1991): A silicon product for use in the production of organosilanes and chlorosilanes together with a method for the production of the silicon product, Norwegian Patent 169 831; German Patent 4 037 021, priority Nov. 22,1989. T. Margaria, M. Rebiere, F. Traversaz, C. Dumay, (1996): “Influence of cooling on silicon structure”, in Silicon for the Chemical Industry III, ed. H. A. Idye, H. M. Rong, B. Ceccaroli, L. Nygaard, J. Kr. Tuset, Norwegian University of Science and Technology, p. 87-94, ISBN

K. R. Forwald, G. Schiissler, (1992): “Aspects upon silicon metal casting techniques”, in Silicon for the Chemical Industry, ed. H. A. Idye, H. Rong, Institute of Inorganic Chemistry, NTH, Trondheim, p. 3946 , ISBN 82-90265-10-7. H. Tveit, E. Myrhaug (2000): Material Balance for Trace Elements for all Elkem Silicon and FeSi-Plant in Norway (in Norwegian), Report to SFT (The Norwegian Pollution Control Authority), February 21. H. K. Delbeck (2000): “Energy recovery from hot waste gas in the ferroalloy and silicon industry”, in Silicon for the Chemical Industry V, ed. H. A. Idye, H. M. Rong, L. Nygaard, G. Schiissler, J. Kr. Tuset, Norwegian University of Science and Technology, p. 71-83, ISBN

H. Rong: personal Communication. A. Schei (1985): High Purity Silicon Production in Refining and Alloying of Liquid Aluminum and Ferro-alloys, ed. T. A. Engh, S. Lyng, H. A. Idye, Aluminum Verlag, Diisseldorf,

G. Halvorsen (1984), Method for production of pure silicon, US patent 577048 (priority 1983). N. Auner (2000): Zeitschrifi Stem, Nov. 9. S. K. Freebume (1996): “The conversion of waste from methylchlorosilane. production into valuable products”, in Silicon for the Chemical Industry III, ed. by H. A. @ye, H. M. Rong, L. Nygaard, B. Ceccaroli,, J. Kr. Tuset, Norwegian University of Science and Technology, p.

J. Rich (2000): “Challenges and opportunities for the silicon industry in the 21st century”, in Silicon for the Chemical Industry V, ed. H. A. Idye, H. M. Rong, L. Nygaard, B. Ceccaroli, G. Schiissler, J. Kr. Tuset, Norwegian University of Science and Technology, ISBN

82-90265-19-0.

82-90265-22-0.

p. 71-89.

303-308, ISBN 82-90265-19-0.

82-90265-22-0.

Reactivity of Doped Silicon in the Direct Synthesis of Methylchlorosilanes

L. Lorey, G. Roewer*

Institut fur Anorganische Chemie, TU Bergakademie Freiberg D-09596 Freiberg, Leipziger Stral3e 29, Germany

Tel.: +49 0373 1 39 3 174 - Fax: +49 0373 1 39 4058 E-mail: Gerhard.Roewer @ chemie.tu-freiberg.de

Keywords: direct synthesis, raw material, electronic state, disilanes

Summary: Photo-EMF measurements provide a useful tool to estimate the electronic state of silicon charges that are used in the direct synthesis of chloromethylsilanes. Relationships were observed between the reactivity of silicon and its electronic state varied by doping with phosphorus, tin, boron, and indium respectively.

Introduction

Silicon alloyed with a copper catalyst and promoter substances reacts with methyl chloride (at temperatures around 300 "C) to give a mixture of methylchlorosilanes in the industrial direct synthesis. Dimethyldichlorosilane represents the most important target in this process. Since Rochow [I] and Miiller [2] discovered this direct synthesis route for the silicon-promoter-catalyst system, many investigations were done to increase the activity as well as the selectivity and to clarify the mechanism. Zinc, tin, and phosphorus, beside other substances, were found to give effects [3-61. The goal of this research work is to find out whether there are relationships between the electronic effect of phosphorus, tin, boron, or indium doping of silicon and its reactivity as well as selectivity in direct synthesis. Characterization of the electronic state of the variously doped silicon relies on photo-EMF measurements.

Photo-EMF as a Useful Tool to Characterize the Reactivity of Silicon

Laser flash irradiation generates electron-hole pairs (e-lh') by excitation of electrons from the valence band into the conduction band of inorganic semiconductors such as silicon (Fig. 1). Due to the local gradient of the light absorption intensity toward the semiconductor bulk, a concentration gradient of the generated electron-hole pairs is engendered. These effects give rise to a transient photo-electromotive-force (photo-EMF) (Eq. 1).

The sign of the generated photo-EMF depends on the difference in the mobilities of electrons

Ovganosilicon Chemisty V Edited by N. Auner and J. Weis

Copyright 0 2003 WILEY-VCH Verlag GmbH & Co. KgaA, Weinheim

510 L. Lorey, G. Roewer

Ole) and holes b h ) (Eq. 1). In the case of n-doping, a positive sign will result. p-Doping engenders a negative photo-EMF (see Fig. 2). As we have proven [7], in n-doped electronic-grade silicon wafers a positive sign photo-EMF arises but a negative one is produced in p-doped samples. The maximal photo-EMF U,,, decays due to subsequent vanishing of charge carriers (recombination via trapping steps) [8]. The photo-EMF decay curve U(t) may be quantified using a biexponential equation as given in Fig. 2, where kl and k2 represent the rate constants of processes in the surface and bulk regions, respectively.

E ,

,cB + ,E=ON~V L a = a m e v silicon surface As

4 = 1.12 eV 4 ......................................... -_____-___

sn - In AE=OXleV

B

VB VB

Fig. lb. Trap depths in silicon generated by doping with

phosphorus, arsenic, tin, indium, and boron.

Fig. la. Generation of electron-hole pairs by

irradiation with laser light.

umax]l decay kinetics: U(t) = Ule-k'' + U2e-"'

Eq. 1.

+ n-doped Si

6 c

p-doped Si -

40 5MO' 1,Mlf 1,Salf ZM$ 2Ma"

tine [s]

Fig. 2. Photo-EMF signals for n- and p-doped silicon.

Technical-grade silicon doped with elements P, Sn, B, In (Table 1) was used (the particle size varied from 70 to 200 pm) to carry out the reactivity screening toward methylene chloride flushing at 350 "C. Trap depths of the doping elements in silicon are given in Fig. 1 b.

Reactivity of Doped Silicon in the Direct Synthesis 511

Relationships Between the Reactivity and Electronic State of Silicon

The as-doped silicon samples containing only a low P concentration engender a negative photo-EMF (Table 2). The charges 9, 10 and 11 exhibited n-type photo semiconductor properties, due to the relatively high P/Sn concentration ratio. Silicon sample 10 exhibited the strongest n-type photo semiconductor properties. The boron- and indium-doped silicon samples 12 and 13 demonstrated p-type semiconductor behavior.

Table 1. Foreign elements content of the doped silicon samples investigated.

Sample Fe[%] Al[%] Ca[%] P[ppm] Sn[ppm] B[ppm] In[ppm] - - - - SiSt. 0.35 0.20 0.036

8 0.45 0.16 0.052 23 49

9 0.58 0.15 0.046 550 61

10 0.40 0.16 0.059 450 <3

11 0.40 0.16 0.054 110 <3 - -

12 0.39 0.16 0.055 - - 153 -

13 0.32 0.15 0.049 - - - 1500

- -

- -

- -

Table 2. Measured photo-EMF of the doped silicon samples.

Sample

SiSt.

8 (P + Sn) 9 (P + Sn)

10 (PI

11 (PI

12 (B)

13 (In)

U I l U X

-11.7

-24.4

19.0

28.0

18.6

-9.0

-32.0

Photo-EMF [mV]

f 0.2

f 0.2

f 0.2

f 0.3

f 0.2

f 0.2

+ 0.3

Treatment of these silicon charges with methyl chloride gave a product mixture of monosilanes (CH3SiCl3, (CH3)2SiC12 (main product), (CH3)3SiCl) as well as disilanes Siz(CH3),Cls_,). These synthesis experiments were carried out in a stirred-bed reactor.

Reactivity depended significantly on the doping state of the silicon charges used. Sample 9 (67 ppm Sn) developed the highest reactivity (Fig. 3). In the case of sample 10 (< 3 ppm Sn combined with relatively high P doping) the reactivity dropped clearly. The standard silicon (SiSt) was nearly as reactive as sample 8 (23 ppm P and 49 pprn Sn). Doping with boron or indium resulted in lower reactivities of silicon.

We were able also to trap the silylenes SiClz and SiCH3C1 by using 3,hlimethylbutadiene

512 L. Lorey, G. Roewer

14-

12-

10-

- . 8 8-

2 z 6

Y

addition in this process. Silylenes may undergo an oxidation addition step with methyl chloride, delivering monosilane products. Alternatively silylenes may enter into insertion steps with adsorbed chloromonosilanes that have already formed, creating disilanes (Eq. 2).

As shown in Fig. 4, the proportion of disilane decreases when using samples whose photo-EMF maximum is moved to a more positive value. This means that the amount of disilane byproduct drops with intensification of the n-type doping quality of the silicon used.

Silylenoids representing key intermediates in the direct synthesis should be stabilized on the surface by n-doping of the silicon. This means that a stronger donor quality of the silicon surface favors the more selective silylene insertion into the C-Cl bond of methyl chloride in comparison with Si-Si bond formation via insertion into a Si-Cl bond or oligomerization of the silylenes.

. , . , ,

+ 'silicon si v Silicon 8 m Pbon 9 - A Silicon 10 X Silicon 12 + Siliconl3 -

+

Eq. 2.

'A 0 2 2 , , , . , , , , , . , . , . , . ,

0 5 10 15 20 25 30 35 40

Silicon-Conversion ["h]

Reactivity of doped silicon.

Fig. 4. Correlation between the electronic state of the educt silicon (described by photo-EMF after irradiation with

laser light) and the disilane content in the crude silane mixture.

Reactivity of Doped Silicon in the Direct Synthesis 513

Conclusions

Phosphorus-, tin-, boron- and indium-doped silicon samples give different photo-EMF signals as well as modified chemical reactivity in the direct synthesis. The actual concentrations of n- and p-dopands control the reactivity of the silylene intermediates. Tests on these silicon samples were demonstrated that the electronic state of the educt silicon significantly affects the selectivity. Thus the amounts of disilanes in the crude silane mixture decreased if the n-type semiconductor behavior of the educt silicon was intensified by doping.

References [I] [2] [3]

E. G. Rochow, N. Y. Schenectady, US Patents 2 380 995 (filed Sept. 26,1941). R. Muller, DDR Pat, (filed June 6,1942). L. Lorey, G. Roewer, C. Damm, in Silicon for the Chemical Industry V, Tromso, Norway, May 29-June 2, 2000, Eds.: H. A. Oye, H. M. Rong, L. Nygaard, G. Schussler, J. Kr. Tuset, Trondheim, Norway, 2000, p. 257-263. R. L. Halm, 0. K. Wilding, US Patent 4946978 (filed Dec. 22,1986). V. D. Dosaj, R. L. Halm, 0. K. Wilding, US Patent 4 898 960 (filed Dec. 22, 1986). G. Rossmy, German Patent 1 165 026 (filed Sept. 14,1959). L. Lorey, Dissertation, TU Bergakademie Freiberg, 2001. G. Israel, F. W. Muller, C . Damm, J. Harenburg, J. In5 Recording 1997,23, 559.

[4] [5] [6] [7] [8]

Solvent Role in the Triethoxysilane Direct Process

Alexander Gorshkov, Victor Kopylov, Anna Markacheva, Alexander Polivanov

Federal State Unitary Enterprise State Research Institute for Chemistry and Technology of Organoelement Compounds, 38 Shosse Entuziastov, Moscow, 1 1 1123, Russia

Tel./Fax: +7 95 273 72 06 E-mail: unkopyl@cityline.ru

Keywords: alkylnaphthalene, alcohol, solubility, direct process

Summary: The solubility of alcohol in termolan was studied. The equilibrium concentration of alcohol in termolan achieves its maximum at 75-80 "C. On the basis of the data obtained an assumption about the possible role of termolan in the triethoxysilane direct process was made.

Alkoxysilanes SiHn(OR)kn are important intermediates in organosilicon production. Triethoxysilane SiH(OCzH5)3, in particular, is used in compound and release agent manufacture, for semi-conducting silicon production, etc.

The etherification of chlorosilane with alcohols is a major industrial method for ethoxysilane production (Eqs. 1 and 2).

S i c4 + 4 CzH50H + Si(OC*H5)4 + 4 HCl

Eq. 1.

SiHCl3 + 3C2H50H + SiH(OC2H& + 3HC1

Eq. 2.

Much research work has been devoted to the direct process for alkoxysilanes, based on silicon interaction with alcohol (Eq. 3) [l-51.

Si + 3 CzH50H + SiH(OCzH5)s + Hz

Eq. 3.

The process is carried out in the presence of copper monochloride catalyst at 180-260 OC in a high-boiling solvent environment that may consist of polyaromatic oils widely used as thermal

Ovganosilicon Chemisty V Edited by N. Auner and J. Weis

Copyright 0 2003 WILEY-VCH Verlag GmbH & Co. KgaA, Weinheim

Solvent Role in the Triethoxysilane Direct Process 515

heat-transfer fluids [6-81. “Terminol” demonstrates the best performance in the alkoxysilane direct process [ 1-31.

We conducted experiments on the triethoxysilane SiH(OCzHs)3 direct process by means of silicon interaction with ethanol. Polymethylsiloxane fluids, vaseline oil, ethyl silicate (an oligoethoxysiloxane mixture of Si,+10,(OC2H5)2(n+2)), alkylbenzene, aloterm (alkyldiphenyl oxide) and termolan (alkylated naphthalene) were tested as a high-boiling solvent.

The best results in terms of the direct process capacity and selectivity were obtained in the termolan environment [9], this being an analogue of foreign “Terminol” and produced in Russia.

A solvent in the alkoxysilane direct process may produce either an inhibiting or a promoting effect on the catalytic reaction between silicon and alcohol. One of the possible effects of the termolan solvent is its adsorption on the silicon surface with the formation of a coating that will modify this surface, thus affecting active reaction centers. When the solvent functions in such a manner, its amount in the system must not influence the reaction rate and even at its low concentration it should provide full silicon surface wetting and the anticipated effect should be manifested. In order to confirm this idea, comparative experiments on triethoxysilane direct synthesis in a silicon suspension in termolan were carried out at a silicon termolan ratio of 1:2, as well as in silicon-termolan paste, when silicon is only wetted with termolan at a silicon termolan ratio of 10:0.8.

The experiments demonstrated that the reaction rate at small termolan quantities is much lower than in a termolan-silicon suspension, testifying to the absence of a termolan promoting effect on alkoxysilane synthesis.

The termolan effect was supposed to result from its specific interaction with alcohol. The major criterion pointing to specific interaction taking place in the above-mentioned solution is the imperfection of the solution and its degree of deviation from Raoult’s law. Therefore a knowledge of the termolan-ethanol system performance could assist comprehension of the termolan part in the direct process.

Besides, to select optimum alkoxysilane direct process conditions, quantitative data on alcohol solubility in termolan should be available, particularly at temperatures relevant to the condition of synthesis.

In order to discover the nature of alcohol interaction with termolan in solution, alcohol solubility in termolan was studied.

Alcohol solubility in termolan at 20 OC amounts to =: 2 %. With temperature elevation, the alcohol solubility first rises, it achieves its maximum at the boiling point of alcohol, and then it drops. Maximum alcohol solubility corresponds to a content of 7.5 % alcohol in termolan.

Table 1 presents experimental data on alcohol solubility in termolan obtained at 80-24OoC. Estimated values of the alcohol partial pressure above the solution were calculated according to Raoult’s law by Eq. 4.

516 A. Gorshkov, V. Kopylov, A. Markacheva, A. Polivanov

p = P x

Eq. 4.

where P = pure alcohol vapor tension and x = alcohol content [mol%] in solution and are also specified in the table.

Table 1. Alcohol solution in temolan at 80-240 "C.

Experimental values

Pure alcohol Alcohol Alcohol concentration Temperature

-n-. vapor tension vapor

[kgf/m2] ['I pressure [wt%l [mol%] L-LI

[kgf/mz] 80 1.07 1 7 0.344

95 2.228 1 4.65 0.253

130 5.685 1 1.43 0.0918

190 23.94 1 0.34 0.0238

240 59.92 1 0.13 0.0094

Estimated values

Alcohol Alcohol

vapor concentration at 1

pressure kgf/mz pressure

[kpUm21 [mol%] 0.367 0.934

0.564 0.449

0.52 0.176

0.569 0.042

0.565 0.0167

[a] Ref. [lo].

The experiments were carried out at atmospheric pressure, but Table 1 shows that the estimated values of the alcohol partial pressure are 2-3 times below atmospheric.

The alcohol used as a reagent in alkoxysilane direct synthesis has a space network of hydrogen bonds just like water, and therefore it forms cyclic and linear associates whose composition is determined by the nature of solvent.

That is the reason for the abnormally high boiling point, heat capacity and values of other physical parameters. One can suppose that in polyaromatics solutions the associates from the alcohol molecules are partly destroyed, the concentration of individual alcohol molecules grows and therefore the volatility of alcohol in the form of individual molecules increases, thus resulting in the increase of the alcohol vapor partial pressure above its solution in termolan. This phenomenon can explain the positive alcohol-termolan system deviation from Raoult's law.

Formation of associates from alcohol and termolan molecules is possible. These associates could possess enhanced reactivity. That may be a reason for the specific effect of termolan on the process.

References [ 11 [2] [3] [4]

US Patent 3 775 457,1973. US Patent 4 999 446, 1991. US Patent 5 211 717, 1992. US Patent 4 21 1 717, 1980.

Solvent Role in the Triethoxysilane Direct Process 517

RU Patent 2 157 375,2000. S. Z. Kagan, A. V. Chechetkin, Organic Thermal Heat-Transfer Agents and Their Use in Industry, Moscow, Leningrad: Goskhimizdat 1951, 171, A. V. Chechetkin, Thermal Heat-Transfer Agents, Moscow, Leningrad: Gosenergoizdat 1962, 496. N. P. Dolinin, Plants for Chemical Device Heating by Thermal Organic Heat-Transfer Agents, Moscow: Mashgiz 1963,292. M. A. Margulis, Yu. E. Anpilov, S. P. Chernykh, New Thermal Organic Heat-Transfer Agent Based on Alkylaromatics, Khimicheskaya Promyshlennost 1993,5,207. V. N. Stabnikov, I. M. Poiter, T. B. Protsjuk, Ethyl Alcohol, Moscow: Pishchevaya Promyshlennost 1976,27 1.

Methylsilane Production by Means of Methyldiethoxysilane Catalytic Disproportionation

Evgenii Belov, Galina Dubrovskaya, Nikoluy Efimov, Salomonida Kleshcevnikova, Evgenii Korobkov, Evgenii Lebedev

Federal State Unitary Enterprise State Research Institute for Chemistry and Technology of Organoelement Compounds, 38 Shosse Entuziastov, Moscow 11 1123, Russia

Tel.: +7 95 273 6338 -Fax: +7 95 913 2538 E-mail: onti@eos.incotrade.ru

Keywords: methylsilane, methyldiethoxysilane, disproportionation, etherification, purification

Summary: Research resulted in basic parameters of disproportionation and etherification; methylsilane and methyldiethoxysilane production techniques were developed. These techniques are based on available raw material, only a small quantity of waste is produced, and the process can be recommended for pilot-scale commercial tests. Recommendations on thorough methylsilane purification are made. The originality of the process is proved by the Russian patent.

Methylsilane (MS) is used for the production of silicon carbide, of new-generation photoresists and thermal protective coatings. Besides, MS is a basic component of highly heat-resistant, stable and lightweight composites.

MS must meet severe requirements for its successful employment in the above-mentioned branches of engineering. Thus, according to Voltaix Inc. (USA) data [ l ] MS must contain a minimum 99.96 % of basic matter.

A literature analysis [2-6] demonstrates that MS is produced by means of the interaction of methyldi- or methyltrichlorosilanes with ordinary or complex hydrides. Usually these reactions are conducted in a solvent medium.

The main drawbacks of these techniques consist in:

0 the high cost of hydrides; inevitable MS contamination due to use of solvents and reduction of foreign matter present in the raw material. Removal of impurities therewith is a great challenge.

For widespread employment of MS a cheaper product is needed. Therefore, the MS production

Ovganosilicon Chemisty V Edited by N. Auner and J. Weis

Copyright 0 2003 WILEY-VCH Verlag GmbH & Co. KgaA, Weinheim

Methylsilane Production 519

technique by means of methyldiethoxysilane (MDES) catalytic disproportionation was taken as the research basis. MDES was obtained by methyldichlorosilane (MDCS) etherification by ethyl alcohol (Eq. 1).

MeSiC12H + 2C2H5OH __ MeSi(OCzH5)zH + 2HCl

Eq. 1.

The hydrogen chloride formed is partly dissolved in the reaction mixture and its remainder is removed from the reactor. One should keep in mind that, in parallel with MDES formation, an undesirable reaction (Eq. 2) proceeds, resulting in a loss of the starting raw material.

MeSi(OCzH5)zH + CzHsOH - MeSi(OCzH5)3 + HZ Eq. 2.

Dissolved hydrogen chloride serves as reaction catalyst. The reaction rate therewith increases significantly with an elevation of temperature and an increase in ethyl alcohol concentration in the reaction mixture.

Therefore a laboratory plant for MDES production was erected in which a two-step film desorber system is used for hydrogen chloride removal from the reaction mixture. This system makes it possible to shorten the reaction mass dwell time in the apparatus, raise the hydrogen chloride desorption rate and thus minimize the role of side reactions.

Our research resulted in determination of the following main process parameters:

MDCS supply rate [dih] 130.0

C2H50H supply rate [dih] 140.0

Nitrogen consumption, [h] 100.0

Temperature of the 1st desorber ["C]

Temperature of the 2nd desorber ["C]

Plant capacity [mL/h] 190-200

50.0

75.0

About 75-80 wt% MDCS conversion into MDES was achieved. MDES samples were used for MS production. The MS production technique was based on the

MDES disproportionation reaction represented by Eq. 3.

cat

3 MeSi(OCzH5)zH - MeSiH3 + 2 MeSi(OC2H&

Eq. 3.

Sodium ethylate (3 wt %) solution in methyltriethoxysilane (MTES) was selected as a catalyst. It

520 E. Belov, G. Dubrovskaya, N. EBmov, S. Kleshcevnikova, E. Korobkov, E. Lebedev

Fractional distillation

was synthesized in a plant consisting of a reactor, refluxer, traps, MS receiver, tube furnace and vacuum pump. The plant is equipped with the required instrumentation. Table 1 presents process parameters.

Absorption

Table 1. Process parameters for the synthesis of sodium ethylate.

~ ~ ~ _ _ _ ~

Expt. no. MDES supply to Temperature in MS content of MDES conversion to reactor [g/min] reactor ["C] product [wt%] MS [%]

1 3.2 25-30 88.8

2 3.4 30-35 96.5

3 6.4 30-35 95.5

75.0

82.0

81.6

It was found that MDES disproportionation with MS production in the presence of sodium alcoholate catalyst is rather intensive at a temperature of 25-30°C. MDES conversion into MS therewith achieved was more than 80 %.

MS samples were analyzed chromatographically. The MS content in the samples amounted to above 95 %. Recommendations for the purification of MS were developed on the grounds of its impurities analysis. All impurities may be subdivided into three groups:

inert gases with low boiling points (hydrogen, nitrogen, oxygen); readily volatile compounds, whose boiling points are lower than that of MS (methane, monosilane, carbon dioxide); high-boiling compounds, whose boiling points are much higher than that of MS (ethanol, methylethoxysilane, polyethoxysiloxanes).

We have developed a main flow diagram of MS purification by sorption (Fig. 1).

Removable Impurities ethanol, methylethoxysilanes,

HP, NP, 02, CH4 CH4, SiH4, COz polysiloxanes

Fig. 1. Flowsheet for MS purification.

MS is fed to a degasifier where it is condensed and exposed to vacuum at a temperature close to the melting point of MS (-156.8 "C). The MS is thus liberated from gaseous impurities (hydrogen, nitrogen, oxygen) and partly from C b . Then the MS is subjected to fractional distillation. The temperature rises to -70°C. CH4, S i b , and COz impurities are removed as well. Then the temperature rises to -20 to +10 O C and MS is distilled to an individual receiver. For more complete

Methylsilane Production 521

MS purification, absorptive purification should be combined with fractional distillation. MTES can be used as an absorbent.

In general, the optimum purification technology will be selected according to commercial factors.

References [ 11 [2] [3] [4] [5] [6]

Catalog of Voltaix, 1996, USA. P. W. Shade, G. D. Cooper, JPhys. Chem. 1958,62, 1467. R. W. Kilb, L. J. Pievce, J. Chem. Phys. 1957,27, 108. T. Wartik, R. K. Pearson, J. Znorg. Nucl. Chem. 1958,5,25a. T. Wartik, R. K. Pearson, J. Znorg. Nucl. Chem. 1958,5,260. SU Patent 193511, 12G, March 26,1967.

Investigations of the Reactivity of Methylchloro- and Methylaminodisilanes toward Alkenes

Cluudia Knopf, Gerhard Roewer

Institut fur Anorganische Chemie, Technische Universitat Bergakademie Freiberg Leipziger Str. 29, D-09596 Freiberg, Germany

Tel.: +49 3731 39 3194 - Fax: 4 9 3731 39 4058 E-mail: Claudia.Knopf@chemie.tu-freiberg.de

Gerd RheinwaM, Heinrich Lang

Institut fur Chemie, Lehrstuhl Anorganische Chemie, Technische Universitat Chemnitz StraSe der Nationen 62, D-09111 Chemnitz, Germany

Tel.: +49 371 531 1200 -Fax: +49 371 531 1833

Keywords: chlorosilane, aminosilane, TDAE, TCNE

Summary: A 9: 1 molar ratio mixture of 1,1,2,2-tetrachlorornethyldisilane 2 plus pentamethyldisilane 4 reacted with tetrakis(dimethy1amino)ethylene (TDAE) to yield, in dimethoxyethane (DME), a crystal mixture of [Si3Me3C16]-.[TDAE]'+ (5a) plus [SisMezCl7]-.[TDAE]'+ (5b). The molecular structure of 5b was determined. Conversely, from donor-substituted disilanes such as 1,1,2-tris(diethylamino)-2-chloro- dimethyldisilane 6 or the N,N'-diphenylethylenediamino-substituted dimethyldisilane 7, electron transfer occurs toward tetracyanoethylene (TCNE).

Introduction

The Si-Si bond of some disilanes undergoes cleavage via a nucleophilic attack of Lewis bases. Some adducts of bidentate ligands with SizCl6 were isolated and characterized by X-ray structure analysis [ 1-31. Disproportionation of the chlorodisilane donor adducts should proceed to a monosilane plus a donor-stabilized silylene [4, 51 that, via insertion into the Si-chlorine bond, leads to oligosilane production. The reactivity of methylchlorodisilanes towards the electron-rich alkene TDAE was investigated to clarity, whether an electron transfer to the disilane is able to enable the disproportionation process of disilanes. A specific N-coordination of the TDAE to a disilane molecule or silylene is sterically hindered by the bulky dimethylamino groups [6, 71. Furthermore aminodisilanes were prepared, to screen their behavior as donors toward the electron-deficient alkene TCNE by means of UV-Vis spectra.

Ovganosilicon Chemisty V Edited by N. Auner and J. Weis

Copyright 0 2003 WILEY-VCH Verlag GmbH & Co. KgaA, Weinheim

Investigations of the Reactivity of Methylchloro- and Methylaminodisilanes toward Alkenes 523

Acceptor-Substituted Disilanes with TDAE

The methylchlorodisilanes (SiClMez-SiClMez (l), SiClzMe-SiClzMe (2), SiCl3-SiCl3 (3) as well as a 9: 1 molar ratio mixture of 2 and SiC13-SiClzMe (4)) were reacted with TDAE in n-hexane as well as DME. The results of the NMR spectroscopic analysis of the product mixtures proved that the electron-rich alkene TDAE is an effective catalyst to disproportionate chloro-substituted disilanes. The disilane reactivity is graduated, depending on the number of methyl- and chloro-substituents as expected from the results of previous investigations. Probably the capability of the catalyst to spend and to transfer electrons has a decisive importance in yielding the disilane disproportionation reaction. Using a mixture of 2 and 4 (molar ratio 9:l) we got an orange to dark red solid (5a + 5b) in addition to the liquid mixture of silanes. The 13C CP MAS NMR signals of 5a + 5b indicate two different kinds of Si-CH3 groups, at 10.1 and 14.5 ppm. Furthermore the (Me2N)ZC groups of the TDAE unit are found at 45.0 ppm. The signal of the two carbon atoms of the C=C unit in TDAE (6 = 131.2 ppm) is shifted to 6 = 156.5 ppm in the product spectrum. This drastic effect should arise from the formation of the radical cation [TDAE]''. The unpaired electron is rather distributed across the whole molecule. Taking into account the six lines in the 29Si CP MAS NMR spectrum, we concluded that two kinds of crystalline compounds coexist in the precipitate. This sophisticated result originated from the fact that the disproportionation process was really started with a mixture of the disilanes 2 plus 4. Both the precipitated products represent [TDAE]" salts of anionic trisilane species. Their middle silicon atoms are pentacoordinated by an additional chloride ligand. The NMR signals of the central Si atoms exhibit the expected downfield shift due to the hypercoordination. Those of the terminal silicon atoms are also shifted downfield relative to the values found in the parent trisilanes SiAC1Me(SiBC12Me2)2 (A: 4 . 9 ppm, B: 23.8 pprn [5]) and SiAC13-SiBC1Me-SiCC1zMe (A: 5.9 ppm, B: 4 . 7 ppm, C: 21.9 ppm [8]).

Cll c12

c12 c2

C7b C12a n 0 "

c9 C9b

C15a

Fig. 1. ORTEP plot of the molecular structure of 5b. Monoclinic, space group P 2 k . Positions of C1, C13 and C14

marked by a prime have an occupation factor of 36.5 %, the positions without a prime have an occupation

factor of 63.5 %.

524 C. KnopJ G. Roewer, G. Rheinwald, H. Lang

The asymmetric unit contains two halves of a [TDAEI- radical cation as well as the trisilane SiCl3-SiClMe-SiC12Me that has picked up an additional chloride ion (C13) at the central silicon atom. The refinement of the structure of 5b (Fig. 1) yielded two positions for the substituents at the central silicon atom (Si2) with occupation factors of 63.5 % and 36.5 % (positions marked by a prime). The ligand geometry at Si2 may be understood as an intermediate between a capped tetrahedron and a distorted trigonal bipyramid with the two chlorine substituents in the axial positions. The positions marked by a prime show a situation where C1' seems to be less strongly coordinated by Si2. The two terminal silyl units as well as the methyl group occupy the equatorial positions at Si2 with smaller angles towards C13 (or C13') than C14 (or C14'). There is one further example of a chlorinated oligosilane coordinated by additional chloride anions, namely the Si6C11:- anion reported by Boudjouk et al. [9]. The C-C distance of the [TDAE]" cation in 5b corresponds to the values calculated by Hino et al. [lo] for the mono cation (1.337 8, in TDAEO, 1.417 8, in TDAE", 1.539 8, in TDAE2+). A further indicator is the dihedral angle N-C-C-N. Mutual repulsion of the methyl groups gives rise to nonzero dihedral angles in TDAEO of 28" [ 111.

Donor-Substituted Methyldisilanes with TCNE

According to Scheme 1 the disilane 2 was reacted with the corresponding amine plus triethylamine, yielding 1,1,2-tris(diethylamino)-2-chlorodimethyldisilane (6) and N,"-diphenylethylenediamino -substituted dimethyldisilane (7) respectively. The results of the GCMS analysis point to the formation of five-membered rings (see Scheme 1).

c1 c1 I. I I I c1 c1

Me-Si-Si-Me -

2

+ 3€ NE32 NEt2

\ I

NEt2 C1

* Me-Si-Si-Me 6 + 3 HNE2 / 3NEt3 - 3NEt3 * HCl I \ - 3NEt3 * HCl I \

NEt2 C1

Scheme 1. Synthesis of the aminodisilanes 6 and 7.

The as-yet unknown aminodisilane 7 was characterized by NMR, GC/MS and IR spectroscopy (Table 1).

Investigations of the Reactivity of Methylchloro- and Methylaminodisilanes toward Alkenes 525

!

Table 1. Spectroscopic identification of the aminodisilane 7.

'H NMR [ppm] 0.75 (s) -SiMe 6.65 (m) q 6 H S

2.91 (m) -CH2- 6.76 (m) 4 6 H s

3.27 (m) -CH2 7.18 (m) -Cd&

I3c NMR [ppm] 4.45 -SiMe 117.9 P-C6HS

44.70 XH2- 129.2 m-Cd5

114.90 o-C~HS 147.3 ipSO-C6Hs

29Si NMR [ppm] -5.0 -%Me

GCNS [ d e ] 506 Si2Me2(PhNCH2CH2NPh)2

253 SiMe(PhNCHzCHzNPh)z

IR [cm-l] 1253 6,SiMe

943 v,SiNSi

UV-Vis Spectroscopic Investigations of Aminodisilanes with TCNE

As described by West et al. [12] as well as Sakurai et al. [13], methyl- and phenylpolysilanes form charge transfer complexes with TCNE exhibiting absorption maxima in the region 417-507 nm in CHC13. Amino-substituted silanes also should be able to give such complexes. As proven by UV-Vis spectroscopic studies, the TCNE bands (253 and 271 nm) disappear in the mixture with 6, but a new band appears simultaneously at 331 nm (Fig. 2).

3 4

A A

2.5

2 6 0.0781 lmKd (6)A CHC13 3 * 0.00781 m l ( 7 ) / 1 CHC13

CI 7.81 m l TCNE (7)A CHCh 0.0781 m l TCNE(6)A 1.5 2

1

0.5 762 nm 1

0 0

The spectrum of the mixture of 7 plus TCNE (Fig. 3) exhibits a CT band (maximum at 762 nm). Its maximum is bathochromically shifted widely, compared with known TCNE complexes [ 12, 141

526 C. KnopJ; G. Roewer, G. Rheinwald, H. Lung

and the [TCNE]" anion radical, which shows broad absorption in acetonitrile from 350 to 500 nm

~151 .

References [ l ] D. Kummer, H. Koster, Angew. Chem. Int. Ed. 1969,81,599 and 878. [2] D. Kummer, A. Balkir, H. Koster, J. Organomet. Chem. 1979,178,29. [3] G. Sawitzki, H. G. v. Schnering, Chem. Ber. 1976,109,3728. [4] R. Richter, G. Roewer, U. Bohme, K. Busch, F. Babonneau, H. P. Martin, E. Muller, Appl.

Organomet. Chem. 1997,11,7 1. [4] U. Herzog, R. Richter, E. Brendler, G. Roewer, J. Organomet. Chem. 1996,507,221. [5 ] N. Wiberg, J. W. Buchler, Chem. Ber. 1963,96,3223. [6] N. Wiberg,Angew. Chem. 1968,20, 809. [7] U. Herzog, N. Schulze, K. Trommer, G. Roewer, Main Group Metal Chem. 1999,22, 19. [8] S.-B. Choi, B.-K. Kim, P. Boudjouk, D. G. Grier,J. Am. Chem. SOC. 2001,123,8117. [9] S. Hino, K. Umishita, K. Iwasaki, K. Tanaka, T. Sato, T. Yamabe, K. Yoshizawa, K.

Okahara, J. Phys. Chem. A 1997,101,4346. [lo] H. Bock, H. Borrmann, Z. Havlas, H. Oberhammer, K. Ruppert, A. Simon, Angew. Chem.

1991,103,1733. [ 111 V. F. Traven, R. West, J. Am. Chem. SOC. 1973,95,6824. [12] H. Sakurai, M. Kira, T. Uchida, J. Am. Chem. SOC. 1973,95,6826. [13] T. L. Cairns, B. C. McKusick, Angew. Chem. 1961,15,520. [I41 0. W. Webster, W. Mahler, R. E. Benson, J. Am. Chem. SOC. 1962,84,3678.

New Organofunctional Silanes for Adhesives, Sealants and Spray Foams

A. Bauer, T. Kammel,* B. Pachaly, 0. Schafer, W. Schindler," V. Stanjek," J. Weis

Consortium fur Elektrochemische Industrie GmbH Zielstattstral3e 20, D-8 1379 Munich, Germany

Tel.: +49 89 74844 0 - Fax: 4 9 89 74844 350 E-mail: andreas.bauer@wacker.com, thomas.kammel@ wacker.com

bernd.pachaly @ wacker.com, oliver.schaefer@wacker.com wolfram.schindler@ wacker.com, volker.stanjek@wacker.com johann.weis @wacker.com

Keywords: a-aminosilanes, a-isocyanatomethylsilanes, adhesives, spray foam, crosslinker

Summary: Besides the well-known application of silanes as adhesion promoters and surface modifiers, organofunctional silanes have also long been applied as terminating agents in organic polymers. The silane condensation in such polymer systems is currently replacing other curing chemistry, such as the use of isocyanates in polyurethanes. For example, silyl-endcapped polyethers and polyurethanes have been developed and their application potentials in sealants, adhesives and coatings have been verified.

This work gives a survey of a new class of a-organofunctional alkoxysilanes and their potential use. These silanes exhibit dramatically enhanced reactivities. Through variation of the organofunctional group (e.g. isocyanato, amino) access to numerous applications is possible.

With isocyanatomethyl alkoxysilanes (a-NCO-silanes) as endcappers, polymers with enhanced properties are feasible. This concept involving a-NCO-silanes is not limited to distinct polymer types and therefore has a broad application base. Additionally, highly reactive ethoxy-functional crosslinking systems are now available. These polymers show low viscosity and consequently a higher formulation flexibility.

An intriguing new application of a-aminosilanes are silane crosslinking spray foams. These foams do not contain any isocyanates which are critical to toxicological and enviromental implications. Due to their high reactivities, a-silanes exhibit excellent curing characteristics. Moreover they provide good mechanical properties and are produced from only low-cost compounds.

Ovganosilicon Chemisty V Edited by N. Auner and J. Weis

Copyright 0 2003 WILEY-VCH Verlag GmbH & Co. KgaA, Weinheim

528 A. Bauer, T. Kammel, B. Pachaly, 0. Schafer, W. Schindler, V. Stanjek, J. Weis

A New Class of Rapid Crosslinkers: a-NCO-silanes

In the area of organofunctional silanes, there is an increasing demand for highly reactive isocyanatosilanes. These compounds are needed in significant amounts for the development of new polmer hybrid systems, e.g. for the functionalization or endcapping of new types of polymers for adhesives [l], sealants [2, 31 or elastomers (see the following sections). Due to the limited availability of suitable, highly reactive and cost-efficient isocyanatosilanes for these applications, a new class of highly reactive NCO-silanes was necessary. Starting from the corresponding chloromethylsilanes these highly reactive isocyanatosilanes can be synthesized as presented in Scheme 1 [4, 51. Compared to the organofunctional silanes with propyl-spacers, these derivatives with methyl spacers start from low-cost precursors, thus facilitating favorable product costs and their usage in additional and new application areas.

CI 0 . CH3 CI 0. CH3 (OW

I CH3OH I

I [ Base ] I * RO-Si-CH&I

I h v IClp I CI 0. CH3-Si-CH3 t CI 0 . CH3-Si-CHzCI

CI 0. CHs CI 0. CH3

Methyl-Silane

M#

Chloromethyl-Sllane

CMM#

Chloromethylalkoxy-Silane

CMM#

(OW I

I (OW

(o)R H KNCOIROH catalyst

t RO-Si-CH2- ' $OR * RO-Si-CHz-N=C=O AT

0 DMFIAT I

Carbamato-Silane Isocyanatomethyl-Silane

Scheme 1. Synthesis of highly reactive isocyanatomethylsilanes.

Methylchlorosilanes, which are one of the major products of every fully integrated silane and silicone producer, are converted to the corresponding chloromethylsilanes by photochlorination. The latter are converted efficiently and in high yields to their alkoxy derivatives. In the third synthetic step, these alkoxy derivatives are reacted with potassium cyanate in the presence of stochiometric amounts of methanol in an aprotic solvent to give the corresponding carbamatoorganosilanes in reasonable to good yields. In the final step, the latter are catalytically converted in excellent yields (see Table 1, below) to the desired isocyanatosilanes, which are subsequently isolated by distillation.

One big advantage of this process is the opportunity to perform the conversion of carbamatoorganosilanes to the corresponding isocyanatosilanes in a continuous manner. In addition, the usage of highly toxic phosgene as reactant can be avoided. Having the same substitution pattern, these a-NCO-silanes exhibit a much higher reactivity compared with their y-NCO derivatives, due

New Organofunctional Silanes for Adhesives, Sealants and Spray Foams 529

to the activation by the so-called alpha-effect [6] of the nitrogen atom (see, e.g., Ref. 6, in which the spectroscopic detection and the interpretation of this effect are convincingly presented).

Table 1. Experimental data for the synthesis of different a-NCO-silanes.

Product Mass Conversion Product Yield balance [%] [%] balance [%] [%]

Precursor Conditions

CH3 H I I I 2 350°C H3CO-Si-CH2-N=C=O

H3CO-Si-CH2-N 90 85 89 > 90 I

CHI

I y O C H 3 0 Catalyst A CHI CHI

H~C-~~--CH,--%OCH~ 86 I Catalyst A

OCH3 0

I H3CO-Si-CH2-N=C=O

84 75 > 90 I CH3

0'3% H ( 7 t h I

'KocH3 Catalyst A 0 ' 3 3 I

t 350 "C > 90 H3CO-Si-CH2-N=C=0

I H3CO-Si-CH2-N 90 77 89

OCH3 0

Using this synthetic route, six different NCO-silanes bearing three, two or one methoxy (TMO, DMO, MMO) or ethoxy group (TEO, DEO, MEO) at the silicon atom are accessible, respectively (see Table 2, below). The dimethoxy (a-NCO-DMO) and the monoethoxy derivatives (a-NCO- MEO) are still not registered in CAS.

Table 2. a-NCO-silanes accessible by the given synthetic pathway.

Derivatives CM CMMl CMM2

OCH3 I

I oC H3

H3CO-Si-CH2-N=C=O Methoxy

[78450-75-61 OCH2CH3

H3CH2CO-Si-CH2-N=C=O

OCHzCH3

I

I Ethoxy

[132112-76-61

oC H3 I

I CH3

H3CO-Si-CHa-N=C=0

NEW !

I I

CH3

OCH2CH3

H3CH2CO-Si-CH2-N=C=0

[20160-30-91

CH3 I

I c H3

[35450-25-01

H3CO-Si-CH2-N=C=0

CH3 I I

CH3

NEW !

H3CH2CO-Si-CH2-N=C=0

In Table 1 typical experimental data are presented for the final synthetic step, the catalytic conversion of the carbamatoorganosilanes to the corresponding NCO derivatives. The unconverted carbamatosilane can by recycled by distillation and re-used in the process. In addition, this technology and synthetic route are applicable to y-NCO-silanes such as trimethoxysilylpropylisocyanate (y-NCO-TMO) as well.

530 A. Bauer, T. Kammel, B. Pachaly, 0. Schafer, W. Schindler, V. Stanjek, J. Weis

Eventually, the complete synthetic process will be executed in a new dedicated production unit as presented in Fig. 1. The necessary amounts of the different chloromethylsilanes and their corresponding precursors (methylchlorosilanes) are already available on an industrial scale within Wacker-Chemie. The alkoxylation process, a well-developed technology, will be run continuously as well whereas the carbamatosilane production will be a batch-type synthesis. The purified carbamatosilane will be fed into the tube reactor, in which the catalytic thermolysis will lead to the desired NCO-silanes. The latter will be isolated in high purity by distillation.

Fig. 1. Draft of the planned production unit for the synthesis of a-NCO-silanes.

NCO-silane-Terminated Copolymers with Tunable Curing Rates

The following results give an overview of the possibilities of new silane-terminated polymers (STP) using isocyanatomethylalkoxysilanes (a-NCO-silanes) for endcapping. These STP systems show dramatically enhanced reactivities and curing rates. Furthermore two possible applications in new fast rtv-silicone copolymer systems were examined.

Figure 2 shows the reactivity, measured as tack-free time in STP-S polymer systems (see below) of systems terminated with different NCO-silanes. The new a-silanes show dramatically enhanced curing rates. Additionally, the curing rates of compounds made from these polymers can be varied over a wide rage, using different amounts of catalyst (usually tin) and water scavengers (usually alkoxysilanes). Comparable results were achieved in silane-terminated polyethers and polyurethanes.

New Organofimctional Silanes for Adhesives, Sealants and Spray Foams 531

OEt OEt

O C f i # i C H 3 OEt O C q i - C H 3 PMe O C 4 i o E t OEt O C f i y i - O M e ?Me

OMe OMe

> 2 h 30 min 5 min c lmin I

I I I w ’ reactivity Et O C T i - O M e ?Me

OMe OC -7 ? i-OEt

OEt

Fig. 2. Reactvity of alkoxysilanes: methylene vs. propylene spacers.

Silane-Terminated Polysiloxanes (STP-S)

Silane-terminated polysiloxanes are a new class of high reactive rtv-1 silicone systems. The synthesis of these polymers is shown in Eq. 1. In a first step a hydroxy-terminated polydimethylsiloxane is converted to the corresponding aminopropyl-polydimethylsiloxane by reaction with stoichiometric amounts of 3-[(2,2-dimethyl- 1,2-azasilolidin-l-yl)dimethylsilyl]-l -propylamine at room temperature. The silane termination reaction in a second step is carried out under the same conditions without adding further catalysts, due to the fast reaction of NCO groups with the primary amino groups.

OR I

I OR

+ R(O)-Si-(CH&,,-N=C=O NHz

n H3C CH3

p” OR I

OR OR CH3 0

Eq. 1. Synthesis of silane-terminated polydiorganosiloxanes (STP-S)

These polymers can be compounded by standard procedures to rtv-1 silicone sealants. The reactivity can be tuned simply by varying the silane used. Using a-NCO-silanes for endcapping there is no need for tin catalysts to accelerate curing time. The properties of cured elastomers of some examples are shown in Table 3.

532 A. Bauer, T. Kammel, B. Pachaly, 0. Schaler, W. Schindler, V. Stanjek, J. Weis

Table 3. Properties of silane-terminated polydiorganosiloxanes (STP-S).

Silicone

n=1000 n = 8 0 0 n=600 n = 2 0 0 n=800 n = 8 0 0

NCO-silane y-TMO y-TMO y-TMO y-TMO ~L-DMO cL-TMO

1.72 1.65 1.49 1.21 1.69 1.70 Tensile stress at break [Mpa s] (DIN 53504)

Tensile strain at break [%] (DIN 53504) 495 395 335 625 507 560

Hardness (Shore A) 17 16 18 20 15 18

Tack-free time [min] 15 15 15 15 7.5 5

[a] Polymers cured in 2 mm thick Teflon molds at defined humidity (50 %) and room temperature. Tensile properties given were measured after storing for 2 weeks (23 W 5 0 % rh).

Silane-Terminated Siloxane-Urea Copolymers

Additionally there are interesting applications using these silanes as crosslinkers in silicone copolymers, e.g. for moisture-curable silicone hot melts. Thermoplastic silicone elastomers can be synthesized via reaction of aminopropyl-terminated silicone oils (for synthesis, see above) with commercially available diisocyanates [e.g. isophorone diisocyanate or methylenediphenyl diisocyanate (IPDI or MDI)]. With an (H2N:NCO) ratio > 1, amino-terminated prepolymers are yielded, which can be endcapped with NCO-silanes as described above. These materials show thermoplastic behavior with processing properties (viscosity, melting point, etc.) depending on the diisocyanate and the molecular weight of the silicone segments. They have good mechanical properties with up to 5 MPa tensile strength and good adhesion to various substrates, even without using standard silane adhesion promoters (Table 4).

Table 4. Properties of silane-terminated siloxane-urea copolymers.

Silicone

n = 2 0 n =50 n = 90 n = 5 0 n = 50

NCO-Silane y-TMO y-TMO y-TMO y-TMO y-TMO

Diisocyanate

Tensile stress at break [Mpa s ] (DIN 53504)

IPDI + 20 % 1,Cbutanediol IPDI IPDI IPDI MDI

3.14 2.59 1.80 4.68 3.67

233 23 1 300 435 507 Tensile strain at break [%I (DIN 53504)

Hardness (Shore A) 48 38 21 48 45 ~ ~~~~ ~~~~ ~

[a] Polymers cured in 2 mm thick Teflon molds at defined humidity (50 %) and room temperature. Tensile properties given were measured after storing for 2 weeks (23 "C/50 % rh).

New Organofinctional Silanes for Adhesives, Sealants and Spray Foams 533

Silane Crosslinking High-Performance Spray Foams

All conventional spray foams are based on polyurethane (PU) systems [7]. They provide excellent mechanical properties and outstanding thermal insulation. Thus they are widely used, especially in construction, i.e. pipe or building insulation and interior applications.

Nevertheless all spray foams have the crucial disadvantage that they contain isocyanates, which are critical with respect to toxicological and enviromental implications [8]. Monomeric isocyanates like TDI (toluene diisocyanate) or MDI (methylenediphenyl diisocyanate) are even suspected of causing cancer [8, 91. Although these foams are used even by nonprofessionals, to date there exist no suitable, less endangering substitutes.

Curing Reactions for Spray Foams

Conventional (single-component) polyurethane spray foams are cured by a reaction of prepolymers containing isocyanate moieties and monomeric isocyanates with moisture to generate a urea unit and COz [7]. The latter serves as propellant or - more often - as co-propellant besides being a physical blowing agent.

To develop isocyanate-free, toxicologically safe spray foams it is necessary to replace this curing reaction by a silane crosslinking system that is not only moisture curing but also possesses high curing rates. This goal is reached by using silane-terminated prepolymers, which are synthesized from a-silanes and therefore exibit a very high reactivity towards moisture. As there are no gasous byproducts generated by silane condensations, the foam must be blown by a physical blowing agent exclusively.

Suitable Silane-Terminated Prepolymers for Spray Foam Applications

The silane-terminated prepolymers are prepared from polypropyleneglycol (M = 450), TDI, and (N- phenylaminomethy1)trimethoxysilane (1; Fig. 3).

TDI 2

Fig. 3. Substrates of the silane-terminated prepolymers for spray foam applications.

The prepolymer can be synthesized by a simple "one-pot synthesis". During a first step an NCO-terminated prepolymer is prepared by a reaction of the polyol with an excess of TDI. The second step comprises the reaction of all remaining NCO groups with the silane 1. Phenyltrimethoxysilane 2 serves to reduce the viscosity of the prepolymer mixture. It has already been added during the prepolymer synthesis.

A foamable coumpound is produced by compounding the prepolymer mixture obtained with a

534 A. Bauer, T. Kammel, B. Pachaly, 0. Schaler, W. Schindler, V. Stanjek, J. Weis

physical blowing agent, a foam stabilizer and further additives (e.g. adhesion promoters, catalysts etc.).

Like the above-mentioned prepolymers endcapped with a-NCO-silanes, the silane-terminated prepolymers generated from the a-silane 1 also show dramatically enhanced reactivities and curing rates. As these prepolymers are much more reactive than conventional silanes, a low-cost compound like phenyltrimethoxysilane 2 can be added to adjust the viscosity of the prepolymer without any decrease in the curing speed.

During foam curing the silane 2 is incorporated into the emerging network. Therefore the content of this silane has only a little effect on the hardness of the cured polymer (Fig. 4).

Fig. 4. Effect of the silane 2 on the viscosity of the prepolymer and the hardness of the cured polymer.

Properties of the Silane Crosslinking Spray Foams

The cured silane crosslinking spray foams exhibit very good mechanical properties. They possess a high hardness and a very good elasticity. The compressive strength is comparable to common polyurethane (PU) spray foams. Foam densities < 40 kg/m3, i.e. the densities of common PU foams, are possible, too.

No shrinking of the foam during curing could be observed. As shown in Fig. 5 the resulting foam provides a good structure.

Due to the high reactivity of the prepolymer, extremely high curing rates (tack-free times < 1 min) can be achieved. Tin-free systems are possible, too (catalyst (e.g.) tertiary amines).

As the silane 1 can be prepared from low-cost raw materials (Eq. 2), the new spray foams are cost-competitive with conventional PU spray foams.

As far as they have yet been tested, the silane-terminated prepolymers for spray foam applications are toxicologically safe and not irritant. Therefore all the demands on silane crosslinking spray foams are met by our system.

New Organofunctional Silanes for Adhesives, Sealants and Spray Foams 535

Fig. 5. Silane-crosslinking spray foams.

Eq. 2. Synthesis of a-silane 1.

References M. Huang, R. Johnston, P. Lehmann, N. Stasiak, B. Waldmann, Adhesive Technol. 1998, 5, 20-25. K. Hashimoto, J. Adhes. Sealant Council, 1997,77-87. M. Probster, Adhasion, 1990, 5, 37-39. a) V. P. Kozyukov, E. V. Muzovskaya; V. F. Mironov, Zh. Obshch. Khim. 1983, 53(5), 1096-1103; b) V. D. Sheludyakov, E. S. Rodionov, G. D. Khatuntsev, V. F. Mironov, Zh. Obshch. Khim. 1971, 41(10), 2340-2341; c) V. D. Sheludyakov, E. X. Rodionov, V. F. Mironov, Zh. Obshch. Khim. 1974, 44(5), 1044-1049; d) V. P. Kozyukov, V. D. Sheludyakov, V. F. Mironov, Zh. Obshch. Khim. 1968,38(5), 1179-1185. a) J. Gulinski, H. Maciejewski, I. Dabek, M. Zaborski, App. Organomet. Chem. 2001, 15, 649-657; b) V. F. Mironov, V. D. Sheludyakov, V. P. Kozyukov, G. D. Khatuntsev, Zh. Obshch. Khim. 1969, 39(4), 813-816; c) V. D. Sheludyakov, F. N. Vishnevskii, E. S. Rodionov, G. D. Khatuntsev, V. F. Mironov, Zh. Obsh. Khim. 1972,42(4), 879-880; d) V. F. Mironov, V. D. Sheludyakov, V. P. Kozyukov, Organomet. Chem. Synth. 1972, I , 329-340; e) A. MacGregor, P. M. Miranda, Polym. Preprints 2001,42(1), 167-168. N . Egorochkin, S. E. Skobeleva, E. I. Sevast'yanova, I. G. Kosolapova, V. D. Sheludyakov, E. S. Rodionov, A. D. Kirilin, Zh. Obsh. Khim. 1976,46(8) 1795-1800 M. Szycher, Szychers 's Handbook of Polyurethanes, Publ.: CRC Press, London, 1999. US Environmental Protection Agency, Toxicological Review of Methylene Diphenyl Diisocyanate (MDI), 1999. TDI already has to be labeled as cancerogenic: (Cancerogenic Category 3).

Isocyanatopropyltrimethoxysilane - Key Intermediate of New Silane Coupling Agents

Hieronim Maciejewski, Bogdan Marciniec"

Adam Mickiewicz University, Faculty of Chemistry Grunwaldzka 6,60-780 Poznab, Poland

Tel.: +48 61 8291366 -Fax: +48 61 8291508 E-mail: marcinb@main.amu.edu.pl

Agnieszka Wyszpolska

Poznan Science and Technology Park, Adam Mickiewicz University Foundation Rubiez 46,61-612 Poznab, Poland

Keywords: silane coupling agents, isocyanato, carbamato, urea, silanes

Summary: Isocyanatopropyltrimethoxysilane is a very useful starting material for the preparation of new compounds. All carbamato- and urea-functional silanes were synthesized by a very simple, convenient and efficient method. These products form a new class of silane coupling agents (for special applications) and are also starting materials for modification of polymers (especially unsaturated compounds) or for other organic syntheses.

Introduction

Organofunctional silanes of the general formula Y-(CH&Si(OR)3, where Y is a functional group and/or, a hydrolyzable group, are the most technologically important group of adhesion promoters in use today. The range of commercially available organosilanes is wide and continuously expanding, depending on the type of application [I, 21. One of the most useful and potentially versatile representatives of this group is 3-isocyanatopropyltrimethoxysilane, which can be used as

0

0

a coupling agent, capable of chemically bonding organic polymers to inorganic substrates [3], a crosslinking agent - it is widely applied to various chemical reactions because of the extreme reactivity of its isocyanate group [4], a valuable substrate for syntheses of silane coupling agents containing new functional groups at carbon and nitrogen (by means of reactions with alcohols [5-71 or amines [S]). High reactivity of the NCO group has led to syntheses of carbamato- or urea-organosilanes.

Ovganosilicon Chemisty V Edited by N. Auner and J. Weis

Copyright 0 2003 WILEY-VCH Verlag GmbH & Co. KgaA, Weinheim

Isocyunatopropyltrimethoxysilune - Key Intermediate of New Silune Coupling Agents 537

The aim of this communication is to present an efficient method of synthesis of 3-isocyanatopropyltrimethoxysilane and its application for preparation of numerous carbamato- and urea-functional silanes.

Synthesis of 3-Isocyanatopropyltrimethoxysilane

A few methods of synthesis of 3-isocyanatopropyltrimethoxysilane have been proposed:

hydrosilylation of ally1 isocyanate (Eq. 1) [9];

CH2=CHCH2NC0 + HSi(OR)3 - (R0)3Si(CH2)3NC0

Eq. 1.

reaction between 3-aminopropyltrimethoxysilane and phosgene (Eq. 2) [ 101;

-HCI * (Me0)3Si(CH2)3NC0

Eq. 2.

reaction between 3-aminopropyltrimethoxysilane and dimethylcarbamate (Eq. 3 ) [ 101.

(MeO)3Si(CH2)3NH- E -0Me -- However, the most convenient and useful method seems to be

nucleophilic substitution of chlorine in 3-chloropropyltrimethoxysilane by isocyanate (Eq. 4)

[ I l l .

cat. (Me0)3Si(CH2)3CI + KOCN DMF * (Me0)3Si(CH2)3NC0 + KCI

Eq. 4.

The above method was also developed [I21 and scaled-up at the Unisil Co, Ltd., Tarn6w

538 H. Maciejewski, B. Marciniec, A. Wyszpolska

(Poland). Contrary to other versions of this method, the catalytic system used in this reaction (tertiary phosphines with KI) made it possible to obtain product in high yield, under mild conditions [ 121. 3-Chloropropyltrimethoxysilane (which is also manufactured by Unisil) is a good starting material for synthesis of various silane coupling agents, such as methacryloxy-, amino-, mercapto- or ureido-functional silanes.

Similarly to other silane coupling agents, 3-isocyanatopropyltrimethoxysilane has a wide range of applications. However, it is worth mentioning that commercially available adhesion promoters do not increase adhesion to all substrates and tend to volatilize significantly before the curing temperature of the adhesives is reached. For this reason, there is a need for new adhesion promoters of higher reactivity and lower volatility than those commercially available at present. Therefore, compounds containing silane and a polar functionality such as carbamate or urea are very promising adhesion promoters for use in coatings and substrates for electronic applications.

Synthesis of Carbamato- and Urea-Functional Silanes

Carbamato- or urea-organosilanes can be synthesized in two ways:

By a direct reaction of 3-isocyanatopropyltrimethoxysilane and an appropriate alcohol (in the presence of organotin compounds as catalysts) or amines (in the absence a catalyst) (Scheme 1).

(CH30)3Si(CHz)3NH- (CH30)3Si(CH2)3NH-

where R = CHz=CHCHz; (CH3)zCH; where R = CH3; (CH3)zCH; CH2CH20CH3; (CH3)3C; CH2CH2NH2 CHzCHzOH; CHz=CHCHz; (CH3)3C

Scheme 1.

In the reactions in scheme 1, the products were synthesized selectively, under very mild conditions (room temperature, 1 h). In this way several new carbamato- or urea-functional organosilanes have been synthesized and characterized, using various linear or branched, as well as aromatic or unsaturated alcohols (amines) as starting materials (see Scheme 1). They are new compounds; therefore each of them was isolated and characterized spectroscopically. The future applications of the unsaturated derivatives, which were obtained in the reactions with allylamine or ally1 alcohol, seem to be very interesting.

Isocyanatopropyltrimethoxysilane - Key Intermediate of New Silane Coupling Agents 539

By the reaction between 3-chloropropyltrimethoxysilane and KOCN in the presence of alcohols or amines (Scheme 2).

(CH30)3Si(CHz)3CI + KOCN

(CH30)3Si(CH2)3NH- (CH30)3Si(CHz)3NH-

where R = CH2=CHCH2; (CH3)zCH; where R = CH3; (CH3)zCH; CHzCHzOCHg (CH3)3C CH2CH2NH2 CHzCHzOH; CH&HCHz; (CH3)3C

Scheme 2.

The latter method resulted in a one-step reaction, in the same products as those presented above but under different reaction conditions. During this process, 3-isocyanatopropyltrimethoxy- silane is formed in situ, and the reaction has to be carried out at a higher temperature (120-130 "C) in DMF as a solvent and in the presence of a catalytic system. After the reaction completed, the solvent has to be removed and generally the product has to be purified; it was not necessary in the former method.

Conclusions

The methodology of synthesis developed makes it possible to obtain optional carbamato- or urea-functional silanes. The choice of synthesis method depends on substrate availability as well as on technological possibilities.

All carbamato- and urea-functional silanes were synthesized by a very simple, convenient and efficient method. These products form a new class of silane coupling agents (for special applications) and also can play the role of starting materials for modification of polymers (unsaturated compounds, in particular) or for other organic syntheses.

Acknowledgments: Financial support for this study by the State Committee for Scientific Research, Poland, Project No. 7 T09B 004 20, is gratefully acknowledged.

References [ 11 [2] [3] [4]

E. D. Plueddeman, Silane Coupling Agents, 2nd edn., Plenum Press, New York, 1991. K. L. Mittal (Ed.), Silanes and Other Coupling Agents, VSP, Utrecht, 1992. A. Pizzi, K. L. Mittal, Handbook of Adhesive Technology, Marcel Dekker, New York, 1994. J. I. Kroschwitz, M. Howe-Grant (Eds.), Kirk-Othmer Encyclopedia of Chemical Technology,

540 H. Maciejewski, B. Marciniec, A. Wyszpolska

John Wiley, New York, 1997. US Patent 5 384 342 (1995). French Patent 2 483 421 (1991). Jpn. Patent 04 235 993 (1992). Jpn. Patent 05 170 777 (1993). US Patent 3 51 1 866 (1970). US Patent 5 218 133 (1995). German Patent 3 524 215 (1987). Polish Patent 182 009 (1996). Polish Patent P-343 141 (1999).

Development of Adhesion Promoters on the Basis of Secondary Reactions of Carbofunctional

Organosilicon Monomers

V. A. Kovyazin, V. M. Kopylov, A. V. Nikitin

Federal State Unitary Enterprise State Research Institute for Chemistry and Technology of Organoelement Compounds, 38 Shosse Entuziastov, Moscow, 11 1123, Russia

Fax: +7 095 273 72 06 E-mail: vlkov@aport.ru

Keywords: adhesion promoters, organosilicon amines

Summary: Investigations have been carried out on development of different types of organosilicon adhesion promoters on the basis of organosilicon carbofunctional amines, for the purpose of expanding their fields of application.

Interaction of 3-aminopropyltriethoxysilane with glycidyl methacrylate at molar ratios 1: 1 and 1: gives adhesion promoters containing one or two methacrylate groups according to Eqs. 1 and 2.

It was shown that the monoacrylate derivative exists as a mixture of two isomers - aminoester (1 and aminoalcohol (11). The products obtained are efficient adhesion promoters for glass-reinforce plastics based on various binders (polyester, polyepoxide, polyacrylate) since they contain three type of functional groups - amino, methacrylate and alcohol - that allow them to enter into reactior with binder functional groups.

(EtO),Si ( C€&),NH2 + CH,-CHCH,OOCC=CH, - \/

0

CH3 I

CH3 0

>CbCHI

(EtO),Si(CH,),NH CH2FHCH200CC=CH2 - (EtO),Si(CH,),N<

I OH

I

‘CH,CHCH,OH I

I1 OH

Eq. 1.

Ovganosilicon Chemisty V Edited by N. Auner and J. Weis

Copyright 0 2003 WILEY-VCH Verlag GmbH & Co. KgaA, Weinheim

542 V. A. Kovyazin, V. M. Kopylov, A. V. Nikitin

CH3 I CH3

I (EtO),Si(CH2),NH2 +2 HCH2OOCd=CH2 + (Et0)3Si(CH2)3N(CHz HCH2OOCC=CH& c

0 OH I11

Eq. 2.

Organosilicon aminoacids and their ammonium salts are obtained by interaction of amines with anhydrides of unsaturated dicarboxylic acids (maleic, phthalic, 3-methyltetrahydrophthalic). Thus, from the reaction of 3-aminopropyltriethoxysilane with maleic anhydride, for example, a neutral product IV was obtained, which according to NMR spectroscopy data is an ammonium salt (Eq. 3 andFig. 1).

0

Eq. 3.

5 8 5 7 5 6 5 5 5 4 5

Fig. 1. 'H NMR spectrum of IV.

u 3 5 2 5 1.5

IV

L 0.5 -05 -1 5 -25

The ammonium form of IV, upon interaction with an equimolar amount of

Development of Adhesion Promoters on the Basis of Secondary Reactions 543

3-aminopropyltriethoxysilane, leads to a neutral product V (Eq. 4).

IV + (EtO),SiCH2CH2CH2NH2 - d e f a b c

(EtO),SiCH2CH2CH2NHCOCH=CHC0~N+H3CH2CH2CH2Si( OEt),

V

Eq. 4.

The attribution of proton signals of compound V (Fig. 2) was done on the basis of 'H NMR data for 3-aminopropyltriethoxysilane and IV.

J I - 0.5 0 . 5 -1.5 -2.5

Fig. 2. 'H NMR spectrum of V.

The signals of the methylene groups in the propylenic bridges are quite separate from each other, as follows: a, f -0.62, b -1.61, e -1.74, c -2.83, d -3.22 ppm. In accordance with integral intensity the extended singlet with 6 = 7.61 ppm shift is assigned to an 'NH3 group and a singlet with 6 = 9.50 pprn to an NHCO group.

By NMR method it has been shown that in some cases for these products characteristic intramolecular salt formation occurs, depending on the nature of the anhydride. The effect of the reagent ratio on salt structures was studied. The products are readily soluble in water and polar organic solvents.

In order to raise the solubility of organosilicon amines and their derivatives in water, their

544 V. A. Kovyazin, V. M. Kopylov, A. V. Nikitin

re-etherification processes by ethyleneglycol monoethyl ether (ethylcellosolve) were investigated. The interaction of aminoalkoxysilanes with ethylcellosolve proceeds by scheme 1.

kl

VIa, VIb, VIc k- 1

b NH2RSi(OR1)3 + CH3CH20CH2CH20H . NH2RSi(OR1)2(0CH2CH20CH2CH3) + R'OH

VIIa, VIIb, VIIc kz

VIIa, VIIb, VIIc k-2

NH2RSi(OR1)2(OCH2CH2OCH2CH3) + CH3CH20CH2CH20H

NH2RSi(OR1) (OCH2CH20CH2CH3)2 + R'OH VIIIa, VIIIb, VIIIc

k3 . NH2RSi(OR1)(OCHzCH2OCH2CH3)2 + CH3CH20CH2CH20H +-. VIIIa, VIIIb, VIIIc k-3

NH~RS~(OCH~CHZOCH~CH~)~ + R'OH, IXa, IXb, IXc

Scheme 1. Re-esterification of organosilicon amines by ethyl cellosolve, where R = (CH& for VIa-IXa, VIc-IXc;

R = (CH2)3NH(CH2)2- for VIb-IXb; R' = Me- for VIb-VIIIb and VIc-VIIIc; R' = Et- for VIa-VIIIa;

kllk.1 = Kpl , kZlk.2 = Kp2, k3lk.3 = Kp3.

Equilibrium compositions of re-etherification products were determined in terms of reagent ratios. Re-etherification rate constant and equilibrium constant ratios were obtained (Table 1).

Table 1. Average values of equilibrium constants.

Starting substance KPl KP2 Kp3

3-Aminopropyltrimethoxysilane 0.925 0.365 0.080

3-Aminopropyltriethoxysilane 1.069 0.394 0.102

3-N-( 2-Aminoethyl)aminopropyltrimethoxysilane 1.139 0.403 0.128

Synthesis of Chiral Amino-Substituted Organosilanes

Uwe Bohme, Betty Giinther, Ben Rittmeister

Institut fur Anorganische Chemie, Technische Universitat Bergakademie Freiberg Leipziger StraBe 29, D-09596 Freiberg, Germany

E-mail: Uwe.Boehme@chemie.tu-freiberg.de

Keywords: amides, silanes, disilanes, aminosilanes, stereochemistry

Summary: Defined crystalline amino-substituted organosilanes were prepared by reaction of N-methylaniline with 1,1,2,2-tetrachlorodimethyldisilane and other chloroorganosilanes. The X-ray structure analyses of 1 ,Zdichloro-1 ,Zbis(N-methyl- ani1ino)dimethyldisilane (l), 1 -chloro- 1,2,2-tris(N-methylanilino)dirnethyldisilane (2) and tetrakis(N-methylanilino)-1,2-dimethyldisilane (3) were performed. The absolute structure of 1 was determined by crystallographic methods. 1,2-Dichloro - 1,2-bis(N-methylanilino)dirnethyldisilane (1) is a useful reagent for the preparation of a variety of other aminoorganodisilanes.

Introduction

The substitution of chlorine with dialkylamides is a known reaction for chlorosilanes. The use of dialkylamides as protecting groups is one important application which allows the preparation of derivatives which are not accessible in a straightforward synthesis [l-31 A number of aminochloroorganosilanes have been prepared, but these are often oils and it is very difficult to separate pure compounds from the mixture of liquid products [ 1, 21. A number of X-ray structures of aminoorganosilanes without chlorine substituents are known [4]. Until now, only very few structural data on aminochloroorganosilanes are available [5].

Results and Discussion

The reaction of N-methylaniline with 1,1,2,2-tetrachlorodimethyldisilane in presence of NEt3 gives the mono-, di- and triaminodisilanes depending on the reactant ratio. The tetraamide 3 is only formed in the reaction with lithium N-methylanilide. The compounds possess very different reactivity. l-Chloro-l,2,2-tris(N-methylanilino)dimethyldisil~e (2) is rather inert towards nucleophilic reagents. 1,2-Dichloro-l,2-bis(N-methylanilino)dimethyldisilane (1) reacts with nucleophiles with substitution of both chlorine atoms (see Scheme 1). The products formed during

Ovganosilicon Chemisty V Edited by N. Auner and J. Weis

Copyright 0 2003 WILEY-VCH Verlag GmbH & Co. KgaA, Weinheim

CI

Me

\I

c''$i

- y

-C'

Me

/N\P

h M

e @

Si) =

21.

4 pp

m, -

0.6

ppm

I l:* ex

cess

6(%

i) =

6.27

ppm

. -7.

94

pprn

Me

ZnF2

(exc

ess)

S("S

i) =

-25.

8 pp

m

+ 2

Me(

C&

)MgB

r

Me

Y\P

h

Me

&("

Si)

= 2.

4 pp

m,

' Jsw

= 3

26 H

z, 'J

&_F

= 63

HZ

qmsi

) = -7.

5 pp

rn, -

7.7

pprn

(rac

, mes

o)

Sche

me

1

+ 2L

iNM

ePh

1 3

Synthesis of Chiral Amino-Substituted Organosilanes 547

A number of oligomeric chloroorganosilanes were reacted with N-methylaniline (Table 1). It is possible to obtain selectively mono- and disubstituted aminosilanes from the shorter a,@ dichlorosilanes. The reactivity of the homologous compounds Cl(SiMe2),Cl decreases with increasing chain length. Therefore the tetrasilane Cl(SiMe2)4Cl gives only the monosubstituted product Cl(SiMe2)4NMePh. The branched chlorosilanes MeClSi(SiC1zMe)z and MeSi(SiClzMe)3 were prepared by disproportionation of 1,1,2,2-tetrachlorodimethyldisilane [6]. The reaction of these compounds with N-methylaniline in different stoichiometric ratios gives product mixtures. It only succeeds to obtain defined reaction products with an excess of N-methylaniline. The products formed under these conditions contain one chlorine atom at every silicon atom. It is not possible to replace all chlorine atoms with the N-methylanilide group in these chlorine-rich organosilanes.

Tablel. Reaction products of oligomeric chloroorganosilanes with N-methylaniline and "Si NMR data.

Reactant ReactanUHNMePh Ratio Product

Cl( SiMe2)zC1 1 1 1 PhMeN(SiMe2)zC1

C1(SiMe2)2CI 1 1 2 PhMeN( SiMe2)zNMePh

CI(SiMe2)3CI 1 1 1 Cl( SiMe&NMePh

CI(SiMe2)3CI 1 1 2 PhMeN(SiMe&NMePh

Cl( SiMe&CI 1 1 2 CI(SiMe&NMePh

MeCISi(SiClzMe)z excess MeClSi[ Si(NMePh)C1MeIz

MeSi(SiClzMe)3 excess MeSi[Si(NMePh)C1MeI3

'JSM, BSi NMR 6 [ppml [Hzl

-1.9,20.9 106

-1.9

25.9,-45.2,2.4

3.0,47.6 80

26.4,-41.3,-43.9, 3.3

-4.3, 3.0 58

12.76, 12.80 (diastereomers) -73.3,-72.9, 12.45, 12.62, 88

X-Ray Structures

Compound 1 was obtained as separated pure enantiomeric crystals in space group a 2 2 1 (Fig. 1). The optical rotation was estimated with a = 377.18 (20 "C, 302 nm, n-pentane). Compound 2 crystallizes in space group P21/n as a racemate (Fig. 2), and compound 3 crystallizes in space group P21/a (Fig. 3).

Conclusions

We were able to show that the N-methylanilino group is a highly useful substituent for chloroorganosilanes in two respects: first, it is possible to obtain crystalline aminoorganodisilanes with this substituent. Second, the N-methylanilino group acts as a protecting group in further substitution reactions. With this strategy we were able to prepare a variety of aminoorganodisilanes starting from 1 ,2-dichloro- 1,2-bis(N-methylanilino)dirnethyldisilane (1).

548 U. Bohme, B. Gunther, B. Rittmeister

Reflections collectedunique 6984 / 2256

Data/restraints/parameters 2256 I 0 I 102

Goodness-of-fit on 2 1.006

Final R indices [I > 20(1)]

R indices (all data)

Absolute structure parameter -0.08(9)

R1= 0.0376

R1= 0.0648

Selected bond lengths [A] and angles ["I for 1.

Si( 1)-Si( la) 2.341(1)

Si( l)-C1( 1) 2.097( 1)

Si(1)-N(1) 1.7 18(2)

N( 1)-Si(l)-Cl( 1) 11 1.88(8)

N( 1)-Si( 1)-Si( la) 109.34(6)

Cl(1)-Si(1)-Si( la) 104.41 (4)

C(1a)-Si(1a)-Sil-Cl 141.9(1)

Fig. 1. Crystal structure of 1,2-dichloro-l,2-bis(N-methylanilino)dimethyldisilane (1).

Reflections collectedunique 5015/4809

Datdrestraintdpararneter s 4809/0/267

Goodness-of-fit on 2 1.056

Final R indices [I > 20(1)]

R indices (all data)

R1= 0.0431

R1= 0.0454

Selected bond lengths [A] and angles ["I for 2.

Si( l)-Si(2) 2.3726(6)

Si( I)-N( 1) 1.75 1( 1)

Si( 1)-N(2) 1.736(1)

Si(2)-N(3) 1.732(1)

Si(2)-CI( 1) 2.1068(7)

N( l)-Si(I)-Si(Z) 109.04(6)

N(2)-Si( 1)-Si(2) 109.26(6)

N(3)-Si(2)-Si( 1) 114.42(6)

Cl( l)-Si(Z)-Si( 1) 104.84(3)

C( I)-%( l)-Si(2)-C(2) -130.4( 1)

Fig. 2. Crystal structure of l-chloro-l,2,2-tris(N-methylanilino)dimethyldisilane (2).

Synthesis of Chiral Amino-Substituted Organosilanes 549

Reflections collectedunique

Data/restraints/parameters 5665/0/331

Goodness-of-fit on 1.062

Final R indices [ I 5 20(l)]

R indices (all data)

863215665 [R(int) = 0.07151

R1 = 0.0572

R1 = 0.0888

Selected bond lengths [A] and angles ["I for 3.

Si( 1)-Si(2) 2.375(1) N( 1)-Si( 1)-Si(2) 115.80(9)

Si( 1)-N( 1) 1.755(2) N(2)-Si( l)-Si(2) 102.92(8)

Si( 1)-N(2) 1.761(3) C( 1)-Si( 1)-Si(2) 11 1.24(10)

Si( I)<( 1) 1.868(3) N(3)-Si(2)-Si( 1) 113.22(8)

Si(2)-C(2) 1.864(3) N(4)-Si(2)-Si( 1) 101.98(9)

Si(2)-N(3) 1.749(2) C(2)-Si(2)-Si( 1) 114.15(11)

Si(2)-N(4) 1.752(2) C( I)-Si( 1)-Si(2)-C(2) 173.6(2)

Fig. 3. Crystal structure of tetrakis(N-methylanilino)-l,2-dimethyldisilane (3).

Acknowledgments: Financial support for this work from the Deutsche Forschungsgemeinschaft and the Fond der Chemischen Industrie is gratefully acknowledged.

550 U. Bohme, B. Giinther, B. Rittmeister

References a) U. Herzog, K. Trommer, G. Roewer, J. Organomet. Chem. 1998,552,99. b) K. Trommer, U. Herzog, G. Roewer, J. Prakt. Chem./Chem.- Ztg. 1997,339,637. a) A. Kawachi, K. Tamao, J. Am. Chem. SOC. 2000,122, 1919. b) A. Kawachi, K. Tamao, J. Organomet. Chem. 2000,601,259. H. Stueger, P. Lassacher, E. Hengge, in Organosilicon Chem. ZZt N. Auner, J. Weis, (Eds.); VCH, Weinheim, 1998, p. 257. a) C. Ackerhans, B. R a e , R. Kratzner, P. Muller, H. W. Roesky, I. U s h , Eur. J. Inorg. Chem. 2000, 827. b) J. Heinicke, S. Mantey, A. Oprea, M. K. Kindermann, P. G. Jones, Heteroatom Chem. 1999, 10, 605. c) M. Veith, A. Rammo, S. Faber, B. Schillo, Pure Appl. Chem. 1999,71,401. d) I. Rietz, E. Popowski, H. Reinke, M. Michalik, J. Organomet. Chem. 1998, 556, 67. e) Th. Schlosser, A. Sladek, W. Hiller, H. Schmidbaur, 2. Natugorsch.Tei1 B 1994,49,1247. U. Bohme, B. Gunther, B. Rittmeister, Inorg. Chem. Commun. 2000,3,428. a) U. Herzog, R. Richter, E. Brendler, G. Roewer, J. Organomet. Chem. 1996,507,221. b) R. Richter, G. Roewer, U. Bohme, K. Busch, F. Babonneau, H. P. Martin, E. Muller, Appl. Organomet. Chem. 1997,l I , 7 1.

Water-Borne Fluoroalkylsilanes: a New Family of Products for Surface Modification

K. WeiJenbach, B. Standke, P. Jenkner

Degussa AG, Germany

Keyword: fluoroalkylsilanes, coatings, surface treament, water-repellents

Summary: There are two hot topics in the field of building protection today: protection against ingress of water, and protection against all kinds of soiling, especially graffiti. Fluoroalkyl-modified silanes produce excellent hydro- and oleophobic effects on a large variety of surfaces. Our first fluoroalkyl silane product, Dynasylan@ F 8261 was introduced commercially in 1997. Since then we have developed new water-based fluoroalkylsilane systems which generate invisible, weather-resistant and gas-permeable coatings on porous mineral surfaces: Dynasylan@ F 8800 and Protectosil@ Antigraffiti. The application fields include easy-to-clean surfaces and permanent anti-graffiti coatings with excellent cleaning properties. The combined application of alkylsilanes with water-borne fluoroalkylsilane systems offers exellent protection against both water ingress and surface soiling.

Introduction

Organofunctional alkoxysilanes have a broad range of classical applications as adhesion promoters, crosslinkers, drying agents and surface modifiers [ l ] and have been used for over 50 years in various industries. Over the last ten years alkoxysilanes with fluorinated organic substituents have attracted particular interest [ 2 ] , The dominant application for such fluorinated compounds is surface modification, the aim being to achieve low-energy surfaces with easy-to-clean, anti-graffiti and anti-fouling properties - keywords in modem coating technology.

Degussa AG manufactures a wide range of standard organoalkoxysilanes under the umbrella brand name SiventoTMSilanes, under which different product tradenames are marketed: Dynasylan@, Dynasil@, and F’rotectosil@. In 1997 the first Dynasylan@ fluoroalkylfunctional alkoxysilane was introduced commercially: Dynasylan@ F 8261 (Fig. 1). This fluoroalkylsilane enables the formation of nanometer-scale, chemically bonded, hydro-/oleophobic, low-energy surface coatings, especially on smooth surfaces such as glass and ceramics [3-81. Several optimized systems based on F 8261 have been developed. Application areas are glass, ceramics, coatings and cosmetics.

A ready-for-use Dynasylan@ F 8263 (a formulation in isopropanol) was created for glass and ceramic applications, especially windshields, facades and shower cabins.

Ovganosilicon Chemisty V Edited by N. Auner and J. Weis

Copyright 0 2003 WILEY-VCH Verlag GmbH & Co. KgaA, Weinheim

552 K. Wegenbach, B. Standke, P. Jenkner

OR

Fig. 1. Structural formula of Dynasylan@ F 8261

Properties and Chemical Structure of Fluoroalkylalkoxysilanes and Corresponding Water-Borne Systems

The new oligomeric water-borne fluoroalkylfunctional siloxanes based on Dynasylan@ F 826 1 include reactive silanol moieties combined with hydrophilic groups X (Fig. 2) that are responsible for solubility in water and chemical reactivity. The film-forming substance is able to react with active hydroxy groups on the surface of the substrate (Fig. 2) and is thus chemically bound after curing at room temperature. The perfluoroalkyl groups (Rf) on the surface of the substrate generate the excellent hydrophobic and oleophobic properties (Fig. 1). The SiventoTMSilanes line includes two oligomeric, water-borne products: Dynasylan@ F 8800 and Protectosil@ Antigraffiti. Dynasylan' F 8800 is primarily used as an additive in coatings based on sol-gel technology.

Fig. 2. Water-borne fluoroalkylsiloxane system chemically bound to a substrate (model).

Performance of Coatings Based on Water-Borne Fluoroalkylsilane Systems

Water-borne fluoroalkylsilane systems are especially suited for the surface modification of porous mineral building materials like concrete. A strong water repellent effect is combined with strong oleophobicity. Surfaces treated with the new water-borne fluoroalkylsilane systems are stain resistant, easy-to-clean, and significantly resistant against infestation with microorganisms such as mildew or algae. Figure 3 shows the oleophobicity of a sand limestone surface protected with Protectosil' Antigraffiti, an oligomeric, water-borne fluoralkylsilane system, and tested with used motor oil.

Water-Borne Fluoroalkylsilanes: Products for Su$ace Modijication 553

Fig. 3. Sand limestone treated with Protectosil". The black droplets are used motor oil.

On many substrates, e.g. concrete, permanent anti-graffiti properties are achievable. Particular advantages of the new protective systems are: lack of volatile organic carbon (VOC) and no evolution of solvent during or after application, a nearly invisible coating, excellent water vapor transmission and very good UV stability. In contrast to surface modification with silicones and many commercially available fluoropolymers, surface coatings with water-borne fluoroalkylsilane systems are long-term UV- and weather-resistant (Fig. 4).

Fig. 4. Water repellency of weathered (one year outdoors,) concrete samples treated with different fluoropolymers

(specially designed for concrete protection, top and middle) and Protectosil" Antigraffiti (bottom).

554 K. WeiJenbach, B. Standke, P. Jenkner

A combined application with alkyltrialkoxysilanes, which are normally used for the water repellent impregnation of concrete, gives an additional benefit: due to their excellent penetration properties alkyltrialkoxysilanes like isobutyltriethoxysilane impart a highly effective barrier against ingress of water-transported pollutants such as chlorides [9, 101. In this way concrete can be protected simultaneously against both graffiti and ingress of water and dissolved chlorides.

Because of the molecular size and remarkable reactivity of water-borne fluoroalkylsilane systems, the penetration depth in concrete and other porous building materials is very low. This desirable effect results in the high efficiency of this special product. Substrates of relatively low porosity such as concrete need about 150-200 g/mz of Rotectosil@ Antigraffiti to impart permanent anti-graffiti properties. Approximately 20-50 g/m2 are necessary for an excellent water and oil- repellent, chemical- and UV-resistant coating (about 200 g/m2 of the aforementioned product).

Application Fields

Protectosil@ Antigraffiti is an oligomeric, water-borne product which has been especially developed for building protection applications. In diluted form this product is used to make porous easy-to- clean mineral surfaces (Fig. 5) [ 11-14].

Fig.5. Two reference objects: the central library in Hong Kong (left), and Luzern, Switzerland (right). Both

buildings are protected with Protectosil" Antigraffiti to obtain an easy-to-clean application.

Water-borne fluoroalkylsilane systems make also permanent antigraffiti coatings possible. Nearly all paint systems and felt-tip markers can be removed without residue from protected surfaces with the aid of commercially available and environmentally friendly graffiti cleaners (Fig. 6) . More than ten cleaning cycles are possible without a significant reduction in cleaning properties [ 151.

Water-Borne Fluoroalkylsilanes: Products for Sudace Modwcation 555

Before cleanina After cleaning

Fig. 6. Test surface for Protectosil@ Antigraffiti: a concrete wall was treated with Protectosil@ Antigraffiti. After

being defaced with graffiti the treated surface was cleaned with a commercially available, environmentally

friendly graffiti cleaner.

Conclusion

Water-borne fluorosilane systems generate invisible, weather-resistant and gas-permeable oil- and water-repellent coatings on porous mineral surfaces and can be used as additives in sol-gel coating systems. The applications range from easy-to-clean to permanent anti-graffiti coatings. Combination with alkylsilane water repellents offers additional protection due to the excellent penetration behavior of monomeric silanes.

References [ 11 E. P. Plueddemann: Silane Coupling Agents, 2nd edn., Plenum Press, New York, 1991. [2] M. J. Owen, D. E. Williams: Silanes and Other Coupling Agents, Elsevier, Utrecht, 1992. [3] P. K. Jenkner: Thin Film Fluoroalkylsilane Coatings: Course of Formation, Structural

Accounts and Practical Applications, SiF News 20,2001. [4] P. K. Jenkner: Second International Symposium on Silanes and Other Adhesion Promoters,

Newark, October 1998. [S] P. K. Jenkner: Thin Film Fluoroalkylsilane Coatings, in Orlando Accounts and Practical

Applications, SiF News Silicon and Fluorine in Coatings January 1999. [6] R. Storger: Vielseitige Einsatzmoglichkeiten fiir fluorhaltige siliciumorganische

Verbindungen, 3rd Seminar Beschichtungen und Bauchemie, Kassel, 26-27 October 1999, p.

P. K. Jenkner, R. Storger: Easy-To-Clean-Surfaces: Fluorosilanes for the Treatment of 117-125.

[7]

556 K. Wegenbach, B. Standke, P. Jenkner

Ceramic Materials, Glazes and Glazing Techniques in the Ceramic Industry, Helsinki, Finland, 20-21 September 1999. P. K. Jenkner: Give Your Surface a Perfect Finish ... with Our Dynasylan F Product Line, Sugace Coatings Australasia (SCAA) Exhibition, Melbourne, Australia, 4-7 September 2000.

[9] M. Wegner, A. Gerdes, F. H. Wittmann: MSR 99, Esslingen, NovJDec. 1999, p. 719-730. [lo] L. Schueremans, D. Van Gemert: Hydrophobe II, Zurich, September 1998, p. 91-106. [ 1 I] B. Standke, P. K. Jenkner, R. Storger: Easy-to-clean and Anti-Graffiti Surfaces: New Invsible

Coatings on Porous Mineral Materials with Huoroalkylsilane Systems, Materials Week 2000, Munich, 2000.

[ 121 WTA Conference Notes, No. 5 [ 131 K. Weissenbach: Anti-Graffiti And Easy-To-Clean Properties on Porous Mineral Surfaces are

Achieved by Using Water-Borne Fluoroalkylsilane Systems, Hydrophobe III, Hannover, 2001.

[ 141 B. Standke: Antigraffiti und seine chemischen Graundlagen aus der Sicht des Bautenschutzes, Dechema Kolloqium No. 544, Frankfurt, 2002.

[ 151 Regelwerk fur die Bewertung von Materialien und Technologien zur Graffitientfernung und Graffitiprophylaxe der Giitegemeinschaft Anti-Graffiti e.V.

[8]

Mineral-Filled Thermoplastics: How Silanes Make the Difference

Helmut Mack

Research - Development - Technical Service Silanes, Degussa AG, Untere Kanalstrasse 3,79618 Rheinfelden, Germany

Tel.: +49 7623 918233 -Fax: +49 7623 918571 E-mail: helmut.mack@degussa.com

Keywords: coupling agent, aminosilane, polyamide, inorganic filler

Summary: For several years, inorganic fillers such as calcined clay and wollastonite have been used to modify the properties of polyamides, and glass fiber-reinforced products are used extensively. Silane coupling agents provide the ability to bond inorganic fillers to organic resins by establishing “molecular bridges”. Current technology utilises primarily y-aminopropyltriethoxysilane as a coupling agent for improving interphase interactions in mineral-filled polyamide systems. Although many mechanical properties are greatly improved by the use of y-aminopropyltriethoxysilane, impact properties generally are not. As a further development N-n-butyl-y- -aminopropyltrimethoxysilane imparts not only excellent processability and rigidity to a composite, but also increased impact strength due to its coupling and dispersion abilities.

Silanes for Inorganic Filler Surface Treatment

Silanes offer tremendous advantages for virtually all market segments involving polymer-filler interactions. Surface modification of fillers with silanes results in such attractive prospects as

outstanding surface characteristics, reduced expansion, improved wet-out between resin and filler, increased process productivity, improved mechanical strength, improved electrical properties, improved filler dispersion, decreased water vapor transmission, controlled rheological properties.

Ovganosilicon Chemisty V Edited by N. Auner and J. Weis

Copyright 0 2003 WILEY-VCH Verlag GmbH & Co. KgaA, Weinheim

558 H. Mack

Silane-Filler Reactivity

Silanes are organofunctional compounds, or coupling agents, that unite different phases in a composite material by forming strong water-resistant and chemical-resistant bonds. These phases are typically organic resins, inorganic fillers, and fibrous reinforcements. Silanes form “molecular bridges” to create a strong, stable bond between two otherwise weak bonding surfaces. The nature of the filler surface plays an important role in determining how effective silane surface treatment will be. Best results are achieved with fillers that have chemically active sites on the surface (Table 1).

Table 1. Reactivities of silanes with various fillers.

Silane-filler reactivity Inorganic filler

Ex c e 11 en t precipitated silica, fumed silica, glass fiber, cristobalite, quartz, sand, kaolin, aluminum trihydrate, wollastonite

Good mica, talc, magnesium dihydrate, other silicate fillers (feldspar, nepheline syenite, etc.), inorganic oxides

Slight

Poor carbon black

barium sulfate, graphite, calcium carbonate

Silane Coupling Mechanism

The properties and effects of silanes are defined by their molecular structure. The silicon at the center is combined with two different functional groups: an organofunctional group Y and a silicon- functional group OR:

The organofunctional group Y is bound tightly to the silicon via a short carbon chain and links with the polymer. This group has to be carefully chosen to ensure maximum compatibility with the resin system. Bonding to the polymer takes place by chemical reactions or physicochemical interactions such as hydrogen bonding, acid-base interaction, interpenetrating polymer network (entanglement), or electrostatic attraction. The silicon-functional groups OR (usually alkoxy) must be activated by moisture. Upon activation, these OR groups are converted to the silanol form OH liberating alcohol, a volatile (VOC) product. If the filler’s surface is receptive to the silanols, stable chemical Si-0-filler bonds are formed. Table 2 lists the silanes shown by experience to provide the best property enhancements in filled polymer systems. This list is not exhaustive - variations are possible.

Mineral-Filled Thermoplastics: How Silanes Make the DifSerence 559

Table 2. Resins and the compatible silane functionalities.

Polymer Silane functionality

Acrylic

Butyl Rubber

EVA

Neoprene

Nitrile

Polyamide

Polyester

Polyolefin

EPR, EPDM,

SBR

methacrylate, vinyl, epoxy

amino, diamino

amino, vinyl

mercapto, diamino

mercapto

amino

methacrylate, polyether

vinyl, alkyl

vinyl, sulfur, mercapto

Application of Silanes to Fillers

In most cases the silane is applied to the filler neat or in diluted form. Typically the silane or silane solution is sprayed onto the filler as it is being agitated. Most important commercial silane coating processes are continuous, with high throughput rates, but application may also be performed in batches. Control of silane addition, dwell time, and temperature within the system is essential for a successful coating. Evolution of volatiles (VOC) must be monitored. To remove reaction byproducts, solvents, and water, and to bond the silane to the filler surface, further heat treatment may be necessary. For those fillers typically processed as a slurry, aqueous treatment with waterborne silanes is also possible.

Silane Loading

In theory, a monomolecular layer of silane on every filler particle is required for optimum adhesion or modification of the surface. In reality, no monomolecular layer of silane on the filler surface has been proven. In most cases 1 % silane based on filler weight is a convenient starting point for filler treatment. However, optimal loading must be determined empirically.

Aminosilanes for Clay-Filled Polyamides

Since the introduction of thermoplastic polyamides in the mid-20th century, polyamide resins have been the most widely used of all engineering plastics with nylon@ 6 and nylon@ 616 being the most common. The regular spacing of the amide groups means that the polymers crystallize with a high

560 H.Mack

intermolecular attraction resulting in high strength with high melting points. These unique properties of polyamide have made it the most versatile and broadly applied plastic material. Its use as an injection molding resin to produce a wide variety of engineering plastic parts in every industry continues to grow. Most polyamides are used unfilled, but there is a significant application sector where higher rigidities and heat deflection temperatures are required than can be achieved with unfilled polymer. Consumption of nylon@s for engineering plastics grows at 7 % annually, mainly driven by functionally filled materials for the automotive and electronics industry. New opportunities for fillers with tailored surface chemistry (lower surface moisture), allowing the control of physical (wet strength and low temperature toughness) and rheological (dispersion, higher output rates) properties, are expected.

Kaolins form the biggest class of white, semi-reinforcing fillers although the coarsest grades do not greatly improve strength properties. When heated to 1000 "C kaolin forms a calcined clay which gives distinctly different properties in polymers. This calcined clay is very reactive to silanes. Aminosilane-modified calcined clays are now included in many nylon@ and thermoplastic polyester compounds because of their superior combination of mechanical properties and favorable economics. y-Aminopropyltriethoxysilane has become an industrial standard for filled nylon@ compounds within record time because of its overall cost-performance properties. However, a compromise must be made between the desired physical strength (stiffness) and impact strength (toughness). Most composites become brittle if the filler loading is raised above 20 vol.%. To improve the interphase interaction between filler and nylon', not only coupling, but also filler dispersion, needs to be optimized. Impact properties are directly related to the filler loading level. The higher the loading level, the lower the impact. Without excellent dipersion of filler in nylon@, there will be filler agglomerates acting as stress concentrators. These will be the sites where impact failure originates. Optimizing filler dispersion therefore minimizes filler agglomerization and yields more homogeneous materials with improved impact properties.

Experimental

This study was undertaken to demonstrate the advantages of using N-n-butyl-y-aminopropyl trimethoxysilane treatment on calcined clay which is compounded into a nylon@ 6/6 matrix. The nylon@ 6/6 selected for this study was a general-purpose, lubricated material that is widely used in injection molding for mechanical parts, consumer products, etc. The filler used was a fine calcined clay. The calcined clay was surface-modified in a laboratory mixer. Surface modification was carried out with 1 wt.% silane by employing a standardized laboratory surface treatment process. The nylon' 6/6 and the silane-treated calcined clay were dried for 24 h at 80 "C in an air-ventilated oven before compounding. Compounds were prepared at a 40 wt.% filler loading level. All of the injection-molded test samples were conditioned for 24 h at 23 "C and 50 % relative humidity before testing. Table 3 summarizes the physical properties obtained for the calcined clayhylon' 6/6 composites described.

Mineral-Filled Thermoplastics: How Silanes Make the Difference 561

Table 3. Physical properties of the experimental calcined clay/nylon" 6/6 composites.

y- Aminopropyl- N-n-Butyl-y-aminopropyl- triethoxysilane trimethoxysilane

Tensile strength 72.0 MPa 69.1 MPa

Flexural modulus 3.7 GPa 3.5 GPa

Charpy impact strength 32.2 kJ m-' 41.7 kJ m-'

Izod impact strength 19.5 kJ m-' 30.1 kJ m-'

Melt Flow Index (maeasured at 275 "C/5 Kg)

43 g 10 min-' 55 g 10 m i d

Aminosilane treatment of calcined clay dramatically improves flexural properties of the filled nylon' 616. By using N-n-butyl-y-aminopropyltrimethoxysilane the overall properties of the final compound can be improved even more. A trade-off between tensile and flexural strength and impact strength need no longer be made. It is well known that optimal impact strength is achieved with a 1 wt.% silane treatment level. Lower levels of silane treatment result in poor impact strength because of calcined clay agglomeration effects. Flexural modulus is less affected by silane treatment level and depends very much on filler particle size and aspect ratio. A pretreated calcined clay achieves superior performance. In-situ silane addition results in poorer impact properties. Coarser calcined clays offer an improvement in flexural modulus and heat deflection temperature as well as a reduced notched sensitivity. Generally, a good calcined clay dispersion in the nylon@ 616 polymer phase leads to excellent compound viscosities (MFI) and a much better processability. N-n-Butyl-y-aminopropyltrimethoxysilane reduces compound viscosity and results in good melt flow rates and a smooth, defect-free surface.

Conclusions

Fillers pretreated with N-n-butyl-y-aminopropyltrimethoxysilane lead to superior mechanical properties of nylon@ composites. N-n-Butyl-y-aminopropyltrimethoxysilane-coated filler particles impart several beneficial properties, including improved wet-out. The goal of a perfectly dispersed discontinuous phase (the filler) in the continuous phase (the polymer matrix) is easily, quickly and perfectly achieved through N-n-butyl-y-aminopropyltrimethoxysilane surface modification. A significant improvement in composite impact properties and processing can be made without detriment to tensile and flexural properties by employing the right surface chemistry of reinforcing fillers.

The Role of Silanes in Filled and Crosslinked Polymers

Peter Kraxner, Louis Boogh, Akin Lejeune

Crompton SA 7, Rue du PrC Bouvier, 1217 Meyrin, Switzerland

Tel: +41 22 989 2247, -2245, -2348 - Fax: +41 22 785 1140 E-mail: kraxnpe@cromptoncorp.com, booghlo@cromptoncorp.com,

lejeual@ cromptoncorp.com

Keywords: silanes, crosslinking, halogen-free flame-retardants, rubber

Summary: Organofunctional silanes (OFS) were introduced in the 1950s as coupling agents for fiberglass-filled composites. The field of application for organofunctional silanes has been growing ever since, at a tremendous pace. Their ambivalent structure is the key reason for their successful use in industry acting as, e.g., stereomodifiers in olefin polymerization and in immobilization of enzymes, protecting agents of mineral and metallic surfaces, adhesion promoters, dispersing and/or coupling agents in mineral-filled polymer systems and in crosslinking of polymers etc. In many cases, this versatility of OFS is used in polymer systems to impart several advantages at the same time. This paper gives a brief update on the current use of OFS for crosslinking polymers, particularly mineral-filled systems.

Introduction

Organofunctional silanes as shown in Fig. 1 have at least one organic group and at least one alkoxy function directly attached to the silicon. However, the vast majority of organofunctional silanes used in industry - and the ones discussed in this paper - have three identical alkoxy groups, i.e., R’ = R” = OR.

Y

Fig. 1. Organofunctional silane.

Ovganosilicon Chemisty V Edited by N. Auner and J. Weis

Copyright 0 2003 WILEY-VCH Verlag GmbH & Co. KgaA, Weinheim

The Role of Silanes in Filled and Crosslinked Polymers 563

Within the whole organic and inorganic silicon industry organofunctional silanes represent only a small part of the cake. Nevertheless, these small molecules are basically present in everyday life, in most cases rather hidden. Figure 2 shows the variety of silane applications in industry.

Fig. 2. Industrial silane applications.

The markets for organofunctional silanes and their total volumes have been growing ever since their first real commercialization in 1950s, when the fiberglass industry was the first to utilize silanes; this was followed by its use in the adhesives and sealants markets, in the rubber and plastics industry, and for coatings. And there are many more applications to come.

The versatility of organofunctional silanes is due to their unique combination of organic activity and silicon reactivity (see Fig. 3); the latter is due the hydrolyzable alkoxy groups.

o m 0 RElCllVE unamo gucoIl n w w n r z m (IROUP DROUP ORQUP

ORGANIC END SILICON END

Fig. 3. Organic and silicon activity of organofunctional silanes.

Bridges can be built between, e.g., organic matter such as polymeric organic resins (thermosets, thermoplastics, and elastomers) and inorganic substrates such as ceramics, metals, and mineral fillers. Silanes can also bridge organic matter itself, thus leading to a crosslinked polymer system via a controlled moisture cure process.

564 P. Kraxner, L. Boogh, A. Lejeune

Control of the reaction and reactivity at both ends is indispensable for basically every industrial application. A whole range of different organic end groups are commercially available, e.g., amine, ureido, epoxy, isocyanato, sulfur, vinyl, methacryl, non-reactive groups, etc. The end group must be chosen according to the functionality of the base resin.

Hydrolysis and Condensation

Reactivity at the silicon end is governed mainly by the choice of the alkoxy group. The number of alkoxy groups on the silicon atom helps to control the reaction speed as well as the crosslinking density. Figure 4 illustrates the relative rates of hydrolysis of trimethoxy, triethoxy and triisopropoxy groups, respectively.

c 0

$2 E

Fig. 4. Relative rates of hydrolysis of trimethoxy, triethoxy and triisopropoxy.

HO \

Y /OH

Y Si - O..H H O . . ~ Si / ‘OH HO

OH HO + H20

/ l Y Si - 0- Si Y

/ ‘OH HO

Fig. 5. Condensation of two silanol moieties.

The Role of Silanes in Filled and Crosslinked Polymers 565

OR OR

Fig. 6. First hydrolysis step of a trialkoxysilane.

The alkoxy groups can be consecutively hydrolyzed to form intermediate silanol structures by releasing the respective alcohol. The silanol groups react in subsequent condensation reactions to yield a flexible three-dimensional network based on the siloxane group as shown in Figs. 5 and 6. It has to be mentioned that not necessarily all three siloxane bonds are actually formed. The reaction rate depends on a number of parameters, such as the choice of the allcoxy group, organic group, catalyst for the hydrolysis and condensation as well as processing conditions (moisture level and temperature).

Coupling and Adhesion

Adhesion between polymer systems and mineral surfaces is significantly increased when using silanes as coupling agents (Fig. 7). The chemical bond on the mineral surface leads to superior adhesion and thus moisture and temperature resistance. The perfect bonding is exemplified in Fig. 8, showing a glass sphere in a polymeric matrix before (left) and after silane treatment.

Mineral Glass Surface

Si- o -Si- 0-Si Si- o -Si- 0-Si I I r 0 0 0

H H H H

'i Polymer

Condensation of Silanols on a Mlneraf Z)urhm yields a Stable Organic Functionallzation

Fig. 7. Silanes as coupling agents between inorganic and organic matter.

Surface Treatment

A special case of coupling is surface treatment in order to change the properties of an inorganic

566 P. Kraxner, L. Boogh, A. Lejeune

surface. One aim might be to compatibilize the surface with the organic matrix - but in this case without actual chemical bonding on the organic side - in order to facilitate dispersion and to reduce the viscosity of such mixture. This is used, e.g., for filler dispersion (Fig. 9) and filler masterbatches. The silane effectiveness depends very much on the surface chemistry of the substrate and can vary from rather poor (carbon black) to excellent in the case of silica (Fig. 10). Another important application is the protection of a surface against environmental influences as for, e.g., metal corrosion or building protection.

Fig. 8. Glass spheres in nylon matrix before (left) and after treatment with aminosilanes.

Fig. 9. Reducing the aqueous dispersion viscosity of fillers by silane treatment.

Silica-Filled Tire Treads

Silica-filled tread compounds for passenger car tires have become state-of-the-art and have taken a significant share from conventional carbon black-filled systems. In addition to the well-known benefits of fillers such as reinforcement (Fig. 11) and increased cut resistance, silica-based compounds are mainly successful because they provide improved rolling resistance (leading to lower fuel consumption), better wet and snow track resistance, and thermal stability.

The Role of Silanes in Filled and Crosslinked Polymers 567

Silica Quartz Glass Aluminum Copper Alumino-Silicates (Clays) Mica Talc Inorganic Oxides (e.g. TiO, Fe203 etc.) Steel, Iron Asbestos Nickel Zinc Lead Chalk (CaCO,) Gypsum (CaSO,)

Graphite Carbon Black

brvter (bW.1

Fig. 10. Silane effectiveness on different substrates.

Fig. 11. Rubber reinforcement through silica.

However, the high polarity of the silica surface structure leads to the formation of strong aggregates preventing good dispersion and processing ease. The hysteresis caused by breaking and reforming such agglomerates would strongly increase energy consumption and might eventually destroy the tire.

Combining silanes with silica led to high-performing silica-filled rubber compounds. Organofunctional silanes with polysulfane groups such as bis-(3-(triethoxysilyl)propyl)tetrasulfane (TESPT) and bis-(3-(triethoxysilyl)propyl)disulfane (TESPD) (Fig. 12) are utilized to improve dispersability of silica in the rubber matrix.

568 P. Kraxner, L. Boogh, A. Lejeune

(OC2H5)3Si\/\/S2\/\/Si(OC2H5)3

Fig. 12. Tetrasulfane TESPT (above) and disulfane TESPD (below) silanes used in tire compounds.

Compounding of tire tread compounds is a complex process and it is crucial to separate the coupling into two steps:

reaction of triethoxysilyl groups with pendant silanol groups on the silica surface in the

reaction of the sulfane groups only during curing in the presence of the crosslinking course of the nonproductive compounding process; and

agents to form the rubber-to-filler bonds.

Figure 13 illustrates the influence of silanes in a silica-rubber system. The graph shows data recorded with a dynamic rheometer, G’ being the elastic modulus and G” being the loss modulus as a function of dynamic strain.

Payne effect Non-linearity

(Rolling Losses)

. I. .. . . . . . .. .-

Fig. 13. Dynamic properties of silica-rubber systems (with and without silane).

There are basically two regions of interest: one at lower strains and the other at very high strains. The region at lower strains is indicative of the filler-filler interaction, whereas the region at high

The Role of Silanes in Filled and Crosslinked Polymers 569

strains is indicative of the coupling strength between filler and rubber. Compounds without silane have very high values at low strains and thus show strong filler-filler

interaction, which leads to high rolling losses and thus high fuel consumption. Compounds comprising silanes have much lower values and thus good rolling characteristics. The world looks different at higher strains: high values of G' for the coupled system thus indicate good reinforcement in comparison with low values for the noncoupled system.

Figure 14 shows a plot of the loss factor tan A against the temperature for three rubber systems: a) carbon black-rubber, b) silica-rubber, and c) silicdsilane-rubber. Time-temperature superposition (TTSP) allows one to allocate some temperature regions to certain properties, e.g., the loss factor should be as low as possible at ca. 60 "C, resulting in low rolling resistance; higher values at ca. 0 "C give good traction and at even lower temperatures (-20 to -30 "C) good wear resistance.

Tempelaturn ["C]

Fig. 14. Relationship between tan A and properties in three different filler-rubber systems.

Even though the final performances of the cured rubber compounds are very important, the process stability is also of great importance in manufacturing and cannot be neglected when choosing the silane structure. Figure 15 shows straidmodulus curves of three silica-filled rubber compounds using TESPT, TESPD, and mercaptopropyltriethoxysilane, respectively.

According to those curves mercaptopropyltriethoxysilane provides the highest coupling strength between filler and rubber, followed by TESPT and TESPD. However, significant differences in reactivity of the sulfur groups prevents tire manufacturer basically from using mercaptopropyltriethoxysilane since it leads very easily to scorch (premature crosslinking) under compounding conditions. TESPT is much more stable but still prone to scorch in the presence of sulfur at temperatures above 120 "C. The disulfane, TESPD, is by far the most stable silane of the three and resistant towards incorporation of sulfur into the S-S bridge, which is necessary to couple later onto the rubber matrix. This lower reactivity increases the processing window and thus improves the processability of the compounds. A tire manufacture thus can either adapt the process

570 P. Kraxner, L. Boogh, A. Lejeune

to the silane or can choose the right silane for the process employed.

No silane

-20% Strain

fl

-300%

Fig. 15. Straidmodulus curves of three different filler-rubber systems.

Halogen-Free Flame Retardant Compounds

Ecological and political requirements have given a strong boost for halogen-free flame retardant (HFFR) compounds to replace polyvinyl chloride (PVC) or polymers containing halogenated flame retardants. They are used in electrical conduits and telecommunication, power, and automotive cables. The exchange is particularly driven by car manufactures such as General Motors who want to have PVC-free cars by 2003. Contrary to public opinion it is less liberation of toxic and acidic gases, e.g., hydrogen chloride, but much more the smoke emission that is greatly reduced when employing HFFR as the base material.

The flame-retardant effect is based on the release of water at elevated temperature from alumina trihydrate (ATH) and magnesium hydroxide (MGH) as well as good char-forming properties. However, in order to pass the stringent flame and bum tests, such HFFR compounds are highly loaded with mineral fillers (60-70 %). Organofunctional silanes are often used as coupling agents in order to improve dispersion of the mineral filler in the respective polymer matrix, which results in improved materials properties (mechanical, electrical, and flame retardancy). The coupling is either carried out prior to compounding (filler treatment) or in situ in a suitable compounding machine, such as a twin-screw extruder, inline blender or Buss Kneader. The choice of silane depends on the chemical structure of the polymer matrix; however, the most commonly used silanes are amino- (particularly for EVA-based systems), methacryloxy-, and vinylsilanes.

HFFR compounds are mainly thermoplastic materials, but one of the latest trends is to manufacture cables and conduits based on crosslinked HFFR compounds. Crosslinking of such

The Role of Silanes in Filled and Crosslinked Polymers 571

compounds further improves the properties of the finished goods, e.g., temperature, creep, and environmental stress crack resistance. Crosslinked materials do not really melt when burning and the drip phenomenon, which often leads to the spreading of an incident, is thus greatly reduced.

Crosslinking of HFFR compounds can be carried out, e.g., by reactive extrusion using suitable crosslinking agents as shown in Fig. 16.

HFFR compound Silane crosslinker

Fig. 16. Reactive extrusion of an HFFR compound and a silane crosslinking agent.

A number of process alternatives are commercially available to run silane crosslinking technology. Reactive extrusion utilizes a specially designed extruder to carry out chemical reactions - in this particular case, grafting of a vinylsilane onto the polymer backbone. Figure 17 illustrates schematically the consecutive steps that are taking place: a) decomposition of a peroxide, b) abstraction of a hydrogen atom, and c) grafting of the vinylsilane onto the polymer backbone. This is actually a chain process, and grafting and a typical radical reaction such as chain extension are concurrent.

energy a) R-0-0-R - 2 R - 0 .

silane grafted polymer

Fig. 17. Grafting of a vinylsilane onto a polyethylene via reactive extrusion.

The presence of water and, in order to accelerate the reaction, a suitable condensation catalyst is required for the crosslinking step itself. Figure 18 illustrates the chemistry involved in the

572 P. Kraxner, L. Boogh, A. Lejeune

crosslinking step.

Fig. 18. Crosslinking step of silane-grafted HFFR compound.

In most cases, silane crosslinking of polyethylenes require hot water baths or steam saunas to obtain the required degrees of crosslinking. In the case of HFFR compounds, however, moisture is generally available as surface-bound water on the ATH or MGH, respectively. Thus, crosslinking of HFFR compounds occurs even under ambient conditions without the need to immerse the finished articles in water. Again, process control is crucial since one has to ensure a) that grafting of the vinylsilane occurs at temperatures below the decomposition temperature of the mineral filler, and b) that the crosslinking reaction is controlled in such a manner that no crosslinking takes place inside the extruder.

Conclusion

Organofunctional silanes can act independently as polymer crosslinkers, dispersing agents, and coupling agents. This versatility and synergistic effects lead to the use of silanes in numerous applications, particularly when interactions between inorganic and organic matter are of great importance.

In most applications control of reactivity of both organic and inorganic groups in the silane molecules is important and has to be adjusted to the chemistries involved. Even more important, however, is the choice of the right silane with respect to the process technologies involved. Industrial tire production and silane crosslinking of halogen-free flame retardant compounds are examples where a perfect match of chemistry and process technology is indispensable to obtain the desired material properties.

Acknowledgments: The authors thank Dr. Fred Osterholtz, Dr. Antoine Guillet, and Dr. Prashant Joshi for their support and helpful discussions.

Hybrid Coatings Based on Manes: Precursor Methods to Make Hybrid

Organic-Inorganic Coatings

B. Borup,* R. Edelmann, J. Monkiewicz

Degussa AG, D-79618 Rheinfelden, Germany Tel.: +49 7623 91 8131 -Fax: +49 723 91 8130

E-mail: bjoern.borup@degussa.com

Keyword: hybrid coatings, sol-gel process, inorganic nanoparticles, organic matrix, organofunctional silane

Summary: Alkoxysilanes hydrolyze and condense with the addition of water. This simple fact has been known for over 150 years. Only much more recently was the chemistry of this reaction elucidated. Initial condensation and hydrolysis lead to sol formation, which, upon further hydrolysis and condensation, yields a gel. This technology is now known as the sol-gel process. Coatings based in part on a sol-gel core involving inorganic particles and alkyl- or organofunctional trialkoxysilanes embedded in an organic matrix, so-called hybrid systems or interpenetrating networks, are described. The properties of these systems are in many ways superior to those of traditional organic-based coatings. Improvements in performance include the mechanical durability (scratch and abrasion resistance), chemical durability, UV stability, and environmental aspects, and the range of applications possible with these materials has increased, in part due to the range of inorganic/organic compositions possible. A new version of the sol-gel process is the reaction of alkyl- and organofunctional trialkoxysilanes with inorganic nanoparticles surrounded by the organic matrix, the so-called in-situ sol-gel process. The chemistry involved in this process is discussed as well as applications resulting from the superior properties of the resulting products.

Introduction

When we take a look around we realize that coatings are ubiquitous. From indoor applications such as on telephones, household appliances (TVs, washing machines), furniture, ceiling panels, and flooring, to outside applications such as on automobiles, houses, even roads, coatings are a normal part of our lives. Up until recently they were exclusively derived from organic resins [l]. Slowly but surely these resins are coming to the end of the possibilities with respect to their performance.

Ovganosilicon Chemisty V Edited by N. Auner and J. Weis

Copyright 0 2003 WILEY-VCH Verlag GmbH & Co. KgaA, Weinheim

574 B. Borup, R. Edelmann, J. Monkiewicz

Only newer technologies can improve on the best we have. We would therefore like to show how improvements can be achieved in coatings by integrating an Si-0-Si (inorganic) network into the coating system.

Hybrid materials are made of a combination of these organic and inorganic components. As there is an especially high demand for chemical and mechanical resistance in coatings, this hybrid technology could have the largest impact in this field.

Unfortunately, it is not quite as simple as adding inorganic and organic components together in order to achieve hybrid materials. These components have to be intimately intertwined. In order to combine the organic and inorganic parts using an organofunctional silane, the sol-gel technology becomes important.

History of Sol-Gel Technology

The history of sol-gel technology can be traced back to 1846 when Ebelmann [2] found that S i c4 could be converted to Si(OR)4 which then gelled upon exposure to air. This is most certainly due to hydrolysis from ambient moisture and subsequent condensation. More important were the commercial applications that were developed at Schott Glass [3] in the 1940s and 1950s (Fig. 1).

SiCI4 HOR - Si(OR)4

atmosphere - gelation

J.J. Ebelrnann, Ann., 57, 1846 , 319

- thin SiOn layers

M = Ti

Schott Glass DE 736 41 1 (1939)

Applications (commercialized): rear view mirrors for automobiles (1953) anti-reflective coatings (1964) solar reflecting coatings (1969)

Fig. 1. Historically important steps in the development of sol-gel technology.

The sol-gel process can be viewed most simply as a series of two reactions (usually hydrolysis and then condensation) where at first the reactants form small particles in a solvent and then condense further to form a gel. Simple alkylsilanes and especially silicic acid esters are often crucial in these reactions as they are the raw materials in this process. Organofunctional silanes have the additional feature in that they can interconnect organic resins with the inorganic part of these

Hybrid Coatings Based on Silanes 575

systems.

Organofunctional Silanes

The contribution that silanes have to the sol-gel process cannot be discounted, as they are the crucial ingredient to combine the organic and inorganic components together. Hydrolysis activates the silicon-functional group of the silane (Eq. 1) [4,5]. This reaction of the alkoxide (Si-OR) group to form a hydroxyl (Si-OH) group can be extended to the other two alkoxide groups on the molecule, as usually all three alkoxide groups are reactive.

OR OR

* Y-' P-OH

+ HzO Y-Si-OR - ROH

OR OR

Eq. 1. Hydrolysis reaction of a silane.

Subsequently, a Si-OH group reacts with the hydroxyl group of the inorganic substrate (Eq. 2 ) to form a chemical bond between the silane and the inorganic substrate. In addition to this chemical bond there is a chemical bond between the binder and the silane through the (Y) organofunctional group. It is important to choose the organofunctional group (Y) for compatibility with the binder system.

I OR

HO

H> -H20 Y-hi-OH + HO - y-hi-0

HO

OR

I OR

Eq. 2. Condensation of a silane with an inorganic substrate.

This reactivity of the silane with an inorganic substrate can be extended to inorganic fillers, pigments and other inorganic particles such as fumed silica.

Fumed Silica

Fumed silica (Aerosil@), a highly dispersed, amorphous silica, also plays an important part in these new hybrid systems as a source of particles. It is composed of agglomerated aggregates. These agglomerates can be decomposed by normal dispersion processes in coatings to aggregates (deaggregation is usually not possible by dispersion in coatings). These aggregates are composed of almost spherical primary particles that are of nanometer size (on the order of approx. 5-50 nm).

576 B. Borup, R. Edelmann, J. Monkiewicz

Fumed silica is characterized by a large surface area (approx. 50-600 m2/g) that is covered with Si-OH groups (Fig. 2) . This is crucial to our technology.

Fig. 2. Surface functionality of an fumed silica (Aerosil@) particle.

These surface hydroxyl groups are also accessible for sol-gel chemistry. For instance, OX-50 suspended in chloroform and gassed with ammonia was successfully converted to glass after sintering (Fig. 3) [6, 71. This example shows the variability of the sol-gel process, as in this case hydrolysis was not necessary. The surface of the fumed silica already contains a large number of Si-OH groups that are accessible for condensation. Upon condensation, the mixture forms a gel which is converted into glass by sintering.

I AerosiCox-w suspension in chloroform I (30 % vhr) and decanoi

gassing with NH

Scherer and Luong, J. Non-Cyst. Solids, 63, 1984,163 Brinker and Scherer, Sol-Gel Sciena,, Academic Press. 1990,292

Fig. 3. Sol-gel chemistry is possible with fumed silica.

It is the aim to use the basics of sol-gel technology in order to improve the performance of coatings. We will thus use hydrolysis and condensation features of the silane to prepare hybrid coating systems that display superior performance characteristics.

In-Situ Hydrolysis and Condensation

In order to obtain hybrid coatings that combine both the advantages of today’s organic materials and the advantages of inorganic materials, sol-gel chemistry is performed in situ on the organic resin. Fumed silica particles (Aerosil@) are modified via condensation of the hydrolyzed organofunctional silane with the Si-OH-rich surface. Scheme 1 describes the condensation of the

Hybrid Coatings Based on Silanes 577

silane with the surface of the fumed silica particle. Due to the presence of the acrylic resin the sol- gel process is stopped from going forward to its natural conclusion: gelation. The resulting product thus contains particles with a siloxane shell derived by sol-gel technology. Low-viscosity formulations are thus obtained [8,9].

OH

HO

HO

OH

H d

Scheme 1. Model reaction of silane with fumed silica to form a siloxane shell.

The thick organophile siloxane layer on the surface of the inorganic particle brings compatibility with the organic acrylic resin (Fig. 4).

Fig. 4. Siloxane shell covering the inorganic particle

UV-Cured Hybrid Systems

The sol-gel hydrolysis and condensation process can easily take place within an acrylic resin matrix. The reaction described in Scheme 1 takes place within an acrylate matrix, allowing for low viscosity (800 to 3000 mPa s ) formulations suitable for coating applications. The low viscosity of these compositions is surprising, as fumed silica (Aerosil@) is often used for rheological purposes, and increases the viscosity of the system upon addition of several percent. In these systems fumed silica is a major component (Fig. 5) .

The resulting coating composition has been designed with several advantages for the user:

100 % solid content; no solvent (VOC-free);

578 B. Borup, R. Edelrnann, J. Monkiewicz

high inorganic content; rapid curing (seconds) due to UV technologylelectron beam technology; intimate interconnection of organic and inorganic components.

Fig. 5. Composition of hybrid organic-inorganic system.

The silane is the essential ingredient that enables this large amount of fumed silica (Aerosil@) to be dispersed into the coating system. How the silane enables the dispersion of the fumed silica in the system is an essential part of the technology. Simple addition of a small amount of silane to an acrylic suspension of fumed silica does not lead to low-viscosity formulations. We believe it is the organophilic siloxane shell (Fig. 4) that has been placed around the fumed silica particles that allows for the high filler content.

Fig. 6. SEM of the coating surface.

Application of the composition to the surface of the substrate is achieved via a roller (be it paper, metal or plastic). Subsequently, curing is performed within seconds using UV or electron beam

Hybrid Coatings Based on Silanes 579

technology. The surface of the resulting clear and scratch-resistant coating is depicted in Fig 6, which shows the homogeneous distribution of the silane-covered silica particles. These measurements indicate that the particle size is smaller than 100 nm.

This homogenous distribution of the hard, silane-modified fumed silica particles results in extremely scratch- and abrasion-resistant formulations. An indication of the performance of this system is given in Fig. 7.

Fig. 7. Scratch resistance of a hybrid coating using different silanes following DIN 53799.

Applications

The coating technology described is actively being introduced into applications. Currently, the most promising applications include:

decorative paper for furniture; decorative trims for furniture; plastic films and boards; metal sheet and objects; parquet flooring (sandable).

References [I] [2] [3] [4]

[5 ]

D. Stoye, W. Freitag eds., Paints, Coatings and Solvents, Wiley-VCH, Weinheim, 1998. J. J. Ebelmann, Annalen 1846,57, 319. Schott Glass, DE 736 41 1,1939. F. Beari, M. Brand, P. Jenkner, R. Lehnert, H. J. Metternich, J. Monkiewicz, W. H. Siesler, J. Organomet. Chem. 2001,625,208-216. M. Brand, A. Frings, P. Jenkner, R. Lehnert, H. J. Metternich, J. Monkiewicz, J. Schram, 2. Natuqorsch. Teil B 1999,54, 155-164.

580 B. Borup, R. Edelmann, J. Monkiewicz

[6] Scherer, Luong, J. Non-Cryst. Solids 1984,63, 163. [ I ] Brinker, Scherer, Sol-gel Science, Academic Press, New York, 1990, p. 292. [8] F. Bauer, H. Emst, U. Decker, M. Findeisen, H.-J. Glasel, H. Langguth, E. Hartmann, R.

Mehnert, C . Peuker, Macromolecular Chemistry and Physics 2000,201,2654-2659. [9] H.-J. Glasel, F. Bauer, H. Emst, M. Findeisen, E. Hartmann, H. Langguth, R. Mehnert, R.

Schubert, Macromolecular Chemistry and Physics 2000,201,2165-2170.

Marketable Products Based on Secondary Raw Materials from

Organosilicon Production Facilities

Anatolii Shapatin

Federal State Unitary Enterprise State Research Institute for Chemistry and Technology of Organoelement Compounds, 38 Shosse Entuziastov, Moscow, 11 1123 Russia

Tel.: +7 095 273 72 79 - Fax: +7 095 273 25 38 E-mail: eos @eos.incotrade.ru

Keywords: silicones, wastes, oil and gas production

Summary: The distinctive features of the processing of silicon wastes into marketable products and their use in the oil and gas industry are surveyed. Examples of such “secondary” organosilicon products and their employment in well drilling and oil production are presented.

Individual problems of the processing of silicone wastes were solved in the past; they were not looked upon as a whole. In the process of analysis it turned out that complex task solutions required specific new research (synthesis; analysis of physical, chemical and application properties of new organosilicon polymers) and solution of both economic and technological problems, - the whole path “from molecules to materials”.

Multi-stage raw material processing into tens and hundreds of various intermediates and commodity products is typical of enterprises with a variety of production facilities ranging from organosilicon monomers to materials production [ 11. A group of different wastes is formed thereby. For large-scale production of commodities, manufacture with the employing of secondary raw material wastes, in other words “secondary organosilicon materials” (SOM), is reasonable. In most cases the SOM price should naturally be below the price of standard “pure” materials. But, first, their sale gives profit instead of inefficient costs on wastes disposal and deactivation. And, secondly, a low price promotes SOM competitiveness.

SOM production is related to complex task solutions. Scientific problems emerge for new polymer synthesis when wastes are used. As a rule there are no strict requirements for the latter in respect of purity and physicochemical properties. But the SOM must be reproducible and possess applied properties therewith.

Another point: wastes are usually produced in small amounts in comparison with basic products. Therefore when a relatively large quantity of SOM is required a diversity of wastes (or part of the standard raw material) should be used for the development of a single material brand.

Ovganosilicon Chemisty V Edited by N. Auner and J. Weis

Copyright 0 2003 WILEY-VCH Verlag GmbH & Co. KgaA, Weinheim

582 A. Shapatin

SOM application in such branches of industry as electronics, aircraft, etc. that require pure polymers with high dielectric properties or enhanced thermal stability etc. is rather problematic. SOM may be characterized by various impurities, pigmenting, absence of extreme stability etc. But they have a series of positive performance characteristics typical of silicones: water repellency, thermal oxidation, radiation stabilities and many others.

Analysis proved that SOM can be used efficiently in the oil and gas industry, for example in: well drilling, construction, over-as well as underground gas storage facilities, oil production chemicalization etc.

Problems that arise are determined by various conditions of such work related to the natural peculiarities of different oil and gas fields, process diversity, problems in drilling and storage facilities maintenance.

In this paper we consider examples of SOM synthesis, analysis of their properties and application of a few relative of SOM.

Production of alkali metal methylsiliconates including methylsilsesquioxane units based on methylchlorosilane derivatives has been described previously. The main application areas of these reagents were building materials and water proofing of smctures [2-4]. We were the first to investigate the synthesis and the physical, chemical and application properties of alkali metal organosiliconates and aluminoorganosiliconates containing dialkylsiloxane or simultaneously both dialkylsiloxane and dialkylsilsesquioxane units. Production of such materials from a raw material base includes organopolysiloxane or organoalkoxysilane alkaline saponification stages principally according to Scheme 1.

Scheme 1. Saponification of organopolysiloxanes and organoalkoxysilanes, where M = Na, K R = CH3, CzH5;

R’ = CzH5: n = 0-80 mol%.

Aluminum was introduced into organosilicon reagents by dissolution of aluminum powder or aluminum compounds in alkali metal organosiliconate solutions.

A study of the physicochemical properties of the new reagents demonstrated the pronounced intermolecular structure and higher viscosity of aluminoorganosiliconate solutions in contrast to organosiliconates. Their parameters (density, viscosity etc.) depend elaborately on the Si/A1 ratio. In the case of sodium aluminodimethylsiliconate, the minimum viscosity value is a function of temperature at Si/Al ratio = 4:3. In the case of aluminomethylsiliconate all extreme values of the solution properties correspond to Si/Al ratio = 3:l. In the case of aluminomethyl(dimethy1)siliconates we obtained a still more elaborate dependence.

The water proofing effect of the new reagents on most building materials is similar to the effect of their analogs containing only alkylsilsesquioxane units. They were found to have an enhanced

Marketable Products Based on Secondary Raw Materials from Organosilicon 583

effect on mud solutions. In Western Siberia consumption of the new reagents amounted to about 100 kg per well in order to provide trouble-free well drilling at 2 500-3 000 m depth. New modifiers with two to fivefold higher efficiency are being developed. SOM application makes possible the achievement of general improvement of drilling clay mud.

Another example of SOM employment is plugging. One of the SOM is an oligomer containing Si-0-Si, Si-Si and Si-R-Si (where R= -CH2-, -C2H4-) groups in the main molecule chain and -CH3, -C2H2, -0C2Hs and -C1 as framing groups at silicon [5 ] . We obtained such material by hydrolytic etherification of a wastes mixture containing the high-boiling fraction (“still bottoms”) of methyl- and ethylchlorosilanes, and other products. Oligomers are soluble in organic solvents and are catalytically cured by water (due to HCl formation) to water-repelling “stone”. The gelation rate depends on the reagenuwater ratio. A maximal curing rate within 0.5-5.0 min (with violent agitation of the tamponage and water) is observed in the case of 15-35 vol% water content per 100 % of reagent. This is explained by formation of rather concentrated hydrochloric acid, which is a curing catalyst. As the water concentration increases, the curing rate is decreasing (about 9-20 min at equal volumes of water and reagent).

When oil-bearing rock is treated, capillaries lose water permeability but retain oil permeability. This selective effect was used with a significant decrease in workover cost for drowned oil producers [6], with an additional production on the average of 400 t of oil per ton of SOM [5].

Recent research allowed us to obtain new SOM with a unique combination of property, namely plugging, water-repellency, acidity and temporary water permeability.

The great opportunities to apply, in such large-capacity branches (hundred of millions of tons per year) as the oil and gas industry, such small-capacity plant as organosilicon ones can evidently be explained by high SOM efficiency as we have emphasized above. For SOM production we used ten different types of wastes that made it possible to produce thousands of tons of products.

This paper is dedicated to the late A. V. Malyarenko, L. Sushon, and Yu. N. Yankovskii, experts in oil industry who made significant contributions in the problem solution under consideration but who passed away untimely. I am thankful to my research colleagues: E. V. Serebryannikova, I. V. Demidov, 0. D. Gracheva and others, employees of the chemical and oil industries who have participated in development and application of SOM production technology.

References [ 11

[2] [3] [4] [5 ] [6]

L. M. Khananashvili, Organoelement Monomers and Polymers Chemistry and Technology, Moscow, 1998, p. 528. A. A, Panchenko, M. G. Voronkov et. al., Hydrophobization, Kiev, 1973, p. 239. M. V. Sobolevskii et. al., Silicones Properties and Application, Moscow, 1972, p. 296. BRD Patent 2 245 927, 1974; Int. C1: C04B 41/28. A. S . Shapatin, V. M. Kopylov, Khim. Prom., 1995,11,57. Yu. V. Zemtzov, A. S . Shapatin et. al., Geology, Geophysics and Development of Oil Fields, Vol. 2, Express-inf., Otechestven. opyt”, 1988, p. 12.

Correlation of the Viscosity and the Molecular Weight of Silicone Oils with the T2 NMR

Relaxation Times

Joachim Gotz, * Horst Weisser

Lehrstuhl fur Brauereianlagen und Lebensmittel-Verpackungstechnik TU Munich Weihenstephan, Weihenstephaner Steig 22, 85350 Freising-Weihenstephan, Germany

Tel.: +49 8161 71 3597 -Fax: +49 8161 71 4515 E-mail: Joachim.Goetz@ wzw.tum.de

Stefan Altmann

Wacker-Chemie GmbH, Burghausen, Germany

Keywords: viscosity, molecular weight, NMR, silicone oils

Summary: Nuclear magnetic resonance (NMR) was used to determine the Tz NMR relaxation times of silicone oils. T2 characterizes the molecular mobility of fluids. The TZ times, furthermore, correlate with both the zero-shear-rate viscosity and the molecular weight of the corresponding silicone oil. Thus NMR provides the possibility of determining simultaneously the zero-shear-rate viscosity and the molecular weight of silicone oils by means of T2 experiments.

Introduction

For pure fluids (e.g. water, glycerine, ethyl alcohol, acetic acid) Bloembergen et al. [ l ] found theoretically and experimentally a correlation between the NMR relaxation times TI and TZ , respectively, and the dynamic viscosity q of Newtonian fluids, which is valid independently of temperature and pressure. Harz [3] could show that this correlation also holds for aqueous solutions like treacles, fruit juices, beer and wine. Further studies on silicone oiVglass sphere suspensions and beer mashes demonstrated that the T2-q correlation, which originally was exclusively derived for Newtonian fluids, can also be applied to suspensions 141. In contrast to solutions, the dependence is nonpotential.

Many experimental data (51 show that the empirically found correlation between a) the molecular structure inclusive average molecular weight and molecular weight distribution in polymer melts, and b) the flow behavior is qualitatively similar for different monomers 161. As silicone oils are of high relevance in technical applications and, furthermore, available within a

Ovganosilicon Chemisty V Edited by N. Auner and J. Weis

Copyright 0 2003 WILEY-VCH Verlag GmbH & Co. KgaA, Weinheim

Correlation of the Viscosity and the Molecular Weight of Silicone Oils 585

wide range of molecular weights, they were chosen as samples in this study. The aim of the present study is to decide whether the viscosity and the molecular weight of silicone oils can be determined with the help of T2 NMR experiments.

Materials

Silicone oils are linear polydimethylsiloxanes with the structure

R3SiO[R2SiOInSiR3 with R = CH3

The dynamic viscosity of the silicone oils is between 1 and lo6 Mpa s at 25 "C according to the corresponding molecular weights (150-250 000). Silicone oils differ from organic materials in their physical and chemical properties such as high- and low-temperature stability, good dielectric properties, low interfacial energy, and chemical inertness. They can be used in a wide variety of applications, ranging from heat transfer media, hydraulic fluids, dielectric fluids, water repellents, polishes, lubricants, antifoams, and mold release agents, to hydrophobizing glass (e.g., in the pharmaceutical industry), ceramics, and stabilizers for varnishes with pigments [7-91. In the cosmetic and pharmaceutical industry silicone oils are used as constituents for toothpaste, lipstick, nail polish, and ointments for skin protection [lo], as a basis for ointments, for the stabilization of hairstyles, and as a fixative of aromatic substances [ l l , 121. In food technology silicones are used as antifoam agents in the desalination of seawater and in fermenters (e.g. for vitamins, monosodium glutamate) [lo].

Flow Behavior and Molecular Weight Distribution

The viscosity and the molecular weight are often used for quality or process control in order to characterize polymer melts or solutions [ 13-16].

Charlesby and Bridges [17] correlated TI and T2 , the respective relaxation times, with the free volume and the viscosity q for cis-polyisoprene solutions. Within the studied concentration range of the polymer Eqs. 1 and 2 hold (M = molecular weight of the polymer).

Eq. 1

T2 - 774'5

Eq. 2.

Gil et al. [18] studied binary mixtures of methanol and tetrahydrofuran by means of NMR,

586 J. Gotz, S. Altmann, H. Weisser

permittivity, viscosity and ultrasonic measurements. Their investigations were focused on the molecular structure of the solution. In Rheo-NMR spectra are used to observe structural changes in complex fluids which undergo a shear flow with constant shear rate [19]. Thebaudin et al. [20] found a linear dependence between TZ and the moduli G' and G" for industrially produced sauces. T2 was thus used to observe the structure of the sauce during and after a thermal treatment.

The flow behavior of silicone oils [5] and silicone oil/glass sphere suspensions [I41 was studied by several authors. One of the most used rheological material parameters to characterize the flow behavior is the zero-shear-rate viscosity qo. The l o value of the linear silicone oils studied are correlated with the relevant weight-average molecular weight M, by Eq. 3 [16], where a = 3.58.

Eq. 3.

Comparable dependences hold for other polymer systems also. For example, the exponent a for polyethylene is 3.4 [21]. In polymer melts and solutions q o changes at a material-specific molecular weight M, from a linear to a 3.4 dependence on M (Eqs. 4 and 5) [22,23].

vo - M for McM,

Eq. 4.

qo - M3.4 for McM,

Eq. 5.

So-called entanglements or temporary physical junctions are assumed to cause the 3.4 dependence [23].

Methods

Rheometry and SEC

The rheometric experiments in this study were performed with a MC 100 Rheolap from Physica MeBtechnik GmbH, Stuttgart. The cone-plate system had an angle of 6" and a radius of 0.1250 x lo-', 0.2500 x

The distribution of the molecular weight was determined with size-exclusion chromatography (SEC) [24, 251. The device consisted of a degasser (Knauer GmbH, Berlin), an HPLC pump (Abimed GmbH, Langenfeld), a refractometer-detector (Bischof GmbH, Leonberg) and separating columns (Polymer Laboratories, Shropshire, UK). The mobile-phase toluene was p.A. For evaluating the experimental data WINGPC 6.2 (Software PSS, Mainz) was applied. Calibration was done with polystyrene standards.

and 0.3750 x lo-' mm, respectively.

Correlation of the Viscosity and the Molecular Weight of Silicone Oils 587

Nuclear Magnetic Resonance

Nuclear Magnetic Resonance (NMR) [26-301 stems from the fact that the nuclei of specific isotopes (e.g. of the isotopes 'H, I9F) possess a magnetic moment (spin) and are processing at a specific angle with respect to an external magnetic field (Eq. 6) .

Eq. 6.

In the case of spin Yi nuclei orientate their components along the field axis (B,) either parallel or anti-parallel to B,. Because the parallel orientation is energetically preferable for 'H, this orientation

is assumed by a larger number of nuclear spins. Despite the fact that this phenomenon can be calculated correctly in physical terms only by the

use of quantum mechanics (e. g. [31]), the macroscopic behavior of the spin ensembles can be described for many of the NMR experiments as a continuous magnetization vector. In this description the magnetization vector in the thermal equilibrium points in the direction of the static magnetic field of the magnet. The thermal equilibrium can be disturbed by an appropriate RF pulse. The subsequent relaxation after the RF field has been switched off can be described by the so-called relaxation times TI and T2 [31]. TI and T2 are measures of the interaction of a spin with its surroundings and the mobility of a spin, respectively.

This NMR principle has a large number of established applications in analytical chemistry known collectively as NMR spectroscopy, due to the fact that the exact resonance frequency for each nucleus is dependent upon the chemical environment of this nucleus [32]. The resonance frequency of 'H, for example, is 42.55 MHz and it is 40.05 MHz for 'F at B, = 1 Tesla.

A low-resolution NMR spectrometer system Minispec mq20 (Bruker Analytik GmbH, Rheinstetten, Germany) was used. The resonance frequency of 'H is 20 MHz. A combination of FID (with a scanning time distance of 5.2 x ms) and subsequently a CPMG sequence with a duration (90-180") of 0.2 ms and 19 600 pulses was applied. Thus the influence of field inhomogeneities and diffusion and chemical exchange on the relaxation could be minimized [33] and in spite of the high scanning rate comparably long total measuring times (- 8 s) could be achieved.

Results

Figure 1 shows the NMR signals of the silicone oils studied at 25 "C. The higher the viscosity VO, 25 OC, the shorter is the relaxation time T2. Obviously this is caused by the increasing molecular weight. Table 1 contains the kinetic viscosity, density and the molecular weight data. In Fig. 2 T2 is illustrated as a function of the molecular weight M, with the temperature as a parameter. The data were fitted to potential function y = Ag. The correlation coefficients R2 obtained are given in Fig. 2.

588 J. Go&, S. Altmann, H. Weisser

A value of -0.631d).053 results for the exponent B which does not deviate much from -0.5 found for other polymer melts (Eq. 1). The correlation of TZ and M , is best for -20 "C. This means that for a reliable determination of the molecular weight it is recommended to cool the sample during the NMR measurement.

100

90

80

70

60

50

40

30

20

10

0

9

\

0 500 1000 1500 2000 2500 3000

t / m s

Fig. 1. NMR signals (FID + CPMG) of the silicone oils studied at 25 "C. The kinetic viscosity vg, 25 -C at 25 "C is

used as a parameter. ax . arbitrary units. FID: scanning rate = 5.2 x lo4 ms; CPMG: duration~~-180~ = 0.2 ms,

19 600 pulses.

Table 1. Kinetic viscosity Vo, 25 oc at 25 "C, number-average molecular weight Mn, weight-average molecular weight

M,, polydispersity M,/M, and density pz5 ac at 25 "C of the silicone oils studied.

Sample vo,zs~c [mm2/s1 pzsec [s/c~'I Mu MW M d M ,

AK 3 3 0.90 600 650 1.08

AK 5 5 0.92 900 1 000 1.11

AK 20 20 0.945 2 200 3 100 1.41

AK 50 50 0.96 3 500 6 800 1.94

AK 1 000 1 000 0.97 14000 30200 2.16

AK 2 000 2 000 0.97 13000 40000 3.07

AK 20 000 20 000 0.97 19200 72800 3.79

AK 500 000 500 000 0.97 23 800 155 000 6.51

AK lo6 1 06 0.97 97628 247000 2.53

Correlation of the Viscosity and the Molecular Weight of Silicone Oils 589

10000

000

100

10

100 1000 10000 100000 1000000

K Fig. 2. The relaxation time T2 as a function of the molecular weight M, of silicone oils. The correlation coefficients

RZ refer to the fit with potential function. Parameter: temperature.

In Fig. 3 the zero-shear-rate viscosity is presented as a function of the molecular weight M,. As predicted by Eqs. 4 and 5, two regimes of M, can be distinguished. The critical molecular weight M, is about 20000. Below the M, the exponent of Mw is 1.238d).O51, whereas above M, the exponent is 3.359 ? 0.088, compared to 1 and 3.4 for polyethylene [22,23].

Figure 4 shows the zero-shear-rate viscosity q o as a function of T2 of the silicone oils studied. The viscosity of a silicone oil changes with temperature as it varies from -20 through 3, 20, 50 to 70 "C. The average of the exponent of the studied samples is 1.196 f 0.203. There is obviously a correlation between qo and Tz. This means for a given, known silicone oil the viscosity can be determined by one NMR experiment.

The question is whether the viscosity can be predicted also for an unknown sample. This was the motivation for another evaluation of the data of Fig. 4. Figure 5 contains the same data as Fig. 4 but in contrast to Fig. 4 values at the same temperature (-20, 20 and 70 "C) are fitted with a potential function. The results of the fitting and the corresponding correlation coefficients are shown. Figure 5 demonstrates that for a constant temperature qo correlates with Tz. This can be expected as both the flow and the NMR relaxation behavior depend on the Ipobility of the molecules in the silicone oils and, thus, on the molecular weight.

That means, if the functions qo, M,, T ( T ~ ) , with Mw and the temperature T as parameters, are known and thus the system is calibrated, the viscosity of the sample for a certain temperature and the molecular weight can be determined simultaneously with two single NMR experiments. One experiment has to be performed at the temperature in question and one at the temperature for which qo, M,, T (T2) is known as a function of the molecular weight.

590 J. Gotz, S. Altmann, H. Weisser

10000000

1000000

100000

10000 E \

2 1000

100

10

1

100 1000 10000 100000 1000000

M w

Fig. 3. The zero-shear-rate viscosity lo as a function of the molecular weight Mw of silicone oils. Parameter:

temperature.

2 a E 1 0 F

1 .WE

1 .OOE

1 .OOE

1 .WE

1 .WE

1 .WE

1 .WE

1 .WE

07

06

05

04

03

02

01

00 1.WE + 01 1 .OOE + 02 1 .OOE + 03 1.WE + 04

T2/ ms Fig. 4. The zero-shear-rate viscosity

changes according to the variable temperature: -20, 3,20,50 and 70 "C. Parameter: kinetic viscosity Vo, 25 oc.

as a function of Tz of the silicone oils studied. The viscosity of a silicone oil

Correlation of the Viscosity and the Molecular Weight of Silicone Oils 591

1 .OoE+ 07

1 .aE+.m

1.OoE+O5 v)

([I a l.OoE+M E

1.00E+03

1.00E+CQ

l.OoE+Ol

l.OoE+OO

1

F

Fig. 5. The zero-shear-rate viscosity qo as a function of Tz of the silicone oils studied, from the same data as Fig. 4.

In contrast to Fig. 4, values for the same temperature are fitted with a potential function. The results and the

corresponding correlation coefficients RZ are shown.

samplel: Tx samplel: Ty sample2 Tx

1.00E+07

1.00E+E

1.00E+05

2 a 1.00E+M E

1.OoE+03

l.OoE+(M

1 .mE+ 01

1.00E+00

1

F

1 .mE+ 01 1.00E+OZ 1 .WE+ 03 l.OoE+M T d m

Fig. 6. Schematic explanation of three position measurement for calibration of qo and Mw by Tz.

If a calibration of the system is not available, a rough estimate of the necessary calibration data can be achieved by means of a three-position measurement. This includes two T2 experiments for the samplel in question at two temperatures Tx (temperature in question) and Ty and one further T2 experiment at Tx for another, different sample2. If q o of sample2 is known, only two experiments

592 J. Gotz, S. Altmann, H. Weisser

are to be done.

Discussion

The simultaneous measurement of q o and M , could be used as an in-line measuring method in production lines as quality or process control. NMR measurements of the sample in a bypass of the production device, which would allow sufficient characterization of the material within a few minutes, are comparable with in-line rheometric techniques such as pressure drop measurements or Rheoswing@ (Physica Messtechnik GmbH, Stuttgart) [34, 351. The pressure measurements could be perturbed by the existence of wall layers consisting of fractions with smaller molecular weight [36, 371. Since for a reliable pressure measurement in a steady, fully developed tube flow a sufficient inlet-flow distance has to be guaranteed, a phase separation with regard to the molecular weight could arise. The problems that might occur using such in-line systems are described in Ref. [38].

For the determination of the molecular weight in-line techniques are not available yet. The measuring time including preparation and evaluation of HPLC experiments to determine the molecular weight comprises several hours. In order to increase the accuracy of the measurements for the M, determination the sample should be cooled down for the NMR experiment for two reasons. Firstly, the fits (with one potential function and two other potential functions that are defined piecewise) at -20 "C are better than for the other temperatures realized. Secondly, due to the low thermal conductivity of silicone oils the temperature distribution is probably not homogeneous in the measuring volume of a conventional in-line rheometric device installed inside tubes of the production process. In order to achieve a homogeneous temperature of the samples, sufficient cooling can be achieved much more easily in the suggested bypass, as NMR experiments require only small sample quantities.

Summary

A comparison of rheological NMR experiments and experiments for the determination of the molecular weight show that there are correlations between the zero-shear-rate viscosity, the T2 relaxation time and the molecular weight of silicone oils. The temperature was varied from -20 to 70 "C, the molecular weight from 650 to 250 000.

A critical molecular weight of approximately 20 000 can be derived from the dependence of q o and M , for the silicone oils studied (Fig. 3). This fact is less pronounced in a T2-Mw plot (Fig. 2 ) . For a rough estimate of the molecular weight the simple potential fit in Fig. 2 can be used. Due to the existence of the critical molecular weight, however, a piecewise fitting of the low and high molecular parts of the curve is recommendable. The T2 experiment should be performed at temperatures sufficiently below 0 "C to increase the accuracy of the measurement.

Summarizing the findings presented, it can be concluded that both the molecular weight and the viscosity of silicone oils could be determined - after calibration - with the help of a less critical

Correlation of the Viscosity and the Molecular Weight of Silicone Oils 593

and fast T2 experiment. This possibility could be quite helpful for process and quality control for the production and handling of silicone oils.

Acknowledgments: We thank Prof. Dr. Ernst Rossler, Universitat Bayreuth, Germany, for helpful discussions.

References N. Bloembergen, E. M. Purcell, R. V. Pound, Nature 1947,160,475. N. Bloembergen, E. M. Purcell, R. V. Pound.: Relaxation effects in Nuclear Magnetic Resonance Absorption, Physical Review 1946, 73(7), p. 679-712. H.-P. Harz: Untersuchungen zum Gefrierverhalten flussiger Lebensmittel im Hinblick auf das Gefrierlagern, Gefriertrocknen und Gefrierkonzentrieren, Dissertation, Universitat Karlsruhe, 1987. J. Gotz, J. Schneider, H. Weisser: Korrelation zwischen der dynamischen Viskositat und der T2-Relaxationszeit aus NMR-Messungen fur reine Flussigkeiten, Liisungen und Suspensionen, CIT Chemie lngenieur Technik, 9/2000,2000. M. Pahl, W. GleiBle, H. M. Laun: Praktische Rheologie der Kunststoffe und Elastomere, VDI Verlag, Dusseldorf, 1991. H. M. Laun: Kautschuk + Gummi, Kunststoffe 1987,40,554-562. Bayer Silicones: Baysilone, Oils M, Product information, Bayer AG, Leverkusen, 1997. Wacker: Wacker Siliconole AK, Product information, Wacker-Chemie GmbH, Burghausen, 2001. J. Falbe, M. Regitz (eds.): Rompp kompakt Basislexikon, Thieme, Stuttgart, 1999. A. Tomanek Silicone & Technik, Hanser, Miinchen, 1990. G. M. Cameron, H. Haala, A. L. Kuo: Der Einsatz von fluchtigen Silikonen fur kosmetische Anwendungen, Parfum. Kosmet. 1986,67,232-239,326-336,384-389. W. Wolfes: Organopolysiloxan-Copolymere fur kosmetische Produkte, Parjkm. Kosmet.

J. Ferry: Viscoelastic Properties of Polymers, Wiley, New York, 1980. E. Sprato: Ermittlung rheologischer Stoffmodelle fiir Silikonole als Grundlage zur Auslegung von Viskose-Drehschwingungsdampfern, Dissertation, TU Berlin, VDI-Verlag, Dusseldorf, 1986. B. Hochstein,: Rheologie von Kugel- und Fasersuspensionen n i t viskoelastischen Matrixflussigkeiten, Dissertation, Universitat Karlsruhe, 1997. L. E. Nielsen: Polymer Rheology, Dekker, New York, 1977. A. Charlesby, B. J. Bridges: Pulsed NMR of cis-Polyisoprene Solutions TI and T2 Relaxations, Free Volume, Viscosity Relationship, European Polymer Journal 1981, 17,

D. S. Gil, J. Singh, R. Ludwig, M. D. Zeidler, J. Chem. Soc., Faraday Trans. 1993, 89(2),

1987,68, 195-203.

6645-6655.

594 J. Gotz, S. Altmann, H. Weisser

3955-3858: P. T. Callaghan, Rep. Prog. Phys. 1999,62,599-670. J.-Y. Thebaudin, A.-C. Lefebvre, A. Davenel, Sciences des Aliments 1998,18,283-291. H. M. Laun, Progr. Coll. Polym. Sci. 1978, 75, 111-139. W. W. Graessley, Fortschr. Hochpo1ym.-Forsch. 1974,16, 1-179. R. B. Bird: Dynamics of Polymeric Liquids, 1 Fluid Mechanics, Wiley, New York, 1987. E. Katz: High-Performance Liquid Chromatography, Wiley Chichester, 1996 R. W. Pekala, E. W. Merrill: GPC Analysis of a Complex Silicone Adhesive System, Int. Lab.

F. Bloch, W. W. Hansen, M. Packard, Phys. Rev. 1946, 70,474. F . Bloch, W. W., Hansen, M. Packard, Phys. Rev. 1946,69, 127. M. Purcell, H. C. Torrey, R. V. Pound, Phys. Rev. 1946,69,37. P. T. Callaghan: Principles of Nuclear Magnetic Resonance Microscopy, Clarendon Press, Oxford, 1991. R. Kimmich: NMR Tomography Dzffusometry Relaxometry, Springer Verlag, Berlin, 1997. A. Abragam: Principles of Nuclear Magnetism, Clarendon Press, Oxford, 1961. R. R. Ernst, G. Bodenhausen, A, Wokaun: Principles of Nuclear Magnetic Resonance in One and Two Dimensions, Clarendon Press, Oxford, 1987. B. Hills, S . Takacs, P. Belton: A New Interpretation of Proton NMR Relaxation Time Measurements of Water in Food, Food Chemistry 1990,37,95-111. D. Hoog, B. Senge, G. Annemuller: Rheologische Kontrolle von Labormaischen, Brauwelt 1997,137(37), 1606-1610. D. Hoog, G. Annemuller, B. Senge: Rheologische Kontrolle des grofltechnischen Brauprozesses, Brauwelt 1998,138(19), 858-865. D. D. Joseph, Y. Y. Renardy: Fundamentals of Two-Fluid Dynamics, Part I: Mathematical Theory and Applications, Springer, Berlin, 1992. D. D. Joseph, Y. Y. Renardy: Fundamentals of Two-Fluid Dynamics, Part 11: Lubricated Transport, Drops and Miscible Liquids, Springer, Berlin, 1992. J. Herrmann, A. Schwill-Miedamer, K. Sommer: Viskositatsmessungen mit dem Physica-Rheoswing RSD l-l@, Brauwelt 1999,139(28/29) 1313-1315.

1983,13(5), 10-24.

Oligoethylsiloxane Modification

Aleksei Gureev, lvladimir Zvered Tat ‘yana Koroleva, Mikhail Lotarev, Sergei Natsjuk

Federal State Unitary Enterprise State Research Institute for Chemistry and Technology of Organoelement Compounds, 38 Shosse Entuziastov, Moscow, 11 1123, Russia

Tel.: +7 95 2737291 - Fax: +7 95 2737982 E-mail: aleksgur@rambler.ru

Keywords: oligoethylsiloxanes, modifications, dimethylsiloxanes, methyldichlorophenylsiloxanes, long-chain alkyls

Summary: Oligoethylsiloxane modification by means of various substituents allowed the development of new siloxane fluids employed as the basis of working fluids, oils, and lubricants.

Oligoorganosiloxane fluids have a variety of valuable properties - low solidification and glass- transition temperatures, viscosity-temperature relationship a flat curve, satisfactory lubricating properties (compared with oligomethyl- and oligomethylphenylsiloxanes), particularly for “steel-steel” tribological situations and good compatibility with mineral and synthetic hydrocarbon environments - which have resulted in their diversified application in engineering in all the climatic zones of Russia. However, their properties should be improved.

The introduction of relatively small quantities of modifying units into a regular siloxane chain is one of the technically valuable oligosiloxane properties, which allows combination of the advantages of both the major chain and the particular benefits of the new units.

Conventional procedures and equipment were used for the chemical reactions, product isolation and sample preparation for analysis. Substance composition and structure were proven by data obtained from exclusion chromatography, elemental analysis and ‘H and 29Si NMR spectra.

Oligoethylsiloxane viscosity, at low temperature in particular, can be improved by means of introduction of a definite number of dimethylsiloxane units into the molecules. The best combination of physical and chemical parameters for oligomethylethylsiloxanes, as previous research has shown, can be obtained at unit statistical distribution in oligomer chains.

Oligomethylethylsiloxanes

Oligomethylethylsiloxanes with kinematic viscosity 500-1000 mm2/s at 20 “C that are currently required for hydraulic systems were produced by catalytic rearrangement of low-molecular

Ovganosilicon Chemisty V Edited by N. Auner and J. Weis

Copyright 0 2003 WILEY-VCH Verlag GmbH & Co. KgaA, Weinheim

596 A. Gureev, V. Zverev, T. Koroleva, M. Lotarev, S. Natsjuk

oligomethyl- and oligoethylsiloxanes at 110-1 15 'C, using cationic catalysts. Then low-molecular fractions with boiling points up to 250 "C/l-3 mm Hg were separated.

The linear a,o-hexamethyl(dimethy1)diethylsiloxanes obtained included from 0 to 27 mol% dimethylsiloxane units. An increase in dimethylsiloxane unit content results in a drop in the solidification temperature from -66 to -1 18 "C, a refractive index drop, a density increase, a molecular weight distribution broadening, and a decrease in the viscosity dependence on temperature.

Branched oligodimethylsiloxanes include from 1.4 to 16.9 mol% ethylsilsesquioxane units. An increase in trifunctional unit amount in oligomers from 0 to 16.9 mol% results in a solidification temperature drop from -91 "C to below -1 18 "C, a density increase, a diminision in the probability of low-molecular ring formation, a narrowing of the molecular weight distribution curves and a rise in viscosity dependence on temperature.

A rise in the trifunctional unit content from 0 to 1.4 mol% in oligomethylethylsiloxanes leads to sharp growth of the kinematic viscosity. There is a great probability therewith of formation of star-like compounds, and lateral branch lengths exceed a definite critical point, thus resulting in a viscosity increase. A further rise in the trifunctional unit content up to 16.9 mol% results in a kinematic viscosity drop.

All the oligomethylethylsiloxanes analyzed demonstrated Newtonian fluid properties in shear rate range from 30 to 2.2 x lo3 s-'.

Oligomethylethylsiloxanes are compatible with mineral and synthetic hydrocarbon oils, additives and some oligoorganosiloxanes.

Within the framework of unconfined space theory (Fulcher-Tamman equation), which describes well the viscosity-temperature relationship of oligomethylethylsiloxanes, the free fluctuation space Cff,) and viscous flow activation energy (EJ of linear and branched oligomethylethylsiloxanes were calculated within the temperature range -20 "C to 90 "C. Introduction of dimethylsiloxane units into an oligoethylsiloxane results in a decrease infk value, and a trifunctional unit content increase from 0 to 1.4 mol% in an oligomethylethylsiloxane results in sharp growth of the free fluctuation space. The share of free fluctuation space in an oligomer with a high ethylsilsesquioxane unit content (16.9 mol%) decreases in comparison with that in an oligomer having a smaller number of branches. Branching evidently increases the internal rotation barrier around particular Si-0 bonds and limits quantity of conformational states. But when the number of branches is small, the internal rotation-inhibiting effect is insignificant, and the prevailing factor consists in failure of atomic group alternation within a molecule in comparison with nonbranched molecules, this results in loosening of the packing.

It was found that introduction of dimethylsiloxane units into an oligodiethylsiloxane chain leads to an insignificant drop in the viscous flow activation energy from 23.3 to 22.6 kJ/mol, and an increase in trifunctional unit content leads to the growth of probability of intermolecular contacts due to molecule branch interactions; this results in viscous flow activation energy growth to 25.5 kJ/mol.

Oligoethylsiloxane Modification 597

Oligoethyl( meth yldichlorophenyl)siloxanes

For practical purposes working fluids combining such properties as high lubricity for steel-steel tribological situations including high temperatures and loading, satisfactory low-temperature parameters and compatibility with mineral, synthetic hydrocarbon media, conventional oligoorganosiloxane working fluids are required. One of the possible trends in such working fluid development is toward synthesis of new oligoethyl(methyldichloropheny1)siloxanes.

Oligoethyl(methyldichlorophenyl)siloxanes were obtained by catalytic rearrangement of low- molecular oligoorganosiloxanes at 85-90 "C, using electrophilic catalysts. Then low-molecular fractions with boiling point below 250 "C/1-3 mm Hg were separated.

The amount of chlorine identified in the end product at methyldichlorophenylsiloxane group contents above 9.3 mol% or end-group contents above 16.9 mol% in the initial low-molecular product mixture exceeded the estimated amount; the difference in the second case was more significant (sometimes double). Evidently, in the first case, the probability of formation of higher-molecular-weight products (with increased chlorine content) that are not distilled off under vacuum was higher. In the second case, a larger amount of short-chain linear ethylsiloxanes was isolated by low-molecular fraction vacuum distillation.

With an increase in the number of methyldichlorosiloxane units the kinematic viscosity rises and low-temperature parameters deteriorate for oligomers with similar chain lengths. A rise in the viscosity dependence on temperature and density is also observed. All the oligomers analyzed demonstrate Newtonian fluid properties in the shear rate range from 30 to 2.2 x lo3 s-'.

Oligoethyl(methyldichlorophenyl)siloxanes show good lubricity under conditions of rolling friction, significantly outperforming oligoethylsiloxane fluid in terms of service life at 200 "C (five-ball friction machine, air, 196 N, 3 000 rev/min, steel-steel). Their service life thus increases as the content of methyldichlorophenylsiloxane units rises in oligomers.

Oligoethyl(methyldichlorophenyl)siloxane anti-welding properties are at the level of oligoethylsiloxane fluid (four-ball friction machine, nitrogen, 200 "C). The anti-wear parameters of oligoethyl(methyldichloropheny1)siloxanes are much better (nitrogen, 200 "C) than those of oligoethylsiloxanes. Oligoethyl(methyldichloropheny1)siloxanes and oligodimethyl(methyldich1oro- pheny1)siloxanes under such conditions achieve similar anti-wear properties when the chlorine content of the latter amounts to 6.7 wt%.

Oligoethyl(methyldichloropheny1)siloxanes are compatible under standard conditions (room temperature and atmospheric pressure) and negative temperatures (-20 "C, atmospheric pressure) with mineral and synthetic hydrocarbon oils, additives and a series of oligosiloxanes.

Introduction of Long-Chain Alkyl Substituents

One of the techniques for improvement of oligoorganosiloxane lubricity under the conditions of the most widespread type of steel-on-steel sliding friction consists of introduction of long-chain aliphatic hydrocarbon groups as substituents at silicon [l, 21. Such substituents can be introduced

598 A. Gureev, V. Zverev, T. Koroleva, M. Lotarev, S. Natsjuk

into oligoorganosiloxane molecules by two methods. The first one consists in the most widely used hydrolytic co-condensation of silane monomeric functional derivatives with long-chain alkyl substituents at silicon [ 2 ] . However, it should be mentioned that synthesis of oligoorganosiloxanes with long chain alkyl substituents employing these monomers is a challenge. The second method of production of oligoorganosiloxanes with long-chain alkyl substituents is of significant scientific and practical interest - namely, hydrosilylation of n-olefins with oligoorganohydridosiloxanes, allowing the introduction of a preset number of long-chain alkyl substituents into the oligomer molecular composition and thus the control of the physical, chemical and performance attributes of the products. Literature data concerning this method are limited in availability and are mostly presented in patents [3-61.

We have produced linear and branched oligodiethylsiloxanes modified by long-chain alkyl substituents at silicon by means of n-olefin hydrosilylation by previously synthesized oligodiethylhydridosiloxanes of the respective structure [7]. Initial oligodiethylhydridosiloxanes were produced by means of diethyldichlorosilane, ethylhydridodichlorosilane and trimethyl chlorosilane hydrolytic co-condensation. For branched oligomer production methyl-, ethyl- or phenyltrichlorosilane or tetraethoxysilane was additionally introduced into the organochlorosilane mixture as a branching center.

Hydrolytic co-condensation products underwent catalytic rearrangement in the presence of electrophilic catalyst presence [7-91. Sulfonic cation-exchange resin or natural aluminosilicate-bentonite activated by mineral acid was used as this catalyst.

The compositions and structures of the oligodiethylhydridosiloxanes synthesized were proven by H and 29Si NMR spectroscopy. 1

Oligodiethyleth yloctylsiloxanes

It has been previously found that if an oligomeric siloxane chain includes more than fifteen silicon atoms, the kinematic viscosity of the hydrosilylation reaction product rises significantly, and if a substituent carbon chain at silicon includes more than eight atoms, solidification temperature of the oligomer increases. Therefore oligodiethylethylhydridosiloxane chain lengths did not exceed fifteen silicon atoms and n-octene was chosen as the n-olefin for hydrosilylation with oligodiethylhydridosiloxanes, which proceeded in the presence of the Speier catalyst at 130- 145 "C in an argon flow. The reaction time was determined by the active hydrogen residue content and bromine number and amounted to 4-6 h (the olefin content in the reaction mixture was 1.1 mol/mol Si-H bonds in the oligodiethylhydridosiloxane). Under such conditions Si-H bond conversion reached 98 % and more. Unreacted and excess n-octene was distilled off from the hydrosilylation products until a temperature of 200 "C was achieved in the liquid at 2-3 mm Hg residual pressure.

The product structure and composition were proven by 29Si NMR spectroscopy. The oligodiethylethyloctylsiloxanes synthesized have low solidification temperatures (below

-80 "C). Their kinematic viscosity depends on the ratio of diethyl- and ethyloctylsiloxy units in the molecular chains and the oligodiethylethyloctylsiloxane molecule structure, and amounts to

Oligoethylsiloxane Modification 599

145-600 mm2/s at 20 "C. The dynamic viscosity dependence on temperature is satisfactorily described by the Fulcher-Tamman equation. Oligodiethylethyloctylsiloxanes have good lubricity in steel-steel friction conditions. The oligomer wear spot diameter determined in a four-ball friction machine (20 "C, air, 196 N load, 60min) amounts to 0.3-0.5 mm, which corresponds to the performance of the best mineral oil.

Oligodiethylethyloctylsiloxanes are compatible with mineral and synthetic hydrocarbon media and a number of oligoorganosiloxanes and additives .

References

[ l ] [2] [3] [4] [5] [6] [7] [8] [9]

K. A. Andrianov, B. A. Ismailov, Zh. Obschhei Khim. 1976,46(1), 109-1 13. K. A. Andrianov, B. A. Ismailov, Zh. Obshchei Khim. 1971,41(8), 1742. Belgian Patent 609 997,1962; Chem. Abstr. 57, 16657,1962. British Patent 1325394; R. Zh. Khim. 1971,23,326. US Patent 3 532 730, 1970. RF Patent 2 101 308, 1995; Bull. Izobretenii, Otkrytii i Tovarnykh Znakov 1, 1998. RF Patent 2 177 484, 2000; Bull. Izobretenii, Otkrytii i Tovarnykh Znakov 36,2001. RF Patent 2 160 747, 1998; Bull. Izobretenii, Otkrytii i Tovarnykh Znakov 35,2000. RF Patent 2 175 334,2000; Bull. Izobretenii, Otkrytii i Tovarnykh Znakov 36, 2001.

Comblike Oligosiloxanes with Higher N-Alkyl Substituents - A Basis for

Lubricants of the New Century

Mikhail Sobolevskii, Vladimir Zverev, Igor Lavygin, Victor Kovalenko

Federal State Unitary Enterprise State Research Institute for Chemistry and Technology of Organoelement Compounds, 38 Shosse Entuziastov, Moscow, 11 1123, Russia

Tel: +7 95 273 7948 -Fax: +7 95 273 7982, E-mail: eosC3eos.incotrade.m

Keywords: silicones, polyaddition, oil, friction

Summary: A procedure for a-olefin polyaddition to oligohydridosiloxanes was developed and a series of comblike oligoorganosiloxanes of general formula R3SiO(R’2Si0),[R’Rf’SiO].SiR3 where R, R‘ = CH3, C2H5; R” = C8H17, C18H37; m = 6, 7; n = 2, 6, 10, 19 with static dimethyl(diethy1)- and methyl(ethy1)alkylsiloxane units distributed in a molecule chain were synthesized. Physical, chemical and rheological properties of the oligomers obtained are presented and their lubricity characteristics (wear spot diameter and critical seizure load) were thoroughly analyzed in comparison with organosilicon fluids and mineral oil.

Introduction

Currently available lubricants have low efficiency in the friction assemblies of modem machines and in mechanisms operating under extreme conditions of “boundary” friction under high loading and within the temperature range 213-523 K. Silicones have excellent physicochemical and thermal characteristics. These qualities provide a good service life of lubricants based on them at both low and high temperatures in the “hydrodynamic” regime of friction. Under “boundary” friction conditions, however, these materials are not efficient due to the low the bearing capacity of the boundary layer. This can be explained by the decreased viscosity of organosilicon fluid boundary phase in comparison with the bulk phase [ 11. Under high loading the boundary layer is destroyed, resulting in the direct contact of the friction surface. Besides, these fluids have poor lubricity for steel-steel pairs.

Meanwhile, mineral oils have good compatibility with steel-steel pairs and serviceable in both friction conditions but they have low thermal characteristics and high-temperature coefficients of viscosity.

Ovganosilicon Chemisty V Edited by N. Auner and J. Weis

Copyright 0 2003 WILEY-VCH Verlag GmbH & Co. KgaA, Weinheim

Comblike Oligosilomnes with Higher N-Alkyl Substituents 601

In this work, we describe the synthesis of oligoorganosiloxanes with higher n-alkyl substituents, and their tribological characteristics.

Synthesis

Comblike oligosiloxanes with higher n-alkyl radicals were produced by polyaddition reaction of a- olefins to oligohydride siloxanes in the presence of a Speier catalyst under an inert environment. It was demonstrated that the nature of the silicon radical, the presence of active hydride, or the lengths of the hydridosiloxane or a-olefin carbon chains do not significantly affect the reaction completeness (degree of conversion 98-99 %, excluding ethyloctadecylsiloxane whose conversion amounted to 90 %). Detailed analysis of production of such material is presented in Ref. [2].

The oligomers were produced under the following conditions.

Conditions of Synthesis

Temperature: 393-413 K; Inert atmosphere (Ar);

0 Spieir catalyst (0.1 % solution of HzRC16 in isopropanol) Olefins: CsH17, C18H37 (98.3-99 % purity);

0 Molar ratio of hydride siloxane Si-H bonds/olefins = 1 : 1.1.

Composition of Comblike Oligosiloxanes

Dimethyl(diethy1)- and methyl(ethy1)alkylsiloxanes with various contents of higher n-alkyl radicals were obtained. The general fomula was

where R, R' = CH3, CzH5; R" = CBH17, C18H37; m = 6,7; n = 2,6, 10, 19.

The dimethyl(diethy1)- and methyl(ethy1)-n-alkylsiloxane unit distribution in oligomer molecule chain was static.

Degree of Conversion of the Si-H Bond

A value of 90-99.8 % was determined by NMR in terms of the diminishing active hydrogen concentration and the occurrence of proton signals from =CH2 units of the octyl radical in the a-position to silicon (0.53-0.56 ppm), =CH2 in the P-position to silicon (0.9-1.0 ppm), and protons of more distant =CH2 units 'y, 6 etc. (1.3 ppm).

602 M. Sobolevskii, V. Zverev, I. Lavygin, V. Kovalenk.

Tribological Chacteristics

Wear Spot Diameter

Wear spot diameter under conditions of hydrodynamic friction and critical seizure load were determined on a four-ball friction testing machine with steel balls of 0.017 mm diameter and a rotation speed of 1 500 revlmin [2].

The lubricity under the conditions of “boundary” friction was determined at a rotation speed of 200 revlmin under the same conditions.

Rheological Properties

cylinders) within ranges for shear rate gradient of 9-1.312 s-’ and for temperature of 288-363 K.

values as well as on the R“ chain length (Table 1).

These were analyzed on a “Rheotest-2.1” rheoviscosimeter with metering N/S devices (coaxial

The oligomers produced are clear colorless fluids whose viscosity depends on their “m” and ”n”

Table 1. Composition and physical and chemical properties of oligoalkylsiloxanes higher n-alkyl substituents of

general formula R3SiO(Rz‘SiO),(RRSiO),SiR3.

~~~~ ~

Composition of oligoalkylsiloxane Physical and chemical properties

2740 523 215 1395

3190 523 <211 462

1150 393 <211 33

- 523 <211 84

480 383 - 23

- 443 307 -

- 453 304 -

M,, Tnwh [K] Tfi,,idlass[K] V ~ O [lo6 M’/c]

At elevated temperatures all oligomers behave as Newtonian liquids. With a temperature drop, the flow type changes, the signs of structural viscosity appear, and when R” =CNHW at 306-305 K the oligomer becomes cloudy and its rheological behavior acquires the features typical of saturated systems: in particular, the yield point shows up. This testifies to the fact that in this temperature range micro-segregation of incompatible siloxane and hydrocarbon chains takes place.

The inverse logarithmic dependence of oligodiethylethyloctylsiloxane (Sample 1,2) viscosity on temperature demonstrated that the slope of their curves was similar to that of polyethylsiloxane fluids (Fig.1).

The flash point was > 523 K, and the fluidity loss around 213 K also compared well with that of polyethylsiloxane fluid (Table 1).

The tribological properties of siloxanes were studied under both “hydrodynamic” and “boundary” friction conditions at various temperatures and loadings. They were compared with

Comblike Oligosiloxanes with Higher N-Alkyl Substituents 603

siloxane fluids of linear polydiethylsiloxane (PES-5), oligodimethylmethylchlorophenylsiloxane (CS-2-1) and mineral oil (MS-20) in Table 2.

Fig. 1. Logarithmic dependence of oligodiethyloctylsiloxane (Sample 1,2), mineral oil (MC-20) and diethylsiloxane

(PES-5) viscosities on temperature.

Just as we anticipated, partial substitution of ethyl radicals for higher n-alkyl substituents (CgH17) significantly improves oligoorganosiloxane lubricity, bringing them (according to the wear spot diameter) to the level of petroleum oil (Tables 2 and 3). The critical seizure load of oligodiethylethyloctylsiloxanes (Table 2) (friction load resulting in lubricating coating destruction) is almost twice under standard conditions and almost three times at 423 K as high as that of mineral oil MS-20

This means that oligodiethylethyloctylsiloxanes form strong films with high bearing capacity and good adhesive and cohesive characteristics on friction surfaces.

The results obtained can be explained by the fact that comblike siloxanes twist in such a way that short radicals are positioned inside a spiral. Therefore only long substituents are capable of

604 M. Sobolevskii, V. Zverev, I. Lavygin, V. Kovalenk.

interaction with each other and the surface of steel ball.

Table 2. Comparison of lubricity of oligoalkylsiloxanes with higher substituents, and that of

standard organosilicon and petroleum oils.

Lubricating Wear spot diameter (D,) [mm] Critical seizure loading [kg]

293 K 423 K 293 K 423 K oil

1 0.22 0.23 126 63

2 100 -

MC-20 0.27 0.32 63 24

IIMC-100 1.03 - <20 -

cs-21 0.8 - <20 -

- -

Table 3. Comparison of lubricity (wear spot diameter D, and friction coefficient kfr) of oligoalkylsiloxanes with

higher substituents, with that of standard organosilicon and petroleum oils (boundary friction conditions,

speed 200 rev/min at ambient temperature)

Viscosity (v) [m%] at 293 K Sample

Loading [kg]

293 K 318 K 363 K

Ow,[-] kh Dw[mml klr Dw [-I ktr

1 1395 - - 0.42 0.12 0.46 0.11

2 462 - - 0.33 0.13 0.45 0.13

3

4

33

84

0.31 0.13 0.37 0.25 - -

- - 0.33 0.12 0.41 0.13

5 2.1 0.45 0.40 - - - -

6 - 0.30 0.12 0.38 0.12 0.43 0.09

7 - - - 0.33 0.12 0.42 0.09

MS-20 - - - 0.34 0.09 0.46 0.10

IIMC-1.5 1.5 0.44 0.35 0.58 >0.2 - -

IIMC-100 100 - - 0.47 >0.2 0.90 0.2

cs-2-1 48 - - 0.82 >0.2 - -

The presence of long substituents at silicon, imparting high conformational mobility and large free volume to a molecule, determines a stable [3] and readily renewable contact in the fluid mass as well as providing good interaction with the steel ball surfaces (at the same level as mineral oil

However, the critical seizure load cannot be explained on the basis of thermophysical and conventional physical and chemical properties of siloxane fluids, as polymethylsiloxane fluids and oligodimethyldichlorophenylsiloxane which compare well with the proposed oils in terms of the

MS-20).

Comblike Oligosiloxanes with Higher N-Alkyl Substituents 605

specified features have much worse properties. Evidently under high loading, just as in case of low temperatures micro-segregation with

formation of blocked systems takes place. These block systems are similar to block copolymer whose strength is much higher.

Under the conditions of boundary friction (200 rev/min and high loading) the oligosiloxanes with higher n-alkyl radicals are comparable with MS-20 mineral oil and significantly outperform other organosilicon oils in terms of lubricity (Table 3).

Under such conditions the presence of dimethyl- and diethylsiloxy units is of particular importance. This is clearly manifested under a 45 kg load (Table 3), as their spirals have the smallest Kuhn number, thus ensuring a more stable structure in respect of external effects.

It should be emphasized that the siloxane oils that have been developed are stable to surface modification, which allows their application in dusty atmospheres [3].

Conclusion

Thus, we present in this study a convenient synthetic technique for comblike organosilicon oligomers with higher alkyl substituents. It is shown that these materials combine the good physical and chemical properties of oligosiloxanes and the high tribological parameters of the mineral oils.

0 It has been emphasized that the comblike organosilicon oligomers are rearranged into block systems at decreased temperature and high pressure, as a result of which their tribological parameters significantly improve. The peculiarities of the interactions of the physical, chemical and tribological properties of these ligomers are presented.

Thus we can conclude that the comblike oligosiloxanes with higher n-alkyls radicals have good physical, chemical and thermophysical characteristics (on the same level of as siloxanes with lower alkyl substituents) and larger unconfined space, and they form strong block structures under high loading. Besides, they are distinguished by only a small viscosity dependence on temperature and good tribological parameters. All these properties indicate that these comblike oligosiloxanes can be classed among the promising materials of the 21 st century.

References [l]

[2]

[3]

B. Deryaguin, V. Karasev, I. Lavygin, I. Skorockodov, E. Khromova, Special discussion on thin films and boundary layers, Cambridge, UK, September 28-30,1970. M. Sobolevsky, I. Lavygin, V. Zverev, V. Kovalenko, M. Legkov, Zhurnal Prikladnoi Khimii 2001, 74(2), 21. M. Sobolevsky, V. Zverev, V. Kovalenko, Yr. Zaslavskii, V. Belyev, Tribology ~ O O O - P ~ U S , Vol. 3, StuttgdOstfidern, Germany, 2000,2151-215.

Permeability of Silicone-Water Interfaces in Water-in-Oil Emulsions

Michael A. Brook," Paul Zelisko, Meaghan Walsh

Department of Chemistry, McMaster University 1280 Main St. W., Hamilton, Ontario, Canada L8S 4M1

Tel.: +1905 525 9140 ext. 23483 -Fax: +1905 522 2509 Email: mabrook@mcmaster.ca

Keywords: water-in-silicone oil emulsion, enzyme stability, protein release

Summary: Water-in-silicone oil emulsions, stabilized by silicone-polyether surfactants, are marginally permeable to polar, but uncharged, molecules such as phenolphthalein and crystal violet. However, charged compounds, including these compounds in basic and acidic pH regimes respectively, and proteins transfer much less readily from the internal water phase to external bulk water. Transfer experiments were followed colorimetrically. These experiments shed light on the possible mechanisms by which proteins may be released from these emulsions in bioactive form: simple breaking of the emulsion does not appear to be the mechanism of action.

Introduction

Silicone polymers, among the most hydrophobic species known [ 11, are often deleterious to protein structure. For example, shaking an aqueous solution of a-chymotrypsin with Dq (octamethylcyclotetrasiloxane) for a few minutes leads to about 85 % loss of enzymatic activity [2]. It was surprising, therefore, to learn that the presence of only a few hydrophilic, functional groups on a silicone surfactant can dramatically both stabilize the emulsion and, in some cases, decrease the rate of denaturation of the enzyme, as measured by changes in enzyme activity [3].

One of the objectives of our current research is to establish the nature of the interactions that lead to stabilization both of the protein structure and of the oiltwater interface. In addition, as there is preliminary evidence that these emulsions, when administered orally to animals, can induce antibody formation [4], we would like to understand better the ease with which different substrates, particularly proteins, can permeate the interface.

A study of the permeability of the silicone/water interface was therefore undertaken. A comparison was made of the ability of neutral and charged substrates, including proteins, to migrate through siliconelwater interfaces to an underlying reservoir of water. It is hoped that an understanding of the behavior of the siliconelwater interface will provide guidance about the

Ovganosilicon Chemisty V Edited by N. Auner and J. Weis

Copyright 0 2003 WILEY-VCH Verlag GmbH & Co. KgaA, Weinheim

Permeability of Silicone-Water Interfaces in Water-in-Oil Emulsions 607

mechanism of protein delivery in vivo.

Results

The experiments described in this report use one member of an extremely interesting class of non- ionic silicone surfactants, the silicone polyethers. DC3225c, which possesses a comb structure with pendant poly(ethy1ene oxide) (PEO) side chains on a silicone backbone, was used exclusively as the surfactant. Water-in-silicone oil (octamethylcyclotetrasiloxane D4, or decamethylcyclopenta- siloxane Dj) emulsions were formed, using a twin blade Caframo mixer, by adding the indicator- containing aqueous solution slowly to the agitated silicone oill3225c mixture. This procedure is compatible with emulsion volumes from about 50 mL up to many hundreds of liters. These emulsions contain water droplets on the order of 2-5 pm in diameter.

The migratory aptitudes of a series of compounds, including the pH indicators phenolphthalein, crystal violet and the acid salicylic acid were tested by placing the water-in-oil emulsions on top of bulk water of different pHs (Fig. 1). The internal phase of the emulsion could contain neutral or charged indicators, as controlled by pH. Several pHs were examined, including experiments where there was a pH gradient (neutral (emulsion) + base (bulk water) or neutral (emulsion) + acid (bulk water)), or the reverse (base (emulsion) + neutral (bulk water)). The degree of compound transfer from the emulsion was determined by measuring the bulk water colorimetrically over time. In a separate set of experiments, the enzyme a-chymotrypsin was dispersed in the emulsion. The bulk water in this case contained a substrate for the enzyme, N-glutaryl-L-phenylalanine p-nitroanilide, which changes color during simple or enzyme-catalyzed hydrolysis; this process was also followed, more sensitively, using UV-visible spectroscopy.

Phenolphthalein Crystal Violet Salicylic Acid

Fig. 1. Indicators and configuration of compound transfer from emulsion to bulk water.

A set of survey experiments was first undertaken. Perhaps not surprisingly, it became clear that neutral molecules move much more efficiently through the silicone/water interface and silicone than negatively or positively charged molecules. Phenolphthalein migrated from neutral water in the internal phase to pH 9.1 (bulk water), where it became purple in color. The migration occurred

608 M. A. Brook, P. Zelisko, M. Walsh

efficiently from bulk to internal water phase or vice versa (Fig. 2). Similarly, salicylic acid in the internal phase at pH 4 migrated with much more efficiency to external water at pH 7 than when the internal phase was also at pH 7, as shown by fluorescence measurements (Fig. 3). In the latter case, there is no pH gradient; salicylate is the (barely) migrating species.

Fig. 2. Phenolphthalein at pH 7 in internal emulsion, pH 12 external emulsion. A: t = 0, B: t = 2 hours.

Phenolphthalein at pH 7 in external emulsion, pH 12 internal emulsion. C: t = 0, D: t = 2 hours.

Fig. 3. Permeability of salicylate versus salicylic acid from internal water phase over 15 hours to bulk water at pH 7

(measured by fluorescence as salicylate after migration at pH 7). Broken line: salicylic acid in internal phase

at pH 4. Solid line: salicylate in internal phase at pH 7.

A series of colorimetric studies was then performed that more carefully tested the effect of pH gradient on migration for both anionic and cationic molecules, including proteins. In order to optimize the rate of transfer (when it became apparent that transfer was very limited in some cases), relatively large surface areas were used. Thus, 38 mL of an indicator-containing water-in-silicone oil emulsion was allowed to spread over bulk water in a Petri dish (10 cm diameter, 79 cm2). The UV-visible spectrum of the underlying bulk water solution was measured approximately ten times over 2-10 days. The changes in the absorption maximum for the most intense peaks are plotted for phenolphthalein (at 555 nm at high pH and 275 at pH 7, Fig. 4) following pH gradients in both

Permeability of Silicone-Water Inte$aces in Water-in-Oil Emulsions 609

directions 7 + 9.1 and 9.1 + 7, and for crystal violet (628 nm at pH 1.6, Fig. 5A). In the former case, the phenolphthalein in the internal phase at pH 7 was placed over a bulk pH 9.1 buffer or vice versa; in the latter case, crystal violet at pH 7 in the internal phase was placed over pH 1.61 buffer. Phenolphthalein samples were run for 2-10 days, depending on the gradient, while the crystal violet samples were run for 9 days.

Phenolphthalein Release from WIO Emulsions

0,045

0,005

0 10 20 30 A Time (hour)

Reversed pH Gradlent for Phenolphthalein Mgratkn Across a Water-In-Silkcm 011

Interface

0,25 0'3 1

0 2 4 6 6 1 0

B Day

Fig. 4. A: Phenolphthalein in internal phase (pH 7) migrates to pH 9.1 bulk water; B: Phenolphthalein in internal

phase (pH 9.1) migrates to pH 7.0 bulk water

A

Crystal V M Release from W/O Emulsbns

3s 1

0 c- 0 - 2

~

* *

- 8 10

B

0- 0

A ' A A

' A

UV of Bulk Phase

2 4 6 8 1 0

Day

Fig. 5. A: Crystal violet in internal phase (pH 7) migrates to pH 2.1 bulk water. B: Comparison of the UV of internal

phase containing a-chymotrypsin (after breaking the emulsion) versus the bulk water phase containing an

emulsion of N-glutaryl-L-phenylalanine p-nitroanilide.

The transfer of a-chymotrypsin was also measured. The protein was placed in a pH 7.8 buffer in the internal phase. Substrate, N-glutaryl-L-phenylalanine p-nitroanilide, was placed in the bulk water

610 M. A. Brook, P. Zelisko, M. Walsh

at pH 7.8. Evolution of substrate cleavage was followed colorimetrically (Fig. 5B). Although there is an increase, over time, in the absorption of the solution, this can be attributed to noncatalyzed hydrolysis of N-glutaryl-L-phenylalanine p-nitroanilide, as shown by comparison with a blank that was never exposed to protein. Thus, this reasonably sensitive experiment showed no evidence of protein transfer.

Discussion

Three observations immediately stand out when examining the results in Figs. 2-5: i) charged small molecules migrate to bulk water much more slowly than their neutral analogues, ii) neutral crystal violet migrated much more slowly than neutral phenolphthalein, and iii) proteins did not migrate at all. Many possible processes could permit the migration of the molecules from the internal water phase to the bulk water phase, including: i) diffusion of the molecules into the silicone oil, followed by transfer to the bulk water, ii) direct diffusion of the internal molecules across the surfactant interface (in a W/O/W configuration), or iii) the breaking of the emulsion, releasing the contents into the bulk water (Fig. 6A-C). The first possibility (Fig. 6A) seems quite remote in light of the very low solubility of even uncharged phenolphthalein in silicone oil. This was demonstrated by adding a methanolic solution of phenolphthalein to D5 (which immediately led to a turbid suspension). After centrifugation and filtration, the UV-visible spectrum showed no UV signal above background attributable to phenolphthalein. The third possibility also seems quite unlikely (Fig. 6C). Breaking of the internal emulsion droplets should have permitted transfer of any of the compounds, charged or not, into the bulk water. However, the absence of protein transfer and the exceptionally low rate of transfer of charged molecules into bulk water is inconsistent with this picture.

B C 6 1 n

Bulk Water

Fig. 6. Diffusion mechanisms: A: internal emulsion + silicone oil -+ bulk water; B: direct diffusion across a

water/oil/water interface; C: breaking of the internal emulsion at the bulk water interface.

The most reasonable explanation for transfer is route Fig. 6B, where the material has only to diffuse through the W/O/W interface. The silicone/polyether surfactant can support relatively efficient transfer of neutral molecules, but not their charged analogues including proteins, which will have difficulty passing through hydrophobic silicones. Note that the more polar the groups, the slower is the transfer, as can be seen by the comparison of rates of transfer of crystal violet and

Permeability of Silicone-Water Interfaces in Water-in-Oil Emulsions 611

phenolphthalein. The former compound, with three dimethylamino groups, is more polar, and more easily ionized, than the latter compound with two phenolic groups. Indeed, crystal violet migrates at a comparable rate to the anion of phenolphthalein.

If this is the valid transfer mechanism, it remains to explain how protein can ultimately be released in vivo. In light of these results, we now believe that demulsification must be taking place, but on a much slower time scale than that under which these experiments were performed. Alternatively, the presentation of the protein at the emulsion interface may be sufficient to induce the measured biological response. Establishing the exact rate of demulsification, and clarifying the structure of proteins at these interfaces, will therefore be the next thrust for this research.

Conclusion

Polar neutral molecules pass relatively easily through silicone/water interfaces stabilized by silicone-polyether surfactants. By contrast, proteins and charged molecules do not. These results are consistent with a direct transfer mechanism through a water/oil/water interface aided by the surfactant.

Acknowledgment: The authors gratefully acknowledge the financial support of the Canadian Institutes of Health Research, the Natural Sciences and Engineering Research Council of Canada and the Department of Chemistry, McMaster University, for awarding a summer research fellowship to MW.

References a) M. J. Owen, Siloxane Surface Activity, in Silicon-based Polymer Science: A Comprehensive Resource (Eds.: J . M. Zeigler, F. W. G. Fearon), American Chemical Society (ACS Adv. Chem. Ser. 224), Washington, DC, 1990, Chap. 40, p. 705. b) M. J. Owen, Surface Chemistry and Applications, in Siloxane Polymers (Eds.: S. J. Clarson, J. A. Semlyen), Prentice Hall, Englewood Cliffs, NJ, 1993, Chap. 7, p. 309. c) M. A. Brook, Silicon in Organic, Organometallic, and Polymer Chemistry, Wiley, New York, 2000, Chap. 9, pp. 256-308. a) L. Sun, H. Alexander, N. Lattarulo, N. C. Blumenthal, J. L. Ricci, G. Chen, Biomaterials 1997, 18, 1593. b) M. A. Brook, P. M. Zelisko, Polymer Prepr. (Am. Chem. SOC., Div. Polym. Chem.) 2001,42(1), 97. P. M. Zelisko, J. N. Crowley, M. A. Brook, Stabilization of a-Chymotrypsin and Lysozyme Entrapped in Water-In-Silicone Oil Emulsions, Lungmuir, 2002,18, 8982. V . Bartzoka, M. R. McDermott, M. A. Brook, Protein-Silicone Interactions at LiquidLiquid Interfaces, in Emulsions, Foams and Thin Films (Eds.: K. L. Mittal, P. Kumar), Dekker, New York, 2000, Chap. 21, pp. 371-380.

New Textile Softener, Rhodorsil@ Hydrosoft

Gilles Lorentz, * Josette Chardon, Martial Deruelle, Carol1 Vergeluti

Rhodia Silicones R&D Applications, 55 rue des Fr6res Perret, BP22,69191 St. Fons Cedex, France

Tel.: + 33 4 12 1 3 16 24-Fax: + 33 4 1213 1440 E-mail: gilles.lorentz@eu.rhodia.com

Keywords: textile, softener, hydrophilic, yellowing, aminosilicone

Summary: Using an experimental design method the structure and functionality of silicone oils was optimized in order to provide the best compromise between softness and hydrophilicity, together with non-yellowing. Molecular modeling and surface science helped in understanding the mechanisms involved in the different properties, so that the final structure of Hydrosoft was finally chosen. Application tests have shown that this new textile softener surpasses any of the Rhodia benchmarks, as well as those of the competitors, providing together the best softness, hydrophilicity, non-yellowing and wash resistance.

The Rhodorsil@ Range of Amino-Silicones for Textile Softening

The market for textile softeners is shared between silicone fluids and organic alternatives, most of them based on quaternary ammonium salts. In general the cosdefficiency ratio is in favor of the silicones, more particularly in favor of amino-silicones.

Many silicone manufacturers and formulators supply current grades of amino-silicones either to textile finishers or to distributors. The drawbacks of this classical offer relate to yellowing, a consequence of thermal oxidation of the amines in the drying oven [ 11, and water repellency which results from the adsorption of polydimethylsiloxane chains onto the fabric. These are particularly detrimental to bath towels and similar goods, for which comfort, soft feel and good appearance are key success factors.

As a response to yellowing, in 1990 Rhodia developed a patented technology of HALS amino- silicones, in which the amines are protected from thermal oxidation by steric hindrance in the piperidinyl group. This proved very efficient in providing both softness and non-yellowing.

Control of the treated fabric hydrophilicity remained the main problem of this market [2], until the release of Rhodorsil@ Hydrosoft, the design of which is presented in this paper.

Ovganosilicon Chemisty V Edited by N. Auner and J. Weis

Copyright 0 2003 WILEY-VCH Verlag GmbH & Co. KgaA, Weinheim

New Textile Softener, Rhodorsip Hydrosoft 613

R&D Methodology

A multidisciplinary approach is mandatory to success in developing a new product to be positioned in a tiny domain of compromise between several antagonistic properties.

Molecular modeling was used to identify the interactions between amino-silicones and cellulose (a model surface for cotton).

The approach to the molecular mechanisms of hydrophilicity or water repellency was attempted in the frame of the theories and experiments of capilarity, wetting and associated physico-chemical disciplines.

Design Of Experiments (DOE) methodologies were employed to optimize the number of synthesis trials in a domain ([N, 0) of amine contenthumber-average molecular weight of the amino-silicone candidate molecules.

Application testing allowed evaluation of the performance of the products from the experimental design, through testing procedures known by the textile softening industry:

first the amino-silicone oils are emulsified and standard fabrics padded with them, then dried in a hot air oven; softness is evaluated by hand sensorial testing with a trained panel; hydrophilicity is measured by the Tegewa method (time for a water drop to be absorbed by the tested fabric); yellow and white indices are evaluated by spectrophotometry; the wash resistance of softness and other benefits is evaluated with European laundry conditions.

Molecular Modeling Evidence of a Strong Interaction between Amino-Silicones and Cellulose

Table 1 shows that despite its bulky structure the HALS amine group experiences a strong interaction with the cellulose (1-10) crystalline face, as calculated in vacuo. It is even higher than the interaction energy of the aminoethyl-aminopropyl reference group currently found in many commercially available textile softeners.

As shown in Fig. 1, the silicones bearing HALS groups optimize their dispersive interaction with the crystalline cellulose surface, meaning that the flexible polydimethylsiloxane backbone can accommodate its configurations to help adsorption.

It should be remembered that the molecular modeling calculations are realized assuming in vacuo conditions with no ionization of any chemical group, so that the pure thermodynamic information is obtained. Actual padding conditions are quite different, with partial quaternization of the amines at acidic pH far below their pK,, but the present molecular modeling information is key to understanding the behavior of treated fabrics in any conditions, for example in laundry at alkaline pH, as will be shown below.

614 G. Lorentz, J. Chardon, M. Deruelle, C. Vergelati

Table 1. Calculated interaction energy of amino-silicones with crystalline cellulose.

Structure (1-10) face (200) face

Specific Dispersive Dispersive Specific Total backbone backbone

ceuulose/PDMS ceuulose/graft Total cellulose/PDMS cellulosdgraft

PDMS -267 -267 0 -225 -225 0

Ref. -370 -246 -133 -306 -205 -101

HALS -388 -269 -118 -332 -222 -1 10

[a1

HALS piperidinyl group Aminoethyl-aminopropyl reference group

Fig. 1. Conformation of HALS-amino-silicone at thermodynamic equilibrium with (1-10) face of crystalline

cellulose.

Physico-Chemistry at Interfaces

The aim was to understand the molecular mechanisms of water repellency following padding treatment of an initially hydrophilic cotton fabric with an amino-silicone. Knowing as much as possible about the molecular mechanisms should help in designing adequate polymer structures to get the best performance.

Figure 2 shows that the contact angle with water of a polymer-treated glass surface is governed by the polymer thickness, the onset of water repellency corresponding to a contact angle of 90" and to a given polymer thickness. We can guess this thickness is such that a continuous hydrophobic film allows no direct contact of the water drop with the hydrophilic silanol groups of the glass surface.

But this conclusion is not sufficient to understand Fig. 3, where very different water-repellent properties are obtained with the same absolute polymer thickness, starting from two different

New Textile Softener, Rhodorsip Hydrosoft 615

polymer structures, A and B. Only the re-scaling of total polymer thickness into the irreversibly adsorbed polymer thickness

in Fig. 4 makes it possible to get a consistent and unique curve with the experimental results of Fig. 3.

2o 0 1 300

0 100 200

Polymer thickness

Fig. 2. Contact angle ["I of water with polymer treated glass, as a function of polymer thickness (arbitrary units).

1

I PolymerA

0 50 100 150

Total polymer thickness

Fig. 3. Water repellency of treated glass as a function of polymer thickness (arbitrary units).

7 3 I I I I I I

Pnlvm

0 20 40 60 80 100

Irreversibly adsorbed polymer thickness

Fig. 4. Water repellency of treated glass as a function of the irreversibly adsorbed polymer thickness.

616 G. Lorentz, J. Chardon, M. Deruelle, C. Vergelati

This clearly shows that in order to provide water repellency with modified polydimethylsiloxanes (PDMS) the monomers should not be allowed to diffuse and fluctuate on too large a length scale. In particular, for linear polymer structures we can expect a very different contribution to water repellency from monomers belonging to loops between two adsorbed amine groups (irreversibly adsorbed) than from monomers belonging to tails between an adsorbed amine group and a chain end, which are much more free to move with a large amplitude. This is particularly relevant to our experimental domain, where the average number of amine-modified monomers per chain is of order of 1 to 5, meaning that the number of loops is close to the number of tail, and where the chain, tail and loop lengths are below the onset of entanglements.

Softness and Hydrophilicity Results from the DOE

Figure 5 shows the response from the DOE methodology, applied to softness as a function of D (number-average molecular weight) and [Nl (nitrogen content) for the amino-silicones studied.

Fig. 5. DOE response for softness as a function of chain length D and amine content N.

Softness depends on molecular weight with a monotonically increasing trend, confirming that oils of higher viscosity provide better handling.

The dependence as a function of nitrogen content is trickier, showing a maximum for any given value of molecular weight. It shows that there exists a limit above which increasing nitrogen becomes detrimental to softness.

This can be interpreted by taking the mechanisms of softness into account :

0 the monotonically increasing softness as a function of viscosity is consistent with a lubrication mechanism, more precisely a mechanism of lubrication by film; the increasing softness as a function of nitrogen content, below the maximum, is consistent

New Textile Softener, Rhodorsil@ Hydrosoft 617

with an electrostatic mechanism, the tribo-electric charges being more and more easily suppressed by an increasing surface conductivity resulting from more and more ionized amines and counter-ions as charge-bearing groups. Above the maximum it is likely that too much adsorption of the PDMS chains occurs, imparting a worse theological behavior and less lubrication.

Figure 6 shows the DOE response for hydrophilicity. In that case the Tegewa test has been used: the lower the time for a drop of water to be completely absorbed, the better the performance.

Fig. 6. DOE response for hydrophilicity as a function of chain length D and amine content N .

Again we can see a monotonic dependence of hydrophilicity upon chain length: here the shorter chains behave the best, providing the more hydrophilic fabric. It should be interpreted with the same topological principles as in the preceding section: for a given nitrogen content, the shorter the chain the lower the number of loops and the higher the number of tails in the sample, so that less irreversibly adsorbed PDMS monomers contribute to water repellency. It is clear that a compromise will have to be found between softness and hydrophilicity.

The way hydrophilicity depends on nitrogen content is also very interesting, with a maximum like that for the softness response. Again we are probably encountering the superposition of two antagonistic effects, the increase in [M leading to more and more irreversibly adsorbed PDMS monomers and more water repellency, then above a certain threshold the increase in polarity and ionized species concentration may lead to a true hydrophilicity of the treated fabric.

With that ensemble of experimental results it is possible to tune the product performance in order to give the best answer to the market needs. In our case we identified hydrophilicity of the treated fabric as the priority, provided that softness was at the same level as for typical amino-silicones. That is why the structure of Rhodorsil@ Hydrosoft was designed as shown in Table 2, together with

618 G. Lmentz, J. Chardon, M. Deruelle, C. Vergelati

the whole commercial range of Rhodorsil@ amino-silicones.

Table 2. The range of Rhodorsil@ amino-silicones.

CH3 I Si-A

I CH3

Reference A B N [%] Viscosity[mPa s] Emulsion

H 21637 -O-CH, -(CHZ)~-NH(CHZ)~-NHZ 0.4 300 21827

H 21642 -0-CH3 -(CHZ)~-NH(CHZ)Z-NHZ 0.2 1400 -

H 21643 -0-CzHS -(CHZ)~-NHZ 0.2 300 21837

Ecosoft -O<H3 -(CHZ)~-NH(CHZ)~-NHZ 0.4 300 -

Extrasoft -CH3 -(CH&-NH(CHz)z-NHz 0.4 4500 -

H 21645 4% HALS 0.25 10 000 S 263

H 21650 -CH3 HALS 0.25 90 000 2 1860

Hydrosoft -CH3 HALS 0.38 250 -

Comparison of Hydrosoft with the other Rhodomil@ Amino-Silicones

Figure 7 shows the Yellow index of fabrics treated with the different Rhodorsil@ amino-silicones, and then subjected to drying under particularly hard conditions (9 minutes at 150 "C) so that thermal and oxidation effects are enhanced.

Fig. 7. Yellow index of fabrics treated with the different amino-silicones of the Rhodorsil" range.

New Textile Softener, Rhodorsip Hydrosojl 619

We can clearly see that for HALS-bearing amino-silicones no yellowing occurs, the level being a little bit less than the blank, showing that some protection of the fabric against thermal oxidation may even be expected. On the other hand a very clear effect of the nitrogen content is observed for conventional amines, as is well known from the literature [l].

Hydrophilicity of the treated fabrics has been evaluated by the Tegewa test as a function of the number of laundry washes in European conditions. Figure 8 shows how close to the blank Hydrosoft behaves, and that any HALS amino-silicone bestows a good hydrophilic property to the treated cotton, while conventional amines lead to truly water-repellent cotton fabrics. The trend to improving hydrophilicity as a function of the number of washes means that some silicone is washed off at each laundering; this effect will be studied more in depth comparing RhodorsilB Hydrosoft with competitor benchmarks regarding softness wash resistance.

Fig. 8. Hydrophilicity of fabrics treated with the different amino-silicones of the Rhodorsil@ range.

Comparison of Rhodorsil@ Hydrosoft with Competitors

Figure 9 shows how Rhodorsil’ Hydrosoft compares to competitors D, M, W and H, and to the blank, for softness and hydrophilicity. Taking into account the antagonistic nature of both properties we can see that a rather good compromise has been found for Hydrosoft: with almost the same soft handling, competitors D and H are far from being as hydrophilic as Hydrosoft (10 seconds is the limit of the “hydrophilic range” as perceived by the market), and hydrophilic competitors M and W are not good softeners.

The Yellow index shown in Fig. 10 confirms the excellent performance of Hydrosoft, and all the competitors look rather good, which may mean they do not bear m i n e groups.

This assumption is consistent with Fig. 11, where it is obvious that Rhodorsil@ Hydrosoft has the best softness resistance to laundry washes: the greater the number of washes the softer the fabric treated with Hydrosoft, compared to the competitors. This result is consistent with a better anchoring of Hydrosoft onto cotton, which we can guess results from the strong thermodynamic

620 G. Lorentz, J. Chardon, M. Deruelle, C. Vergelati

interaction of the piperidinyl-modified PDMS with cellulose as seen in the molecular modeling study.

Fig. 9. Softnesss (panel test) and hydrophilicity (Tegewa test) for RhodorsilCO Hydrosoft compared to competitors

and to a blank.

Yellow index Fig. 10. Yellow index (spectrophotometric) of Rhodorsil@ Hydrosoft compared to competitors and to a blank.

Conclusion

We have tried to show how a multidisciplinary approach can result in the design of a truly differentiated product. Taking into account the unmet customer needs make it possible to focus the product design onto a very precise target in a performance domain where several antagonistic properties should be optimized: softness, hydrophilicity, and wash resistance together with non- yellowing.

New Textile Softener, Rhodorsip Hydrosoft 621

Understanding the molecular mechanisms of the performance led to the design of candidate molecules, and DOE methodologies helped save time and trials, and provided a quantitative interpretation of the experimental results so that Rhodorsil@ Hydrosoft could be launched with an optimum time to market.

When compared to the other Rhodorsil@ amino-silicones Hydrosoft shows the best possible hydrophilicity, given a very good level of softness and absolutely no yellowing. This positioning is the key to the comparison to hydrophilic competitors, so that Hydrosoft occupies a unique place in the performance domain, being altogether non-yellowing and a very good wash-resistant softener, and leaving the initial cotton hydrophilicity almost unchanged.

Fig. 11. Softness as a function of the number of laundry washes, for Rhodorsil@ Hydrosoft compared to competitors

and to a blank.

References [ 11 [2]

J. M. Pujol, Yellowing in Silicones, in Silicones in Coatings ZZZ, Conference Papers, 2000. A. Van Der Spuy, New Concept Hydrophilic Softners, Textile Technology International, 2000,104.

Nature Meets Silicones - Synthesis and Properties of Modern Organomodified Silicones

Philipp C. Tomuschut

Research Oligomers & Silicones, Goldschmidt AG, 45 127 Essen, Goldschmidtstrasse 100, Germany Tel.: +49 201 1732219 -Fax: 4 9 201 1731839

E-mail: philipp.tomuschat@degussa.com

Keywords: organomodified silicones, carbohydrates, cholesterol, hydrosilylation

Summary: Silicones with substituents from natural sources can be synthesized following simple protocols. The modification can be either hydrophilic, oleophilic or both. As hydrophilic substituents carbohydrates and polyethers are used, as oleophilic substituents cholesterol and simple alkyl chains. The physicochemical properties, such as viscosity, reduction of surface tension and emulsification performance, of silicones with standard modifications and of silicone modifications with substituents from natural sources are significantly different.

Introduction

One of the most unusual properties of silicones is their high surface activity. This is a result of their limited ability to establish attractive interactions combined with their high molecular flexibility. To utilize these properties in technical applications [l], the siloxane has often to be modified with substituents that are compatible with one of the ambient phases in order to form amphiphilic siloxanes. Due to the flexibility of the chemistry that can be performed with silicones, including reactions such as equilibration, hydrosilylation and alcoholysis, a broad variety of siloxanes bearing different organic residues can be realized. Recently, due to increasing environmental awareness, the focus has been directed to substituents derived from natural sources. Hydrophilic, oleophilic as well as amphiphilic [2] silicones can be synthesized using naturally occurring compounds such as carbohydrates or cholesterol. To develop a general understanding of differences and similarities between standard modified silicones and silicones modified with substituents from natural sources, different physicochernical properties such as viscosity, solubility in polar and nonpolar phases, the ability to reduce the surface tension of water and oils or emulsifying properties were determined.

Herein we present the synthesis and the physicochemical properties of these silicones compared to standard polyether and alkyl-modified silicones.

Ovganosilicon Chemisty V Edited by N. Auner and J. Weis

Copyright 0 2003 WILEY-VCH Verlag GmbH & Co. KgaA, Weinheim

Nature Meets Silicones 623

Synthesis of Carbohydrate-Modified Silicones

Carbohydrates and polyoxyethylene polyethers are both classes of hydrophilic compounds that are easily soluble in polar solvents. Therefore, it should be possible to use carbohydrates as potential substitutes for polyethers in order to provide silicones with improved biodegradability and good compatibility with polar phases.

The synthesis of carbohydrate-modified silicones is a two-step process (Scheme 1). The synthesis starts with a comb-like SiH-silicone which reacts in the first step with allylglycidyl ether via a Pt-catalyzed hydrosilylation [3] to form an epoxy-functionalized silicone. An excess of allylglycidyl ether is used for the hydrosilylation to achieve 100 % conversion of the SiH groups. After completion of the reaction the excess of allylglycidyl ether is removed in vacuo. In the second step the epoxide rings are readily opened by the nucleophilic nitrogen atom of N-methylglucamine. At the beginning of the reaction the mixture of silicone and carbohydrate is turbid due to the different polarities of carbohydrate and silicone. However, the mixture becomes clear at about 80 % conversion is reached, which can be ascribed to a self-emulsifying effect. This protocol guarantees a high amount of covalently bound carbohydrate.

Scheme 1. Synthesis of carbohydrate-modified silicones.

Silicones modified with polyethers and carbohydrates can be obtained in a very similar way (Scheme 2). Analogously, the synthesis starts with a comb-like SiH silicone which reacts easily in a co-hydrosilylation with an allyl-terminated pol yether and allylglycidyl ether. Again, the epoxide rings of the resulting silicone are opened with N-methylglucamine.

624 P. C. Tomuschat

silicone backbone

hydrophilic carbohydrate and polyether

moieties

j $ $ Scheme 2. Synthesis of carbohydrate- and polyether-modified silicones.

Physicochemical Properties of Carbohydrate- and/or Polyether-Modified Silicones

As mentioned before, the control of interfacial phenomena with surfactants requires a general understanding of the surfactant's basic physicochemical properties.

The following parameters of three different compounds (a polyether-modified silicone, a silicone with mixed substituents and a carbohydrate-modified silicone) were investigated and compared: solubility in water, cloud point [4], reduction of surface tension in water (Table 1) and the viscosity as function of the shear rate in bulk (Fig. 1).

Table 1. Properties of polyether- and carbohydrate-modified silicones

Polyether-modified Polyether-and carbohydrate Carbohydrate-modified silicone -modified silicone (1:l) silicone Property

Solubility in water ++ ++ ++ Cloud point (water, 1 %) 83°C none none

Surface tension (water, static, 1 %) 28.9 mN/m 32.6 mN/m 40.2 mN/m

Table 1 shows that all three compounds have excellent solubility in water. A cloud point is only observed for the pure pol yether-modified silicone at 83°C. The ability to reduce the surface tension of water is excellent for the polyether-modified silicone (28.9 mN/m), moderate for the silicone

Nature Meets Silicones 625

Y .- (Ti 0 3 10 .- >

with carbohydrate and polyether substituents (32.6 mN/m) and poor for the carbohydrate-modified silicone (40.2 mN/m).

The viscosities of the three compounds are approximately on the same level over the whole detection range. Only a slight decrease in viscosity is observed when the shear rate (Fig. 1) is increased; all three compounds exhibit no shear thinning behavior [5 ] . In contrast, the viscosity levels are quite different. Whereas the carbohydrate-modified silicone exhibits a high viscosity (100 Pa s) the viscosity of the polyether-modified silicone is much lower (1.8 Pa s). The value observed for the silicone with mixed substituents is in between (50 Pa s).

- = . Carbohydrate and p o l y e t h e m

modified silicone

Polyether modified silicone L - - C - - * - - t - t C ~ - - * ~ ~ ~ - ~ ~ ~

1000 1

1 10 shear rate [ l/s]

100

Fig. 1. Viscosity as a function of the shear rate of polyether- and/or carbohydrate-modified silicones.

Fig. 2. Calculated structures of polyether- (left) and carbohydrate- (right) modified silicones.

626 P. C. Tomuschat

The observed differences and similarities of carbohydrate- and polyether-modified silicones can be attributed to the fact that carbohydrates, in contrast to polyethers, are able to form a high number of strong hydrogen bonds. Therefore, the carbohydrate-modified silicone exhibits no cloud point and because of its extensive interaction with water it does not behave as a strong amphiphile. Hence, the reduction of the surface tension of water is poor. In bulk the intermolecular hydrogen bonds cause a significantly higher viscosity than for the polyether-modified silicone. The values for the silicone with mixed substituents lie in between these extremes for all properties investigated.

The observed effects are in accordance with molecular modeling experiments. As can be seen from semiempirical calculations in vacuo (PM3) the silicone backbone of the carbohydrate- modified silicone is strongly deformed and twisted because of attractive interactions between the carbohydrate moieties. In contrast, the silicone backbone of polyether-modified silicone is not distorted (Fig. 2).

Synthesis of Cholesterol-Modified Silicones

Cholesterol and linear alkyl chains are both lipophilic and are easily soluble in nonpolar solvents. In contrast to simple alkyl chains, cholesterol has the ability to form strong van der Waals interactions. Moreover, cholesterol and its derivatives can form liquid crystals (LCs). These liquid crystals can form either thermotropic or - if the cholesterol is part of an amphiphile - lyotropic types of LC structures [6] .

cholesterol moieties

Scheme 3. Synthesis of cholesterol-modified silicones.

Nature Meets Silicones 627

The synthesis of cholesterol-modified silicones is accomplished in a two-step process (Scheme 3). In the first step undecylenic acid is esterified with cholesterol via catalysis with methanesulfonic acid. The resulting ester bearing a terminal double bond then reacts with a comb-like SiH-silicone to yield the cholesterol-modified silicone. Two different cholesterol-modified silicones were synthesized, one with 40 % and one with 70 % (i.e. wt.%) silicone.

Properties of Cholesterol-Modified Silicones

The properties of the two silicones with different contents of cholesterol ester have been compared with those of a silicone bearing exclusively alkyl chains. Table 2 summarizes the properties of the three compounds.

Table 2. Properties of cholesterol- and alkyl-modified silicones.

Property

~ ~~~ ~~ ~~ ~ ~ _ _ _ _ _

Highly cholesterol Slightly cholesterol Alkyl-modified -modified silicone modified silicone silicone

Silicon content (wt.%)

Physical appearance

40

pasty

70 85

honey-like liquid

LC structures Yes Yes no

Solvent tolerance ( W E ) 88% > 95% -

Surface tension (RME, dynamic, 1%)[" 28.6 mN/m 22.5 mN/m 22 mN/m

Spreading (RME on PP, 10 min, 1%)'"' 35 mm 33 mm 36 mm

[a] RME = rape-seed fatty acid methyl ester; surface tension: 30.1 mN/m, spreading on P P 12 mm.

The physical appearance of the compounds varies from pasty for the highly cholesterol-modified silicone and honey-like for the slightly cholesterol-modified silicone to liquid for the alkyl-modified one. As expected, the two cholesterol-modified silicones exhibit LC structures. The highly modified silicone tolerates more solvent (RME = rape seed acid methyl ester) than the slightly modified one until the LC structures disappear. A complete loss of LC structures is observed at RME concentrations of 12 % and below 5 % respectively. In a standard application a concentration of 1 % silicone additive, corresponding to a solvent concentration of 99 %, is used. That means the concentration of cholesterol-modified silicones which is necessary for the formation of LC structures is not reached in a standard application.

Whereas the surface tension of RME is reduced in an appropriate way only by the slightly cholesterol-modified and the alkyl-modified silicone, all three compounds have an excellent ability to increase the spreading of RME on polypropylene. In general these two properties are closely related [7].

Again, attractive interactions are responsible for the different physicochemical properties of

628 P. C. Tomuschat

cholesterol- and alkyl-modified silicones. The anisotropically shaped cholesterol skeleton is able to form attractive van der Waals interactions, as can be seen from the formation of LC structures (Fig. 3). This perception is in accordance to PM3 calculations which have shown that the interactions are strong enough to force a distortion of the silicone backbone (Fig. 3).

Fig. 3. Calculated structure (left) and LC structures of slightly and highly cholesterol-modified silicones (pictures

were taken between crossed polars).

Synthesis of Polyether- and Cholesterol-Modified Silicones

Silicones bearing both hydrophilic and lipophilic substituents are amphiphilic compounds. They are well known as W/O emulsifiers that are used in a broad range of cosmetic applications [8]. When cholesterol is used as a lipophilic substituent, an amphiphilic molecule is formed that has the ability to form both lyotropic and thermotropic LC structures at the water/oil interface. These structures are in principle able to stabilize an emulsion e~cellently[~]. Silicones with polyether and cholesterol substituents can be obtained by a co-hydrosilylation of an allyl-modified pol yether and the undecylenic acid cholesterol ester with a comb-like SiH-silicone (Scheme 4).

Nature Meets Silicones 629

Scheme 4. Synthesis of polyether- and cholesterol-modified silicones.

Cholesterol modified emulsifier

Fig. 4. Structures of established and cholesterol-modified emulsifiers.

Properties of Pol yether- and Cholesterol-Modified Silicones

A cholesterol- and pol yether-modified silicone was used for the stabilization of a W10 emulsion. The same W10 emulsion was made using one of our well established W10 emulsifiers and the two emulsions were compared. To our surprise the cholesterol-modified emulsifier was as efficient as our standard product, which was the result of an extensive optimization procedure. Parameters that are generally used to characterize emulsions are ease of preparation, dispersity and long-term as

630 P. C. Tomuschat

well as temperature stability. Thereby, the molecular structures of both emulsifiers are significantly different. Neither the substitution pattern nor the balance of hydrophilic to lipophilic substituents is equal, even with respect to the different molecular weights of alkyl chains and cholesterol esters (Fig. 4).

However, in the emulsion no LC structures were found at the oivwater interface that could explain the unusual stability of the emulsion.

Conclusions

The physicochemical properties of silicones with substituents from natural sources are significantly different compared with silicones with well established substituents. In the case of hydrophilic substituents (pol yethers, carbohydrates) strong hydrogen bonds are responsible for the observed differences, whereas the differences observed for the lipophilic alkyl chain and cholesterol substituents are based on shape anisotropy and attractive van der Waals interactions. Therefore, a simple “one to one“ replacement of synthetic with natural substituents does not yield silicones with identical properties. To utilize silicones with substituents from natural sources in industrial applications further investigations have to be done. In this context, we are trying to develop a deeper understanding of the major physicochemical properties that determine the behavior of compounds that are used for the control of interfacial phenomena.

At the same time, the observed differences are a chance to open up new fields of applications in which silicones functionalized with “traditional” substituents can not be used so far.

Acknowledgments: The author thanks Dr. T. Dietz for fruitful discussions and S. Volkmer and W. Miiller for the preparation and characterization of the compounds described.

References [ l ] T. C. Kendrick, B. Parbhoo, J. W. White, Siloxane polymers and copolymers, in The

Chemistry of Organic Silicon Compounds (Eds.: S . Patai, Z. Rappoport), Part 2, Wiley, New York, 1989, p.1289.

[2] G. Feldmann-Krane, I. Schlachter, Silicone surfactants, in Novel Surfactants (Ed.: K. Holmberg), Surfactant Science Series, Vol. 74, Marcel Dekker, New York, 1998, p. 201.

[3] B. Marciniec, Hydrosilylation and related reactions of silicon compounds, in Applied Homogeneous Catalysis with Organometallic Compounds (Eds.: B. Cornils, W. A. Henmann), Vol. 1, VCH, Weinheim, 1996, p. 487. P. D. T. Huibers, D. 0. Shah, A. R. Katritzky, J. Colloid Interface Sci. 1997,193, 132. T. Mezger, The Rheology Handbook, Vincentz, Hanover, 2002. G. Decher, H. Ringsdorf, Liq. Cryst. 1993,13(I), 57. K. Grundke, Wetting, Spreading and penetration, in Applied Surface and Colloidal Chemistry

[4] [5] [6] [7]

Nature Meets Silicones 631

(Ed.: K. Holmberg), Vol. 2, Wiley, New York, 2002, p. 119. B. Griining, A. Bungard, Silicone surfactants: emulsification, in Silicone Suflactants (Ed.: R. M. Hill), Surfactant Science Series, Vol. 86, Marcel Dekker, New York, 1999, p. 209. T. Engels, W. von Rybinski, J. Muter. Chem. 1998,8(6), 1313.

[8]

[9]

Organo-Modified Hydropolysiloxanes for Release Control in Silicone Paper Coatings

Christine Strissel, Oskar Nuyken

Lehrstuhl fur Makromolekulare Stoffe, TU Munchen D-85747 Garching, Germany

Tel.: +49 89 289 13570 -Fax: +49 89 289 13562 E-mail: 0skar.Nuyken @ ch.tum.de

Jochen Dauth, Christian Herzig, Hans Luutenschliiger

Wacker-Chemie GmbH, D-84480 Burghausen, Germany

Keywords: hydrosilylation, paper coatings, release control, silicone

Summary: Poly(dimethylsi1oxane-co-methylhydrosi1oxane)s with an organo-modified siloxane backbone were synthesized by alternating copolymerization of various diolefinic compounds with 1,1,3,3-tetramethyldisiloxane. The copolymers were used as crosslinkers in silicone paper coatings. An increase in the amount of organic compounds in the polysiloxane backbone was supposed to be accompanied by the suppression of silicone-typical release properties, thereby improving the adhesion of pressure-sensitive adhesives. It was demonstrated that the synthesized organo-modified poly(dimethylsi1oxane-co-methylhydrosi1oxane)s can be used as crosslinkers in silicone coating composites, partially leading to an increase in the release force values.

Introduction

A well-known application for crosslinkable silicone polymers are silicone release papers, which are used for labels as well as for industrial coatings. The necessity for a broad variety in the properties of the siloxane-containing coatings, accompanied by some shortcomings of pure polysiloxanes, such as weak mechanical characteristics and incompatibility with almost all organic polymers, led to the incorporation of polysiloxane in copolymer structures [l]. First approaches resulted in the development of siloxane-containing block, random and graft copolymers using different techniques such as living anionic polymerization [24], free [5] and living [6] radical polymerization and coupling techniques [7, 81. Among the latter, the hydrosilylation reaction was one of the earliest reaction types used in the synthesis of siloxane-containing copolymers [9]. More recently, the hydrosilylation reaction was used to synthesize alternating copolymers of diolefinic compounds and tetramethyldisiloxane via a polyaddition reaction [ 10-131.

Ovganosilicon Chemisty V Edited by N. Auner and J. Weis

Copyright 0 2003 WILEY-VCH Verlag GmbH & Co. KgaA, Weinheim

Organo-modified Hydropolysiloxanes for Release Control in Silicone Paper Coatings 633

The aim of the present work was the synthesis of alternating copolymers of tetramethyldisiloxane and different diolefinic molecules via hydrosilylation polyaddition followed by an equilibration reaction with SiH-containing polysiloxanes (PMHS) and poly(dimethylsi1oxane) (PDMS) (see Schemes 1 and 2).

Me Me n bQ + i . i n H-StO-Si-H

Scheme 1. Hydrosilylation polyaddition reaction resulting in alternating copolymers (Pl-P5).

I I E 3, H

[PNCIzldPDMS/PMHS I

cmsslinker IC1

Scheme 2. Equilibration reaction resulting in crosslinkers (C l-C.5).

The resulting organo-modified poly(dimethylsi1oxane-co-methylhydrosi1oxane)s were supposed to possess tunable release behavior. Additionally, the incorporation of the organic compound into the siloxane backbone should help to avoid compatibility problems.

Experimental

Typical Procedure for Polymerization of Hydrosilyl-Terminal Poly( 1,3-diisopropenylbenzene -aZt-tetramethyldisiloxane)

39.5 g (252 mmol) of 1,3-diisopropenylbenzene was placed under nitrogen in a dry three-neck flask equipped with a mechanical stirrer and a dropping funnel. 67 mg (20 ppm) of Karstedt catalyst (1.18 % solution in poly(dimethylsi1oxane)) was dissolved in the 1,3-diisopropenylbenzene. The reaction mixture was heated to 145 "C and 37.1 g (277 mmol) of 1,1,3,3-tetramethyldisiloxane was added dropwise over a period of 6 h. After half of the tetramethyldisiloxane has been added, its addition was stopped for 0.5 h to avoid an accumulation of unreacted SiH compound. An additional 67 mg (20 ppm) of Karstedt catalyst was put into the reaction mixture and the addition of tetramethyldisiloxane was continued. The conversion was controlled by the disappearance of all

634 C. Strissel, J. Dauth, C. Herzig, H. Lautenschlager, 0. Nuyken

vinyl proton signals in 'H NMR. The product was isolated as a yellowish, highly viscous fluid in quantitative yield.

The polyaddition hydrosilylation reactions with the other monomers were carried out under the same conditions except for a polymerization temperature of 110 "C in the case of 5-vinylnorbornene.

Equilibration Reaction

Poly(dimethylsi1oxane) and the hydrosilylation polyaddition product were placed in a three-neck flask. After they has been heated to 60 "C 200 ppm of (PNClZ), was added and the mixture was heated to 100 "C. At a temperature of 80-100 "C poly(dimethylsi1oxane-co-methylhydrosiloxane) was mixed with the other components. The equilibration process is accompanied by a visible homogenization and decrease in viscosity of the reaction mixture. The viscosity should reach a value below 180 mm2/s. The equilibration was complete after approximately 2 h and was stopped by neutralization with 1 % (w/w) MgO. After filtration and the removal of low molecular weight cyclic siloxanes in vacuum a fluid transparent oil was obtained and the SiH content was calculated by 29Si NMR spectroscopy.

Release Force Measurements

Release force measurements were carried out after mixing the equilibration product with an a,m-divinyl(polydimethylsi1oxane) at the desired ratio of SiWvinyl and 100 ppm of platinum as a 1 % Karstedt catalyst solution in poly(dimethylsi1oxane). The crosslinker thereby obtained was coated onto paper. Acrylate- (A7475, Beiersdorf) and rubber- (RP51, Raflatec) based adhesive tapes were stuck on the paper inline (immediately after coating) and offline (after four weeks) and release force measurements were carried out at a speed of 0.3 drnin.

Results and Discussion

Due to their expected low compatibility with poly(dimethylsi1oxane) aromatic compounds were preferred for the hydrosilylation polyaddition with tetramethylsiloxane. The monomers used are presented in Fig. 1.

M1 M2 M3 M4 M5

Fig. 1. Monomers chosen for hydrosilylation polymerization (M1 = 1,3-diisopropenylbenzene, M2 =

5-vinylnorbornene, M3 = bisphenol A diallyl ether, M4 = 2,2'-diallyloxybiphenyl, MS = 2,3-

diallyloxynaphthalene).

Organo-modijied Hydropolysiloxanes for Release Control in Silicone Paper Coatings 635

The hydrosilylation polyaddition resulted in polymers of a degree of polymerization of 9-20 (see Table 1). Except for the polyaddition product P4 the measured values are in good accordance with those calculated with Carothers equation (Eq. l), where r = c(monomer l)/c(monomer 2) < 1 and p = conversion.

l + r DP(calc.)=

r + l - 2 p r

Eq. 1.

Table 1. Results of hydrosilylation polyaddition.

MJ[g/mol] PDI DP DP Viscosity Conversion (‘H NMR) (calc.) rmm2/sl (‘H NMR) 1%1

Diisopropenylbenzene P1 3880 3.44 -14 18 3960 >99

Vinylnorbornene P2 5120 1.71 -20 18 7800 >99

Bisphenol A diallyl ether P3 4450 1.97 -10 12 - 96.5

2,2’-Diallyloxybiphenyl P4 3740 1.50 -9 17 - 98.9

2.3-Diallyloxynaphthalene P5 6740 4.14 -18 15 - 97.9

[a] GPC in CHCI,, PS standard, RI detection.

In equilibration reactions (Scheme 2) with SiH containing polysiloxanes and poly(dimethylsi1oxane) catalyzed by (PNCl&, the amount of SiH per polymer chain can be adjusted to the desired values.

These crosslinkers were used in a curing system containing a,a-divinyl(polydimethylsi1oxane) and diluted Karstedt catalyst. The crosslinkers were also supposed to function as controlled release additives (CRA) in the paper coatings. The controlled release effect of the crosslinkers was analyzed as a function of the SiWvinyl ratio, of the organic content and of the organic monomer used. In all cases an increase in the release force was observed with rising SiWvinyl ratio, whereas only the crosslinker C2 showed the expected behavior of increasing release force values with a rise in the content of the organic compound in the backbone of the crosslinker. This is shown in Fig. 2 for inline sticking of an acrylate-based adhesive tape and a SiWvinyl ratio of 6: 1 for the crosslinker composition.

Surprisingly, using an acrylate-based adhesive (A 7275, Beiersdorf), the crosslinkers containing aromatic compounds did not show the expected increase in release force values with a higher content of organic compounds (Fig. 2). This was confirmed in various measurements in which the SiWvinyl ratio as well as the type of adhesive and the method of sticking (inline or offline) were modified. When a rubber-based adhesive (RP 51, Raflatec) was stuck onto the coated surfaces, an increase in the release force values was also observed in case of C2, whereas the other crosslinkers showed approximately the same release force values for all compositions.

Additionally to the behavior described regarding the content of the organic compound, the long-term stability of all crosslinkers was analyzed. Similarly to unmodified crosslinkers containing

636 C. Strissel, J. Dauth, C. Herzig, H. Lautenschlager, 0. Nuyken

SiH bonds, the organomodified crosslinkers showed a clear decrease in the release force values during a period of four weeks. This can probably be ascribed to hydrolysis of the SiH bonds.

0 1 0 5 10 15

Content of organic compounds [wt%]

Fig. 2. Development of release force with content of organic compounds (SiWvinyl = 6:l) (inline sticking, after

20 h storage at 70 "C under pressure (70 g/cm2); crosslinker C1 = 1,3-diisopropenylbenzene, C2 =

5-vinylnorbornene, C3 = bisphenol A diallyl ether, C4 = 2.2'-diallyloxybiphenyl, C5 =

2,3-diallyloxynaphthaline.)

Conclusion

It could be shown that a broad range of diolefinic comonomers are suited to copolymerization with 1,1,3,3-tetramethyldisiloxane resulting in alternating copolymers with SiH terminal groups. Following an equilibration reaction with SiH-containing polysiloxanes and poly(dimethylsiloxane), organo-modified crosslinkers for paper coating were obtained. With 5-vinylnorbornene as the comonomer an increase in the release force values with a higher content of the organic compound was observed.

References [l] [2] [3] [4] [5]

[6] [7] [8]

I. Yilgor, J. E. McGrath, Adv. Polym. Sci. 1988, 86, 1. M. Morton, A. Rembaum, E. E. Bostick, J. Appl. Polym. Sci. 1964,8,2707. P. Bajaj, S. K. Varshney, A. Misra, J. Appl. Polym. Sci. 1980,18,295. J. C. Saam, D. J. Gordon, S. Lindsey, Macromolecules 1970,3,458. T. Noguchi, T. Mise, H. Yoshikawa, J. Inoue, A. Ueda (Showa Highpolymer, Ltd.) JP 04 372 675 A2, 1992; Chem. Abstr. 1992,120,32966. P. J. Miller, K. Matyjaszewski, Macromolecules 1999,32, 8760. P. Chaumont, G. Beinert, J. Herz, P. Rempp, Eur. Polym. J. 1979,15,459. J. P. Mason, T. Hattori, T. E. Hogen-Esch, Polym. Prepr. (Am. Chem. SOC., Polym. Div.) 1989,30(1), 259.

Organo-modified Hydropolysiloxanes for Release Control in Silicone Paper Coatings 637

[9] M. Galin, A. Mathis, Macromolecules 1981,14,677. [lo] F. Tronc, L. Lestel, L., S . Boileau, Polym. Prepr. (Am. Chem. SOC., Polym. Div.) 1998, 39(1),

583. [ 113 F. Tronc, L. Lestel, S . Boileau, Polymer 2000,41,5039. [ 121 L. J. Mathias, C. M. Lewis, Macromolecules 1993,26,4070. [13] C. Herzig, B. Deubzer, D. Huttner (Wacker-Chemie GmbH), EP 0523660, 1993; Chem.

Abstr. 1993,119, 118574.

Catalytic Hydrosilylation of Fatty Compounds

Arno Behr," Franz Naendrup, Dietmar Obst

Chair of Technical Chemistry A (Process Development) Department of Chemical Engineering, University of Dortmund

D-44221 Dortmund, Germany Tel.: +49 231 755 2310 -Fax: +49 231 755 231 1

E-mail: behr@ct.uni-dortmund.de

Keywords: hydrosilylation, oleochemicals, homogeneous catalysis, hexachloroplatinic acid, catalyst recycling

Summary: The hydrosilylation of unsaturated fatty acid esters with different hydrosilanes was carried out under mild conditions using hexachloroplatinic acid as homogeneous catalyst. The catalyst was recycled using biphasic liquid-liquid solvent systems. The consecutive chemistry of the oleochemical silicon compounds was examined with respect to solvolysis reactions.

The use of renewable resources, especially of fats and oils, as feedstocks leads to products that are interesting in an economical as well as a technical way [ 11. By the introduction of branching in the fatty acid carbon chain, products with special physical and chemical properties can be synthesized.

The hydrosilylation of methyl undec-10-enoate 1 or methyl linoleate 2 (Scheme 1) was carried out with different hydrosilanes using dried hexachloroplatinic acid as homogeneous catalyst. The results are given in Tables 1 and 2.

1 [H,PtCld

R,Si

0 (4) + regioisomers (5)

Scheme 1. Hydrosilylation of methyl undec-10-enoate 1 or methyl linoleate 2 with different hydrosilanes.

Ovganosilicon Chemisty V Edited by N. Auner and J. Weis

Copyright 0 2003 WILEY-VCH Verlag GmbH & Co. KgaA, Weinheim

Catalytic Hydrosilylation of Fatty Compounds 639

Table 1. Hydrosilylation of methyl undec- 10-enoate 1 with different hydrosilanes (conditions: stoichiometric

amount of hydrosilane, 1 mol% H2PtCI6, 40 "C, 2 h, solvent: cyclohexane/propylene carbonate).

Expt. Hydrosilane Product Fatty ester conversion [%I Product yield ['I [%]

1 HSiMezCl 4a

2 HSiMeClz 4b

3 HSi(OEt), 4c

4 HSiEt, 4d

5 HSiPh, 4e

71

92

77

30

15

59

41

77

16

4

[a] Relative to the fatty acid ester.

Table 2. Hydrosilylation of methyl linoleate 2 with different hydrosilanes (conditions: 1.8-fold hydrosilane excess,

2 mol% H2PtCI6, 40 "C, 4 h, no solvent).

~

Expt. Hydrosilane Product Fatty ester conversion [%] Product yield ['I [ %]

1 HSiMezCl 5a 95

2 HSiMeCI2 5b 98

3 HSi(OEt)3 5C 74

4 HSiEt, 5d 61

5 HSiPh, 5e 69

51

25

0

5

0

[a] Relative to the fatty acid ester.

In the hydrosilylation of 1, the best yield and selectivity were achieved using triethoxysilane, whereas moderate yields were observed using chloromethylsilanes. The hydrosilylation of 2 gave moderate yields only with chloromethylsilanes. In both cases, the reactions could be carried out at low temperatures and under ambient pressure.

The recycling of the homogeneous catalyst was examined using a biphasic liquid-liquid solvent system of cyclohexane and propylene carbonate. The reaction of 10-undecenoate 1 with triethoxysilane 3c was chosen as test reaction. The results are shown in Fig. 1.

Fig. 1. Conversion of 1 and yield of 4c in five recycles. Conditions as given in Table 1.

640 A. Behr, F. Naendrup, D. Obst

Yield and selectivity of the hydrosilylation reaction remained largely constant even in the fifth run, which demonstrates the successful application of the liquid-liquid biphasic technique [2] for the catalyst recycling.

We also examined the consecutive chemistry of oleochemical silanes (Scheme 2).

H,C&? COOMe

p/ ROHI \&I

H,C- I-CH, L C O O M e H,C- I-CH,

H,C- I-CH, L C O O M e $: H,C- I-CH,

COOMe J

COOMe

H,C- I-CH, MeoocT

Scheme 2. Consecutive chemistry of hydrosilylated methyl linoleate 5a.

The Si-Cl bond readily undergoes solvolysis reactions with water or alcohols to yield the silanol and silyl ether compounds, respectively. The silanol dimerizes giving a disiloxane with two fatty acid ester units. Using methanol and an excess of hydrosilane under reaction conditions, fatty acid esters with siloxane branches are accessible.

The combination of hydrophilic carboxyl and siloxane groups with hydrophobic carbon chains in the same molecule leads to a number of possible product applications. The oleochemical silicon compounds may be suitable as formulation compounds for cosmetics, as water repellents for building protection, or as special lubricants with high thermal stability and good biodegradability.

Acknowledgments: The authors thank Cognis Deutschland GmbH for financial support and for the donation of fatty compounds. The donation of silanes by Wacker Chemie and the donation of hexachloroplatinic acid by Degussa Metal Catalysts Cerdec (now a division of the OMG group) are gratefully acknowledged.

References [l]

[2]

U. Biermann, W. Friedt, S. Lang, W. Liihs, G. Machmiiller, J. 0. Metzger, M. Riisch gen. Klaas, H. J. Schafer, M. P. Schneider, Angew. Chem. Int. Ed. 2000,39,2206-2224. A. Behr, Chem.-hg.-Tech. 1998, 70,685-695.

Polycarbosilanes as Precursors of Novel Membrane Materials

Hieronim Maciejewski, Piotr Pawluk, Bogdan Marciniec, * Zreneusz Kownacki, Wioletta Maciejewska, Mariusz Majchnak

Adam Mickiewicz University, Faculty of Chemistry Grunwaldzka 6,60-780 Pomari, Poland

Tel.: +48 618291366 -Fax: +48 618291508 E-mail: marcinb@ amu.edu.pl

Keywords: hydrosilylation, polycarbosilane, Karstedt catalyst, membrane materials

Summary: Polycarbosilanes and crosslinked polycarbosilanes were obtained by using a polyhydrosilylation reaction between difuncional vinylsilanes and difunctional hydrosilanes or in the intermolecular hydrosilylation of monomers containing an Si-H bond and an SiCH=CHT group in the same molecule. Poly(sily1ene-ethy1enearylene)s were obtained with the highest molecular weight. They seem to be potential parent substances for future applications as a membrane materials.

Introduction

Over the last decade or so, the separation or fractionation of gas mixtures by means of polymeric membranes has become an established energy-saving and environment-friendly technology. Most of the R&D effort devoted to the identification of membrane materials suitable for this purpose has hitherto been focused on glassy polymers [l]. Much less attention has been paid to rubbery polymeric materials, which could increase membrane selectivity for the heavier components due to higher solubility of the latter in the membrane. There are important applications involving elimination of heavier component fractions present in relatively small amounts in the mixture which require membranes of this type. So far, only siloxane, crosslinked siloxane or siloxane-containing block copolymers have been proposed for organic vapor separation. Siloxane-based polymers have the advantage of high permeability, which is unlikely to be surpassed by new rubbery materials of higher glass transition temperature T,. On the other hand, siloxane-based polymer materials suffer from low stability for separation of hydrocarbons in natural and petroleum gas under inclement conditions and are sensitive even to traces of hydrogen sulfide, mercaptans or thiophene, which are likely to be present in the gas streams processed. This point is not of particular importance to the separation of gasoline or solvent vapors from air but can be significant in the case of applications to the gas and oil industry. Therefore the idea of using polycarbosilanes instead of polysiloxanes was

Ovganosilicon Chemisty V Edited by N. Auner and J. Weis

Copyright 0 2003 WILEY-VCH Verlag GmbH & Co. KgaA, Weinheim

642 H. Maciejewski, P. PawluC, B. Marciniec, I. Kownacki, K Maciejewska

the starting point of the search for efficient synthetic routes to silylene-alkenylene(ary1ene) polymers which could be good precursors of membranes used in this field of industry.

Polyhydrosilylation is a well-known reaction leading to polycarbosilane and related polymers [2-51. The above-mentioned reaction has been extensively studied because of its versatile application to polymer chemistry [6,7]. The polymeric products obtained by means of polyhydrosilylation - polycarbosilanes - are suitable substrates for high-permeability membrane materials because of their high silicon content [8].

Linear polycarbosilanes of general formula -[RR'Si(CH2)2]- have interesting gas penetration and separation properties [9]. This work was aimed at synthesizing and characterizing polycarbosilanes, mainly silylene-alkenylene(ary1ene) polymers, as potential membrane materials for the separation of hydrocarbons present in natural and petroleum gas.

Synthesis of Silylene-alkenylene Polymers

Silylene-alkenylene polymers can be obtained via three different routes, using one substrate (intermolecular hydrosilylation) or two substrates (polyaddition reaction), which are presented in Scheme 1.

Scheme 1. Synthesis of polysilylethylene.

The right choice of reaction conditions and the nature of catalyst affect the molecular weight of the polymeric product as well as the presence of short-chain oligomers and cyclic byproducts.

On the basis of literature data [lo-121, it is very difficult to obtain high molecular weight polymer using vinyldimethylsilane as a substrate. Our experiments with vinyldimethylsilane were carried out in various solvents and in the presence of [Pt2((CH2=CHSiMe2)20}3] (Pt-Karstedt catalyst). The molecular weights of the oligomers were rather low: the highest molecular weight M, was 4 100.

An alternative route for polysilylethylene synthesis consists in the reaction between divinyldimethylsilane and 1,2-bis(dimethylsilyl)ethane in the presence of Pt-Karstedt catalyst. This kind of polyaddition, which starts from two different difunctional monomeric substrates, seems to

Polycarbosilanes as Precursors of Novel Membrane Materials 643

be more useful. The hydrosilylation reactions were carried out with or without a solvent. In all cases a 10 % excess of 1,2-bis(dimethylsilyl)ethane was used. In this case the highest molecular weight M , was 9 000.

The most attractive and promising method seems to be the reaction of polyaddition between 1,2- bis(dimethylsily1)ethane and 1,2-bis(dimethylvinylsilyl)ethane. In this reaction polysilylethylene of molecular weight M , as high as 24 800 was obtained. It is worth adding that similar results were published previously by Boileau and co-workers [ 131.

Synthesis of Silylene-alkenylene-arylene Polymers

Condensation polymers containing aromatic units, synthesized by means of hydrosilylation, are also good candidates for membrane materials of high permeability. They can be synthesized in four different ways which generally consist in two types of reactions - intermolecular hydrosilylation and polyaddition (Schemes 2 and 3).

Me Me

n

My

Me Me

Scheme 2. Synthesis of poly(ethy1ene-phenylene-silylene)

Scheme 3. Synthesis of poly(ethy1ene-phenylene-silylene).

All reactions proceeded in solvents (benzene, toluene, xylene), with or without a crosslinking agent (trivinylmethylsilane), in the presence of Pt-Karstedt catalyst. In order to determine optimum reaction conditions, different molar ratios of substrates and crosslinking agent concentrations were used.

One method of synthesis of poly(ethy1ene-phenylene-sily1ene)s consists in the reaction between

644 H. Maciejewski, P. PawluC, B. Marciniec, I. Kownacki, W Maciejewska

divinyldimethylsilane and 1,4-bis(dimethylsilyl)benzene in the presence of trivinylmethylsilane. Unfortunately, low molecular weight polymers were obtained (M, = 17 800) and the results obtained were not as satisfactory as reported previously [8]. Therefore, the idea of using one monomer containing both an Si-H bond and an SiCH=CH2 group in the same molecule, instead of two substrates, was applied. The replacement of 1,4-bis(dimethylsilyl)benzene and divinyldimethylsilane by 4-(dimethylsily1)- 1 - [ (2- { (dimethylvinylsily1)ethyl ] dimethylsilyl]benzene and intermolecular hydrosilylation of the latter in the presence of trivinylmethylsilane have led to a polymer of molecular weight M, = 78 000. Poly(sily1ene-phenylene-ethy1ene)s were also prepared using hydrosilylation of 1,4-bis(dimetylvinylsilyl)benzene with 1,2-bis(dimethylsilyl)ethane in the presence of trivinylmethylsilane. The highest molecular weight, which was obtained using these two substrates, was M, = 37 000.

When 1,2-bis(dimethylvinylsilyl)ethane and 1,4-bis(dimethylsilyl)benzene were used in the presence of trivinylmethylsilane, the result was not so good because a polymer of lower molecular weight (M, = 9 800) was obtained.

Acknowledgment: Financial support by the NATO (Project No. 972638 “Novel Membrane Materials and Membranes for Separation of Hydrocarbons in Natural and Petroleum Gas”) is gratefully acknowledged.

References Y. P. Yampolskii (Ed.), Polymeric Gas Separation Membranes, CRC Press, Boca Raton, 1993. E. N. Znamenskaya, N. S. Nametkin, N. A. Pritula, V. D. Oppengeim, T. I. Chernysheva, Neftekhimiya 1964,4,487. Y. Pang, S. Iljadi-Maghssodi, T. J. Barton, Macromolecules 1993,26,5671. P. R. Dvornic, V. V. Gerov, Macromolecules 1994,27, 1068. M. Tsumura, T. Iwahara, Polym. J. 1999,31,452. B. Marciniec (Ed.), Comprehensive Handbook on Hydrosilylation, Pergamon Press, Oxford, 1992. B. Marciniec, J. Gulinski, H. Maciejewski, Encyclopedia of Catalysis (Ed. Horvath), 2003, p.

G. K. Rickle, J. Appl. Polym. Sci. 1994,51, 605. M. A. Brook, Silicon in Organic, Organometallic and Polymer Chemistry, J. Wiley, New York, 2000.

107-152.

[lo] J. W. Curry, J. Org. Chem. 1961,26, 1308. [ l l ] J. W. Curry, J. Am. Chem. SOC. 1956, 78, 1686. [12] R. J. P. Corriu, D. Leclercq, P. H. Mutin, J. M. Planeix, A. Vioux, Organometallics 1993, 12,

454. [13] A. Jallouli, L. Lestel, F. Tronc, S . Boileau, Macromol. Symp. 1997, 122, 223.

Innovative Hybrid Coatings for Faqades

U. Posset, K. Rose

Fraunhofer-Institut Silicatforschung (ISC) Neunerplatz 2, D-97082 Wurzburg, Germany

Tel.: +49 931 4100 626 -Fax: +49 931 4100 698 E-mail: klaus.rose@isc.fraunhofer.de

Keywords: hybrid polymers, sol-gel, binders, masonry paints, low emissivity

Summary: This paper describes the synthesis of novel inorganic-organic (hybrid) polymers to be used as binder systems for functional masonry paints. Commercially available polymers are covalently reacted with organofunctional silanes and subsequently crosslinked via the sol-gel process. The resulting dispersions form tack-free coatings without the supply of additional energy. The novel binders show good compatibility with IR-reflective metallic pigments and allow high degrees of filling. The resulting IR optical properties in the range between 8 and 14 pm allow the preparation of IR-reflective paints with very low IR emissivities.

Introduction and Background

By incorporation of infrared (1R)-reflective metallic pigments into masonry paints, the heat emission from buildings can be efficiently reduced [l]. As a result the wall will cool down less at night, which prevents the formation of dew on the surface. Thus, corrosion by humidity is minimized as well as soiling due to humidity-induced growth of microorganisms.

For the development of long-term protective paints for faGades to be used on plaster or masonry with the properties mentioned above, hybrid polymers, ORMOCER@s, were considered as promising new binder systems (ORMOCER@s is a trademark of Fraunhofer-Gesellschaft zur Fordemng der angewandten Forschung e.V., Germany). Via the specific selection of starting compounds, ORMOCER@s can be synthesized by sol-gel processing and their properties tailored within a wide range. They have been optimized for many applications [2, 31. A main advantage of these materials is that, depending on the proportion of inorganic and organic structural elements, the miscibility with pigments, the optical, infrared optical, and chemical properties of the resulting materials can be adjusted. It is obvious that any new binder system for masonry paints has to have key properties like those of the commonly used binders, i.e. self-curing, a water-based nature and high pigment acceptance.

This paper describes the synthesis of the novel hybrid binders and demonstrates their usefulness with respect to incorporation of IR-reflective metallic pigments.

Ovganosilicon Chemisty V Edited by N. Auner and J. Weis

Copyright 0 2003 WILEY-VCH Verlag GmbH & Co. KgaA, Weinheim

646 U. Posset, K. Rose

Synthesis of Hybrid Copolymers as Binders

Typical Ormocer@ materials developed so far are cured thermally or by UV radiation [2, 31. For masonry paints a room-temperature self-curing system is required, that forms dry films on evaporation of the dispersion medium. A prerequisite for a self-curing film-forming paint is a precrosslinked polymer with a high molecular weight. In order to synthesize highly crosslinked inorganic-organic composites, organic polymers were modified by organofunctional alkoxysilanes. Commercially available polymers such as styrene-ally1 alcohol copolymer or vinylbutyral-vinyl alcohol-vinyl acetate copolymer (polyvinyl butyral) bearing free OH groups were found to be suitable to react with 3-isocyanatopropyl triethoxysilane (IPTES) and 3-(triethoxysily1)propyl succinic anhydride (TESSA) (Scheme 1). The modified silylated polymers (Fig. 1) were then co- reacted via the sol-gel process (hydrolysis and polycondensation of alkoxysilanes to polysiloxanes) with hydrophobizing and silicate network forming precursor compounds such as Si(OEt)4, MeSi(OMe)3 or MeZSi(0Me)z. Due to the high molecular weight of these silicate crosslinked, long- chain polymers, the resulting dispersions were forming tack-free coatings within a few hours after application without the supply of additional energy.

J I!

(EtO)3Si-\C/ ' 0 -polymer-OH ( E t 0 ) 3 S i d C / 0 -polymer-OH

a 8 Scheme 1. Alkoxysilane modification of organic polymers. Left: silylation with IPTES; Right: silylation with

TESSA.

The reaction of both the isocyanate and the anhydride functions with the hydroxyl groups of the polymers were followed by R-IR spectroscopy (Fig. 2). During isocyanate addition the band at 2269 cm-' attributed to the asymmetric stretching mode v(N=C=O),, disappeared while a new peak due to an N-C vibration of the urethane group arose at 1529 cm-'. The corresponding urethane carbonyl band could not be detected, as strong solvent (acetone or acetic acid esters) carbonyl bands were present in the spectral region around 1710 cm-'. The addition of the succinic anhydride group involving ring opening was followed by means of the characteristic carbonyl stretching modes at 1783 cm-' and 1859 cm-'. While both bands disappeared during the reaction, a new band at 1732 cm-' appeared, that was assigned to the carbonyl vibration of an ester.

Innovative Hybrid Coatings for Fapdes 647

0

( i i )

Fig. 1. Silylated styrene-ally1 alcohol copolymer (i) and polyvinyl butyral (ii).

50 - - 40-

?? 30-

9 2 0 -

0 .

5 -

10 - 0 -

I 1 I . , '

2400 2200 2000 1800 1600 1400 2000 I900 I800 1700 Wavenumbers [cm '1 Wavenumbers [cm-'1

Fig. 2. FT-IR transmittance spectra obtained during the silylation of styrene-ally1 alcohol copolymer with I R E S

(left spectrum) and TESSA (right spectrum). The dotted and solid lines demonstrate the situations before and

after the reaction was completed, respectively.

Infrared Optical Properties of Hybrid Binders

Metallic pigments reflect infrared radiation and hence are able to reduce the heat emission from a surface coated therewith, according to Kirchhoff's law 1-R(h) = &(A), where R = reflectance and E = emissivity. Hybrid polymers are superior to commercially available binders, considering both miscibility with IR-reflective pigments and the achievable degree of filling. However, in order not to affect the optical function of the pigments, the infrared absorption of the binder should also be as

648 U. Posset, K. Rose

low as possible between 8 and 14 pm, the spectral range where masonry emits to outer space most of its heat upon cooling [4]. Figure 3 shows the IR absorbance spectra (as measured by means of an ATR accessory) of three typical hybrid binder compositions consisting of silylated polymer, tetraethoxysilane and dimethyldimethoxysilane. The corresponding relative amounts of the constituents are given in Table 1. As can be deduced from the spectra, the intensity of the IR absorptions in the region of interest mainly depends on the relative content of silicate-forming constituents. Thus, the chosen hybrid polymer concept allows the tuning of IR optical properties between those of the pure organic polymers and those of silicone resins.

1 " ' " " " " " '

4 6 8 10 12 14 16

Wavelength [pm]

Fig. 3. ATR spectra of the hybrid binder systems (for the compositions of a-c see Table 1).

Table 1. Relative composition of different hybrid binder systems.

Composition [w/w%] Binder system

Polymer (type [bl) Si(OEt)., MezSi(OMe)z

27 (ii) 0 56

21 (i) 15 40

18 (i) 53 9

[a] As in Fig. 3. [b] As in Fig. 1.

Pigmented coatings with emissivities as high as 0.45 (45 %) were prepared [ 5 ] . Common masonry without IR-selective coatings typically show emissivities above 0.9. Applied on faGades the new IR functional coatings promise to have a considerable energy- and cost-saving potential. Simulations performed recently by means of the building simulation program ESP-r have demonstrated that the use of an IR-reflective paint with an emissivity value of 0.5 results in a 6 % saving of energy on walls with low heat insulation (in a normal climate). Moreover, dew point

Innovative Hybrid Coatings for FaFades 649

occurrences will be reduced by 45 % in a normal climate and up to 80 % in a dry climate (for masonry with k = 0.42 and absorption factor 0.3) [5]. Thus, damage by frost, mold formation, humidity-induced corrosion, and growth of microorganisms are efficiently minimized. Additionally, hydrophobic and/or oleophobic components can optionally be incorporated in order to produce a significant anti-soiling effect [6].

References [ 11 [2 ] [3] [4] [5] [6]

K. Gertis, H. Erhorn, Haustechnik-Bauphysik-Umwelttechnik-gi 1982,103, 20. G. Schottner, Chem. Mater. 2001,13, 3422. K.-H. Haas, Adv. Eng. Mater. 2000,2,571. G. Finger, Schweiz. Ing. Architek. 1979,17, 287. K. Rose, U. Posset, K.-H. Haas, M. Kohl, Farbe Luck 2002,108,29. K. Rose, K.-H. Haas, European Coatings Conference: Anti-GrafJiti Coatings, Tagungsband, Berlin, 1999.

Adhesion of Silicone Coatings to Plastic Films

Lesley-Ann O’Hare, Stuart R. Leadley

Dow Corning Plasma Solutions, Unit 12 Owenacurra Business Park, Midleton, Co. Cork, Republic of Ireland

Tel.: +353 21 4621526 -Fax: +353 21 4631960

Bhukan Parbhoo, John G. Francis

Dow Coming Ltd., Cardiff Road, Barry, Vale of Glamorgan, CF63 2YL, UK Tel.: +44 1446 723812 -Fax: +44 1446 730495

Keywords: adhesion, silicone, corona, surface, contact angle, XPS, AFM, PP, PET

Summary: This work aimed to elucidate the mechanisms of adhesion of a silicone coating to corona discharge treated polypropylene (PP) and poly(ethy1ene terephthalate) (PET) films. A physico-chemical study of the PP and PET surface utilizing contact angle measurements, X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM) was carried out. Surface energies investigated the role of physisorption, whilst chemisorption was examined by the surface chemistry changes. The presence of mechanical interlocking was studied by observing changes in the morphology of the films, and a combination of these techniques on washed samples made possible the scrutinizing of a weak boundary layer. These results, in addition to practical adhesion measurements, have enabled identification of the dominant mechanisms of adhesion of silicones to PP and PET films. Physisorption and mechanical interlocking are not dominant causes of enhanced adhesion after corona discharge treatment in this system. It is believed that the principal mechanisms of adhesion of silicone to corona treated PP and PET are chemisorption and weak boundary layer formation.

Introduction

The aim of this work was to elucidate the mechanisms of adhesion of a silicone coating to corona discharge treated polypropylene (PP) and poly(ethy1ene terephthalate) (polyester, or PET) films. A physico-chemical study of the PP and PET surfaces utilizing contact angle measurements, X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM) was carried out. These techniques were used to probe changes in the surface energy, surface chemistry and surface morphology, and the results were related to practical adhesion data. Surface energies elucidated the

Ovganosilicon Chemisty V Edited by N. Auner and J. Weis

Copyright 0 2003 WILEY-VCH Verlag GmbH & Co. KgaA, Weinheim

Adhesion of Silicone Coatings to Plastic Films 651

role of physisorption, whilst chemisorption was examined by the surface chemistry changes. The presence of mechanical interlocking was studied through changes in the morphology, and a combination of these techniques on washed samples made possible the scrutinizing of a weak boundary layer.

Experimental

Materials

The PP film used in this study was 30 pm thick, biaxially oriented polypropylene. The film was previously untreated, with no surface contamination observed by XPS. The PET films studied were both biaxially oriented, with one 50 pm thick, known commercially as Melinex S (DuPont Films), and the other of thickness 23 pm (Goodfellow). Again, no surface contamination was observed prior to treatment.

The contact liquids used were water (HPLC grade; Fisher); formamide (99.5+ %; Acros); diiodomethane (> 98 %; Fluka); ethane-l,2-diol(> 99.9 %; Acros).

The silicone coating system used reacted via addition cure chemistry; comprised a vinyl- and a hydride-functional silicone polymer.

Equipment

Corona discharge treatment (CDT) was carried out on a GXlO corona treater, manufactured by Sherman Treaters, Thame, UK. The films were exposed to one pass under an electrode of width 0.4 m, in ambient air at a speed of 10 d m i n . The various power settings used were converted to energy values.

Contact angles of sessile drops were measured using an Advanced Surface Technology video contact angle VCA2500 system. Contact angles were measured on the modified side of the plastic films within 20 min of treatment. The method of calculation used in this study was the geometric mean approach of Owens-Wendt [ 11 and Kaeble [2].

XPS was performed using a Kratos Analytical Axis Ultra instrument. A monochromated A1 Ka X-ray source was used at a nominal power of 300 W to record spectra at normal emission. All the samples under consideration required charge compensation.

AFM was carried out on a Digital Instruments Dimension 3100, in the TappingMode@ using a silicon tip. Both height and phase images of scanned areas 1 pm2, 25 pm2 and 400 pm2 were captured.

Results and discussion

Physical Adsorption Theory

Surface energy (y) measurements interrogate the polar (f) and dispersive (4) components of a

652 L.-A. O’Hare, S. R. Leadley, B. Parbhoo, J. G. Francis

I

P +Polar Contribution [mJ m-*] $ t D i e p e n i v e Contribution [ml m?]

+Total Surface Energy [mJ m-’1

m 10 -

substrate. The role of the physical adsorption theory in adhesion of silicone to PP and PET films can thus be investigated. Figure 1 shows the values obtained for y, f and 4 plotted as a function of the energy delivered to the surface of the polypropylene film by corona discharge. The values of y, f and 4 calculated for the PET film are presented in Fig. 2.

1)

0

- 1 I

0 5 10 15

Corona Energy [kJ m-’1

Fig. 1. Surface energy (y) and its polar (Ip) and dispersive (4) components of corona discharge treated

polypropylene film as a function of corona energy.

+Surface Energy [mJ m?] +Polar Contribution [mJ m-’] +Dieyrsive Contribution [I@ m”] ,

30 P

W

I 0 2 4 6 8 1 0 1 2 1 4

CDT Energy [kJ m-’]

Fig. 2. Surface energy (y) and its polar (Ip) and dispersive (4) components of corona discharge treated polyester film

as a function of corona energy.

Before CDT, the surface energy of the polypropylene film is composed solely of a dispersion component. As expected, y and increase with increasing energy of CDT. However, the shape of the curve for f is not the same as that of y. This indicates that at certain CDT energy levels (2.25 to 4.05 kJ m-*), the surface energy does not correlate with the increase in polarity alone. For PET, y

Adhesion of Silicone Coatings to Plastic Films 653

increased regularly with increasing CDT energy; f , however, again deviated from the regular trend in the region of 3.9-5.7kTm-’. For both films, the values of y, even for the untreated film, demonstrated that a silicone coating would wet the surface of the plastic with no requirement for additional treatment. This indicates that since the physisorption mechanism is always fulfilled, enhanced adhesion after CDT is caused by another mechanism.

Chemical Adsorption Theory

The chemisorption theory was probed by monitoring the changes in the surface chemistry of the PP and PET due to CDT. The introduction to the surface of the plastic films of any functional groups that could interact with the silicone coating would imply that chemical adsorption was an important mechanism of adhesion.

a b

Fig. 3. High resolution spectra of polypropylene film: a) untreated film; b) treated film, corona energy = 15 M m-’.

’ *i I

a b

High resolution spectra of polyester film: a) untreated film, b) treated film, corona energy = 15 M m-’. Fig. 4.

654 L.-A. O’Hure, S. R. Leudley, B. Purbhoo, J. G. Francis

Survey spectra acquired from the corona treated films showed carbon and oxygen to be the only elements present at the surface. The amount of oxygen incorporated typically increased with increasing corona energy. By fitting peaks to the high-resolution C 1s spectra, it was also possible to identify the specific type of functional groups introduced at the surface by CDT. Corona treatment introduces a shoulder on the high binding energy side of the C 1s spectrum of the PP film (Fig. 3). In agreement with studies on PP published elsewhere [3-51, this work has assigned the functional groups introduced by CDT as hydroxyl, carbonyl, peroxy, ester, carboxylic acid and anhydride groups. The changes in the C 1s spectrum for PET after CDT were more subtle, but it was still clear that at least four additional peaks were required to ensure a good fit (Fig. 4). The functionalities introduced by CDT to PET were assigned as CHZOH, phenolic OH, peroxy, carboxylic acids and anhydrides, in agreement with much work found in the literature [6-81.

3

2-5

2

I ,5

1

O S 0

-+- c-on i coo

-8- c=o + c-oc*=o + COOH

o=coc=o

0 5 10 15

Corana Energy [kJ m-2]

Fig. 5. Relative concentration of functional groups introduced onto the surface of polypropylene film by corona

discharge treatment.

0 5 10 15

Corana Energy [kJ m?]

Fig. 6. Relative concentration of functional groups introduced onto the surface of polyester film by corona discharge

treatment.

It was observed that the number of peaks that could be fitted, and their relative areas, varied

Adhesion of Silicone Coatings to Plastic Films 655

depending on the energy of corona discharge. For the PP film, it was not possible to fit all the additional peaks at lower energy levels. It is proposed, therefore, that the type of functional groups introduced to the surface of the PP and PET films, and their relative concentrations, are dependent on the energy delivered to the surface by CDT. The changes in the relative concentrations of the species introduced to PP and PET are presented as a function of the energy of the corona in Figs. 5 and 6, respectively.

The chemisorption mechanism of enhanced adhesion must be of importance; the nature of such functional groups, as discussed previously, may affect the performance of any particular adhesive system. The highly reactive Si-H species in the adhesive can react with functional groups introduced to the surface of the plastic film by CDT.

Mechanical Interlocking

Increased practical adhesion after CDT has often been attributed to mechanical interlocking due to increased roughness. AFM was used to monitor any morphological changes induced by CDT, and then to relate these to any variation in the adhesive properties of the plastic films.

The biaxial orientation of the untreated PP film, and the defined fibrillar structure, as observed by Boyd et al [9] were apparent in the 1 km2 image, Fig. 7a. After CDT, however, a different morphology emerged; globular features of 50-100 nm diameter were observed (Fig. 7b). At energy levels between 0 and 5.7 kJ m-’, both the fibrillar and globular morphologies could be observed, due to the heterogeneity of the treatment at low energy levels. Figure 7c shows a different morphology. In this image, a branchlike structure is observed. It may be surmised that at this high energy level the treatment is exposing the underlying polymer structure. The low molecular weight boundary layer common to polyolefins, together perhaps with parts of the amorphous regions, has been ablated under these conditions.

a b C

Fig. 7. AFM images of polypropylene film: a) 1 pm x 1 p m untreated; b) 1 pm x 1 pm treated film, corona energy =

5.70 kJm-*; c) 5 pm x 5 pm treated film, corona energy =15.0 k.Tm-*.

A similar observation was made for the PET film; globular features were found on the surface at

656 L.-A. O’Hare, S. R. Leadley, B. Parbhoo, J. G. Francis

all energy levels of CDT. With increasing energy, these globules decreased in size, and were more uniformly distributed over the surface of the film (Fig. 8).

In addition to observing the changes in morphology, the root mean square roughness (RRMS) was calculated as shown in Table 1.

a b C

Fig. 8. AFM images of polyester film: a) 1 pm x 1 pm untreated; b) 1 pm x 1 Fm treated film, corona energy =

5.70 Urn-’; c) 1 Fm x 1 pm treated film, corona energy = 15.0 kJm-*.

Table 1. Roughness values of polypropylene and polyester film with corona discharge treatment.

Polypropylene Polyester

Corona energy [ k ~ m-’] 0 3.15 5.70 15.0 0 3.15 5.70 15.0

RRMS [nml 2.4 2.2 2.6 4.2 1.0 1.3 1.0 1.6

To try to avoid ambiguities regarding the fractal nature of the surface, and any changes in roughness that may incur, all roughness values were calculated on a 1 pm2 area. It is clear that under these conditions no significant increase in roughness occurs. Previous studies that reported an increase in roughness by CDT utilized longer treatment times and higher powers. This suggests that, in the case of silicone adhering to plastic films, mechanical interlocking is not a dominant mechanism of adhesion under these conditions.

Weak Boundary Layer

The formation of a layer of low molecular weight, oxidized material (LMWOM) on the surface of PP and PET films after CDT is widely accepted [lo]. It is the effect of the material on adhesion that is debated. The formation of a weak boundary layer may be beneficial or detrimental to adhesion, depending on its solubility in the adhesive matrix. If the layer is soluble in the matrix, no decrease in adhesion may be observed. If, however, the layer is not soluble, this may reduce adhesion due to the presence of this cohesively weak layer at the interface. Water washing experiments were carried out to evaluate the effect of LMWOM on the surface chemistry and morphology. XPS and AFM

Adhesion of Silicone Coatings to Plastic Films 657

analyses were carried out under the same conditions as for the unwashed samples. XPS identified that up to 50 % of the oxidized material on the surface is in the form of water-soluble LWMOM. Since after washing the atomic oxygen content does not decrease to that of the untreated film, this indicates that some of the oxygen incorporated is firmly bound to the polymer backbone.

AFM also reinforces the theory that not all the oxidized material is removed by washing, since although the globular morphology observed is removed, the structure does not return to that of the untreated film. The amount of LMWOM removed by water washing varies with the energy imparted to the surface of the film. The greatest change is observed on the film treated at the highest corona energy. It is possible that in the presence of this material, a cohesively weak layer may be formed at the interface that is insoluble in the silicone adhesive matrix. Thus formation of weak boundary layer is an important mechanism in the adhesion of silicone to CDT plastic films.

Practical Adhesion Measurements

The anchorage of the silicone coating to the film has been evaluated using an anchorage index test. The test was carried out on films treated at all CDT energy levels, with siliconizing of films being carried out both with and without water washing. The results clearly showed that removal of LMWOM from PP film by water washing before siliconizing is beneficial to adhesion over a one month time period, particularly at the highest treatment energy (Fig. 9).

0 5 10 15

Corana Energy [kl m-’1

Fig. 9. Anchorage of silicone coating on corona discharge treated polypropylene film siliconized before and after

10 s water wash.

Conclusions

The effect of corona discharge treatment on the surface physico-chemistry of PP and PET films has been investigated using surface energy measurements from contact angles, XPS and AFM. The information gathered from these techniques, in addition to practical adhesion measurements, has enabled identification of the dominant mechanisms of adhesion of silicones to these plastic films. The physisorption mechanism and the mechanism of mechanical interlocking are not the cause of

658 L.-A. O’Hare, S. R. Leadley, B. Parbhoo, J. G. Francis

enhanced adhesion after corona discharge treatment in this system. It is believed that the dominant mechanisms of adhesion of silicone to corona treated plastic films are chemisorption and weak boundary layer formation.

Acknowledgments: The authors thank Andy J. Goodwin (Dow Coming Plasma Solutions) and Steve Cray (Dow Coming Ltd) for encouragement, support and valuable discussions over the course of this work.

References [l] Owens, D. K.; Wendt, R. C. J. Appl. Polym. Sci. 1969,13, 1741. [2] Kaeble, D. H. J. Adhesion 1970,2,66. [3] Foersch, R.; McIntyre, N.S. J. Polym. Sci.: Part A: Polym. Chem. 1990,28, 193. [4] Comyn, J. Adhesion Science, Paperbacks, Royal Society of Chemistry, Cambridge, 1997, Ch.

1. [5] Mayoux, C.; Garcia, G.; Sarlaboux, J. ZEEE Trans, Elec. Insulation 1982,17(2), 156. [6] Briggs, D.; Rance, D. G.; Kendall, C. R.; Blythe, A. R. Polymer 1980,21,895. [7] Pochan, J. M.; Gerenser, L. J.; Elman, J. F. Polymer 1986,27, 1058. [8] Leadley, S. R.; Watts, J. F. J. Adhesion 1997,60, 175. [9] Boyd, R. D.; Kenright, A. M.; Badyal, J. P. S . Macromolecules 1997,30,5429. [lo] Strobel, M.; Lyons, C. S.; Strobel, J. M.; Kapaun, R. S . J. Adhesion Sci. Techno1 1992, 6(4),

429.

Thermoplastic Silicone Elastomers

Andreas Bauer,* 0. Schafer, J. Weis

Consortium fur Elektrochemische Industrie GmbH Zielstattstrasse 20, D-8 1371 Munich, Germany Tel.: +49 89 74844 0 - Fax: +49 89 74844 350

E-mail: andreas.bauer@ wacker.com

Keywords: silazane, aminosilane, silicone, thermoplastic, urea, urethane, isocyanate, rubber. elastomer

Summary: Various amino fluids were prepared by utilization of a new cyclic disilazane which offers an easy and cost-efficient way to introduce aminopropyl functionality into various systems containing SiOH groups. The reaction of the disilazane is complete in minutes even in the absence of a catalyst and proceeds without release of unwanted byproducts. These amino-functional fluids were reacted with diisocyanates to obtain siloxane-urea copolymers. These materials are solid “two-phase” systems and they exhibit hard block segments embedded in a siloxane matrix. These segments allow the modified rubber to be melted reversibly at elevated temperatures and are thus to be regarded as nonpermanent crosslinking sites. By carefully choosing the appropriate organic segment, it is possible to get silicone rubbers with melting or softening points from 50 “C up to 170 “C. By modifying the amount of organic segments it is also possible to get very soft or very hard materials. The materials display tensile strengths up to 12 MPa and do not need additional fillers.

Introduction

One of the most important commercial applications of silicones today is their use as silicone rubber. PDMS rubbers require extremely high molecular weights to develop useful properties. Additionally they have to be chemically crosslinked by heating with peroxides or Pt compounds, for example. Unfortunately this crosslinking reaction is not reversible and the network cannot be recycled in a useful way.

Thermoplastic elastomers such as polyurethane elastomers have an entirely different structure. They consist of two “phases”, an “elastomeric matrix” with embedded “hard-block” segments, which act as physical crosslinking sites. It is possible to soften the hard blocks at elevated temperatures to obtain a “single-phase melt”, which can be processed easily. Upon cooling the two- phase nature is restored and the material solidifies again. To avoid the additional crosslinking step in producing silicone rubbers we modified the PDMS backbone (soft block) with certain organic

Ovganosilicon Chemisty V Edited by N. Auner and J. Weis

Copyright 0 2003 WILEY-VCH Verlag GmbH & Co. KgaA, Weinheim

660 Andreas Bauer, 0. Schafer, J. Weis

segments, which tend to crystallize at room temperature or at least tend to separate from the siloxane matrix. These hard-block segments are able to melt reversibly at elevated temperatures and act as nonpermanent crosslinking sites in the modified rubbers. For the formation of such a heterogenic silicone block copolymer we used very simple building blocks. By choosing aminopropyl- or hydroxypropyl-terminated silicones/oligomers we were able to obtain poly(urea/urethane) siloxane block copolymers via a simple polyaddition reaction with diisocyanates, e.g. functionalized silicone fluids and diisocyanates [ 11. These poly(urea/urethane) siloxane block copolymers are based on commercially available chemicals such as diisocyanates or polydimethylsiloxanes (commodities). However the key step of this type of isocyanate chemistry is the generation of the so called linker, an aminopropyl end group.

Although, a,waminopropyl-functionalized polydimethylsiloxanes (PDMS) are industrially produced materials, their manufacturing costs are still prohibitively high for most potential applications, e.g. in copolymers, textiles and cosmetics. Manufacturing processes currently in use for those compounds are multistep syntheses involving costly precursors, particularly allylamine- and hydrogen-terminated siloxanes.

Synthesis and Characterization

The new compound 3-[(2,2-dimethyl-1,2-azasilolidin-l-yl)dimethylsilyl]-l-propanamine 1 is readily prepared from (3-chloropropyl)dimethylchlorosilane or bis(3-chloropropy1)tetramethyl- disilazane, respectively. The educts are reacted with excess ammonia (molar ratios 1:20 to 1:150) at pressures ranging from 30 to 200 bar and temperatures of 60 to 200 "C (Scheme 1) [2].

Scheme. 1. Synthesis of 1.

Thermoplastic Silicone Elastomers 661

The cyclic structure was derived by NMR spectroscopy and confirmed by GC-MS and NMR analysis (Fig. 1) of derivatives. The proton resonances (C6D6, RT) of the propylene unit in the five-membered ring show a characteristic pseudo first-order splitting (triplet, quintet, triplet), while the propylene group in the side chain exhibits the common peak pattern of 3-substituted propylsilanes.

a

Fig. 1. 'H NMR spectrum of 1 (C6D6, 500 MHz, RT).

Upon distillation of the cyclic disilazane an unexpectedly low boiling point was found. The distillate collected was very viscous and had a higher temperature than it should have. These observations led to the assumption that the compound is monomeric in the gas phase and that the dimerization is exothermic. Ab-initio calculations (BP8616-3 1G*, Gaussian 98, Revision A.7, Gaussian, Inc., Pittsburgh PA, 1998) of the reaction (Eq. 1) support this assumption.

Eq. 1. Dimerization of two monomers yielding 1.

H O - f j ~ ~ ~ i i O H CH3

n+l

Eq. 2. Functionalization of a SiOH substrate with 1.

662 Andreas Bauer, 0. Schajer, J. Weis

The reaction of the cyclic silazane with almost any type of SiOH group (Eq. 2) is complete within minutes, even at room temperature, due to the high reactivity of the cyclic disilazane. The products obtained are 100 % functionalized. Furthermore, no catalyst is needed and no unwanted byproducts are released. So these precursors are the best choice to prepare silicone-urea block copolymers.

Synthesis and Characterization of Silicone-Urea Block Copolymers

The reaction between aminopropyl-terminated fluids and diisocyanates (Eq. 3), on the other hand, proceeds smoothly either in solution or in the bulk within a few minutes. We were able to obtain molecular weights up to 150000 Da. For economic reasons we favor the synthesis of the block copolymers in the melt. By using a twin-screw extruder we are able to produce the material simply by feeding the starting materials with separate pumps into the first zone of the extruder [3].

Eq. 3. Reaction of aminopropyl-terminated fluid with an isocyanate group.

Fig. 2. Various glass transition temperatures with different diisocyanates.

The material is mixed within the extruder at elevated temperatures and leaves the machine as a string, which is then cooled to room temperature. The extruded string is subsequently chopped into granules. These granules can be mixed with pigments or fillers in conventional extruders or injection-molding machines to obtain products based essentially on thermoplastic silicone elastomers. By carefully choosing the appropriate organic segment which is defined by the

Thermoplastic Silicone Elastomers 663

corresponding diisocyanate, it is possible to get silicone rubbers with melting or softening points from 50 “C up to 170 “C (Fig. 2).

By variation of the proportion of organic segments the mechanical properties can be tuned between very soft and very hard (Fig. 3). Those materials display tensile strengths up to 12 MPa and do not need additional fillers. The mechanical properties can also be modified by introducing additional “chain extenders” or mineral fillers.

Fig. 3. Influence of molecular weight on mechanical properties.

References [ 11 [2] [3]

EP 250 248, Leir et al., Minnesota Mining & Manufacturing, 1986. DE 10 049 183, Bauer et al., Consortium fur Elektrochem. Znd., 2002. US 6 355 759, Scherman et al., Minnesota Mining & Manufacturing, 1996.

LC Silicones Improving the Temperature- Dependent Optical Performance of STN Displays

Eckhard Hanelt, Thomas Kammel

Consortium fur Elektrochemische Industrie GmbH, ZielstattstraBe 20, D-81379 Munich, Germany

Tel.: +49 89 74844 23 1 - Fax: +49 89 74844 242 E-mail: eckhard.hanelt@ wacker.com

Masato Kuwabara

Tsukuba Research Laboratory, Sumitomo Chemical Co. Ltd. 6 Kitahara, Tsukuba, Ibaraki 300-32 Japan

Keywords: liquid-crystal polymers, retarder films, liquid-crystal displays

Summary: A new retarder film was developed which shows a reversible retardation change with temperature to reduce the loss of contrast of film-compensated STN-mode LCDs with increasing temperature. The film consists of polycarbonate blended with a liquid-crystalline silicone. The improvement of the LCD’s contrast over a wide temperature range and the stability of the film satisfy the requirements for car equipment and other mobile applications.

Introduction

The performance of STN (supertwisted nematic)-mode LCDs has been improved so far that STN displays can be used for many applications with low and high informational content [l]. In particular, in price-sensitive applications they have proven to be competitive with the progressive TFT (Thin Film Transistor) technology. However, the usage of reasonable film-compensated STN displays has almost been restricted to environments with rather stable surrounding temperatures, which still is an obstacle for their widespread use, e.g. for car equipment or mobile applications.

In order to obtain images with a high contrast and to reduce the viewing-angle dependence, all STN displays need at least one retarder film which compensates the optical retardation of the switchable cell filled with a low-molecular-weight liquid-crystalline mixture. The conventional polymer retardation films used in these displays have a nearly constant optical retardation as a function of temperature. Its retardation value is usually adapted to the retardation of the STN cell at room temperature. However, the cell’s retardation decreases with increasing temperature due to the thermal motion of the LC molecules in the cell. This leads to a mismatch between the retarder film

Ovganosilicon Chemisty V Edited by N. Auner and J. Weis

Copyright 0 2003 WILEY-VCH Verlag GmbH & Co. KgaA, Weinheim

LC Silicones Improving STN Displays 665

and the cell which results in a loss of the display's contrast at higher temperature. A known technical solution to obtain retardation compensation over a wider temperature range is

to replace the retarder film by a second STN cell filled with LC molecules of low molecular weight showing a temperature dependence of the optical retardation similar to the switchable cell [2]. However, these DSTN-mode displays are heavier and more expensive. These disadvantages are removed by a new retarder film which shows a reversible retardation change with temperature adapted to the given cell's retardation change.

The film is made from a mixture of a high-T, polymer such as polycarbonate (PC) or poly(viny1 alcohol) (PVA) with a nematic liquid-crystalline silicone. After casting a solution of both components in an organic solvent and subsequent drying of the film, the polymer chains and the mesogenic groups of the LC silicone are oriented by stretching the film at the glass transition temperature. The optical anisotropy induced in this way has two contributions. The polycarbonate's contribution to the total optical retardation is independent of the temperature up to about 130 "C, whereas the liquid-crystalline component's contribution decreases with increasing temperature and finally stays zero above its clearing point.

This new type of temperature-compensating retarder film (TCR) is now commercially available. Its first applications have already shown that the temperature-dependent performance of displays for mobile applications can be considerably improved with only low additional costs. In this paper we report some results on the properties of these TCR films.

Sample Preparation

Different types of liquid-crystalline side-chain polymers based on siloxane backbones were synthesized by hydrosilylation reactions as described in Refs. [3] and [4]. The resulting nematic LC silicones have a broad chain length distribution. The length of the backbones are controlled by GC, 'H NMR and 29Si NMR. An example of an LC silicone used for the TCR films is shown in Fig. 1. Its degree of polymerization is about 14 and the phase transition temperatures measured by a differential scanning calorimeter are a glass transition temperature T, of 18 "C and an isotropic transition temperature Tc of 68 "C.

Fig. 1. Chemical structure of a nematic LC silicone used for the production of TCR films.

666 E, Hanelt, T. Kammel, M. Kuwabara

A polycarbonate was mixed with 2 to 15 wt% LC silicone and dissolved in methylene chloride. The solutions were cast and dried to produce films. After that, the films were stretched at about 180 "C and subsequently cooled down to room temperature. The resulting films have a thickness of about 100 pm and a low haze of less than 3 %. Their optical retardation as a function of temperature was measured by the Senarmont method using a laser beam and a hot stage. Typical retardation values of the TCR films produced with the LC silicone of Fig. 1 are 400 nm to 600 nm at 30 "C and 200 nm to 400 nm at 80 "C.

Optical Retardation Properties

The measured data of the temperature dependence of a TCR film's optical retardation are shown in Fig. 2. The same film is compared to a conventional stretched polycarbonate (PC) film in a wider temperature range in Fig. 3. The contributions of the LC silicone and the PC to the total retardation can be distinguished clearly. The reversible retardation change in the temperature range up to 100 "C corresponds to the temperature dependence of the order parameter of the liquid crystalline phase.

The second decrease starting above 120 "C is irreversible and finally leads to destruction of the film above 180 "C. This decrease, which is observed in both films, is caused by the relaxation of the oriented PC chains at the PC glass transition around 150 "C. The constant retardation in the temperature range from 100 "C to 120 "C is due to only the stretched PC, because the LC material is in the isotropic phase whereas the PC is in the glassy state, still keeping its retardation induced by the stretching process.

20 40 60 80 100 120 Temperature ["C]

Fig. 2. Temperature dependence of the optical retardation of a TCR film. The contributions of the polycarbonate

(dashed line) and the LC silicone are indicated schematically. Measured data are marked by diamonds.

LC Silicones Improving STN Displays 667

+ TCR

-A- PC

0 1 I I I

0 50 100 150 200 Temperature ["C]

Fig. 3. Temperature dependence of the optical retardation of an TCR film and a pure polycarbonate retarder film.

The retardation is normalized against its value at 30 "C.

The temperature dependence of the TCR film's retardation can be adapted to the given retardation curve of any STN cell by adjusting the film thickness, the content of LC silicone and the isotropic transition temperature T, of the LC. The influence of the LC content is demonstrated in Fig. 4. The slope of retardation vs. temperature curve increases with increasing LC content. However, there is an upper limit of the LC content given by the requirement of a high mechanical stability of the film. Another way to control the slope or the curvature of the retardation change is by adjusting the T, of the LC silicone. Some examples covering the range of interest for most applications between room temperature and 80 "C are given in Fig. 5. Finally, the film thickness is used to control the magnitude of the optical retardation.

100 - 8 Y

c 8o 0 m m

.- c E 60

?? -0 40 w -A- low LC content

20

I

.- - + high LC content m

g z o ! , I I I I

0 20 40 60 80 100 120 140 Temperature ["C]

Comparison of the retardation of TCR films with low and high content of the LC silicone of Fig. 1. The

measured retardation curves were normalized against their values at 30 "C.

Fig. 4.

668 E. Hurzelt, T. Kunzmel, M. Kuwuhum

--t 3°C 168°C

+ 4°C 182°C

Z t- 7°C 192°C

0 1

0 20 40 60 80 100 120 140 Temperature ["C]

Fig. 5. Comparison of TCK film containing LC silicones with different isotropic transition temperatures Tc. The

measured retardation curves were normalized against the retardation at 20 "C

The performance of a TCR film in an STN display is demonstrated in Fig. 6. The TCR film was mounted on the left-hand side o f the screen and a conventional PC retarder film with corresponding retardation at room temperature on its right-hand side. Both parts of the screen show a good contrast at room temperature. At 80 " C , the readability is still sufficient on the left-hand side but it diminishes on the right-hand side of the display.

Fig. 6. An STN display containing a TCK film on the left-hand side of the screen and a conventional PC retarder

fi lm on its right hand side ;II 25 "C and XO "C (photograph by courtesy of Optrex Co., Tokyo, Japan).

LC Silicones Improving STN Displays 669

Film Durability

The film is mechanically stable up to temperatures of about 130 "C. To test the durability of its optical properties a sample was kept in an oven at 100 "C. The film was taken out of the oven and the retardations at 30 "C and 80 "C were remeasured after the time intervals shown in Fig. 7. The change of retardation was negligible during 1 000 hours of heat treatment at 100 "C.

500

- 400 L E -A- R(30"C)

--t R(80"C) 300

5 e 200

100

0

0 .-

([I a [r

-

0 200 400 600 800 1000 Storage time [hours]

Fig. 7. Stability of the optical retardation of a TCR film after storage at 100 "C. The diamonds and triangles mark

the retardation values measured at 80 "C and 30 "C, respectively.

The UV stability of the TCR film was tested in an environment similar to an LCD. The TCR was laminated with a polarizer and exposed to a high luminous intensity in a sunshine tester. It showed sufficient stability after 265 hours of irradiation with a total light dose of 67 kWh/mz, which corresponds, for example, to 30 days of sunshine in Tokyo at August.

Conclusion

The concept of using a second layer containing LC molecules as in a DSTN display to compensate for the temperature-dependent retardation of STN cells is now applicable also to film-compensated displays. The TCR films can be produced to improve the performance of any STN display in a wide range from room temperature up to 80 "C. The durability of the TCR films is comparable to that of conventional PC retarder films. The advantage of applying the well established and reliable production process for retarder films from polycarbonate by adding a low content of LC silicone is reasonable from the economic point of view also. Thus, the field of application of STN displays, especially STN for car equipment and mobile applications, is expected to be considerably expanded by using these films.

670 E. Hanelt, T. Kammel, M. Kuwabara

References [l]

[2]

[3]

[4]

T. J. Scheffer, J. Nehring, in: SID Seminar Lecture Notes San Jose 1999, Society for Informational Displays, Santa Ana, CA, USA, 1999, Seminar M-6. K. Katoh, Y. Endo, M. Akatsuka, M. Ohgawara, K. Sawada, Jpn. J. Appl. Phys. 1987, 26,

H. Finkelmann, G. Rehage, in: Advances in Polymer Science 60/61 (Ed.: M.Gordon), Springer-Verlag, Berlin, 1984, p. 99-172. H. Finkelmann, G. Rehage, Makromol. Chem., Rapid Commun. 1980,1,31-34.

L 1784-L 1786.

Self- Adhesive Liquid Silicone Rubbers (LSRs) for the Injection

Molding of Rigid Flexible Combinations

Stephan BoJhammer

GE Bayer Silicones GmbH & Co. KG, Gebaude R 20, D-51368 Leverkusen, Germany Tel.: +49 214 3025143 -Fax: +49 214 3028474

E-mail: Stephan.Bosshammer@ gesm.ge.com

Keywords: liquid silicone rubber, injection molding, adhesion, rigid flexible combination

Summary: Liquid Silicone Rubbers (LSRs) exhibit outstanding material properties at extreme temperatures and are processed fully automatically by injection molding. Therefore, LSRs offer high productivity for the manufacturer. Self-adhesive LSRs as presented here are designed in a way that no mold coating is necessary during the injection molding process, and are ideally suitable where high performance combinations of rigid flexible combinations for technical applications are required. The adhesion properties of a broad range of engineering thermoplastics have been improved and a highly reproducible method for testing the adhesion properties of injection-molded rigid flexible parts is presented.

Background

One big area where silicones have found their way into technical products with a wide range of applications is silicone rubbers. The first products were introduced into the market in the 1950s [l]. This was possible due to the unique properties of silicones (also known as polysiloxanes) based on their molecular structure with a silicodoxygen backbone, where the remaining valences of the silicon atom are saturated by hydrocarbon radicals (mainly methyl groups). Both silicones and silicone rubbers exhibit outstanding properties such as:

high-temperature resistance (up to 200 "C), maintenance of elasticity even at low temperatures (down to -50 "C), low temperature dependence of physical properties in general, good aging stability and weather resistance, physiological inertness.

Ovganosilicon Chemisty V Edited by N. Auner and J. Weis

Copyright 0 2003 WILEY-VCH Verlag GmbH & Co. KgaA, Weinheim

672 S. BoJhammer

If necessary, these properties can even be improved by modifying the silicone polymers or by using special additives during compounding [2]. At the end of the 1970s a new kind of silicone rubber was introduced into the market, the so-called Liquid Silicone Rubber (LSR). The LSR silicone polymers have a lower viscosity compared to the raw material of the older High-Temperature Vulcanizing Rubbers (HTVs). In contrast to HTVs, which are normally processed by techniques known from the manufacturing of organic rubbers, a modified injection molding process is used for LSRs, made possible by their lower viscosity as well as by their high reactivity based on a platinum-catalyzed hydrosilylation reaction [ 11. The so-called Liquid Injection Molding (LIh4) process uses a special mixing and metering technology for the two-component LSR system (catalyst and siliconhydrogen-crosslinker have to be stored separated), an injection molding machine and a mold with very low tolerances [3]. The whole manufacturing process is fully automated and, together with very low cycle times due to the high reactivity of LSR, the LIh4 process provides high productivity for the custom molder (Fig. 1).

color/additive batch pump

0°C

dosing apparatus injection molding machine

Fig. 1. Machine concept for the processing of Liquid Silicone Rubber (LSR).

For the manufacture of rigid flexible combinations the use of the two-component injection molding technology, where the injection molding of a thermoplastic part is combined with the injection molding of an elastomeric component, is becoming more and more attractive. The high level of automation provides high productivity, at the same time guaranteeing a high level of quality. Application areas of rigid flexible parts can be found, e.g. where the haptical properties of rigid parts need to be improved, where a seal with special geometry needs to be fixed to a rigid part, or where a flexible connection between rigid parts is required. When properties as mentioned above are required, i.e. especially the application of a flexible part at extreme temperatures [4], the use of LSR instead of organic rubbers or thermoplastic elastomers as the soft component is preferred.

Generally, there are three different technologies for the manufacture of a rigid flexible combination, where an LSR represents the flexible part:

Self-Adhesive Liquid Silicone Rubbers for the Injection Molding 673

0 mechanical combination of the two components by design of the part (reduced flexibility in design and functional integration), adhesion by using an external adhesion promoter on the rigid part (requires an additional manufacturing step and handling of solvents) and adhesion by using an internal adhesion promoter in either the rigid or the flexible component.

This presentation deals with the third possibility. Figure 2 illustrates the combining strategies and the difference between mechanical combination of an LSR to a rigid component and use of a self-adhesive LSR.

Fig. 2. Combining strategies for rigid flexible combinations from thermoplastics and LSR.

Objective

The adhesion properties of a self-adhesive LSR to different engineering thermoplastics should be improved to make it possible to benefit from all the above-mentioned advantages of the two-component injection molding process and the properties of the LSR wherever needed. A new test method using an injection-molded test specimen should be developed to make it possible to investigate a wide range of material combinations and to consider also the influence of injection molding process parameters. Furthermore, to keep the high productivity, the use of mold coating techniques should be avoided because mold coatings have to be renewed from time to time and therefore lead to manufacturing interruptions and additional costs for the custom molder.

674 S. Boj'hammer

Development of a Test Method for Testing the Adhesion Strength of LSR to Rigid Components

Several ways for testing adhesion strength between a rigid and a flexible part are known [5 ] . For screening experiments in the laboratory, standard rubber molds of 6 mm thickness are used. Substrate sheets of thermoplastics or metals (thickness 3 mm), which are supplied as molded sheets, are put into the mold. These are then overmolded with a self-adhesive LSR, with which the rigid sheets are in contact during vulcanization in a laboratory press. Vulcanization is performed at 135 "C for 20 min. After vulcanization a peeling test specimen is prepared by cutting out the substrate with its layer of LSR overmolded. The rubber layer is then separated partly with a knife by hand. This prepared specimen undergoes a peeling test according to DIN 53289.

Although for screening experiments this is an acceptable method, there are several factors which cannot be controlled, but have a significant influence on the adhesion results, as is known from experience. Variation in the results can be caused by the substrate sheet itself (e.g. age, water content, surface), by the preparation of the test specimen (much manual work), and even by interpretation of the diagram resulting from the peeling test. The results have a more qualitative character and care should be taken when interpreting the numbers. Furthermore, the molding conditions in a laboratory press are different from those in an injection molding machine and conclusions for planning a production process based on laboratory adhesion test results alone are not possible.

In order to minimize uncontrollable factors as described above and to get reproducible quantitative results, and also where a potential adhesion to the mold could be detected, a variable mold system for injection molding has been designed, with which it is possible to produce substrate sheets starting from pellets of thermoplastic materials. In a second step the sheets are overmolded with a self-adhesive LSR. A test specimen is delivered which can be used directly for a peeling test. Figure 3 gives a schematic overview of this method. The results of a gage study presented in Table 1 show the high level of repeatability and reproducibility.

Table 1. Gage study of peeling test for adhesion strength with injection-molded parts (substrate: special PA grade).

Adhesion [N/mm]

Operator A Operator B

1 5.38 5.15

2 5.19 5.30

3 5.34 5.25

4 5.43 5.21

5 5.51 5.29

6 5.20 5.25

Mean 5.34 5.24

Stand. dev. 0.12 0.06

SelfAdhesive Liquid Silicone Rubbers for the Injection Molding 675

Fig. 3. Schematic overview of peeling test for adhesion strength.

This method makes it possible to compare quantitatively the adhesion strength results of a wide range of material combinations under real manufacturing conditions. The influence of material formulations and injection molding process parameters can now be investigated with a high level of reliability.

Investigation of Adhesion Results of Different LSIUThermoplastic Combinations

In order to achieve self-adhesive properties with an LSR, the use of functional silanes as additional ingredients together with an excess of siliconhydrogen compounds is well known [6]. This technology leads to good adhesion to thermoplastics, but requires a special mold coating, because these systems also exhibit good adhesion to metals. The 40 Shore A LSR described herein has been modified by functional silanes, but at the same time the formulation has been adjusted so that no mold coating is necessary and consistent manufacture of test specimen as described above is possible.

For our investigation we have chosen four different thermoplastic grades:

a 30 % glassfiber-filled polyamide 6, a 30 % glassfiber-filled polyamide 6.6, a poly(buty1ene terephthalate) and a 40 % glassfiber-filled poly(pheny1ene sulfide).

676 S. Boj’hammer

These grades have been delivered by different suppliers as pellets. Sheets of equal dimensions have been produced using the mold system described above, after drying the pellets according to the recommendations given by the suppliers. Before overmolding the sheets with the self-adhesive 40 Shore A LSR, the sheets had been stored at ambient temperature for several days.

Before the overmolding by injection molding, laboratory trials with the injection-molded thermoplastic sheets were performed. The results are shown in Table 2. Within the range of the combinations investigated, the one with the polyphenylene grade led to the lowest level of adhesion. In this case it was possible to separate rigid and flexible components by hand, whereas this was not possible with all three other combinations.

Table 2. Adhesion strength results with self-adhesive LSR using

a laboratory press (vulcanization 20 min, 135 “C).

Substrate Adhesion [N/mm] with silane-modified 40 Shore A LSR

PA 6.6 GF 30

PA 6 GF 30

PBT

PPS GF 40

4.8

4.2

4.9

1.3

Then injection-molded test specimen using the two polyamide grades were produced and the peeling tests were performed. During injection molding the process parameters heating time and mold temperature were varied in order to investigate their influence on the adhesion strength. The results are shown in Table 3. Demolding of the parts was easy and no adhesion to the mold could be detected at any time. For both polyamide grades the same influence of the process parameters is detected. Lower mold temperature and longer heating time lead to improved adhesion results. This knowledge is useful for adjusting the injection molding process parameters in order to optimize the production process for soft rigid technical parts. It has also been found that the adhesion strength can be improved by post-curing the parts or by storage at ambient temperature for several days.

Table 3. Adhesion strength results after injection molding using different process parameters.

Adhesion [N/mm]

Substrate mold temp [“Clheating time [s] PA 6.6 GF 30 PA 6 GF 30

140/80 2.6 2.5

140/160

165/120

3.8 2.4

3.4 1.6

190/80 2.5 1.3

190/160 2.2 1.6

Self-Adhesive Liquid Silicone Rubbers for the Injection Molding 677

Conclusion

A reliable method for testing the adhesion strength between rigid thermoplastics and flexible LSRs has been developed by designing a variable mold system, which delivers overmolded test specimen for a peeling test starting from thermoplastic pellets and raw LSR. It has been shown that this method leads to highly reproducible results. A 40 Shore A self-adhesive LSR has been designed and tested as the flexible component with different engineering thermoplastics. The influence of the injection molding process parameters heating time and mold temperature on adhesion strength has been investigated. It was found that a lower mold temperature and an extended heating time improved the adhesion strength significantly, while a safe demolding was guaranteed at any time.

References [ 11 [2] [3] [4] [5]

[6]

D. Wrobel, Silicones - Chemistry and Technology, Vulkan Verlag, Essen, 1991. W. Noll, Chemie und Technologie der Silicone, Verlag Chemie, Weinheim, 1968. K. Pohmer, G. Schmidt, H. Steinberger, T. Briindl, T. Schmidt, Kunststoffe 1997,87, 1396. R. Steger, Hartmeich Verbindungen im SpritzgieJverfahren, SKZ-Seminar, Stuttgart, 1999. A. V. Pocius, Adhesion and Adhesives Technology, HansedGardner Publications, Cincinnati, 1997. T. Suzuki, A. Kasuya, J. Adhesion Sci. Technol. 1989,3,463.

Oil-Bleeding Properties of Self-Lubricating Liquid Silicone Rubbers

Klaus Pohmer

Wacker-Chemie GmbH, Burghausen Plant, Elastomers Business Unit, Rubber Fabricators Business Team, Automotive Rubber Market Segment,

D-84480 Burghausen, Germany Tel.: + 49 8677 83 38 50 -Fax: + 49 8677 83 41 42

E-mail: klaus.pohmer@ wacker.com

Keywords: silicone rubber (MVQ), liquid silicone rubber (LR), liquid injection molding (LIM), oil-bleeding, diffusion, methylphenyl silicone fluid

Summary: Liquid silicone rubber accounts for only a thousandth of the worlds total rubber production, and is thus a specialty among rubber types. Nevertheless, LR products boast a large number of applications in many different fields. There is a particularly high demand for LR products in the automotive sector, where they are highly valued because of their low-temperature flexibility and thermal stability. Applications include radiator gaskets, exhaust pipe suspension elements, O-rings, seals and membranes, and insulating coverings for spark plugs. It has become standard practice to use weather packs made of special silicone rubber grades for sealing cable connectors for wiring harnesses. These silicone rubbers contain a silicone fluid which is incompatible with rubber and bleeds or exudes slowly after the product has cured.

Introduction

The term Liquid Silicone Rubber (LR) covers a whole range of platinum-catalyzed heat-curing products. They are readily processed by means of Liquid Injection Molding (LIM), a technique which resembles the injection molding of thermoplastics [ 11. First introduced around 1980, these products now make up roughly 10 % of the silicone rubber market, with annual consumption running to approximately 25 000 t/a [2]. Liquid rubber’s high-tech properties make it indispensable in a large number of fields, the most important of which are:

electronics and electrical engineering, household and sanitary applications,

automotive, railway and airplane industries,

medical engineering and pharmaceutical industries,

Ovganosilicon Chemisty V Edited by N. Auner and J. Weis

Copyright 0 2003 WILEY-VCH Verlag GmbH & Co. KgaA, Weinheim

Oil-Bleeding Properties of Self-Lubricating Liquid Silicone Rubbers 679

food sector, sports and leisure activities.

Composition of LR

Liquid silicones are nearly always offered as two-part 1:l systems, and consist of the following main components [3]:

silicone polymers, silicone crosslinkers , catalysts, inhibitors, fillers, plasticizers, pigments, additives.

Silicones are essentially quartz-like structures in which the three-dimensional SiOz backbone has been modified by incorporation of methyl groups. This progressive saturation leads ultimately to low-molecular weight polymers [4].

The silicone polymers used are generally linear, unbranched polydimethylsiloxanes with vinyl groups (Fig. 1) and a molecular weight between 10 000 und 100 000 glmol, since these are well suited to LIM processing. Polymers with a heterogeneous vinyl group distribution improve tear strength and are therefore also used [5 ] .

CH3 I I 7H3

CHz=CH -S i -0 -Si-0 -Si-CH=CHp I I

CH3 CH3 "CH3

CH3 I

CH3 CH3 CH3

! I

m CH3

R-Si -0 ! [ -f.-30]n [ - i i - O ] -Si-R

CH=CH* CH3

R=CH3, CH=CHP & . 100 = 1 to10

Fig. 1. Chemical structure of polydimethylsiloxanes (LR polymers) [5 ] .

680 K. Pohmer

The polymers are typically tangled, with the methyl groups able to rotate freely about the -Si-0-Si- chain. LR polymers, which have an average of 1 000 siloxy units, are water-white, self- flowing substances with viscosities ranging from 5 to 100 Pa s. Since there is little interaction among the molecules, the polymers behave as Newtonian fluids at low flow rates (D = lo-' S-') .

The reaction between polymers containing vinyl groups and crosslinker molecules carrying Si-H functional groups (Fig. 2) produces three-dimensional elastomeric networks. Platinum catalysts are added to lower the activation energy and speed up curing, while inhibitors ensure that the two-part compound can still be processed after a weekend.

Fig. 2. Chemical structure of the Si-H crosslinker [3].

Since most technical applications require better mechanical properties than those displayed by pure polydimethylsiloxane polymer/crosslinker networks, liquid silicone systems also contain fillers. These are classified as inert, i.e. nonreinforcing, or active, i.e. reinforcing. Inert fillers such as quartz powder, diatomaceous earths, calcium silicates, calcium carbonates, and iron oxides interact neither chemically nor physically with the polydimethylsiloxane network. They mainly influence the silicone rubber's hardness and swelling properties.

Mechanical strength is influenced exclusively by the use of active fillers such as fumed silicas with a BET specific surface area > 125 mVg, whose silanol groups (3-4.5 SiOH groups/mm2) can form hydrogen bonds with the polymer's oxygen atoms. In addition to improving the product's mechanical properties, these filer-polymer interactions increase the viscosity and effect changes in the glass transition temperature and crystallization properties.

+ 2 R-Si-OH+NH3 W02l I

CH3

R = CH3, CH CH2

1 I Si-OH+R-Si-OH -

I "H31 CHa CH3

C Fig. 3. Imparting water repellency to fumed silica with hexaalkydisilazanes [5 ] .

Oil-Bleeding Properties of Self-Lubricating Liquid Silicone Rubbers 681

As a result of primary-particle agglomeration and the tertiary structures formed by way of hydrogen bonds, fumed silicas normally have a bulk density of only about 50 g/L. For this reason, special agents are added which largely prevent the formation of agglomerates and thus facilitate incorporation of the filler. Hexaalkydisilazanes have proved the most effective agents for this purpose. They render the surface of the fumed silica almost 100 % hydrophobic [5] . The hexaalkydisilazanes are converted into trimethylsilanol or dimethylvinylsilanol, the actual water repellents, which then react with the filler’s SiOH groups (Fig. 3). Ammonia is released as a byproduct. Such silylated fillers have much less of a thickening action, so that liquid silicone rubbers remain pumpable.

LR compounds can also contain silicone fluids, i.e. polydimethylsiloxanes without reactive groups. These lower the viscosity of the uncured rubber and/or the hardness of the cured product. Pigments are used to color the transparent-to-translucent liquid silicones, to which heat stabilizers and antistatic agents, for example, may also be added.

Curing of LR

Curing is effected by way of a platinum-catalyzed hydrosilylation reaction in which the crosslinker’s Si-H groups add across the polymer’s vinyl groups (Fig. 4). The crosslinking reaction involves several repetitions of the catalysis cycle. The platinum catalyst, whose oxidation state changes a number of times between 0 and +II, reduces the activation energy for the crosslinking reaction but is not itself consumed during the process. The reactivity is controlled by means of the catalyst concentration (usually between 5 and 10 ppm platinum) and the amount of inhibitor.

. ,c-c: I I

“ - - C - G . S , L

I I .. elastomer

complex formation

-Pt-

u-slg cross-linker

reductive elimination Pt’”+ Pt‘

migration

Fig. 4. Catalysis cycle of the hydrosilylation reaction [6].

682 K. Pohmer

Processing of LR

On account of their low viscosity, liquid silicone rubbers are processed chiefly by liquid injection molding (LIM), a technique which resembles the injection molding of plastics and is combined with a meter/mix unit developed specifically for the process [7]. Additional homogenization is effected by a static mixer located between the mixing station and the feed screw of the injection molding machine (Fig. 5).

Mode of operation of an injection molding machine

ELASTOSIL@ LR is suitable for virtually all metering and mixing equipment.

1. Metering device for Components A and B of the liquid silicone rubber

Metering device for pigment (if required)

3. Mixing devece

4. Mold

5.

2.

Heated injection mold with injection mold part

Fig. 5. Mixing and metering technology for processing LR [S].

Short curing times (about 5 s/mm wall thickness at 170 to 200 "C) and excellent flow properties permit flawless, cost-efficient production even of complex parts, for example parts involving long flow paths in multicavity molds with undercuts. The manufacture of LR parts is characterized by easy demolding and short cycle times. This permits efficient mold utilization without the need for release agents. The moldings exhibit high fidelity of reproduction, making post-finishing operations such as deflashing unnecessary. Another advantage is the virtual elimination of scrap due to the use of cold runner systems. Thanks to the low viscosity of LR, sprues and runners can be designed with very small diameters and thus account for very little volume.

For many applications, especially in the automotive industry, precision parts can only be mass-produced competitively and cost-effectively by using liquid silicone rubber. Successful applications include seals, O-rings, membranes including central locking membranes, switch

Oil-Bleeding Properties of Self-Lubricating Liquid Silicone Rubbers 683

covers, bellows and rubber springs. Special ready-to-use compounds that already contain heat stabilizers and colorant have been developed for protective sheaths for spark plug boots.

Self-Lubricating LR

Special self-lubricating liquid silicone rubber grades containing added methylphenylsilicone fluids are available for weather packs. These silicone fluids disperse homogeneously throughout the uncured rubber but are incompatible with the cured product. Once the rubber has vulcanized, the silicone fluid starts to exude from the surface of the part. Here the fluid film fulfills two functions. Firstly, by reducing insertion resistance or sticking friction, it facilitates assembly of the various components which make up the weather pack. Secondly, it also provides lasting protection against corrosion and the ingress of water and dirt [9].

The incompatibility of these special silicone fluids derives from the fact that some of the methyl groups have been replaced by phenyl groups. The methylphenylsilicone fluids used comprise a random mixture either of methylphenylsiloxy/dimethylsiloxy units or of dimethylsiloxy/diphenylsiloxy units (Fig. 6) [ 101.

Fig. 6. Methylphenylsilicone fluids for self-lubricating LR [ 101.

The incorporation of phenyl groups in the polysiloxane chain causes strong intermolecular interaction, which results in high viscosity despite the relatively low weight (Fig. 7) [l 11. The reasons for the intermolecular interactions are to be found in the different electronic effects of the methyl and phenyl groups, such as induction and resonance [ 11:

by definition, methyl groups show neither an inductive nor a resonance effect. unsaturated groups always show an inductive effect. phenyl groups show a resonance or mesomeric effect caused by x-electron delocalization.

The higher viscosity of self-lubricating liquid silicones - it is roughly twice that of equally hard standard products containing no silicone fluid - is evident from their thixotropic behavior.

684 K. Pohmer

I Me I II Ph Me Me 111 C F 3 C H 2 C H 2

104

mPa s

103

2; 3; 4; 5; 6; -0c 3.4 3.3 3.2 3.1 3.0 C- VK.10’

Fig. 7. Viscosity of polydiorganylsiloxanes as a function of temperature [l 11.

Transport Phenomena During the Vulcanization of Self-Lubricating LR

The incompatibility of the silicone fluid (liquid phase) in the silicone rubber (solid phase) derives from the differences in chemical potential p between the two phases in the system. There is a natural movement of silicone fluid in response to the potential gradient, i.e. there is a steady nonequilibrium state which results in a transport of mass, known as diffusion.

The rate of diffusion is expressed by the amount of substance n flowing across a plane of given cross-section per unit time t . The concentration gradient causes a particle flow J,, according to Eq. 1, which we express as the number of particles passing per unit time across a plane A perpendicular to the x-axis.

J, = dnlAdt

Eq. 1.

Equation 2 is known as Fick’s first law of diffusion, established empirically in 1855.

dnldt = -AD dcldx

Eq. 2.

Oil-Bleeding Properties of Self Lubricating Liquid Silicone Rubbers 685

The proportionality constant D is referred to as the diffusion constant and is calculated according to Eq. 3 for a given temperature T and given substances defined by their viscosity q and particle size r.

Eq. 3.

The constant NA is Avogadro’s or Loschmidt’s number. If we look at the nonsteady state, i.e. the change in concentration with time, we arrive at Eq. 4,

Fick’s second law of diffusion.

dnldt = D d2nldx2

Eq. 4.

This second-order differential equation contains an essential statement about the stationary nonequilibrium state. In this state, there should be no concentration changes with time, i.e. dnldt = 0. As a necessary consequence, d2Nldx2 = 0 or d2Nldx2 = constant. That means, however, that in the steady nonequilibrium state, concentration must vary with the space coordinate.

The diffusion laws which apply to gases are not valid for the condensed phase, where diffusion is governed by migration processes. These require activation energy E, and therefore the temperature dependency of the diffusion coefficient can be described by the Arrhenius-type Eq. 5, where R is the universal gas constant.

Eq. 5.

The decrease in viscosity at elevated temperatures is neglected in this equation since it is assumed to be constant. The equation is not valid for methylphenylsilicone fluids, as was shown before (Fig. 7).

Practical Considerations

The use of self-lubricating silicone rubber grades thus necessitates controlling a large number of parameters, such as particle density, pressure and temperature. If these are given sufficient consideration when an article is being designed, and particularly during processing and assembly, weather packs with oil-bleeding LR seals will do an excellent job throughout the service life of an automobile.

Particle density is a function of the crosslinking density in the silicone rubber and is influenced by the

686 K. Pohmer

curing conditions (injection time, mold temperature and heating time), hardness of the rubber (filler and crosslinker content), and color (added pigment).

However, provided an appropriate rubber compound is used, today’s injection molding technology makes it easy to keep the particle density constant once the processing window has been established (Fig. 8).

Fig. 8. Oil distribution on a test plate of 2 mm thickness (cured for 5 min at 165 “C), after five days’ conditioning at

room temperature (left to right: Elastosil@ LR 3080/50, LR 3081/50 and LR 3082/50, manufacturer: Wacker-

Chemie GmbH, Munich) as seen under a photoelectron microscope (magnification x 50).

When, by contrast, the influence of pressure and temperature on the exudation of silicone fluid is considered, it is evident that these parameters are much more difficult to control. The pressure depends not only on the amount of silicone fluid in the rubber compound (Fig. 8) but also on the article design and the storage conditions. Given that molded articles are often stored in tightly packed plastic bags for long periods, they may well be expected to “leak” silicone fluid.

The storage temperature has a considerable influence on the diffusion rate, firstly because viscosity decreases with increasing temperature, and secondly because migration processes are initiated. This dependency of the diffusion coefficient on temperature means that diffusion is much faster at higher than at lower temperatures. There is clear experimental evidence of this increase in diffusion rate at elevated temperatures (Fig. 9).

Where finished articles are not stored under controlled conditions, seasonal temperature fluctuations can influence oil-bleeding to the extent that a silicone rubber article which exudes hardly any silicone fluid in winter could be well wetted in summer.

In cases where single wire seals or cable bushing mats are fabricated in automated processes, such fluctuations in oil-bleeding can cause considerable difficulties. Since not only temperature and pressure but also storage duration will influence the amount of silicone fluid on the surface of the article, it is essential to optimize storage conditions.

Oil-Bleeding Properties of Self-Lubricating Liquid Silicone Rubbers 687

Fig. 9. Oil-bleeding properties of Elastosif' LR 3088/50 (manufacturer: Wacker-Chemie GmbH, Munich) at

different temperatures (shown as weight loss over time) [12].

Self-Adhesive, Self-Lubricating LR

It is becoming more and more common to fabricate entire weather packs as multi-component composites in order to avoid the cost-intensive process of assembling individual plastic and silicone rubber components [ 131.

There are two basic ways of joining the hard and soft components of a composite, chemical and mechanical [14]. A purely mechanical joint is obtained by overmolding apertures or undercuts in the hard component. The use of internal or external adhesion promoters permits the production of composites with liquid-tight frictional joints [15]. A bond formed by chemical cohesion has a number of advantages over a mechanical joint formed by interlocking [ 161:

no perforations and therefore no concentration of stress, greater fatigue resistance, lower weight, the bond can serve simultaneously as a seal, silicones can be joined to shock-sensitive substrates, silicones can be joined to metals with electrochemically treated surfaces, and usually more economical.

Novel self-adhesive, self-lubricating liquid silicones such as Elastosil@ LR 3072/30 and LR 3072/40 from Wacker-Chemie GmbH in Munich, Germany, are particularly suitable for

688 K. Pohmer

manufacturing entire weather packs. Since there is no sticking friction to be overcome, these products require only a very low oil content. The oil film is only needed to protect against corrosion and to impart water repellency. Because silicone fluids show release properties, products have been developed which bond firmly to the substrate before any oil is exuded; in other words, the adhesion promoter starts diffusing towards the surface before the silicone fluid does.

References

[ 11 [2] [3]

K. Pohmer, GAK Kautschuk Fasern Kunststoffe 2000,53, 709. B. Ganther, E. Box, Rubber World 2001,224,3130. G. Kollmann, The Fine Art of Molding: Flexible Molds of RTV-2 Silicone Rubber, in Organosilicon Chemistry IV- From Molecules to Materials (Eds.: N. Auner, J. Weis), VCH, Weinheim, 2000, p. 7 10. A. Tomanek, Silicones and Industry - A Compendium for Practical Use (Ed.: Wacker- Chemie GmbH, Munich), Carl Hanser, Munich, 1991. D. Wrobel, Structure and Properties of Hot-Vulcanized Silicone Rubbers, in Silicones - Technology and Chemistry, Vulkan, Essen, 1991, p. 61. K. Pohmer, H. Steinberger, Silicone Rubbers - Innovative - High Performance - Efficient, in Organosilicon Chemistry IV - From Molecules to Materials (Eds.: N. Auner, J. Weis), VCH, Weinheim, 2000, p. 699.

[7] K. Pohmer, G. Schmidt, H. Steinberger, T. Briindl, T. Schmidt, Kunststoffe 1997, 87, 10; Kunststoffe Plast Europe 1997,87,2146.

[8] P. Jerschow, KGK Kautschuk Gummi Kunststoffe 1998,51,410. [9] K. Pohmer, KunststofJberater 2001,46,3/35. [lo] P. Preiss, Seifen- Ole - Fette- Wachse 1990,116, 175. [ 111 J. Burkhardt, Chemistry and Technology of Polysiloxanes in Silicones - Technology and

Chemistry, Vulkan, Essen, 1991, p. 21. [ 121 K. Pohmer, Rubber News 2001,224,3134. [ 131 K. Pohmer, Kunststoffe 2000,90,2/94; Kunststoffe Plast Europe 2000,90,2/32. [ 141 C. Freyer, K. Pohmer, KunststofJberater 2000,45,7-8/27. [ 151 E. Haberstroh, E. Henze, K. C. Ronnewinkel, Kunststoffe 1999,89,410. [ 161 A. V. Pocius, Adhesion and Adhesion Technology - An Introduction, Carl Hanser, Munich,

Hanser-Gardner, Cincinatti, 1997.

[4]

[5]

[6]

PDMS4-PEO Block Copolymers as Surfactants in the Synthesis of Mesostructured Silica:

A Theoretical and Practical Approach

Dietmar Sturmayr, Josef Bauer, Beatrice Luunay, Guido Kickelbick, Nicola Hiising

Institute of Inorganic Chemistry, Vienna University of Technology Getreidemarkt 9, A-1060 Wien, Austria

E-mail: nhuesing@mail.zserv.tuwien.ac.at

Anthony P. Malanoski, Dhaval A. Doshi, Frank van Swol

University of New Mexico and Sandia National Laboratories Albuquerque, NM 87185 USA

Keywords: block copolymers, silicones, mesostructured silica, Monte Car10 simulations

Summary: Well-defined amphiphilic block copolymers containing a poly(dimethy1 siloxane) (PDMS) and a poly(ethy1ene oxide) (PEO) block were synthesized by ring- opening polymerization of hexamethylcyclotrisiloxane, followed by chain extension with a PEO block of a defined length. These amphiphilic molecules were used as structure-directing agents in a solvent evaporation-driven synthesis approach to self-assembled mesostructured silica films. In addition, a general theoretical approach for an understanding and ultimately a prediction of the phase behaviour of block copolymers is presented.

Introduction

The discovery of a novel family of mesoporous molecular sieves (M41-S) has opened the way to new mesostructured materials [l]. The synthesis involves a templating mechanism in which not single molecules, but supramolecular arrangements such as the liquid crystalline phases of surfactant molecules, serve as structure-directing agents.

Most often, commercially available and purely organic amphiphilic, self-assembling molecules are applied in the synthesis of mesostructured materials such as ionic surfactants or block copolymers, i.e. Pluronic@ surfactants (PEO-b-PPO-b-PEO with PPO = poly(propy1ene oxide)) or poly(ethy1ene oxide) alkyl ether surfactants (Brij@). However, due to the restricted availability of amphiphilic block copolymers, not only are the accessible pore sizes and phases limited, but commercial products are sometimes inhomogeneous and have high molecular weight distributions [2].

Ovganosilicon Chemisty V Edited by N. Auner and J. Weis

Copyright 0 2003 WILEY-VCH Verlag GmbH & Co. KgaA, Weinheim

690 D. Stumayr, J. Bauer, B. Launay, G. Kickelbick, N. Hiising, A. P. Malanoski

In this study we present the synthesis of amphiphilic block copolymers based on inorganic- organic hybrid systems, with poly(dimethy1 siloxane) (PDMS) as the hydrophobic component and poly(ethy1ene oxide) (PEO) as the hydrophilic part of the polymer, and their application as templates in the synthesis of mesostructured silica. The formation of novel block copolymers with inorganic polysiloxane units is of interest because of a) their strong hydrophobic character, high flexibility and biocompatibility, and b) their potential to be transformed into silica upon heat treatment, and therefore to allow for different ways to control structural parameters such as the wall thickness. Despite all these advantages, major problems related to using novel block copolymers are a) their controlled preparation and b) information concerning their phase behavior. For the controlled preparation of such block copolymers only a limited number of synthetic methods can be used, i.e. ionic polymerizations, and the synthesis has to be carried out under very constrained conditions. The phase behavior of complex mixtures has not been investigated very well, and therefore their use as templating agents is somewhat limited and arbitrary. This is especially true for the formation of thin films in which the phase behavior is governed not only by the presence of water and silica but also by the presence and the evaporation of alcohol. As a supporting instrument for the investigation of the phase behavior we started to carry out simulation experiments on the phase behavior of block copolymers in watedethanol solutions based on Monte Car10 calculations.

Synthesis of PDMS-b-PEO Diblock Copolymers

The end-functionalized PEO block was prepared via an etherification reaction between an HO-terminated poly(ethy1ene oxide) of a defined block length and ally1 bromide. The SiH-functionalized polysiloxane was prepared by anionic ring-opening polymerization (ROP) of hexamethylcyclotrisiloxane (D3) and quenching of the chain end with chlorodimethylsilane (Scheme 1) [3].

2. 7H3 CI-SI-H

I CH3

In situ I R T l A r ITHF

1.THFI

FH3 ?Ha

CH, CHa

Buf~I-oj-;sl-o- Ll+ 1. KOHlToluene

2. -Br I Scheme 1. Synthesis of diblock copolymers from PEO and PDMS.

PDMS-PEO Block Copolymers as Su$actants in the Synthesis of Mesostructured Silica 691

The two blocks were coupled by hydrosilylation reaction using a Pto catalyst [4]. The block lengths, and therefore the relative molar ratio of the blocks were varied. The resulting polymers were characterized by NMR and size exclusion chromatography (SEC) (Table 1).

Table 1. Examples of SEC analyses data for EO-b-DMS diblock copolymers.

M . Ratioby Calculated (MJM,,) 'HNMR ratio Conversion M" M"

Composition (MJM,,) rn (MJM,,) n (PDMS) (PEO) ['I (diblock) [PDMSPEO] [PDMSPEO]

PDMS-PEO-Me

PDMS-PEO-Me

PDMS-PEO-Me

PDMS-PEO-Me

PDMS-PEO-Me

PDMS-PEO-Me

1320 18

( 1.24)

730 10

(1.10)

1730 24

(1.21)

1320 18

(1.14)

2400 32

(1.30)

2400 32

(1.30)

580 12 79

(1.15)

580 12 83

(1.15)

410 8 93

(1.13)

410 8 96

(1.13)

4620 105 91

(1.36)

750 16 96

(1.18)

1570 2.30 2.64

(1.52)

1160 1.51 1.46

(1.41)

1890 4.97 4.79

(1.24)

1100 3.75 3.68

(1.30)

3690 0.45 0.52

(1.41)

3380 3.04 3.20

(1.26)

Preparation of Mesostructured Silica Films

The diblock copolymers synthesized (Table 1) are structurally related to the commercially available poly(oxyethy1ene-alkyl ether) systems (Brij@) that are built from PEO blocks and hydrocarbon chains. These systems have been successfully applied as templates in the synthesis of mesostructured silicate materials [2]. PDMS-PEO-based block copolymers are also known to exhibit a two-phase morphology in a certain set of solvents, which may be ascribed, to a f i s t approximation, to the large difference in the solubility parameters of the PDMS and PEO blocks

Precursor solutions were prepared by mixing tetraethoxysilane (TEOS), ethanol, H20 and hydrochloric acid in a molar ratio of 1:20:4:0.004 [6]. The PEOd-PDMS diblock copolymer surfactant (1 to 20 wt%) was added to the sol, the mixture was stirred until a homogeneous solution was obtained and films were deposited on silicon wafers by dip coating [7].

The synthesis of powders is well established using hydrothermal processing strategies [l, 21. For the synthesis of thin films novel pathways have been developed that are based on an evaporation- induced self-assembly (EISA) process of amphiphilic molecules with inorganic species using mild sol-gel processing conditions [6]. Starting with a homogeneous solution of the amphiphilic polymer

~ 1 .

692 D. Sturmayr, J. Bauer, B. Launay, G. Kickelbick, N. Husing, A. P. Malanoski

and the siliceous precursor, evaporation during dip-coating is used to induce continuous self- assembly to liquid crystalline mesophases. Simultaneously the polymeric and inorganic constituents organize into the desired nanocomposite form and, finally, polycondensation reactions of the siliceous precursor are used to “lock in” the preformed composite structure. Therefore, the formation of liquid crystal phases in the final film is governed by the concentration of the siliceous precursor, the polymer, water and ethanol in the initial solution.

2 3 I 5

028

Fig. 1. X-ray diffraction pattern (left) of an as-synthesized (bottom) and calcined (top) film from E012-b-DMS18 and

the corresponding TEM image (right) for the calcined sample.

For the PDMS-b-PEO diblock copolymer surfactant, mesostructured silica films were successfully synthesized. Figure 1 shows examples of the X-ray diffraction patterns of a film prepared with EOs-b-DMS18 a) prior to and b) after calcination at 450 “C/3 h. The reflections at a d- spacing of about 48 nm clearly indicate the presence of ordered domains in the film.

The TEM image of a calcined EOl~-b-DMSl&ilica film with 15 wt% of surfactant shows a layered structure corresponding to a [I101 orientation of an one-dimensional hexagonal film or a [OOl] orientation of a lamellar mesostructure. The layer spacing of about 5 nm, obtained from the TEM image, is consistent with the long-range order parameter obtained from the XRD pattern.

Dip coating allows a fast and easy access to mesostructured silica thin films; however, a major drawback, as already mentioned above, is that many trial and error experiments are necessary to define the parameters necessary for the formation of mesophases. Therefore, theoretical investigations were performed with respect to the phase behavior of amphiphilic block copolymers in ethanoYwater solutions.

Theoretical Modeling of the Phase Behavior of Block Copolymers

Theoretical modeling of the phase behavior of block copolymers is not a trivial task, due to the size of the system to be calculated. Therefore the system has to be simplified to a high extent (Fig. 2).

The interactions employed are short-range, square-well like, and extend to the nearest,

PDMS-PEO Block Copolymers as &&actants in the Synthesis of Mesostructured Silica 693

next-nearest and third-nearest neighbours. The attractive interaction between the different species i and j is characterized by EW Values for the latter, relative to the water-water interaction (which is denoted by E ~ O % O ) , are listed in Table 2. Using the critical temperatures, the ethanol-ethanol

parameter (EEtOmtOH) can be calculated to be -0.8. The solvent-head and solvent-tail interactions are considerably more qualitative. The choices listed in the table reflect the hydrophilic nature of the head sites and the hydrophobic nature of the tail sites. In contrast, the alcohol-surfactant interaction is expected to be somewhat less discriminating, as these molecules themselves display more mixed characteristics, the OH group favoring head sites, the alkane chains prefemng tail sites. The values listed in Table 2 were chosen to reflect these tendencies.

Fig. 2. Schematic description of a block copolymer. Grey represents the hydrophilic head group (H, in our case

PEO) and black the hydrophobic tail (T, PDMS).

Table 2. Energy parameters for the water-ethanol-polymer mixture, in units of the absolute water-water interaction value ( I ~ H p r n ~ O I ).

H20 EtOH Head(H) Tail (T)

H2O -1 -0.86 -1 0

EtOH -0.8 -0.7 4.1

Head (H) -1 0

Tail (T) -1

To simulate the behavior of the block copolymer during the dip coating procedure, not only has the phase diagram of the different liquid-crystal like species in water to be calculated, but also the concentration gradient from evaporation of ethanol has to be taken into account. The evaporation of the solvent, and therefore the concentration, are mimicked by calculating points in the polymerEtOWwater phase diagram. The simulations were performed with a 3D lattice Monte Car10 method using simple effective pair-potentials of a three-component mixture of ethanol, water and polymer. The polymer chains were modeled using a Gaussian bead, head (H) and tail (T), representation. This approach follows that of Larson, who f is t employed it in the description of a (symmetric) system of oil, water and surfactant [8]. The details in brief are that water and ethanol species each occupies a single lattice site, while the surfactant is a polymer molecule that consists of connected beads each occupying a lattice site. In this paper we will restrict ourselves to linear chains and here, in particular, we consider a chain with two heads and six tail beads (H2T6).

A large variety of structures is encountered when the three-component phase diagram is traversed (Fig. 3). At the lowest surfactant concentrations, aggregations of the surfactant into single spherical micelles can be found. As the concentration is increased these micelles become elongated and then form an “infinite” cylinder, due to the periodic boundary conditions. This regime and its

694 D. Sturmayr, J. Bauer, B. Launay, G. Kickelbick, N. Hiising, A. P. Malanoski

evolution are of course strongly influenced by the periodic boundary conditions and the number of surfactants. Thus, the cylinder is not necessarily an indication of a hexagonal phase, for instance. Increased surfactant concentrations first give rise to two-dimensional structures that exhibit intersections involving two cylindrical tubes. Usually these structures display large variations and substantial curvature of the cylindrical portions. As the surfactant concentration goes up, these structures smoothly morph into layers with holes. Further increase leads to the emergence of three- dimensional connectivity, with three cylindrical tubes meeting at an intersection. Ultimately, this leads to perfect cubic structures of three tubes intersecting from orthogonal directions. As the concentration is further increased, lamellar phases form. The structure of lamellar layers depends on the amount of ethanol present. In the bilayers that make up the lamellar phase, ethanol is found intruding between the heads and tails. This arrangement helps to shield the tails from the water phase, and hence increased amounts of ethanol decrease the thickness of the bilayer. This corresponds to a reduced area density of head sites in the bilayer.

Fig.3. Calculated phase diagram for an HzT6 block copolymer using the method described in the text, with

examples of the most stable structures at the relevant concentrations.

These calculations are a first step towards the prediction of the phase behavior of the three-component system water, ethanol and amphiphilic block copolymer. Hopefully, results based on this modeling approach can lead to a simplified synthesis and more precise tailoring of the structural features in meso-ordered thin films.

Conclusions

The synthesis of inorganic materials is not a problem which is associated with inorganic chemistry

PDMS-PEO Block Copolymers as Surjiictants in the Synthesis of Mesostructured Silica 695

alone. Most often, only a combination of organic, polymer, inorganic, theoretical etc. chemistry can address all the aspects required for a tailored design of certain materials properties. In this work we showed the successful preparation of novel mesostructured inorganic films by a combination of polymer chemistry (well-defined inorganic-organic (PDMS-b-PEO) diblock copolymers have been synthesized), inorganic chemistry (the sol-gel process has been applied for the preparation of mesostructured thin silica films), and computer simulations which provided a better insight into the role of ethanol and water in the phase diagram of amphiphilic block copolymers.

Acknowledgments: We gratefully acknowledge the financial support by the Fonds zur Forderung der wissenschaftlichen Forschung, Austria, and Wacker Chemie, Germany for their kind donation of chemicals. We also thank Dr. E. Halwax and Prof. C. J. Brinker for their support with the XRD and the TEM experiments, respectively.

References [l] C. Kresge, M. Leonowicz, W. Roth, C. Vartuli,

Curr. Opin. Solid State Muter. Sci. 1996, 1, 798. Beck, Nature 1992,359, 710; C. Brinker,

[2] D. Zhao, J. Feng, Q. Huo, N. Melosh, G. H. Fredrickson, B. F. Chmelka, G. D. Stucky, Science 1998, 279, 548; P. Schmidt-Winkel, W. W. Lukens, D. Zhao, P. Yang, B. F. Chmelka, G. D. Stucky, J. Am. Chem. SOC. 1999,121,254. J. Bauer, N. Husing, G. Kickelbick, Chem. Commun. 2001, 137-138; G. Kickelbick, J. Bauer, N. Husing, Mat. Res. SOC. Symp. Proc. 2000,628, CC3.l.l-CC3.1.7. P. J. Miller, K. Matyjaszewski, Macromolecules 1999, 32, 8760; G. Belorgey, G. Sauvet in Silicon-Containing Polymers (R. G. Jones, W. Ando, J. Chojnowski, Eds.), Kluwer Academic Publishers, Netherlands, 2000, p. 43. H. W. Haesslin, Makromol. Chem. 1985, 186, 357; P. R. Dvomic in Silicon-Containing Polymers (R. G. Jones, W. Ando, J. Chojnowski, Eds.), Kluwer Academic Publishers, Netherlands, 2000, p. 185. C. J. Brinker, Y. Lu, A. Sellinger, H. Fan, A h . Muter. 1999, 11(7), 579; Y. Lu, R. Ganguli, C. A. Drewien, M. T. Anderson, C. J. Brinker, W. Gong, Y. Guo, H. Soyez, B. Dunn, M. H. Huang, I. J. Zink, Nature 1997,389, 364. N. Husing, B. Launay, J. Bauer, G. Kickelbick, D. Doshi, J. Sol-Gel Sci. Technol. 2003, 26, 609. R. G. Larson, J. Chem. Phys. 1989,91,2479; R. G. Larson, J. Chem. Phys. 1992,96,7904.

[3]

[4]

[5]

[6]

[7]

[8]

Preparation and Properties of Porous Hybrids Silicone Resin for Interlayer Dielectronic Application

P. M. Chevalier," D. L. Ou, I . MacKinnon, K. Eguchi, R. Boisvert, K. Su

New Venture R&D, Dow Coming Ltd., Barry, CF63 2YL, UK Tel: +44 1446 723 504 - Fax: +44 1446 730 495

E-mail: p.chevalier @ dowcorning.com

Keywords: silicone resin, porosity, dielectric constant, low k, modulus

Summary: Silicon oxide dielectric films have traditionally been used in microelectronic fabrication for integrated circuits having dielectric constants (k) near 4.0. However, as the feature size has been continuously scaling down, the relatively high k of such silicon oxide films have become inadequate to provide efficient electrical insulation. As such, there has been an increasing market demand for materials with an even lower dielectric constant for Interlayer Dielectric (ILD) applications, yet retaining thermal and mechanical integrity. We report our investigations on the preparation of ILD materials using a sacrificial approach whereby organic groups are burnt out to generate low-k porous silicone resin films. We have been able to prepare a variety of hybrid silicone resin compositions leading to highly microporous thin films, exhibiting ultra-low k from 1.8 to 2.9, and good to high modulus, 1.5 to 5.5 GPa. Structure-property influences on porosity, dielectric constant and modulus are discussed.

Introduction

Moore's law states that the computing power of chip manufacturing doubles every 18 months. This improvement in performance is achieved by continually shrinking the device size and so increasing the speed and the number of functions on a single chip. However, as device dimensions shrink to <0.25 pm, problems such as propagation delay, crosstalk noise and power consumption become significant. The industry is, therefore, actively seeking new inter layer dielectric (ILD) materials with ever-lower dielectric constants (k) . The requirement for the ILD materials used in the current 0.13 pm device should have k as low as 2.5; the requirement for the ILD materials used in the future 0.10 pm device should have k between 1.6 to 2.2. Yet the dielectric constant k of most of the commercial dielectric materials was below this demanding requirement (i.e., CVD SiOz: 4.0; FOX@ siloxane resin: 2.75; SILK@ organic resin: 2.65).

Ovganosilicon Chemisty V Edited by N. Auner and J. Weis

Copyright 0 2003 WILEY-VCH Verlag GmbH & Co. KgaA, Weinheim

Organic-Inorganic Hybrids towards the Preparation of Porous Silicone Resin Thin Films 697

Introduction of porosity is a key approach to reduce the dielectric constant. Porous silica demonstrated the ability to “tune” dielectric constants over the range of 1.3 to 2.5. In general, k is reduced with an increment of percentage porosity [I]. Aerogel-type materials have a very high percentage porosity, which leads to an ultra-low k. However, the feature size in integrated circuits is approaching 0.18 pm and most of the aerogel pores are reaching a limit for this type of application. Moreover, aerogels have lower mechanical strength and are difficult to fabricate for micron-sized items. The next-generation ILD material should be highly porous, and the pore size should not exceed 50 A in order to avoid electric breakdown.

Several attempts have been made to obtain porous films by thermal degradation of organosilsesquioxane resins [2-51. The loss of organic groups in the form of several vapor species led to the formation of void space in the resin matrix during the pyrolysis process. In the present work, we incorporated seven distinct types of organic functions into a siloxane resin network by covalent bonding. We report here our studies on the thermal degradation of these sacrificial organic groups and the porosity generated as well as the dielectric and mechanical properties associated with them.

Results and Discussion

A wide range of organic functions could be incorporated into the siloxane resin matrix. Seven distinct types of organic groups R (R1 to R7) were selected for this study, which were chemically bonded into a T-siloxane network as c o n f i e d by 29Si NMR spectroscopy. Truly homogeneous, organically modified siloxane resins were synthesized, containing a thermally labile group R and a R that was thermally stable up to 450 “C. These resins, which were soluble in most of the common organic solvents, were analyzed by GPC, which revealed that the molecular weights of resins modified with the smaller organic groups R4 and R5 were very low, whilst the resins modified with the larger organic groups R6 and R7 had much higher molecular weights with larger polydispersity.

All of the above seven organically modified siloxane resins were subject to pyrolysis at 450 “C for 2 h. Porosity measurements were performed on the pyrolyzed resins using the nitrogen sorption method (Table 1). The surface areas, calculated using the BET equation [6], were considered to give the total internal and external surface area of the material. The micropore surface areas and micropore volume were calculated using the de Boer t-method.

The BET surface areas of the first three pyrolyzed resins were lower than 20 m2/g due to the type of the organic group R1, R2 and R3. The R4-modified resin gave a slightly higher level of porosity after pyrolysis, whereas R5-, R6- and R7-modified siloxane resins led, after pyrolysis, to a porosity higher than 400m2/g and up to 900 m2/g, which rendered them attractive candidates as ILD materials and they were selected for further porosity studies.

The pore size distribution, BJH plots [7], of the last four pyrolyzed porous materials obtained from organically modified siloxane resins (R4 to R7) revealed that mainly microporous materials were generated through pyrolysis and that most of the pore diameters were below 50 A. HK pore size distributions confirmed that the majority of the pores in the R6- and R7-modified siloxane

698 P. M. Chevalier, D. L. Ou, I. MacKinnon, K. Eguchi, R. Boisvert, K. Su

resins were centered to 12 8, with some minor contribution near 20 A. Density functional theory (DFT) [8, 91 was also used for the calculation of the pore size distribution of the last four selected porous materials. This method indicated that most of the pores sizes are in the range of 5 to 25 A, with absence of pores over 50 A, thus confirming both the BJH and the HK distributions.

Table 1. Porosity measurements.

BET surface area Micropore surface area Micropore volume Total pore volume [m2/g1 [m2/s1 [cm3/gl [cm3/sl

Organic group

R1 18

R2 19

R3 20

R4 136

R5 424

R6 686

R7 904

413

670

879

- 0.030

- 0.050

- 0.050

- 0.178

0.239 0.270

0.378 0.409

0.527 0.577

Thin Film Property Evaluation

The thin film properties, including k and modulus, of the four porous resins R4 to R7, spun onto silicon wafers and pyrolyzed at 450 OC under an inert atmosphere, were evaluated (Table 2).

Table 2. Thin film characterization.

~ ~~ ~~~~

Organic group Thickness [A] Refractive index k Modulus [GPa] Hardness [GPa]

R4 3 900 1.356 2.87 - -

R5 5 544 1.301 2.36 5.3 0.69

R6 6 309 1.260 2.09 5.5 0.72

R7 10 163 1.211 1.80 1.5 0.26

The thickness of these good-quality and crack-free films were in the range of 5 500A to 7 2008,, with a standard deviation below 4 %. The dielectric constants (k ) observed for these four resin films were in the range of 1.80 to 2.90. R7-modified siloxane giving the highest porosity after pyrolysis at 45OoC, and leading to the lowest k = 1.80 and refractive index of the series, confirms the relationship between k, the porosity and the refractive index. The mechanical integrity of these thin films, in place of a modulus as high as 5.5 GPa, was also good and of prime importance for interlayer dielectric application.

Organic-Inorganic Hybrids towards the Preparation of Porous Silicone Resin Thin Films 699

Conclusion

Various types of porogen organic functions can be homogenously incorporated into siloxane resins, by covalent bonding. The thermal degradation of these organic groups was observed through the evolution of vapor species leading to the generation of pores more often smaller than 50 A. Various pore characteristics were evidenced, depending on the pyrolysis temperature. Lower k thin film values from 1.8 to 2.9 were obtained for the highly porous, pyrolyzed, organically modified resins; thus the mechanical integrity of the film was not compromised, as conf i i ed by their high modulii up to 5.5 GPa.

References [l]

[2] [3] [4] [5] [6] [7] [8] [9]

T. Ramos, K. Roderick, A. Maskara, D. M. Smith Muter. Res. SOC. Symp. Proc. 1997, 443,

S. Mikoshiba, S . Hayase, J. Muter. Chem. 1995,9,591-598. L. Figge, D. Jude. S. Wang, WO 00/75979,2000. N. P. Hacker, T. Krajewski, S. Lefferts, WO 98/47942,1998. R. Y. Leung, T. Nakano, WO 98/47943,1998. E. P. Barett, L. G. Joyner, P. H. Haleuda, .I. Am. Chem. Soc. 1951,73,373. S . Brunauer, P. H. Emmett, E. J. Teller, J. Am. Chem. Soc. 1938,60,309. R. Evans, U. M. B. Marconi, P. Tarazona, J. Chem. SOC. Faruday Trans. II 1986,82, 1763. C . Zhang, F. Babonneau, C. Bonhomme, R. M. Laine, C. L. Soles, H. A. Hristov, A. F. Yee, J. Am. Chem. Soc. 1998,120,8380.

91-98.

Control of the Dispersion of Metal Oxide Phases in Silica Gels via

Organically Modified Alkoxysilanes

Wolfgang Rupp, Gregor Trimmel, Nicoh Hiising, Ulrich Schubert

Institute of Materials Chemistry, Vienna University of Technology Getreidemarkt 9, A-1060 Wen, Austria

E-mail: uschuber@mail.zserv.tuwien.ac.at

Keywords: nanoparticles, sol-gel processing, silica-titania mixed oxides, inorganic-organic hybrid materials

Summary: A new approach for the preparation of homogeneously dispersed silicdmetal oxide systems by sol-gel processing is presented which consists in using a new kind of single-source precursor in which the components are tethered by an organic spacer group. For this purpose, alkoxysilanes of the type (R0)3Si(CH&X are employed, where X is a group capable of coordinating the metal oxide precursor.

Introduction

An inherent problem in the preparation of sol-gel materials from more than one precursor is that the different reaction rates cause a non-statistical arrangement of the building blocks. This may result in a separation of the molecular units on any length scale from the molecular level to macroscopic phase separations. The ordering of the building blocks may be beneficial for the material properties (see Ref. [ l ] for an example). However, for many applications homogeneous distribution of the building blocks during sol-gel processing is necessary. There are several methods to avoid macroscopic phase separations, such as pre-hydrolysis of the slower-reacting component or lowering the reactivity of faster-reacting components by chemical additives.

Our approach to control the mutual arrangement of the building blocks in mixed oxide systems is to tether the components by an organic spacer group that may be removed after sol-gel processing, i.e. to use a special kind of single-source precursors. If silica is one of the oxides, organically substituted alkoxysilanes of the type (R0)3Si(CH2),,X are employed, where X is a coordinating group, such as an amino or acetylacetonate group. These groups allow to tether metal compounds to the silicate network during sol-gel processing. This results in a homogeneous dispersion of the metal compound in the obtained silica gel.

Ovganosilicon Chemisty V Edited by N. Auner and J. Weis

Copyright 0 2003 WILEY-VCH Verlag GmbH & Co. KgaA, Weinheim

Control of the Dispersion of Metal Oxide Phases in Silica Gels 701

Silica/Titania Mixed Oxides

A first example given here are the bimetallic compounds (R0)3Si(CH2)3C[C(Me)O]2Ti(OR)3 and [ (R0)3Si(CH2)3C[C(Me)O]2]2Ti(OR)2, which are prepared by reacting Ti(O'Pr), with one or two equivalents of 3-(propyltrimethoxysilyl)acetylacetone [2], a molecule that consists of both a complexing group and a hydrolyzable alkoxysilane (Scheme 1) [3].

/ M =Ti, Zr

Scheme 1.

Hydrolysis of a mixture of Si(OR)4 and Ti(OR)4 results in the immediate precipitation of titania, because Ti(OR)4 is much more reactive than Si(OR)4. When (R0)3Si(CH2)3C[C(Me)O]2Ti(OR)3 or { (RO)$i(CH2)3C[C(Me)0]2)2Ti(OR)2 is hydrolyzed under basic conditions, transparent and crack- free monolithic xerogels are formed instead. Mixed-oxide powders with the nominal composition TiOz.SiO2 or Ti02.2Si02 and a low crystallization tendency are obtained when the organic groups were removed by calcination at 550 "C.

L u v

1200 "C 1140 "C 1100 "C 1040 "C lo00 "C 950 "C 900 "C 850 "C 800 "C

20 25 30 35 20 ["I

Fig. 1. Development of TiOz and SiOz phases from (RO)3Si(CH2)3C[C(Me)O]2Ti(OR)3 (J = cristobalite, u =

anatase, V = rutile).

702 W. Rupp, G. Trimmel, N. Hiising, U. Schubert

X-ray diffraction studies carried out at different temperatures showed that the onset temperature for the crystallization of anatase was unusually high (800-850 "C) in both Ti02.SiO~ (Fig. 1) and Ti02.2Si02, and cristobalite started to crystallize at about 1050 "C. The anatase/rutile transformation also occurred at higher temperatures than usual: in TiOz.SiOz at about 1000-1200 "C, and in Ti02.2SiOz no rutile phase was observed until 1200 "C. The crystallite diameter of anatase in Ti02.Si02, as calculated by the Scherrer equation, was 5.9 nm at 850 "C and that of rutile 12.2 nm at 1000 "C.

The low crystallization tendency is probably due to the intimate mixing of the two metals in the gel. This approach can be easily extended to other silica/metal oxide systems, because most transition metal oxides form stable complexes with P-diketonates.

Metal Oxide and Metal Nanoparticles in a Silica Matrix

A second example is the preparation of metal oxide/silica or metdsilica nanocomposites from amino-substituted alkoxysilanes, e.g. (RO)~S~(CHZ)~NR~ or (R~)~S~(CH~)~NHCHZCHZNHZ, and transition metal salts [4-61.

The dispersed metals are prepared by a three-step procedure. A solution of a metal salt (MYm), the silane (R0)3Si(CHz)nX and, optionally, Si(OR)4 is processed by the sol-gel method in the f is t step. Complexes of the type [(R0)3Si(CH2),X],MYm are formed in situ, which are identified by their characteristic UV spectra. The metal coordination is retained during sol-gel processing. The metal complexes are tethered to the silicate matrix via the (CH~)~Si03n groups, and aggregation of the metal ions is thus prevented. The gelling behavior of the mixtures is influenced by the presence of metal ions and depends very strongly on the kind of complexing silane. The resulting gels have the idealized composition [03/2Si(CH2),X].MYm.xSiOz. The metal complex-containing gels are then dried and heated in air to oxidize all the organic moieties. Due to the high dispersion of the metal ions in the first step, nano-sized metal oxide particles (i.e., the nanocomposites MO,.(x+n)SiOZ) are formed, which are then reduced to metal particles (composites M.(x+n)SiOZ). The composition of the final composite is determined by the M:Si ratio of the starting compounds.

The metal oxide or metal particles obtained by this approach are highly dispersed, not agglomerated, and homogeneously distributed throughout the SiOz matrix, even in the materials with high metal loadings. The particle size distributions are very narrow. The metal dispersion obtained by this method depends on the kind of metal, on the reaction conditions, and for some metals also on the metal loading. An example is given in Scheme 2.

If one wants to get carbon-free composites, the oxidation temperature has to be high enough to ensure complete oxidation of all the organic components, but should not be higher than necessary, to avoid excessive sintering of the metal particles. The particle size does not change very much during the reduction step, because in most cases Tred < Tax. Some noble metals will already have given metal particles in the oxidation step, due to the thermal instability of the oxides.

The method also allows generation of the metal nanoparticles on a solid support (e.g., silica). In this case the support is impregnated with the gelling solution, and the subsequent steps (calcination

Control of the Dispersion of Metal Oxide Phases in Silica Gels 703

and reduction) are carried with the supported gel as described above.

12+ (R0)3Si N NH2 + pt2+ - (RO)3Si NnNH2

'R '' NH Si(OR)3 H

H z N d

+ Si(OR)4 I + H20 I L

Scheme 2.

The average metal particle size is also influenced by the complexing silane and the metal salt used as precursors (Table 1) [5 ] . Particularly high metal dispersions are obtained when the organic groups are removed, without generating too much local heat by exothermic events.

Table 1. Average metal particle diameters (from XRD) for some metal precursodsilane combinations [AEAPTS =

3-(2-aminoethylamino)propyltriethoxysilae, APS = aminopropyltriethoxysilane, TAS = (3-trimethoxy-

silylpropy1)-diethylentriamine)].

Silane Mean particle Metal precursor (5-6 molar equiv.) diameter [nm]

Pt(acac)2

Pt(acac)2

Pt(acac)2

PtC12

Ptc12

PtClZ

H2PtC16

Na2PtC16

Na2PtC16

Pt(CN)z

P t N U W "

AEAPTS

APS

TAS

AEAPTS

APS

TAS

AEAPTS AEAPTS

APS (1.2 equiv.)

AEAPTS

AEAPTS

7.1

3.5

4.0

5.7

5.6

10.2

22.2

16.6

4.2

13.8

6.4

The complexing silane also has a large influence on the porosity and the pore type of the xerogels, both before and after calcination. While xerogels prepared from M(TAS)F/TEOS

704 W. Rupp, G. Trimmel, N. Hiising, U. Schubert

mixtures are essentially nonporous before calcination, the xerogels prepared from M(AEAPTS)p/TEOS mixtures are porous and already have high surface areas in the xerogel stage after drying. In each series, calcination results in an increase of porosity, mainly by creation of micropores due to the removal of the organic groups [6].

Acknowledgment: This work was supported by the Fonds zur Forderung der wissenschaftlichen Forschung.

References

[l] N. Husing, U. Schubert, K. Misof, P. Fratzl, Chem. Muter. 1998, 10, 3024; N. Husing, U. Schubert, R. Mezei, P. Fratzl, B. Riegel, W. Kiefer, D. Kohler, W. Mader, Chem. Muter. 1999,II, 45 1. W. Urbaniak, U. Schubert, Liebigs Ann. Chem. 1991, 1221. W. Rupp, N. Husing, U. Schubert, J. Muter. Chem. 2002,12,2594.

Schwertfeger, C. Gorsmann, ACS Symp. Ser. 1996,622,366. C. Lembacher, U. Schubert, New J. Chem. 1998,22,721. G. Trimmel, U. Schubert, J. Non-Cryst. Solids 2001,296, 188.

[2] [3] [4] B. Breitscheidel, J. Zieder, U. Schubert, Chem. Muter. 1991, 3, 559. U. Schubert, F.

[5] [6]

Interaction of Silica Particles in a Model Rubber System: The Role of Silane Surface Treatments

Antoine Guillet

Crompton SA, 7, Rue du Pr6 Bouvier, 1217 Meyrin, Switzerland Tel: +41 22 989 2241 - Fax: +41 22 785 1140

E-mail: antoine-guillet @club-internet.fr

Jacques Persello

University of Franche-Cornt6,25030 Besanqon Cedex, France E-mail: rei @ net.asi.fr

Jean- Claude Morawski

Ressources en Innovation, 49, rue Edouard Herriot, 69002 Lyon, France Tel: +33 4 78 37 54 69 -Fax: +33 4 78 37 54 29

E-mail: rei@net.asi.fr

Keywords: silica, structure, rheology, silane

Summary: The nature of the silica surface has a strong influence on the rheology of filled rubber compounds. By reacting silica with silane coupling agents its surface properties can be controlled. A simple rubber model was used to understand the influence of organosilanes used as coupling agents in controlling the dispersibility and agglomeration of silica. The model system consists of monodisperse non-aggregated silica particles, silanes and squalene. Rheological properties were measured using steady shear and creep techniques at high filler volume fraction. Particular emphasis was placed on observing the transition between liquid-like and solid-like behavior at the maximum packing volume. Results obtained on the rubber model indicate that it is possible to measure and observe the silica agglomeration process at very low shear. Under these conditions the rheology and thixotropic behavior of silane-treated silica are highly depending on surface treatment. The method particularly allows understanding the widely different processing behavior of various organofunctional silanes. The observed trends are consistent with those observed in rubber compounds. Small-angle neutron scattering (SANS) techniques were used to study the short-range arrangement of silica aggregates and the structure factor of the concentrated silica-squalene system. Light diffraction was used to measure the occurrence of larger agglomerates. Finally it is shown how the silane structure affects both the dispersion of silica particles and the

Ovganosilicon Chemisty V Edited by N. Auner and J. Weis

Copyright 0 2003 WILEY-VCH Verlag GmbH & Co. KgaA, Weinheim

706 A . Guillet, J . Persello, J.-C. Morawski

agglomeration-deagglomeration process

Introduction

The properties of silica-filled rubber and composites depends primarily on the association of individual primary silica particles in the final material. The association of primary particles is believed to be a reversible process responsible for the physical properties of the filled material at low strain levels. This process is furthermore believed to an essential mechanism explaining the improvement of dynamic properties of filled rubber, as opposed to carbon black, silica particles are characterized by a strongly polar surface able to generate a strong interaction. The interaction is reversible and leads to reduced hysteresis.

A negative result of the strong polarity of silica surfaces is the high viscosity developed during compounding. However, it may be used to measure the interaction of silica particles. We have used rheology and silane treatments to understand and control silica particle interactions.

Experiment

The model consists of a concentrated suspension of monodisperse silica in squalene. The silica was prepared from sodium silicate [ I ] and treated as described by Philipse and Vrij [2]. Its particle size was about SO nm. In the preparation of monodisperse silica samples, the silane was added in the squalene solvent phase. To assess the completeness of the silane treatment, adsorption was followed by recording the compacted silica density against silane addition. This provided evidence that the silane/silica reaction had reached completion at silane concentrations of about 2 %. However, silica samples were treated with 8 % silane throughout the study.

The surface-modified silica samples were separated from the squalene medium by centrifugation at 15000 rpm. They were placed in a cone-plane rheometer and the shear rate was recorded at constant shear stress. The equipment used could record shear rates of lo-* to lo4 s-I.

Measurements by differential scanning calorimetry showed that, at the temperature used, no reaction could take place between the silanes and the squalene model. For the study of commercial silica, a narrower shear rate rangc was investigated, using a conventional cone-plane rheometer. In this case xylene was used as the liquid medium.

Several silane treatments were compared to evaluate the role of surface modifications in the formation of the silica structure in a rubber compound.

The shear curves show three regions:

A solid flow region where the material behaves like a dry compacted powder. Slow rearrangements allow for the packed spheres to yield without creating any large separation. A fracture region where the material breaks like a solid. The rupture plane is filled with liquid and the shear rate rises sharply without any increase in shear stress.

Interaction of Silica Particles in a Model Rubber System 707

A liquid flow region where the rheology follows a quasi-Newtonian regime.

It is proposed to consider the fracture shear stress as a measure of the strength of the filler

The compounding trials were based on a rubber formulation described in a patent for low rolling structure and hence of particle-particle interactions.

resistance tire treads [3, 41.

Silanes

The following silanes were compared in this study: triethoxysilylpropylmercap (SiSH); bis(triethoxysilylpropyltetrasu1fane) (SiS4); bis(triethoxysilylpropyldisu1fane) (SiS2); Triethoxy- silyloctadecane (Sic 18); triethoxysilylmethane (Sic). SiSH and SiS4 are commercial products of Crompton Corporation, A-1891 and A-1289 respectively. Sic& S ic and SiS2 were laboratory samples.

Results and Discussion

Figure 1 shows the results of the rheology study at a filler volume fraction @ of 0.11. Untreated silica is not represented because at such volume fractions the solid structure is too strong to be measured with the available equipment.

80.-

lo--

f 60.- B - 50.- b { 40.-

"i 10

@ = O . l l TI

-8 -6 -4 -2 0 2 4 log shear rate [s-'1

Fig. 1. Rheology study of the silica-squalene model.

Sic18 SiSH SiS2

The disulfide silane (SiS2) shows the strongest silica-silica interaction, followed by the tetrasulfide (SiS4) and mercapto (SiSH) silanes. The alkylsilanes (Sic and SiC18) provide very low interaction. The silica-silica interactions are due to polar and hydrogen-bonding forces. Adsorption of molecules with surface-active properties results in modifications of these forces. The silane molecules chemisorb through the SiOR moieties and the structure of the organic part of the

708 A. Guillet, J. Persello, J.-C. Morawski

molecule imparts properties related to this organic structure. The low-polarity methylsilane S ic and octadecylsilane S ic 18 provide, as expected, the silica

particles with very low surface polarity resulting in low viscosity, even at low shear rate. The slurry behaves as a liquid over the complete shear rate range.

On the contrary, the mercaptosilane SiSH makes it possible to form a strong solid structure characterized by a high shear stress for the solid-liquid transition. This is consistent with the strong polarity and hydrogen-bonding ability of the mercapto group.

Surprisingly the disulfide silane SiS2 and the tetrasulfide silane SiS4 give rise to a strong solid structure, despite moderate polarity and hydrogen-bonding ability. (Note: in a previous publication [4], we reported results obtained with a nonrefined disulfide silane product. The results showed very low silica-silica interactions, due to foreign matter. The current study was made using distilled disulfide silane.)

It has been suggested that polysulfide silanes may be adsorbed onto vicinal OH groups of the silica surface through both silane groups of the same molecule [ 5 ] . This mechanism would lead to a very low polarity due to the exposed sulfur chain. Also, the polarity should decrease with increasing length of the sulfur chain. A rough representation of this concept is shown in Fig. 2.

on I

OH- Si - O H

s - 5 - s - s I-5-5-5

i 2- -Si 2 1 ' /

Fig. 2. Possible bis-silane attachment to silica surface.

Using a molecular design program (Alchemy), we could sketch some model structures to visualize adsorbed molecules. Figures 3 and 4 show SiS2 and SiS4 bis-silanes with one siloxane group attached to a polysilicic acid oligomer and one silanol group protruding, unreacted.

Several assumptions support this model:

The rate-determining step is the hydrolysis reaction. Condensation with the surface takes place immediately after hydrolysis. The hydrolysis rate for the second alkoxy group on the same silicon is five times lower than for the first alkoxy. After one alkoxy group is hydrolyzed the most likely candidate is therefore one of the three alkoxy groups on the second silyl group. If the second silanol group is restricted in finding an adsorption site, it may protrude in a fairly stable condition.

Interaction of Silica Particles in a Model Rubber System 709

~~ ~

Property A A1589-4.AL2: SI

Brutto formula

Molecular weight

Volume [A3]

Ovality

Surface [A*]

Dipole [D]

%C

%H

%O

%Si

%S

CioH33018Si6Sz

674.006

495.523

1.92

581.411

3.467

17.82

4.93

42.73

25

9.52

Fig. 3. Disulfide silane SiS2 adsorbed through one Si-0-Si link to silicic acid oligomer with one hydroxyl group

protruding out unreacted.

Property A1289-4.AL2: S3

Brutto formula CloH35016Si6S4

Molecular weight 708.155

Volume [A3] 533.895

Ovality 1.982

Surface [AZ] 630.665

Dipole [D] 6.166

%C 16.96

%H 4.98

%O 36.15

%Si 23.8

%S 18.11

Fig. 4. Tetrasulfide silane SiS4 adsorbed through one Si-0-Si link to silicic acid oligomer with one hydroxyl group

protruding unreacted.

710 A. Guillet, J. Persello, J.-C. Morawski

1000 -

n 100 -

0 10 m -

The models indicate that the reactivity of the second silanol group is restricted. The distance to the next “surface hydroxyl” is large and would imply some constraint (about 5 kcallmol). The calculated dipoles give an indication of the character of the silylorganic residue that protrudes, unreacted. When it is hydrolyzed the dipole is slightly increased. However, the dominant parameter is the number of sulfur atoms, as suggested by the difference between SiS2 and SiS4. The molecular volume of the disulfide structures is of course significantly smaller than that of the tetrasulfide.

Based on these models one would expect disulfide silanes, and moreover tetrasulfide silanes to build a hydrophobic surface, providing excellent dispersibility and low silica-silica interaction (resulting eventually in a lower Payne effect). The rheology study suggests the contrary.

Small-angle neutron scattering (SANS) spectra (Fig. 5) were recorded at low wavenumbers to characterize small particles and the possible occurrence of aggregates.

Sic

A Sic1 8

SiSH

4

++

a RC2 exp

mSiS4

6 Untreated 0 7 10000

- - SiS2 1

0.001 0.01 0.1 octadecyichlorosilane

log Q [k’]

Fig. 5. S A N S spectra of silane-treated silica samples at high concentration.

At high wavenumbers (0.01 to 0.1 A-’) we observed the elemental particles. They are not affected by the silane treatment, as evidenced by the superposing traces. At intermediate wavenumbers, the plateau indicates that there is no aggregation. At low wavenumbers (0.001 to O.OIA-’) we observed the formation of agglomerates. Again all curves superpose. The agglomeration process is similar for all silanes.

Light diffraction makes it possible to measure larger agglomerates. In Fig. 6 we compare disulfide and tetrasulfide silanes.

Here again there is no major difference; we can only conclude from the strong absorption at low wavenumbers that there is significant agglomeration, excluding however the formation of strongly bonded particles.

Mechanical properties are summarized for di- and tetrasulfide bis-silanes in Table 1. We observed a significant difference in viscosity, with SiS2 showing a lower Mooney viscosity than SiS4. However, the storage modulus G‘ that is representative of the Payne effect is inverted: SiS2

Interaction of Silica Particles in a Model Rubber System 711

provides a stronger Payne effect than SiS4.

18000 16000

3 14000 'E 12000

8000 5 6000

4000 2000

0

- ? 10000 x SiS4

0.000001 0.0001 0.01 1

log Q [A-;']

Fig. 6. Light-scattering spectra of silane-treated silica after dilution.

Table 1. Mechanical properties of rubber compounds with S2 and S4 silanes.["]

Silane None SiS4 SiS2 SiS2

A1289 -

A1589 -

Additional S to S2 -

Mooney viscosity at 100 "C

ML14 124

Mooney scorch at 135 "C

Mv

MSl+, t3 [min] 9.5

MSl+, tI8 [min] 11.0

ODR at 149 "C, 1' arc, 30-minute timer

ML [in.-lb.] 23.8

MH [in.-lb.] 39.4

t S l [mini 5.4

t90 10.5

Physical properties, cured ts0 at 149 "C

Hardness, Shore A 64

Elongation [ %] 830

100% modulus [psi] 170

200% modulus [psi] 280

300% modulus [psi] 470

Tensile strength [psi] 2170

DIN abrasion [ m 3 ] 160

7.0

-

-

84

52

6.8

8.7

10.4

28.8

5.0

16.0

57

400

280

830

1830

2820

114

-

6.2

-

71

23

9.5

11.8

8.2

26.6

5.6

15.8

58

540

230

560

1220

3130

101

-

6.2

-

70

34

8.8

10.8

8.2

31.8

5.3

16.1

60

430

280

800

1740

3070

89

712 A. Guillet, J. Persello, J.-C. Morawski

Heat build-up at 212 OF, 17.5 % compression, 143 psi static load

AT [ O F ] +66 +26

Permanent set [%I delam. 6 min. 13.1

Dynamic properties at 0.15% strain, lOHz, torsion mode

G' at o "c, x lo7 26.8 4.94

G'at 60 "C, x lo7 12.7 2.40

at o "c, x 10' 2.87 1.03

G"at 60 "C, x lo6 11.2 2.43

tan 6 at 0 "C 0. I070 0.2086

tan 6 at 60 "C 0.0876 0.1012

ratio, 0 "C/60 "C 1.22 2.06

+32

14.9

9.75

3.63

1.99

4.39

0.2039

0.1211

1.68

+22

8.5

9.88

3.86

2.04

3.78

0.2066

0.0981

2.1 1

[a] Formulation: 75 Solflex 1216 sSBR, 25 Budene 1207 BR, 80 Zeosil 1165MP, 32.5 Sundex 3125 process oil, 2.5 Kadox 720C zinc oxide, 1 .O Industrene R stearic acid, 2.0 6PPD antiozonant, 1.5 M4067 microwax, 3.0 N330 carbon black, 1.4 Rubbermakers sulfur 104, 1.7 CBS, 2.0 DPG, silane as shown.

If we consider that the amount of sulfur is the most important factor governing the polar character of the surface layer, we should have observed both a higher viscosity and a stronger Payne effect with SiS2. However, if the adsorbed silane concentration is the dominant factor, then SiS2 leads to a higher surface population and enhanced silane effect and the results are consistent.

Conclusion

We have seen that using a simple rheology model it is possible to measure the changes in physical characters of the silica surface during the silanization treatment.

We observed that while alkylsilanes provide very important reductions in the filler-filler attraction forces, conventional sulfur silanes do not. Polysulfide silanes and mercaptosilanes are about equivalent. However, very good filler dispersion is observed in all cases and the filler-filler interactions are essentially reversible.

These observations are not consistent with the occurrence of a looped, nonpolar, hydrophobic layer as implied by the complete reaction of both silyl groups with the silica, an adsorption mechanism proposed earlier. They are not consistent either with the formation of chemical bonds between filler particles.

To unify the different observations and reports on surface properties of silica during the rubber compounding process, a new adsorption mechanism is necessary.

The proposed mechanism is the formation of a silane ester layer extending out of the silica surface (Fig. 7) which converts into a silanol layer upon hydrolysis (Fig. 8).

This layer is first hydrophobic and helps to disperse the silica, later it becomes hydrophilic and eventually it is able to react into a strongly adherent layer. Depending on the processing conditions, low viscosity and excellent dispersibility may be achieved unless premature hydrolysis occurred. In

Interaction of Silica Particles in a Model Rubber System 713

all cases a strong filler network is built through the conversion of alkoxy to silanol groups, leading to a significant Payne effect. Both steps are enhanced when using disulfide silanes, because their smaller size allows for better surface coverage in comparison to tetrasulfide silanes. This mechanism also explains why processing conditions are so important for the final properties of silica-filled rubber.

Property A1289-5.AL2: S3

Brutto formula

Molecular weight

Volume [.A3]

Ovality

Surface [A'] Dipole [D]

%C

%H

%O

%Si

%S

CioH35016Si6S4

647.02

478.573

1.912

565.675

6.106

16.7 1

4.67

37.09

21.7

19.82

Fig. 7. Formation of a nonpolar layer through reaction of a SiS4 silane with a single alkoxysilane group.

Property A1289-5.AL2: S3

Brutto Formula CioH35016Si6S4

Molecular weight 647.02

Volume [.A3] 478.573

Ovality 1.912

Surface [A'] 565.675

Dipole [D] 6.106

%C 16.71

%H 4.67

%O

%Si

37.09

21.7

%S 19.82

Fig. 8. Conversion of the nonpolar layer to a polar hydrophilic layer through SiS4 silane hydrolysis.

714 A. Guillet, J. Persello, J.-C. Morawski

The proposed mechanism renders obsolete the advantage of bis-silanes for the treatment of silica in automotive tires. It suggests that only 50 % of the silane groups are effectively grafted onto the silica surface, necessitating high levels of addition. It is consistent with the observation that monofunctional silanes such as mercaptosilanes are more efficient than bis-silanes as their optimum loading is only 3 % against 8 %.

References [I] Ralph K. ller, The Chemistry ofSilica, John Wiley, NY, 1979. [2] A. P. Philipse, A. Vrij, J. Colloid Inte$ace Sci. 1989,128 ( I ) , 121-136. [3] R. Rauline, USP 5227425 to Michelin & Cie, 1993. [4] A. Guillet, J. C. Morawski, J Persello, Paper 58, 15gh ACS Rubber Div. Meeting, Cincinnati

[5] S. Wolff, Paper 13, Tyretech 93 Basel, Switzerland, Oct. 1993. OH, Oct. 2000.

The Structure of a PDMS Layer Grafted onto a Silica Surface Studied

by Means of DSC and Solid-state NMR

V. M. Litvinov

DSM Research, P.O.Box 18,6160 MD Geleen, the Netherlands

H. Barthel; J. Weis

Wacker-Chemie GmbH, Werk Burghausen, D-84480 Burghausen, Germany E-mail: herbert.barthel@ wacker.com

Keywords: silica, PDMS, adsorption, solid-state NMR, T2 relaxation

Summary: The structure of a poly(dimethylsi1oxane) (PDMS) layer at the surface of hydrophilic silica has been studied by means of DSC, proton NMR T2 relaxation experiments, and ‘H and 29Si NMR spectroscopy. The samples were prepared using 1) the adsorption of PDMS from a PDMS solution onto the silica surface followed by a thermal treatment and 2) mechanical mixing of PDMS with the silica followed by the separation of bound rubber. It was shown that these procedures caused the formation of chain loops chemically attached to the silica surface at both chain ends. The average length of the grafted chains varied from about four to eight Si-0 bonds. The experiments provided information on the mobility of the grafted chains, which is related to the structure of the grafted layer. The grafted PDMS layer was found to consist of immobilized chain segments at the PDMS-silica interface and mobile chain portions outside the interface. The chain immobilization at the interface caused a substantial decrease in the heat capacity at Tg and suppressed crystallinity of the grafted PDMS. The interface fraction increased proportionally with a decreasing chain length. About four dimethylsiloxane pendant chain units next to the grafting site were immobilized due to chain anchoring to the silica surface. A small fraction of -SiO(CH&- chain units were immobilized as a result of physical adsorption at the silica surface. The fraction of physically adsorbed chain units appears to be proportional to the number of residual silanol groups at the silica surface. The mobility of the chain portions outside the interface was found to differ significantly in the various samples studied and to increase with an increasing average length of the grafted chains. The NMR method allowed us to make a distinction between a dense “brush-like’’ structure of the grafted layer containing grafted chains of a fairly uniform length and a layer containing a significant fraction of long chain loops outside the densely grafted layer. It was found that the

Ovganosilicon Chemisty V Edited by N. Auner and J. Weis

Copyright 0 2003 WILEY-VCH Verlag GmbH & Co. KgaA, Weinheim

716 V. M. Litvinov, H. Barthel, J. Weis

structure of the grafted layer is to a great extent dependent on the grafting procedure employed.

Introduction

Chain grafting is widely used to improve the miscibility of components in polymer mixtures and blends. Understanding the effect of chain grafting on the structure of the interfacial layer is of importance in many polymeric applications. Polymer brushes, densely grafted polymer chains, are the systems that have been studied the most both experimentally and theoretically [l-121. The majority of the polymer brush studies carried out so far have focused on long chains grafted to the surface of solids at their ends. Several structural characteristics of the grafted layers have been analyzed, i.e. the average thickness of the grafted layer, the monomer density profiles, the distribution of chain ends, and the effect of solvents and the geometry of the solid surface on the aforementioned characteristics. Less information is available on short grafted polymeric chains. The studies focusing on these issues have concentrated mainly on modified types of porous silica used as supports in high-performance liquid chromatography or as solid-supported catalysts [ 13-1 61.

Fumed silicas find wide application in the reinforcement of silicon rubbers and the thickening of polymeric liquids. Grafting polymer chains onto the surface of fumed silicas allows modification of silica-silica and silica-polymer interactions [ 171. Variations in the grafting density, its heterogeneity, and the average length of the grafted chains have a significant effect on silica-silica interactions and interactions of silica particles with the matrix material. It follows that knowledge of the molecular structure of the grafted layer is needed for a complete understanding of the molecular mechanisms responsible for the reinforcing and thickening effects. Solid-state NMR spectroscopy and NMR relaxation studies are among the most informative methods to obtain information on the chemistry of grafting, chain order, chain conformation, and molecular mobility of short grafted chains [10-16, 18, 191.

The molecular mobility of high-molecular-mass poly(dimethy1 siloxane) (PDMS) chains grafted at one end, and of PDMS that is physically adsorbed at the silica surface, has previously been studied with the aid of various solid-state NMR techniques [lo-12, 18, 191. It was found that long PDMS chains which are chemically attached to the silica surface undergo uniaxial reorientations around the direction normal to the solid-polymer interface [ 10-121. Physically adsorbed PDMS chains form an immobilized layer at the silica surface [18, 191. These studies showed that NMR relaxation methods are highly sensitive to heterogeneous mobility of PDMS chains at a silica surface. Since molecular mobility is closely coupled to chain length, grafting density, and chain length distribution, NMR relaxation methods can provide valuable information on the structure of grafted layers.

The present study focuses on the structure of PDMS layers on the surface of fumed silicas. Two different methods were used for sample preparation. One method was based on the adsorption of PDMS from a PDMS solution onto the silica surface followed by a thermal treatment. The other method was based on mechanical mixing of PDMS with silica followed by the separation of bound

The Structure of a PDMS Layer Grafted onto a Silica Surface 717

rubber. The effects of the amount of PDMS on the silica surface and specifically of the preparation method on the structure of the PDMS layer have been analyzed. Several techniques were used in this study: 1) DSC experiments provided information on glass transition and crystallinity, 2) 'H and 29Si NMR spectroscopy were used to analyze the chemical structure of the samples, and 3) low- resolution 'H T2 relaxation experiments were performed to analyze the PDMS-silica interface, the chain length distribution of PDMS loops at the silica surface, and the effect of physical adsorption on the immobilization of PDMS at the silica surface.

Samples Studied

Preparation of Silylated Silicas

Surface modification was applied to hydrophilic fumed silica with a BET surface area of 300 m2/g. Controlled surface coverage was realized using two different techniques.

Method I is based on the adsorption of PDMS from a PDMS solution onto the silica surface followed by a thermal treatment. An aerosol of 10 wt% PDMS solution in tetrahydrofuran (THF) was added to the silica fluidized by mechanical stirring. For this method, trimethylsiloxy-end- capped PDMS fluid with the gross formula (CH~)~S~O[(CH~)~S~O]IO&(CH~)~ was used. The viscosity of the PDMS fluid was 100 P a s at 25 "C. Several cycles of aerosol addition and subsequent drying of the mixture from THF were performed. The drying was effected at 120 "C under a stream of nitrogen. The PDMS content after the last cycle was about 30 wt%. The silica particles coated with the PDMS layer were subsequently heated at 300 "C for 4 h.

Method 2 is based on mechanical mixing of PDMS with silica, followed by the separation of bound rubber. The conditions were similar to those employed during the processing of a mixture of high-temperature-curing silicon rubbers. The initial mixture consisted of 60 wt% trimethylsiloxy- end-capped PDMS rubber, 10 wt% OH-terminated PDMS fluid and 30 wt% silica. The PDMS rubber and the fluid had the gross formulae (CH~)~S~O[(CH~)~S~O]~,WS~(CH~)~ and HO[(CH3)$30]15H, respectively. Their viscosities at 25 "C were lo4 Pa s and 0.025 Pa s, respectively. The mixture was kneaded for 0.5-2 h at temperatures ranging from 50 "C to 150 "C (see Table 1). The rubbers were stored for approximately two months. Afterwards, the mixtures were dispersed in THF for 48 h using a magnetic stirrer. The dispersion was then kept without stirring for 24 h, which resulted in sedimentation of silica aggregates coated with a PDMS layer. The supernatant, consisting of PDMS solution and a small amount of silica particles, was removed by means of decantation. A fresh portion of THF was added a few times followed by stirring and decantation until the concentration of PDMS in the supernatant THF solution was below 1 wt%. The total amount of silica in the PDMS/silica mixture (bound rubber) obtained was more than 90 %, relative to the concentration in the initial mixture.

The following experiments were performed to characterize silylated silicas (see Table 1). The concentration of PDMS grafted onto the silica surface was inferred from the carbon content

(% C) by determining the COz content of combustion products using IR spectroscopy.

718 V. M. Litvinov, H. Barthel, J. Weis

The degree of silylation of the silica surface was determined by calculating the ratio (as % SiOH) of the density of silanol groups at the silica surface after and before silylation.

The density of silanol groups was measured by means of an acid-base titration using a solution of NaOH in a watedmethanol mixture (5050 % w/w), following a procedure described by Sears [20]. It should be noted that only silanol groups on the silica surface are measured with this method.

The average number of siloxane bonds, N,, in the grafted chains was determined from the PDMS content and the number of reacted SiOH groups. A value of Nt corresponds to the total length of chains anchored at one end, if present, or the half-length of chain loops and chains attached to neighboring silica particles.

The structural characteristics of the silylated silicas are presented in Table 1.

Table 1. Structural characteristics of silylated silicas.

Sample 1A 2A 2B 2c 2D

Preparation method 1 2 2 2 2

Reaction temperature ["C] 300 150 100 150 50

Reaction time [h] 4 0.5 2 0.5 2

Carbon content [wt%] 9.62 10.25 14.2 11.4 8.6

PDMS content [wt%]["I 29.7 31.6 44.0 35.2 26.5

Residual SiOH [%Iu 5.0 14.2 12.7 12.3 10.0

Average chain length, Nt[cl 4.2 5.5 7.1 6.2 4.5

[a] Based on the carbon content. [b] With respect to the SiOH content of an initial hydrophilic silica, which is about 1.76 SiOH per nm2. [c] The average number of siloxane bonds per grafted site, Nt, was inferred from the PDMS content and the number of reacted SiOH groups. The Nt value corresponds to the total length of the chains grafted at one end, if present, or the half-length of chain loops and chains attached to neighboring silica particles.

Sample Preparation for NMR Experiments

About 0.37 g of each sample was placed into an 18 mm-diameter NMR tube with a length of 180 mm. The NMR experiments were performed using a) the samples as a whole, b) the samples swollen in C2C4 and c) the samples swollen in CzCl4 in an ammonia atmosphere. Swollen samples formed turbid gels or suspensions. The volume fraction of PDMS, V,, in swollen samples was 5.15 vol.%. The V, value was calculated using the specific density of PDMS of 0.98 g/cm3 and C2C14 of 1.623 g/cm3. The volume fraction of SiOz was subtracted to determine the V, value. In order to prevent evaporation of the solvent, a Teflon plug was inserted so that the bottom was just above the sample. The swollen samples were stored for one day before the experiments. Swelling in the presence of ammonia was performed in uncovered NMR tubes containing swollen samples. The tubes were placed in a flask containing a 25 wt% ammonia solution in water. The flask was then sealed with a stopper and the samples were stored under ammonia for one day prior to the experiments.

The Structure of a PDMS Layer Grafted onto a Silica S u ~ a c e 719

The DSC experiments were performed using a Perkin-Elmer Pyris-1 equipped with a cryofill. The purge gas was a mixture of 10 % helium and 90 % neon at a flow rate of 25 mumin. The sample pans, which were supplied by v.d. Boel Eng. NL, had a volume of 40pL. The temperature calibration of the analyzer was performed with the aid of adamantane, water, indium, and lead. The sample weight was in the range of 10 to 20 mg. The samples were kept for 20 min at -150 "C before being heated at a rate of 20 "Clmin.

Solid-state NMR Experiments and Data Analysis

Equipment

The proton NMR T2 relaxation experiments were performed using a Bruker Minispec PC-120 spectrometer. This spectrometer operates at a proton resonance frequency of 20 MHz. The length of the 90" and 180" pulses and the dead time were 2.5 ps, 5.2 ps and 10 ps, respectively. The T2 relaxation experiments were performed at 40 "C.

'H MAS and 29Si MAS NMR spectra were recorded with the aid of a Varian Inova-400 MHz wide-bore NMR spectrometer operating at a 'H and 29Si frequency of 400 MHz and 79.5 MHz, respectively, using a 7 mm CP/MAS probe. The 'H and 29Si 90" pulse widths were 1.5 ps and 5 ps, respectively. All the spectra were recorded at room temperature.

'H NMR Tz Relaxation Experiments and Data Analysis

T2 Relaxation Experiments

Two different pulse sequences were used to record the decay of the transverse magnetization (T2 decay) from both (semi)rigid and mobile fractions of the samples as previously described [21]. The solid-echo pulse sequence (SEPS), go", - tse - go", - t,, - [acquisition: Alf(t)], with tse = 20 ps was used to determine the T2 relaxation time and the proton content of the (semi)rigid fraction of the samples. The time after the first pulse t = (2t,, + t9&) was taken to be zero, with t90 being the duration of the 90" pulse. The Hahn-echo pulse sequence (HEPS), go", - tHe - 180", - tHe -

[acquisition], was used to record the slow decay of the mobile fraction of the samples. The second pulse in the HEPS inverts nuclear spins of mobile molecules only and an echo signal is formed with a maximum at time t = (2fHe + tl*o/2) after the first pulse, where t1g0 is the duration of 180" pulse. By varying the pulse spacing tHe in the HEPS from 0.035 ms to 67.5 ms, the amplitude of the transverse magnetization, A(t), is measured as a function of time t. The HEPS makes it possible to eliminate magnetic field and chemical shift inhomogeneities, and to accurately measure the T2 relaxation time of mobile materials. The analysis of data points measured separately by the SEPS and the HEPS showed that in the time interval ranging from 0.07 to 0.14 ms both experiments yielded similar values for the amplitude of the transverse magnetization. The total decay of the transverse magnetization was then reconstructed by combining the SEPS data in the time domain

720 V. M. Litvinov, H. Barthel, J. Weis

ranging from 0 to 0.14 ms with the HEPS data in the time interval from 0.07 to 135 ms. The time constants (T2 relaxation time), which are characteristic for different slopes in the

magnetization decay curve, were obtained by performing a least-squares fit of the T2 decay using a linear combination of two or three exponential functions. The results obtained with these functions were statistically relevant, unlike those obtained with several other functions used. The relative fraction of the relaxation components, designated in the text by %TzindeX, represents the fraction of hydrogen in chain portions with different molecular mobilities that are responsible for the relaxation components.

The error in the relaxation parameters consisted of a) experimental errors (about 2 %), b) an error ascribable to the chosen fitting function (estimated to be about 5 %) and c) uncertainties of the fitting (about 0.5 %). Repeated experiments using the same sample showed that the standard deviation of the results was smaller than 2-3 %.

'H and 29Si NMR Spectroscopy

All the 'H and 29Si spectra were recorded using magic-angle spinning (MAS) with a MAS frequency of 7.6 kHz. Additionally, high-power proton decoupling was used to record the 29Si spectra. The 29Si spectra were referenced to the trimethylsilyl resonance of the cubic octamer silicic acid trimethylsilyl ester (@&) observed at 11.7 ppm relative to tetramethylsilane (TMS). 'H resonances were referred to TMS. Recycle delay times of 4 and 60 s were used in the 'H and 29Si experiments, respectively. 1000 scans were accumulated for each spectrum.

Results and Discussion

Glass Temperature and Crystallinity as Determined by DSC

DSC yields information on overall chain immobilization via the glass transition temperature, Tg, and a step in the heat capacity at Tg (ACJ. It is well known that PDMS shows a glass transition at about -123 "C and at higher temperatures phase transitions caused by crystallization and melting. The thermal behavior of PDMS is compared with that of silylated silicas in Fig. 1 (see Table 2). All the silica samples showed a minor glass transition from about -118 "C to -123 "C. PDMS chains grafted onto a silica surface showed ACp to be 13 to 50 times smaller than that of pure PDMS. This suggests substantial immobilization of a large fraction of the PDMS chains at the silica surface. It was thought that a gradual change in ACp above -60 "C could be related to a second glass transition of the PDMS-silica interface. It will be shown below that an immobilized PDMS layer formed at the silica surface and that chain mobility in this layer was strongly constrained.

Pure PDMS reveals an exothermic peak of cold crystallization at -85 "C and a melting endotherm with a maximum at -36 "C. The crystallization behavior of PDMS chains at the silica surface is related to topological constraints due to chain anchoring to the silica surface, which hamper crystallization. The only silylated silica that showed crystallinity in our examples was sample 2B. In the series of samples studied the amount of PDMS in silica was largest in 2B. The

The Structure of a PDMS Layer Grafted onto a Silica S u ~ a c e 721

i+ r 2 P.

heat of melting of this sample was about ten times smaller than that of pure PDMS. This means that just a small fraction of PDMS chain units in this sample could form a crystalline phase. Apparently, only fairly long PDMS chains, which are virtually not constrained by chain anchoring, are capable of forming crystallites. The low melting temperature of PDMS crystals in silica 2B suggests that the crystals were smaller than those in pure PDMS.

c) 2D

Tmpanture, *C

DSC continuous specific heat capacity curves of a) pure PDMS; b) silylated silica 2B; c) 2A, 2C and 2D. Fig. 1.

722 V. M. Litvinov, H. Barthel, J. Weis

Table 2. The thermal behavior of pure PDMS and silylated silicas.

PDMS -123 0.40 0.40 -36 27.2 27.2

2A -123 0.03 0.0095 - - -

2B -121 0.07 0.031 4 8 5.4 2.4

2 c -123 0.05 0.018 - -

2D -118 0.03 0.0080 - - -

[a] Temperature at which 50% increase in ACp has occurred. [b] Per gram of the sample. [c] Per gram of PDMS.

Chemical Structure of Silylated Silicas Studied by NMR Spectroscopy

The fraction of physically adsorbed PDMS chain units in the samples studied is small, as will be shown on p. 729. That fact that the samples studied contain only a small fraction of residual silanol groups on the silica surface (see Table 1) suggests that PDMS chains react with silanol groups at the silica surface. In order to determine the chemical structure of silylated silicas, 'H and 29Si NMR spectra were recorded. The 29Si NMR spectrum obtained for silica 2A is shown as an example in Fig. 2. The spectra obtained for the other samples were similar. The narrow resonance at -22.2 ppm and broad resonance at about -110 ppm are attributable to -SiO(CH3)2- chain units and Si02, respectively. A broad shoulder at a lower field observed for the resonance at -22.2 ppm may be ascribable to the effect of the Si-0-Si bond angle or bond distance distribution of -SiO(CH3)2- chain unit(s) adjacent to the silica surface. It is well known that the 29Si chemical shift is strongly affected by the geometry of the Si-0-Si bond [22, 231. The samples were found to contain only a small fraction of -OSi(CH3)2-OH pendant groups, because no intensive 29Si resonance corresponding to this group was observed at about -12 ppm [24]. Thus, the grafted chains formed loops andor interconnected neighboring silica particles, and the fraction of dangling chain ends is small. This conclusion is supported by the 'H NMR spectra obtained for the samples (Fig. 3). The spectra reveal an intensive resonance for -SiO(CH&- chain units at about 0 ppm and broad resonances at about 3 ppm and 5-6 ppm, which are ascribable to residual silanol groups and adsorbed water molecules [25]. No signal was observed at 1.2 ppm corresponding to CH3 hydrogen of -OSi(CH3)2-OH pendant groups. Thus, the sample preparation causes depolymerization of the PDMS chain followed by its grafting to the silica surface. The mean length of the grafted chains in the samples varies from 4 to 8 Si-0 bonds.

The fact that the -SiO(CH&- chain units that were grafted to the silica surface and those in the middle portion of the chain show the same chemical shift in the 'H and 29Si spectra precluded the use of chemically selective NMR relaxation techniques for characterizing the PDMS-silica interfacial layer. We therefore performed nonselective low resolution NMR relaxation experiments to characterize the structure of the PDMS grafted layer.

The Structure of a PDMS Layer Grafed onto a Silica S u ~ a c e 723

Structure of the Grafted Layer Studied in 'H TZ Relaxation Experiments

Proton NMR relaxation is caused mainly by dipole-dipole interactions between proton spins and by the averaging of these interactions due to molecular mobility. The T2 relaxation is highly sensitive to chain dynamics involving large spatial-scale chain motion at temperatures above Tg Since chain motion is closely coupled to the length of the grafted chains, grafting density, and sterical constrains from surrounding chains, 'H T2 relaxation yields information on the structure of the grafted layer.

Fig. 2.

X50

7.. . . I - - . - , . . . . , . . . . , . . . - . , . , . . , . . . , . ,

-20 -10 -80 -80 -102 -120 ppm

29Si NMR MAS spectrum obtained for silylated silica 1A. The resonance at -48.4 ppm is ascribable to silica

atoms of the silicon nitride rotor.

2A

12 8 4 0 -4 PPm 12 a 4 0 -4w

Fig. 3. 'H NMR MAS spectra obtained for the silylated silicas 1A and 2A.

The decay of the transverse magnetization of two silica samples is shown in Fig. 4. The T2 decay for the samples as a whole consists of two distinct components with characteristic decay times, T2, of 0.08 ms and 1-3 ms (Table 3). As mentioned above, the T2 values are related to molecular mobility. The larger the amplitude and/or the frequency of chain motions, the longer the T2 will be. The large difference in the decay time of these components suggests that these relaxation

724 V. M. Litvinov, H. Barthel, J. Weis

1 ,oo

0,75 n 0

0,50 % z

0,25

0,oo 0 50 100

time, ms 1,oo I I I

Fig.

0 50 100 time, ms

"1 0 i 2

Nme, me

0.00 0 50

time, ms

The decay of the transverse magnetization (points) measured at 40 "C a) for silicas 2B an 2D as a whole; b)

for the samples swollen in C2CI4; c) for those swollen in CzC14 in the presence of NH3. The decay was

measured with the aid of the HEPS. The solid lines represent the result of a least-squares adjustment of the

decay with a linear combination of two/three exponential functions. The dotted lines show the individual

components. The initial part of the decay, measured with the aid of the SEPS, is shown in the insert on the

right-hand side of the plots.

The Structure of a PDMS Layer Grafed onto a Silica Surface 725

components are related to PDMS chain portions with significantly different local and large spatial- scale chain mobility. The Tz value of the component with the short decay time,T;" = 0.08 ms is in the range typical of polymers that are in the proximity of their Tg This component is ascribed to low-mobility chain portions adjacent to the silica surface - the semi-rigid PDMS-silica interface [18, 19, 261. Local chain mobility in the interface is strongly hindered due to chain anchoring to the silica surface and adsorption interactions of monomer units with the silica surface. It has previously been shown that T2in relaxation at the polymer-solid interface is not caused by the magnetic field gradients introduced by the filler particles [18, 211. This is also confirmed by the 'H and 29Si MAS spectra of the silica samples. If there were any magnetic field inhomogeneities at the silica surface, they would be averaged out by the MAS frequency of 7.6 kHz. The fact that the spectra reveal significant line broadening suggests immobilization of grafted chains.

726 V. M. Litvinov, H. Barthel, J. Weis

Q = SiO,, D = -O-Si(CH,),-

T p highly mobile

T,mO

mobile

T,'" interface

(3) Sample swollen in CzCL in NH3 atmosphere

Schematic representation of the structure of a grafted PDMS layer at the surface of silicas 2A-D: (1) samples

as a whole; (2) samples swollen in C2C14; (3) those swollen in C2Cb in an ammonia atmosphere. A solid

ellipse indicates adsorption bonds at the silica surface. The chain portions responsible for the T2 relaxation

components are indicated by dashed lines.

Fig. 5.

The component with the longer decay time, Tzmo = 1-3 ms, presumably derives from mobile chain portions outside the intei$ace. Since the statistical segment length of PDMS is about three -SiO(CH&- units [27], the rotational and translational chain mobility increases rapidly as the distance from the grafting site increases. This results in a sharp border between the interfacial and outer soft PDMS layer, as reflected by the distinct difference in Tzin and Tzm0 relaxations. Long spatial-scale chain mobility in the outer layer is to some extent hindered, as follows from the relatively small Tzm0 values that are comparable with those obtained for crosslinked elastomers of about 0.5-2 ms [28]. Constrained chain mobility outside the silica-PDMS interface is caused by anchoring of PDMS chains at both chain ends and probably by extension of the chain loops in the direction perpendicular to the silica surface reducing the number of possible chain conformations. A schematic representation of the grafted layer in relation to the relaxation parameters is shown in Fig. 5.

The third relaxation component with a characteristic time constant, T:"', of 30-100 ms was detected in the swollen samples 2A-D. The Tz values of this component are in the range typical of polymer solutions.29 Since all the extractable PDMS had been removed in the exhaustive extraction of silylated silicas, this relaxation component can be ascribed to highly mobile chain portions of long chain loops in the space above the densely grafted layer and/or in areas of low grafting density. The relaxation parameters obtained for the samples swollen without and in the presence of ammonia (see Table 3) differ substantially. The fraction of highly mobile chain units and values of T:"' of ammonia-treated samples are larger than the original ones, corresponding to an increase in

The Structure of a PDMS Layer Grafed onto a Silica Surface 727

the mobility of grafted chains. Since ammonia is preferentially adsorbed on the silica surface [18], its molecules cause desorption of physically adsorbed PDMS chains, resulting in the observed increase in chain mobility. Moreover, these changes suggest that areas with a relatively low grafting density which are available for physical adsorption of PDMS chains were present in silicas 2A-D.

The density of chain units at the PDMS-silica interface, in the densely grafted layer covering the interface, and in long-chain portions in the space above the densely grafted layer can be inferred from the values of %Tzi", %T2"'O and %T2hm, respectively (see Table 3).

Table 3. The T2 values [ps] and the relative fraction of the relaxation components, %T2, of the samples as a whole

(bulk), samples swollen in C2C14 (sw) and samples swollen in C2CL in an ammonia atmosphere (sw+NH,).

Silica 1A

bulk

sw

SW+NH~

Silica 2A

bulk

sw

SW+NHS

Silica 2B

bulk

sw

SW+NH~

Silica 2C

bulk

sw

SW+NH~

Silica 2D

bulk

sw

SW+NH~

76

86

82

81

78

79

75

91

78

84

87

87

75

73

70

980

1030

1300

1960

780

960

3010

2570

1560

2340

1000

1180

1100

1300

930

-

-

-

-

31000

91000

-

96000

300000

-

43000

100000

-

-

100000

47

53

50

70

77

65

36

54

48

62

72

62

85

87

75

53

47

50

30

13

23

64

19

23

38

12

24

15

13

20

-

-

-

- -10

-12

-

27

29

-

16

15

-

-

4

Molecular Structure of the Grafted Layer

The structure of the grafted PDMS layer can be generally characterized with the aid of the following parameters: 1) molecular mobility (stiffness) in the interfacial layer; 2) the fraction of immobilized PDMS-silica interface and the interface thickness; 3) the fraction of immobilized interface deriving from chain anchoring by means of chemical bonding of chain ends to the silica

728 V. M. Litvinov, H. Barthel, J. Weis

sugace and from physical adsorption of PDMS at the silica sugace (the fraction of physically adsorbed chains is apparently related to the grafting density and its heterogeneity); 4) the number of monomer units along the chain in the interface which are immobilized due to chain anchoring to the silica surface and physical adsorption; 5) the chain length distribution of grafted chains; and 6 ) the fraction of long chain loops in the space above the densely grafted layer. The results of the NMR experiments in which these issues were studied are discussed below.

The PDMS/Silica Interface

Molecular mobility in the interfacial layer: It was found that chain mobility in the interfacial layer is not affected by the length of grafted chains and the presence of solvent, since the relaxation time measured for the PDMS-silica interface, T P , was almost the same in all the samples (see Table 3). This suggests that the chain immobilization is caused by a loss of configurational entropy due to chain anchoring to the silica surface and by excluded volume effects from the silica surface and neighboring chains. It is known that grafted chains are the most elongated close to the surface when the surface area per polymer chain is much smaller than the characteristic size of an isolated chain [30], which was the case in the samples studied. The small Tzin value indicates that the chain units in the interfacial layer were substantially immobilized. The chain mobility at the interface is comparable with the mobility of pure PDMS at temperatures slightly above Tg [ 181.

The total interface fraction and its thickness: The fractions of the interfaces (%Tzin) of the samples studied were found to differ substantially. The dependence of %Tzi" on the average length of PDMS chains is shown for samples swollen in C2CL before and after ammonia treatment in Figs. 6 and 7, respectively. In the case of the samples swollen in the presence of ammonia, %Tzin was slightly smaller, suggesting chain desorption. Thus, chain immobilization at the silica surface is caused not only by chemical attachment of PDMS chains to the silica surface but also by physical adsorption. The results shown in Figs. 6 and 7 clearly illustrate that the interfacial fraction of silicas 2A-D was strongly determined by the average length of the grafted chains and decreased with an increasing chain length. The slight increase in the interface content observed upon swelling (see Table 3) may have been caused by a reduction in the number of chain conformations in the interfacial layer due to some upward extension of chains from the silica surface in the swollen samples.

The thickness of the PDMS-silica interface was determined from the interface fraction and the specific surface of the hydrophilic silica, on the assumption of complete coverage of the silica surface and an interface with a uniform thickness. Taking into account the experimental error, the estimated average thickness of the interfaces of silicas 2A-D for all samples equals about 1 nm (see Table 4). The above results suggest a high degree of similarity in the structure of the interfaces in silicas 2A-D. If the grafting density was the only parameter determining the interfacial fraction, one would expect the largest interface fraction to be that of silica sample lA, since this sample has the highest grafting density, as can be inferred from the number of residual SiOH groups at the silica surface (Table 1). However, the interface fraction of silica 1A is significantly lower than the fractions of silicas 2A, 2C and 2D, as can be seen from Tables 3 and 4 and Figs. 6 and 7. This

The Structure of a PDMS Layer Grafed onto a Silica Su$ace 729

suggests that the structures of the interfaces in silicas 2A-D differ substantially from the structure of that of silica 1A. This difference could be attributable to a difference in chain length distribution. Apparently, the molecular mobility of the short loops, which spread over the silica surface, is hindered to a greater extent than that of the long ones. The fact that the interfacial fraction in silicas 2A-D is significantly larger than the fraction in silica 1A could be due to a larger fraction of short chain loops in silicas 2A-D.

Table 4. Estimated molecular characteristics of the grafted layers of silicas la'.

Molecular characteristics

Average thickness of the grafted layer in the samples as a whole, rg [ I I ~ ] ' ~ ]

Interface fraction in the samples as a whole, %TZh Average thickness of the interface in the sample as a whole, rin, [nm]lb'

Interface fraction ascribable to:

chemical grafting (fchem [%I)

physical adsorption (fd [%I)"'

Fraction of highly mobile portions of chain loops and/or tails,"' % T : ~

1A 2A 2B 2c 2D

1.4

41

-0.7

-94

-6

-0

1.6 2.1

70 36

-1.1 -1.0

-84 -89

-1 1 -16

-12 -29

1.9

62

-1.1

-86

-14

-15

1.2

85

-1.0

-86

-14

-4

[a] The NMR data and structural characteristics of the samples presented in Tables 1 and 3 were used in estimating the molecular characteristics. [b] In order to estimate the rg and ri. values, complete coverage of the silica surface and a uniform thickness of the grafted layer and the PDMS-silica interface were assumed. [c] The estimated absolute error in a value offd is about 2-3 %. [d] Obtained for samples swollen in C2Cl4 in an ammonia atmosphere.

The interfacialfraction caused by physical adsorption: The treatment of the swollen samples with ammonia resulted in a decrease in %T?. This decrease was of course caused by chain desorption due to preferential adsorption of ammonia at the silica surface [18]. The fraction of the interface resulting from physical adsorption ( f a d ) was determined from Eq. 1 on the basis of the percentages of %Tzin obtained for the swollen samples before (%Tp)b and after (%Tz~")~ ammonia treatment, suggesting that all the adsorption bonds were cleaved by ammonia.

Eq. 1.

The density of physically adsorbed PDMS chain portions seems to be proportional to the number of residual silanol groups at the silica surface, as can be seen in Fig. 8 and Tables 1 and 4.

Immobilization of siloxane chains due to chain anchoring and physical adsorption: The number of immobilized -SiO(CH3)2- chain units adjacent to the silica surface in silicas 2A-D was determined from the relationship between %T.in and the average number of Si-0 bonds in grafted

730 V. M. Litvinov, H. Barthel, J. Weis

chains, Nt (see Figs. 6 and 7). It can be easily shown that the fraction of immobilized chain units at the PDMS-silica interface, %T2in, is related to the length of the grafted chains by Eq. 2, where Nad is

Eq. 2.

the number of immobilized -SiO(CH+ chain units at the PDMS-silica interface and the Nt value corresponds to the total length of the chains grafted at one chain end, if present, or the half-length of chain loops and chains attached to neighboring silica particles. It is suggested that the length of the grafted chains does not affect Nad. Extrapolation of this relationship to %T? = 100% yielded the average number of immobilized chain units adjacent to the silica surface. The interface in the swollen samples 2A-D consisted of about 4.1M.2 -SiO(CH&- chain units, whose mobility was hindered to a greater extent than that of the distant units. The number of low-mobility chain units decreased slightly to about 3 .6a .2 upon ammonia adsorption. This was caused by desorption of chain portions from the silica surface and possibly by a decrease in chain-surface attractions.

100

8 80

r" 2 E

8 60 .-

40

Fig. 6. The fraction of PDMS-silica interface (%Tp)

as a function of Nt determined for silica

samples swollen in CZCl4. The line represents

the result of a linear regression analysis of the

data obtained for samples 2A-D using Eq. 2.

The correlation coefficient equals 0.90.

100

8 80

f U

S

8 60 .-

40 2 4 6 8

semi-rigid K O f i

1A \ 0

0 2 4 6

N, I

Fig. 7. The fraction of PDMS-silica interface (%Tp)

as a function of Nt determined for silica

samples swollen in CzC14 in the presence of

NH3. The line represents the result of a linear

regression analysis of the data obtained for

samples 2A-D using equation 2. The

correlation coefficient equals 0.89.

The interface thickness of silica IA was determined from the area of interface and the average length of the grafted chains. The obtained value of about two -SiO(CH&- chain units is smaller than the value obtained for silicas 2A-D. This agrees with the above suggestion of a smaller

The Structure of a PDMS Layer Grafted onto a Silica Sugace 731

fraction of short chain loops in silica 1A than in the other samples.

Chain Length Distribution of Grajled Chains

The mobility of chain portions outside the interface increases with an increasing average chain length, as can be inferred from the dependence of Tzm0 on the chain length observed in the samples as a whole (see Fig. 9). It is apparently caused by a decrease in chain elongation with an increasing distance from the surface [30]. With a decreasing chain length, Tzm0 would have reached the Tzin value at about 3 -SiO(CH&- bonds if a linear dependence of Tzm0 on the chain length is assumed. This value is in good agreement with the number of immobilized chain units in the interfacial layer estimated above (see Figs. 6 and 7).

Residual SOH, %

Fig. 8. The fraction of the interface caused by physical

adsorption (fad ) as a function of the fraction of

residual S O H groups in silylated silicas. The line

represents the result of a linear regression

analysis of the data: intercept = 2+4 %; slope =

0.9+0.3. The correlation coefficient and the

standard deviation equal 0.87 and 2.2,

respectively.

E n

8 bN

3-

2-

1 - 1A

I ' I ' I '

4 5 6 7 I

Fig. 9. The dependence of TZmo on Nt determined

for the samples as a whole. The line

represents the result of a linear regression

analysis of the data: intercept = -1 S 6 . 3 %;

slope = 0.606.5. The correlation

coefficient and the standard deviation equal

0.989 and 0.15, respectively.

Due to the swelling, the T2mo relaxation component is split into two components, described by Tzmo and T t m values (see Table 3). The Tzm0 of the swollen samples is similar to that of the bulk samples. This component seems to originate from the densely grafted layer outside the interface. The chain mobility in this layer was hardly affected by the swelling, which can be explained as follows. In the samples as a whole, translational mobility of these chain portions was already hindered because they had extended upwards from the silica surface and suffered steric hindrance from surrounding chains, which affected largely long-scale chain mobility. Swelling evidently does

732 V. M. Litvinov, H. Barthel, J. Weis ~ ~~ ~~~~~

not lead to a significant increase in the number of possible chain conformations in the densely grafted layer. The TZhm relaxation component originates from long chains outside the densely grafted layer and/or chains in areas of low grafting density. The mobility and the relative fraction of highly mobile chain portions increased with an increasing average length of grafted chains in silicas 2A-D, as can be seen in Figs. 10 and 11, respectively. The increase in mobility was apparently caused by a decrease in hindrance from anchoring sites with an increasing distance from the grafting site. The T2hm increased significantly upon adsorption of ammonia, whose molecules cleaved adsorption bonds between the silica surface and the PDMS chains [ 181. The desorbed chain portions apparently suffered negligible steric hindrance from grafting sites and other segments in comparison with the short grafted chains.

400 7 300 -

l! - 200- c bN

100-

4 5 6 7 8

Nt

Fig. 10. The dependence of T;”’ on Nt determined for

the swollen samples before (circles) and after

(triangles) adsorption of ammonia. The solid

lines have been included as guidelines.

I 300 -

200- r bN

100-

/_I 0 1

4 5 6 7

Nt I

Fig. 11. The dependence of %T:”’ on Nt determined for

samples swollen in the presence of ammonia.

The line represents the result of a linear

regression analysis of the data: intercept = -3li4 %; slope = 7.7k0.6. The correlation

coefficient and the standard deviation equal

0.994 and 1.4, respectively.

The difference in the mobility of the long grafted chains in the samples as a whole and the swollen samples may have been caused by a change in the chain conformation upon swelling. The long grafted chains in the samples as a whole were flattened at the surface of the grafted layer due to van der Waals interactions with other PDMS chains. These chains probably take a so-called “pancake” configuration [31]. Swelling caused the long grafted chains to extend and to stick out of the densely grafted layer, resulting in increased chain mobility. These changes in polymer density profiles can be regarded as “pancake-to-brush” conformational transitions [ 1,321.

The Structure of a PDMS Layer Grafted onto a Silica Sugace 733

The Structure of the Grafted Layer with Respect to Grafting Conditions

The structure of the grafted layer is largely determined by the grafting method employed, as can be inferred from Tables 1 and 4. It was found that a grafted layer in silica 1A has a dense structure, containing chains of a fairly uniform length in comparison with the other samples. Despite end- capping of the initial high-molecular-mass PDMS used in the sample preparation, the chains were broken into short fragments of about eight -SiO(CH+- units. Apparently, silanol groups at the silica surface caused depolymerization of the PDMS chain. According to the Nt value of silica lA, eight chain units correspond to thermodynamically the most stable length of chain loops. The silica samples prepared using method 2 showed a broad distribution of the length of grafted chains and a less uniform grafting density, suggesting incomplete chain depolymerization. The depolymerization was apparently favored by the long-term exposure of the silica-PDMS mixtures to a high temperature, as can be inferred from the structure of the grafted layer in silica 1A in comparison with that of samples 2A-D. It was rather surprising that silica 2D contained a large fraction of chemically attached PDMS chains despite the low temperature used during kneading (50 "C) and the large fraction of end-capped PDMS (86 wt%) used. Since a long kneading time was used during the preparation of this sample, this suggests that kneading causes significant mechanical breakdown of PDMS chains followed by the formation of chemical bonds between chain ends and silanol groups at the silica surface. Perhaps silanol groups at the silica surface facilitate PDMS depolymerization. Apparently, mechanical-chemical chain scission is determined by local shear forces during kneading [33], the viscosity of the mixture and the strength of silica-PDMS interactions, which are greatly affected by temperature [ 181.

Conclusions

Low-resolution 'H NMR T2 relaxation experiments provide valuable information on the structure of PDMS layers grafted onto silica surfaces. The method presents several advantages for the characterization of the structure of the grafted layer in materials of this type in relation to high- resolution NMR experiments because only a single resonance is observed for all chain units, which limits the execution of selective relaxation experiments. Our experiments showed that about four -SiO(CH&- chain units adjacent to the grafting site form a semi-rigid PDMS-silica interface. When samples are swollen, NMR experiments can be used to determine the fraction of chain units within and outside the densely grafted layers, which is related to the chain length distribution of grafted chains. When swelling is performed in the presence of ammonia, such experiments can be used to determine the fraction of physically adsorbed chain units. Our experiments showed that the grafting conditions strongly affect the chain length distribution, the mean length of grafted chains, and the fraction of residual silanol groups at the silica surface.

Acknowledgments: The authors are grateful to M. F. J. Pijpers for providing the results of the DSC

734 V. M. Litvinov, H. Barthel, J. Weis

experiments, and greatly appreciate E. Currie's comments on the manuscript.

References S . Alexander, J. Phys. (Paris) 1977,38,983. P. G. de Gennes, Macromolecules 1980,13, 1069. S . T. Milner, T. A. Wittem, M. E. Cates, Macromolecules 1988,21,2610. Y. B. Zhulina, 0. V. Borisov, V. A. Pryamitsyn, T. M. Birshtein, Macromolecules 1991, 24, 140. N. Semenov, Sov. Phys. JETP 1985,61,733. F. D. Blum, B. R. Sinha, F.C. Schwab, Macromolecules 1990,23,3592. T. P. Lodge, G. H. Fredrickson, Macromolecules 1992,25,5643. M. Murat, G. S . Grest, Macromolecules 1991,24,704. P. -Y. Lai, K. Binder, J. Chem. Phys. 1991,95,9288. M. Zeghal, P. Auroy, B. Deloche, Phys. Rev. Lett. 1995,75,2140. M. Zeghal, B. Deloche, P. Auroy, Macromolecules 1999,32,4947. M. Zeghal, B. Deloche, P. -A. Albouy, P. Auroy, Phys. Rev. E 1997,56,5603. R. K. Gilpin, M. E. Gangoda, Anal. Chem. 1984,56, 1470. E. C. Kelusky, C. A Fyfe, J. Am. Chem. SOC. 1986,108,1746. Tuel, H. Hommel, A. P. Legrand, H. Balard, M. Sidqi, E. Papirer, Colloids and Sur$aces 1991,58, 17. M. Pursch, R. Brindle, A. Ellwanger, L. C. Sander, C. M. Bell, H. Hiindel, K. Albert, Solid State Nucl. Magn. Res. 1997, 9, 191. H. Barthel, L. L. Rosch, J. Weis, in Organosilicon Chemistry Il , From Molecules to Materials, Eds. N. Auner, J. Weis, VCH, Weinheim, 1996, p. 779-814. V. M. Litvinov, in Organosilicon Chemistry II. From Molecules to Materials, Eds. N. Auner, J. Weis, VCH, Weinheim, 1996, p. 779-814, and references therein. J. Van Alsten, Macromolecules 1991,24,5320. G. W. Sears, Anal. Chem. 1956,28,1981. V. M. Litvinov, P. A. M. Steeman, Macromolecules 1999,32, 8476, and references therein. G. Engelhardt, D. Michel, High Resolution Solid-state NMR of Silicates and Zeolites, John Wiley & Sons, New York, 1987. J. Neuefeind, K. -D. Liss, Ber. Bunsenges. Phys. Chem. 1996,100, 1341. R. K. Harris, M. L. Robins, Polymer, 1978,19, 1123. D. R. Kinney, I-S. Chuang, G. E. Maciel, J. Am. Chem. SOC. 1993,115,6786. The contribution to this component from protons of residual silanol groups at the silica surface and from those of adsorbed water molecules is below a few percent, as follows from the 'H NMR spectra shown in Fig. 3, which is in agreement with the results of T2 experiments using hydrophilic fumed silica. The density of silica was taken to be equal to that in silylated silicas. The T2 decay of the hydrophilic silica can be described by a single exponential function with a characteristic decay time of 0.18 ms. The amplitude of the decay was smaller

The Structure of a PDMS Layer Grafted onto a Silica Surface 735

by a factor of ten than that of the silylated silicas despite the large fraction of surface silanol groups in hydrophilic silica that are sites for water adsorption.

[27] S. Aharoni, Macromolecules 1983,16, 1722. [28] V. M. Litvinov, W. Barendswaard, M. van Duin, Rubber Chem. Technol. 1998, 71, 105. [29] Cuniberti, J. Polym. Sci.: PartA-2 1970,8,2051. [30] Y. B. Zhulina, V. A. Pryamitsyn, 0. V. Borisov, J. Coll. Interface Sci. 1990,31,495. [31] P. G. De Gennes, Adv. Colloid Interface Sci. 1987,27, 189. [32] M. Aubouy, 0. Guiselin, E. Raphael, Macromolecules 1996,29,7261, and references therein. [33] H. Barthel, K. Nikitina, V. M. Litvinov, Rubber World, submitted.

Novel Routes for the Preparation of Nanoporous Silica Particles

P. M. Chevalier,* D. L Ou

New Venture R&D, Dow Corning Ltd., Barry CF63 2YL, UK Tel: +44 1446 723 504 -Fax: +44 1446 730 495

E-mail: p.chevalier@dowcorning.com

Keywords: hybrid organic-inorganic silicate, silica, porous particles

Summary: Organic-inorganic silicate particles containing labile organic templates have been prepared by either controlled sol-gel processing or co-condensation of colloidal silica particles with organosilicon precursor. Chemical removal of the templating organic spacer led to monomodal, nanoporous silica particles exhibiting up to 50 % porosity. Particle size and distribution, and porosity, varied depending upon the process and the chemistry used to remove the porogenic organic groups.

Introduction

Dense silica nanoparticles have received considerable attention since monodisperse colloidal silica spheres, obtained from ammoniacal TEOS solution, were reported in the late 1960s by Stober et al. [I]. These dense particles, more often being monodispersed with controlled particle size, a well- defined morphology, and a surface with silanol groups by which they could be functionalized, are used for a variety of commercial applications including colorants, fillers and pigments.

The engineering of porosity in silica has been emerging as a new area of interest, particularly since the development of the MCM-type of materials [2]. Indeed, tailor-made pore sizes and shapes are particularly important in applications where molecular recognition is needed, such as shape-selective catalysis, molecular sieving, chemical sensing and selective adsorption [3].

However, it recently became desirable to be able to synthesize porous silica nanoparticles, having both a narrow monodispersity and a well-defined pore size, for a wide range of applications: catalysis, chromatography, controlled release, custom-designed pigments, and optical hosts.

We report here two routes for the preparation of porous silica particles through the temporary incorporation of chemically labile organic templating fragments. ORMOSIL particles are prepared in a first step by either a “controlled” hydrolysis-condensation process or by co-condensation of colloidal silica nanoparticles with an organosilicon precursor leading, after chemical removal of the templating organic spacer, to porous monomodal silica particles.

Ovganosilicon Chemisty V Edited by N. Auner and J. Weis

Copyright 0 2003 WILEY-VCH Verlag GmbH & Co. KgaA, Weinheim

Novel Routes for the Preparation of Nanoporous Silica Particles 737

Results and Discussion

The first multi-step route towards the preparation of nanoporous silica particles involved the formation of ORMOSIL particles through "controlled" hydrolysis-condensation of the bridged organosilicon precursor 1,4-bis(trimethoxysilylethynyl)benzene (Scheme 1). The colloidal particles were quenched at the sol stage using N, 0-bis(trimethylsily1)carbamate capping agent, leading to hybrid particles H. The organic spacer was chemically removed by Si-C, bond cleavage under nucleophilic catalysis, leading to particles A, followed by further calcination at 600 "C under air, yielding carbon-free porous silica particles B.

Quench d

BSC

w<3 w= r+y

SlO,.an(OSIMe,),(OMe~ 1

Hybrid particles H

Scheme 1. Organic-inorganic hybrid route to porous silica particles.

The characterization of the hybrid particles H by NMR and FTIR spectroscopies confirmed that the organic structure was maintained during the hydrolysis-condensation and quenching reaction. The surface area of these hybrid particles, determined by BET Nz sorption [4], were low (52 m2/g) (Table 1).

Table 1. BET surface area, porosity and maximum particle diameters of particles H, A and B.

Maximum particle diameter [p]

Volume [ %] Total pore Surface area volume Porosity

[%I Particles

10.00 25.00 50.00 75.00 90.00 [m2/gI [cm3/gl

~ ~~

- 0.48 0.54 0.62 0.71 0.79 H 52 -

A 402 0.326 32.9 9.90 15.55 25.11 37.06 47.25

B 339 0.243 26.7 1.61 2.56 4.28 6.83 9.62

The particle size distribution of H, determined by laser particle size analyzer, was monomodal, very narrow and centered at 600 nm (Table 1, Fig. 1).

Removal of the porogen organic spacer by Si-C bond cleavage under nucleophilic catalysis led to silica particles A. However, residual free organic groups resulting from the chemical cleavage were observed by NMR spectroscopy, confirming the need for a subsequent thermal treatment at 600 "C under an air atmosphere to get carbon-free silica particles B.

738 P. M. Chevalier, D. L. Ou

Fig. 1. Particle size distributions of particles H, A and B.

As a result of the removal of the templating organic groups, a large increase in surface area of up to 400 m2/g was observed for particles A (Table l), versus the low porosity of the starting hybrid particles H. The subsequent thermal treatment, leading to particles B, did not greatly affect the porosity with a total pore volume of 0.24 cm3/g. The pore size distribution of the silica particles B was also determined by the BJH [5] calculation method, revealing the formation of mainly microporous particles, with some contribution in the mesopore region, being mostly below 50 A.

Similarly, the mild removal of the organic spacer strongly modified the size of the particles (Table 1, Fig. 1). The chemical treatment of the sub-micron particles H led to particles A with a much broader monomodal size distribution of higher particle sizes centered at 25 pm. This phenomenon could be explained easily by the hydrolytic Si-C,, cleavage under nucleophilic catalysis, leading to the removal of the organic spacer with rearrangement of the silicate framework as well as further hydrolysis-condensation of remaining methoxy groups, both responsible for the particle aggregation. The calcination of particles A led to carbon-free particles B, still having a monomodal size distribution, but of lower particle sizes centered at 4 pm.

A second multi-step route towards the preparation of porous silica particles was also investigated. ORMOSIL particles were prepared through the co-condensation of pre-formed colloidal Ludox TMA silica particles (size < 150 nm), with 1,4-bis(trichlorosilyl)but-2-ene. Chlorotrimethylsilane was used to cap the remaining hydroxy functionalities, minimizing the particle aggregation, thus leading to particles C (Scheme 2). Oxidative treatment using H202 in slightly basic medium (NaHC03) [6] was further performed, leading to particles D. The characterization by solid-state NMR spectroscopy confirmed the formation of hybrid particles C and silica particles D.

HO ,OH

Ludox TMA n colloidal silica partlcles

Co-condensstion H # J W MeOHtTHF - Hybrid particles C - Silica particles D + H,O/IPA/HCI NHCO, OH

Me,SiCI/CI,SiCH,CH=CHCH~SiCI,

Scheme 2. From dense colloidal silica particles to porous silica particles.

85'82 68'81 58'01 29's 26'2 6PE 95E'O 66E a E6'PI SP'II OP'8 109 LET L8P 9E9'0 OL 3

€1'0 ZI'O 11'0 01'0 01'0 1-1 02 VMIL x0pn-I - -

740 P. M. Chevalier, D. L. Ou

Conclusion

We reported here two simple routes for the preparation of stable porous silica particles. ORMOSIL particles were prepared in a first step either by a controlled sol-gel process or by co-condensation of colloidal silica nanoparticles with an organosilicon precursor. Stable sub-micron hybrid particles of low porosity or mesoporous microparticles were then easily isolated. Removal of the templating organic spacer by various chemical treatments, completed by calcination, led to carbon-free silica particles. High porosity and surface area, up to 420 mZ/g, and monomodal particle size distributions were demonstrated by both routes. These porous silica particles of average particle size below 5 pm, being thermally stable up to 600 "C under air and dispersible in common organic solvents, are believed to offer interesting properties for a wider range of applications than conventional dense particles or porous gels.

References

[ 11 [2] [3] [4] [5] [6]

W. Stober, A. Fink, E. Bohn, J. Colloid Integace Sci. 1968,26,62. C. T. Kresge, M. E. Leonowicz, W. J. Roth, J. C. Vartuli, J. S. Beck, Nature 1992,359,710. G. Cao, Y. Lu, G. P. Lopez, C. J. Brinker,Adv. Muter. 1996,8,588. S. Brunauer, P. H. Emmett, E. J. Teller, J. Am. Chem. SOC. 1938,60,309. E. P. Barett, L. G. Joyner, P. H. Halenda, J. Am. Chem. SOC. 1951, 73,373. K. Tamao, N. Ishida, T. Tanaka, M. Kumada, Organometallics 1983,2, 1694.

Particle Size Distribution of Fumed Silica Agglomerates at Low Shear Stress

Michael Stink * Technische Universitat Dresden, Institut fiir Verfahrenstechnik,

Zentrum fur PartikeltechniWGranulometrie, D-01062 Dresden, Germany Tel.: 4 9 351 463 35176 -Fax: 4 9 351 463 37058

E-mail: Michae1.S tintz @mailbox.tu-dresden.de

Herbert Barthel, Mario Heinemann

Wacker-Chemie GmbH, Werk Burghausen, D-84480 Burghausen, Germany

Johann Weis

Consortium fur elektrochemische Industrie GmbH, Zielstattstr. 20, D-8 1379 Munchen, Germany

Keywords: particle size, fumed silica, powder handling

Summary: An experimental study has been undertaken to give guidelines for assessing particle size measurements for non-monodisperse, non-spherical, highly agglomerated particles such as pyrogenic (fumed) silicas under technical handling conditions, i.e., high solid concentrations and low shear stress. Contrary to common dry powders, the agglomerates have the relevant particle sizes to describe this colloidal system best. Conventional particle size methods often lead to destruction of the sample by applying high dispersion forces, or they are performed at extreme dilution. None of them reproduces the relevant particle size of pyrogenic silicas.

There is a wide range of industrial handling situations of fumed silica powder, for example, dosing, transport, and storage, represented by agglomerate sizes between 100 and 2000 pm and corresponding higher settling rates (> 0.05 d s ) . Figure 1 shows their constituents. In a flame, primary particles generated in the 10-20 nm size range form stable three-dimensional chain-like structures up to 500 nm, so-called aggregates (Fig. 2). It is noteworthy that isolated primary particles do not exist in fumed silica. As is known from electron microscopic images, these aggregates built up large agglomerates (Fig. 1) with porosity 90-100 %, held together by adhesion forces. Shear stress and particle concentration have been figured out to be the dominant influences on the agglomerate size distribution, varying in the median size from 7 pm up to 0.7 mm [l].

Ovganosilicon Chemisty V Edited by N. Auner and J. Weis

Copyright 0 2003 WILEY-VCH Verlag GmbH & Co. KgaA, Weinheim

742 M. Stintz, H. Barthel, M. Heinemann, J. Weis

Figs. 1 and 2. SEM images of an agglomerate (left), prepared in an air-jet under special conditions (high shear

stress, high electrostatic loading, very low solids concentration), and of aggregates (right).

Fig. 3. Particle size distributions of agglomerates of the same fumed silica sample, resulting from different

measuring conditions and different methods. Aerodynamic diameters that were measured initially have been

converted into geometric diameters.

Comparison of Sizing Methods

Particle size measurement techniques (Fig. 1) with various higher dispersion energies at very low (time-of-flight method) and at high silica aerosol concentrations (cascade impactor, laser diffraction method with air-jet injection) seem to be a powerful tool to investigate particle interactions.

Particle Size Distribution of Fumed Silica 743

However, the related actual agglomerate sizes, mainly between 1 and 90 pm, do not represent the usual state during industrial handling.

Laser difSraction (LD) analysis (IS0 13320-1) was performed using Sympatec Helos, along with a Rodos dispersion unit and a Gradis disperser. Fraunhofer theory describes the case when spherical particles of equal size are exposed to parallel monochromatic light. The light which is diffracted by the particles through a very specific angle is also parallel and is therefore focused by a lens onto a particular point in the focal plane. The intensity of the diffracted monochromatic light is plotted for individual particles against the measured radius in the plane of observation. The position of individual particles and their state of motion have no effect on the diffraction pattern. Thus, the measurement can be carried out on a free beam of particles in an aerosol or on a flowing particle suspension.

Laser light diffraction using air-jet injection is a suitable technique for studying particle size distributions of dispersed solids in air at different dispersion forces. This equipment uses a pressure drop in an injection orifice to apply a high shear force including particle impacts on the aerosol in order to achieve effective dispersion. The particles leave the dispersion stage in an ejector air stream. At a point downstream the laser beam intercepts the solids-loaded free jet, with a volume of intersection forming the measuring zone. A feeding and dispersing system Rodos has been developed which allows mass throughputs of between 0.1 and 20 kg/h. It has been used as a feeder unit for dry particle size analysis from diffraction patterns.

A sedimentation shaft (downpipe) Gradis instead of the air-jet injection is used for feeding the silica particles as well. They fall onto two sloping (45") planes and then through the laser beam. A low-vibration 1 mm sieve was used for dosing the laser beam with the agglomerates (sedimentation height: 5 1 cm free fall in air ).

The cascade impactor (CI) used was built for the particle size range 0.35-30 pm, calculated from the Strokes equation and slip correction, verified with monodisperse particles. CI was used at an air flow of 1100 dm3/h. The particles are accelerated in a nozzle but after this acceleration the large particles collide with an impactor plate whereas the small particles follow the streamlines of the gas flow to the next nozzle with a smaller diameter than the first. A cascade of nozzles and collection plate pairs can be used to determine the mass distribution of the aerodynamic particle size in an aerosol by weighing the plates after the experiment.

Particle image velocimetry (PIV) was used to measure the sedimentation speed of the silica agglomerates in air. Two lasers generated a light plan perpendicular to the velocity direction of moving particles. Two flashes with an ultra-short time delay were triggered to a CCD camera, connected to a computer calculating the direction and quantity of each particle velocity vector. From the known time difference of the laser pulses and taking into account the known reproduction scale the velocity (V) of the particle(s) is (are) calculated. The aerodynamic diameter D,, in the Stokes regime (g 9.81 d s 2 ; temperature 293.15 K; air viscosity 1.8071~10-~ Pa s; pressure 101.3 kPa) is given by Eq. 1.

The Time-of-Flight (TOF) method was applied using an API Aerosizer, Armherst. The underlying principle is based on accelerating particles through a sonic nozzle. A gas containing the entrained particles is allowed to expand through a nozzle into a partial vacuum, contained within a

744 M. Stintz, H. Barthel, M. Heinemann, J. Weis

barrel shock envelope, at supersonic velocities. The exit velocity of a particle depends on its density and size. Smaller particles are accelerated rapidly with the flow, larger ones more slowly. The velocity is calculated from the time-of-flight between two laser beams, separated by a discrete distance, which is detected by two photomultipliers separately. From the measured velocity and the known material density the system calculates the particle size.

v x 18 x viscosity

Dae= [ 3 g x density

Eq. 1.

Relationship between Different Particle Equivalent Diameters

The direct comparison of the results from optical methods with those from aerodynamic methods requires knowledge of the density of the particles to convert the aerodynamic diameter into the geometric diameter. Because of the unknown porosity of the silica agglomerates, no valid conversion was known. Experiments using a direct optical method to measure fractions of aerodynamic classified silica agglomerates [l] led to an effective particle density of a silica agglomerate of about 0.075 g/cm3.

Discussion

Conventional particle size methods often lead to destruction of the sample by applying high dispersion forces (LD - air-jet injection), or are performed at extreme dilution. Neither of them reproduces the relevant particle size of pyrogenic silicas. Contrary to common dry powders, the agglomerates are the relevant particle sizes for best describing pyrogenic (fumed) silicas under technical handling conditions, i.e., high solid concentrations.

Only laser diflraction allows different kinds of sample feeding and dispersion. Therefore the first on-line applications were realized using this method. Typical solid concentration is about 0.1 ~ 0 1 % .

A cascade impactor needs an aerosol of 1 000 times smaller concentration to classify individual particles according to their aerodynamic diameter.

A typical solid volume concentration is again about 10 times smaller for TOF and PZV.

Test of Measuring Methods with Low Shear Stress

Appropriate particle size analysis methods with very low shear stress (free settling with particle image velocity, field emission scanning electron microscopy, fluidized bed or vibrating sieve feeding with laser diffraction) have been tested for their reproducibility and minimal dispersion energy.

The following fumed silica samples were used as test materials:

Particle Size Distribution of Fumed Silica 745

Wacker HDK H15 is dimethyldichlorosilane-treated fumed silica; H15 was taken directly from the process. H2000 and SKS300 are hexamethyldisilazane-treated highly hydrophobic fumed silica; H2000 was desagglomerated (and milledclassified in addition). T30 is hydrophilic, untreated, fumed silica.

Dry Sieving

The use of this method to give reliable data for the reference samples requires a rigidly standardized operating procedure. Maximum allowed oversize volumes must be observed according to DIN 66 165 (e.g., max. 4 cm3/dmz at a 90 pm sieve; that is, a layer height of 0.4 mm). Therefore typically no more than 3 g of fumed silica and 10 sieves must be used. A special balance with a resolution of 0.01 g and a housing box is necessary.

To apply DIN 66165 for the reference materials properly, the effective bulk densities of the products after passing the corresponding vibrating sieves have to be determined. These were measured by using a 1 mm mesh sieve, a powder funnel, and a volume-graduated cylinder (beaker). The effective bulk density for reference products T30, H15 and TS610 is 0.035-0.040 g/cm3. For products such as SKS300, and H2000 the values are determined as 0.074-0.08 g/cm3.

Best reproducibility was observed using moderate sieve vibration amplitudes. A sieving time of 5 min was sufficient to get constant oversize mass (Fig. 4). The maximum standard deviation SD was 5 %.

Q

30 - g 20 0

10

0

7- 7

5 rnin 2/9 H2000

5 rnin 2/9 T30

5 rnin 2/9 HI5

+5 rnin 50%A Interv. H15

~

0 200 400 600 800 1000 1200 1400 1600 1800 2000

mesh size [pm]

Fig. 4. Dry Sieving of SKS300, H2000, T30 and H15. Particle size distributions for dry vibrating tower sieving at

two different sieving machines (Retsch - amplitude 9; Haver & Boecker - amplitude 50 %).

746 M. Stintz, H. Barthel, M. Heinemann, J. Weis

Laser Diffraction

A sedimentation shaft (downpipe) Gradis was used instead of the air-jet injection for feeding the silica particles. They fell onto two sloping planes (45") and then through the laser beam. A low- vibration 1 mm sieve was used for dosing the down-pipe with the agglomerates.

This combination of LD and sample feeding allows high reproducibility at close to technical handling conditions (Fig. 5). This method is nondestructive, i.e., silica agglomerates are mostly preserved and the aerosol concentration is sufficiently high.

0 100 200 300 400 500 600 700 800 900 1000 1100 1200 1300 1400 1500 1600

laser diffraction equivalent diameter [pm]

Fig. 5. Geometric particle size distribution of different silica samples as obtained by LD Gradis.

Reference [l] H. Barthel, M. Heinemann, M. Stintz, B.Wessely, Particle sizes of fumed silica. Chem. Eng.

Technol. 1998, 21(9), 745-752; H. Barthel, M. Heinemann, M. Stintz, B.Wessely, Part. Part. Syst. Charact. 1999,16, 169-176.

Hydroxylation of Amorphous Fumed Silicas Demonstrated by IGC, Solid-state NMR and IR

Spectroscopies

H. Barthel"

Wacker-Chemie GmbH, 84480 Burghausen Germany E-mail: herbert-barthel@ wacker.com

H. Balard

ICSI-CNRS, 15 rue Starky, 68057 Mulhouse, France E-mail: h.balard@univ-mulhouse.fr

B. Bresson

ESA 7069, Laboratoire de Physique Quantique, ESPCI, 10 rue vauquelin 75005 Paris. France

A. Burneau, C. Carteret, A. P. Legrand

UMR 7564. LCPE, UniversitC Henri PoincarC, Vandoeuvre-les-Nancy; France E-mail: carteret @lcpe.cnrs-nancy.fr, andre-pierre.legrand@espci.fr

Keywords: fumed silica, IGC, solid-state NMR, IR spectroscopy, surface morphology

Introduction

Evidenced has been obtained previously that the reinforcing effect of different grades of fumed silicas on silicone elastomers is influenced by the surface fractality [l] and that the surface roughness increases with the specific surface energy. The aim of the present work is to demonstrate variations by calling on NMR and infrared spectroscopic methods, which are applied to fumed silica samples that have been carefully characterized through adsorption methods including IGC analysis.

Experimental Part

Silicas

Industrial products (HDK-S 13, HDK-T30, HDK-T40) and specially prepared microporous silicas

Ovganosilicon Chemisty V Edited by N. Auner and J. Weis

Copyright 0 2003 WILEY-VCH Verlag GmbH & Co. KgaA, Weinheim

748 H. Barthel, H. Balard, B. Bresson, A. Burneau, C. Carteret, A. P. Legrand

(HDK-T30F1, HDK-T30F2) were from Wacker-Chemie and Aerosil A200 was from Degussa. Their main characteristics are given in Table 1.

Inverse Gas Chromatography

Inverse gas chromatographic measurements were performed on a Fison chromatograph fitted with two FID detectors. Chromatographic columns (length 20 cm, inner diameter 2 mm) were filled with silica particles. Pure n-alkanes and cyclooctane were used as probes. The measurements were performed at 150 "C under helium at a flow rate of about 20 mllmin.

NMR Spectroscopies

To analyse the hydroxyl content through 'H NMR analysis, samples were outgassed under secondary vacuum for 12 h and then heat-treated at 150 "C. Rotors were rapidly loaded in the glove box under a dry nitrogen atmosphere (RH < 3 %). 'H MAS NMR measurements were carried out with ASX500 and ASX300 Bruker spectrometers. All spectra were carried out using a spinning speed of 10 kHz.

Results

The specific surface areas of the samples were determined by nitrogen and cetyltrimethyl ammonium bromide (CTAB) adsorption whereas their surface roughness Ds was assessed in terms of surface fractal dimensions by relating the CTAB surface area to the nitrogen one (SET) [ 11 or by small-angle X-ray scattering (SAXS) [l]. Their specific surface areas determined by CTAB and nitrogen adsorption, the ratio ( R c T A B ~ ~ ) and their surface fractal dimensions are reported in Table 1.

Table 1. Surface area and fractal dimensions of fumed silica samples.

Samples Surf. Area [m2/g] DS

RCATBIN2 N2 CTAB CTAB-N2 SAXS

HDK-S13 standard 131 133 1.02 2.0 2.0

HDK-N20 standard 199 200 1.01 2.0 2.0

HDK-T30 standard 300 245 0.82 2.5 2.2

HDK-T40 standard 380 262 0.69 2.9 2.5

HDK-T30F1 microporous 278 176 0.63 3.1 2.5

HDK-T30F2 microporous 336 83 0.25 5.6 2.9

A 200 standard 205 202 0.99 2.0 -

It was observed that the two silica samples with a specific surface area lower or equal to 200 m2/g could considered as flat at a molecular level either by CTAB adsorption measurements or

Hydroxyltion of Amorphous Fumed Silicas 749

by SAXS. Above the critical specific surface area of 200 m2/g, the surface roughness increases readily in such way that it becomes close to 3 for the HDK-T30 microporous sample when using the SAXS technique whereas CTAB/N2 comparison delivers a value that exceeds 3 that has no physical meaning.

Another way for testing the surface roughness andor the microporosity of a solid is to call on size-exclusion inverse gas chromatography [ 2 ] . When stereochemistry hinders branched or cyclic isomers from entering a structure in which linear alkanes could be absorbed, much lower retention times are observed for nonlinear isomers than for the linear ones. This size-exclusion effect leads to a decrease (AGaM) of the free energy of adsorption compared to the n-alkane ones quantified through a morphological index IM which is given by Eq. 1.

n fMCyclooc tane

0 Ds SAXS

/ \ .

fMCyclooc tane 0

0

0 RCTABINZ

IM = exp(-AGaM/RT)

- 1.0

. 0.8

. 0.6

0.4

- 0.2

Eq. 1.

The higher the IM value is, the lower the size-exclusion effect. It will equal 1 on a flat solid surface. The variation of the cyclooctane index of morphology ( I M ) with the CTAB/N2 surface area ratio and the SAXS fractal dimensions of the samples studied is shown in the Fig. 1.

1 .o

0.8

0.6

0.4

0.2

Fig. 1. Variation of the cyclooctane index of morphology ( f , ) with the CTAB/Nz surface area ratio and the SAXS

fractal dimensions of the fumed silica samples.

The morphological index of cyclooctane was observed to vary linearly either with the CTAB/N2 surface area or with the SAXS fractal dimensions, a proof that the size-exclusion effect is directly related to the surface morphology. It was proposed that the surface roughness originates from a partial coalescence of protoparticles of around 1 nm in diameter that would be formed during the first stage of the silicon tetrachloride combustion [3] as schematized in Fig. 2.

If this partial coalescence obviously influences the fumed silica surface geometry, it would certainly also modify its functionality and the spatial distribution of the silanol groups because the high viscosity of the fused silica will mean that there will be a high temperature inside the particle during its cooling. Hence, the hydration reaction will take place mainly on the outer most part of the cooling particle, leading to an irregular distribution of the silanol groups. NMR spectroscopies or IR

750 H. Barthel, H. Balard, B. Bresson, A. Bumeau, C. Carteret, A. P. Legrand

spectroscopy were then called upon to provide evidence of this nonstatistical distribution.

Fig. 2. Primary particle mechanism of formation according to the flame temperature.

HDK ,913, HDK T30, HDK T30 F1 and HDK T30 F2 were examined using 29Si NMR and 'H NMR techniques. The fumed samples were examined using 29Si NMR spectroscopy using cross polarization techniques and 'H NMR spectroscopy. The spectra are depicted in Fig. 3.

Fig. 3.

1.5 65 5.5 4.5 3.5 2 5 1.5 05

29Si NMR (CP/MAS) spectra and 'H spectra of some silica samples.

One observes clearly on the 29Si NMR (CPMAS) spectra that for both silica samples, if the Q2/Q3 ratio remains relatively constant, the number of Q4 surface silicon atoms is significantly lower for the HDK-T30 silica than for the HDK-S13 one. In other words, we could find fewer 4 4 atoms in the vicinity of the T30 surface protons than in the vicinity of the S13 ones. Because of the relative stability of the global silanol density of the silica samples studied (about 1.8 OWnm2), this must correspond to a highest local density of silanol groups on the T30 silica surface. On the other hand, 'H spectra demonstrate that the relative concentration of isolated silanol groups corresponding to the sharp peak component of the spectra decreases with increasing surface area and is no longer observed for the microporous HDK-T30 sample.

Finally, the same samples were examined by IR spectroscopy. Figure 4 shows the IR spectra of some fumed silica samples in the characteristic region of the OH vibrations.

Again, it was observed that the relative concentration of isolated silanol groups decreases when the specific surface area increases and that the microporous HDK-T30F2 sample no longer exhibits isolated silanol, evidencing an augmentation of the local silanol surface density.

Hydroxyltion of Amorphous Fumed Silicas 751

Fig. IR spectra of some fumed

3000 3200 3400 36003720 37

silica samples in the silanol domain.

Conclusion

Either 29Si and ‘H NMR or IR spectroscopies confirm that when one lowers the flame temperature in order to obtain a high specific surface area, one increases the surface roughness but also one changes the surface functionality through the spatial distribution of the silanol groups on the silica surface. This occurs especially when the specific surface area exceeds a critical value of 200 m2/g.

References [ l ] [2]

[3]

[4]

[ S ]

[6]

H. Barthel, F. Achenbach, H. Maginot, MOFFZS 93 -Proc., Namur, 1993, p. 301-303. H. Balard, E. Brendle, E. Papirer, Acid-Base Interactions: Relevance to Adhesion Science and Technology, K. Mittal (ed.), VSP, Utrecht 2000, p. 316. H. Balard, E. Papirer, A. Khalfi, H. Barthel, J.Weis, Organosilicon ZV, N. Auner, J.Weis (eds.), Wiley-VCH, Weinheim, 2000, p. 773. A. P. Legrand, H. Hommel, J.-B. d’Espinose de la Caillerie, J. Coll. S u ~ A : Phys. Eng. Aspects. 1999, 158. H. Hommel, A. P. Legrand, C. Doremieux, J.-B. d’Espinose de la Caillerie, The surjiace properties of silicas. A. P. Legrand (ed.), John Wiley, 1998, p. 273. A. Burneau, J. P. Gallas, The surjiace properties of silicas. A. P. Legrand (ed.), John Wiley, 1998, p. 147.

Fumed Silica - Rheological Additive for Adhesives, Resins, and Paints

Herbert Barthel, * Michael Dreyer, Torsten Gottschalk-Gaudig*

Wacker-Chemie GmbH, D-84480 Burghausen, Germany E-mail: herbert.barthel@ wacker.com, torsten.gottschalk-gaudig@ wacker.com

Victor Litvinov

DSM Research, NL-6160 MD Geleen, The Netherlands

Ekaterina Nikitina

Institute of Applied Mechanics, RAS, Moscow, Russia

Keywords: fumed silica, unsaturated polyester resins, vinyl ester resins, intermolecular interactions, rheology

Summary: Fumed silica, a synthetic silicon dioxide, is a powerful rheological additive for resins and paints to introduce thixotropy or even a yield point. The rheological effectiveness of fumed silica is based on its ability to form percolating networks which immobilize large volumes of liquid. By a combination of advanced rheological experiments, spectroscopic investigations, and quantum chemical calculations it could be demonstrated that the formation and stability of the silica network is strongly influenced by particle-resin interactions. The results can be used to develop comprehensive models, which explain the rheological performance of different grades of fumed silica in different resins.

Introduction

Fumed silica is a synthetic amorphous silicon dioxide produced by hydrothermal hydrolysis of chlorosilanes in an oxygen-hydrogen flame. In this process, as a first step, SiOz molecules are formed which collide and react to nano-size proto particles, which by further collision in a second step form primary particles of around 10 nm in size. The flame process itself leads to the formation of smooth particle surfaces, which provides fumed silica with a high potential for surface interactions [l, 21. At the high temperatures of the flame, primary particles are not stable but are fused together to form space-filling aggregates. Leaving the flame, at lower temperatures the silica aggregates stick together by physico-chemical forces, building up large micron-sized agglomerates

Ovganosilicon Chemisty V Edited by N. Auner and J. Weis

Copyright 0 2003 WILEY-VCH Verlag GmbH & Co. KgaA, Weinheim

Fumed Silica - Rheological Additive for Adhesives, Resins, and Paints 753

and finally fluffy flocks [3,4]. Fumed silica is widely used in industry as an efficient thickening agent providing shear thinning

and thixotropy to liquid media like adhesives, resins, paints, and inks. Various parameters control the rheological performance of fumed silica:

the smoothness of the primary particle surfaces, which provides a maximum contact area for various types of interactions, such as H-bonding and van der Waals interactions of dipolar and dispersive character; the space-filling structure of the aggregates with a mass fractal dimension of D, < 2.7, leading to a fluffy structure of agglomerates, typically with a “density” d = 50-100 g/L (note: density of amorphous silicon dioxide dsioz = 2200 g/L), and agglomerate sizes > 1 pm;

the high physico-chemical interaction potential of the fumed silica surface, which is based on its reactive surface silanol groups (surface density 1.8 SiOWnmz), and on its polar Si-0 bonds of the particle back-bone. By surface modification, most commonly surface silylation, these interactions can be controlled precisely.

Particle interactions are the driving force for agglomerate and network formation, enabling fumed silica to form percolating networks in liquid media [5-71. Basically, two kinds of n’etworks are possible: firstly, a network of fumed silica particles or aggregates originating from direct particle-particle contacts and, secondly, a network based on polymer bridging where aggregates interact with polymers at least on two particles. Real systems may consist of both, and mixed types. At rest or very low shear rates these networks are able to immobilize large volume fractions of liquids even at low fumed silica loading (< 5 wt.%), resulting in very high viscosities or a high yield point, respectively. Upon application of shear forces the network structure is reversibly destroyed and the apparent viscosity of the mixture decreases with increasing shear rate. When the shearing stops the system is able to recover the network structure. Figure 1 depicts this process schematically.

? Fig. 1. Dependence between shear rate, network structure, and viscosity.

754 H. Barthel, M. Dreyer, T. Gottschalk-Gaudig, V. Litvinov, E. Nikitina

Obviously, the rheological performance of fumed silica in adhesives, resins, and paints is mainly determined by the stability of the colloidal network. This raises the question of which parameters influence the formation and stability of a colloidal silica network. In order to answer this question we have to understand a) the nature of interactions in silicdresidsolvent mixtures in terms of the silica-silica, silica-resin, and silica-solvent interactions, and b) how these interactions influence the rheological behavior of the mixtures. From this information we should be able to develop a comprehensive model that explains the rheological performance of different grades of fumed silica in different resin types.

The aim of this paper is to present comprehensive models of interactions by comparing rheological results, analytical data from spectroscopic methods such as solid-state NMR and IR, and quantum chemical calculations. Furthermore we demonstrate how these interactions influence the rheology of fumed silica in unsaturated polyester (UP) and vinyl ester (VE) resins.

Experimental

Two grades of fumed silica with a different degree of surface treatment and different polarity have been used: a nontreated, hydrophilic Wacker HDK@ N20 (BET surface area 200 mVg, 1.8 SiOWnmz equivalent to 100 % residual SiOH); and a fully silylated hydrophobic Wacker HDK@ H18 (carbon content 4.5 % C, 15 % residual SOH), the latter being covered by a chemically grafted PDMS layer. Both silicas are products from Wacker-Chemie GmbH, Germany. The resins used are an unsaturated polyester resin Palatal P4 (UP resin), a co-condensate of a diol, maleic acid and orthophthalic acid, and the vinyl ester resin Atlac 590 (VE resin), a co-condensate of glycidine, methacrylic acid and bisphenol A. Both resins have a styrene content of 35 % and are provided by DSM, The Nertherlands.

Rheological studies have been performed using three techniques:

measuring the step profile of the shear viscosity at a controlled but constant shear rate D at 1 s-', 10 s-' and 100 s-', each 120 s, respectively; recording the dynamic behavior and thixotropy using a shear rate controlled relaxation experiment at D = 0.5 s-' (conditioning), 500 s-' (shear thinning), and 0.5 s-' (relaxation); oscillation at a frequency of 1.6 s-' and a deformation sweep between 0.001 to 10; determination of the yield point from a log-log plot of deformation y vs. shear stress z. The yield point is defined as the stress z where the corresponding y z curve exhibits a deviation > 5 % from a tangent defined by data points at low shear stresses.

All samples contained 3 %wt of fumed silica. I3C CPMAS spectra were recorded with cross-polarization (CP) times of 0.5 and 8 ms for

IR spectra were obtained from liquid samples using NaCl plates on a standard FT-IR samples with a silica content of 4 % [8].

spectrometer with compensation of the solvent signal.

Fumed Silica - Rheological Additive for Adhesives, Resins, and Paints 755

Quantum chemical calculations were performed using cluster approach methods in a software Cluster Z1 and a modified PM3 parameter set.

Results and Discussion

Rheological Studies

It is well known that both hydrophilic and highly hydrophobic fumed silica are efficient rheological additives for unsaturated polyesters. In more polar systems such as vinyl esters, however, only highly hydrophobic fumed silica is suitable. This observation is illustrated by the viscosity step profile of Wacker HDK N20 and H18 in Palatal P4 (UP) and in Atlac 590 (VE), respectively, depicted in Fig. 2.

100 .--.-.--+-.-.- -1- N20; Palatal P4 -- H 18; Palatal P4 -- N20; Atlac 590 1 re1 1 A & & L L - - A - A i -- H 1 8; Atlac 590

1zj=wzw111-

1 ! , . , . , . , . , . ! . , ' , . 1 0 50 100 150 200 250 300 350 400

time [s]

Fig. 2. Relative viscosity in a shear rate step profile at D =I s-', 10 s-' and 100 s-', each 120 s, respectively, of four

resin systems: Wacker HDK N20 and HDK H18 in Palatal P4 (UP resin) and in Atlac 590 (VE resin); 35

wt% styrene; 3 wt% fumed silica.

The step profile reveals that N20 and H18 dispersed in Palatal P4 exhibit an almost identical rheological behavior, whereas the apparent relative viscosities of N20 and H18 dispersed in Atlac 590 at low and moderate shear rates are distinctly different. To explain this behavior it is necessary to consider the polarity, functional groups, and chain length of the resins but also the surface properties of the fumed silica. All parameters together will influence the nature and strength of the colloidal forces, which govern the rheology of the mixtures.

Firstly we investigated the rheology of Wacker HDK N20 and H18 in pure styrene. Figure 3a shows the relaxation test experiment, Fig. 3b the deformation sweep, and Fig. 3c the determination of the yield point.

Hydrophilic silica N20 forms an extremely stable and rigid network with a yield point of 84 Pa at 3 wt.% loading in pure styrene. Both experiments, relaxation and oscillation, reveal an

756 H. Barthel, M. Dreyer, T. Gottschalk-Gaudig, V. Litvinov, E. Nikitina

lo4 lo3 1 o2 10' loo

b lo-*

- g

instantaneous breakdown of the percolating structure upon exceeding the yield point, resulting in a low relative apparent viscosity qrel, and a fast and complete recovering (< 3 s). However, under similar conditions, fully silylated silica exhibits a much weaker network with a yield point of around 0.06 Pa. The network is more elastic according to the deformation sweep experiment but recovers its structure after shear thinning markedly more slowly (ca. 40 s). The difference can be understood by H-bonding particle-particle interactions in the case of hydrophilic silica, which can recover the network structure almost instantaneously. For fully silylated silica we suggest a combination of hydrophobic interactions (interactions of hydrophobic particles in a hydrophilic environment are related to phase separation phenomena) [9] and entanglement of the grafted PDMS chains. This process of network forming can be seen as a kind of phase separation between the grafted PDMS and the styrene matrix. In this context it is important to know that styrene is a worse- than-8 solvent for PDMS [lo]. However, phase separations between compounds of comparable polarity are inherently slow. The occurrence of a yield point for H18 in styrene is a strong indication of a combination of hydrophobic interactions and chain entanglement. Systems flocculated exclusively by hydrophobic interactions are supposed to show no yield point [ 111.

- G'; 3 % N20kyrene - G'; 3 % H 1 Wstyrene * * . . 1 .

zy&N20)=84Pa

D = 0.5 s ' = o.5 < I

1 i".- ................. \ / ...... ...........................

m5-

1051 1 o4 t,(N20) < 3 s trec(H18) = ca. 40 s ,D = 500 s '

Fumed Silica - Rheological Additive for Adhesives, Resins, and Paints 757

j / \ : , ' ' ""." ' ' ""'.' ' ' . ' ' ' '.I ' ' " " "~ ' ' ....:.I ' ' "

1E-3 0.01 0.1 1 10 100 shear stress z [Pa]

Fig. 3. a) Controlled shear rate: relaxation experiment (profile D = 0.5 s-', 500 s-', and 0.5 s-I) of N20 and H18

dispersed in styrene; b) oscillation: deformation sweep at a frequency of 1.6 s-' and a deformation of 0.001 to

10 of N20 and H18 dispersed in styrene; c) determination of the yield point of N20 and HI8 dispersed in

styrene: data from deformation sweep.

In the presence of resin oligomers the situation is distinctly changed. Mixtures of UP resin Palatal P4/N%O/styrene exhibit a much weaker but more flexible network than N%O/styrene mixtures, as indicated by a yield point of about 10 Pa and a slow degradation of the network structure by increasing the deformation in the oscillation experiment (Fig. 4b). More interestingly, this structure degradation occurs in a step wise manner which can be explained by a polymer bridging of the resin molecules between the fumed silica particles. The relaxation time of the N20 network structure after shearing is dramatically increased in the presence of Palatal P4 (Fig. 4a).

This effect can be explained with a reversible adsorption of resin molecules to the freshly provided surfaces of silica particles produced by shearing down the cluster structure of the N20 network. The re-formation of the network requires at least a partial desorption of the resin molecules which is a slow and time-consuming process due to the multi-point interaction of the resin chains with the silica surface [12].

For H18 we observe an increase of the network stability (yield point 10 Pa) in the oscillation experiment and a decrease of the relaxation time to less than 3 s in the presence of Palatal P4. Both effects can be put down to the fact that the polarity of the mixture is enhanced by the resin oligomers, which increases the interaction energy with respect to PDMS-PDMS entanglement and the phase separation between the grafted PDMS layer and the surrounding medium occurs faster.

When hydrophilic fumed silica N20 is dispersed in a vinyl ester Atlac 590/styrene mixture the oscillation experiment reveals that the fumed silica is not able to build up a percolating network, as indicated by the lack of a yield point (Fig. 5b). This is also supported by the observation that the relaxation time of the N%O/Atlac 590/styrene system is markedly longer than the time frame of the viscosity relaxation experiment of 500 s (Fig. 5a).

758 H. Barthel, M. Dreyer, T. Gottschalk-Gaudig, V. Litvinov, E. Nikitina

/ D = 0.5 s-' ........................................... #

- 3 % N20Palatal P4/35 % styrene - 3 % H18Palatal P4/35 % styrene %el lo1

t,(N20) = ca. 120 s tm(H18) < 3 s 1 I I D = 500 SI

time [s]

b)

1007

10:

-A- G'; 3 % H18/Palatal P4/35 % styrene

1 E-3 0.01 0.1 1 10

deformation

0.1

Fig. 4. a) Controlled shear rate: relaxation experiment (profile D = 0.5 s-', 500 SKI, and 0.5 SKI) of N20 and H18

dispersed in Palatal P4ktyrene; b) oscillation: deformation sweep at a frequency of 1.6 s-' and a deformation

of 0.001 to 10 of N20 and H18 dispersed in Palatal P4ktyrene.

Vinyl esters resins are characterized by pendant OH groups in the chain which are able to form strong H-bonds to the silanol groups of silica particles. Due to the strength of this interaction the adsorption of vinyl ester molecules at silica surfaces should be almost irreversible and result in a kind of steric stabilization of the silica particles. The formation of a silica network is suppressed and the relative viscosities achieved remain low.

In the case of H18/Atlac 590/styrene we observe a behavior in terms of network stability and relaxation which is comparable to the H18Palatal P4/styrene system. This reveals that the net polarity of the medium is probably a more important driving force for the network stability of fully hydrophobic fumed silica than the specific chemical structure of the single components of the medium.

Fumed Silica - Rheological Additive for Adhesives, Resins, and Paints 759

D = 0.5 S-' , D = 0 . 5 s 1 .............................................. - 3% N20/Atlac 590/35% styrene -3% HWAtlac 590/35% styrene 1 mmmmI 1

l o l l " ........................

I I /m'"""' [./ tJN20) > 300 s tJH18) < 3 s

150 200 250 300 350 400 450 500

time [s]

b)

1

\..*-.....-.-. ..... -1- G'; 3 % N20/Atlac 590/35 % styrene

10.' 1 o-2 10-1 1 oo lo1

deformation

Fig. 5. a) Controlled shear rate: relaxation experiment (profile D = 0.5 s-I, 500 sd, and 0.5 SKI) of N20 and H18

dispersed in Atlac 590ktyrene; b) oscillation: deformation sweep at a frequency of 1.6 sd and a deformation

of 0.001 to 10 of N20 and H18 dispersed in Atlac 590htyrene.

NMR and IR Spectroscopies

The results of our rheological study demonstrate that interactions between resin molecules and fumed silica particles significantly influence the network stability and its formation. In order to get a deeper understanding of the nature of such interactions a I3C CPMAS study at different cross -polarization times zcp from 0.5 to 8 ms has been performed. At short T~~ spectra intensities are enhanced by I3C resonances of the least mobile chain fragments. This study includes two different fumed silicas, hydrophilic Wacker HDK N20 and fully silylated Wacker HDK H18, with an unsaturated polyester resin Palatal P4 and a vinyl ester resin Atlac 590, respectively. Figure 6 depicts the I3C CPMAS spectra of N20 and H18, respectively, in Palatal P4/35 % styrene at

760 H. Barthel, M. Dreyer, T. Gottschalk-Gaudig, V. Litvinov, E. Nikitina

different cross-polarization times T ~ ~ .

I

180 160 140 120 100 80 60 40 20 0 ppm

PDMS I b)

PDMS

z,, = 0.5 ms

Fig. 6. I3C CPMAS spectra at different cross-polarization times T~~ of a) N20 in Palatal P4/35 % styrene and b) H18

in Palatal P4/35 70 styrene.

The N20Palatal P4 I3C CPMAS spectra exhibit an enhanced intensity of the C=O and C=C signals at short zCp, whereas Palatal P4 without added N20 shows no signals in the same experiment. This indicates that specific interactions of carbonyl and C=C-C=O groups of the resin oligomers

Fumed Silica - Rheological Additive for Adhesives, Resins, and Paints 761

with the fumed silica surface result in an immobilization of the resin molecules [8, 131. This finding supports our interpretation of the rheological relaxation experiment, where we suggested that the increased relaxation times in the presence of Palatal P4 are related to the reversible adsorption of the resin oligomers at the silica surface after shear deformation.

The I3C CPMAS spectra of H18 in Palatal P4/35 % styrene show an enhanced intensity of the grafted PDMS chains at short zcp. In a 13C MAS experiment no signals for the PDMS chains of H18 could be detected. The fact that it was possible to detect the grafted PDMS by 13C CPMAS indicates a strong immobilization of the chains, which is in agreement with our suggestion of H18 silica-silica network formation by a combination of hydrophobic interactions and chain entanglement. Both mechanisms are supposed to immobilize the PDMS chains in the grafted layer

Further support for a specific interaction between the silanol groups of hydrophilic fumed silica and the C=O groups of resin molecules comes from IR spectroscopy, where additionally to the carbonyl band of free Atlac 590 oligomers at 1724 cm-', a second band at 1704 cm-' appears under adsorption at the N20 surface (Fig. 7). This indicates an interaction of the C=O function with the silica surface and particularly with the silanol groups of the silica [14]. At very low resin concentrations of less than 1.0 wt.% the fraction of adsorbed oligomer is approximately 60 % of the total resin, as seen from the IR intensities. Upon increasing the amount of resin the fraction of adsorbed resin oligomer remains small and does not exceed 5-8 wt.% relative to the silica. The fact that only a small portion of the resin molecules is immobilized is in accordance with our model of thickening by hydrophilic fumed silica, based on a) direct particle-particle interactions, b) polymer bridging, and c) steric stabilization.

3 % N20 + 1 % Atlac 590 in CCl, -. - - - - - 3 % N20 + 0.5 % Atlac 590 in CCl, I -

1600 iioo 1800

wave number [cm-'1 Fig. 7. IR spectra of 3 % N20 in CC4 after addition of 0.5 and 1 % Atlac 590.

Quantum Chemical Calculations

Quantum chemical modeling is a suitable tool to elucidate the microscopic mechanisms of the adsorption processes of polymers on silica surfaces [15]. In the current study quantum chemical

762 H. Barthel, M. Dreyer, T. Gottschalk-Gaudig, V. Litvinov, E. Nikitina

modeling has been used to compare energies of interactions quantitatively in the system of silica and resin molecules.

For modeling of hydrophilic silica HDK N20 particles, a hydroxylated silica cluster [Si0248-OH9] [15], containing 48 silicon dioxide units and nine surface silanol groups, has been used; grafting two five-membered dimethylsiloxy (DMS) chains loopwise (bonded at both ends) on it provided the model for the fully silylated silica HDK H18, silica cluster [Si0248-OH5-DMS52]. Two resin models have been simulated representing all the typical functional groups of unsaturated polyester resins (UP) and vinyl ester resins (VE):

Sequence of UP model: methyl ether of 1,2-propanediol - maleic acid - 1,2-propanediol - orthophthalic acid - methy lether of 1,2-propanediol; Sequence of VE model: methacrylic acid - glycidine - bisphenol A - glycidine - methacrylic acid.

A special study has been dedicated to the nature of bonds in the system, in particular in the silica-resin system. The interaction energies of the different kinds of H-bonds decrease in the order C-O-H...O(H)-Si > C=O...H-O-Si > C-(H)O.-.H-O-Si, which is summarized in Table 1.

Table 1. Energies of specific interactions: silica model [Si0218-OH5] [15] and

VE model: methyl ether of glycidine-methacrylic acid.

System Energy of interaction Bond distance

[ kcallmol] [nml

- 2.80

- 4.46

- 5.60

- 9.51

0.283

0.185

0.183

0.177

The oxygen atom of Si-OH bears a higher negative charge than that of C-OH, but the hydrogen of the latter is slightly more positively charged than in Si-OH; in consequence -C-OH-O(H)-Si is the strongest H-bond, but it occurs only in a VE system. Surprisingly, the carbonyl function also shows a rather high H-bond energy - linking an important interaction energy to carbonyl group immobilization as seen by 13C CPMAS NMR, and carbonyl band red shift as observed by IR. In summary this indicates that a VE-like resin structure shows strong adsorption affinity towards hydrophilic fumed silica.

Additionally to the superior interaction energy of -C-OH-O(H)-Si vs. -C-(H)O-HO-Si, steric considerations suggest that end-of-the-chain carbinols in a resin are forming stronger H-bonds with a silica surface than in-chain carbinols; however, this suggestion has to be verified in future studies. Figure 8 shows fully optimized structures of adsorption complexes of VE and UP resin models with the [Si0248-OH9] and [Si0248-OH5-DMS52] silica clusters.

Fumed Silica - Rheological Additive for Adhesives, Resins, and Paints 763

Calculated interaction energies of silica-silica, silica-resin, and resin-resin adsorption complexes are given in Fig. 9.

Fig. 8. Fully optimized structures of the adsorption complexes of a) hydrophilic silica and vinyl ester

[Si0248-OH9]NE; b) hydrophilic silica and unsaturated polyester and [Si0248-OH9]luP; c) hydrophobic

silica and vinyl ester [SiO248-0H5-DMSs2]NE; d) hydrophobic silica and unsaturated polyester [Si0248-

OH5-DMSs2]/UP.

VE System

Hydrophilic silica particles interact strongly with the VE molecule ([SiO248-OH9]NE) due to H- bonds of the form -C-OH-.O(H)-Si in addition to H-bonds of the form C=O-.H-O-Si and dispersion interactions - it is the most favorable interaction in all system combinations in terms of energy. As it is even stronger than the silica-silica interaction ([SiO248-OH9]/[SiO248-OH9]), strong adsorption is expected, leading to steric stabilization of the colloidal system by hampering direct particle-particle contacts. Rheologically, we would interpret this as low thickening efficiency

764 H. Barthel, M. Dreyer, T. Gottschalk-Gaudig, V. Litvinov, E. Nikitina

- indeed, experiments show that N20 is not a stable thickener for VE resin systems. The weakest interaction in the system is that of silylated silica with a VE molecule

([Si0248-DMS52]NE). It is even weaker than the interaction of silylated silica with itself ([Si0248-DMS52]/[Si0248-DMS52] ). The medium interaction energies of the latter is surprising, as only nonspecific components are involved - however, modeling makes evident a particle-to- particle entanglement of the five-membered DMS chains on the silica clusters: in fact, the additional immobilization of PDMS chains on H18 as seen by I3C CPMAS NMR seems to be correlated with energies of interaction. The strongest complexes in the VE and silylated silica system are those of the VE resin molecules with each other. In consequence, the interface of highly silylated silica towards a VE phase is energetically less favorable: in order to optimize VE-VE contacts the silylated silica particle surfaces separate from the system by direct silylated silica- silylated silica particle contacts - a phenomenon which is well known as “hydrophobic interaction” [9]. However, real systems contain monostyrene, which is not or only very weakly adsorbed on a silica surface according to calculations and spectroscopic experiments. Hence, styrene is enhancing the effect of silylated silica separation from the VE-styrene phase. In fact, HDK H18 is an excellent and stable thickener and rheology control additive for all VE and epoxy resin-like systems.

-10

-20

-30

W/mol

-40

-50

Si0248-OH5-2DMS52 / VE

Si0248-OH9 / UP Si0248-OH5-2DMS52 / Si0248-OH5-2DMS52

UP / UP Si0248-OH9 / Si0248-OH9 VE 1 $E

Si0248-OH5-2DMS52 / UP - Si0248-OH9 / VE

-60

Fig. 9. Calculated energies of intermolecular interactions in the system comprising silica particles and resin

molecule. The contact areas a = 0.82 nm2 are identical ( 2 4 % deviation) for resin-resin and resin-silica

interaction for both UP and VE, and hydrophilic and silylated silica, and therefore interaction energies are

taken as directly comparable. Silica-silica contact areas are both larger by a factor of 3.2; the interaction

energies are therefore normalized by the ratio of the contact areas.

Fumed Silica - Rheological Additive for Adhesives, Resins, and Paints 765

UP System

The complex UP on hydrophilic silica is rather weak, but the interaction of two hydrophilic silica clusters is quite strong, even in competition with the resin-resin interactions, which are of comparable strength or weaker. Additionally, not depicted in Fig. 9, modeling shows that styrene is not adsorbed at all on hydrophilic silica. In consequence, following the interpretations given above, hydrophilic silica HDK N20 is an excellent and powerful thickener and thixotropic agent for UP resins, as strong particle-particle interactions occur. However, spectroscopic and rheological results indicate an adsorption of UP oligomers onto hydrophilic silica surfaces. In accordance with our calculations this adsorption is only weak and reversible and therefore influences only the rate of gelation but not the network stability.

Surprisingly, the adsorption complex of UP molecules on the silylated silica cluster shows a distinctly higher interaction energy. Following the interpretations given above, this should result in wetting, adsorption, steric stabilization, and a weak colloidal network. However, as shown by the rheological data given above, HDK H18 is an excellent thickener for UP resins, too. It seems that in a system of real UP resins, the solvent styrene plays a key role: being a bad or worse-than8 solvent for (the) PDMS (layer on silica), styrene is the driving force of hydrophobic interactions. In the case of silylated silica in UP resins the adsorption of UP molecules on the silylated silica surface seems not to hinder hydrophobic interaction - a reasonable assumption as the latter is driven not by particle-particle contact but by phase separation.

Conclusion

In order to develop a comprehensive model for the thickening behavior of different grades of fumed silicas, advanced rheological and spectroscopic experiments as well as quantum chemical calculations have been performed. The results indicate that fundamentally different mechanisms are responsible for the formation of colloidal networks of hydrophilic and fully silylated silica.

Hydrophilic silica forms a strong and rigid network by strong short-range H-bonds between silica particles. Addition of UP resin reduces the network stability but increases network flexibility and relaxation time after shearing. VE resins hamper the formation of a percolating network due to irreversible adsorption of OH end groups on the silica surface.

Fully silylated silica exhibits hydrophobic interactions between the PDMS-covered surface and the solvent as well as entanglement of the PDMS layers. Addition of UP or VE resins increases the polarity of the medium. As a result, the mismatch between the matrix and the PDMS layer is increased, which favors PDMS-PDMS interactions and therefore enhances the network stability and decreases the network relaxation time.

For both grades of silica the formation and strength of their percolating networks in terms of both particle-particle and particle-polymer-particle interactions is strongly influenced by the polarity of the surrounding medium and the nature of the resins. In particular, adsorption processes have been identified as playing an important role in the rheological performance of hydrophilic

766 H. Barthel, M. Dreyer, T. Gottschalk-Gaudig, V. Litvinov, E. Nikitina

fumed silica. From these results recommendations for the application of fumed silica in different types of resins have been derived, and are summarized in Table 2.

Table 2.

and the potential to form strong specific interactions.

Recommendations for the application of fumed silica in different resin types depending on their polarity

Resin oligomer Polarity and type of interaction Rheological additives

Alkyd resins

Saturated polyester resins non polar; no strong specific interactions hydrophilic fumed silica HDK N20

hydrophilic fumed silica HDK N20

fully silylated fumed silica HDK H18 Unsaturated polyester resins Polar to medium polarity; no strong

specific interactions

fully silylated fumed silica HDK H18 Vinyl ester resins Melamine-polyesters interactions

medium to highly polar; strong specific

Epoxy Resins

Polyurethane System

Acrylate Systems

polar; strong specific interactions fully silylated fumed silica HDK H18

References [I] F. Achenbach, H. Barthel, H. Maginot, Proc. Int. Symp. on Mineral and Organic Functional

Fillers in Polymers (MOFFIS 93) 1993, p. 301. [2] G. D. Ulrich, Chem. Eng. News 1984,62,22. [3] H. Barthel, L. Roesch, J. Weis, Organosilicon Chemistry II (Eds.: N. Auner, J. Weis), VCH,

Weinheim, 1996, p. 761. [4] H. Barthel, Colloids SurJ:, A: Physicochemical and Engineering Aspects 1995,101,217. [5] D. Quemada, Prog. Colloid Polym. Sci. 1989, 79, 112. [6] W. B. Russel, J. Rheol. 1980,24,287. [7] T. F. Tadros, Chem. Ind. 1985, 7,210. [8] D. G. Cory, W. M. Ritchey, Macromolecules 1989,22, 161 1. [9] J. N. Israelachvili, R. M. Pashley, J. Colloid Interjke Sci. 1984,98, 500. [lo] D. W. Van Krevelen, in Properties of Polymers, 3rd edn. (Ed.: D. W. Van Krevelen),

Elsevier, Amsterdam, 1990, p. 774. [ 1 11 E. Killmann, J. Eisenlauer, EfJ: Polym. Dispersion Prop. [Proc. Int. Symp.] 1981,66, 36. [12] Y. Otsubo, Adv. Colloid Integkce Sci 1994,53, 1. [13] V. M. Litvinov, A. W. M. Braam, A. F. M. J. van der Ploeg, Macromolecules 2001,34,489. [ 141 G. R. Joppien, K. Hamann, J. Oil Colour Chem. Assoc. 1977,60,412. [15] E. Nikitina, V. Khavryutchenko, E. Sheka, H. Barthel, J. Weis, in Organosilicon Chemistry

IV (Eds.: N. Auner, J. Weis), Wiley-VCH, Weinheim, 2000, p. 745.

Morphology of Toner-Silica Interfaces

Sabine Hild * Experimental Physics University of Ulm, D-89069 Ulm, Germany

Tel: +49 731 5023016 - Fax: +49 731 5023036 E-mail: sabine.hild@physik.uni-ulm.de

Herbert Barthel, Mario Heinemann, Ute Voelkel

Wacker-Chemie GmbH, Werk Burghausen D-84480 Burghausen, Germany

Johann. Weis

Consortium fur elektrochemische Industrie GmbH, Zielstattstr. 20, D-81379 Miinchen, Germany

Keywords: toner, silica, SFh4, material contrast, adhesive properties

Summary: In this study Scanning Force Microscopy (SFM) is used for the investigation of toner particles and toner-silica particle interfaces. The focus is on the simultaneous mapping of topographic features and material properties such as local stiffness and adhesion. Topographic images of toner particles of different toner binder compositions were recorded in addition with phase images of hard and soft domains. A series of toner-silica blends were prepared by using a common powder blender. The toner binder resin was styrene acrylics or polyester, respectively. In the topographic images the toner surface is mapped as a rough surface. The mapping of the elastic signals of the silica free toner surface reveals the arrangement of hard and soft domains. With this technique single silica aggregates have been imaged at the toner surface and are mainly located on topographic rough spots.

Introduction

Fumed silicas are key ingredients in electrophotographic toners and developers. They are widely used as surface additives for toners and developers. The interaction of silica particles with the toner surface is of high importance for the toner performance [ l , 21. Nevertheless, the adhesion of silica particles at toner particles is still an open topic of discussion. Thus it is necessary to visualize the distribution of silica particles and the silica particle size distribution on toner particle surfaces and

Ovganosilicon Chemisty V Edited by N. Auner and J. Weis

Copyright 0 2003 WILEY-VCH Verlag GmbH & Co. KgaA, Weinheim

768 S. Hild, H. Barthel, M. Heinemann, U. Voelkel, J. Weis

to investigate the adhesive interactions between silica particles and the toner surface. Scanning force microscopy (SFM) is an effective tool for such investigations [3-51. The SFM enables one to image simultaneously surface topography and adhesive properties on nanometer scale [6].

The working principle of this type of microscope differs from that of conventional microscopes in the method of generating images: a small tip with a radius of about 5-10 nm mounted on a flexible cantilever beam is scanned over the surface at a constant distance. To control the distance between the surface and tip a laser beam is focused at the end of the cantilever. The reflected beam is detected in a four-segment photodiode; thus a change in sample height causes a change in cantilever deflection. A feedback-controlled piezo will readjust the position of the tip to a given height. This working principle is shown schematically in Fig. 1 (left-hand side). Recording the piezo movement results in a topography image in the z-axis; combing this with the x-, y-scanning of the cantilever a 3-D image is recorded. In the images, differences in height are mapped in different colors, with the dark areas representing low z values and bright areas the high z values.

Fig. 1. Schematic view of the experimental SFM set-up and working principle of tapping mode imaging.

For sensitive samples such as polymers or resins, dynamic techniques like the tapping-mode or pulse force mode were developed, where the tip is periodically brought into contact with the surface during the imaging process [5, 7, 81. This allows one to image soft surfaces without destructive shear forces. During the last few years the tapping mode has become a standard technique for the investigation of polymers. Here a cantilever with a high spring constant of about 50 Nlm is oscillated near its resonant frequency, typically in the range of 100-300 kHz with amplitude values of 30-100 nm. To control the distance between the tip and surface, constant damping of the amplitude is used (Fig. 1, center).

Comparison of the freely oscillating cantilever and the cantilever tapping the surface shows a shift of the resonance frequency. This shift depends on the viscoelastic properties of the sample and

Morphology of Toner-Silica Inte~aces 769

on the adhesive force between the sample and the tip. Measuring at a given constant frequency, this shift in resonance frequency can be detected as a variation in the corresponding phase (Fig. 1, center). Working in the repulsive force regime, the shift in resonance frequency is dominated by the elastic modulus of the sample. A harder material will cause a larger phase shift than a softer one (Fig. 1, right-hand side). Thus, phase imaging can be used to investigate mechanical, e.g., elastic, properties of the sample or distinguish different materials by their mechanical properties. A detailed description of this technique is given in Ref. [7]. As in the topography images, in the phase images different phase shifts are mapped as different colors: dark areas are the relatively soft materials and bright areas are relatively hard materials.

The focus of this study is on characterization of the toner surface morphology and description of the morphology of toner-silica interfaces. To determine the adhesion forces at toner-silica interfaces the distribution of silica particles on toner surfaces has been analyzed with respect to the toner composition and the chemical properties of the silica particles.

Experimental

Raw materials

Two types of toner were used: toner 1 (monocomponent magnetic; resin: styrene-acrylic; dso =

13 pm; Tsoft = 100 "C, crushed type) and toner 2 (nonmagnetic; colored (magenta); resin: polyester-epoxy; d50 = 12 pm; Tsoft = 95 "C, crushed type). These toners were blended with silica. For blending silica and toner a low-rotating mixer was used; silica loading was 0.4 wt% and mixing time t = 60 min. Two types of silica were applied for the blending: Silica 1 (Wacker HDK H2050EP, tribocharge: positive) and Silica 2 (Wacker HDK H30TX, tribocharge: negative).

SFM experiments

Samples were prepared by dispersing silica, toner, or toner-silica blends on mica. For all samples a suspension of toner or silica-loaded toners in ethanol were prepared. This suspension was spin-coated onto the freshly cleaved mica. For all measurements a commercial stand-alone SFM (D3100, Veeco Instruments, Santa Barbara, CA) was used. Images were formed under ambient conditions using a normal Olympus Tapping Mode cantilever with a resonance frequency of 270 kHz. A ratio of about 0.75 between the tapping amplitude and the free cantilever amplitude was used. To get a flat image of the spherical toner particle, second-order surface leveling has been applied to the raw topography data. Thus, surface structures appeared that reflected the roughness of the toner sphere.

Results and Discussion

Figure 2 shows the topography (left) and phase shift (right) images of silica-free toner surfaces of

770 S. Hild, H. Barthel, M. Heinemann, U. Voelkel, J. Weis

toner 1 recorded in tapping mode. Obviously, the toner surface is relatively rough: on a z-scale the maximum height is in the rage 0.1-0.25 pm. Additionally, the phase shift reveals two incompatible phases of different stiffness which we attribute to a hard polystyrene phase (isolated) in a poly(alky1 methacrylate) matrix (continuous). Contrary to the immiscible blend of polyalkyl-methacrylics/polystyrene, where a dispersion of hard polystyrene is observed in a softer continuous polyalkylmethacrylics phase, polyester-epoxy resins appear to be a homogeneous. Additionally, compared to the styrene-acrylics-based toner, the surface of the crushed polyester-epoxy resin appears to be less rough (Fig. 3).

Fig. 2. The surface of toner1 shows areas with different

particle sizes and z ranges.

Fig. 3. The surface of toner 2 appears to be smoother

than that of toner 1.

Figure 4 shows the topography (left) and phase shift (right) images of toner 1 loaded with silica 1. It turns out that phase imaging is an effective tool to locate the hard silica particles on the softer polymer matrix. In particular the phase shift images easily distinguish the silica on the toner background. The silica particles seem to adhere preferentially at the hot spots, i.e., at edges and holes of the toner surfaces. This suggestion is supported by the observation that on the smooth surface of toner 2 only very few silica particles can be found (Fig. 5). The influence of the silica particle dimension on the surface of the toner particle is still not fully understood and needs further investigation.

Fig. 4. Surface of toner 1 loaded with silica 1. Fig. 5. Surface of toner 2 loaded with silica 2.

Morphology of Toner-Silica Intelfaces 771

Conclusion and Outlook

SFM is an ideal tool for estimating the morphology of toner and toner-silica interfaces. Silica particles are clearly identified due to differences in hardness with respect to the resin. SFM resolves different material properties at the resin surfaces (hardsoft domains) which we relate to polyacrylic and polystyrene domains in the case of the styrene-acrylics toner. Obviously, silica particles are mainly located at edges and holes on the resin surface (silica loading 0.4 wt%). For the polyester-epoxy based toner the investigation has been extended to smaller and larger silica particle sizes. Both silicas are clearly detected on the resin surface as particles of a stiffer material. Both silicas show aggregates and small agglomerates which tend to adhere to edges and craggy areas of the resin surface. Further work will be dedicated to more detailed investigation of the influence of the particle size on the adhesive properties of silica particles to toner surfaces. Also, adhesion forces will be directly measured at the nanoscale level using the SFM.

References [ 11 [2] [3] [4] [5] [6]

[7] [8]

H. Barthel, M. Heinemann, Pun-Puciflc Imaging Conference/Jupan Hurcopy '98, 1998,428. W. H. Barthel, M. G. Heinemann, Electrophotogruphy 1995,34(4), 401. G. Binning, C. F. Quate, Ch. Gerber, Phys. Rev. Lett. 1986,56,930. Th. Stifter, E. Weilandt, 0. Marti, S. Hild, Appl. Phys. A 1998, 66, 597. D. Sarid, V. Elings, J. Vuc. Sci. Technol. 1991, B9(2), 331. H. A. Mizes, K.-G. Loh, R. J. D. Miller, S. K. Ahuji, E. F. Grabowski, Appl. Phys. Lett 1991, 59, 2901. S. N. Magonov, V. Elings, M.-H. Whangbo, Surf: Sci. 1997, L385-L391,375. H.-U. Krotil, E. Weilandt, Th. Stifter, 0. Marti, S. Hild, Surf: Interface Anal. 1999,27, 341

Selective Surface Deposition of Colloidal Particles

Christian Kriiger, Esther Barrena, Ulrich Jonas"

Max Planck Institute for Polymer Research Ackermannweg 10,55128 Mainz, Germany

E-mail: jonas@mpip-mainz.mpg.de

Keyword: fabrication, silane layer, colloid particle, polymer latex, regioselective adsorption, deposition, particle pattern, self-assembly

Summary: The fabrication of complex surface patterns by selective assembly of polymer latex particles onto laterally patterned silane layers on silicon dioxide substrates is described. For this purpose poly(buty1 acrylate) (PBA) particles with a narrow size distribution and negative surface charge were synthesized by emulsion polymerization under monomer-starved conditions, which can form well defined colloid crystals. As a second class of colloid materials polystyrene (PS) latex particles with a corrugated surface and low overall surface charge were prepared under soap-free conditions. The laterally structured silane layers for the regioselective adsorption of the colloidal particles were fabricated by deposition of reactive silanes carrying different functionalities in combination with photolithographic patterning techniques. The particle patterns were prepared by colloid assembly from aqueous suspension due to specific interactions between the latex surfaces and the functionalities in the silane layers. The particles, silane surfaces, and assembly structures were characterized by optical microscopy, scanning force microscopy, and low voltage scanning electron microscopy.

Introduction

With the accelerated miniaturization tendency found in various fields of technology, new methods for the controlled assembly of the individual components constituting a functional device become increasingly relevant. When the size of these component falls below millimeter dimensions such objects cannot be positioned easily by common handling procedures (controlled by humans or robots). One strategy to surpass this problem is the application of self-assembly concepts known from biology and supramolecular chemistry to mesoscale objects, sometimes referred to as fluidic assembly [ 1-81. This technique allows the assembly of tiny objects into complex structures by self- organization using attractive (and preferably) reversible interactions between opposing object surfaces to control the assembly process. The method is highly parallel so that many objects can be assembled simultaneously (high throughput) and bears no principal size limitation; this makes the

Ovganosilicon Chemisty V Edited by N. Auner and J. Weis

Copyright 0 2003 WILEY-VCH Verlag GmbH & Co. KgaA, Weinheim

Selective Sugace Deposition of Colloidal Particles 773

technique applicable to objects and patterns in the size range of nanometers to millimeters. Colloidal particles are of particular interest as model objects since they can be made of a large

variety of materials in different sizes, and due to their intrinsic property to form ordered crystals they may be used in optical devices as photonic bandgap filters. The specific adsorption of polymeric latices from suspension onto unpatterned surfaces was achieved by employing attractive Coulomb interactions between negatively charged particles and positively charged substrates, leading to porous colloid layers with irregular morphologies [9-151. Using a complex procedure to deposit charged thiols onto gold surfaces by microcontact printing and subsequent adsorption of polyelectrolyte multilayers, patterned substrates were obtained which allowed the regioselective adsorption of oppositely charged latex particles, again by Coulomb interactions [ 16, 171.

In order to use technologically important substrates with silicon dioxide surfaces (like silicon, glass, quartz) we have investigated the possibilities of fabricating laterally patterned surface properties with alkoxysilanes and photolithographic patterning techniques already well established in industrial processes (microelectronics, printing, etc.). By this approach we can tailor the surface properties and interactions by regioselective deposition of silane molecules with specific functional groups in only a few processing steps. The patterning procedure can in principle also be extended to electron beam lithography and microcontact printing to yield very high resolution or throughput, respectively.

Additionally, we have synthesized poly(buty1 acrylate) (PBA) and polystyrene (PS) latices with specific surface charge, size, and morphology. The tailored synthesis of polymer colloids allows in general the control of particle size, morphology, crosslinking density, and surface functions (type of function, surface density of functions) for a large number of monomers. It is thus possible to produce and study colloids with various complementary surface functions as model objects for the assembly process.

Experimental Section

Materials

All chemicals were used as received (unless stated otherwise): hydrogen peroxide solution -30 % (Merck); conc. sulfuric acid, acetone (Riedelde Haen); Microposit S 1805, Microposit ma-T 1041, Microposit MF CD26, Microposit 1 165 (Microresist Technology); methylene chloride, cyclohexane (Fisher Scientific); hexamethyldisilazane (HMDS, 98 %), octadecyltriethoxysilane (OTE, 95 %), aminopropyltriethoxysilane (APTE, 98 %), N-trimethoxysilylpropyl-N,N,N-trimethylammonium chloride (N&+, 50 % in methanol) (ABCR); fluoresceinisothiocynate (FITC, 90 %), n-butyl acrylate (nBA, 99 %), ally1 methacrylate (AMA, 98 %), methacrylic acid (MAA, 98 %), styrene (99 %), sodium dodecylsulfate (SDS, 98 %) (Fluka), divinylbenzene (DVB, 80 %) (Polyscience), Dowfax 2A1 (Dow, 45 %) (Dow Chemical), potassium persulfate (KPS, 98 %) (Aldrich). Deionized water was passed through a Millipore Gradient device (R = 18.2 MQ cm) and used throughout.

774 C. Kriiger, E. Barrena, U. Jonas

Patterned Silane Layers

Silicon wafers (Wacker Siltronic, with native oxide layer, 1.6 nm thickness) and glass substrates (Menzel) were cleaned with Piranha solution (H2SOdH202, 5:l v/v) according to Goss et a1 [18] before spin coating (4000 rpm, 1 min) with a 1:l mixture of a (+)-photoresist (Microposit S 1805) and thinner (Microposit ma-T 1041). The resulting resist layer (110 nm thickness from SFM measurement) was exposed to UV light (254 nm, 40 s for Si wafers, 30 s for glass substrates, penray 2 W) through a gold mask on quartz and then developed (1 min, Microposit MF CD26) and washed with water.

Vapor Deposition of HMDS

The photoresist-patterned Si substrates were placed in a sample holder inside an appropriate reaction vessel (approx. 500 mL, home-built) and 2 g HMDS was added to the bottom of the glass vessel. The reaction took 30 min at room temperature. After silanization the substrates were immersed in a remover solution (5 min, Microposit 1 165), washed with methylene chloride/acetone (1:l v/v), then wiped with ethanol-soaked tissue (KimWipe), rinsed with water, and sonified in cyclohexane (sonification repeated three times, replacing the cyclohexane). Water contact angle: 84"/75"/65" (advancing/static/receding).

Solution Deposition of APTE

A 1 % silane solution in water was stirred for 15 min and filtered through a 0.2 pm PTFE filter. The substrates were immersed for 30 min and, after washing with MilliQ-water, tempered at 110 "C for 1 h. The final remover and cleaning procedure is described above (HMDS). Water contact angle: 58"155"/26" (advancinghtaticlreceding).

Labeling of APTE Layers with FITC

Na2C03 (25 mg) and NaHC03 (134 mg) were dissolved in 25 g MilliQ-water. After adding 10 mg FITC the solution was stirred for 5 min and filtered through a 0.2 pm F'TFE filter. The substrates were immersed for 30 min in the 1:lOO dilution of the filtrate and then washed with a solution of 7.04 g NaCl, 40 g MilliQ-water and 200 mL ethanol. Finally, the labeled substrates were flushed with MilliQ-water and stored in the dark.

Vapor Deposition of OTE

The photoresist-patterned Si substrates were placed in a sample holder inside an appropriate reaction vessel (approx. 500 mL, home-built). OTE (2 g) was added to the bottom of the glass vessel and after evacuation (0.03 mbar) the whole container was heated to 130 "C for 1.5 h. The final remover and cleaning procedure is described above (HMDS). Water contact angle: 1 1 1 "/109"/8 1 " (advancing/static/receding).

Solution Deposition of N-Trimethoxysilylpropyl-N,N, N-trimethylammonium Chloride (NRd')

An aqueous 2 % silane solution (from stock solution of 50 % silane in methanol) was stirred for 15 min and filtered through a 0.2 pm PTFE filter. The substrates were immersed for 30 min, then

Selective Surface Deposition of Colloidal Particles 775

washed with water, tempered at 110 "C for 1 h, and then cleaned as described above (HMDS). Water contact angle: 72"/69"/47" (advancing/static/receding).

Silane Layer Characterization

SFh4 images were taken in air at room temperature on a Digital Instruments Nanoscope IIIa multimode surface probe microscope operated in contact and tapping mode with a silicon cantilever (spring constant 14 N/m). Ellipsometric film thickness measurements were carried out on a home- built variable-null ellipsometer [19] with a He-Ne laser (h = 632.8 nm) light source at 70" incidence on unpatterned samples. The film thickness (0.1 nm standard deviation) was calculated from the measured relative phase shift A (average of three measurements at three positions) using a clean Si substrate as reference. Water contact angle measurements were performed on a Kriiss Contact Angle Meter GI (goniometer type) at ambient temperature. Fluorescence microscopy images were taken with an Axioskop microscope (Zeiss) equipped with a reflection mode setup (50 W HBO lamp), fluorescence optics (L = 450-490 nm, 5, >515 nm), and a digital AxioCam camera (Zeiss). The images were contrast enhanced by expanding the maximum range of existing gray levels in the raw image to the maximum range of representable gray scales in the final image using the auto levels feature of the image processing software GIMP (http://www.gimp.org).

Synthesis and Characterization of Carboxylated Latex Spheres

PBA(~1)250-C00H

Table 1. Recipe for the preparation of PBA(x1)250-COOH particles.

Basis Monomer feed Initiator feed

Seed stage 5 0 g H20

3.5 g BMA -

0.1 g SDS

Core stage 28.67g seed 12.87g BMA

1 5 g H20

Shell stage 1 2 g core 7.27g nBA

7.5 g H20 0.91 g AMA

0.91 g MAA

60mg KF'S

l o g H2O

27mg KF'S

0.13 g SDS

11.7g HzO

45mg KPS

0.22 g Dow

5 0 g H2O

[a] The initiator feed was added during the seed stage in one portion after reaching 80°C. All other feeding times were 5 h each.

The core-shell particles were prepared by seeded semi-continuous emulsion polymerization under monomer-starved conditions. A detailed experimental procedure for similar latices is given by Winnik et al. [20] and Kriiger et al. [21]. The seed was prepared by batch emulsion polymerization. After synthesizing the PBMA core a highly crosslinked PnBA-shell with MAA comonomer

776 C. Kruger, E. Barrena, U. Jonas

(10 mol % of total monomer feed) was introduced by an additional semi-continuous step. The shell represents 78 % of the total monomer mass added. The recipe is given in Table 1. From the dynamic light scattering measurements a diameter of 250 nm was obtained for the PBA(x1)250- COOH particles with a narrow polydispersity index of 0.03, also visible in Fig. 1 (below). The number of surface groups was determined by polyelectrolyte titration under shear flow (experimental details below) yielding a surface requirement (parking area) of 7 A2 per acid group. This corresponds to a very high acid concentration at the particle surface, probably due to branched poly-methacrylic acid,

PS-DVB-SO4

The microgels were prepared by soap-free emulsion polymerization in a batch process starting from 3.4 g styrene, 69 mg DVB and 15 g H20, which were introduced into the reaction flask and purged with nitrogen for 15 min. After heating to 80 "C under nitrogen 90 mg K&08 was added. Stirring at 80 "C was continued for an additional 20 h before cooling to room temperature. Dynamic light scattering gave a diameter of 593 nm and a polydispersity index of 0.205. No apparent surface charge could be detected by polyelectrolyte titration for these particles.

PuriJication of Latex Dispersions

All dispersions (sample volumes 10 mL, solid content = 1 %) were purified by ultrafiltration through a Millipore Ultrafee 15 centrifugal filter device (exclusion size = 30000 D) at a centrifugation speed of 2000 min-' for 30 min to remove excess monomer, surfactant and other water-soluble residues. The purification step was repeated with fresh water until the dispersion reached constant pH.

Particle Characterization

Particle diameter and particle size distribution (polydispersity index) were measured by dynamic light scattering on a Zetasizer 5000 (Malvern). Low voltage scanning electron microscope (LV- SEM) images of unsputtered latex samples on adhesive carbon foil were taken with an LEO Gemini 1530 at acceleration voltages of 0.2-1 keV. The particle surface charge (average from three measurements) was determined by quantitative polyelectrolyte titration on unshielded latices on a PCD 03 pH titrator (Mutek), which shears the diffuse counter-ion cloud from the particle surface, inducing a flow potential. The latex solution is adjusted to pH 10 with 10 % aqueous NaOH and titrated with a 0.001 M PDADMAC (polydiallyldimethylammonium chloride) solution.

Particle Assembly

A droplet of the latex suspension in water (solid content 0.1-1 %) was placed onto the patterned substrate, covered to prevent evaporation, and incubated for 23 h (preferably over night) at 4 "C (refridgerator). Excessive liquid was then slowly removed with a pipet and some patterns (when stable enough, e.g. carboxylated latex spheres on m+) were carefully rinsed with water and ethanol. We refer to this procedure as "drop coating". The assembly structures were investigated immediately after preparation with the optical microscope (Axioskop, Zeiss) in reflection mode,

Selective Su~ace Deposition of Colloidal Particles 777

using differential interference contrast (DIC) and dark field optics.

Results and Discussion

Latex Particles

Since polymeric latex particles can be synthesized in a wide range of sizes and with varying monomer composition, they represent ideal model objects for a study of their self-organization at patterned surfaces.

The PBA particles used in this study were obtained by seeded semi-continuous emulsion polymerization under monomer-starved conditions in a multistep process. In this respect they are similar to polymer latices used for other surface assembly experiments [21] but with a much higher surface charge. In general, seed particles are synthesized as a batch in the first step, then the core particles are grown from the seed, and in the last step a shell with a high content of carboxylic acid functions is added. By applying a simple model in which all added monomer is distributed among the seed particles and no second initiation step occurs (monomer-starved conditions), the particle sizes can be predicted very accurately for the second and third polymerization steps. The core particles obtained after the second step (Fig 1, left) have a diameter of 159 nm (predicted: 160 nm), contain pure poly(buty1 methacrylate) (PBMA) and do not possess a significant surface charge (Figure 1, left).

Fig. 1. LV-SEM of PBA(x1)250-COOH latex particles after second polymerization stage (left, diameter 159 nm) and

third stage (right, diameter 250 nm).

When a shell of PBA in the third stage is added with 10 % free methacrylic acid and 10 % crosslinker, highly charged (7 w2 per acid group) latex particles with a diameter of 250 nm (predicted: 258 nm) are obtained that show a very narrow size distribution and a strong tendency to crystallize into well ordered arrays (Fig. 1, right).

778 C. Kriiger, E. Barrena, U. Jonas

A second class of colloids was synthesized in a one-step batch process from styrene under soap- free emulsion polymerization conditions. This procedure resulted in PS particles with a diameter of about 590 nm and a very interesting morphology. The particles have a highly corrugated surface (Fig. 2) and at higher resolution a few smaller particles with a diameter of around 100 nm can be observed (Fig. 2, right), explaining the somewhat larger size distribution found by dynamic light scattering (polydispersity index: 0.205). The irregular shape and the presence of smaller particles suggest that the large PS particles are formed by aggregation of small particles during the emulsion polymerization step. Since the polymerization is carried out in the absence of a stabilizing emulsifier, aggregation of small particles before complete polymerization is certainly possible. Due to the corrugated morphology and the broader size distribution the PS colloids cannot crystallize and form only highly disordered aggregates in the solid (Fig. 2, left).

Fig. 2. LV-SEM of PS-DVB-S04 latex particles with corrugated surface (diameter approximately 590 nm).

Patterned Silane Layers

To control the regioselective adsorption of latex particles at predefined positions on glass, quartz, and silicon surfaces (with a native oxide layer) different silanes were chemisorbed onto the silicon oxide surfaces. The various silanes carry different substituents that form different types of interactions with the colloid particle surface. The regioselective deposition of the silanes is achieved by masking the substrate surface with a photolithographic film of the desired pattern. After development of the photoresist film the free surface areas are reacted with the first silane via the vapor phase. In the current case we used hexamethyldisilazane (HMDS) or octadecyltriethoxysilane (OTE) to obtain a square pattern of hydrophilic squares (40 pm x 40 pm) of silicon oxide surface with silanol functions (SiOH) in a hydrophobic matrix of SiMe3 (HMDS) or Si(CH&CH3 (OTE) groups. The process of vapor phase deposition is favored over the common solution deposition of silanes since the solvent used in the latter process could negatively interfere with the photoresist (swelling, delamination), leading to imperfect silane patterns.

After removal of the photoresist the remaining SiOH surface can reacted with a second silane from the vapor phase or from solution. We have used aqueous solutions of

Selective Sulface Deposition of Colloidal Particles 779

aminopropyltriethoxysilane (APTE) to generate squares with primary amino functions (Fig. 3, left) or N-trimethoxysilylpropyl-N,N,N-trimethylammonium chloride (Nk') to yield squares with a permanent positive charge. The polarity difference of the HMDS and APTE surfaces, for example, can be visualized by water condensation patterns in the optical microscope (Fig. 3, left). In this experiment the substrate is cooled down with a Peltier element under the microscope (bright field optics) until water condensation occurs. On the more polar APTE squares with the primary amino groups condensation occurs first, leading to an optical contrast to the water-free methyl-terminated HMDS surface.

The free amino groups can be chemically modified, which was done with fluorescein isothiocyanate (FITC) to visualize the surface layer pattern in fluorescence microscopy (Fig. 3, middle, and right). Interestingly, an inverted fluorescence contrast is observed where the reactive amino squares with high FITC concentration show lower fluorescence intensity than the surrounding trimethylsilyl regions. This effect might be due to self-quenching of the close-packed FITC molecules in the amino squares, whereas the unspecifically bound FITC on the methyl surface is at such a low concentration that the individual fluorescein molecules are well enough separated not to show energy transfer [22 ] . The presence of surface-bound FITC in the amino regions can be proven by bleaching experiments with the microscope light at high magnification, leading to a complete decease of the fluorescence in the illuminated region (Fig. 3, right).

Fig.3. Left Optical micrograph of a water condensation pattern on APTE (bright) squares surrounded by a

trimethylsilane layer (HMDS, dark). Middle: FITC-stained APTEMh4DS surface pattern with inverted

fluorescence contrast. Right: Photobleaching of the fluorescence at the FITC-stained surface pattern.

To assess the quality and morphology of the silane layers, scanning force microscopy (SFM) was used in contact and tapping mode. Figure 4 shows the topographic image of an ATPE layer on silicon in tapping mode. The surface is smooth over a large area of 4 pm x 4 pm with a weak granular profile of about 1 nm in height. The absence of large three-dimensional aggregates and the small surface roughness (about twice that of the substrate) speak for a well defined monolayer of APTE in accordance with the layer thickness of 0.6 nm measured by ellipsometry.

In the case of an OTE pattern with square holes exposing the free silicon oxide surface the height of the OTE layer can be measured in contact mode as being 0.6 nm (Fig. 5, left and middle). The ellipsometric measurements give a height of 1.1 nm for such an OTE layer. The lower height

780 C. Kriiger, E. Barrena, U. Jonas

observed in the contact SFM might be due to a mechanical deformation of the monolayer with the SFM tip. These data demonstrate that well defined monolayers of OTE can be deposited via the vapor phase. The high contrast in the friction image on the right of Fig. 5 indicates a strong difference in surface properties, which is desirable for the selective adsorption of colloidal particles. When the squares are covered with the quaternary ammonium silane (N&+) no height difference is visible any more in the topography, but the different material properties of OTE and w+ can be visualized in phase mode (measuring the phase lag between the driving signal of the oscillating cantilever and its response signal).

nn

.- 0

I

0 L O O 2.OO 3 .00 Lln

Fig. 4. AFM topography image (tapping mode) of a silicon substrate modified with APTE and corresponding line

scan (right).

Fig. 5. SFM line scan (left) and images in contact mode of a rectangular hole in an OTE layer on silicon.

Particle Assembly

The assembly of the colloidal particles on the patterned silane layers was achieved through adsorption from a droplet of latex suspension deposited onto the horizontal substrate (drop coating method). For the interaction of the particles with the silane surface several scenarios have to be considered. The PBA particles are composed of an intrinsically hydrophobic polymer, but they

Selective Sugace Deposition of Colloidal Particles 781

carry carboxylic acid functions on their surface which are highly polar, can be deprotonated to form anions (depending on the pH of the medium), and can form hydrogen bonds. The PS colloids carry only very minute amounts of anionic sulfate groups introduced by the initiator.

The CH~terminated silane layers (HMDS and OTE) are extremely apolar and form van der Waals interactions with molecules in close proximity only. A weak polar influence of the SiO2 substrate on the surrounding medium through the very thin SiMe3 layer of HMDS can be expected, as indicated by the lower water contact angle of 84" (advancing) than the thick OTE layer with a contact angle of 1 1 1 O (advancing). With these apolar surfaces the colloid particles should form only weak van der Waals interactions with low preference for adhesion.

The free silica surface is polar and can form hydrogen bonds via its silanol functions SiOH. At very low pH (12-3) the silica surface can be slightly positively charged, with the charge increasing with decreasing pH [23]. At higher pH (29) the silica surface can be significantly negatively charged, with the charge increasing with pH. The polar polymer particles can adhere to the silica surface by polar and charge interactions depending on the pH of the medium.

The APTE surface with primary amine functions possesses polar groups which can be protonated at lower pH of the surrounding medium and that can form hydrogen bonds. These groups serve as good complements for the adhesion of the carboxylated polymer spheres in the assembly process at intermediate to low pH.

The NR4' layers carry a permanent positive charge on the nitrogen that cannot be altered by pH changes of the medium (only the double layer charge can be altered). This polar surface is the ideal complement for the carboxylated colloids at high pH due to strong Coulomb interactions of the deprotonated acid groups with the positive charge of the ammonium function.

In the particle adsorption experiments at varying pH it was found that the polymer particles (both polyacrylate and polystyrene) adhere to the SiOH squares at pH I 5 (Fig. 6, right). Weakly charged PBMA particles preferred the hydrophobic surface at pH above 9 when suspended in the aqueous medium [21]. Upon drying, these inverted patterns were often destroyed by the water meniscus retracting from the hydrophobic surface regions and adhering to the hydrophilic regions, dragging the particles along by capillary forces. The highly charged PBA(x1)250-COOH particles could be selectively adsorbed to the amino surface of an HMDS/APTE pattern at pH 3.5 (Fig. 6, left).

Fig. 6. Left: PBA(x1)250-COOH particles adsorbed to the amino surface of an HMDS/APTE substrate. Middle:

PBMA particles with COOH surface groups assembled onto an OTE I m+ pattern. Right: Regioselective

assembly of PS particles on an OTE/SiOH pattern.

782 C. Kriiger, E. Barrena, U. Jonas

At pH 5 the negatively charged PBMA particles adhered strongly to the permanently positively charged N&+ surfaces by strong Coulomb interactions, leading to a complete coverage of the ammonium squares with particles (Fig. 6, middle). Based on this strong preference of the particles for the charged surface over the apolar surface, it was attempted to create assembly patterns in the dimension of the particles. This is shown for individual lines with a thickness of about 500 nm in Fig. 7. The particles follow the line shape, with partial interruptions along the lines. In the SFM image in Fig. 7, right, individual particles are resolved within a line. The tendency to cluster in a hexagonal packing with a line thickness of two to five particles can be seen at this high resolution.

Fig. 7. Carboxylated PBMA particles selectively adsorbed on a line pattern of N%+ surrounded by an OTE surface.

Right: Optical microscopy image in DIC mode. Left: SFM image in tapping mode.

Conclusions

It could be shown that controlled surface deposition of colloid particles is possible by selective interaction of colloid surface groups with specific functions in silane layers. This was demonstrated for polyacrylate, polymethacrylate and polystyrene latices with COOH and S04H functions, respectively, on silica, amino, and quaternary ammonium surface patterns.

The selectivity of the particles depends not only on the surface functions of the particles and the substrate, but also on the pH of the adsorption medium.

Utilizing the high attractive interactions of carboxylated PBMA particles with positively charged ammonium silanes, assembly patterns with dimensions down to several particle diameters were achieved, promising the possibility of generating latex surface patterns in the dimension of individual particles with this method.

Further work is in progress to investigate the role of charge density, nature of functional groups, pH, and salt concentration of the medium in the assembly process, and to explore the possibilities of chemical reactions between the assembled objects and the substrate surface.

Selective Su$ace Deposition of Colloidal Particles 783

Acknowledgments: The authors thank Prof. Hans Wolfgang Spiess for his helpful discussion and continued support of this research. Furthermore the authors thank Wacker Siltronic for the generous gift of the silicon wafers. Dr. Kenichi Morigaki is acknowledged for his support in fabricating the photolithography masks, Uta Pawelzik for the latex preparation and dynamic light scattering analysis, Volker Scheumann for the SFM measurements, and Gunnar Glaljer and Dr. Giinter Lieser for the SEM images. Financial support by the Max-Planck-Society, the Fonds der Chemischen Industrie, and the Gesellschaft Deutscher ChemikerDr. Hermann Schnell Stiftung is highly appreciated.

References M. B. Cohn, C.-J. Kim, A. P. Pisano; “Self-Assembling Electrical Networks: An Application of Micromachining Technology”, Proc. 6th Int. Con$ Solid-state Sensors and Actuators (Transducers ’91), Sun Francisco, CA, USA, 24-28 June 1991, p. 49W93. J. S. Smith, H.-J. H. Yeh; “Fluidic Self-Assembly of Microstructures and its Application to the Integration of GaAs on Si“, Sensors and Materials 1994,6(6), 319-332. K. Hosokawa, I. Shimoyama, H. Miura; “Two-Dimensional Micro-Self-Assembly Using the Surface Tension of Water”, Sensors and Actuators 1996, A57, 117-125. N. Bowden, A. Terford, J. Carbeck, G. M. Whitesides; “Self-Assembly of Mesoscale Objects into Ordered Two-Dimensional Arrays”, Science 1997,276,233-235. J. Tien, A. Terfort, G.M. Whitesides; “Microfabrication through Electrostatic Self- Assembly”, Langmuir 1997, I (20), 5349-5355. S. C. Esener, D. Hartmann, S. Giincer, C. Fan, M. Heller, J. Cable; “DNA-Assisted Assembly of Photonic Devices and Crystals” in: S.D. Fantone (ed), Proc. Spatial Light Modulators. Topical Meeting. OSA Trends in Optics and Photonics Series 1997,14,65-68. U. Srinivasan, R. T. Howe, D. Liepmann; “Fluidic Microassembly Using Patterned Self- assembled Monolayers and Shape Matching”, Proc. Int. Con$ on Solid-State Sensors and Actuators, Sendai, Japan, June 7-10 1999, 1170-1 173. T. Nakakubo, I. Shimoyama; “Three-Dimensional Micro Self-Assembly Using Bridging Flocculation”, Sensors and Actuators 2000,83, 161-166. R. K. Iler; “Multilayers of Collidal Particles”, J. Colloid Inte$uce Sci. 1966,21,569-594. C. A. Johnson, A. M. Lenhoff; “Adsorption of Charged Latex Particles on Mica Studied by Atomic Force Microscopy”, J. Colloid Integace Sci. 1996,179,587-599. S . Slomkowski, B. Miksa, M. Trznadel, F.W. Wang; “Synthesis, Morphology, and Properties of Latex Monolayers at the Quartz-Liquid Interface”, ACS Polym. Prep. 1996,3(2), 747-748. V. N. Bliznyuk, V. V. Tsukruk; “Composite Self-Assembled Films from Charged Latex Nanoparticles”, ACS Polym. Prep. 1997,38(1), 963-964. T. Serizawa, H. Takeshita, M. Akashi; “Electrostatic Adsorption of Polystyrene Nanospheres onto the Surface of an Ultrathin Polymer Film Prepared by Using an Alternate Adsorption Technique”, Langmuir 1998,14(15), 40884094.

[14] A. Kampes, B. Tieke; “Self-Assembly of Carboxylated Latex Particles at Charged Surfaces:

784 C. Kriiger, E. Barrena, U. Jonas

Influences of Preparation Conditions on the State of Order of the Monolayers”, Mater. Sci. Eng. 1999, C8-9, 195-204.

[ 151 H. Hattori; “Anti-Reflection Surface with Particle Coating Deposited by Electrostatic Attraction”, Adv. Mater. 2001, 13(1), 51-54.

[I61 J Aizenberg, P. V. Braun, P. Wiltzius; “Patterned Colloidal Deposition Controlled by Electrostatic and Capillary Forces”, Phys. Rev. Lett. 2000,84(13), 2997-3000.

[I71 K. M. Chen, X. Jiang, L. C. Kimerling, P. T. Hammond; “Selective Self-organization of Colloids on Patterned Polyelectrolyte Templates”, Lungmuir 2000, I6(20), 7825-7834.

[ 181 C. A. Goss, D. H. Charych, M. Majda; “Application of (3-Mercaptopropy1)tnmethoxysilane as a Molecular Adhesive in the Fabrication of Vapor-Deposited Gold Electrodes on Glass Substrates”, Anal. Chem. 1991,63(1), 85-88.

[ 191 M. Biesalski; “Terminal an Festkorperoberflachen gebundene Polyelektrolytbursten: S ynthese und Quellverhalten”, PhD Thesis, Johannes Gutenberg-Universitat Mainz, Germany 1999.

[20] M. A. Winnik, P. Pinenq, C. Kriiger, J. Zhang, P. V. Yaneff; “Crosslinking Versus Interdiffusion Rates in Melamine-Formaldehyde Cured Latex Coatings: A Model for Waterborne Automotive Basecoat”, Journal of Coating Technology 1999, 71,47-60.

[21] C. Kriiger, H. W. Spiess, U. Jonas; “Controlled Assembly of Carboxylated Latex Particles on Patterned Surface Layers”, Proceedings PARTEC 2001, International Congress for Particle Technology 2001,17/2,1-8.

[22] R. D. Haugland; Handbook of Fluorescent Probes and Research Chemicals, Molecular Probes, Eugene 1996.

[23] R. K. Iler; The Chemistry of Silica, Wiley, New York 1979.

Synthesis and Functionalization of Monodisperse Nanoparticles with High Optical Density Based

on Inorganic Networks

Carsten Blum*, Heinrich Marsmann

Inorganic and Analytical Chemistry, Universiat Paderborn Warburger Str. 100, D-33098 Paderborn, Germany

Klaus Huber

Physical Chemistry, Universiat Paderborn Warburger Str. 100, D-33098 Paderborn, Germany

Siegmund Greulich- Weber, Holger Winkler

Experimental Physics, Universiat Paderborn Warburger Str. 100, D-33098 Paderborn, Germany

Keywords: nanospheres, photonic crystals, sol-gel process

Summary: For the preparation of photonic crystals, colloid particles with high refractive indices are formed with diameters from a few nanometers up to several hundred nanometers. Whereas the optical density of Ti02 is already given by the titanium atom itself, the incorporation of dyes in these SiOz systems increases the refractive indices (near the absorption wavelength). The colloid particles are characterized by dynamic and static light scattering, SEM, MAS-NMR, UVNis, and optical transmission/reflection spectroscopy.

Introduction

According the method of Stober et al. [ 11 colloids with a narrow size distribution are prepared via ammonium hydroxide-catalyzed hydrolysis and condensation of tetraalkoxysilanes and titanium tetraethoxide. The particle size can also be adjusted by variation of several reaction parameters (monomer, NH3 and H20 concentration; solvent; temperature) and by controlled cocondensation.

The last method enables the incorporation of chromophores in two different ways:

cocondensation of chromophores with organyltrialkoxysilane precursors; or functionalization of the particle surface with trialkoxysilanes, followed by surface reaction

Ovganosilicon Chemisty V Edited by N. Auner and J. Weis

Copyright 0 2003 WILEY-VCH Verlag GmbH & Co. KgaA, Weinheim

786 C. Blum, H. Marsmann, K. Huber, S. Greulich-Weber, H. Winkler

with suitable chromophores.

Examples

cl

cl

111

Fig. 1. Corekhell design of nanospheres.

ClCH2CH2CH2SiO 1.5

SiO2

SiO2

H2NCH2CH2CH2SiO 1.5

surface modification of H2NCH2CH2CH2SiOl.5 with Fthodamine B dye

Particles are characterized by dynamic (DLS) and static light scattering (SLS). The ratio of hydrodynamic radius to radius of gyration (R&) (Fig. 2) correlates with spherical shape. Size distribution is defined by the polydispersity index (PDI).

Synthesis and Functionalization of Monodisperse Nanoparticles 787

180 -

160-

140 -

120 -

E 100-

IF 80-

..

60 -

40 -

20 -

90 -

E 80:

70 -

60-

50-

2

k

linear fit: Rg = 0,8413'Rh - 2,8304 r = 0,99025

I ~ , - , - , . , . , . I

4 , . , . , . , , , , , , , . , . , .

Rh /nm 40 60 80 100 120 140 160 180 200

Fig. 2.

loo I

Fig. 3. Controlled cocondensation of nanoparticles.

788 C. Blum, H. Marsmann, K. Huber, S. Greulich-Weber, H. Winkler

200 -

180-

160-

E ' 140-

[r r

120-

100-

-1

. .

. I ' I ' I ' I ~ I ' /

1.5 2.0 2.5 3.0 3.5 4.0 4.5

Table 1. Samples of SiOz and TiOz spheres: Properties.'"]

Sample R, (SLS) [nm] Rh (DLS) [nm] p = R$Rh PDI

TEOS + MEMO 109 136 0.81 1.03

TEOS -+ APS 125 157 0.81 1.03

TEOS + FLPS 135 171 0.79 I .02

Ti(Oet), + MEMO 93 112 0.83 1.07

Ti(OsiMe3)4 + MEMO 71 92 0.77 1.09

Ti(OEt)4 + PrS 121 153 0.79 1.01

[a] Abbreviations:

TEOS tetraethoxysilane ClPS 3-chloropropyltriethoxysilane

APS 3-aminopropyltrimethoxysilane Rh hydrodynamic radius

radius of gyration PrS n-propyltrimethoxysilane R,

FLPS 3-fluorenylpropyltrichlorsilane PDI polydispersity index

MEMO 3-methacryloxypropyltrimethoxysilane

References [l] [2] [3]

W. Stober, A. Fink, E. Bohn, J. Coll. Inte6 Sci. 1968,26,62. R. D. Badley, W. T. Ford, F. J. McEnroe, R. A. Assink, Lungmuir 1990,6,792. S. L. Chen, P. Dong, G. H. Yang, J. Coll. Interf Sci. 1997,189,268.

Oxidation States of Si and Ge Sheet Polymers

Gunther Vogg, Martin S. Brandt,. Martin Stutzmann

Walter Schottky Institut, Technische Universitat Miinchen Am Coulombwall, D 85748 Garching, Germany

Tel.: +49 89 289 12758 -Fax: +49 89 289 12737 E-mail: mbrandt @physik.tu-muenchen.de

Keywords: sheet polymers, siloxene, polygermyne, topotactic transformation

Summary: The chemistry of the topotactic transformation of the Zintl phases CaSi2 and CaGe2 into sheet polymers is discussed. While silicon sheet polymers are predominantly obtained in the form of the partially hydroxyl-substituted siloxene, germanium sheet polymers are found to be pure polygermyne. Only after extended reaction times, the Ge-analogue germoxene is formed.

Introduction

In the chemistry of silicon and oxygen, sheet polymers play a special role due to their characteristic structure. On the other hand, this structure also leads to characteristic physical properties such as electronic band structures with a direct bandgap, which result in a strong visible luminescence and therefore make these materials potentially useful in optoelectronic applications. Starting from the well known transformation of CaSi2 into siloxene [l], we have shown that thin films of Si-based sheet polymers can be formed epitaxially on crystalline Si substrates [2] and have extended this to the first preparation and characterization of Ge and SiGe-alloy sheet polymers [3-51. In this contribution, we discuss the chemistry of the topochemical transformation of the layered Ca-Zintl phases of Si and Ge in more detail. While both CaSi2 and CaGe2 react with aqueous HCl to form sheet polymers, the polymers obtained exhibit a marked difference with respect to their oxidation state. In particular, silicon sheet polymers are predominantly obtained in the form of the partially hydroxyl-substituted siloxene (SizHOH),. In contrast, germanium sheet polymers are found to be pure Ge-H compounds. We also present here evidence that germoxene, the Ge-analogue of siloxene, can be formed under prolonged exposure of the Ge sheet polymers to aqueous HC1.

Siloxene and Polygermyne

The layered Zintl phases CaSi2 and CaGe2 are ideally suited for topotactic chemical reactions, since the ionic Ca interlayers separating the group-14 backbone sheets can act as built-in layers of

Ovganosilicon Chemisty V Edited by N. Auner and J. Weis

Copyright 0 2003 WILEY-VCH Verlag GmbH & Co. KgaA, Weinheim

790 G. Vogg, M. S. Brandt, M, Stutzmann

cleavage. The simplest process one can imagine is the removal of the Ca sheets, leaving the unsaturated (Si), or (Ge), layers behind. Indeed, there have been some efforts to prepare such two-dimensional active silicon [6,7], e.g., by treating a suspension of CaSiz and CC4 with gaseous Clz according to Eq. 1 [8]. As expected, the resulting products are extremely reactive and unstable, and therefore are difficult to be handled and characterized.

(CaSiZ), + n Clz -+ (Si)z, + n CaC12

Eq. 1.

In contrast, the basic reaction of Cash with aqueous HC1 is a simple ion exchange with the Ca atoms being replaced by H and OH groups. During the reaction, the negatively charged dangling bonds of the polyanion (Si-), are transformed into covalent bonds with H and OH, and the former ionic interlayer bonding is replaced by van der Waals or hydrogen bonding. This process is accompanied by an increase of the Si layer distance from 5.1 A in CaSi2 [9] to 5.4-6.2 A in the resulting product [ 10, 111, depending on the exact oxygen content. The general reaction is given by Eq. 2 with 0 I x I 2. The topotactic character of this reaction, leaving the Si backbone unchanged, has been confirmed by several authors [lo, 121. The resulting structural similarity of CaSi2 and of the reaction educt, siloxene, even includes the exact stacking of the corresponding single layers [2].

(CaSiz), + 2n HC1 + xn H2O + (SiZH2.x(OH)x)n + n CaC12 + xn H2

Eq. 2.

The parameter x in reaction 2 describes the degree of hydrolysis taking place, and therefore the amount of oxygen incorporated into the final product. For siloxene in its ideal composition, x = 1 leads to (ShHOH),, which has also been observed, e.g. according to Ref. [ 131. Thus, in the case of siloxene, Eq. 2 simplifies to Eq. 3.

(CaSiz), + 2nHC1 + nH20 + (SizHOH), + nCaC12 + nH2

Eq. 3.

Our own data obtained from energy dispersive X-ray (EDX) analysis on epitaxial siloxene films confirm this general result. Figure 1 shows the typical EDX spectrum of a 1 pm-thick siloxene film on Si(l1 l), obtained by exposure to concentrated HCl at 0 "C for one hour and washed in acetone. Whereas Ca is obviously completely removed from the sample, the film still contains some C1 not entirely washed out. From the spectrum, the Si:O ratio is determined as 2.1: 1. Thus, the formation of (SizHOH), is confirmed to be favored compared to that of related compounds with a lower or higher oxygen content. Indeed, siloxene is relatively stable in ambient atmosphere, in contrast to polysilyne (SiH),, which is reported to combust spontaneously in air [lo]. Since in the ideal siloxene structure the single Si layers are exclusively terminated by H on one side and by OH on the

Oxidation States of Si and Ge Sheet Polymers 791

other, as shown in Fig. 2a, the structure with x = 1 exhibits the highest symmetry possible, and therefore seems to be energetically favored. In fact, the binding energy of adjacent layers is found to be larger for (SizHOH), than for (SiH),, which is attributed to the interaction between H atoms bonded to Si on one layer and H atoms bonded to 0 on the adjacent layer [14]. Therefore, the formation of this special structure may well be the result of a certain feedback during the topotactic reaction.

- Siloxene _._._.__._ Polygerrnyne

Si

0 1 2 3 4

Energy (keV)

Fig. 1. X-ray emission spectra of 1 pm-thick siloxene and polygermyne films as obtained by energy dispersive

X-ray analysis (EDX). The peaks correspond to the K, lines of the respective elements, with the exception of

Ge, which is detected by the La line.

In contrast to CaSi2, we find epitaxial CaGe2 to be transformed by concentrated HCl without notable hydrolysis (x = 0) , but also topotactically as discussed in detail in Ref. [3]. The corresponding simplified version of Eq. 2 is Eq. 4, leading ideally to the oxygen-free product

nCaGe2 + 2nHCl + 2(GeH), + nCaC12

Eq. 4.

polygermyne (GeH),. In fact, no hydrogen evolution is detected during the reaction of CaGez with HC1, in contrast to what is generally observed during the preparation of siloxene. This qualitative observation is confirmed by the EDX spectrum given in Fig. 1, where only a negligible amount of oxygen (< 5 at.%) is found in polygermyne. The corresponding idealized structure of polygermyne is shown in Fig. 2b. Contrary to (SiH),, (GeH), is stable when exposed to ambient atmosphere, thereby indicating a qualitative difference between the Ge and the Si sheet polymer systems. This stronger tendency of Si to show hydrolysis in sheet polymers can be rationalized, e.g. by comparing the relevant binding energies involved: whereas the Si-0 bond (8.0 eV) is significantly stronger

792 G. Vogg, M. S. Brandt, M. Stutzmann

than the Ge-0 bond (6.6 eV), the Si-H bond (3.0 eV) is only slightly weaker than the Ge-H bond (3.2 eV) [15].

Fig. 2. Structure of group-14 sheet polymers. a) partially hydroxyl-substituted siloxene, (Si2HOH),; b) pure

polygermyne, (GeH),.

Influence of Reaction Conditions

Considering the topotactic reaction of CaSiz with aqueous HC1 solutions in more detail, we obtain the possible partial reactions in Eqs. 5a-d.

a) Si-Ca-Si + 2HCl + Si-H + H-Si + CaC12 ionexchange b) Si-H + H20 + Si-OH + Hz hydrolysis c) Si-Si-OH _j Si-0-Si-H in-plane oxidation d) Si-OH + HO-Si + Si-0-Si + H2O condensatiodcrosslinking

Eqs. 5.

Here, the reactions 5a) and 5b) together describe the total process leading to the ideal structure of siloxene. On the other hand, the latter reactions represent the incorporation of oxygen into the Si backbone as well as the crosslinking of adjacent layers and, therefore, the destruction of the siloxene crystal lattice.

It is clear that all these reactions are influenced by the specific reaction conditions used. In contrast to the ion exchange 5a), the hydrolysis reaction 5b) requires higher thermal activation. Indeed, it is known that reaction 5b) can be suppressed significantly by reducing the reaction temperature, leading to a decrease in OH present in the product [lo]. By additionally limiting the reactions 5c) and 5d), a reduced reaction temperature will affect the crystalline quality of the product positively. The same argument holds for the reaction time, too, which should be as short as possible to suppress the processes 5b-d).

Furthermore, the reactivity of water is also influenced by the properties of the acid solution itself. When concentrated aqueous HCl reacts with CaSiz, water plays a double role as solvent for HC1 and as reactant participating in the chemical reaction itself. In concentrated aqueous HCl, the H20

Oxidation States of Si and Ge Sheet Polymers 793

molecules are not free, but are bonded to the dissociated HCl molecules as hydrate shells, limiting their own mobility and reactivity. Thus, the degree of hydrolysis taking place can be influenced significantly by the HCVH20 ratio used during the topochemical reaction of CaSi2 with HCl.

Thus, as a general rule, a high quality topotactic transformation of layered Zintl phases requires low temperatures, short reaction times and high HC1 concentration, as well as the exclusion of water during washing and storage of the samples.

Polysilyne and Germoxene

Whereas siloxene and polygermyne are stable in the ambient atmosphere, the question arises whether the corresponding compounds polysilyne (SiH), and, in particular, germoxene (GezHOH), can be synthesized. For the preparation of (SiH),, the oxygen-free counterpart of siloxene, two approaches have been described so far: a reduction of the reaction temperature to below -20 "C results in a nearly oxygen-free compound [16] which, however, can contain a significant chlorine fraction attributed to intercalated HC1 molecules [17]. On the other hand, preparation at 0 "C followed by a treatment in dilute HF to remove silicon oxides is also reported to reduce the oxygen content of the resulting compound significantly [13].

We have applied both methods to our thin film samples as well. However, in the corresponding IR absorption and XRD spectra, almost no difference to siloxene with the ideal stoichiometric composition could be detected, in agreement with quantitative EDX measurements which find an Si/O ratio of about 2:l for all samples. This demonstrates how easily Si-based sheet polymers are subject to hydrolysis and that an even better protection from the ambient atmosphere and a more complete removal of dissolved oxygen in the aqueous solution might be necessary to achieve pure polysilyne, especially as an epitaxial film.

In this respect, the possible formation of oxygen-containing Ge sheet polymers, i.e. germoxene as the Ge counterpart of siloxene, could be quite informative. As discussed above, the hydrolysis 5b) required in this case could be enhanced either by higher reaction temperatures or longer reaction times. In contrast to the Si system, we have observed that the reaction temperature does not have a significant influence on the oxygen content of the product: both for 0 and -30 "C no notable oxygen fraction is detectable in the product. On the other hand, with increasing reaction time at 0 "C, an increase of the c lattice constant of the reaction product is observed by XRD, as shown in Fig. 3. From the corresponding IR absorption measurements presented in Fig. 4, it can be seen that this increase directly correlates with related vibrational modes of oxygen, in particular with a broad band around 3500 cm-' representing the Ge-OH stretching mode [18]. Therefore, we attribute the increase of the layer distance observed in Fig. 3 to the substitution of H by OH bonded to the Ge backbone, thereby gradually transforming polygermyne into germoxene.

In contrast to polygermyne, the layer distance of siloxene decreases slightly with the reaction time (Fig. 3), which can be rationalized by two effects, i.e. by the incorporation of oxygen from the OH ligands into the Si backbone represented by reaction 5c), and by layer crosslinking caused by the condensation reaction 5d). Indeed, it is clear that a further addition of OH to the comparatively

794 G. Vogg, M. S. Brandt, M. Stutzmann

- Siloxene

1h

stable composition of (Si2HOH), will favor both reactions. However, the overall effect is not very large for the reaction times discussed here.

6 2 1 ................................... . ._ ._____________ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ . _ _ _ _ _ _ _ _ _ _ _ _ _ _ a , .

T

L Polygerr-- .- - a,

J 2 5.8

5.6

0 60 120 180

Reaction Time (min)

Fig. 3. Layer distance of Si and Ge sheet polymers as a function of the reaction time in concentrated HCI at 0 "C as

determined by X-ray diffraction.

Reaction time 180 rnin

Reaction time 20 min

Ge-OH stretching bending

Stretching Ge-H y : x10 stretching .-

l i l . I , I , l . I I I !

500 1000 1500 2000 2500 3000 3500 4000

Wavenumber (cm-')

Fig. 4. IR absorption spectra of CaGea films which have been immersed in concentrated aqueous HCl at 0 "C for

different times.

Oxidation States of Si and Ge Sheet Polymers 795

For germoxene, the situation is rather different. Already at low oxygen contents this compound is found to be extremely unstable with respect to destruction of the crystal lattice. This is confirmed by the fact that the intensities of the XRD reflections decrease significantly with increasing reaction time. Therefore, the increased hydrolysis obtained at longer reaction times seems to be immediately followed by the incorporation of oxygen into the Ge backbone. Consequently, the structural stability of the sheet polymers depends on the degree of hydrolysis: while pure polygermyne is stable in air, partial OH substitution in germoxene leads to a destruction of the crystal structure. The situation is reversed in Si sheet polymers, where siloxene is found to be stable, while polysilyne spontaneously combusts in air.

Conclusions

To summarize, we find the oxygen-containing sheet polymer siloxene (SbHOH), on the one hand, and the oxygen-free polygermyne (GeH), on the other, to be the most stable compounds of the corresponding sheet polymer systems. This behaviour is linked to the significantly different affinity of Si and Ge toward oxygen and also leads to the characteristic dependence of the oxygen concentration in SiGe-alloy sheet polymers [4]. The easy destruction of the Ge layers upon substitution with OH suggests that other ligands might be better suited than hydroxyl groups to influence the luminescence properties of Ge sheet polymers.

Acknowledgment: This work was supported by Deutsche Forschungsgemeinschaft within Schwerpunktprogramm “Silicium-Chemie”.

References F. Wohler, Liebigs Ann. 1863,127,257. G. Vogg, M. S. Brandt, M. Stutzmann, M. Albrecht, J. Crystal Growth 1999,203,570. G. Vogg, M. S. Brandt, M. Stutzmann, Adv. Muter. 2000,12, 1278. G. Vogg, A. J.-P. Meyer, C. Miesner, M. S. Brandt, M. Stutzmann, Appl. Phys. Lett. 2001, 78, 3956. G. Vogg, A. J.-P. Meyer, C. Miesner, M. S. Brandt, M. Stutzmann, Chem. Monthly 2001,132, 1125. H. Kautsky, Z. Nutu$orsch. 1952, 7b, 174. E. Bonitz, Angew. Chem. Znt. Ed. 1966,5,462. E. Bonitz, Chem. Ber. 1961, 94,220. J. Bohm, 0. Hassel, Z. unorg. allg. Chem. 1927,160, 152. J. R. Dahn, B. M. Way, E. Fuller, Phys. Rev. B 1993,48, 17872. H. Ubura, T. Imura, A. Hirak, I. Hirabayashi, T. Morigaki, J. Non-Cryst. Solids 1983, 59-60, 645.

796 G. Vogg, M. S. Brandt, M. Stutzmann

[12] A. Weiss, G. Beil, H. Meyer, Z. Natugorsch. 1979,35b, 25. [ 131 U. Dettlaff-Weglikowska, W. Honle, A. Molassioti-Dohms, S . Finkbeiner, J. Weber, Phys.

Rev. B 1997,56,13 132. [ 141 C. G. Van de Walle, J. E. Northrup, Phys. Rev. Lett. 1993, 70, 1 1 16. [ 151 CRC Handbook of Chemistry and Physics, CRC, Boca Raton, 1989. [ 161 J. He, J. S. Tse, D. D. Klug, K. F. Preston, J. Muter. Chem. 1998,8,705. [17] S . Yamanaka, H. Matsu-ura, M. Ishikawa, Muter. Res. Bull. 1996,31, 307. [18] Y. P. Chou, S . C. Lee, Solid State Commun. 2000,113,73.

Light-Emitting Properties of Size-Selected Silicon Nanoparticles

F. Huisken,* G. bdoux

Max-Planck-Institut fur Stromungsforschung Bunsenstr. 10, D-37073 Gottingen, Germany

Tel.: +49 551 5176 575-Fax.: +49 551 5176 607 E-mail: fhuiske@ gwdg.de

0. Guillois, C. Reynaud

CEA/DSM/DRECAM/SPAM, CE Saclay F-91191 Gif-sur-Yvette Cedex, France

Keywords: silicon quantum dots, quantum confinement, photoluminescence

Summary: Crystalline Si nanoparticles with diameters between 2.5 and 8 nm were prepared by COz laser-induced decomposition of silane in a gas flow reactor. A small portion of the products created in the reaction zone was extracted through a nozzle into a high-vacuum apparatus to form a freely propagating molecular beam of clusters and nanoparticles. This technique enables us to select the Si particles according to their size, to deposit them on a suitable substrate, and to study their photoluminescence (PL) as a function of their size. In another experiment, the evolution of the PL was monitored as a function of the time the samples were exposed to air. With increasing oxidation time, the PL became more efficient and shifted to smaller wavelengths. In a final experiment, the Si nanoparticle samples were treated with HF to remove the oxide layer and to study the effect on the PL properties. All observations can be explained in terms of quantum confinement as the origin for the PL behavior.

Introduction

Silicon (Si) is the most widely used material of the electronics industry. Unfortunately, it is an indirect gap semiconductor and, thus, the efficiency to emit photons upon electronic excitation or charge carrier injection is extremely low since the radiative recombination of the electron-hole pair is not allowed without the assistance of a momentum-conserving phonon. Moreover, the existence of defects leads to an almost total quenching of this already rather unlikely process. As a result, one would like to develop techniques to make silicon an efficient emitter of visible photons. This will be the requirement if one wants to employ silicon-based devices for optoelectronic applications.

Ovganosilicon Chemisty V Edited by N. Auner and J. Weis

Copyright 0 2003 WILEY-VCH Verlag GmbH & Co. KgaA, Weinheim

798 F. Huisken, G. Ledoux, 0. Guillois , C. Reynaud

In the early 1990s, the studies of Canham [ l l and Lehmann and Gosele [2] demonstrated that a silicon wafer could be made to emit visible light when it was electrochemically etched in hydrofluoric acid (HF), thus producing a porous nanostructured surface. The observation of this photoluminescence (PL) was explained on the basis of the quantum confinement leading to a widening of the bandgap and a partial relaxation of the selection rules making silicon a somewhat more direct-gap material [3, 41. An even more important reason for the enhanced efficiency is an effect termed “spatial confinement” that prevents the diffusion of the carriers to nonradiative recombination centers. Due to the reduced size, the probability of the carriers finding a defect in the core is drastically reduced. However, for this to occur it is important that the crystalline Si core is either isolated or surrounded by a higher bandgap material and that the silicon nanoparticle is perfectly crystalline and does not have any dangling bonds.

Since the discovery of the intense red photoluminescence of porous silicon [ 1,2], much work has been devoted to this particular nanostructured material [4, 51 and, in the meantime, also to silicon nanoparticles [6,7] . An important issue of current studies is the influence of the surface passivation on the photoluminescence properties. It has already been said that, in the quantum confinement model, it is essential that the surface is well passivated to avoid any dangling bonds [8]. Being middle-gap defects, these dangling bonds will quench the PL. On the other hand, the surface itself may lead to surface states that can be the origin of another kind of photoluminescence [9, 101.

Recently, we have studied silicon nanocrystals produced by C02 laser pyrolysis of silane and we have been able to show that, in these experiments, the PL characteristics can be unambiguously explained by quantum confinement effects [ 11-13]. However, to observe the photoluminescence with the naked eye, we had to wait for a few hours or even a few days. It appeared that the silicon nanocrystals were passivated by natural oxidation in air and that, as time progressed, the photoluminescence became more and more intense.

In this contribution, we present our most recent results on the photoluminescence of size-separated Si nanocrystals and report on experiments devoted to the initial steps of oxidation and its influence on the PL properties of Si nanoparticles. This gives us new insight into the PL behavior of aged samples. We also demonstrate the effect of HF attack on the oxide shell of aged samples and the following oxidation as far as the PL of these samples is concerned. Short accounts of these studies have been given [14, 151. Using the etching technique with HF vapor and following oxidation, it is also possible to shift the size distribution of macroscopic samples of silicon nanoparticles collected in the exhaust line of the flow reactor to smaller sizes and to shift their PL from the near-IR to the visible. All observations presented here can be explained in the framework of the quantum confinement model. Other origins of photoemission need not be invoked.

Results and Discussion

Thin films of non-interacting silicon nanoparticles were produced by pulsed C02 laser pyrolysis of silane in a dedicated gas flow reactor incorporated into a molecular beam apparatus. Details of the experimental setup have been described [7, 11, 16, 171. Distinctive features of the apparatus are the

Light-Emitting Properties of Size-Selected Silicon Nanoparticles 799

preparation of the Si nanoparticles in a freely propagating pulsed “molecular beam” and the possibility of exploiting the fact that the velocity of the silicon nanoparticles correlates inversely with their size. Larger and heavier nanoparticles are significantly slower than the smaller and lighter ones. Therefore, by introducing a properly synchronized chopper into the molecular beam, it is possible to distribute the Si nanoparticles spatially according to their size on a nearby substrate, and thus prepare size-selected samples.

Fig. 1. PL study of a sample of Si nanoparticles produced by cluster beam deposition using a chopper for size

separation. The upper panel (a) shows a photo of the deposit when it was illuminated by a simple UV lamp. The lower panel (b) reports a selected set of PL spectra recorded at the positions indicated by the

arrows.

The size selection capability of the cluster beam apparatus is demonstrated in Fig. la, which shows a luminescent sample of silicon nanocrystals when it was exposed to the light of a laboratory UV lamp (h = 254 nm). The sample was prepared with a clockwise rotating molecular beam chopper that distributed the transmitted nanoparticles from left to right according to their size. Details of the experiment are given in a recent publication [15]. Within the deposited film, the size of the Si nanoparticles varies from 2.5 nm (on the left) to 8 nm (on the right), as was determined in situ with the time-of-flight mass spectrometer of the cluster beam apparatus. Accordingly, the color of the photoluminescence varies from yellow-orange to the IR (the color may not be properly reproduced in the figure).

In order to obtain quantitative information on the photoluminescence of the silicon nanoparticles as a function of their size, we have measured their light emission along a horizontal line with a calibrated PL spectrometer [ l l , 131. A selected set of the PL curves that were measured in steps of

800 F. Huisken, G. Ledoux, 0. Guillois , C. Reynaud

0.25 mm (on a horizontal line of the sample) is plotted in Fig. lb. In Fig. 2, we have plotted the peak positions (in eV) of the entire set of PL spectra, extracted

from the sample shown in Fig. 1, as a function of the nanocrystal size (d in nm) [15]. The data points follow nicely the inverse power law (Eq. 1) derived by Delerue and co-workers [ 181 on the basis of the quantum confinement model. The bandgap of bulk silicon enters this formula as EO = 1.17 eV. The scatter of the experimental data is much less than in an earlier comparison [l l] . This can be ascribed to the fact that all particle sizes are contained in a single sample that was prepared in a single run, thus avoiding any difference in the production conditions or the oxidation history after deposition. The discrepancy between experiment and theory observed for crystallites with diameters smaller than 3 nm can be partly explained by the finite size distribution of the particles contributing to the signal at a given position on the sample. As a result of their larger absorption cross-section and measured higher PL yield (see below), the larger particles in the tail of the size distribution shift the maximum of the PL curve to smaller energies. Besides that, another explanation can be given. Recently, Wolkin et al. [9] showed that, in oxidized porous silicon, the PL energy of very small nanocrystals (d c 2.5 nm) does not increase any more, as one would expect from the quantum confinement model. This behavior is explained with the appearance of an oxide- related surface state within the bandgap of the Si nanocrystal. The deviation from theory observed in our study for small Si nanoparticles could very well be partly due to this effect.

diameter (nm)

Fig. 2. Correlation between average diameter and PL peak energy. It is seen that, except for very small

diameters, the experimental data points compare nicely with the theory of Delerue et al. [18], which is

represented by the solid curve.

Ep&) = EO + 3.13ld 1'39

Eq. 1.

Light-Emitting Properties of Size-Selected Silicon Nanoparticles 801

In the same study, we have also determined the efficiency of the PL process by measuring carefully the ratio between emitted and absorbed energy. A pronounced maximum was found for crystallite sizes around 3.5 nm. For this size, a PL yield of 30 % was observed. Going to larger sizes, the PL yield decreases exponentially to reach a value of only 1 % for 8-nm particles. On the other side, the PL yield of 2.5-nm particles is reduced to 10 %. The extraordinarily high quantum yield of 0.3 measured for 3.5-nm Si nanocrystals is only a lower limit. Atomic force microscopy studies of our samples have shown that the size selection is not perfect. It always happens that a few Si particles with diameters larger than 10 nm, which do not exhibit visible luminescence, escape the size selection and are deposited on the substrate. These larger particles absorb the light of the exciting laser but do not contribute to the PL signal. Taking into account this effect, the corrected PL yield will exceed even 90 % [13]. This indicates that our Si nanoparticles are nicely passivated and that they have no dangling bonds at their surface. As stated before, any dangling would give rise to a radiationless recombination of the charge carriers and an effective quenching of the luminescence.

The conclusion that can be drawn from the experiments just discussed is that, except for the very small particles, the photoluminescence of our Si nanocrystals, which are produced by COz laser-assisted pyrolysis of silane and which are gently oxidized in air under normal conditions, can be perfectly explained on the basis of the quantum confinement model, that is, by the radiative recombination of electron-hole pairs confined in the nanocrystals [ 151. In order to obtain high quantum yields, the nanoparticles must be defect-free; in particular, they must be perfectly monocrystalline and all dangling bonds must be passivated, for example by a silicon oxide layer. Indeed, high-resolution electron microscopy (HREM) studies have shown that our Si nanoparticles are composed of a perfect diamond-phase crystalline core and a surrounding layer of SiO, [ 191.

Early investigations showed that freshly prepared samples that are taken out of the vacuum apparatus do not show any visible photoluminescence [20]. Although the nanoparticles are immediately covered by an oxide layer, this oxidation is not yet sufficient to passivate all dangling bonds. It always takes some time for the luminescence to appear, and the PL becomes stronger and stronger as time passes. To put these observations on a more quantitave basis, we have performed a set of dedicated experiments [ 141.

For the following study we prepared a size-selected sample with an average size of ( d ) = 3.6 nm and a rather narrow size distribution with a full width at half maximum (FWHM) of Ad = 0.6 nm. The freshly prepared sample was transferred under vacuum into the analysis chamber, where we tried to measure a PL spectrum. However, no PL could be detected unless we exposed the sample for some time to air (under normal conditions). After 20 min, some rather weak photoluminescence could be observed. The maximum position of the corresponding PL curve was at 1.72 eV. This PL curve and the following results are displayed in Fig. 3. With increasing time of exposure to air, the PL became more intense and the PL peak position further shifted to the blue. After one day, the maximum position had already experienced a shift to 1.85 eV. At later times, the effect became smaller and seemed to go into saturation. Finally, after 25 days, the maximum position was found at 1.87 eV.

802 F. Huisken, G. Ledoux, 0. Guillois , C. Reynaud

PL wavelength (nm) 900 800 700 600

1

Fig.3. Evolution of the amplitude-normalized PL spectra of a size-selected sample exposed to air for the

different cumulative times given in the figure.

1 hour 1 day 1 week 1 month 0.35 I I I I

Time (in min)

Fig. 4. Evolution of the full width at half maximum (upper panel) and peak position (lower panel) of the PL

bands as a function of the time for which the sample was exposed to air.

While the maximum of the PL band shifts to higher energies it also becomes wider. Thus, for the

Light-Emitting Properties of Size-Selected Silicon Nanoparticles 803

same period, the FWHM varies from 0.23 eV for the first spectrum to 0.31 eV after 25 days. The evolution of the two parameters, position and width of the PL band, as a function of time is summarized in Fig. 4. In this representation, it is also clearly seen that the time dependence of these two parameters saturates after approximately one week (lo4 min). In any case, all studies have shown that completely stable PL properties are achieved after six months.

For another, non-size-selected and thicker sample ( ( d ) = 4.4 nm; Ad = 2 nm), we paid particular attention to keeping the power of the exciting laser at a constant level. Therefore, the measured spectra can be readily compared as far as their PL efficiencies are concerned. As for the sample discussed above, no PL could be observed directly after preparation. This time, however, the process of complete passivation seemed to take longer since the PL could not be detected before two days. Subsequently, we observed a strong increase in the PL intensity. From two days to one month, it increased by a factor of 16.

Fig. 5. PL spectra of a non-size-selected sample after different successive treatments: after passivation in air for

two months (thick solid curve), after exposure to HF vapor for 40 min (thin and dotted gray curves), after

reexposure to air for 1 h (thin and dashed black curves), and after continued exposure to air for 2 d (dash-

dotted curve). The sketch in the upper part of the figure illustrates, from right to left, the effect of the

various treatments on the core and oxide shell of the nanoparticles (schematic).

In the final experiment to be presented here, we modified the oxide layer by HF attack and studied the effect of this treatment on the photoluminescence properties of the sample. The results are summarized in Fig. 5. We started from an already passivated sample whose PL spectrum is given by the black solid curve peaking at approximately 775 nm or 1.6 eV. Then the sample was

804 F. Huisken, G. Ledoux, 0. Guillois , C. Reynaud

exposed for 40 min to the vapor of HF. The spectrum recorded after this procedure is shown by the thin gray curve peaking near 1.55 eV. Due to the low signal, the measured PL band is rather noisy. Therefore, we have fitted the experimental curve by a Gaussian that is presented in the figure by the dotted gray curve. Then the sample was again exposed to air for definite periods of time. After 1 h the spectrum represented by the thin black curve and the dashed fit was measured. Finally, the spectrum representing the latest stage of the evolution after two days in air, has been plotted by the dash-dotted line. This rather broad curve peaks at approximately 2 eV.

The attack of HF results in a complete or almost complete removal of the oxide layers surrounding the Si nanoparticles. As is seen in Fig. 5, this treatment results in a substantial narrowing of the band (from 0.4 to 0.26 eV) while the position of the band maximum is not changed. On the other hand, the integrated intensity of the PL is considerably lower, having decreased by a factor of 70. This is the reason why the spectrum is rather noisy after HF treatment. The following oxidation of the Si nanoparticles has the same effect as the oxidation of freshly prepared samples. The peak shifts to higher energies (from 1.59 through 1.81 to 1.97 eV), and the width increases from 0.26 through 0.28 to 0.55 eV. At the same time, the integrated PL intensity finally increases by a factor of 5.

In a previous study [19], we showed by high-resolution transmission electron microscopy that aged silicon nanocrystals are surrounded by an oxide shell whose thickness corresponds to approximately 10 % of the total particle diameter. It was found that, for a given particle size, the spacing of the { 11 1 ) lattice plane fringes varies by -2 %. This variation, which can be explained by different degrees of oxidation and thus different stresses exerted on the crystalline lattice, results in an inhomogeneous “oxide-induced” PL bandwidth of 0.25 eV for a given particle size in an aged sample [ 111. The final PL response of a given sample can be calculated by transforming the particle size distribution into a PL band (taking the correlation between particle size and bandgap given by Delerue et al. [ 181) and convoluting the resulting curve with a Gaussian line shape function with a FWHM of 0.25 eV to account for the oxide-induced inhomogeneous broadening [ 111. Since the oxide layer is clearly less pronounced for fresh samples we expect much narrower PL bandwidths for these samples, provided that the size distribution is not too broad.

If we look at the results of the present study, we indeed find that, for rather fresh samples, the spectra are quite narrow, ranging in width from 0.2 to 0.23 eV (FWHM). These values are even smaller than the oxide-induced width of 0.25 eV of an ensemble of single-sized aged nanocrystals studied in Ref. [ l l ] . On the other hand, after one month of exposure to air, we end up with PL bandwidths of 0.31 to 0.4 eV. This is clearly due to an enlargement of the oxide-induced width. Another confirmation of this interpretation is given by the last experiment. Starting from an aged sample whose PL peak is rather broad, the width is significantly reduced when the oxide layer is removed. This results in a relaxation of the stress exerted by the oxide layer. Conversely, when the sample is oxidized again, the width increases and even exceeds 0.55 eV.

In the same frame, we can also understand the evolution of the peak position. As we have shown before, the position of the PL of aged samples is well correlated with the average size of the nanocrystallites in accordance with the theoretical law established by Delerue et al. [18]. In the present experiment, we find that, as time progresses, the PL peak gradually shifts to the blue. On the

Light-Emitting Properties of Size-Selected Silicon Nanoparticles 805

basis of the inverse power law [ 181 (see Eq. 1) and its experimental verification, one can derive, for the first sample reported in Fig. 3, the variation of the crystalline core from di = 3.96 nm at the beginning to df= 3.33 nm in the final state. This corresponds to a shrinking by a factor of 0.84.

Taking an average molecular weight of SiO, as M = 52 (corresponding to x =1 .5) and assuming that the density of silicon oxide is quite close to that of silicon (2.33 g/cm3) [ 111, it follows that the volume containing the same number of Si atoms is a factor of 1.86 larger for the oxide shell (SiO,) than for crystalline silicon. Knowing furthermore, that the thickness of the oxide layer of an aged Si nanoparticle is approximately 10 % of the total diameter [ 191, we can calculate that, after complete passivation, the size of the crystalline core is reduced by a factor of 0.87. This number compares quite favorably with the experimental value (0.84) derived in the previous paragraph.

The results obtained for the sample exposed to HF give new insight into the characteristics of the photoluminescence of Si nanocrystals. The initially measured PL curve (the black curve of Fig. 5) could be fitted rather well by assuming a nanocrystal size distribution with an average diameter (d) = 4.1 nm and having a width of 2.2 nm (FWHM) [14]. Corresponding to our earlier investigations [l 11, the convolution was carried out with an oxide-induced width of w = 250 meV. The treatment with HF did not change the nanocrystal size distribution as evidenced by the fact that the PL maximum did not shift. While keeping the original size distribution, we had to reduce the individual width w of a single nanocrystal from 250 to less than 10 meV to obtain good agreement between simulation and measurement [14]. This gives us an upper limit for the intrinsic width of the PL response of a single Si nanocrystal of 10 meV. Upon oxidation, the crystalline cores of the nanoparticles are further reduced. An attempt to fit the experimental curve of the last measurement (dash-dotted curve) with the same log-normal distribution shifted to smaller sizes and an oxide-induced width of w = 250 meV resulted in an average particle size (d) = 2.8 nm. However, the agreement with the experimental data was not completely satisfactory, especially at higher energies.

Recently, Wolkin et al. [9] observed in oxidized porous silicon an upper limit of the PL emission energy of 2.1 eV even if the nanocrystals became smaller than 2 nm. This behavior, which seems to contradict quantum confinement, was explained by the formation of stabilized electronic states on Si=O bonds at the surface. For nanocrystals with diameters smaller than 2.8 nm, the widening of the bandgap due to quantum confinement makes them appear as inner bandgap states. Including the results of Wolkin et al. into our model calculations, we now obtained nice agreement with the experimental data [ 141.

The behavior of the PL band of silicon nanoparticles with diameters between 2 and 5 nm after exposure to air can be summarized as follows. The maximum position of the PL band shifts to higher energies while its intensity and bandwidth both increase with the time. All observations can be explained as resulting from a shrinking crystalline core and a growing oxide shell. From the fact that the time dependencies go into saturation, it can be concluded that the oxidation of Si nanoparticles in this size regime is a self-limiting process. This is in agreement with our earlier electron microscopy study [19] and with previous studies on silicon nanowires being subjected to progressive oxidation by Liu et al. [21]. Finally, it should be stressed that the PL behavior of the Si nanoparticles investigated in the present study can be fully understood in terms of quantum

806 F. Huisken, G. Ledoux, 0. Guillois , C. Reynaud

confinement. Only if the nanocrystal size drops below approximately 2.8 nm does a new state within the bandgap seem to evolve, limiting the maximum PL energy to 2.1 eV. This latter observation is in perfect agreement with the combined experimental and theoretical study of Wolkin et al. [9].

Conclusions

COz laser pyrolysis of silane in a gas flow reactor and the extraction of the resulting silicon nanoparticles into a cluster beam apparatus has been shown to present an excellent means for the production of homogeneous films of size-separated quantum dots. Their photoluminescence varies with the size of the crystalline core. All observations are in perfect agreement with the quantum confinement model, that is, the photoluminescence is the result of the recombination of the electron-hole pair created by the absorption of a UV photon. Other mechanisms involving defects or surface states are not operative in our samples.

We have shown that, in order to exhibit intense PL, the Si nanocrystals must be perfectly passivated. A simple way to achieve this is by natural oxidation. We have followed this process by measuring the photoluminescence as a function of time. Completely stable conditions are achieved after approximately six months. This indicates that the oxidation of Si nanoparticles is a self- limiting process.

The oxide shell of silicon nanoparticles can be etched away by exposing the samples to the vapor of HF. The following oxidation reduces the size of the cristalline core and shifts the PL of the nanoparticles to shorter wavelengths. Very recently, we found that the same technique can also be applied to reduce the size of the larger Si nanoparticles collected in the exhaust line of the flow reactor and to shift their photoluminescence, which is normally in the IR, into the visible. This opens the way to producing much larger quantities of strongly luminescent silicon nanoparticles for various applications.

Acknowledgments: The authors are grateful to the Deutsche Forschungsgemeinschaft for support in the framework of the Schwerpunktprogramm Silicon Chemistry. This work was also supported by PROCOPE, a bilateral cooperation between France and Germany. G.L. thanks the Alexander- von-Humboldt Foundation for a fellowship.

References [l] [2] [3] [4] [5]

L. T. Canham, Appl. Phys. Lett. 1990,57, 1046. V. Lehmann, U. Gosele, Appl. Phys. Lett. 1991,58, 856. L. T. Canham, Phys. Stat. Sol. (b) 1995,190,9. A. G. Cullis, L. T. Canham, P. D. J. Calcott, J. Appl. Phys. 1997,82,909. P. M. Fauchet, J. Luminescence 1996, 70,294.

Light-Emitting Properties of Size-Selected Silicon Nanoparticles 807

[6] L. E. Brus, P. F. Szajowski, W. L. Wilson, T. D. Harris, S . Schuppler, P. H. Citrin, J. Am. Chem. SOC. 1995,117,2915.

[7] M. Ehbrecht, B. Kohn, F. Huisken, M. A. Laguna, V. Paillard, Phys. Rev. B 1997,56,6958. [8] R. B. Wehrspohn, J.-N. Chazalviel, F. Ozanam, I. Solomon, Eur. Phys. J. B 1999,8, 179. [9] M. V. Wolkin, J. Jorne, P. M. Fauchet, G. Allan, C. Delerue, Phys. Rev. Lett. 1999,82, 197. [lo] S. M. Prokes, J. Mater. Res. 1996, 11,305. [ 111 G. Ledoux, 0. Guillois, D. Porterat, C . Reynaud, F. Huisken, B. Kohn, V. Paillard, Phys. Rev.

B 2000,62, 15942. [12] G. Ledoux, 0. Guillois, C. Reynaud, F. Huisken, B. Kohn, V. Paillard, Mater. Sci. Eng. B

[13] G. Ledoux, 0. Guillois, F. Huisken, B. Kohn, D. Porterat, C. Reynaud, Astron. Astrophys. 2001,377,707.

[14] G. Ledoux, J. Gong, F. Huisken, Appl. Phys. Lett. 2001, 79,4028. [15] G. Ledoux, J. Gong, F. Huisken, 0. Guillois, C. Reynaud, Appl. Phys. Lett. 2002,80,4834. [16] M. Ehbrecht, H. Ferkel, V. V. Smirnov, 0. M. Stelmakh, W. Bang, F. Huisken, Rev. Sci.

Instrum. 1995,66, 3833. [ 17) M. Ehbrecht, F. Huisken, Phys. Rev. B 1999,59,2975. [ 181 C. Delerue, G. Allan, M. Lannoo, Phys. Rev. B 1993,48,11024. [19] H. Hofmeister, F. Huisken, B. Kohn, Eur. Phys. J. D 1999,9, 137. [20] F. Huisken, B. Kohn, V. Paillard,Appl. Phys. Lett. 1999, 74,3776. [21] H. I. Liu, D. K. Biegelsen, F. A. Ponce, N. M. Johnson, R. F. W. Pease, Appl. Phys. Lett.

1994,64, 1383.

2000,69-70,350.

Spinel-SiAlONs - A New Group of Silicon-Based Hard Materials

Marcus Schwarz," Rama S. Komaragiri, Andreas Zerr, Edwin Kroke, R a y Riedel

Fachgebiet Disperse Feststoffe, Fachbereich Material- und Geowissenschaften Technische Universitat Darmstadt

PetersenstraBe 23, D-64287 Darmstadt, Germany Tel.: +49 6151 16 6347 -Fax: +49 6151 16 6346

E-mail: mrschw@tu-darmstadt.de

Gerhard Miehe

Fachgebiet Strukturforschung Technische Universitiit Darmstadt Petersenstraae 23, D-64287 Darmstadt, Germany

James E. Lowther

School of Physics, University of the Witwatersrand P. 0. Wits. Johannesburg, South Africa.

Keywords: ceramics, high-pressure chemistry, oxide-nitrides, sialons, spinel phases

Summary: The synthesis of multinary oxide-nitride spinels, ySiZAION3, ySi1.9A11.101.1N2.9, ySiA1202N2 and pSio.9Alz.102.1N1.9, is presented. These phases belong to a solid solution series ySi3-xAlxOxN~x between the new high-pressure modification of silicon nitride (ySi3N4) and a hypothetical yA1303N. The spinel-sialons were characterized using optical and scanning electron microscopy, electron probe microanalysis, X-ray powder diffractometry and micro-Raman spectroscopy. The spinel structure was also investigated using techniques of ab-initio electronic structure theory, revealing a bulk modulus Bo of about 270 GPa. The Vickers microhardness of ySi2A10N3 was measured to be 27 GPa and a mean fracture toughness of 4.6 Mpa mH was obtained from the post-indentation crack length (ICL) method. These excellent mechanical properties make ysialons a promising new class of hard ceramic materials.

Introduction

Sialons are nitride-xide hybrid ceramics based on the system Si-A1-0-N. a- and psialons, especially, are used for metal-cutting and engineering applications [l]. They can be formally

Ovganosilicon Chemisty V Edited by N. Auner and J. Weis

Copyright 0 2003 WILEY-VCH Verlag GmbH & Co. KgaA, Weinheim

Spinel-SiAlONs - A New Group of Silicon-Based Hard Materials 809

derived by charge neutral substitution of [Si4+N3-]"-' pairs by [A13+02-p within the respective a- and PSi3N4 structures. In our recent work we discovered and characterized a third, cubic polymorph with a spinel-type structure ( Y - S ~ ~ N ~ = C & N ~ ) , space group Fd3m, No. 227 [2, 31. ySi3N4 is one of the first inorganic solids known to contain SiNs octahedra. It forms at pressures >12 GPa, and is 26 % more dense and significantly harder than PSi3N4 [4, 51, with which it shares a common phase boundary. Also, the other group 14 element nitrides, Ge3N4 and Sn3N4, were discovered to crystallize in the spinel structure [6-8]. In a recent study we synthesized a first example of a ysialon, ySi3-,AlXO,N~,, with x = 1.0, by a static high-pressure technique [9]. The existence of a series of spinel-sialon compounds was evidenced by sucessive syntheses of ysialons with stoichiometries x = 1.1,2.0 and 2.1.

HP/HT Synthesis of y-Sialons

For all experiments we started from small cylinders (0 x h = 1.1 x 3.5 mm) of psialon with identical composition and used a multi-anvil high-pressure technique [ 10, 161. The maximum applied pressure and temperature were 130 kbar and 1800 "C respectively.

Sample Characterization

The octahedral pressure cells containing the samples were recovered from the multi-anvil experiment and either broken or cut in half, using a diamond wiresaw. In the latter case, the ysialon samples were also halved in the axial direction. The specimens were then characterized with optical and scanning electron microscopy (SEM), transmission electron microscopy (TEM), electron probe microanalysis (EPMA), powder X-ray diffractometry (XRD), and microhardness testing using the Vickers method.

Results and Discussion

SEM studies of fractured and polished surfaces revealed homogeneous microstructures of the ysialons with equi-axed grains and shapes similar to Kelvin polyhedra. The grain size was below 1 pm for the x = 1.0 and 1.1 samples and 1-4 pm for the more highly substituted ysialons. The chemical composition, as determined by EPMA, was identical to that of the starting materials and no indications of a phase separation were found by scanning 100 pm x 100 pm areas in the element mapping mode. The spinel structure was verified by XRD and TEM. Reflections of a-quartz appeared in the XRD pattern after the first powder sample had been prepared by grinding y'SiZAlON3 in an agate mortar (Fig.1). The volume fraction of the quartz was as high as 20 % and could be traced to abraded debris from the mortar. Sucessive samples were prepared by grinding powders with a diamond tool or by X-raying small pieces of the material directly in transmission

810 M. Schwarz, R. S . Komaragiri, A . Zerr, G. Miehe, E . Kroke, R . Riedel, J . E . Lowther

geometry or with a Gandolfi camera.

8000

7000

6000

5000

d000

' 3 0 0 0

c c 1

2000

1000

0

Q

533 73 1

I I I I I I I I I I I I I I I I I I I I I I I II I I I I I l l I I I II I I I IIIII I l l I I I l l I 11111II

1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 100105

2theta(deg.)

Fig.1. Powder diffraction pattern and Rietveld difference plot of y-Si2A10N3 and a-quartz (lines marked with Q)

which had been abraded from the agate mortar used for powder preparation. Inset: Quartz-free diffraction

pattern obtained from a monolithic piece of y-Si2AION3.

Figure 2 shows the dependency of the lattice parameter a0 with substitution level x and includes also data for shock-synthesized ysialons from Ref. [ 1 11. For direct comparability, structural data of the equivalent psialons with hexagonal lattice symmetry are plotted as the cube root of the volume of eight formula units (for spinel Z = 8). The same trend of a lattice expansion upon [Al-01 insertion is evident for both p and psialons. In the spinel structure, two different cation environments, a tetrahedral and an octahedral, exist. Rietveld structure refinement of gSizAlON3 revealed that the size of the octahedral cation site had slightly increased with respect to ySi3N4, while the tetrahedral site had decreased. This indicates a site preference of the bulkier A1 ion for the octahedral coordination, which would mean that ySiZAlON3 is a partially inverse spinel [9].

In order to elucidate the energetics of the cation site occupation, we have applied ab-initio techniques of density functional theory within the local density approximation. Two possible configurations corresponding to normal and (partially) inverse spinel were considered (Figs. 3a and b). The results are given in Tablel, with the bulk moduli obtained by fitting the Birch equation of state.

Spinel-SiAlONs - A New Group of Silicon-Based Hard Materials 811

E 8.25:

m P 8.00- e, 0

m

E

3 7.75- - a, (y-sialons)

**. In H \ / [31 [$I1

7.503 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0 1 2 3

Si,N, Si,AION, SiAI,O,N, AI,O,N

substitution level x [-]

Fig. 2. Variation of lattice parameters with increasing [A141 content ( substitution level x ) in

from Ref. [3], [ l l ] and [13] are included.

and ysialons. Data

Fig. 3. The 56-atom unit cell of spinel-ySi~AlON3 in two different configurations. a) Normal spinel, with the

lower-charged A?+ ions occupying only the lowe-coordinated tetrahedral sites. b) Inverse spinel

configuration, in which some A13+ ions occupy octahedral sites, but the compositon requires at least half of

the Si4+ ions to remain on octahedral sites.

Table 1. Experimentally determined lattice constant and calculated properties of two possible configurations of

ySi2A10N3. Bo and B' are the bulk modulus and its pressure derivative, Ecoh the cohesive energy per

formula unit.

a0 1 Bo [GPal, B' Ecah [eV/f.u.l

y-Si2A10N3 (experiment) 7.8234(3) - -

y-SizA10N3 (normal) 7.824 260,4.80 -365.2424

y-Si2A10N3 (inverted) 7.806 27 1.4.23 -365.2518

812 M . Schwarz, R . S . Komaragiri, A . Zerr, G. Miehe, E. Kroke, R . Riedel, J . E. Lowther ~~

A small energy difference between the normal and inversed cation topology is found - but the inverted distribution is marginally lower in energy. Obviously, the energies of quaternary Si-Al-0-N spinels display little dependence on the relative cation occupancy of the tetrahedral or octahedral sites.

Vickers microhardness was measured for all four ysialons at loads ranging from 25g to 1 kg. The small sample sizes and cracks within the polished surfaces, however, resulted in a considerable data scatter and prohibited indentations at all loads for every sample. It should also be noted that the less and more highly substituted ysialons had different grain sizes. The results are summarized in Fig. 4, which also shows hardness values for the psialons and the parent phases pSi3N4 and ySi3N4, for which the first hardness measurements have been published recently [4, 5, 151. The ysialons are significantly harder than their pphase counterparts and the low load values for the ySi~.~A1~.10~.1N2.~ material demonstrate that their hardness can reach up to 3300 HVo.05. For ySizAlON3, an overall mean hardness of 2808 f 60 HV(o.5 + 1) ( = 27.5 GPa) was determined [9]. Comparatively high toughness values (4.6 f 0.14 MPam’’ for ySi2AlON3 [9] and -3.4 MPam*’ for ySiA1202N2 ) were obtained from the crack systems of selected indentations, using the corellation established by Shetty et al. (see Ref. [ 151). These superior mechanical properties may be expected for a ceramic that can be said to have been ”hot istostatically pressed” at 13 Gpa; however, the fact that ysialons were also synthesized in shock-wave experiments [ 111 shows that there may be a realistic chance of making these materials available on an industrial scale.

v 1000 . . . . , . . . . , . . . . , . . . . I . . . . , . .

0 1 2

substitution level x I-1

Fig. 4. Hardness of the synthesized ysialons at different loads in comparison to ySi3N4. psialon, and PSi3N4. Data

from Ref. [4], [5] and [12-141 are included.

Acknowledgments: We thank M. Zenotchkine (University of Pennsylvania) for the synthesis of Psialon starting materials and M. Heck and B. Thybusch (TU-Darmstadt) for EPMA characterization and analysis. The Department of High-pressure Mineral Physics, Max-Planck- Institut fur Chemie, Mainz, is acknowledged for providing technical support. All high-pressure experiments were conducted at the Bayerisches Forschungsinstitut fiir Experimentelle Geochemie

Spinel-SiAlONs - A New Group of Silicon-Bused Hard Materials 813

und Geophysik, Universitat Bayreuth. We are further grateful for the financial support of this work provided by the Deutsche Forschungsgemeinschaft and the Fonds der Chemischen Industrie. M. Schwarz thanks the KSB-Stiftung for financing a conference participation in the U.S.

References F. L. Riley, J . Am. Cerum. SOC. 2000,83,246. A. Zerr, G. Miehe, G. Serghiou, M. Schwarz, E. Kroke, R. Riedel, H. FueS, P. Kroll, R. Boehler, Nature 1999,400, 340. M. Schwarz, G. Miehe, A. Zerr, E. Kroke, B. Poe, H. Fuess, R. Riedel, Adv. Muter. 2000,12, 883. A. Zerr, M. Kempf, M. Schwarz, E. Kroke, M Goken, R. Riedel, J . Am. Ceram. Soc. 2002, 85,86. I. Tanaka, F. Oba, T. Sekine, E. Ito, A. Kubo, J . Muter. Res. 2002,17,731. G. Serghiou, G. Miehe, 0. Tschauner, A. Zerr, R. Boehler, J . Chem. Phys. 1999,111,4659. K. Leinenweber, M. O'Keeffe, M. Somayazulu, H. Hubert, P. F. McMillan, G. H. Wolf, Chem. Eur. J . 1999,5,3076. N. Scotti, W. Knockelmann, J. Senker, St. TralSel, H. Jacobs, Z. Anorg. Allg. Chem. 1999, 625, 1435. M. Schwarz, A. Zerr, E. Kroke, G. Miehe, I.-W. Chen, M. Heck, B. Thybusch, B. T. Poe, R. Riede1,Angew. Chem. Int. Ed. 2002,41,789. D. C . Rubie, Phase Transitions 1999,68,43 1. T. Sekine, H. L. He, T. Kobayashi, M. Tansho, K. Kimoto, Chem. Phys. Lett. 2001,344,395. J. Z. Jiang, F. Kragh, D. J. Frost, K. StAhl, H. Lindelov, J . Phys.: Condens. Matter 2001, 13, L515. T. Ekstrom, P. 0. Kall, M. Nygren, P. 0. Olsson, J . Muter. Sci. Lett. 1989,24, 1853. D. Chakraborty, J. Mukerji, J . Muter. Sci. 1980,15,3051. J. Dusza, Scr. Metull. Muter. 1992,26,337.

[I61 E. Kroke, Angew. Chem. 2002,114,81; Angew. Chem. Int. Ed. 2002,41,77.

Aluminosiloxanes as Molecular Models for Aluminosilicates

Roisin Reilly * Department of Chemistry, Trinity College Dublin, Dublin 2, Ireland

Tel.: +353 16082032 E-mail: rreilly@ tcd.ie

Keywords: aluminosiloxanes, aluminosilicates, zeolites, aluminosilsesquioxanes

Summary: The reaction of AlzC16 with an excess of PhzSi(0H)z in THF in the presence of pyridine yielded new anionic and cyclic aluminosiloxanes. Interactions of Al(Et)3 with PhnSi(0H)z in a 1:2 ratio respectively yielded an Ak(OH)4 eight-membered ring in a molecular aluminopolysiloxane. Another reaction involving A12C16 with two trisilanols (c-C,H,,),Si,0g(OH)}3 in diethyl ether assisted by NEt3 yielded the anionic Complex [HNEt,] (Al( (c-C~H,,),S~,O~~(OH)}~] with two free OH functionalities.

Aluminosiloxanes

Anionic [PyH][A1{OSiPh2(OSiPh,)20}2] (1) and Cyclic [AICI(THF){O(Ph2SiO)2}]z (2)

Ph Ph Ph Ph

2 / "*,, c1 THF

Scheme 1. Synthesis of [PyH][A1[OSiPh2(OSiPh2)20]z] (1) and [AICl(THF)~O(Ph2SiO)2)]2 (2).

Ovganosilicon Chemisty V Edited by N. Auner and J. Weis

Copyright 0 2003 WILEY-VCH Verlag GmbH & Co. KgaA, Weinheim

Aluminosiloxanes as Molecular Models for Aluminosilicates 815

The reaction of aluminium chloride with four equivalents of Ph,Si(OH), in THF in the presence of

an excess of pyridine afforded two new aluminosiloxanes 1 and 2 (Scheme. 1) [ 11. In the anion 1, the A1 atom is coordinated to four oxygen atoms of two cyclic siloxane fragments having a slightly distorted tetrahedral arrangement (Fig. 1). The eight-membered aluminosiloxane rings may also be considered as four-membered, if one is counting only tetrahedral atoms (A1 and Si) such as are referred to in zeolites. The central fragment of 2 is a twelve-membered Al,Si,O, ring, which occurs

in natural aluminosilicates [2] (Fig. 2). Compound 2 has interesting host-guest possibilities. The diameter of the aluminosilicate rings in 1 is about 4.4-4.3 A, i.e. they are comparable to those in zeolites such as NaY [2]. The average Si-0 bond lengths in both 1 and 2 are comparable to the idealized Si-0 distances in natural tetrahedral aluminosilicates (1.603 A). The average A1-0 bond length in 1 is also comparable to the idealized A 1 4 distance (1.761 A) in natural tetrahedral aluminosilicates [3].

Fig. 1. Crystal structure of [Al( OSiPh,(OSiPh,),O],]~ (1).

Fig. 2. Crystal structure of [AlCl(THF)( O(Ph,SiO),]], (2).

Aluminosiloxane (C9SH84Ab016Si8) (3)

The reaction of Al(Et), with Ph,Si(OH), resulted in the formation of an aluminopolysiloxane (Scheme 2). The OSi(Ph,)OSi(Ph,)O units present in the molecule can be attributed to the Lewis acid catalyzed condensation of the Ph,Si(OH), monomer. There is an A1,0, eight-membered ring in

the center of the structure. The Al(0H)Al edges are spanned by four disiloxane bridges resulting in

816 R. Reilly

a framework of five annelated eight-membered rings (Fig. 3). A similar yet different structure was obtained by Veith et al. [4].

A1(Et)3 + 2Ph2Si(OH)z

b I I

Investigation of Siloxane-Pyridine Systems in the Presence of Lewis Acids: (CZ,H,,NO,SiZ) (4)

The reaction of AlCl, and Ph,Si(OH), (1 : 1) with pyridine assistance in diethyl ether formed the

intermediate ligand in the reaction mechanism for the formation of the fully condensed trisiloxane (Ph,SiO),, i.e. Lewis acid (Al,Cl,J catalyzed condensation of Ph,Si(OH), was followed by a

reaction with the amine to form the tetraphenylsiloxan-1-01 coordinated to pyridinium (Fig. 4). The average Si-0 bond length in 4 is comparable to the idealized Si-0 (1.603 A) distances in natural tetrahedral silicates, but the Si-0 bond distance for the Si-0 attached to the pyridinium is much higher (1.641 A) than this idealized Si-0 bond distance, due to the steric effect of the pyridinium.

Fig. 4. X-ray structure of (Cz9H,6N03Si,) (4).

Aluminosiloxanes as Molecular Models for Aluminosilicates 817

Anionic Aluminosilsesquioxane: [HNEtJ{Al{ (c-C,Hll),Si,Oll(OH)},] (5)

Interactions of A12C16 with two trisilanols ( C - C ~ H , ~ ) ~ S ~ ~ ~ ~ ( O H ) } ~ [S] in diethyl ether with

triethylamine yielded the anionic complex 5, containing two free nonreacted OH functionalities (Scheme 3). 29Si NMR revealed six signals for the different silyl fragments present. See Fig. 5 for the X-ray crystal structure of the anion. All A 1 4 bond lengths (av. 1.738 A) are shorter than those found in natural tetrahedral aluminosilicates [3]. The range of the 0-A1-0 bond angles indicates a slightly distorted tetrahedral arrangement.

29Si NMR, 6: -58.94, -52.55, -63.99, -65.92.67.85.69.29

Scheme 3. Synthesis of [HNEt,]{Al( (c-C,H,,),Si,O,,(OH)),] (5).

(Al-0) av. = 1.738 A, (0-Al-0) = 106.7(5)-112.5(5)"

Fig. 5. Structure of [(Cy,Si,O,(OH)O,],Al]- anion (5). Cyclohexyl groups are omitted for clarity.

In summary, both aluminosiloxanes 1 and 2 appear to be interesting molecular models for some fragments in aluminosilicate minerals. The aluminopolysiloxane 3 obtained is remarkably stable in aerobic conditions, unlike the aluminopolysiloxane Veith et al. [4] obtained, and also it crystallized free of base donor molecules. The interaction of disilanols and trisilanols with pyridine goes with the formation of ionic (pyridinium cation-siloxane anion) salts. Pyridinium salt 4 is the first example of a siloxane-amine adduct. We also hope to further metallate the product [HNEt3] (A1{ ( C - C ~ H ~ ~ ) ~ S ~ ~ O ~ ~ ( O H ) ) ~ ] (5) using its two free OH functionalities.

818 R. Reilly

Acknowledgments: Gratitude goes to Enterprise Ireland and Trinity College Dublin for funding this work.

References [ 13 Y. Gun’ko, R. Reilly, V. G. Kessler, New J. Chem. 2001,25,528-530. [2] J. V. Smith, Chem. Rev. 1988,88,149. [3] J. B. Jones, Acta Crystallogr. Sect. B, 1968,24, 355. [4] M. Veith, M. Jarczyk, V. Huch, Angew. Chem. Int. Ed. 1998,37, 105. [5] F. J. Feher, D. A. Newman, J. F. Walzer, J. Am. Chem. SOC. 1989,111, 1741-1748. [6] P. S. Gradeff, K. Yiinlii, T. J. Deming, J. M. Olofson, R. J. Doedens, W. J. Evans, Inorg.

Chem. 1990,29,420424. [7] D. A. Foucher, A. J. Lough, I. Manners, Inorg. Chem. 1992,31,3034. [ 8 ] F. T. Edelmann, Y. Gun‘ko, S. GielJmann, F. Olbrich, Inorg. Chem. 1999,38,210. [9] R. Duchateau, R. J. Harmsen, H. C. L. Abbenhuis, R. A. van Santen, A. Meetsma, S. K.-H.

Thiele, M. Kranenburg, Chem. Eur. J. 1999,5, 3130. [lo] P. B. Venuto, Microporous Mater. 1994,2,297. [11] F. J. Feher, D. Soulivong, A. G. Eklund, Chem.Commun. 1998,399-100. [ 121 F. J. Feher, T. A. Budzichowski, J. Organomet. Chem. 1989,3340. [13] Y. Gun’ko, R. Reilly, F. T. Edelmann, H. G. Schmidt, Angew. Chem. Int. Ed. 2001,40, 1279.

Investigation of Silicone-Modified Photocatalytic Ti02 Formation by Solid-Liquid Reaction and Its

Structural Changes under Irradiation

Akira Nakabayashi

Performance Chemicals R&D Department, Asahi Kasei Corporation Kawasaki-ku, Kanagawa 210-0863, Japan

Keyword: photocatalytic titanium dioxide, solid-liquid reaction, silicone-modified, H-siloxane, irradiation

Summary: The kinetic study of the solid-liquid reaction between photocatalytic titanium dioxide (photo-Ti02) and H- siloxane was investigated. The results showed that the solid-liquid reaction was inhibited in the presence of water, alcohol, ether, or other polar molecules, and supported its characterization as a dehydrogenation condensation reaction. The synthesized silicone-modified photo-Ti02 by the solid-liquid reaction was initially hydrophobic, but became super-hydrophilic after irradiation by BLB light. Both ESR and Si-NMR studies suggested that this effect was caused by the photocatalytic oxidation of the silicone present on the photo-Ti02.

Introduction

Photocatalytic titanium dioxide (photo-Ti02) shows excellent potential as a photocatalyst for decomposition of undesirable substances from the environment. In essence, this is based on the strong redox power of the hole-electron pairs generated in photo-Ti02, when its valence-band electrons are converted to conducting-band electrons by photo-illumination. The oxidizing strength of the generated hole is high enough to oxidize completely a wide range of organic substances, thus converting them to inorganic compounds.

Recently, various studies have focused on increasing the photocatalytic activities by modifying the surface of the photo-TiOz. In one of these, Dr. Fukui obtained silicone-modified photo-Ti02 by a solid-vapor reaction between photo-Ti02 powder and H-siloxane [ 11. He also obtained silica- coated photo-Ti02, by baking the silicon-modified photo-Ti02 at 500 "C. The silica-coated photo- Ti02 reportedly showed strong Lewis acidity and an increased capability for decomposing organic compounds.

The surface-modification reaction between photo-Ti02 and H-siloxane is thought to be a dehydrogenation-condensation reaction, involving the Ti-OH group of the photo-Ti02 surface and the Si-H group of the H-siloxane [2], but confirmation has been slow because of the difficulty of

Ovganosilicon Chemisty V Edited by N. Auner and J. Weis

Copyright 0 2003 WILEY-VCH Verlag GmbH & Co. KgaA, Weinheim

820 A. Nakabayashi

analyzing solid-vapor reactions. We therefore used a solid-liquid reaction between photo-Ti02 and H-siloxane to gain further

insight into the process of silicone-modified Ti02 formation. In this paper, we report on our kinetic analysis of silicone-modified photo-Ti02 formation by the solid-liquid reaction between photo- Ti02 and H-siloxane. We also discuss the change induced in its surface properties by irradiation.

Results and Discussion

Silicone-Modified Photo-Ti02 Formation by Reaction of Photo-Ti02 and H-Siloxane

The photo-Ti02 used throughout this work was STOl (Ishihara Sangyo Kaisha Ltd.). For H -siloxane, we used H-silicone oil, such as KF99 (Shin-Etsu Chemical Co. Ltd.), with a structural formula as shown in Fig. 1. We added a toluene solution of H-siloxane (20 wt%) to a toluene dispersion of photo-Ti02 (20 wt%) at constant temperature, and measured the rate of H2 gas evolution with a gas burette.

Figure 2 shows plots of the H2 gas evolution versus time, as obtained from the solid-liquid reaction of the H-siloxane and the

the quantity of H-siloxane added was increased, up to a ratio of about two parts H-siloxane to eight parts ~ig.1. SmmdforrmlaofH-siloxan.

photo-TiO2. Beyond that ratio, however, increasing the H-siloxane addition did not bring any further increase in Hz gas evolution.

We confirmed that the IR absorption peak (3635 cm-') of the Ti-OH group of photo-Ti02 was eliminated by the reaction with H-siloxane. Both the loss of this peak and the limiting ratio of the H-siloxane addition indicate that the solid-liquid reaction between photo-Ti02 and H-siloxane is actually a dehydrogenation-condensation reaction, involving the Ti-OH group and the Si-H group.

'q#" Si-Me I Me

I Me photo-Ti02 at 15 "C. The quantity of H2 gas evolved increased as

H-Siloxane/photo-Ti02 = 039.5 (15 "C: H-Siloxandphoto-TiOz = 1/9 (15 "C) ~

H-Siloxandphoto-Ti02 = U8 (15 "C *H-Siloxandphoto-Ti02 = 3/7 (15 "C)

0 ; " " ' " ' 0 50 100 150 200 250 3M) 350 400

H 2 0 Addition (3 wt.%) II/--- 0 50 100 150 200 250 300 350

Reaction Time (min) Reaction Time (min)

- 400

Fig. 2. Results of solid-liquid reaction of photo-Ti02 Fig. 3. Influence of HzO on silicone-modified

and H-siloxane (KF99) in various H- photo-Ti02 formation at 15 "C (H-siloxane

siloxane/photo-TiOa ratios at 15 "C (40 % RH). (KF99)/photo-Ti02 = 3:7 wuwt).

Investigation of Silicone-Modi$ed Photocatalytic Ti02 Formation 821

In the course of our study, we also found that the solid-liquid reaction of photo-Ti02 and H-siloxane in toluene was influenced by the humidity in which the photo-Ti02 powder had been handled before it was placed in toluene. As shown in Fig. 3, the reaction speed was clearly decreased by prior exposure to high humidity, and the reaction was strongly inhibited by the addition of water to the toluene. We found a similar inhibiting effect when we added other polar molecules, such as alcohol or ether. To elucidate this effect, we performed a kinetic study of the solid-liquid reaction.

As shown in Scheme 1, when there is a large excess of the Si-H group over the Ti-OH group throughout the solid-liquid reaction, it can be treated as a first-order reaction. Thus, a straight line is obtained by plotting In( I/( 1-x)) against the time t, and its slope represents the apparent rate constant

(kapp).

a( 1 -x) A -ax ax ax

a& = ka(1- x)(A - ax) dt

A>> ax dJC=kA(l-x)=k ( 1 - X ) dt aPP j d x = k a p p J ' d t , 1 h L = k t 1 - x 1 - x aPP

Scheme 1. Kinetic analysis of the solid-liquid reaction between photo-Ti02 and H-Silicone.

To simplify the reaction system in this experiment, we used bis-(trimethylsi1oxy)methylsilane (monoSiH), which contained only one Si-H group in the molecule, for the H-siloxane. Figure 4 shows plots of H2 gas evolution versus time for the solid-liquid reaction between photo-Ti02 and monoSiH at various temperatures. The plots indicate that the concentration of the Ti-OH groups on photo-Ti02 with which the monoSiH can react is 0.68 mmoUg-TiO2

Figure 5 shows the plot of In( 1/( 1-n)) versus time for the solid-liquid reaction of photo-Ti02 and monoSiH at 50 "C. The results indicate the existence of two distinct reaction regions. In the first region, the reaction speed is relatively high; in the second region, it is lower.

TG-DTA analysis of the photo-Ti02 showed that it contained about 10 wt% physically adsorbed water, and that this water content changed with the ambient humidity.

To examine the influence of adsorbed water on the reactivity of the photo-Ti02 with H-siloxane, we added water to the toluene dispersion (water/photo-TiOz = 2:8 wt/wt)) just before mixing the two reactants. As shown in Fig. 6, the water addition substantially decreased the reaction rate in the first reaction region.

Conversely, as shown in Fig. 7, the reaction quantity in the first reaction region clearly increased

822 A. Nukabuyashi

when we heated the photo-Ti02 for 12 hours at 120 "C, resulting in its dehydration by 8 wt%. The increase in the reaction quantity was presumably a result of the increased proportion of water-free Ti-OH groups.

1.4

1.2

~

-

2 1 . -

Y 0.8

......

~

0 100 200 300 400 500 600 700 0 100 200 300 400 500 600 700

Time (min)

Fig. 4. Solid-liquid reaction of photo-Ti02 and

monoSiH at various temperatures

(monoSiWphoto-Ti02 = 1:2 wtlwt).

1.4

1.2

h - 7 ' 5 0.8 v -

0.6

0.4

0.2

*'i

photo-TiOZ A photo-Ti02 (H20 Addition (20 wt%N

100 200 300 400 500 600 700

Time (min)

Fig. 6. Influence of H 2 0 addition on formation of

silicone-modified photo-Ti02

(monoSiWphoto-Ti02 = 1:2 wuwt at 50 "C).

Time (min)

Kinetic analysis of solid-liquid reaction of

photo-Ti02 and monoSiH (monoSiWphoto

-Ti02 = 1:2 wt/wt at 50 "C).

Fig.5.

2.5 I I

2 - h

" 0 100 200 300 400 500 600 700

Time (min)

Fig. 7. Influence of dehydration on formation of

silicone-modified photo-Ti02

(monoSiWphoto-Ti02=l/2 wt/wt at 50 "C).

We formulated a putative mechanism for the slowing of the reaction speed between photo-Ti02 and H-siloxane by water, as shown in Scheme 2. In this mechanism, the

Investigation of Silicone-Modified Photocatalytic Ti02 Formation 823

dehydrogenation-condensation reaction of the Ti-OH group and the Si-H group is a nucleophilic substitution reaction. If water is present, the nucleophilicity of the Ti-OH group is decreased, which may be expected to lower the reaction speed. This implies that there are two types of reactions in the overall solid-liquid reaction, one governed by k(kee), and the other by k(H,o).

From the slopes in the first and second regions shown in Figs. 6 and 7, representing k, in each region, we obtained a k(free) value of 710 g mol-' m i d and a k(H,O) value of 4.3 g mol-' min-' at 50 "C. The value was thus larger by an order of two than the k(HzO) value.

Dehydrogenation condensation reaction between TiOH group and SiH group

i \ / + H-Si- -Ti-0-Si- + H 2 f I \ / \

E) Nucleophilic substitution reaction

6+ b e e ) I -* S i - 0 - T i + H2

Si +:O-H I H 6- Ti

\ 6+ H b-H k&Zo) -* S i - 0 - T i + H2

Si + y - H

H 6- Ti I

Decreasing nucleophilicity of TiOH group by hydration

Scheme 2. Proposed mechanism of the solid-liquid reaction betweeh photo-TiO, and H-silicone.

We performed analogous experiments with other polar solvents in place of the toluene, at 50 "C. We expected the solid-liquid reaction to be slowed by the same mechanism, in which the polar molecule in the solvated Ti02 tends to decrease its nucleophilicity, and thus impede the interaction between the TiOH groups and the H-siloxane. From Fig. 8, we obtained a k(dioxane) value of 3.1 g mol-' m i d ) and a ~ ( B c ) value of 1.3 g mol-' min-' at 50 "C. The slowing of the reaction between the photo-Ti02 and the monoSiH presumably corresponds to the solvation strengths of the solvents for Ti-OH.

0.3 1 J /

butyl cellosolve (BC) 0.1

0 100 200 300 400 500 600 700 Time (min)

Fig. 8. Influence of various polar molecules on rate

of dehydrogenation- condensation reaction

at 50 "C (monoSiWphoto-Ti02 = 1:2 ( w t m ) .

824 A. Nakabayashi

a c! 0, d

5

8

0

W m -

- 0 m C -

Fig. 9. Changes in water contact angle of silicone-modified photo-

TiOz with increasing irradiation time. Irradiation: BLB light

(1 mW/cm2).

d I Before irradiation

-=o -.20

I After irradiation Q-unit I

Fig. 10. Si NMR spectra of the silicone-modified

photo-Ti02, before and after the irradiation

(KF99/photo-TiO2 = 1:9 wt/wt)

a Q-unit structure by irradiation.

Effects of Irradiation

~

Figure 11 shows the ESR spectrum of the silicone-modified photo-Ti02 powder during irradiation at the temperature of liquid nitrogen. It shows that a methyl radical is evolved during the irradiation, as well as Ti3+ and OH radicals. In all of the experiments, the results indicate that the silicone-modified photo-Ti02 surface was transformed from hydrophobic to hydrophilic under

We reacted photo-Ti02 powder (9 g) and H-silicone oil (KF99; 1 g) in toluene (40 g) at 50 "C for 8 h, and then removed the toluene to obtain the silicone-modified photo-Ti02 powder. We pressed the powder into a tablet, and then measured the water contact angle before and after irradiation. The silicone-modified photo-Ti02 was initially hydrophobic, but it became highly hydrophilic after irradiation by BLB light (Fig. 9).

Figure 10 shows the Si NMR spectra of the silicone-modified photo-Ti02 powder before and after the irradiation. They indicate that the silicone of the photo-Ti02 particle is transformed from a T-unit structure to

2.011 1.998

Fig. 11. ESR spectrum of the silicone-modified

photo-Ti02 under irradiation (liq. NJ (KF99/photo-Ti02 = 1:9 wt/wt).

Investigation of Silicone-Mod$ed Photocatalytic Ti02 Formation 825

irradiation by a mechanism in which the irradiation induced the formation of Ti3+ and a negative hole in the photo-TiO2, the negative hole reacted with water to form an OH radical, and the OH radical attacked the silicone on the surface of the photo-TiO2, generating the Si-OH group and a methyl radical.

Under irradiation, in short, the silicone-modified photo-Ti02 oxidized silicon present on the photo-Ti02, thus yielding the hydrophilic silica.

Conclusions

We investigated the formation of silicone-modified photo-Ti02 in solid-liquid reactions under various conditions, measuring the rate of H2 gas evolution. The results show that the solid-liquid reaction is inhibited in the presence of water, alcohol, ether, or other polar molecules, and support its characterization as a dehydrogenation-condensation reaction.

The synthesized silicone-modified photo-Ti02 was initially hydrophobic, but became hydrophilic after irradiation by BLB light. To investigate this effect, we measured the ESR spectrum during its irradiation at liquid Nz temperatures, and compared the Si NMR spectra before and after the irradiation. These experiments resulted in two findings: the evolution of methyl radicals during the irradiation, presumably derived from the silicone on the surface of the photo- TiO2, and the transformation of the silicone on the surface of the photo-Ti02 from T-unit silicone before irradiation to Q-unit silicone after irradiation.

References [ l ] H. Fukui, Hyomen 1994,32, 131-140. [2] H. Tada, Langmuir 1996,12,966-971.

Organosilicon Chemistry V Edited by N. Auner and J. Weis

Copyright © 2003 WILEY-VCH Verlag GmbH & Co. KgaA, Weinheim

Author Index

Ahlbrecht, Hubertus 207 Altmann, Stefan 584 Andres, Katrin 329 Antolini, Fiona 27 Arnason, Ingvar 135 Auer, Dominik 150, 167 Auner, Norbert 1, 139, 180, 334

B

Backer, Michael 145 Balard, H 747 Barrena, Esther 772 Barthel, Herbert 715, 741, 747, 752, 767 Bauer, Andreas 527, 659 Bauer, Josef 689 Baumer, Ute 82 Baumgartner, Judith 171, 186, 294 Becker, G 307 Behr, Arno 638 Belov, Evgenii 518 Bera, Holger 486 Berner, Jan Uwe 312 Bertermann, Riidiger 329 Beyer, Christian 467 Binnewies, Michael 126, 130 Blazejewska-Chadyniak, Paulina 415 Blum, Carsten 785 Bock, Hans 66 Bockholt, Andreas 50 Bohme, Uwe 277, 282, 317, 467, 545 Boisvert, R 696 Bolte, Michael 139 Bones, Simeon J 339 Boogh, Louis 562 Borup, B 573 BoBhammer, Stephan 671 Boury, Bruno 389 Brandt, Martin S 789 Braun, Thomas 50 Bresson, B 747 Brook, Michael A 606

Buchold, Daniel H. M 155 Burneau, A 747

C

Cai, Xiaoping 27 Carteret, C 747 Chadyniak, Dariusz 415 Chardon, Josette 612 Chernyshev, Evgenii 360 Chevalier, P. M 435, 696, 736 Corriu, Robert 389

D

D'yakov, V. M 348, 352, 356 Dauth, Jochen 632 Deruelle, Martial 612 Diedrich, Friedhelm 246 Ditten, G 307 Djakov, Valerii 344 Don Tilley, T 379 Doshi, Dhaval A 689 Drake, Robert A 339 Dreyer, Michael 752 Dubrovskaya, Galina 518 du Mont, Wolf-Walther 210, 213

E

Ebker, Christina 246 Edelmann, R 573 Efimov, Nikolay 518 Egenolf, Heiko 5, 15 Eguchi, K 696 El-Sayed, Ibrahim 78, 375

F

Fallmann, Helmut 490 Fischer, Roland 186, 190, 294 Francis, John G 650 Frank, Dieter 186 Frauenheim, Thomas 324 Fujdala, Kyle L 379 FiirpaB, Gottfried 490

828 Author Index

G

Gehrhus, Barbara 27 Glatthaar, Jorg 5, 11, 15 Goodgame, David M. L 447 Gorshkov, Alexander 514 Gottschalk-Gaudig, Torsten 752 Gotz, Joachim 584 Greulich-Weber, Siegmund 785 Grogger, Christa 490 Gudnason, Palmar 1 135 Guillet, Antoine 705 Guillois, 0 797 Guliashvili, Tamaz 78, 375 Gunther, Betty 545 Gureev, Aleksei 595 Gust, Thorsten 210,213 Guzei, Ilia 19

H

Haaf, Michael 19 Halvorsen, Gunnar 495 Hanelt, Eckhard 664 Harloff, Jorg 175 Hassler, Karl 294 Heine, Thomas 324 Heinemann, Mario 741, 767 Helmol, Nina 261 Herzig, Christian 632 Herzog, Uwe 282, 288 Hild, Sabine 767 Hitchcock, Peter B 27 Hoffmann, D 420 Hofmann, Marco 473 Horner, S 307 Hornig, Jan 150,167 Huber, Klaus 785 Hubler, Klaus 312 Huisken, F 797 Htising, Nicola 689, 700

I

Ivanov, Vladimir 344

J

Jenkner, P 551 Jerzembeck, Marion 126

Jonas, Ulrich 772 Jonsdottir, Sigridur 135 Jutzi, Peter 50,429

K

Kalikhman, Inna 55, 61 Kammel, Thomas 527, 664 Karsch, Hans H 270 Kaupp, Martin 329 Kayser, Christian 186 Kempe, Rhett 82 Kickelbick, Guido 294,462, 689 Kingston, Vijeyakumar 55, 61 Kireev, Vyacheslav 344 Kleshcevnikova, Salomonida 518 Kliem, Susanne 254 Klingebiel, Uwe 233, 246,254,261 Knopf, Claudia 522 Kochina, T. A 321 Komaragiri, Rama S 808 Konig, H. J 425 Kopylov, V. M 541 Kopylov, Victor 344, 514 Kornick, Andreas 126 Korobkov, Evgenii 518 Koroleva, Tat'yana 595 Kost, Daniel 55, 61 Kovalenko, Victor 600 Kovyazin, V. A 541 Kownacki, Ireneusz 641 Kraxner, Peter 562 Krempner, C 217,420 Kretschmer, Axel 339 Kroke, Edwin 808 Krompiec, Stanislaw 415 Kruger, Christian 772 Kujawa-Welten, Malgorzata 415 Kuwabara, Masato 664 Kvaran, Agust 135

L

Lang, Heinrich 522 Lange, Heike 288 Lappert, Michael F 27 Launay, Beatrice 689 Lautenschlager, Hans 632 Lavygin, Igor 600

Author Index 829

Leadley, Stuart R 650 Lebedev, Evgenii 518 Ledoux, G 797 Lee, Vladimir Ya 92 Legrand, A. P 747 Lejeune, Alain 562 Letzel, M. C 425 Lickiss, Paul D 45, 447 Liebau, Verena 261 Lindemann, Hans Martin 429 Litvinov, V. M 715 Litvinov, Victor 752 Loginov, S. V 352 Lorentz, Gilles 612 Lorey, L 509 Losehand, Udo 226 Lotarev, Mikhail 595 Lowther, James E 808

M

Maciejewska, Wioletta 641 Maciejewski, Hieronim 536, 641 Mack, Helmut 557 MacKinnon, 1 696 Mahnke, Jens 213 Maier, Giinther 5, 11, 15 Majchrzak, Mariusz 641 Malanoski, Anthony P 689 Malisch, Wolfgang 473, 486 Marciniec, Bogdan 363, 415, 536, 641 Markacheva, Anna 514 Marschner, Christoph 171, 186, 190 Marsmann, H. C 425 Marsmann, Heinrich 324, 785 Masangane, Phindile C 45 Maulitz, A. H 307 Mechtler, Christian 171 Merkulov, Alexei A 451 Merz, Steffen E. F 312 Mickoleit, Martin 82 Miehe, Gerhard 808 Milbrad, Marc 324 Mitzel, Norbert W 226 Monkiewicz, J 573 Morawski, Jean-Claude 705 Moser, Daniel F 19 Mountford, Philip 451

Miiller, Lars 210, 213 Muller, Thomas 34, 139, 334

N

Naendrup, Franz 638 Nakabayashi, Akira 819 Natsjuk, Sergei 595 Neidhoefer, Michael 339 Neumann, Beate 50,429 Nikitin, A. V 541 Nikitina, Ekaterina 752 Nikonov, Georgii I 451 Nuyken, Oskar 632

O

O'Hare, Lesley-Ann 650 Oberhammer, Heinz 135 Obst, Dietmar 638 Oehme, Hartmut 82, 202 Ofitserov, Evgenii 356 Ottosson, Henrik 78, 375 Ou, Duan Li 435, 696, 736

P

Pachaly, B 527 Parbhoo, Bhukan 650 Parrucci, Massimo 27 Pawluc, Piotr 641 Pernisz, Udo 145 Persello, Jacques 705 Pfeiffer, Jurgen 456 Pietzsch, Claus 462,467 Pohmer, Klaus 678 Polivanov, Alexander 514 Popowski, Eckhard 175 Posset, U 645 Potter, Matthias 82

Q Quellhorst, Heike 126, 130

R

Rasulov, Maksud 356 Rautz, Hermann 462 Reiche, Clemens 254 Reilly, Roisin 814

830 Author Index

Reinke, H 202, 217,420 Reisenauer, Hans Peter 5, 11, 15 Reynaud, C 797 Rheinwald, Gerd 282, 522 Riedel, Ralf 808 Rittmeister, Ben 545 Robert K. Szilagyi 277 Roewer, G 317, 509 Roewer, Gerhard 288, 467, 522 Rooke, Stephanie J 447 Rose, K 645 Rupp, Wolfgang 700

S

Sakurai, Hideki 195 Schafer, 0 527, 659 Schildbach, Daniel 155 Schindler, W 527 Schluttig, Birgit 277 Schmatz, Stefan 233, 246,261 Schmedake, Thomas A 19 Schmohl, Kathleen 82, 202 Schneiderbauer, Stefan 303 Schollmeier, Thorsten 299 Schubert, Ulrich 456, 700 Schumacher, Dirk 473,486 Schurmann, Markus 299 Schiissler, Gunnar 495 Schwarz, Marcus 808 Segimiller, Thomas 270 Seifert, Gotthard 324 Sekiguchi, Akira 92 Seppala, Emma 210, 213 Shapatin, Anatolii 581 Shchogolev, B. F 321 Sinotova, E. N 321 Slootweg, J. Chris 27 Sobolevskii, Mikhail 600 Soger, Nicola 130 Sohal, Wazir 45 Sohns, Andreas 486 Sommer, Joachim 207 Spiess, Hans Wolfgang 339 Stalke, Dietmar 61 Stammler, Anja 50,429 Stammler, Hans-Georg 50, 429 Standke, B 551

Stanjek, V 527 Steinberger, Hans-Uwe 180 Stintz, Michael 741 Stohr, Frank 456 Strissel, Christine 632 Strohfeldt, Katja 329 Strohmann, Carsten 150, 155, 167, 329 Stuger, Harald 462, 490 Sturmayr, Dietmar 456, 689 Stutzmann, Martin 789 Su,K 696

T

Thompson, Susan 456 Tomuschat, Philipp C 622 Trimmel, Gregor 700 Tsantes, Georgios 334

U

Uhlig, Frank 299 Uhlig, Wolfram 222

V

van Swol, Frank 689 Veneziani, Guilaine L 45 Vergelati, Caroll 612 Voelkel, Ute 767 Vogg, Gunther 789 Vogler, Matthias 473 Vojinovic, Krunoslav 226 Vrazhnov, D. V 321

W

Wack, Eric 329 Wagler, J 317 Walczuk, Edyta 415 Walfort, Bernhard 61 Walsh, Meaghan 606 Weidenbruch, Manfred 114 Weinrich, Sabine 303 Weis, Johann ......... 1, 527, 659, 715,741, 767 Weiltenbach, K 551 Weisser, Horst 584 West, Robert 19 Westerhausen, Matthias 303 White, Andew J. P 447

Author Index 831

Wiberg,Nils 101 Wild, Kerstin 155 Wilhelm, Manfred 339 Wilkening, Andreas 126 Williams, David J 447 Winkler, Holger 785 Wurthwein, E.-U 307 Wyszpolska, Agnieszka 536

Y

Yan, Duanchao 139, 180

Z

Zelisk, Paul 606 Zerr, Andreas 808 Zheneva, Marina 344 Zhun, Alia 360 Zhun, Vladimir 360 Zverev, Vladimir 595, 600

Organosilicon Chemistry V Edited by N. Auner and J. Weis

Copyright © 2003 WILEY-VCH Verlag GmbH & Co. KgaA, Weinheim

Subject Index

ab initio calculations 167, 277, 312 acetoxysilane 344 acrylic acids 352 activation parameters 190 acyclic 254 acylbis(trimethylstannyl)phosphanes 307 adamantanes 282 addition reactions 114 adhesion 650, 671 adhesion promoters 541 adhesive properties 767 adhesives 527 adsorption 715 aerogel 435 aerosil 126 AFM 650 agosticbond 451 alcohol 344,514 alkaline earth metals 303 alkylation 207 alkylidenediphosphane 210 alkylnaphthalene 514 alkynes 171 allyl derivatives 415 aluminosilicates 814 aluminosiloxanes 814 aluminosilsesquioxanes 814 ambient pressure drying 435 amides 545 amines 321 1-aminoallyl anions 207 amino-^3-phosphaalkynes 307 amino-functionalised silyl anions 27 aminosilane 277, 522, 545, 557, 659

cx-aminosilanes 527 aminosilicone 612

B

binders 645 binuclear silicon complexes 61 bioactive silicon 348

block copolymers 339, 689 burns 352

C

cages 254, 294, 303 calculations 246 carbamato 536 carbanions 180 carbocations 34 carbohydrates 622 carbothermic process 495 catalysis 363,456 catalyst recycling 638 ceramics 808 charge transfer 467 chelates 317 chemical shift 329 chemical shift calculation 324 chirality 167, 207 chlorosilanes 217, 490, 522 chlorosiloxanes 126, 130 cholesterol 622 coatings 551 co-condensation 5, 15 colloid particle 772 computational chemistry 34, 334 condensation 473 conformational analyses 135 contact angle 650 corona 650 cosmetics 348 coupling agent 557 crosslinker 527 crosslinking 562 cross-metathesis 363 crystal structure 329, 447, 462

see X-ray structures

CVD process 126 cyclic silane 299 cyclic voltammetry 490 cycloadditions 114 cyclopentadienyl-silanetriols 429 cyclosilazanes 261

834 Subject Index

cyclotrimetallene 92

D

density functional theory 233 deposition 772 DFT 66, 78, 277, 375 DFTB 324 dielectric constant 696 diffusion 678 1,2-TV, Af-dimethylaminomethylferrocenyl ..467 dimethylsiloxanes 595 diphosphene 210,213 R3,3^3-diphosphetanes 307 direct process 15, 514 direct synthesis 509 disilanes 509,545 disilene 101,114 disilyne 101 disproportionate 518 domain size 339 donor-acceptor systems 82 doubly bonded silicon 92

E

elastomer 659 electrochemistry 490 electron diffraction 135 electronic state 509 enamines 317 enantiomerically enriched 167 energy carrier 495 enzyme stability 606 equilibria 45 equilibrium reaction 55 esterification 344 etherification 518 ethoxysiloxanes 130 exchange reactions 456 expansions 233

F

fabrication 772 fast gelation 435 ferrocene 462, 467 fluorescence 139 fluoroalkylsilanes 551

fluorosilanes 217 force field 277 formal hydroxylation 207 friction 600 fumed silica 741, 747, 752

GC-MS 130 germanes 312 germanium amidinates 270 germanium compounds 114 granulation-fibroid tissue 352 growth pattern 126

H

half-sandwich complexes 486 halocarbons 19 halogen-free flame-retardants 562 hepatoprotection effect 356 herbs 348 heteroatomic siloxanes 254 heterogeneous catalysis 379 hexachloroplatinic acid 638 hexacoordinate silicon 55,61 high energy consumption 495 highly porous hybrid 435 high-pressure chemistry 808 homoenolate 207 homogeneous catalysis 638 H-siloxane....... 819 hybrid coatings 573 hybrid organic-inorganic silicate 736 hybrid polymers 645 hydrazine 226 hydride 451 hydrogen migration 180 hydrogensilsesquioxanes 435 hydrolysis 130 hydrophilic 612 hydrosilylation 415, 622, 632, 638, 641 hydroxylamine 226 hypercoordinate organosilicon compounds ..66 hypercoordination 226 hypersilyl 288 hypersilyl alcohols 202 hypervalency 270 hypervalent bond 451

Subject Index 835

hypervalent compounds 317

I

IGC 747 imino-methylidenephosphanide anion 307 injection molding 671 inorganic filler 557 inorganic nanoparticles 573 inorganic-organic hybrid materials 700 insertion 27 intensification 360 intensity contour plots 145 intermolecular interactions 752 inversion barrier 190 ionic dissociation 55 ion-molecule reactions 321 IR spectroscopy 747 iron 473,486 irradiation 11, 114, 139,360,819 isocyanate 659 isocyanato 536 a-isocyanatomethylsilanes 527 isomerization 233, 261

K

Karstedt catalyst 641

liquid injection molding (LIM) 678 liquid silicone rubber 671, 678 liquid-crystal displays 664 liquid-crystal polymers 664 lithiosilanes 167 lithium 150, 155 long-chain alkyls 595 long-range coulomb interactions 66 low emissivity 645 low k 696 low valency 270

M

MALDI-TOF-MS 425 masked disilene 195 masonry paints 645 material contrast 767 matrix isolation 5, 11, 15

medicine 348 membrane materials 641 mesostructured silica 689 metal complexes 447 metal siloxides 420 metallo-silanols 486 metallurgical-grade silicon 495 metathesis 167 methyl cations 321 methyldichlorophenylsiloxane s 595 methyldiethoxysilane 518 methylphenyl silicone fluid 678 methylsilane 518 modifications 595 modulus 696 molecular modeling 277 molecular precursors 379 molecular weight 584 Monte Carlo simulations 689 Mossbauer spectroscopy 462, 467 multiple bonds 114

N

nanoparticles 700 nanospheres 785 negative entropy 55 neutral hexacoordinate complexes 55 NMR spectroscopy 34, 135, 294, 334, 584

NMR shift calculations 294 29Si 324 29Si NMR 50, 329, 334 29Si NMR spectroscopy 425 !H solid-state NMR 339 31PNMR 210 dynamic NMR spectroscopy 190 solid-state NMR 715,747

nonclassical interactions 451 nucleophile-catalyzed 210

O

octasilsesquioxane 425 oil 600 oil and gas production 581 oil-bleeding 678 oleochemicals 638 oligoethylsiloxanes 595 oligosilanes 171, 217,420

836 Subject Index

oligosilyl anions 217 organic matrix 573 organofunctional silane 573 organomagnesiumchlorides 360 organomodified silicones 622 organosilanes 155 organosilicon amines 541 oxide-nitrides 808 oxygenation 473

P

paper coatings 632 particle pattern 772 particle size 741 PDMS 715 PET 650 pharmacokinetics 356 phase-dependent structures 226 phenols 344 phenylsiloxanes 145 phosphaalkenes 213 phosphanediides 303 photocatalytic titanium dioxide 819 photoisomerizations 5, 11, 15 photoluminescence 145, 797 photonic crystals 785 pincer ligands 50 platinum complexes 456 polyaddition 600 polyamide 557 polycarbosilane 641 polycycles 282 polygermyne 789 polyhedral zirconasiloxanes 429 polymer latex 772 polymeric complexes 447 polymerization 19, 363 polysilanes 195, 462 polysilylene-ethynylenes 222 polysilylene-phenylenes 222 porosity 696 porous particles 736 potassium 186 powder handling 741 PP 650 protein release 606 purification 518

pyrazoles 312

quantum chemical calculations.. 155, 261, 329 quantum confinement 797 quantum mechanical calculations 34

R

raw material 509 reactive intermediates 34 rearrangements 78, 202, 246, 375 regioselective adsorption 772 regiospecific polymerization 195 release control 632 retarder films 664 reversed polarization 78, 375 rheology 705,752 rhodium(i) siloxide complex 415 rigid flexible combination 671 rings 233,254 rubber 562,659

schiff bases 317 selenium 282, 288 self-assembly 772 self-condensation 175 SFM 767 sheet polymers 789 sialons 808 Si-C cleavage 155 Si-C cleavage 150 silacycles 329 silacyclobutanes 180 silacyclohexane 135 silane 705 silane coupling agents 536 silane layer 772 silanediols 145 silanes 82, 139, 202, 312, 536, 545, 562 silanols 420,473 silatranes 348, 352, 356 silazane 659 silenes 78, 82, 375 silica 705, 736, 767 Silica 715

Subject Index 837

silica-titania mixed oxides 700 silicocations 45 silicon 101, 294, 317, 451, 467 silicon amidinates 270 silicon atoms 11 silicon dioxide 126 silicon quantum dots 797 silicon solar cell 495 silicone 581, 600, 632, 650, 659, 689 silicone oils 584 silicone resin 696 silicone rubber (MVQ) 678 silicone-based product 495 silicone-modified 819 siliconium cations 61 siliconium compounds 55 silicon-metal complexes 19 silicon-tin bond 299 silocanes 348, 352 siloxanes 254, 334, 447, 473 siloxanoles 130 siloxene 789 siloxypyridines ....447 silthianes 282 silyl anions 27, 150, 171, 186, 190 silyl cations 34, 50 silyl complexes 456 silyl triflates 222 silylation 207, 363 silylative coupling 363 silylcarbenium ions 202 silylenes 27, 101, 114,213 silylenoids 175 silylgermanes 213 silylhydrazines 233 silylhydroxylamines 246 silyllithium compounds 150 silylphosphanes 213 silyl-stabilization 180 Si-N bonds 66 Si-O-Zr frameworks 429 small ring 92 softener 612 sol-gel process 573, 645, 700, 785 solid-liquid reaction 819 solid-state structure 50 solubility 514

spin diffusion 339 spinel phases 808 spray foam 527 stereochemistry 155, 545 stereoselectivity 195 stilbene 139 structure 45, 705 structure elucidation 82 sulfur 288 surface... 650 surface morphology 747 surface treament 551 sustainable development 495 synthesis 45

T

T2 relaxation 715 tailored materials 379 TCNE 522 TDAE 522 tellurium 282,288 tetraorganosilanes 150 tetrasilabutadiene 114 textile 612 thermoplastic 659 thioether 425 three-membered rings 226 tin 288,303 tin-substituted oligosilane 299 titanium 420 toner.. 767 topotactic transformation 789 transition metal silyl compounds 186 transition states 233 trialkylsilyl substituents 303 trimethylsiloxydisilanes 175 trimethylsiloxysilyllithiums 175 trimethylsiloxytrisilanes 175 1,2-¾ C-trimethylsilyl migration 202 trityl salts .....45 tungsten 486

U

ultrasound 360 unsaturated polyester resins 752 urea 536, 659 urethane 659

838 Subject Index

V

vinyl ester resins 752 vinylsilanes 363 viscosity 584

W

wastes 495, 581 water-in-silicone oil emulsion 606 water-repellents 551 wounds 352

X

XPS 650

X-ray structure analysis 101 X-ray structures 294, 312

see crystal structure

Y

yellowing 612

Z

zeolites 814 zirconium 420