paramagnetic iron dinitrosyl complexes...table 4.3 no stretching frequencies of paramagnetic iron...

STRUCTURE AND REACTIVITY OF PARAMAGNETIC

IRON DINITROSYL COMPLEXES

by

TRACI R. BRYAR, B.Sc.

A thesis

Submitted to the Faculty of Graduate Studies

In Partial Fulfilment of the Requirements

for the Degree

Doctor of Philosophy

McMaster University

October, 1990

(c) Copyright by Traci A. Bryar, 1990.

PARAMAGNETIC IRON DINITROSYL COMPLEXES

DOCTOR OF PHILOSOPHY (1990)

((;hemistry)

McMASTER UNIVERSITY

Hamilton, Ontario

TITLE: Structur~ and Reactivity of Paramagnetic Iron Dinitrosyl Complexes

AUTHOR: Traci R. Bryar, B.Sc. (McMaster University)

SUPERVISOR: Professor D.R. Eaton

NUMBER OF PAGES: xvi,167

iI

ABSTRACT

Paramagnetic iron dinitrosyl complexes have been investigated since 1968.

The compounds are not easy to isolate and the majority of studies have concentrc.ted on

solution EPR measurements. As a result ambiguities remain, which concern both the

geometries and electronic structures of these complexes. More recently, their catalytic

properties in reactions involving the polymerization and oligimerization of olefinic

compounds have been reported. Questions remain regarding the effectiveness of the

complexes as catalysts, the mechanism of catalysis and the variation in catalytic activity

with changes in the ligands.

This thesis reports research aimed at clarifying both the structural and

catalytic chemistry of this class of compounds. The major tool used in the structural work

is solid s~ate EPR spectroscopy. although this has been supplemented by infra-red

spectroscopy and X-ray crystallography. The conclusions reached favour a description

of the electronic configuration which differs from that adopted by most previous

researchers. The emphasis in the catalytic work has been the determination of the

mechanism of polymerization. The effect of electron-withdrawing or -donating substituents

on the rate of polymerization was studied. In addition, the tacticity of the polymeric

products was characterized using high resolution 13C NMR spectroscopy. Different

microstructures are predicted for polymers produced using a metal-coordinated catalyst

iii

or using free-radical, anionic or cationic initiators.

It was determined that the paramagnetic Fe(NO)2 compounds are best

described as 17-electron complexes with a rf. rather than a d7, electronic configuration

on iron and a distorted tetrahedral geometry. The nature of the distortion is dependent

on the type of ligands bonded to iron. When the Iipands include hard, nonpolarizable

donor atoms such as oxygen or fluoride, the complex distorts towards a square-planar

geometry. The spin-containing molecular orbital is predominantly dx2.y2. When atoms such

as sulfur and phosphorus are bonded to iron the geometry of the complex can be

described as a trigonal bipyramid with one axial ligand missing. The dz2 orbital now

contains the unpaired electron. When the ligands are halides or N-bonded species, the

distortions from tetrahedral are less extreme and the spin-containing MO is comprised of

a mixture of d orbitals. The crystal structure of [Fe(NO)212r has been determined to be

tetrahedral with a slight distortion towards the tbp geometry. This is the first example of

an unrestricted complex with a tetrahedral geometry. The factors inf;uencing these

structural changes will also be discussed.

The paramagnetic Fe(NO}2 complexes are labile in solution and one

complex can often be converted to another simply by addition of excess ligand to the

reaction mixture. The reactivity of these compounds is dependent on the availability of

a vacant coordination site on iron, therefore the more labile complexes are more reactive,

while complexes missing one ligand have significantly greater reactivity than those which

are four-coordinate. The paramagnetic dinitrosyl complexes effectively initiate the

iv

polymerization of styrene. The efficiency of initiation does not depend markedly on the

geometry of the complex. A mechanism is suggested in which the alkene coordinates to

iron; this is followed by electron transfer which formally reduces the iron. This process

creates an organic ligand which bears a formal positive charge and which subsequently

participates in a cationic polymerization mechanism. This mechanism is unusual in that

a paramagnetic complex is involved, but the mechanism is not a free radical one.

Although the 17-electron complexes react quite well with alkenes they do not react with

cyclooctatetraene or dienes such as norbomadiene an~ isoprene.

v

to Kirt,

I could never thank you enough.

vi

ACKNOWLEDGEMENTS

I wish to express my deepest gratitute to my supervisor Dr. Don Eaton for

his guidance throughout the course of this project. I appreciate his creative ideas,

constant support and his relaxed, enjoyable approach to research. I feel that I could not

have chosen a more suitable supervisor for my graduate studies. I sincerely hope he

enjoys his retirement.

I would also like to thank the members of my supervisory committee, Drs.

J. Warkentin and M.J. McGlinchey for their useful suggestions in the last three yea":.'. Dr.

Warkentin has kindly reminded me that I should not forget organic chemistry simply

because my specialty is transit:::::n metal chemistry. Dr. McGlinchey has often provided

the assistancE:. normally given by a supervisor and he has adopted me unofficially into his

research croup.

It is often said that a thesis could not be completed without the help of

many others in the department. I am deeply indebted to Ian Thompson and Dr. Bruce

Fulton for their assistance with the EPR spectrometer and related software. I wish to

thank Dr. Don Hughes and Brian Sayer for providing the NMR spectra of my polymer

samples and I sincerely appreciate the time DaVE! Adams and Joe Vetrone spent to obtain

Mossbauer spectra of my compounds. I would also like to thank Romolo Faggiani for his

assistance with X-ray crystallography. Special thanks go to Dr. Chris Frampton for always

taking the time to answer my questions about crystallography and Mossbauer

vii

spectroscopy. Finally, I would like to thank Carol Dada, Paula Martin and the staff in the

departmental office for their help, and the Department of Chemistry for the award of a

teaching assistantship.

On a more personal note I would like to thank Vic Pavski for his friendship

over the years. His unusual sense of humour helped me througi-, those times when no

experiment would work, no matter how trivial. As Dr. Eaton has moved closer to

retirement he has generously donater' I~b space to those in need. So, to Richard, Bavani

and Lijuan of the McGlinchey connection, and to lan, Bruce and Jan from the Bain group;

a special thank you for your support and friendship.

viii

TABLE OF CONTENTS

CHAPTER 1: INTRODUCTION TO NITROSYL COMPLEXES

PAGE

1.1 Introduction 1

1.2 Structure and Bonding 2

1.3 Historical Background 8

.1 Synthesis 8

.2 Spectroscopic Characterization 10

.3 Reactivity 26

1.4 Research Objectives 30

CHAPTER 2: THEORY

2.1 Introduction 32

2.2 Electron Paramagnetic Resonance 33

.1 Introduction 33

.2 The Zeeman Hamiltonian 33

.3 Hyperfine Coupiing 36

.4 Anisotropic Effects 39

.5 EPR of Transition Metal Complexes 42

2.3 Infra-red Spectroscopy 46

ix

CHAPTER 2: THEORY

PAGE

NMR of Polymers2.4

.1

.2

Configurational Microstructurl:l

13C NMR of Vinyl Homopolymers

47

48

48

CHAPTER 3: EXPERIMENTAL PROCEDURES

3.1 Structural Characterization 56

.1 Synthesis 56

.2 Infra-red Spectroscopy 60

.3 EPR Spectroscopy . 60

.4 Mossbauer Spectroscopy 62

.5 X-ray Crystallography 62

.6 EHMO Calculations 67

3.2 Reactivity Investigations 72

.1 Polymeri~ation of Styrenes 72

.2 13C NMr~ of Polymers 74

.3 Reaction with Nitrosobenzene 74

.4 Reaction& with Dienes 75

x

PAGE

CHAPTER 4: STRUCTURAL CHARACTERIZATION OF IRON

DINITROSYL COMPLEXES

4.1 Results 76

.1 Solution EPR Spectra 76

.2 EPR of Frozen Solutions 82

.3 Infra-red Spectra 82

.4 Mossbauer Spectra 88

.5 Crystal Structure of NPP[Fe(NO)2'2] 88

4.2 Discussion 92

.1 Solution EPR 92

.2 EPR of Frozen Solutions 93

.3 Electron Configuration of Iron 94

.4 Structure of Iron Dinitrosyl Complexes 97

4.3 Molecular Orbital Description 104

4.4 Conclusions 106

xi

PAGE

CHAPTER 5: REACTIONS OF THE PARAMAGNETIC IRON

DINiTROSYL COMPLEXES

5.1 Polymerization of Styrene 108

.1 Activity of Paramagnetic Complexes as Catalyst 108

.2 Mechanism of Polymerization 111

5.2 Reaction with Nitrosobenzene 125

5.3 Reaction with Dienes 126

5.4 Conclusions 127

CHAPTER 6: SUMMARY

6.1 Summary

6.2 Future Work

REFERENCES

APPENDIX

xii

128

129

132

142

LIST OF TABLES

PAGE

Table 1.1 EPR of Iron Dinitrosyl Complexes in Solution 12

Table 1.2 Solid State EPR of Iron Dinitrosyl Complexes 15

Table 1.3 Structural Data for Iron Dinitrosyl Complexes 18

Table 1.4 Comparison of Fe(NOMPPh3)CI Bond Angles to the Idealized

Trigonal Bipyramidal Geometry 21

Table 1.5 NO Stretching Frequencies tor Paramagnetic Iron Dinitrosyl

Comp!exes and the Diamagnetic Dimers 23

Table 1.6 Mossbauer Data for Paramagnetic Iron Dinitrosyl Complexes

and Diamagnetic Dimers 25

Table 2.1 ProgressuJn from Dyad to Tetrad to Hexad Sequences tor CH2

Carbons in Vinyl Homopolymers 50

Table 2.2 Progression from Triad to Pentad Sequences for CH and

R-group Carbons in Vinyl Homopolymers 51

Table 2.3 Relative Peak Intens!t!~s for an Atactic Polymer with a

Random Distribution (Pm-=O.5) and a Polymer Which Follows

Markov Statistics (Pm=O.75) 53

Table 3.1 Paramagnetic Iron Dinitrosyl Complexes Prepared from Iron (II)

Salts and Anionic Ligands 57

xiii

PAGE

Table 3.2 Crystal Data for NPP[Fe(NO)212] 63

Table 3.3 Positional Parameters (x105) and Ueq (x104

) for NPP[Fe(NO)212] 68

Table 3.4 Standard HiI's and Slater Exponents and Coefficients Used in

Extended Huckel Calculations 71

Table 3.5 Polym(~rization Reactions 73

Table 4.1 Solution EPR Data for Iron Dinitrosyl Complexes 77

Table 4.2 Solid State EPR Data 83

Table 4.3 NO Stretching Frequencies of Paramagnetic Iron Dinitrosyl

Complexes 87

Table 4.4 Selected Bond Lengths (A) and Bond Angles for the [Fe(NO)212r

Anion 90

Table 5.1 (J Values of Common Substituents 112

Table 5.2 13C NMR Chemical Shifts of Methyl Substituted Polystyrenes 115

Table 5.3 13C NMR Chemical Shifts of poly(p-Methoxystyrene) 119

Table 5.4 Microtacticity of Poly(p-Methoxystyrene) 122

Table A1 Anisotropic Temperature Factors (x1 05) for NPP[Fe(NO)2IJ 142

Table A2 Hydrogen Atom Positional Parameters (x105) for NPP(Fe(NO)2121 144

Table A3 Observed and Calculated Structure Factors for NPP[Fe(NO)2121 145

xiv