paramagnetic iron dinitrosyl complexes...table 4.3 no stretching frequencies of paramagnetic iron...
TRANSCRIPT
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
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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
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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.
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to Kirt,
I could never thank you enough.
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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
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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.
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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
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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
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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
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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
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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
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