facilitating multi-electron reactivity at low-coordinate cobalt complexes using redox-active ligands

183 Pages·2011·2.89 MB·English

FACILITATING MULTI-ELECTRON
REACTIVITY AT LOW-COORDINATE
COBALT COMPLEXES USING
REDOX-ACTIVE LIGANDS

A Thesis
Presented to
The Academic Faculty

by

Aubrey L. Smith

In Partial Fulfillment
of the Requirements for the Degree
Doctorate in the
School of Chemistry and Biochemistry

Georgia Institute of Technology
December, 2011
FACILITATING MULTI-ELECTRON
REACTIVITY AT LOW-COORDINATE
COBALT COMPLEXES USING
REDOX-ACTIVE LIGANDS

Approved by:

Dr. Jake D. Soper, Advisor Dr. L. Andrew Lyon
School of Chemistry and School of Chemistry and
Biochemistry Biochemistry
Georgia Institute of Technology Georgia Institute of Technology

Dr. E. Kent Barefield Dr. Christopher W. Jones
School of Chemistry and School of Chemical and
Biochemistry Biomolecular Engineering
Georgia Institute of Technology Georgia Institute of Technology

Dr. Z. John Zhang Date Approved: August 16, 2011
School of Chemistry and
Biochemistry
Georgia Institute of Technology
ACKNOWLEDGEMENTS

Throughout this journey, the list is long of those who deserve acknowledgement
and thanks. First, great appreciation goes to my advisor, Professor Jake D. Soper, for
five years of advice, guidance and support. I hope I am a better chemist for all you have
taught me. Words cannot express the recognition which “the group” deserves. My
friendship formed with Clarence Rolle, Michael Bayless, and Cameron Lippert made
each day more enjoyable and successful. Many thanks for your support which ranged
from scientific advice and collaboration to comic relief and a shoulder to lean on. To my
life-long friend, Kris Hammitt, countless thanks for always making me laugh, standing
beside me throughout the years, and giving me valuable outside perspective on the real
priorities in life. My sister, Ashley Smith, deserves immeasurable gratitude for always
listening to all my “boring chemistry”. I count myself lucky that you are not only my sister
but became my best friend. My parents Terry and Beverly Smith built a solid and
valuable foundation on which I could grow. They then trusted me to find my own way
through life. For this I became who I am and will spend the rest of my life trying to
express my appreciation. And finally to my husband, Dean Shahriari, my most heartfelt
thanks goes to you for always being there for me on good days and bad, for keeping me
focused on the end goal, reminding me I could succeed when I doubted myself, and for
helping me be the person and make the life I want to have. You each helped me reach
my goals and without you, I would not have completed this process and presented this
body of work today.
iii
TABLE OF CONTENTS
Page
Acknowledgements iii
List of Tables ix
List of Figures xi
List of Symbols and Abbreviations xiii
Summary xvi
Chapter
1. Background and introduction 1
1.1. Carbon-carbon coupling reactions 1
1.2. Design and characterization of redox-active ligands 4
1.3. Project aim 7
1.4. References 8
2. Synthesis and characterization of square planar cobalt(III) complexes and
their ligand mediated Co-X bond forming reactivity 13
2.1. Introduction 13
2.2. Experimental Details 17
2.2.1. General considerations 17
2.2.2. Methods and materials 18
2.2.3. Preparation of CoIII(isqPh)(apPh) 19
2.2.4. Preparation of CoIII(isqiPr)(apiPr) 20
2.2.5. Preparation of CoIII(isqCl)(apCl) 20
2.2.6. Preparation of Na[CoIII(apPh) ] 21
2
2.2.7. Preparation of Na[CoIII(apiPr) ] 22
2
2.2.8. Preparation of Na[CoIII(apCl) ] 22
2
2.2.9. Preparation of (Cp* Co)[CoIII(apiPr) ] 22
2 2
iv
2.2.10. X-ray crystallography 23
2.3. Results 25
2.3.1. Preparation and characterization of CoIII(isqAr)(apAr) 25
2.3.2. Preparation and characterization of [CoIII(apAr) ]– 27
2
2.3.3. Behavior of [CoIII(apAr) ]– in various solvents 31
2
2.3.4. One electron Co-Cl bond-forming reaction 32
2.3.5. Two electron Co-Cl bond-forming reaction 34
2.3.6. Two-electron Co-Br bond-forming reaction 35
2.4. Discussion 36
2.4.1. Structure and electronic assignment of four-coordinate
complexes 36
2.4.2. Changes in [CoIII(apAr) ]– in various solvents 39
2
2.4.3. Proof-of-principle bond-forming reactions at cobalt(III) 40
2.4.4. Mechanistic implications of reactions with [CoIII(apAr) ]– 41
2
2.5. Conclusions 42
2.6. References 43
3. Pseudo-oxidative addition of alkyl halides to nucleophilic cobalt(III) 48
3.1. Introduction 48
3.2. Experimental Details 50
3.2.1. General considerations 50
3.2.2. Methods and materials 51
3.2.3. Synthesis of CoIII(CH Cl)(isqPh) 52
2 2
3.2.4. Synthesis of CoIII(Et)(isqPh) 53
2
3.2.5. Synthesis of CoIII(Me)(isqPh) 53
2
3.2.6. X-ray crystallography 54
3.3. Results 55
3.3.1. Synthesis and characterization of CoIII(CH Cl)(isqPh) 55
2 2
v
3.3.2. Synthesis and characterization of CoIII(Et)(isqPh) 58
2
3.3.3. Reaction scope and relative rates with [CoIII(apPh) ] 59
2
3.3.4. Stability of the organometallic products 61
3.3.5. Kinetic study of the reaction with CH Cl 62
2 2
3.3.6. Reaction with alkyl halides radical clocks 64
3.3.7. Lack of reactivity of CoIII(Et)(isqPh) with CO 65
2
3.4. Discussion 65
3.4.1. Pseudo-oxidative addition product 65
3.4.2. Mechanistic implications of pseudo-oxidative addition 66
3.5. Conclusions 68
3.6. References 69
4. Homogeneous carbon-carbon coupling facilitated by cobalt(III) metal
centers 72
4.1. Introduction 72
4.2. Experimental Details 76
4.2.1. General considerations 76
4.2.2. Methods and materials 77
4.2.3. Preparation of [CoIII(CH CN)(isqPh) ][BF ] 77
3 2 4
4.2.4. Preparation of [CoIII(CH CN)(isqiPr) ][BF ] 78
3 2 4
4.2.5. Cross-coupling reactions and quantification of organic products 79
4.2.6. Homocoupling reactions and quantificiation of organic products 79
4.2.7. Quantification of metal containing species after coupling
reactions 80
4.2.8. Preparation and characterization of CoIII(nPr)(isqiPr) 81
2
4.2.9. X-ray crystallography 81
4.3. Results 84
4.3.1. Step-wise cross-coupling facilitated by [CoIII(apAr) ]– 84
2
vi
4.3.2. Identification of the source of homocoupling side-product 97
4.3.3. Preparation and characterization of [CoIII(CH CN)(isqAr) ]+ 99
3 2
4.3.4. Homocoupling facilitated by [CoIII(CH CN)(isqAr) ]+ 101
3 2
4.4. Discussion 108
4.4.1. Characterization of [CoIII(CH CN)(isqAr) ]+ and implications of
3 2
its electronic structure 108
4.4.2. Similarities between the two coupling reactions 109
4.4.3. Limitations of catalytic turnover 110
4.4.4. Mechanistic implications of coupling reactions 112
4.4.5. Challenges to coupling using [CoIII(apAr) ]– and [CoIII(isqAr) ]+ 114
2 2
4.5. Conclusions 116
4.6. References 118
5. Synthesis and reactivity of low-coordinate cobalt complexes containing
only one redox-active ligand 124
5.1. Introduction 124
5.2. Experimental Details 128
5.2.1. General considerations 128
5.2.2. Methods and materials 128
5.2.3. Direct synthesis of (triphos)CoII(cat) 129
5.2.4. Synthesis of [(triphos)CoII(sq)]BF 130
4
5.2.5. Reaction of (triphos)CoII(cat) with electrophilic Cl source 130
5.2.6. Attempted synthesis of (triphos)Co(apAr) 130
5.2.7. Attempted synthesis of (dppe)Co(cat) 131
5.2.8. Attempted synthesis of (dppe)Co(apAr) 132
5.2.9. Synthesis and reactivity of (Tp)Co(apAr) 132
5.2.10. X-ray crystallography 133
5.3. Results 134
vii
5.3.1. Preparation and characterization of (triphos)CoII(cat) 134
5.3.2. Alternate synthetic procedure and characterization of
[(triphos)CoII(sq)]BF 136
4
5.3.3. Co-Cl bond forming reactivity at (triphos)CoII(cat) 138
5.3.4. Attempted synthesis of (triphos)Co(apAr) 140
5.3.5. Attempted synthesis of (dppe)Co(cat) and (dppe)Co(apAr) 140
5.3.6. Synthesis and characterization of (Tp)Co(apAr) 141
5.4. Discussion 143
5.4.1. Characterization of triphos-cobalt species 143
5.4.2. Multi-electron reactivity with reduced (triphos)CoII(cat) 145
5.4.3. Instability of other complexes with tridentate phosphine
ligand 146
5.4.4. Instability of other complexes containing bidentate phosphine
ligand 147
5.4.5. Unstable complex with stabilizing tridentate nitrogen ligand 149
5.5. Conclusions 150
5.6. References 151
6. Future work and conclusions 155
6.1. Conclusions 155
6.2. Future work 159
6.3. References 163
Vita 165
viii
LIST OF TABLES
Page
Table 2.1: Crystallographic data and structure parameters for CoIII(apPh)(isqPh),
(Ph P)[CoIII(apPh) ] and (Cp* Co)[CoIII(apiPr) ]2CH CN 24
4 2 2 2 3
Table 2.2: Electrochemical midpoint potentials for [CoIII(apAr) ]– 30
2
Table 3.1: Crystallographic data and structure parameters for CoIII(CH Cl)(isqPh)
2 2
and CoIII(CH CH )(isqPh) 55
3 2 2
Table 3.2: Relative reaction rates of organohalides with Na[CoIII(apPh) ] 60
2
Table 4.1: Crystallographic data and structure parameters for
[CoIII(CH CN)(isqiPr) ](Ph B)2CH CN and CoIII(nPr)(isqiPr) 83
3 2 4 3 2
Table 4.2: Cross-coupling yields from reaction of CoIII(Et)(isqPh) with equivalents of
2
PhZnBr or HxZnBr 88
Table 4.3: Cross-coupling yields of CoIII(Et)(isqPh) and PhZnBr in THF and 1:1
2
THF:CH CN solvent 89
3
Table 4.4: Cross-coupling yields of CoIII(Et)(isqPh) with RLi, RMgX and RZnX 90
2
Table 4.5: Cross-coupling yield of reaction of CoIII(nPr)(isqPh) with PnZnBr and
2
CoIII(Pn)(isqPh) and nPrZnBr 91
2
Table 4.6: Cross-coupling yield of reaction of CoIII(Hx)(isqPh) and
2
CoIII(4MePn)(isqPh) with nPrZnBr 92
2
Table 4.7: Cross-coupling yields from reaction of CoIII(Oc)(isqPh) with equivalents
2
of HxZnBr 94
Table 4.8: Cross-coupling yield of reaction of CoIII(Et)(isqAr) with PhZnBr where
2
Ar=Ph, iPr, and Cl 96
Table 4.9: Cross-coupling yields from reaction of [CoIII(apPh) ]– with EtBr and
2
PhZnBr 97
ix
Table 4.10: Yield of homocoupling of PhZnBr by [CoIII(apPh) ]–, CoIII(isqPh)(apPh)
2
and [CoIII(CH CN)(isqPh) ]+ 98
3 2
Table 4.11: Homocoupling yields from reaction of [CoIII(CH CN)(isqPh) ]+ with
3 2
equivalents of PhZnBr or HxZnBr under nitrogen 102
Table 4.12: Homocoupling yields from reaction of [CoIII(CH CN)(isqPh) ]+ with
3 2
equivalents of PhZnBr or HxZnBr with air as catalytic oxidant 103
Table 5.1: Crystallographic data and structure parameters for (triphos)CoII(cat)
and [(triphos)CoII(sq)][BF ]THF 134
4

x