Pioneering Investigations into Organometallic Electrochemistry Thomas Dann A thesis submitted in accordance with the requirements for the degree of Doctor of Philosophy by research at the University of East Anglia School of Chemistry University of East Anglia March 2014 © This copy of the thesis has been supplied on condition that anyone who consults it is understood to recognise that its copyright rests with the author and that use of any information derived there from must be in accordance with current UK Copyright Law. In addition, any quotation or extract must include full attribution. Abstract Recently there has been a large effort to take advantage of electrochemistry to develop and understand novel chemical processes and reactivity. This thesis contains investigations into the synthesis and electrochemical properties of three diverse organometallic systems. In chapter 2, the first experimental determination of a an unsupported AuII-AuII bond is reported. The electrochemical characterisation of gold(III) hydride, hydroxide and chloride pincer complexes based on a backbone of a doubly cyclometalated 2,6-bis(4’-tert-butylphenyl) pyridine ligand was performed, in which it was determined that upon reduction of the AuIII complexes, an unsupported AuII dimer is formed, confirmed by characterisation of an authentic sample of the dimer. Using digital simulation, the reduction potentials of the hydride and hydroxide along with the oxidation potential of the dimer were determined, allowing the construction of a Hess cycle, from which the bond energy of the gold-gold bond in the dimer and the difference of the Au-OH and Au-H bond energies could be estimated. In chapter 3, the redox non-innocent behaviour of the ligands in zinc(II) bis(formazanate) complexes is investigated. These complexes have been electrochemically characterised by cyclic voltammetry, showing remarkably facile reduction to a radical anion, and further reduction to a dianion. Simulation of the cyclic voltammetry recorded for these compounds yielded optimised values of formal potentials, E 0, and electron transfer rate constants, k0. In chapter 4, the synthesis and electrochemical characterisation of the first known examples of triazole-substituted cymantrene and cyrhetrene complexes are reported. The compounds h5-(-phenyltriazol-1-yl)cyclopentadienyl tricarbonyl manganese(I), with a phenyl, 3-aminophenyl or 4-aminophenyl substituent on the 4-position of the triazole ring were prepared via the copper(I)-catalyzed azide-alkyne cycloaddition (1,3-CuAAC) reaction. Cyclic voltammetric characterization of the redox behavior of each of the three cymantrene–triazole complexes is presented together with digital simulations, in-situ i CHAPTER 0. ABSTRACT infrared spectroelectrochemistry, and DFT calculations to extract the associated kinetic and thermodynamic parameters. The synthesis and characterisation of the rhenium(I) analogues of the phenyl and 4-amino triazole substituted complexes are also reported. In chapter 5, the use of diazirines as carbene precursors for carbon surface modification is investigated, via the synthesis and characterisation of diazirine derivatised cymantrene and cyrhetrene. The surface modification of glassy carbon electrodes was attempted via irradiation of the half-sandwhich diazirine bearing complexes, resulting in oxidation waves visible on the electrode by cyclic voltammetric analysis. ii Acknowledgements Firstly I would like to express my gratitude to my supervisor, Dr. Gregory G. Wildgoose for the opportunity to be in his reseach group and embark on this exciting and groundbreaking research, and I am thankful for his enthusiastic guidance throughout. I am also grateful to a number of individuals who made this work possible. In particular Professor Manfred Bochmann and Dr Simon Lancaster for access and use of their resources and guidance on this research project; Dr. Edwin Otten and Mu-Chieh Chang for their collaboration, Dr. David Day for his collaboration and guidance; Dr. Robin Blagg for his advice and crystallography expertise and Elliot Lawrence and James Butress for their support and friendship. Thanks also goes out to the students and post-docs within 1.34b whose help has been outstanding. I would also like to acknowledge Dr. David Huges for his expert elucidation of crystal structures and Dr. Joseph Wright and Dr. Vasily Oganesyan for their input and DFT calculations. Finally I would like my family for their tremendous support throughout all my studies, with special thoughts to my parents Chris and Mary who are always there for me. This research was funded by a Dean’s Studentship from the University of East Anglia. iii Contents Abstract i Acknowledgements iii List of Abbreviations ix 1. Introduction to Organometallic Electrochemistry 1 1.1. Early Organometallic Electrochemistry . .1 1.1.1. Electrochemical Studies of Sandwich Compounds . .2 1.2. Mechanistic Electrochemistry . .3 1.2.1. 17 and 19 Electron Complexes . .7 1.2.2. Dimerisation . .8 1.2.3. Electrochemistry with Chemical Redox Agents . .9 1.2.4. Ligand exchange . 10 1.2.5. Electrochemical effects of ligand derivatisation . 11 1.2.6. Catalysis of organic molecule transformations . 12 1.2.7. Mixed-valence compounds . 14 1.3. Electrochemical Techniques and Analysis . 16 1.3.1. Diffusion . 16 1.3.2. Migration . 16 1.3.3. Convection . 17 1.3.4. Electrode Kinetics . 18 1.3.5. Cyclic voltammetry . 20 1.3.6. Digital Simulation in Electrochemistry . 23 iv Contents 1.4. Surface Bound Electrochemistry . 24 1.4.1. Graphite . 24 1.4.2. Glassy carbon electrodes . 26 1.4.3. Carbon nanotubes . 27 1.4.4. Modification methods . 28 1.4.5. Diazonium salt reduction . 29 1.4.6. Lithium alkyne salt oxidation . 31 1.4.7. Exploitation of CNT edge plane defects . 32 1.5. Modification with Non-heterocyclic Carbenes . 33 1.5.1. Diazirines as carbene precursors for surface modification . 35 1.6. Surface Characterisation Techniques . 36 1.6.1. X-ray Photoelectron Spectroscopy . 36 2. The electrochemistry of gold(III) pincer complexes 43 2.1. Introduction to the organometallic chemistry of gold . 43 2.2. Electrochemistry of organogold compounds . 47 2.3. Gold pincer complexes . 48 2.4. Gold(II) chemistry . 49 2.5. Electrochemical characterisation of Gold(III) hydroxide, hydride and chloride complexes . 50 2.6. Electrochemical Simulations . 59 2.7. Thermodynamic Cycle of Reductive Condensation . 63 2.8. DFT Calculations . 65 2.9. Conclusions . 67 3. Bis(formazanate) zinc compounds 71 3.1. Introduction to redox non-innocent ligands . 71 3.2. Synthesis of bis(formazanate) zinc compounds . 73 3.3. Cylic Voltammetric Analysis . 75 3.3.1. Chemical reduction of zinc formazante compounds . 80 3.3.2. DFT calculations . 83 3.4. Conclusion . 84 v Contents 4. Piano stool–triazole “Click” Products 88 4.1. Introduction to cymantrene . 88 4.2. Redox Chemistry . 89 4.3. Ligand Exchange - Replacing the Carbonyls . 90 4.4. Cyrhetrene . 90 4.5. Redox Chemistry . 91 4.6. Surface bound organometallic chemistry . 92 4.7. Introduction to triazole chemistry . 93 4.8. Organometallic triazole chemistry . 96 4.9. Synthesis and characterisation of cymantrene triazole derivatives . 97 4.10. Cyclic voltammetric characterisation . 104 4.11. Synthesis and characterisation of cyrhetrene triazole complexes . 118 4.12. Crystallographic analysis of triazole derivatised cyrhetrene . 119 4.13. Electrochemical characterisation of cyrhetrene triazole derivatives . 121 4.14. Surface bound electrochemistry . 124 4.14.1. Triazole derivatised phenyldiazonium reduction method . 124 4.14.2. Surface coverage estimation . 125 4.14.3. Electrode surface 1,3-CuAAC of cymantrene azide . 126 4.15. Conclusions . 129 5. Diazirine derivatised cymantrene and cyrhetrene 134 5.1. Using carbene forming diazirines for organometallic electrode modification . 134 5.2. Preparation and characterisation of cymantrene derivatives . 136 5.2.1. Preparation and characterisation of methylcarboxylate cymantrene . 137 5.2.2. Preparation and characterisation of carboxyl cymantrene . 139 5.2.3. Preparation and characterisation of diazirine derivatised cymantrene . 140 5.3. Cyrhetrene derivatives . 146 5.3.1. Cyrhetrene carboxylic acid . 146 5.3.2. Cyrhetrene diazirine . 146 5.3.3. Cyclic voltammetric characterisation . 148 vi Contents 5.4. Surface Bound Electrochemistry . 149 5.4.1. Modification with cymantrene derivatives - diazirine method . 149 5.4.2. Characterisation with X-ray Photoelectron Spectroscopy . 151 5.4.3. Surface coverage estimation: carbene insertion method . 154 5.4.4. Modification with cyrhetrene diazirine derivative . 155 5.5. Characterisation with X-ray Photoelectron Spectroscopy . 156 5.6. Conclusion . 158 6. Conclusion 161 7. Methods and Materials 163 7.1. General Considerations . 163 7.2. X-ray Crystallography . 163 7.3. Electrochemistry . 164 7.3.1. 2,2,2-Trifluoro-1-(3-methoxyphenyl)ethanone . 165 7.3.2. 2,2,2-Trifluoro-1-(3-methoxyphenyl)ethanone oxime . 165 7.3.3. O-Tosyl-2,2,2-trifluoro-1-(3-methoxyphenyl)ethanone oxime . 166 7.3.4. 3-(Trifluoromethyl)-3-(3-methoxyphenyl)diaziridine . 166 7.3.5. 3-(Trifluoromethyl)-3-(3-methoxyphenyl)diazirine . 167 7.3.6. 3-(Trifluoromethyl)-3-(3-hydroxyphenyl)diazirine . 167 7.3.7. Cymantrene azide CymN3 12 ....................... 167 7.3.8. Cyrhetrene azide, CyrN3 19 ........................ 168 7.4. Synthesis of cymantrene derivatives . 168 7.4.1. h5-methylcarboxylate cyclopentadienyl tricarbonyl manganese(I) 23 .. 168 7.4.2. h5-[carboxyl]cyclopentadienyl tricarbonyl manganese(I) 24 - method I . 169 7.4.3. h5-[carboxyl]cyclopentadienyl