N-Heterocyclic Carbene Complexes: Mechanisms in Catalysis and Electrochemical Reactivity

This thesis is concerned with the chemistry of N-heterocyclic carbene complexes. There are two parts to this work, the first describes the synthesis and catalytic activity of Ir-NHC complexes, the second part details the electrochemical synthesis of Cu-NHC complexes derived from triazolium salts.
Iridium complexes bearing NHC-carborane ligands are active in the catalytic transfer hydrogenation of acetophenone. These complexes were synthesised and characterised. The known catalytic activity was probed by studying the chemical reactivity of these complexes under a range of conditions. This led to the characterisation of a hydride species by various analytical techniques. The proposed mechanism of catalysis has been investigated using computational methods (DFT). No evidence to support the previously proposed mechanism was found, but there was some evidence for the operation of an unprecedented alternative. The catalytic reaction was monitored using on-line NMR spectroscopy. This confirmed the presence of hydridic species under catalytic conditions, however their exact identity could not be determined. Combined mass spectroscopic data indicates the presence of both monomeric and dimeric species.
Transition metal NHC complexes can be synthesised electrochemically, by reduction of the NHC precursor and concomitant oxidation of a sacrificial metal anode. Triazolium salts, which are known precursors to NHCs, have not previously been applied to this method. Triazolium salts were prepared and fully characterised. Several copper-NHC complexes bearing triazolium derived NHCs have been prepared both chemically and electrochemically, and characterised by a range of analytical techniques. An unexpected ring-opening reaction was observed under electrochemical conditions in one case, and various factors affecting this reaction have been investigated.