The Water Oxidation Reaction Catalyzed by Homogeneous Iridium Carbene Complexes
The efficient conversion of solar energy into a chemical fuel represents a technological milestone that would engender society with a clean and renewable source of energy orders of magnitude more powerful than our current consumption. By dividing the process into two complementary half reactions each step of the conversion can be assessed and modified independently. Of the potential oxidants, few are better than the currently employed dioxygen gas but the formation of oxygen from reduced species is plagued by high thermodynamic barriers and requires multiple concerted proton and electron transfers. Homogeneous catalysis is an excellent platform from which to study these reactions and iridium complexes in particular have been shown to be highly active and long-lived in contrast to several other water oxidation catalysts. Building on the wealth of knowledge already contained in the literature, this thesis describes the in situ speciation and provides evidence for the homogeneity of a class of iridium complexes capable of water oxidation over an extended period of time. In contrast to initial evidence that a single site was sufficient, spectroscopic and computational investigations support the formation of a dimeric species at the expense of a sacrificial place-holder ligand, Cp*. The dimeric complex then undergoes several sequential proton coupled electron transfer steps to generate at least one iridium(V) species followed by nucleophilic attack of a solvent water to form the difficult O-O bond. An additional consequence of dividing the overall reaction into two complementary half reactions is the choice of oxidant. Investigations also support the non-innocence of the oxidants employed underpinning the importance of reaction conditions. Despite these complications, these catalysts are robust and highly active.