Credit: © 2010 ACS

One thrust of research aimed at cleanly reducing the increasing gap between our energy needs and energy stores is to mimic the chemical processes of photosynthesis. The aim for artificial photosynthesis is to use sunlight and water to produce hydrogen and oxygen, and to do so, catalysts that efficiently 'split' water are needed. An initial step in this process is the oxidation of water and this is carried out in nature by a manganese–oxo cluster within photosystem II. This occurs in a hydrophobic environment with water as a limiting reagent, but previous experiments to mimic this process have been carried in aqueous solution, with water obviously in abundance.

Now, Thomas Meyer and colleagues at the University of North Carolina at Chapel Hill have studied1 the water oxidation reaction catalysed by a ruthenium complex in a non-aqueous environment — with water as a limiting reagent. The catalyst used was a ruthenium complex coordinated by both bipyridine and a tridentate ligand made up of two benzimidazol ligands covalently linked through pyridine.

From cyclic voltammograms of the catalyst in propylene carbonate — at varying concentrations of water — the team calculated a rate enhancement for water oxidation of a factor of about 300 relative to the same catalyst in aqueous solution at pH 1. The results from Meyer and colleagues also provide evidence for a different reaction pathway compared with that in aqueous solution, supporting a direct oxygen transfer to water to give a hydrogen peroxide intermediate, rather than oxygen attack alongside simultaneous proton transfer.