Mechanistic Approaches to Molecular Catalysts for Water Oxidation

Water oxidation, in which two water molecules undergo the coupled loss of four electrons and four protons to form an O-O bond, is one of the most appealing target reactions for molecular catalysts in view of hydrogen production by electrolytic or photolytic water splitting to cope with urgent energy and environmental problems. Inspired by a natural oxygen-evolving multinuclear manganese cluster, a number of water oxidation catalysts based on multinuclear transition-metal complexes have been developed over the last three decades. In recent years, in parallel with the discovery of mononuclear oxygen-evolving complexes, both experimental and theoretical studies have yielded important insight into O-O bond-formation pathways in these water-oxidizing complexes. In this microreview, we will present an updated view of selected current literature focusing on the working mechanism of ruthenium-based water oxidation catalysts and on the development of rationally designed ruthenium complexes that activate water at mild potentials.