Molecular Catalysts and Organometallics for Water Oxidation

Water can be used as a cheap and renewable source of electrons and protons to make nonfossil fuel-based chemical energy carriers for a sustainable power supply. However, water oxidation is an intricate chemical process and an energy-intensive reaction involving the removal of four electrons with the release of four protons at the same time. Inside the thylakoid membrane in plant leaves is embedded a manganese-calcium molecular cluster in natural photosystem II (PS-II), which represents an excellent model for designing an artificial equivalent of the photosynthesis for light-to-fuel conversion via water splitting. Inspired by the natural PS-II, the scientific community has been striving hard during the last two decades to develop a bio-inspired catalytic system for water oxidation. However, a truly biomimetic catalytic system matching the performance of photosystem for efficient water splitting operating with four consecutive proton-coupled electron transfer (PCET) steps to generate oxygen and hydrogen for hundred thousands of cycles at high rate is yet to be demonstrated. In this chapter, we provide an insight regarding the biomimetic approaches to make molecular and organometallic water oxidation complexes that have been investigated recently in homogeneous solution catalysis using chemical oxidants or as surface-immobilized heterogeneous species for electro-assisted catalytic systems. After comparing their catalytic activities and stabilities, an overview of the mechanistic aspects is also discussed.