Mononuclear first-row transition-metal complexes as molecular catalysts for water oxidation

Abstract

Water oxidation is significant in both natural and artificial photosynthesis. In nature, water oxidation occurs at the oxygen-evolving center of photosystem II, and leads to the generation of oxygen, protons, and electrons. The last two are used for fixation of carbon dioxide to give carbohydrates. In artificial processes, the coupling of water oxidation to evolve O2 and water reduction to evolve H2 is known as water splitting, which is an attractive method for solar energy conversion and storage. Because water oxidation is a thermodynamically uphill reaction and is kinetically slow, this reaction causes a bottleneck in large-scale water splitting. As a consequence, the development of new and efficient water oxidation catalysts (WOCs) has attracted extensive attention. Recent efforts have identified a variety of mononuclear earth-abundant transition-metal complexes as active and stable molecular WOCs. This review article summarizes recent progress in research on mononuclear catalysts that are based on first-row transition-metal elements, namely manganese, iron, cobalt, nickel, and copper. Particular attention is paid to catalytic mechanisms and the key O–O bond formation steps. This information is critical for designing new catalysts that are highly efficient and stable.