The development of technologies for the production of chemical fuels or useful compounds with renewable energy (e.g., sunlight) and inexpensive feedstocks relies on an abundant supply of protons and electrons to form the reduced products. In natural oxygenic photosynthesis, a possible blueprint for artificial photosynthesis, the only suitably abundant source for the needed protons and electrons is water. Oxidation of water liberates protons and electrons and gives off oxygen gas.
    However, the water-oxidation reaction is both thermodynamically and kinetically demanding, resulting in slow kinetics without the use of a catalyst. At an electrode, the slow uncatalyzed kinetics are observed in the form of high overpotentials. In both the chemical and the electrochemical cases, the sluggish kinetics are likely due to the many intermediates required to accomplish the complete 4H+/4e– oxidation of water to dioxygen. An effective catalyst stabilizes these intermediates, resulting in a lower kinetic barrier and, consequently, faster rates of oxygen production.
   Here, we review the field of molecular catalysts for water oxidation. Inspired by the oxygen-evolving complex in Photosystem II, many researchers have endeavored to develop well-defined catalysts for water oxidation that operate in homogeneous solution. Such efforts have important consequences for understanding natural water oxidation and also possible applications in artificial photosynthesis, envisioned as a future source of clean energy for society. Thus, we briefly review the motivations for studying water oxidation, as well as the basic details of water oxidation in Photosystem II. We then turn our attention to artificial, molecular catalysts for water oxidation.
   The catalysts described in this review are organized by elements, focusing first on manganese, ruthenium, and iridium. Each of the elements in this privileged series shows good activity in a variety of compounds and ligand environments for water oxidation, suggesting that the diagonal relationship between these elements in Groups 7, 8, and 9 is key to their activity. We then review cobalt, iron, and other catalysts. Emphasis in this review is placed on synthetic, kinetic, and electrochemical data, as they pertain to the mechanism of action of each family of catalysts. Special attention is also paid to distinguishing homogeneous, molecular catalysis from heterogeneous catalysis of metal oxide solids or particles that can form in situ during studies of molecular precatalysts.

http://pubs.acs.org/doi/full/10.1021/acs.chemrev.5b00122