Recent advances in mononuclear water oxidation catalysts based on the first row transition metal elements

Abstract

Water oxidation is a key process in photosynthesis. It provides electrons and protons for the fixation of carbon dioxide and also releases dioxygen that is essential for the biosphere. In photosystem Ⅱ, the oxygen evolution center, a complex composed of four manganese atoms and one calcium atom, catalyzes the oxidation of water. Design and synthesis of efficient and stable water oxidation catalysts (WOCs) has attracted major attention in the field of artificial photosynthesis. Previous studies focused on nano-structured metal oxides and/or polynuclear metal complexes based on ruthenium and iridium. Mononuclear WOCs containing Ru or Ir have also been extensively developed since 2008, because (1) they can catalyzes the oxidation of water as efficiently as polynuclear systems; (2) their synthesis, characterization and modification are more straightforward; (3) mononuclear WOCs provide a better system for mechanistic studies by both experimental and theoretical methods. Very recently, mononuclear metal complexes with the first row transition metal elements have been shown to catalyze water oxidation efficiently. In addition to the fact that the first row transition metals are earth-abundant and cheap, they also have variable oxidation states for the activation of water. Therefore, mononuclear WOCs based on the first row transition metal elements have the potential for future application in artificial photosynthesis. This article reviewed recent advances in mononuclear WOCs based mainly on the first row transition metal elements. By briefly discussing the formation of oxygen-oxygen bonds, it sheds light on catalytic water oxidation and should benefit the development of novel WOCs that are highly efficient and stable.