Oxygen evolution from water catalyzed by mononuclear aquaruthenium complexes

[Ru(terpy)(bpy)(OH₂)]²⁺ and its analogues were found to be highly active as catalysts toward water oxidation in the presence of Ce⁴⁺ as an oxidizing reagent in acidic aqueous media. These findings were considered as a significant breakthrough in this field because there had been a long belief that the four-electron process (i.e., 2H₂O → O₂+ 4H⁺ + 4e^-) is much more effectively accelerated by dinuclear or tetranuclear metal complexes. The kinetics of O ₂ evolution is investigated as a function of either the catalyst concentration or the oxidant concentration, revealing that these catalysts can be classified into two groups exhibiting different rate laws for O₂ evolution. Moreover, the singlet biradical character of the hydroxocerium(IV) ion is realized, indicating that the radical coupling of the oxygen atoms of a Ru^v= O species and a hydroxocerium (IV) ion is the key step for the catalysis. Several important insights into the mechanism of oxygen evolution from water catalyzed by the mononuclear aquaruthenium complexes will also be discussed.