Mechanistic Insight into Water Oxidation Catalysis by a Mononuclear Ruthenium Complex

A reaction mechanism of water oxidation involving a mononuclear Ru^IV-oxo complex (<b>1</b>) as an intermediate with use of (NH₄)₂[Ce^IV(NO₃)₆] (CAN) as an oxidant has been scrutinized to provide a clear view of O-O bond formation and O₂ release. This work includes the spectroscopic and theoretical characterization of an end-on Ru^III-superoxo complex (<b>3</b>), together with the crystallographic characterization of a side-on Ru^IV-peroxo complex (<b>4</b>) which should be in equilibrium with <b>3</b> in an aqueous solution. The formation of the Ru^V-oxo intermediate as a responsible species for the water oxidation was supported by a square wave voltammogram of <b>1</b> in an aqueous solution, showing an oxidation wave at +1.52 V (vs NHE) which is accessible with use of excess CAN through an electron-transfer equilibrium. Kinetic analysis and isotope labeling experiments supported a water nucleophilic attack (WNA) mechanism in the water oxidation. The stability of <b>3</b> as a product of WNA allowed us to detect it in aqueous solution. The diamagnetic character of <b>3</b> enabled the detailed kinetic investigation of O₂-releasing from the intermediate to determine activation parameters. Herein, a new insight was gained into the O₂ release from <b>3</b> as the final step of water oxidation by the mononuclear Ru catalyst.