Mechanistic Insight into Water Oxidation Catalysis by a Mononuclear Ruthenium Complex

Abstract

A reaction mechanism of water oxidation involving a mononuclear Ru^IV-oxo complex (1) 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 (3), together with the crystallographic characterization of a side-on Ru^IV-peroxo complex (4) which should be in equilibrium with 3 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 1 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 3 as a product of WNA allowed us to detect it in aqueous solution. The diamagnetic character of 3 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 3 as the final step of water oxidation by the mononuclear Ru catalyst.