Mechanistic Insight into Water Oxidation Catalysis by a Mononuclear Ruthenium Complex

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A reaction mechanism of water oxidation involving a mononuclear Ru IV -oxo complex ( 1 ) as an intermediate with use of (NH 4 ) 2 [Ce IV (NO 3 ) 6 ] (CAN) as an oxidant has been scrutinized to provide a clear view of O–O bond formation and O 2 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 O 2 -releasing from the intermediate to determine activation parameters. Herein, a new insight was gained into the O 2 release from 3 as the final step of water oxidation by the mononuclear Ru catalyst.