Mononuclear Ruthenium-Based Water Oxidation Catalyst Supported by Anionic, Redox-Non-Innocent Ligand: Heterometallic O-O Bond Formation via Radical Coupling Pathway

Cerium(IV)-driven water oxidation catalysis mediated by a mononuclear ruthenium(III) complex, [Ru(L)(pic)(3)] (H3L = 2,2'-iminodibenzoic acid, pic = 4-methylpyridine), has been demonstrated in this work. The mechanistic details of water oxidation have been investigated by the combined use of spectroscopy, electrochemistry, kinetic analysis, and computational studies. It was found that water oxidation proceeds via formal high-valent Ru-VII species. The capability of accessing such a high-valent state is derived from the non-innocent behavior of the anionic tridentate ligand frame which helps in accumulation of oxidative equivalents in cooperation with metal center. This metal-ligand cooperation facilitates the multi-electron-transfer reaction such as water oxidation. Kinetic analysis suggests water oxidation at a single site of Ru where O-O bond formation occurs via radical-radical coupling pathway between the oxygen atom of ruthenium-oxo species and the oxygen atom of the hydroxocerium(IV) ion.