Homogeneous Photocatalytic Water Oxidation with a Dinuclear Co^III-Pyridylmethylamine Complex

A bis-hydroxo-bridged dinuclear Co^III-pyridylmethylamine complex (1) was synthesized and the crystal structure was determined by X-ray crystallography. Complex 1 acts as a homogeneous catalyst for visible-light-driven water oxidation by persulfate (S₂O₈^2-) as an oxidant with [Ru^II(bpy)₃]²⁺ (bpy = 2,2′-bipyridine) as a photosensitizer affording a high quantum yield (44%) with a large turnover number (TON = 742) for O₂ formation without forming catalytically active Co-oxide (CoO_x) nanoparticles. In the water-oxidation process, complex 1 undergoes proton-coupled electron-transfer (PCET) oxidation as a rate-determining step to form a putative dinuclear bis-μ-oxyl Co^III complex (2), which has been suggested by DFT calculations. Catalytic water oxidation by 1 using [Ru^III(bpy)₃]³⁺ as an oxidant in a H₂¹⁶O and H₂¹⁸O mixture was examined to reveal an intramolecular O-O bond formation in the two-electron-oxidized bis-μ-oxyl intermediate, prior to the O₂ evolution.