The Catalytic Mechanism of a Highly Active Cobalt\nChlorin Complex for Photocatalytic Water Oxidation

Highly active catalysts for electrocatalytic\nand photocatalytic\nwater oxidation are strongly demanded to realize artificial photosynthesis.\nA cobalt complex with a chlorin derivative ligand (Co<sup>II</sup>(Ch)) exhibited high activity for electrocatalytic water oxidation\nwith an overpotential of 0.45 V at pH 9.0. Spectroelectrochemistry\n(UV–vis) unveiled the formation of two intermediates by successive\none-electron oxidations. Also, the Pourbaix diagram depicted by the\npH dependence of redox potentials indicated that the water oxidation\nproceeded after the oxidation of both the central cobalt ion and chlorin\nligand with proton-coupled electron transfer (PCET). Then, the photocatalytic\nactivity of Co<sup>II</sup>(Ch) was examined for water oxidation using\n[Ru<sup>II</sup>(bpy)<sub>3</sub>]<sup>2+</sup> (bpy: 2,2′-bipyridine)\nand S<sub>2</sub>O<sub>8</sub><sup>2–</sup> as a photosensitizer\nand a sacrificial electron acceptor, respectively. The turnover number,\nturnover frequency, and oxygen yield reached as high as 980, 5.2 s<sup>–1</sup>, and 98%, respectively, under optimized conditions.\nThe O<sub>2</sub>-evolution rates increased in proportion to the square\nof the catalyst concentration in the reaction solution, suggesting\nthat the formation of the O–O bond regarded as the rate-determining\nstep of water oxidation proceeded by the interaction of two metal\ncenters (I2M) mechanism in which two molecules of high-valent metal\noxo or oxyl radical species react with each other.