Visible Light-Driven Water Oxidation by a Molecular Ruthenium Catalyst in Homogeneous System

Abstract

Discovery of an efficient catalyst bearing low overpotential toward water oxidation is a key step for light-driven water splitting into dioxygen and dihydrogen. A mononuclear ruthenium complex, Ru(II)L(pic)(2) (1) (H(2)L = 2,2'-bipyridine-6,6'-dicarboxylic acid; pic = 4-picoline), was found capable of oxidizing water eletrochemically at a relatively low potential and promoting light-driven water oxidation using a three-component system composed of a photosensitizer, sacrificial electron acceptor, and complex 1. The detailed electrochemical properties of 1 were studied, and the onset potentials of the electrochemically catalytic curves in pH 7.0 and pH 1.0 solutions are 1.0 and 1.5 V, respectively. The low catalytic potential of 1 under neutral conditions allows the use of [Ru(bpy)(3)](2+) and even [Ru(dmbpy)(3)](2+) as a photosensitizer for photochemical water oxidation. Two different sacrificial electron acceptors, [Co(NH(3))(5)Cl]Cl(2) and Na(2)S(2)O(8), were used to generate the oxidized state of ruthenium tris(2,2'-bipyridyl) photosensitizers. In addition, a two-hour photolysis of 1 in a pH 7.0 phosphate buffer did not lead to obvious degradation, indicating the good photostability of our catalyst. However, under conditions of light-driven water oxidation, the catalyst deactivates quickly. In both solution and the solid state under aerobic conditions, complex 1 gradually decomposed via oxidative degradation of its ligands, and two of the decomposed products, sp(3) C-H bond oxidized Ru complexes, were identified. The capability of oxidizing the sp(3) C-H bond implies the presence of a highly oxidizing Ru species, which might also cause the final degradation of the catalyst.