Synthesis, structure, and water oxidation activity of ruthenium(II) complexes: influence of intramolecular redox process on O2 evolution

Abstract

The synthesis and characterization of two new RuII complexes, a mononuclear complex [RuII(MeMPTP)(bpp)Cl]PF6 (1) and a dinuclear complex [RuII(MeMPTP)(bpp)RuII(bpy)2Cl](PF6)3 (2), [where MeMPTP = 4′-(4-methylmercaptophenyl)2,2′:6′2′′-terpyridine, bpp = 2,3-bis(2-pyridyl)pyrazine, bpy = 2,2′-bipyridine] is reported. Single-crystal X-ray structural determination of the compounds unambiguously confirms the formation of the complexes 1 and 2. Catalytic investigations of the complexes for the chemical oxidation of water in the presence of Ce4+ as a sacrificial oxidant reveals a higher rate of O2 evolution by the dinuclear complex 2 over the physical mixture of complex 1 and a redox mediator, [Ru(bpy)3]2+ (med), followed by the mononuclear complex 1. The enhancement in the catalytic rate of O2 evolution by 2 is ascribed to being due to the intramolecular redox process, which facilitates the facile formation of a high-valent ruthenium–oxo species that is required for water oxidation through a cooperative effect. The present study highlights the importance of the intramolecular redox process over the intermolecular redox process.