A ruthenium catalyst linked to a redox-active ruthenium polypyridine for water oxidation

Abstract

Chromophore–catalyst assemblies are interesting benchmark molecules for photocatalysis. We have prepared two examples of these assemblies and characterized their behaviour as catalysts for the water oxidation reaction. In the bimetallic complexes [Ru(tpy)(4,4′-X₂-bpy)(μ-CN)Ru(bda)(DMSO)](PF₆) (X = –H (1), –OCH₃ (2), tpy = 2,2′:6′,2′′-terpyridine, bpy = 2,2′-bipyridine, H₂bda = 2,2′-bipyridine-6,6′-dicarboxylic acid and DMSO = (CH₃)₂SO), a Ru(II)-polypiridine chromophore {Ru(tpy)(4,4′-X₂-bpy)} is linked by a cyanide bridge to a {Ru(bda)} water oxidation catalyst. Both complexes catalyze water oxidation both chemically, using Ce(IV) as the terminal oxidant, and electrochemically. Electrochemical and spectroelectrochemical studies, aided with (TD)DFT calculations, allowed us to study the different species involved in the catalytic cycle. For the Ru^IIRu^III, an intense MMCT (metal-to-metal charge transfer) band suggests a geometrical reorganization during the oxidation process. The wave associated with oxidation to the Ru^IIIRu^IV species shows an increased current that suggests that it may be a catalyst for the water oxidation reaction. Further oxidation results in the Ru^IIIRu^V redox state which is an active catalyst for the water oxidation reaction as demonstrated by the evolution of oxygen.