A Strongly Coupled Biruthenium Complex as Catalyst for the Water Oxidation Reaction

Abstract

The catalytic activity towards water oxidation of a strongly coupled bimetallic ruthenium complex, trans-[Ru(tpy)(bpy)(μ-CN)Ru(py)4(OH2)]3+ (RuIIRuIIOH2), where tpy=2,2′:6′,2’’-terpyridine, bpy=2,2′-bipyridine and py=pyridine are presented. At pH 1 the first two oxidation reactions are centred at the aquo fragment and result in the RuIIRuIIIOH2 and RuIIRuIVO redox states as confirmed by its spectroscopy and DFT calculations. Oxidation by an additional electron is followed by an irreversible step and a catalytic wave associated with the water oxidation reaction. At pH 1 the reaction with an excess of Ce(IV) results in the generation of an stoichiometric amount of oxygen based on molar amounts of the added Ce(IV). The dominant species during the catalytic cycle is the three-electron oxidized product, RuIIIRuIVO. The reduction of the concentration of Ce(IV) monitored at 370 nm follow the rate equation, −d[Ce(IV)]/dt=kox[Ce(IV)][RuIIRuIIOH2] with a kox=82±3 M−1s−1 at T=298 K. The RuIIIRuIVO species is not stable and reacts to give RuIIRuIVO.