Sequential proton coupled electron transfer events from a tetraruthenium polyoxometalate in photochemical water oxidation

Abstract

The tetraruthenium polyoxometalate {Ru^IV(H₂O)₄(μ-OH)₂(μ-O)₄[SiW₁₀O₃₆]₂}¹⁰⁻ (Ru₄POM) shows multiple oxidative proton coupled electron transfer (PCET) events in a [Ru(bpy)₃]²⁺/S₂O₈²⁻ photochemical cycle for catalytic water oxidation, with electrons conveyed to the photogenerated [Ru(bpy)₃]³⁺ oxidant and protons transferred to aqueous bases. As shown by laser flash photolysis, in aqueous phosphate buffer the consumption of the [Ru(bpy)₃]³⁺ oxidant by Ru₄POM shows bi-exponential kinetics with a fast component and a slow component that feed the Ru₄POM catalyst with up to 6 oxidative equivalents through PCET in ca. 50 ms. The apparent rates of both the fast and slow components depend linearly on HPO₄²⁻ and on the pH of the aqueous medium, suggesting the involvement of the buffer base, of water and of OH⁻ in assisting removal of the protons from Ru₄POM. In particular, the beneficial role of HPO₄²⁻ is reflected in a proportional improvement in the oxygen evolution activity, reaching quantum efficiency approaching 14%, although an excessive increase of buffer concentration is detrimental to the [Ru(bpy)₃]³⁺ stability and leads to the abatement of the O₂ evolution.