Direct Evidence for Buffer-Enhanced Proton-Coupled Electron Transfer Generation of a High-Valent Metal-Oxo Complex

The oxidation of metal-aquo and -hydroxo complexes to generate the high-valent metal-oxo species used in oxidative catalysis is often kinetically slow due to sluggish proton transfer between ligated -H₂O/-OH in the proton-coupled electron transfer (PCET) chemistry. In this research, a ruthenium water oxidation catalyst anchored to a conductive tin-doped indium oxide (ITO) thin film, abbreviated ITO|Ru^II-OH₂, was characterized by spectroscopic and electrochemical methods in acetate or phosphate buffers. The deprotonated intermediate, Ru^II-OH, was observed spectroscopically in the PCET half-reaction ITO(e^-)|Ru^III-OH + H⁺ → ITO|Ru^II-OH₂ indicating an underlying stepwise ET-PT mechanism. In contrast, at elevated buffer concentrations, this intermediate was absent, and a 2-4 order of magnitude increase in the proton transfer rate constant was observed. Kinetic data for this PCET reaction measured as <i>a</i> function of the driving force provided the reorganization energy λ = 1.05 eV and was assigned to <i>a</i> concerted electron-proton transfer (EPT) mechanism. In addition, the standard heterogeneous rate constants for two PCET equilibria, Ru^III-OH + H⁺ + e^- ⇌ Ru^II-OH₂ and Ru^IV = O + H⁺ + e^- ⇌ Ru^III-OH were enhanced by these same buffers. Collectively, the data show that the added buffers can enhance the kinetics and thermodynamics for PCET reactions relevant to oxidative catalysis.