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|RuII-OH2, was characterized by spectroscopic and electrochemical methods in acetate or phosphate buffers. The deprotonated intermediate, RuII-OH, was observed spectroscopically in the PCET half-reaction ITO(e-)|RuIII-OH + H+ → ITO|RuII-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 a function of the driving force provided the reorganization energy λ = 1.05 eV and was assigned to a concerted electron-proton transfer (EPT) mechanism. In addition, the standard heterogeneous rate constants for two PCET equilibria, RuIII-OH + H+ + e- ⇌ RuII-OH2 and RuIV = O + H+ + e- ⇌ RuIII-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.