Products Cited in Publications Worldwide Direct Evidence for Buffer-Enhanced Proton-Coupled Electron Transfer Generation of a High-Valent Metal-Oxo Complex

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

Kessinger, Matthew; Whittemore, Thomas; Grandi, Silvia; Danilov, Evgeny O; Caramori, Stefano; Castellano, Felix N; Meyer, Gerald

DOI: PMID:

Abstract

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.

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