Direct Evidence for Buffer-Enhanced Proton-Coupled Electron Transfer in Metal Aquo Bond Formation

Proton-coupled electron transfer (PCET) reactions play a crucial role in the interconversion of metal-aqua and metal-hydroxo species present in transition metal complexes and oxide surfaces (M(III)-OH + e− + H+ M(II)-OH2). For ruthenium-based water oxidation catalysts, PCET reactions involved in the mechanism of oxygen evolution have demonstrated a strong dependence on the identity and concentration of the proton donor and acceptor with significant rate enhancements observed for electrocatalysis performed in acetate, phosphate, and borate buffered electrolytes. However, the systematic study of this phenomenon has been hampered by the inability to independently measure discrete rates for electron transfer (ET) and proton transfer (PT) under electrochemical applied potentials. Herein, the PCET kinetics and mechanism of metal aqua bond formation in a ruthenium water oxidation catalyst [Ru(tpy)(bpy′)H2O]2+, Ru(II)−OH2 where tpy is 2,2′:6′,2″-terpyridine and bpy′ is 4,4′-diaminopropylsilatrane-2,2′-bypyridine were investigated at a conductive metal oxide interface as a function of buffer identity and concentration. The reaction of interest was triggered by visible light excitation of the catalyst and the kinetics of the independent ET and PT steps of the PCET mechanism were determined through nanosecond transient absorption spectroscopy. Kinetic measurements performed in aqueous acetate, phosphate, or borate buffer solutions revealed two distinct regimes of PT kinetics solely dependent on the buffer concentration. At the greatest buffer concentrations investigated (2 M acetate) spectral signals corresponding to the discreet ET and PT steps were absent indicative in a change in underlying PCET mechanism. Likewise, kinetic modeling indicated that PT from protonated acetate or phosphate occurred with rate constants that were 2-4 orders of magnitude greater than those for bulk water. In all, these results suggest that the presence of buffer-bases can significantly enhance PCET rates and, in this reaction, may alter the underlying mechanism.