Direct Evidence for a Sequential Electron Transfer–Proton Transfer Mechanism in the PCET Reduction of a Metal Hydroxide Catalyst

The proton-coupled electron transfer (PCET) mechanism for the reaction M_ox–OH + e– + H+ → M_red–OH₂ was determined through the kinetic resolution of the independent electron transfer (ET) and proton transfer (PT) steps. The reaction of interest was triggered by visible light excitation of [RuII(tpy)(bpy′)H₂O]2+, RuII–OH₂, where tpy is 2,2′:6′,2″-terpyridine and bpy′ is 4,4′-diaminopropylsilatrane-2,2′-bipyridine, anchored to In₂O₃:Sn (ITO) thin films in aqueous solutions. Interfacial kinetics for the PCET reduction reaction were quantified by nanosecond transient absorption spectroscopy as a function of solution pH and applied potential. Data acquired at pH = 5–10 revealed a stepwise electron transfer–proton transfer (ET–PT) mechanism, while kinetic measurements made below pK_a(RuIII–OH/OH₂) = 1.3 were used to study the analogous interfacial reaction, where electron transfer was the only mechanistic step. Analysis of this data with a recently reported multichannel kinetic model was used to construct a PCET zone diagram and supported the assignment of an ET–PT mechanism at pH = 5–10. Ultimately, this study represents a unique example among M_ox–OH/M_red–OH₂ reactivity where the protonation and oxidation states of the intermediate were kinetically and spectrally resolved to firmly establish the PCET mechanism.

Kessinger, M. C.; Xu, J.; Cui, K.; Loague, Q.; Soudackov, A. V.; Hammes-Schiffer, S.; Meyer, G. J. Direct Evidence for a Sequential Electron Transfer–Proton Transfer Mechanism in the PCET Reduction of a Metal Hydroxide Catalyst, J. Am. Chem. Soc., 2024, 146 (3) 1742-1747. https://doi.org/10.1021/jacs.3c10742