Strategies for switching the mechanism of proton-coupled electron transfer reactions illustrated by mechanistic zone diagrams.

The mechanism by which proton-coupled electron transfer (PCET) occurs is of fundamental importance and has great consequences for applications, e.g. in catalysis. However, determination and tuning of the PCET mechanism is often non-trivial. Here, we apply mechanistic zone diagrams to illustrate the competition between concerted and stepwise PCET-mechanisms in the oxidation of 4-methoxyphenol by Ru(bpy) 3 3+ -derivatives in the presence of substituted pyridine bases. These diagrams show the dominating mechanism as a function of driving force for electron and proton transfer (Δ G 0 ET and Δ G 0 PT ) respectively [Tyburski et al. , J. Am. Chem. Soc. , 2021, 143 , 560]. Within this framework, we demonstrate strategies for mechanistic tuning, namely balancing of Δ G 0 ET and Δ G 0 PT , steric hindrance of the proton-transfer coordinate, and isotope substitution. Sterically hindered pyridine bases gave larger reorganization energy for concerted PCET, resulting in a shift towards a step-wise electron first-mechanism in the zone diagrams. For cases when sufficiently strong oxidants are used, substitution of protons for deuterons leads to a switch from concerted electron-proton transfer (CEPT) to an electron transfer limited (ETPT lim ) mechanism. We thereby, for the first time, provide direct experimental evidence, that the vibronic coupling strength affects the switching point between CEPT and ETPT lim , i.e. at what driving force one or the other mechanism starts dominating. Implications for solar fuel catalysis are discussed.