Strategies for Switching the Mechanism of Proton-Coupled Electron Transfer Reactions Illustrated by Mechanistic Zone Diagrams

The mechanism under which proton-coupled electron transfer (PCET) occurs has great consequences for applications 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 PCET-mechanisms in the oxidation of 4-Methoxyphenol by Ru(bpy)33+-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 (ΔG0ET and ΔG0PT) respectively [Tyburski et al., J. Am. Chem. Soc., 2021, 143, 560]. Within this framework, we demonstrate strategies for mechanistic tuning, namely balancing of ΔG0ET and ΔG0PT, steric hindrance of the proton-transfer coordinate, and isotope substitution. For cases when strong oxidants are used, substitution of protons for deuterons leads to a switch from concerted electron-proton transfer (CEPT) to an electron transfer limited (ETPTlim) mechanism. We thereby, for the first time, provide direct experimental evidence, that vibronic coupling affects the switching point between CEPT and ETPTlim, i.e. at what driving force one or the other mechanism starts dominating. Implications for solar fuels catalysis are discussed