Molecularity Control in Electrochemical Multi-Site Proton-Coupled Electron Transfer Reactions.

Multi-site proton-coupled electron transfer (MS-PCET) is a powerful strategy for activating strong X─H bonds and forging M─H bonds at mild driving force. Translating this logic to electrosynthesis is attractive from a sustainability perspective because better coupling between proton and electron flow can lower overpotentials, suppress wasteful side reactions, and improve selectivity. Yet, electrochemical MS-PCET remains underexploited, particularly beyond O─H and N─H activation. One of the persistent barriers is molecularity: productive oxidative PCET often requires the simultaneous engagement of three components, a substrate (X-H), an oxidant (Y), and a base (B). Termolecular encounters are entropically disfavored, so practical systems often require molecularity to be engineered down through preorganization. Here, we take a molecularity-based view of electrochemical PCET and classify oxidative PCET into three types based on which pair is preassembled: Type I (X─H⋯B), Type II (X─H⋯Y), and Type III (Y⋯B). For each type, reducing molecularity opens a concerted channel, with feasibility determined by the BDFE gap between the donor/acceptor pair and the target X─H bond. We then discuss representative advances in electrochemical N─H/O─H activation, M─H generation, and emerging C-H PCET, and conclude with prospects for C─H bond, electrosynthesis, molecularity, PCET, metal hydride interfacial PCET, and asymmetric catalyst design enabled by molecularity engineering.