Redox-Mediated Reversible C-C Bond Cleavage in Aqueous Electrochemical Systems.

Redox-active organic molecules provide a versatile platform for applications such as organocatalysis, electrocatalysis, and energy storage; however, parasitic reactions can yield competing bond-forming pathways that trap the molecule in electrochemically inaccessible states, passivating the redox electrochemical reversibility. Reductive radical coupling exemplifies this challenge. Although carbonyl reduction to an alcohol is a standard chemically reversible process, electrochemical reduction can induce radical coupling that leads to C-C bond formation, effectively diverting the reaction from the canonical two-electron, two-proton pathway and from electrochemical reversibility. Here, we demonstrate an electron-abstraction mechanism in which the mediator activates chemically reversible, yet electrochemically irreversible C-C bond scission with an estimated apparent rate constant of 3.54 × 10-6 M-1 s-1 under the tested conditions, which supports the restoration of "net" electrochemical reversibility and bypasses the large kinetic activation barrier in electrochemical systems. A flow battery configuration is used to illustrate how this mediating strategy enhances energy efficiency in electrochemical systems by bypassing high-kinetic barrier reaction pathways. Spectroscopic analyses directly capture both C-C bond formation and dissociation, while density functional theory calculations suggest an electron-abstraction mechanism for C-C bond cleavage. The demonstrated mediated, reversible C-C bond cleavage highlights a mechanistic proof-of-concept for overcoming high kinetic activation barriers in otherwise sluggish chemical transformations under electrochemical conditions.