Mechanistic Divergence in TEMPO-Mediated Electrocatalytic Formate Oxidation

Abstract TEMPO (2,2,6,6-tetramethylpiperidin-1-oxyl) has been widely used as a mediator for electrocatalytic oxidation of alcohols and amines at inert electrode surfaces, operating through an established mechanism in which an exogenous base facilitates the final proton-coupled electron transfer step that regenerates TEMPO from its N–OH intermediate. Here, we report a detailed mechanistic study of TEMPO-mediated electrocatalytic formate oxidation in aprotic, nonaqueous environments, which proceeds with ∼87% Faradaic efficiency toward CO2 in acetonitrile. Cyclic voltammetry further reveals catalytic turnover at formate concentrations as low as 1 mM, with a 14-fold current enhancement at 100 mM formate, indicative of exceptionally fast catalytic rates. Unlike the established alcohol oxidation pathway, formate oxidation proceeds through a mechanistically distinct, base-independent route: formate itself serves as the terminal base, and the reaction is sensitive to the H-bond donating character of the medium. DFT calculations reveal that a key structural signature of this divergence is the close N–Oformate contact in the TEMPO+-formate adduct intermediate, which is disrupted in polar protic solvents that compete for H-bonding. Finite-element simulations of the CV data corroborate the proposed mechanisms and quantify the rate constants for the elementary steps.