Abstract:
Oxidation and reduction reactions are crucial to the synthesis of organic chemicals, and they also provide the basis for energy production. Electrochemistry is the archetypal method for the removal and delivery of electrons in oxidation and reduction reactions, but electrochemical processes face numerous challenges. Most of the important redox processes involving organic molecules and energy-related small molecules (e.g., H2, O2, CO2, N2) feature the addition or removal of an even number of electrons and protons: 2e–/2H+, 4e–/4H+, 6e–/6H+. Such reactions are not well suited for a direct electrochemical processes, and catalysts are required to enable these reactions proceed with high efficiency and controlled selectivity. This talk will present our recent efforts to develop electrochemical transformations and electrocatalytic methods inspired by biological energy transduction and enzymatic redox processes. Specifically, we take advantage of electron-proton transfer mediators (EPTMs) that couple the movement of both electrons and protons. These mediators avoid unfavorable charge separation associated with independent electron and proton transfer steps, and they introduce new mechanistic pathways to achieve electrode-driven redox reactions. Quinones and organic nitroxyls are especially promising EPTMs, as they mediate hydrogen-atom or other proton-coupled electron transfer reactions with molecules or catalysts in solution, and then are capable of efficient regeneration via proton-coupled electron-transfer at an electrode. These mediator concepts and their use in electrocatalytic reactions will be illustrated through a series of case studies related to chemical synthesis (alcohol oxidation, C–H functionalization) and energy conversion (the oxygen reduction reaction).