Proton Relaying in Carbenium-Based Oxygen Reduction

The catalytic reduction of oxygen is at the heart of many energy-related and biological processes. Traditionally mediated by transition metals, metal-free systems have emerged as a powerful alternative. In this study, we describe newly prepared quinoline-carbenium species as catalysts for the two-electron reduction of oxygen and show that the quinoline nitrogen atom, depending on its position, can act as a proton relay, thereby boosting the H2O2 production rate. Kinetic studies, H/D kinetic isotope effect measurements, and cyclic voltammetric investigations support a mechanism that involves intramolecular proton transfer from the protonated quinoline moiety to a carbenium hydroperoxide intermediate in the rate-determining step. Additional kinetic investigations indicate that, in contrast to the proton relaying system, simple carbenium ions use acid from the bulk of the solution to heterolytically cleave the C–O bond of the key carbenium hydroperoxide intermediate. Finally, we uncover the effect of the proton relay on the overpotential, affording an ηeff as low as 80 mV.