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.