Decoding Proton-Coupled Electron Transfer with Potential–pK_a Diagrams

Aqueous potential–pH diagrams, commonly called Pourbaix diagrams, were originally developed to study metal corrosion in the 1930s and 1940s. Pourbaix diagrams have since been widely adopted for use across chemistry disciplines, particularly for the study of aqueous proton-coupled electron transfer reactions. Despite this enormous versatility, a clear extension of analogous diagrams to nonaqueous solvents is lacking. The problem hinges on the difficulty of defining the nonaqueous solution pH. Here, we address this issue by reporting the development of diagrams based on nonaqueous pK_a scales. We experimentally construct diagrams for two transition-metal complexes that undergo proton-coupled electron transfer reactivity by recording their reduction potentials in the presence of acids with varying pK_a values. These experimental diagrams validate the potential–pK_a theory and provide valuable thermochemical information for proton-coupled electron transfer reactions, including for fleetingly stable species.