Proton-Coupled Electron Transfer: O–H bonds, C-H bonds, and Molecular Electrocatalysis
A wide range of chemical processes proceed by proton-coupled electron transfer (PCET), from hydrogen atom transfer in combustion to electrochemical catalysis in fuel cells to redox processes at the surface of minerals. 1e–/1H+ transfers can occur in stepwise mechanisms, electron-first or proton-first, or they can occur without an intermediate, by concerted proton-electron transfer (CPET). CPET can occur with the 1e– and 1H+ transferring ‘together,’ perhaps as a hydrogen atom, or being separated in the reactants or products. As an example of the latter, the reaction cleavages a C–H bond with proton transfer to a base concerted with electron transfer to a separate oxidant. The similarities and differences between O–H and C–H bonds in these reactions will be discussed using case studies. The possibility that a concerted reaction can be asynchronous or imbalanced in the transfer of the two particles will be discussed, starting from rate/driving force relationships. These linear free energy relationships will be extended to multistep electrocatalytic reactions, with the specific example of the oxygen reduction reaction by soluble iron porphyrin catalysts (bottom figure). The power and limitations of the derived ‘molecular scaling relationships’ will be discussed.
Hosted by Professor Mark Lipke