Unimolecular and Bimolecular Pathways in Bidirectional Redox Molecular Catalysis

The expression "catalytic reversibility" describes the situation where a bidirectional molecular redox catalyst can function in either direction of the reaction under near equilibrium conditions, as observed with various redox enzymes and with an increasing number of synthetic inorganic complexes. Understanding how this property emerges is crucial to the design of efficient catalysts. Here, for a two-electron reaction, we compare the voltammetric responses of the catalysts for two distinct general mechanisms: the unimolecular pathway includes two successive electron transfers with the electrode and pseudo-first-order chemical steps and was described before (Fourmond et al., J. Am. Chem. Soc. 141, 11269 (2019)); the bimolecular mechanism includes a single interfacial electron transfer and one intermolecular electron transfer in solution. The general theory of the latter is derived here. The predictions of the kinetic models are compared to data available in the literature, and diagnosis criteria are discussed with emphasis on the shapes of the sigmoidal steady-state catalytic voltammograms. We describe the kinetic and thermodynamic requirements for redox molecular catalysts to work reversibly.