Electronic and Electrochemical Control of Isostructural Ruthenium Hydricities and the Implications for Catalytic Overpotentials

Electronic tuning of metal hydrides enables precise control over potentials, mechanisms, selectivity, and rates of electrocatalytic reactions by regulating bond dissociation free energies such as the hydricity (.G degrees H-) and pKa of the catalyst. Here, we investigate a series of electronically tuned ruthenium hydrido complexes that are isostructural at the metal center: [Ru(4,4'-R-2-bpy)2(CO)H]+ (R = CF3, Cl, H, CH3, and CH3O; bpy = 2,2'bipyridine) (denoted as (R)Ru-H+). A substantial 22 kcal mol-1 hydricity range is available across five complexes in three stable oxidation states: (R)Ru-H+, (R)Ru-H0, and (R)Ru-H-. Thermodynamic and mechanistic predictions of electrocatalytic proton reduction were tested experimentally by reducing protons from weak acids to H2. Two mechanisms are observed, depending on the acid strength and the catalyst hydricity. The rate constants for hydride transfer and protonation of the catalyst were, in some cases, extracted from the analysis of cyclic voltammetry data. A key finding is a 400 mV decrease in the catalytic overpotential for H-2 production by using a doubly reduced electron-poor metal hydride instead of a singly reduced electron-rich metal hydride. The former also exhibits a higher rate constant for hydride transfer, representing a strategy to disconnect rate and free energy relationships.