Metal hydrides are the mechanistic branchpoint for H₂ evolution vs CO₂ reduction to formate, and the catalytic activity may be predicted by the thermodynamic parameters of hydricity (ΔG^o_H_-), the pK_aof the catalyst, and the pK_aof the proton source. We hypothesized that electronic tuning of a series of isostructural metal hydrides with reversible redox properties would enable precise potential control of catalytic H₂ evolution rates and overpotentials without changing the nature of the transition states. Confirmation of this hypothesis may facilitate the prediction of selectivity for H⁺ vs CO₂ reduction based on rates and thermodynamic parameters. In this context, we investigated a series of electronically-tuned ruthenium hydrido complexes: [Ru(4,4-R₂-bpy)₂(CO)H]⁺ (R = CF₃, Cl, H, CH₃, CH₃O; bpy = 2,2-bipyridine), denoted ^RRu(H)⁺. A substantial 18 kcal mol^-¹ hydricity range is available across five complexes in three oxidation states, ^RRu(H)⁺, ^RRu(H)⁰, and ^RRu(H)^-for catalyzing a wide pK_a range of proton sources to H₂. Electrocatalytic reduction of protons from acetic acid buffer, pure acetic acid and nonafluoro-tert-butanol was investigated at a mercury drop working electrode. The observed rate, k_obs and the catalytic rate constant, k_cat. were computed using the foot-of-the-wave analysis (FOWA). Two unique kinetic regimes are observed with linear free-energy dependence on the reduction potential: one for singly reduced catalysts and a second for doubly reduced catalysts. Importantly, the versatile reduction states across the series enable a significant decrease in the overpotential by operating an electron-poor catalyst, e.g. ^CF3Ru(H)⁺, or ^ClRu(H)⁺, at its doubly reduced potential vs an electron-rich catalyst at its singly reduced potential.

Gerald Manbeck

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