Thermodynamics of H⁺/H^•/H^-/e^- Transfer from [CpV(CO)₃H]^-: Comparisons to the Isoelectronic CpCr(CO)₃H

Hydrogen atom (H^•) donors generated from H₂ facilitate the atom efficient reduction of small molecule substrates. However, generating H^• donors with X-H bond dissociation free energies (BDFEs) below 52 kcal mol^-1 is especially challenging because they thermodynamically favor the bimolecular evolution of H₂. We have recently proposed that [CpV(CO)₃H]^- catalyzes the conversion of H₂ into a proton, an electron, and a hydrogen atom in the presence of a sacrificial base. In order to understand the driving force for H^• transfer, the free energies of H⁺/H^•/H^-/e^- transfer from [CpV(CO)₃H]^- have been evaluated using solution phase techniques and state-of-the-art quantum chemical calculations. Thermochemical cycles have been constructed in order to anchor the computational values against experimental observations. This facilitates a quantitative comparison of the thermodynamic driving force for H⁺/H^•/H^-/e^- transfer between isoelectronic anionic/neutral hydrides of the same row (the corresponding values are already available for CpCr(CO)₃H). The overall charge greatly influences the thermodynamics of transferring H⁺, H^-, and e^- (i.e., [CpV(CO)₃H]^- is a much weaker acid, a stronger hydride donor, and a stronger reductant than CpCr(CO)₃H); there is almost no change in the thermodynamics of H^• transfer (V-H BDFE 54.7 kcal mol, Cr-H BDFE 57.0 kcal mol^-1). In MeCN, the one electron oxidation of [CpV(CO)₃H]^- (-0.83 V vs Fc/Fc⁺) generates CpV(CO)₃H, which spontaneously evolves H₂. The resulting CpV(CO)₃ is trapped as the solvent adduct CpV(CO)₃(MeCN). Because H^• transfer is now coupled to metal-solvent binding, the V-H bond is substantially weakened for CpV(CO)₃H (V-H BDFE 36.1