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 kcal mol–1), amounting to a strategy for obtaining very reactive H atoms from H2.