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

Hydrogen atom (H • ) donors generated from H 2 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 H 2 . We have recently proposed that [CpV(CO) 3 H] − catalyzes the conversion of H 2 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) 3 H] − 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) 3 H). The overall charge greatly influences the thermodynamics of transferring H +, H –, and e – (i.e., [CpV(CO) 3 H] − is a much weaker acid, a stronger hydride donor, and a stronger reductant than CpCr(CO) 3 H); 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) 3 H] − (−0.83 V vs Fc/Fc + ) generates CpV(CO) 3 H, which spontaneously evolves H 2 . 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) 3 H (V–H BDFE 36.1 kcal mol –1 ), amounting to a strategy for obtaining very reactive H atoms from H 2 .