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

Jonathan L. Kuo, Thilina Gunasekara, Andreas Hansen, Hunter B. Vibbert, Fabian Bohle, Jack R. Norton, Stefan Grimme, Patrick J. Quinlivan

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Abstract

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

Original languageEnglish (US)
Pages (from-to)4319-4328
Number of pages10
JournalOrganometallics
Volume38
Issue number21
DOIs
StatePublished - Nov 11 2019

All Science Journal Classification (ASJC) codes

  • Physical and Theoretical Chemistry
  • Organic Chemistry
  • Inorganic Chemistry

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