Thermodynamic Studies of Hydride- and Proton-Donor Abilities of Heteroleptic Transition Metal Complexes Containing Triphosphine and Monophosphine Ligands
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Transition metal hydrides are reactive intermediates in many catalytic reactions, including the hydrogenation of carbon dioxide. These intermediates can react as proton donors or as hydride donors, characterized by thermodynamic parameters such as pKa and ΔG°H–, for acidities and hydricities, respectively. In this study, the reactivity of a series of transition metal hydride complexes containing a combination of triphosphine (PP2) and monophosphine (PR3) ligands was studied to identify promising catalysts for CO2 hydrogenation. The structures and energies of group 9 and 10 transition metal hydrides were determined by density functional theory (DFT) calculations to estimate the pKa, ΔG°H–, and the free energy of H2 activation (ΔG°H2) of these species. The DFT-calculated structures are largely similar to those of the analogous bis(diphosphine) complexes, with the notable exception of the Pd(II) and Pt(II) hydride cations. The structures of the Pd(II) and Pt(II) hydrides were virtually four-coordinate and square planar, resulting from dissociation of one of the terminal phosphines of the triphosphine ligand from the metal center. Based on the energies of the calculated species, rhodium and cobalt complexes containing trialkyl or alkyldiarylphosphines were identified as promising catalysts for CO2 hydrogenation. The monohydride complexes with these ligand combinations have sufficient hydricity to reduce CO2 to formate (ΔG°H- ≤ 44 kcal/mol), the metal(I) cations have free energies for H2 activation that are thermally accessible at low H2 pressures (ΔG°H2 ≲ 0), and the metal(III) dihydride complexes with these ligands have pKa values between 19 and 27, allowing them to be deprotonated by a range of bases commonly used for CO2 hydrogenation. Based on these promising leads, the rhodium(I) complexes [Rh(PP2)(PPh3)]+, [Rh(PP2)(LPhenH)]+, [Rh(PP2)(LPhen+)]2+, [HRh(PP2)(PPh3)], and [HRh(PP2)(LPhenH)] were synthesized, where LPhenH and LPhen+ are diphenylethylphosphine ligands with an embedded phenanthridinium-based organic hydride donor in its reduced and oxidized forms. Studies of H2 activation and hydride transfer were conducted using these complexes, giving a preliminary experimental estimate for the hydricity of [HRh(PP2)(PPh3)], (ΔG°H– = 39.5 kcal/mol). Preliminary catalytic studies of [Rh(PP2)(LPhenH)]+ showed it was active for CO2 hydrogenation under very mild conditions, with an initial turnover frequency (TOF) as high as 90 h-1 at 1.8 atm H2/CO2 at ambient temperature.