Structure and Thermodynamic Hydricity in Cobalt(triphosphine)(monophosphine) Hydrides

The mononuclear cobalt hydride complex [HCo(triphos)(PMe₃)], in which triphos = PhP(CH₂CH₂PPh₂)₂, was synthesized and characterized by X-ray crystallography and by ¹H and ³¹P NMR spectroscopy. The geometry of the compound is a distorted trigonal bipyramid in which the axial positions are occupied by the hydride and the central phosphorus atom of the triphos ligand, while the PMe₃ and terminal triphos donor atoms occupy the equatorial positions. Protonation of [HCo(triphos)(PMe₃)] generates H₂ and the Co(I) cation, [Co(triphos)(PMe₃)]⁺, and this reaction is reversible under an atmosphere of H₂ when the proton source is weakly acidic. The thermodynamic hydricity of HCo(triphos)(PMe₃) was determined to be 40.3 kcal/mol in MeCN from measurements of these equilibria. The reactivity of the hydride is, therefore, well suited to CO₂ hydrogenation catalysis. Density functional theory (DFT) calculations were performed to evaluate the structures and hydricities of a series of analogous cobalt(triphosphine)(monophosphine) hydrides where the phosphine substituents are systematically changed from Ph to Me. The calculated hydricities range from 38.5 to 47.7 kcal/mol. Surprisingly, the hydricities of the complexes are generally insensitive to substitution at the triphosphine ligand, as a result of competing structural and electronic trends. The DFT-calculated geometries of the [Co(triphos)(PMe₃)]⁺ cations are more square planar when the triphosphine ligand possesses bulkier phenyl groups and more tetrahedrally distorted when the triphosphine ligand has smaller methyl substituents, reversing the trend observed for [M(diphosphine)₂]⁺ cations. More distorted structures are associated with an increase in ΔG_H^°, and this structural trend counteracts the electronic effect in which methyl substitution at the triphosphine is expected to yield smaller ΔG_H^° values. However, the steric influence of the monophosphine follows the normal trend that phenyl substituents give more distorted structures and increased ΔG_H^° values.