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

The mononuclear cobalt hydride complex [HCo(triphos)(PMe 3 )], in which triphos = PhP(CH 2 CH 2 PPh 2 ) 2 , was synthesized and characterized by X-ray crystallography and by 1 H and 31 P 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 PMe 3 and terminal triphos donor atoms occupy the equatorial positions. Protonation of [HCo(triphos)(PMe 3 )] generates H 2 and the Co(I) cation, [Co(triphos)(PMe 3 )] + , and this reaction is reversible under an atmosphere of H 2 when the proton source is weakly acidic. The thermodynamic hydricity of HCo(triphos)(PMe 3 ) 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 CO 2 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)(PMe 3 )] + 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 Δ G H - ° , and this structural trend counteracts the electronic effect in which methyl substitution at the triphosphine is expected to yield smaller Δ G H - ° values. However, the steric influence of the monophosphine follows the normal trend that phenyl substituents give more distorted structures and increased Δ G H - ° values.