Triphosphine-Ligated Copper Hydrides for CO₂ Hydrogenation: Structure, Reactivity, and Thermodynamic Studies

The copper(I) triphosphine complex LCu(MeCN)PF₆ (L = 1,1,1-tris(diphenylphosphinomethyl)ethane), which we recently demonstrated is an active catalyst precursor for hydrogenation of CO₂ to formate, reacts with H₂ in the presence of a base to form a cationic dicopper hydride, [(LCu)₂H]PF₆. [(LCu)₂H]⁺ is also an active precursor for catalytic CO₂ hydrogenation, with equivalent activity to that of LCu(MeCN)⁺, and therefore may be a relevant catalytic intermediate. The thermodynamic hydricity of [(LCu)₂H]⁺ was determined to be 41.0 kcal/mol by measuring the equilibrium constant for this reaction using three different bases. [(LCu)₂H]⁺ and the previously reported dimer (LCuH)₂ can be synthesized by the reaction of LCu(MeCN)⁺ with 0.5 and 1 equiv of KB(OⁱPr)₃H, respectively. The solid-state structure of [(LCu)₂H]⁺ shows threefold symmetry about a linear Cu-H-Cu axis and significant steric strain imposed by bringing two LCu⁺ units together around the small hydride ligand. [(LCu)₂H]⁺ reacts stoichiometrically with CO₂ to generate the formate complex LCuO₂CH and the solvento complex LCu(MeCN)⁺. The rate of the stoichiometric reaction between [(LCu)₂H]⁺ and CO₂ is dramatically increased in the presence of bases that coordinate strongly to the copper center, e.g. DBU and TMG. In the absence of CO₂, the addition of a large excess of DBU to [(LCu)₂H]⁺ results in an equilibrium that forms LCu(DBU)⁺ and also presumably the mononuclear hydride LCuH, which is not directly observed. Due to the significantly enhanced CO₂ reactivity of [(LCu)₂H]⁺ under these catalytically relevant conditions, LCuH is proposed to be the catalytically active metal hydride.