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)2HPF6. (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(O(i)Pr)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.