PNNL

Abstract

The copper(I) triphosphine complex LCu(MeCN)PF6, which we recently demonstrated is an active catalyst for hydrogenation of CO2 to formate, reacts with H2 in the presence of a base to form a cationic dicopper hydride, [(LCu)2H]PF6¬. The thermodynamic hydricity of [(LCu)2H]+ was determined to be 41.0 kcal/mol by measuring the equilibrium constants 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 equivalent of KB(OiPr)3H, respectively. The solid-state structure of [(LCu)2H]+ shows three-fold 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)+. [(LCu)2H]+ is also an active precursor for catalytic CO2 hydrogenation, with equivalent activity to that of 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. The formation of LCuH is proposed as the reason for the enhanced CO2 reactivity of [(LCu)2H]+ under these catalytically relevant conditions, and LCuH is proposed to be the catalytically active metal hydride. This work was supported by the US Department of Energy, Office of Science, Office of Basic Energy Sciences, Division of Chemical Sciences, Geosciences, and Biosciences. Pacific Northwest National Laboratory (PNNL) is a multiprogram national laboratory operated for DOE by Battelle.