Direct evidence for ligand-enhanced activity of Cu(i) sites.

Little is known about the strong mediating effect of the ligand sphere and the coordination geometry on the strength and isotopologue selectivity of hydrogen adsorption on the undercoordinated copper(i) site. Here, we explore this effect using gas-phase complexes Cu(HO)(H) (with ≤ 3) as model systems. Cu(HO) attracts dihydrogen (82 kJ mol ) more strongly than bare Cu (64 kJ mol ) does. Combining experimental and computational methods, we demonstrate a high isotopologue selectivity in dihydrogen binding to Cu(HO), which results from a large difference in the adsorption zero-point energies (2.8 kJ mol between D and H, including an anharmonic contribution of 0.4 kJ mol). We investigate its origins and the bond strengthening between Cu and H upon addition of a single HO ligand. We discuss the role of the environment and the coordination geometry of the adsorption site in achieving a high selectivity and the ramifications for identifying and designing future materials for adsorptive dihydrogen isotopologue separation.