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

Abstract

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⁺(H₂O)(H₂)_n (with n ≤ 3) as model systems. Cu⁺(H₂O) attracts dihydrogen (82 kJ mol⁻¹) more strongly than bare Cu⁺ (64 kJ mol⁻¹) does. Combining experimental and computational methods, we demonstrate a high isotopologue selectivity in dihydrogen binding to Cu⁺(H₂O), which results from a large difference in the adsorption zero-point energies (2.8 kJ mol⁻¹ between D₂ and H₂, including an anharmonic contribution of 0.4 kJ mol⁻¹). We investigate its origins and the bond strengthening between Cu⁺ and H₂ upon addition of a single H₂O 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.