Toward Hydrogen Isotope Separations through Strong Hydrogen Adsorption at Open Copper(I) Sites in an Ultramicroporous Metal–Organic Framework

Metal–organic frameworks with coordinatively unsaturated metal sites (open metal sites) capable of engaging in orbital interactions with π-acidic gases are of interest for enabling ambient-temperature gas separations, such as hydrogen isotope separations. In view of the weakly π-acidic nature of H 2, we sought to strengthen π-backbonding-mediated H 2 adsorption through pore confinement effects. Toward that end, we synthesized and characterized the ultramicroporous metal–organic framework Cu x Zn 5– x Cl 4– y H z (bbta) 3 (Cu I Zn-MFU-4; H 2 bbta = 1 H,5 H -benzo(1,2- d:4,5- d ′)bistriazole), featuring π-basic trigonal pyramidal Cu I sites that reside within 7 Å of one another at their closest. Gas adsorption measurements reveal an H 2 adsorption enthalpy of −38 kJ/mol, exceeding that of the larger-pore analog (Cu I Zn-MFU-4 l; −33 kJ/mol) and representing the strongest H 2 adsorption yet achieved in a metal–organic framework. The stronger H 2 adsorption in Cu I Zn-MFU-4 is attributed to a combination of pore confinement effects and the increased σ-accepting nature of the Cu I sites caused by a more electron-withdrawing bbta 2– linker, as supported by structural, spectroscopic, and computational evidence. With the strongest H 2 adsorption, equilibrium isotope effects in Cu I Zn-MFU-4 lead to a D 2 /H 2 selectivity (as estimated by ideal adsorbed solution theory) of 1.35 even at 298 K, approaching the values reported below 200 K for conventional porous materials.