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 H₂, we sought to strengthen π-backbonding-mediated H₂ adsorption through pore confinement effects. Toward that end, we synthesized and characterized the ultramicroporous metal−organic framework Cu_xZn_5−xCl_4−yH_z(bbta)₃ (Cu^IZn-MFU-4; H₂bbta = 1H,5H-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 H₂ adsorption enthalpy of−38 kJ/mol, exceeding that of the larger-pore analog (Cu^IZn-MFU-4l;−33 kJ/mol) and representing the strongest H2 adsorption yet achieved in a metal−organic framework. The stronger H2 adsorption in Cu^IZn-MFU-4 is attributed to a combination of pore confinement effects and the increased σ-accepting nature of the CuI sites caused by a more electron-withdrawing bbta²⁻ linker, as supported by structural, spectroscopic, and computational evidence. With the strongest H₂ adsorption, equilibrium isotope effects in Cu^IZn-MFU-4 lead to a D₂/H₂ 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.