Toward Hydrogen Isotope Separations through Strong Hydrogen Adsorption at Open Copper(I) Sites in an Ultramicroporous...

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 Cu^I sites caused by a more electron-withdrawing bbta^2- 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.