Investigating H 2 Adsorption in Isostructural Metal-Organic Frameworks M-CUK-1 (M = Co and Mg) through Experimental and Theoretical Studies.

A combined experimental and theoretical study of H 2 adsorption was carried out in Co-CUK-1 and Mg-CUK-1, two isostructural metal-organic frameworks (MOFs) that consist of M 2+ ions (M = Co and Mg) coordinated to pyridine-2,4-dicarboxylate (pdc 2- ) and OH - ligands. These MOFs possess saturated metal centers in distorted octahedral environments and narrow pore sizes and display high chemical and thermal stability. Previous experimental studies revealed that Co-CUK-1 exhibits a H 2 uptake of 183 cm 3 g -1 at 77 K/1.0 atm [ Angew. Chem., Int. Ed. 2007, 46, 272-275, DOI: 10.1002/anie.200601627], while that for Mg-CUK-1 under the same conditions is 240 cm 3 g -1 on the basis of the experimental measurements carried out herein. The theoretical H 2 adsorption isotherms are in close agreement with the corresponding experimental measurements for simulations using electrostatic and polarizable potentials of the adsorbate. Through simulated annealing calculations, it was found that the primary binding site for H 2 in both isostructural analogues is localized proximal to the center of the aromatic rings belonging to the pdc 2- linkers. Inelastic neutron scattering (INS) spectroscopic studies of H 2 adsorbed in both MOFs revealed a rotational tunnelling transition occurring at around 8 meV in the corresponding spectra; this peak represents H 2 adsorbed at the primary binding site. Two-dimensional quantum rotation calculations for H 2 localized at the primary and secondary binding sites in both MOFs yielded rotational energy levels that are in agreement with the transitions observed in the INS spectra. Even though both M-CUK-1 analogues possess different metal ions, they exhibit similar electrostatic environments, modeled structures at H 2 saturation, and rotational potentials for H 2 adsorbed at the most favorable adsorption site. Overall, this study demonstrates how important molecular-level details of the H 2 adsorption mechanism inside MOF micropores can be derived from a combination of experimental measurements and theoretical calculations using two stable and isostructural MOFs with saturated metal centers and small pore windows as model systems.