Large Degree of Variation in Sorption Properties within a System of Isoreticular, Mixed-Ligand Ni- and Co-Based 2-Periodic Metal-Organic Frameworks

Two Ni-based and two Co-based mixed-ligand metal-organic frameworks (MOFs), which are all isoreticular with each other, are shown to have a large variation in nitrogen, carbon dioxide, hydrogen, and water vapor sorption properties, despite their similar structures. The MOFs have 1,2-bis(4-pyridyl)ethane (bpe) as the common ligand with the second ligand being either 2-nitroterephthalate (2-nta) or 2-bromotereph-thalate (2-bta) for the Ni-based MOFs or 2-nta or 2-amino-terephthalate (2-ata) for the Co-based MOFs. The four MOFs have the formula [Ni(2-nta)(bpe)(H2O)(2)](n.)2(DMA)(n) (1), [Ni(2-bta)(bpe)(H2O)(2)](n.)2.8(DMF)(n) (2), [Co(2-nta)(bpe)(H2O)(2)](n.)2-(DMA)(n) (3), and [Co(2-ata)(bpe)(H2O)(2)](n.)3(DMA)(n.)(H2O)(n) (4), respectively, where DMA = N,N'-dimethylacetamide and DMF = N,N'-dimethylformamide. Variable-temperature powder X-ray diffraction (VT-PXRD) studies indicate that the activated MOF structures are different from those of the as-synthesized MOFs, showing that the 2-periodic layers are dynamic upon desolvation. For the same metal ion, the MOF involving the 2-nta ligand has higher sorption, while sorption for the Ni-based MOFs is higher than for the Co-based MOFs. MOF 1 has the highest sorption within the isoreticular series and also has the highest sorption (the highest for water vapor) compared to other MOFs with the same or related ligands.