Construction of structural diversity and fine-tuned porosity in acylamide MOFs by a synthetic approach

Abstract

Four new metal–organic frameworks (MOFs), namely, Zn(L1)(AzDC)·(DMF)₃ (1), Zn(L2) (AzDC)·(DMF)_0.5(H₂O) (2), Zn₂(AzDC)₂(L3)·(DMF)(H₂O)_0.5 (3), and Zn(AzDC)(L3)·(DMF) (4), (L1 = N¹,N⁴-di(pyridin-4-yl)terephthalamide, L2 = N¹,N⁴-di(pyridin-3-yl)terephthalamide, L3 = N⁴,N^4′-di(pyridin-4-yl)-[1,1′-biphenyl]-4,4′-dicarboxamide, H₂AzDC = 4,4′-(diazene-1,2-diyl)dibenzoic acid, DMF = N,N-dimethylformamide) were prepared by the solvothermal reaction of Zn(NO₃)₂, H₂AzDC, and acylamide ligands. Polymers 1–3 show the common layer-pillared pcu-type net with four- or two-fold interpenetration, while polymer 4 displays an unprecedented four-connected net with the 6³8²10 topology symbol. Note that the difference in the pillar of L1 for 1 and L2 for 2, related to the position isomers, caused significant alterations, including a reduction in the interpenetration number from four to two, enhanced potential porosity from 47.1% to 66.6%, and a change in the pore configuration from a smaller triangle window to a larger quadrangle window. Furthermore, controlling the volume of the reaction solvent, such as 5 mL for 3 and 15 mL for 4, also resulted in significant alterations, including a change in the structural diversity from a four-fold interpenetrating pcu net to a two-fold interpenetrating 6³8²10 net, enhanced potential porosity from 49.1% to 57.2%, and a change in the pore configuration from a smaller rectangle window to a larger quadrangle window. Most importantly, all these polymers display highly selective adsorption of CO₂ over other gases, such as N₂, CH₄, CO, and O₂, most likely due to the free standing acylamide groups the polymers contain, which afford unique affinity towards CO₂.