Low-Dimensional Porous Coordination Polymers Based on 1,2-Bis(4-pyridyl)hydrazine: From Structure Diversity to Ultrahigh CO2/CH4 Selectivity

Solvothermal reactions of metal salts, benzenedicarboxylic acids, and 4,4′-azopyridine (azpy) in different conditions produced four coordination polymers, namely, [Zn3(bdc)3(bphy)3]·2DMF·10H2O (3; H2bdc = 1,4-benzenedicarboxylic acid, bphy = 1,2-bis­(4-pyridyl)­hydrazine, and DMF = N,N-dimethylformamide), [Ni­(bdc)­(bphy)]·DMF·3.5H2O (4), [Zn­(nipa)­(bphy)]·EtOH (5; H2nipa = 5-nitroisophthalic acid), and [CoBr­(bdc)0.5(bphy)]·2DMA·H2O (6; DMA = N,N-dimethylacetamide), in which the azpy ligand was in situ reduced. Structural determination reveals that 3–5 consist of the same metal/ligand ratio and similar coordination modes, as well as similar two-dimensional square-grid networks, but differ from their packing/interpenetration modes. 3 consists of alternately arranged single layers and interweaved double layers. Single layers in 4 directly stack in an offset fashion, while 5 is constructed of interdigitated double layers. 6 is a one-dimensional ladderlike structure, which could be regarded as that half of the bridging benzenedicarboxylate ligands in 3–5 are replaced by monodentate bromide ions. Interestingly, the crystal structures of these low-dimensional coordination polymers contain considerable solvent-accessible voids. Thermogravimetric curves, powder X-ray diffraction, and gas sorption experiments were used to study the potential porosity of these structures, which indicated that they can all reversibly desorb and adsorb solvent molecules. In particular, 4 showed gated sorption behavior and high CO2/CH4 selectivity because of its flexible structure.