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, [Zn-3(bdc)(3)(bphy)(3)]center dot 2DMF center dot 10H(2)O (3; H(2)bdc = 1,4-benzenedicarboxylic acid, bphy = 1,2-bis(4-pyridyl)hydrazine, and DMF = N,N-dimethylformamide), [Ni(bdc)(bphy)]center dot DMF center dot 3.5H(2)O (4), [Zn(nipa)(bphy)]center dot EtOH (5; H(2)nipa = 5-nitroisophthalic acid), and [CoBr(bdc)(0.5)(bPhY)]center dot 2DMA center dot 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.