Assembly of metal-organic frameworks based on 3,3',5,5'-azobenzene-tetracarboxylic acid: photoluminescences, magnetic properties, and gas separations.

We systematically studied the solvothermal reactions of transition-metal ions and H4abtc ligand and successfully isolated five metal-organic frameworks with various characterized tools, which are formulated as {[Mn2(abtc)(DMA)2.75]·1.25(DMA)}n (1), {[NH2(CH3)2][Co3(COOH)2.5(abtc)(H2O)2]2·H2O}n (2), {[Zn3(abtc)1.5(DMF)3]·1.75(DMF)}n (3), {[Zn2(abtc)(H2O)0.75(DMA)0.5]·3(DMA)·(H2O)}n (4), and {[Cd2(abtc)(DMA)2]·2(DMA)}n (5), (H4abtc = 3,3',5,5'-azobenzenetetracarboxylic acid, DMF = N,N-dimethylformamide, and DMA = N,N-dimethylacetamide). 1-5 all consist of {Mn(CO2)4}-type clusters and H4abtc ligands; however, they exhibit four distinct architectures resulting from different coordinated modes of H4abtc ligand. A pair of Mn(2+)ions in 1 forms a {Mn2(CO2)4} cluster, which further results in the "PtS"-type three-dimensional (3D) framework. In 2, three independent Co(2+) ions are bridged by COOH(-) groups to afford a {Co3} core, and {Co3} cores are connected by abtc(2-) to generate a "ZSW1"-type topology. Two types of {Zn2(CO2)4} secondary building units in 3 linked by abtc(2-) give the 3D "NbO"-type cage. When DMF is replaced by DMA and CH3OH, the scarce "nou"-type topology of 4 is obtained. And 5 is isomorphous to 1. Photoluminescence properties of 1-5 were characterized. Magnetic measurements demonstrate that dominant antiferromagnetic interactions exist in 1 and 2. In addition, 3 exhibits significant adsorption capability of CO2 and highly selective sorption of CO2 over N2.