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)}_<i>n</i> (<b>1</b>), {[NH₂(CH₃)₂]­[Co₃(COOH)_2.5(abtc)­(H₂O)₂]₂·H₂O}_<i>n</i> (<b>2</b>), {[Zn₃(abtc)_1.5(DMF)₃]·1.75­(DMF)}_<i>n</i> (<b>3</b>), {[Zn₂(abtc)­(H₂O)_0.75(DMA)_0.5]·3­(DMA)·(H₂O)}_<i>n</i> (<b>4</b>), and {[Cd₂(abtc)­(DMA)₂]·2­(DMA)}_<i>n</i> (<b>5</b>), (H₄abtc = 3,3′,5,5′-azobenzenetetracarboxylic acid, DMF = <i>N</i>,<i>N</i>-dimethylformamide, and DMA = <i>N</i>,<i>N</i>-dimethylacetamide). <b>1</b>–<b>5</b> all consist of {<i>M</i>_n(CO₂)₄}-type clusters and H₄abtc ligands; however, they exhibit four distinct architectures resulting from different coordinated modes of H₄abtc ligand. A pair of Mn²⁺ions in <b>1</b> forms a {Mn₂(CO₂)₄} cluster, which further results in the “<b>PtS</b>”-type three-dimensional (3D) framework. In <b>2</b>, three independent Co²⁺ ions are bridged by COOH^– groups to afford a {Co₃} core, and {Co₃} cores are connected by abtc^2– to generate a “<b>ZSW1</b>”-type topology. Two types of {Zn₂(CO₂)₄} secondary building units in <b>3</b> linked by abtc^2– give the 3D “<b>NbO</b>”-type cage. When DMF is replaced by DMA and CH₃OH, the scarce “<b>nou</b>”-type topology of <b>4</b> is obtained. And <b>5</b> is isomorphous to <b>1</b>. Photoluminescence properties of <b>1</b>–<b>5</b> were characterized. Magnetic measurements demonstrate that dominant antiferromagnetic interactions exist in <b>1</b> and <b>2</b>. In addition, <b>3</b> exhibits significant adsorption capability of CO₂ and highly selective sorption of CO₂ over N<sub>2