Solvent- and pH-Dependent Formation of Four Zinc Porous Coordination Polymers: Framework Isomerism and Gas Separation

Four new zinc porous coordination polymers (PCPs) were synthesized from a linear ligand of [1,1'-biphenyl]-3,3',5,5'-tetracarboxylic acid (H₄L) and zinc nitrate by the strategy of high throughput solvothermal syntheses. Their structural differences arise from different pH of the medium, as well as the composition of the solvent system. Very interesting is that although NTU-34, -35, and -37 possess the same 3,3,4-connections, they have completely different porous frameworks, along with varied cavity size and shape. In addition, given their same framework formula [Zn₂L], these three PCPs can be considered to be a new group of framework isomers. Further, the coordination between distorted Zn₄O clusters and linear ligands with partial intermolecular π···π interaction formed the other porous framework (NTU-36), which has a new topology with 3,3,3,6-connection and point symbol of {4.6²}4{4².5.6⁴.8⁸}2{5 × 10²}{5².8}). More importantly, gas adsorption and breakthrough experiments revealed that the microporous nature enabled NTU-36 to selectively remove CO₂ from its CH₄ or N₂ mixtures under flowing conditions at ambient temperature.