Pore Design of Two-Dimensional Coordination Polymers toward Selective Adsorption

We have synthesized four porous coordination polymers (PCPs) using Zn²⁺, 4,4′-sulfonyldibenzoate (sdb), and four types of dinitrogen linker ligands, 1,4-diazabicyclo­[2,2,2]­octane (dabco), 1,4-bis­(4-pyridyl)­benzene (bpb), 3,6-bis­(4-pyridyl)-1,2,4,5-tetrazine (bpt), and 4,4′-bipyridyl (bpy). The bent sdb ligands form a rhombic space connected by zinc paddle-wheel units to form a one-dimensional double chain, and each dinitrogen ligand linked the one-dimensional double chains. There are different assembled structures of two-dimensional sheets with the same connectivities between Zn²⁺ and the organic ligands. [Zn₂(sdb)₂(dabco)]_<i>n</i> (<b>1</b>) has a noninterpenetrated and noninterdigitated structure, [Zn₂(sdb)₂(bpb)]_<i>n</i> (<b>2</b>) and [Zn₂(sdb)₂(bpt)]_<i>n</i> (<b>3</b>) have interdigitated structures, and [Zn₂(sdb)₂(bpy)]_<i>n</i> (<b>4</b>) has an interpenetrated structure. The length of the dinitrogen ligands dominated their assembled structures and flexibility, which influence the adsorption properties. The flexible frameworks of <b>2</b> and <b>3</b> provide different stepwise adsorption behaviors for CO₂, CH₄, C₂H₆, and C₂H₄ affected by their pore diameters and the properties of the gases. Their different adsorption properties were revealed by IR spectroscopy and X-ray analysis under a gas atmosphere. The framework of <b>4</b> possesses less flexibility and a smaller void space than the others and a negligible amount of CH₄ was adsorbed; however, <b>4</b> can adsorb either C₂H₆ or C₂H₄ through the gate-opening phenomenon. Measurement of the solid-state ²H NMR was also carried out to investigate the relationship between the framework structure and the dynamics of bpy with regard to the lower flexibility of <b>4</b>. We have demonstrated a strategy to control the pore size and assembled structures toward selective adsorption properties of PCPs