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)]n (1) has a noninterpenetrated and noninterdigitated
structure, [Zn2(sdb)2(bpb)]n (2) and [Zn2(sdb)2(bpt)]n (3) have interdigitated structures,
and [Zn2(sdb)2(bpy)]n (4) 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 2 and 3 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 4 possesses less flexibility and a smaller
void space than the others and a negligible amount of CH4 was adsorbed; however, 4 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 4. We have demonstrated a strategy to control the
pore size and assembled structures toward selective adsorption properties
of PCPs.