Construction of Non-Interpenetrated Charged Metal–Organic Frameworks with Doubly Pillared Layers: Pore Modification and Selective Gas Adsorption

The
rigid and angular tetracarboxylic acid 1,3-bis­(3,5-dicarboxyphenyl)­imidazolium
(H4L+), incorporating an imidazolium group, has been used with different
pyridine-based linkers to construct a series of non-interpenetrated
cationic frameworks, {[Zn2(L)­(bpy)2]·(NO3)·(DMF)6·(H2O)9}n (1), {[Zn2(L)­(dpe)2]·(NO3)·(DMF)3·(H2O)2}n (2), and
{[Zn2(L)­(bpb)2]·(NO3)·(DMF)3·(H2O)4}n (3) [L = L3–, DMF = N,N′-dimethylformamide,
bpy = 4,4′-bipyridine, dpe = 1,2-di­(4-pyridyl) ethylene, bpb
= 1,4-bis­(4-pyridyl)­benzene]. The frameworks consist of {[Zn2(L)]+}n two-dimensional layers that are further pillared by the linker
ligands to form three-dimensional bipillared-layer porous structures.
While the choice of the bent carboxylic acid ligand and formation
of double pillars are major factors in achieving charged non-interpenetrated
frameworks, lengths of the pillar linkers direct the pore modulation.
Accordingly, the N2 gas adsorption capacity of the activated
frameworks (1a–3a) increases with increasing pillar
length. Moreover, variation in the electronic environment and marked
difference in the pore sizes of frameworks permit selective CO2 adsorption over N2, where 3a exhibits
the highest selectivity. In contrast, the selectivity of CO2 over CH4 is reversed and follows the order 1a > 2a > 3a. These results demonstrate
that
even though the pore sizes of the frameworks are large enough compared
to the kinetic diameters of the excluded gas molecules, the electronic
environment is crucial for the selective sorption of CO2.