Exploration of Structural Topologies in Metal–Organic Frameworks Based on 3‑(4-Carboxyphenyl)propionic Acid, Their Synthesis, Sorption, and Luminescent Property Studies

Four new compounds (two coordination
polymers (CPs) and two metal
organic frameworks (MOFs)), namely, [Zn­(cpp)­(H2O)]n (1), [Cu­(cpp)­(4-bpmh)]n­·nH2O­·nMeOH (2), [Cd2(cpp)2­(3-bpmh)2]n­4nH2O­·2nMeOH (3), and [Cd­(cpp)­(bpy)­(H2O)2]n­·2nH2O (4), have been synthesized
through the slow diffusion technique using cpp ligand and different
neutral linkers (H2cpp = 3-(4-carboxyphenyl)­propionic
acid, 4-bpmh = N,N-bis-pyridin-4-ylmethylene-hydrazine,
3-bpmh = N,N-bis-pyridin-3-ylmethylene-hydrazine,
bpy = 4,4-bipyridine). Single crystal X-ray analysis of compounds 1–4 reveals their structural diversities
which might have been generated due to both rigidity (aryl carboxylate)
and flexibility (aliphatic carboxylates) of cpp ligands as well their
bridging modes and the orientation of nitrogen atoms of the neutral
linkers. In addition, the dihedral angle in the aliphatic carboxylates
of cpp ligand is also playing an important role in directing the final
structural arrangement. Compound 1 exhibits a uninodal
6-connected three-dimensional (3D) coordination polymer with point
Schälfli symbol {33.59.63}
and shows an uncommon lcy; 6/3/c1 topological type.
Compound 2 reveals a 6-connected uninodal 3D framework
with point Schälfli symbol {48.66.8}
and shows a rare rob topology. Compound 3 formed a 4-connected uninodal two-dimensional framework with point
Schälfli symbol {44.62} and displays
a sql/Shubnikov tetragonal plane net topology, whereas 4 forms a one-dimensional CP which subsequently extended to
3D supramolecular networks through hydrogen bonding interactions.
Gas adsorption studies reveal that compounds 1 and 2 show selective adsorption of CO2 over other gases
(N2, CH4) at low temperature, whereas 3 and 4 show no uptake. Vapor sorption studies
reveal that compounds 1, 2, and 4 show high uptake capacities for H2O over MeOH and EtOH.
Solid state luminescence studies of compounds 1, 3, and 4 display significant red shifts compared
to free ligands.