Metal–Organic Frameworks Built from a Linear Rigid Dicarboxylate and Different Colinkers: Trap of the Keto Form of Ethylacetoacetate, Luminescence and Ferroelectric Studies

The ligand 2,6,2′,6′-tetranitro-biphenyl-4,4′-dicarboxylic
acid (H2L) has been
used alone or with a coligand to construct a number of metal organic
frameworks (MOFs) with different metal ions that are X-ray crystallographically
characterized. The porous three-dimensional (3D) MOF {[Gd2(L)3­(DMF)4]­·(4DMF)­·(3H2O)}n (1) (DMF = N,N′-dimethylformamide) is found to be an excellent
host to the keto form of ethylacetoacetate to produce {[Gd­(L)1.5­(DMF)2­(H2O)2]­(S)­(H2O)}n (1a) (S = ethyl 3-oxobutanoate) in single crystal to single crystal
(SC–SC) transformation. This involves drastic rearrangement
of the channels including several carboxylate shifts and concomitant
movement of water molecules from the cavity to the metal center. Interestingly,
the daughter framework 1a reverts back to the mother
framework 1 upon keeping it in DMF for 3 days at room
temperature, suggesting that the framework 1 can be used
as a container. The linker H2L also forms the MOFs {[(Cd)4(L)3­(HlL1)2­(DMF)­(H2O)2]­(DMF)3­(H2O)2}n (2), {[Cd­(L)­(L2)]}n (3), {[(Cd)1.5­(L)1.5­(L3)]}n (4), and {[Cd­(L)­(L4)­(H2O)]}n (5) in the presence of different colinkers. Solid-state photoluminescence
studies performed on MOFs 2–5 at
room temperature showed intraligand (π–π*) emission.
The MOF 2 being a chiral compound has been subjected
to ferroelectric measurements. All the compounds (1–5) have been characterized by X-ray crystallography, elemental
analysis, powder X-ray diffraction patterns, thermogravimetry, and
infrared spectroscopy.