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 {[Gd-2(L)(3)(DMF)(4)]center dot(4DMF)center dot(3H(2)O)}(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 la 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)(H1L1)(2)(DMF)(H2O)(2)](DMF)(3)(H2O)(2)}(n) (2), {[Cd(L)(L-2)]}(n) (3), {[(Cd)(1.5)(L)(1.5)(L-3)]}(n) (4), and {[Cd(L)(L-4)(H2O)]}(n) (5) in the presence of different colinkers. Solid-state photoluminescence studies performed on MOFs 2-5 at room temperature showed intraligand (pi-pi*) emission. The MOP 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.