Structural Diversity, Thermal Studies, and Luminescent Properties of Metal Complexes of Dinitrobenzoates: A Single Crystal to Single Crystal Transformation from Dimeric to Polymeric Complex of Copper(II)

Seventeen complexes of 3,5- and 2,4-dinitrobenzoates (L1–L2) with alkali, alkaline, and transition metals have been synthesized and characterized by the single crystal X-ray diffraction, IR spectroscopy, elemental analysis, and thermal studies. Diverse structural topologies have been achieved due to various coordination modes of the benzoates, resulting in five new topologies. Interesting architectures such as zero-dimensional (0D) monomers and paddle-wheel dimers; pseudocubane, double helices, ladders and linear one-dimensional (1D) tapes; pseudodiamondoid and brick-wall type two-dimensional (2D); and chiral three-dimensional (3D) networks were generated. The latter three are formed by the coparticipation of −NO2 groups in the coordination, while 1D complexes are formed by the coordination of water/solvent. Thermogravimetric analysis studies show that the 3D complexes are more stable than 2D; however, 1D complexes become more stable than 2D after the loss of the solvent. The effects of positional isomerism and the nature of the metal ions on the topology also have been observed. The ligands are nonemissive but nine complexes have shown a moderate amount of photoluminescence, owing to the rigidity conferred by the crystal structure of the complex, which largely reduces the radiation less decay and results in enhancement of the intensity of the ligand to ligand charge transfer (LLCT) band. A relatively much larger photoluminescence in the polymeric complex (VIII) of copper(II), however, is a combination of enhanced LLCT due to the double helical 1D crystal structure and chelation enhanced fluorescence (CHEF) phenomenon. A single crystal to single crystal supramolecular transformation of a paddle-wheel complex of copper(II) with guest solvent molecules in the lattice to a desolvated 1D polymer is achieved for the CuL1 complex. Because of self-assembly, six of these complexes crystallize as homochiral, single, double, or triple helical conglomerates, which constitute the most active expression of chirality.

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