Calcium coordination compounds of dipicolinic and quinolinic acids: synthesis, characterization, crystal structures and DFT studies

Abstract

In the past two decades various N-heterocyclic carboxylate coordination compounds have been synthesized with intriguing architectures and potential applications as functional materials. In this regard, two novel calcium coordination compounds [Ca(2,6-Hpdc)2(H2O)2] 1 and [Ca2(2,3-pdc)2(H2O)6]n 2 (where 2,6-H2pdc = 2,6-pyridinedicarboxylic acid and 2,3-H2pdc = 2,3-pyridinedicarboxylic acid) were grown at room temperature. The compounds were characterized using elemental analysis, Fourier Transform infrared (FT-IR) spectroscopy, thermogravimetric analysis, powder X-ray diffraction and single crystal X-ray crystallographic analysis. Compounds 1 and 2 crystallize in orthorhombic space group Pccn and monoclinic space group Pc, respectively. The structures of both compounds are stabilized by a network of hydrogen bonds arising from coordinated water molecules and carboxylate groups. Computational analysis revealed that compound 1 has a large energy gap (9.221 eV) suggesting high excitation energies and chemical hardness making it a better electron acceptor while compound 2 displayed a smaller energy gap (5.156 eV) which is indicative of a softer molecule with better polarizability and reactivity.