Abstract A novel compound with formula unit (pipzH2)[Cd(pydc)2]·6H2O, was synthesized and characterized by IR, 1H NMR and 13C NMR spectroscopy, elemental analysis, single crystal X-ray diffractometry. Moreover, the solution behavior was studied, and ab initio HF, DFT calculations were carried out. The compound belongs to a great family of supramolecular metal complexes derived from a proton transfer ion pair i.e. (pipzH2)(pydc), where pipz is piperazine and pydcH2 is pyridine-2,6-dicarboxylic acid. The compound shows a distorted octahedral geometry around the six-coordinated CdII atom. A variety of intermolecular OH···O, NH···O and CH···O hydrogen bonds are responsible to extend the supramolecular network of the compound. The geometry parameters of [Cd(pydc)2]2− complex and free (pydc)2− ligand have been optimized with the B3LYP method of density functional theory (DFT) and ab initio Hartree-Fock (HF) methods for comparison. The effect of basis sets has been investigated using four combination basis sets. The electronic properties of the CdII compound and free (pydc)2− ligand have been investigated based on the natural bond orbital (NBO) analysis at the B3LYP/A level of theory which verifies that the synergistic effect have been occurred in the compound. In solution study, the protonation constants of pipz and pydc, the equilibrium constants for pydc/pipz proton transfer system and the stoichiometry and stability of the system with Cd2+ ion in aqueous solution were investigated by potentiometric pH titrations. The stoichiometry of one of the most abundant complexed species in solution was found to be the same as that of the crystalline cadmium complex.

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