The novel title compounds, (pipzH2)1.5(pydcH)33.7H2O, 1, (pipzH2)[Zr(pydc)3]8H2O, 2 and (pipzH2)[ Ce(pydc)3]8H2O, 3 in which pydcH2 is pyridine-2,6-dicarboxylic acid and pipz is piperazine were obtained in aqueous solution. The compounds were characterized by IR, 1H NMR and 13C NMR spectroscopy, elemental analyses, and X-ray crystallography. Compound 1 is resulted from proton transfer between pydcH2 and pipz. However, compounds 2 and 3 are resulted from complexation of 1 and corresponding metallic salts. Both compounds 2 and 3 contain three pyridine-2,6-dicarboxylate species as tridentate ligands, one piperazinediium as counter ion, and eight-uncoordinated water molecules in the asymmetric unit. In both structures each M(IV) is coordinated in a distorted tricapped trigonal prism geometry by three nitrogen and six oxygen atoms of carboxylate groups of three (pydc)2 fragments. In the crystal structures of 1, 2 and 3, extensive O–HO, N–HO and C–HO hydrogen bonds as well as electrostatic forces, C–Hp, C–Op and p–p stacking play important roles in stabilizing structures. The geometrical parameters of the [M(pydc)3]2 anionic complexes, where M = Ce(IV), Zr(IV) have been optimized with the B3LYP method of density functional theory (DFT) and ab initio Hartree–Fock (HF) methods for comparison. In addition, we have studied the structures of (pydc)2 anion and its mono and doubly protonated forms, (pydcH) and pydcH2. The electronic properties of the anionic complexes and ligands have been investigated based on the natural bond orbital (NBO) analysis at the B3LYP method which verifies that the synergistic effect has been occurred in the title complexes. In solution study of 2, the stoichiometry and stability constant of complexation of pipz, pydc, pydc–pipz proton transfer system and Zr(IV) ion in aqueous solution were investigated by potentiometric method.
