A novel supramolecular structure of an s-Block metal ion, Ca(II) atom, formulated as (pnH2)2[Ca2(H2O)2(Hhypydc)4]·4H2O (1), was synthesized and characterized by elemental analysis, IR, 1H and 13C NMR spectroscopies and single crystal X-ray diffraction. Compound 1 is a member of a large family of supramolecular metallic compounds recently derived from a proton transfer ion pair, (pnH2)(Hhypydc), where pn is propane-1,3-diamine and (Hhypydc)2- is 4-hydroxypyridine-2,6-dicarboxylate ion. Based upon the molecular structure analysis of the binuclear anionic complex, the coordination environment around each Ca(II) atom is pentagonal bipyramidal with seven-coordination number. The strong hydrogen bonding between hydroxyl groups of two neighboring anionic complexes produce the nice tape as X-like supramolecular structure. Optimization was carried out using two standard basis sets of 6-31G (d,p) for the single anionic complex (S) and LanL2MB basis set for the double anionic complexes (D) which describe the electrons of all atoms. As an interesting finding from our theoretical calculations, when we reoptimized the double complexes in the presence of strong hydrogen bond interaction between two hydroxyl groups, O3-H3A···O8ii (ii = x-1, y, z-1), it was revealed that the parallel position of ligands to each other was exactly like that of the solid state. The complexation reaction of H3hypydc with Ca2+ in aqueous solution was investigated by potentiometric pH titrations, the equilibrium constants and species distribution in various pHs. The for major complexes formed are described.
