A novel proton transfer compound containing 2,6-pyridinedicarboxylic acid and creatinine and its zinc(II) complex - Synthesis, characterization, crystal structure, and solution studies

The 1:1 proton transfer compound LH2 (creatH)(+) (pydcH)(-), has been prepared from the reaction of creatinine, creat, and dipicolinic acid, pydcH(2), (2,6- pyridinedicarboxylic acid) and characterized using IR, H-1 and C-13 NMR spectroscopy. The first coordination complex (creatH)[Zn(pydc)(pydcH)]center dot 4H(2)O, was prepared using LH2 and zinc(II) nitrate, and characterized using IR, H-1 and C-13 NMR spectroscopy and single crystal X-ray crystallography. The crystal system is triclinic with space group P-1 with two molecules per unit cell. The unit cell dimensions are a 8.085(2) angstrom, b = 10.802(4) angstrom, c = 13.632(4) angstrom, alpha = 104.98(2)degrees, beta = 90.31(2)degrees and gamma = 92.55(3)degrees. The structure has been refined to a final value for the crystallographic R factor of 0.0381 based on 3003 reflections. The zinc atom is six-coordinated with a distorted octahedral geometry. The (pydc)(2-) and (pydcH)(-) units are almost perpendicular to each other. Extensive hydrogen bondings between carboxyl-ate groups, (creatH)(+) and water molecules throughout the zinc(II) complex as well as pi-pi stacking and ion pairing play important roles in stabilizing the corresponding lattices. The protonation constants of the building blocks of the pydcH(2)-creat adduct, the equilibrium constants for the reaction of (pydc)(2-) with creat and the stoichiometry and stability of the Zn-II complex with LH2 in aqueous solution were accomplished by potentiometric pH titration. The solution studies support a self-associated (creatH)(+)(pydcH)(-) as the most abundant species at pH = 3.4. The stoichiometry of the crystalline complex (i.e. (creatH) [Zn(pydc)(pydcH)])and that of the most abundant species detected in solution were found the same.