A novel proton transfer self-associated compound from dipicolinic acid and guanidine and its cadmium(II) complex: Synthesis, characterization, crystal structure, and solution studies

A novel 1:2 proton transfer self-associated compound LH(2), (GH(+))(2)(pydc(2-)), was synthesized from the reaction of dipicolinic acid, pydcH(2), (2,6-pyridinedicarboxylic acid), and guanidine hydrochloride, (GH(+))(Cl(-)). The characterization was performed using IR, (1)H and (13)C NMR spectroscopy and single-crystal X-ray diffraction. LH(2) (.) H(2)O crystallizes in the space group C2/c of the monoclinic system and contains eight molecules per unit cell. The unit cell dimensions are: a = 26.480(5) Angstrom, b = 8.055(2) Angstrom, c = 14.068(3) Angstrom. The first coordination complex (GH)(2)[Cd(pydc)(2)](.)2H(2)O, was prepared using LH(2) and cadmium(II) iodide, and characterized by (1)H and (13)C NMR spectroscopy and X-ray crystallography. The crystal system is triclinic with space group P (1) over bar with one molecule per unit cell. The unit cell dimensions are: a = 8.5125(7) Angstrom, b = 11.0731(8) Angstrom, c = 13.2404(10) Angstrom. The cadmium(II) atom is six-coordinated with a distorted octahedral geometry. The two pydc(2-) units are almost perpendicular to each other. The protonation constants of the building blocks of the pydc-guanidine adduct, the equilibrium constants for the reaction of pydc(2-) with guanidine and the stoichiometry and stability of the Cd(2+) complex with LH(2) in aqueous solution were accomplished by potentiometric pH titration. The solution studies strongly support a self-association between pydc(2-) and GH(+) with a stoichiometry for the Cd(II) complex similar to that observed for the isolated crystalline complex. In fact, the [Cd(pydc)(2)](2-) complex was found as the most abundant species in solution (> 90%) at a pH >5.