A Novel Proton Transfer Self‐Associated Compound from Dipicolinic Acid and Guanidine and Its Cadmium(II) Complex: Synthesis, Characterization, Crystal Structure, and Solution Studies

Abstract

Abstract 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 C 2/ c of the monoclinic system and contains eight molecules per unit cell. The unit cell dimensions are: a = 26.480(5)Å, b = 8.055(2)Å, c = 14.068(3)Å. 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¯ with one molecule per unit cell. The unit cell dimensions are: a = 8.5125(7)Å, b = 11.0731(8)Å, c = 13.2404(10)Å. 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 p H 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 p H >5.