Synthesis, characterization, and X-ray crystal structures of Co(II) and La(III) complexes of a pyridine containing self-assembling system and solution studies of the Co(II) complex

The complexes [pyda.H](2)[Co(pydc)(2)].H2O (I) and [pyda.H](2)[La-2(pydc)(4)(H2O)(4)].2H(2)O (II) were prepared from the complexation of a novel pyridine containing self-assembling system LH2, ([pyda.H-2](2+)[pydc](2-)) (pyda = 2,6-pyridinediamine and pydc.H-2 = 2,6-pyridinedicarboxylic acid), with cobalt(II) and lanthanum(III) salts in 82% and 74% yields, respectively. The characterizations were performed using elemental analysis, ESI mass spectroscopy as well as H-1 and C-13 solution NMR spectroscopy, and X-ray crystallography. The crystal systems I and II are monoclinic and triclinic with space groups P2(1)/n and P (1) over bar respectively. Coordination number around each Co atom is six with distorted octahedral geometry, while coordination number around each La atom is nine with highly distorted tricapped trigonal prism geometry. The paramagnetic Co(II) and diamagnetic La(III) complexes show H-1 NMR resonances of both anionic [pydc](2-) and cationic counter ion [pyda.H](+) in DMSO-d(6). The complexation reactions of the complex I in aqueous solution were investigated by potentiometric pH titrations and the equilibrium constants for all major complexes formed are described. The results are presented in the form of distribution diagrams revealing the concentrations of individual complex species as a function of pH. The results revealed that at a pH range 4.0-4.5 the major complex species is [(pyda.H)](2)[Co(pydc)(2)].