X-ray crystal structure and solution studies of hexacoordinated mercury(II) complex of a pyridine containing proton transfer compound

Solution studies and X-ray crystallography were used to investigate the complexation of Hg(II) by the proton transfer compound (pyda(.)H(2)) (pydc) (pyda = 2,6-pyridinediamine and pydc(.)H(2) = 2,6-pyridinedicarboxilic acid), LH2. The protonation constants of the building blocks of the LH2 adduct, the equilibrium constants for the reaction of pydc(.)H(2) with pyda and the stoichiometry and stability of the Hg(II) complex with LH2 on aqueous solution were accomplished by potentiometric pH titration. The solution studies strongly support a self-association between (pydc)(2-) and (pyda(.)H(2))(2+). The most aboundant ternary complex of mercury(II) formed in aqueous solution is Hg(pydc)(2)(pyda)(2) which existed at pH > 9.0 by an extent of 86.5%, while a (pyda(.)H)(2)[Hg(pydc)](2) complex species exists in about 35% in a pH range of 3.5-4.5. The complexation reactions of LH2 with HgCl2 lead to the formation of a crystalline anionic {(pyda(.)H)(2)[Hg(pydc)Cl](2)(.)2H(2)O}(n) complex. The Hg(II) complex shows H-1 and C-13 NMR resonances of cationic counter ion (pyda(.)H)(+) and signals corresponding to the coordinated ligands (pydc)(2-). This complex crystallizes in the triclinic space group P (1) over bar with a = 7.078(2) Angstrom, b = 10.152(3) Angstrom, c = 10.784(3) Angstrom, alpha = 96.107(6)degrees, beta = 99.163(7)degrees, gamma = 101.792(6)degrees and Z = 1. Coordination number around each Hg(II) atom is six, with distorted octahedral geometry. The binuclear units of the polymeric complex are linked into infinite network via additional Hg-O bonds. (C) 2004 Elsevier B.V. All rights reserved.