J. Iran. Chem. Soc., Vol. 7, No. 3 September 2010, pp.727~739.

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A Proton Transfer Self-Associated Compound from Benzene-1,2,4,5-Tetracarboxylic Acid and Piperazine and Its Cobalt(II) Complex: Syntheses, Crystal Structures and Solution Studies

H. Aghabozorga,* F. Mahfoozib, M.A. Sharifc, A. Shokrollahid, S. Derkid, M. Shamsipure and H.R. Khavasif

aFaculty of Chemistry, Tarbiat Moallem University, 15614, Tehran, Iran
bDepartment of Chemistry, Islamic Azad University, Neiriz fars Branch, Neiriz fars, Iran
cDepartment of Chemistry, Islamic Azad University, Qom Branch, Qom, Iran
dDepartment of Chemistry, Yasouj University, Yasouj, Iran
eDepartment of Chemistry, Razi University, Kermanshah, Iran
fDepartment of Chemistry, Shahid Beheshti University, Tehran, Iran

Two compounds containing a proton transfer ion pair (pipzH2)(btcH2), 1, and its Co(II) complex, were synthesized and characterized using IR spectroscopy and single crystal X-ray diffraction method. The chemical formula is {(pipzH2)[Co(btc)(H2O)4].4H2O}n for Co(II) complex, 2, (pipz:piperazine, btcH4:benzene-1,2,4,5-tetracarboxylic acid). The space group and crystal system of both compounds are P1and triclinic. The proton transfer ion pair 1 was prepared by the reaction between piperazine and benzene-1,2,4,5-tetracarboxylic acid, and its Co(II) complex, 2, was synthesized by the reaction of 1 with cobalt(II) sulphate. The complex 2 is a polymeric substance with Co(II) atoms linked by two O atoms from one benzene-1,2,4,5-tetracarboxylate. In both crystal structures, a wide range of noncovalent interactions consisting of hydrogen bonding (types of O-H∙∙∙O, N-H∙∙∙O and C-H∙∙∙O) and ion pairing interactions connecting the various components into a supramolecular structure. There are also C-H···O and C-H∙∙∙π intermolecular interactions in compound 2 which take part in the stabilization of molecular packing. The protonation constants of pipz and btcH4, the equilibrium constants for the btcH4-pipz proton transfer system and the stoichiometry and stability of this system with Co2+ ion in aqueous solution were investigated by potentiometric pH titrations. The stoicheiometry of one of the most abundant complexed species in solution was found to be the same as that of the crystalline cobalt(II) complex.

Keywords: Cobalt(II) complex, Benzene-1,2,4,5-tetracarboxylic acid, Piperazine, Crystal structure, Solution study, Hydrogen bond

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