Syntheses, crystal, molecular structures, and solution studies of Cu(II), Co(II), and Zn(II) coordination compounds containing pyridine-2,6-dicarboxylic acid and 1,4-pyrazine-2,3-dicarboxylic acid: comparative computational studies of Cu(II) and Zn(II) complexes

Three new coordination compounds of Cu(II), Co(II), and Zn(II) based on different dicarboxylic acids formulated as (AcrH)[Cu(pydc)(pydcH)]center dot 5H(2)O (1) (2a-4mpyH)(2)[M(pyzdc)(2)(H2O)(2)]center dot 6H(2)O; M = Co(II) (2) and Zn(II) (3) have been synthesized and structurally characterized by elemental analyses, IR spectroscopy, and single crystal X-ray diffraction (where pydcH(2) = pyridine-2,6- dicarboxylic acid; Acr = acridine; 2a-4mpy = 2amino- 4-methyl pyridine; pyzdcH(2) = 1,4-pyrazin-2, 3-dicarboxylic acid). In all cases, the metal centers have distorted octahedral coordination geometries. Through hydrogen bonding (such as O-H center dot center dot center dot O and N-H center dot center dot center dot O) and/or slipped or offset p-p stacking interactions, 3D supramolecular networks are constructed in these complexes. In the crystalline network, O-H center dot center dot center dot O hydrogen bonding create (H2O)(n) water clusters, so the hydrogen bond interactions play an important role in sustaining of the supramolecular solid-state architectures in compounds 1-3. The species in the solution media were studied by potentiometric method. The protonation constants of 2a-4mpy, 2-apy = 2-aminopyridine, pydc and pyzdc in aqueous solution, pydc and Acr in a 50% dioxane-50% water (v/v) solvent, as well as the equilibrium constants for three proton-transfer systems, pyzdc-2a-4mpy, pydc-2-apy, and pydc-Acr were calculated using potentiometrical method. The stoichiometry and stability of complexation during the first proton-transfer system with Cu2+, Co2+, and Zn2+ ions and also the second proton-transfer system with Fe3+ and Cr3+ ions in aqueous solution were investigated by potentiometric pH titration method, from point of comparison view of their behaviors in the solution state. The stoichiometry and stability of complexation of third system with Cu2+ and Zn2+, metal ions in 50% dioxane-50% water (v/v) solvent were also investigated by the cited method. The stoichiometry of the most complex species in solution were compared with corresponding crystalline metal ion complexes. Furthermore, DFT calculations have been carried out on the Cu(II) and Zn(II) complexes in the presence of pydcH(2) and pyzdcH(2) in order to better understanding of their molecular orbital structures of HOMO and LUMO.