Different Complexation Behavior of Fe(III), Co(II) and Ni(II) with Pyridine-2,6-dicarboxylic Acid and 4,4′-Bipyridine Adduct: Syntheses, Crystal Structures and Solution Studies

Three complexes (bpyH)[Fe(pydc)(2)]center dot 4H(2)O (1), [Co-2(pydc)(2)(mu-bpy)(H2O)(4)]center dot 4H(2)O (2) and [Ni(pydc)(bpy)(mu-bpy)](4)center dot 8H(2)O (3) were prepared from the reaction between pyridine-2,6-dicarboxylic acid, pydcH(2), and 4,4'-bipyridine, bpy, with FeCl2 center dot 4H(2)O, CoCl2 center dot 6H(2)O and NiSO4 center dot 6H(2)O, respectively. The complex (1) crystallizes in the space group P (1) over bar of the triclinic system and contains two molecules per unit cell. The structure has been refined to a final value for R factor of 0.0355 based on 4922 reflections. The crystal structure shows that Fe(III) is coordinated to two pyridine-2,6-dicarboxylate ligands. The complexes (2) and (3) crystallize in the space group P2(1)/c of the monoclinic system and contain two molecules per unit cell. The structure (2) has been refined to a final value for R factor of 0.0295 based on 2249 reflections. The molecular structure (2), which is a binuclear complex, contains two pyridine-2,6-dicarboxylate ligands, and a 4,4'-bipyridine bridge ligand. The structure (3) has been refined to a final value for R factor of 0.0579 based on 7721 reflections. In this structure, which is a tetramer complex, pyridine-2,6-dicarboxylate and 4,4'-bipyridine fragments act as a tri- and bidentate ligands, respectively. The protonation constants of 4,4'-bipyridine and pyridine-2,6-dicarboxylate, the equilibrium constants for the pydc-bpy system and the stoichiometry, and stability of complexation of this system with Fe3+, Co2+ and Ni2+ ions in a aqueous solution were investigated by potentiometric pH titration method. The stoichiometry of the most complexes species in solution was found to be very similar to the crystalline cited metal ion complexes.