Thermodynamic study of axial coordination reaction of zinc porphyrin with metal Schiff base and imidazole complex

A thermodynamic study is reported for the coordination reaction of ZnT (m-X ) PP derivatives (X = NO2, Cl, OCH3, H or CH3) with various ligands L (L = imidazole (Im), 2-methylimidazole (MeIm), clotrimidazole (CM), imidazole-4- carboxaldehyde (4-CHOIm), unsymmetrical tetradentate copper Schiff base, Cu Im(p-Cl), Culm(p-Br), and nickel Schiff base, NiIm(p-Cl)), in dichlorometha ne solvent. Conversion of the four-coordinated ZnT( m-X) PP to the five-coo rdinated species is followed and isosbestic behavior is exhibited in the re gion among 450 and 700 nm. The reaction of a copper(II) or nickel(II) imida zolate Schiff base with ZnT( m-X)PP results in the formation of an imidazol ate bridged heterobinuclear complex. The stoichiometric number is unity for all axial ligands. The equilibrium constants were determined using the bet a band of ZnT(m-X)PP in the 293-308 K range by the method of Rose and Drago . It increases with decrease in temperature, and Delta H-0 < 0, Delta S-0 < 0. The stronger the nucleophilic ability of the axial ligand is, the large r the stability of the axial coordination product is. Hammett Linear relati onships and isoequilibrium relationships exist in the system studied.