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 (Melm), clotrimidazole (CIM), imidazol-4-carboxaldehyde (4-CHOIm), unsymmetrical tetradentate copper Schiff base, Culm(p-C1), Ch(p-Br), and nickel Schiff base, Nilm(p-CI)), in dichloromethane solvent. Conversion of the four-coordinated ZnT( m-X)PP to the five-coordinated species is followed and isosbestic behavior is exhibited in the region among 450 and 700 nm. The reaction of a copper(II) or nickel(II) imidazolate Schiff base with ZnT( m-X)PP results in the formation of an imidazolate bridged heterobinuclear complex. The stochiometric number is unity for all axial ligands. The equilibrium constants were determined using the β 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 ΔH⁰ 0, Δ⁰9 0. The stronger the nucleophilic ability of the axial ligand is, the larger the stability of the axial coordination product is. Hammet linear relationships and isoequilibrium relationships exist in the system studied.