Axial Coordination Reactions of Substitutive Derivates of Manganese Porphyrins and Determination of Equilibrium Constants

Axial coordination reactions between 4 substitutive derivates of manganese porphyrins and 4 small molecular ligands of imidazole (Im), 4-methylimidazole (4-MeIm), pyridine (Py) and 4-methylpyridine (4-MePy) in CHCl₃ medium were studied and the equilibrium constants of the coordination reactions were determined by micro UV-Vis spectrophotometric titration. It was found that each of the 4 manganese porphyrins with different substitution forms coordination complex with either of the 4 guest ligands, with their formation ability and stability related to the position of substituents in the manganese porphyrin molecules. The manganese porphyrins with substitution of -NO₂^- group at the β position, form more stable complexes with the guest ligands due to the strong electron affinity of the NO₂^- group; different structures of the guest ligands also show significant effects on the coordination reaction. A 5-membered complex with mole ratio of manganese porphyrins and pyridine of 1 to 1 is formed; and 6-membered complexes with mole ratio of manganese porphyrins and the remainder 3 guest ligands of 1 to 2 are formed. Reaction abilities of the 4 guest ligands in the coordination reactions have the following order:Im > 4-MeIm > 4-MePy > Py.