Kinetics of the accelerator ligand effect for the complex formation of Cd(II) ion with a cationic water-soluble porphyrin

A kinetic study of the acceleration activities of inorganic and organic ligands for the incorporation of a Cd(II) ion into 5,10,15,20-tetrakis(4-N -methylpyridyl)porphine (TMPyP) has been performed. The acceleration activities of the inorganic ions decreased in the order View the MathML sourceI->NO2->Br->Cl->SO42->OH-. The logarithmic values of the rate constants of the Cd-TMPyP formation were proportional to the values of the nucleophilic constant. This fact suggests that the acceleration of the Cd(II) incorporation into TMPyP is mainly due to the enhancement of the water exchange rate in the inner coordination sphere of the Cd(II)–accelerator complex. Furthermore, the acceleration effects of organic ligands increased with the hydrophobicities of the accelerator ligands. In addition, accelerators possessing negative charges, which are capable of interacting with the positive charges of the N-methylpyridyl groups of TMPyP, significantly enhanced the incorporation of Cd(II) into TMPyP. The rate constant of the metal ion exchange reaction of Cd(II) with Pb-TMPyP in the presence of bathophenanthroline sulfonic acid was 1 400 000-fold greater than the reaction of Cd(II) with TMPyP in the absence of an accelerator. The acceleration effect of organic ligands was due to the enhancement of the hydrophobic interaction and the electrostatic interaction between the Cd(II)–accelerator complex and Pb-TMPyP in the outer coordination sphere.