Kinetics of the Demetallation of Water Soluble Lanthanide Porphyrins by Ethylenediaminetetraacetic Acid

Abstract

Abstract The kinetics of the demetallation of six water soluble trivalent lanthanide tetrakis (iV-methyl-4-pyridyl)-porphyrins (Lu, Yb, Dy, Gd, Sm and Nd-TMPyP) by EDTA (ethylenediaminetetraacetic acid) were studied at 25°C between pH 5 and 10. The reactions were first order in porphyrin, and the kinetics indicated complexation between the HY3- form of EDTA and the monohydroxylanthanide porphyrin. The rate determining step was either the dissociation of this complex into products (k1), or, if the complex was a dead-end intermediate, the reaction between the uncomplexed reactants, k2. The rate laws for either pathway were the same, and thus the mechanisms kinetically indistinguishable. For all of the lanthanide porphyrins, an equation of the form log (k2) = 44.9 (Ro)—39.4 described the results, with Ro the ionic radii of the lanthanide ions in Angstroms. A linear relationship was also found between log (k2) for EDTA and log (kH+) for the acid catalyzed solvolysis reactions of the complexes.