Kinetics and mechanisms of the complexation of aqueous lanthanide ions by 4-(2-pyridylazo)resorcinol†

Abstract

Three kinetic steps were observed for the complexations of Eu³⁺ and UO₂²⁺ by PAR [4-(2-pyridylazo)resorcinol] or PAN [1-(2-pyridylazo)-2-naphthol] in different buffered solutions. The first step can be assigned to the co-ordination of the nitrogen donor from the pyridine moiety of the ligand based on the dependencies of [Eu³⁺], pH, pressure, nature and concentration of the buffer. The rate-determining step is the release of water molecules from the co-ordination sphere of the lanthanide ion. Variations in the rate of the first step with different lanthanide ions indicated that a co-ordination number changeover is involved in this lanthanide series. For the second step the formation of a “hydroazone–Ln³⁺ chelate” intermediate accounts for all of the observed kinetic behaviors. The kinetic investigations of the third step show that there is a deprotonation preequilibrium preceding the transition state of the final product with two chelated 5-membered rings involved. Surprisingly, the rate constants of the three steps for the complexation of UO₂²⁺ by PAR are very close to those for 18-crown-6 and diaza-18-crown-6 reacting with uranyl ion. The differences in the kinetics between PAR and PAN can be related to the difference in their structures.