Studies on the complexation of Pr(III) and Nd(III) with glycyl-glycine (gly-gly) using spectral analysis of 4f-4f transitions and potentiometric titrations.

The interaction of gly-gly with Pr(III) and Nd(III) have been studied in different aquated organic solvents like CH(3)OH, CH(3)CN, dioxane and DMF by using 4f-4f transitions spectra. Various energy interaction parameters like Slater Condon (F(k)), Racah (E(k)), Lande (ξ(4f)), nephelauxetic effect (β), bonding (b(1/2)) and percent covalency (δ) parameters have been calculated to explain the nature of complexation. The intensity parameters like oscillator strength (P) and Judd-Ofelt parameters (T(λ), λ=2, 4, 6) also support the mode of binding of gly-gly to metal ions. The results show that Pr(III) and Nd(III) with gly-gly form complexes by ionic linkages with carboxylate anion with weak covalency. The protonation constants and metal-ligand stability constants have also been determined by potentiometric measurements in aqueous medium at different temperatures (290, 300 and 310 K) at constant ionic strength, 0.1 mol dm(-1). The results show the formation of metal-ligand complexes in the stoichiometric ratio 1:1. The stability of complexes is more in Nd(III) complexes as compared to Pr(III) complexes. Thermodynamic parameters (ΔG, ΔH and ΔS) of complexes were also calculated and the negative values of ΔG and ΔH show that the complex reactions are spontaneous and exothermic. The positive values of ΔS indicate high stability of complex reactions which are enthalpy-driven.
(Copyright © 2012 Elsevier B.V. All rights reserved.)