Structural and thermodynamic aspects of water-carbonate exchange equilibrium for M-III/IV-EDTA-carbonate systems

摘要

The results of a thermodynamic description of ternary M-EDTA-carbonate (where M = Er(III), Th(IV)) complex formation are presented. The crystal structures of the novel [C(NH2)(3)](3)[Er(EDTA)(CO3)]center dot H2O (I) and [C(NH2)(3)](4)[Th(EDTA)(CO3)(2)]center dot 5H(2)O (II) compounds were determined. The structural and UV-vis-NIR spectroscopic results of crystal I served as the model data to analyze the stoichiometry and stability of the Er(III)-EDTA-carbonate system in aqueous solutions. The formation of the [Er(EDTA)(CO3)](3-) complex in solution under different conditions was examined by complementary techniques including temperature dependent UV-vis-NIR and NMR spectroscopy as well as potentiometry. It was established that the affinity of the carbonate for the Er(III)-EDTA chelate is strongly pH dependent. Thus reaction (A) [Er(EDTA) (H2O)(2)](-) + CO32- reversible arrow [Er(EDTA)(CO3)](3-) + 2H(2)O is more favoured at near neutral pH, while reaction (B) [Er(EDTA)(OH)(H2O)(2)](2-) + CO32- reversible arrow [Er(EDTA)(CO3)](3-) + 2H(2)O + OH- occurs under more basic conditions. The log beta values were found to be 3.66 +/- 0.07 and 0.20 +/- 0.12 for reactions (A) and (B), respectively. The temperature dependence of log beta allowed the determination of the enthalpy and entropy changes of both reactions for the first time (Delta H-(A) = -2.8 +/- 0.8 kJ mol(-1), Delta S-(A) = 62 +/- 3 J mol(-1) K-1 and Delta H-(B) = 28.1 +/- 4.6 kJ mol(-1), Delta S-(B) = 92 +/- 15 J mol(-1) K-1). These data indicate that the carbonate anion more readily displaces H2O than the OH- ligand. The obtained results are important not only from the point of view of environmental lanthanide and actinide mobility, but also for designing MRI contrast agents.