Structural and thermodynamic aspects of water–carbonate exchange equilibrium for MIII/IV–EDTA–carbonate systems

Abstract

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)]·H₂O (I) and [C(NH₂)₃]₄[Th(EDTA)(CO₃)₂]·5H₂O (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]⁻ + CO₃^2– ⇄ [Er(EDTA)(CO₃)]^3– + 2H₂O is more favoured at near neutral pH, while reaction (B) [Er(EDTA)(OH)(H₂O)₂]^2– + CO₃^2– ⇄ [Er(EDTA)(CO₃)]^3– + 2H₂O + OH⁻ occurs under more basic conditions. The log β 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 β allowed the determination of the enthalpy and entropy changes of both reactions for the first time (ΔH_(A) = −2.8 ± 0.8 kJ mol⁻¹, ΔS_(A) = 62 ± 3 J mol⁻¹ K⁻¹ and ΔH_(B) = 28.1 ± 4.6 kJ mol⁻¹, ΔS_(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.