Complexation of a Nitrilotriacetate-Derived Triamide Ligand with Trivalent Lanthanides: A Thermodynamic and Crystallographic Study.

Non-heterocyclic N-donor nitrilotriacetate-derived triamide ligands are one of the most promising extractants for the selective extraction separation of trivalent actinides over lanthanides, but the thermodynamics and mechanism of the complexation of this kind of ligand with actinides and lanthanides are still not clear. In this work, the complexation behaviors of ,,',',″,″-hexaethylnitrilotriacetamide (NTAamide(Et)) with four representative trivalent lanthanides (La, Nd, Eu, and Lu) were systematically investigated by using H nuclear magnetic resonance (H NMR), ultraviolet-visible (UV-vis) and fluorescence spectrophotometry, microcalorimetry, and single-crystal X-ray diffractometry. H NMR spectroscopic titration of La and Lu indicates that two species of 1:2 and 1:1 metal-ligand complexes were formed in NO and ClO media. The stability constants of NTAamide(Et) with Nd and Eu obtained by UV-vis and fluorescence titration show that the complexing strength of NTAamide(Et) with Nd is lower than that with Eu in the same anionic medium, while that of the same lanthanide complex is higher in ClO medium than in NO medium. Meanwhile, the formation reactions for all metal-ligand complexes are driven by both enthalpy and entropy. The structures of lanthanide complexes in the single ClO and NO medium and the mixed one were determined to be [LnL(MeOH)](ClO) (Ln = La, Nd, Eu, and Lu), [LaL(EtOH)][La(NO)], and [LaL(NO)](ClO), separately. The average bond lengths of lanthanide complexes decrease gradually with the decrease in ionic radii of Ln, indicating that heavier lanthanides form stronger complexes due to the lanthanide contraction effect, which coincides with the trend of the complexing strength obtained by spectroscopic titration. This work not only reveals the thermodynamics and mechanism of the complexation between NTAamide ligands and lanthanides but also obtains the periodic tendency of complexation between them, which may facilitate the separation of trivalent lanthanides from actinides.