Unraveling the Extraction and Complexation Trends of Rare-Earth Elements with a Nitrilotriacetamide Ligand

The nitrilotriacetamide <i>n</i>-octyl derivative ligand (NTAamide(<i>n</i>-Oct)) is promising for minor actinide/lanthanide separation, yet periodic trends in its extraction and complexation across the lanthanide series remained unclear. This study systematically investigated its behavior with trivalent rare-earth ions (Ln³⁺, Y³⁺, and Sc³⁺) using solvent extraction, ¹H NMR titration, UV-vis spectrophotometric titration, microcalorimetric titration, single-crystal X-ray diffractometry, ESI-MS, and FTIR. Key findings reveal opposing ionic radius dependencies. Solvent extraction showed that extraction efficiency negatively correlates with the ionic radius, decreasing with the ionic radius. Slope analysis indicated Sc³⁺ forms 1:2 metal-ligand extracted complexes, while other RE³⁺ ions form 1:1 complexes. Conversely, homogeneous and solid-state studies uniformly identified 1:2 stoichiometry for all RE³⁺ complexes, with complexation strength positively dependent on the ionic radius, increasing as the ionic radius decreases. Overall, the extraction and complexation ability trends of NTAamide(<i>n</i>-Oct) toward RE³⁺ exhibit opposing tendencies. Combined with the results of solvent extraction and complexation studies, the compositions of the complexes formed by NTAamide(<i>n</i>-Oct) with Sc³⁺ or other RE³⁺ are [ScL₂](NO₃)₃ and [REL₂][RE(NO₃)₆] (L = NTAamide(<i>n</i>-Oct)), respectively. Furthermore, an ion-pair extraction model is also proposed and discussed.