Unassisted and Efficient Actinide/Lanthanide Separation with Pillar[5]arene-Based Picolinamide Ligands in Ionic Liquids

Actinide/lanthanide separation is crucial for the development of nuclear industries, while this is largely hindered by their similar chemical and physical properties. In this work, we report solvent extraction systems comprising different pillar[5]-arene-based picolinamide ligands with various substituted groups and the ionic liquid, C4mimNTf2, for efficient actinide/lanthanide separation from acidic solutions. Changing the pyridine nitrogen basicity by substituting the chlorine group (-I effect) or methoxy group (+I effect) at the para-position changes the extraction performance of the ligands substantially. The chlorine group gave negative extraction performance while the methoxy group enhanced the metal ion extraction ability by three orders of magnitude. Extraction kinetics for Am3+ and Eu3+ was fast, and equilibrium distribution values were reached within 15 min with all three ligands. Slope analyses suggested that the stoichiometry of the extracted complexes of Am3+ and Eu3+ with the three ligands is M/L 2/1, and each metal ion is extracted following the cation exchange mechanism involving the transfer of three C4mim+ cations into the aqueous phase. Fluorescence spectroscopic investigation indicated that the extracted complexes were much distorted and ligands were directly coordinated to the metal ion with no sign of water molecules in the primary coordination sphere. The possible complexation structures of the ligands and the Eu3+ ion were optimized by density functional theory calculations, which confirmed that the differences in the extraction ability of the ligands originate from different bonding strength between pyridine nitrogens and metal ions.