Am(III)/Eu(III) Extraction and Separation with N-heterocyclic-Derived Unsymmetric Amide-triazine Extractants

Due to the highly similar coordination chemical properties of trivalent actinides (An(III)) and lanthanides (Ln(III)) in aqueous solution, the development of multidentate chelating extractants with selective An(III)/Ln(III) separation ability holds significant research and application value. This work conducted an in-depth study on the extraction and separation abilities of Am(III) and Eu(III) by two unsymmetric extractants featuring nitrogen heterocyclic skeletons and amide-triazine side chains (2-N-ethyl-N-(tolyl)carboxamide-6-(5,5,8,8-tetramethylcyclohexyl-[1,2,4]triazin-3-yl)-pyridine (Et-Tol-CyMe4-ATP, L1) and 6-N-ethyl-N-(tolyl)carboxamide-6′-(5,5,8,8-tetramethylcyclohexyl-[1,2,4]triazin-3-yl)-2,2′-bipyridine (Et-Tol-CyMe4-ATBP, L2)) and discovered that both two extractants exhibit excellent separation performance under high acidity conditions. Subsequently, the extraction mechanisms and complex structures of L1/L2 with An(III)/Ln(III) were investigated by slope analysis, NMR titration, UV–vis and fluorescence spectroscopy, and single-crystal X-ray diffraction. Finally, through DFT calculations, the influence of extractant structures and complex species on the selectivity between Am(III)/Eu(III) was discussed at the theoretical level. This work not only systematically investigated the properties of two novel unsymmetric extractants but also offered guiding value for the mechanism research of other nitrogen heterocyclic extractants utilized for An(III)/Ln(III) separation.