Selective Am(III)/Eu(III) Separation by Asymmetric Phenanthroline Derivatives with Lateral Phosphonate and Pyrazole Groups

The separation of trivalent americium from lanthanides poses a significant challenge for nuclear waste management. In this study, we present a series of novel asymmetric tetradentate ligands PO-PzPhen, i.e., diethyl (9-(1H-pyrazol-1-yl)-1,10-phenanthrolin-2-yl)phosphonate ( L1 ), dibutyl (9-(1H-pyrazol-1-yl)-1,10-phenanthrolin-2-yl)phosphonate ( L2 ), bis(2-ethylhexyl) (9-(1 H -pyrazol-1-yl)-1,10-phenanthrolin-2-yl)phosphonate ( L3 ), diphenyl (9-(1H-pyrazol-1-yl)-1,10-phenanthrolin-2-yl)phosphonate ( L4 ), and diethyl (9-(4-methyl-1 H -pyrazol-1-yl)-1,10-phenanthrolin-2-yl)phosphonate ( L5 ), which integrate lateral phosphonate and pyrazolyl groups on a phenanthroline core. By introducing hard/soft donor synergy, L1 – L5 demonstrated robust extraction capabilities, high selectivity for Am(III) (SF Am/Eu reaching 49 for L3 ), and rapid extraction (equilibrium achieved within 10 min) under conditions of high acidity and favorable solubility in common diluents as well. The compositions of the extracted complexes, i.e., [ML(NO 3 ) 3 ] (M = Am(III) or Eu(III); L = L1 – L5 ), were characterized using slope analysis, high-resolution mass spectrometry, and time-resolved fluorescence spectroscopy. X-ray crystallographic analysis established the precise 1:1 (M:L) coordination geometry, demonstrating that all N- and O-donors are involved in the metal–ligand interaction. On the basis of these structural insights, DFT calculations were employed to further elucidate the mechanisms for the Am(III) selectivity and relatively rapid extraction kinetics, highlighting the lower rotation energy and enhanced intrinsic binding affinity toward actinides. This study demonstrates that the asymmetric ligands with multiple functional groups can synergistically optimize solvent extraction performance, providing an efficient framework for the advanced partitioning of trivalent minor actinides from high-level radioactive wastes.