A comparative spectroscopic study of U(III)/Am(III) and Ln(III) complexed with N-donor ligands

Selected soft N-donor ligands are highly effective extractants for the separation of trivalent actinides (An(III)) from lanthanides (Ln(III)) in the partitioning and transmutation (P&T) strategy. Comparative structural investigations using X-ray absorption spectroscopy (EXAFS) on An(III) and Ln(III) complexed with tris[(2-pyrazinyl)methyl]amine (TPZA) and 2,6-di(5,6-dipropyl-1,2,4triazin-3-yl)pyridine (BTP) are performed in order to evaluate if ligand extraction performance is reflected in coordination structure differences, specifically in variations in the metal cation-N bond lengths. We observe U(III) to generally exhibit a decrease in the bond distance over that expected for pure ionic binding for the ligand complexes investigated and interpret this as higher covalent character of the U(III)-N binding. In contrast, no measurable differences for Am(III), Cm(III), and the Ln(III) elements in the middle of the 4f series are observed. Time-resolved laser fluorescence spectroscopy (TRLFS) investigations on solution BTP complexed with Cm(III) and Eu(III) reveal that the selective extraction behavior of this ligand can be explained by the large difference in conditional stability constants of their 1:3 complexes (K-13). The K-13 value for Cm-BTP3 is a number of orders of magnitude larger than that for Eu-BTP3.