Remarkable effect of stereoisomerism on the Am(III)/Ln(III) solvent extraction. New ligands for highly efficient separation of americium.

Two novel 1,10-phenanthroline-2,9-diamide ligands were constructed based on 2-phenylpyrrolidine and obtained as pure diastereomers. These ligands demonstrated advanced properties in liquid-liquid extraction tests. They revealed high efficiency of americium(III) extraction alongside with the record values of selectivity in the separation of americium from light lanthanides from strongly acidic media. An abrupt increase of extraction efficiency when moving along the lanthanide series from lanthanum to lutetium was observed. The examination of the extraction behavior of pure diastereomeric forms revealed noticeable differences in their selectivity while maintaining the overall extraction trend. The explanation of the discovered patterns was elucidated by a comprehensive study of the ability of the ligands to bind lanthanide nitrates in solutions. All the data collected (UV-vis and NMR titration, X-ray analysis of resulting complexes, solvation numbers estimation) were supported by quantum chemical calculation. These data clearly indicated that in case of light lanthanides the formation of 1:1 complexes is most preferable. In the same time, complexes with heavy lanthanides, such as ytterbium and lutetium, exist as ionic pairs which may consist of [L2M]z+ cations counterbalanced by metallates anions, which may result in formation of unusual composition species L2M2 or even L4M5 clusters.