The extraction of americium and lanthanides by diamides of 2,2′-dipyridyl-6,6′-dicarboxilic acid solutions in polar diluents was investigated. The dependence of extraction properties of the diamides on their structure was studied and the distribution ratios of americium and lanthanides from nitric acid solutions were determined. The highest extraction was found for the compounds with the ethyl-group and the alkyl-substituted aryl-group on the amidic nitrogen. The Am/Ln separation factors higher than 10 were achieved for extraction from high acidity media.

N,N′-Dialkyl-N,N′-diaryl-1,10-phenanthroline-2,9-dicarboxamides (IV) were predicted (DFT simulation) and then were proved experimentally to be efficient donor ligands with high and unusual selectivity for the extraction separation of lanthanides. Distribution coefficients D of lanthanide cations in two-phase aqueous solution–polar organic solvent decrease with increasing Ln³⁺ atomic number. The selectivity factors SF_Ln1/Ln2 for adjacent lanthanide ions were found to be about 3.

A new phosphine oxide ligand demonstrates high selectivity for the Am–Cm pair with SF = 2.9–3.5 and the Am–Eu pair with SF = 7.3–8.5 in a range of 0.1–3 M nitric acid. Thermodynamic measurements show that the entropy factor is responsible for selectivity observed in the extraction experiment. The most prevalent complexes of all three metal ions were (Ph₂PyPO)₂M(NO₃)₃. According to their DFT modelling, the M–N distances for the Cm ion were larger than those for Am, so the last ion enters deeper into the pseudo-cavity of the ligand, which causes the observed selectivity.