A novel highly selective ligand for separation of actinides and lanthanides in the nuclear fuel cycle. Experimental verification of the theoretical predictionElectronic supplementary information (ESI) available: Synthetic protocols, formation energies, and Cartesian coordinates of the calculated structures. See DOI: 10.1039/c7dt01009e

We have predicted earlier by DFT simulation that tridentate O,N,O-donor cyclic dilactams ( B ) belonging to the family of pyridine-2,6-dicarboxamides are much more selective and efficient extractants for the separation of lanthanides and actinides than open-structure pyridine-2,6-dicarboxamides due to the higher degree of "ligand preorganization". In the present work, three new ligands of type ( B ) were synthesized. Extraction experiments showed that, in line with the data from DFT simulation, these ligands have 5-6-fold higher selectivity for the separation of an Am 3+ /Eu 3+ pair and provide distribution coefficients D which are by three orders of magnitude higher than those for the related parent ligands with an open structure. Determination of the solvate numbers (SNs) for Eu 3+ and Am 3+ cations by slope analysis has shown that the stoichiometry of complexes, in the form of which these ions pass from the aqueous into the organic phase, depends to a considerable extent on the polarity of the organic solvent. Strongly polar solvents ( > 20) extract these cations mainly in the form of 1 : 1 complexes LM(NO 3 ) 3 having according to the DFT simulation the largest dipole moments ( μ = 18.6-19.7 D). The solvents of low polarity ( ≤ 10) extract these cations mainly in the form of less polar 2 : 1 complexes L 2 M(NO 3 ) 3 ( μ 1.6 D). For solvents of intermediate polarity fractional values of solvate numbers were obtained which indicates the coexistence of complexes LM(NO 3 ) 3 and L 2 M(NO 3 ) 3 in the organic phase. Predicted by DFT simulation dilactams ( B ) are selective and efficient extractants for the separation of Eu 3+ and Am 3+ .