Résumé
Organic extractants used in liquid-liquid extraction have conformational and functional properties that are essential for the selective complexation of metals. Although little discussed in the literature, the stereochemistry of chiral ligands has a major impact on the affinity and organisation of extractant molecules. In the context of the separation of uranium and plutonium from spent fuel, enantiopure monoamides were synthesized to evaluate their extraction performance and to study the mechanisms underlying the observed differences between their regio- and stereoisomers. The enantio- and diastereoselective synthesis of 8 linear or cyclic monoamides was carried out. The regio and stereoisomer of the N,N-di(2-éthylhexyl)butanamide (DEHBA) showed differences in extraction, with a greater effect of regio- than stereoisomerism. X-ray absorption and scattering methods showed that uranium is extracted by a UO2(NO3)2L2 complex, which is not very sensitive to steric hindrance, while plutonium is extracted by two complexes, Pu(NO3)4L2 and Pu(NO3)6(HL)2, which are affected differently by stereo- and regioisomerism. The Pu(NO3)4L2 complex is disadvantaged by the bulkiness of the extractants, while the Pu(NO3)6(HL)2 complex is favoured by the preformation of larger supramolecular aggregates. Cyclic butanamides showed uranium precipitation properties closely dependent on cis/trans stereoisomerism. An octanoyl chain allowed them to be used in pure form, giving high extraction performances for uranium and plutonium without any stereoisomerism effect. As a preliminary study, the different synthesised ligands were tested for the extraction of uranium in the presence of various competitor metals. Promising effects on the extraction of metals were observed, allowing the synthesised monoamides to be considered in the context of upstream or downstream of the fuel cycle.