Molecular and Supramolecular Study of Uranium/Plutonium Liquid–Liquid Extraction with <i>N</i>,<i>N</i>-Dialkylamides

In the context of the separation of uranium and plutonium from spent fuel allowed by <i>N,N</i>-dialkylamides, three regioisomers of <i>N,N</i>-di(2-ethylhexyl) butyramide (DEHBA or ββ) and the diastereopure isomers of <i>N</i>-(2-ethylhexyl)-<i>N</i>-(oct-3-yl)butyramide (EHOBA or αβ) were synthesized to assess their extraction performance and to study the mechanisms at the origin of the differences observed between the stereo- and regioisomers. The <i>N,N</i>-dialkylamides showed differences in extraction, with a greater effect of regio- than stereoisomerism. A mechanistic study at both the molecular and supramolecular scales was initially applied to explain these effects. X-ray absorption and UV-vis spectroscopy 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 differently affected by stereo- and regioisomerism. Investigations at the supramolecular scale also showed that Pu(NO₃)₄L₂ complexes are disadvantaged by the bulkiness of the extractants, while Pu(NO₃)₆(HL)₂ is favored by the preformation of larger supramolecular aggregates.