Design and synthesis of a new class of diamides for selective extraction of plutonium

The immense interest in purification of plutonium (Pu) originated from its dual use both in domestic power production as well as in strategic applications. As on today, the most prevalent application for ²³⁸Pu is its use as an important fuel for heat and power sources for space exploration. Recently, a useful Pu selective diamide solvent (BenzoDODA) in n-dodecane diluent has been reported with a relatively high distribution coefficient (6.74 at 3M HNO₃ and 9.5 at 6M HNO₃). This ligand showed a distribution ratio ∼ 2 at 0.5M HNO₃ and stripping was carried out with a mixture of N₂H₄ (0.2M), HNO₃ (0.6M) and oxalic acid (0.1M). In diamide based solvents choice of spacer between two amide groups is important as the presence of coordinating group in the spacer may further augment its selectivity in metal ion extraction. This has been substantiated in the case of phenanthroline based diamides used in fuel reprocessing. Recently, triarylpyridine based ligands has generated interest as fluorophores for making metal ion chemo-sensors. Keeping in view of special attributes of heterocycle spacered diamides, we designed and synthesized a new class of diamides with 2,4,6-triarylpyridine as a spacer wherein amide functions are anchored in the aromatic rings at 2- and 6-positions for its use as solvent in nuclear fuel reprocessing. The triarylpyridine spacered diamide (TAP-diamide) was synthesized from 2,6-bis(3-hydroxyphenyl)-4-phenylpyridine using classical synthetic methodologies and its extraction potential was evaluated in the separation of Pu and other relevant metal ions including Pu after long term storage. Higher D_Pu and separation factors (higher than the reported values for Pu with TAP-diamide using nitrobenzene as diluent over Am, Eu, Th, Zr were observed. However, due to toxicity of nitrobenzene, efforts are on to modify the structure of diamide so that dodecane/phase modifier can be used as a diluent towards Pu extraction. Following is the predominant working mechanism for Pu(IV) extraction by TAP-diamide solvent from HNO₃ acid medium. (author)