Piperazinyl-Based Diamide Ligand for Selective Precipitation of Actinyl (UO₂²⁺/PuO₂²⁺) Ions with Fast Kinetics

A novel ligand N,N'-bis(N″,N″-diethyl carbamoyl) piperazine (BDECP), L1, is synthesized as a selective precipitant for hexavalent actinyl (UO₂²⁺ and PuO₂²⁺) ions from an aqueous nitric acid medium. The ligand BDECP forms an infinite one-dimensional coordination polymer with uranyl nitrate and behaves as a bridging bidentate neutral donor. There is an alternate repetition of [UO₂(NO₃)₂] and BDECP units as evidenced by single-crystal X-ray diffraction. Uranyl ion (UO₂²⁺) can be precipitated in >99% yield from an aqueous nitric acid medium. L1 shows fast kinetics of precipitation of uranyl ion as compared to those of other reported ligands like N-alkyl pyrolidone and N-(1-adamantyl) acetamide. Avrami's coefficient, obtained from the Avrami-Erofe'ev equation, shows that the precipitation mechanism is controlled by the phase boundary and not governed by diffusion. Theoretical studies of the uranyl complex of L1 show that there is no thermodynamic preference for L1 as compared to other potential amide-based precipitants. The principal factors that govern the fast kinetics of precipitation are the aqueous solubility and higher charge density on the amide oxygen of L1.