Molecular and Crystal Structures of Uranyl Nitrate Complexes with N-Alkylated 2-Pyrrolidone Derivatives: Design and Optimization of Promising Precipitant for Uranyl Ion

Abstract

Molecular and crystal structures of UO2(NO3)2(NRP)2 (NRP = N-alkylated 2-pyrrolidone derivative) have been investigated by using single crystal X-ray analysis. All UO2(NO3)2(NRP)2 complexes have typical structural properties of UO2(NO2)2(L)2 (L = unidentate ligand), i.e., hexagonal-bipyramidal geometry, two NRP and two NO3 − located in trans positions in an equatorial plane of the uranyl moiety, UOyl ≃ 1.76 Å, U−ONRP = 2.38−2.41 Å, U−ONO3 = 2.50−2.54 Å, and a bond angle between the U−ONRP bond and the carbonyl group of NRP (ca. 135°). In the crystals of the uranyl nitrate complexes with N-n-propyl-2-pyrrolidone (L3a) and N-iso-propyl-2-pyrrolidone (L3b), polymorphism between monoclinic (293 K) and triclinic forms (173 K) was observed, indicating the presence of significant voids in the crystal lattices of these compounds. From this result, an approach for construction of efficient packing of UO2(NO2)2(NRP)2 was proposed, i.e., the efficient packing would be built up when an alkyl chain of NRP fills the voids in the crystal lattice of UO2(NO3)2(L3a)2 or UO2(NO3)2(L3b)2 without significant deformation of their crystal structures. On this basis, the molecular and crystal structures of UO2(NO3)3(NRP)3 with C4- and C5-alkyl chains were examined. Consequently, it was found that N-iso-butyl-2-pyrrolidone (L4b) completely satisfies that requirement. The high packing efficiency of UO2(NO3)2(L4b)2 was demonstrated quantitatively by introducing “compactness parameter” (C p) defined as a mean volume occupied by one carbon atom of the alkyl chain in the crystal of UO2(NO3)2(NRP)2.

Solubility of Uranyl Nitrate Precipitates withN-Alkyl-2-pyrrolidone Derivatives (Alkyl =n-Propyl,n-Butyl,iso-Butyl, and Cyclohexyl)

“…This result may arise from the more effective packing efficiency of UO 2 (NO 3 ) 2 (NiBP) 2 than UO 2 (NO 3 ) 2 (NBP) 2 , as revealed in our recent work. 6) …”

“…N-iso-butyl-2-pyrrolidone (NiBP) was synthesized by the method reported in our previous article. 6 After vigorous stirring, the mixtures were stored for 1 h in a room maintained at 293 K. The supernatants were separated from the precipitate by filtration. The concentration of total uranium in the supernatants was determined by inductively coupled plasma atomic emission spectroscopy (ICP-AES, PerkinElmer Optima 3000).…”

“…6) The molecular and crystal structures of various UO 2 (NO 3 ) 2 (NRP) 2 complexes [R = H, n-propyl, iso-propyl, iso-butyl, (AE)sec-butyl, tert-butyl, iso-amyl, (1-ethyl)propyl, (AE)(1-methyl)butyl, tert-amyl, neopentyl, and 1,2-(dimethyl)propyl] on the nitrogen atom of the pyrrolidone ring were determined by single-crystal X-ray analysis. As a result, the packing efficiency of UO 2 (NO 3 ) 2 (NRP) 2 molecules in their crystal structures is also found to be an important factor for construction of a more stable precipitate.…”

A study on selective precipitation ability of cyclic urea to U(VI) for developing reprocessing system based on precipitation method

“…Hence, we newly selected 3,4,5,6-tetrahydro-1,3-dimethyl-2(1H)-pyrimidinone (N,N 0 -dimethylpropyleneurea, DMPU) as the precipitant, which is a same cyclic urea compound as DMI. It has been already clarified that DMPU precipitates U(VI) in HNO 3 solution to form UO 2 (NO 3 ) 2 (DMPU) 2 , which is similar structure to those of U(VI) precipitate with NRPs and DMI [14,20,22]. The DMPU molecule is expected to have the similar precipitation property to DMI.…”

“…We have also proposed a reprocessing system for spent FBR (Fast Breeder Reactor) fuels based on the precipitation method using NRPs (N-alkyl-2-pyrrolidones) as precipitants [9][10][11][12][13][14][15]. In this system, U and Pu are recovered through two precipitation processes after dissolution of spent FBR fuels in HNO 3 solution.…”