Molecular and Crystal Structures of Plutonyl(VI) Nitrate Complexes with N-Alkylated 2-Pyrrolidone Derivatives: Cocrystallization Potentiality of UVI and PuVI for Uniform MOX Fuel Precursor

Plutonyl(VI) nitrate complexes with N-cyclohexyl-2-pyrrolidone (NCP) and N-neopentyl-2-pyrrolidone (NNpP) were prepared, and their molecular and crystal structures were determined by single crystal X-ray analysis. The obtained compounds have the similar composition, PuO2(NO3)2(NRP)2 (NRP = NCP, NNpP), which are analogous to the corresponding UVI complexes. Both PuO2(NO3)2(NRP)2 complexes show typical structural properties of actinyl(VI) nitrates, i.e., hexagonal-bipyramidal geometry consisting of two NRP molecules and two NO3– ions located in trans positions in the equatorial plane of PuO22+ moiety, Pu=Oax = 1.73 Å, Pu–ONRP = 2.38 Å, Pu–ONO3 = 2.50 Å, and a bond angle between the U–ONRP bond and the carbonyl group of NRP ≈. 135°. The lattice constants and molecular arrangement of PuO2(NO3)2(NCP)2 were completely different from those of UO2(NO3)2(NCP)2. In contrast, these properties of PuO2(NO3)2(NNpP)2 are the same as those of UO2(NO3)2(NNpP)2. These findings provide one of criteria in selection of suitable NRP as a precipitation agent for the spent nuclear fuel reprocessing based on the precipitation method from a viewpoint of crystal engineering.