Molecular and Crystal Structures of Plutonyl(VI) Nitrate Complexes with <i>N</i>-Alkylated 2-Pyrrolidone Derivatives: Cocrystallization Potentiality of U^VI and Pu^VI

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 a 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, that is, hexagonal-bipyramidal geometry consisting of two NRP molecules and two NO3− ions located in trans positions in the equatorial plane of the PuO22+ moiety, PuOax = 1.73 Å, Pu−ONRP = 2.38 Å, Pu−ONO3 = 2.50 Å, and a bond angle between the Pu−ONRP bond and the carbonyl group of NRP ≈ 135°. PuO2(NO3)2(NNpP)2 is isostructural with the corresponding uranium compound, whereas PuO2(NO3)2(NCP)2 is not. 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.本研究合成了由N-环己基-2-吡咯烷酮（N-cyclohexyl-2-pyrrolidone，NCP）与N-新戊基-2-吡咯烷酮（N-neopentyl-2-pyrrolidone，NNpP）作为配体构筑的钚(VI)酰硝酸盐配合物（Plutonyl(VI) nitrate complexes），并通过单晶X射线衍射分析（single crystal X-ray analysis）测定了其分子与晶体结构。所获两种配合物的组成相近，通式为PuO₂(NO₃)₂(NRP)₂（其中NRP指代NCP或NNpP），与相应的铀(VI)配合物结构具有相似性。两种PuO₂(NO₃)₂(NRP)₂配合物均展现出锕酰(VI)硝酸盐（actinyl(VI) nitrates）的典型结构特征：以PuO₂²⁺（钚酰阳离子）为中心，在其赤道平面的反式位置排布两个NRP分子与两个硝酸根离子，形成六方双锥几何构型（hexagonal-bipyramidal geometry）；轴向Pu=O键长（PuOax）约为1.73 Å，Pu与NRP配位氧原子间的键长（Pu−ONRP）为2.38 Å，Pu与硝酸根配位氧原子间的键长（Pu−ONO₃）为2.50 Å，且Pu−ONRP键与NRP的羰基之间的键角约为135°。PuO₂(NO₃)₂(NNpP)₂与对应的铀化合物同构（isostructural），而PuO₂(NO₃)₂(NCP)₂则不具备该同构特性。上述研究结果为基于沉淀法的乏燃料后处理（spent nuclear fuel reprocessing）中，从晶体工程（crystal engineering）视角筛选适宜的NRP类沉淀剂提供了参考依据之一。