Crystallization in Ionic Liquids: Synthesis, Properties, and Polymorphs of Uranyl Salts

Crystallizations in uranyl-containing ionic liquids yielding crystals {X}a{[UO2]b[Y]c} with selected anions and cations (X = [Bmim]+ for Y = Cl–, NO3–, and SCN–; Y = Cl– for X = [Emim]+, [Emmim]+, [Bmim]+, and [Bmmim]+; Emim = l-ethyl-3-methyl-imidazolium, Emmim = l-ethyl-2,3-dimethylimidazolium, Bmim = l-butyl-3-methylimidazolium, and Bmmim = l-butyl-2,3-dimethylimidazolium) were performed herein. Through standard crystallographic analyses, Hirshfeld surface analyses, and multiple characterization techniques, compounds with common cations/anions were investigated. For compounds [Bmim]2[(UO2)2­(μ-OH)2(NO3)4] (1), [Bmim]3­[UO2(NCS)5] (2), and [Bmim]2­[UO2Cl4] (3TT and 3TG), common [Bmim]+ cations with different conformations were studied with respect to packing and interactions (for 1). The coordinated [(UO2)2(μ-OH)2­(NO3)4]2–, [UO2(NCS)5]3–, and [UO2Cl4]2– anions that have historically been related to nuclear fuel cycles were demonstrated with respect to geometry and distortion. For compounds with common [UO2Cl4]2– anions [Emim]2­[UO2Cl4] (4), [Emmim]2­[UO2Cl4] (5), [Bmmim]2­[UO2Cl4] (6), 3TT, and 3TG, observed interionic interactions that have been previously impeded by limited structural information were discussed fully in relation to different cations and temperatures. Moreover, multistep phase transformations of 2, which have been undefined in solution studies, have been identified through differential scanning calorimetry analyses and polarizing optical microscopy. The polymorph transformations between 3TT and 3TG in solution, as controlled by uranyl concentration, were studied using optical microscopy and powder X-ray diffraction. The thermal stability, IR/Raman, and UV–vis/luminescence spectra of these compounds were also investigated.