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.