Speciation of Uranyl Complexes in Ionic Liquids by Optical Spectroscopy

Uranyl complexes dissolved in room-temperature ionic liquids have diagnostic absorption and emission spectra
which reflect the molecular symmetry and geometry. In particular, the characteristic vibrational fine structure of the
absorption spectra allows identification of the molecular symmetry of a uranyl complex. The concept of speciation
of uranyl complexes is illustrated for the hydrated uranyl ion, the tetrachloro complex [UO2Cl4]2-, the trinitrato
complex [UO2(NO3)3]-, the triacetato complex [UO2(CH3COO)3]-, and the crown ether complex [UO2(18-crown-6)]2+ in imidazolium and pyrrolidinium bis(trifluoromethylsulfonyl)imide ionic liquids. The competition between 18-crown-6 and small inorganic ligands for coordination to the uranyl ion was investigated. The crystal structures of
the hydrolysis product [(UO2)2(μ2-OH)2(H2O)6] [UO2Br4](18-crown-6)4 and imidazolium salt [C6mim]2[UO2Br4] are
described.

溶解于室温离子液体（room-temperature ionic liquids）中的铀酰配合物（uranyl complexes）具有特征性的吸收与发射光谱，其光谱特征可反映其分子对称性与空间几何构型。具体而言，吸收光谱中特有的振动精细结构可用于判定铀酰配合物的分子对称性。本文通过水合铀酰离子、四氯合铀酰配离子[UO₂Cl₄]²⁻、三硝酸根合铀酰配离子[UO₂(NO₃)₃]⁻、三乙酸根合铀酰配离子[UO₂(CH₃COO)₃]⁻以及冠醚合铀酰配离子[UO₂(18-冠-6)]²⁺在咪唑鎓（imidazolium）与吡咯烷鎓（pyrrolidinium）双(三氟甲基磺酰)亚胺（bis(trifluoromethylsulfonyl)imide）类离子液体中的实例，阐释了铀酰配合物的物种组成（speciation）概念。本研究考察了18-冠-6与小型无机配体在与铀酰离子配位时的竞争作用，并对水解产物[(UO₂)₂(μ₂-OH)₂(H₂O)₆][UO₂Br₄]·4(18-冠-6)以及咪唑鎓盐[C₆mim]₂[UO₂Br₄]的晶体结构进行了描述。