A Structural and Electrochemical Investigation of 1-Alkyl-3-methylimidazolium Salts of the Nitratodioxouranate(VI) Anions [{UO2(NO3)2}2(μ4-C2O4)]2-, [UO2(NO3)3]-, and [UO2(NO3)4]2-

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Abstract
The properties of the 1-butyl-3-methylimidazolium salt of the dinuclear μ4-(O,O,O‘,O‘-ethane-1,2-dioato)bis[bis(nitrato-O,O)dioxouranate(VI)] anion have been investigated using electrochemistry, single-crystal X-ray crystallography, and extended X-ray absorbance fine structure spectroscopy: the anion structures from these last two techniques are in excellent agreement with each other. Electrochemical reduction of the complex leads to the a two-electron metal-centered reduction of U(VI) to U(IV), and the production of UO2, or a complex containing UO2. Under normal conditions, this leads to the coating of the electrode with a passivating film. The presence of volatile organic compounds in the ionic liquids 1-alkyl-3-methylimidazolium nitrate (where the 1-alkyl chain was methyl, ethyl, propyl, butyl, pentyl, hexyl, dodecyl, hexadecyl, or octadecyl) during the oxidative dissolution of uranium(IV) oxide led to the formation of a yellow precipitate. To understand the effect of the cation upon the composition and structure of the precipitates, 1-alkyl-3-methylimidazolium salts of a number of nitratodioxouranate(VI) complexes were synthesized and then analyzed using X-ray crystallography. It was demonstrated that the length of the 1-alkyl chain played an important role, not only in the composition of the complex salt, but also in the synthesis of dinuclear anions containing the bridging μ4-(O,O,O‘,O‘-ethane-1,2-dioato), or oxalato, ligand, by protecting it from further oxidation.