Speciation of Uranyl Nitrato Complexes in Acetonitrile and in the Ionic Liquid 1-Butyl-3-methylimidazolium Bis(trifluoromethylsulfonyl)imide

Complex formation between the uranyl ion and nitrate ions in acetonitrile and the ionic liquid 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([C4mim][Tf2N]) has been studied by absorption, magnetic circular dichroism (MCD) and uranium LIII EXAFS spectroscopy. The experimental results point to the existence of a trinitrate species [UO2(NO3)3]– with D3h symmetry in both solvents. The atomic distances in the uranium(VI) coordination sphere for the trinitrato complex in acetonitrile are U–Oax = 1.770.01 Å and U–Oeq = 2.480.01 Å. EXAFS data show that the uranyl ion in the ionic liquid is surrounded by six oxygen atoms in the equatorial plane at a distance of 2.490.01 Å. The U–N distance of 2.920.01 Å indicates a bidentate coordination of the nitrate group in both solvents. A structural comparison is made between the uranyl trinitrato complex anion [UO2(NO3)3]– and the uranyl tricarbonato complex anion [UO2(CO3)3]4–. No evidence is found for the presence of uranyl nitrato complexes in aqueous solution. The optical absorption, MCD and EXAFS spectra resemble those of the hydrated free uranyl ion. There are two axial oxygen atoms at 1.770.01 Å and five equatorial oxygen atoms at 2.410.01 Å. These values agree well with structural parameters obtained for the uranyl aqua ion.