Production of Gas-Phase Uranium Fluoroanions Via Solubilization of Uranium Oxides in the [1-Ethyl-3-Methylimidazolium]⁺[F(HF)_2.3]⁻ Ionic Liquid

A new methodology for gas-phase uranium ion formation is described in which UO₂ is dissolved in neat N-ethyl,N′-methylimidazolium fluorohydrogenate ionic liquid [EMIm⁺][F(HF)_2.3 ⁻], yielding a blue-green solution. The solution was diluted with acetonitrile and then analyzed by electrospray ionization mass spectrometry. UF₆ ⁻ (a U(V) species) was observed at m/z = 352, and other than cluster ions derived from the ionic liquid, nothing else was observed. When the sample was analyzed using infusion desorption chemical ionization, UF₆ ⁻ was the base peak, and it was accompanied by a less intense UF₅ ⁻ that most likely was formed by elimination of a fluorine radical from UF₆ ⁻. Formation of UF₆ ⁻ required dissolution of UO₂ followed by or concurrent with oxidation of uranium from the + 4 to the + 5 state and finally formation of the fluorouranate. Dissolution of UO₃ produced a bright yellow solution indicative of a U(VI) species; however, electrospray ionization did not produce abundant U-containing ions. The abundant UF₆ ⁻ provides a vehicle for accurate measurement of uranium isotopic abundances free from interference from minor isotopes of other elements and a convenient ion synthesis route that is needed gas-phase structure and reactivity studies like infrared multiphoton dissociation and ion-molecule dissociation and condensation reactions. The reactive fluorohydrogenate ionic liquid may also enable conversion of uranium in oxidic matrices into uranium fluorides that slowly oxidize to uranyl fluoride under ambient conditions, liberating the metal for facile measurement of isotope ratios without extensive chemical separations. [Figure not available: see fulltext.].