Uranyl complexation by chloride ions. Formation of a tetrachlorouranium(VI) complex in room temperature ionic liquids [Bmim][Tf2N] and [MeBu3N][Tf2N].

The tetrachlorouranium(VI) complex is formed in [Bmim][Tf2N] and [MeBu3N][Tf2N] from a uranium(VI) solution in the presence of a stoichiometric quantity of chloride ions. The [UVIO2Cl4]2- absorption and emission spectra show bands splitting in comparison with the [UVIO2]2+ spectra, as observed in the solid state, organic solvents, and chloroaluminate-based ionic liquids. The fluorescence lifetime of [UO2Cl4]2- in [MeBu3N][Tf2N] is 0.7 +/- 0.1 mus. The reduction potential of this complex is -1.44 and -1.8 V vs Ag/Ag+ respectively in [Bmim][Tf2N] and [MeBu3N][Tf2N] and does not depend on the chloride concentration. The mechanism proposed for the redox process is a monoelectronic reduction to form [UVO2Cl4]3-, followed by a chemical reaction. The tetrachlorouranium(V) complex seems more stable in [Bmim][Tf2N] than in [MeBu3N][Tf2N]. The electrochemical analysis put in evidence specific interactions of the ionic liquid cation with the uranium anionic species.