Competitive Complexation of Nitrates and Chlorides to Uranyl in a Room Temperature Ionic Liquid

By coupling EXAFS, UV-vis spectroscopy and molecular dynamics and quantum mechanical calculations, we studied the complexation of uranyl cations with nitrate and chloride ions in a water immiscible ionic liquid, C4mimTf2N. Both anions are shown to be stronger uranyl ligands than the ionic liquid Tf2N- or triflate anions. Structural parameters of the trinitrate-complex of uranyl are similar to those obtained in organic solvents like acetonitrile. When chloride and nitrate anions are simultaneously present, one never observes “limit” complexes like UO2(NO3)3- or UO2Cl42- alone. At a U/ NO3 /Cl ratio of 1 /2 /2, the dominant species is likely UO2Cl(NO3)2-. When chlorides are in excess over uranyl with different nitrate concentations (U/ NO3 /Cl ratio of 1/2/6, 1/4/4, 1/12/4) the solution contains a mixture of UO2Cl42- and UO2Cl3(NO3)2- species. Furthermore, it is shown that the experimental protocol for introducing these anions (either as uranyl counterion, as added salt, or as ionic liquid component) influences the UV-vis spectra, pointing to kinetic equilibration issues in the ionic liquid.