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 competitive complexation of uranyl cations with nitrate and chloride ions in a water immiscible ionic liquid (IL), C4mimTf2N (C4mim+: 1-butyl-3-methyl-imidazolium; Tf2N− = (CF3SO2)2N)−: bis(trifluoromethylsulfonyl)imide). Both nitrate and chloride are stronger ligands for uranyl than the IL Tf2N− or triflate anions and when those anions are simultaneously present, neither the limiting complex UO2(NO3)3 − nor UO2Cl4 2− alone could be observed. At a U/NO3/Cl ratio of 1/2/2, the dominant species is likely UO2Cl(NO3)2 −. When chloride is in excess over uranyl with different nitrate concentrations (U/NO3/Cl ratio of 1/2/6, 1/4/4, and 1/12/4) the solution contains a mixture of UO2Cl4 2− and UO2Cl3(NO3)2− species. Furthermore, it is shown that the experimental protocol for introducing these anions to the solution (either as uranyl counterion, as added salt, or as IL component) influences the UV−vis spectra, pointing to the formation of different kinetically equilibrated complexes in the IL.