Solvation of uranium hexachloro complexes in room-temperature ionic liquids. A molecular dynamics investigation in two liquids

We report a molecular dynamics study of the solvation of UCl6-, UCl62-, and UCl63- complexes in the [BMI][Tf2N] and [MeBu3N][Tf2N] ionic liquid cations based on the same anion (bis(trifluoromethylsulfonyl)imide (Tf2N-)) and the butyl-3-methyl-imidazolium(+) (BMI+) or methyl-tri-n-butyl-ammonium (MeBu3N+) cation, respectively. The comparison of two electrostatic models of the complexes (ionic model with -1 charged halides versus quantum mechanically derived charges) yields similar solvation features of a given solute. In the two liquids, the first solvation shell of the complexes is positively charged and evolves from purely cationic in the case of UCl63- to a mixture of cations and anions in the case of UCl6-. UCl63- is exclusively coordinated to BMI+ or MeBu3N+ solvent cations that mainly interact via their CH aromatic protons or their N-Me group, respectively. Around the less charged UCl6- complex, the cations interact via the less polar moieties (butyl chains of BMI+ or MeBu3N+) and the anions display nonspecific interactions. In no case does the uranium atom further coordinate solvent ions. According to an energy components analysis, UCl63- interacts more attractively with the [BMI][Tf2N] liquid than with [MeBu3N][Tf2N], while UCl6- does not show any preference, suggesting a significant solvation effect of the redox properties of uranium, also supported by free energy perturbation simulations. The effect of ionic liquid (IL) humidity is investigated by simulating the three complexes in 1:8 water/IL mixtures. In contrast to the case of naked ions (e.g., lanthanide(3+), UO22+, alkali, or halides), water has little influence on the solvation of the UCl6n- complexes in the two simulated ILs, as indicated by structural and energy analysis.