Chloride complexation by uranyl in a room temperature ionic liquid. A computational study

The stepwise addition of 1 to 4 Cl- anions to the uranyl cation has been studied via potential of mean force (PMF) calculations in the [BMI][Tf2N] ionic liquid based on the 1-butyl-3-methylimidazolium cation (BMI+) and the bis(trifluoromethylsulfonyl)imide anion (Tf2N-). According to these calculations, the four Cl- complexation reactions are favored and UO2Cl42- is the most stable chloride complex in [BMI][Tf2N]. The solvation of the different chloro-complexes is found to evolve from purely anionic (ca. 5 Tf2N- ions around UO22+) to purely cationic (ca. 8.5 BMI+ cations around UO2Cl42-), with onion-type alternation of solvent shells. We next compare the solvation of the UO2Cl42- complex to its reduced analogue UO2Cl43- in the [BMI][Tf2N] and [MeBu3N][Tf2N] liquids that possess the same anion, but differ by their cation (imidazolium BMI+ versus ammonium MeBu3N+). The overall solvation structure of both complexes is found to be similar in both liquids with a first solvation shell formed exclusively of solvent cations (about 9 BMI+ cations or 7 MeBu3N+ cations). However, a given complex is better solvated by the [BMI][Tf2N] liquid, due to hydrogen bonding interactions between Cl- ligands and imidazolium-ring C-H protons. According to free energy calculations, the gain in solvation energy upon reduction of UO2Cl42- to UO2Cl43- is found to be larger in [BMI][Tf2N] than in [MeBu3N][Tf2N], which is fully consistent with recent experimental results.