Solvation of Uranium Hexachloro Complexes in Room-Temperature Ionic Liquids. A Molecular Dynamics Investigation in Two Liquids

Abstract

We report a molecular dynamics study of the solvation of UCl(6)(-), UCl(6)(2-), and UCl(6)(3-) complexes in the [BMI][Tf(2)N] and [MeBu(3)N][Tf(2)N] ionic liquid cations based on the same anion (bis(trifluoromethylsulfonyl)imide (Tf(2)N-)) and the butyl-3-methyl-imidazolium+ (BMI+) or methyl-tri-n-butyl-ammonium (MeBu(3)N+) 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 UCl(6)(3-) to a mixture of cations and anions in the case of UCl(6)(-). UCl(6)(3-) is exclusively "coordinated" to BMI+ or MeBu(3)N+ solvent cations that mainly interact via their CH aromatic protons or their N-Me group, respectively. Around the less charged UCl(6)(-) complex, the cations interact via the less polar moieties (butyl chains of BMI+ or MeBu(3)N+) and the anions display nonspecific interactions. In no case does the uranium atom further coordinate solvent ions. According to an energy components analysis, UCl(6)(3-) interacts more attractively with the [BMI][Tf(2)N] liquid than with [MeBu(3)N][Tf(2)N], while UCl(6)(-) 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+), UO2(2+), alkali, or halides), water has little influence on the solvation of the UCl(6)(n-) complexes in the two simulated ILs, as indicated by structural and energy analysis. This is in full agreement with the experimental observations (Nikitenko, S. I.; et al. Inorg. Chem. 2005, 44, 9497).

Chloride Complexation by Uranyl in a Room Temperature Ionic Liquid. A Computational Study

“…However, recent calculations on UCl 6 2-complexes with the two types of charges have been shown to yield very similar results. 55 The mutation of UO 2 Cl 4 2-to UO 2 Cl 4 3-was achieved in 21 steps, i.e. with increments ∆λ of 0.05.…”

Ionic Liquids for Lanthanide and Actinide Chemistry

“…By dissolving uranyl perchlorate in [C 4 mim][NfO] (NfO: nonafluorobutanesulfonate), the formation of a fourfold complex of [UO 2 ] 2+ and OPPh 3 can be detected in the liquid phase by 31 P NMR spectroscopy and in the solid by single‐crystal X‐ray structure analysis 34. Molecular dynamic studies of homoleptic hexachlorido complexes of U V , U IV and U III in [C 4 mim][Tf 2 N] or [N 1114 ][Tf 2 N] reveal that [UCl 6 ] 3– is surrounded by a first coordination shell exclusively formed by [C 4 mim] + or [N 1114 ] + , whereas in the cases of [UCl 6 ] 2– and [UCl 6 ] – , the Tf 2 N – anion is able to enter the first coordination sphere 35. It is noteworthy that [C 4 mim] + shows a stronger interaction with [UCl 6 ] 3– than [N 1114 ] + due to its higher hydrogen bonding capability.…”

Molecular Dynamics Simulations of Ionic Liquids and Electrolytes Using Polarizable Force Fields

“…Numerous studies indicated that additional attention should be paid to polarization treatment in systems with high ionic concentrations. 3−41 While simulations using nonpolarizable models provided important insight into molecular level correlations and structure in ionic systems, 42,43 they also demonstrated that thermodynamic and transport properties predicted from MD simulations are often inconsistent with experiments showing much slower dynamics, with the agreement becoming worse at higher salt concentrations. 43−46 For example, recent simulations by Rajupt et al of 3 M lithium bis(trifluoro-methanesulfonyl)imide (referred as NTf 2 or TFSI or TFSA) solution in 1,3-dioxolane and 1,2-dimethoxyethane (DOL:DME) mixture predicted Li + and NTf 2 ion diffusion coefficients more than 100 times slower than experiments.…”