Solvation of M3+ lanthanide cations in room-temperature ionic liquids. A molecular dynamics investigation

We report a molecular dynamics study of the solvation of M3+ lanthanide cations (La3+, Eu3+ and Yb3+) in two room-temperature ionic liquids: [BMI][PF6] based on 1-butyl-3-methyl-imidazolium(+), PF6- and [EMI][TCA] based on 1-ethyl-3-methyl-imidazolium(+), AlCl4-. They reveal specific solvation as a function of the cation size and on the solvent. In [BMI][PF6] solution, the three studied M3+ cations are surrounded by six PF6- anions, while in [EMI][TCA] solution, they are surrounded by eight AlCl4- anions. The precise binding mode of the AlCl4- or PF6- solvent anions (denticity, number of coordinated halides and dynamics) depends on the M3+ cation size. The first shell PF6- anions rotate markedly during the dynamics while AlCl4- do not. In both liquids, this first ionic shell is surrounded by about 11 - 13 imidazolium cations. The solvation of the neutralizing NO3- counterions is similar in both liquids, and involves mainly imidazolium(+) cations in the first shell (approximate to5.0 in [BMI][PF6] and 4.3 in [EMI][TCA]). When simulated in the gas phase, the first anionic shells are similar, but more rigid than in the solutions. Finally, free energy calculations are performed to compare the Eu3+/La3+ and Yb3+/ Eu3+ solvation. In both ionic liquids, the smaller cations are better solvated, but the differences from one cation to the other are weaker than in water.