M3+ Lanthanide Chloride Complexes in “Neutral” Room Temperature Ionic Liquids: A Theoretical Study

The solvation of MCl n 3- n lanthanide chloride salts (La 3+, Eu 3+, and Yb 3+; n = 3, 6, and 8) is studied by molecular dynamics simulations in two room-temperature ionic liquids which are “neutral” (Lewis acidity): [BMI][PF 6 ] composed of 1-butyl-3-methyl-imidazolium +,PF 6 - ions and [EMI][TCA] composed of 1-ethyl-3-methyl-imidazolium +,AlCl 4 - ions. The simulations reveal the importance of the MCl 6 3- complex, commonly observed in solid-state structures and in chloride-containing solutions. This contrasts with the gas phase where, according to QM calculations, MCl 6 3- is unstable toward the dissociation of 1 to 2 Cl - ions. In the two studied solvents, the MCl n 3- n complexes with n = 3 to 6 remain bound during the simulation, while MCl 8 5- complexes lose two Cl - anions and form MCl 6 3- . The only exception concerns LaCl 8 5- which dissociates to LaCl 7 4- in [EMI][TCA] solution. The first shell of MCl 3, MCl 4 -, and MCl 5 2- is mainly completed by solvent anions (about 3, 2, 1 PF 6 - and 4, 3, 1 AlCl 4 - anions, respectively for M = Eu), while the three studied MCl 6 3- complexes are surrounded by a cage of 9−10 BMI + or EMI + cations, into which some solvent anions can be inserted. The results are important for our understanding of the solution state of trivalent actinide or lanthanide ions in room temperature ionic liquids.