M3+ lanthanide chloride complexes in "neutral" room temperature ionic liquids:: A theoretical study

The solvation of MCln3-n lanthanide chloride salts (La3+, Eu3+, and Yb3+; n = 3, 6, and 8) is studied by molecular dynamics simulations in two room-temperature ionic liquids which are "neutral" (Lewis acidity): [BMI][PF6] composed of 1-butyl-3-methyl-imidazolium(+),PF6- ions and [EMI][TCA] composed of 1-ethyl-3-methyl-imidazolium(+),AlCl4- ions. The simulations reveal the importance of the MCl63- complex, commonly observed in solid-state structures and in chloride-containing solutions. This contrasts with the gas phase where, according to QM calculations, MCl63- is unstable toward the dissociation of 1 to 2 Cl- ions. In the two studied solvents, the MCln3-n complexes with n = 3 to 6 remain bound during the simulation, while MCl85- complexes lose two Cl- anions and form MCl63-. The only exception concerns LaCl85- which dissociates to LaCl74- in [EMI][TCA] solution. The first shell of MCl3, MCl4-, and MCl52- is mainly completed by solvent anions (about 3, 2, 1 PF6- and 4, 3, 1 AlCl4- anions, respectively for M = Eu), while the three studied MCl63- 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.