Solvation of Ln((III)) lanthanide cations in the [BMI][SCN], [MeBu(3)N][SCN], and [BMI](5)[Ln(NCS)(8)] ionic liquids: a molecular dynamics study.
We report a molecular dynamics study on the solvation of trivalent lanthanide cations Ln((III)) (Ln = La, Eu and Yb) in the [BMI][SCN] and [MeBu(3)N][SCN] ionic liquids (ILs), based respectively on 1-methyl-3-butylimidazolium (BMI(+)) and tributylmethylammonium (MeBu(3)N(+)) cations, and on thiocyanate (SCN(-)) anions. For this purpose we first derive a force field representation of SCN(-) that simultaneously describes the [BMI][SCN] liquid and SCN(-) as a ligand for Ln((III)) ions and, in particular, for the energy difference between N- vs S-coordination, as compared to QM calculated values. In liquid simulations, we compare different initial states where the solute is either the "naked" Ln((III)) ion or its anionic Ln(NCS)(8)(5-) complex. In all cases, the first solvation shell of Ln((III)) is found to be purely anionic, with 6 to 8 N-coordinated ligands, depending on the nature of Ln((III)) and the immersed solute. This first shell is surrounded by 13 - 14 BMI(+) or 8 - 9 MeBu(3)N(+) cations, leading to an "onion type" solvation of Ln((III)). The comparison of gas phase optimized structures (that are all unstable from n = 5 NCS(-) ligands) to those observed in ILs points to the importance of solvation forces on the nature of the Ln((III)) complex, with a marked contribution of the IL cation. A given Ln(NCS)(n)(3-n) complex is found to be better stabilized by the imidazolium than by the ammonium based IL. Furthermore, according to free energy PMF (potential of mean force) calculations, the imidazolium based liquid favors somewhat higher coordination numbers (CNs), compared to the ammonium based IL. For instance, the coordination of an eight SCN(-) ligand to Eu(NCS)(7)(4-) is favored in the [BMI][SCN] liquid, but not in [MeBu(3)N][SCN]. For the La((III)) and Yb((III)) cations, the CNs are the same in both liquids (8 and 7, respectively), but the free energy differences between the two types of complexes differ markedly. The final part of the paper is devoted to the [BMI](5)[Ln(NCS)(8)] pure ILs, based on BMI(+) as cation and on the Ln(NCS)(8)(5-) complex (Ln = La((III)) and Yb((III))) as anionic component. In these liquids, Ln((III)) CNs are found to be similar to those found in the [BMI][SCN] solutions, and the dynamics is characterized by a fluid behavior of the BMI(+) ions diffusing around a quasi frozen network of anionic complexes.
Institut de Chimie, Université Louis Pasteur, 4 rue B. Pascal, Strasbourg 67 000, France.
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Chaumont
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Wipff