Bromide Complexation by the EuIII Lanthanide Cation in Dry and Humid Ionic Liquids: A Molecular Dynamics PMF Study

Authors

  • Dr. Alain Chaumont,

    1. Laboratoire MSM, UMR CNRS 7177, Institut de Chimie, 1 rue B. Pascal, 67 000 Strasbourg (France)
    Search for more papers by this author
  • Prof. Dr. Georges Wipff

    Corresponding author
    1. Laboratoire MSM, UMR CNRS 7177, Institut de Chimie, 1 rue B. Pascal, 67 000 Strasbourg (France)
    • Laboratoire MSM, UMR CNRS 7177, Institut de Chimie, 1 rue B. Pascal, 67 000 Strasbourg (France)
    Search for more papers by this author

Abstract

We report a molecular dynamics study on the EuBrn3-n complexes (n=0 to 6) formed upon complexation of Br by Eu3+ in the [BMI][PF6], [BMI][Tf2N] and [MeBu3N][Tf2N] ionic liquids (ILs), to compare the effect of the IL anion (PF6 versus Tf2N), the IL cation (BMI+ versus MeBu3N+) and the “IL humidity” on their solvation and stability. In “dry” solutions all complexes remain stable and the first coordination shell of Eu3+ is purely anionic (Br and IL anions), surrounded by IL cations (BMI+ or MeBu3N+ ions). Long range “onion type” solvation features (up to 20 Å from Eu3+), with alternating cation-rich and anion-rich solvent shells, are observed around the different complexes. The comparison of gas phase-optimized structures of EuBrn3-n complexes (that are unstable for n=5 and 6) with those observed in solution points to the importance of solvation forces on the nature of the complex, with a higher stabilization by imidazolium- than by ammonium-based dry ILs. Adding water to the IL has different effects, depending on the IL. In the highly hygroscopic [BMI][PF6] IL, Br ligands are displaced by water, to finally form Eu(H2O)93+. In the less “humid” [BMI][Tf2N], the EuBrn3-n complexes do not dissociate and coordinate at most 1–2 H2O molecules. We also calculated the free-energy profiles (Potential of Mean Force calculations) for the stepwise complexation of Br, and found significant solvent effects. EuBr63− is predicted to form in both [BMI][PF6] and [BMI][Tf2N], but not in [MeBu3N][Tf2N], mainly due to weaker interactions with the cationic solvation shell. First steps are found to be more exergonic in the PF6- than in the Tf2N-based IL. Molecular dynamics (MD) comparisons between ILs and classical solvents (acetonitrile and water) are also reported, affording good agreement with the experimental observations of Br complexation by trivalent lanthanides in these classical solvents.

Ancillary