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.
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