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

We report a molecular dynamics study on the EuBr n 3‐ n complexes ( n =0 to 6) formed upon complexation of Br − by Eu 3+ in the [BMI][PF 6 ], [BMI][Tf 2 N] and [MeBu 3 N][Tf 2 N] ionic liquids (ILs), to compare the effect of the IL anion (PF 6 − versus Tf 2 N − ), the IL cation (BMI + versus MeBu 3 N + ) and the “IL humidity” on their solvation and stability. In “dry” solutions all complexes remain stable and the first coordination shell of Eu 3+ is purely anionic (Br − and IL anions), surrounded by IL cations (BMI + or MeBu 3 N + ions). Long range “onion type” solvation features (up to 20 Å from Eu 3+ ), with alternating cation‐rich and anion‐rich solvent shells, are observed around the different complexes. The comparison of gas phase‐optimized structures of EuBr n 3‐ 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][PF 6 ] IL, Br − ligands are displaced by water, to finally form Eu(H 2 O) 9 3+ . In the less “humid” [BMI][Tf 2 N], the EuBr n 3‐ n complexes do not dissociate and coordinate at most 1–2 H 2 O molecules. We also calculated the free‐energy profiles (Potential of Mean Force calculations) for the stepwise complexation of Br − , and found significant solvent effects. EuBr 6 3− is predicted to form in both [BMI][PF 6 ] and [BMI][Tf 2 N], but not in [MeBu 3 N][Tf 2 N], mainly due to weaker interactions with the cationic solvation shell. First steps are found to be more exergonic in the PF 6 − ‐ than in the Tf 2 N − ‐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.