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.