Solvation of fluoro and mixed fluoro/chloro complexes of Eu-III in the [BMI][PF6] room temperature ionic liquid. A theoretical study

We report a molecular dynamics study on the solvation of EuFn(3-n) complexes in the [BMI][PF6] ionic liquid, composed of 1-butyl-3-methyl-imidazolium(+) cations and PF6- anions. It is found that the most fluorinated complex in the liquid should be the EuF63- species. In solution the EuF107- and EuF74- complexes indeed loose, respectively, 4 and 1 F- anion to form the EuF63- complex, while the first solvation shell of the less fluorinated complexes (n = 1 to 5) is completed with 5 to 1 PF6- anions to form an octahedral first shell around Eu3+. There is one case (simulations with a small'' F- model) where the EuF74- complex remains stable, and cannot therefore be fully precluded. The anionic complexes are embedded in a cage formed by 6-9 BMI+ cations at ca. 8 angstrom, hydrogen-bonded by imidazolium-CH...F- interactions. Simulations on the mixed EuFnCl6-n3- complexes in solution and in gas phase also reveal the highest stability of EuF63- compared to the mixed or the EuCl63- complexes. This is confirmed by free energy perturbation calculations and results from the stronger coordination of F-, compared to Cl- ligands, as well as from better solvation of the fluoro complexes by the ionic liquid. In the gas phase, however, QM and MM calculations indicate that EuF63- is unstable towards the dissociation of 1 to 2 F- ions, which points to the importance of environment and solvation forces on the stability of this octahedrally coordinated lanthanide complex.