Solvation of fluoro and mixed fluoro/chloro complexes of EuIII 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 EuF6(3-) species. In solution the EUF10(7-) and EuF7(4-) complexes indeed loose, respectively, 4 and 1 F anion to form the EuF6(3-) 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 EuF7(4-) 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 A, hydrogen-bonded by imidazolium-CH..F- interactions. Simulations on the mixed EuF(n)Cl(6-n)3- complexes in solution and in gas phase also reveal the highest stability of EuF6(3-) compared to the mixed or the EuCl6(3-) 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 EuF6(3-) 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.