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

Abstract

We report a molecular dynamics study on the solvation of EuF_n⁽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 Å, hydrogen-bonded by imidazolium–CH⋯F⁻ interactions. Simulations on the mixed EuF_nCl_6−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.