Halide anion solvation and recognition by a macrotricyclic tetraammonium host in an ionic liquid: a molecular dynamics study

Abstract

We report a molecular dynamics study of halide anions X⁻ and their inclusion complexes X⁻ ⊂ L⁴⁺ with a macrotricyclic tetrahedral host L⁴⁺ built from four quaternary ammonium sites, in an ionic liquid (IL) based on the 1-butyl-3-methylimidazolium (BMI⁺) cation and the PF₆⁻ anion. The “dry” and “humid” forms of the [BMI][PF₆] IL are compared, showing the importance of IL ions in the “dry” IL and, in some cases, of water molecules in the “humid” IL. In the “dry” IL the F⁻, Cl⁻, Br⁻ and I⁻ uncomplexed halides are surrounded by 4–5 BMI⁺ cations whose binding mode evolves from hydrogen bonding to facial coordination along this series. Solvent humidity has the largest impact on the solvation of F⁻ whose first shell BMI⁺ cations are all displaced by H₂O molecules, while the first solvation shell of Cl⁻, Br⁻ and I⁻ comprises 3–4 BMI⁺ cations plus ca. 4 H₂O molecules. The solvation of the L⁴⁺ host and of its X⁻ ⊂ L⁴⁺ complex mainly involves PF₆⁻ anions in the “dry” IL, and additional H₂O molecules in the “humid” IL. The question of anion binding selectivity is addressed by free energy perturbation calculations which predict that, in the “dry” liquid, F⁻ is preferred over Cl⁻, Br⁻ and I⁻, which contrasts with the aqueous solution where L⁴⁺ is selective for Cl⁻. In the “humid” liquid however, there is no F⁻/Cl⁻ discrimination, showing the importance of “small amounts” of water on the complexation selectivity.