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

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.