Computational Study of Halogen-Halogen Interactions in Polyhalide Ionic Liquids

Recent years have seen many specific applications of polyhalide ionic liquids (ILs) such as oxidizing solvents for metals and alloys, immersion fluids for optical mineralogy, and electrolyte components for dye-sensitized solar cells. In this work, interhalogen interactions in a set of polyhalide ILs composed of polyhalide anions, [X3]−, [X5]− and [X7]− (X = I or Br), with two typical cations, tetramethylammonium [NMe4]+ and 1,3-dimethylimidazolium [DMIM]+, were thoroughly studied from a computational point of view. In addition, a halogen-bonded supramolecular anion, [C6F13-I∙∙∙I∙∙∙I-C6F13]−, was also taken into account for comparison. Unlike those in bare polyhalide ions, halogen-halogen interactions in ionic pairs for the investigated ILs are somewhat asymmetric caused by the interactions between the cations and the anions. Most interhalogen contacts in ionic pairs have some covalent content, while I∙∙∙I interactions in the complexes of the supramolecular anion are purely noncovalent. In general, there are two classes of interhalogen bonds in ionic pairs: one class with longer X∙∙∙X distances shows primarily ionic character, while the other with shorter distances has a larger degree of covalency, i.e. intermediate ionic/covalent nature.