Superhalogens as Building Blocks of Ionic Liquids

Ionic liquids (ILs) are composed of large asymmetric organic cations with a wide range of anions. The simple anions, e.g., halogen, result in less stable ILs, and therefore, ILs generally consist of complex anions such as BF₄ and PF₆. These anions coincidently belong to a special class known as superhalogen. This prompted us to enquire whether the concept of superhalogen can be exploited to design new ILs. We study the complexes of 1-butyl-3-methylimidazolium (BMIM) cation and typical superhalogen (X) anions such as LiF₂, BeF₃, BO₂, NO₃, BF₄, and PF₆ including Cl using density functional theory and the quantum theory of atoms in molecule. Our ωB97XD/6-311++G(d,p) calculations suggest that the BMIM-X complexes are stable in which the charge transfer of 0.90-0.97 e takes place from BMIM to X. The charge-transferred tends to delocalize as the size of X increases. These complexes are stabilized by several ionic and/or covalent intramolecular interactions (H-bonds). The BMIM-X complexes prefer to dissociate into ionic fragments (BMIM⁺ + X^-) than neutral fragments (BMIM + X). The dissociation energy and energy gap of BMIM-X complexes are closely related to the electron affinity of superhalogens (X). These findings not only reveal the superhalogens as building blocks of ILs but also suggest the design of highly stable ILs by employing the superhalogens with higher electron affinities.