Halide Anions as Halogen Bond Acceptors

A survey of the Cambridge Structural Database, followed by a detailed statistical analysis, has been performed to study the frequency of occurrence and structural properties of halogen bonds with halide anions as halogen bond acceptors. The results of these studies confirm that halogen bonds C–X···F– are very rare in the crystal structure in comparison with contacts C–X···Y–, for Y = Cl, Br, or I. To verify the hypothesis that the strength and nature of halogen bonds with fluoride anions as acceptors differ significantly from the properties of analogous halogen bonds with other halides, extensive computational analyses have been performed for archetypal halogen-bonded model complexes F3C–X···Y– (X, Y = F, Cl, Br, and I). To describe the trends in halogen bonding strength for the consistent set of data and to understand the driving force behind them, a canonical energy decomposition analysis, together with the quantitative Kohn–Sham molecular orbital model and Voronoi deformation density charge analysis, has been employed. The results of the quantum chemical calculations show that fluoride anions form much stronger halogen bonds than do other halides. This phenomenon is traced to more pronounced electrostatic and stabilizing orbital interactions in the case of F–. Additionally, a linear trend between the difference in orbital energies after the formation of complexes F3C–X···Y–, and the orbital interaction energies is demonstrated.