Comparison of halide receptors based on H, halogen, chalcogen, pnicogen, and tetrel bonds.

A series of halide receptors are constructed and the geometries and energetics of their binding to F, Cl, and Br assessed by quantum calculations. The dicationic receptors are based on a pair of imidazolium units, connected via a benzene spacer. The imidazoliums each donate a proton to a halide in a pair of H-bonds. Replacement of the two bonding protons by Br leads to binding via a pair of halogen bonds. Likewise, chalcogen, pnicogen, and tetrel bonds occur when the protons are replaced, respectively, by Se, As, and Ge. Regardless of the binding group considered, F is bound much more strongly than are Cl and Br. With respect to the latter two halides, the binding energy is not very sensitive to the nature of the binding atom, whether H or some other atom. But there is a great deal of differentiation with respect to F, where the order varies as tetrel > H ∼ pnicogen > halogen > chalcogen. The replacement of the various binding atoms by their analogues in the next row of the periodic table enhances the fluoride binding energy by 22-56%. The strongest fluoride binding agents utilize the tetrel bonds of the Sn atom, whereas it is I-halogen bonds that are preferred for Cl and Br. After incorporation of thermal and entropic effects, the halogen, chalcogen, and pnicogen bonding receptors do not represent much of an improvement over H-bonds with regard to this selectivity for F, even I which binds quite strongly. In stark contrast, the tetrel-bonding derivatives, both Ge and Sn, show by far the greatest selectivity for F over the other halides, as much as 10, an enhancement of six orders of magnitude when compared to the H-bonding receptor.