Assembly of Effective Halide Receptors from Components. Comparing Hydrogen, Halogen, and Tetrel Bonds.

Receptors for halide anions are constructed based on the imidazolium unit and then replacing the H-bonding C-H group first by halogen-bonding C-I and then by tetrel-bonding C-SnH and C-SiF. Attaching a phenyl ring to any of these species has little effect on its ability to bind a halide, but incorporation of a second imidazolium to the benzene connector, forming a bidentate dicationic receptor, greatly enhances the binding. Addition of electron-withdrawing F atoms to each imidazolium adds a further increment. F consistently binds more strongly to the various receptor models than does Cl. Whereas replacement of the H atom on the imidazolium groups with the halogen-bonding I has an inconsistent perturbing effect, tetrel-bonding SnH significantly enhances the binding with either halide, and SiF even more so. Placement of the various complexes into aqueous solution reduces binding energies but the trends that occur in the gas phase are largely reproduced in water. The tetrel-bonding receptors are the most selective for F over Cl with an equilibrium ratio on the order of 10 for SnH and 10 for SiF. When combined with their strong halide binding, SiF-ImF-Bz-ImF-SiF bipodal receptors represent an optimal choice in terms of both binding strength and selectivity.