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-SnH₃ and C-SiF₃. 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 SnH₃ significantly enhances the binding with either halide, and SiF₃ 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 10¹⁴ for SnH₃ and 10²⁸ for SiF₃. When combined with their strong halide binding, SiF₃-ImF₃-Bz-ImF₃-SiF₃⁺² bipodal receptors represent an optimal choice in terms of both binding strength and selectivity.