Tetrel Bonding as a Vehicle for Strong and Selective Anion Binding

Tetrel atoms T (T = Si, Ge, Sn, and Pb) can engage in very strong noncovalent interactions with nucleophiles, which are commonly referred to as tetrel bonds. The ability of such bonds to bind various anions is assessed with a goal of designing an optimal receptor. The Sn atom seems to form the strongest bonds within the tetrel family. It is most effective in the context of a -SnF₃ group and a further enhancement is observed when a positive charge is placed on the receptor. Connection of the -SnF₃ group to either an imidazolium or triazolium provides a strong halide receptor, which can be improved if its point of attachment is changed from the C to an N atom of either ring. Aromaticity of the ring offers no advantage nor is a cyclic system superior to a simple alkyl amine of any chain length. Placing a pair of -SnF₃ groups on a single molecule to form a bipodal dicationic receptor with two tetrel bonds enhances the binding, but falls short of a simple doubling. These two tetrel groups can be placed on opposite ends of an alkyl diamine chain of any length although SnF₃⁺NH₂(CH₂)_nNH₂SnF₃⁺ with n between 2 and 4 seems to offer the strongest halide binding. Of the various anions tested, OH⁻ binds most strongly: OH⁻ > F⁻ > Cl⁻ > Br⁻ > I⁻. The binding energy of the larger NO₃⁻ and HCO₃⁻ anions is more dependent upon the charge of the receptor. This pattern translates into very strong selectivity of binding one anion over another. The tetrel-bonding receptors bind far more strongly to each anion than an equivalent number of K⁺ counterions, which leads to equilibrium ratios in favor of the former of many orders of magnitude.