Competitive Halide Binding by Halogen Versus Hydrogen Bonding: Bis-triazole Pyridinium

The binding of F-, Cl-, Br-, and I- anions by bis-triazole-pyridine (BTP) was examined by quantum chemical calculations. There is one H atom on each of the two triazole rings that chelate the halide via Hbonds. These H atoms were replaced by halogens Cl, Br, and I, thus substituting Hbonds by halogen bonds. I substitution strongly enhances the binding; Br has a smaller effect, and Cl weakens the interaction. The strength of the interaction is sensitive to the overall charge on the BTP, rising as the binding agent becomes singly and then doubly positively charged. The strongest preference of a halide for halogenated as compared to unsubstituted BTP, as much as several orders of magnitude, is observed for I-. Both unsubstituted and I-substituted BTP could be used to selectively extract F- from a mixture of halides.