Monitoring fluoride binding in DMSO: why is a singular binding behavior observed?

Fluoride recognition by molecular receptors is attracting much interest in the scientific community and a variety of approaches are used to achieve recognition with high affinity and selectivity. Most of the synthetic systems reported have been extensively studied in organic solvents by using tetrabutylammonium fluoride (TBAF) as the source of fluoride. In many cases, titration behaviors are observed that cannot be ascribed to a classical 1:1 binding isotherm. By using UV/Vis, 19F NMR, and 1H NMR spectroscopy we have been able to highlight that the equilibrium between the fluoride and the corresponding bihalide ion, HF2-, which is inevitably generated in situ due to trace amounts of water, can be at the origin of this singular behavior. Our results highlight that when undertaking titrations with fluoride in DMSO, the data can be affected by the fluoride-bihalide equilibrium and that the latter species can even be the dominating species at low TBAF concentrations.