Complexation of fluoride anion and its ion pairs with alkali metal cations by tetra-substituted lower rim calix[4]arene tryptophan derivative

Abstract

Selective binding of fluoride anion and its alkali metal cation ion pairs by a fluorescent calix[4]arene (L) derivative bearing tryptophan moieties was studied in acetonitrile at 25 °C. It was found that LF− and LF22− complexes were formed and their stability constants were determined by means of UV and 1H NMR titrations. Both amide and indole NH groups were involved in the stabilisation of the fluoride complexes, i.e. L afforded two anion-binding sites. 1H NMR titrations provided evidence of simultaneous complexation of both Na+ cation and fluoride anion resulting in the formation of a ternary NaLF complex. In this ion pair complex, the Na+ cation was most probably bound primarily by the calixarene ether and amide oxygen atoms, whereas the indole NH groups interacted with the fluoride ion. A highly favourable ion pairing between Na+ and F– in acetonitrile was studied using potentiometric measurements. The results pointed out the importance of fluoride pairing with alkali metal cations in aprotic organic solvents and the necessity of taking these reactions into account in the course of speciation studies of such solutions.