Induction-Driven Stabilization of the Anion-pi Interaction in Electron-Rich Aromatics as the Key to Fluoride Inclusion in Imidazolium-Cage Receptors

Abstract

Intermolecular interactions that involve aromatic rings are key processes in both chemical and biological recognition. It is common knowledge that the existence of anion-pi interactions between anions and electron-deficient (it-acidic) aromatics indicates that electron-rich (pi-basic) aromatics are expected to be repulsive to anions due to their electron-donating character. Here we report the first concrete theoretical and experimental evidence of the anion-pi interaction between electron-rich alkylbenzene rings and a fluoride ion in CH3CN. The cyclophane cavity bridged with three naphthoimidazolium groups selectively complexes a fluoride ion by means of a combination of anion-pi interactions and (C-H)(+)center dot center dot center dot F--type ionic hydrogen bonds. H-1 NMR, F-19 NMR, and fluorescence spectra of 1 and 2 with fluoride ions are examined to show that only 2 can host a fluoride ion in the cavity between two alkylbenzene rings to form a sandwich complex. In addition, the cage compounds can serve as highly selective and ratiometric fluorescent sensors for a fluoride ion. With the addition of 1 equiv of F-, a strongly increased fluorescence emission centered at 385 nm appears at the expense of the fluorescence emission of 2 centered at 474 nm. Finally, isothermal titration calorimetry (ITC) experiments were performed to obtain the binding constants of the compounds 1 and 2 with F- as well as Gibbs free energy. The 2-F- complex is more stable than the 1-F- complex by 1.87 kcal mol(-1), which is attributable to the stronger anion-pi interaction between F- and triethylbenzene.