Induction‐Driven Stabilization of the Anion–π 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-π interactions between anions and electron-deficient (π-acidic) aromatics indicates that electron-rich (π-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-π interaction between electron-rich alkylbenzene rings and a fluoride ion in CH(3)CN. The cyclophane cavity bridged with three naphthoimidazolium groups selectively complexes a fluoride ion by means of a combination of anion-π interactions and (C-H)(+)···F(-)-type ionic hydrogen bonds. (1)H NMR, (19)F 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-π interaction between F(-) and triethylbenzene.

Putting the Anion into the Cage – Fluoride Inclusion in the Smallest Trisimidazolium Macrotricycle

“…[16] Anion-π interactions could not be established by 3a 3+ , which contained plain phenyl caps and, in the absence of such an additional energy contribution, anion inclusion could not take place.…”

“…[16] On the basis of titration experiments in MeCN solution (fluorescence emission, 1 H and 13 C NMR spectroscopy, isothermal calorimetry), it was assessed that 3b 3+ forms a 1:1 complex with fluoride, in which F -is included into the cage and partakes in three equivalent H-bonds from the three C-H fragments. On the other hand, 3a 3+ does not include fluoride, but interacts electrostatically with two exterior F -ions to form 1:1 and 1:2 ion pairs in sequence, according to two stepwise equilibria.…”

Chromogenic and fluorogenic chemosensors and reagents for anions. A comprehensive review of the years 2010–2011

“…146), bearing three naphthoimidazolium moieties, showed a broad emission at 474 nm (excitation at 326 nm). 191 F NMR and isothermal titration calorimetry (ITC) studies. Chakraborty and coworkers designed different receptors (305-308, see Fig.…”

Time-Dependent Density Functional Theory Study on a Fluorescent Chemosensor Based on C–H•••F Hydrogen-Bond Interaction

“…It has been used as ionic liquids and precursors to stable carbenes [12]. As mentioned above, the C-H moiety between two nitrogen atoms in imidazolium cation is likely to form H-bond with electronegative atoms or anions [13][14][15][16]. The H-bond could be strengthened or weakened upon excitation [17][18][19] and can be monitored by fluorescent spectra.…”