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

Abstract

AbstractThe trisbenzimidazolium cyclophane receptor 23+ incorporates the F– anion in MeCN solution, which was inferred by spectrophotometric and 1H NMR titration experiments, with an association constant log K > 7. On the basis of geometric considerations, it is assumed that F– lies in the middle of the triangle 3C, whose vertices are the carbon atoms of the three imidazolium C–H fragments, and profits from three C–H···F H‐bonds. No other anion is encapsulated by 23+ for reasons of size. The parent trisbenzimidazolium tripodal receptor 43+ does not exert size exclusion selectivity and forms 1:1 complexes not only with F–, but also with other mono‐ (Br–) and polyatomic anions (NO3–). X‐ray diffraction studies on [4···Br]2+ and [4···NO3]2+ complexes indicated that, due to the steric restraints of the tripodal receptor, the anion is not positioned in the middle of the 3C triangle but stays below it, profiting from H‐bonds. Cage effects are observed in the higher thermodynamic stability of the [2···F]2+ complex with respect to [4···F]2+ and in its resistance to excess F–. In fact, on addition of excess fluoride, the tripodal [4···F]2+ complex decomposes with deprotonation of a C–H fragment and formation of the very stable HF2– complex.