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

The trisbenzimidazolium cyclophane receptor 2(3+) incorporates the F-anion in MeCN solution, which was inferred by spectrophotometric and H-1 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 center dot center dot center dot F H-bonds. No other anion is encapsulated by 2(3+) for reasons of size. The parent trisbenzimidazolium tripodal receptor 4(3+) 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 center dot center dot center dot Br](2+) and [4 center dot center dot center dot 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 center dot center dot center dot F](2+) complex with respect to [4 center dot center dot center dot F](2+) and in its resistance to excess F-. In fact, on addition of excess fluoride, the tripodal [4 center dot center dot center dot F](2+) complex decomposes with deprotonation of a C-H fragment and formation of the very stable HF2- complex.