Putting the anion into the cage - fluoride inclusion in the smallest trisimidazolium macrotricycle

The trisbenzimidazolium cyclophane receptor 23+ incorporates the F- anion in MeCN soln., which was inferred by spectrophotometric and 1H NMR titrn. expts., with an assocn. const. 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 polyat. 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 obsd. in the higher thermodn. stability of the [2•••F]2+ complex with respect to [4•••F]2+ and in its resistance to excess F-. In fact, on addn. of excess fluoride, the tripodal [4•••F]2+ complex decomps. with deprotonation of a C-H fragment and formation of the very stable HF2- complex.