Geometrically diverse anions in anion-π interactions

The role of different anion geometries in anion-pi interactions is discussed. The chemistry described herein is different to the interaction of spherical cations with aromatics. The influence of different geometries makes selective anion recognition more complicated than respective cation sensing. The present structural study reveals attractive interactions between pentafluorophenyl units and geometrically diverse anions (linear, trigonal planar, tetrahedral and octahedral). Due to the electrostatic nature of anion-pi interactions, the anion geometry seems to be irrelevant. The size of the anion controls the relative orientation of the anion and the pi system (e. g. in compounds 1-3). The dimeric solid-state structure of ammonium tetrafluorophenolate betaine 4 shows pi-pi as well as anion-pi interactions. In the solid-state structure of 5, the linear BrIBr anion is panelled by three pentafluorophenyl units.