CH-Directed Anion-π Interactions in the Crystals of PentafluorobenzylSubstituted Ammonium and Pyridinium Salts

Simple pentafluorobenzyl-substituted ammonium and pyridinium salts with different anions can be easily obtained by treatment of the parent amine or pyridine with the respective pentafluorobenzyl halide. Hexafluorophosphate is introduced as the anion by salt metathesis. In the case of the ammonium salt 4, water co-crystallisation seems to suppress effective anion it interactions of bromide with the electron-deficient aromatic system, whereas with salts 5 and 6 such interactions are observed despite the presence of water. However, due to asymmetric hydrogen-bonding interactions with ammonium side chains, the anion of 5 is located close to the rim of the pentafluorophenyl group (ti' interaction). In 6 the CH anion hydrogen bonding is more symmetric and fixes the anion on top of the ring (16). A similar structure-controlling effect is observed in case of the 1,4-diazabicyclo[2.2.2loctane derivatives 7. Here the position of the anion (Cl, Br, I) is shifted according to the length of the weak CH halide interaction. The hexafluorophosphate 7d reveals that this "non-coordinating" anion can be located on top of an aromatic it system. In the methyl-substituted pyridinium salts 9 and 10 different locations of the bromide anions with respect to the it system are observed. This is due to different conformations of the monoversus disubstituted pyridine, which leads to different directions of the weak, but structurally important, Hme Br bonds.