CH-Directed anion-π Interactions in the crystals of pentafluorobenzyl- substituted ammonium and pyridinium salts

摘要

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. Hexafluoro-phosphate is introduced as the anion by salt metathesis. In the case of the ammonium salt 4, water co-crystallisa-tion seems to suppress effective anion-π interactions of bromide with the elec-tron-deficient aromatic system, where-as with salts 5 and 6 such interactions are observed despite the presence of water. However, due to asymmetric hydrogen-bonding interactions with am-monium side chains, the anion of 5 is located close to the rim of the penta-fluorophenyl group (η~1 interaction). In 6 the CH-anion hydrogen bonding is more symmetric and fixes the anion on top of the ring (η~6). A similar struc-ture-controlling effect is observed in case of the 1, 4-diazabicyclo-[2.2. 2]octane derivatives 7. Here the position of the anion (Cl, Br, I) is shift-ed according to the length of the weak CH-halide interaction. The hexafluoro-phosphate 7d reveals that this " non-co-ordinating" anion can be located on top of an aromatic it system. In the methyl-substituted pyridinium salts 9 and 10 different locations of the bro-mide anions with respect to the π system are observed. This is due to dif-ferent conformations of the mono-versus disubstituted pyridine, which leads to different directions of the weak, but structurally important, H_(Me)-Br bonds.