Simplified bicyclic cage-type molecule as a C 3 -symmetric host: X-ray and FTIR characterization of encapsulation of a nitrile molecule

We synthesized a new amine-type host molecule 2 by 2 steps, using 1,3,5-tris(aminomethyl)-2,4,6-trimethylbenzene and 2,6-pyridinedicarboxaldehyde as starting materials. Recrystallization of 2·6H2O from hot acetonitrile, hot propionitrile and hexane/benzonitrile solutions gave colorless crystals of MeCN@2·MeCN·2H2O, EtCN@2·EtCN, and 8H2O@2·PhCN. In the former two crystals, the nitrile compounds were captured inside of the cavity of 2. On the other hand, in the latter one, the benzonitrile was laid outside 2. Infrared spectral measurements of MeCN@2·MeCN·2H2O, EtCN@2·EtCN, and 8H2O@2·PhCN showed that absorption bands assignable to the C ≡ N stretching vibrations of nitrile compounds were observed at 2240, 2241, and 2226 cm−1 for MeCN@2·MeCN·2H2O, EtCN@2·EtCN, and 8H2O@2·PhCN, respectively. The former two peaks shifted to a lower energy region by 6–13 and 21–26 cm−1 than those of liquid and gas phases of MeCN and EtCN, respectively. That for 8H2O@2·PhCN shifted to a slightly lower region by 2 and 12 cm−1 those of liquid and gas states of PhCN, respectively, indicating that the outer benzonitrile molecule dose not so much interact with 2 in the crystal.