Self-Assembly of Azulene-Based Imine Cages: Synthesis and Supramolecular Organization

New organic cages built from the reaction of tris(2-aminoethyl)amine and azulene-1,3-dicarboxaldehyde through the Schiff base condensation reaction are reported. The equimolecular reaction of the dialdehyde with the tris-amine in the presence of trifluoroacetic acid yielded the cationic [1 + 1]2+ tetraimine cage, 1, whereas a 3:2 ratio led to the cyclic hexaimine cage, 2, both in the presence and in the absence of trifluoroacetic acid. The molecular structure determinations show pi-pi interactions established between azulene moieties, intracage in the tetraimine structure and intercage in the cyclic hexaimine compound. The cryptand type hexaimine cyclic cage can accommodate silver cations through a template synthetic approach, resulting in a disilver-metallacage. The complexation induces significant changes in the conformation of the macrobicycle from eclipsed to a staggered orientation of the bridgehead N-CH2- bonds. Furthermore, the metallacage assembles into a supramolecular network with a honeycomb topology via a pi-pi stacking interaction between azulene moieties. The hexaimine cage functions as a halogen bond acceptor, forming halogen-bonded chains with triiodoperfluorobenzene. Owing to the azulene motif, these cages show fluorescent properties, especially in the solid state, the intensity and position of the emission peaks depending on the molecular environment and solid-state packing of the discrete molecules.