Synthesis, characterization of calix[5]azulene and its complexation with pyridinium salts

Abstract Calix[5]azulene (CA5) was synthesized from bis(guaiazulene-3-yl)methane and azulene in the presence of sulfuric acid. Three crystal structures were collected from different solvent systems, of which the conformation of CA5 molecules resemble to each other. The complexation behavior of calixazulenes with using pyridinium salts were investigated and the respective binding constants were determined using non-linear fitting of the spectral data from UV-vis absorption titration studies. CA5 showed higher affinity toward pyridinium salts compared to calix[4]azulene, and unusual preference to bromide salt over chloride was observed. In addition, more obvious substituent effect on the pyridine ring was observed for the complexation of CA5. The possible structure of host-guest complexes was studied by DFT calculation.