6-Azahemiporphycene: A new member of the porphyrinoid family

The reaction of 5,10,15-triarylcorrole with 4-amino-4H-1,2,4-triazole provides another example of corroie ring expansion to give the corresponding 6-azahemiporphycene, a novel porphyrin analogue. The facile oxidation of the corroie ring is a required step for the ring expansion and for this reason the reaction fails in the case of corroles bearing meso-phenyl groups carrying electron-withdrawing substituents. Sterlc requirements also limited the scope of the reaction, which is not successful In the case of 2,6-disubstituted meso-aryl corroles. The occurrence of an initial oxidation is further supported by formation of the 6-azahemiporphycene derivative when the reaction is carried out under the same conditions, using a 5- or a 10-isocorrole as starting material. 1H NMR spectra and X-ray crystal characterization of 6-azahemlporphycene evidenced the presence of an Intramolecular N-H • • • N hydrogen bond In the Inner core of the macrocycle, while photophysical characterization confirmed the aromatic character of the novel macrocycle, showing an intense Soret-like band around 410 nm in the absorption spectrum. The fluorescence emission Is very modest, and 6-azahemiporphycene showed higher photostabllity than the corresponding corroie species. Different metal complexes of 6-azahemlporphycene were prepared following synthetic protocols usually exploited for the preparation of metalloporphyrlns, demonstrating good coordination properties for the macrocycle. Both the free-base and metal derivatives were characterized by cyclic voltammetry and spectroelectrochemlstry In dichloromethane and benzonitrile. To further detail the behavior of this novel macrocycle, density functional theory (DFT) calculations were carried out on the basic structure of 6-azahemiporphycene with the aim of assessing aromaticlty and tautomerism, as well as calculating its stability with respect to the 5-aza isomer. © 2009 American Chemical Society.