26π aromatic core-modified hexaphyrins: syntheses, characterization, and structural diversities

Abstract

Synthesis and characterization of several 26π core-modified hexaphyrins are reported. The synthetic methodology involved a well-known acid-catalyzed MacDonald-type condensation reaction of the required tripyrrane with electron deficient pentafluorobenzaldehyde. The nature of the product and yield depends on the nature of the acid catalyst and its concentration. Dioxahexaphyrin 9 was isolated only when 0.5 equiv of TFA was used as a catalyst, while dithiahexaphyrin 10 and diselenahexaphyrin 11 were formed with TFA, PTSA, and even in the absence of catalyst. The detailed ¹H and 2-D COSY as well as HSQC experiments reveal the solution structure as well as the conformational mobility of hexaphyrins. In the tetracationic state, 10 and 11 exhibit a four heterocyclic ring inverted structure, while only two completely inverted heterocyclic rings were observed for 9. The other four heterocyclic rings are only partially inverted in 9. All the hexaphyrins reported here show aromatic character inferred from large Δδ values (difference in chemical shift between the most shielded and the most deshielded protons). Electronic absorption spectral studies also support the conformational changes observed upon protonation.