meso-substituted aromatic 34π core-modified octaphyrins: Syntheses, characterization and anion binding properties

Modified octaphyrins with 34π electrons have been synthesized and characterized following a simple synthetic methodology. An acid-catalyzed α,α coupling of tetrapyrranes containing furan, thiophene and selenophene rings resulted in the formation of the respective octaphyrins in relatively good yield. Solution studies by ¹H NMR and 2D NMR methods and single crystal X-ray structural characterization reveal an almost flat structure with two heterocyclic rings inverted. Specifically, in 14 two selenophene rings (one on each biselenophene unit) are inverted while in 15 two furan rings (one on each bifuran unit) are inverted when the meso substituent are mesityl groups. On changing the mesityl substituent to m-xylyl group as in 19, the location of ring inversion shifts to pyrrole rings (one on each bipyrrole unit) indicating the dependence of structure on the meso substituents. UV/Vis studies, both in freebase and protonated forms reveal typical porphyrinic character and the aromatic nature of the octaphyrins. The Δδ values evaluated by ¹H NMR spectroscopy also support their aromatic nature. The protonated forms of octaphyrins bind TFA anion in a 1:2 ratio. The TFA anions are located one above and below the plane of the octaphyrin macrocycle and they are held by weak electrostatic N-H-O interactions similar to that observed for protonated rubyrins. However, in the present case, there is an additional non-electrostatic C-H-O interaction involving β-CH of the inverted heterocyclic ring and the carbonyl oxygen of the TFA. Furthermore, inter molecular interactions between the C-H of the meso-mesityl group and the fluorine of CF3 group of bound TFA leads to the formation of one-dimensional supramolecular arrays with interplanar distance of 13 Å between two octaphyrins.