28π Non‐Fused Hexaphyrin: Synthesis, Protonation Triggered Möbius Aromaticity and Structural Properties

Abstract

Synthesis of core-modified 28π non-fused hexaphyrin is reported. The optical and 1H NMR spectral analysis reveals the non-aromatic nature of the macrocycle in its freebase form at 298 K. However, the non-aromatic nature of the macrocycle remains unaltered upon lowering the temperature. The structural elucidations for the freebase macrocycle also justify the non-aromaticity of the freebase 8, where the thiophene units of the 1,2 diphenyl 1,2 dithienyl ethene moiety is deviated maximum from the overall macrocyclic framework. Protonation on the imino pyrrolic nitrogens results in single–crystal X-ray structures of 8 which show essential distortion in macrocyclic framework needed for the effective π-electron delocalization, attaining Möbius aromaticity. The Nucleus Independent Chemical Shift (NICS) values for the freebase and protonated macrocycles are −2.0 ppm and −5.6 ppm. The Anisotropy Induced Current density shows the clockwise delocalization of current density vectors. Overall, the spectral, structural and theoretical analysis suggests nonaromatic in freebase and Möbius aromatic nature of the protonated hexaphyrin.