24π Core‐Modified Nonfused Pentaphyrin with Möbius Aromaticity.

Syntheses of core‐modified 24π pentaphyrin 7 and 20π homoporphyrin 8 are reported. The freebase form of 7 exhibits optical and 1H NMR spectroscopic behavior corresponding to a 4nπ nonaromatic topology at 298 K. However, upon lowering the temperature to 188 K, 7 exhibits weak aromaticity because of a small conformational change. Protonation of the imine pyrrolic nitrogen atoms results in sharpening of the absorption band with more than 100 nm red shift and two‐fold increase in ϵ values. The 1H NMR spectra show large shielding and deshielding of specific protons accompanied by twisting of pyrrole and thiophene rings from the mean plane of the macrocycle. These observations suggest a major structural change upon protonation. The single‐crystal X‐ray structure of protonated 7 a reveals a severe twisting of the macrocycle, especially at the ethene bridge, and the two thiophene rings adjacent to the ethene bridge are pointed above and below the plane of the ethene bridge (49.07° and 49.08°). The nucleus‐independent chemical shift (NICS) values calculated at the center of the macrocycle for the freebase and protonated derivatives of 7 a and 7 b are −5.6, −8.3 ppm and −5.3, −8.4 ppm, respectively. The anisotropy‐induced current density (AICD) plots suggest a clockwise circulation of π electrons. Taken together, the changes occurring in the optical spectra, 1H NMR spectra, single‐crystal X‐ray structure and NICS calculation reflect a Möbius aromatic topology for protonated 7. However, the homoporphyrin 8 remains nonaromatic both in the freebase and protonated form. Nonaromatic–aromatic: The protonation‐triggered conversion of a core‐modified 24π nonaromatic pentaphyrin (see graphic) to a 24π Möbius aromatic pentaphyrin is described. The aromaticity of the protonated form is revealed by 1H NMR spectroscopy, X‐ray structure analysis and nucleus‐independent chemical shift (NICS) calculations. [ABSTRACT FROM AUTHOR]