24π Core-Modified Nonfused Pentaphyrin with Möbius Aromaticity

Chemistry : a European Journal
Arindam GhoshTavarekere K Chandrashekar

Abstract

Syntheses of core-modified 24π pentaphyrin 7 and 20π homoporphyrin 8 are reported. The freebase form of 7 exhibits optical and 1 H 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 1 H 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 densi...Continue Reading

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Jun 9, 2016·Chemistry : a European Journal·Arindam GhoshTavarekere K Chandrashekar
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Citations

Dec 21, 2019·Chemical Communications : Chem Comm·Pinky YadavZeev Gross
Jan 17, 2020·ChemPlusChem·Bulti Abdisa KerayuChen-Hsiung Hung
Nov 9, 2021·Chemical Society Reviews·Rima SenguptaTavarekere K Chandrashekar

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