24π Core‐Modified Nonfused Pentaphyrin with Möbius Aromaticity

Abstract

Syntheses of core-modified 24π pentaphyrin 7 and 20π homoporphyrin 8 are reported. The freebase form of 7 exhibits optical and 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 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 density (AICD) plots suggest a clockwise circulation of π electrons. Taken together, the changes occurring in the optical spectra, H 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.

Core-Modified Pentaphyrins(2.1.1.1.1) and Bis(difluoroborane) Complex: Synthesis, Structure, and Spectral and Redox Properties

“…Similarly, the eight protons of four pyrrole rings, which appeared as four sets of resonances in 8b , appeared as six resonances in 8b·2H 2+ , which were downfield-shifted compared to 8b and appeared in the region of 6.77–8.11 ppm. All of these changes in the chemical shifts of various protons in the protonated macrocycle 8b·2H 2+ were attributed to the presence of Mobius aromaticity, which is in line with the Mobius aromaticity observed for the protonated macrocycle 5a·2H 2+ .…”

“…Similar kinds of macrocycles were antiaromatic in neutral free base form (10) or showed Mobius aromatic behavior in the protonated form (5a). 26 Hence, we recorded the 1…”

“…The NMR studies also indicate inversion of the furan ring in macrocycles 7a – 7c and the thiophene ring in macrocycles 8a – 8c . Similar kinds of macrocycles were antiaromatic in neutral free base form ( 10 ) or showed Mobius aromatic behavior in the protonated form ( 5a ) . Hence, we recorded the 1 H NMR spectrum of the protonated macrocycle 8b·2H 2+ generated by the addition of TFA to a CDCl 3 solution of free base 8b (Figures S25–S29).…”

Protonation-Triggered Hückel and Möbius Aromatic Transformations in Nonaromatic Core-Modified [30]Hexaphyrin(2.1.1.2.1.1) and Annulated [28]Hexaphyrin(2.1.1.0.1.1)

“…The structure of [ 8 ·2H] 2+ (Figure b) reveals substantial distortion of the individual rings relative to the freebase form. The distortion of the macrocycle triggers the molecule to attain Möbius topology with distinct aromatic character . The tilt angles of DTT, pyrrole and thiophene units from the mean macrocyclic (C9, C14, C25, C30) plane are 9.03° (DTT), 32.33° (N1), 14.43° (N2), 36.02° (S4), and 35.58° (S5), respectively.…”

Annulated Isomeric, Expanded, and Contracted Porphyrins

“…Again in 2016, the group of Chandrashekar reported the first annulated nonaphyrins 171a−171b (Scheme 36) via condensation between 160a−160b and pentapyrromethanes 170a−170b in the presence of pentafluorobenzaldehyde. 124 Like the octaphyrins 169a−169b reported by the same group, nonaphyrins 171a−171b also displayed twisted figureeight conformations with nonaromatic behavior in their free base forms, while protonation led to open extended conformations corresponding to 40π Huckel antiaromatic macrocycles. The structural changes that took place upon protonation could be followed by 1 H NMR studies, as well as by absorption spectroscopy (Figure 38a,b).…”