5,20-Bis(ethoxycarbonyl)-Substituted Antiaromatic [28]Hexaphyrin and Its Bis-NiII and Bis-CuII Complexes

5,20-Bis(ethoxycarbonyl)-[28]hexaphyrin was synthesized by acid catalyzed cross-condensation of meso-diaryl-substituted tripyrrane and ethyl 2-oxoacetate followed by subsequent oxidation. This hexaphyrin was found to be a stable 28π-antiaromatic compound with a dumbbell-like conformation. Upon oxidization with PbO₂ , this [28]hexaphyrin was converted into an aromatic [26]hexaphyrin with a rectangular shape bearing two ester groups at the edge side. The [28]hexaphyrin can incorporate two Ni^II or Cu^II metals by using the ester carbonyl groups and three pyrrolic nitrogen atoms to give bis-Ni^II and bis-Cu^II complexes with essentially the same dumbbell-like structure. The antiaromatic properties of the [28]hexaphyrin and its metal complexes have been well characterized.

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 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 density (AICD) plots suggest a clockwise circulation of π electrons. Taken together, the changes occurring in the optical spectra, ¹ 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.

The Curious Case of a Parasitic Twin of the Corroles

An expanded porphyrinoid has been obtained by a simple ring expansion from a contracted porphyrinoid, namely corrole. Spectroscopic, structural, and computational investigations reveal peculiar π-conjugation and geometry. The effect of extended π-conjugation is evident from perturbed redox behavior and photophysical properties. Owing to the strong diatropic ring current of the corrole and cross-conjugation, the molecule exhibits a non-aromatic nature for the expanded π-circuit, as evident from NMR studies.

meso-Aryl [20]π Homoporphyrin: The Simplest Expanded Porphyrin with the Smallest Möbius Topology

An unstable conjugated homoporphyrin was successfully stabilized by introducing meso-aryl substitutents. It was evident from the moderate diatropic ring current found by NMR analysis that the newly formed 20π conjugated free base and its protonated form exhibited Möbius aromatic character. Furthermore, complexation as a ligand with an Rh^I ion afforded a unique binding mode and retained the Möbius aromaticity. Overall, these compounds are the smallest Möbius aromatic molecules, as confirmed by spectral and crystal-structure analysis and supported by theoretical studies.

Conformational Fixation of a Rectangular Antiaromatic [28]Hexaphyrin Using Rationally Installed Peripheral Straps

A rectangular [28]hexaphyrin bearing outer straps at the long side has been synthesized by SN Ar reaction of [26]hexaphyrin with allyl alcohol, intramolecular olefin metathesis by using Hoveyda-Grubbs second-generation catalyst, and reduction with NaBH4 . The peripheral straps enforce a rectangular conformation for the [28]hexaphyrin, which shows Hückel antiaromatic character, as confirmed by its planar X-ray structure, a strong paratropic ring current, characteristic UV/Vis/NIR absorption features, small electrochemical HOMO-LUMO gap, and very fast S1 decay.

Diprotonated [28]hexaphyrins(1.1.1.1.1.1): triangular antiaromatic macrocycles

Protonation of meso-aryl [28]hexaphyrins(1.1.1.1.1.1) triggered conformational changes. Whereas protonation with trifluoroacetic acid led to the formation of monoprotonated Möbius aromatic species, protonation with methanesulfonic acid led to the formation of diprotonated triangular antiaromatic species. A peripherally hexaphenylated [28]hexaphyrin was rationally designed and prepared to undergo diprotonation to favorably afford a triangular-shaped antiaromatic species.