I have discussed Möbius aromatic systems in the book and in the blog. A new Möbius aromatic platform has been synthesized, where the porphyrin π-system is appropriately twisted. Osuka has prepared the hexaphyrins 1 and 2.1 These possess a double-twist structure, and with its 28 π-electrons 1 should be antiaromatic and 2, having 26 π-electrons should be aromatic.
In fact, the x-ray structure of 1 displays significant bond alternation and the NH protons (in the interior of the molecule) have chemical shift far downfield (δ 14.95 and 12.35 ppm) – all consistent with antiaromatic character. On the other hand¸ while 2 exhibits little bond alternation, the NH protons are seen at 11.1 ppm, too far downfield for the interior positions of an aromatic compound!
Rzepa2 has computed 1 and 2 at MPW1PW91/6-31G(d,p) for X=H and CF3; the latter matches the experimentally prepared compounds. (Rzepa supplies very nice web-enabled access to his results through the supporting materials, and so I do not repeat his structures here. Please also see comments to this post.) As expected, both optimized structures of 1(X=H or X=CF3) shows distinct bond localization and positive NICS values. The chemical shifts of the NH protons are far downfield, and in reasonable agreement with the experimental shifts. The optimized structures of 2 display bond delocalization and negative NICS values, indicative of aromaticity, as do the NH chemical shifts of 5.2 ppm (X=CF3) or 3.8 ppm (X=H). These chemical shifts differ from the experiment. Rzepa locates a second less stable conformation 3, but its NH chemical shifts are at 10.9 and 10.1 ppm, in reasonable agreement with experiment. So, he concludes that 1 is antiaromatic and 2 is aromatic and both have a double-twist Möbius topology.
Tanaka, et al have reported the structure of the octaphyrin held in place by a complexed
metal, such as 4.3 A number of analogues have been prepared and their x-ray structure shows the single twist needed for Möbius topology. The NMR spectra are consistent with an aromatic system. And relevant to this blog, B3LYP/6-31G(d) (SDD for the heavy metals) NICS computations reveals a large negative value, -14.6 ppm for 5.
(1) Shimizu, S.; Aratani, N.; Osuka, A., "meso-Trifluoromethyl-Substituted
Expanded Porphyrins," Chem. Eur. J., 2006, 12, 4909-4918, DOI: 10.1002/chem.200600158
(2) Rzepa, H. S., "Lemniscular Hexaphyrins as Examples of Aromatic and Antiaromatic
Double-Twist Möbius Molecules," Org. Lett. 2008, DOI: 10.1021/ol703129z
(3) Tanaka, Y.; Saito, S.; Mori, S.; Aratani, N.; Shinokubo, H.; Shibata, N.; Higuchi, Y.; Yoon, Z. S.; Kim, K. S.; Noh, S. B.; Park , J. K.; Kim , D.; Osuka, A., "Metalation of Expanded Porphyrins: A Chemical Trigger Used To Produce Molecular Twisting and Möbius Aromaticity," Angew. Chem. Int. Ed., 2008, 47, 681-684, DOI: 10.1002/anie.200704407
1(X=CF3): InChIKey: XMLQOLTZAIQLSA-UFROFZBYBD
2(X=CF3): InChIKey: FFCNJJQCGZCFGC-FJVZODTDBS