Substituent Effect on Structures of Hexaphyrins(2.1.2.1.2.1) and Their Metal Complexes
Authors: Xue Songlin , Kuzuhara Daiki , Aratani Naoki , Yamada Hiroko 
Published: July 2021

Porphyrins are key materials in nature, as well as having applications as dyes, catalysts, or electron donors and acceptors, among others. Compared with common porphyrins, porphyrins containing vinylene bridges have been less studied, but they have unique properties, such as lower symmetry, flexibility of their frameworks, and cis/trans-isomerization reaction. The symmetry of the porphyrin framework is strongly related to the optical/electronic properties of porphyrins and the flexibility of the framework is related to the switching of aromaticities and coordination to metal ions [1].

Hexaphyrins and larger expanded porphyrins consisting of six or more pyrrole rings are significant research targets owing to their large and flexible structures, their optical and electrochemical properties, and their diverse coordination abilities. Although hexaphyrins(1.1.1.1.1.1) have been reported by several groups, hexaphyrins with ethylene bridges have been less frequently reported, because of the limitation of synthetic methods. However, the incorporation of double bonds within porphyrin frameworks markedly affects their molecular structures because cis- and trans-isomers produce different geometries.

To synthesize expanded porphyrins incorporating vinylene bridges, we focused on 1,2-di(pyrrol-2-yl)ethene (DPE) derivatives. We found that treatments of
trans-DPE with pentafluorobenzaldehyde under acidic conditions selectively gave the [30]hexaphyrin(2.1.2.1.2.1) 1 [2]. In particular, protonated 13+
forms a highly planar structure with strong 30p-electron aromaticity. The free base 1 can be converted into the trinuclear rhodium(I) complex Rh-1. Planar 1 and Rh-1 display a sharp intense Soret-like band and a weak Q-like band in the visible and near infrared (NIR) region, respectively, as a result of their aromatic characteristics.

Treatment of phenyl-substituted DPEs (trans- and cis-Ph-DPE) with pentafluorobenzaldehyde in the presence of BF3·OEt2, followed by oxidation with DDQ, gave three expanded porphyrins: the cis,cis-porphyrin(2.1.2.1) 2, the cis,trans,trans-hexaphyrin(2.1.2.1.2.1) 3, and the all-trans-octaphyrin(2.1.2.1.2.1.2.1) 4 [3]. These expanded porphyrins have highly distorted structures, as revealed by X-ray crystallography. Reflecting their structures, these expanded porphyrins exhibit nonaromatic properties.

By treatment of hexaphyrin 3 with Zn(OAc)2, Zn-3 was obtained in 83% yield. The Zn-3 is categorized as a nonaromatic compound because of its non-planar structure. Treatment of hexaphyrin 3 with Cu(OAc)2 in CHCl3– MeOH gave Cu-3 in 50% yield, whereas in MeOH, Cu2-3 was obtained in 30% yield. Interestingly, the second copper ion (Cu+) of Cu2-3 is coordinated to the remaining two nitrogen atoms on the dipyrrin unit and to the pyrrolic sp
2 carbons in an η2-coordination manner. Additionally, the molecular structures are drastically changed from the distorted structure of 3 to the figure-of-eight structures of Cu-3 and Cu2-3. This structural change induces a change in electronic properties from the nonaromatic property of hexaphyrin 3 to the aromatic properties of the copper complexes.

In this presentation, we will discuss the single crystal structures as well as the optical properties of hexaphyrins(2.1.2.1.2.1) and their metal complexes.

References

[1] Kuzuhara, D.; Yamada, H. Synlett
2020, in press.

[2] Xue, S.; Kuzuhara, D.; Aratani, N.; Yamada, H. Inorg. Chem.
2018, 57, 9902.

[3] Xue, S.; Kuzuhara, D.; Aratani, N.; Yamada, H. Angew. Chem. Int. Ed.
2019, 58, 12524.

Figure 1