Pillar[n]arene (n: number of units), macrocyclic para-arylene methylene molecules, have attracted attention owing to their symmetrical structures, host-guest properties, and original supramolecular assembly characteristics.¹ Similarly, their quinone counterparts, pillar[n]quinone (P[Q]_n), are also fascinating macrocycles containing electron-deficient quinone units, and therefore have potential application for novel host-guest chemistry and redox active materials. A hexagonal molecule, pillar[6]quinone (P[Q]₆), is expected to form densely packed structures and a candidate for organic active material but the synthesis of P[Q]₆ still remains a challenge.

We previously demonstrated that the electrochemical oxidation (1.0 V vs. SCE) of 1,4-dihydroxypillar[6]arene (P[HQ]₆) in methanol afforded micrometer scale hexagonal cylindrical depositions on electrode surfaces, which were composed of partially oxidized P[HQ]_6-m[Q]_m (composed of both hydroquinone and benzoquinone units) aggregating via quinhydrone formation.²

In this work, we successfully synthesized P[Q]₆ for the first time by oxidation of its hydroquinone precursor P[HQ]₆. Electrochemical oxidation (1.2 V vs. SCE) of P[HQ]₆ in methanol gave the similar hexagonal cylindrical crystal of P[Q]₆ evidenced by single crystal X-ray diffraction, NMR and HRMS analyses. In the crystallographic data, all quinone moieties seem to have intermolecular CH-O interaction between adjacent macrocycles, resulting in the formation of a hexagonal packing structure. In addition, we also found that scalable synthesis of P[Q]₆ was possible by chemical oxidation of P[HQ]₆ with phenyliodine(III)bis(trifluoroacetate) in 1,1,1,3,3,3-hexafluoro-2-propanol.

To understand the electrochemical properties and electron-transfer behavior of P[Q]₆, various voltametric studies were carried out. We revealed that three electrons are injected first, followed by stepwise three one-electron reductions due to the electrostatic repulsion in the latter electron-transfer process.

Reference

1. T. Ogoshi, T. Yamagishi, Y. Nakamoto, Chem. Rev., 116, 7937 (2016).
2. C. Tsuneishi, Y. Koizumi, R. Sueto, H. Nishiyama, K. Yasuhara, T. Yamagishi, T. Ogoshi, Tomita and S. Inagi, Chem. Commun., 53, 7454 (2017).
3. T. Hirohata, N. Shida, H. Uekusa, N. Yasuda, H. Nishihara, T. Ogoshi, I. Tomita, S. Inagi, Chem. Commun., 57, 6360 (2021).