(Anti-)aromatic anions of a cyclophane hydrocarbon: solid state structures and manifestation of aromaticity in battery materials Wojciech Stawski 1 , Z. Wei 2 , M. A. Petrukhina 2 , H. L. Anderson 3 1 University of Oxford, UK, 2 Department of Chemistry, University at Albany, State University of New York, USA, 3 Department of Chemistry, University of Oxford, UK Paracyclophanetetraene is a classic example of a macrocyclic hydrocarbon that becomes globally aromatic on reduction to the di-anion, and switches to globally antiaromatic in the tetra-anion. This redox activity makes it promising as an electrode material for batteries. Work with crystals of these salts is challenging because they immediately decompose upon contact with air. Perhaps it is the reason why they have never been prepared and successfully analysed before, even though the red-ox behavior of PCT has been known for over 40 years. [2] Here, we report the solid-state structures of the di- and tetra-anions of this cyclophane, in several coordination environments. The changes in bond length on reduction yield insights into the global aromaticity of the dianion (26 π electrons), and anti-aromaticity of the tetra-anion (28 π electrons), that were previously deduced by NMR spectroscopy. The experimental geometries of the aromatic di-anion and anti-aromatic tetra-anion match well with gas-phase calculated structures, and reproduce the low symmetry expected in the anti-aromatic ring. Comparison of coordinated and naked anions demonstrates that metal coordination has little effect on the bond lengths. The UV-vis-NIR absorption spectra show a sharp intense peak at 878 nm for the di-anion, whereas the tetra-anion gives a broad spectrum typical of an anti-aromatic system.
Figure 1. Alkali metal reduction of PCT together with molecular structure of one of the salts of the dianion and appearance of single crystals. References 1. W. Huber, K. Müllen, O. Wennerström, Angew. Chem. Int. Ed. Engl. 19 , 624–625 (1980). 2. S. Eder, D.-J. Yoo, W. Nogala, M. Pletzer, A. Santana Bonilla, A. J. P. White, K. E. Jelfs, M. Heeney, J. W. Choi, F. Glöcklhofer, Angew. Chem. Int. Ed. 59 , 12958–12964 (2020); Angew. Chem. 132 , 13058–13064 (2020).
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