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[8]Cyclo‐para‐phenylmethine as A Super‐Cyclooctatetraene: Dynamic Behavior, Global Aromaticity, and Open‐Shell Diradical Character in The Neutral and Dicationic States

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A super-cyclooctatetraene, one new core-expanded analogue of cyclooctatetraene, has been synthesized by insertion of a benzenoid/quinoid C6H4 unit into each C−C bond. The neutral molecule shows three dynamic processes on the NMR time scale and non-aromatic character, whereas its dication exhibits enhanced planarity, global aromatic, and open-shell diradical character.


Abstract

An [8]cyclo-para-phenylmethine derivative ([8]CPPM-Mes) was synthesized. X-ray analysis revealed a tub-shaped geometry similar to the cyclooctatetraene, with alternating benzenoid/quinoid structure. Variable-temperature NMR measurements disclosed a slow valence tautomerization process with an interconversion energy barrier of about 11.7 kcal mol−1 at coalescence temperature (273 K), and two more lower-barrier dynamic processes involving flipping of the 1,4-phenyl rings on the backbone and rotation of the mesityl substituents. Its dication ([8]CPPM-Mes2+ ) adopts a bowl-like geometry with a smaller depth of the cavity, and a slow bowl inversion process was observed by dynamic NMR. The bond lengths of the benzenoid/quinoid rings are more averaged via electron delocalization and the molecule shows global aromaticity, which was further validated by NMR and theoretical analysis. [8]CPPM-Mes2+ exhibits open-shell diradical character with a small singlet-triplet energy gap (−1.8 kcal mol−1).

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