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

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).