[8]Cyclo‐<i>para</i>‐phenylmethine as a Super‐Cyclooctatetraene: Dynamic Behavior, Global Aromaticity, and Open‐Shell Diradical Character in the Neutral and Dicationic States
Zhengtao Li, Xudong Hou, Yi Han, Wei Fan, Yong Ni, Qifeng Zhou, Zhu Jun, Shaofei Wu, Kuo‐Wei Huang, Jishan Wu
Abstract
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‐Mes 2+ ) 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‐Mes 2+ exhibits open‐shell diradical character with a small singlet‐triplet energy gap (−1.8 kcal mol −1 ).