[8]Cyclo-para-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; Jun Zhu; Shaofei Wu; Kuo-Wei Huang; Jishan Wu

doi:10.1002/anie.202210697

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

Angew Chem Int Ed Engl. 2022 Oct 24;61(43):e202210697. doi: 10.1002/anie.202210697. Epub 2022 Sep 14.

Authors

Zhengtao Li¹, Xudong Hou¹, Yi Han¹, Wei Fan¹, Yong Ni¹, Qifeng Zhou^{1

2}, Jun Zhu¹, Shaofei Wu¹, Kuo-Wei Huang^{3

4}, Jishan Wu^{1

2}

Affiliations

¹ Department of Chemistry, National University of Singapore, 3 Science Drive 3, 117543, Singapore, Singapore.
² Joint School of National University of Singapore and Tianjin University, International Campus of Tianjin University, Binhai New City, Fuzhou, 350207, China.
³ KAUST Catalysis Center and Division of Physical Science and Engineering, King Abdullah University of Science and Technology, Thuwal, 23955-6900, Saudi Arabia.
⁴ Institute of Materials Research and Engineering and Institute of Sustainability for Chemicals, Energy and Environment, Agency for Science, Technology, and Research (A*STAR), Singapore, 138634, Singapore.

PMID: 36008354
DOI: 10.1002/anie.202210697

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 ).

Keywords: Annulene; Aromaticity; Diradicaloid; Dynamic Behavior; Valence Tautomerization.

Abstract

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