Enhanced Aromaticity and Open‐Shell Diradical Character in the Dianions of 9‐Fluorenylidene‐Substituted Expanded Radialenes

• Published in: Angewandte Chemie 2022-09, Vol.134 (39), p.n/a
• Main Authors: Xin, Shan, Han, Yi, Fan, Wei, Wang, Xuhui, Ni, Yong, Wu, Jishan
• Format: Article
• Language: English
• Subjects: Anions; Aromatic compounds; Aromaticity; Chemistry; Cross-Conjugation; Crystallography; Dianion; Diradicaloid; Energy gap; Magnetic resonance spectroscopy; NMR; NMR spectroscopy; Nuclear magnetic resonance; Optical properties; Radialene; Substitutes; Synthesis
• ISSN: 0044-8249; 1521-3757
• DOI: 10.1002/ange.202209448

Radialenes and expanded radialenes are cross‐conjugated macrocycles displaying poor aromatic character. In this work, three 9‐fluorenylidene substituted expanded [n]radialenes (ER‐n, n=3–5) with a diacetylene spacer were synthesized and their structures were confirmed by X‐ray crystallographic analysis and NMR spectroscopy. They all can be easily reduced into relatively stable dianions. Detailed experimental measurements and theoretical calculations suggest that their dianions (ER‐n2−, n=3–5) are stabilized by both the aromatic fluorenyl anion substituents and the central aromatic rings with formally [4n+2] delocalized π electrons. In addition, the dianions of the extended radialenes (ER‐42− and ER‐52−) show unique open‐shell diradical character with a small singlet‐triplet energy gap. For comparison, their linear counterparts (L‐3 and L‐4) were also synthesized; their dianions exhibit very different redox and optical properties from their respective macrocycles. Three 9‐fluorenylidene‐substituted expanded radialenes with a diacetylene spacer were synthesized. The neutral compounds are cross‐conjugated but their dianions are stabilized by the aromaticity of the central cores and fluorenyl anion substituents. The larger sized macrocyclic dianions exhibited unique open‐shell diradical character.