A Negatively Curved π‐Expanded Pyracylene Comprising a Tropylium Cation

Abstract

We disclose π‐expanded pyracylenes and their cationic species comprising 7‐membered rings. The compounds were synthesized by stepwise oxidative cyclodehydrogenation to monitor the effect of successive cyclization on the structural and optoelectronic properties. As shown by X‐ray crystallography the complete cyclization leads to a boat‐shaped scaffold featuring negative curvature provided by the 7‐membered ring. The embedded tropone unit enabled the convenient generation of a stabilized tropylium cation, showing bathochromically shifted absorption bands reaching into the near‐infrared region beyond 1000 nm. The altered structural features supported by theoretical calculations point towards aromatic character of the positively charged 7‐membered ring.

Synthesis and Properties of Azahomocorannulenyl Cations and Radicals

“…Upon reduction from 2 a + to 2 a⋅ , significant changes in bond lengths were observed in the fused 6–7–6 membered ring unit (shown in bold in Figure 2C). The harmonic oscillator model of aromaticity (HOMA) analysis [39] as one of the aromaticity indices (Figures 2C,D and Figure S24) revealed the HOMA values of the 6‐ and 7‐membered rings are 0.61 (0.53) and 0.38, respectively, which are in good agreement with those of a polycyclic aromatic tropylium cation (0.63 and 0.36) [7] . Upon conversion from cation to radical, the value in the 7‐membered ring decreased to 0.10, indicating less efficient delocalization, while the aromaticity of its adjacent benzene rings was slightly enhanced.…”

“…The tropylium cation (C 7 H 7 + ; A ; Figure 1A), first discovered in 1891, [1,2] stands as a cyclic, planar molecule endowed with 6π electrons and a positive charge delocalized over the entire 7‐membered ring. According to the Hückel rule, this species is classified as aromatic and displays notable stability, as demonstrated by the isolation of stable tropylium compounds [3–7] . Despite their aromaticity, tropylium cations exhibit electrophilic reactivity as Lewis acids and oxidants, which has led to their utilization as catalysts and stoichiometric reagents in a diverse array of organic transformations [8,9] and as functional organic dyes/chromophores in material sciences [10–17] …”

On‐Surface Synthesis of Non‐Benzenoid Nanographenes Embedding Azulene and Stone‐Wales Topologies

“…Nevertheless, one of the main challenges limiting the study of non‐benzenoid molecules is their difficult synthesis and separation, and only a few examples have been successfully reported [9–11,13–22] . In contrast, the “bottom‐up” approach, combining modern solutions and on‐surface syntheses to create NGs with atomically precise edge structures, has recently appeared as an alternative [23] …”

Synthesis and Properties of Azahomocorannulenyl Cations and Radicals

“…According to the Hückel rule, this species is classified as aromatic and displays notable stability, as demonstrated by the isolation of stable tropylium compounds. [3][4][5][6][7] Despite their aromaticity, tropylium cations exhibit electrophilic reactivity as Lewis acids and oxidants, which has led to their utilization as catalysts and stoichiometric reagents in a diverse array of organic transformations [8,9] and as functional organic dyes/chromophores in material sciences. [10][11][12][13][14][15][16][17] In contrast, the neutral tropyl radical (C 7 H 7 * ; B), first experimentally observed in 1960, [18] represents a more elusive species due to its structural and electronic complex-ity primarily attributed to the presence of 7π electrons.…”

“…The harmonic oscillator model of aromaticity (HOMA) analysis [39] as one of the aromaticity indices (Figures 2C,D and Figure S24) revealed the HOMA values of the 6-and 7-membered rings are 0.61 (0.53) and 0.38, respectively, which are in good agreement with those of a polycyclic aromatic tropylium cation (0.63 and 0.36). [7] Upon conversion from cation to radical, the value in the 7membered ring decreased to 0.10, indicating less efficient delocalization, while the aromaticity of its adjacent benzene rings was slightly enhanced.…”