Aromaticity is an important concept in chemistry. The commonly known Hückel aromatics contain planar structures with 4n+2 π-electrons. In contrast, Craig and Möbius aromaticity describe molecules with 4n π-electrons. Möbius aromaticity is characterized by a “twisted” structure with an inversion in the phases of the involved p-orbitals. Craig aromatics feature a pπ–dπ interaction for the 180° phase shift and can, thus, retain planar structures.

Dafa Chen, Xiamen University, China, Haiping Xia, Xiamen University and Southern University of Science and Technology, Shenzhen, China, have synthesized the first monometallic aromatic metalla-[n]annulenes with a ring size larger than six (example pictured). The team prepared a series of osmadehydro[11]annulenes via a simple [10+1] strategy, incorporating a metal fragment into conjugated, chain-like, ten-carbon precursors in a one-pot reaction.

The researchers treated such a precursor with OsCl₂(PPh₃)₃ in the presence of acetic acid and PPh₃ in CH₂Cl₂ at room temperature, followed by the addition of P(OMe)₃, 2-isocyanonaphthalene, or 2,2’-bipyridine and AgBF₄. This led to three different isolable osma-dehydro[11]annulenes in yields of 86–93 %. The team also prepared osma-dehydrobenzo[11]-annulenes and osma-dehydrodibenzo[11]-annulenes using the corresponding precursors.

The products show Craig–Hückel hybrid aromaticity and have good thermal stability in air. According to the researchers, the compounds have potential uses in materials science with, e.g., adjustable UV/Vis-NIR absorption and electrochemical properties.

• Craig‐Hückel Hybrid Aromatic Metalla‐dehydro[11]annulenes Constructed by a Formal [10+1] Cycloaddition Reaction,
  Kaidong Ruan, Zhengyu Lu, Ren Rao, Jiao-Jiao Liu, Dafa Chen, Haiping Xia,
  Angew. Chem. Int. Ed. 2023.
  https://doi.org/10.1002/anie.202316885