Silicon-containing heterocycles can have applications, e.g., in organic photovoltaic cells, organic light-emitting diodes, or sensors. Various cyclic aza-sila compounds have also been found to have pharmaceutical relevance. However, transformations of silicon-containing heterocycles lag behind those of analogous purely organic compounds. Similarly, carbene chemistry is well-explored, including mesoionic carbenes, which feature both a negative and a positive formal charge. Less is known about the corresponding compounds of heavier main-group elements, such as silicone analogues of mesoionic carbenes.

Qiuming Liang, Southern University of Science and Technology, Shenzhen, China, Great Bay University, Dongguan, China, Liu Leo Liu, Southern University of Science and Technology and Nankai University, Tianjin, China, and colleagues have synthesized a mesoionic 1,4,2-diazasilole (pictured) featuring a low-valent silicon center that exhibits properties of both a mesoionic silylene and a silylone. Silylenes are carbene analogues, and silylones are analogues of carbones (two-coordinate formal C(0) species). The team started from an imino-N-heterocyclic carbene (NHC) ligand with a bulky 2,6-diisopropylphenyl (Dipp) substituent, which was reacted with SiCl₄ in n-pentane to form the desired Si-containing ring.

The product was characterized using X-ray crystallographic analysis and ²⁹Si NMR spectroscopy, as well as studied using density functional theory (DFT) calculations. The results indicate an electronic structure that sits between between a mesoionic N-heterocyclic silylene (pictured) and a cyclic silylone. The researchers performed preliminary reactivity studies, which gave a variety of new silicon-based heterocycles and confirmed the dual nature of the product, which was converted to both a silylene iridium complex and a silylone diiron complex.

• A Crystalline Mesoionic Diazasilole Featuring Low‐Valent Silicon,
  Xiaofang Lan, Hongyu Wang, Qiuming Liang, Liu Leo Liu,
  Angew. Chem. Int. Ed. 2024.
  https://doi.org/10.1002/anie.202415246