One Collision - Two Heteroatoms: Gas-Phase Preparation of Azasilabenzenes

Nature has long favored cyclic, particularly heterocyclic, molecules in the synthesis of key biomolecules. Silicon-substituted hydrocarbons have gained increasing attention due to their unique chemical bonding and electronic structure compared to their iso-valent carbon counterparts. However, the synthesis of silicon-containing heterocyclic molecules remains a significant challenge. In this work, we investigate the reaction mechanism leading to a sparsely explored class of silicon- and nitrogen-containing aromatic heterocycles, in which silicon and nitrogen atoms are positioned adjacently within an aromatic, six-membered ring: azasilabenzenes. This is achieved through the reaction of the silicon nitride (SiN) radical with 1,3-butadiene (C₄H₆) via single collision conditions, exploiting a crossed molecular beam experiment combined with electronic structure calculations and statistical analysis. Our results reveal the formation of two novel cyclic products: 1-aza-2-silacyclohexa-3,5-dien-2-ylidene and 1-aza-2-silabenzene. Interestingly, this reaction contrasts with the iso-valent system involving the cyano radical (CN) and 1,3-butadiene (C₄H₆), which predominantly yields an acyclic product (1-cyano-1,3-butadiene) via a simple addition-elimination pathway. In addition to elucidating the reaction pathways, this study also provides insights into the nature of bonding and atomic interactions in the resulting products, offering a deeper understanding of structure-stability relationships in silicon-nitrogen heterocycles and the counterintuitive concept of iso-valency.