Boron-Rich Inorganic Benzene

Benzene (C₆H₆) is an aromatic molecule. The aromaticity of "inorganic benzene", i.e., borazine (B₃N₃H₆), remains controversial, mainly due to the significantly polarized B–N bonds. Thus, the synthesis of aromatic inorganic benzene derivatives is still a challenge. Verified aromatic character is limited to a few heavier analogues.

Kei Ota and Rei Kinjo, Nanyang Technological University, Singapore, have synthesized a neutral, boron-rich inorganic benzene analogue (pictured). The team first synthesized a cyclic diazatetraborinane (B₄N₂) precursor via the deprotonation of a bis(anilide)bis(dimethylamido)diborane with n‐butyllithium, followed by treatment with bis(dimethylamido)diborondichloride. The resulting cyclic intermediate was chlorinated using boron trichloride (BCl₃). Finally, the researchers reduced the chlorinated precursor with potassium graphite in the presence of trimethyl phosphine (PMe₃). They obtained deep blue crystals of the desired 1,4,2,3,5,6-diazatetraborinine derivative.

This product was characterized by NMR spectroscopy, UV/Vis spectroscopy, and X-ray diffraction. The bonding situation was investigated using density functional theory (DFT) calculations. The structural and electronic properties of the molecule confirm that the planar central B₄N₂ ring has a pronounced aromatic nature, similar to benzene. According to the researchers, this result could aid the design of new aromatic materials.

• A Neutral and Aromatic Boron-Rich Inorganic Benzene,
  Kei Ota, Rei Kinjo,
  Angew. Chem. Int. Ed. 2020.
  https://doi.org/10.1002/anie.201915790