Boron-nitrogen compound is close cousin of benzene

A cunningly modified ring of boron and nitrogen atoms is the closest inorganic analog of benzene yet created, its creators say (Angew. Chem., Int. Ed. 2020, DOI: 10.1002/anie.201915790). The first inorganic version of benzene was reported almost a century ago. Borazine (B₃N₃H₆) contains a flat six-membered ring and a delocalized cloud of six π electrons but is only moderately aromatic. Since borazine’s discovery, a handful of other inorganic benzenes have been created, none of which seem to have much aromaticity. Rei Kinjo and Kei Ota at Nanyang Technological University have now created a 1,4,2,3,5,6-diazatetraborinine derivative that is much more benzene-like than its predecessors. The blue solid is a valence isomer of benzene, with each ring atom bearing only one substituent. Unlike borazine’s alternating pattern of elements, diazatetraborinine’s ring includes four boron atoms that sit in pairs and bond to either chlorine or trimethylphosphine. X-ray crystallography shows that the molecule is flat and has a perfectly hexagonal central ring, while nuclear magnetic resonance measurements demonstrate the magnetic deshielding effects typical of aromatic rings. The researchers’ theoretical calculations show that the molecule’s six π electrons are highly delocalized and that its aromaticity lies between that of benzene and borazine. “By modulating the position of inorganic elements, we have demonstrated that even inorganic benzenes can exhibit a pronounced aromatic nature,” Kinjo says.