A borabenzene is a heteroaromatic compound that has a boron atom instead of the carbon atom of a benzene molecule. A free borabenzene, which has no donor ligand on the boron atom, has not been isolated yet despite its simple structure and the chemical robustness of boron-carbon bonds. The instability of this molecule results from the high Lewis acidity of the boron atom due to its electron deficiency. All attempts to create a free borabenzene have resulted in complex formations with anionic or neutral ligands (even a nitrogen molecule can coordinate to a borabenzene). However, the complexes with anionic ligands, called boratabenzenes, show rich chemistry including the coordination chemistry as anionic π-type ligands.

Propeties as ligands

Boratabenzenes have a negative charge on their six-membered rings and act as strong π-donating ligands like a cyclopentadienyl anion, which is often used in transition metal complexes. In fact, sandwich or half-sandwich type complexes of many transition metals have been reported using boratabenzenes, and the zirconium sandwich complexes exhibit catalytic activity in ethylene polymerization similar to zirconocenes, which are the cyclopentadienyl counterparts.

Family of boratabenzenes

Several boracyclic compounds are involved in the family of boratabenzenes: 1-boratanaphthalene, 9-borathaanthracene, boracyclooctatetraene, and 2,2’-diboratabiphenyl. Among these compounds, 2,2’-diboratabiphenyl is the first bidentate Lewis acid based on a borabenzene framework, so it is regarded as a Lewis acid analogue of 2,2’-bipyridine , a common bidentate ligand. This compound forms polycyclic aromatic hydrocarbon analogues containing boron and nitrogen atoms, which show characteristic optical and electrochemical properties due to the charge transfer between the boron and nitrogen atoms.