Boraacenes

This results in slightly less negative charge within the ring, smaller HOMO-LUMO gaps, as well as differences in redox chemistry when compared to their acene analogues.

[2] Due to this empty p orbital, however, it is also highly reactive when exposed to nucleophiles like water or normal atmosphere, as it will readily be attacked by oxygen, which must be addressed to maintain its stability.

[4][5] Synthesis of boraacenes presents a unique challenge due to boron’s incredible Lewis acidic character.

[10] However, they introduced a bulky mesityl ligand to the structure via a Grignard reaction which sterically shielded the highly electropositive boron.

The stabilized product was then treated with the sterically hindered base, tert-butyllithium in benzene and a stable anionic 9-boraanthracene compound was formed.

A silicon containing, tricyclic stannacycle is reacted with excess boron trichloride followed by the addition of H2IMes, an N-heterocyclic carbene (NHC), to stabilize the product.

To accomplish this, the structure of the starting stannacycle was altered with the addition of either one or two more flanking benzene rings to synthesize the naphthacene and pentacene boron analogues respectively.

This allows for some interesting chemistry where, since the electrophilic boron is protected by the mesityl group, the molecule may act as a nucleophile.

Van Veen and Bickelhaupt characterized reaction products with carbon dioxide, trimethylchlorosilane, ethyl chloroformate, iodomethane, formaldehyde, and deuterium oxide.

By irradiating the diazo product in benzene and adding various substituted alkenes, they were able to isolate molecules with cyclopropane adducts to the nucleophilic carbon.

[15] Owing to boron’s Lewis acidic nature, in the presence of molecular oxygen, the neutral 9-boranthracene rapidly forms a peroxide.

Ishikawa et al demonstrated that mesityl; 1,3,5-trichlorobenzene; trifluorotoluene; 1,3-Bis(trifluoromethyl)benzene; anthracene; and ethane could be used in place of the NHC ligand.

[14] As a consequence of its instability in solution, its charge transfer band of electrons going from the HOMO of the boraacene to the LUMO of pyridine could only be estimated at 486 nm.

[19] If boron is introduced into graphene it has also been shown to convert it from a semi-metal to a half-metal, giving it properties more akin to a semiconductor.

[23] Bis-(dimesitylboryl)-azaborine, a boron nitrogen aromatic compound similar in structure to 9-aza-10-boraanthracene, was shown to fluoresce at a specific wavelength in its native state.

This was also shown to occur with cyanide ions, which means it could be utilized to detect anionic poisons in possibly hazardous areas.