Boraacenes are polycyclic aromatic hydrocarbons containing at least one boron atom. Structurally, they are related to acenes, linearly fused benzene rings. However, the boron atom is electron deficient and may act as a Lewis Acid when compared to carbon. 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. When incorporated into acenes, Boron maintains the planarity and aromaticity of carbon acenes, while adding an empty p-orbital, which can be utilized for the fine tuning of organic semiconductor band gaps. 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.
Synthesis
Synthetic strategies to incorporate boron into a polycyclic carbon backbone aim to sterically protect the atom from nucleophilic attack while ensuring that favorable π-stacking interactions are still preserved to ensure electron mobility in the ring and compound stability within organic electronics. Synthesis of boraacenes presents a unique challenge due to boron’s incredible Lewis acidic character. Borabenzene, an analog of benzene, cannot exist independently, despite obeying Huckel’s Rule for aromaticity with 6 π electrons. However, it has been characterized with various neutral and anionic metal complexes that help stabilize boron’s empty p orbital. Interestingly, when synthesized with metal ligands, the compound is quite inert and will usually maintain the metal ligand bond, even towards nucleophiles. Though it has been shown to undergo some Diels-Alder reactions.
Boranaphthalene
Synthesis of the simplest boraacene, boranaphthalene, was achieved in 1985. This was achieved by reacting benzyldibromoborane and acetylene to form 2-bromo-1,2-dihydro-2-boranaphthalene, which was mixed with diethyl ether to form an ethoxy analogue. This was then deprotonated with the sterically hindered base lithium 2,2,6,6-tetramethylpiperidine, and silylated with chlorotrimethylsilane. The product was reacted with boron trichloride to replace the ethoxy group with chlorine. After this, pyridine was added yielding the final product, pyridine-2-boranaphthalene.
9-Boraanthracene
An early synthesis of 9-boraanthracene was done by Van Veen and Bickelhaupt in 1974. Magnesium was added to a two-boron containing dibenzene to form a Grignard reagent. Diethylaminoboron was then added to form the initial skeleton, after which, hydrochloric acid was used to convert the product to an anhydride. The anhydride was treated with boron trichloride to cleave the anhydride and form the chloride adduct. Interestingly, the group failed to aromatize this compound with a strong base initially due to the highly Lewis acidic nature of boron, as the base would prefer to attack the boron, forming the adduct, rather than remove the hydrogen across from it, which is necessary to aromatize the compound. 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.
The first synthesis of neutral 9-boraanthracene was initially reported by in 2009. When compared to the 1974 synthesis of a charged boraanthracene, the boron is uncharged and the carbon opposite it is not as Lewis Basic. The neutral molecule was achieved by transmetalation of silicon, as opposed to a Grignard Reaction. 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. Following this, lithium 2,2,6,6-tetramethylpiperidine was used to deprotonate it and form the fully aromatic, neutral 9-boraanthracene, in a similar manner to Van Veen.