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Borepins are a class of boron-containing heterocycles used in main group chemistry. They consist of a seven-membered unsaturated ring with a tricoordinate boron in it. Simple borepins are analogues of cycloheptatriene, which is a seven-membered ring containing three carbon-carbon double bonds, each of which contributes 2π electrons for a total of Pi bond. Unlike other seven-membered systems such as and , boron has a vacant p-orbital that can interact with the π and π* orbitals of the cycloheptatriene. This leads to an Isoelectronicity akin to that of the tropylium cation, aromatizing the borepin while also allowing it to act as a Lewis acid. The aromaticity of borepin is relatively weak compared to traditional aromatics such as benzene or even cycloheptatriene, which has led to the synthesis of many fused, π-conjugated borepin systems over the years. Simple and complex borepins have been extensively studied more recently due to their high fluorescence and potential applications in technologies like organic light-emitting diodes () and Solar cell.
More recently a method for a minimally substituted borepin was developed by Ashe and Drone. They proceeded from 1,2-dibromocyclopentene and performed a van der Kerk method for boron heterocycle preparation. Next, they initiated a Cyclic compound to form a 7-membered tin complex. Finally, they completed a tin-boron exchange reaction to afford the bicyclic borepin on the right.
Previous synthetic methods yielded heavily substituted and bulky borepin compounds such as heptaphenyl borepin. These routes, while generating very stable complexes, made it difficult to analyze the properties of the borepin ring. Minimal substitution allowed scientists like Ashe to confirm the presence of aromaticity and ring currents within the borepin system.
As more modern methods appeared, the tin-boron exchange reaction has become more commonly used as tin can act as a placeholder in the seven-membered ring, reacting with boryl halides quite easily.
Chemists like Ashe were able to utilize this knowledge in the 1990s to functionalize borepins as a compound, leading to the formation of many Lewis acid-base adducts. The most common borepin precursor used by chemists is a borepin-halide complex as halides are a good leaving group. The borepin-hydride complex has not been able to be isolated due to its instability, whereas the boron-doped Spiro compound on the right side satisfies boron's octet, forming a zwitterion between boron and nitrogen.
Most recently, in 2022 Gilliard et al. were able to apply similar principles from their cationic borepins to form and characterize the first instance of isolated borepin radicals. These radicals were also capable of being reduced to the first instance of a borepin anion where there is multiple bonding between a boron-carbon center. The generation of the radical comes from the strong Pi backbonding ability of the carbene carbon. The electron density shared with the boron center back bonds slightly with the carbon atom, leading to the single-electron radical species.
As a result, chemists sought ways to increase the aromatic character of borepins. The tried-and-true method by which chemists stabilize borepins is phenyl-borepin ring fusion (annulation). The addition of two fused phenyl rings increases the 6π borepin system to a 14π fused system.
A complication that arises with fusion of the phenyl rings is their positioning. When synthesizing dibenzob,fborepins (b is the carbon next to the boron atom) they are perfectly aligned for conjugation of the borocycloheptatriene ring. However, if the phenyls are positioned in a c,e fashion (see below) then the resulting compound is less stable than dibenzob,fborepins by around 34 kcal/mol, quite a large energy difference. These results explained by Schulman and Disch have been applied many times over to modify borepin frameworks. Some common examples include increasing the number of rings—making boron-doped polycyclic-aromatic hydrocarbons (PAHs), adding additional R groups to the framework such as and long-chain alkanes, and even introducing electron-rich heteroatoms such as nitrogen or sulfur in order to further stabilize the borepins. Some examples of these compounds can be seen in the image below:
The rapid development of borepin stabilization and functionalization since the 2000s has catapulted studies of complex and versatile molecules. Like many other main group compounds, borepins have been in the field since the mid-late 1900s yet lay dormant until more modern methods could utilize them.
The initial excitement behind these results was the potential for use in electronic materials such as organic light-emitting diodes (). If the fluorescence "switch" could be controlled, in addition to having stable borepin complexes, then it would be relatively easy and cheap to achieve bright fluorescent lights, potentially of any color.
Another potential of redox chemistry is the use of boron-containing polycyclic aromatic hydrocarbons as semiconductors. Because of borepins' low-lying LUMO, it can act as an electron acceptor to participate in electron transport. The Wagner group as well as Toscano and co-workers showed computationally and experimentally the potential applications for these complexes.
On another note, scientists have sought to utilize borepins as potential anion sensors. In the past, tri-coordinate boranes have been used to detect anions like fluoride, cyanide, and even ammonia. Scientists like Adachi and Ohshita have demonstrated that upon coordination of fluoride (F−) fluorescence increases by many magnitudes.
In contrast to that example, upon addition of cyanide to one of their borepin analogues to tetrathienoanthracence, Adachi and Ohshita saw a loss of fluorescence. However, upon cooling, there was a noticeable phosphorescence in solution.
Fluorescence is not only limited to outside coordination. Upon insertion of nitrogen into the borepin ring, Li et al. were able to observe Solvatochromism effects. Upon addition of the borepin to , toluene, tetrahydrofuran (THF), dichloromethane (DCM) and acetonitrile (MeCN), rather drastic changes in color were observed.
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