Boraacenes

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9-Boraanthracene Boraanthracene.svg
9-Boraanthracene

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. [1] 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. [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. [3]

Contents

Synthesis

Borabenzene Borabenzene.svg
Borabenzene

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. [4] [5] 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. [6] 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. [7] Though it has been shown to undergo some Diels-Alder reactions. [8]

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]

Synthesis of boranaphthalene-pyridine. Boranaphthalene Synthesis.svg
Synthesis of boranaphthalene-pyridine.

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. [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. [11]

Synthesis of anionic 9-boraanthracene with stabilization by a mesityl group. Charged Boraanthracene Synthesis 2.svg
Synthesis of anionic 9-boraanthracene with stabilization by a mesityl group.

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. [12]

Synthesis of neutral 9-boraanthracene with stabilization by an NHC ligand. Neutral Boraanthracene Synthesis 2.svg
Synthesis of neutral 9-boraanthracene with stabilization by an NHC ligand.

Higher Order Boraacenes

The above technique was expanded on in later studies, and created a generalized approach to synthesize 9-boraanthracene, as well as the higher order boraacenes: 5-boranaphthacene and 6-borapentacene. 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. The stannacycle used to prepare 9-boraanthracene was prepared by reacting 1,2-dibromobenzene with isopropyl magnesium chloride, and quenching the product with 2-bromobenzaldehyde, in a Grignard reaction. The product was reduced with hydroiodic acid and silylated with dichlorodimethylsilane producing the initial stannacycle. [1]

Synthesis of stannacycle reactants used in the production of 9-boraanthracene. Synthesis of the stannacycle reactants used in the production of 9-boraanthracene 2.svg
Synthesis of stannacycle reactants used in the production of 9-boraanthracene.

The stannacycle used for 5-boranaphthacene was synthesized similarly, except the dibromobenzene was substituted with a naphthalene analogue as the Grignard reagent, with the same aldehyde. While the stannacycle used to synthesize 6-borapentacene was synthesized by with a dibromonaphthalene and a naphthalene analogue of the aldehyde. The structures of the molecules used are shown below. Following synthesis of the stannacycles, transmetalation was done utilizing the same reagents to synthesize 9-boraanthracene. [1]

A: Dibromide and aldehyde used to produce B: Tetracyclic stannacycle. C: Dibromonaphthalene and naphthaldehyde used to produce D: Pentacyclic stannacycle Reactants for Boraacene stannacycle synthesis.svg
A: Dibromide and aldehyde used to produce B: Tetracyclic stannacycle. C: Dibromonaphthalene and naphthaldehyde used to produce D: Pentacyclic stannacycle

9-Aza-10-boraanthracene

More recently, a boron acene analogue containing nitrogen was synthesized via a similar transmetalation and stabilization process, this time utilizing a nitrogen bridge between the two benzene groups instead of a carbon with the product being stabilized by an NHC. Adding a nitrogen opposite the boron allows the nitrogen’s lone pair to contribute to the molecules aromaticity and serves to stabilize the empty p orbital on boron. This was achieved by lithiating the nitrogen containing bromophenyl reactant with n-BuLi, followed by a reaction with boron trichloride. The resulting hydroxide is reacted with one more equivalent of boron trichloride to yield the chlorine adduct. Finally, the NHC was added to stabilize the product. [13] [14]

Synthesis of 9-aza-10-boraanthracene via lithiation and subsequent stabilization with an NHC ligand 9-aza-10-boraanthracene synthesis.svg
Synthesis of 9-aza-10-boraanthracene via lithiation and subsequent stabilization with an NHC ligand

Reactivity

Charged 9-Boraanthracene as a Lewis Base

The anionic 9-boraanthracene has a resonance structure characterized by a lone pair on the carbon opposite boron. 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. The products formed were racemic mixtures of adducts that were the result of nucleophilic attack by the nucleophilic carbon. [11]

The anionic 9-boraanthracene acting as a Lewis Base. Adducts include: carbon dioxide, trimethylchlorosilane, ethyl chloroformate, iodomethane, formaldehyde, and heavy water. 9-Boraanthracene as a Lewis Base.svg
The anionic 9-boraanthracene acting as a Lewis Base. Adducts include: carbon dioxide, trimethylchlorosilane, ethyl chloroformate, iodomethane, formaldehyde, and heavy water.

One other interesting reaction they noted was a ring expansion that occurred when formaldehyde was added and the reaction was allowed to proceed at reflux instead of room temperature. [11]

Ring expansion reaction of charged 9-boraanthracene with formaldehyde at reflux. 9-Boraanthracene Ring Expansion.svg
Ring expansion reaction of charged 9-boraanthracene with formaldehyde at reflux.

Photochemical Reactions of Diazoboraanthracene

The non-aromatic precursor developed by Van Veen was characterized in several reactions developing novel products. One such reaction was with tosyl azide in diethylamine to yield an aromatic diazo product. 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]

Photochemical addition of alkenes to diazoboraanthracene forming cyclopropane adducts. Photochemical addition of alkenes to diazoboraanthracene.svg
Photochemical addition of alkenes to diazoboraanthracene forming cyclopropane adducts.

A similar irradiation method, but with cyclohexane lead to the rapid formation of a cyclohexane adduct in a manner similar to the Lewis acid mechanism demonstrated by Van Veen, but it also formed dimers as well as regenerated the starting material. [15]

Photochemical addition of cyclohexane to diazoboraanthracene. Photochemical addition of cyclohexane to diazoboraanthracene.svg
Photochemical addition of cyclohexane to diazoboraanthracene.

Endoperoxide Formation

3D Structure of the boraanthracene endoperoxide adduct 3D Structure of boraanthracene endoperoxide.svg
3D Structure of the boraanthracene endoperoxide adduct

Owing to boron’s Lewis acidic nature, in the presence of molecular oxygen, the neutral 9-boranthracene rapidly forms a peroxide. This reaction occurs quickly and spontaneously, without any energy input. A similar reaction occurs in anthracene; [16] however, the oxygen has to be excited with a photosensitizer from its triplet state to its singlet state, and the reactions are significantly slower than its boraacenes analogue. [1] Due to this reaction, the product must be kept under inert conditions so that it does not react with oxygen in the air.

Endoperoxide formation from exposure to molecular oxygen Boraanthracene endoperoxide formation from exposure to molecular oxygen.svg
Endoperoxide formation from exposure to molecular oxygen

9-Aza-10-boranthracene

Along with the NHC ligand, 9-aza-10-boranthracene, many aryl and alkyl groups could be introduced as a stabilizing ligand. 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. Furthermore, the lone pair on nitrogen can act as a Lewis Base and form adducts with alkyl halides. The compound overall is highly reactive to air and needs to be stored under inert atmosphere. [14]

Properties

Neutral Boraacenes

For the boraacenes possessing an NHC ligand, the peak absorption of 9-boraanthracene, 5-boranaphthacene, and 6-borapentacene is 485 nm, 608 nm, and 800 nm respectively. While the anthracene and naphthacene analogues are weakly fluorescent, the pentacene analogue is not. The HOMO-LUMO gaps are 2.25, 1.74, and 1.28 eV respectively, which are smaller overall when compared to the carbon acene’s gaps, however the boron HOMO and LUMO energies, individually, are higher. This gap is associated with a lower LUMO energy rather than a higher HOMO energy, as each boron compound has a relatively similar HOMO energy. 9-boraanthracene and 5-boranaphthacene are irreversibly oxidized by cyclic voltammetry while 6-borapentacene is quasi-irreversibly oxidized. This essentially qualitatively characterizes the kinetics of the redox reaction, so the boron containing anthracene and naphthacene undergo a slow electron transfer rate, while the pentacene has a faster electron transfer rate, but not fast enough to be classified as reversible. For comparison, their carbon analogues are reversibly oxidized. [1]

9-Aza-10-boranthracene

9-aza-10-boraanthracene has peak absorption around 427 nm and broad emission around 513 nm, with HOMO-LUMO gap of 3.075 eV, again the individual HOMO and LUMO energies are higher than anthracene, with an overall lower band gap. [14]

Boranaphthalene

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. [9]

Applications

Boron containing polycyclic aromatic compounds are particularly important in organic electronics. The ability to easily manipulate their absorption and emission wavelengths, as well as the conjugated system’s HOMO and LUMO, through the addition and removal of boron [17] makes them likely candidates for organic semiconductors, utilized in light sensitive systems like photovoltaic cells [18] and light emitting diodes. [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. [20] [21]

Due to its incredibly fast reaction with molecular oxygen, even in darkness at room temperature, Wood et al have proposed that 9-boraanthracene could be used as a highly sensitive oxygen sensor. [1] Current oxygen sensors are used in medicine as well as for environmental precautions. However, they do need to be highly specific and stable, sometimes for temperatures up to 1000°C. While 9-boraanthracene will react with oxygen rapidly, it will also react with any water present, and 9-boraanthracene will break down at high temperatures, which would need to be considered when utilizing it as an oxygen sensor. [22]

The emissive properties of 9-aza-10-boraanthracene also may have potential use as an optical anion sensor. [14] Useful fluorescent anion sensors will generate sensitive, and selective variance in fluorescence in response to specific anions. This is so it can be determined if the analyte is present, what it is specifically, and how much of it there is. [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. However, when fluoride ions were bound to it, the compound fluoresces in a different color, [24] which can be useful for a simple optical anion sensor, without lab quality fluorescence detection. This was also shown to occur with cyanide ions, which means it could be utilized to detect anionic poisons in possibly hazardous areas. [25]

Related Research Articles

Mesitylene or 1,3,5-trimethylbenzene is a derivative of benzene with three methyl substituents positioned symmetrically around the ring. The other two isomeric trimethylbenzenes are 1,2,4-trimethylbenzene (pseudocumene) and 1,2,3-trimethylbenzene (hemimellitene). All three compounds have the formula C6H3(CH3)3, which is commonly abbreviated C6H3Me3. Mesitylene is a colorless liquid with sweet aromatic odor. It is a component of coal tar, which is its traditional source. It is a precursor to diverse fine chemicals. The mesityl group (Mes) is a substituent with the formula C6H2Me3 and is found in various other compounds.

<span class="mw-page-title-main">Silabenzene</span> Chemical compound

A silabenzene is a heteroaromatic compound containing one or more silicon atoms instead of carbon atoms in benzene. A single substitution gives silabenzene proper; additional substitutions give a disilabenzene, trisilabenzene, etc.

<span class="mw-page-title-main">Organoboron chemistry</span> Study of compounds containing a boron-carbon bond

Organoboron chemistry or organoborane chemistry studies organoboron compounds, also called organoboranes. These chemical compounds combine boron and carbon; typically, they are organic derivatives of borane (BH3), as in the trialkyl boranes.

<span class="mw-page-title-main">Borazine</span> Boron compound

Borazine, also known as borazole, is an inorganic compound with the chemical formula B3H6N3. In this cyclic compound, the three BH units and three NH units alternate. The compound is isoelectronic and isostructural with benzene. For this reason borazine is sometimes referred to as “inorganic benzene”. Like benzene, borazine is a colourless liquid with an aromatic odor.

Organogermanium chemistry is the science of chemical species containing one or more C–Ge bonds. Germanium shares group 14 in the periodic table with carbon, silicon, tin and lead. Historically, organogermanes are considered as nucleophiles and the reactivity of them is between that of organosilicon and organotin compounds. Some organogermanes have enhanced reactivity compared with their organosilicon and organoboron analogues in some cross-coupling reactions.

<span class="mw-page-title-main">Acene</span> Class of chemical compounds

In organic chemistry, the acenes or polyacenes are a class of organic compounds and polycyclic aromatic hydrocarbons made up of benzene rings which have been linearly fused. They follow the general molecular formula C4n+2H2n+4.

<span class="mw-page-title-main">Borirene</span> Chemical compound

Borirenes are a unique class of three-membered heterocyclic compounds characterized by an unsaturated boron atom within their ring structure. First computationally predicted by John Pople and Paul von Rague Schleyer in 1981, the simplest borirene, (CH)2BH, is isoelectronic with the cyclopropenium cation and exhibits Hückel aromaticity. Borirenes undergo ring-opening reactions with polar reagents and form Lewis adducts, due to the significant ring strain in its three-membered structure and the presence of an empty p orbital on the boron atom. The balance of these two properties leads to unique properties as a ligand for transition metals, in addition to observation of photochemical rearrangement and ring expansion. While borirenes were first discovered in the 1980s, new derivatives such as benzoborirenes have led to renewed interest in the field, with their potential applications yet to be fully explored.

Carbene analogs in chemistry are carbenes with the carbon atom replaced by another chemical element. Just as regular carbenes they appear in chemical reactions as reactive intermediates and with special precautions they can be stabilized and isolated as chemical compounds. Carbenes have some practical utility in organic synthesis but carbene analogs are mostly laboratory curiosities only investigated in academia. Carbene analogs are known for elements of group 13, group 14, group 15 and group 16.

Boroles represent a class of molecules known as metalloles, which are heterocyclic 5-membered rings. As such, they can be viewed as structural analogs of cyclopentadiene, pyrrole or furan, with boron replacing a carbon, nitrogen and oxygen atom respectively. They are isoelectronic with the cyclopentadienyl cation C5H+5 or abbreviated as Cp+ and comprise four π electrons. Although Hückel's rule cannot be strictly applied to borole, it is considered to be antiaromatic due to having 4 π electrons. As a result, boroles exhibit unique electronic properties not found in other metalloles.

The retro-Diels–Alder reaction is the reverse of the Diels–Alder (DA) reaction, a [4+2] cycloelimination. It involves the formation of a diene and dienophile from a cyclohexene. It can be accomplished spontaneously with heat, or with acid or base mediation.

<span class="mw-page-title-main">Borylene</span>

A borylene is the boron analogue of a carbene. The general structure is R-B: with R an organic moiety and B a boron atom with two unshared electrons. Borylenes are of academic interest in organoboron chemistry. A singlet ground state is predominant with boron having two vacant sp2 orbitals and one doubly occupied one. With just one additional substituent the boron is more electron deficient than the carbon atom in a carbene. For this reason stable borylenes are more uncommon than stable carbenes. Some borylenes such as boron monofluoride (BF) and boron monohydride (BH) the parent compound also known simply as borylene, have been detected in microwave spectroscopy and may exist in stars. Other borylenes exist as reactive intermediates and can only be inferred by chemical trapping.

<span class="mw-page-title-main">1,3-Diphenylisobenzofuran</span> Chemical compound

1,3-Diphenylisobenzofuran is a highly reactive diene that can scavenge unstable and short-lived dienophiles in a Diels-Alder reaction. It is furthermore used as a standard reagent for the determination of singlet oxygen, even in biological systems. Cycloadditions with 1,3-diphenylisobenzofuran and subsequent oxygen cleavage provide access to a variety of polyaromatics.

A phosphetane is a 4-membered organophosphorus heterocycle. The parent phosphetane molecule, which has the formula C3H7P, is one atom larger than phosphiranes, one smaller than phospholes, and is the heavy-atom analogue of azetidines. The first known phosphetane synthesis was reported in 1957 by Kosolapoff and Struck, but the method was both inefficient and hard to reproduce, with yields rarely exceeding 1%. A far more efficient method was reported in 1962 by McBride, whose method allowed for the first studies into the physical and chemical properties of phosphetanes. Phosphetanes are a well understood class of molecules that have found broad applications as chemical building blocks, reagents for organic/inorganic synthesis, and ligands in coordination chemistry.

<span class="mw-page-title-main">Trisilaallene</span> Class of silicon chemical compounds

Trisilaallene is a subclass of silenes derivatives where a central silicon atom forms double bonds with each of two terminal silicon atoms, with the generic formula R2Si=Si=SiR2. Trisilaallene is a silicon-based analog of an allene, but their chemical properties are markedly different.

<span class="mw-page-title-main">Tellurophenes</span> Chemical compound

Tellurophenes are the tellurium analogue of thiophenes and selenophenes.

Aluminium(I) nucleophiles are a group of inorganic and organometallic nucleophilic compounds containing at least one aluminium metal center in the +1 oxidation state with a lone pair of electrons strongly localized on the aluminium(I) center.

<span class="mw-page-title-main">9-Borafluorene</span> Class of chemical compounds

9-borafluorenes are a class of boron-containing heterocycles consisting of a tricyclic system with a central BC4 ring with two fused arene groups. 9-borafluorenes can be thought of as a borole with two fused arene rings, or as a trigonal planar boron atom with an empty p orbital bridging two biphenyl rings. However, 9-borafluorenes are generally less reactive than boroles due to less antiaromatic character and Lewis acidity. Containing highly conjugated π systems, 9-borafluorenes possess interesting photophysical properties. In addition, 9-borafluorenes are good Lewis acids. This combination of properties enables potential uses such as in light-emitting materials, solar cells, and sensors for some molecules.

<i>N</i>-Heterocyclic carbene boryl anion Isoelectronic structure

An N-heterocyclic carbene boryl anion is an isoelectronic structure of an N-heterocyclic carbene (NHC), where the carbene carbon is replaced with a boron atom that has a -1 charge. NHC boryl anions have a planar geometry, and the boron atom is considered to be sp2-hybridized. They serve as extremely strong bases, as they are very nucleophilic. They also have a very strong trans influence, due to the σ-donation coming from the boron atom. NHC boryl anions have stronger electron-releasing character when compared to normal NHCs. These characteristics make NHC boryl anions key ligands in many applications, such as polycyclic aromatic hydrocarbons, and more commonly low oxidation state main group element bonding.

<span class="mw-page-title-main">Borepin</span> Aromatic, boron-containing rings

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 6π electrons. Unlike other seven-membered systems such as silepins and phosphepins, boron has a vacant p-orbital that can interact with the π and π* orbitals of the cycloheptatriene. This leads to an isoelectronic state 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 (OLEDs) and photovoltaic cells.

<span class="mw-page-title-main">Aluminylene</span>

Aluminylenes are a sub-class of aluminium(I) compounds that feature singly-coordinated aluminium atoms with a lone pair of electrons. As aluminylenes exhibit two unoccupied orbitals, they are not strictly aluminium analogues of carbenes until stabilized by a Lewis base to form aluminium(I) nucleophiles. The lone pair and two empty orbitals on the aluminium allow for ambiphilic bonding where the aluminylene can act as both an electrophile and a nucleophile. Aluminylenes have also been reported under the names alumylenes and alanediyl.

References

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  2. ↑ Farrell, Jeffrey M.; Mützel, Carina; Bialas, David; Rudolf, Maximilian; Menekse, Kaan; Krause, Ana-Maria; Stolte, Matthias; Würthner, Frank (2019-06-05). "Tunable Low-LUMO Boron-Doped Polycyclic Aromatic Hydrocarbons by General One-Pot C–H Borylations". Journal of the American Chemical Society. 141 (22): 9096–9104. doi:10.1021/jacs.9b04675. ISSN   0002-7863. PMID   31117551. S2CID   162181390.
  3. ↑ Kawai, Shigeki; Saito, Shohei; Osumi, Shinichiro; Yamaguchi, Shigehiro; Foster, Adam S.; Spijker, Peter; Meyer, Ernst (November 2015). "Atomically controlled substitutional boron-doping of graphene nanoribbons". Nature Communications. 6 (1): 8098. Bibcode:2015NatCo...6.8098K. doi:10.1038/ncomms9098. ISSN   2041-1723. PMC   4560828 . PMID   26302943.
  4. ↑ Entwistle, Christopher D.; Marder, Todd B. (2004-11-01). "Applications of Three-Coordinate Organoboron Compounds and Polymers in Optoelectronics". Chemistry of Materials. 16 (23): 4574–4585. doi:10.1021/cm0495717. ISSN   0897-4756.
  5. ↑ Chen, Jinhui; Kampf, Jeff W.; Ashe, Arthur J. (2008-08-01). "Syntheses and Structures of 6,13-Dihydro-6,13-diborapentacenes: π-Stacking in Heterocyclic Analogues of Pentacene". Organometallics. 27 (15): 3639–3641. doi:10.1021/om8005068. ISSN   0276-7333.
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  10. ↑ Van Veen, R.; Bickelhaupt, F. (1970-10-01). "Attempted synthesis of dibenzo[b,e]borin (9-boraanthracene) and its formation under electron impact". Journal of Organometallic Chemistry. 24 (3): 589–594. doi:10.1016/S0022-328X(00)84487-3. ISSN   0022-328X.
  11. 1 2 3 Veen, R. Van; Bickelhaupt, F. (1974-09-10). "Reactions of the 9-mesityl-9-boraanthracene anion". Journal of Organometallic Chemistry. 77 (2): 153–165. doi:10.1016/S0022-328X(00)81314-5. ISSN   0022-328X.
  12. ↑ Wood, Thomas K.; Piers, Warren E.; Keay, Brian A.; Parvez, Masood (2009-05-18). "9-Boraanthracene Derivatives Stabilized by N-Heterocyclic Carbenes". Angewandte Chemie International Edition. 48 (22): 4009–4012. doi:10.1002/anie.200901217. PMID   19402082.
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  24. ↑ Agou, Tomohiro; Sekine, Masaki; Kobayashi, Junji; Kawashima, Takayuki (2009). "Synthesis and reactivity of a bis(dimesitylboryl)azaborine and its fluoride sensing ability". Chemical Communications (14): 1894–1896. doi:10.1039/b818505k. ISSN   1359-7345. PMID   19319437.
  25. ↑ Agou, Tomohiro; Sekine, Masaki; Kobayashi, Junji; Kawashima, Takayuki (November 2009). "Multi-step detection of cyanide ion by a bis(dimesitylboryl)dibenzoazaborine". Journal of Organometallic Chemistry. 694 (23): 3833–3836. doi:10.1016/j.jorganchem.2009.07.046.
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