Chemistry:Borazine

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Short description: Boron compound
Borazine
Borazine-dimensions-2D.svg
Borazine-elpot-3D-vdW.png
Borazine-3D-vdW.png
Names
Preferred IUPAC name
1,3,5,2,4,6-Triazatriborinane (only preselected[1])
Other names
Cyclotriborazaneborazol
Inorganic benzene
Borazole
Identifiers
3D model (JSmol)
ChEBI
ChemSpider
UNII
Properties
B3H6N3
Molar mass 80.50 g/mol
Appearance Colorless liquid
Density 0.81 g/cm3
Melting point −58 °C (−72 °F; 215 K)
Boiling point 53 °C (127 °F; 326 K) (55 °C at 105 Pa)
-49.6·10−6 cm3/mol
Hazards
NFPA 704 (fire diamond)
Flammability code 2: Must be moderately heated or exposed to relatively high ambient temperature before ignition can occur. Flash point between 38 and 93 °C (100 and 200 °F). E.g. diesel fuelHealth code 2: Intense or continued but not chronic exposure could cause temporary incapacitation or possible residual injury. E.g. chloroformReactivity code 1: Normally stable, but can become unstable at elevated temperatures and pressures. E.g. calciumSpecial hazards (white): no codeNFPA 704 four-colored diamond
2
2
1
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
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Infobox references

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[2] with an aromatic odor.

Synthesis

The compound was reported in 1926 by the chemists Alfred Stock and Erich Pohland by a reaction of diborane with ammonia.[3]

Borazine can be synthesized by treating diborane and ammonia in a 1:2 ratio at 250–300 °C with a conversion of 50%.

3 B2H6 + 6 NH3 → 2 B3H6N3 + 12 H2

An alternative more efficient route begins with sodium borohydride and ammonium sulfate:[4]

6 NaBH4 + 3 (NH4)2SO4 → 2 B3N3H6 + 3 Na2SO4 + 18 H2

In a two-step process to borazine, boron trichloride is first converted to trichloroborazine:

3 BCl3 + 3 NH4Cl → Cl3B3H3N3 + 9 HCl

The B-Cl bonds are subsequently converted to B-H bonds:

2 Cl3B3H3N3 + 6 NaBH4 → 2 B3H6N3 + 3 B2H6 + 6 NaCl

Structure

Borazine is isoelectronic with benzene and has similar connectivity, so it is sometimes referred to as "inorganic benzene". This comparison is not rigorously valid due to the electronegativity difference between boron and nitrogen. X-ray crystallographic structural determinations show that the bond lengths within the borazine ring are all equivalent at 1.429 Å, a property shared by benzene.[5] However, the borazine ring does not form a perfect hexagon. The bond angle is 117.1° at the boron atoms and 122.9° at the nitrogens, giving the molecule the D3h symmetry point group.

The electronegativity of boron (2.04 on the Pauling scale) compared to that of nitrogen (3.04) and also the electron deficiency on the boron atom and the lone pair on nitrogen favor alternative mesomer structures for borazine.

Borazin Mesomers1.svg

Boron behaves as a Lewis acid and nitrogen behaves as a Lewis base.

Aromaticity

Due to its similarities to benzene, there have been a number of computational and experimental analyses of borazine's aromaticity. The number of pi electrons in borazine obeys the 4n + 2 rule, and the B-N bond lengths are equal, which suggests the compound may be aromatic. The electronegativity difference between boron and nitrogen, however, creates an unequal sharing of charge which results in bonds with greater ionic character, and thus it is expected to have poorer delocalization of electrons than the all-carbon analog. Borazine, with a standard enthalpy change of formation ΔHf of −531 kJ/mol, is thermally very stable.

Natural bond orbitals (NBO)

Natural bond orbital (NBO) analysis suggests weak aromaticity in borazine.[6] In the NBO model, B-N bonds in the ring are slightly displaced from the nuclear axes, and B and N have large differences in charge. Natural chemical shielding (NCS) analysis provides some further evidence for aromaticity based on a contribution of the B-N π bond to magnetic shielding. Computations based on NBO orbitals show that this π bond allows for weak ring current which somewhat counteracts a magnetic field simulated at the center of the borazine ring. A small ring current does suggest some delocalization.

Electron localization function (ELF)

Topological analysis of bonding in borazine by the electron localization function (ELF) indicates that borazine can be described as a π aromatic compound. However, the bonding in borazine is less delocalized than in benzene based on a difference in bifurcation values of the electron basins. Larger bifurcation values indicate better electron delocalization, and it is argued that when this bifurcation value is greater than 0.70, the delocalization is sufficient to designate a compound aromatic.[7] For benzene, this value is 0.91, but the borazine π system bifurcates at the ELF value 0.682.[8] This is caused by the difference in electronegativity between B and N, which produces a weaker bond interaction than the C-C interaction in benzene, leading to increased localization of electrons on the B-H and N-H units. The bifurcation value is slightly below the limit of 0.70 which suggests moderate aromaticity.

Reactivity

Hydrolysis

Borazine hydrolyzes readily, yielding boric acid, ammonia, and hydrogen.

Polymerization

Polyborazylene

Heating borazine at 70 °C expels hydrogen with formation of polyborazylene:

n B3N3H6 → 1/n[B3N3H4]n

With hydrogen halides and halogens

With hydrogen chloride it forms an adduct.

B3N3H6 + 3 HCl → B3N3H9Cl3
Addition reaction of borazine with hydrogen chloride
B3N3H9Cl3 + NaBH4 → (BH4N)3
Reduction with sodium borohydride

The addition reaction with bromine does not require a catalyst. Borazines undergo nucleophilic attack at boron and electrophilic attack at nitrogen.

Ceramic precursor

Boron nitride can be prepared by heating polyborazylene to 1000 °C.[4]

Borazines are also starting materials for other potential ceramics such as boron carbonitrides. Borazine can also be used as a precursor to grow hexagonal boron nitride (h-BN) thin films and single layers on catalytic surfaces such as copper,[9] platinum,[10] nickel[11] iron[12] and many more, with chemical vapor deposition (CVD).

synthetic route to boron carbonitrides, first step a hydroboration reaction to an oligomeric precursor followed by step two: pyrolysis

Polyborazylene has been proposed as a recycled hydrogen storage medium for hydrogen fuel cell vehicle applications, using a "single pot" process for digestion and reduction to recreate ammonia borane.[13]

Among other B-N type compounds mixed amino-nitro substituted borazines have been predicted to outperform carbon based explosives such as CL-20.[14][15]

Related compounds

(C2H2B2N2) is a six-membered aromatic ring with two carbon atoms, two nitrogen atoms, and two boron atoms in opposing pairs.[16][17]

1,2-Dihydro-1,2-azaborine (C4BNH6) is a six-membered ring with four carbon atoms, one nitrogen atom, and one boron atom.

See also

Further reading

References

  1. ↑ "Front Matter". Nomenclature of Organic Chemistry : IUPAC Recommendations and Preferred Names 2013 (Blue Book). Cambridge: The Royal Society of Chemistry. 2014. p. 968. doi:10.1039/9781849733069-FP001. ISBN 978-0-85404-182-4. 
  2. ↑ Duward Shriver; Peter Atkins (2010). Inorganic Chemistry (Fifth ed.). New York: W. H. Freeman and Company. p. 328. ISBN 978-1429218207. 
  3. ↑ Stock, Alfred; Pohland, Erich (October 1926). "Borwasserstoffe, VIII. Zur Kenntnis des B2H6 und des B5H11" (in German). Berichte 59 (9): 2210–2215. doi:10.1002/cber.19260590906. 
  4. ↑ 4.0 4.1 Wideman, Thomas; Fazen, Paul J.; Lynch, Anne T.; Su, Kai; Remsen, Edward E.; Sneddon, Larry G. (1998). "Borazine, Polyborazylene, β-Vinylborazine, and Poly(β-Vinylborazine)". Borazine, Polyborazylene, β‐Vinylborazine, and Poly(β‐Vinylborazine). Inorganic Syntheses. 32. pp. 232–242. doi:10.1002/9780470132630.ch39. ISBN 9780470132630. 
  5. ↑ "Solid-State Borazine: Does it Deserve to be Entitled "Inorganic Benzene"?". Chemische Berichte 127 (10): 1887–1889. 1994. doi:10.1002/cber.19941271011. 
  6. ↑ "Theoretical study of borazine and its derivatives". Inorg. Chim. Acta 360 (2): 619–624. 2007. doi:10.1016/j.ica.2006.08.028. 
  7. ↑ "Sigma-pi separation of the electron localization function and aromaticity". The Journal of Chemical Physics 120 (4): 1670–3. January 2004. doi:10.1063/1.1635799. PMID 15268298. Bibcode: 2004JChPh.120.1670S. 
  8. ↑ "Borazine: to be or not to be aromatic". Struct. Chem. 18 (6): 833–839. 2007. doi:10.1007/s11224-007-9229-z. 
  9. ↑ "In Situ Observations during Chemical Vapor Deposition of Hexagonal Boron Nitride on Polycrystalline Copper". Chemistry of Materials 26 (22): 6380–6392. November 2014. doi:10.1021/cm502603n. PMID 25673919. 
  10. ↑ "Growth of high-crystalline, single-layer hexagonal boron nitride on recyclable platinum foil". Nano Letters 13 (4): 1834–9. April 2013. doi:10.1021/nl400559s. PMID 23527543. Bibcode: 2013NanoL..13.1834K. 
  11. ↑ Chatterjee, Shahana; Luo, Zhengtang; Acerce, Muharrem; Yates, Douglas M.; Johnson, A. T. Charlie; Sneddon, Larry G. (2011-10-25). "Chemical Vapor Deposition of Boron Nitride Nanosheets on Metallic Substrates via Decaborane/Ammonia Reactions" (in en). Chemistry of Materials 23 (20): 4414–4416. doi:10.1021/cm201955v. ISSN 0897-4756. 
  12. ↑ "Nucleation control for large, single crystalline domains of monolayer hexagonal boron nitride via Si-doped Fe catalysts". Nano Letters 15 (3): 1867–75. March 2015. doi:10.1021/nl5046632. PMID 25664483. Bibcode: 2015NanoL..15.1867C. 
  13. ↑ "Efficient regeneration of partially spent ammonia borane fuel". Angewandte Chemie 48 (37): 6812–6. 2009. doi:10.1002/anie.200900680. PMID 19514023. 
  14. ↑ "Boron-Based High Explosives.". Propellants, Explosives, Pyrotechnics 37 (3): 335–344. 2012. doi:10.1002/prep.201100157. 
  15. ↑ "Polymorphism, fluorescence, and optoelectronic properties of a borazine derivative". Chemistry: A European Journal 19 (24): 7771–9. June 2013. doi:10.1002/chem.201204598. PMID 23616404. http://qmro.qmul.ac.uk/xmlui/handle/123456789/12638. 
  16. ↑ Srivastava, Ambrish Kumar; Misra, Neeraj (2015). "Introducing "carborazine" as a novel heterocyclic aromatic species". New Journal of Chemistry 39 (4): 2483–2488. doi:10.1039/c4nj02089h. 
  17. ↑ "Boron-nitrogen doped carbon scaffolding: organic chemistry, self-assembly and materials applications of borazine and its derivatives". Chemical Communications 51 (83): 15222–36. October 2015. doi:10.1039/C5CC06611E. PMID 26411675. 

Further reading

External links



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