Borazine

{{chembox

| Watchedfields = changed

| verifiedrevid = 444958440

| ImageFileL1 = Borazine-elpot-3D-vdW.png

| ImageFileR1 = Borazine-3D-vdW.png

| ImageFile = Borazine-dimensions-2D.svg

| PIN = 1,3,5,2,4,6-Triazatriborinane (only preselected{{cite book | title = Nomenclature of Organic Chemistry : IUPAC Recommendations and Preferred Names 2013 (Blue Book) | publisher = The Royal Society of Chemistry | date = 2014 | location = Cambridge | page = 968 | doi = 10.1039/9781849733069-FP001 | isbn = 978-0-85404-182-4| chapter = Front Matter }})

| OtherNames =Cyclotriborazaneborazol
Inorganic benzene
Borazole

|Section1={{Chembox Identifiers

| ChemSpiderID_Ref = {{chemspidercite|correct|chemspider}}

| ChemSpiderID = 122374

| InChI = 1/B3H6N3/c1-4-2-6-3-5-1/h1-6H

| InChIKey = BGECDVWSWDRFSP-UHFFFAOYAU

| StdInChI_Ref = {{stdinchicite|correct|chemspider}}

| StdInChI = 1S/B3H6N3/c1-4-2-6-3-5-1/h1-6H

| StdInChIKey_Ref = {{stdinchicite|correct|chemspider}}

| StdInChIKey = BGECDVWSWDRFSP-UHFFFAOYSA-N

| CASNo_Ref = {{cascite|correct|??}}

| CASNo = 6569-51-3

| UNII_Ref = {{fdacite|correct|FDA}}

| UNII = CEM55A0275

| PubChem = 138768

| ChEBI_Ref = {{ebicite|correct|EBI}}

| ChEBI = 33119

| SMILES = [BH-]1-[NH+]=[BH-]-[NH+]=[BH-]-[NH+]=1

}}

|Section2={{Chembox Properties

|Formula = B₃H₆N₃

|MolarMass = 80.50 g/mol

|Appearance = Colorless liquid

|Density = 0.81 g/cm³

|MeltingPtC = −58

|BoilingPtC = 53

|BoilingPt_notes = (55 °C at 105 Pa)

|MagSus = −49.6·10⁻⁶ cm³/mol

}}

|Section3={{Chembox Hazards

| NFPA-H = 2

| NFPA-F = 2

| NFPA-R = 1

}}

Borazine, also known as borazole, inorganic benzene, 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{{cite book |author=Duward Shriver |author2=Peter Atkins |title=Inorganic Chemistry |edition=Fifth |publisher=W. H. Freeman and Company |location=New York |year=2010 |page=328 |isbn=978-1429218207}} 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.{{cite journal |title=Borwasserstoffe, VIII. Zur Kenntnis des B₂H₆ und des B₅H₁₁ |language=German |trans-title= Boric acid solution, VIII Regarding knowledge of B₂H₆ and B₅H₁₁ |last1= Stock |first1=Alfred |last2=Pohland |first2=Erich | name-list-style = vanc |journal=Berichte |volume=59 |issue=9 |pages= 2210–2215 |date= October 1926 |doi=10.1002/cber.19260590906}}

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:{{cite book |series=Inorganic Syntheses|volume=32|year=1998|first1=Thomas|last1= Wideman|first2=Paul J.|last2= Fazen|first3=Anne T.|last3= Lynch|first4=Kai|last4= Su|first5=Edward E.|last5=Remsen|first6=Larry G.|last6=Sneddon|title=Borazine, Polyborazylene, β‐Vinylborazine, and Poly(β‐Vinylborazine)

|chapter=Borazine, Polyborazylene, β-Vinylborazine, and Poly(β-Vinylborazine)|pages=232–242|doi=10.1002/9780470132630.ch39|isbn=9780470132630}}

: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.{{cite journal| vauthors = Boese R, Maulitz AH, Stellberg P |date=1994|title=Solid-State Borazine: Does it Deserve to be Entitled "Inorganic Benzene"? |journal=Chemische Berichte|volume=127|issue=10|pages=1887–1889|doi=10.1002/cber.19941271011}} 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 D_3h 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.

Image: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 Δ_fH of −531 kJ/mol, is thermally very stable.

=Natural bond orbitals (NBO)=

Natural bond orbital (NBO) analysis suggests weak aromaticity in borazine.{{cite journal| vauthors = Shen W, Li M, Li F, Wang S |title=Theoretical study of borazine and its derivatives|journal=Inorg. Chim. Acta|date=2007|volume=360|issue=2|pages=619–624|doi=10.1016/j.ica.2006.08.028}} 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.{{cite journal | vauthors = Santos JC, Tiznado W, Contreras R, Fuentealba P | title = Sigma-pi separation of the electron localization function and aromaticity | journal = The Journal of Chemical Physics | volume = 120 | issue = 4 | pages = 1670–3 | date = January 2004 | pmid = 15268298 | doi = 10.1063/1.1635799 | bibcode = 2004JChPh.120.1670S| hdl = 10533/175272 | hdl-access = free }} For benzene, this value is 0.91, but the borazine π system bifurcates at the ELF value 0.682.{{cite journal|title=Borazine: to be or not to be aromatic| vauthors = Islas R, Chamorro E, Robles J, Heine T, Santos JC, Merino G | s2cid = 95098134 |journal=Struct. Chem.|date=2007|volume=18|issue=6|pages=833–839|doi=10.1007/s11224-007-9229-z}} 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=

Image:Polyborazylene Polymer.svg

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.

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,{{cite journal | vauthors = Kidambi PR, Blume R, Kling J, Wagner JB, Baehtz C, Weatherup RS, Schloegl R, Bayer BC, Hofmann S | display-authors = 6 | title = In Situ Observations during Chemical Vapor Deposition of Hexagonal Boron Nitride on Polycrystalline Copper | journal = Chemistry of Materials | volume = 26 | issue = 22 | pages = 6380–6392 | date = November 2014 | pmid = 25673919 | pmc = 4311958 | doi = 10.1021/cm502603n }} platinum,{{cite journal | vauthors = Kim G, Jang AR, Jeong HY, Lee Z, Kang DJ, Shin HS | title = Growth of high-crystalline, single-layer hexagonal boron nitride on recyclable platinum foil | journal = Nano Letters | volume = 13 | issue = 4 | pages = 1834–9 | date = April 2013 | pmid = 23527543 | doi = 10.1021/nl400559s | bibcode = 2013NanoL..13.1834K }} nickel{{Cite journal|last1=Chatterjee|first1=Shahana|last2=Luo|first2=Zhengtang|last3=Acerce|first3=Muharrem|last4=Yates|first4=Douglas M.|last5=Johnson|first5=A. T. Charlie|last6=Sneddon|first6=Larry G. | name-list-style = vanc |date=2011-10-25|title=Chemical Vapor Deposition of Boron Nitride Nanosheets on Metallic Substrates via Decaborane/Ammonia Reactions |journal=Chemistry of Materials|language=en|volume=23|issue=20|pages=4414–4416|doi=10.1021/cm201955v|issn=0897-4756}} iron{{cite journal | vauthors = Caneva S, Weatherup RS, Bayer BC, Brennan B, Spencer SJ, Mingard K, Cabrero-Vilatela A, Baehtz C, Pollard AJ, Hofmann S | display-authors = 6 | title = Nucleation control for large, single crystalline domains of monolayer hexagonal boron nitride via Si-doped Fe catalysts | journal = Nano Letters | volume = 15 | issue = 3 | pages = 1867–75 | date = March 2015 | pmid = 25664483 | pmc = 4358078 | doi = 10.1021/nl5046632 | bibcode = 2015NanoL..15.1867C }} and many more, with chemical vapor deposition (CVD).

Image:Boroncaronitrides.svg

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.{{cite journal | vauthors = Davis BL, Dixon DA, Garner EB, Gordon JC, Matus MH, Scott B, Stephens FH | title = Efficient regeneration of partially spent ammonia borane fuel | journal = Angewandte Chemie | volume = 48 | issue = 37 | pages = 6812–6 | year = 2009 | pmid = 19514023 | doi = 10.1002/anie.200900680 | osti = 960522 }}

Among other B-N type compounds mixed amino-nitro substituted borazines have been predicted to outperform carbon based explosives such as CL-20.{{cite journal | vauthors = Koch EC, Klapötke TM |year=2012 |title=Boron-Based High Explosives. |volume=37 |issue=3 |pages=335–344 |doi=10.1002/prep.201100157 |journal=Propellants, Explosives, Pyrotechnics|doi-access=free }}{{cite journal | vauthors = Kervyn S, Fenwick O, Di Stasio F, Shin YS, Wouters J, Accorsi G, Osella S, Beljonne D, Cacialli F, Bonifazi D | display-authors = 6 | title = Polymorphism, fluorescence, and optoelectronic properties of a borazine derivative | journal = Chemistry: A European Journal | volume = 19 | issue = 24 | pages = 7771–9 | date = June 2013 | pmid = 23616404 | doi = 10.1002/chem.201204598 | s2cid = 9774352 | url = http://qmro.qmul.ac.uk/xmlui/handle/123456789/12638 | doi-access = free }}

Related compounds

({{chem|C|2|H|2|B|2|N|2}}) is a six-membered aromatic ring with two carbon atoms, two nitrogen atoms, and two boron atoms in opposing pairs.{{cite journal | last1 = Srivastava | first1 = Ambrish Kumar | last2 = Misra | first2 = Neeraj | name-list-style = vanc | s2cid = 94792421 | year = 2015 |title = Introducing "carborazine" as a novel heterocyclic aromatic species | journal = New Journal of Chemistry | volume = 39 | issue = 4| pages = 2483–2488 | doi=10.1039/c4nj02089h}}{{cite journal | vauthors = Bonifazi D, Fasano F, Lorenzo-Garcia MM, Marinelli D, Oubaha H, Tasseroul J | title = Boron-nitrogen doped carbon scaffolding: organic chemistry, self-assembly and materials applications of borazine and its derivatives | journal = Chemical Communications | volume = 51 | issue = 83 | pages = 15222–36 | date = October 2015 | pmid = 26411675 | doi = 10.1039/C5CC06611E }}

1,2-Dihydro-1,2-azaborine ({{chem|C|4|B|N|H|6}}) is a six-membered ring with four carbon atoms, one nitrogen atom, and one boron atom.