Borazine

Borazine is an inorganic compound composed of the elements boron, nitrogen and hydrogen. In this cyclic compound three hydroborane (BH) units and three amino units (NH) alternate. The compound was synthesised in 1926 by the chemists Alfred Stock and Pohland by a reaction of diborane with ammonia. ^[1] The structure is isoelectronic and isostructural with benzene and for this reason borazine is called inorganic benzene by a proposal of Nils Wiberg and the compound also goes by the name of borazol from the German name for benzene which is benzol.

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Synthesis

Borazine is synthesized from 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 lithium borohydride and ammonium chloride with improved chemical yield:

3 LiBH₄ + 3 NH₄Cl → B₃H₆N₃ + 3 LiCl + 9 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:

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

Properties

Borazine is a colourless liquid with an aromatic smell. In water it decomposes to boric acid, ammonia, and hydrogen. Borazine, with a standard enthalpy change of formation ΔH_f of -531 kJ/mol, is thermally very stable.

Structure

Borazine is isostructural with benzene and bond lengths are identical just as in benzene. The distance between boron and nitrogen in the ring is 0.1436 nm, the carbon carbon bond in benzene has a length of 0.1397 nm. The boron nitrogen bond is between that of the boron nitrogen single bond with 0.151 nm and the boron nitrogen double bond which is 0.131 nm. This suggests partial delocalisation of nitrogen lone pair electrons.

Mesomers

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.

Boron is the Lewis acid and nitrogen is the Lewis base.

Reactions

Borazine is more reactive than benzene. It reacts with hydrogen chloride in an addition reaction. If borazine were truly aromatic like benzene this reaction would not occur without a Lewis acid catalyst.

B₃N₃H₆ + 3HCl → B₃N₃H₉Cl₃

Addition reaction of borazine with hydrogen chloride

B₃N₃H₉Cl₃ + NaBH₄ → (BH₄N)₃

reduction with sodium borohydride

The addition reaction with bromine takes place without catalyst. Borazines interact with nucleophilic attack at boron and electrophilic attack at nitrogen. Heating borazine at 70 °C expulses hydrogen with formation of a borazinyl polymer or polyborazylene in which the monomer units are coupled in a para fashion by new boron - nitrogen bonds.

Applications

Borazine and borazine derivatives are potential precursors to boron nitride ceramics. 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 boron nitride thin films on surfaces, such as the nanomesh structure which is formed on rhodium.

References

• Polymeric precursors to boron based ceramics Larry G. Sneddon, Mario G. L. Mirabelli, Anne T. Lynch, Paul J. Fazen, Kai Su, and Jeffrey S. Beckdon Pure & Appl. Chem., Vol. 63, No. 3, pp. 407-410, 1991. Article
• Synthesis of Novel Amorphous Boron Carbonitride Ceramics from the Borazine Derivative Copolymer via Hydroboration Jong-Kyu Jeon, Yuko Uchimaru, and Dong-Pyo Kim Inorg. Chem., 43 (16), 4796 -4798, 2004. Abstract
• New perspectives in boron-nitrogen chemistry - I P. Paetzold Pure & Appl. Chern., Vol. 63, No. 3, pp. 345-350, 1991. Article