Borazine

Borazine is an inorganic compound with the chemical formula (BH)₃(NH)₃. 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"

Synthesis

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

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:

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

Borazine is a colourless liquid with an aromatic smell. In water it hydrolyzes 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 and bonding

Borazine is isostructural with benzene. The six B-N bonds have length of 1.436 Å. The carbon-carbon bond in benzene is shorter length at 1.397 Å. The boron-nitrogen bond length 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. 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. Borazine is more reactive than benzene. It reacts with hydrogen chloride in an addition reaction. Benzene, in contrast, is unreactive toward HCl.

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 interact via nucleophilic attack at boron and electrophilic attack at nitrogen. Heating borazine at 70 °C expels 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.

Borazine and its 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.
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.