Borazine thermolysis

IV-substituted and B-substituted borazines are readily prepared by suitable choice of amine and borane starting materials or by subsequent reaction of other borazines with Grignard reagents, etc. Thermolysis of monocyclic borazines leads to polymeric materials and to polyborazine analogues of naphthalene, biphenyl, etc. ... 

Poly[/i-(alkylamino)borazines] can be prepared through a multistep thermolysis process involving tris(monoalkylamino)boranes of the type B(NHR)3 (R = Me, Pr ). Heating these molecular precursors under reduced pressure or in an inert atmosphere... 

Borazine, B3N3H6, was first prepared by thermolysis of the diborane ammonia adduct [(BH2)(NH3)2] [BH4] . More convenient procedures for the laboratory preparation of this important ring system in multigram quantities involve either (a) the decomposition of ammonia- borane [eqn (9.1)] or (b) the reaction between ammonium sulfate and sodium borohydride [eqn (9.2)]. The latter method provides a convenient and economical synthesis of borazine. 

Borazines, particularly polymeric compounds, have been extensively investigated as preceramic materials from which coatings and fibers of boron nitride can be produced upon thermolysis. B-aryl and halogeno-amino borazines are reported to have use as fire retardants in cotton and nylon textiles. Other reported uses for borazines are as epoxy resin catalysts, polymerization inhibitors of unsaturated alcohols and esters, and catalysts for polymerization of alkenes (95). 

Unfortunately, borazine itself is quite volatile and any thermolysis reactions must be conducted in a pressure vessel with provision for removal of hydrogen as the pyrolysis proceeds. 

As shown by Framery and Vaultier, N-substituted borazines are easily accessible by thermolysis of primary amine-borane complexes RNH2 BH3 [10], Based on this work, N-trimethylborazine (N-TMB) and A-triethylborazine (N-TEB) were synthesized pure and in excellent yields (Scheme 1). 

The complexes were obtained after the removal of thf and SMe2 under vacuum. Thermolysis of these complexes at 120 °C led to borazanes as intermediates. A further thermolysis at 200 °C gave the desired borazines, which were purified by distillation. 

Scheme 1. Synthesis of A/ alkyl-substituted borazines by thermolysis of primary amine-borane complexes...

This graphite form of BN transforms into a cubic, diamondlike form under pressure at 1800°C, which is claimed to surpass diamond in mechanical strength [30]. Combinations of B-trichloroborazine and hexamethyl-disilazane lead to the formation of gels, which upon thermolysis give a hexagonal BN material. Similarly, on pyrolysis B-trianilinoborazine, B-triamino-A-triphenyl-borazine, and B-triaminoborazine also give BN ceramics with the presence of C impurities. The pyrolysis of the B-triamino-N-triphenylborazine (3) is [26,31]... 

Figure 14.2 Biphenyl and naphtalen-type units formed during thermolysis of borazine...

In another paper [15], these authors have prepared the first inorganic mesophase by low-temperature thermolysis of the borazine. Following the synthesis procedure reported by Sneddon etal. [11-13] (Eq. 14.1), the prediction made on the formation of biphenyl and naphtalen-type units and requirements for the formation of pitch mesophases [16], the authors have modified the thermolysis rate to maintain a certain mobility in as-formed molecules, and therefore obtain optically anisotropic phases during thermolysis of borazine. The formation of a liquid-crystalline phase during thermolysis provides an efficient way to produce a final BN material with a high degree of crystalline order by heating to 1800 °C. 

Thermolysis of ammonium borohydride produces nearly pnre hydrogen, with only a trace of borazine evolved at higher temperatures. The primary disadvantage of ammonium borohydride for many practical applications is its poor thermal stability. Ammonium borohydride slowly decomposes even at room temperature. Thus, methods to stabilize ammonium borohydride are needed to make this material practical for use in hydrogen storage applications. Ammonium borohydride also serves as a synthetic precnrsor for ammonia borane (AB), as discussed below. 

---