Pyrolysis of borazine

Alternative routes to boron nitride have been examined that do not require those challenging reaction conditions. These fall into three categories, which are (1) the pyrolysis of borazines (2) pyrolysis of organic or inorganic polymers that bear borazine groups as side units and (3) pyrolysis of polyhedral borane derivatives. 

Apart from the above methods, some of which employ drastic conditions, processes close to equilibrium conditions such as pyrolysis and chemical vapor deposition (CVD) have also been employed to prepare BN nanotubes. The CVD growth of hollow, crystalline BN nanotubules by the pyrolysis of borazine on nickel boride catalyst particles maintained at 1270-1370 K, produced nanotubes with bulbous or flag-like caps (Fig. 38). The reaction is given by,... 

The borazine rings coupled at ring boron atoms by NH can be prepared" by a controlled pyrolysis of boranes such as B(NHR)3i polymers thus linked can be formed by pyrolysis of borazines that have two B-amino substituents and all other ring atoms alkylated. An amine molecule is eliminated and NR links the rings ... 

The pyrolysis of bis(trimethylsilyl)aminodichloroborane to B-trichloro-N-tris(trimethylsilyl)borazine, contrary to literature (in. did not take place in boiling xylene. Temperatures above 150 C were necessary for trimethylchlorosilane elimination. The highest yield of the relatively pure product was around 20%. The transformation of the B-trichloro-N-tris(trimethylsilylJborazine to B-triamino-N-tris(trimethylsilyl)borazine proceeded readily using liquid ammonia. 

Fig. 8. Data showing (a) the pressure dependence of the quantum yield of H2 produced in the 184.9 nm pyi dysis of borazine and (b) the build-up of solid polymer during the pyrolysis reaction...

Ceramic-type materials that contain no organic linkage units can be prepared by the pyrolysis of cyclic or high polymeric aminophosphazenes. An example is shown in reaction (44). Under appropriate conditions, pyrolysis products that correspond to phosphorus-nitride are formed. Polyphosphazenes that contain both amino and borazine side groups yield phosphorus-nitrogen-boron ceramics following pyrolysis 94,95 The conversion of a formable polymer into a ceramic has many potential advantages for the controlled synthesis and fabrication of advanced ceramics. This principle is discussed in more detail in Chapter 9. 

The tetramer is said to be irreversibly converted into the trimeric product by heating above its melting point. In contrast to an earlier report claiming the synthesis of linear N-isobutyl-B-phenyl-polybora-zynes 86> only the normal borazine-formation could be observed on the pyrolysis of bis(isobutylamino)phenylborane 87> ... 

Steric hindrance in aryl-substituted borazines depends on o-substitu-tion of the aryl groups. Significant results concerning the reaction of primary arylamines with trihaloboranes, have recently been discussed in detail 89>9°). While pyrolysis of aniline-trihaloboranes or o-toluidine-trihaloboranes yields the corresponding B-trichloroborazines, the base-promoted dehydrohalogenation of the latter results in the formation of a l,3-diazaro-2,4-dibora-naphthalene compound besides a minor amount of borazine. 

A new class of boron compounds has been prepared by the reaction of pentacarbonyliron with dichloro(dimethylamino)borane to form (CO)4FeBN(CH3)2, a borylene complex.10 The mono- and dihaloamino-boranes can be prepared from the corresponding tris(amino)borane.11,12 Pyrolysis of the tris(primary amino)borane B(NHR)3 yields the borazine B3(NHR)3N3R3.3... 

Scheme 2. Fonnation (pyrolysis) of the ceramic material from B-tris(tricbIorosilylvinyl)borazine (2) via different synthetic pathways.

Pyrolysis techniques that involve the elimination of hydrogen or alkanes provide facile routes to boron- or aluminum-containing small-ring systems. For example, borazines are synthesized by the pyrolysis of mixtures of boranes and amines ... 

Boron nitride can be prepared by many routes such as CVD (NH3 + organoborane), 2 pyrolysis of preceramic polymers derived from borazine, 2 and solid-state metathesis... 

Initially, BN was synthesized by the pyrolysis of boric acid and urea in the presence of ammonia [27]. In this reaction, boric acid presumably reacts with urea to form a urea-boric acid complex gel, which on pyrolysis in an ammonia atmosphere results in the formation of BN [28]. Precursors for BN recently received the greatest attention, with emphasis on borazine-derived polymers. It has been recognized that borazine (1) can form polymeric chains of B and N, which can form BN ceramics on thermal pyrolysis [29]. The polymerization gives a graphitelike hexagonal layered structure (2) ... 

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]... 

While borazine is analogous in shape and structure to benzene, the behavior observed here differs sharply from that of benzene. During the pyrolysis of benzene at temperatures in excess of 500 C, linear biphenyl, terphenyl and polyphenyl units are formed which do not exhibit liquid crystallinity [22]. In contrast, when naphthalene or anthracene are heated, they will enter a liquid crystalline mesophase and will produce graphitizable carbon upon continued pyrolysis (Figure 2.3). This mesophase consists of a wide range of planar, polynuclear species which have a tendency to stack, thereby forming a discotic phase. The mesophase is also observed to form when a 50 50 mixture of benzene and naphthalene is heated [23]. 

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