Boron borane pyrolysis

Polyhedral Expansion. The term polyhedral expansion is used to describe a host of reactions in which the size of the polyhedron is increased by the addition of new vertex atoms whether boron, heteroelements, or metals. In the case of the boranes, the pyrolysis of B2H has been used to obtain B H and industrially. Although a subject of much study, the mechanism of such pyrolytic expansions is not well understood. 

Innovatory boronated carbons (manufactured in the Institute of Chemistry and Technology of Petroleum and Coal, Wroclaw University of Technology, Poland) were obtained by co-pyrolysis of coal-tar pitch with a pyridine-borane complex. In the first stage of pyrolysis (520°C) the so-called semi-coke is obtained. Further carbonization at 2500°C leads to obtaining boron-doped carbonaceous material (sample labeled 25B2). 

Boron-containing carbons synthesized by co-pyrolysis of coal-tar pitch with pyridine-borane complex (series 25Bn) have already been considered as hosts for lithium insertion [4], Unlike the commercial graphites described above, the boron-doped carbon 25B2 (WUT) as received was not suitable for direct use in the cylindrical cell due to very large and hard particles. This feature makes the coating process very difficult. 

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. 

A sacrificial carrier polymer approach was reported by Sneddon in 1988.64 Here, an organic polymer with a pentaborane side group was prepared by addition polymerization as shown in reaction (18). Treatment of this polymer with ammonia converted the borane side group to a fused borazine (naphthalene-type) side unit. Subsequent pyrolysis in ammonia resulted in loss of the organic polymer and hydrogen and the formation of boron nitride. 

The third approach, this one by Seyferth, is based on the coordinative linkage of borane cage structures by diamines or phosphines to form soluble oligomers (9.21), followed by pyrolysis (reaction (20)).66-68 Pyrolysis in an inert atmosphere gives a 70-90% ceramic yield of a black boron carbonitride, but pyrolysis in ammonia yields white BN in 70% yield. A disadvantage of a process based on a borane starting material is its high cost. 

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... 

The form of the polymer powder particles is maintained during pyrolysis. Nanosized Si-C-N-powder is obtained after pyrolysis. Furthermore, the high reactivity of the carbodiimide group provides an easy introduction of other elements, e.g., boron. As an example the hydroboration of bis(trimethyl-silyl)carbodiimide (4) with dimethylsulfide-borane in toluene has been investigated ... 

Machnikowski, J., Frackowiak, E., Kierzek, K., et al. (2004). Lithium insertion into boron containing carbons prepared by co-pyrolysis of coal-tar pitch and borane-pyridine complex./. Phys. Chem. Solids, 65, 153-8. 

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 ... 

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 ... 

Pyrolysis gives a mixture of various mixed trialkyl boranes, because of the alkyl exchange on boron 8). For example, as a result of thermal isomerization the propyl groups on boron will be both 1- and 2-propyl. No B—H-containing products are obtained, but they appear as reactive intermediates. 

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