Boron nitride borazine

The emphasis in the approaches to boron nitride [10043-11 -5] BN, precursors has been concentrated on cycHc compounds. There have been recent reports of trimethylsilyl-substituted aminoboranes being evaluated as B—N precursors. These are linear borylamines containing up to four boron atoms. Compounds were also synthesized with free —NH2 groups amenable to condensation with either dihaloboranes or dihaloborazines (65) and offering suitable monomers for linear B—N polymer synthesis and borazine-ring-linking appHcations. 

Borazine Pyrolysis Studies. In a simplistic manner, one can visualize the formation of boron nitride via stepwise eliminations between rings, i.e.,... 

There is a great deal of potential interest in borazine as a precursor of boron nitride, since it offers the advantages of being a single source of boron and nitrogen with the correct B/N ratio and a high ceramic yield. In addition, borazine contains the elementary BN building block as its substituted derivatives. This is described later. 

Heating borazine in vacuo at 70°C yields poly(borazylene) polymers, which are soluble in solvents such as tetrahydrofuran or glyme and could be thermolyzed to boron nitride in good yields (120). Other soluble preceramic polymers were produced by transition-metal catalyzed formation of B-alkenylborazines (eq. 34) which were thermally polymerized under mild conditions to poly(alkenylborazines). The latter yielded boron nitride having low carbon contents when thermolyzed in an ammonia atmosphere (121). 

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). 

Renewed interest in borazine derivatives has resulted from their possible application as precursors to boron nitride ceramics. For example, the inorganic analogue of styrene, (H->C=CH)B3NjH5i has been polymerized and decomposed to produce BN.72... 

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 decrease in volatility can be achieved by the linkage of borazine rings to form a cyclolinear oligomer or polymer of the type shown in 9.19. Oligomers that approximate to this structure have been produced by the controlled dehydrogenolysis of borazine (reaction (15)) and these yield /-boron nitride when heated to 1,000 °C.55... 

Other approaches to boron nitride, by Paine, Paciorek, and Sneddon and their coworkers are shown in reaction sequences (16) and (17), processes that involve the loss of Me3Si, NH2, or Cl side units from borazine rings as pyrolysis proceeds in the temperature program leading up to 1,000 °C.61-63... 

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. 

A related synthetic strategy by Allcock and Welker65 is illustrated in reaction (19). Here, borazine side groups are linked to a polyphosphazene chain. This reduces the volatility of the borazine and allows facile fabrication of the linear high polymer. Subsequent pyrolysis at temperatures up to 1,000 °C results in breakdown of the phos-phazene carrier backbone (possibly via the formation of volatile phosphorus nitride) and formation of boron nitride. 

In retrospect, it is ironic to it that when I met Ernst Schumacher in 1969 (he was then Professor at the University of Bern in Switzerland) we did not talk about the experiments he did at Zurich in the same building where I was at that time. Instead, his interest focussed on our work on borazine transition metal compounds and we discussed in some detail whether it would be possible to incorporate metal atoms like chromium or molybdenum between the layers of hexagonal boron nitride (BN) in a similar way as it can be done with graphite. In the course of these discussions I did not mention that, after I had moved to Zurich, we had begun to investigate the reactivity of nickelocene towards both nucleophilic and electrophilic substrates. The reason was that we were still at the beginning, and while we had been able to prepare a series of monocyclopentadienyl nickel complexes from Ni(C5H5)2 and Lewis bases, our attempts to obtain alkyl- or acyl-substituted nickelocenes by the Friedel-Crafts reaction failed. 

At a slightly higher temperature of 700°C, the single-source precursor borazine can be decomposed on a surface to form boron nitride ... 

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