B-Trichloroborazine

Submitted by KURT NIEDENZU and JOHN W. DAWSON Checked by A. W. LAUBENGAYEHf 

The original method reported for the preparation of trichloroborazine involves the use of borazine. However, this procedure is not recommended because of the difficulty of preparing this heterocycle and its hydrolytic instability. More recently, this trichloroborazine has been prepared directly from ammonium chloride and boron trichloride at elevated temperatures with or without a solvent. Ammonium chloride may be replaced by a primary aliphatic or aromatic amine or its hydrochloride, and organodihaloboranes can be used in place of the boron trichloride to give the respective N- and -substituted borazines. Boron tribromide has been used in analogous reactions to yield B,B, B -tribromoborazines.  

The ammonium chloride is dried at 110° for 24 hours. Reagent-grade chlorobenzene dried over sodium sulfate is distilled prior to use. 

i -TrichIoroborazine forms white crystalline needles, m.p. 83.9 to 84.5°. It is extremely sensitive to moisture and decomposes violently in water. It is soluble without decomposition in anhydrous nonprotonic organic solvents such as benzene, ethyl ether, and carbon tetrachloride. 

5 -Trichloroborazine is readily reduced with alkali metal hydroborates to yield borazine, (HBNH)3. The boron-bonded chlorine can be replaced by fluorine through the action of metal fluorides upon the trichloroborazine. With amines, the latter reacts to yield B,B, B -triaminoborazines. The substitution of chlorine in iV-substituted B,B, B -trichloro-borazines by organic groups is most easily achieved by the Grignard method but has also been effected by the use of organolithium compounds. 

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Borazine originally was obtained by the reaction of ammonia with diborane.1 Mixtures of lithium or sodium tetrahydro-borate with ammonium chloride also have been pyrolyzed to yield this product.2 More recently, the reduction of B,B, B"-trichloroborazine with alkali metal hydroborates has proved to be a convenient laboratory-scale method for the preparation of this compound.3-7 The procedure described herein is a variation of the last method as reported by Dahl and Schaeffer.7 This method is effective for the synthesis of iV-substituted alkyl- and arylborazines, i.e., compounds of the formula (HBNR)3 where R is CH3, C2H6, C6H , C6H5, p-C6H4CH3, or p-C6H4OCH3.5... 

The trichloro derivative having a chlorine atom bound to each of the boron atoms is known as B-trichloroborazine. Borazine can be prepared by several different methods, one of which is the reduction of the trichloro compound ... 

Synthesis. The parent compound, borazine [6569-51-3], is best prepared by a two-step process involving formation of B-trichloroborazine followed by reduction with sodium borohydride. These reactions have been studied in some detail (96). 

Hydrolysis. Borazine is slowly hydrolyzed by water at ambient or higher temperatures to boric acid. Substituted borazines react with water to give ring-cleavage products, eg, boronic acids and amines (122). B-Trichloroborazine reacts with water to give boric acid and chloramine (123). 

Addition Reactions. In general, polar molecules such as hydrogen halides add across the B—N bonds, the more electronegative group bonding to boron (91). The adducts are cydotriborazanes such as the product formed by reaction of B-trichloroborazine and hydrogen chloride (eq. 35). X-ray crystal analysis shows the structure exists in a chair conformation (124). 

Complex Formation. B-Trichloroborazine was reported to readily form crystalline adducts of uncertain structure with pyridine (131). The Lewis acids aluminum tribromide or gallium trichloride form 1 1 adducts with hexamethylborazine (eq. 36) in which the metal atom coordinates with a nitrogen with loss of planarity of the ring (132,133). 

Amorphous boron nitride (a-BN) can be synthesized by decomposition of B-trichloroborazine and cesium [23]. Such layers are applied during production of semiconductor devices [24]. 

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. 

The literature contains many references to the, /V-alkylated B-trichloroborazines, (BC1NR)3, as a starting material in the synthesis of boron-substituted borazines.1 The procedure described is a modification of syntheses originally used in the preparation of 2,4,6-trichloroborazine.2... 

Borazine can be prepared in a variety of ways, including the reaction of NH3 with BC13 to form B,B, B"-trichloroborazine, C13B3N3H3, which is then treated with NaBHi to yield B3H3H6. However, these and other literature methods are hampered by limitations of scale and other problems. Improved routes that utilize reactions of ammonia-borane (H3N BH3) at 140-160°C or alternatively, (NH4)2S04 and NaBIL at 120-140°C, to afford very pure borazine on a multigram scale have been reported recently.25... 

S13.4 A, A, fV"-trimethyl-5,R, R"-trimethylborazine is hexamethyiborazine. The reaction of ammonium chloride with boron trichloride yields B,5, B"-trichloroborazine, whereas the reaction of a primary ammonium chloride with boron trichloride yields N-alkyl substituted R,5, R"-trichloroborazine, as shown below ... 

E13.23 (a) PhjNaBjClj The reaction of a primary ammonium salt with boron trichloride yields A -substituted B-trichloroborazines ... 

B-Trichloroborazine [B-trichloroborazole) [933-18-6] M 183.1, m 83.9-84.5 (sealed evacuated cappillary), 87 , b 88-92 /21mm, df 1.58. Purily it by distillation from mineral oil. It sublimes at 70°/lmm. [Brown Laubangayer 7 Am Chem Soc 77 3699 1955, Emeleus Videla 7 Chem Soc 1306 7959.] It is extremely sensitive to moisture and reacts with H2O exothermically to give boric acid and NH4CI. Store it in sealed tubes. It is soluble in C6H6, cyclohexane, CS2, CHCI3, CCl, and CgHsCl... 

However, the method is inefficient, and borazine itself is better prepared by NaBH4 reduction of the B—Cl bond in the preformed ring compound (HNBC1)3. The 2,4,6-trichloroborazine can be prepared by reaction of NH4CI with BCI3 or the acetonitrile adduct of BCI3 in chlorobenzene This and other B-trichloroborazines... 

Three chlorines replace the three hydrogens on the three borons of the borazine molecule. Notice that the hydrogens on the nitrogens can be replaced as well, therefore we must use B s to indicate that the chlorines are attached to the boron. Its name is B,B,B-trichloroborazine. 

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]... 

Because borazine is a poorly handling compound that is difficult to work with, most of the researchers focused their work on the use of B-trichloroborazine (Figure 14.4). 

In a two-step synthesis procedure, the B-trichloroborazine reacts with ammonia or amine derivatives as linking reagents to form B-aminoborazines which are subsequently thermolyzed at moderate temperatures to generate poly[B-aminoborazine] [25 7]. Pioneering works in this chemistry have described the preparation of B-aminoborazine-type polymers through a two-step procedure [31,32,34], but no attention has been given to the materials as BN precursors. 

A similar approach has been reported by Paciorek et al. [45 16] Poly[B-ami-noborazines] have been prepared through nucleophilic reactions of the chlorine atoms in the B-trichloroborazine (Eq. 14.12). 

It is prepared by heating boron trichloride and ammonium chloride and then reducing the B-trichloroborazine with sodium borohydride. 

Auwarter, W., Suter, H.U., Sachdev, H., and Greber, T. (2004) Synthesis of one monolayer of hexagonal boron nitride on Ni(lll) from B-trichloroborazine (C1BNH)3. Chem. Mater., 16, 343-345. 

Stolle, R. and Wahl, G. (1995) Deposition of boron nitride films from BB B"-trichloroborazine. European Conference on CVD, 10, Venice, Italy. J. Phys. IV, 5 (2), C5.761-C5.768. 

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