B- borazine

Fig. 16.22 Contour map of the charge density in the molecular plane of (a) benzene (b) borazine. [From Boyd. R. J. Choi. S. C. Hale. C. C. Chem. Phys. Li ii. 1984. 112, 136-141. Reproduced with permis.sion.]...

For which compound would an HF/6-31G calculation be most likely to result in an accurate optimized geometry (a) F3BNH3, (b) borazine, B3N3H6 or (c) [Fe(OH2)6] ... 

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. 

The cyclic borazine (-BH-NH-)3 and its derivatives form one of the largest classes of B-N compounds. The parent compound, also known as inorganic benzene , was first isolated as a colourless liquid from the mixture of products obtained by reacting B2H6 and NH3 (A. Stock and E. Pohland, 1926) ... 

Numerous other reactions have been documented, most of which are initiated by nucleophilic attack on B. There is no evidence that electrophilic substitution of the borazine ring occurs and conditions required for such reactions in benzenoid systems disrupt the borazine ring by oxidation or solvolysis. However, it is known that the less-reactive hexamethyl derivative B3N3Mee (which can be heated to 460° for 3 h without significant decomposition)... 

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. ... 

B-C bonds, 3, 97 B-N bonds, 3, 97 B-O bonds, 3,94 B-P bonds, 3, 97 B-Si bonds, 3, 97 oxo acid anion complexes, 3, 96 Borates, alkoxo-, 3, 94 Borates, amidotrihydro-, 3,92 Borates, aryloxo-, 3, 94 Borates, carboxylato-, 3,96 Borates, catechol, 3,95 Borates, chlorosulfato-, 3,97 Borates, dicarboxylato-, 3,96 Borates, dipyrazol-l-yl-, 3, 92 Borates, halogeno-, 3,92 Borates, halogenohydro-, 3,90 Borates, hydro-, 3,90 Borates, hydrohydroxo-, 3,90 Borates, hydropyrazol-l-yl-, 3, 92 Borates, hydroxo-, 3,94 Borates, hydroxycarboxylato-, 3,96 Borates, inositol, 3, 95 Borates, monoalkyl-, 3, 92 Borates, monophosphido-, 3, 92 Borates, peroxohydroxo-, 3, 94 Borates, polyol, 3, 95 Borates, pyrrol-l-yl-, 3, 92 Borates, sulfato-, 3, 97 Borates, tetrabromo-, 3, 92 Borates, tetrachloro-, 3, 92 Borates, tetrafluoro-, 3, 92 minerals, 6, 847 Borates, tetrahalogeno-mixed, 3, 93 nB NMR, 3, 92 Borates, tetraiodo-, 3, 92 Borates, tetranitrato-, 3, 96 Borates, tetraperchlorato-, 3, 97 Borates, tripyrazol-l-yl-, 3, 92 Borax, 3,101 Borazines... 

Borazine, B3N3Hft, a compound that has been called inorganic benzene because of its similar hexagonal structure (but with alternating B and N atoms in place of C atoms), is the basis of a large class of boron—nitrogen compounds. Write its Lewis structure and predict the composition of the hybrid orbitals used by each B and N atom. 

Borazine is isoelectronic and isostructural with benzene and may act as a six-electron donor in complex chemistry. In contrast to arene ligands of arene-transi-tion-metal complexes, coordinated borazines lose their planarity and are slightly puckered . Nevertheless, the B atoms show interactions with metal atoms. 

The cyclic conjugation and orbital phase are both continuous in benzene. Electrons delocalize in a cyclic manner. The cyclic conjugation is discontinuous in borazine in Scheme 33). Electrons cannot delocalize in a cyclic manner, but only between the neighboring pairs of donors and acceptors. There arises a fundamental question how electrons delocalize in the isoelectronic molecules where C=C bonds are replaced with N-B bonds. 

Borazlne synthesis. The ease of borazine ring condensation would be expected to depend on the nature of the substituents. To investigate these aspects, a series of compounds were synthesized, namely B-tr i chloro-N-tr 1 phenyl borazlne, B-triamino-N-trlphenylborazine, B-trianilinoborazine, B-tris[di(trimethylsilyl)amino]borazlne, B-trlchloro-N-tris(trimethylsilyl)borazine, and B-triamino-N-tris-(trimethylsilyl)borazine. 

B-trichloro-N-triphenylborazine, mp 290-292°C, was obtained in 86 yield following the procedure of Groszos and StafieJ (6). This material was then transformed into B-triamino-N-triphenyl-borazine in 67 yield using the method of Toeniskoetter and Hall (2)- B-trianilinoborazine and the novel B-tris[di(trimethylsilyl)-amlno]borazlne (mp, 131.5-132°C characterized by GC/MS, molecular ion 558 amu, and elemental analysis) were synthesized in 76 and 71 yields, respectively, by interaction of aniline and hexamethyl-disllazane with chloroborazine in the presence of triethylamine. 

The synthesis of B-trichloro-N-tris(trimethylsilyl)borazine was much more complicated than that of the other borazines. The overall scheme is given below ... 

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. 

The behavior of B-triamino-N-tris(trimethylsilyl)borazine was in good agreement with the ring opening mechanism. This compound was much more reactive than its phenyl analogues thus, the pure monomer could not be Isolated. The product obtained from the... 

B. Poly(/i-borylamino)borazines Prepared in a One-Step Process 159... 

The area of organoboron polymers containing borazine and its derivatives is covered in Chapter 5 of this book by Miele and co-workers. Miele and Bernard also describe the utilization of these polymers in ceramics, fibers, and so on, in Chapter 3 of this book. In this section, the utilization of polymers containing borazine or in some cases the bicyclic boron ligand, 9-BBN, for the production of SiC or Si/C/B fibers is briefly described. Recent advances in polypyrazolylborate or pyrazabole-containing polymers and other boron ring system-derived polymers also have been briefly described. 

The introduction of small amounts of boron into precursors that produce silicon nitride have been known to improve the ceramic yields of silicon nitride and Si—B—C—N ceramics as first reported in 1986.110 Several reports have appeared in the past couple of years alone that utilize borazine precursors such as 2,4-diethylb-orazine and other cyclic boron precursors, such as pinacolborane, 1,3-dimethyl-1, 3-diaza-2-boracyclopentane, for their reactions with silanes, polysilazanes, and polysilylcarbodiimides for the high-yield production of Si—B—N—C ceramics.111... 

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... 

B. tris(2 -Alkylamino)borazine-Derived Poly Z -(alkylamino)borazines]... 

We have reported that the reaction of 2,4,6-tri(chloro)borazine CI3B3N3H3 with three equivalents of tris(isopropylamino)borane B(NHPr )3 led to the formation of 2,4,6-tri[bis(isopropylamino)boryl(isopropyl)amino]borazine 11 [Fig. 5(a)] in quantitative yield.26,27 The subsequent polycondensation carried out in vacuo up to 150°C, led to the formation of the polymer 12 containing B3N3 rings linked either through direct B—N interring bonds (minor pathway) or three-atoms —N—B—N— bridges... 

The nB NMR spectrum of the product showed two broad signals at 27.7 ppm and 24.7 ppm, which are attributed to boron atoms of the borazine ring and the boryl substituents, respectively. These results are consistent with the formation of the /i-tri [bis( methy lam ino (boryl (methyl jam ino]borazine 13 as the main product (Fig. 6). No free B(NHCH3)3 was detected. 

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