Compounds of boron with nitrogen

Key points: Compounds containing BN, which is isoelectronic with CC, include the ethane analogue ammonia borane H3 NBH₃, the benzene ana Logue H₃N₃B₃H₃, and BN analogues of graphite and diamond.

The thermodynamically stable phase of boron nitride, BN, consists of planar sheets of atoms like those in graphite (Section 14.5). The planar sheets of alternating B and N atoms consist of edge shared hexagons and, as in graphite, the B N distance within the sheet (145 pm) is much shorter than the distance between the sheets (333 pm, Fig. 13.4). The difference between the structures of graphite and boron nitride, however, lies in the register of the atoms of neighbouring sheets: in BN, the hexagonal rings are stacked directly over each other, with B and N atoms alternating in successive layers; in graphite, the hexagons are staggered. Molecular orbital calculations suggest that the stack ing in BN stems from a partial positive charge on B and a partial negative charge on N. This charge distribution is consistent with the electronegativity difference of the two elements (X_P(B) = 2.04, XP(N) = 3.04).

As with impure graphite, layered boron nitride is a slippery material that is used as a lubricant. Unlike graphite, however, it is a colourless electrical insulator, as there is a large energy gap between the filled and vacant π bands. The size of the band gap is consistent with its high electrical resistivity and lack of absorption in the visible spectrum. In keeping with this large band gap, BN forms a much smaller number of intercalation compounds than graphite (Section 14.5). In contrast to graph-ite, layered boron nitride is stable in air up to 1000°C, making it a useful refractory material.

Layered boron nitride changes into a denser cubic phase at high pressures and temperatures (60 kbar and 2000C, Fig. 13.5).

Figure 13.4 The structure of layered hexagonal boron nitride. Note that the rings are in register between layers.

Figure 13.5 The sphalerite structure of cubic boron nitride.

Figure 13.6 The correlation of hardness with lattice enthalpy density (the lattice enthalpy divided by the molar volume of the substance). The point for carbon represents diamond; that for boron nitride represents the diamond-like sphalerite structure.

This phase is a hard crystalline analogue of diamond but, as it has a lower lattice enthalpy, it has a slightly lower mechanical hardness (Fig. 13.6). Cubic boron nitride is manufactured and used as an abrasive for certain high-temperature applications in which diamond cannot be used because it forms carbides with the material being ground. The fact that BN and CC are isolectronic suggests that there might be analogies between these compounds and hydrocar bons. Many amine-boranes, the boron nitrogen analogues of saturated hydrocarbons, can be synthesized by reaction between a nitrogen Lewis base and a boron Lewis acid:

However, although amine-boranes are isoelectronic with hy drocarbons, their properties are significantly different, in large part due to the difference in electronegativities of B and N. For example, whereas ammoniaborane, H₃ NBH₃ , is a solid at room temperature with a vapour pressure of a few pascals, its ana logue ethane, H₃CCH₃, is a gas that condenses at 89C. This difference can be traced to the difference in polarity of the two molecules: ethane is nonpolar, whereas ammoniaborane has a large dipole moment of 5.2 D (20).

Several BN analogues of the amino acids have been prepared, including ammoniacarboxyborane, H3 NBH2 COOH, the ana logue of propionic acid, CH3 CH2 COOH. These compounds display significant physiological activity, including autoinhibition and reduction of serum cholesterol. The simplest unsaturated boron nitrogen compound is amino borane, H2 NBH2 , which is isoelectronic with ethene. It has only a transient existence in the gas phase because it readily forms cyclic ring compounds such as a cyclohexane analogue (21). However, the amino boranes do survive as monomers when the double bond is shielded from reaction by bulky alkyl groups on the N atom and by Cl atoms on the B atom (22). For instance, monomeric amino boranes can be synthesized readily by the reaction of a dialkylamine and a boron halide:

The reaction also occurs with xylyl (2,4,6-trimethylphenyl) groups in place of isopropyl groups.

Apart from layered boron nitride, the best-known unsaturated compound of boron and nitrogen is borazine, B3 N3 H6 (23), which is isoelectronic and isostructural with benzene. Borazine was first prepared by Alfred Stock in 1926 by the reaction between diborane and ammonia. Since then, many symmetrically trisubstituted derivatives have been made by procedures that depend on the protolysis of BCl bonds of BCl₃ by an ammonium salt (24):

The use of an alkylammonium chloride yields N-alkyl substi-tutedB-trichloroborazines.

Despite their structural resemblance, there is little chemical resemblance between borazine and benzene. Once again, the difference in the electronegativities of boron and nitrogen is influential, and BCl bonds in trichloroborazine are much more labile than the CCl bonds in chlorobenzene. In the borazine compound, the π electrons are concentrated on the N atoms and there is a partial positive charge on the B atoms that leaves them open to electrophilic attack. A sign of the difference is that the reaction of a chloroborazine with a Grignard reagent or hydride source results in the substitution of Cl by alkyl, aryl, or hydride groups. Another example of the difference is the ready addition of HCl to borazine to produce a trichlorocyclohexane analogue (25):

The electrophile, H, in this reaction attaches to the partially negative N atom and the nucleophile Cl attaches to the partially positive B atom.

اخر الاخبار

اخبار العتبة العباسية المقدسة

شعبة الخطابة تقيم مجلس عزاء بذكرى شهادة الإمام الحسن العسكري (عليه السلام)

قسم الشؤون الطبية يُنهي استعداداته لإحياء شهادة الإمام الحسن العسكري (عليه السلام) في سامراء

بالصور.. المؤمنون يتوافدون إلى سامراء لإحياء ذكرى شهادة الإمام الحسن العسكري (عليه السلام)

العتبة العباسية المقدسة تنشر لافتات العزاء بذكرى شهادة الإمام الحسن العسكري (عليه السلام)

المزيد

الآخبار الصحية

لا تبحث عن الدواء.. هذه الأطعمة كفيلة بإيقاف ألم الرأس

لا تترك زجاجة الماء في السيارة.. احذر شربها

عادة يومية.. كيف يؤثر تصفيف الشعر على جهازك التنفسي؟

احذر هذه الأطعمة.. تسرع الشيخوخة وتفقد بشرتك شبابها

المزيد

الآخبار التكنلوجية

الأوكتان وتأثيره: نصائح لحماية محرك سيارتك بكفاءة وأمان

حرب شهر سبتمبر تشتعل.. ماذا في جعبة أبل وسامسونغ؟

واتساب يطلق ميزة لتعديل الرسائل بالذكاء الاصطناعي

على الكوكب الأحمر.. ناسا تعثر على "قبعة الساحرة"

المزيد

مواضيع ذات صلة

Extended silicon oxygen compounds

Boron oxygen compounds

Uses of the elements and their compounds

Occurrence and recovery

Boron clusters

Symmetry-adapted linear combinations

The assignment of molecular symmetry from vibrational spectra

Information from the symmetries of normal modes

The exclusion rule

Molecular vibrations

Chiral molecules

Polar molecules