The B-N System

With respect to the technical importance of c-BN its stabilization by pressurizing is of great interest. The first version of the BN pressure vs. 

Solozhenko (1988) [176] concluded that kinetic factors influenced the transformation significantly and that the true equilibrium line can only be determined by a thermodynamic approach. From data on heat capacities [177-189], relative enthalpies (heat contents) [190-199], enthalpies of formation data [177, 201-207], equations of state and thermal expansion data for all BN modifications, Solozhenko [176] derived a calculated new phase diagram which significantly differed from the Corrigan and Bimdy (1975) [174] version. An overview of somces of thermodynamic data is given in Table 15. A review of calorimetric studies was given by Gavrichev et al. (1994) [212]. Vaporization studies of boron nitride were made by [208-211]. 

A striking feature of the version according to Solozhenko [175, 176] is the large region of stability of c-BN with normal pressure stability up to a 

Type of experiment Type of data Pressure and temp, range Literature 

High pressure Triple point h-BN-c- 0-10 GPa Bundy and Wentorf 

---

The B-N system is accepted ITom [2003Rec]. Only one intermediate phase BN exists in this system. Boron nitride has four crystalline stractural modifications cubic (cBN), wurtzite (wBN), hexagonal (ABN) and rhombohedral (rBN). Except for that, there are two other ordered BN phases EBN, obtained by explosion (E) of a mixture of ABN and aBN, compressed ABN attributable to a monoclinic lattice distortion of ABN and two disordered BN phases turbostratic BN (tBN) and amorphous BN (uBN). 

The chemistry and stereochemistry of aminoboranes containing the siLicon—nitrogen—boron linkage have been the subject of numerous studies. Many of these compounds are useful precursors to other B—N systems including diboryl-amines (45) and B—H substituted aminoboranes (46). A series of... 

In an analogous fashion to the hydroboration reaction, a variety of boron-containing substrates react with iminoboranes. Addition of X2B—Cl, X2B—N3, X2B—SR, X2B—NR2, and X2B—R to the unsaturated B—N system is called chloro-, azido-, thio-, amino-, and alkyloboration, respectively. The azidoboration and chloroboration of two iminoboranes are shown ia equations 23 and 24 (72). 

The B-Na system includes two phases with different thermal stabilities. Either of these two borides can be obtained by direct synthesis on adjusting the alkali-metal pressure in the vapor phase. Thus, the preparation of NaB can be carried out in isothermal reactors at < 1100°C (p a = 45 X 10 N m" ) where the equilibrium... 

The compounds that are being reported here are mostly those with combined anions, such as BN anions in nitridoborates. On the other hand, we know of compounds with mixed anions, like the well-known Ti(N,C) in which anions share the same lattice site, or NbBN with separate B and N sublattices. How can these compounds be explained in comparison with compounds having C-N or B-N bonding Are there separate strategies for their synthesis, what are the differences in stability, what are their properties, and what are their structures like These and some other questions will be discussed, mainly for Ca-(B,C,N) and La-B-N systems. 

Mixed compounds, e.g., in the M-B-N system, where M is a transition metal, are well known with separate B and N sublattices or atoms. Prominent examples are NbBN [22] containing kinked B-chains and isolated nitride ions or Ti(N,C) with a NaCl-like structure containing disordered C and N ions sharing... 

Fig. 7.6a and b. The mode of conjugation of the cyclopropane ring with the adjacent n system... 

Many X-ray data are given for these bicyclic 5-6 systems, as a method for structure confirmation or elucidation of unexpected reaction products. Analysis of compound 25 (Scheme 5) revealed a planar molecule, in which the B-N distances (1.420 and 1.429 A) indicate a multiple bond character, since in diazaboroles they range from 1.395 to 1.450 A... 

This reversal of diastereoselectivity i.e., high selectivity in favor of the. mt-alkylation product 19b has been explained by the authors by an allylic strain effect65. If R3 is bulky, then in A and B the preferred entry is syn to R1. In these conformations the C—H bond in the exn allylic position is coplanar with the enolate n-system and R2 is antiperiplanar to R1 to minimize steric interaction. 

Figure 11.7. Interaction diagrams comparing (a) ethylene to (b) benzyne. Note that (i I < lAxl-The aromatic n system is largely unperturbed with nearly degenerate orbitals at a — /icc and a + Pcc -...

A possible mechanism that features a six-membered transition state is given in Scheme 18 (38, 43). The high catalytic activity stems from the excess strain of the B=N bond in the 5/5-fused bicyclic ring system, which results in strong coordination of the angular nitrogen atom to BH3 to form the active dinuclear species. The rate and stereoselectivities of the reaction of trihalomethyl ketones has been explained on the basis of the X-ray analysis of the substrates. 

Figure 4.7(a,b) shows EPMA scans across phase bands shown in Figure 4.10(a,b) which reveal the homogeneity ranges of the occurring phases. The phase diagram of the Ta-N system was corrected for the present findings (Figure 4.8). These are in excellent agreement with published data.10...

Similar to the 6 phase in the Nb-N system, the mononitride b-TaNj is also a high-temperature phase, but the nitrogen equilibrium pressure is even higher than that for 6-NbN. The main purpose of the work performed here was to determine the eutectoid decomposition temperature... 

Some hydrolysis and oxidation reactions of s-diazadiborines have been described 169>, but no further data are available. This lack of information seems to confirm the view that the stability of the s-diazadi-borine ring systems rests primarily on steric effects. While substitution with bulky substituents — even assuming that the B—N four-membered ring is stable in the transition state — presents a reaction which is not readily performed due to steric hindrance, substitution by smaller groups will not maintain the ring size. Unequivocal preparative proof will not be possible until more effective syntheses are developed for these compounds. 

---