The Si-N System

An assessment of the Si-N system was presented by Carlson (1990) [114], Thermodynamic calculations were published by Kaufman (1979) [115], Dorner et al. (1981) [116] and Weiss et al. (1981) [117] and later by HiUert et al. (1992) [37]. Fig. 5 shows the calculated phase diagram at normal pressure using the data of HiUert et al. (1992 [37]) for Si3N4. 

The silicon-rich part of the phase diagram according to Yatsurugi et al. (1973) [119] was accepted in the evaluation by Carlson (1990) [114]. Further nitrogen solubility data were provided [71, 120]. Melting experiments were carried out by Kostanovskii and Evseev (1994) [121] and Cerenius (1999) 

Different types of experimental thermodynamic data are available for Si3N4 and are listed in Table 10. Data for heat capacities [94, 123-125], relative 

Type of experiment Type of data Temp, range [K] Cone, range, Xn Literature 

Zone melting experiments Solvus, liquidus 1687 0-0.0002 Kaiser and Thurmond (1959) [118] 

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The structure property relationships observed in the Si-N system [17], are reminiscent of those observed for silicon nitride whiskers grown by metal catalyzed chemical vapor deposition (Chapter 2.2). The difficulty in obtaining binary (i.e., boron or silicon nitride) fibers with exactly required stoichiometry seems to have so far precluded the production of single crystal fibers by this route. 

While r 2-coordination of silanimines has been realized in species such as Cp2Zr(r 2-SiMe2=Nt-Bu)(PMe3) [13], no -(silanimine) transition metal complexes are known so far [14 - 16]. Access to these Si=N systems is opened up by treatment of 19b,c with Me3P=CH2 at low temperature leading to elimination of hydrogen chloride and formation of the fert-butyl and mesityl-N-derivative 23a,b. These species are stable only for a short period in solution (two hours in toluene at -30°C), but can be... 

Comparison of all reported cleavages gives the apparent ordering H > Ph (or aryl) > alkyl in terms of tendency for groups to be cleaved from silicon, with the exception that at least one aryl substituent is retained on silicon in the anion VI. It is clear that reduction of aryl silanes leads to significant weakening of Si—R bonds that have appropriate symmetry for hyperconjugative interactions with the reduced n systems, a point that has been cited as evidence for the importance of n — type interactions in unsaturated silanes (86). 

The preparation of silyl derivatives with a Si—C bond between the silicon atom and the drug is not so easy to realize as compared with Si—O and Si—N systems. In most cases the Si-C bond must be constructed at the beginning of synthesis. This is shown for the triethylsilyl derivative (4) of phenethylamine (3) in Scheme 1 ... 

Substitution of oxygen in the Si—0—C system by the isoelectronic CH2 group leads to stable compounds with a decreased but prolonged antihistaminic, anticholinergic and antitremorine activity. Replacement of the oxygen atom by a NH group leads to compounds, showing no spasmolytic activity at all because of rapid hydrolysis of the Si—N bond. 

The nomenclature of Si=N-systems used in the literature is somewhat confusing, especially for unusual systems. The nomenclature used in this review corresponds with the most commonly used description. 

Other neutral reaction partners used were CO2, CS2, O2, CgFg and alcohols. Computational studies of the [H2,Si,N] system show the cis and turns isomers 11 ISiNI 11 to be more stable than either [H2SiN] (by 24 kcalmol-1), or [SiNFh]- (by 24 kcal mol 1). 

The reactivity of SiN multiply bonded species has in part been discussed in Section IV. B, dedicated to synthesis, because the existence of reactive and short-lived species is often proven by trapping experiments. This part of the review therefore focusses on more comprehensive reactivity studies of Si=N systems. 

The addition of donors to the Lewis acidic sp2-hybridized silicon in Si=N systems is a general method to stabilize the multiple bond, even when non-bulky substituents are used. In most cases the donor adduct reacts just like the free Si=N compound, obviously by initial dissociation of the donor molecule. In the absence of trapping agents and when the steric bulk allows it, silanimines dimerize300,308,311, e.g. 672 gives cyclodisilazane 789 (equation 265). 

Insertion of the Si=N bond into polar bonds is the most used reaction for the characterization of very reactive and transient silanimines. Especially, the reaction with alcohols is often the first reaction to be carried out with silicon-nitrogen multiple bond systems. Other reagents used for insertion reactions are amines, water and alkoxysilanes (equation 269)300,303,306,311,351-353. The insertion into E—X bonds (E = Si, Ge, Sn X = Cl, OR, NR2, N3) has been shown earlier in this review for the insertion into the Si—N bond of silyl azides310,351. A reaction reported is the insertion of 678 into the C—H... 

The six-membered ring B can be lithiated at two different positions. Endocyclic lithiation yields structure B, which is 6.1 kcal/mol above the eight-membered anion A, comparable to the neutral system. The Si-N-Si angle at the anionic center measures 130.4° it is more than 23° smaller than in A because of the smaller ring... 

Fig. 5. Temperature-concentration section (isopleth) through the Si-N-0 phase diagram from Si02 to Si3N4 [69]. Below 2114 K it is a quasi-binary system. G = gas phase LS = oxide nitride liquid LM = metallic liquid...

Nowadays polyorganosilazanes are used more and more often. Until recently, due to the small hydrolytic stability (in acid and neutral media) of compounds with a Si—N bond, the possibility of their practical application seemed doubtful. However, the increased ability to hydrolyse and the chemical activity of the Si—N bond in polyorganosilazanes predetermined their industrial applications. It was found that polyorganosilazanes are hydrolysed when kept in air even at room temperature, with 80-85% of si-lazane bonds replaced by siloxane bonds. Probably, due to the high gas permeability of siloxane film the ammonia released during the hydrolysis is withdrawn out of the system without disrupting the film, even in case of considerable thickness (1-2 mm). 

The last section of this chapter deals with cyclic silylhydrazines. The synthesis of the first cyclic silylhydrazines were reported as early as 1958.55 Since then, a great diversity of such ring systems containing the Si—N—N unit have been synthesized. These can be classified as follows. 

Electrochemical equivalent — Amount of a chemical substance deposited, dissolved, or transformed in an electrochemical redox reaction with exchange of one unit of electric charge. In the SI unit system the electrochemical-equivalent unit is in kgC-1, or alternatively, in molC-1. It means that in a n-electron redox reaction (Ox + ne Red) the electrochemical equivalent is equal to the molar mass M of the reacting compound divided by n times the - Faraday constant (M/nF). In some sources the electrochemical equivalent is defined as the mass of the substance transformed by electric charge corresponding to the Faraday constant. 

Silicon has an affinity both for B (at 1500°C Si dissolves up to 17 at.% B (Massalski 1990)) and N (although Si3N4 is less stable than BN). Naidich (1981) achieved stationary contact angles of 95° and 110° for Si on cubic and hexagonal BN at 1500°C in a high vacuum in a few minutes. These values are much higher than those observed for Al, but significantly lower than the 140° or so observed for non-reactive metals. A detailed interpretation of these values is not possible in the absence of information on interfacial reactions which could occur in the Si/BN system. 

This method is most practical for complexes where methane elimination is facile and occurs below room temperature. Zirconaaziridines such as those in Fig. 1 (with N-aryl and N-silyl [19] substituents) have been readily prepared in this manner [20-22]. Buchwald noted that methane elimination from 1 is facile when the availability of the lone pairs on nitrogen is reduced [20] Whitby suggests that such reduced availability... [is] due to conjugation with the aromatic n system, or overlap with the Si d orbitals (or Si-C o orbitals) [23]. The rate of zirconaaziridine formation from le, for example, is faster by a factor of 1,000 than the rate of the formation from If (Fig. 2). 

ILECTRONIC TRANSPORT IN AMORPHOUS SOLIDS and in random organic media in particular has been the subject of vigorous scientific activity for more than 2 decades. The approach taken in this chapter is to integrate recent studies of electronic transport in polysilanes with the extensive earlier work on poly(N-vinylcarbazole) (PVK) and molecularly doped polymers (MDPs) (i,e., systems in which transport-active molecular species are dispersed in an inert binder) and thereby allow the distinctive features of transport in the Si-based systems to emerge. 

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