Silicon elemental

F. Liebau, Silicon, element 14, in K. H. Wedepohl (ed.). Handbook of Geochemistry, Vol. 11-2, Chap. 14, Springer-Verlag, Berlin, 1978. F. Liebau, Structural Chemistry of Silicates, Springer-Verlag, Berlin, 1985, 347 pp. 

This fundamental discovery dramatically affected the whole chemistry of main-group elements. Subsequently, a series of new compounds with silicon element multiple bonds has been introduced. Within only a few years, stable silenes (silaethenes with a Si = C double bond) [8-11], silaimines Si = N [12-14], and silaphosphenes Si = P [15] were synthesized. As a pacemaker, silicon chemistry has exerted a strong influence on further areas of main-group chemistry a variety of stable molecules with Ge = Ge [16], P = P [17], As = As [18], P = C and P = C [19-22] bonds were subsequently isolated, and systems with cumulated double bonds P = C = P [23-25] are also known today. 

Another important aspect is the very simple preparation of the silyltriflates. Systematic investigations of the cleavage of the silicon element bond (Si-E) by CF3SO3H have shown that the reaction rate decreases in the sequence (E=) a-naphthyl > phenyl > Cl > H > alkyl [3]. Therefore especially pure silyltriflates result from protodesilylation of arylsilanes with CF3SO3H. On the basis of these general results the synthesis of a large number of variously substituted silyltriflates [4,5] can be planned. This is of particular interest in the chemistry of oligosilanes. 

On the other hand polysilylalkynes with phenyl or allyl substituents are converted with triflic acid into polymeric alkynylsilyltriflates. These polymers react with many acidic element hydrogen compounds or lithium element compounds with formation of silicon element bonds. Thus we found an easy approach to numerous new functional substituted alkynes [12], Eq.(9) shows selected examples of this reaction type. 

A prerequisite for all etch-stop techniques discussed so far is an electrical connection to an external power supply. However, if the potential required for passivation in alkaline solutions is below 1 V, it can be generated by an internal galvanic cell, for example by a gold-silicon element [As4, Xil]. An internal galvanic cell can also be realized by a p-n junction illuminated in the etchant, as discussed in the next section. Internal cells eliminate the need for external contacts and make this technique suitable for simple batch fabrication. 

The stability of silicon electrodes contacting an aqueous electrolyte is a severe problem in regenerative solar systems. As mentioned previously, the standard electrode potential of a silicon element is negative enough to induce an electrochemical reaction mechanism, giving rise to an insulating surface silicon oxide in the absence of complexing reactants. On the... 

Physical Properties of Carbon-Element and Silicon-Element Bonds... 

A comparison of carbon-element bonds (C—El) inside the most important structural frameworks of living matter (and of the most important drugs) with the corresponding silicon-element bonds (Si—El) is given in Table 1. 

Table 1. Comparison of carbon- and silicon-element bonds...

Carbon- and silicon-element single bonds in 4-coordinated carbon and silicon compounds. Polarities of the bonds. 

S- and 6-coordinated silicon compounds appear often as reaction intermediates. In several cases such compounds are isolable. The known structure frameworks with carbon- and silicon element bonds, which are stable under normal conditions, are represented in Table 2. 

Preparation and properties of silicon. Elemental silicon of about 98% purity may be produced by the reduction of silicon dioxide by aluminum. 

If formation of silicon-element bonds by a reaction of the alkali-organosilicon compound and a halide of the corresponding element is attempted a disilane is usually formed ... 

The class of phosphaalkenes with isolated P=C double bonds was first synthes ized by Becker.33 His synthetic strategy starting from trimethylsilylphosphines and acyl chlorides is still the most versatile (Protocol 3). The principle is based on the easily achievable, 1,3-silatropic migration of a silyl group bonded to phosphorus to a doubly bonded element such as nitrogen, oxygen, or sulfur. The process is favoured energetically by the construction of the P=C double bond with concomitant formation of a very stable silicon-element bond. 

Table 5 lists selected bond lengths of tetracoordinated silicon-element and the corresponding carbon-element bonds. As noted before, the bond lengths for silicon bonded to elements more electronegative than carbon tend to be shorter than the calculated values. 

To understand Group 14 - and especially organosilicon - chemistry some comparisons between silicon and carbon have to be considered. There are two major properties that distinguish silicon from carbon. Silicon atoms are about 50 % larger than carbon atoms and this increased size will have some ramifications and consequences, such as lower barriers to silicon-element bond rotations and less stable n-bonds. Furthermore, the smaller Pauling electronegativity of silicon results in differently polar silicon-element bonds compared to carbon and thus will change its reactivity and enable reactions not possible in carbon chemistry. 

A variety of catalytic bis-silylation reactions, i.e., addition of Si-Si bonds across multiple bonds, have been reported. Generally the reaction mechanism can be presented as follows (1) formation of bis(organosilyl) transition-metal complexes through activation of Si-Si bonds, (2) insertion of unsaturated organic molecules into the silicon-transition-metal bonds, and (3) reductive elimination of the silicon-element (mostly carbon) bonds giving bis-silylation products. The final step regenerates the active low-valent transition-metal complexes. Not only appropriate choice of transition metal, but also choice of suitable ligand on the transition metal is crucially important for the success of the bis-silylation reaction. In addition, substituents on the silicon atoms of disilane are also of importance. 

Fig. 7.3.20 shows the results of prototype evaluation for Nip, TNO, and TNS of the pressure sensor with the optimal gage layout on its (100) silicon element. Nip, TNO, and TNS are nearly zero, as expected. 

At the same time, synthetic methods using the efficacy of nucleophilic agents in the activation of various silicon-element bonds, such as Si-H, Si-O, Si-N, and Si-C, were developed. The activation of these bonds was used as a tool for reduction of carbonyl compounds (271-273), Michael condensations (274,275), and formation of heterodienes by cleavage of the Si-N bond (276). The activation of the Si-C bond has received particular attention, as illustrated by the cleavage of the Si-allyl bond (277) and the use of pentafluorosilicates as synthetic intermediates (278). 

TABLE 1. Comparison of selected carbon-element and silicon-element bond distances and barriers to rotation... 

TABLE 3. Comparison of selected silicon-element bond lengths at different coordination numbers (CN)fl... 

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