Monosilane

More recent studies, particularly with slower hafnium complexes, have provided more detailed mechanistic insight As a step polymerization, the reaction is "nonideal" in that inequivalent reactivities for different Si-H functional groups in the system are observed. For exaniple, disilanes tend to be more reactive than monosilanes. Beyond disilane formation, the preferred dehydrocoupling reaction appears to involve addition of one silicon at a time to the growing chain, via M-S1H2R intermediates (n = 1 above). The Si-Si bond-forming reactions are also reversible. 

Disproportionation is also an important method of forming polysilanes. The method is used on an industrial scale. It is important that disilanes are much easier to disproportionate than monosilanes. The reason seems to be the strong nucleophilic reactivity of the SiSi bond. It has been shown that different substituents exhibit different reactivities and this reactivity sequence is the one that we have found in equilibrium reactions. Investigations of several substituents show a series with a decreasing exchange rate valid for equilibrium and disproportionation reactions [16]. 

In 1984 Harrod found that monosilanes with more than one Si-H function react to form oligomers and hydrogen in the presence of catalysts like dimethyltitaiocene or dimethylzirconocene [21]. We have now found that H-containing methyldisilanes react very rapidly with the same catalysts to form polymers with interesting properties. The corresponding chloromethyldisilanes are by-products in the direct synthesis process, and are available on an industrial scale. For use in this polymerization reaction they must be hydrated. 

Copolymerisation is also possible (Fig. 4). Dimethyldisilane reacts with diphenylsilane with formation of a copolymer with the composition H[(MeSiHx)(PhSiHy)]nH. This copolymer is a viscous liquid and is spinnable. By heating to 180° C the polymerization continues and a solid results [23]. The presence of branched structures, which were not found with the polymerization of monosilanes, the very rapid polymerization rate achievable, and the observable SiSi cleavage points to another mechanism, as was postulated for monosilanes. 

For monosilanes, a metathetical mechanism was postulated by Tilley [24] and an a-elimination mechanism was postulated by Harrod [25]. Neither mechanism was able to explain the experimental results on disilanes. Therefore we have postulated a new mechanism via silylenes, shown in Fig. 5 it seems to be a special P-elimination mechanism called p -elimination [26]. The starting reaction for the generation of the silylene is also shown in Fig. 5. Small amounts of methane should be formed, and we found it experimentally. 

Fig. 10. Product-time curves for the initial stages of pyrolysis of 77 torr monosilane at 430 °C. (3 min = 20 % decomposition.) (From Purnell and Walsh67.)...

Stokland74 found that SiD4 decomposed more slowly than SiH4. There seemed to be a systematic increase in the ratio of the rate coefficients, kD/kH (for the second stage of the pyrolysis), from about 0.54 at 648 °K to ca. 0.7 at 764 °K. The experimental conditions used by all investigators of monosilane pyrolysis, and their results, are summarized in Table 3. 

Their pressure records at the beginning of the reaction show that no induction period similar to that observed during the pyrolysis of monosilane occurs. This suggests that the overall reaction... 

Name silane Synonyms silicane, monosilane, silicon tetrahydride CAS No. [7803-62-5] Formula SiH4 MW 32.12... 

Silicon hydrides can be prepared by several methods. A few methods are outlined below. Silane and its higher homologs can be made by treating magnesium silicide, Mg2Si with 20% hydrochloric acid in an atmosphere of hydrogen. An equation for monosilane is given below ... 

Silanes are pyrophoric substances igniting and exploding spontaneously in air. They also liberate toxic hydrogen chloride gas. The gaseous monosilane and the vapors of higher silanes are irritants to the respiratory tract. Chronic exposure to low concentration can cause pulmonary edema. 

Iodosilane has been prepared by the cleavage of phenylsilane10 or chloro-phenylsilane11,12 with hydrogen iodide, and by the reaction of monosilane with hydrogen iodide in the presence of a catalytic amount of aluminum triiodide.4 The latter method has been extended to prepare the methylated iodosilanes.5,13 Iodotrimethylsilane has also been prepared by the cleavage of trimethylphenyl-silane with iodine.14... 

Insertion into Si H and Si Si Bonds. Silylenes, generated by thermolysis of cyclotrisilanes, inserted into the Si—Cl or Si—H bonds of monosilane to yield a variety of disilanes, which could be further functionalized. In contrast to carbenes, the insertion of silylenes into C—H bonds has not been observed. However, the insertion into Si—H bonds has been studied extensively. The occurrence of direct insertion has been indicated by formation of nongeminate homocoupling products. ... 

A detailed mechanism was proposed for 69a as an example. The overall reaction (equation 40) finally produced the disilyl compound 74a with a yield not much higher than 50%. However, unexpected resnlts were obtained nsing nltrasonic irradiation during the electrolysis. The apparent cnrrent efficiency increased np to 200% giving, for example, 71% of disilane 74b (and by-prodncts) bnt no monosilane 71b or 72b after passage of 2.5 Faradays per mole of 69b. 

Tri ethyl monosilane. (C2H5)3SiH mw 88.52 colorl liq mp -156.90° bp 107.7°, 108.77° (sep vals) d 0.7318g/cc RI 1.4100. Sol in 95% ethanol insol in w. Prepn is by partial decompn of triethylmonosilylethylether by treatment with diethyl zinc and Na. The silane decomps explosively when reacted with fuming nitric acid... 

The properties of ionized monosilane, S i FT4+, have been reevaluated by Berkowitz and coworkers6. In agreement with earlier studies2,7 it was found that ionized monosilane,... 

The clustering reactions of SiD + (n = 0-3) and Si2D + (n = 0-6) cations with deuterated disilane, Si2D6, have been measured in a FTMS study110. The dominant pathway for these reactions was found to correspond to silylene transfer and SiD4 elimination. The overall reactivity of disilane compared to monosilane was found to be higher, and this was explained by the fact that the silicon-silicon bond in disilane is considerably weaker (76 kcalmol-1) than the Si—H bond of SiFLt (88 kcalmol-1)111. Thus the insertion of Si+ into the Si—Si bond was calculated to be 17 kcalmol-1 more favorable than Si+ insertion into the Si—H bond of SiFFt106,112. 

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