Doubly bonded silicon compounds

Polyolefins are, of course, usually synthesized by the catalyzed polymerization of alkenes. Why is an analogous route, polymerization of disilenes, not employed to prepare polysilanes The reason is paradoxical. The energy barrier to polymerization of doubly bonded silicon compounds is simply too low, so that in most cases they polymerize (or oligomerize) immediately when they are generated ... 

FIGURE 15. Schematic description of the energy profiles for addition reactions to doubly bonded silicon compounds. Reproduced by permission of Kluwer Academic Publishers from Ref. 175b. 

Both experimentally and theoretically triply bonded silicon compounds proved to be a harder nut to crack than doubly bonded silicon compounds. Strong experimental evidence for the existence of triply bonded silicon comppunds such as RSi=CR or RSi=SiR is not yet available (however, HN=Si has been identified, see below), although some indications that they may exist as transient species have been presented19. In this situation, theory remains the only reliable source of information for these molecules that allow one to study their fundamental properties and reactions. Furthermore, the calculations can be used to develop and test ideas, which may eventually lead to the synthesis of stabilized molecules of this family that may have a better chance to be observed experimentally. As will be demonstrated in the discussion below, theory has faced many difficulties in studying these molecules and it seems that only in the last two years the calculations are converging towards a consensus about the structures and relative stabilities of these molecules. 

Of particular interest is the recent synthesis of iron complexes of the rjM-silapropenyl ligand. These are the first stable compounds of doubly bonded silicon [H. Sakurai, Y. Kamiyama, and Y. Nakadaira, J. Am. Chem. Soc. 98, 7453 (1976)]. The compounds [Fe(r)3-CH2CHSiMe2) (SiMe2R) (CO)3] (R = Me or CH CH2) have been prepared by reacting Fe2(CO)9 with vinylpentamethyldisilane or 1,2-divinyltetramethyldisi-lane. 

Whereas the analogous carbenes easily isomerize wherever possible to compounds containing doubly bonded carbon atoms even under the conditions of matrix isolation, silylenes are almost as stable as the corresponding substances with doubly bonded silicon atoms. For example, methyl- and silylsilylene lie just 4 and 8 kcalmol-1 above silaethene and disilene, whereas the difference between ethene and methylcarbene is as high as 70 kcalmol-1 149-151 As a consequence, silylenes are often key intermediates on the way to other highly reactive silicon compounds discussed above. 

On the theoretical side, Schaefer reviewed the early studies on silene that contributed greatly to the theoretical interest in these compounds (35). The review by Raabe and Michl (79) on multiply bonded silicon included a comprehensive examination of the early theoretical work. Gordon (85,86) has summarized multiply bonded silicon, and the studies of Luke et al. deserve mention as a comprehensive study of multiply bonded silicon at a uniform level of theory (87,88). Nagase et al. (89) have reviewed their studies on doubly bonded silicon and germanium, with particular emphasis on reactivity. Most recently, Apeloig (90) has provided encyclopedic coverage of theoretical studies of silicon compounds through the middle of 1987. 

Corriu and coworkers found that the reaction of the pentacoordinated functionalized silane 6 (see also Sections II.B.l and II.C.l) with elemental selenium leads to a stable silaneselone 140, which is stabilized by intramolecular coordination of the nitrogen-containing substituent to the doubly-bonded silicon (Scheme 42) . Although the structure of 140 was supported by Si and NMR and MS spectra, including a downfield Si chemical shift (S = -1-29.4) and a high coupling constant with Se (JseSi = 257 Hz), neither the crystallographical structure analysis nor the reactivity of this isolable Si=Se compound has been reported. 

Disilenes exhibit the relatively low-field (8 = 49-155) 29Si chemical shifts characteristic of low-coordinate silicon compounds (Table I) thus 29Si NMR spectra are very important in their characterization. This deshielding is similar to that observed in the 13C chemical shifts of doubly bonded carbons relative to those of their saturated counterparts. 

Among the silicon-chalcogen double-bond compounds, the silicon-sulfur doubly-bonded compounds (silanethiones) are considered to be easier to synthesize, since it has been predicted by the theoretical calculations that a silicon-sulfur double bond is thermodynamically and kinetically more stable than a silicon-oxygen double bond (silanone)13,14. According to the calculations, the lower polarization of Si=S compared to Si=0 should lead to a lower reactivity of Si=S. In addition, H2Si=S (1) is calculated to be by 8.9 kcal mol-1 more stable than its divalent isomer, H(HS)Si , whereas H2Si=0 (2) is by 2.4 kcal mol-1 less stable than H(HO)Si . 

As described in the preceding reviews on this field, most of the early work on silicon-sulfur doubly-bonded compounds was restricted to simple dialkylsilanethiones, which are all transient in solution or in the gas phase4. However, in contrast to the successful matrix isolation and spectroscopic identification of dimethylsilanone 1023, no spectroscopic detection of transient dialkylsilanethiones in matrices has been reported up to now, although the matrix isolation of Cl2Si=S50 and Cl(H)Si=S51, the silicon analogues of thiophosgene and thioformyl chloride, has been reported. 

As mentioned in this chapter, in recent years much progress has been made in the chemistry of silicon-chalcogen multiple bonds. For silicon-sulfur doubly-bonded compounds, we have now several isolated examples, both kinetically stabilized and thermodynamically stabilized. Furthermore, there have been reports of the synthesis and characterization of stable compounds with silicon-nitrogen double bonds (i.e. silanimines or iminosilanes) as well as their heavier group 15 element analogues such as phosphasilenes and arsasilenes. 

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