Silicon analogue

Besides stmctural variety, chemical diversity has also increased. Pure silicon fonns of zeolite ZSM-5 and ZSM-11, designated silicalite-l [19] and silicahte-2 [20], have been synthesised. A number of other pure silicon analogues of zeolites, called porosils, are known [21]. Various chemical elements other than silicon or aluminium have been incoriDorated into zeolite lattice stmctures [22, 23]. Most important among those from an applications point of view are the incoriDoration of titanium, cobalt, and iron for oxidation catalysts, boron for acid strength variation, and gallium for dehydrogenation/aromatization reactions. In some cases it remains questionable, however, whether incoriDoration into the zeolite lattice stmcture has really occurred. 

At one time it was felt that it would be possible to produce silicon analogues of the multiplicity of carbon compounds which form the basis of organic chemistry. Because of the valency difference and the electropositive nature of the element this has long been known not to be the case. It is not even possible to prepare silanes higher than hexasilane because of the inherent instability of the silicon-silicon bond in the higher silanes. 

Silicon analogues of imidazole-2-ylidenes are stable carbenes that form adducts where the metal-silicon bond is relatively weaker than that between metal and carbon atoms. 

It is somewhat disappointing to realize that the thermochemistry of germanium, tin and lead organometallic compounds is still at the level achieved ten years ago, in contrast to the considerable recent efforts to probe the energetics of the silicon analogues. The data analysis in the previous sections shows that many key values are either missing or require experimental confirmation. To a certain extent, an overall discussion of the thermochemical data for Ge, Sn and Pb is therefore hindered by the probable inaccuracies and the uncertainties that affect those values. 

The rate constants for these reactions are outlined in Table 1, and in all cases indicate that the tin and germanium radicals are less reactive than the silicon analogue. The rate constants for the addition of these radicals with other unsaturated materials are also presented. 

The silicon analogue 74 u5) also appears to be a potentially useful conjunctive reagent in this sequence even though silicon appears to retard the rearrangement71 It is generated by silylation of 37a followed by reductive lithiation (Eq. 90). The... 

The silicon analogue of carbene is now called silylene and it was discovered by the thermally-induced alpha elimination of methoxydisilanes. The most common method of silylene generation is ... 

The silicon analogue can also be prepared by an analogous reaction but it is less stable. I.r. data suggest the presence of isomers in which the (jt-Cp)... 

This section provides a discussion of RsSi", the silicon analogue of the carbenium ion. While such species do exist in the gas phase, their existence in condensed... 

Strained cyclotrisilane derivatives, which are silicon analogues of cyclopropane, are prepared by the reductive coupling of overcrowded dichlorodiarylsilanes using lithium naphthalenide (Scheme 14.9). 

Cyclic siloxanes (R2SiO) (see Section 6.2.1) may be considered as silicon analogues of crown ethers (see the introduction to Chapter 6). Note, however,... 

The sesquisiloxanes, (RSiOi,s)n (n = 4, 6, 8, 10, 12), adopt a variety of molecular cage structures (Figure 13).404-408 The ferr-butylgermanium sesquioxide, (Bu Ge Og, has the same structure as its silicon analogue.409... 

Explain why the size of the silicon atom does not permit a silicon analogue of the graphite structure. 

KEY CONCEPT PROBLEM 19.3 The organic solvent acetone has the molecular formula (CH3)2CO. The silicon analogue, a thermally stable lubricant, is a polymer, [(CH3)2SiO] . Account for the difference in structure ... 

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