Polysilanes trisilanes

Lithiotrisilanes, which are used as precursors to polysilane dendrimers, have been obtained by a mercury-lithium exchange reaction (equation 25)56,61. The terminally metalated trisilane Me3Si(Me2Si)2Li has been synthesized analogously56. 

As a result of several decades of research it is now known that a polysilane of three or more contiguous silicon atoms is susceptible to reaction by one or more of several pathways when photolyzed, each associated with cleavage of a silicon-silicon bond. The two most common processes observed are the homolysis of a silicon-silicon bond to yield a pair of silyl radicals, and the elimination of a silicon atom from the chain in the form of a silylene. As discussed in Section VII, the use of trisilanes, particularly where the central silicon atom bears aryl groups, has become an important route for the preparation of a wide variety of diarylsilylenes, A Si , many of which have been captured in glasses at low temperature, or have been allowed to dimerize to disilenes by warming. 

Both 1,2-dimethoxytetramethyldisilane and 1,3-dimethoxyhexamethyl-trisilane also undergo thermolysis at higher temperatures (225° and 175° C, respectively) to give dimethyldimethoxysilane and the homologous series of ,a>-dimethoxypermethylated polysilanes. In contrast to the base-catalyzed... 

Doubtless, in these reactions unstable intermediate silylmercury compounds such as [H2(CH3)Si]2Hg and [H(CH3)2Si]2Hg are first produced which then undergo decomposition. The formation of the trisilane and higher polysilanes from the reaction of (CH3)2SiHBr with Na/Hg may be rationalized by the insertion into [H(CH3)2Si]2Hg of dimethylsilylene, (CH3)2Si , which is probably produced during the reaction through decomposition of an unstable intermediate species (CH3)2SiHg, followed by decomposition of the adducts. The generation of dimethylsilylene and its insertion into the... 

The proton NMR spectral data of organopolysilanes have often been published incidental to preparative studies (51, 54, 62, 74, 108, 119, 177, 187, 190). Only recently has a systematic investigation to determine the chemical shifts and coupling constants in linear and cyclic permethylated polysilanes, and to study the effects of substituents on the NMR properties of methyl derivatives of disilane and trisilane been reported (see Table V-VII) (206). 

The contraction of the silicon-silicon chain of a higher polysilane molecule to give the trisilane can be explained as a result of extrustion of di-... 

Mislow et al. have used empirical force fields to study structures and conformations of the noncarbon members of group 14. Initially, parameterization of MMP was extended to cover compounds containing Si—Si bonds in order to treat polysilanes.Tbe types of compound considered were disilane, trisilane, tetrasilane, hexaphenyldisilane, cyclopentasilane, and 1,3,5,7-tetramethyltetrasilaadamantane. The minimum energy conformations calculated with their parameters matched experimental data well. The bond lengths are accurate within 0.05 A and bond angles within 2°. 

The reactions of halo polysilanes with organometallic reagents usually proceed with some Si—Si bond cleavage 17, 26, 29, 30, 31), and these reactions have little synthetic value for the preparation of higher polysilanes. Thus, from the reaction between ClgSi3 and phenylmagnesium bromide no trisilane product was obtained, and only tetraphenylsilane and hexaphenyl-disilane were isolated (25). On the basis of reactions of this type, it was postulated (26) that the reactivity of the Si—Si bond in halogenated polysilanes increases as the number of silicon atoms increases. 

Before we consider the synthesis of polysilanes in the next section, let us consider if addition polymerization involving disilene monomers can be used for the preparation of polysilanes. Robert West from the University of Wisconsin prepared the first stable disilene in 1981. His synthetic route consisted of photolyzing an acyclic trisilane. Loss of McsSi-SiMcs leads to a silylene intermediate that dimerizes to the stable disilene (Fig. 7.4) [12-13]. 

As discussed in Section VI.2.C below, the insertion of SiMeOMe into the Si-Si bonds of alkoxy-substituted polysilanes was also not observed. In 1972, Ishikawa, Takaoka, and Kumada attempted the insertion of SiMej into the Si-Si bonds of 1,2-diethyltetramethyldisilane. No 1,3-diethylhexamethyl-trisilane was observed, denying the insertion of SiMej into Si-Si bonds ... 

Even in the presence of such negative evidence, the insertion of silylenes into Si-Si bonds is not necessarily an absolutely impossible reaction. The breeding modes of producing silylenes from trisilanes and polysilanes as observed by Sakurai and coworkers and by Ishikawa, Kumada, and coworkers are actually the reverse of such Si-Si bond insertion reactions, and they point to the possibility of a transition state as shown in equation (50). 

Me Si. has C, symmetry, and the vibration spectra of trisilanes have been analysed. Aryl polysilanes give a strong 2p charge transfer in the excited state, which affects both photochemical reaction and photophysical processes, while phenyl substituted... 

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