Disilane Derivatives

Harald Stilger, Paul Lassacher, Edwin Hengge 

When Si2Cl6 reacts with one or two equivalents of LiN(SiMe3)2 under mild reaction conditions (-40°C, heptane solution), the corresponding mono- or diaminochlorodisilanes bis(trimethyl-silyl)aminopentachlorodisilane (1) or l,2-bis[bis(trimethylsilyl)amino]tetrachlorodisilane (2), are obtained nearly exclusively. 

Attempts to attach more than two (Me3Si)2N- groups to the disilane moiety by the application of more severe reaction conditions (excess LiN(SiMe3)2, refluxing THF or DME) have not been successful. Even after reaction times of up to 48 h only the starting materials could be recovered. 

The stability of the Si-N bonds in 1 towards nucleophilic attack subsequently allows the introduction of further functional groups, which leads to the formation of multifunctional disilane derivatives simultaneously bearing three different kinds of substituents. Thus, for instance, 1 can react with /-butyllithium or the transition metal anion [Fe(CO)2Cp] without any detectable scission of the Si-N linkages  

Hydrogenation of the remaining Si-Cl bonds in 1-4 using LiAlHa in diethyl ether at 0°C additionally affords the corresponding disilanyl hydrides 5-8 in excellent yields. Neither Si-Si, Si-Fe or Si-N bond cleavage is observed  

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Correlation of the SiSi bond lengths in disilane derivatives X3Si-SiX3 with X = CH3, C(CH3)3 and Si(CH3)3 together with those of sterically overcrowded cyclic [7a] and linear [7b] trisilane derivatives versus their Pauling bond orders [5a,6b,7], expectedly, produces a linear regression (Fig. 4). 

One alternative route to the observed disilane derivatives could be via the silylenes by hydrogen abstraction and dimerization. This process, although we consider it to be less likely, might be one pathway anticipated if triplet silylenes are generated in the reaction media. 

The possibilities of synthesis are limited by the general properties of the Si-Si bond to a small number of reaction types. Disilane derivatives were usually synthesized by a Wurtz-like reaction of monosilane halide derivatives with alkali or alkaline-earth elements. Some pyrolysis reactions that have yielded disilane derivatives are also known. Often, new disilane derivatives were prepared from common derivatives by an exchange of substituents. Some rearrangements have been observed in recent years. 

These difficulties and generally low yields in the preparation of organic disilane derivatives by the reaction of disilane hexahalide with organometallic compounds lead to the conclusion that the better method for preparation of these compounds is the Wurtz-type synthesis starting from organohalomono-silane. 

However, the limitations of these methods were soon discovered, and Kipping385) found that the reaction of i/j2(CM) SiCl with Na does not yield the expected disilane derivative. A Si—0—C cleavage occurs and some reaction products (SM2)4, (ty (SM2) CM and (CV)2Si result, perhaps via radicals. Similar observations were sometimes made on the cleavage reaction of the Si—0-C bond systems ... 

The formation of disilane derivatives is not possible only with metals organometallic compounds also react with halosilanes. West et al.509 found formation of disilanes in the reaction with Grignard compounds ... 

Under suitable reaction conditions, the Grignard reaction is a useful means of forming disilane derivatives. Some examples following the general reaction ... 

Mixed disilane derivatives with Si—me and Si—Cl bonds were prepared, usually by the methods of Kumada et al. These methods were recently reviewed368. Two principal methods are known, firstly methyl exchange by H2S04 and subsequent chlorination or fluorination by NH4C1 or NH4F,... 

Like all silicon-halogen bonds, halogenated disilanes are hydrogenated by complex hydrides. A series of new disilane derivatives are thereby synthesized ... 

Kumada, Ishikawa and Tamoa617, 618 623) reported the preparation of disilane derivatives with cis-trans isomerism. A mixture of cis-trans 1,2-diphenyl-1,2-dimethyl-1,2-disilacyclohexane was obtained from the following reaction ... 

The NMR data for most disilane derivatives are known. However, only a small number of studies have been published relating NMR data to the properties of the Si-Si bond. The chemical shift of the protons of Si2me6 shows only a very small difference in comparison with Sime4 (Si2me6 9.963 0.02 ppm59)), indicating the small influence of the Si-Si bond. 

Die Bezeichnungen a)—f) werden im folgenden so verwendet, daB sie fiir einen bestimmten Reaktionstyp charakteristisch sind und Vergleiche zwi-schen den Darstellungsverfahren der Disilan-Derivate gestatten. 

This liberation of energy by the reaction just shown was found by Schumb 4 and his coworkers also to be sufficient to cleave the silicon-silicon bond. The action of sodium on mixtures of ethyl bromide, chlorobenzene, or n-amyl chloride with hexachlorodisilane resulted only in tetrasubstituced monosilanes and no disilane derivatives. In the same way, the silicon-oxygen-silicon bond of hexachlorodisiloxane was split by sodium in the presence of ethyl bromide or chlorobenzene and yielded only silicon tetraethyl or tetraphenyl. 

Because of the close relationship between silicon and carbon, many attempts have been made to try to synthesize species containing multiple bonds to silicon (Si=C, Si=0, Si=Si, etc.). However, it was not until 1967 that compelling evidence was presented that Si=C might exist in the thermal reaction of 1,1-dimethyl-1-silacyclobutane (equation 90). The first evidence for the existence of Si=Si as transient intermediate was provided in the thermolysis of bridged disilane derivatives (equation 91). Since then, many studies have been published on these unsaturated species, but it was in 1981 that synthesis and characterization of relatively stable crystalline compounds containing Si=C (silene) (equation 92) and Si=Si (disilene) (equation 85) were reported (equations 90-92). 

Silacyclopropenes were reviewed as intermediates in the photolysis of several alkynyl-substituted disilane derivatives <1996CC2609>. Photolysis of the disilane derivatives gives highly reactive silacyclopropene intermediates. 

Si-Si Bond Homolysis. Ishikawa and co-workers (50, 51) have also studied the photolysis of a number of polymeric disilane derivatives. These workers have examined both derivatives with pendant silicon substituents (50) and materials in which the disilane moiety is incorporated into the polymeric backbone (5i). For derivatives with pendant silicon substituents (Scheme II), irradiation destroys the characteristic phenyldisilane absorption... 

Degradation of Larger Silicon Catenates. Although it seems clear that polymeric disilane derivatives photodecompose by bond homolysis to produce silyl radicals, model studies on larger silicon catenates indicate that their photochemistry may be more complex (Scheme IV). Cyclic silane derivatives seem to extrude monomeric silylenes upon irradiation to produce smaller cyclic silanes (52). The proposed silylene intermediates have been identified spectroscopically 49, 53), and trapping adducts have been isolated in solution. Exhaustive irradiation ultimately results in acyclic silanes, which... 

Table 1. NMR chemical shifts 6[ppm) of multifunctional disilane derivatives 1-10 (CsDs solution vs. ext. TMS).

The heterogeneously catalyzed disproportionation of 1,1,2,2-tetrachlorodimethyldisilane leads via oligo(chloromethylsilane)s to highly branched poly(chloromethylsilane)s. The disilane derived oligomer formation can be controlled by the nature of the Lewis base catalyst. 

The Si-Si bond cleavage has been poorly explored. Nevertheless, a new disilane derivative (10.31) has been very recently proposed as a very efficient cleavable photoinitiator [55]. 

In view of the vast number of disilane derivatives which have been prepared, a comprehensive tabulation of these compounds in this work was considered impractical. We have, however, attempted to include in Table I 71-100, etc.) all disilanes prepared since 1958 together with numerous historically or synthetically significant derivatives prepared prior to this date. This table together with that previously reported (24) should, however, provide a comprehensive survey of the known higher polysilanes. 

As we have previously remarked, the majority of the studies in the field of polysilanes have been, until recently, confined to the disilane derivatives. 

UV-Vis data of compounds 1-12 are summarized in Table 2 Figure 2 exhibits the absorption spectra of the donor-acceptor-substituted disilane derivatives 1 and 11 together with appropriate model compounds containing the donor and acceptor moieties in the absence of interaction. The absorption spectrum of the Fc derivative 11, exhibiting the acceptor band near 335 nm and the weak ferrocene band near 450 nm, apparently fulfills the classical expectation for nonconjugatively connected chromophores, being identical to the sum spectrum of the isolated donor and acceptor models (Fig. 2A). Other compounds containing Fc as a donor behave similarly [4]. 

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