Metal silanes

A recent study by DePalma and Tillman [ 10] also demonstrates the potential of surface modification by self-assembled monolayers of low surface energy fiuoroalkyl-containing silanes. Fatty acids, amines and alcohols have long been known to adsorb as monomolecular films on metals. Silane coupling agents have featured strongly in new studies to develop more robust films, covalently bound together and to the metal substrate. 

The substrate/silane interphase and the silane/matrix interphase are equally important in considering the mechanism of reinforcement by silane coupling agents in composites. The mineral oxide/silane interphase is more well defined than a metal/silane or a silane/matrix interphase. For example, in the case of a metal substrate, surface oxides may dissolve into the silane layer or form a complex. In the case of the silane/matrix interphase, a diffuse boundary layer may exist due to dispersion of physisorbed silanes in the matrix phase or penetration of the matrix resin into chemisorbed silane layers. Many features of the interaction of a silane coupling agent with a polymer matrix are specific to the system, and thus the chemistry of the silane/matrix interphase must be characterized and defined for each system. 

The chemistry of metalated organosilicon compounds has been the subject of several reviews1, the most recent ones by Lickiss and Smith13 and Tamao and Kawachilb, which cover the literature up to the year 1994. This chapter will now take into account the developments in the chemistry of metalated silanes up to the middle of 1996 however, for completeness there will be some overlap with former reviews. The emphasis of this review is on the synthesis and structure of these metalated silanes. However, some examples of their utilization for synthetic purposes will also be given where appropriate. For more information about synthetic applications of silyl anions the reader is referred to some leading references in this fieldla,b h k. 

Another point which needs to be clarified from the start is the nomenclature of metalated silanes We will frequently use the term silyl anion in this chapter when we talk about metalated silanes. Although the term anion defines, literally taken, an ionic compound, this expression, when used by us, does not necessarily imply that the compound in question is of ionic nature, but covers, as well, in analogy to the use of the term carbanion , silicon compounds with a polarized covalent silicon-metal bond. 

This reaction, which initially was assumed to be a simple deprotonation8a,b, was shown later by Corriu and coworkers9 to proceed via a two-step mechanism. The initial step is the reversible formation of a pentacoordinated species, which decomposes irreversibly under formation of molecular hydrogen and the metalated silane. 

The first step of this conversion is assumed to be the formation of the silyl anion, which undergoes a subsequent nucleophilic attack on the starting material283. The resulting disilane may be isolated, when stoichiometric amounts of metal are used. Flowever, in contrast to peralkylated disilanes, disilanes which bear at least one aryl substituent at each silicon are susceptible to further reduction. Accordingly, the Si—Si bond of the fully or partially arylated disilane is easily cleaved under the reaction conditions by slow electron transfer from excess metal, eventually transforming both silyl units of the disilane into the desired metalated silane. 

Metalated silane Precursor silane Solvent Reference... 

X3SiL ligand silane + M - metal precursor XjSiLM metal-silane complex... 

SiOII surface silane + X3SiLM metal-silane complex - SiOSiLM + IE... 

Moran et al. 41 attached the Cr(CO)3 moiety to tetrakis(phenylsilane) 32 (prepared by the hydrosilylation of tetraallylsilane with four equivalents of dimethylphenylsilane) by treatment with excess Cr(CO)6 in dibutyl ether—THF at 140 °C affording the air-stable, crystalline tetrakis(chromium carbonyl) dendrimer 33, which was also prepared by reaction of tetraallylsilane with [r76-C6H5Si(Me)2H]Cr(CO)3) (Scheme 8.9). 42a Reaction of the corresponding eight phenyl-terminated analogues afforded the partially metalated silane dendrimer 34 as the major product, even with an excess of Cr(CO)6. 

Figure 9. Reaction of alcohol on penta-coordinated metal-silane complex.

Polyurethane Methanol 1. Abrasion followed by brushing. Grit or vapor blast or 280-grit emery cloth followed by solvent wipe. 2. Incorporation of a chlorosilane into the adhesive elastomer system 1% by weight is usually sufficient. Chlorosilane is available commercially. Addition to adhesive eliminates need for priming and improves adhesion to glass and metals. Silane may be used as a surface primer... 

Si-Si bonds are formed when 1 is reacted with phenylthiooligosilanes. Alkali metal thiophenolates, which are byproducts in the reaction can be easily removed. Also reaction of triethyl- or tributylstannyl-undecamethylcyclohexasilane with alkali metal silanes leads to the formation of Si-Si bonds. While the reaction of phenylthiooligosilanes proceeds at room temperature the conversion of the tin compound requires temperatures below 0 °C to avoid decomposition of the product. 

A well established method for the preparation of metalated silanes is the cleavage of Si-Si a-bonds by alkali metals [1]. The reaction of arylsubstituted cyclosilanes yields the corresponding a,(0-dilithiated oligosilanes. Whereas the reaction of cyclopenta- and cyclotetrasilanes with lithium is well known [2, 3], a metal mediated cleavage of a cyclotrisilane was not described up to now. Here we report the reaction of cyclotrisilane 1 with lithium, which affords, depending on the conditions, either 1,3-dilithiotrisilane 2-Li or l,2-dilithiodisilane3-Li. 

Although there are numerous reactions in which transition metal-silane systems catalytically reduce unsaturated molecules (Section VI.E), the degree to which insertions into M-Si bonds contribute to this reactivity is unclear, since few insertions involving M-Si bonds have been directly observed. This low reactivity toward insertions is consistent with observations that transition metal-silicon bonds are usually shorter than expected, presumably strengthened by rc-bonding between the transition metal and silicon (Section V). Clearly, much work is needed to determine conditions that make insertion reactions favorable. 

Solutions of 2 left at room temperature for 2 h show reduced ee values of 53 % after trapping with MesSiCl. In order to determine stabilizing effects on the configuration at the silicon center, we performed a metathesis reaction with [Mg(thf)4]Br2 at -70 °C. After 2 h at room temperature no significant racemization of the resulting metalated silane could be observed (ee > 98 %). 

This increase in stability caused by the change of the metal is in contrast to the proposed mechanism of racemization for metalated silanes. Since the rate-determining step of the racemization process is discussed in the literature [3a, 8-11] as the inversion of the free silyl anion, no drastic effect of transmetalation should be expected. 

Trihalogenosilyl compounds are of general importance as trifunctional precursors for the synthesis of highly functionalized silicon compounds such as branched silicones and silsesquioxanes. Trihalogenosilylstannanes and related germanes, being a kind of a-halogeno(metal)silane, would be most desirable precursors for further transformation. 

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