Metal-silicon bond compounds

Acidic transition-metal hydrides have been used to prepare metal-silicon bonded compounds by an interesting condensation reaction that proceeds with amine elimination (equation 30)66. An attempt to carry out a similar reaction between Ph3SiH and the zirconium amide Cp2Zr(NMe2)2 failed67. 

Transition-metal chemistry is currently one of the most rapidly developing research areas. The record of investigation for compounds with metal silicon bonds is closely comparable to that for silicones it was in 1941 when Hein discovered the first metal silicon complex, followed by Wilkinson in 1956. A milestone in the development of this chemistry was Speier s discovery of the catalytic activity of nobel metal complexes in hydrosilylation reactions in 1977. Hydrosilylation is widely used in modem organic syntheses as well as in the preparation of organo functionalized silicones. Detailed investigations of the reaction mechanisms of various catalysts continue to be subject of intense research efforts. 

Most of the above reactions occur via a mechanism involving intermediates with a metal-silicon bond (i.e. silicometallics) and a metal-hydrogen bond, accompanied (or sided) only occasionally by compounds containing metal-carbon bonds (i.e. organometallics) that are characteristic of the key intermediates of transition-metal-catalyzed transformations of organic compounds (for recent reviews, see Refs [11, 13]). 

The silicon-metal doubly bonded compounds can further react to yield desired derivatives. Recently, vinyl magnesium bromide was reported to react with chloro complexes containing chromium or iron double bonds to silicon, to yield the l-metalla-2-sila-l,3-diene compounds (115, equation 34), which can potentially be used to functionalize silicon polymers in a desired fashion121. 

During the last five years, a vast amount of research has been focused on the chemistry of group-8 silicon complexes. This section takes into account only those complexes containing well-characterized metal-silicon bonds but does not take into account intermediary compounds in stoichiometric or catalytic reactions in which such species may be formed. 

The intermediate of the reaction may be the sUyl-alkali metal species, so that the reaction of the silyl-alkah metal derivatives with chlorosUane resulted in the formation of the silicon-silicon bonded compound (equation 58). 

Summary This conttibution describes two general approaches to the development of new transition metal-based silicon chemistry. In one approach, the focus is on early transition metals and the chemistry of d metal-silicon bonds. A second approach targets metal-silicon multiple bonds, which are expected to exhibit rich reaction chemistry. Various new compounds and chemical processes are discussed. 

Metallo-silanols represent a special class of silanols which is characterized by a remarkably high stability with respect to self-condensation. This property, which is even valid for silanetriol derivatives [2], and the generally stable metal-silicon bond makes these compounds useful precursors for controlled condensation with chlorosilanes to build up unusual arrangements of fimctionalized siloxanes at metal centers. The metal fragment is responsible for highly hydridic Si-H units, creating the conditions for electrophilic oxygenation with DMD [3]. 

Common syntheses of these compounds are the alkaline salt elimination reaction, the insertion of unsaturated metal fragments into Si-H bonds, or metal-silicon bond formation by elimination of small molecules [1, 2],... 

This chapter presents the hydrosilylation processes as well as the main types of reactions related to hydrosilylation (except double silylation) in which intermediates with metal-silicon bonds (i. e., silicometallics vs. organometallics) play a decisive role with mechanistic implications for both new and well-known catalytic reactions leading to formation of organosilicon compounds [6]. Double silylation (bissilylation) of unsaturated organic compounds has recently been reviewed comprehensively [7]. 

II. THE FORMATION OF COMPOUNDS CONTAINING TRANSITION-METAL-SILICON BONDS... 

An early report of Ti(SiPh3)476, prepared from TiCl4 and KSiPh3, has since been refuted and the compound reformulated as the siloxide Ti(OSiPh3)467. Further studies have shown that early metal-silicon bonds are quite reactive toward oxygen77, underscoring the importance of excluding air in preparations in early metal silyls. 

The first observation of CO insertion into a transition metal-silicon bond was made recently83,249. Zirconium silyl Cp2Zr(SiMe3)Cl reacts rapidly with carbon monoxide with precipitation of a bright pink complex that has been identified as the silaacyl compound 46 (equation 94). An X-ray crystal structure of 46 confirmed the formation of a... 

Chapter III basically collects contributions on the role of transition metals in organosilicon-based chemistry. The review of the Wacker-Silicone Awardee T. Don Tilley on Transition Metals in Organosilicon Chemistry ftindamentally describes two different strategies toward the exploitation of reactive metal-silicon bonds. One type involves the early transition metals, since d° metal-silicon single bonds are unsupported by metal-to-silicon 7t-backbonding and are therefore weaker than other transition metal-silicon bonds these M-Si bonds activate various unsaturated compounds via insertion and participate in the activation of single bonds (e.g., C-H and Si-C), possibly being responsible for the action as catalysts for the dehydropolymerization of hydrosilanes to poly silanes. Most recently, even the catalytic silylation of methane has been observed. 

This artiele is focused on common features and principal ddferences between earbene and silylene eomplexes of early and late transition metals from a quantum ehemical point of view. The bonding properties of the metal earbon and metal silicon bond were investigated for that purpose at the model compounds 1 to 4 (Fig. 1). The molecules were optimized with DFT methods and the electronie properties were analyzed with CDA, NBO and the EHT-method. 

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