Metal silyl complexes

The growing interest in volatile silyl-metal complexes for chemical vapor deposition reactions should also be mentioned. This technique is extremely useful for the preparation of silicide films in microelectronic devices. Further examples of applications of silicon-metal compounds are given in the appropriate sections. 

The first silyl-metal complex was synthesized in 1956 with the compound CpFe(CO)2SiMe3 [93, 94]. Since then, methods for the formation of M-Si bonds have been investigated systematically and developed to an effective set of tools. Numerous articles covering this subject give evidence for the continuous development of this area [64, 95],... 

Employing ketones or aldehydes as starting materials, the corresponding silylethers are obtained. Thereafter, the oxidation or hydrolysis of the obtained silylethers gives the corresponding alcohols (Scheme 17). In most cases, a hydride (silyl) metal complex H-M-Si (M = transition-metal), which is generated by an oxidative addition of H-Si bond to the low-valent metal center, is a key intermediate in the hydrosilylation reaction. 

In spite of the many known silicon-transition metal complexes (15,16), little systematic work has appeared on the reaction mechanisms of silyl metal complexes. This state of affairs is in marked contrast to the current work on cr-alkyl transition metal complexes, where much emphasis is placed on determining detailed decomposition mechanisms (75-76). The reason for the interest in decomposition mechanisms is that the products of a transition metal-catalyzed reaction are released by the decomposition of the product-metal complex. Thus, to understand a catalytic process, one must have knowledge not only of the interaction of the reactants with the metal (leading to a substrate-metal complex), but also of the mechanisms whereby substrates are transformed on the metal and the manner in which the products are released. 

Chalk and Harrod provided the first mechanistic explanation for the transition metal catalyzed hydrosilation as early as in 1965. Their mechanism was derived from studies with Speier s catalyst and provided a general scheme, which could be used also for other transition metals. The catalytic cycle consists of an initial oxidative addition (see Oxidative Addition) of the Si-H bond, followed by coordination of the unsaturated molecule, a subsequent migratory insertion (see Insertion) into the metal-hydride bond and eventually a reductive elimination (see Reductive Elimination) (Scheme 3 lower cycle). The scheme provides an explanation for the observed Z-geometry in the hydrosilation of alkynes, which is a consequence of the syn-addition mechanism. The observation of silated alkenes as by-products in the hydrosilation of alkenes along with the lack of well-established stoichiometric examples of reductive elimination of aUcylsilanes from alkyl silyl metal complexes... 

S.2.9.3. t] o-Alkyl-, -Aryl-, Acyl- and Silyl Metal Complexes by Reaction of Metal-Atom Vapors. 

Unusual Double Silylation Reactions of Bis(silyl)metal Complexes with o-Carboranyl Unit... 

Thermal elimination of silyl and hydride moieties from silyl-metal complexes yields silanes ... 

The double silylation of unsaturated organic conq)ounds catalyzed by group 10 metals is a convenient synthetic route to disilacyclic conqx)unds. Nickel and platinum complexes, in particular, are excellent catalysts for the transformation of disilanes. Cyclic bis(silyl)metal complexes have been in licated as key intermediates in the metal-catalyzed double silylation of alkynes, alkenes, and aldehydes however, the intermediates have not been isolated due to their instability. We now describe (i) the isolation of the reactive intermediates cyclic bis(silyl)metal compounds (1) with bulky o-carborane unit (ii) the generation of a new class of heterocyclic con unds (4-5) by the stoichiometric reaction of the intermediates with a variety of substrates such as an alkyne, dione, and nitrile and (iii) the facile double silylation of alkenes and alkynes (10,12-14) catalyzed by the intermediate under mild conditions. ... 

The synthesis of this special type of metallo-silanol starts most efficiently with an appropriate silyl metal complex, such as Cp(OC)2Fe-SiMe2R (R = H, OMe). An anionic shift of the silyl group from the iron to the cyclopentadienyl unit can be induced with lithium diisopropylamide, leading to the metallates 21a,b. Methylation with methyl iodide produces the neutral methyl iron complexes (C5H4SiMe2R)(OC)2Fe-Me (R = H, OMe), which can be converted either, for R = H, by the Co2(CO)8 method, or, for R = OMe, by hydrolysis in the presence of acetic acid, into the corresponding silanol 22. 

Figure 17. Examples of synthetic methods for the preparation of silylated metal complexes.

In method b) of Fig. 18, the silylated metal complex is stirred with the support material to allow for covalent attachment to the silica. 

Introduction of an alkyl silicate component into a sol-gel mixture creates an extremely complicated kinetic situation (192, 193). Typically, these network modifiers are diluted with an orthosilicate species such as TMOS (network formers) to afford a porous material. Condensation of pure aUcyltriaUcoxysilanes tends to result in nonporous materials, as is apparent from the plot of surface area versus silylated metal complex TEOS ratio shown below (Fig. 19). 

---