Silicon compounds halosilanes

The properties and applications of commercially important hydride functional silanes, ie, compounds having a Si—H bond halosilanes, ie, compounds having a Si—X bond and organosilanes, ie, compounds having a Si—C bond, are discussed hereia. Compounds having Si—OSi bonds are called sdoxanes or sihcones. Those having a Si—OR bond are called siUcon esters. Sdoxanes and siUcon esters are discussed elsewhere ia the Eniyclopedia (see Silicon COMPOUNDS, SILICON ESTERS SILICON COMPOUNDS, SILICONES). 

An important reaction of silicon compounds containing electronegative substituents, such as the halogens or a Group 15 or 16 element, is hydrolysis, which often occurs very readily even with atmospheric moisture. The primary reaction in silicone production is the hydrolysis of halosilanes, usually the readily available chlorosilanes, to give silanols, which then undergo acid-catalyzed condensation with loss of water, as shown in Scheme 1. 

Although halosilanes undergo smooth nucleophilic substitution at the silicon, they are rather inert to redox reactions. Therefore, chlorosilanes are usually used as trapping agents of anionic intermediates generated by electroreduction of organic compounds. However, in the absence of other reactive substrates halosilanes are reduced electrochemically to form Si-Si bonds. Indeed, there are a number of reports in the literature of the cathodic reduction of chlorosilanes (Sect. 4.2). 

Halosilane. A halogen derivative of a silane a compound of silicon containing at least one Si—X bond, where X represents a halogen. 

Activation parameters at coalescence temperature show that the coordinate interaction in these compounds is not a function of the electronegativity of X but is controlled by the ability of the nitrogen atom to stretch the Si—X bond. The tendency of the silicon atom to increase its valency decreases in the order X = OCOR, Br, Cl > SR F > OR, H This sequence corresponds directly to the rate of racemization of halosilanes and to the substitution of R3SiX with inversion of configuration Although no intramolecular coordination was observed in solutions of acetoxysilanes (CH3) Si(OCOR)4 by the Si NMR method the shape of the H NMR spectra of these compounds with chiral silicon atom points to Si -0 interaction... 

The reaction between halosilanes and ammonia or primary or secondary amines is the most widely used synthetic route to compounds containing 8i-N bonds. 8uch reactions are greatly controlled by steric effects if large groups are present on either the silicon or the amine, then less than full substitution may occur. For example, ammonia reacts with chlorosilanes to give a variety of products depending on the size of the chlorosilane (equation 32). 

Since the preparation of anionic species or organosilicon compounds is largely restricted by its reaction conditions, the synthetic utility of silyl anions for the synthesis of organosilicon compounds is limited. However silyllithiums possessing one or more phenyl groups on a silicon atom are readily prepared by the reductive metallation of the corresponding halosilanes with lithium. Allylsilanes are synthesized by the reaction of allyl acetates with the cuprate prepared from such a reagent (eq (47)) [43]. 

The silicon-halogen bonds, on the other hand, occupy a position of great importance in organosilicon chemistry as most all organosilicon compounds must be synthesized ultimately through halosilane intermediates. The three general methods available for the preparation of halosilanes from silicon are listed in Scheme IV. 

Aminosilanes are valuable compounds in modem silicon chemistry, not only as alternative precursors for chemical vapor deposition of Si3N4, but also because of their many interesting chemical features. The amino groups can be easily attached to silicon by the reaction of halosilanes with amines or alkali metal amides using standard literature procedures [I ]. 

Only recently, nitriles were shown to increase the silicon coordination number to up to five in cationic complexes (18) [95], even though nitriles had already been shown to form adducts with sihcenium ions, thus functioning as a donor moiety in a tetrahedral Si coordination sphere [96]. Amines are scarcely encountered in silicon coordination compounds, only few examples of crystallographically evidenced silicon complexes with monodentate amines have been reported so far, which include the adduct SiF4(NH3)2 [97-100], Imines, however, are well known to add to various halosilanes. Especially IV-heterocycles with imine functionahty, such as pyridines [101-103], imidazoles [104, 105], pyrazoles [106], and related compounds, can be found as ligands in various silicon complexes (e.g., in 19 and 20). 

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