Silanols silanetriols

Modern metallosiloxane chemistry mainly deals with three types of silanols silanetriols,11 incompletely condensed silasesquioxanes,15 and tri(tert-butyl) ester of orthosilicic acid.16 Research on these compounds has contributed to the development of model compounds for zeolites, new homogeneous catalysts, and low-temperature pathways to metal—silica materials. 

As mentioned above, hydrolysis of bulky RSiCls precursors tends to give a variety of silanols, these may be isolated and used for the preparation of Tg derivatives. Thus, simple silanetriols RSifOHls or disiloxanetetraols [RSi(OH)2l20 (R = fBu, fHexyl, or 2,4,6-iPr3C6H2) may be dehydrated and... 

Organosilanols are analogous to alcohols the most common are simple silanols of the type R3SiOH, but compounds of the types R2Si(OH)2, silanediols, and RSi(OH)3, silanetriols, are also well known. Between... 

The increasing tendency of silanols to condense as the number of OH groups on a particular silicon increases means that relatively few silanetriols have been isolated and even fewer structurally characterized. A wide range of substituted arylsilanetriols have been prepared (102), but none have been studied by x-ray crystallography. The first silanetriol to be structurally characterized was c-C6HuSi(OH)3, which has a sheet structure comprising double layers of molecules joined in a... 

For a very recent review article on the chemistry of silanols, silanediols and silanetriols see the following and references cited therein Chandrasekhar, V. Boomishankar, R. Nagendran, S. Chem. Rev. 2004, 104, 5847. 

The exact mechanism of silanization depends on the reaction conditions. It is generally accepted that silanization in a Uquid solution is a three-step process [20]. In a first step, the silanes mentioned before form silanetriols by hydrolyzation in the presence of water on the surface or in the solvent. These silanetriols attach themselves by physiosorption via hydrogen bonds onto the substrate surface. Subsequently, the silanol groups react with the free hydroxyl groups on the surface according to a Sn2 reaction mechanism (see Fig. 5). 

Individual examples of monomeric silanols such as triethylsilanol [10] and phenylsilanetriol [II] have been prepared under regimens different from those used for surface treatments and they exhibit extended stability. To date, no monomeric silanetriols have been isolated from aqueous hydrolysates of alkoxysilanes. The kinetics of silanetriol condensation have been studied [12]. The conditions which promote the hydrolysis of alkoxysilanes also promote condensation of silanols the persistence of monomeric silanetriols for more than a few hours in typical solutions is unlikely. However, the persistence of silanols in reaction mixtures containing condensed structures have been observed empirically [ 13] and by JVSi-NMR [ 14,15],... 

The equilibrium as well as the kinetics of hydrolysis and condensation of trialkoxysilanes is influenced by the organic substitution. A special case exists for many aminosilanes such as aminopropyltriethoxysilane, where a stable zwitterionic silanolate forms that is stable in solution [17-19]. It has been reported that at concentrations lower than 0.2% the aminosilane exists as a monomeric silanetriol [19]. 

Figure 7. A plot of the logarithm of the experimentally determined rate consiants for silanol condensation divided by the rate constant for the methyl substituted silanol vs. the modified Taft equation for a series of alkyl substituted methylsilanediols in dioxane at 25°C ( ) and for a series of alkyl substituted silanetriols in deuterium oxide at 35 C (A).

Novel Metallo-Silanols, -Silanediols, and -Silanetriols of the Iron and Chromium Group ... 

Oxofunctionalization with dimethyldioxirane can be additionally applied for the synthesis of a vast number of chromium-, molybdenum-, and tungsten-silanols (lOa-lOh), -silanediols (11a, 11b), and -silanetriols (12a, 12b). These compounds are usually not available via the hydrolysis route due to the electronrichness of the metal fragments, for which the Mc3P- and CsMcs-ligand are responsible, creating a silicon unsusceptible towards nucleophilic attack. 

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]. 

Towards this end, we have been recently successful in synthesizing a series of silanetriols with diverse substituents and studying their chemistry in detail. The various sections of this article summarize the following aspects of silanol chemistry (a) various synthetic routes available for the preparation of soluble and stable silanetriols starting from commonly available starting materials, (b) common spectroscopic and structural properties of silanetriols, (c) reactions of silanetriols with various metal precursors leading to the preparation of novel polyhedral metallasiloxanes, (d) structures of metallasiloxanes, and (e) possible applications. 

The preparation of stable silanetriols of the type RSi(OH)3 has proven to be difficult, compared to the other types of silanols, in view of their well known tendency to self-condense and result in siloxane rings and polymers [9]. However, in recent years, the use of very bulky R groups on silicon and carefully chosen experimental conditions has led to the isolation of many stable silanetriols. While silanetriols such as [Pg.377]

While the chemistry of other types of silanols has been explored in somewhat detail [30], the reactivity studies of silanetriols, until recently, have mainly been limited to a few esterification reactions [31], The recent objective in this area is to see whether the three OH groups on silicon can be reacted in concert with suitable metal and metalloid precursors to result in three-dimensional polyhedral cages containing a high metal to silicon ratio. 

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