Si-F bond

Trimethylsilyl enol ethers can also be cleaved by tetraalkylammonium fluoride (Entry 2) The driving force for this reaction is the formation of the very strong Si-F bond, which has a bond energy of 142 kcal/mol.31 These conditions, too, lead to enolate equilibration. 

Compounds 17 were formed by the reaction of Is(R)SiF2 (R = Is, /Bu) with two equivalents of [LiPH2(DME)] (DME = 1,2-dimethoxyethane). They do not show any tendency to eliminate LiF in THF solutions, surely because of the strong Si—F bond. Similar stability was observed for the nitrogen analogs.13... 

It is interesting to note that the strongly polar B—F and Si—F bonds have the largest known single-bond enthalpies, which must be a reflection of the nature of the bonds, as we will discuss in Chapters 8 and 9. 

More illustrative is the structure of the fluorosilane which exhibits the same tricapped tetrahedron geometry. The three N-Si interactions take place in the frontal position towards the Si-F bond instead of the rear coordination observed in penta- and hexacoordinated structures. Furthermore, the benzylamino groups have the possibility not to be coordinated at silicon since these groups have the capability of free rotation around the carbon-nitrogen bond. 

In contrast, when boron trifluoride etherate is substituted for the free boron trifluoride, only a trace of the hydrocarbon is formed, even after weeks of reaction.143 The unique effectiveness of boron trifluoride gas in promoting these reductions is believed to be due to several factors, including the ability of the coordinatively unsaturated boron center to rapidly and tightly coordinate with oxygen centers and to the thermodynamically favorable creation of a Si-F bond.1 A slight pressure of boron trifluoride gas must be maintained over the surface of the solution throughout the reaction because boron trifluoride has only limited solubility in the weakly coordinating dichloromethane solvent. 

For rigid Si-F bonds, the 19F CSA is characterized by a span, Q = Sn - S33, of about 80 ppm [150]. This parameter describes the total static linewidth of 19F NMR of rigid Si-F bonds in zeolites. Line narrowing (reduced span values) is observed, when the fluoride ion carries out motional jumps between two or more Si sites. 

The reactivity of organosilicon compounds is influenced by the more electropositive nature of the silicon atom as compared to carbon and hydrogen and also by the availability of empty d orbitals, although the latter point has now become controversial. On this basis Si-0 and Si-F bonds are stronger than the C-0 and C-F bonds. The Si-H bonds are weaker than C-C and H-C bonds. 

This fast removal of Si-F species can be ascribed to the weakening of the Si backbonds induced by the strong polarizing effect of F [Ubl], The weak back-bonds are then attacked by HF or H20. This reaction scheme for the dissolution process is supported by quantum-chemical calculations [Trl]. The observed dissolution valence of two for Jelectron injection current and Si-F bond density [Be22] are experimental findings that are in support of the divalent dissolution mechanism, as shown in Fig. 4.3 [Lei, Ge7, Ho6]. 

The current transients observed upon immersion of n-type Si electrodes in HF of low concentration (2%) depend on the sample pretreatments. Immersion in 50% HF prior to anodization, for example, leads to current densities of up to 30 pA cm-2, which decreases within 1 s to the steady-state dark current value, producing a charge density of 24 pC cnrf 2. This effect has been ascribed to the density of Si-F bonds present on the electrode surface electrode prior to anodization [Be22],... 

As demonstrated for compounds 56 and 57 in Scheme 11 (method b cleavage of two Si-O bonds and one Si-C bond method c cleavage of four Si-F bonds) and Scheme 12 (method b cleavage of three Si-N bonds),... 

The formation of silicon-flvxyride bonds on the surface of silica after treatment with hydrogen fluoride was never proven directly. However, there is a pronounced change in the adsorption and wetting properties. The silica becomes hydrophobic as was mentioned in a patent to Kimberlin (279a). Neimark and collaborators (279b) found a type V isotherm in the methanol adsorption on silica gel which had been treated with a solution of SiF in absolute alcohol. Wilska (280) obtained a water-repellent silica when solutions of HaSiPg were precipitated with ammonia. The Si—F bond is hydrolyzed only slowly. A considerable fluorine content of 7-10% F was reported in an older patent (281) for a silica that had been prepared by hydrolysis of SiF. ... 

The Fourth-Group Elements.—The C—F bond, with 44 percent ionic character, is the most ionic of the bonds of carbon with nonmetallic elements. The Si—F bond has 70 percent ionic character, and Si—Cl 30 percent. The Si—O bond is of especial interest because of its importance in the silicates. It is seen to have 50 percent ionic character, the value of Xo — ssi being 1.7. 

The Si—Cl distance in SiCIF is small—only 1.99 A, corresponding to 31 percent double-bond character. The increase over the amount for the other molecules is probably to be attributed to the release of bond orbitals by the largely ionic (70 percent) Si—F bonds, permitting the Si—Cl bond to assume the amount of double-bond character that completely neutralizes the transfer of electric charge corresponding to its normal partial ionic character. 

It is interesting that the internuclear distance of the gas molecule SiF is 1.603 A presumably the bond is rather similar to the Si—F bonds in stable molecules of quadrivalent silicon. 

The surface area of sample B decreases sharply with treatment duration after 2 days in boiling water, the surface area drops from 1146 to 743 m2/g. This is not surprising as it has been known that Si-Cl bonds are easily hydrolyzed by water molecules to form Si-OH groups [11], On the contrary, the surface area of sample A remains almost unchanged (around 1210 m2/g) even after treatment in boiling water for 2 days. This shows that the presence of fluoride anions on the surface prevents the framework from collapsing in the presence of water molecules. The remarkable improvement of the hydrothermal stability of sample A may be attributed to the formation of surface Si-F bonds, which are known to be quite resistant to be attacked by water molecules [11]. 

However, examination of Tables I and II shows clearly that this explanation is not sufficiently general. For instance, the Si—SR and Si—F bonds show very similar stereochemical behavior, leading either to retention or inversion. Following the Sommer s rule, the SR group (for instance MeSH, pKa 10) would be a poor leaving group and be displaced mainly with retention. At the opposite extreme, the fluoro substituent (HF, pKa ... 

A satisfactory explanation also cannot be made in terms of polarizability of the leaving group. The highly polarizable Si—Cl and Si—SR bonds are always displaced with inversion. However, the Si—F bond, which is far less polarizable than Si—OR and Si—H bonds, is more easily cleaved by nucleophiles with inversion of configuration. It is also impossible to explain the closely similar behavior of F and SR groups. However, the relative tendency of Si—X bonds to undergo inversion and the polarizability order run parallel in the same group of the periodic table. The order of polarizability and ability for inversion are... 

The valence orbitals around F are contracted. The o-g, F MO is centered on silicon (Scheme 12). These two factors favors retention. But because the Si—F bond is short [1.54 A (41)], the unfavorable X—Nu overlap increases. Both inversion and retention may be possible. In contrast, valence orbitals on sulfur are diffuse the o-gj SR MO has more p character (Scheme 13). These two factors favors inversion. But the Si—SR bond is long [2.14 A (41)]. The unfavorable SR—Nu overlap in a RN attack decreases and retention may be also possible. 

This very versatile method for the synthesis of organoimido functions makes use of the great thermodynamic strength of Si—O and Si—F bonds. With metal oxides two synthetic strategies can be used leading to either complete removal of the oxygen or its conversion to a siloxy derivative (equations 45 and 46). 

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