Fluorosilicates pentacoordinate

Several mechanisms for the peroxide oxidation of organosilanes to alcohols are compared. Without doubt, the reaction proceeds via anionic, pentacoordinate silicate species, but a profound difference is found between in vacuo and solvated reaction profiles, as expected. In the solvents investigated (CH2CI2 and MeOH), the most favorable mechanism is addition of peroxide anion to a fluorosilane used as starting material or formed in situ, followed by a concerted migration and dissociation of hydroxide anion. In the gas phase, and possibly in very nonpolar solvents, concerted addition-migration of H2O2 to a pentacoordinate fluorosilicate is also plausible. ... 

The possibility of pseudorotation at silicon was first suggested by Klanberg and Muetterties31 in their study of pentacoordinate fluorosilicates SiFsA RSiF4- and R2SiF3-. Recently there have been a number of studies on the pseudorotation of pentacoordinate silicon species. 

Hexacoordinate fluorosilicates were obtained from neutral pentacoordinate complexes by addition of K[18-crown-6]F191,192. In this way compounds 168-173 were prepared from their pentacoordinate precursors. These compounds, in addition to being fluorosilicates, contain in each chelate ring a dative Si—N bond. 

The 29 Si NMR (Table 20) supports a hexacoordinate structure in solution which is similar to that in the solid state. (529Si for the hexacoordinate fluorosilicates is ca 50 ppm shifted upheld relative to the pentacoordinate precursors. The multiplicity of the 29 Si resonances indicates the nonequivalence of fluorine ligands at low temperatures (between 25 and — 60 °C). These become equivalent as the temperature is raised, such that a symmetrical 29Si multiplet is observed. 

Berry pseudorotation as the mechanism of intramolecular exchange of ligands in pentacoordinate fluorosilicate anions represents a possible pathway. However, no definitive evidence has been presented for systems with mono-dentate ligands, and a pathway involving an intermediate hexacoordinated species can constitute an alternative mechanism. 

The axial Si-F bonds in the fluorosilicate anions listed in Table 21 are between 0.040 and 0.080 A longer than the respective equatorial Si-F bonds. An increase in the silicon coordination number is associated with an elongation in the Si-X bond distances, as exemplified by a comparison of SiF4311 and [SiF5] The equatorial Si-F bonds in the pentacoordinated anion are respectively 0.023 and 0.046 A longer than the Si-F bonds in the tetrahedral molecule SiF4. 

The lower reactivity of the Si-C bond requires the use of activating reagents to enhance the reactivity of silanes and to promote the Si-Pd transmetallations. Fluoride is the common additive, although other nucleophiles such as hydroxide, metal oxides, and aUcoxides are also effective [30, 293, 294]. Fluoride converts the starting silanes into pentacoordinate fluorosilicates, which are the actual transmetallation reagents. 

LXIX. Steric hindrance in pentacoordinated fluorosilicates - synthesis and molecular-structure of the diphenyl-1-naphthyldifluorosilicate anion and the phenylmethyltrifluorosilicate anion, Inorg. Chem. 26 (5), 760-765 (1987). 

Scheme 1 Formation of pentacoordinated fluorosilicates from di- and trifluorosilanes...

---