_P.-Powell-auth.-Principles-of-Organometallic-Chemistry-Springer-Netherlands-1988

Catenated organic derivatives of Group IV
Me 2
SiCI 2 --"a--->(Me 2 Si)" ~ ( ~e-CH,-~e-CH 2 \ ~ {J-SiC
autocla\'t' L~O C \ H ... H 7
11 vac.
Planned syntheses have been devised for the preparation of specific linear
polysilanes. This is illustrated by the following example starting from the cyclic
hexamer.
CI,
MeMgX
(Me 2 Si) 6 -----+ Cl(SiMeJ 6 Cl -----> Me,Si(SiMe 2 ) 6 Cl
Linear polysilanes show strong absorption in the ultraviolet. The wavelength
at which maximum absorption occurs and the molar absorbance both increase
with increcsing chain length in a way resembling conjugated polyenes.
The Si--Si bonds in aryldisilanes and in polysilanes are cleaved by lithium in
tetrahydrofuran to give lithio derivatives. Triphenylsilyllithium can be obtained
in solution either from Ph,SiCI or from Ph,SiSiPh,. The Ph,Si- anion is
isoelectronic with triphenylphosphine and has a similar pyramidal structure
which undergoes slow inversion ( barrier > l 00 kj moi- 1 ) (p. 12 7 ). It is a strong
nucleophile; some of its reactions are shown below (Fig. 4. 7).
Alkali metals do not cleave hexaalkyldisilanes. although alkoxides in the
presence of crown ethers do. The usual route to Me,SiLi is via the mercury
derivative.
In the solid state trimethylsilyllithium exists as hexameric units (Me,SiLi) 6 • There
are six lithium atoms in a highly folded chair arrangement and each trimethylsilyl
group bridges three lithium atoms. The structure is very similar to that of
cyclohexyllithium (p. 3 7).
119