A New Method for the Synthesis of Branched Polysilane Anions

Lithiation by MeLi of three families of branched polysilanes (Me 3 Si) 3 Si−X {( 1 ) X = SiMe 2 H, t -BuMe 2 Si, SiMe 2 SiMe 3, t -BuSiMe 2 SiMe 2; ( 2 ) X = SiMe(SiMe 3 ) 2, Si(SiMe 3 ) 3, SiMe 2 Si(SiMe 3 ) 3, SiMeHSi(SiMe 3 ) 3; ( 3 ) X = Si(SiMe 3 ) 2 Cl, Si[Si(SiMe 3 ) 3 ]MeCl, Si[Si(SiMe 3 ) 3 ]HCl} yields, depending on X, one of the following: (a) novel branched polysilyllithium compounds; (b) the known (Me 3 Si) 3 SiLi; (c) various mixtures. The cleavage of family 1 can be understood in terms of steric effects, i.e., assuming that MeLi attacks preferentially the sterically less hindered Si atom leading to the formation of new polysilyllithium compounds, e.g., t -BuMe 2 Si(Me 3 Si) 2 SiLi·3THF and t -BuMe 2 SiSiMe 2 (Me 3 Si) 2 SiLi·3THF, which were isolated in a crystalline form. The X-ray structure of the former is presented. In contrast to 1 the lithiation of the branched polysilanes 2 leads exclusively (mostly with (Me 3 Si) 3 SiSiMe(SiMe 3 ) 2 ) to the known (Me 3 Si) 3 SiLi. Lithiation of 3 leads to a number of novel polysilyllithium compounds. We tentatively propose that the lithiation of 3 proceeds by a two step reaction in which a disilene is formed as an intermediate, and this is followed by fast addition of a second molecule of MeLi to produce the final silyllithium product.