1, 3-Diazabutadienes and Amidinates as Ligands to Silicon Centers

A novel strategy for the access to dichlorosilylene SiCl₂, and its incorporation into silaheterocycles has been worked out successfully by performing the Benkeser reaction in the presence of dienes and heterodienes. For example, a 1,3-diazabutadiene is used to prepare the five membered heterocycle tBuN-C(Ph)=N-C(CF₃)₂SiCl₂ the molecular structure of which has been determined by X-ray diffraction. It turned out to be isomorphous to the germanium analog, obtained from GeCl₂ and the respective 1,3-diazabutadiene. The silacycle reacts with water to give the first silanol with pentacoordinated silicon and an amidinate ligand, thus demonstrating the "Umpolung" of the 1,3-diazabutadiene on complexation. The X-ray structure determination of the silanol reveals very short ligand to silicon bonds. Obviously, amidinate ligands are ideal for the stabilization of high coordination numbers at silicon. Nevertheless, an amidinate, prepared from PhCN and LiNHtBu in reaction with SiCl₄ (2:l) does not give a bisamidinate silicon complex with hexacoordinated silicon, but rather leads to the novel amidine derivative [H(tBu)N-C(Ph)=N]₂SiCl₂ (X-ray). In order to extend the amidinate ligand series to mixed N/P systems, [Ph-C(PSiMe₃) (NSiMe₃)]Li was synthesized from PhCN and LiP(SiMe₃)₂. Its reaction with Me,SiCl gives a novel access to a phosphaalkene, for example (Me₃Si)P=C(Ph)[N(SiMe₃)₂].