Synthesis, structure and spectroscopic properties of branched oligosilanes

A number of various branched heptasilanes [(Me3Si)2MeSi]2MeSiX (2: X=Ph; 3: X=Me; 6: X=Br; 7: X=I; 8: X=Cl; 9: X=F; 10: X=OH) bearing two (Me3Si)2MeSi groups as well as branched decasilanes [(Me3Si)2MeSi]3SiX (12: X=H; 13: X=Br; 14: X=Cl; 15: X=F; 16: X=OH) bearing three (Me3Si)2MeSi groups at one silicon centre were synthesized. The structures of the compounds prepared were elucidated on the basis of comprehensive NMR and MS studies. Additionally, the molecular structures of the decasilanes 12 and 14–16 were obtained from X-ray diffraction data, which verify a ca. spherical shape with the core silicon atom at the centre of the sphere. The spatial demand of three (Me3Si)2MeSi groups forces a widening of the SiSiSi angles of the XSiSi3 tetrahedra toward the group X (X=H, Cl, F, OH) and a remarkable elongation of the central SiX bonds. The analysis of the SiSiSiSi dihedral angles of the tetrasilane subunits in the decasilanes 12 and 14–16 indicates the existence of three conformations denoted as anti, ortho and gauche and four different arrangements of pentasilane subunits, denoted as anti–gauche, anti–ortho, ortho–gauche and ortho–ortho. The absorption spectra of the heptasilanes 2–10 exhibit broadened absorption maxima, shifted strongly to the red relative to those of the decasilanes 11–16. As a result of strongly limited conformational flexibility due to steric overcrowding, the singlet excitation in 11–16 is at much higher energy than in linear pentasilanes. A comparison of NMR chemical shifts of 2–16 with those of branched silanes of the type (Me3Si)3SiX and (Me3Si)2MeSiX reveals that the replacement of methyl groups by additional SiMe3 groups in the β-position leads to a strong low-field shift of the signal of the central nucleus.