Adamantanes, nortricyclenes, and dendrimers with extended silicon backbones

By reaction of the hexabromoheptasilane MeSi(SiMe(2)SiMeBr(2))(3) (1 a) with H(2)O, H(2)S, NH(3), and H(2)NMe the heptasilaadamantanes MeSi(SiMe(2)SiMeO)(3) (4), MeSi(SiMe(2)SiMeS)(3) (5), MeSi(SiMe(2)SiMeNH)(3) (6 a), and MeSi(SiMe(2)SiMeNMe)(3) (6 b), respectively, were prepared in good to moderate yields. Molecular structures of 4, 5, 6 a, and 6 b were determined by X-ray crystallography. The symmetry of the cages is approximately C(3v), and the geometry around the nitrogen atoms is essentially planar. Ab initio SCF/HF calculations with the 6-31G* basis set confirm these results. Reduction of MeSi(SiMe(2)SitBuBr(2))(3) (1 b) with lithium naphthalenide afforded the heptasilanortricyclene MeSi(SiMe(2)SitBu)(3) (7). The (29)Si NMR spectrum of 7 consists of three signals with chemical shifts that agree closely with values predicted by ab initio calculations. (29)Si INADEQUATE spectra also strongly support the nortricyclene structure. Ab initio SCF/HF calculations were performed for the parent molecule Si(7)H(10), and the ring strain of the cage was estimated as 168.8 kJ mol(-1) by using the homodesmic reaction Si(7)H(10) + 3 Si(2)H(6)-->Si(13)H(28). Compound 1 a also served as the starting material for the preparation of first-generation dendrimer 2 a by reaction with six equivalents of Ph(2)MeSiLi. Subsequent protodearylation with HBr and reaction with (Me(2)PhSi)(2)SiMeK afforded second-generation dendrimer 3. All dendrimers were characterized by multinuclear NMR spectroscopy.