Pentacoordinated organo and hydrido cyclic silanes via sulfur donor action

The new cyclic organosilanes S(C10H6O)(2)SiMe2 (1) and S(C10H6O)(2)SiPh2 (2 ) were obtained by the reaction of R2Si(NMe2)(2) (R = Me, Ph) with thiobis( 2,2'-naphthol), while S[Me2C6H2O](2)SiH(Me) (3) and S[(t-Bu)MeC6H2O](2)SiH( Me) (4) were formed by reaction of Cl2SiH(Me) with the appropriate diol in the presence of Et3N. Characterization was achieved by Si-29 and H-1 soluti on NMR, Si-29 solid-state NMR, and X-ray structural studies. As a result of sulfur donor interaction, pentacoordinate geometries were obtained where 3 and 4 were displaced further toward a trigonal bipyramid (TBP) compared to 1 and 2. Variable-temperature NMR spectra showed 1 to be fluxional with an activation energy of 9.4 kcal/mol. An exchange mechanism involving rupture of the Si-S donor interaction accompanying a ring pseudorotation is postul ated. The less symmetric silanes 3 and 4 do not exchange, as indicated by t he H-1 NMR data. Comparison of the degree of Si-S donor interaction of 1-4 with that of related cyclic organosilanes possessing Si-O and Si-N donor ac tion allows a correlation of the degree of geometrical displacement toward a TBP, which increases as ring substituents enhance the Lewis basicity of t he donor group and the electrophilicity at the silicon center is enhanced b y its ligand attachments.