Pentacoordinated Organo and Hydrido Cyclic Silanes via Sulfur Donor Action1

The new cyclic organosilanes S(C 10 H 6 O) 2 SiMe 2 ( 1 ) and S(C 10 H 6 O) 2 SiPh 2 ( 2 ) were obtained by the reaction of R 2 Si(NMe 2 ) 2 (R = Me, Ph) with thiobis(2,2‘-naphthol), while S[Me 2 C 6 H 2 O] 2 SiH(Me) ( 3 ) and S[( t -Bu)MeC 6 H 2 O] 2 SiH(Me) ( 4 ) were formed by reaction of Cl 2 SiH(Me) with the appropriate diol in the presence of Et 3 N. Characterization was achieved by 29 Si and 1 H solution NMR, 29 Si 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 postulated. The less symmetric silanes 3 and 4 do not exchange, as indicated by the 1 H 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 action allows a correlation of the degree of geometrical displacement toward a TBP, which increases as ring substituents enhance the Lewis basicity of the donor group and the electrophilicity at the silicon center is enhanced by its ligand attachments.