CYCLIC SILANES - SULFUR-INDUCED PENTACOORDINATION IN A DISILOXANE

Reactions of chlorosilanes with diols capable of forming eight-membered-ring systems led to new cyclic silanes. With tetramethylenedichlorosilane ((CH2)4SiCl2) and S[(t-Bu)2C6H2OH]2, the bicyclic silane S[(t-Bu)2C6H2O]2Si(CH2)4 (2) was obtained in 89 % yield, whereas with SiCl4, a hydrolysis reaction dominated, yielding the cyclic disiloxane {S[(t-Bu)2C6H2O]2Si(OCH2CF3)}2O (1) in 62% yield. A similar reaction with a related diol having a methylene group in place of sulfur gave CH2[(t-Bu)MeC6H20]2Si(CH2)4 (3). X-ray studies of 1 and 3 are reported, as well as NMR spectral data on all three new cyclic silicon compounds. Structural parameters of 1 indicate silicon-sulfur interactions with the geometry at the silicon atoms displaced about halfway from a tetrahedron toward a trigonal bipyramid. The upfield Si-29 chemical shift for 1 in the solid state is shown to be in the pentacoordinate region and thus confirms the presence of a sulfur-silicon donor interaction. The rings of both 1 and 3 have boat or tublike conformations. Comparison of ring distortions of the cyclic silicon compounds with analogous ring geometries at related cyclic phosphoranes indicates that decreasing ring distortion parallels decreasing central atom-sulfur distance, leading to increased coordination. Disiloxane 1 crystallizes in the monoclinic space group P2(1)/n with a = 10.490(3) angstrom, b = 25.599(2) angstrom, c = 24.089(3) angstrom, beta = 96.64(1)degrees, and Z = 4. Silane 3 crystallizes in the triclinic space group P1BAR with a = 10.149(4) angstrom, b = 10.586(4) angstrom, c = 12.392(3) angstrom, a = 104.65(3)degrees, beta = 94.81(2)degrees, gamma = 102.52(3)degrees, and Z = 2. The final conventional unweighted residuals are 0.159 (1) and 0.057 (3).