CYCLIC SILANES - SULFUR-INDUCED PENTACOORDINATION IN A DISILOXANE

Reactions of chlorosilanes with diols capable of forming eight-membere d-ring systems led to new cyclic silanes. With tetramethylenedichloros ilane ((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-B u)MeC6H20]2Si(CH2)4 (3). X-ray studies of 1 and 3 are reported, as wel l as NMR spectral data on all three new cyclic silicon compounds. Stru ctural parameters of 1 indicate silicon-sulfur interactions with the g eometry at the silicon atoms displaced about halfway from a tetrahedro n 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 th us confirms the presence of a sulfur-silicon donor interaction. The ri ngs of both 1 and 3 have boat or tublike conformations. Comparison of ring distortions of the cyclic silicon compounds with analogous ring g eometries at related cyclic phosphoranes indicates that decreasing rin g distortion parallels decreasing central atom-sulfur distance, leadin g to increased coordination. Disiloxane 1 crystallizes in the monoclin ic space group P2(1)/n with a = 10.490(3) angstrom, b = 25.599(2) angs trom, c = 24.089(3) angstrom, beta = 96.64(1)degrees, and Z = 4. Silan e 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).