Sulfur-Induced Pentacoordination in Cyclic Silanes¹

Reaction of dichlorosilanes with diols in the presence of Et₃N in toluene solution gave the monocyclic silanes S[(t-Bu)₂C₆H₂O]₂Si(Ph)Me (1), S[(t-Bu)₂C₆H₂O]₂SiPh₂ (2), S[t-BuMeC₆H₂O]₂Si(Ph)CHCH₂ (3), and S[Me₂C₆H₂O]₂SiPh₂ (4). X-ray structural analysis revealed that the silanes 1−3 exhibited varying degrees of sulfur−silicon donor coordination, giving geometries intermediate between a tetrahedron and a trigonal bipyramid. The eight-membered ring, common to 1−3, which contained the sulfur atom as a bridge between two aryl groups, resided in a syn or boatlike conformation. For the cyclic silane 4 having the same type of ring composition, X-ray analysis showed no sulfur−silicon interaction, and the ring in this case was in an anti or chairlike conformation. Similar to that observed with pentaoxyphosphoranes possessing the same type of sulfur-containing eight-membered ring, sulfur donor ability increased with an increase in alkyl substitution on the ring aryl components (t-Bu > Me). Sulfur coordination in the phosphorus compounds gave geometries along a coordinate from square pyramidal to octahedral. Retention of the solid state structures in solution is indicated by comparison of ²⁹Si NMR data on 1−4 with solution state and solid state ²⁹Si NMR data on related silanes. The ability of the sulfur atom of reduced electronegativity to promote silanes to a higher coordinate state shows that hypervalency is readily accessible for silicon. Silane 1 crystallizes in the monoclinic space group P2₁/n with a = 10.133(3) Å, b = 20.839(4) Å, c = 16.181(3) Å, β = 99.47(2)°, and Z = 4. Silane 2 crystallizes in the monoclinic space group Cc with a = 10.006(2) Å, b = 42.478(6) Å, c = 17.917(4) Å, β = 99.25(1)°, and Z = 8. Silane 3 crystallizes in the orthorhombic space group Pna2₁ with a = 20.735(2) Å, b = 12.724(1) Å, c = 10.387(1) Å, and Z = 4. Silane 4 crystallizes in the monoclinic space group P2₁/c with a = 10.786(2) Å, b = 26.053(7) Å, c = 8.876(1) Å, β = 100.57(1)°, and Z = 4. The final conventional unweighted residuals are 0.059 (1), 0.035 (2), 0.027 (3), and 0.050 (4).