Sulfur-Induced Pentacoordination in Cyclic Silanes1

Reaction of dichlorosilanes with diols in the presence of Et 3 N in toluene solution gave the monocyclic silanes S[( t -Bu) 2 C 6 H 2 O] 2 Si(Ph)Me ( 1 ), S[( t -Bu) 2 C 6 H 2 O] 2 SiPh 2 ( 2 ), S[ t -BuMeC 6 H 2 O] 2 Si(Ph)CH CH 2 ( 3 ), and S[Me 2 C 6 H 2 O] 2 SiPh 2 ( 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 29 Si NMR data on 1 − 4 with solution state and solid state 29 Si 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 P 2 1 / 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 Pna 2 1 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 P 2 1 / 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 ).