The monophenylhydrosilasesquioxanes PhHn–1SinO1.5n where n= 8 or 10

Abstract

The first monosubstituted decasilasesquioxane, PhH₉Si₁₀O₁₅, and the analogous PhH₇Si₈O₁₂ molecule have been prepared and characterized by IR and Raman spectroscopy and X-ray crystallography. Both have crystallographic C₁ symmetry, but their cages exhibit an approximate C_s, and an effective C₃, symmetry, respectively in the crystalline state. The IR and Raman spectra of the two molecules are very similar and reflect the close similarity observed for the spectra of H₈Si₈O₁₂ and H₁₀Si₁₀O₁₅. They have been treated as a superposition of the spectra of the siloxane cage H_n-1Si_nO_1.5n, n= 8 or 10, the phenyl substituent and the connecting moiety O₃Si–C(CH)₂ and assigned on the basis of spectral correlation and normal coordinate analysis. The siloxane cage vibrations are best understood by correlation with those of the unsubstituted cages, indicating that distortions of the Si₈O₁₂ and Si₁₀O₁₅ cages caused by the substituent are small. A comparison of the Si–C stretching force constants indicates that Si–C_vinyl and Si–C_phenyl are of similar strength while the Si–C_alkyl bond is weaker. The notion of ring-opening vibrations, introduced for (HSiO_1.5)_2n, n= 2, 3, 4, etc., is also applicable to PhH₇Si₈O₁₂ and PhH₉Si₁₀O₁₅. The phenyl substituent does not influence the frequency range of four- and five-membered ring-opening vibrations, however the number of such vibrations is increased.