Structural and vibrational properties of the octanuclear silasesquioxane C6H13(H7Si8O12)

Abstract

The crystal structure of hexylheptahydrooctasilasesquioxane C₆H₁₃(H₇Si₈O₁₂) has been determined by single-crystal X-ray diffraction and compared with the structures of H₈Si₈O₁₂, [Co(CO)₄(H₇Si₈O₁₂)] and (CH₃Si₈O₁₂. The monosubstituted Silasesquioxane molecules have a crystallographic C₁symmetry. The local symmetry of their octanuclear Si cage, however, is close to C_3v. They are closer to C_3v than H₈Si₈O₁₂ which has an effective molecular symmetry of T_h in the crystal. The major distortions originate in intermolecular O ⋯ Si contacts. The larger the number of such contacts per molecule, the larger is the distortion of the local symmetry of the cage. The structure of C₆H₁₃(H₇Si₈O₁₂) compares well with that of [Co(CO)₄(H₇Si₈O₁₂)], and it is clearly the least distorted of the two due to a reduced influence of the n-hexyl ligand on the cage geometry and due to a smaller number of intermolecular interactions. The IR spectrum of C₆H₁₃(H₇Si₈O₁₂) has been interpreted by correlating it with the three molecules H₈Si₈O₁₂, C₆H₁₃[Si(OSiMe₃)₃] and HSi(OSiMe₃)₃.