cis,cis-1,3,5-Trihydroxynonamethylcyclohexasilane: A Cyclopolysilane with Unusual Properties

cis,cis-Trihydroxynonamethylcyclohexasilane (2) is easily accessible by the controlled hydrolysis of Cl3Si6Me9 in the presence of Et3N as an auxiliary base. The crystal structure of 2, as determined by single-crystal X-ray crystallography, exhibits “barrel-type” face-to-face dimeric aggregates held together by six intermolecular hydrogen bonds. NMR and IR spectra suggest that these OH-bonded aggregates are also present in nonpolar solutions. Full geometry optimization (B3LYP/TZVP) of the gas-phase structure of 2 further reveals unusually high energy differences between different conformers and a considerable stabilization of the molecule upon dimerization due to O−H hydrogen bonding. Time-dependent DFT B3LYP/TZVP calculations allow for a detailed interpretation of the UV absorption spectra of 2. The electronic transitions occur between occupied molecular orbitals with predominant σ(Si−Si) character and a small contribution of the oxygen lone pairs, and virtual MOs with contributions from σ*(Si−C), σ*(Si−O), and σ*(Si−Si) type orbitals. If 2 is reacted with MeSiCl3/Et3N, the adamantane-like cage MeSi(O3Si6Me9) (4) is obtained without the formation of considerable amounts of polymeric material, which is most likely a consequence of the preferred conformation of 2 with three adjacent OH groups in axial positions.

顺,顺-三羟基九甲基环六硅烷(2)可通过在辅助碱三乙胺（Et3N）存在下，对Cl3Si6Me9进行可控水解而便捷制备。通过单晶X射线晶体学确定的化合物2的晶体结构，呈现出由6个分子间氢键维系的桶型面对面二聚体聚集体。核磁共振（NMR）与红外（IR）光谱表明，这类以O−H氢键结合的聚集体在非极性溶液中同样存在。对化合物2的气相结构进行全几何优化（B3LYP/TZVP）后进一步发现，不同构象异构体之间存在异常显著的能量差，且由于O−H氢键作用，二聚过程可使该分子得到显著稳定。采用含时密度泛函理论（TD-DFT）结合B3LYP/TZVP基组进行的计算，能够对化合物2的紫外吸收光谱做出详细解析。电子跃迁发生在以σ(Si−Si)轨道特征为主、且带有少量氧孤对电子贡献的占据分子轨道，与带有σ*(Si−C)、σ*(Si−O)及σ*(Si−Si)轨道贡献的虚分子轨道之间。若将化合物2与MeSiCl3/Et3N进行反应，可得到类金刚烷笼状结构的MeSi(O3Si6Me9)(4)，且未生成大量聚合物产物，这一结果很可能源于化合物2中三个相邻羟基处于轴向位置的优势构象。