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 sigma(Si-Si) character and a small contribution of the oxygen lone pairs, and virtual MOs with contributions from sigma*(Si-C), sigma*(Si-O), and sigma*(Si-Si) type orbitals. If 2 is reacted with MeSiCl3/Et3N, the adamantane-like cage MeSi(O3Si5Me9) (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.