Monocyclic Zwitterionic λ⁵<i>Si</i>-Silicates with an <i>Si</i>O₂FC₂ Framework: Syntheses and Structural Characterization in the Solid State and in Solution

Treatment of the acyclic zwitterionic pentacoordinate silicate F3MeSiCH2NMe2H with 1 molar equiv of Me3SiOC6H4OSiMe3, Me3SiOCH2C(O)OSiMe3, Me3SiOC(Ph)NOSiMe3, or Me3SiOC(O)C(O)OSiMe3 (solvent CH3CN, room temperature) yielded the respective monocyclic zwitterionic pentacoordinate silicates (11a), (12a), (13a), and (14a), along with 2 molar equiv of Me3SiF. The derivatives 11b−14b with a 2,2,6,6-tetramethylpiperidinio substituent instead of the dimethylammonio group were prepared analogously, starting from F3MeSiCH2NR2H (NR2H = 2,2,6,6-tetramethylpiperidinio). Single-crystal X-ray diffraction studies showed that the Si-coordination polyhedra of 11a·1.5CH3CN, 12a−14a, and 11b−14b are distorted trigonal bipyramids, the axial positions being occupied by the fluorine atom and one of the two oxygen atoms (12a/12b, carboxylate oxygen atom; 13a/13b, carbon-linked oxygen atom). These results are in agreement with the NMR data (1H, 13C, 19F, 29Si) obtained for these compounds in solution. The chiral (C1 symmetry) zwitterions 11a−14a and 11b−14b exist as pairs of (A)- and (C)-enantiomers in solution. VT 1H NMR studies with 11b−14b in CH3CN in the temperature range 25−85 °C gave no indications for an enantiomerization process [(A)/(C)-enantiomerization] at the silicon atom.