Monocyclic Zwitterionic λ⁵Si-Silicates with an SiO₂FC₂ 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.