Inorganic Chemistry, Vol.40, No.11, 2520-2527, 2001
Monocyclic zwitterionic lambda Si-5-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)OSiM3 (solvent CH3CN, room temperature) yielded the respective monocyclic zwitterionic pentacoordinate silicates FMe(OC6H4O)SiCH2NMe2H (11a), FMe(OCH2C(O)O)SiCH2NMe2H (12a), FMe(OC(Ph)=NO)SiCH2NMe2H (13a), and FMe(OC(O)C(O)O)SiCH2NMe2H (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(.)1.5CH(3)CN, 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 (H-1,C-13, F-19,Si-19) obtained for these compounds in solution. The chiral (C-1 symmetry) zwitterions 11a-14a and 11b-14b exist as pairs of (A)- and (C)-enantiomers in solution. VT H-1 NMR studies with 11b-14b in CH3CN in the temperature range 25-85 degreesC gave no indications for an enantiomrrization process [(A)/(C) enantiomerization] at the silicon atom.