Monocyclic zwitterionic lambda(5)Si-silicates with an SiO(2)FC(2) framework: syntheses and structural characterization in the solid state and in solution.
Treatment of the acyclic zwitterionic pentacoordinate silicate F(3)MeSiCH(2)NMe(2)H with 1 molar equiv of Me(3)SiOC(6)H(4)OSiMe(3), Me(3)SiOCH(2)C(O)OSiMe(3), Me(3)SiOC(Ph)=NOSiMe(3), or Me(3)SiOC(O)C(O)OSiMe(3) (solvent CH(3)CN, room temperature) yielded the respective monocyclic zwitterionic pentacoordinate silicates (11a), (12a), (13a), and (14a), along with 2 molar equiv of Me(3)SiF. The derivatives 11b-14b with a 2,2,6,6-tetramethylpiperidinio substituent instead of the dimethylammonio group were prepared analogously, starting from F(3)MeSiCH(2)NR(2)H (NR(2)H = 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 ((1)H, (13)C, (19)F, (29)Si) 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 (1)H NMR studies with 11b-14b in CH(3)CN in the temperature range 25-85 degrees C gave no indications for an enantiomerization process [(A)/(C)-enantiomerization] at the silicon atom.
Tacke
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Institut für Anorganische Chemie, Universität Würzburg, Am Hubland, D-97074 Würzburg, Germany. r.tacke@mail.uni-wuerzburg.de
Bertermann
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Dannappel
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Neugebauer
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Pülm
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Willeke
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