Synthesis, structures, and stereodynamic behavior of novel pentacoordinate fluorosilanes: fluorosilyl derivatives of proline

Nikolin, Alexei A.; Kramarova, Evgeniya P.; Shipov, Aleksander G.; Baukov, Yuri I.; Negrebetsky, Vadim V.; Korlyukov, Alexander A.; Arkhipov, Dmitry E.; Bowden, Allen; Bylikin, Sergey. Yu; Bassindale, Alan R. and Taylor, Peter G. (2012). Synthesis, structures, and stereodynamic behavior of novel pentacoordinate fluorosilanes: fluorosilyl derivatives of proline. Organometallics, 31(14) pp. 4988–4997.

DOI: https://doi.org/10.1021/om3002697

Abstract

The (O→Si)-chelate N'-(dimethylfluorosilylmethyl))-N'-methyl-N-(organosulfonyl)prolinamides RSO₂-Pro-N(Me)CH₂SiMe₂F (2a-f, R = Me (a), Ph (b), 4-MeC₆H₄ (c), 4-ClC₆H₄ (d), 4-BrC₆H₄ (e), 4-NO₂C₆H₄ (f)) were synthesized from the corresponding disiloxanes 1a-f using Et₂O•BF3. According to the NMR and IR data, the extent of dimerization of fluorosilanes 2a-f in solution is negligible, while the O→Si coordination in solution is weaker than that in the solid state. Comparative CP/MAS NMR and X-ray diffraction studies revealed that in solution the coordination Si-O bond length varies in a narrow range (2.22-2.24 angstrom) that is 0.02-0.11 angstrom longer than in the crystalline state. Dynamic NMR (DNMR) studies of the fluorides revealed a fine structure of the ¹⁹F signals in the 0-20°C temperature range, which was related to the structural features of the coordination set in these complexes. The temperature dependence of the SiMe₂ signals in the ¹H DNMR spectra was attributed to a permutational isomerization process involving a positional exchange of equatorial ligands. The narrow range of activational barriers of the process (23-24 kcal mol^-1 and more) and high negative values of the entropy of activation are similar to those observed earlier for Si-substituted N-(dimethylsilylmethyl) and N-(methylphenylsilylmethyl) amides and lactams, which suggests similar permutational processes in all cases. Gas-phase quantum chemical studies demonstrate that the solvation of F^- reduces the activation barrier.

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