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

Article abstract of DOI:10.1021/om3002697

The (O→Si)-chelate N′-(dimethylfluorosilylmethyl))-N′- methyl-N-(organosulfonyl)prolinamides RSO₂-Pro-N(Me)CH ₂SiMe₂F (2a-f, R = Me (a), Ph (b), 4-MeC₆H ₄ (c), 4-ClC₆H₄ (d), 4-BrC₆H ₄ (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 A) that is 0.02-0.11 A 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 SiMe ₂ 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.