Synthesis, Structures, and Stereodynamic Behavior of Novel Pentacoordinate Fluorosilanes: Fluorosilyl Derivatives of Proline

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 Å) that is 0.02–0.11
Å 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.