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 center dot 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 F-19 signals in the 0-20 degrees 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 H-1 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.