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

The (O→Si)-chelate N'- (dimethylfluorosilylmethyl)) -N'- methyl -N- (organosulfonyl)prolinamides RSO 2 -Pro-N(Me)CH 2 SiMe 2 F (2a-f, R = Me (a), Ph (b), 4-MeC 6 H 4 (c), 4-ClC 6 H 4 (d), 4-BrC 6 H 4 (e), 4-NO 2 C 6 H 4 (f)) were synthesized from the corresponding disiloxanes 1a-f using Et 2 O•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 19 F 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 1 H 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.