Pentafluoroethyl-substituted α-silanes: model compounds for new insights.

To further investigate the α-effect in silanes bearing a geminal donor atom, the model compounds (C2F5)3SiCH2NMe2, (C2F5)3SiCH2OMe and (C2F5)3SiONMe2 were prepared by introduction of pentafluoroethyl groups via nucleophilic substitution of the corresponding chloro-derivatives with pentafluoroethyl lithium. The substances were characterised by NMR spectroscopy and X-ray diffraction via in situ crystallization techniques. The solid state structures of these highly electronegatively substituted α-silanes contain monomeric molecules. The Si-C-N angle in (C2F5)3SiCH2NMe2 shows a value of 115.3(2)° and the Si-C-O angle in (C2F5)3SiCH2OMe a value of 105.4(1)°. Both values are smaller than the Si-C-C angle of the reference compound (C2F5)3SiCH2CH3 with a value of 118.6(2)° indicating attractive interaction between the silicon atom and the respective donor atoms. The Si-O-N angle in (C2F5)3SiONMe2 is extremely narrow at 82.0(1)°. This behaviour was further investigated by gas electron diffraction and by quantum-chemical calculations. The NBO method finds no significant orbital interactions between Si and N/O atoms in the Si-C-N, Si-C-O and Si-O-N units. The IQA model describes the compounds as strongly stabilised by electrostatic interactions between formally non-bonded silicon and donor atoms.