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

Abstract

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.