β-Donor Bonds in SiON Units: An Inherent Structure- Determining Property Leading to (4 + 4)-Coordination in Tetrakis-(<i>N</i>,<i>N</i>-dimethylhydroxylamido)silane

Abstract

Tetrakis(N,N-dimethylhydroxylamido)silane, Si(ONMe2)4 (1), and the analogous ethyl compound, Si(ONEt2)4 (2), have been prepared by the reaction of silicon tetrachloride with 4 equiv of the corresponding lithium N,N-dialkylhydroxylamide. The compounds have been characterized by elemental analysis, IR spectroscopy, NMR spectroscopy of the nuclei 1H, 13C, 15N [including the determination of 1J(29Si15N) coupling constants], 17O, and 29Si, and nominal and high-resolution mass spectrometry. A crystal structure determination on 1 showed the molecule to be (4 + 4)-coordinate with short Si···N contacts [2.514(3)−2.555(3) Å] and narrow SiON angles [109.1° on average]. For comparison, the crystal structure of the isoelectronic Si(OCHMe2)4 (in situ grown crystal) was determined, showing the absence of β-donor interactions and an SiOC angle of 124.7(1)°. Ab initio calculations have been performed on the model compounds Si(ONH2)4 and H3SiONMe2 as well as the electronegatively substituted fluoro derivatives FH2SiONMe2 and F3SiONMe2 up to the MP2/6-311G** level of theory. Si(ONH2)4 is predicted to adopt C2 symmetry and to have an SiON angle of only 109.5°, while the interactions in the other model compounds are predicted to be much stronger, as indicated by their SiON angles: H3SiONMe2 102.5°, FH2SiONMe2 90.0°, and F3SiONMe2 93.8°. The results are discussed with respect to the consequences of enlargement of the coordination sphere at the silicon atom, the unique chemical behavior of O-silylhydroxylamines (in particular their fast nucleophilic substitutions reactions), and the resulting use of this class of compounds in the silicone rubber industry.