beta-donor bonds in SiON units: An inherent structure-determining property leading to (4+4)-coordination in tetrakis-(N,N-dimethylhydroxylamido)silane

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 H-1, C-13, N-15 [including the determination of (1)J((SiN)-Si-29-N-15) coupling constants], O-17, and Si-29, 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) Angstrom] and narrow SiON angles [109.1 degrees on average]. For comparison, the crystal structure of the isoelectronic Si(OCHMe2)(4) (in situ grown crystal) was determined, showing the absence of beta-donor interactions and an SiOC angle of 124.7(1)degrees. 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 C-2 symmetry and to have an SiON angle of only 109.5 degrees, while the interactions in the other model compounds are predicted to be much stronger, as indicated by their SiON angles: H3SiONMe2 102.5 degrees, FH2SiONMe2 90.0 degrees, and F3SiONMe2 93.8 degrees. 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.