O-OXIMATOSILANES - WEAK BETA-DONOR INTERACTIONS AS SECONDARY BONDS

The oximatosilanes H3SiON=CMe2, ClH2SiON=CMe2 and H2Si(ON=CMe2)(2) wer e prepared from the reaction of lithiated acetone oxime with bromosila ne and dichlorosilane respectively. They have been characterised by NM R (H-1, C-13, N-15, O-17 and Si-29) and by gas-phase IR spectroscopy. The molecular structures of H3SiON=CMe2 and H2Si(ON=CMe2)(2) have been determined by low-temperature X-ray crystallography. Small Si-O-N ang les have been detected in these structures, which are indicative of th e formation of weak secondary interactions of the beta-donor-acceptor type between silicon and nitrogen atoms. Important geometrical paramet ers: H3SiON=CMe2, Si-O 1.673(1), N-O 1.433(1) Angstrom, Si-O-N 106.0(1 )degrees, N=C 1.273(2) Angstrom; H2Si(ON=CMe2)(2), Si-O 1.657(1), 1.65 0(1), N-O 1.466(2), 1.447(2), N=C 1.271(2), 1.266(2) Angstrom, Si-O-N 102.5(1), 107.5(1)degrees. The geometries of H2SiON=CMe2, FH2SiON=CMe2 and ClH2SiON=CMe2 were calculated by optimisations at the MP2/6-311G level of theory. The Si-O-N angles in FH2SiON=CMe2 and ClH2SiON=CMe2 are predicted to be smaller than that in H3SiON=CMe2. A natural bond orbital analysis describes the beta-donor interaction as weak negative hyperconjugation lp(O)-->sigma(SiO) and lp(O)-->sigma*(SiH). In gene ral the beta-donor bonds in oximatosilanes are found to be weaker than in the related hydroxyaminosilanes.