O-Oximatosilanes: weak β-donor interactions as secondary bonds
The oximatosilanes H3SiONCMe2, ClH2SiONCMe2 and H2Si(ONCMe2)2 were prepared from the reaction of lithiated acetone oxime with bromosilane and dichlorosilane respectively. They have been characterised by NMR (1H, 13C, 15N, 17O and 29Si) and by gas-phase IR spectroscopy. The molecular structures of H3SiONCMe2 and H2Si(ONCMe2)2 have been determined by low-temperature X-ray crystallography. Small Si–O–N angles have been detected in these structures, which are indicative of the formation of weak secondary interactions of the β-donor–acceptor type between silicon and nitrogen atoms. Important geometrical parameters: H3SiONCMe2, Si–O 1.673(1), N–O 1.433(1) Å, Si–O–N 106.0(1)°, NC 1.273(2) Å; H2Si(ONCMe2)2, Si–O 1.657(1), 1.650(1), N–O 1.466(2), 1.447(2), NC 1.271(2), 1.266(2) Å, Si–O–N 102.5(1), 107.5(1)°. The geometries of H3SiONCMe2, FH2SiONCMe2 and ClH2SiONCMe2 were calculated by optimisations at the MP2/6-311G** level of theory. The Si–O–N angles in FH2SiONCMe2 and ClH2SiONCMe2 are predicted to be smaller than that in H3SiONCMe2. A natural bond orbital analysis describes the β-donor interaction as weak negative hyperconjugation lp(O)→σ*(SiO) and lp(O)→σ*(SiH). In general the β-donor bonds in oximatosilanes are found to be weaker than in the related hydroxyaminosilanes.