PUB - Publikationen an der Universität Bielefeld

The compounds ClH2SiONMe2 and ClH2SiONEt2 have been prepared by the reaction of the corresponding O-lithiated hydroxylamines and dichlorosilane. Their identity has been proved by gas-phase IR and solution NMR (H-1, C-13, N-15, O-17, and Si-29) spectroscopy. In contrast to CM2SiONMe2, ClH2SiONEt2 is unstable at ambient temperature and decomposes to give H2SiCl2 and H2Si(ONEt2)(2). ClH2SiONEt2 shows dynamic behavior in the solution as shown by low-temperature NMR. In the crystal ClH2SiONMe2 (low-temperature crystallography) is present as anti-conformer (torsional angle Cl-Si-O-N 180 degrees) and shows an exceptionally small Si-O-N angle of 79.7(1)degrees and a Si ... N distance of 2.028(1) Angstrom, corresponding to a relatively strong Si-N-beta-donor interaction. Ab initio calculations predict a wider Si-O-N angle of 91.6 degrees (MP2/6-311G**). A gas-phase structure determination by electron diffraction shows the presence of two conformers (anti and gauche), which are equal in potential energy. The Si-O-N angle in the anti-conformer is 87.1(9)degrees and that in the gauche-conformer 104.7(11)degrees [Cl-Si-O-N torsion angle: 72.9(28)degrees]. Calculations (MP2/6-31G*) have shown the angle Si-O-N and the energy of the system to be largely dependent on the torsion angle Cl-Si-O-N, with a variation of the angle Si-O-N over a range of 30 degrees during a full rotation of Cl-Si-O-N. Analysis of the calculated charges on the Si and N atoms show the beta-donor interaction not to be simply caused by electrostatic interaction. Natural bond orbital analysis describes a Ip-N --> sigma(Si-X)* type negative hyperconjugation as an important contribution to beta-donor bonding. The silicon substituent in anti position to nitrogen exerts the major effect on the strength of the beta-donor bond. Simultaneous analysis of negative hyperconjugation Ip-O --> sigma(Si-X)* shows the Si-O-N compounds to have similar contributions to this interaction as H3SiOCH3 and questions negative hyperconjugation to be quoted in isolation for the rationalization of the wide bond angles in silyl ethers.