β-Donor Interactions of Exceptional Strength in N,N-Dimethylhydroxylaminochlorosilane, ClH2SiONMe2

• Published in: Journal of the American Chemical Society 1998-07, Vol.120 (29), p.7320-7327
• Main Authors: Mitzel, Norbert W, Losehand, Udo
• Format: Article
• Language: English
• ISSN: 0002-7863; 1520-5126
• DOI: 10.1021/ja980933j

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 (1H, 13C, 15N, 17O, and 29Si) spectroscopy. In contrast to ClH2SiONMe2, 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°) and shows an exceptionally small Si−O−N angle of 79.7(1)° and a Si···N distance of 2.028(1) Å, corresponding to a relatively strong Si−N-β-donor interaction. Ab initio calculations predict a wider Si−O−N angle of 91.6° (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)° and that in the gauche-conformer 104.7(11)° [Cl−Si−O−N torsion angle:  72.9(28)°]. 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° during a full rotation of Cl−Si−O−N. Analysis of the calculated charges on the Si and N atoms show the β-donor interaction not to be simply caused by electrostatic interaction. Natural bond orbital analysis describes a lp-N → σ*Si - X type negative hyperconjugation as an important contribution to β-donor bonding. The silicon substituent in anti position to nitrogen exerts the major effect on the strength of the β-donor bond. Simultaneous analysis of negative hyperconjugation lp-O → σ*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.