BETA-DONOR INTERACTIONS OF EXCEPTIONAL STRENGTH IN N,N-DIMETHYLHYDROXYLAMINOCHLOROSILANE, CLH2SIONME2

The compounds ClH2SiONMe2 and ClH2SiONEt2 have been prepared by the re action of the corresponding O-lithiated hydroxylamines and dichlorosil ane. 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 CM2SiON Me2, 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 ClH2SiON Me2 (low-temperature crystallography) is present as anti-conformer (to rsional 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) Ang strom, corresponding to a relatively strong Si-N-beta-donor interactio n. Ab initio calculations predict a wider Si-O-N angle of 91.6 degrees (MP2/6-311G*). A gas-phase structure determination by electron diffr action shows the presence of two conformers (anti and gauche), which a re equal in potential energy. The Si-O-N angle in the anti-conformer i s 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) hav e shown the angle Si-O-N and the energy of the system to be largely de pendent 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 bet a-donor interaction not to be simply caused by electrostatic interacti on. Natural bond orbital analysis describes a Ip-N --> sigma(Si-X) ty pe negative hyperconjugation as an important contribution to beta-dono r 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 isol ation for the rationalization of the wide bond angles in silyl ethers.