A Steeply Pyramidal Silylamine:  N,O-Dimethyl-N-silylhydroxylamine

Abstract

N,O-Dimethyl-N-silylhydroxylamine (H(3)SiMeNOMe) has been prepared by reaction of HMeNOMe with H(3)SiBr and 2,6-lutidine as an auxiliary base. Its identity has been proved by gas-phase IR and solution NMR spectroscopy of the nuclei (1)H, (13)C, (15)N, (17)O, and (29)Si and by mass spectrometry. The solution NMR data indicate aggregation of the compound in solution. H(3)SiMeNOMe decomposes within weeks at ambient temperature, and an extrusion of methylnitrene is probably the mechanistic pathway involved. The final products of this decomposition are an insoluble precipitate and SiH(4). The energy of H(3)SiMeNOMe relative to its potential rearrangement isomer MeHN-H(2)Si-OMe has been estimated by ab initio calculations to be 289 kJ mol(-)(1). The molecular structure of H(3)SiMeNOMe has been determined by gas-phase electron diffraction and by ab initio calculations. The results show H(3)SiMeNOMe to possess a steeply pyramidal nitrogen atom and to adopt a trans conformation. It is thus the first silyl nitrogen compound that adopts a typically pyramidal nitrogen coordination for purely electronic reasons. The Si-N bond is found to be only slightly elongated with respect to those of comparable compounds with planar nitrogen coordination. Important bond length and angles are Si-N 1.742(1), N-O 1.449(4), N-C 1.460(3), and O-C 1.425(4) Å and Si-N-C 121.8(5), Si-N-O 104.3(4), C-N-O 106.2(12), and N-O-C 103.2(12) degrees. Results of an NBO analysis show the silyl group to be bound by a nitrogen sp(2) hybrid, the d-orbital contribution at silicon to be less than 2%, and the N-O bond to comprise mainly p-orbital contribution. The presence of a marked p(lp-N)-sigma(Si-H) hyperconjugation in the NBO description shows that this effect cannot be solely responsible for the generally observed flattening of the nitrogen coordination in silylamines.

A Simple Electrostatic Model for Trisilylamine: Theoretical Examinations of the n→σ* Negative Hyperconjugation, pπ→dπ Bonding, and Stereoelectronic Interaction

“…For the OH- and F-substituted compounds, both of which have higher electronegativity than nitrogen, the fluorine atom and the hydroxyl group have negative partial charges, resulting in an electrostatic attraction with the silyl group and net stabilization of the pyramidal geometry. The computed θ t angle of 336° for HN(OH)SiH 3 is in good accord with the experimental value of 332.3° for N(CH 3 )(SiH 3 )(OCH 3 ) 4 Computed θ τ (deg) and the NPA Partial Charges for Substituted Silylamines (NHXSiH 3 ) at the HF/6-31G(d) and MP2/6-311G(dp) Levels HF/6-31G(d)MP2/6-311G(dp)Xθ t q (X) a θ t q (X) a SiH 3 360.0 0.5909 359.8 0.5749 PH 2 360.0 0.4630 360.0 0.4440 SH 359.8 0.3075 359.6 0.2893 NH 2 357.2 0.0242 352.5 0.0184 CH 3 355.7 0.2057 351.5 0.1981 H 352.2 0.4092 349.1 0.3784 Cl 345.7 0.1031 342.9 0.0948 OH 335.7 −0.1451 332.8 −0.1443 F 329.2 −0.3061 327.9 −0.3167 a Partial charges range from 0.39 to 0.43 e on the hydrogen attached to nitrogen and from 0.56 to 0.60 e on the silyl group for all compounds. …”

“…To corroborate recent theoretical findings that d orbitals play a minor role in bonding for main group elements, Schleyer and co-workers 7 proposed that the n→σ XH * negative hyperconjugation effect may be responsible for the difference between the physical and structural properties of first- and second-row elements. In other studies, it has been suggested 1c, that the planarity may be caused by the electrostatic repulsion among the silyl groups since the Si−N bond is more polar than the C−N bond.…”

“…On the basis of this electrostatic picture for N(SiH 3 ) 3 , it may be anticipated that a pyramidal structure for silylamine can be obtained if a substituent with strong eletron-withdrawing power is attached to the nitrogen. Indeed, Mitzel and Oberhammer recently reported the X-ray crystal structure of N(CH 3 )(SiH 3 )(OCH 3 ), which is nonplanar at the nitrogen center . To further elaborate on our electrostatic model for N(SiH 3 ) 3 , we carried out calculations on a series of substituted silylamines, HXNSiH 3 (X = H, CH 3 , NH 2 , OH, F, SiH 3 , PH 2 , SH, Cl), at the HF/6-31G(d) and MP2/6-311G(dp) levels.…”

Molecular structure of N-trimethylsilylaziridine in the gas phase

“…The observation was ascribed to the influence of the high electronegativity of the alkoxy substituent bound to the nitrogen atom which causes a high barrier to inversion, thus overcoming the counter influence of the silicon substituent. Examples are provided by (H 3 Si) 2 NOMe 5 and (H 3 Si)MeNOMe 6 which have slightly and steeply pyramidal cores respectively.…”