Methylsilylhydroxylamines: Preparative, spectroscopic and ab initio studies

Methylsilylhydroxylamines [(MeH₂Si)₂NOMe, (MeH₂Si)MeNOMe, Me₂NOSiH₂Me] have been prepared from bromo(methyl)silane and the corresponding methylhydroxylamines in the presence of an auxiliary base (triethylamine or N,N,N′,N′-tetramethylethylenediamine). The compounds were studied by NMR spectroscopy of all elements present (¹H, ¹³C, ¹⁵N, ¹⁷O, ²⁹Si). The magnitude of the one-bond coupling constants ¹J(¹⁵N²⁹Si) is interpreted in terms of the hybridization associated with the pyramidal co-ordination of nitrogen, a unique structural feature in Si/N chemistry, Ab initio studies confirmed these structural predictions. Singly silylated hydroxylamines have been shown to be more strongly pyramidal than doubly silylated ones. Calculations on the model compound (H₃Si)₂NOMe gave a barrier to inversion at nitrogen of 9.7 kcal mol^-1. This inversion is accompanied by a partial rotation around the N-O bond. The NMR chemical shifts of the compounds have been calculated and the results are in good agreement with the experimental data. The unusually low chemical shifts δ(¹⁷O) of hydroxylamines have thus been confirmed by theory. A comparison of the calculated normal modes of vibration with experimental data leads to a complete assignment of the IR spectra.