THE STRUCTURAL CHEMISTRY OF AZIRIDINO-SILANE AND AZETIDINOSILANE

In a continuation of structural investigations of poly(amino)silanes, a series of silicon derivatives of aziridine and azetidine have been p repared. Analogies with the isoelectronic phosphorus ylide species and the high barrier to inversion at nitrogen in small N heterocycles wer e suggestive of steeply pyramidal and rather rigid configurations at t he N atoms in the title compounds. Tetrakis (N-aziridino)silane (1) an d tetrakis(N-azetidino)silane (2) have been synthesized from SiCl4 and LiN(CH2)(x) (x = 2, 3). Compound 1 is also formed when LiN(CH2)(2) an d HSiCl3 are used as starting materials, but with free aziridine a non -volatile product (la) is obtained. In neither case could any trace of HSi[N(CH2)(2)](3) be detected. In contrast, RSiCl3 (R = Me, Ph) could readily be converted into the corresponding tris(N-aziridino)silanes (3, 4) by treatment with excess aziridine. Tris(N-azetidino)silane (5) was accessible from HSiCl3 and excess azetidine, but the product was found to contain an unknown impurity. In order to determine the local symmetry and the dynamics of the aziridine rings, H-1-NMR spectra were recorded at low temperature (-80 degrees C). No splitting of the sign als was observed, indicating that the inversion barriers are extremely low, even in the highly strained three-membered heterocycles. Neverth eless, single-crystal X-ray diffraction studies of the N-triphenylsily l derivatives of aziridine (6) and azetidine (7) revealed an aziridiny l group with a steeply pyramidal configuration at nitrogen in 6 (sum o f the angles at N 313.32 degrees), and an azetidinyl group with a flat geometry in 7 (sum of the angles at N 350.96 degrees). The Si-N bond is significantly shorter in 7 as compared to that in 6.