The Structural Chemistry of Aziridino‐ and Azetidinosilanes

In a continuation of structural investigations of poly(amino)silanes, a series of silicon derivatives of aziridine and azetidine have been prepared. Analogies with the isoelectronic phosphorus ylide species and the high barrier to inversion at nitrogen in small N heterocycles were suggestive of steeply pyramidal and rather rigid configurations at the N atoms in the title compounds. Tetrakis( N ‐aziridino)silane ( 1 ) and tetrakis( N ‐azetidino)silane ( 2 ) have been synthesized from SiCl 4 and LiN(CH 2 ) x ( x = 2, 3). Compound 1 is also formed when LiN(CH 2 ) 2 and HSiCl 3 are used as starting materials, but with free aziridine a non‐volatile product ( 1a ) is obtained. In neither case could any trace of HSi[N(CH 2 ) 2 ] 3 be detected. In contrast, RSiCl 3 (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 HSiCl 3 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, 1 H‐NMR spectra were recorded at low temperature (−80°C). No splitting of the signals was observed, indicating that the inversion barriers are extremely low, even in the highly strained three‐membered heterocycles. Nevertheless, single‐crystal X‐ray diffraction studies of the N ‐triphenylsilyl derivatives of aziridine ( 6 ) and azetidine ( 7 ) revealed an aziridinyl group with a steeply pyramidal configuration at nitrogen in 6 (sum of the angles at N 313.32°), and an azetidinyl group with a flat geometry in 7 (sum of the angles at N 350.96°). The Si−N bond is significantly shorter in 7 as compared to that in 6 .