Synthesis of Volatile Cyclic Silylamines and the Molecular Structures of Two l-Aza-2,5-disilacyclopentane Derivatives

An optimized synthetic procedure for α,ω-bis(bromosilyl)alkanes, BrH₂Si(CH₂)_nSiH₂Br (with n = 2 and 3), is proposed. l,2-Bis(bromosilyl)ethane reacts with ammonia to give l,4-bis(l-aza-2,5-disilacyclopentane-l-yl)-l,4disilabutane, traces of l,6-diaza-2,5J,10,11,14-hexasilabicyclo[4.4.4]tetradecane and nonvolatile products. The primary reaction products undergo slow redistribution reactions whereby (l-aza-2,5-disilacyclopentane-l-yl)-l,4disilabutane is formed as the major product. Reactions of α,ω-bis(bromosilyl)alkanes, BrH₂Si(CH₂)wSiH₂Br (with n = 2 and 3), with isopropylamine afford the heterocycles l-isopropyl-l-aza-2,5-disilacyclopentane and 1-isopropyll-aza-2,6-disilacyclohexane, whereas the analogous reaction with bis(bromosilyl)methane gives 1,5-diisopropyl-l,5-diaza-2,4,6,8-tetrasilacyclooctane rather than a four-membered ring compound. All compounds have been characterized by elemental analysis, mass spectrometry, and IR and NMR spectroscopy [¹H, ¹³C, ¹⁵N and ²⁹Si including the measurement of ¹(²⁹Si¹⁵N) coupling constants]. The molecular structure of l-isopropyl-l-aza-2,5-disilacyclopentane, determined by analysis of gas-phase electron-diffraction data augmented by restraints derived from ab initio calculations, is compared with the molecular structure of the isoelectronic l-(dimethylamino)-l-aza-2,5-disilacyclopentane. The latter also was determined by gas-phase electron-diffraction (supported by ab initio calculations) and by low-temperature crystallography. The presence of a β-donor Si⋯N interaction in the latter compound, leading to a narrow Si-N-N angle, is apparent from a significant distortion of the molecular structure as compared with the isoelectronic reference compound.