Synthesis of Volatile Cyclic Silylamines and the Molecular Structures of Two 1-Aza-2,5-disilacyclopentane Derivatives

An optimized synthetic procedure for alpha,omega-bis(bromosilyl)alkanes, BrH(2)Si(CH(2))(n)()SiH(2)Br (with n = 2 and 3), is proposed. 1,2-Bis(bromosilyl)ethane reacts with ammonia to give 1,4-bis(1-aza-2,5-disilacyclopentane-1-yl)-1,4-disilabutane, traces of 1,6-diaza-2,5,7,10,11,14-hexasilabicyclo[4.4.4]tetradecane and nonvolatile products. The primary reaction products undergo slow redistribution reactions whereby (1-aza-2,5-disilacyclopentane-1-yl)-1,4-disilabutane is formed as the major product. Reactions of alpha,omega-bis(bromosilyl)alkanes, BrH(2)Si(CH(2))(n)()SiH(2)Br (with n = 2 and 3), with isopropylamine afford the heterocycles 1-isopropyl-1-aza-2,5-disilacyclopentane and 1-isopropyl-1-aza-2,6-disilacyclohexane, whereas the analogous reaction with bis(bromosilyl)methane gives 1,5-diisopropyl-1,5-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 [(1)H, (13)C, (15)N and (29)Si including the measurement of (1)J((29)Si(15)N) coupling constants]. The molecular structure of 1-isopropyl-1-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 1-(dimethylamino)-1-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 beta-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.