Synthesis of Volatile Cyclic Silylamines and the Molecular Structures of Two 1-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. 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
α,ω-bis(bromosilyl)alkanes,
BrH2Si(CH2)nSiH2Br
(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 [1H, 13C, 15N and
29Si
including the measurement of
1J(29Si15N)
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 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.