Hydroxylaminosilanes:  Compounds with β-Donor−Acceptor Bonds

Abstract

Two series of compounds, H x Si(ONMe 2 ) 4 - x ( 3a, 2a, 1a ) and H x Si(ONEt 2 ) 4 - x ( 3b, 2b, 1b ) (with x = 1, 2, 3), have been prepared by either the condensation of N, N -dialkylhydroxylamines with halogenosilanes in the presence of 2,6-lutidine as an auxiliary base or by the more selective reaction of the O -lithiohydroxylamines with halogenosilanes at low temperatures. The compounds are very sensitive to hydrolysis, but are not pyrophoric, and are stable at ambient temperature to a potentially very exothermic rearrangement into aminosilanoles. The compounds have been characterized by gas-phase IR and solution NMR spectroscopy ( 1 H, 13 C, 15 N, 17 O, 29 Si) and by mass spectrometry. The IR frequencies of the simplest compound H 3 SiONMe 2 ( 1a ) have been assigned by comparison with ab initio frequencies. The NMR data are discussed in the light of β-donor interactions. The 29 Si NMR shifts of the series H x Si(ONMe 2 ) 4 - x are compared with those of the isoelectronic isopropoxysilanes, H x Si(OCHMe 2 ) 4 - x, which have been prepared for this purpose. Single crystals of H 3 SiONMe 2 ( 1a ), H 3 SiONEt 2 ( 1b ), H 2 Si(ONMe 2 ) 2 ( 2a ), and HSi(ONMe 2 ) 3 ( 3a ) have been grown by in situ methods, and their structures have been determined by X-ray diffraction. All compounds have small Si−O−N angles, with the minimum of 95.2° occurring with H 2 Si(ONMe 2 ) 2 ( 2a ). The crystallographic data are compared with the results of ab initio calculations (MP2/6-311G**) and the crystal structure of the isoelectronic H 3 SiOCHMe 2, which has now been determined, and the earlier reported Si(OCHMe 2 ) 4 . H 3 SiOCHMe 2 crystallizes in a transition state geometry according to MP2/6-311G** calculations and has a Si−O−C angle of 118.4(1)°. The data show clearly the large differences between Si−O−N and Si−O−C angles, which are attributed to the occurrence of β-donor interactions in Si−O−N units. The strength of this interaction has been estimated to be 15 kJ mol - l from ab initio calculations.