β-Donor Interactions of Exceptional Strength in N,N-Dimethylhydroxylaminochlorosilane, ClH₂SiONMe₂

Abstract

The compounds ClH2SiONMe2 and ClH2SiONEt2 have been prepared by the reaction of the corresponding O-lithiated hydroxylamines and dichlorosilane. Their identity has been proved by gas-phase IR and solution NMR (1H, 13C, 15N, 17O, and 29Si) spectroscopy. In contrast to ClH2SiONMe2, ClH2SiONEt2 is unstable at ambient temperature and decomposes to give H2SiCl2 and H2Si(ONEt2)2. ClH2SiONEt2 shows dynamic behavior in the solution as shown by low-temperature NMR. In the crystal ClH2SiONMe2 (low-temperature crystallography) is present as anti-conformer (torsional angle Cl−Si−O−N 180°) and shows an exceptionally small Si−O−N angle of 79.7(1)° and a Si···N distance of 2.028(1) Å, corresponding to a relatively strong Si−N-β-donor interaction. Ab initio calculations predict a wider Si−O−N angle of 91.6° (MP2/6-311G**). A gas-phase structure determination by electron diffraction shows the presence of two conformers (anti and gauche), which are equal in potential energy. The Si−O−N angle in the anti-conformer is 87.1(9)° and that in the gauche-conformer 104.7(11)° [Cl−Si−O−N torsion angle: 72.9(28)°]. Calculations (MP2/6-31G*) have shown the angle Si−O−N and the energy of the system to be largely dependent on the torsion angle Cl−Si−O−N, with a variation of the angle Si−O−N over a range of 30° during a full rotation of Cl−Si−O−N. Analysis of the calculated charges on the Si and N atoms show the β-donor interaction not to be simply caused by electrostatic interaction. Natural bond orbital analysis describes a lp-N → σ*Si - X type negative hyperconjugation as an important contribution to β-donor bonding. The silicon substituent in anti position to nitrogen exerts the major effect on the strength of the β-donor bond. Simultaneous analysis of negative hyperconjugation lp-O → σ*Si - X shows the Si−O−N compounds to have similar contributions to this interaction as H3SiOCH3 and questions negative hyperconjugation to be quoted in isolation for the rationalization of the wide bond angles in silyl ethers.

Strong Intramolecular Secondary Si⋅⋅⋅N Bonds in Trifluorosilylhydrazines

“…We could only prove the presence of strong non-classical Si¥¥¥N interactions that lead to three-membered SiON rings in O-silylhydroxylamines that contain SiON units. The strongest b-donor bonds detected so far were found in H 2 Si(ONMe 2 ) 2 , [9] ClH 2 SiONMe 2 , [10] and F 3 SiONMe 2 .…”

N,N‐Dimethylaminopropylsilane: A Case Study on the Nature of Weak Intramolecular Si⋅⋅⋅N Interactions

“…This work was done experimentally and theoretically by analysing model compounds including F 3 SiCH 2 NMe 2 , [6] (F 3 C)F 2 SiONMe 2 , [7] F 3 SiN(R)-NMe 2 (R = Me, SiMe 3 , SnMe 3 ), [8] MeHSi[ONA C H T U N G T R E N N U N G (BH 3 )Me 2 ] 2 [9] and ClH 2 SiONMe 2 . [10] It transpires that the composition of the spacer function is highly important for enabling or disabling a silicon-donor interaction. No example of an attractive silicon-donor interaction in geminally donor substituted silanes with a methylene unit (CH 2 ) as spacer function is known.…”

(Dimethylaminomethyl)trifluorosilane, Me2NCH2SiF3—A Model for the α‐Effect in Aminomethylsilanes

“…In recent years we have established that related systems containing SiNN (silylhydrazine) and SiON (hydroxylaminosilane) units can indeed form stable three-membered ring systems. The most intriguing examples are ClH 2 SiONMe 2 [7] and F 3 SiONMe 2 [8] for the SiON systems and F 3 SiN(SiMe 3 )NMe 2 and F 3 SiN(SnMe 3 )NMe 2 for the SiNN systems. [9] Our investigations into the SiCN systems have so far been restricted to the parent systems H 3 SiCH 2 NMe 2 [10] and Cl 3 SiCH 2 NMe 2 , [11] but these compounds do not show significant attractive interactions between the Si and N atoms, which would lead to compression of the Si-C-N angle.…”