Direct EPR evidence for Si–N dπ–pπ bonding in silylamine radical cations

Abstract

A series of amine radical cations with α-silyl substitutents has been studied by EPR spectroscopy, following γ-radiolysis of dilute solutions of the parent amines in CFCl₃ at 77 K. It is clear that the nitrogen centred singly occupied molecular orbitals (SOMOs) are delocalised and extend appreciably onto the silyl groups, as shown by a fall in the magnitude of the anisotropic ¹⁴N couplings from those found in alkylamine radical cations, and thus the concept of Si–N dπ–pπ bonding in silylamines is strongly supported. The results contrast with prior conclusions for the isoelectronic alkyl radicals i.e. the extent of spin delocalisation onto the silyl group is negligible.

Additionally, there is considerable variation in the effective McConnell Q_H-values throughout the series, and we propose that this is due to variations in the electron charge on the nitrogen atom centres.

The radical cation of tetrakis(trimethylsilyl)hydrazine, (Me₃Si)₂N–N(SiMe₃)₂˙⁺, was found to adopt a twisted geometry with an angle between the two ¹⁴N tensors [and hence the N(2p_z) orbitals] of <24°. This is essentially in accord with the conclusions drawn from a liquid phase study of this cation by Bock et al., but the twist angle is smaller than their estimate of 45°. This indicates that the N–N torsional potential may be quite shallow so that the cation is undergoing torsional averaging at the higher temperature used in the liquid state study.