Silylaminyl radicals. Part 3. Electron spin resonance studies of alkyl(trialkylsilyl)aminyl radicals

Abstract

A series of N-alkyl-N-trialkylsilylaminyl radicals R¹ṄSiR₃²(R₃²Si = Me₃Si, Et₃Si, Prⁱ₃Si, or Bu^tMe₂Si) have been generated photochemically in solution and characterised using e.s.r. spectroscopy. The radicals PhṄSiBu^tMe₂ and Bu^tṄGeMe₃ were also studied. The well resolved spectra and low magnitudes of a(N)(ca. 12.5 G) indicate that these aminyls are π radicals and this is confirmed by the magnitudes of the β-proton splittings for a series of radicals in which the conformation about the C–N bond may be inferred on the basis of steric considerations. The +l/–M substituent effect of the trialkylsilyl ligand is thus insufficient to bring about a switch from a π to a σ SOMO. However, the g-factors for the alkyl (silyl) aminyls (2.0061—2.0075) are significantly larger than those of dialkylaminyls (typically 2.0047) and also increase with the bulk of the substituents at nitrogen. This effect is interpreted in terms of increased effectiveness of spin-orbit coupling for the unpaired electron on nitrogen in RṄSiR₃, as a result of a reduction in the energy required to promote a σ lone-pair electron into the π SOMO. These conclusions are supported by the results of MNDO molecular orbital calculations. The lifetimes of the alkyl (silyl) aminyls increase with the extent of steric shielding of the nitrogen radical centre. The aminyl cyclo-C₄H₇ṄSiBu^tMe₂ undergoes rapid ring-opening at 195 K, whilst the cyclopentyl analogue does not rearrange at 265 K.