An electron spin resonance study of phosphine-boryl radicals; their structures and reactions with alkyl halides

Abstract

A number of phosphine-boryl radicals X₃[graphic omitted]–[graphic omitted]H₂˙(2; X = MeO, CF₃CH₂O, Me₂N, Et, Buⁿ, Bu^t) have been generated in solution by hydrogen abstraction from the phosphine-boranes X₃[graphic omitted]–[graphic omitted]H₃ and characterised by e.s.r. spectroscopy. The boron radical centre in (2) is effectively planar and the unpaired electron is contained mainly in a B-2p_π orbital, although the low g-factors (ca. 2.0020) provide some evidence for the energetic proximity of an excited state in which the unpaired electron occupies a B–P σ* orbital. The phosphine-boryl radicals are less reactive than the borane radical anion H₃B^–z.rad; and fail to add to benzene, ethylene, or vinyltrimethylsilane. They do abstract halogen atoms from alkyl bromides but not, in general, from alkyl chlorides, although (2; X = alkyl) does abstract chlorine from benzyl chloride. Selectivity in bromine abstraction increases in the order H₃B^–z.rad; < H₂ḂCN^–≃(2; X = Buⁿ, Me₂N) < (2; X = MeO) < (2; X = CF₃CH₂O) and the relatively high selectivity of the fluorine-containing radical is attributed to polar effects.