Electron spin resonance studies of thiophosphoranyl radicals. The mechanism of ligand permutation in phosphoranyl radicals

Abstract

A series of thiophosphoranyl radicals have been generated in solution by addition of thiyl radicals (RS˙, from photolysis of RSSR or H₂S) to phosphorus(III) compounds and their e.s.r. spectra have been studied. The phosphoranyl radicals [graphic omitted](OR′)SR have trigonal bipyramidal (TBP) structures in which the ring bridges apical and equatorial sites: the apicophilicity of RS is greater than that of R′O. Apical–equatorial (a–e) ring proton exchange is evident from e.s.r lineshape effects and it is proposed that the exchange takes place via a σ*(P–S) intermediate or transition state. In general, the rate of a–e ligand exchange in TBP phosphoranyl radicals appears to increase as the energy difference between TBP and σ* structures decreases. TBP (C_2v) and σ*(C_3v) configurations represent limiting structures for hypervalent phosphoranyl radicals and the geometry between these two extremes adopted by a particular radical appears to depend upon the nature of the ligands. It is suggested that the apicophilicity of ligands in TBP or σ* isomers is related to the ease of heterolytic dissociation of the P–ligand bonds, rather than to ligand electronegativity alone. The phosphoranyl radicals Ph₃ṖSR are hypervalent and probably have σ*(P–S) structures, whereas the RS ligands are equivalent in (MeS)₂ṖPh₂ and (RS)₂ṖH₂, indicating TBP structures. The radical [Bu^tOPPh₃]˙ has a ligand–π-electronic structure and the low-temperature e.s.r. spectrum shows that the unpaired electron is centred on one ring in a π-type orbital. At high temperatures electron exchange between the rings renders them magnetically equivalent.