An electron spin resonance study of fluoroalkoxy- and fluoroalkylphosphoranyl radicals

Abstract

Two series of phosphoranyl radicals, containing fluoroalkoxy or trifluoromethyl ligands, respectively, have been generated in solution by radical addition to trivalent phosphorus compounds, and their e.s.r. spectra have been studied. The spectra of the radicals R_FOṖ(OR)₃ and R_FOṖMe(OR)₂exhibit large isotropic phosphorus hyperfine splittings and long-range fluorine coupling with (in many cases) fewer than the maximum number of chemically equivalent ¹⁹F nuclei. Temperature-dependent line-shape changes were observed and interpreted in terms of the presence of rapid conformational equilibria. The e.s.r. spectra were consistent with trigonal bipyramidal structures for the phosphoranyl radicals, in which the R_FO ligands are apical and the CH₃ groups equatorial.

The structures of the trifluoromethylphosphoranyl radicals, assigned on the basis of their e.s.r. spectra, show that the apicophilicity of the CF₃, group is less than that of Cl and usually greater than that of RO. Phosphoranyl radicals containing an apical CF₃ ligand exhibit lower values of a(P) than their methylphosphoranyl analogues, although the group electronegativity of CF₃ is much greater than that of CH₃. Temperature-dependent line-shape effects demonstrate that rotation about the P–CF₃ bond is restricted to a degree dependent upon the nature of the other substituents. Trifluoromethyl radicals add reversibly to trialkyl phosphites.