Evidence for variable metal‐radical spin coupling in oxoferrylporphyrin cation radical complexes

Oxoferrylporphyrin cation radical complexes were generated by m‐chloroperoxybenzoic acid oxidation of the chloro and trifluoromethanesulfonato complexes of tetramesitylporphyrinatoiron(III) [(TMP)Fe] and the trifluoromethanesulfonato complex of tetra(2,6‐dichlorophenyl)porphyrinatoiron(III) [TPP(2,6‐Cl)Fe]. Coupling between ferryl iron (S= 1) and porphyrin radical (S′= 1/2) spin systems was investigated by Mössbauer and EPR spectroscopy. The oxoferrylporphyrin cation radical systems generated from the TMP complexes show strong ferromagnetic coupling. Analysis of the magnetic Mössbauer spectra, using a spin Hamiltonian explicitly including a coupling tensor J̃ suggests an exchange‐coupling constant J > 80 cm⁻¹. The EPR spectra show non‐zero rhombicity, the origin of which is discussed in terms of contributions from the usual zero‐field effects of iron and from iron‐radical spin‐dipolar interaction. A consistent estimate of zero‐field splitting parameter D∼+ 6 cm⁻¹ was obtained by EPR and Mössbauer measurements. EPR and Mössbauer parameters are shown to be slightly dependent on solvent, but not on the axial ligand in the starting (TMP)Fe complex. In contrast to the TMP complex, the oxoferrylporphyrin cation radical system generated from [TPP(2,6‐Cl)FeOSO₂CF₃] exhibits Mössbauer and EPR spectra consistent with weak iron‐porphyrin radical coupling of |J|∼ 1 cm⁻¹.