Spin coupling in distorted high-valent Fe(IV)-porphyrin radical complexes

In order to study structural influences on the interaction of Fe(IV) (S=1) and porphyrin cation radical (S′=1/2) in high-valent iron porphyrin complexes of the type |X-(TMP)Fe=O|⁺(Cl^-), X=I, Br₂, Br₄ were generated by mCPBA oxidation of corresponding Fe(III) porphyrins. The halogen substitution at the peripheral positions of the porphyrin leads to distortion of the planar porphyrin ring of |(TMP)Fe=O|⁺. The new species have beeen investigated by temperature-dependent EPR and field-dependent Mössbauer spectroscopy; for the evaluation of spectra, we adopted the spin-Hamiltonian formalism including exchange interaction explicitly. As in |(TMP)Fe=O|⁺, strong ferromagnetic spin coupling was observed with|J₀|D=0.9-1 and a zero-field spltting of D∼32 cm^-1. For consistent parametrization of EPR and Mössbauer results, anisotropic coupling had to be introduced. Compared to |(TMP)Fe=O|⁺ [1], analysis of the spectroscopic data shows that zero-field splitting and spin coupling is only slightly affected by the halogen distortion of the porphyrin structure.