Markus Müther

Generation of oxoiron(IV) tetramesitylporphyrin π-cation radical complexes by m-CPBA oxidation of ferric tetramesitylporphyrin derivatives in butyronitrile at −78 °C. Evidence for the formation of six-coordinate oxoiron(IV) tetramesitylporphyrin π-cation radical complexes FeIV=O(tmp)X (X=Cl−, Br−), by Mössbauer and X-ray absorption spectroscopy

Abstract The generation of six-coordinate oxoiron(IV) tetramesitylporphyrin π-cation radical complexes by m -CPBA ( meta -chloroperbenzoic acid) oxidation of ferric tetramesitylporphyrin derivatives in butyronitrile at −78 °C was investigated. UV–Vis and EPR spectroscopies indicate that the axial ligand present in the ferric starting derivatives is retained in the high-valent iron complexes. Indirect evidence for the formation of six-coordinate oxoiron(IV) tetramesitylporphyrin complexes Fe IV =O(tmp )X (X=Cl − , Br − ) by m -CPBA oxidation of FeX(tmp) (X=Cl − , Br − ) in butyronitrile at −78 °C was also obtained by Mossbauer spectroscopy. Direct confirmation of the presence of a halide ion as second axial ligand of iron in these high-valent iron species was obtained by X-ray absorption spectroscopy. The EXAFS spectra of the samples obtained by m -CPBA oxidation of FeX(tmp) (X=Cl − , Br − ) were refined using two different coordination models including both four porphyrinato-nitrogens and the axial oxo group. The two models include (model I) or exclude (model II) the axial halogen. The statistical tests indicate the presence of a halide ion as second axial ligand of iron in both derivatives. The refinements led to the following bond distances: Fe IV O(tmp )Cl ( 3 ): Fe–O=1.66(1), Fe–Cl=2.39(2) and Fe–N p =1.99(1) A; Fe IV =O(tmp )Br ( 4 ): Fe–O=1.65(1), Fe–Br=2.93(2), Fe–N p =2.02(1) A. The lengthening of the Fe–X (X=Cl − , Br − ) distances relative to those occurring in the ferric precursor porphyrins is, most probably, related to the strong trans influence of the oxoiron(IV) fragment present in 3 and 4 .

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 ofD∼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.