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 Fe IV=O(tmp [rad])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 Mössbauer 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) Å; Fe IV=O(tmp )Br ( 4): Fe–O=1.65(1), Fe–Br=2.93(2), Fe–N p=2.02(1) Å. 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.