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^.)X (X=Cl^-, Br^-), by Mossbauer and X-ray absorption spectroscopy

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) 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. Copyright (C) 2000 Elsevier Science Inc.