PREPARATION, CHARACTERIZATION, AND REACTIONS OF NOVEL IRON(III) PORPHYRIN DICATION COMPLEXES

A dication complex of (meso-tetramesitylporphynato)iron(III) (Fe(III)TMP) has been prepared by the reaction of Fe(III)TMP N-oxide with trifluoroacetic acid in toluene at low temperature. The electronic structure of the complex is characterized to be ferric high spin with rhombic symmetry on the basis of iodometric titration and UV-vis, H-2 NMR, and EPR spectroscopies. A value of E/D, which shows rhombicity of the heme environment, was estimated to be 0.29 by EPR measurement. An unusually high E/D value indicates that the symmetry of the ligand field around the iron is rather similar to those of nonheme ferric complexes, and the porphyrin ring could exhibit large nonplanarity. While tetraphenylporphyrin dication complexes of Zn, Mg, and Ni are known to react with methanol to afford the corresponding isoporphyrins, the dication complex of Fe(III)TMP was found to react with methanol to yield Fe(III)TMP N-methoxide, possibly due to the sterric barrier of the o-methyl groups of mesitylene. The Fe(III)TMP dication is not thermally stable and is readily reduced to the Fe(III)TMP pi-cation radical even at -25-degrees-C in toluene. The dication complex of (meso-tetrakis(2,6-dichlorophenyl)porphynato)iron(III) (Fe(III)TDCPP) was also prepared by the same reaction. The preparation of the dication complex of Fe(III) porphyrin demonstrates possible manipulation of the two-electron-oxidized equivalent in various forms by Fe porphyrin, i.e., the O=Fe(IV) porphyrin pi-cation radical, O=Fe(V) porphyrin, Fe(III) porphyrin N-oxide, and the Fe(III) porphyrin dication.