Effects of the Electron-Withdrawing Power of Substituents on the Electronic Structure and Reactivity in Oxoiron(IV) Porphyrin π-Cation Radical Complexes

Abstract

The effects of the electron-withdrawing power of the substituents bound to a porphyrin ring on the electronic structures and the reactivities of oxoiron(IV) porphyrin π-cation radical complexes were studied by using 2,7,12,17-tetramethyl-3,8,13,18–tetraarylporphyrins (aryl = (1) mesityl, (2) 2-chloro-6-methylphenyl, (3) 2,6-dichlorophenyl, or (4) 2,4,6-trichlorophenyl) and tetrakis-5,10,15,20-tetraarylporphyrins (aryl = (5) mesityl, (6) 2-chloro-6-methylphenyl, (7) 2,6-dichlorophenyl, or (8) 2,4,6-trichlorophenyl). The electronic structures of oxoiron(IV) porphyrin π-cation radicals were investigated by low-temperature UV-visible absorption spectroscopy and proton NMR measurements. The absorption spectra of oxoiron(IV) porphyrin π-cation radicals of compounds 1-4 changed with an increase of the electron-withdrawing power of ring substituents, while those of compounds 5-8 did not. Proton NMR measurements demonstrated that oxoiron(IV) porphyrin π-cation radicals of compounds 1-4 have an a1u radical character and that those of compounds 5-8 are better described as an a2u radical species. The reactivities of oxygen atoms of oxoiron(IV) porphyrin π-cation radicals were examined by competitive epoxidation of cyclohexene by two oxoiron(IV) porphyrin π-cation radicals with different radical orbital occupancies or oxidation potentials. The oxygen atom with the higher oxidation potential was more reactive than that with the lower oxidation potential. Furthermore, the oxygen atom with the a1u radical state was almost as reactive as that with the a2u radical state. The results indicate that the reactivity of the oxygen atom of the oxoiron(IV) porphyrin π-cation radical depends on its oxidation potential and is not affected by the a1u/a2u orbital occupancy. © 1993, American Chemical Society. All rights reserved.