Elucidation of Factors Affecting the Electronic Structures of Magnesium(II) and Zinc(II) Tetraarylporphyrin Radical Cations

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Abstract
A series of magnesium and zinc tetraarylporphyrins and their porphyrinoxidized derivatives were studied by UV/Vis, ESR, and resonance Raman spectroscopy at various temperatures. The series included tetra(meta‐dichlorophenyl)porphyrinatozinc (5), tetra(ortho‐dichlorophenyl)porphyrinatozinc (6), tetra(ortho‐difluorophenyl)porphyrinatozinc and ‐magnesium (9 and 10), and tetra(pentafluorophenyl)porphyrinatozinc and ‐magnesium (7 and 8). The radical cations (3a‐10a) were isolated by chemical one‐electron oxidation of their neutral precursors (3‐10). Despite the structural similarity of all these radicals, their electronic ground state varied within the series. The position of the chloro groups was found to play a key role. While the radical cation of the meta‐dichloro‐substituted derivative 5a exhibited A2u spectroscopic features, the ortho‐dichlorophenyl derivative (6a) showed A1u spectral features. Radicals of the fluoro‐substituted porphyrins, especially that of 10, were found to have state‐admixed (A1u/A2u) electronic structures, and the relative contributions of the two states was found to vary with temperature and to depend on the axial ligand. The results indicate that the fluoro‐substituted porphyrins are primarily A2u at low temperature, even though their room temperature spectroscopic features resemble those of A1u cations. The elucidation of factors that affect the electronic structures of the radicals in the present series is helpful in providing a greater understanding of the spin‐spin interactions in the intermediates of heme‐dependant enzymatic reactions and their synthetic analogues.