Triarylporphyrin meso-Oxy Radicals: Remarkable Chemical Stabilities and Oxidation to Oxophlorin π‑Cations

5-Hydroxy-10,15,20-triarylporphyrin (oxophlorin) and its Ni­(II) and Zn­(II) complexes were oxidized with PbO2 to give the corresponding porphyrin meso-oxy radicals as remarkably stable species. These radicals were fully characterized with X-ray diffraction analysis, UV/vis/NIR absorption and ESR spectroscopies, magnetic susceptibility measurement, electrochemical studies, and theoretical calculations. Free-base radical and its Ni­(II) complex have been shown to exist as a monoradical in solution, while the Zn­(II) complex exists in an equilibrium between monomer (doublet monoradical) and dimer (a non-Kekulé singlet biradicaloid) with a dimerization constant of KD = 3.0 × 105 M–1 in noncoordinating CH2Cl2 but becomes a pyridine-coordinated monoradical upon addition of pyridine. Variable temperature magnetic susceptibility measurements of these radicals revealed different magnetic interactions in the solid-states, which has been interpreted in terms of their different packing structures in a microscopic sense. These radicals undergo one-electron oxidation and reduction in a reversible manner within narrow potential windows of 0.57–0.82 V. Finally, one-electron oxidation of Ni­(II) and Zn­(II) porphyrin meso-oxy radicals with tris­(4-bromophenyl)­aminium hexachloroantimonate furnished oxophlorin π-cations, which displayed nonaromatic closed-shell character, NIR absorption, and significant double bond character of the C–O bond.