Triarylporphyrin meso-Oxy Radicals: Remarkable Chemical Stabilities and Oxidation to Oxophlorin π-Cations
Journal of the American Chemical Society
by Daiki Shimizu, Juwon Oh, Ko Furukawa, Dongho Kim and Atsuhiro Osuka
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Journal of the American Chemical Society
DOI: 10.1021/jacs.5b11223
abstract
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.
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