Clarification of the Oxidation State of Cobalt Corroles
single CO molecule22–24,27–30 or act as a catalyst for the
electroreduction of O2 in acid media.21,31–33 Doubly reduced
Co(III) corroles, which formally contain Co(I), have been
spectroscopically characterized in acetonitrile. The Co(I)
form of the corrole is stable in polar organic solvents but
has been shown to catalytically reduce CO2 in nonaqueous
media.20
There have been extensive studies on the catalytic reduc-
tion of O2 using cobalt porphyrins,34–39 cobalt corroles,31
and dyads containing one porphyrin and one cobalt corrole
linked in a face-to-face arrangement.21,31–33 The main
difference between the cobalt corrole and cobalt porphyrin
catalysts is the oxidation state of the central metal ion in the
neutral compound. Uncharged cobalt porphyrins generally
contain a Co(II) ion in their air stable form, whereas cobalt
corroles contain Co(III) as a result of the -3 charge on the
macrocycle. In the case of the Co(II) porphyrins, the oxidized
species are well established to be Co(III) porphyrins rather
than a radical cation of the porphyrin with the same metal
oxidation state, that is, Co(II). In the case of Co(III) corroles,
however, it has not been clearly established whether the
oxidized species is in all cases a Co(IV) corrole as has been
previously observed4,5 or a radical cation of the corrole
containing Co(III).
We clarify this point in the present study by examining
the electrochemistry and spectroscopic properties of the three
meso-substituted corroles shown in Chart 1. The compounds
electrogenerated in each redox reaction can exist with Co(I),
Co(II), Co(III), or Co(IV) central metal ions, and the site of
each electron addition or abstraction was examined by
UV-vis spectroelectrochemistry. These compounds differ
from a previously examined compound, (Me4Ph5Cor)Co,7,31
in that they contain bulky substituents at the three meso-
positions of the macrocycle, which influence not only the
half-wave potentials for oxidation and reduction but also
diminish the ability of the corroles to dimerize in solution
or at an electrode surface. The ability of 1–3 to act as
homogeneous and heterogeneous catalysts in the reduction
of O2 is also investigated in acid media, and the singly
oxidized cobalt corrole products are characterized by UV-vis
and/or ESR spectroscopy.
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