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moiety [Barraclough et al., 1978], and lower than those in singly bridged [L
8
Co
2
(m-
O
2
)]
5
þ
complexes (about 1100 - 1125 cm
21
;L¼ various neutral ligands) [Strekas
and Spiro, 1975]. Although no unambiguous examples of CoO
2
units containing a
side-on O
2
moiety are known, the n(O
2
) value of side-on O
2
in (TPP)FeO
2
is 89
cm
21
lower than that in the end-on isomer (1195 vs. 1106 cm
21
) [Proniewicz et al.,
1991]. Finally, n(Co - O) in [(DPA)Co
2
O
2
] (about 628 cm
21
in CH
2
Cl
2
) is also com-
parable to those in [(NH
3
)
8
Co
2
(m-O
2
)(m-NH
2
)]
4
þ
.
As mentioned early, the O
2
chemistry of (FTF3)Co
2
is distinct from that of other
group 2 cofacial porphyrins, which may result from the ( presumed) short separation
of the two porphyrin macrocycles in FTF3. Exposure of solutions of (FTF3)Co
2
containing excess N-methylimidazole to O
2
, followed by I
2
, did not generate an
EPR-active species [Collman et al., 1983a], in contrast to other group 2 porphyrins.
It was proposed that (FTF3)Co
2
might have no affinity to O
2
under the experimental
conditions and could not be oxidized with I
2
, thereby remaining fully reduced and
antiferromagnetically coupled throughout the manipulations. Alternatively, O
2
bind-
ing outside the cavity followed by formation of an intermolecular m-peroxo complex
and its decomposition ( possibly assisted by the excess imidazole and moisture) can
eventually yield a doubly oxidized diamagnetic [(FTF3)Co
2
]
2
þ
. Since FTF3Co
2
is a
poor ORR catalyst, its O
2
chemistry has been little studied.
In summary, the cooperative behavior of the two metal ions determines the dioxygen
chemistry of group 2 bis-Co porphyrins and certain flexible group 1 analogs (e.g., a
mixture of (FTF6)Co
2
and [(FTF6)Co
2
]
2
þ
in benzonitrile conproportionates in the pre-
sence of O
2
to yield a stable [(FTF6)Co
2
O
2
]
þ
adduct containing a bridging superoxide
ligand [LeMest et al., 1997]). Unlike monoporphyrin analogs, which bind O
2
only in
the reduced (Co
II
) form bearing an appropriate axial ligand, singly oxidized cofacial
bis-Co porphyrins manifest unusually high O
2
affinities even in the absence of
nitrogenous heterocycles. In all studied cases, O
2
binding to singly oxidized group 2
porphyrins (except FTF3) yields bridging m-superoxo adducts, although at present
the binding mode(s) of the m-O
2
2
moiety is unknown. Whereas O
2
adducts of mono-
metalloporphyrins rapidly autooxidize in the presence of protic sources, [(FTF4)Co
2
(O
2
)]
þ
undergoes reversible protonation (pK
a
of the conjugate acid in benzonitrile is
12 [Fukuzumi et al., 2004]) and reversible one-electron reduction and (possibly)
one-electron oxidation [LeMest et al., 1998].
Little is known about the O
2
adducts of cofacial corrole/porphyrin, cofacial biscor-
role, or cofacial phthalocyanine derivatives (Fig. 18.13).
18.5.3 ORR Catalysis by Cofacial Diporphyrins: Selectivities
and Structure
/
Activity Relationships
The selectivities of metal complexes of cofacial porphyrinoids ( porphyrins, corroles,
and phthalocyanines) reported in the literature by mid-2007 are summarized in
Fig. 18.15. The data are organized by the type of catalyst as well as in order of decreas-
ing n
av
. Whereas ORR catalysis by certain cofacial porphyrins, such as (FTF4)Co
2
and
(DPY)Co
2
(Y ¼ A, B) has been studied extensively by a number of groups, and the
values of n
av
are known with high degree of confidence, those for most other catalysts
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