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an optimal orbital overlap in these octahedral complexes (Figure 8.12). The ground state is consequently
(S
t
=
1) for [Mn
III
(L
14
ISQ
)
••
2
(L
14
AP
300 cm
−
1
4cm
−
1
)
](J
=−
and D
t
=
3
.
)
, whereas it is (S
t
=
/
2
)
with
1
470 cm
−
1
in the case of [Mn
IV
(L
14
ISQ
)
••
2
(HL
14
AP
a stronger J of
].
Other octahedral manganese complexes have been obtained from the two tridentate ligands H
2
qui
L
14
and H
2
L
15
Se
/
S
−
)
in the presence of one metal,
67,68
or three tetradentate ligands H
4
mPh
L
16
N
in the presence
of two equivalents of manganese.
69
)
2
]
−
affords the correspond-
ing
o
-iminobenzosemiquinonate radical, but only as a transient species (its existence has been evidenced
indirectly by cyclic voltammetry measurements) in the course of formation of the thermodynamically
stable manganese(IV) complex [Mn
IV
(L
15
Se
One-electron oxidation of [Mn
III
(L
15
Se
)
2
]. In contrast, a manganese -radical complex could be iso-
lated from H
2
qui
L
14
AP
, in particular as crystals. The structures of [Mn
IV
qui
L
14
ISQ
)
•
(
qui
L
14
AP
]
+
(
)
and
[Mn
IV
qui
L
14
AP
)
2
] are very interesting, as no significant change in the bond distances is observed in spite
of changes in oxidation state of one subunit. This shows that the unpaired
(
π
-radical electron is
delocalized
over both ligands, in contrast with [Cr
III
(L
3
tBuOMe
)
•
]
+
for which the unpaired electron is
localized
on a
single ring.
[Mn
IV
)
••
2
] is a dimetallic complex in which each manganese ion is coor-
dinated to two
o
-iminobenzosemiquinonate radical moieties. Its electronic structure is complicated and
can be summarized by strong antiferromagnetic interactions between the radicals and each metal, which
in turn coupled weakly antiferromagnetically (intermetallic distance of 6.7
mPh
L
16
ISQ
)
••
(
mPh
L
16
N,ISQ
(
A).
70
It is noticeable that
[Mn
IV
mPh
L
16
ISQ
)
••
(
mPh
L
16
N,ISQ
)
••
2
], as [Mn
IV
(L
14
ISQ
)
••
2
(L
14
AP
], exhibits a significant catalytic activity
for the aerobic oxidation of catechols into quinones, with 500 turnovers achieved within 24 hours for the
former. [Mn
IV
(
)
)
••
2
] also catalyses the aerobic oxidation of hindered phenols to
diphenoquinones in a mechanism that involves an exclusively ligand-based redox chemistry.
mPh
L
16
ISQ
)
••
(
mPh
L
16
N,ISQ
(
8.4.5 Iron complexes
In the usual potential range, iron can exist both at the
+
II and
+
III redox state (and even redox states as
VI have been described).
71
Hence, in iron phenolate complexes, the redox processes could be
either metal-centered or ligand-centered according to Equation 8.6:
Fe
II
high as
+
Fe
III
or Fe
II
Fe
II
(
phenolate
)
(
phenolate
)
(
phenolate
)
(
phenoxyl
)
(8.6)
In fact, coordination of the phenolate moieties to the iron center lowers the iron(III)/iron(II) redox potential,
and thus stabilizes the (
III) redox state. Consequently, the iron(II) ion is oxidized prior to the phenolate,
and all the phenoxyl radical complexes of iron involve a metal at the
+
+
III redox state at least.
Depending on the ligand properties, different spin states can be attained for the iron(III) ion: S
Fe
=
/
2
,
1
/
2
. Furthermore, because the radical and the metal ion spins are magnetically interacting, the
number of possible ground states is even higher. This versatility, combined with the fact that only the
total (and not local) spin state is accessible by techniques such as EPR or magnetic measurements, creates
challenges in obtaining the correct description of the electronic structure of these complexes. As it turns
out, determination of the local spin state is much easier than expected, since iron exhibits a M ossbauer
effect. The local state of the iron can, therefore, be easily and specifically probed with this technique, thus
greatly facilitating the chemist's task.
Historically, the first coordinated phenoxyl radical described was generated from an iron complex of a
tris-phenolate TACN ligand in 1993.
72
The expected spin state for a iron(III) ion in the N
3
O
3
octahedral
environment provided by the tris-phenolate TACN ligand is (S
Fe
=
/
2
or
3
5
5
/
2
). This can be confirmed by the
X-ray crystal structures of some complexes like [Fe
III
(L
3
tBu,OMe
], which show the typical Fe-O and Fe-N
bond lengths of 1.92 and 2.22 A, respectively. In addition, EPR reveals a typical signal at g
)
=
4.3 and
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