Chemistry Reference
In-Depth Information
Table8.2
Redoxpotentialsofrepresentativemanganesecomplexes
2
[V vs Fc
+
/Fc]
Complex
E
1
/
Ref.
[Mn
III
(
Me
L
1
OMe
)(Bu
2
acac)]
+
−
0.61,* 0.42, 0.89(irr)
[64]
[Mn
IV
(L
3
tBu,OMe
)]
+
−
0.92,*
−
0.55,* 0.56, 0.74 (irr), 0.89 (irr)
[17]
[Mn
III
(
et,py
L
10
)]
+
−
0.65, 0.36, 1.15
[65]
[Mn
IV
(
qui
L
14
AP
)
2
]
−
1.10,
−
0.33, 0.09
[67]
[Mn
IV
(
mPh
L
16
ISQ
)
••
(
mPh
L
16
N,ISQ
)
••
2
]
−
1.23,
−
1.02 (red),
−
0.34,
−
0.12, 0.10 (irr)
[70]
[Mn
III
(L
15
Se
)
2
]
−
05
#
−
0
.
[68]
∗
: metal-centered process; irr: irreversible couple, E
p
a
is given
#
:E
◦
value attributed to the phenoxyl/phenolate redox wave
The manganese(III) - radical - rather than manganese(IV) - phenolate - character of the dication is evident
from UV-Vis spectroscopy (bands at 428 and 1015 nm) and resonance Raman (rR) measurements. Upon
excitation in resonance with the
π
-
π
* transition of the phenoxyl (380 - 430 nm), the rR band originat-
C stretching) is observed at the expected value (1619 cm
−
1
ing from the mode
ν
8a
(C
=
)
for phenoxyl
radicals. The ground state is S
t
2
as a result of a strong antiferromagnetic coupling between the rad-
ical and the octahedral paramagnetic metal ion (S
Mn
=
/
3
=
2). The zero field splitting parameters are E/D
0.33 cm
−
1
,D
t
5cm
−
1
, with g
x
=
=
=
g
y
=
2
.
05 and g
z
=
1
.
85. It is instructive to compare the prop-
erties of [Mn
III
Me
L
1
OMe
(Bu
2
acac)]
+
with those of [Mn
III
(L
3
tBu,OMe
]. In both cases the first electron
removal is reversible; however, while the former complex affords a manganese(III) - phenoxyl radical
species, oxidation of the latter affords a manganese(IV) - phenolate species. An increasing number of phe-
nolate oxygens in the metal coordination sphere thus favors metal-centered rather than ligand-centered
redox process, as also observed for [Mn
III
(
)
)
et,py
L
10
)
].
65
(
The additional redox wave observed at
+
0.56 V for
[Mn
III
(L
3
tBu,OMe
] is attributed to the formation of the Mn(IV) - phenoxyl species [Mn
IV
(L
3
tBu,OMe
)
•
]
2
+
)
according to Equation 8.5:
[Mn
II
L
3
tBu,OMe
]
−
[Mn
III
L
3
tBu,OMe
[Mn
IV
L
3
tBu,OMe
]
+
[Mn
IV
L
3
tBu,OMe
)
•
]
2
+
(
)
(
)
]
(
)
(
(8.5)
From the aminophenolate ligand H
2
L
14
AP
, two diradical complexes, namely [Mn
III
(L
14
ISQ
)
••
2
(L
14
AP
)
]and
[Mn
IV
(L
14
ISQ
] can be isolated.
66
Interestingly, the redox state of the metal center is evident
in the structure, not only from the shortening of the mean Mn - N/O bond lengths (2.017 and 1.934 A
in the manganese(III) and manganese(IV) complexes, respectively) but also from the deformation of the
coordination polyhedron, because high spin d
4
ions (such manganese(III)) are known to exhibit a strong
Jahn - Teller distortion, whereas the d
3
(manganese(IV)) configuration does not exhibit such distortion.
The coupling between the radical spins and the manganese ion is, as expected, antiferromagnetic, due to
)
••
2
(HL
14
AP
)
z
z
x
x
y
y
Figure8.12
Overlapbetweenthe
π
andselecteddorbitalsinoctahedralandsquareplanarcomplexes.
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