Chemistry Reference
In-Depth Information
=
EPR data are available for the former, showing that the (S
t
1) system is in the limit of strong coupling
52 cm
−
1
,E/D
(
0.05).
As pointed out in the introduction, amino substituents are known to greatly stabilize phenoxyl radicals.
For instance the
triradical
[Cr
III
(L
14
ISQ
|
D
|=
0
.
=
)
•••
3
] could be prepared from H
2
L
14
AP
and chromium without
specific experimental conditions. This is due to the very low redox potentials of this class of ligands.
The first iminobenzosemiquinone/aminophenolate redox couple is, for example, too low to be measured in
dichloromethane, whereas the di and tri-radicals are successively formed at
−
1.77 and
−
1.26 V (remember
that the first phenoxyl/phenolate redox wave of [Cr
III
(L
3
tBu,OMe
0.03 V).
62
)
] is observed at
−
The redox
activity of [Cr
III
(L
14
ISQ
)
•••
3
] is summarized below (Equation 8.3):
[Cr
III
L
14
AP
L
14
ISQ
)
•
]
2
−
[Cr
III
L
14
AP
L
14
ISQ
)
••
2
]
−
[Cr
III
L
14
ISQ
)
•••
3
]
(
)
2
(
(
)(
(
[Cr
III
L
14
ISQ
)
••
2
(
L
14
BQ
]
+
[Cr
III
L
14
ISQ
)
•
(
L
14
BQ
)
2
]
2
+
[Cr
III
L
14
BQ
)
3
]
3
+
(
)
(
(
(8.3)
The magnetic orbitals of the radical ligands in [Cr
III
(L
14
ISQ
)
•••
3
] adopt an arrangement relatively similar to
those found in [Cr
III
(L
3
tBu,OMe
)
•••
]
3
+
. As a consequence, antiferromagnetic coupling between the radicals
and the metal ion also exists in [Cr
III
(L
14
ISQ
)
•••
3
], which is consequently diamagnetic (J
436 cm
−
1
=−
)
.
)
•••
3
], as well as those of many other complexes involv-
ing
o
-iminobenzosemiquinonate ligands, has been solved. A quinoid distribution of bond lengths is
observed in the
o
-iminobenzosemiquinonate ring (as for the tri-
tert
-butylphenoxyl) with shortening of
the carbon - oxygen bond, thus emphasizing its double bond character, and elongation of the adjacent
carbon - carbon bonds. Interestingly no substantial change in these bond lengths is observed upon substi-
tution of the chromium by other metal ions. The mean bond distances given in Figure 8.11 are thus valid
for most of the complexes described below.
The X-ray crystal structure of [Cr
III
(L
14
ISQ
8.4.4 Manganese complexes
The manganese ion normally adopts an octahedral geometry, as chromium, but in contrast it is redox
active, and can exist at the (
IV) redox states in the usual potential range. The study of
manganese complexes is, therefore, much more complicated than that of the chromium analogues.
The complex [Mn
III
+
II,
+
III and
+
Me
L
1
OMe
(Bu
2
acac)]
+
(
)
has been prepared from the monophenolate TACN ligand
H
Me
L
1
OMe
.
64
It exhibits both a metal-centered reduction wave at
−
0.61 V and a ligand-centered oxidation
one at
+
0.42 V (Table 8.2) according to Equation 8.4:
[Mn
II
Me
L
1
OMe
[Mn
III
Me
L
1
OMe
]
+
[Mn
III
Me
L
1
OMe
)
•
(
]
2
+
(
)(
Bu
2
acac
)
]
(
)(
Bu
2
acac
)
(
Bu
2
acac
)
(8.4)
t
-Bu
t
-Bu
t
-Bu
t
-Bu
2H
+
+M,
−
e
−
+e
−
e
−
+e
−
1.40
1.40
1.42
−
1.38
1.43
1.46
−
1.36
1.46
1.52
1.41
1.43
1.46
1.40
1.38
1.36
+2H
+
,
−
M
t
-Bu
H
t
-Bu
N
t
-Bu
N
t
-Bu
N
1.41
1.43
1.46
OH
O
1.37
O
1.35
O
1.30
M
M
M
1.35
1.30
1.24
(a)
(b)
(c)
(d)
Figure8.11
Schematicrepresentationandmetricalparametersforthecoordinated o-iminobenzosemiquinone
unitunderdifferentredoxstates:(a)aminophenol(H
2
L
14
AP
);(b)deprotonatedaminophenolate(L
14
AP
)complex;
(c) o-iminobenzosemiquinonate (L
14
ISQ
) complex; and (d)o-iminobenzoquinone (L
14
BQ
) complex. ([67] Repro-
ducedbypermissionoftheRoyalSocietyofChemistry.)
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