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In-Depth Information
30
20
e
10
0
300
400
500
600
700
800
900
1000 1100
l
(nm)
t
-Bu
Ph
t
-Bu
Ph
O
O
t
-Bu
t
-Bu
N
N
h
n
Ni
II
Ni
II
N
N
t
-Bu
O
t
-Bu
O
Ph
t
-Bu
Ph
t
-Bu
Excited state
Figure8.15
Intraligandcharge transfertransitionin [Ni
II
(
14
L
ISQ
)
••
2
].
inmM cm
−1
. (Adaptedwith permission
ε
from[96].Copyright2001AmericanChemicalSociety.)
are orthogonal to the half-filled ligand
orbitals. Then, the coupling between the
o
-iminobenzosemiquinone
moieties and the metal is expected to be strongly ferromagnetic. EPR spectroscopy confirms this assign-
ment, an (S
t
=
π
5cm
−
1
3
/
2
) signal being detected at g
x
=
g
y
=
2
.
10 and g
z
=
2
.
06 with
|
D]
3
/
2
=
10
±
and
0.21. Interestingly, this complex is the only one in the H
2
L
14
AP
E/D
=
series that could be finally oxidized
into a (S
=
/
2
) nickel(III) complex, perhaps due to its octahedral preorganization (Equation 8.12):
1
[Ni
II
L
14
AP
[Ni
II
L
14
ISQ
]
+
[Ni
II
L
14
BQ
]
2
+
[Ni
III
L
14
BQ
]
3
+
(8.12)
(
)(
tren
)
]
(
)(
tren
)
(
)(
tren
)
(
)(
tren
)
8.4.8 Copper complexes
The copper complexes were those that attracted the greatest and earliest interest due to the
occurence of the copper-radical entity in Galactose Oxidase. The copper(II)-phenolate complex
[Cu
II
(
iPr
L
1
tBu
)(CH
3
CN)]
+
98,99
exhibits in its cyclic voltammetry curve a reversible signal at 0.18 V
attributed to the phenoxyl/phenolate redox couple (Table 8.6). The electrogenerated radical species is
relatively stable and EPR silent, with absorption bands at 410 (4000) and 672 nm (1000 M
−
1
cm
−
1
typical
for phenoxyl radicals. Interestingly, the copper(II) - phenolate complex could also be isolated as a benzyl
alcoholate adduct [Cu
II
)
iPr
L
1
tBu
(OCH
2
Ph)]. The electrochemical behavior of this complex is characterized
by a total loss of reversibility of the redox wave corresponding to the phenoxyl/phenolate couple, as
expected for an electrocatalytic process. In addition, bulk electrolysis of [Cu
II
(
)
iPr
L
1
tBu
(benzyl alcoholate)]
affords benzaldehyde in a 46% yield showing that the catalyst is only active as its Cu
II
-phenoxyl radical
form as galactose oxidase. The TACN scaffold has been developed concomitantly by Wieghardt
et al
.
100
:
from H
Me
L
1
OMe
and in the presence of acetylacetonate, a copper(II) - phenolate complex in which the metal
is six-coordinate was isolated.
101
(
)
Interestingly, the Cu
II
- O-C bond angle (
)
between the x,y plane at the copper(II) ion and the phenyl ring of the phenolate ligand differ significantly
when the coordination number increases to six (Figure 8.16). This results in changes in orbital overlap that
are sufficient to invert the sign of the magnetic coupling between the metal and the radical. For instance,
it is antiferromagnetic in [Cu
II
α
) and the dihedral angle (
β
iPr
L
1
tBu
)
•
(Cl)]
+
whereas it is ferromagnetic in [Cu
II
Me
L
1
OMe
)
•
(Ph
2
acac)].
(
(
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