Chemistry Reference
In-Depth Information
Table8.7
Redoxpotentialsofrepresentativezinccomplexes
[V vs Fc
+
/Fc]
Complex
E
1
/
2
Ref.
[Zn
II
(
iPr
L
1
tBu
)(Cl)]
0.21
[99]
[Zn
II
(
iPr
L
1
SMe
)(Cl)]
0.18
[99]
[Zn
II
(
Me
L
1
tBu
)(Ph
2
acac)]
−
0.09
[139]
[Zn
II
(
Me
L
1
OMe
)(Ph
2
acac)]
−
0.28
[139]
[Zn
II
(
iPr
L
2
tBu
)]
0.17, 0.50
[99]
[Zn
II
(
Et
L
2
OMe
)]
−
0.10, 0.15
[102]
[Zn
II
(L
3
tBu,OMe
)]
−
−
0.63,
−
0.28,
−
0.06
[139]
[Zn
II
(HL
3
tBu,OMe
)]
−
0.28,
−
0.05, 0.80(irr)
[139]
[Zn
II
(H
2
L
3
tBu,OMe
)]
+
0.17, 0.94(irr)
[139]
[Zn
II
(H
2
L
3
tBu
)]
+
0.39, 1.40(irr)
[139]
[Zn
II
(L
5
SMe
)(CH
3
CN)]
+
0.42
[104]
[Zn
II
(L
5
tBu
)(CH
3
CN)]
+
0.41
[104]
[Zn
II
(L
7
SMe
)(Pyr)]
0.41(irr)
[109]
[Zn
II
2
(L
7
tBu
)
2
]
0.20, 0.37
[112]
[Zn
II
(L
9
)(EtOH)]
0.45, 0.67
[140]
[Zn
II
(H
et,py
L
10
)]
+
0.39
[121]
[Zn
II
(L
13
Ph
)
2
]
0.23, 0.51
[127]
[Zn
II
(L
13
Bz
)
2
]
0.41, 0.58
[128]
[Zn
II
(L
13
PhOMe
)
2
]
0.18, 0.47
[128]
[Zn
II
(
AP
L
15
ISQ
)
•
(NEt
3
)]
−
1.28,
−
0.29
[141]
[Zn
II
(
AP
L
15
BQ
)
2
]
−
1.28,
−
1.06, 0.43, 0.83
[141]
[Zn
II
(
Ph
L
16
AP
)]
−
1.29,
−
0.64, 0.03, 0.37
[133]
irr: irreversible couple, E
p
a
is given
The first zinc(II) - phenoxyl radical complexes to be described were those from TACN ligands.
99,102,139
As with their copper(II) analogues, the complexes can be isolated with different protonation states. The
electrochemically generated zinc(II) - radical species (Table 8.7) usually exhibit a green-blue color, with
the
π
-
π
* transitions of the phenoxyl radicals being observed at
≈
400 nm (their intensities increase with
the number of coordinated phenoxyl moieties). Interestingly, the
λ
max
is only slightly shifted when it is
compared to that of the copper analogue. The monoradicals exhibit in their EPR spectra an (S
=
1
/
2
) signal
at the expected g
iso
value of
2.005. A strong hyperfine coupling with one of the benzylic hydrogens (up
to 6 G) evidences a rigid six-membered coordination ring, thus demonstrating that the radical remains
coordinated. The diradical [Zn
II
≈
)
••
]
2
+
can be obtained, and the two unpaired electrons interact
one with each other via exchange and weak dipolar coupling. The magnitudes are
Et
L
2
OMe
(
3 and 0.01 cm
−
1
−
0.014 cm
−
1
0), thus the ground state is diamagnetic.
102
Tripodal ligands involving a 2-methylthio-4-
tert
-butylphenolate (with a N
3
OorN
2
O
2
coordination
sphere) have been developed in order to better understand the influence of the Cys-Tyr cross-link
in
respectively (with D
=
and E/D
=
GO.
104,109
While
the
monoradical
species
generated
from
the
zinc(II)
bis-phenolate
complex
[Zn
II
2
(L
7
SMe
(CH
3
CN)]
+
,
which contain a single phenolate moiety, can be oxidized to stable monoradical species. Both radicals
exhibit the typical
)
2
]
2
+
[Zn
II
(L
5
SMe
(CH
3
CN)]
+
[Zn
II
(L
5
tBu
is
poorly
stable in
solution,
)
and
)
π
-
π
* phenoxyl bands at
≈
410 nm, but there is a remarkable difference between
[Zn
II
(L
5
SMe
(CH
3
CN)]
+
and [Zn
II
(L
5
tBu
(CH
3
CN)]
+
)
)
in that only the former exhibits a transition at
(510 M
−
1
cm
−
1
)
887 nm
similar
to
that
reported
in
GO.
This
transition
has been
attributed
to
an
intramolecular charge transfer from the benzene ring to the methylthio group. [Zn
II
(L
5
SMe
)
•
(NO
3
)
]
+
,
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