Chemistry Reference
In-Depth Information
acetylacetonate (acac) or the semiquinone (SQ) radical. The phenolate complex [Co
III
(L
4
tBu
(acac)]
+
)
can
be oxidized to the corresponding phenoxyl radical species [Co
III
(L
4
tBu
)
•
(acac)]
2
+
, which is characterized
by an absorbance maximum at 412 nm and an (S
=
1
/
2
) EPR signal at g
≈
2.00. Oxidation of the
(SQ)]
+
complex occurs at E
p
=
paramagnetic
mer
-[Co
III
(L
4
tBu
)
0
.
50 V, and also affords a radical species,
namely
mer
-[Co
III
(L
4
tBu
)
•
(SQ)]
2
+
. In contrast, oxidation of
fac
-[Co
III
Me2
L
4
tBu
(SQ)]
+
(
)
does not afford
a phenoxyl radical, but instead a
fac
-[Co
III
(quinone)]
2
+
species with two closed shell ligands,
illustrating that the isomerism and the steric bulk around the pyridine nitrogen are non-innocent in the
redox process.
Complexes isolated from aminophenolate ligands are mainly [Co
III
(L
14
ISQ
Me2
L
4
tBu
(
)
)
•
(tren)]
2
+
(monoradical),
76
[Co
III
(L
14
ISQ
)
••
2
X] (diradical)
85
and [Co
III
(L
14
ISQ
)
•••
3
] (triradical).
62,86
Of these, [Co
III
(L
14
ISQ
)
•
(tren)]
2
+
exhibits in its EPR spectrum an (S
1.997 with hyperfine splitting due to the inter-
action of the electron spin with the diamagnetic
59
Co (14 G) and the
14
N (8.8 G) nuclei. Its UV-Vis
spectrum (Figure 8.13) reveals
o
-iminobenzosemiquinonate bands at 476 (3100), 497 (3200) and 851 nm
(980 M
−
1
cm
−
1
=
1
/
2
)signalatg
=
.As[Fe
III
(L
14
ISQ
)
•
(tren)]
2
+
)
it could be one-electron oxidized or reduced according to
Equation 8.9:
[Co
III
L
14
AP
]
+
[Co
III
L
14
ISQ
)
•
(
]
2
+
[Co
III
L
14
BQ
]
3
+
(
)(
tren
)
(
tren
)
(
)(
tren
)
(8.9)
In [Co
III
(L
14
ISQ
)
••
2
X] (where X
=
iodine or chloine) the metal ion adopts a square pyramidal geometry,
similarly to [Fe
III
(L
14
ISQ
)
••
2
X]. The UV-Vis features of these two complexes are also similar, with a strong
670 nm.
85
band at
0) (antiferromagnetic coupling between
the radicals) regardless of the identity of X and no temperature dependence of S
t
has been reported, unlike
[Fe
III
(L
14
ISQ
≈
Nevertheless, the ground state remains (S
t
=
)
••
2
X].
In contrast to [Co
III
(L
14
ISQ
)
••
2
X], the ground state of the triradical [Co
III
(L
14
ISQ
)
•••
3
]is(S
t
=
3
/
2
),
that is the radicals couple
ferromagnetically
rather than
antiferromagnetically
in the former case.
86
What
is the reason for the difference in magnetic coupling? The three
π
* magnetic orbitals of the radical
ligands in [Co
III
(L
14
ISQ
)
•••
3
] are orthogonal to one each other (octahedral metal) and consequently cannot
couple antiferromagnetically one to each other (Figure 8.12). A new question then arises: Why does the
isostructural [Fe
III
(L
14
ISQ
)
•••
3
] exhibit a (S
t
=
1) ground state resulting from strong
antiferromagnetic
coupling? The reason is obvious, as the interactions involve both the ligands and the paramagnetic iron in
this complex, and not solely the ligand radical spins. The subsequent orbital overlap is higher, resulting in
a stronger and antiferromagnetic coupling. The redox chemistry of [Co
III
(L
14
ISQ
)
•••
3
] is summarized by
Equation 8.10:
[Co
III
L
14
AP
L
14
ISQ
)
•
]
2
−
[Co
III
L
14
AP
L
14
ISQ
)
••
2
]
−
[Co
III
L
14
ISQ
)
•••
3
]
(
)
2
(
(
)(
(
2
]
2
+
(8.10)
A surprising example in this series is the complex formed with the H
2
qui
L
14
AP
ligand.
68
No X-ray crystal
structure could be obtained, but its magnetic properties suggest the presence of a cobalt(II) ion rather
than cobalt(III) antiferromagnetically coupled to a ligand radical (
[Co
III
L
14
ISQ
)
••
2
(
L
14
BQ
]
+
[Co
III
L
14
ISQ
)
•
(
L
14
BQ
(
)
(
)
200 cm
−
1
|
J
|
>
)
. This complex, namely
[Co
II
qui
L
14
ISQ
)
•
(
qui
L
14
BQ
]
+
shows a rich redox chemistry, as does [Co
III
(L
14
ISQ
)
•••
3
] but attribution of
(
)
the oxidation/reduction sites is less detailed.
8.4.7 Nickel complexes
The nickel ion is a redox active metal that can exist either in the (
III) redox state in the usual
potential range. In the former state the nickel can be either paramagnetic (octahedral) or diamagnetic
(square planar).
+
II) or (
+
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