Chemistry Reference
In-Depth Information
[Co
III
(en)
3
]
3+
/[Co
II
(en)
3
]
2+
couple,
E
¼
0.02 V versus the normal hydrogen
electrode (NHE) [
18
]. Such an electron transfer is especially slow. In particular,
for the self-exchange process:
3
þ
2
þ
2
þ
3
þ
Co
III
Co
II
Co
II
Co
III
½
ð
Þ
3
þ½
ð
Þ
3
!½
ð
Þ
3
þ½
ð
Þ
3
en
en
en
en
10
5
M
1
s
1
[
18
]. As a consequence, all reductions
processes involving [Co
III
(en)
3
]
3+
, as well as oxidation processes involving
[Co
II
(en)
3
]
2+
, are slow. Sluggishness of the Co
III
/Co
II
electron transfer process is
well interpreted in terms of the Marcus theory: on moving from the [Co
III
(en)
3
]
3+
complex (
d
6
, low-spin) to the [Co
II
(en)
3
]
2+
(
d
7
, high-spin), a substantial increase in
the Co-N bond distance from 1.96 to 2.16
˚
is observed. This makes the
reorganisation energy and the free energy of activation
the rate constant
k
11
is 2.9
G
{
especially high and
D
the electron transfer slow.
On the other hand, while the
E
value for the [Co
III
(sep)]
3+
/[Co
II
(sep)]
2+
(
0.30 V vs. NHE) is close to that observed for the en analogue, the rate constant
of the self-exchange process is more than five orders of magnitude higher
(
k
11
¼
5.1 M
1
s
1
)[
18
]. Such a behaviour is surprising in that the [Co
III
(sep)]
3+
/
[Co
II
(sep)]
2+
reduction process also involves a low-spin to high-spin conversion
and a drastic increase of the bond distances (from 1.99 to 2.16
˚
), to which a
similarly high value of reorganisation energy and of
G
{
should correspond.
However, attention should be addressed not only to the metal-ligand interaction,
but also to the overall energy of the complex, including the strain energy of
the ligating framework [
19
]. The strain energy, which is intuitively larger in the
cage system than in a bidentate ligand, brings the Co
III
and Co
II
complexes closer to
the transition state, making
D
G
{
much lower and the electron transfer much faster.
As a further advantage, the [Co
II
(sep)]
2+
complex can be generated in aqueous
solution by reducing the Co
III
complex with zinc dust or zinc amalgam in the 5-7
pH range and investigated for its redox properties. For instance, [Co
II
(sep)]
2+
reacts
with dioxygen to give hydrogen peroxide [
20
], according to the a fast process:
D
Co
II
2
þ
2H
þ
!½
Co
III
3
þ
½
ð
sep
Þ
þ
O
2
þ
ð
sep
Þ
þ
H
2
O
2
Non-caged Co
II
polyamine complexes, e.g. [Co
II
(NH
3
)
6
]
2+
, do not give such
a reaction, but form stable Co
III
peroxy dimers, [(NH
3
)
5
Co
III
-O-O-Co
III
(NH
3
)
5
]
4+
.
Moreover, encapsulation provides a redox couple whose oxidised and reduced form
are protected from undesired side reactions. This makes the [Co
III
(sep)]
3+
/
[Co
II
(sep)]
2+
a convenient and robust electron relay in many processes. For
instance, it can successfully replace methylviologen (MV
2+
) as an efficient electron
transfer agent from photoexcited [Ru
II
(bpy)
3
]
2+
to H
+
in the photoreduction of
water (in the presence of colloidal platinum, and excess EDTA as a sacrificial
electron donor) [
21
]. In particular, it works for more than 5,000 turnovers, as
compared with the 55 turnovers of the more fragile MV
2+
. [Co
III
(sep)]
3+
can act
as a photosensitiser itself and, when irradiated at 331 nm in a solution adjusted to
