Chemistry Reference
In-Depth Information
complexes [57]. Reactions of copper complexes with superoxide were postu-
lated to proceed via an inner-sphere mechanism with the formation of a
CuO
+
transient [57].
Reactions of various iron compounds and its complexes with
HO /O
2
• •−
have been carried out for the last several decades in order to evaluate the
adverse effects of superoxide in the absence and presence of iron catalysts.
The iron complexes allowed to carry out studies in neutral and basic solutions
because corresponding free (or uncomplexed) iron ions form insoluble Fe(III)
hydroxides at pH > 2. The reactivity between Fe(II)/Fe(III) complexes and
HO /O
2
2
• •−
, where iron is complexed to polyaminocarboxylate ligands, is
presented in Table 4.1 [54, 59-62]. The most studied complex is iron-
ethylenediaminetetracetate (Fe-EDTA) complex. The oxidation-reduction
potential also changes when iron is chelated, hence the rates. The reaction
between the Fe
2+
-aquo complex and hydrogen peroxide was found be about
two orders of magnitude times slower than the Fe
2+
EDTA complex [63, 64].
The type of ligand determined the rate constants of iron(III) complexes with
O
•−
(Table 4.1). Reactions (4.13) and (4.14) may explain the catalytic dismuta-
tion of
O
•−
[59]:
2
(4.13)
Fe EDTA O
3
+
+
•−
→
Fe EDTA O
2
+
+
2
2
2
+
•−
3
+
2
−
.
(4.14)
Fe EDTA O
+
→
Fe EDTA-O
2
A recent study using nuclear magnetic resonance (NMR) spectroscopy and
pulse radiolysis technique suggested the reactions of Fe
III
complexes with
O
•−
occur via inner-sphere mechanisms where the
metal/O
•−
association is the
rate-limiting step [65]. The formation of a purple peroxo complex, [Fe
III
(EDTA)
(η
2
-O
2
)]
3−
, in the reaction of Fe
3+
EDTA and related complexes with H
2
O
2
has
TABLE 4.1. Rate Constants (/M/s) of Ferrates with Hydrogen Peroxide and
Superoxide
Species
Species
HO
•
O
•−
O
•−
Fe(II)
Fe(V)
Fe
2+
1.2 × 10
6
1.0 × 10
7
HFeO
2−
1.0 ± 0.3 × 10
7
a
Fe
2+
EDTA
-
2.0 × 10
6
Fe(III)
Fe(VI)
Fe
3+
<10
3
1.5 × 10
8
H
2
FeO
4
7.0 × 10
8
Fe
3+
EDTA(H
2
O)
−
-
1.9 × 10
6
1.7 × 10
7
HFeO
−
Fe
3+
DTPA
−
-
4.4 × 10
5
≤1.0 × 10
2
FeO
2−
Fe
3+
DTPA
2−
-
6.0 × 10
3
-
1.2 × 10
4
Fe O TTHA
2
3
+
(
)
2
−
a
pH 8.2.
Data were taken from References 54, 59, 65, and 78-80.
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