Chemistry Reference
In-Depth Information
Mn II Mn III OOR
(
)
+
(
)
→
2
Mn II ROO
(
)
+
•
.
(6.58)
The reaction of Mn(III) with benzyl radicals proceeded at a fast rate in
aqueous solution (reaction 6.59):
Mn III
(
)
+
PhCH H O Mn II
•
+
→
(
)
+
PhCH OH H
+
+
k
= ×
1 10
7
/M/s
.
2
2
2
59
(6.59)
The rate constants for reaction (6.59) were 2.3 × 10
8
/M/s and 3.7 × 10
8
/M/s
in glacial acetic acid and 95% acetic acid, respectively.
6.2.1.2 Mn(IV).
Mn(IV) in aqueous acetic acid has recently been prepared
[182]. uV-vis spectroscopy was used to characterized Mn(IV) acetate. Mag-
netic susceptibility data of Mn(IV) acetate (µ = 3.57 BM) helped to assign the
+4 oxidation state of Mn. The reactivity of Mn(IV) acetate was studied with
Br
−
, which gave first-order dependence on [Br
−
]. The reaction produced 1 equiv
of Br
2
/Br
−
[182].
Mn(IV) has also generated in alkaline solution from the reduction of
Mn(V) with
e
a
−
and
•
CO
2
in an argon-saturated 10 M NaOH solution contain-
ing formate (reaction 6.60) [183]:
Mn V e
(
)
+ →
−
Mn IV
(
)
k
60
=
5 0 10
.
×
9
/M/s
.
(6.60)
aq
The spectrum of Mn(IV) is presented in Figure 6.17, which suggests that
Mn(IV) would most likely appear as a blue or blue-green color. As shown in
the inset of Figure 6.17, a similar spectrum was also obtained when dissolving
pyrolusite (MnO
2
) in concentrated base [184]. It is likely that the Mn(IV)
species in the solution mixture were polymeric. This was supported by the
ESR-silent property of dimeric or polymeric species. For example, Mn(IV)
oxalate complexes were ESR silent due to the possible presence of µ-oxo-
bridged Mn(IV) dimers [185]. Generally, the yellow/brown precipitates
obtained in the reduction of permanganate in various reactions are highly
polymeric.
6.2.1.3 Mn(V) and Mn(VI).
Mn(V) and Mn(VI) have been observed in the
reactions of Mn(VII) with As(III) and propane-1,2-diol, respectively, in acidic
solutions [186-189]. In alkaline solutions, tetraoxyanions of Mn(VI) and
Mn(V) were conveniently generated by pulse radiolysis in which Mn(VII) and
Mn(VI) ions were reduced by hydrated electrons and radicals, respectively
(Table 6.2) [183, 190-192]. Both Mn(VII) and Mn(VI) are strong oxidizing
agents and react with the hydrated electron at the diffusion-controlled rates
(Table 6.2). However, the rate constant for the reduction of Mn(VI) by O
−
was
somewhat slower than the rate of diffusion-controlled reactions. The reaction
of Mn(VI) with
O
−
was relatively slow (reaction 6.61) (see Table 6.2):
MnO
2
−
+ →
O
−
MnO
3
−
.
(6.61)
4
2
4
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