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O MnO
2
−
+
•
CH C OH CH
(
)(
)
→
[
O Mn OC H OH
(
)]
2
−
+
OH /H O
−
→
A
3
2
3 2
3
4
8
2
MnO /O Mn OH
3
−
(
)
2
−
+
OHCH C OH CH
(
)(
)
→
B
(6.64)
4
3
2
3 2
Oxo-Mn(V) porphyrins have also been generated by the oxidation of
Mn(III) porphyrins in aqueous solutions using peroxides and hypobromite
[194, 195]. Mn
V
-oxo complexes (TBP
8
Cz)Mn
V
(O); TBP
8
Cz = octa-
tert
-
butylphenylcorrolazinato) have also been isolated in order to examine high-
valent manganese reactions with one-electron reductants [196]. This complex
was recently converted to Mn
V
(O) π-cation radical porphyrinoid complex,
which was successfully characterized by uV-vis, EPR, and laser deposition
and ionization mass spectrometry (MDI-MS) [197]. The Mn
V
(O) π-cation
radical porphyrinoid complex was found to be more reactive than the TBP
8
Cz)
Mn
V
(O) complex. The Mn(V)-imido complex ((TBP
8
Cz)Mn
V
(NMes) has also
been synthesized [198]. The complex, (PPh
4
)
2
[Mn
V
(N)(CN)
4
], has been shown
to catalyze epoxidation of alkenes and oxidation of alcohols by H
2
O
2
[199].
The Mn(VI)-ester was observed in the interaction of Mn(VII) with the
tert
-butyl alcohol radicals at pH 9.4 [183]. Initially, a spectrum, which appeared
to be similar to
MnO
2−
, shown in Figure 6.19b, was observed (reaction 6.65).
This species decayed by first order to produce a second transient (reaction
6.66). The decay of this second species finally yielded
MnO
2−
and an oxidized
product of
tert
-butyl alcohol (reaction 6.67):
MnO
−
+
•
CH C OH CH
(
)(
)
→
[
O Mn OCH C OH CH
VI
(
)(
) ]
−
(6.65)
4
2
3 2
3
2
3 2
k
65
=
1 85 0 2 0 10
.
±
.
×
9
/M/s
[
O Mn OCH C OH CH
VI
(
)(
) ]
−
→
[
O Mn O C H chelate OH
(
)](
)
+
−
3
2
3 2
2
2
4
8
(6.66)
k
66
=
5 5 1 0 10
.
±
.
×
1
/s
[
O Mn O C H chelate MnO
(
)](
)
→
2
−
+
OHCH C OH CH
(
)(
)
(6.67)
2
2
4
8
4
2
3 2
k
67
=
0
.
8
±
0
.
2/s
.
The formation of Mn(V) and Mn(VI) has also been seen in the spectral
changes during the reactions of sulfite with Mn(VII) in alkaline solutions
[200-202].
6.2.2 Reactivity of Complexes of Hypervalent Mn
In a recent study, the activation of a water molecule by electrochemical
oxidation of the Mn-aquo complex has been reported [203]. The oxidation
of a non-porphyrinic six-coordinated Mn(II)(OH)
2
complex resulted in a
Mn(O) complex, which involved the sequential (2 × 1 electron/a proton) and
direct (2 electron/2 proton) proton-coupled transfers. The intermediate Mn(III)
(OH)
2
and Mn(III)OH complexes were analyzed [203]. Complexes of salen,
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