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IV
nated product was detected for the Mn =O system when
keeping a similar concentration of chloride anion at pH 13.4
as at pH 1.5 (HCl and NaOH were used to adjust the pH as
needed). Formation of monochlorinated anthracene is typical
evidence for an electron-transfer process in which the
anthracene cation intermediate has been generated. Accord-
IV
ingly, the anthraquinone formed by the Mn ÀOH at pH 1.5 is
most probably the product of sequential oxidation of
anthracene by electron transfer, whereas anthracene is not
an intermediate for anthraquinone formation by oxidation by
Figure 1. Manganese(IV) species under three pH conditions:
a) pH 1.5, b) pH 4.0, and c) pH 13.4.
IV
the Mn =O functional group at pH 13.4.
IV
pH conditions. Hydrogen abstraction reactions from 9,10-
dihydroanthracene substrate were performed in acetone/
water (4:1) under nitrogen, and the product distributions are
summarized in Table 1. It was found that the product
Electron-transfer ability of the Mn ÀOH functional
group at pH 1.5 was further supported by oxygenation of
IV
thianthrene. It was found that, while the Mn =O group at
IV
pH 13.4 is incapable of oxygenating thianthrene, the Mn À
OH group at pH 1.5 provides 35.1% yield of thianthrene-S-
oxide, and a yield of 12.1% at pH 4.0. The solvent kinetic
isotope effect for this oxygenation in neutral acetone/water
Table 1: Hydrogen abstraction from 9,10-dihydroanthrancene by the
[
a]
manganese(IV) complex under different pH conditions.
(
ratio 4:1) revealed an inverse KIE value of 0.604 (in acetone/
IV
pH
Mn moiety Product distribution [%]
À5 À1
H O: kobs = 2.33 ꢁ 10 s , in acetone/D O: kobs = 3.86 ꢁ
1
implying that protonation of [Mn (Me EBC)(OH) ] to
generate [Mn (Me EBC)(OH)(H O)] is essential for elec-
2
2
anthra- anthra-
anthrone monochlorinated
anthracene
À5 À1
0
s ; see Figure S3 in the Supporting Infomration),
cene
quinone
IV
2+
2
2
IV
1
4
.5 Mn ÀOH
3.0
17.6
6.3
8.4
0.8
21.8
trace
none
trace
trace
none
none
IV
3+
IV
2
2
.0 Mn ÀOH
IV
tron transfer as has previously been demonstrated for the
1
3.4 Mn =O
[14,15]
oxygenation of triphenylphosphine.
Such an inverse KIE
[a] Reaction conditions: acetone/water (4:1) 5 mL, manganese(IV)
value also suggests that the formation of minor amounts of
anthraquinone from 9,10-dihydroanthracene at pH 4.0 could
complex 5 mm, 9,10-dihydroanthrancene 2.5 mm, stirring under nitrogen
at 298 K for a) 24 h at pH 1.5 and 4.0; or b) 6 h for pH 13.4.
IV
be attributed to electron transfer by the Mn ÀOH, present in
IV
3+
trace amounts as [Mn (Me EBC)(OH)(H O)] . The redox
2
III
2
IV
distributions are different under these three pH conditions.
potentials of the Mn /Mn couple for the manganese(IV)
species under the hydrogen-abstraction conditions are 0.54 V,
0.46 V, and 0.10 V (vs SCE) at pH 1.5, 4.0, and 13.4,
respectively, which are also consistent with the different
IV
IV
At pH 4.0, hydrogen abstraction by Mn ÀOH in [Mn -
2
+
(
Me EBC)(OH) ] provides anthracene as the dominant
2 2
product (17.6%) with only a trace of anthraquinone (0.8%).
IV
IV
IV
In strong base at pH 13.4, where Mn =O is in large excess in
electron-transfer capabilities between Mn ÀOH and Mn =
IV
[
Mn (Me EBC)(O) )], only 6.3% yield of anthracene was
O (see Figure S10). Very interestingly, the sole difference
2
2
IV
IV
found with the 21.8% yield of anthraquinone being dominant.
between the Mn ÀOH moiety in the species [Mn -
IV
IV
3+
2+ IV 3+
The Mn ÀOH moiety in [Mn (Me EBC)(OH)(H O)] at
(Me EBC)(OH) ] and in [Mn (Me EBC)(OH)(H O)]
2 2 2 2
2
2
pH 1.5 gives a 3.0% yield of anthracene and an 8.3% yield of
anthraquinone, a surprisingly similar product distribution to
that for the Mn =O moiety at pH 13.4. Although the product
species is one proton and one unit of net charge (2 + vs. 3 +)
because of the different protonation states of the mangane-
se(IV) complex. This increase of one unit of positive charge
IV
IV
yields are relatively low because of the sluggish oxidizing
power of the manganese(IV) species, the obtained mecha-
nistic information is fruitful (see below).
has made it possible for the manganese(IV) in [Mn -
3
+
(Me EBC)(OH)(H O)] to perform electron transfer in
2
2
addition to hydrogen abstraction from 9,10-dihydroanthra-
IV
IV
To clarify whether the anthraquinone product at both high
and low pH is due to a sequential oxidation of the anthracene
produced during the reaction, anthracene was tested as the
substrate under identical conditions to those used in the 9,10-
dihydroanthracene oxidation. The yields of anthraquinone
were 33.4% at pH 1.5, 2.5% at pH 4.0, and 2.9% at pH 13.4,
indicating that the formation of anthraquinone from 9,10-
cene substrate, whereas the Mn ÀOH center in [Mn -
2+
(Me EBC)(OH) ] can only perform hydrogen abstraction
2
2
even though their redox potentials are very similar (0.54 V vs.
0.46 V). Thus, modulating the net charge has the power to
IV
significantly change the reactivity of the Mn ÀOH moiety in
the same manganese(IV) complex.
IV
The different electron-transfer capabilities of the Mn À
IV
IV
dihydroanthracene substrate by the Mn ÀOH center at
OH and Mn =O functional groups were definitively distin-
IV
pH 1.5 and by the Mn =O center at pH 13.4 proceed through
guished through the tests with 2,2’-azino-di-(3-ethylbenzthia-
different pathways. In GC–MS analysis of the products from
zoline-6-sulphonic acid) (ABTS), a relatively facile electron-
transfer reagent. At pH 4.0 and 13.4, ABTS is in the same
[16]
9
,10-dihydroanthracene oxidation, in addition to minor yields
of anthrone, identified as the intermediate for anthraquinone
protonation state (pK = 2.2) with identical electrochemical
a
[16c]
formation at pH 1.5 and 13.4, a trace of monochlorinated
properties,
and the UV/Vis spectra of ABTS are also
IV
anthracene was observed for the Mn ÀOH at pH 1.5
identical in the pH range 3–13.4 (see Figure S11). In aqueous
solution, electron transfer from ABTS to the manganese(IV)
(
monochlorinated anthracene product was also observed for
+
anthracene as the substrate at pH 1.5). However, no chlori-
species can be completed in minutes to form ABTS C with the
7322
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2011, 50, 7321 –7324