obtained by other methods. Also, identical results were obtained
with several different sources of HOAc.
ion pairing and complex formation despite an increase in solvent
polarity.
The effect of solvent on the lifetime and thermodynamic stability
The potential role of Mn(IV) in Mn-catalyzed autoxidation of
aromatics will depend on reaction conditions and concentrations
of various species involved. At low concentrations of Mn(II) and
NaBr, the rate laws for the reactions with Mn(IV) have the same
general form (mixed second-order kinetics), and the observed rate
constants and activation parameters are similar. Thus, the two
reactions will be competitive in a wide range of temperatures if
the concentrations of bromide and Mn(II) are low and not too
different from each other.
2
+
of soluble Mn(IV) is remarkable. In aqueous solutions, Mnaq
O
is believed to be too short-lived to be observed or chemically
31
trapped. In AcOH, on the other hand, soluble form(s) of Mn(IV)
persist for several hours even in the presence of 5% water. In
2
+
2+
5
aqueous solutions, the very fast MnaqO /Mnaq reaction, k >10
-
1
-1
M
(
1
s , is believed to be the main pathway for the loss of the
2
+ 31
transient) MnaqO . In AcOH, reaction 9 is much slower (k =
9
10 M- s ), and not significantly affected by small amounts of
1
-1
-
1
-1
water (k
9
= 149 M
s
in 95% AcOH). The different behaviour
At high and comparable concentrations of Mn(II) and bromide,
the reaction of Mn(IV) with Mn(II) will benefit from the fact that
the dependence on Mn(II) remains first order, whereas bromide
of Mn(IV) in the two solvents must be related to the differences
in polarity, hydrogen bonding, and ion-pairing, as already noticed
26,32
and discussed for Mn(II) and Mn(III).
exhibits saturation kinetics. The Mn(IV)/Mn(II) reaction generates
∑
The oxidation of NaBr with Mn(III) is fast regardless of the
source of Mn(III). Minor differences do exist between different
sources, but the second-order dependence on bromide persists,
and the rate constant determined for freshly generated solution
species is almost the same as that for samples prepared by
Mn(III) which in turn oxidizes bromide to HBr
2
, eqn (11), a
key oxidant and hydrogen atom abstracting species in the chain
oxidation of aromatic substrates. The Mn(IV)/bromide reaction,
on the other hand, produces bromine, which may be involved in the
undesirable formation of bromoaromatics and methyl bromide, as
well as in the oxidation of solvent. The conditions favouring the
Mn(II)/Mn(IV) route should be therefore more productive in the
process of catalytic autoxidation of alkylaromatics.
dissolution of commercial Mn(OAc) . Under most conditions,
3
the reaction is much faster than the Mn(IV)/NaBr reaction. This
makes the mechanism for the latter difficult to assign because the
involvement of Mn(III) as an intermediate is hard to establish.
None the less, the different kinetic dependencies, i.e. first order
in [NaBr] in Mn(IV) reaction, and second-order in [NaBr] in
Mn(III) reaction do suggest a change in mechanism. From the
Conclusions
Four independent reactions, including oxidation of Mn(II) with
data on Mn(III)/bromide reaction in 10% H
2
O, it was concluded
oxygen atom donors and reduction of KMnO by acetic acid
4
that thereactioninvolved one-electron intramolecular oxidation of
are shown to generate a Mn(IV) species. In addition to the
∑
-
5
coordinated bromide followed by elimination of Br
2
, eqn (11).
most reasonable interpretation of the chemistry generating the
manganese product, the assignment of the 4+ oxidation state
is based on the magnetic susceptibility data (m = 3.57 BM) and
This mechanism is also applicable to the reaction in this work
which retained the second-order dependence on [NaBr] in both
glacial and 95% AcOH with two other sources of Mn(III).
stoichiometry and products of oxidation of Mn(OAc) and NaBr.
2
The former reaction produces two equivalents of Mn(III) for each
III
III
HOAc
II
Mn + 2NaBr ←⎯ ⎯⎯⎯→ Mn Br ⎯⎯ ⎯→ Mn + HBr2
(11)
equivalent of Mn(IV). The reaction with NaBr generates one
2
-
equivalent of Br
2
/Br
3
with a rate law that exhibits first-order
-
If a similar, 1 e mechanism were utilized by the Mn(IV)/NaBr
reaction, the observed first order dependence on [NaBr] in
glacial acetic acid would require that the much more energetic
dependence on NaBr.
The data are insufficient to infer the structure of Mn(IV)
generated in this work, but, as pointed out by a reviewer, the
∑
∑
bromine atom, Br , be released instead of HBr
2
. Even though this
ready oxidation of Mn(OAc) suggests similar structures for the
3
IV
+
possibility cannot be completely excluded, it is considered unlikely,
especially in 95% AcOH, where the binding of bromide to Mn(IV)
is so strong that the kinetics exhibit saturation. Small amounts
of dibromo complex(es) should be easily available, but are clearly
not kinetically relevant, suggesting that another mechanism must
be at work. We favour a two-electron process of eqn (6) which
two and points to Mn (OAc) as the most reasonable possibility.
3
Acknowledgements
Support for this project from BP Amoco is gratefully acknowl-
edged. The research was carried out in the facilities of the Ames
Laboratory [under contract number DE-AC02-07CH11358 with
the U. S. Department of Energy-Basic Energy Sciences].
bypasses halogen radicals and produces HOBr and Mn(II) instead.
-
The observed Br
2
/Br
3
is formed rapidly from HOBr and NaBr in
reaction 7. The source of oxygen in HOBr may be a molecule of
water from the first or second coordination sphere of manganese.
It is less likely that the molecule contains a Mn-oxo group, see
later, which rules out a mechanism proposed for the oxidation
References
1 W. Partenheimer, Catal. Today, 1995, 23, 69–158.
2 W. Partenheimer and R. K. Gipe, ACS Symp. Ser., 1993, 523, 81–88.
33
of nitrite by MnO . Also, as mentioned above, the reaction of
2
3
R. Landau and A. A. Saffer, Mod. Chem. Ind., Proc. Int. Union Pure
Appl. Chem. Symp., 1968, 185–190.
Br(I) with NaBr in glacial acetic acid must proceed without prior
hydrolysis.
4 A. Saffer, R. S. Barker 1958, vol., p US 2833816.
The observation of saturation kinetics in 95% AcOH, but not
in glacial acetic acid is believed to be caused by weak dissociation
of NaBr into free ions in glacial acetic acid. In the presence of
5 X.-D. Jiao and J. H. Espenson, Inorg. Chem., 2000, 39, 1549–1554.
6
P. D. Metelski, V. A. Adamian and J. H. Espenson, Inorg. Chem., 2000,
3
9, 2434–2439.
7
P. D. Metelski and Espenson J. H., J. Phys. Chem. A, 2001, 105, 5881–
5% water, greater percentage of NaBr is ionized, which facilitates
5884.
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Dalton Trans., 2010, 39, 11636–11642 | 11641