Kinetics and mechanism of benzene oxidation by peroxymonosulfate catalyzed with a binuclear manganese(IV) complex in the presence of oxalic acid

Article abstract of DOI:10.1134/S003602441303028X

It is established that Oxone (peroxymonosulfate, 2KHSO₅· KHSO₄·K₂SO₄) oxidizes benzene to p-quinone very efficiently and selectively in a homogeneous solution in aqueous acetonitrile in the presence of a catalyst, i.e., dimeric manganese(IV) complex [LMn(O)₃MnL](PF₆)₂ where L is 1,4,7-trimethyl-1,4,7-triazacyclononane, and a cocatalyst, i.e., oxalic acid. The dependences of the maximum rate of quinone accumulation on the initial concentrations of reagents are studied. It is proposed that benzene is oxidized by the manganyl particle containing the Mn(V)=O fragment that forms upon the reaction of the reduced form of the starting dimeric manganese complex with Oxone.

Full text of DOI:10.1134/S003602441303028X

ISSN 0036ꢀ0244, Russian Journal of Physical Chemistry A, 2013, Vol. 87, No. 3, pp. 393–396. © Pleiades Publishing, Ltd., 2013.
Original Russian Text © L.S. Shul’pina, Yu.N. Kozlov, T.V. Strelkova, G.B. Shul’pin, 2013, published in Zhurnal Fizicheskoi Khimii, 2013, Vol. 87, No. 3, pp. 414–417.
CHEMICAL KINETICS
AND CATALYSIS
Kinetics and Mechanism of Benzene Oxidation
by Peroxymonosulfate Catalyzed with a Binuclear Manganese(IV)
Complex in the Presence of Oxalic Acid
L. S. Shul’pinaa, Yu. N. Kozlovb, T. V. Strelkovaa, and G. B. Shul’pinb
aA. N. Nesmeyanov Institute of Organoelement Compounds, Russian Academy of Sciences, Moscow, 119991 Russia
bN. N. Semenov Institute of Chemical Physics, Russian Academy of Sciences, Moscow, 119991 Russia
eꢀmail: yunkoz@mail.ru
Received February 21, 2012
Abstract—It is established that Oxone (peroxymonosulfate, 2KHSO5
· KHSO4 · K2SO4) oxidizes benzene to
p
ꢀquinone very efficiently and selectively in a homogeneous solution in aqueous acetonitrile in the presence
of a catalyst, i.e., dimeric manganese(IV) complex [LMn(O)3MnL](PF6)2 where L is 1,4,7ꢀtrimethylꢀ1,4,7ꢀ
triazacyclononane, and a cocatalyst, i.e., oxalic acid. The dependences of the maximum rate of quinone
accumulation on the initial concentrations of reagents are studied. It is proposed that benzene is oxidized by
the manganyl particle containing the Mn(V)=O fragment that forms upon the reaction of the reduced form
of the starting dimeric manganese complex with Oxone.
Keywords: benzene oxidation, kinetics and mechanism, catalysis, manganyl particle, Oxone.
DOI: 10.1134/S003602441303028X
INTRODUCTION
Me
Me
N
N
MnIV
The oxidation of benzene to phenol and quinone
(as well as other arenes to phenols and quinones),
which is of great importance from a practical point
of view, can be performed selectively via reactions
with peroxy compounds catalyzed using transition
metal complexes [1–5]. We found earlier that binuꢀ
clear manganese(IV) complex with the macrocyclic
nitrogenꢀcontaining ligand 1,4,7ꢀtrimethylꢀ1,4,7ꢀ
triazacyclononane (L) [LMn(O)3MnL](PF6)2
O
MnIV
Me N
N Me (PF6)2
O
O
N
N
Me
Me
(1)
EXPERIMENTAL
Our experiments were performed in air in thermoꢀ
stated glass vessels at 50°С. The volume of the reacꢀ
tion solution was 5 mL. The substrate (benzene),
(compound
such as oxalic acid (compound
1
) in the presence of an organic acid
) catalyzes the effiꢀ
2
cient oxidation of saturated hydrocarbons, olefins,
sulfides, and secondary alcohols by hydrogen perꢀ
oxide in acetonitrile at room temperature [6–13].
Unfortunately, this catalytic system using hydrogen
peroxide as the oxidizing agent allowed no highꢀ
yield oxidation of aromatic compounds. In this
work, we studied the possibility of using Oxone
(peroxymonosulfate, 2KHSO5
instead of hydrogen peroxide to oxidize benzene.
This oxidizing agent was found to transform benꢀ
catalyst
umes of the starting compound (benzene) or prepreꢀ
pared starting solutions of and in acetonitrile. The
1, and cocatalyst 2 were added as definite volꢀ
1
2
reaction was initiated after the addition of solid
Oxone, which dissolves quickly in our chosen solvent.
(Note: Mixing peroxy compounds with organic subꢀ
stances, especially at elevated temperatures, can result
in an explosion!) The solvent was aqueous acetonitrile.
(Oxone is insoluble in dry acetonitrile.) For NMR
control of the reaction, D2O was used (MeCN : D2O
ratio, 2 : 1 (v/v). Samples (0.3 mL) of the reaction
solution were taken at intervals and definite amounts
of a solution of the internal standard (1,4,ꢀdinitrobenꢀ
· KHSO4 · K2SO4)
zene to pꢀquinone efficiently. Our study of the proꢀ
cess allowed us propose the following mechanism
for it:
393

394
SHUL’PINA et al.
×
102, M
excited acceleration, the reaction delay period diminꢀ
c
ishing as the initial concentration of the catalyst
increases. This period can be reduced considerably via
the addition of hydroquinone. We showed in special
experiments that hydroquinone is also oxidized rapꢀ
idly to quinone using the described system.
4
8
Phenol (0.03 M) transforms almost quantitatively to
3
quinone under the action of catalyst 1 (3.0 × ,
10–5 M)
6
4
2
0
1
oxalic acid (0.05 M), and Oxone (0.032 M). In our
experiment, the yield of quinone was 0.027 M and, in
addition, catechol was obtained in small amounts
(0.003 M). Such high selectivity of oxidation of phenol
to pꢀbenzoquinone, which is atypical of hydroxyl radꢀ
2
icals, was observed earlier upon the oxidation of pheꢀ
nol with xenon difluoride in aqueous solutions [14].
According to [14], phenol transformation is in this
case induced by xenon monoxide (XeO) formed upon
the hydrolysis of xenon difluoride.
Our preliminary kinetic data show that the maxiꢀ
mum oxidation rate of phenol in the narrow range of
variation in reagent concentration is proportional to
3''
3'
the concentrations of the substrate, catalyst
Oxone; this corresponds to the concept of the complex
formation between oxalic acid and catalyst
1, and
1.
To obtain additional data allowing conclusions on
the nature of the oxidizing particle in the investigated
system, we also studied oxidation of saturated hydrocarꢀ
0
4
8
12
, h
t
bons by the catalyst 1–oxalic acid–Oxone system. It
was found that the selectivity parameters (regioselectivꢀ
ity, bonding selectivity, and stereoselectivity) in the oxiꢀ
Accumulation of
(initial concentration, 0.1 M) with Oxone (initial concenꢀ
tration, 0.032 M) catalyzed by complex in the presence
of oxalic acid (0.05 M) at different concentrations of
pꢀquinone upon oxidation of benzene
1
1
:
dation of nꢀheptane, methycyclohexane, and cisꢀ and
–5
–5
–4
0.5
×
10
(
1
), 1.0
' is the experiment of curve
'' is the experiment of curve
but acetic acid (0.1 M) was used instead of oxalic acid. The
maximum initial reaction rate was determined from
×
10
(
2
), 1.0
×
10
(3), and 2.0 ×
transꢀ1,2ꢀdimethylcyclohexanes was considerably
higher than those found for systems that oxidize by
involving hydroxyl radicals (e.g., systems based on
vanadium complexes [15–18]). In addition, these
parameters are close to those obtained for the oxidation
4
10⎯
(
4);
3
3
, but performed in
the absence of oxalic acid;
3
3,
W
0
the slope of the tangent to this kinetic curve at the point of
the maximum rate.
with the catalyst 1–oxalic acid–hydrogen peroxide sysꢀ
tem [6–13]. It should be noted that there are differences
in the behavior of systems containing Oxone or hydroꢀ
gen peroxide. For example, the chromatography of the
reaction products before and after reduction with triphꢀ
enylphosphine (we developed this procedure in [7, 8,
10, 19, 20]) show that the oxidation of cyclohexane with
zene) in acetone were added immediately after cooling
to room temperature. All salts, including the unreꢀ
acted Oxone, were precipitated; as was shown in speꢀ
cial experiments, the concentration of the product in
the sample remained unchanged over time. The conꢀ Oxone yields only slight amounts of cyclohexylhydropꢀ
centration of p
ꢀquinone was determined by 1H NMR
spectroscopy (Bruker AVꢀ300, 300 MHz).
eroxide, while alkylhydroperoxide is the main intermeꢀ
diate product in the reaction with hydrogen peroxide at
the initial moment [6–13].
The features found for oxidation of benzene with
Oxone allow us to propose the mechanism shown in
RESULTS AND DISCUSSION
Scheme 1. In the first step (
)– transforms to catalytically active species (CAS), i.e.,
) system oxidizes benzene to quinone effiꢀ partially reduced dimeric Mn(III)Mn(IV) derivatives.
a), the catalyst 1 precursor
We found that the catalyst
Oxone (
1–oxalic acid (2
3
ciently in a homogeneous solution of aqueous acetoniꢀ Complexes of this type were suggested and recorded as
trile. The highest yield of quinone under our experiꢀ intermediates in catalytic reactions, and some of them
mental conditions was 0.25 mol/mol of Oxone. The were isolated and characterized [21–23]. The reducꢀ
turnover number per catalyst 1 reached 1150. The figꢀ tion of one of the Mn(IV) ions to Mn(III) proceeds
ure shows examples of the kinetic time curves of during the induction period under the action of oxalic
quinone formation at different concentrations of catꢀ acid, the reductive properties of which are known.
alyst 1. As can be seen, the reaction proceeds with selfꢀ Indeed, upon replacement of oxalic acid for acetic
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KINETICS AND MECHANISM OF BENZENE OXIDATION
395
2+
HO
O
MnV MnIV
KHSO4
KSO3–O–OH
O
O
(b)
2+
2+
H+
2
CO2
HO
O
(A)
(c)
C6H6
MnIII MnIV
MnIV MnIV
C6H5–OH
(a)
O
O
O
O
(d)
R–H
(1)
(CAS)
R•
OH
MnIV MnIV
2+
(e)
HO
R–OH
O
O
(B)
Scheme 1.
acid possessing no reductive properties, the induction ganese oxoperoxide complex formed in the case of
period increases drastically (curve '' in figure). Virtuꢀ hydrogen peroxide and the dimeric manganese oxohyꢀ
ally the same induction period was noted for the reacꢀ droxide complex formed in the case of Oxone (comꢀ
tion in the absence of cocatalyst ( ) (curve in figꢀ plex in the scheme).
ure). In the case of reactions depicted by curves and
'', a very slow reduction of one of the Mn(IV) ions of
3
2
3
'
А
3
'
3
ACKNOWLEDGMENTS
the dimeric complex appears to occur under the action
of hydrogen peroxide formed upon hydrolysis of
Oxone during the induction period. We showed in [6–
13] that hydrogen peroxide can reduce Mn(IV) to
Mn(III). In contrast to hydrogen peroxide, Oxone
itself has no reductive properties. The formation of a
catalytically active complex containing Mn(III) is also
possible due to the slow disproportionation of the
Mn(IV) ions in the initial dimeric Mn(IV)–Mn(IV)
complex to the Mn(III)–Mn(V) complex.
This work was financially supported by the Russian
Foundation for Basic Research, project nos. 06ꢀ03ꢀ
32344ꢀa and 12ꢀ03ꢀ00084ꢀa.
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