Oxidations by the "hydrogen peroxide-manganese(IV) complexcarboxylic acid" system. Part 4. Efficient acid-base switching between catalase and oxygenase activities of a dinuclear manganese(IV) complex in the reaction with H2O2 and an alkane

Article abstract of DOI:10.1039/b110594a

Whereas the dinuclear manganese(IV) complex [LMn(O)₃MnL](PF₆)₂ (1a, L = 1,4,7-trimethyl-1,4,7-triazacyclononane) does not react with H₂O₂ in acetonitrile solution containing cyclohexane, acetic acid added to this mixture in small amounts induces the catalytic decomposition of hydrogen peroxide to O₂ and H₂O (catalase activity) and the transformation of cyclohexane to cyclohexyl hydroperoxide (oxygenase activity). Addition to the acetic acid containing solution only 2 equivalents (relative to the Mn catalyst) of a base enhances the catalase activity and suppresses the oxygenase activity. The proposed mechanism includes the formation of dinuclear dihydroperoxy derivatives of manganese, which can be transformed under the action of acetic acid to O=Mn(V)-Mn(IV)-OOH species. The latter can abstract a hydrogen atom from an alkane. The interaction of the so-formed R^. radical with Mn(IV)-OOH can give the alkyl hydroperoxide, ROOH, which is the main primary product of the oxidation process.

Full text of DOI:10.1039/b110594a

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Oxidations by the ‘‘hydrogen peroxide–manganese(IV) complex–
carboxylic acid’’ system.
Part 4.y Efficient acid-base switching between catalase and
oxygenase activities of a dinuclear manganese(IV) complex
in the reaction with H O and an alkane
2
2
Georgiy B. Shul’pin,* Galina V. Nizova, Yuriy N. Kozlov and Irina G. Pechenkina
Semenov Institute of Chemical Physics, Russian Academy of Sciences, ul. Kosygina, dom 4,
Moscow 119991, Russia. E-mail: Shulpin@chph.ras.ru, http://members.fortunecity.com/shulpin
Received (in Montpellier, France) 19th November 2001, Accepted 22nd April 2002
First published as an Advance Article on the web 18th July 2002
Whereas the dinuclear manganese(IV) complex [LMn(O)
3
MnL](PF
6
)
triazacyclononane) does not react with H O in acetonitrile solution containing cyclohexane, acetic acid added
2
(1a, L ¼ 1,4,7-trimethyl-1,4,7-
2
2
to this mixture in small amounts induces the catalytic decomposition of hydrogen peroxide to O and H O
2
2
(
catalase activity) and the transformation of cyclohexane to cyclohexyl hydroperoxide (oxygenase activity).
Addition to the acetic acid containing solution only 2 equivalents (relative to the Mn catalyst) of a base
enhances the catalase activity and suppresses the oxygenase activity. The proposed mechanism includes the
formation of dinuclear dihydroperoxy derivatives of manganese, which can be transformed under the action of
acetic acid to O=Mn(V)–Mn(IV)–OOH species. The latter can abstract a hydrogen atom from an alkane. The
ꢀ
interaction of the so-formed R radical with Mn(IV)–OOH can give the alkyl hydroperoxide, ROOH, which is
the main primary product of the oxidation process.
stereoselectively: the reaction with decalin isomers gives (after
Introduction
treatment with triphenylphosphine) alcohols hydroxylated in
the tertiary positions, the cis/trans ratio being ꢁ2 in the case
of the oxidation of cis-decalin, and the trans/cis ratio being
Manganese-containing enzymes as well as synthetic manga-
nese-containing complexes are known to catalyse very effi-
ciently the decomposition of hydrogen peroxide. The
mechanism of H
dation of a dimanganese(II) intermediate followed by reaction
with a second hydrogen peroxide molecule, leading to O for-
1
8c,d
ꢁ30 in the case of trans-decalin.
2 2
O dismutation by Mn catalase involves oxi-
2
2
mation (Scheme 1). Added bases accelerate significantly the
H O decomposition catalysed by manganese complexes. It
3
2
2
has been demonstrated that the lack of proton donor and
acceptor groups around the catalytic centre of synthetic man-
ganese complexes may be the reason for the intense suppres-
3
a
sion of the rate compared to the natural Mn enzyme. The
photosynthetic water oxidation enzyme contains a polynuclear
manganese core in the oxygen-evolving center (OEC).
4
Complexes of certain transition metals with 1,4,7-triazacy-
clononane (TACN) and its derivatives, for example with
Here, we report the study of unusual catalase and oxygenase
activities of complex 1a. Note that enzymes called catalases
disarm hydrogen peroxide by converting it into oxygen and
water (dismutation reaction) according to:
1
,4,7-trimethyl-1,4,7-triazacyclononane (TMTACN), have
been used as models of metal-containing proteins and are
5
known to catalyse oxidations of organic compounds. Note
6
also that a dinuclear iron complex with TACN is a model
2 H2O2 ¼ 2 H2O þ O2
7
2
for O -transporting hemerythrin proteins.
Metal complexes that decompose hydrogen peroxide mimic
catalase (exhibit catalase activity). Enzymes that catalyse oxy-
gen atom incorporation into the substrate are known as oxyge-
nases. For example, methanemonoxygenase transforms
methane into methanol in the presence of a reducing agent
Recently, we discovered that the manganese(IV) complex
(TMTACN)Mn(O) Mn(TMTACN)](PF (1a) catalyses the
[
3
6 2
)
efficient oxidation of saturated hydrocarbons with hydrogen
peroxide in acetonitrile at room temperature only in the pre-
sence of a small amount of acetic acid. The reaction in the
8
9
2
AH :
beginning yields predominantly alkyl hydroperoxides. The
oxidation of alkanes containing tertiary C–H bonds proceeds
CH4 þ O2 þ AH2 ¼ CH3OH þ H2O þ A
Thus, metal complexes that take part in oxygen incorporation
catalysis exhibit oxygenase activity.
y For parts 1–3 see ref. 8(b)–(d), respectively. A preliminary communi-
8
a
cation of previous parts has been also published.
1
238
New J. Chem., 2002, 26, 1238–1245
DOI: 10.1039/b110594a
This journal is # The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2002

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Scheme 1 Key steps proposed for the hydrogen peroxide dismutation
by Mn catalase.
Experimental
ꢂ
All reactions were carried out in MeCN at 25 C in thermo-
ꢃ3
Fig. 1 Decomposition of H
2
O
2
(0.25 mol dm ) in the presence of
cyclohexane (0.46 mol dm ) and cyclohexane oxidation catalysed
stated Pyrex cylindrical vessels with vigorous stirring. The total
volume of the reaction solution was 5 mL. In a typical experi-
ment, initially, a portion of 35% aqueous solution of hydrogen
peroxide was added to the solution of the catalyst and cyclo-
hexane in acetonitrile.
The volume of dioxygen evolved was measured using a ther-
mostated burette. The reaction system was connected to a
manometric burette with water that was saturated with oxygen
prior to use. After certain time intervals, the pressure was equi-
librated using a separation funnel by adjusting the water level
to the same heights.
ꢃ3
ꢃ5
ꢃ3
by complex 1a (5 ꢅ 10 mol dm ). Kinetic curves are presented for
the O evolution (1) and for the sum of cyclohexanol and cyclohexa-
2
3
none (after PPh reduction) accumulation (2) in the absence of any
additive (curves a) and in the presence of acetic acid
ꢃ3
(
neously of acetic acid ([CH
[CH CO
3 2
H] ¼ 0.05 mol dm ) (curves b), in the presence simulta-
ꢃ3
3
CO
2
H] ¼ 0.05 mol dm
) and of
(curves c) {at
ꢃ5
n-Bu
4
NOH at various concentrations: 10 ꢅ 10
ꢃ3
0
ꢃ5
ꢃ3
[H
2
O
2
]
0
¼ 0.75 mol dm (curve c )}, 20 ꢅ 10 mol dm (curves
ꢃ5
ꢃ5
ꢃ3
d), or of TMTACN (10 ꢅ 10 mol dm ) (curves e), as well as in
ꢃ3
the presence of HClO
neously CH CO H (0.05 mol dm ) and HClO
curve g).
4
(10 ꢅ 10 mol dm ) (curves f) and simulta-
ꢃ3
ꢃ5
ꢃ3
3
2
4
(5 ꢅ 10 mol dm
)
(
In order to determine concentrations of all cyclohexane oxi-
dation products the samples of reaction solutions were ana-
lysed twice (before and after their treatment with PPh ) by
3
GC (LKhM-80-6, 2 m columns with 5% carbowax 1500 on
0
.25–0.315 mm Inerton AW-HMDS; argon carrier gas), mea-
suring concentrations of cyclohexanol and cyclohexanone.
This method (an excess of solid triphenilphosphine is added
to the samples 10–15 min before the GC analysis) described
8
–10
by us earlier
allows the detection of alkyl hydroperoxides
and also to measure the real concentrations of all three pro-
ducts (alkyl hydroperoxide, alcohol and aldehyde or ketone)
present in the reaction solution, because alkyl hydroperoxides
are usually decomposed in the gas chromatograph to produce
mainly the corresponding alcohol and ketone.
Synthesis of the complexes 1 has been described in the
literature.
5
c,11
Results
We have found that, in the absence of a carboxylic acid, com-
ꢂ
pound 1a in acetonitrile solution at 25 C does not catalyse
alkane hydroperoxidation (oxygenase activity), and catalyses
very slow the decomposition of hydrogen peroxide to molecu-
lar oxygen (catalase activity). However, addition of a small
ꢃ3
amount (only 0.02–0.05 mol dm ) of acetic acid leads to a
noticeably more rapid evolution of molecular oxygen (see
ꢃ3
Fig. 1, curve 1b and Fig. 2).
At [CH CO H] < 0.1 mol dm , added cyclohexane has
Fig. 2 Decomposition of H
2
O
2
(0.25 mol dm ) catalysed by com-
plex 1a (5 ꢅ 10 mol dm ). The dependence of the O evolution rate
on [CH CO H] in the absence (curve 1) and in the presence (curve 2) of
cyclohexane (0.46 mol dm ), as well as the analogous dependence for
the initial rate of cyclohexanol and cyclohexanone (after PPh reduc-
ꢃ3
ꢃ5
ꢃ3
3
2
2
practically no effect on the initial rate of O evolution (com-
2
pare curves 1 and 2 in Fig. 2) and this rate is higher than that
for the cyclohexane oxidation (curve 3 in Fig. 2). However, in
3
2
ꢃ3
3
tion) accumulation (curve 3) are shown. Curves of oxygenate accumu-
lation with time in the cyclohexane oxidation at various acetic acid
concentrations are shown in the inset at the top of the figure. Initial
rates of these reactions are plotted as curve 3 in the main graph.
2
the presence of cyclohexane, the rate W(O ) passes through a
maximum while W(CyH) rises monotonously; W(O ) ¼
2
ꢃ3
W(CyH) at [CH
3
CO
2
H] ꢄ 0.4 mol dm . Perchloric acid
New J. Chem., 2002, 26, 1238–1245
1239

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ꢃ3
ꢃ3
Fig. 3 Electronic spectra of compound 1a (1 ꢅ 10 mol dm ) in
CH CN in the absence of additives (curve 1), and in the presence of
HClO
) (D ¼ optical density).
3
ꢃ4
ꢃ3
ꢃ4
ꢃ3
4
: 1 ꢅ 10 mol dm (curve 2) and 7 ꢅ 10 mol dm (curve
3
accelerates neither catalase nor oxygenase reactions if acetic
acid is absent (Fig. 1, curves 1f and 2f, respectively). Even in
the presence of acetic acid, addition of a low concentration
ꢃ5
ꢃ3
of HClO
gives rise to a much lower oxygenase activity of the system
compare curves 2g and 2b in Fig. 1). The activity loss under
4
(5 ꢅ 10 mol dm ; 1 equiv. relative the catalyst)
(
the action of the very strong perchloric acid could be explained
by the transformation of complex 1a, for example, into a
mononuclear species that is not capable of catalysing both
the peroxide dismutation and the alkane oxygenation. Fig. 3
shows changes in the visible spectrum caused by addition of
low concentrations of perchloric acid. Addition of acetic acid
does not change the spectrum of the 1a solution, however, if
both acetic acid and H O are added some changes can be
2
2
noticed.
It is noteworthy that in the presence of both acetic acid (0.05
ꢃ3
ꢃ5
ꢃ
ꢃ3
4
mol dm ) and HO anion, used in the form of n-Bu NOH
(
20 ꢅ 10 mol dm , see Fig. 1, curve 1d) the W(O ) value
2
is much higher than that for reactions in the presence of acetic
acid only.
The second very interesting feature is that when only 2
ꢃ
equiv. of HO , relative to the catalyst, are added to the reac-
tion solution containing acetic acid and [H
dm the oxygenase activity of 1a is suppressed very signifi-
cantly. The yield of oxygenates from the reaction with cyclo-
hexane after 20 min is 85 times lower when only 4 equiv. of
2
O
2
]
0
< 0.3 mol
ꢃ3
Fig. 4 Plots of the cyclohexane oxygenation rate (graph A) and the
evolution rate (graph B) versus initial concentration of catalyst 1a
and versus initial concentration of cyclohexane (graph C). Conditions:
O
2
n-Bu
(
4 3 2
NOH are added to the solution containing CH CO H
compare curves 2b and 2d in Fig. 1). Thus, it can be con-
ꢃ3
ꢃ5
cyclohexane (0.46 mol dm for graphs A and B), 1a (5 ꢅ 10 mol
ꢃ3 ꢃ3
cluded that the system requires the presence of either a car-
boxylic acid or a base (free amine TMTACN has been found
to affect the reaction analogously, see Fig. 1, curve 1e) to exhi-
bit catalase activity whereas only acetic acid in relatively high
concentration induces oxygenase activity. The highest catalase
activity has been found when an acetate buffer at very low con-
centration was used. It should be noted that at relatively high
concentrations of hydrogen peroxide ([H O ] ¼ 0.75 mol
dm for graph C), H O (0.25 mol dm ), CH CO H (0.05 mol
2 2 3 2
dm ).
ꢃ3
ꢃ5
ꢃ3
4 ꢅ 10 mol dm ) a sigmoidal shape of the dependence for
the O evolution rate versus H O concentration was found
2
2
2
[Fig. 5(B)]. It should be noted that the sigmoidal shape of
the dependence of the initial cyclohexane oxygenation rate
on [H O ] was not found under the conditions used. This
2
2 0
ꢃ3
dm ) the addition of small amounts of Bu
4
NOH affects the
2
2 0
oxygenase activity less dramatically (compare curves 2b and
0
could be due to the relatively low accuracy of the ROOH ana-
lysis. Effective activation energies have been measured to be 8
2
and 14 kcal mol for oxygenation and O evolution, respec-
2
c in Fig. 1).
The rate dependences of both cyclohexane oxygenation and
ꢃ1
O
2
evolution catalysed by 1a in the presence of CH
first-order for the initial (Mn) complex [Fig. 4(A,B)] in rela-
3
CO
2
H are
tively (Fig. 6). Comparison of the initial oxygenation rates of
cyclohexane and cyclohexane-d12 allowed us to estimate the
kinetic isotope effect at 1.3. This value is only slightly higher
than that for alkane oxidation with participation of non-selec-
2
tively low concentrations and for cyclohexane [Fig. 4(C)]. It
is, however, noteworthy that for both catalase and oxygenase
routes a second-order dependence has been found for hydro-
gen peroxide [Fig. 5(A)]. When lower concentrations of the
tive hydroxyl radicals (k /kD ¼ 1.1) so such a small difference
H
requires additional investigation and explanation. It is note-
1
2a
catalyst were used in the presence of n-Bu NOH (only
4
worthy that, according to our
H D
preliminary results, k /k
1
240
New J. Chem., 2002, 26, 1238–1245

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Fig. 6 Dependence of the O
2
evolution rate W(O
2
) (graph A) and the
cyclohexane oxygenation rate W(CyH) (graph B) on temperature.
ꢃ3
ꢃ5
ꢃ3
Conditions: cyclohexane (0.46 mol dm ), 1a (5 ꢅ 10 mol dm ),
ꢃ3 ꢃ3
2 2 3 2
H O (0.25 mol dm ), CH CO H (0.05 mol dm ). Curves of dioxy-
Fig. 5 Dependence of the O
2
evolution rate (curves 1) and the cyclo-
hexane oxygenation rate (curves 2) on initial concentration of hydro-
gen evolution and oxygenate accumulation with time at various tem-
peratures are shown in the insets. Initial rates of these reactions are
plotted in the main graphs.
ꢃ3
gen peroxide. Graph A: CH
3
CO
2
H (0.05 or 0.5 mol dm ), 1a
mol dm ). Graph B: CH CO H (0.05 mol dm ), 1a
ꢃ5
ꢃ5
ꢃ3
ꢃ3
ꢃ3
(
(
2
5 ꢅ 10
3
2
ꢃ5
ꢃ3
1 ꢅ 10 mol dm ), n-Bu
4
NOH (4 ꢅ 10 mol dm ) and (for curve
ꢃ3
ꢃ3
) cyclohexane (0.46 mol dm ). On graph B bold curves for the O
2
dm after 11 min) has been found for this catalyst in the pre-
sence of CH CO H (0.05 mol dm ). This argues that a binuc-
evolution (curve 1) and for the cyclohexane oxygenation (curve 2) were
simulated using eqns. (1)–(7) and the conditions in eqns. (8)–(11).
Curves of dioxygen evolution with time in the hydrogen peroxide
decomposition at various hydrogen peroxide concentrations are shown
in the inset at the top of the figure. Initial rates of these reactions are
plotted as curve 1 in graph A.
ꢃ3
3
2
lear structure of the manganese complex is necessary for
oxygenase activity.
Discussion
attained a value of 2.8 in the cyclohexane oxidation catalysed
with the Mn(IV) complex of a polymeric ligand described in ref.
On the basis of the obtained data we propose the mechanism
shown in Scheme 2. In the first step of the process acetic acid
protonates one of the oxygen bridges between two mangane-
se(IV) centres, resulting in the formation of a vacant site at
one Mn(IV). Although acetic acid is a weak protonating agent
we believe that such a process is possible in acetonitrile solu-
tion (in aqueous or methanol solutions oxo bridges can be
5v.
The data presented in Fig. 4(C) are primordial for under-
standing the process mechanism: the linear dependence of
the ROOH formation rate on the initial cyclohexane concen-
tration up to almost 1 mol dm testifies that the oxidizing spe-
ꢃ3
3
e,13a
cies in the system under consideration is not hydroxyl radical.
protonated by strong acids;
see also a very recent paper
1
2b,c
13b
Indeed, it is known
hydroxyl radicals, the ROOH formation rate becomes practi-
that in the case of systems generating
on Mn–OH complexes ). ‘‘Formation of a weak Mn–
O(hydroxo) bond instead of a strong Mn–O(oxo) bond at
the active site of OxMnCAT will greatly reduce the relative
stability of the oxidized site and thus ensure the necessary driv-
cally independent of the hydrocarbon concentration at
ꢃ3
[
cyclohexane]
0
> 0.3 mol dm , and this is in accordance with
1
3c
a competition between cyclohexane and acetonitrile for the
reaction with the hydroxyl radical.
ing force for the peroxide oxidation reaction’’. The formed
complex then adds one hydrogen peroxide molecule, acetate
anion being the proton acceptor. The hydroperoxo derivative
3 eliminates hydroperoxyl radical to afford the catalytically
active Mn(III)Mn(IV) species, 4. For oxidations of hydrogen
peroxide by transition metal complexes see, for example, ref.
13d,e. Binuclear Mn(III)Mn(IV) derivatives with various ligands
We have also found that mononuclear complex 1b exhibits
only very low oxygenase activity (the yield of cyclohexane oxy-
ꢃ4
ꢃ3
genates was 3 ꢅ 10 mol dm after 15 min, which is 45 times
less than that in the reaction catalysed by 1a). Also negligible
catalase activity (O2 concentration was only 0.6 ꢅ 10 mol
ꢃ2
New J. Chem., 2002, 26, 1238–1245
1241

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2
Scheme 2 Mechanism proposed for hydrogen peroxide decomposition and alkane hydroperoxidation catalysed by a (Mn) complex in the pre-
sence of acetic acid.
are well known and in some cases were obtained by the reduc-
tion of Mn(IV)Mn(IV) species.
formation of the dihydroperoxo complex of Mn(III)Mn(IV), 6.
Acetate anion and acetic acid (both in low concentrations) cat-
alyse the formation and decomposition of complex 6, yielding
higher concentration, the protonation of the –OOH ligand in
V
3g,13a,f,g
Steps 4 and 5 lead to the
6 can occur and the dinuclear Mn =O derivative 8 is formed
V
(such a mononuclear Mn =O species has been proposed and
1
4
even detected in the literature ). The high-valent oxoman-
ganese species abstracts a hydrogen atom from the alkane
RH (step 9) to produce an alkyl radical, R , and the
ꢀ
H
2
O and O
2
and the derivative 11. If acetic acid is present in
2 2
Table 1 Activities of some Mn-containing systems in H O dismutation
a
System (Mn-containing catalyst)
ꢃ1
Entry
k
cat/s
Reference
5
5
4
1
2
3
4
5
6
7
8
9
Thermus thermophilus
Lactobacillus plantarum
Thermoleophilum album
Liver arginase
2.6 ꢅ 10
2a, 15a
2a, 15b
2a, 15c
2a, 15d
3f
2.0 ꢅ 10
2.6 ꢅ 10
30
3+
[Mn
4
IV
O
6
(bpea)
[Mn (salpn)O]
[(TMTACN)Mn(O)
4
]
–TMTACN
405
250
2
15e, 16
This work
16a
2
+
3
Mn(TMTACN)] –n-Bu
4
NOH
> 140
22
13
III
[Mn (2-OHsalpn)]
2
III
III
/
IV
2+
[Mn
(TMTACN)(O)
2
(OAc)(bpy)]
(m-OAc)(OAc)
16a, 17a
3d, 17b
16a, 17a
3d
10
11
12
13
[Mn (saltnO)]
2
13
5
III IV
/
[Mn
(TMTACN)(O)
2
2
]
[Mn(salen)Cl]
[Mn(TPP)Cl]
0.24
0.013
3d, 17c
a
Abbreviations: bpea ¼ N,N-bis(2-pyridylmethyl)ethylamine; salpn ¼ 1,3-bis(salicylidenamino)propane; 2-OHsalpn ¼ 1,3-bis(salicylidenea-
0
0
mino)-2-propanol; H
tetraphenylporphyrinate.
2
saltn ¼ N,N -propane-1,3-diylbis(salicylideneamine); H
2
salen ¼ N,N -ethylenebis(salicylideneamine); TPP ¼ tetraphenyl-
1
242
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Table 2 Comparison of various systems for the oxidation of organic compounds with hydrogen peroxide that are based on Mn–TACN derivatives
as catalysts
No Catalytic system
Oxidation reaction
Reference
1
2
1a and Mn complexes with related ligands
Bleaching of stains, epoxidation of 4-vinylbenzoic acid and
a
5a
ꢂ
styrene in water at pH 8–10 (40 C)
ꢂ
1b
Oxidation in water at pH 9 (25 C) of 4-vinylbenzoic acid to
the corresponding epoxide and of styrylacetic acid to a mixture
5c
b
of the epoxide, diol and a ring-closed lactone
Epoxidation of styrene and cyclohexene in acetone (0 C)
ꢂ
ꢂ
c
c
3
4
5
6
7
8
TMTACN–MnSO
4
ꢆH
2
O
A manganese faujasite containing TMTACN
5d
5d
5e
5f
Epoxidation of styrene and cyclohexene in acetone (0 C)
d
2+
ꢂ
TMTACN–Mn
TACN derivative–MnSO
Chiral TACN derivative–Mn(OAc)
Epoxidation of olefins in acetone (0 C)
Epoxidation of olefins in acetone or methanol (0 C)
ꢂ
e
4
ꢆH O
2
ꢂ
f
2
ꢆ4H
2
O
Enantioselective epoxidation of olefins in methanol (0 C)
Epoxidation of styrene and cyclohexene in acetone or
5g
5h
TACN covalently anchored on silica–MnSO
4
ꢆH
2
O
ꢂ
g
methanol (0 C)
Oxidation of benzyl alcohols to benzaldehydes in acetone at
9
1a
5i
h
room temperature
Oxidation of olefins and alkanes with H
2+
10
11
12
13
14
15
16
17
Zeolite-exchanged Mn –TMTACN
Mn
2
O
2
and t-BuOOH
Allylic oxidations by t-BuOOH–O (fluorous biphasic catalysis)
5j
R
f
TACN–[R (CH
f
2
)
2
2
CO ]
2
2
5k
5l,m
5n
5o
5p
5q
5r
1a
1a
Oxidation of phenols in water at pH 10.5
Oxidation of sulfides to sulfones by periodic acid in pyridine
ꢂ
i
TMTACN–MnSO
4
ꢆH
2
O–oxalic acid–Na oxalate
ꢆH
(L–TACN derivative)
Epoxidation of olefins in aqueous acetonitrile (5 C)
cis-Dihydroxylation and epoxidation of olefins in acetonitrile (0 C)
ꢂ
TACN derivative anchored on solid–MnSO
4
2
O
IV
[LMn (O)
IV
Mn L](PF
j
3
6
)
2
Two-phase epoxidation of styrene and dodecene
1
[L Mn
IV
(O) ](ClO
2 3
4
)
2
Oxidation of lignin models (epoxidation of double bonds and
ꢂ
k
oxidation of hydroxyl groups) in acetone–water (30–60 C)
l
III
[L*Mn (O)(AcO)
III
ꢂ
1
1
8
9
2
Mn L*](PF
ꢆ4H O–sodium ascorbate
6
)
2
Enantioselective epoxidation of olefins in acetone (ꢃ25 C)
5s
5t
TMTACN–Mn(OAc)
2
2
Epoxidation of olefins and oxidation of alcohols to ketones and
ꢂ
carboxylic acid in acetonitrile–water solution (0 C)
m
ꢂ
2
0
1
[TPTNMn
2
(O)(OAc)
2
](ClO
4
)
2
Epoxidation of olefins in acetone (0 C)
O– Epoxidation of olefins and oxidation of alcohols in
5u
5v
2
Polymer-bound TACN derivative–Mn(OAc)
oxalic acid–sodium oxalate
2
ꢆ4H
2
ꢂ
acetone–methanol–water (0 C)
Hydroperoxidation of alkanes with partial retention of
2
2
3
4
1a–carboxylic acid
8a–d
8d
ꢂ
n
configuration in acetonitrile or nitromethane (ꢃ22 to +25 C)
2
1a–carboxylic acid
1a–carboxylic acid
Epoxidation of olefins, oxidation of alcohols to ketones and
n
sulfides to sulfoxides at room temperature in acetonitrile
Oxidation of alkanes by t-BuOOH at room temperature in acetonitrile 8d,e
o
2
a
b
–substrate–catalyst ratio was 10 000:100:1; reaction time 2 h; conversion 98–99%. The H
The H
2
O
2
2
O
2
–substrate–catalyst ratio was
c
0 000:100:1; reaction time 1–3 h; conversion 69–98%. The H
1 –substrate ratio was 1:1; hydrogen peroxide was gradually added; reaction time
ca. 12 h; in methanol at 25 C almost no epoxidation. Hydrogen peroxide (2 mmol, 30%) was added gradually to the solution of olefin (1 mmol),
2
O
2
ꢂ
d
2+
TMTACN (1.5 mmol) and Mn (1.0 mmol); turnover numbers (TON) were up to 1000; in methanol, isopropanol or acetonitrile, only small
e
amounts of the epoxide were obtained; the oxidation of cyclohexane in acetone gave very low yield of cyclohexanol (TON ¼ 9). Epoxide yields
f
were up to 98%; yields in acetonitrile or THF were much lower. The epoxidation of cis-b-methylstyrene gave mainly (1R,2R)- trans-epoxide with
g
h
i
5
5% ee. TON were up to 50 per hour. TONs were up to 380. Aqueous H
mmol of MnSO O, 1.5 mmol of TMTACN, 1.5 mmol of oxalic acid, 1.5 mmol of sodium oxalate and 0.67 mmol of an olefin; reaction time 0.3–2
ꢆH
h; yields 55–99%; the epoxidation occurs if the oxalate buffer is replaced with oxalic acid or sodium oxalate; no epoxide was found if acetic acid was
2 2
O was injected over 300 s into the reaction mixture contained 1
4
2
j
k
0
added. Reaction in dichloroethane–water–methanol; reaction time 16 h. L is 1,2-bis-(4,7-dimethyl-1,4,7-triazacyclonon-1-yl)ethane; 1-(3,4-
dimethoxyphenyl)ethanol, 1-(3,4-dimethoxyphenyl)-1-propene (mixture of E and Z isomers) and E-1,2-diphenylethene were used as model com-
l
pounds; L* is enantiopure TACN derivative; the epoxidation of styrene gave after 2 h the corresponding epoxide with 24% ee at 28% conversion
m
0
0
n
2 2
of the styrene. TPTN is N,N,N ,N -tetrakis(2-pyridylmethyl)propane-1,3-diamine; the H O –substrate ratio was 8:1. TONs attain 3300; reac-
o
tion time 1–2 h; no reaction occurs in the absence of carboxylic (usually acetic) acid. In the absence of acetic acid the reaction proceeds but less
efficiently; olefins are not epoxidised under these conditions even in the presence of acetic acid.
þ
þ
Mn(IV)-hydroxy-Mn(IV)-hydroperoxy derivative, 10. Finally,
recombination of the alkyl radical R and hydroperoxy ligand
HOO occurs in the solvent cage to give the reaction product,
ðMnÞ ꢆðH2O2Þ þ H Ð ðMnÞ ꢆðH2O2Þ ꢆH
K3ðstep 6Þ
ð3Þ
2
2
2
2
ꢀ
ꢀ
alkyl hydroperoxide, ROOH. The alkyl radicals can partially
leave the solvent cages and react with molecular oxygen. The
recombination step within the solvent cage with partial leaving
of the alkyl radicals into the solution explains the partial reten-
tion of configuration in the alkane hydroperoxidation.
ðMnÞ ꢆðH2O2Þ ! ðMnÞ þ O2 þ 2H2O k1ðstep 6aÞ ð4Þ
2
2
2
þ
ðMnÞ ꢆðH2O2Þ ꢆH þ CyH ! oxygenates k2ðsteps 7 9Þ
2
2
ð5Þ
We have carried out the kinetic analysis of the scheme
described above taking [CH CO H] ¼ const. Let us consider
Assuming that steps 6a and (7 + 8) (see Scheme 2) are rate-lim-
iting steps and that the concentrations of complexes 5, 6 and 7
are at quasi-equlibrium, we obtain eqns. (6) and (7) for the
rates of the catalase and oxygenase reactions, respectively:
3
2
the following simplified kinetic scheme [eqn. (1)–(5)], where
(
Mn) is a dinuclear catalytically active species.
2
2
ðMnÞ þ H2O2 Ð ðMnÞ ꢆH2O2 K1ðstep 4Þ
ð1Þ
d½O ꢇ
k K K ½H O ꢇ ½ðMnÞ ꢇ
2
2
2
1
1
2
2
2
2 0
¼
ð6Þ
2
þ
dt
1
þ K1½H2O2ꢇ þ K1K2½H2O2ꢇ ð1 þ K3½H ꢇÞ
ðMnÞ ꢆH2O2 þ H2O2 Ð ðMnÞ ꢆðH2O2Þ K2ðstep 5Þ ð2Þ
2
2
2
New J. Chem., 2002, 26, 1238–1245
1243

View Article Online
2
þ
d½RO2Hꢇ
dt
k2K1K2K3½H2O2ꢇ ½H ꢇ½ðMnÞ ꢇ ½RHꢇ
Conclusions
2
0
¼
ð7Þ
2
þ
1
þ K1½H2O2ꢇ þ K1K2½H2O2ꢇ ð1 þ K3½H ꢇÞ
We have demonstrated that only in the presence of small
amounts of acetic acid can a dinuclear manganese complex
catalyse both hydrogen peroxide decomposition and cyclohex-
ane oxygenation by H O . The reaction pathway can be effec-
tively controlled by addition of acids or bases in very small
concentrations. The system described here mimics certain
enzymes that can be ‘‘switched on’’ or ‘‘switched off’’ in living
cells by the action of base or acid ‘‘switchers’’ such as amino
acids.
These kinetic equations adequately describe the experimentally
determined dependencies of both reaction rates on the initial
concentrations of the catalyst [Fig. 4(A,B)] and of the cyclo-
hexane [Fig. 4(C)] at a constant concentration of acetic acid.
Using eqns. (6) and (7) and accepting the following values
for the combinations of constants that are included in these
rate equations:
2
2
ðk1K1K2Þꢃ1 ¼ 0:01 mol dm
K1½H2O2ꢇ ꢈ 1
k1 ð1 þ K3½H ꢇÞ ¼ 7 ꢅ 10
2
ꢃ6
s
ð8Þ
ð9Þ
ð10Þ
ð11Þ
Acknowledgements
ꢃ1
þ
ꢃ3
s
We thank the Russian Basic Research Foundation (Project
No. 98–03–32015a) for support. The authors are grateful to
Prof. John R. Lindsay Smith for valuable discussions.
þ
ꢃ3
9
ꢃ1
k2K1K2K3½H ꢇ ¼ 13 mol dm s
we calculated theoretical curves for the dependencies of the
rates of O evolution and cyclohexane oxidation on the initial
2
hydrogen peroxide concentration (Fig. 5, curves 1 and 2,
respectively). It can be seen that although the simulated curves
correctly correspond to the main features of both processes,
there is not a very close coincidence of the experimental points
and the theoretical curves. From this we can assume that the
proposed scheme contains some simplifications and that in rea-
lity the mechanism is more complex and can involve additional
steps. Nevertheless, this mechanism is consistent with all fea-
tures found for the system. It follows from eqn. (10) that the
constant for the monomolecular decomposition of the dimeric
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