Kinetics and mechanism of (salen)MnIII-catalysed hydrogen peroxide oxidation of alkyl aryl sulphides

Article abstract of DOI:10.1002/poc.1160

The kinetics of (salen)Mn^III complexes catalysed oxidation of aryl methyl and alkyl phenyl sulphides with hydrogen peroxide have been investigated at 25 °C in 80% acetonitrile - 20% water spectrophotometrically. The reaction follows first-order

Full text of DOI:10.1002/poc.1160

JOURNAL OF PHYSICAL ORGANIC CHEMISTRY
J. Phys. Org. Chem. 2007; 20: 255–263
Published online in Wiley InterScience
(www.interscience.wiley.com) DOI: 10.1002/poc.1160
Kinetics and mechanism of (salen)MnIII–catalysed
hydrogen peroxide oxidation of alkyl aryl sulphides
1
1
2
*
Arunachalam Chellamani, Nainamohamed Ismail Alhaji and Seenivasan Rajagopal
1Department of Chemistry, Manonmaniam Sundaranar University, Tirunelveli – 627 012, India
2School of Chemistry, Madurai Kamaraj University, Madurai – 625 021, India
Received 22 November 2006; revised 9 January 2007; accepted 9 January 2007
ABSTRACT: The kinetics of (salen)MnIII complexes catalysed oxidation of aryl methyl and alkyl phenyl sulphides
with hydrogen peroxide have been investigated at 258C in 80% acetonitrile – 20% water spectrophotometrically. The
reaction follows first-order kinetics in (salen)MnIII complex and zero-order kinetics in hydrogen peroxide. The order
of the reaction with respect to sulphide is fractional and saturation in reaction rate occurs at higher sulphide
concentrations. The pseudo first-order rate constants have been analysed as per Michaelis–Menten kinetics to obtain
the values of k2, the oxidant-substrate complex decomposition rate constant, and K, the oxidant-substrate complex
formation constant. The effects of nitrogenous bases, free radical inhibitor and changes in solvent composition have
also been studied. A suitable mechanism, supported by electronic-oxidant and electronic-substrate effect studies,
involving a manganese(III)-hydroperoxide complex as reactive species has been proposed. Copyright # 2007 John
Wiley & Sons, Ltd.
KEYWORDS: (salen)MnIII complexes; hydrogen peroxide; organic sulphides; catalysed oxidation; mechanism
INTRODUCTION
are known29 to produce different oxidising species.
Although the Jacobsen-type (salen)MnIII complexes have
been used as catalysts for the H2O2 oxidation of
sulphides, no detailed kinetic study has so far been
attempted. Also, it will be interesting to know whether the
(salen)MnIII complexes-catalysed oxidation of sulphides
by PhIO and H2O2 follows similar type of mechanism or
not. Herein we report the results obtained from a detailed
kinetic study on the oxidation of several alkyl aryl
sulphides with hydrogen peroxide catalysed by (sale-
n)MnIII complexes (1a–f).
Reactions of oxygen atom transfer from H2O2 and alkyl
hydroperoxides are usually very slow1 and such reactions
are reported to be catalysed by an acid,2 a solid support3
and transition metal complexes.4–8 In recent years, studies
of metal complexes-catalysed oxygenations of alkanes,
alkenes and sulphides by dioxygen and/or peroxides are
being given considerable attention9–17 in view of their
relevance in understanding the mechanism of biological
processes.18 There are a number of reports on the
peroxide oxidation of organic sulphides catalysed by
metals and metal complexes.12,14,19 Among the various
peroxides employed in these studies, H2O2 is an attractive
oxidant since it is less expensive, environmentally benign
and gives only water as a by-product. We have recently
investigated20–28 the (salen)MIII complexes (salen ¼
N,N’-ethylenebis(salicylideneaminato) and M ¼ Mn,
Cr, Fe or Ru) mediated oxygen atom transfer from PhIO
and NaOCl to organic sulphur compounds. Compared to
PhIO, H2O2 has been reported to afford higher yields of
sulphoxides and minimal over oxidation to sulfone.17
Moreover, with the same metal complex, PhIO and H2O2
4
−
3
PF
+
6
5
O
N
O
2
Mn
6
1
N
7
8
1
a: Unsubstituted
b: 5,5'-(OCH )
3 2
c: 5,5'-Cl
2
d: 5,5'-(NO )
2 2
e: 7,7'-(CH )
3 2
*Correspondence to: A. Chellamani, Department of Chemistry,
Manonmaniam Sundaranar University, Tirunelveli – 627 012, India.
E-mail: achellamani@yahoo.co.in
f: 7,7'-(C H )
6
5 2
Copyright # 2007 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2007; 20: 255–263

256
A. CHELLAMANI, N. M. I. ALHAJI AND S. RAJAGOPAL
EXPERIMENTAL
sulfone, which established a 1:1 stoichiometry between
MPS and H2O2 as represented by Eqn (1).
Materials
ðsalenÞMnIII
H2O2 þ PhSMe ꢂ! PhSOMe þ H2O
ð1Þ
All the sulphides were prepared by known methods20 and
were purified by distillation under reduced pressure or
recrystallisation from suitable solvents. Their purity was
checked by comparing the boiling or melting points with
the literature values. Further, the sulphides showed no
impurity peaks in 1H NMR spectra, and the HPLC
analyses proved the presence of a single entity in each
sulphide. (Salen)MnIII complexes 1a–f were prepared
by the reported method.10a,20 Acetonitrile (GR, E.Merck
India) was first refluxed over P2O5 for 5 h and then distilled.
EPR spectra were recorded with a JEOL JES-TE 100 X-
band EPR spectrometer in CH3CN.
Product analyses were done using a Shimadzu LC-8A
modular HPLC system (reverse phase column (ODS),
UV-detector at 258 nm) using 70% methanol as the mobile
phase. A mixture of 1a (0.0006 M), MPS (0.08 M) and H2O2
(0.006 M) in 80% acetonitrile – 20% water was allowed to
stand overnight. The reaction mixture was extracted with
chloroform and dried over anhydrous Na2SO4. The solvent
was removed under reduced pressure. GC and TLC analyses
of the residue showed the product to be methyl phenyl
sulphoxide (ꢁ90%) with negligible amount of sulfone.
After the organic product was chloroform extracted the
left behind brown residue gave an absorption spectrum
which resembled that of (salen)MnIII complex. The
productanalyses carried out with other complexes
and other sulphides also showed the formation of corres-
ponding sulphoxide as the product. The percentage yields of
sulphoxides determined from HPLC analyses ranged
between 75 and 95 depending on the complex and the
sulphide employed.
Kinetic measurements
The kinetic runs of the oxidation of aryl methyl and alkyl
phenyl sulphides with hydrogen peroxide in the presence
of 1a–f were conducted in 80% acetonitrile-20% water at
25 ꢀ 0.1 8C under pseudo first-order conditions using 10-
to 100-fold excess of sulphide over H2O2. Reaction
mixtures for the kinetic runs were prepared by mixing the
standard solutions of (salen)MnIII, sulphide and H2O2 in
varying volumes so that in each run the total volume did
not exceed 5.0 ml. The reaction mixture was shaken well
and quickly transferred to the 1 cm quartz cuvette. The
progress of the reaction was monitored by following the
changes in absorbance values at 400 nm in a Perki-
n–Elmer UV–Visible spectrophotometer (Lambda 3B)
fitted with thermostated cell compartments.
RESULTS AND DISCUSSION
Dismutation of hydrogen peroxide by
(salen)MnIII complexes
The (salen)MnIII complexes 1a–f show catalase-like
activity with hydrogen peroxide. When an aqueous
solution of H2O2 is added to a clear brown solution of 1a
(lmax ꢁ350 nm) at 258 C, the solution gets darkened
associated with a brisk evolution of dioxygen as
represented in Eqn (2).
The pseudo first-order rate constants (kobs) for the initial
15–20% of the reaction were estimated from the slopes of
linear least square plots of ln (At–A ) versus time, where At
1
1
H2O2 ! 2H2O þ O2
ð2Þ
is the absorbance at time t and A is the experimentally
determined infinity point. The precision of kobs values is
given in terms of 95% confidence limit of Student’s t-test.30
The oxidant-substrate complex decomposition rate constant
(k2) and the complex formation constant (K) were then
obtained from the slope and intercept of double reciprocal
plot of kobs versus [sulphide]o.
Similar catalase-like activity has been reported for mono-
and di-nuclear manganese,13,31,32 iron11,12 and cobalt17b
complexes in acetonitrile. The oxygen evolution observed
here stops after sometime and restarts on addition of a fresh
quantity of H2O2, thereby establishing the catalyst-like
nature of the complex. However, with subsequent additions
of H2O2, the oxygen evolution becomes less and less vigour
and after 10th or so addition the complex gets decomposed
to unknown products as indicated by the bleaching
of the complex solution. The dismutation of H2O2 is
accelerated by the addition of nitrogenous bases such as 1-
methylimidazole, 2-methylimidazole or pyridine as shown
by the more vigorous evolution of oxygen. This observation
is consistent with previous reports that nitrogenous bases
stimulate catalase-like activities of iron and manganese
complexes by binding to the metal centre.33 On the other
hand, addition of MPS greatly diminishes H2O2 dismuta-
tion and oxygen evolution.
Stoichiometry and product analysis
To a solution of 1a (0.0004 M) and methyl phenyl sulphide
(MPS) (0.01 M) in acetonitrile was added an aqueous
solution of H2O2 (0.0004 M) and the composition of
acetonitrile and water in the solvent mixture was maintained
at 80:20 by adding the solvents in the required proportions at
258C under nitrogen atmosphere. The reaction gave methyl
phenyl sulphoxide in ꢁ90% yield with negligible amount of
Copyright # 2007 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2007; 20: 255–263
DOI: 10.1002/poc

(SALEN)MnIII–CATALYSED OXIDATION OF ALKYL ARYL SULPHIDES
257
Figure 1. Spectral changes occurred during the mixing of H2O2 with an acetonitrile solution of 1a at 58 C with time interval of
2 min
Under the present experimental conditions employed
(i.e. lower complex concentration and [complex]/[H2O2]
ꢃ1:30), the dismutation of hydrogen peroxide is
insignificant as evidenced by the iodometric estimation
of residual hydrogen peroxide.
MPS (0.15 M) is added to this mixture, the spectral
analysis shows the reverse changes (Fig. 2) that is, the
species gets slowly decayed to finally give the original
spectrum of 1a. From these spectral observations it is
clear that mixing of solutions of 1a and H2O2 produces an
intermediate species which is responsible for the
oxidation of sulphide.
Preliminary spectral studies
Kinetics of oxidation of aryl methyl sulphides
Addition of an aqueous solution of hydrogen peroxide
(0.004 M) to a clear brown solution of 1a (0.0004 M) at
258 C causes an instantaneous darkening in the brown
colour of the solution. But, the darkening is not as much
as intense as observed20 when PhIO is stirred with the
complex. The UV–Vis spectral analysis of the dark
solution shows an absorption band with lmax ꢁ420 nm
indicating the formation of an intermediate complex. The
increase in concentration of this species with time is
evident from the spectra recorded at 58C (Fig. 1). When
The kinetics of oxidation of aryl methyl sulphides
with H2O2 in presence of 1a–f was studied spectro-
photometrically in 80% acetonitrile – 20% water at 258C
by monitoring the reaction at 400 nm. The pseudo first-
order rate constants (kobs) obtained for the oxidation of
MPS are listed in Table 1. Comparison of rate constants
for the uncatalysed and catalysed oxidation of MPS (first
two entries) indicates that the presence of 1a accelerates
the reaction by a factor of ꢁ45 times and hence the
contribution of uncatalysed reaction between MPS
and H2O2 can be neglected. Furthermore, if sulphide is
added sometime after mixing H2O2 and complex, the rate
constants are lower than those obtained when sulphide is
added immediately by only 5–10% depending on the
length of the interval period. Therefore, the decay of the
complex in the absence of sulphide seems not to be
substantial.
Inspection of rate data in Table 1 reveals that the rates
are almost independent of initial complex concentration.
This, coupled with the excellent linearity (r > 0.995)
observed in ln (At–A ) versus time plots ensures the order
1
of the reaction with respect to complex is one. Also, the
constant kobs values at different initial concentration
of H2O2 show that the reaction rate is independent of
[H2O2]o. The interesting feature of the present system is
the fractional-order dependence of the reaction on the
Figure 2. The absorption spectra showing the decay of the
species formed from a mixture of 1a and H2O2 in the
presence of 0.15 M of MPS at 258 C in 80% acetonitrile-20%
water
Copyright # 2007 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2007; 20: 255–263
DOI: 10.1002/poc

258
A. CHELLAMANI, N. M. I. ALHAJI AND S. RAJAGOPAL
Table 1. Pseudo first-order rate constants for the oxidation
of MPS with H2O2 catalysed by 1a–f at 258 C in 80%
acetonitrile-20% watera
The double reciprocal plots of kobs versus [sulphide]o
(Lineweaver–Burk plots) are excellently linear (Fig. 3,
r > 0.991) thereby indicating that the reaction follows
Michaelis–Menten type kinetics involving an intermedi-
ate complex formation between the oxidant and the
sulphide.
102[MPS] (M) 103[H2O2] (M) 104[1]o (M) 104kobs (sꢂ1
)
1a
15.0
15.0
8.0
8.0
8.0
8.0
8.0
8.0
8.0
8.0
8.0
4.0
8.0
15.0
30.0
40.0
10.0
10.0
6.0
6.0
6.0
6.0
6.0
2.0
3.0
4.0
5.0
4.0
4.0
4.0
4.0
4.0
—
10.0
2.0
3.0
4.0
5.0
6.0
2.0
2.0
2.0
2.0
4.0
4.0
4.0
4.0
4.0
0.15 ꢀ 0.02b
Addition of a small amount of 2-methylimidazole, as
low as 0.0004 M, to a solution containing 1a and H2O2 in
a 1:10 ratio, induced the dismutation of H2O2. Other
nitrogenous bases, such as 1-methylimidazole and
pyridine also showed a similar effect. Spectral analysis
of the solution showed similar changes as depicted in
Fig. 1, but with a much faster rate. When the experiment
was repeated in the presence of MPS (0.15 M), the
increasing trend of the absorbance continued, implying
that the oxidation of MPS did not occur. These results
suggest that the nitrogenous bases bind to the manganese
centre irreversibly. It has been reported12,13,32 that bases
such as imidazole stimulate catalase-like activity of iron
and manganese complexes through binding to the metal
centre. The fact that the oxidation of sulphide does not
occur in the presence of 2-methylimidzole implies that
the nitrogenous base has greater binding affinity for metal
centre than MPS, thereby indicating that the coordination
of sulphide to the metal centre is a required condition for
oxidation to occur. The reaction rate for the 1a catalysed
oxidation of MPS with H2O2 is almost unaffected
(Table 3) by the presence of N-phenyl-1-anthranilic acid,
a free-radical inhibitor. The retardation of reaction rate by
this inhibitor in the metal-cyclam complex catalysed
epoxidation34 and by ionol in the (salen)CoII catalysed
epoxidation29b with TBHP have been taken in favour of
free-radical oxidising species. Consequently, it may be
inferred that free radicals are not involved in the
rate-controlling step of the present reaction. The rate
of the reaction increases with increase in water content in
the solvent mixture (Table 3). A plot of log kobs versus 1/e
is linear35 (r ¼ 0.994). The observed effect is similar to
that reported in the oxidation of organic sulphides
by H2O2 in the presence of a diiron complex.12
6.87 ꢀ 0.76b
2.07 ꢀ 0.04
2.12 ꢀ 0.04
2.10 ꢀ 0.12
2.14 ꢀ 0.13
2.08 ꢀ 0.15
1.91 ꢀ 0.11
1.89 ꢀ 0.08
2.01 ꢀ 0.13
2.09 ꢀ 0.14
1.48 ꢀ 0.12
1.95 ꢀ 0.16
2.48 ꢀ 0.26
3.08 ꢀ 0.27
3.29 ꢀ 0.34
1b
1c
1d
1e
1f
4.0
8.0
15.0
30.0
40.0
4.0
4.0
4.0
4.0
4.0
4.0
4.0
4.0
4.0
4.0
1.23 ꢀ 0.11
1.63 ꢀ 0.17
1.99 ꢀ 0.15
2.50 ꢀ 0.20
2.63 ꢀ 0.22
4.0
8.0
15.0
30.0
40.0
4.0
4.0
4.0
4.0
4.0
4.0
4.0
4.0
4.0
4.0
2.31 ꢀ 0.22
3.14 ꢀ 0.28
3.89 ꢀ 0.34
4.65 ꢀ 0.45
5.03 ꢀ 0.41
4.0
8.0
15.0
30.0
40.0
4.0
4.0
4.0
4.0
4.0
4.0
4.0
4.0
4.0
4.0
5.84 ꢀ 0.37
7.36 ꢀ 0.64
9.55 ꢀ 0.78
11.6 ꢀ 1.0
12.2 ꢀ 1.1
4.0
8.0
15.0
30.0
40.0
4.0
4.0
4.0
4.0
4.0
4.0
4.0
4.0
4.0
4.0
1.14 ꢀ 0.12
1.56 ꢀ 0.13
2.01 ꢀ 0.17
2.46 ꢀ 0.18
2.60 ꢀ 0.25
4.0
8.0
15.0
30.0
40.0
4.0
4.0
4.0
4.0
4.0
4.0
4.0
4.0
4.0
4.0
0.85 ꢀ 0.04
1.17 ꢀ 0.09
1.64 ꢀ 0.12
2.00 ꢀ 0.15
2.09 ꢀ 0.13
Mechanism
In previous reports of metal complexes catalysed
oxidation studies with peroxides, metal-oxo,10b metal-
peroxo9,36 and in some cases metal-independent peroxo
radicals11,17,29a have been discussed as reactive species.
The non-involvement of metal-independent peroxo
reactive species in the present reaction is strongly
indicated by the absence of any effect of N-phenyl-
1-anthranilic acid. Furthermore, manganese complexes,
among a set of metal complexes containing the same
ligand, behave peculiarly by not preferring the meta-
l-independent reactive species in oxidation reactions
catalysed by them.34,37 The fact that the oxidising species
in the present study is a metal-derived one is also
a The kobs values were determined by a spectrophotometric technique
following the absorbance changes at 400 nm over 15–20% of reaction;
the error quoted in kobs values is 95% confidence limit of Student’s t-test.
b Estimated iodometrically by following the unreacted H2O2.
initial concentration of MPS. The rate constants obtained
for the oxidation of MPS catalysed by 1a–f do not
increase linearly with increasing concentration of
sulphide (Table 1). It may be seen from Table 2 that
this is the case with other aryl methyl sulphides also. The
fractional-order dependence on [sulphide]o is further
evidenced from the fractional slopes (Table 2) shown by
the double logarithmic plots between kobs and [sulphide]o.
Copyright # 2007 John Wiley & Sons, Ltd.
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DOI: 10.1002/poc

(SALEN)MnIII–CATALYSED OXIDATION OF ALKYL ARYL SULPHIDES
259
Table 2. Pseudo first-order rate constants for the oxidation of p-XC6H4SMe with H2O2 catalysed by 1a at 258 C in 80%
acetonitrile-20% watera
104kobs (sꢂ1
)
[sulphide]o (M)
0.15
X
0.04
0.08
0.30
0.40
Order in sulphideb
OMe
Me
H
F
Cl
Br
Ac
NO2
2.99 ꢀ 0.28
1.95 ꢀ 0.17
1.48 ꢀ 0.12
1.41 ꢀ 0.14
1.03 ꢀ 0.06
1.01 ꢀ 0.07
0.59 ꢀ 0.06
0.34 ꢀ 0.02
3.91 ꢀ 0.28
2.62 ꢀ 0.21
1.95 ꢀ 0.16
1.79 ꢀ 0.14
1.38 ꢀ 0.11
1.34 ꢀ 0.09
0.77 ꢀ 0.06
0.45 ꢀ 0.05
4.75 ꢀ 0.42
3.15 ꢀ 0.30
2.48 ꢀ 0.26
2.29 ꢀ 0.18
1.75 ꢀ 0.11
1.73 ꢀ 0.13
0.98 ꢀ 0.08
0.55 ꢀ 0.03
6.00 ꢀ 0.54
3.95 ꢀ 0.33
3.08 ꢀ 0.27
2.80 ꢀ 0.23
2.15 ꢀ 0.19
2.17 ꢀ 0.18
1.19 ꢀ 0.12
0.68 ꢀ 0.06
6.51 ꢀ 0.61
4.12 ꢀ 0.35
3.29 ꢀ 0.34
2.95 ꢀ 0.31
2.25 ꢀ 0.19
2.26 ꢀ 0.21
1.25 ꢀ 0.11
0.72 ꢀ 0.06
0.34
0.33
0.35
0.33
0.34
0.36
0.33
0.33
a General conditions: [1a]o ¼ 0.0004 M; [H2O2]o ¼ 0.004 M.
b Slope values of double logarithmic plots of kobs versus [sulphide]o.
Figure 3. Lineweaver–Burk plots for the reactions involving (a) p-NO2C6H4SMe and 1a, (b) MPS and 1b, (c) MPS and 1a,
(d) p-OMeC6H4SMe and 1a, (e) MPS and 1d with H2O2
established by the changes in the reaction rates with
respect to changes in electronic and steric environment of
the (salen)MnIII complexes (vide infra).
excluded because the salen ligand cannot adopt the
cis-b-binding mode and also the steric demands of the
salen ligand are too great.38 One electron oxidised
metal-peroxo complexes have also been shown as reactive
The development of an absorption band with lmax
ꢁ420 nm on mixing H2O2 with an acetonitrile solution of
(salen)MnIII complexes and the subsequent decay of this
band on adding MPS (Figs 1 and 2) indicate that this new
species may be the active species responsible for the
oxidation of sulphides. The absence of a spectral band at
lmax ꢁ530 nm, which has been reported to be formed in
the (salen)MnIII/PhIO and (salen)MnIII/NaOCl sys-
tems,20,25 clearly shows that the oxidising species here
is not an oxo(salen)manganese(V) species. An oxidi-
sed dimeric intermediate, [(salpn)MnIVO]2 has been
proposed38 as the reactive species for the [(salpn)MnIII(a-
cac)] catalysed epoxidation with H2O2. But this type of
species, [(salen)MnIVO]2 in the present system may be
Table 3. Effects of N-phenyl-1-anthranilic acid and solvent
composition in the 1a catalysed oxidation of MPS by H2O2 at
258 Ca
103[Inhibitor]o (M) 104kobs (sꢂ1
)
% Waterb 104kobs (sꢂ1
)
0.0
2.0
4.0
8.0
2.48 ꢀ 0.26 10
2.40 ꢀ 0.17 20
2.38 ꢀ 0.11 30
2.32 ꢀ 0.15 40
50
1.67 ꢀ 0.09
2.48 ꢀ 0.26
3.69 ꢀ 0.28
5.61 ꢀ 0.36
7.85 ꢀ 0.55
a General conditions: [1a]o ¼ 0.0004 M; [H2O2]o ¼ 0.004 M; [MPS]o ¼
0.15 M.
b Rest was acetonitrile.
Copyright # 2007 John Wiley & Sons, Ltd.
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DOI: 10.1002/poc

260
A. CHELLAMANI, N. M. I. ALHAJI AND S. RAJAGOPAL
intermediates in some manganese11,34,39 and cobalt5,40
complexes catalysed oxidation reactions using oxygen
and/or peroxide. The formation of such a one-electron
oxidised metal-peroxo complex, [(salen)MnIVOOH]þ, in
the present study would result in the generation of
free radicals which would complicate the reaction.
But from the observed neat kinetics and the absence of
free radicals, it can be assumed that the MnIV-peroxo
complex is not the reactive species in the present
system. Also, the one-electron oxidised metal-peroxo
species have been postulated mostly in reactions
involving alkyl hydroperoxides39,40 or oxygen,34 while
in reactions involving H2O2,9,10c,41 HOO—coordinated
complexes are shown as reactive species. On the
basis of foregoing arguments, it may be said that the
active species responsible for the oxidation of sulphi-
des in the present study is presumably a MnIII-hydroper-
oxide, [(salen)MnIIIOOH], the formation of which can be
shown as,
Sawyer and coworkers.44,45 They have also proposed
that when the reaction is carried out in an oxygen
atmosphere or in the presence of excess ROOH
(R ¼ But or H), the reactive intermediates, [(sale-
n)MnIII(OOBut)(HOAc)] produce an oxygen adduct,
[(salen)MnIV(OOBut)(O2)(HOAc)] which oxidises
cyclohexane to cyclohexanone. Participation of O2
derived either from the dismutation of H2O2 or from the
oxygen atmosphere in the reaction process has been
suggested in some other studies also.11,46 Involvement
of such oxygen adduct as reactive species in the present
system can be ruled out on the facts that (a) under the
present experimental combinations of complex
and H2O2 (between 1:10 and 1:30) the catalase activity
and hence the generation of O2 in solution was
negligible, (b) the reaction was not carried out under
oxygen atmosphere and (c) sulphoxide formation was
noted even when the reaction was conducted in an inert
(nitrogen) atmosphere. The observation that the
evolution of dioxygen is greatly diminished in the
presence of sulphide suggests that the reactive
species C1 is responsible for the catalase activity also.
In the presence of excess H2O2 and in the absence of
sulphide, the access of a second molecule of H2O2 to
the (salen)MnIIIOOH species leads to catalase activity
as represented in Eqn (4).
The intermediate, [(salen)MnV(OH)(OOH)]þ, is
formed only in the presence of excess H2O2 and
intermediacy of such species has been proposed in the
reactions45 of hydroperoxides with (salen)MnIII(OAc)
and (PA)2FeII (PAH ¼ picolinic acid) to explain the
observed catalase activity. In the presence of sulphide,
there arises a competition between H2O2 and sulphide
for the active species (C1) and hence the catalase activity
is diminished. Similar explanation has been offered for
the dismutation of H2O2 and other alkyl hydroperoxides
by Fe(II)14b,,47 and Mn(III)11,31 complexes. Thus we can
conclude that this metal-based oxidant (C1) is responsible
for sulphide oxidation at lower H2O2 concentrations.
Based on the observed experimental features, the
following mechanism (Scheme 1) is proposed for
½ðsalenÞMnIIIꢄþ þ H2O2 þ H2O
! ½ðsalenÞMnIIIOOHꢄ þ H3Oþ
(3)
ðC1Þ
EPR analysis of the mixture of H2O2 and 1a at 58 C
gave no signal as expected for a low-spin MnIII-OOH
intermediate. It may be pointed out here that intermediacy
of similar MnIIIOOH42 and MnIII-peracid43 complexes as
reactive species has been postulated in the (TPP)MnIIICl
catalysed epoxidation of olefins with H2O2 and peracid,
respectively. Moreover, an EPR active mononuclear
low-spin FeIII-OOH complex has been characterized as
reactive intermediate in the oxidation of alkyl phenyl
sulphides by H2O2 catalysed by a diiron complex.12
The reactivity of MnIII-OOH intermediate (C1)
seems to be dependent on the complex/H2O2 ratio,
that is, when the ratio was maintained between 1:10 and
1:30 smooth kinetics was obtained; at ratios less than
1:10 there was no reaction, while at ratios higher than
1:30 dismutation of H2O2 occurred to evolve O2. These
observations are in consistent with those made by
Scheme 1
Copyright # 2007 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2007; 20: 255–263
DOI: 10.1002/poc

(SALEN)MnIII–CATALYSED OXIDATION OF ALKYL ARYL SULPHIDES
Table 4. Values of k2 and
261
the oxidation of sulphides by hydrogen peroxide in the
K
for the oxidation of
presence of (salen)MnIII complexes.
p-XC6H4SMe with H2O2 catalysed by 1a–f at 258 C in
80% acetonitrile-20% water
The proposed mechanism involves the reversible
formation of a ternary complex (C2) in which both
peroxide and the sulphide are bound to the complex. Then
the monooxidation of sulphide occurs by a nucleophilic
attack of the sulphide to the peroxide followed
by H2O release, within the manganese peroxide sulphide
complex.
S.No.
Complex
X
104k2,a (sꢂ1
)
K,a (Mꢂ1
)
1
2
3
4
5
6
7
8
9
10
11
12
13
1a
1a
1a
1a
1a
1a
1a
1a
1b
1c
1d
1e
1f
OMe
Me
H
6.86
4.49
3.57
3.17
2.51
2.50
1.36
0.78
2.85
5.49
13.2
2.92
2.45
18.6
18.7
17.1
19.1
16.7
16.3
18.4
18.9
18.5
17.8
19.1
15.7
12.9
F
Cl
Br
Ac
NO2
H
H
H
H
H
Based on kinetic observations and the mechanism
proposed, the following rate expression can be derived.
d½C1ꢄo k2K½C1ꢄo½R2Sꢄo
ꢂ
¼
(8)
dt
1 þ k1½R2Sꢄo
where K, equal to k1/k-1, is known as oxidant-substrate
complex formation constant and gives a measure of the
binding affinity of the substrate to the oxidant and k2
is the complex decomposition rate constant. Such
Michaelis–Menten kinetics have been observed in the
(TPP)MnIIICl and (TMP)MnIIICl complexes catalysed
LiOCl epoxidation of olefins48 and pyridinium chlor-
ochromate,49 bis(2,2’-bipyridyl)copper(II) permanga-
a Calculated from the slope and intercept of Lineweaver–Burk plots.
phides with H2O2 in the presence of 1a are included in
Table 2. Electron-donating substituents accelerate the
rate, while electron-withdrawing substituents retard it.
Using these rate constant values, the values of K and k2
have been estimated for each sulphide from Line-
weaver–Burk plots. An analysis of K and k2 values listed
in Table 4 suggests that though k2 is highly sensitive to the
nature of the substituents, K remains almost constant,
thereby establishing that the decomposition of the ternary
complex is the rate-determining step. The Hammett
correlation of log k2 with sp values shows excellent
linearity (r ¼ 0.994) yielding a r value of ꢂ 0.85 ꢀ 0.09
(Fig. 4). The correlation of log k2 with sþ/sꢂ
(slope ¼ ꢂ0.51 ꢀ 0.08, r ¼ 0.983) is not better than the
sp correlation. The negative r value indicates that the
sulphur atom of the sulphide is more positively charged in
the transition state than it is in the reactant. The lower r
value ꢂ0.85 observed in this reaction may be due to the
weaker electrophilic character of MnIII-OOH than the
oxo(salen)manganese(V) complex,20 which gave a r
nate50 and diiron complex catalysed H2O2 oxidations
12
of organic sulphides. As H2O2 is present always in excess
over 1, the conversion of 1 to C1can be considered
quantitative on the facts that (a) the absorption changes
observed during the mixing of H2O2 and 1 was
instantaneous and no further increase in absorbance with
time was noted at 258 C, (b) the initial absorbance of C1
remained constant at varying [H2O2]o, and (c) kobs is
independent of H2O2 concentration (cf. Table 1).
Thus, Eqn (8) can be written as
d½1ꢄo k2K½1ꢄo½R2Sꢄ
ꢂ
¼
(9)
dt
1 þ K½R2Sꢄ o
o
As [R2S]o >> [1]o
k2K½R2Sꢄo
kobs
¼
(10)
(11)
1 þ K½R2Sꢄo
1
1
1
1
Hence;
¼
þ
kobs k2K ½R2Sꢄo k2
Equation (11) suggests that a plot of 1/kobsversus 1/
[R2S]o should be a straight line with definite intercept on
the ordinate. The excellent linearity observed in the
Lineweaver–Burk plots in the present study supports the
given mechanism. Also the proposed mechanism
(Scheme 1) gets further support from substituent and
steric effect studies carried out with alkyl aryl sulphides
and substituted (salen)MnIII complexes.
Substitutent-effect studies
Figure 4. Hammett plot for the 1a catalysed oxidation of
aryl methyl sulphides with H2O2. The points are referred to by
the same numbers as in Table 4
The pseudo first-order rate constants obtained for the
oxidation of several p-substituted phenyl methyl sul-
Copyright # 2007 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2007; 20: 255–263
DOI: 10.1002/poc

262
A. CHELLAMANI, N. M. I. ALHAJI AND S. RAJAGOPAL
Steric effect studies
The oxidation of alkyl phenyl sulphides (C6H5SR’;
R’ ¼ Me, Et, Prn, Pri and But) with H2O2 in presence of 1a
was studied by measuring kobs at different initial
concentrations of each sulphide. An analysis of the data
given in Table 5 points out that the reactivity decreases in
the order of increasing bulkiness of R’. The double
logarithmic plots of kobs versus [PhSR’] show that the
order of the reaction increases from 0.35 for MPS to 0.63
for PhSBut. Here it may be pointed out that in the
oxidation of sulphides with H2O2 catalysed by TiO(a-
cac)2,52 a fractional order in MPS and a first-order in
PhSBut has been found and this observation has been
attributed to the fact that the binding of sulphide with
metal complex becomes difficult due to bulkiness of the
t-butyl group. The steric effect exerted by these sulphides
is also reflected in the magnitude of K values (Table 5),
which decreases from MPS to PhSBut. Thus, it is clear
that the binding affinity of these sulphides decreases with
increase in the bulkiness of R’. Finally, the correlation of
log(k/kMe) versus Es, as per Taft’s linear free energy-steric
energy relationship53 shows an excellent linearity
(r ¼ 0.994) yielding d value of 0.34 ꢀ 0.07. All these
facts indicate that the reaction is sensitive to steric
crowding at the reaction centre.
Substituents at the 7 and 8 positions of salen ligand in
metal-salen complexes are reported to modulate signifi-
cantly the steric environment of the complexes.51 The kobs
values measured at different initial concentrations of MPS
for the complexes 1a, 1e and 1f show that the presence of
methyl or phenyl group at 7 and 7’ positions of salen
ligand significantly reduces the rate of the reaction. The
values of k2 and K were also estimated and are included in
Table 4. The lower values of K for 1e and 1f than 1a
implies that the association between the complex and
sulphide is sensitive to steric environment of the
complexes.
Figure 5. Hammett plot for the oxidation of MPS with H2O2
catalysed by 1a–d
value of ꢂ1.85 in the oxidation of aryl methyl sulphides
with (salen)MnIII-PhIO system. Also, this r value is
comparable to that of ꢂ0.55 obtained in the diiron
complex catalysed oxidation of sulphides with H2O2,
where intermediacy of a similar peroxide comp-
lex, FeIII-OOH has been envisaged.12
The substituents present on 5 and 5’-positions of salen
ligand greatly influence the electronic nature of (sale-
n)MnIII complexes and hence the rate of reactions they are
catalysing.20,51 Using the pseudo first-order rate constants
obtained at different initial concentrations of MPS in
presence of each of the complexes 1a–d, K and k2 values
for each complex were estimated from Lineweaver–Burk
plots and they are collected in Table 4. The Hammett
correlation of log k2 with 2sp shows an excellent linearity
with a r value of 0.33 ꢀ 0.11 (Fig. 5, r ¼ 0.994). The
positive r value indicates a build-up of negative charge on
metal centre in the transition state of the rate-determining
step. This r value is lower than that of 0.48 obtained in the
(salen)MnIII-PhIO system,20 which indicates a lower
charge transfer in the rate-determining step.
Table 5. Pseudo first-order rate constants for the oxidation of alkyl phenyl sulphides with H2O2 catalysed by 1a at 258 C in 80%
acetonitrile-20% watera
104kobs (sꢂ1
)
[PhSR’] (M)
R’ ¼ Me
Et
Prn
Pri
But
0.04
0.08
0.15
0.30
1.48 ꢀ 0.12
1.95 ꢀ 0.16
2.48 ꢀ 0.19
3.08 ꢀ 0.27
3.29 ꢀ 0.34
0.35
1.19 ꢀ 0.09
1.62 ꢀ 0.11
2.11 ꢀ 0.16
2.62 ꢀ 0.22
2.87 ꢀ 0.27
0.38
0.73 ꢀ 0.08
1.05 ꢀ 0.07
1.50 ꢀ 0.10
1.79 ꢀ 0.13
2.07 ꢀ 0.17
0.44
0.57 ꢀ 0.05
0.78 ꢀ 0.06
1.19 ꢀ 0.09
1.53 ꢀ 0.14
1.89 ꢀ 0.18
0.52
0.14 ꢀ 0.01
0.26 ꢀ 0.02
0.41 ꢀ 0.03
0.53 ꢀ 0.05
0.62 ꢀ 0.05
0.63
0.40
Order inꢂs1ulphideb
104k2,c s
K,c Mꢂ1
3.57
17.1
3.16
14.6
2.40
10.7
2.10
8.85
1.04
3.89
a General conditions: [1a]o ¼ 0.0004 M; [H2O2]o ¼ 0.004 M.
b Slope values of double logarithmic plots of kobs versus [sulphide]o.
c Calculated from the slope and intercept of Lineweaver–Burk plots.
Copyright # 2007 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2007; 20: 255–263
DOI: 10.1002/poc

(SALEN)MnIII–CATALYSED OXIDATION OF ALKYL ARYL SULPHIDES
263
Acknowledgements
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Copyright # 2007 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2007; 20: 255–263
DOI: 10.1002/poc