Selective H2O2 oxidation of organic sulfides to sulfoxides catalyzed by cobalt(III)-salen ion

Article abstract of DOI:10.1016/j.poly.2015.07.020

Abstract The catalytic activity of cobalt(III)-salen ion catalyzed selective H₂O₂ oxidation of organic sulfides to sulfoxides is examined using spectrophotometric technique. The catalytic reaction proceeds through Michaelis-Menten kinetics and the rate of the reaction is highly sensitive to the nature of the substituent present in the substrate as well as in the salen ligand. The product analyses show that the aryl methyl sulfides are selectively oxidized to the corresponding sulfoxides. Based on the spectral and kinetic studies two possible mechanisms have been proposed.

Full text of DOI:10.1016/j.poly.2015.07.020

Accepted Manuscript
Selective H2O2 Oxidation of Organic Sulfides to Sulfoxides Catalyzed by Co-
balt(III)-Salen Ion
A. Mary Imelda Jayaseeli, Arumugam Ramdass, Seenivasan Rajagopal
PII:
S0277-5387(15)00375-7
DOI:
http://dx.doi.org/10.1016/j.poly.2015.07.020
POLY 11406
Reference:
Polyhedron
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Accepted Date:
20 April 2015
7 July 2015
Please cite this article as: A. Mary Imelda Jayaseeli, A. Ramdass, S. Rajagopal, Selective H2O2 Oxidation of Organic
Sulfides to Sulfoxides Catalyzed by Cobalt(III)-Salen Ion, Polyhedron (2015), doi: http://dx.doi.org/10.1016/j.poly.
2015.07.020
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Selective H2O2 Oxidation of Organic Sulfides to Sulfoxides Catalyzed by
Cobalt(III)-Salen Ion
A. Mary Imelda Jayaseeli,a,b Arumugam Ramdass,b and Seenivasan Rajagopal*b,c
a Department of Chemistry, Jayaraj Annapackiam College for women, Periyakulam-
625 601, India.
bSchool of Chemistry, Madurai Kamaraj University, Madurai – 625 021, India.
cPresent Address: Postgraduate and Research Department of Chemistry,
Vivekananda College, Tiruvedagam West – 625 234, Madurai, India.
*Corresponding Author
E-mail: rajagopalseenivasan@yahoo.com (S. Rajagopal)
Tel: 91-452-2458246; Fax: 91-452-2459139
1

Abstract
The catalytic activity of cobalt(III)-salen ion catalyzed selective H2O2 oxidation
of organic sulfides to sulfoxides is examined using spectrophotometric technique. The
catalytic reaction proceeds through Michaelis-Menten kinetics and the rate of the reaction
is highly sensitive to the nature of the substituent present in the substrate as well as in the
salen ligand. The product analyses show that the aryl methyl sulfides are selectively
oxidized to the corresponding sulfoxides. Based on the spectral and kinetic studies two
possible mechanisms have been proposed.
Keywords: Cobalt(III)-Salen, H2O2 Oxidation, Michalis-Menten kinetics, Organic
Sulfides, Sulfoxides
2

1. Introduction
Salen complexes [salen=N,N’bis(salicylidene)ethylenediaminato] of transition
metals (metal – Fe, Ru, Mn, Cr, Co, V, Ti, Mo, Zr, U, Y, Pt) attract considerable interest
because of their catalytic activity and easy methods of syntheses [1-11]. In the past
decades, metal–salen complexes have been extensively used as catalysts for epoxidation
[4,12], sulfoxidation [2,13], ring opening reaction [14], fixation of CO2 in the form of
carbonates [15] and as models for enzymes [16]. In our laboratory, detailed kinetic
studies using spectrophotometric technique were carried out to propose the mechanism
for the oxidation of aryl methyl sulfides to sulfoxides and amines to N-oxides using
metal-salen complexes (Metal = Fe, Mn, Cr, V) as enzyme models and PhIO and H2O2 as
the oxidants [2]. By introducing various substituents in the two aromatic rings of the
salen ligands, the electronic and steric factors affecting the stability of active oxidants
generated from metal-salen complexes and oxygen sources H2O2, PhIO, tert-BuOOH and
their reactivity have been studied [2].
Cobalt being a biologically relevant metal, it is desirable to study the role of
cobalt ion as catalyst. The oxygen binding abilities of cobalt-salen complexes stimulated
the research towards reversible O2 binding properties of cobalt(II)-salen ions and their
utility as catalysts [17,18]. Punniyamurthy et al [19] have used cobalt(II)-salen complexes
as efficient catalysts for the oxidation of wide range of organic substrates (e.g, alkenes,
alcohols, benzylic compounds and aliphatic hydrocarbons) with dioxygen in the presence
of aliphatic aldehydes or ketones or ketoesters. Cobalt-salen complexes have been
endowed as catalysts for epoxidation reactions [20,21], ring opening [22], oxidation of
3

phenols and anilines [23-25], cyclopropanation [26], carbon dioxide fixation [27],
asymmetric catalysis [28-30] and catalysts for other reactions [31-33].
As far as we know, there is no literature available on the catalytic role of cobalt-
salen complexes for the oxidation of biologically important organic sulfides. The study of
oxidation of organic sulfides is of wide interest because of the important role of sulfur
compounds and in particular, sulfur centered radicals in biological systems [34]. As
sulfoxidation reactions play vital role in biological systems because of their antioxidant
nature [6,35-39], it is desirable to study the catalytic role of cobalt(II)-salen complexes
for the oxidation of sulfides. As far as the nature of the active oxidant species generated
from the catalyst cobalt(II)-salen complex and H2O2 is concerned much effort has been
attempted in recent years [32,40]. From a detailed investigation Thomas and coworkers
[3c] have reported that Co(III)-tris(phenolate) species as the active catalyst of the
reaction. The authors examined the pathways possible for one electron oxidation of
phenolate complex (CoII-L) either via metal ligand radical (CoII-L•) or (CoIII-L) and
concluded the active catalyst is cobalt(III)-bis(phenolate) salen. To accomplish the
catalytic role of Co(II) ion on sulfoxidation reaction we have synthesized five Co(II)-
salen complexes (I-V, Chart 1) and used them as catalyst. Seven para- substituted phenyl
methyl sulfides (1-7, Chart 2) are used as the substrates and H2O2, the eco-friendly
reagent, has been used as the oxidant. On addition of H2O2 to cobalt(II)-salen complexes,
the obvious spectral changes explain the formation of active oxidant species whose
spectral changes are followed to study the kinetics of sulfide oxidation.
4

2. Experimental
2.1. Materials
Salicylaldehyde and the substituted salicylaldehydes (5-chloro, 5-methyl, 5-methoxy,
3,5-di-tert-butyl) were purchased from Aldrich and used as such. The para-substituted
phenyl methyl sulfides (p-methoxy, p-methyl, p-flouro, p-chloro, p-bromo and p-nitro)
were also purchased from Aldrich and used as such. HPLC grade acetonitrile,
dichloromethane and 30% H2O2 were used as received.
2.2. Instrumentation
The UV-vis absorption spectral measurements have been carried out using
Analytik Jena Specord S 100 diode array spectrophotometer with a constant
accumulation of 10 and a constant integration time of 40 ms over a range of 200-1000
nm. The infra red spectra of the Co(II)-salen complexes and the products have been
recorded in a JASCO FT-IR–410 spectrophotometer. 1H NMR for the salen ligands have
been acquired on a Bruker 300 MHz NMR spectrometer with CDCl3 as the solvent. The
redox potentials of the complexes are measured in CH3CN medium using BAS CV-50W
Voltammetric Analyzer at a platinum electrode containing 0.1 M tetrabutylammonium
perchlorate as the supporting electrolyte.
2.3. Synthesis of ligands and cobalt(II)-salen complexes (I-V)
The general procedure [41,42] for the preparation of salen ligands involves the
condensation of salicylaldehyde and ethylenediamine in the ratio of 2:1 in an alcoholic
medium. A mixture of salicylaldehyde (2.44g, 19.9 mmol) and ethylenediamine (0.60g,
9.99 mmol) in 100 ml of ethanol is refluxed for one hour. The reaction mixture is cooled
to room temperature. The yellow solid is collected at the pump, and washed with cold
5

methanol and dried in air. Yield: 97%; m.p. 128 ºC, reported 128-129 ºC [42]. 5,5’-
(CH3)2-salen, 5,5’-(OCH3)2-salen and 5,5’-(Cl)2-salen are prepared by condensing the
corresponding substituted salicylaldehyde with ethylenediamine in ethanol and
recrystallized from cold methanol. The ligand 3,3’5,5’-tetra-tert-butylcyclosalenH2 is
1
prepared by a published procedure [43]. The H NMR spectra of all substituted salen
ligands are shown in our previous report [2e].
Cobalt(II)-salen complexes are synthesized by established procedures [44]. Parent
salen ligand 1.8 mM (0.5g in ethanol) is degassed under N2. Co(OAc)2.4H2O (0.46 g, 1.8
mmol) in warm water is added to salen solution in small quantities at a time under N2
atmosphere. Immediately the yellow color was changed into the dark brown. The reaction
mixture is refluxed under N2 for one hour, cooled and dried in vacuum. The product is
extracted with CHCl3 and dried in vacuum. Dark brown colored cobalt(II)–salen complex
I is obtained. The similar procedure is followed to the synthesis of complexes II–V using
appropriate salen ligand.
2.4. General procedure for sulfide oxidation
The concentrations of cobalt(II)-salen complexes and H2O2, have been
maintained as 2×10-4 M and 5×10-3 M respectively. The strength of H2O2 is carefully
monitored in all the experiments. The kinetics of H2O2 oxidation of organic sulfides
catalyzed by cobalt(II)-salen ion in 90% CH3CN and 10% DCM are followed under
pseudo first order conditions. The progress of the reaction is monitored by maintaining
the concentration of sulfide 10 times greater over the cobalt(III)-salen complex.
6

2.5. Product analysis
In a typical experiment involving oxidation of organic sulfides, 0.2 mM
cobalt(II)-salen is treated with 5 mM MPS and 5 mM H2O2 in CH3CN/CH2Cl2 (90/10
v/v) medium. The solution is stirred for 1-2 h at 298 K, depending on the nature of the
reactants. The solvent is removed under reduced pressure and the organic product is
extracted with chloroform. It is dried over sodium sulfate and evaporated. The residual
mass is subjected to IR and NMR studies. The details of the product analyses are
elaborated in the discussion section.
3. Results and Discussion
The structures of metal-salen complexes and the substrates used in the present
study are depicted in Charts 1 and 2 respectively. The synthesized cobalt(II)-salen
complexes are characterized by UV-vis, FT-IR and CV studies and data are collected in
supporting information (Table S1). The spectra and CV of some complexes are shown in
the supporting information (Figs. S1–S6). The spectral data obtained in this study are
similar to the previous reports [45].
3.1. Cobalt(II)-salen complexes not suitable catalysts for sulfide oxidation
When we attempted to use cobalt(II)-salen complexes as catalysts for the
oxidation of organic sulfides, there was no reaction even if we follow the course of the
reaction for several hours. From this experimental observation we concluded that
cobalt(II)-salen complexes were not serving as the catalysts for the oxidation of organic
sulfides. According to Jacobsen and co-workers [46] and Thomas and co-workers [3c]
cobalt(II)-salen complexes were catalytically inactive for hydrolytic kinetic resolution but
they became suitable catalysts if [Co(II)-salen] ions were subjected to one electron
7

oxidation to produce Co(III)-salen-X (X= anionic ligand). Fukuda and Katsuki [47]
achieved asymmetric cyclopropanation of styrene using optically active cobalt(III)-salen
complexes by in situ generation from the reaction of cobalt(II)-salen with Br2. With this
background we wanted to try Co(III)-salen complexes as catalysts for the H2O2 oxidation
of organic sulfides.
3.2. Generation of cobalt(III)-salen ion
In the present study, cobalt(III)-salen complex is generated in situ using H2O2 as the
oxidant. At this juncture, it is worth mentioning that H2O2 is the green oxidant that
converts the cobalt(II)-salen complex into cobalt(III)-salen complex [48]. The attractive
spectral changes observed by the addition of H2O2 to the parent cobalt(II)-salen complex
(I) is shown in Fig.1. It is interesting to note that the intensity of the three peaks at λmax
=
346 nm (n–π*), λmax= 403 nm (d–π*) and λmax= 489 nm (d–d) decreases with clear
isosbestic points at λmax= 327 and 515 nm for the 2×10-4 M (I) on adding 5×10-3 M H2O2.
The changes in the absorption spectrum are measured for every second using the diode
array spectrophotometer. The conversion of Co(II) to Co(III) is achieved in less than 10
seconds. The intensity of the three peaks corresponding to cobalt(II)-salen decreases and
becomes a single peak at 376 nm (for complex I) on adding H2O2. The obvious decrease
in the absorbance at λmax= 489 nm (d–d) indicates that cobalt ion attains +3 state [48].
The similar behavior is observed with the Co(II) complexes carrying substituted salen
ligands (II–V) also. The spectral changes observed for complex V are shown in Fig. S7.
In principle one electron oxidation of Co(II)-salen complex could afford either
metal-ligand radical CoII-(salen•+) or (CoIII-salen) complex according to eq 1.
8

Determining the electronic structure of one electron oxidized Co(II)-salen complexes is
of prime importance to understand the catalytic pathway and mechanism of the reaction
[3c,49]. Recent studies show that the absorption in the whole visible region as well as an
intense absorption band at > 900 nm indicate the formation of [CoII-(salen•+)] as the
oxidized product of [CoII-salen] complex [49c]. As shown in Fig 1 there is no significant
absorption at > 600 nm for [CoII-salen] in the presence of H2O2 which compels us to
conclude that [CoIII-salen] complex is the catalyst for the catalysed oxidation of organic
sulfides.
3.3. Kinetic studies
The in situ generated cobalt(III)-salen complexes catalyze the H2O2 oxidation of
organic sulfides. When phenyl methyl sulfide is injected into the reaction mixture, there
is a substantial shift in λmax towards red region and also there is increase in intensity. For
parent cobalt(III)-salen (I), the λmax value shifts from 376 nm to 383 nm. The increase in
the absorbance (OD) is to the extent of 0.1 to 0.3 depending on the nature of the
substituents in the salen ligand (Fig. S8). The increase in the absorption intensity is
instantaneous and decreases fast with time. The rate of the reaction is measured following
the decay of absorbance of the complex at appropriate wavelength (λmax) with time
intervals of 5 minutes at 298 K. The sample spectral changes with time are shown in Figs.
2 and S9.
When we monitored the progress of the reaction spectrophotometrically, an
induction period of about 20 to 30 minutes was observed followed by decrease in the
intensity at the appropriate λmax. It is evident from Fig. 3 that there is negligible change in
OD initially for about 25 minutes and then there is a linear decrease in log OD with time.
9

It is interesting to point out that similar induction periods have been observed for
the Co(II)-salen complexes catalyzed O2 oxygenation of cyclohexane [50]. The induction
period is used for the generation of the active oxidant species [Co(III)-(salen)-OOH]. It is
generally accepted that co-coordinated peroxy complexes are the active oxygen transfer
species formed during propagation of the oxidation reactions catalyzed by Co complexes
[50].
The inset in Fig. 3 shows the linear part of the plot signifying the first order kinetics
of the reaction on the oxidant. The rate measurement is followed for the linear part of the
plot giving the appropriate range for measurement of rate constant using the inbuilt
program Win Aspect. The first order rate constant, k1, values measured for seven para-
substituted aryl methyl sulfides catalyzed by cobalt(III)-salen complexes I and II are
tabulated in Tables S2 and S3.
The influence of substrate concentration on the rate of reaction is studied by
measuring the rate of the reaction at different [substrate]. The kinetic results are shown in
Tables S2 and S3 and in Fig. 4. These results indicate that the rate of the reaction
increases with increase in [substrate] and attains saturation at high [substrate]. The
inference from the saturation kinetics is that the substrate binds to the oxidant before the
rate controlling step. The binding of substrate with the oxidant is evident with an initial
increase in the absorbance (OD) of oxidant on the addition of substrate (Fig. S8).
Thus the redox reaction proceeds through Michaelis-Menten kinetics which can be
treated through equations (2) and (3).
Substrate
Oxidant
Complex
Product
+
(2)
(3)
k
Complex
10

From these equations the rate law can be derived and the rate expression is given by
equation (4).
k1 = kobs = k [substrate] / (KM + [substrate])
The inverse of eqn. (4) becomes eqn. (5)
1 / k1 = (1 / k ) + (KM / k [substrate])
(4)
(5)
When 1/k1 is plotted against 1/[substrate], linear plots are obtained with finite intercepts.
A sample plot is shown in Fig. S10. From the slope and intercept values, k and KM have
been evaluated and are given in Tables 1 and 2.
3.4. Substituents effects
The data provided in Table 1 give a clear picture on the substituent effect. The
substituents present in 5,5’-positions of salen ligand and the aryl moiety of ArSMe
influence the rate of the reaction enormously. The rate of the reaction is enhanced by the
introduction of electron donating substituents in the phenyl ring of PhSMe and the rate is
retarded by electron withdrawing substituents in the same moiety. The opposite effect is
noticed when the substituents are introduced in the 5,5’-positions of salen ligand. In order
to understand the extent of charge separation in the transition state of the reaction, the
rate constant values collected in Table 1 are analyzed in terms of the Hammett equation
[51] (Eq 6).
log k/k0 = σ ρ
(6)
where k0, the rate constant when X = H, k, the rate constant when X = substituents, ρ =
reaction constant, σ = substituent constant.
The correlation of log k values of phenyl methyl sulfides with Hammett sigma
constant is good (Figs. 5 and S11) and the reaction constant (ρ) values range from –0.8 to
11

–1.0 for different cobalt(III)-salen complexes. The correlation is not improved if sigma+ /
sigma– values are used instead of sigma (r = 0.96, n = 7, ρ+ = -0.7 for complex I).
The effect of introducing substituents in the salen ligand of cobalt(III)-salen
complex on the rate of oxidation of phenyl methyl sulfides is also investigated and the
rate data are collected in Table 1. These kinetic data are also treated using Hammett
equation by plotting log k verses 2σ (Fig. 6). The ρ value is positive and it is in the range
of +0.4 to +0.7.
Jayaseeli and Rajagopal [2e] followed the iron(III)-salen ion catalyzed H2O2
oxidation of organic sulfides based on spectrophotometric measurements. H2O2 is used as
oxidant and the active oxidant, oxo iron radical cation is generated in situ with iron(III)-
salen complexes. At this juncture, it is worth mentioning that the H2O2 oxidation
reactions are attractive because of its eco-sustainability. Sivasubramanian et al [2h]
separated the [oxo(salen)iron] ion on treating iron(III)-salen complexes with PhIO and
followed the kinetics of oxidation of sulfides. Michaelis-Menten saturation kinetics are
observed when H2O2 as well as PhIO are used as oxidants. The ρ value obtained for the
iron(III)-salen ion catalyzed H2O2 oxidation of seven para-substituted methyl phenyl
sulfides is –1.5. Interestingly similar ρ value was obtained when PhIO was used as the
oxidant for the oxidation of nine para - substituted methyl phenyl sulfides. In this present
work, the Michaeleis-Menton kinetics was followed for the 5,5’-substituted cobalt(III)-
salen complexes catalyzed H2O2 oxidation of para substituted methyl phenyl sulfides.
The reaction constant, ρ, value lies between -0.8 to -1.0 for Co(III)-salen catalyzed
oxidation. The reported ρ value is -0.85 for the kinetics of Mn(III)-salen complexes
catalyzed oxidation of aryl methyl sulfides with hydrogen peroxide [52]. The ρ values
12

obtained for the Cr(III)-salen catalyzed oxidation of sulfides ranges between -1.5 to -1.8
[2g,2i,53]. The comparison of ρ values observed with different metal-salen complexes
indicates that the reaction constant observed with cobalt-salen complexes is comparable
with the value reported on the Mn(III)-salen catalyzed reaction but less compared to
Fe(III)- and Cr(III)-salen complexes. The inference from this comparison is that though
the electrophilicity of Co(III)-salen ion is comparable with Mn(III)-salen but less
compared to Fe(III)- and Cr(III)-salen ions. This is also evident from the ρ values
observed with these complexes when the effect of changing the substituent in the salen
ligand is tested on the catalytic activity of the sulfoxidation reaction. As the magnitude
of ρ is related to the development of charge during the rate determining step, mechanisms
can be devised based on this information.
3.5. Product analysis
In order to analyze the products formed during the course of the reaction, FT-IR
and 1H NMR techniques have been used. FT-IR analysis of the product obtained from the
oxidation of MPS shows S=O stretching frequency at 1049 cm-1, indicating the formation
of sulfoxide from sulfide. There is a band at 692 cm-1 corresponding to C-S stretching
frequency. Since there is no stretching frequency at 1150 cm-1 corresponding to sulfone,
we understand that sulfide is not oxidized to sulfone under the present experimental
conditions. The product analysis study demonstrates that sulfoxide is the only product
formed under the present reaction conditions.
1
When H NMR spectral study is used for the product analysis, it also gives similar
results as observed with FT-IR study. From the available literature, it is known that the
methyl groups of C6H5SCH3, C6H5S(O)CH3 and C6H5S(O2)CH3 in deuterated solvents
13

show distinctive peaks at δ 2.4, 2.72 and 3.08 ppm respectively [54]. In the present study,
CDCl3 is used as the solvent.
1
The H NMR peak obtained at 2.7 ppm (Fig. S12) is a strong evidence for the
selective oxidation of sulfide to sulfoxide. The aromatic protons appear as multiplets
centered at 7.2 and 7.3 ppm for sulfide and at 7.6 and 7.7 ppm for sulfoxide respectively
[55]. These studies strongly confirm that the major product of the reaction is the
corresponding sulfoxide.
The percentage yield of the products obtained during the course of the reaction is
carried out based on GC analysis as well as NMR spectral data. The results obtained in
the H2O2 oxidation of seven para substituted phenyl methyl sulfides in the presence of
complex I are given in Table 3 and sample GC-MS spectrum is shown in supporting
information (Fig. S13). It is evident that the percentage yield lies in the range 50 to 64 %
in accordance with the effect of substituents on the reactivity of the reaction. Organic
sulfides carrying electron donating substituents in the aryl moiety form higher yield
compared to electron withdrawing substituents. The turnover number (TON) calculated
from the percentage yield obtained from different organic sulfides is in the range of 10 –
13.
3.6. Mechanism of the reaction
The initial reaction between Co(II) and H2O2 leads to the formation of Co(III)
which catalyzes the H2O2 oxidation of organic sulfides. Co(III) ion reacts with H2O2 to
produce cobalt(III)-hydro peroxide adduct [CoIII-OOH] similar to compound 0 in iron(III)
catalyzed oxidation reaction [2e]. After the formation of [CoIII-OOH], the nature of its
interaction with organic sulfides is speculative. Whether [CoIII-OOH] itself is the active
14

•
oxidant or short lived radicals OH and OOH generated from the adduct are the active
•
oxidant is not understood at this stage.
When the substrate phenyl methyl sulfide is added to [CoII-salen]-H2O2 system
there is an initial increase in OD at 383 nm (corresponding to Co(III)) indicating the
binding of substrate with the oxidant. Thus the initial step involves the binding of
substrate to the oxidant similar to enzyme catalyzed reaction. After the formation of
oxidant-substrate complex the product sulfoxide is formed through oxygen atom transfer
from the oxidant to the substrate via the ligand coupling mechanism as shown in
Scheme 1.
In this proposal both –OOH and ArSMe serve as ligands for CoIII and oxygen
atom transfer takes place through ligand coupling reaction. The importance of this ligand
coupling reaction has been extensively advocated by Oae and co-workers [36b]. We have
proposed such ligand coupling mechanism for the picolinic acid catalyzed Cr(VI)
oxidation of organic sulfides [56]. We also wish to propose alternative mechanism for
this [CoIII-salen] catalyzed oxidation. After the formation of [CoIII(salen)-OOH] it may
form [oxo(salen)cobalt(IV)] as the active oxidant similar to compound I type species
postulated for iron(III)-salen and iron(III)-porphyrin catalysed H2O2 oxidation of organic
substrates [2e,57]. This active species can oxidize organic sulfides through electron
transfer mechanism. The mechanism for iron(III)-salen ion catalysed H2O2 oxidation of
sulfides via active oxidant species [FeIV=O(salen)]•+ followed by electron transfer (ET)
was proposed by Rajagopal and coworkers [2e]. In a similar way, the reactive species
[CoIV=O(salen)] •+ formed from [CoIII(salen)-OOH] combines with the substrate to form
the oxidant-substrate complex. After the formation of this oxidant-substrate adduct the
15

reaction proceeds through a series of electro transfer processes. In the first step the
electron transfer from the substrate to the oxidant results in the formation of compound II
and sulfide radical cation. The compound II and sulfide radical cation produce the
corresponding sulfoxide and CoIII-salen complex as shown in Scheme 2.
4. Conclusion
The oxidation of aromatic sulfides with H2O2 catalysed by cobalt(II)-salen
complexes is an eco-friendly efficient method for the synthesis of sulfoxide. Cobalt(III)-
salen complexes are generated in situ from cobalt(II)-salen complexes using H2O2 as the
oxidant. The reaction follows Michaelis-Menten kinetics and the rate is highly sensitive
to the change of substituent in the salen moiety of Co(III) complex and aryl moiety of
ArSMe. The product analysis study using FT-IR and NMR techniques shows that aryl
methyl sulfides are selectively oxidized to the corresponding sulfoxides under the present
experimental conditions. Based on these spectral, kinetic and product analysis study, two
possible mechanisms have been proposed.
Acknowledgement
Prof. S.R thanks UGC, New Delhi for sanctioning UGC-BSR Faculty and UGC
Emeritus Fellowships. The authors thank UGC and DST for providing funds to School of
Chemistry, Madurai Kamaraj University, Madurai through DRS and IRHPA projects to
procure the instruments which are used to carryout our studies. A.M.I.J thanks the UGC
and the Management of Jayaraj Annapackiam College for Women (Autonomous),
Periyakulam-625 601, for sanctioning permission to avail the benefits of FDP under X
Plan. A.R is the recipient of UGC Meritorious fellowship under the Basic Scientific
Research (BSR) Scheme.
16

Appendix A. Supplementary data
Supplementary data associated with this article can be found, in the online version, at
http://dx.doi.org/10.XXXXXXXXXXX
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Table 1 Rate constants 104 k, s-1 for the cobalt(III)-salen catalyzed H2O2 oxidation of p-
XC6H4SCH3 in 90% CH3CN-10% DCM at 298 K.
X=
H
I
II
III
IV
ρ
r
V
3.30
5.90
5.50
2.30
2.70
2.12
0.72
-0.9
0.98
4.90
7.60
6.40
3.60
3.00
2.90
1.20
-0.8
0.99
1.98
3.40
2.90
0.99
1.00
0.91
0.40
-1.0
0.97
1.93
3.30
2.70
0.92
0.93
0.88
0.39
-0.9
0.97
0.4
0.4
0.4
0.7
0.6
0.6
0.5
--
0.98
0.97
0.97
0.97
0.98
0.97
0.97
--
1.40
2.30
2.10
0.91
0.90
0.85
0.35
-.0.9
0.97
OCH3
CH3
F
Cl
Br
NO2
ρ
r
--
--
26

Table 2 Michaelis-Menten constants, 103 KM obtained from Michaelis-Menten kinetics of
cobalt(III)-salen complexes catalyzed H2O2 oxidation of p-XC6H4SCH3 in 90% CH3CN-
10% DCM system at 298 K.
X=
H
I
II
III
IV
V
9.50
6.50
9.80
6.40
12.9
13.0
35.0
18.1
8.10
12.2
7.00
4.80
8.40
15.5
19.6
13.6
14.5
19.2
20.8
15.1
12.7
19.2
17.0
14.5
20.4
16.2
11.2
9.50
6.20
4.80
5.80
8.90
8.50
9.50
13.5
OCH3
CH3
F
Cl
Br
NO2
Table 3 Percentage yield of sulfoxide formed from the complex I catalyzed H2O2
oxidation of p-XC6H4SCH3.
X
Sulfoxide (yield %)
H
60
64
62
53
55
52
50
OCH3
CH3
F
Cl
Br
NO2
27

Figure Captions
Scheme 1. Mechanism for the [Co(III)-salen]+ catalyzed H2O2 oxidation of organic
sulfides.
Scheme 2. Electron transfer mechanism for the [Co(III)-salen]+ catalyzed H2O2 oxidation
of organic sulfides.
Chart 1. Structure of Cobalt(II)-salen complexes.
Chart 2. Aryl methyl sulfides (1-7).
Fig. 1. UV-vis spectral changes observed for complex I in the presence of H2O2 in 90%
CH3CN and 10% DCM at 298 K. [I] = 2×10-4 M, [H2O2] = 5×10-3 M.
Fig. 2. Change of absorbance of I–H2O2 system with time in the presence of C6H5SMe
(1). [I] = 2x10-4 M, [H2O2] = 5x10-3 M, [1] = 4x10-3 M.
Fig. 3. Log OD vs time plot. [I] = 2×10-4 M, [H2O2] = 5×10-3 M, [1] = 2 x10-3 M.
Fig. 4. Plot of k1vs [substrate] for a reaction catalyzed by complex II.
Fig. 5. Hammett plot for the H2O2 oxidation of ArSMe catalyzed by II.
Fig. 6. Hammett plot for the H2O2 oxidation of 1 catalyzed by cobalt(III)-salen complexes
I-IV.
28

Scheme 1. Mechanism for the [Co(III)-salen]+ catalyzed H2O2 oxidation of organic
sulfides.
Scheme 2. Electron transfer mechanism for the [Co(III)-salen]+ catalyzed H2O2 oxidation
of organic sulfides.
29