New polymer-immobilized peroxotungsten compound as an efficient catalyst for selective and mild oxidation of sulfides by hydrogen peroxide

Article abstract of DOI:10.1016/j.molcata.2011.12.025

A new polymer-bound peroxotungstate(VI) catalyst of the type [W(O) ₂(O₂)(CN)₂]-PAN [PAN = poly(acrylonitrile)] (PANW) has been prepared by reacting H₂WO₄ with 30% H ₂O₂ and the macromolecular ligand, PAN at pH 5.0. The compound was characterized by elemental analysis, SEM, EDX, TGA and spectral studies (FTIR and ¹³C NMR). Clean conversion of a variety of sulfides and dibenzothiophene (DBT) to the corresponding sulfoxide or sulfone, using H₂O₂ as oxidant, could be achieved in the presence of the heterogeneous catalyst PANW, by a versatile variation of reaction conditions. The reactions proceed under mild conditions to afford the resulting products with impressive turn over frequency (TOF). The catalyst exhibit complete chemoselectivity toward sulfur group of substituted sulfides with other oxidation prone functional groups. Easy regeneration and reusability of the catalyst for at least up to seven catalytic cycles with consistent activity and selectivity is an important attribute of the catalyst.

Full text of DOI:10.1016/j.molcata.2011.12.025

Journal of Molecular Catalysis A: Chemical 356 (2012) 36–45
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Journal of Molecular Catalysis A: Chemical
journal homepage: www.elsevier.com/locate/molcata
New polymer-immobilized peroxotungsten compound as an efficient catalyst
for selective and mild oxidation of sulfides by hydrogen peroxide
Siva Prasad Das, Jeena Jyoti Boruah, Niharika Sharma, Nashreen S. Islam∗
Department of Chemical Sciences, Tezpur University, Tezpur 784 028, Assam, India
a r t i c l e i n f o
a b s t r a c t
Article history:
A
new polymer-bound peroxotungstate(VI) catalyst of the type [W(O)2(O2)(CN)2]–PAN
Received 2 October 2011
Received in revised form
23 December 2011
Accepted 26 December 2011
Available online 3 January 2012
[PAN = poly(acrylonitrile)] (PANW) has been prepared by reacting H2WO4 with 30% H2O2 and the
macromolecular ligand, PAN at pH 5.0. The compound was characterized by elemental analysis, SEM,
EDX, TGA and spectral studies (FTIR and 13C NMR). Clean conversion of a variety of sulfides and diben-
zothiophene (DBT) to the corresponding sulfoxide or sulfone, using H2O2 as oxidant, could be achieved
in the presence of the heterogeneous catalyst PANW, by a versatile variation of reaction conditions.
The reactions proceed under mild conditions to afford the resulting products with impressive turn over
frequency (TOF). The catalyst exhibit complete chemoselectivity toward sulfur group of substituted
sulfides with other oxidation prone functional groups. Easy regeneration and reusability of the catalyst
for at least up to seven catalytic cycles with consistent activity and selectivity is an important attribute
of the catalyst.
Keywords:
Tungsten(VI) catalyst
Polymer-immobilized peroxotungsten(VI)
Dibenzothiophene oxidation
Chemoselective sulfoxidation
© 2012 Elsevier B.V. All rights reserved.
1. Introduction
a promoter or a co-catalyst, chlorohydrocarbon solvents and
excessive H2O2, which limit their practical utility. Therefore,
The contemporary interest in the selective oxidation of sulfur
containing compounds has to a great extent been fueled by the util-
ity of sulfoxides as fine chemicals, pharmaceuticals and as valuable
intermediates for the construction of chemically and biologically
active molecules [1–5]. Moreover, oxidative desulfurization pro-
cesses are emerging as important and sustainable procedures for
obtaining ultra low sulfur fuels [6–12].
Aqueous H2O2 of less than 60% concentration has been recog-
nized as ideal, clean and green oxidizing agent [13–15] for oxidation
of sulfides under acidic conditions [16]. However, owing to its being
a mild oxidant the H2O2 mediated oxidations are usually very slow
and often require to be activated by homogeneous or heteroge-
neous catalysts [16–18]. This feature has spurred the development
of a plethora of useful and interesting catalysts, including vanadium
[19–21], rhenium [22], iron [23–25], manganese [26,27], titanium
[8,28] and tungsten [16,17,29–36] based systems, for oxidation
of sulfides by H2O2. Despite these important progresses, scope
for improvement still remains owing to some of the disadvan-
tages associated with the available protocols viz., over oxidation,
low TON, high cost, difficulty in regeneration of the catalyst,
and requirements such as high temperature, long reaction time,
search for newer catalysts, active under improved environmentally
acceptable conditions, for selective oxidation of sulfide continues
unabated.
It is pertinent here to mention that we have recently developed a
series of polymer anchored peroxovanadium and peroxomolybde-
num compounds [37,38], as well as a set of monomeric and dimeric
peroxotungsten (pW) complexes [39–42], a few of which proved to
be effective oxidants of bromide with good activity at ambient tem-
perature and neutral pH, an essential requirement of a biomimetic
model [38,41]. Also, the compounds efficiently mediated bromi-
nation of organic substrates in aqueous organic media [38,41,43].
This finding is significant particularly because, contrary to natural
vanadium bromoperoxidases which are most efficient at pH 5.5–7
several model complexes were found to be catalytically active only
in acid medium [44–46].
Inspired by the above observations, in the present work, we
endeavored to generate polymer supported peroxotungsten com-
pound with an ability to serve as heterogeneous catalyst for
selective oxidation of organic sulfide to sulfoxide or sulfone, under
mild reaction conditions. Peroxotungsten complexes (pW) have
been receiving continued attention mainly owing to their appli-
cations as efficient and versatile catalysts in a variety of organic
oxidations [6,13,34,47–51] including sulfide oxidation [6,13,48,49].
Much of the stimulations for research in the area of polymer-
anchored catalytically active transition metal complexes have
come from the advantages associated with converting selec-
tive homogeneous catalysts to heterogeneous polymer supported
∗
Corresponding author at: Department of Chemical Sciences, Tezpur University,
Napaam, Tezpur 784 028, Assam, India. Tel.: +91 3712 267007 (O)/+91 9435380222;
fax: +91 3712 267006.
E-mail addresses: nsi@tezu.ernet.in, nashreen.islam@rediffmail.com (N.S. Islam).
1381-1169/$ – see front matter © 2012 Elsevier B.V. All rights reserved.
doi:10.1016/j.molcata.2011.12.025

S.P. Das et al. / Journal of Molecular Catalysis A: Chemical 356 (2012) 36–45
37
systems, including enhancement of stability, easy work up of reac-
2.3. Elemental analysis
tion mixture, regeneration and reusability of such systems [52–54].
Polymer-immobilized peroxometallates are yet to be fully explored
for their potential as environmentally benign heterogeneous cata-
lysts in organic oxidations. A few peroxometal systems supported
on insoluble cross-linked polymers were reported recently which
were found to exhibit good activity as catalytic or stoichiometric
oxidant in organic oxidations [55–58]. Proper choice of the func-
tional group is an important prerequisite in order to obtain a stable
polymer–metal linkage [38,52,54]. Further, the task of establishing
a robust polymer–metal bond which will be retained after repeated
catalytic cycles is particularly challenging in the regeneration of
supported catalytic species [52,56,58].
Here, we present the preparation and characterization of a
new polymer-bound pW compound, prepared by incorporating
peroxotungsten species into the poly(acrylonitrile) matrix (PAN),
and its activity as a heterogeneous catalyst in selective oxidation
of thioethers and dibenzothiophene (DBT) by H2O2, to the cor-
responding sulfoxide or sulfone. We selected PAN as a suitable
support for the purpose of this investigation due to its being a non-
toxic, cheap and commercially available reagent. Most importantly,
it provides suitable pendant functional groups for easy attachment
of transition metals [59,60]. In addition, acrylonitrile polymers
have attracted much attention for their application in diverse areas
that include medicine [61], antioxidants [61], surface coatings [62],
catalysis [60], textiles treatment [61], binders [61] and as adsorbant
for removal of heavy metal ions from water [59,63].
The compounds were analyzed for C, H and N by using ele-
mental analyzer Perkin Elmer 2400 series II. Tungsten content was
determined by EDX analysis as well as by gravimetric estimation
as BaWO4. Peroxide content was determined by adding a weighed
amount of the compound to a cold solution of 1.5% boric acid
(w/v) in 0.7 M sulfuric acid (100 mL) and titration with standard
Cerium(IV) solution [64].
2.4. Physical and spectroscopic measurements
The IR spectra were recorded with samples as KBr pellets in
a Nicolet model 410 FTIR spectrophotometer. The spectra were
recorded at ambient temperature by making pressed pellets of
the compounds. Spectra in the visible and ultraviolet region were
recorded in a Cary 100 Bio, Varian spectrophotometer equipped
with a peltier controlled constant temperature cell. Thermogravi-
metric analysis was done in SHIMADZU TGA-50 system at a heating
rate of 10 ◦C/min under an atmosphere of nitrogen using alu-
minum pan. The SEM characterization was carried out using the
JEOL JSM-6390LV Scanning Electron Micrograph attached with an
energy-dispersive X-ray detector. Scanning was done at 1–20 ␮m
range and images were taken at a magnification of 15–20 kV. Data
were obtained using INCA software. The standardization of the
data analysis is an integral part of SEM-EDX instrument employed.
The 13C NMR spectra for PAN and PANW were recorded in a JEOL
JNM-ECS400 spectrometer at carbon frequency 100.5 MHz, 32,768
X-resolution points, number of scans 10,000, 1.04 s of acquisi-
tion time and 2.0 s of relaxation delay with 1H NMR decoupling
method in DMSO-d + DMF (1:4). The 1H and 13C NMR of organic sul-
fides, sulfoxides and sulfones were recorded in a JEOL JNM-ECS400
spectrometer (CDCl3 as solvent and TMS as an internal standard).
Melting points were determined in open capillary tubes on a Büchi
Melting Point B-540 apparatus and are uncorrected. GC analysis
was carried out on a CIC, Gas Chromatograph model 2010 using a
SE-52 packed column (length 2 m, 1/8 in. OD) with a Flame Ioniza-
tion Detector (FID), and nitrogen as carrier gas (30 mL/min).
2. Experimental
2.1. Materials
The chemicals used were all reagent grade products. The
sources of chemicals are given below: Tungstic acid (Himedia
laboratories, Mumbai, India), acetone, hydrogen peroxide, ace-
tonitrile, methanol, ethylacetate, petroleum ether, diethyl ether,
dichloromethane (RANKEM), silica gel (60–120 mesh), sodium
hydroxide, sodium sulfate (E. Merck, India). The poly(acrylonitrile)
(PAN) (Mw = 48,200), methyl phenyl sulfide (MPS), dimethyl sulfide
(DMS), dibutyl sulfide (DBS), butyl propyl sulfide (BPS), dibenzoth-
iophene (DBT), phenylvinyl sulfide (PVS), 2-(phenylthio)ethanol
(PTE) and allyl phenyl sulfide (APS) were obtained from
Sigma–Aldrich Chemical Company, Milwaukee, USA. The water
used for solution preparation was deionized and distilled.
2.5. General procedure for oxidation of sulfides to sulfoxides
In a representative procedure, organic substrate (5 mmol) was
added to a mixture of PANW (13.2 mg, containing 0.005 mmol of W)
and 30% H2O2 (1.13 mL, 10 mmol) in methanol (5 mL), maintaining
molar ratio of W:substrate at 1:1000 and substrate:H2O2 at 1:2, in
a 50 mL two-necked round-bottomed flask. The reaction was con-
ducted at room temperature (RT) under continuous stirring. The
progress of the reaction was monitored by thin layer chromatog-
raphy (TLC) and GC. After completion of the reaction, the catalyst
was separated by filtration and washed with acetone. The product
as well as unreacted organic substrates were extracted with diethyl
ether from the filtrate and dried over anhydrous Na2SO4 and dis-
tilled under reduced pressure to remove excess diethyl ether. The
corresponding sulfoxide obtained was purified by column chro-
matography on silica gel using ethyl acetate and n-hexane (1:9).
The products obtained were characterized by IR, 1H NMR, 13C
NMR spectroscopy and in case of solid sulfoxide products, in addi-
tion to the above spectral analysis, we have carried out melting
point determination (see Supporting Information).
2.2. Synthesis of [WO2(O2)(CN)2]–PAN [PAN = poly(acrylonitrile)]
(PANW)
In a typical reaction, 1.17 g of H2WO4 (4.72 mmol) was dis-
solved in minimum volume of H2O2 (30% solution, 10 mL) in a
250 mL beaker with constant stirring at room temperature. The pH
of the clear solution obtained was recorded to be 1.21. Concentrated
sodium hydroxide (ca. 8 M) was then added to the above solution
dropwise with constant stirring to raise the pH of the reaction
medium to 5.0. Keeping the temperature of the reaction mixture
below 4 ◦C in an ice bath, 1.0 g of poly(acrylonitrile) was added to
it. The suspended polymer beads were allowed to swell in the reac-
tion mixture under continuous stirring for 24 h. The supernatant
liquid was then decanted and the white residue was repeatedly
washed with pre-cooled acetone. The product was separated by
centrifugation and dried in vacuo over concentrated sulfuric acid.
Found: C, 63.41; H, 2.86; N, 25.37; W, 6.84; O22−, 1.20%. The
metal loading calculated from the observed tungsten content is
0.38 mmol g−1 of polymer for [WO2(O2)(CN)2]–PAN.
2.6. General procedure for oxidation of sulfides to sulfones
To a stirred solution of sulfide (5 mmol) in acetonitrile (5 mL),
50% H2O2 (1.36 mL, 20 mmol) and the catalyst (13.2 mg, containing
0.005 mmol of W) were added successively maintaining molar ratio
of W:substrate at 1:1000 and substrate:H2O2 at 1:4. The resulting

38
S.P. Das et al. / Journal of Molecular Catalysis A: Chemical 356 (2012) 36–45
Table 1
reaction mixture was stirred at room temperature. After comple-
tion of the reaction, the sulfone obtained was isolated, purified
and characterized by methods similar to those mentioned under
procedure for oxidation of sulfide to sulfoxide (Section 2.5).
Infrared spectral data for PANW.
Assignment
νsym (W-O2)
νasym(W-O2)
PANW
535(m)
611(m)
858(m)
939(m)
2247(vs)
2.7. Regeneration of the catalyst
The regeneration of the catalyst for reuse was tested for the reac-
tion using methyl phenyl sulfide (MPS) as substrate. Since we have
optimized two types of reaction conditions for selective transfor-
mation of sulfide to sulfoxide or sulfone, the recyclability of the
reagent was examined for the afore mentioned reactions sepa-
rately. In the sulfoxidation reaction, the reaction mixture contained
the same recipe mentioned under Section 2.5. The catalyst was sep-
arated by centrifugation after being used in the reaction (35 min)
washed in acetone and dried in vacuo over concentrated sulfuric
acid. The catalyst was then placed into a fresh reaction mixture
containing 30% H2O2 (1.13 mL, 10 mmol), substrate (5 mmol) and
methanol (5 mL). The progress of the reaction was monitored by
thin layer chromatography (TLC) and GC.
ν
ν
(O-O)
(W=O)
ν
(C≡N)
2366(m)
2867(m)
2937(m)
νsym (CH2)
νasym(CH2)
counter ion in the compound indicated the charge neutrality of the
nitrile bound monoperoxotungsten(VI) species, consistent with the
formula assigned. The EDX spectral data obtained on the composi-
tion of the compound were in good agreement with the elemental
analysis values.
In case of the reaction of oxidation of MPS to sulfone, the
separated catalyst after completion of the reaction (90 min) was
similarly subjected to further catalytic cycle by placing it into a fresh
reaction mixture of 50% H2O2 (1.36 mL, 20 mmol), MPS (5 mmol)
and acetonitrile (5 mL) as mentioned under Section 2.6.
3.2.1. IR and electronic spectral studies
In an alternative procedure, regeneration of the spent catalyst
was achieved by charging the spent reaction mixture, remaining in
the reaction vessel after separating the organic reaction product,
with fresh H2O2 and substrate and then repeating the experiment.
Each of the procedures was repeated for seven cycles.
The significant features of IR spectrum of the polymer-anchored
peroxo complex PANW are summarized in Table 1 and presented
in Fig. 2. The presence of side-on bound peroxo ligand in the
compound was evident from the observance of the characteristic
ꢀ(O O), ꢀasym(W O2) and ꢀsym(W O2) modes in the vicinity of
ca. 860, ca. 610 and ca. 530 cm−1, respectively [65,66]. The strong
absorption at ca. 940 cm−1 have been assigned to ꢀ(W O) mode of
terminally bonded W O group [65,66].
3. Results and discussion
3.1. Synthesis
By comparison of IR spectrum of the anchored complex to the
spectrum of pure poly(acrylonitrile) [67] and the available liter-
ature data on metal compounds with co-ordination environment
comprising nitrile ligand as well as PAN [60,68,69], fairly reliable
empirical assignments could be derived for the IR bands observed
for PANW. The pristine polymer has a strong ꢀ(C N) absorption
at 2247 cm−1, apart from the characteristic bands at 2937 and
2867 cm−1 due to ꢀasym(CH2), ꢀsym(CH2), respectively (Fig. 2). The
spectrum of the compound after incorporation of pW species dis-
played, in addition to the band at 2247 cm−1 for the free nitrile
groups, a new medium intensity band at 2366 cm−1. This latter
band is attributable to a shift of ꢀ(C N) to a higher frequency,
resulting from co-ordination of the W(VI) ion with the pendant
nitrile group of the polymer. It has been documented that in simple
N-bonded nitrile complexes there is usually an increase in ꢀ(C N)
upon coordination [60,68,69].
In the present work the anchoring of peroxotungsten species to
the poly(acrylonitrile) matrix to obtain the heterogeneous catalyst
PANW, could be achieved easily by stirring the mixture contain-
ing the polymer and peroxotungsten species, generated in situ by
reacting H2WO4 with H2O2 at pH ca. 5, in an ice-bath for ca. 24 h.
The strategically maintained pH of ca. 5 was found to be optimal for
the desired synthesis of the polymer anchored pW compound. An
essential component of the procedure is to limit water to that con-
tributed by 30% H2O2 and alkali hydroxide solution. The catalyst is
stable, non-hygroscopic and can be stored for a prolonged period
without any change in its catalytic efficiency.
3.2. Characterization
The elemental analysis data for the compound indicated the
presence of one peroxide group per metal center. The tungsten
loading on the compound calculated on the basis of tungsten con-
tent obtained from chemical analysis as well as EDX analysis was
found to be 0.38 mmol per gram of the polymeric support.
Scanning electron microscopy was employed to study the parti-
cle size as well as morphological changes occurring on the surface
of the polymer after loading of the peroxotungstate to the poly-
mer matrix. The SEM micrographs revealed that the metal ions
are distributed across the surface of the polymer in PANW. The
average particle size of the pristine PAN beads was recorded to be
1.85 ␮m (Fig. 1(A)). The SEM micrograph of the catalyst (Fig. 1(B))
showed considerable enhancement of the average particle size after
incorporation of pW moieties into the PAN matrix. Energy disper-
sive X-ray spectroscopy of the compounds, which provides in situ
chemical analysis of the bulk, clearly showed W, C, N and O as the
constituents of the anchored complexes. The absence of sodium as
A band has been expected in the electronic spectra of the PANW
in the range of 320–330 nm, attributable to LMCT transitions orig-
inating from co-ordinated peroxide of monoperoxo derivatives of
tungsten [70,71]. However, no such peak was observed in the spec-
trum of the catalyst probably due to the low metal loading on the
polymer matrix.
3.2.2. 13C NMR studies
The study of co-ordination induced 13C NMR chemical shift
has been recognized as an important tool in understanding the
mode of coordination of the co-ligands in peroxo metal compounds
[37,66,72–76]. The 13C NMR spectra of the pure PAN polymer dis-
plays resonance due to pendant nitrile group at 120.21 ppm and
the characteristic signals corresponding to chain carbon atoms at
33.35 (for CH2) and 27.97 (for CH) ppm. The assignments are on the
basis of the available literature data [77]. The spectrum of PANW

S.P. Das et al. / Journal of Molecular Catalysis A: Chemical 356 (2012) 36–45
39
Fig. 1. Scanning electron micrographs of (A) PAN and (B) PANW. EDX spectra of (C) PANW.
exhibited a new signal at 128.48 ppm attributable to tungsten
bound nitrile group in addition to the signal at ı 120.15 ppm owing
to free nitrile group of the polymer (Fig. 3). It has been documented
that on co-ordination to a metal the nitrile carbon resonance under-
goes a downfield shift [78]. The spectrum of PANW thus evidenced
for the presence of both coordinated and free nitrile groups. The
substantial downfield shift, ꢁı (ıcomplexed nitrile − ıfree nitrile) ≈ 8 ppm
in the metal anchored compound relative to the free nitrile peak
of the pristine polymer suggests strong metal–ligand interaction.
The findings from the 13C NMR spectral analysis of the compound
is consistent with retention of its solid-state structure in solution.
3.2.3. Thermal analysis
The TG–DTG plots for the compound PANW presented in Fig. 4
shows that the compound undergoes multistage decomposition
on heating up to a temperature of 750 ◦C. It is noteworthy that
unlike pure PAN or some monomeric peroxometal compounds
[79], the polymer-anchored catalyst does not explode on heating
and remains stable up to a temperature of 115 ◦C. The degra-
dation of PAN in the temperature range 250–400 ◦C has been
reported to be explosive in nature [67]. In PANW, the first stage
Fig. 2. FTIR spectra of (A) PAN, (B) PANW and (C) PANW after 7th cycle of reaction.
Fig. 3. 13C NMR of (A) PAN and (B) PANW in DMSO-d + DMF (1:4).

40
S.P. Das et al. / Journal of Molecular Catalysis A: Chemical 356 (2012) 36–45
3.3. Catalytic activity of PANW
3.3.1. Oxidation of sulfides to sulfoxides
The efficacy of the polymer-immobilized pW compound, PANW
as catalyst in the oxidation of organic sulfides, using H2O2 as ter-
minal oxidant, has been explored. In order to optimize the reaction
conditions, several reaction runs were performed using methyl
phenyl sulfide (MPS) as model substrate (S) in the presence of
different solvents as shown in Table 2.
The reactions were conducted at room temperature (r.t.) under
magnetic stirring. In a preliminary experiment, MPS was allowed
to react with H2O2 (1 equiv.) maintaining the W:substrate molar
ratio at 1:1000 in methanol. Under these conditions MPS has been
rapidly oxidized initially to sulfoxide and then to sulfone to afford
ultimately a mixture of 1a and 1b in the ratio of 77:23 (Table 2,
entry 1). The initial fast reaction was observed to slow down on
prolonged reaction time and remained incomplete even after 5 h.
The significant finding of the reaction run was the facile formation
of sulfoxide as the exclusive product within the initial period of
ca. 30 min of starting the reaction. The observation led us to exam-
ine the possibility of achieving selective sulfoxidation of MPS by
terminating the reaction at specific time after the initial formation
of sulfoxide in order to prevent over oxidation to sulfone. Indeed,
when a reaction carried out separately under analogous condition
was terminated after 35 min of its starting, pure sulfoxide could be
isolated in ca. 49% yield (Table 2, entry 2). The yield of 1a could
be improved further to nearly 72% by increasing the amount of
H2O2 to 2 equiv., leading to reasonably high TOF (Table 2, entry 3).
A 1:2 molar ratio of oxidant:substrate, thus appeared to be opti-
mal in order to achieve high TOF without affecting the selectivity.
Allowing a similar reaction to run for 50 min although led to com-
plete conversion of sulfide with a nearly 97% isolated product yield
(Table 2, entry 4), however, 16–20% of sulfone was also formed
along with sulfoxide, thereby reducing the selectivity of the oxi-
dation. The data obtained thus evidenced for higher selectivity at
lower conversions. The effect of catalyst amount was then evalu-
ated (Table 2, entries 5 and 6). A 5 or 10-fold increase in the amount
of catalyst, albeit led to an enhancement in the rate of the reac-
tion. However, the corresponding TOF was found to decrease. We
have therefore decided to maintain a W:S molar ratio at 1:1000 for
subsequent reactions.
Fig. 4. TG–DTG plot of PANW.
decomposition occur in the temperature range of 115–141 ◦C
with the corresponding weight loss of 1.21%, attributable to loss
of peroxide ligand. The absence of peroxide in the decompo-
sition product, isolated at this stage, was confirmed from the
IR spectral analysis. The next decomposition occurred between
305 and 360 ◦C which appeared as a single peak in DTG (weight
loss 12.83%). The degradation further continued up to 750 ◦C.
IR spectrum of the residue obtained at 360 ◦C showed bands at
1578 cm−1 and 1625 cm−1 characteristic of ꢀ(C C) and ꢀ(C N)
stretching in addition to ꢀ(W O) at 957 cm−1. The results are in
general agreement with previous work where it was found that
the degradation of PAN below 400 ◦C is accompanied by elim-
ination of HCN, NH3 and H2O and concomitant intramolecular
polymerization of nitrile groups to form conjugated polyamine
(–C N)n and that the loss of nitrogen commences at 750 ◦C
[80]. The total weight loss which occurred during the course
of the overall decomposition process on heating the compound
up to a final temperature of 750 ◦C was recorded to be 59.82%.
Thermogravimetric analysis data of the compound thus pro-
vided further evidence in support of its composition and formula
assigned.
Based on the above data, a structure of the type shown in Fig. 5,
has been proposed for the compound PANW which includes tung-
The nature of the solvent was found to play a crucial role in
determining the activity and selectivity of the catalyst in the oxi-
dation reactions. We were particularly interested to carry out the
reactions in environmentally acceptable common organic solvents
as well as in water. The data in Table 2 clearly shows that both
methanol and ethanol, fully miscible with the organic substrate
and H2O2, are highly effective as a solvent affording product selec-
tivity as well as high TOF in the presence of the catalyst. The results
also indicated a favorable effect of the protic solvent. The obser-
vation is in agreement with previous report that solvents of high
hydrogen bonding ability favor the formation of sulfoxide with
high chemoselectivity [20,24,81]. No appreciable conversion was
noted in aqueous medium in the present case probably due to
the insolubility of the catalyst as well as the substrate in water.
Reactions were also conducted in solvents such as chloroform or
dichloromethane which yielded low conversions of MPS within the
range of 7–11% after 2 h of reaction time. It is interesting to note
that when the reaction was conducted in acetonitrile under anal-
ogous condition, the concomitant formation of the corresponding
sulfone was observed along with sulfoxide (Table 2, entries 8 and
9). Therefore, selective sulfoxidation of MPS could not be attained
in acetonitrile even by stopping the reaction at lower conversion.
It is noteworthy that on increasing the reaction temperature up
to 65 ◦C, a 3 to 4-fold increase in TOF could be achieved without
affecting the selectivity (Table 2, entries 10 and 11). This striking
sten(VI) atom with a side-on bound peroxo and terminal W
O
groups, bonded to the polymer matrix via the pendant nitrile
groups. Simultaneous bond formation of W(VI) to the two neigh-
boring nitrile groups of the polymer chain appears to complete the
hexa co-ordination around each tungsten atom.
Fig. 5. Proposed structure of PANW. “
” represents polymer chain.

S.P. Das et al. / Journal of Molecular Catalysis A: Chemical 356 (2012) 36–45
41
Table 2
Optimization of reaction conditions for PANW catalyzed selective oxidation of methyl phenyl sulfide (MPS) by 30% H2O2.a
O
O
O
PANW
Solvent,
S
S
S
+
Ph
Me
30% H2O2
Ph
Me
Ph
Me
1
1b
1a
.
Entry
Molar ratio W:MPS
H2O2 (equiv.)
Solvent
Temperature
Time (min)
Isolated yield (%)
1a:1b
TOF (h−1
)
1
2
3
4
5
6
7
8
9
1:1000
1:1000
1:1000
1:1000
1:500
1
1
2
2
2
2
2
2
2
2
2
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
EtOH
MeCN
MeCN
MeOH
MeOH
r.t.
r.t.
r.t.
r.t.
r.t.
r.t.
r.t.
r.t.
r.t.
300
35
35
50
20
10
35
120
45
15
10
79
49
72
97
73
71
70
96
67
73
74
77:23
100:0
100:0
84:16
100:0
100:0
100:0
46:54
75:25
100:0
100:0
158
840
1235
1164
1095
426
1200
480
893
2920
4440
1:100
1:1000
1:1000
1:1000
1:1000
1:1000
10
11
50 ◦
65 ◦
C
C
a
All reactions were carried out with 5 mmol of substrate in 5 mL of solvent.
O
S
O
O
S
S
[W(O)2(O2)(CN)2 ] PAN
[W(O)2(O2)(CN)2 ] PAN
50% H2O2, r.t.,90 min
CH3CN
30% H2O2, r.t., 35 min
CH3OH
-1
-1
TOF=653 h
TOF=1235 h
Scheme 1.
feature of the protocol indicated its synthetic utility. In a control
experiment conducted in the absence of the catalyst, very little con-
version of MPS to sulfoxide or sulfone was observed (<10%) under
similar reaction condition. It is thus evident that the catalyst plays
a crucial role in facilitating the desired reactions.
Having optimized the right conditions for sulfoxidation (see
Scheme 1), a series of various types of structurally diverse sul-
fides were subjected to the oxidation reaction using PANW–H2O2
system. Aliphatic and aromatic sulfides as well as DBT underwent
clean and selective oxidation to the corresponding sulfoxide under
the optimum condition, in impressive yields and TOFs. The results
are summarized in Table 3. Significantly, the maximum selectivity
of 100% was achieved at 70–75% of conversion for the substrates
(Table 3, entries 1–8).
employed in case of DBT oxidation since the reaction was relatively
sluggish at room temperature. Also, a higher amount of catalyst
used (W:substrate ratio of 1:100) further enhanced the rate of the
reaction (Table 3, entry 8).
The catalyst exhibited excellent chemoselectivity toward sul-
fur group of substituted sulfides containing other oxidation prone
functional groups such as
C
C
and OH (Table 3, entries 5–7).
Allylic and vinylic sulfoxides were obtained without epoxidation
products under such conditions. It is pertinent to highlight herein
that the methodology worked well for a relatively larger scale oxi-
dation using 5 g of MPS to give sulfoxide (Table 3, entry 1e) in high
yields, proving its potential for scaled-up synthetic applications.
3.3.2. Oxidation of sulfides to sulfones
Termination of the reactions at these conversions thus afforded
sulfoxides of high purity. It is pertinent to mention here that no
trace of sulfone was detected in the isolated products at this stage
of the reactions. The conversion:selectivity as a function of time was
investigated for each of the substrates (see Supporting Information
Table S1). Although increased conversions were noted with increas-
ing reaction time for each of the substrates tested however, the
selectivity was found to be reduced due to the expected overox-
idation to sulfone. The observation is consistent with the earlier
findings of Choudary et al. [32] pertaining to activity of a W based
LDH catalyst in sulfide oxidation.
The rate of the oxidation of thioethers were observed to vary
depending on the nature of the substrates and the substituents
attached (Table 3, entries 1–8). Allylic and vinylic sulfides (Table 3,
entries 5 and 6) were found to be less readily oxidized by H2O2
then the dialkyl sulfides probably due to effect of conjugation.
These observations are in agreement with the earlier findings that
rate of oxidation of sulfides by H2O2 increases with the increased
nucleophilicity of the sulfide [32,35]. The present protocol is also
effective for much less nucleophillic sulfide, such as DBT. Signifi-
cantly, selective oxidation of DBT, a refractory sulfide, to sulfoxide
has been reported to be difficult to achieve with most of the
available reaction procedures [11,13,35,82]. In the present work
a moderately higher temperature of 65 ◦C was required to be
We have observed that formation of sulfone is favored along
with sulfoxide when oxidation of MPS was carried out in ace-
tonitrile (Table 2, entries 8 and 9). We have therefore decided
to explore the possibility of achieving selective oxidation of sul-
fides to sulfones by conducting the reaction in acetonitrile. Our
initial attempts to obtain pure sulfone using MPS as a model
substrate were however, unsuccessful when we used 30% H2O2
as the oxidant. Even at a high S:oxidant molar ratio of 1:5 a
mixture of 1a and 1b were obtained. On the other hand, clean
conversion to sulfone could be achieved for MPS, in excellent
yield, and at room temperature within a reasonably short reac-
tion time by using 50% H2O2 and maintaining the W:S ratio at
1:1000. It is evident from the data presented in Table 4 that
the selectivity toward sulfone gradually increases with increas-
ing concentration of H2O2. A substrate:oxidant molar ratio of
1:4 was optimum for the complete conversion of MPS into
sulfone under the reaction conditions used (Table 4, entry 4).
Apart from MPS, the protocol was also found to be effective
for wide range of aromatic and aliphatic substrates, the latter
being more reactive as expected. The results are summarized in
Table 5.
Most importantly, the TOF of the conversions could be sub-
stantially increased by carrying out the reactions under reflux
at 78 ◦C. Even under such relatively drastic conditions viz., the

42
S.P. Das et al. / Journal of Molecular Catalysis A: Chemical 356 (2012) 36–45
Table 3
Selective oxidation of sulfides to sulfoxides with 30% H2O2 using PANW as catalysta at room temperature or 65 ◦Cb (values within parenthesis).
O
(W:S=1:1000)
S
PANW
S
30% H2O2(2 equivalent),
CH3OH
R1
R2
R1
R2
.
Entry
1
Substrate
Time (min)
35 (10)
Product
Isolated yield (%)
72 (74)
TOFc (h−1
)
O
S
S
1235 (4440)
35 (10)d
35 (10)e
69 (72)
73 (70)
1182 (4320)
1251 (4200)
O
S
S
2
30
71
1420
O
S
3
4
30 (10)
30 (10)
72 (75)
68 (74)
1440 (4500)
1360 (4440)
S
O
S
S
S
O
S
5
6
7
45 (12)
180 (55)
70 (20)
65 (70)
76 (74)
74 (76)
866 (3500)
253 (807)
634 (2280)
O
S
S
S
O
S
OH
OH
O
S
S
8f
(420)
(71)
(10)
a
Reaction conditions: substrate (5 mmol), catalyst (13.2 mg, 0.005 mmol of W), 30% H2O2 (1.13 mL, 10 mmol), methanol (5 mL).
Reaction at 65 ◦C in refluxing methanol.
b
c
d
e
f
TOF (Turnover frequency) = mmol of product per mmol of catalyst per hour.
Yield of 7th reaction cycle.
Yield at 5 g scale.
Substrate (5 mmol), catalyst (132 mg, 0.05 mmol of W), methanol (5 mL).
Table 4
Optimization of reaction conditions for PANW catalyzed selective oxidation of methyl phenyl sulfide (MPS) to sulfone by 50% H2O2.a
Entry
Molar ratio W:MPS
H2O2 (equiv.)
Solvent
Temperature
Time (min)
240
180
120
90
60
40
25
Isolated yield (%)
1a:1b
TOF (h−1
)
1
2
3
4
5
6
7
1:1000
1:1000
1:1000
1:1000
1:500
1
2
3
4
4
4
4
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
r.t.
r.t.
r.t.
r.t.
r.t.
r.t.
55
97
96
98
99
97
96
46:54
37:63
13:87
0:100
0:100
0:100
0:100
137
323
480
653
495
145
2304
1:100
1:1000
78 ◦
C
a
All reactions were carried out with 5 mmol of substrate in 5 mL of solvent.
presence of reasonable excess of 50% H2O2 and high temperature,
the co-existing alcohol and olefinic moieties in substituted sulfides
remained unaffected (Table 5, entries 5–7). The results presented
in Table 5 show that the oxidation of sulfides to the correspond-
ing sulfones has been achieved with complete chemoselectivity
(Table 5, entries 5–7). The suitability of the methodology for scaled
up synthesis was also ascertained by conducting the reaction at a
5 g scale (Table 5, entry 1e).
3.3.3. Catalyst recycling
The recyclability of the catalyst was assessed by using MPS as the
substrate. The catalyst afforded regeneration and could be reused
without further conditioning after separating the oxidized product
and unreacted sulfide from the reaction mixture. The regenera-
tion was accomplished by charging the spent catalyst with H2O2, a
fresh lot of substrate and the respective solvent after each cycle of
reaction. The catalyst was reused for up to seven catalytic cycles.

S.P. Das et al. / Journal of Molecular Catalysis A: Chemical 356 (2012) 36–45
43
Table 5
Oxidation of sulfides to sulfones using 50% H2O2 catalyzed by PANWa at room temperature or 78 ◦Cb (values within parenthesis).
O
O
S
PANW(W:S=1:1000)
S
R1
R2
50%H2O2(4 equivalent),
CH3CN
R1
R2
.
Entry
1
Substrate
Time (min)
90 (25)
Product
Isolated yield (%)
98 (96)
TOFc (h−1
)
O
O
S
S
653 (2304)
90 (25)d
90 (25)e
93 (91)
94 (97)
620 (2184)
626 (2328)
O
O
S
S
2
72
96
800
O
O
S
3
4
85 (25)
85 (25)
97 (98)
95 (97)
684 (2352)
670 (2328)
S
O
O
S
S
S
O
O
O
O
O
O
O
S
5
6
7
100 (30)
360 (130)
270 (80)
96 (98)
95 (97)
96 (98)
576 (1960)
158 (447)
213 (735)
S
S
S
S
OH
OH
O
S
S
8f
(570)
(93)
(9)
a
Reaction conditions: substrate (5 mmol), catalyst (13.2 mg, 0.005 mmol of W), 50% H2O2 (1.36 mL, 20 mmol), acetonitrile (5 mL).
b
Reaction at 78 ◦C in refluxing acetonitrile.
cTOF (Turnover frequency) = mmol of product per mmol of catalyst per hour.
d
Yield of 7th reaction cycle.
Yield at 5 g scale.
Substrate (5 mmol), catalyst (132 mg, 0.05 mmol of W), acetonitrile (5 mL).
e
f
Data presented in Table 3 (entry 1d) and Table 5 (entry 1d) show
that catalyst remain effective with consistent activity and selec-
tivity even after seventh cycle of oxidation. The IR spectrum of the
spent catalyst resembled closely the corresponding spectrum of the
original starting complex showing the presence of peroxo group,
terminal oxo and co-ordinated as well as free nitrile groups
indicating that tungsten centers are intact and the coordina-
tion environments were not altered during the catalytic process
(Fig. 2(C)). Although a slight loss in peroxide content was noted,
however, the EDX analysis showed no significant decrease in the
metal loading value of the spent catalyst compared to the origi-
nal catalyst. These findings further indicate that the catalyst PANW
remains stable after several cycles of oxidations.
It is important to note that the overall TOF after seven cat-
alytic cycles of oxidation of MPS was recorded to be 8671 h−1 and
4431 h−1 for sulfoxide and sulfone, respectively, at room temper-
ature. These values could further be increased to 30,300 h−1 (for
sulfoxide) and 15,792 h−1 (for sulfone) by increasing the reaction
temperature (Table 3, entry 1d and Table 5, entry 1d). A perusal
of the available literature revealed that polymer supported cata-
lyst PANW displayed superior activity in terms of TOF as well as
selectivity, achieved under mild reaction condition, over majority
of tungsten containing catalysts reported so far for sulfide oxidation
[6,13,16,17,29–36,49].
3.3.4. Test for heterogeneity of the reaction
In order to examine the leaching of the metal complex from
the polymer-bound catalyst into the reaction medium during the
oxidation reactions, separate experiments were carried out using
MPS as the substrate. After completion of the reaction, the catalyst
was separated by filtration. The filtrate collected was transferred
to a reaction vessel and the reaction was allowed to continue for
another 2 h, by adding a fresh MPS–H2O2 mixture. In the absence of
the catalyst, under analogous conditions the sulfide conversion was
noted to be ca. 5%, in line with the value obtained in the absence of
any catalyst. This demonstrates that the reaction did not proceed
on the removal of the solid catalyst. These data are in agreement
with the absence of catalyst leaching and the occurrence of a purely
heterogeneous catalytic process.
3.3.5. The proposed mechanism
A scheme of reactions, shown in Fig. 6, is proposed which satis-
factorily describes the principal features of our results. It is plausible
that the polymer-bound monoperoxotungsten species I reacts with

44
S.P. Das et al. / Journal of Molecular Catalysis A: Chemical 356 (2012) 36–45
Fig. 6. The proposed mechanism. (a) The polymer-bound monoperoxotungsten species I reacts with H2O2 to yield the reactive diperoxotungsten intermediate II. (b) Transfer
of electrophilic oxygen from II to the substrate V takes place to yield sulfoxide VI with concomitant regeneration of the original catalyst I. (c) The sulfoxide VI may further
react with active diperoxotungstate species II leading to the formation of sulfone VII and simultaneous regeneration of the original catalyst I.
H2O2 to yield the reactive diperoxotungsten intermediate II (reac-
tion a). The electrophilicity of peroxotungstate intermediate being
much higher than that of H2O2 [33], it is reasonable to expect that
facile transfer of electrophilic oxygen to the substrate V would take
place to yield sulfoxide VI (reaction b) with concomitant regenera-
tion of the original monoperoxo W(VI) catalyst I. The sulfoxide may
further react with the active diperoxotungstate species II (reaction
c) generated from another cycle of reaction between catalyst I and
H2O2 (reaction a) to yield sulfone. The reaction leading to the for-
mation of sulfone thus appears to be a typical two step process. The
facile oxidation of nucleophilic sulfide to sulfoxide is apparently
an easier process than the second oxidation of the resulting less
nucleophilic sulfoxide to sulfone [32,33]. Although further studies
are necessary to establish the exact identity of the reactive diper-
oxotungsten(VI) species II, support for the proposed mechanism
comes from the earlier findings that, for a peroxotungstate species
to be catalytically active in oxidation, an oxo-diperoxo configura-
tion may be a pre-requisite [41,83,84].
India for providing Research Fellowship to SPD (RGNSRF). We thank
Dr. G. V. Karunakar and Dr. R. Borah, Department of Chemical
Sciences, Tezpur University, Tezpur, Assam, India for valuable dis-
cussion and suggestions.
Appendix A. Supplementary data
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.molcata.2011.12.025.
References
[1] M.C. Carreno, Chem. Rev. 95 (1995) 1717–1760.
[2] H.L. Holland, Chem. Rev. 88 (1988) 473–485.
[3] H. Cotton, T. Elebring, M. Larsson, L. Li, H. Sorensen, S. Unge, Tetrahedron:
Asymmetr. 11 (2000) 3819–3825.
[4] S. Patai, Z. Rappoport, Synthesis of Sulfones, Sulfoxides and Cyclic Sulfides, J.
Wiley, Chichester, 1994.
[5] J. Legros, J.R. Dehli, C. Bolm, Adv. Synth. Catal. 347 (2005) 19–31.
[6] W. Zhu, G. Zhu, H. Li, Y. Chao, Y. Chang, G. Chen, C. Han, J. Mol. Catal. A: Chem.
347 (2011) 8–14.
[7] M.R. Maurya, A. Arya, A. Kumar, M.L. Kuznetsov, F. Avecilla, J. Costa Pessoa,
Inorg. Chem. 49 (2010) 6586–6600.
4. Conclusions
[8] V. Hulea, F. Fajula, J. Bousquet, J. Catal. 198 (2001) 179–186.
[9] A.V. Anisimov, A.V. Tarakanova, Russ. J. Gen. Chem. 79 (2009) 1264–1273.
[10] F. Figueras, J. Palomeque, S. Loridant, C. Feche, N. Essayem, G. Gelbard, J. Catal.
226 (2004) 25–31.
[11] F.M. Collins, A.R. Lucy, C. Sharp, J. Mol. Catal. A: Chem. 117 (1997) 397–403.
[12] P.S. Raghavan, V. Ramaswamy, T.T. Upadhya, A. Sudalai, A.V. Ramaswamy, S.
Sivasanker, J. Mol. Catal. A: Chem. 122 (1997) 75–80.
[13] R. Noyori, M. Aoki, K. Sato, Chem. Commun. (2003) 1977–1986.
[14] C.W. Jones, Applications of Hydrogen Peroxide and Derivatives, Royal Society
of Chemistry, Cambridge, 1999.
A heterogeneous catalyst has been developed for oxidation of
organic sulfides by hydrogen peroxide under mild reaction con-
ditions, by immobilizing peroxotungstate on poly(acrylonitrile)
matrix. It is remarkable that the catalyst can be effectively used
for obtaining high purity sulfoxide as well as sulfone from the cor-
responding aryl or alkyl sulfide and DBT in excellent yield and high
TOF by a versatile variation of reaction conditions.
The significant advantages offered by the developed protocol
include (i) control over degree of oxidation of products; (ii) chemos-
electivity of the catalyst toward sulfur group of substituted sulfides
and sulfoxides with other oxidation prone functional groups; (iii)
avoidance of chlorinated solvent or other additives; (iv) easy regen-
eration and reusability of the catalyst for several catalytic cycles
without any change in its activity. Moreover, simplicity in the
method of preparation of the catalyst involving easily available
starting materials and its non-hygroscopic and stable nature are
additional attractive features associated with the catalyst. The find-
ings confirm that immobilization of pW species on the PAN matrix
enhances its stability, and leads to an improvement in the over-
all efficiency of the pW catalyst. It is reasonable to expect that
the synthesized catalyst may emerge as an useful addition to the
range of ecologically suitable oxidation catalysts with the poten-
tial for accomplishing some of the goals of green chemistry. The
work on other applications of PANW is now underway in our
laboratory.
[15] B.S. Lane, K. Burgess, Chem. Rev. 103 (2003) 2457–2473.
[16] K. Kaczorowska, Z. Kolarska, K. Mitka, P. Kowalski, Tetrahedron 61 (2005)
8315–8327.
[17] B. Karimi, M. Ghoreishi-Nezhad, J.M. Clark, Org. Lett. 7 (2005) 625–628.
[18] R.A. Sheldon, J.K. Kochi, Metal Catalyzed Oxidations of Organic Compounds,
Academic Press, London, 1981.
[19] V. Conte, F. Fabbianesi, B. Floris, P. Galloni, D. Sordi, I.C.W.E. Arends, M. Bonchio,
D. Rehder, D. Bogdal, Pure Appl. Chem. 81 (2009) 1265–1277.
[20] F. Gregori, I. Nobili, F. Bigi, F. Maggi, G. Predieri, J. Mol. Catal. A: Chem. 286
(2008) 124–127.
[21] F. Di Furia, G. Modena, R. Curci, Tetrahedron Lett. 17 (1976) 4637–4638.
[22] K.J. Stanger, J.W. Wiench, M. Pruski, J.H. Espenson, G.A. Kraus, R.J. Angelici, J.
Mol. Catal. A: Chem. 243 (2006) 158–169.
[23] H. Egami, T. Katsuk, J. Am. Chem. Soc. 129 (2007) 8940–8941.
[24] E. Baciocchi, M.F. Gerini, A. Lapi, J. Org. Chem. 69 (2004) 3586–3589.
[25] A. Marques, M. Marin, M.F. Ruasse, J. Org. Chem. 66 (2001) 7588–7595.
[26] F. Xie, Z. Fu, S. Zhong, Z. Ye, X. Zhou, F. Liu, C. Rong, L. Mao, D. Yin, J. Mol. Catal.
A: Chem. 307 (2009) 93–97.
[27] J.E. Barker, T. Ren, Tetrahedron Lett. 46 (2005) 6805–6808.
[28] M. Iwamoto, Y. Tanaka, J. Hirosumi, N. Kita, S. Triwahyono, Micropor. Mesopor.
Mater. 48 (2001) 271–277.
[29] X.Y. Shi, J.F. Wei, J. Mol. Catal. A: Chem. 280 (2008) 142–147.
[30] D.H. Koo, M. Kim, S. Chang, Org. Lett. 7 (2005) 5015–5018.
[31] Y.M.A. Yamada, H. Tabata, M. Ichinohe, H.T.S. Ikegami, Tetrahedron 60 (2004)
4087–4096.
[32] B.M. Choudary, B. Bharathi, C.V. Reddy, M.L. Kantam, J. Chem. Soc. Perkin Trans.
1 (2002) 2069–2074.
Acknowledgements
[33] K. Sato, M. Hyodo, M. Aoki, X.-Q. Zheng, R. Noyori, Tetrahedron 57 (2001)
2469–2476.
[34] N.M. Gresley, W.P. Griffith, A.C. Laemmel, H.I.S. Nogueira, B.C. Parkin, J. Mol.
Catal. A: Chem. 117 (1997) 185–198.
Financial support from the Department of Science and Tech-
nology, New Delhi, India is gratefully acknowledged. We are also
grateful to the University Grants Commission (UGC), New Delhi,

S.P. Das et al. / Journal of Molecular Catalysis A: Chemical 356 (2012) 36–45
45
[35] V. Hulea, A.L. Maciuca, F. Fajula, E. Dumitriu, Appl. Catal.
200–207.
[36] H.S. Schultz, H.B. Freyermuth, S.R. Buc, J. Org. Chem. 28 (1963) 1140–1142.
[37] J.J. Boruah, D. Kalita, S.P. Das, S. Paul, N.S. Islam, Inorg. Chem. 50 (2011)
8046–8062.
A
313 (2006)
[59] L.C.S. Maria, M.C.V. Amorim, M.R.M.P. Aguiar, P.I.C. Guimaraes, M.A.S. Costa, A.P.
Aguiar, P.R. Rezende, M.S. Carvalho, F.G. Barbosa, J.M. Andrade, R.C.C. Ribeiro,
React. Funct. Polym. 49 (2001) 133–143.
[60] Z.M. Michalska, B. Ostaszewski, J. Zientarska, B.N. Kolarz, M. Wojaczyn´ ska,
React. Polym. 16 (1992) 213–221.
[38] D. Kalita, S. Sarmah, S.P. Das, D. Baishya, A. Patowary, S. Baruah, N.S. Islam,
React. Funct. Polym. 68 (2008) 876–890.
[39] D. Kalita, S.P. Das, N.S. Islam, Biol. Trace Elem. Res. 128 (2009) 200–219.
[40] P. Hazarika, D. Kalita, N.S. Islam, J. Enzyme Inhib. Med. Chem. 23 (2008)
504–513.
[41] P. Hazarika, D. Kalita, S. Sarmah, R. Borah, N.S. Islam, Polyhedron 25 (2006)
3501–3508.
[42] P. Hazarika, D. Kalita, S. Sarmah, N.S. Islam, Mol. Cell. Biochem. 284 (2006)
39–47.
[43] S. Sarmah, D. Kalita, P. Hazarika, R. Borah, N.S. Islam, Polyhedron 23 (2004)
1097–1107.
[44] A. Butler, Coord. Chem. Rev. 187 (1999) 17–35.
[45] D. Rehder, M. Bashirpoor, S. Jantzen, H. Schmidt, M. Farahbakhsh, H. Nekola, in:
A.S. Tracey, D.C. Crans (Eds.), Vanadium Compounds, Chemistry, Biochemistry
and Therapeutic Applications, Oxford University Press, New York, 1998, pp.
60–70.
[61] F.M. Peng, in: H.F. Mark, N.M. Bikales, C.G. Overberger, G. Menges (Eds.), Ency-
clopedia of Polymer Science and Engineering, vol. 1, John Wiley and Sons, New
York, 1985, pp. 426–470.
[62] J.C. Petropoulos, L.E. Cadwell, W.F. Hart, Ind. Eng. Chem. 49 (1957) 379–381.
[63] W.E. Rayford, R.E. Wing, W.M. Doane, J. Appl. Polym. Sci. 24 (1979) 105–113.
[64] M.K. Chaudhuri, S.K. Ghosh, N.S. Islam, Inorg. Chem. 24 (1985) 2706–2707.
[65] N.J. Campbell, A.C. Dengal, C.J. Edwards, W.P. Griffith, J. Chem. Soc. Dalton Trans.
(1989) 1203–1207.
[66] A.C. Dengel, W.P. Griffith, R.D. Powell, A.C. Skapski, J. Chem. Soc. Dalton Trans.
(1987) 991–995.
[67] S.M. Badawy, A.M. Dessouki, J. Phys. Chem. B 107 (2003) 11273–11279.
[68] K. Nakamoto, Infrared and Raman Spectra of Inorganic and Co-ordination Com-
pounds, Part B, 5th ed., Wiley and Sons, New York, 1997, pp. 113–115.
[69] S.F. Rach, F.E. Kühn, Chem. Rev 109 (2009) 2061–2080.
[70] S.K. Maiti, K.M.A. Malik, R. Bhattacharyya, Inorg. Chem. Commun. 7 (2004)
823–828.
[46] A. Butler, M.J. Clague, G.E. Meister, Chem. Rev. 94 (1994) 625–638.
[47] J. Tang, L. Wang, G. Liu, Y. Liu, Y. Hou, W. Zhang, M. Jia, W.R. Thiel, J. Mol. Catal.
A: Chem. 313 (2009) 31–37.
[48] M.H. Dickman, M.T. Pope, Chem. Rev. 94 (1994) 569–584.
[49] O. Bortolini, F. Di Furia, G. Modena, R. Seraglia, J. Org. Chem. 50 (1985)
2688–2690.
[71] M.R. Maurya, N. Bharti, Transition Met. Chem. 24 (1999) 389–393.
[72] D. Bayot, B. Tinant, M. Devillers, Inorg. Chim. Acta 357 (2004) 809–816.
[73] L.L.G. Justino, M.L. Ramos, M.M. Caldeira, V.M.S. Gil, Inorg. Chim. Acta 356 (2003)
179–186.
[74] L. Pettersson, I. Andersson, A. Gorzsas, Coord. Chem. Rev. 237 (2003) 77–87.
[75] V. Conte, F.D. Furia, S. Moro, J. Mol. Catal. 94 (1994) 323–333.
[76] S.E. Jacobson, R. Tang, F. Mares, Inorg. Chem. 17 (1978) 3055–3063.
[77] J. Schaefer, Macromolecules 4 (1971) 105–107.
[50] S. Kenneth, K.S. Krishenbaum, K.B. Sharpless, J. Org. Chem. 50 (1985)
1979–1982.
[51] S.E. Jacobson, D.A. Mccigrosso, F. Mares, J. Org. Chem. 44 (1979) 921–924.
[52] A.D. Pomogalilo, Catalysis by Polymer Immobilized Metal Complexes, Gordon
and Breach Sci. Publ., Amsterdam, 1998.
[53] D.C. Sherrington, in: B.K. Hodnett, A.P. Kybett, J.H. Clark, K. Smith (Eds.),
Supported Reagents and Catalysts in Chemistry, Royal Society of Chemistry,
Cambridge, 1998, p. 220.
[54] A. Skorobogaty, T.D. Smith, Coord. Chem. Rev. 53 (1984) 55–226.
[55] M.R. Maurya, M. Kumar, S. Sikarwar, React. Funct. Polym. 66 (2006) 808–818.
[56] B. Tamami, H. Yeganeh, React. Funct. Polym. 50 (2002) 101–106.
[57] K. Vassilev, R. Stamenova, C. Tsvetanov, React. Funct. Polym. 46 (2000) 165–173.
[58] B. Tamami, H. Yagenh, Eur. Polym. J. 35 (1999) 1445–1450.
[78] J.H. Kim, J. Britten, J. Chin, J. Am. Chem. Soc. 115 (1993) 3618–3622.
[79] T.T. Bhengu, D.K. Sanyal, Thermochim. Acta 397 (2003) 181–197.
[80] T.J. Xue, M.A. McKinney, C.A. Wilkie, Polym. Degrad. Stab. 58 (1997) 193–202.
[81] T. Patonay, W. Adam, A. Levai, P. Kover, M. Nemeth, E.-M. Peters, K. Peters, J.
Org. Chem. 66 (2001) 2275–2280.
[82] S. Hussain, S.K. Bharadwaj, R. Pandey, M.K. Chaudhuri, Eur. J. Org. Chem. 20
(2009) 3319–3322.
[83] M.S. Reynolds, S.J. Morandi, J.W. Raebiger, S.P. Melican, S.P.E. Smith, Inorg.
Chem. 33 (1994) 4977–4984.
[84] A.F. Ghiron, R.C. Thompson, Inorg. Chem. 28 (1989) 3647–3650.