A chitosan supported peroxidovanadium(V) complex: Synthesis, characterization and application as an eco-compatible heterogeneous catalyst for selective sulfoxidation in water

A chitosan supported peroxidovanadium(V) complex: synthesis,

characterization and application as an eco-compatible heterogeneous

catalyst for selective sulfoxidation in water

Gangutri Saikia, Kabirun Ahmed, Sandhya Rani Gogoi, Mitu Sharma, Hiya Talukdar and

Nashreen S. Islam*

Department of Chemical Sciences, Tezpur University, Tezpur 784028, Assam, India.

*

To whom correspondence should be addressed:

Fax: +91-3712-267006

E-mail: nsi@tezu.ernet.in

nashreen.islam@rediffmail.com

Address: Prof. Nashreen S. Islam

Dept. of Chemical Sciences

Tezpur University

Napaam, Tezpur-784028

Assam, India

Phone : +91-9435380222 (Off.)

+91-3712-267007

1

Highlights



A new polymer immobilized peroxidovanadium(V) complex has been synthesized by

reacting V O with 30% H O and the biopolymer chitosan.

2

5

2

The compound served as a heterogeneous catalyst for the selective oxidation of

sulfide with H O in aqueous medium.

2

The catalytic protocol provided sulfoxide with an impressive yield and high TOF or

TON.

The catalyst is chemoselective towards the sulfur group in the presence of other

oxidation prone functionalities.

The recyclability of the catalyst for several reaction cycles renders the protocol

ecologically sustainable.

2

Abstract

Anchoring of peroxidovanadium(V) (pV) species to chitosan, a natural chelating polymer,

afforded a new and structurally defined immobilized complex, VO(O ) (NH )-chitosan

2

(ChpV) which was comprehensively characterized by elemental analysis (CHN, ICP-OES,

1

3

energy dispersive X-ray spectroscopy), spectral studies (FT-IR, Raman, C NMR, diffuse

reflectance UV-Vis and XPS), SEM, XRD, Brunauer–Emmett–Teller (BET) and

thermogravimetric analysis (TGA). Apart from being thermally stable, the compound is

water-tolerant and non-hygroscopic. The compound effectively facilitated chemoselective

oxidation of thioethers with H O in aqueous medium to the respective high purity sulfoxide,

2

-1

in impressive yield (TOF 5628 h ) and with high H O efficiency, under mild reaction

2

condition. In addition to water, the heterogeneous catalyst is highly compatible in a variety of

organic solvents and is recyclable for several catalytic cycles without significant loss in

selectivity. The catalytic procedure is amenable to ready scalability and is free from halogen,

co-catalyst and acidic or any other hazardous additive.

Keywords: Peroxidovanadium(V) complex; Chitosan immobilized vanadium(V);

Heterogeneous catalysis; Green oxidation catalyst; Aqueous sulfide oxidation.

3

1

. Introduction

Currently, with an ever increasing emphasis on the development of cleaner and sustainable

chemical processes, efforts towards heterogenization of homogenous catalysts by

immobilization of active transition metal complexes on insoluble polymer supports appear to

have intensified. Various advantages associated with such systems, including their enhanced

stability, specific site selectivity, ease of handling of toxic materials, facile separation and

recovery of the catalysts etc., ultimately contributes toward enhancing the overall

sustainability of a chemical transformation by minimizing its environmental impact [1].

Apart from cross-linked synthetic organic polymers, natural materials, viz. cellulose,

chitin and chitosan, are receiving tremendous contemporary interest as potential polymer

supports for immobilization and heterogenization of metal catalysts [1e,2]. Chitosan, the

copolymer

of

2-amino-2-deoxy-D-glucopyranose

and

2-acetamido-2-deoxy-D-

glucopyranose, [3] with amino and hydroxyl groups in its structure, exhibits an enormous

ability to form metal complexes and has been extensively used in the extraction of metallic

impurities from wastewater [4]. Moreover, owing to its attractive physical and mechanical

properties, chitosan is also becoming increasingly relevant in a wide variety of applications,

including biomedical products [4b,c,5], cosmetics [4b,6], food processing [4b,c,7] etc. Some

of the unique features of chitosan and its derivatives which boost their applicability as green

supports for heterogenization of metal complexes [2b,3b,4d,8] include: (i) biocompatibility

and biodegradability, (ii) non-toxicity, (iii) low cost and eco-friendly, (iv) high metal

dispersion on the surface of chitosan, (v) high thermal stability and durability, (vi) physical

and chemical versatility, and (vii) insolubility in organic solvents [1e,2b,3b,4d,8]. Several

studies have demonstrated chitosan-metal complexes as catalysts in wide range of organic

transformations, including C-C bond formation reactions [1e,4d], hydration [1e,4d,9],

polymerization [1e], water splitting reactions [9] etc. Nevertheless, reports dealing with the

application of chitosan immobilized metal complexes as heterogeneous catalysts in the field

of organic oxidation appear to be still scarce [10-12].

It is pertinent to mention here that our group has developed in recent years a series of

0

novel polymer immobilized peroxidometal complexes using d metal ions belonging to group

5

and 6, viz. vanadium [13], niobium [14], molybdenum [15] and tungsten [15c, 16], which

display excellent activity in a variety of organic oxidations, including phenol hydroxylation

[

13a], oxidative bromination [13c,15c,16a] and sulfide oxidation [14,15a,16b,16c], under

4

eco-sustainable reaction conditions. It is however intriguing to note that, despite numerous

reports available in the literature on the synthesis and application of peroxidometal

complexes as highly efficient and versatile catalysts for organic oxidations [17], we have

come across only two reports dealing with the activity of chitosan supported peroxidometal

derivatives as oxidation catalysts [18]. For example, Zhu et al. reported a chitosan supported

protonated peroxidotungstate (pW) system which efficiently catalysed the oxidation of a

variety of organic substrates in acetonitrile [18a].

In the present work we have explored the scope of generating a new heterogeneous

oxidation catalyst, by immobilizing peroxidovanadate (pV) species on chitosan. Vanadium

has been considered an adequate choice for our study, as vanadium based catalysts are known

to support environmentally benign aerobic oxidation conducted in water [17a,19].

Furthermore, vanadium, with its well-established biological and therapeutic roles, exhibits

diverse chemistry owing to its easily accessible oxidation states and rich coordination

chemistry, which have led to its application in various fields including catalysis, both

homogeneous and heterogeneous [12b,17a,19,20].

Among the great variety of organic oxidations, one of the industrially as well as

fundamentally most important transformations is known to be the chemoselective oxidation

of organic sulfides. Sulfoxides and sulfones serve as versatile synthetic intermediates for the

construction of chemically and biologically significant molecules, including chiral auxiliaries

and therapeutic agents [21]. Since the earliest synthesis of sulfoxides via the oxidation of

sulfides by Maercker in 1865 [22], numerous oxidation strategies have been introduced for

the successful synthesis of sulfoxides or sulfones using a vast range of traditional oxidants

[

22a,23]. The identification of aqueous H O as the ideal non-polluting and green oxidizing

2 2

agent [20e,21e,24] for the oxidation of sulfides spurred the development of a plethora of

useful and promising transition metal catalysts over the years

12a,b,c,14,15a,b,20,21f,g,24b,25-29] to facilitate H O induced sulfide oxidation. However,

[

2

many of these methodologies utilized hazardous and toxic reagents and solvents or complex

procedures often resulting in unwanted by-products [30]. So, the ever increasing demand for

newer alternative catalysts and sustainable oxidation procedures that can operate under

environmentally benign reaction conditions, implementing safer reagents and non-polluting

reaction media, continues.

Owing to the recognition of water as a natural green solvent [31] which can often be

used as an alternative to traditional organic solvents [31a-d,32], an upsurge in contemporary

interest in water-centred organic synthesis has been witnessed recently [32]. Due to its

5

abundance, low cost and inherent properties, such as polarity, viscosity, high heat capacity,

environment compatibility, non-toxicity and non-flammability, [31a,32] water can be a great

asset to the reaction. Along with the increasing importance of water-based organic reactions,

the need for the development of water-tolerant and water compatible catalysts to support such

transformations is rising tremendously. It is notable that whereas the majority of the available

studies on metal catalysed sulfide oxidations conducted in aqueous medium focused on

developing water soluble catalysts [14,15a,20a,b,25a,f,28], reports pertaining to

heterogeneously catalysed thioether oxidation in water are still very limited.

[

24b,27b,c,d,31f]

Herein, we discuss the preparation and characterization of a new chitosan anchored

pV complex, as well as its performance as a heterogeneous catalyst for the controlled

oxidation of sulfide to sulfoxide in aqueous medium, in terms of selectivity, reusability, TOF,

scalability and sustainability. To the best of our knowledge, this is the first example related to

the preparation and use of a well-defined peroxidovanadate compound supported on chitosan

as a catalyst for organic oxidation.

2

. Experimental

2

.1 Materials

Acetone, methanol, ethylacetate, ethanol, hydrogen peroxide, petroleum ether (RANKEM),

diethyl ether, sodium hydroxide (E. Merck, India), silica gel (60-120 mesh), vanadium

pentoxide (Sisco Research Laboratories), chitosan (medium molecular mass, deacetylated

degree 85%, viscosity 200-800 cps), methyl phenyl sulfide (MPS), methyl p-tolyl sulfide

(

MpTS), ethyl phenyl sulfide (EPS), dimethyl sulfide (DMS), dibutylsulfide (DBS), 2-

phenylthio)ethanol (PTE), dihexylsulfide (DHS), diphenylsulfide (DPS) 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

.2 Synthesis of the chitosan anchored peroxidovanadium complex VO(O ) (NH )-

2 2 2

chitosan (ChpV)

The procedure adopted for the synthesis of ChpV is as follows: V O (0.56 g, 3.08 mmol)

2

5

was dissolved in 30% H O (10 mL, 89.28 mmol) maintaining the temperature below 4 ºC.

2

The initial pH of the resulting clear yellow solution was found to be ca. 1. Concentrated

6

potassium hydroxide (ca. 8 M) was then added dropwise to raise the pH of the solution up to

ca. 7 with constant stirring. Keeping the reaction mixture stirred in an ice-bath, 1 g of

chitosan was added to it. The resulting reaction mixture was allowed to stand for 24 h under

constant stirring, maintaining the temperature below 4 °C. Subsequently, the supernatant

liquid was decanted and the resulting yellowish residue was washed repeatedly with acetone.

The product was then dried in vacuo over concentrated sulfuric acid and kept in refrigerator.

The catalyst was found to be non-hygroscopic and stable for several weeks.

2

-

Found: C, 28.78; H, 5.55; N, 6.32; K, 9.67; V, 11.90; O , 14.98%. The metal loading

-1

calculated from the observed vanadium content is 2.34 mmol g of polymer for

VO(O ) (NH )-chitosan (ChpV).

2

.3 Elemental analysis

The vanadium content of the synthesized compound was determined by atomic absorption

spectroscopy (AAS) employing a Thermo iCE 3000 series atomic absorption

spectrophotometer (model analyst 200) and an inductively coupled plasma optical emission

spectrophotometer (ICP-OES) (Perkin Elmer, OPTIMA 2100 DV), along with volumetrically

by the iodometric method described in an earlier paper [33]. The potassium content was also

measured by ICP-OES. Elemental analysis for C, H and N was carried out on an elemental

analyzer (Perkin-Elmer 2400 series II). The peroxide content of the compound was

determined by the procedure reported previously [14,15a,b].

2

.4 Physical and spectroscopic measurements

The IR spectra were recorded by making pressed pellets of samples with KBr disks using a

Perkin-Elmer spectrum 100 FTIR spectrophotometer. The raman spectrum of the compound

was recorded using a Renishaw InVia Raman microscope equipped with an argon ion laser

with an excitation wavelength of 514 nm and a laser maximum output power of 20 mW. The

diffuse reflectance electron absorption spectra were measured using a Hitachi U-3400

spectrophotometer equipped with a diffuse reflectance accessory with an integrating sphere

of 60 mm inner diameter and using BaSO as the standard. The powder X-ray diffraction

4

(XRD) patterns were recorded on a Rigaku X-ray diffractometer (Miniflax, UK) using Cu Kα

−

1

(

λ = 0.154 nm) radiation at a scanning rate of 10 °C min in the 2θ range 10–70°. XPS

measurements were carried out by using a Thermo Fisher Scientific Instrument’s X-ray photo

spectrometer equipped with an Al Kα (monochromatic) X-ray source. The source was

7

-9

operated at 12 kV. The base pressure was maintained at 2 x 10 mbar in the analysing

chamber. Charging of the samples was corrected by setting the binding energy of the

adventitious carbon (C 1s) at 284.6 eV. Thermogravimetric analysis (TGA) was carried out

-1

on a SHIMADZU TGA-50 system at a heating rate of 10 °C min under a N atmosphere

2

using an aluminium pan. Scanning electron microscopy (SEM) and Energy-dispersive X-ray

analysis of the compound were obtained using a JEOL JSM-6390LV scanning electron

micrograph attached with an energy-dispersive X-ray detector. Scanning was done in the 1–

2

0 µM range, and images were taken at a magnification of 15–20 kV. Data were obtained

using INCA software. Standardization of the data analysis is an integral part of the SEM-

1

3

EDX instrument employed. The C NMR spectra for chitosan and ChpV were recorded

using a JEOL JNM-ECX400II spectrometer at a carbon frequency of 100.5 MHz, 1024 X-

resolution points, number of scans 1300, 29 ms of acquisition time and 5 s of relaxation

5

1

delay. V NMR spectra were recorded on a JEOL JNM-ECS400 spectrometer at a vanadium

frequency of 105.154 MHz with the samples in a 10 mm spinning tube using a sealed co-

axial tube containing D O, which provided the lock signal. The chemical shift data were

2

recorded as negative values of ppm (δ) in the low-frequency direction with reference to

VOCl at 293 K. The surface areas were determined by nitrogen adsorption/desorption

3

measurements based on the Brunauer-Emmett-Teller (BET) method at 77.3 K on a standard

module NOVA 1000E, Quantachrome Instrument. The pore size and pore volumes were

measured by the Barrett-Joyner-Halenda (BJH) model in the same Quantachrome Instrument.

Magnetic susceptibilities of the complex were measured by the Gouy method, using

Hg[Co(NCS)] as the calibrant. The melting points of the products were determined in open

capillary tubes on a Büchi Melting Point B-540. The GC analysis was done using a CIC

model 2010 gas chromatograph and an SE-52 packed column (length 2 m, 1/8 inch OD) with

−

1

a flame ionization detector (FID) and nitrogen as the carrier gas (30 mL min ).

2

.5 General procedure for the catalytic oxidation of sulfides to sulfoxides

In a representative procedure, the organic substrate MPS (5 mmol) was added to the reaction

mixture consisting of the catalyst (21.00 mg, containing 0.005 mmol of V) in 5 mL of H O.

2

Then 30% H O (2.82 mL, 25 mmol) was added to the reaction system, contained in a round

2

bottom flask. The molar ratio of V:substrate and that of substrate:H O was maintained at

2

1

:1000 and 1:5, respectively. For the reaction conducted in acetonitrile, the V:substrate and

the substrate:H O ratios were adjusted to 1:1000 and 1:2, keeping the other reaction

2

conditions identical to the reaction conducted in water. The reaction was conducted at

8

ambient temperature under continuous magnetic stirring. The progress of the reaction was

monitored by thin layer chromatography (TLC) and GC. After completion of the reaction, the

catalyst was separated by filtration and washed with acetone. The product and the unreacted

substrates were extracted with diethyl ether from the filtrate, dried over anhydrous sodium

sulphate and distilled under reduced pressure to remove excess solvent. The obtained product

was purified by column chromatography on silica gel using ethyl acetate-hexane (1:9) as the

1

13

eluent. The product was characterized by IR, H NMR, C NMR and melting point

determination for solid products [Text S1 (Supplementary Information)].

2

.6 Regeneration of the catalyst

The recyclability of the catalyst was tested using MPS as a model substrate. After completion

of the reaction, the catalyst was separated by centrifugation, washed with acetone and dried in

vacuo over concentrated sulfuric acid. Diethyl ether was used to extract the unreacted

substrate and product. To repeat the experiment, the recovered catalyst was added to a fresh

batch of MPS (0.59 mL, 5 mmol) and 30% H O (2.82 mL, 25 mmol in water or 1.13 mL, 10

2

mmol in acetonitrile) in 5 mL solvent (water or acetonitrile). The reaction was allowed to

proceed under the optimized conditions. The reusability of the catalyst was tested up to 5

reaction cycles independently in both CH CN and H O. The recycled catalyst was

3

2

characterized by IR spectroscopy and ICP-OES.

3

. Result and Discussion

3

.1 Synthesis and characterization

In the present study, the synthetic strategy developed for obtaining the desired immobilized

catalyst, ChpV, was based on the reaction of in-situ generated pV species with chitosan

under fairly mild reaction conditions (Scheme 1). The reaction was carried out in water at

neutral pH and at ice-bath temperature. As chitosan is known to be water soluble under acidic

conditions [34], the pH of the reaction medium was stringently maintained above ca.7 in

order to obtain the solid catalyst in a water insoluble form. The alkali used to adjust the pH of

the reaction solution also served as the source of the counter cation for the pV complex

anions formed. It is notable that in contrast to the highly hygroscopic nature usually observed

for monomeric peroxidovanadium complexes [13c,35], the supported catalyst ChpV has been

observed to be non-hygroscopic and stable for weeks, without any change in its catalytic

activity.

9

The newly synthesized catalyst has been characterized by elemental analysis, TG-

DTG analysis and a combination of spectroscopic techniques, such as IR, Raman and

1

3

electronic spectral studies, SEM-EDX, ICP-OES, powder XRD, XPS, BET analysis and C

NMR study etc. The presence of two peroxido groups per vanadium centre in the compound

2

-

was confirmed from elemental analysis data, showing the V:O₂ratio as 1:2. The elemental

analysis data complemented by inductively coupled plasma-optical emission

spectrophotometer (ICP-OES) analysis and EDX spectral analysis results showed the

vanadium loading on chitosan to be 2.34 mmol metal per gram of the polymeric support. The

occurrence of vanadium in the compound in its +5 oxidation state was confirmed by

magnetic susceptibility measurement, which showed the diamagnetic nature of the complex

ChpV.

3

.1.1 SEM and energy dispersive X-ray (EDX) analysis

The scanning electron micrograph of the compound ChpV showed considerable roughening

of the surface compared to the free polymer chitosan, as seen in Figs. 1(a) and (b). This

morphological change of the surface of the compound is due to the adsorption or dispersion

of the peroxidometallates on the pristine polymer surface.

<Fig. 1>

The presence of K and V along with C, O and N in the supported pV catalyst has been

confirmed further from the energy dispersive X-ray analysis results [Fig. 1(c)]. EDX analysis

has been carried out by focusing on multiple regions over the surface of the catalyst. The

composition of the compound as obtained by EDX analysis is in close agreement with the

elemental analysis data.

3

.1.2 X-ray diffraction studies

The X-ray diffraction patterns of chitosan and the metal incorporated polymer immobilized

catalyst, ChpV, are presented in Fig. 2. The X-ray diffractogram of chitosan displayed three

main diffraction peaks at 2Ɵ values of ca. 12.7, 19.97 and 26.7º, which correspond to the

semi-crystalline nature of the polymer [36]. In the diffractogram of ChpV, the intensities of

the characteristic peaks of chitosan were observed to weaken with the concomitant

appearance of many new diffraction peaks, indicative of the formation of a new crystalline

1

0

phase [4a,e,37]. The decrease in intensity of the peaks corresponding to chitosan is likely to

be due to the disruption of hydrogen bonds within chitosan owing to incorporation of the

metal complex [4e,37,38]. This may be a plausible factor responsible for the relatively lower

thermal stability of the catalyst, as has been demonstrated by TGA results. The additional

sharp diffraction peaks in the diffractogram of the ChpV complex, were located at 2Ɵ values

of 14.4, 15.3, 17.7, 25.2 and 28.6°. These values are close to the ones observed for

peroxidovanadium species (PDF 81-2392) and may be ascribed to the (001), (200), (201),

(110) and (202) planes, respectively. These observations confirmed the anchoring of the

peroxidovanadium moiety to the polymer matrix.

<Fig. 2>

3

.1.3 X-ray photoelectron spectroscopy

The XPS spectrum of ChpV is presented in Fig. 3. This technique is important for analyzing

the electronic properties of the species on the surface. The complex displayed characteristic

peaks for vanadium (2p₃_/₂and 2p ) at 517.5 and 525.0 eV, respectively, with a splitting

1/2

difference of ca. 7.5 eV, which is close to the value reported for V O [39]. The binding

2

5

energy values suggest the presence of vanadium in the + 5 oxidation state in the complex on

the basis of the available literature [39,40]. The O 1s peak is observed at 533.0 eV with a

shoulder at 530.9 eV. This may be due to oxygen from both the chitosan support and the

metal peroxido system, respectively [39d,41]. Thus, the XPS study confirms the

immobilization of the pV complex on chitosan.

<Fig. 3>

3

.1.4 BET analysis

The surface areas of the polymer, chitosan and metal incorporated complex ChpV were

measured using BET analysis with the nitrogen adsorption method and the pore volume was

determined by the BJH model [42]. The N adsorption/desorption isotherm of the compound

2

showed a typical type II adsorption (Fig. 4) of an IUPAC standard for particles which have

macropores or non-pores showing poor adsorption [43]. The average specific surface area of

2

the vanadium incorporated metal complex ChpV was found to be 43.6 m /g, which is

2

considerably greater than that of the pristine polymer (3.0 m /g) (Table 1). Thus, it is evident

that complexation of chitosan with vanadium results in affirmative changes in the surface

1

morphology of the polymer. The value of the constant C in the BET equation was in the

range (2-200), which is characteristic of multilayer adsorption (type II isotherm) [44].

<Fig. 4>

3

.1.5 IR and Raman spectral studies

Significant features of the IR spectra of chitosan and ChpV are summarized in Table S1

Supplementary Information]. Characteristic differences were observed between the spectral

patterns originating from the vanadium anchored chitosan and free chitosan. As seen in Fig.

[

5

3

, the IR spectrum of the initial free chitosan exhibits typical peaks located at approximately

434 (O-H stretch and N-H stretch), 2921 (-CH stretch), 2876 (-CH stretch), 1592 (N-H

3

2

-1

bending), 1154 (bridge O stretch) [38a,45] and 1646 cm (C–O stretching along with N–H

deformation mode, amide I) [4e,46]. In the spectrum of the compound ChpV, there is no

distinct shift observed in the position of the amide I band of the complex compared to the

untreated polymer, indicating that the amide group is not involved in coordination with the

metal. The possibility of coordination through the N atom of the amide group is also not

likely as such coordination causes a considerable decrease in the carbonyl stretching

-1

frequency, which is not the case for the ChpV complex [16a,47]. The peak at 3434 cm ,

corresponding to the stretching vibrations of the –NH and –OH groups, showed a

2

-1

considerable shift to a lower frequency of 3199 cm , suggesting that either the -NH or –OH

2

group takes part in complexation [4a,b,37a,38a,45,48]. However, the peak attributed to the

bending vibration of the –OH group and the band assigned to the secondary –OH group at

-1

1

421 and 1089 cm , respectively remained unaltered in the spectrum of the complex,

negating the possibility of the –OH group taking part in chelation [4a,b,34b,37a,48,49].

However, the –NH bending vibration of the free amine group in chitosan is shifted to a lower

wavenumber in the complex, ChpV. It was observed that both the stretching and bending

vibrations of the –NH group shifted to a lower value in the complex, because coordination of

2

chitosan through the nitrogen centre with the metal ion would possibly reduce the electron

density of the amino group [4a,b,e,12a,37a,38a,46a,48].This confirmed the coordination of

the vanadium(V) center with chitosan through the -NH group [4a,b,e,37a,38a]. The band at

2

-1

6

42 cm , ascribed to V-N bond, further signifies the complexation of the metal with chitosan

-1

[

4b,8b,37,38,50] through the –NH group. Moreover, the band at 1038 cm of chitosan

2

remain unaffected in ChpV, which means that the primary -OH group is not involved in

1

2

complexation [4e,48]. The characteristic bands of chitosan, the β-(1–4) glycoside bridge

−1

bands (at around 1,154 and 894 cm ), were also seen in the FT-IR spectra of ChpV,

indicating that the metal complex successfully anchored to chitosan without altering the main

backbone of chitosan [4e,38b,46c,50a].

Apart from the typical bands due to the chitosan support, the IR spectrum of ChpV

enabled the clear identification of diagnostic peaks attributable to the terminal oxido and

-1

peroxido groups of the [VO(O ) ] moiety, with an intense band at 939 cm and absorptions at

2

-1

ca. 869, ca. 616 and ca. 525 cm representing the ν(V=O), ν(O-O), ν (V-O ) and ν (V-

asym

2

sym

O ) modes, respectively [13b,51]. The Raman spectrum of the catalyst complemented the IR

2

results, displaying well-resolved bands corresponding to ν(O-O), ν_a_s_y_m(V-O ), ν (V-O ) and

2

sym

2

-1

ν(V= O) in the vicinity of ca. 895, ca. 628, ca. 542 and ca. 956 cm , respectively (Fig. 6).

<

Fig. 5>

Fig. 6>

<

3

.1.6 Electronic spectral studies

The diffuse reflectance UV-visible spectrum of the compound ChpV showed two bands at

around 350 and 245 nm (Fig. 7). The band appearing at 350 nm is assigned to a peroxido to

metal transition (LMCT), which is characteristic of a diperoxido species of vanadium(V)

[

13b,c]. The band at 245 nm in the spectrum of ChpV is assigned to the typical n→π* and

π→π* transitions of the chitosan support [38a,48,52].

Fig. 7>

<

1

3

51

3

.1.7 C and V NMR studies

1

3

The C NMR spectrum of the catalyst along with the spectrum of free chitosan recorded in

the solid state are presented in Fig. 8. The corresponding resonances are listed in Table 2 for

comparison. The assignments are on the basis of available literature data [3a,53], however,

the exact resonance position may vary slightly according to the NMR technique used (solid or

1

3

liquid state analysis). The C NMR spectrum of chitosan displays characteristic resonances

at δ 58.7, 62.3, 85.1 and 106.3 ppm attributable to the C-2, C-6, C-4, and C-1 carbon atoms,

respectively. The resonance centered at δ 76 ppm probably originates from a combination of

the C-3 and C-5 signals, as has been reported [53d]. As the chitosan used was highly

deacetylated, no observable peaks at around δ 174 and 24 ppm, characteristic of the carbon

1

3

atoms of the –C=O and –CH groups, were found in the spectrum, as anticipated [53a,b]. The

3

comparison of the NMR spectra before and after loading of the pV moiety showed distinct

changes in the spectrum of the catalyst, (as seen in Table 2) although the overall spectral

pattern of the catalyst ChpV resembled closely the spectrum of free chitosan by exhibiting

major peaks at δ 57.7, 62.7, 85.8 and 100.5 ppm due to the C-2, C-6, C-4, and C-1 carbon

atoms. However, the spectrum showed a distinct shift of the resonance positions of the carbon

atoms of chitosan along with a broadening of the signals in comparison to the original

chitosan spectrum, indicating a change in environment of the C atoms after metal anchoring

[

53b,c]. The intensity of the signal corresponding to the C-2 atom attached to the –NH group

2

was shifted by nearly 1 ppm and appeared as a weaker intensity shoulder along with the peak

at δ 62.7 ppm in the spectrum of the catalyst. Moreover, the C-1 signal showed a considerable

shift and the spectrum of the catalyst enabled clear identification of the C-3 and C-5 signals at

δ 100.5 and 76.8 ppm, respectively. It is significant to note that no new resonance was seen in

the spectrum at around δ 175 ppm, attributable to the –COOH group. Thus, from the NMR

analysis it has been confirmed that although the chitosan structure undergoes modification

owing to anchoring of the pV species, the main backbone structure of chitosan does not

change during complexation with V(V) [54].

<Fig. 8>

5

1

The V NMR spectral analysis of the compound ChpV furnished further evidence to

suggest anchoring of perxidovanadium moieties to the chitosan polymer. The spectrum of the

compound shown in Fig. 9, displayed a resonance at δ -690 ppm, which is in the region

typical of a diperoxidovanadate species. The peak has been assigned on the basis of available

literature [13c,15b,35b].

<Fig. 9>

3

.1.8 Thermal analysis

The thermal degradation behaviour of chitosan, as well as chitosan-metal complexes, has

been extensively investigated in recent years [4b,e,11,12a,37b,38b,c,55]. Apart from the

initial water loss stage below 100 °C, chitosan exhibits a single stage decomposition in the

range 270-315 ºC, with a mass loss of 33.1% due to the degradation of the polymer, including

deacetylation. The polymer undergoes further degradation up to 700 °C. In the majority of

1

4

the reports, it has been observed that complexation of chitosan with metals usually lowers the

thermal stability of the polymer [4b,e,11,12a,37b,38b,c,55].

The TG-DTG profile of the chitosan anchored pV compound showed a multistage

degradation upon heating up to 700 °C (Fig. 10, Table 3). After the first decomposition step

due to dehydration occurring in the temperature range below 100 °C, the subsequent

degradation occurred in the temperature range 139-182 °C with a mass loss of 12.7%. This

step has been ascribed to the loss of peroxido groups from the chitosan anchored pV species.

Thereafter, on increasing the temperature, the compound undergoes further degradation in the

range 226-310 °C due to the cleavage of the glycosodic linkage in the chitosan backbone

[

4e,38c] with a mass loss of 21.6%. Thus the step corresponding to degradation of the

chitosan support occurs at a lower temperature in the catalyst ChpV, compared to free

chitosan, indicating a relatively lower thermal stability of the pV-chitosan complex, in

agreement with the previous finding [4b,e,11,12a,37b,38b,c,55]. The black residue remaining

after complete degradation of the catalyst ChpV was found to be 59.8%. The FTIR spectrum

of the residue displayed the typical ν(V=O) bands of oxidovanadium species. The chitosan

bands of the original compound nearly disappeared in the spectrum of the residue, which is in

accord with a previous report showing that the characteristic bands of chitosan disappear in

the char residue obtained after thermal degradation of chitosan on heating up to a temperature

of 600 ºC [56]. Thus, it has been confirmed that the residue from the chitosan immobilized

pV complex ChpV consists of an oxidovanadium compound along with the char residue of

the polymer.

<Fig. 10>

Based on the above collective evidences, a structure of the type shown schematically

in Fig. 11 has been envisaged for the macromolecular complex ChpV. It is worthy to

mention that different models have been proposed previously [4a,b,f] to describe the mode of

metal ion-chitosan coordination, such as the ‘bridge model’ and ‘pendant model’. In the

bridge model, the metal ion is usually bound with four nitrogen atoms from the same chain or

from different chains, whereas in the pendant model, the metallic ion is linked to the amino

group as a pendant. In case of the catalyst ChpV, the structure shows the coordination of the

V(V) centre of the pV moiety to the amino group of chitosan to form a ‘pendant complex’.

1

5

<Fig. 11>

3

.2 Catalytic activity of the supported complex ChpV

.2.1 Oxidation of sufides to sulfoxides

We have explored the catalytic performance of the chitosan immobilized pV compound,

ChpV, as a heterogeneous catalyst in the selective oxidation of organic sulfides to sulfoxides

using 30% H O as a terminal oxidant, under a variety of reaction conditions. In order to

2

optimize the reaction conditions to achieve the best conversion and selectivity, the influence

of key factors, such as type of solvent, substrate:H O stoichiometry, catalyst concentration

2

etc., were investigated using methyl phenyl sulfide as a representative substrate. Details of

the study are shown in Table 4.

3

.2.1.1 Effect of concentration of the oxidant

In an exploratory experiment, the reaction of MPS with H O , maintaining an MPS:H O

2

molar ratio of 1:2 and a catalyst:substrate ratio of 1:1000, was conducted in aqueous medium

at ambient temperature under magnetic stirring. As evident from the data presented in Table 4

(entry 1), under these conditions MPS was completely transformed into the sulfoxide with

1

00% selectivity. The TOF could be improved further by increasing the amount of oxidant

gradually from 2 to 5 equivalents. More than a 10-fold rise in the TOF (Table 4, entry 4)

could be achieved with 5 equivalents of H O , without affecting the selectivity of the

2

reaction. Interestingly, a further increase in the oxidant amount to 6 equivalents led to over

oxidation of sulfoxide to sulfone, rendering the reaction non-selective, although a substantial

increase in TOF occurred (Table 4, entry 5).

3

.2.1.2 Effect of catalyst amount

Apart from the oxidant concentration, the amount of catalyst was observed to have a

significant influence on the rate and selectivity of the sulfide oxidation. As illustrated in

Table 4 (entry 6), the catalyst was found to be effective even at a V:substrate molar ratio of

1

7

:2000, affording a reasonably good TOF. An increase in the catalyst amount (Table 4, entry

) accelerated the process as expected, however, the reaction resulted in a reduced TOF and

1

6

partial over oxidation of sulfoxide to sulfone. Thus, for achieving a high conversion without

compromising the sulfoxide selectivity in aqueous medium, a catalyst:substrate molar ratio of

1

:1000 and a substrate:oxidant ratio of 1:5 have emerged to be optimal.

The importance and role of the catalyst in facilitating the formation of the target

product was confirmed by conducting a control experiment in the absence of the catalyst. The

reaction under such conditions was found to be rather slow and non-selective, leading to the

formation of both sulfoxide and sulfone in < 12% yield (Table 4, entries 16 and 17) within

the stipulated reaction time. With the aim of comparing the activity of the heterogeneous pV

catalyst with the homogeneous one, we have conducted the oxidation reaction using a

previously reported water soluble monomeric diperoxidovanadate complex,

K[VO(O ) (H O)] (DPV) [57], in lieu of the heterogeneous pV catalyst, ChpV, maintaining a

2

similar catalyst concentration (V:MPS of 1:1000). As can be seen from the results in Table 4

-

(

entries 18,19), in the presence of the free complex species, [VO(O ) (H O)] , the TOF

2

obtained was nearly half of that afforded by the polymer supported heterogeneous catalyst

ChpV under otherwise analogous reaction conditions. Moreover, the DPV catalysed reaction

provided a mixture of sulfoxide and sulfone and hence was found to be less selective.

3

.2.1.3 Effect of solvent

We have also screened the performance of the catalyst in common organic solvents, in

addition to water. In order to assess the solvent effect, we have chosen relatively safer

organic solvents such as methanol, ethanol and acetonitrile for the oxidation of MPS [58]. It

is pertinent to mention that in the present study, we have strategically avoided the use of

hazardous chlorinated solvents. To our pleasure, as seen from the data depicted in Table 4,

the catalytic protocol for sulfoxidation was observed to be compatible with each of the tested

organic solvents. In fact, the catalyst turned out to be more potent in organic solvents

compared to water, providing the best results in acetonitrile with respect to both product

selectivity and TOF, even with 1 equivalent of H O (Table 4, entry 8). This may not be

2

surprising in view of the complete solubility of the organic substrate in these solvents. Nearly

a 4-fold improvement in the TOF could be achieved in acetonitrile simply by increasing the

amount of oxidant to 2 equivalents and maintaining the catalyst:substrate ratio as 1:1000

(Table 4, entry 9). A further increase in amount of oxidant, however, led to a loss of

selectivity of the reaction (Table 4, entry 10). Thus, in acetonitrile a substrate:oxidant molar

ratio of 1:2 was found to be optimal (Table 4, entry 9). Moreover, as can be seen from the

1

7

results presented in Table 4 (entry 9), a catalyst:substrate ratio of 1:1000 also proved to be

conducive to attain the best conversion in acetonitrile. It is thus remarkable that the same

catalyst enabled us to achieve selective oxidation of sulfides in aqueous medium as well as in

organic solvents by versatile variation of the reaction conditions, as shown in Table 4.

Subsequently, we have extended the study to a range of structurally diverse sulphides,

such as dialkyl, diaryl, aryl alkyl, aryl allyl, aryl alcohol etc., in experiments conducted

independently in water as well as acetonitrile under the respective standardized reaction

conditions. The results, depicted in Tables 5 and 6, demonstrate that excellent yields with

complete selectivity were possible to be attained with the series of substrates examined in the

presence of the catalyst. Importantly, no over oxidation of sulfoxide to sulfone was observed

in any of the tested substrates under the investigated conditions.

That the nature of the substituent influenced the rate of oxidation was evident from

the variations in the TOF values within the substrates examined. Dialkyl sulfides were

oxidized by H O at a faster rate with a higher TOF compared to allylic sulfides. In fact,

2

-1

dimethyl sulfide was oxidized with the highest TOF of 5628 and 5688 h in water and

acetonitrile, respectively (Table 5, entry 2 and Table 6, entry 2). On the other hand, in case of

a conjugated system, such as diphenylsulfide, the reaction was found to be rather sluggish

with a relatively poor TOF (Table 5, entry 9 and Table 6, entry 9). The observed trend in the

rates of oxidation is in accord with the decreasing nucleophilicity of the thioethers examined.

As it is known that sulfide oxidation by H O generally occurs via an electrophilic addition

2

reaction of an oxygen atom to the substrate [59], it is expected that thioethers such as alkyl

sulfides with a higher electron density on the sulfur atoms would react faster compared to

conjugated systems, such as aromatic and allylic sulfides.

<

Table 5>

Table 6>

<

An additional attractive feature of the methodology, irrespective of the solvent used,

is the excellent chemoselectivity of the catalyst towards the sulfur group of the substituted

sulfides with other oxidation prone functional groups. Thus, allylic and alcoholic sulfoxides

were obtained without affecting any other functional group transformation (Table 5, entries

7

,8 and Table 6, entries 7,8). The protocol for sulfide oxidation was also applied for the high

c

scale synthesis of sulfoxide up to ten-fold under optimized conditions (Table 5, entry 1 and

1

8

c

Table 6, entry 1 ). The H O efficiency in the oxidation reaction under the optimized

2

conditions was found to be greater than 90% for all substrates, except for diphenyl sulfide

67% in H O and 73% in CH CN), complementing the higher activity of the catalyst within a

(

2

3

reasonably short reaction time in both CH CN and H O. The efficiency was measured as 100

3

2

×

moles of H O consumed in the formation of the oxyfunctionalized products per mole of

2 2

H O converted [Text S2 (Supplementary Information)] [60].

2

3

.2.2 Test for the heterogeneity of the reaction

To confirm the heterogeneity of the reaction, separate experiments were carried out using

MPS as te substrate under standard protocols. After completion of the reaction, the solid

catalyst was separated by filtration and the filtrate was treated with a fresh batch of MPS and

3

0% H O . The reaction was allowed to continue for another 1h. It was found that the

2 2

conversion was very low, only about 11% in H O (9% in CH CN), which is close to the

2

3

conversion obtained from the control experiment conducted in the absence of a catalyst

Table 4, entries 16,17). That the reaction did not proceed on removal of the catalyst was thus

(

evident. Moreover, the absence of vanadium in the filtrate obtained after separating the solid

catalyst was ascertained by ICP-OES analysis. These observations refute the possibility of

leaching of the metal complex from the polymer incorporated catalyst into the reaction

medium during the oxidation reaction, which further proves the heterogeneity of the catalytic

process.

3

.2.3 Recyclability of the catalyst

Owing to the heterogeneous nature of the catalyst, it could be easily separated from the spent

reaction mixture and recharged in the subsequent reaction run. The recyclability of the

catalyst was tested for five reaction cycles using MPS as the substrate. Recycling experiments

were performed by charging the spent catalyst with a fresh batch of substrate, H O and the

2

respective solvent (CH CN or H O) after completion of each reaction cycle. The catalyst

3

2

could be reused for at least up to five reaction cycles with consistent activity and selectivity

when acetonitrile was used as the solvent (Table 6 and Fig. 12). The FT-IR spectrum of the

spent catalyst showed the characteristic bands corresponding to chitosan and the metal-

peroxido stretching, as has been observed in the spectrum of the original catalyst (Fig. 13).

Moreover, no significant decrease in the vanadium content value of the recovered catalyst

was indicated by the ICP-OES analysis and EDX spectral data compared to the starting

catalyst. In the case of the reaction conducted in water, the catalyst could be reused with

1

9

rd

consistent activity and selectivity up to the 3 cycle, after which a slight decrease in activity

was noted, indicating a small amount of leaching of the pV species (Table 5 and Fig. 12).

<

Fig. 12>

Fig. 13>

<

3

.2.4 Proposed catalytic cycle

In order to explain our observations pertaining to the catalytic activity displayed by the

chitosan immobilized catalyst in the sulfoxidation reaction, a plausible catalytic cycle, as

shown in Fig.14, is proposed. Our previous findings on the oxidant activity of pV

complexes, as well as ample evidences available in the literature, have shown that

vanadium(V) peroxido complexes act as electrophilic oxygen transfer agents, reacting

preferentially with the most nucleophilic group of the substrate molecule [13b,c,17a-

d,58c,d,61]. It is therefore expected that the first step in the proposed scheme of reaction

would be the facile transfer of the electrophilic oxygen atom from the diperoxido vanadium

species (I) to the sulphide, yielding sulfoxide as the product, with the concomitant

transformation of the catalyst into a dioxidemonoperoxidovanadium intermediate II (reaction

a). The monoperoxidovanadate intermediate subsequently combines with peroxide to

regenerate the original catalyst in the presence of excess H O , thereby completing the

2

catalytic cycle (reaction b).

<Fig. 14>

4

. Conclusions

To conclude, this report highlights the preparation and characterization of a new

peroxidovanadium(V) compound immobilized on chitosan and its use as a cost-effective

environmentally safe heterogeneous catalyst for the selective oxidation of sulfides to their

respective sulfoxides in aqueous medium. The compatibility of the catalyst in relatively safer

organic solvents as well, under mild conditions, to afford desired products with impressive

yields, is an additional important attribute of the catalyst. Apart from the high activity and

product selectivity displayed in both aqueous as well as organic media, other significant

strengths of the protocol which make it attractive from both economic and ecological

2

0

perspectives include (i) chemoselectivity and easy recyclability of the catalyst for several

catalytic cycles; (ii) high H O efficiency percentage; (iii) ready scalability; (iv) simplicity in

2

operation and work-up procedure.

Acknowledgement

Financial support from the Department of Science and Technology, New Delhi, India under

Grant No. SR/WOS-A/CS-35/2017, is gratefully acknowledged. The authors are thankful to

Mr. D. Shriniwas, National Chemical Laboratory (NCL), Pune, India for XPS analysis.

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A chitosan supported peroxidovanadium(V) complex: Synthesis, characterization and application as an eco-compatible heterogeneous catalyst for selective sulfoxidation in water

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