Schiff base Mn(III) and Co(II) complexes coated on Co nanoparticles: An efficient and recyclable magnetic nanocatalyst for H2O2 oxidation of sulfides to sulfoxides

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PAPER

Schiﬀ base Mn(III) and Co(II) complexes coated on

Co nanoparticles: an eﬃcient and recyclable

magnetic nanocatalyst for H O oxidation of

Cite this: RSC Adv., 2018, 8, 3889

2

sulﬁdes to sulfoxides

ab

a

c

Shokoufeh Ghahri Saremi,* Hassan Keypour, Mohammad Noroozi

b

and Hojat Veisi

In this paper, an eﬀective and selective heterogeneous catalyst was produced by immobilization of

manganese and cobalt Schiﬀ base-complexes on Co magnetite nanoparticles (MNP). The catalysts

3

Co@SiO

2

[(EtO)

3

Si–L ]/M (M

¼

Mn(III) and Co(II)) were synthesized using Co@SiO

2

core–shell

3

nanoparticles and amino-functionalized Co@SiO

2

. The Schiﬀ base ligand Co@SiO [(EtO)

2

3

Si–L ] was

synthesized by reacting Co@SiO

2

core–shell nanoparticles with 2-hydroxy 1-naphthaldehyde for the

Received 11th October 2017

Accepted 5th January 2018

3

2 3

synthesis of Co@SiO [(EtO) Si–L ]/M. The catalysts were characterized by several techniques, such as

FT-IR, TEM, XRD, TGA and VSM. The catalytic activities of the prepared catalysts were studied by

oxidation of sulﬁdes to the sulfoxides under diﬀerent conditions. These catalysts can be easily recovered

and reused in at least seven sequential cycles without considerable leaching and loss of reactivity.

DOI: 10.1039/c7ra11225d

rsc.li/rsc-advances

22

Mn(III) complex supported on magnetic cobalt nanoparticles,

Schiﬀ base complexes of Ni, Co, Cr, Cd and Zn supported on

Introduction

23

3 4

Since the oxidation reactions have potential for generating Fe O magnetic nanoparticles, magnetic nanoparticle immo-

24

25

a wide range of drugs and ne chemicals, they are very impor- bilized N-propylsulfamic acid,

Au/CTN–silica catalyst,

tant to the chemical industry. Conventionally, oxidation reac- cobalt(II), copper(II), zinc(II) and palladium(II) Schiﬀ base

26

tions have been carried out using environmentally unfavorable complexes, 1,4-bis(3-methylimidazolium-1-yl) butane ditri-

28

27

reagents, solvents and catalysts. Hence, eﬀorts have been made bromide, carboxylated multi-walled carbon nano tubes,

towards the development of eﬃcient, easily recoverable, highly 1,4-bis(3-methylimidazolium-1-yl) butane ditribromide

ionic liquid reagent and Mo(VI) complex sup-

29

selective and reusable heterogeneous catalysts. Recently [bMImB](Br

magnetic nanomaterials have attracted signicant attention in ported on Fe

oxidation reactions because of their catalytic activity and easy neous nanocatalysts were synthesized by chemical modication

3 2

)

30

3 4

O nanoparticles. In this study, new heteroge-

1–5

separation.

of surface magnetic nanoparticles of Co with Schiﬀ-base

Schiﬀ base transition metal complexes as catalysts have been ligands, followed by complex formation through the reaction

widely used because of their potential use in numerous reac- with Co(II) and Mn(III) salts. Furthermore, the catalytic behavior

6

,7

8

tions, such as hydrogenation of organic substrates, epoxi- of these complexes was investigated by carrying out the oxida-

9

10,11

dation of olens, conversion of epoxides into halohydrines,

asymmetric ring opening of terminal epoxides and oxidation

reactions.

tion of suldes to sulfoxides under mild conditions.

12

13–15

In recent years some publications have been re-

ported suldes-oxidation using phthalazine-based di-iron Experimental

1

6

complexes,

b-brominated

meso-tetraphenylporphyrinato

Materials and equipments

17

18

manganese(III) acetate, copper Schiﬀ base complex, immo-

2 5

All solvents used in this study such as THF, C H OH, DMSO,

19

bilized metalloporphyrins, TsOH by phenyliodine diacetate as

2 2 3

DMF, CH Cl , ethyl acetate, MeOH, CHCl , acetone, and MeCN

2

0

21

an oxidant, dendritic bis(acylamino) pyridines, Schiﬀ base of

(analytical grade) were purchased from Fluka or Merck and

used without further purication. In addition, cobalt chloride

hexahydrate, citric acid, sodium borohydride, 3-aminopropyl-

triethoxysilane (APTES), tetraethoxysilane (TEOS), 2-hydroxy

a

Faculty of Chemistry, Bu-Ali Sina University, Hamedan 65174, Iran

b

Department of Chemistry, Payame Noor University, Tehran, Iran. E-mail: sho.

1

-naphthaldehyde, Mn (acac) , H O (analytical grade 30%

saremi@gmail.com

3 2 2

c

3 2

aqueous solution), and Co(CH COO) were purchased from

Center for Research and Development of Petroleum Technologies at Kermanshah,

Research Institute of Petroleum Industry (RIPI), Iran

Fluka or Merck. HPLC grade methyl phenyl sulde and other

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sulde compounds were purchased from Aldrich in order to test water, 0.07 g of citric acid was added and the solution was

the performance of the catalysts. stirred for 2 min under nitrogen atmosphere. Then, a solution

Nanocatalysts were characterized using a Holland Philips of 3 g sodium borohydride in 100 mL distilled water was added

ꢀ

PW-1840 with monochromatic Cu Ka radiation and l 1.54 A dropwise to the stock solution under nitrogen atmosphere.

4

X-ray powder diﬀraction (XRD) diﬀractometer at a scanning During the addition of NaBH , cobalt nanoparticles were

ꢀ

ꢁ1

ꢀ

speed of 2 min from 5 to 80 (2q). The particle size and formed and the color of the solution changed to black. The

morphology were investigated using scanning electron micros- nanoparticles were rst stabilized by the addition of citrate.

copy (SEM) (Holland Philips XL30 microscope) at an accelerating Aer reduction, the particles remained stable for several hours.

ꢁ

1

voltage of 25 kV. The FT-IR measurements (4000–400 cm ) were A perfect monomer formation is generally favored through

performed using KBr disc on a Shimadzu Fourier Transform decreased concentration of cobalt nuclei and good long-time

1

13

Infrared spectra (FT-IR) 8400. The H- and C-spectra were dispersion aer citrate stabilization to ensure that the core

recorded at 90 MHz, on a Bruker DRX 500-Avance FT NMR particles are well separated before coating begins.

instrument with CDCl

3

as the solvent. Elemental analysis for C,

Second, the cobalt magnetic core was coated with the SiO

. The solu-

2

H and N were performed using a Perkin-Elmer 2400 series shell to obtain core–shell nanoparticles Co@SiO

2

analyzer. Thermogravimetric Analysis (TGA) was performed on tion containing Co nanoparticles was stirred for 30 min. Then,

ꢀ

a Mettler Toledo TGA SDTA 85-e instrument at 30–800 C with under nitrogen atmosphere, 200 mL ethanolic solution of

ꢀ

ꢁ1

a temperature gradient of 10 C min

.

3-aminopropyl-triethoxysilane (APTES) and 800 mL tetrae-

Preparation of the magnetic Co@SiO –NH amine (3). The thoxysilane (TEOS) was sequentially added to the solution. The

2

overall synthesis process of the nanocatalyst Co@SiO [(EtO) - nitrogen atmosphere was discontinued and the resultant

2

3

Si–L ]/M (M ¼ Co(II) and Mn(III)) (6) is shown in Scheme 1. solution was vigorously stirred for 24 h. Finally, the obtained

Magnetic Co@SiO –NH amine was synthesized according to black precipitate was collected with an external magnet,

the procedure previously reported in literature. First, to washed twice with deionized water and ethanol and dried at

2

15

ꢀ

a solution of 1 g cobalt chloride hexahydrate in 60 mL distilled 50 C in the oven.

3

Scheme 1 Preparation of Co@SiO

2

[(EtO)

3

Si–L ]/M (M ¼ Mn(III) and Co(II)) nanocatalysts.

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Further, 3.0 g of the resultant Co@SiO

50 mL toluene for 50 min via sonication. Then, 11.25 mL

48.06 mmol) of APTES was added dropwise to the mixture with

2

was dispersed in

1

(

vigorous mechanical stirring. The mixture was reuxed for 15 h.

Eventually, the reaction mixture was cooled and the resultant

precipitate was collected with an external magnet, washed twice

ꢀ

with deionized water and ethanol and the dried at 55 C in the

oven.

3

Preparation of Co@SiO

2

[(EtO)

3

Si–L ] (5). For the preparation

of the Schiﬀ base ligand (5), the obtained amine in the previous

step was reacted with the 2-hydroxy-1-naphthaldehyde.

First, 1.0 g of Co@SiO –NH was dispersed in 60 mL of dry

2

methanol by sonication for 20 min. Then, 1.20 g (7 mmol) of 2-

hydroxy 1-naphthaldehyde was added to the mixture. Aer

ꢀ

stirring at 50 C for 24 h, the reaction ask was le undisturbed

to be cooled. Then, the obtained precipitate was collected with

an external magnet and washed twice with dry methanol and

dried under vacuum for 20 h.

3

Preparation of Co@SiO [(EtO) Si–L ]/M (M ¼ Co(II) and

2

3

Mn(III)) (6). Initially, 300 mg of Co@SiO [(EtO) Si–L ] was

2

3

ultrasonically dispersed in 30 mL ethanol for 30 min. Then,

Mn (acac) (1 mmol) was dissolved in 10 mL ethanol. This

solution was added to the dispersion of Co@SiO

The mixture was reuxed for 10 h. Then, the Co@SiO

3

2

[(EtO)

3

Si–L ].

3

Fig. 1 FT-IR spectra of (a) Co@SiO

2

–NH

2

, (b) Co@SiO

2

[(EtO)

3

Si–L ],

3

2 3 2 3

3

2

[(- (c) Co@SiO [(EtO) Si–L ]/Mn(III) and (d) Co@SiO [(EtO) Si–L ]/Co(II).

3

EtO) Si–L ]/Mn(III) was isolated by an external magnet and

washed with ethanol, water and acetone, in sequence. All the

above steps were also performed for the preparation of

ꢁ1

peak at 1082 cm indicates the formation of silica layer on

magnetic cobalt NPs.

3

Co@SiO [(EtO) Si–L ]/Co with Co(CH COO) instead of Mn

2

3

2

(

acac)

The Mn content in Co@SiO

determined to be 4.41 wt% by atomic absorption spectroscopic

3

.

In the FT-IR spectrum of 2-hydroxy-1-naphthaldehyde,

a band appears at 1652 cm , which is assigned to (C]O)

vibration of carbonyl group. In the FT-IR spectrum of

Si–L ] (curve b), this band was shied to

(lower wavenumber), which is attributed to the

formation of the imine bond (C]N) on the surface of the

magnetic cobalt NPs. Vibrations in the range of 1480–1600 cm

can be assigned to the aromatic ring.

The IR spectrum of Co@SiO [(EtO) Si–L ]/Mn(III) complex

curve c) shows a vibrational peak at 1638 cm , corresponding

to (C]N) bond. The shi in stretching vibration to lower

frequency in the complex in comparison to free ligand

1641 cm ) indicates the formation of metal–ligand bonds.

2 3

This band in the complex Co@SiO [(EtO) Si–L ]/Co(II) (curve d)

3

2

[(EtO)

3

Si–L ]/Mn(III) catalyst was

ꢁ1

2

(

AAS) analysis. The percent cobalt in the Co@SiO

2

[(EtO)

3

Si–L ]/

3

Co@SiO

2

ꢁ1

[(EtO)

3

Co(II) was determined to be 12.43 wt%. This value is attributed

to the cobalt magnetic core and metal in the complex. The

prepared catalysts were characterized by FT-IR, XRD, FESEM,

EDX, TEM, and VSM.

1641 cm

ꢁ1

General procedure for oxidation of suldes to sulfoxides. In

3

2

3

a typical reaction, a 25 mL reaction ask was charged with 0.02 g

ꢁ1

(

3

of the catalyst Co@SiO

2

[(EtO)

3

Si–L ]/M, 1 mmol of the methyl

phenyl sulde, 3 mL of ethyl acetate and 60 mL H

reaction mixture was stirred at 50 C. The progress of reaction

was monitored by thin layer chromatography (TLC).

2

O

2

(30%). The

ꢀ

ꢁ1

(

3

ꢁ

1

appears at 1637 cm . From the comparison between these

spectra, it can be concluded that rst, the reaction for the

formation of an imine has occurred, and second, the reaction

for the formation of the complex occurred at the surface of the

nanoparticles.

Results and discussion

Characterization of the catalysts

Fig. 1 shows the FT-IR absorption spectrum of Co@SiO

2

–NH

2

Fig. 2 illustrates the TGA curves, showing the mass loss of

the samples as they decompose upon heating. The heating

3

(

a), Co@SiO

and Co@SiO

pure Co@SiO

445 and 1627 cm that are characteristic of symmetrical and

asymmetrical modes of the O–H bonds on the surface cobalt

2

[(EtO)

–NH

3

Si–L ] (b), Co@SiO

2 3

[(EtO) Si–L ]/Mn(III) (c),

3

2

3

Si–L ]/Co(II) (d). The FT-IR spectrum for

2

(curve a) shows a stretching vibration at

ꢀ

ꢁ1

ꢀ

rate is 10 C min between 30 and 800 C. The metal ions

cobalt, manganese and silicone) and organic materials of the

15

2

(

ꢁ

1

3

samples were completely converted into metals, metal oxides

and burn to generate gaseous products at elevated tempera-

tures. The rst mass loss in the thermal analysis of Co@SiO₂

curve a) is observed in the temperature range 90–250

4.35%) due to removal water molecules present at the surface

ꢁ1

nanoparticles, respectively. The peaks at 2901 and 2872 cm

are assigned to the methylene (–CH ) of the propyl group,

2

ꢀ

(

C

respectively, which indicates that APTES molecules were

attached to the surface of the silica-coated MNPs. The main

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because of the existence of amorphous silica. XRD pattern of

3

Co@SiO₂[(EtO) Si–L ]/Mn(III) complex (curve a) shows three

3

ꢀ

major peaks in the range of 20–40 . XRD pattern of

Co@SiO [(EtO) Si–L ]/Co(II) (curve b) can be assigned to the

diﬀraction from the three major peaks with 2q at 19 , 30 and

0 reection indexes of (100), (200) and (220) planes, respec-

3

2

3

ꢀ

4

15

tively (card 25-0250 in the JCPDS le).

3

Fig. 4 shows the TEM images of Co@SiO

2

[(EtO)

3

Si–L ]/Mn(III)

3

(

a) and Co@SiO [(EtO) Si–L ]/Co(II) (b). As observed from the

2 3

images, the Schiﬀ base complex Mn(III) and Schiﬀ base complex

Co(II) shell (black color) were surrounded by the Co nano-

particles (ash color) and core–shell-type supports were ob-

tained. This indicates the successful coating on the surface of

the magnetic nanoparticles. It can be observed that the size of

Fig.

Co@SiO

2

TGA spectra of (a) Co@SiO

2

,

(b) Co@SiO

2

–NH

2

(c)

3

3 2 3

3

2

[(EtO) Si–L ]/Co(II) and (d) Mn@SiO [(EtO) Si–L ]/Co(III).

of nano cobalt. The second small weight loss (5.52%) occurred Co nanoparticles is in range of 20–30 nm.

ꢀ

in the region of 280–480 C, which is related to the removal of

2 3

As shown in Fig. 5, the morphology of the Co@SiO [(EtO) Si–

3

trapped water molecules from the lattice. The subsequent L ]/Mn(III) and Co@SiO [(EtO) Si–L ]/Co(II) was evaluated using

weight loss (5.16%) appears in the range of 500–750 C that is scanning electron microscopy. SEM images Fig. 5a and b indi-

2

3

ꢀ

related to the decomposition of SiO . In the TGA curve of cate that the catalysts were composed of nanometer-sized

2

Co@SiO –NH (curve b), the small weight loss is observed in particles. It can be observed that the particles are not fully

2

ꢀ

the range of 40–150 C (1.10%) and 150–280 C (3.55%), which spherical. In addition, some particle aggregations are observed,

is assigned to the physical loss of the adsorbed or trapped which are likely to be caused by the magneto static interactions

lattice water, respectively. A weight loss is detected in the between particles.

ꢀ

range of 300–450 C and 500–750 C, which are predominantly

2 3

To investigate the catalyst composition, Co@SiO [(EtO) Si–

3

attributed to the decomposition of organic substances in L ]/Mn(III) and Co@SiO [(EtO) Si–L ]/Co(II) were analyzed using

magnetic Co nanoparticles.

Co@SiO [(EtO) Si–L ]/Co(II) and Co@SiO [(EtO) Si–L ]/Mn(III) spectrum of Co@SiO [(EtO) Si–L ]/Co(II) showed the presence

2

3

1

5

The loss in mass for energy dispersive X-ray spectroscopy (EDX) (Fig. 6). The EDX

3

2

3

2

3

2

3

catalysts according to curves c and d is 31.17% and 33.77%, of Si, Co, C, O, and N atoms (curve b). In the EDX spectrum of

3

respectively. These results are related to the loss of [(EtO)

3

Si– Co@SiO [(EtO) Si–L ]/Mn(III), in addition to the mentioned

2

3

L ] coated on the Co nanoparticles in the complexes and prove atoms, the signals of manganese is also seen (curve a). These

3

the attachment of [(EtO)

3

Si–L H] moiety on to the surface of results once again conrm that the nanoparticles are

Co nanoparticle.

completely covered with functional groups containing nitrogen,

Therefore, the results of the TG curves correspond to the oxygen, etc.

results of the infrared spectra, which indicate that the surface

Magnetic characterization of the Co@SiO [(EtO) Si–L ]/

of the nanoparticles is covered with the ligand functional Mn(III) and Co@SiO [(EtO) Si–L ]/Co(II) was performed using

3

2

3

2

3

groups.

As shown in Fig. 3 (curve a and b), for core magnetic cobalt versus magnetic eld at 300 K for the two catalysts are shown in

a vibrating sample magnetometer (VSM). Plots of magnetization

ꢀ

NPs, diﬀraction peaks with 2q at 44.2 , 51.5 and 75.9 corre- Fig. 7. As shown in the gure, the plots of the catalysts indicate

spond to (111), (200) and (220) Braggs reection, respectively

superparamagnetic behavior. The measured saturation

indicative of an fcc metallic cobalt structure of the magnetite magnetization value (36.28 emu g for Co@SiO [(EtO) Si–L ]/

a

ꢁ

1

3

2

3

ꢁ

1

(

card 15-0806 in the JCPDS le). The XRD patterns of Mn(III) and 35.98 emu g

2 3

for Co@SiO [(EtO) Si–L ]/Co(II))

3

Co@SiO

2

[(EtO)

3

Si–L ]/Mn(III) and Co@SiO

2

[(EtO)

ꢀ

3

Si–L ]/Co(II)

show a visible diﬀusion peak at 2q ¼ 15–25 that appeared

2

3

Fig. 3 The X-ray diﬀraction patterns of (a) Co@SiO

2

[(EtO)

3

Si–L ]/ Fig.

4

TEM image of (a) Co@SiO

2

[(EtO)

3

Si–L ]/Mn(III) and (b)

2

3

Mn(III) and (b) Co@SiO

2

3

[(EtO) Si–L ]/Co(II).

2 3

Co@SiO [(EtO) Si–L ]/Co(II).

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Fig. 5 SEM image of (a) Co@SiO

2

[(EtO)

3

Si–L ]/Mn(III) and (b) Co@SiO

2

[(EtO)

3

Si–L ]/Co(II).

3

Fig. 6 Energy-dispersive X-ray spectroscopy (EDX) of (a) Co@SiO

2

[(EtO)

3

Si–L ]/Mn(III) and (b) Co@SiO

2

[(EtO)

3

Si–L ]/Co(II).

3

C, H, and N content. Anal. found for Co@SiO

C, 21.53; H, 1.70; N, 2.01 and for Co@SiO [(EtO)

2.08; H, 1.74; N, 2.11. The calculated percentage of elemental

2

[(EtO)

3

Si–L ]/Mn(III):

3

2

3

Si–L ]/Co(II): C,

2

3

analysis for ligand L is: C, 71.00; H, 6.36; N, 5.52. Aer analyzing

these results, it can be concluded that the ratio of the percentage

of the elements in the two complexes corresponds to the ratio of

3

these elements in the ligand L .

Catalytic activities of the synthesized catalysts

3

The catalytic activity of Co@SiO

2

[(EtO)

3

Si–L ]/M (M ¼ Mn(III),

Co(II)) was examined through the oxidation of suldes to sulf-

[(- oxides (Scheme 2).

Fig. 7 Room temperature magnetization curves of (a) Co@SiO

EtO) Si–L ]/Mn(III) and (b) Co@SiO [(EtO) Si–L ]/Co(II).

3 2 3

2

3

demonstrate smaller saturation magnetization values of the

ꢁ

1

15

bare Co nanoparticles (39.78 emu g ). This could be due to

3

2 3

SiO [(EtO) Si–L ] coating on the surface of Co NPs. Neverthe-

less, their magnetization is suﬃcient and these magnetic

nanoparticles can be quickly separated from their dispersion by

using an external magnetic eld.

3

The complexes of Co@SiO [(EtO) Si–L ]/Mn(III) and

2

3

Co@SiO [(EtO) Si–L ]/Co(II) were analyzed by a Perkin-Elmer 2400

2

3

series analyzer and elemental analysis results showed a signicant Scheme 2 Co@SiO [(EtO) Si–L ]/M (M ¼ Mn(III), Co(II)) catalysts for

2

3

oxidation of sulﬁde.

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First, the oxidation reaction of the methyl phenyl sulde to calculated. In order to analyze the products formed during the

methyl phenyl sulfoxide was selected as the model reaction. reaction, FT-IR technique was used. FT-IR analysis of the ob-

The catalysts were tested under the following reaction condi- tained product from the oxidation of sulde compounds

ꢁ1

tions: methyl phenyl sulde (0.5 mmol), hydrogen peroxide shows S]O stretching frequency at 1049 cm , indicating the

0% (90 mL, 0.881 mmol), catalyst 0.02 g, reaction time 50 min, formation of sulfoxide from sulde. There is a band at

3

ꢀ

ꢁ1

reaction temperature 50 C and various solvents without 692 cm , corresponding to C–S stretching frequency. Since

optimized amounts. The results are summarized in Table 1. As there is no stretching frequency band observed at 1150 cm

ꢁ

1

,

indicated by the data, the catalysts are highly active and corresponding to sulfone, we understand that sulde is not

selective. In addition, it can be seen that excellent yield of oxidized to sulfone under the present experimental condi-

product was obtained in acetonitrile. To determine the reac- tions. The product analysis study demonstrates that sulfoxide

tion yield, the following procedure was adopted. Aer is the only product formed under the present reaction

completion of reaction and separation of catalyst using conditions.

a magnet, the organic phase was extracted with ethanol. Then,

The eﬀect of temperature was investigated by carrying out

it was dried over sodium sulfate and evaporated. In the next the model reaction at diﬀerent temperatures in acetonitrile. As

ꢀ

step, the obtained product was recrystallized with ethanol. The shown in Fig. 8a, the best result was achieved at 65 C.

resultant sulfoxide was weighed and the reaction yield was Furthermore, the diﬀerent reaction times were examined at

a

2 2

Table 1 Catalytic results oxidation of methyl phenyl sulﬁde to methyl phenyl sulfoxide with H O by various catalysts and diﬀerent solvents

Entry

Catalyst

Solvent

THF

C H OH

2 5

DMSO

DMF

Selectivity (%)

Isolated yield

3

1

2

3

4

5

6

7

8

9

Co@SiO

2

[(EtO)

3

Si–L ]/Mn(III)

100

52

77

45

84

49

69

73

77

89

84

41

72

35

82

48

65

69

75

84

80

3

Si–L ]/Mn(III)

3

Si–L ]/Mn(III)

3

Si–L ]/Mn(III)

3

Si–L ]/Mn(III)

CH

Ethyl acetate

CHCl

2 2

Cl

3

Si–L ]/Mn(III)

3

Si–L ]/Mn(III)

3

Si–L ]/Mn(III)

Acetone

MeCN

CH OH

3

THF

MeCN

DMSO

3

Si–L ]/Mn(III)

3

10

11

12

13

14

15

16

17

18

19

20

Si–L ]/Mn(III)

3

Si–L ]/Co(II)

3

Si–L ]/Co(II)

3

Si–L ]/Co(II)

3

Si–L ]/Co(II)

C

CH

2

H

5

OH

Cl

Ethyl acetate

CHCl

3

Si–L ]/Co(II)

2

3

Si–L ]/Co(II)

3

Si–L ]/Co(II)

3

Si–L ]/Co(II)

Acetone

MeCN

CH OH

3

Si–L ]/Co(II)

3

Si–L ]/Co(II)

a

Reaction condition: methyl phenyl sulde (0.5 mmol), hydrogen peroxide 30% (90 mL, 0.881 mmol), catalyst 0.02 g, reaction time: 50 min, reaction

ꢀ

temperature 55 C.

2 3

Fig. 8 Eﬀect of temperature (a) and time (b) on the oxidation of methyl phenyl sulﬁde to methyl phenyl sulfoxide catalyzed by Co@SiO [(EtO) Si–

3

2 3

L ]/Mn(III) (red) and Co@SiO [(EtO) Si–L ]/Co(II) (blue).

3894 | RSC Adv., 2018, 8, 3889–3898

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Table 2 The optimized amounts of the catalyst and oxidant in

a

oxidation of methyl phenyl sulﬁde to methyl phenyl sulfoxide

3

Co@SiO

Mn(III) (g)

2

[(EtO)

3

Si–L ]/

H

(mL)

2

O

2

30%

Selectivity

(%)

Isolated

yield

Entry

1

2

3

4

5

6

7

8

9

1

2

0.02

0.03

0.04

0.05

0.06

Not used

60

70

80

90

60

70

80

90

60

70

80

90

60

70

80

90

60

70

80

90

100

61

73

82

87

70

78

84

92

81

88

90

93

86

92

94

97

91

95

97

99

8

Fig. 9 Recycling of the two synthesized magnetic nanocatalysts used

the oxidation of methyl phenyl sulﬁde to methyl phenyl sulfoxide.

0

1

2

3

4

5

6

7

8

9

0

1

oxidation reactions (Table 3). The results indicated

that the catalyst is capable of oxidizing of suldes with high

yield.

Very recently some studies have been reported for suldes

17,22

oxidation using manganese(III) complexes.

Therefore, the proposed mechanism for the performance of

these complexes is shown in the Scheme 3.

a

Reaction condition: methyl phenyl sulde (0.5 mmol), reaction time:

0 min, reaction temperature 65 C.

ꢀ

5

Comparison with other catalysts

Comparative data on the performance of various catalysts in

the oxidation of suldes are shown in Table 4. Although Schiﬀ

base complexes are oen used as eﬃcient homogeneous

catalysts for the oxidation reaction, some problems oen exist

in these types of oxidation studies, such as (1) deactivation, (2)

instability and (3) expensive recycling of these homogeneous

systems. Therefore, for improving the performance of catalytic

activity, scientists tried to convert them into heterogeneous

catalysts using a variety of methods. The greatest advantage of

heterogeneous catalysis is the facile separation of catalysts

from the reaction media and products. In order to x catalysts

onto heterogeneous supports, the diﬀerent methods can be

used, including graing the catalyst onto inorganic supports

such as silica. This is a very good method for synthesis of

heterogeneous catalysts because of it eliminates some of the

synthesis steps. However, a major challenge is the design of

insoluble catalysts with high yields, selectivity, and cost

eﬀectiveness. According to all the above discussion, insoluble

Schiﬀ base complexes with good stability and catalytic activity

are still rare and lack generality.

ꢀ

a constant temperature of 65 C and the reaction progress was

monitored by TLC. Finally, the reaction time of 50 min was

obtained as optimum time (Fig. 8b).

In addition to the solvent, time and temperature of the

reaction, the other conditions such as catalyst amount and

oxidant amount were investigated (Table 2). The amount of

3

Co@SiO

2

[(EtO)

3

Si–L ]/Mn(III) catalyst used in the reaction was

varied in the range 0.02–0.06 (g). As shown in Table 2,

increasing the amount of catalyst improved the yield and the

maximum yield of the product was obtained with 0.06 g of

catalyst. Also increasing the amount of H O (30%), as an

oxidizing agent, to 90 mL, increases the reaction yield (Table 2,

entry 20).

To evaluate reusability of the catalyst, the oxidation of

methyl phenyl sulde was studied under optimum condition.

Aer each cycle, the catalyst was separated by an external

magnet, and washed several times with deionized water and

2

ꢀ

ethanol. Then, it was dried in an oven at 50 C and used in the

It is noteworthy that the reaction tolerates oxidation-

sensitive functional groups and the sulfur atom is selec-

tively oxidized. Synthetic processes for magnetic nano-

materials are currently undergoing rapid improvement and

have opened up tremendous possibilities for the fabrication

of magnetically-recoverable catalysts. Separation and

recovery of catalysts by external magnetic eld is an envi-

ronmentally friendly alternative because not only does it

minimize the use of auxiliary materials, but also prevents

mass loss and reduces the operation time. We synthesized

a highly active catalyst based on Schiﬀ base complexes of

Mn(III) or Co(II) ion supported on magnetic cobalt

next run. The results show that the synthesized catalysts can be

reused seven times without any considerable loss in activity

(Fig. 9).

Due to the relatively high magnetic capability of the present

catalysts, it is easy to separate each of them by an external

magnet. Moreover, due to the strong bond between functional

groups with core and the presence of Schiﬀ base groups,

a strong bond with transition metals are formed in the

complexes.

Performance capability of the catalyst Co@SiO [(EtO) -

2

3

Si–L ]/Mn(III) was evaluated by using diﬀerent suldes in

RSC Adv., 2018, 8, 3889–3898 | 3895

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Table

Co@SiO

3 Oxidation of the variety of sulﬁdes to sulfoxides by Table 3 (Contd.)

3

2 3

[(EtO) Si–L ]/Mn(III) nanocatalyst

Isolated

yield (%)

Isolated

yield (%)

Entry Suldes

Sulfoxides

Entry Suldes

Sulfoxides

1

99

96

14

90

2

3

1

5

6

87

92

90

95

91

17

97

4

5

nanoparticles, which have been synthesized in a limited

number in recent years. This synthesized catalysts were

1

5

capable of catalyzing the oxidation reactions of alcohols and

suldes with high yield. Further investigation of other cata-

lytic reactions with this synthesized nanocatalyst is in prog-

ress. Other desirable properties of synthesized catalysts that

are included in this publication are the high yield, short

reaction time, selectivity, mild reaction conditions, and good

catalyst recovery.

The eﬃciency of this method is demonstrated by comparing

our results on the oxidation of benzyl phenyl sulde with data

from the literature. As shown in Table 4, the previously reported

procedures suﬀer from one or more disadvantages such as

6

7

8

9

97

90

86

96

89

90

3

1–36

33–35

longer reaction times,

low yield,

using a transition

31–35

metal,

solvents.

and the need for volatile and toxic organic

31–34,37

1

0

1

2

3

93

95

Scheme 3 Electron transfer mechanism for the Co@SiO

2

[(EtO)

3

Si–

3

2 2

L ]/M (M ¼ Mn(III) and Co(II)) catalyzed H O oxidation of organic

sulﬁdes.

3896 | RSC Adv., 2018, 8, 3889–3898

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2 2

Table 4 Comparison of the activity of various catalysts in the oxidation of benzyl methyl sulﬁde using H O

Entry

Catalyst

Conditions

Time (min)

Yield (%)

Ref.

1

2

3

4

5

6

7

8

9

MNPs–PA

SSA

NBS

Solvent-free, rt

10

40

15

20 h

150

330

240

360

180

40

95

96

90

52

96

92

83

53

83

92

100

99

24

37

36

33

34

31

32

35

CH

3

CN, rt

CN, 35–40 C

CN, 20 C

ꢀ

Cu(II) complex, TEMPO

TaCl

Cp/Mo(CO)

Silica-based ammonium tungstate

ZnBr , pyridine-2-3-carboxylic acid

Schiﬀ base/Cu(II) salen/Fe

Co@SiO @[Mn(III)SBC]

Au/CTN-silica

Co@SiO [(EtO)

5

MeOH, 45 C

3

Cl

CH

3

COCH

Cl :MeOH, rt

2

3

–MeOH

2

MeOH, rt

EtOH, 60 C

ꢀ

3

O

4

23

24

22

ꢀ

10

11

12

2

Solvent-free, 45 C

ꢀ

60 C

120

50

3

ꢀ

2

3

Si–L ]/Mn(III)

55 C

This work

4

M. J. Jacinto, H. C. F. Santos, R. F. Jardim, R. Landers and

L. M. Rossi, Appl. Catal., A, 2009, 360, 177.

Conclusion

3

In this study, the catalysts Co@SiO

2

[(EtO)

3

Si–L ]/Mn(III) and

5 R. L. Oliveira, P. K. Kiyohara and L. M. Rossi, Green Chem.,

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101, 37.

3

Co@SiO [(EtO) Si–L ]/Co(II) were successfully prepared via

2 3

3

immobilizing the Schiﬀ base ligand [(EtO) Si–L H] on the Co

3

nanoparticles coated with SiO₂and then reacting them with

metal salts viz. Mn(III) and Co(II). These catalysts were employed

for the oxidation of the suldes to sulfoxides and showed high

catalytic activity and selectivity. It was further found that the

9 B. S. Lane and K. Burgess, Chem. Rev., 2003, 103, 2457.

catalyst can be easily isolated using an external magnet and 10 G. Righi and C. Bonini, Synthesis, 1994, 3, 225.

reused seven times without signicant catalytic deactivation.

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1992, 57, 5140.

These newly developed heterogeneous catalysts were simple

to prepare. These catalysts were found to be easily reusable 12 L. P. C. Nielson, C. P. Stevenson, D. G. Backmond and

several times aer reaction with an external magnet and drying

E. N. Jacobsen, J. Am. Chem. Soc., 2004, 126, 1360.

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due to the leaching of the active species or degradation of the

4199.

structure. It appears that the Schiﬀ base complex has played an 14 A. M. Daly, C. T. Dalton, M. F. Renehan and D. G. Gilheany,

important role in the stabilization of metal catalyst particles.

Tetrahedron Lett., 1999, 40, 3617.

These unique results open new perspectives for the application 15 H. Keypour, S. G. Saremi, H. Veisi and R. Azadbakht, RSC

of these types of magnetic catalysts in other organic reactions.

Adv., 2016, 6, 77020.

16 M. Szavuly, S. D. Szilvasi, R. Csonka, D. Klesitz, G. Speier,

M. Giorgi and J. Kaizer, J. Mol. Catal. A: Chem., 2014, 393,

Conﬂicts of interest

3

17.

17 S. Rayati, F. Nejabat and S. Zakavi, Inorg. Chem. Commun.,

014, 40, 82.

There are no conicts to declare.

2

1

8 P. Gogoi, M. Kalita, T. Bhattacharjee and P. Barman,

Tetrahedron Lett., 2014, 55, 1028.

Acknowledgements

1

2

9 X. T. Zhou and H. B. Ji, Catal. Commun., 2014, 53, 29.

0 B. Yu, C. X. Guo, C. L. Zhong, Z. F. Diao and L. N. He,

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1 Y. Imada, T. Kitagawa, S. Iwata, N. Komiya and T. Naota,

Tetrahedron, 2014, 70, 495.

We thank the Payame Noor University, Bu-Ali Sina University

and Center for Research and Development of Petroleum Tech-

nologies at Kermanshah, Research Institute of Petroleum

Industry (RIPI), Iran for their partial support on this project.

2

2 A. R. Judy Azar, E. Safaei and S. Mohebbi, Mater. Res. Bull.,

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3898 | RSC Adv., 2018, 8, 3889–3898

Article Doi

Schiff base Mn(III) and Co(II) complexes coated on Co nanoparticles: An efficient and recyclable magnetic nanocatalyst for H2O2 oxidation of sulfides to sulfoxides

DOI: 10.1039/c7ra11225d

Source and publish data:

Authors:

Article abstract of DOI:10.1039/c7ra11225d

Full text of DOI:10.1039/c7ra11225d

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