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Journal of Organometallic Chemistry 774 (2014) 61e69

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Journal of Organometallic Chemistry

journal homepage: www.elsevier .com/locate/ jorganchem

Synthesis of a reusable polymer anchored cobalt(II) complex forthe aerobic oxidation of alkyl aromatics and unsaturated organiccompounds

Sk Manirul Islam a, *, Kajari Ghosh a, Rostam Ali Molla a, Anupam Singha Roy a,Noor Salam a, Md Asif Iqubal b

a Department of Chemistry, University of Kalyani, Kalyani, Nadia, 741235 West Bengal, Indiab Department of Chemistry, IIT Roorkee, Roorkee, 247667 Uttarakhand, India

a r t i c l e i n f o

Article history:Received 21 August 2014Received in revised form29 September 2014Accepted 4 October 2014Available online 15 October 2014

Keywords:Polymer anchoredCobalt(II) complexAerobic oxidationH2O2

* Corresponding author. Tel.: þ 91 33 2582 8750; fE-mail address: manir65@rediffmail.com (S.M. Isla

http://dx.doi.org/10.1016/j.jorganchem.2014.10.0100022-328X/© 2014 Elsevier B.V. All rights reserved.

a b s t r a c t

Polymer anchored cobalt(II) catalyst was synthesized and characterized. The solid catalyst was charac-terized by fourier transform infrared spectroscopy (FT-IR), UVevis diffuse reflectance spectroscopy (DRS),thermogravimetric analysis (TGA) and scanning electron microscope (SEM). The catalytic activity of thecomplex was investigated for the oxidative functionalization of alkyl aromatics to benzylic ketones usingO2 (1 atm) with 1 mmol H2O2. The oxidation of organic compounds with carbonecarbon double bondwith molecular oxygen under atmospheric pressure in the presence of this catalyst has also been studied.Oxidation of alkenes, cycloalkenes and terpenes gave corresponding epoxy derivatives. The developedcatalyst can be facilely recovered and reused six times without significant decrease in its activity andselectivity.

© 2014 Elsevier B.V. All rights reserved.

Introduction

The development of efficient and cheap catalytic systems for theselective oxidation of organic substrates under mild and environ-mentally benign conditions represents one of the major challengesin organic synthesis. Oxidation of aromatic compounds containingbenzylic CeH bonds into corresponding benzylic ketones is one ofthe important transformations in organic synthesis, since theoxidation products are essential intermediates for the manufactureof high-value fine chemicals, agrochemicals, pharmaceuticals andhigh-tonnage commodity chemicals [1,2]. In the last few years,many catalytic methods have been developed for the oxidation ofbenzylic CeH bonds using H2O2 or O2 as the oxidant due to theirinexpensiveness, cleanliness, easy handling, availability, and pro-duction of water as their only by-product in the oxidation reaction[3]. Recently, there has been an increased interest in the oxidationof benzylic CeH bonds using transition metal Schiff base complexesimmobilized on supports such as polymers [4], silica matrices [5],

ax: þ91 33 2582 8282.m).

mesoporous materials [6] and SiO2eAl2O3 mixed-oxide [7] as het-erogeneous catalysts, which can be separated by simple filtration atthe end of the reaction. However, there are still some drawbackssuch as the use of costly oxidant [8], harsh reaction conditions andpoor conversion and low selectivity [4]. Therefore, it is still highlydesirable to develop a cheap, catalytic and environmental friendlymethod for the oxidation of benzylic CeH bonds that can beoperated under mild conditions with excellent conversion and highselectivity.

The selective oxidation of alkenes to epoxides is one of the mostimportant and fundamental reaction in organic chemistry both atthe laboratory and industrial scale. Numerous catalysts have beendeveloped using a variety of reagents to achieve this purpose[9e14]. However, most of these reagents are required in stoichio-metric quantities. Moreover these reagents are sometimes expen-sive and toxic. From an environmental as well as economic point ofview, catalytic oxidation processes, especially those, in which mo-lecular oxygen is used as primary oxidant, are particularlyattractive.

Homogeneous and heterogeneous catalysts offer their owndistinct advantages. Heterogeneous catalysts have an advantagethat at the end of reaction the catalyst can be removed by simplefiltration [15,16]. In principle the product is un-contaminated with

S.M. Islam et al. / Journal of Organometallic Chemistry 774 (2014) 61e6962

a transition metal or ligand and allows the catalyst to be recycledinto the next reaction. Cobalt-based catalysts, due to their plausiblepotential as commercial catalysts, are the ones among the mostinvestigated. In this regard Co(II) complexes are an important classof oxidants for these types of reactions and quite a number ofstudies have been conducted [17,18]. This includes NHPI (N-hy-droxyl-phthalimide)/Co(OAc)2/O2 [19], cobalt(II) phthalocyanine/([bmim]Br)/O2 [20], [CoTPP-P(4VP-co-St)/SiO2]/O2 [21], [cobalt(II)porphyrin/CPS(crosslinked polystyrene)]/O2 [22], silica gel-cobalt(II) Schiff base/O2 [23], Co(II)-Schiff base/O2 [24] and Co(cy-clam)-SBA-15/H2O2 [25].

In this work, we report an efficient procedure for the selectiveoxidation of alkyl aromatics and selective epoxidation of alkenesunder oxygen atmosphere (1 atm) using polymer supportedcobalt(II) complex as an effective and reusable catalyst. Comparisonof homogeneous and supported catalyst for oxidation reaction wasalso done. The catalytic activities were also tested with the recycledcatalyst.

Experimental

Materials and instrumentation

Analytical grade reagents and freshly distilled solvents wereused throughout. All reagents and substrates were purchased fromMerck. Liquid substrates were pre-distilled and dried by molecularsieve and solid substrates were recrystallized before use. Distilla-tion, purification of the solvents and substrates were done bystandard procedures [26]. Chloromethylated polystyrene and 4-(2-pyridylazo) resorcinol were purchased from Aldrich ChemicalCompany, USA. Cobalt acetate was procured from Merck and usedwithout further purification.

The FTIR spectra of the samples were recorded from 400 to4000 cm�1 on a PerkineElmer FTIR 783 spectrophotometer usingKBr pellets. UVeVis spectra were recorded using a Shimadzu UV-2401PC doubled beam spectrophotometer having an integratingsphere attachment for solid samples. Thermogravimetric analysis(TGA) was carried out using a Mettler Toledo TGA/DTA 851e. Sur-face morphology of the samples was measured using a scanning

N

N

N

HO

OH

CH2Cl

DMF/NaH0-5 0C

PAR

Scheme 1. Synthesis of polyme

electron microscope (SEM) (ZEISS EVO40, England) equipped withEDX facility. Cobalt content in the catalyst was determined using aVarian AA240 atomic absorption spectrophotometer (AAS). Thereaction products were quantified (GC data) by Varian 3400 gaschromatograph equipped with a 30 m CP-SIL8CB capillary columnand a flame ionization detector and identified by Trace DSQ IIGCeMS equipped with a 60 m TR-50MS capillary column.

Preparation of catalyst

An outline of the preparation of polymer anchored Co(II) PARcomplex catalyst is given in Scheme 1.

Catalyst was readily prepared in two steps. At first N, N-dime-thylformamide (DMF) containing 145 mg of sodium hydride (NaH)was placed in a round bottom flask and stirred for 30min at 0e5 �C.Then 4-(2-Pyridylazo) resorcinol (PAR) (2.5 g) was added slowly insmall portions. After 30 min chloromethylated polystyrene (1 g)was added to the above solution and stirred overnight at roomtemperature. The deep violet polymer anchored PS-PAR ligand wasfiltered out, washed thoroughly with methanol and dried undervacuum. This polymer anchored PS-PAR ligand (1 g) in acetic acid(20 mL) was treated with 5 mL 1% (w/v) acetic acid solution ofcobalt acetate over a period of nearly 30 min under constant stir-ring. Then the reaction mixture was refluxed for 24 h. The deepbrown cobalt(II) catalyst (PSeCo(II)-PAR) thus formed was filteredand washed thoroughly with ethanol and dried at room tempera-ture under vacuum (yield of catalyst is 50%).

General procedure for oxidation of alkyl aromatics

Alkyl aromatics (5 mmol) in 10 mL acetonitrile and the polymersupported cobalt catalyst (60 mg) were in turn introduced into a50 mL round bottomed flask equipped with a condenser and athermometer in an oil bath. The reaction was carried under oxygenatmosphere (1 atm) with 1 mmol H2O2. Then, the catalyst wasremoved by filtration, and the solvent was evaporated in vacuum.The product was filtered, washed thoroughly with water andextracted with ether, then dried with anhydrous Na2SO4 andanalyzed by GC and GCeMS.

NN

N

HO

O

NN

N

O

O

CoAcO

PS-PAR

Co(OAc)2CH3COOHReflux/24 h

PS-Co(II)-PAR

r anchored Co(II) complex.

Table 1Chemical composition and chemical analysis of different compounds.

Compound Color C% H% N% Co%

PS-PAR Orange 72.68 5.38 9.4 e

PS-Co(II)-PAR Deep brown 67.56 4.18 8.45 3.80

S.M. Islam et al. / Journal of Organometallic Chemistry 774 (2014) 61e69 63

General procedure for oxidation of alkenes

Alkenes (5 mmol) in 10 mL acetonitrile were taken in roundbottomed flask along with 50 mg of the polymer supported cobaltcatalyst. Molecular oxygen at atmospheric pressure as an oxidizingagent was bubbled through the reaction medium. 2-methylpropanal (1 mmol) was added to the reaction medium whichplays the role both reductant of the catalytic centre and co-catalystof epoxidation reaction. Whole reaction mixture was stirred untilcompletion of the reaction. Then the solvent was evaporated toyield a residue, which was dissolved in ethyl acetate. After thatmixture was extracted with aqueous NaHCO3 and then with water.To obtain the final product, organic layer should be dried and thesolvent should be evaporated to give the desired product andanalyzed by GC and GCeMS.

Results and disscussion

Characterization of catalyst

Due to insolubilities of the polymer anchored copper catalyst inall common organic solvents, its structural investigation waslimited to its physicochemical properties, chemical analysis, SEM,TGA, IR and UVeVis spectroscopic data. Elemental analysis data ofthe polymer anchored ligand, PS-PAR (C ¼ 72.68%, H ¼ 5.38% andN ¼ 9.40%) and polymer anchored cobalt catalyst, PS-Co(II)-PAR(C ¼ 67.56%, H ¼ 4.18% and N ¼ 8.45%) support the formulation ofthe catalyst. The metal content determined by atomic absorptionspectroscopy suggests 3.80% Co in the immobilized complex(Table 1).

The mode of attachment of cobalt metal onto the heterogeneoussystem was confirmed by the FTIR spectral bands. In 4-(2-

Fig. 1. FT-IR spectra of (a) polymer anchored PS-PAR ligan

Pyridylazo) resorcinol (PAR) ligand, the infrared band for N]Nappeared at 1479 cm�1. The IR spectrum of pure chloromethylatedpolystyrene has shown absorption band at 1261 cm�1, due to thepresence of CeCl group in polymer beads, which has got reducedintensity in the polymer anchored PAR-ligand. The N]N stretchingfrequency for the immobilised PAR ligand at 1444 cm�1 is shifted to1402 cm�1 in the Co(II) complex [27] (Fig. 1). In the polymeranchored PAR-ligand, the presence of the bands at 3414 (in com-plex this peak is shifted toz 3200 cm�1) and 1320 cm�1 have beenassigned to the stretching vibrational frequency of phenolic eOHand free CeO stretching. The FTIR spectra of the PS-PAR ligandshows one medium to sharp band around at 1265 cm�1, which maybe attributed to the stretching vibration of phenolic oxygenattached with polymer. In the immobilised PS-Co(II)-PAR catalyst, anew peak appears at 1297 cm�1. This could be attributed to a shiftin free CeO stretching as a result of coordination of phenolic oxy-gen with copper metal [28,29]. In addition, the bands at 519 and460 cm�1 may be assigned to the CoeO and CoeN [30] stretchingvibration respectively. An appearance of a shoulder at 1753 cm�1

may be assigned to C]O stretching of the acetate group of cobaltacetate. This confirms the metal loading in the polymer supportedPAR ligand. The FT-IR spectrum of the recycled complex is alsogiven in Fig. 2; the stretching vibrations do not change significantly,suggesting the existence of all the properties in the recycledcatalyst.

The electronic spectrum of the polymer anchored PAR ligandand the immobilized PS-Co(II)-PAR catalyst has been recorded inthe diffuse reflectance mode (Fig. 3). The spectra of the PS-PARligand and PS-Co(II)-PAR catalyst exhibit few spectral bandsbelow 250 nm, which may be assigned to pep* intra-ligandcharge transfer and pep* transition of the benzenoid moiety.The shoulder peaks nearer to 315 nm are attributed to the nep*transition of the azo (eN]Ne) group [31]. The high intensitybands in the visible region, at 400e520 nm involving the wholeelectronic system of the PS-Co(II)-PAR catalyst are assigned to thepep* transition [32].

Field emission-scanning electron micrographs for single beadsof polymer supported ligand and complex were recorded to un-derstand the morphological changes occurring on the polystyrenebeads at various stages of the synthesis. In Fig. 4 the SEM images of

d and (b) polymer anchored PS-Co(II)-PAR complex.

Fig. 2. FT-IR spectra of recycled polymer anchored PS-Co(II)-PAR complex.

S.M. Islam et al. / Journal of Organometallic Chemistry 774 (2014) 61e6964

polymer anchored PS-PAR ligand (A) and the immobilized cobaltcomplex, PS-Co(II)-PAR (B) are shown. The pure chloromethylatedpolystyrene bead has a smooth surface (not shown). Introduction ofligands into polystyrene beads causes the light roughening of thetop layer of polymer beads. After metal loading on polymeranchored ligand, changes in morphology of the ligand surface wasobserved by SEM pictures. As expected, the smooth and flat surfaceof polymer bead shows slight roughening on complexation. Also,presence of cobalt metal along with nitrogen and chlorine can befurther proved by energy dispersive spectroscopy analysis of X-rays(EDAX) (Fig. 5) which suggests the formation of metal complexwith the polymer anchored ligand.

Thermal stability of the complex was investigated using TGA at aheating rate of 10 �C/min in air over a temperature range of30e600 �C. TGA curves of the polymer anchored PAR-ligand andsupported PS-Co(II)-PAR catalyst are shown in Fig. 6. The PS-PAR-ligand is stable up to 300 �C and thermal decomposition of theligand starts above this temperature. The initial decomposition of

300 400 500 6001.0

1.1

1.2

1.3

1.4

1.5

(b)

(a)

Abso

rban

ce

Wavelength (nm)

Fig. 3. DRS-UV-visible absorption spectra of (a) polymer anchored PS-PAR ligand and(b) polymer anchored Co(II) complex.

the catalyst occurred at 250 �C. Finally, degradation of the Co-PARcatalyst occurred above 350e380 �C temperature. So, we can sug-gest that the complex is stable up to 250 �C and above this tem-perature it starts to decompose. Thermogravimetric study suggeststhat the polymer anchored Co-PAR catalyst degrades at consider-ably higher temperatures.

Catalytic activity

Catalytic oxidation of alkyl aromaticsOxidation of ethylbenzene in acetonitrile was selected as a

model reaction and the oxidation was carried out in differentconditions by varying the oxidants, temperature and catalystamount.

In the catalytic oxidation of ethylbenzene the choice of oxygendonor is important. In this study, we investigated the ability ofdifferent oxidants for the oxidation of ethylbenzene catalyzed bythe heterogeneous Co(II) complex, PS-Co(II)-PAR. The results aresummarized in Table 2. There was almost no conversion of ethyl-benzene to acetophenone using air as oxidant (Table 2, entry 1).When molecular oxygen (1 atm) was taken as oxygen source,moderate conversion was detected (Table 2, entry 2). Good con-version was obtained with 1 mmol H2O2 (Table 2, entry 3). Theexperiment was then conducted using O2 with 1 mmol H2O2(Table 2, entry 4). The results show that in the presence of thecatalyst, excellent conversion with high selectivity was obtainedwhen O2 with 1 mmol H2O2 was chosen as oxidant.

Effect of temperature on the oxidation of ethylbenzene wasstudied at five different temperatures, 40, 50, 60, 70 and 80 �C.While reaction temperature increased from 40 �C (Table 3, entry 1)to 50 �C (Table 3, entry 2), there was a slight increase in conversion.From Table 3 (entry 4) it is seen that the best working temperatureis 70 �C. At this temperature best conversion with highest selec-tivity was obtained. Above 70 �C no such increment in conversionas well as in selectivity was observed. After the optimization of thereaction temperature, the experiment was repeated by varying thecatalyst amount (Table 3, entries 4 and 6e8). From Table 3 it is seenthat at 70 �C, the best conversion and selectivity was obtained with60 mg catalyst (entry 4).

The same reaction conditions were used for the conversion ofother alkyl aromatics to their corresponding carbonyl compounds ingood to excellent yields (Table 4). It was observed that diphenyl-methane was selectively oxidized to benzophenone with a

Fig. 4. FE SEM images of polymer anchored PAR ligand (A) and polymer anchored Co(II)-PAR complex (B).

S.M. Islam et al. / Journal of Organometallic Chemistry 774 (2014) 61e69 65

conversion of 86% and selectivity of 97% (entry 1). The aromaticscontaining two benzene rings like fluorene and acenaphthene un-derwent oxidation to the corresponding ketones with good conver-sions and selectivity (entries 2, 3). Cycloalkylarenes such as tetralene

Fig. 5. EDX images of polymer anchored PAR ligand (A

and indane are readily oxidized to their corresponding carbonylcompounds 1-tetralone and 1-indanone and the oxidation tookplace selectively at the benzylic CeH bond and few oxidation by-products were observed during the reaction (entries 4, 5).

) and polymer anchored Co(II)-PAR complex (B).

100 200 300 400 500 600

Wei

ght l

oss

(%)

Temperature (0C)

PS-PAR ligand PS-Co(II)-PAR complex

Fig. 6. Thermogravimetric weight loss plots for polymer anchored ligand PS-PAR andpolymer anchored complex PS-Co(II)-PAR.

Table 3Effect of temperature and catalyst amount on the oxidation of ethylbenzene.a

Entry Temperature(�C)

Catalyst amount(mg)

Conversion(%)b

Selectivity(%)b

1 40 60 66 852 50 60 72 883 60 60 78 904 70 60 89 995 80 60 90 976 70 40 75 877 70 50 79 928 70 70 91 96

a Reaction conditions: ethylbenzene (5 mmol), 30% aq H2O2 (1 mmol), O2 (1 atm),acetonitrile (10 mL), time (6 h).

b Determined by GC and GCeMS analysis.

S.M. Islam et al. / Journal of Organometallic Chemistry 774 (2014) 61e6966

Anthrone and xanthene were readily oxidized with 92% conversionand 100% selectivity (entries 6, 7). Alkylbenzenes containing openchain alkyl group such as n-propylbenzene, n-butylbenzene andisobutyl benzene underwent the reaction with 86%, 84% and 83%conversion providing corresponding benzylic ketones with 95%, 92%and 91% selectivity respectively (entries 8, 9 and 10). It is worthmentioning that no oxidation occurred on the aromatic ring of all thesubstrates.

Catalytic epoxidation of alkenesTo study the epoxidation of alkenes, dodec-1-ene (5 mmol) in

acetonitrile (10 mL) was selected as the model substrate. Theoxidation was examined in the presence of the polymer supportedcobalt catalyst in the presence of 2-methyl propanal (1 mmol)under oxygen pressure (1 atm). The reaction was carried out indifferent conditions by varying the reaction temperature andcatalyst amount.

The effect of temperature on the epoxidation of dodec-1-enewas studied at four different temperatures, 40, 50, 60 and70 �C. When the reaction temperature increased from 40 �C(Table 5, entry 1) to 50 �C (Table 5, entry 2), there was a slightincrease in conversion. From Table 5 (entry 3) it is seen that thebest working temperature is 60 �C. At this temperature thehighest conversion was obtained. Over 60 �C no such incrementin conversion was observed. After the optimization of the reac-tion temperature, the experiment was repeated by varying thecatalyst amount (Table 5, entries 3, 5 and 6). From Table 5, it is

Table 2Oxidation of ethylbenzene to acetophenone with different oxidants catalyzed bypolymer anchored Co(II) catalyst.a

Entry Oxidant Conversion (%)b Selectivity (%)b

1 Air 04 802 O2 (1 atm) 42 863 1 mmol H2O2 67 884 O2 (1 atm) þ 1 mmol H2O2 89 99

a Reaction conditions: ethylbenzene (5 mmol), catalyst (60 mg), acetonitrile(10 mL), temperature (70

�C) time (6 h).

b Determined by GC and GCeMS analysis.

seen that at 60 �C the best conversion was observed at 50 mgcatalyst (entry 3).

To study the scope of this procedure, the oxidation of broadgroup of organic compounds with carbonecarbon double bondswas next studied (Table 6). Oxidation of 4-methyl-1-cyclohexenewas converted to corresponding mono-epoxide with 100% yield,after 3 h of reaction (Table 6, entry 1). Oxidation of terpenes(Table 6, entry 2, 3) occurs also very easily, as a result correspondingepoxides are formed as main products. However, on account ofpeculiarity of this group of compounds and the consecutive re-actions, selectivity is lower than for alkenes and cycloalkenes. As itwas pointed out, oxidation of terpenes in reaction conditions oc-curs very easily, therefore it is important to carry out the reactionvery carefully. Because, too long reaction time cause consecutivereactions of epoxy derivatives to undesired products. 3-(prop-1-en-2-yl)cyclohexanone was oxidized to its epoxide with a moderateyield of 72% (Table 6, entry 4). Prop-1-en-2ylbenzene was oxidizedto its epoxide (Table 6, entry 5) with good yield and selectivity.Oxidation of acenaphthylene affords corresponding ketone (ace-naphenone) in high yield and selectivity with only traces (<2%) ofby-products (Table 6, entry 6). High reactivity was observed withcyclic unsaturated hydrocarbon (1-Cyclooctene, Table 6, entry 7)with one double bond in its structure, which was oxidized to cor-responding epoxide after 6 h with 95% yield. Oxidation of alkeneswith terminal carbonecarbon double bond (Table 6, entries 8e10)occurs with high yield and selectivity. Corresponding epoxides areformed as an only product. It was also observed that in case of al-kenes with single terminal carbonecarbon double bond, increasingof aliphatic chain length leads to a decrease in the yield of oxidationreaction.

Comparison with other reported systems

The catalytic activity of the polymer anchored cobalt(II) complexwas compared with other reported systems for the oxidation ofalkyl aromatics and epoxidation of alkenes (Table 7). It was foundthat the polymer anchored cobalt(II) complex catalyst was superiorto many other reported catalysts in respect of conversion, selec-tivity, reaction temperature and reaction time.

Test for heterogeneity

The leaching of cobalt from polymer anchored cobalt(II) com-plex was confirmed by carrying out analysis of the used catalyst(EDX, IR) as well as the product mixtures (AAS, UVevis). Analysisof the used catalyst did not show appreciable loss in the cobaltcontent as compared to the fresh catalyst. IR spectrum of therecycled catalyst was quite similar to that of fresh sample indi-cating the heterogeneous nature of this complex. Analysis of the

Table 4Oxidation of alkyl aromatics catalyzed by polymer supported Co(II) catalyst.a

Entry Reactant Time (h) Conversion (%)b Product Selectivity (%)b TON

1 6 86 O 97 111

2 7 88 O 98 114

3 7 80 O 97 104

4 8 87 O 93 113

5 8 93 O 90 120

6 O 6 92 O

O

100 119

7

O

6 92

O

O 100 119

8 7 92 O 97 119

9 7 90 O 96 116

10 7 87 O 96 113

a Reaction conditions: substrate (5 mmol), 30% aq H2O2 (1 mmol), O2 (1 atm), acetonitrile (10 mL), temperature (70�C), catalyst (60 mg, 0.03864 mmol).

b Determined by GC and GCeMS analysis.

S.M. Islam et al. / Journal of Organometallic Chemistry 774 (2014) 61e69 67

Table 5Effect of temperature and catalyst amount on the aerobic epoxidation of dodec-1-ene catalyzed by polymer anchored Co(II) catalyst.a

Entry Temperature (�C) Amount of catalyst (mg) Conversion (%)b

1 40 50 542 50 50 623 60 50 944 70 50 955 60 60 966 60 40 82

a Reaction conditions: dodec-1-ene (5 mmol), O2 (1 atm), acetonitrile (10 mL), 2-methyl propanal (1 mmol), time (4 h).

b Determined by GC and GCeMS analysis.

Table 6Aerobic oxidation of unsaturated organic compounds using polymer supportedCo(II) catalyst.a

Entry Reactant Time(h)

Product Conversion(%)b

TON

1 3

O

100 194

2 2O

54 105

3 2O

78 151

4 O 3

O

O

72 140

5 3 76 148

S.M. Islam et al. / Journal of Organometallic Chemistry 774 (2014) 61e6968

product mixtures showed that if any ironwas present it was belowthe detection limit. The UVevis spectroscopy was also used todetermine the stability of this catalyst. The UVevis spectra of thereaction solution, at the first run, did not show any absorptionpeaks characteristic of cobalt metal, which indicates that theleaching of metal did not take place during the course of the re-action. The metal content of the recycled catalyst was determinedwith the help of AAS and it was found that metal content of therecycled catalyst remained almost unaltered. These observationsstrongly suggest that the present catalyst is truly heterogeneous innature.

Heterogeneity test was carried out for the oxidation of ethyl-benzene as an example. For the rigorous proof of heterogeneity, atest was carried out by filtering catalyst from the reaction mixtureat reaction temperature after 4 h and the filtrate was allowed toreact up to the completion of the reaction (6 h). The reactionmixture of 4 h and the filtrate were analyzed by gas chromatog-raphy. No change in the conversion (%) as well as selectivity (%) wasfound which indicates that the present catalyst was heterogeneousin nature.

O

6 2 o 98 190

7 3

O

95 184

Recycling of catalyst

The catalyst remains insoluble in the present reaction condi-tions and hence can be easily separated by simple filtration fol-lowed by washing. The catalyst was washed with dichloromethaneand dried at 100 �C. Oxidation of ethylbenzene and dodec-1-enewas carried out with the recycled catalyst under the optimizedreaction conditions. The catalyst was recycled in order to test itsactivity as well as stability. The obtained results are presented inFig. 7. As seen from Fig. 7, the recycled catalyst did not show anyappreciable change in the activity which indicates that the catalystis stable and can be regenerated for repeated use.

8 ( )5

4 ( )5

O

95 184

9 ( )44 ( )4

O

97 188

10( )3

4 ( )3O

99 192

a Reaction conditions: dodec-1-ene (5 mmol), O2 (1 atm), acetonitrile (10 mL), 2-methyl propanal (1 mmol), temperature (60

�C), catalyst (50 mg, 0.02576 mmol).

b Determined by GC and GCeMS analysis.

Conclusions

Chloromethylated polystyrene anchored cobalt(II) complex wasprepared and characterized. This complex proved to be active andreusable catalyst for the oxidation of alkyl aromatics and alkenes.The catalyst is stable and recyclable under the used reactionconditions. This heterogeneous catalyst shows no significant lossof activity in the recycling experiments. The active sites do notleach out from the support and thus can be reused withoutappreciable loss of activity, indicating that the anchoring proce-dure was effective. The reusability of this catalyst is high and canbe reused six times without significant decrease in its initialactivity.

Table 7Comparison of the present catalyst with other reported systems.

Reaction Catalyst Reaction conditions Conversion/Yield (%) Ref.

Oxidation ofethylbenzene

Polymer-anchored Co (II) catalyst O2 (1 atm) with 0.5 mmol 30% H2O2,CH3CN, 70 �C, 6 h

89% (99% acetophenoneselectivity)

This work

Silica gel supported Co (II) Schiff base catalyst O2 (1 atm), NHPI,CH3COOH, 100 �C, 24 h

98% (99% acetophenoneselectivity)

[23]

SBA-15 Supported Cobalt Oxide H2O2, CH3CN, 70 �C, 6 h 75.2% (87.4% acetophenoneselectivity)

[33]

Epoxidation ofdodec-1-ene

Polymer-anchored Co (II) catalyst O2 (1 atm), CH3CN, 2-methyl propanal, 60 �C, 4 h 94% This workPolyaniline (PANI) supported Co (II) catalyst O2 (1 atm), CH3CN, 2-methyl propanal, 60 �C, 24 h 89% [13]Poly-o-anisidine (POA) supported Co (II) catalyst O2 (1 atm), CH3CN, 2-methyl propanal, 65 �C, 16 h 65% [9]

Fig. 7. Recycling efficiency for the oxidation of ethylbenzene and dodec-1-ene with polymer anchored Co(II) complex.

S.M. Islam et al. / Journal of Organometallic Chemistry 774 (2014) 61e69 69

Acknowledgment

SMI acknowledges Department of Science and Technology (DST)(SB/S1/PC-107/2012, dated: 10.06.13) and Council of Scientific andIndustrial Research (CSIR), New Delhi, India (02(0041)/11/EMR-II,dated: 19.12.2011) for funding. RAM acknowledges UGC, NewDelhi,India for his Maulana Azad National Fellowship.

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