Organic inorganic hybrid cobalt phthalocyanine ...

Accepted Manuscript Organic inorganic hybrid cobalt phthalocyanine/polyaniline as efficient catalyst for aerobic oxidation of alcohols in liquid phase Vineeta Panwar, Pawan Kumar, Siddharth S. Ray, Suman L. Jain PII: DOI: Reference:

S0040-4039(15)00792-3 http://dx.doi.org/10.1016/j.tetlet.2015.05.003 TETL 46275

To appear in:

Tetrahedron Letters

Received Date: Revised Date: Accepted Date:

11 February 2015 30 April 2015 2 May 2015

Please cite this article as: Panwar, V., Kumar, P., Ray, S.S., Jain, S.L., Organic inorganic hybrid cobalt phthalocyanine/polyaniline as efficient catalyst for aerobic oxidation of alcohols in liquid phase, Tetrahedron Letters (2015), doi: http://dx.doi.org/10.1016/j.tetlet.2015.05.003

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Graphical Abstract

Organic inorganic hybrid cobalt phthalocyanine/polyaniline as efficient catalyst for aerobic oxidation of alcohols in liquid phase Vineeta Panwar, a Pawan Kumar a Siddharth S. Raya and Suman L. Jaina

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1

Tetrahedron Letters j o ur n al h o m e p a g e : w w w . e l s e v i e r . c o m

Organic inorganic hybrid cobalt phthalocyanine/polyaniline as efficient catalyst for aerobic oxidation of alcohols in liquid phase Vineeta Panwar,a Pawan Kumara Siddharth S. Raya and Suman L. Jaina a

Chemical Sciences Division, CSIR-Indian Institute of Petroleum, Mohkampur, Dehradun-248005 (India)

AR T IC LE IN F O

A B S TR A C T

Article history: Received Received in revised form Accepted Available online

Organic inorganic hybrid catalyst synthesized by doping of cobalt phthalocyanine (CoPc) on polyaniline support (CoPc/PANI) exhibited higher activity for the oxidation of various alcohols to the corresponding carbonyl compounds in high to excellent yield using molecular oxygen as oxidant and isobutyraldehyde as a sacrificial agent. Notably, the synthesized catalyst was found to be truly heterogeneous in nature and could be easily recovered, recycled for several recycling runs without loss of catalytic activity.

Keywords: Hybrid material Heterogeneous catalyst Oxidation Alcohol Polyaniline

Catalytic liquid phase oxidation of compounds particularly oxidation of alcohols is an important synthetic transformations in organic chemistry.1-5 This transformation is traditionally carried out using stoichiometric quantities of inorganic oxidants, which are relatively expensive, highly toxic and environmentally undesirable.6-13 Therefore, development of non-precious polymer supported catalysts with high reactivity and efficient recycling ability for this transformation is an area of tremendous importance in recent decades.14-18 The use of polymer-supported catalysts offers several advantages such as they increases the stability19-21 of the catalyst, combine the advantages of both homogeneous catalysts like higher reactivity and selectivity as well as heterogeneous catalyst such as facile recovery and recycling ability.22-24 Numerous materials such as mesoporous silica, activated carbon, organic and bio-polymers have been employed as support for aerobic oxidation of alcohols.25-29 Polyaniline (PANI) is one of the polymers which have been used as a matrix for grafting of homogeneous metal complexes. It is cheap, easy to synthesize and insoluble in a large majority of commonly used solvents, which is the main advantage of supported catalysts.30 Moreover, PANI is thermally stable to 300°C and chemical resistant to oxidation.31 Polyaniline involves both electron and proton exchange and exist in three most stable forms as shown in Scheme 1.

2009 Elsevier Ltd. All rights reserved.

Scheme 1 Redox forms of polyaniline

Pielichowski and Iqbal have reported first time the use of polyaniline supported catalytic systems based on cobalt (II) salts and its complexes for the oxidation of alkenes.32-35 Inspired with their reports, herein we report an efficient, easily recyclable organic inorganic hybrid material fabricated by doping of tetrasulfonated cobalt(II) phthalocyanine (CoPcS) on to polyaniline (CoPc/PANI) as catalyst for the oxidation of alcohols using molecular oxygen as oxidant and isobutyraldehyde as sacrificial agent under milder reaction conditions.

2

Tetrahedron

Scheme 2: CoPc/PANI-catalyzed oxidation of alcohols

Synthesis and characterization of the catalyst Synthesis and characterization of CoPc/PANI catalyst The desired CoPc/PANI catalyst was synthesized from emeraldine salt form of polyaniline and using tetrasulfonated cobalt phthalocyanine as dopant ion as shown in Scheme 3.

Fig. 1: SEM images of a) PANI and b) CoPc/PANI and EDX pattern of c) PANI and d) CoPc/PANI

The successful doping of cobalt phthalocyanine on polyaniline matrix was further confirmed by FTIR studies. Fig 2 shows the FTIR absorption spectrum for CoPc, PANI, CoPc/PANI samples.In case of FTIR spectrum of PANI, the characteristic bands due to quinoid ring at 1562 cm-1 and benzoid ring at 1471 cm-1 clearly indicated the existence of these two states in the PANI polymer chains. In the presence of phthalocyanine the intensity of quinoid ring and benzoid ring shifts to 1594 cm-1 and 1498 cm1 respectively. Further the C-N stretching vibrations are appeared at 1301 cm-1 in pure PANI which was shifted to 1307 cm-1 in CoPc/PANI. The absorption at 1110 cm-1 in PANI is due to N=Q=N (where N is for quinoid) which was shifted to1166 cm -1 in CoPc/PANI and N-H stretching shifted from 802 to 827 cm-1. The data represented for these polymers is well matched with the reported literature IR data for polyaniline and phthalocyanine.36-41

Scheme 3: Synthesis of cobalt phthalocyanine doped polyaniline (CoPc/PANI)

The surface morphology of polyaniline and the CoPc/PANI catalyst was determined with the help of SEM as shown in Fig 1. In the SEM image of polyaniline (PANI) many nanofibrillar structures can be seen which confirms the fibrous polymeric structure of polyaniline Fig 1a. After the doping with CoPcS, the fibre like morphology changed to granular structures which are most likely due to the ionic interaction between CoPc and polyaniline Fig 1b. Energy dispersive X-ray spectroscopy analysis (EDX) pattern of polyaniline(PANI) shows absence of any metal ions but in CoPc/PANI, the presence of cobalt confirmed the successful doping of support matrix with metal complex units (Fig 1c,d). The low intensity of peak clearly indicated the lower loading of cobalt in CoPc/PANI.

Fig. 2: FTIR Spectra of a) CoPc and b) PANI and c) PANI-CoPc

Fig 3 shows UV-Vis spectra of CoPc, PANI and CoPc/PANI. The UV-Vis spectrum of CoPc shows its characteristics intense peaks at 670 nm (Q band) and 320 nm (Soret band) due to specific macrocyclic π-π* ring transitions (Fig 3a).42 In pure PANI the band observed at 330-370 nm corresponds to n-π* transition of aniline and broad band at around 630 nm represent the transition of the quinoid rings in long PANI chains (Fig 3b).43-44 Interestingly, in CoPc/PANI, the main absorption band of PANI at 320 nm and 670 nm were affected due to the presence of CoPc in the polymer matrix

3 (Fig 3c). The observed enhancement in the absorption profile of CoPc/PANI was most likely due to the presence of CoPc complex units.

Fig. 3: UV-Vis Absorption Spectra of a) CoPc and b) PANI and c) CoPc/PANI Fig. 5: TGA thermogram of a) CoPc and b) PANI and c) CoPc/PANI

Fig 4 depicted the XRD pattern of PANI and CoPc/PANI.45,46 The XRD diffractogram of PANI shows a broad peak at 2θ value ≈ 25 o due to the 200 plane of carbon of aromatic sheets (Fig 4a). This pattern was specific for aromatic graphitic type sheet structure. Further absence of any sharp peak confirms that material was amorphous in nature. In CoPc/PANI the incorporation of CoPc in polymeric matrix altered the graphitic structure and therefore the peak at 2θ value ≈ 25o was found to be diminished (Fig 4b).

Fig. 4: XRD Diffractogram of a) PANI and b) CoPc/PANI

Thermal stability of the synthesized materials was determined by thermo gravimetric analysis (Fig 5). In case of CoPc (Fig. 5a), the first weight loss at around 100 ᵒC was due to the evaporation of water. The second very minor weight loss around 220 ᵒC was assumed due to the breaking of –SO2Cl moieties presented in phthalocyanine ring. The third major weight loss at 400 ᵒC was observed due to the degradation of phthalocyanine ring structure after that a linear weight loss was observed for continuous degradation of material (Fig 5a). For PANI the small weight loss at 100 ᵒC was due to the evaporation of water and later on two major weight losses at around 175 ᵒC and 520 ᵒC were specific to the PANI polymer chains as suggested in literature (Fig 5b).47-50 However, in CoPc/PANI, the additional weight loss at 400 ᵒC was observed due to the degradation of doped CoPc units in polymer chains (Fig 5c).

Catalytic activity After the successful synthesis of CoPc/PANI catalyst, we aimed to explore the catalytic activity of the catalyst for oxidation of alcohols using molecular oxygen as oxidant and isobutyraldehyde as a sacrificial agent. Benzhydrol was chosen as a model substrate to perform the optimization experiments by varying the reaction parameters. The results of these experiments are summarized in Table 1. Initially the reaction was carried out in different solvents such as acetonitrile, DMF, toluene and p-xylene. Aromatic solvents such as toluene and p-xylene were found to be almost ineffective and no reaction did occur in these solvents (Table 1, entries 1 and 2). Further, the reaction was found to be slow in water under reflux conditions and gave moderate yield of the product (Table 1, entry 3). Aprotic polar solvents such as acetonitrile and DMF were found to be most effective and afforded higher yield of the product (Table 1, entry 4-5). Further, the effect of reaction temperature was investigated. The reaction was found to be very slow at room temperature; however at 65 °C using acetonitrile as solvent was found to be optimum which gave highest value of turnover frequency (TOF) and 76 % isolated yield of the benzophenone in 3.5 h (Table 1, entry 4). The presence of isobutyraldehyde was found to be vital and in its absence very poor yield of benzophenone as well as TOF was obtained (Table 1, entry 6). Further, we checked the effect of catalyst loading by varying the catalyst amount from 0.5 to 5 mol % under otherwise identical experimental conditions. The reaction was found to be increased with the catalyst amount from 0.5 to 2 mol % and gave desired product in lesser reaction time. However, the turnover frequency was found to be decreased with increasing the catalyst amount as shown in Table 1 (entry 7-9). Further increase in catalyst amount from 2 to 5 mol % gave lowest TOF with the marginal increase in the product yield (Table 1, entry 7-9). As the difference in the product yields between 1 and 2 mol % was marginal, we have considered the 1 mol % catalyst loading as the optimum amount for the reaction. Furthermore, we also performed the oxidation of benzhydrol in the absence of oxygen (inert condition) under otherwise identical reaction conditions

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Tetrahedron

(Table 1, entry 10). The reaction did not proceed and the original substrate could be recovered at the end.

Table 2: CoPc/PANI-catalyzed oxidation of alcohols using molecular oxygen as an oxidanta

Table 1: Results of the optimization experimentsa

Entr y

Entry

Solvent

1 2 3 4

Toulene p-xylene H2 O CH3CN

5 6 7 8 9 10

DMF CH3CN CH3CN CH3CN CH3CN CH3CN

Temp. (oC) reflux 60 reflux rt 50 65 65 65 65 65 65 65

Cat. (mol%) 1.0 1.0 1.0 1.0

1.0 1.0 0.5 2.0 5.0 1.0

Time (h) 6.0 5.0 6.5 3.5 3.5 3.5 3.5 3.5 5.0 3.0 2.5 6.0

Yield (%)b 20 10 40 15 24 76 72 15c 70 77 77 -

TOF (h-1 ) 3.3 2.0 6.1 4.2 6.8 21.7 20.6 28.0 12.8 6.16 -

Substrate

Product

1 2

OH

84

24

4

83

20.7

4

87

21.7

6.5

77

11.8

88

25.1

3.5

87

24.8

3.5

86

24.5

3.5

85

24.2

82

20.5

3

80

26.6

4

72

18.0

4

78

19.5

4

75

18.75

5

Turnover frequency=number of moles of the product/ number of moles of cobalt in catalyst x reaction time Next, the scope of the developed catalytic system was studied by performing the oxidation of a variety of primary and secondary alcohols under the described reaction conditions. The results are presented in Table 2. A variety of secondary alcohols including benzoins could be oxidized to the corresponding ketones in good to excellent yields under the described reaction conditions (Table 2, entries 1–9). The presence of catalyst was crucial for the oxidation of alcohols and in its absence the reaction did not take place even in a prolonged reaction time of 12 h (Table 2, entry 1). We also investigated the catalytic activity of homogeneous CoPcS catalyst for oxidation of benzhydrol under otherwise similar reaction conditions. The reaction was found to be slow and gave comparatively poor product yield (Table 2, entry 1). These findings showed the merits of supported catalyst exhibiting higher catalytic activity with the additional benefits of facile recovery and recycling of the catalyst. The developed catalytic system demonstrated higher reactivity and selectivity for the oxidation of benzyl alcohols to corresponding benzaldehydes without any evidence for the formation of over-oxidation products, i.e. carboxylic acids (Table 2, entry 10-15). Among the various benzylic alcohols, those substituted with electron-donating groups were found to be more reactive as compared to those having electron withdrawing substituents (Table 2, entry 14-15). Furfuryl alcohol was selectively converted to furfuraldehyde in good yield under the developed reaction conditions (Table 2, entry 16). Under the described reaction conditions, primary aliphatic alcohols (1-octanol, 1-decanol) gave an intricate mixture of the unidentified products, which were difficult to analyze by GC (Table 2, entry 17-18). Furthermore, we performed the oxidation of benzyl alcohol under drastic conditions i.e. 10 mol % catalyst, 80 oC temperature and 10 h as the reaction time. Under these conditions, benzyl alcohol was quantitatively converted to the corresponding benzoic acid. These results suggested that the reaction parameters such as catalyst loading, temperature and reaction time played a crucial role in the selectivity of the reaction.

3.5

O

a

Reaction conditions: benzhydrol(1 mmol), isobutarldehyde (1.5 mmol), catalyst (1 mol%, 0.01 mmol) in solvent (5 mL) under oxygen atmosphere; bIsolated yield; cIn the absence of isobutyraldehyde; d

TOF (h-1) 21.7 20.6

O

OH

4

Yield (%)b 76 -c 72d

O

OH

3

T/ h 3.5 12 3.5

OH

O

O

OH

6

OH

O

OH

7

3.5

O

CH3 OH

H3C

CH 3 O

H 3C

8

O

9

OMe

O OMe

O

MeO

OH

MeO

CHO

CH2 OH

10

4

11

12

13

14

H3 C

CH2 OH

CHO

CH3

CH3

CH2OH CH3

CHO H3C

CH3

CH3

CH3

CH2OH

CHO

OCH3

OCH3

CH2 OH

CHO

Cl

Cl

5 15

16

CH2OH

CHO

NO2

NO2

70

15.5

4

85

21.25

-

6

-

-

6

-

OH CHO

O

O

17e

OH

e

OH

18

4.5

a

Reaction conditions: substrate (1 mmol), isobutarldehyde (1.5 mmol), catalyst (1 mol % 0.01 mmol) in acetonitrile (5 mL) at 65 o C under oxygen atmosphere, bIsolated yield; cin the absence of catalyst; d using homogeneous CoPcS as catalyst; e mixture of unidentified products was obtained.

Furthermore, we have tested the recycling of the developed heterogeneous catalyst by choosing the benzhydrol as the model substrate. After the completion of the reaction, the catalyst could be easily recovered by centrifugation followed by washing with methanol and reused for subsequent runs. The recovered catalyst showed an efficient recycling ability without giving any change in the reaction time and the yield of the product (Fig. 6). After six recycling the cobalt content of catalyst was determined with ICP-AES was found to be 1.46 wt% that was comparable to 1.5 wt% for freshly synthesized catalyst.

catalyst with the added benefits of facile recovery and recycling of the catalyst. Notably, benzylic alcohols were oxidized to their corresponding carbonyl compounds selectively in high to excellent yields without giving any over-oxidation product. The catalyst was found to be highly stable and could be recycled several times without any significant loss in catalytic activity

Acknowledgments We are thankful to the Director, IIP for his permission to publish these results. PK and VP are thankful to CSIR, New Delhi for providing research fellowships.

References and notes Synthesis of polyaniline (PANI)52 The polymerization of aniline has been carried out by free radical chemical oxidative polymerization method by using ammonium persulphate (APS) as an oxidant in non-oxidizing protonic acid like HCL. In a chemical reaction 1ml of aniline is dissolved in 50 ml HPLC water and 2.8 g of ammonium persulphate in 50 mL HPLC water. Then both the solutions were mixed with constant stirring at room temp. Then 0.2 M HCL in 50 mL HPLC water is added drop wise to the above reacting mixture for 3-4 h and allowed to stir overnight. Then the dark green precipitate so obtained was filtered and washed repeatedly with distilled water till the pH of the filtrate became neutral. This precipitate was then dried in an oven overnight. Synthesis of tetrasulfonated cobalt phthalocyanine (CoPcS)53

Fig. 6: Results of catalytic recycling experiments

Tetrasulfonated cobalt phthalocyanine (CoPcS) was prepared by following the literature procedure. In briefly cobalt phthalocyanine was added slowly to 10 times by weight chlorosulfonic acid with stirring. Then temperature of reaction mixture was increased in steps up to 130- 135 °C and maintained for 4 h. The obtained reaction mixture was cooled to 60 °C and little more than two-fold excess by weight of thionyl chloride were slowly added. This mixture was heated to 79 °C and maintained at this temperature for 1 h.Reaction mixture, after cooling at room temperature, was slowly added to crushed ice by keeping the temperature preferably below 5 °C. The precipitated cobalt phthalocyanine tetrasulfonyl chloride was filtered and washed thoroughly with cold water and dried in vacuum and stored in air tight vial. Synthesis of CoPc/PANI catalyst54

Although the mechanism of this reaction is not clear at this stage, however in analogy to the mechanism suggested by Mukaiyama et al,51 the initial step might involve the free radical autoxidation of the aldehydes to generate an acyl radical. The acyl radical RC(O)· then reacted with O2 to give an acylperoxyl radical RC(O)OO· (dioxygen activation step), which subsequently abstract hydrogen from alcohol to convert it into corresponding carbonyl compound. In conclusion, we have demonstrated an efficient organicinorganic hybrid fabricated by doping of easily accessible cobalt phthalocyanine on to polyaniline support as catalyst for the oxidation of alcohols using molecular oxygen as oxidant. The developed catalyst exhibited superior activity as comparable to the homogeneous cobalt phthalocyanine

Tetrasulfonated cobalt phthalocyanine (CoPcS) (0.125 g) was dissolved in a 10 mL of DMF, then polyaniline (0.5 g) is added to it and reaction was stirred at room temperature for five min. Then 2 mL of triethyl amine was added to the above reacting mixture. Now this solution is allowed to stir for 24 h at 80 ºC. After thorough mixing of the solution, the solution was filtered, washed with distilled water and ethanol. The obtained precipitate was dried at 120 ºC overnight then grinded for further use. Finally the black green colored polyaniline doped cobalt phthalocyanine (CoPc/PANI) was obtained. The percentage of cobalt in the synthesized catalyst was found to be 1.5 % as determined by ICPAES analysis. General procedure for the oxidation A mixture of benzhydrol (1 mmol, 0.184 g), isobutyraldehyde (1.5 mmol, 0.108 g) and catalyst (1 mol % 0.01 mmol) in acetonitrile (15 mL) was heated at 60 °C under stirring by using an oxygen balloon. The progress of the reaction was monitored by thin layer chromatography on silica gel. On completion, the reaction mixture

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Tetrahedron

was cooled to room temperature and centrifuged to separate the catalyst. The product was identified with GCMS. The solvent was removed under reduced pressure and the product was obtained by passing it through a short column of silica gel using EtOAc– hexane (1: 9) as eluent. The identity of the product was confirmed by comparing the physical and spectral data (1H & 13C NMR) with the reported compound. The recovered catalyst was dried at 50 °C for 2 h and can be reused for recycling experiments. 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16.

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