Chemoselective Reduction of Nitro and Nitrile Compounds with

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Chemo-selective reduction of nitro and nitrile compounds with magnetic carbon nanotubes-supported Pt(II) catalyst under mild conditions Seyed Jamal Tabatabaei Rezaei, Hossein Khorramabadi, Ali Hesami, Ali Ramazani, Vahid Amani, and Roya Ahmadi Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b02795 • Publication Date (Web): 09 Oct 2017 Downloaded from http://pubs.acs.org on October 10, 2017

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Industrial & Engineering Chemistry Research

Chemo-selective reduction of nitro and nitrile compounds with magnetic carbon nanotubes-supported Pt(II) catalyst under mild conditions Seyed Jamal Tabatabaei Rezaei,†,* Hossein Khorramabadi,† Ali Hesami,† Ali Ramazani,† Vahid Amani,§ Roya Ahmadi ‡

†

Department of Chemistry, Faculty of Science, University of Zanjan, P.O. Box 45195-313, Zanjan, Iran §

‡

Department of Chemistry, Farhangian University, Tehran, Iran

Department of Chemistry, Yadegar-e-Imam Khomeini (RAH) Branch, Islamic Azad University, Tehran, Iran

* Corresponding author: E-mail address: [email protected]

ABSTRACT: Multi-walled carbon nanotubes (MWNTs) decorated with Fe3O4 nanoparticles has been prepared using a coprecipitation technique and were surface modified using poly(citric acid) (PC) dendrimer. These PC-functionalized magnetic carbon nanotubes (MWCNT/MA@PC) have been used as a novel support for Pt(II) complex immobilized as magnetic nanocatalyst (MWCNT/MA@PC/Pt(II)). The morphology and structural feature of the magnetic nanocatalyst was characterized using different microscopic and spectroscopic techniques such as FT-IR, TEM, EDX, XRD, TGA, ICP and VSM. The nitro and nitrile groups in aromatic and aliphatic 1 ACS Paragon Plus Environment


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compounds containing various reducible substituents such as carboxylic acid, ketone, aldehyde and halogen are selectively reduced to the corresponding amines in water as a eco-friendly solvent with excellent yields by employing NaBH4 in the presence of MWCNT/MA@PC/Pt(II). In addition, the Pt(II) magnetic nanocatalyst can be simply separated from the mixture of reactants by using an external magnetic field and could be reused up to five runs continuously without considerable loss of catalytic activity. KEYWORDS: Magnetic carbon nanotubes; Pt(II) complex; Reduction; Nitro and Nitrile compounds

1. INTRODUCTION The using of catalysis technology is one of best choices to achieve a milder process for the chemical transformations and has a very important role in energy resource management and protecting the environment and human health.1 Amines are one of the most important building blocks in the chemical structure of pharmaceuticals, dyes, surfactants, anti-foam agents, corrosion inhibitors, polymers and agrochemicals.2 However, there are various methods to synthesis of aromatic and aliphatic amines, most of them are not proper due to the formation of harmful waste chemicals for the environment and human health.3 The reductions of nitro groups with metal complexes (or metal powder) catalysts have diverse practical disadvantages, for instance formation of harmful waste chemicals and difficulty in reuse of catalyst

4

and or

reduction of nitrile groups with LiAlH4 and NaBH4 often lead to a mixture of type I, II and III amines.5 Therefore, from an industrial and environmental point of view, the development of green and efficient catalytic systems is a most important issue. Also, the designed catalytic

2 ACS Paragon Plus Environment

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systems should have highly selective performance, so as to prevent the reduction of other reducible groups along with nitro or nitrile group. In these cases, a variety of transition-metal supported catalytic systems have been developed for reduction of nitro compounds (e.g. Pt/CNTs, Pt/C, Pt@Gn-NAs, Graphene/NiPd and P4VP/Cu).6 Several research teams have used diverse homogeneous catalysts, especially based on complexes of transition metals, such as Ni, Cu, Zn, Pd, Pt, Cr, and Ru.7 Although homogeneous catalytic systems exhibit higher catalytic activity and selectivity than supported heterogeneous catalysts, their separation from the products, recovery and reusability are challenging. This leads not only to loss of expensive metal and ligands, but also to contamination of the final product by leached metal, which makes the homogeneous catalysts unsuitable in particular for industrial large-scale production. In order to overcome these limitations, research groups investigated the catalytic performance of transition metal complexes supported on semi-heterogeneous support, e.g., surface-modified carbon nanotubes (SMCNTs).8 A semi-heterogeneous catalytic system simultaneously has the same nature of homogeneous and heterogeneous catalysts.8b, 9 SMCNT has a high potential for use as a semi-heterogeneous catalyst supports due to its high surface-tovolume ratio, high stability and dispersion in polar solvents and unique physical and chemical properties. However, SMCNTs remain suspended for long periods of time, so separation of these modified nanomaterials from reaction mixture is a very difficult and time consuming procedure.8a To overcome this drawback and to induce constant semi-heterogeneous ability, it seems an effective way is induction of the nature of magnetism into SMCNTs. These catalysis systems were highly active, dispersible, and recovered using an external magnet, which results in



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conservation of energy and environment, and production of target products with a low cost in mild and green conditions.10 The aim of this study is to introduce an efficient semi-heterogeneous nanocatalyst for the selective hydrogenation of nitro and nitrile groups in aromatic and aliphatic compounds using a milieu friendly process. To reach this target, we present here our studies on the preparation, characterization, activity, and selectivity of new magnetic carbon nanotubes-supported Pt(II) catalyst

(MWCNT/MA@PC/Pt(II)),

as

semi-heterogeneous

system

for

the

selective

hydrogenation of nitro and nitrile compounds in the presence of various reducible substituents such as carboxylic acid, ketone, aldehyde and halogen to the respective amines under mild and green conditions (room temperature and atmospheric pressure).

2. EXPERIMENTAL SECTION 2.1. Apparatus and reagents All chemicals were purchased from Sigma-Aldrich or Merck Chemical Companies. The FT-IR spectra were recorded on a Jasco 6300 apparatus. Crystallographic assay measurement was performed on prepared samples using an X-ray powder diffractometer (XRD-6000) with Cu Ka radiation (wave length = 0.154056 nm). 1H NMR spectra were recorded on a BRUKER DRX250 AVANCE spectrometer at 250.0 MHz using deuterated chloroform as the solvent. The content of platinum in the magnetic catalyst was determined by inductively coupled plasma mass spectrometry (ICP-MS, Agilent ICP-MS 7500 Series) and thermogravimetric analysis (TGA) (STA 1500 instrument at a heating rate of 10 ºC/min in air). The magnetic properties of catalyst were determined at room temperature using vibrating sample magnetometer (VSM, Meghnatis Kavir Kashan Co., Kashan, Iran). The size and morphological characterization of the particles


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were carried out using a Zeiss-EM10C transmission electron microscope (TEM) operating at 80 kV. Ultrasonic generator was carried out on a TECNO-GAZ, S.p.A., Tecna 6, input: 50–60 Hz/305 W, and uniform sonic waves to disperse materials in solvents. N2 adsorption-desorption (ASAP 2020 (Micromeritics) instrument) at 77 K were used to determine specific surface area and pore size of the catalysts.

2.2. Synthesis of cis-[Pt(1,7-phenanthroline)(DMSO)Cl2] A solution of 1,7-phenanthroline (0.20 g, 1.10 mmol) in 30 ml methanol was added to a solution of K2PtCl4 (0.46 g, 1.10 mmol) in distilled water (15 ml) at room temperature and stirred for 10 min until a precipitate was formed. The precipitate started to disappear gradually when DMSO (20 ml) was added to the solution and the mixture was stirred at 50 °C for 1 h. The solution was filtered and allowed to crystallize at room temperature, whereupon pale yellow prismatic crystals of complex formed after five weeks (yield 0.42 g, 72.8%, m.p: 244 °C).

2.3. Synthesis of MWCNTs-(COOH)n The MWCNTs-(COOH)n was synthesized in the presence of HNO3 and H2SO4 (v:v/1:3) as described previously.8a

2.4. Synthesis of MWCNTs/MA The aqueous solution of the FeSO4.7H2O/FeCl3 (2.11 mmol, 0.585 g/4.47 mmol, 0.727 g in 150 mL) and MWCNTs-(COOH)n (0.125 g in 100 mL) were mixed and subjected to ultrasonic waves for 10 min to completely spread metal ions over the surface of MWCNTs-(COOH)n under N2 atmosphere, then the resulting mixture was stirred at 90 °C for 30 minutes. The ammonia



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aqueous solution (1.5 M) was then added dropwise to the above mixture and reaction continued further for two hours at 90 °C. After completion, the solvent was removed via magnetic decantation and the resulting solid was purified from water and then ethanol to afford MWCNTs/MA magnetic support.

2.5. Synthesis of MWCNTs/MA@PC 0.1 g of MWCNTs/MA and 2 g of monohydrate citric acid were added to a reaction ampoule equipped with a magnetic stirrer and vacuum inlet and it was sealed under vacuum. The reaction mixture was then stirred at 120 ° C for 30 min. Subsequently, after removal of water by vacuum inlet, the reaction temperature increased to 140 °C and the reaction continued for 1 hour at this temperature. Then, the same procedure was repeated for 160 °C once again. The resulting PC functionalized magnetic MWCNTs (MWCNTs/MA@PC) were purified from THF and then ethyl acetate and dried for characterization.

2.6.

Synthesis

of

Pt(II)

complex

immobilized

on

MWCNTs/MA@PC

(MWCNTs/MA@PC/Pt(II)) The aqueous solution of cis-[Pt(1,7-phenanthroline)(DMSO)Cl2] complex (0.15 g in 5 ml) and MWCNTs/MA@PC (1 g in 10 mL) were mixed and subjected to ultrasonic waves for 10 min to completely spread complexes over the dendritic (PC) shell of MWCNTs/MA@PC support. Then the mixture was stirred at room temperature for 48 h. The Pt(II) supported MWCNTs/MA@PC nanocatalyst were magnetically separated and the resulting solid were purified from water and then ethanol (2 × 50 mL) and dried under vacuum (at 40 °C) for characterization.


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2.7. General procedure for hydrogenation of nitro and nitrile compounds with MWCNTs/MA@PC/Pt(II) as catalyst 10 mmol nitro or nitrile compounds was added to 5 mL distilled water and then 0.004 g ultrasonically dispersed MWCNTs/MA@PC/Pt(II) catalyst (0.04 mol% Pt(II)) in water (5 mL) was introduced to this solution. Then 10 mmol NaBH4 was added and the mixture was stirred in 25 °C. The progress of reaction was detected by TLC (or GC). After completion of the reaction, the nanocatalyst was magnetically removed and washed several times with ethanol and used after drying in subsequent reactions and then, the residual solvent was evaporated under vacuum to obtain the pure amines. The conversions were determined by the gas chromatography (GC) analysis. All of the synthesized amines were characterized by comparison of NMR spectral data with the reported values in literatures (Supporting Information).

3. RESULTS AND DISCUSSION The modification of the surface of the MWCNTs with magnetite nanoparticles and PC, and subsequently the conjugation of the platinum complex, is depicted in the Scheme 1. The carboxyl-functionalized MWCNTs-(COOH)n were prepared via a simple acid oxidation method.8a For easy separating the CNTs-based catalyst from the reaction mixture by using an external magnetic field, the incorporation of CNTs with magnetic moieties are required. Magnetite nanoparticles possess good magnetic property and thus have been used as the magnetic cores for design of many catalyst systems.11 So, we were chosen the MWCNTs/MA nanocomposites as the original hybrid material for the design of magnetic nano-support in this study. Consequently, in order to increase the loading-capacity and water-solubility of magnetic



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nanocomposites, the surface of MWCNTs/MA was further modified using PC as a hydrophilic polymer by using a divergent strategy.12

Scheme 1. Synthesis of MWCNT/MA@PC/Pt(II) magnetic nanocatalyst.

Meanwhile, a novel Pt(II) complex (cis-[Pt(1,7-phenanthroline)(DMSO)Cl2]) was prepared using commercially available 1,7-Phenanthroline ligand. This ligand combines a free pyridine group for conjugating on MWCNT/MA@PC and a good conjugation site for platinum species. The integrity of the chemical structure presented for the Pt(II) complex was investigated by

1

H NMR and X-ray diffraction techniques (Supporting Information). In

1

H NMR

spectroscopy in DMSO-d6, the corresponding proton signals were detected at δ: 7.80 (m, 1H), 8.89 (m, 1H), 8.17 (d, J = 9.0, 1H), 8.52 (d, J = 9.0, 1H), 8.66 (d, J = 7.5, 1H), 9.08 (d, J = 3.5, 1H), 9.18 (d, J = 3.5, 1H) and 9.79 (d, J = 7.5, 1H). The ORTEP view for cis-[Pt(1,7phenanthroline)(DMSO)Cl2] complex is depicted in Figure 1. As seen in this Figure 1, the


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platinum(II) cation is four-coordinated in a square-planar configuration by one S atom from one dimethyl sulfoxide ligand, one N atom from one 1,7-phenanthroline ligand and two chloride anions.

Figure 1. The ORTEP diagram of cis-[Pt(1,7-phenanthroline)(DMSO)Cl2] complex.

Finally,

the

cis-[Pt(1,7-phenanthroline)(DMSO)Cl2]

complex

was

conveniently

conjugated to the MWCNT/MA@PC by the hydrogen-bonding interactions of unreacted pyridine group of Pt(II) complex with the abundant free carboxyl groups of PC dendrimer (Scheme 1).13 The content of Pt(II) complex in the MWCNTs/MA@PC/Pt(II) sample was confirmed by measuring Pt of the sample by the ICP-MS technique (11.85% Pt(II) complex) that indicated a high loading capacity for this nanocatalyst. Also, Figure 2 represents the EDX spectrum of MWCNT/MA@PC/Pt(II) magnetic nanocatalyst that could obviously indicated its principal elements. Accordingly, the C, N, O, Fe, Pt and Cl signals can be observed as the principal elements of MWCNT/MA@PC/Pt(II).



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Figure 2. EDX spectrum of MWCNT/MA@PC/Pt(II) nanocatalyst.

In order to prove the successful preparation of MWCNTs/MA@PC/Pt(II) magnetic nanocatalyst, the prepared materials were characterized by FT-IR, TGA and XRD techniques. Figure 3 represented the IR spectrum of MWCNTs, MWCNTs/MA, MWCNTs/MA@PC and MWCNTs/MA@PC/Pt(II). In the all spectra of MWCNT/MA, MWCNT/MA@PC, and MWCNTs/MA@PC/Pt(II), the characteristic absorption band of Fe3O4 is appeared in the range 565-585 cm-1.11 In Figure 3c and d, the peaks in the range of 3050-3630 cm−1 were ascribed to carboxylic acid groups (O–H stretching) of grafted PC. These bands could be associated to different hydrogen bonding interactions. The absorbance band of C=O groups of PC dendrimer is appeared from 1507 to 1736 cm−1.12 Also, the FT-IR spectrum of MWCNTs/MA@PC/Pt(II) (Figure 3d) has almost similar peaks with cis-[Pt(1,7-phenanthroline)(DMSO)Cl2] (Supporting Information), which indicate the surface functionalization of MWCNTs/MA@PC magnetic nano-support with cis-[Pt(1,7-phenanthroline)(DMSO)Cl2].


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Figure 3. FTIR spectra of MWCNTs-(COOH)n (a), MWCNTs/MA (b), MWCNTs/MA@PC (c) and MWCNTs/MA@PC/Pt(II) (d).

Thermogravimetry analysis was used to determine the weight percent of polymer bound to the magnetic MWCNTs surface (Figure 4). Comparing the TGA curves indicates that the weight percent of grafted PC on the surface of MWCNTs/MA is about 15.63% (Figure 4c). Also, comparing the TGA curves of MWCNT/MA@PC and MWCNTs/MA@PC/Pt(II) (Figure 4c and d) demonstrates that the weight percent of immobilized Pt ions on the surface of magnetic nanosupport is about 4.47% (equal with 12.01% cis-[Pt(1,7-phenanthroline)(DMSO)Cl2]), Which is in full compliance with the results of the ICP-Ms technique.



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Figure 4. TGA curves of MWCNTs-(COOH)n (a), MWCNTs/MA (b), MWCNTs/MA@PC (c) and MWCNTs/MA@PC/Pt(II) (d).

As seen in the samples XRD patterns, Fe3O4 magnetic nanoparticles have crystalline structure and MWCNTs with amorphous structure, which is consistent with the reported results for each of these systems in the literatures (Figure 5).8a, 14 The broad peaks at 2θ = 26.23 can be assigned to the (0 0 2) planes of the multi-walled carbon nanotubes with graphite like structure. The other six peaks of the samples are characteristic of crystalline Fe3O4 magnetic nanoparticles, corresponding to the planes (220), (311), (400), (422), (511) and (440) at 2θ values of about 30.2°, 35.6°, 43.3°, 53.6°, 57.2° and 62.9°, respectively.


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Figure 5. X-Ray powder diffraction of MWCNTs-(COOH)n (a), MWCNTs/MA (b), MWCNTs/MA@PC (c) and MWCNTs/MA@PC/Pt(II) (d).

Figure 6a shows the TEM image of unmodified MWCNTs. As shown in Figure 6a, the MWCNTs have a smooth and uniform surface before the modification with a diameter in the range of 10–20 nm. In comparison, the MWCNTs/MA@PC/Pt(II) nanocatalyst surface shows small spots (related to Fe3O4 and PC) that are uniformly distributed on the MWCNTs surface (Figure 6b and 6c). The magnified image reveals that the Pt(II) complexes are strongly adhered into the hyperbranched polymers on the surface of MWCNTs (Figure 6d).



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Figure 6. TEM image of unmodified MWNTs (a), MWCNTs/MA@PC/Pt(II) (b, c and d) nanocatalyst in different magnifications.

In

order

to

characterize

the

magnetic

properties

of

the

synthesized

MWCNTs/MA@PC/Pt(II), a vibrating sample magnetometer (VSM) was used to record hysteresis loops of the nanocatalyst. As shown in the Figure 7, the saturation magnetizations were found to be 16.11 emu/g for the MWCNTs/MA@PC/Pt(II), which means that these nanocatalyst have superparamagnetic properties and can be effectively aggregated at a special site by application of an external magnetic field.


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Figure 7. Hysteresis loops for the MWCNTs/MA@PC/Pt(II) at 25 °C.

To study the porosity of MWCNTs/MA@PC/Pt(II) catalyst, Brunauer–Emmett–Teller (BET) method was used. Comparison of nitrogen adsorption-desorption isotherm for MWCNTs/MA@PC/Pt(II) with various type isotherms classified by the IUPAC shows that the isotherm of the catalyst is type IV.15 Increasing gas adsorption with a steep slope by the sample at a relatively big pressure range (p/p0 = 0.6-1) proves that the catalyst structure has a high degree of mesoporosity. The specific surface area and the total pore volume of the MWCNTs/MA@PC/Pt(II) catalyst are 32.65 cm2g-1 and 0.133 cm3g-1, respectively. As shown in the corresponding Barrett–Joyner–Halenda (BJH) pore size distribution curves (Figure 8b), the highest distribution of catalyst pores are in the range of 2-10 nm, with an average of 7.65 nm in diameter. The high specific surface area and high degree of mesoporosity are two important parameters for a catalytic system that can offer an ample interface for efficient and rapid interaction of reactants with catalytic sites.



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Figure 8. Nitrogen adsorption/desorption isotherms of MWCNTs/MA@PC/Pt(II) (a) and corresponding BJH pore size distribution (b).

In order to evaluate the efficiency of the designed Pt(II) catalyst in the reduction reactions of nitro and nitrile compounds and obtain optimal reaction conditions, 4-nitroaniline was used as a model nitro compound. To optimize the amount of the catalyst, the model reaction was performed with various amounts of the catalyst in water at 25 ºC (Table 1, Entries 1-3). Within 15 min and with 0.04 mol% of Pt(II), >99% yield of p-phenylenediamine was obtained (Table 1, Entry 6). However, the reaction did not happen in the absence of catalyst (Table 1, Entry 1). The effect of reaction time (Table 1, Entries 4 and 5) was also investigated by carrying out the model reaction at different times and the highest yield was obtained at 15 min. Also, we tested the reactions at 25 ºC in diverse solvents including water, water/ethanol (1/2), ethanol, toluene, THF, DMSO, and the results showed that the highest yield for the product was obtained in water. Hydrophobic nature of the dendrimers' interior void space ensures that the reactants tend to


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assemble at those areas rather than reaction solvent. Since Pt(II) complexes as catalyst also exist in these spaces, therefore, it is predictable that a very efficient reaction will result.

Table 1. Optimization of the reaction conditions for the reduction of 4-nitroaniline with MWCNTs/MA@PC/Pt(II) as the catalyst system.a

a b

Entry

Solvent

Catalyst amount (Pt (II) content, mol%)

Time (MIN)

Temp. (°C)

Yield b(%)

1

H 2O

-

15

25

-

2 3 4 5 6

H 2O H 2O H 2O H 2O H 2O

0.02 0.05 0.04 0.04 0.04

15 15 5 25 15

25 25 25 25 25

85 >99 69 >99 >99

7

H2O/EtOH (1/2)

0.04

15

25

96

8 9 10 11

EtOH Toluene THF DMSO

0.04 0.04 0.04 0.04

15 15 15 15

25 25 25 25

92 65 70 72

Reaction conditions: 10 mmol of 4-nitroaniline and10 mmol of NaBH4. Yield of isolated product.

Under optimal catalytic reaction conditions, we studied the reduction reactions of various functionalized nitro substrates to the respective amines (Table 2). Some of these amines, such as chloro-substituted aromatic amines, industrially and biologically are important. The m- and pnitroaniline were cleanly converted to the corresponding aromatic 1,3- and 1,4-diamines (Table 2, Entries 1−2). The Chloro- and fluoro-substituted nitroarenes (Table 2, Entries 3 and 4) were 17 ACS Paragon Plus Environment


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selectively converted to the corresponding haloaromatic amines without any dehalogenation.16 A glimpse into the results of the Table 2 shows that the best yields were obtained for derivatives that have electron-withdrawing substitutions (Entries 5-8). Also, nitro compounds contain other functional groups such as ketone, aldehyde and carboxylic acid, are selectively converted to the corresponding amines without changing other groups by this procedure (Table 2, Entries 5-8). The nitroethane was successfully reduced to ethanamine by this catalyst system (Table 2, Entry10). As shown in Table 2, in all reactions, nitro compounds with 100% selectivity were converted into corresponding amines.


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Table 2. The reduction reaction of nitro compounds with MWCNTs/MA@PC/Pt(II) as the catalyst system.a Yield b (%)

Selectivity c (%)

1

>99

100

2

98

100

3

97

100

4

>99

100

5

96

100

6

97

100

7

>99

100

8

98

100

9

>99

100

10

94

100

Entry

Nitro compound

Product

a

Reaction conditions: 10 mmol nitro compound, 10 mmol NaBH4, 10 mL H2O, MWCNTs/MA@PC/Pt(II) (0.04 mol % Pt(II)) at 25 ˚C for 15 min. b

Yield of isolated product.

c

Calculated by gas chromatography.



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Encouraged by the excellent results obtained by using MWCNTs/MA@PC/Pt(II) as a catalyst for the reduction of nitro compounds in aqueous media, we set out to optimize the reaction conditions for the reduction of nitrile compounds in aqueous media. To obtain the optimum reaction conditions for the hydrogenation of nitrile compounds was used 4pyridinecarbonitrile as a model compound (Table 3). As can be seen in Table 3 (Entries 1-3), the yield of the product increased with increasing the amounts of catalyst. While a further increase in the amount of catalyst to 0.05 mol% did not have a significant effect on the product yield (Table 3, Entry 3). According to the results of the effect of temperature and reaction time on the reduction of 4-pyridinecarbonitrile, the highest yield was obtained at 95 ºC and 30 min (Table 3, Entry 6). After the amount of catalyst, time and temperature for the reaction were optimized; then the effect of different solvents on the reaction was investigated (Table 3, Entries 10-14). The results showed that the best conditions for the reaction were water as solvent using 0.04 mol% of catalyst at 95 ºC and a time of 30 min.

Table 3. Optimization of the reaction conditions for the reduction of 4-pyridinecarbonitrile with MWCNTs/MA@PC/Pt(II) as the catalyst system.a

Entry

Solvent

Catalyst amount (Pt(II) content, mol%)

Time (MIN)

Temp. (°C)

Yield b(%)

1 2 3 4 5 6

H 2O H 2O H 2O H 2O H 2O H 2O

0.02 0.05 0.04 0.04 0.04

30 30 30 15 45 30

95 95 95 95 95 95

81 99 87 98 98


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7

H 2O

0.04

30

75

83

8

H 2O

0.04

30

55

79

9

H 2O

0.04

30

25

66

10

H2O/EtOH (1/2)

0.04

30

Reflux

94

11 12 13 14

EtOH Toluene THF DMSO

0.04 0.04 0.04 0.04

30 30 30 30

Reflux 95 Reflux 95

86 47 73 58

a

Reaction conditions: 10 mmol of 4-pyridinecarbonitrile and10 mmol of NaBH4.

b


Because benzyl amines compounds are important biologically, the reduction of benzonitriles is a phenomenal issue. The results in Table 4 show that the best yields were obtained for benzonitrile derivatives that have one or more electron-withdrawing substitutions under optimum condition (Table 4, Entries 1-3). In contrast, benzonitriles with electron-donating substitution (Table 4, Entries 4-6) converted to the corresponding amines in moderate yields. The relatively lower yield of the amine product obtained from 2-methoxybenzonitrile in comparison with the 4-methoxybenzonitrile (Table 4, Entry 4 and 5) is related to the steric hindrance around the reaction site. It is necessary to mention that many developed catalytic systems are not able to reduce

aliphatic

nitrile

even

under

extended

reflux

conditions.17

However,

MWCNTs/MA@PC/Pt(II) is able to reduce aliphatic nitriles in good yields (Table 4, Entries 9 and 10). As shown in Table 4, in all reactions, nitrile compounds with 100% selectivity were converted into corresponding amines.



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Table 4. The reduction reaction of nitrile compounds with MWCNTs/MA@PC/Pt(II) as the catalyst system.a Yield b (%)

Selectivity c (%)

99

100

2

97

100

3

98

100

4

91

100

5

93

100

6

94

100

7

98

100

8

96

100

9

89

100

10

90

100

Entry

Nitrile compound

Product F3C NH2

1

a

Reaction conditions: 10 mmol nitrile compound, 10 mmol NaBH4, 10 mL H2O, MWCNTs/MA@PC/Pt(II) (0.04 mol % Pt(II)) at 95 ˚C for 30 min. b


c

Calculated by gas chromatography.

Recycling and reusability of a catalyst is particularly important in industrial applications. For this purpose, the recycling and reusing capability was investigated in 5 consecutive reduction reaction of 4-nitroaniline and 4-pyridinecarbonitrile in optimum condition for the as-prepared


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MWCNTs/MA@PC/Pt(II) catalyst. As the results obtained in Figure 9 shown, the MWCNTs/MA@PC/Pt(II) catalyst, after being used five cycles in successive reactions, still has a high catalytic performance. In another experiment, after ~50% progressed of the reaction the catalyst was removed from the reaction mixture (at the reaction temperature). The analysis of reaction mixture showed that further performing of the reaction under optimum conditions in the absence of catalyst did not show any significant progress. Also, ICP-Ms analysis of the reaction mixture demonstrated that the amount of Pt in the solution was less than the detection limit. Therefore, we can conclude that the designed catalyst has a truly heterogeneous nature.

Figure 9. Effect of recycling on the catalytic activity of MWCNTs/MA@PC/Pt(II); (a) 4-nitroaniline (red) and (b) 4-pyridinecarbonitrile (blue).

Comparing the results obtained with the MWCNTs/MA@PC/Pt(II) catalyst for the reduction of nitro and nitrile groups with other reported catalysts for this reaction indicates that it is able to achieve better efficacy in a milder and greener conditions (Table 5, Entry 1). These systems require longer reaction time (Entries 2-7), higher amount of additives (Entries 2-5 and



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Page 24 of 31

7), and difficulties of catalyst separation from the reaction mixture (Entries 4-7) and higher amounts of catalyst (Entries 2-7).

Table 5. Comparison of the results for the reduction of nitrobenzene (A) and benzonitrile (B) with some other reported catalysts.

Entry

Catalyst

1

MWCNTs/MA@PC/Pt(II)

2

Reactant

Solvent

Catalyst/ metal content, mol%

Additive/mmol

Atm.

A

Water

Pt(II)/0.04

NaBH4/1

‒

NaBH4/1

‒

95

30

96

A

Glycerol

Ni(0)/8.85

KOH/2

‒

80

180

94

Not tested Water

Au(0)/1

NaBH4 /10

N2

25

70

96

‒

25

30

92

‒

25

300

50

H2 (1 atm)

25

60

>99

21

‒

80

240

100

22

MNPs@PIL@AuNPs

EtOH

Au(0)/0.1

NH3BH3/1.5

Au/TiO2

A

Water

Co(0)/20

B A

N2H4·H2O/excess

A

6d

Not tested 2-PrOH

Pd/NiO/1

B

CeO2 Nanorods

20

Not tested

CoNPs/PVP

Pd/NiO

19

Not tested

B

7

Present work

18

Fe3O4–Ni MNPs

A

6

Ref.

>99

Pt(II)/0.04

B

5

15

Water

A

4

25

B

B

3

Temp. Time Yield (°C) (min) (%)

‒ Not tested

EtOH/ Water (1/1)

CeO2/5

B

N2H4·H2O/2.5 Not tested


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4. CONCLUSIONS In this study, we have introduced a new magnetically recyclable platinum-based semiheterogeneous catalytic system (MWCNTs/MA@PC/Pt(II)) for the selectively reduction of nitro and nitrile groups in aromatic and aliphatic compounds containing various reducible substituents such as carboxylic acid, ketone, aldehyde and halogen to the corresponding amines in water as a green solvent. The dendritic layers on MWCNTs/MA magnetic nano-support with unique properties such as large surface area, multi-functionality cause to enhance the dispersibility of the magnetic supports in polar solvents and stabilize the Pt(II) complexes. Other remarkable advantages of this methodology include high stability and reusability of catalyst, clean reactions, easy workup, short reaction times, and cost-effective.

ASSOCIATED CONTENT Supporting Information This information is available free of charge via the Internet at http://pubs.acs.org/. 1

H-NMR and FT-IR spectra and crystals data of cis-[Pt(1,7-phenanthroline)(DMSO)Cl2]

complex. 1H-NMR spectra of the amine products

ABBREVIATIONS MWNTs PC

multi-walled carbon nanotubes poly(citric acid)

SMCNTs

surface-modified carbon nanotubes

DMSO

dimethyl sulfoxide

THF

tetrahydrofuran

PIL

poly ionic-liquid

PVP

poly(vinylpyrrolidone) 25 ACS Paragon Plus Environment


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AUTHOR INFORMATION Corresponding author * Fax: +98 24 32283203. Email: [email protected]. ORCID: 0000-0002-1065-752X

ACKNOWLEDGMENT We are grateful to University of Zanjan and Farhangian University Research Council for partial support of this study.

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For Table of Contents Only

In this work, we have developed a novel magnetically retrievable platinum-based semiheterogeneous catalytic system (MWCNTs/MA@PC/Pt(II)) for the selectively reduction of nitro and nitrile groups in aromatic and aliphatic compounds containing various reducible substituents such as carboxylic acid, ketone, aldehyde and halogen to the corresponding amines in water as a green solvent.


Chemoselective Reduction of Nitro and Nitrile Compounds with

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