Research Article pubs.acs.org/journal/ascecg
Immobilized Iron Metal-Containing Ionic Liquid-Catalyzed Chemoselective Transfer Hydrogenation of Nitroarenes into Anilines Nilesh M. Patil,† Takehiko Sasaki,‡ and Bhalchandra M. Bhanage*,† †
Department of Chemistry, Institute of Chemical Technology, N. Parekh Marg, Matunga, Mumbai-400019, India Department of Complexity Science and Engineering, Graduate School of Frontier Sciences, The University of Tokyo, 5-1-5, Kashiwanoha, Kashiwa, Chiba 277-8561, Japan
‡
S Supporting Information *
ABSTRACT: The chemoselective transfer hydrogenation of nitroarenes into anilines was investigated using immobilized iron metal containing ionic liquid (ImmFe-IL) as a versatile heterogeneous catalyst. The present protocol has high atom efficiency with excellent chemosectivity and regioselectivity. Selective reduction of the nitro group was observed in the presence of other reducing functional groups such as nitrile, ketone, acid, amide, ester, halogens, hydroxyl, olefin, and heterocycles. The developed protocol is also highly selective toward conversion of dinitroarenes into nitroanilines. The present catalytic system uses environmentally benign ethylene glycol as a solvent which is inexpensive, easily available, nontoxic, nonvolatile, nonflammable, thermally stable, and biodegradable. The catalyst was reused up to four consecutive cycles with high activity and selectivity. The fresh catalyst and recycle catalyst was well characterized by XPS analysis. KEYWORDS: Iron, Immobilization, Heterogeneous catalyst, Chemoselective hydrogenation, Nitroarenes
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INTRODUCTION Aromatic amines play an important role in agrochemicals, pharmaceuticals, fine chemicals, dyes, pigments, material science, and biotechnology.1,2 The catalytic hydrogenation or transfer hydrogenation of nitroarenes is one of the fundamental reactions for the synthesis of primary amines.3−6 The chemoselective reduction of nitro compounds is a key step in the synthesis of pharmaceutically active compounds such as antibiotic linezolid (Zyvox) and sildenafil (Viagra).7,8 Traditionally, reduction of nitroarnes was carried out by using Fe/HCl system. This process is not environmentally friendly as it requires the use of stoichiometric amount of reagents which also generates large quantity of secondary waste materials. Another widely used conventional catalyst is the RANEY nickel catalyst which also suffers from several drawbacks such as pyrophoric nature, moisture sensitivity, and difficulty in handling.9,10 In the past decade, alternatively, several other methods have been developed for reduction of nitro compounds like (i) sodium borohydride/ metal catalyst,11,12 (ii) zinc and tin catalyzed,13,14 (iii) Mo(CO)6/1,8-diazabicyclo-[5.4.0]undec-7-ene(DBU),15 (iv) Cu nanoparticles with HCOONH4,16 (v) metal-catalyst with hydrazine,17−19 (vi) silane/oxorhenium complexes,20 (vii) HI,21 and (viii) Pd(OAc)2 with polymethylhydrosiloxane (PMHS).22 In particular, various iron-catalyzed reduction of nitroarens has also been demonstrated e.g. Fe with CaCl2 and NH4Cl, Fe/Zn in DMF-H2O system, Fe(EDTA)Na2 complex in water, iron-oxide/ hydroxide/hydrazine hydrate in ethanol, Fe(acac)3/TMDS, COH2O with Au−Fe(OH)3, and FeS/NH4Cl/CH3OH-H2O.23−29 However, in spite of their potential utility, these reports have one or more drawbacks such as less chemoselectivity, need of © 2015 American Chemical Society
stoichiometric amount of metal hydrides, the use of acid or base, preactivation of catalysts. In addition, some of these developed protocols are found to be less chemoselective toward a nitro group in the presence of other reducing functional groups such as acid, ketone, ester, olefin, halide, benzyl, and nitrile and also shows less regioselectivity in the reduction of dinitro compounds. Further, hydroxylamines, hydrazines, azoarenes, or azoxyarenes are known to be formed as byproducts during the reduction of nitroarnes.30,31Palladium−ruthenium and silver catalyst are also tested for nitro reduction.32−35 Chemoselective reduction of nitro groups into corresponding amines is a challenging task in the organic chemistry. Recently, gold catalyst has been found to be highly selective and active toward the nitro reduction.36,37 However, gold is an expensive noble metal which makes the protocol economically unfavorable. Therefore, the efforts have been made to develop cost-effective, heterogeneous, and reusable catalytic system which can efficiently catalyzed chemoselective transfer hydrogenation of nitroarenes. Considering this aspect, many groups have reported iron-catalyzed nitro reductions in the presence of hydrogen source. Of note, Beller and co-workers have described various iron catalysts such as FeBr2/P(cy)3 with silane, Fe(BF4)·6H2O/PP3 with formic acid, nano based Fe2O3 with molecular hydrogen, and Fe-phenanthroline/C with hydrazine hydrate as a hydrogen Special Issue: Ionic Liquids at the Interface of Chemistry and Engineering Received: November 7, 2015 Revised: December 4, 2015
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DOI: 10.1021/acssuschemeng.5b01453 ACS Sustainable Chem. Eng. 2016, 4, 429−436
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ACS Sustainable Chemistry & Engineering source for hydrogenation of nitroarenes.38−41 Although, these catalytic systems showed high conversion and selectivity, but the long reaction times, use of phosphine or nitrogen containing ligands, high pressure system, and use of organic solvents are some limitations. Recently, many researchers have focused the reduction of nitro compounds using green reaction media. Ranu and co-workers successfully applied an iron nanoparticle catalyst for selective nitro reduction using water as a hydrogen source.42 Singh and co-workers have also explored chemoselective transfer hydrogenation of nitro compounds using various homogeneous phthalocyanine metal complexes with hydrazine hydrate as hydrogen source in green solvents (ethynene glycol and PEG 400).43−45 Nowadays, immobilization of metal has gained great prominence in catalysis. Not only does it avoid contamination
of product with metal−ligand but it also minimizes the organic waste from work up. In addition, the reusability of the immobilized catalyst makes the process green and cost-effective.46,47 Hence, various methods have been developed for immobilization of metals on to the surface of inorganic/organic solid supports.48 Recently, eco-friendly ionic liquids supported on solid materials have been used in many organic transformations.49,50 In continuation of our ongoing research toward the development of efficient and environmentally benign heterogeneous catalysts,51−56 here we have first synthesized immobilized iron metalcontaining ionic liquid [ImmFe-IL] and successfully applied chemoselective transfer hydrogenation of nitroarenes into anilines using hydrazine hydrate as a hydrogen source in ethylene glycol as an environmentally benign solvent (Scheme 1).
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Scheme 1. Chemoselective Transfer Hydrogenation of Nitroarenes Using ImmFe-IL
EXPERIMENTAL SECTION
Materials and Method. N-Methylimidazole (99+%) and 3-trimethoxysilylpropyl chloride (97+%) were procured from Aldrich. FeCl2 was purchased from WAKO. Dried redistilled 1-methylimidazole (99+%) was obtained from Aldrich. All the dehydrated solvents were purchase from WAKO. Aerosil 300 (300 m2/g) was acquired from Japan Aerosil Co. and calcined at 573 K for 1.5 h in air and 30 min in vacuum before use as a support. Loading of the catalyst was calculated by XRF measurements (SEA-2010, Seiko Electronic Industrial Co.). The XPS of ImmFe-IL was measured using a PHI5000 Versa Probe with monochromatic focused (100 μm × 100 μm) Al Kα X-ray radiation (15 kV, 30 mA) and dual beam neutralization using a combination of argon ion gun and electron irradiation. The products are well-known in the literature and were confirmed by GC (PerkinElmer, Clarus 400) (BP-10 GC column, 30 m × 0.32 mm ID, film thickness 0.25 mm) and GCMS (Shimadzu GC-MS QP 2010), 1H NMR, and 13C NMR spectroscopy. Catalyst Preparation (ImmFe-IL). Immobilized metal ioncontaining ionic liquid catalyst was prepared as shown in (Scheme 2), as reported previously. 57 1-Methyl-3-(3-trimethoxysilylpropyl) imidazolium chloride was synthesized by mixing N-methylimidazole (0.690 mol) and 3-trimethoxysilylpropyl chloride (0.690 mol) in a dry 300 mL flask under a nitrogen atmosphere and the resulting reaction mixture refluxed for 48 h. After completion, the reaction mixture was cooled to room temperature and fully washed with dry ethyl acetate. The obtained residue was dried at room temperature under reduced pressure for 48 h and then stored at 253 K under dry nitrogen. Silica (Aerosil 300, surface area 300 m2/g, calcined at 573 K for 1.5 h in air) and 1-methyl-3-(3-trimethoxysilylpropyl) imidazolium chloride (weight ratio 1:1) were mixed in dehydrated toluene and then the reaction mass was refluxed for 48 h under nitrogen. After the completion of reaction, toluene was removed by filtration using glass filter and the excess of ionic liquid was removed by washing with hot dichloromethane several times. The resultant solid is denoted as Imm-IL. In the next step, Imm-IL was added to an acetonitrile solution of anhydrous FeCl2 (Wako Chemical 98%) and refluxed for 24 h. The resulting mass was separated by using glass filter and wash with acetone until removal of excess acetonitrile and metal chloride. The metal loading of ImmFe-IL was 3.1 wt % as determined by XRF measurements (SEA-2010, Seiko Electronic Industrial Co.). Catalyst Characterization. Fe K-Edge EXAFS Measurements for [Bmim]2FeCl4 and ImmFe-IL. EXAFS measurements of the Fe K-edge were carried out at the High Energy Accelerator Research Organization (KEK-IMMS-PF). The measurement was carried out in transition mode and the spectra were taken at BL-9C. The electron storage ring was
Scheme 2. Preparation of Iron Metal-Ion-Containing Immobilized Ionic Liquid
Figure 1. k3-Weighted Fourier transform of Fe K-edge EXAFS for the [Bmim]2FeCl4 sample. The amplitude and imaginary part are traced by solid curve and dotted curve, respectively. Observed data are shown with thick lines, and fitting data are shown with thin lines.
Table 1. Summary of the Fe K-Edge-EXAFS Fitting Results for [Bmim] 2FeCl4a
[Bmim]2FeCl4 a
path
R bond distance (10−1nm)
S02
CN
DW (10−5nm2)
Δk (10 nm−1)
ΔR (10−1nm)
ΔE0 (eV)
Rf (%)
Fe−Cl
2.240 ± 0.007
0.505 ± 0.065
4.0 (fixed)
3.3 ± 0.8
3−14
1.5−3.0
3.6 ± 1.7
3.2
CN: coordination number. DW: Debye−Waller factor. Δk: wavenumber range included in the fitting. ΔR: R range included in the fitting. 430
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ACS Sustainable Chemistry & Engineering operated at 2.5 GeV and 450 mA. The synchrotron radiation from the storage ring was monochromatized by a Si (111) channel cut crystal. The ionized chambers, used as detectors for the incident X-ray (Io) and transmitted X-ray (I), were filled with N2 gas and N2 and Ar gas mixture (75:25), respectively. Cu foil was used to calibrate the angle of the monochromator. UWXAFS analysis package,58 including background subtraction program AUTOBK59 and curve fitting program FEFFIT60 was used to analyze the EXAFS raw data. The backscattering amplitude and phase shift were calculated theoretically by FEFF 8.4 code.61 EXAFS for [Bmim]2FeCl4, of which structure was determined by single crystal X-ray diffraction analysis,62 was also measured as a reference sample. Observed and fitted k3-weighted Fourier transform of Fe K-edge EXAFS for [Bmim]2FeCl4 are shown in (Figure 1). In the crystal structure of [Bmim]2FeCl4 the Fe atom is coordinated by four Cl anions in tetrahedral symmetry. Therefore, fitting of EXAFS function was achieved by fixing the coordination number of Fe as four and the Cl was chosen as the X-ray scattering atom. The fitted results are summarized in Table 1. The amplitude reducing factor, S02, was obtained as 0.505 ± 0.065. This value was used in the fitting procedure of ImmFe-IL. The Fe−Cl bond distance was obtained as 2.240 ± 0.007 Å, in agreement with the XRD determined structure. Observed and fitted k3-weighted Fourier transform of Fe K-edge EXAFS for ImmFe-IL are shown in (Figure 2).The EXAFS spectrum
Table 3. Effect of Catalyst and Catalyst Loading on Chemoselective Transfer Hydrogenation of 2Chloronitrobenenea entry
catalyst
catalyst loading (mol %)
conversion (%)
selectivity (%)
00 99 59 60 20 28
00 99 99 99 99 99
22 40
99 99
79 30
99 99
Catalyst Screening 1 2 3 4 5 6 7 8
no catalyst ImmFe-IL FeCl3 FeCl3·6H2O FeSO4·7H2O Fe(NO3)3· 9H2O Fe(C2H3O2)2 Fe(C5H7O2)3
9 10
ImmFe-IL ImmFe-IL
3 3 3 3 3 3 3 Catalyst Loading 2 1
a Reaction conditions: 2-choloronitrobenzene (1 mmol), NH2NH2·H2O (3 mmol), ethylene glycol (8 mL), temperature (110 °C), time (12 h). Conversion and selectivity based on GC and GC-MS analysis.
Table 4. Effect of Reaction Parameter on Chemoselective Transfer Hydrogenation of 2-Chloronitrobenenea entry
Figure 2. k3-Weighted Fourier transform of Fe K-edge EXAFS for the ImmFe-IL sample. Amplitude and imaginary part are traced by solid curve and dotted curve, respectively. Observed data are shown with thick lines, and fitting data are shown with thin lines.
1 2 3 4 5 6 7
water DMF ethanol THF toluene p-xylene ethylene glycol
8
ethylene glycol ethylene glycol ethylene glycol
9 10
can be fitted with a single Fe−Cl path, indicating that the Fe species is homogeneous in the sample. Fitted results are given in (Table 2). Fe−Cl bond distance was obtained as 2.247 ± 0.006 Å and the coordination number was 2.9 ± 0.3. These results indicate that Fe species uniformly distributed in the fresh ImmFe-IL catalyst as the form of [FeCl3]− as a counteranion for imidazoium cation, the valence of Fe2+ is retained in the fresh catalyst in agreement with XPS results. ImmFe-IL Catalyzed Chemoselective Transfer Hydrogenation of Nitroarenes. To a mixture of nitro compound (1 mmol) and catalyst (3 mol %) in ethylene glycol (8 mL) was added hydrazine hydrate (3 mmol) and then the reaction mixture was stirred at 110 °C for 12 h. The progress of reaction was monitored by thin layered chromatography (TLC) and gas chromatography (GC). After completion of the reaction, the catalyst was separated by filtration method. The product was extracted with ethyl acetate (3 × 5 mL) and dried over anhydrous Na2SO4 and then evaporated under vacuum. All the obtained products are well-known in literature and were analyzed by GC (PerkinElmer, Clarus 400) (BP-10 GC column, 30 m × 0.32 mm ID, film thickness
11
ethylene glycol
12
ethylene glycol ethylene glycol
13
NH2NH2·H2O (mmol)
time (h)
conv/selec (%)b
12 12 12 12 12 12 12
12/99 34/99 22/99 32/99 74/99 85/99 99/99
Effect of Hydrogen Donor Concentration 1.5 110 12
45/99
solvent
3 3 3 3 3 3 3
temp (°C)
Effect of Solvent 100 110 80 80 110 110 110
3
110
12
99/99
5
110
12
99/99
12
67/99
Effect of Temperature 3 90
3
Effect of Time 110
8
74/99
3
110
6
42/99
a
Reaction conditions: 2-choloronitrobenzene (1 mmol), ImmFe-IL (3 mol %), NH2NH2·H2O (3 mmol), solvent (8 mL), temperature (110 °C), time (12 h). bConversion and selectivity based on GC and GC-MS analysis. 0.25 mm) and GCMS (Shimadzu GC-MS QP 2010), 1H NMR, and 13 C NMR spectroscopy. General Experimental Procedure for Recycling of ImmFe-IL. After completion of the reaction, the reaction mixture was cooled to room
Table 2. Summary of the Fe K-Edge-EXAFS Fitting Results for ImmFe-ILa ImmFe-IL a
path
R:bond distance (10−1 nm)
S02
CN
DW (10−5 nm2)
Δk (10 nm−1)
ΔR (10−1 nm)
ΔE0 (eV)
Rf (%)
Fe−Cl
2.247 ± 0.006
0.505 (fixed)
2.9 ± 0.3
3.9 ± 0.7
3−14
1.2−2.5
8.8 ± 1.3
2.1
CN: coordination number. DW: Debye−Waller factor. Δk: wavenumber range included in the fitting. ΔR: R range included in the fitting. 431
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ACS Sustainable Chemistry & Engineering Table 5. ImmFe-IL Catalyzed Chemoselective Transfer Hydrogenation of Various Substitute Nitroarenesa
Reaction conditions: nitroarenes (1 mmol), ImmFe-IL (3 mol %), NH2NH2·H2O (3 mmol), ethylene glycol (8 mL), temperature (110 °C), time (12 h). bConversion and selectivity based on GC and GC-MS analysis. cIsolated yield.
a
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temperature, and the catalyst was separated by filtration. The separated catalyst was washed with distilled water and methanol to remove all traces of product or reactant present. The separated catalyst was then dried under reduced pressure. The dried catalyst was then used for the next run.
RESULTS AND DISCUSSION
Optimization Study for Chemoselective Transfer Hydrogenation of Nitroarenes. The reaction conditions were optimized and for which preliminary studies were 432
DOI: 10.1021/acssuschemeng.5b01453 ACS Sustainable Chem. Eng. 2016, 4, 429−436
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ACS Sustainable Chemistry & Engineering Table 6. Various Reported Iron Catalyst Tested for Chemoselective Reduction of Nitroarenes into Anilinesa
a
entry
catalyst
H2 source
solvent
temp (°C)
time (h)
recycle study
selectivity
ref
1 2 3 4 5 6 7 8 9 10 11
FeS FeII/EDTANa2 Fe(acac)3 Au/Fe(OH)x iron oxide/hydroxide Fe-phosphine Fe(BF4) 6H2O/PP3 Fe-phen/C-800 Fe-phenanthroline/C iron phthalocyanine ImmFe-IL
NH4Cl H2 (400 psi) TMDS CO-H2O NH2NH2·H2O PhSiH3 HCOOH H2 (50 bar) NH2NH2·H2O NH2NH2·H2O NH2NH2·H2O
CH3OH-H2O water THF water ethanol toluene ethanol H2O-THF THF H2O-ethanol ethylene glycol
reflux 150 60 100 70 110 40 120 100 120 110
4−8 2−8 24 1.5 4−12 16 1−2 15 10 7 12
ND recycle ND ND ND ND ND recycle recycle ND recycle
ND high moderate high low high high high high high high
24 26 27 28 29 38 39 40 41 43 this work
ND: not demonstrated.
performed using ImmFe-IL as a catalyst, NH2NH2·H2O as a hydrogen source, and ethylene glycol as a solvent for the chemoselective transfer hydrogenation of 2-choloronitrobenzene as a model reaction at 110 °C for 12 h. The influence of various reaction parameters has been investigated on the model reaction such as catalyst screening, catalyst loading, effect of solvent, and effect of hydrogen donor concentration, reaction temperature, and time. The obtained results are summarized in Tables 3 and 4. Initially, the reaction was performed without use of catalyst, but the reaction did not proceed (Table 3, entry 1). Next, we studied the activity of synthesized ImmFe-IL catalyst and other iron catalyst such as FeCl3, FeCl3·6H2O, FeSO4·7H2O, Fe(NO3)3·9H2O, Fe(C2H3O2)2, and Fe(C5H7O2)3 (Table 3, entries 2−8). Among them heterogeneous ImmFe-IL catalyst was found to be the best catalyst providing 99% conversion and excellent selectivity toward the desired product (Table 3, entry 2). Next, we checked the effect of catalyst loading and it was found that conversion decreases with decrease in catalyst loading (Table 3, entries 9−10). In the next set of experiments, it was found that other solvents such as water, DMF, ethanol, THF, toluene, and p-xylene provided the desired product in poor to moderate yields (Table 4, entries 1−6). Interestingly, when ethylene glycol was employed as a solvent, 99% conversion of 2-chloronitrobenzene was noted along with an excellent selectivity (Table 4, entry 7). The effect of hydrogen donor concentration was also studied and it was noticed that the use of 1.5 mmol of NH2NH2·H2O furnished 45% conversion with 99% selectivity (Table 4, entry 8). On the other hand, the use of 3 mmol of NH2NH2·H2O furnished 99% conversion with 99% selectivity (Table 4, entry 9). Further, increase in hydrogen donor concentration up to 5 mmol has found no effect on selectivity (Table 4, entry 10). Next, the decreasing reaction temperature up to 90 °C led to a decrease in the conversion of 2-chloronitrobenzene (Table 4, entry 11). The time study shows that 12 h is optimum time required for the completion of reaction (Table 4, entries 12−13). Therefore, the optimized reaction conditions for chemoselective transfer hydrogenation of nitroarene are nitroarene (1 mmol), NH2NH2·H2O (3 mmol) and ImmFe-IL (3 mol %), ethylene glycol as a solvent, and temperature 110 °C for 12 h. With optimized conditions in hand, we have extended our protocol for transfer hydrogenation of wide range of aromatic nitro compounds with diverse substituent groups (Tables 5 and 6). In all the entries the developed catalyst furnished high conversion with excellent selectivity without forming any side products such as hydroxylamines, hydrazines, azoarenes, or azoxyarenes. First, we checked halogen-substituted chloro, bromo, fluoro and iodo
Figure 3. Recyclability study of ImmFe-IL catalyst.
nitroarenes and it was observered that in all the halogenated nitrobenzenes cleanly reduced to the respective primary amines without the formation of any dehalogenation products (Table 5, entries 1−6). The nitrobenzene easily reduced to aniline in the excellent yields (Table 5, entry 7). Nitrobenzene containing electron donating groups such as o-, m-, and p-methyl and p-methoxy was successfully reduced into corresponding amines in high yields (Table 5, entries 8−11). The reduction of m-nitro and p-nitro phenol selectivity reduced into the desire products without affecting the −OH groups (Table 5, entries 12−13). The 2-nitroaniline and 2-amino benzyl alcohol was also reduced easily into corresponding amines in excellent yields (Table 5, entries 14−15). Only selective reduction of nitro groups into corresponding amines was observed in case 2-nitro-9H-fluorene and nitro-1,4-benzodioxane (Table 5, entries 16−17). Interestingly, ImmFe-IL catalyst was also active for the reduction of heterocyclic nitroarenes (Table 5, entries 18−19). Furthermore, we have also extended our protocol for the chemoselective reduction of nitro compounds that contain cyano group and interestingly it was observed that only selective reduction of nitro groups into corresponding amines with the cyano group remain intact (Table 5, entries 20−22). Selective reduction of dinitro group has been investigated, and it was found that only one nitro group was reduced into the respective amine without affection another nitro group (Table 5, entries 23−25). Next we have studied regioselective reduction of dinitro toluene, chlorodinitrobenzene, and dinitro aniline, and it was observered that only the o-nitro group reduced into corresponding amines (Table 5, entries 26−28). From the synthesis point of view, only chemoselective nitro reduction is very important while presence other reducible functional groups in substrate like acid, amide, ester, ketone and olefin. To our delight, the present catalytic 433
DOI: 10.1021/acssuschemeng.5b01453 ACS Sustainable Chem. Eng. 2016, 4, 429−436
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ACS Sustainable Chemistry & Engineering
Figure 4. (a) Wide scan survey. (b) XPS of Fe 2p region for ImmFe-IL catalyst.
functional groups such as halo, cyano, hydroxyl, acid, ketone, ester, amide, olefin, and heterocycles. The developed protocol uses of ethylene glycol as a solvent and recyclable catalytic system making this process cost-effective and greener. The fresh and reused catalyst is well characterized with XPS analysis.
system is very selective toward nitro group while other reducible functional groups remained unaffected (Table 5, entries 29- 33). Recycle Study. In an effort to make the present catalytic system more inexpensively, the recycling of heterogeneous is definitely important in the catalytic reactions. Hence we studied the recyclability of ImmFe-IL for chemoselective transfer hydrogenation of 2-chloro nitrobenzene as model reaction. After completion of reaction, ImmFe-IL catalyst was separated by filtration method. The filtrate catalyst was washed with distilled water by several time and methanol to remove the rest of organic compounds. The filtrated catalyst dried in oven and reused for next run. The ImmFe-IL catalyst was found to be efficiently recycled up to four successive cycles with continuing high activity and selectivity (Figure 3). XPS Analysis of the ImmFeIL Catalyst. XPS spectra were measured for the fresh ImmFe-IL, the first recycle, and the fourth recycle catalysts to find out valence states and elemental composition of the catalyst (Figure 4). The wide scan survey for fresh and recycled (1st and fourth) ImmFeIL catalyst are shown in Figure 4a. The wide scan study shows that no large change in the fresh and recycled catalysts. In fresh catalyst, peak observed for Fe 2p region at 709.6 for Fe2+ species. Next, first recycle peak was slightly shifted at 710.9 for same region. In case of fourth recycled, XPS peak shift toward at 711.5 for Fe3+ species in the same region. The XPS study reveals that ImmFe-IL catalyst changes the oxidation sate from Fe2+ to Fe3+ which indicating that catalyst gets self-oxidized because it reduces the nitroarenes (Figure 4b). We tested various iron precursors having various oxidation state such as Fe2+ and Fe3+ (Table 3, entries 2−8). But among them ImmFe-IL catalyst having Fe2+ oxidation state was found to be the best catalyst providing 99% conversion and excellent selectivity toward the desired product (Table 3, entry 2). In fourth recycled catalyst having Fe3+ oxidation state also shows good conversion and excellent selectivity (Figure 4) may be due to Fe species lying in immobilized imidazolium groups as characteristics of Imm-IL or synergic effect of catalyst.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.5b01453. Spectral data and copies of 1H and 13C NMR of the products (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected];
[email protected]. Tel.: +91 2233612603. Fax: +91 2233611020. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The author (Nilesh. M. Patil) is greatly thankful to the University Grant Commission (UGC-SAP), India for providing the Senior Research Fellowship. XPS analysis was conducted at the Research Hub for Advanced Nano Characterization, the University of Tokyo, supports by the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan. This work has been supported by DST and JSPS under the IndiaJapan Science Cooperative Program (Project No. DST/INT/ JSPS/P-152/2013). T.S. acknowledges the support by MEXT through a Grant-in-Aid for Scientific Research on Innovative Areas “Initiative for High-Dimensional Data-Driven Science through Deepening of Sparse Modeling” (No. 26120509). EXAFS measurements were performed at KEK-IMSS-PF with the approval of the Photon Factory Advisory Committee (project 2014 G 070).
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CONCLUSION In summary, an inexpensive and recyclable iron catalyst system has been developed for the efficient reduction of nitroarenes into amines using hydrazine hydrate as a hydrogen donor. The developed ImmFe-IL catalyst furnishes high yield with excellent chemo and regio-selectivity. Notably, selectively hydrogenate aryl nitro groups in the presence of other reducible
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DOI: 10.1021/acssuschemeng.5b01453 ACS Sustainable Chem. Eng. 2016, 4, 429−436
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