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Article

A Reusable FeCl3∙6H2O/Cationic 2,2′-Bipyridyl Catalytic System for Reduction of Nitroarenes in Water

by
Tsai-Yu Hung
†,
Wen-Sheng Peng
†,
Jing-Wen Tang
and
Fu-Yu Tsai
*
Institute of Organic and Polymeric Materials, National Taipei University of Technology, 1, Sec. 3, Chung-Hsiao E. Rd., Taipei 10608, Taiwan
*
Author to whom correspondence should be addressed.
†
These authors contributed equally to this work.
Catalysts 2022, 12(8), 924; https://doi.org/10.3390/catal12080924
Submission received: 16 July 2022 / Revised: 6 August 2022 / Accepted: 19 August 2022 / Published: 21 August 2022
(This article belongs to the Special Issue Catalysis in Green Chemistry and Organic Synthesis)
Browse Figure
Versions Notes

Abstract

:
The association of a commercially-available iron (III) chloride hexahydrate (FeCl3∙6H2O) with cationic 2,2′-bipyridyl in water was proven to be an operationally simple and reusable catalytic system for the highly-selective reduction of nitroarenes to anilines. This procedure was conducted under air using 1–2 mol% of catalyst in the presence of nitroarenes and 4 equiv of hydrazine monohydrate (H2NNH2∙H2O) in neat water at 100 °C for 12 h, and provided high to excellent yields of aniline derivatives. After separation of the aqueous catalytic system from the organic product, the residual aqueous solution could be applied for subsequent reuse, without any catalyst retreatment or regeneration, for several runs with only a slight decrease in activity, proving this process eco-friendly.

1. Introduction

Aniline and its derivatives are important key intermediates in the chemical industry, widely-used for the preparation of dyes, drugs, agrochemicals, and polymers [1,2,3]. The common process for the preparation of anilines is the reduction of nitroarenes utilizing transition metals as catalysts, with various hydrogen sources [4]. Catalyzation of nitroarene reduction by precious metal complexes, such as Re [5], Ru [6,7,8,9,10,11,12,13,14,15,16,17,18,19], Rh [20,21], Pd [20,22,23,24,25], Ir [20], Pt [20,26,27], Au [28,29], and Ir-Au bimetallic [30], and first series metal complexes, such as Cr [31], Mn [32], Co [33,34,35], Ni [36,37], Cu [35,38], and Zn [39], has been well-documented. Alternatively, transition metal nanoparticles have also been widely applied to catalyze the reduction of nitroarenes recently [4,40,41,42,43,44,45,46,47,48,49,50,51,52].
Iron, as the most abundant, cheapest, and nontoxic transition metal, is an ideal catalyst candidate instead of other transition metals for nitroarene reduction. Its single atom [53,54,55,56,57], powder [58,59,60,61,62,63,64], salts [65,66,67,68,69,70,71], and complexes [6,22,72,73,74,75,76,77,78,79,80,81] are widely applied to mediate or catalyze the reduction of nitroarenes to anilines in organic or organic/H2O mixed solvents. Recently, iron-based heterogeneous catalysts have also been applied for the reduction of nitroarenes [40,41,42,43,44,45,46,47,48,49,50,51]. However, such nanocatalysts are usually obtained through precursor hydrothermal treatment, complex pyrolysis, or treatment with moisture-sensitive reagents, which may limit their widespread application. On the other hand, water is an idea solvent to reduce the environmental impact and costs due to its environmental compatibility, nontoxicity, abundance, and low cost. When neat water has been used as the reaction medium, however, an excess amount of iron was usually required for nitroarene reduction [82,83]. Only rare examples, including Fe(II)-citrate in situ forming nanoscale zero-valent iron (nZVI) with sodium borohydride (NaNH4), catalyzing the reduction of p-nitrophenol [84], and iron carbonyl clusters (Fe3E2(CO)9, E = S, Se, Te) with hydrazine monohydrate (N2H4∙H2O) catalyzing the reduction of nitroarenes in neat water [85], have been reported.
Iron (III) chloride hexahydrate (FeCl3∙6H2O) is one of the most readily available iron sources; however, its reduction of nitroarenes usually requires stoichiometric or excess amounts to accomplish the transformation, leading to wastage of the metal [66,71]. An example of successful catalysis employed largely excess amounts of N,N-dimethylhydrazine against nitroarenes under refluxed methanol [65]. Therefore, the development of an eco-friendly protocol using this commonly-available iron source, without the requirement for high temperature or moisture-sensitive reagent pretreatment, in neat water to reduce wastage of metal and eliminate the use of an organic solvent as the reaction medium, is highly desirable (See Table 1 for the comparison of the Fe catalysts). As part of our interest in FeCl3∙6H2O catalysis under an aqueous phase [86,87,88], in this report, the association of FeCl3∙6H2O with a water-soluble cationic bipyridyl ligand acted as a green catalytic system to accomplish nitroarene reduction for the formation of anilines in neat water. In order to avoid the manipulation of hazardous H2 under high pressure at high temperature, the safe-to-handle and low-cost dihydrogen precursor hydrazine monohydrate (N2H4∙H2O) was employed as the reducing agent, because H2 and N2 are generated in situ in the presence of a catalytic amount of transition metal, leaving no residual waste [89]. Moreover, after separation of the catalytic system from the organic products by simple extraction, the residual aqueous phase could be reused for the next run immediately without any retreatment or regeneration (Scheme 1).

2. Results and Discussion

First, various readily-available iron salts were associated with the water-soluble bipyridyl ligand, L, in water in order to evaluate the efficiency for reduction of 1-nitronaphthalene 1a. The reaction was conducted using 1a (1.0 mmol) and N2H4∙H2O (4.0 mmol) as the reducing agent in water (2 mL) at 100 °C for 6 h (Table 2, Entries 1–7). Among the iron salts, FeCl3∙6H2O (≥99% purity) was found to be the best catalyst, which rendered 1-naphthylamine 2a in a 69% yield (Entry 7). As expected, the reduction did not take place in the absence of the iron salt (Entry 8). Therefore, the FeCl3∙6H2O/L system (1 mol%) was then selected for further optimization. A reaction duration of 12 h was found to be sufficient to obtain a near-quantitative yield of 2a (Entries 9 and 10). Reducing the amount of N2H4∙H2O or lowering the reaction temperature led to decreasing product yields (Entries 11 and 12). Ligand L plays a decisive role in obtaining a high yield of 2a in the reaction and, hence, an inferior yield of 2a resulted when the reaction was conducted in the absence of L (Entry 13). This result was consistent with a published paper stating that the combination of FeCl3·6H2O and N2H4·H2O for nitroarene reduction is ineffective [65]. In addition, only 43% of 2a was furnished when L was replaced with neutral 2,2′-bipyridine (Entry 14). Lipshutz et al. reported that Fe/ppm Pd nanoparticles prepared from commercially-available FeCl3 (≥97% purity, contains 300 to 350 ppm Pd) or doped with 350 ppm Pd(OAc)2 were able to catalyze Suzuki–Miyaura coupling [90]. Alternatively, Fe/80 ppm Pd nanoparticles can be applied to reduce nitroarenes to anilines in the presence of NaBH4 as the reducing agent [91,92]. In order to exclude catalysis resulting from contaminated metals in commercially-available sources, the palladium impurity was analyzed by inductively coupled plasma mass (ICP-MASS) spectrometry, which showed that FeCl3∙6H2O (≥99% purity) contained only 0.8 ppm of Pd [87]. A further reaction performed with a 99.99% purity of FeCl3 as the catalyst provided 2a in a 98% yield, which indicated that this reduction is indeed catalyzed by iron (Entry 15). Other common reducing agents, such as NH4Cl and HCOONH4, failed to reduce 1a and, hence, 1a remained intact (Entries 16 and 17). It is worth mentioning that a large-scale reaction employing 10 mmol of 1a and 40 mmol of N2H4∙H2O under the conditions of Entry 9 was achieved, giving rise to 2a in a 97% isolated yield (Entry 18).
Following identification of the optimal conditions, a variety of nitroarenes were applied to the FeCl3∙6H2O/L system to assess the scope and limitations of this process (Table 3). It was found that this catalytic system reduced 4-halonitrobenzenes 1b–1d efficiently, producing corresponding 4-haloanilines 2b–2d in excellent yields with no side products of hydrodehalogenation compounds (Entries 1–3), which has been observed in several hydrogenation processes [93,94,95]. When applying activating groups at the 4-position, the reduction took place smoothly under 1–2 mol% catalyst loading, and high yields of corresponding aniline derivatives 2e–2l were obtained (Entries 4–11). This catalytic system also worked efficiently with nitroarenes bearing cyano, ester, and amide groups (1m–1o), giving 2m–2o in excellent yields (Entries 12–14). Procainamide 2p, a drug for the treatment of cardiac arrhythmias, could be synthesized using this protocol in an excellent yield (Entry 15). These results indicated that the FeCl3∙6H2O/L system possessed excellent tolerance to a wide variety of reducible functional groups. Multi-substituted nitroarenes 1q–1v, except for sterically-hindered 1v, were reduced to the corresponding products smoothly (Entries 16–21). In general, this reduction was clean. For instance, no intermediates or by-products were detected after 12 h in the reduction of 1v, which gave only 2v and unreacted 1v (Entry 21). This may indicate that the intermediates were more reactive than the nitroarene in our system [64]. This protocol was applicable to the reduction of heterocyclic substrates, which are important intermediates for pharmaceuticals [96]. Hence, 1w–1z were reduced smoothly, giving 2w–2z in high yields (Entries 22–25). Nitroarenes bearing formyl (3a) and keto (3b) groups can react with hydrazine to give hydrazone compounds [19,35,65]; 4a and 4b were, therefore, obtained in 83% and 90% yields, respectively (Entries 26 and 27). The reduction of a nitroarene with a terminal C=C bond showed no selectivity, and both nitro group and π-bonds were reduced (Entry 28). With internal alkene 3d, the double bond was partially protected, which gave 4d/4d′ in a 7.9/1 ratio (Entry 29). Similar results have also been observed in a FePc/FeSO4·7H2O system using N2H4·H2O as the reducing agent [74]. Dinitro compounds, such as 5a and 5b, were reduced efficiently and, hence, furnished 4,4′-oxydianiline 6a and diaminodiphenyl sulfone (Dapsone) 6b in high yields (Entries 30 and 31).
It is recognized that the reusability of a catalytic aqueous solution is one of the major advantages of performing the reaction in neat water, thus, reducing the environmental impact and cost of the procedure. The reusability of this green catalytic system was, therefore, examined, and 1a and 1h were selected as the representative reactants. As shown in Table 4, the reduction of 1a with 1 mol% catalyst loading resulted in the formation of 2a in a 98% yield in the initial run. After extracting the reaction mixture with EtOAc (3 × 5 mL), the residual aqueous phase was applied for subsequent reuse studies without any retreatment or regeneration. It was observed that, at the third reuse run, an 80% isolated yield of 2a could still be achieved (Entry 1). In addition, the catalyst reuse studies for 1h were performed with a 2 mol% catalyst loading, and a 78% yield of 2h was obtained in the third reuse run (Entry 2). The gradual decrease in catalytic activity in reuse studies might be due to the deactivation of the catalyst or a gradual decrease in the catalyst concentration upon successive extraction of the aqueous solution.

3. Materials and Methods

3.1. Instruments and Reagents

Iron salts and most nitroarenes were acquired from commercial suppliers and were used without further purification. Here, 1j [97], 1k [98], 1n [99], 1p [100], and 3d [101] were synthesized according to published procedures. The water-soluble bipyridyl ligand L was obtained by our published method [102]. The 1H- and 13C-NMR spectra were obtained at 25 °C in CDCl3, DMSO-d6 or acetone-d6 solution on a Bruker Biospin AG 300 NMR spectrometer (Bruker Co., Faellanden, Switzerland), in which the chemical shifts (δ in ppm) were established with respect to CHCl3, non-deuterated DMSO, and acetone, which were employed as the reference (1H-NMR as follows: CHCl3 at 7.24, non-deuterated DMSO at 2.49, and non-deuterated acetone at 2.05 ppm; 13C-NMR as follows: CDCl3 at 77.0, DMSO-d6 at 39.5, and acetone-d6 at 29.9 ppm). The spectral data of all nitroarene reduction products and copies of their 1H and 13C NMR spectra can be found in the Supplementary Materials.

3.2. Experimental Method

3.2.1. General Procedure for Reduction of Nitroarenes

A 20 mL sealable glass tube equipped with a magnetic stirrer bar was charged with FeCl3∙6H2O (2.7 mg for 1 mol% or 5.4 mg for 2 mol% reactions), L (4.6 mg for 1 mol% or 9.2 mg for 2 mol% reactions), and H2O (2 mL). This mixture was stirred at room temperature for 30 min to give a wine-red solution. After the addition of nitroarene (1 mmol) and H2NNH2∙H2O (0.2 mL, 4 mmol), the tube was sealed under air and stirred at 100 °C for 12 h. After cooling the reaction to room temperature, the aqueous solution was extracted with EtOAc (3 × 5 mL), the combined organic phase was dried over MgSO4, and the solvent was removed under vacuum. Column chromatography on silica gel eluted with n-hexane/EtOAc (2/1) provided the desired products.

3.2.2. General Procedure for Catalyst Reuse Studies

After finishing the initial run and separating the product from the aqueous phase as mentioned above, the residual aqueous solution was recharged with nitroarene (1 mmol) and H2NNH2∙H2O (0.2 mL, 4 mmol). The tube was then sealed and stirred at 100 °C for 12 h for the reuse run.

4. Conclusions

In conclusion, we have successfully developed an operationally simple and reusable protocol for the reduction of nitroarenes to anilines catalyzed by a green catalytic system in water. This procedure features (i) inexpensive, nontoxic, and commonly-available iron salt as the catalyst, and the greenest solvent, water, as the sole reaction medium; (ii) compatibility with a broad spectrum of functional groups; and (iii) potential for reuse of the catalytic aqueous solution several times without any retreatment or regeneration, proving it an eco-sustainable process.

Supplementary Materials

The following supporting information can be downloaded at https://www.mdpi.com/article/10.3390/catal12080924/s1: spectral data and copies of 1H- and 13C-NMR spectra for all products.

Author Contributions

Experimental design and performance, T.-Y.H. and W.-S.P.; spectral analysis, T.-Y.H., W.-S.P. and J.-W.T.; writing and editing, F.-Y.T. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the Ministry of Science and Technology of Taiwan (MOST), MOST 108-2113-M-027-003.

Acknowledgments

We are grateful for the financial support provided by the Ministry of Science and Technology of Taiwan (MOST).

Conflicts of Interest

The authors declare no conflict of interest.

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Catalysts 12 00924 sch001 550
Scheme 1. Iron–catalyzed reduction of nitroarenes in water.
Scheme 1. Iron–catalyzed reduction of nitroarenes in water.
Catalysts 12 00924 sch001
Table
Table 1. Comparison of the reaction conditions of iron catalysts for the reduction of nitroarenes.
Table 1. Comparison of the reaction conditions of iron catalysts for the reduction of nitroarenes.
Type of Iron CatalystH2 SourceSolventTemp. (°C)Ref.
Iron single atom site
FeSA@NC-20A (0.42 mol%)N2H4·H2O (3 equiv)EtOHrt[53]
Fe1/N-CH2 (5 bar)iPrOH160[54]
FeSAs/Fe2O3ACs/NPCN2H4·H2O (40 equiv)EtOHrt[55]
Fe-P900-PCCH2 (4 Mpa)Heptane150[56]
Fe1/N−CN2H4·H2O (5 equiv)EtOH60[57]
Iron powder (stoichiometric or excess)
Fe/NH4Cl MeOH/H2OReflux[58]
Fe/CaCl2 EtOH/H2O60[59]
Fe/HCl EtOH70[60]
Fe/NH4Cl H2O/AcetoneReflux[61]
Fe/AcOH EtOH/H2OSonication[62]
Activated Fe H2O210[63]
Fe/HCl EtOH/H2O65[64]
Iron salts
FeCl3∙6H2O (1.33 mol%)H2NNMe2 (10.5 equiv)MeOHReflux[65]
FeCl3∙6H2O (3 equiv)/Zn DMF/H2O100[66]
Fe(acac)3 (10 mol%)TMDS (4 equiv)THF60[67]
FeS2 (0.83 equiv)H2 (50 bar)THF/H2O120[68]
Fe(OTf)3 (10 mol%)NaBH4 (20 equiv)EtOHrt[69]
FeS (5 equiv)/NH4Cl MeOH/H2OReflux[70]
FeCl3∙6H2O (1 equiv)/In MeOH/H2OSonication[71]
Iron complex
Fe(CO)3(PPh3)2 (0.5 mol%) or
Fe(CO)3(AsPh3)2 (0.5 mol%)		H2 (80 atm)C6H6/EtOH125[6]
FeSO4∙7H2O/Na2EDTA (0.075 mol%)H2 (400 psi)CH3C6H5/H2O150[72]
FeBr2/PPh3 (10 mol%)PhSiH3 (2.5 equiv)CH3C6H5110[73]
FePc/FeSO4·7H2O (0.5 mol%)N2H4·H2O (2 equiv)H2O/EtOH120[74]
Fe(BF4)2 6H2O/PP3 (4 mol%)HCO2H (4.5 equiv)EtOH40[75]
[FeF(PP3)][BF4] (2 mol%)H2 (20 bar)t-AmOH120[76]
Fe(III)(Furf) (2 mol%)HSi(OEt)3 (4 equiv)CH3CN80[77]
Fe(CO)4(IMes) (5 mol%)PhSiH3 (3 equiv)CH3C6H590, hν[78]
ImmFe-IL (3 mol%)N2H4·H2O (3 equiv)Ethylene glycol110[79]
(TPP)Fe(III)Cl (0.06 mol%)NaBH4 (1.6 equiv)Diglyme30[80]
PcFe(II) (2 mol%)NaBH4 (2 equiv)Diglymert[81]
Carbonyl iron powder (5 equiv)NH4Cl (3 equiv)H2O45[83]
FeSO4/Citrate (1 mol%)NaBH4 (400 equiv)H2Ort[84]
Fe3Se2(CO)9 (3 mol%)N2H4·H2O (2 equiv)H2O110[85]
This work
FeCl3∙6H2O/Cationic 2,2′-
bipyridyl (1–2 mol%)		N2H4·H2O (4 equiv)H2O100
Abbreviations are as follows: TMDS = 1,1,3,3-tetramethyldisiloxane; Pc = phthalocyanine; PP3 = tetraphosphine; Furf = tetrahydro-2-furanyl; IMes = 1,3-bis(2,4,6-trimethyl-phenyl)imidazol-2-ylidene; ImmFe-IL = immobilized iron metal-containing ionic liquid; TPP = tetraphenylporphyrin.
Table
Table 2. Iron-catalyzed hydrogenation of 1-nitronaphthalene in water a.
Table 2. Iron-catalyzed hydrogenation of 1-nitronaphthalene in water a.
Catalysts 12 00924 i001
EntryIron SaltDuration (h)Yield (%) b
1FeCl2∙4H2O634
2FeBr2635
3FeC2O4∙2H2O633
4FeSO4∙7H2O611
5FeBr3632
6Fe2O3∙2H2O60
7FeCl3∙6H2O669
8none60
9FeCl3∙6H2O1298
10FeCl3∙6H2O2498
11 cFeCl3∙6H2O2473
12 dFeCl3∙6H2O1241
13 eFeCl3∙6H2O1218
14 fFeCl3∙6H2O1243
15 gFeCl31298
16 hFeCl3∙6H2O120
17 iFeCl3∙6H2O120
18 jFeCl3∙6H2O1297
a Reaction conditions are as follows: 1-nitronaphthalene 1a (1.0 mmol), Fe/L (1 mol%), N2H4∙H2O (4.0 mmol), H2O (2 mL) at 100 °C for 12 h. b Isolated yields. c 3 equiv of N2H4∙H2O were employed. d At 80 °C. e In the absence of L. f Neutral 2,2′-bipyridine was employed instead of L. g 99.99% purity of FeCl3 was used. h HCOONH4 was used as the reducing agent. i NH4Cl was used as the reducing agent. j 1-Nitronaphthalene 1a (10 mmol), FeCl3∙6H2O/L (1 mol%), N2H4∙H2O (40 mmol), H2O (20 mL) at 100 °C for 12 h.
Table
Table 3. Iron–catalyzed hydrogenation of substituted nitroarenes in water a.
Table 3. Iron–catalyzed hydrogenation of substituted nitroarenes in water a.
EntryNitroareneProductYield (%) b
1 Catalysts 12 00924 i002 1b Catalysts 12 00924 i003 2b99
2 Catalysts 12 00924 i004 1c Catalysts 12 00924 i005 2c97
3 Catalysts 12 00924 i006 1d Catalysts 12 00924 i007 2d97
4 Catalysts 12 00924 i008 1e Catalysts 12 00924 i009 2e83
5 Catalysts 12 00924 i010 1f Catalysts 12 00924 i011 2f92
6 c Catalysts 12 00924 i012 1g Catalysts 12 00924 i013 2g99
7 c Catalysts 12 00924 i014 1h Catalysts 12 00924 i015 2h88
8 c Catalysts 12 00924 i016 1i Catalysts 12 00924 i017 2i72
9 c Catalysts 12 00924 i018 1j Catalysts 12 00924 i019 2j78
10 c Catalysts 12 00924 i020 1k Catalysts 12 00924 i021 2k70
11 c Catalysts 12 00924 i022 1l Catalysts 12 00924 i023 2l89
12 Catalysts 12 00924 i024 1m Catalysts 12 00924 i025 2m90
13 Catalysts 12 00924 i026 1n Catalysts 12 00924 i027 2n86
14 Catalysts 12 00924 i028 1o Catalysts 12 00924 i029 2o92
15 Catalysts 12 00924 i030 1p Catalysts 12 00924 i031 2p98
16 c Catalysts 12 00924 i032 1q Catalysts 12 00924 i033 2q95
17 Catalysts 12 00924 i034 1r Catalysts 12 00924 i035 2r95
18 Catalysts 12 00924 i036 1s Catalysts 12 00924 i037 2s95
19 Catalysts 12 00924 i038 1t Catalysts 12 00924 i039 2t86
20 Catalysts 12 00924 i040 1u Catalysts 12 00924 i041 2u96
21 c Catalysts 12 00924 i042 1v Catalysts 12 00924 i043 2v49
22 Catalysts 12 00924 i044 1w Catalysts 12 00924 i045 2w82
23 Catalysts 12 00924 i046 1x Catalysts 12 00924 i047 2x93
24 Catalysts 12 00924 i048 1y Catalysts 12 00924 i049 2y95
25 Catalysts 12 00924 i050 1z Catalysts 12 00924 i051 2z92
26 Catalysts 12 00924 i052 3a Catalysts 12 00924 i053 4a83
27 Catalysts 12 00924 i054 3b Catalysts 12 00924 i055 4b90
28 Catalysts 12 00924 i056 3c Catalysts 12 00924 i057 4cʹ79
29 Catalysts 12 00924 i058 3d Catalysts 12 00924 i059 4d + 4d′77 d
30 e Catalysts 12 00924 i060 5a Catalysts 12 00924 i061 6a80
31 e Catalysts 12 00924 i062 5b Catalysts 12 00924 i063 6b87
a Reaction conditions are as follows: nitroarene (1.0 mmol), FeCl3∙6H2O/L (1 mol%), N2H4∙H2O (4.0 mmol), H2O (2 mL) at 100 °C for 12 h. b Isolated yield. c 2 mol% FeCl3∙6H2O/L was used. d 4d/4d′ = 7.9/1. e 2 mol% FeCl3∙6H2O/L and 8.0 mmol N2H4∙H2O were used.
Table
Table 4. Reuse studies of the Fe–catalyzed reduction of nitroarenes.
Table 4. Reuse studies of the Fe–catalyzed reduction of nitroarenes.
Catalysts 12 00924 i064
EntryProductIsolated Yield (%)
Initial Run1st Reuse Run2nd Reuse Run3rd Reuse Run
1 a2a98948780
2 b2h99938678
a FeCl3∙6H2O/L (1 mol%). b FeCl3∙6H2O/L (2 mol%).
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Hung, T.-Y.; Peng, W.-S.; Tang, J.-W.; Tsai, F.-Y. A Reusable FeCl3∙6H2O/Cationic 2,2′-Bipyridyl Catalytic System for Reduction of Nitroarenes in Water. Catalysts 2022, 12, 924. https://doi.org/10.3390/catal12080924

AMA Style

Hung T-Y, Peng W-S, Tang J-W, Tsai F-Y. A Reusable FeCl3∙6H2O/Cationic 2,2′-Bipyridyl Catalytic System for Reduction of Nitroarenes in Water. Catalysts. 2022; 12(8):924. https://doi.org/10.3390/catal12080924

Chicago/Turabian Style

Hung, Tsai-Yu, Wen-Sheng Peng, Jing-Wen Tang, and Fu-Yu Tsai. 2022. "A Reusable FeCl3∙6H2O/Cationic 2,2′-Bipyridyl Catalytic System for Reduction of Nitroarenes in Water" Catalysts 12, no. 8: 924. https://doi.org/10.3390/catal12080924

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