A copper acetate/2-aminobenzenthiol complex supported on magnetite/silica nanoparticles as a highly active and recyclable catalyst for 1,2,3-triazole synthesis

Abstract

A novel and recoverable Cu(II)/2-aminobenzenthiol complex immobilized on magnetite/silica nanoparticles (Fe₃O₄/SiO₂ NPs) catalyzed the synthesis of 1,2,3-triazole derivatives via three-component condensation of terminal alkynes, benzyl or alkyl halides, and sodium azide in wet polyethylene glycol (PEG 400/H₂O) as a green medium at ambient temperature. The catalytic potentials of some transition metal salt complexes adhered on Fe₃O₄/SiO₂ NPs (5–10 nm) were employed for the one pot 1,2,3-triazole synthesis. The order of catalytic activity of the various transition metal salt complexes for a model reaction was found to be Cu(II) > AgI ∼ CuI > CuCl > Ni(OAc)₂ ∼ Co(OAc)₂ > ZnCl₂. Moreover, the order of catalytic activity of Cu(II) salts supported on Fe₃O₄/SiO₂ NPs was found to be Cu(OAc)₂ > CuSO₄ ≫ CuCl₂ > Cu(NO₃)₂. The catalytic potential of the complex has been improved significantly when core–shell magnetite/silica has been used instead of magnetite for successful wide-spread syntheses of 1-aryl-1,2,3-triazoles under mild reaction conditions. 1-Aryl-1,2,3-triazoles syntheses require harsh reaction conditions.

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Introduction

The 1,2,3-triazole moiety, a key structural element of many natural products, pharmaceuticals and synthetic agents, has increasingly attracted the synthetic pursuit of chemists because of its important biological properties.^1–4 So far, numerous synthetic methods have been developed and reported in the literature.^5–9 One of the most popular protocols for the preparation of these compounds is the 1,3-dipolar “Huisgen” cycloaddition reaction which is catalyzed by Cu(I) and ruthenium(II).^10,11 By this reaction, 1,4- or 1,5-disubstituted 1,2,3-triazoles are prepared with high regiospecificity. But the inherent thermodynamic instability of Cu(I) enforces conditions like inert atmosphere, anhydrous solvents along with the formation of undesired by-products.^12,13 Development of new methods and catalysts for the synthesis of the aforementioned target molecules under cleaner and milder reaction conditions is of demands. Along this interest, protocols such as; the in situ generation of Cu(I) catalyst by the reduction of Cu(II) or oxidation of Cu(0) to Cu(I) or using stable Cu(II) complexes of oligopyridines with electron-donating groups have been developed.^14–17 Meanwhile, a few strategies using heterogeneous copper catalysts are available in which the copper moiety is immobilized on zeolite, activated carbon and polymer compounds. By this mean, the catalytic activity of the catalysts has been improved and their recovery has been more facilitated.^5,8,15–20 But in practice, separation of the catalysts from the reaction mixture still encounter difficulties and is a time- and an energy-consuming process.

Recently, some attempts have started to prepare catalysts supported on magnetic nanoparticles (MNPs) which are easily separable by an external magnetic field from the reaction mixtures. This is highly beneficial for large-scale operation in pharmaceutical and food industries and minimizes the contamination of the final products with the metal catalysts. In this respect, we have recently reported new magnetic catalysts and reagents used in basic organic reactions.^21–26

However, it should be declared that, the naked MNPs are extremely chemically active and are easily oxidized in the air.²⁷ Therefore, their protection and the coating of the naked MNPs with organic or inorganic layers is a valuable strategy which also provides useful sites for further functionalization of the material.^28,29

Considering the less costs with much higher purity of Cu(II) salts in comparison with the corresponding Cu(I) salts, herein we report preparation of a novel and a magnetically recoverable Cu(OAc)₂/2-aminobenzenthiol complex immobilized on magnetite/silica nanoparticles (Fe₃O₄/SiO₂NPs). This magnetic composite has been successfully used as a catalyst for the synthesis of 1,2,3-triazole derivatives via a three-component condensation of terminal alkynes, benzyl or alkyl halides, in the presence of sodium azide under mild conditions.

Results and discussion

Magnetite/silica nanoparticles (Fe₃O₄/SiO₂NPs) of 5–10 nm size were prepared according to the reported reverse micelle procedure as presented in ESI (S1†).³⁰ By this method, a micro emulsion was prepared by dissolving sodium dodecylbenzenesulfonate in xylene by sonication. Then a solution composed of FeCl₂·4H₂O, Fe(NO₃)₃·9H₂O was added to the micro emulsion under vigorous stirring with heating under argon atmosphere. Hydrazine was injected into the reverse-micelle solution. The resulting mixture was heated at 90 °C and then was cooled down to 40 °C in an appropriate time. To the resulting mixture, tetraethoxy silane (TEOS) was added with stirring. The resulting solid material was washed with sodium phosphate buffer solution (pH 7.4) to give Fe₃O₄@SiO₂ NPs which were dried at room temperature under vacuum and characterized by XRD, FTIR.³⁰

The preparation steps of the Cu(OAc)₂/2-aminobenzenethiol complex immobilized on magnetite/silica nanoparticles (Fe₃O₄@SiO₂/ABT–Cu(OAc)₂) are shown in Scheme 1.

Scheme 1

The progress of the reaction steps, as demonstrated in Scheme 1, are in accord with their FT-IR spectra as presented in Fig. 1.

Fig. 1 FT-IR spectra of Fe₃O₄@SiO₂ (I), Fe₃O₄@SiO₂/(3-chloropropyl)trimethoxysilane (II), Fe₃O₄@SiO₂/ABT (III), Fe₃O₄@SiO₂/ABT–Cu(OAc)₂ (IV) and Fe₃O₄@SiO₂/ABT–Cu(OAc)₂ after the 1^st run of recycling for the synthesis of 1-benzyl-1,2,3-triazole as a model reaction (V) which is similar to (IV).

The vibration bands at 557 and 1035 cm⁻¹ are the typical IR absorbance induced by Fe–O vibration and stretching vibrations of the Si–O respectively (Fig. 1I). By supporting (3-chloropropyl) trimethoxysilane moiety on Fe₃O₄@SiO₂, the intensity of O–H band at 3000–3600 cm⁻¹ has been decreased. The vibration band of CH₂ aliphatic at 2921 cm⁻¹ shows that the (3-chloropropyl)trimethoxysilane moiety has been functionalized on the surface of the Fe₃O₄@SiO₂ (Fig. 1II). The increased intensity of the bands at 1035, 1100, 1254, and the broadening of the peak at 1632 cm⁻¹ is due to nucleophilic substitution of Cl with S atom of 2-aminobenzthiol. The band in 3000–3600 cm⁻¹ region has become sharper which is due to the presence of NH₂ functional group (Fig. 1III). By the copper acetate complex formation with Fe₃O₄@SiO₂/ABT, the band intensity at 1632 cm⁻¹ has been increased and the band in the region of 3000–3600 cm⁻¹ has become broader (Fig. 1IV). The resulting new magnetic composite has been further characterized by XRD, XRF, TEM and VSM techniques. The amount of copper(II)acetate supported on Fe₃O₄/SiO₂NPs was determined by atomic absorption method to be in the range of 2.8–3.0 (mmol g⁻¹) (Fig. S1, ESI†). X-ray diffraction analysis confirms the structural order of the Fe₃O₄@SiO₂ and Fe₃O₄@SiO₂/ABT–Cu(OAc)₂ compounds and approves the existence of copper acetate on Fe₃O₄@SiO₂ material (Fig. S2†). X-ray fluorescence analysis of Fe₃O₄@SiO₂/ABT–Cu(OAc)₂ also (Table S1†) demonstrates the presence of Cu(OAc)₂ moiety in the material attached to the magnetite surface.

The magnetization curves of the Fe₃O₄, Fe₃O₄@SiO₂ and Fe₃O₄@SiO₂/ABT-Cu(OAc)₂ nanoparticles recorded at room temperature exhibit a super paramagnetic behavior (zero coercivity and remanence on the magnetization loop) (Fig. 2). The saturation magnetization value of the Fe₃O₄@SiO₂/ABT–Cu(OAc)₂ is 17 emu g⁻¹ (Fig. 2c). This number is three times lower than that of naked Fe₃O₄@SiO₂ (Fig. 2b, 55 emu g⁻¹). This is due to the shielding of the Fe₃O₄@SiO₂ surface with the coating material.

Fig. 2 Comparison of magnetization loops of (a) Fe₃O₄ with (b) Fe₃O₄@SiO₂ (ref. 30) and with (c) Fe₃O₄@SiO₂/ABT–Cu(OAc)₂ composite.

The thermogravimetric analysis of Fe₃O₄@SiO₂/ABT–Cu(OAc)₂ as shown in Fig. S4,† indicates the weight loss (2.27%) in the temperature range of 120–180 °C. This weight loss is mainly related to dehydration of the composite. When the temperature is raised to the range of 240–280 °C, another weight loss (3.21%) is observed, which is attributed to the decomposition of AcO. The third mass loss (22.15%) is observed when the temperature reaches to the range of 300–600 °C. This corresponds to the thermal decomposition of ABT.

Transmission electron microscopy (TEM) image of Fe₃O₄@SiO₂/ABT–Cu(OAc)₂ shows the spherical morphology of the material with an average size of 5–10 nm (Fig. 3a). The TEM image of the composite used as a catalyst for the synthesis of 1-benzyl 1,2,3-triazoles as a model reaction after the 1^st run of its recycling Fig. 3b showed conservation of its morphology and the size of the particles size (5–10 nm) (Fig. 3).

Fig. 3 TEM images of Fe₃O₄@SiO₂/ABT–Cu(OAc)₂ before (a) and after the 1^st run of recycling (b) for the synthesis 1-benzyl-1,2,3-triazole.

When the core–shell magnetite/silica was employed as a support instead of magnetite, the amount of the supported copper and also the catalytic activity of the complex was significantly improved. A good connectivity of silane group of ABT ligand with SiO₂ improves the number of sites for the complex formation with Cu(OAc)₂ and as a result, improving the amounts of the copper moiety supported on the shell–core and improving the catalytic activity of the composite (Scheme 2). We have also performed the control experiment with copper salt complex with ABT is immobilized on silica particles for the model reaction. The results obtained for the catalytic activity of silica catalyst was similar with the magnetic one.

Scheme 2

The stability of the complexes of Cu(OAc)₂ with electron-donating ligands such as 3-amino propyl (AP), 2-iminomethylphenol (IMP) and 2-aminobenzenethiol (ABT) (Scheme 3), attached to the core of magnetite/silica nanoparticles were examined by measuring the amounts of leaching of Cu(II) into the reaction mixtures.

Scheme 3

This was studied by graphite furnace atomic absorption analysis and hot filtration test. The ABT ligand (Scheme 3) showed the least amount of leaching in comparison with the other two ligands; IMP and AP in this study (Table S2†). The hot filtration test was also studied for Fe₃O₄@SiO₂/ABT–Cu(OAc)₂ catalyst in the reaction of 1,2,3-triazol synthesis. The reaction was not proceeded after separating the catalyst by a permanent magnet. It was compulsory to analyze the filtered reaction mixture by atomic absorption analysis to confirm the absence of metal.

The recycling ability of the complexes with various ligands (Scheme 3, IMP, AP, ABT) was investigated for the synthesis of 1-benzyl-1,2,3-triazole starting from benzyl bromide, phenyl acetylene and sodium azide as a model reaction (Fig. 4). The recycling of the catalysts was repeated for 5 consecutive runs for the reaction. In this regard, ABT ligand proved to be the best reusable catalyst in this respect. We have also found that the life span of the catalyst was long enough to be considered as a stable catalyst. The reaction of benzyl bromide, phenyl acetylene and sodium azide as the model reaction was proceeded efficiently in the presence of the catalyst which was prepared and stored for sixty days (Fig. 5).

Fig. 4 The recycling of the catalysts (a) Fe₃O₄@SiO₂/ABT–Cu(OAc)₂, (b) Fe₃O₄@SiO₂/IMP–Cu(OAc)₂ and (c) Fe₃O₄@SiO₂/AP–Cu(OAc)₂ for the synthesis of 1-benzyl-1,2,3-triazole as a model reaction.

Fig. 5 The life span of Fe₃O₄@SiO₂/ABT–Cu(OAc)₂ showing the stability of the catalyst.

The catalytic activity of various metal salts such as Cu(NO₃)₂, Cu(SO₄)₂, CuCl₂, AgI, CuI, CuCl, Ni(OAc)₂, Co(OAc)₂, ZnCl₂ complexed with Fe₃O₄@SiO₂/ABT for the synthesis of 1-benzyl-1,2,3-triazole as a model reaction at ambient temperature in different media was also investigated (Table 1) (Fig. S3†). The best result was obtained when Fe₃O₄@SiO₂/ABT–Cu(OAc)₂ was used in PEG/water (9 : 1) as the solvent (Table 1, Entry 18).

Table 1 The optimization results obtained for the reaction of benzyl bromide with phenyl acetylene and sodium azide

Entry	M^a	Solvent^b	Time (h)	Yield (%)
a Catalyst (2 mol%).b Solvent (1 mL).c Under N2 atmosphere.d By-product observed.e Catalyst (1 mol%).f Catalyst (4 mol%).g At 50 °C.h At 100 °C.
1	—	—	8	20
2	CuI^c^,^d	—	1.5	80
3	CuCl^c^,^d	—	2	85
4	AgI^c^,^d	—	1.15	80
5	Ni(OAc)2	—	8	50
6	Co(OAc)2	—	8	50
7	ZnCl2	—	8	50
8	Cu(NO3)2	—	3	91
9	CuSO4	—	1.5	92
10	CuCl2	—	2	93
11	Cu(OAc)2	—	1	92
12	Cu(OAc)2	H2O	8	30
13	Cu(OAc)2	EtOH	8	10
14	Cu(OAc)2	PEG	8	30
15	Cu(OAc)2	PEG/H2O(1 : 1)	5	96
16	Cu(OAc)2	PEG/H2O(2 : 1)	3	94
17	Cu(OAc)2	PEG/H2O(4 : 1)	1	95
18	Cu(OAc)2	PEG/H2O(9 : 1)	0.25	95
19	Cu(OAc)2^e	PEG/H2O(9 : 1)	0.5	97
20	Cu(OAc)2^f	PEG/H2O(9 : 1)	0.2	92
21	Cu(OAc)2^g	PEG/H2O(9 : 1)	0.08	93
22	Cu(OAc)2^h	PEG/H2O(9 : 1)	0.05	92
23	Cu(OAc)2	EtOAc	8	10
24	Cu(OAc)2	CH3CN	8	10
25	Cu(OAc)2	n-Hexane	8	5

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Then, the obtained optimized conditions were applied for the reaction of different alkynes with various benzyl halides and sodium azide as summarized in Table 2.

Table 2 1-Benzyl-1,2,3-triazoles synthesis using benzyl bromides, terminal acetylenes and sodium azide

Entry	Benzyl halide	Acetylene	Product	Time (min)	Yield (%)
1				15	95
2				60	90
3				120	85
4				60	92
5				100	90
6				10	90
7				10	95
8				10	90

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We have also extended the scope of the use of the catalyst to the reaction of varieties of diazonium salts with various terminal acetylenes and sodium azide under optimized conditions (Table S3†). As shown in Table 3, diazonium salts bearing either electron-withdrawing or electron-donating substituents on the benzene ring afforded the corresponding 1-aryl-1,2,3-triazole products in excellent yields (Table 3, Entries 1, 2, 6 and 7).

Table 3 1-Aryl-1,2,3-triazole synthesis using diazonium salts, terminal acetylenes and sodium azide

Entry	Diazonium salt	Acetylene	Product	Time (min)	Yield (%)
1				60	95
2				60	85
3				45	90
4				35	95
5				50	92
6				240	90
7				300	90
8				120	92

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The catalytic activity of Fe₃O₄@SiO₂–ABT/Cu(OAc)₂ in comparison with some other reported catalysts for the synthesis of 1-benzyl-1,2,3-triazole is presented. In the presence of this catalyst, the reaction proceeded at ambient temperature within 0.25 h in 95% yield of the desired product (Table 4, Entry 9). Comparison of the results in Table 4, shows an obvious advancement of using Fe₃O₄@SiO₂–ABT/Cu(OAc)₂ catalyst with respect to its easy separation by an external magnetic field without using filtration, centrifugation or sedimentation techniques, which are all energy and time-consuming processes. The catalyst recycling with good recycling cycles along with a negligible amount of leaching of the metal with a long life span of the catalyst, are the advancements observed by using this catalyst for the reaction. The media in which the catalyst is highly active is wet PEG that is an eco-friendly media. In addition, in the presence of this catalyst the target molecules are produced with excellent yields.

Table 4 Comparison of the catalytic activity of different catalysts and conditions for the synthesis of 1-benzyl-1,2,3-triazole

Entry	Catalyst	Solvent	Temp. (°C)	Time (h)	Yield (%)
a Under N2 atmosphere.
1	Cu/charcoal^5	H2O	100	0.6	91
2	CuO^31	THF/H2O (0.4 M)	60	24	95
3	Tris(triazolyl)methanol–Cu(I)^a ^32	MeOH : H2O (1 : 1)	40	4	99
4	Cu3N/Fe3N@SiO2 (ref. 33)	Et3N/CH3CN	25	3	84
5	Cu-apatite^34	H2O	100	1.5	99
6	Cu NPs/carbon^35	THF/NEt3	65	0.16	98
7	Amberlyst A-21-CuI^a ^36	CH2Cl2	25	18	97
8	SiO2–NHC–Cu(I)^a ^37	—	25	0.5	93
9	Fe3O4@SiO2–ABT/Cu(OAc)2	PEG/H2O(9 : 1)	25	0.25	95

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Experimental

Materials and methods

All commercially available chemicals were obtained from Merck and Fluka Chemical Companies, and used without further purification unless otherwise stated; solvents were distilled before their use. The products were characterized by FTIR, ¹H NMR (500 MHz) and ¹³C NMR (125 MHz) spectroscopy. All the reactions were monitored by thin layer chromatography (TLC) coated with silica gel illuminated with UV light and iodine as the reagent. All yields refer to the isolated products.

Synthetic procedures

Preparation of Fe₃O₄@SiO₂-(3-chloropropyl)trimethoxysilane. Silica-coated Fe₃O₄ NPs that was prepared by reverse micelle method³⁰ (1 g) was added to toluene (5 mL) and under nitrogen atmosphere. Then, (3-chloropropyl)trimethoxysilane (0.4 mL) was added and the mixture was slowly heated to 90 °C and was kept at this temperature for 6 h. The brown precipitates were collected with an external magnet and washed with aqueous buffer (100 mm sodium phosphate, pH 7.4). The resulting precipitates were dried for 12 h at room temperature under vacuum and characterized by FTIR (Scheme 1).

Preparation of Fe₃O₄@SiO₂–ABT. Fe₃O₄@SiO₂-(3-chloropropyl)trimethoxysilane (1 g) as prepared in the above section, was added to ethanol (5 mL) and dispersed under ultrasound irradiation for 15 min. Then, 2-aminobenzenethiol (1.1 mL) was slowly added to the mixture under vigorous stirring for 6 h. The brown precipitates were collected with an external magnet and washed with ethanol (10 mL). The resulting precipitates were dried for 12 h at room temperature under vacuum and characterized by FTIR (Scheme 1).

Preparation of the Fe₃O₄@SiO₂–ABT/Cu(OAc)₂. Fe₃O₄@SiO₂–ABT as prepared (1 g) in the preceding section, was added to ethanol (5 mL) and dispersed under ultrasound irradiation for 15 min. Then copper acetate (0.6 g) was added to the mixture under vigorous stirring for 6 h. The light brown precipitates were collected with an external magnet and washed with ethanol (10 mL). The resulting precipitates were dried for 12 h at room temperature under vacuum and characterized by FTIR, XRD, XRF, VSM, TEM (Scheme 1).

Synthesis of 1-benzyl-1,2,3-triazole, a model reaction. Fe₃O₄@SiO₂–ABT/Cu(OAc)₂ (0.02 mmol, 0.04 g) was added to a stirring mixture of acetylene (1.05 mmol), benzyl halides (1 mmol) and sodium azide (1.1 mmol, 0.07 g) in H₂O/PEG 600 (1 : 9, 1 mL) mixture at room temperature. The progress of the reaction was monitored by TLC (hexane/ethylacetate eluent 4 : 1). After completion of the reaction, the desired 1-benzyl-1,2,3-triazole product was separated by extraction with diethyl ether from the reaction mixture. The product was obtained after concentration of the diethyl ether as highly pure crystalline compounds in good to excellent yields (Table 2).

General procedure for the synthesis of 1-aryl-1,2,3-triazoles. Fe₃O₄@SiO₂–ABT/Cu(OAc)₂ (0.02 mmol, 0.04 g) was added to a stirring mixture of acetylene (1.05 mmol), diazonium salt (1 mmol) and sodium azide (1.1 mmol, 0.07 g) at 70 °C in H₂O (1 mL). The progress of the reaction was monitored by TLC (hexane/ethylacetate eluent 3 : 2). After completion of the reaction, ethanol (2 mL) was added to the reaction mixture. The catalyst was separated by a permanent magnet from the reaction mixture. The desired 1-aryl-1,2,3-triazole product were obtained after concentration of the reaction mixture under diminished pressure as highly pure powder compounds in good to excellent yields (Table 3).

Conclusions

In conclusion, in this study, a facile preparation of a new magnetic composite in which Fe₃O₄ NPs are walled with SiO₂ particles which carrying many branches composed of a siloxypropyl aminobenzenethiol moiety. This solid magnetic composite has a high capacity for the preparation of many different transition metal complexes. In this paper, we have reported the preparation of Cu(OAc)₂ complex which has been applied as a high capacity catalyst with a long endurance catalytic activity for the preparation of 1,2,3-triazole compounds with excellent yields.

The results of this study show advances for the use of Fe₃O₄@SiO₂–ABT/Cu(OAc)₂ with respect to the other catalysts used for the similar reactions discussed in this article. The catalyst can be easily separated from the reaction mixture by an external magnetic field. This property saves time and energy with respect to filtration or decantation techniques. The reactions proceeded at room temperature in wet PEG400 as an entirely green and safe media in a short reaction time. All the reactions proceeded well with excellent isolated yields.

Acknowledgements

The authors are appreciative to the Research Council of Yazd University for financial support. H. Mahmoudi is grateful to Dr Mohammad Javad Faghihi for his generous supports during this work and H. F. is appreciative for the grant (BN048) from National Elite Foundation of Iran.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra22909j

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