Fe₃O₄/PANI/CuI as a sustainable heterogeneous nanocatalyst for A³ coupling

Abstract

The prepared copper iodide nanoparticles were impregnated on the support of ferrite nanoparticles functionalized with polyaniline, resulting in a magnetically recoverable heterogeneous nanocomposite. The activity of the prepared nanocomposite was investigated in the synthesis of propargylamine derivatives via A³ coupling under mild conditions. Techniques such as FESEM, EDAX, XRD, XPS, TEM, BET and FTIR were used to characterize the effective and unique heterogeneous Fe₃O₄/PANI/CuI nanocomposite developed in this work. This method used in the current study has several advantages, including a short reaction time, neat conditions, good product yield, ideal green matrices values, reusability for up to seven cycles, and magnetic retrievability.

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Introduction

Magnetic nanoparticles (MNPs) are of great importance due to their various applications in catalysis, magnetic resonance imaging (MRI), magnetic fluids, and biotechnology.¹ MNPs can serve as magnetically recoverable catalysts for a variety of catalytic reactions because of their insoluble and paramagnetic character, which allows for easy separation from the reaction medium. Moreover, magnetic separation has evolved into one of the most significant and well-known catalytic methods in organic chemistry without the need for filtering, centrifugation, or other laborious workup procedures, simply by using an external magnet.² However, bare MNPs have some limitations such as the tendency to easily agglomerate, colloidal instability, and dissolution in acids.³ The colloidal instability of MNPs leads to their agglomeration due to magnetic dipole–dipole interaction.^4,5 This issue can be resolved by surface functionalization of MNPs using protective shells or coatings such as silica, carbon, or organic polymers.³ Furthermore, these coatings allow the covalent attachment of organic compounds on distinct nanoparticles, facilitating applications such as drug carriers, heterogeneous catalysis, and absorption media.³ The magnetic nature of nanoparticles permits facile recovery of nanocatalysts from reaction mixtures through magnets.⁵

One of the polymeric shells synthesized via oxidative polymerization is polyaniline (PANI).⁶ The choice of polyaniline is because of its various properties such as facileness of synthesis, conductivity as a polymer, low cost, and a porous structure that can enhance the catalytic activity of nanoparticles. The Fe₃O₄/PANI hybrid shell can be considered a multifunctional support for metal nanocatalysts with significant catalytic performance.⁷

The benefits of a metal nanoparticle supported on nano-size heterogeneous material include good selectivity, minimal accumulation of metal nanoparticles, high dispersion in a liquid medium, and excellent reusability.⁸ Copper-based nanoparticles not only enhance the physicochemical characteristics of the nanoparticles but also reinforce the interface between the metal and the support.⁹ Cu-based nanocatalysts have abundant applications in nanotechnology due to their special properties and features such as catalysing organic transformations, electrocatalysis, and photocatalysis.¹⁰ Supported copper nanoparticles, such as CuO/NiO,⁹ CuO/Al₂O₃,¹¹ ZnO/CuI/PPy,¹² and Cu–MgO,¹³ have been used in many organic transformations. Copper-based nanocatalysts are found to be useful in various reactions including C–H activation of alkynes, oxygen arylation reaction, Suzuki reaction, Click reaction, Knoevenagel condensation-Michael addition cyclization reaction, Heck reaction and producing copper-acetylated species in situ to afford propargylamines.¹⁴

Propargylamines are crucial building blocks for organic synthesis because they can be utilized as synthetic precursors for synthesizing various medicinally essential compounds.⁸ Propargylamines are formed via a three-component reaction known as A³ coupling, which comprises a terminal alkyne, an aldehyde, and an amine.¹⁵ Moreover, a variety of propargylamines have been used to cure neuropsychiatric conditions like anxiety, Parkinson's disease, and depression.¹⁶ Various approved drugs, such as pargyline, selegiline, and rasagiline (Fig. 1), have a propargylamine scaffold.^8,17 Late transition metals such as Au, Cu, and Ag are used to catalyse A³ reactions via one-pot synthesis.¹⁸

Fig. 1 Examples of approved drugs containing propargylamine skeletons.

Over the past two decades, a catalytic variation of A³ coupling has attracted chemists' attention¹⁵ A³ coupling catalysed by various nanocatalysts such as Fe₃O₄@R. tinctorum/Ag,¹⁹ Cu/C,¹⁶ Fe₃O₄@SiO₂@DNHCS-Tr@CuI,²⁰ Au nanoparticles,²¹ Fe₃O₄–MoO₃ (ref. 22) and CuO/GNS²³ have been reported. However, these methods involve use of harmful reagents, prolonged reaction time, use of additives and costly reagents. Hence, there is a need for a sustainable heterogeneous nanocatalyst for the facile synthesis of propargylamines via A³ coupling.

In the current work, we successfully develop a novel heterogeneous nanocatalyst, Fe₃O₄/PANI/CuI, for the synthesis of propargylamines via A³ coupling using pyrrolidine, phenylacetylene, and different benzaldehydes under neat conditions at 80 °C in a N₂ atmosphere. The reaction was completed in 10 min with a high yield of the desired product. The fabricated nanocatalyst was easily recoverable and reusable with high catalytic efficiency for the synthesis of propargylamines.

Results and discussion

Synthesis and characterisation

Synthesis of Fe₃O₄ nanoparticles. Fe₃O₄ nanoparticles were synthesised using the co-precipitation approach. To a 250 mL round bottom flask containing 100 mL of water, 4.2 g of FeSO₄·7H₂O and 6.1 g of FeCl₃·6H₂O were added. The mixture was stirred at 80 °C for 1 h. Then, 10 mL of ammonia solution (25%) was added dropwise into the reaction mixture with continuous stirring. Then, the reaction was continuously stirred for another 1.5 h at the same temperature. The Fe₃O₄ nanoparticles were collected using a magnet and washed with water many times and then with ethanol. Finally, the synthesised nanoparticles were dried in an oven at 50 °C.⁵

Synthesis of Fe₃O₄/PANI nanoparticles. The obtained Fe₃O₄ nanoparticles were dispersed in 10 mL of deionized water. Subsequently, 0.3 mL of HCL (0.1 M) and 0.2 mL of aniline were added to this solution. Then, the solution was stirred for 1 h at room temperature. Then, the aqueous solution of ammonium persulfate (5 mL) was poured dropwise to the above reaction under ultrasonic irradiation. The stirring was then continued for 3 h in an ice bath. Then, nanoparticles were collected with a magnet and then washed many times with water and three times with ethanol, and further dried in an oven at 50 °C.

Synthesis of Fe₃O₄/PANI/CuI nanoparticles. The prepared Fe₃O₄/PANI nanoparticles were dispersed in water via stirring for 10 min. Then, copper iodide nanoparticles were added to this solution and it was stirred overnight at room temperature. Then, nanoparticles were washed with water and ethanol and dried in an oven overnight at 50 °C (Fig. 2).

Fig. 2 Schematic diagram for the synthesis of nanocomposite.

Characterisation of developed Fe₃O₄/PANI/CuI nanocatalyst. X-ray diffraction analysis of the Fe₃O₄/PANI/CuI nanocomposite is shown in Fig. 3. The diffraction angles (2θ) at 35.46° and 57.06° correspond to the crystal planes (311) and (511), respectively, of the Fe₃O₄ nanoparticles.^24,25 The peaks at 2θ = 25.42°, 30.06°, 42.2°, 49.86°, 57.14°, 61.22°, and 67.3° correspond to the crystal planes (200), (311), (111), (420), (222), (220), and (420), respectively, of the cubic phase of CuI.²⁶

Fig. 3 XRD pattern of Fe₃O₄/PANI/CuI nanocatalyst.

The field-emission scanning electron microscopy (FESEM) technique reveals the spherical morphology of the nanocomposite (Fig. 4). The transmission electron microscopy (TEM) analysis of the Fe₃O₄/PANI/CuI nanocomposite indicates that CuI is well embedded over the core–shell structure of Fe₃O₄ nanoparticles and the average size of nanoparticles is 42.6 nm, as shown in Fig. 5.

Fig. 4 FESEM analysis of Fe₃O₄/PANI/CuI nanocatalyst.

Fig. 5 TEM analysis of Fe₃O₄/PANI/CuI nanocatalyst.

The energy-dispersive X-ray analysis of the Fe₃O₄/PANI/CuI nanocatalyst revealed the presence of iron (42.74 wt%), nitrogen (1.47 wt%), copper (11.7 wt%), oxygen (15.29 wt%), iodine (19.9 wt%), and carbon (8.9 wt%), as can be seen in Fig. 6.

Fig. 6 EDAX analysis of Fe₃O₄/PANI/CuI nanocatalyst.

Fig. 7 illustrates the X-ray photoelectron spectra (XPS) of the Fe₃O₄/PANI/CuI nanocomposite. The spectra revealed the presence of Cu 2p_1/2 and Cu 2p_3/2 with binding energies at 952.88 and 932.44 eV, respectively, and the presence of I 3d_3/2 and 3d_5/2 with binding energies at 631 and 619.23 eV, respectively. The values for copper and iodine resemble the reported binding energy values of CuI, which confirm the +1 oxidation state of copper in the nanocomposite.²⁷ The peak at 284.78 eV corresponds to the binding energy value of C 1s. The values of binding energies at 724.53 and 710.63 eV resemble the reported values of Fe 2p_3/2 and Fe 2p_1/2, respectively, while the peak at 530.17 eV corresponds to O 1s, confirming the presence of Fe₃O₄ in the nanocomposite. The broadness of the iron peaks indicates the presence of both oxidation states (Fe²⁺ and Fe³⁺) in Fe₃O₄.²⁸

Fig. 7 XPS of Fe₃O₄/PANI/CuI nanocatalyst.

Fig. 8 shows the FTIR spectrum of Fe₃O₄/PANI/CuI; it depicts a peak at 3311 cm⁻¹, which is attributed to the presence of the surface OH group in the nanocomposite.²⁹ The peaks at 1598 and 1494 cm⁻¹ are attributed to the C=C stretching vibrations of a quinoid and benzenoid ring, respectively.³⁰ A peak that appeared at 1374 cm⁻¹ is similarly typical of polyaniline and is considered to be a consequence of C–N stretching vibrations near a quinonoid ring.³¹ The peak at 1161 cm⁻¹ is due to the C–N stretching vibration.³⁰ The peak at 553 cm⁻¹ is the characteristic peak of ferrite nanoparticles.³⁰

Fig. 8 FTIR of Fe₃O₄/PANI/CuI nanocatalyst.

N₂-Adsorption desorption isotherm was collected using the Brunauer–Emmett–Teller (BET) technique, which is portrayed as a H3 hysteresis loop of isotherm and shows a surface area of 38.471 m² g⁻¹, pore radius of 2.16 nm, and pore volume of 0.076 cm³ g⁻¹ (Fig. 9).

Fig. 9 N₂ adsorption–desorption isotherm of nanocatalyst.

Fe₃O₄/PANI/CuI as heterogeneous nanocatalysts for the synthesis of propargylamine derivatives

We synthesized propargylamine derivatives using Fe₃O₄/PANI/CuI to investigate its catalytic properties in organic transformations (Scheme 1). For optimization, a model reaction was performed involving phenylacetylene (1), pyrrolidine (2), and 4-methyl benzaldehyde (3) using the nanocatalyst in various solvents or under neat conditions at 80 °C in a nitrogen atmosphere for the preparation of the desired product 4b, as shown in Table 1. We examined how different catalyst loading amounts, solvent concentrations, and temperatures affected the reaction kinetics, as presented in Table 1. Initially, the model reaction was performed in toluene (Table 1, entry 1), resulting in a 47% yield of the product. Subsequently, the reaction was carried out in polar aprotic solvents such as THF, acetonitrile, DMSO, and DMF. The desired product did not form in both acetonitrile and THF (Table 1, entries 2 and 3). However, the product was obtained with a 40% yield in DMF (Table 1, entry 4). The reaction was then monitored in environmentally friendly solvents such as ethanol, water, and ethylene glycol (EG), yielding no product in water (Table 1, entry 5), trace amounts of product in ethanol, and 30% product yield in EG (Table 1, entry 7). The product was isolated in good yield in neat conditions (Table 1, entry 8). By altering the catalyst loading, the % yield was found to remain unchanged on lowering or increasing the catalyst amount respectively (Table 1, entries 9 and 10). Further, we studied the influence of temperature on development of reaction. On raising the temperature, there was no change in product yield (Table 1 entry 11), while on decreasing the temperature, there was a reduction in the product yield (Table 1, entry 12).

Scheme 1 Fe₃O₄/PANI/CuI catalysed synthesis of propargyl derivatives via A³ coupling. Reaction conditions: nanocatalyst (10 mg), phenylacetylene (1 mmol), pyrrolidine (1 mmol), aromatic aldehyde (1 mmol), neat, 80 °C, N₂ atm, 10 min.

Table 1 Optimization of nanocatalyst for the synthesis of propargyl derivatives via A³ coupling using phenyl acetylene (1), pyrrolidine (2), and 4-methylbenzaldehyde (3)^a

S. no.	Nanocatalyst (mg)	Solvent	Temp. (°C)	Time (min)	Yield (%)
a Reaction conditions: catalyst (5–20 mg), 1 (1.0 mmol), 2 (1 mmol), 3 (1 mmol), solvent (2–3 mL), N2 atm, 80 °C, 10 min.
1	Fe3O4/PANI/CuI (10)	Toluene	80	10	47
2	Fe3O4/PANI/CuI (10)	CH3CN	80	10	—
3	Fe3O4/PANI/CuI (10)	THF	80	10	—
4	Fe3O4/PANI/CuI (10)	DMF	80	10	40
5	Fe3O4/PANI/CuI (10)	Water	80	10	—
6	Fe3O4/PANI/CuI (10)	Ethanol	80	10	Trace
7	Fe3O4/PANI/CuI (10)	EG	80	10	30
8	Fe3O4/PANI/CuI (10)	Neat	80	10	96
9	Fe3O4/PANI/CuI (5)	Neat	80	10	53
10	Fe3O4/PANI/CuI (20)	Neat	80	10	96
11	Fe3O4/PANI/CuI (10)	Neat	110	10	96
12	Fe3O4/PANI/CuI (10)	Neat	50	10	22
13	CuI (10)	Neat	80	10	41

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Under optimized conditions, we examined the recyclability of the catalyst to produce the product 4b. Once the reaction was completed, the catalyst was recovered from the reaction using a magnet and then washed many times with water and ethanol before being dried in the oven. The recovered catalyst was then used for seven cycles (Fig. 10). The stability of the recycled catalyst after seven cycles was confirmed by XRD, SEM, FTIR, EDAX and TEM, which confirmed that there was no change in the activity and morphology of the catalyst (ESI Fig. S1–S5†). An ICP study of the filtrate was done after catalyst recovery and showed the leached metal concentrations of copper and iron ion to be 2.08 and 0.12 ppm, respectively, which are lower than the authentic values of the respective ions according to WHO terms.³²

Fig. 10 Catalyst recyclability test.

The existing methodology demonstrates sustainability and eco-friendliness, as evidenced by the green metrics values, as shown in Table 2 (refer to calculations in the ESI†), which closely approach the ideal values.

Table 2 Green matrices values

Catalyst	Reaction mass efficiency	E-Factor	Process mass intensity	Carbon efficiency
Fe3O4/PANI/CuI	90%	0.10	1.10	96%

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Table 3 provides a summary of the literature review, listing the previously established methods for producing propargyl derivatives, including the reaction conditions and corresponding yields.

Table 3 Comparative analysis of various catalysts for the synthesis of propargylamine derivatives

S. no.	Nanocatalyst	Reaction conditions	Time	% Yield	Ref.
1	Fe3O4@SiO2–Se-T/CuI	Neat, 80 °C	2 h	95	33
2	ZSM-5/APTMS/(E)-4-((pyridine-2-ylimino)methyl)benzaldehyde/Cu-NPs	K2CO3, H2O, 60 °C	2 h	94	34
3	UIO-66-NH2G1@PdNPs	Toluene, N2 gas, 110 °C	3 h	93	35
4	[Fe3O4@bisimidazolium-Pd]^2Cl−	PEG-400, 100 °C	2 h	98	36
5	Fe3O4@starch-Acr@Cu(II)	H2O, reflux	35 min	99	37
6	g-C3N4-TCT-2AEDSEA-Ag-Cu-Ni	Toluene, 80 °C	8 h	91	38
7	Fe3O4@SiO2-di-(pyridin-2-yl)amine-Cu	H2O, reflux	2 h	99	39
8	Co^2+-Cu@SA(0)-600	Toluene, 110 °C	1 h	89	40
9	MMT-K10/Fe3O4/CuO	Toluene, 80 °C	8 h	91	41
10	o-Cu2O-PVP	Neat, 100 °C	5 min	80	42
11	Fe3O4/PANI/CuI	Neat, 80 °C	10 min	96	Our work

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The plausible mechanism for the synthesis of propargylamine via A³ coupling catalysed by the Fe₃O₄/PANI/CuI nanocomposite is shown in Fig. 11. The copper-based nanocatalyst activates the phenylacetylene ring and proceeds through an attack on the carbon of the iminium ion, which is formed from the aldehyde and amine and results in the formation of the desired product as well as catalyst regeneration.^33,39

Fig. 11 Mechanism for Fe₃O₄/PANI/CuI catalysed synthesis of propargylamine derivative via A³ coupling.

General procedure for the synthesis of propargyl derivatives

In general, a mixture of phenylacetylene (1 mmol), pyrrolidine (1 mmol), aromatic aldehyde (1 mmol), and catalyst (10 mg) was added to a 50 mL round-bottom flask and stirred continuously at 80 °C. TLC was used to monitor the progress of the reaction. After the completion of the reaction, the reaction mixture was cooled and diluted with ethyl acetate, and the catalyst was separated with the aid of a magnet. The crude product was extracted with ethyl acetate and purified by column chromatography using basic alumina as a stationary phase and ethyl acetate : hexane as an eluent. The obtained pure product was confirmed by ¹H and ¹³C NMR spectroscopy.

Conclusion

In summary, we have developed a sustainable heterogeneous copper-based magnetic nanocatalyst for the one-pot synthesis of propargylamine derivatives under solvent-free conditions with a short reaction time. The designed nanocatalyst is easily magnetically recoverable and can be recycled for up to seven runs without any drastic reduction in product yield. This protocol provides a shorter reaction time to obtain products with high yield and good catalytic activity under mild reaction conditions as compared to previously reported methods.

Data availability

The data that support the findings of this study are available from the corresponding author following reasonable request.

Conflicts of interest

The authors declare no conflicts of interest.

Acknowledgements

RC and SS acknowledges the Institution of Eminence at the University of Delhi and the Institute of Nanomedical Science (INMS) for their assistance. Nisha is obliged to USIC, University of Delhi for instrumental facilities and CSIR for awarding her a Junior Research Fellowship (09/045(1792)/2020-EMR-I). RC and SS are thankful to Indo-Russia DSTRFBR: INT/RUS/RFBR/389 and SS is thankful to SERB-TARE: TAR/2022/000618 for their financial assistance.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4na00448e

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