Introduction

Palladium-catalyzed cross-coupling reactions have long been considered as efficient methods for C–C, C–N, C–O, and C–S bond formations [1, 2]. In 2010, Suzuki, Heck, and Negishi won the Nobel Prize for palladium-catalyzed cross-coupling synthesis [3,4,5]. Coupling reactions play strategic roles in synthesizing natural products and pharmacological compounds [6,7,8]. Developing and designing new catalytic systems for these reactions is an attractive research field in organic synthesis.

Cross-coupling reactions have recently focused on magnetic nanocarrier-supported palladium catalysts [9,10,11]. These catalysts, as one of the efficient metal–organic catalytic systems, have benefits such as high surface‐to‐volume ratio, eco-friendliness, high dispersibility, high coercivity, convenient recoverability by an external magnet, and reusability [12,13,14]. The Fe3O4 nanoparticle is used as the magnetic core of these systems and is usually covered with some coating materials such as silica [15, 16]. This coat protects the magnetic core against agglomeration and oxidation and helps maintain the magnetic properties [17,18,19]. The silica shell has other advantages, such as easy functionalization and readying the surface of the core for more modifications [20]. Suzuki coupling reaction of organoboron compounds with aryl halides can be carried out in the presence of versatile functional groups [21, 22]. This reaction is considered an efficient and flexible method for synthesizing biaryls, which are found in the structure of conducting polymers, natural products, and herbicides [23, 24]. Modifications to Suzuki coupling include the design of novel catalysts, solid-phase synthesis, and the use of biocompatible solvents.[25,26,27,28] Given the facts mentioned above and in line with our continued work on nanocatalyst [29,30,31,32,33]. Here, we have immobilized palladium (II) onto magnetic nanoparticles stabilized by tartaric acid and applied it as a nanocatalyst in the Suzuki cross-coupling reaction (Scheme 1).

Scheme 1
scheme 1

Our Fe3O4/tart/Pd-NPs as a nanocatalyst for the Suzuki cross-coupling (R = MeO2, MeO, CHO, Methyl, Ethyl, H, Br, NH2, NO2)

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Results and discussion

Magnetic nanoparticles (Fe3O4 MNPs) were obtained from the Fe2+/Fe3+ salt solution in a basic medium, then functionalized with tartaric acid and anchored by palladium (Scheme 2).

Scheme 2
scheme 2

The presence of Fe3O4/tartaric acid/Pd(0) NPs

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The Fe3O4/tart/Pd-NPs nanocatalyst and its components were characterized using FT-IR, XRD, EDS, SEM, and VSM spectroscopic analysis. After characterization of the nanocatalyst, its catalytic activity is examined in the Suzuki coupling reaction. Phenylboronic acid and iodobenzene are selected as model substrates to optimize the reaction conditions (Table 1). Initially, several solvents such as toluene, DMF, EtOH, water, and EtOH–H2O (1:1) were tested. The best solvent turns out to be H2O–EtOH (1:1) (Table 1, entry 1–6). To optimize the loading amount of the catalyst, different quantities (5–20 mg) of the catalyst were employed (Table 1, entries 6–9). The best result is obtained with 0.01 g of the catalyst (Table 1, entry 7).

Table 1 The optimization of reaction conditions
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The catalyst's efficiency is investigated by using a range of substituted aryl halides subjected to the optimized reaction conditions (Table 1, entry 7 Table). Aryl iodide shows better reactivity and product yields compared to aryl bromide (Table 2, entries 1, 7). This is due to the strength of the halogen-carbon bond, which weakens from bromine to iodine. In addition, the electronic properties of substitute rings influence the reaction yield. Efficiency is higher in the presence of electron-withdrawing groups than electron‐donating ones (Table 2, entries 1–5, 7–13).

Table 2 Scope of the reactiona
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Ortho‐substituted derivatives show a lower yield compared with the para‐substituted ones, indicating the sensitivity of the reaction to steric effects (Table 2, entries 5 and 6). Based on the catalytic cycle for the Suzuki coupling reaction and the aforementioned results, we suggest a plausible mechanism for the formation of biaryls in the presence of Fe3O4/tartaric acid/Pd(0) NPs (Scheme 3).

Scheme 3
scheme 3

A Proposed catalytic cycle for the Suzuki cross-coupling reaction in the presence of Fe3O4/tartaric acid/Pd(0) NPs

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The FT-IR spectrum of Fe3O4 nanoparticles, tartaric acid, Fe3O4/tartaric acid nanoparticles, and Fe3O4/tartaric acid/Pd(0) nanoparticles are obtained (Fig. 1). The FT-IR spectrum of Fe3O4 NPs shows peaks at 567, 1619, and 3427 cm−1, indicating the basic nature of the NPs obtained. The FT-IR spectrum of tartaric acid shows a strong peak at 1739 cm−1, along with a broad peak at 2600–3600 cm−1, corresponding to the characteristics of a carboxylic acid group. Absorption at 1066–1132 cm−1 shows stretching for the C–O single bonds. In the IR spectrum of Fe3O4/tartaric acid NPs, peaks of C=O stretchings appear at 1654 and 1630 cm−1. Also, this spectrum shows peaks at 1364 and 1365 cm−1 related to the C–O stretchings of carboxylate groups. These observations revealed the chemisorption of tartaric acid on Fe3O4 NPs as a carboxylate. In the Fe3O4/tartaric acid/Pd(0) NPs spectrum, a shift in peaks of C=O and C–O to lower wavenumber, a shift to higher wavenumber for the OH of carboxyl groups, and also an increase in the intensity of C–O peak are observed. This evidence proves the interaction between the carboxyl group of tartaric acid and palladium (Fig. 1).

Fig. 1
figure 1

FT-IR spectrum of Fe3O4 nanoparticles, tartaric acid, Fe3O4/tartaric. The structural composition is examined using EDS, which displays the catalyst containing Pd, Si, Fe, O, and C (Fig. 2)

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Fig. 2
figure 2

EDS pattern of our catalyst (Fe3O4/tart/Pd(0)-NPs)

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The size and morphology of the catalyst nanoparticles were evaluated via SEM (Fig. 3) and TEM analyses (Fig. 4). Spherical particles with an average size of 25 nm are observed with weak agglomeration. TEM image shows magnetic nanoparticles covered with organic groups.

Fig. 3
figure 3

SEM image of the catalyst (Fe3O4/tart/Pd(0)-NPs)

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Fig. 4
figure 4

TEM images of the Fe3O4/tart/Pd(0)-NP catalysts

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The magnetization vs. applied magnetic field curves at the room temperature range from + 10,000 to − 10,000 Oersted is illustrated for the Fe3O4 NPs and the catalyst (Fig. 5). The magnetic saturation (Ms) value is varied from 63 emug−1 for Fe3O4 NPs to 41 emug−1 for the catalyst. The decrease in magnetic saturation for Fe3O4/tart/Pd(0)-NPs can be attributed to the modification on the surface of the Fe3O4 with tartaric acid.

Fig. 5
figure 5

The VSM curves of the Fe3O4, Fe3O4/tart/Pd(0)-NPs and recycled catalyst after last run

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The properties of crystallinity and phase purity of the as‐synthesized Fe3O4/tart/Pd(0)-NPs were investigated by wide angle XRD. (Fig. 6). The decrease in the intensity of peaks is related to the functionalization and modification of the Fe3O4 surface. Both the catalyst and recycled catalyst exhibit the six characteristics.

Fig. 6
figure 6

a The XRD pattern of the (Fe3O4/tart/Pd-NP) and b recycled catalyst after last run

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The organic content and the thermal stability on the surface of the magnetic NPs are determined by TGA whose thermogram shows the initial weight loss is below 200 °C, which is attributed to the removal of physically adsorbed solvent and water. Also, the mass loss at the temperature range 200–400 °C is due to the decomposition of l-(+)-tartaric acid, grafted on the surface of the magnet (Fig. 7).

Fig. 7
figure 7

Thermogravimetric analysis (TGA) and Differential thermal analysis (DTA) of Fe3O4/tart/Pd(0)-NP

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To check leaching of the Pd NPs, after completion of reaction, the catalyst is separated from the solution by an external magnet, analysis of the aqueous phases of reaction mixture using inductively coupled plasma analysis show that the leaching of Palladium is negligible and the catalyst is mainly heterogeneous in nature.

Given commercial and industrial applications and the principles of green chemistry and environmentally friendly processes, the recyclability and reusability of catalysts are very important. The reusability of our catalyst is probed through the model reaction, where the catalyst is simply removed from the reaction medium using an external magnet. Before each run, the catalyst is washed thoroughly with ethanol and acetone and dried in an oven. The results showed that the catalyst could be reused at least five times with a slight decrease in its catalytic activity (Fig. 8).

Fig. 8
figure 8

Reusability of magnetic Fe3O4/tart/Pd(II) nanocatalyst

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Experimental

Chemical reagents in high purity were purchased from Merck and Aldrich and used without further purification. Melting points were determined in open capillaries using an Electrothermal 9100 apparatus and are uncorrected. Fourier transform infrared (FT‐IR) spectra were recorded using KBr pellets in the range of 400–4000 cm−1 on a Nicolet IR‐100 infrared spectrometer. NMR spectra were recorded in DMSO‐d6 on Brucker DRX 500‐Avance spectrometer. The particle morphology is examined by SEM (KYKY EM3200–25 kV). X‐ray diffraction (XRD) is performed on Philips XPert 1710 diffractometer. The latter appears with Co Kα (α = 1.79285 Å) with 40 kV voltage.

Preparation of Fe3O4 magnetic nanoparticles (MNPs)

FeCl3⋅6H2O (5.4 g, 20 mmol) and FeCl2⋅4H2O (1.98 g, 10 mmol) were dissolved in 100 ml deionized water at 80 °C under Ar atmosphere by vigorous mechanical stirring. Then, 10 ml of 28% aqueous ammonia solution was quickly injected into the reaction mixture in one portion. The addition of the base to the Fe2+/Fe3+ salt solution resulted in the immediate formation of a black precipitate of MNPs. The stirring continued for another 2 h, and the mixture was cooled to room temperature. Subsequently, the produced ultra-fine magnetic particles were separated by a magnet and washed several times with deionized water.[49] The resulting nanoparticle weighed 2.1 g.

Fe3O4/tart-NPs preparation

A round-bottom flask was charged with Fe3O4 (500 mg) in EtOH (50 ml), and the flask was sonicated for 30 min. Then, the sonication was stopped, and l-(+)-tartaric acid (1 mmol) was added. The mixture was stirred for 12 h at 40 °C. Finally, Fe3O4/tart NPs were separated by an external magnet washed with ethanol and dried in an oven. The resulting nanoparticle weighed 700 mg.

Fe3O4/tart/Pd(0)-NPs preparation

A round-bottom flask was charged with Fe3O4/tart NPs (500 mg) and sonicated in ethanol for half an hour. Then, the flask was removed, followed by the addition of palladium acetate (50 mg), and the mixture was stirred at 50 °C for 4 h. NaBH4 (100 mg) was added to the mixture, and the stirring continued for 2 h at 40 °C. The resulting magnetic nanoparticles were washed with ethanol and dried in an oven at 50 °C for 4 h. The resulting nanoparticle weighed 540 mg.

General procedure for the Suzuki cross-coupling reaction

A round-bottom flask was charged with MNP supported palladium (0) (Fe3O4/tart/Pd(0)-NPs) (10 mg), aryl halide (1.0 mmol), phenylboronic acid (1.2 mmol), K2CO3 (1.5 mmol) and EtOH:H2O (1:1, 3 ml). Then, the reaction mixture was stirred at 60 °C for 20 min. The reaction progress was monitored by TLC. After completion of the reaction, the mixture was cooled to room temperature, and the catalyst was separated by an external magnet. The product was purified by thin-layer chromatography using n-hexane and ethyl acetate (5:2) as eluents. Here we have immobilized palladium (II) onto magnetic nanoparticles stabilized by tartaric acid and applied it as a nanocatalyst in Suzuki cross-coupling (Scheme 1).

4-methoxy-1,1′-biphenyl (Table 2, Entry 3): 1H NMR (300 MHz, CDCl3) δ 7.60–7.54 (m, 4H), 7.467.41 (m, 2H), 7.32–7.26 (m, 1H), 7.02–6.99 (m, 2H), 3.87 (s, 3H) ppm; 13C NMR (75 MHz, CDCl3) δ 159.15, 140.82, 133.78, 128.70, 128.14, 126.72, 126.64, 114.20, 55.35 ppm.

1,1′-biphenyl (Table 2, Entry 1): 1H NMR (300 MHz, CDCl3) δ 7.66–7.63 (dd, 4H), 7.49–7.42 (t, 4H), 7.42–7.26 (t, 2H) ppm; 13C NMR (75 MHz, CDCl3) δ 141.26, 128.77, 127.26, 127.18 ppm.

Conclusions

Here, we present a recoverable heterogeneous catalyst by stabilizing palladium on the surface of tartaric acid-activated magnetic iron. The catalytic activity of the synthesized magnetic nanoparticles in the Suzuki reaction, i.e., the interaction of aryl halides with phenylboronic acid, was investigated. The advantages of this new heterogeneous nanocatalyst include easy separation by magnets, compatibility with aquatic environments, easy access, thermal stability, high efficiency, recyclability, green conditions, and short reaction time.