A nanomagnetic triazole-based Schiff-base complex of palladium(0) as an efficient heterogeneous catalyst for the Mizoroki–Heck C–C cross-coupling reaction under green conditions

Abstract

A novel magnetically recoverable palladium(0) catalyst, [Fe₃O₄@triazole-Schiff-base-Pd(0)], was synthesized via a simple three-step functionalization. Magnetite nanoparticles were first modified with a haloalkane silane linker to introduce C–Cl functionalities, followed by nucleophilic substitution with a triazole-Schiff-base ligand derived from 3-amino-1,2,4-triazole and salicylaldehyde. Finally, coordination with palladium chloride and in situ reduction to Pd(0) afforded a stable, magnetically separable catalyst. Comprehensive characterization (FT-IR, XRD, TGA, EDX, ICP-OES, WDX, FE-SEM, TEM, and VSM) confirmed its successful synthesis. The catalyst demonstrated outstanding activity in the Mizoroki–Heck C–C cross-coupling reaction, resulting in efficient coupling of a diverse range of aryl iodides, bromides, and chlorides with butyl acrylate in PEG-400 as a green solvent and co-catalyst. It exhibited superior selectivity for aryl iodides and bromides and maintained high efficiency over eight consecutive cycles with minimal metal leaching. The use of cost-effective materials, facile magnetic recovery, and scalability make this catalyst an eco-friendly and economically viable alternative to conventional palladium-based methodologies.

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1 Introduction

The Heck cross-coupling reaction, a pivotal tool in modern organic synthesis, facilitates the formation of carbon–carbon bonds via the palladium-catalyzed coupling of aryl or vinyl halides with alkenes.¹ This reaction was initially developed by Richard F. Heck in the late 1960s and later recognized with the 2010 Nobel Prize in Chemistry, and this synthetic tool has become indispensable in the production of pharmaceuticals, agrochemicals, and advanced materials.^2,3 Despite the well-established efficiency and selectivity of homogeneous palladium catalysts with phosphine ligands, significant challenges remain, including catalyst recovery, metal leaching, ligand cost, and alkene stability issues.^4–9 A significant hurdle lies in the practical handling of the alkene reactants. Olefins used in Heck coupling pose challenges, including difficulties in synthesis and purification, as well as a tendency to polymerize during storage or at high reaction temperatures. Even commercially available alkenes often require excess amounts to counteract these stability issues.^10,11 This, coupled with concerns regarding residual palladium contamination, has spurred significant research. Current efforts are focused on the development of heterogeneous catalysts, including those supported on zeolites, metal oxides and polymeric frameworks, and the application of palladium nanoparticles.^8,12–14 These advancements aim to enhance sustainability and recyclability, and mitigate the challenges associated with alkene instability, broadening the industrial applicability of the Heck reaction and ensuring its continued relevance in contemporary chemical research and production.^14–16

To improve sustainability and economic efficiency, heterogeneous catalysis has emerged as a compelling alternative to traditional homogeneous methods.¹⁷ The inherent advantages of heterogeneous systems, notably enhanced recyclability and selectivity, are largely attributed to the strategic use of solid supports that anchor active catalytic species.¹⁸ Among various catalytic supports, magnetic metal oxide nanoparticles, particularly spinel ferrites, have gained significant attention due to their unique combination of catalytic activity, stability, and ease of recovery.¹⁹ Within this class, magnetic metal oxide nanoparticles, particularly spinel ferrites, have garnered significant attention as exceptionally effective catalytic supports.^20,21 They uniquely combine the benefits of green catalytic activity with the practical advantages of facile magnetic separation, high stability, and reusability, thereby streamlining catalyst recovery and significantly reducing waste generation across various applications.²² Among these, Fe₃O₄ stands out for its widespread adoption, balancing magnetic separability with excellent catalytic support properties. Crucially, the surface hydroxyl groups present on Fe₃O₄ nanoparticles offer a versatile platform for effective functionalization through chemical bonding, enabling the tailored design of catalysts for specific applications.^23,24

To further enhance catalytic performance, Schiff-base ligands, formed by condensing amines and carbonyls, have been explored due to their ability to form stable metal complexes. Their applications include biological studies, materials science, and catalysis, where they exhibit significant activity and stabilize various metal oxidation states.^25–27 While the convenient synthesis and solvent compatibility of these ligands drive their catalytic utility, the immobilization of these ligands on solid supports, though a burgeoning field, continues to demand novel strategies and refined methodologies for enhanced performance.^28–30

This study aims to overcome key limitations of traditional Heck catalysts including high costs, environmental concerns, and catalyst recovery issues by developing a novel immobilization strategy using chlorine-functionalized Fe₃O₄ MNPs. This study employs chlorine-functionalized Fe₃O₄ MNPs as a stable, cost-effective support for a triazole-Schiff-base ligand derived from 3-amino-1,2,4-triazole and salicylaldehyde, along with its corresponding palladium(0) complex. This approach yields a magnetically recoverable, low-cost, and environmentally benign palladium catalyst designed to facilitate Heck coupling reactions under mild, sustainable conditions. By leveraging the unique properties of MNPs, we aim to demonstrate enhanced catalyst stability, recyclability, and activity. Furthermore, this study will investigate the catalyst's performance across a range of substrates, exploring its scope and limitations.

2 Experimental

2.1. Typical procedure for synthesis of the [Fe₃O₄@triazole-Schiff-base-Pd(0)] complex

Fe₃O₄ MNPs were synthesized according to a previously reported method.³¹ Subsequently, 10 g of the Fe₃O₄ MNPs were dispersed in 200 mL of anhydrous toluene and sonicated for 1 h. 20 mL of (3-chloropropyl)trimethoxysilane (CPTMS) was then added dropwise to the vigorously stirred suspension. The resulting mixture was heated to 80 °C and stirred for 48 h. After the reaction, the Fe3O4@CPTMS MNPs were magnetically separated, washed with n-hexane, and dried under vacuum at 80 °C overnight. In the subsequent step, 5 g of the Fe₃O₄@CPTMS MNPs were dispersed in 150 mL of anhydrous acetonitrile and sonicated for 1 h to afford a homogeneous suspension. A solution of 3-amino-1,2,4-triazole Schiff-base (prepared by reacting equimolar amounts (10 mmol) of 3-amino-1,2,4-triazole and salicylaldehyde in acetonitrile (25 mL) under reflux conditions for 4 h (ref. 32–34)) and 5 mmol of anhydrous K₂CO₃ were added to the Fe₃O₄@CPTMS MNP suspension. The reaction mixture was stirred under reflux conditions under a nitrogen atmosphere for 24 h. The resulting Fe₃O₄@triazole-Schiff-base product was magnetically separated, washed with hot ethanol, and dried at 80 °C for 4 h. Finally, 2 g of Fe₃O₄@triazole-Schiff-base was dispersed in 100 mL of ethanol and stirred for 30 minutes. 4 mmol of PdCl₂ was then added to this suspension, and the mixture was refluxed for 20 h. Finally the reaction mixture was cooled to room temperature and 10 mmol of NaBH₄ was added to the reaction and refluxed for an additional 4 h. The resulting [Fe₃O₄@triazole-Schiff-base-Pd(0)] complex was magnetically separated, washed sequentially with hot deionized water and ethanol, and dried at 80 °C for 6 h.

2.2. General procedure for the Mizoroki–Heck reaction catalyzed by [Fe₃O₄@triazole-Schiff-base-Pd(0)]

In a reaction vessel, a mixture of an aryl halide (1 mmol), an alkene (1 mmol), potassium carbonate (3 mmol), and a [Fe₃O₄@triazole-Schiff-base-Pd(0)] catalyst (5 mg) was suspended in 3 mL of polyethylene glycol (PEG-400). The reaction was stirred at 100 °C until complete, with progress tracked by thin-layer chromatography (TLC). Upon completion, the reaction mixture was diluted with hot water, the catalyst was separated by using a magnetic field and the product was extracted into ethyl acetate and washed with warm water. The organic extract was dried over sodium sulfate and filtered. After rotary evaporation to remove the solvent, the crude butyl cinnamate product was subjected to preparative silica gel TLC for purification.

3 Results and discussion

In this study, a novel palladium triazole-Schiff-base complex was successfully grafted onto Fe₃O₄ MNPs through a three-step post-synthetic functionalization protocol. This process was initiated with the synthesis and surface modification of Fe₃O₄ MNPs using the CPTMS linker, introducing reactive chloropropyl functionalities. Subsequently, an in situ generated 3-amino-1,2,4-triazole Schiff-base ligand was used to perform a nucleophilic substitution reaction on this substrate. This ligand was synthesized via a condensation reaction (Hoeg's reaction) between 3-amino-1,2,4-triazole and salicylaldehyde. Finally, the immobilized Schiff-base ligand was complexed with palladium(II) chloride, followed by in situ reduction with NaBH₄ to generate the [Fe₃O₄@triazole-Schiff-base-Pd(0)] catalyst (Scheme 1). The synthesis and immobilization of the targeted catalytic complex were rigorously characterized using a series of analytical techniques, including Fourier-transform infrared spectroscopy (FT-IR), X-ray diffraction (XRD), thermogravimetric analysis (TGA), energy-dispersive X-ray spectroscopy (EDX), inductively coupled plasma optical emission spectrometry (ICP-OES), elemental mapping, field-emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM), and vibrating sample magnetometry (VSM) to comprehensively evaluate its structural, compositional, and magnetic properties.

Scheme 1 Stepwise synthesis of the [Fe₃O₄@triazole-Schiff-base-Pd(0)] catalyst.

3.1. Catalyst characterization

The FTIR spectra of pristine Fe₃O₄, Fe₃O₄@CPTMS, Fe₃O₄@triazole-Schiff-base, and the [Fe₃O₄@triazole-Schiff-base-Pd(0)] catalyst are shown in Fig. 1. The FTIR spectrum of pristine Fe₃O₄ exhibits an Fe–O stretching band at 562 cm⁻¹. Broad peaks at 3436 cm⁻¹ and 1625 cm⁻¹ correspond to O–H bond vibrations from hydroxyl groups on the Fe₃O₄ surface.³⁵ In the spectrum of Fe₃O₄@CPTMS, additional peaks at 1042 cm⁻¹ and 2929 cm⁻¹ are attributed to the Si–O–Si stretching vibrations and aliphatic C–H bonds, respectively, confirming the successful functionalization with CPTMS.³⁶ The FTIR spectrum of Fe₃O₄@triazole-Schiff-base shows new peaks at 1438, 1559, and 1624 cm⁻¹, which are assigned to the aromatic C–C and C=N stretching vibrations in the Schiff base and triazole ring.³³ These peaks further support the successful immobilization of the Schiff base ligand onto the haloalkene-modified magnetic support. Finally, the FTIR spectrum of the [Fe₃O₄@triazole-Schiff-base-Pd(0)] catalyst reveals several changes such as some shifts in the C=N and N–H peaks, likely due to the π-back-bonding effect of the immobilized Pd(0) ions, confirming the formation of the targeted catalyst.

Fig. 1 FT-IR spectra of (a) Fe₃O₄, (b) Fe₃O₄@CPTMS (c) Fe₃O₄@triazole-Schiff-base and (d) [Fe₃O₄@triazole-Schiff-base-Pd(0)].

Powder X-ray diffraction (P-XRD) was employed to characterize the crystalline phases present in the [Fe₃O₄@triazole-Schiff-base-Pd(0)] catalyst. The diffraction pattern exhibited sharp reflections at 2θ values of 30.10°, 35.24°, 43.05°, 53.75°, 57.14°, and 62.83°, which correspond to the (220), (311), (400), (422), (511), and (440) crystallographic planes of cubic Fe₃O₄, consistent with JCPDS 88-0866.³⁷ This confirmed the formation and stability of the magnetite nanoparticles during the multistep synthetic procedure. Furthermore, reflections at 2θ values of 37.52°, 45.44°, 50.14°, and 65.02° were attributed to metallic Pd(0),³⁸ indicating the successful incorporation of palladium within the catalyst structure. Additionally, the approximate crystallite size calculated from the (311) peak using the Debye–Scherrer equation is about 15.2 nm (Fig. 2).

Fig. 2 XRD pattern of the [Fe₃O₄@triazole-Schiff-base-Pd(0)] catalyst.

The thermal behavior of the [Fe₃O₄@triazole-Schiff-base-Pd(0)] catalyst was evaluated via the TGA technique, as shown in Fig. 3. The TGA curve reveals an initial weight loss of approximately 4.9% in the temperature range of 25–200 °C, attributed to the evaporation of physically and chemically adsorbed moisture and organic solvents. A significant weight loss of around 29% occurs between 200 °C and 600 °C, corresponding to the decomposition of the supported organic components, including the CPTMS linker and Schiff-base ligand, through pyrolysis reactions involving oxygen, heat and the hydrocarbon. These results confirm the presence of organic moieties on the catalyst surface and indicate its thermal stability up to 200 °C, making it suitable for reactions conducted at temperatures below this threshold.

Fig. 3 TGA curve of the [Fe₃O₄@triazole-Schiff-base-Pd(0)] catalyst.

The elemental composition of the [Fe₃O₄@triazole-Schiff-base-Pd(0)] catalyst was investigated using energy dispersive spectroscopy (EDS) (Fig. 4). The presence of characteristic signals for iron (40.38 wt%, 20.58 at%) and oxygen (19.63 wt%, 34.93 at%) confirmed the incorporation of iron oxide magnetic nanoparticles within the composite matrix. The detection of silicon (1.66 wt%, 1.68 at%) validated the successful silane coating and attachment of the CPTMS linker to the support. Carbon (12.37 wt%, 29.30 at%) and nitrogen (3.58 wt%, 7.27 at%) signals also provided evidence for successful surface functionalization with the CPTMS linker and the triazole-Schiff-base ligand. Finally, the presence of palladium (15.87 wt%, 4.25 at%) and chlorine (2.51 wt%, 2.02 at%) signals demonstrated the effective coordination of palladium chloride to the surface-bound ligands, confirming the synthesis of the desired palladium catalytic complex. The concentration of palladium within the [Fe₃O₄@triazole-Schiff-base-Pd(0)] catalyst, as determined by ICP-OES, was measured to be 1.67 × 10⁻³ mol g⁻¹.

Fig. 4 EDX analysis of the [Fe₃O₄@triazole-Schiff-base-Pd(0)] catalyst.

Elemental mapping of the [Fe₃O₄@triazole-Schiff-base-Pd(0)] catalyst demonstrated a high surface density of iron and oxygen, consistent with the Fe₃O₄ core (Fig. 5). The homogeneous distribution of silicon, carbon, and nitrogen confirmed the uniform coverage of the silane linker and further Schiff-base ligand immobilization. Furthermore, the homogeneous dispersion of palladium and chlorine provided evidence for the accessibility of the active catalytic species across the nanoparticle surface. These results confirm the successful functionalization of Fe₃O₄ MNPs and formation of the [Fe₃O₄@triazole-Schiff-base-Pd(0)] catalyst.

Fig. 5 EDX elemental mapping images of the [Fe₃O₄@triazole-Schiff-base-Pd(0)] catalyst.

The morphology of the [Fe₃O₄@triazole-Schiff-base-Pd(0)] catalyst was characterized using SEM and TEM analysis, as shown in Fig. 6. The SEM image (Fig. 6a) demonstrates that the catalyst particles exhibit a quasi-spherical shape with uniform dispersion, which is consistent with the typical morphology of magnetic nanoparticles reported in the literature. This observation further substantiates the stability of the catalytic support throughout the post-synthetic modification process.³⁹ However, some agglomeration was also observed, which can be attributed to the intrinsic magnetic properties of Fe₃O₄ and the surface modification with polar groups. This agglomeration is indicative of successful catalyst synthesis, confirming the effectiveness of the synthesis method and the integrity of the catalyst structure. The TEM micrographs clearly show that the spherical shape particles are uniformly distributed with good dispersion, ranging from 15 to 20 nm in size. Additionally, the lighter layer of supported groups around the darker Fe₃O₄ core is clearly visible, confirming the successful immobilization of the targeted complex onto the MNP support (Fig. 6b).

Fig. 6 (a) SEM and (b) TEM images of the [Fe₃O₄@triazole-Schiff-base-Pd(0)] catalyst.

The magnetic behavior of the [Fe₃O₄@triazole-Schiff-base-Pd(0)] catalyst was characterized at ambient temperature via vibrating sample magnetometry (VSM). The obtained hysteresis loop (Fig. 7) revealed a ferromagnetic response with a saturation magnetization (M_s) of approximately 28 emu g⁻¹. This value represents a decrease compared to the higher M_s values reported for unmodified Fe₃O₄.⁴⁰ This decrement is attributed to the presence of the non-magnetic moieties, which effectively decrease the overall magnetic moment per unit mass of the composite material, providing evidence of successful surface functionalization with the [triazole-Schiff-base-Pd(0)] complex. Nonetheless, the catalyst exhibited sufficient magnetic properties to enable efficient magnetic separation from reaction media using an external magnetic field.

Fig. 7 VSM analysis of the [Fe₃O₄@triazole-Schiff-base-Pd(0)] catalyst.

The nitrogen adsorption–desorption isotherm of the [Fe₃O₄@triazole-Schiff-base-Pd(0)] complex was a type IV isotherm according to the IUPAC classification, accompanied by an H3-type hysteresis loop (Fig. 8). The Brunauer–Emmett–Teller (BET) specific surface area was determined to be 56.36 m² g⁻¹, and the average pore diameter was calculated to be 17.33 nm, unequivocally confirming the mesoporous nature of the material. Furthermore, the total pore volume (V_p) was found to be 0.115 cm³ g⁻¹, suggesting a substantial pore network capable of facilitating the efficient diffusion of reactants and products during catalytic applications.

Fig. 8 VSM analysis of the [Fe₃O₄@triazole-Schiff-base-Pd(0)] catalyst.

3.2. Catalytic study

After the complete characterization of [Fe₃O₄@triazole-Schiff-base-Pd(0)] its catalytic efficiency was examined in the Mizoroki–Heck C–C coupling reaction of aryl halides and butyl acrylate as an industrially important reaction. For this purpose initially the coupling of iodobenzene and butyl acrylate was selected as a model reaction to optimize reaction parameters, including the catalyst amount, solvent, base type and reaction temperature (Table 1). Initially, we conducted the reaction under catalyst free conditions, but after 1 day no product was observed (Table 1, entry 1). Then the catalytic activity of Fe₃O₄, Fe₃O₄@CPTMS and Fe₃O₄@triazole-Schiff-base MNPs was investigated, but in this case as monitored by TLC no detectable product was formed even after 4 h (Table 1, entries 2–4). But when the reaction was conducted in the presence of [Fe₃O₄@triazole-Schiff-base-Pd(0)] MNPs reaction progress was observed but the catalytic efficiency was observed to be quite low (35%) when the reaction was performed using 1 mg of the catalyst (Table 1, entry 5). So the reaction was conducted by varying the catalyst amounts from 1–6 mg, resulting in enhanced yields of the coupling reaction (Table 1, entries 5–9). The evaluation determined that using 5 mg of the catalyst is the most appropriate catalytic amount for the per 1 mmol scale reaction with 98% yield (Table 1, entry 8). Furthermore, increasing the catalyst loading to 6 mg did not result in improved yields or reaction time (Table 1, entry 9).

Table 1 Optimization of synthesis of butyl cinnamate via the Mizoroki–Heck reaction of iodobenzene and butyl acrylate in the presence of the [Fe₃O₄@triazole-Schiff-base-Pd(0)] catalyst

Entry	Catalyst	Catalyst amount (mg)	Base	Solvent	Temperature (°C)	Time (min)	Yield^a^,^b (%)
a Isolated yield.b Conditions: iodobenzene (1 mmol), butyl acrylate (1 mmol), base (3 mmol), catalyst (mg) and solvent (3 mL).
1	—	—	K2CO3	PEG-400	120	1 day	NR
2	Fe3O4	5	K2CO3	PEG-400	120	4 h	NR
3	Fe3O4@CPTMS	5	K2CO3	PEG-400	120	4 h	NR
4	Fe3O4@triazole-Schiff-base	5	K2CO3	PEG-400	120	4 h	NR
5	[Fe3O4@triazole-Schiff-base-Pd(0)]	1	K2CO3	PEG-400	120	25	35
6	[Fe3O4@triazole-Schiff-base-Pd(0)]	3	K2CO3	PEG-400	120	25	87
7	[Fe3O4@triazole-Schiff-base-Pd(0)]	4	K2CO3	PEG-400	120	25	95
8	[Fe3O4@triazole-Schiff-base-Pd(0)]	5	K2CO3	PEG-400	120	25	98
9	[Fe3O4@triazole-Schiff-base-Pd(0)]	6	K2CO3	PEG-400	120	25	98
10	[Fe3O4@triazole-Schiff-base-Pd(0)]	5	K2CO3	Water	Reflux	180	78
11	[Fe3O4@triazole-Schiff-base-Pd(0)]	5	K2CO3	Ethanol	Reflux	135	93
12	[Fe3O4@triazole-Schiff-base-Pd(0)]	5	—	PEG-400	80	2 h	Trace
13	[Fe3O4@triazole-Schiff-base-Pd(0)]	5	Na2CO3	PEG-400	80	2 h	95
14	[Fe3O4@triazole-Schiff-base-Pd(0)]	5	KOH	PEG-400	120	25	49
15	[Fe3O4@triazole-Schiff-base-Pd(0)]	5	Et3N	PEG-400	120	25	93
16	[Fe3O4@triazole-Schiff-base-Pd(0)]	5	K2CO3	PEG-400	60	25	47
17	[Fe3O4@triazole-Schiff-base-Pd(0)]	5	K2CO3	PEG-400	r.t.	25	Trace
18	[Fe3O4@triazole-Schiff-base-Pd(0)]	5	K2CO3 (1 mmol)	PEG-400	120	25	87
19	[Fe3O4@triazole-Schiff-base-Pd(0)]	5	K2CO3 (4 mmol)	PEG-400	120	25	98

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Then the effect of more green solvents including water and ethanol; however, the outcomes were unsatisfactory and these solvents led to a decrement in the reaction efficiency, so PEG-400 was used as the optimal solvent. Then an investigation was conducted to optimize the alkaline media of the reaction, and base-free conditions and several bases including; K₂CO₃, Na₂CO₃, KOH and organic triethylamine base were tested (Table 1, entries 8 and 12–15). Furthermore, in the absence of a base, only a trace amount of product was obtained (Table 1, entry 12). Additionally, reducing the base amount from 3 mmol to 1 mmol resulted in a decrease in yield, while increasing it to 4 mmol produced similar results to 3 mmol (Table 1, entries 18 and 19). Among the tested bases, 3 mmol of K₂CO₃ provided the best efficiency, making it the optimal base for this coupling reaction methodology (Table 1, entry 8). Furthermore, the optimal conditions were found to be 5 mg of the [Fe₃O₄@triazole-Schiff-base-Pd(0)] nanocatalyst and K₂CO₃ (3 mmol) in PEG-400 at 120 C (Table 1 entry 8).

To assess the scope and applicability of the optimized methodology, a series of aryl halides, encompassing unsubstituted and substituted aryl iodides, bromides, and chlorides bearing a variety of functional groups, were subjected to the reaction conditions (Table 2). The methodology proved robust, affording butyl cinnamate products in good to excellent yields across all tested substrates. A clear trend in halide reactivity was observed, with aryl iodides exhibiting the highest reactivity, followed by aryl bromides, and aryl chlorides requiring prolonged reaction times to achieve comparable conversions. This trend is consistent with the expected oxidative addition rates to the catalytic species. The influence of electronic effects was also investigated, revealing that electron-withdrawing substituents on the aryl ring facilitated the reaction, likely due to enhanced oxidative addition, while electron-donating groups retarded it. Notably, in the case of dihaloarenes, the reaction demonstrated exceptional chemoselectivity, selectively activating C–I and C–Br bonds in preference to C–Cl bonds (Table 2, entries 18 and 21). This selectivity is highly advantageous for synthetic applications where the chlorine functionality must be retained, enabling selective coupling at the more reactive halide sites. Finally, to confirm the practical applicability and scalability of the optimized method, a gram-scale experiment was successfully performed. The reaction between iodobenzene and butyl acrylate, conducted at a 30 mmol scale, efficiently yielded the desired product in 95% within only 40 min, highlighting the method's high efficiency and robustness for larger-scale synthesis.

Table 2 The scope of synthesis of butyl cinnamate via the Mizoroki–Heck C–C coupling reaction catalyzed by [Fe₃O₄@triazole-Schiff-base-Pd(0)]

Entry	Aryl halide	Product	Time (min)	Yield^a^,^b (%)	TON	TOF (min^−1)	Melting point (°C)
a Isolated yield.b Conditions: aryl halide (1.0 mmol), butylacrylate (1 mmol) and the [Fe3O4@triazole-Schiff-base-Pd(0)] catalyst (5 mg) in PEG-400 (3 mL) at 120 °C.
1			25	98	11 676	467.04	Oil^41
2			45	96	11 566	257.02	Oil^41
3			160	91	10 963	68.52	Oil^41
4			25	99	11 927	477.08	62–64 (ref. 42)
5			30	99	11 927	397.56	53–55 (ref. 43)
6			15	99	11 927	795.13	58–63 (ref. 41)
7			20	98	11 807	590.35	39–42 (ref. 41)
8			50	93	11 204	224.08	Oil^43
9			35	96	11 566	330.45	Oil^41
10			105	87	10 481	99.82	Oil^41
11			85	93	11 204	131.82	Oil^41
12			110	83	10 000	90.90	Oil^44
13			95	89	10 722	112.87	74–76 (ref. 45)
14			70	89	10 722	153.18	Oil^41
15			20	96	11 566	587.31	39–42 (ref. 41)
16			25	96	11 566	462.65	62–64 (ref. 42)
17			20	97	11 686	584.33	58–63 (ref. 41)
18			60	92	11 084	184.73	Oil^41
19			110	95	11 445	104.05	58–63 (ref. 41)
20			120	93	11 204	93.37	39–42 (ref. 46)
21			25	96	11 566	462.65	Oil^41

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The proposed mechanism for the Mizoroki–Heck cross-coupling reaction over the [Fe₃O₄@triazole-Schiff-base-Pd(0)] catalyst is outlined in Scheme 2. The reaction initiates with the oxidative addition of aryl halide to a Pd(0) catalyst, generating a Pd(II) intermediate. Subsequently, the activated alkene coordinates to the palladium center, forming a π-complex. This is followed by a migratory insertion, where the aryl group inserts across the alkene double bond. The resulting alkylpalladium(II) species then undergoes β-hydride elimination, leading to the formation of the desired alkene product and a Pd(II)-hydride complex. Finally, reductive elimination of HX in the presence of the base regenerates the Pd(0) catalyst, completing the catalytic cycle.⁴⁷

Scheme 2 Possible mechanism for synthesis of butyl cinnamate via the Mizoroki–Heck C–C coupling reaction catalyzed by [Fe₃O₄@triazole-Schiff-base-Pd(0)].

3.3. Reusability of the catalyst

The development of sustainable catalytic processes necessitates heterogeneous catalysts with high recyclability and stable active sites, especially when employing precious metals. These attributes are intrinsically linked: effective recycling hinges on minimizing active site loss or deactivation. Addressing challenges such as metal leaching and structural degradation is crucial for creating robust, reusable catalysts that enhance both economic and environmental viability.⁴⁸ To evaluate these critical properties, we conducted a comprehensive reusability study, encompassing catalyst recovery, hot filtration, and leaching tests. To assess catalyst reusability, the optimized model reaction was conducted across multiple reaction cycles. Upon completion of each cycle, the catalyst was isolated via the procedure described in the Experimental section. The recovered catalyst was subjected to rigorous washing with ethyl acetate, water, and ethanol, followed by drying at 80 °C. Subsequently, the catalyst was employed in subsequent reaction cycles. Over eight consecutive cycles, the catalyst exhibited remarkable stability, demonstrating minimal activity loss (Fig. 9). These results confirm the robust reusability and recyclability of the synthesized catalyst under the established optimized conditions.

Fig. 9 The reusability of the [Fe₃O₄@triazole-Schiff-base-Pd(0)] complex in the model reaction of butyl cinnamate synthesis.

The structural stability of the [Fe₃O₄@triazole-Schiff-base-Pd(0)] catalyst after multiple reaction cycles was rigorously evaluated using FT-IR, VSM, and ICP-OES analyses. The consistent FT-IR spectral pattern observed after eight cycles (Fig. 10) confirms the exceptional chemical stability and excellent recyclability of the catalyst. Additionally, VSM analysis (Fig. 11) showed that the M_s value of the spent catalyst was approximately 24.91 emu g⁻¹, indicating no significant decrease compared to the fresh catalyst. Furthermore, ICP-OES analysis revealed that the Pd content in the spent catalyst was 1.63 × 10⁻³ mol g⁻¹, which is very similar to that of the fresh catalyst, thereby providing strong evidence for the catalyst's stability throughout the reaction and recycling processes.

Fig. 10 FT-IR spectra of the recycled catalyst.

Fig. 11 VSM analysis of the recycled catalyst.

3.4. Leaching and the hot filtration test

The heterogeneous nature of the catalyst was further confirmed through a hot filtration test. Given the high viscosity of PEG-400, the reaction was conducted in ethanol. At half the reaction time of the model synthesis, the catalyst was removed via magnetic separation, yielding a 64% product conversion. Subsequent continuation of the reaction for an additional 4 h, under catalyst-free conditions, showed no significant increase in product conversion. This observation definitively establishes the catalyst's heterogeneous character. Furthermore, ICP-OES analysis of the filtrate demonstrated the absence of detectable palladium in the mixture. This result demonstrates the robust stability of the catalytic species anchored to the catalyst surface, indicating negligible leaching during the reaction.

3.5. Comparison study of catalytic activity

In recent years, various catalytic approaches have been developed for the Mizoroki–Heck C–C coupling reaction due to its crucial role in organic synthesis. However, a comparison between [Fe₃O₄@triazole-Schiff-base-Pd(0)] and the most effective methods documented in the literature reveals several drawbacks of conventional catalysts (Table 3). Many traditional catalysts depend on expensive, toxic, and phosphine-based ligands, with most being homogeneous and therefore non-reusable. Moreover, these homogeneous catalysts typically require high-cost metals such as palladium, iridium, rhodium, and ruthenium, extended reaction times, hazardous reagents or solvents, and additional purification steps. In contrast, [Fe₃O₄@triazole-Schiff-base-Pd(0)] presents a more sustainable solution. It functions effectively under eco-friendly conditions, achieving high yields within short reaction times. Additionally, its synthesis involves inexpensive, green reagents and ligands through a straightforward and scalable process while exhibiting minimal metal leaching. These benefits make it a highly promising, efficient, and environmentally friendly alternative to traditional catalytic methods.

Table 3 Comparison of the [Fe₃O₄@triazole-Schiff-base-Pd(0)] catalytic activity in synthesis of butyl cinnamate via the Mizoroki–Heck C–C coupling reaction

Entry	Catalyst	Time (min)	Yield (%)	Reference
1	PdBr2-bis(N-heterocyclic carbene)	1440	99	49
2	Cyclometallated Pd(II) complex	25	97	50
3	Pd(II)–SBA-16	60	97	51
4	Bis(oxamato)palladate(II)	180	94	52
5	Fe3O4@MCM-41@Pd(0)–P2C	20	94	53
6	Spent automotive catalyst (s, containing Rh, Pd and Pt nanoparticles)	240	100	54
7	[{IrCl(cod)}2]	24	71	55
8	(SP-5-41)-[1,3-Bis(2,4,6-trimethylphenyl)-2-imidazolidinylidene]dichloro[[2-(1-methylethoxy-κO)phenyl]methylene-κC]ruthenium	720	82	56
9	SiO2–Rh^0	600	82	57
10	[Fe3O4@triazole-Schiff-base-Pd(0)]	25	98	This work

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4 Conclusions

In conclusion, a novel, magnetically recoverable palladium(0) Schiff base nanocomposite was synthesized via a facile post-synthetic modification. The resulting catalyst, comprising a magnetite core functionalized with a Schiff base ligand for palladium coordination, demonstrated effective performance as a reusable heterogeneous catalyst. Comprehensive characterization confirmed successful synthesis and structural integrity. FT-IR validated the ligand immobilization, XRD revealed high crystallinity, and FE-SEM and TEM confirmed uniform, spherical nano-sized particles. TGA indicated robust thermal stability up to 200 °C, while EDX, ICP-OES, elemental mapping, and VSM analyses substantiated the anticipated elemental composition and superparamagnetic properties. The [Fe₃O₄@triazole-Schiff-base-Pd(0)]catalyst exhibited exceptional catalytic activity in the Mizoroki–Heck C–C coupling reaction, efficiently synthesizing butyl cinnamates from aryl halides and butyl acrylate in PEG-400 at 120 °C, achieving high yields. Notably, the catalyst displayed excellent reusability with minimal palladium leaching, demonstrating its stability and practical applicability. These results highlight the potential of this nanomagnetic palladium catalyst for a range of catalytic applications.

Data availability

The authors declare that all the data are available within the paper.

Author contributions

Asma Ahmad Nashawi and Jawza A. Almutairi: characterization of the catalyst and laboratory studies. Fatimah Mohammed Alshamsan: writing the original draft and laboratory studies. Yassin T. H. Mehdar: conceptualization, analysis, laboratory studies, review of the draft, acquiring research funding and supervision. Hussein Eledum and Ahmed Mohajja Alshammari: conceptualization, analysis, software and review/editing.

Conflicts of interest

The authors declare that they have no competing interests.

Acknowledgements

The authors are thankful to Princess Nourah Bint Abdulrahman University for financially supporting this work through the Researchers Supporting Project Number PNURSP2025R892.

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