Palladium catalyst supported on N-aminoguanidine functionalized magnetic graphene oxide as a robust water-tolerant and versatile nanocatalyst

Abstract

A novel heterogeneous Pd catalyst has been developed by decorating palladium onto the surface of N-aminoguanidine functionalized magnetic graphene oxide nanosheets (denoted as Pd@AGu-MGO), while the diethylene glycol (DEG) group has been applied as an organic spacer. The usefulness of the Pd@AGu-MGO nanocatalyst was investigated in palladium catalyzed organic reactions including Heck/Suzuki couplings of aryl halides and reduction of 4-nitrophenol (4-NP) to 4-aminophenol (4-AP). The nanocatalyst showed high efficiency and thermal stability (up to 300 °C). The catalyst recovery test was performed using an external magnet device, and showed that this catalyst can be reused several times without a significant decrease in its performance and catalytic activity. The loading level of Pd in the Pd@AGu-MGO catalyst was assessed as 0.9 mmol g⁻¹ by ICP-AES and elemental analysis. This catalyst is more remarkable from the environmental and economic points of view because of its key properties such as high efficiency and turnover frequency (TOF), mild reaction conditions, utilization of green solvent, simple product work-up, and easy catalyst recovery.

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Introduction

An interesting goal in clean chemistry is the development of simple synthetic methods to provide new and improved catalysts.¹ Among the different metal catalysts, palladium is increasingly used for different carbon–carbon bond forming reactions such as the Mizoroki–Heck and Suzuki–Miyaura coupling reaction etc.^2–4 The catalyst recovery as well as recycling of the expensive palladium catalyst are of great importance from the economic and environmental perspective. Therefore, recently, a number of strategies have been used to develop an immobilized Pd catalyst on solid supports in order to take advantage of both homogeneous and heterogeneous catalysts to overcome the drawbacks of homogeneous Pd catalysts.^5–7 Among these materials, graphene oxide (GO), a one-atom-thick two dimensional sheet having honeycomb sp²-bonded carbon, has attracted a great deal of interest for different potential utilizations in theoretical and experimental studies in catalysis, electronics, and material sciences.^8–12 Magnetic graphene oxide (MGO), one of the hottest materials in chemistry and material sciences, has been studied extensively due to its fantastic thermal, mechanical, and chemical properties and its unique potential technical applications.^13–20 Its remarkable characteristics such as numerous oxygen-containing groups (e.g., –OH, –COOH, and epoxy), and extremely high surface areas amenable to ligand conjugation and other manipulations enable it to be a promising candidate as a brilliant support to immobilize metal nanoparticles (NPs) for applying in catalyst development. However, after their discovery, constructing highly efficient magnetic graphene oxide immobilized nanohybrids remains a great challenge to date.

Guanidine and its derivatives are very important biologically active organic compounds. Guanidine (Gu) and aminoguanidine (AGu) are highly basic amines with pK_a values of 13.5 and 11.04, respectively.^21,22 Accordingly, aminoguanidine appearing as an effective ligand in aminoguanidine functionalized persimmon tannin gel, shows remarkable selectivity towards Au(III) followed by Pd(II) and Pt(IV).²³ The key features of palladium–guanidine complex including the air-stability, low toxicity, low cost and easy accessibility (via chemical synthesis) are advantages that make it more efficient and potential ligand for practical applications compared with the well-established N-heterocyclic carbine, phosphine, and palladacyclic complex systems. However, to the best of our knowledge, there has been little report about the heterogeneous Pd–aminoguanidine complex catalyst. As regards to the utility of the C–C coupling reactions such as the Mizoroki–Heck reaction²⁴ and the Suzuki–Miyaura reaction,^25,26 several Pd nanoparticle catalytic system have been developed in the literature.^27–29 There are numerous immobilization or heterogenization methodologies of Pd NPs based on polymers,^30–32 glass–polymer composite materials,³³ ionic liquids,^34,35 and organic–inorganic fluorinated hybrid materials³⁶ as well as common inorganic materials such as alumina,³⁷ carbon^38–40 or silica.^41,42 Also, recently several examples of Pd catalysts such as Pd–rGO–CNT nanocomposite, and etc. has shown excellent catalytic properties; they also catalyzed the reduction of 4-nitrophenol (4-NP) by NaBH₄ to 4-aminophenol (4-AP).^43–47 However, the facile and green synthesis of novel Pd–NP catalysts having further improved catalytic activity remains a challenge. This study aims to develop a more efficient and recyclable heterogeneous catalyst for the Mizoroki–Heck, Suzuki–Miyaura reaction, and also for the reduction of 4-nitrophenol (4-NP). Therefore, it was tried to create a novel heterogenous palladium catalyst based on aminoguanidine functionalized magnetic graphene oxide (AGu@MGO), and evaluate its performance in aforementioned reactions.

Experimental section

Preparation of graphene oxide

According to the literature, the GO was manufactured from graphite using modified Hummer's method.⁴⁸ Briefly, 1 g of graphite powder was added to a continuous stirring solution of 50 mL H₂SO₄ 98% in an ice bath and then, 2 g KMnO₄ was gently added to it. It should be noted that the rate of addition was carefully controlled to avoid a sudden increase of mixture temperature. The reaction mixture was left to stir for 2 h at temperatures below 10 °C, and followed by another 1 h at 35 °C. Then the reaction mixture was diluted with 50 mL of deionized water (DW) in an ice bath and the temperature was kept below 100 °C. After 1 h of stirring, the mixture was further diluted to 150 mL with DW. Then, 10 mL of H₂O₂ 30% was added to the mixture which changing its color to brilliant yellow. The resulting solid was separated by centrifuge and washed thoroughly with 5% HCl aq. solution and DW to neutralize. Finally the resultant was dried at 60 °C for 24 h.

Preparation of magnetic graphene oxide (MGO)

The MGO nanosheets were synthesized by co-precipitation of Fe³⁺ and Fe²⁺ in the presence of GO.⁴⁹ The solution of Fe³⁺ and Fe²⁺ was prepared in a 2 : 1 mole ratio. 50 mg of GO was dispersed in 40 mL of water by sonication for 45 min. Then a 50 mL solution of FeCl₃ (1 g) and FeCl₂ (375 mg) in DW was added to the later solution. The reaction temperature was raised to 85 °C and ammonia solution 30% was added to it increasing the pH to 10. After 45 min of vigorously stirring, the solution was cooled to r.t., and finally the obtained black precipitate was centrifuged at 6000 rpm for 10 min and washed thoroughly with DW and dried at 60 °C for 24 h.

Preparation of aminoguanidine functionalized magnetic graphene oxide (AGu@MGO)

To produce the aminoguanidine functionalized magnetic graphene oxide (AGu@MGO) nanosheets, we first modified the surface of MGO with DEG (diethylene glycol). In order to this purpose, the necessary carboxylic acid functional groups were introduced by taking 50 mg of MGO in DW (∼4 mg mL⁻¹) and sonicating it for 1 h and then 50 mL NaOH 1 M was added to it and sonicated for 3 h. Afterwards, HCl was added to neutralize and the solution filtered and then rinsed.⁵⁰

Then, 50 mg of the modified MGO was homogenized in 70 mL DW by ultrasonication for 10 min. Next, 10 mg of 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide (EDC) and 8 mg of N-hydroxysuccinnimide (NHS) were added into the later solution. The above mixture was stirred for 1 h and homogenized by ultrasonication for another 30 min. Then, 2.5 mmol of H₂N–DEG–OTs was added into the suspension and sonicated for 30 min. Finally, the reaction was carried out at 90 °C for 1 h under stirring. The obtained OTs–DEG–MGO was purified by magnetic separation and washed with water several times. At the end, 50 mg of OTs–DEG–MGO product, 2.5 mmol of aminoguanidine hydrochloride (AGu· HCl), and 2.0 g of sodium carbonate in 50 mL DMF were stirred at 90 °C overnight. The obtained solid was separated by an external magnet. The product was successively washed with DW until neutralization and dried under vacuum at 60 °C for 24 h to achieve aminoguanidine functionalized magnetic graphene oxide (AGu@MGO) nanosheets.

Preparation of Pd catalyst supported on aminoguanidine functionalized graphene oxide (Pd@AGu@MGO)

1 mmol of PdCl₄²⁻ was added to a solution of 100 mg of AGu@MGO in dry acetone under inert atmosphere. The mixture was stirred at 40 °C for 24 h and then cooled to r. t. The final product, which named as Pd@AGu@MGO was collected by centrifugation, washed thoroughly with EtOH and diethyl ether, and dried under vacuum at r.t. for 12 h.

General procedure for Heck reaction

The mixture of aryl halide (1.0 mmol), alkene (1.1 mmol), K₂CO₃ (2.0 mmol) and catalytic amount of Pd@AGu@MGO (10 mg) was taken in a round-bottom flask, was stirred in EtOH–H₂O (1 : 1) (3.0 mL) at r.t. and the reaction progress was monitored by TLC. At the end of reaction, liquid was isolated by magnetic decantation. The liquid was poured into distilled water (20 mL) and the product was extracted with ethyl acetate. The organic phase was dried over Na₂SO₄. The combined organic phase was evaporated and the column chromatography on silica gel using n-hexane–ethyl acetate (5 : 1) as eluent gave the purified coupled products. The recovered catalyst was rinsed with water and EtOH, dried at r.t. and used without any pretreatment for the next runs. The recycling test of the catalyst was performed in the reaction between bromobenzene and n-butyl acrylate according to the above procedure.

General procedure for Suzuki reaction

The mixture of aryl halide (1.0 mmol), aryl boronic acid (1.1 mmol), K₂CO₃ (2.0 mmol) and the catalytic amount of Pd@AGu@MGO (10 mg) was taken in a round-bottom flask, allowed to stirred in H₂O (3.0 mL) at r.t. and then, TLC was monitored the reaction progress. After the reaction, the catalyst was separated from reaction mixture by an external magnet device. Then, the residual mixture was poured into distilled water (20 mL) and the product was extracted with ethyl acetate. The organic phase was washed with brine, dried by anhydrous sodium sulfate (Na₂SO₄), filtered, and then passed through celite. Next, the organic phase was evaporated and the residue was purified by column chromatography on silica gel using n-hexane–ethyl acetate (5 : 1) as eluent to obtain the corresponding products.

Catalytic reduction of 4-nitrophenol (4-NP) to 4-aminophenol (4-AP)

2.5 mL of fresh NaBH₄ (1.2 M) was added to a 10 mL solution of 3.4 mmol 4-NP which lead to a color change from light yellow to yellow-green. Then was added 0.1 mg of nanocatalyst to it and stirred it. The color of the later solution was faded as the reaction proceeded. To UV-visible spectroscopy measurement the supernatant was transferred to a quartz cuvette. To monitor the progress of the reaction, UV-vis spectra were recorded at short intervals (a few seconds). This procedure was repeated for blank experiments to show that the reactions do not proceed without this catalyst only in the presence of NaBH₄.

Results and discussion

Catalyst preparation and characterization

The construction process of Pd@AGu@MGO nanosheets as a robust magnetic catalyst is schematically depicted in Scheme 1. The GO was prepared from purified natural graphite using modified Hummer's method,⁴⁸ and then the Fe₃O₄@GO NPs was manufactured from obtained GO nanosheets following a previously reported protocol.⁴⁹ Then the surface of magnetic graphene oxide (MGO) was organically modified by grafting the aminoguanidine using diethylene glycol (DEG) moiety as a proper linker. Aminoguanidine ligand is capable to form 5-membered chelate complex with metals due to its brilliant structurally ability. The electroneutral N-aminoguanidine can coordinate to metal by the N₄ amino group and the deprotonated N₁ imino group that lead to create 5-membered chelate complex with Pd chloro complexes.²³

Scheme 1 Schematic representation of the construction procedure for Pd@AGu@MGO.

The Pd content of the obtained solid catalyst was 0.9 mmol per gram of Pd@AGu@MGO catalyst using the inductively couple plasma-atomic emission spectrometry (ICP-AES). The morphology of Pd@AGu@MGO was characterized by transmission electron microscopy (TEM). As shown in the TEM images, magnetic NPs with diameters less than 10 nm are deposited on the surface of MGO nanosheets (Fig. 1).

Fig. 1 TEM image of AGu@MGO nanosheets.

The magnetic hysteresis measurement was done by vibrating sample magnetometer (VSM) in an applied magnetic field at r.t, with the field sweeping from −8000 to +8000 Oe to assay the magnetic behaviour of Pd@AGu@MGO nanosheets. The superparamagnetic nature of Pd@AGu@MGO has been shown by its S-type magnetization hysteresis loop (Fig. 2(a)). The images of Pd@AGu@MGO in an aqueous solution with and without external magnet, firmly demonstrate the excellent and sufficient magnetization for its magnetic separation with a conventional magnetic field (Fig. 2(b)).

Fig. 2 (a) The magnetization hysteresis loop of AGu@MGO nanosheets, and (b) magnetic separation of Pd@AGu@MGO nanosheets.

The structural properties of the synthesized Pd@AGu@MGO catalyst was analyzed and compared with the graphene oxide (GO) by XRD (Fig. 3(a)). The XRD pattern of GO shows two diffraction characteristic peaks at 10.58° and 22.29° which are corresponding to the (001) and (002) planes of GO. The XRD peaks of the Pd@AGu@MGO nanohybrid are indexed to 220, 311, 400, 422, 511 and 440 planes of a cubic unit cell of magnetite, appearing at 34.97°, 43.53°, 50.41°, 57.81°, 67.29°, and 72.32°, respectively. Moreover, the appeared peaks at 2θ of 40.1°, 47.5° and 68.0° which match to 111, 200 and 220 crystalline planes of Pd in XRD pattern, confirm the successfully Pd catalyst supported on AGu@MGO.

Fig. 3 (a) The XRD pattern, (b) N₂ adsorption–desorption isotherm, and (c) FTIR spectra of nanoparticles.

The surface area of the synthesized solids was determined by BET (Brunauer–Emmett–Teller). The N₂ adsorption–desorption isotherm of GO and AGu@MGO is shown in Fig. 3(b). The surface area of GO and AGu@MGO are 50.96 m² g⁻¹ and 31.56 m² g⁻¹, respectively. The GO, GO functionalized with Fe₃O₄ NPs (MGO), and aminoguanidine functionalized MGO (AGu@MGO) are confirmed by the FTIR spectroscopy (Fig. 3(c)). In the IR spectrum of GO, the strong bands at 3430 and 1246 cm⁻¹ are attributed to the stretching and bending of the O–H bond, respectively. The bands appeared at 1725 and 1630 cm⁻¹ can be assigned to the vibration of C=O bond in carboxylic acid (or carbonyl moieties) and aromatic C=C bond in unoxidized graphitic domains, respectively. The peak of bending absorption of carboxyl group O=C–O appears at 1365 cm⁻¹. The existence of Fe₃O₄ is confirmed by the appearance of the vibration band of Fe–O around 572 cm⁻¹, in the FTIR spectrum of GO functionalized with Fe₃O₄ NPs (MGO). Further, the observed decreases in the intensity of the peaks of C=O and O–H at 1725 and 3430 cm⁻¹ confirm the existence of Fe₃O₄. The appearance of such bands in FTIR spectrum of AGu@MGO is in agreement with the attachment of AGu to the MGO. The bands at 2950, 2870, and 1025 cm⁻¹ are due to the stretching and bending vibration of aliphatic CH₂ in the organic pendant group attached to the surface, and the band at 1470 cm⁻¹ assigned to the C–N stretching of guanidine.

Catalyst activity in the Heck/Suzuki reactions

The catalytic performance of Pd@AGu@MGO was surveyed in the Heck coupling reaction. The Heck reaction between bromobenzene and styrene was considered as a model reaction to assay the catalytic potential of Pd@AGu@MGO and determine the optimized reaction conditions (Table 1).

Table 1 The optimization of the Heck reaction^a

Entry	Solvent	Base	T (°C)	T (h)	Yield^b (%)
a Reaction conditions: bromobenzene (1.0 mmol), alkene (1.1 mmol), base (2.0 mmol), solvent (3.0 mL), cat. (10 mg).b Isolated yields.c Cat. (20 mg).
1	DMF	K2CO3	120	0.5	95
2	DMF	K2CO3	RT	1	90
3	EtOH	K2CO3	70	2	95
4^c	EtOH	K2CO3	70	2	97
5	EtOH	K2CO3	RT	3	94
6	H2O	K2CO3	RT	3.5	80
7	EtOH–H2O (1 : 1)	K2CO3	RT	2	92
8	EtOH–H2O (1 : 1)	NEt3	RT	3	85
9	EtOH–H2O (1 : 1)	NaOH	RT	3	90

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Clearly, in the Heck C–C coupling reaction, bases have important roles in catalyst regeneration and beta hydrogen elimination. Therefore, different bases such as NEt₃, K₂CO₃, and NaOH were examined. Among the screened bases, K₂CO₃ was found to be the best proper base for this catalytic system. Also, the solvent screening exhibited that among different solvents, EtOH–H₂O (v/v = 1 : 1) was the most efficient solvent for this purpose. The best results for C–C coupling reaction catalyzed by Pd@AGu@MGO (0.01 g, 0.009 mmol Pd) were achieved in EtOH–H₂O (v/v = 1 : 1) solvent in the presence of K₂CO₃ at room temperature for 2 h. The optimized reaction condition was applied to the reaction of different aryl iodides and bromides with arylboronic acids. According to the results shown in Table 2, the generality of this protocol using the synthesized catalytic system is confirmed and excellent yields are obtained in the presence of 0.01 g of Pd immobilized AGu@MGO nanosheets.

Table 2 Heck reaction between different substrates using Pd@AGu@MGO as catalyst^a

Entry	Z	R	X	Yield^b (%)
a Reaction conditions: aryl halide (1.0 mmol), alkene (1.1 mmol),K2CO3 (2.0 mmol), solvent (EtOH–H2O, 3.0 mL), cat. (10 mg).b Isolated yields.
1	Ph	H	Br	92
2	Ph	4-NO2	Br	94
3	Ph	4-Me	Br	93
4	Ph	4-OMe	I	96
5	Ph	2-Me	Br	92
6	CO2Bu	H	Br	93
7	CO2Bu	4-Me	Br	95
8	CO2Bu	4-OMe	I	96
9	CO2Bu	2-Me	Br	92
10	CO2Bu	4-NO2	Br	91

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Inspired by the high activity and stability of Pd@AGu@MGO, the Suzuki coupling reaction of bromobenzene with phenylboronic acid was further employed as another model reaction to test the further performance of Pd@AGu@MGO nanocatalyst. To optimize the reaction conditions, series of experiments under different solvent, temperatures and quantities of catalyst was carried out (Table 3). According to the optimization results and the pivotal goal of green chemistry, the ideal result is obtained by using catalytic amount of Pd@AGu@MGO (0.01 g, 0.009 mmol Pd) and H₂O as a green solvent at room temperature; which the yield can reach as high as 100% in 1 h.

Table 3 Optimization of the Suzuki reaction^a

Entry	Solvent	T (°C)	T (h)	Yield^b (%)
a Reaction conditions: bromobenzene (1.0 mmol), phenyl boronic acid (1.1 mmol), K2CO3 (2.0 mmol), solvent (3.0 mL), cat. (10 mg).b Checked by GC analysis.c Cat. (20 mg).
1	DMF	120	>0.5	97
2	EtOH	70	1.0	94
3^c	EtOH	70	1.0	92
4	EtOH	RT	2.0	89
5	EtOH–H2O (1 : 1)	70	1.5	95
6	H2O	70	1.0	95
7	H2O	RT	1.0	90

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The generality of this protocol is confirmed and excellent yields are achieved in the presence of 0.01 g of Pd@AGu@MGO nanosheets (Table 4). Furthermore, there is no byproduct in this reaction using Pd@AGu@MGO as catalyst. Both aryl halides bearing electron-withdrawing or electron-donating groups can be converted into the corresponding coupling products with conversion yields ranging from good to excellent. The Pd@AGu@MGO nanohybrid retains a reasonable performance after ten cycles. The recovery of Pd@AGu@MGO nanocatalyst can be easily achieved by an external magnetic device and washing with acetone and water for several times. The yield is as high as 90% after ten cycles. The morphologies of the recovered catalysts were examined by TEM; the typical images indicate that NPs are well dispersed on graphene surfaces after ten cycles.

Table 4 The Suzuki reaction using Pd@AGu@MGO as catalyst^a

Entry	R^1	X	R^2	Yield^b (%)
a Reaction conditions: bromobenzene (1.0 mmol), aryl boronic acid (1.1 mmol), K2CO3 (2.0 mmol), solvent (3.0 mL), cat. (10 mg).b Isolated yields.
1	4-Me	Br	4-OMe	94
2	4-NO2	Br	4-OMe	90
3	4-Me	Br	2-NO2	87
4	4-Me	Br	4-F	86
5	4-NO2	Br	4-F	85
6	4-NO2	Br	4-NO2	90
7	H	Br	4-OMe	90
8	H	I	4-OMe	96
9	4-Me	I	3-NO2	91
10	4-NO2	I	4-OMe	86
11	4-OMe	I	4-OMe	94
12	2-Me	I	4-OMe	91
13	2-CF3	I	4-OMe	85
14	2-Me	I	H	93
15	H	Br	H	96
16	H	I	H	95
17	4-Me	I	H	94
18	2-Me	Br	4-OMe	91
19	4-NO2	Br	H	96
20	4-NO2	I	H	92
21	4-Me	Br	H	93
22	4-OMe	I	H	86

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Catalyst activity in the reduction of 4-nitrophenol

As 4-nitrophenol (4-NP) has carcinogenic, mutagenic, and cyto- and embryonic toxic effects has been classified as a priority pollutant by US Environmental Protection Agency (EPA), there is an urgent need to develop an efficient and environmentally clean method for its degradation. Therefore, to reveal the potential of the Pd@AGu@MGO catalyst, we evaluated the catalytic ability of this catalyst in the reduction of 4-NP to 4-AP by NaBH₄ in an aqueous medium. The addition of Pd@AGu@MGO nanocatalyst led to fade and ultimately a bleaching of the yellow colour of the aqueous solution of 4-NP at room temperature. The absorption of 4-NP at 400 nm decreased rapidly, with a concomitant increase in the intensity of a new peak at 290 nm in UV-vis spectra; this peak is assigned to 4-AP (Fig. 4). The reduction reaction with the Pd@AGu@MGO nanocatalyst carried out in only 70 s, even though the Pd content of the catalyst was as low as 0.001 g (∼0.0009 mmol of Pd). We investigated the stability of Pd@AGu@MGO nanocatalyst by performing the same reduction reaction with the same catalyst 20 times. This catalyst also shows high activity after twenty successive cycles of reactions, with 99% conversion within a reaction period of 200 s, which demonstrates the performance of the Pd@AGu@MGO nanocatalyst.

Fig. 4 Successive UV-vis spectra of the reduction of 4-NP by Pd@AGu@MGO nanocatalyst, condition: cat. (0.001 g), NaBH₄ and H₂O as reducing agent and solvent, respectively, and at r.t.

The reaction is of second order, but since the concentration of NaBH₄, [NaBH₄], is higher than that of 4-NP, the reduction rate can be regarded independent from [NaBH₄]. Therefore, this exclusivity permits that the rate constants of 4-NP reduction to evaluate by the pseudo-first-order kinetics. As regards, the absorbance of 4-NP indicates its concentration in the reaction medium, and where the absorbance at time “t” and t = 0 is respectively showed by A_t and A₀, and C_t and C₀ were the concentration at time “t” and t = 0, respectively, therefore the ratio of A_t to A₀ is proportional to C_t to C₀ and the kinetic equation for the reduction can be written as

dc_t/dt = k_appC_t or ln C_t/C₀ = ln A_t/A₀ = k_appt.

As shown in Fig. 5, there is a linear relationship between ln(A_t/A₀) and time reaction in the reduction reaction of 4NP catalyzed by Pd@AGu@MGO nanosheets, which the k_app (apparent rate constant) is estimated directly from the slope of the straight line obtained by plotting ln(A_t/A₀) as a function of time. For Pd@AGu@MGO nanocatalyst, the rate constant k is calculated to be 0.49 s⁻¹.

Fig. 5 The plot of ln(A_t/A₀) versus time, the rate constant correspond to the slope.

These results show that Pd@AGu@MGO catalyst possesses a green, unique mechanical strength, and excellent durability, and hereupon this report exhibits remarkably high and stable catalytic activity toward different reactions such as Heck/Suzuki reaction and reduction reaction of 4-NP. The catalyst recovery results confirms the excellent stability of the Pd@AGu@MGO, allowing 10 successive cycles of Heck/Suzuki reactions and also 20 successive cycles of 4-NP reduction reaction that leads to the cumulative turnover numbers (TONs) around 1005 and 9565, respectively. Indeed, after mentioned successive cycles, the obtained cumulative turnover frequencies (TOFs) is approximately 500, 1000, and 22 770 for the Heck, Suzuki, and 4-NP reduction reactions, respectively. TON was calculated using the following formula.

TON = (moles of substrate converted/mole Pd).

And the cumulative TON for the recycle sequences at any given time is calculated by adding values of TON for all cycles. Afterwards, mean TOF is calculated as the ratio of cumulative TON/time taken to calculate cumulative TON, (TON per h).

It was investigated that the highly active Pd species are generated in situ by dissolution of Pd species from the support and stabilized against agglomeration via re-precipitation on the surface of the support.^51–53 To rule out the existence of leached Pd species in during the reaction process, a sample of 10 mg of Pd@AGu@MGO catalyst in 10 mL water prepared and left to stir at 90 °C for 1 h, and then the catalyst was removed (by filtration or magnetically separation). Furthermore, we found that the filtrate showed no catalytic activity in the Heck reaction (the model reaction) under the optimal reaction condition. It is also worth mentioning that the amount of leached Pd was less than 1 ppm for the separated liquid (measured by ICP-AES).

Conclusions

In this study, we have reported the preparation of palladium immobilized on N-aminoguanidine functionalized magnetic graphene oxide (Pd@AGu@MGO) nanosheets. The study has been focused on the preparation and characterization of the Pd@AGu@MGO nanosheets and finally, its catalytic activity has been evaluated in the Pd catalyzed organic reactions. Here, the significance of the structure of the supporting material for dispersion of the catalyst has been fully concerned. It is found that AGu@MGO appears as an impressive and efficient supporting material for immobilization of Pd catalyst. Aminoguanidine as a brilliant chelating group was grafted to the surface of magnetic graphene oxide with diethylene glycol as spacer. We believe that this unique and reliable route to manufacture Pd@AGu@MGO as catalyst can successfully combine the pivotal properties of GO, magnetic nanoparticles, and homogenous Pd catalyst in the nanohybrid catalyst. It is notable that TOF values evidence that the developed nanocatalyst has an enhanced catalytic behavior with respect to literature data. The resultant Pd@AGu@MGO catalyst possesses high efficiency, coupling with the simplicity of the production, the suitability of this method for large-scale production, and the green synthesis approach, allow a variety of industrial applications in the catalysis, environmental protection, electrocatalysis, and biomedical applications.

Acknowledgements

This research was supported by grants from the research council of Tehran University of Medical Sciences and INSF (Iran National Science Foundation).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: NMR spectral data and catalyst characterization data. See DOI: 10.1039/c4ra07130a

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