Leila Ma'mania,
Simin Mirib,
Mohammad Mahdavia,
Saeed Bahadorikhalilib,
Elham Lotfia,
Alireza Foroumadia and
Abbas Shafiee*c
aPharmaceutical Science Research Center, Tehran University of Medical Sciences, Tehran, 14176, Iran
bDepartment of Chemistry, College of Science, University of Tehran, P. O. Box: 14155-6455, Tehran, Iran
cDepartment of Medical Chemistry, Faculty of Pharmacy and Pharmaceutical Science Research Center, University of Medical Sciences, Tehran, 14176, Iran. E-mail: shafieea@tums.ac.ir; Fax: +98 21-66461175
First published on 22nd September 2014
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.
Guanidine and its derivatives are very important biologically active organic compounds. Guanidine (Gu) and aminoguanidine (AGu) are highly basic amines with pKa 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 carbon38–40 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.
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.
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).
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)).
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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.
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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 CO 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.
Entry | Solvent | Base | T (°C) | T (h) | Yieldb (%) |
---|---|---|---|---|---|
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 |
4c | EtOH | K2CO3 | 70 | 2 | 97 |
5 | EtOH | K2CO3 | RT | 3 | 94 |
6 | H2O | K2CO3 | RT | 3.5 | 80 |
7 | EtOH–H2O (1![]() ![]() |
K2CO3 | RT | 2 | 92 |
8 | EtOH–H2O (1![]() ![]() |
NEt3 | RT | 3 | 85 |
9 | EtOH–H2O (1![]() ![]() |
NaOH | RT | 3 | 90 |
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.
Entry | Z | R | X | Yieldb (%) |
---|---|---|---|---|
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 |
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.
Entry | Solvent | T (°C) | T (h) | Yieldb (%) |
---|---|---|---|---|
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 |
3c | EtOH | 70 | 1.0 | 92 |
4 | EtOH | RT | 2.0 | 89 |
5 | EtOH–H2O (1![]() ![]() |
70 | 1.5 | 95 |
6 | H2O | 70 | 1.0 | 95 |
7 | H2O | RT | 1.0 | 90 |
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.
Entry | R1 | X | R2 | Yieldb (%) |
---|---|---|---|---|
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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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 At and A0, and Ct and C0 were the concentration at time “t” and t = 0, respectively, therefore the ratio of At to A0 is proportional to Ct to C0 and the kinetic equation for the reduction can be written as
dct/dt = kappCt or ln![]() ![]() |
As shown in Fig. 5, there is a linear relationship between ln(At/A0) and time reaction in the reduction reaction of 4NP catalyzed by Pd@AGu@MGO nanosheets, which the kapp (apparent rate constant) is estimated directly from the slope of the straight line obtained by plotting ln(At/A0) as a function of time. For Pd@AGu@MGO nanocatalyst, the rate constant k is calculated to be 0.49 s−1.
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 22770 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).
Footnote |
† Electronic supplementary information (ESI) available: NMR spectral data and catalyst characterization data. See DOI: 10.1039/c4ra07130a |
This journal is © The Royal Society of Chemistry 2014 |