Preparation and characterization of Pd supported on 5-carboxyoxindole functionalized cell@Fe3O4 nanoparticles as a novel magnetic catalyst for the Heck reaction

Pd supported on 5-carboxyoxindole functionalized cell@Fe3O4 nanoparticles (Pd@CAI@cell@Fe3O4), a new magnetic nanocatalyst, was prepared and characterized using inductively coupled plasma atomic emission spectroscopy, Fourier-transform infrared spectroscopy, X-ray diffraction, scanning electron microscopy, transmission electron microscopy, thermogravimetric analysis, and energy-dispersive X-ray spectroscopy techniques. The synthesized nanocatalyst (Pd@CAI@cell@Fe3O4) was employed for Heck-type arylation of different substituted maleimides with iodoarenes in good to excellent yields. This green catalyst was easily recovered and reused several times with no substantial loss of activity, providing a clean and efficient synthetic procedure with excellent yield and reduced time.


Introduction
Nanocatalysis has emerged as a eld at the interface between homogeneous and heterogeneous catalysis and suggests exceptional solutions to the various industries for catalyst enhancement. 1,2 In the meantime, heterogeneous catalysis is one of the oldest commercial implementations of nanoscience, and nanoparticles of metals, semiconductors, oxides, and other compounds have been extensively used for signicant chemical reactions. The principal focus of this eld is the evolution of catalysts that include both metal nanoparticles and a nanomaterials as the main support. In fact, these nanocatalysts have high specic surface area and surface energy, which result in their high catalytic activity. Also, they have other properties, such as improving the selectivity of the reactions while reducing the reaction temperature, minimizing side reactions, and with higher recycling rates. Hence, these catalysis are of great interest for researchers in the synthesis of many organic compounds. 3 Recently, the majority of scientists increasingly employ biopolymer supports from renewable, durable, and abundant resources as attractive materials for catalysis reactions. 4,5 Nanocellulose is the most noteworthy and abundant biopolymer, and it is obtained from plants, bacteria and algae; its known properties include hydrophobicity, biodegradability, economy, biocompatibility, and wide chemical-functionalization capacity. 6 These abilities of cellulose make it an interesting support, and its application as an efficient support for the catalytic processes in the synthesis of many organic compounds has been studied. 6 In fact, supported nanocellulose is emerging as an attractive protocol to stabilize some transition metal complexes. [6][7][8][9][10][11][12] In recent times, nanocellulose supporting palladium, platinum, zirconium, copper and nickel nanoparticles have been studied and reported. [13][14][15][16] Also, nanocellulose is one of the most perfect coating supports for magnetic nanoparticles (MNPs) due to its ability to not only stabilize nanoparticles in solution but also promote functionalization to produce biopolymer-based catalysts. 17 In fact, MNPs have attracted great attention due to their unique properties, including eco-friendliness, high exibility, low toxicity, low Curie temperature, easy preparation and functionalization, large surface-to-volume ratio, facile separation using an external magnet, and a high degree of chemical stability. 18,19 The design of new, magnetically separable systems has generated a lot of attention in recent years as an attractive candidate to improve the efficient separation of heterogeneous nanocatalysts from products by their response to an external magnetic eld. 20,21 In other words, in order to control the fast oxidation and the tendency of MNPs to agglomerate, their surface is generally protected with organic, inorganic or biopolymeric materials to form core shell structures, which have thus been named bio-based magnetic nanocatalysts. 22 Heck cross-coupling reactions [23][24][25] involve the coupling of an unsaturated halide with an alkene in the presence of palladium catalyst (or palladium nanomaterial-based catalyst) to form a substituted alkene; they are also referred to as Mizoroki-Heck reactions. 26 The rst work of Heck-type coupling was reported by Hacksell and Daves in 1985, 27 with a new update in 1990. 28 In fact, Heck reactions are very signicant in industry, since substitution reactions can be accomplished on planar centers. 29 The rst time the Heck reaction was discovered it was by the American chemist Richard F. Heck, 29 and also Mizoroki, who considerably developed this reaction in organic chemistry. 30 The C-H functionalization of alkenes is presently under intensive investigation as a direct carbon-carbon bond-forming procedure for the preparation of higher substituted alkenes. The prevalently used Heck reaction enables direct C-H to C-C functionalization of alkenes with aryl and vinyl halides, but is rarely performed with alkyl halide coupling partners due to the propensity of the alkyl group to undergo b-hydride elimination. 31,32 In continuation of our research on nanomagnetic supports, [33][34][35] according to the above-mentioned explanations, in this work, the connected palladium supported on 5-carboxyoxindole functionalized cell@Fe 3 O 4 (Pd@CAI@cell@Fe 3 O 4 ), an efficient and interesting bio-based magnetic nanocatalyst, was prepared. Then, this nanocatalyst was employed for the Heck-type arylation of maleimides with iodoarenes (Scheme 1).

Chemicals and apparatus
Powder X-ray diffraction (PXRD) of the prepared catalyst was performed on a Philips PW 1830 X-ray diffractometer using a Cu Ka source (l ¼ 1.5418Å) in a Bragg's angle range of 10-80 at 25 C. Fourier-transform infrared (FT-IR) spectroscopy was carried out using a FT-IR spectrometer (Vector 22, Bruker) in the range of 400-4000 cm À1 at room temperature. Scanning electron microscopy (SEM) analysis was recorded using a VEGA// TESCAN KYKY-EM 3200 microscope (acceleration voltage 26 kV). Transmission electron microscopy (TEM) experiments were done on a Philips EM 208 electron microscope. The elemental analysis spectrum of the catalyst was assessed by energy dispersive X-ray (EDX) spectroscopy (VEGA3 XUM/TESCAN). Thermogravimetric analysis (TGA) was performed on a Stanton Red cra STA-780 (London, UK). Nuclear magnetic resonance (NMR) spectra were measured using a Bruker DRX-400 AVANCE instrument (300.1 MHz for 1 H, 75.4 MHz for 13 C) in DMSO-d 6 as a solvent. Magnetic measurements were carried out using a vibration sample magnetometer (VSM, MDK, and Model 7400). The metal loading was determined by inductively coupled plasma-atomic emission spectrometry (ICP-AES). Melting points were evaluated on electrothermal 9100 apparatus.
2.2. General procedure 2.2.1. Preparation of cellulose nanocrystals (cell). To prepare cellulose nanocrystals (cell), rst, acidic hydrolysis of Whatman lter paper was employed as reported in the literature with a slight modication. Hydrolysis of the cellulose was obtained aer 3 h at 100 C using 100 mL of 2.5 M HBr and alternating ultrasonication. Aer dilution with twice-distilled water, the mixture was subjected to ve washing/ centrifugation cycles to remove excess acid and water-soluble residues. Aer neutralization to around pH 5, the ne cellulose nanoparticles started to disperse into the aqueous supernatant and were collected by centrifugation at 12 000 rpm for 60 min to remove the ultrane particles.

Preparation of cell@Fe 3 O 4 .
Here, nanocellulose (5.0 g) was rst added to a solution containing FeCl 3 $6H 2 O (4.865 g, 0.018 mol), FeCl 2 $4H 2 O (1.789 g, 0.0089 mol) and 100 cm 3 of CH 3 OH/deionized water (50/50), then stirred for 3 h. Next, 10 mL of 25% NH 4 OH (10 mL) was added immediately into the reaction mixture in one portion under a N 2 atmosphere at 80 C, followed by vigorous stirring for around 30 min using a magnetic stirrer. The product was washed with deionized water four times (Scheme 2).
2.2.3. Preparation of CAI@cell@Fe 3 O 4 . 400 mg of Fe 3 -O 4 @cell was mixed with 400 mg of 5-carboxyoxindole (CAI) in 20 mL of DMSO and stirred for 24 h at 100 C. Aer the reaction was completed, the mixture was cooled to room temperature, and the obtained precipitate was ltered using an external magnet and rinsed with ethanol several times, then placed in an oven for 24 h to dry, thus the CAI@ Fe 3 O 4 @cell nanoparticles were obtained (Scheme 2).

Preparation of Pd@CAI@cell@Fe 3 O 4 nanomagnetic catalyst.
To prepare the catalyst, 0.50 g of CAI@Fe 3 O 4 @cell in DMF was added to a solution of Pd(Cl) 2 (0.10 g, 0.45 mmol) in 10 mL DMF under N 2 atmosphere, and the obtained mixture was stirred for 24 h at 60 C. Aer the reaction was completed, Scheme 1 Heck-type arylation with Pd@CAI@cell@Fe 3 O 4 as a nanomagnetic catalyst.
the mixture was cooled to room temperature, and the resulting product was collected using an external magnet. The solid black product was washed carefully with deionized water (3 Â 25 mL), absolute ether (2 Â 25 mL) and absolute ethyl alcohol (2 Â 25 mL), then dried in a vacuum oven at room temperature (Scheme 2).

Pd@CAI@cell@Fe 3 O 4 -catalyzed Heck-type arylation of maleimides with iodoarenes
A vial equipped with a stirrer bar was charged with different substituted maleimides (0.1 mmol, 1.0 eq.), iodoarene (0.2 mmol, 2.0 eq.), triethylamine (2.0 eq.) and Pd@CAI@cell@Fe 3 O 4 (10 mol%). Acetonitrile (10 mL) was then added, and the reaction mixture was vigorously stirred at 80 C for different lengths of time, according to each substrate. Upon completion of the reaction (as monitored by thin layer chromatography (TLC)), the reaction mixture was cooled to room temperature. Aer that, the reaction mixture was dissolved in dichloromethane (10 mL) and, subsequently, the Pd@CAI@cell@Fe 3 O 4 nanoparticle catalyst was separated by an external magnet at 5 min. Removal of the solvent under reduced pressure yielded a crude mixture, which was puried by column chromatography (hexanes/EtOAc gradient) to provide the desired product (Scheme 1).
Spectroscopic data for the unknown products are as follows: 1-Methyl-3,4-diphenyl-1H-pyrrole-2,5-dione (   Fig. 1. As can be seen in Fig. 1 for the nanocell, the adsorption peak at 1058 cm À1 exhibits vibration of the C-O-C in the pyranone ring. The peaks for C-H and C-O vibrations in the polysaccharide rings of cellulose are around 1200-1050 cm À1 . The absorption bands at 3420 cm À1 and 2998-2850 cm À1 are attributed to the O-H and C-H stretching vibrations, respectively. Also, the adsorption bands at 586 and 520 cm À1 are ascribed to Fe-O stretching band, and those at 3442 cm À1 correspond to broad OH groups on the magnetic surface of the MNPs.

Results and discussion
Aer synthesis of CAI@cell@Fe 3 O 4 , two absorption peaks were observed at 2990 and 2930 cm À1 , which can be assigned to symmetric stretching of the C-H group. The bands situated at 1460 and 1629 cm À1 display symmetric and asymmetric stretching adsorption peaks, and one at 1700 cm À1 can be assigned to the C]O group related to 5-carboxyoxindole connected to the cell@Fe 3 O 4 nanoparticles.
Also, the intensity of the adsorption peak at 586 cm À1 , which is assigned to the bending vibration of C-H in the pyridine heterocyclic ring, reduces aer the formation of CAI@cell@Fe 3 O 4 complex with Pd. Comparison of FTIR spectra results conrm the well graed 5-carboxyoxindole (CAI) as ligand connected to the Pd metal on CAI@cell@Fe 3 O 4 .
The PXRD spectrum of Pd@CAI@cell@Fe 3 O 4 is shown in Fig. 2. Wide-angle PXRD measurements were performed to affirm the presence of palladium on the CAI@cell@Fe 3 O 4 . The wide peak at 2q ¼ 22.5 present in both the Pd@CAI@cell@Fe 3 O 4 and cell@Fe 3 O 4 spectra is concluded to be from nanocellulose. Also, the PXRD pattern revealed that standard Fe 3 O 4 crystal has six diffraction peaks, namely, (220), (311), (400), (422), (511) and (440) at 2q ¼ 30, 35.5, 43.5, 54, 57, 63 (Fig. 10). Also, the index peaks at 2q ¼   shown in Fig. 3, the rst weight loss at almost 100 C (3%) was allotted to the vaporization of adsorbed water molecules. According to the obtained results, Pd@CAI@cell@Fe 3 O 4 is stable before 200 C. The mass weight loss of nearly 50% between 200-500 C is attributed to thermal decomposition of the organic group. Fig. 4. According to the SEM image, the nanoparticles have a spherical morphology. Besides this, the size distribution is narrow, and the mean size of the nanocomposite is around 15-30 nm.

Scanning electron microscopy (SEM). The SEM image of Pd@CAI@cell@Fe 3 O 4 is presented in
3.1.5. Transmission electron microscopy (TEM). The morphology of the prepared catalysts consisting of Pd@CAI@cell@Fe 3 O 4 nanoparticles was studied by TEM, which is displayed in Fig. 5, showing particles have spherical morphology. According to TEM image, the average particle size is estimated to be about 17 nm for Pd@CAI@cell@Fe 3 O 4 nanoparticles, which is in good agreement with the crystallite size estimated from PXRD, at 15 nm. As shown in Fig. 5, a fundamentally core-shell structure (dark core for Pd and Fe 3 O 4 nanoparticles, and light shell for the organic group) was concluded. This is a presentation of the almost single crystalline character of the Pd@CAI@cell@Fe 3 O 4 nanoparticles.
The EDX analysis of Pd@CAI@cell@Fe 3 O 4 is shown in Fig. 6. As can be seen, Pd@CAI@cell@Fe 3 O 4 is composed of Fe, C, O and Pd, indicating that Pd has been inserted in the desired catalyst. In other words, there is a Pd peak, which is consistent with the ICP-AES results.    (Fig. 7). The magnetization curves for these nanoparticles display no hysteresis in their magnetization. As can be seen in Fig. 7 In other words, these differences arise from the different coating layers and their thicknesses on the surface of the MNPs. The prepared catalyst reveals excellent magnetic characteristics and thus can be quickly and completely separated from the reaction media using an external magnet.

Catalytic application of Pd@CAI@cell@Fe 3 O 4 as a nanomagnetic catalyst for the Heck-type arylation of maleimides with iodoarenes
First, to nd the optimum conditions, the reaction of maleimides (1 mmol) and 4-iodotoluene (2 mmol) in the presence of the Pd@CAI@cell@Fe 3 O 4 as nanocatalyst was selected as a model reaction. The reaction was performed using various    Aerwards, different substituted maleimides and iodoarene derivatives were applied in the Heck-type arylation to prepare the corresponding products, which led to high to excellent yields ( Table 2). As can be observed from Table 2, the starting materials, either with electron-donating or electronwithdrawing substituents on the maleimides and iodoarenes, provided the desirable products in high to excellent yields ( Table 2).
In order to show the accessibility of the present work in comparison with the only reported result in the literature, we summarized some of the results for the Heck-type arylation of maleimide with 4-iodotoluene. The results show that the Pd@CAI@cell@Fe 3 O 4 (10 mol%) at 80 C (reaction time 12 h and yield 84%) is the better catalyst relative to PdCl 2 (10% mol) at 100 C (reaction time 24 h and yield 28%) 36 due to the reusability of the catalyst several times with no substantial loss of activity, its short time reaction and good yields of the product.
Additionally, the recycling and reusability of the Pd@CAI@cell@Fe 3 O 4 nanoparticles were studied using a model reaction (Experimental section). According to the observed results, the recovered catalyst was reused for ve runs with no loss of activity (Fig. 8).
Moreover, the FT-IR, PXRD and ICP analysis of recycled Pd@CAI@cell@Fe 3 O 4 (aer recycling ve times) were conducted, as shown in Fig. 9 and 10. The ICP-AES analysis shows the weight percentage of the Pd to be 10% in the Pd@CAI@cell@Fe 3 O 4 NPs. As can be seen, the structure of the recycled nanocatalyst has not changed and is quite similar to that of the newly prepared catalyst.

Conclusions
In conclusion, we report the preparation of Pd supported on 5carboxyoxindole functionalized cell@Fe 3 O 4 nanoparticles (Pd@CAI@cell@Fe 3 O 4 ) as an efficient, novel and reusable heterogeneous nanomagnetic catalyst. The prepared nanomagnetic catalyst was characterized by XRD, ICP-AES, SEM, TEM, FT-IR, TGA, VSM and EDX techniques and was used successfully for the Heck-type arylation between different substituted maleimides with iodoarenes. The Pd@CAI@cell@Fe 3 O 4 nanomagnetic catalyst demonstrated an average particle size of about 15 nm. The nanocatalyst was recovered by simple separation using an external magnet and reused for subsequent cycles. The prepared nanocatalyst exhibited several advantages, including high specic surface area, more active sites, prominent chemical and thermal stability, decrease in the leaching (the release) of the nanocatalyst into the bioenvironment (ecosystem), the presence of organic groups for easier modication, and lower accumulation with respect to other nanocatalysts.

Conflicts of interest
There are no conicts to declare.