RETRACTED ARTICLE: Synthesis and characterization of ZrFe2O4@SiO2@Ade-Pd as a novel, recyclable, green, and versatile catalyst for Buchwald–Hartwig and Suzuki–Miyaura cross-coupling reactions

This work presents the simple synthesis of a green and novel Palladium based magnetic nanocatalyst with effective catalytic properties and reusability. These heterogeneous catalysts were prepared by the anchoring of Pd(0) on the surface of ZrFe2O4 MNPs coated with a di-substituted adenine (Ade) compound as a green linker. The as-synthesized ZrFe2O4@SiO2@Ade-Pd MNPs were methodically characterized over different physicochemical measures like VSM, EDX, Map, SEM, TEM, ICP, and FT-IR analysis. The catalytic activity of ZrFe2O4@SiO2@Ade-Pd was carefully examined for the room-temperature Carbon–Carbon coupling reaction in acetonitrile as a solvent. It is worth noting that the synthesized solid catalyst can be easily recovered with a bar magnet and reused for five cycles without decrease of catalytic activity.


Experimental
Preparation of ZrFe 2 O 4 @SiO 2 @Ade-Pd For the synthesis of ZrFe 2 O 4 @SiO 2 @Ade-Pd nanocatalyst, first ZrFe 2 O 4 @SiO 2 @n-Pr-Cl were synthesized by the co-precipitation method.To synthesize the ZrFe 2 O 4 @SiO 2 @Ade-Pd complex, 3 g of the prepared ZrFe 2 O 4 @ SiO 2 @n-Pr-Cl was dispersed in 50 mL toluene by sonication for 65 min.Then, 5 mmol of adenine and Et 3 N (10 mmol) were added into the flask and stirred at 90˚C for 60 h at 60 °C under N 2 conditions.After the reaction completion, the obtained ZrFe 2 O 4 @SiO 2 @Ade were separated by magnetic decantation, washed with EtOAc and n-hexane, and then dried at 80 °C in an oven for 21 h.Finally, to prepare ZrFe 2 O 4 @SiO 2 @Ade-Pd organometallic catalytic complex, a mixture of ZrFe 2 O 4 @SiO 2 @Ade (2.0 g), Pd(OAc) 2 (0.1 g) and 75 ml ethanol was added into the flask and, then, N 2 gas was charged into the flask and, finally, it was stirred at 75 °C for 48 h under reflux conditions.Then, NaBH 4 (8 mmol) was added into the flask and stirred for 8 h.Next, after completion of the reaction, the final complex ZrFe 2 O 4 @SiO 2 @Ade-Pd was separated and washed with H 2 O and ethanol and, finally, dried under vacuum at 55 °C (Scheme 1).

Preparation of Buchwald-Hartwig cross-coupling
In a typical run, iodoarene (0.5 mmol), Et 3 N (0.5 mmol), and arylamine (0.6 mmol) were dissolved in EtOH (4 mL).The ZrFe 2 O 4 @SiO 2 @Ade-Pd (0.01 g) was added to the mixture, and the resultant mixture was stirred at reflux conditions.The reaction progress was monitored by TLC.Upon the reaction completion, the ZrFe 2 O 4 @ SiO 2 @Ade-Pd was separated using a magnet, and then, EtOAc (3 × 4 mL) was added to the reaction mixture.

A R T I C L E
www.nature.com/scientificreports/Then the organic phase was separated and dried with the MgSO 4 , and then the organic solvent was removed at room temperature.The products were purified by silica gel column chromatography (Scheme 2).

Preparation of C-C cross-coupling reaction
A mixture of PhB(OH) 2 (0.6 mmol), aryl halides (0.5 mmol), and K 2 CO 3 (0.7 mmol) in the presence of ZrFe 2 O 4 @ SiO 2 @Ade-Pd (0.01 g) was dissolved in Acetonitrile and stirred at room temperature as the mild conditions for appropriate time which is shown in Table 3. Once the reaction was completed, was monitored by TLC technique in Hexane.After the completion of the reaction, the nanocatalyst was separated by magnetic filtration and the product was washed with EtOAc, and the organic layer was dried over anhydrous sodium sulfate.Then EtOAc was evaporated and the pure products were obtained in excellent yields (Scheme 3).
As shown in Fig. 1, the FTIR spectra of different intermediates and desired catalysts were investigated in the range 400-4000 cm −1 .The addition of each layer to the previous one leads to the observation of characteristic peaks of newly added functional groups in the spectrum which proves the formation of a new layer.The absorption bands for ZrFe 2 O 4 (Fig. 1a), ZrFe 2 O 4 @SiO 2 (Fig. 1b), and ZrFe 2 O 4 @SiO 2 @n-Pr (Fig. 1c) were consistent with our previous reports respectively.Figure 1d, the successful substitution reaction of NH 2 with terminal Cl groups on the modified surface of ZrFe 2 O 4 MNPs was confirmed through the presence of absorption bands in the range of 1530 and 1097 cm −1 assigned to Carbon-Nitrogen and Nitrogen-Hydrogen vibration bands, respectively.These spectra reveal that the surface of ZrFe 2 O 4 MNPs is successfully modified with organic layers.More importantly, the bending vibration of NH 2 in the FTIR spectrum of ZrFe 2 O 4 @SiO 2 @Ade-Pd support (Fig. 1e), is shifted which confirmed the coordination of nitrogen atom of amino groups to Pd (0).
XRD analytical techniques are used to identify the phase of crystalline materials.Also, the obtained results are consistent with the standard patterns of the ZrFe 2 O 4 crystal structure.Then, the XRD pattern of the nanocatalyst shows that the ZrFe 2 O 4 phase has not been changed during the modifications with a different organic functional group (Fig. 2).
The TGA was investigated for the quantitative determination of the complex (Ade-Pd) supported on the surface of ZrFe 2 O 4 magnetic nanoparticles.The little quantity of weight loss (3%) below 250 °C is due to the removal of adsorbed solvents and OH groups (Fig. 3a).As shown in Fig. 3b, for ZrFe 2 O 4 @SiO 2 @Ade-Pd, there is a weight loss of 8% between 250 and 700 °C related to the breakdown of the ZrFe 2 O 4 @SiO 2 @Ade-Pd moieties.The obtained results from TGA analysis confirmed the successful support of Ade-Pd on the surface of ZrFe 2 O 4 MNPs.
The EDS technique was used to analyze the elemental components of the synthesized catalyst.EDS spectrum of ZrFe 2 O 4 @SiO 2 @Ade-Pd indicates the existence of Zr, O, N, Fe, Si, C, and Pd elements (Fig. 4a).Also, the Morphology of ZrFe 2 O 4 @SiO 2 @Ade-Pd was studied by SEM (Fig. 4b-d).The average size of magnetic nanoparticles calculated from images taken from SEM microscopy was estimated at around 156 nm with the least possible agglomerate (Fig. 4b) Therefore, highly active sites are available for the immobilization of Pd on the surface of ZrFe 2 O 4 nanoparticles.

R E T R A C T E D A R T I C L E
www.nature.com/scientificreports/After the five cycles, the recovered nanocatalyst was characterized by SEM, showing almost the same results as the freshly synthesized nanocatalyst with no significant changes (Fig. 5e-h).A comparison of FT-IR spectra of the nanocatalyst after recycling is shown in Fig. 5I.There are no changes in the FT-IR of ZrFe 2 O 4 @SiO 2 @ Ade-Pd after recovery, which confirmed the stability of this nanocatalyst.
The scanning electron microscopy images show that the size of the nanocatalyst particles is in the nanometer range (134-178 nm) with a sphere-like structure.These observations were verified with the achieved data from transmission electron microscopy images (Fig. 6j-m).
The surface area and pore structure of the ZrFe 2 O 4 @SiO 2 @Ade-Pd were probed with N 2 adsorption − desorption isotherm measurements.The obtained results show that the BET-specific surface area is 11.7 m 2 g −1 for the catalyst (Fig. 7).
Also, with the help of ICP analysis, the amount of palladium in the primary nanocatalyst and the amounts of Pd leaching after recycling of the nanocatalyst were studied by using ICP analysis.Based on such analysis, the amounts of Pd in fresh and reused catalysts are 2.4 × 10 −4 mol g −1 and 2.2 × 10 −4 mol g −1 respectively, which shows that Pd leaching from the ZrFe 2 O 4 @SiO 2 @Ade-Pd framework is very low.
As illustrated in Fig. 8, the magnetic behavior of a) ZrFe 2 O 4 , and (b) ZrFe 2 O 4 @SiO 2 @Ade-Pd was also investigated using VSM technics.As expected, the decrease in saturation magnetization from about 41 emu/g to about 29 emu/g, is related to the newly coated layer and confirmed the successful formation of the desired catalyst.

R E T R A C T E D A R T I C L E
www.nature.com/scientificreports/

Catalytic studies
Buchwald-Hartwig cross-coupling reaction catalyzed by ZrFe 2 O 4 @SiO 2 @Ade-Pd In the next step, to determine the optimum conditions, the catalytic activity of ZrFe 2 O 4 @SiO 2 @Ade-Pd in the reaction between aniline with iodoarene was studied as a model reaction (Table 1).The effects of different factors such as the amount of base, temperature, catalyst, and different solvents on the reaction result were investigated (Table 1).The progress of the reaction became checked by way of TLC, and after completion, the reaction product was extracted with ethyl acetate.The reaction was then screened for a range of catalyst loading in Et 3 N base, with the use of EtOH solvent at 60 °C, and 0.02 g gave the best result.Among different bases, Et 3 N was the most   In the proposed mechanisms illustrated in Scheme 4, a plausible reaction mechanism for promoting the Buchwald − Hartwig amination reaction under ZrFe 2 O 4 @SiO 2 @Ade-Pd nanocatalysis can be proposed.More precisely, the reaction of the aryl halide with the nanocatalyst could also promote the formation of an intermediate, ArPdX (I).Then from the intermediate reaction with an arylamine to form intermediate (II).Finally, for Buchwald − Hartwig amination reactions, the product is formed through the base-assisted elimination of X-H from intermediate (III) by latter reductive elimination and regenerates ZrFe 2 O 4 @SiO 2 @Ade-Pd MNPs.
The ZrFe 2 O 4 @SiO 2 @Ade-Pd complex was then utilized as catalyst for the C-C cross-coupling reaction.Initial experiments with iodobenzene (1 mmol) as the substrate was conducted to optimize the reaction conditions including the amount of nanocatalysts, solvents, and base (Table 3).Also, the effect of different catalyst loadings on the model reaction was examined in Acetonitrile as solvent with phenylboronic acid as the phenylating agent at room temperature.in the absence of nanocatalyst, the reaction did not give the biphenyl product after 2 days.It should be noted that changing the amount of catalyst has a very important effect on the reaction.When 0.005-0.03g of nanocatalyst based on Pd content was used as the nanocatalyst, the reaction afforded the desired product in 30-99% yields and 0.02 g gave the best result.whilst increasing the amount of the catalyst to 0.03 g could enhance the reaction rate significantly, but did not increase the yield of the product obviously.Further Table 1.Optimization of reaction conditions for Buchwald-Hartwig coupling reaction of iodoarene with arylamine.a Reaction conditions: iodoarene (0.5 mmol), arylamine (0.6 mmol), ZrFe 2 O 4 @SiO 2 @Ade-Pd (0.01 g), Et 3 N (0.5 mmol), EtOH (2 mL), temperature.www.nature.com/scientificreports/screening of the nanocatalyst quantities revealed that the use of 0.02 g ZrFe 2 O 4 @SiO 2 @Ade-Pd was the optimal choice.The use of PEG-400, EtOH, or MeOH as the solvent also produced the desired product in very good yields of 81-92%, but Acetonitrile was the best choice.Replacement of K 2 CO 3 with Na 2 CO 3 , Cs 2 CO 3 , Et 3 N, t-BuOK, NaOH, or KOH as the base resulted in a decreased yield of the corresponding product, and a long reaction time was needed.According to the observed results, the optimal conditions for this reaction are ZrFe 2 O 4 @SiO 2 @ Ade-Pd nanocatalyst (0.02 g), K 2 CO 3 (2 mmol) as the base in the Acetonitrile at 25 °C.After obtaining the optimal conditions for the coupling reaction of C 6 H 5 B(OH) 2 with C 6 H 5 I (Iodobenzene), the scope of the catalytic activity of ZrFe 2 O 4 @SiO 2 @Ade-Pd was extended to the coupling of other aryl halides with phenylboronic acid.Therefore, various aryl chlorides, aryl iodides, and aryl bromides were investigated in the cross-coupling reaction with phenylboronic acid in the presence of ZrFe 2 O 4 @SiO 2 @Ade-Pd MNPs (Table 4).For example, both electron-deficient and electron-rich aryl halides underwent Suzuki reaction smoothly under mild conditions to afford the desired biphenyl derivatives in 57-99% yield.Table 2. ZrFe 2 O 4 @SiO 2 @Ade-Pd catalyzed Buchwald-Hartwig reaction of various halides with amines.a Isolated yield.b Reaction conditions: arylamine (0.6 mmol), aryl halide (0.5 mmol), ZrFe 2 O 4 @SiO 2 @Ade-Pd (0.01 g), Et 3 N (0.5 mmol), in EtOH (3 ml) at 60 °C.www.nature.com/scientificreports/ The proposed mechanism for the Suzuki reaction in the presence of ZrFe 2 O 4 @SiO 2 @Ade-Pd is shown in Scheme 5.The general catalytic cycle for Suzuki cross-coupling involves three fundamental steps: oxidative addition, transmetalation, and reductive elimination as demonstrated in Scheme 5, The oxidative addition of aryl halides to ZrFe 2 O 4 @SiO 2 @Ade-Pd complex is the initial step to give intermediate I, a ZrFe 2 O 4 @SiO 2 @Ade-Pd species.The presence of amines as the electron-donating groups on the Adenine ligand can activate the Pd (0) catalyst such that the Ar-X bond can be easily broken along with the formation of the Pd-Ar and Pd-X bond.Next, under the participation of the base, an organoborane compound reacts with intermediate I in transmetalation to afford intermediate II.
This is followed by reductive elimination to regenerate the original ZrFe 2 O 4 @SiO 2 @Ade-Pd species to give the desired product.

Catalyst recyclability
In another check, to investigate the recyclability of the magnetic nanocatalyst, the Iodobenzene with phenylboronic acid was examined as a model reaction in EtOH using 0.02 g of ZrFe 2 O 4 @SiO 2 @Ade-Pd.The nanocatalyst was separated by a bar magnet from the reaction mixture and washed several times with EtOH and then with H 2 O several times.The nanocatalyst was used over five runs without any significant loss of activity (Fig. 9).
In the next step, to demonstrate the merit of ZrFe 2 O 4 @SiO 2 @Ade-Pd we compared the results of the Suzuki reaction with the previously reported in the literature.As shown in Table 5, in comparison with other nanocatalysts employed for the synthesis of biphenyl, by iodobenzene, ZrFe 2 O 4 @SiO 2 @Ade-Pd indicated significant catalytic activity in terms of the high yield and best reaction time.

Conclusion
This work reports the investigation of an efficient procedure to prepare ZrFe 2 O 4 @SiO 2 @Ade-Pd a green, novel, recoverable magnetic nanocatalyst.The prepared catalyst, ZrFe 2 O 4 @SiO 2 @Ade-Pd, was identified via EDS, SEM, VSM, TEM, XRD, TGA, and FTIR.The new nanocatalyst was used for the synthesis of the Buchwald-Hartwig Cross-coupling and Suzuki reaction.Furthermore, this new ZrFe 2 O 4 @SiO 2 @Ade-Pd can be readily prepared from commercially available materials.Also, it can be mentioned good catalytic activity, easy separation, and reusability of the introduced nanocatalyst.

Figure 2 .Figure 3 .
Figure 2. XRD spectrum of (a) ZrFe 2 O 4 @SiO 2 @Ade-Pd.R E T R A C T E D A R T I C L E

Figure 5 .
Figure 5. SEM (e-h) images and FT-IR spectra(I) of the recovered catalyst after five cycles.R E T R A C T E D A R T I C L E

Scheme 4 .
Scheme 4. Possible mechanism for Buchwald − Hartwig reaction.R E T R A C T E D A R T I C L E

Table 3 .
Optimization of the reaction conditions for the Carbon-Carbon coupling of phenylboronic acid with iodobenzene.

Table 4 .
Catalytic C-C cross-coupling reaction of various aryl halides with PhB(OH) 2 in the presence of catalytic amounts of ZrFe 2 O 4 @SiO 2 @Ade-Pd.

Table 5 .
Comparing catalytic activity of ZrFe 2 O 4 @SiO 2 @Ade-Pd with previously reported methods in the C-C cross-coupling reaction.