Palladium catalyst immobilized on functionalized hyper-crosslinked polymers with 8-hydroxyquinoline as monomer for Suzuki-Miyaura coupling reactions

An e�cient and stable palladium species catalyst immobilized on functionalised hyper-crosslinked polymers (HCPs-Pd) has been successfully developed and applied in the Suzuki-Miyaura coupling reaction of diverse types of aromatic halides with aromatic boric acid in this work. The results demonstrated that HCPs-Pd exhibited high catalytic activity, which bene�ted from the microporous structure of the catalyst guaranteed high dispersibility of active palladium, and high speci�c surface area, large pore volume, good chemical stability. Under optimal reaction conditions, 93.0% of biphenyl yield in the Suzuki-Miyaura reaction with bromobenzene (C 6 H 5 Br) and phenylboric acid (C 6 H 7 BO 2 ) as raw material was obtained. The good stability of the HCPs-Pd catalyst was veri�ed by �ve cycles experiments. Perhaps this work provides new insights into the high-yield preparation of biphenyl aromatic compounds employing HCPs-Pd as an e�cient and stable catalyst in the Suzuki-Miyaura reaction.


Introduction
As an essential part of the porous organic polymers (POPs), the hyper-crosslinked polymers (HCPs) have been shown to display superior properties, including high surface area (S BET ) and pore volume, special porosity and easy to prepare and chemical modify.Therefore, they have remarkable potentials for the adsorption materials and catalytic materials [1][2][3][4].Compared with other materials, the diversity of synthesis is the great advantage of HCPs, which can have different structures through connecting different parts [5,6].
Compared with the material HCPs, the catalytic activity and stability of the immobilized precious metal were signi cantly improved, because of its organic cage frameworks and the coordination between the heteroatoms in HCPs and metal ions, which could x the ultra-ne metal nanoparticles in the cages and make the metal particles rmly anchored to maintain an excellent activity in the long term, as well as serving as a catalyst support for the production of heterogeneous catalytic converters.[7][8][9][10].
In recent years, precious metal immobilized on functionalized HCPs have attracted interested due to a range of signi cant advantages, including the ability to utilize a variety of synthesis methods, the presence of small pore size, excellent stability, low economic cost and optimum processing conditions [11,12].It was widely used in organic synthesis [13,14], light/heterogeneous catalysis [15,16], adsorption and separation [17,18], CO 2 conversion [19,20] and other elds.Especially, as a heterogeneous catalyst in the eld of organic synthesis, immobilization HCPs catalyst with precious metals has more advantages than traditional homogeneous catalysts, such as stable catalyst, mild reaction conditions, low emissions of three wastes, and low environmental pollution [21,22].Gu [23] synthesized the hypercrosslinked organic polymers (HDS-3.6)by Friedel-Crafts alkylation, it exhibited the best results for ethyl levulinate formation with the optimal yield of 70.3%, and it did not signi cantly decrease in their catalytic properties.Lyubimov [24] developed a novel palladium nanoparticles catalyst by reduction of [Pd(πallyl)Cl] 2 with hydrogen in a hyper-crosslinked polystyrene matrix.it showed high catalytic activity in the hydrogenation of benzene (benzene conversion was 100%).Nobre [25] obtained active catalysts by combining Pd(OAc) 2 and iminophosphine ligands, which indicated high catalytic activity in Suzuki-Miyaura coupling reactions, high-yield coupling products can be obtained under mild conditions (25-50°C) using aryl bromide, iodide or benzyl chloride as substrates.Jia [26] prepared (HCP-PPh 3 -Ru) catalyst by a one-step external crosslinking reaction.It exhibited excellent catalytic activities in the synthesis of 2,4-diaryl substituted pyridine and diazodicarbonyl cycloaddition reaction.The results revealed that the ligand PPh 3 and Ru immobilised on HCP materials was quite robust in the organic transformations reaction.
Up to now, based on above mentioned investigated and our earlier experimental research on Suzuki-Miyaura coupling reactions [27][28][29][30], an e cient and stable palladium catalyst immobilized on functionalized hyper-crosslinked polymers had been successfully exploited.A convenient and environment-friendly method for the e cient synthesis of biphenyl and its derivatives from the organic coupling reaction had been successfully developed over HCPs-Pd catalyst.The research results from the catalytic Suzuki-Miyaura coupling reactions over palladium catalyst immobilized on functionalized hypercrosslinked polymers are thoroughly reported in this paper.

Catalyst Preparation
( ) Synthesis of HCPs- [31]: AlCl 3 (anhydrous, 4.3601 g, 32.70 mmol ) and 8-hydroxyquinoline (0.5975 g, 4.12 mmol) were added to a solution of benzene (1.2165 g, 15.60 mmol) in 40 mL chloroform.The above formative mixture was stirred su ciently at 20°C, then stirred at 45°C for 5.0 h in order to obtain the initial network, continued to raise the temperature to 80°C for 48 h to complete the reaction.The obtained precipitate after the reaction was washed 3 times with CH 3 OH, followed by hydrochloric acid in a Soxhlet for 24 h.pH of the product was modulated to neutralized with NaOH solution, then washed repeatedly with CH 3 OH in a Soxhlet for 55 h at 105°C, lastly vacuum dried at 60°C for 24 h (De ned as HCPs-).
( ) Synthesis of HCPs- [32]: ( ) Preparation of HCPs-Pd [33]: The immobilized catalyst was obtained by the coordination of Palladium(II) ions with the HCPs ligand.Typically, HCPs (0.25 g) were added to the tetrahydrofuran solution (10 mL) containing Pd(OAc) 2 (0.037 g, 0.17 mmol), then re uxed at 60°C for 24 h.The next experimental steps are similar to ( ) Synthesis of HCPs-, and the catalyst as prepared was designated as HCPs--Pd.For comparison, the HCPs--Pd were prepared in the same way (As shown in Scheme 1).

Catalyst characterizations
XPS spectroscopy was performed using an ESCALAB 250Xi spectrometer.The textural properties of the samples were afforded using the N 2 physisorption on an ASAP 2020 at 77K.The elemental content of the samples was analysed on a Thermo Scienti c iCAP 7400 ICP-OES.FT-IR spectra of the samples were collected by using KBr matrix on a Nicolet-380 instrument (500-4000 cm − 1 ).The HRTEM and SEM images were afforded on a JEM-2100 and Sigma HD, Carl Zeiss, Germany (FE-SEM), respectively.

Typical Suzuki-Miyaura coupling reaction
The reactions were performed in a Schlenk reactor [34].Typically, HCPs-Pd (20 mg) catalyst, bromobenzene (0.7801 g, 5.0 mmol), phenylboronic acid (0.7930 g, 6.5 mmol), and anhydrous K 3 PO 4 (1.6960 g, 8.0 mmol) were joined to solvent (15 mL, EtOH: H 2 O (4:1 v/v)) and reacted for 3.0 h at 80°C under N 2 .Bromobenzene was consumed as indicated by TLC, and poured into 100 mL ice water with stirring.The mixture was alternately washed with dichloromethane and water by turn (3×20 mL).The combined organic phase was ltered and concentrated under reduced pressure after MgSO 4 has adsorbed trace amounts of water.The obtained product was puri ed by ash chromatography (silica gel, light petroleum: diethyl ether = 20: 1) until biphenyl was obtained as a white solid (716.0 mg, 93.0%) (As shown in Scheme 2).

Effects of reaction conditions on the reaction with bromobenzene and phenylboric acid as raw material
The catalytic performances of HCPs-Pd catalyst with different solvent and base for the Suzuki-Miyaura reaction were examined (Table 1).Firstly, the in uence of base species on the reaction process was investigated (entries 1-3), it shows that K 3 PO 4 has better effects than K 2 CO 3 and NaOH.Subsequently, different solvents were used for the reaction, it showed that 4/1 (v/v) CH 3 CH 2 OH and H 2 O solutions were the best with a yield of 93.0% (entries 4-7).Finally, other reaction conditions were explored (entries 8-13), including catalyst dosage, reaction temperature and time, no nitrogen protection and room temperature reaction.The optimal reaction conditions were obtained, 5.0 mmol bromobenzene, 6.5 mmol phenylboronic acid, 8.0 mmol base, 20 mg HCPs-Pd catalyst, 3.0 h, 80°C, the product yield reached 93.0%.

The recycle of HCPs-Pd catalyst and thermal ltration experiment in the reaction
The recycle of HCPs-Pd catalyst was studied under above optimum reaction conditions.The used catalyst was separated by centrifugation and washed several times in acetone and methanol.It was dried at 105°C and reused for the next reaction.Figure 1 shows the reusable results of the catalyst.It was evident that HCPs-Pd catalyst exhibited good stability in the reaction, 89.5% of biphenyl yield with high TON ( 2400) was still obtained after ve runs.
In addition, in order to verify the absence of free palladium in the catalytic reaction system, two comparative experiments (normal reaction and reaction after thermal ltration of the catalyst) were conducted to further verify the stability of the catalyst [35].For normal reactions, quenching was occurred after 1.0 h (Table 1, entry 11), and the nal yield of biphenyl was 65.2%.Comparative experiment showed that after 1.0 h of reaction, the catalyst was removed by thermal ltration, and the reaction continued for 3.0 h.After quenching the reaction, the nal yield of biphenyl was 66.7%.Furthermore, ICP testing was conducted on the ltered reaction solution, and the results showed that it did not contain Pd.Comparing two experiments, it was found that the change in product yield was relatively small, and the HCPs Pd catalyst did not shed palladium species, indicating good stability of the catalyst [36].

Exploration of reactions with different substrates
Under the above conditions, a series of exploratory reactions were conducted to explore the catalytic reaction range of the catalyst (Scheme 3).As shown in Table 2, the catalytic reaction of HCPs-Pd has been proven to be suitable for the reactions of some compounds 1 and 2 containing different functional groups, resulting in the synthesis of product with high catalytic activity.Catalyst characterization N 2 adsorption desorption isotherms of four samples are shown in Fig. 2A.The typical type-isotherm with H4 hysteresis loop was observed for all samples [37].In addition, the Non-Local Density Functional Theory method was used to get the pore diameter distribution curves (Fig. 2B).It can be evident that a regular distribution of the pore diameters in the micro-and meso-sizes is found for all samples [38].
Furthermore, the chemical composition and structural characteristics of the samples are listed in Table 3.
The Pd content of the HCPs--Pd catalyst is essentially the same as that of the HCPs--Pd catalyst from the results of ICP test.The increased in pore volume and the decreased in BET speci c surface area of HCPs-Pd may be due to the modi cation of Pd, which blocks the small pore size of HCPs [39].These results indicated that hyper-crosslinked polymers containing palladium Pd components have been successfully prepared [39,40].[13,15,41].It can be inferred that Pd has successfully coordinated with HCPs to form HCPs-Pd [41,42].
SEM-Mapping was used to study the morphologies of four materials.As shown in Fig. 4, it indicated that HCPs--Pd and HCPs--Pd have no speci c morphology, as if they are irregularly stacked spherical particles.To provide direct evidence of the successful introduction of Pd into HCPs, new evidence was obtained by elemental mapping of SEM images of two samples (Fig. 5) [43].It indicates that the distribution of C, N, O and Pd can be clearly observed, con rming the existence and uniform distribution of Pd in two samples.It is worth noting that one of the reasons for its superior performance in the reaction process is the homogeneity of the four elements [44].

TEM
Figure 6 depicts the TEM images of four samples.The presence of Pd on hyper-crosslinked polymers is determined, and the lattice spacings of around 0.2 nm can be detected and attributed to the Pd species [45], as shown in the HRTEM images (Figs.6B, 6D).The results demonstrate that the distribution of Pd species in HCPs-Pd samples is uniform.The uniform dispersion of active Pd species in the catalyst may be one reason for the good catalytic activity in the reaction [46-48].
FT-IR shows a large number of distinctive peaks of four samples (Fig. 7).Compared two curves of HCPsand HCPs--Pd, it is found that the intensity of the infrared peaks at 1624, 1492, 877 and 736 cm − 1 positions increased [49,50].Interestingly, compared the curves of HCPs-and HCPs--Pd, it is found that the infrared peak of HCPs--Pd shifted red at positions 782 and 736 cm − 1 , and is a separate infrared peak (741 cm − 1 ).In addition, the peak intensity also weakens at 606 cm − 1 [51,52].These changes may be caused by the coordination of Pd on HCPs [53,54].

Conclusions
In conclusion, two porous hyper-crosslinked polymers (HCPs-, HCPs-) were successfully designed and prepared by a one-pot Friedel-Crafts alkylation reaction employing anhydrous FeCl 3 or AlCl 3 in the presence or absence of FDA.Further, a "braided" strategy was used for the preparation of Pd metalorganic catalysts on functionalized HCPs.The new polymer (HCPs-Pd) has excellent catalytic activity, good porosity with high speci c surface area, large pore volume, good stability and highly dispersed palladium.Consequently, in the Suzuki-Miyaura coupling reaction of aryl bromides, the HCPs-Pd showed good performance in combination with the mild conditions and hydrophilic reaction media.Furthermore, the HCPs-Pd catalyst can be recycled and reused for multiple runs with no apparent obvious loss of catalytic e ciency.This work also shows that the microporous polymers can not only act as supporting materials, but can also protect the catalyst and have a positive effect on the catalytic activity.In addition, we also expect the work to open up a new path of designing and constructing excellent heterogeneous solid catalysts on a molecular scale.

Table 3
Chemical composition and structural properties of samples Figure3mainly explains XPS characterization of the Pd element in two HCPs-Pd samples.Figure3Ashows the characteristic peaks of C 1s and Pa 3d at binding energies 284.8 and 338.1 eV, respectively.It can be observed that both HCPs--Pd and HCPs--Pd exhibit the characteristic peaks of Pd.The complex Pd 3d spectra are shown in Fig.3B.The presence of Pd( ) is correlated with the peaks at 343.7 and 338.2 eV, and the other peak at 342.2 and 336.6 eV are attributed to Pd(0), respectively