Li-Ping
Jing
a,
Jin-Shi
Sun
a,
Fuxing
Sun
a,
Peng
Chen
*a and
Guangshan
Zhu
*ab
aState Key Laboratory of Inorganic Synthesis and Preparative Chemistry, College of Chemistry, Jilin University, 2699 Qianjin Street, Changchun 130012, China. E-mail: pengchen@jlu.edu.cn
bKey Laboratory of Polyoxometalate Science of the Ministry of Education, Faculty of Chemistry, Northeast Normal University, Changchun 130024, China. E-mail: zhugs100@nenu.edu.cn
First published on 2nd March 2018
A strategy using a mesoporous amine-tagged porous aromatic framework (PAF70-NH2) to immobilize a palladium (Pd)-based molecular catalyst has been developed. The resulting immobilized catalyst PAF70-Pd, in which the framework is entirely constructed by phenyl rings linked with stable carbon–carbon bonds, has high structural rigidity and stability. Compared with the known porous organic material immobilized Pd-based catalysts, PAF70-Pd has the highest Pd content so far. Moreover, PAF70-Pd has extremely high catalytic activity with good size selectivity and very easy recyclability in catalyzing the Suzuki–Miyaura coupling reaction. In the current system, the catalyst loading could be as low as 0.001 mol% and the TOF value could go up to 28800 h−1 which is far higher than those of the known porous organic material immobilized Pd-based catalysts. In order to elucidate the particularly high catalytic efficiency of PAF70-Pd, we prepared PAF1-Pd from PAF1-NH2 for comparison. PAF1-Pd has a higher Pd content than PAF70-Pd. However, due to the absence of large enough mesopores in PAF1-NH2, PAF1-Pd has almost no catalytic activity under the same conditions, which definitely demonstrated that the intrinsic mesoporosity of PAF70-NH2 plays a crucial role in the superb catalytic efficiency of PAF70-Pd. This strategy to immobilize Pd-based molecular catalysts has very good expansibility to be applied in the immobilization of different organometallic catalysts into the pores of PAFs, which also has very high potential in the chemical and pharmaceutical industry.
Using porous materials such as metal–organic frameworks (MOFs), covalent organic frameworks (COFs) or porous organic polymers (POPs) as supported materials began to appear in the last few decades, which is a good idea because of their porosity and high surface area.6,7,30–36 However, the intrinsic instability of MOFs and COFs or the flexibility of the frameworks of POPs makes this related research field still face many difficulties. In 2009, a new type of porous organic material with robust regular frameworks constructed entirely from rigid aromatic building blocks linked by stable covalent bonds, named porous aromatic frameworks (PAFs), was developed by our group and achieved intensive interest from researchers in diverse fields due to their wide range of structures and potential applications in gas sorption,37–45 separation,46,47 catalysis,48–54etc. Owing to their robust structure together with high stability in most organic solvents, PAFs are extremely suitable platforms for the catalysis of organic reactions. It’s worth noting that, due to the presence of the Pd center and organic ligand, Pd-based organometallic catalysts usually have relatively large sizes. Hence, immobilization of Pd-based molecular catalysts into the porous materials often needs a large enough pore size. Most of the reported porous organic material immobilized Pd-based catalysts always suffer from low Pd utilization efficiency which might be due to that the pore space after introduction of the Pd-based catalyst is too small to accommodate the catalytic reaction. Apparently, for application of PAFs as the platforms for Pd-based organometallic catalysts, PAFs with large enough mesopores are needed. However, the synthesis of narrowly distributed mesoporous PAFs is still a challenge because of the interpenetration while using large-size monomers. Thus using PAFs as the platforms for covalent anchoring of organometallic catalysts into the pores still remains rare up to now. In this paper, we will make an attempt in this area.
Considering the need for large enough pore space for accommodating Pd-based molecular catalysts and the subsequent catalysis, in this paper, PAF70-NH2, an amine-tagged PAF with narrowly distributed mesopores which was recently reported by our group,55 was selected as the platform for Pd-based organometallic catalysis. In this paper, a strategy involving two post-synthesis modification steps for the introduction of the Pd-based organometallic catalyst into the pores of PAF70-NH2 was used for the synthesis of our desired material, and the catalytic performance of the desired material (PAF70-Pd) was systematically studied. In order to further demonstrate the importance of the mesopores in PAF70-NH2, another amine-tagged PAF (PAF1-NH2) without mesopores was also used as a platform to immobilize the same Pd-based molecular catalyst, affording PAF1-Pd for comparison with PAF70-Pd.
Nitrogen adsorption–desorption isotherms for the obtained materials were measured at 77 K. As shown in Fig. 1b, at low relative pressures, PAF70-NH2, PAF70-NCPy and PAF70-Pd all showed sharp uptakes, indicating the existence of micropores in the materials. It’s worth noting that, in the desorption branch of PAF70-NH2, a relatively sharp hysteresis demonstrated the presence of narrowly distributed mesopores. Compared with PAF70-NH2, the corresponding hysteresis disappeared in the desorption branches of PAF70-N
CPy and PAF70-Pd, which indicated the disappearance of the mesopores after post-modification of PAF70-NH2. The apparent surface area calculated from the Brunauer–Emmett–Teller (BET) model was 599 m2 g−1 for PAF70-NH2, 263 m2 g−1 for PAF70-N
CPy, and 172 m2 g−1 for PAF70-Pd. Through the change of pore size distributions calculated by non-local density functional theory (NLDFT), it was clear that the mesopores with a pore width of 3.8 nm of PAF70-NH2 disappeared in PAF70-Pd (see Fig. S10 in the ESI†). The decrease of the BET surface area and the disappearance of mesopores from PAF70-NH2 to PAF70-Pd further proved the successful introduction of the Pd-based functional groups into the pores of the PAF.
Thermogravimetric analysis (TGA) was performed to test the thermal stabilities of the above PAF materials. As shown in Fig. 1c, PAF70-NH2 (black curve) and PAF70-NCPy (red curve) showed similar TGA curves. There is almost no weight loss before 300°C, which suggested the high thermal stability of PAF70-NH2 and PAF70-N
CPy. At about 400°C, the framework decomposition started and when the temperature was above 500°C the decomposition became obvious. The 3.96 wt% residue for PAF70-NH2 and 2.15 wt% residue for PAF70-N
CPy at 800°C could be ascribed to some palladium oxide residue which originated from the Pd catalysts in the preparation process of PAF70-NH2. As shown in Fig. 1c, PAF70-Pd (blue curve) had a 56% weight loss at 277–320°C. This weight loss could be attributed to the decomposition of both N,N-bidentate ligand and AcO− species which were directly connected to the Pd center. Compared with PAF70-N
CPy, PAF70-Pd showed lower stability, which might be due to that the Pd species could catalyze the cleavage of carbon–carbon bonds around the Pd centers in the PAF material.56 After a further obvious decomposition of the framework that started at 450°C, there was a 26.2 wt% palladium oxide residue left at 800°C. In addition, all the three PAF materials could not be dissolved or decomposed in almost all common solvents such as water, ethanol, dichloromethane, toluene, tetrahydrofuran, ethyl acetate, hexane, diethyl ether, etc. The high thermal stability and chemical stability made PAF70-Pd fully satisfy the demands of catalysis. The TGA analysis of PAF1-Pd can be found in the ESI (Fig. S17†). The Pd content was further determined by inductively coupled plasma (ICP) analysis, which revealed that 23.0 wt% of Pd was incorporated into PAF70-Pd and 25.1 wt% of Pd was incorporated into PAF1-Pd. These were in agreement with the TGA analysis. Importantly, to the best of our knowledge, PAF70-Pd and PAF1-Pd have higher Pd contents than other reported porous organic material immobilized Pd catalysts, which significantly profits from that the pores of our PAF materials could endow high surface area for immobilizing the Pd coordination system.
In order to further investigate the incorporation of palladium within PAF70-NCPy and PAF1-N
CPy, X-ray photoelectron spectroscopy (XPS) was performed. As shown in Fig. 1d, the binding energy (BE) at 337.80 eV, assigned to the Pd3d5/2 orbital, indicated that the Pd species in PAF70-Pd and PAF1-Pd are present in the +2 state. Compared with the BE of 338.55 eV for free Pd(OAc)2, the BE for Pd species in PAF70-Pd and PAF1-Pd negatively shifted by 0.75 eV. This negative shift indicated that Pd(OAc)2 has strong coordination with the N,N-bidentate ligand in PAF70-N
CPy and PAF1-N
CPy.11,26
Transmission electron microscopys (TEM) images obviously showed the successful introduction of Pd species into the PAF materials. As shown in Fig. 2a and Fig. 2b, compared with PAF70-NCPy, some evenly distributed black dots with a mean diameter of about 1 nm emerged in the TEM images of PAF70-Pd, indicating that the Pd species are uniformly dispersed in the frameworks of the PAF material, which was in accordance with the above analysis of the TGA curve of PAF70-Pd. Similarly, compared with PAF1-N
CPy (Fig. 2c), the TEM image of PAF1-Pd (Fig. 2d) also showed uniformly dispersed Pd species. This demonstrated that the Pd-based catalyst could also be anchored into the pores of the PAF1-NH2 material.
Entry | Catalyst (catalyst loading) | Solvent | T [°C] | Time | Yieldb |
---|---|---|---|---|---|
a Reaction conditions (unless otherwise noted): a solution of 1a (0.5 mmol), phenylboronic acid (0.75 mmol), K2CO3 (1.0 mmol), and the catalysts (for entry 10, no catalyst was added; for entry 11, 1.2 mg PAF70-N![]() |
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1 | PAF70-Pd (0.5 mol%) | CH2Cl2 | 40 | 12h | 0 |
2 | PAF70-Pd (0.5 mol%) | p-Xylene | 150 | 4h | 92% |
3 | PAF70-Pd (0.5 mol%) | EtOH | 25 | 1h | 92% |
4 | PAF70-Pd (0.5 mol%) | EtOH | 40 | 45min | 96% |
5 | PAF70-Pd (0.5 mol%) | EtOH | 60 | 20min | 95% |
6 | PAF70-Pd (0.5 mol%) | EtOH | 80 | 7min | 97% |
7c | PAF70-Pd (0.1mol%) | EtOH | 80 | 15min | 96% |
8c | PAF70-Pd (0.01 mol%) | EtOH | 80 | 25min | 97% |
9d | PAF70-Pd (0.001 mol%) | EtOH | 80 | 4h | 93% |
10 | No catalyst | EtOH | 80 | 12h | 0 |
11 |
PAF70-N![]() |
EtOH | 80 | 12h | 0 |
12e | The supernatant liquid of the EtOH suspension of PAF70-Pd and K2CO3 | EtOH | 80 | 12h | 0 |
13 | PAF1-Pd (0.01 mol%) | EtOH | 80 | 25min | <5% |
For comparison, PAF1-Pd was then employed as the catalyst for the current Suzuki–Miyaura coupling reaction under the best conditions as shown in entry 8 of Table 1. Compared with PAF70-Pd which gave a 97% yield (Table 1, entry 8), PAF1-Pd showed almost no catalytic activity (<5% yield, Table 1, entry 13) under the same conditions which should be due to that the remnant space in the pores after introducing the Pd-catalyst was too small to accommodate the current catalysis. This comparison fully demonstrated the importance of the large enough mesopores in PAF70-NH2 for its application in immobilizing large-size metal-based molecular catalysts.
The catalytic performance of PAF70-Pd was further tested using a series of aryl bromides as the reaction substrates at a 0.01 mol% catalyst loading. As shown in Table 2, bromobenzene 9a (entry 9) or the substituted aryl bromides with either an electron-withdrawing group (such as –NO2, CHO, –Br, –F and –CN, entries 1–5) or an electron-donating-group (such as –OMe, –Me and –(OH)CHCH3, entries 6–8) afforded the cross-coupling products in excellent yields (up to >99%) with high turnover frequency (TOF) values (all ≥14700 h−1), demonstrating the wide generality and functional tolerance of the current system.
Entry | Ar–Br | Product | Time (min) | Yieldb | TOF (h−1)c |
---|---|---|---|---|---|
a Reaction conditions: a solution of 2.5 mmol 1a, 3.75 mmol phenylboronic acid, 5.0 mmol K2CO3 and PAF70-Pd (0.01 mol%) in 10 mL of EtOH was stirred at 80°C for the indicated time. b The isolated yield. c TOF = (moles of product)/(moles of Pd in the catalyst × reaction time). d For entry 11, 4-biphenylboronic acid was used instead of phenylboronic acid. | |||||
1 |
![]() |
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25 | 97% | 23![]() |
2 |
![]() |
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30 | 96% | 18![]() |
3 |
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30 | 95% | 19![]() |
4 |
![]() |
![]() |
30 | 90% | 18![]() |
5 |
![]() |
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40 | >99% | >14![]() |
6 |
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40 | 98% | 14![]() |
7 |
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35 | 92% | 15![]() |
8 |
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40 | >99% | >14![]() |
9 |
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20 | 96% | 28![]() |
10 |
![]() |
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20 | 35% | — |
11d |
![]() |
![]() |
20 | <5% | — |
For the porous material immobilized catalysts with narrowly distributed pore size, the size selectivity is a signature feature of the catalyzed reaction which could occur in the pores. In order to investigate the size selectivity of PAF70-Pd, some contrast tests were performed as shown in entries 9–11 of Table 2. Compared with 9a, which could smoothly transform to 9b completely (Table 2, entry 9), the larger-size 10a reacted more slowly under the same conditions in the same time (35% yield, Table 2, entry 10). When 10a reacted with the larger-size 4-biphenylboronic acid, the reaction rate further decreased and even almost no product was obtained under the same conditions in the same time (Table 2, entry 11). The above size selectivity obviously indicated that the catalytic reaction could occur inside the pores of PAF70-Pd.
For heterogeneous catalysts, recyclability is an important factor. Hence the recyclability of PAF70-Pd as the catalyst was tested by subjecting it to 3 cycles of the Suzuki–Miyaura coupling reaction of 4-bromonitrobenzene 1a and phenylboronic acid (Table S1 in the ESI†). After each cycle, PAF70-Pd was easily recovered by centrifugation followed by washing and could be directly used in the next cycle for cycles 2–3, in which the substrate dosages were the same as that in cycle 1. After 3 cycles, the recovered PAF70-Pd was dried in vacuo at 120°C for 18 h. ICP analysis showed that the recovered PAF70-Pd had 22.7 wt% of Pd content, which had no obvious change compared with the fresh PAF70-Pd (23.0 wt% of Pd content). These indicated that there is very low metal leaching during the reaction process. The results demonstrated that PAF70-Pd could undergo at least 3 cycles of the reaction without obvious loss of catalytic activity.
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Fig. 3 Pd contents and the catalytic performances in the Suzuki–Miyaura coupling reaction of PAF70-Pd, PAF1-Pd and other reported porous organic material (POPs and COFs) immobilized Pd catalysts. |
Footnote |
† Electronic supplementary information (ESI) available: Experimental details including synthesis and experimental methods. See DOI: 10.1039/c8sc00510a |
This journal is © The Royal Society of Chemistry 2018 |