Palladium supported on structurally stable phenanthroline-based polymer nanotubes as a high-performance catalyst for the aqueous Suzuki-Miyaura coupling reaction

Article abstract of DOI:10.1039/d0nj04864j

Though the Suzuki-Miyaura coupling reaction has intrinsic advantages in organic synthesis, it is still a challenging task to develop a highly active and truly heterogeneous catalyst for the aqueous Suzuki-Miyaura coupling reaction (SMR). In this work, a series of phenanthroline-based polymers (PBPs; PBP1 to PBP8) were synthesized by a simple one-step AlCl₃-catalyzed Friedel-Crafts polymerization method. Systematic measurements of PBPs by N₂adsorption-desorption isotherms, scanning electron microscopy (SEM), transmission electron microscopy (TEM), Fourier transform infrared spectroscopy (FT-IR) and thermogravimetric analysis (TGA) show that most of the PBPs have a nanosheet morphology, except PBP8 which has both one-dimensional nanotubular morphology and large surface area (745 m²g^?1). Benefitting from the porous nanotube morphology and the two N atoms contained in the phenanthroline unit of the polymer structure, polymer PBP8 shows adsorption effects and strong chelating stabilization on the Pd active metal (size, 2-5 nm). The Pd/PBP8 catalyst exhibits superior catalytic activity within 2 h (TOF value: 3077 h^?1) and reusability (7 cycles) in the SMR with typical reactants such as bromobenzene, phenylboronic acid and the base of K₃PO₄.3H₂O at 30 °C in a solvent mixture of water and ethanol (V_H2O?:?V_ethanol= 3?:?2).

Full text of DOI:10.1039/d0nj04864j

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Palladium supported on structurally stable
phenanthroline-based polymer nanotubes as a
high-performance catalyst for the aqueous
Suzuki–Miyaura coupling reaction†
Cite this: New J. Chem., 2021,
45, 12120
Didi Yang,‡ Shuhui Wang,‡ Ting Dan, Dashuang Gao, Chaktong Au, Wanju Zhang*
and Yan Zhang
*
Though the Suzuki–Miyaura coupling reaction has intrinsic advantages in organic synthesis, it is still a
challenging task to develop a highly active and truly heterogeneous catalyst for the aqueous Suzuki–
Miyaura coupling reaction (SMR). In this work, a series of phenanthroline-based polymers (PBPs; PBP1 to
PBP8) were synthesized by a simple one-step AlCl3-catalyzed Friedel–Crafts polymerization method.
Systematic measurements of PBPs by N2 adsorption–desorption isotherms, scanning electron micro-
scopy (SEM), transmission electron microscopy (TEM), Fourier transform infrared spectroscopy (FT-IR)
and thermogravimetric analysis (TGA) show that most of the PBPs have a nanosheet morphology, except
PBP8 which has both one-dimensional nanotubular morphology and large surface area (745 m2
g
À1).
Benefitting from the porous nanotube morphology and the two N atoms contained in the phenanthroline
unit of the polymer structure, polymer PBP8 shows adsorption effects and strong chelating stabilization on
the Pd active metal (size, 2–5 nm). The Pd/PBP8 catalyst exhibits superior catalytic activity within 2 h
(TOF value: 3077 hÀ1) and reusability (7 cycles) in the SMR with typical reactants such as bromobenzene,
Received 3rd October 2020,
Accepted 29th April 2021
DOI: 10.1039/d0nj04864j
phenylboronic acid and the base of K3PO4.3H2O at 30 1C in a solvent mixture of water and ethanol (VH O
:
2
rsc.li/njc
Vethanol = 3 : 2).
ethynylarenes, haloarenes, benzyl halide, and so on),7,16 well-
designed templates (SiO2),11 and expensive Pd(0) or Pd(II)
1. Introduction
The design of materials with special morphologies and functions catalysts.7,19
at the molecular level to achieve a green and sustainable
For example, Zhao, et al.11 synthesized a tube-in-tube structured
synthesis of materials has become an important topic in carbon nanomaterial by coating acid-treated multiwalled carbon
research fields1–4 such as catalysis,5–7 gas capture8,9 and energy nanotubes with porous SiO2 layers, which was designed as the
storage.10–12 By means of bottom-up polymerization strategies, template for trapping octadecyltrimethoxysilane (C18TMS). After
porous polymer materials can be tailored into designated micro- C18TMS was converted into carbon within the porous SiO2 via
morphologies.5,13–15 In particular, tubular polymer materials chemical treatment and high-temperature calcinations, a tube-
have attracted wide attention due to their special nanofiber in-tube carbon nanostructure was obtained by etching the SiO2
morphology, large specific surface area (4600 m2 gÀ1), structure layer with NaOH solution. The synthesis strategy of the tube-in-
adjustability (polymer structure could be changed by replacing tube structured carbon nanomaterial involves hard templates
its structural units with other organic units) and functional and a high-temperature carbonization process, which could be
diversity (including adsorbed gas, catalyst supporter, energy interminable. Zhu, et al.7 synthesized nanotube-like porphyrin-
storage, etc.).6,9,16–18 Nonetheless, it is not easy to synthesize based polymers as nanoporous nitrogen-rich metal-free electro-
tubular polymer materials. This involves harsh polymerization catalysts using a one-step cross-coupling reaction under the
conditions, including diversity of monomers (such as porphyrin, condition of noble metal Pd(0)/CuI as the catalyst. Chun,
et al.19 reported a tubular-shape microporous organic network,
Hubei Key Laboratory for Processing and Application of Catalytic Materials,
which was synthesized in need of the noble metal catalyst PdCl2/
CuI. Compared with the above methods, the one-step
AlCl3-catalyzed Friedel–Crafts polymerization reaction has the
Huanggang Normal University, Huanggang 438000, China.
E-mail: wanjuzhang@126.com, zhangyan@hgnu.edu.cn
† Electronic supplementary information (ESI) available. See DOI: 10.1039/
advantages of simple synthesis and cheap and easily available
non-precious metal salt catalysts.
d0nj04864j
‡ Both authors have contributed equally to this work.
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Tubular polymeric materials with extra functionalities can
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be fabricated by adjusting the physicochemical properties of
original polymeric nanotubes by means of heteroatom
modification.20–24 For instance, introducing phenolic hydroxyls
into the skeleton of nanotube-like organic porous materials
could improve the hydrophilic performance, which in turn
improves the adsorption performance of amphiphilic–hydro-
philic organics in the water phase.4,11,25–27 The introduction of
pyrrolic nitrogen into the framework of hollow copolymer tubes
could enhance the adsorption selectivity of small functional
peptides.1,28 The doping of catechol in porous polymers could
enhance the cycling stability and provide Lewis acid active sites
in a Fe catalyst.5,29 Besides, it is reported that heteroatomic
organic ligands in polymer materials are beneficial to the
adsorption of metal nanoparticles.30–33 Moreover, the SMR
reaction system is composed of lipophilic aromatic halogen
derivatives, hydrophilic phenylboronic acid derivatives, water-
soluble basic inorganic salts and a Pd catalyst. There are two
advantages of introducing heteroatoms into tubular polymeric
materials: (1) anchoring the active Pd nanoparticles and (2)
enhancing the interactions of the compounds in the SMR
reaction system.34 Since the pincer-like nitrogen and cis-
chelating structure in 1,10-phenanthroline ligands have strong
coordination ability with metal nanoparticles, phenanthroline-
based organic porous materials (POPMs) show excellent
catalytic performance in heterogeneous catalysis.33,35–37 Due to
the multiple reaction sites in the Friedel–Crafts polymerization
reaction, it is difficult for phenanthroline monomers to form a
long-range ordered structure like inorganic materials, and the
phenanthroline-based polymers mainly showed random block
shapes and rare ordered morphology.33,35–37 Currently, it is
difficult to synthesize POPMs with a nanotube morphology.
So far, heteroatoms (nitrogen and oxygen) have been
successfully doped into the framework of tubular
polymers,16,19,38–40 showing a large surface area. Nevertheless,
mostly tubular polymers were synthesized by using a tedious
template method,16,38 multi-step reactions,39 and a noble metal
catalyst.19,40 In this work, a new type of 1,10-phenanthroline-
based tubular polymer was easily synthesized through a one-
step Friedel–Crafts polymerization reaction with a non-noble
metal catalyst AlCl3. When used as a support for Pd-catalyzed
Fig. 1 Schematic for the synthesis of 1,10-phenanthroline-based
materials.
tetraphenylmethane and 30 mL of CH2Cl2 were added into an
eggplant flask and stirred at 600 rpm for 30 minutes at room
temperature to form a homogenous solution. Under the protection
of argon, 3.206 g of anhydrous AlCl3 was added to the above
solution, and the mixture was heated to 60 1C and stirred at
600 rpm for 48 h. After cooling to room temperature, the
collected precipitate was first washed with 15 mL of 2 M HCl
and then with 100 mL of ethanol. Finally, the residual AlCl3 was
removed by subsequent washing with ethanol, tetrahydrofuran
and methylene dichloride for 24 h using a Soxhlet extractor.
The final solid product was dried in a vacuum oven to give a
brown powder (yield: 96%), which is herein named PBP8.
Similarly, phenanthroline-based polymers PBP1, PBP2, PBP3,
PBP4, PBP5, PBP6, and PBP7 were synthesized following the
steps for the synthesis of PBP8 but adopting fluorene (0.333 g),
perylene (0.505 g), 1,3,5-triphenylbenzene (0.817 g), triptycene
(0.678 g), poly(styrene) (0.832 g), tetraphenylethylene (0.665 g),
and anthracene (0.357 g) as co-monomers, respectively.
At the same time, TMP was synthesized from tetraphenyl-
methane and phenanthrene via the same synthesis steps
of PBP8.
2.2 Synthesis of the PBP8-supported Pd catalyst
SMRs, the Pd nanoparticles (2–5 nm) are dispersed in the With many aromatic rings and pyridinic nitrogen atoms in the
tubular polymer support, and the tubular polymer support skeleton of 1,10-phenanthroline-based polymers, the as-
catalyst Pd/PBP8 displayed excellent selectivity, activity, and synthesized materials are endowed with hydrophilic and lipo-
recyclability.
philic properties. Consequently, the 1,10-phenanthroline-based
polymer materials can be dispersed in polar organic solvents.
The PBP8-supported Pd catalyst was prepared in acetonitrile
solution with strong polarity. In a typical run, 0.100 g of
palladium chloride powder and 30 drops of concentrated HCl
were dissolved into 30 mL of acetonitrile to obtain a clear
2. Experimental
2.1 Preparation of PBPs and TMP
As depicted in Fig. 1, all 1,10-phenanthroline-based polymer solution. Next, 1.000 g of PBP8 material was dispersed into the
(PBP) materials were synthesized by the AlCl3-catalyzed Scholl above solution and the mixture was heated at 70 1C in a round
reaction1,13,41–44 of 1,10-phenanthroline with a series of aromatic bottom flask for 12 h under Ar atmosphere. Finally, the solid
co-monomers in CH2Cl2 solvent, and the preparation details are was filtered out and rinsed with dichloromethane in a Soxhlet
provided in Table S1 of the ESI.† In a typical synthesis, 0.793 g of extractor for 24 h to remove the poorly adsorbed PdCl2, and
1,10-phenanthroline monohydrate monomer, 0.641
g of then dried in a vacuum oven at 50 1C for 12 h to obtain the
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supported Pd catalyst. The acquired catalyst is herein named system, and the corresponding activity was measured using a
Pd/PBP8. The loading of Pd in the corresponding catalyst GC9790-II gas chromatograph (Fuli GC9790-II, Zhejiang
is 0.14 wt%, which was measured by atomic absorption analytical instrument Inc., China) with naphthalene (0.5 mmol)
spectroscopy.
as an external standard. The products in the effluent were analyzed
using a flame ionization detector (FID) and an appropriate column
(HP-5, China) using a nitrogen carrier gas. The analysis was
2.3 Characterization of materials
The porosity, microstructure, thermal stability and other properties performed under the condition that the temperature was
of PBP and TMP materials were investigated by Fourier transform maintained at 80 1C for 3 minutes and then was heated to
infrared spectroscopy (FT-IR), thermogravimetric analysis 260 1C (at the rate of 15 1C minÀ1) and maintained for 3 min.
(TGA), scanning electron microscopy (SEM), transmission electron The collected catalyst was washed with ca. 50 mL of deionized water
microscopy (TEM), and N2 adsorption–desorption isotherms. and 50 mL of ethanol to remove impurities, and then dried in a
Among the prepared PBPs, PBP8 is the only material that has vacuum oven at 50 1C for 12 h before being used for the next cycle.
the features of both nanotube morphology and large specific
surface area. Hence, polymer nanotube PBP8 was selected as
the support for the loading of Pd (Pd/PBP8 catalyst) to catalyze
SMRs. The Pd/PBP8 catalyst was investigated by SEM, TEM, N2
3. Results and discussion
adsorption–desorption isotherms and X-ray photoelectron The pore structures of PBPs and TMP were investigated on the
spectroscopy (XPS). The Pd contents on both the fresh Pd/PBP8 basis of N2 adsorption–desorption isotherms. The isotherms
and Pd/TMP catalysts were analysed by measuring the precursor exhibit type-I isotherms with steep N2 uptake at a low relative
solutions of the Pd-loaded catalysts using atomic absorption pressure of 0 o P/P0 o 0.1, reflecting microporosity.4,44,46 Also,
spectroscopic analysis (AAS) and the Pd contents in the reaction the presence of H4-type hysteresis (0.1 o P/P0 o 0.9) suggests
mother liquids after the solid catalysts are removed from the the rich existence of mesopores.8,38 Moreover, the isotherms
Suzuki reaction were analysed by inductively coupled plasma– with steep N2 uptake at 0.9 o P/P0 o 1.0 indicate the macro-
optical emission spectroscopy (ICP-OES).
porosity. Thus, the N2 adsorption–desorption isotherms in
Fig. S1 and S2 (ESI†) show PBP1 with rich mesopores, PBP2
and PBP4 mainly with macropores, PBP3, PBP5, PBP6, and
2.4 General procedure for Suzuki–Miyaura reaction (SMR)
In a typical run, arylborane compound (0.75 mmol), base PBP8 with rich micropores and mesopores, PBP7 mainly with
(1.50 mmol) and Pd/PBP8 (4.9 mg, Pd : naryl mesopores and TMP with rich micropores. The BET surface
n
=
halide
0.00013 : 1) catalyst were dispersed in 3 mL of a solvent in a areas were calculated using the P/P0 values ranging from 0.05 to
10 mL brown glass sample bottle with PP cap and stirred with 0.35: 997 m2 gÀ1 (PBP1), 614 m2 gÀ1 (PBP3), 865 m2 gÀ1 (PBP5),
the rate of 600–700 rpm for 10 min to achieve a uniform 725 m2 gÀ1 (PBP6), 756 m2 gÀ1 (PBP8) and 629 m2 gÀ1 (TMP) are
dispersion, and then aryl halide (0.5 mmol) was added to the larger than 31 m2 gÀ1 (PBP2), 32 m2 gÀ1 (PBP4) and 192 m2 gÀ1
above dispersion to start the SMR. The progress of the reaction (PBP7). Despite PBP8 and TMP are similar in co-monomer
was monitored using TLC plates, in which petroleum ether was structure and specific surface area, the heteroatom distribution
used as the solvent at room temperature. The activity of the Pd/ and pore size distribution of PBP8 (rich in both micropores and
PBP8 is the isolated yield, which is the ratio of the actual mass mesopores: 0.77 nm and 2.43 nm) (Fig. S2b and d, ESI†) are
of the product separated by the TLC plates to the theoretical superior to those of TMP (only rich in micropores: 0.5 nm,
mass of the complete transformation of the reactants.
0.85 nm and 1.15 nm). Besides, it has been reported that proper
pore size distribution (2–50 nm) of support and proper palladium
nanoparticle size (o10 nm) distribution of the highly active sites
2.5 Hot filtration experiment
The hot filtration experiment was conducted by the reported are helpful for improving the catalyst performance.47,48
method,45 which was used to prove that the catalyst is a
The structure effect of co-monomers on the morphology and
heterogeneous catalyst. After the reaction of bromobenzene microstructure of PBPs is shown in Fig. S3 and S4 (ESI†). The
and phenylboronic acid under optimal conditions for 0.5 h, SEM images of phenanthroline-based polymers, except PBP8,
0.2 mL of the reaction liquid was taken to analyze the yield after display similar morphology with the variation of co-monomers.
the removal of the heterogeneous catalyst by filtration. The It is PBP8 from 1,10-phenanthroline and tetraphenylmethane
mother liquid was allowed to react continuously under the copolymerization that shows a nanotube morphology, while the
same conditions for another 3 h and then the yield was others display morphologies of stacked nanosheets. As revealed
analyzed again. Then, the Pd concentrations in the reaction in TEM analysis, PBP8 (Fig. 2a) and Pd/PBP8 (Fig. 2b) display
mother liquids were analysed by ICP-OES. The hot filtration tubular morphologies, and the outer and inner diameters of the
experiment was performed in three-fold.
tubes are ca. 100 nm and 11 nm, respectively. There are four Pd
nanoparticles with an average size of 3 nm in the Pd/PBP8-fresh
catalyst (as shown in the red ring of Fig. 2b), indicating that a
part of Pd(II) was reduced to Pd nanoparticles during
2.6 Recycling experiment for the SMR of bromobenzene with
phenylboronic acid
After the coupling reaction of bromobenzene and phenylboronic the synthesis of Pd/PBP8, which may be attributed to the
acid, the Pd catalyst was separated by filtration from the reaction electron-donating properties of PBP8. Besides, there are five
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before performance investigation. As listed in Table S2 (ESI†), upon
changing the bases as a single variable, Pd/PBP8 exhibited the best
activity (78%) when K3PO4Á3H2O was used as a base in ethanol–
water solution (VEtOH :VH O = 3:2) at 30 1C in air (Table S2, ESI,†
2
entries 1–7). To promote reactivity, solvents or mixtures of solvents
of medium polarity are often used to dissolve bases and organic
reactants. Due to phenylboronic acid derivatives and bases with
good hydrophilicity and aromatic halogen derivatives with good
lipophilicity, the influence of mixed solvents of water and different
organic solvents (entries 7–14) and mixed solvents of different
ratios of water and ethanol (entries 15–18) on SMRs yields has
been studied (Table S2, ESI†). Across the screened solutions, Pd/
PBP8 achieved the highest activity in an ethanol–water mixture
when the volume ratio of water-to-organic solvent was 2 : 3 (entry 7).
Additionally, the volume ratio of ethanol and water was also
screened (entries 15–18), and Pd/PBP8 exhibited even better activity
when the water-to-ethanol volume ratio was 3 : 2. In the absence of
Fig. 2 TEM images of (a) PBP8, (b) Pd/PBP8-fresh, (c) Pd/PBP8-1st used,
(d) TMP, (e) Pd/TMP-fresh and (f) Pd/TMP-1st used.
Pd nanoparticles with an average size of 4 nm in the Pd/PBP8- a base or catalyst, no biphenyl product was detected (entries 3 and
used catalyst (as shown in the red ring of Fig. 2c), indicating 19), indicating the necessity of a base and catalyst in the synthesis
that another part of Pd(II) was reduced to Pd nanoparticles of biphenyl products.30 As depicted in Table S3 (ESI†), Pd/PBP8
in SMRs.
To confirm the successful synthesis of PBP8, 13C-NMR and under optimized reaction conditions.
FT-IR spectra were collected. As shown in Fig. S5 (ESI†), the
According to the literature,51–55 the heterogeneity of the
with a nanotube morphology exhibits a high TOF value (3077 hÀ1
)
chemical shift at 39, 63 and 190 ppm can be attributed to C1, catalyst is usually evaluated by investigating the leaching
C2, and C6 in the tetraphenylmethane, methylene and phenan- behaviour in the catalytic reaction. To some extent, the hetero-
throline blocks, respectively,5,8,38 suggesting the successful geneity of the catalyst in the SMRs could be confirmed by
linking of tetraphenylmethane and phenanthroline by methylene. separated soluble Pd and solid Pd species at a certain time after
Compared to the FT-IR spectra of 1,10-phenanthroline and tetra- the reaction started. If the SMRs were catalysed on the surface
phenylmethane, the new peaks at 2850 cmÀ1 and 2930 cmÀ1 of solid Pd species, the yield of the reaction will not increase
detected over PBP8 (Fig. S6, ESI†) can be attributed to the with time delay once the solid Pd species were removed. At the
methylene groups,38 again indicating the successful connection same time, in our work, when the conversion of bromobenzene
of 1,10-phenanthroline and tetraphenylmethane by methylene via was around 58%, the Pd/PBP8 catalyst was removed from the
Friedel–Crafts polymerization. Thermogravimetric analysis reaction system and the mother liquids continued to react for
(Fig. S7, ESI†) revealed that PBP8 is thermally stable up to another 3 h. The result exhibited that there was no significant
280 1C. The residual mass at 850 1C was 15.1%, indicating the change in either the bromobenzene conversion or the product
high carbonization efficiency of PBP8 in an inert atmosphere. yield. Besides, after 0.5 h of the reaction, the Pd/PBP8 catalyst
The XRD pattern of PBP8 displays a broad peak at 211 (Fig. S8, removed from the reaction system was washed with water,
ESI†), suggesting that PBP8 is amorphous in nature.
ethanol and dichloromethane successively and the Pd content
To find out the merit of nanotube morphology on catalytic of the Pd/PBP8 catalyst was 0.14% as measured by AAS, which
performance, we selected PBP8 to support Pd to generate the was consistent with that of the fresh Pd/PbP8 catalyst. In the
Pd/PBP8 catalyst. The fixed pincer-like pyridine nitrogen atoms above mother liquids, no Pd was detected by ICP-OES, indicating
in 1,10-phenanthroline can coordinate with cations, giving a that no Pd species were leached from the Pd/PBP8 catalyst.
cis-chelating structure.33 The Pd-catalysed SMR, which is a Thus, we can conclude that the catalysis on the Pd/PBP8 catalyst
powerful tool to construct C–C bonds in organic synthesis, is indeed heterogeneous in nature.
was used as the model reaction to investigate the performance
After having the reaction conditions optimized, the Pd/PBP8
of Pd/PBP8. As seen in Fig. S4a and b (ESI†), the nanotube catalyst was tested for SMRs of different substrates and the
morphology of PBP8 remained after the loading of Pd results are summarized in Table 1. The phenyl-boronic acids
(0.14 wt.%). Besides, both Pd/PBP8-fresh (Fig. S4b, ESI†) and substituted with electron-donating (entries 2, 3, 6, 7, 8 and 9) or
Pd/PBP8-1st used (Fig. S4c, ESI†) samples have similar nano- electron-withdrawing groups (entries 1, 4, 5 and 10) afford
tube morphology. The results of SEM suggest that the structure excellent yields in the coupling with bromobenzene (80–98%,
of PBP8 is stable.
Table 1, entries 1–10) in 5–12 h at room temperature.
According to the mechanism of SMRs, the type and solubility of To demonstrate the universality of the Pd/PBP8 catalyst for SMRs,
bases strongly affect the SMRs, because bases act as both the co- the coupling of aryl bromide derivatives with phenylboronic acid
reactant and promoting agent to initiate the trans-metalation step was conducted. As shown in Table 1 (entries 11–15), the ortho-,
and promote the reductive elimination step, respectively.32,33,49 meta- and para-substituted bromobenzenes with electron-
Therefore, it is necessary to conduct optimization experiments donating groups (such as –CH3 and) achieved high yields
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Table 1 Suzuki–Miyaura reaction catalysed by Pd/PBP8a
Yieldb (%)
96
Entry
1
Reactant 1
Reactant 2
Temperature (1C)
Time (h)
5
TOFd (hÀ1
1478
)
30
2
3
4
30
30
30
7
7
5
97
96
97
1066
1055
1492
5
6
30
30
5
6
96
98
1478
1256
7
8
30
30
6
5
97
97
1244
1492
9
30
30
12
5
80
94
513
10
1446
11
12
13
30
30
70
9
98
99
98
838
846
558
9
13.5
14
30
9
97
829
15
16
30
70
9
98
97
838
553
13.5
17c
90
15
87
45
18c
19c
90
90
15
15
98
99
503
508
20c
90
15
95
487
a
Pd % = 0.013 mol% of catalyst, 0.5 mmol of aryl halide, 0.75 mmol of arylboronic acid, 1.5 mmol of base (K3PO4ÁH2O), 3 mL of mixture solvent
b
c
d
(VH O : Vethanol = 3 : 2), in air. Isolated yield. 90 1C, 15 h, in Ar. The TOF value from the beginning of the reaction until it reaches equilibrium.
2
(497%) after 9 h of reaction at room temperature. Generally, the that of C–Br. Therefore, the coupling of chlorobenzene is more
coupling of phenylboronic acid and bromobenzene derivatives difficult than that of bromobenzene. Nevertheless, chlorobenzene
with electron-withdrawing groups requires a high reaction and chloromethylbenzene give product yields of 99% and 95% at
temperature or long reaction time to obtain satisfactory 90 1C in 15 h, respectively (Table 1, entries 19 and 20). Compared
performance.30 For example, 4-hydroxy-bromobenzene afforded with some reported Pd catalysts, the Pd/PBP8 catalyst shows
98% yield at 70 1C in 13.5 h (Table 1, entry 16), and 4-nitro- excellent catalytic performance in SMRs.56,60,61 Besides, the
substituted bromobenzene obtained 98% yield at 90 1C in 15 h 1H-NMR spectra of the target products in Table 1 are shown in
(Table 1, entry 18). As for 2-nitrobromobenzene, the product yield Fig S9 (ESI†), which proved that SMRs went successfully with the
was 87% at 90 1C in 15 h, plausibly due to the combined effects of Pd/PBP8 catalyst.
electron-withdrawing groups and steric hindrance (Table 1, entry
Although the mechanism of SMRs involves a three-step cycle
17). In SMRs, the dissociation of C–Cl bonds is more difficult than of oxidative addition, transmetallation and reductive elimination,
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the oxidation addition reaction of aryl halide on the Pd(0) surface
plays a key role in SMRs.32,53,57–59 To illustrate the change of the
Pd/PBP8 catalyst before and after the reaction, TEM and XPS were
used to analyse the morphology and chemical state, respectively.
As shown in Fig. 2b and c, in the TEM images of both the fresh
Pd/PBP8 catalyst and the Pd/PBP8-1st used, there are several black
spherical particles of size 2–5 nm (in the red ring) that dispersed
in the tubular polymer. In the XPS investigation shown in Fig. 3a,
the two Pd 3d peaks at 335.9 eV and 341.2 eV can be attributed to
the Pd(0) species, while those at 338.1 eV and 343.1 eV to the Pd(II)
species.30 The fitting results reveal that there is 59.9 atom% of
Pd(0) species in the fresh Pd/PBP8 catalyst. It is deduced that the
black spots observed in Fig. 2b (B2 nm) and Fig. 2c (B3 nm)
could be related to the palladium nanoparticles [48]. In the
synthesis of fresh Pd/PBP8, the reduction of Pd(II) ions to Pd(0)
species may be attributed to the electron-donating ability of the
phenanthroline-based support.50
As shown in Fig. 3a, there is an increase of Pd(0) species
from 59.9% of the fresh Pd/PBP8 to 79.4% after 7 cycles of
reaction, which may be the result of the reduction of Pd(II) by
phenylboronic acid during the SMRs.61,62 Meanwhile, after
being reused 7 times (Fig. 4), the Pd/PBP8 catalyst only showed
a slight decrease of 4.2% in activity, implying good recyclability.
According to the results of AAS, the change of the Pd content
between Pd/PBP8-fresh and Pd/PBP8-used (for 7 times) is
insignificant, both with a value of ca. 0.14 wt.%. The results
clearly indicate that the Pd species are firmly anchored on the
Fig. 4 Catalytic activity of Pd/PBP8 and Pd/TMP in 7 cycles of SMRs.
1,10-phenanthroline units and there is almost no Pd leaching
during the reaction. The N atoms in the phenanthroline
monomer can form a stable chelate with metal ions like a
crab’s tongs to adsorb the metal ions, and the framework of the
organic porous material can confine the palladium nano-
particles. These two effects can inhibit the loss of palladium
active species (including metal ions and nanoparticles).
Catalysed by the Pd/PBP8-fresh and Pd/PBP8-7th used catalysts,
the SMR of bromobenzene and phenylboronic acid gave yields
of 97% and 93% within 5 h at 30 1C, respectively. The above
results indicate that the Pd/PBP8 catalyst has good catalytic
performance which is not inferior to that reported in the
literature (listed in Table S5, ESI†).
Compared to Pd/PBP8, Pd/TMP gives a higher product
yield (Table S3, ESI†) in the first cycle but with much poorer
recyclability (Fig. 4), which is mainly due to Pd leaching.
Without the help of heteroatoms for palladium anchoring,
there is an aggregation (see in the red ring of Fig. 2e) and
leaching (Fig. 2f) of the Pd nanoparticles. In addition, the XPS
spectrum of the ‘‘Pd/TMP-1st used’’ sample exhibited no Pd 3d
peaks, indicating the near-complete loss of the surface Pd
species (Fig. 3b). Because TMP and PBP8 were synthesized by
the polymerization of tetraphenylmethane with phenanthrene
and 1,10-phenanthroline, respectively, it can be deduced that
the excellent reusability of Pd/PBP8 is mainly due to the
inclusion of the heteroatom N in the PBP8 structural unit.
4. Conclusion
In summary, a series of PBPs were synthesized by a one-pot
Friedel–Crafts polymerization method. Among them, most of
the PBPs displayed a nanosheet morphology, except PBP8
which displayed both one-dimensional nanotubular morphology
and a large surface area of 745 m2 gÀ1. Due to the presence of N
atoms in the structural units of PBP8, the Pd nanoparticles (size
2–5 nm) reduced from Pd(II) were highly dispersed in the PBP8
nanotube in the synthesis of Pd/PBP8 (as shown in the TEM
image). At room temperature and in aqueous solutions, Pd/PBP8
catalyzed the SMRs of bromobenzene and 10 benzenboric acid
derivatives, respectively, and all their yields were more than 80%
Fig. 3 XPS Pd 3d spectra of (a) Pd/PBP8-fresh and Pd/PBP8-used and (b)
Pd/TMP-fresh and Pd/TMP-1st used.
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Conflicts of interest
There are no conflicts to declare.
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Acknowledgements
This work was supported by the PhD Start-Up Fund (30002/
2042019026) for Young Teachers of Huanggang Normal
University.
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