A quinoxaline-based porous organic polymer containing copper nanoparticles CuNPs@Q-POP as a robust nanocatalyst toward C-N coupling reaction

Article abstract of DOI:10.1039/d0ra10741g

A novel porous organic polymer (denoted by Q-POP) was successfully fabricated by free-radical copolymerization of allyl-substituted 2,3-di(2-hydroxyphenyl)1,2-dihydroquinoxaline, and divinylbenzene under solvothermal conditions and used as a new platform for immobilization of copper nanoparticles. The CuNPs@Q-POP nanocatalyst was prepared via incorporating of Cu(NO3)2 into the polymeric network, followed by the reduction of Cu2+ ion with hydrazine hydrate. The obtained materials were characterized through FT-IR, XRD, N2 adsorption-desorption isotherms, ICP, TGA, SEM, HR-TEM, EDX, and the single-crystal X-ray crystallography. The results displayed that Q-POP and CuNPs@Q-POP possessed high surface area, hierarchical porosity, and excellent thermal and chemical stability. The as-synthesized catalyst was utilized for the Ullmann C-N coupling reaction of aromatic amines and different aryl halides to prepare various diarylamine derivatives. All types of aryl halides (except aryl fluorides) were screened in the Ullmann C-N coupling reaction with aromatic amines to produce diaryl amines in good to excellent yields (50-98%), and it turned out that aryl iodides have the best results. Besides, due to the strong interactions between CuNPs, N, and O-atoms of quinoxaline moiety existing in the polymeric framework, the copper leaching from the support was not observed. Furthermore, the catalyst was recycled and reused for five consecutive runs without significant activity loss.

Full text of DOI:10.1039/d0ra10741g

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A quinoxaline-based porous organic polymer
containing copper nanoparticles CuNPs@Q-POP as
a robust nanocatalyst toward C–N coupling
reaction†
Cite this: RSC Adv., 2021, 11, 3655
*
Forough Gorginpour, Hassan Zali-Boeini
and Hadi Amiri Rudbari
A novel porous organic polymer (denoted by Q-POP) was successfully fabricated by free-radical
copolymerization of allyl-substituted 2,3-di(2-hydroxyphenyl)1,2-dihydroquinoxaline, and divinylbenzene
under solvothermal conditions and used as a new platform for immobilization of copper nanoparticles.
The CuNPs@Q-POP nanocatalyst was prepared via incorporating of Cu(NO3)2 into the polymeric
network, followed by the reduction of Cu2+ ion with hydrazine hydrate. The obtained materials were
characterized through FT-IR, XRD, N2 adsorption–desorption isotherms, ICP, TGA, SEM, HR-TEM, EDX,
and the single-crystal X-ray crystallography. The results displayed that Q-POP and CuNPs@Q-POP
possessed high surface area, hierarchical porosity, and excellent thermal and chemical stability. The as-
synthesized catalyst was utilized for the Ullmann C–N coupling reaction of aromatic amines and
different aryl halides to prepare various diarylamine derivatives. All types of aryl halides (except aryl
fluorides) were screened in the Ullmann C–N coupling reaction with aromatic amines to produce diaryl
amines in good to excellent yields (50–98%), and it turned out that aryl iodides have the best results.
Besides, due to the strong interactions between CuNPs, N, and O-atoms of quinoxaline moiety existing
in the polymeric framework, the copper leaching from the support was not observed. Furthermore, the
catalyst was recycled and reused for five consecutive runs without significant activity loss.
Received 22nd December 2020
Accepted 11th January 2021
DOI: 10.1039/d0ra10741g
rsc.li/rsc-advances
disqualify them for wide application in industry. In recent years,
heterogeneous catalysts are attracting substantial attention due
1. Introduction
to the role they played for synthetic organic transformations
owing to the advantages of good recyclability, reusability, and
ease of catalyst separation from the reaction mixture.21–26
Nevertheless, it is difficult to design and synthesize the
heterogeneous catalysts with uniform active centers similar to
homogeneous ones. Thus, the development of heterogeneous
catalysts, which cover the benets of both heterogeneous (ease
of recyclability) and homogeneous catalysts (easily accessible
active sites), have received much attention during recent
years.27–33 This can be done by introducing suitable active
centers on porous materials. Indeed, the porosity inside porous
materials makes their active centers more accessible and
increases reactivity between reactants and active sites. There-
fore, these materials can be considered as hosts for guest
encapsulation with unique functions such as energy storage,
separation, catalysis, sensing, drug delivery, and so on.34–36
Recently, metal–organic frameworks (MOFs) and covalent
organic frameworks (COFs), as porous materials, have been
utilized in the above-mentioned elds.37–45 Despite the extraor-
dinary porosity and large surface areas in MOFs and COFs, they
oen suffer from low thermal and chemical stability issues
under drastic reaction conditions due to weak or reversible
Ullmann coupling reaction is an effective organic synthetic tool
to construct carbon–heteroatom bonds leading to valuable
compounds with fascinating applications in various elds,
including pharmaceutical, biological, and material chem-
istry.1–9 In particular, due to the low toxicity and cost of copper-
catalyzed Ullmann coupling reactions, they have gained
considerable attention compared to other noble metal cata-
lysts.10–12 However, implementation of the stoichiometric
quantities of copper at harsh conditions (>200 ꢀC) and long
reaction times are required to run copper-mediated classical
Ullmann reactions.13,14 In this context, diverse copper catalysts
with various ligand groups have been developed for carbon–
heteroatom bond formation under relatively mild and homog-
enous conditions.15–20 Although signicant progress has been
attained in the homogeneous Cu-catalyzed coupling systems,
there are some limitations such as the use of expensive ligands,
low recyclability, and metal contamination of products that
Department of Chemistry, University of Isfahan, 81746-73441, Isfahan, Iran. E-mail: h.
zali@chem.ui.ac.ir; Tel: +98-37934925
† Electronic supplementary information (ESI) available. CCDC 1953774. For ESI
and crystallographic data in CIF or other electronic format see DOI:
10.1039/d0ra10741g
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Scheme 1 Synthetic strategy for fabrication of Q-POP and CuNPs@Q-POP.
chemical bonds in their structure.46 Given these constraints, the polymerization of divinylbenzene to nanoporous poly-
constructive units of porous organic polymers (POPs), as supe- divinylbenzene under solvothermal conditions, which was used
rior porous materials, are connected through irreversible cova- as an adsorbent for removal of organic pollutants from
lent bonds. They dedicate permanent porosity, high surface water.66,67 Based on the high reactivity of organic frameworks in
area, and excellent stability toward harsh reaction conditions, porous polydivinylbenzene, a Schiff base-modied PDVB has
which as a result, they have successfully gained wide applica-
tions in various elds of science.47–57 Additionally, numerous
organic functional groups can be integrated into the framework
of porous organic polymers to act as a proper anchorage to
immobilize catalytically active sites and provide heterogeneous
catalytic systems. Compared with inorganic porous materials,
porous organic polymers that can apply as platforms for
heterogeneous catalysis exhibit better catalytic performance.
Porous organic polymers contain many organic pores in their
structure; therefore, they provide an environment similar to
homogeneous conditions. Porous organic polymers can be
achieved in different ways,58–65 but most of the polymerization
methods occur in the presence of metals that are costly unfa-
vorable, and environmentally incompatible. Free radical poly-
merization of organic monomers with double bonds is
a suitable strategy for the synthesis of porous organic polymers
by taking advantage of low-costs, metal-free, and green strate-
gies. Due to these merits, the preparation of porous organic
polymers on a large scale is conceivable and highly desirable.
Fig. 1 ORTEP representation of compound B. Displacement ellipsoids
are drawn at the 30% probability level and H atoms are shown as small
spheres of arbitrary radii. The unit cell contains two independent
Recently, the free radical strategy was utilized for the
molecules. Only one molecule is showed for clarity.
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Fig. 2 (a) FT-IR spectra of Q-POP and CuNPs@Q-POP; (b) wide-angle powder XRD patterns of Q-POP and CuNPs@Q-POP; (c) TGA curves of
Q-POP and CuNPs@Q-POP.
been prepared through consecutive reactions on the PDVB successfully obtained through pre-functionalized strategies by
phenyl ring, in which incorporation of copper cations and free radical co-polymerization of functional monomers con-
complexation leads to PDVB–SB-Cu. The prepared material taining double bonds and divinylbenzene and used for various
served as a heterogeneous catalyst for the Ullmann C–O applications.69–74 Mondal et al. reported the fabrication of
coupling reaction using Cs2CO3 as an expensive base in DMSO a nanoporous polymer DVAC-1 through copolymerization of
as the solvent.68 Despite the fruitful achievements in the context acrylic acid and divinylbenzene under hydrothermal condi-
of post-modication of porous organic polymers, difficulties tions. The polymer was applied as a support for the deposition
such as reduced surface area and pore blocking during the post- of Cu0 nanoparticles.75 The resulting Cu0 supported nano-
synthetic strategies restrict their widespread use. To this end, porous polymer was employed as a heterogeneous nanocatalyst
a variety of functionalized porous organic polymers have been for Ullmann coupling of aryl halides and amines in water.
Fig. 3 (a) N2 sorption isotherms of Q-POP and CuNPs@Q-POP at 77 K; (b) pore size distribution of Q-POP and CuNPs@Q-POP calculated by
BJH model.
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Table 1 Textural parameters for Q-POP and CuNPs@Q-POP
activity and recyclability for C–N coupling reactions of amines
with aryl halides.
Material
SBET (m2 gꢁ1
)
VP (cm3 gꢁ1
)
DP (nm)
Q-POP
CuNPs@Q-POP
632.1
555.9
1.40
1.36
8.9
9.8
2. Results and discussion
2.1 Synthesis of Q-POP mesoporous polymer and CuNPs@Q-
POP catalyst
Notwithstanding the aforementioned developments in the area
of POPs, there is still a drastic need to prepare novel POPs
containing metal nanoparticles to overcome the previously
mentioned limitations and to improve their catalytic activities
and recyclability.
Herein, we describe the preparation of a quinoxaline-based
porous organic polymer (Q-POP) as a new functionalized mes-
oporous organic polymer with extraordinary porosity and high
stability for immobilization of Cu0 nanoparticles (CuNPs) and
the preparation of CuNPs@Q-POP catalyst (Scheme 1). This
novel heterogeneous catalyst demonstrated superior catalytic
Scheme 1 represents the synthetic pathway of Q-POP and
CuNPs@Q-POP. Initially, the bis(salicylidene)-o-phenylenediamine
(salophen) A was prepared through the reaction of O-phenyl-
enediamine with salicylaldehyde.76 Then, 2,3-di(2-hydroxyphenyl)
1,2-dihydroquinoxaline B was obtained via cyanide-catalyzed
cyclization reaction of the salophen under the argon atmo-
sphere. Aer that, allylation of B with allyl bromide produced
diallyl derivative of 2,3-di(2-hydroxyphenyl)1,2-dihydro quinoxa-
line C monomer as a new ligand. Subsequently, the desired Q-POP
was synthesized by the free-radical copolymerization of ligand C
and divinylbenzene (DVB) as the cross-linker, using
Fig. 4 SEM images of Q-POP (a and b), and CuNPs@Q-POP (c and d).
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azobisisobutyronitrile (AIBN) as the radical initiator under sol- 1167.6 cmꢁ1 and 1061.6 cmꢁ1 assigned to the C–N and C–O
vothermal conditions in the form of yellow solid. stretching vibrations emerged in the Q-POP structure compared
The resultant polymer was not soluble in the common with the FT-IR spectra of PDVB, which implies that ligand C was
organic solvents such as MeOH, THF, acetone, acetonitrile, successfully copolymerized with divinylbenzene.
DMF, DMSO, and water. Finally, aer the treatment of Q-POP
The wide-angle X-ray diffraction (WAXRD) pattern of Q-POP
with a solution of Cu(NO3)2$3H2O in methanol, Cu2+@Q-POP and CuNPs@Q-POP are demonstrated in Fig. 2b. The Q-POP
was yielded as a green solid. Indeed, Cu2+ ions were immobi- illustrates only one broad diffraction peak, indicating that the
lized on Q-POP through strong coordination bonds with N and as-synthesized polymer possesses an amorphous framework.
O-atoms present in Q-POP. In the end, CuNPs@Q-POP catalyst On the other hand, the XRD pattern of CuNPs@Q-POP displays
was produced as a deep brown solid through the reduction three additional diffraction peaks at 2q ¼ 43.45ꢀ, 50.5ꢀ, and
reaction of Cu2+ ions to CuNPs using hydrazine hydrate as the 74.6ꢀ related to diffractions of the (111), (200), (220) planes,
reducing agent. The copper content in the CuNPs@Q-POP was along with one broad peak as compared with Q-POP XRD
determined by inductively coupled plasma (ICP) analysis.
pattern. These observations validated the existence of CuNPs in
CuNPs@Q-POP.
Thermal stabilities of Q-POP and CuNPs@Q-POP were deter-
mined by thermal gravimetric analysis (TGA) under an argon
atmosphere. Fig. 2c displays that both ꢀof these compounds have
excellent thermal stabilities up to 300 C, which could be attrib-
uted to the existence of a cross-linked network.
2.2 Characterizations of DHQ ligand, Q-POP polymer, and
CuNPs@Q-POP catalyst
The formation of compound B as the key structure of the
1
CuNPs@Q-POP was evidenced by its H and 13C NMR spectra
The porosity of Q-POP and CuNPs@Q-POP were measured by
nitrogen adsorption and desorption isotherms at 77 K (Fig. 3a).
The isotherms for both Q-POP and CuNPs@Q-POP illustrate
a slight nitrogen uptake at low relative pressure and a steep
uptake with a vivid hysteresis loop at a high relative pressure (P/
and nally conrmed by its X-ray single crystallography analysis
(SCXRD, CCDC 1953774, Fig. 1).
Fourier transform infrared spectroscopy (FT-IR) was applied
to conrm the ligand C's incorporation in the polymeric
network (Fig. 2a). The appearance of extra new bands at
Fig. 5 Elemental mapping and EDX spectrum of N, O, and Cu for CuNPs@Q-POP.
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Fig. 4 displays the scanning electron microscopy (SEM)
images of Q-POP and CuNPs@Q-POP. Q-POP shows a spherical
morphology with spheres involving tiny irregular particles
interconnected with each other. In addition, agglomerated
particles and abundant nanoporosity can be seen in the SEM
image (Fig. 4a and b). It should be mentioned that SEM images
of CuNPs@Q-POP are approximately similar to Q-POP, indi-
cating that morphology is well preserved aer Cu loading
(Fig. 4c and d).
To evidence the presence of C, O, N and Cu elements in the
CuNPs@Q-POP, the energy-dispersive X-ray spectroscopy (EDX)
and elemental mapping were carried out (Fig. 5). The elemental
mapping images indicate that Cu nanoparticles are rather
uniformly dispersed in the porous organic polymer DHQ.
The high-resolution transmission electron microscopy (HR-
TEM) is shown in Fig. 6. As revealed in HR-TEM, CuNPs@Q-
POP are composed of polymeric frameworks in which the
CuNPs are consistently distributed on the Q-POP. Additionally,
the HR-TEM image demonstrates that the copper nanoparticles
(average size below 6 nm) are well dispersed in the Q-POP
texture without the obvious agglomeration.
Fig. 6 HR-TEM image of CuNPs@Q-POP.
P0) of above 0.6, implying type-IV plus type-I isotherm behavior. 2.3 N-Arylation of anilines with aryl halides catalyzed by
Therefore, according to the hysteresis loop present in N2 sorp- CuNPs@Q-POP
tion isotherm and the distribution curves of the pore size using
Initially, we surveyed the reaction of aniline with iodobenzene
the BJH model (Fig. 3b), both compounds possess hierarchical
as model substrates for the synthesis of diphenylamine. As
porosity structure with dominant mesopores, which is highly
indicated in Table 1, a series of parameters, including the
inuence of Cu loading into Q-POP, amount of catalyst, base
type, solvent, temperature, and the reaction time were investi-
desirable for promoting the mass transfer of reactants and
better catalytic activity.
The BET analysis of Q-POP and CuNPs@Q-POP showed a high
gated to nd out the optimum reaction conditions. The reaction
surface area 632.1 and 555.9 m2 gꢁ1, respectively (Table 1).
in the presence of CuNPs@Q-POP (7.3 mol% Cu0 loading) gave
Table 2 Optimization of various parameters on the cross-coupling of iodobenzene and anilinea
Entry
Catalyst
Cat. (mg)
Solv.
Base
Convb. (%)
Selb. (%)
1
2
3
4
CuNPs@Q-POP (7.3%)
CuNPs@Q-POP (2.8%)
CuNPs@Q-POP (2.8%)
CuNPs@Q-POP (2.8%)
CuNPs@Q-POP (2.8%)
CuNPs@Q-POP (2.8%)
CuNPs@Q-POP (2.8%)
CuNPs@Q-POP (2.8%)
CuNPs@Q-POP (2.8%)
CuNPs@Q-POP (2.8%)
CuNPs@Q-POP (2.8%)
No catalyst
75
75
50
25
75
75
75
75
75
75
75
—
75
75
75
PEG-200
PEG-200
PEG-200
PEG-200
DMF
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
NaOH
K3PO4
Et3N
98
98
81
63
60
25
0
70
70
30
0
100
100
100
100
100
100
0
100
100
100
0
5
6c
7
H2O
Tolu.
8
9
PEG-200
PEG-200
PEG-200
PEG-200
PEG-200
PEG-200
PEG-200
PEG-200
10
11
12
13
14d
15e
—
K2CO3
K2CO3
K2CO3
K2CO3
0
0
70
80
0
0
100
100
Q-POP
CuNPs@Q-POP (2.8%)
CuNPs@Q-POP (2.8%)
a
Reaction condition: iodobenzene (1 mmol), aniline (1.2 mmol), catalyst, solvent (2.5 mL), base (2 mmol), 110 ꢀC, 24 h. b Conversion and selectivity
were determined by GC. c The reaction was performed in an autoclave. d 90 ꢀC. e 12 h.
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diphenylamine in 98% conversion (Table 2, entry 1). We were (Table 2, entry 13) showed no conversion. Additionally, inves-
surprised by the fact that CuNPs@Q-POP catalyst could also tigation of reaction temperature on the progress of the reaction
perform the same reaction with almost complete conversion, indicated that with a decrease in temperature from 110 to 90 ꢀC,
even when a 2.8 mol% of copper was loaded into Q-POP (Table the reaction conversion was diminished from 80 to 70% (Table
2, entry 2). In the reaction of iodobenzene (1 mmol) and aniline 2, entries 14 and 15). Moreover, our careful studies claried that
(1.2 mmol) when the amount of catalyst was decreased from 75 the best reaction conversion was achieved aer 24 h. Conse-
to 50, and to 25 mg, the reaction conversion was decreased from quently, the best conversion and selectivity for the N-arylation of
98 to 81 and 63% under the same reaction times, respectively iodobenzene with aniline was obtained using K2CO3 as the
(Table 2, entries 3 and 4). Screening of solvents in the reaction base, PEG-200 as the solvent, and in the presence of CuNPs@Q-
0
ꢀ
revealed that PEG-200 was considered the best solvent for the POP (2.8% Cu , 75 mg) at 110 C for 24 h.
C–N cross-coupling reaction using CuNPs@Q-POP catalyst.
Considering the optimized condition, various aniline derivatives
Moreover, the investigations showed that the presence of a base bearing both electron-donating and electron-withdrawing substitu-
in the reaction mixture was essential, and K2CO3 had the ents were utilized in the coupling reaction with iodobenzene in the
highest reaction conversion compared to other bases such as presence of CuNPs@Q-POP (2.8% Cu) as the catalytic system. As
NaOH, K3PO4, and Et3N (Table 2, entries 8–10). We also found illustrated in Table 3, anilines possessing electron-donating
that performing the reaction at 110 ꢀC for 24 h without catalyst substituents such as methoxy and/or a methyl group at para posi-
(Table 2, entry 12) or even in the presence of 75 mg Q-POP tion produced the corresponding diarylamines in excellent
Table 3 Ullmann N-arylation reaction catalyzed by CuNPs@Q-POP(2.8% Cu)a
Entry
1
Aryl halides
Nucleophiles
Products
Conversionb (%)
98
Selectivityb (%)
100
2
3
4
5
6
7
8
95
97
90
90
89
91
50
100
100
100
100
80
85
100
9
96
100
10
90
69
100
100
11
a
Reaction condition: aryl halides (1 mmol), amines (1.2 mmol), K2CO3 (2 mmol), PEG-200 (2 mL), CuNPs@Q-POP (2.8% Cu, 75 mg), 110 ꢀC, 24 h,
b
under N2 atmosphere. Conversion and selectivity were determined by GC.
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Table 4 Comparison of catalytic performance of CuNPs@Q-POP in the N-arylation with the other reported method
Catalyst
Condition
N-arylation
Ref.
CuNPs@Q-POP
Meso Cu/MnOx
K2CO3, PEG-200, 110 ꢀC
K2CO3, DMF, 140 ꢀC
Conversion (98%)
Selectivity (100%)
Conversion (5%)
Selectivity (90%)
Conversion (26%)
Yield (93%)
This work
77
CuO/AB
PS-Cu-furfural
KOtBu, toluene, 180 ꢀC
78
79
Cs2CO3, DMSO, t-Bu4NBr, 120 ꢀ
C
conversions, and selectivity (Table 3, entries 2 and 3), while anilines Hg poisoning test using an excess amount of Hg0 was per-
with a methoxy group at ortho or meta position on the phenyl ring formed. When the optimal conditions was used to perform the
gave slightly lower conversions with also 100% selectivity (Table 3, reaction of aniline and iodobenzene (entry 1, Table 3) using
entries 4 and 5). 4-Nitro and 4-chloroaniline showed very good CuNPs@Q-POP catalyst in the presence of excess amount of
conversions with 80 and 85% selectivity aer 24 h, respectively metallic mercury, the reaction conversion was dropped drasti-
(Table 3, entries 6 and 7). Furthermore, when 2-chloroaniline was cally from 100 to <10% as a result of Hg–Cu amalgam forma-
applied as substrate, the conversion rate was 50% and the desired tion. This clearly approve that, the CuNPs@Q-POP is
product was afforded with complete selectivity (Table 3, entry 8). heterogeneous. A hot ltration test was also accomplished to
Indole as a heterocyclic amine was also coupled well with iodo- further survey the heterogeneous nature of separated
benzene to generate an N-arylated product with excellent selectivity CuNPs@Q-POP. In the N-arylation of iodobenzene with aniline
(100%, Table 3, entry 9). The coupling reaction of aniline with under optimized conditions, the catalyst was ltered from the
bromobenzene and chlorobenzene was also successful, with reaction mixture aer 8 h (57% conversion). Then, the reaction
conversion rates of 90 and 69%, respectively, along with 100% of the ltrate was further continued at the same reaction
selectivity for both of them (Table 3, entries 10 and 11).
conditions for an additional 16 h. No changes in conversion and
selectivity were observed aer removing the catalyst. According
to the results of this experiment, we concluded that there were
no copper nanoparticles in the ltrate and the atomic absorp-
tion spectroscopy conrmed it. Nevertheless, a closer investi-
gation of the used catalyst by ICP showed that a very little
leaching of the copper nanoparticles (0.7%) was seen in the
solution and the nal content of copper in the CuNPs@Q-POP
catalyst was 2.78%. High catalyst stability and low copper
leaching can be attributed to the strong chemical interaction
between CuNPs and N and O binding sites in Q moiety present
in the polymer network.
2.4 The comparison of our method with some reported
method in the literature
In Table 4, the catalytic activity of our present catalytic system
with various reported heterogeneous catalysts in the N-arylation
of iodobenzene with aniline is compared.77–79 Our synthesized
CuNPs@Q-POP catalyst demonstrates better or comparable
performance over reported catalysts. It is more likely, high
catalytic activity and durability of our catalyst can be ascribed to
the extraordinary stability of the polymeric network as a support
along with large pore size playing the role of nanoreactor and
the facilitated diffusion of reactants inside the polymeric
network of catalyst. On the other hand, the N-arylation of
iodobenzene with aniline occurred in PEG-200 as a safe and eco-
friendly solvent over our CuNPs@Q-POP catalyst.
To check the recyclability of the catalyst, CuNPs@Q-POP as
a solid was separated from the reaction mixture by ltration and
then washed with water, ethyl acetate, and methanol, respec-
tively, and dried under vacuum. Ultimately, the isolated and
rened catalyst was applied for numerous consecutive recycle
runs under the optimized reaction conditions in the N-arylation
of iodobenzene with aniline. It was found that the recycled
catalyst for the C–N cross-coupling reaction could be reused ve
times without appreciable reduction in the conversion rate and
selectivity (Fig. 7). Wide-angle X-ray diffraction (WAXRD)
patterns of the reused CuNPs@Q-POP aer the h run were
similar to the fresh nanocatalyst (Fig. S1†). The SEM images
indicate that the morphology of CuNPs@Q-POP is well main-
tained aer ve runs (Fig. S2†). Furthermore, ICP analysis
reveals that the reused CuNPs@Q-POP possesses Cu content of
2.74 mol%, which is close to fresh CuNPs@Q-POP (2.8 mol%).
Fig. 7 Recyclability of CuNPs@Q-POP Ullmann N-arylation. Reaction
conditions for cross-coupling of iodobenzene and aniline: iodo-
benzene (1 mmol), aniline (1.2 mmol), K2CO3 (2 mmol), PEG-200 (2
mL), CuNPs@Q-POP (2.8% Cu, 75 mg), 110 ꢀC, 24 h, under N2
To conrm that the CuNPs@Q-POP is a heterogeneous catalyst, atmosphere.
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recycling ve times. We found that the as-synthesized
CuNPs@Q-POP catalyst might be used in other synthetic
transformations. Meanwhile, Q-POP polymer has the potential
to be used in many other research elds such as the light-
emissive layer in OLEDs (due to its intense uorescence),
making different catalysts with changing copper with other
metals, removing of organic chemical pollutants, and toxic
heavy metal ions from wastewater. The aforementioned studies
are currently underway in our laboratory.
Conflicts of interest
Authors hereby conrm that there is no competing interests to
declare.
Acknowledgements
The authors are grateful to the University of Isfahan research
council for providing instrumental facilities and partial nan-
cial support for this work.
Scheme 2 The plausible reaction mechanism for the N-arylation of
iodobenzene with aniline catalysed by CuNPs@Q-POP.
The feasibility of a multi-gram scale synthesis was also
checked for the catalyst. When the C–N coupling reaction of
aniline was performed with iodobenzene in a 50 mmol scale,
the resulting diphenylamine compounds were produced with
a nearly complete conversion.
A plausible mechanism for the N-arylation of iodobenzene
with aniline is shown in Scheme 2. The catalytic cycle
commences with the oxidative addition of iodobenzene to the
Cu0 nanoparticles of CuNPs@Q-POP to give the intermediate
(I). Subsequently, the intermediate (I) is reacted with the
nucleophile to form the intermediate (II) followed by reductive
elimination to generate the desired product.
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