Synergistic effect of copper nanocrystals-nanoparticles incorporated in a porous organic polymer for the Ullmann C-O coupling r–eaction

Article abstract of DOI:10.1016/j.mcat.2021.111460

A quinoxaline-based porous organic polymer (Q-POP) as a mesoporous organic copolymer was developed as a new platform for the immobilization of CuNPs and copper nanocrystals. The prepared materials were characterized by FT-IR, XRD, N₂ adsorption-desorption isotherms, ICP, TGA, SEM, HR-TEM, EDX, and single-crystal X-ray crystallography. The obtained catalyst presented extraordinary catalytic activity towards Ullmann C–O coupling reactions with high surface area, hierarchical porosity, and excellent thermal and chemical stability. Due to its high porosity, and synergistic effect of copper nanocrystals incorporated in the polymer composite, the as-synthesized catalyst was successfully utilized for the Ullmann C–O coupling reaction of phenols and different aryl halides to prepare various diaryl ether derivatives. All types of aryl halides (except aryl fluorides) were screened in the Ullmann C–O coupling reaction with phenols to produce diaryl ethers in good to excellent yields (70–97 %), and it was found 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:10.1016/j.mcat.2021.111460

Molecular Catalysis 504 (2021) 111460
Contents lists available at ScienceDirect
Molecular Catalysis
journal homepage: www.journals.elsevier.com/molecular-catalysis
Synergistic effect of copper nanocrystals-nanoparticles incorporated in a
–
porous organic polymer for the Ullmann C-O coupling r eaction
Forough Gorginpour, Hassan Zali-Boeini*
Department of Chemistry, University of Isfahan, 81746-73441, Isfahan, Iran
A R T I C L E I N F O
A B S T R A C T
Keywords:
A quinoxaline-based porous organic polymer (Q-POP) as a mesoporous organic copolymer was developed as a
new platform for the immobilization of CuNPs and copper nanocrystals. The prepared materials were charac-
terized by FT-IR, XRD, N2 adsorption-desorption isotherms, ICP, TGA, SEM, HR-TEM, EDX, and single-crystal X-
Heterogeneous catalysis
Mesoporous materials
Copper nanoparticles
Ullmann C-O coupling
–
ray crystallography. The obtained catalyst presented extraordinary catalytic activity towards Ullmann C
O
coupling reactions with high surface area, hierarchical porosity, and excellent thermal and chemical stability.
Due to its high porosity, and synergistic effect of copper nanocrystals incorporated in the polymer composite, the
–
as-synthesized catalyst was successfully utilized for the Ullmann C O coupling reaction of phenols and different
aryl halides to prepare various diaryl ether derivatives. All types of aryl halides (except aryl fluorides) were
–
screened in the Ullmann C O coupling reaction with phenols to produce diaryl ethers in good to excellent yields
(70–97 %), and it was found that aryl iodides have the best results. Besides, due to the strong interactions be-
tween 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.
1. Introduction
free radical polymerization of organic monomers with double bonds has
been used as a cost-effective, and metal-free strategy for the synthesis of
In recent years, porous materials have attracted enormous attention
in different areas such as catalysis, energy storage, separation, sensing,
drug delivery, and so on [1–3]. To date, metal-organic frameworks
(MOFs) and covalent organic frameworks (COFs), as porous materials,
have been also employed in the various fields [4–12]. However, the
reversible nature of chemical bonds in their structure leads to the low
thermal and chemical stability under the harsh reaction conditions that
restrict their real utilization [13]. In this regard, porous organic poly-
mers (POPs) are an excellent class of porous materials because of their
permanent porosity, large surface area, and strong irreversible covalent
bonds in the network that result in their superior physical-chemical
stability, make them appropriate for the potential applications in the
various fields of science [14–24]. Therefore, the design and synthesis of
porous organic polymers with organic functional groups in the frame-
work have drawn remarkable research endeavors. For this mean, several
synthetic strategies have been developed for the fabrication of POPs
[25–32], but most of polymerization methodologies, suffer from
expensive noble-metal catalysts, and high temperature. Alternatively,
POPs. A nanoporous polydivinylbenzene was prepared via free radical
polymerization of divinylbenzene under solvothermal conditions and
used as an adsorbent for removing the pollutants from water [33,34].
Using pre-functionalized strategies, a variety of functionalized porous
organic polymers have been acquired via free radical co-polymerization
of functional monomers containing double bonds and divinylbenzene
and demonstrated wide applications in various areas [35–40].
Ullmann cross-coupling for carbon-heteroatom bonds formation is
one of the most powerful and convenient approaches for the synthesis of
valuable compounds with rich applications in the areas of pharmaceu-
tical, biological, and material chemistry [41–49]. The homogeneous
copper-catalyzed Ullmann coupling in the presence of diverse ligand is
one of the conventional methods for carbon-heteroatom bonds forma-
tion [50–55]. However, these homogeneous Cu-catalyzed coupling
systems suffer from the difficulties including separation from the prod-
ucts, non-reusability, and the use of expensive ligands, which limit their
wide usage. By consideration of all of the abovementioned drawbacks of
homogeneous catalysis, remarkable attention has been focused on
* Corresponding author.
E-mail address: h.zali@chem.ui.ac.ir (H. Zali-Boeini).
https://doi.org/10.1016/j.mcat.2021.111460
Received 16 November 2020; Received in revised form 3 February 2021; Accepted 4 February 2021
Available online 15 February 2021
2468-8231/© 2021 Elsevier B.V. All rights reserved.

F. Gorginpour and H. Zali-Boeini
Molecular Catalysis 504 (2021) 111460
developing the heterogeneous catalysis due to simplicity of workup,
reusability, and minimization of metal contamination of products
[56–61]. Recently, the porous organic polymers (POPs) have been uti-
lized as promising host and support to immobilize active catalytic metal
species due to their unique properties [62–64]. For instance, Xiao et al.
[65] reported incorporation of copper cations into Schiff-based modified
PDVB that indicated excellent performance for Ullmann biaryl ether
coupling reaction. Mondal et al. [66] successfully deposited Cu nano-
particles on nanoporous polymer DVAC-1 and found that Cu◦ supported
on nanoporous polymer showed extraordinary catalytic activity for
Ullmann coupling of aryl halides and amines in water.
Bruker X-ray diffractometer using Cu K
α
(λ = 1.54◦A) radiation. Nitro-
gen sorption isotherms were carried out using a BELsorb-mini 2 at 77 K.
The specific surface areas and the pore size distributions of materials
were determined by the Brunauer-Emmett-Teller (BET) method and the
Barrett-Joyner-Halenda (BJH) model, respectively. Before analysis, the
samples were outgassed at 120 ◦C for 4 h under vacuum. The Fourier
transform infrared (FT-IR) spectra were recorded on Jasco FTIR-
6300spectrometer as KBr pellets. Thermal gravimetric analysis (TGA)
was obtained using an SDT Q600 instrument by heating samples from 25
to 600 ◦C in a dynamic argon atmosphere with a heating rate of 10 ◦C
minꢀ 1. Field-emission scanning electron (FE-SEM) microscopy and
energy-dispersive X-ray spectroscopy (EDX) analysis were carried out on
a Tescan Mira3 scanning electron microscope. Transmission electron
microscopy (TEM) images were performed on Philips-CM120. Induc-
tively coupled plasma (ICP) was measured on an Analytic Jena PQ9000
for the determination of Cu content. 1H NMR (400 MHz) and 13C NMR
(100 MHz) spectra were recorded on a Bruker Avance 400 MHz NMR
spectrometer.
In this contribution, we describe the preparation of a quinoxaline-
based porous organic polymer (Q-POP) as a new functionalized meso-
porous 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 activity and recyclability for C
O
coupling reactions of phenols with aryl halides.
2. Experimental section
2.2. Synthesis of bis(salicylidene)-O-phenylene diamine (A)
2.1. Materials and instruments
This compound was prepared according to the procedure reported in
the literature [67]. A solution of o-phenylenediamine (2.16 g, 20 mmol)
in dry methanol (30 mL) was added to a solution of salicylaldehyde
(4.24 mL, 40 mmol) in dry methanol (20 mL) at room temperature. After
stirring for 16 h, the resulting solid was separated by filtration. The
obtained solid was washed with methanol and dried in vacuo to afford
All chemicals were purchased from commercial sources and used
without further purification. Solvents used in this study were dried and
purified using standard procedures before usage. The powder X-ray
diffraction (XRD) patterns of samples were performed on a D8 Advance
Scheme 1. Synthetic strategy for fabrication of Q-POP and CuNPs@Q-POP.
2

F. Gorginpour and H. Zali-Boeini
Molecular Catalysis 504 (2021) 111460
compound (A) as yellow powders (6 g, 95 %).
green Cu2+@Q-POP was isolated by filtration, washed with MeOH for
removal of excess Cu(NO3)2.3H2O, and then dried in vacuo for overnight.
Next, CuNPs@Q-POP was synthesized by the reduction of Cu2+@Q-POP.
An aqueous solution of hydrazine hydrate (10 mL, 35 %) was added
dropwise with vigorous stirring under argon atmosphere to a mixture of
Cu2+@Q-POP (1.5 g), EtOH (17.5 mL), H2O (10 mL), and NH4OH (17.5
mL, 25 %). Afterward, the mixture was transferred into a 100 mL
stainless steel autoclave and treated at 100 ◦C for 4 h. Finally, the
mixture was filtered and washed with water and methanol successively.
The resultant brown solid was dried under vacuum to obtain CuNPs@Q-
POP. Inductively coupled plasma (ICP) analysis displayed a Cu content
of 7.3 % in the CuNPs@Q-POP.
2.3. Synthesis of 2,3-di(2-hydroxyphenyl)1,2-dihydro quinoxaline (B)
Under an argon atmosphere, NaCN (0.226 g, 4.6 mmol) was intro-
duced into a solution of bis(salicylidene)-o-phenylenediamine (3.7 g,
11.8 mmol) in dry DMF (35 mL), and the mixture was stirred at room
temperature for 48 h. Subsequently, the reaction mixture was poured
into ice water (50 mL). The resultant solid was filtered via Buchner
funnel, washed with water for several times, and dried. The crude
product was purified by recrystallization in acetonitrile. Finally, the
pure desired product was obtained as an orange solid (2.96 g, 80 %). 1H-
NMR (400 MHz, CDCl3): δ 14.92 (s, 1 H), 10.18 (s, 1 H), 7.44 (d, J = 8
Hz, 1 H), 7.33ꢀ 7.29 (m, 1 H), 7.24 (d, J = 8 Hz, 1 H), 7.08ꢀ 7.04 (m, 1
H), 7.01ꢀ 6.96 (m, 1 H), 6.92ꢀ 6.86 (m, 4 H), 6.81ꢀ 6.77 (m, 1 H), 6.66
(s, 1 H), 6.64 (s, 1 H), 6.62ꢀ 6.59 (m, 1 H), 6.225 (d, J = 4 Hz, 1 H); 13C-
NMR (100 MHz, CDCl3): δ 161.7, 161.1, 152.6, 136.8, 132.6, 129.3,
129.1, 128.9, 127.4, 127.0, 125.7, 119.4, 118.5, 117.5, 117.2, 117.1,
115.7, 114.1, 45.5.
2.7. Synthesis of CuNPs@Q-POP(2.8 % Cu)
To a round-bottom flask, Q-POP (1.5 g) was suspended in a mixture
of MeOH (50 mL) and Et3N (1 mL). The resultant suspension was
allowed to stir at room temperature for 1 h. Then, Cu(NO3)2.3H2O (0.2
g) in MeOH (6 mL) was added and refluxed for about 12 h and the
resultant green Cu2+@Q-POP precipitate was filtered, washed with
MeOH, and dried in vacuo overnight. Next, an aqueous solution of hy-
drazine hydrate (5 mL, 35 %) was added dropwise with intense stirring
under argon atmosphere to the mixture of Cu2+@Q-POP (1.5 g), EtOH
(17.5 mL), H2O (10 mL), and NH4OH (17.5 mL, 25 %). Then, the mixture
was heated at 100 ◦C for 4 h in an appropriate stainless steel autoclave.
Finally, the mixture was filtered and successively washed with water
and methanol. Thereafter, the brown solid was dried under vacuum to
obtain CuNPs@Q-POP. Inductively coupled plasma (ICP) analysis
revealed that the Cu content of CuNPs@Q-POP was 2.8 %.
2.4. Synthesis of allyl-functionalized 2,3-di(2-hydroxy phenyl)1,2-
dihydroquinoxaline (C)
A solution of 2,3-di(2-hydroxyphenyl)1,2-dihydro quinoxaline (2.96
g, 9.4 mmol) in acetone (60 mL) was shaken with K2CO3 (3.9 g, 28.2
mmol). Allyl bromide (2.44 mL, 28.2 mmol) was then added slowly to
the solution. After the mixture was stirred at room temperature for 48 h,
water was added (20 mL) and the product was extracted with ethyl
acetate (3 × 30 mL) and dried over MgSO4. The solvent was evaporated
under vacuum and the crude product was purified by column chroma-
tography on silica gel (EtOAc/hexane 1:4) to yield allyl functionalized
2,3-di(2-hydroxyphenyl)1,2-dihydroquinoxaline monomer as an orange
powder (1.86 g, 50 %). 1H-NMR (400 MHz, CDCl3): δ 14.98 (s, 1 H), 7.31
(d, J = 8 Hz, 1 H), 7.29 (d, J = 8 Hz, 1 H), 7.17ꢀ 7.02 (m, 4 H), 6.88 (d, J
= 8 Hz, 1 H), 6.79 (d, J = 8 Hz, 1 H), 6.71ꢀ 6.64 (m, 3 H), 6.55 (d, J = 8
Hz, 1 H), 6.30 (s, 1 H), 6.14ꢀ 6.07 (m, 1 H), 5.61ꢀ 5.54 (m, 1 H), 5.46
(dd, J = 18 Hz, J = 4 Hz, 1 H), 5.32 (dd, J = 10 Hz, J = 4 Hz, 1 H), 5.03
(dd, J = 18 Hz, J = 4 Hz, 1 H), 4.97 (dd, J = 12 Hz, J = 4 Hz, 1 H),
4.63ꢀ 4.62 (m, 2 H), 3.92ꢀ 3.91 (m, 2 H). 13C NMR (100 MHz, CDCl3): δ
161.5, 160.0, 152.3, 135.8, 132.6, 131.9, 131.3, 129.9, 128.9, 128.1,
127.9, 126.4, 126.3, 125.5, 120.8, 117.2, 116.9, 116.6, 116.5, 115.5,
111.3, 110.5, 68.0, 50.6, 50.5.
2.8. General procedure for O-arylation of phenol with aryl halides
catalyzed by CuNPs@Q-POP(7.3 % Cu)
In a typical reaction in a 25 mL round bottom flask (two necked-
flask) a mixture of aryl halide (1 mmol), phenol (1.2 mmol), K2CO3 (2
mmol), DMF (2 mL) and CuNPs@Q-POP(7.3 % Cu) catalyst (75 mg) was
stirred under nitrogen atmosphere at 110 ◦C for 24. After the reaction,
ethyl acetate (10 mL) was added to the reaction mixture and the catalyst
was separated by filtration and washed with ethyl acetate. After water
(10 mL) was added to the filtrate, extracted with ethyl acetate (2 × 10
mL) and the organic phase was dried with anhydrous MgSO4. The
organic phase was analyzed by GC to determine conversion and selec-
tivity. Then the solvent was removed under reduced pressure and the
crude product was purified by column chromatography over silica gel to
obtain the desired product. The product was analyzed by 1H-NMR and
13C-NMR.
2.5. Synthesis of Q-POP
Radical copolymerization of ligand C with divinylbenzene (DVB) as
cross-linker, in the presence of azobisisobutyronitrile (AIBN) as a radical
initiator, was used to synthesis the nanoporous polymer under sol-
vothermal conditions. Typically, the monomer C (1 g, 2.52 mmol) and
divinylbenzene (1.43 mL, 10.1 mmol) were dissolved in dry DMF (10
mL), followed by the addition of AIBN (0.05 g). The reaction mixture
was degassed with argon gas for 30 min, transferred into a 50 mL
stainless steel autoclave, and heated at 100 ◦C for 24 h. After cooling to
room temperature, water (30 mL) was added to the reaction mixture and
the resulting polymer was filtered and washed with excess water.
Additionally, the as-prepared polymer was further washed with meth-
anol for the elimination of any unreacted monomer C. Finally, the
desired polymer was obtained as a yellow powder and dried in an oven
at 90ꢀ 100 ◦C overnight.
3. Results and discussion
3.1. Synthesis of Q-POP mesoporous polymer and CuNPs@Q-POP
catalyst
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-phenylenediamine with
salicylaldehyde.
Then, 2,3-di(2-hydroxyphenyl)1,2-dihydroquinoxaline B was ob-
tained via cyanide-catalyzed cyclization reaction of the salophen under
the argon atmosphere. After that, allylation of B with allyl bromide
produced diallyl derivative of 2,3-di(2-hydroxyphenyl)1,2-dihydro
quinoxaline 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 azobisisobutyroni-
trile (AIBN) as the radical initiator under solvothermal conditions in the
form of yellow solid.
2.6. Synthesis of CuNPs@Q-POP(7.3 % Cu)
To a round-bottom flask were introduced Q-POP (1.5 g), MeOH (50
mL), and Et3N (1 mL). The resultant suspension was allowed to stir at
room temperature for 1 h. Then, Cu(NO3)2.3H2O (0.8 g) in MeOH (20
mL) was added to the above suspension and refluxed for about 12 h. The
The resultant polymer was insoluble in water and common organic
3

F. Gorginpour and H. Zali-Boeini
Molecular Catalysis 504 (2021) 111460
solvents such as MeOH, THF, acetone, acetonitrile, DMF, and DMSO.
Finally, after the treatment of Q-POP with a solution of Cu(NO3)2.3H2O
in methanol, Cu2+@Q-POP was yielded as a green solid. Indeed, Cu2+
ions were immobilized on Q-POP through strong coordination bonds
with N and O-atoms present in Q-POP. In the end, CuNPs@Q-POP
catalyst was produced as a deep brown solid through reduction reac-
tion of Cu2+ ions to CuNPs using hydrazine hydrate as the reducing
agent. The copper content in the CuNPs@Q-POP was determined by
inductively coupled plasma (ICP) analysis.
uptake at low relative pressure and a steep uptake with a vivid hysteresis
loop at a high relative pressure (P/P0) of above 0.6, implying type-IV
plus type-I isotherm behavior. Therefore, according to the hysteresis
loop present in N2 sorption isotherm and the distribution curves of the
pore size using the BJH model (Fig. 3b), both compounds possess hier-
archical porosity structure with dominant mesopores, which is highly
desirable for promoting the mass transfer of reactants and better cata-
lytic activity.
The BET surface area of Q-POP and CuNPs@Q-POP were 632.1 and
555.9 m2 gꢀ 1, respectively (Table 1).
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,
indicating that morphology is well preserved after Cu loading (Fig. 4c
and d).
3.2. Characterizationsof DHQ ligand, Q-POP polymer, and CuNPs@Q-
POP catalyst
The formation of compound B as the key structure of the CuNPs@Q-
POP was evidenced by its 1H and 13C NMR spectra and finally confirmed
by its X-ray single crystallography analysis (SCXRD, CCDC 1953774,
Fig. 1).
Fourier transform infrared spectroscopy (FT-IR) was applied to
confirm the incorporation of ligand C in the polymeric network (Fig. 2a).
The appearance of extra new bands at 1167.6 cmꢀ 1 and 1061.6 cmꢀ 1
To evidence the presence of C, O, N and Cu elements in the
CuNPs@Q-POP, the elemental mapping and energy-dispersive X-ray
spectroscopy (EDX) were carried out (Fig. 4e-h). The elemental mapping
images indicate that Cu nanoparticles are rather uniformly dispersed in
the porous organic polymer.
––
assigned to the CN and CO
stretching vibrations emerged in the Q-
POP structure compared with the FT-IR spectra of PDVB, implies that
ligand C was successfully copolymerized with divinylbenzene.
The wide-angle X-ray diffraction (WAXRD) pattern of Q-POP and
CuNPs@Q-POP are demonstrated in Fig. 2b. The Q-POP illustrates only
one broad diffraction peak, indicating that the as-synthesized polymer
possesses an amorphous framework. On the other hand, the XRD pattern
of CuNPs@Q-POP displays three additional diffraction peaks at 2θ =
43.45˚, 50.5˚, and 74.6˚related to diffractions of the (111), (200), (220)
planes, along with one broad peak as compared with Q-POP XRD
pattern. These observations validated the existence of CuNPs in
CuNPs@Q-POP.
The high-resolution transmission electron microscopy (HR-TEM) is
shown in Fig. 4i. As revealed in TEM image CuNPs@Q-POP are
composed of polymeric frameworks in which the CuNPs are consistently
distributed on the Q-POP without the obvious agglomeration. Addi-
tionally, the TEM image demonstrates that copper nanocrystals were
distributed between the polymeric networks to form a composite.
3.3. O-arylation of phenol with aryl halides catalyzed by CuNPs@Q-POP
The catalytic efficiency of CuNPs@Q-POP was examined as an active
–
Thermal stabilities of Q-POP and CuNPs@Q-POP were determined
by thermal gravimetric analysis (TGA) under argon atmosphere. Fig. 2c
displays that both of these compounds have excellent thermal stabilities
up to 300 ◦C, which could be attributed to the existence of a cross-linked
network.
catalyst for C O cross-coupling reaction of phenols with aryl halides. At
the outset of our study, we selected the reaction of phenol with iodo-
benzene in PEG-200 at 110 ◦C with CuNPs@Q-POP (7.3 mol% Cu◦
loading) for the synthesis of diphenyl ether as the model reaction. The
reaction proceeded with a little yield within 24 h (Table 2, entry 1).
Then, other solvents such as DMF, toluene, and H2O were screened to
obtain the highest conversion (Table 2, entries 2–4). When this reaction
was performed in DMF in the presence of CuNPs@Q-POP (7.3 mol% Cu◦
loading), nearly complete conversion (97 %) was obtained and hence,
DMF was chosen as the reaction solvent. To investigate the effect of Cu
loading amount of CuNPs@Q-POP in the course of the reaction, the
process was carried out with CuNPs@Q-POP (2.8 mol% copper loading).
It was found that in this case, the conversion rate for the formation of
diphenyl ether was considerably reduced (Table 2, entry 5). Therefore,
CuNPs@Q-POP (7.3 mol% Cu◦ loading) was considered to attain the
The porosity of Q-POP and CuNPs@Q-POP were measured by ni-
trogen adsorption and desorption isotherms at 77 K (Fig. 3a). The iso-
therms for both Q-POP and CuNPs@Q-POP illustrate a slight nitrogen
–
highest conversion for the C O coupling reactions. To determine the
effect of the amount of catalyst on the cross-coupling reaction of phenol
and iodobenzene, the reaction was also performed at various amounts of
catalyst. We found that the conversion rate of the reaction was decreased
from 97 to 70 and 55 % when the amount of catalyst was reduced from
75 to 50, and to 25 mg respectively (Table 2, entries 6 and 7). Addi-
tionally, in the arylation of phenol with iodobenzene, K2CO3 was
discovered to be the most appropriate base, whereas other bases such as
NaOH, K3PO4, and Et3N were not suitable and offered a conversion rate
of 60 %, 70 %, and 10 % for diphenyl ether respectively (Table 2, entries
8–10). The control experiment without Cu loading into Q-POP was
accomplished to confirm the role of CuNP in this reaction and unveiled
that, no desired product was generated (Table 2, entry 14). Screening of
the reaction temperature also revealed that decreasing the temperature
from 110 ◦C to 90 ◦C reduces the reaction conversion from 97 to 54 % at
the same reaction time (Table 2, entry 15). The reaction at a shorter time
(12 h) resulted in lower conversion (Table 2, entry 16). In light of all the
above mentioned findings, the best conversion and selectivity for the O-
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 molcules. Only one
molcule is showed for clarity.
4

F. Gorginpour and H. Zali-Boeini
Molecular Catalysis 504 (2021) 111460
Fig. 2. (a) FT-IR spectra of PDVB and 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.
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.
arylation of iodobenzene with phenol was obtained using K2CO3 as the
Table 1
base, DMF as the solvent, and in presence of 75 mg of CuNPs@Q-POP
Textural Parameters for Q-POP and CuNPs@Q-POP.
(7.3 mol% Cu◦ loading) at 110 ◦C for 24 h.
Material
SBET (m2/g)
VP (cm3/g)
DP (nm)
Afterward, with the optimum conditions in hand, the scope of the
–
C
O coupling reaction of phenols with iodobenzene was extended using
Q-POP
632.1
555.9
1.40
1.36
8.9
9.8
CuNPs@Q-POP
several types of substituted phenols in presence of CuNPs@Q-POP, and
the results are exhibited in Table 3. Para-substituted phenols containing
5

F. Gorginpour and H. Zali-Boeini
Molecular Catalysis 504 (2021) 111460
Fig. 4. SEM images of (a and b) Q-POP and (c and d) CuNPs@Q-POP, (e) Cu, (f) N, and (g) O elemental mapping of CuNPs@Q-POP, (h) EDX spectrum of Cu, N, and
O, and (i) HR-TEM image of CuNPs@Q-POP.
electron donating such as –Me, –OMe, and –t-Bu were completely con-
verted to the corresponding diaryl ethers with exceptional selectivity
(Table 3, entries 2–4). The conversion for phenols bearing electron-
withdrawing substituents such as 3-NO2, 4-Cl, and 4ꢀ CHO was lower
than electron-donating phenols but the selectivity of products was again
100 % (Table 3, entries 5–7). 1-naphthol and 2-naphthol also demon-
strated excellent conversions (92 and 94 % respectively) with 100 %
selectivity of the desired ethers under the optimized conditions (Table 3,
entries 8 and 9). Similarly, 8-hydroxyquinoline was converted into an O-
arylated product with excellent conversion rate and the selectivity
(Table 3, entry 10). Besides, 2,7-dihydroxynaphthalene generated mono
aryl ether product with 92 % conversion and 80 % selectivity while the
stoichiometric ratio of iodobenzene: 2,7-dihydroxynaphthalene was 1:1
(Table 3, entry 11). Interestingly, when the same reaction was con-
ducted at iodobenzene to the 2,7-dihydroxynaphthalene ratio (3:1) at
prolonged reaction time (48 h), the bis-arylated product was produced
with 88 % conversion and 75 % selectivity (Table 3, entry 12). This
distinct trend for large molecules was probably ascribed to the high
surface area and hierarchical porosity present in the catalyst that facil-
itates the diffusion of bulky reactant molecules into the pores and in-
creases the chance of reactants to have efficient collision and interaction
with catalytic active centers. Furthermore, the reaction with bromo-
benzene instead of iodobenzene led to the formation of diphenyl ether
with 80 % conversion and 100 % selectivity (Table 3, entry 13), while
6

F. Gorginpour and H. Zali-Boeini
Molecular Catalysis 504 (2021) 111460
Table 2
Table 3
Optimization of various parameters on the cross coupling of iodobenzene and
Ullmann O-arylation reaction catalyzed by CuNPs@Q-POP(7.3 % Cu)a.
phenola.
Entry
Aryl
Nucleophiles
Products
Conv.
Sel.
halides
(%)b
(%)b
1
2
3
4
97
97
91
90
100
100
100
100
Entry
Catalyst
Cat.
Solv.
Base
Con.
(%)b
Sel.
(mg)
(mmol)
(%)b
1
CuNPs@Q-
POP (7.3 %)
CuNPs@Q-
POP (7.3 %)
CuNPs@Q-
POP (7.3 %)
CuNPs@Q-
POP (7.3 %)
CuNPs@Q-
POP (2.8 %)
CuNPs@Q-
POP (7.3 %)
CuNPs@Q-
POP (7.3 %)
CuNPs@Q-
POP (7.3 %)
CuNPs@Q-
POP (7.3 %)
CuNPs@Q-
POP (7.3 %)
CuNPs@Q-
POP (7.3 %)
CuNPs@Q-
POP (7.3 %)
CuNPs@Q-
POP (7.3 %)
Q-POP
75
75
75
75
75
50
25
75
75
75
75
75
75
PEG-
200
K2CO3 (2)
K2CO3 (2)
K2CO3 (2)
K2CO3 (2)
K2CO3 (2)
K2CO3 (2)
K2CO3 (2)
NaOH (2)
K3PO4 (2)
Et3N (2)
25
97
<10
0
100
100
–
2
DMF
5
6
7
8
85
70
89
92
100
100
100
100
3
Tolu.
H2O
4 c
5
–
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
55
70
55
60
70
10
83
45
trace
100
100
100
100
100
100
100
100
–
6
9
94
95
100
100
7
10
8
9
11
12c
13
14
92
80
75
100
–
77
10
11
12
13
80
K2CO3
(1.5)
K2CO3
(0.5)
–
<10
a
Reaction condition: aryl halide (1 mmol), phenol (1.2 mmol), K2CO3 (2
mmol), DMF (2 mL), CuNPs@Q-POP (7.3 % Cu, 75 mg), 110 ◦C, 24 h, under N2
atmosphere. b) Conversion and selectivity were determined by GC. c) Reaction
conditions for the synthesis of bis-arylated product: iodobenzene (3 mmol), 2,7-
dihydroxynaphthalene (1 mmol), K2CO3 (4 mmol), DMF (4 mL), CuNPs@Q-POP
(7.3 % Cu,150 mg), 110 ◦C, 48 h, under N2 atmosphere.
14
75
75
DMF
DMF
K2CO3 (2)
K2CO3 (2)
0
–
100
15 d
CuNPs@Q-
POP (7.3 %)
CuNPs@Q-
POP (7.3 %)
54
16e
75
DMF
K2CO3 (2)
70
100
a
Reaction condition: iodobenzene (1 mmol), phenol (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.
Table 4
Comparison of catalytic performance of CuNPs@Q-POP catalyst in the O-ary-
lation with the other reported Cu-based catalysts.
Catalyst
Condition
O-arylation
Ref.
CuNPs@Q-
POP
K2CO3, DMF, 110 ◦
C
C
Conversion (97 %),
Selectivity (100 %)
Yield (94 %)
This
work
[63]
[65]
[68]
very little amount of diphenyl ether was detected (<10 %) when chlo-
robenzene was used (Table 3, entry 14).
Cu@MCTP-1
PDVB-SB-Cu
Meso Cu/
MnOx
Cs2CO3,DMF, 130 ◦
Cs2CO3, DMSO, 115 ◦
C
Yield (91 %)
K2CO3, DMF, 140 ◦
C
Conversion (88 %),
Selectivity (>99 %)
Yield (96 %)
3.4. The Comparison of our method with some reported method in the
literature
PS-Cu-
Cs2CO3, DMSO, t-
[69]
[70]
Furfural
Cu2O/
Bu4NBr, 120 ◦
C
Cs2CO3, THF, 150 ◦
C
Yield (96 %)
In Table 4, the catalytic performance of CuNPs@Q-POP in the O-
arylation of iodobenzene with phenol are compared with other reported
heterogeneous catalysts [63,65,68–70]. Compared to other catalysts,
our synthesized CuNPs@Q-POP catalyst demonstrates better or com-
parable performance. It can be probably attributed to the synergistic
effects of high surface area and the hierarchical porosity together with
the superior stability, which enhance diffusion of reactants inside the
polymeric network and efficient access to catalytic sites.
graphene
separated CuNPs@Q-POP. In the case of the O-arylation of iodobenzene
with phenol under optimized conditions, the catalyst was filtered from
the reaction mixture after 8 h (48 % conversion). Then, the reaction of
the filtrate was further continued at the same reaction conditions for an
additional 16 h. No changes in conversion and selectivity were observed
after removing the catalyst (Fig. 5b). This fact implies that no Cu species
were detectable in the filtrate and the atomic absorption spectroscopy of
the filtrate confirm that. This can be attributed to the strong chemical
interaction between CuNPs and N and O binding sites in quinoxaline
moiety present in the polymer network, which prevents leaching of
CuNPs from the polymeric network.
To check the recyclability of the catalyst, CuNPs@Q-POP as a solid
was separated from the reaction mixture by filtration and then washed
with water, ethyl acetate, and methanol, respectively, and dried under
vacuum. Ultimately, the isolated and refined catalyst was applied for
numerous consecutive recycle runs under the optimized reaction con-
–
ditions. It was found that the recycled catalyst, for the C O cross-
coupling reaction could be reused five times without appreciable
reduction in the conversion rate and selectivity (Fig. 5a). Wide-angle X-
ray diffraction (WAXRD) patterns of the reused CuNPs@Q-POP after the
fifth run were similar to the fresh nanocatalyst (Fig. S2). The SEM im-
ages indicate that the morphology of CuNPs@Q-POP is well maintained
after five runs (Fig. S3). A hot filtration test was further accomplished to
remove CuNPs@Q-POP and survey the heterogeneous nature of
The feasibility of a multi-gram scale synthesis was also checked for
–
the catalyst. When the C O coupling reaction of phenol was performed
with iodobenzene in a 50 mmol scale, the resulting diphenyl ether was
produced with nearly complete (96 %) conversion.
A reaction pathway is proposed for the N-arylation of iodobenzene
with aniline catalysed by CuNPs@Q-POP, and shown in Scheme 2 [71,
7

F. Gorginpour and H. Zali-Boeini
Molecular Catalysis 504 (2021) 111460
Fig. 5. (a) Recyclability of CuNPs@Q-POP in the Ullmann O-arylation reactions, (b) Hot filtration test for CuNPs@DHQ-POP. Reaction conditions for the Cross-
Coupling of iodobenzene and phenol: Iodobenzene (1 mmol), phenol (1.2 mmol), K2CO3 (2 mmol), DMF (2 mL), CuNPs@Q-POP (7.3 % Cu, 75 mg), 110 ◦C, 24
h, under N2 atmosphere.
catalytic activity after recycling five times. In one hand we also found
that the as-synthesized CuNPs@Q-POP catalyst can be used in other
synthetic transformation and on the other hand, Q-POP copolymer has
the potential to be used in many other research fields such as the light-
emissive layer in OLEDs (due to its intense fluorescence), as adsorbant
for the removing of organic chemical pollutants, and toxic heavy metal
ions from wastewater.
CRediT authorship contribution statement
Forough Gorginpour: Methodology, Validation, Formal analysis,
Investigation, Writing - original draft, Visualization. Hassan Zali-
Boeini: Supervision, Project administration, Funding acquisition,
Writing - review & editing.
Declaration of Competing Interest
The authors report no declarations of interest.
Acknowledgments
Scheme 2. A plausible mechanism for the O-arylation of iodobenzene with
phenol using CuNPs@Q-POP.
The authors are grateful to the University of Isfahan research council
for providing instrumental facilities and partial financial support for this
72]. At first, an oxidative addition of iodobenzene to Cuo nanoparticles
work.
occurs, leading to the formation of the intermediate (I). Subsequently,
the in-situ produced phenolate ion reacts with the intermediate (I) to
give the intermediate (II). Finally, the desired product is obtained by the
reductive elimination of intermediate (II).
Appendix A. Supplementary data
Supplementary material related to this article can be found, in the
online version, at doi:https://doi.org/10.1016/j.mcat.2021.111460.
4. Conclusion
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tion and used for immobilization of copper nanoparticles and nano-
crystals to synergistically enhance its catalytic activity in the Ullmann
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