Electrochemical coupling halobenzene into biphenyl on a reusable Pd nanoparticle-coated carbon-paper electrode at ambient conditions

Article abstract of DOI:10.1039/d0nj06027e

Electrochemical organic synthesis (EOS) employing electrons to directly activate the reactants can readily complete the chemical conversion under mild conditions. Here, it presented an efficient electrochemical coupling halobenzene into biphenyl on a Pd nanoparticle-coated cathode. The biphenyl product can be obtained with a yield up to 77% at 35 mA, 6 h (3.9 F mol^?1). In addition, after consecutive fifth run of the coupling reaction, the yield still remained atca.40%, suggesting its considerable recyclable capacity. In addition, the preliminary kinetics studyviathe off-line gas chromatography analysis of the reaction mixture shows a two-section reaction process, including the introduction process (IP) and fast conversion process (FCP). Further, the estimated reaction kinetics constant value of 0.196 min^?1for FCP suggests a more effective conversion than that obtained by the previous study. This study adopts a simple way to fabricate a low-cost and reusable Pd electrode, achieving a high-efficiency electrochemical strategy for the Ullmann-type coupling reaction at mild conditions, and holds a great promise to extend this synthesis route to other important organic synthesis.

Full text of DOI:10.1039/d0nj06027e

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Electrochemical coupling halobenzene into
biphenyl on a reusable Pd nanoparticle-coated
carbon-paper electrode at ambient conditions†
Cite this: New J. Chem., 2021,
45, 3997
Chao-Nan Wang,‡ Yong-Heng Lu,‡ Yue Liu, Jun Liu, Yao-Yue Yang * and
Zhi-Gang Zhao*
Electrochemical organic synthesis (EOS) employing electrons to directly activate the reactants can
readily complete the chemical conversion under mild conditions. Here, it presented an efficient
electrochemical coupling halobenzene into biphenyl on a Pd nanoparticle-coated cathode. The biphenyl
product can be obtained with a yield up to 77% at 35 mA, 6 h (3.9 F molÀ1). In addition, after
consecutive fifth run of the coupling reaction, the yield still remained at ca. 40%, suggesting its
considerable recyclable capacity. In addition, the preliminary kinetics study via the off-line gas
chromatography analysis of the reaction mixture shows a two-section reaction process, including the
introduction process (IP) and fast conversion process (FCP). Further, the estimated reaction kinetics
constant value of 0.196 minÀ1 for FCP suggests a more effective conversion than that obtained by the
previous study. This study adopts a simple way to fabricate a low-cost and reusable Pd electrode,
achieving a high-efficiency electrochemical strategy for the Ullmann-type coupling reaction at mild
conditions, and holds a great promise to extend this synthesis route to other important organic
synthesis.
Received 10th December 2020,
Accepted 14th January 2021
DOI: 10.1039/d0nj06027e
rsc.li/njc
receive better efficiency of Ullmann coupling reactions, homo-
geneous palladium (Pd) and nickle (Ni) catalysts were intro-
1. Introduction
The derivatives of a biaryl structure have wide applications in duced into these reactions. However, the cost and toxicity of
agrochemical, pharmaceutical and material industries.1,2 It is these metal catalysts limited their utilization and
known that the Ullmann coupling reaction is one of the development.11,12 Furthermore, numerous groups have tried
frequently used protocols to architecture the chemicals of these to modify the ligands of Cu homogeneous catalysts, which led
biaryl-based structures.3–5 Specifically, the Ullmann coupling to the enhancement of the catalytic performance of copper
reaction makes use of aryl halides as reaction substrates to metals,13–15 so that they can use fewer metal catalysts to
convert them into biaryl products, as shown in Scheme 1, where complete the transformation, dramatically decreasing the
normally successful C–C bond coupling needs a temperature as consumption of catalysts. Among these, Ma et al. demonstrated
high as more than 373 K3,6,7 and essential high-alkali solution a mild method for Ullmann coupling reaction using either
environment.8,9 Moreover, the copper (Cu) catalysts should be N-methyl glycine or L-proline as the ligand of copper
equivalent or even excessive to the amount of aryl halides to catalyst.16 Moreover, in recent years, numerous advanced mate-
obtain a considerable yield.2–4,10 Obviously, the harsh reaction rials such as biodegradable microbeads/microcapsules,17,18
conditions of the traditional Ullmann coupling reaction con- magnetic lignin,19,20 modified kaolin,21 modified boehmite,22
flictwith the green chemistry concept, which should be urgent polymeric nanocomposite,23 graphene oxide24 and other
to promote.
Actually, in the past decades, numerous efforts were made to
simplify the conditions of Ullmann coupling reactions.1,4,5 To
Key Laboratory of Basic Chemistry of State Ethnic Commission, School of Chemistry
and Environment, Southwest Minzu University, Chengdu 610041, Sichuan, China.
E-mail: yaoyueyoung@swun.edu.cn, zzg63129@163.com
† Electronic supplementary information (ESI) available. See DOI: 10.1039/
Scheme 1 The typical chemical reaction formula of the traditional Ull-
d0nj06027e
mann coupling reaction.
‡ These authors contributed equally.
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materials25,26 were used to load Pd nanoparticles for synthesizing
biphenyl molecules. These support materials can efficiently
modulate the catalytic performance of Pd catalysts. However, they
still need high temperature and strong alkaline environment,
which limit their further development.
Fortunately, in recent years, electrochemical organic
synthesis27–30 has obtained increasing attention because of
the growing environmental protection concept. This method
directly employs electrons and/or protons as the key reagent to
activate the substrate molecules; thus, it can ready complete the
organically chemical conversions under mild conditions (at
room temperature and standard atmospheric pressure).
Moreover, the reaction conventionally occurs without any other
additives, so there will be little extra by-products. Actually,
several important molecules such as azaindoles,31 imidazole
Scheme 2 The schematic of the synthesis process of Pd/C and the
fabrication process of the working electrode.
and its derivatives,32–34 diamine,35,36 hindered dialkyl ether,37 the designated pH value (ca. 9.5) using a 0.1 M Na2CO3
O-heterocycles,38 and amidyl compounds39 have been obtained solution. Successively, a 10 mL 3 mg mLÀ1 NaBH4 + 0.05 M
via the electrochemical organic synthesis. Regarding the Na2CO3 mixture was added dropwise to the suspension under
Ullmann-type reactions, Rothenberg et al.40 presented a room- vigorous stirring by a peristaltic pump at 0.5 mL minÀ1. After
temperature electrocatalytic alternative to the traditional 12 h stirring, the suspension was filtered and washed with
Ullmann coupling reaction, obtaining the biphenyl product copious amounts of Milli-Q water and then dried in vacuum at
from iodobenzene with a yield up to 80%. However, they used 323 K overnight.41 The obtained catalysts are named as Pd/C.
high-purity Pd wires as the sacrificial anode to generate the Pd The as-prepared Pd/C catalysts were detected by a SPECTRO
nanoclusters to promote the C–C coupling of aryl halides, ARCOS inductively coupled plasma-atomic emission spectro-
which likewise leads to much waste of the noble metals. meter (ICP-AES) to analyse the metal loading. The morphology
Moreover, the Pd cluster catalysts that are randomly dispersive and size distributions of Pd nanoparticles were characterized
in the reaction solution, thus the Pd clusters are disposable. by a Tecnai G2F20 transmission electron microscope (TEM).
Besides, the ionic liquids serving as the electrolyte are The electronic and lattice structure was analysed by a Thermo
conceivably expensive. Therefore, their synthesis strategy is Fisher K-Alpha X-ray photoelectron spectrometer (XPS) and a
high-cost and non-recyclable, which should be further D8 QUEST X-ray diffractometer with Cu Ka radiation
improved to meet the high-efficiency and low-cost principles.
(0.15418 nm), respectively. The Cyclic voltammograms (CVs)
Considering the above-mentioned facts, in this study, we of Pd/C were obtained on a CHI 660E electrochemistry work
fabricated a reusable carbon paper cathode through simply station using an untreated carbon paper (1 cm  6 cm) as the
coating the premade carbon-supported Pd nanoparticles. In counter electrode and Ag/AgCl as the reference electrode.
addition, an electrochemical cell was assembled using a carbon
paper as the anode and the conventionally used organic solvent
such as acetonitrile associated with certain amount of quaternary
2.2 Electrochemical organic synthesis
ammonium as the electrolyte. The electrochemical Ullmann-type The electrochemical synthesis was performed in
a two-
C–C coupling of iodobenzene (PhI) into biphenyl molecule was electrode cell, taking a Pd/C-coated carbon Paper (Toray) elec-
successfully carried out at ambient conditions, with a production trode (designated as PdCPE hereafter) and carbon paper elec-
yield up to 77%. In addition, a 40% yield was still obtained after a trode as the cathode and anode, respectively. As shown in
consecutive five-time recyclable use of the Pd nanoparticles- Scheme 2, PdCPE was fabricated through brushing the catalyst
coated cathode. The similar coupling reactions that employed ink on the carbon paper (8 cm  1.5 cm) surface on a heating
other monosubstituted aryl halides as reactants were also per- plate at ca. 50 1C to dry the solvent. The catalyst ink including
formed with a considerable yield. This study may illuminate a 22 mg of the as-prepared Pd/C dispersed in the solution of
promising route towards the low-cost and high-efficiency green 0.5 mL water + 0.5 mL ethanol + 50 mg 5% Nafion ethanol
synthesis of valuable molecules.
solution. The coating area was controlled to be 2 cm  1.5 cm,
and the coating amount of Pd/C was 22.0 Æ 1.0 mg determined
by weighing. Then, the PdCPE and same-size carbon paper were
symmetrically assembled into the electrochemical cell. 20 mL
of acetonitrile (MeCN, KESHI, AR) or dimethylformamide
(DMF, KESHI, AR) or CH2Cl2 (KESHI, AR) containing 0.05 M
2. Experimental methods
2.1 Preparation and characterization of Pd catalysts
As shown in Scheme 2, 43 mg of Vulcan XC-72 and 2.23 mL of tetrabutylammonium tetrafluoroborate (TBATFB, Adamas 98% +)
45 mM PdCl2 (Sinopharm Chemical Reagent) were added into were added into the cell to serve as the electrolyte, while the
40 mL of Milli-Q water and the mixture was kept under vigorous amount of reactant aryl halides was kept as 2 mmol. A LP220DE
stirring for ca. 4 h. Then, the solution alkalinity was adjusted to direct-current power supply purchased from Shenzhen Lodestar
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CO., Ltd was used to provide the cell voltage and current to ensure (111) peak. This result is in reasonable agreement with the
that the reaction occurs.
value estimated by TEM. The ICP-AES detection of the
as-prepared catalysts obtained a ca. 19.1 wt% of Pd loading
for the as-prepared Pd/C catalysts, which is approximately
corresponding to our expectation. The ex situ XPS spectra of
Pd 3d in Fig. 1c show the Pd 3d core levels split into 3d5/2
(ca. 335 eV) and 3d3/2 (ca. 340 eV) states, and both the Pd 3d5/2
and the Pd 3d3/2 peaks can be deconvoluted into two peaks of
Pd0 and PdII species. It is worthwhile to note that the catalysts
were exposed under ambient condition before XPS measure-
ments. Hence, the presence of PdII species may be attributed to
the oxidation of Pd active sites.44,45
The electrochemical response of the as-prepared Pd/C
towards the C–C coupling of iodobenzene was roughly investi-
gated via cyclic voltammetry. Typically, the cyclic scanning was
performed in a three-electrode cell containing 10 mL MeCN +
0.05 M TBA TFB + 0.01 M iodobenzene, employing the Pd/
C-coated glassy carbon, carbon paper (1 cm  6 cm) and Ag/
AgCl electrode as the working electrode, counter electrode and
reference electrode, respectively. As shown in Fig. 2, during the
negative scanning, the peak at À0.75 V could be related to the
reduction of iodobenzene into the radical intermediates (vide
infra), and the peak at À1.75 V could be attributed to the C–C
coupling of the intermediates. The peak at À1.75 V was only
observed on the Pd/C surface, which suggested that the C–C
coupling of radical intermediates could occur on the Pd/C
surface rather than that on the glassy carbon surface. To
confirm this, we also performed the chromatographic separa-
tion of reaction residual and undetectable biphenyl product
was obtained when the cyclic scanning was carried out on the
bare glassy carbon electrode.
2.3 Product separation and analysis
After the determined reaction time, the reaction solution was
pumped out to separate effectively through a chromatographic
separation in a home-made silica gel column (200–300 mesh
column chromatography silica gel). Typically, the petroleum
ether (KESHI, AR) was chosen as the mobile phase to extract the
products. When DMF is used as solvent, it should be removed
before column chromatographic separation by washing with
water because of its high boiling point. The molecule structures of
as-obtained products were confirmed by an Agilent 400-MR DD2
400 MHz nuclear magnetic resonance (HNMR) spectrometer and
an Agilent Cary 660 FTIR spectrometer. The deuterated chloro-
form (CCl3D) with tetramethylsilane (TMS) was chosen to dissolve
ca. 20–25 mg products to carry out the HNMR measurements.
All the chemicals were obtained commercially and used
without further purification. All experiments were carried out
at room temperature (25 Æ 2 1C) and atmosphere pressure.
3. Results and discussion
3.1 Catalyst’s Characterization
It is generally known that particle sizes and distribution of
nanoparticles play a key role in their catalysis process.42,43 It
can be seen in Fig. 1a that Pd nanoparticles are evenly dis-
tributed on the carbon surface, without distinct aggregation.
The histograms of the particle size and distribution are
obtained based on the measurements of ca. 400 nanoparticles
in a randomly selected area, showing the average particle size of
2.88 Æ 0.5 nm. Fig. 1b shows the XRD patterns of the as-
prepared Pd/C catalysts, featuring the face-center-cubic (fcc)
crystalline structure of Pd (PDF#46-1043). Thus, the average
particle size of Pd nanoparticles can also be estimated as ca.
2.83 nm according to the Scherrer equation D = 0.89 l/(b cos y),
where D is the average particle diameter, l is the wavelength of
X-ray (0.15418 nm), b is the half-height width of the Pd (111)
diffraction peak in radians and y is the diffraction angle of the
3.2 Electrochemical C–C coupling on Pd/C
The homo-coupling of PhI was chosen as the model reaction for
optimizing and screening the reaction conditions, which
Fig. 2 Cyclic voltammograms (CVs) collected on a bare (dash line) and
Pd/C-coated glassy carbon electrode (solid line) in an electrolyte of 10 mL
MeCN + 0.05 M TBA TFB (0.05 M) + 0.01 M iodobenzene at room
Fig. 1 (a) TEM image of Pd NPs loaded on the activate carbon surface,
(b) X-ray diffraction patterns of as-prepared Pd/C catalysts. (c) Core-level
XPS spectrum in the Pd 3d region for Pd/C catalysts.
temperature. Scan rate, 50 mV sÀ1
.
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effective for this electrochemical coupling reaction, which is in
line with previous investigation on electrochemical organic
reactions.32,33,38,47
Furthermore, the applied cell current was taken as an
important factor to probe since this C–C coupling reaction
was electrochemically triggered here. A higher current makes
this reaction a higher yield. When the current was 10 mA, PhI is
successfully turned into biphenyl with the yield of 34%. With
the current rising to 35 mA, it can be obviously seen that the
product yield rapidly increases to 77%. However, it should
be noticed that excessive current may conversely inhibit the
reaction. When the applied current increased to 45 mA, the yield
of biphenyl decreased to 64% (entries 5–8, Table 1). Thus, 35 mA
current was finally determined as the best condition in this
study. In addition, the coating quantity of Pd/C catalysts on the
carbon paper surface also exhibited influence to this reaction
(entries 10 and 11 Table 1). Among these, we successfully trans-
form the substrate to the biphenyl product with the highest yield
of 77%. As a result, we get optimized reaction conditions in this
electrochemical organic synthesis system, being the solvent of
MeCN, 22 mg Pd/C (20 wt%) catalysts, 35 mA cell-current and a
6 h reaction time (3.9 F molÀ1). This result is pretty effective
compared with those obtained in the traditional catalysis pro-
cess in Table S2 in the ESI.†
To evaluate the recyclable capacity of PdCPE, we conducted
the same electrochemical reaction on PdCPE for five consecutive
times (Fig. 3). In the second run, under same optimized reaction
conditions, the biphenyl product yield was dipped to ca. 58%. In
the next few cycles, the product yield changed a little according
to the experimental data. However, the yield dropped down to
ca. 38% in the 5th run. According to these results, we speculate
that the Pd nanoparticles may perform migration and accumula-
tion due to the so-called Ostwald ripening effect.48,49
Fig. 3 Recycling experiments for the electrochemical Ullmann reaction
on the as-prepared Pd/C catalysts. Reaction conditions: 20 mL MeCN,
0.1 M PhI, 0.05 M supporting electrolyte, room temperature, 35 mA, 6 h.
included solvent and applied cell current. Initially, we found
that the solvent has a significant influence to this reaction. It is
clear that the yield of product gradually decreased with the
sequence of the usage of the solvents: MeCN (77% yield), DMF
(63% yield), MeOH (42% yield) and CH2Cl2 (35% yield) under
the same reaction conditions (entries 1–3,7 Table 1). In the
traditional Ullmann reaction, an aprotic solvent such as DMF,
MeCN or tetrahydrofuran (THF) has been frequently used, and
similarly, the product yield was usually different with the varia-
tion of solvent.4,5 Moreover, in the organic electrochemistry,
MeCN and DMF have the widest electrochemical working win-
dows; therefore, these solvents are the best organic solvents used
for electrochemical studies.46 In our study, the experimental
results apparently show that MeCN could be more suitable and
Then, we extend this Ullmann-type electrochemical coupling
to other substrates on the Pd surface, particularly other mono-
substituted halobenzenes such as bromobenzene (PhBr) and
chlorobenzene (PhCl) were first selected as the reaction sub-
strates. As shown in Table 2, the yield of biphenyl transformed
from PhBr can reach to 66% (entry 2, Table 2). There was no
biphenyl product obtained in the case of PhCl (entry 3, Table 2).
It is still a knotty problem to finish the chemical conversion of
PhCl in organic reactions under simple conditions.50,51 Then,
we employ other para-position substituted compounds to
further extend our synthesis method. According to the experi-
mental results, the electron-donating groups (here –CH3, –NH2
groups were used as the model) can inhibit this reaction. The
stronger the electron-donating effect, the more obvious the
inhibition appearance (entries 4, 5 and 10, 11, Table 2). In
contrast, the electron-withdrawing groups (here –COOH,
–COCH3 groups were used as the model) can slightly promote
the conversion efficiency, leading to an increase in the yield to
88% (entries 8, 9 and 12, 13, Table 2). The enhancement effect
of electron-withdrawing groups could be related to the stability
of intermediates of the Ph radical anion. In addition, 4-tert-
butyl-iodobenzene with a large steric hindrance surprisingly
obtained a 42% yield (entry 7, Table 2). It means the
Table 1 Optimization of the electrochemical homo-coupling reaction of
iodobenzenea
Current
(mA)
Weight of Pd/C
(20 wt%)
Time
(h)
Yieldb
(%)
Entry
Solvent
1
2
3
4
5
6
7
8
DMF
35
35
35
0
10
25
35
45
35
35
35
22 mg
22 mg
22 mg
22 mg
22 mg
22 mg
22 mg
22 mg
6
6
6
8
8
6
6
6
6
6
6
63
35
42
NR
34
58%
77c
64
0
CH2Cl2
MeOH
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
9
10
11
0 mg
15 mg
22 mg (10 wt%)
66
54
a
Reaction conditions: 20 mL solvent, 0.1 M iodobenzene, 0.05 M
b
TBATFB, room temperature. Isolated yields of products after column
chromatography. NR means no reaction. 6 h (3.9 F molÀ1
)
c
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Table 2 Pd/C NP-catalysed electrochemical homocoupling of some alkyl
halidesa
Entry
1
Aryl halide
Yieldb (%)
77
Time (h)
6
2
66
0
8
8
8
8
8
8
6
6
8
8
Fig. 4 (a) Reaction time-dependent off-line GC of the reaction mixture
withdrawing from the reaction solution. (b) Reaction time-dependent
variation of the integrated GC peak intensity of the PhI substance and
biphenyl product, which is normalized to that obtained at 0 min.
3
4
72
61
14
42
75c
58c
38
17
syringe, and then injected into the GC to analyse the product
distribution with reaction proceeding.
5
As shown in Fig. 4a, the gradual consumption of the PhI
substrate (retention time centered at ca. 6 min) and formation
of the biphenyl product (retention time centered at ca. 10 min)
can be clearly observed. More directly, as shown in Fig. 4a, the
integrated GC peak intensity variation apparently shows that
more than 90% PhI was electrochemically transferred into the
biphenyl product after a 6 h reaction. More importantly, this
off-line reaction process monitoring could provide a possibility
to preliminarily study the reaction kinetics shown in Table 1. As
shown in Fig. 4b, the reaction process could be readily divided
into two sections, where the apparent reaction kinetics
constant (k) was estimated by the slope to be k1 = 0.075 minÀ1
(1–3 h, R2 = 0.982) and k2 = 0.196 minÀ1 (3–5 h, R2 = 0.984),
respectively. Note that, here this PhI coupling reaction was
treated with a zero-order reaction kinetics similar to that
adopted in the literature40 although the reaction mechanism
was not manifested clearly. Moreover, the two-section actions
of reaction kinetics could suggest that this coupling reaction of
PhI should include an introducing process (IP) and a fast
conversion process (FCP) similar to that observed by Rothen-
berg et al. in an electrochemical organic synthesis system
assembled by a Pd wire anode and a Pt wire cathode.
Nevertheless, they claimed that the first process should be
related to the formation of the Pd cluster, which could not take
place in our system with a Pd-nanoparticle cathode. Thus, we
speculated that IP could mainly be due to the activation of PhI
molecules and formation of Phꢀ radical anion (see below), and
FCP could be mainly connected with the generation of the
biphenyl product. Therefore, it can be claimed that a compara-
6
7
8
9
10
11
12
88
69
6
6
13
a
Reaction conditions: 20 mL MeCN, 0.1 M aryl halide, 0.05 M support-
b
ing electrolyte, constant current, room temperature. Isolated yields of
products after column chromatography. The solvent is DMF because
of the poor solubility in MeCN.
c
electrochemical tactics may be a promising route to overcome tively smaller k2 value than that in the previously reported work
the steric effect, which commonly exists in organic synthesis.27 (0.38 minÀ1 40 and it demonstrated a more effective strategy for
)
electrochemical coupling. Note that, this hypothesis of the
reaction mechanism should be further investigated and
verified.
3.3 Preliminary clarification of the reaction mechanism
In order to better understand the reaction process, we used the
Moreover, to obtain further evidence of the mechanism
off-line FULI 9790II gas chromatography (GC) with a 30 m about this homo-coupling reaction, we chose 2,6-di-tert-butyl-
capillary column (KB-5) to monitor the PhI coupling reaction 4-methylphenol (BHT, 1.0 equiv.) as the radical scavenger
process. The initial column temperature of 80 1C was used, and (Table S1, ESI†). Interestingly, the desired product yield of
it was then increased to 250 1C at a rate of 10 1C minÀ1. The biphenyl was only 12%. This result implied that this reaction
2 mL reaction mixture was withdrawn per hour via a disposable might involve a radical intermediate. Therefore, a preliminary
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Conflicts of interest
The authors declare no conflict of interest.
Acknowledgements
This work was supported by NSFC (No. 21603177), CSC (No.
201908510084), open project of key laboratory of fundamental
chemistry of state ethnic commission of SMU (No.
2020PTJS20003), and students’ innovation program (No.
S202010656084)
Notes and references
Scheme 3 Schematic of the proposed reaction mechanism.
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4. Conclusions
In summary, we have presented an efficient electrochemical
Ullmann-type coupling reaction from halobenzenes into a
biphenyl product on the Pd nanoparticle surface at room
temperature. The carbon-supported Pd nanoparticles with a
ca. 2.8 nm diameter was simply prepared by a NaBH4 reduction
method. Additionally, the carbon-supported Pd nanoparticles
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