Controlled Synthesis of Palladium Nanocubes as an Efficient Nanocatalyst for Suzuki-Miyaura Cross-Coupling and Reduction of p-Nitrophenol

Article abstract of DOI:10.1021/acs.langmuir.0c00526

Anisotropic nanocatalysts have attracted considerable attention in comparison to bulk/nanocatalysts for their enhanced activity and reactivity. The demand toward anisotropic palladium (Pd) nanostructures has increased rapidly in the field of catalysis. Pd is a well-known active catalyst for several carbon-carbon (C-C) cross-coupling reactions; among them, the Suzuki-Miyaura cross-coupling reaction is one of the most versatile and dominant methods for constructing the extraordinarily useful unsymmetrical biaryls and also for hydrogenation of organic contaminants like p-nitrophenol (p-NP). This paper provides a brief explanation about the controlled synthesis, characterization, and catalytic activity of well-defined palladium nanocubes (Pd NCs) prepared by a seed-mediated method. The synthesized monodispersed Pd NCs were characterized by spectroscopic and microscopic tools such as UV-visible, XRD, FESEM, HRTEM, and EDS analyses. Pd NCs proved as an efficient catalyst for Suzuki-Miyaura cross-coupling reactions and p-NP reduction. The catalyst shows enhanced activity, greater stability, and higher selectivity with remarkable recyclability up to 92% for five consecutive cycles. The catalytic performance of the synthesized Pd NCs was also studied in the reduction of the organic contaminant p-NP, which showed an excellent performance screening of 99% conversion in 6 min.

Full text of DOI:10.1021/acs.langmuir.0c00526

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Article
Controlled Synthesis of Palladium Nanocubes as an Efficient Nanocatalyst
for Suzuki-Miyaura Cross-Coupling and Reduction of p-Nitrophenol
Swarnalata Swain, Bhavya M B, Vishal Kandathil, Prangya Bhol, Akshaya K Samal, and Siddappa A Patil
Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.0c00526 • Publication Date (Web): 22 Apr 2020
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Controlled Synthesis of Palladium Nanocubes as an Efficient Nanocatalyst for Suzuki-Miyaura
Cross-Coupling and Reduction of p-Nitrophenol
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Swarnalata Swain,† Bhavya M. B.,† Vishal Kandathil, Prangya Bhol, Akshaya K. Samal,*
Siddappa A. Patil*
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Centre for Nano and Material Sciences, Jain University, Jain Global Campus, Kanakapura,
Ramanagaram, Bangalore 562112, India
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Email: s.akshaya@jainuniversity.ac.in, p.siddappa@jainuniversity.ac.in
Abstract
Anisotropic nanocatalysts have attracted a considerable attention in comparison to bulk/nano catalysts for
their enhanced activity and reactivity. The demand towards anisotropic palladium (Pd) nanostructures have
been increased rapidly in the area of catalysis. Pd is a well-known active catalyst for several carbon-carbon
(C-C) cross-coupling reactions, among them Suzuki-Miyaura cross-coupling reaction is one of the most
versatile and dominant method for constructing the extraordinarily useful unsymmetrical biaryls and also
for hydrogenation of organic contaminant like p-nitrophenol (p-NP). This paper provides a brief
explanation about the controlled synthesis, characterization and catalytic activity of well defined palladium
nanocubes (Pd NCs) prepared by the seed mediated method. The synthesized monodispersed Pd NCs were
characterized by spectroscopic and microscopic tools such as UV-Visible, XRD, FESEM, HRTEM and
EDS analyses. Pd NCs proved as an efficient catalyst for Suzuki-Miyaura cross-coupling reactions and p-
NP reduction. The catalyst shows enhanced activity, greater stability and higher selectivity with remarkable
recyclability upto 92% for five consecutive cycles. The catalytic performance of the synthesized Pd NCs
have also studied in the reduction of the organic contaminant p-NP which showed an excellent performance
screening of 99 % conversion in 6 minutes.
Keywords: Pd Nanocubes; C-C coupling reaction; Suzuki-Miyaura cross-coupling; Anisotropic
nanostructures; p-Nitrophenol reduction.
Introduction
Anisotropic nanostructures have emerged as a significant research area due to the unusual, enriched
and unique properties associated with it and attracts new fundamental applications in the field of
health, energy, water and environmental research. The structural features of anisotropic
nanostructures play an important role, due to the shape, size and the enhanced selectivity and
activity based on morphology. Isotropic nanoparticles have only one uniform surface to provide its
activity but anisotropic nanostructures obtain asymmetric axes, and this transformation in symmetry
is responsible for the enhanced properties of the synthesized nanocrystals.1,2 Several approaches
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have been employed for the synthesis of anisotropic nanostructures such as template assisted
synthesis, electrochemical synthesis and photochemical synthesis etc. Synthesis of Pd tetrahedral
and trigonal bipyramidal nanocrystals with concave surfaces using solvothermal process in the
presence of formaldehyde has been performed.3-6 Xia et al. successfully used polyol based synthesis
in the presence of poly(vinyl pyrrolidone) (PVP) to form varieties of Pd nanostructures such as
nanobars, short nanorods, nanocubes, octahedra, icosahedra, and nanoplates.7,8 In spite of these
reported procedures, it is simpler and easier to follow water-based seed mediated synthesis due to
high yield and better control over the size and shape evolution of metal nanostructures.9,10 In this
paper, we report the synthesis of structurally well-defined monodisperse Pd nanostructures using
seed mediated wet chemical synthesis. Cetyltrimethylammonium bromide (CTAB) used as capping
or stabilizing agent in order to prevent the agglomeration during the formation of Pd NCs. Pd is
well known catalyst for catalysing many industrial applications from a long time. Literature shows
60% of Pd used as catalyst instead of Pt noble metal catalyst to minimize the cost effect and serves
as an efficient metal substitution instead of Pt for performing various applications.11-17 Some of the
notable applications of Pd materials include sensing through surface enhanced Raman Scattering
(SERS),18-21 hydrogenation/dehydrogenation reactions, low-temperature reduction of automobile
pollutants, and petroleum cracking.22-24 Among all applications performed using Pd as a catalyst,
C-C bond formation reactions has been attracted worldwide attention. Some of the examples of C-
C bond forming reactions are Suzuki-Miyaura, Mizoroki-Heck, and Stille, Hiyama and Negishi
cross-coupling reactions.25-28 The biaryl products obtained plays characteristics role for the
production of polymers, agrochemicals, pharmaceutical intermediates and high-tech materials.
Suzuki-Miyaura cross-coupling reactions of aryl boronic acids with aryl halides recommend a
valuable procedure to get biaryl compounds. This well-known reaction is mainly associated because
of the utilization of mild reaction conditions and their tolerance towards wide variety of functional
groups.29-33 Balanta et al. elucidated a broad explanation towards Pd nanoparticles catalyzed C-C
bond-forming processes, taking into account the recent work progress in this area and looking into
stabilizing agent’s performance, catalytic upshot and recycling prospective.34 A wide summary of
nanocatalysts for Suzuki-Miyaura cross-coupling reactions, emphasizing their performance,
stability and recyclability has reported.35-38 In this article, controlled growth of monodisperse Pd
NCs along (100) plane attributed excellent catalytic behaviour for Suzuki-Miyaura cross-coupling
reaction and 99% conversion for p-NP reduction. Being heterogeneous in nature, Pd NCs catalyst
can be recovered easily from the reaction mixture and recycled for further rounds of analogous
organic transformations. It has been observed that Pd NCs shows high product yields even for five
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consecutive cycle of reaction, suggesting that this can serve as an excellent recyclable catalyst. p-
NP is one of the harmful contaminants among the organic pollutants. In living organisms, it is
recognized for causing adverse health issues. The anthropogenic and toxic p-NP is used for
manufacturing most of the pesticides, insecticides, fungicides and also used in case of hair dyes,
pharmaceuticals purpose and to darken leather.39,40 Human exposure to p-NP takes place by three
pathways, namely inhalation, ingestion (eating, drinking) and dermal contact. Immediate exposure
of p-NP by the pathways of inhalation or ingestion causes symptoms such as headaches, drowsiness,
nausea, and cyanosis.41 p-NP has the property of highly solubilized and more stable in water bodies
which creates adverse effect for marine organisms. Hence, it is difficult to remove p-NP from
contaminated waste water by traditional waste water treatment. Therefore, researchers have been
used many techniques for the removal of p-NP. The techniques such as adsorption, microwave-
assisted catalytic oxidation, microbial degradation, photo catalytic degradation, electro coagulation,
and electrochemical treatment.42-45 On the other hand, all these techniques have some drawbacks
like energy-consuming and include the use of organic solvents. Therefore, in the present scenario,
it is very important to develop and employ aqueous phase conversion of p-NP to p-AP under mild
conditions by catalytic reduction method. The product (p-AP) obtained from the catalytic reduction
of p-NP is effective in many useful applications such as analgesic and antipyretic drugs, photo-
graphic developer, corrosion inhibitor, anticorrosion lubricant etc..46
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Experimental
Materials and methods
Palladium chloride, Cetyltrimethylammonium bromide, p-nitrophenol and NaBH4 were purchased
from SD Fine Chemical Limited, India and used without further purification. Ascorbic acid (L-AA)
was purchased from Merck. All the glass wares were cleaned with aqua regia and finally rinsed
with deionized water. UV-Visible analysis was carried out with the help of PerkinElmer Lambda
360 UV-Visible spectrophotometer. FESEM analysis was done with the instrument JEOL,
Singapore. TEM and HRTEM measurements were carried out using Thermo Scientific TALOS
F200S G2 (200 KV), FEG, CMOS Camera 4K x 4K. Samples for HRTEM were prepared by drop
casting on 300 mesh carbon coated copper grid. XRD analysis was performed using the instrument
Rigaku Ultima-IV from Japan. The unit cell parameters can be calculated from the formula a0
=2d10/√3. The metallic Pd sizes can be estimated using Scherrer formula: size = 0.89λ/β cos θ.47
Zeta potential and DLS (Dynamic Light Scattering) measurement were performed using Anton Paar
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Lite sizer 500 particle size analyser. H NMR spectra were recorded at 400 MHz in deuterated
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chloroform (CDCl3, δ = 7.27 ppm). 1H NMR coupling constants (J) are reported in Hertz (Hz) and
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multiplicities are given as follows: s (singlet), d (doublet), t (triplet), m (multiplet).
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Preparation of Pd seed
Pd NCs are synthesized using seed mediated protocol with slight modification in the temperature
reported by Anna Klinkova’s group.48 Initially, H2PdCl4 (20 mM) was prepared by taking 0.115 g
of PdCl2 with 6.5 mL 0.2 M conc. HCl and stirred for 3 h to obtain a clear orange solution of
H2PdCl4, followed by dilution with distilled water to make 20 mM H2PdCl4 solution. For the
synthesis of seed solution; 0.045 g CTAB was dissolved in 10 mL of distilled water in a 25 mL
round bottom flask equipped with a stirring bar and heated to 95 °C under stirring for 5-10 min.
After that, 0.25 mL of H2PdCl4 was added followed by the addition of 0.02 mL of ascorbic acid (0.4
M) solution at a stretch. The reaction mixture was stirred for 10 minutes and then cooled to 30 °C.
The solution kept undisturbed 2 h for the complete growth of seed particles (Figure 1, step 1). The
solution turns blackish brown from light orange.
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Preparation of growth solution
9.0 g of CTAB taken in a round bottom flask containing 90 mL of distilled water and stirred for 10-
15 min at 70 °C. 3.2 mL of 20 mM H2PdCl4 was added to it. Then, 2 mL of aged Pd seeds were
added for the growth solution under vigorous stirring, followed by the addition of 2.5 mL of 0.4 M
ascorbic acid. The resulting reaction mixture was kept undisturbed 12 h at 70 °C for the growth of
Pd NCs. The resulting Pd NCs were settled at the bottom of the flask and the solution was sonicated
for a minute to remove the settled particles from the flask (Figure 1, step 2). Then it was centrifuged
twice at 6500 rpm for 15 minutes to remove the unreacted ions and excess surfactant. The residue
was redispersed in deionised water and dried at 70 °C over night for further characterization.
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Pd seed
H2PdCl4, AA
nucleation
2 h
CTAB
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Step. 1 Synthesis of smaller size Pd NC seed by the reducing agent L-AA.
Pd NCs
H2PdCl4
Seed, AA
growth
12 h
CTAB
Step. 2 Synthesis of Pd NCs by the addition of Pd seed.
Figure 1. Schematic representation of synthesis protocol of nucleation and growth of nuclei into
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desired Pd NCs.
Procedure for Suzuki-Miyaura cross-coupling reaction
To a mixture of aryl halide (1 equiv.), phenylboronic acid (1.1 equiv.), and potassium carbonate
(2.2 equiv.) in ethanol:water (EtOH:H2O) (1:1) solvent system, Pd NCs (4 wt%) were added and
stirred at room temperature for designated hours. TLC was employed to evaluate the progress of
the reaction. After the reaction completion, Pd NCs were separated through centrifugation and the
separated Pd NCs were washed with EtOH and water and then dried. To the filtrate,
dichloromethane (DCM) and deionized water were added. DCM layer was separated from the water
layer using a separating funnel and dried with sodium sulphate (Na2SO4). The separated DCM layer
was concentrated in a rotary evaporator to obtain the crude product which was purified through
column chromatography using hexane and ethyl acetate as eluting solvent to get the corresponding
products in good to excellent yields. All the coupled products were reported molecules and were
confirmed by comparing the 1H NMR spectral data with those of the authentic samples.
1. 1,1'-Biphenyl (Table 2, entries 1 & 11): Colourless crystals. 1H NMR (400 MHz, CDCl3) δ 7.60
(d, J = 7.4 Hz, 4H), 7.45 (t, J = 7.6 Hz, 4H), 7.35 (t, J = 7.3 Hz, 2H).
2. [1,1'-Biphenyl]-4-ol (Table 2, entries 2 and 12): Off-white solid. 1H NMR (400 MHz, CDCl3) δ
7.53 (d, J = 7.8 Hz, 2H), 7.47 (d, J = 8.6 Hz, 2H), 7.40 (t, J = 7.6 Hz, 2H), 7.29 (s, 1H), 6.90 (d, J
= 8.6 Hz, 2H), 4.94 (s, 1H).
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3. [1,1'-Biphenyl]-2-carboxylic acid (Table 2, entry 3): Off-white solid. H NMR (400 MHz,
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CDCl3) δ 7.93 (d, J = 7.4 Hz, 1H), 7.55 (d, J = 7.5 Hz, 1H), 7.54 - 7.34 (m, 7H).
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4. 4-Methyl-1,1'-biphenyl (Table 2, entries 4 and 13): Colourless crystals. H NMR (400 MHz,
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7.3 Hz, 1H), 7.23 (s, 2H), 2.39 (s, 3H).
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5. 4-Methoxy-1,1'-biphenyl (Table 2, entries 5 and 14): White solid. 1H NMR (400 MHz, CDCl3)
δ 7.47 (t, J = 9.3 Hz, 4H), 7.34 (t, J = 7.3 Hz, 2H), 7.23 (t, J = 7.2 Hz, 1H), 6.90 (t, J = 9.6 Hz, 2H),
3.78 (s, 3H).
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6. [1,1'-Biphenyl]-4-carbonitrile (Table 2, entry 6): Pale yellow crystals. H NMR (400 MHz,
CDCl3) δ 7.72 (d, J = 8.4 Hz, 2H), 7.67 (d, J = 8.4 Hz, 2H), 7.60 – 7.56 (m, 2H), 7.48 (d, J = 10.0
Hz, 2H), 7.42 (d, J = 8.4 Hz, 1H).
7. [1,1'-Biphenyl]-4-yl(phenyl)methanone (Table 2, entry 7): Off-white solid. 1H NMR (400 MHz,
CDCl3) δ 7.89 (d, J = 8.3 Hz, 2H), 7.86 – 7.81 (m, 2H), 7.70 (d, J = 8.3 Hz, 2H), 7.65 (d, J = 7.3
Hz, 2H), 7.60 (t, J = 7.4 Hz, 1H), 7.49 (d, J = 7.8 Hz, 4H), 7.40 (t, J = 7.3 Hz, 1H).
8. 4-Nitro-1,1'-biphenyl (Table 2, entry 8): Yellow crystals. 1H NMR (400 MHz, CDCl3) δ 8.29 (d,
J = 8.8 Hz, 2H), 7.73 (d, J = 8.8 Hz, 2H), 7.65 – 7.59 (m, 2H), 7.50 (d, J = 11.4 Hz, 2H), 7.46 –
7.41 (m, 1H).
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9. 1-([1,1'-Biphenyl]-4-yl)ethanone (Table 2, entry 9): Off-white solid. H NMR (400 MHz,
CDCl3) δ 8.03 (d, J = 8.4 Hz, 2H), 7.68 (d, J = 8.4 Hz, 2H), 7.62 (d, J = 7.3 Hz, 2H), 7.47 (t, J =
7.4 Hz, 2H), 7.39 (t, J = 7.3 Hz, 1H), 2.63 (s, 3H).
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10. [1,1'-Biphenyl]-2-carbaldehyde (Table 2, entry 10): Pale yellow liquid. H NMR (400 MHz,
CDCl3) δ 9.98 (s, 1H), 8.02 (d, J = 7.8 Hz, 1H), 7.64 (t, J = 7.5 Hz, 1H), 7.52 – 7.41 (m, 5H), 7.38
(d, J = 7.5 Hz, 2H).
Procedure for p-NP to p-AP reduction
Further to investigate the catalytic activity of synthesized Pd NCs catalyst, a standard reduction test
was carried out and the reaction progress was monitored through UV-Visible spectra. First, 0.01 M
p-NP aqueous solution and 0.5 M NaBH4 solution were prepared and the reduction process was
observed by the addition of synthesized Pd NCs. 20 L (0.01 M) p-NP was mixed with 3 mL of
H2O and then 10 L of ice-cold aqueous NaBH4 solution (0.5 M) was added which result in deep
yellow colour solution. Then different amount of catalyst (mg/mL) was added to the reaction
mixture and the progress of reaction was observed through UV-Visible analysis until the solution
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became colourless. The maximum amount of reactant converted to product has been calculated by
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using the formula.49,50
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퐶0 − 퐶푡
% reduction =
× 100
퐶0
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Where C0 is the initial concentration of phenolate ion and Ct is the concentration of phenolate ion
at time t.
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Results and Discussion
Synthesis and characterization
Solution-phase synthesis of Pd NCs is typically conducted without using any inert environment. Pd
precursor is reduced in the presence of stabilizer, CTAB. The overall synthesis protocol has been
known as wet chemical seed mediated approach. Among all the synthetic methods, seed-mediated
growth approach has been proven to be the finest method because it separates the nucleation and
growth stages of nanoparticles, which provides better control over the size distribution, and shape
evolution of nanoparticles. In this protocol, smaller size Pd NCs were formed as seed. These were
synthesized through one-step reduction of H2PdCl4 in aqueous solution with ascorbic acid in
presence of CTAB at 95 °C. The colour of the solution turned to brownish black from light orange
and it confirmed that the reduction process was rather fast at higher temperature (shown in Fig. 1).
The reaction was kept undisturbed for 2 h to continue the aging process of the Pd seed. These seed
particles were added to growth solution for the formation of Pd NCs with well distinct growth.
FESEM images and corresponding histogram of the diagonal length of Pd NCs seed are given in
Fig. 2(a) and 2(b), respectively. At ambient condition, a rapid reduction process was followed for
the formation of smaller nuclei cubic Pd seeds. The seed nanocubes were produced in high yield
and size of Pd NC seeds were 22 ± 5 nm confirmed from the histogram shown in Fig. 2(b). Energy-
dispersive X-ray spectroscopy (EDS) analysis used for the elemental analysis of desired samples in
a selected surface area and provides information about the existence of each elements present in the
sample. EDS has been performed for the seed nanocubes as shown in Fig. 2(c). The characteristic
signals revealed the presence of Pd in the Pd NCs and inset shows the approximate wt. % and atomic
% obtained from the corresponding FESEM image. The above characterized smaller size Pd seed
particles were then injected into the reaction mixture during the preparation of growth solution for
the formation of well grown Pd NCs in high yield by the help of stabilizing agent. Twinning of the
nanostructures were completely avoided starting from nucleation stage to the growth process. The
facets of Pd NCs proceeding at relatively lower temperatures has been reported previously.51
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a
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Pd seed
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100 nm
Length (nm)
c
Energy, keV
Figure 2. (a) FESEM image of cubic Pd seed particles, (b) corresponding histogram of seed
particles shows the average size around 22 ± 5 nm and (c) EDS analysis small cubic seed confirming
the elements present in the selected area.
The growth Pd NCs were performed at 70 °C using 20 mM H2PdCl4 solution in presence of ascorbic
acid as reductant. With slight modification in the temperature, multilayer and monodisperse
formation of desired nanostructure was obtained in high yield compared to the previous report.48-51
The obtained Pd NCs were purified using centrifugation after 12 h of cooling as discussed in the
experimental section. Synthesized Pd NCs were preliminarily subjected to UV-Vis analysis. Fig.
3(a) represents the UV-Visible spectrum shows one broad peak of Pd NCs having λmax at 416 nm.
The peak position Pd NCs shift towards higher wavelength (bathochromic shift) or lower
wavelength (hypsochromic shift) depending upon the size of the Pd NCs formed. Well defined,
uniform size and shapes of Pd NCs are formed using smaller Pd seeds as mentioned wet chemical
seed-mediated growth approach. FESEM analysis was carried out for Pd NCs on silicon wafer and
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sonicated before the drop casting for better dispersity as shown in Fig. 3(b) and 3(c) which shows
monodisperse morphology of Pd NCs. FESEM images show the multilayer formation and well
dispersity of the sample. The diagonal length of the synthesized Pd NCs was 60 ± 5 nm as shown
in the histogram in Fig. 3(d). The characteristic signals observed for Pd NCs from EDS analysis
represented in Fig. S1. The hydrodynamic diameter of the particles was found to be 126.25 nm from
dynamic light scattering (DLS) measurement. To confirm the surface charge of Pd NCs, Zeta
potential measurement was carried out. Zeta potential of Pd NCs was +41.4 mV due to the presence
of cationic surfactant, CTAB. DLS and Zeta potential data were presented in the supporting
information S2 and S3, respectively. The zeta potential of CTAB was found +44.4 mV and
decreased in the case of Pd NCs.
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b
a
2.0
Pd NCs
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416
1.0
0.5
0.0
200
400
600
800
1000
100 nm
Wavelength(nm)
c
d
Pd NCs
40
20
0
50
60
70
100 nm
Length (nm)
Figure 3. (a) UV-Visible spectrum of Pd NCs, (b) FESEM image Pd NCs showing multilayer
growth, (c) high resolution FESEM image of monodispersed Pd NCs, (d) size distribution histogram
of Pd NCs.
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d=0.19nm
(200)
a
b
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2nm
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50 nm
10 nm
c
(200)
d
(111)
(111)
(200)
(220)
(311)
(220)
20
30
40
50
60
70
80
2 (degree)
5.00 1/ nm
Figure 4. (a) TEM image of Pd NCs, (b) HRTEM image of the selected square corner of an
individual Pd NCs having d spacing 0.19 nm corresponds to (200) lattice plane of Pd NCs. (c)
SAED pattern of Pd NCs and (d) XRD pattern of Pd NCs.
Transmission electron microscopy (TEM) images were in well agreement with the morphologies
observed in FESEM. Fig. 4(a) shows the TEM image of monodisperse Pd NCs. The high resolution
TEM image mentioned in Fig. 4(b) clearly shows lattice fringes with a measured d-spacing of 0.19
nm corresponds to (200) plane of Pd. The selected area electron diffraction (SAED) pattern in Fig.
4(c) shows the Debye-Scherrer rings of Pd NCs corresponds to (111), (200), (220) and (311)
reflection planes of the Pd nanocrystal.6 The SAED pattern is suitably supported by the powder X-
ray diffraction (PXRD) and confirmed the crystalline lattice structure of the Pd NCs. The diffraction
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pattern of the Pd NCs is shown in the Fig. 4(d). The spacing value d = 0.19 nm was calculated on
the basis of Bragg’s law following the formula n λ = 2d sinθ. The diffraction peaks at 2θ values
obtained are 40.09°, 46.57° and 68.87° corresponds to the (111), (200) and (220) crystal planes of
Pd NCs, respectively. The sharp intense peak of the above-mentioned planes confirms the
crystalline nature of the Pd NCs. The diffraction planes of Pd NCs are well matches with the JCPDS
(file no. 87-0638) and face centered cubic (fcc) arrangements.
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Application of the Pd NCs in Suzuki-Miyaura cross-coupling reactions
The Suzuki-Miyaura cross-coupling is a simple and reliable organic transformation for the synthesis
of C-C cross-coupled products. The ability to tolerate a wide range of functional groups, mild
reaction conditions and easily available reagents are some of the key advantages of employing
Suzuki-Miyaura cross-coupling reaction. Thus, following the successful characterization of Pd NCs
by various analytical techniques, we have proceeded with the catalytic activity study in Suzuki-
Miyaura cross-coupling reaction. The cross-coupling between iodobenzene and phenylboronic acid
was chosen as the model reaction for optimization of the reaction conditions (Scheme 1). Following
a series of reactions by altering various parameters, the Suzuki-Miyaura cross-coupling reaction
was optimized as shown in Table 1.
Scheme 1. Suzuki–Miyaura cross‐coupling reaction of iodobenzene with phenylboronic acid.
Among the bases used, K2CO3 (Table 1, entry 2) was found to be effective when compared with
other bases like Na2CO3 (Table 1, entry 3), NaOH (Table 1, entry 4) and KOH (Table 1, entry 5).
In case of solvent system, H2O:EtOH (1:1) system was found to be superior (Table 1, entry 2).
When only EtOH (Table 1, entry 7) and MeOH (Table 1, entry 7) were used, the yields obtained
were not appreciable. Also, there was no improvement in the yield when acetonitrile (ACN) (Table
1, entry 8) or toluene (Table 1, entry 10) was employed. The optimum reaction temperature was
found to be room temperature (Table 1, entry 2) since there was no any improvement in the yield
with increase in temperatures (Table 1, entries 11-13)
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Table 1. Optimization of reaction conditions for model Suzuki-Miyaura cross-coupling reaction in
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5
the presence of Pd NCsa
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9
Temp.
(°C)
Time
(h)
0.5
0.5
0.67
0.5
0.67
0.67
0.5
1
Yield
(%)b
95
Entry Base
Solvent
Catalyst (mg)
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1
K2CO3
K2CO3
H2O:EtOH(1:1) R.T
4
2
2
2
2
2
2
2
2
2
2
2
2
3
1
1
2
2
2
H2O:EtOH(1:1) R.T
95
3
Na2CO3 H2O:EtOH(1:1) R.T
90
4
NaOH
KOH
H2O:EtOH(1:1) R.T
H2O:EtOH(1:1) R.T
80
5
70
6
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
H2O
R.T
R.T
R.T
R.T
R.T
80
7
EtOH
ACN
75
8
75
9
MeOH
Toluene
1
72
10
11
12
13
14
15
16
17
18
1
80
H2O:EtOH(1:1) 40
H2O:EtOH(1:1) 50
H2O:EtOH(1:1) 60
H2O:EtOH(1:1) R.T
H2O:EtOH(1:1) R.T
H2O:EtOH(1:1) R.T
H2O:EtOH(1:1) R.T
H2O:EtOH(1:1) R.T
0.5
0.5
0.5
0.5
0.5
1
95
95
95
95
90
90
1
95
0.25
90
aReaction conditions: iodobenzene (0.27 mmol), phenylboronic acid (0.30 mmol), base (0.60 mmol)
and solvent (5 mL) in air. bIsolated yield after separation by column chromatography.
Table 2. Suzuki-Miyaura cross-coupling reactions of different aryl halides with phenylboronic acid
catalyzed by Pd NCsa
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Entry
Aryl halide
Product
Time (h) Yield (% )b
TON
5
6
7
8
9
01
0.5
0.5
95
92
1242
02
03
1115
968
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49
50
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52
53
54
0.5
90
04
05
06
0.5
0.5
0.5
93
92
90
1137
1048
1319
07c
1
85
868
08
0.5
2
91
82
1201
1099
09c
10c
2
80
1153
11
12c
13c
14c
1
2
1
1
92
88
86
88
1563
1356
1341
1255
aReaction conditions: aryl halide (0.27 mmol), phenylboronic acid (0.30 mmol), Pd NCs (2 mg),
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60
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base (0.60 mmol) and EtOH: H2O (1:1) (5 mL) in air. Isolated yield after separation by column
chromatography. cReaction carried out at 50 °C.
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The cross-coupling yield increased with increase in the catalyst ratio from 1 mg to 2 mg (Table 1,
entries 15 and 2). But further increase in the catalyst quantity (Table 1, entries 1 and 14) does not
have any effect in the yield and hence 2 mg of the catalyst was found to be most favourable Pd NC
ratio (Table 1, entry 2). Increase in the reaction time from 0.25 h (Table 1, entry 18) to 0.5 h (Table
1, entry 2) increased the yield but further increase in time to 1 h (Table 1, entry 17) does not increase
the yield and hence 0.5 h was finalized to be the optimum time duration for the Suzuki-Miyaura
cross-coupling with Pd NCs. After optimizing the reaction conditions, the scope of Pd NCs was
studied for various aryl halides to understand the general applicability of the catalyst which is
represented in Table 2. From the results obtained it can be generalized that Pd NCs are highly active
and could catalyse the cross-coupling reaction very efficiently for aryl halides with different
electron withdrawing as well as electron donating groups to give good to excellent yields. Also,
aryl iodides were faster to react when compared with their bromide counterparts. In addition,
external heating was required in most of the cases for aryl bromides to undergo cross-coupling with
phenylboronic acid.
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Table 3. Comparison of results for Pd NCs catalyst with other Pd catalysts for Suzuki–Miyaura
cross-coupling reaction between iodobenzene and phenylboronic acid.
Entry
Catalyst
Solvent
Temp.
(°C)
Time
(h)
Yield
(%)
Ref.
1
EtOH:H2O
(4:1)
85
4
92
6
Pd NRs
2
3
Pd Nanoclusters
Pd NPs
NMP:H2O
CH3OH:CH3CN
(1:1)
100
R.T
5
2
89
99
52
53
4
5
6
Pd-acac-am-
Fe3O4@SiO2
Pd NPs@CS
DMF:H2O
(8:2)
80
65
2
3
2
98
98
98
54
55
56
EtOH:H2O
(1:1)
Pd/AlO(OH)
NPs
IPA:H2O
(1:1)
R.T
7
8
MUA‐Pd
DMF
90
85
8
99
99
57
58
C/Co@PNIPAM
Toluene:H2O
(2:1)
16
9
Pd(II)-NiFe2O4
EtOH:H2O
(9:1)
80
3
96
59
10
Pd NCs
EtOH:H2O
(1:1)
R.T
0.5
95
Present
work
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In order to understand the exceptionality and unique activity of the synthesized Pd NCs, the catalytic
properties of synthesized catalyst was compared with other reports towards C-C cross coupling
reactions between substituted aryl iodide/bromide with phenylboronic acid. The results of Suzuki-
Miyaura cross-coupling shows a better catalytic activity with high yield in shorter reaction time and
milder reaction conditions for Pd NCs catalyst compared to other Pd NPs supported on different
supports as shown in Table 3.
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Recyclability of the Pd NCs in Suzuki-Miyaura cross-coupling reactions
Recyclability is a vital factor which assesses the greenness and the sustainability of every catalyst
in any organic transformation. Hence, Pd NCs were subjected to recyclability study on the model
of Suzuki-Miyaura cross-coupling reaction and the results obtained are depicted in Fig. 5.
Figure 5. The recycling efficiency of Pd NCs in Suzuki-Miyaura cross-coupling reaction.
The results obtained from it can be understood that there is no significant change in the catalytic
activity of Pd NCs with the yield reaching up to 92 % from 95 % by the end of fifth recycle. The
Pd NCs characterized by FESEM given in Figure S4 after the fifth recycle of Suzuki-Miyaura cross-
coupling shows that there is slight characteristic change in the morphology of Pd NCs.
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Reduction of p-NP
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The catalytic reduction of p-NP to p-AP was initially evaluated by the addition of pale-yellow
coloured aqueous solution of p-NP which exhibits a characteristic absorption peak at 317 nm. The
addition of NaBH4 to p-NP, colour of the solution changes to dark yellow and shows a characteristic
peak at 400 nm due to the formation of phenolate ion as shown in Fig. 6 (a). In order to understand
the role of catalyst, the reaction proceeded up to 1h without the addition of Pd NCs as shown in Fig.
6 (b). Interestingly, it has seen that the reduction of nitro-group did not proceed in the absence of
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-
catalyst because the kinetic barrier between the two negative ions of p-NP and BH4 is extremely
high and NaBH4 is not able to overcome the barrier alone.49 However, the addition of Pd NCs a new
absorption band appeared at 313 nm with the simultaneous decrease in the absorption peak of
phenolate ion and peak for p-NP at 317 nm vanishes completely.
1.8
1.5
1.2
0.9
0.6
0.3
0.0
1.8
1.5
1.2
0.9
0.6
0.3
0.0
Phenolate ion
Phenolate ion (1h)
4-NP
Phenolate ion
a
b
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300
350
400
450
500
250
300
350
400
450
500
Wavelength(nm)
Wavelength(nm)
2.0
1.5
1.0
0.5
0.0
0.0
-0.7
-1.4
-2.1
-2.8
-3.5
y = -0.73x + 0.67
R2 = 0.97
c
d
6 min
k = 7.3* 10-1 min-1
250
300
350
400
450
500
1
2
3
4
5
Wavelength(nm)
Time (min)
Figure 6. (a) Absorption peaks at 317 and 400 nm represents the characteristic peak for p-NP and
phenolate ions, respectively, (b) represents the progress of reaction without addition of catalyst, (c)
successive reduction of p-NP to p-AP with 70 L Pd NCs catalyst and (d) kinetic plot of ln (Ct/C0)
vs time for the reduction of p-NP.
In order to study the doses of catalyst, primarily 1 mg/mL of catalyst concentration was prepared.
It was seen that even after the addition of 100 L of catalyst the reduction proceeded slowly, with
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rate constant, k = 0.4*10-1 min-1 and 85 % conversion obtained after 40 min of reaction shown in
Fig. S5. Hence, the catalyst concentration was increased. Initially, different volumes of 10, 30, 50
and 70 L Pd NCs catalyst of concentration 2 mg/mL were added and allowed to perform the
reduction. It was seen that 70 L of Pd NCs performed maximum conversion up to 99 % in 6 min
as shown in Fig. 6 (c). The comparison of different loading of the catalyst shown in Fig. S6 and
Table S1 shows the variations of rate of the reaction, time and conversion % with stepwise increase
in the catalyst amount. The colour changes from light yellow to dark yellow due to the addition of
NaBH4 gives the information about the alkalinity of the solution and a colourless solution was
observed after reduction on the surface of catalyst is shown in Figure S7. The pathway of catalysed
reduction reaction follows first order kinetics which has confirmed through the linear correlation
curve obtained from the plot ln (Ct/C0) versus time based on the absorbance. The rate constant, k
was calculated and found to be 7.3*10-1 min-1 for 70 L of Pd NCs whereas for 10, 30 and 50 L,
the rate constant was observed 10-2, 2.4*10-1 and 3.8*10-1 min-1, respectively.
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+
+
Figure 7. Steps involved in the reduction of p-NP to p-AP using Pd NCs as catalyst, NaBH4 as
hydrogen source under RT using aqueous solvent.
-
The progress of the reduction reaction described as, adsorption of both the donor BH4 and acceptor
p-NP molecules to the catalytic surface which led electron transfer from donor to acceptor and
provide the reduction product, p-AP. Addition of NaBH4 to the reactant leads the formation of
phenolate ion which retain its position in absence of catalyst. Whereas subsequently addition of
-
catalyst, leads transformation of hydride from BH4 to the reactant. Fig. 7 represents the stepwise
plausible reduction mechanism from p-NP to p-AP using Pd NCs as a catalyst to overcome
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activation energy barrier of two repelling negatively charged anions.60 Comparison of the literature
results are summarized in Table 4 for the hydrogenation of p-NP using various Pd nanocatalyst,
which concern the importance of anisotropic Pd NCs catalyst towards the reaction.
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9
Table 4. Comparison of Pd Nanocatalyst used in p-NP reduction reaction.
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Entry
Catalyst
Pd/MPC
Time (min)
k (min-1)
1.2*10-2
Ref.
49
1
2
3
4
5
6
7
12
24
7
Pd–graphene nanohybrid
PdNPs stabilized in XG
UiO-66-NH2@COP@Pd
Pd NP/CNT
1.4*10-1
61
1.84.1*10-1
3.1*10-1
62
63
7
6.32*10-1
7.3*10-1
64
Pd NCs
6
Present
work
Conclusion
Anisotropic monodisperse Pd NCs were synthesized using seed mediated wet chemical synthesis
in two step process. Smaller cubic Pd seeds were synthesized for the growth of Pd NCs of sharp
edges with high yield. The synthesized Pd NCs shows well-defined morphology throughout the
sample. Spectroscopic and microscopic tools were used for the characterization of the synthesized
Pd NCs. These Pd NCs were found to perform highly efficient heterogeneous catalyst for Suzuki-
Miyaura cross coupling reaction as well as for p-NP reduction. It exhibited enhanced activity,
greater selectivity and stability of the Pd NCs. The recyclability study of Pd NCs showed a high
catalytic activity even after performing five consecutive cycles. The better catalytic activity of Pd
NCs can be attributed to its increase in surface energy associated with the different planes of the
nanocubes or increase in coordination atom on the low index crystallographic facets sites which
help them to act as the surface-active agent. These catalysts may attract useful attention for further
applications.
Associated Content
Supporting Information
The Supporting Information is available free of charge at http://pubs.acs.org/doi/xxxxxx
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EDS spectrum, DLS measurement, Zeta potential measurement, FESEM images of Pd NCs and 1H
NMR spectra of cross-coupled products.
6
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Author Contributions
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9
The manuscript was written through contributions of all authors. All authors have given approval
to the final version of the manuscript.
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† These authors contributed equally.
Acknowledgements
BMB acknowledges JAIN University for junior research fellowship. S.S and AKS are grateful to
the SERB, New Delhi, India for funding to conduct the research (CRG/2018/003533). The authors
thankful to Nano Mission (SR/NM/NS-20/2014) for FESEM facility at Jain University.
Notes
The authors declare no conflict of interest.
References
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