Highly Active Bimetallic Nickel-Palladium Alloy Nanoparticle Catalyzed Suzuki-Miyaura Reactions

Article abstract of DOI:10.1002/cctc.201500145

A bimetallic Ni-Pd alloy nanoparticle catalyst with a low palladium content (Ni_0.90Pd_0.10 nanocatalyst) was prepared and its catalytic performance in Suzuki-Miyaura reactions was evaluated along with that of other Ni-Pd nanocatalysts (with varying Ni/Pd molar ratios in the range of ≈0.25-0.75) and the corresponding monometallic, Ni and Pd, analogues. Notably, the bimetallic Ni_0.90Pd_0.10 alloy nanocatalyst performed exceptionally well for the synthesis of biaryls by employing a wide range of substituted aryl halides and arylboronic acids having electron-donating and electron-withdrawing groups, and they exhibited high recyclability in water/ethanol solution at moderate reaction temperatures. Catalyst poisoning tests and leaching experiments inferred the heterogeneous nature of the Ni_0.90Pd_0.10 nanocatalysts. The significant synergistic interactions between Ni and Pd account for the observed high catalytic efficacy of the Ni_0.90Pd_0.10 nanocatalyst. Up, up, and alloy! Highly active bimetallic Ni-Pd alloy nanocatalysts with high Ni/Pd molar ratios facilitate the Suzuki-Miyaura reactions of a wide range of aryl iodides/bromides and arylboronic acids to biaryls in high yields at moderate temperatures in water/ethanol solution. High synergistic interaction between Ni and Pd accounts for the observed high activity and recyclability of the bimetallic Ni-Pd nanocatalysts. FG=functional group.

Full text of DOI:10.1002/cctc.201500145

DOI: 10.1002/cctc.201500145
Communications
Highly Active Bimetallic Nickel–Palladium Alloy
Nanoparticle Catalyzed Suzuki–Miyaura Reactions
Rohit Kumar Rai,[a] Kavita Gupta,[a] Silke Behrens,[b] Jun Li,[c] Qiang Xu,[c] and
Sanjay Kumar Singh*[a, d]
Dedicated to Professor Daya Shankar Pandey on the occasion of his 54th birthday.
A bimetallic Ni–Pd alloy nanoparticle catalyst with a low palla-
dium content (Ni0.90Pd0.10 nanocatalyst) was prepared and its
catalytic performance in Suzuki–Miyaura reactions was evaluat-
ed along with that of other Ni–Pd nanocatalysts (with varying
Ni/Pd molar ratios in the range of ꢀ0.25–0.75) and the corre-
sponding monometallic, Ni and Pd, analogues. Notably, the bi-
metallic Ni0.90Pd0.10 alloy nanocatalyst performed exceptionally
well for the synthesis of biaryls by employing a wide range of
substituted aryl halides and arylboronic acids having electron-
donating and electron-withdrawing groups, and they exhibited
high recyclability in water/ethanol solution at moderate reac-
tion temperatures. Catalyst poisoning tests and leaching ex-
periments inferred the heterogeneous nature of the Ni0.90Pd0.10
nanocatalysts. The significant synergistic interactions between
Ni and Pd account for the observed high catalytic efficacy of
the Ni0.90Pd0.10 nanocatalyst.
Palladium metal (as metal complexes or nanoparticles) re-
mains, unarguably, the first choice catalyst for various cross-
coupling reactions, such as the Suzuki–Miyaura reaction, for
the synthesis of symmetrical and unsymmetrical biaryl com-
pounds owing to its high activity for a wide range of sub-
strates.[3–8] However, Pd nanoparticles suffer from significant
deterioration in catalytic activity owing to leaching or aggrega-
tion of the Pd species and to the formation of inactive Pd
black particles.[9] Moreover, considering the high cost associat-
ed with Pd metal, replacing Pd partially or completely with
low-costing or inexpensive metals is indeed a demanding and
challenging task. Several recent findings evidenced the impor-
tance of bimetallic nanoparticle catalysts, such as Pd–Au, Pd–
Ni, and Pd–Cu as active catalysts for the Suzuki–Miyaura reac-
tion.[10–14] Among the reported bimetallic Pd catalysts, Pd–Cu
and Pd–Ni catalysts are of particular interest, because these
metals (Ni or Cu) are inexpensive and are abundantly avail-
able.[13,14] Moreover, environmentally friendly aqueous-phase
(or mixtures of water/organic solvents, for example, ethanol
and methanol) catalytic reactions have significant impor-
tance.[15] Unfortunately, most reactions in which Ni–Pd catalysts
are used for the Suzuki–Miyaura reaction are performed in
high-boiling-point solvents, for example, dioxane or DMF, and/
or are performed at high reaction temperatures of approxi-
mately 50–1208C.[14] Bimetallic Ni–Pd nanoparticles and other
alloy nanoparticles have been reported to be very active for
the Suzuki–Miyaura reaction, and in a few cases, they are even
superior to monometallic Pd nanoparticles. The excellent syn-
ergic interactions between Ni and Pd in bimetallic Ni–Pd nano-
catalysts are considered to play a crucial role in the observed
high catalytic activity. Despite these important observations, so
far, only bimetallic Ni–Pd nanoparticles with equimolar Ni and
Pd contents have been explored, and surprisingly, hardly any
attempt, unless reported otherwise, has been made to investi-
gate the effect of a high Ni/Pd molar ratio (higher than 1) on
the catalytic performance of Ni–Pd nanocatalysts. We believed
that there was no reason why Ni–Pd nanocatalysts with high
Ni/Pd ratios would not be active; therefore, we synthesized the
Ni0.90Pd0.10 nanocatalyst and extensively investigated its catalyt-
ic activity for the Suzuki–Miyaura cross-coupling reaction in
water/ethanol solution (Scheme 1).
A tremendous effort has been made towards the development
of highly active, selective, stable, and cost-effective catalysts—
an integral part of any efficient catalytic chemical process. Bi-
metallic nanocatalysts, as alloys or core–shell states, are known
to show catalytic performance that is superior to that of its
monometallic analogous for many catalytic reactions.[1,2] The
enhanced catalytic performance of bimetallic systems results
from favorable synergistic interactions between the metals, for
which strong metal–metal interactions presumably tune the
bonding pattern of the reactants and stabilize the important
reaction intermediates on the catalyst surface.[1]
[a] R. K. Rai, K. Gupta, Dr. S. K. Singh
Discipline of Chemistry, School of Basic Sciences,
Indian Institute of Technology (IIT) Indore
Indore 452020 (India)
E-mail: sksingh@iiti.ac.in
[b] Dr. S. Behrens
Institut für Katalyseforschung und Technologie (IKFT)
Karlsruher Institut für Technolgie (KIT)
Hermann-von-Helmholtz-Platz 1,
76344 Eggenstein-Leopoldshafen (Germany)
[c] J. Li, Prof. Q. Xu
National Institute of Advanced Industrial Science and Technology (AIST)
Ikeda, Osaka 563-8577 (Japan)
[d] Dr. S. K. Singh
Herein, we report the enhanced heterogeneous catalytic
performance of the Ni0.90Pd0.10 alloy nanocatalyst for the
Suzuki–Miyaura reactions of a wide range of aryl halides and
arylboronic acids in water/ethanol solutions under aerobic con-
ditions. Further, to have a wide overview and better under-
Center for Material Science and Engineering
Indian Institute of Technology (IIT) Indore
Indore 452020 (India)
Supporting Information for this article is available on the WWW under
http://dx.doi.org/10.1002/cctc.201500145.
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Scheme 1. Catalytic Suzuki–Miyaura reaction over bimetallic Ni–Pd nano-
catalysts.
Figure 1. a) TEM image, b) line scan compositional profile corresponding to
the HAADF-STEM image (inset), c) EDX point analysis, and d) SEM image and
e,f) corresponding EDX elemental mapping showing the Ni (green) and Pd
(red) elements of the Ni0.90Pd0.10 nanocatalyst.
standing of the effect of varying the Ni/Pd molar ratio on the
catalytic performance of the Ni–Pd nanocatalysts, the catalytic
performance of the synthesized Ni0.90Pd0.10 nanocatalyst for the
Suzuki–Miyaura reaction was compared with that of other Ni–
Pd nanocatalysts (with Ni/Pd molar ratios varying from 0.25 to
0.75) synthesized analogously to the Ni0.90Pd0.10 nanocatalyst.
Bimetallic Ni1ÀxPdx (x=0.10–0.75) alloy nanoparticles were
obtained by reducing varying molar ratios of nickel(II) chloride
and potassium tetrachloropalladate(II) by adding an aqueous
solution of sodium borohydride, as a reducing agent, in the
presence of polyvinylpyrrolidone. Using a procedure analogous
to that described above for the Ni–Pd alloy nanocatalysts, the
respective monometallic Ni and Pd nanoparticles were also
prepared from nickel(II) chloride and potassium tetrachloropal-
ladate(II), respectively. A physical mixture of monometallic Ni
and Pd nanoparticles with the appropriate Ni/Pd molar ratio
was obtained by mixing the separately prepared monometallic
Ni and Pd nanoparticles. Transmission electron microscopy
(TEM) and high-angle annular dark-field scanning TEM
(HAADF-STEM) images of the Ni0.90Pd0.10 nanocatalyst revealed
a particle size of approximately 10 nm (Figure 1, see also Fig-
ure S1 in the Supporting Information). A uniform composition
of the Ni0.90Pd0.10 alloy nanoparticles was demonstrated by TEM
energy-dispersive X-ray spectroscopy (EDX) point analysis of
randomly chosen nanoparticles (Figure S1). Moreover, the
cross-sectional compositional line scan EDX profiles of the Ni–
Pd nanoparticles, as shown in Figure 1, revealed the presence
of both metals (i.e., Ni and Pd), which suggests the alloy char-
acter of the bimetallic nanoparticles. This was further support-
ed by the appearance of both metals, Ni and Pd, in the ele-
mental mapping of the bimetallic Ni0.90Pd0.10 nanoparticles
(Figure 1). According to the powder X-ray diffraction (XRD) pat-
tern, the only major peak for the Ni0.90Pd0.10 nanoparticles
emerged at 2q=42.158, and it can be assigned to the (111)
plane diffraction, similar to Ni (JCPDS 04-0850, see Figure S2).
The shift in the diffraction peaks of all the Ni–Pd nanocatalysts
towards high-angle values relative to the original peaks of Pd
(JCPDS 05-0681) suggests formation of an alloy, in which lattice
contraction occurred as a result of substitution of the Pd
atoms by Ni atoms. Moreover, it was noted that no individual
Ni or Pd diffraction peaks were observed in the XRD pattern,
which is indicative of the formation of a Ni–Pd alloy with no
segregation of Ni or Pd atoms. Consistent with the XRD results,
analysis of the Ni0.90Pd0.10 nanocatalyst by X-ray photoelectron
spectroscopy (XPS) revealed characteristic signals for metallic
Ni[2p] and Pd[3d], and this indicated the coexistence of both
metals in the bimetallic Ni0.90Pd0.10 nanocatalyst, as shown in
Figure 2. A thin oxide cover observed on the surface of the
Ni0.90Pd0.10 nanocatalyst, presumably formed during exposure
of the sample to air, was readily removed by argon sputtering.
The signals observed at binding energies of 853.62 and
336.28 eV for the Ni0.90Pd0.10 nanocatalyst can be attributed to
metallic nickel (Ni2p3/2 core level) and palladium (Pd3d5/2 core
level), respectively. The observed shift in the peaks correspond-
ing to Pd3d5/2 levels to higher binding energies relative to the
peaks for the monometallic Pd sample and a shift in the
Ni2p3/2 levels to lower energies relative to the peaks for the
monometallic Ni sample suggested alloy formation. Moreover,
the presence of a uniform alloy composition for the Ni0.90Pd0.10
nanocatalyst was inferred by the appearance of no significant
change in the relative intensities of the features of Ni0 and Pd0
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boronic acid as model substrates to establish the most suitable
reaction conditions (Table 1). Upon screening various bases, we
observed that the conversions and selectivities of the reac-
tions, towards biphenyl and phenol, were significantly depen-
dent on the base, for which NaOH (Table 1, entry 3) facilitated
complete conversion with >99% selectivity for biphenyl. How-
ever, other bases did not perform well within the evaluated
time slot (Table 1, entries 1, 2, 5–7). Moreover, complete con-
version with >99% selectivity for the cross-coupled product,
biphenyl, was also achieved by using a Ni0.90Pd0.10 nanocatalyst
loading of 2 mol% (Table 1, entry 4).
To further investigate the dependence of the formation of
the cross-coupled product on the Ni/Pd ratio, Ni–Pd nanocata-
lysts with varying Ni/Pd molar ratios (Ni0.75Pd0.25, Ni0.50Pd0.50, and
Ni0.25Pd0.75) along with the monometallic counterparts, Ni and
Pd, were analogously synthesized and their catalytic per-
formance was examined under the optimized reaction condi-
tions, as shown in Figure 3. To our surprise, despite the fact
Figure 2. XPS profiles corresponding to the Ni2p and Pd3d core levels of
the Ni0.90Pd0.10 nanocatalyst a,b) before and c,d) after the catalytic reaction
with time-dependent Ar sputtering.
Figure 3. Comparison of the catalytic conversions of the cross-coupled prod-
uct in the Suzuki–Miyaura reaction of 4-iodoanisole with phenylboronic acid
in the presence of 2 mol% of various catalysts at room temperature in
water/ethanol solution (5 h). [a] Reaction with 0.2 mol% Pd nanocatalyst
(10 h), equal to the Pd content present in the highly active 2 mol%
Ni0.90Pd0.10 nanocatalyst. [b] Reaction with 0.2 mol% Pd/C (10 wt% Pd), 10 h.
[c] Reaction with 2 mol% of a physical mixture of Ni and Pd monometallic
nanoparticles in a 9:1 molar ratio, 6 h.
of the Ni0.90Pd0.10 nanocatalyst during XPS analysis with argon
sputtering for 180 min.
The catalytic performance of the synthesized Ni0.90Pd0.10
nanocatalyst for the Suzuki–Miyaura reaction in water/ethanol
solution was evaluated by using iodobenzene and phenyl
Table 1. Reaction optimization for the Ni0.90Pd0.10 nanoparticle catalyzed
Suzuki–Miyaura reaction in water/ethanol solution.[a,b]
that Ni itself was inactive for the studied catalytic reaction, the
presence of a higher concentration of Ni in the bimetallic
Ni0.90Pd0.10 nanocatalyst had no adverse effect on the selectivity
and conversion of the catalytic reaction. In the specified time,
it was found that the Ni0.90Pd0.10 nanocatalyst with a higher Ni
content exhibited high activity, whereas an increase in the Pd
content decreased the conversion (Figure 3, see also Fig-
ure S3). Moreover, by performing the reaction with a Pd nano-
particles loading of 0.2 mol% (equivalent to the Pd content in
the active 2 mol% Ni0.90Pd0.10 nanocatalyst), a very poor conver-
sion of only 2% was obtained. Although commercial Pd/C
(10 wt% Pd, 0.2 mol%) showed higher catalytic activity than
the monometallic Pd nanocatalyst, a conversion of only 52%
was achieved, even after a prolonged reaction time of 10 h.
Notably, unlike the high activity observed for the Ni0.90Pd0.10
nanocatalyst, the physical mixture of monometallic Ni and Pd
Entry
Base
Conv. [%]
Select. [%]
1
2
3
4[c]
5
Na2CO3
K2CO3
NaOH
NaOH
KOH
88
73
99
99
80
60
41
92
81
99
99
81
76
88
6
7
Na3PO4
K3PO4
[a] Reaction conditions: Phenylboronic acid (1.2 mmol), iodobenzene
(1.0 mmol), base (2.0 mmol), catalyst (0.2 mmol), H2O/EtOH (1:1 v/v,
20 mL), room temperature, 1 h. [b] Conversions and selectivities were de-
termined by 1H NMR spectroscopy based on iodobenzene. [c] Reaction
with 2 mol% catalyst, 6 h.
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(in a 9:1 molar ratio of Ni/Pd) was only poorly catalytically
active (conversionꢀ1%). The observed results inferred the in-
volvement of a bimetallic phase as the active catalytic species,
for which strong synergistic interactions between nickel and
palladium facilitated the observed high catalytic activity of the
Ni0.90Pd0.10 nanocatalyst. To further evaluate this, an approxi-
mate ninefold excess amount of the Ni nanoparticles was
added to the catalytic reaction performed in the presence of
the Pd nanocatalyst (2 mol%), and no inhibition of the catalytic
activity was observed. Moreover, the catalytic behavior of
a physical mixture of Ni and Pd was analogous to that of the
reaction performed with 0.2 mol% Pd, which further inferred
that Ni had no inhibitory effect on the catalytic reaction
(Figure 3).
then subjected to 1H NMR spectroscopy (after workup). The
1H NMR spectroscopy results showed no remarkable enhance-
ment in the conversion to the biaryl product in the reaction
for these two fractions, even after the extended reaction time
of 5.5 h, which indicated that the heterogeneity of the
Ni0.90Pd0.10 nanocatalyst remained intact. In contrast to these
observations, it is noteworthy that a quantitative yield of the
coupled product, 4-iodoanisole, was obtained with the
Ni0.90Pd0.10 nanocatalysts in 4 h (Table 2, entry 14). Moreover,
analysis of the mixture at 80% conversion and upon comple-
tion of the catalytic reaction by inductively coupled plasma
atomic emission spectroscopy (ICP-AES) demonstrated no sig-
nificant leaching of Pd (1.28 and 1.18 ppm, respectively; see
also Table S2). Advantageously, the observed value is well
below the minimum acceptable range of Pd (3–5 ppm), if Pd
catalysts are used for active pharmaceutical constituents.[16]
Moreover, Ni leaching was also not detected during ICP-AES
measurements from the Ni0.90Pd0.10 nanocatalysts. Furthermore,
catalyst poisoning tests were performed by using Hg0 and CS2
to demonstrate the heterogeneous nature of the studied
Ni0.90Pd0.10 nanocatalyst (Figure S5).[17] A significant decrease in
catalytic activity was observed if the catalytic reaction was per-
formed with an excess amount of Hg0 in the presence of the
Ni0.90Pd0.10 nanocatalyst. Moreover, experiments performed with
0.5 equivalents of carbon disulfide (CS2, corresponding to the
actual Pd content in 2 mol% Ni0.90Pd0.10 nanocatalyst) also dis-
played significant poisoning of the Ni0.90Pd0.10 nanocatalyst.
These results further indicated the heterogeneous nature of
the bimetallic Ni0.90Pd0.10 nanocatalyst, for which the Ni–Pd
alloy nanoparticles remain the active catalytic species.
To extend the generality and scope of the Ni0.90Pd0.10 nano-
catalyst, the Suzuki–Miyaura reaction of a wide range of aryl io-
dides (at room temperature)/aryl bromides (at 508C), and aryl-
boronic acids was investigated under the optimized reaction
conditions over the Ni0.90Pd0.10 nanocatalyst (2 mol%). Moder-
ate to high yields of the coupled biaryl products were ob-
tained, and the results are summarized in Table 2. The results
show that by using NaOH as the base for the aryl bromides
the selectivity towards the formation of biaryls was substantial-
ly affected, whereas by replacing NaOH with K2CO3 high selec-
tivity for the biaryls was achieved (Figure S4). Therefore, cata-
lytic reactions with aryl bromides were investigated with the
use of K2CO3 as the base instead of NaOH. Several attempts
were made with various aryl chlorides under varying condi-
tions, but the aryl chlorides did not undergo the catalytic reac-
tion even at 808C (Table S1). As shown in Table 2, substituted
aryl iodides/bromides having electron-withdrawing groups
were effectively involved in the coupling reaction with arylbor-
onic acids to give the corresponding biaryl products in low to
excellent yields (32–95%), irrespective of their positions
(Table 2, entries 3–5, 7–10, 12, 13, 18, 20, 21). Similarly, aryl io-
dides with electron-donating substituents also gave good
yields (Table 2, entries 6, 14, 16), whereas aryl bromides with
electron-donating substituents gave moderate yields (Table 2,
entries 11, 15, 17, 19). Notably, the electronic effects (electron-
donating or electron-withdrawing) of the substitutes of the ar-
Interestingly, the studied Ni0.90Pd0.10 nanocatalyst exhibited
outstanding stability upon recovery from the reaction mixture
and was recycled over more than five consecutive catalytic
runs, as shown in Figure 4. The observed marginal loss in cata-
lytic activity was possibly due to agglomeration of the nano-
particles and loss of nanoparticles during centrifugation after
each catalytic run. Moreover, analysis of the Ni0.90Pd0.10 nano-
catalyst by ICP-AES before and after the catalytic reaction
showed no significant change in the Ni/Pd ratio (Table S2).
These results indicate that the alloy composition of the
Ni0.90Pd0.10 nanocatalyst remained intact during the catalytic re-
action.
Notably, Ni0 is considered a good nucleophile and can facili-
tate aromatic coupling through Suzuki–Miyaura-type reac-
tions.[18] Therefore, the presence of Ni in the Ni–Pd alloy nano-
catalyst may exhibit a synergistic effect, presumably through
activation of Pd by Ni, to enhance the catalytic activity of the
Ni–Pd nanocatalysts for the Suzuki–Miyaura reactions in water/
ethanol solutions. Indeed, through our experiments we ob-
served that the studied bimetallic Ni–Pd alloy nanocatalyst
having a low Pd content displayed higher catalytic activity
than the other analogous Ni–Pd nanocatalysts having high Pd
contents. This behavior is not random; rather, there is a clear
relation between the Ni/Pd ratio and the catalytic activity (as
shown in Figure 3, see also Figure S3). With an increase in the
Ni/Pd molar ratio in the Ni–Pd alloy nanocatalyst, the catalytic
activity of the Ni–Pd alloy nanocatalyst also increased, and
ylboronic acids had
a negligible influence on reactivity
(Table 2, entries 20, 21). Our results indicate that the reported
bimetallic Ni0.90Pd0.10 nanocatalyst works significantly well for
a wide range of aryl iodides/bromides and arylboronic acid to
give the desired biaryls in water/ethanol solution.
To investigate the nature of the active catalytic species fur-
ther, a series of reactions were performed, including a leaching
experiment and catalyst poisoning test of the Ni0.90Pd0.10 nano-
catalyst for the Suzuki–Miyaura reaction. Under the optimized
reaction conditions, the reaction of 4-iodoanisole with phenyl-
boronic acid was performed in the presence of the Ni0.90Pd0.10
nanocatalyst (2 mol%) in water/ethanol solution; after stirring
the mixture for 2 h, it was centrifuged to separate the
Ni0.90Pd0.10 nanoparticles. The supernatant liquid was then sepa-
rated into two fractions; the first fraction was worked up and
1
analyzed by H NMR spectroscopy, whereas the content of the
second fraction was allowed to stir for an additional 3.5 h and
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Table 2. Ni0.90Pd0.10 nanoparticle catalyzed Suzuki–Miyaura reactions.[a,b]
Entry
Boronic acid
Aryl halide
t [h]
6
Biaryl product
Yield[c] [%]
1
2
90
65
7
3
4
5
6
7
7
9
54
95
68
90
74
7.5
8
10
8
5
6.5
7
89
82
73
77
70
9
10
11
12
24
24
13
24
70
14
15
16
17
4
92
85
72
71
6.5
8.5
24
18
24
32
19
20
15
47
95
8.5
21
22
95
[a] Reaction conditions: Boronic acid (1.2 mmol), aryl halide (1.0 mmol), NaOH (2.0 mmol), catalyst (0.02 mmol), H2O/EtOH (1:1 v/v, 20 mL), room tempera-
ture. [b] For aryl bromides: K2CO3 (2.0 mmol), 508C. [c] Yield of isolated product.
Ni0.90Pd0.10 was found to be the most effective ratio. These ob-
servations can be explained by taking into account possible
electron transfer from Ni to Pd.[14b] Given that the standard re-
duction potential of Ni (À0.23 V) is lower than that of Pd
(+0.83 V), there is a possibility of easy oxidation of Ni through
donation of electrons to Pd. As a result of this transfer of elec-
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tween Ni and Pd that accounted for the observed high stability
and excellent recyclability (low Pd leaching) of the reported
Ni0.90Pd0.10 nanocatalyst, the activity of which is superior to that
of the monometallic counterparts synthesized under analogous
conditions and even better than that of a commercial Pd/C
catalyst. Our attempt to develop bimetallic alloy nanoparticle
catalysts with a low noble metal content and a high-per-
formance catalytic system will offer new opportunities for the
development and application of low-costing catalysts for vari-
ous other related reactions. Further investigation into the use
of the Ni0.90Pd0.10 nanocatalyst for other coupling reactions is
underway.
Figure 4. Recyclability of the Ni0.90Pd0.10 nanocatalyst for the Suzuki–Miyaura
reaction of 4-iodotoluene with phenylboronic acid at room temperature in
water/ethanol solution.
Experimental Section
trons, the Pd center becomes more negative and, therefore,
enhances the facile oxidative addition of the aryl halides. This
effect may be expressed more prominently in Ni–Pd nanocata-
lysts with high Ni/Pd molar ratios, because with an increase in
the Ni content in the Ni–Pd nanocatalyst, the probability of
having Pd surrounded by more Ni also increases, and this re-
sults in the accumulation of a more negative charge on Pd. To
further evaluate this, a Ni–Pd nanocatalyst with an even higher
Ni/Pd molar ratio (Ni0.95Pd0.05) was synthesized in a manner
analogous to that used to prepare Ni0.90Pd0.10, and its catalytic
efficacy for the catalytic Suzuki–Miyaura reactions of various
aryl halides was examined. The Ni0.95Pd0.05 nanocatalyst exhibit-
ed even better catalytic performance (Table S3). These observa-
tions are consistent with the enhancement in the catalytic per-
formance of Ni–Pd nanocatalysts with an increase in Ni/Pd
molar ratio. Moreover, the process of electron transfer from Ni
to Pd also suppresses leaching of Pd from the surface of the
catalyst by preventing Pd oxidation.[14b] The ICP-AES data of
the Ni0.90Pd0.10 catalyst is also consistent with the above explan-
ation, which implies that there is very little leaching of Pd
during the reaction (Table S2). Further, to evaluate the superi-
ority of Ni0.90Pd0.10 versus higher Pd-containing Ni–Pd nanocata-
General
High-purity metal salts and chemicals were used for the experi-
ments. NMR spectra were recorded with a Bruker Avance 400
(400 MHz) spectrometer. Chemical shifts are referenced to internal
solvent resonances and are reported relative to tetramethylsilane.
Electron microscopy (TEM and HAADF-STEM) experiments and ele-
mental analysis (EDX) were performed with a FEI Tecnai F20 ST
TEM (operating voltage 200 kV) equipped with a field-emission
gun and an EDAX EDS X-ray spectrometer [Si(Li) detecting unit,
super-ultrathin window, active area 30 mm2, resolution 135 eV (at
5.9 keV)]. Scanning electron microscopy (SEM) images and elemen-
tal mapping data were collected with a Carl Zeiss supra 55 (operat-
ing voltage 15 kV) equipped with an Oxford Instrument EDS X-ray
spectrometer. For TEM and SEM analyses, a few droplets of a nano-
particle suspension were deposited onto amorphous carbon-
coated 400 mesh copper grids and eventually air dried. Powder X-
ray diffraction (XRD) measurements were performed on the dried
particles with a Rigaku SmartLab, Automated Multipurpose X-ray
Diffractometer at 40 kV and 30 mA by using CuKa radiation (l=
1.5418 Š). Inductively coupled plasma-atomic emission spectro-
scopic (ICP-AES) analysis for Ni and Pd (ppm level) were performed
by using ARCOS (Spectro, Kleve, Germany). X-ray photoelectron
spectroscopic (XPS) analyses were performed with a Shimadzu
ESCA-3400 X-ray photoelectron spectrometer by using a MgKa
source (10 kV, 10 mA). The Ar sputtering experiments were per-
formed under the conditions of background vacuum 3.2”10À6 Pa,
sputtering acceleration voltage 1 kV.
lysts, we performed
a recyclability experiment for the
Ni0.25Pd0.75 nanocatalyst in the reaction of 4-iodotoluene with
phenylboronic acid and compared its performance with that of
Ni0.90Pd0.10 over three consecutive catalytic runs (Figure S6). A
gradual decrease in activity of the Ni0.25Pd0.75 nanocatalyst was
observed, whereas no significant loss in activity was observed
for the Ni0.90Pd0.10 catalyst. These results further support the
fact that a large Ni content (alloyed with Pd) in the Ni–Pd
nanocatalyst not only enhances the catalytic performance of
the Ni–Pd nanocatalyst but also enhances the stability of the
Ni–Pd nanocatalyst, and therefore, the bimetallic Ni–Pd alloy
nanocatalyst remains active over several catalytic cycles.
General procedure for preparation of the Ni–Pd nano-
catalysts
Bimetallic Ni1ÀxPdx (x=0.10–0.75) nanoparticles were prepared by
using a polyvinylpyrrolidone (PVP) stabilized aqueous-phase core-
duction process by varying the molar ratios of the NiII and PdII
salts. In a typical procedure, a solution of NaBH4 (0.050 g) in water
(5 mL) was added dropwise to a solution of potassium tetrachloro-
palladate(II) (0.006 g, 0.020 mmol), nickel(II) chloride hexahydrate
(0.043 g, 0.180 mmol), and PVP (0.100 g) in water (5 mL) at room
temperature. The contents of the flask were sonicated for 10 min
to obtain a black suspension of the Ni0.90Pd0.10 nanocatalyst, and
the same (specified amounts in volume) was used directly for the
catalytic reactions.
In summary, we demonstrated the use of the highly active
Ni0.90Pd0.10 nanocatalyst for the Suzuki–Miyaura reaction of
a wide range of substituted aryl iodides/bromides and arylbor-
onic acids having electron-donating and electron-withdrawing
functional groups at moderate reaction temperatures in water/
ethanol solution. A high Ni content in the reported Ni0.90Pd0.10
nanocatalyst facilitated a remarkable synergistic interaction be-
ChemCatChem 2015, 7, 1806 – 1812
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Communications
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lyst (2 mol%) and water/ethanol (1:1 v/v, 20 mL). The arylboronic
acid (1.2 mmol) was added, followed by the aryl iodide (1.0 mmol),
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Acknowledgements
This work was financially supported by the Indian Institute of
Technology (IIT), Indore, Council of Scientific and Industrial Re-
search (CSIR), New Delhi, and Science and Engineering Research
Board (SERB) (DST), New Delhi. R.K.R. and K.G. thank CSIR, New
Delhi, for their SRF and JRF fellowships, respectively. We also
thank the Sophisticated Instrument Centre (SIC), IIT, Indore; So-
phisticated Analytical Instrument Facility (SAIF), IIT, Bombay; In-
stitut für Katalyseforschung und Technologie (IKFT), Karlsruher In-
stitut für Technolgie (KIT), Karlsruhe, Germany; and National Insti-
tute of Advanced Industrial Science and Technology (AIST), Ikeda,
Japan, for extended instrumentation facilities.
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Keywords: biaryls · cross-coupling · heterogeneous catalysis ·
nickel · palladium
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Received: February 13, 2015
Revised: April 7, 2015
Published online on May 26, 2015
ChemCatChem 2015, 7, 1806 – 1812
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ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim