Nanosized Bispyrazole-Based Cryptand-Stabilized Palladium(0) Nanoparticles: A Reusable Heterogeneous Catalyst for the Suzuki-Miyaura Coupling Reaction in Water

Article abstract of DOI:10.1021/acs.inorgchem.8b03015

A macrobicyclic cryptand with a long rigid cavity incorporating a chelating bispyrazole moiety in each of the three bridges was synthesized. The multiple chelating metal binding sites were utilized for the controlled synthesis and stabilization of ultrafine palladium nanoparticles (Pd NPs) of nearly ～2 nm size. The as-synthesized Pd NPs were characterized by X-ray photoelectron spectroscopy, transmission electron microscopy, and powder X-ray diffraction. The well-dispersed cryptand-stabilized nanoparticles are found to catalyze the C-C bond-forming Suzuki-Miyaura reaction heterogeneously using water as a green solvent.

Full text of DOI:10.1021/acs.inorgchem.8b03015

Communication
pubs.acs.org/IC
Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
Nanosized Bispyrazole-Based Cryptand-Stabilized Palladium(0)
Nanoparticles: A Reusable Heterogeneous Catalyst for the Suzuki−
Miyaura Coupling Reaction in Water
Ashish Verma, Kapil Tomar, and Parimal K. Bharadwaj*
Department of Chemistry, Indian Institute of Technology Kanpur, Kanpur 208016, India
S
* Supporting Information
envisioned that cryptands bearing chelating moieties in the
ABSTRACT: A macrobicyclic cryptand with a long rigid
cavity incorporating a chelating bispyrazole moiety in each
of the three bridges was synthesized. The multiple
chelating metal binding sites were utilized for the
controlled synthesis and stabilization of ultrafine palla-
dium nanoparticles (Pd NPs) of nearly ∼2 nm size. The
as-synthesized Pd NPs were characterized by X-ray
photoelectron spectroscopy, transmission electron micros-
copy, and powder X-ray diffraction. The well-dispersed
cryptand-stabilized nanoparticles are found to catalyze the
C−C bond-forming Suzuki−Miyaura reaction heteroge-
neously using water as a green solvent.
bridges could serve as exocyclic binding sites for MNPs, leading
to the controlled synthesis of MNPs, and such cryptand-capped/
stabilized nanoparticles will be ideal candidates for catalysis.
Herein, we report a cryptand incorporating bispyrazole
moieties that can serve as anchoring sites for the controlled
synthesis of palladium nanoparticles (Pd NPs; Scheme 1). The
cryptand-capped/stabilized Pd NPs are further utilized as
heterogeneous catalysts for the C−C bond-forming Suzuki−
Miyaura coupling reactions in water as a green solvent.
Scheme 1. Schematic Diagram Showing the Process of Pd NP
Synthesis at the Bispyrazole Site of PzC
ryptands are the first studied examples of functional cage
C
molecules, which initially were used for the selective
recognition of cations.1 At the beginning of the development of
these organic compounds, bond formation was rather
irreversible in nature, affording the desired product at yields.2
The reversible nature of imine bonds was then observed, and
after that, organic cage molecules can be synthesized in high
overall yields.3 This kind of synthetic approach, which is also
called dynamic covalent synthesis,4,5 is nowadays most
frequently used for the synthesis of various sizes and shapes of
organic molecules of high and complex structures from simple
building blocks because a one-pot step can be achieved.6 An
attractive feature of these cage molecules is their solubility in
common organic solvents, which makes their processing easier
compared to insoluble porous solids. These cage molecules have
attracted enormous attention in recent years with applications in
wide areas like, sensing,7 gas storage/separation,8 catalysis,9
drug delivery,10 molecular separation,11 light harvesting,12 and
proton conduction.13
Metal nanoparticles (MNPs) exhibit high surface area,14,15
which results in a very large number of catalytically active sites,
making them excellent heterogeneous catalysts for various
organic transformation reactions.16 However, unstabilized
MNPs can aggregate, leading to impairment of the catalytic
activities.17 Hence, there have been constant efforts to
investigate different solid support materials for the fabrica-
tion/stabilization of MNPs over the past few years.18−24 The use
of cryptands and other porous organic cage molecules to
stabilize the MNPs is a recent advancement in this area.25−30
Our interest in the synthesis of novel cryptands prompted us to
investigate new porous organic cage molecules for the capping/
stabilization of MNPs for heterogeneous catalytic purposes. We
The highly porous cryptand PzC was prepared via the [2 + 3]
Schiff base condensation reaction of the bispyrazole-based
dialdehdye with a tripodal aromatic amine, as described in our
recent report.30 This cryptand was found to stabilize gold
nanoparticles.30 Another cryptand, where the bridgehead was
nitrogen in place of the aromatic ring as in the present case, was
found through crystallographic investigation to bind metal ions
Received: October 24, 2018
© XXXX American Chemical Society
A
DOI: 10.1021/acs.inorgchem.8b03015
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Communication
at the bispyrazole moiety that turned31 outward upon metal
complexation (Figure S2b). Therefore, it was presumed that
when the cryptand was reacted with Pd(OAc)2, the bispyrazole
moiety turned31 outward and acted as a nucleation site for metal
aggregation (Scheme 1). The length of the cryptand from the
crystal structure was found to be around 2 nm, while the average
size of the Pd(0) NPs was 2.24 nm, which suggested that about
five or more cryptand molecules formed an intermolecular cavity
via aggregation to stabilize the nanoparticle.
3d3/2, respectively. Also, peaks at 2θ values of ∼40, 47, 68, 82,
and 87° in the powder X-ray diffraction (PXRD) pattern could33
be assigned to the (111), (200), (220), (311), and (222) lattice
planes, respectively, consistent with the face-centered-cubic
structure of palladium (Figure 2). The observed peak at 39.8°
The cryptand PzC-stabilized Pd(0) NPs were prepared by
stirring the cryptand with Pd(OAc)2 vigorously in methanol and
CHCl3 (1:1) for 2 h at room temperature, which afforded a
yellow solution. Solid NaBH4 was added portionwise until the
solution turned black with the appearance of a precipitate. Upon
completion of the reaction, the solvent was removed, and the
black precipitate that remained was washed several times with
deionized water and methanol via centrifugation, followed by
drying under vacuum to obtain Pd(0)·PzC as a black solid.
The high-resolution transmission electron microscopy
(HRTEM) images of the as-obtained material showed well-
dispersed nanoparticles of mean size 2.24 nm and a narrow
distribution of the particle size (Figure 1a). With the internal
Figure 2. HAADF-STEM image and corresponding EDX mapping of
Pd(0)·PzC (color code: C, red; N, green; Pd, cyan) and PXRD pattern
of Pd(0)·PzC.
was due to metallic palladium, indicating the formation of a Pd
NP cryptand composite for the reduced material. Further, to
explore the role of PzC in the stabilization of nanoparticles, the
syntheses of Pd NPs were performed in the presence of
bispyrazole dialdehyde (precursor of the cryptand) and also in
the absence of PzC. The TEM images (Figures S16 and 17) in
both cases revealed the agglomeration of reduced palladium
particles. The result showed the role of PzC in stabilization of
the nanoparticles. We have also performed the thermal-
annealing experiment of the as-synthesized Pd(0)·PzC material
at 100 and 200 °C to test the stability of the nanoparticles. No
considerable change in the size/shape of Pd NPs with any
agglomeration could be observed in the HRTEM images
(Figures S18 and S19).
The C−C bond-forming reactions are important reactions in
organic chemistry because they can easily link several functional
moieties together. One such reaction is the Suzuki coupling
between aryl halides and boronic acids. A number of
homogeneous palladium catalysts were explored for this purpose
with high selectivity, activity, and low catalyst loading.34 Also, in
some cases, water was used as a green solvent by several
workers.35 However, recovery of the catalyst and its reuse
remained a big issue here. To circumvent this shortcoming,
palladium complexes or nanoparticles were immobilized on
solid supports like silica,36 activated carbon (charcoal),37
organic polymers,38 and metal−organic frameworks.39 Most of
these methods still suffer from drawbacks like the use of
expensive phosphines, additives, and severe reaction conditions
besides the lack of stability. This scenario prompted us to further
develop new active and stable heterogeneous catalysts for these
important organic transformations.
In this paper, the catalytic activity of the cryptand-anchored
Pd NPs was explored for the Suzuki−Miyura C−C coupling
reaction. The reaction between bromobenzene and phenyl-
boronic acid was selected as the model. For this, a catalytic
amount of Pd(0)·PzC was placed in a 5 mL round-bottom flask
followed by the addition of the two reactants along with KOH.
The reaction was performed in water as the solvent at 90 °C in
air. The progress of the reaction was monitored by thin-layer
Figure 1. (a) TEM images of Pd(0)·PzC at different magnifications.
(b) Particle-size distribution and XPS spectrum of Pd(0)·PzC.
cavity size of PzC (1.2 nm) being much smaller compared to
that of the as-obtained Pd(0) NPs (2.24 nm), the nanoparticles
would reside outside the cryptand. As observed from the CoCl2
complex of PzC (see the Supporting Information), where the
metal ions were bound in an exocyclic manner to the bispyrazole
unit, stabilization of Pd(0) NPs anchoring at the bispyrazole unit
from outside the cavity is well justified (Scheme 1). Loading of
palladium was measured by inductively coupled plasma mass
spectrometry to be ∼34 wt %. High-angle annular dark-field
scanning transmission electron microscopy (HAADF-STEM)
imaging and energy-dispersive X-ray (EDX) mapping analysis
showed that carbon and nitrogen were homogeneously
distributed around the Pd NPs (Figure 1). Additionally, X-ray
photoelectron spectroscopy (XPS) data (Figure 1b) were
consistent with the formation of Pd(0) NPs, showing binding
energies of 334.4 and 339.2 eV, attributable32 to Pd(0) 3d5/2 and
B
DOI: 10.1021/acs.inorgchem.8b03015
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Inorganic Chemistry
Communication
chromatography, and formation of the desired products could
1
be confirmed by the H NMR spectra (Figures S6−S15). It
showed that bromobenzene and phenylboronic acid afforded a
good yield of the product, which encouraged us to investigate
this reaction with substrates having different substituents. A look
at the results of the reactions summarized in Table 1 revealed
Table 1. Results of the Suzuki−Miyaura Coupling Reactions
a
in Water Catalyzed by Pd(0)·PzC
Figure 3. HRTEM image of Pd(0)·PzC and the particle-size
distribution after three cycles of catalysis.
b
% yield
strong binding affinity of the Pd2+ ions toward the bispyrazole
moiety of the cryptand is considered to be the governing factor
during the nucleation of Pd NPs, whereas aromatic moieties
protect the nanoparticles from agglomeration.
The cage-capped Pd NPs were found to be excellent catalysts
for the Suzuki−Miyuara reaction in water with a variety of
functional groups. The promising features of the system include
a wide variety of functional group tolerances, reusability, high
yield, and water as the solvent.
halobenzene
(R1)
boronic acid (R2,
R3, R4)
Pd(0)
Pd(0)·
entry
particles Pd(OAc)2
PzC
1
2
3
4
5
6
7
H
R2, R3, R4 = H
R2, R3, R4 = H
R2, R3, R4 = H
R2, R3, R4 = H
R2, R3, R4 = H
7
5
8
6
9
32
21
33
26
31
33
22
94
84
97
93
95
96
85
−CH3
−OH
−NO2
−CHO
−C(O)CH3 R2, R3, R4 = H
−C(O)CH3 R2, R4 = −CH3, R3
10
6
= H
ASSOCIATED CONTENT
* Supporting Information
8
−C(O)CH3 R2, R4 = −C(O)
9
8
31
28
34
94
92
98
■
OEt, R3 = H
S
9
−C(O)CH3 R2, R4 = H, R3 =
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.8b03015.
−C(O)OEt
10
−C(O)CH3 R2, R4 = H, R3 =
−OCH3
11
a
Experimental details, characterization procedures, IR,
NMR, TGA, and PXRD patterns (PDF)
Reaction conditions: RBr (1.0 mmol), boronic acid (1.2 mmol),
KOH (2.0 mmol), tetrabutylammonium bromide (0.6 mmol), catalyst
(3 mol %), and water (3 mL) in air at 90 °C for 2 h. Isolated yields
after silica gel chromatography.
b
Accession Codes
CCDC 1883649 contains the supplementary crystallographic
data for this paper. These data can be obtained free of charge via
www.ccdc.cam.ac.uk/data_request/cif, or by emailing data_
request@ccdc.cam.ac.uk, or by contacting The Cambridge
Crystallographic Data Centre, 12 Union Road, Cambridge
CB2 1EZ, UK; fax: +44 1223 336033.
that both electron-donating and -withdrawing substituents gave
fairly good yields. The catalyst after one catalytic cycle could be
easily separated by filtration and washed with water and acetone,
followed by drying at room temerature to regenerate the catalyst.
We also performed these reactions by replacing the Pd(0)·PzC
catalyst with cryptand-free Pd(0) particles, a Pd(OAc)2 salt, and
metal-free cryptand PzC only. The free Pd(0) particles were
prepared by reducing Pd(OAc)2 with NaBH4. The resultant
black precipitate of Pd(0) was characterized by TEM (Figure
S17). In the case of free Pd(0) particles, very low yields were
found, while with Pd(OAc)2, about 34% product formation was
observed (Table 1). Metal-free cryptand PzC failed to afford any
product.
AUTHOR INFORMATION
Corresponding Author
*E-mail: pkb@iitk.ac.in.
■
ORCID
Parimal K. Bharadwaj: 0000-0003-3347-8791
Notes
The authors declare no competing financial interest.
The performance of the catalyst was checked for up to three
cycles to evaluate its reusability, and no significant loss of activity
was found at least up to three cycles (Figure S4). The TEM
images of the recovered material after three cycles suggest no
considerable change in the nanoparticle size and morphology
(Figure 3). PXRD patterns of the material after each cycle were
also recorded, which showed negligible change after each
catalytic cycle (Figure S3). Besides, the hot filtration test carried
out with the system indicated negligible leaching of palladium
particles (Figure S5) into the solution phase, proving the
heterogeneous nature of the reactions.
ACKNOWLEDGMENTS
■
We gratefully acknowledge financial support from the DST,
New Delhi, India (to P.K.B.), and SRF from the IIT Kanpur,
India (to A.V.) and financial support in the form of a postdoc
fellowship from SERB (DST) (YSS/2015/001088) to K.T.
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C
DOI: 10.1021/acs.inorgchem.8b03015
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