Palladium nanoparticles capped by thermoresponsive N-heterocyclic carbene: Two different approaches for a comparative study

Article abstract of DOI:10.1002/aoc.6166

In this article, we reported the synthesis of thermoresponsive palladium nanoparticles, stabilized by polymer-tagged N-heterocyclic carbenes (NHCs), using two different approaches. In one case, the nanoparticles were synthesized from the NHC–palladium complex, while in other cases, palladium nanoparticles and NHC were synthesized simultaneously for the in situ cappings of the nanomaterial. While the thermoresponsive nature of the nanomaterials was observed in both cases, differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA) analyses showed distinct characteristics in the thermal behavior of those materials. High-resolution transmission electron microscopic (HRTEM) and scanning electron microscopic studies separately revealed temperature-dependent aggregation in both cases although separate patterns were observed when the nanomaterials were synthesized using different approaches. Finally, both the nanomaterials were successfully used as a recyclable catalyst for Suzuki reactions in an aqueous medium, with slightly different catalytic activity, plausibly due to variations in the size of the nanoparticles.

Full text of DOI:10.1002/aoc.6166

Received: 7 August 2020
DOI: 10.1002/aoc.6166
Revised: 27 December 2020
Accepted: 2 January 2021
F U L L P A P E R
Palladium nanoparticles capped by thermoresponsive
N-heterocyclic carbene: Two different approaches for a
comparative study
1,2
1,2
1,2
Varnakumar Gayathri
| Nagaraju Pentela
| Debasis Samanta
1
Polymer Science & Technology
In this article, we reported the synthesis of thermoresponsive palladium
nanoparticles, stabilized by polymer-tagged N-heterocyclic carbenes (NHCs),
using two different approaches. In one case, the nanoparticles were
synthesized from the NHC–palladium complex, while in other cases, palla-
dium nanoparticles and NHC were synthesized simultaneously for the in situ
cappings of the nanomaterial. While the thermoresponsive nature of the
nanomaterials was observed in both cases, differential scanning calorimetry
Department, CSIR-CLRI, Chennai, India
2
Academy of Scientific and Innovative
Research, Ghaziabad, India
Correspondence
Debasis Samanta, Polymer Science &
Technology Department, CSIR-CLRI,
Adyar, Chennai 600 020, India.
Email: debasis@clri.res.in
(
DSC) and thermogravimetric analysis (TGA) analyses showed distinct
Funding information
DST-INSPIRE, Grant/Award Number:
IF170645
characteristics in the thermal behavior of those materials. High-resolution
transmission electron microscopic (HRTEM) and scanning electron
microscopic studies separately revealed temperature-dependent aggregation in
both cases although separate patterns were observed when the nanomaterials
were synthesized using different approaches. Finally, both the nanomaterials
were successfully used as a recyclable catalyst for Suzuki reactions in an
aqueous medium, with slightly different catalytic activity, plausibly due to
variations in the size of the nanoparticles.
K E Y W O R D S
N-heterocyclic carbenes, palladium nanoparticles, poly(N-isopropylacrylamide), Suzuki coupling
reaction, thermoresponsive behavior
1
| INTRODUCTION
phosphine ligands, it enhances the crucial steps of cou-
pling reactions such as oxidation addition and reductive
Organometallic compounds of palladium and suitable
ligands can be conveniently used as a catalyst for
elimination, which, in turn, increases the efficiency of
such catalysts towards cross-coupling reactions.
[10,11]
[1–4]
various coupling reactions.
Among the ligands,
After the first report on water-soluble NHC–Pd complex,
N-heterocyclic carbene (NHC) ligands are considered
particularly attractive because of their versatility in
chemical structures, tolerance against moisture, air, and
NHC has been subsequently modified to attain homoge-
[
12–14]
neous catalysts for cross-coupling reactions.
Despite
showing high reactivity, they required either high tem-
perature, a mixture of organic solvent/additives, and in
some cases, the catalyst lacked easy recoverability and
recyclability, which leads to the metal contamination in
[5,6]
ease of synthesis.
Additionally, by being a strong σ
donor and weak π acceptor, it formed a strong
ligand–metal bond that enhanced the stability and
reactivity of transition metal catalysts in organometallic
[
15–18]
the product.
Hence, to address environmental and
[7–9]
transformations.
Furthermore, owing to its strong σ
economic demands, the catalysts were immobilized
[19–21]
donating and weak accepting ability than the commercial
on different heterogeneous surfaces like silica,
Appl Organomet Chem. 2021;35:e6166.
wileyonlinelibrary.com/journal/aoc
© 2021 John Wiley & Sons, Ltd.
1 of 14
https://doi.org/10.1002/aoc.6166

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GAYATHRI ET AL.
[
22]
[23]
polymers,
work (COF),
metal oxides,
covalent organic frame-
(NIPA-co-PMA)
demanded an elevated temperature (90 C) to catalyze the
Suzuki coupling reaction in a water medium.
despite having recyclability, these reported catalysts
required either mixture of solvents, high temperature, or
stabilizers to perform catalytic reactions effectively.
To overcome those problems, we recently designed
hydrogel-based
catalyst,
which
[
24]
[25]
ꢀ
and nanoparticles (NPs)
for recyclabil-
[37]
ity. In most of the cases, solid-supported heterogeneous
catalysts showed less activity, although recyclability was
achieved for several cycles.
Responsive polymers are labeled as “smart materials,”
which shows unique reversible changes concerning
external stimuli like temperature, pH, and magnetic field.
Thermoresponsive polymers are smart polymers that
undergo a phase transition from the hydrophilic coil to
hydrophobic globular at a lower critical solution temper-
Thus,
[29,38]
various thermoresponsive organometallic catalysts,
which offer the advantage of both homogeneous and
heterogeneous catalysts where the catalytic reactions can
be performed in the homogeneous condition under
ambient temperature followed by recovery at elevated
temperature due to temperature-dependent phase separa-
tions in water. To improve the recovery, herein we report
palladium NPs with thermoresponsive NHC ligands
using two different methods. Although there are a few
reports of the synthesis of thermoresponsive palladium
NPs, to the best of our knowledge, this is the first report
of the synthesis of thermoresponsive NHC-capped
palladium NPs. We used two different approaches in
synthesis for a detailed comparison of their properties
and catalytic activities.
[26]
ature (LCST).
The vital criteria for a polymeric system
to show responsive behavior in an aqueous solution are
the equilibrium between hydrophobic and hydrophilic
moiety located through the polymeric system. Hence
LCST point of a thermoresponsive polymer can be easily
tuned by disturbing the balance between hydrophobic
[
27]
and hydrophilic moieties by incorporating copolymers.
In addition to that, separating of polymer above the LCST
was caused by the involvement of secondary forces such
as hydrogen bond, van der Waals forces, electrostatic
attraction, and hydrophobic interactions within the poly-
meric system. For example, PNIPAM is a well-known
thermoresponsive polymer that shows a phase transition
ꢀ
at LCST of 32 C due to the involvement of hydrogen
2 | EXPERIMENTAL
2.1 | Materials and instrumentation
2.1.1 | Materials
bond between polymer and water molecules. Hence,
below the LCST, these polymers will be soluble in water
medium whereas elevation of temperature will result in
breaking of a hydrogen bond and will cause the polymer
[28]
to precipitate out from the aqueous medium.
Having
such a unique responsive behavior in an aqueous
medium, these thermoresponsive polymers were widely
used in the organometallic field for catalytic applications
Palladium diacetate, palladium (II) chloride, sodium
ascorbate, phenylboronic acid, 4-iodoacetophenone,
4-bromoacetophenone,
4-bromobenzaldehyde,
4-
[29,30]
as well as in the biofield for drug delivery.
iodobenzoic acid, 4-iodoaniline, 4-chloroacetophenone,
4-bromophenol, 4-bromobenzoic acid, 4-bromotoluene,
Exterior to other solid supports, NPs have been used
extensively as a catalyst due to their high surface-
and deuterated chloroform (CDCl ) were purchased from
3
[18]
to-volume ratio than the bulk catalytic systems.
Naked
Aldrich, USA and used as received. Potassium carbonate
was purchased from Emplura and used as such.
NPs tended to undergo aggregation that may hinder the
catalytic activity; hence, various supports have been
reported to prevent the aggregation of NPs during cata-
[6,31–33]
lytic reactions.
For example, Wang et al. reported
2.1.2 | Instrumentations
microporous organic polymer-based NHC tagged Pd NPs
as an efficient catalytic system for the Suzuki coupling
Thin-layer chromatography was performed using Merck
60 F254 precoated silica gel aluminum sheets. Column
chromatography was performed employing Silica Gel
Merck (100–200 mesh). NMR spectra were recorded at
400 MHz using Bruker Avance III spectrometer. Termi-
nologies used for describing nuclear magnetic resonance
(NMR) signals were singlet (s), doublet (d), triplet (t),
multiplet (m), and broad (br). Fourier transform infrared
(FTIR) was carried out using JASCO 4700 with the wave-
length range between 4,000 and 400 nm or Perkin Elmer
UATR Two. BET measurements were carried out using
[34]
reaction.
In another case, Heidari et al. reported that
magnetically recoverable cyclodextrin@Grapheneoxide
supported the Pd nanocomposite system for the Suzuki
ꢀ
[35]
coupling reaction in water medium at 40–80 C.
Hong et al. reported Pd NPs incorporated thermo-
responsive hydrogel (PNIPAM-4-VP [vinyl pyridine])
ꢀ
where LCST of PNIPAM was increased to 48 C due to
the presence of vinyl pyridine; hence, it required high
ꢀ
temperature (60 C) to perform the reaction in a water
[36]
medium.
Similarly, Sividu et al. reported the Pd@Poly

GAYATHRI ET AL.
3 of 14
Belsorb-mini II analyzer at 77 K, and the samples were
preheated at 200 C for 3 h. Thermogravimetric analysis
2.3 | Synthesis of thermoresponsive
NHC-capped Pd NPs (catalyst 2) using
method A
ꢀ
(
4
TGA) was executed using a NETZSCH Jupiter STA
49 F3 instrument with a heating rate of 10 or 5 C/min
ꢀ
under the nitrogen atmosphere. Differential scanning
calorimetry (DSC) experiments were performed by
using NETZSCH DSC 214 Polyma with a heating rate
To the 10-ml reaction flask under N atmosphere, NHC
2
precursor (200.66 mg, 0.05 m.mol) and Pd (OAc)2
(206.94 mg, 0.8908 m.mol) were added and degassed by
purged with nitrogen. To it five ml of DMSO was added,
after which the reaction mixture became orange in color.
ꢀ
of 5 C/min under an N2 atmosphere. Ultraviolet
(UV) experiments were carried out using Agilent, Cary
ꢀ
8
454 with a wavelength range between 190 and 1100 nm.
The reaction mixture was allowed to stir at 50 C for 3 h
Powder X-ray diffraction (XRD) was performed by table-
top Rigaku X-ray diffractometer using Cu/30 kV/15 mA
as a source at a scan rate of 4 /min over a scan range of
and 30 min during which darkening of color was
observed followed by stirring at 100 C for 30 min resulted
ꢀ
ꢀ
in a dark brownish-black solution. Then reprecipitation
of reaction mixture with excess ethanol leads to the
precipitation of NHC capped Pd NPs (catalyst 2), which
was separated using a centrifuge and dried in an oven at
5–80 2Ɵ degrees. High-resolution scanning electron
microscopy (HRSEM) images were taken by FEI Quanta
FEG 200F microscope using FEG assembly with Schottky
emitter (−200 V to 30 kV) as a source with >100-nA
beam current. High-resolution transmission electron
microscopy (HRTEM) was performed using a JEOL 3010
instrument with a UHR polepiece operates at an acceler-
ating voltage of 300 kV. JASCO V-750 spectrophotometer
was used for the temperature-dependent UV–visible (vis)
analysis to find the cloud point of the compounds by
ꢀ
40 C for 30 min to give NHC-capped Pd NPs as a fine
black powder.
2.4 | Synthesis of thermoresponsive
NHC-capped Pd NPs (catalyst 4) using
method B
ꢀ
using a controlled temperature program from 20 to 40 C
ꢀ
with a scan rate of 2 C/min at 480-nm wavelength.
2.4.1 | Synthesis of PNIPAM–NHC–Pd
(II) complex (3)
2
.2 | Synthesis of thermoresponsive
To the 10-ml reaction flask, NHC precursor (0.056 m.
mol, 150 mg) was added under a nitrogen atmosphere
and dissolved using 5 ml of pyridine. To the precursor
polymer-based NHC-precursor (1)
In a separate vial, initiator (275 mg, 0.48 m.mol) and
NIPAM (monomer) (1.08 g, 9.6 m.mol) were dissolved
using degassed anhydrous DMF, and the solutions were
degassed for 2 h. To the 25 ml, two necked round bottom
flask (RBF) Cu (0) catalyst (59.22 mg, 0.94 m.mol) and
solution, PdCl (0.056 m.mol, 10 mg,) was added and
stirred at 100 C for 24 h under nitrogen condition. Over
2
ꢀ
time, the reaction mixture's color changed from yellowish
orange to dark orange. After the completion of the
reaction, the solvent was removed using rotor vapor. The
orange color solid obtained was dissolved completely
using DCM and filtered by passing through the celite
pad. The collected liquid was concentrated and dried at
ꢀ
degassed initiator solution were added at 40 C under N2
atmosphere. Followed by the monomer, solution was
ꢀ
added to the reaction flask at 68 C. Then the temperature
ꢀ
ꢀ
of the reaction medium was increased to 110 C at which
60 C for 2 h to obtain NHC–Pd (II) complex (3) as an
PMDETA ligand (0.5 ml) was added to the reaction
mixture. During the addition of the ligand, blue color
was appeared and changed into greenish brown after
stirring for a few minutes. The reaction mixture was
orange color solid (80.91 mg).
2.4.2 | Synthesis of PNIPAM–NHC-
ꢀ
allowed to stir for 24 h at 110 C. After 24 h, the reaction
capped Pd NPs (catalyst 4)
mixture's color changed into dark green. The catalyst was
removed by using flash column chromatography (silica/
methanol). The yellow compound obtained was repre-
cipitated using THF/diethyl ether and dried to afford
NHC–Pd (II) complex (50 mg) and ascorbic acid
(0.5677 m.mol, 100 mg) were taken in a reaction flask. To
that, 50 ml of distilled water was added and sonicated; a
yellow color dispersed solution was obtained, which was
PNIPAM-based NHC-precursor (1) as a white solid.
1
ꢀ
(
Yield = 595 mg). H NMR (400 MHz, D O): δ = 7.82 (s),
stirred at 90 C for 1.5 h. After the reaction, the reaction
2
7
.03 (s), 3.79 (s), 1.33–2.27 (br, m), 1.04 (s) ppm; gel per-
mixture darkened to a dark blackish-brown solution. Pd
NPs formed were separated using a centrifuge and dried
to obtain the product as a black color solid (catalyst 4).
meation chromatography (GPC) (THF) M = 2107 g/mol,
M = 2703 g/mol, polymer dispersity index (PDI) = 1.2.
n
w

4
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GAYATHRI ET AL.
2.5 | General procedure for Suzuki
3 | RESULTS AND DISCUSSIONS
coupling reaction
As depicted in Scheme 1, thermoresponsive NHC-capped
palladium NPs were synthesized using PNIPAM-tagged
NHC precursor (1), which was synthesized using the
To the 6 ml, RBF aryl halides (0.25 m.mol), phe-
nylboronic acid (0.375 m.mol, 45.9 mg), catalyst (6 mg),
and potassium carbonate (0.5 m.mol, 72.3 mg) were
added together and dissolved with 2 ml of distilled water.
[
38]
procedure borrowed from our previous work.
Two dif-
ferent “Bottom-up” approach has been employed for the
successful formation of thermoresponsive NHC-capped
Pd NPs (catalysts 2 and 4) from the NHC precursor (1).
The reaction mixture was stirred at room temperature
ꢀ
(
29–30 C) for 24 h. After the completion of the reaction,
the organic layer was extracted using DCM. The solvent
was evaporated by using rotor vapor, and the compound
was dried in an oven under vacuum to afford product,
which was confirmed by H NMR. The percentage of
conversion was calculated from the H NMR spectrum.
In method A, Pd (OAc) and DMSO were used to synthe-
2
34]
[
size palladium NPs,
where both the formation of
NHC–Pd (II) complex and reduction into PNIPAM–
NHC–Pd (0) NPs (catalyst 2) take place in a single step.
Whereas method (B) consisted of two steps such as the
formation of palladium (II) complex from the NHC
1
1
precursor using PdCl in pyridine solvent and followed
2
2
.6 | Sample preparation for
by the reduction of PNIPAM–NHC–Pd (II) complex using
characterizations
ascorbic acid in an aqueous medium to form the
[
39]
catalyst 4.
2.6.1 | HRSEM analysis
1
Synthesized palladium NPs were completely dispersed in
water by keeping at 8 C for 5–10 min (for micrographs a
and c). Then the prepared sample was coated on alumi-
3.1 | H NMR analysis
ꢀ
1
In the H NMR spectrum of NHC precursor (1)
num foil and dried. Similarly, for micrographs b and d,
samples were kept at 42 C for 5 min and coated immedi-
ately on aluminum foil and dried. (Both catalyst 2 and
(Figure S1), the appearance of a peak at 8.07 ppm, which
corresponds to the hydrogen at the precarbenic carbon
center, confirmed the formation of the NHC precursor.
Similarly, the presence of broad peaks between 1 and
ꢀ
4
samples were prepared in the same manner).
2
ppm and the absence of a monomer (NIPAM) peak
between 5 and 6 ppm confirmed the presence of
polymeric moiety in the NHC precursor. From the GPC
analysis (Figure S2), M , M , and PDI of the synthesized
2.6.2 | HRTEM analysis
n
w
Synthesized palladium NPs were completely dispersed in
polymer were found to be 2107 g/mol, 2703 g/mol, and
1.28, respectively.
ꢀ
water by keeping at 8 C for 5–10 min. Then the solution
was coated on a copper grid and dried using a vacuum.
ꢀ
For micrographs, b, d, and f samples were kept at 42 C
for 5 min and coated immediately on a copper grid
followed by dried using vacuum. (Both catalyst 2 and
3.2 | FTIR analysis
4
samples were prepared in the same manner).
In the IR spectrum of NHC precursor, the presence of a
peak at 1773 cm− corresponding to the C N bond
1
+
confirmed the structure of NHC precursor (Figure 1a,
[
40]
2.6.3 | Cloud point measurement
red curve).
After the binding of NHC precursor with
+
Pd metal center, C N bond disappeared, and a new
peak corresponds to the C–N bond appeared at 1726
(Figure 1a, black curve) and 1736 cm− (Figure 1b, black
curve) for catalysts 2 and 4, respectively, confirming the
binding of NHC with Pd metal center. IR spectrum of
NHC precursor (Figure 1a, red curve) showed the
PNIPAM and NHC moiety characteristic peaks at
The sample solution for the temperature-dependent
1
UV–vis studies was prepared by dissolving the com-
ꢀ
pounds in distilled water, which was kept at ꢁ10 C to
get a clear solution. Then % transmittance (%T) was
measured at a wavelength of 480 nm with temperature
ꢀ
ꢀ
varying from 20 to 50 C at a scan rate of 2 C/min.
Samples were equilibrated for 60 s before each measure-
ment. UV–vis graph was obtained by plotting %T versus
temperature, and point with the drastic change in %T
was taken as an LCST point.
3434 cm− (νstretching of free NH bond), 3299 cm
1
−1
−1
(νstretching of bound NH bond), 3070 cm (Amide “B” of
−1
PNIPAM), 2974 and 2935 cm (νasym.stretching of CH3
−
1
and CH bond), 2875 cm (νsym.stretching of CH bond),
2
3

GAYATHRI ET AL.
5 of 14
SCHEME 1 Synthetic methodology
for PNIPAM-NHC capped palladium
nanoparticles (catalysts 2 and 4)
2+
FIGURE 1 Fourier transform infrared (FTIR) spectrum of N-heterocyclic carbene (NHC) precursor and catalyst 2 (a); NHC–Pd
complex and catalyst 4 (b)
1
−1
−1
1
(
652 cm− (νstretching of Amide I of PNIPAM), 1544 cm
black curve) displayed peaks at 3447 cm (νstretching of
−
1
−1
νsym.stretching of amide II of PNIPAM), 1457 cm (νasy.
free NH of PNIPAM), 2922 cm (νasym.stretching of CH2
(νsym.stretching of CH3 stretching),
1383 cm (νsym.bending of CH ), 1629 cm (C O of
(bending of CH2 bond),
respectively, and catalyst 4 (Figure 1b, black curve) dis-
(νstretching of free NH of
(νasym.stretching of CH2 bond),
2849 cm (νsym.stretching of CH stretching), 1386 and
−
1
−1
of CH bond), 1385 and 1365 cm (νsym.bending −of
bond), 2853 cm
bending
2
1
−1
−1
CH and CH bond), and peaks at 1269 and 1173 cm
3
2
3
[
41]
−1
correspond to amide III and CH skeletal vibrations.
PNIPAM), and 1461 cm
3
2+
For the NHC–Pd complex (Figure 1b, pink curve),
appearance of new peaks at 3063, 3022, (due to C–H of
pyridine ring), and 1453 cm (C N of pyridine ring)
−
1
played peaks at 3429 cm
−
1
−1
PNIPAM), 2920 cm
2+
−1
confirms the formation of the NHC–Pd –pyridine
3
[
42]
−1
complex.
The other characteristic peak of PNIPAM
1367 cm
(νsym.bending of CH3 and CH bond),
2
moiety appeared in the respective region for both cata-
lysts 2 and 4. The IR spectrum of catalyst 2 (Figure 1a,
1633 cm− (C O of PNIPAM), and 1461 cm (bending
1
−1
of CH bond), respectively.
2

6
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GAYATHRI ET AL.
3
.3 | Thermal analysis—TGA and DSC
NHC-capped Pd NPs (catalysts 2 and 4) showed
decreased glass transition temperature than the NHC
precursor. Catalyst 2 showed characteristic Tg of
TGA and DSC analyses were carried out to illustrate the
thermal properties of the synthesized compounds, and
results were provided in Figure 2a,b. TGA of NHC pre-
cursor showed major decomposition (75.01%) at 395 C
with a 21.06% residual mass (Figure 2a, red curve) due to
ꢀ
PNIPAM polymer backbone at 116.4 C (Figure 2b, black
ꢀ
curve), whereas catalyst 4 showed Tg at 130.5 C
ꢀ
(Figure 2b, blue curve). In both the cases of NHC capped
Pd NPs (catalyst 2 and 4), decrease in Tg suggested that
the incorporation of metal NPs with the NHC precursor
acted as plasticizer for the polymer matrix and increased
[41]
the breakage of the polymeric unit.
Unlike NHC
precursor, catalysts 2 and 4 showed more than one stage
of decomposition with a minimal loss of weight at a
lower temperature. Catalyst 2 showed different trends of
the TGA curve than catalyst 4 due to their difference in
the method of preparation.
first decomposition at a much lower temperature
[
48–50]
the mobility of the polymer chain.
Hence, the incor-
poration of Pd NPs in the polymer network had increased
the degree of freedom as well as the entropy of the system,
which, in turn, resulted in a decrease in glass transition
[43]
Catalyst 2 displayed the
[
51]
(Tg) of PNIPAM.
On comparison of glass transition
ꢀ
(around 146 C) with 11.78% weight loss, which was
(Tg) point between Pd NPs method A and Pd NPs
method B, it was observed that method A showed slightly
higher Tg value than method B, which was due to the dif-
ference in Pd NPs size between catalysts 2 and 4 as well as
the presence of high content of Pd metal in catalyst 2 than
attributed to the loss of free NHC precursor possibly
adsorbed on the surface of palladium NPs (Figure 2a,
black curve).
because it involved a reduction of NHC–Pd complex
where Pd metal is already bounded with NHC, the TGA
plot does not show any degradation that corresponds to
the adsorption of free NHC on the surface of Pd NPs at
the earlier stages (Figure 2a, blue curve). In the case of
catalysts 2 and 4, breakings of NHC–Pd bond were
observed as a major weight loss at 246 C (25.84% weight
loss) and 298 C (29.3% weight loss).
catalysts 2 and 4 showed about 49.07% and 40.62% as a
residual content, which concluded the presence of NPs in
the catalytic system.
[44]
Whereas in the case of method B,
2+
[
49–51]
catalyst 4 (evident from TGA graph).
3.4 | UV analysis
ꢀ
UV–vis studies were employed to confirm the formation
of palladium NPs, and the results were provided in
Figure 3. The characteristic π ! π* transition of PNIPAM
ꢀ
[45]
Additionally,
moiety appeared at around λ
of 263 nm for the NHC
On the other hand, for
max
[52]
[46]
precursor (Figure 3, red curve).
2+
DSC analysis of the NHC polymer and NHC-capped
Pd NPs was shown in Figure 2b. The glass transition
temperature (Tg) for the PNIPAM-tagged NHC precursor
was observed at 136.3 C, which was well matched
with the reported Tg of PNIPAM homopolymer
the NHC–Pd complex addition to the PNIPAM charac-
teristics band (at around 263 nm), a broad hump
appeared at 340 nm due to the presence of d–d transition
(metal to ligand charge transfer—MLCT) of Pd
ꢀ
[
53]
(II) (Figure 3, pink curve).
In the case of NHC-capped
[47]
(Figure 2b, red curve).
Whereas both PNIPAM-tagged
Pd NPs (catalysts 2 and 4), appearance of broad
FIGURE 2 (a) Thermogravimetric analysis (TGA) of N-heterocyclic carbene (NHC) precursor and NHC-capped Pd nanoparticle (NPs)
catalysts 2 and 4); (b) differential scanning calorimetry (DSC) of NHC precursor and NHC-capped Pd NPs (catalysts 2 and 4)
(

GAYATHRI ET AL.
7 of 14
HRSEM analysis was taken below the LCST as well as
above the LCST, and the results obtained were provided
in Figures 5a–d and 6a–d. Considering the presence of
the thermoresponsive PNIPAM chain, it is expected that
ꢀ
below the LCST point (8 C), synthesized NPs will be less
aggregated due to the coil conformation of PNIPAM moi-
ꢀ
ety. Whereas at above the LCST (42 C), because PNIPAM
will change into globule conformation, as a result, these
NPs will be highly aggregated. HRSEM results hold a
good agreement with the above statement by showing
less aggregated Pd NPs for catalyst 2, which was kept
below LCST (micrographs 5a and 5c), and in the case of
catalyst 2 at above LCST temperature, the NPs were
highly aggregated (micrographs 5b and 5d). In Figure 5
and 6a, even the samples below the LCST showed some
aggregation, which may due to the aggregation caused by
evaporation of water molecules from the sample at room
FIGURE 3 Ultraviolet–visible (UV–vis) spectrum of N-
2
+
heterocyclic carbene (NHC) precursor, NHC–Pd complex, Pd
OAc) , and NHC-capped Pd nanoparticles (NPs) (catalysts 2 and 4)
ꢀ
temperature (around 30–31 C), which is slightly close to
(
2
the LCST point of the NHC-capped Pd NPs (catalyst 2).
Similar results (difference in aggregation) were observed
in the case of catalyst 4; thus, Pd NPs were less aggre-
gated below the LCST (micrographs 6a and 6c) than
above LCST (micrographs 6b and 6d).
continuous bands at lower wavelength confirmed the
[54]
presence of Pd (0) Nps (Figure 3, black curve).
Addi-
tionally for catalyst 4, the formation of Pd (0) NPs was
confirmed by observing Pd (0) characteristic peak at
[55]
around 307 nm.
Along with that, the hump was
observed for catalyst 4 (Figure 3, red curve), which may
be owing to the presence of ascorbic acid.
3.7 | HRTEM analysis
[56]
The size of the formed palladium NPs and their thermo-
responsive nature were further confirmed using HRTEM
analysis, and the results were provided in Figures 7a–d
and 8a–d. HRTEM results showed the synthesized NPs
were spherical, and the size of the synthesized NPs was
3
.5 | XRD analysis
In NHC precursor (Figure 4), peak of PNIPAM moiety
appeared at 20.7 (Figure 4, black curve); however, in the
ꢀ
case of the NHC–Pd (II) complex (Figure S3), the peak of
ꢀ
[57]
PNIPAM was shifted to high order 21.1 .
In catalyst 2,
in addition to PNIPAM peaks, new peaks that correspond
to fcc crystalline planes (111), (200), and (222) of Pd
ꢀ ꢀ ꢀ
(
0) NPs were observed at 39.8 , 46.3 , and 67.8 ,
[58]
respectively (Figure 4, red curve).
In the case of
catalyst 4 (Figure 4, blue curve), broad fcc crystalline
planes of Pd (0) NPs were observed in the same region as
for catalyst 2, and peak broadening may be attributed to
the smaller size of NPs. Similar results were also noticed
[39,58,59]
in some of the reported literature.
3.6 | HRSEM analysis
The surface morphology of the synthesized catalysts was
studied using HRSEM, and the results were given in
Figures 5 and 6. HRSEM results showed both the cata-
lysts were close to spherical shape. Furthermore, to study
the thermoresponsive nature of these synthesized NPs,
FIGURE 4 X-ray diffraction (XRD) patterns of N-heterocyclic
carbene (NHC) precursor and NHC-capped Pd nanoparticles (NPs)
(catalysts 2 and 4)

8
of 14
GAYATHRI ET AL.
FIGURE 5 High-resolution scanning electron microscopy (HRSEM) micrographs of N-heterocyclic carbene (NHC)-capped Pd
ꢀ
ꢀ
nanoparticles (NPs)—catalyst 2 (a,c—samples at 8 C; b,d—samples at 42 C)
FIGURE 6 High-resolution scanning electron microscopy (HRSEM) micrographs of N-heterocyclic carbene (NHC)-capped Pd
ꢀ
ꢀ
nanoparticles (NPs)—catalyst 4 (a,c—samples at 8 C; b,d—samples at 42 C)
ꢀ
in the range of 2–3 nm for catalyst 2 (Figure S4) and
showed a similar drop of %T at 35.8 C, that is, slightly
5
–6 nm for catalyst 4 (Figure S5). Furthermore, it also
higher than the precursor, which was possibly due to the
presence of pyridine moiety in the complex. Unlike precur-
sor and complex, these NPs showed a reduced amount of
initial %T, which implies the presence of NPs within the
system caused scattering of the light, which, in turn,
supported the thermoresponsive behavior of these palla-
dium NPs thus showing less aggregated NPs below the
LCST (a and c micrographs of Figures 7 and 8) and
highly aggregated images above the LCST (b and d micro-
graphs of Figures 7 and 8). Both catalysts showed similar
kinds of results, and this difference in aggregation proved
the thermoresponsive property of the synthesized
PNIPAM tagged NHC-capped Pd NPs.
[60]
resulted in the reduced %T at the initial stages.
How-
ever, catalyst 2 showed an initial %T of about 34.1%,
slightly higher than catalyst 4, which may due to the high
content of Pd NPs that causes settling of some of the nps
at the bottom thus showed increased %T at the initial stage
(
Figure 9a). Furthermore, catalyst 2 displayed a drop in
ꢀ
3
.8 | Temperature-dependent UV–vis
%T up to 28.2 C; afterward, increase in %T was observed
due to the enhanced aggregation of catalyst 2 owing to the
presence of Pd NPs that caused the settling down of the
catalyst 2 at the bottom of the cuvette. Settling down of
the catalyst was further proved in the right side inset
image in Figure 9a. As shown in Figure 9b, catalyst
study for LCST point measurement
[
60]
To find the LCST point of the synthesized catalysts
(
(
2 and 4), %T of NHC precursor, PNIPAM–NHC–Pd
II) complex, and catalysts 2 and 4 was measured using
ꢀ
temperature-dependent UV–vis spectroscopy, and the
results were plotted as temperature against %T, which
were provided in Figures S6 and S7 and Figure 9a,b. In all
the cases, the temperature corresponds to the sudden
change in %T taken as the LCST point of the compound.
As shown in Figure S6, NHC precursor shows a drastic
change in %T at around 33.8 C, which was taken to be the
characteristics LCST point of the NHC precursor. While,
the respective PNIPAM–NHC–Pd (II) complex (Figure S7)
4 showed a drastic change in transmittance at 34.3 C,
which is a characteristic LCST point of catalyst 4. Unlike
catalyst 2, catalyst 4 does not get settled down at the bot-
tom of the vial (right side inset image in Figure 9b); thus,
it does not display an increase in %T after LCST. On com-
paring with catalyst 2, catalyst 4 showed a higher LCST
point, which was attributed to the changes in the size of
their NPs. Thus, by having a slightly bigger size of palla-
ꢀ
[61]
dium NPs, catalyst 4 attained a higher LCST point.

GAYATHRI ET AL.
9 of 14
FIGURE 7 High-resolution transmission electron microscopy (HRTEM) micrographs of N-heterocyclic carbene (NHC)-capped Pd
ꢀ
ꢀ
nanoparticles (NPs)—catalyst 2 (a,c—samples at 8 C; b,d—samples at 42 C)
3
.9 | Catalytic activity
carried out under the same condition and their percent-
age of conversion was tabulated in Table 1. As expected,
chloro-based substrate gave very low conversion
To examine the catalytic efficiency of the herein reported
PNIPAM–NHC-capped palladium NPs (catalysts 2 and 4),
coupling reactions (shown in Scheme 2) were employed
using a variety of aryl halides. As shown in Scheme 2,
catalytic reactions were executed using catalyst 2 or 4 in
a neat water medium at room temperature (29–30 C)
without any additional ligands or additives, and the
percentage conversion was measured using H NMR
[
11]
(Entry 3) due to the strong C–Cl bond.
In both the cat-
alysts, catalytic studies showed iodo- and bromo-based
aryl substrates with an electron-withdrawing group
(Entries 1, 2, 6, and 7) and showed a higher percentage of
conversion than the strong electron-donating group
substituted aryl halides (Entry 4). Similarly, Entry 5 also
displayed relatively high reactivity due to the weak
ꢀ
1
spectroscopy (Figure S8–S33), and the outcomes were
offered in Table 1.
For 4-iodoacetophenone substrate (Entry 1—Table 1),
both the catalysts showed high catalytic activity to yield
electron-donating ability of the CH group. Results dis-
3
played that the reactivity of both the catalysts was com-
parable, whereas in some cases, catalyst 4 showed
slightly higher catalytic activity, which may be attributed
to the difference in their LCST point as well as the size of
palladium NPs. For instance, catalyst 2 showed LCST
>
90% of conversion, which was comparatively higher
[38]
than our previous work.
Furthermore to extend the
ꢀ
scope of the substrates, coupling reactions of different
aryl substrates such as with activating R groups (OH,
CH ) or deactivating R groups (COCH , COOH) were
behavior at around 28 C, which is very close to room
ꢀ
temperature (29–30 C); on the other hand, catalyst
ꢀ
4 showed LCST behavior at 34 C. By having an LCST
3
3

10 of 14
GAYATHRI ET AL.
FIGURE 8 High-resolution transmission electron microscopy (HRTEM) micrographs of NHC-capped Pd nanoparticles (NPs)—catalyst
ꢀ
ꢀ
4
(a,c—samples at 8 C; b,d—samples at 42 C)
FIGURE 9 Ultraviolet–visible (UV–vis) spectrum of catalyst 2 (a); catalyst 4 (b) at the variable temperature and 480-nm wavelength.
inset images: left side: catalytic solution at below LCST; right side: at above LCST)
(
point close to the experiment temperature (room temper-
ature), catalyst 2 tends to undergo higher aggregation
lead to show lesser efficiency for catalyst 2 towards
Suzuki coupling reactions under ambient conditions.
Furthermore, in catalyst 4, the size of Pd NPs was in the
range of 5–6 nm, which is slightly bigger than Pd NPs in
(as evident from temperature-dependent UV–vis) behav-
ior in the reaction medium than catalyst 4. Thus, it may

GAYATHRI ET AL.
11 of 14
1
FIGURE 10 Crude H nuclear magnetic resonance (NMR) of the catalytic cycle of 4-iodoacetophenone performed using catalyst
(a) and catalyst 4 (b)
2
catalyst 2. Thus, hence by having a smaller size of Pd
NPs, catalyst possessed higher surface area
Figure S34 and S35 and Table S1), which may undergo
Recyclable studies were performed for PNIPAM-
tagged NHC-capped Pd NPs (catalyst 2 as well as
catalyst 4) using 4-iodoacetophenone as a substrate, and
the results were provided in Table 2. The percentage of
2
a
(
strong adsorption of reaction intermediates on the sur-
face during the reaction, thus may result in the decrease
in catalytic activity towards coupling reaction by reduc-
ing the catalytically active surface as well as displayed
1
conversion for each cycle was determined from H NMR
(Figures S8–S14 and S21–S27) where the decrease in the
intensity of product peak and increase in the intensity of
starting material peak were observed as it goes from the
[62,63]
uncertain reactivity.

12 of 14
GAYATHRI ET AL.
TABLE 2 Recyclability studies of catalyst 2 and 4 for
-iodoacetophenone substrate
4
%
of conversion
Cycles
1
2
3
4
5
6
7
SCHEME 2 Suzuki coupling reaction between aryl halides and
Catalyst 2
Catalyst 4
92%
97%
50%
83%
28%
68%
44%
57%
33%
66%
19%
34%
6%
12%
phenylboronic acid using catalyst 2 or 4
first cycle to the last cycle (Figure 10a,b). As shown in
Table 2, catalyst 2 showed a decrease in reactivity from
the first cycle to the last cycle due to the possible loss of
catalyst during the phase separation of the catalyst after
We have compared the efficiency of the synthesized
catalysts with the previous reports of similar catalysts
and provided the comparative table in Table S2. From
the comparative studies, we found these catalysts showed
higher catalytic activity than the similar kind of catalytic
systems under the ambient conditions (Table S2: Entries
1–4) and also maintained similar catalytic activity and
[60]
each cycle.
On the other hand, owing to its aggrega-
tion behavior, catalyst 2 showed a sudden drop in the
[
64]
conversion,
and it showed a slight fluctuation in the
[
65]
percentage of conversion during recyclability studies.
TABLE 1 Catalytic activities of catalysts 2 and 4
%
conversion (first cycle conversion)
Entry
1
Substrate
Coupling product
NHC-capped Pd NPs catalyst 2
NHC-capped Pd NPs catalyst 4
98
95
78
<5
2
3
91
<5
4
5
6
40
55
95
48
66
98
7
95
99
ꢀ
Note: Reaction conditions: 0.8 mol % of catalyst, 0.25 m.mol of aryl halides, 0.37 m.mol of phenylboronic acid, 0.5 m.mol of K
% conversion—an average of two separate runs).
Abbreviations: NHC, N-heterocyclic carbene; NPs, nanoparticles.
2
CO
3
, 2 ml of H
2
O, rt (29–30 C),
(

GAYATHRI ET AL.
13 of 14
recyclability with the reported catalysts (Table S2:
Entries 5 and 6).
REFERENCES
[1] N. Miyaura, K. Yamada, A. Suzuki, Tetrahedron Lett. 1979, 20,
437.
3
[
[
[
2] I. Maluenda, O. Navarro, Molecules 2015, 20, 7528.
3] D. Roy, Y. Uozumi, Adv. Synth. Catal. 2018, 360, 602.
4] H. Peng, Y.-Q. Chen, S.-L. Mao, Y.-X. Pi, Y. Chen, Z.-Y. Lian,
T. Meng, S.-H. Liu, G.-A. Yu, Org. Biomol. Chem. 2014, 12,
4
| CONCLUSIONS
Using two different “bottom-up” approaches, thermo-
responsive PNIPAM–NHC-capped palladium NPs have
been successfully synthesized and characterized using
various techniques. Owing to the presence of PNIPAM,
herein reported catalysts 2 and 4 attained thermo-
responsive behavior thus showed LCST behavior in a
water medium. As a result, using these catalysts, toxic
phosphine-free Suzuki coupling reactions have been
effectively performed in a neat water medium at room
temperature without any external additives. The coupling
reaction was carried out below the LCST in the water
medium while the recovery of the catalyst was performed
above the LCST point and adequately reused for further
cycles. Catalytic studies concluded though both the
NHC-capped Pd NPs (catalysts 2 and 4) showed relatively
similar catalytic activities towards Suzuki coupling under
ambient conditions, catalyst 4 exhibited relatively high
catalytic activity due to the difference in the size of Pd
Nps and LCST point between catalysts 2 and 4. In conclu-
sion, in this work, by having PNIPAM–NHC as a capping
agent for highly active Pd (0) NPs that act as a catalytic
center, we achieved high reactivity and recyclability of
the catalyst towards the Suzuki coupling reaction under
ambient conditions.
6
944.
5] T. Strassner, in Organometallic Oxidation Catalysis, Springer,
006, 125.
[
2
[6] E. Niknam, F. Panahi, A. Khalafi-Nezhad, Appl. Organomet.
Chem. 2020, e5470.
7] H. D. Velazquez, F. Verpoort, Chem. Soc. Rev. 2012, 41,
[
[
[
7032.
8] R. Zhong, A. Pöthig, Y. Feng, K. Riener, W. A. Herrmann,
F. E. Kühn, Green Chem. 2014, 16, 4955.
9] W. Mansour, M. Fettouhi, B. El Ali, Appl. Organomet. Chem.,
e5636.
[10] J. B. Ernst, C. Schwermann, G.-I. Yokota, M. Tada, S.
Muratsugu, N. L. Doltsinis, F. Glorius, J. Am. Chem. Soc. 2017,
139, 9144.
[
11] R. Sedghi, B. Heidari, H. Shahmohamadi, P. Zarshenas, R. S.
Varma, Molecules 2019, 24, 3048.
[
[
[
12] F. Rajabi, W. R. Thiel, Adv. Synth. Catal. 2014, 356, 1873.
13] S. Paul, M. M. Islam, S. M. Islam, RSC Adv. 2015, 5, 42193.
14] V. Ramakrishna, N. D. Reddy, Dalton Trans. 2017(46),
8598.
[15] P. D. Stevens, G. Li, J. Fan, M. Yen, Y. Gao, Chem. Commun.
2005, 4435.
16] G. Liu, M. Hou, J. Song, T. Jiang, H. Fan, Z. Zhang, B. Han,
[
Green Chem. 2010, 12, 65.
[
[
17] N. Liu, C. Liu, Z. Jin, Green Chem. 2012, 14, 592.
18] A. Fihri, M. Bouhrara, B. Nekoueishahraki, J.-M. Basset,
V. Polshettiwar, Chem. Soc. Rev. 2011, 40, 5181.
19] A. R. Hajipour, P. Abolfathi, F. Mohammadsaleh, RSC Adv.
2016, 6, 78080.
[
ACKNOWLEDGMENT
The author acknowledges the DST-INSPIRE program for
providing the funds under the grant number IF170645.
CSIR-CLRI communication No: A/2020/PLY/DST/1488.
[20] G. Li, H. Yang, W. Li, G. Zhang, Green Chem. 2011, 13, 2939.
[
[
[
21] W. R. Reynolds, P. Plucinski, C. G. Frost, Catal. Sci. Technol.
2014, 4, 948.
22] R. Lambert, A.-L. Wirotius, J. Vignolle, D. Taton, Polym.
Chem. 2019, 10, 460.
23] A. Taher, J.-B. Kim, J.-Y. Jung, W.-S. Ahn, M.-J. Jin, Synlett
AUTHOR CONTRIBUTIONS
Varnakumar Gayathri: Formal analysis; funding
acquisition; methodology; validation. Nagaraju Pentela:
Formal analysis; investigation; methodology; validation.
Debasis Samanta: Conceptualization; investigation;
supervision; validation.
2
009, 2477.
[24] S.-Y. Ding, J. Gao, Q. Wang, Y. Zhang, W.-G. Song, C.-Y. Su,
W. Wang, J. Am. Chem. Soc. 2011, 133, 19816.
25] A. R. Siamaki, S. K. A. El Rahman, V. Abdelsayed, M. S.
El-Shall, B. F. Gupton, J. Cat. 2011, 279, 1.
26] V. Gayathri, N. Pentela, D. Samanta, in Advances in Sustain-
able Polymers, ed. K. A. Katiyar V., Mulchandani N., Springer,
Singapore, 2020, 145.
[
[
DATA AVAILABILITY STATEMENT
Data are available in article supplementary material.
[
27] M. Shibayama, Y. Suetoh, S. Nomura, Macromolecules 1996,
29, 6966.
ORCID
[28] Y. Zhang, S. Chen, M. Pang, W. Zhang, Polym. Chem. 2016, 7,
Varnakumar Gayathri https://orcid.org/0000-0002-
6880.
[
29] N. Pentela, P. Murugan, S. N. Jaisankar, D. Samanta, A. B.
Mandal, J. Organomet. Chem. 2015, 778, 42.
30] N. Pentela, S. Rainu, N. Duraipandy, A. Boopathi, M.
Kiran, S. Sampath, D. Samanta, SN Applied Sciences 2019,
4
927-2424
Nagaraju Pentela https://orcid.org/0000-0003-2068-
477
Debasis Samanta https://orcid.org/0000-0002-3043-
033
[
9
1
, 448.
8

14 of 14
GAYATHRI ET AL.
[
[
31] P. P. Mpungose, Z. P. Vundla, G. E. Maguire, H. B. Friedrich,
Molecules 2018, 23, 1676.
32] A. Pourjavadi, A. Motamedi, Z. Marvdashti, S. H. Hosseini,
Catal. Commun. 2017, 97, 27.
33] M. O. Sydnes, Catalysts 2017, 7, 35.
34] C.-A. Wang, Y.-W. Li, X.-M. Hou, Y.-F. Han, K. Nie, J.-P.
Zhang, ChemistrySelect 2016, 1, 1371.
A. N. Ozerin, A. M. Muzafarov, Polym. Compos. 2016, 37,
1978.
[52] S. Makharza, J. Auisa, S. A. Sharkh, J. Ghabboun, M. Faroun,
H. Dweik, W. Sultan, M. Sowwan, Int. J. Polym. Anal. Ch.
2010, 15, 254.
[
[
[53] M. Esmaeilpour, A. R. Sardarian, H. Firouzabadi,
J. Organomet. Chem. 2018, 873, 22.
[
[
[
35] B. Heidari, M. M. Heravi, M. R. Nabid, R. Sedghi, S. E.
Hooshmand, Appl. Organomet. Chem. 2019, 33, e4632.
36] M. C. Hong, M. C. Choi, Y. W. Chang, Y. Lee, J. Kim, H. Rhee,
Adv. Synth. Catal. 2012, 354, 1257.
37] K. S. Sivudu, N. M. Reddy, M. N. Prasad, K. M. Raju, Y. M.
Mohan, J. Yadav, G. Sabitha, D. Shailaja, J. Mol. Catal. A:
Chem. 2008, 295, 10.
[54] S. Chatterjee, S. K. Bhattacharya, ACS Omega 2018, 3, 12905.
[55] M. Nasrollahzadeh, S. M. Sajadi, E. Honarmand, M. Maham,
New J. Chem. 2015, 39, 4745.
[56] E. A. Larios-Rodríguez, F. Castillón-Barraza, D. J. Borbón-
González, R. Herrera-Urbina, A. Posada-Amarillas, Adv. Sci.
Eng. Med. 2013, 5, 665.
[57] A. M. Atta, G. A. El-Mahdy, H. A. Al-Lohedan, A. O. Ezzat,
Molecules 2014, 19, 6246.
[58] S. Ghasemi, Z. A. Harandi, RSC Adv. 2018, 8, 14570.
[59] S. Gurunathan, E. Kim, J. W. Han, J. H. Park, J.-H. Kim, Mole-
cules 2015, 20, 22476.
[
[
[
[
38] N. Pentela, V. Gayathri, D. Samanta, J. Organomet. Chem.
2
020, 913, 121196.
39] N. Shang, S. Gao, C. Feng, H. Zhang, C. Wang, Z. Wang, RSC
Adv. 2013, 3.
40] A. Z. Wilczewska, I. Misztalewska, Organometallics 2014, 33,
[60] Q. Wang, J. Wang, D. Wang, M. Turhong, M. Zhang, Chem.
Eng. J. 2015, 280, 158.
5
203.
41] C. A. Ribeiro, M. V. S. Martins, A. H. Bressiani, J. C. Bressiani,
[61] S. Gupta, A. Singh, N. Matsumi, ACS Omega 2019, 4, 20923.
[62] Y. Li, E. Boone, M. A. El-Sayed, Langmuir 2002, 18, 4921.
[63] M. Pérez-Lorenzo, J. Phys. Chem. Lett. 2012, 3, 167.
[64] K. Song, P. Liu, J. Wang, B. Tan, T. Li, J. Porous Mater. 2016,
23, 725.
M. E. Leyva, A. A. A. de Queiroz, Mater. Sci. Eng. C 2017,
8
1, 156.
[
[
[
[
[
42] C. Boztepe, A. Künkül, S. Ya s¸ ar, N. Gürbüz, J. Organomet.
Chem. 2018, 872, 123.
43] S. N. Jadhav, A. S. Kumbhar, S. S. Mali, C. K. Hong, R. S.
Salunkhe, New J. Chem. 2015, 39, 2333.
[65] M. Zeltner, A. Schätz, M. L. Hefti, W. J. Stark, J. Mater. Chem.
2011, 21, 2991.
44] J.-T. Lu, J. C. Lin, M.-C. Lin, N. D. Khupse, I. J. Lin, Langmuir
2
014, 30, 10440.
SUPPORTING INFORMATION
Additional supporting information may be found online
in the Supporting Information section at the end of this
article.
45] E. C. Hurst, K. Wilson, I. J. Fairlamb, V. Chechik, New
J. Chem. 2009, 33, 1837.
46] J. M. Giussi, C. Von Bilderling, E. Alarcón, L. I. Pietrasanta, R.
Hernandez, P. Rafael, M. Vázquez, C. Mijangos, M. L. Cortez,
O. Azzaroni, Nanoscale 2018, 10, 1189.
[
[
47] X. Su, Y. Shuai, Z. Guo, Y. Feng, Molecules 2013, 18, 4599.
48] F. Chen, A. Clough, B. Reinhard, M. Grinstaff, N. Jiang, T.
Koga, O. Tsui, Macromolecules 2013, 46, 4663.
How to cite this article: Gayathri V, Pentela N,
Samanta D. Palladium nanoparticles capped by
thermoresponsive N-heterocyclic carbene: Two
different approaches for a comparative study. Appl
Organomet Chem. 2021;35:e6166. https://doi.org/
[
49] O. A. Serenko, V. I. Roldughin, A. А. Askadskii, E. S.
Serkova, P. V. Strashnov, Z. B. Shifrina, RSC Adv. 2017, 7,
5
0113.
[
50] T. Hanemann, D. V. Szabó, Materials 2010, 3, 3468.
10.1002/aoc.6166
[
51] V. I. Roldughin, O. A. Serenko, E. V. Getmanova, N. A.
Novozhilova, G. G. Nikifirova, M. I. Buzin, S. N. Chvalun,