Ultra-small palladium nano-particles synthesized using bulky S/Se and N donor ligands as a stabilizer: Application as catalysts for Suzuki-Miyaura coupling

Article abstract of DOI:10.1039/c9ra03498f

Two chalcogenated ligands L1 and L2 containing anthracene core and amine functionality have been synthesized. Both the ligands have been characterized using ¹H and ¹³C{¹H} NMR techniques. The structure of L1 has also been corroborated by single crystal X-ray diffraction. Application of L1 and L2 as stabilizers for palladium nano-particles (NPs) has been explored and six different types of NPs 1-6 have been prepared by varying the quantity of stabilizer. The nano-particles have been characterized by PXRD, EDX, and HRTEM techniques. The size of NPs has been found to be in the range of ～1-2 nm, 2-3 nm, 4-6 nm, 1-2 nm, 1-2 nm and 3-5 nm for 1-6 respectively. The catalytic activities of 1-6 have been explored for Suzuki-Miyaura coupling of phenyl boronic acid with various aryl halides. These NPs showed good catalytic activity for various aryl chlorides/bromides at low catalyst loading (5 mg). Among 1-6, the highest activity has been observed for NPs 1, probably due to their relatively small size and high uniformity in the dispersion. The recyclability of the NPs upto 5 catalytic cycles is a distinct advantage.

Full text of DOI:10.1039/c9ra03498f

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PAPER
Ultra-small palladium nano-particles synthesized
using bulky S/Se and N donor ligands as a stabilizer:
application as catalysts for Suzuki–Miyaura
coupling†
Cite this: RSC Adv., 2019, 9, 22313
a
a
a
b
Preeti Oswal, Aayushi Arora, Jolly Kaushal, Gyandshwar Kumar Rao,
a
c
a
Sushil Kumar,
Ajai K. Singh
and Arun Kumar
*
Two chalcogenated ligands L1 and L2 containing anthracene core and amine functionality have been
1
13
1
synthesized. Both the ligands have been characterized using H and C{ H} NMR techniques. The
structure of L1 has also been corroborated by single crystal X-ray diffraction. Application of L1 and L2 as
stabilizers for palladium nano-particles (NPs) has been explored and six different types of NPs 1–6 have
been prepared by varying the quantity of stabilizer. The nano-particles have been characterized by PXRD,
EDX, and HRTEM techniques. The size of NPs has been found to be in the range of ꢀ1–2 nm, 2–3 nm,
4
–6 nm, 1–2 nm, 1–2 nm and 3–5 nm for 1–6 respectively. The catalytic activities of 1–6 have been
Received 9th May 2019
Accepted 17th June 2019
explored for Suzuki–Miyaura coupling of phenyl boronic acid with various aryl halides. These NPs
showed good catalytic activity for various aryl chlorides/bromides at low catalyst loading (5 mg). Among
DOI: 10.1039/c9ra03498f
1–6, the highest activity has been observed for NPs 1, probably due to their relatively small size and high
rsc.li/rsc-advances
uniformity in the dispersion. The recyclability of the NPs upto 5 catalytic cycles is a distinct advantage.
can be modied. In order to improve and expand the scope of
Suzuki–Miyaura coupling, intensive research efforts are being
Introduction
Palladium-catalyzed C–C coupling reactions are impressive carried out. In this regard, the development of new catalyst
1
synthetic methods in organic transformations. The tolerance systems with high efficiency and recyclability is an important
10
of palladium compounds towards a wide range of functional area that has received particular attention.
groups and the easy interchange between Pd(0)/Pd(II) or Pd(II)/
In the last few decades, nano-sized palladium particles
Pd(IV) oxidation states are mainly responsible for their versa- stabilized with diverse agents, have drawn continuous attrac-
2
11,12
tility. Of the various C–C bond-forming reactions, Suzuki– tion.
Nanoparticles play remarkable roles in a variety of
Miyaura coupling is one of the most important methods for the elds due to their large surface to volume ratio, reactive surface
syntheses of diversied and unsymmetrical biaryls (which have molecules, and the capability of tuning the properties of
1
3–16
numerous applications in synthesis of drugs, natural products, material etc.
High surface to volume ratio of nanoparticles
3
agrochemicals, advanced material etc.) compared to other has constrained the researchers to stabilize them by utilizing
coupling reactions, mainly due to the low cost of reagents and appropriate stabilizers. The stabilizing ligands play an impor-
ease in handling and removal of reagents and products. The tant role in dictating nanoparticles' size, shape, dispersion, and
highly efficient Suzuki–Miyaura catalysts include complexes of inter-particle spacing. These ligands also affect the catalytic
Pd(II) with phosphines, N-heterocyclic carbenes, and orga- efficiency, recyclability and solubility of synthesized NPs
nochalcogens with an advantage of the ease with which they through ligation. For efficient organic synthesis, generally
4,5
6,7
8,9
surfactants, organic ligands, dendrimers, and polymers have
17–19
been used to stabilize metal nanoparticles.
The stabilizer
a
Department of Chemistry, School of Physical Sciences, Doon University, Dehradun, molecules generally have functional groups like amine, alcohol,
2
48012 India. E-mail: arunkaushik@gmail.com; akumar.ch@doonuniversity.ac.in
20
carbonyl or hydroxyl, thiol and phosphine, which limits the
b
Department of Chemistry, Amity School of Applied Sciences, Amity University
Haryana, Gurgaon, Haryana, 122413, India
21
catalytic activity because of strong chemisorption. Thus, the
stabilizing ligand must be strong enough to stabilize nano-
particles. It must readily allow access of reactant to its surface so
that they can be used for multiple reaction cycles. In the
c
Department of Chemistry, India Institute of Technology Delhi, New Delhi, 110016,
India
†
Electronic supplementary information (ESI) available. Spectral data of L1 and
L2; single crystal data of L1 (CCDC 1887878), SEM-EDS data of NPs 1–6. For ESI absence of a stabilizing agent, there is a possibility of aggrega-
and crystallographic data in CIF or other electronic format see DOI:
tion or precipitation of nanoparticles which results in the loss of
1
0.1039/c9ra03498f
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their catalytic activity. Therefore, it is worthy to explore new with the structures given in Scheme 1. The CH (imine) signal in
1
0
0
ligands as stabilizers suitable for ensuring minimum surface the H NMR spectra of precursor Schiff bases L1 and L2
deactivation. appeared at 9.4 and 9.31 ppm respectively, supporting the
Sulphur and selenium donor groups may serve as good formation of pC]N bond. This signal disappeared on their
candidates for designing new ligand systems for stabilization of subsequent reduction to L1 and L2 and a new signal of –NCH
2
–
nanoparticles. The strong electron donating ability of sulphur emerged at 4.66 and 4.56 ppm respectively in case of L1 and L2.
and selenium and the advantage of their ligands for being air The needle-shaped light yellow single crystals of L1 were ob-
stable and moisture insensitive has made them attractive for tained by slow evaporation of its solution in ethanol. The
designing catalyst frameworks. Such type of ligands have been molecular structure of L1 is given in Fig. 1 with 50% probability
reported in the recent past as an important building block for ellipsoids and with selected bond lengths and bond angles.
2
2–29
a variety of catalysts.
These have shown good to excellent Both L1 and L2 have been used to stabilize Pd(0) NPs 1–6 as
22–29
catalytic activity for the coupling of aryl halides.
Presence of summarized in Scheme 1. Some organic compounds have been
neutral chalcogen donor in stabilizers affects the catalytic effi- used to develop NPs with good catalytic efficiency, their
ciency, composition, size, recyclability, and dispersion of the synthesis are very cumbersome along with the difficulty in
30,31
32–35
nanoparticles to a great extent.
The use of organochalcogen separation, and consequently low yields.
Only a few reports
ligands in the development of molecular catalysts has been are available on the use of an organic ligand with neutral
2
2–29
studied in a variety of cases.
However, the use of such chalcogen donor in the stabilization and development of cata-
30,31
ligands in the stabilization of nanoparticles is limited.
To lytically active Pd(0) NPs. The distinct advantage of the ligands
the best of our knowledge, no such compound containing in the present study is their simple and easy preparation in
a combination of hard–hard or hard–so donors have been excellent yields.
reported for this purpose. Thus, in this article, we are reporting
the synthesis (Scheme 1) and characterization of palladium Na
The Pd(0) NPs 1–6 have been prepared (Scheme 1) using
PdCl as a palladium precursor and L1 or L2 as a stabilizer
2
4
nano-particles stabilized with protecting ligands (L1 an L2) in varying ligand to metal ratio. The L1 : Pd molar ratio is 1 : 1
containing a combination of sulphur/selenium and ‘N’ donors. for 1, 1 : 4 for 2, and 4 : 1 for 3. The L2 : Pd molar ratio is 1 : 1
The catalytic potential of these NPs has been explored for for 4, 1 : 4 for 5, and 4 : 1 for 6. The maximum quantity of ligand
Suzuki–Miyaura coupling reaction and also found to be prom- has been used for the purpose of stabilization in the case of 3
ising. Additionally, they have also been found to be recyclable and 6. All the NPs have been subjected to powder XRD (Fig. 2),
up to ve cycles.
HRTEM (Fig. 3), and SEM-EDX (ESI Fig. S9–S13†) techniques for
characterization. PXRD data of 1–6 showed the presence of
a peak at 40.0 (2q) corresponding to (111) plane of Pd(0) (d ꢀ
Results and discussion
Syntheses and characterization
2
.26) (JCPDS 88-2335) with face-centered cubic structure. The
peaks at 27.3, 31.7, 45.5, 56.4 and 66.2 which correspond to
111), (200), (220), (222), and (400) planes of the palladium oxide
0
0
(
The syntheses of L1 and L2 have been carried out using
a method similar to reported procedures. Ligands L1 and L2
have been synthesized using a methodology shown in Scheme 1.
36
24
were observed in the PXRD pattern of NPs 2–4 (Fig. 2). Palla-
dium oxide was also present in NPs 5, but the palladium(0)
content was found to be higher. Small amount of palladium
oxide was observed in the case of NPs 6 (Fig. 2). In the case of
NPs 1, pure phase palladium was observed as revealed by PXRD
1
13
Ligands L1 and L2 have been characterized using H and
C
1
{
H} NMR techniques. The spectra were found to be consistent
(
(
Fig. 2). The SEM-EDX studies have revealed that Pd : S ratio
atom%) in NPs 1–3 are 82 : 18, 76 : 24 and 79 : 21 respectively.
The Pd : Se ratio in the case of NPs 4–6 has been found to be
69 : 31, 73 : 27 and 71 : 29 respectively.
The HRTEM images of NPs 1–6 shown in Fig. 3, conrm the
size and nature of dispersion of the nano-particles. The size of
Fig. 1 ORTEP of L1 (ellipsoid of 50% probability). Selected bond
lengths ( A˚ ): S(2)–C(27) 1.765(6), S(2)–C(13) 1.826(6), N(3)–C(45)
ꢁ
1
.436(8), N(3)–C(33) 1.492(7). Selected bond angles ( ): C(27)–S(2)–
Scheme 1 Syntheses of ligands and nano-particles.
C(13) 104.5(3), C(45)–N(3)–C(33) 113.9(4).
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possibly, the reduction is slowed down due to increase in the
number of ligand molecules in the reaction mixture leading to
an increase in the size as compared to NPs 1, 2, 4, 5.
Application in Suzuki–Miyaura coupling reaction
The NPs 1–6 have been investigated for the Suzuki–Miyaura
coupling reaction of several aryl halides including aryl iodides,
bromides, and chlorides. To optimize the reaction conditions,
the coupling reaction of 4-bromobenzaldehyde with phenyl-
boronic acid in the presence of NPs 1 was studied (Table 1)
2 3
using various bases and solvents. K CO has been found to be
the most appropriate base as the yield of cross-coupled product
is higher than that obtained with the use of other alternative
bases (Table 1, entry no 4). Of the various solvents investigated,
a mixture of DMF and water was found to be the best solvent
system for the reaction (Table 1, entry no 4) (Scheme 2).
The results of investigations on catalytic activities of NPs 1–6
under optimized parameters have been summarized in Tables 2
and 3. In the coupling reaction of 4-bromobenzaldehyde under
Fig. 2 PXRD pattern of (a) NPs 1, (b) NPs 2, (c) NPs 3, (d) NPs 4, (e) NPs
ꢁ
optimum conditions for 6 hours at 100 C in presence of NPs 1,
5, (f) NPs 6.
2,
3
and 4, the cross-coupled product biphenyl-4-
carboxaldehyde was obtained in more than 99% yield (Table
: entry 7 and Table 3: entry 6). 1-Bromo-4-nitrobenzene was
successfully converted in to 4-nitrobiphenyl in presence of NPs
, 2, 3, 5 as a catalyst in 99%, 99%, 71% and 99% respectively
Table 2: entry 3, Table 3: entry 7). More than 99% yield has been
2
1
(
observed for 4-bromotoluene, 4-bromoaniline and 4-bromo-
benzoic acid with NPs 4 in 6 hours (Table 2: entry 1, 2 and 3). It
has also been assessed that among 1, 2, 3 and 5, NPs 1 (Pd : L ¼
1
: 1) showed maximum efficiency in the coupling of 4-bromo-
phenol (Table 2: entry 5), a very less reactive substrate.
Success has also been achieved in the coupling of chloroar-
enes (least reactive halides), and that too at the low catalyst
loading in 12 hours. For instance, 4-chlorobenzophenone is
converted to 4-phenyl benzophenone in the presence of NP's 1
with 90% yield in 6 hours (Table 2: entry 1). In addition to that,
4
-chlorotoluene is also successfully coupled to yield 4-methyl-
biphenyl in 71%, 62%, 68% with NPs 1, 4 and 6 respectively
Table 2: entry 8, Table 3: entry 4). Interestingly, NPs 1 have been
(
found to be more efficient than NPs 2, 4 and 6 in converting 4-
chlorobenzaldehyde to biphenyl-4-carboxaldehyde in 75% yield
(Table 2: entry 2). NPs
6
were also explored for 4-
Fig. 3 HRTEM images of Pd NPs. (a) 1 (scale bar 10 nm), (b) 2 (scale bar
a
Table 1 Optimization of reaction conditions using NPs 1
50 nm), (c) 3 (scale bar 20 nm), (d) 4 (scale bar 20 nm), (e) 5 (scale bar
0 nm), (f) 6 (scale bar 10 nm).
5
S. no.
Solvent
Base
Yield (%)
1
2
3
4
5
6
7
8
DMF : water (3 : 2)
DMF : water (3 : 2)
DMF : water (3 : 2)
DMF : water (3 : 2)
DMF
Toluene
THF
EtOH : water (3 : 1)
NaOH
42
85
52
95
79
38
47
68
NPs of Pd have been found to be ultra-small which has been
Cs
CH
2
CO
COONa
3
calculated to be ꢀ1–2 nm, 2–3 nm, 4–6 nm, 1–2 nm, 1–2 nm, 3–
3
5
nm respectively for 1–6. The reason for small size and good
K
2
K
2
K
2
K
2
K
2
CO
CO
CO
CO
CO
3
3
3
3
3
dispersion of NPs 1–6 can be attributed to the fast reduction
process by sodium borohydride used for the reduction of Pd(II)
salt. The stabilizing ligands for Pd NPs containing bulky and
so donor sites have been reported to prevent their aggregation
a
21
Reaction
conditions:
4-bromobenzaldehyde
(1.0
mmol),
during growth. The slightly larger size in case of NPs 3 and 6
might be due to high ligand : Na PdCl ratio. In these cases,
phenylboronic acid (1.1 mmol), base (2.0 mmol), solvent (5.0 mL),
temperature; 100 C, time; 6 h.
ꢁ
2
4
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NPs with high catalytic activity. The size of the nanoparticles
1, 2, 4 and 5 is in the range of 1–5 nm. NPs 1 were found to be
the most catalytically active owing to the fact that they have
uniformity in their size and dispersion and the complete
conversion of Pd(II) salt to Pd(0) NPs without the formation of
the PdO NPs as evident from XRD data (Fig. 2). The NPs 3 and 6
showed lower catalytic efficiency, which may be due to their
large size as evident from TEM images (Fig. 3) and high ligand
ratio in comparison to palladium. It might be possible that the
lower number of active catalytic sites is available for coupling
reaction since most of them are blocked by an excess of ligand
molecules.
Scheme 2 Suzuki–Miyaura reaction.
a
Table 2 Suzuki–Miyaura reaction catalyzed by NPs 1, 2 and 3
Ar–X + PhB(OH)
2
/ Ar–Ph (X ¼ Cl or Br)
Yield%
The recyclability of the NPs is one of the distinct advantages
which was explored for 1 and 3. For this purpose, Suzuki
coupling of 4-bromobenzaldehyde for 1 and 1-bromo-4-nitro
benzene for 3 was carried out under optimized reaction condi-
tions. Aer carrying out the reaction, the NPs were centrifuged,
washed with methanol and dried in vacuo. These were reused
for further catalytic runs and it was found that NPs do not lose
their catalytic efficiency even aer 5 runs. The results are
summarized in Table 4.
Entry no.
Aryl halide
NPs 1
NPs 2
NPs 3
b
1
4-Chlorobenzophenone
4-Chlorobenzaldehyde
1-Bromo-4-nitrobenzene
4-Bromobenzonitrile
4-Bromophenol
4-Bromobenzophenone
4-Bromobenzaldehyde
4-Chlorotoluene
90%
75%
99%
99%
91%
88%
99%
71%
NT
NT
NT
b
2
68%
99%
99%
50%
NT
3
4
5
6
7
71%
NT
29%
86%
75%
NT
99%
NT
b
8
a
Some Pd(0) NPs stabilized with mono dentate organo-
Reaction conditions: catalyst, 5 mg; time 6 h; aryl halide 1 mmol;
CO
30,31
K
2
3
, 2 mmol; phenylboronic acid, 1.1 mmol; time 6 h; aqueous chalcogen ligands
are reported earlier. These NPs showed
b
DMF, 5.0 mL. Time 12 h. NT, reaction not tested.
excellent activity towards the coupling of aryl bromides and
iodides. However, the Suzuki–Miyaura coupling of aryl chlo-
rides is less explored. In the present study, Pd NPs have been
found to show the efficient catalytic activity for the coupling of
a wide range of aryl chlorides substrates in addition to aryl
bromides and iodides.
a
Table 3 Suzuki–Miyaura reaction catalyzed by NPs 4, 5 and 6
Ar–X + PhB(OH) / Ar–Ph (X ¼ Cl or Br)
Yield%
NPs
To further understand the catalysis by these Pd(0) NPs,
a mercury poisoning, PPh poisoning and hot ltration tests
3
were conducted. The mercury poisoning test was performed for
a coupling reaction of 1-bromo-4-nitrobenzene with phenyl-
boronic acid using NPs 1 as catalyst under optimum conditions.
The biaryl product was obtained in 99% yield even in the
presence of an excess of mercury. It suggests that the NPs are
protected by ligand and Hg does not have access to interact with
them and catalysis is proceeding via a homogeneous pathway.
Entry no.
Aryl halide
4
NPs 5
NPs 6
1
2
3
4-Bromotoluene
4-Bromoaniline
1-Bromobenzoic acid
4-Chlorotoluene
4-Chlorobenzaldehyde
4-Bromobenzaldehyde
1-Bromo-4-nitrobenzene
4-Bromobenzophenone
4-Bromophenol
99%
99%
99%
62%
84%
99%
NT
NT
NT
NT
NT
NT
NT
NT
NT
NT
NT
99%
99%
66%
99%
NT
NT
99%
99%
NT
61%
NT
68%
71%
NT
NT
NT
NT
NT
b
4
5
b
6
7
8
9
3
For the PPh test, coupling reaction of 1-bromo-4-nitrobenzene
was carried out with phenylboronic acid using NPs 1 as a cata-
lyst. There was only a slight and insignicant decrease in the
yield of the cross-coupled product corroborating the afore-
mentioned homogeneous nature of the catalysis. These results
are also concluding the assumption that the steric effect of
1
1
1
1
1
0
1
2
3
4
4-Bromobenzonitrile
4-Chlorobenzophenone
4-Chlorobenzonitrile
4-Iodoacetophenone
4-Iodoanisole
b
b
72%
76%
NT
NT
NT
NT
NT
a
Reaction conditions: catalyst, 5 mg; time 6 h; aryl halide 1 mmol;
CO
, 2 mmol; phenylboronic acid, 1.1 mmol; time 6 h; aqueous Table 4 Recyclability of NPs 1 and 3
a
K
2
3
b
DMF, 5.0 mL. Time 12 h. NT, reaction not tested.
Reaction cycle
chlorobenzonitrile and 76% conversion was achieved under
standard reaction conditions (Table 3, entry 12).
The results of catalysis corroborate on the observation that
ligand : metal ratio used in the synthesis of NPs affect their
catalytic activity. The reason for good efficiency of each of NPs 1,
Entry no. Aryl bromide
1
2
3
4
6
5
6
b
1
2
4-Bromobenzaldehyde t (h)
6
6
6
Yield (%) 99 99 99 95 95
4-Bromonitrobenzene t (h)
Yield (%) 99 99 95 92 85
c
6
6
6
6
6
2, 4 and 5 may be attributed to the large surface to volume ratio
a
Reaction conditions: catalyst, 5 mg; time 6 h; aryl halide 1 mmol;
CO , 2 mmol; phenylboronic acid, 1.1 mmol; time 6 h; aqueous
and greater shielding of NPs by weakly coordinating ligand
molecules which results in the development of well-dispersed
K
2
3
b
c
DMF, 5.0 mL. NPs 1. NPs 3.
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stabilizing ligands does play a signicant role and prohibit Hg Synthesis of L1 and L2
or PPh to reach the surface of NPs of palladium. Hot ltration
0
0
3
The ligand L1 (0.345 g, 1.0 mmol)/L2 (0.387 g, 1.0 mmol) was
test for the coupling of 4-bromobenzaldehyde with phenyl-
boronic acid using NPs 1 as catalyst under optimum conditions
was carried out in order to understand the leaching of palla-
dium. The reaction mixture was hot ltered aer 10 minutes of
reaction and half of the reaction mixture was quenched and
subjected to NMR. The other half was stirred for another 3 h
under optimum reaction conditions and subjected to NMR. It
was found that the 84% of 4-bromobenzaldehyde was converted
into the product in 10 minutes and aer 3 h a maximum
conversion of 92% was obtained. It suggests that the leaching of
Pd is very low as they are efficiently stabilized by ligands and
hence the reaction in the ltrate continued at an insignicant
rate.
ꢁ
stirred in 20 mL dry ethanol for 30 minutes at 70 C. NaBH
0.079 g, 2.1 mmol) was added and the mixture then reuxed for
0 h. The solvent was removed under reduced pressure on
a rotary evaporator. The residue was extracted with chloroform
and distilled water (100 mL) followed by drying over anhydrous
sodium sulphate. The solvent was evaporated off under reduced
pressure on a rotary evaporator to obtain the product L1 and L2
as light yellow oil.
4
(
1
1
L1. Yield: 0.29 g (83%); light yellow oil. H NMR (500 MHz,
ꢁ
CDCl
3
, 25 C, TMS): d (ppm): 3.01 (t, 2H, J ¼ 12.4 Hz SCH
2
), 3.10
–N), 4.66 (s, 2H, anthra–CH –N),
.14–7.19 (m, 1H), 7.20–7.25 (m, 2H), 7.26 (m, 2H), 7.41–7.44
(
t, 2H, J ¼ 13.05 Hz, C–CH
2
2
7
(
m, 2H), 7.47–7.51 (m, 2H), 7.95 (d, 2H, J ¼ 8.25 Hz), 8.26 (d, 2H,
1
3
1
ꢁ
J ¼ 8.95 Hz), 8.35 (s, 1H). C{ H}NMR (125 MHz, CDCl , 25 C,
3
TMS): d (ppm): 34.1 (SCH ), 45.1 (anthra–CH –N), 48.1 (C–CH –
Experimental section
Physical measurements
2
2
2
N), 124, 124.9, 126.1, 126.2, 127.2, 128.1, 128.8, 130.2, 131.1,
1
31.4, 135.3.
1
13
1
1
ꢁ
The H and C{ H} spectra were recorded on a JNM ECX-500
L2. H NMR (500 MHz, CDCl , 25 C, TMS): d (ppm): 2.98 (m,
3
NMR spectrometer at 500 and 125 MHz respectively. The 4H, SeCH and C–CH –N), 4.56 (s, 2H, anthra–CH –N), 7.09–
2
2
2
chemical shis are reported in ppm relative to the internal 7.13 (d, 3H, J ¼ 4.8 Hz), 7.34–7.38 (m, 4H), 7.42–7.45 (m, 2H),
standard tetramethylsilane. Single crystal X-ray diffraction 7.87 (d, 2H, J ¼ 8.25 Hz), 8.21 (d, 2H, J ¼ 8.95 Hz), 8.25 (s, 1H,
1
3
1
ꢁ
studies were performed on Supernova X-ray diffractometer Ar–H). C{ H} NMR (125 MHz, CDCl , 25 C, TMS): d (ppm):
3
˚
system at 150 K using Mo Ka radiation (0.710 A). CrysalisPro 28.3 (SeCH ), 44.9 (anthra–CH –N), 48.8 (C–CH –N), 123.9,
2
2
2
Soware (online version) was used for data collection. The 124.7, 125.9, 126.8, 127.0, 128.9, 129.2, 130.0, 131.0, 131.1,
structure was solved by direct methods using olex2, SHELXS-97 131.2, 132.9.
and rened by full matrix least-squares with SHELXL-97,
2
rening on F . The image was created using the program
Diamond.
Syntheses of NPs 1–6
2 4
TEM studies were carried out with an FEI Tecnai G2-S-twin The Na [PdCl ] (0.147 g, 0.5 mmol) was dissolved in 50 mL of
electron microscope operated at 200 kV. The specimen for methanol. The solution of ligand L1 [0.173 g, 0.5 mmol (L : Pd ¼
TEM was prepared by dispersing the powder in methanol by 1 : 1)] and L2 [0.194 g, 0.5 mmol (L : Pd ¼ 1 : 1)] for NP's 1 and 4;
ultrasonic treatment, dropping slurry onto a porous carbon lm [0.689 g, 2 mmol (L : Pd ¼ 1 : 4)] of L1 and [0.778 g, 2 mmol of
supported on a copper grid, and then drying in air. Elemental L2 (L : Pd ¼ 1 : 4)] for NPs 2 and 5; [0.0865 g, 0.125 mmol (L : Pd
composition of NPs was analyzed by using an EDX system ¼ 4 : 1)] of L1 and [0.097 g, 0.125 mmol (L : Pd ¼ 4 : 1)] of L2 for
(
model: JSM 6100). The samples were scanned in different NPs 3 and 6 made in 50 mL dichloromethane was added to
regions. Powder X-ray diffraction (PXRD) studies were carried above solution with vigorous stirring. The mixture was further
out on Panalytical XPert diffractometer with Cu ltered radia- stirred for 1 h. A solution of NaBH (2 equivalents) made in
tion using a scan speed of 2 degrees per min and scan step of methanol (5 mL) was added as quickly added to the reaction
.02 degree. The products of catalytic reactions were authenti- mixture and was further stirred for 2 h. The solvent was reduced
4
0
cated by matching their NMR spectral data with those reported to 15 mL on a rotary evaporator. The residue was centrifuged
in the literature. The commercial nitrogen gas was used as and washed three times with methanol. The resulting nano-
received. Nitrogen atmosphere if required was created using particles 1–6 were dried in vacuo.
Schlenk techniques. All reactions were carried out in glassware
dried in an oven, under ambient conditions.
General procedure for catalysis of Suzuki–Miyaura C–C
coupling
Chemicals
To a 100 mL round bottom ask equipped with a reux
2
-(Phenylthio)ethyl amine and 2-(phenylseleno)ethyl amine was condenser, aryl halide (1.0 mmol), phenylboronic acid
37–40
synthesized by reported methods.
2 3
Diphenyldisulphide, (1.1 mmol, 0.133 g), K CO (2.0 mmol, 0.276 g), aqueous
diphenyl diselenide, 2-chloroethylamine hydrochloride, dimethylformamide (5.0 mL) and 5 mg of catalyst were added.
ꢁ
anthracene carboxaldehyde, sodium tetrachloropalladate, aryl The reaction mixture was reuxed at 100 C and the reaction
bromides, iodides and chlorides, phenyl boronic acid, K CO
progress was monitored with TLC. Aer the maximum conver-
2
3
and NaBH were procured from Sigma-Aldrich (USA) and used sion of aryl halide into the product has been accomplished, the
4
as received. The solvents were dried and distilled before use by reaction mixture was allowed to cool to room temperature. It
standard procedures.
was mixed with water and extracted with diethyl ether. The
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2 4
organic phase was dried over anhydrous Na SO . The solvent
Conclusions
was removed on a rotary evaporator under reduced pressure and
1
product was subjected to H NMR studies. In the case of impure In summary, ligands L1 and L2 have been synthesized and
1
13
1
product, further purication was carried out by column chro- characterized by H and C{ H} NMR spectroscopy. The
matography on silica gel using an ethyl acetate and hexane structure of L1 has also been supported by single crystal X-ray
mixture as the eluent.
diffraction studies. The ligand L1 and L2 were further used as
stabilizers for syntheses of catalytically active and recyclable
palladium NPs 1–6. The powder X-ray diffraction (PXRD),
HRTEM, and SEM-EDX techniques were used to authenticate
NPs 1–6. HRTEM study of NPs 1–6 suggests the formation of
ultra-small nanoparticles of size between 1–6 nm. These NPs
have been explored as catalysts for Suzuki–Miyaura cross
coupling reaction and found to be efficient for coupling of aryl
bromides and chlorides. NPs 1 stabilized by thioether ligand
have shown maximum catalytic efficiency amongst all due to
the relatively small size (ꢀ1–2 nm), uniformity in the dispersion
and high purity. The NPs stabilized by secondary amine con-
taining sulphur donor atom are slightly more efficient than that
of selenium analogue. The distinct advantage of these nano-
particles as a catalyst is their recyclability. The catalytic pathway
Catalytic recyclability
An oven-dried round bottom ask was charged with 4-bromo-
benzaldehyde (1 mmol), phenylboronic acid (1.1 mmol, 0.133
g), K CO (2.0 mmol, 0.276 g) and aqueous DMF (5.0 mL). Nano-
2 3
particles (10 mg of 1), were added to the reaction mixture and
ꢁ
the temperature was maintained at 100 C. The progress of the
reaction was monitored with TLC. Aer completion of the
reaction, water and ethyl acetate were added and the mixture
was centrifuged. The black residue was collected and washed
with methanol in order to remove the organic content and base.
The resulting nanoparticles were dried in vacuo. The organic
layer from water–ethyl acetate was separated and the solvent
was evaporated off. The residue was subjected to H NMR for
conversion estimation. The NPs separated were reused for the
next cycle and this procedure was repeated for ve times.
1
3
as suggested by the Hg and PPh poisoning tests is proceeding
via homogeneous mode. These tests also suggest that the
anthracene unit being bulky in nature stabilize the NPs so
efficiently that Hg/PPh does not reach the catalyst surface to
3
poison them. At the same time, they allow the reactant mole-
cules to reach the NPs surface.
Hg poisoning test
To carry out the Hg poisoning test, catalyst (NPs 1; 5 mg) was
stirred with an excess of Hg (Hg : Pd; 500 : 1) in an oven dried

ask for 15 minutes before the addition of the coupling reac-
Conflicts of interest
tants. Thereaer, 4-bromobenzaldehyde (1.0 mmol, 0.185 g),
phenylboronic acid (1.1 mmol, 0.133 g) and K CO (2 mmol)
There are no conicts to declare.
2
3
were added to the ask and reaction was carried out under
optimum conditions. Reaction progress was monitored with
TLC. The reaction was further continued for 10 hours at 100 C.
Acknowledgements
ꢁ
The used mercury was collected and stored safely.
Authors acknowledge SAIF, PU for EDS facility. P. O. acknowl-
edge DST for INSPIRE Fellowship [DST/INSPIRE Fellowship/
2
017/IF170491]. A. A. and A. K. also acknowledge DST SERB
PPh
3
poisoning test
[ECR/2016/001549] for fellowship and research grant. A. K. also
PPh
3
(5 equivalents) was added under optimum conditions to
thanks University Grants Commission for nancial support in
the form of UGC-BSR-Start-Up Research Grant Vide Letter No.
F.30-371/2017 (BSR), dated 10 July 2017. SK would like to thank
DST for INSPIRE Faculty Award [DST/INSPIRE/04/2015/002971].
the catalyst (1 equivalent) and stirred to 15 minutes before the
addition of the coupling substrates 4-bromobenzaldehyde
(1.0 mmol, 0.185 g) and phenylboronic acid (1.1 mmol, 0.133 g).
Aer 10 hours, the cross-coupled product was obtained in 99%
yield.
Notes and references
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