Support-Free Pd3Co NCs as an Efficient Heterogeneous Nanocatalyst for New Organic Transformations of C?C Coupling Reactions

Article abstract of DOI:10.1002/chem.201901834

A support-free heterogeneous Pd₃Co nanostructured composite (NC), synthesized through a hydrothermal route, acted as an effective catalytic system in multivariate Heck-, Sonogashira-, and Suzuki-type coupling reactions of iodonium ylides. The XPS analysis of the bimetallic Pd₃Co NCs confirmed the elemental composition as 75 % palladium and 25 % cobalt. Furthermore, high-resolution (HR) TEM analysis confirmed the spherical morphology of the Pd₃Co bimetallic nanoparticles. The average diameter of the NCs is 14.8 nm. The coupling reaction proceeded through the generation of α-iodoenones with simultaneous migration of the phenyl group, thereby giving a scaffold with higher atom economy. The heterogeneous Pd₃Co NCs were recycled and reused without any significant change in catalytic ability for up to five reaction cycles. The high concentration of Pd and association of cobalt into the lattice of palladium appears to enhance its catalytic ability for the diverse coupling reactions in comparison with its monometallic counterparts as well as with bimetallic NCs with a comparatively lesser amount of Pd.

Full text of DOI:10.1002/chem.201901834

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Accepted Article
Title: Support-free Pd3Co NCs as an efficient heterogeneous
nanocatalyst for novel organic transformations of C-C coupling
reactions
Authors: Samjhana Pradhan, Kanchan Mishra, and Yong Rok Lee
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Support-free Pd3Co NCs as an efficient heterogeneous
nanocatalyst for novel organic transformations of C-C coupling
reactions
Samjhana Pradhan, Kanchan Mishra and Yong Rok Lee*
Dedication ((optional))
Abstract: A support-free heterogeneous Pd3Co nanostructured
composite (NC), synthesized via hydrothermal route, acted as an
effective catalytic system in multivariate Heck, Sonogashira and
Suzuki-type coupling reactions of iodonium ylides. The XPS analysis
of the bimetallic Pd3Co NCs confirmed the elemental composition as
75% palladium and 25% cobalt. Furthermore, HRTEM analysis
confirmed the spherical morphology of Pd3Co bimetallic
nanoparticles. The average diameter of the NCs is 14.8 nm. The
coupling reaction proceeded via the generation of α-iodoenones with
simultaneous migration of the phenyl group thereby giving a scaffold
with higher atom economy. The heterogeneous Pd3Co NCs were
[19a] Dabiri et al. [19b] and Xu et al. [19c] have reported separately on
the synthesis of Pd-Co bimetallic NCs with graphene as a
support material for Sonogashira and Suzuki reactions. However,
the preparation of such supported nanostructured catalysts
[20]
involves complicated multistep synthesis procedures.
In
addition, the catalytic behaviour of the supported nanostructured
catalysts is affected by various factors, such as, choice of
[21a]
support materials, particle size and preparation methods.
Nonetheless, such kind of effects are negligible in support-free
nanostructured composites, and henceforth, these are still
considered as an important facet in the field of heterogeneous
recycled and reused without any significant change in catalytic ability, catalysis. [22]
for up to five reaction cycles. The high concentration of Pd content
and association of cobalt into the lattice of palladium appears to
enhance its catalytic ability for the diverse coupling reactions in
comparison with its monometallic counterparts as well as bimetallic
NCs with comparatively lesser amount of Pd.
Introduction
C-C bond formation is one of the most important processes in
synthetic organic chemistry. The most common and well-known
types are Suzuki,[1-3] Heck, [4,5] Sonogashira coupling reactions, [6]
and so on. These provide facile and highly efficient route for the
synthesis of natural products, polymers, [7,8] agrochemicals and
pharmaceuticals. [9,10] Since its discovery innumerable methods
have been reported for coupling reactions with burgeoning use
of palladium bimetallic nanostructured composites. The
synergistic effect imparted by the bimetallic nanostructured
composites (NCs) due to their tunable components and
morphologies have attracted significant interest in the field of
catalysis. The latter is attributed to the electronic interaction that
interplays between the two metal atoms. [11,12]
Scheme 1. Synthesis of Pd3Co NCs via hydrothermal route and their catalytic
application as C-C coupling of iodonium ylides.
Various studies have revealed that the incorporation of
cheap as well as easily available 3d – transition metals with Pd
to form Pd-M NCs (M = Co, Fe, Cu, Ni) [13-17] is one of the most
efficient approaches to increase the catalytic activity and
utilization of Pd metal.[18] In this context, Pd-Co bimetallic NCs
have shown remarkable performance as heterogeneous
nanostructured catalyst in coupling reactions. Shaaboni et al.,
A number of studies have been reported previously on the
synthesis of naked/ unsupported nanostructured particles (NPs)
for organic reactions [21b-21d]. Zhang et al. [22] and Ordomsky et al.
[23]
have recently designed naked IrO2 NPs and unsupported Ru
NPs for hydrogenation of nitrogen heterocycles and amination of
alcohol, respectively. In this case, the NPs were reported to
exhibit high selectivity and stability without loss of activity or
metal leaching. The support-free nanostructured catalyst
possesses large number of free metallic surface sites that
provides a platform rendering the catalytic activity more feasible
by the elimination of support effect. [24] Similarly, our group have
also previously designed transition metal based bimetallic
S. Pradhan, K. Mishra, Prof. Y. R. Lee
School of Chemical Engineering Yeungnam University, Gyeongsan, 38541
Republic of Korea
E-mail: yrlee@yu.ac.kr
Supporting information for this article is given via a link at the end of the
document. ((Please delete this text if not appropriate))
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nanoparticles. These heterogeneous composites proved to be
highly effective catalyst for various organic transformations.
not observed. To the best of our knowledge, there have been no
reports of transition metal-catalyzed or nanocatalyzed phenyl
migration and direct coupling reactions of iodonium ylides for the
construction of diverse organic compounds with higher atom
economy. The present work is the first of its kind to synthesize
support-free Pd3Co NCs for the simultaneous phenyl migration
and direct Heck-, Sonogashira-, and Suzuki-type coupling
reactions of iodonium ylides with acrylates, arylalkynes, and
boronic acids, respectively (Scheme 2d–2f).
[25]
Based on these approaches, herein, we have developed a facile
and green approach for the one pot synthesis of support-free,
palladium rich bimetallic Pd3Co NCs via a hydrothermal method
using ethylene glycol as a mild reducing agent (Scheme 1).
Pd3Co NCs have been used mostly to enhance the
rechargeability of Li-O2 cell, [26a] for oxygen reduction reaction on
[26b]
[26c]
Li-O2 cathode cell,
and as hydrogen storage material.
However, reports on the use of support-free Pd3Co NCs as a
catalyst in organic transformations are rarely known. In this
regard, we have attempted to use Pd3Co NCs as a catalyst for a
variety of coupling reactions of iodonium ylides.
Results and Discussion
Synthesis of Pd3Co NCs
Pd3Co NCs were synthesized via one pot hydrothermal method
using palladium(II) acetate and cobalt(II) acetate tetrahydrate as
the metal precursors, and ethylene glycol as the reducing agent.
The hydroxyl groups of ethylene glycol reduced the Pd2+ and
Co2+ to its respective metallic forms, forming Pd3Co NCs.
Characterization of Pd3Co NCs
To identify the crystalline phase and composition of Pd3Co NCs,
XRD patterns of the catalysts were obtained (Figure 1a). They
displayed typical Pd face-centred cubic (fcc) features without
[33]
separate individual peaks for cobalt and its oxide.
The
diffraction peaks corresponding to (111), (200), and (220) planes
are consistent with those of pure Pd metals with an fcc structure
(JCPDS no. 46-1043). The shift in the peak positions, compared
to that of pure Pd, is subtle resulting in a bimetallic composition
[34]
with small lattice contraction.
As the nanocomposites were
not treated at high temperatures, a prominent shift in the relative
peak angle did not occur.
Scheme 2. Reported and novel strategies for the C-C coupling reactions of
iodonium ylides.
Iodonium ylides comprise an important branch of organic
[27]
hypervalent iodonium chemistry.
They are widely used as
[28]
analogues of the corresponding diazo compounds
and are
highly resourceful compounds in organic synthesis. [29] Recently,
direct cyclization of iodonium ylides with tertiary aryl amines for
N-heterocycles has been reported by Wang group (Scheme 2a).
[30] In addition, Takemoto and co-workers have demonstrated the
Umpolung alkylation of silyl enol ethers with iodonium ylides
[31]
(Scheme 2b).
However, direct coupling reactions of such
Figure 1. (a) XRD patterns (inset: high resolution peak), (b) XPS peaks of
Pd3Co, PdCo, and Pd, (c) Survey scan, inset: atomic percentage of Pd and Co
in Pd3Co NCs, and (d) Co 2p electrons of Pd3Co NCs.
iodonium ylides are relatively unknown. In 2002, Yang and co-
workers reported the Pd-catalyzed reaction of iodonium ylides
with aryl boronic acids for the synthesis of 3-aryl-4-
hydroxycoumarins (Scheme 2c).[32] In this case, coupled
products via simultaneous migration of the phenyl group were
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XPS studies of support-free Pd3Co NCs were carried out to
determine the elemental composition of the nanocatalyst and the
valence state of the metals in the nanoparticles. The XPS
spectra over a full range scan revealed the presence of Pd, Co,
and Oxygen, with an elemental composition of Pd and Co as
75% and 25% (Figure 1c). The high resolution XPS spectrum of
the Pd 3d was deconvoluted into Pd 3d5/2 and Pd 3d3/2 peaks.
Each subshell exhibits two different valence states. i.e. Pd2+ and
Pd0. The characteristic peaks at 335.2 and 340.5 eV explain the
presence of metallic Pd in the nanocomposites with a spin orbit
coupling constant at 5.3 eV. While the peaks observed at 337.2
and 342.4 eV can be attributed to the presence of a small
amount of Pd (II) in the catalyst (Figure 1b). Similarly, the peak
at 797.4 and 781.5 eV confirmed the formation of Co (0) in the
NCs (Figure 1d). Furthermore, we have compared the XPS
spectra of PdCo and Pd with that of the Pd3Co NCs. The peak
shifting towards the higher binding energies, with decrease in
cobalt content, indicate the synergistic effect of Pd with Co in
Pd3Co (Figure 1b).[35]
The morphology and structure of the nanocomposites were
determined by transmission electron microscopy (TEM). HRTEM
images of Pd3Co, and PdCo NCs, and of Pd NPs are included
as Figures 2a–2c, respectively. In all three cases, the spherical
shape of the particles is evident. Furthermore, Pd3Co NCs have
spherical shapes (Figure 3a) with diameters ranging from 12 to
18 nm, with an average size of approximately 14.8 nm (Figure
3b). In addition, the HRTEM images clearly depict the lattice
fringes of palladium and cobalt. The lattice fringe with a
measured spacing of 0.22 nm corresponds to the (111) atomic
plane of Pd, whereas the lattice fringe of the (101) plane (d =
0.19 nm) can be attributed to Co (Figure 3c). The high-angle
annular dark-field scanning TEM (HAADF-STEM) elemental
map reveals a homogeneous distribution of Pd (blue color) and
Co (yellow color). This indicates the spatial arrangement of
bimetallic Pd3Co NCs (Figure 3d). Likewise, the energy-
dispersive X-ray spectroscopy (EDS) coupled with TEM
displayed characteristic peaks for Pd, Co, and O, as well as Cu
and C due to the carbon-coated copper grid, which agrees with
the compositional data from XPS (Figure 3e).
The initial weight loss (1.53 %) below 200 °C can be
attributed to the loss of volatile material and adsorbed moisture
on the Pd3Co NCs, whereas the abrupt weight loss (20.38 %)
after 200 °C may be due to the removal of the residual ethylene
glycol adhered to the surface of the NCs.
Catalytic activity of Pd3Co NCs
Figure 2. HRTEM image of (a) Pd3Co NCs (b) PdCo NCs and (c) Pd NPs at 5
nm scale. Inset: corresponding SAED pattern.
In order to demonstrate the applications and catalytic activity of
the Pd3Co NCs, Heck, Sonogashira, and Suzuki coupling
reactions of iodonium ylides were carried out as model reactions.
For these reactions, a variety of cyclic six-membered iodonium
ylides 1a–1m (Chart 1) was prepared (see the SI).
Chart 1. Six-membered cyclic iodonium ylides 1a-1m.
To optimize the reaction conditions, we investigated the coupling
of 5,5-dimethyl-2-(phenyl-λ3- iodanylidene)cyclohexane-1,3-
dione (1a) and ethyl acrylate (2a) via the α-iodoenone
intermediate (3a′), using different catalysts, solvents, and bases
(Table 1). First, commercially available catalysts were screened.
After refluxing 1a in toluene for 20 min, under inert conditions,
2a was added, followed by the addition of 2 mol% Pd(OAc)2 and
2.0 equiv. of TEA, which afforded 3a, 3a′, 3a″, and 3a‴ in 15, 10,
59, and 12%, respectively, after 4 h (entry 1). Meanwhile, using
2 mol% Pd/C furnished 50, 30, 10, and 15% yields, respectively,
(entry 2). Interestingly, the unexpected side product 3a‴ was
generated by the involvement of reaction of only phenyl group
moiety on ylide 1a with 2a in low yield. With monometallic Pd
Figure 3. (a) TEM image of Pd3Co NCs at 20 nm (b) Histogram showing
average diameter of Pd3Co NCs (c) HRTEM showing d-spacing (d) HAADF-
STEM image of Pd3Co NCs along with the EDS elemental maps of Pd (blue
color) and Co shell (yellow color) and (e) EDX peaks of Pd3Co NCs.
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and Co NPs, compounds 3a, 3a′, and 3a‴ were obtained in 42,
30, and 10%, and 30, 51, and 7% yields, respectively (entries 3
and 4). Interestingly, the use of bimetallic PdCo NCs (2 mol%)
afforded 3a in increased yield (80%), together with 3a′ (9%) and
3a‴ (4%) (entry 5). This may be attributed to the coordination
between Pd and Co atoms in the PdCo NCs that allows the high
mobility of the electrons to vacant 5s, 5d and 5f-orbitals of Pd,
partially filled 3d, and vacant 4p, 4d and 4f orbitals of Cobalt
atom. The best yield of 3a (92%) was obtained using Pd3Co NCs
(2 mol%) and 2.0 equiv. of TEA (entry 6) due to the synergistic
effect of Pd with Co metal in the Pd3Co NCs. However, lowering
Table 1. Optimization of Heck coupling of iodonium ylidesa
.
o
the catalyst loading to 1 mol% and temperature to 80 C did not
improve the yield of desired product 3a. These results indicated
the importance of the amount of catalyst loading and optimum
temperature (entries 7-8). When TEA was replaced with
inorganic bases K2CO3 and Cs2CO3, 3a was not isolated; instead
3a′ was produced in 72-75% yields together with 3a‴ in 15-25%
yields (entries 9 and 10). In the absence of TEA, only 3a′ was
isolated, in 85% yield (entry 11). Changing the solvent to DMF
under reflux conditions afforded 3a (81%), and 3a‴ (15%) (entry
12). However, refluxing in THF did not produce 3a, affording
instead 3a′ and 3a‴ in 80 and 5% yields, respectively (entry 13).
Less coordinating solvents such as DMF and toluene are
preferable for the formation of desired product 3a. Likewise,
coordinating solvent THF provides stabilization to the
intermediate 3a’ which prevents further formation of product
3a.[36] Furthermore, performing the reaction as a one-pot
procedure afforded, not only the desired product 3a but also 3a‴
in 65 and 20% yields, respectively (entry 14).
a Reaction conditions: 1a (0.25 mmol) in toluene (3 mL) was refluxed for 20
min under N2 atmosphere, followed by the addition of 2a (0.75 mmol). Then,
catalyst and base (2.0 equiv.) were added. b Isolated yields. c1a (0.25 mmol),
2a (0.75 mmol), catalyst (2 mol%), and TEA (2.0 equiv.) were refluxed in
toluene (3 mL) for 4 h under N2 atmosphere.
Table 2. Substrate scope of Heck coupling.
To explore the generality of the Heck-type coupling
reaction, further reactions of different iodonium ylides and
various acrylates were examined under the optimized conditions
(Table 2). For example, the reaction of 5,5-dimethyl-2-(phenyl-
λ3-iodanylidene)cyclohexane-1,3-dione (1a) with methyl acrylate
(2b), t-butyl acrylate (2c), or phenyl acrylate (2d) afforded the
desired products 3b–3d in 89–91% yields. Similarly, treatment of
iodonium ylides 1b or 1e, bearing no substituent or phenyl group
on the cyclohexanedione ring, with 2a, produced 3e and 3f in 89
and 96% yields, respectively. In addition, the reaction of 7-
(phenyl-λ3-iodanylidene)spiro[3.5]nonane-6,8-dione (1h) bearing
cyclobutyl group on the cyclohexanedione ring with ethyl
acrylate (2a) gave the desired product 3f in 72% yield.
Combination of 3-(phenyl-λ3-iodanylidene)chromane-2,4-dione
(1k) with ethyl acrylate (2a), phenyl acrylate (2d), or phenyl
methacrylate (2e) afforded compounds 3h–3j in 92–97% yields,
and that of 1-methyl-3-(phenyl-λ3-iodanylidene)quinoline-
2,4(1H,3H)-dione (1l) with 2b produced 3k in 78% yield.
Reaction of 2-(phenyl- λ3-iodanylidene)-1H-phenalene-1,3(2H)-
dione (1m) with 2a successfully produced 3l in 96% yield.
Moreover,
treatment
of
5,5-dimethyl-2-(p-tolyl-λ3-
iodanylidene)cyclohexane-1,3-dione (1g) bearing 4-Me group on
the benzene ring with 2a provided the desired product 3m (78%).
aReaction conditions: 1 (0.25 mmol) in toluene (3 mL) was refluxed for 20 min
under N2 atmosphere, followed by the addition of 2 (0.75 mmol). Then, Pd3Co
NCs (2 mol%) and TEA (2.0 equiv.) were added and refluxed for 4h. bIsolated
yields.
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Having proved the generality of this protocol in C-C bond
forming reaction, we investigated the possibility of Sonogashira
type coupling reactions of iodonium ylides (1a, or 1d–1f, or 1j-
1m) with phenylacetylene (4a) and arylacetylenes 4b–4d (Table
3). Combination of iodonium ylides 1a, 1d, 1e, 1f, or 1j, bearing
various substituents on the cyclohexanedione ring, with 4a,
successfully produced the desired products 5a–5e in 80–90%
yields. Treatment of 3-(phenyl-λ3-iodanylidene)chromane-2,4-
dione (1k) with phenylacetylene (4a), 4-ethynyltoluene (4b), or
1-ethynyl-4-fluorobenzene (4c) afforded products 5f–5h in 95,
91, and 90% yields, respectively, whereas that with 2-
ethynylthiophene (4d) produced 5i in 85% yield. Combination of
1-methyl-3-(phenyl-λ3-iodanylidene)quinoline-2,4(1H,3H)-dione
(1l) with 4a afforded 5j in 90% yield, and that of 1m with 2a
produced 5k in 86% yield.
combined with 1a, products 7d and 7e were isolated in 91 and
79% yields, respectively. Further combination of 1b-1c, 1e, 1g-
1i, or 1k-1m, with 6a or 6b afforded 7f–7n in 75–97% yields.
To illustrate the mechanism of these coupling reactions,
control experiments were conducted on 5,5-dimethyl-2-(phenyl-
λ3-iodanylidene)cyclohexane-1,3-dione, 1a (Scheme 3). When
1a was refluxed in toluene for 20 min, 2-iodo-3-
phenoxycyclohex-2-en-1-one (3a′) was isolated in 90% yield
(Scheme 3, eq. a). Further reaction of 3a′ with 2a, under the
optimized reaction conditions, afforded 3a in 93% yield (Scheme
3, eq. b). These results suggest that 3a′ is the intermediate, and
that the coupling reaction proceeds via the generation of α-
iodoenone.
Table 4. Substrate scope of Suzuki coupling.
Table 3. Substrate scope of Sonogashira coupling.
aReaction conditions: 1 (0.25 mmol) in toluene (3 mL) was refluxed for 20 min
under N2 atmosphere, followed by the addition of 4 (0.375 mmol). Then,
Pd3Co NCs (2 mol%) and TEA (2.0 equiv.) were added and refluxed for 4h.
bIsolated yields.
aReaction conditions: 1 (0.25 mmol) in toluene (3 mL) was refluxed for 20 min
under N2 atmosphere, followed by the addition of 6 (0.3 mmol). Then, Pd3Co
NCs (2 mol%) and K2CO3 (2.0 equiv.) were added and refluxed for 4h.
bIsolated yields.
To investigate the versatility of the catalyst, Suzuki-type coupling
reactions were next attempted with arylboronic acids (Table 4).
In contrast with the results of the Heck and Sonogashira-type
coupling reactions, treatment of 1a with phenylboronic acid (6a),
under the optimized reaction conditions, using TEA as base,
produced lower yield of the product 7a (40%). However, when
TEA was replaced with 2 equiv. of K2CO3, 7a was produced in
high yield (93%). For reactions of 1a with 4-
methoxyphenylboronic acid (6b) and 4-fluorophenylboronic acid
(6c), bearing electron-donating and withdrawing groups, the
desired products 7b and 7c were isolated in 91 and 80% yields,
respectively. As observed for other arylboronic acids, when 2-
naphthylboronic acid (6d) and 2-thienylboronic acid (6e), were
On the basis of this control experiment, and the literature
concerning Heck, Sonogashira, and Suzuki coupling reactions, a
plausible mechanism for the coupling of iodonium ylides 1a with
2a, 4a, or 6a is depicted in Scheme 4. For the Heck coupling
cycle, first, when refluxed, iodonium ylide 1a produces stable
[37]
intermediate 3a′ through 1,3-phenyl migration.
Then, the
Pd3Co NCs undergo oxidative addition to 3a′ to form complex 8.
The coordination of olefin 2a with 8 produces π-complex 9,
which undergoes olefin insertion, followed by β-H elimination, to
afford 11. Then, the active catalyst is regenerated through
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reductive elimination with the basic triethylamine, and 3a is
isolated. In Sonogashira coupling, the intermediate 8 reacts with
phenylacetylene (4a) to produce complex 12. The terminal
alkyne of complex 12 is deprotonated to yield 13, which affords
final product 5a and regenerates the active catalyst through
reductive elimination. In the same way, for Suzuki coupling, in
the presence of K2CO3, complex 8 yields the intermediate 14,
which reacts with the boronate complex 15, formed by in-situ
reaction of boronic acid and K2CO3, to form complex 16 via
transmetallation. Reductive elimination liberates the active
nanocatalyst and affords final product 7a.
Recyclability and reusability test
One of the important properties of the prepared Pd3Co
nanocatalyst is its recoverability and reusability. The catalyst
was recycled and reused five times. The recyclability test was
carried
out
using
5,5-dimethyl-2-(phenyl-λ3-
iodanylidene)cyclohexane-1,3-dione (1a) in 1 mmol scale with
corresponding amount of 2a for the Heck coupling reaction. In
each cycle, after the completion of the reaction, the mixture was
decanted. The mother liquor was transferred into a conical flask
and the catalyst was recovered by centrifugation and
subsequently washed with ethanol and dried in a hot air oven,
prior to use for another catalytic cycle. The ethanolic wash was
concentrated and added to the mother liquor for yield
determination by column chromatography. However, after the
fifth cycle, there was a slight decrease in the yield, which may be
due to blocking of the active sites of the nanocomposites during
the coupling reaction (Figure S2, see the SI). The nanocatalyst
displayed high catalytic efficiency without any morphology and
structural changes, which was confirmed by the respective TEM
(Figure S3a, see the SI) and XRD analyses (Figure S3b, see SI).
Scheme 3. Control experiments.
Conclusions
Support-free Pd3Co nanostructured composites were prepared
via a hydrothermal approach, using readily available ethylene
glycol as a mild reducing agent. The structural and catalytic
properties of the nanostructured composites were investigated.
XPS studies revealed that the elemental composition of the
nanocomposites was 75% palladium and 25% cobalt. The
Pd3Co nanostructured composite exhibited enhanced selectivity
with highest yield of coupled products. Moreover, the catalytic
performance of support-free Pd3Co has an edge over its
monometallic counterparts and bimetallic PdCo catalyst
particularly in view of Heck, Sonogashira, and Suzuki-type
coupling reactions of iodonium ylides. Pd3Co NCs has higher
percentage of Pd indicating an enhanced amount of Pd in the
catalyst can induce effective synergistic effect with Co metal
thereby enhancing the overall catalytic efficiency. The coupling
reaction hence becomes highly atom economy, which proceeds
via the generation of α-iodoenones along with simultaneous
migration of the phenyl group.
Experimental Section
Materials and Methods
All chemicals were commercially available and used as received, without
further purification. Palladium(II) acetate, cobalt(II) acetate tetrahydrate,
iodobenzene, ethyl acrylate, phenyl boronic acid, phenylacetylene,
potassium carbonate, triethylamine, and toluene were obtained from
Sigma-Aldrich.
Scheme 4. Mechanistic pathway for the multivariate C-C coupling of iodonium
ylides.
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Synthesis of support-free bimetallic Pd3Co NCs
Pd3Co nanocatalyst (2 mol%), arylalkynes (4a–4d), (1.2 equiv.), and TEA
(2.0 equiv.) were added. After completion of the reaction, determined by
TLC, the solvent was removed and the residue was purified by column
chromatography on silica gel using hexane:ethyl acetate = 19: 1 as the
solvent system, to afford the desired product. The structures of the
products were assigned by spectroscopic analysis (see SI).
Pd3Co NCs were prepared using one pot hydrothermal method with
palladium(II) acetate (Pd(OAc)2) and cobalt(II) acetate tetrahydrate
(Co(OAc)2∙4H2O) as the metal precursors, and ethylene glycol as the
reducing agent. The calculated molar ratios of the precursor salts were
added to a conical flask containing deionized water. The resulting mixture
was sonicated for approximately an hour and transferred into a Teflon
cup with a 50 mL inner volume. The autoclave was sealed carefully and
heated to 120 °C, for 5 h, in a hot air oven. After completion of the
hydrothermal process, the reaction mixture was allowed to cool at room
temperature, and the resulting suspension was centrifuged at 12,000 rpm
for 15 min. The residue was washed several times with DI water and
ethanol, and dried in a hot air oven, under vacuum. The dried material
was collected in a storage vial for further use. A similar synthetic
procedure was adapted for the preparation of Pd NPs, Co NPs and PdCo
NCs, with the use of 12 mg of Co(OAc)2∙4H2O for the latter case.
General procedure for the Suzuki coupling between iodonium ylides
and boronic acids
2-(Phenyl-λ3-iodanylidene)cyclohexane-1,3-dione (1a–1c, 1e, 1g–1i, 1k-
1m) (0.25 mmol) and toluene (3 mL) were added to a round bottom flask
equipped with a magnetic stirrer and reflux condenser. The mixture was
refluxed in an oil bath for 20 min under nitrogen atmosphere, then the
Pd3Co nanocatalyst (2 mol%), boronic acid (6a–6e) (1.2 equiv.), and
K2CO3 (2.0 equiv.) were added. After completion of the reaction,
determined by TLC, the solvent was removed and the residue was
purified by column chromatography on silica gel using hexane:ethyl
acetate = 19: 1 as the solvent system, to afford the desired product. The
structures of the products were assigned by spectroscopic analysis (see
SI).
Characterization of support-free bimetallic Pd3Co NCs
X-ray diffraction (XRD) was performed on powdered samples on a
PANalytical X'Pert PRO MPD (operating at 40 kV and 30 mA, with Cu Kα
as the X-ray source (λ = 1.5406 Å) over the 2θ angle range of 20° to 90°,
and
a scan rate of 1.2°/min). The elemental composition of the
Conflicts of interest
nanoparticles and the chemical state of each element were analyzed by
X-ray photoelectron spectroscopy (XPS) using a Thermo Scientific, K-
Alpha system fitted with an Al Kα X-ray light source. The ion source
energy ranged from 100 eV to 3 KeV for the survey. The size of the
nanoparticles was determined by field emission transmission electron
microscopy (FE-TEM, FEI Tecnai F20). The samples were prepared by
placing a drop of the nanoparticle solution on a carbon-coated copper
grid and allowing the solvent to evaporate in air at ambient temperature.
The elemental make-up of the bimetallic nanoparticles was analyzed by
high-angle annular dark-field scanning TEM energy-dispersive X-ray
spectroscopy (HAADF-STEM-EDS) at 200 kV with a point resolution of
0.24 nm, a Cs and Cc of 1.2 mm each, and a focal length of 1.7 mm,
using a Genesis liquid nitrogen cooled EDS detector with an ultrathin
window. The thermal properties of the nanoparticles were analyzed by
thermogravimetric analysis coupled with differential scanning calorimetry
(DSC-TGA SDT-Q600 V20.5 Build 15) by heating under a N2 atmosphere
to 1000 °C at a heating rate of 10 °C min-1.
There are no conflicts to declare.
Acknowledgements ((optional))
This work was supported by the National Research Foundation
of Korea (NRF) grant funded by the Korea government (MSIT)
(2018R1A2B2004432) and the Priority Research Centers
Program (2014R1A6A1031189). This research was supported
by the Nano Material Technology Development Program of the
Korean National Research Foundation (NRF) funded by the
Korean Ministry of Education, Science, and Technology
(2012M3A7B4049675).
Keywords: Pd3Co • Iodonium ylide • C-C coupling •
Nanocatalyst • Heterogeneous
General procedure for the Heck coupling between iodonium ylides
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Entry for the Table of Contents (Please choose one layout)
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A green protocol for support-free and
versatile Pd3Co nanocomposites is
designed using ethylene glycol. The
synergistic effect of the
nanocomposites further shows
enhanced catalytic activity for the
multivariate Heck, Sonogashira and
Suzuki-type coupling reactions of
iodonium ylides
S. Pradhan, K. Mishra, Y. R. Lee*
Page No. – Page No.
Support-free Pd3Co NCs as an
efficient heterogeneous nanocatalyst
for novel organic transformations of
C-C coupling reactions
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