Ligand and additive free aerobic synthesis of diynes using Pd-CuFe2O4magnetic nanoparticles as an efficient reusable catalyst

Article abstract of DOI:10.1039/d0nj04133e

Herein, we present the synthesis of Pd-CuFe2O4 magnetic nanoparticles as an efficient and recyclable catalyst for the oxidative homocoupling of various terminal alkynes to form symmetric 1,3-diynes. The catalyst was found to be effective with very low palladium loading. The protocol has the advantages of easy synthesis of the catalyst, mild reaction conditions, short reaction times, excellent product yields and enhanced recyclability of the catalyst.

Full text of DOI:10.1039/d0nj04133e

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Ligand and additive free aerobic synthesis of
diynes using Pd–CuFe O magnetic nanoparticles
2
4
Cite this:DOI: 10.1039/d0nj04133e
as an efficient reusable catalyst†
Rituparna Chutia
and Bolin Chetia
*
Received 16th August 2020,
2 4
Herein, we present the synthesis of Pd–CuFe O magnetic nanoparticles as an efficient and recyclable
Accepted 26th September 2020
catalyst for the oxidative homocoupling of various terminal alkynes to form symmetric 1,3-diynes. The
catalyst was found to be effective with very low palladium loading. The protocol has the advantages of
easy synthesis of the catalyst, mild reaction conditions, short reaction times, excellent product yields and
enhanced recyclability of the catalyst.
DOI: 10.1039/d0nj04133e
rsc.li/njc
2
9–36
copper
But these processes involve multiple steps in the synthesis
Diynes are useful building blocks in the synthesis of organic of the catalyst and this increases the overall cost, making the
along with some high molecular weight ligands.
1
. Introduction
1
2–4
5
conductors, macrocyclic annulenes, carbon-rich materials,
process tedious and expensive; the reactions also normally required
supramolecular switches, conducting polymermaterials, etc. longer reaction times with sufficiently high temperatures.
These compounds possess prominent biological activities such In accordance with the principles of Green chemistry,
as antimicrobial, antitumor, antibacterial, and anti-HIV, to scientists have always tried to find an alternative to toxic
name a few; thus, significant attention has been devoted to the chemicals using recyclable, low cost catalyst systems. Thus,
6
7
3
7
8
synthesis of diynes by many chemists.
nowadays, nanoparticles serve as an excellent heterogeneous
The earliest and most remarkable methods for the synthesis catalyst in various organic transformations, because of their
9
,10
of diynes include Glaser–Hay coupling
and Cadiot–Chodkie- high surface area to volume ratio, enhanced stability, selectiv-
1
1
wicz coupling. But these processes require an excess of ity, chemical inertness etc. But, because of their nanosize, the
alkynes and toxic organic amine bases. Although the homo- recovery and isolation of these tiny nanoparticles after the
coupling of terminal alkynes has been investigated for more completion of the reaction is a difficult task. This difficulty
than 140 years, from the economic and environmental view- has been overcome by the use of Magnetic Nanoparticles
points, the development of new approaches continues to be of (MNPs) in present day catalysis reactions. In addition, they
great importance and interest. Among all the methods for the show high chemical reactivity and thermal stability.
synthesis of diynes, the palladium-catalyzed homocoupling
Among the magnetic compounds, the nano magnetic ferrites
1
2–14
reactions of terminal alkynes
coupling reactions of aryl-halides with terminal alkynes
and Sonogashira cross are very good catalyst systems and serve as excellent catalytic
1
5,16
38
supports. They have shown remarkable catalytic applications
39
have achieved much significance. Several mild and efficient such as the synthesis of thiazolidin-4-ones; benzoimidazo[1,2-a]
methods have been reported for the synthesis of diynes using pyrimidines and tetrahydrobenzo[4,5]imidazo-[1,2-d]quina-
1
7
18
40
41
different Pd catalysts, e.g., Pd(dba) –Bu NBr; PdCl (PPh ) ;
zolin-1(2H)-one; and 2,4,5-trisubstituted imidazoles.
Therefore, a more economical approach using magnetic
2
n
4
2
3 2
21
1
9
20
PdCl
2
CuI; Pd(OAc)
2
CuI; (PPh
3
)
2
PdCl
2
, CuI, PPh
3
;
and
2
2
Pd Au.
2
However, these reactions require high loadings nanoparticles on Pd has nowadays served as an alternative
of palladium, which often cannot be recovered. Scientists method for various C–C bond formation reactions due to their
therefore tried to perform these reactions using different easy recovery and recyclability.
catalysts in the absence of palladium using less expensive
2 4
Recently, many researchers have combined CuFe O mag-
2
3,24
25
26
27,28
metals like nickel,
cobalt, titanium, gold
and netic nanoparticles and palladium, which was shown to exhibit
high activity in various transformations such as the selective
4
2
hydrogenation of arylacetylenes and the cyanation of aryl
Department of Chemistry, Dibrugarh University, Dibrugarh-786004, Assam, India.
4
3
halides. This catalyst has shown efficient catalytic activity
compared to other nanoparticles because of the synergistic
effect between palladium and copper nanoparticles.
E-mail: bolinchetia@dibru.ac.in; Fax: +91-373-237-0323; Tel: +91-373-237-0210
†
Electronic supplementary information (ESI) available. See DOI: 10.1039/
d0nj04133e
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2 4
Scheme 2 Synthesis of Pd–CuFe O MNPs.
2.2 General procedure for homocoupling reactions
The prepared Pd–CuFe MNPs were used as the catalyst for
2 4
O
the homocoupling of various terminal alkynes. The reactions
were performed in a 50 ml round bottom flask at a temperature
of 60 1C. 1 mmol of terminal alkyne, 2 mol% Pd–CuFe
mmol of triethyl amine (TEA) and 3 ml of n-butyl alcohol
n-BuOH) were subjected to magnetic stirring for an appropri-
2 4
O ,
4
(
ate time period (Scheme 3). The progress of the reaction was
monitored using TLC. After the completion of the reaction, the
products were diluted with 30 ml of distilled water and
extracted with ethyl acetate, and the combined organic layer
was washed with brine, dried over anhydrous Na SO and
2 4
evaporated in a rotary evaporator under reduced pressure.
The products were purified using column chromatography on
silica gel with hexane as the eluent to afford the corresponding
1
1
,3-diynes. The products were confirmed using GC-MS, H-
1
3
NMR, and C-NMR analyses.
Scheme 1 Different schemes for diyne synthesis.
3
. Results and discussion
3
.1 Catalyst characterization
In this context, we attempt to synthesize Pd–CuFe O mag-
netic nanoparticles (MNPs) with a very low amount of Pd
loading and use them as a catalyst for the synthesis of various
2
4
The crystallite phase and purity of the synthesized Pd–CuFe
2 4
O
MNPs were characterized using powder X-ray diffraction (XRD)
analysis (Fig. 1). The Bragg reflections and crystallographic
faces of magnetite Pd–CuFe O appeared at 2y: 23.8, 27.2,
symmetric 1,3-diynes. The CuFe
using the biogenic route according to our previous report,
2 4
O MNPs were synthesized
4
4
2
4
2
5
9.92, 32.72, 35.3, 40.1 (Pd), 43, 46.06 (Pd), 49.06, 53.64,
6.96, 58.48, 62.28, 63.7, 68.34 (Pd), and 82.26 (Pd), which are
without using any harsh reaction conditions or any reducing
agent, using Camellia sinensis var. Assamica (Tea) leaves. Tea
leaves contain various polyphenolic compounds that might be
mainly responsible for the synthesis of biogenic nanoparticles.
The catalyst proved to be highly efficient, yielding almost all the
substrates in excellent to good yields even after five recycles. To
the best of our knowledge, no such usage of MNPs for the
synthesis of diynes has been reported so far (Scheme 1).
consistent with the (111), (202), (220), (113), (311), (111), (400),
200), (331), (422), (333), (511), (404), (440), (220), and (311)
planes, respectively. These values are comparable with the XRD
information of JCPDS card numbers 034-0425 (t-CuFe ) and
9-4897 (Pd). Thus, it was confirmed that Pd was doped in the
lattice of tetragonal CuFe O and formed solid Pd–CuFe O
(
2 4
O
4
8
8
2
4
2 4
4
9
MNPs. By using the Scherrer equation, the average crystallite
size of Pd–CuFe MNPs was found to be 5.228 nm. Moreover,
2 4
O
by considering the FWHM of the Pd peaks from the XRD
analysis and by using the Scherrer equation, the average crystal-
lite size of Pd in the nanoparticle was found to be 2.618 nm.
The SEM images exhibited the agglomerated nature of Pd–
2
. Experimental
2
.1 Synthesis of Pd–CuFe
For the synthesis of Pd–CuFe
Firstly, we prepared magnetic nanoparticles according to our CuFe
previously reported method using Camellia sinensis var. Assa- nature. Fig. 2(b) shows that the precious Pd metal homoge-
2
O
4
MNPs
2 4
O we adopted a two-step process.
O [Fig. 2(a)]. This might be because of its magnetic
2 4
4
4
mica (Tea) leaf extract. In the second step, we coated Pd with neously covered the surface of CuFe
O which was revealed by
2 4
the prepared MNPs in which 100 mg of the MNPs was dispersed
in ethanol and then 100 mg of Pd(OAc)2 was quickly added
under ultrasonication for 1 h to obtain a fine mix. Finally 1 ml
of 80% NH ÁNH ÁH O was added dropwise into the mixture.
2
2
2
After 30 min, the solid products were collected using a magnet
and washed thoroughly with distilled water and ethanol several
times (Scheme 2).
Scheme 3 General scheme for the homocoupling of various terminal
alkynes.
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2 4
Fig. 1 Powder-XRD patterns of the synthesized Pd–CuFe O MNPs.
the presence of smaller particles on the surface of the Pd-doped
sample. The energy dispersive X-ray (EDX) analysis [Fig. 2(c)]
shows the presence of Pd, Cu, Fe and O with an elemental
distribution of Cu = 19.42%, Fe = 26.02%, O = 52.58%, and Pd =
Fig. 3 (a) Low resolution TEM image; (b) SAED pattern showing different
crystallographic planes; (c) HRTEM image; and (d) particle size histogram
1
.98%. Thus, it was confirmed that no impurity was present in of the TEM image.
the nanoparticle.
From the low resolution TEM image, the nanoparticle was
found to be spherical in shape [Fig. 3(a)]. Bright fringes were indicated that the nanoparticles were polycrystalline in nature.
observed in the SAED pattern, which also confirmed the highly From the size distribution histogram, determined using a large
crystalline nature of the MNPs as revealed by the XRD analysis number of particles in the TEM image, the average particle size
[Fig. 3(b)]. The crystallographic (111), (202), (220), (113), and of the nanoparticle was found to be 6.02 nm [Fig. 3(d)].
(
311) planes corresponding to tetragonal CuFe O along with
The ICP-AES analysis showed that the amounts of Pd, Fe and
2
4
the (111) plane of palladium were clearly indexed in the SAED Cu loading in 57 mg of the compound are 0.000324 mmol,
pattern. The fringe separations of 0.202 nm and 0.193 nm were 0.03629 mmol, and 0.02265 mmol, respectively.
present in the HR-TEM image [Fig. 3(c)], which were in good
agreement with the (400) plane of CuFe and the (200) plane of Pd in the sample along with the proper stoichiometry of Cu
XPS analysis was performed to determine the oxidation state
2 4
O
of Pd. The observance of the fringes in different directions and Fe [Fig. 4]. In the survey scan [Fig. 4(a)], the peaks
corresponding to Cu, Fe, O and Pd were found. The Cu 2p scan
[Fig. 4(b)] shows two distinct peaks with binding energies of
934.19 eV and 954.26 eV, which can be attributed to Cu 2p3/2
and Cu 2p1/2, respectively. The presence of two strong satellite
peaks at 941.97 and 962.69 eV is characteristic of Cu in the +2
5
0
oxidation state. The Fe 2p scan [Fig. 4(c)] depicts the 2p3/2
peak at 711.60 eV and 2p1/2 peak at 724.56 eV with its satellite
peak at 719.68 eV, which is characteristic of Fe in the +3
5
1
oxidation state. Fig. 4(d) clearly shows the presence of a peak
for Pd 3d5/2 at 335.90 eV and 3d at 341.14 eV, which shows
3/2
5
2
the 0 (zero) oxidation state of Pd. Thus, all the above results
confirmed the formation of Pd–CuFe MNPs.
2 4
O
The magnetic study of the catalyst was performed using VSM
analysis at room temperature [Fig. 5]. The M–H curve showed
an S-type plot. The sample showed a coercivity (Hci) of 37.66 Oe,
À1
a magnetization (M
0
s
) of 26.82 emu g and a retentivity (M
r
) of
À1
À1
.72004 emu g . The magnetization value of 26.82 emu g
proved that the sample was ferromagnetic in nature. Thus, it
could be established that a magnetically retrievable sample
formed that could be used for catalytic applications.
By using the Brunauer–Emmett–Teller (BET) method, the
2 4
nitrogen adsorption–desorption isotherms of both Pd–CuFe O
Fig. 2 (a) SEM image; (b) SEM image showing Pd particles on the surface
of CuFe ; and (c) EDX analysis showing the presence of O, Cu, Fe and Pd
in the synthesized Pd–CuFe MNPs (inset shows the compositional
analysis of the elements present).
2
O
4
2
O
4
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Fig. 6
2 2 4 2 4
N adsorption–desorption isotherm of CuFe O and Pd–CuFe O
samples obtained at 77.3 K.
2 4
Fig. 4 XPS spectra: (a) survey scan of Pd–CuFe O ; (b) Cu 2p; (c) Fe 2p;
and (d) Pd 3d.
increases. However, pore diameter decreases in Pd–CuFe O .
2
4
The results are tabulated in Table S1 (ESI†). This also indicates
directly that CuFe is wrapped by Pd, as shown in the SEM
image in Fig. 2(b).
2 4
Furthermore, we compared the Pd–CuFe O MNPs with
2 4
O
previously reported catalyst systems in terms of the amount
of Pd metal contained (Table 1).
3
.2 Homocoupling of different substrates using Pd–CuFe
MNPs
.2.1 Optimization of the reaction conditions. We synthe-
sized various diynes using the prepared Pd–CuFe as the
2 4
O
3
2 4
O
catalyst. Our prime goal was to screen different solvents for
the reaction (Table 2) and, thus, we took phenylacetylene as the
model substrate. It was found that n-BuOH was the best solvent
for the reaction (Table 2, entry 10). The reaction temperature
was set at around 60 1C, and the reaction worked well for
most of the substrates, making the process highly efficient
and comparatively quicker compared to previously reported
Pd-catalyst systems.
We then optimized the amount of catalyst for the model
reaction of phenylacetylene and found that only 2 mol%
(6.89 mg) catalyst was sufficient for the smooth progress of
the reaction (Table 3, entry 8).
We further optimized our reaction using different alcohols
as solvents and varying the temperature (Table 4). From our
investigations, it was found that both EtOH and n-butyl
alcohol provided good results for the reaction, but compared
to n-BuOH, EtOH required more time and the product yield was
Fig. 5 (a) VSM spectra of Pd–CuFe
2
O
4
MNPs; (b) enlarged M–H curve
Table
1
2 4
A comparison study of Pd–CuFe O MNPs with previous
showing coercivity and retentivity.
reported catalysts
S. no.
Catalyst
Metal (mol%)
Ref.
and CuFe
Fig. 6. The BET analysis showed that when Pd is incorporated
on CuFe , the surface area of CuFe increases from
0.309 m g to 39.748 m g and the total pore volume also
2
O
4
were studied at a temperature of 77.3 K, see
1
2
3
4
5
Fe
Fe
Fe
Fe
3
3
3
3
O
O
O
O
4
4
4
4
/SiO
@PUVS-Pd
@PUNP-Pd
/P(GMA-AAMMA-Pd)
2
–NH
2
/SA/Pd
Pd (0.05)
Pd (0.09)
Pd (0.1)
Pd (0.2)
Pd (0.0324)
53
54
55
56
2
2
O
4
À1
2 4
O
2
À1
2
Pd–CuFe
2
O
4
This work
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a
a
Table 2 Optimization of the solvent
Table 4 Optimization using different alcohols
Entry Solvent Base (mmol) Catalyst (mg) Time (h) Isolated yield (%)
Entry
Solvent
Time (h)
Isolated yield (%)
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
n-BuOH 1
n-BuOH —
n-BuOH 1
n-BuOH 1
n-BuOH 1
n-BuOH 1
n-BuOH 2
n-BuOH 3
n-BuOH 4
n-BuOH 4
—
1
1
1
2
3
6
6
6
6
6
6
6
6
6
48
48
7
24
6
6
6
—
—
50
93
60
72
80
95
95
1
2
3
4
5
6
7
MeOH
EtOH
n-BuOH
Polyethylene glycol
Isopropanol
Triethylene glycol
Ethylene glycol
5
2
0.5
5
3
3.5
5
No product
80
95
75
80
70
No product
b
b
c
4
3
a
Phenylacetylene (1 mmol), Pd–CuFe O4 MNPs (2 mol%, 6.89 mg,
2
0
1
2
3
4
5
45 min 95
0.000324 mmol of Pd per 57 mg), 60 1C, solvent (3 ml), TEA (4 mmol).
CH
3
CN
4
24
24
24
24
24
78
60
73
80
60
Toluene 4
DMF
DMSO
4
4
4
H
2
O
Table 5 Optimization of bases for the homocoupling of phenylacetylene
a
2 4
catalyzed by Pd–CuFe O MNPs
a
Phenylacetylene (1 mmol), solvent (3 ml), base (TEA = triethylamine),
, 0.000324 mmol of Pd per 57 mg), r.t. = 23 1C
2 4
catalyst (Pd–CuFe O
unless otherwise noted. 40 1C. 60 1C.
b
c
b
Entry
Base
—
Triethylamine
Diethylamine
Pyridine
Isolated yield (%)
Table 3 Optimization of the amount of catalyst for the homocoupling of
1
2
3
4
5
6
7
8
0
95
69
12
24
38
20
14
a
phenylacetylene
2 3
K CO
Piperidine
tert-Butylamine
NaOH
b
Entry
Amount of catalyst (mol%)
Time (h)
Yield (%)
a
1
2
3
4
5
6
7
8
9
0
24
5
4
3.5
2.5
2.5
45 min
30 min
30 min
—
70
75
75
80
80
95
95
95
Phenylacetylene (1 mmol), Pd–CuFe
2
O
4
MNPs (2 mol%, 6.89 mg,
0.05
0.1
0.5
1
1.5
1.74
2
0.000324 mmol of Pd per 57 mg), 60 1C, air, solvent (3 ml), base
(4 mmol). Isolated yield after column chromatography.
b
(Table 6). The exact reason for this is not known and further
studies on this are under progress. But it clearly indicated that
Cu only synergistically supports the activity of Pd and is not
active by itself for the reaction alone.
5
8
2.5
a
Phenylacetylene (1 mmol), 60 1C in air, n-BuOH (3 ml), catalyst (Pd–
MNPs, 0.000324 mmol of Pd per 57 mg), TEA (4 mmol).
Isolated yield after column chromatography.
3.2.2 Substrates scope. After optimization of the reaction
2 4
CuFe O
b
conditions, we used the catalyst for the homocoupling of
different substituted terminal alkynes (Table 7). Within the
stipulated time, the catalytic oxidative homocoupling of all
the phenylacetylenes (Table 7, entries 1–8), which contain
electron-withdrawing as well as electron-donating substituents,
proceeded readily to afford the corresponding diyne derivatives
also low (Table 4, entry 2). Moreover, as can be found in the
literature, both EtOH and n-BuOH are considered as green
solvents. Thus, we chose n-BuOH as the best solvent for the
reaction at a temperature of 60 1C (Table 4, entry 3).
5
7
As it was observed that the presence of a base is essential for
the formation of diynes, we optimized the reaction conditions
with various bases in air (Table 5). Compared to the inorganic
bases, the organic bases including primary amines, secondary
amines and tertiary amines were more effective, with triethyl-
amine being the best (Table 5, entry 2) at the temperature of
a
Table 6 Optimization using different catalyst systems
b
6
0 1C.
We also tested the homocoupling reaction of phenylacety-
lene under the optimized conditions using CuFe MNPs as
Entry
Catalyst
Blank
Time (h)
Yield (%)
1
2
5
24
18
0.5
—
—
95
2
O
4
2 4
CuFe O
the catalyst, but it failed to show a positive homocoupling
reaction. It was witnessed that when Pd was supported on
CuFe O MNPs, the coupling reaction afforded the best result (2 mol%), TEA (4 mmol). Isolated yield after column chromatography.
2 4
Pd/CuFe O
2 4
a
Phenylacetylene (1 mmol), 60 1C in air, n-BuOH (3 ml), catalyst
b
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a
Table 7 Synthesis of various diynes
b
Entry
R
Time (min)
Yield (%)
1
2
3
4
5
6
7
8
9
H
30
25
25
30
30
30
30
30
60
60
95
98
95
97
96
88
85
80
70
63
p-OMe
m-OMe
p-Me
m-Me
p-Br
p-NO
3-NH
2
2
1-Heptyne
(2-Propynyl)cyclohexane
1
0
a
Alkynes (1 mmol), Pd–CuFe
2
O
4
MNPs (2 mol%, 6.89 mg,
0
.000324 mmol of Pd per 57 mg), 60 1C, air, n-BuOH (3 ml), TEA
b
(4 mmol). Isolated yield after column chromatography, 60 1C.
2 4
Fig. 7 XPS spectra: (a) survey scan of recycled Pd–CuFe O ; (b) Cu 2p; (c)
Fe 2p; and (d) Pd 3d of the recycled catalyst after the 4th cycle.
in 80–98% yields, thus indicating the fact that the electronic
nature of the substituents had little influence on the yield.
Moreover, steric hindrance also did not obviously affect the
reaction yields and catalytic activity (Table 7, entries 2 vs. 3 and
entries 4 vs. 5). However, alkyl substrates required longer
reaction times compared to the aryl substrates and the yields
were also low (Table 7, entries 9 and 10).
analysis of the recycled catalyst was performed after the 4th
cycle to check if there was any change in the oxidation state of
the catalyst (Fig. 7). The amount of catalyst found in the 4th
cycle was 4.27 mg (0.000324 mmol of Pd per 57 mg). It was
observed that the oxidation state of all three metals remained
intact with slight changes in the binding energy. Thus, the
3.3 Catalyst leaching and recyclability test
2+
oxidation states of the metal in the recycled catalyst were Cu ,
3
+
0
3.3.1 Hot filtration test. Considering the economic and Fe and Pd . Therefore, with a very low amount of Pd loading,
green potential, recyclability is an important factor where our catalyst proved to be very efficient for the synthesis of
5
9
heterogeneous catalysis is concerned. Thus, it was necessary various diynes.
to know the working principle of the active catalyst, i.e. whether
it is operating heterogeneously or homogeneously. To observe
3
.4 Mechanism
The exact mechanism of alkyne homocoupling reaction is not
2 mol%, 6.89 mg) at a temperature of 80 1C in a 50 ml round clearly known. Thus, based on the above results and according
this, a hot filtration test was performed using phenylacetylene
(
(
2 4
1 mmol), TEA (4 mmol), n-BuOH (3 ml), and Pd–CuFe O
6
0
bottom flask. The reaction was stopped after 10 min, the to a literature survey, we would like to propose a brief
catalyst was separated by using an external magnet (40% yield mechanistic pathway for the Pd–CuFe catalyzed homocou-
O
2 4
was determined using GC-MS analysis), and the reaction was pling of terminal alkynes (Fig. 8). For our convenience, to
allowed to continue under the same reaction conditions for 3 h provide the reaction mechanism, we would like to take phenyl-
without the solid catalyst. It was observed that this reaction acetylene as the model substrate.
did not proceed and the reagents remained intact up till 3 h
of reaction time (using GC-MS analysis), which ruled out
the possibility of homogeneous/semi-heterogeneous catalysis.
Thus, it was proved that the working mode of our catalyst was
heterogeneous.
3.3.2 Recyclability test. Moreover, the recyclability of the
catalyst was tested using the model substrate of phenylacety-
lene under the optimized conditions. After the completion of
the reaction, the catalyst was easily removed from the reaction
mixture using an external magnet and dried at 40 1C and reused
for a fresh batch of reaction [Fig. S1 (ESI†)]. The catalyst was
easily recyclable up to the 5th cycle without any significant
loss in the catalytic activity [Fig. S2 (ESI†)]. However, after the
5
th cycle, there was a slight loss in the catalytic activity, as we
weighed out the catalyst after each cycle and no significant
leaching of the catalyst was observed during catalysis. XPS homocoupling reaction.
2 4
Fig. 8 Proposed mechanism for the Pd–CuFe O catalyzed oxidative
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Table 8 Comparison of the efficiency of the Pd–CuFe
2
O
4
MNPs with those of some reported catalyst systems for the homocoupling reaction of
phenylacetylene
Catalyst
CuCl
Á2H
Reaction conditions
NaOAc, PEG
Temp. (1C)
Time (h)
Yield (%)
Ref.
2
2
O (0.1 mmol)
(5 mol%)
(5 mol%)
120
rt
rt
rt
100
rt
1.5
10
4
2
4
7
5
0.5
99
95
97
90
97
98
92
95
62
63
64
65
66
67
68
Cu ophen
Ni(dppe)Cl
Pd(OAc) (2 mol%), CuI (2 mol%)
2
2
TBAB (5 mol%), H
2
O
2
Ag O (1 equiv.), 1,2-dichloroethane
2
2
DABCO, MeCN
DMSO, air
Cu(II)-SBA-15 (50 mg)
CuCl (5 mol%)
CH
3
CN, O
2
, blue LED light
Cu(OTf)
Pd–CuFe
2
(5 mol%)
MNPs (2 mol% i.e. 6.89 mg)
DBU, acetone
n-Butanol, air, TEA
rt
60
2
O
4
This work
For heterogeneous catalysed reactions, it is widely accepted Conflicts of interest
that Pd in solution as a colloid or complex is the true cataly-
6
1
There are no conflicts to declare.
tically active species. We believe that Pd is activated by
CuFe , which helps in better desorption and leads to faster
formation of complexes or colloids in the solution, and on
2 4
O
completion of the reaction, the palladium is redeposited onto Acknowledgements
the support or Pd clusters. Thus, CuFe
the Pd species.
Initially, oxygen from air dissolved in the reaction solution,
oxidizing the Pd(0) nanoparticles on the surface of CuFe O
2 4
O acts as a reservoir for
The authors acknowledge SAIF NEHU Shillong, CIF IIT Guwa-
hati, CSIC Dibrugarh University, SAIF IIT Bombay, IIT Kanpur,
and IIT Bombay for providing the analytical services.
2
4
MNPs to Pd(II). The n-BuOH solvent quenches the resulting
oxygen anion, but the quantities of this one-time reaction are
too small to be determined. The deprotonation of the terminal Notes and references
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