Tetragonal Cu2Se nanoflakes: Synthesis using selenated propylamine as Se source and activation of Suzuki and Sonogashira cross coupling reactions

Article abstract of DOI:10.1039/c4dt03320e

The metastable tetragonal Cu₂Se phase as nanoflakes has been synthesized for the first time by treating CuCl₂ taken in a mixture (1:1) of 1-octadecene and oleylamine with H₂N-(CH₂)₃-SePh dissolved in

Full text of DOI:10.1039/c4dt03320e

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Tetragonal Cu2Se nanoflakes: synthesis using
selenated propylamine as Se source and activation
of Suzuki and Sonogashira cross coupling
reactions†
Cite this: Dalton Trans., 2015, 44, 725
Ved Vati Singh and Ajai Kumar Singh*
The metastable tetragonal Cu2Se phase as nanoflakes has been synthesized for the first time by treating
CuCl2 taken in a mixture (1 : 1) of 1-octadecene and oleylamine with H2N–(CH2)3–SePh dissolved in
1-octadecene. Powder X-ray diffraction (PXRD), HRTEM, SEM-EDX and XPS have been used to authenti-
cate the nanoflakes. The XPS of Cu2Se nanoflakes indicates oxidation states of Cu and Se as +1 and −2
respectively. The size of the majority of Cu2Se nanoflakes was found to be between 12 and 14 nm. The
nanoflakes have been explored for Suzuki and Sonogashira cross coupling reactions in the presence of
TBAB in DMF and DMSO respectively. For Suzuki coupling conversion was found upto ∼86% in 15 h at
110 °C when loading of Cu was 1 mol%. In case of Sonogashira coupling conversion was found upto 82%
in 15 h at 160 °C (Cu loading: 1 mol%). The catalytic efficiency of Cu2Se nanoflakes for Suzuki coupling
reaction is greater than that for Sonogashira coupling. These nanoflakes have been found reusable for a
second time. Most probably TBAB facilitates the release of CuBr from the nanoflakes which catalyze both
reactions, as catalytic efficiency is very low in the absence of TBAB and CuBr has been found to activate
readily both the coupling reactions. In comparison to many other copper based nano-crystals, the
present nanoflakes are a better activator.
Received 28th October 2014,
Accepted 28th October 2014
DOI: 10.1039/c4dt03320e
www.rsc.org/dalton
tance. Thus attempts are being made to employ less expensive
and toxic transition metal complexes as catalyst.5 Among
Introduction
Carbon–carbon cross-coupling reactions viz. Heck, Suzuki– them, copper-based alternatives are particularly attractive, due
Miyaura and the Sonogashira catalyzed with palladium are to their low cost and harmfulness to the environment.6 Of the
among the most important protocols, used abundantly in various copper based options its salts are not reusable whereas
organic syntheses and widely studied in recent decade1,2 as the such possibility exists with copper chalcogenides, which have
resulting biaryl and acetylenic products are extremely valuable shown potential for efficient solar cells due to their capability
intermediates. These reactions are key steps in the total syn- of bandgap engineering apart from the advantages of low-
thesis of a large number of natural products, biologically temperature synthesis, economical and convenient post-pro-
active compounds and a variety of organic building blocks cessing.7 Among these chalcogenides, copper selenides are
having applications in several fields, such as pharmaceutical, unique8 as they are found in many phases and structural
agrochemical and fine-chemical industry,3 polymer and forms with different stoichiometries such as CuSe, Cu2Se,
materials science.4 Further these three coupling procedures CuSe2, Cu3Se2, Cu5Se4 and Cu7Se4 as well in non-stoichio-
generally involve the use of complexes of phosphine ligands metric form such as Cu2−xSe. Out of these selenides Cu2Se has
with palladium. The drawbacks of such Pd based catalyst attracted considerable attention due to its unique properties
systems are their sensitivity for moisture/air, high cost and like large thermal power, a direct band gap of ∼2.2 eV and
toxicity. Therefore, the task of developing palladium and phos- superionic conductivity.9 Recently, efforts have been made to
phine free catalytic systems is a challenge of current impor- synthesize Cu2Se as advanced nano-architectures with
different morphologies, including particles, flakes, tubes,
cages, and dendrites10 because size, shape and the phase
Department of Chemistry, Indian Institute of Technology Delhi, New Delhi 110016,
structure have a great impact on the performance of a desired
India. E-mail: aksingh@chemistry.iitd.ac.in, ajai57@hotmail.com;
material.11 The Cu2Se is also envisaged as an interesting
Fax: +91-01-26581102; Tel: +91-011-26591379
†Electronic supplementary information (ESI) available: SEM-EDX images, and
material as it can be synthesized in various crystallographic
forms, monoclinic, cubic, tetragonal, hexagonal and ortho-
spectral data of coupled products. See DOI: 10.1039/c4dt03320e
This journal is © The Royal Society of Chemistry 2015
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rhombic. Among them, the cubic phase is thermodynamically dried and distilled before use by standard procedures.23 The
stable while the tetragonal phase is metastable.12 progress of the catalytic reactions was monitored with 1H NMR
Copper based catalysts (nano-clusters as well as salts) are spectroscopy. The products obtained in Suzuki–Miyaura and
not unknown for chemical synthesis.13 There are few reports Sonogashira coupling were authenticated by matching their
also on the use of copper nanocatalysts/salts for the Suzuki– proton NMR spectroscopic data with the literature values.24,25
Miyaura and Sonogashira coupling reactions.14 Rothenberg
and co-workers15 have reported copper containing mixed
nanocluster catalysts for both Suzuki–Miyaura and Sonoga-
Physical measurements
shira cross-coupling. Li et al.16 have reported reusable nano 1H and 13C{1H} NMR spectra were recorded on a Bruker Spec-
Cu2O–PPh3–TBAB system for cross coupling of aryl and hetero- trospin DPX 300 NMR spectrometer at 300.13, and 75.47 MHz
aryl halides with terminal alkynes. They have also reported respectively. The chemical shifts are reported in ppm relative
CuI-DABCO (ligand) catalyzed Suzuki–Miyaura and Sonoga- to the normal standards. All reactions were carried out in an
shira cross coupling reactions in the presence of TBAB. Zhang oven dried round bottom flask under ambient conditions.
et al.17 have reported ligand free copper(II) oxide nanoparticle Powder X-ray diffraction (PXRD) studies were carried out on a
catalyzed Sonogashira coupling reactions. Basu et al.18 have Bruker D8 advance diffractometer using Ni-filtered Cu-Kα radi-
explored Pd–Cu bimetallic nanoparticles embedded in macro- ations with scan speed of 1 s and scan step of 0.05°. HRTEM
porous ion-exchange resins as heterogeneous catalyst for Sono- studies were carried out using an instrument TECNAI G2 20
gashira coupling. Miura and co-workers19 have reported operated at 200 KV to understand nature of nanoparticles and
CuI-PPh3 as catalyst for Sonogashira cross coupling reactions. their elemental compositions. The specimens for these studies
Ma and Liu20 have catalyzed coupling reactions of aryl halides were prepared by dispersing the powdered sample in toluene
with terminal alkynes with CuI-N,N-dimethylglycine.
by an ultrasonic treatment, placing few drops of dispersed
Monnier et al.21 have used Cu(acac)2-β-diketone system for sample on a porous carbon film supported on a copper grid
Sonogashira type cross coupling of aryl iodides with phenyl- and drying it in air. SEM-EDX studies were carried out using
and hexylacetylene, which affords disubstituted alkynes in Bruker-AXS Energy Dispersive X-ray System (Model QuanTax
good yield. To the best of our knowledge, the synthesis of tetra- 200) for elemental compositions. For X-ray photoelectron spec-
gonal Cu2Se nanoflakes and its catalytic properties (probably troscopic (XPS) measurements a commercial electron energy
not of many other nanostructures also) for Suzuki and Sonoga- analyzer (Phoibus 150 from Specs GmbH, Berlin, Germany)
shira coupling have not been investigated so far. Thus for the and a nonmonochromatic Mg Kα X ray source were used. The
first time, we have designed a synthetic method for Cu2Se 1253.6 eV energy was used to excite photoelectrons.
nanoflakes and investigated their catalytic applications for
Synthesis of 3-(phenylseleno)propylamine (L). Diphenyl-
Suzuki and Sonogashira coupling reactions. The easy route for diselenide (0.312 g, 1.0 mmol) dissolved in 30 mL of EtOH was
the synthesis of Cu2Se tetragonal nanoflakes developed by us stirred under reflux in N2 atmosphere. Sodium borohydride
is based on 3-(phenylseleno)propylamine (H2N–(CH2)3–SePh) (0.080 g, 2.0 mmol) was added to it as a solid so that it became
as a selenium source which has been prepared by a slight colorless due to the formation of PhSeNa. A solution of
modification of the method reported in the literature.22 It is 3-chloropropylamine hydrochloride (0.520 g, 4 mmol) dis-
reacted with CuCl2 using non-coordinating 1-octadecene solved in 10 mL of EtOH was added to it drop wise with stir-
(ODE) as solvent and oleylamine as both the reductant for Se ring. The resulting mixture was further refluxed for 5 h.
and the ligand for the nanostructures. The Cu2Se has been Thereafter, it was stirred for overnight at room temperature.
found to catalyze both coupling reaction at 1 mol% (Cu) in the After cooling, cold water (30 mL) was poured into it and the
presence of TBAB efficiently and is reusable. These results are mixture extracted with chloroform (4 × 25 mL). The extract was
described in this paper.
washed with water (3 × 40 mL) and dried over anhydrous
sodium sulphate. The solvent was evaporated off under
reduced pressure on a rotary evaporator resulting L as light
yellow viscous oil.22
Synthesis of Cu2Se nanoflakes. Copper(II) chloride
(1.0 mmol) was added to oleylamine and ODE (5 mL each)
Experimental
Chemicals and reagents
3-Chloropropylamine hydrochloride, diphenyldiselenide, pot- taken in a flask and the mixture heated for 1 h at 80 °C on a
assium carbonate, caesium carbonate, NaBH4, tetra-n-butylam- Schlenk line under nitrogen atmosphere. Thereafter, tempera-
monium bromide (TBAB), phenylboronic acid, phenyl ture was increased to 250–280 °C and a suspension of 3-(phe-
acetylene, copper(I) bromide, copper(II) chloride, oleylamine nylseleno)propylamine, (0.5 mmol) made in 1 mL of ODE was
(OAm), 1-octadecene (ODE), 4-bromobenzaldehyde, 1-bromo- quickly injected with a syringe. The temperature was allowed
4-nitrobenzene, 4-bromo-benzonitrile, 4-bromoaceto-phenone, to rise to the pre-injection level. The reaction after injection
4-bromobenzoic acid, bromobenzene and 4-bromotoluene pro- was continued for 30 min. Thereafter the reaction flask was
cured from Sigma-Aldrich (USA) were used as such. All solvents cooled rapidly to room temperature and 5 mL of toluene
(AR grade) viz. toluene, acetone, chloroform, acetonitrile, added. The crude product was separated by centrifugation at
dimethyl formamide, dimethyl sulfoxide and ethanol were 5680 rpm for several minutes. The crude product was washed
726 | Dalton Trans., 2015, 44, 725–732
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with methanol (20 mL) for 3 times to remove the byproducts.
Finally, the product was dried and stored in vacuo.
General procedure for catalysis of Suzuki Miyaura and
Sonogashira cross coupling reaction
Aryl bromide (1 mmol), phenylboronic acid/phenylacetylene
(1.2 mmol), K2CO3 (2.0 mmol), DMF/DMSO (2 mL) and nano-
flakes (equivalent to 1 mol% of Cu) and 1 mmol of tetra-n-
butylammonium bromide (TBAB) were taken in a round
bottom flask, which was placed on an oil bath under anaerobic
condition at 110/160 °C respectively for Suzuki/Sonogashira
coupling reactions. The mixture was stirred and the progress
of reaction was monitored with TLC. When maximum conver-
sion of ArBr into product occurred, the reaction mixture was
cooled to room temperature. It was mixed with water (20 mL)
and extracted with diethyl ether/ethyl acetate (3 × 10 mL)
respectively for Suzuki/Sonogashira coupling. Organic phase
was dried over anhydrous Na2SO4. Its solvent was removed on
Fig. 1 X-ray diffraction (XRD) patterns of Cu2Se nanoflakes.
1
a rotary evaporator and the product subjected to H NMR. In
case of impure product, further purification by column chrom-
atography on silica gel using ethyl acetate and hexane mixture
as an eluent, was carried out and NMR of the purified product
recorded.
Results and discussion
An easy route has been developed for the synthesis of Cu2Se
nanoflakes by treating a suspension of 3-(phenylseleno)-propyl-
amine with copper(II) chloride, in oleylamine and ODE at
250–280 °C. The present synthesis of Cu2Se avoids the use of
toxic, pyrophoric and expensive TOP used earlier.26 The Cu2Se
nanoflakes have been characterized with SEM-EDX, powder
X-ray diffraction (PXRD), high-resolution transmission electron
microscopy (HRTEM) and X-ray photoelectron spectroscopy
(XPS). The SEM-EDX pattern of the tetragonal Cu2Se nano-
flakes is shown in Fig. S1 of ESI.† Except the small C and O
peaks originated from the adsorbed organic capping reagent(s),
only Cu and Se peaks are exhibited. The atomic ratios of
Cu and Se for the sample was found close to 2 : 1, implying
that nanoflakes are chemically stoichiometric Cu2Se. Its PXRD
pattern, shown in Fig. 1 has peaks consistent with those
reported for standard tetragonal phase of Cu2Se (space group:
P42/n) with the lattice parameters of a = 11.521 and c = 11.740
(JCPDS 29-0575). In Fig. 1 the observed PXRD pattern, given in
JCPDS 29-0575 are shown. The matching between the two is
good.
Fig. 2 HRTEM of Cu2Se nanoflakes (a) and (b) scale bar 100 nm (c)
scale bar 50 nm and (d) scale bar 20 nm.
On the basis of SEM-EDX and PXRD analysis the presently
synthesized sample of Cu2Se is its pure tetragonal phase. The
size and shape of the present Cu2Se nanoflakes have been
examined with high-resolution transmission electron
microscopy (HRTEM). The Fig. 2 showing the HRTEM of the
present tetragonal phase of Cu2Se reveals its shape as nano-
sized flake-like. Fig. 3 shows the size distribution patterns of
Cu2Se nanoflakes and their average size as ∼12–13 nm.
Fig. 3 Size distribution patterns of Cu2Se nanoflakes.
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monium bromide (TBAB) was used for screening of catalysis
with Cu2Se nanoflakes. In absence of TBAB the catalytic reac-
tion was very slow. The presence of TBAB improved the conver-
sions in the present ligand-free copper-catalyzed cross-
coupling reactions. The DMF, DMSO, EtOH and toluene as
solvent and K2CO3/Cs2CO3,/Na2PO4 as base have been explored
for the two coupling reactions. The catalyst loading was equi-
valent to 1 mol% of Cu and exploratory reactions were carried
out at 100–160 °C for 24 h under N2 atmosphere.
The results given in Tables 1 and 2 exhibit optimum con-
ditions for catalysis with Cu2Se nanoflakes. For Suzuki–
Miyaura coupling reaction at 110 °C under N2 atmosphere
(Table 1) the conversions were found ∼89% and 86% in 24 and
15 h respectively using DMF as a solvent, K2CO3 as a base and
nanoflakes equivalent to 1 mol% of Cu as a catalyst. In Sono-
gashira coupling with Cu2Se nanoflakes equivalent to 1 mol%
Table 1 Screening of bases and solvents for Suzuki–Miyaura cross
coupling reaction catalyzed with Cu2Se nanoflakesa
Fig. 4 High-resolution XPS Cu 2p and Se 3d spectra of Cu2Se
nanoflakes (a) Cu 2p and (b) Se 3d.
S. no.
Catalyst
Solvent
Base
Time (h)
Yield (%)
1.
2.
3.
4.
5.
6.
7.
8.
9.
Cu2Se
Cu2Se
Cu2Se
Cu2Se
Cu2Se
Cu2Se
Cu2Se
Cu2Se
Cu2Se
DMF
DMF
DMF
DMF
DMF
DMF
DMSO
Toluene
EtOH
K2CO3
Cs2CO3
Na2PO4
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
24
24
24
24
15
9
24
24
24
46
23
39
89
86
34
63
16
31
To identify the valence states of Cu and Se in the nano-
flakes of Cu2Se, XPS measurements were performed on them.
The XPS spectra are shown in Fig. 4. As shown in Fig. 4(a), the
sample only consists of copper and selenium. The Cu 2p spec-
trum illustrates that the observed values of binding energies
for Cu 2p3/2 and 2p1/2 peaks appearing at 932.4 and 952.4 eV
are symmetric, narrow, and devoid of satellite. These values of
binding energies are consistent with the recent literature
report26,27 and indicate the valences of Cu as +127 in the
present tetragonal phase of Cu2Se nanoflakes. In Se 3d spec-
trum, only a single broad peak at 52–57 eV has been observed
(Fig. 4(b)). This broad peak of Se 3d spectrum can be fitted
a Reaction
conditions:
4-bromobenzaldehyde
(1.0
mmol);
phenylboronic acid, 1.2 equiv.; base, 2.0 equiv.; Cu2Se nanoflakes:
1 mol% of Cu; TBAB, 1.0 mmol; solvent, 2 ml; Temp, 110 °C; Time
24 h; Yield: NMR based.
with two symmetric peaks assignable to Se 3d5/2 and 3d3/2
respectively (Fig. 4(b)).
,
Table 2 Screening of bases and solvents for Sonogashira cross coup-
ling reaction catalyzed with Cu2Se nanoflakesa
The binding energies corresponding to these two peaks are
54.4 and 55.2 eV, respectively, characteristic of Se2− 28
In Cu 2p
.
region a small peak (Fig. 4(a)) marked with arrow appears due
to some oxidation of Cu+ to Cu2+ on the surface of the nano-
flakes.28 Similarly in Se 3d region the small peak also pointed
out by arrow in Fig. 4(b) probably arise due to oxidation of Se2−
ions on the surface of nanoflakes.
S. no.
Catalyst
Solvent
Base
Time (h)
Yield (%)
1.
2.
3.
4.
5.
6.
7.
8.
9.
Cu2Se
Cu2Se
Cu2Se
Cu2Se
Cu2Se
Cu2Se
Cu2Se
Cu2Se
Cu2Se
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMF
K2CO3
Cs2CO3
Na2PO4
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
24
24
24
24
24
15
9
33
12
—
83
71
83
43
50
12
Catalysis of Suzuki–Miyaura and Sonogashira cross coupling
reactions
As Cu nanoparticles are known as efficient catalysts for both
Suzuki–Miyaura and Sonogashira coupling reactions,14 we
have explored Cu2Se nanoflakes as catalysts for both Suzuki–
Miyaura and Sonogashira coupling reactions. The reaction
between 4-bromobenzaldehyde with phenylboronic acid/
Toluene
EtOH
24
24
a Reaction
conditions:
4-bromobenzaldehyde
(1.0
mmol);
phenylacetylene, 1.2 equiv.; base, 2.0 equiv.; Cu2Se nanoflakes:
1 mol% of Cu; TBAB, 1.0 mmol; solvent, 2 ml; Temp, 160 °C; Time
phenyl acetylene in the presence of 1 mmol of tetra-n-butylam- 24 h; Yield: NMR based.
728 | Dalton Trans., 2015, 44, 725–732
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ability experiments carried out for Sonogashira coupling are
given in Table 4.
Table 3 Suzuki–Miyaura and Sonogashira cross-coupling reactions
catalyzed with Cu2Se nanoflakes
Yielda (%)
Proposed mechanism
Suzuki–Miyaura
Sonogashira
Copper(I) iodide has been reported earlier to catalyze both
Suzuki15,16 and Sonogashira coupling16–21 reactions. The cata-
lysis of both the coupling reactions with present nanoflakes is
very slow in the absence of TBAB. Therefore it appears that
probably copper(I) bromide is a real catalytic species which
carries forward the reaction. This is supported by the fact that
CuBr (without TBAB) catalyze Suzuki and Sonogashira coup-
ling reactions of 4-bromobenzaldehyde.
S.N.
Aryl halide
cross-couplingb
cross-couplingc
1.
2.
3.
4.
5.
6.
7.
4-Bromobenzaldehyde
1-Bromo-4-nitrobenzene
4-Bromobenzonitrile
4-Bromoacetophenone
4-Bromobenzoic acid
Bromobenzene
86
74
84
92
90
80
66
83
83
84
64
60
84
62
4-Bromotoluene
a Yield: NMR % yield. Time 15 h. b Reaction condition: aryl bromide,
1.0 mmol; phenylboronic acid, 1.2 equiv.; K2CO3, 2.0 equiv.; Cu2Se
nanoflakes: 1 mol% of Cu; TBAB, 1.0 mmol; DMF, 2 ml; Temp, 110 °C.
c Reaction condition: aryl bromide, 1.0 mmol; phenylacetylene, 1.2
Based on the earlier mechanisms14c,19,21 proposed for cata-
lytic activation of the two coupling reactions with copper(I)
halides, the catalytic process activated with Cu2Se + TBAB may
equiv.; K2CO3, 2.0 equiv.; Cu2Se nanoflakes: 1 mol% of Cu; TBAB, be envisaged as shown in Schemes 1 and 2 and for Suzuki and
1.0 mmol; DMSO, 2 ml; Temp, 160 °C.
Sonogashira coupling reactions respectively.
of Cu as a catalyst ∼83% conversion was achieved in 24 h
when DMSO as a solvent and K2CO3 as base were used at
160 °C under N2 atmosphere (Table 2). The scope of Suzuki–
Miyaura and Sonogashira coupling reactions with Cu2Se nano-
flakes as catalyst was investigated using various substrates
(Table 3) under optimum conditions spelled in Tables 1 and 2.
The performance of Cu2Se nanoflakes has been found slightly
better for Suzuki–Miyaura coupling compared to Sonogashira.
Reusability of catalyst
The recyclability of Cu2Se nanoflakes as catalyst (Table 4) was
examined for the coupling of 4-bromobenzaldehyde with
phenyl boronic acid/phenyl acetylene under N2 atmosphere. In
case of Suzuki reaction (carried out with optimized procedure)
after complete conversion (91%), the fresh lot of reactants as
per standardized procedure was added to the reaction flask
Scheme 1 Proposed mechanism of Suzuki coupling with Cu2Se
nanoflakes.
along with 1.0 mmol of TBAB and the coupling reaction
continued.
After 24 h the conversion of the added reactants was found
to be 68%. The reactants were added for third cycle in a
similar manner but without additional TBAB. The conversion
(Table 4) in 24 h dropped to 19%. The results of similar recycl-
Table 4 Recycling experiment with Cu2Se nanoflakes
Yielda (%)
Suzuki–Miyaura
Sonogashira
couplingc
S.N.
Run
couplingb
1.
2.
3.
1
2
3
91
68
19
87
42
36
a Yield: NMR based. b Reaction conditions for Suzuki coupling:
4-bromobenzaldehyde (1.0 mmol); phenylboronic acid, 1.2 equiv.;
base, 2.0 equiv.; Cu2Se nanoflakes: 1 mol% of Cu; TBAB 2.0, 4.0 mmol;
DMF, 2 ml; Temp, 110 °C; Time 24 h. c Reaction conditions for
Sonogashira coupling: 4-bromobenzaldehyde (1.0 mmol); phenylacetylene,
1.2 equiv.; base, 2.0 equiv.; Cu2Se nanoflakes: 1 mol% of Cu; TBAB,
2.0, 4.0 mmol; DMSO, 2 ml; Temp, 160 °C; Time 24 h.
Scheme 2 Proposed mechanism of Sonogashira coupling with Cu2Se
nanoflakes.
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Nanomaterials are effective catalysts for many organic Thus, Cu2Se nanoflakes appear to be an attractive alternative
syntheses due to their high surface area and reactive mor- to many palladium and Cu based catalytic systems.
phologies. They also offer the advantages of easy recovery of
products and recyclability of catalyst.29 Copper(I) halides have
been more explored for catalytic organic synthesis than nano-
Conclusion
particles containing Cu(I) which are more likely to be air stable
An easy method of synthesis of Cu2Se nanoflakes using orga-
and recyclable. Addition of external ligand(s) is a common
noselenium ligand has been developed. Powder XRD, XPS,
feature reported in case of their applications in cross-coupling
SEM-EDX and HRTEM have been used to authenticate the
reactions of aryl halide. On the other hand copper(II) oxide
phase of these flakes as tetragonal, which have been success-
fully applied as a catalyst for Suzuki–Miyaura and Sonogashira
cross coupling reactions of different aryl bromides. The Cu2Se
nanoflakes have been found efficient catalyst (1 mol% loading)
for both Suzuki–Miyaura and Sonogashira coupling reactions
of aryl halides with arylboronic acid and phenylacetylene
respectively. The catalytic efficiency of Cu2Se for Suzuki coup-
ling is slightly greater than that of Sonogashira coupling.
These nanoflakes have been found reusable for second time.
Most probably TBAB facilitates the release of CuBr from nano-
flakes which catalyze both reactions, as catalytic efficiency is
very low in the absence of TBAB and CuBr has been found to
nanoparticles have been used under ligand free conditions.30
It is therefore worthwhile to see present catalytic activation
with respect to literature reports on other copper containing
nano-phases which have catalyzed such coupling reactions.
Their number is of course small. Rothenberg et al.15 have uti-
lized heterogeneous catalysis with copper nanoclusters for pal-
ladium and phosphine free Sonogashira coupling of phenyl
acetylene with aryl halides. Rothenberg et al.15 have also
reported Suzuki cross-coupling using Cu and mixed (with Pd/
Ru) nanoclusters as catalyst. The 2 mol% of catalyst is needed
at temperature similar to ours but good conversion takes
longer time than the present catalysts for some substrates.
activate readily both the coupling reactions. In comparison to
Unlike these nanocatalysts, in the activation with Cu2Se homo-
many other copper based nano-crystals, the present nanoflakes
coupling of phenylboronic acid does not occur. For substrate
are better activator.
p-nitrobromobenzene complication of hydrogenation resulting
4-phenylaminobenzene was found absent in the present case.
Zhang et al.16 have reported the combination of Cu2O nano- Acknowledgements
particles with several ligands and TBAB as an efficient and
The authors are thankful for the support of this work by
reusable system for the Sonogashira coupling of aryl and
Department of Science and Technology New Delhi-110016,
heteroaryl halides with terminal alkynes under solventless
India under the project no. SR/WOS-A/CS-57/2012 (G). Dr
Rajendra S. Dhaka, Department of Physics, IIT Delhi, Delhi is
conditions. Out of PPh3, P(o-tol)3, PCy3, P(2,6-diMeC6H4)3,
P(n-Bu)3, POPh3, 1,1-bipyridinyl and DABCO explored PPh3 has
gratefully acknowledged for XPS measurements and discus-
been found best as ligand. With Cu2Se there is no requirement
sions. The XPS facility at Department of Physics, IIT Delhi is
of external ligand, a distinct advantage. Ranu et al.31 have
reported shape-dependent catalytic activity of copper oxide-
partially funded by FIST grant of DST.
supported on Pd(0) nanoparticles for Suzuki reactions and
found that oval shaped species catalyze Suzuki coupling
Notes and references
better. Zhang et al.17 have explored ligand free copper(II) oxide
nanoparticle catalyzed Sonogashira coupling reactions
between terminal alkynes and aryl halides. For good conver-
sion 10 mol% of copper(II) oxide nano particles as catalyst
have been reported as optimum. The reaction performed for
12 hours in the presence of potassium carbonate as base in
dimethyl sulfoxide at 160 °C gives good conversion. The CuI,
Cu(phen)(PPh3)Br and copper nanoclusters have been
employed in the Sonogashira cross-coupling reaction.19,20,32
Okuro et al.19 demonstrated the first copper catalyzed Sonoga-
shira reaction, but the scope was limited to aryl iodides. Even
1.10-phenanthroline was used as a ligand in the Sonogashira
coupling reaction catalyzed with Cu(phen)(PPh3)Br or [Cu-
(phen)(PPh3)2]NO3. Ma and Liu20 have found that CuI com-
bined with N,N-dimethylglycine as a catalytic system can
couple the deactivated aryl bromides with terminal alkynes.
The reactions conditions with present nanoflakes of Cu2Se are
less harsh in comparison to many Cu based catalytic systems
reported earlier for Suzuki and Sonogashira coupling. Further
the present nanoflakes of Cu2Se are reusable second time.
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