Disproportionation route to monodispersed copper nanoparticles for the catalytic synthesis of propargylamines

Article abstract of DOI:10.1039/c3ra43341b

By taking advantage of the coordination between a monovalent Cu⁺ precursor and trioctylphosphine, monodisperse Cu nanoparticles were synthesized via a disproportionation reaction. A Cu@SiO₂ nanocatalyst was formed by supporting Cu na

Full text of DOI:10.1039/c3ra43341b

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Disproportionation route to monodispersed copper
nanoparticles for the catalytic synthesis of
propargylamines†
Cite this: RSC Adv., 2013, 3, 19812
a
a
a
a
a
Received 1st July 2013
*
Huizhang Guo, Xiang Liu, Qingshui Xie, Laisen Wang, Dong-Liang Peng,
Accepted 12th August 2013
b
b
*
Paula S. Branco and Manoj B. Gawande
DOI: 10.1039/c3ra43341b
www.rsc.org/advances
By taking advantage of the coordination between a monovalent Cu+ growth of the Cu nanocrystals would be lost in the presence of
precursor and trioctylphosphine, monodisperse Cu nanoparticles these strong reducing conditions.18 Therefore, for the synthesis
were synthesized via a disproportionation reaction. A Cu@SiO2 of monodisperse Cu nanoparticles in solution, it is imperative
nanocatalyst was formed by supporting Cu nanoparticles onto a silica to focus our attention towards the mechanism by which Cu ions
aerogel, which showed a high surface area (779.53 m2 gÀ1) and are reduced into zero-valent Cu(0) atoms and controlling the
excellent catalytic activity for the synthesis of propargylamines.
nucleation and growth rate with stabilizing agents.19
Controllable synthesis of monodisperse Cu nanoparticles
has been a long-standing challenge posed to chemists because
of their relatively low redox potential and ease of oxidation
when exposed to air. In the quest for new nanomaterials and
applications in catalysis and sustainable protocols, some
effective methods have been developed in our groups for the
preparation of nanocatalysts.5,20,21 Herein, we present the
synthesis of monodisperse Cu nanoparticles by taking advan-
tage of the synergetic effect of trioctylphosphine (TOP) and
bromide ion (BrÀ) in a disproportionation reaction. Then, a
SiO2@Cu nanocatalyst was synthesized by supporting the Cu
nanoparticles on a silanized silica aerogel. The as-synthesized
and well characterized SiO2@Cu nanocatalyst was employed for
A3 coupling reactions. The synthesis of propargylamines is one
of the most demanded and valuable for various drug interme-
diates and other important valuable adducts.22–24 Generally,
propargylamines are prepared by stoichiometric amounts of
traditional reagents, which produce waste at the end of the
reaction, but in recent years, propargylamines have been syn-
thesised by a new one-pot A3 coupling reaction, which gives
only water as a by-product. Hence, this coupling synthesis of
propargylamines is an important alternative reaction pathway.
In addition, various methodologies for the catalytic synthesis of
propargylamines have been recently reported.25–34 However, in
most of these methodologies, gold or silver catalysts were used
in the A3 coupling reactions, which are expensive. From an
economic point of view, replacing noble metal catalysts with low
cost and robust transition metal catalysts with equal or similar
activity is also crucial in green chemistry.35
Well-dened nanostructures have emerged as versatile alter-
natives in the elds of science and technology, especially in
catalysis.1 These nanocatalysts have been extensively employed
in various catalytic processes2,3 and environmental remedia-
tion,4 as well as magnetically separable nanocatalysts.5 Among
these, supported nanocatalysts have been explored extensively
by chemists, because of their signicant contribution to the
eld of nanocatalysis. Various transition metal nanostructures
have been studied and employed for several applications. Since
the emergence of nanotechnology, Cu nanostructures have
shown a variety of applications ranging from conductive
features in electronic devices6–9 to catalysis.10–13 Meanwhile,
copper nanostructures have shown a great potential to replace
noble metals such as Ag and Au in many elds as they are
inexpensive. However, compared to their noble metal counter-
parts (namely, Pt,14 Au,15 and Ag16) which have been successfully
synthesized with various morphologies, the preparation of Cu
nanocrystals has not been equally successful.13,17 The reduction
of the hitherto considered difficult transition metals such as Ni,
Cu and Co nanocrystals in solution requires strong reducing
agents and high temperatures because of their relatively low
redox potential.7 Contradictorily, control of the nucleation and
aDepartment of Materials Science and Engineering, College of Materials, Xiamen
University, Xiamen 361005, People's Republic of China. E-mail: dlpeng@xmu.edu.
cn; Fax: +86 592 2183515; Tel: +86 592 2180155
bREQUIMTE, Department of Chemistry, Faculty of Science and Technology, New
University of Lisbon, Quinta da Torre, 2829-516 Caparica, Lisbon, Portugal. E-mail:
m.gawande@fct.unl.pt; Fax: +351 21 2948550; Tel: +351 96 4223243
Typically, CuBr was dissolved in oleylamine under strong
magnetic stirring and Ar purging. Aer that, TOP was injected
† Electronic supplementary information (ESI) available: Experimental details,
SEM and TEM images, BET data. See DOI: 10.1039/c3ra43341b
ꢀ
into the mixed solution rapidly, followed by stirring at 100 C
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for 5 min and then heating up to 260 ꢀC for the preparation of
monodisperse Cu nanoparticles. By aging the mixed solution
for 3 h, monodisperse Cu nanoparticles with an average diam-
eter of 23 nm were obtained (see Fig. S1 in ESI† for more
detailed information on size distribution). As demonstrated in
the representative TEM image in Fig. 1a, the nanoparticles are
all of spherical morphology with a narrow size distribution,
indicating a good monodispersity on carbon grids. The selective
area electron diffraction (SAED) pattern acquired from these Cu
nanoparticles (Fig. 1b) exhibits a series of diffraction rings
corresponding to the face-centered cubic (FCC) structure of Cu,
and no other diffraction rings belonging to CuO are observed,
illustrating the pure nature of the Cu nanoparticles. The freshly
prepared Cu nanoparticles were collected by centrifugation and
washed with a mixture of anhydrous hexane and acetone. Aer
dispersion in a toluene solution via ultrasound sonication for
30 min, a bright reddish solution was obtained (Fig. 1c inset).
The solution displays an absorption peak at 574 nm in the vis-
NIR extinction spectrum (Fig. 1c) due to the surface plasmon
resonance of the Cu nanoparticles. The sharp absorption peak
also indicates the narrow size distribution of the Cu nano-
particles. These freshly prepared Cu nanoparticles could be
assembled into a two dimensional superlattice on a carbon grid
(Fig. 1d) or silicon wafer (Fig. S2†) via slow evaporation of the
toluene solution of the Cu nanoparticles in a thermostat.
Furthermore, the synergetic or additive effect of TOP and BrÀ
was found to be necessary for the formation of monodisperse
Cu nanoparticles. Control experiments were carried out to verify
this conclusion. The blue solution of CuBr and oleylamine at
100 ꢀC turned achromatous aer the injection of TOP revealing
the coordination between TOP and the Cu+ ions while, in the
absence of TOP, a reddish solutiꢀon was observed when the
reaction temperature reached 210 C, indicating the formation
of Cu seed particles, which was a much lower temperature than
Fig. 2 SiO2@Cu nanocatalyst. (a) Photography of the SiO2@Cu nanocatalyst
after 2 h standing in the toluene solution. (b) SEM image. (c) TEM image. (d) XRD
pattern.
in the presence of TOP (it turned red at about 250 ꢀC). The fast
disproportionation reaction of Cu+ without the coordination of
TOP resulted in large Cu nanocrystals with chaotic morphol-
ogies (Fig. S3†). Preventing the metal ions from fast reduction to
Cu(0) seems to be necessary to accumulate enough reactant
species to reach the critical nucleation concentration to achieve
instantaneous nucleation, which is particularly important to
obtain highly monodisperse Cu nanoparticles. The formation of
Cu+–TOP complexes in the reaction solution hindered the
disproportionation reaction even under highly favorable
conditions (210 ꢀC in oleylamine solution).18 Similarly, by
taking advantage of the coordination between Pt2+ and a
surfactant, Lee et al. have successfully synthesized mono-
disperse polyhedral Pt nanocrystals.36 Meanwhile, the BrÀ ions
from the CuBr precursor are essential as well. In our previous
Table 1 Optimization of the reaction conditionsa
Entry
Catalyst
Solvent
Time (h)
Yield (%)
1
2
3
4
5
6
7
—
—
CH3CN
Water
Dioxane
AcOEt
Solvent-free
Toluene
24
6
12
7
7
5
NR
65
30
60
58
72
95
SiO2@Cu
SiO2@Cu
SiO2@Cu
SiO2@Cu
SiO2@Cu
SiO2@Cu
5
Fig. 1 Monodisperse Cu nanoparticles. (a) TEM image; (b) SAED pattern; (c)
visible-NIR extinction spectrum of the Cu nanoparticles dissolved in toluene; (d)
TEM image of the two dimensional supperlattice assembled on a carbon grid;
inset in (c) shows the photography of the toluene solution of Cu nanoparticles.
a
Reaction conditions: 4-methoxy benzaldehyde (1 mmol), phenyl
acetylene (1.5 mmol), morpholine (1.1 mmol), SiO2@Cu (50 mg),
temperature 110 ꢀC.
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Table 2 SiO2@Cu catalyzed A3 coupling reactionsa
No.
Aldehyde
Amine
Product
Time (h)
5
Yieldb (%) (TON)c
95 (1032)
1
2
3
4
5
5
5
6
6
94 (1021)
93 (1010)
86 (934)
82 (891)
6
6
85 (891)
a
Reaction conditions: aldehyde (1 mmol), phenyl acetylene (1.5 mmol), amine (1.2 mmol), SiO2@Cu (50 mg), toluene (1.5 mL), temp. 110 ꢀC.
b
Isolated yield. c Turn over number.
studies, CuCl37 and CuCl238were used as precursors to synthe- FCC Cu (JCPDS #03-1018), revealing the high stability of this
size Cu nanowires or nanocubes in oleylamine solution. If we SiO2@Cu nanocatalyst. The SiO2@Cu nanocatalyst exhibits a
used Cu(NO3)2 instead of CuBr in the typical procedure without relatively large Brunauer–Emmett–Teller (BET) specic surface
changing other factors, the Cu nanoparticles displayed a wide area of 779.53 m2 gÀ1 and many homogeneous mesopores
size distribution (Fig. S4†). This may be due to the adsorption of (Fig. S5†).
BrÀ on the surface of the seed particles which ensures a slow
and synchronous nanocrystal growth process.
The as-synthesized and well characterized SiO2@Cu nano-
catalyst was further investigated in A3 coupling reactions. The
We next supported the as-synthesized Cu nanoparticles on a catalytic activity of SiO2@Cu was initially tested for the model
silanized silica aerogel to fabricate an oleophilic SiO2@Cu reaction between 4-methoxy benzaldehyde, phenyl acetylene and
nanocatalyst (15 wt% Cu). The silica aerogel was dissolved in morpholine in various possible reaction conditions (Table 1).
the toluene solution of Cu nanoparticles by ultrasound soni-
First, we tested the reaction under catalyst-free and solvent-
cation (see ESI† for the catalyst preparation). Aer two hours free conditions at room temperature and at 110 ꢀC, but no
standing, delamination was observed in the mixed solution. product formation was observed (Table 1, entry 1). Aer opti-
The contrast between the colorless supernate and the reddish mization of the reaction conditions, it was noted that a fair yield
colloid (Fig. 2a) indicates that all Cu nanoparticles have been was obtained under solvent-free conditions (Table 1, entry 6),
adsorbed onto the SiO2 support and that strong interactions but in toluene the yield of product was excellent (Table 1, entry
between the Cu nanoparticles and the SiO2 support have been 7). This could be attributed to the high dispersibility of
formed. Fig. 2b and c display the representative SEM and TEM SiO2@Cu in toluene leading to a high surface area. In other
images of the Cu nanoparticles deposited on the SiO2 support, solvents such as water, ethyl acetate, dioxane and acetonitrile, a
respectively, from which a good dispersion of Cu nanoparticles low to moderate yield was observed (Table 1, entries 2–5) owing
is observed. The X-ray diffraction (XRD) pattern acquired from to the poor dispersibility of the SiO2@Cu catalyst in polar
the as-prepared Cu nanocatalyst aer exposure to air for one solvents. Further exploration of the substrate scope of the A3
month shows a series of diffraction peaks in accordance with coupling reaction is depicted in Table 2.
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