Selective synthesis of Cu2O nanocrystals as shape-dependent catalysts for oxidative arylation of phenylacetylene

Article abstract of DOI:10.1002/chem.201200567

Shape-dependent nanocatalysis: Monodisperse cubic, rhombic dodecahedral, and octadecahedral Cu₂O nanocrystals (NCs) were selectively synthesized. Cu₂O octadecahedra exhibited the best activity upon recycling, though etched on nanoframes, in the ligand-free oxidative arylation of phenylacetylene. Rhombic dodecahedra had moderate activity and optimal stability, whereas cubes suffered significant loss of activity during recycling. Copyright

Full text of DOI:10.1002/chem.201200567

COMMUNICATION
DOI: 10.1002/chem.201200567
Selective Synthesis of Cu O Nanocrystals as Shape-Dependent Catalysts for
2
Oxidative Arylation of Phenylacetylene
[
a]
Lingling Li, Caiyun Nan, Qing Peng,* and Yadong Li
Copper(I) oxide is a typical p-type semiconductor with
a band-gap energy of 2.17 eV. In recent years, great effort
NCs provides an ideal model catalyst with well-defined size
and geometric structures for organic reactions. Herein, we
has been put into the synthesis of Cu O nanocrystals (NCs)
report the selective synthesis of monodisperse Cu O cubes,
rhombic dodecahedra, and octadecahedra in an octadecyl-
ACHTUNGTRENNUNaG mine (ODA) system as well as their catalytic properties in
2
2
[1]
and their applications such as photocatalysis, CO oxida-
[2]
[3]
tion, and gas sensing. In particular, Cu O (bulk or in NC
2
form) was reported as an efficient catalyst for CÀC, CÀN,
our designed aerobic oxidative coupling of arylboronic acids
and phenylacetylene (Scheme 1).
[4]
and CÀO bond-formation cross-coupling reactions. Lately,
internal alkynes have been produced by cross-coupling of al-
kynes with arylboronic acids, which are less toxic and more
[5]
temperature- and air-stable. However, most of these stud-
ies involved noble palladium salts or copper halides as cata-
lysts and/or the addition of ligands and additives. Cu O was
2
Scheme 1. The designed aerobic oxidative coupling reaction.
also demonstrated to have highly efficient catalytic activity
for the synthesis of internal alkynes via aerobic oxidative ar-
[
5d]
ylation of terminal alkynes using pyridine as additive.
Cu O NCs were prepared by using a modified protocol
2
[3c,9]
However, several problems arise by the addition of ligands
and additives: 1) it is difficult to recycle the catalysts, 2) cost
and waste are greatly increased, and 3) research into their
shape-dependent catalytic properties is limited owing to co-
ordination of the catalysts with ligands or additives in the
reaction system.
for alkylamine systems.
Scanning electron microscopy
(SEM) investigation revealed three types of NCs: cubes,
rhombic dodecahedra, and octadecahedra, all of which were
obtained with narrow size distribution (Figure 1a–c). Their
X-ray diffraction (XRD) patterns were all indexed to pure
cubic-phase Cu O (JCPDS No. 78-2076, Pn3m, a=
2
The advantages of nanoparticles have been extensively
0.4267 nm; Figure 1d). Notably, the intense ratio of (110)
and (200) varied among the three samples. Rhombic dodec-
ahedra and cubes are unifaceted with {110} and {100}, re-
[6]
recognized in organic catalysis. Cu O nanoparticles can
2
have enhanced activity even in the absence of ligands or ad-
ditives thanks to their large surface-to-volume ratio and
good dispersion in catalytic reaction systems; this provides
an opportunity to solve the above-mentioned problems and
to serve as a complement to traditional reaction conditions.
Moreover, the catalytic properties of nanoparticles depend
on the type and distribution of crystal planes, edges, corners,
[3c]
spectively, as previously reported.
Cu O octadecahedra
2
were single crystalline, each composed of six {100} and
twelve {110} facets as demonstrated by high-resolution trans-
mission electron microscopy (HRTEM; Supporting Informa-
tion, Figure S1a), which is in accordance with the moderate
intense ratio of (110) and (200) in the above XRD patterns.
[7]
and individual chemicals in the reaction system. Therefore,
control over nanoparticle size and morphology may in turn
allow control in the specific activity and selectivity for or-
ganic catalysis. That is, ligand- and additive-free synthesis of
internal alkynes may be realized with arylboronic acids as
The Cu O octadecahedron is essentially a truncated rhombic
2
dodecahedron with six of its fourteen vertices truncated by
[8c,10]
{100} planes.
This is the first report on the synthesis of
Cu O octadecahedra with such a narrow size distribution,
2
and sizes could be easily tuned from 125 to 500 nm by con-
one of the substrates catalyzed by Cu O NCs of various
trolling formation times at 2008C (Figure S2).
2
shapes and sizes.
To understand the synthetic mechanism of Cu O NC for-
2
Over the past decades, much progress has been made in
mation, we carried out two proof experiments without the
addition of ODA. Polydisperse microcrystals of 1.5–2 mm
and nanoparticles were obtained in n-hexadecane and 1-oc-
tadecene (ODE), respectively (Figures S3a and S3b). XRD
[1–3,8]
the controlled synthesis of Cu O.
Fine-tuning of Cu O
2
2
[
a] L. Li, C. Nan, Dr. Q. Peng, Prof. Y. Li
Department of Chemistry
Tsinghua University, Beijing, 100084 (P.R. China)
patterns confirmed them to be cubic-phase Cu O (Fig-
2
ure S3c). It is well known that thermal decomposition of Cu-
AHCTUNGERTNNUN(G CH COO) ·H O (Cu ACHTUNRTGUNNEGN( OAc) ·H O) produces cuprous salt
3 2 2 2 2
Fax : (+86) 10-62788765
above a certain temperature (1708C, for instance), depend-
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201200567.
[11]
ing on the atmosphere, heating rate, etc. Therefore Cu O
2
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cross-linking interactions are
[12]
commonly used.
ODA has
good solubility in ethanol, yet is
hardly soluble in n-hexane at
room temperature. Cross-link-
ing interactions between the
dissolved ODA molecules and
those tethered to the surfaces
of Cu O octadecahedra caused
2
their ordered self-assembly. The
magnitude of cross-linking de-
pends on the concentration of
linkers in solution and the bond
[12]
energies. As a result, self-as-
sembly was difficult in n-
hexane due to the poor solubili-
ty of ODA.
Having established that, we
tested the catalytic properties
of Cu O octadecahedra. Cu O
2
2
octadecahedra
with
size
~
125 nm were chosen as the
model catalyst. The reaction
shown in Scheme 1 was carried
Figure 1. Typical SEM images of Cu
c) octadecahedra. d) XRD patterns of as-obtained Cu O NCs.
2
2
O NCs with various morphologies: a) cubes, b) rhombic dodecahedra, and out without the addition of li-
gands (see the Experimental
Section below for details). Phe-
nylboronic acid (1a) and phe-
NCs were actually obtained by thermal decomposition of
Cu(OAc) ·H O in the present work. Herein, ODA served as
an intermediate for electron transfer while facilitating the
nylacetylene (2) were chosen as model substrates to opti-
mize reaction conditions by altering the bases (Table 1).
ACHTUNGTRENNUNG
2
2
N,N’-Dimethylethylenediamine (DMEDA) and Cs CO3
2
[9]
nucleation and growth kinetics of Cu O NCs. In addition,
gave disappointing results (entries 1 and 2), whereas product
1,2-diphenylethyne (3a) was obtained in 89% yield with
tBuOK as the base (Table 1, entry 3). Two additional sub-
strates of arylboronic acids 1b and 1c were tested under the
optimized reaction conditions, and both provided excellent
yields (Table 1, entries 4 and 5).
2
the shapes of Cu O NCs were inherently sensitive to the
2
molar ratio of cupric precursor to ODA. Cubes, octadecahe-
dra, and rhombic dodecahedra were successively obtained
upon decreasing the above-mentioned molar ratio. In other
words, {100} facets are more stable than {110} in high con-
centrations of ODA. This could be ascribed to the aniso-
tropic adsorption capacity of ODA on different facets of
Table 1. Cu
ic acid and phenylacetylene.
2
O octadecahedra-catalyzed oxidative arylation of arylboron-
Cu O NCs.
[a]
2
As shown in Figure S1b, 2D arrays of Cu O octadecahe-
2
dra with 100% morphology yield had a tendency to lie on
{
110} facets when doped on the silicon wafer. Viewed in the
[b]
Entry
Ar (1)
(1a)
Base
Product
Yield [%]
direction of <110>, subunits of the building blocks adopted
a quasi-hexagonal configuration (labeled by a white square
in Figure S1b). It should be noted that Cu O octadecahedra
tend to disassemble when dispersed in n-hexane (Fig-
ure S1d). In the context of self-assembly, it was reported
that van der Waals (vdW) interaction among the surface al-
kylamine molecules give rise to the formation of Cu O
nanocrystal superlattices. However, the size of Cu O octa-
1
2
3
4
5
C
6
H
5
DMEDA
Cs CO
3a
3a
3a
3b
3c
13
trace
89
C H (1a)
6
6
5
5
2
3
C
4-CH
H
(1a)
(1b)
(1c)
tBuOK
tBuOK
tBuOK
2
[
c]
3
OC
6
H
5
92
92
[
c]
4-ClC
6
H
5
[
1
a] Reactions were carried out at 608C under ambient conditions for
8 h; catalyst: 0.02 mmol Cu O, solvent: 2 mL dioxane and 0.5 mL
iPrOH. [b] GC yield using n-hexadecane as the internal standard.
2
2
[3c]
1
2
[c] H NMR yield using trichloroethylene as the internal standard.
decahedra is much larger than the length scale of vdW
forces (<100 nm) in our work, which made it rather difficult
to form large areas of Cu O superlattice self-assembly by
2
vdW forces. At present, a wide variety of interparticle inter-
actions (vdW, hydrogen bonds, electrostatic and molecular
surface forces) have been studied extensively, among which
Thereafter we studied the catalytic properties of the three
differently shaped Cu O NCs (~125 nm) upon recycling
2
(Figure S4). As shown in Table 2, all showed excellent yields
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Shape-Dependent Cu O Nanocrystal Catalysts
2
COMMUNICATION
Table 2. Cu
and phenylacetylene upon recycling.
2
O NC-catalyzed oxidative arylation of phenylboronic acid
Cu O rhombic dodecahedra, no change was observed in
either size or morphology during the reaction (Figure 2e,f
and Figures S6a and S6b). Considering the above SEM
2
[
a]
images, {100} facets of Cu O cubes are speculated to be vul-
nerable to perturbation of the organic reaction, whereas
2
[
b]
Entry
Catalyst
Cycle No.
Yield [%]
1
2
3
4
5
6
7
8
9
cubes
cubes
cubes
1
2
3
1
2
3
4
1
2
3
4
1
1
1
94
47
32
84
87
96
90
89
93
92
97
47
–
{
110} planes of Cu O rhombic dodecahedra were quite
2
stable. SEM images of Cu O octadecahedra during recycling
2
further confirmed this speculation. As shown in Figure 2h,
rhombic dodecahedra
rhombic dodecahedra
rhombic dodecahedra
rhombic dodecahedra
octadecahedra
octadecahedra
octadecahedra
octadecahedra
a large portion of Cu O octadecahedra were distorted slight-
2
ly concave on certain facets after the first run (marked with
white squares in Figure 2h); 0.012 mmol of Cu O octadeca-
2
hedra remained, as confirmed by inductively coupled
plasma (ICP) mass spectrometry, and were nearly etched to
10
11
12
13
14
nanocages after the fourth cycle (Figure 2i). Cu O octadeca-
2
commercial Cu
no catalyst
CuO nanoleaves
2
O
hedra were carefully examined after the third run, to ad-
dress detailed shape evolution of the catalyst during recy-
–
cling. Under closer observation of a single Cu O octadeca-
2
[
a] Reactions were carried out at 608C under ambient conditions for
8 h; catalyst: 0.02 mmol Cu O, solvent: 2 mL dioxane and 0.5 mL
hedron, all the concave surfaces could be ascribed to {100}
facets (Figure 3a,b).
1
2
iPrOH. [b] GC yield using n-hexadecane as the internal standard.
In previous reports, Cu O nanoframes could be prepared
2
by particle aggregation and acidic etching or selective oxida-
[
1b,8c]
in the first run (entries 1, 4, and 8), but there was considera-
ble distinction in their activities in the following cycles (en-
tive etching.
To understand the formation of Cu O octa-
2
decahedral nanoframes in our catalytic reaction, X-ray pho-
toelectron spectroscopy (XPS) was performed to detect the
valence of catalysts. XPS results show only two peaks at
932.2 and 951.9 eV in the Cu 2p photoelectron spectrum of
tries 2, 3, 5, 6, 7, 9, 10, and 11). Cu O octadecahedra and
2
rhombic dodecahedra could be reused at least four times
without appreciable loss of activity, whereas Cu O cubes
2
had very poor recyclability, with half of the activity lost in
the second run (entry 2). Compound 3a was obtained in
Cu O octadecahedra before being subjected to the catalytic
reaction, indicating their surfaces remained Cu . However,
2
I
4
7% yield by using commercial Cu O microcrystals as the
after the third catalytic cycle for Cu O octadecahedra, two
2
2
catalyst (Table 2, entry 12). Importantly, no product could
be detected in the absence of catalyst (entry 13) by GC. In
additional peaks at 933.5 and 953.9 eV appeared, and could
be indexed to Cu 2p3/2 and 2p1/2 of CuO, respectively. This
view of the above results, Cu O octadecahedra were the
means that partial Cu O octadecahedra were oxidized to
2
2
most robust catalysts with the highest activity and could
remain consistently active in the recycled runs of the ligand-
free oxidative arylation reaction.
CuO during aerobic oxidative arylation. The three different
Cu O NC shapes naturally displayed distinct shape evolu-
2
tion (or stability) during the catalytic reaction, as illustrated
in Figure 3d. Accordingly, a primary consideration was
The present Cu O NC-catalyzed aerobic oxidative cou-
2
pling reaction could be defined as semi-homogeneous, given
taken into the formation mechanism of Cu O octadecahe-
2
the good dispersion of Cu O NCs in the reaction system
dral nanoframes. Active copper atoms are thought to be
more readily “pulled” from {100} facets than {110} and oxi-
dized to CuO in the present reaction environment (namely,
leaching). It should be noted that partial oxidative etching
2
(
insets at top right of Figure 2). The three different Cu O
2
NC shapes showed similar catalytic activity in the first run,
despite their variations in morphologies and exposed facets.
Their specific surface area was preliminarily calculated on
the assumption that they all have symmetrical geometries
of Cu O octadecahedra did not elicit loss of activity for the
2
catalytic reaction. We prepared CuO nanoleaves of 100 nm
length and 50 nm width (Figure S7). They did not show ac-
tivity in the arylation of phenylacetylene, as detected by
(
Figure S5). The reaction appeared not to be sensitive to
[13]
nanocatalyst morphology, as previously reported. Howev-
er, the activity of Cu O NCs upon recycling (or stability) de-
GC. As shown in Figure 3c, only partial Cu O octadecahe-
2
2
pends heavily on their morphology. It is expected that sur-
face reconstruction or dissolution of active atoms at corners
or edges by the reactants or even the solvent occurs during
dra were oxidized to CuO in the catalytic reaction, which
may decrease the amount of Cu O catalyst. However, the
2
exposure of more active facets (the concave counterparts)
after being etched could finally improve their catalytic activ-
[14]
catalysis. After each cycle, Cu O NCs were examined by
2
SEM (Figure 2 and Figure S6). Cu O cubes had the lowest
ity. For Cu O rhombic dodecahedra, no loss of activity was
2
2
stability, with their surfaces etched rough after the first cycle
of oxidative arylation (Figure 2b). During the second run,
severe aggregation was caused by close linking of smaller
Cu O fragments. The decreased activity of Cu O cubes
caused, as their size and morphology did not change during
recycling. The slight improvement might be attributed to the
[15]
removal of surfactant on Cu O NCs in the recycled runs.
2
However, nanoframes could not be formed, as they are uni-
2
2
could be due to the loss of active atoms into the reaction
system and their difficulty in being recycled. In the case of
faceted with {100} for Cu O cubes; the severe leaching of
active atoms brought about a significant loss of activity
2
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Q. Peng et al.
Figure 2. Typical SEM images of Cu
2
O NCs before and after the catalytic reaction: cubes a) before and after the b) first and c) second cycles. Rhombic
dodecahedra d) before and after the e) first and f) fourth cycles. Octadecahedra g) before and after the h) first and i) fourth cycles. The catalytic reaction
systems are shown in the insets.
during recycling. We are currently working on further mech-
anistic investigations.
In conclusion, cubic, rhombic dodecahedral, and octadeca-
catalytic performance in practical organic reactions, which
could aid in the selection of suitable nanocatalysts with high
activity upon recycling for idiographic reactions.
hedral Cu O NCs were selectively synthesized in the ODA
2
system assisted by ODE, among which Cu O octadecahedra
2
with such a narrow size distribution are reported herein for
the first time. The 2D self-assembly and disassembly of
Experimental Section
Cu O octadecahedra can be easily controlled by their disso-
lution in ethanol and n-hexane, respectively. Meanwhile, we
designed a ligand-free arylation of phenylacetylene with ar-
Typical procedure for the synthesis of Cu
0%; Alfa Aesar) was introduced to molten ODA (4.04 g, AR; Sino-
pharm Chemical Reagent Co. Ltd.) at 808C in a capped vial. After stir-
ring for 10 min, Cu(OAc) ·H
the mixture; stirring was continued at 1008C for 15 min to ensure com-
plete dissolution. The mixture was then heated at 2008C for 25 min.
2
O octadecahedra: ODE (2 mL,
2
9
A
H
U
G
R
N
U
G
2
2
O (0.1 g, ꢀ98%; Alfa Aesar) was added to
ylboronic acids catalyzed by as-obtained Cu O NCs. They
2
all show good to excellent yields, while displaying distinct
shape evolution (or stability) during the catalytic reaction.
General procedure for the oxidative arylation of phenylacetylene: Sub-
Cu O octadecahedra have the best catalytic activity upon re-
2
strates 1 (0.3 mmol) and 2 (0.2 mmol), base (1 mmol), catalyst (Cu O
2
NCs, 0.02 mmol), and solvent (2.5 mL) were added to a 10 mL two-
necked flask. The reaction mixture was stirred at 608C for the indicated
time. After reaction, EtOAc (10 mL) was added to the flask, and the re-
sulting mixture was centrifuged at 6000 rpm (room temperature, 5 min).
The supernatant was collected, and EtOAc (5 mL) was then added to the
cycling, although they were etched on {100} facets in the for-
mation of nanoframes, possibly through oxidative etching.
This work provides not only a facile and selective synthesis
of Cu O NCs, but also offers a better understanding of their
2
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Shape-Dependent Cu O Nanocrystal Catalysts
2
COMMUNICATION
Figure 3. a) A typical TEM image of Cu
Cu O octadecahedra before the catalytic reaction and after the third cycle; the inset shows a model of a Cu
of the three different Cu
2
O octadecahedra after the third cycle of the catalytic reaction and b) their corresponding SEM image. c) XPS of
2
2
O octadecahedral nanoframe. d) Evolution
2
O NC shapes during the catalytic reaction; molecular O
2
is displayed as ball-and-stick models.
precipitate for the next centrifugation; after three further cycles of centri-
fugation (same conditions as above), the combined supernatant was con-
centrated under reduced pressure, and later dissolved in EtOAc (2 mL)
for further analysis. Thin-layer chromatography (TLC) was used to moni-
tor all reactions. GC was operated on an SP-6890 instrument. Yields
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Revised: May 15, 2012
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5
, 1600–1630.
Published online: && &&, 0000
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6
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www.chemeurj.org
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 0000, 00, 0 – 0
ÝÝ
These are not the final page numbers!

Shape-Dependent Cu O Nanocrystal Catalysts
2
COMMUNICATION
Shape-dependent nanocatalysis: Mon-
odisperse cubic, rhombic dodecahe-
Nanocrystals
dral, and octadecahedral Cu O nano-
crystals (NCs) were selectively synthe-
2
L. Li, C. Nan, Q. Peng,*
Y. Li ............................ &&&& — &&&&
sized. Cu O octadecahedra exhibited
2
Selective Synthesis of Cu O Nanocrys-
the best activity upon recycling though
etched nanoframes in our ligand-free
oxidative arylation of phenylacetylene.
Rhombic dodecahedra had moderate
activity and optimal stability, whereas
cubes suffered significant loss of activ-
ity during recycling.
2
tals as Shape-Dependent Catalysts for
Oxidative Arylation of Phenylacety-
lene
Chem. Eur. J. 2012, 00, 0 – 0
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
&
7
&
ÞÞ
These are not the final page numbers!