Surfactant-free single-nano-sized colloidal Cu nanoparticles for use as an active catalyst in Ullmann-coupling reaction

Article abstract of DOI:10.1039/c2cc30975k

Surfactant-free, single-nano-sized copper nanoparticles (Cu NPs) (size: about 2 nm) were prepared by the DMF reduction method. The Cu NPs showed high catalytic activity (with a turnover number (TON) of up to 2.2 × 10 ⁴) in Ullmann-type cross-coupling of aryl halides with phenols under ligand-free conditions. The Royal Society of Chemistry 2012.

Full text of DOI:10.1039/c2cc30975k

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COMMUNICATION
Surfactant-free single-nano-sized colloidal Cu nanoparticles for use as an
active catalyst in Ullmann-coupling reactionw
Yuto Isomura,a Takashi Narushima,b Hideya Kawasaki,a Tetsu Yonezawab and
Yasushi Obora*a
Received 10th February 2012, Accepted 27th February 2012
DOI: 10.1039/c2cc30975k
Surfactant-free, single-nano-sized copper nanoparticles (Cu NPs)
(size: about 2 nm) were prepared by the DMF reduction method.
The Cu NPs showed high catalytic activity (with a turnover number
(TON) of up to 2.2 Â 104) in Ullmann-type cross-coupling of aryl
halides with phenols under ligand-free conditions.
Reflecting on these studies using an inexpensive Cu-catalyzed
reaction, we became aware of the necessity of investigating
catalytically active support/ligand/surfactant-free single-nano-sized
Cu nanoparticles in a cross-coupling reaction.
More recently, we have studied the surfactant-free solution
synthesis of noble-metal (Au, Pt and Pd) nanoclusters (NCs)
(diameter 1–1.5 nm) using the DMF reduction method.10 We
found that the use of colloidal Pd NCs exhibits in highly
catalytic cross-coupling reactions because of the ultra-small
sizes and the surfactant free-active surfaces.10c The Pd NCs
had a high turnover number, up to 6.0 Â 108 was achieved.
Inspired by these studies on these noble metal NCs as active
catalysts, we report the solution synthesis of surfactant-free,
single-nano-sized Cu NPs (size: about 2 nm) using the DMF
reduction method. We first found that the tiny Cu NPs showed
high catalytic activity under unusual ligand-free conditions in
Ullmann-type O-arylation reactions.
The interest and use of transition metal nanoparticles (NPs)
(diameter B100 nm) has shown an exponential growth in the past
few years.1 In particular, transition metal NPs have become a very
important class of catalysts owing to their inherently large surface
area, being different from that of the bulk metal.1 To date, various
synthetic methods of colloidal metal NPs have been developed and
used as catalysts in chemical transformations.1b,c Such metal NPs
are prone to lose reactivity in their bulk metal form; therefore,
several stabilizers such as functionalized polymers,2 dendrimers,3
inorganic solids,4 ligands,5 and ionic surfactants6 are generally
prerequisites to prevent aggregation of the metal NPs in these
preparations.
The Cu NPs can be prepared simply according to the
following one-step reaction. A solution of 15 mL of 0.1 M
aqueous CuCl2 was added to 15 mL of DMF at 140 1C; the
DMF solution was heated under reflux at 140 1C for 8 h.
The transmission electron microscopy (TEM) image of the
Cu NPs shows numerous nanoparticles of size approximately
2 nm (Fig. 1, left). The high-magnification TEM image of Cu
NPs shows that the crystal lattice fringes are 0.2 nm apart,
which agrees with the d value of the (111) planes of the metallic
Cu crystal (see ESIw). Dynamic Light Scattering (DLS)
measurement of the Cu NPs reveals that the average diameter
of the Cu NPs is around 2 nm (Fig. 1), which is in good
agreement with the HRTEM image. The resulting light-yellow
solution was photoluminescent and the maximum emission
wavelength was around 420 nm for UV excitation at 350 nm
On the other hand, the use of precious metals limits the
attractiveness in industrial applications owing to their high
cost. As an alternative, Cu is widely used industrially and is an
inexpensive metal, and considerable efforts have been devoted
to explore its use in catalysis.7 For instance, Cu-catalyzed
arylation of phenols (Ullmann condensation) is an important
organic transformation.8 However, these examples generally
require large Cu loadings (5–20 mol%) and are highly dependent
on the use of pyridine- or phosphine-type ligands to achieve
high catalytic activity. The employment of nano-particulate Cu
catalysts represents a much more recent research trend. Various
exploitable methods exist for the synthesis of Cu NPs and their
reactions.8l,m,p–r,9 These Cu nanoparticle catalysts are attractive
both from economic and industrial points of view, as compared
to those of the precious metals.
a Department of Chemistry and Materials Engineering, Faculty of
Chemistry, Materials and Bioengineering, Kansai University, Suita,
Osaka 564-8680, Japan. E-mail: obora@kansai-u.ac.jp;
Fax: +81-6-6339-4026; Tel: +81-6-6368-0876
b Division of Materials Science and Engineering, Faculty of
Engineering, Hokkaido University, Sapporo, Hokkaido 060-8628,
Japan
w Electronic supplementary information (ESI) available: Photo-
luminescence emission spectrum; XPS spectrum; experimental and
characterization data and 1H and 13C NMR spectra for products
3a–3i. See DOI: 10.1039/c2cc30975k
Fig. 1 Left: HRTEM image of DMF-protected Cu NPs. Bar length:
5 nm. Right: DLS spectrum of Cu NPs.
c
3784 Chem. Commun., 2012, 48, 3784–3786
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Table 1 Cu-NPs-catalyzed Ullmann O-arylation of iodobenzene (1a)
with 3,5-dimethyl phenol (2a)a
Table 2 Cu-NP-catalyzed Ullmann O-arylation of aryl halides (1)
with various phenols (2)a
Entry
Solvent
T/1C
Yield 3ab (%)
TONc
Entry Ar–X
1d
1
Ar’–OH
2
Yieldb (%)
TONc (Â103)
1
2
3
4
DMF
DMF
DMF
MeCN
NMP
DMF
DMF
80
7
7.0 Â 102
1.0 Â 104
8.8 Â 103
5.1 Â 103
1.2 Â 103
2.2 Â 104
—
110
140
110
110
110
110
499(88)
88
51
12
22
1a
2b 93(85)
2c 85(81)
3b 9.3
5
2d
3d
1a
1a
3c 8.5
3d 7.1
6d
7e
n.d.
a
Conditions: 1a (1.5 mmol) was allowed to react with 2a (1.0 mmol) in
the presence of 1 mL of Cu NP catalyst solution (0.1 mM) and Cs2CO3
2d 71(65)
b
(2.0 mmol) in solvent (1 mL) at T 1C for 24 h. GC yields based on 2a
used. The number in the parentheses shows the isolated yield. TON =
c
3a (mol)/Cu NPs (mol). d 1 mL of Cu NP catalyst solution (10 mM) and 1a
4d
5d
1a
1a
2e 64(60)
2f 70(66)
3e 6.4
3f 7.0
(15 mmol) used. e Without Cu NPs.
because of the ultra-small sizes of around 2 nm (see ESIw).11
The chemical composition of the Cu NPs was examined by
X-ray photoelectron spectroscopy on a dried sample (see ESIw).
The major contribution of the Cu 2p3/2 peak (932.0 eV) is that
associated with zerovalent copper12 However, the minor shoulder
peak with the smaller contribution at 934.4 eV is consistent with
CuO,12 suggesting the existence of a thin copper oxide layer around
the particle surfaces.
6
7
8
9
1b
1c
1d
1e
1f
2a 89(83)
2a 87(80)
2a 90(75)
2a n.d.
3g 8.9
3he 8.7
3i 9.0
The Cu NPs prepared by the DMF reduction method were
evaluated as catalysts for arylation of phenols, which is an efficient
catalytic method for aryl–oxygen bond formation in organic
synthesis.8 As shown in Table 1, the reaction conditions were
optimized by using the reaction between iodobenzene
(1a: 1.5 mmol) and 3,5-dimethylphenol (2a: 1.0 mmol) as the
model reaction. For instance, Cu NPs (1 Â 10À2 mol% based on
2a used) in DMF were used at 80 1C, and the desired product 3a
was obtained in only 7% yield (entry 1, Table 1). Then, when the
reaction temperature was raised to 110 1C, quantitative yield and
88% isolated yield was achieved (entry 2). In this reaction, Cs2CO3
was found to be a suitable base, whereas other bases such as
Na2CO3, KOH, K3PO4, and NEt3 totally inactivated the catalytic
activity in this reaction under these conditions. Next, the solvent
was evaporated under vacuum from a DMF solution of Cu NPs,
and the residue was re-dissolved in other polar-solvents such as
acetonitrile (MeCN) and N-methylpyrrolidone (NMP); then these
solutions were used in the reaction. The reactions using MeCN
and NMP were not effective and gave 3a in 51% and 12% yield,
respectively (entries 4 and 5). Here, the conversions of 1a and 2a
were low and remained unchanged. Therefore, DMF was
considered to be the best solution as a solvent in this reaction.
When the catalyst loading was decreased to 1 Â 10À3 mol%, the
highest turnover number (TON, 2.2 Â 104) was achieved in the
presence of 15 mmol of 1a (entry 6). Needless to say, no reaction
was induced in the absence of Cu NPs (entry 7). The present
support/ligand/surfactant-free Cu NPs showed remarkable
catalytic activity which showed almost an order of magnitude
higher turnover number than Cu2O nanocube.8p
3j
—
10d,f
2a 93(85)
2b 70(66)
3a 9.3
11d,f
1f
1f
3b 7.0
3c 8.9
12d,f
2c 89(84)
a
b
Conditions: same as entry 2, Table 1. GC yields based on 2 used.
c
The numbers in the parentheses show the isolated yields. TON =
d
e
3 (mol)/Cu NPs (mol). Reaction temperature was 140 1C. White
solid: mp: 65–67 1C (Lit:8e 67 1C). Bromobenzene (15 mmol) used.
f
Table 2. In the reaction of 1a with various substituted phenols,
the corresponding diaryl ethers were obtained in good to
excellent yields (entries 1, 2 and 4–7, Table 2). Phenols bearing
methyl (2b) and methoxy (2c) substituents provided arylation
products in high yields (entries 1 and 2). Reaction of o- (2d),
m- (2e), p-cresol (2f) gave the corresponding products (3d–3f)
in good yields (entries 3–5). To further explore the scope of
this reaction, several aryl halides were used (entries 6–12).
The substrates bearing electron-donating and electron-
withdrawing groups were tolerated as coupling partners
(entries 6–8). The use of iodothiophene (1e) resulted in total
On the basis of the optimized reaction conditions, we
expanded the scope of the reaction substrates, as shown in
c
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catalytic inactivity due to aggregation of the Cu NPs during
the reaction course (entry 9). When the amount of bromobenzene
(1f) was increased to 15 mmol, it also afforded arylation products
in excellent yields (entries 10–12).
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The reusability of the Cu NPs catalyst was examined under
the conditions given in entry 2, Table 1. In the first cycle, the
product 3a was obtained in quantitative yield with complete
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catalytic sequence by adding 1a, 2a and Cs2CO3. The Cu
NPs could be re-used as a catalyst for the coupling reaction at
least three times.
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In conclusion, we have developed Cu NPs with high surface
area for effective utilization of metallic-resources in chemical
transformations. The Cu NPs were firstly prepared by surfactant-
free, DMF reduction methods, the particle size of which (about
2 nm) was estimated by HRTEM as well as DLS spectrum.
These tiny Cu NPs served as an efficient catalyst in the cross-
coupling reaction of aryl halides with phenols under low catalyst
loading and ligand-free conditions.
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This work was supported by the Strategic Project to Support
the Formation of Research Bases at Private Universities,
matching fund subsidy from the Ministry of Education,
Culture, Sports, Science and Technology, Japan. This study
was partially supported by Grants-in-Aid for Scientific
Research (B) (No. 23360361 (to HK), 21310072 (to TY) and
ALCA (to YO) from the Japan Society for the Promotion of
Science (JSPS), the Ministry of Education, Culture, Sports,
Science and Technology (MEXT), Japan, and Japan Science
and Technology Agency (JST).
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