Nanoporous copper metal catalyst in click chemistry: Nanoporosity-dependent activity without supports and bases

Article abstract of DOI:10.1002/adsc.201100760

Nanoporous copper (CuNPore) catalysts with tunable nanoporosity were fabricated from Cu₃₀Mn₇₀ alloy by controlling the de-alloying temperature under free corrosion conditions. The tunable nanoporosity of CuNPore led to a significant

Full text of DOI:10.1002/adsc.201100760

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DOI: 10.1002/adsc.201100760
Nanoporous Copper Metal Catalyst in Click Chemistry:
Nanoporosity-Dependent Activity without Supports and Bases
Tienan Jin,a,* Mei Yan,b Menggenbateer,c Taketoshi Minato,d Ming Bao,e
and Yoshinori Yamamotoa,*
a
WPI–Advanced Institute for Materials Research (WPI–AIMR), Tohoku University, Sendai 980-8577, Japan
Fax : (+81)-22-217-5129; e-mail: tjin@m.tohoku.ac.jp or yoshi@m.tohoku.ac.jp
Department of Chemistry, Graduate School of Science, Tohoku University, Sendai 980-8578, Japan
Center for Integrated NanoTechnology Support, Tohoku University, Sendai 980-8577, Japan
Institute for International Advanced Interdisciplinary Research (IIAIR), International Advanced Research and
b
c
d
Education Organization, Tohoku University, Sendai, 980-8578, Japan
State Key Laboratory of Fine Chemicals, Dalian University of Technology, Dalian 116012, Peopleꢀs Republic of China
e
Received: September 30, 2011; Published online: November 16, 2011
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/adcs.201100760.
Abstract: Nanoporous copper (CuNPore) catalysts
with tunable nanoporosity were fabricated from
Cu30Mn70 alloy by controlling the de-alloying tem-
perature under free corrosion conditions. The tuna-
ble nanoporosity of CuNPore led to a significant
enhancement of catalytic activity in click chemistry
without using any supports and bases. Characteriza-
tion of CuNPore surface, high reusability, leaching
experiment, and formation of nanostructured
copper acetylide revealed that the click reaction oc-
curred at the catalyst surface.
methodology. Moreover, in contrast to the supported
nanoparticles (NPs) catalyst, a nanoporous metal
without supports should be a challenging catalyst
system to understand the relevant catalytic mecha-
nism more easily and to extend the catalytic applica-
tion widely by elimination of the support effect prob-
lems and relaxation of aggregation. In this regard, we
have focused on the study of the catalytic properties
of unsupported nanoporous metals in organic molecu-
lar transformations under liquid-phase conditions.[6]
Nanoporous gold exhibited a remarkable catalytic ef-
ficiency and reusability in the oxidation of organosi-
lanes into silanoles with water,[6a] and non-porous pal-
ladium without ligands and supports showed high cat-
alytic activity and reusability in the Suzuki cou-
pling.[6b] Therefore, the design and synthesis of new
and efficient nanoporous metal catalytic systems for
the development of molecular transformations are
highly desirable.
Keywords: click reaction; heterogeneous catalysis;
high reusability; nanoporous copper catalyst; tuna-
ble nanoporosity
Nanoporous metals are promising materials for catal-
In 2002, Sharpless[7a] and Meldal[7b] reported inde-
ysis,[1–3] sensing,[4] and actuation[5] applications due to pendently the Huisgen [3+2]cycloaddition of termi-
their interesting structural, optical and surface proper- nal alkynes and organic azides catalyzed by homoge-
ties. In spite of the growing number of studies on vari- neous Cu(I) salts, the so-called click reaction. This
ous applications of nanoporous metals, the catalytic methodology has been emerged as the most powerful
properties in chemical reactions are still less explored, tool for connecting the two useful functional groups.
although the interesting features of nanoporous During the last few years, numerous applications of
metals make them potentially attractive candidates the click reaction have been reported in the field of
for new heterogeneous catalysts;[1–3] the three-dimen- material sciences, bio- and medicinal chemistry.[8] The
sional, open-pore network structures and metal liga- click reaction can be catalyzed also by heterogeneous
ments allow the transport of molecules and ions, a copper(I), such as immobilized copper nanoclusters
high surface-to-volume ratio in comparison with bulk and copper/cuprous oxide NPs, copper-in-charcoal
metals results in outstanding catalytic efficiency; fur- NPs, copper zeolites, and copper nitride NPs support-
thermore, the potentially high reusability and rather ed on mesoporous SiO2 with or without bases.[9]
simple work-up are favorable for practical synthetic Copper metal alone also has been reported to cata-
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lyze this click reaction, while the reaction is relatively electrolyte concentration under an electrochemical
slow and requires high catalyst loading.[10] Recyclabili- method.[11e] However, the small thickness (~20 mm)
ty of these heterogeneous copper catalysts was rarely and width (~1 mm) of the used ribbon would be brit-
reported.[9i] Recently, it has been reported that the tle during the catalytic transformations. After screen-
monolithic CuNPore is a promising high strength/low ing a number of de-alloying conditions, we fabricated
density material, which can be synthesized through CuNPore catalysts with tunable nanoporosity by
the chemical or electrochemical de-alloying of various changing the de-alloying temperature under a modi-
single-phase Cu/M (M=Al, Mg, Mn) alloys.[11] fied Hayes method;[11a] free corrosion of a homogene-
Herein, we report an efficient approach for the syn- ous Cu30Mn70 alloy with a thickness of ~200 mm and
thesis of CuNPore with tunable nanoporosity and its size of ~5ꢂ5 mm by using (NH4)2SO4 (1M) and
remarkable catalytic properties in click chemistry MnSO4 (0.01M) as electrolytes [see Figure S1 in Sup-
without using any supports and bases [Eq. (1)]. To porting Information]. Scanning electron microscopy
the best of our knowledge, this is the first report on a (SEM) images in Figure 1 (a)–(e) showed that liga-
monolithic CuNPore-catalyzed molecular transforma- ments and nanopore channels were formed uniformly
tion, while the non-monolithic CuNPore in the form across the entire CuNPore. The average ligament di-
of Raney copper is a well known catalyst in the ameters of CuNPore were estimated as ~25 nm (08C,
water-gas shift reaction.[12]
cat-1), ~30 nm (108C, cat-2), ~40 nm (258C, cat-3),
~50 nm (408C, cat-4), ~70 nm (608C, cat-5) (Table 1).
Energy dispersive X-ray (EDX) studies showed that
the residual manganese composition of CuNPore for
the 08C de-alloyed sample (cat-1) is ~4 at%, and re-
mains approximately constant at ~2 at% for cat-2 to
cat-5 ( Supporting Information, Figure S2).
The catalytic activities of various fabricated CuN-
Initially, we attempted to fabricate uniform CuN- Pore materials were examined in the click reaction of
Pore catalysts having different ligament diameters be- phenylacetylene (1a) and benzyl azide (2a) in toluene
cause it has been proved that the tunable size and at 658C for 2 h without using any supports and bases
shape of nanostructured materials may provide vari- (Table 1). The reactions using CuNPore catalysts with
ous physical and chemical properties.[11d,13] To date, ligament sizes of about ~25 nm and ~30 nm (cat-1
less attention has been paid to the synthesis of CuN- and cat-2) gave moderate yields of the corresponding
Pore with a tunable nanoporosity by changing the de- triazole 3a (entries 1 and 2). Surprisingly, the use of
alloying conditions. Recently, Chen and co-workers cat-3 having a ligament size about ~40 nm afforded
obtained CuNPores with tunable nanopore sizes from 3a in almost quantitative yield (entry 3). The specific
Cu/Mn ribbons by controlling de-alloying time and surface area of cat-3 was measured by the BET
Figure 1. SEM images of CuNPore fabricated by de-alloying of Cu30Mn70 in 1M (NH4)2SO4 and 0.01M MnSO4 at (a) 08C (6
days, cat-1), (b) 108C (6 days, cat-2), (c) 258C (6 days, cat-3), (d) 408C (4 days, cat-4), (e) 608C (4 days, cat-5); (f) SEM
image of nanostructured copper acetylide.
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Nanoporous Copper Metal Catalyst in Click Chemistry: Nanoporosity-Dependent Activity
Table 1. Screening of copper catalysts.[a]
The XPS spectra of cat-1 to cat-5 having different lig-
ament sizes did not show a significant difference on
the surface component (Supporting Information, Fig-
ure S3). Moreover, after reaction, the satellite peaks
of Cu(II) were almost disappeared while Cu(I) re-
mained predominantly (Supporting Information, Fig-
ure S4). This result suggested that some amounts of
Cu metal might be leached to the organic solvent.
Indeed, the inductively coupled plasma (ICP-AES)
analysis showed that only 0.2 ppm of Cu was leached
after reaction. To clarify whether the leached Cu
metal catalyzes the present reaction or not, we moni-
tored the reaction with or without catalyst. The reac-
tion of 1a and 2a was carried out by using 5 mol% of
cat-3 in toluene (0.5M) at 658C. After 3 h, CuNPore
catalyst was removed from the reaction vessel and 3a
was produced in 38% 1H NMR yield at this time
Entry
Cu catalyst (ligament size)
Yield of 3a [%][b]
1
2
3
4
5
6
7
8
CuNPore (cat-1, ~25 nm)
CuNPore (cat-2, ~30 nm)
CuNPore (cat-3, ~40 nm)
CuNPore (cat-4, ~50 nm)
CuNPore (cat-5, ~70 nm)
Cu30Mn70 alloy
42
51
99[c]
69
48
0
Cu2O
CuO
3
3
[a]
The reaction of 1a and 2a (1 equiv.) was carried out in
the presence of 2 mol% of Cu catalyst in toluene (2M)
at 658C for 2 h.
&
[Figure 2, (a), symbols]. The reaction mixture was
[b]
[c]
1H NMR yield determined using CH2Br2 as an internal
standard.
continuously heated in the absence of the catalyst for
1 h, affording 3a in 40% yield, while the reaction was
completely stopped in the next 1 h. The reaction re-
started when cat-3 was put back into the mixture and
finally gave 3a in 99% yield within 4 h. On the other
Isolated yield.
method to be 14 m2 gÀ1, the turnover frequency (TOF) hand, the control reaction without removal of catalyst
thus reached to 0.26 sÀ1. It should be noticed that the was complete in 6 h, giving 3a in 98% yield [Figure 2,
*
reaction can be catalyzed by cat-3 at ambient temper- (a) symbols]. These results indicated that the leach-
ature, although it required a longer reaction time ed copper was mainly Cu(II) species which was
(30 h). The reaction using cat-4 or cat-5 having liga- unable to catalyze the present click reaction, suggest-
ment sizes larger than cat-3, resulted in a dramatically ing that the reaction is catalyzed by Cu(I) ions on the
decreased yield of 3a (entries 4 and 5). Cu-Mn alloy CuNPore surface. It should be noted that Cu2O or
was totally inactive as a catalyst (entry 6). These re- CuO powder cannot catalyze this reaction without a
sults indicate that the tunable nanoporosity of CuN- base (Table 1, entries 7 and 8). These results further
Pore has a strong influence on catalytic activity in the support that the nanoporous structure plays a crucial
click reaction. The remarkable difference in catalytic role in the current catalysis.
activities cannot simply be attributed to the difference
The CuNPore (cat-3) catalyst was reused for multi-
in surface areas, because the CuNPore catalyst with a ple cycles without significant loss of catalytic activity
ligament size smaller than 40 nm exhibited a lower in the reaction of 1a and 2a; the yield was still 98% in
catalytic activity. A similar effect of nanopore size the tenth cycle [Figure 2, (b)]. After simple filtration
was observed in CuNPore-based surface-enhanced of the reaction mixture, the catalyst was washed with
Raman scattering (SERS); CuNPore with nanopore acetone and reused without further purification. The
sizes about 30–50 nm significantly enhanced SERS in- product was produced almost quantitatively in every
tensity while a dramatic loss of the SERS effect was
observed for nanopore sizes larger or smaller than
that.[11e]
A number of homogeneous and heterogeneous ex-
amples proved that the click reaction was catalyzed
by Cu(I) species.[7–9] X-ray photoelectron spectroscopy
(XPS) spectra showed that the fresh CuNPore surface
is composed of Cu(0), Cu(I), and Cu(II) species,
while Cu(I) is a predominant component (Supporting
Information, Figure S4).[14] The Cu 2p peaks at
932 eV and 953 eV and Cu LMM Auger peak at
568 eV were assigned to Cu(0); peaks at 570 eV,
932 eV, and 953 eV were ascribed to Cu(I); several
satellite peaks at 935 eV, 944 eV, 955 eV, and 963 eV
correspond to Cu(II). The Cu(I) and Cu(II) ions exist
Figure 2. (a) Leaching test of CuNPore for the reaction 1a
with 2a was performed by using 5 mol% of cat-3 in toluene
(0.5M) at 658C. (b) Reusability study of CuNPore (cat-3)
for the reaction of 1a with 2a was carried out in toluene
as the native Cu2O and CuO on CuNPore surface. (0.5M) at 658C for 6 h.
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Table 2. CuNPore (cat-3)-catalyzed synthesis of various sub-
cycle and the turnover number (TON) reached up to
8200. It is noteworthy that the nanostructure of the
recovered catalyst did not show significant changes
after the fifth cycle, however, the nanostructure on
surface was not clear after the tenth cycle while the
cross-sectional nanostructure survived well (Support-
ing Information, Figure S5).
stituted triazoles.[a]
Entry R1 1
R2 2
Time [h] Yield [%][b]
Involvement of copper(I) acetylide species in the
click reaction is well demonstrated by computational
study and kinetic experiments.[15,16] During this study
of the catalytic properties of CuNPore materials, we
found that when cat-3 was treated with an excess
amount of phenylacetylene in toluene without organic
azides at room temperature, the CuNPore catalyst
surface was turned to yellow after washing with ace-
tone and dichloromethane. The SEM image showed
that the cat-3 surface was covered with a uniform
grass-like material [Figure 1, (f) and Supporting Infor-
mation, Figure S6]. A binding energy peak appeared
in between those of CuO and Cu2O, and peaks of
Cu(II) and Cu(0) were not detected in the XPS spec-
tra, suggesting that the new peaks (marked by *) of
the grass-like material surface should be assigned to
Cu(I) species (Supporting Information, Figure S7).
However, treatment of cat-3 with benzyl azide with-
out phenylacetylene did not change the nanoporous
structure of the catalyst. These results prompted us to
1
2
3
4
5
6
2-pyridyl 1b
Bn 2a
Bn 2a
Bn 2a
Bn 2a
Bn 2a
Bn 2a
Ph 2b
cinnamyl 2c
4.5
7
6
3b, 96
3c, 99
3d, 99
3e, 97
3f, 99
3g, 94
3h, 95
3i, 99
3j, 99
3k, 95
3l, 98
3-thienyl 1c
n-pentyl 1d
c-hexyl 1e
t-Bu 1f
HOCH2 1g
Ph 1a
Ph 1a
Ph 1a
BnNHCH2 1h Ph 2b
TsNHCH2 1i Ph 2b
4
22
19
22
19
7[c]
8[c]
9
EtO2CCH2 2d 3
10
3.5
6
11[d]
[a]
Reaction conditions: to a toluene (2M) solution of cat-3
(2 mol%) was added alkyne 1 and azide 2 (1 equiv.). The
mixture was stirred at 658C for the time shown in
Table 2.
Isolated yield.
2 equiv. azide were used.
[b]
[c]
[d]
1.2 equiv. azide were used.
examine the possibility of the nanostructured copper protected amines were tolerated under the present
acetylide formation. When the grass-like material heterogeneous conditions (entries 8–11). Under neat
[shown in Figure 1, (f)] was treated with benzyl azide conditions, the catalytic loading can be decreased to
at 658C for 2 h, the grass-like material disappeared 0.1 mol% without any significant influence on the re-
from the surface, the nanoporous structure of cat-3 action efficiency, and the TOF was increased up to
was recovered, and a very small amount of the corre- 5.2 sÀ1 [Eq. (2)].
1
sponding triazole 3a was detected by H NMR and
GC-MS. Moreover, the recovered CuNPore retained
its high catalytic activity in the reaction of 1a and 2a
(99% of 3a), although the reaction with the grass-like
material as a catalyst gave 58% of 3a. These results
implied that the grass-like material should be the
nanostructured polymeric copper acetylide which was
less active than the copper acetylides with lower
order aggregates generated in situ. The results were in
good agreement with the previous DFT studies and
In conclusion, we have developed an efficient ap-
experimental investigation reported by Fokin[15] and proach for the fabrication of nanoporous copper ma-
Straub.[16] It is noteworthy that the nanostructured terials with tunable nanoporosity, and demonstrated
copper acetylide was very stable in air compared to that CuNPore has an outstanding catalytic activity for
the air-sensitive fresh CuNPore, suggesting that the click chemistry without using any supports and bases.
polymeric copper acetylide can be used as a CuNPore The catalytic activity was highly in dependence on the
metal storage to protect the nanoporosity of CuNPore ligament (or pore) sizes of the CuNPore materials;
from air.
ligament size about ~40 nm significantly enhanced
The catalytic activity of CuNPore (2 mol% cat-3) the catalytic efficiency. Characterization of the cata-
was further examined with various terminal alkynes lyst surface, leaching experiments, and the formation
and organic azides in toluene (2M) at 658C (Table 2). of nanostructured copper acetylide revealed that the
Not only aromatic and heteroaromatic alkynes but present click reaction is catalyzed by Cu(I) species on
also alkylalkynes were catalyzed regioselectively, af- the CuNPore surface. The CuNPore catalyst exhibited
fording 1,2,3-triazoles in excellent yields (entries 1–7). a high reusability; it can be recycled for ten times
Various functional groups, such as alkene, ester, and without significant loss of activity. A wide range of al-
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Nanoporous Copper Metal Catalyst in Click Chemistry: Nanoporosity-Dependent Activity
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kynes and azides can be tolerated, giving the corre-
sponding triazoles in excellent yields. Further studies
on exploring new catalytic activities of CuNPore ma-
terials and extension of its utility to organic synthesis
are in progress.
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The rolled alloy Cu30Mn70 with 200 mm thickness and 5 mmꢂ
5 mm size was placed directly in 1M (NH4)2SO4 and 0.01M
MnSO4 electrolytes at room temperature (258C) for 6 days.
All electrolytes were prepared with deionized (DI) water
and reagent grade chemicals. After de-alloying, the sample
was removed from electrolytes and rinsed in DI water for
few minutes. The sample was then washed with acetone and
dried under vacuum. The dried CuNPore was stored in a
glove box. Using the same electrolytes, cat-1 and cat-2 were
fabricated at 08C and 108C for 6 days, respectively. Cat-4
and cat-5 were de-alloyed at 408C and 608C for 4 days.
Representative Procedure for the CuNPore (cat-3)-
Catalyzed Click Reaction
To a toluene (2M, 0.5 mL) solution of CuNPore (cat-3,
2 mol%, 1.3 mg) were added phenylacetylene 1a (1 mmol,
112 mL) and benzyl azide 2a (1 mmol, 125 mL) in a V-shaped
reactor vial. The reaction mixture was stirred at 658C for
2 h by using a bulky round-shaped magnetic stirring bar.
After consumption of 1a and 2a which were monitored by
TLC, the reaction mixture was cooled to room temperature.
The mixture was filtered and washed with dichloromethane.
The recovered CuNPore catalyst was washed with acetone
and dried under vacuum. After concentration of the filtrate,
the white solid was purified via short silica gel chromatogra-
phy by using a 3:1 mixture of hexane and ethyl acetate as
an eluent, to afford 1-benzyl-4-phenyl-1H-1,2,3-triazole 3a
as a white solid; yield:233 mg (99%).
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This work was supported by a Scientific Research (A) from
Japan Society for Promotion of Science (JSPS) (No.
23245020). The authors thank Dr. Aiko Nakao (Cooperative
Support Team, RIKEN Advanced Science Institute) for assis-
tance with the XPS experiments.
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