Silver nanoparticles supported by novel nickel metal-organic frameworks: An efficient heterogeneous catalyst for an A3 coupling reaction

Article abstract of DOI:10.1055/s-0028-1087540

A novel heterogeneous catalyst of silver nanoparticles was prepared by using a nickel metal-organic framework (Ni-MOF) as a template. The catalyst has a very high catalytic activity and a strict structural selectivity towards linear aliphatic aldehydes in

Full text of DOI:10.1055/s-0028-1087540

LETTER
447
Silver Nanoparticles Supported by Novel Nickel Metal-Organic Frameworks:
3
An Efficient Heterogeneous Catalyst for an A Coupling Reaction
3
C
S
atalyst for an
u
A
Coupling
j
React
i
ion ng Wang, Xiaoxiao He, Lexin Song,* Zhiyong Wang*
Hefei National Laboratory for Physical Science at Microscale, Joint Lab of Green Synthetic Chemistry and Department of Chemistry,
University of Science and Technology of China, Hefei 230026, P. R. of China
Fax +86(551)3603185; E-mail: zwang3@ustc.edu.cn.
Received 23 September 2008
It should be noted that, before this investigation, a few
Abstract: A novel heterogeneous catalyst of silver nanoparticles
templates had been tested in the preparation of heteroge-
neous silver catalysts in our initial experiments.
was prepared by using a nickel metal-organic framework (Ni-MOF)
as a template. The catalyst has a very high catalytic activity and a
strict structural selectivity towards linear aliphatic aldehydes in an
Firstly, natural starch was employed as a stabilizer to sup-
3
A coupling reaction. The catalyst could be recovered by simple
phase separation and reused for many times without obvious loss of
activity.
6
port silver nanoparticles. However, it was found that for
3
the starch-stabilized Ag catalyst in the A coupling reac-
tion of aldehyde, amine and alkyne, no propargylamine
products were observed, suggesting that the silver parti-
cles supported by natural starch had no catalytic activities
Key words: silver nanoparticles, template, metal-organic frame-
3
works, heterogeneous catalyst, A coupling
3
for this A reaction, possibly due to the strong binding af-
finity between the starch and the silver nanoparticles.
Therefore, it is important to select a suitable template with
a moderate binding ability to silver nanoparticles.
In the past few years, metal-organic frameworks (MOFs)
showed a great advantage as an efficient artificial tem-
plate for the preparation of metal nanoparticles. The dis-
tinct advantage of MOFs acting as templates is due to their
nanoscale channels or cavities, by which small monodis-
persed particles are formed instead of the aggregation of
Secondly, diatomite, a natural porous material, was em-
3
ployed in the catalysis of this A reaction after it was treat-
7
ed with silver nanoparticles (Ag/diatomite). Initially, the
1
reaction of 4-fluorobenzaldehyde with diethylamine and
phenylalkyne was used as a model reaction. After the re-
action conditions were optimized, the best result was
achieved with the reaction temperature of 80 °C, solvent
of acetonitrile and catalyst loading of 0.34 mol% (see
Supporting Information 1.4). Subsequently, various alde-
hydes, amines, and alkynes were employed to extend the
scope of the reaction substrates. The results were summa-
rized in Table 1.
nanoparticles into large size. Some recent researches in-
dicated that palladium particles encapsulated on different
supporting materials possessed high catalytic activities
2
and good recycle abilities at the same time. Furthermore,
some supporting materials have been employed as tem-
plates to prepare silver nanoparticles in different forms.3
However, a heterogeneous catalyst based on MOF-sup-
ported silver nanoparticles has not been reported so far.
In the view of consideration above, we designed and syn-
4
As shown in Table 1, both aliphatic aldehydes (entries 1–
thesized a novel MOF, that is, Ni-MOF, in which there
3
) and aromatic aldehydes (entries 4–10), as reaction sub-
were many free thiophene sulfur atoms from a newly syn-
5
strates, smoothly afford the corresponding products with
moderate to good yields. For the three aliphatic aldehydes,
the reaction was finished within shorter time and gave
corresponding propargylamines in higher yields in com-
parison with the aromatic aldehydes, indicating the influ-
ence of electronic effect, structural rigidity, and steric
thesized ligand, H DTDC (Figure 1), to provide a suit-
2
3
able template for silver nanoparticles. Furthermore, an A
reaction of aldehyde, amine, and alkyne was employed as
a model reaction to evaluate the catalytic effect of the con-
structed heterogeneous catalyst involved silver nanoparti-
cles supported by Ni-MOF.
3
hindrance on the A reaction. For examples, the presence
of electron-withdrawing groups (F and Br) on the aromat-
ic rings (entries 6 and 7) accelerated the reaction and led
to better results than benzaldehyde (entry 4). In contrast,
the electron-rich aromatic aldehyde (entry 5) took a longer
reaction time to generate the product with a lower yield.
In addition, a substituent at the ortho position (entry 8) or
a substituent with a bigger volume (entry 10), or a het-
eroatom in an aromatic ring (entry 9) would result in a
negative effect on the yield of the coupling reaction.
Figure 1 Structure of H DTDC and one 2D layer of the novel MOF
2
(
Ni-MOF); coloring scheme: C, gray; O, red; S, yellow; Ni, green
Being different from starch-stabilized Ag, the Ag/dia-
tomite exhibited a considerable catalytic activity in the A3
coupling reaction, implying that a suitable template is a
SYNLETT 2009, No. 3, pp 0447–0450
Advanced online publication: 21.01.2009
DOI: 10.1055/s-0028-1087540; Art ID: W14908ST
x
x.
x
x.
2
0
0
9
©
Georg Thieme Verlag Stuttgart · New York

4
48
S. Wang et al.
LETTER
Table 1 Coupling Reaction of Aldehyde, Amine, and Alkyne Cata-
lyzed by Ag/Diatomite
size about 1 nm, which were much smaller than the dia-
tomite-supported silver nanoparticles (see Supporting In-
formation 2.7). The results from UV-vis spectrometry and
cyclic voltametry (see Supporting Information 2.4 and
a
Et
Et
Ag/diatomite
0.34 mol%)
N
(
RCHO + Et NH
+
Ph
2
2
.5) of Ni-MOF, Ag(I)/Ni-MOF, and Ag/Ni-MOF sug-
MeCN, 80 °C
N2 (1 atm)
R
gested that the silver ions in Ag(I)/Ni-MOF were com-
pletely converted into silver nanoparticles. And XPS
analysis showed a 7.03 wt% of Ag in the Ag/Ni-MOF cat-
alyst.
Ph
Entry
R
Time (h)
Yield (%)
Hb
1
2
3
4
5
6
7
8
9
3
82
74
68
63
56
71
73
51
48
47
Afterwards, Ag/Ni-MOF was used as a heterogeneous
Me(CH2)2
6
3
catalyst in the A reaction. Initially, it was used to catalyze
Me CH
4
the reaction of butyraldehyde with diethylamine and phe-
nylalkyne. After the reaction conditions were optimized,
the reaction afforded the corresponding product with a
yield of 94% (see entry 4 in Table 2). What is more impor-
tant, the reaction time was shortened largely to 50 minutes
in comparison with six hours of entry 2 in Table 1, as well
as with those of the other catalytic systems.8
2
Ph
11
12
6
4-MeOC H
6
4
4-FC H4
6
4-BrC H
10
12
6
6
4
4
2-BrC H
6
nicotinaldehyde
1
0
1-naphthaldehyde
6
a
Conditions: aldehyde (1 mmol), amine (1.2 mmol), and alkyne (1.5
mmol), Ag/diatomite.
b
(
HCHO)n.
crucial point for the performance of a heterogeneous cat-
alyst based on silver nanoparticles.
Subsequently, the recycle of the Ag/diatomite heteroge-
neous catalyst was estimated in the reaction of polyform-
aldehyde, diethylamine, and phenylalkyne. A significant
decrease of yield (34%) and much longer reaction time (8
h) were observed even for the second run of the Ag/dia-
tomite catalysis. Moreover, it gave only a trace of the final
product for the third cycle of the catalysis. The finding
suggested that the silver nanoparticles of this catalyst
were not immobilized steadily on diatomite. Therefore,
the chase for shorter reaction time, higher yields, and
Figure 2 (a) General view of the silver nanoparticles supported by
Ni-MOF; (b) HR-TEM image of Ag/Ni-MOF with larger magnifica-
tion
Subsequently, various aldehydes, amines, and alkynes
were applied to extend the scope of reaction substrates.9
The results are listed in Table 2. From this table, it can be
seen that many important and significant phenomena were
observed.
good recycle ability of a heterogeneous silver catalyst for First, shapes and sizes of the reaction substrates have a
3
the selected A reaction inspired us to attempt the building great influence on the reaction. As for the reaction of phe-
of Ni-MOF as a template.
nylacetylene with polyformaldehyde and diethylamine
entry 1), in which polyformaldehyde has little steric hin-
drance, the reaction was finished within ten minutes and
generated the corresponding product with a high yield of
(
Nickel-MOF was synthesized solvothermally through the
reaction of H DTDC (0.035 mmol) and Ni(ClO ) ·6H O
2
4 2
2
(
0.069 mmol) in the presence of DMSO (0.2 mL) and py-
9
5%. With the volume increase of the aldehyde (entry 4)
ridine (0.2 mL). The X-ray analysis showed that the struc-
ture of Ni-MOF was a 2D-coordination network
crystallizing in the monoclinic space group C2/c (see Sup-
porting Information 2.1). As shown in Figure 1, the dinu-
clear nickel(II) ions in Ni-MOF were interlinked through
the oxygen atoms of DTDC, resulting in many free
or amine (entry 3), it took longer time to complete the re-
action. For the reaction of heptaldehyde with piperidine in
the presence of phenylacetylene (entry 10), it took two
hours to complete the reaction with a lower yield, perhaps
due to their bigger size.
thiophene-sulfur atoms, which should be helpful for the Second, either aromatic aldehydes or branch aliphatic al-
immobilization of silver nanoparticles.
dehydes hardly gave any reaction products, strongly sug-
gesting the size selectivity of Ni-MOF to reaction
substrates. For instance, when the mixture of a linear ali-
phatic aldehyde and an aromatic aldehydes or a branched
aliphatic aldehyde was added to the reaction system, only
the corresponding product of a linear aliphatic aldehyde
The synthesis of the silver nanoparticles supported by Ni-
MOF was carried out according to a simple process (see
Supporting Information 1.2). The TEM images (Figure 2)
clearly illustrated the formed silver nanoparticles with a
Synlett 2009, No. 3, 447–450 © Thieme Stuttgart · New York

3
LETTER
Catalyst for an A Coupling Reaction
449
Table 2 Coupling Reaction of Aldehyde, Amine, and Alkyne Cata-
was detected (entries 18–25). Therefore, the strict struc-
tural selectivity of the Ni-MOF template to linear aliphatic
aldehydes implies that Ag/Ni-MOF could be a potential
candidate for the use of separation between linear aliphat-
ic aldehydes and aromatic or branch aliphatic aldehydes.
lyzed by Ag/Ni-MOFa
R2
R1
R2
Ag/Ni-MOF
0.3 mol%)
N
(
1
R CHO + R22NH
R3
+
MeCN, 70–80 °C
N2 (1 atm)
R3
Third, the presence of a bulky alkyl or aryl group on the
alkyne moiety also affected the reaction. When hexyne
was selected as a reaction substrate, the reactions were ex-
perienced in longer time with lower yields (entries 11–17)
in comparison with the phenylacetylene reactions. This
indicated that phenylacetylene had a higher reactivity than
hexyne in this reaction.
Entry
R1
R2
R3
Time
Yield
(
min)
(%)
95
93
92
94
92
94
89
86
87
85
92
90
88
82c
84c
77c
81
94
85
92
75
95
87
90
78
1
2
3
4
5
6
7
8
9
Hb
Hb
Hb
Et
Ph
10
pyrrolidine
piperidine
Ph
30
Ph
30
3
It should be stressed that the reaction time of the A cou-
pling catalyzed by Ag/Ni-MOF was shortened largely in
comparison with the catalytic results previously reported,8
indicating that the silver nanoparticles supported by Ni-
MOF has an excellent reactivity due to a larger interaction
area resulted from the unique environment of the silver
nanoparticles on/in Ni-MOF.
Me(CH2)2 Et
pyrrolidine
Me(CH2)2 piperidine
Ph
50
Me(CH2)
2
Ph
60
Ph
60
Me(CH2)
Me(CH2)
2
5
morpholine
Et
Ph
60
Ph
90
Table 3 Recycling of Ag/Ni-MOFa
Me(CH2)5 pyrrolidine
Me(CH2)5 piperidine
Ph
120
120
30
No. of cycles
Yield (%)b
Run 1 Run 2 Run 3 Run 4 Run 5
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
Ph
95
10
96
10
94
10
95
15
93
15
Hb
Et
n-Bu
n-Bu
n-Bu
n-Bu
n-Bu
n-Bu
n-Bu
Ph
Time (min)
Hb
piperidine
morpholine
60
a
Reaction conditions: (HCHO) (1 mmol), Et NH (1.2 mmol), phenyl-
n
2
Hb
acetylene (1.5 mmol), Ag/Ni-MOF (0.3 mol%), MeCN, 75 °C, nitro-
gen (1 atm).
60
b
Me(CH2)2 Et
120
120
120
120
10
Isolated yield.
Me(CH2)2 piperidine
Me(CH2)5 Et
Furthermore, the recycle ability of the Ag/Ni-MOF cata-
lyst was investigated. As shown in Table 3, the heteroge-
neous catalyst could be reused several times without
obvious loss in catalytic activity. Even in the fifth round,
a good yield of the reaction product still can be achieved
by slightly prolonging the reaction time.
Me(CH2)5 piperidine
Hb,d
Me(CH2)5d Et
Hb,d
Et
Me(CH2)5d Et
Hb,e
Et
Me(CH2)5e Et
Hb,e
Et
Me(CH2)5e Et
Et
Ph
90
In summary, a heterogeneous catalyst of silver nanoparti-
cles was prepared via the selection of supporting tem-
plates. The experimental results showed that the moderate
binding affinity between template and nanoparticles, as
well as the size of the nanoparticles supported on the tem-
plate, is a key factor for the catalytic efficiency of a hetero-
geneous catalyst. Furthermore, the good selectivity of the
Ni-MOF template to linear aliphatic aldehydes implies
that Ag/Ni-MOF could be a potential candidate for the use
of separation between linear aliphatic aldehydes and aro-
matic or branch aliphatic aldehydes. By means of metal-
organic frameworks, we can design and adjust the void
cavities and coordination atoms to nanoparticles. This
preparation provides us a new method to design and syn-
thesize heterogeneous catalysts and to develop the cataly-
sis of metal nanoparticles. The application of the
constructed heterogeneous catalyst for other organic reac-
tions is in progress in our laboratory.
n-Bu
n-Bu
Ph
30
120
10
Ph
90
n-Bu
n-Bu
30
2
120
a
Conditions: aldehyde (1 mmol), amine (1.2 mmol), and alkyne (1.5
mmol), Ag/Ni-MOF.
b
(
HCHO)n.
c
Only 0.6 mol% of catalyst was used.
The mixture of 1 mmol of benzaldehyde and 1 mmol of linear ali-
d
phatic aldehydes. Only the corresponding products of linear aliphatic
aldehydes were obtained.
e
The mixture of 1 mmol of isobutyraldehyde and 1 mmol of linear
aliphatic aldehydes. Only the products of linear aliphatic aldehydes
were obtained.
Synlett 2009, No. 3, 447–450 © Thieme Stuttgart · New York

4
50
S. Wang et al.
LETTER
structure was solved by direct method.10
Crystallographic Parameters
Supporting Information for this article is available online at
http://www.thieme-connect.com/ejournals/toc/synlett.
C H N Ni O S , M = 1088.49, monoclinic, space group
4
0
34
4
2
13 6
C2/c, a = 22.7475, b = 19.5874, c = 12.0742 Å, a = 90.000°,
3
Acknowledgment
b = 99.593°, g = 90.000°, V = 5304.318 Å , Z = 4,
3
–1
D = 1.363 g/cm , F(000) = 2232, m = 1.004 mm , crystal
c
The authors are grateful to the Natural Science Foundation of China
size: 0.161 × 0.088 × 0.066, goodness of fit: 0.914, R [I >
1
(30572234) and for support from the Chinese Academy of Sciences.
2
s(I)] = 0.0677, wR [I > 2s(I)] = 0.1742, R (all
2
1
–
3
data) = 0.1275, wR (all data) = 0.2036, residuals (e Å ):
2
0
.546, –0.648. CCDC 675102.
References and Notes
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(
9) General Procedure for the A Coupling Reaction
To a solution of aldehyde (1.0 mmol) and amine (1.2 mmol)
in MeCN (5.0 mL) was added a certain loading of catalyst
2
2
(5.0 mg). Then the mixture was degassed in vacuum and put
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into a nitrogen atmosphere. After adding alkyne (1.5 mmol)
into the mixture, it was heated at 70–80 °C for a required
reaction time. Then the solid phase was filtered off and
residue solvent was evaporated under vacuum. Product was
obtained after purification by column chromatography.
(
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(
2
(
10) SHELXL97. Program for the Solution of Crystal Structures;
4) X-ray diffraction data of Ni-MOF were collected on a
Bruker SMART APEX II CCD diffractometer equipped
with a graphite crystal and incident beam monochromator
using Mn Ka radiation (l = 0.71073 Å) at 293 K. The
Sheldrick, G. M., Ed.; University of Göttingen: Germany,
1997.
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