Engineering metal-organic frameworks immobilize gold catalysts for highly efficient one-pot synthesis of propargylamines

Article abstract of DOI:10.1039/c2gc35284b

Engineering metal-organic frameworks (MOF) for heterogeneous catalysts have been of extreme interest since they could bridge the gap between homogeneous and heterogeneous catalysis. We have designed and synthesized gold functionalized IRMOF-3 catalysts by post-covalent modification (PM) and one-pot (OP) synthesis methods. The gold functionalized IRMOF-3 catalysts provide an efficient, economic, and novel route for the one-pot synthesis of structurally divergent propargylamines via three component coupling of alkyne, amine, and aldehyde (A³) without any additives or an inert atmosphere. The catalysts were characterized in depth to understand their structure-property relationship. It was shown that the 4.6%Au/IRMOF-3 catalyst, prepared by the PM method, contains a fraction of cationic gold (Au³⁺/Au⁰ = 0.2), which shows much higher catalytic activity than that of 3.2% or 0.6%Au/IRMOF-3 prepared by OP method, although the former exhibits much lower crystallinity than the latter two catalysts. Notably, the catalytic activity of the Au/IRMOF-3 catalysts could be significantly enhanced at a moderate reaction temperature (150°C). All the Au/IRMOF-3 catalysts can be easily recycled and used repetitively at least 5 times, especially the catalysts prepared by the OP method, which showed no drop in activity for the successive 5 uses. These features render the catalysts particularly attractive in the practice of propargylamines synthesis in an environmentally friendly manner. The Royal Society of Chemistry.

Full text of DOI:10.1039/c2gc35284b

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PAPER
Engineering metal–organic frameworks immobilize gold catalysts for highly
efficient one-pot synthesis of propargylamines
Liu Lili,a Zhang Xin,*a,b Gao Jinsena and Xu Chunming*a,b
Received 24th February 2012, Accepted 15th March 2012
DOI: 10.1039/c2gc35284b
Engineering metal–organic frameworks (MOF) for heterogeneous catalysts have been of extreme interest
since they could bridge the gap between homogeneous and heterogeneous catalysis. We have designed
and synthesized gold functionalized IRMOF-3 catalysts by post-covalent modification (PM) and one-pot
(OP) synthesis methods. The gold functionalized IRMOF-3 catalysts provide an efficient, economic, and
novel route for the one-pot synthesis of structurally divergent propargylamines via three component
coupling of alkyne, amine, and aldehyde (A3) without any additives or an inert atmosphere. The catalysts
were characterized in depth to understand their structure–property relationship. It was shown that the
4.6%Au/IRMOF-3 catalyst, prepared by the PM method, contains a fraction of cationic gold (Au3+/
Au0 = 0.2), which shows much higher catalytic activity than that of 3.2% or 0.6%Au/IRMOF-3 prepared
by OP method, although the former exhibits much lower crystallinity than the latter two catalysts.
Notably, the catalytic activity of the Au/IRMOF-3 catalysts could be significantly enhanced at a moderate
reaction temperature (150 °C). All the Au/IRMOF-3 catalysts can be easily recycled and used repetitively
at least 5 times, especially the catalysts prepared by the OP method, which showed no drop in activity for
the successive 5 uses. These features render the catalysts particularly attractive in the practice of
propargylamines synthesis in an environmentally friendly manner.
exhibited high activity for the A3 coupling reaction.4 Recent pro-
Introduction
gress in this area has been reported using homogeneous catalysts
such as organic gold complexes,5 silver salts,6 copper salts,7
Propargylamines are major skeletons, synthetically versatile and
Hg2Cl2,8 and a Cu–RuII bimetallic system,9 among which the
are key intermediates for the preparation of many nitrogen-con-
taining biologically active compounds, such as β-lactams, oxo-
tremorine analogues, conformationally restricted peptides,
isosteres, and important structural elements of natural products
and therapeutics drug molecules.1 Conventionally, propargyla-
mines are synthesized by nucleophilic attack of lithium acety-
lides or Grignard reagents to imines or their derivatives.2
However, these methods require the use of stoichiometric
amounts of organometallic reagents, strictly controlled reaction
conditions, and sensitive functional groups (such as aldehydes)
which should be well-protected during the reaction process. An
alternative atom-economical approach is to perform this type of
reaction by a catalytic coupling of aldehydes, alkynes, and
amines (A3 coupling) by C–H activation, where water is the only
theoretical by-product.1e,3,4 The significant and pioneering work
made by Wei and Li has shown that gold salts (AuBr3 or AuCl)
cationic gold species showed the highest catalytic activity.
However, besides the potential limitations of homogeneous cata-
lysis for achieving a sustainable catalytic process, the rapid
reduction of cationic gold species into inactive metallic atoms is
unavoidable when gold salts activate alkynes/alkenes.10
Heterogeneous catalysis offers the opportunity for easy separ-
ation and recycling of the catalyst. Thus, the development of
improved synthetic methods for the preparation of propargyl-
amines remains an active research area. Remarkably, Kidwai
et al. reported that a one-pot synthesis of propargylamines can
be readily accomplished by using unsupported Au0 nanoparticles
as catalyst.11 The report is indeed significant because the results
from the study demonstrate that the Au0 nanoparticles exhibit
excellent alkynophilicity, which is similar to those observed with
gold complexes. Recently, Datta et al. reported Au nanoparticles,
embedded in a mesoporous carbon nitride support with a very
low loading of gold function, as an efficient recyclable hetero-
geneous catalytic system for the one-pot synthesis of propargyla-
mines under atmospheric conditions.12 Layek et al. reported an
efficient and highly catalytic system containing Au0 nanoparti-
cles of the size 10–12 nm that were supported on commercial
nano active magnesium oxide plus (NAP–MgO) with ultra low
loadings of Au (0.236 mol%) for the one-pot synthesis of
aState Key Laboratory of Heavy Oil Processing, China University of
Petroleum, Beijing 102249, PR China. E-mail: zhangxin@cup.edu.cn;
Fax: +86-10-69724721; Tel: +86-10-18911226208
bThe Key Laboratory of Catalysis, China University of Petroleum,
Beijing 102249, PR China. E-mail: xcm@cup.edu.cn;
Fax: +86-10-69724721; Tel: +86-10-89733392
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Scheme 1 The strategy for gold functionalized IRMOF-3 catalysts by (i) post covalent modification (PM) and (ii) one-pot synthesis (OP).
propargylamines.13 Zhang and Corma found that cationic gold
species stabilized on CeO2 and ZrO2 have much higher catalytic
activity in A3 coupling reactions than Au0.14
Results and discussion
Characterization of catalysts
Metal–organic frameworks (MOFs) are a newly emerging
important class of materials, comprising metal “nodes” linked
with “rod” shaped organic linkers, which have attracted consider-
able attention because of their profound range of applications
such as in catalysis, separations, and gas storage.15 Unlike other
materials (such as metal oxides), the lack of non-accessible bulk
volume and the fully exposed metal sites in MOFs could provide
an ultimately high degree of metal dispersion. MOFs supported
Au catalysts could help to bridge the gap between homogeneous
and heterogeneous Au catalysis for the one-pot synthesis of
propargylamines.
The well-known IRMOF-3 consists of Zn4O clusters
linked by 2-amino-1,4-benzendicarboxylic acid (NH2–BDC)
(Scheme 1a).16 The high surface area and pore volume observed
for IRMOF-3, coupled with uncoordinated amino groups, made
it an ideal candidate for devising MOFs with a variety of
functionalities.17,18 In the present study, IRMOF-3 was func-
tioned with Au by a post-covalent modification (PM) and one-
pot (OP) methods. The functionalized IRMOF-3 catalysts were
explored in one-pot synthesis of propargylamines by the A3
coupling reaction of aldehydes, alkynes, and amines. We attempt
to establish a relationship between the catalytic performance and
catalyst structure, in which the effect of catalyst preparation
method, as well as the scope of substrates, recyclability of the
catalysts, and the mechanism of A3 coupling reaction were
investigated.
The IRMOF-3 structure is made of Zn4O tetranuclear clusters
connected by rigid dicarboxylic linkers NH2–BDC to generate a
cubic framework as shown in Fig. 1a and Scheme 1a. The
pendant amino groups may serve as a chemical handle that
can be manipulated via post-synthetic modification with a
diverse series of anhydrides, isocyanates and salicylaldehyde.18
Recently, IRMOF-3 was prepared by different research
groups.17–19 However, the texture properties of IRMOF-3 were
different with each research group. In preliminary work
performed by Rowsell et al., IRMOF-3 was solvothermal syn-
thesized with NH2–BDC and Zn(NO3)2·4H2O in N,N′-diethyl-
formamide (DEF) with a BET surface area of 2446 m2 g−1 17
.
Gascon et al. also solvothermal synthesized with a high BET
surface area of 3130 m2 g−1 in the N2 protection.19b One of our
authors prepared IRMOF-3 with a BET surface area of 750 m2
g−1 by a “direct mixing” synthesis strategy at room temperature,
following a modification of the procedure described by Huang
et al.18b,20 In the present study, IRMOF-3 was synthesized by a
solvothermal method with NH2–BDC and Zn(NO3)2·6H2O in
DMF without N2 protection. The nitrogen adsorption–desorption
isotherms of the solvothermal synthesized IRMOF-3 are dis-
played in Fig. 1b. The isotherms showed the type IV isotherm
with a hysteresis, which indicated this IRMOF-3 material pos-
sessed mesoporosity. The mesoporosity should arise from the
secondary particle piled pores of the IRMOF-3 crystals. The
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Fig. 2 Powder X-ray diffraction patterns of the different MOFs
samples: (a) IRMOF-3, (b) IRMOF-3-SI, (c) 4.6%Au/IRMOF-3, (d)
3.2%Au/IRMOF-3, and (e) 0.6%Au/IRMOF-3.
To prepare the gold functionalized IRMOF-3 catalyst, a strat-
egy including PM (i) and OP (ii) was employed as pictorially
shown in Scheme 1. The PM method was followed the procedure
as described earlier.18b As can be seen in Scheme 1, it can be
comprehended by three steps: (1) IRMOF-3 was firstly syn-
thesized by solvothermal with NH2–BDC and Zn(NO3)2·6H2O
in DMF; (2) the IRMOF-3 was then modification with salicyl-
aldehyde to yield IRMOF-3-SI, and salicylideneimine was
formed by reaction of the amine of the NH2–BDC ligand with
aldehyde group of salicylaldehyde; (3) finally, the NaAuCl4 was
added and 4.6%Au/IRMOF-3 was produced. The detailed
describe of each step has been reported earlier.18b The simple
and elegant synthesis of gold functionalized IRMOF-3 by OP
method is developed as shown in Scheme 1. Typically, a mixture
of NH2–BDC, Zn(NO3)2·6H2O, salicylaldehyde, gold precursor
(AuCl/NaAuCl4) and DMF with certain ratio of amount was sealed
in a teflon-lined stainless steel vessel and heated for 24 h at 100 °C.
The XRD patterns of IRMOF-3, IRMOF-3-SI, 4.6%Au/
IRMOF-3, 0.6%Au/IRMOF-3, and 3.2%Au/IRMOF-3 are
shown in Fig. 2. It should be noted that the pattern of IRMOF-3
is very similar to those reported in the literature (Fig. 2a).18c,f
Apart from the fact that the intensity of the peak decreases for
the IRMOF-3-SI samples, the crystal structure seems to remain
unchanged through the post-synthetic modification with salicy-
laldehyde (Fig. 2b). After IRMOF-3-SI was further functiona-
lized with the solution of NaAuCl4, the intensity ratio of typical
peaks located at 6.8° and 9.6° became remarkably different from
that of IRMOF-3, i.e., the intensity of the peak centered at 2θ =
6.8° for 4.6%Au/IRMOF-3 was severely weakened (Fig. 2c).
However, the peak located at 2θ = 6.8° was the strongest one for
IRMOF-3. This result could mean that there are some difference
for the crystalline structure between 4.6%Au/IRMOF-3 and
IRMOF-3. We noted that IRMOF-3 modified by anhydrides with
PM method also affected the variance of the intensity ratio for
the typical two peaks (see Fig. 4 in ref. 18a, and Fig. 3 in
ref. 18b). The change in XRD pattern could be due to the devi-
ation or shrinkage, and even the collapse of pores during the
modification process. Wang et al. found that a remarkable vari-
ation in XRD pattern over Ag/Ni–MOF prepared by the impreg-
nation method compared with Ni-MOF.22 Compared with the
sample prepared with PM method, 0.6%Au/IRMOF-3 and 3.2%Au/
Fig. 1 The optical microscope photograph (a) of IRMOF-3, and N2
adsorption–desorption isotherms (b), pore size distribution (c).
average pore size of IRMOF-3 was ca. 1.3 nm via the SF
method according to a model of cylinder, as shown in Fig. 1c.
The surface area determined by the BET method was
1212 m2 g−1. This value is 62% higher than that (750 m2 g−1
)
prepared by the “direct mixing” synthesis,18b but it is still much
lower than the 2446 m2 g−1 reported by researchers using DEF
as solvent.17 It was reported that higher BET surface areas could
be obtained by using DEF rather than DMF as solvent.19b More-
over, our procedure for preparing IRMOF-3 was not under N2
protection, although it is well known that MOFs adsorb moisture
very quickly upon exposure to ambient air.21 Finally, the solvent
molecules such as DMF and CHCl3 could be not completely
removed at 50 °C before our BET measurement. We have made
attempts to improve the BET surface area by changing the degas-
sing conditions, e.g., by degassing at 100–200 °C for 8–12 h or
at 50–60 °C for 24–72 h to remove the physically adsorbed
DMF, CHCl3, and water before BET analysis, however, the BET
surface area was not improved. Specifically, when the IRMOF-3
was degassed at ca. 200 °C for 12 h, the BET surface area even
less than 10 m2 g−1, and XRD patterns of the sample indicated
that the crystalline structure was decomposed completely.
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Fig. 3 Infrared spectra of (a) IRMOF-3, (b) IRMOF-3-SI, (c) 4.6%Au/
IRMOF-3, (d) 0.6%Au/IRMOF-3, and (e) 3.2%Au/IRMOF-3.
Fig. 4 TG-DTA curves of different samples: (a) IRMOF-3-SI, (b)
4.6%Au/IRMOF-3, (c) 0.6%Au/IRMOF-3, and (d) 3.2%Au/IRMOF-3.
0.6%Au/IRMOF-3 and 3.2%Au/IRMOF-3. Prior to measure-
ment, the samples of 0.6%Au/IRMOF-3 and 3.2%Au/IRMOF-3
were dried in air without further pretreatment. IRMOF-3-SI and
4.6%Au/IRMOF-3 were vacuumed in a rotator at 30–50 °C for
3–5 h before test. The weight loss of the second stage is depen-
dent on the crystalline of the samples. The high crystallinity of
0.6%Au/IRMOF-3 and 3.2%Au/IRMOF-3 led to low weight
loss (less than 8.0%), while the low crystallinity of IRMOF-3-SI
and 4.6%Au/IRMOF-3 led to a much higher weight loss (up to
27.4%). This result indicates that the samples prepared via the
one-pot synthesis have higher thermal stabilities compared with
the samples prepared by the post-covalent modification. For all
the samples, a significant exothermic peak can be observed in
their DTA curves for the sharp weight loss of the third stage.
TG-DTA experiments showed that 4.6%Au/IRMOF-3, 0.6%Au/
IRMOF-3, and 3.2%Au/IRMOF-3 decompose when the temp-
erature is higher than ca. 380 °C, which showed lower thermal
stabilities than IRMOF-3 with a decomposition temperature of
ca. 430 °C.18a,d
TEM measurements were conducted to analyze the size and
morphology of the Au in 4.6%Au/IRMOF-3, 0.6%Au/
IRMOF-3, and 3.2%Au/IRMOF-3. Fig. 5 shows the representa-
tive TEM images and particle size distribution. It was shown that
the Au nanoparticles were close to spherical with average diam-
eters of 3.3, 1.7, and 2.5 nm for 4.6%Au/IRMOF-3, 0.6%Au/
IRMOF-3, and 3.2%Au/IRMOF-3, respectively. After four suc-
cessive cycles, the Au nanoparticles of 4.6%Au/IRMOF-3
appeared significantly agglomerated with maximum sizes up to
34.5 nm, while the average size increased to 13.8 nm (Fig. 5d).
The Au nanoparticles of 3.2%Au/IRMOF-3 also appeared
agglomerated after four successive cycles, and the average size
of Au nanoparticles was increased from 2.5 to 12.0 nm (Fig. 5e).
We anticipated the preparation of a MOF containing a Au(III)
Schiff base complex lining the pore wall. However, the agglom-
eration of the Au nanoparticles is clear, as depicted from TEM
analysis (Fig. 5). Nevertheless, we cannot rule out the possibility
of cationic gold species existing on functionalized IRMOF-3,
since the cationic gold species cannot be detected by TEM and
the exposure to electron beams under high vacuum during TEM
measurements might also effect changes in the gold oxidation
state.25
IRMOF-3 prepared by the OP method maintains a higher crystal-
linity (Fig. 2d and e). It should be pointed out that three
additional peaks at 2θ = 38.2°, 44.4°, and 64.5° were observed
for 4.6%Au/IRMOF-3 and 3.2%Au/IRMOF-3, which were
indexed to the (111), (200), and (220) reflection of face
centered cubic (fcc) crystalline Au, respectively (JCPDS, card
no. 7440-57-5).
Fig. 3 shows the IR spectra of IRMOF-3, IRMOF-3-SI, and
gold functionalized IRMOF-3 samples. The IR spectra of
IRMOF-3 show two peaks in 3473 and 3358 cm−1 due to the
existence of the amino groups of the NH2–BDC ligand.23
However, the N–H stretches at 3473 and 3358 cm−1, associated
with the amine of the NH2–BDC ligand, are notably diminished
for IRMOF-3-SI and gold functionalized IRMOF-3 samples,
which indicated the formation of salicylideneimine by reaction
of the amine of the NH2–BDC ligand with the aldehyde group
of the salicylaldehyde. For all the samples, the broad bands at
3600–2800 cm−1 could be assigned to the stretching vibrations
of the O–H in the carboxyl acid and/or phenol groups which
originate from NH2–BDC and salicylaldehyde, respectively. The
two sharp bands at 1574 and 1382 cm−1 corresponds to asym-
metric (νas(C–O)) and symmetric (νs(C–O)) vibrations of car-
boxyl groups, respectively.24 The peaks centered at 1658, 1501,
and 1427 cm−1 were ascribed to the CvC stretching vibrations
of the aromatic. The 1256 cm−1 frequency can be assigned to
C–N vibrations for all the samples. The bands at ca. 759 cm−1
are ascribed to C–H bending vibrations of the monosubstituted
benzene.
Fig. 4 shows the TG-DTA curves of IRMOF-3-SI, 4.6%Au/
IRMOF-3, 0.6%Au/IRMOF-3, and 3.2%Au/IRMOF-3. The
weight loss of these samples could be divided into three stages.
The first stage ranged from 50 to 150 °C was due to the release
of physically adsorbed solvent molecules such as CHCl3 and
DMF, which is accompanied with an endothermic peak centered
at ca. 100 °C in the DTA curve. The second stage from 150 to
380 °C was ascribed to the further release of solvent physically
absorbed within pores or on the surface of the material. The
third stage of weight loss over temperatures higher than 380 °C
was due to the decomposition of the crystal structure. The
weight loss in the first stage is dramatically influenced by the
pretreatment of the sample, and can be higher up to 40% for the
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Fig. 6 TPR profiles of (a) IRMOF-3-SI, (b) 4.6%Au/IRMOF-3, (c)
0.6%Au/IRMOF-3, (d) 3.2%Au/IRMOF-3, and (e) 4.6%Au/IRMOF-3
after four successive cycles.
heavily disturb the XPS features of Au and the exposure to
photoelectrons under high vacuum during XPS measurements,
which might also effect changes in the gold oxidation state.25c
Excellent work by Wang et al. has shown that the oxidation
states of silver supported on Ni-MOF could be discerned using
UV-vis spectrometry and cyclic voltametry,22 by which,
however, it is still difficult to quantitatively measure the exact
amount of gold species with a mixture of metallic and cationic
ions.
The TPR technique is usually used to quantitatively measure
the reduction of oxidized gold species and thereby calculate the
distribution of different gold oxidation states in supported Au
catalysts.18f,26 Fig. 6 shows H2-TPR profiles of IRMOF-3-SI,
4.6%Au/IRMOF-3, 0.6%Au/IRMOF-3, and 3.2%Au/IRMOF-3.
IRMOF-3-SI exhibits only one peak in the range of 300–400 °C.
The TPR profile of 4.6%Au/IRMOF-3 features two hydrogen
consumption peaks, one is located at a relatively low temperature
range (190–270 °C) and another is located at temperatures
higher than 300 °C, while only one peak, located at higher temp-
eratures, was found for 0.6%Au/IRMOF-3 and 3.2%Au/
IRMOF-3. There was also no H2-consumption peak at low temp-
eratures (190–270 °C) for the catalyst of 4.6%Au/IRMOF-3 after
four successive cycles. One of our authors has previously
observed two reduction peaks over IRMOF-3-SI-Au, while only
one reduction peak located at a high temperature range was
found on IRMOF-3-SI.18b Similar to the previous work, we
presume that the low temperature peak was due to the reduction
of Au3+ ions into Au0 atoms, and the ratio of Au3+/Au0 for 4.6%
Au/IRMOF-3 is 0.2. The higher temperature peak was ascribed
to the decomposition of the support of IRMOF-3. The catalyst
model can be illustrated as in Scheme 1. It should be pointed out
that a mixture of Au0 and Au3+ was obtained by using the post-
covalent modification, however, only cationic Au3+ species were
found in the earlier report.18b No peak that was characteristic of
Au3+ reduction was found on 0.6%Au/IRMOF-3 and 3.2%Au/
IRMOF-3 samples which were prepared by OP method at
100 °C for 24 h, as well as for the 4.6%Au/IRMOF-3 after four
successive cycles. We presume that the cationic gold species
could be reduced into metallic gold atoms during the synthesis.
It has been reported that cationic gold species are easily reduced,
Fig. 5 TEM images and particle size distribution of (a) 4.6%Au/
IRMOF-3, (b) 0.6%Au/IRMOF-3, (c) 3.2%Au/IRMOF-3, (d) 4.6%Au/
IRMOF-3 after four runs, and (e) 3.2%Au/IRMOF-3 after four runs.
We have made an attempt to characterize the gold oxidation
states of 4.6%Au/IRMOF-3 and 3.2%Au/IRMOF-3 by XPS.
However, the overlap of binding energies between Au 4f and Zn
3p made it very difficult to discern the exact binding energy of
Au 4f. Moreover, it is understood that quantitative assessment of
the gold oxidation state by XPS characterization is limited
because the final state effects associated with particle size could
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even at room temperature, and that alkynes, alkenes, alcohols,
CO, and phosphines present in the reaction media are the redu-
cing agents.27
the reaction rates from the maximum conversions were calcu-
−1
lated to be 120.5, 9.2, and 3.6 mmol gAu h−1 for 4.6%Au/
IRMOF-3, 0.6%Au/IRMOF-3, and 3.2%Au/IRMOF-3, respect-
ively, and the TON numbers calculated were 12, 9, and 4,
respectively. It is evident that the reaction rate of 4.6%Au/
IRMOF-3 is one order of magnitude higher than those of 0.6%
Au/IRMOF-3 and 3.2%Au/IRMOF-3. We noted that the benzal-
dehyde conversions over 0.6%Au/IRMOF-3 and 3.2%Au/
IRMOF-3 can be remarkably enhanced by increasing the reac-
tion temperature from 120 to 150 °C. As shown in Fig. 8, the
conversions were continuously increased to ca. 98% within 12 h
on both catalysts. The reaction rates calculated on the basis of
We suggest that the above results of the characterizations with
IRMOF-3 immobilized gold catalysts can be understood on the
basis of models as depicted in Scheme 1. For 4.6%Au/IRMOF-3
prepared by PM method, the active catalyst contains both met-
allic gold nanoparticles and cationic Au3+ ions (Scheme 1c).
Since the average diameter of gold nanoparticles on the 4.6%Au/
IRMOF-3 catalyst is 3.3 nm, which is clearly much larger than
that of the average pore size of IRMOF-3 (1.3 nm), the gold
nanoparticles should be deposited on the surface of IRMOF-3 as
shown in Scheme 1c. The Au3+ ions are coordinated by O, N
and Cl atoms and are present as a Schiff base complex.18b In the
cases of the 3.2%Au/IRMOF-3 and 0.6%Au/IRMOF-3 catalysts
prepared by the OP method, only metallic gold nanoparticles
were verified by the H2-TPR analysis. Due to their larger diam-
eters (2.5, 1.7 nm) of gold nanoparticles compared with the
average pore size of IRMOF-3, the gold nanoparticles should be
also deposited on the surface of IRMOF-3 (Scheme 1d). The
structures of Au/IRMOF-3 catalysts are not, however, fixed: (i)
the gold nanoparticles are clearly aggregated during the A3 coup-
ling reaction, (ii) the cationic Au3+ ions on the 4.6%Au/
IRMOF-3 catalyst may change during the A3 coupling reaction
and were found to be completely reduced into metallic gold
atoms after four successive cycles.
total gold weight at 150 °C are 47.3 and 10.9 mmol gAu h−1
−1
for 0.6%Au/IRMOF-3 and 3.2%Au/IRMOF-3, respectively, and
the TON numbers are 112 and 22, respectively.
To examine the scope of the A3 coupling reaction, we
extended our studies to different combinations of aldehydes,
amines, and alkynes. The results are summarized in Table 1.
Aromatic aldehydes including those bearing functional groups
such as alkoxy, alkyl, and chloro were able to undergo the corre-
sponding three-component-coupling, and afforded good conver-
sions of aldehydes in the A3 coupling reaction (Table 1, entries
1–6, 15–24). It was found that aryl aldehydes possessing elec-
tron-withdrawing groups (Table 1, entry 4) afforded a higher
conversion than those with an electron-donating group bound to
the benzene ring (Table 1, entries 2 and 3) over 4.6%Au/
IRMOF-3 catalyst. However, aryl aldehydes with electron-donat-
ing groups gave higher conversions than aryl aldehydes with
electron-withdrawing groups for the 0.6%Au/IRMOF-3 (Table 1,
entry 16 vs. 17) and 3.2%Au/IRMOF-3 (Table 1, entry 21 vs.
22) catalysts. Notably, aliphatic aldehydes such as cyclohexane-
carboxaldehyde and n-octaldehyde display higher activities than
aromatic aldehydes with the 4.6%Au/IRMOF-3, 0.6%Au/
IRMOF-3, and 3.2%Au/IRMOF-3 (Table 1, entries 5, 6, 18, 19,
23, 24). While unwanted trimerization of aliphatic aldehydes is a
major limitation of the A3 coupling reactions catalyzed by homo-
geneous catalysts,5,9,28 no trimer could be detected with the sup-
ported Au catalyst. A referee of the present paper noticed that
the activity difference between 4.6%Au/IRMOF-3 and 0.6%Au/
IRMOF-3, 3.2%Au/IRMOF-3 catalysts, i.e., aromatic aldehydes
Catalytic studies
Fig. 7 shows the kinetic curves for 4.6%Au/IRMOF-3, 0.6%Au/
IRMOF-3, and 3.2%Au/IRMOF-3 for the A3 coupling reaction
of benzaldehyde, phenylacetylene, and piperidine with 1,4-
dioxane as solvent at 120 °C. It can be seen that 3.2%Au/
IRMOF-3 has a higher conversion of benzaldehyde than
0.6%Au/IRMOF-3. Maximum conversions of 8.0% and 16.0%
were obtained within 5 h over 0.6%Au/IRMOF-3 and 3.2%Au/
IRMOF-3, respectively. However, the conversion reached 77.0%
in 0.5 h over 4.6%Au/IRMOF-3 at 120 °C, and then remained
constant. Taking into account the total Au content of the catalyst,
Fig. 7 Comparison of the catalytic activity over 4.6%Au/IRMOF-3
(●), 0.6%Au/IRMOF-3 (▲), and 3.2%Au/IRMOF-3 (4) for A3 coup-
ling reaction of benzaldehyde (0.25 mmol), phenylacetylene
(0.33 mmol), and piperidine (0.30 mmol) with 1,4-dioxane (1.0 mL) as
solvent at 120 °C.
Fig. 8 Comparison of the catalytic activity over 0.6%Au/IRMOF-3
(▲) and 3.2%Au/IRMOF-3 (4) for A3 coupling reaction of benzal-
dehyde (0.25 mmol), phenylacetylene (0.33 mmol), and piperidine
(0.30 mmol) with 1,4-dioxane (1.0 mL) as solvent at 150 °C.
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Table 1 Coupling of aldehyde, alkyne, and amine catalyzed by Au/IRMOF-3 in 1,4-dioxanea
Entry
Cat.
R1
R2R3NH
R4
Tb (h)
Conv./Yieldc (%)
1
2
3
4
5
6
7
8
4.6%Au/IRMOF-3
4.6%Au/IRMOF-3
4.6%Au/IRMOF-3
4.6%Au/IRMOF-3
4.6%Au/IRMOF-3
4.6%Au/IRMOF-3
4.6%Au/IRMOF-3
4.6%Au/IRMOF-3
4.6%Au/IRMOF-3
4.6%Au/IRMOF-3
4.6%Au/IRMOF-3
4.6%Au/IRMOF-3
4.6%Au/IRMOF-3
4.6%Au/IRMOF-3
0.6%Au/IRMOF-3
0.6%Au/IRMOF-3
0.6%Au/IRMOF-3
0.6%Au/IRMOF-3
0.6%Au/IRMOF-3
3.2%Au/IRMOF-3
3.2%Au/IRMOF-3
3.2%Au/IRMOF-3
3.2%Au/IRMOF-3
3.2%Au/IRMOF-3
Ph
Piperidine
Piperidine
Piperidine
Piperidine
Piperidine
Piperidine
Pyrrolidine
Morpholine
Diethylamine
Piperidine
Piperidine
Piperidine
Piperidine
Piperidine
Piperidine
Piperidine
Piperidine
Piperidine
Piperidine
Piperidine
Piperidine
Piperidine
Piperidine
Piperidine
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
0.5
12
12
12
12
12
12
12
12
12
12
12
12
12
12
24
24
24
24
10
24
24
24
24
77.0
70.6
38.6
95.4
98.9
96.3
91.2
54.6
26.6
86.4
70.6
28.5
62.1
56.6
98.0
75.0
40.3
98.2
97.6
98.0
65.2
30.0
98.5
96.8
4-MeC6H4
4-MeOC6H4
3-ClC6H4
Cyclohexyl
Heptyl
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
4-MeC6H4
3-ClC6H4
Cyclohexyl
Heptyl
Ph
4-MeC6H4
3-ClC6H4
Cyclohexyl
Heptyl
9
Ph
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
4-MeC6H4
4-EtC6H4
4-ButC6H4
Hexyl
(CH3)3Si
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
a Reaction conditions: aldehyde (0.250 mmol), amine (0.300 mmol), alkyne (0.325 mmol), catalyst (0.07 g), the reaction temperatures are 120, 150,
and 150 °C on 4.6%Au/IRMOF-3, 0.6%Au/IRMOF-3, and 3.2%Au/IRMOF-3 catalysts, respectively. b The reaction time was not optimization. c GC
yield or conversion based on aldehydes, the product was determined by GC-MS, and 1H-NMR.
bearing electron-withdrawing groups (Table 1, entry 4) afforded
a better yield than those bearing electron-donating groups
(Table 1, entries 2 and 3) when 4.6%Au/IRMOF-3 catalyst was
used, however, the employment of 0.6%Au/IRMOF-3 (Table 1,
entry 17 vs. 16) and 3.2%Au/IRMOF-3 (Table 1, entry 22 vs.
21) gave the opposite results. The A3 coupling reaction was
reported to be highly affected by the nature of aldehydes, and
the reactivity of aldehydes could be affected by the employed
catalyst.4,6,14,22,29 Kidwai et al. found that aryl aldehydes posses-
sing electron-withdrawing groups afforded a better yield than
that with an electron-donating group bound to the benzene ring
which required longer reaction time with metallic Au nanoparti-
cles as catalyst.11 Wei and Li reported that both aromatic and ali-
phatic aldehydes were able to undergo the corresponding A3
coupling with the catalyst of AuBr3.4 They found that electron-
withdrawing groups displayed high reactivities, however, elec-
tron-rich groups bound to the benzene ring decreased the reactiv-
ity and required a longer reaction time. The decreased yield for
aliphatic aldehydes was caused by the trimerization of aliphatic
aldehydes with homogeneous gold salts. Zhang and Corma
found that benzaldehyde with electron-donating groups react
smoothly, while substitution of electron-withdrawing groups on
the benzene ring decreases the reactivity with the catalyst of
Au/CeO2.14 Based on the above results, we may presume that
the activity difference could be due to the different properties
(e.g., different oxidation states) between 4.6%Au/IRMOF-3 and
0.6%Au/IRMOF-3, 3.2%Au/IRMOF-3 catalysts.
With the catalyst of 4.6%Au/IRMOF-3, the reaction with sec-
ondary amines proceeded smoothly to afford the corresponding
propargylamines (Table 1, entries 1, 7–9). Among the various
amines tested, alicyclic amines such as piperidine, pyrrolidine,
and morpholine gave good conversions of benzaldehyde,
whereas the diethylamine afforded a lower conversion. This was
probably due to the iminium ions generated from alicyclic
amines and benzaldehyde which are more stable than that gener-
ated from dialkyl amine and benzaldehyde.
Avariety of aromatic alkynes were coupled with benzaldehyde
and piperidine in the presence of the 4.6%Au/IRMOF-3 catalyst.
It was found that straight-chain alkyl substituted phenylacetylene
showed decreasing conversion of benzaldehyde with increasing
chain length, the conversions were 86.4%, 70.6%, and 28.5% for
methyl, ethyl, and butyl phenylacetylene, respectively (Table 1,
entries 10–12). Clearly, when increasing the steric hindrance of
alkynes, it gave a decrease in conversion. The reaction was
found to be highly affected by the steric hindrance of the
alkynes, which was probably ascribed to the limitation effect of
the IRMOF-3 channels. Aliphatic alkynes such as 1-octyne and
(trimethylsilyl)acetylene also afforded moderate conversions of
benzaldehyde with 62.1% and 56.6%, respectively (Table 1,
entries 13 and 14).
The reusability studies of 4.6%Au/IRMOF-3, 0.6%Au/
IRMOF-3, and 3.2%Au/IRMOF-3 were carried out on the A3
coupling reaction of benzaldehyde, piperidine, and phenylacety-
lene. The results are summarized in Table 2. The fresh 4.6%Au/
1716 | Green Chem., 2012, 14, 1710–1720
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IRMOF-3 gave a benzaldehyde conversion of 77% within 0.5 h.
After four successive cycles with intermediate extensive washing
with 1,4-dioxane, the conversions over 4.6%Au/IRMOF-3 were
75%, 73%, 62%, and 66% at reaction times of 5, 12, 12 and
13 h, respectively. Clearly, 4.6%Au/IRMOF-3 features a slight
deactivation mode. However, no drop in activity was found over
0.6%Au/IRMOF-3 and 3.2%Au/IRMOF-3 which were prepared
by the one-pot synthesis. Specifically, the conversions over 3.2%
Au/IRMOF-3 for the fresh and three successive cycles are 97%,
98%, 98%, and 97% at reaction time of 10, 7, 9, and 10 h,
respectively. The fourth reuse of 0.6%Au/IRMOF-3 showed a
slight decrease in the conversion. Considering that the sizes of
the gold nanoparticles for the recycled 4.6%Au/IRMOF-3 and
3.2%Au/IRMOF-3 catalysts were aggregated from 3.3, 2.5 nm to
13.8, 12.0 nm, respectively (Fig. 5), we can rule out the possi-
bility that the deactivation mode of 4.6%Au/IRMOF-3 is due to
the conglomeration of Au nanoparticles. This conclusion agrees
very well with the work of Kidwai et al., who found that unsup-
ported gold nanoparticles with average sizes of 10, 20, or 30 nm
gave very similar benzaldehyde conversions (ca. 90%).11 As it
was well established that cationic gold species showed higher
catalytic activity than that of metallic gold counterpart for the A3
coupling reaction,4,14 the deactivation of 4.6%Au/IRMOF-3
could be due to the reduction of Au3+ as demonstrated by the
TPR analysis (Fig. 6). It should be pointed out that there is a
slight leaching of gold (ca. 5%) over the recycled 4.6%Au/
IRMOF-3, however, there is no leaching of gold over the
recycled 3.2%Au/IRMOF-3 and 0.6%Au/IRMOF-3 catalysts
evidenced by ICP-AES analysis. The slight leaching of gold
could also be responsible for the deactivation of 4.6%Au/
IRMOF-3. We presume that the high crystallinity and thermal
stability of the 3.2%Au/IRMOF-3 and 0.6%Au/IRMOF-3 cata-
lysts provide the advantage of resisting gold leaching.
Wei and Li have initially reported the A3 coupling catalyzed
using homogeneous gold catalysts (e.g., AuBr3),4 while silver,
copper, and mercury salts were also found to be able to catalyze
the reaction.6–8 However, as mentioned earlier, the limitations
and the deactivation for homogeneous catalysts make it imposs-
ible for sustainable catalytic processes. By investigating several
heterogeneous gold catalysts for the A3 coupling reaction, using
benzaldehyde, phenylacetylene and piperidine as a model reac-
tion, we could evaluate their catalytic performance as summar-
ized in Table 3. Unsupported Au nanoparticles (ca. 20 nm)
showed good catalytic activity, i.e., 97% conversion of benzal-
dehyde in 5 h at 75–80 °C.11 The reaction rate and TON based
−1
on total Au contents were only ca. 9.8 mmol gAu h−1 and 9,
respectively. Gold on layered double hydroxide (LDH–AuCl4)
afforded the desired propargylamines with a reaction rate and
−1
TON of ca. 31.0 mmol gAu h−1 and 30, respectively, in
refluxing THF (ca. 80 °C).30 However, the activity of the cata-
lysts sharply decreased in the successive reuse (92% yield of the
fresh to 40% in the second use). The reaction rate of Au nano-
particles (ca. 7 nm) encapsulated over mesoporous carbon
nitride afforded a 96% yield at 100 °C in 24 h, whereas the cata-
lytic efficiency has only been demonstrated for very few sub-
strates and its reuse has not been reported.12 Layek et al.
reported that Au0 nanoparticles (10–12 nm) supported on com-
mercial nano active magnesium oxide plus showed high yields
of 96% in 24 h at 100 °C with a reaction rate and TON of
−1
436.7 mmol gAu h−1 and 407, respectively.13 CeO2 supported
gold nanoparticles (2–5 nm) gave benzaldehyde conversion up
to 99% in 6 h at 100 °C with a TON as high as 788, however,
the leaching of gold was unavoidable for the first reuse.14 Based
on the above results, our 4.6%Au/IRMOF-3 catalyst shows
Table 2 Recyclability of Au/IRMOF-3 in A3 coupling reaction of
benzaldehyde, piperidine, and phenylacetylenea
−1
much higher reaction rates (120.5 mmol gAu h−1) than unsup-
4.6%Au/IRMOF-3b 0.6%Au/IRMOF-3c 3.2%Au/IRMOF-3c
ported gold nanoparticles and LDH–AuCl4. Although its TON
number is little smaller than LDH–AuCl4, the catalyst shows
good stability and can be reused at least 4 times. The 0.6%Au/
IRMOF-3 and 3.2%Au/IRMOF-3 catalysts prepared by the one-
pot synthesis method showed improved catalytic activity at a
moderate reaction temperature (150 °C). Specifically, the
Time
Run (h)
Conv.
(%)
Time
(h)
Conv.
(%)
Time
(h)
Conv.
(%)
Fresh 0.5
77
75
73
62
66
12
7
7
8
9
98
98
94
93
90
10
7
9
97
98
98
97
1
2
3
4
5
12
12
13
10
0.6%Au/IRMOF-3 shows
a reaction rate of 47.3 mmol
gAu−1 h−1 and TON number of 112. Although the activity is still
less than those of NAP–MgO and Au/CeO2, the excellent per-
formance of the catalyst reuse shows the advantage of using the
IRMOF-3 as a stabilizer.
a Reaction conditions: benzaldehyde (0.250 mmol), piperidine
(0.300 mmol), phenylacetylene (0.325 mmol), and 1,4-dioxane (1.0 g),
catalyst (0.07 g). b 120 °C. c 150 °C.
Table 3 The reaction rate and TON with different heterogeneous gold catalysts in A3 coupling reaction
Average size of Au
(nm)
Time
(h)
Conv./Yield
(%)
Reaction rate
Catalyst
Au oxidation states
(mmol gAu−1 h−1
)
TON
Ref.
4.6%Au/IRMOF-3
0.6%Au/IRMOF-3
3.2%Au/IRMOF-3
Unsupported Au nanoparticles
LDH–AuCl4
Au–MCN
NAP–MgO
3.3
1.7
2.5
20
—
7
Au3+, Au0
Au0
0.5
12
10
5
77
98
97
97
92
96
96
99
120.5
47.3
10.9
9.8
31.0
—
12
112
22
9
30
—
This work
This work
This work
11
30
12
13
14
Au0
Au0
Au3+
5
Au0
24
24
6
10–12
2–5
Au0
436.7
659.5
407
788
Au/CeO2
Au3+, Au0
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Taking into account our reaction results and the previous
reports, we may infer that the A3 coupling reaction is insensitive
for the size of gold nanoparticles. One evidence comes from the
fact that high catalytic activity could be obtained on various
sizes of gold nanoparticles (1.7–50.0 nm).11–14 Another evidence
may lie in the fact that the activity is not changed for the three
successive reused 0.6%Au/IRMOF-3 and 3.2%Au/IRMOF-3
catalysts, although their sizes of gold nanoparticles significantly
aggregated (Fig. 5). Finally, Kidwai et al. found that unsupported
gold nanoparticles with average sizes of 10, 20, 30, and 50 nm
exhibited very similar yields (73%–92%), although the gold
nanoparticles with ca. 20 nm showed a little higher reaction
rates.11
Based on the above discussion, and taking views that the size
of gold nanoparticles is insensitive for the A3 coupling reaction,
we presume that the higher reaction rate and TON number of
4.6%Au/IRMOF-3 than 0.6%Au/IRMOF-3 and 3.2%Au/
IRMOF-3 at 120 °C were most probably due to the effect of
gold oxidation states. The appearance of Au3+ ions on 4.6%Au/
IRMOF-3, while only metallic gold on the latter two catalysts as
demonstrated by TPR was the prominent difference on the three
catalysts. Wei and Li found that metallic gold sponge showed
not any activity for the A3 coupling reaction at 100 °C.4 Kantam
et al. found that the activity of LDH–AuCl4 sharply decreased
with the reduction of Au3+ ions into Au+ and Au0.30 Zhang and
Corma also found that the catalytic activity of Au3+ supported
on nanocrystalline ZrO2 and CeO2 is much higher than Au0 in
A3 coupling reaction.14 Therefore a fraction of cationic Au3+
(Au3+/Au0 = 0.2) in the 4.6%Au/IRMOF-3 catalyst should
responsible to the higher catalytic activity than those of 0.6%Au/
IRMOF-3 and 3.2%Au/IRMOF-3 with only metallic gold
counterpart.
frameworks into the C–H bond. The alkenyl–Au intermediate
then reacted with the iminium ion generated in situ from the
aldehydes and secondary amines to give the corresponding pro-
pargylamines and to regenerate the Au3+ and/or Au0 nanoparti-
cles catalysts for further reactions.
Conclusions
In conclusion, we have shown that gold functionalized
IRMOF-3 catalysts can be achieved by using the strategies of
post-covalent modification and one-pot synthesis. The 4.6%Au/
IRMOF-3 catalyst prepared by the PM method contains gold
nanoparticles with an average size of 3.3 nm and features a
mixture of Au0 and Au3+. The 0.6%Au/IRMOF-3 and 3.2%Au/
IRMOF-3 catalysts prepared by the OP method show higher
crystallinity and contain metallic gold nanoparticles with average
sizes of 1.7, and 2.5 nm, respectively. These catalysts were
explored in A3 coupling reactions of aldehydes, amines, and
alkynes, and showed good to excellent conversions of aldehydes
and 100% selectivity for propargylamines. It was also shown
that the catalytic activity of 4.6%Au/IRMOF-3 was higher than
0.6%Au/IRMOF-3 and 3.2%Au/IRMOF-3 at 120 °C. The reac-
tion rate and TON can be greatly enhanced by increasing the
reaction temperature of 150 °C for 0.6%Au/IRMOF-3 and 3.2%
Au/IRMOF-3 catalysts. The appearance of cationic Au3+ on
4.6%Au/IRMOF-3 is the main reason for the higher catalytic
activity. The size of the gold nanoparticles is insensitive for the
catalytic activity of A3 coupling reaction. The A3 coupling reac-
tion catalyzed by IRMOF-3 immobilized gold catalysts renders
broad substrate applicability. And the catalysts can readily be
recovered and reused for at least 4 cycles. No loss of activity was
found for the 0.6%Au/IRMOF-3 and 3.2%Au/IRMOF-3 cata-
lysts with higher crystallinity and thermal stability as compared
with 4.6%Au/IRMOF-3. These features render the catalysts par-
ticularly attractive in the practice of propargylamines synthesis in
an environmentally friendly manner.
We speculate that the reaction mechanism of these supported
Au catalysts (Scheme 2) could be the same as that with cationic
gold and/or Au0.4,11,14 A tentative mechanism was proposed
involving the absorption of alkyne followed by the insertion of
Au3+ and/or Au0 nanoparticles stabilized by the metal–organic
Scheme 2 Probable mechanism of the A3-coupling catalyzed by Au/IRMOF-3.
1718 | Green Chem., 2012, 14, 1710–1720
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Dried IRMOF-3 (2.000 g, 2.450 mmol) was dispersed in
15 mL CHCl3. A solution of salicylaldehyde (1.050 g,
8.600 mmol) in CHCl3 (15 mL) was dropwise added at room
temperature, and stored seven days at room temperature.18b The
samples were centrifuged, washed thrice with CHCl3 and dried
in vacuum at 50 °C to yield IRMOF-3-SI.
Experimental
Materials and methods
All the chemicals purchased are of reagent grade and were used
without further purification. The crystal structures of the samples
were characterized by powder X-ray diffraction (XRD) on a
Brüker D8 Advance diffractometer at 40 kV and 40 mA for Cu
Kα, with a scan speed of 10° min−1 and a step size of 0.02° in
2θ. Surface areas of the samples were measured with nitrogen
adsorption at 77 K on a Quantachrome instrument. For each
measurement, the sample was degassed at 50 °C for 13 h to
remove any loosely held adsorbed species, then analyzed at
77 K with N2 analysis gas. The surface area and pore volume
were determined by the Brunauer–Emmett–Teller (BET)
method, and the average pore size was calculated from the deso-
rption branch by using the Staito–Foley (SF) method. Thermo-
gravimetric analysis (TGA) and differential thermal (DTA)
analyses were measured in a Mettler Toledo TGA/SDTA 851
instrument in flowing nitrogen (50 mL min−1) with a rate of
10 °C min−1. The samples of IRMOF-3-SI and 4.6%Au/
IRMOF-3 were vacuumed in a rotator at 30–50 °C for 3–5 h
before test. The TG-DTA curves were obtained with the
as-synthesized 0.6%Au/IRMOF-3 and 3.2%Au/IRMOF-3
drying in air without further pretreatment. Infrared (IR) spectra
(400–4000 cm−1) were recorded from KBr pellets in a
2000FT-IR spectrometer. The metal (Au) contents were deter-
mined by inductively coupled plasma (ICP) on an Optima 7000
DV instrument. Transmission electron microscopy (TEM) was
used to determine the Au particle size distribution and mor-
phology of the samples. The specimens were prepared by grind-
ing with a mortar and the resulting powder was then sonicated in
ethanol to achieve good dispersion. The solution was dropped
onto a holey carbon coated 300 mesh copper grid (SPI). The
samples were dried and then analyzed with JSM-200CX trans-
mission electron microscope. X-Ray photoelectron spectroscopy
(XPS) measurements were carried out in an ARL-9800 instru-
ment. The temperature-programmed-reduction (TPR) measure-
ment was conducted on a home-made apparatus equipped with a
TCD detector. The catalysts were pretreated at 50 °C for 0.5 h
under argon. The catalysts were subsequently contacted with a
6.0%H2/Ar mixture and heated, at a rate of 10 °C min−1, to a
final temperature of 400 °C. Water, which is the only volatile
product of the reduction reaction was removed from the exit gas
with a cold molecular sieve trap at −17 °C to avoid its interfer-
ence with the TCD detector.
Preparation of Au catalysts. For the synthesis of Au/
IRMOF-3 by post-covalent modification,
a solution of
NaAuCl4·2H2O (0.071 g) in 0.5 mL MeCN was dropwise added
to IRMOF-3-SI (0.690 g) at room temperature and was stored
overnight. Then the sample was dried in a vacuum at 30 °C for
3 h. The catalyst was denoted as 4.6%Au/IRMOF-3.
The typical procedure for the one-pot synthesis of the Au/
IRMOF-3 catalysts: a mixture of NH2–BDC (0.68 mmol,
0.124 g), salicylaldehyde (0.220 mmol, 0.027 g), and DMF
(18 mL) were stirred for 0.5
h at room temperature.
Zn(NO3)2·6H2O (2.000 mmol, 0.600 g) was added to the
mixture and was stirred for a further 0.5 h. Then a solution of
AuCl (0.020 mmol, 0.007 g) in DMF (1 mL) was dropwise
added. The mixture was transferred into a Teflon-lined stainless
steel vessel and was heated for 24 h at 100 °C. After cooling of
the vessel to room temperature, the resulting green solid was col-
lected by centrifugation and was washed thrice with DMF and
CHCl3 over three days. The catalysts were denoted as 0.6%Au/
IRMOF-3. 3.2%Au/IRMOF-3 was synthesized using the same
molar ratios as for 0.6%Au/IRMOF-3, but using NaAuCl4·2H2O
(0.020 mmol, 0.015 g) as the Au precursor.
Catalytic measurements. Typical procedure for the A3 coup-
ling reaction: a mixture of catalyst (0.070 g), benzaldehyde
(0.250 mmol, 0.027 g), piperidine (0.300 mmol, 0.026 g), phe-
nylacetylene (0.325 mmol, 0.034 g), and 1,4-dioxane (1.000 g)
were put into a closed glass reactor (2 mL, SUPELCO) and were
extensively stirred (ca. 500 rpm) at 120 or 150 °C. After the
reaction, the catalyst was removed from the solution by centrifu-
gation at 6000 rpm for 10 minutes. The product was analysed by
GC (SP6890, capillary column, SE-30). And the recovered cata-
lyst was thoroughly washed with 1,4-dioxane and used for the
next run.
Acknowledgements
We thank the National Natural Science Foundation of China (no.
20903119, 21173269, and 91127040) and Programme for New
Century Excellent Talents in University.
References
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vated according to the procedure from literature with slight mod-
ifications.17,18a In a typical catalyst preparation, Zn(NO3)2·6H2O
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were dissolved in 200 mL N,N′-dimethylformamide (DMF) and
stirred for 0.5 h at room temperature in air. The solution was
transferred and sealed in a 500 mL Teflon-lined autoclave, and
kept at 100 °C for 18 h. The resulting brown solid was collected
by centrifugation and washed thrice with DMF and CHCl3 over
three days, and the solid was finally dried in a vacuum at 50 °C.
This journal is © The Royal Society of Chemistry 2012
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1720 | Green Chem., 2012, 14, 1710–1720
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