Synthesis of a heterogeneous Cu(OAc)2-anchored SBA-15 catalyst and its application in the CuAAC reaction

Article abstract of DOI:10.1039/c7nj04495j

A novel dinuclear Cu(OAc)₂-anchored SBA-15 catalyst was synthesized by simple proton exchange of a carboxyl functionalized mesoporous SBA-15 silica with Cu(OAc)₂ in water. XAFS and EPR spectra were utilized to probe its structure. Su

Full text of DOI:10.1039/c7nj04495j

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ISSN 1144-0546
PAPER
Jason B. Benedict et al.
The role of atropisomers on the photo-reactivity and fatigue of
diarylethene-based metal–organic frameworks
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Synthesis of a heterogeneous Cu(OAc)2-anchored SBA-15 catalyst
and its application in CuAAC reaction
Nan Sun,*a Zhongqi Yu,a Hong Yi,b Xiayue Zhu,a Liqun Jin,a Baoxiang Hu,a Zhenlu Shen,a and
Xinquan Hu *a
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
A
novel dinuclear Cu(OAc)2-anchored SBA-15 catalyst was attention.10 On the other hand, great progress has been made
synthesized by simple proton exchange of carboxyl on the mechanism research in this field over the past years, a
a
functionalized mesoporous SBA-15 silica with Cu(OAc)2 in water. majority of which suggested that the reaction was preferably
XAFS and EPR spectra were utilized to probe its structure. Such cooperative catalyzed by two copper atoms on the basis of a
11
12
Cu(II) solid material exhibited excellent catalytic activity for azide- series of experimental and computational studies as well
alkyne cycloaddition reaction in water without addition of either as the isolation of key intermediates.13 In contrast to
reductant or base.
monocopper catalytic pathway, transition state with dicopper
significantly reduced the activation energy of the reaction,
thereby promoting the catalysis. With the idea of the
synergistic cooperation of two copper atoms in CuAAC
catalysis, more recently, a few structure-defined dinuclear
copper catalysts were designed, synthesized and consequently
applied in CuAAC reaction.14 However, all of these catalytic
systems were homogeneous and inherently suffered from the
difficulty in removing the catalysts from product streams and
incapability of reuse. Furthermore, a majority of them required
expensive and/or special ligands to stabilize copper species
and limited the application in organic solvent under inert gas
atmospheric condition.
Since the seminal works of the independent groups of
2
1
Sharpless
and Meldal
in 2002, the copper-catalyzed
cycloaddition reaction of organic azides and terminal alkynes
to form 1,4-disubstituted 1,2,3-triazoles (CuAAC reaction) have
gained tremendous attention among the scientists from
different disciplines. It provides a robust and regio-selective
route to link two functionalized building blocks together under
mild conditions, and to date has become a prevailing reaction
with valuable applications in the fields of medicinal chemistry,3
biochemistry,4 polymer and material science,5 as well as
catalyst design.6
Commonly, CuAAC reaction was performed with Cu(I)
species as the real catalyst, which could be either directly
derived from Cu(I) salts, or in situ generated through reduction
of Cu(II) salts, or oxidation of Cu(0) clusters.7 Nevertheless,
other than Cu(I) catalyst systems, there also have been some
In this letter, we reported the synthesis of a novel dinuclear
Cu(II) solid catalyst (Cu@SBA-15-PTAA) and its application in
azide-alkyne cycloaddition reaction in water without addition
of any reductant and base. The detailed preparing procedure
of Cu@SBA-15-PTAA was elucidated in Scheme 1, which was
confirmed by FT-IR method (Figure S1, ESI+). Firstly, the
surface of SBA-15 silica was modified with organic tert-butyl
ester groups through a conventional silane coupling reaction,
followed by hydrolyzation with H3PO4 to provide a carboxylic
8
reports of the direct use of Cu(II) salts (or complexes) and
9
copper nanoparticles as catalysts for the titled reaction.
Taking cosideration of simply product(s)/catalyst seperation,
potential catalyst reuse as well as great decrease of the toxic
copper metal remain in end products, in recent years, the
development of copper-supported solid materials as
heterogeneous catalyst for CuAAC reaction was received much
functionalized mesoporous SBA-15 silica
(
SBA-15-PTAA).15
Then, the resulted SBA-15-PTAA underwent
a
proton
exchange process with Cu(OAc)2 in water, affording the
desired solid catalyst Cu@SBA-15-PTAA as a bluish solid. The
supported Cu(II) amount was 1.29 mmol/g measured by
atomic absorption spectrophotometer. In combination of acid
capacity of SBA-15-PTAA (1.28 mmol/g based on thermo-
gravimetric and potentiometrictitration analyses, see Figures
S2 and S3, ESI+), we could find that the anchoring of Cu(OAc)2
on SBA-15-PTAA to form Cu@SBA-15-PTAA with
a 1:1
complex.
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the EPR experiment of Cu@SBA-15-PTAA, a strong EPR signal
was observed (Figure 2c), which was similar to the previous
reported Cu(II) species.18a This result suggested that single-
electron existed, which was in accordance with the Cu(II) in the
heterogeneous catalyst .
(b)
(c)
(a)
10
8
1.6
1.2
0.8
0.4
0.0
Cu -edge
K
Cu K-edge
Cu(OAc)2
Cu@SBA-15-PTAA
6
Cu@SBA-15-PTAA
4
Cu(OAc)2
Cu@SBA-15-PTAA
Scheme 1 The synthesis of Cu(OAc)2-anchored SBA-15 catalyst (Cu@SBA-15-
PTAA).
2
0
3.0
4
2.8
2.6
2.4
2.2
2.0
1.8
1.6
1.4
8960
9000
9040
9080
9120
0
1
2
3
g
Energy (eV)
(Å)
R
The architecture information of this novel catalyst was
characterized by X-Ray reflective diffraction (XRD), high
resolution transmission electron microscopy (HR-TEM) and N2
sorption measurements. The small-angle XRD and TEM in
Figure 1 showed that the structure of the well-ordered
mesoporous SBA-15 remained intact after the immobilization
process. Nevertheless, the XRD peak intensity of Cu@SBA-15-
PTAA was a slight lower than those of SBA-15 and SBA-15-
PTAA, which could be attributed to the lowering local order
after anchoring Cu(OAc)2 at the surface. The N2 sorption
isotherms of Cu@SBA-15-PTAA showed a typical type IV
isotherm with a very large hysteresis loop in the 0.5 to 0.8 P/Po
range (Figure S4, ESI+), also accounting for the ordered
mesopore. Using a BJH model, the BET surface area, pore
volume and pore size of Cu@SBA-15-PTAA was calculated to
be 411 m2/g, 0.58 cm3/g and 49.6 Å, respectively. These values
were obviously less than those of their parent SBA-15 and
SBA-15-PTAA (Table S1, ESI+), suggesting the Cu-sites inside
the channels of SBA-15 in some extent.
Figure 2 XAFS spectra of Cu@SBA-15-PTAA and Cu(OAc)2 (a) XANES spectra, (b)
EXAFS spectra, (c) EPR spectrum.
We were delighted to find that the newly prepared
dinuclear copper(II) solid exhibited excellent catalytic activity
in the cycloaddition reaction of azides and terminal alkynes in
water without the requirement of any external reductant and
base.19 As the results shown in Scheme 2, with 2 mol% of
Cu@SBA-15-PTAA as catalyst, a wide range of substituted
phenylacetylenes, aliphatic acetylenes and heteroaromatic
acetylenes could smoothly react with benzyl azide and phenyl
azide at 50 oC to afford their corresponding cycloaddition
product, 1,4-disubstituted 1,2,3-triazoles, in excellent yields
(91-99%) with appropriate reaction time. A diversity of
functional groups, such as NH2, Br, NO2, OH, alkynyl, and
COOEt, were well tolerated and could be potentially utilized
for further transformations. Interestingly, by employing 1,4-
diethynyl benzene as substrate, only mono-cyclization addition
product was obtained. The exclusive chemoselectivity on the
mono-cyclization of 1,4-diethynyl benzene could be attributed
to the pore size control of catalyst that restrained the
formation of much larger di-cyclization addition product. The
cycloaddition product 2ag could be potentially used for further
CuAAC reaction to achieve unsymmetric di-1,2,3-triazoles.
When 2-ethynylpyridine and 2-ethynylthiophene were used as
Figure 1 XRD patterns and HR-TEM images of Cu@SBA-15-PTAA
cycloaddition substrates,
a
relatively higher reaction
o
temperature (70 C) was required for the cmpletion reaction.
We reckoned that the lower reactivity of these two substrates
might be ascribed to the coordination action of the generated
products 3ap and 3aq with catalytic Cu(II) species, which
restrained the reaction to some extent. We also found that the
catalyst Cu@SBA-15-PTAA was competent in one-pot
multicomponent reaction of benzyl bromide/chloride, NaN3
and phenyl alkyne for 1,4-disubstituted 1,2,3-triazole synthesis
in water (Scheme 3). It was noteworthy to point out that
irrespective of the nature of alkynes and azides, an exclusive
1,4-regioselectivity was observed in all these reactions under
the catalysis of Cu@SBA-15-PTAA in water.
In addition, we also used XAFS spectroscopy, which serves
as a powerful tool for the metal structure characterization,16 to
study the local structure of the resulted copper catalyst. As
shown in Figure 2a, the XANES spectrum of Cu@SBA-15-PTAA
was similar to that of Cu(OAc)2, which indicated the valency of
copper in Cu@SBA-15-PTAA was +2. Then, the comparison of
Cu(OAc)2 and Cu@SBA-15-PTAA in EXAFS parts shown in
Figure 2b revealed that the coordination environments of
Cu(OAc)2 and Cu@SBA-15-PTAA were similar from the local
structure parts. Furthermore, we also fitted the EXAFS spectra
of Cu@SBA-15-PTAA. The fitting results (Table S2, ESI+)
revealed that four oxygen atoms coordinated to one Cu(II)
species. The average bond length of Cu-O bond was 1.96 Å.
The recyclability of Cu@SBA-15-PTAA was further studied.
Since the catalytic reaction was heterogeneous and performed
in neat water without any additive reagent, therefore, the
yielded product could be conveniently separated from the
reaction mixture by simple extraction with common organic
solvent (such as ethyl acetate, CH2Cl2), while the catalyst
remaining in aqueous phase could be served for next run. It
was observed that the catalyst could be reused at least 5 times
Based on the well-known dimeric structure of Cu(OAc)2,17 the
18
literature reports
and our fitting results, the structure of
Cu@SBA-15-PTAA was assigned in Scheme 1, which was a
dinuclear species and each Cu(II) was coordinated with four
oxygen atoms. Furthermore, we also used EPR spectroscopy to
probe the heterogeneous copper catalyst. When performing
2 | J. Name., 2012, 00, 1-3
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(Table S4, ESI+). Furthermore, we characterized the recovered results were shown in Scheme 4. One experiment, that using
catalyst by XRD and EPR analyses, performed the leaching test 1-deuterium-substituted phenyl alkyne as substrate, led to a
and measured the Cu content of the reused catalyst.20 All of remarkably lower yield (63%) and the percentage of 5-
these experiments signified that the catalyst was very stable deuterated 1,2,3-triazole in final product was about 2% (eq. 1,
during the course of reaction and Cu(II) was tightly grafted on Scheme 4). In another experiment, replacement of H2O with
the SBA-15-PTAA support. In order to check the D2O as reaction medium did not affect the yield. However, the
heterogeneous nature of the catalyst, a hot filtration test was ratio of 5-D/5-H products reached to 9 : 1 (eq. 2, Scheme 4).
also carried out. The result showed that nearly no further These phenomena indicated that the reaction involved C-D
reaction proceeded after filtering the catalyst from the bond cleavage and it might be the rate-determining step in the
reaction mixture.20
catalytic cycle.
Scheme 4 Deuteration experiments
Based on above experimental results and clued by previous
reports,21 a possible stepwise mechanism was proposed. As
shown in Figure 3, firstly, benzyl azide and phenyl acetylene
respectively coordinated with two adjacent Cu(II) cations of
o
Cu@SBA-15-PTAA. Then, under the reaction conditions (50 C
in water), the C-H band of phenyl acetylene was cleaved to
form a Cu(II)-acetylide intermediate, accompanying with the
diassociation of Cu-O bond of Cu@SBA-15-PTAA. In this case,
the solvent water acted as base and promoted the conversion.
As the Cu(II)-acetylide intermediate generated, it immediately
reacted with the neighboring benzyl azide, forming a copper
metallocycle species. The latter was unstable and immediately
contracted to afford 1,2,3-triazole ring. Finally, proto
demetallation occurred through internal proton delivery to
release the produced 1,2,3-triazole and regenerate the
catalyst. If the reaction was carried out in D2O, D-H exchange
occurred before the protodemetallation step and the
corresponding deuterium product was resulted. The exclusive
regio-selectivity to 1,4-isomers was attributed to the selective
coordination of Cu(II) cation to N3 of benzyl azide.
Scheme 2 Cycloaddition reaction of azides and alkynes. Reaction conditions: 1a
or 1b (1.0 mmol),
2
(1.1 mmol), catalyst, H2O (2 mL), isolated yields.
Cu@SBA-15-PTAA
H2O, 50 oC, 8 h
2 mol%
X
N
N
NaN3
+
+
N
1.0 mmol
1.1 mmol
2a (1.1 mmol)
3aa
X=Br, 98%
X=Cl, 89%
Scheme 3 One-pot multicomponent azide-alkyne cycloaddition reaction
In order to well understand this Cu@SBA-15-PTAA-catalyzed
cycloaddition process, we then turned our attention to
mechanistic studies. At first, kinetic experiments of the
cycloaddition reaction were carried out with the reaction of
benzyl azides and phenyl acetylene. The results (Figure S4,
ESI+) showed that no obvious induction period was observed
for the reaction. In combination with the fact that the color of
Cu@SBA-15-PTAA maintained blue during the overall reaction
process and no a threshold amount of phenyl acetylene homo-
coupling product was detected in product profiles based on
HPLC analyses, we can conclude that the possibility of
generation of Cu(I) species as real catalyst during this catalytic
process could be convincingly ruled out.14a Furthermore, two
deuterium-exchange experiments were performed and the
Figure 3 Plausible reaction mechanism for 1,4-disubstituted 1,2,3-triazole
formation with Cu@SBA-15-PTAA catalyst
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Jia, N. Guo, G. Xu and W. Zhang, Chin. J. Chem., 2017, 35
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In summary, we have obtained a new dinuclear copper(II)
solid catalyst (Cu@SBA-15-PTAA) by simple proton exchange
of carboxyl functionalized mesoporous SBA-15 silica with
Cu(OAc)2 in water at room temperature. Its actual structure
and architecture information were characterized by FT-IR,
XRD, HR-TEM, N2 sorption, EXAFS and EPR. The resulted
Cu@SBA-15-PTAA could be directly utilized for the catalysis of
CuAAC reaction in water without the requirement of any
reductant and base. Moreover, we found that the Cu@SBA-15-
PTAA in water could be reused at least for five times. The
mechanism studies revealed that the cycloaddition process
was directly catalyzed by Cu(II) species and the unique
dinuclear copper(II) structure of Cu@SBA-15-PTAA played a
crucial role on its catalytic activity. The convenient operation,
easy preparation of Cu@SBA-15-PTAA as well as its excellent
recyclibility make this method attractive for practical
applications.
9
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Conflicts of interest
There are no conflicts to declare.
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The authors gratefully acknowledge the National Natural
Science Foundations of China (21473160, 21603190). We also
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Table of Contents
A dinuclear Cu(OAc)2-anchored SBA-15 silica was synthesized, confirmed and showed
excellent catalytic activity for azide-alkyne cycloaddition reaction in water.