Dual-functional click-triazole: A metal chelator and immobilization linker for the construction of a heterogeneous palladium catalyst and its application for the aerobic oxidation of alcohols

Article abstract of DOI:10.1039/c2cc18023e

A novel SBA-15 supported catalyst PdL_n@SBA-15 containing a 2-pyridyl-1,2,3-triazole ligand framework was prepared via a "click" route, in which the click-triazole acted as both a stable linker and a good chelator. The catalyst was characterized and applied for the aerobic oxidation of alcohols, and the product was obtained in up to 98% yield.

Full text of DOI:10.1039/c2cc18023e

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Dual-functional click-triazole: a metal chelator and immobilization linker
for the construction of a heterogeneous palladium catalyst and its
application for the aerobic oxidation of alcoholsw
Guofu Zhang, Yong Wang, Xin Wen, Chengrong Ding* and Ying Li
Received 22nd December 2011, Accepted 2nd February 2012
DOI: 10.1039/c2cc18023e
4e,k
ions like Cu,
4c,j,i
Ru,
4b,h
Pd,
A novel SBA-15 supported catalyst PdL @SBA-15 containing a
n
etc. have been successively
2
-pyridyl-1,2,3-triazole ligand framework was prepared via a
explored. Inspired by its attractive features, we were intrigued
to design solid catalysts based on the utilization of the versatile
click-triazole, not only as a stable linker to graft catalysts onto
the supports but also as a good chelator to participate in the
‘
‘click’’ route, in which the click-triazole acted as both a stable
linker and a good chelator. The catalyst was characterized and
applied for the aerobic oxidation of alcohols, and the product
was obtained in up to 98% yield.
5
catalytic reactions.
Herein, we report a novel SBA-15 supported catalyst
0
PdL @SBA-15 containing a ‘‘2,2 -bipyridine analogue’’ ligand
n
During the past decades, palladium-mediated catalysis has been
recognized as one of the most powerful synthetic tools and as a
fascinating area in organic transformations for both academic
framework via the copper-catalyzed ligation of 2-ethynylpyridine
with azide functionalized SBA-15, and its application for the
aerobic oxidation of alcohols.
1
research and industrial processes. Various palladium complexes
Our investigation began with the preparation of PdL @SBA-15
n
with original ligand frameworks have been well designed and
successfully applied in organic reactions. However, these homo-
geneous catalysts often suffer from catalyst separation and
catalyst recycling. To overcome these problems, palladium
complexes have been subsequently immobilized onto various
inorganic or organic supports to provide heterogeneous catalysts
which are easier for product isolation and catalyst recycling.
However, in most cases, the special section that connects the
catalysts and supports in these solid palladium catalysts acts only
as a linker, and its effects on the catalytic reaction are often
negligible. In these regards, we believe that the design of a novel,
facile grafting approach to generate heterogeneous catalysts with
a multifunctional linker is still desirable in heterogeneous
catalysis.
4
by immobilizing the palladium(II) acetate into mesoporous silica
SBA-15, which was modified with bidentate 2-pyridyl-1,2,3-
triazole ligand in advance. The detailed route is shown in
Scheme 1. The azide group was first introduced into SBA-15
through a simple procedure and verified by the appearance of the
ꢀ
1
N
3
band at 2112 cm in FT-IR spectra. Then it underwent a
‘‘click’’ process with commercially available 2-ethynylpyridine in
the presence of copper sulfate and sodium ascorbate in aqueous
methanol. The resulting functionalized SBA-15 3 was further
treated with Pd(OAc) in toluene to provide the desired solid
2
catalyst PdL @SBA-15 4, the palladium amount of which
n
measured by inductively coupled plasma atomic emission spectro-
ꢀ
1
.
metry (ICP-MS) was 0.138 mmol g
X-Ray reflective diffraction (XRD), transmission electron
microscopy (TEM) and nitrogen sorption measurements were
carried out to get detailed information about the architecture
of the novel catalyst. The small-angle XRD (Fig. S10, ESIw)
and TEM images (Fig. S12a–c, ESIw) have shown that the
structure of the well-ordered mesoporous SBA-15 remained
Copper-catalyzed Huigen-type [3+2] azide–alkyne cyclo-
2
addition (CuAAC) known as the ‘‘click reaction’’, because of
its high efficiency, simple procedure, absence of side products and
mild reaction conditions, has shown promising applications in
drug discovery, biochemistry, dendrimers, material and polymer
science during the past decade. In addition, the 1,2,3-triazoles
generated in the CuAAC reaction as a high stable linker have
3
also drawn much attention for catalyst immobilization.
Recently, the ‘‘1,2,3-triazole’’ unit has also emerged as a chelator
4
showing great potential for metal coordination, and many metal
College of Chemical Engineering and Materials Science, Zhejiang
University of Technology, Hangzhou 310014, People’s Republic of
China. E-mail: dingcr2004@yahoo.com.cn; Fax: +86-571-8832-0147;
Tel: +86-571-8832-0147
w Electronic supplementary information (ESI) available: Detailed
experimental procedures and analysis, BET data, XRD patterns,
TEM images and NMR data for products. See DOI: 10.1039/
c2cc18023e
Scheme 1 Synthesis of PdL
n
@SBA-15 4. Reaction conditions:
CN, TBAB, reflux, 24 h; (b) SBA-15, toluene, reflux,
/NaASc, MeOH/H O, rt, 3 d;
(
a) NaN
3
, CH
3
2
4 h; (c) 2-ethynylpyridine, CuSO
4
2
(d) Pd(OAc)
2
, toluene, rt, 2 h.
This journal is c The Royal Society of Chemistry 2012
Chem. Commun., 2012, 48, 2979–2981 2979

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was coordinated with both units of the ligand and the triazole unit
worked as a part of the bidentate chelator. Meanwhile, because
the triazole ligand could bind with the palladium metal through
0
either N2 or N3 atom, the spectra of PdL ðOAcÞ showed two
n
2
sets of proton signals, the ratio of which was around 1 : 3. To the
best of our knowledge, the formation of five-membered chelate
rings by binding with the N3 atom and the nitrogen atom of
pyridine is considered to be the predominant structure.
The selective oxidation of alcohols, especially utilizing
oxygen as the terminal oxidant, has been considered one of the
7
most important transformations in organic synthesis. With the
SBA-15 supported palladium catalyst in hand, we attempted to
evaluate its catalytic activity for the aerobic oxidation of
alcohols, and the selected examples are summarized in Table 1.
Initially, the benzyl alcohol was chosen as the model substrate,
and the oxidation reaction was conducted in toluene at 100 1C
using K CO as the base. Within 4 h, 95% isolated yield of the
0
0
Fig. 1 UV-Vis spectra of Ln (in methanol), PdLn (in methanol),
@SBA-15 and PdL @SBA-15.
2
3
L
n
n
benzaldehyde was obtained and the selectivity was more than
9% (Table 1, entry 1). Notably, the starting material could also
completely transform either under air atmosphere or at a low
9
intact after the immobilization process. The TEM images also
suggested that no palladium clusters were formed during the
temperature (80 1C) although the reaction was prolonged slightly
n
preparation. While the PdL @SBA-15 material analyzed by
(
Table 1, entries 2 and 3). In addition, the catalyst PdL
showed good activity in the oxidation of other primary benzyl
alcohols containing substituents such as 4-Me, 4-MeO, 4-F and
4-NO groups (Table 1, entries 4–8). No carboxylic acids or esters
were detected for all substrates investigated. The oxidation of
-substitute benzyl alcohol was achieved in a moderate yield
n
@SBA-15
nitrogen sorption isotherm measurements (Fig. S11, ESIw) showed
less surface area, pore volume and nanopore size compared with
starting material SBA-15, which might be ascribed to the
4
0
2
occupation of the ‘‘2,2 -bipyridine analogue’’ ligand and
palladium that blocked a part of pores of the material.
To further investigate the coordination state of the catalyst 4,
1
UV-Vis, H NMR and FT-IR were performed, meanwhile, its
2
(Table 1, entry 9), which may be ascribed to the steric hindrance
by the quasi-two-dimensional surface of the palladium sites. For
heterocyclic and secondary alcohols, the catalyst also showed
satisfactory performances, and the desired products were isolated
in moderate to excellent yields (Table 1, entries 10–15).
0
homogeneous ligand counterpart 1-methyl-4-(2 -pyridyl)-1,2,
0
0
3
-triazole (L ) and corresponding Pd complex (PdL ðOAcÞ )
n
n
2
were also designed and synthesized for easy comparison (see ESIw
for details). From the UV-Vis spectra (Fig. 1), it was found that
0
the ligands in solution (L ) and on the solid surface (L @SBA-15)
n
n
showed a distinct p–p* transition around 280 nm. After coordi-
nation, a slight red shift from 280 nm to 293 nm was observed
Table 1 The aerobic oxidation of alcohols with PdL @SBA-15 4 as
n
the catalyst
a
0
from the spectra of the palladium complexes (PdL ðOAcÞ and
n
2
n
PdL @SBA-15), which was probably assigned to the ligand to
6
metal charge transfer transition (LMCT). And the similar
absorption bands of the palladium complexes might suggest that
palladium existing on the solid surface was in a coordinated
fashion. To further test if either the triazole or the pyridine or
both units would coordinate to palladium cations, we have
R
1
R
2
b
Yield (%) Selec. (%)
c
Entry
1
R
x
t/h
Ph
Ph
Ph
H
H
H
H
H
H
H
H
H
H
Me
Et
Ph
Me
0.5
0.5
0.5
0.5
0.5
0.5
0.5
4
95
6.5 94
499
499
499
499
499
499
499
499
499
499
499
499
499
499
499
1
6
d
carefully examined the H NMR, COSY spectra (d -DMSO,
0
2
3
e
8
3
94
95
2
98K) of the ligand (L ) and those of the palladium complex
0
n
4
5
6
7
8
9
4-MePh
4-MeOPh
3,4,5-triMeOPh
4-FPh
(
PdL ðOAcÞ ). It was clear that the proton signals on pyridine
n
2
2.5 97
0
and triazole rings of PdL ðOAcÞ in general were shifted down-
4
9
96
94
98
65
65
96
95
92
96
98
n
2
field apparently (Fig. 2), which indicated that the palladium ion
4-NO
2
Ph
0.5 10
2-OMePh
3-Pyridine
Ph
Ph
Ph
1
1
1
1
1
1
24
20
18
21
24
12
10
11
12
13
14
15
4-MeOPh
PhCO
Ph 0.5 10
a
All the reactions were conducted with substrate/K
@SBA-15 4 (the molar ratios of substrate/K CO /Pd were
: 1 : x%) at 100 1C in toluene (3.0 mL) under an oxygen atmosphere
2 3
CO /
PdL
n
2
3
1
b
c
unless otherwise noted. Isolated yield. The selectivity was detected
e
by GC-MS. Air was used instead of oxygen. Under O at 80 1C.
2
1
Fig. 2 Partial H NMR spectra (500 MHz, d -DMSO, 298 K) of the
6
d
0
0
ligand L (a) and the palladium complex PdL ðOAcÞ (b).
n
n
2
2
980 Chem. Commun., 2012, 48, 2979–2981
This journal is c The Royal Society of Chemistry 2012

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Further experiments were performed to verify the catalyst
recyclability. After the first use, the catalyst was recovered by
simple filtration and reused in the next run after a simple workup.
We were pleased to find that the recovered catalyst was successfully
reused in the subsequent seven cycles with a consistent catalytic
activity, giving the products in excellent yields (92–95%, Fig. S13,
ESIw). To further rule out the contribution of the homogeneous
catalysis to the results, the solid catalyst was hot-filtered off from
the benzyl alcohol oxidation system after reacting for 0.5 h and the
conversion was found to be 22%. Then the liquid filtration was
performed for reaction under the same conditions for 3.5 h,
however no further reaction was observed. Meanwhile, the
ICP-MS analysis of the palladium metal in the filtration showed
that the amount of palladium leaching in solution was 0.043 ppm.
Moreover, it was noteworthy to mention that no formation of
apparent agglomerated palladium was detected after the first cycle
from the TEM image (Fig. S12d, ESIw). But we did notice a slight
decline in the quality of ordered channel frameworks of SBA-15,
which might be attributed to its poor hydrothermal stability under
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2
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supported catalyst PdL @SBA-15 4 via a ‘‘click’’ route, in
which the click-triazole acts as a stable linker as well as a good
chelator to participate in the catalytic reactions. The obtained
solid catalyst has demonstrated a promising catalytic activity
for the aerobic oxidation of benzyl alcohols. The novel design
and facile synthesis of the supported metal catalyst described
here might be widely applied to derive diverse catalysts based
on the dual-functional click-triazole. Currently, we are investigating
2
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0
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2
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We gratefully acknowledge the financial support from the
National Natural Science Foundation of China (No.
3
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Molecules, 2010, 15, 4947.
2
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Y 200907685) and the Key Scientific Technological Innovation
(
Team of Zhejiang Province (No. 2010R50018).
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Chem. Commun., 2012, 48, 2979–2981 2981