Ag-NHC anchored on silica: An efficient ultra-low loading catalyst for regioselective 1,2,3-triazole synthesis

Article abstract of DOI:10.1039/c9nj03892b

A silica-supported silver complex, Ag-NHC@SiO₂, was prepared by an anchoring coordination technique, which was successfully employed for the click reaction under mild reaction conditions. The protocol offered the remarkable advantages of operat

Full text of DOI:10.1039/c9nj03892b

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Ag–NHC anchored on silica: an efficient ultra-low
loading catalyst for regioselective 1,2,3-triazole
synthesis†
Cite this: New J. Chem., 2019,
43, 19331
a
b
c
a
Anirban Garg,
Nagesh Khupse,
Ankur Bordoloi
and Diganta Sarma
*
Received 26th July 2019,
Accepted 10th November 2019
A silica-supported silver complex, Ag–NHC@SiO2, was prepared by an anchoring coordination technique,
which was successfully employed for the click reaction under mild reaction conditions. The protocol
offered the remarkable advantages of operational simplicity, water as a reaction medium, ultra-low loading
of the metal and easy recovery of the supported catalyst.
DOI: 10.1039/c9nj03892b
rsc.li/njc
The azide–alkyne cycloaddition (AAC) reaction, popularly known
N-Heterocyclic carbenes (NHC) have become a popular choice
as the click reaction, has been a promising tool for drug of ligands for organic and inorganic chemists in synthesizing
discovery, molecular biology and proteomic applications.1 The highly efficient homogeneous catalysts ever since Arduengo and
original Huisgen cycloaddition of organic azides and alkynes co-workers isolated the first stable N-heterocyclic carbene (NHC)
can be successfully applied for triazole synthesis but has been in 1991.18,19 With strong s-donation and weaker p-acceptor
found to have several demerits, such as the requirements of properties, NHCs can stabilize metal centres and hence are
long reaction times and high temperatures as well as the lack of capable of forming complexes with almost all transition metals
control in the regiochemical outcome of the product. However, and many main group elements.20 Attributing to these properties,
the introduction of a Cu(I) catalyst rejuvenates this reaction to metal NHCs are being used in different key catalytic steps
accomplish regioselectivity in a short reaction time.2 Since then, of various organic transformations such as hydrosilylation
the concept of click chemistry has been widely utilized in every of carbonyl compounds, intra and intermolecular C–H bond
field of chemistry. After the successful application of Cu cata- activation, and regioselective azide–alkyne 1,3-dipolar cyclo-
lysts, attempts have also been made by several researchers to addition reactions.21 Silver–NHC compounds are effective NHC
replace copper, due to its cytotoxicity, with other metals such as transfer agents owing to their straightforward synthesis, avoiding
ruthenium,3 zinc,4 iridium,5 nickel,6 and palladium.7 Silver the requirement for free NHC isolation and the relative stability
sources in various forms have long been used in different chemical of the Ag–NHC complexes towards light and air.22 However, the
transformations, like the synthesis of furan and its derivatives,8 use of these catalysts in aqueous media is quite limited.
the total synthesis of (+)-asimicin and pyrroloistatin,9 and the
The heterogenization of homogenous catalysts has evolved
dearomatizing spirocyclization of alkyne-tethered aromatics.10 as an attractive tool to acquire recoverable and reusable catalysts.
The possibility of silver acting as a substitute to copper in AAC Although heterogeneous catalysts lack the distinct selectivity
has been demonstrated by McNulty and coworkers11 using offered by homogeneous catalysts, they are indeed beneficial
P,O-type silver complexes for the first time, while Tuzun et al. considering the depletable nature, complicated recycling
provided mechanistic insights to the reaction through DFT problems and metal contamination of the products associated
studies.12 In addition, Ag-based complexes,13 Ag nanoparticles with homogeneous catalysts. Silica remains a popular choice
supported on graphene,14 Al2O3@Fe2O3,15 ceria,16 and AgCO3 in of support to prepare heterogeneous catalysts owing to its
micellar media17 have been employed successfully in AAC. Although easy availability, low cost and excellent porosity. The use of
there are a few reports on Ag-catalyzed azide–alkyne cycloaddition silica-supported catalysts in triazole syntheses has been well
(AgAAC), there is a possibility for further improvements.
explored; however, the existing methods demand the use of
high temperatures, organic solvents, longer reaction times or a
high loading of the catalysts.10,23 To the best of our knowledge,
there is no report using silica-supported Ag–NHC complexes
as catalysts for the click reaction let alone in aqueous media.
In the search of an effective methodology for Cu-free click
reactions, herein, we reported an ultra-low loading of an Ag(I)–
a Department of Chemistry, Dibrugarh University, Dibrugarh – 786004, Assam,
India. E-mail: dsarma22@gmail.com
b Center for Materials for Electronics Technology, Pashan Road, Pune – 411008,
India
c Nano Catalysis, Catalytic Conversion and Process Division, CSIR – Indian Institute
of Petroleum, Mohkampur, Dehradun 248005, India
† Electronic supplementary information (ESI) available. See DOI: 10.1039/c9nj03892b NHC complex supported on silica as an efficient heterogeneous
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Scheme 1 Preparation of Ag–NHC@SiO2.
catalyst for the regioselective synthesis of triazoles in water
under ambient conditions.
Materials and methods
Fig. 2 N2 adsorption–desorption isotherms of various silica samples.
The preparation of Ag–NHC@SiO2 was performed by a three-
step procedure (Scheme 1) following the report by Lei Wang
et al. for the synthesis of a silica-immobilized Ag–NHC complex
for the A3-coupling reaction.24 The supported catalyst was prepared
from commercial chromatography grade silica gel (60–120 Mesh),
which was first functionalized with imidazole-based ionic liquids
by the reaction with (3-chloropropyl)trimethoxysilane and then
with 1-methyl imidazole in refluxing dry toluene for 8 h under a
nitrogen atmosphere to obtain a silica-immobilized ionic liquid.
After filtration and drying of the functionalized silica support,
it was treated with AgBr in the presence of KOtBu in THF under
light protection, which was again filtered and dried to obtain
Ag–NHC@SiO2 as a light grey coloured solid.
Results and discussion
Ag–NHC@SiO2 was characterized by FT-IR, TGA, SEM-EDX, and
XRD. To validate the formation of the N-heterocyclic carbene
complex and its effective immobilization on the silica surface,
an infrared spectroscopic study was conducted. The FTIR
spectra of the prepared samples are shown in Fig. 1.
For the samples, the bands around 1637 cmÀ1 and 3430 cmÀ1
can be ascribed to the nO–H stretching and bending vibrations of
the adsorbed water, respectively. The absorption peaks observed
around 808 and 1092 cmÀ1 were due to the Si–O–Si structure
of the silica framework. A characteristic band at 965 cmÀ1
was found due to the silanol group of silica, indicating the
formation of an –Si–O– bond, which was formed by the reaction
of the silanol groups of silica with the (CH3O)3Si– group
of CPTMS. However, after the functionalization of silica with
CPTMS, 1-methyl imidazole and insertion of the metal,
the intensity of this band progressively decreased for all the
samples with each step due to a decrease in the availability of
free silanol groups after the successful functionalization of the
silica material. Comparing untreated silica with Ag–NHC@SiO2,
important features of the Ag catalyst were observed in the
spectrum. The peaks at 3167 (sp2 C–H stretching vibration of
the imidazole moiety), 2887 (N–CH2 stretching vibration) and
1572 cmÀ1 (C–N and CQC vibrations of the imidazole ring)
clearly indicate the distinct attachment of all the materials
and the successful attachment of the metal complex with the
support.
N2 adsorption–desorption is a common method to char-
acterize porous materials, which can be helpful in providing
insights into specific surface area, average pore diameter, pore
volume, etc. The N2 adsorption–desorption isotherms of various
Fig. 1 FTIR spectra of various silica samples.
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Table 1 Surface area, pore size and volume distribution of the silica-
based samples
Surface areaa
Pore volumeb Pore diameterc
Entry Material
(ABET) (m2 gÀ1
)
(cm3
g
À1) (Vp) (nm)
1
2
3
4
SiO2
509.02
0.558
0.368
0.248
0.187
4.39
3.98
4.05
6.31
CPTMS@SiO2 369.98
NHC@SiO2 244.73
Ag–NHC@SiO2 184.19
a
b
BET method used in N2 sorption. Single-point pore volume at P/P0 =
c
Fig. 5 SEM images of (a) silica and (b) Ag–NHC@SiO2.
0.994. Adsorption average pore diameter (by BET method).
A further decrease in ABET for the metal-immobilized silica
sample to 184.19 m2 gÀ1 is a clear indication of the successful
anchoring of the Ag metal onto the functionalized support.
Similarly, a gradual decrease in pore volume was observed from
silica (0.558 cm3 gÀ1) to Ag–NHC@SiO2 (0.187 cm3 gÀ1), further
validating the formation of a silica-supported Ag–NHC complex.
Fig. 3 shows the X-ray diffraction pattern of Ag–NHC@SiO2.
It shows reflections at the 2y values of 26.84, 31.04, 44.42,
55.15, 64.57 and 73.32, corresponding to the (110), (200),
(220), (222), (400) and (420) planes of Ag; the reflection at 2y
of B22.4 was attributed to the silica component, indicating its
amorphous nature.
The comparison of the EDX analyses (Fig. 4a and b) of silica
with Ag–NHC@SiO2 confirmed the presence of silver in the
catalyst along with the other elements, indicating the formation
of the desired metal complexes with the anchored ligand. The
SEM images (Fig. 5a and b) of silica and the immobilized silver
Fig. 3 Powder XRD image of the catalyst.
silica samples measured at 77.15 K are shown in Fig. 2. Accord- catalyst clearly indicate the changes in the morphologies of the
ing to the IUPAC classification, all the samples are type IV with a catalyst after introduction of the metal.
typical hysteresis loop, featuring mesoporous materials with
Considering the results of ICP-AES and SEM-EDX, we could
highly uniform pore size distribution.25 The BET surface area, determine the attachment of the metal on the surface of the
pore size and pore volume for different materials are presented solid supports. The amount of the metal in the Ag–NHC@SiO2
in Table 1. The BET surface area for unfunctionalized SiO2 was catalyst was determined by ICP-AES analysis and the loading
found to be 509.02 m2 À1, while after functionalization with of the metal was found to be 0.002 mmol gÀ1 of the catalyst
g
CPTMS and 1-methyl imidazole to generate silica-anchored (0.022 wt%). X-ray photoelectron spectroscopy (XPS) confirmed
NHC, a subsequent decrease in the ABET value to 369.98 and the presence of Ag(I) species along with other elements
244.73 m2 gÀ1 was observed, respectively.
(Fig. 6). The binding energy of the O 1s component was found
Fig. 4 SEM-EDX patterns of (a) silica and (b) Ag–NHC@SiO2.
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Fig. 6 XPS analysis of Ag–NHC@SiO2 for (A) O 1s, (B) N 1s, (C) Br 3d, (D) Ag 3d.
to be 532.0 eV, which was obtained due to the presence of silica-
bound silanol groups. The presence of the imidazole moiety
even after incorporating Ag was validated from the N 1s curve
(Fig. 6B), which had a peak at 401.3 eV. Similarly, the presence
of Br was also confirmed by the Br 3d curve (Fig. 6C), exhibiting
a peak due to the 3d5/2 component at 67.9 eV. Furthermore, the
Ag 3d curve (Fig. 6D) could be fitted with peaks at the binding
energies of 367.14 eV and 373.12 eV, corresponding to the 3d5/2
and 3d3/2 components for the Ag+ oxidation state,26 which was
consistent with the formation of a silica-bound Ag–NHC complex.
The transmission electron microscopy images (Fig. 7) show
a well dispersed distribution of spherical Ag–NHC@SiO2 nano-
particles with a diameter of about 45.7 nm. The SAED diffraction
pattern (Fig. 7d) indicates the amorphous nature of our catalyst.
To examine the thermal stability of the catalyst, thermogravi-
metric analysis (TGA) was performed from 35 to 880 1C at a ramp
rate of 20 1C minÀ1 with a PerkinElmer STA 8000 instrument under
nitrogen atmosphere (N2 gas flow rate: 20 mL minÀ1) with
alumina as the reference. The TGA curve (Fig. 8) of the catalyst
demonstrates the superior thermal stability of the catalyst up to
750 1C, indicating that it can be used over a wide temperature
Fig. 7 (a–c) TEM images of Ag–NHC@SiO2 at different magnifications;
range for any chemical reaction.
(d) SAED pattern of the catalyst.
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study their effect in suppressing the formation of the 1,5-regiomer
(Table 2, entries 2–5). In this context, quinine was found to
produce the best result. We also changed the solvent from water
to toluene as in the majority of the AgAAC reported, it is the
solvent of choice. However, in our case, toluene turned out to be
ineffective; DMF, THF, DMSO, EG, and binary solvent mixtures
like water : DMSO (1 : 1), water : t-butanol (1 : 1), and water : CAN
(1 : 1) failed to produce better results as well (Table 2, entries 6–13).
It was also speculated that lowering the temperature as well as
the catalyst loading largely affects the catalytic performance of
the reaction, generating a lower product yield (Table 2, entries
14 and 15). The necessity of a catalyst was confirmed by a
control experiment, which showed that in the absence of
Ag–NHC@SiO2, only a fraction of azide cyclized to triazole as
a mere outcome of thermal dipolar cycloaddition with a clear
lack of regioselectivity (Table 2, entry 16). Furthermore, it was
Fig. 8 TGA–DTG plot for the catalyst.
To investigate the catalytic performance of the Ag–NHC@SiO2 confirmed that the Ag–NHC complex present in the supported
catalyst towards azide–alkyne cycloaddition, benzyl azide and catalyst was the active catalyst and the functionalized support
phenyl acetylene were chosen as the test substrates. The without the metal played no role in initiating the reaction as no
optimization studies are summarized in Table 2. Considering improvement was observed relative to the reaction performed
abundance and economic point of view, water was chosen as without the catalyst when NHC@SiO2 alone was used to promote
the medium to perform the click reaction with 15 mg catalyst at cycloaddition (Table 2, entry 17). It is worth noting that the
60 1C and the reaction produced about 94% of the desired reaction also proceeded at room temperature although it required
product in 6 hours. It was seen that a small amount of 1,5-regiomer much longer time to achieve a reasonable yield.
was also produced along with the 1,4-disubstituted triazole, which
To examine the general applicability and feasibility of trans-
encouraged us to further examine the feasibility of this protocol. formation, various azides and alkynes were subjected to azide–
The use of an additive is substantial to enhance the regioselectivity alkyne cycloaddition reaction under similar conditions. The results
of this reaction; hence, we introduced different organic bases to are summarized in Table 3. Benzyl azide was found to be the
Table 2 Screening of solvent and additive for cycloaddition of benzyl azide (1a) to phenylacetylenea
Entry
Solvent
Additive
Time (h)
Isolated yield 3a(3b)
1
2
3
4
5
6
7
8
H2O
H2O
H2O
H2O
H2O
Toluene
DMF
THF
DMSO
EG
H2O/DMSO (1 : 1)
H2O/t-BuOH (1 : 1)
—
DIPEA
Quinine
L-Proline
Xphos
Quinine
Quinine
Quinine
Quinine
Quinine
Quinine
Quinine
Quinine
Quinine
Quinine
—
6
6
6
6
6
6
6
6
6
6
6
6
6
6
6
6
6
79(15)
90(5)
98(0)
85(0)
60(35)
15(0)
32(0)
22(0)
10(0)
31(0)
56(0)
60(0)
27(0)
91b(0)
89c(0)
12d(10)
10(10)e
52 f(0)
9
10
11
12
13
14
15
16
17
18
H2O/ACN (1 : 1)
H2O
H2O
H2O
H2O
H2O
Quinine
Quinine
12
a
Reaction conditions: benzyl azide (0.5 mmol), phenyl acetylene (1.2 eq.), AgNHC 15 mg (0.006 mol%) with respect to benzyl azide, and additive
b
c
d
(1 eq.) are stirred in 1 mL solvent at 60 1C. Reaction mixture is stirred at 50 1C. 12 mg of AgNHC was used. Reaction carried out in the absence
of catalyst and additive. Reaction is carried out with NHC@SiO2 instead of Ag–NHC@SiO2. Reaction is carried out at room temperature.
Percentage of 1,5-regiomer is determined by GCMS.
e
f
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Table 3 Scope of the catalytic systema
Entry
Azide (R1)
Alkyne
Products
Time (h)
Yieldb (%)
TONc
TOFd (hÀ1
)
1
2
3
4
5
6
7
8
Benzyl (1a)
Phenyl (1b)
4-Chlorophenyl
4-Fluorophenyl
4-(Difluoromethoxy)phenyl
3-Chlorophenyl
2-Methylphenyl
4-Nitrophenyl
4-Cyanophenyl
Octyl
1a
1a
1a
1a
1a
1b
1b
1b
1b
1b
1b
1b
Phenyl acetylene
Phenyl acetylene
Phenyl acetylene
Phenyl acetylene
Phenyl acetylene
Phenyl acetylene
Phenyl acetylene
Phenyl acetylene
Phenyl acetylene
Phenyl acetylene
2-Ethynyl-6-methoxynaphthalene
1-Iodo-4-(prop-2-yn-1-yloxy)benzene
1-Ethynyl-4-methoxy-2-methylbenzene
Propargyl alcohol
Ethyl propiolate
Propargyl benzoate
Propiolic acid
Dimethylacetylene dicarboxylate
Diethylacetylene dicarboxylate
1-Hexyne
3a
3b
3c
3d
3e
3f
3g
3h
3i
5
6
6
6
6
6
6
6
6
6
6
5
6
6
6
6
6
6
6
5
5
6
98
89
85
91
85
92
90
82
78
91
87
98
75
91
93
91
61
94
92
90
95
87
8166
7416
7083
7583
7083
7666
7500
6833
6500
7583
7250
8166
6250
7583
7750
7583
5083
7833
7666
7500
7916
7250
1633
1236
1180
1263
1180
1277
1250
1138
1083
1263
1208
1633
1041
1264
1291
1264
847
9
10
11
12
13
14
15
16
17
18
19
20
21
22
3j
3k
3l
3m
3n
3o
3p
3q
3r
3s
3t
3u
3v
1305
1277
1500
1583
1208
1-Octyne
3-Ethynylthiophene
a
Reaction condition: azide (0.5 mmol), alkyne (1.2 eq.), quinine (1 eq.), AgNHC 15 mg (0.006 mol%) with respect to azides are stirred in 1 mL
b
c
d
solvent at 60 1C. Isolated yields. mmol of product per mmol of catalyst. TON per unit time.
easiest to cyclize, while aromatic azides also reacted efficiently. It we examined the cycloaddition of benzyl azide and phenyl
was seen that all the aromatic azides with electron-donating acetylene as a test reaction and the previously described optimized
substituents reacted efficiently, affording the desired triazoles in conditions were maintained throughout each cycle of reuse. For
good to excellent yields (Table 3, entries 2–7). The position of the this purpose, after the completion of the reaction, the catalyst was
substituents do not at all affect reactivity. However, aromatic first separated by simple filtration, washed alternatively with ethyl
azides with strong electron-withdrawing groups (Table 3, entries acetate and water for two cycles, dried and reused for subsequent
8 and 9) were found to react sluggishly. Interestingly, octyl azide runs. It was observed that the catalyst was reusable up to the
produced the corresponding triazole in a high yield (Table 3, entry 5th cycle with only a small decline in the catalytic performance
10). Similarly, the scope of alkyne substrates was also explored. It (1st cycle 98% to 5th cycle 86%; Fig. 9). The slight decrease in yield
was observed that all alkynes irrespective of whether they were may be due to a gradual physical loss during separation, filtration
aromatic or aliphatic or contained diverse functionalities, viz., acids, and washings between each run. The ICP-AES analysis of the
alcohols, esters, and ethers underwent a swift transformation to the
desired products (Table 3, entries 11–21). Heterocyclic alkynes
(Table 3, entry 22) also showed tolerance towards this protocol,
reflecting the wide applicability of our catalytic system.
The excellent catalytic performance of the low-loading Ag
catalyst may be attributed to the presence of the NHC ligand,
which may have enhanced the stability of some intermediates
like Ag-acetylide as well as the stability of the metal centre during
the course of the reaction.11b In this context, the silica support
also played an important role. Keeping in mind that the NHC
complexes are not very stable, the support material indeed
played a dominant role in terms of inducing the stability and
compatibility of the NHC catalyst in water, which is rare.
Reusability of the catalyst
Reusability is a key factor in determining the potential of a hetero-
Fig. 9 Reusability of the catalyst.
geneous catalyst. To study the reusability of our catalytic system,
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S. Lee, J. Lee, B. Choi, P. M. S. D. Cal, S. Kang, J.-M. Kee,
G. J. L. Bernardes, J.-U. Rohde, W. Choe and S. Y. Hong, J. Am.
Chem. Soc., 2017, 139, 12121–12124.
reused catalyst revealed an extremely small amount of decrease in
the metal loading. Furthermore, XRD analysis (ESI†) also showed
almost no change in the diffraction pattern and the intensity of the
peaks. These facts validate the strong anchoring of the NHC
complex onto the silica surface.
ˇ
´
7 J. Barluenga, C. Valdes, G. Beltran, M. Escribano and
F. Aznar, Angew. Chem., Int. Ed., 2006, 45, 6893–6896.
8 (a) J. A. Marshall and C. A. Sehon, J. Org. Chem., 1995, 60,
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M. O’Halloran and W.-F. Tan, Tetrahedron Lett., 2007, 48,
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Conclusion
In summary, an efficient, highly effective, low-loading silica-
supported Ag–NHC catalyst was developed for the synthesis of
triazoles through AgAAC. The method offered significant
advantages over the existing methods employing silver catalysts
in terms of broad substrate scope, easy handling, and most
importantly ultra-low loading of silver metal, which could be
reused for up to 5 cycles without losing its catalytic efficiency
appreciably. Furthermore, water as a reaction medium and
quinine as an additive contributed to the cleaner chemistry
and greener credential of this protocol.
9 (a) J. A. Marshall and K. W. Hinkle, J. Org. Chem., 1997, 62,
5989–5995; (b) D. G. Dunford and D. W. Knight, Tetrahedron
Lett., 2016, 57, 2746–2748.
10 A. K. Clarke, M. J. James, P. O’Brien, R. J. K. Taylor and
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J. Org. Chem., 2012, 5462–5470.
12 E. Boz and N. S. Tuzun, Dalton Trans., 2016, 45, 5752–5764.
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13 (a) A. I. Ortega-Arizmendi, E. Aldeco-Perez and E. Cuevas-Yanez,
Sci. World, 2013, 1–8; (b) A. A. Ali, M. Chetia, B. Saikia, P. J. Saikia
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14 N. Salam, A. Sinha, A. Singha Roy, P. Mondal, N. R. Jana and
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Conflicts of interest
There are no conflicts of interest to declare.
15 P. Basua, P. Bhanjab, N. Salamc, T. K. Deya, A. Bhaumikb,
D. Dasc and S. M. Islama, Mol. Catal., 2017, 439, 31–40.
16 S. Das, P. Mondal, S. Ghosh, B. Satpati, S. Deka, S. M. Islam
and T. Bala, New J. Chem., 2018, 42, 7314–7325.
17 J. Sultana, N. D. Khupse, S. Chakrabarti, P. Chattopadhyay
and D. Sarma, Tetrahedron Lett., 2019, 60, 1117–1121.
18 A. J. Arduengo, R. L. Harlow and M. Kline, J. Am. Chem. Soc.,
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Acknowledgements
DS is thankful to DST, New Delhi, India for a research grant
[No. EMR/2016/002345]. AG thanks DST, New Delhi for Research
Fellowship. The financial assistance of DST-FIST and UGC-SAP
programme to the Department of Chemistry, Dibrugarh University
is also gratefully acknowledged.
19 N-Heterocyclic carbenes in transition metal catalysis. Topics in
organometallic chemistry, ed. F. Glorius, Springer, Berlin,
2007, p. 21.
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