Zinc(II) complex immobilized on amine functionalized silica gel: A novel, highly efficient and recyclable catalyst for multicomponent click synthesis of 1,4-disubstituted 1,2,3-triazoles

Article abstract of DOI:10.1080/00958972.2016.1165807

We report a highly efficient and recyclable heterogeneous zinc catalytic system via covalent immobilization of 2-hydroxyacetophenone (2-HAP) onto an amine functionalized silica gel followed by metallation with zinc chloride and its catalytic application in three component click synthesis of 1,4-disubstituted 1,2,3-triazoles. The structure of the synthesized organic-inorganic hybrid material (SiO₂@APTES@2HAP-Zn) has been confirmed by various physicochemical characterization techniques, such as solid-state ¹³C CPMAS NMR spectroscopy, Fourier transform infrared spectroscopy (FT-IR), X-ray diffraction analysis (XRD), Brunauer-Emmett-Teller (BET) surface area analysis, scanning electron microscopy (SEM), atomic absorption spectroscopy (AAS), energy-dispersive X-ray fluorescence spectroscopy (ED-XRF), and elemental analysis. The newly designed catalyst works under mild reaction conditions and also exhibits excellent performance in terms of good product yield and high turnover number (TON). One of the most important attributes of the present methodology is that the catalyst can be recycled several times without appreciable loss in its activity as proved by FTIR spectroscopy and SEM analysis. Besides, the heterogeneity test also confirms that no leaching of active catalytic species occurs from the silica supported zinc catalyst which confirms its remarkable structural stability under the reaction conditions. (Figure Presented).

Full text of DOI:10.1080/00958972.2016.1165807

Journal of Coordination Chemistry
ISSN: 0095-8972 (Print) 1029-0389 (Online) Journal homepage: http://www.tandfonline.com/loi/gcoo20
Zinc(II) complex immobilized on amine
functionalized silica gel: A novel, highly efficient
and recyclable catalyst for multicomponent click
synthesis of 1,4-disubstituted 1,2,3-triazoles
R.K. Sharma, Mallika Mishra, Shivani Sharma & Sriparna Dutta
To cite this article: R.K. Sharma, Mallika Mishra, Shivani Sharma & Sriparna Dutta (2016):
Zinc(II) complex immobilized on amine functionalized silica gel: A novel, highly efficient and
recyclable catalyst for multicomponent click synthesis of 1,4-disubstituted 1,2,3-triazoles,
Journal of Coordination Chemistry, DOI: 10.1080/00958972.2016.1165807
To link to this article: http://dx.doi.org/10.1080/00958972.2016.1165807
Accepted author version posted online: 16
Mar 2016.
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Date: 21 March 2016, At: 11:43

Publisher: Taylor & Francis
Journal: Journal of Coordination Chemistry
DOI: http://dx.doi.org/10.1080/00958972.2016.1165807
Zinc(II) complex immobilized on amine functionalized silica gel: A novel, highly
efficient and recyclable catalyst for multicomponent click synthesis of 1,4-
disubstituted 1,2,3-triazoles
R.K. SHARMA*, MALLIKA MISHRA, SHIVANI SHARMA and SRIPARNA DUTTA
Green Chemistry Network Centre, Department of Chemistry, University of Delhi, Delhi-110007, India
We report a highly efficient and recyclable heterogeneous zinc catalytic system via covalent
immobilization of 2-hydroxyacetophenone (2-HAP) onto an amine functionalized silica gel
followed by metallation with zinc chloride and its catalytic application in three component click
synthesis of 1,4-disubstituted 1,2,3-triazoles. The structure of the synthesized organic-inorganic
hybrid material (SiO2@APTES@2HAP-Zn) has been confirmed by various physico-chemical
characterization techniques such as solid-state 13C CPMAS NMR spectroscopy, Fourier transform
infrared spectroscopy (FT-IR), X-ray diffraction analysis (XRD), Brunauer-Emmett-Teller (BET)
surface area analysis, scanning electron microscopy (SEM), atomic absorption spectroscopy (AAS),
energy dispersive X-ray fluorescence spectroscopy (ED-XRF), and elemental analysis. The newly
designed catalyst works under mild reaction conditions and also exhibits excellent performance in
terms of good product yield and high turnover number (TON). One of the most important attributes
of the present methodology is that the catalyst can be recycled several times without appreciable
loss in its activity as proved by FTIR spectroscopy and SEM analysis. Besides, the heterogeneity
test also confirms that no leaching of active catalytic species occurs from the silica supported zinc
catalyst which confirms its remarkable structural stability under the reaction conditions.
Keywords: Silica gel; Recyclable catalyst; Click reaction; Triazoles synthesis; Zinc
*Corresponding author. Email: rksharmagreenchem@hotmail.com
1

1. Introduction
1,2,3-Triazole derivatives represent an important class of heterocyclic compounds that have large
scale industrial applications as agrochemicals, photostabilizers, corrosion inhibitors, dyes and
optical brightners [1-3]. These motifs also produce large numbers of peptides, oligosaccharides and
natural product analogues having exceptional biological properties including anti-HIV, antitumor,
anticonvulsant and antimicrobial activities [4, 5]. A few substituted triazoles have been effectively
used as deoxyribose nucleic acid cleaving agents and potassium channel activators [6, 7].
Consequently, several methodologies have been developed for the synthesis of 1,2,3-triazole
derivatives. However, the well-established methodology is the Huisgen 1,3-dipolar cycloaddition of
azides and alkynes which was pioneered by Huisgen in 1967 [8]. Although the Huisgen approach
shows high chemoselectivity, it suffers from several limitations such as formation of a mixture of
regioisomers, requirement of high temperature and prolonged reaction time, which restricts the
scope of its synthetic utility. To circumvent these drawbacks, Sharpless and Meldal introduced the
concept of click chemistry wherein a copper based catalyst was used to accelerate the yield of 1,4-
disubstituted 1,2,3-triazoles [9, 10]. Later, several other research groups also utilized the click
concept for the synthesis of these industrially significant triazole derivatives with slight
modifications in the employed reaction conditions [11-14]. Unfortunately, most of these protocols
are catalyzed by homogeneous metal complexes which hinder their practical applications and
feasibility on a large scale due to metal contamination and recyclability issues. Thus, in order to
overcome these difficulties, the fabrication of recoverable and reusable catalysts which combine the
best attributes of homogeneous as well heterogeneous catalytic systems is desirable.
Organic-inorganic hybrid catalysts based on solid support materials such as silica, zeolite,
alumina, titania, ceria, etc. have gained importance because of the increasing worldwide momentum
towards the development of environmentally friendly reactions [15-23]. An extensive literature
survey reveals that silica gel is an efficient solid support as it is chemically inert, readily available,
inexpensive, non-toxic and thermally stable. A large number of organic or organometallic moieties
can be robustly anchored on its surface due to the presence of highly reactive silanol groups
[24-27]. In this perspective, the grafting of active catalytic species onto silica gel by means of
covalent linkage is considered to be the best technique because it minimizes leaching as a
consequence of which the catalyst becomes sufficiently robust to survive the harsh reaction
conditions.
2

Thus, in continuation of our ongoing research on the design and synthesis of silica based
organic-inorganic hybrid materials and their applications as metal scavengers, sensors, and catalysts
for various organic transformations [28-41], herein, we report the fabrication of an efficient and
highly recyclable silica supported organic-inorganic hybrid zinc catalyst (SiO2@APTES@2HAP-
Zn) via the covalent immobilization of 2-hydroxyacetophenone onto amine functionalized silica
followed by metallation with zinc chloride. The resultant heterogeneous zinc catalyst has been
applied for synthesis of industrially significant 1,4-disubstituted 1,2,3-triazole derivatives using
click chemistry.The results reveal that this catalyst not only exhibits excellent selectivity and
activity, but can also be easily recovered from the reaction mixture and reused several times without
any appreciable loss in catalytic activity. In addition, excellent product yield, high turnover number,
mild reaction conditions, simple work-up procedure and intrinsic stability of the catalyst are
features of this protocol that make it indispensable to address industrial prerequisites.
2. Experimental
2.1. Materials and reagents
Silica gel (Qualigens), 3-aminopropyltriethoxysilane (APTES) (Fluka), 2-hydroxyacetophenone
(Sisco Pvt. Ltd, India) and zinc chloride (Thomas Baker) were commercially obtained and used as
received. All other starting materials and reagents used in the reactions were procured from
Spectrochem and Alfa Aesar.
2.2. Physicochemical characterizations
Powder X-ray diffraction (XRD) patterns of the samples were recorded on a Bruker D8 ADVANCE
X-ray diffractometer using graphite monochromated Cu Kα radiation (k = 1.54183 Å) over 2 h
range 10-60° at a scan rate of 2 h min-1. Scanning electron microscopy (SEM) images were obtained
using a ZEISS EVO 40 instrument. The IR spectra were obtained on a Perkin Elmer Spectrum 2000
FT-IR spectrometer. The BET surface area analysis was carried out using a Gemini-V2.00
instrument (Micromeritics Instrument Corp.) and solid samples were outgassed under vacuum in
order to eliminate physisorbed moisture. Elemental analysis (CHN) was performed using an
Elementar Analysensysteme GmbH VarioEL V3.00. Solid-state 13C (CPMAS) NMR spectra were
recorded on a Bruker DSX-300 NMR spectrometer. Energy dispersive X-ray fluorescence
spectroscopy was done on a Fischer scope X-Ray XAN-FAD BC. The amount of zinc content was
3

detected using LABINDIA AA 7000 Atomic Absorption Spectrometer. The triazole products
obtained were analyzed and confirmed on Agilent gas chromatography (6850 GC).
2.3. Catalyst preparation
Surface functionalization of silica gel was achieved by suspending 5 g of activated silica gel in
20 mL of 3-aminopropyltriethoxy silane (APTES) using dry toluene as the solvent. The mixture was
then refluxed for 10 h in an inert atmosphere with constant stirring [42]. The resultant aminopropyl
silica gel (SiO2@APTES) was filtered, washed thoroughly with toluene, ethanol and acetone,
respectively, and dried in a vacuum oven. Thereafter, the silica supported Schiff base ligand
(SiO2@APTES@2-HAP) was synthesized by dispersing SiO2@APTES and 2-hydroxy
acetophenone (2-HAP) in ethanol followed with continuous stirring for 12 h [43]. Finally, the
ligand grafted amine functionalized silica support was metallated by adding zinc chloride
(scheme 1) to a solution of acetone containing 2 g of SiO2@APTES@2-HAP to afford the final
catalyst (SiO2@APTES@2-HAP-Zn).
4

Scheme 1. Synthesis of silica supported organic-inorganic hybrid zinc catalyst
(SiO2@APTES@2HAP-Zn).
2.4. Typical procedure for the synthesis of 1,4-disubstituted 1,2,3-triazoles catalyzed using the
silica based organic-inorganic hybrid zinc catalyst
A solution containing alkyne (1.0 mmol), organic halide (1 mmol), sodium azide (1 mmol), 1:1
mixture of water/tBuOH (2 mL) and SiO2@APTES@2HAP-Zn (20 mg) catalyst was stirred at
55 °C. After completion of the reaction as monitored through Thin-Layer Chromatography (TLC),
the catalyst was removed by filtration. The reaction mixture was then extracted with ethyl acetate
and dried over anhydrous Na2SO4. Finally, the GC-MS analysis of the products was done to confirm
the structures of the resultant 1,4-disubstituted 1,2,3-triazole derivatives.
3. Results and discussion
3.1. Catalyst characterization
3.1.1. Fourier-transform infrared spectroscopy. Figure 1 shows the FT-IR spectra of SiO2,
SiO2@APTES, SiO2@APTES@2HAP and SiO2@APTES@2HAP-Zn. The FT-IR spectrum of
5

SiO2 exhibits strong bands in the low-frequency region (1000-500 cm-1) ascribed to the silica
framework such as the Si–O–Si asymmetric stretching band at 1095 cm-1 and Si–O symmetric
stretching band at 805 cm-1. Additionally, a band at 1649 cm-1 is observed in the spectrum due to
H-O-H bending vibrations of physically adsorbed water and a broad band centered around
3426 cm-1 due to O-H stretching vibrations of hydrogen-bonded surface silanol groups. The surface
functionalization of the silica support with the linking agent is confirmed by the appearance of two
new peaks at 2928 and 2852 cm−1 that are attributed to the CH2 groups of APTES [44-46]. Finally,
for SiO2@APTES@2HAP-Zn, a strong band at 1626 cm−1 due to C=N stretching vibration affirms
the Schiff condensation between the NH2 group and carbonyl group of ligand [47-49]. Further, it
should be noted that there is not much difference between the spectra of recovered and fresh
catalyst, suggesting that the catalyst is not poisoned during the reaction.
3.1.2. 13C CPMAS NMR spectroscopy. Cross-polarization magic angle spinning 13C nuclear
magnetic resonance study is an invaluable tool as it often provides information regarding chemical
structure of the organic functional groups present in the hybrid frameworks. Hence, the surface
modification of the SiO2 surface with silane coupling agent has been confirmed using solid state
13C CPMAS NMR spectroscopy. In figure 2, the 13C CPMAS NMR spectrum of SiO2@APTES is
shown. The spectrum exhibits the expected signals at δ = 9.3, 22.3 and 42.9 ppm which can be
assigned the three methylene groups (Si–CH2–, –CH2– and –N–CH2–) of the propyl chain of
APTES [50]. This validates that the APTES linking agent has been covalently grafted onto the
surface of SiO2.
3.1.3. SEM analysis. Scanning electron microscopy has been utilized to determine the morphology
of the synthesized silica based organic-inorganic hybrid materials and the corresponding SEM
images are presented in figure 3. The appearance and size of silica gel and the catalyst are very
similar. Even the beads of silica gel do not possess any cracks after the surface functionalization
during the catalyst preparation. This provides a clear indication that the SiO2 particles have not been
destroyed during the modification reaction, owing to their excellent mechanical stability.
3.1.4. Surface area and elemental analysis. The specific surface areas of the synthesized silica
based samples have been calculated using the Brunauer-Emmett-Teller (BET) method and the
results are presented in table 1. As expected, the surface area of SiO2 gets reduced from 232.45 to
6

155.11 m2g-1 after functionalization of the APTES and it was further reduced to 81.23 m2g-1 upon
anchoring the 2-hydroxyacetophenone zinc complex. The reduction in the surface area is due to
immobilization of organic moieties onto the porous silica matrix which eventually blocks access of
nitrogen gas molecules. This observation directly confirms the successive anchoring of APTES and
zinc complex onto the surface of SiO2. Table 1 also lists the chemical compositions of all the
synthesized silica based materials. Increased content of both carbon and nitrogen were detected on
moving from SiO2 to SiO2@APTES. Further, the elemental analysis data of SiO2@APTES shows
that carbon and nitrogen contents are 5.17% and 1.81%, respectively. Based on these results the
C/N ratio has been calculated to be ~2.86 which is close to the theoretical value, thereby confirming
incorporation of APTES linking agent into the silica matrix. In addition, the
SiO2@APTES@2HAP-Zn was subjected to AAS analysis after being digested in aqua regia which
gives 0.116 mmol of Zn per gram of catalyst. Apart from this, the well-resolved peak of zinc in the
ED-XRF spectrum of SiO2@APTES@2HAP-Zn (figure 4) also provides clear evidence of the
immobilization of zinc complex on the modified silica gel.
3.2. Catalytic performance
Catalytic performance of silica supported organic-inorganic hybrid zinc complex was evaluated in
the one pot synthesis of 1,4-disubstituted triazole derivatives using phenyl acetylene and benzyl
bromide as test substrates. To obtain an optimum reaction profile for SiO2@APTES@2HAP-Zn
mediated triazole synthesis, the effect of different reaction parameters such as amount of catalyst,
reaction time and temperature, substrate molar ratio and type of solvent have been studied.
3.2.1. Effect of amount of catalyst. In our initial investigations, the efficiency of the click reaction
is affected by the amount of SiO2@APTES@2HAP-Zn and the results are presented in figure 5.
Increase in amount of catalyst to 20 mg resulted in enhancement of the conversion percentage of the
reactants. A further increase in the catalytic amount beyond this value did not cause considerable
increase in the conversion. The optimal amount of SiO2@APTES@2HAP-Zn was fixed to 20 mg
for the rest of the experiments.
3.2.2. Comparison of various catalysts. In order to examine the effect of catalyst on the click
synthesis, the model reaction was carried out in the presence of different catalysts. As evident from
table 2, the use of simple silica gel failed to render the desired product, thereby showing poor
7

catalytic activity. Use of the homogeneous 2HAP-Zn complex gave higher conversion in
comparison to heterogenized SiO2@APTES@2HAP-Zn catalyst. However, the homogeneous metal
catalyst decomposed immediately after the reaction and could not be reused for further catalytic
cycles. In particular, SiO2@APTES@2HAP-Zn system has been fabricated via immobilization onto
silica gel to acquire the advantages of heterogeneous catalytic system so that it could be easily
recovered and reused for several catalytic cycles. In addition, the heterogenized catalyst is devoid of
separation problems typically associated with homogeneous catalytic systems.
3.2.3. Effect of time and temperature. In order to examine the effect of time and temperature, the
model reaction was performed under a diverse range of temperatures (25-65 °C) for different time
periods (0.5-2.5 h) and the results are summarized in figure 6. The reaction time had a remarkable
influence on the one pot synthesis of triazoles. Detailed examination of the results revealed that a
time period of 2 h was appropriate for the maximum conversion of the reactants when the reaction
was carried out at 55 °C.
3.2.4. Effect of various solvents. Subsequently, the activity of the silica supported heterogeneous
catalyst was evaluated in the click reaction using different solvents in the presence of the model
substrates. The effect of solvents on the synthesis of triazole derivatives is illustrated in figure 7. It
can be seen that the conversion percentage obtained in these solvents increases in the order
acetonitrile < THF < DCM < toluene <ethanol < water < water/tBuOH. Thus, the best catalytic
performance was observed when a 1:1 mixture of water/tBuOH was employed as solvent.
3.2.5. Effect of substrate molar ratio. Having established 1:1 mixture of water/tBuOH as the best
solvent for the click reaction, we then investigated the influence of substrate ratio and the results are
summarized in figure 8. Different molar ratios of phenyl acetylene, benzyl bromide and sodium
azide (1:0.5:0.5, 1:1:1 and 1:2:2) were considered while keeping the other parameters fixed. Upon
utilizing 1:0.5:0.5 substrate ratio, a moderate conversion was observed. When equal molar ratio of
all three substrates was applied, the reaction time was shortened and maximum conversion was
observed. Hence, 1:1:1 molar ratio was the minimum requirement for the effective one pot synthesis
of triazoles. It was also noticed that further increase in the molar ratio did not have much effect on
the conversion percentage.
8

3.2.6. Scope and versatility of the protocol. To further explore the potential of this catalyst for
1,4-disubstituted 1,2,3-triazole synthesis, a series of terminal alkynes and organic halides were
subjected to the click reaction under the optimized reaction conditions using
SiO2@APTES@2HAP-Zn (table 3). Analysis of the results showed that most of the substrates
produced the expected triazole products with good to excellent conversion, indicating remarkable
catalytic efficacy of the silica based zinc catalyst. For example, phenyl acetylene was almost
quantitatively converted to the desired product. The most important attributes of the present
methodology are that the click reaction is devoid of unwanted products and the catalyst can be
recycled for six consecutive cycles without appreciable loss in its performance.
3.2.7. Mechanism. A probable reaction mechanism for the synthesis of 1,4-disubstituted 1,2,3-
triazoles catalyzed by the silica based organic-inorganic hybrid zinc catalyst has been proposed and
illustrated in scheme 2. Initially, zinc coordinated acetylide complex is generated in step I via the
interaction of zinc catalyst and terminal alkyne species. The complex thus formed reacts with azide
to produce complex B which gets transformed into triazolide intermediate C. This triazolide
intermediate produces heterocyclic complex D which subsequently undergoes protonolysis,
delivering the desired product and regenerating the active catalyst [51, 52].
9

H
Zn(Cl)2
R1
H
R1
N
R2
SiO2
N
N
Product
SiO2
SiO2
R1
ZnCl2
A
R2-N3
ZnCl2
MECHANISM
R1
N
R2
N
N
SiO2
D
ZnCl2
R1
N
N
R2
SiO2
R1
ZnCl2
N
B
N
N
R2
N
C
Scheme 2. Proposed mechanism for synthesis of 1,4-disubstituted 1,2,3-triazoles.
3.2.8. Hot Filtration test. A hot-filtration test was performed using the model substrates in order to
test the heterogeneous nature of the newly fabricated catalyst. At about half the reaction time, the
solid catalyst was separated from the reaction mixture through filtration and the conversion
percentage obtained after GC-MS analysis of the resultant supernatant was found to be 67%.
Thereafter, the supernatant was subjected to further heating under the same reaction conditions. No
increment in the conversion of the reactants was observed. This supported the observation that the
10

active metal species did not leach from the solid catalyst. Further, the atomic absorption
spectrometric analysis of the filtrate also confirmed the absence of metal in the reaction mixture.
These studies clearly demonstrated that the zinc(II) complex was intact with the support, confirming
the true heterogeneous nature of the catalyst. Complexes of 2-hydroxyacetophenone with metals
such as Cu(II), Ni(II), Co(II) and Zn(II) exhibit remarkable stability as evident from the literature
[53]. In fact, the binding constant value of the 1:1 metal complex of 2-hydroxyphenone with Zn(II)
has been found to be 6.30 using Calvin Bjerrum titration technique as modified by Irving
and Rossotti.
Equation used: log K = pL + log [ nꢀ/(1 − nꢀ)]
The structure of 2HAP-Zn complex is as follows:
O
Cl
Zn
O
Cl
3.2.9. Catalyst recycling test. One of the main reasons for immobilizing homogeneous metal
complexes onto solid supports is to enhance the lifetime of the resultant catalyst, thereby rendering
it suitable for industrial applications. In order to investigate the reusability of the
SiO2@APTES@2HAP-Zn catalyst, the model reaction was performed under the optimized reaction
conditions. Upon completion of the reaction, the catalyst was separated by filtration after each run,
dried under vacuum and then subjected to the next reaction cycle under the same conditions. It is
evident from figure 9 that there was no significant decrease in conversion percentage even after six
runs, indicating excellent durability and reusability of the catalyst. SEM analysis of the reused
catalyst after the sixth run (figure 3(c)) showed no significant difference in the SEM images of the
fresh and the reused catalyst which provided evidence for the good structural stability of
SiO2@APTES@2HAP-Zn under the employed reaction conditions.
3.2.10. Comparison with the literature precedents. The efficiency of the silica based organic-
inorganic hybrid catalyst (SiO2@APTES@2HAP-Zn) was compared with the previously reported
catalytic system for the synthesis of triazole derivatives. From table 4, it is evident that the newly
fabricated zinc catalyst is far more proficient in terms of product yield and cost. Besides, the
SiO2@APTES@2HAP-Zn can be easily recovered through simple analytical techniques such as
11

filtration or centrifugation, and reused for six consecutive cycles without any significant loss in its
activity making it an interesting alternative for zinc catalyzed click promoted triazole synthesis.
4. Conclusion
We have developed a silica based organic-inorganic hybrid zinc catalyst (SiO2@APTES@2HAP-
Zn) with good activity and recyclability through the covalent immobilization of 2-
hydroxyacetophenone on the surface of the amine functionalized silica support followed by its
metallation with zinc chloride. The resultant heterogeneous catalytic system exhibits remarkable
efficiency in terms of product yield and high turnover number in the synthesis of 1,4-disubstituted
1,2,3-triazole derivatives. The hot filtration test ensures that the catalyst works in a heterogeneous
manner and is devoid of leaching problem. The facile recovery, coupled with the intrinsic stability
of the support material, allows the catalyst to be recycled six times without any discernible loss in
its activity. Simple work-up procedure, mild reaction condtions and cost effectiveness are additional
attributes of the present protocol that render it suitable for addressing various economic and
industrial concerns.
Acknowledgements
The authors thank IISc, Bangalore, India for carrying out solid state NMR analysis and USIC-CLF,
DU, Delhi, India for XRD data.
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14

Table 1. Physico-chemical parameters of SiO2, SiO2@APTES and SiO2@APTES@2HAP-Zn.
Elemental analysis
Material
BET surface area (m2g-1)
C(%)
H(%)
N(%)
SiO2
-
-
-
232.54
155.11
81.23
SiO2@APTES
5.17
1.26
2.59
1.81
3.51
SiO2@APTES@2HAP-Zn 14.94
15

Table 2. Model reaction performed using various catalysts.a
Entry
Catalyst
Conversion (%)b
TONc
1
2
3
SiO2
2HAP-Znd
-
-
97
95
418
409
SiO2@APTES@2HAP-Zn
a Reaction conditions: phenyl acetylene (1 mmol), benzyl bromide (1 mmol) and
sodium azide (1 mmol), 1:1 mixture of Water/tBuOH (2 mL), catalyst (20 mg)
stirring at 55 °C. b Conversion was determined by GC-MS. c TON is the number of
moles of product per mol of catalyst and TOF = TON per hour. d 0.0023 mmol.
16

Table 3. SiO2@APTES@2HAP-Zn catalyzed one pot synthesis of 1,4-disubstituted 1,2,3-triazoles.
Conversionb
(%)
Entry Halide
1.
Alkyne
Time (h)
2
TONc
409
95
2.
3.
4.
2
96
90
92
88
94
96
91
414
388
397
379
405
413
392
2.5
2
5.
6.
7.
8.
3
1.5
2
2.5
9.
2
3
85
90
366
388
10.
17

11.
12.
3
91
86
392
371
2.5
13.
14.
15.
3
88
80
82
379
345
353
3.5
3.5
a Reaction conditions: terminal acetylene (1 mmol), organic halides (1 mmol) and sodium azide
(1 mmol), catalyst (20 mg), water/tBuOH (2 mL), stirring at 55 °C. b Conversion (%) was determined
using GC-MS. c TON is the number of moles of product per mol of catalyst.
18

Table 4. Comparison of the catalytic activity of SiO2@APTES@2HAP-Zn catalyst with some
recently reported zinc catalysts for the synthesis of 1,4-disubstituted 1,2,3-triazoles.
Yield
(%)
Entry Organic halides
1.
Terminal alkynes
Catalytic conditions
Reference
[14]
zinc(II) L-prolinate
(2 mol %), water
reflux, 2 h
91
zinc(II) L-prolinate
(2 mol %), water
reflux, 2 h
91
[14]
2.
Zn/C (10 mol %),
81
76
[13]
[13]
3.
4.
DMF, 50 °C, 15 h
Zn/C (10 mol %),
DMF, 50 °C, 15 h
SiO2@APTES@2HAP-
Zn, water/tBuOH,
55 °C
93
92
Present
study
5.
6.
SiO2@APTES@2HAP-
Zn, water/tBuOH,
55 °C
Present
study
19

(d)
(c)
1626
(b)
(a)
2852
2928
796
974
3426
1088
4000
3500
3000
2500
2000
1500
1000
500
Wavenumber (cm-1)
Figure 1. FT-IR spectra of (a) SiO2, (b) SiO2@APTES, (c) SiO2@APTES@2HAP-Zn,
(d) recovered SiO2@APTES@2HAP-Zn.
20

Figure 2. 13C CPMAS NMR spectrum of SiO2@APTES.
21

Figure 3. SEM images of (a) silica gel, (b) SiO2@APTES@2HAP-Zn catalyst, (c) recovered
SiO2@APTES@2HAP-Zn catalyst.
22

Figure 4. ED-XRF spectrum of SiO2@APTES@2HAP-Zn catalyst.
23

100
90
80
70
60
50
40
30
20
10
5
10
15
20
25
Amount of catalyst (mg)
Figure 5. Effect of amount of catalyst on the model reaction (reaction conditions: phenyl acetylene
(1 mmol), benzyl bromide (1 mmol) and sodium azide (1 mmol), 1:1 mixture of water/tBuOH (2
mL), catalyst, stirring at 55 °C).
24

65
)
o
(
C
55
T
e
r
i
m
u
45
t
e
a
(
h
r
e
)
35
p
m
e
T
25
Figure 6. Effect of time and temperature on the model reaction (reaction conditions: phenyl
acetylene (1 mmol), benzyl bromide (1 mmol) and sodium azide (1 mmol), 1:1 mixture of
water/tBuOH (2 mL), catalyst (20 mg), stirring).
25

CH3CN
THF
DCM
Toluene
Ethanol
Water
Water/tBuOH
0
20
40
60
80
100
Conversion (%)
Figure 7. Effect of solvent on the model reaction (reaction conditions: phenyl acetylene (1 mmol),
benzyl bromide (1 mmol) and sodium azide (1 mmol), catalyst (20 mg), solvent (2 mL), stirring at
55 °C).
26

100
90
80
70
60
50
40
30
20
10
0
Molar ratio
1:0.5:0.5
1:1:1
1:2:2
0.5
1.0
1.5
2.0
2.5
Time (h)
Figure 8. Effect of substrate molar ratio on the model reaction (reaction conditions: phenyl
acetylene (1 mmol), benzyl bromide (1 mmol) and sodium azide (1 mmol), catalyst (20 mg),
water/tBuOH (2 mL), stirring at 55 °C).
27

100
80
60
40
20
0
1
2
3
4
5
6
7
Number of runs
Figure 9. Catalytic recyclability test for multicomponent click synthesis of 1,2,3-triazoles (reaction
conditions: phenyl acetylene (1 mmol), benzyl bromide (1 mmol) and sodium azide (1 mmol), 1:1
mixture of water/tBuOH (2 mL), catalyst (20 mg) stirring at 55 °C).
28

Graphical abstract
SiO2
N
R1
N
N
+ NaN
R2
R1X
+
3
Water/ tBuOH (1:1)
55oC
R2
O
Si
N
O
SiO
SiO22
O
SiO2
Cl
Zn
O
Cl
Silica based organic-inorganic hybrid zinc catalyst
Excellent product yield
Recyclable & reusable catalyst
Ease of catalyst separation
Simple workup procedure
High Turnover number
Mild reaction conditions
29