Cube-octameric silsesquioxane-mediated cargo copper Schiff base for efficient click reaction in aqueous media

Article abstract of DOI:10.1016/j.molcata.2015.12.022

The azide-alkyne cycloaddition reaction was investigated under catalytic conditions involving a copper(II) - polyhedral oligomeric silsesquioxane (POSS) - bridged Schiff base. This material demonstrated a high catalytic activity in organic synthesis of 1,4-triazoles. No additive such as a base or a reductant was required. Finally, recoverability and reusability of the POSS-bridged Schiff base-Cu(II) catalyst was analyzed.

Full text of DOI:10.1016/j.molcata.2015.12.022

Journal of Molecular Catalysis A: Chemical 414 (2016) 47–54
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Journal of Molecular Catalysis A: Chemical
journal homepage: www.elsevier.com/locate/molcata
Cube-octameric silsesquioxane-mediated cargo copper Schiff base for
efficient click reaction in aqueous media
Ali Akbaria,c,1, Nasser Arsalania,∗, Mojtaba Aminib,∗, Esmaiel Jabbaric,∗
a Polymer Research Laboratory, Department of Organic and Biochemistry, Faculty of Chemistry, University of Tabriz, Tabriz, Iran
b Department of Chemistry, Faculty of Science, University of Maragheh, P.O. Box. 55181-83111731, Maragheh, Iran
c Department of Chemical Engineering, University of South Carolina, Columbia, SC, United States
a r t i c l e i n f o
a b s t r a c t
Article history:
The azide–alkyne cycloaddition reaction was investigated under catalytic conditions involving a
copper(II)—polyhedral oligomeric silsesquioxane (POSS)—bridged Schiff base. This material demon-
strated a high catalytic activity in organic synthesis of 1,4-triazoles. No additive such as a base or a
reductant was required. Finally, recoverability and reusability of the POSS-bridged Schiff base–Cu(II)
catalyst was analyzed.
Received 16 October 2015
Received in revised form
17 December 2015
Accepted 27 December 2015
Available online 2 January 2016
© 2015 Elsevier B.V. All rights reserved.
Dedicated to the memory of Professor Ali
Akbar Entezami who passed away in the
summer of 2015.
Keywords:
Silsesquioxane
Copper
Azide–alkyne cycloaddition
1. Introduction
thermally stable network of POSS make these compounds a promis-
ing precursor for the synthesis of new hybrid materials [20–22].
The copper-catalyzed azide–alkyne cycloaddition (CuAAC) is
one of the most attractive synthetic tools for convenient and reli-
able construction of 1,2,3-triazole derivatives, whose subunits are
often found in biologically active molecules [1–7]. In recent years,
due to higher stability and dispersibility of nanoparticles as well
as the demand for higher activity and recyclability of catalysts,
heterogeneous nano-catalysis has emerged as a sustainable and
competitive alternative to conventional catalysis [6–14].
Silsesquioxanes, the three-dimensional oligomeric compounds
with the general formula (RSiO1.5)2n, where R can be any func-
tional group (alkyl, hydrogen, halogen, aryl, etc.), show unique
properties. A number of silicon-based compounds have recently
been developed as substrates for catalyst immobilization in a wide
range of biochemical reactions [15–17]. Among them, polyhe-
dral oligomeric silsesquioxane (POSS) cage molecules, (RSiO1.5)8,
have received a great deal of attention for the synthesis of
organic–inorganic hybrid materials with precise control of nanoar-
Recently, there has been a strong interest in the synthesis of
functional polysilsesquioxanenano-building blocks as a substrate
for immobilization of organo-metallic catalysts, thereby build-
ing a bridge between homogeneous and heterogeneous catalysts
[23–27]. The formation of stable bonds between metal ions of the
catalyst and functionalized silsesquioxanes prevents catalyst leach-
ing [28]. Therefore, several ligands bearing a silsesquioxane moiety
have been synthesized and utilized as precursors to the preparation
of heterogeneous catalysts with transition metal complexes [29]
or nanoclusters [30–34]. Recently, there has been interest in func-
tionalization of POSS with heteroatom ligands as chelating agents
for metal ions such as thiourea [35], 3-amino-1,2,4-triazole [36],
2-amino-1,3,4-thiadiazole [37], and 2,2-dipyridylamine [38].
In this work, the application of POSS as a nano-platform for
the synthesis of POSS-bridged Schiff base ligands is explored
by functionalizing the cage containing eight propylammonium
chloride pendant arms with salicylaldehyde. The heterogeneous
POSS-bridged Schiff base-Cu(II) complex is obtained by the reaction
between POSS-bridged Schiff base ligand and the metal precur-
sor Cu(OAc)2·H2O. To the best of our knowledge, POSS has not
been used as a catalyst for the azide–alkyne cycloaddition reac-
tion. Therefore, the produced POSS–SAL–Cu was tested for the first
time as a catalyst to carry out the cycloaddition reaction. The two
chitecture [18,19]. The Si
O Si backbone and the chemically and
∗
Corresponding authors.
E-mail addresses: arsalani@tabrizu.ac.ir (N. Arsalani), mamini@maragheh.ac.ir
(M. Amini), jabbari@cec.sc.edu (E. Jabbari).
1
Fax: +98 4133340191.
http://dx.doi.org/10.1016/j.molcata.2015.12.022
1381-1169/© 2015 Elsevier B.V. All rights reserved.

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A. Akbari et al. / Journal of Molecular Catalysis A: Chemical 414 (2016) 47–54
Scheme 1. Synthetic route and the proposed chemical structure for POSS–SAL–Cu complex.
step synthetic pathway for POSS–SAL–Cu is depicted in Scheme 1.
POSS (1) was prepared via hydrolyses and condensation reactions
using 3-aminopropyltriethoxysilane (ATPES) as the sol–gel precur-
sor in acidic condition [39]. The temperature was increased to 90 ◦C
to reduce the reaction time from one week to 16 h [40]. Imine
bonds were readily formed via condensation of salicylaldehyde
with POSS (1) in the presence of triethylamine in methanol to yield
POSS–SAL (2) in nearly quantitative yield. Finally POSS–SAL–Cu (3)
was synthesized as a green solid by the reaction of POSS–SAL with
Cu(OAc)2·H2O in ethanol.
2. Experimental
2.1. Materials and analyses
Fig. 1. FT-IR spectra of POSS (a), POSS–SAL (b) and POSS–SAL–Cu (c).
All chemicals and solvents were purchased from Merck and
Fluka and were used without further purification. Benzyl azides
were prepared according to a previously reported procedure

A. Akbari et al. / Journal of Molecular Catalysis A: Chemical 414 (2016) 47–54
49
Scheme 2. Cycloaddition of alkyl halides with phenyl acetylene and NaN3.
Varian Mercury-3001H NMR (Varian, Palo Alto, CA) spectrometer.
The scanning electron microscopy (SEM) image was taken with Tes-
can, Kotoutovice, Czech Republic at accelerating voltage of 8 keV.
Transmission electron microscopy (TEM) was studied on a Hitachi
H-8000 transmission electron microscope. MALDI-MS measure-
ments were performed on Bruker Autoflex III MALDI-TOF MS.
2.2. Synthesis of POSS–NH3+, compound 1
Octa (3-aminopropyl) octasilsequioxane octahydro chloride
(hereafter named POSS–NH3+ for simplicity) was prepared accord-
ing to the method described in literature [42]. Briefly in 1000 mL
two-neck round bottom flask equipped with a reflux condenser and
a magnetic stirrer, a solution of 350 mL methanol and concentrated
HCl (30 mL, 35–37%) was charged, and then (3-aminopropyl)-
triethoxysilane (15 mL, 60 mmol) was added dropwise at 60 ◦C. The
reaction mixture refluxed in oil bath at 90 ◦C for 16 h with vigor-
ous stirring and then cooled to ambient condition and 300 mL THF
was added to precipitate. The white precipitate was collected by
+
filtration and washed three times with THF to obtain POSS–NH3
as a white powder with a yield of 33%. The characterization data are
as follows: FT-IR (KBr, cm−1): 3224, 3025, 2920, 1604, 1494, 1125,
1107, 935, 705 cm−1 1H NMR (300 MHz, D2O): ı 2.91(t, CH2NH3,
;
16 H), 1.68 (m, SiCH2CH2, 16 H), 0.68 (t, SiCH2, 16 H). 13CNMR
(100 MHz, D2O): ı 41.04 (s, CH2NH3), 20.01 (s, SiCH2CH2), 8.55 (s,
SiCH2).
2.3. Synthesis of POSS–SAL, compound 2
To a solution of POSS–NH3+ (1.0 g, 0.85 mmol) and triethylamine
(0.97 mL , 0.7 mmol) in methanol (20 mL ), salysilaldehyde (4.88 g,
40 mmol) was added, and the reaction mixture was stirred at ambi-
ent condition for 2 h and then condensed by evaporation. Then the
crude product was washed with diethyl ether (4 × 35 mL) to yield
1.21 g. FT-IR (KBr, cm−1): 3021, 2938, 2883, 1630, 1498, 1277, 1124,
760 cm−1 1H NMR (300 MHz, DMSO): ı 13.30 (b, OH, 4 H), 8.45
;
(s, NCH , 3.95 H), 6.83–7.30 (b, Aromatic protons, 16 H), 2.91(t,
CH2N, 8 H), 1.68 (m, SiCH2CH2, 8 H), 0.68 (t, SiCH2, 8 H). 13CNMR
Table 1
The effect of reaction conditions on the azide–alkyne cycloaddition.
Entry
Amount of Cu in
Temperature (◦C)
Time (h)
Yield (%)a
nanocatalyst (mmol)
1
2
3
4
5
6
7
8
–
70
70
70
70
70
70
r.t.
40
50
60
70
70
70
12
12
12
12
12
12
12
12
12
12
2
0
0.002
0.004
0.006
0.008
0.01
0.008
0.008
0.008
0.008
0.008
0.008
0.008
34
45
69
81
95 (37)b
8
16
33
87
29
53
80
Fig. 2. (a–d) 1HNMR (a,c) and 13CNMR (b,d) spectra of POSS and POSS–SAL.
9
10
11
12
13
[41]. Fourier transform infrared (FT-IR) spectra was carried out
on a Bruker FTIR spectrometer (model Tensor 27) over the
range of 500–3500 cm−1 using spectroscopic grade KBr pow-
der at room temperature. 1HNMR spectra were recorded on a
6
8
a
Isolated yield.
b
In the presence of 0.02 mmol of Cu(OAc)2·H2O as a catalyst.

50
A. Akbari et al. / Journal of Molecular Catalysis A: Chemical 414 (2016) 47–54
Table 2
Cycloaddition of alkyl halides with terminal alkynes in the presence of POSS–SAL–Cu.
Entry
Substrate
Alkyne
Yield (%)a
95
1
2
3
4
95
89
94
5
6
91
93
96
92
90
95
7
8
9
10
11
95
96
12
13
91
93
14
a
Isolated yield.

A. Akbari et al. / Journal of Molecular Catalysis A: Chemical 414 (2016) 47–54
51
Fig. 3. MOLDI-TOF of POSS–SAL (a) and POSS–SAL–Cu (b).
(100 MHz, DMSO): ı 165.68 (s, phenolic OH), 161.01 (s, CH NH),
2.4. Synthesis of POSS–SAL–Cu, compound 3
132.44–116.75 (s, Aromatic carbons), 39.57 (s, CH2NH3), 24.39 (s,
SiCH2CH2), 9.53 (s, SiCH2).
A solution of Cu(OAc)2·H2O (8 mmol) in 10 mL C2H5OH was
added to a solution of 1.0 mmol POSS–SAL in 10 mL CH2Cl2. The

52
A. Akbari et al. / Journal of Molecular Catalysis A: Chemical 414 (2016) 47–54
Fig. 4. SEM (a) and TEM (b) images of POSS–SAL–Cu.
mixture was stirred for 24 h at ambient condition to form a green
solid. The solid was filtered off, washed with ethanol and finally
with diethyl ether and dried in vacuum. Elemental analysis for
POSS–SAL–Cu (by the weight percentage): C 51.48%, N 5.57%, and
H 5.47%, and the loading amount of Cu in the catalyst POSS–SAL–Cu
is calculated to be about 16.7%.
2.5. General procedure for Cu(II) POSS-bridged Schiff base
azide–alkyne cycloaddition
Alkyne (0.5 mmol), the organic halide (0.55 mmol), and NaN3
(0.55 mmol) were added to a suspension of POSS–SAL–Cu (5 mg)
in H2O (2 mL). The reaction mixture was heated to 70 ◦C
and monitored by TLC until total conversion of the starting

A. Akbari et al. / Journal of Molecular Catalysis A: Chemical 414 (2016) 47–54
53
Fig. 6. IR spectrum of the reused catalyst POSS–SAL–Cu.
We also characterized the new products POSS–SAL and
POSS–SAL–Cu by MOLDI-TOF mass spectrometry. In both cases,
the observed mass was lower than the calculated mass. According
to Fig. 3a and b, the observed mass for POSS–SAL (calcu-
lated m/z = 1664) and POSS–SAL–Cu (calculated m/z = 2072) were
m/z = 1615.35 and 1906.91, respectively. The discrepancy can be
attributed to the partial cleavage of imine bonds of the Schiff bases
in the structure of the products by high-energy laser with acid as
the matrix. The same results were observed previously by Wu et al.
[45].
Fig. 4a and b shows the SEM and TEM images of POSS–SAL–Cu
(3). These images show spherical aggregate of nanoparticles with
average size of 100 nm. It was not possible to image the individ-
ual POSS nanoparticles (1–3 nm) because of the complexation of
copper metals with POSS–SAL–Cu.
Fig. 5. Product yield with recycling of the catalyst POSS–SAL–Cu in the azide–alkyne
cycloaddition reaction.
materials. Water (5 mL) was added to the resulting mixture, the
catalyst was recovered by filtration and the product was extracted
with EtOAc (2 × 10 mL). The collected organic phases were dried
with anhydrous Na2SO4 and the solvent was removed under vac-
uum to give the corresponding triazoles, which did not require
any further purification. The catalyst was washed with ethanol and
dried in vacuum in order to use the next run without the addition
of any fresh catalyst.
3. Results and discussion
3.1. Complex characterization
3.2. Catalytic effects
The two step synthetic pathway for POSS–SAL–Cu is depicted in
Scheme 1. POSS–NH3 was prepared via hydrolyses and conden-
To optimize the reaction conditions, a series of experiments
were carried out with varying catalyst loading, temperature and
time for representative reaction of benzyl chloride, phenyl acety-
lene and NaN3 to synthesize 1-benzyl-4-phenyl-1,2,3-traizole with
POSS–SAL–Cu as the catalyst (Scheme 2). Water was used as the
standard “green” solvent. All reactions were performed in air with-
out any additive. In a typical reaction, alkyne, organic halide and
NaN3 in a ratio of 1:1.1:1.1 were mixed in a flask, followed by the
addition of water and catalyst. The mixture was heated to 70 ◦C
for a certain period (Table 1). The organic product was isolated by
extraction and analyzed by elemental analysis and 1H NMR. Results
are summarized in Table 1.
When the amount of catalyst was increased from 0 to 5 mg, the
product yield increased sharply from 0 to 95%. No product was
obtained in the absence of catalyst. As indicated in Table 1, the reac-
tion did not initiate at ambient temperature with magnetic stirring
for 12 h, but the product yield increased significantly at higher tem-
peratures. Also, the yield increased significantly from 29% to 95%
with increasing reaction time from 2 to 12 h. Catalytic activity of
the reaction system decreased dramatically when POSS–SAL–Cu
was replaced by Cu(OAc)2, as shown in Table 1 (entry 6).
A wide range substituted phenyl acetylenes were reacted with
the mixture of benzyl bromides/chlorides and sodium azide using
this procedure to produce the corresponding 1,4-disubstituted-
1,2,3-triazoles. The results are summarized in Table 2. The
substitution of electron withdrawing or electron donating groups
on the phenyl ring of benzyl halides and phenyl acetylenes did not
have an appreciable effect on the reaction outcome (entries 2–6).
Furthermore, steric hindrance of the ortho substituents on benzyl
chlorides affected progress of the reaction. The benzyl chloride sub-
stituted with methyl group at the ortho position was less reactive
toward the cycloaddition reaction than the para derivative (entries
2 and 3). However, the cycloaddition of benzyl bromide had a lower
yield under similar reaction conditions (entries 7–9). The results
of Cu(II)-catalyzed cycloaddition of various terminal alkynes with
benzyl azide are summarized in Table 2. The reactions of substi-
tuted phenyl acetylenes and benzyl azides in water with 5 mg of
POSS–SAL–Cu resulted in 91–96% products (entries 10–14), while
a blank experiment under similar conditions provided no product.
+
sation reaction by using 3-aminopropyltriethoxysilane (ATPES) as
the sol–gel precursor in acidic condition [43]. The reaction was car-
ried out at 90 ◦C to reduce the reaction time (one week at ambient
condition) to 16 h [42]. Imine bonds were readily formed via con-
+
densation of salicylaldehyde with POSS–NH3 in the presence of
triethylamine in methanol and produced the POSS–SAL, compound
2, in nearly quantitative yield. Finally POSS–SAL–Cu, compound
3, was prepared as a green solid. All compounds were character-
ized by 1HNMR, 13CNMR, FT-IR, MOLDI-TOF, SEM and TEM. The
resulting solid catalyst was insoluble in common organic solvents,
such as THF, DMF, ethanol, acetonitrile, and chloroform, thus the
synthesized catalyst is a good candidate with respect to solubility
for heterogeneous catalysis. FT-IR was used for structure eluci-
dation. Fig. 1 illustrates the FT-IR spectra of POSS (a), POSS–SAL
(b) and POSS–SAL–Cu (c). Fig. 1a shows the most prominent
bands of the POSS cage, Si
O
Si asymmetric stretching absorption
Si bridges
between 1107 and 1125 cm−1 and deformation of Si
O
at 705 cm−1 [44]. In addition, the peaks at 2893 and 2955 cm−1 in
the FT-IR spectra confirmed the presence of propyl chains in POSS.
After the reaction of POSS with salicylaldehyde, the structure of the
new compound, POSS–SAL, as a free ligand exhibited many new
absorption peaks including those at 1630 and 1277 cm−1 which
were attributed to C N azomethine and C O phenolic vibrations,
respectively. After complexation with copper, the azomethine
vibration of the free ligand at 1630 cm−1 was moved to a lower fre-
quency, i.e., 1620 cm−1. Additionally, the C O phenolic vibration
at 1277 cm−1 in the free ligand was shifted to a higher frequency
by 47 cm−1 after complexation. These shifts confirmed the forma-
tion of POSS–SAL–Cu by coordination of azomethine nitrogen and
phenolic oxygen with copper ions.
+
We have characterized the POSS–NH3 and POSS–SAL by NMR
spectroscopy (1HNMR and 13CNMR) as shown in Fig. 2a–d. Based
on the 1HNMR spectrum in Fig. 2c, the ratio of the peaks for methy-
lene groups of POSS, as the base with integral value of 8 protons, to
that of the imine protons (8.45 ppm, 3.95 H) was 2.02. As a result, 8
ammonium groups of POSS were successfully reacted with salicy-
laldehyde and converted to imine groups.

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A. Akbari et al. / Journal of Molecular Catalysis A: Chemical 414 (2016) 47–54
Higher reaction conversions compared to the blank experiments
clearly demonstrated the activity of POSS–SAL–Cu catalyst in the
reactions.
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azide–alkyne cycloaddition reaction of benzyl chloride, phenyl
acetylene and NaN3 at 70 ◦C for 12 h. For each cycle, the catalyst
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All eight propylammonium side chains of POSS 1 reacted with
salicylaldehyde, producing the new compound 2 (POSS–SAL) with
eight salicylaldehyde arms. Further, the reaction of compound 2
with the metal precursor Cu(OAc)2 produced the new derivative
POSS–SAL–Cu (3). Based on the SEM and TEM images, the derivative
3 contained an aggregate of spherical nanoparticles. The synthe-
sized POSS-bridged copper Schiff base was a highly efficient catalyst
for azide–alkyne cycloaddition in aqueous media. Superior cat-
alytic activity, easy recovery and reusability are the advantages of
compound 3. To our knowledge, this is the first POSS cage function-
alized with eight salicylaldehydes as a ligand for coordination with
copper metal to produce model nano-platforms in heterogeneous
catalysis. The application of POSS–SAL as a Schiff base free ligand
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Acknowledgments
Partial funding support was provided by the Research Council
of University of Tabriz (N.A.) and University of Maragheh (M.A.).
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