Synthesis, characterization, antibacterial and catalytic activity of a nanopolymer supported copper(ii) complex as a highly active and recyclable catalyst for the formamidation of arylboronic acids under aerobic conditions

Article abstract of DOI:10.1039/c4ra02052a

This paper reports on the synthesis and use of a nanopolymer supported copper(ii) complex, as a separable catalyst for the formamidation of arylboronic acids under aerobic conditions. The catalyst was characterized using powder XRD, SEM, EDS, TGA-DTG and FT-IR spectroscopy. This method has the advantages of high yields, elimination of homogeneous catalysts, simple methodology and easy work up. Catalytic efficiency remains unaltered even after several repeated cycles. The synthesized catalyst is found to be more highly toxic towards Gram-positive bacteria than Gram-negative bacteria. This journal is the Partner Organisations 2014.

Full text of DOI:10.1039/c4ra02052a

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PAPER
Synthesis, characterization, antibacterial and
catalytic activity of a nanopolymer supported
copper(II) complex as a highly active and recyclable
catalyst for the formamidation of arylboronic acids
under aerobic conditions
Cite this: RSC Adv., 2014, 4, 20351
a
Mahmoud Nasrollahzadeh, Ali Zahraei,b Ali Ehsania and Mehdi Khalajc
*
This paper reports on the synthesis and use of a nanopolymer supported copper(II) complex, as a separable
catalyst for the formamidation of arylboronic acids under aerobic conditions. The catalyst was
characterized using powder XRD, SEM, EDS, TGA-DTG and FT-IR spectroscopy. This method has the
advantages of high yields, elimination of homogeneous catalysts, simple methodology and easy work up.
Catalytic efficiency remains unaltered even after several repeated cycles. The synthesized catalyst is
found to be more highly toxic towards Gram-positive bacteria than Gram-negative bacteria.
Received 9th March 2014
Accepted 8th April 2014
DOI: 10.1039/c4ra02052a
www.rsc.org/advances
Introduction
The ability to construct formamides efficiently is currently an
active area in organic synthesis due to the prevalence of this
structural motif in a myriad of bioactive important products
and pharmaceutically interesting compounds.1–4 Among the
various strategies developed to date, the copper-catalyzed
Chan–Lam coupling reaction has proven to be one of the most
convenient synthetic routes for the synthesis of formamides.5
Chan–Lam coupling allows aryl carbon-heteroatom bond
formation via an oxidative coupling of arylboronic acids, stan-
nanes or siloxanes with N–H or O–H containing compounds in
air. The reaction is induced by a stoichiometric amount of
copper(II) or a catalytic amount of copper catalyst which is
reoxidized by atmospheric oxygen.6–8
Recently, less expensive Cu catalysts has found much
interest for the C–N bond formation.6–9 However, traditional Cu-
catalyzed reactions also require the use of stoichiometric
amounts of homogeneous copper catalysts, use of toxic ligands,
harsh reaction conditions, strong bases and oen use of toxic
polar solvents. As a result, these drawbacks have limited their
large scale applications in industry. Therefore, it is desirable to
develop more efficient and convenient methods for the for-
mamidation of arylboronic acids using heterogeneous catalysts
under aerobic conditions.
Scheme 1 Copper-catalyzed formamidation of arylboronic acids.
In continuation of our researches on the synthesis of tetra-
zoles and application of heterogeneous catalysts,10–15 we report a
new protocol for the preparation of the nanopolystyrene-
anchored Cu(II) thiotetrazole complex [PS–ttet–Cu(II)] (2) and its
antimicrobial and catalytic applications as a novel and stable
heterogeneous catalyst for the formamidation of arylboronic
acids under aerobic conditions at 60 ꢀC (Scheme 1). The PS–
ttet–Cu(II) has become an affordable catalyst, as it can be
prepared from inexpensive and readily available materials.
Environmental acceptability, economic viability, and recycla-
bility of the PS–ttet–Cu(II) is the advantages of this novel
catalyst.
Result and discussion
Catalyst was readily prepared in two steps. The synthesis of the
polymer-anchored Cu(II) complex catalyst is shown in Scheme 2.
Characterization of catalyst
The catalyst was characterized using the powder XRD, SEM,
EDS, TGA-DTG and FT-IR spectroscopy.
aDepartment of Chemistry, Faculty of Science, University of Qom, Qom 37185-369,
Iran. E-mail: mahmoudnasr81@gmail.com; Fax: +98 25 32103595; Tel: +98 25
32850953
bYoung Researchers and Elite Club, Qom Branch, Islamic Azad University, Qom, Iran
cDepartment of Chemistry, Buinzahra Branch, Islamic Azad University, Buinzahra,
Qazvin, Iran
Presence of the 1-phenyl-1H-tetrazole-5-thiol ligand on
chloromethylated polystyrene and formation of PS–ttet–Cu(II) 2
was conrmed by FT-IR spectra. The FT-IR spectrum of the 1-
phenyl-1H-tetrazole-5-thiol and chloromethylated polystyrene
was compared with the polymer-supported thiotetrazole ligand
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Scheme 2 Preparation of copper tetrazole-supported complex.
and nanopolymer-supported [PS–ttet–Cu(II)] catalyst in order to C–Cl peak (due to CH2Cl groups) at 698 cmꢁ1 and the strong
conrm the coordination of the copper with the 1-phenyl-1H- peak at 1274 cmꢁ1 corresponding to the H–C–Cl wagging modes
tetrazole-5-thiol ligand (Fig. 1). As shown in Fig. 1B, the sharp in the starting polymer practically disappeared aer
Fig. 1 FT-IR spectra of: (A) 1-phenyl-1H-tetrazole-5-thiol, (B) polystyrene–thiotetrazole ligand and (C) supported Cu–thiotetrazole complex.
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introduction of the thiotetrazole ligand on the polymer. The
stretching vibrations of the N]N double bonds appear at 1449
cmꢁ1 and 1403 cmꢁ1 for polystyrene–tetrazole ligand (Fig. 1B)
and complex (Fig. 1C), respectively. Signals ranging under 3000
to 2800 cmꢁ1 identify the methylene stretching bonds. The
functionalized beads exhibited a band at 1652 cmꢁ1 due to
stretching vibrations of the C]N double bond, which was
shied to 1598 cmꢁ1 in the anchored beads, indicating that the
nitrogen atoms of the thiotetrazole is coordinated to copper.
This suggests that the ligand acts as chelating agent coordi-
nated through ‘N’ atom.
Phase investigation of the crystallized product was per-
formed by powder XRD measurements and the powder
diffraction pattern of PS–ttet–Cu(II) 2 is presented in Fig. 2.
Polystyrene exhibits a broad peak at about of 2q ¼ 19.0ꢀ and
22.7ꢀ.16 The diffraction peaks at 38.2ꢀ and 46.5ꢀ correspond to
copper.
Fig. 3 SEM image of PS–ttet–Cu(II) 2.
Scanning electron micrograph (SEM) image of PS–ttet–Cu(II)
2 is shown in Fig. 3. As expected, the pure polystyrene bead had
a smooth and at surface,17 while the anchored complex
showed roughening of the top layer. The presence of Cu has
caused changes, demonstrated by change in the polymer
particle size and roughness of the surface.
We used Energy Dispersive X-ray Spectroscopy (EDS) to
determine chemical composition of catalyst. The presence of
the metals is conrmed by EDS. EDS results show that carbon,
copper and oxygen concentrations are about 79.8%, 4.57% and
11.89%, respectively (Fig. 4). The excess oxygen is due to phys-
ical absorption of oxygen from environment during sample
preparation for SEM experiment.
Thermal stability of polymer-bound complex was investi-
gated using TGA-DTG at a heating rate of 10 ꢀC minꢁ1 in air over
ꢀ
a temperature range of 30–800 C. The TGA curve of the poly-
mer-supported copper complex is shown in Fig. 5. The complex
is stable up to 200 ꢀC, and above this temperature it decom-
poses. The polymer-supported Cu(II) complex decomposed at
ꢀ
Fig. 4 EDS spectrum of PS–ttet–Cu(II) 2.
300 C.
Fig. 2 XRD pattern of PS–ttet–Cu(II) 2.
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increased under thermal conditions (Table 1, entry 7). A
decrease in the catalyst loading from 0.05 to 0.03 g afforded the
product in lower yield (Table 1, entry 13). No signicant
improvement on the yield was observed using higher amounts
of the catalyst (Table 1, entry 14) and 0.05 g of the catalyst was
found to be optimum. The best result was obtained with 1.0
mmol of phenylboronic acid, 2.5 mmol of NH2CHO, 0.05 g of
ꢀ
PS–ttet–Cu(II) (2) and 1.3 mmol of K2CO3 at 60 C, which gave
the product in an excellent yield.
Next, the reactivity of several arylboronic acids was tested in
the formamidation reaction under at 60 ꢀC and the results were
indicated in Table 2. To study the effects of the nature of the
substituent groups on the benzene ring of phenylboronic acids,
various formamidation were performed from different phenyl-
boronic acids containing both electron-releasing and electron-
withdrawing groups with NH2CHO in high yields under thermal
conditions (Table 2). The formation of formamides was
conrmed by melting point and IR spectra, which showed two
characteristic peaks, one between 3300 and 3400 cmꢁ1
(secondary NH) and the other between 1640 and 1680 cmꢁ1 (N-
formyl, C]O).3,4,18
Fig. 5 Thermal studies of PS–ttet–Cu(II) 2.
Activity of PS–ttet–Cu(II) (2) catalyst for the formamidation of
arylboronic acids
The catalytic behavior of the PS–ttet–Cu(II) (2) was studied for
the formamidation of arylboronic acids.
Initially, we employed NH2CHO and phenylboronic acid as
model substrates for the development of optimized conditions.
Several solvents such as water, toluene, MeCN, MeOH and
CH2Cl2 were examined. Control experiments show that there is
no reaction in the absence of catalyst (Table 1, entry 12).
However, addition of the catalyst to the mixture has rapidly
increased the formamidation of phenylboronic acid in high
yields. According to data given in Table 1, when the reaction was
carried out in the absence of solvent and only in the presence
NH2CHO (as both solvent & formylating agent), excellent
conversion was achieved (Table 1, entries 6 and 7). The reactivity
of the catalyst in the presence of different bases was also
investigated (Table 1, entries 8–10). The results indicated that
base had a demonstrative effect on the yield of product (Table 1,
entry 11). Among the tested bases (K2CO3, Et3N, Na2CO3,
NaOAc), K2CO3 was found to be superior for the highest yield of
N-phenyl formamide (Table 1, entry 7). The experimental results
show that the reaction times are reduced and the yields
Antibacterial studies
In vitro antibacterial activity of the PS–ttet–Cu(II) (2) was evalu-
ated against two bacterial species using the standard well
diffusion method. The results presented in Table 3 showed the
antibacterial effects of PS–ttet–Cu(II) against E. coli (Escherichia
coli) and S. aureus (Staphylococcus aureus). The antibacterial
activity is estimated by the zone of inhibition. The diameter of
the zone is measured to the nearest millimeter (mm). The
standard error for each assay is presented in the parenthesis
(Table 3). As shown in Table 3, PS–ttet–Cu(II) (2) has shown high
antibacterial effect. The present study clearly indicates that the
PS–ttet–Cu(II) (2) has good antibacterial action against Gram
positive (Staphylococcus aureus) organism than Gram negative
(Escherichia coli) organisms. It is possible that PS–ttet–Cu(II) not
only interact with the surface of membrane, but can also
penetrate inside the bacteria.
Table 1 The formamidation of phenylboronic acid with NH2CHO under different reaction conditionsa
Entry
PS–ttet–Cu(II) (g)
Base
Solvent
Temperature (ꢀC)
Time (h)
Yieldb (%)
1
2
3
4
5
6
7
8
0.05
0.05
0.05
0.05
0.05
0.05
0.05
0.05
0.05
0.05
0.05
—
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
NaOAc
Na2CO3
Pyridine
—
H2O
MeOH
CH2Cl2
MeCN
Toluene
—
—
—
—
—
r.t.
r.t.
r.t.
r.t.
r.t.
r.t.
60
60
60
60
60
60
60
60
20
20
20
20
20
8
5
5
5
5
37
67
33
60
58
89
93
42
81
79
32
0
9
10
11
12
13
14
—
—
—
—
5
5
5
5
K2CO3
K2CO3
K2CO3
0.03
0.08
80
92
a
Reaction conditions: phenylboronic acid (1.0 mmol), NH2CHO (2.5 mmol), base (1.3 mmol), PS–ttet–Cu(II) (0.05 g), solvent (2 mL). b Isolated yield.
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washed with ethyl acetate and dried in a hot air oven at 100 ꢀC
for 2 h and employed for the next run of the reaction. The
catalytic activity did not decrease considerably aer ve cata-
lytic cycles (Fig. 6). This reusability demonstrates the high
stability and turnover of catalyst under operating condition.
Table 2 Formamidation of different arylboronic acids by the PS–ttet–
Cu(II)
Entry
R
Time (h)
Yielda%
Conclusions
1
2
3
4
5
6
7
8
C6H5
5
5
5
5
5
9
9
9
9
9
9
5
93
90
91
91
93
88
91
92
81
80
79
92
In conclusion, we have developed an efficient and simple
procedure for preparation of nanopolystyrene-anchored Cu(II)
thiotetrazole complex [PS–ttet–Cu(II)] (2) as an efficient, easily
recoverable and reusable catalyst. The catalyst was character-
ized by SEM, XRD, EDS, TGA-DTG and FT-IR analysis. This
catalyst demonstrated a high catalytic activity in promoting
formamidation of phenylboronic acids to provide the form-
amides in high yields. This method has the advantages of high
yields, elimination of homogeneous catalysts, simple meth-
odology and easy work up. Another important factor is the
stability and recyclability of the catalyst under the reaction
conditions used. The results show that the immobilized cata-
lyst is slightly more active than its homogeneous analogue.
This heterogeneous catalyst shows no signicant loss of
activity in the recycling experiments. The active sites do not
leach out from the support and thus can be reused without
appreciable loss of activity, indicating that the anchoring
procedure was effective. The antibacterial activity of PS–ttet–
Cu(II) was evaluated against E. coli and S. aureus, showing
effective bactericidal activity.
2,4-(Me)2-C6H3
2-Me-C6H4
4-Me-C6H4
4-OMeC6H4
2-Cl-C6H4
4-Br-C6H4
4-Cl-C6H4
4-COMe-C6H4
3-CF3-C6H4
4-NO2-C6H4
1-Naphthyl
9
10
11
12
a
Yields are aer work-up.
Table 3 Zone of inhibition (mm) of PS–ttet–Cu(II) against bacterial
pathogens
Diameter of inhibition zone (mm)
Bacterial
E. coli
(Gram ꢁve)
S. aureus
(Gram +ve)
100 ppm 200 ppm 300 ppm 400 ppm 500 ppm
12 (ꢂ3)
16 (ꢂ3)
21 (ꢂ3)
25 (ꢂ3)
+35
17 (ꢂ3)
22 (ꢂ3)
23 (ꢂ3)
28 (ꢂ3)
+35
Experimental section
All reagents were purchased from the Merck and Aldrich
chemical companies and used without further purication.
Chloromethylated polystyrene (4–5% Cl and 2% cross-linked
with divinylbenzene) was purchased from Merck company.
Products were characterized by different spectroscopic
methods (FT-IR and 1H NMR spectra), elemental analysis
(CHN) and melting points. 1H NMR spectra were recorded on
a Bruker Avance DRX 90 and 300 MHz instrument. The
chemical shis (d) are reported in ppm relative to the TMS as
internal standard. J values are given in Hz. IR (KBr) spectra
were recorded on a Perkin-Elmer 781 spectrophotometer.
Melting points were taken in open capillary tubes with a
BUCHI 510 melting point apparatus and were uncorrected.
The elemental analysis was performed using Heraeus CHN-
O-Rapid analyzer. TLC was performed on silica gel polygram
SIL G/UV 254 plates. X-ray diffraction measurements were
performed with a Philips powder diffractometer type PW
1373 goniometer. It was equipped with a graphite mono-
Catalyst recyclability
The reusability of the catalyst is one of the most important
benets and makes them useful for commercial applications.
From an economic point of view, the stability and sustained
activity of the catalysts are of great importance. Thus, the
recovery and reusability of the PS–ttet–Cu(II) (2) catalyst was
examined by applying it to the C–N coupling of NH2CHO with
phenylboronic acid under the present reaction conditions. Aer
the rst run completed, the catalyst was separated by ltration,
˚
chromator crystal. The X-ray wavelength was 1.5405 A and the
diffraction patterns were recorded in the 2q range (10–60)
with scanning speed of 2ꢀ per min. Morphology and particle
dispersion was investigated by scanning electron microscopy
(SEM) (Cam scan MV2300). The chemical composition of
the prepared nanostructures was measured by EDS per-
formed in SEM.
Fig. 6 Reusability of PS–ttet–Cu(II) for the formamidation of phenyl-
boronic acid.
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N-(4-Nitrophenyl) formamide (Table 2, entry 11): 1H NMR (90
MHz, acetone-d6): dH 7.91 (d, J ¼ 8.7 Hz, 2H), 8.25 (d, J ¼ 9.0 Hz,
2H), 8.49 (s, 1H), 9.77 (s, 1H).
N-(1-Naphthyl) formamide (Table 2, entry 12): H NMR (300
MHz, CDCl3): dH 7.27–8.06 (m, 7H), 8.65 (d, 1H), 8.83 (s, 1H).
Preparation of nanopolystyrene-anchored Cu(II) thiotetrazole
complex 2
The reaction was carried out in a round-bottomed ask of 250
mL capacity. Chloromethylated polystyrene (2.0 g, 1.25 mmol
gꢁ1 of Cl) was stirred in 50 mL of DMF. Then 1-phenyl-1H-tet-
razole-5-thiol (5.0 mmol) and K2CO3 (5.0 mmol) was added to
the above solution of the polymer, and the mixture was heated
for 24 h at 100 ꢀC. The polymer-anchored ligand (1) was ltered
out, washed thoroughly with DMF and dried under vacuum for
12 h. Polymer-anchored thiotetrazole ligand (1.0 g) was added to
ethanol (50 mL). CuCl2$2H2O (0.5 g) was added to the above
suspension with constant stirring and then reuxed for 24 h.
Aer cooling the reaction mixture to room temperature, the
separated bright green solid was ltered out, washed thor-
oughly with ethanol and dried under vacuum to give PS–ttet–
Cu(II) 2.
1
Procedure for antibacterial studies
Antibacterial activity of the catalyst was tested by using the
standard well diffusion method. The bactericidal activities of
PS–ttet–Cu(II) was investigated against E. coli (ATCC 25922) and
S. aureus (ATCC 25923) bacterial as Gram negative and Gram
positive models, respectively. Before each experiment, all the
ꢀ
samples and glassware were sterilized by autoclaving at 120 C
for 10 min. Mueller-Hinton agar (MHA) (Merck) was used as
base medium and sterile saline was used for the preparation of
inoculum. Various concentrations (100, 200, 300, 400 & 500
ppm) of each compound were made. For each concentration
separate Petri plate with preinoculated bacteria was used. Four
to ve isolated colonies of tested organisms were picked by
sterile inoculating loop and inoculated in tubes of sterile saline
(5 mL in each). The inoculated tubes were incubated at 35–37 ꢀC
for 10 minutes and matched with 0.5 McFarland nephelometer
turbidity standards. The surface of MHA was completely
cultured using a cotton swab which steeped in prepared
suspension of bacterium. On each plate ve cups were made
and 25 mL of each test compound were poured on each cup
carefully. Aer incubation at 35–37 ꢀC for 24 hours, the different
levels of zone of inhibition of bacteria were measured in milli-
meter (mm), and were recorded as mean ꢂ SD of the triplicate
experiment.
General procedure for the formamidation of arylboronic acids
A mixture of the appropriate arylboronic acid (1.0 mmol),
NH2CHO (2.5 mmol), base (1.3 mmol) and PS–ttet–Cu(II) (0.05 g)
was stirred at 60 ꢀC for the appropriate time. Aer completion of
reaction (as monitored by TLC), the reaction mixture was cooled
to room temperature, ethyl acetate and water was added and
organic layer was separated. Then, aqueous layer was again
extracted with ethyl acetate three times. The combined organic
layers were washed with water, dried over MgSO4, ltered and
evaporated under reduced pressure. The residue was puried by
column chromatography to give the desired pure products. The
1
physical and spectral (IR, H NMR and 13C NMR) data of the
known products were found to be identical with those reported
in the literature.3,4,18
N-Phenyl formamide (Table 2, entry 1): 1H NMR (90 MHz,
CDCl3): dH 7.61–7.12 (m, 5H), 8.29 (s, 1H, cis), 8.66 (d, 1H, trans),
8.99 (brs, 1H, cis), 9.38 (brs, 1H, trans).
Acknowledgements
We gratefully acknowledge the Iranian Nano Council and
University of Qom for the support of this work.
N-(2,4-Dimethylphenyl) formamide (Table 2, entry 2): 1H
NMR (300 MHz, DMSO-d6): dH 2.28 (s, 3H), 2.29 (s, 3H), 7.66 (d,
1H, J ¼ 8.7), 8.43 (d, 1H, J ¼ 15.5).
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4H), 2.29 (s, 3H).
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