Palladium-free and phosphine-free Sonogashira coupling reaction of aryl halides with terminal alkynes catalyzed by boehmite nanoparticle-anchored Cu(I) diethylenetriamine complex

Article abstract of DOI:10.1007/s11164-017-3079-0

In this work, a novel diethylenetriamine-functionalized-boehmite nanoparticle-supported Cu(I) catalyst is synthesized. The catalyst prepared is then characterized by FT-IR spectroscopy, scanning electron microscopy, transmission electron microscopy, X-ray

Full text of DOI:10.1007/s11164-017-3079-0

Res Chem Intermed
DOI 10.1007/s11164-017-3079-0
Palladium-free and phosphine-free Sonogashira
coupling reaction of aryl halides with terminal alkynes
catalyzed by boehmite nanoparticle-anchored
Cu(I) diethylenetriamine complex
Mohammad Bakherad1 Rahele Doosti1 Mahdi Mirzaee1
•
•
•
Khosrow Jadidi2 Amir H. Amin1 Omid Amiri2
•
•
Received: 4 May 2017 / Accepted: 12 July 2017
Ó Springer Science+Business Media B.V. 2017
Abstract In this work,
a novel diethylenetriamine-functionalized-boehmite
nanoparticle-supported Cu(I) catalyst is synthesized. The catalyst prepared is then
characterized by FT-IR spectroscopy, scanning electron microscopy, transmission
electron microscopy, X-ray diffraction spectroscopy, and inductively coupled
plasma techniques. The catalyst prepared shows great activity in the Sonogashira
coupling reaction of aryl halides with terminal alkynes in DMF under an argon
atmosphere. Moreover, it could be removed from the reaction mixture by means of
simple filtration and used again for up to six runs without an appreciable loss in its
activity.
Keywords Boehmite nanoparticles Á Palladium-free Á Sonogashira
reaction Á Aryl halide
Introduction
From the time it was discovered in 1975, Sonogashira coupling reaction has been
widely applied to the synthesis of both the synthetically and biologically significant
aryl acetylenes [1, 2]. This reaction generally makes use of Pd(PPh3)2Cl2 or
Pd(PPh3)4 and copper(I) iodide as the catalytic system, which is not favored for
industrial applications because of the high price of palladium [3, 4]. From the
&
Mohammad Bakherad
m.bakherad@yahoo.com
Mahdi Mirzaee
mmirzaee@shahroodut.ac.ir
1
2
School of Chemistry, Shahrood University of Technology, Shahrood 3619995161, Iran
Department of Chemistry, Shahid Beheshti University, Tehran 1983963113, Iran
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M. Bakherad et al.
standpoint of modern organic synthesis, employing costly palladium catalysts and
the demand for a copper (I) salt as a co-catalyst is less appealing, particularly for
industrial-scale syntheses. Consequently, some studies of great importance have
concentrated on the invention of new palladium- and phosphine-free catalytic
systems with notable activities [5–8]. According to these research works, various
catalytically active metals have been proposed for the Sonogashira coupling
reactions, which include the ionic complexes of cobalt [9], iron [10], silver [11],
nickel [12], ruthenium [13, 14], and gold [15, 16] with different ligands. Moreover,
the use of copper salts and complexes have attracted much attention for these types
of coupling reactions due to their lower toxicity and price. Recently, various
phosphine-free ligands such as N,N-dimethylglycine [17], 1,10-phenanthroline [18],
1,4-diazabicyclo[2.2.2]-octane (DABCO) [19], N,N0-dimethylethylenediamine (DMEDA)
[20], 1,10-binaphthyl-2,20-diol (rac-BINOL) [21], diamines [22], pyrimidines [23],
ß-diketones [24], 8-hydroxyquinoline [25], and salicylic acid [26] have been investigated
for this purpose. However, in most of the procedures employed, either a high loading of
copper salts or complexes have been used or they have been applicable only to special
substrates. Furthermore, the question remains how to activate the less reactive aryl
bromides and chlorides for the Sonogashira cross-coupling reactions under phos-
phine- and palladium-free conditions. Additionally, the copper-catalyzed Sonogashira
reactions are conducted using a catalyst produced in situ by mixing the copper salts
with excess free ligands. Although these procedures are reasonably beneficial in both
the small-scale and process chemistry operations, they suffer from many disadvan-
tages. For instance, an excess amount of ligand frequently stops the activity of the
catalytic system, it is not cost-efficient, and it gives rise to a lot of problems in the
succeeding purification of the reaction product from the reaction mixture. As a
consequence, the establishment of a new catalytic system that is readily available, eco-
friendly, easily separable, and mild is still a challenge in modern synthesis.
The heterogeneous metal-supported catalysts have acquired much interest due to
their operational benefits in comparison with their homogeneous counterparts
[27–29]. Nevertheless, only a few examples of the Sonogashira reactions with
heterogeneous copper supported catalysts have been reported in the literature.
Rothenberg and co-workers have introduced cross-coupling of aryl halides with
alkynes using copper nano-clusters as the catalyst in DMF at 110 °C for 24 h [30].
Biffis et al. [31] have reported copper(II) oxide and copper particles supported on
alumina as effective pre-catalysts for the Sonogashira coupling reactions of aryl
iodides in toluene at 110 °C. Furthermore, Saha and co-workers have proposed
successful Sonogashira reactions of alkynes with cis- and trans-styrenyl bromides
catalyzed by a heterogeneous hydroxyapatite-supported Cu(I) catalyst in DMF at
120 °C [32]. Recently, Zhao et al. [33] have reported the Schiff base-pyridine
bidentate copper (I) complex anchored onto MCM-41, which shows high reactivity
and recyclability in a Sonogashira reaction. Although these procedures all give good
product yields, on the one hand, some are sluggish and require at least 24 h and high
temperature for completion, and, on the other hand, as mentioned earlier, the
reactions have to be conducted in a relatively great quantity of copper.
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Palladium- and phosphine-free Sonogashira coupling…
Boehmite is an aluminum oxide hydroxide (c-AlOOH) mineral, a constituent of
aluminum ore bauxite comprising additional hydroxyl groups on its surface. Among
the various techniques used for the production of boehmite nanoparticles (BNPs),
the hydrothermal-assisted sol–gel method has some benefits such as production in a
one-pot process and processing at low temperatures. The most encouraging property
of the hydrothermal-processed BNPs is the development of a highly crystalline
single-phase product with no organic residues [34]. Our strategy for the
establishment of a multi-purpose heterogeneous catalyst was based upon the notion
of immobilizing an organometallic complex onto the BNPs since using the BNPs
catalyst frequently provides an increased catalytic activity due to its high surface
area.
Recently, Ghorbani et al. [35–37] have reported some heterocyclic synthetic
reactions catalyzed by BNPs. Moreover, Mirzaee et al. [38–40] have reported the
epoxidation of different olefins using oxosulfate vanadium(IV) and hexacarbonyl
molybdenum complexes anchored onto the amine and/or Schiff base functionalized
BNPs. We have recently reported the use of unfunctionalized BNPs for the synthesis
of highly substituted imidazoles [41] and 3,4-dihydropyrimidin-2-(1H)-ones [42].
These results encouraged us to establish an additional application for BNPs toward
advanced chemical conversions, particularly the carbon–carbon bond formation
reactions.
In continuation of our interest in the generation of new catalysts for the C–C
bond formation reactions [43–46], herein we report the synthesis and characteri-
zation of the boehmite nanoparticle-anchored Cu(I) diethylenetriamine complex
[BNPs-DETA-Cu(I)], and demonstrate its use in the Sonogashira coupling reactions
in DMF at 70 °C. This catalyst exhibits a high activity in the coupling reactions of
various aryl halides. Moreover, the simplicity of its recovery and its notable recy-
cling effectiveness make it an appropriate system for coupling reactions.
Experimental
General
Materials and methods
Solvents and reagents were supplied from Merck or Fluka and used without any
further purification. Gas chromatographic (GC) analyses were performed using an
Agilent Technologies 6890N chromatograph equipped with a flame ionization
detector (FID). Powder X-ray diffraction (PXRD) patterns were collected on a
Philips PW-1800 or STOE diffractometer with Cu Ka radiation. Scanning electron
microscopy (SEM) studies were conducted on a MIRA3TESCAN-XMU instrument.
Transmission electron microscopy (TEM) was performed using a JEOL JSM-
6360LV transmission electron microscope. Brunauer–Emmett–Teller (BET) surface
area and porosity of nanoparticles were determined with a micromeritics ASAP
2000 automatic analyser. Thermogravimetric analysis (TGA) curves were recorded
using a BAHR STA 503.
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M. Bakherad et al.
Preparation of catalyst
(a) Preparation of BNPs Aluminium 2-butoxide was synthesized from alu-
minium and 2-butanol according to the general method used for the synthesis
of aluminium alkoxides. Aluminium alkoxide thus prepared was vacuum-
distilled in order to obtain a high-quality aluminium alkoxide precursor. For
the hydrothermal-assisted hydrolysis, according to an earlier report [41],
aluminium-2-butoxide (2 M, 10 mL) in 2-butanol was placed in a 300 mL
stainless steel autoclave that contained 50 mL deionized water. The autoclave
was heated for 5 h at 100 °C in an oven. After cooling the autoclave, the
powder produced was filtered off and dried at 100 °C overnight. This powder
was used as the catalyst in the organic reactions.
(b) Preparation of BNPs-Cl For the preparation of the functionalized BNPs,
1.00 g BNPs was refluxed with MSPCl (400.0 lL, 2.2 mmol) in dry toluene
(50.0 mL) for 24 h (Scheme 1). The functionalized BNPs (BNPs-Cl), as a
white solid, was filtered off, washed three times with dry toluene, and dried at
100 °C overnight.
(c) Preparation of BNPs-DETA 1.00 g BNPs-Cl was refluxed with diethylene-
triamine (320 lL, 3 mmol) in dry toluene (50.0 mL) for 48 h (Scheme 1) in
order to replace the terminal chlorine atoms. Finally, diethylenetriamine
functionalized BNPs (BNPs-DETA) was filtered off, washed three times with
dry toluene, and dried at 100 °C.
(d) Loading with CuI In a 50 mL round-bottom flask, BNPs-DETA (1.00 g)
was mixed with CuI (0.1 g, 1.6 mmol) in DMF (5 mL). The mixture was
stirred for 5 h under an N2 atmosphere at room temperature. Then it was
filtered, and the solid obtained was washed with dry toluene and dried at
100 °C overnight to give BNPs-DETA-Cu(I). The copper loading of BNPs-
DETA-Cu(I) was 3.2% (0.5 mmol g-1), determined by inductively coupled
plasma (ICP).
OH
OH
OH
OH
O
O
OCH3
Si
H3CO
Si
OCH3
Cl
+
Cl
Dry toluene
Reflux 24h
DETA,dry toluene
reflux, 48h
OH
OCH3
OH
OH
OH
MSPCl
BNPs
BNPs-Cl
OH
O
O
OH
O
Si
OCH3
N
H
DMF,rt
CuI
Si
OCH3
N
H
O
N
HN
OH
OH
N
H
OH
NH2
OH
ICu
BNPs-DETA-Cu(I)
BNPs-DETA
Scheme 1 Preparation of BNPs-DETA-Cu(I)
123

Palladium- and phosphine-free Sonogashira coupling…
Sonogashira coupling reaction
The typical reaction was carried out with a terminal alkyne (1.0 mmol), an aryl
halide (1.0 mmol), Et3N (2.0 mmol), and the catalyst (0.1 g, 0.05 mmol based on
Cu) in DMF (3.0 mL). The reaction mixture was stirred for 10 h at 70 °C under an
Ar atmosphere. After completion of the reaction, the mixture was filtered to recover
the catalyst. The catalyst was washed with dry toluene, vacuum dried, and stored for
a new run. After GC analysis, the solvent was removed under vacuum, and the crude
product was subjected to silica gel column chromatography using CHCl3/CH3OH
(97:3) as eluent to afford the pure product.
Catalyst recycling
In a typical experiment, after recovering the catalyst from the reaction mixture
under the optimum conditions, it was washed several times with dry toluene, and
then used in the same procedure under the same conditions.
Results and discussion
BNPs-DETA-Cu(I) was synthesized as shown in Scheme 1, wherein the first step is
the synthesis of BNPs via a hydrothermal-assisted sol–gel process [34]. The second
step is the preparation of BNPs-Cl by grafting 3-(tri-methoxy silyl) propyl chloride
(MSPCl) on the surface of BNPs by refluxing the mixture of BNPs and MSPCl in
dry toluene for 24 h [37].
Elemental analysis shows that the carbon content of BNPs-Cl was 10.25%, which
means that 2.1 mmol/g of pending groups were covalently bonded to the surface of
1.00 g BNPs (Table 1). We also determined the loading level of Cl by
potentiometric titration of the chlorine content. For this purpose, 0.03 g of the
sample was added to 5.0 mL of deionized water and titrated by Ag? standard
solution using a potentiometer (Model: LT, Lutron, PH-207). The results showed
that the loading level of Chlorine was *2.1 mmol g-1. In the third step, the
pending chlorine atoms of BNPs-Cl were changed to amine by refluxing with
Table 1 Elemental analysis of BNPs, BNPs-DETA, and BNPs-DETA-Cu(I) samples
Entry
Sample
Analysis
C%
H%
N%
Cu%
–
1
2
3
4
5
BNPs
CHN
CHN
CHN
ICP
NF
10.25
29.31
–
NF
4.24
5.49
–
NF
–
BNPs-Cl
BNPs-DETA
BNPs-DETA-Cu(I)
BNPs-DETA-Cu(I)
6.90
–
–
3.2
3.12a
ICP
–
–
–
NF not found
a
Recycled catalyst
123

M. Bakherad et al.
diethylenetriamine (DETA) in toluene for 48 h to afford BNPs-DETA. The loading
amount of the DETA ligand on BNPs was determined by the C, H, and N elemental
analysis, which reveals that the DETA loading is around 1.64 mmol/g or near 78%
of pending chlorine atoms were changed to DETA by this reaction (Table 1).
Finally, BNPs-DETA and CuI were stirred in DMF at room temperature for 5 h to
generate the BNPs-immobilized copper(I) complex [BNPs-DETA-Cu(I)]. The
loading amount of copper salt was determined by ICP. The details of the analysis
data are presented in Table 1, which reveals that the loading of copper is
approximately 0.5 mmol/g.
The FT-IR spectra for (a) BNPs, (b) BNPs-Cl, (c) BNPs-DETA, and (d) BNPs-
DETA-Cu(I) are shown in Fig. 1. The broad band observed in 3086–3369 cm-1 in
all samples typically indicates the stretching vibrations of the boehmite surface
hydroxyl groups accompanied by a low intensity band in 1073–1155 cm-1 due to its
bending vibrations. The absorption band at around 1630 cm-1 in all samples is due
to the adsorbed water molecules. The new bands observed in the ranges of
2864–2932 and 1384–1474 cm-1 for BNPs-Cl, BNPs-DETA, and BNPs-DETA-
Cu(I) correspond to the CH2 stretching and bending vibrations, respectively. The
newly generated band at 1557 cm-1 in BNPs-DETA and BNPs-DETA-Cu(I) cor-
responds to the N–H bending vibrations in amine grafted groups [47]. The NH2
stretching vibration in the FT-IR spectrum of these amine-grafted samples should be
seen in 3135 cm-1 , but it is masked with a broad band for the hydroxyl group
stretching vibrations of BNPs in this region.
The PXRD pattern for BNPs confirmed its crystallization as a single-phase
boehmite (Fig. 2a). The PXRD patterns for BNPs-DETA and BNPs-DETA-
Cu(I) (Fig. 2b, c) confirm the retention of the crystalline structure after the
functionalization and metal loading. Calculation of the particle size from the PXRD
pattern using the Scherer equation showed 10-nm particles for all the three samples
without any change during functionalization and metal loadings. This is confirmed
by the transmission electron microscopy images for BNPs and BNPs-DETA-
Cu(I) (Fig. 3), which show needle-shaped particles up to 30-nm long and 5-nm
Fig. 1 FT-IR spectra for BNPs (a), BNPs-Cl (b), BNPs-DETA (c), and BNPs-DETA-Cu(I) (d)
123

Palladium- and phosphine-free Sonogashira coupling…
Fig. 2 PXRD patterns for BNPs (a), BNPs-DETA (b), BNPs-DETA-Cu(I) (c)
Fig. 3 Transmission electron microscopy (TEM) images for BNPs (a) and BNPs-DETA-Cu(I) (b)
wide. The surface area of the boehmite nanoparticles was 326 m2/g, according to
the BET experiment.
In order to reveal the surface morphological changes of these samples, the
scanning electron microscopy (SEM) studies were engaged. The SEM micrographs
for the BNPs and BNPs-DETA-Cu(I) samples are shown in Fig. 4. We were
delighted to find that the rod-shaped morphology of BNPs was replicated for the
BNPs sample (Fig. 4a). The coagulate particles of BNPs were also seen in the
BNPs-DETA-Cu(I) sample (Fig. 4b) but the surface morphology was changed after
metal loading. The EDX analysis of BNPs-DETA-Cu(I) (Fig. 5) also confirmed the
presence of Cu, I, Al, Si, O, and C atoms on the surface of this sample.
Figure 6 showsthe TG/DTA thermogram of the BNPs-DETA-Cu(I) catalyst. There
are three important weight-loss regions in the TG curve. The first one is below 100 °C
accompanied by an endothermic peak in the DTA curve, which should be related to the
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M. Bakherad et al.
Fig. 4 Scanning electron microscopy (SEM) images for BNPs (a) and BNPs-DETA-Cu(I) (b)
AlKα
13000
12000
11000
10000
9000
8000
7000
6000
5000
4000
3000
2000
1000
0
O Kα
SiKα
CuLα
C Kα
I Lβ
I Lα
CuKα
CuKβ
keV
0
5
10
Fig. 5 EDX analysis of BNPs-DETA-Cu(I)
elimination of physically adsorbed water. Another weight loss is in the temperature
range of 110–360 °C accompanied by two exothermic peaks in the DTA curve, which
may be related to the decomposition of the organic residue anchored onto the surface
of BNPs-DETA-Cu(I). The last weight loss was observed in the temperature range of
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Palladium- and phosphine-free Sonogashira coupling…
Fig. 6 TG/DTA thermogram of BNPs-DETA-Cu(I) catalyst
470–560 °C accompanied by another endothermic peak in the DTA curve, which
could be related to the dehydroxylation of BNPs and the crystallization of c-alumina.
Catalytic activity
To test the catalytic activity of BNPs-DETA-Cu(I) in the Sonogashira reaction, the
reaction between phenylacetylene and iodobenzene was chosen as a model reaction
using 5.0 mol% of the catalyst in DMF at 70 °C under an Ar atmosphere. The
influences of different parameters were examined to obtain the best possible
combination. The parameters included solvent, base, reaction temperature, and
catalyst concentration. As shown in Table 2 (entries 1–4), the coupling reaction
yields were susceptible to temperature changes with 5.0 mol% catalyst in DMF. The
increase in temperature up to 70 °C led to an excellent yield (95%) after 10 h
heating (Table 2, entry 3). Decreasing the loading of the catalyst to 2.0 mol%
lowered the yield dramatically (Table 2, entry 5). However, increasing the amount
of catalyst to 10 mol% showed no substantial improvement in the yield (Table 2,
entry 6).
The effect of various solvents was then investigated. The model reaction was
examined in the presence of 5.0 mol% of the catalyst at 70 °C in several solvents
including CH3CN, THF, EtOH, and 1,4-dioxane (Table 2, entries 7–10). Among all
these solvents, DMF was found to be the best one, affording the highest yield
(Table 2, entry 3). Moreover, when the reaction was carried out under solvent-free
conditions, the targeted product was obtained with a low yield (Table 2, entry 11).
Our next study focused on the effect of a base on the model reaction. Among the
bases examined, Et3N was found to be the most effective one (Table 2, entry 3).
Diisopropyl ethylamine (DIPEA), pyrrolidine, and pyridine also gave good yields
(Table 2, entries 12–14), whereas inorganic bases such as Na2CO3 and KOH were
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Palladium- and phosphine-free Sonogashira coupling…
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M. Bakherad et al.
substantially less effective (Table 2, entries 18 and 19). To find out the roles of
BNPs and BNPs-DETA-Cu(I) during the Sonogashira coupling reaction, the model
reaction was examined in the presence of BNPs. As shown in Table 1, in the
presence of BNPs, Sonogashira coupling could not be achieved (Table 2, entry 20).
Thus, the optimized reaction conditions for this Sonogashira coupling reaction were
BNPs-DETA-Cu(I) (5.0 mol%) in DMF using Et3N as base at 70 °C under an Ar
atmosphere for 10 h (Table 2, entry 3).
Under the optimized reaction conditions, the scope of the reaction was explored
with various terminal alkynes and aryl halides. As shown in Table 3, the coupling of
phenylacetylene with phenyl iodide took place smoothly at 70 °C in the presence of
5 mol% of the catalyst to give a quantitative yield of diphenylacetylene (entry 1). It
could be concluded that aryl iodides bearing an electron-donating or electron-
withdrawing group were obtained in high yields. For example, the Sonogashira
coupling reaction of phenylacetylene with p-nitroiodobenzene, p-chloroiodoben-
zene, p-iodo toluene, and p-iodoanisol gave the corresponding biarylacetylenes in
97, 96, 92, and 90% yields, respectively (Table 3, entries 2–5). When the less
reactive acetylene, 1-hexyne, 1-octyne, and propargyl alcohol were used, the
coupling product was produced efficiently. The coupling of para-substituted
iodobenzene having nitro, chloro, and methyl groups took place with 1-hexyne or
1-octyne to give the corresponding products in high yields (Table 3, entries 7–9 and
11–13). The coupling reaction of propargyl alcohol with the more reactive electron-
withdrawing p-nitroiodobenzene gave a good yield (90%) (entry 15). Under the
same conditions, the less active electron-rich p-iodotoluene produced a lower yield
(entry 16).
To extend the scope of our work, we next investigated the coupling of various
aryl bromides with terminal alkynes. As expected, aryl iodides were more reactive
than aryl bromides, and the substituent effects in the aryl iodides appeared to be less
significant than in the aryl bromides. However, as shown in Table 3, a high catalytic
activity was observed in the coupling of unactivated aryl bromides such as
p-nitrobromobenzene (entries 18, 23, 25, and 27), as well as activated
p-nitroiodobenzene (entries 2, 7, 11, and 15). Although aryl iodides and bromides
were found to be effective substrates, unfortunately, this catalyst failed to produce
biaryl compounds when chlorobenzene was used as a substrate (Table 3, entries 29
and 30). The low reactivity of chlorides resulted from the strength of the C–Cl bond,
whose bond dissociation energy was 96 kcal/mol [48].
We also investigated the recyclability of BNPs-DETA-Cu(I) using the coupling
reaction of iodobenzene with phenylacetylene. As shown in Fig. 7, the recovered
catalyst was added to the reaction mixture under the same conditions for five cycles
without significant losses in the yield and catalytic activity. In addition, copper
leaching from the supported catalyst was determined. The copper content (3.12
wt%) of the recovered catalyst remained just about the same as the fresh catalyst,
indicating that Cu leaching was negligible (Table 1, entry 4). Hot filtration test was
also conducted and confirmed negligible leaching in this catalytic protocol because
by removing the catalyst in the middle of reaction progress after 5 h, the reaction
stopped and the yield did not raised anymore (Table 2, entry 21). In addition, in this
catalytic coupling reaction a single product with high yield was produced which
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Palladium- and phosphine-free Sonogashira coupling…
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M. Bakherad et al.
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Palladium- and phosphine-free Sonogashira coupling…
Fig. 7 Synthesis of product 1 catalyzed by recycled catalyst
Scheme 2 Proposed mechanism
along with above results could proposed the presence of static single type active
center in this catalytic protocol according to the Eremin, and Ananikov report [49].
We suggest that a terminal alkyne coordinates to the copper (I) species BNPs-
DETA-CuI (I) to provide a BNPs-immobilized copper acetylide (II) under the basic
conditions. Then intermediate (II) reacts with the aryl halide via a four-centered
transition state (III) [50] to afford the desired coupling product (3) and regenerate
BNPs-DETA-CuI (I) (Scheme 2).
Conclusion
We described the synthesis and characterization of a novel diethylenetriamine-
functionalized-boehmite nanoparticle-supported Cu(I). The catalyst was character-
ized by various methods including XRD, TEM, SEM, EDX, and FT-IR spectroscopy.
The catalytic efficacy of the BNPs-DETA-Cu(I) catalyst was evaluated in Sonogashira
coupling reactions, which led to better yields under moderate conditions compared
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M. Bakherad et al.
with those given in the literature. Furthermore, aryl iodides and aryl bromides showed
high yields, satisfactory in Sonogashira coupling reactions, using this catalyst. The
recycling experiments showed that the catalyst could be used for several times without
a significant loss in its catalytic activity.
Acknowledgements We gratefully acknowledge the financial support of the Research Council of the
Shahrood University of Technology.
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