Anchoring Mn(IV) in multi pyridine modified mesoporous silica SBA-15: An efficient nanocatalyst for selective oxidation of sulfides to sulfoxides

Article abstract of DOI:10.1016/j.poly.2020.114359

In this current study we report the implementation of Mn(IV) grafted multipyridine linked melanine functionalized SBA-15/CCPy/Mn(OAc)₂ as a novel high surface area nanocatalyst. The melamine linked multipyridine ligand was introduced over mesop

Full text of DOI:10.1016/j.poly.2020.114359

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Anchoring Mn(IV) in multi pyridine modified mesoporous silica SBA-15 : An
efficient nanocatalyst for selective oxidation of sulfides to sulfoxides
Saba Hemmati, Malak Hekmati, Delafarin Salamat, Mohammad Yousefi,
Bikash Karmakar, Hojat Veisi
PII:
S0277-5387(20)30016-4
DOI:
https://doi.org/10.1016/j.poly.2020.114359
POLY 114359
Reference:
Polyhedron
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28 August 2019
31 December 2019
11 January 2020
Please cite this article as: S. Hemmati, M. Hekmati, D. Salamat, M. Yousefi, B. Karmakar, H. Veisi, Anchoring
Mn(IV) in multi pyridine modified mesoporous silica SBA-15 : An efficient nanocatalyst for selective oxidation of
sulfides to sulfoxides, Polyhedron (2020), doi: https://doi.org/10.1016/j.poly.2020.114359
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Anchoring Mn(IV) in multi pyridine modified mesoporous silica SBA-15 : An
efficient nanocatalyst for selective oxidation of sulfides to sulfoxides
Saba Hemmati,*,a Malak Hekmati,b Delafarin Salamat,c Mohammad Yousefi,*,d Bikash Karmakar,*,e Hojat
Veisia
aDepartment of Chemistry, Payame Noor University, Tehran, Iran
bDepartment of Organic Chemistry, Faculty of Pharmaceutical Chemistry, Tehran Medical Sciences, Islamic Azad
University, Tehran, Iran
cDepartment of Chemistry, Science and Research Branch, Islamic Azad University, Tehran, Iran
dDepartment of Chemistry, Yadegar-e-Imam Khomeini (RAH) Shahr-e-Rey Branch, Islamic Azad University, Tehran,
Iran
eDepartment of Chemistry, Gobardanga Hindu College, 24-Parganas (North), India
E-mails:
s_organo2007@yahoo.com
(S.
Hemmati),
myousefi50@hotmail.com
(M.
Yousefi),
bikashkarm@gmail.com (B. Karmakar)
Abstract
In this current study we report the implementation of Mn(IV) grafted multipyridine linked
melanine functionalized SBA-15/CCPy/Mn(OAc)2 as a novel high surface area nanocatalyst. The
melamine linked multipyridine ligand was introduced over mesoporous SBA-15 to ensure the
strong chelation of Mn(OAc)2 in order to deploy as active site. The structural features of the so
designed material were properly analyzed with SEM, TEM, EDX, ICP-AES and N2 adsorption-
desorption isotherm techniques. Catalytic behavior of the nanocomposite was investigated in the
chemoselective oxidation of sulfides to sulfoxides using H2O2 as a mild oxidant under solvent-free
conditions at room temperature resulting very good yields. The catalyst was reused up to 8
consecutive cycles without significant leaching and reduced catalytic performance.
Keywords: Mesoporous, poly-ligand, Mn(IV), Hydrogen peroxide, oxidation, Sulfides,
Sulfoxides, reusable
1

1. Introduction
The metal ion-Schiff base complexes have been of special interest in academia and industries due
to their profuse applications in different arena like catalysis, biomedical investigations and
electrochemistry [1-4]. Among different metals, the transition metals acquire great affinity to form
broad range of complexes with different kinds of organic ligands. Especially, their Schiff base
complexes have been largely applied as effective homogeneous catalysts. Nevertheless, the issues
of catalyst isolation, regeneration and reusability have been the major drawbacks during the
reactions [5-12]. Thereby, heterogenization of those catalysts by immobilizing over different solid
supports like organosilicon compounds [13,14], Fe3O4 nanoparticles [15], carbon nanotubes [16],
mesoporous silica [17,18], zeolites [19,20] and layered materials [21] has been a significant area
of concern. In other words, technically, the surface engineering on solid supports by covalent
bonding of active catalytic sites has come into prominence to design a quasi-heterogeneous
catalytic system in order to adopt several advantages like introducing milder environments,
evading the direct contact of hazardous substances and easy recovery and reuse of catalyst
following green chemistry standards [22-26].
Now, among various solid heterogeneous supports [27-30], the ordered mesoporous silica (OMS)
matrixes acquire special priority. In the last few decades, the catalysis fraternity has extensively
accepted SBA-15 as heterogeneous support because of its highly ordered hexagonal honeycomb
like morphology with horizontally packed arrays, significantly high surface area, large pore sizes
appropriate for organic molecules to fit in, high thermal stability and rigid configuration [31-34].
Moreover, the external surface hydroxyl groups of SBA-15 could be utilized for surface
functionalization.
2

As part of our ongoing research works based on the development of novel heterogeneous catalysts
[35], we exploited herein the multipyridine linked melamine organo-moiety as ligand to form
covalent bonding with the mesoporous silica surface and thereafter effectively chelated the
Mn(OAc)2 salt as a secondary shell. Such surface modifications decrease the affinity of active sites
to agglomerate and thereby make them significantly stable, prevent their oxidation and decrease
solubility in organic solvents [36, 37]. Subsequently, we investigated the catalytic performance of
the so developed material in the chemo-selective oxidation of sulfides to sulfoxides using H2O2 as
a green oxidant.
In recent years sulphoxide chemistry has got tremendous impetus. Organic sulfoxides are found
important as chemical intermediates, in medicinal biology as cardiotonic agents, anti-hypertensive,
vasodilators, antibacterial, antifungal and anti-atherosclerotics [38-41]. In this current study, we
report the chemoselective synthesis of organic sulphoxides catalyzed over Mn(IV) catalyst
anchored with multi-pyridine linked melanine functionalized mesoporous silica SBA-15 [SBA-
15/CCPy/Mn(OAc)2] (Scheme 1). Heterogeneity of the novel material was investigated via
leaching test. It was reused for several times as well with minimal change in its catalytic activity.
3

H2N
H2N
NH2
Si
Si
O
Si
O
O
O
O
O
OH OH OH OH OH
Si
Si
Si
Si
Si
Si
Si
Si
Si
Si
O
O
O
O
O
O
O
APTES
Toluene, 24 h, reflux
SBA-15
SBA-15
SBA-15
SBA-15
DIEA
Cyanuric chloride (CC)
N
N
THF, 0 C
N
N
N
N
Cl
N
N
Cl
N
N
N
N
N
N
N
N
N
N
N
N
Cl
H2N
N
HN
H2N
HN
N
NH
N
Cl
NH
N
Si
O
Si
O
Si
O
Si
Si
O
Si
O
O
O
O
O
O
O
O
O
Si
Si
Si
Si
Si
Si
Si
Si
Si
Si
Dipyridylamine
DIEA
CH3CN
r.t-reflux/15 h
O
O
O
O
O
SBA-15
SBA-15
SBA-15
SBA-15
Mn(OAc)2
H2O
AcO
N
OAc
OAc
AcO
N
Mn
Mn
N
N
N
N
N
N
N
N
N
N
N
H2N
HN
OAc
N
N
NH
N
Mn
AcO
N
Mn
OAc
N
AcO
Si
O
Si
O
Si
O
O
O
O
O
O
Si
Si
Si
Si
Si
O
SBA-15
SBA-15
SBA-15
Scheme 1. Preparation of SBA-15/CCPy/Mn(OAc)2.
4

2. Experimental
2.1 Materials and methods
All commercially available chemicals required for the preparation of catalyst were purchased from
Aldrich and Fluka. For organic synthesis the corresponding chemicals and different solvents were
bought from Merck. All of them were used as such without further purification. Morphology of
the mesostructured material were determined by SEM technique using FESEM-TESCAN MIRA3.
TEM analysis was carried out with a Philips CM10 microscope operating at 200 Kv. The elemental
analysis of the samples was done by Energy dispersive X-ray spectroscopy (EDAX, TSCAN).
2.2 Synthesis of functionalized organo-functionalized SBA-15
The mesoporous silica SBA-15 was synthesized according to our previously reported method [42].
3.0 g SBA-15 was suspended in 30 mL anhydrous toluene under stirring in a 100 round bottomed
flask. For amino functionalization 1.5 mmol of aminopropyl triethoxy silane (APTES) was then
added to it dropwise and the mixture was refluxed for 24 h at N2 atmosphere. It was then filtered
and washed successively with toluene, dichloromethane and methanol. The aminopropyl
derivative of SBA-15 was obtained as a white powder after drying under vacuum at 80 oC for 10
h. Subsequently, 3.0 g of the initial derivative was suspended in 35 mL tetrahydrofuran (THF) and
then 0.5 mL diisopropyl ethylamine (DIPEA) was added. A 5 mL THF solution of cyanuric
chloride (CC) was added to the mixture at 0 oC and then stirred at room temperature for another 2
h. Then the solution was decanted and the residue was washed with 50 mL THF. Without isolation,
the material was again suspended in 25 mL acetonitrile and 1.0 mL DIPEA was added at room
temperature. 1.2 g of dipyridylamine (Py) (7.0 mmol) was further added at room temperature and
the mixture was refluxed for 15 h. Finally, the resulting solid was filtered, washed with DI water
5

followed by acetone. Thereby, the SBA-15/CCPy nanocomposite was obtained after drying at 50
oC for 12 h.
2.3 Synthesis of Mn(OAc)2 immobilized SBA-15/CCPy [SBA-15/CCPy/Mn(IV)]
For the Mn anchoring procedure, 1.0 g of SBA-15/CCPy was suspended in 50 mL deionized water
via sonication. A 10 mL aqueous solution containing 0.17 g of Mn(OAc)2 salt (1.0 mmol) was
added slowly to the suspension and the mixture was stirred at 25 °C for 24 h. The solid product
was isolated by filtration and washed several times with water and ethanol successively. Finally
the nanocomposite was dried under vacuum at 100 ˚C for 24 h to have SBA-15/CCPy/Mn(OAc)2.
Elemental analysis of the material via ICP-AES technique revealed 0.71 mmol/g loading of Mn.
2.4. General procedure to synthesize sulfoxides
To 1.0 mmol organic sulfide 1.5 mmol 30% (w/w) H2O2 was added at solvent-free condition in
presence of 15 mg SBA-15/CCPy/Mn(OAc)2 (1 mol%) catalyst. The mixture was stirred for
requisite time at ambient temperature. Progress of the reaction was monitored by thin layer
chromatography. After completion of the reaction, the catalyst was isolated from the mixture by
filtration and the products were extracted with Et2O and concentrated to afford the pure products.
3. Results and discussion
The novel material SBA-15/CCPy/Mn(OAc)2 was synthesized through post-functionalization over
the high surface matrix. The structural morphology and physicochemical behaviors of the material
were investigated through different analytical techniques like Scanning Electron Microscopy
(SEM), Transmission Electron Microscopy (TEM), Energy Dispersive X-ray Spectroscopy
(EDX), ICP-AES and BET surface area analysis. The SEM image demonstrates a rod-shaped
morphology being aggregated into bundles (Fig. 1a). The small dots distributed over the surface
are indication of possible surface modifications and metal doping. A better structural insight can
6

be observed from TEM images. The three dimensional parallel channel of hexagonal pores,
characteristic of SBA-15, are clearly visible from Fig. 1b. The dark shades over the surface are
possibly due to the surface modifications and Mn insertion. It clearly reveals that functionalization
over SBA-15 did not affect the basic morphology of the material.
Fig. 1. a) SEM and b) TEM images of SBA-15/CCPy/Mn(OAc)2.
The EDX analysis confirms the presence of N, O, C, Si and Mn as elements present in the prepared
material (Fig. 2). N, O and C elements justifies the surface functionalization over the Si surface.
7

Fig. 2. EDX analysis of SBA-15/CCPy/Mn(OAc)2 nanocomoposite.
While dealing with mesoporous materials surface analysis is essential criteria. Thereby, nitrogen
gas adsorption-desorption isotherm analysis was performed for the SBA-15/CCPy/Mn(OAc)2
material and the corresponding data have been depicted in Table 1. The bare SBA-15 matrix exerts
an ample surface area of 752 m2/g as determined by BET method. However, due to surface loading
it was enormously reduced to 371 m2/g. The pore volume and mean pore diameter also were
decreased because of the modification.
Table 1. The structure parameters of SBA-15 and SBA-15/CCPy/Mn(OAc)2
Entry Samples
SBET
(m2g-1)
752
Total pore volume
(cm3g-1)
Mean pore diameter
(nm)
7.02
1
2
SBA-15
0.810
SBA-15/ CCPy/Mn(OAc)2
371
0.619
4.62
8

After the meticulous structural analysis, it was the turn of investigating catalytic performance of
SBA-15/CCPy/Mn(OAc)2 which was done in the selective oxidation of organic sulphides. In view
of maintaining green chemistry criteria we used 30% H2O2 as non-hazardous oxidant and carried
out the reactions at room temperature under solvent-free conditions (Scheme 2).
AcO
N
OAc
OAc
AcO
N
Mn
Mn
N
N
N
N
N
N
N
N
N
N
N
H2N
HN
OAc
N
N
NH
N
Mn
AcO
N
Mn
OAc
N
AcO
Si
Si
O
Si
O
O
O
O
O
O
Si
Si
Si
Si
Si
O
O
SBA-15
SBA-15
SBA-15
O
S
S
R2
R1
R1
R2
H2O2 (30%), r.t.
Solvent-free
Scheme 2. Catalytic application of SBA-15/CCPy/Mn(OAc)2 towards the synthesis of sulfoxides
from sulfides using H2O2.
However, prior to any generalized reaction protocol, it was obligatory to investigate the best
possible conditions and thereby the oxidation of methyl phenyl sulfide was selected as a reference
reaction. Different reaction parameters like catalyst mol%, H2O2 mol% and solvent were imposed
thereupon. The results have been documented in Table 2. The reaction was not much successful in
the absence of SBA-15/CCPy/Mn(OAc)2 catalyst even after a prolonged run (Table 2, entries 1-2)
ensuring the active role of Mn. 1 mol% Mn loaded catalyst was found to be optimum among
different concentrations (Table 2, entries 3-6). Again, keeping the best catalyst a screening of
solvent was carried out and among them solvent-free condition resulted the best (Table 2, entries
9

8-11). Ultimately, 1 mol% catalyst (15 mg), 2.0 mmol H2O2 and solvent-free conditions at room
temperature were chosen as optimum conditions to convert methyl phenyl sulfide to corresponding
sulfoxide with excellent yields (Table 2, entry 5).
Table 2. Optimization of conditions for the conversion of methyl phenyl sulfide to methyl phenyl
sulfoxide over SBA-15/CCPy/Mn(OAc)2 and H2O2
Entry Catalyst (mol%)
H2O2 (mmol) Solvent
Time (min) Yield (%)a
1
Catalyst-free
2.0
2.0
2.0
2.0
2.0
2.0
1.0
3.0
2.0
2.0
2.0
Solvent-free
12 h
12 h
2 h
45
35
40
95
95
95
95
88
80
95
95
85
2
SBA-15/CCPy (10)
Solvent-free
Solvent-free
Solvent-free
Solvent-free
Solvent-free
Solvent-free
Solvent-free
EtOH
3
SBA-15/CCPy/Mn(OAc)2 (0.25)
SBA-15/CCPy/Mn(OAc)2 (0.50)
SBA-15/CCPy/Mn(OAc)2 (1.00)
SBA-15/CCPy/Mn(OAc)2 (1.25)
SBA-15/CCPy/Mn(OAc)2 (1.00)
SBA-15/CCPy/Mn(OAc)2 (1.00)
SBA-15/CCPy/Mn(OAc)2 (1.00)
SBA-15/CCPy/Mn(OAc)2 (1.00)
SBA-15/CCPy/Mn(OAc)2 (1.00)
4
5
10
6
10
7
45
8
10
9
45
10
CH3CN
45
11
CH2Cl2
3 h
aDetermined by GC.
After settling the optimum conditions, the scope and generality of the recommended approach as
an easy and effective technique for the oxidation of aliphatic and aromatic sulfides was studied
and the results are shown in Table 3. A wide variety of aliphatic and aromatic sulphides are well
compatible under the protocol affording good to excellent yields. The sulfides with sensitive
functional groups (OH, CN and COOMe) could be oxidized with tremendous chemo-selectively
(Table 3, entries 6-8). The turnover frequencies (TOF) and turnover numbers (TON) in all the
cases were found to be outstanding at very short reaction times.
10

Table 3. Selective oxidation of sulfides to sulfoxides in the presence of SBA-15/CCPy/Mn(OAc)2
under optimized conditions.a
Sulfide
Sulfoxide
Time Yield
(min) (%)b
Entry
1
TOF TON
(h-1)c (h-1)d
S
O
S
10
10
10
15
15
95
95
96
85
90
570
95
S
O
S
2
3
4
5
570 95
S
O
S
Ph
Ph
576
340
360
96
S
O
S
85
90
S
O
S
O2N
O2N
O
S
S
6
7
8
8
95
88
75
712.5
528
90
88
75
OH
OH
O
S
S
CN
CN
10
12
O
S
S
375
COOMe
COOMe
O
S
S
S
8
12
12
85
88
425
440
85
88
O
S
10
aReaction conditions: sulfide (1 mmol), 30% H2O2 (2.0 mmol), SBA-15/CCPy/Mn(OAc)2(15 mg, 1.0 mol%), rt.
bIsolated yield.
cTOF = (Yield/Time)/Amount of catalyst (mol).
dTON = Yield/Amount of catalyst (mol).
In order to explore the reusability of SBA-15/CCPy/Mn(OAc)2, the catalyst was taken out of the
reaction mixture after completion, regenerated after drying and applied once again for the next
11

run. To our anticipation, the catalyst could be used up to 8 successive runs in oxidizing methyl
phenyl sulfide to the its sulfoxide (Fig. 3). After the successful 7 runs, the catalyst was carefully
filtered out of the reaction mixture and leaching of Mn into the solution was evaluated by ICP-
AES check which resulted only a 1.12%, showing the robustness of catalyst.
Fig. 3. Reusability of SBA-15/CCPy/Mn(OAc)2 nanocatalyst.
To determine the heterogeneity of the catalyst, a hot filtration test was conducted for the same
reaction under the optimized conditions. After a half time of the reaction, the catalyst was isolated
from the reaction mixture by filtration and the reaction was allowed to continue. There was no
further increase in the yield, verifying the heterogeneity of the catalyst.
The viability of our protocol in the oxidation of sulfides was further justified by comparison with
literature reports. As shown in Table 4, the previously reported procedures suffer from several
drawbacks like longer reaction times, use of costlier transition metals, need of volatile and/or toxic
organic solvents and much lower TOF. Evidently, our method is superior to them in overcoming
all the said drawbacks which ascertains the efficiency of SBA-15/CCPy/Mn(OAc)2 catalyst for
selective sulfide oxidations.
Table 4. Comparison of our work with reported procedures in the synthesis of methyl phenyl
sulfuxide.
12

Entry Reaction conditions
Time Yield TOF
Ref.
(min)
%
(h-1)
570
1
2
3
4
5
6
7
8
9
10
SBA-15/CCPy/Mn(OAc)2, solvent-free, R.T.
10
95
This work
[43]
MNPs-Pr-NHSO3H, solvent-free, R.T.
MCM-Mo, CH3CN, 40 ºC
80
100
99
78.9
839
27.3
24.5
98
60
[44]
SiO2-based tungstate interphase, CH3OH/CH2Cl2, R.T.
Peroxotungstate/SiO2, CH3OH/CH2Cl2, 8 ºC
VO(acac)2/Resin, H2O, R.T.
90
82
[45]
150
10
92
[46]
98
[47]
Fe3O4@SiO2/Mo, CH3CN, 50 ºC
90
92
20.4
[48]
c
VO(IV)–MCM-41, EtOH, R.T.
480
10
95
-
[49]
c
VO(IV) Schiff base complexes, EtOH, R.T.
VO(IV) Schiff base complex, solvent-free,45 °C
100a
-
[50]
b
c
50
-
-
[51]
aConversion (%).
bThe major product is sulfone.
cThe catalyst’s mol% is not reported.
4. Conclusion
In conclusion, we have developed a post grafting approach to synthesize the novel Mesoporous
nanocatalyst of Mn(OAc)2 immobilized on multipyridine linked melamine modified SBA-15, i.e.
SBA-15/CCPy/Mn(OAc)2. We applied the material in the selective oxidation of various sulfides
to their mono-sulfoxides using H2O2 as green oxidant at room temperature under solvent-free
conditions. All the products were obtained in high yield with excellent TOF. The catalyst was
sufficiently stable as it could be reused as much as eight times. There was negligible amount of
leaching of active species into the reaction mixture as well, as confirmed from ICP-AES studies.
A comparison study with the reported procedures also justified the uniqueness of this protocol.
Acknowledgments
The authors gratefully acknowledge the financial and other support of this research, provided by
Payame Noor University, Tehran Medical Sciences, Islamic Azad University and Yadegar-e-Imam
13

Khomeini (RAH) Shahr-e-Rey Branch, Islamic Azad University, Tehran, Iran. B.K. thanks
Gobardanga Hindu College for providing research facilities.
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16

Graphical Abstract
Anchoring Mn(IV) in multi pyridine modified mesoporous silica SBA-15: An
efficient nanocatalyst for selective oxidation of sulfides to sulfoxides
Saba Hemmati,*Malak Hekmati, Delafarin Salamat, Mohammad Yousefi,*Bikash Karmakar,*
Hojat Veisi*
AcO
OAc
OAc
AcO
N
Mn
N
Mn
N
N
N
N
N
N
N
N
N
N
N
N
H2N
HN
OAc
N
NH
N
Mn
AcO
N
Mn
OAc
N
AcO
Si
Si
O
Si
O
O
Si
O
Si
O
O
Si
O
Si
Si
O
O
SBA-15
SBA-15
SBA-15
O
S
H2O2 (30%), r.t.
Solvent-free
S
R2
R1
R1
R2
Conflict of Interest
The authors report no conflicts of interest in this work.
17

AcO
N
OAc
OAc
AcO
N
Mn
Mn
N
N
N
N
N
N
N
N
N
N
N
H2N
HN
OAc
N
N
NH
N
Mn
AcO
N
Mn
OAc
N
AcO
Si
Si
O
Si
O
O
O
O
O
O
Si
Si
Si
Si
Si
O
O
SBA-15
SBA-15
SBA-15
O
S
H2O2 (30%), r.t.
Solvent-free
S
R2
R1
R1
R2
Graphical Abstract
Anchoring Mn(IV) in multi pyridine modified mesoporous silica SBA-15: An
efficient nanocatalyst for selective oxidation of sulfides to sulfoxides
Saba Hemmati,*Malak Hekmati, Delafarin Salamat, Mohammad Yousefi,*Bikash Karmakar,*
Hojat Veisi*
18

AcO
N
OAc
OAc
AcO
N
Mn
Mn
N
N
N
N
N
N
N
N
N
N
N
N
H2N
HN
OAc
N
NH
N
Mn
AcO
N
Mn
OAc
N
AcO
Si
Si
O
Si
O
O
Si
O
Si
O
O
Si
O
Si
Si
O
O
SBA-15
SBA-15
SBA-15
O
S
H2O2 (30%), r.t.
Solvent-free
S
R2
R1
R1
R2
Dear Editor
Please attached find a copy of the manuscript entitled “Anchoring Mn(IV) in multi pyridine
modified mesoporous silica SBA-15: An efficient nanocatalyst for selective oxidation of sulfides to
sulfoxides” which I would like you to consider it to be published in the Polyhedron.
Due to the aforementioned motives and also as a component of our enduring research program on
the utilization of catalysts for the development of beneficial new synthetic methodologies [36], we
reporting on the technique implemented in SBA-15/CCPy/Mn(OAc)2 preparation as a novel
Mesoporous nanocatalyst, while melamine bearing pyridine groups on SBA-15 were applied for
securing Mn(OAc)2 catalyst as capping agent. TEM, SEM, ICP, BET, and EDX analysis were
done to specify the resultant catalyst. The chemoselective oxidation of sulfides was catalyzed by
SBA-15/CCPy/Mn(OAc)2 to sulfoxides with hydrogen peroxide as a green oxidant in high yields
at room temperature. The catalyst was recycled up to 7 times with no significant leaching and
reduced catalytic activity.We believe that this paper is well-suited for publication in the
Polyhedron.
19

I have to add that this submission is original and all co-authors are aware of the submission and
agree to its publication in the Journal. This work is novel/original and is not under consideration
for publication elsewhere.
Thank you, in anticipation, for your consideration of this matter.
With the sincere regards
Hojat Veisi,
Associat. Prof. of Org. Chem.
Department of Chemistry, Payame Noor University (PNU), Iran
E-mail: hojatveisi@yahoo.com
20