Synthesis, characterization, density functional theory studies and antibacterial activity of a new Schiff base dioxomolybdenum(VI) complex with tryptophan as epoxidation catalyst

Article abstract of DOI:10.1002/aoc.3782

A cis-dioxomolybdenum(VI) complex was prepared with MoO₂(acac)₂ and a Schiff base ligand (2-((2-hydroxybenzylidene)amino)-3-(1H-indol-3-yl)propanoic acid) derived from salicylaldehyde and l-tryptophan in ethanol and designated as [MoO₂(Sal-Tryp)(EtOH)]. It was characterized using several techniques including thermogravimetric and elemental analyses and mass, Fourier transform infrared and UV–visible spectroscopies. Theoretical calculations were performed using density functional theory for studying the molecular structure. An in vitro antibacterial activity evaluation showed that [MoO₂(Sal-Tryp)EtOH] complex exhibits good inhibitory effects against Gram-positive (Bacillus subtilis, Staphylococcus aureus) and Gram-negative (Escherichia coli, Pseudomonas aeruginosa) bacteria in comparison to standard antibacterial drugs. It was also found that [MoO₂(Sal-Tryp)EtOH] complex successfully catalyses the epoxidation of cyclooctene, norbornene, cyclohexene, styrene, α-methylstyrene and trans-stilbene, with 45–100% conversions and 64–100% selectivities. Based on the obtained results, the heterogeneity and reusability of the catalyst seem promising.

Full text of DOI:10.1002/aoc.3782

Received: 29 November 2016
DOI 10.1002/aoc.3782
Revised: 15 January 2017
Accepted: 1 February 2017
F U L L PA P E R
Synthesis, characterization, density functional theory studies and
antibacterial activity of a new Schiff base dioxomolybdenum(VI)
complex with tryptophan as epoxidation catalyst
1
1
1
2
Zeinab Asgharpour | Faezeh Farzaneh
| Mina Ghiasi | Mohammad Azarkish
1
Department of Chemistry, Faculty of
A cis‐dioxomolybdenum(VI) complex was prepared with MoO (acac) and a Schiff
base ligand (2‐((2‐hydroxybenzylidene)amino)‐3‐(1H‐indol‐3‐yl)propanoic acid)
derived from salicylaldehyde and L‐tryptophan in ethanol and designated as
Physics and Chemistry, Alzahra University,
PO Box 1993891176, Vanak, Tehran, Iran
2
2
2
Department of Chemistry, Payame Noor
University (PNU), 19395‐4697 Tehran, Iran
[
MoO (Sal‐Tryp)(EtOH)]. It was characterized using several techniques including
2
Correspondence
thermogravimetric and elemental analyses and mass, Fourier transform infrared
and UV–visible spectroscopies. Theoretical calculations were performed using den-
sity functional theory for studying the molecular structure. An in vitro antibacterial
Faezeh Farzaneh, Department of Chemistry,
Faculty of Physics and Chemistry, Alzahra
University, PO Box 1993891176, Vanak,
Tehran, Iran.
Email: faezeh_farzaneh@yahoo.com;
Farzaneh@alzahra.ac.ir
activity evaluation showed that [MoO (Sal‐Tryp)EtOH] complex exhibits good
2
inhibitory effects against Gram‐positive (Bacillus subtilis, Staphylococcus aureus)
and Gram‐negative (Escherichia coli, Pseudomonas aeruginosa) bacteria in com-
parison to standard antibacterial drugs. It was also found that [MoO (Sal‐Tryp)
2
EtOH] complex successfully catalyses the epoxidation of cyclooctene, norbornene,
cyclohexene, styrene, α‐methylstyrene and trans‐stilbene, with 45–100% conver-
sions and 64–100% selectivities. Based on the obtained results, the heterogeneity
and reusability of the catalyst seem promising.
KEYWORDS
amino acid Schiff base ligand, antibacterial activity, dioxomolybdenum(VI)complex, epoxidation catalyst
1
| INTRODUCTION
of molybdenum(VI) Schiff base complexes possessing
[
6]
Mo═O in electrochemistry, particular attention has been
[
7,8]
Molybdenum is an essential metal that is less toxic than many
other metals of industrial importance. The coordination
chemistry of molybdenum(VI) has attracted considerable
attention due to the enzymatic role of molybdenum in reduc-
tion reactions of molecular nitrogen and nitrate in plants and
paid to applications such as antibacterial
and
[
9]
[10–12]
antitumour
sulfide oxidation,
of benzyl alcohols.
agents, olefin epoxidation catalysts,
[
13,14]
[15]
hydrogen generation and oxidation
Schiff base complexes derived from
[
16]
amino acids have attracted much attention because of their
[1,2]
[17–19]
hydroxylation of xanthine and aldehydes in animals.
inorganic and biological importance.
Schiff base
The ability of molybdenum to form stable complexes
complexes derived from salicylaldehyde or 2‐
hydroxynaphthaldehyde with amino acids can be used as
non‐enzymatic models analogous to intermediates in meta-
bolic reactions of amino acids such as transamination, race-
mization, decarboxylation and α‐ or β‐elimination.
The epoxidation of alkenes is one of the most widely
studied reactions in organic chemistry, since epoxides are
with oxygen‐, nitrogen‐ and sulfur‐containing ligands led to
[
3]
development of molybdenum Schiff base complexes.
Molybdenum dioxo Schiff base complexes are excellent
enzyme model systems for the active sites of transferase
enzymes like nitrate reductase, the active sites of which con-
[
20,21]
[4,5]
sist of a cis‐MoO moiety.
In addition to the application
2
Appl Organometal Chem. 2017;e3782.
wileyonlinelibrary.com/journal/aoc
Copyright © 2017 John Wiley & Sons, Ltd.
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https://doi.org/10.1002/aoc.3782

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ASGHARPOUR ET AL.
key starting materials for forming a wide variety of products
such as alcohols, polyethers and aldehydes which have wide-
spread application in the chemical and pharmaceutical indus-
tries, and in epoxy resins, epoxy paints, surfactants,
pesticides, agrochemicals, perfume materials and sweet-
phenylmethylsiloxane capillary and an Agilent 5973 Net-
work, mass selective detector, HP‐5 MS 6989 Network GC
system.
2
.2 | Synthesis of [MoO (Sal‐Tryp)(EtOH)]
[
22,23]
2
eners.
The type of ligand structure present in the com-
plex and the catalytic reaction conditions have a significant
effect on the catalytic activity of complexes involving the
Salicylaldehyde (0.1 ml, 1 mmol, in 2 ml of ethanol) was
introduced into a tryptophan solution (204 mg, 1 mmol, in
2 ml of water). Upon addition of an aqueous sodium acetate
solution (164 mg, 2 mmol, in 1 ml of water), the colour
[24]
Mo(VI) metal centre.
As a result, several research groups
have focused on the design of new dioxo Mo(VI) complexes
and investigation of their catalytic behaviour as epoxidation
[
39]
changed to yellow. Then, MoO (acac)2
solution
2
[
25–37]
catalysts.
(247 mg, 1 mmol, in 10 ml of ethanol) was prepared and
added to the mixture followed by heating at reflux for 3 h
while stirring. The white resultant solid was filtered, washed
with water, ethanol and diethyl ether, and dried in air at room
temperature. Yield 70%; decomposition point 288–290°C.
In many cases, density functional theory (DFT) calcula-
tions have been used in understanding the molecular proper-
ties of compounds, providing good assignment of
experimental spectra and illustration of reaction
[
38]
−1
mechanisms.
Anal. Calcd for C H MoN O (M = 480.32 g mol ) (%):
20
20
2 6
In the study reported here, attempts were made to synthe-
size a new amino acid Schiff base complex of Mo with tryp-
tophan and salicylaldehyde. Details of DFT studies in order
to compare theoretical and experimental results together with
an investigation of epoxidation catalytic behaviour and anti-
bacterial activity of the complex are described in this paper.
Mo, 19.97; C, 50.01; H, 4.20; N, 5.83. Found (%): Mo,
20.14; C, 50.63; H, 3.98; N, 6.11. FT‐IR (KBr pellet, cm ):
−
1
1658 (C═N), 1557, 1307 (COO), 896, 871 (cis‐MoO ), 513
2
[
40,41]
(Mo─O), 422 (Mo─N).
UV–visible (DMSO, λ
nm): 275 (ε = 22 500 M cm ), 283 (ε = 24 000 M cm ),
,
max
−1
−
1
−1
−1
290 (ε = 21 167 M− cm ).
1
−1 [42,43] 1
H NMR (250 MHz,
DMSO‐d , δ, ppm): 1.70 (3H, CH ), 2.49–2.96 (2H, CH ),
6
3
2
3
.26–3.43 (4H, CH , CH, OH), 6.92–7.55 (9H, Ar), 10.92
2
2
2
| EXPERIMENTAL
(1H, NH).
.1 | Materials and methods
2
.3 | General procedure for oxidation of
All materials were of commercial reagent grade and used
without further purification. t‐Butyl hydroperoxide (TBHP;
alkenes
7
0% in H O) and cyclooctene were purchased from
To a mixture of catalyst (0.020 g) in CCl (5 ml) was added
2
4
Fluka and Sigma‐Aldrich, respectively. Salicylaldehyde, L‐
tryptophan, sodium acetate, norbornene, cyclohexene, sty-
rene, α‐methylstyrene, trans‐stilbene, acetonitrile, ethanol,
chloroform, dichloromethane and carbon tetrachloride were
purchased from Merck.
the desired alkene (10 mmol) and TBHP (12 mmol, 1.6 ml)
in a round‐bottom flask. The suspension was then heated at
reflux for 8 h. After filtration and washing the catalyst with
CCl , the solution was subjected to GC and GC–MS.
4
Fourier transform infrared (FT‐IR) spectra were recorded
with a Bruker Tensor 27 FT‐IR spectrometer using KBr pel-
lets over the range 400–4000 cm . Elemental analyses were
performed with a Heraeus CHN‐O‐Rapid elemental analyser.
UV–visible spectral measurements were performed with a
2
.4 | Ab Initio molecular orbital calculation
−
1
All calculations were performed using Gaussian 98 soft-
ware.
[
44]
The geometries of the ligand and complex were
fully optimized using the Hartree–Fock density functional
scheme, the adiabatic connection method, Becke three‐
parameter with Lee–Yang–Parr (B3LYP) functional of
DFT with the standard 6‐311G** basis set. Restricted for-
malism was applied to all closed‐shell systems. Full optimi-
zations were performed without any symmetry constrains.
Harmonic vibrational frequencies were computed to confirm
that an optimized geometry correctly corresponded to a local
minimum that had only real frequencies. The solvent effects
on the conformational equilibrium and contribution to the
double‐beam PerkinElmer Lambda 35 spectrophotometer.
1
The H NMR spectrum was recorded in DMSO‐d with a
6
[
45]
Bruker DPX spectrometer. Chemical analyses of samples
were determined with a PerkinElmer atomic absorption spec-
trometer. Thermogravimetric analysis (TGA) was carried out
with a PerkinElmer Pyris Diamond TG/DTA, with a nitrogen
−
1
atmosphere, the flow rate and heating rate being 80 mg min
−1
and 10 °C min , respectively. The mass spectrum of the
complex was obtained with an HP (Agilent Technologies)
[
46]
5
937 Mass Selective Detector. Oxidation products were
analysed using GC and GC‐MS with an Agilent 6890 Series
with flame ionization detector, HP‐5, 5%
total enthalpies were investigated with the PCM method
at the B3LYP/6‐31G** level. Tomasi's polarized continuum
model defines the cavity as the union of a series of
a

ASGHARPOUR ET AL.
3 of 10
interlocking atomic spheres. The effect of polarization of the
solvent continuum was represented numerically. Solvation
calculations were carried out for ethanol with the geometry
optimization for this solvent. NMR computations of absolute
shieldings were performed using the GIAO method for the
1
DFT optimized structure in the presence of solvent. The H
chemical shifts were calculated by using the corresponding
absolute shielding calculated for Me Si at the same level of
4
theory for both ligand and complex in DMSO as solvent.
3
3
| RESULTS AND DISCUSSION
.1 | Characterization of [MoO (Sal‐Tryp)
2
(
EtOH)] complex
Salicylaldehyde and L‐tryptophan were treated with
MoO (acac) in ethanol and water. As seen in Scheme 1,
2
2
the initially generated N‐salicylidene‐L‐tryptophan Schiff
base ligand undergoes complexation with MoO (acac)2
2
affording [MoO (Sal‐Tryp)(EtOH)].
2
MS was used to determine the molecular ion mass as well
as molecular fragments to confirm the proposed structure of
[
MoO (Sal‐Tryp)(EtOH)] complex. This method is particu-
FIGURE 1 FT‐IR spectra: (a) MoO (acac) ; (b) Sal‐tryp Schiff base
2
2
2
larly useful when a poorly crystalline nature of a complex
ligand; [MoO
2
(Sal‐tryp)(EtOH)] complex (c) before and (d) and after
[
47,48]
prevents its X‐ray crystallography characterization.
The mass spectrum of the complex shown in Figure S1
supporting information) shows m/z 480 corresponding to
epoxidation
(
stretching bands generated by coordination of O and N atoms
+
the complex M + 1. The fragments appearing at m/z 309
base peak) and 204 are attributed to Schiff base and L‐
−1
[50]
to Mo appear at 513 and 424 cm , respectively.
(
To evaluate the thermal stability, the thermal decomposi-
+
tryptophan ions, generated from parent M + 1 followed
by sequential loss of [MoO (EtOH)] and [C H MoO ]
tion of [MoO (Sal‐Tryp)EtOH] complex was studied
2
2
9
11
4
(Figure 2; Table 1). TGA carried out from 25 up to 500 °C
indicates that [MoO2(Sal‐Tryp)EtOH] complex decomposes
in three steps. The first step occurring at 30–130 °C corre-
sponds to the liberation of solvent molecule (obs. 9.62%;
calc. 9.59%). The second stage appears in the range
radicals. Therefore, the proposed complex structure of
MoO (Sal‐Tryp)EtOH] is confirmed.
[
2
The FT‐IR spectra of MoO (acac), (Sal‐Tryp) Schiff base
2
ligand and [MoO (Sal‐Tryp)(EtOH)] before and after reac-
tion are shown in Figure 1, respectively. The bands appearing
at 1658, 1577 and 1307 cm (Figure 1a, b, c, d)
to the C═N and COO asymmetric and symmetric stretching
of Schiff base ligand coordinated to the metal. The rather
large difference between asymmetric and symmetric COO−
frequencies indicates a monodentate coordination of this
2
−
1
[49]
are due
−
−1
group. Two bands at 896 and 871 cm regions are due to
the Mo═O vibrations (Figure 1b). Mo─O and Mo─N
SCHEME 1 Preparation of [MoO
2
(Sal‐Tryp)(EtOH)] complex
2
FIGURE 2 TGA curve of [MoO (Sal‐Tryp)(EtOH)] complex

4
of 10
ASGHARPOUR ET AL.
TABLE 1 TGA results for complex decomposition
Removed
Entry
Complex fragments
fragment
Calc. Obs.
1
EtOH
9.59 9.62
2
3
C
C
11
H
10
N
2
O
2
41.91 41.88
21.89 21.92
FIGURE
complex in solvent
3
Optimized structure of [MoO
2
(Sal‐Tryp)(EtOH)]
7
H
5
O
indicate that the complex is stabilized by 45.17 kcal mol−
in ethanol solvent, perhaps due to the high dipole moment
of the complex (11.88 D).
1
The calculated net charges on [MoO (Sal‐Tryp)
EtOH)] complex are presented in Table S3. These partial
2
(
1
50–247 °C, representing the decomposition of organic com-
pounds as well as solvent removing (obs. 41.88%; calc.
1.99%). The last step observed in the range 250–400 °C
obs. 21.92%; calc. 21.89%) indicates the decomposition of
charges are derived from natural bond orbital calculations.
The net negative charge on O (−0.644), O (−0.478), O
1
3
4
4
(
(
−0.559), O (−0.383), O (−0.522), N (−0.585) and N
5
6
1
2
(
−0.807) atoms in (Sal‐Tryp) molecule is more than
the remaining organic compounds with the formation of
molybdenum oxide. Finally above 500 °C, the weight of the
remaining complex remains constant.
The electronic absorption spectrum of the complex in
DMF exhibits three absorption bands at 275, 283 and
that on other atoms. The net charge on Mo is 1.116
when (Sal‐Tryp) is coordinated. Significantly, whereas
the calculated electron density on the donor oxygen and
nitrogen atoms is less than expected, it is more than
expected on the central ion. Thus, the net charge results
confirm an electron transfer from the donor atoms of
2
90 nm due to the benzene π → π*, imine π → π* and
[42,51]
1
n → π* transitions of Schiff base of azomethine.
the (Sal‐Tryp) ligand to the central Mo ion. H NMR
13
and
C
NMR spectra of [MoO (Sal‐Tryp)(EtOH)]
2
complex and (Sal‐Tryp) Schiff base were measured in
3
3
.2 | Theoretical
DMSO‐d and are shown in Figures S2 and S3, respec-
6
.2.1 | Geometry optimization of (Sal‐Tryp)
tively (supporting information).
Schiff base ligand
The structure of the (Sal‐Tryp) Schiff base ligand was fully
optimized with the B3LYP method using 6‐311G** basis
set, with no initial symmetry restrictions and assuming C1
point group. The optimized geometries of the (Sal‐Tryp)
Schiff base ligand in the gas phase were re‐optimized by
considering the solvent effect using the PCM method.
The optimized geometry of the ligand and some structural
details are depicted in Figure S4 and given in Table S1,
respectively.
3.2.3 | Molecular orbital analysis
The model [MoO (Sal‐Tryp)EtOH] complex used in this
2
study was generated when one optimized (Sal‐Tryp) mole-
[26]
cule come close to optimized [MoO (acac) ] complex
in
2
2
EtOH solvent:
Â
Ã
ðSal‐TrypÞ þ MoðacacÞ ðO Þ →
(1)
2
2
½
MoO2ðSal‐TrypÞEtOHꢀ þ acac
3
.2.2 | Geometry optimization of [MoO2
To determine the nature of the bonds in [MoO (Sal‐Tryp)
2
(
Sal‐Tryp)(EtOH)] complex
EtOH] complex, molecular orbital analysis was employed
since this method is useful for presenting the factors influenc-
ing the stability of this complex. The highest occupied molec-
ular orbital (HOMO) and lowest unoccupied molecular
In the next step, the fully optimized geometries of
MoO (Sal‐Tryp)(EtOH)] complex in gas phase were re‐
[
2
optimized by considering the solvent effect using PCM
method (Figure 3 and Table S2). The calculated results
orbital (LUMO) of [MoO (Sal‐Tryp)EtOH] complex are
2

ASGHARPOUR ET AL.
5 of 10
shown in Figure 4. The energy gap or EHOMO − ELUMO, an
with the most (1.6 D) and least (0 D) dipole moments is con-
sistent with a rather non‐polar transition state.
To test the generality of the method, epoxidation of a
range of alkenes such as cyclooctene, norbornene, trans‐
stilbene, styrene and α‐methyl styrene was investigated
under optimized reaction conditions (Table 2). Whereas
cyclooctene exclusively gives the corresponding epoxide
[
55]
important stability index in characterizing the chemical reac-
[
51–53]
tivity and kinetic stability of a molecule,
is 0.11 au.
1
Moreover, the H NMR chemical shifts obtained for
MoO (Sal‐Tryp)EtOH] complex via calculation
[54]
[
have
2
been compared to those obtained experimentally (Table S4,
supporting information).
(Table 2, entry 1), cyclohexene undergoes allylic oxidation
3
.3 | Catalytic studies
Cyclooctene was used for our early exploration of epoxida-
tion under the catalytic effect of [MoO (Sal‐Tryp)EtOH]
2
complex. To determine the optimum reaction conditions, var-
ious parameters such as amount of catalyst, reaction time and
effect of solvent were studied (Figures 5–7). As seen in
Figure 5, increasing the amount of catalyst from 3 to 20 mg
increases the conversion from 25 to 100% with 100% selec-
tivity. Therefore, all epoxidation reactions were carried out
using 20 mg of catalyst.
In the next step, it was found that increasing the reaction
time from 1 to 8 h using 20 mg of catalyst increases the
cyclooctene conversion from 10 to 100% with 100% selectiv-
ity (Figure 6).
FIGURE 5 Effect of amount of catalyst on the epoxidation of
cyclooctene. Reaction conditions: cyclooctene (10 mmol), TBHP
4
(14 mmol), solvent (CCl , 5 ml), under reflux condition, 8 h
Compared to solvents such as EtOH, CH CN and ethyl
3
acetate which are totally inefficient for epoxidation reaction,
CHCl , CH Cl and CCl were found to be suitable solvents
3
2
2
4
in the presence of 20 mg Schiff base complex within 8 h
Figure 7). The inefficiency of EtOH, CH CN and ethyl ace-
(
3
tate may be rationalized by hard–soft acid base theory. Coor-
dination of these solvents to hard Mo(VI) ion via hard O and
N atoms retards reaction perhaps by inhibiting the coordina-
tion of TBHP to the metal centre. On the other hand, non‐
coordinative chlorinated solvents such as CCl , CHCl and
4
3
CH Cl make more efficient the coordination of TBHP to
Mo(VI) ion followed by oxygen transfer to alkene. Obtaining
FIGURE 6 Effect of time on epoxidation of cyclooctene. Reaction
conditions: cyclooctene (10 mmol), TBHP (14 mmol), solvent (CCl4,
5 ml), under reflux condition
2
2
4
7 and 100% conversions respectively in CH Cl and CCl
2 2 4
FIGURE 4 Generation of [MoO (Sal‐
2
Tryp)(EtOH)] complex with HOMO and
LUMO views of complex

6
of 10
ASGHARPOUR ET AL.
chair conformation in which allylic carbons lie in the plane
[
57]
of the double bond (Scheme 2b).
Therefore, α‐H
abstraction by radical species competes with epoxidation,
leading to 2‐cyclohexene‐1‐ol/2‐cyclohexene‐1‐one and
cyclohexene epoxide, respectively.
Particularly significant is norbornene which selectively
affords the corresponding epoxide (Table 2, entry 2). If
norbornene undergoes allylic‐site oxidation through α‐H
abstraction by radical species, it affords a bridgehead radical
[
58]
which has been shown to be unstable due to Bredt's rule.
Therefore, norbornene conclusively undergoes epoxidation
reaction, affording the corresponding epoxide. In the case
of styrene, trans‐stilbene and α‐methylstyrene, epoxidation
is partly accompanied by generation of benzaldehyde,
benzoic acid and acetophenone (Table 2, entries 4, 5 and
FIGURE 7 Effect of solvent on catalytic epoxidation of cyclooctene.
Reaction conditions: catalyst(20 mg), cyclooctene (10 mmol), TBHP
(14 mmol), solvent (5 ml), reflux condition, 8 h
6
). A further nucleophilic attack of TBHP on the styrene, stil-
and epoxidation, concomitantly (Table 2, entry 3). It has
been reported that the high epoxidation selectivity typically
observed for cyclooctene is related to a poor σC─αH–πC═C
orbital overlap in the predominant conformation
bene and α‐methylstyrene oxides seems to be responsible for
the formation of benzaldehyde and acetophenone. Benzalde-
hyde in turn is readily oxidized to benzoic acid.
The recyclability of [MoO (Sal‐Tryp)EtOH] complex as
2
(
Scheme 2a). Therefore, such poor overlap disfavours the
catalyst was investigated under optimum reaction conditions.
After each reaction cycle with cyclooctene, the catalyst was
recovered by centrifugation, washed three times with CCl4
and dried under vacuum. It was found that after using the
[56]
α‐H abstraction by radical species.
As such, allylic‐site
oxidation of cyclooctene is not in competition with epoxi-
dation. On the other hand, cyclohexene exists in the half‐
TABLE 2 Results obtained for epoxidation of different alkenes in the presence of Schiff base complex
a
TONb
TOF (h−1)
c
Entry
Substrate
Conversion (%)
Selectivity to epoxide (%)
1
100
100
244
30.48
2
3
78
50
100
190
122
23.78
15.24
d
64
e
4
45
85
70
110
21
13.71
2.59
f
5
78
g
6
72
93
177
21.95
a
4
Reaction conditions: catalyst (20 mg), substrate (alkenes (10 mmol), trans‐stilbene (1 mmol)), TBHP (14 mmol), solvent (CCl , 5 ml, reflux condition), time (8 h).
b
TON: mmol of epoxide to the mmol molybdenum present in catalyst.
cTOF: turnover frequency which is calculated by the expression [epoxide]/[catalyst] × time (h−1).
d
2
‐Cyclohexene‐1‐ol and 2‐cyclohexene‐1‐one identified as byproduct.
e
f
Benzaldehyde and benzoic acid identified as byproduct.
Benzaldehyde and benzoic acid identified as byproduct.
g
Acetophenone identified as byproduct.

ASGHARPOUR ET AL.
7 of 10
SCHEME 2 Suggested mechanism for
oxidation of (a) cyclooctene and (b)
cyclohexene
catalyst for five runs, there was a decrease in conversion from
100 to 94% without any changes in selectivity (Figure 8).
Since no catalytic activity is observed when the filtrate of
each run is subjected to epoxidation conditions in the absence
of the catalyst, it can be concluded that the catalyst is truly func-
tioning heterogeneously. The Mo content in the catalyst before
and after use in reaction is 20.14 and 20.12%, respectively. No
significant desorption is observed during the course of reaction.
The XRD patterns of the complex before and after use as
a catalyst are shown in Figure 9. The similarity in XRD pat-
terns is evidence for the stability and heterogeneous character
of the catalyst.
Finally, results of epoxidation of cyclooctene catalysed by
[
MoO (Sal‐Tryp)(EtOH)] in the present study have been
2
compared to those recently reported for some Mo catalytic
epoxidations (Table 3). As is evident, results obtained for
cyclooctene in the current work are comparable or even better
in some cases.
FIGURE 9 XRD patterns of [MoO
before and (b) after epoxidation reaction
2
(Sal‐Tryp)(EtOH)] complex (a)
activity of two standard antibacterial drugs: nalidixic acid
and vancomycin. The microorganisms used in this study were
Bacillus subtilis and Staphylococcus aureus (as Gram‐
positive bacteria) and Escherichia coli and Pseudomonas
aeruginosa (as Gram‐negative bacteria). The tests were
3
.4 | Antibacterial activity
The in vitro antibacterial activity of the prepared [MoO (Sal‐
Tryp)EtOH] complex was studied and compared with the
2
−
1
done at compound concentrations of 15 and 25 mg ml . As
evident from Table 4 and shown in Figure 10, the complex
at 15 mg ml− exhibits good activity against B. subtilis,
S. aureus, E. coli and P. aeruginosa. With increasing complex
1
−
1
concentration to 25 mg ml , the inhibition effect of the
complex increases against B. subtilis and leads to increasing
inhibition zone for E. coli, P. aeruginosa and S. aureus. The
antibacterial activity effect of the complex may be rationalized
by chelation of ligand with metal. Partial sharing of ligand
donor groups with the positive charge of the metal ion
increases the delocalization of electrons over the whole
ligand, which reduces the polarity of the metal ion and
increases the lipophilic character of the central metal ion.
The lipophilic character of the central metal ion favours
FIGURE 8 Effect of recycling of [MoO
as catalyst in epoxidation of cyclooctene. Reaction conditions: catalyst
20 mg), cyclooctene (10 mmol), TBHP (14 mmol), solvent (CCl
ml), reflux condition, 8 h
2
(Sal‐Tryp)(EtOH)] complex
(
5
4
,

8
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ASGHARPOUR ET AL.
TABLE 3 Comparison of epoxidation of cyclooctene catalysed by [MoO
2
(Sal‐Tryp)EtOH] complex with some previously reported epoxidations
catalysed by Mo compounds with TBHP
Entry
Catalyst
Conversion (%) Selectivity (%) TOF(h−1)
Ref.
1
2
3
4
5
6
[MoO
Mo Ar/ZAPS‐PVPA
MCM‐41‐SB‐MoO (acac)
[Mo(O) (salen)–POM]
MoO salen‐SBA‐15
2
(Sal‐Tryp)EtOH]
100
99
100
>99
100
100
49.1
97
30.48
6.25
14.7
8.33
25.6
126
This work
[
[
[
[
[
59]
60]
61]
62]
63]
2
2
94
2
100
49.1
97
2
Supported bis‐acetylacetonatodioxo molybdenum complex on boehmite
nanoparticles functionalized with imidazole
[63]
7
Supported bis‐acetylacetonatodioxo molybdenum complex on boehmite
97
97
89
nanoparticles functionalized with amine
[
[
[
[
64]
65]
66]
66]
8
9
MCM‐41‐supported molybdenum/bisdithiocarbamate
96
97
100
99
184
100
124
66
MoL
1
100
87
1
1
0
1
MoO
MoO
2
alaacacAmpMCM‐41
valacacAmpMCM‐41
2
35
99
TABLE 4 Quantitative antimicrobial assay results (zone of growth inhibition) of metal salt and [MoO
2
(Sal‐Tryp)(EtOH)] complex
Inhibition zone (mm)
Concentration
Compound
MoO (Sal‐Tryp)(EtOH)]
(mg ml−
1
)
E. coli (−)
P. aeruginosa (−)
S. aureus (+)
B. subtilis (+)
[
2
25
32
29
30
28
32
30
30
29
1
5
2
MoO (acac)
2
25
9
8
14
8
21
19
8
7
1
5
Nalidixic acid
Vancomycin
25
25
24
13
n.a.
n.a.
12
17
22
23
permeation through the lipid layer of cell membranes and
deactivating cellular enzymes that play important role in the
[
67–69]
metabolic pathways of microorganisms.
It is significant
that the complex has good inhibition activity against
P. aeruginosa while standard drugs show no activity against
it. Also, the complex has greater antibacterial activity against
B. subtilis, S. aureus and E. coli than the standard drugs.
4
| CONCLUSIONS
In this study, [MoO (Sal‐Tryp)EtOH] complex was prepared
2
using MoO (acac) and (Sal‐Tryp) Schiff base ligand. The
2
2
molecular structure was determined with FT‐IR, UV–visible
and mass spectroscopies and TGA. The optimized geometric
parameters (bond lengths and bond angels) were theoretically
determined. The complex was used as a catalyst for the epox-
idation of several olefins with TBHP with high conversion
FIGURE 10 Antibacterial activities of (1) [MoO
2
(Sal‐Tryp)(EtOH)]
−
1
(Sal‐Tryp)(EtOH)] (15 mg ml−1), (3)
(
25 mg ml ), (2) [MoO
2
2 2 2 2
(acac) (acac) (15 mg ml ), (5)
(25 mg ml− ), (4) MoO
1
−1
MoO
nalidixic acid standard (25 mg ml ) and (6) vancomycin standard
−
1
−
1
(
25 mg ml )

ASGHARPOUR ET AL.
9 of 10
yields and selectivities. The reusability of the catalyst was
studied to evaluate its practical potential. Finally, the antibac-
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Industrial Chemistry, Wiley‐VCH, Weinheim 2000.
terial activity of the [MoO (Sal‐Tryp)EtOH] complex against
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2
Gram‐positive and Gram‐negative bacteria in comparison to
standard nalidixic acid and vancomycin antibiotics was
evaluated.
[
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Ghiasi M, Azarkish M. Synthesis, characterization,
density functional theory studies and antibacterial
activity of a new Schiff base dioxomolybdenum(VI)
complex with tryptophan as epoxidation catalyst.
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