Synthesis, characterization, and application of a new nanohybrid Schiff base polyoxometalate in epoxidation of olefins in the presence of tert-butyl hydroperoxide

Article abstract of DOI:10.1080/00958972.2020.1727453

The new molybdenyl acetylacetonate Schiff base complex with mono-lacunary polyoxometalate (1) was synthesized in the reaction of [PMo₁₁O₃₉]^7-, molybdenum acetylacetonate (MoO₂(acac)₂), and ethylenediamine. The catalyst was characterized by elemental analysis and various routine techniques like FT-IR, XRD, FESEM, EDX, UV–Vis, and TGA. The catalytic activities of this nanocatalyst were examined for epoxidation of linear, cyclic and phenyl-substituted olefins using tert-butyl hydroperoxide (tert-BuOOH) as oxidant.

Full text of DOI:10.1080/00958972.2020.1727453

Journal of Coordination Chemistry
ISSN: 0095-8972 (Print) 1029-0389 (Online) Journal homepage: https://www.tandfonline.com/loi/gcoo20
Synthesis, characterization, and application of a
new nanohybrid Schiff base polyoxometalate in
epoxidation of olefins in the presence of tert-butyl
hydroperoxide
Samira S. Hosseinyzade, Farrokhzad M. Zonoz & Bahram Bahramian
To cite this article: Samira S. Hosseinyzade, Farrokhzad M. Zonoz & Bahram Bahramian (2020):
Synthesis, characterization, and application of a new nanohybrid Schiff base polyoxometalate
in epoxidation of olefins in the presence of tert-butyl hydroperoxide, Journal of Coordination
Chemistry, DOI: 10.1080/00958972.2020.1727453
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JOURNAL OF COORDINATION CHEMISTRY
https://doi.org/10.1080/00958972.2020.1727453
Synthesis, characterization, and application of a new
nanohybrid Schiff base polyoxometalate in epoxidation
of olefins in the presence of tert-butyl hydroperoxide
Samira S. Hosseinyzadea, Farrokhzad M. Zonoza and Bahram Bahramianb
b
aDepartment of Chemistry, Hakim Sabzevari University, Sabzevar, Iran; College of Chemistry,
Shahrood University of Technology, Shahrood, Iran
ABSTRACT
ARTICLE HISTORY
Received 21 July 2019
Accepted 23 January 2020
The new molybdenyl acetylacetonate Schiff base complex with
mono-lacunary polyoxometalate (1) was synthesized in the reaction
of [PMo11O39]7-, molybdenum acetylacetonate (MoO2(acac)2), and
ethylenediamine. The catalyst was characterized by elemental ana-
lysis and various routine techniques like FT-IR, XRD, FESEM, EDX,
UV–Vis, and TGA. The catalytic activities of this nanocatalyst were
examined for epoxidation of linear, cyclic and phenyl-substituted
olefins using tert-butyl hydroperoxide (tert-BuOOH) as oxidant.
KEYWORDS
Schiff base;
polyoxometalate; molybde-
nylacetylacetonate; olefin
epoxidation; nanohybrid;
nanocatalyst
1. Introduction
Lacunary Keggin-type polyoxometalates (POMs) are anionic clusters of nanosize transi-
tion-metal oxides that display strong coordination with a wide range of organic com-
pounds which leads to formation of new compounds [1–3]. Organic–inorganic hybrid
polyoxometalates have been extensively examined [4, 5]. The properties of hybrid
materials are the sum of properties of organic and inorganic components [6].
CONTACT Samira S. Hosseinyzade
samir.hosseinyzade89@gmail.com
ß 2020 Informa UK Limited, trading as Taylor & Francis Group

2
S. S. HOSSEINYZADE ET AL.
Physicochemical properties, such as molecular structure, surface charge densities, and
strong redox ability [7], make these compounds appropriate for application in magnet-
ism, electronics, electro, and photochromic systems, medicine, molecular batteries, and
catalysis [8–11]. Many organic conversions can be catalyzed by POMs due to their
redox and acidic properties [12, 13], but due to the good solubility of POMs in polar
solvents, it is difficult to isolate the compounds from the reaction mixture [12, 14].
Many reports have been presented on solid catalysts containing metal complexes
immobilized on organic or inorganic solid supports as heterogeneous catalysts
[15–17], however, slow reaction rates, leaching of the active species, or method of
preparation of the complicated catalyst are disadvantages of this method [7]. Hybrid
POMs displayed some attractive aspects in contrast to the widely studied supported
heterogeneous catalysts. Several catalytic studies have been reported based on these
compounds as heterogeneous catalysts with high activity and reusability [12, 18, 19].
Because of the importance of oxidation reactions in industry, many efforts have
been made to improve catalytic oxidation systems [20–22]. Epoxidation of alkenes is
one of the most important industrial reactions that lead to the preparation of com-
pounds such as surfactants, detergents, lubricating oils, textiles, cosmetics, rubber pro-
moters, epoxy paints, and dyestuff products [23–25].
Dioxomolybdenum(VI) complexes have been shown to be important catalysts or
catalyst precursors for oxygen-transfer reactions in chemical and biological systems
[26, 27] with many reports on the use of dioxomolybdenum(VI) complexes as catalysts
in alkene epoxidation [28–30]. In this article, we synthesized a new nanohybrid Schiff
base polyoxometalate by combining the properties of a dioxomolybdenum(VI) com-
plex and a mono lacunary Keggin-type polyoxometalate (PMo11O397-). The synthesized
compound was characterized by routine techniques. We have applied 1 as a heteroge-
neous catalyst for epoxidation of olefins and studied the effect of different solvents,
temperatures, oxidant, catalyst, and time on the activity of the catalysts.
2. Experimental
2.1. Instruments and reagents
All solvents and materials were purchased from Merck and Fluka companies and used
without purification. [MoO2(acac)2] was synthesized from the reported procedure [31]
and identified by FT-IR spectroscopy. FT-IR spectra were recorded on a Shimadzu
8400S spectrometer using KBr pellets from 400 to 4000 cmꢀ1. Carbon, hydrogen, and
nitrogen elemental analyses were determined on a Perkin-Elmer CHN analyzer. ICP
analyses were performed on an ICP-Spectrociros CCD instrument. X-ray powder diffrac-
tion (XRD) patterns were obtained by using a Phillips PW-1730 or STOE diffractometer
with Cu Ka radiation. UV–Vis spectra were recorded on a Perkin-Elmer Lambda 25
spectrophotometer. FESEM equipped by EDS microanalysis was done with a
MIRA3TESCAN-XMU instrument. TGA of the sample was carried out using a TA instru-
ment DSC SDT Q600 with a heating rate of 10 ꢁC/min under nitrogen. Gas chromatog-
raphy (GC) was accomplished on a Shimadzu GC-16A instrument using a 2 m column
packed with silicon DC-200. In the GC experiments, n-decane was used as an internal
standard. The operating parameters were as follows: injector temperature, 180 ꢁC;

JOURNAL OF COORDINATION CHEMISTRY
3
Figure 1. FT-IR spectra of (a) [MoO2(acac)2], (b) [PMo11O39]7-, and (c) 1.
detector temperature, 200 ꢁC; oven temperature, 70–200 ꢁC. The conversions and yields
were calculated by normalizing the areas of the GC peaks by means of the area of the
internal standard peak (n-decane).
2.2. Synthesis of mono-lacunary phosphomolybdate molybdenyl
acetylacetonato Schiff base
Ethylenediamine (0.05 mL, 0.75 mmol) was added to a stirring solution of 0.456 g
(0.25 mmol) of H3PMo12O40 in 15 mL of distilled water. After 30 min, methanol solution of
[MoO2(acac)2] (0.246 g, 0.75 mmol) was added and the mixture refluxed for 5 h at 80 ꢁC.
The light violet precipitate was filtered, washed with methanol, and dried at 80 ꢁC. IR
spectrum t, cmꢀ1: 2893 (C-H), 1620 (C ¼ N), 1602 (C ¼ O), 1320 (d C-H), 1060 (P-O), 946
(Mo ¼ O), 785 and 650 (Mo-Oc-Mo). Found, %: C12.35; H 1.59; Mo 53.38 N 2.31; P 1.34.
C24H36Mo13N4O43P. Calculated, %: C, 12.28; H, 1.53; Mo, 53.16; N, 2.39; P 1.32. M 2347.
2.3. General procedure for the epoxidation of alkenes
All reactions were performed in 10 mL round bottom flasks equipped with a magnetic
stir bar and a condenser under reflux. The mixture of alkene (0.5 mmol), catalyst (7 mg,

4
S. S. HOSSEINYZADE ET AL.
Figure 2. UV–Vis spectra of (a) [PMo11O39]7-, (b) [MoO2(acac)2], and (c) 1.
3 mmol), and tert-BuOOH (0.4 mL) was refluxed in 1,2-dichloroethane as solvent. The
progress of the reaction was monitored by gas chromatographic analysis. Blank experi-
ments were performed in the absence of catalyst under the same experimental condi-
tions. Since different alkenes have various reactivities toward oxidation, the reactions
were continued until no further progress was observed.
3. Results and discussion
3.1. Characterization of the nanohybrid catalyst
POMs are unstable in basic media, therefore [PMo12O40]3- changes to mono-lacunary
[PMo11O39]7- upon reaction with ethylenediamine. In 2014, Memon and Coworkers
reported preparation of a Schiff base in the reaction of acetylacetone and ethylenedi-
amine [32]. The Schiff base complex, 1, is then obtained in the reaction of molyb-
denum acetylacetonate, ethylenediamine, and mono-lacunary POMs as shown in
Scheme 1.
Because efforts to obtain suitable crystals for single crystal analysis were unsuccess-
ful, we used the other techniques for characterization of 1. FT-IR spectra of
[MoO2(acac)2], [PMo11O39]7-, and 1 are compared in Figure 1. Characteristic bands at
2893 (m(C-H)), 1620 (m(C═N)), 1602 (m(C═O)), 1320 (m(C-H)), and 1060 cmꢀ1 (m(P–Oa)) are
attributed to formation of 1 [33–35]. Peaks of [MoO2(acac)2] and [PMo11O39]7- at
960–860 cmꢀ1 were overlapped.
UV–Vis spectra of [MoO2(acac)2], H3PMo12O40, and 1 are shown in Figure 2. The
characteristic absorption peak of 1 can be seen at 318 nm which is attributed to O
(pp) ! Mo (dp) charge transfer transition. This peak is displaced to higher wave-
lengths than the observed peak (271 nm) in [MoO2(acac)2] [36]. The presence of
mono-lacunary POM is established by a peak at 214 nm which is attributed to O !
Mo charge-transfer band [37]. This result confirms an intramolecular interaction
between [MoO2(acac)2] and POM in 1.
The XRD patterns of Keggin-type polyoxoanions show two typical refluxing areas
(2h ¼ 2–10ꢁ and 25–35ꢁ) [38]. The XRD patterns of 1 (Figure 3) show the existence of

JOURNAL OF COORDINATION CHEMISTRY
5
Figure 3. XRD pattern of (a) PMo11O397-, (b) [MoO2(acac)2], and (c) 1.
Figure 4. FE-SEM images of 1 nanocatalyst.
polyoxometalate and [MoO2(acac)2] in this compound. According to these data, 1 has
crystalline characteristics after metal loading. By using the Williamson–Hall equation (b
cos h ¼ Kk=D þ 4e sin h, b is line broadening at half of the maximum intensity, K is a
dimensionless shape factor (0.9), h is the Bragg angle, k is the X-ray wavelength for Cu
Ka radiation, D is the average crystallite size, and e is the lattice strain), the average
size of the particles was estimated to be 50 nm [39].
FE-SEM is used to examine three important factors: surface crystallography (i.e. the
surface formation of atoms), surface morphology (surface structural features based on
shape and size), and surface composition (the composition of surface, in terms of ele-
ments and compounds). The morphology of 1 is shown in the FE-SEM images (Figure
4). These images show row-like and spherical shapes in 1 with particle sizes of 75–100
and 42–46 nm, respectively, which is in agreement with XRD results. There are slight
differences between the particle sizes obtained from FE-SEM and XRD techniques
because XRD provides information of the volume and SEM of the surface. Therefore
we have a mixture of particles of various sizes.
The EDX spectrum of 1 indicates the existence of C, N, O, P, and Mo elements
(Figure 5). These results can be confirmed in the proposed structure of 1.

6
S. S. HOSSEINYZADE ET AL.
Figure 5. EDX spectra of 1.
Figure 6. TGA analysis of 1.
The DTG–TGA curve of 1 shows three main weight losses at 200–280 ꢁC,
300–500 ꢁC, and 650–1000 ꢁC (Figure 6). These weight losses correspond to the
removal of imine groups in the Schiff base (267.3 ꢁC, 5.62%), decomposition of molyb-
denum acetylacetonate (in two steps) (403.6 and 454.9 ꢁC, 8.59%), and molybdophos-
phate to P2O5 and MoO3, respectively (13.58%).
The results of FT-IR spectra, XRD patterns, and DSC-TGA analysis can confirm the
proposed structure (Scheme 1).
3.2. Catalytic applications of 1 in the epoxidation of alkenes
The epoxidation of cis-cyclooctene with 1 using tert-BuOOH as oxidant was studied as
the model reaction under optimized conditions under reflux (Table 1). The effects of
different solvents (Table 1, entries 3–7), mg of catalyst (Table 1, entry 7 and entries
12–15), oxidant type (Table 1, entries 7–10), mol % for catalyst loading (0.35–1.375 mol
%), and oxidant to substrate molar ratio (0.2–2.24) for 1 were investigated. The results
show that TBHP is the only oxidizing system, which can epoxidize cyclooctene in

JOURNAL OF COORDINATION CHEMISTRY
7
Scheme 1. Reaction scheme for the synthesis of 1 nanohybrid.
Table 1. Epoxidation of cis-cyclooctene at refluxed conditions.a
Catalyst
Oxidant/Substrate
molar ratio
Epoxide
Entry
Catalyst (mg)
Solventb
Oxidant
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
H2O2
loading (mol%)c
yield (%)d
1
2
3
4
5
6
7
8
Catalyst-free
H3PMo12O40
I(15)
I(15)
I(15)
I(15)
I(15)
I(15)
1(15)
I(15)
I(15)
I(10)
I(7)
I(5)
I(3)
I(7)
I(7)
I(7)
I(7)
C2H4Cl2
C2H4Cl2
CH3OH
CCl4
CH2Cl2
CHCl3
C2H4Cl2
C2H4Cl2
C2H4Cl2
C2H4Cl2
CH3CN
C2H4Cl2
C2H4Cl2
C2H4Cl2
C2H4Cl2
C2H4Cl2
C2H4Cl2
C2H4Cl2
CH3OH
–
–
2.24
2.24
2.24
2.24
2.24
2.24
2.24
2.24
2.24
2.24
2.24
2.24
2.24
2.24
2.24
2.24
1.68
1.12
0.2
–
trace
18
95
78
87
99
23
15
trace
57
99
99
75
42
99
75
51
30
1.375
1.375
1.375
1.375
1.375
1.375
1.375
1.375
1.375
0.917
0.642
0.458
0.275
0.642
0.642
0.642
0.35
9
H2O2.CON2H4
e
10
11
12
13
14
15
16
17
18
19
NaIO4
H2O2
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
aThe time of reactions is 30 min.
bThe amount of solvent is 2 mL.
cCalculated for 0.5 mmol cyclooctene.
dGC yields are based on the starting cyclooctene.
eTetra-butyl phosphonium bromide was used as phase transfer reagent.
Figure 7. Epoxidation of 0.5 mmol cis-cyclooctene with 1.12 mmol TBHP catalyzed by 7 mg of 1 in
2 mL C2H4Cl2 under reflux.

8
S. S. HOSSEINYZADE ET AL.
Table 2. Epoxidation of some olefins with TBHP catalyzed by 1 under reflux.a
Conversion
Entry
1
Alkene
(epoxide) (%)b
99(99)
Selectivity (%)
100
Time (min)
30
Cyclooctene
2
98(90)c
92
30
Cyclohexene
1-Octene
3
4
5
58(58)
63(63)
85(78)d
100
100
92
90
90
90
1-Hexene
Styrene
6
90(82)e
91
60
a-Methylstyrene
a-Pinene
7
8
87(80)
92(92)f
92
60
60
100(trans)g
trans-Stilbene
cis-Stilbene
9
(90)f
78(cis)
and
60
12 (trans)h
aReaction conditions: alkene (0.5 mmol), TBHP (1.12 mmol), catalyst (7 mg), C2H4Cl2 (2 mL).
bGC yield based on the starting alkenes.
cThe by-product is 8% for the catalyst.
dThe by-product is 7% for the catalyst.
eThe by-product is 8% for the catalyst.
1
fBoth H NMR and GC data approved the reported yields.
gTrans isomer is the only product according to 1H NMR data.
1
hCis and trans isomers are both produced according to H NMR data.
C2H4Cl2. Other oxygen sources like hydrogen peroxide, sodium periodate (in the pres-
ence of 0.01 g tetra-butyl phosphonium bromide as phase transfer reagent) and urea-
hydrogen peroxide in C2H4Cl2 do not show activity (Table 1, entries 7–11). Figure 7
shows the effect of reaction time on the epoxidation of cyclooctene with TBHP.
Reactivity of 1 achieves the maximum and becomes constant after 30 min.
Finally epoxidation of several olefins including linear, cyclic, and phenyl-substituted
olefins with tert-butyl hydroperoxide as oxidant were performed in the presence of 1
(Table 2); electron-rich olefins are more reactive than electron-poor ones. Comparison

JOURNAL OF COORDINATION CHEMISTRY
9
Table 3. The catalytic activity of various catalysts for the epoxidation of cyclooctene.
Entry no.
Catalyst
Substrate
Solvent
Time (h) TOF (h–1) Conv. (%) Sel. (%)
Ref.
1
2
3
4
1
Cyclooctene C2H4Cl2
0.5
5
1.5
9
396
6.83
ꢀ
99
96
99
99
99
>99
100
This work
[40]
[41]
PW4–C18
Cyclooctene
Cyclooctene
Cyclooctene
CH3CN
CH3CN
CH3CN
BTE-PW11O39
As8V14O42(H2O)]
[Cu(2,20-bpy)2]4
[As8V14O42(H2O)]
[Cu(1,10-phen)2]4
PW12/MIL-101
MimAM(H)-PW
ꢀ
65.5
[42]
5
Cyclooctene
CH3CN
9
ꢀ
33
100
[42]
6
7
8
Cyclooctene
Cyclooctene
CH3CN
CH3CN
CH3CN
3
4
3
0.38
ꢀ
76
95
96
99
100
–
[43]
[12]
[44]
f[Ag2(trz)2] [Ag24(trz)18]g Cyclooctene
–
[PMo12O40]2
9
10
[CuI 6(3atrz)6][PMo12O40 2
]
Cyclooctene 1,4-dioxane
Cyclooctene ethanol
10
3.25
–
–
66
82
99
98
[45]
[46]
[HxPMo12O40ꢂH4Mo72
Fe30O254(CH3COO)15(H2O)98
]
Table 4. Epoxidation of cis-cyclooctene with TBHP under reflux using recycled catalysts.a
Number of cycles
Epoxide yield (%)b
TOFc (h–1
)
Amount of Mo leached (%)d
1
2
3
4
5
6
98
95
93
90
90
88
392
380
372
360
360
352
1
0.5
0
0
0
0
aReaction conditions: 0.5 mmol cyclooctene, 1.12 mmol TBHP, 7 mg catalyst, 2 mL C2H4Cl2, 30 min.
bGC yields are based on the starting cyclooctene.
cTOF ¼ (Number of molecules reacted)/(Number of sites) ꢃ (time).
dDetermined by ICP.
of 1 with other reported catalysts in the epoxidation of cyclooctene is given in Table
3. Swalus et al. reported epoxidation of cyclooctene catalyzed by PW4–C18 compound
[40]. The epoxidation reaction was performed using CH3CN as a solvent giving epoxide
products with a yield of 96% and excellent selectivities (Table 3, entry 2). Zhang et al.
applied BTE-PW11O39 as a catalyst for olefin epoxidation using H2O2 as an oxidant in
CH3CN (Table 3, entry 3) [41]. Conversion and selectivity toward the epoxide were
99%. Guo et al. found that two complexes [As8V14O42(H2O)][Cu(2,20-bpy)2]4 and
[As8V14O42(H2O)][Cu(1,10-phen)2]4, which were synthesized by the hydrothermal
method, were applied to the selective epoxidation of olefins [42]. In acetonitrile as
solvent, 64.5% conversion and 100% selectivity for compound 1 and for compound 2,
33% conversion and 100% selectivity were reported (Table 3, entries 4 and 5).
Maksimchuk et al. reported that PW12/MIL-101 used as a true heterogeneous catalyst
can be recycled four times without loss of activity (Table 3, entry 6) [43]. Leng et al.
demonstrated that the epoxidation of cyclooctene can be made with MimAM (H)-PW
[12]. This group of researchers performed the oxidation reaction in acetonitrile as a
solvent, and 95% conversion and 100% selectivity of epoxide were reported (Table 3,
entry 7). Du et al. in 2019 reported that the oxidation reaction performed in aceto-
nitrile with f[Ag2(trz)2][Ag24(trz)18]g[PW12O40]2, giving 3 h 96% conversion [44]. Jia et al.
reported the catalytic activity of [CuI 6(3atrz)6][PMo12O40]2 in the epoxidation of vari-
ous alkenes (Table 3, entry 9) [45]. Taghiyar and Yadollahi carried out the epoxidation
of cyclooctene catalyzed by [HxPMo12O40ꢂH4Mo72Fe30O254(CH3COO)15(H2O)98] in

10
S. S. HOSSEINYZADE ET AL.
Figure 8. Reusability of 1 for the oxidation of cis-cyclooctene at optimum conditions. Reaction con-
ditions: 0.5 mmol cyclooctene, 1.12 mmol TBHP, 7 mg catalyst, 2 mL C2H4Cl2, 30 min.
Figure 9. FT-IR spectra of recovered 1 nanocatalyst.
ethanol with conversion and selectivity toward epoxide of 82% and 98% [46]. In the
present work, 1 gives 100% conversion and 100% selectivity for epoxidation of cyclo-
octene in 30 min (Table 3, entry 1). Therefore, the present catalysts display the desired
catalytic activity for epoxidation of alkenes compared to other catalysts listed in
Table 3.
The reusability of 1 was studied for oxidation of cis-cyclooctene as a model reaction
using tert-butyl hydroperoxide at optimum conditions. In each experiment, after com-
pletion of the reaction, the catalyst was separated from the reaction mixture by centri-
fugation, washed several times with 1,2-dichloroethane, dried, and then, reused in the
same reaction. The results are given in Table 4. As shown in Figure 8 the recovered
catalysts were reused at least six times without losing activity. These results prove that
the catalytic reactions are retained and the catalysts are not leached to the reaction
medium. The decrease in cyclooctene conversion could be attributed mainly to

JOURNAL OF COORDINATION CHEMISTRY
11
unavoidable loss of the catalyst during collection. The average values of cyclooctene
conversion and epoxide selectivity were 93% and 99%, respectively. The results con-
firm that 1 has good stability for the oxidation of cyclooctene under our investigation
conditions. Furthermore, ICP has been employed to detect/determine the amount of
metal that leached out into the reaction mixture. No detectable molybdenum was
found in the reaction mixtures. The IR spectra of recovered catalysts did not show any
obvious differences from fresh catalysts (Figure 9).
4. Conclusion
We have prepared a nanohybrid heterogeneous catalyst by combination of molybde-
nylacetylacetonate and Schiff base with polyoxometalates and tested its catalytic activ-
ity in the synthesis of various epoxides carried out via reaction olefins with 1
nanocatalyst and using tert-butyl hydroperoxide as oxidant. This catalytic system com-
bines the benefits of biphasic and heterogeneous catalysis, allowing easy separation
of products and catalysts. This catalyst is recyclable, reused at least six times without
significant loss in its catalytic activity.
Acknowledgement
The authors would like to thank the Vice-President’s Office for Research Affairs at Islamic Azad
University, Shahrood for supporting this work.
Disclosure statement
No potential conflict of interest was reported by the authors.
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