Co(II) Schiff base complex decorated on polysalicylaldehyde as an efficient, selective, heterogeneous and reusable catalyst for epoxidation of olefins in mild and self-coreductant conditions

Article abstract of DOI:10.1007/s11164-017-3027-z

Abstract: A new Co(II)-Schiff base complex was decorated on a polysalicylaldehyde (PSA) framework and used as a selective and efficient catalyst for the epoxidation of alkenes in the presence of O₂ as a green source of oxygen without aco-reductant. The catalyst was characterized step by step by FTIR, UV–Vis, ¹H NMR, TGA, CHN, XPS and EDX analyses. Loading an amount of Co ions in the catalyst as well as its leaching amount were studied by an ICP-OES instrument. The catalyst demonstrated excellent activity for the of a variety of alkenes in a mild, inexpensive and efficient protocol. Also, the catalyst can be simply recovered from the reaction mixture and reused for several times without any noteiceable loss in its activity. Reaction parameters including temperature, oxygen flow volume, catalyst amount and solvent were screened by reaction time. Catalytic studies including XPS spectroscopy and some blank experiments were performed to obtain an initial insight into the reaction mechanism. Also, the reactivity of the novel catalyst was evaluated as the turnover frequency. Graphical Abstract: A new and efficient protocol have been developed for the selective epoxidation of olefins using a heterogeneous recyclable Co(II)-Schiff base complex of polysalicylaldehyde in mild and self-coreductant conditions by molecular oxygen.

Full text of DOI:10.1007/s11164-017-3027-z

Res Chem Intermed
DOI 10.1007/s11164-017-3027-z
Co(II) Schiff base complex decorated
on polysalicylaldehyde as an efficient, selective,
heterogeneous and reusable catalyst for epoxidation
of olefins in mild and self-coreductant conditions
Milad Kazemnejadi1 Alireza Shakeri2 Mahsa Nikookar1
•
•
•
Mohammad Mohammadi1 Mohsen Esmaeilpour3
•
Received: 3 April 2017 / Accepted: 26 June 2017
Ó Springer Science+Business Media B.V. 2017
Abstract A new Co(II)-Schiff base complex was decorated on a polysalicylaldehyde
(PSA) framework and used as a selective and efficient catalyst for the epoxidation of
alkenes in the presence of O2 as a green source of oxygen without aco-reductant. The
1
catalyst was characterized step by step by FTIR, UV–Vis, H NMR, TGA, CHN, XPS
and EDX analyses. Loading an amount of Co ions in the catalyst as well as its leaching
amount were studied by an ICP-OES instrument. The catalyst demonstrated excellent
activity for the of a variety of alkenes in a mild, inexpensive and efficient protocol. Also,
the catalyst can be simply recovered from the reaction mixture and reused for several
times without any noteiceable loss in its activity. Reaction parameters including tem-
perature, oxygen flow volume, catalyst amount and solvent were screened by reaction
time. Catalytic studies including XPS spectroscopy and some blank experiments were
performed to obtain an initial insight into the reaction mechanism. Also, the reactivity of
the novel catalyst was evaluated as the turnover frequency.
Electronic supplementary material The online version of this article (doi:10.1007/s11164-017-3027-z)
contains supplementary material, which is available to authorized users.
&
&
Milad Kazemnejadi
miladkazemnejad@yahoo.com
Mohsen Esmaeilpour
m1250m551085@yahoo.com
Alireza Shakeri
Alireza.shakeri@khayam.ut.ac.ir
1
2
3
Department of Chemistry, College of Sciences, Golestan University, Gorgan, Iran
Faculty of Chemistry, University College of Science, University of Tehran, Tehran, Iran
Department of Chemistry, College of Sciences, Shiraz University, Shiraz 71946 84795, Iran
123

M. Kazemnejadi et al.
Graphical Abstract A new and efficient protocol have been developed for the
selective epoxidation of olefins using a heterogeneous recyclable Co(II)-Schiff base
complex of polysalicylaldehyde in mild and self-coreductant conditions by molecular
oxygen.
Keywords Epoxidation Á Polysalicylaldehyde Á Self-coreductant Á Solid
support Á Co-polymeric Schiff base complexes
Introduction
Due to the high reactivity of epoxides, they can be used as useful intermediates,
building blocks and precursors in different industries as well as in organic synthesis
[1, 2]. Epoxide derivatives have been largely used in surfactants, paints, and epoxy
resins and have the potential for the production of bulk materials [3]. Epoxidation of
alkenes is one of the most important and applicable reactions in organic synthesis
[4]. So, finding suitable catalytic systems to provide desirable amounts of
conversion and selectivity seems necessary [5]. The Mukaiyama reaction is a
general method for the aerobic epoxidation of olefins by metal catalysts in the
presence of an aldehyde [6]. Also, Halcon–Acro and Sumitomo processes have been
used for the epoxidation of alkenes [5], but these methods have different by-
products.
Various homogeneous and heterogeneous catalysts have been reported for the
epoxidation of alkene in the presence of molecular oxygen as an inexpensive and
safe oxidant compared to peroxides [7–12]. Due to the ability of homogeneous
catalysts in reversible oxygen bonding and also their high activities, they have
attracted significant attention for the oxidation of alkenes [13–15], but, in a
homogeneous reaction, the isolation of the products and the reuse of the catalyst
after the reaction become difficult [16]. Recently, attention has been focused on
metal–Schiff base complexes for aerobic oxidation of olefins [17]. A diversity of
transition metal complexes have been used as heterogeneous catalysts for alkene
epoxidation [18], including Au, Ti [19], Fe/V [20, 21], Mn [22, 23], Ni [24], Yb [25]
and W [26]. Co complexes are known as efficient catalysis for the epoxidation of
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Co(II) Schiff base complex decorated on…
olefins [27], and the potential of Co complexes for the aerobic epoxidation of olefins
has been known for decades [28]. Previously, Salavati-Niasari et al. [29] have
reported the epoxidation of olefins by Co-Schiff base complex-supported
MWCNTs.
Lately, Hu et al. [30] have shown the catalytic activity of Co(OAc)2 in ionic
liquid with hydrogen peroxide. Various heterogeneous Co-catalysts have been
reported for the successful aerobic epoxidation of olefins, including: the cobalt-
based metal–organic framework STA-12 [31], Co-substituted zeolite [32], CoAlPO-
36 [33], CoOx-MCM-41 [34], cobalt complexes immobilized on HMS modified by
organic amines [35], Co-hydroxyapatite [36], and so on [37]. Immobilization of Co
complexes on solid polymer supports is another strategy for the preparation of
heterogeneous catalysts that address some technical and economical obstacles [38].
Most recently, much attention has been paid to this strategy in order to catalyze
various types of reactions [39]. Some reports showed higher epoxidation selectivity
for polymer-supported metals than unsupported methods [40]. Nevertheless, not
much has so far been reported for the use of polymer-bounded cobalt complexes for
the epoxidation of olefins [41], among whichare: (1) Co complex immobilized on
polyaniline which selectively epoxidized N-cinnamoyl amino acid in the presence of
molecular O2 and 2-methylpropanal [42], (2) silica-supported Co carboxylate [43],
(3) polybenzimidazole-supported Co salt [44], and (4) Co complex supported on
conductive polymer [45]. Previously, Kowalski et al. used cobalt-supported
polyaniline, poly-o-toluidine, and poly-o-anisidine as heterogeneous efficient
catalysts for the epoxidation of trans-stilbene [46]. Recently, a polyoxometalate-
based framework has been developed by Haddadi et al. [3] as a heterogeneous
catalyst for the epoxidation of alkene with aqueous H2O2 in acetonitrile. Their
system also showed higher catalytic activity for heterogeneous catalysts compared
to corresponding homogeneous catalysts.
Due to environmentally benign and cost-effective aspects, the aerobic epoxida-
tion of olefins without using co-reductants has attracted a large volume of attention
from scientists and, from this point of view, cobalt-based catalysts showed a better
performance among the other transition metals [47–49]. The Co-reductant is usually
used as an oxygen acceptor in order to achieve satisfactory yields and selectivity
[36]. First, free co-reductant Co-catalyst aerobic epoxidation of olefins was reported
by Tang et al. [50] through introducing Co2? into zeolite. Epoxidation of alkenes
with free co-reductants has also been reported with Fe [51] and Ru [52, 53]. A
sacrificial reductant mixed with O2 such as H2 [54], Zn powder [55] and N2O
[56, 57] have been other attempts for the environmental epoxidation of alkenes.
However, due to environmental, safety, and economic considerations, the utilization
of molecular oxygen alone for a catalytic epoxidation is more desirable.
Most of the mentioned methods for the aerobic epoxidation of olefins suffer from
a lack of selectivity, the need for toxic materials as co-reductant (aldehydes, acetals,
and b-ketoesters), toxic solvents, toxic and harmful peroxides, long reaction times
and unsatisfactory yields. Therefore, the development of a new catalytic system to
overcome these shortcomings and fulfill the criteria of a mild and efficient protocol
for the epoxidation of olefins is still necessary. Salicylaldehyde is a safe, readily
available organic compound from a health and reactivity point of view and can be
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M. Kazemnejadi et al.
extracted from natural sources such as buckwheat [58]. Also, salicylaldehyde is
known as one of the constituents of beaver Castoreum (Fig. 1) [59]. In the present
work, we synthesized polysalicylaldehyde by self-polycondensation reaction of
2-hydroxy-5-chloromethyl-benzaldehyde, and then designed a polymer-Co complex
through complexation of a Schiff base ligand on polysalicylaldehyde chains to
Co(II) metals as a new heterogeneous, low-cost polymeric catalyst for the
epoxidation of different alkenes in the presence of molecular oxygen as a green
oxidant and acetonitrile as a solvent.
Experimental
Materials and instrumentation
All materials were purchased from Sigma Aldrich and Merck and used as received
without any purification. FT-IR spectra were obtained using a Bruker Equinox 55
FT-IR spectrophotometer using KBr pellets. The 1H-NMR spectra were recorded on
a Bruker Avance DPX 250 MHz spectrometer in DMSO-d6 using tetramethylsilane
(TMS) as an internal reference. Elemental analyses (C, H, N) were performed on a
Perkin Elmer-2004 instrument. UV–Vis analyses were performed on a UV
Spectrolab BEL photonics. Thermal studies were performed on a NETZSCH
STA 409 PC/PG in a nitrogen atmosphere with a heating rate of 20 °C/min in the
temperature range of 25–750 °C. ICP analysis was performed by a VARIAN
VISTA-PRO CCD simultaneous ICP OES instrument. Gel permeation chromatog-
raphy (GPC) was acquired by a Knauer advanced scientific instrument (Germany),
with a RI detector (Smartline 2300) PL gel 10 lm, 10E3 A° column. Monodis-
persed poly(methyl methacrylate), PMMA standards were used for calibration; the
injected volume was 20 lL. An Oxford INCA 350 energy-dispersive X-ray
microanalysis system connected to the Hitachi S-4800 field emission scanning
electron microscope was used for the energy dispersive X-ray spectrometry (EDX)
measurement. The reaction progress was monitored by gas chromtography (GC).
X-ray photoelectron spectroscopy (XPS) measurements were performed using an
XR3E2 (VG Microtech) twin anode X-ray source with AlKa = 1486.6 eV. GC
analyses were recorded on a Shimadzu-14B gas chromatography instrument
Fig. 1 Images of buckwheat (a) and Castoreum (b)
123

Co(II) Schiff base complex decorated on…
equipped with a HP-1 capillary column (30 m, 0.25 mm, 0.25 lm) and N2 and
anisole as carrier gas and internal standard, respectively.
Preparation of PSA and PSA–Schiff base ligand
2-hydroxy-5-chloromethyl-benzaldehyde were synthesized and purified according
to the previousely described procedure [60]; salicylaldehyde (10 mmol),
paraformaldehyde (0.49 g, 16.4 mmol) and HCl 37% (80 mmol) along with several
drops of concentrated H2SO4 as a catalyst were mixed together at 70 °C and stirred
for 20 h (Scheme 1). The reaction mixture was cooled to room temperature and then
water (20 mL) was added to the mixture and the product was extracted into CH2Cl2
(20 mL). Some anhydrous Na2SO4 was used for drying the organic phase. The
CH2Cl2 was removed with a rotary evaporator and the thick purple oil was set aside
overnight, solidified upon standing and was isolated as a powder (9.5 mmol, 95%
yield). Polymerization of 2-hydroxy 4-chloromethyl benzaldehyde 1 (Scheme 1)
(6 mmol), was performed in the presence of concentrated KOH (50%, 20 mmol) at
80 °C which, after 16 h stirring, produced a dark yellow solid (PSA). This was
filtered and washed with distilled water (3 9 10 mL) in order to completely
eliminate acid and inorganic salts impurities (including KOH and KCl which can be
formed during the polycondensation reaction with the release of HCl molecules) and
placed in an oven (50 °C) overnight. The final mass for 2 was 0.83 g
(m.p. [ 400 °C). PSA (2 g, Mw * 2190) along with 2-aminophenol (6 mmol)
were added to 20 mL of ethanol. The reaction was stirred at room temperature for
12 h until a pale orange product was obtained which shows the formation of the
Schiff base. The product was filtered, washed with methanol (2 9 5 mL) and
isolated (1.6 g) after drying in an oven (50 °C).
Scheme 1 General scheme for preparation of the PSA–Schiff base-Co(II) complex 4 (proposed
structure)
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M. Kazemnejadi et al.
Molecular weight studies of PSA
Analytical method The average molecular weight of the PSA was measured
through acylation of the hydroxyl groups (end group analysis); an analytical method
determined the hydroxyl number (%OH) by following Eq. (1) [61, 62]:
ðV1 À V2Þ Â f  0:0085  100
%OH ¼
ð1Þ
mp
where V1 (mL) and V2 (mL) are the consumed volumes of potassium hydroxide
required for titration of the blank and polymer samples, respectively, f is the
coefficient of KOH solution 0.5 N thwhichat is equal to 0.732 [63], and finally mp
(g) is the mass of the polymer sample weighed for titration. The molecular weight
ꢀ
obtained by this method will be the numerical molecular weight (Mn) which was
measured by insertion of %OH into Eq. (2) [61, 62]:
56:11 Â 1000
ꢀ
Mn ¼
ð2Þ
%OH
To measure the hydroxyl number of the PSA, 0.017 g was weighed and dissolved
in pyridine/acetic anhydride mixture (88:12) along with stirring at 50 °C for 8 h.
One or two droplets of phenolphthalein indicator were added and the reaction
mixture was then titrated with KOH (0.5 N). The hydroxyl number of the PSA was
ꢀ
measured as 25.62 (V2 - V1 = 0.7 mL, Eq. 1). So the Mn was calculated by Eq. 2,
equal to 2190 for PSA.
GPC analysis The molecular weight of the PSA was also studied with gel
permeation chromatography (GPC). The results obtained from GPC are tabulated in
ꢀ
Table 1. The numerical molecular weight obtained (Mn) by GPC was equal to 2226.
Small differences (just 36 units) between the analytical and instrumentational
ꢀ
measuring (GPC) for the numerical average molecular weight (Mn) of the PSA
ꢀ
exhibited the accuracy of the methods as well as the obtained amount for Mn. PSA
has a well-polydisperse index (PDI = 1.43). An explanation for this low PDI was
Table 1 Data obtained from GPC analysis of PSA
Types of molecular weight
Weight average molecular weight
Numerical average molecular weight
Z average molecular weight
Z ? 1 average molecular weight
Polydispersity index
2278
2226
2328
2377
1.430
2244
1.028
1.056
Peak molecular weight
Z average/weight average
Z ? 1 average/weight average
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Co(II) Schiff base complex decorated on…
the step-growth polymerization of 2-hydroxy-5-chloromethyl-benzaldehyde in
alkali medium. The alkali medium neutralized the HCl molecules that were formed
during the condensation reaction. This acts as a driving force for promoting the
polymerization reaction and providing such a low PDI. The obtained molecular
weight for the PSA clearly confirms the formation of PSA through polycondensation
reaction.
Complexation of Co to PSA–Schiff base ligand
The PSA–Schiff base Co(II) complex was prepared by the addition of ethanol
(15 mL) to the PSA–Schiff base ligand (0.25 g) at 70 °C (underwent reflux). The
reaction was stirred for 20 min, then Co(OAc)2 (2.5 mmol) was added slowly to the
mixture during 10 min. Stirring lasted for another 4 h. The brown sediments were
filtered and washed with methanol (2 9 5 mL). The resultant product was placed in
the oven overnight and then isolated as ba rown powder (final isolated mass was
0.50 g).
Elemental analysis along with the color of the samples are shown in Table 2. The
percentages of C and H of 2-hydroxy-5-chloromethyl-benzaldehyde clearly proved
its formation. Due to polymerization, the percentage of C in the PSA was raised to
73.8%. The PSA–Schiff base ligand shows 5.84% for N which was the result of
imine formation. Also, after the formation of the metal complexes, the C, H and N
percentages reduced which demonstrated the insertion of Co in the PSA–Schiff base
ligand. Table 2 shows the physical properties of the samples including their
elemental analysis and colors.
General procedure for catalytic epoxidation of alkenes
Epoxidation of olefins was carried out in a 25-mL three-necked round-bottom flask
equipped with O2 gas (as an oxidant) inlet (*1 atm., bubbling 15 mL/min), water
condenser and magnet stirrer bar. In a typical run, a mixture of alkene (10 mmol)
and 50 mg of catalyst 4 (2 mol%) were added to acetonitrile (20 mL) and the
reaction was kept in a constant temperature oil bath at 60 °C. Molecular oxygen
with a rate of 15 mL/min-1 was bubbled during the reaction. The reaction mixture
was stirred vigorously for a sufficient time. After completion of the reaction, the
Table 2 Elemental analysis and colors of the compounds
Compounds
Color
Elemental analysis
%C
%H
%N
2-Hydroxy-5-chloromethyl-benzaldehyde
PSA
Purple
56.82 (56.33)a
70.11
4.25 (4.14)a
4.35
–
Yellow
–
PSA–Schiff base ligand
PSA–Schiff base Co(II) complex
Orange
73.81
5.27
5.84
7.02
Dark brown
65.27
3.31
a
Theoretically calculated
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M. Kazemnejadi et al.
catalyst was filtered off and the liquid organic product was quantified by GC. Also,
the pure product was separated from the reaction mixture by silica-gel plate or
silica-gel column chromatography, then their structures were identified by 1H NMR
and 13C NMR spectra. Activity of the catalyst was expressed as the turnover
frequency (TOF) and calculated from the moles of cyclohexene converted per mole
of cobalt contained in the catalyst using the following equation:
Moles ðConverted cyclohexeneÞ
TOF ¼
ð3Þ
Moles of Co (Active site) Â Reaction time (h)
The conversion and selectivity of the cyclohexene epoxide were calculated by the
following equations:
ðInitial mol%Þ À ðFinal mol%Þ
Conversion (mol%) =
 100
ð4Þ
Initial mol%
GC peak area of cyclohexene epoxide
GC peak area of all products
Cyclohexene epoxide selectivity =
 100
ð5Þ
The recovered catalyst which was obtained by simple filtration was washed with
acetone (2 9 10 mL), dried at 50 °C for 1 h and then reused for the next run in the
same conditions.
Results and discussion
Characterization of PSA and catalyst
FTIR spectra
Figure 2 shows the FTIR spectra of the samples. A new peak which appeared at
1481 cm-1 in the spectrum of 2-hydroxy-5-chloromethyl-benzaldehyde 1 was
attributed to the bending vibration of the methylene groups (-CH2-) (Fig. 2a).
Another characteristic peak for 1 was 725 cm-1 for the vibration of C–Cl, which
demonstrated successfull formation of the chloromethylene substitution on the
salicylaldehyde. After polymerization of 1, the stretching vibration of C–Cl in all
the spectra at *760 cm-1 showed the presence of the chloromethylene group in the
terminal of the PSA chains (Fig. 2b). Moreover, the spectra showed C-H bending
vibrations at 1400–1480 cm-1 for the chloromethylene group and confirmed the
introduction of a –CH2Cl group at the para-position compared with the hydroxyl
group. Aromatic and aliphatic stretching vibrations of C-H can be seen in all the
spectra in the ranges of *3030 to 3710 and *2746 to 3000 cm-1, respectively. The
changes in the stretching vibration of the hydroxyl groups in the samples gives
useful information about the structure of the samples. Due to the intense
polymerization of the salicylaldehyde, the stretching vibration of hydroxyl groups
at 3200 cm-1 has been largely reduced (Fig. 2, dotted circle). Formation of the
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Co(II) Schiff base complex decorated on…
Fig. 2 FTIR spectrum of a 2-hydroxy-5-chloromethyl-benzaldehyde 1, b PSA 2, c PSA–Schiff base
ligand 3 and d PSA–Schiff base Co(II) complex 4
Schiff base moiety on the PSA in the spectrum of the PSA–Schiff base ligand
(Fig. 2c) provided a broad peak at 3200–3300 cm-1 representing the O–H phenolic
groups with H-bonding.
The main characteristic peak for the PSA–Schiff base ligand was 1612 cm-1
related to the vibration of the Schiff base (C=N) bond which corroborated the imine
formation. Coordination of Co(II) to N and O atoms in the PSA–Schiff base ligand
causes the shift of C=N stretching vibrations to lower wavenumbers at 1589 cm-1
(Fig. 2d) [14]. Vibrations corresponding to Co–O and Co–N were found at 540 and
123

M. Kazemnejadi et al.
617 cm-1, respectively [52, 64]. Furthermore, this chelation reduced the intensity of
the hydroxyl stretching vibration (greatly eliminating the H-bonding) at catalyst 4
and illustrated that chelation of Co metals also occurs through oxygen atoms
(Fig. 2d).
1H NMR spectroscopic studies
2-Hydroxy-5-chloromethyl-benzaldehyde 1 has been previously synthesized and,
due to the presence of two directing groups on the phenyl, the chloromethylene
group substituted at the para-position hydroxy group [65]. 1H NMR of 2-hydroxy-5-
chloromethyl-benzaldehyde 1 and PSA are shown in Fig. 3. Resonance for
methylene protons was found at 4.58 ppm. Chemical shifts for aldehyde and
hydroxyl protons for 1 were found at 9.88 and 11.06 ppm, respectively (Fig. 3a).
Methylene protons for PSA 2 are represented with three peaks at 4.09, 4.74 and
4.98 ppm (Fig. 3b). Aromatic protons were set in the region of 6.97–7.57 ppm for 1
1
and 7.11–8.33 ppm for PSA 2. H NMR spectrum of PSA shows two peaks for
aldehyde at 10.22 and 10.41 ppm due to different chemical environments. Hydroxyl
groups at terminal PSA chains exhibited a single peak at 11.33 ppm (Fig. 3b).
Electronic spectra
The electronic spectra of the samples were scrolled in the region of 200–800 nm.
Figure 4a shows UV–Vis spectra for the PSA, PSA–Schiff base ligand and PSA-
Co(II) complex. A single peak at *336 nm with a shoulder at 296 nm in the
electronic spectrum of PSA was assigned to p ? p* and n ? p* transitions related
to the phenyl and carbonyl groups, respectively. The PSA–Schiff base ligand 3
exhibited three adsorption bands at 298, 356 and 444 nm [66, 67]. These
adsorptions were due to n ? p* and p ? p*, resulting in the formation of an
imine bond and also p ? p* transitions for the benzene rings. The PSA–Schiff base
Co(II) complex showed a broad peak at 440 nm resulting in d–d transitions for
Co(II) [52]. Adsorption of p electrons related to the benzene rings appear as a
shoulder at 374 nm for the catalyst 4 (Fig. 4a). UV–Vis spectra reported six-
coordinated Co complexes and showed an absorption band at 400–500 nm which
confirms that the coordination of Co in the catalyst 4 was six [52].
Thermal analysis
Thermal studies of the PSA, PSA–Schiff base ligand and PSA–Schiff base Co(II)
complex are shown in Fig. 4b. The spectra showed good thermal stability for all
three samples. Thermal decomposition of PSA (Fig. 4b) occurs only in one step in
the temperature span of 200–525 °C corresponding to a 47% weight loss. After that,
the weight loss continues to 750 °C with a gentle slope which showed carbonization
of the polymer. Advent of an additional weight loss step in the TGA curve of PSA–
Schiff base ligand was attributed to the decomposition of the Schiff base moiety on
the PSA chain and confirmed the Schiff base formation on the PSA. This
decomposition occurs in the temperature range of 99–360 °C (8% weight loss). The
123

Co(II) Schiff base complex decorated on…
Fig. 3 1H NMR spectrum of a 2-hydroxy-5-chloromethyl-benzaldehyde 1 and b PSA 2
second stage, which starts at 420 °C, shows decomposition of the PSA. Insertion of
Co in the PSA–Schiff base ligand leads to about a 32% (300–500 °C) weight loss in
the thermal spectrum of the PSA–Schiff base-Co(II) complex (Fig. 4b) in the first
123

M. Kazemnejadi et al.
Fig. 4 a UV–Vis spectra of PSA, PSA–Schiff base ligand and PSA–Schiff base Co(II) complex; b TGA
analysis of PSA, PSA–Schiff base ligand and PSA–Schiff base Co(II) complex
stage. According to the thermal degradation of the Schiff base moiety in the PSA–
Schiff base ligand, this amount was in accordance with the loading amount of Co
obtained from inductive coupled plasma (ICP) analysis. The second step also
belonged to PSA decomposition.
ICP meseaurment
The loading amount of Co on the PSA–Schiff base ligand was measured by ICP
analysis. The experiments indicated that 23.57 mg (in 100 mg of catalyst) of Co
was loaded on 100 mg of the PSA–Schiff base ligand framework.
EDX analysis
The elements of the catalyst 4 were detected by energy dispersive spectroscopy
(EDX) analysis. Related information including intensity and weight of the elements
are shown in the inset table of Fig. 5. The EDX spectrum clearly confirmed the Co
complex formation on the PSA framework. Also, the spectrum shows the attendance
of Cl atoms set at the end chains of the PSA framework.
Fig. 5 EDX spectra of the
PSA–Schiff base-Co complex
123

Co(II) Schiff base complex decorated on…
On the basis of the reported structure for the Co(II) complex by Refat et al. [52]
along with the achieved results from FTIR, UV–Vis, TGA and EDX analyses, the
most probable structure for catalyst 4 is shown at Scheme 1.
Investigation of effective parameters for oxidation reactions
The influence of reaction parameters has been thoroughly investigated on the
epoxidation of cyclohexene. The solvent, temperature, catalyst amount and
molecular oxygen flow rate affected epoxide selectivity and conversion of the
reactions and the corresponding results are shown in Fig. 6a–d. The solvent plays an
important role in an epoxidation reaction [68]. High possible conversion (88%) and
epoxide selectivity (89%) were achieved in acetonitrile as solvent. Other tested
solvents (acetone, EtOH, MeOH, CHCl3 and DCM) did not show a good
performance compared to acetonitrile (Fig. 6a). Increasing the temperature to
60 °C enhanced the cyclohexene conversion, and the conversion increased almost
proportionally with increasing temperature (Fig. 6b). No more conversion could be
achieved at a temperature higher than 60 °C (Fig. 6b). The most epoxide selectivity
was also obtained at 60 °C (88%) for 6 h. The chart in Fig. 6b (right) shows that the
epoxide selectivity strongly depended on the temperature and that the catalyst did
not have good selectivity at low temperatures. Also, the catalyst amount had an
impressive role in the progress of the reaction (Fig. 6c). The reaction did not make
any progress in the absence of the catalyst 4 (zero conversion, not shown in Fig. 6).
Oxygen flow rate had the least effect on the reaction conversion and selectivity
(Fig. 6d) among the other reaction parameters. This phenomenon demonstrates that
the reaction rate was slower than the consumption of oxygen, proving that no large
difference is seen between air and O2 as oxidative agents. Above 15 mL/min, the O2
flow rate had little influence on the epoxide selectivity. A flow rate of 15 mL/min of
oxygen was sufficient to reach the highest possible conversion (Fig. 6d, 88%).
Increasing the flow rate above 15 mL/min did not enhance the conversion.
The reactions were performed in the absence of any aldehyde as coreductant. It is
believed that the free aldehyde groups in the PSA framework in catalyst 4 play the
role of co-reductant in the epoxidation of olefins.
TOF amounts for each effective parameter of the epoxidation reaction were also
measured in order to measure the activity effect of the catalyst on temperature,
solvent, molecular oxygen flow rate and catalyst amount in the reaction (Fig. 7).
There was a good correlation between epoxide selectivity, percentage of conversion
and TOF amounts in all cases. Optimization of each effective parameter causes the
enhancement of TOF.
Catalytic epoxidation of olefins
We then submitted various alkenes to the optimized reaction conditions: cyclohex-
ene (10 mmol), catalyst 4 (50 mg, 2 mol%), acetonitrile (20 mL), O2 (*1 atm.,
bubbling 15 mL/min), 60 °C, Time.
The catalytic activity of the PSA–Schiff base Co complex was investigated in the
epoxidation of various types of olefins in O2 atmosphere, and the corresponding
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M. Kazemnejadi et al.
Fig. 6 Influence of (a) solvent, (b) temperature, (c) catalyst amount and (d) air flow volume on
conversion in the epoxidation of cyclohexene. The reaction selectivity corresponding to each parameter
are depicted as right column charts. a Reaction conditions: cyclohexene (10 mmol), catalyst 4 (50 mg,
2 mol%), solvent (20 mL), O2 (*1 atm., bubbling 15 mL/min), 60 °C, 5 h. b Reaction conditions:
cyclohexene (10 mmol), catalyst 4 (50 mg, 2 mol%), solvent (20 mL), O2 (*1 atm., bubbling 15 mL/
min), temperature, 6 h. c Reaction conditions: cyclohexene (10 mmol), catalyst 4, solvent (20 mL), O2
(*1 atm., bubbling 15 mL/min), 60 °C, 6 h. d Reaction conditions: cyclohexene (10 mmol), catalyst 4
(50 mg, 2 mol%), solvent (20 mL), O2 (*1 atm., bubbling), 60 °C, 6 h
results are tabulated in Table 3. Among the endocyclic alkenes, cyclooctene with 92
and 98% conversion and selectivity, respectively, were the most reactive of all. This
reactivity arises from the stability of cyclooctene as well as the high reactivity of its
double bond [69]. Cyclohexene showed 89% epoxide selectivity. The potential of
active allylic hydrogens to oxidation was responsible for reducing the epoxide
123

Co(II) Schiff base complex decorated on…
Fig. 7 Calculated TOF amounts for the epoxidation of cylcohexene for a different solvents, b different
temperatures, c various amounts of catalyst 4 and d various air flow volumes. For reaction conditions, see
caption to Fig. 6
selectivity for cylcohexene. Open-chain alkenes such as styrene (Table 3, entry 1)
and 1-octene (Table 3, entry 7) are epoxidized with moderate efficiency. The
present method demonstrated low side reaction as shown for most of the entries in
Table 3. Furthermore, catalyst 4 performed completely chemoselectively for the
epoxidation of terminal alkene compared with the terminal a,b-unsaturated terminal
alkene demonstrated for 2-methylhexa-1,5-dien-3-one (Table 3, entry 12). Simi-
larly, 1H-indene epoxidized with low efficiency (Table 3, entry 5). These results
indicated that conjugated C=C bonds caused lower efficiencies compared to non-
conjugated types. This property provided selectivity in the case of 2-methylhexa-
1,5-dien-3-one (Table 3, entry 12) in which just terminal alkene was bearing the
epoxidation. Chemoselectivity behavior of the catalyst 4 was also tested with
substrates involving two C=C functional groups (Table 3, entries 10, 13). The
method presented good selectivity toward the epoxidation of one equivalent C=C
bond for the mentioned substrates. Also, high selectivity was achieved for the other
substrates (Table 3). Steric hindrance on the C=C bond not only increased the
recation time but also decreased the conversion of the olefins such as 3,3-
dimethylhex-1-ene (Table 3, entry 8) and 1-methoxy-2-methylprop-1-ene (Table 3,
entry 11).
Recycling study of the catalyst
It is well known that the advantages of a heterogeneous catalyst lie in the ease of
recoverability, long-term stability, minimum leaching and minimum change in its
activity [27]. Stability and recoverability of the catalyst were studied in the
epoxidation of cyclohexene in the obtained optimum conditions. Figure 8 shows the
123

M. Kazemnejadi et al.
Table 3 Oxidation of alcohols to carbonyl compounds
(a)
(b)
Epoxide
TOF
(%)a
Alkene
(a)
Product
(b)
Time Conversion Yield
Entry
1
selectivity
(h–1)
(h)
(%)a
(%)b
5.5
84
80
92
7.6
2
3
4.2
1.0
88
89
82
83
89
96
10.5
44.5
4
5
6
10.2
6.0
86
55
87
79
50
85
92
76
95
4.2
4.5
4.2
10.4
7
8
10.0
27.0
90
80
85
73
75
66
4.5
1.4
9
3.0
3.0
92
88
78
90
83
70
98
77
80
15.4
14.6
2.8
10
11
14.0
12
13
2.2
1.5
83
87
76
85
81
79
18.9
29.0
Reaction conditions: alkene (10 mmol), catalyst 4 (50 mg, 2 mol%), 60 °C, CH3CN (20 mL), O2
(*1 atm. bubbling 15 mL/min), time
a
GC analysis; using an internal standard technique (Anisole) and are based on alkenes
b
Isolated yield
results obtained from the retrieval ability of the catalyst 4 including the leaching
amount of the catalyst, conversion of cyclohexene and epoxidation selectivity. The
catalyst was successfully recovered after completion of the reaction and charac-
terized with FTIR spectra and reused for 5 runs without noticeable reactivity or
selectivity loss. Also, amounts of metal leaching were measured by ICP analysis
after every run. The results showed 3.5% cobalt leaching after the 5th run which
corresponds to 80% cyclohexene conversion. Also, the experiments did not show
123

Co(II) Schiff base complex decorated on…
Fig. 8 Conversion and selectivity percent as well as leaching amount percent of spent catalysis in
reaction cycles. Reaction conditions: alkene (10 mmol), catalyst 4 (50 mg, 2 mol%), 60 °C, CH3CN
(20 mL), O2 (*1 atm, bubbling 15 mL/min), 4.2 h
any considerable drop of selectivity during the reaction cycles and just 2% loss
occurred in selectivity after 5 runs. These data strongly confirm the stability and
reactivity of the PSA–Schiff base-Co complex as an efficient catalyst for the
epoxidation of alkenes.
Mechanism studies
Heterogeneous salen and salen-type cobalt complexes have been reported to show
catalytic activity for the epoxidation of olefins in the presence of O2, although a
sacrificial co-reductant, such as isobutyraldehyde [11] or DMF as a solvent and co-
reductant [31], was necessary.
For elucidation of the sacrificial co-reductant activity of the catalyst (substrate),
we prepared the monomeric analogue of the catalyst and performed the epoxidation
reaction under the same conditions as described for the epoxidation of cyclohexene
in Table 3. As shown at Scheme 2, N-salicylidene-o-aminophenol showed that the
reaction of 2-aminophenol with salicylaldehyde was prepared with the previously
described procedure [52], and complexed to Co and used as a catalyst for the
epoxidation of cyclohexene in optimum conditions (cyclohexene (10 mmol),
catalyst 5 (50 mg), 60 °C, CH3CN (20 mL), O2 (*1 atm., bubbling 15 mL/min),
4.2 h).
Scheme 2 Preparation of N-salicylidene-o-aminophenol (saphH2)
123

M. Kazemnejadi et al.
Fig. 9 XPS spectra of the catalyst (blue curve) and the recovered catalyst result in the epoxidation of
cyclohexene (green curve). (Color figure online)
The results demonstrated just 17% conversion for cyclohexene. It is believed that
the absence of any reductant stops the epoxidation cycle. Surprisingly, the addition
of the polymeric ligand, PSA 3, to the reaction mixture caused the progress of the
reaction to 56% after 6 h, which was evidence for the effect of the substrate as a
sacrificial agent.
Furthermore, in order to be completely sure about the oxidation state of Co in the
complex, we used X-ray photoelectron spectroscopy (XPS) analysis. The XPS
spectra demonstrated that all the cobalt species in the fresh and recovered catalyst 4
presented in the ?2 oxidation state, corresponding to the binding energy of 797 (Co
2p1/2) and 781 eV (Co 2p3/2) [70–72]. The results clearly proved the sacrificial co-
reductant activity of the catalyst 4 which can completely reduce the Co(III) to Co(II)
with free aldehyde groups in its framework (Fig. 9).
According to the reported mechanisms in the literature for Co complexes
catalyzed epoxidation [31, 69, 73] along with our observations, we propose a
mechanism for the PSA–Schiff base Co(II) complex-catalyzed epoxidation of
alkenes (Scheme 3). In this mechanism, the catalyst 4 is assumed to play two roles.
First, epoxidation of alkene through Co-superoxo radicals (which are sensitive to
radical scavengers) results in the reaction of Co(II) sites with molecular oxygen;
second, the presence of the free aldehyde group in the framework of the catalyst 4
causes the reduction of Co(III) to Co(II) species (Scheme 3). The reaction of the
catalyst 4 with molecular oxygen gave Co-superoxo species [31, 69, 73]. Epox-
idation of the substrate was performed by Co-superoxo species and gave an epoxide
product and LCo(III)O. species. Then, the reduction of Co(III) to Co(II) species
occurred by the oxidation of free aldehyde groups on the catalyst 4. This mechanism
explains both the presence of the Co(II) and Co(III) sites in the catalyst in the
reaction mixture and the role of the substrate as an activating agent for the reduction
of Co(III) to Co(II) species.
In order to demonstrate the presence of radical species (LCo(III)-O.) in our
reactions, hydroquinone, as a radical scavenger, was added after 2 h of the reaction,
and the reaction progress was monitored by GC. It was observed that the reaction
stopped immediately upon addition of the radical scavenger (Fig. 10, blue curve),
123

Co(II) Schiff base complex decorated on…
Scheme 3 Proposed mechanism for PSA–Schiff base Co(II) complex
Fig. 10 Comparison between
reactions with hydroquinone as a
radical scavenger (blue curve)
and a control reaction without
adding a radical scavenger
(orange curve). (Color figure
online)
which suggests that the process indeed proceeds via formation of LCo(III)-O.
species.
To test the worth of the present work, we comprised our catalyst with some of the
reported catalysts in the literature for the epoxidation of cyclooctene (Table 4).
Cyclooctene oxide was formed by the PSA–Schiff base Co(II) complex in a short
reaction time in CH3CN at 60 °C.
Also, the advantage of the present method is that epoxidation occurred by a
heterogeneous Co(II) catalyst without the need for an external co-reductant in mild
conditions by using O2 as a green oxidant in short reaction times. It uses a readily
123

M. Kazemnejadi et al.
Table 4 Comparison of catalytic activity of the present catalyst with the other reported catalysts for the
epoxidation of cyclooctene
Entry
Catalyst
Conditions
Time (h)
Epoxide
yield (%)
References
1
2
3
POMs-MOF
CH3CN/H2O2/50 °C
CH3CN/anisole/H2O2/r.t
DMF/O2/100 °C
5
24
8
82
75
47
[3]
Amberlite-HCO3
[74]
[11]
Cobalt(II)-
exchanged
zeolite X
4
5
Cobalt(III) Schiff
base complex
CH3CN/isobutyraldehyde/air/
60 °C
DMF/H2O2/air/100 °C
8
6
92
11
[69]
[75]
Co(II)-exchanged
SAPO-34
6
7
8
Ti-LHMS-3
MnO
CH3CN/TBHP/Reflux
DMF/NaHCO3/H2O2/r.t
22
5
68
19
24
[76]
[77]
[78]
Co(ac)2.4H2O
CH3CN/isobutyraldehyde/air/
60 °C
CH3CN/isobutyraldehyde/air/
8
9
Co(acac)3
8
3
25
90
[78]
60 °C
CH3CN/O2/60 °C
10
PSA–Schiff base
Co(II) complex
Present work
available, safe and environmental friendly material for the preparation of the
catalyst. High selectivity, no tedious work-up and no by-product are other
advantages of the method. Furthermore, a comparison of the catalytic efficiency
of 4 with other cobalt salts (cobalt(II) acetate and cobalt(III) acetylacetonate) in a
homogeneous medium (Table 4, entries 9, 10) clearly indicates that the PSA–Schiff
base Co(II) complex performs as a more efficient reusable catalyst than simple
cobalt salts.
Conclusion
The PSA–Schiff base-Co(II) complex was synthesized through a simple procedure,
and cheap and commercially accessible organic material which can catalyze the
epoxidation of various olefins through a free co-reductant protocol in the presence
of molecular oxygen. The catalyst was characterized with FTIR, UV–Vis, 1H NMR,
CHN, ICP-OES, XPS, EDX and TGA instruments. Average molecular weight of
PSA was found to be 2278. The method demonstrated good chemoselectivity
toward thermnal and on-conjugated olefins. Furthermore, the catalyst can be simply
recovered from the reaction mixture and reused for 5 runs without any
notable reactivity, selectivity and efficiency loss.
Acknowledgement The authors are grateful to Golestan University research council for financial support
of this work.
123

Co(II) Schiff base complex decorated on…
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