Synthesis, characterization, and catalysis of recyclable new piperazine-bridged Mo(VI) polymers [MoO 2(Salen)(piperazine)] n in highly selective oxygenation of alkenes and sulfides

Article abstract of DOI:10.1080/00958972.2013.818671

This study reports synthesis and characterization of polymeric [MoO2(Salen)] complexes that combine MoO2(acac)2 and polymeric ligands bearing N2O2 coordination sites, abbreviated herein as [piperazinCH2{MoO2(Salen)}]n (6), [piperazinCH2{MoO2(Salophen)}]n (7), and [piperazinCH2{MoO2(Salpn)}]n (8). The epoxidation of alkenes using tert-butyl hydroperoxide and oxidation of sulfides to sulfoxides by urea hydrogen peroxide were enhanced with excellent selectivity under the catalytic influence of these coordination polymers. No relevant difference in activity was found due to change in the diamine in the Schiffs base ligands. The stability of polymeric catalysts was assessed by recycling a sample five times in small batch reactions in both epoxidation and sulfoxidation. In the case of cyclooctene epoxidation, catalytic activity of [piperazinCH2{MoO2(Salen)}]n increased without losing the selectivity, whereas the catalytic activity and selectivity of the spent catalyst decreased in sulfoxidation of methyl phenyl sulfide in consecutive runs. Loss of Mo from the coordination polymer has been investigated using the filtrates as potential catalysts in sulfoxidation reactions. Molybdenum leaching degree decreases through five cycles. Moreover, the catalyst can be recovered from the reaction mixture unchanged as determined by IR and UV-Vis spectra.

Full text of DOI:10.1080/00958972.2013.818671

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Synthesis, characterization and catalysis of recyclable
new piperazine-bridged Mo(VI) polymers [MoO2(Salen)
(piperazine)]n in highly selective oxygenation of
alkenes and sulfides
MOJTABA BAGHERZADEH a & MARYAM ZARE a
a Chemistry Department , Sharif University of Technology , Tehran , P.O. BOX 11155-3615 ,
Iran
Accepted author version posted online: 21 Jun 2013.
To cite this article: Journal of Coordination Chemistry (2013): Synthesis, characterization and catalysis of recyclable new
piperazine-bridged Mo(VI) polymers [MoO2(Salen)(piperazine)]n in highly selective oxygenation of alkenes and sulfides, Journal
of Coordination Chemistry, DOI: 10.1080/00958972.2013.818671
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Synthesis, characterization and catalysis of recyclable new piperazine-bridged
Mo(VI) polymers [MoO2(Salen)(piperazine)]n in highly selective oxygenation
of alkenes and sulfides
MOJTABA BAGHERZADEH∗ and MARYAM ZARE
Chemistry Department, Sharif University of Technology, Tehran, P.O. BOX 11155-3615, Iran
This paper reports synthesis and characterization of polymeric [MoO2(Salen)] complexes which
combine MoO2(acac)2 and polymeric ligands bearing N2O2 coordination sites, abbreviated herein
as [piperazinCH2{MoO2(Salen)}]n (6), [piperazinCH2{MoO2(Salophen)}]n (7) and
[piperazinCH2{MoO2(Salpn)}]n (8). The epoxidation of alkenes using tert-butylhydroperoxide
(TBHP) and oxidation of sulfides to sulfoxides by urea hydrogen peroxide (UHP) were enhanced
with excellent selectivity under the catalytic influence of these coordination polymers. No
relevant difference in activity was found due to change in the diamine in the Schiff-base ligands.
The stability of polymeric catalysts was assessed by recycling a sample five times in
small batch reactions in both epoxidation and sulfoxidation. In the case of cyclooctene
epoxidation, catalytic activity of [piperazinCH2{MoO2(Salen)}]n increased without losing the
selectivity whereas the catalytic activity and selectivity of the spent catalyst decreased in
sulfoxidation of methyl phenyl sulfide in consecutive runs. Loss of Mo from the coordination
polymer has been investigated using the filtrates as potential catalysts in sulfoxidation reactions.
Molybdenum leaching degree decreases through five cycles. Moreover, the catalyst can be
recovered from the reaction mixture unchanged as determined by IR and UV-Vis spectra.
Keywords: Oxidation; Heterogeneous catalysis; Molybdenum; Polymeric complexes
1. Introduction
Coordination chemistry of molybdenum(VI) has attracted interest due to a large number of stable
and variable oxidation states as well as coordination numbers which can vary from four to eight.
Dioxomolybdenum(VI) complexes are model systems for active sites of molybdoenzymes such
as nitrogenase, aldehyde oxidase, xanthine oxidase, sulfite oxidase, nitrate reductase and
xanthine dehydrogenase [1-3]. Molybdenum Schiff-base complexes were efficient homogeneous
∗Corresponding author. Email: bagherzadeh@sharif.edu
1

catalysts for olefin epoxidation [4, 5], oxidation of saturated hydrocarbons [6] and oxidations of
sulfides [7, 8], but there are serious problems including the difficulty in separation and recycling
of expensive catalysts, as well as leaching of the active metal into the solvent and insufficient
stability of the catalysts. An approach to solve these problems involves incorporation of metals
into polymers. Polymers containing transition metals in the backbone offer properties of solid
heterogeneous catalysts while retaining most of their original character. The relatively easy
preparation, the local C2 symmetry in the individual Salen units and wide variety of Salen-type
ligands have provided opportunities to prepare Schiff base polymers and to develop catalytic
processes for various reactions. There have been extensive studies on metal-containing polymers.
Reaction conditions such as condensation strategies, linker units, use of endcaps, reaction solvent
and temperature have been studied in polymerization of Salen units [9-11]. Kureshy et al.
incorporated a whole range of Salen complexes {e.g., [Mn(Salen)], [Cu(Salen)] and [V(Salen)]}
into polymers and studied their catalytic activities for various reactions [12-14]. Jacobsen et al.
synthesized the polymeric Co(Salen) catalysts for asymmetric epoxide ring-opening reactions
[15].
In continuation of our research on transition metal complexes with Schiff bases, we
report the synthesis and characterization of polymeric [MoO2(Salen)] complexes and test their
catalytic activity for the oxidation of a range of olefins and sulfides. The reusability of cyclic
polymeric [MoO2(Salen)] complex was also investigated in the reaction of cyclooctene with
TBHP in 1,2-dichloroethane and reaction of methyl phenyl sulfide with UHP in mixture of
methanol/dichloromethane. IR and UV-Vis spectral data of freshly prepared and recovered
catalysts are identical, which indicate the metal complex is intact during the catalytic reaction.
2. Experimental
2.1. Materials and methods
All reagents were used as received from Aldrich unless otherwise noted. MoO2(acac)2 has been
synthesized according to published procedure [16]. FT-IR spectra were recorded on a Unicam
Matson 1000 FT-IR paragon 1000 spectrophotometer. 1H spectra of the monomer and polymeric
ligand were recorded on a Bruker FT-NMR 500 MHz spectrometer. Electronic spectra of
polymers in the UV-Vis region were recorded in DMSO using a Shimadzu UV-Vis scanning
spectrophotometer. The loading and leaching of molybdenum was determined by inductively
2

coupled plasma atomic emission spectroscopy which was recorded on a Perkin-Elmer AA-300
spectrophotometer. Gas chromatography (GC) analyses were performed on an Agilent
Technologies 6890N, 19019 J-413 HP-5, 5% phenyl methyl siloxane, capillary
60.0m×250μm×1.00μm.
2.2. Sample preparation
2.2.1. Preparation of 5-(chloromethyl)salicylaldehyde (1). 1 was prepared by synthetic
methodology previously reported in literature with little modification [17]. To a mixture of
salicylaldehyde (5.8 g, 53 mmol) and paraformaldehyde (1.6 g, 53 mmol) was dropwise added
concentrated HCl (100 mL) at 0 °C. The mixture was stirred at 20 °C for 5 h and extracted with
CH2Cl2 (20 mL), and the organic phase was separated and washed with saturated sodium
hydrogen carbonate solution and deionized water and was dried over anhydrous Na2SO4. The
CH2Cl2 was then removed under reduced pressure to afford the product as a white solid. Yield:
1
94%. H NMR [CDCl3, 500 MHz; δ(ppm)]: 11.10 (s; 1H; OH); 9.93 (s; 1H; CHO); 7.03-7.63
(3H; aromatic ring); 4.63 (s; 2H; CH2).
2.2.2. Preparation of piperazinCH2(sal)2 (2). A solution of 1 (1.50 g, 5.54 mmol) in dry
CH2Cl2 (30 mL) was added dropwise to a stirred solution of Et3N (1.11 g, 11.0 mmol) and
piperazine (0.24 g, 2.77 mmol) in dry CH2Cl2 (15 mL). The yellow solution was stirred for 24 h
at room temperature. The solution was washed with water and the organic phase dried over
anhydrous Na2SO4. Removal of the organic solvent afforded a cream solid. Yield: 68%. 1H NMR
[CDCl3, 500 MHz; δ(ppm)]: 10.94 (s, 2H, OH); 9.93 (s, 2H, CHO); 6.93-7.54 (6H; aromatic
ring); 3.53 (s, 4H, CH2), 2.52 (br s, 8H, CH2 in piperazine). IR (KBr, cm-1): 3113 (br, ν(OH +
H-bonding)); 2939, 2810, 2771(m, νC-H); 1651(s, νC=O); 1592, 1483 (s, νC=C vibration band
of the aromatic ring), UV–vis (DMSO), λmax (ε): (nm (105 dm3 mol-1 cm-1) 260 (0.214), 328
(0.137). Anal. Calcd (found) for C20H22O4N2·¼ H2O: C, 67.00 (67.20); H, 6.30 (6.25); N, 7.80
(7.77).
2.2.3. Preparation of [piperazinCH2(Salen)]n (3). A solution of ethylenediamine (0.06 g,
1 mmol) was added dropwise to a stirred solution of 2 (0.354 g, 1 mmol) in CH2Cl2 (40 mL) and
the mixture was refluxed for 6 h. The solvent was removed under vacuum to produce the yellow
3

solid, which was washed with 5 mL of cooled methanol and then dried in air. Yield: 90%.
1H NMR [CDCl3, 500 MHz; δ(ppm)]: 13.13 (s, 2H, OH); 8.37 (s, 2H, C=N); 6.9-7.3 (6H;
aromatic ring); 3.94 (s, 4H, CH2-N), 3.50 (s, 4H, CH2 attached with benzene ring), 2.52 (br s,
8H, CH2 in piperazine). IR (KBr, cm-1): 3415 (br, ν(OH + H-bonding)); 2928, 2804, 2775 (w,
νC-H); 1634 (s, νC=N); 1583, 1490 (s, νC=C vibration band of the aromatic ring). UV–vis
(DMSO), λmax (ε): (nm (105 dm3 mol-1 cm-1) 260 (0.183), 320 (0.084), 405 (0.007). Anal. Calcd
(found) for C22H26N4O2·CH3OH: C, 67.29 (67.63); H, 7.37 (7.31); N, 13.65 (13.56).
2.2.4. Preparation of [piperazinCH2(Salophen)]n (4). A solution of orthophenylenediamine
(1 mmol) was added dropwise to a stirred solution of 2 (0.108 g, 1 mmol) in CH2Cl2 (10 mL) and
the mixture was refluxed for 6 h. The solvent was removed under vacuum to produce the yellow-
orange solid, which was washed with 5 mL of cooled methanol and then dried in air. Yield: 88%.
1H NMR [CDCl3, 500 MHz; δ(ppm)]: 12.98 (s, 2H, OH); 8.64 (s, 2H, C=N); 6.96-7.35 (10H;
aromatic ring); 3.48 (s, 4H, CH2 attached with benzene ring), 2.36 (br s, 8H, CH2 in piperazine).
IR (KBr, cm-1): 3415 (br, ν(OH + H-bonding)); 2926, 2800, 2762 (w, νC-H); 1619 (s, νC=N);
1577, 1483 (s, νC=C vibration band of the aromatic ring). UV–vis (DMSO), λmax (ε) (nm
(105 dm3 mol-1 cm-1): 276 (0.121), 297 (0.112), 325 (0.118), 337 (0.122). Anal. Calcd (found) for
C26H26N4O2·CH2Cl2: C, 63.39 (63.75); H, 5.52 (5.54); N, 10.93 (11.01).
2.2.5. Preparation of [piperazinCH2(Salpn)]n (5). A solution of 1,2-propanediamine (1 mmol)
was added dropwise to a stirred solution of 2 (0.074 g, 1 mmol) in CH2Cl2 (10 mL) and the
mixture was refluxed for 6 h. The solvent was removed under vacuum to produce the yellow
solid, which was washed with 5 mL of cooled methanol and then dried in air. Yield: 94%.
1H NMR [CDCl3, 500 MHz; δ(ppm)]: 13.19 (br s, 2H, OH); 8.37 (s, 1H, C=N); 8.33 (s, 1H,
C=N); 6.9-7.3 (6H; aromatic ring); 3.72-3.85 (m, 3H, CH-N), 3.5 (br s, 4H, CH2 attached with
benzene ring), 2.46 (br s, 8H, CH2 in piperazine), 1.42 (s, 3H, CH3). IR (KBr, cm-1): 3445 (br,
ν(OH + H-bonding)); 3009, 2803, 2768 (w, νC-H); 1635 (s, νC=N); 1586, 1493 (s, νC=C
vibration band of the aromatic ring). UV–vis (DMSO), λmax (ε) (nm (105 dm3 mol-1 cm-1): 260
(0.191), 320 (0.080), 400 (0.0049). Anal. Calcd (found) for C23H28N4O2·CH3OH: C, 67.90
(67.83); H, 7.60 (7.51); N, 13.20 (13.26).
4

2.2.6. Preparation of [piperazinCH2{MoO2(Salen)}]n (6). Dichloromethane solution of 3
(0.378 g, 1 mmol) was refluxed with MoO2(acac)2 (0.326 g, 1 mmol) in 10 mL of methanol
under air for 12 h. The desired MoO2 complex was filtered off, washed with dichloromethane
and methanol, and dried in vacuum. Yield: 83%. IR (KBr, cm-1): 3415 (br, ν(OH + H-bonding));
2929, 2810, 2765 (w, νC-H); 1631 (s, m, νC=N); 1487, 1457 (s, νC=C vibration band of the
aromatic ring), 948 (w, νMo=O), 900 (m, νMo=O). UV–vis (DMSO), λmax (ε) (nm (105 dm3
mol-1 cm-1): 260 (0.178), 327 (0.088). Anal. Calcd (found) for C22H24N4O4Mo·(3H2O·1/3
CH3OH): C, 47.12 (46.75); H, 5.54 (5.49); N, 9.84 (9.84).
2.2.7. Preparation of [piperazinCH2{MoO2(Salophen)}]n (7). The procedure was similar to the
one used for synthesis of 6. Dichloromethane solution of 4 (0.424 g, 1 mmol) was refluxed with
MoO2(acac)2 (0.326 g, 1 mmol) in methanol (10 mL) under air for 12 h. The desired MoO2
complex was filtered off, washed with dichloromethane and methanol, and dried in vacuum.
Yield: 79%. IR (KBr, cm-1): 3435 (br, ν(OH + H-bonding)), 2926, 2813, 2790 (w, νC-H); 1621
(s, νC=N), 1485 (s, νC=C vibration band of the aromatic ring), 933 (w, νMo=O), 897 (m,
νMo=O). UV–vis (DMSO), λmax (ε) (nm (105 dm3 mol-1 cm-1): 260 (0.310), 323 (0.210), 370
(0.106), 500 (0.021). Anal. Calcd (found) for C26H24N4O4Mo·(3H2O·1/3 CH3OH): C, 51.24
(51.01); H, 5.12 (5.18); N, 9.08 (9.12).
2.2.8. Preparation of [piperazinCH2{MoO2(Salpn)}]n (8). The procedure was similar to the
one used for synthesis of 6. Dichloromethane solution of 5 (0.392 g, 1 mmol) was refluxed with
MoO2(acac)2 (0.326 g, 1 mmol) in methanol (10 mL) under air for 12 h. The desired MoO2
complex was filtered off, washed with dichloromethane and methanol, and dried in vacuum.
Yield: 82%. IR (KBr, cm-1): 3432 (br, ν(OH + H-bonding)); 2929, 2964, 2819, 2772 (s, νC-H);
1631 (s, νC=N); 1492, 1454 (s, νC=C vibration band of the aromatic ring); 948.7 (w, νMo=O);
899 (m, νMo=O). UV–vis (DMSO), λmax (ε) (nm (105 dm3 mol-1 cm-1): 260 (0.185), 327 (0.088).
Anal. Calcd (found) for C23H26N4O4Mo·(3H2O·1/3 CH3OH): C, 48.00 (47.82); H, 5.76 (5.71); N,
9.61 (9.62).
5

2.3. Catalytic reactions
2.3.1. General procedure for oxidation of olefins. A mixture of the substrate (1 mmol),
polymeric MoO2Salen complexes (0.005 mmol) and C2H4Cl2 (2 mL) was stirred in a 5 ml tube
for 10 min at 75 °C. The oxidant TBHP (80% in di-tert-butyl peroxide/water 3:2) (1 mmol) was
then added and the system was stirred for 4-12 h at 75 °C and the reaction progress was
monitored by GC. Assignments of products were made by comparison with authentic samples.
All reactions were run at least in duplicate.
2.3.2. General procedure for oxidation of sulfides. A mixture of methyl phenyl sulfide
(3 mmol), polymeric MoO2Salen complexes (0.005 mmol) and (1:1) mixture of CH3OH/CH2Cl2
(2 mL) was stirred in a 5 ml tube for 10 min at room temperature. The oxidant urea-hydrogen
peroxide (UHP) (3.75 mmol) was then added and the system was stirred at room temperature for
approximately 2 h and the reaction progress was monitored by GC. Assignments of products
were made by comparison with authentic samples. All reactions were run at least in duplicate.
3. Results and discussion
3.1. Synthesis and characterization
Scheme 1 shows the synthesis of polymeric [MoO2(Salen)] complexes with ethylenediamine,
orthophenylenediamine and 1,2-propanediamine. Compound 1, a starting compound in this
approach, was synthesized by reacting salicaldehyde with paraformaldehyde (CH2O)n in the
presence of HCl. The reaction of 5-(chloromethyl)salicylaldehyde with piperazine yielded 5,5’-
(N,N-piperazine bis(methylene))-bis-(salicylaldehyde). The polymeric ligands (3, 4, and 5) were
easily prepared by condensation of dialdehyde with a diamine in excellent yield. Mo(VI)
complexes (6, 7, and 8) were synthesized by reaction of corresponding polymeric ligands with
MoO2(acac)2 under reflux in air in mixture of MeOH and dichloromethane for 12 h. The
compounds were characterized by various spectroscopic and analytical techniques (data are
given in the experimental section).
Polymeric ligands 3, 4, and 5 are very slightly soluble in THF, methanol and ethanol but
soluble in chloroform, dichloromethane, DMF and DMSO. Polymers 6, 7, and 8 are insoluble in
common solvents but very slightly soluble in DMF and DMSO. All compounds are air-stable
solids. These compounds are formulated on the basis of elemental analysis and a variety of
6

1
physical measurements. In H NMR spectrum of 1, the resonance of CH2 attached to benzene
appears at 4.63 ppm. The 1H NMR spectrum of 2 shows a singlet at 2.5 ppm for CH2 protons of
piperazine as linker. The signal at δ 9.92 ppm (proton of aldehyde) in the spectrum of 2
disappears because of formation of ligand. A peak for HC=N is at 8.37 and 8.64 ppm in the case
of 3 and 4, respectively, whereas for 5 two peaks are at 8.33 and 8.37 ppm. The splitting of the
1
signals in the H NMR spectrum of 5 confirms the non-equivalence of C=N bands [18-20]. IR
spectra of ligands show intense bands at 1634, 1619 and 1634 cm-1 for 3, 4 and 5 associated with
the C=N stretch of the free ligands, which shift to 1631, 1621 and 1631 cm-1, respectively, in the
corresponding complexes (6, 7 and 8, respectively), indicating coordination of azomethine
nitrogen [21]. Molybdenum complexes exhibit two (O=Mo=O) vibrations at 930-950 cm-1 and
890–900 cm-1, confirming the presence of a cis-MoO2 structure [20, 22]. UV-Vis spectra of 3
and 5 in DMSO exhibit two bands at 260 and 320 nm due to π–π* and n–π* transitions,
respectively. The π–π* band has no definite trend in metal complexes while n–π* transition shifts
to higher wavelengths, indicating coordination of the azomethine nitrogen to Mo(VI). The band
due to ligand to metal charge transition in 6 and 8 is generally not observed. Complex 7 shows a
band at 500 nm due to ligand to metal charge transfer [21], in addition to bands in the UV region,
which correspond to the π–π* and n–π* transitions of the ligand.
The inductively coupled plasma atomic emission spectroscopy (ICPAES) data confirm
complete metalation of the ligands to give the polymeric complexes.
3.2. Catalytic activities
3.2.1. Oxidation of olefins. To test the catalytic potential of the complexes prepared,
cyclooctene was chosen as a model substrate. Thus, the oxidation of cyclooctene catalyzed by 6
was carried out using TBHP as an oxidant. The effect of various reaction media have been
studied on catalytic epoxidation of cyclooctene catalyzed by 6 (table 1). It is apparent that the
efficiency of the catalysts in different solvents decreases in the order dichloroethane >
acetonitrile > dichloromethane > toluene > methanol > DMF. No clear relationship exists
between the dielectric properties of these solvents and the catalytic activity, which may be at
least partly because of the major differences in polarity between the olefin and the oxidant. The
detrimental effect of methanol on the reaction rate may be related with its polar protic nature
inhibiting the reaction by competing with the oxidant for coordination to the metal centre similar
7

to tert-butyl alcohol, a by-product of the decomposition of TBHP [23]. Use of acetonitrile, an
aprotic coordinating solvent, led to a quite reasonable epoxide yield of 86% after 12 h. The
poorer yields obtained with CH2Cl2 are probably caused by the lower reaction temperature at
reflux.
Several TBHP sources were tested in oxidation of cyclooctene, TBHP (70%) in water,
TBHP (80%) in di-tert-butyl peroxide/water 3:2, TBHP (5.0-6.0 M) in decane and TBHP (70%)
in water in the presence of di-tert-butyl peroxide (figure 1) with catalyst 6 in 1,2-dichloroethane.
Both TBHP in di-tert-butyl peroxide/water and TBHP in decane show the highest yields (98%)
after 12 h at 75 °C; TBHP in water and TBHP in water in the presence of di-tert-butyl peroxide
show much lower yields (<32%). The significant difference in the cyclooctene oxide yield in
case of various oxidants can be explained by an inhibition due to water during the catalytic test.
Furthermore the results show that di-tert-butyl peroxide does not accelerate the oxidation of
cyclooctene.
Catalytic activities of the prepared catalysts were studied through oxidation of olefins
with TBHP in 1,2-dicholoroethane. The results are given in table 2. Generally, excellent yields
and selectivity towards epoxide are observed for cyclooctene and cyclohexene.
To provide evidence about mechanism, oxygenations of cyclohexene were performed.
The data for oxygenation of cyclohexene show very low selectivity with allylic attack, showing
that catalytic reaction occurs via nonradical processes [24].
Furthermore, molybdenum catalysis of the oxidation of cyclooctene and cyclohexene was
carried out with tert-butyl hydroperoxide in the presence of ionol (2,6-di-tertbutyl-p-cresol) as
radical scavenger (table 2). Since the catalytic epoxidation were both slightly inhibited in the
presence of ionol, a free radical chain mechanism is excluded.
Based on the result of the study, we suggest a mechanism via a seven-coordinate
intermediate (scheme 2) similar to the mechanism outlined [25].
It is assumed that the first step in the proposed sequence is formation of a seven-
coordinate species as intermediate (possibly the catalytically active species) containing
coordinated peroxide and a protonated oxo. The subsequent step in the sequence proceeds by
approach of olefin to coordinated oxygen (*). Herein, the metal center is a Lewis acid, removing
charge from the O–O bond, facilitating its dissociation, and activating the nearest oxygen (*) for
insertion into the olefin double bond. The intermediate exhibits a hydrogen bond between the
8

oxygen and coordinated OH, favoring formation of epoxide and release of alcohol from the
precursor complex.
In the case of styrene epoxidation more than 90% yield was achieved at the end of the
first run. However, polymers were decomposed by benzoic acid formed during the oxidation
reaction.
The major advantages of heterogeneous catalysis over its homogeneous counterpart are
the ease of recovering and reusability of the catalyst. To confirm that reusability of
[MoO2(Salen)]n arises from the polymer-bound metal, after completion of the first cycle the
catalyst was recovered from the reaction mixture by centrifugation, washed sequentially with
dichloroethane and dried for 3 h at 40 °C. Then, the recovered catalyst was added to fresh
substrates under identical conditions for five consecutive runs in the epoxidation of cyclooctene
at 75 °C. The results are summarized in figure 2, where it is seen that in the first run the catalyst
exhibits less activity in epoxide formation in comparison to the following runs. This may be due
to swelling of polymeric support which leads to change in the degree of accessibility of
catalytically active sites. The conversion curves for runs four and five look very similar.
Therefore, it can be concluded that the activity of the catalyst in the epoxidation of cyclooctene
remains stable after run four.
To find whether molybdenum leaching occurs during the reaction we have estimated the
amount of molybdenum of the catalyst by inductively coupled plasma atomic emission
spectroscopy (ICPAES). In this manner the filtrates were collected after each run and checked
for activity and quantities of Mo (which could have leached from the catalyst) (table 3). Increase
in the amount of leached Mo was observed through five runs. The results show the Mo species
leached out during the catalytic reaction slightly increase through five cycles.
Furthermore, to understand the nature of catalyst, IR and UV-Vis spectrophotometric
measurements were carried out for fresh and used catalyst (after the 5th reaction). Figure 3
shows the UV-vis spectra of as-prepared and spent catalyst, lines a and c, respectively. The UV-
vis spectrum of the catalyst after oxygenation shows only the original absorption bands with
almost the same intensities.
As shown in figure 4c, the IR spectrum of the catalyst after use for oxygenation of
cyclooctene shows the characteristic ν(C=N), ν(C-H) and ν(MoO2) absorption bands, and the
positions are in close agreement with the values for the as-prepared sample shown in figure 4a.
9

The small change in the signal intensity and position is probably due to band overlapping or
shifting by coexisting tert-BuOH, and cyclooctene oxide. On the basis of these experiments, it
appears that heterogeneous catalyst is stable under reaction condition.
3.2.2. Oxidation of sulfide. In order to find optimized conditions for oxidation of sulfides with
UHP catalyzed by [piperazinCH2{MoO2(Salen)}]n, methyl phenyl sulfide is used as the
substrate. The molar ratio 1:600:750 of the catalyst:methyl phenyl sulfide:UHP is optimum
molar ratio for the catalytic system. Different solvents such as acetone, dichloromethane,
acetonitrile, DMF, CH3OH, 1,2-dichloroethane and mixture of CH3OH/CH2Cl2 were also
examined for the oxidation of methyl phenyl sulfide by UHP. Mixture of CH3OH/CH2Cl2 gave
the highest conversion of methyl phenyl sulfide. Urea hydrogen peroxide is not completely
soluble in less polar organic solvents such as acetone, dichloromethane, 1,2-dichloroethane and
acetonitrile, so the oxidation yield is low in these solvents (entries 1-4, table 4).
To improve the conversion rate and yield of sulfoxide, various molar ratios of
UHP/sulfide in the catalytic oxidation of methyl phenyl sulfide were examined in mixture of
CH3OH/CH2Cl2 (figure 5). The conversion of methyl phenyl sulfide oxidation improves from
87% to 96% as the molar ratio increases from 1 to 2. Simultaneously, the selectivity of sulfoxide
drops remarkably from 89% to 64%. Thus 1.25 ratio of UHP/sulfide that affords sulfoxide as
major product is used as optimum condition for oxidation of other substrates.
The effect of substrate to catalyst ratios (S/C) on the turnover number (TON) is illustrated
in table 5. The reactions proceed at S/C ratio from 80 to 1000 with increasing turnover number.
The selectivity of sulfoxide increases until substrate to catalyst ratio reach to 600 and then
decreases.
Encouraged by the excellent catalytic performance for the methyl phenyl sulfide
oxidation to sulfoxide, the oxidations of various sulfides were carried out with UHP catalyzed by
polymeric complexes of molybdenum. All polymeric complexes are active (table 6) in the
oxidation of sulfides with similar reactivity; changing diamine moieties does not have significant
effect on the catalyst activity.
Based on Chand et al. [26], organic sulfides are oxidized by hydrogen peroxide in a
catalytic cycle involving nucleophilic attack of sulfur on oxygen in the oxoperoxo intermediate
where positive charge is induced by the Lewis acidic metal center or coordination of the sulfide
10

to the metal. Obviously, the rate of reaction can be controlled by sulfide electronic and steric
effects. The yield of the oxidation of sulfides increases with nucleophilic addition of sulfur on
oxoperoxo intermediate. Then replacement of phenyl or benzyl with alkyl decreases the reaction
rate.
In order to evaluate the stability of the catalyst, a sample of the catalyst was employed in
five consecutive runs in the reaction of methyl phenyl sulfide with UHP in mixture of
CH3OH/CH2Cl2 at room temperature for 2 h (table 7). The heterogenized catalyst was reused
several times with a conversion ranging from 95% at the beginning to 86% at the fifth run, with a
decrease in the selectivity of sulfoxide. In addition, filtration of [piperazinCH2{MoO2(Salen)}]n
from the reaction mixtures followed by ICPAS of the reaction solution showed that Mo leaching
into solution amounted to < 0.2 wt %. Therefore, metal loss should not be the main factor
responsible for the observed decrease of catalytic activity and sulfoxide selectivity. It is probably
due to competitive adsorption of reactant and product at the catalytically active sites.
Furthermore, blocking of the active sites of catalyst by the solvent may also contribute to the
reduction in conversion with recycling.
We examined the filtrate of reactions without adding fresh catalyst to see if these display
catalytic activity (figure 6). The conversion of curves derived from sulfoxidation using the
filtrate as a potential catalyst show that active Mo species are present in all the filtrates. For run
one, methyl phenyl sulfide could be converted at a relatively high rate in filtrates, which means
that leaching of active species occurs during the reaction and thus plays an important role for
catalytic performance of this material. For runs three and five sulfoxidation is continued at low
rate after the solid catalyst is removed, which means that a small part of the active species is
leached from the catalyst. In general the activity displayed by the filtrates decreases
progressively, so that conversions achieved in the fifth run are typically only 20% above that of
the control reaction measured in the absence of the catalyst. It seems clear that in MoO2(Salen)
catalysis arises from heterogeneous complex incorporated in polymer and Mo species lost from
the support. However, contribution of the heterogeneous catalyst to species lost from the support
increases through five cycles. Similar behavior has been observed for polybenzimidazole-
supported molybdenum catalysts for oxidation of alkene by Sherrington et al. [27] Therein, both
polymer-supported Mo and Mo species leached out during the reaction contribute to catalysis
after ten runs.
11

IR and UV-Vis spectra confirm the similarity of structures of the starting and recovered
materials. UV spectra of the fresh and used catalysts show two broad bands at 260 and 327 nm
due to n→π* and π→π* transitions and IR spectra of the spent catalyst show a similar pattern to
fresh catalyst, indicating the stability of the catalyst under relatively harsh reaction conditions.
For comparison, some of the most relevant systems reported to date have been included
in table 8. These results clearly point out the efficiency of the proposed catalytic systems
(table 8, first and sixth entries) in both activity and selectivity in epoxidation and oxidation of
sulfide reactions as compared to literature reports involving several heterogeneous systems under
various conditions (table 8, entries 2-5 and entries 7-9).
4. Conclusion
Mo-Schiff base polymers, where the polymer linkage is formed by piperazine, have been
synthesized and characterized by elemental analyses and IR, UV-Vis and 1H NMR
spectroscopies. Linked Schiff base polymers have been studied as hetereogeneous catalysts for
epoxidation of olefins and sulfoxidation. All compounds were active and selective catalysts.
According to our data for oxidation of alkenes and sulfides with catalysts precursors 6-8, no clear
trend of different catalytic reactivity was found upon change of the diamine of the Schiff base.
Investigation about reusability of [piperazinCH2{MoO2(Salen)}]n indicated that in the first run
the incorporated catalyst exhibited a smaller activity in epoxide formation in comparison to
following runs. The polymeric complex could be reused at least five times with a decrease in
activity and selectivity in sulfoxidation. On the basis of methyl phenyl sulfide oxidation results,
in addition to polymer-supported Mo, species leached out in solution contribute to catalysis but
decrease through consecutive runs. Furthermore, the fresh and recovered catalysts were studied
by IR and UV-vis spectra with little differences in the IR and UV-Vis spectra, indicating that the
catalyst has a similar composition and good stability after recycle.
Acknowledgements
Financial support of this work by the Research Council of Sharif University of Technology is
greatly appreciated.
12

References
[1]
[2]
R. Hille. Chem. Rev., 96, 2757 (1996).
M.J. Romao, M. Archer, I. Moura, J.J.G. Moura, J. LeGall, R. Engh, M. Schneider, P.
Hof, R. Huber. Science, 270, 1170 (1995).
[3]
S.P. Kramer, J.L. Johnson, A.A. Ribiero, D.S. Millington, K.V. Rajagopalan. J. Biol.
Chem., 262, 16357 (1987).
[4]
[5]
M. Bagherzadeh, R. Latifi, L. Tahsini. J. Mol. Catal. A: Chem., 260, 163 (2006).
M. Bagherzadeha, M. Aminia, H. Parastara, M. Jalali-Heravia, A. Ellernc, L.K. Woo.
Inorg. Chem. Commun., 20, 86 (2012).
[6]
[7]
[8]
[9]
M. Bagherzadeh, M. Amini, A. Ellern, L.K. Woo. Inorg. Chem. Commun., 15, 52 (2012).
R. Debel, A. Buchholz, W. Plass. Z. Anorg. Allg. Chem., 634, 2291 (2008).
I. Sheikhshoaie, A. Rezaeifard, N. Monadi, S. Kaafi. Polyhedron, 28, 733 (2009).
M.R. Maurya, A. Kumar, P. Manikandan, Shri Chand. Appl. Catal. A: Gen., 277, 45
(2004).
[10] M.R. Maurya, A. Kumar. J. Mol. Catal. A: Chem., 250, 190 (2006).
[11] A.R. Paital, D. Mandal, X. Huang, J. Li, G. Aromi, D. Ray. Dalton Trans., 1352 (2009).
[12] N.C. Maity, S.H.R. Abdi, R.I. Kureshy, N.H. Khan, E. Suresh, G.P. Dangi, H.C. Bajaj.
J. Catal., 277, 123 (2011).
[13] R.I. Kureshy, A. Das, N.H. Khan, S.H.R. Abdi, H.C. Bajaj. ACS Catal., 1, 1529 (2011).
[14] N.H. Khan, A. Sadhukhan, N.C. Maity, R.I. Kureshy, S.H.R. Abdi, S. Saravanan, H.C.
Bajaj. Tetrahedron, 67, 7073 (2011).
[15] J.M. Ready, E.N. Jacobsen. J. Am. Chem. Soc., 123, 2687 (2001).
[16] M.M. Jones. J. Am. Chem. Soc., 81, 3188 (1959).
[17] L.J. Chen, F.M. Mei, G.X. Li. Catal. Commun., 10, 981 (2009).
[18] K. Ambroziak, R. Pelech, E. Milchert, T. Dziembowska, Z. Rozwadowski. J. Mol.
Catal. A: Chem., 211, 9 (2004).
[19] K. Yamanouchi, S. Yamada. Inorg. Chim. Acta, 9, 161 (1974).
[20] M. Gullotti, A. Pasini, G.M. Zanderighi, G. Ciani, A. Sironi. J. Chem. Soc., Dalton
Trans., 902 (1981).
[21] W.E. Hill, N. Atabay, C.A. McAuliffe, F.P. McCullough, S.M. Razzoki. Inorg. Chim.
Acta, 35, 35 (1979).
13

[22] M. Cindric, N. Strukan, V. Vrdoljak, T. Kajfez, B. Kamenar. Croat. Chem. Acta, 76, 257,
(2003).
[23] C.D. Nunes, A.A. Valente, M. Pillinger, A.C. Fernandes, C.C. Romao, J. Rocha, I.S.
Goncalves. J. Mater. Chem., 12, 1735 (2002).
[24] H. Mimoun. Catal. Today, 1, 281 (1987).
[25] F.E. Kühn, M. Groarke, E. Bencze, E. Herdtweck, A. Prazeres, A.M. Santos, M.J.
Calhorda, C.C. Romão, I.S. Gonçalves, A.D. Lopes, M. Pillinger. Chem. Eur. J., 8, 2370
(2002).
[26] R.D. Chakravarthy, K. Suresh, V. Ramkumar, D.K. Chand. Inorg. Chim. Acta, 376, 57
(2011).
[27] R. Mbeleck, K. Ambroziak, B. Saha, D.C. Sherrington. React. Funct. Polym., 67, 1448
(2007).
[28] P.C. Bakala, E. Briot, L. Salles, J.M. Bregeault. Appl. Catal. A: Gen., 300, 91 (2006).
[29] A. Bezaatpour, M. Amiri, V. Jahed. J. Coord. Chem., 64, 1837 (2011).
[30] A.C. Coelho, S.S. Balula, S.M. Bruno, J.C. Alonso, N. Bion, P. Ferreira, M. Pillinger,
A.A. Valente, J. Rocha, I.S. Goncalves. Adv. Synth. Catal., 352, 1759 (2010).
[31] A. Castro, J.C. Alonso, P. Neves, A.A. Valente, P. Ferreira. Eur. J. Inorg. Chem., 602
(2010).
[32] A. Fuerte, M. Iglesias, F. Sanchez, A. Corma. J. Mol. Catal. A, 211, 227 (2004).
[33] M. Bagherzadeh, M.M. Haghdoost, A. Shahbazirad. J. Coord. Chem., 65, 591 (2012).
[34] B. Karimi, M. Ghoreishi-Nezhad, J.H. Clark. Org. Lett., 7, 625 (2005).
14

Table 1. Effect of solvent on conversion in oxidation of cyclooctene with TBHP.a
Entry
Solvent
Con %b
Selectivity to epoxide %b
1
2
3
4
5
6
DMF
CH3OH
CH3CN
CH2Cl2
C2H4Cl2
toluene
0
-
28
86
12
95
39
>99
98
>99
98
>99
aReaction condition: The molar ratio for catalyst:alkene:TBHP are
1:200:200. The reactions were run for 12 h at 75 °C.
bGC conversion based on starting alkene.
15

Table 2. Catalytic oxidation of various alkenes with TBHP by polymeric [MoO2(Salen)] complexes.a
Catalyst
Substrate
Con %b (Time)
Selectivity to epoxide % (Time)
95 (12 h)
98 (12 h)
[PiperazinCH2{MoO2(Salen)}]n
73 (12 h)
94 (18 h)
92 (12 h)
86 (18 h)
89 (12 h)
97 (12 h)
[PiperazinCH2{MoO2(Salphen)}]n
[PiperazinCH2{MoO2(Salpn)}]n
68 (12 h)
92 (18 h)
92 (12 h)
84 (18 h)
93 (12 h)
97 (12 h)
82 (12 h)
97 (18 h)
99 (12 h)
92 (18 h)
85 (12 h)
98 (12 h)
c
[PiperazinCH2{MoO2(Salen)}]n
60 (12 h)
96 (12 h)
aReaction condition: The molar ratio for catalyst:alkene:TBHP are 1:200:200. The reactions were run at 75 °C in 2 mL of
1,2-dichloroethane.
bGC conversion based on starting alkene.
cGC conversion in the presence of the radical scavenger.
16

Table 3. Recycling of catalyst in oxidation of cyclooctene with TBHP.a
Run
Con % (Time)
Selectivity to epoxideb
Leaching to Mo (wt %)c
1
2
3
4
5
95 (12 h)
98 (7 h)
99 (5 h)
97 (4 h)
98 (4 h)
98
98
98
98
98
0.071
-
0.143
-
0.178
aReaction condition: The molar ratio for catalyst:cyclooctene:TBHP are 1:200:200. The
reactions were run for 12 h at 75 °C in 2 mL of 1,2-dichloroethane.
bGC conversion based on starting styrene.
cDetermined by ICPAES.
17

Table 4. The oxidation of sulfide by [piperazinCH2{MoO2(Salen)}]n with UHP in various solvents.a
Entry
Solvent
Con %b
Selectivity to sulfoxide %b
1
2
3
4
5
6
7
CH2Cl2
C2H4Cl2
2
100
65
62
67
60
89
84
17
64
37
30
87
75
CH3CN
CH3COCH3
DMF
CH2Cl2:CH3OH
CH3OH
aConditions: The molar ratio for catalyst:methyl phenyl sulfide:UHP is 1:80:80. The
reactions were run for 2 h at room temperature.
bGC yields based on the starting methyl phenyl sulfide.
18

Table 5. Effect of methyl phenyl sulfide/catalyst molar ratios on the catalytic
performance of heterogeneous catalytic systems in sulfide oxidation.a
Substrate to catalyst ratio
Con %b
Selectivity to sulfoxide %b
TONb
80
100
200
600
1000
94
95
94
95
87
83
81
81
84
76
75
95
188
570
870
aReaction condition: The molar ratio for methyl phenyl sulfide:UHP is 1:25. The
reactions were run for 2 h at room temperature in 2 mL of CH3OH/CH2Cl2.
bTurnover number (TON) is mole of product per mole catalyst. GC conversion based
on starting methyl phenyl sulfide.
19

Table 6. Oxidation of various sulfides catalyzed by polymeric [MoO2(Salen)] complexes.b
O
O
O
S
S
S
R
+
R
catalyst, UHP
R
r.t, CH3OH/CH2Cl2
Selectivity to
sulfoxide %b ( )c
Catalyst
R
Con %b ( )c
CH3
95
84
-
CH3CH2
Ph
77
[PiperazinCH2{MoO2(Salen)}]n
20 (55)
21 (62)
85 (88)
83 (84)
Ph-CH2
CH3
CH3CH2
Ph
96
86
-
84
[PiperazinCH2{MoO2(Salphen)}]n
[PiperazinCH2{MoO2(Salpn)}]n
26 (60)
28 (66)
84 (84)
86 (88)
Ph-CH2
CH3
CH3CH2
Ph
98
93
-
80
22 (56)
23 (64)
84 (86)
83 (83)
Ph-CH2
aConditions: The molar ratio for catalyst:methyl phenyl sulfide:UHP is 1:600:750. The reactions were run for
2 h at room temperature in 2 mL of CH3OH/CH2Cl2.
bGC yields based on the starting sulfide.
cThe reactions were run for 6 h.
20

Table 7. Recycling of catalyst in oxidation of methyl phenyl sulfide with UHP. a
Run
Con %b
Selectivity to sulfoxide %b
Leaching to Mo (wt %)c
1
2
3
4
5
95
94
90
88
86
84
82
81
74
65
0.191
-
0.178
-
0.036
aConditions: The molar ratio for catalyst:methyl phenyl sulfide:UHP is 1:600:750. The
reactions were run for 2 h at room temperature in 2 mL of CH3OH/CH2Cl2.
bGC yields based on the starting methyl phenyl sulfide.
cDetermined by ICPAES.
21

Table 8. Comparison of literature reports on the selective oxidation of methyl phenyl sulfide (MPS) and oxidation of
cyclooctene under various conditions.
Molar ratio
cat:sub:ox
Temp.
(oC)
Selectivitya
Time
(h)
Conv.
%
Entry
Catalyst
Substrate
Ref.
%
1
2
3
6
1:200:200
1:68:205
1:150:150
cyclooctene
cyclooctene
cyclooctene
75
40
12(4)b
3
95
97
98
90
65
this work
Mo-MCM-41
[Ni(L1)]@Mont
28
29
67.6
reflux
12
(CH3CN)
4
5
6
7
8
PMO-phMo(CO)3
LP-TEPPPO-Mo
6
1:100:154
0.1:42:64
1:600:750
1:200:300
1:12.9:14.5
cyclooctene
cyclooctene
MPS
55
55
r.t
40
r.t
0.5
24
2
100
87
100
100
84
30
31
95
this work
32
MPS
MCM-Mo
1
99
99
Fe3O4@SiO2@
VO(salen)
MPS
6
93
95
33
r.t
9
silica-based
1:50:75
MPS
1.5
99
82
34
tungstate interphase
aSelectivity to cyclooctene epoxide for cyclooctene; selectivity to methyl phenyl sulfoxide for methyl phenyl sulfide.
bReaction time for fifth run.
22

Figure 1. Epoxidation of cyclooctene catalyzed using [piperazinCH2{MoO2(Salen)}]n with various TBHP at
75 °C for 12 h in 2 mL of 1,2-dichloroethane.
23

100
80
60
40
20
0
Run 1
Run 2
Run 3
run 4
run 5
0
2
4
6
8
10
12
Time/h
Figure 2. Conversion curves for the epoxidation of cyclooctene catalyzed using
[piperazinCH2{MoO2(Salen)}]n with TBHP in five consecutive reactions.
24

Figure 3. UV-Vis spectra of the samples: (a) fresh catalyst, (b) used catalyst in sulfoxidation
after run five, (c) used catalyst in epoxidation after run five.
25

Figure 4. FT-IR spectra of the samples: (a) fresh catalyst, (b) used catalyst in sulfoxidation after
run five, (c) used catalyst in epoxidation after run five.
26

100
80
60
40
20
0
selectivity to sulfoxide
conversion
1
1.25
1.5
2
Figure 5. Effect of UHP/sulfide molar ratios on the catalytic performance of
[piperazinCH2{MoO2(Salen)}]n with UHP in 2 mL of CH3OH/CH2Cl2.
27

100
80
60
40
20
0
without cat
run1
filterated run1
filterated run 3
filterated run 5
0
50
100
150
Time/ min
Figure 6. Conversion curves for the oxidation of methylphenylsulfide at room temperature
catalyzed using filtrates with UHP.
28

O
H
OH
a
H2
C
O
Cl
H
OH
(1)
b
O
CH
OH
H2
C
H2
C
(2)
HO
HC
N
N
N
CH3
N
HO
OH
N
e
c
O
CH2
N
N
(5)
N
(3)
OH
d
HO
H2C
f
N
*
f
N
N
CH3
N
N
N
N
CH2
N
N
C
OH HO
N
N
Mo
O
n
O
Mo
H2
N
O
O
O
O
(6)
(4)
O
O
n
(8)
H2C
N
f
*
N
O O
Mo
N
N
O
N
N
C
O
H2
n
(7)
Scheme 1. Procedure for the synthesis of molybdenum-containing polymers. The process starts
with chloromethylation of salicylaldehyde to form 5-(chloromethyl)salicylaldehyde (1) and then
reaction with piperazine for formation of dialdehyde. This was followed by addition of
ethylenediamine, orthophenylenediamine or 1,2-propanediamine to dialdehyde for formation of
polymeric ligands (3, 4 and 5), and finally the binding of MoO2 to the polymeric ligands (6, 7
and 8). Reagents and conditions: a) CH2O,HCl, stirring at 20 °C, 5 h; b) piperazine,
triethylamine, CH2Cl2, stirring at room temparature, 24 h; c) 1,2-propanediamine, CH2Cl2,
reflux, 6 h; d) orthophenylenediamine, CH2Cl2, reflux, 6 h; e) ethylenediamine, CH2Cl2, reflux,
6 h; f) MoO2(acac)2, CH3OH,CH2Cl2, reflux, 12 h.
29