Synthesis, structure characterization and study of a new molybdenum Schiff base complex as an epoxidation catalyst with very high turnover numbers

Article abstract of DOI:10.1016/j.inoche.2017.07.025

The reaction between [MoO₂(acac)₂] and an ONO type Schiff-base ligand (L?=?4-bromo-2-((2-hydroxy-5-methylphenylimino)methyl)phenol) resulted a new oligomer molybdenum complex ([MoO₂(L)]_n). The oligomer complex was characterized by elemental analysis, FT-IR, ¹H and ¹³C NMR spectroscopies. A suitable single crystal of the complex was grown in DMSO and characterized by X-ray single crystal diffraction as monomer stabilized by one DMSO molecule, [MoO2L(DMSO)]. The [MoO₂(L)]_n complex was used as a catalyst in epoxidation of olefins. Besides the high activity and selectivity, very high turnover numbers were a remarkable advantage of the catalytic system.

Full text of DOI:10.1016/j.inoche.2017.07.025

Accepted Manuscript
Synthesis, structure characterization and study of a new
molybdenum Schiff base complex as an epoxidation catalyst with
very high turnover numbers
Mojtaba Bagherzadeh, Saeed Ataie, Hamed Mahmoudi, Jan
Janczak
PII:
DOI:
Reference:
S1387-7003(17)30525-7
doi: 10.1016/j.inoche.2017.07.025
INOCHE 6722
To appear in:
Inorganic Chemistry Communications
Received date:
Revised date:
Accepted date:
19 June 2017
15 July 2017
18 July 2017
Please cite this article as: Mojtaba Bagherzadeh, Saeed Ataie, Hamed Mahmoudi, Jan
Janczak , Synthesis, structure characterization and study of a new molybdenum Schiff base
complex as an epoxidation catalyst with very high turnover numbers, Inorganic Chemistry
Communications (2017), doi: 10.1016/j.inoche.2017.07.025
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ACCEPTED MANUSCRIPT
Synthesis, Structure Characterization and Study of a New Molybdenum Schiff
Base Complex as an Epoxidation Catalyst with Very High Turnover Numbers
Mojtaba Bagherzadeh,a* Saeed Ataie,a Hamed Mahmoudia and Jan Janczakb
a
Chemistry Department, Sharif University of Technology, Tehran, P.O. Box 11155-3615, Iran;
bagherzadeh@sharif.edu
b
Institute of Low Temperatures and Structure Research, Polish Academy of Sciences, 2 Okólna
St., P.O. Box 1410, 50-950 Wrocław, Poland.
______________________________________________________________________________
Abstract: The reaction between [MoO2(acac)2] and an ONO type Schiff-base ligand (L= 4-bromo-
2-((2-hydroxy-5-methylphenylimino)methyl)phenol)) resulted a new oligomer molybdenum
complex ([MoO2(L)]n). The oligomer complex was characterized by elemental analysis, FT-IR,
13
1H and C NMR spectroscopies. A suitable single crystal of the complex was grown in DMSO
and characterized by X-ray single crystal diffraction as monomer stabilized by one DMSO molecule,
[MoO2L(DMSO)]. The [MoO2(L)]n complex was used as a catalyst in epoxidation of olefins. Besides
the high activity and selectivity, very high turnover numbers were a remarkable advantage of the
catalytic system.
Keywords: Mo(VI) complex, Schiff base ligand, Olefin epoxidation, Oligomer complex, Crystal
structure
During recent decades, epoxidation of alkenes has attracted certain attention, due to the useful
products such as epoxy resins, perfumes, cosmetics, pharmaceuticals and polymers in different
areas like industry and medicine [1-5]. Molybdenum complexes, as an epoxidation catalyst, play
a great role [6-9]. The presence of an active site of Mo=O grants this catalytic activity to
molybdenum complexes, like molybdenum enzymes [10,11]. Also, employing non-toxic
procedures and materials having friendly treatment with the environment is very important [12].
The nature of ligands directly affect the catalytic activity and selectivity [13] and Schiff-bases can
be referred as one of the efficient ligands [14-16]. In this study, a new oligomer molybdenum
complex has been synthesized [17], (Fig. 1.), through the reaction of [MoO2(acac)2] [18] and
Schiff-base ligand (4-bromo-2-((2-hydroxy-5-methylphenylimino)methyl)phenol) [19].
The oligomer complex was rearranged to monomer structure in DMSO solution in the suitable
crystalline form stabilized by one DMSO molecule and characterized by X-ray crystallography.
The oligomer complex was utilized as a homogeneous catalyst in the epoxidation of olefins. The
presence of a band at 3031 cm-1 (OH) in the ligand IR spectrum [19] and disappearance of this
band in the complex spectrum, and also a shift in ν(N=C) band from 1627 cm-1 to 1610 cm-1
suggests the ligand coordination to the metal through the oxygen and nitrogen atoms. The oligomer
complex IR spectrum shows one band at 925 cm-1 related to Mo=O group and one band in 807 cm-
1 belonged to Mo─O─Mo [20]. On the other hand, the IR spectrum belonged to an orange crystal
of monomer complex indicates two close bands in 898 and 918 cm-1 related to two Mo=O groups
[20]. Two singlet peaks (1H, OH) at 13.89 and 9.55 ppm in ligand 1H NMR spectrum indicate two
phenolic protons [19], and they are disappeared in the complex spectrum. As well as, the shift of
singlet (1H, N=CH) from 8.96 to 9.21 ppm proves that the complexation happens through the

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ligand nitrogen and oxygen atoms. The NMR spectra showed that no solvent is coordinated to
molybdenum and confirmed IR data based on the oligomeric structure of the complex. 1H and 13C
NMR and IR spectra are illustrated in supplementary material.
The crystal data of complex are demonstrated in Table 1. Refinement of the crystal structure of
mononuclear complex established that the complex is a hexacoordinated. both the ligand and the
coordinated DMSO molecule are disordered. The ligand has statistically occupied two positions,
whereas the coordinated DMSO molecule has two orientations with occupation factors of 0.25 and
0.75 as illustrated in Fig. 2. The CH3 groups of coordinated DMSO molecules are shared for both
orientations. The central molybdenum(VI) is a six-coordinated, in a distorted octahedral fashion
including a deprotonated ligand, an oxido ligand in the equatorial plane, an oxido ligand and an
oxygen from DMSO in the apical sites. Probably due to the coordination requirements, the ligand
as a 4-bromo-2-((2-hydroxy-5-methylphenylimino)methyl)phenolato anion is more planar than in
the crystal of its free neutral form (10.02(14)o) [21], the dihedral angle between the phenyl rings
are only 5.6(2)o. Two chelate rings made by the coordinated tridentate ligand are five- and six-
membered and the mean planes of them have 4.7(2)o angle relative to each of the rings. However,
bite angles of these rings are 64.1(3)o and 94.0(3)o respectively, which are different from the
ideality. The equatorial MoNO3 unit with only 0.215(3) Å deviation is planar. It can be also
expressed that there is a difference between the Mo—O bonds; the shortest Mo—O bond is formed
between the molybdenum and oxido ligands intermediated between the molybdenum and
phenolate oxygen and the longest between the molybdenum and the oxygen of DMSO molecule
(Table 2).
In order optimize the catalytic system [22] including the complex ([MoO2L]n) as catalyst and
TBHP as the oxidant, Table 3 was obtained. Different amounts of the catalyst were used (entries
1-8). Also, various amounts of TBHP was tested (entries 2, 9-11), and the highest yield was
eventuated for the ratio 1:2 for substrate and TBHP (entry 2).The activity of catalytic system was
changeable in various solvents (entries 2, 16-21). The low conversion was resulted for coordinating
solvents, but actually, it was expected according to the competition between solvent with TBHP
for coordinating to the molybdenum. On the other hand, in noncoordinating solvents, the high
conversion is significant. But for dichloromethane, as a non-coordinating solvent, the low
conversion was reported. It can be because of dichloromethane low boiling point, which is not able
to prepare the activation energy for the reaction. For DCE, chloroform and dichloromethane
highest selectivity is manifest. Considering both the conversion and TON, entry 2 was chosen as
the optimum condition containing DCE as a solvent, 1 mmol of TBHP, 23×10-3 mol% catalyst and
80 °C. Epoxidation of different alkenes was evaluated under the optimized reaction condition
(Table 4). Cyclic olefins showed significant conversion and selectivity (entries 1,2). For styrene
and its derivatives (entries 3-6) selectivity showed a reasonable decrease according to the side
products (benzaldehyde, 4-methylbenzaldehyde, and acetophenone). Between cis and trans-
stilbenes, trans-stilbene showed lower conversion, according to more steric hindrance nearing the
activated intermediate catalyst (entries 7,8). In linear substrates, internal C=C (entry 11) undergoes
more conversion compared with terminal C=C (entry 10), and the catalytic yield is higher for
shorter chains than longer chains (entry 9, 10). As shown in the reaction profile (Fig. 3), after 3 h
the reaction is almost stopped. Based on previous studies published, a generalized mechanism for
epoxidation of alkenes in the presence of TBHP and [MoO2L]n were illustrated in Fig. 4 [23-25].
At the beginning, a proton is transferred from TBHP to Mo=O group and TBHP concurrently is
coordinated to the central metal. Herein, an activated intermediate is formed, including peroxo
group, and Lewis acidity property of molybdenum amplifies the oxidizing power of this peroxo

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group. After interaction between an alkene and the activated intermediate, the final products are
the epoxy compound and t-BuOH. The resulted t-BuOH has coordinating ability to fill free sites
of catalyst and the output is an inactive compound, causing a deceleration in the progress of the
catalytic reaction. This mechanism explains well why the reaction is stopped before completing,
after 3 h.
Finally, a comparison was done between presented work and recently published works (Table
5). In general, a brief comparison shows that the presented catalytic system behaves more effective
than previously published work in the same condition and also much higher TONs are obtained
for presented catalytic system.
In summary, the oligomer Mo-complex ([MoO2L]n) was obtained and characterized. The Mo-
complex was evaluated in epoxidation reaction of olefins. In addition, high dependence of the
catalytic system on the temperature and solvents nature was obvious in the results. Notable yields
were reported for non-coordinating solvents compared with coordinating solvents. The complex
showed very high activity and TONs in catalytic system.
Supplementary material
Experimental section, characterization data, additional spectra and full details of the X-ray data
collection and final refinement parameters including anisotropic thermal parameters and the full
list of the bond lengths and angles have been deposited with the Cambridge Crystallographic Data
Center in the CIF format as supplementary publication no. CCDC 1542849. Copies of the data can
be obtained free of charge on the application to CCDC, 12 Union Road, Cambridge, CB21EZ, UK,
(fax: (+44) 1223-336-033; email: deposit@ccdc.cam.ac.uk ).
Acknowledgements
M.B. acknowledges the research council of the Sharif University of Technology for the research
funding of this work.
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(1.83 g) was added a methanolic solution of [MoO2(acac)2] (1.95 g, 6 mmol). The mixture was
refluxed for 24 h under magnetic stirring condition. A brown precipitate was obtained. The
precipitate was filtered and washed with hot methanol and dried at the atmosphere.
Yield: 1.9 g, 73%. IR (KBr) (max/cm-1): 1610 ν(N=C), 925 ν(Mo=O), 807 ν(Mo─O─Mo). Anal.:
1
Calcd. For C14H10BrNO4Mo: C, 38.92; H, 2.33; N, 3.24. Found: C, 38.60; H, 3.15; N, 3.18. H
NMR (500 MHz, 25 °C, DMSO-d6): δ=9.21 (s, 1H; CH=N), 6.70-8.00 (m, 6H, aromatic rings),
2.31 ppm (s, 3H, (CH3)). 13C NMR: δ=161.1 (C=N), 159.1 and 156.1 (C2, C8), 112.1-138.3 (other
aromatic carbons), 21.3 (CH3).

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and 1,2-dichloroethane (DCE) as solvent (1 mL) was heated up to 80 °C under magnetic stirring
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Figure captions
Figure 1. Synthesis of complex
Figure 2. A perspective view of the complex [MoO2(L)(DMSO)].(DMSO)] together with the
labeling scheme showing the disorder of the ligand and coordinated DMSO molecule.
Figure 3. Reaction profile of epoxidation of cyclooctene under the optimum condition of catalytic
system
Figure 4. Proposed mechanism for epoxidation of olefin in the presence of TBHP and [MoO2L]

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Br
Br
N
N
[MoO2(acac)2]
O
Methanol
Reflux
Mo
HO
O
n
OH
O
O
Figure 1. Synthesis of complex

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Figure 2. A perspective view of the complex [MoO2(L)(DMSO)].(DMSO)] together with the
labeling scheme showing the disorder of the ligand and coordinated DMSO molecule.

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100
90
80
70
60
50
40
30
20
10
0
Conv (%)
0
1
2
3
4
5
6
Time/h
Figure 3. Reaction profile of epoxidation of cyclooctene under the optimum condition of catalytic system
(solvent: 1,2-dichloroethane, Cat.: 23×10-3 mol%, Temperature: 80 °C, TBHP: 1 mmol)

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Br
N
O
Mo
O
O
O
R1
R3
O
R2
R4
OH
+
O
OH
Br
R3
R1
R4
O
N
O
M
o
O
O
N
O
R2
o
OH
O
M
O
O
HO
O
O
Br
R1
R3
R2
R4
Figure 4. Proposed mechanism for epoxidation of olefin in the presence of TBHP and [MoO2L]
(Only one active site in the oligomer complex is showed)

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Table 1. Crystallographic data for MoO2(C14H10NO2)(DMSO).DMSO.
Empirical formula
Formula weight (g·mol–1)
Crystal system
C18H22BrMoNO6S2
588.33
monoclinic
Space group
I 2/a (No 15)
a (Å)
26.553(3)
b (Å)
c (Å)
6.8777(5)
24.707(3)
103.978(4)
4378.5(8)
8
β (º)
V (Å3)
Z
Dcalc (g·cm–3)
1.785
2.649
μ (mm–1)
F(000)
508
Crystal size (mm)
Radiation type, λ (Å)
θ range(o)
Absorption correction
Tmin/Tmax
0.31  0.24  0.14
Mo Kα , 0.71073
3.07 - 29.356
multi-scan
0.75458/ 1.000
46304 / 5748 / 4715
0.0376
Refls collected / unique / observed
Rint
Refinement on
F2
R[F2>2σ(F2)]
wR(F2 all reflections)*
Goodness-of-fit, S
Δρmax, Δρmin (e Å–3)
0.0641
0.1277
1.071
+1.308, -1.184
*wR={Σ [w(Fo –Fc2)2]/ΣwFo } ; w–1=σ2(Fo ) + (aP)2 +(bP) where a=0.0286 and b=51.5011, P = (Fo + 2Fc2)/3.
2
4
½
2
2

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Table 2. Important bond lengths [Å] and angles [°] for complex [MoO2L(DMSO)]
Mo1—O1
Mo1—O3
Mo1—O6
O1—Mo1—O2
1.688(3)
1.934(4)
2.281(4)
Mo1—O2
Mo1—O4
Mo1—N2D
1.707(3)
1.962(5)
2.299(8)
98.52(17)
104.67(17) O1—Mo1—O3
O1—Mo1—O4
O2—Mo1—O4
O3—Mo1—O4
O4—Mo1—O6
97.60(18)
98.1(2)
O1—Mo1—O6
O2—Mo1—O6
O3—Mo1—O6
166.83(17)
86.67(18)
85.95(19)
152.4(2)
73.83(19)
O1—Mo1—N2D 88.9(2)
O3—Mo1—N2D 64.1(3)
O2—Mo1—N2D 160.4(2)
O4—Mo1—N2D 94.0(3)
O6—Mo1—N2D 81.9(2)

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Table 3. Effect of the catalyst amount, TBHP amount, Temperature and different solvents on the
epoxidation of cyclooctene using [MoO2L]n
Entry Mol% Cat TBHP
Solvent
Temp (°C)
Conv%a
Sel%
TON
×10-3
46.2
(mmol)
1
1
1
1
1
1
1
1
DCE
DCE
DCE
DCE
DCE
DCE
DCE
DCE
DCE
80
80
80
80
80
80
80
80
80
80
80
70
60
50
r.t.
95
93
86
70
65
44
41
28
83
80
64
76
47
32
14
89
19
53
0
100
100
100
100
100
100
100
100
100
100
100
100
100
100
100
100
100
61
2056
4043
7439
15151
28260
39497
89130
121739
3608
3478
2782
3304
2043
1391
608
1
2
3
4
5
6
7
8
9
23
11.6
4.6
2.3
1.1
0.5
0.2
23
0.815
23
23
23
23
23
23
23
23
23
23
23
23
0.75
0.5
1
1
1
1
1
1
1
DCE
DCE
DCE
DCE
DCE
DCE
10
11
12
13
14
15
16
17
18
19
20
21
Chloroform
1,2-Dichloromethane
Ethanol
Methanol
Toluene
Acetonitrile
Reflux
Reflux
Reflux
Reflux
80
3869
826
2652
-
2739
913
1
1
1
-
90
90
63
21
Reflux
a Time: 2 h. Cyclooctene: 0.5 mmol. (DCE): 1,2-dichloroethane. Solvent: 1 mL. GC yield based on starting alkene.

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Table 4. Epoxidation of multiple alkenes using TBHP and [MoO2L]n catalyst.
Entry
Substrate
cyclooctene
cyclohexene
styrene
Time (h) Conv%a Sel% TON
1
2
3
4
5
6
7
8
9
3
5
5
5
4
5
5
4
4
4
5
94
92
91
52
30
88
90
54
81
57
91
100
100
41
46
60
72
55
59
70
74
90
4087
4000
3956
2260
1304
3826
3913
2347
3521
2478
3956
α-methylstyrene
4-methoxylstyrene
4-methylstyrene
Cis-stilbene
Trans-stilbene
1-heptene
10
11
1-octene
Trans-2-octene
a Reaction condition: 23×10-3 mol% cat, 0.5 mmol alkene, 1 mmol TBHP, 1 mL DCE (solvent) and temperature 80
°C. GC yield based on starting alkene.
Table 5. Recently reported catalytic results for the cyclooctene epoxidation
Entry
Catalyst
Time
(h)
5
Cyclooctene/TBHP Temp (°C) Conv(%)
(mmol)
TON TOF
Ref.
[MoO2LMeOH]
[MoO2(ONOtBu)(
MeOH)].MeOH
1
2
1/2
80
87
362
107
-
-
[26]
[27]
2
1/2
60
51
Mo-
DAPSH@APTES
@SiO2
[Mo(O)2(salen)-
POM]
3
2
1/2
1/3
60
75
100
100
-
28
[13]
4
6
50
8.33
[28]
Mo(O)2(salen)
[MoO2(L1)(MeO
5
6
6
6
1/3
1/2
75
80
46
97
23
1940
-
-
[28]
[9]
H)] (1a*)
[MoO2L]n
7
2
1/2
80
93
4043
-
Present
work

ACCEPTED MANUSCRIPT
Graphical abstract

ACCEPTED MANUSCRIPT
Highlights:
1. Efficient catalytic system with good results
2. New molybdenum ONO Schiff base complex
3. Full characterization
4. Very high turnover numbers