Synthesis, x-ray studies, and catalytic activity of tridentate schiff base dioxo-molybdenum(VI)

Article abstract of DOI:10.1080/00958972.2014.939076

The reaction of a solution of MoO₂(acac)₂ in CH₃OH and salicylidene 2-picoloyl hydrazone as a tridentate ONO donor Schiff base (ONO) afford a six-coordinated Mo(VI) complex [MoO₂(ONO) (CH₃OH)], with a distorted octahedral configuration. [MoO₂(ONO)(CH₃OH)] was isolated as an airstable crystalline solid and fully characterized by single-crystal X-ray structure analysis. [MoO₂(ONO)(CH₃OH)] shows reactivity in the oxidation of sulfides to their corresponding sulfoxides using urea hydrogen peroxide as the oxidant at room temperature under air.

Full text of DOI:10.1080/00958972.2014.939076

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Synthesis, X-ray studies, and catalytic
activity of tridentate Schiff base dioxo-
molybdenum(VI)
a
b
b
Mojtaba Amini , Mostafa Khaksar , Davar M. Boghaei , Mojtaba
b
c
c
Bagherzadeh , Arkady Ellern & L. Keith Woo
a
Faculty of Science, Department of Chemistry, University of
Maragheh, Maragheh, Iran
b
Chemistry Department, Sharif University of Technology, Tehran,
Iran
c
Chemistry Department, Iowa State University, Ames, IA, USA
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online: 21 Jul 2014.
To cite this article: Mojtaba Amini, Mostafa Khaksar, Davar M. Boghaei, Mojtaba Bagherzadeh,
Arkady Ellern & L. Keith Woo (2014) Synthesis, X-ray studies, and catalytic activity of tridentate
Schiff base dioxo-molybdenum(VI), Journal of Coordination Chemistry, 67:14, 2435-2444, DOI:
10.1080/00958972.2014.939076
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Journal of Coordination Chemistry, 2014
Vol. 67, No. 14, 2435–2444, http://dx.doi.org/10.1080/00958972.2014.939076
Synthesis, X-ray studies, and catalytic activity of tridentate
Schiff base dioxo-molybdenum(VI)
MOJTABA AMINI*†, MOSTAFA KHAKSAR‡, DAVAR M. BOGHAEI*‡,
MOJTABA BAGHERZADEH‡, ARKADY ELLERN§ and L. KEITH WOO§
†
Faculty of Science, Department of Chemistry, University of Maragheh, Maragheh, Iran
Chemistry Department, Sharif University of Technology, Tehran, Iran
Chemistry Department, Iowa State University, Ames, IA, USA
‡
§
(
Received 4 March 2014; accepted 23 May 2014)
The reaction of a solution of MoO
dentate ONO donor Schiff base (ONO) afford a six-coordinated Mo(VI) complex [MoO
CH OH)], with a distorted octahedral configuration. [MoO (ONO)(CH OH)] was isolated as an air-
stable crystalline solid and fully characterized by single-crystal X-ray structure analysis.
MoO (ONO)(CH OH)] shows reactivity in the oxidation of sulfides to their corresponding sulfox-
2
(acac)
2
in CH
3
OH and salicylidene 2-picoloyl hydrazone as a tri-
2
(ONO)
(
3
2
3
[
2
3
ides using urea hydrogen peroxide as the oxidant at room temperature under air.
Keywords: Dioxo-Mo(VI) complex; Schiff base; Crystal structure; Sulfide oxidation
1. Introduction
Organic sulfoxides are important compounds to synthesize various chemically and biologi-
cally significant molecules [1–5]. Oxidation of sulfides is a common method for preparing
sulfoxides [6–9]. There are several reagents for this conversion, but many have the problem
of over oxidation of sulfoxides to sulfones. Different solvents, oxidants, and catalysts are
used in improving the procedure [10–12]. The use of transition metal complexes as catalytic
*
Corresponding authors. Email: mamini@maragheh.ac.ir (M. Amini); dboghaei@sharif.edu (D.M. Boghaei)
2014 Taylor & Francis
©

2
436
M. Amini et al.
system is based on readily available, inexpensive, and environmentally benign oxidants like
hydrogen peroxide is very attractive [13–15].
Metal complexes with Schiff bases as ligands play an important part in coordination
chemistry as a whole, and there have been many reports on their applications in homoge-
neous and heterogeneous catalysis [16, 17]. Among various transition metal complexes,
Schiff basedioxo-Mo(VI) complexes are one of the most efficient catalysts for oxidation
reactions [18–21].
Recently, we used Schiff basedioxo-Mo(VI) complexes as efficient catalysts for oxidation
of olefins and sulfides [22, 23]. Here, we describe the synthesis of a new dioxo-Mo(VI)
complex [MoO (ONO)(CH OH)] ((ONO) = salicylidene 2-picoloyl hydrazone) and the use
2
3
of this complex as a catalyst in oxidation of sulfides in the presence of urea hydrogen per-
oxide (UHP) as an oxidant at room temperature under air (scheme 1).
2
2
. Experimental
.1. General procedure
Chemicals and solvents were purchased from Fluka and Merck Chemical companies. Sali-
cylidene 2-picoloyl hydrazone (ONO) and dioxo-molybdenum acetylacetonate, MoO2
(
acac) , were synthesized according to published procedures [22, 24]. The elemental analy-
2
sis (carbon, hydrogen, and nitrogen) of the compound was obtained from a Carlo ERBA
Model EA 1108 analyzer. FT-IR spectrum was obtained by using a Unicam Matson 1000
1
FT-IR spectrophotometer using KBr disks at room temperature. H spectra of the free ligand
and complex in DMSO-d were obtained using a Bruker FT-NMR 500 MHZ spectrometer.
6
The products of oxidation reactions were determined and analyzed by an HP Agilent 6890
gas chromatograph equipped with a HP-5 capillary column and a flame-ionization detector.
Crystal evaluation and data collection were performed on a Bruker APEX diffractometer
(Mo Kα radiation, λ = 0.71073 Å).
2
.2. Synthesis of [MoO (ONO)(CH OH)]
2 3
To a solution of MoO (acac) (1.00 mM) in 10 mL of methanol was added a solution of sal-
2
2
icylidene 2-picoloyl hydrazone (ONO) (1.00 mM) in 10 mL of methanol. The above solu-
tion gave a yellow solid when stirred. The solid was filtered off, washed with methanol,
and dried. Crystals of the complex were obtained by diffusion of methanol into a solution
of the complex in DMSO. Yield: 0.311 g, 78%. Anal. Calcd for C H MoN O : C, 42.12;
1
4
13
3 5
H, 3.28; N, 10.52. Found: C, 41.89; H, 3.17; N, 10.60.
O
S
O
S
[
MoO (ONO)(CH OH)], UHP
2 3
R
S
R'
R
R'
+
R
R'
CH Cl / CH OH, r.t.
2
2
3
O
2 3
Scheme 1. The oxidation of sulfides by the [MoO (ONO)(CH OH)]/UHP catalytic system.

Dioxo-molybdenum(VI) complex
2437
2.3. General procedure for sulfide oxidation
To a solution of sulfide (0.2 mM), chlorobenzene (0.2 mM) as internal standard, and
[
MoO (ONO)(CH OH)] (0.01 mM) in a 1 : 1 mixture of CH OH/CH Cl (1 mL) was added
2 3 3 2 2
0
.2 mM UHP as an oxidant. The mixture was stirred at room temperature and the reaction
progress was monitored by GC. Assignments of products were made by comparison with
authentic samples.
3
3
. Results and discussion
.1. Complex characterization
The complex was obtained by reaction of 1 equiv. of salicylidene 2-picoloyl hydrazone and
MoO (acac) (scheme 2). The fast color change from a pale yellow to a yellow-orange solu-
2
2
tion during the synthesis indicated the coordination of ligand. The new tridentate Schiff
base dioxo-molybdenum(VI) complex is yellow, stable to air and light, and soluble in DMF
and DMSO, but has low solubility in methanol, ethanol, and acetonitrile.
Elemental analysis and spectral data confirm the assigned composition of [MoO (ONO)
2
(
CH OH)]. IR spectra of the complex clearly showed the coordination of the ligands, as
3
exemplified by the shift of the absorption band assigned to C=N bonds from 1621 (free
−
1
ligand) to 1607 cm (Mo complex) [25, 26]. Also, the ν(Mo=O) bands of complex are at
−
1
9
16 and 981 cm [22].
Molar conductivities of ligand and its complex in DMSO were 3.7 and 2.3 Ω cm M ,
−
1
2
−1
respectively. These results show that the ligand coordinates to the metal ions to form neutral
coordination compound. Their molar conductances are much smaller than those of 1 : 1
electrolytes [27], thus they are all considered to be nonelectrolytes.
1
H NMR spectrum of the complex has been recorded in d -DMSO solution. A sharp sin-
6
glet appeared for OH proton of phenolic group of the ligand in the region δ = 11.49 ppm is
absent in the complex, and also a significant downfield shift (Δδ = 0.71 ppm) of the signal
for the azomethine (–CH=N–) proton in the complex relative to the corresponding ligand
demonstrates the coordination of ligand through oxygen of the deprotonated phenolic group
and azomethine nitrogen [28]. The complex shows multiplets at δ 6.88–8.73 ppm due to the
aromatic protons. The coordinated methanol molecule displayed two signals at around
1
δ = 3.15 and 4.08 ppm in the H NMR spectrum of complex [26].
To confirm the coordination mode of salicylidene 2-picoloyl hydrazone in this complex
as well as the stereochemistry of the complex, the structure of the complex has been deter-
mined by X-ray crystallography, and an ORTEP view of the complex is shown in figure 1.
N
N
N
H
[
MoO (acac) ]
N
N
2
2
N
CH OH
O
3
O
O
OH
Mo
O
O
HOCH3
2 3
Scheme 2. Synthesis of [MoO (ONO)(CH OH)].

2
438
M. Amini et al.
2 3
Figure 1. ORTEP diagram of [MoO (ONO)(CH OH)] with thermal ellipsoids drawn at a 50% probability limit.
The crystal lattice is stabilized by intermolecular hydrogen bonds O–H⋯N between
hydrogen of coordinated methanol molecule and the aromatic N of Schiff base ligand
belonging to the two neighboring complexes, to form an infinite 1-D polymeric array along
the a axis (figure 2).
The summary of single-crystal X-ray structure refinement is shown in table 1. The com-
plex crystallizes in the monoclinic crystal system and space group P121/n1. The structure
of the complex is mononuclear, consisting of discrete monomeric units of [MoO (ONO)
2
(
CH OH)], with a distorted octahedral arrangement. The structure shows that the ligand is
3
Figure 2. Depicting weak intermolecular O–H⋯N interactions of the 1-D polymeric array along the a axis for
MoO (ONO)(CH OH)].
[
2
3

Dioxo-molybdenum(VI) complex
2439
Table 1. Crystal data and structure refinement for [MoO
MoO (ONO)(CH
12MoN
2
(ONO)(CH
3
OH)].
2
3
OH)
Empirical formula
Formula weight
Temperature
C
14
H
3 5
O
398.21
173(2) K
0.71073 Å
Monoclinic
P1 21/n 1
Wavelength
Crystal system
Space group
Unit cell dimensions
a = 11.477(3) Å
α = 90°
b = 10.124(3) Å
β = 108.505(5)°
c = 13.213(4) Å
γ = 90°
3
Volume
1455.9(7) Å
Z
4
3
Density (calculated)
Absorption coefficient
F (0 0 0)
1.817 g/cm
0.931 mm
796
−
1
Crystal size
0.080 × 0.140 × 0.200 mm
Theta range for data collection
Index ranges
2.05°–28.70°
−15 ≤ h ≤ 15, −13 ≤ k ≤ 13, −17 ≤ l ≤ 17
Reflections collected
Independent reflections
Completeness to theta = 28.70°
Absorption correction
Max. and min. transmission
Refinement method
Data/restraints/parameters
19,814
6374 [R(int) = 0.0383]
99.7%
Multi-scan
0.8920 and 0.7850
Full-matrix least-squares on F
3761/0/209
1.083
2
2
Goodness-of-fit on F
Final R indices [I > 2σ(I)]
R indices (all data)
Largest diff. peak and hole
R
R
1
= 0.0573, wR
= 0.0788, wR
2
= 0.1431
= 0.1554
1
2
−
3
1.814 and −1.761 e Å
2
2
2
2
2 2
Notes: Rint = Σ|F
o
− F − F
o
(mean)|/Σ[F
o
]. R
1
= Σ||F | − |F ||/Σ|F |. GOOF = S = {Σ[w(F
o
c
o
o
c
) ]/
1/2
2
2 2
2 2 1/2
o c o o
2
2
(
n − p)} . wR
2
= {Σ[w(F − F ) ]/Σ[w(F ) ]} . w = 1/[σ(F ) + (aP) + bP] where P is
2F + Max(F , 0)]/3.
c o
2
2
[
coordinated to molybdenum in the expected tridentate fashion through O, N, and O, in addi-
tion to two cis-oxido groups and one methanol. The ligand binds the metal center at O, N,
and O forming five-membered and six-membered chelate rings with a bite angle of
O(1)–Mo(1)–N(1) = 81.53(15)° and O(2)–Mo(1)–N(1) = 71.70(14)° and a bond length of
Mo(1)–O(1) = 1.925(3) (Å), Mo(1)–O(2) = 2.005(3) (Å), Mo(1)–O(3) = 1.686(4) (Å), Mo
(1)–O(4) = 1.699(4) (Å), Mo(1)–O(5) = 2.320(4) (Å) and C(8)–N(2) = 1.293(6) (Å).
3
.2. Catalytic activity
In order to evaluate the catalytic activities of [MoO (ONO)(CH OH)] for the oxidation of
2
3
sulfides, the reactions were optimized according to the oxidation of methylphenyl sulfide
MPS) through investigation of the influence of solvent, the amount of the catalyst, and the
(
amount of the UHP.
To find the best solvent for sulfide oxidation, dichloromethane, chloroform, acetonitrile, ace-
tone, methanol, and 1 : 1 mixture of CH OH/CH Cl were employed as solvents. Among the
3
2
2
solvents examined, 1 : 1 mixture of CH OH/CH Cl was the best for this protocol (table 2).
3
2
2
The effect of amount of catalyst on the conversion and the selectivity of the MPS oxidation at

2
440
M. Amini et al.
Table 2. The effect of various conditions in the oxidation of MPS by [MoO
2
(ONO)(CH
3
OH)]/UHP.
Amount of catalyst
(mM)
Amount of UHP
(mM)
Solvent
(1 mL)
Conversion
Selectivity to sulfoxide
a
b
Entry
(%)
(%)
1
2
3
4
5
6
0.01
0.01
0.01
0.01
0.01
0.01
0.4
0.4
0.4
0.4
0.4
0.4
CH
CHCl
2
Cl
3
2
33
28
66
74
54
100
100
99
98
100
51
CH
CH
CH
CH
CH
CH
CH
CH
CH
CH
CH
CH
CH
CH
CH
CH
CH
3
3
3
2
3
2
3
2
3
2
3
2
3
2
3
2
3
CN
OH
COCH
3
Cl
OH
Cl
OH
Cl
OH
Cl
OH
Cl
OH
Cl
OH
Cl
OH
2
:
:
:
:
:
:
:
100
7
8
9
0
0.4
0.4
0.4
0.1
0.2
0.3
2
Trace
88
–
0.005
0. 015
0.01
2
96
46
99
94
68
2
100
65
10
2
11
0.01
2
97
12
0.01
2
100
a
TON = (mM of sulfoxide + mM of sulfone)/mM of catalyst.
Selectivity to sulfoxide = (sulfoxide%/(sulfoxide% + sulfone%)) × 100.
b
room temperature for 30 min in CH OH/CH Cl was also studied. The conversion of MPS
3
2
2
monotonously increases with the addition of catalyst of 0 to 0.01 mM (table 2). When the
amount of catalyst is increased to 0.015 mM, the selectivity of the MPS oxidation reduces
from 94 to 83%, where the conversion of MPS was increased from 97 to 100% (table 2, entry
9
). Reaction without catalyst proceeds very slightly. Hence, the amount of catalyst enhances
the reaction rate for selective oxidation of sulfides. The amount of UHP could significantly
affect the MPS conversion and methylphenylsulfoxide selectivity (table 2, entries 10–13).
When the amount of UHP increased from 0 to 0.2 mM, the conversion of MPS increased from
0
to 97%. With a further increase of UHP to 0.3 mM, the selectivity to methylphenylsulfoxide
decreases from 94 to 68%, while the conversion of MPS increased from 97 to 100%. In other
words, selectivity to sulfoxide would appear to be better for reactions with 1 equiv. of the
oxidant compared to 1.5 equiv. of UHP.
A series of structurally diverse sulfides were subjected to the oxidation using
[
MoO (ONO)(CH OH)] as the catalyst and UHP as oxidant. Arylalkyl (table 3, entries 1
2 3
and 2), arylbenzyl (table 3, entry 3), dibenzyl (table 3, entry 4), diaryl (table 3, entry 5),
and dialkyl (table 3, entries 6–8) sulfides underwent clean and selective oxidation to the
corresponding sulfoxide under air, in impressive selectivity (91–100%). The very good con-
versions of substrates, depending on the nature of the sulfide of 71–97% (TON =
1
4.2–19.4), were obtained for all cases. In the case of dialkyl sulfides (table 3, entries 6–8),
no over-oxidation to sulfone was observed. The highest and the lowest conversions were
obtained for methylphenyl sulfide (97%) and diphenyl sulfide (71%), respectively (table 3,
entries 1 and 5).
The mechanism for oxidation of various sulfides to sulfoxides using [MoO (ONO)
2
(
CH OH)] has been proposed by comparison with the literature report (scheme 3) [4]. The
3
changes of UV–vis absorbance spectra (figure 3) of the complex before and after adding
UHP indicate that the reaction starts with nucleophilic attack of H O to dioxo-Mo(VI)
2
2

Dioxo-molybdenum(VI) complex
2441
a
2 3
Table 3. Oxidation of sulfides catalyzed by [MoO (ONO)(CH OH)]/UHP.
b
c
d
Entry
1
Substrate
Conversion (%) (TON)
97 (19.4)
Selectivity (%)
S
94
S
2
3
91 (18.2)
90 (18)
96
97
S
4
5
88 (17.6)
71 (14.2)
97
91
S
S
S
6
7
8
89 (17.8)
90 (18)
100
100
100
S
S
84 (16.8)
C
8
H
17
8 17
C H
a
The molar ratios for [MoO
2 3
(ONO)(CH OH)] : substrate : oxidant are 1 : 20 : 20. The reactions were performed in (1 : 1) mixture
of CH
2
Cl /CH OH (1 mL) under air at room temperature.
2
3
b
The GC yields (%) are measured relative to the starting sulfide.
c
TON = (mM of sulfoxide + mM of sulfone)/mM of catalyst.
d
Selectivity to sulfoxide = (sulfoxide%/(sulfoxide% + sulfone%)) × 100.
Figure 3. UV–vis absorption spectra: (a) absorbance of complex, (b) spectral changes for the reaction of complex
and UHP.
complex and the conversion of dioxo-Mo(VI) complex to oxo-peroxo Mo(VI) complex.
The attack of the uncoordinated sulfide on one oxygen of the peroxido ligand, yielding sulf-
oxide and generating the molybdenum(VI) dioxido complex completes the catalytic cycle
(scheme 3).

2
442
M. Amini et al.
N
N
N
O
O
Mo
O
O
O
HOCH3
UHP
R
R
S
R'
R'
S
OH + urea
2
N
O
N
N
O
O
O
Mo
HOCH3
O
2 3
Scheme 3. Proposed mechanism for sulfide oxidation by the [MoO (ONO)(CH OH)]/UHP catalytic system.
In order to show the merit and efficiency of the present catalytic system in comparison
with recently reported protocols, we compared the results of the methylphenyl sulfide oxida-
tion in the presence of other catalysts. As shown in table 4, our catalytic system is superior
to some of the previously reported catalysts in terms of reaction conditions and conversion.
In contrast to similar, previously reported systems, the catalytic system presented in this
paper does not suffer from the harsh reaction conditions, such as using large amount of cata-
lyst (table 4, entry 2), long reaction time (table 4, entries 3 and 4), and using an additive
(table 4, entries 3 and 4). As illustrated in table 4, all dioxo-Mo(VI) complexes are almost
equally effective sulfide oxidation catalysts in different conditions (table 4, entries 1–4).
Table 4. Recently reported catalytic systems for oxidation of sulfides by dioxo-Mo(VI) catalytic systems.
Conversion
(%)
Selectivity
(%)
Entry Catalyst
1
Condition
Ref.
N
1 M% of catalyst/UHP/rt/30 min
97
94
Present
work
N
N
O
O
Mo
O
O
HOCH3
2
5 M% of catalyst/UHP/rt/30 min
92
100
[29]
N
O
MeOH
O
O
Mo
O
OH
3
4
1 M% of catalyst/H
10 °C/2 h
2
O
O
2
2
/TMB/
/TMB/
100
100
100
100
[30]
[30]
N
N
O
O
Mo
O
OMe
MeOH
O
OMe
1 M% of catalyst/H
10 °C/2 h
2
N
N
O
O
Mo
O
OMe
MeOH
O

Dioxo-molybdenum(VI) complex
2443
4. Conclusion
We have synthesized and characterized a dioxo-Mo(VI) complex and have used it as cata-
lyst in sulfide oxidation. The complex is catalytically active in the oxidation of sulfides to
their respective sulfoxides under homogeneous conditions. This study affords new vistas of
designing of new dioxo-Mo(VI) complexes for use as homogeneous oxidation catalysts.
Supplementary material
The CIF file of crystal structure of complex has been deposited with the CCDC, No.
986254. This data can be obtained free of charge via http://www.ccdc.cam.ac.uk/conts/
retrieving.html, or from the Cambridge Crystallographic Data Center, 12 Union Road,
Cambridge CB2 1EZ, UK; Fax: (+44) 1223 336 033; or Email: deposit@ccdc.cam.ac.uk.
Acknowledgements
Mojtaba Amini, Mojtaba Bagherzadeh and L. Keith Woo thank the Research Council of
University of Maragheh, Sharif University of Technology, and the National Science Foun-
dation for funding of this work.
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