Molybdenum(VI)-oxodiperoxo complex containing an oxazine ligand: Synthesis, X-ray studies, and catalytic activity

Article abstract of DOI:10.1080/00958972.2014.915525

A new mononuclear molybdenum(VI)-oxodiperoxo complex [MoO(O ₂)₂(phox)] with a simple bidentate ligand, 2-(2′-hydroxyphenyl)-5,6-dihydro-1,3-oxazine (Hphox), has been synthesized and characterized by X-ray structure analysis, elemental analysis, infrared, and ¹H NMR spectroscopy. A triclinic space group P-1 was determined by X-ray crystallography from single-crystal data of this complex. The resulting complex functioned as a facile sulfide oxidation catalyst with urea hydrogen peroxide as terminal oxidant at room temperature. The catalyst showed efficient reactivity in oxidation of sulfides giving high yield and selectivity.

Full text of DOI:10.1080/00958972.2014.915525

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Molybdenum(VI)–oxodiperoxo complex
containing an oxazine ligand: synthesis,
X-ray studies, and catalytic activity
Mojtaba Aminia, Mojtaba Bagherzadehb, Behnam Atabakib, Parviz
Gohari Derakhshandehb, Arkady Ellernc & L. Keith Wooc
a Faculty of Science, Department of Chemistry, University of
Maragheh, Maragheh, Iran
b Department of Chemistry, Sharif University of Technology,
Tehran, Iran
c Department of Chemistry, Iowa State University, Ames, IA, USA
Accepted author version posted online: 16 Apr 2014.Published
online: 13 May 2014.
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To cite this article: Mojtaba Amini, Mojtaba Bagherzadeh, Behnam Atabaki, Parviz Gohari
Derakhshandeh, Arkady Ellern & L. Keith Woo (2014) Molybdenum(VI)–oxodiperoxo complex
containing an oxazine ligand: synthesis, X-ray studies, and catalytic activity, Journal of
Coordination Chemistry, 67:8, 1429-1436, DOI: 10.1080/00958972.2014.915525
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Journal of Coordination Chemistry, 2014
Vol. 67, No. 8, 1429–1436, http://dx.doi.org/10.1080/00958972.2014.915525
Molybdenum(VI)–oxodiperoxo complex containing an
oxazine ligand: synthesis, X-ray studies, and catalytic activity
MOJTABA AMINI*†, MOJTABA BAGHERZADEH*‡, BEHNAM ATABAKI‡,
PARVIZ GOHARI DERAKHSHANDEH‡, ARKADY ELLERN§ and L. KEITH WOO§
†Faculty of Science, Department of Chemistry, University of Maragheh, Maragheh, Iran
‡Department of Chemistry, Sharif University of Technology, Tehran, Iran
§Department of Chemistry, Iowa State University, Ames, IA, USA
(Received 17 January 2014; accepted 25 March 2014)
A new mononuclear molybdenum(VI)–oxodiperoxo complex [MoO(O2)2(phox)] with a simple
bidentate ligand, 2-(2′-hydroxyphenyl)-5,6-dihydro-1,3-oxazine (Hphox), has been synthesized and
1
characterized by X-ray structure analysis, elemental analysis, infrared, and H NMR spectroscopy. A
triclinic space group P-1 was determined by X-ray crystallography from single-crystal data of this
complex. The resulting complex functioned as a facile sulfide oxidation catalyst with urea hydrogen
peroxide as terminal oxidant at room temperature. The catalyst showed efficient reactivity in oxida-
tion of sulfides giving high yield and selectivity.
Keywords: Molybdenum complex; Oxodiperoxo; Oxazine; Crystal structure; Oxidation
*Corresponding authors. Email: mamini@maragheh.ac.ir (M. Amini); bagherzadeh@sharif.edu (M. Bagherzadeh)
© 2014 Taylor & Francis

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M. Amini et al.
1. Introduction
Selective oxidation of sulfides to sulfoxides is important for producing sulfoxides as
chemical intermediates to afford useful chemicals [1–7]. The application of transition metal
complexes as catalysts in organic synthesis is an exciting field of research, employed in a
wide range of areas of preparative organic chemistry [8–15]. More generally, in the
presence of hydrogen peroxide, metals form oxido–peroxido complexes as the basis for the
synthesis of a number of fine chemicals [16–19].
The chemistry of transition metal oxido–peroxido complexes has received special atten-
tion due to their importance in industrial, pharmaceutical, and biological processes [20–22].
Coordination to a metal center activates peroxido groups towards oxidation of a variety of
substrates, rendering oxido–peroxido metal complexes important as intermediates in
biological and synthetic catalysis [23–29].
We recently reported a highly efficient method for oxidation reactions by using Mo(VI)
oxo–peroxo complexes with oxazoline and Schiff base ligands [30, 31]. In order to study
the effect of complex structure on catalytic reactivity, herein, we describe the synthesis of a
new mononuclear molybdenum(VI)–oxodiperoxo complex [MoO(O2)2(phox)] ((Hphox) =
2-(2′-hydroxyphenyl)-5,6-dihydro-1,3-oxazine) and the use of this complex as a catalyst in
oxidation of sulfides in the presence of urea hydrogen peroxide (UHP) as an oxidant under
air at room temperature (scheme 1).
2. Experimental setup
2.1. General procedure
Chemicals and solvents were purchased from Fluka and Merck Chemical companies. The
ligand 2-(2′-hydroxyphenyl)-5,6-dihydro-1,3-oxazine (Hphox) was synthesized according to
a published procedure [32].
Elemental analyses (carbon, hydrogen, and nitrogen) of the compound were obtained from
a Carlo ERBA Model EA 1108 analyzer. FT-IR spectrum was obtained by using a Unicam
Matson 1000 FT-IR spectrophotometer using KBr disks at room temperature. MS spectra
were recorded on a Finnigan MAT95 spectrometer. 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. The crystal evaluation and data collection
were performed on a Bruker APEX diffractometer (Mo Kα radiation, λ = 0.71073 Å).
2.2. Synthesis of [MoO(O2)2(phox)]
[MoO(O2)2(phox)] was synthesized by dissolving MoO3 (2.5 mM, 0.36 g) in H2O2 (30%,
5 cm3) by stirring at room temperature. Addition of 2-(2′-hydroxyphenyl)-5,6-dihydro-1,3-
oxazine (Hphox; 2.5 mM, 0.44 g) dissolved in a minimum volume of ethanol (ca. 15 cm3)
O
S
O
S
[MoO(O2)2(phox)], UHP
CH2Cl2/ CH3OH, r.t.
+
R'
R
R'
R
R
S
R'
O
Scheme 1. Oxidation of sulfides by the [MoO(O2)2(phox)]/UHP catalytic system.

Molybdenum(VI)–oxodiperoxo complex
1431
to the above solution under stirring gave a yellow solid. The solid was filtered off, washed
with water, methanol, and finally with diethyl ether, and dried in vacuum. Orange crystals
of the complex were obtained with slow evaporation from an acetonitrile solution of the
product. Yield 0.57 g (65%). C10H10MoNO7 (352.13): Calcd C 34.07, H 2.84, N 3.98;
found C 33.92, H 3.06, N 4.05; Selected IR frequency (KBr disk, cm−1): 1644(νC=N),
1
966 (ν(Mo=O)) and 860 (ν(O–O)); H NMR (500 MHz, DMSO-d6, δ): 1.96 (q, 2H), 3.54 (d,
2H), 4.41 (t, 2H), 6.70–7.63 (m, 4H).
2.3. General procedure for sulfide oxidation
To a solution of sulfide (0.2 mM), chlorobenzene (0.2 mM) as internal standard and [MoO
(O2)2(phox)] (0.015 mM) in a 1 : 1 mixture of CH3OH/CH2Cl2 (1 mL), 0.4 mM UHP was
added as the 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. Results and discussion
3.1. Complex characterization
[MoO(O2)2(phox)] was synthesized by dissolving MoO3 (freshly precipitated) in H2O2 and
treating the resulting solution with 2-(2′-hydroxyphenyl)-5,6-dihydro-1,3-oxazine dissolved
in ethanol. The complex is air stable and was fully characterized by spectroscopy (1H NMR
and IR), elemental analysis, and X-ray crystallography. In order to study the binding of the
ligand to molybdenum in the new complex, the IR spectrum of the free ligand was com-
pared with the spectrum of the molybdenum complex. The absorption of the complex at
1644 cm−1 assigned to the C=N of the ligand did not shift showing that the imine nitrogen
is not coordinated [30]. The ν(Mo=O) and ν(O–O) vibrations of the complex appear around
966 and 860 cm−1, respectively [30]. The coordinating modes of the ligand were confirmed
1
by comparing H NMR patterns of the ligand and the complex. The signal appearing at
δ = 14.2 ppm, due to the phenolic OH of the ligand, disappears in the spectrum of the com-
plex. The 1H NMR spectrum of the other protons of the complex exhibits almost no
changes in comparison with the ligand and all the signals in the complex were very similar
to that of the ligand and appear nearly at the same positions. Each of the hydrogens in the
oxazine ring and aromatic ring show distinct peaks at 1.96–4.41 and 6.70–7.63 ppm,
respectively [32].
The ORTEP view of the complex with atom numbering scheme is shown in figure 1. A
summary of the single-crystal X-ray structure refinement is shown in table 1, and selected
bond lengths and angles for the complex are listed in table 2.
The geometry around Mo can best be described as distorted pentagonal pyramidal with
the axial site occupied by oxo (O7) (Mo–O7 distance 1.6694(19) Å). The phenolate oxygen
(O2) with significantly longer bond length of 2.0229(19) Å and the peroxo moieties
(Mo–O3, Mo–O4 distances 1.9850(19) and 1.9333(19) Å; Mo–O5, Mo–O6 distances
1.926(2) and 1.923(2) Å) define the equatorial plane. The structural parameters given in
figure 1 are consistent with those reported for oxodiperoxo–Mo(VI) complexes [33, 34].

1432
M. Amini et al.
Figure 1. ORTEP diagram of [MoO(O2)2(phox)] with thermal ellipsoids drawn at a 50% probability limit.
The observation that greater stability of the oxodiperoxo–Mo(VI) complexes is attained
when the two peroxo groups coordinate in the equatorial plane is consistent with our
structure [35].
3.2. Catalytic activity
In order to evaluate the catalytic activities of [MoO(O2)2(phox)] for oxidation of sulfides,
the reactions were optimized according to the oxidation of methylphenyl sulfide (MPS)
through investigation of the factors that influence oxidation, such as solvent, the amount of
catalyst, and the amount of UHP.
In order to understand the effect of solvent on the catalytic reactivity, a series of reactions
was investigated by performing the model reaction in dichloromethane, acetonitrile, metha-
nol, dichloroethane, and (1 : 1) mixture of CH2Cl2/CH3OH. Among the solvents examined,
the (1 : 1) mixture of CH2Cl2/CH3OH was found to be best for this protocol (table 3). The
conversion of MPS monotonously increases with the addition of catalyst from 0.0025 to
0.015 mM. When the amount of catalyst is increased to 0.02 mM, the conversion of MPS
oxidation reduces from 87 to 81%. The amount of H2O2 could also affect the conversion
and methylphenylsulfoxide selectivity (table 3). When the amount of H2O2 was increased
from 0.1 to 0.4 mM, the conversion of MPS increased from 25 to 94%. With a further
increase of H2O2 to 0.5 mM, the selectivity to methylphenylsulfoxide decreased from 95 to
88%.

Molybdenum(VI)–oxodiperoxo complex
1433
Table 1. Crystal data and structure refinement for [MoO(O2)2(phox)].
Empirical formula
Formula weight
Temperature
C10H10MoNO7
352.13
173(2) K
Wavelength
0.71073 Å
Crystal system
Space group
Triclinic
P-1
Unit cell dimensions
a = 8.4403(11) Å
α = 65.1050(17)°
b = 8.6815(12) Å
β = 68.9470(18)°
c = 9.5957(13) Å
γ = 68.6800(18)°
576.18(13) Å3
2
Volume
Z
Density (Calcd)
Absorption coefficient
F (0 0 0)
2.030 g cm−3
1.169 mm−1
350
Crystal size
θ Range for data collection
Index ranges
0.240 × 0.270 × 12.000 mm
2.41°–25.03°
−10 ≤ h ≤ 10, −10 ≤ k ≤ 10, −11 ≤ l ≤ 11
4731
2034 [Rint = 0.0211]
99.9%
multi-scan
0.7670 and 0.0250
Full-matrix least-squares on F2
2034/0/172
Reflections collected
Independent reflections
Completeness to θ = 28.70°
Absorption correction
Max. and min. transmission
Refinement method
Data/restraints/parameters
Goodness-of-fit on F2
Final R indices [I > 2σ(I)]
R indices (all data)
Largest diff. peak and hole
1.121
R1 = 0.0220, wR2 = 0.0536
R1 = 0.0245, wR2 = 0.0550
0.928 and −0.315 eÅ−3
Note: Rint = Σ|Fo2 − Fo2(mean)|/Σ[Fo2];
R1 = Σ||Fo| − |Fc||/Σ|Fo|;
GOOF = S = {Σ[w(Fo2 −Fc2)2]/(n −p)}1/2
;
wR2 = {Σ[w(Fo2 −Fc2)2]/Σ[w(Fo2)2]}1/2
;
w = 1/[σ(Fo2) + (aP)2 + bP], where P is [2Fc2 + Max(Fo2, 0)]/3.
Table 2. Significant bond lengths (Å) and angles (°) for [MoO(O2)2(phox)].
Bond lengths (Å)
Bond angles (°)
Mo1–O7
Mo1–O6
Mo1–O5
Mo1–O4
Mo1–O3
Mo1–O2
O3–O4
1.6694(19)
1.923(2)
1.926(2)
1.9333(19)
1.9850(19)
2.0229(19)
1.478(3)
O7–Mo1–O6
O6–Mo1–O5
O6–Mo1–O4
O7–Mo1–O3
O6–Mo1–O2
O4–Mo1–O2
O3–O4–Mo1
O5–O6–Mo1
104.59(10)
44.95(8)
130.19(8)
107.65(9)
88.42(8)
130.95(8)
69.71(11)
67.65(11)
O7–Mo1–O5
O7–Mo1–O4
O5–Mo1–O4
O6–Mo1–O3
O5–Mo1–O2
O3–Mo1–O2
O4–O3–Mo1
O6–O5–Mo1
103.30(9)
100.34(9)
87.75(8)
147.72(9)
132.50(8)
86.69(8)
66.00(10)
67.40(11)
O5–O6
1.472(3)
A series of structurally diverse sulfides were subjected to oxidation reaction using [MoO
(O2)2(phox)] as catalyst and UHP as oxidant (table 4). The reactions were run with molar
ratio of 1 : 13 : 26 for catalyst : substrate : UHP that was found to be optimum. Arylalkyl
(table 4, entries 1,2), arylbenzyl (table 4, entry 3), dibenzyl (table 4, entry 4), diaryl (table 4,
entry 5) and dialkyl (table 4, entries 6–8) sulfides underwent clean and selective oxidation to
the corresponding sulfoxides in air, with impressive selectivity (63–100%). Very good

1434
M. Amini et al.
Table 3. The effect of various conditions in oxidation of MPS by [MoO(O2)2(phox)].
Amount of catalyst
(mM)
Amount of UHP
(mM)
Conversion
(%)a
Selectivity to sulfoxide
(%)b
Entry
Solvent (1 mL)
1
2
3
4
5
6
7
8
0.01
0.01
0.01
0.01
0.2
0.2
0.2
0.2
0.2
0.2
0.2
0.2
0.2
0.1
0.3
0.4
0.5
0.4
CH2Cl2
CH3OH
C2H4Cl2
CH3CN
Trace
72
Trace
55
77
75
71
87
81
25
–
85
–
75
96
98
98
97
89
100
96
95
88
100
0.01
CH3OH/CH2Cl2
CH3OH/CH2Cl2
CH3OH/CH2Cl2
CH3OH/CH2Cl2
CH3OH/CH2Cl2
CH3OH/CH2Cl2
CH3OH/CH2Cl2
CH3OH/CH2Cl2
CH3OH/CH2Cl2
CH3OH/CH2Cl2
0.0025
0.005
0.015
0.02
0.015
0.015
0.015
0.015
–
9
10
11
12
13
14
90
94
95
11
aThe GC conversion (%) are measured relative to the starting substrate.
bSelectivity to sulfoxide = (sulfoxide/(sulfoxide + sulfone))% × 100.
Table 4. Oxidation of sulfides catalyzed by [MoO(O2)2(phox)]/UHP.a
Entry
1
Substrate
Conversion (%)b (TON)c
Selectivity (%)d
95
94(12.5)
2
3
94(12.5)
90(12)
67
91
4
5
97(12.9)
71(9.5)
91
63
6
7
8
100(13.3)
100(13.3)
100(13.3)
100
100
100
aThe molar ratios for [MoO(O2)2(phox)] : substrate : oxidant are 1 : 13 : 26. The reactions were performed in (1 : 1) mixture of
CH2Cl2/CH3OH (1 mL) under air at room temperature within 20 min.
bThe GC yield (%) are measured relative to the starting sulfide.
cTON = (mM of sulfoxide + mM of sulfone)/mM of catalyst.
dSelectivity to sulfoxide = (sulfoxide/(sulfoxide + sulfone))% × 100.
conversions of substrates depending on the nature of the sulfide from 71 to 100% were
obtained for all cases. Aromatic and aliphatic sulfides undergo oxidation reactions more
easily than diaryl substrates. The lowest conversion was obtained for diphenyl sulfide.

Molybdenum(VI)–oxodiperoxo complex
1435
O
O
O
O
O
CH3
Mo
L
H2O, urea
UHP
S
Ph
A
L= ligand
O
O
O
O
O
O
O
Mo
L
O
B
C
Mo
L
O
Ph
S
CH3
O
S
Ph
CH3
Scheme 2. Proposed mechanism for sulfide oxidation by the [MoO(O2)2(phox)]/UHP catalytic system.
The mechanism of oxidation in the presence of oxodiperoxo–Mo(VI) complexes is widely
accepted even though detailed mechanistic investigations have led to a more refined under-
standing in recent years [1]. The mechanism for the catalytic oxidation of sulfides catalyzed
by the oxodiperoxo–Mo(VI) complex [MoO(O2)2(phox)] has been proposed (scheme 2) by
comparison with literature reports [1]. The catalytic cycle begins with the addition of sulfide
to complex A to form intermediate B that can transfer an oxygen atom to sulfide. Then, UHP
can react with generated intermediate C to regenerate the catalyst.
4. Conclusion
The six-coordinate oxodiperoxo–Mo(VI) complex [MoO(O2)2(phox)] has been synthesized
and characterized by physico-chemical methods. Solid state studies further exemplified the
distorted pentagonal pyramidal geometry around Mo. In this study, we have demonstrated
the effectiveness of this complex as a catalyst for the oxidation of sulfides to their corre-
sponding sulfoxides. Easy preparation, mild reaction conditions, high yields of the products,
short reaction time, and high selectivity make this catalytic system a useful method for oxi-
dation reactions.
Supplementary material
The CIF file of crystal structure complex [MoO(O2)2(phox)] has been deposited with the
CCDC, No. 980856. 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 E-mail: deposit@ccdc.cam.ac.uk.

1436
M. Amini et al.
Acknowledgements
M. Amini, M. Bagherzadeh and L.K. Woo thank, respectively, the Research Council of
University of Maragheh, Sharif University of Technology and the National Science Founda-
tion for funding of this work.
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