Synthesis, crystal structure and catalytic activity of a new Mo Schiff base complex with Mo histidine immobilized on Al-MCM-41 for oxidation of sulfides

Article abstract of DOI:10.1016/j.ica.2014.01.028

A cis-dioxomolybdenum(VI) complex with Schiff base ligand derived from salicylaldehyde and l-histidine was prepared and designated as MoO ₂(Sal-His). Characterization of MoO₂(Sal-His) was accomplished with elemental analysis, FT-IR, UV-Vis and TGA. X-ray crystallography studies revealed that the coordination of Mo in MoO ₂(Sal-His) is a distorted octahedron formed by tetradentate histidine Schiff base ligand and two binding oxygen atoms. MoO₂(Sal-His) was used either alone or immobilized within Al-MCM-41 as sulfoxidation catalyst for a variety of sulfides with H₂O₂ as oxidant. The latter proved to be a very active and reusable heterogeneous sulfoxidation catalyst, providing 70-99% conversion and 80-95% selectivity.

Full text of DOI:10.1016/j.ica.2014.01.028

Inorganica Chimica Acta 414 (2014) 63–70
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Inorganica Chimica Acta
journal homepage: www.elsevier.com/locate/ica
Synthesis, crystal structure and catalytic activity of a new Mo Schiff base
complex with Mo histidine immobilized on Al-MCM-41 for oxidation
of sulfides
a
a,
⇑
b
a
Elham Zamanifar , Faezeh Farzaneh , Jim Simpson , Mahboobeh Maghami
a
Department of Chemistry, Alzahra University, PO Box 1993891176, Tehran, Iran
Department of Chemistry, University of Otago, PO Box 56, Dunedin 9054, New Zealand
b
a r t i c l e i n f o
a b s t r a c t
Article history:
A cis-dioxomolybdenum(VI) complex with Schiff base ligand derived from salicylaldehyde and
was prepared and designated as MoO (Sal-His). Characterization of MoO (Sal-His) was accomplished
with elemental analysis, FT-IR, UV–Vis and TGA. X-ray crystallography studies revealed that the coordi-
nation of Mo in MoO (Sal-His) is a distorted octahedron formed by tetradentate histidine Schiff base
ligand and two binding oxygen atoms. MoO (Sal-His) was used either alone or immobilized within Al-
MCM-41 as sulfoxidation catalyst for a variety of sulfides with H as oxidant. The latter proved to
be a very active and reusable heterogeneous sulfoxidation catalyst, providing 70–99% conversion and
0–95% selectivity.
L-histidine
Received 27 October 2013
Received in revised form 3 January 2014
Accepted 15 January 2014
Available online 8 February 2014
2
2
2
2
2 2
O
Keywords:
Dioxomolybdenum complex
Histidine
Schiff base ligand
Al-MCM-41
8
Ó 2014 Elsevier B.V. All rights reserved.
Sulfoxidation
1
. Introduction
The coordination chemistry of molybdenum(VI) has attracted
these complexes varies with the ligand type, coordination sites
and so on.
Sulfoxidation is one of the fundamental reactions in industrial
organic synthesis as sulfoxides and sulfones are biologically impor-
tant compounds and have significant role in the synthesis of many
organic compounds [24]. In fact, the development of novel, eco-
nomical and energy efficient routes for the selective and green oxi-
dative transformation of sulfides represents a challenge to diverse
areas of contemporary chemistry. However, the homogeneous cat-
alytic systems suffer from problems associated with the separation
and recovery of the active catalyst as well as instability at high
temperatures [19–23,25]. These drawbacks have so far precluded
their industrial utilization. Moreover, metal contamination of
products is inevitable when using homogeneous catalysts. There-
fore, all strategies including catalyst reactivity, recycling, economy
and environmental concerns are needed to be modified [26].
Herein, we report the synthesis and characterization of a new
considerable interest due to the significant enzymatic role of
molybdenum in biochemical reactions such as oxotransfer like
nitrate reduce, xanthine oxidase/dehydrogenase, and DMSO reduce
[
1–3]. Molybdenum biochemical role is based on its ability to
facilitate electron exchanges, forming stable complexes with
oxygen-, nitrogen- and sulfur-containing ligands. This has created
a tremendous impetus in the synthesis of a number of model
complexes mimicking the oxotransferase molydoenzymes [3,4].
Therefore, researchers were encouraged to use molybdenum com-
plexes as biomimetic catalysts for oxidation of aldehydes, purines
and sulfides [5,6], epoxidations and hydroxylations of olefins [5–9],
oxidation of alcohols [10,11] or oxygen atom transfer reactions
[
12].
In order to mimic biological systems, a number of dioxomolyb-
denum complexes have been synthesized and characterized
6,7,11,13–15]. Molybdenum Schiff base complexes continue to
cis dioxomolybdenum complex with N-salicylidene-L-histidine
[
Schiff base ligand, followed by immobilization within mesoporous
support. The limited reports presented on molybdenum-based cat-
alysts performing the oxidation of sulfides together with several
disadvantages such as long reaction times, inconvenient condi-
tions, expensive oxidants and low yields prompted us to investi-
serve as one of the most important models of molybdoenzymes
due to their preparative accessibility [5,16–23]. The activity of
⇑
Corresponding author. Address: Department of Chemistry, University of
Alzahra, P.O. Box 1993891176, Vanak, Tehran, Iran. Tel.: +98 21 88258977.
2
gate the catalytic activity of MoO (Sal-His) on sulfide oxidation.
E-mail addresses: faezeh_farzaneh@Yahoo.com, farzaneh@alzahra.ac.ir (F. Farzaneh).
http://dx.doi.org/10.1016/j.ica.2014.01.028
0
020-1693/Ó 2014 Elsevier B.V. All rights reserved.

6
4
E. Zamanifar et al. / Inorganica Chimica Acta 414 (2014) 63–70
2
. Experimental
2.4. General procedure for the oxidation of sulfides
2.1. Materials and characterization
A mixture of Mo-catalyst (40 mg) and substrate (20 mmol in
cc acetonitrile) was slowly stirred in a round bottom flask.
5
All materials were commercial reagent grade and used without
2 2
H O (30% aqueous solution, 20 mmol) was then added and the
further purification. Except the t-butyl hydroperoxide (TBHP) pur-
chased from Fluka, salicylaldehyde, -histidine, sodium acetate,
acetonitrile, acetone, methy phenyl sulfide, diphenyl sulfide, di-
methyl sulfide, p-tolyl-methyl sulfide, hydrogen peroxide (30%),
commercial reagent grade diethyl ether and ethanol were pur-
chased from Merck Chemical Company.
mixture stirred for a few minutes at room temperature. After filtra-
tion and washing the solid with fresh acetonitrile, the filtrate was
subjected to GC and GC Mass analysis.
L
2.5. X-ray crystallography
FT-IR spectra were recorded on a Bruker Tensor 27 FT-IR spec-
2
Yellow crystals of [MoO (Sal-His], 1, were grown by diffusion of
ꢀ
1
trometer using KBr pellets over the range of 4000–400 cm . The
UV–Vis measurements were performed on double beam
ether into a DMF complex solution. X-ray data for 1 was collected
on an Agilent SuperNova Dual single crystal diffractometer. Mo K
a
a
UV–Vis Perkin Elmer Lambda 35 spectrophotometer. X-ray powder
diffraction (XRD) data were recorded on a diffractometer type, Seif-
ert XRD 3003 PTS, using Cu Ka radiation (k = 1.5406 Å). Single
1
crystal measurement was performed on an Agilent SuperNova Dual
single crystal diffractometer. Intensity data were collected using
radiation (k = 0.71073 Å) was used for the collection which was
controlled by CRYSALISPRO [32] with data collected at 100(2) K. Data
was corrected for Lorentz and polarization effects using multi-scan
absorption corrections were applied using CRYSALISPRO [32]. The
structure was solved by direct methods (SHELXS-97) [33] and refined
using full-matrix least-squares procedures (SHELXL-97 [33] and
TITAN2000) [34]. All non-hydrogen atoms were refined anisotropi-
cally and those hydrogen atoms bound to carbon were placed in
the calculated positions, and their thermal parameters were
refined isotropically with Ueq = 1.2 Ueq(C). The hydrogen atom
bound to N1 was located in a difference Fourier map and its coor-
dinates were refined with Ueq = 1.2 Ueq (N). All molecular plots
and packing diagrams were drawn using MERCURY [35] and addi-
tional metrical data was calculated using PLATON [36]. Crystallo-
graphic data and details of collected data and structure
refinement are listed in Table 1. Selected bond lengths and angles
are shown in Table 2.
graphite monochromatised Mo
Ka radiation (k = 0.71073 Å).
Chemical analyses of samples were determined with a Perkin-
Elmer atomic absorption spectrometer (AAS). TGA/DTS was carried
out by Perkin Elmer, Pyrisdiamond TG/DTA. Oxidation products
were analyzed by GC and GC–MS using an Agilent 6890 Series with
FID detector, HP-5, 5% phenylmethyl siloxane capillary and an
Agilent 5973 Network, mass selective detector, HP-5 MS 6989
Network GC system.
2
2
.2. Preparation of [MoO (Sal-His)]
[
MoO
2
(Sal-His)] was prepared using the reported procedure
[
23]. Salicylaldehyde (10 mmol in 20 mL ethanol) was introduced
3. Results and discussion
into the histidine solution (10 mmol in 20 mL water). Upon addi-
tion of an aqueous sodium acetate solution (0.02 mol in 10 mL
Salicylaldehyde and
in ethanol and water. As seen in Scheme 1, the initially generated
N-salicylidene- -histidine Schiff base ligand from condensation of
2
L-histidine were treated with MoO (acac)2
water), the color was changed to deep yellow. MoO
2
(acac)
2
(
8.5 mmol, prepared according to the literature method [27] in
L
1
0 mL ethanol) was then added and the mixture was heated at re-
salicylaldehyde and L-histidine undergoes complexation with
flux for 5 h while stirring. The yellow resultant solid was filtered,
washed with water, acetone, and diethyl ether, and dried in air
at room temperature. X-ray quality crystals of the complex
Table 1
Selected crystallographic data and details for the structure refinement of [MoO2(Sal-
His].
[
MoO
into the saturated [MoO
Anal. Calc. for C13
0.62; H, 2.60; N, 10.94. Found: Mo, 25.19; C, 40.67; H, 2.61; N,
2
(Sal-His)] was obtained by slow diffusion of diethyl ether
2
(Sal-His)]solution in dimethylformamide.
H N O Mo (M = 385 g mol ): Mo, 25.00; C,
11 3 5
ꢀ1
Empirical formula
Formula weight
Crystal system; space group
C13H11MoN O5
385.19
3
4
1
1
monoclinic, P 21/n
7.43173(17)
10.7623(2)
17.3644(4)
101.948(2)
1358.77(5)
4
0.994
1.883
0.0311
ꢀ1
ꢀ
0.93%. FT-IR (KBr pellet) cm : 1631 (C@N), 1603, 1347 (COO ),
556 (C@C), 1315 (O–Ph), 910, 930 (cis-MoO ), which are
a (Å)
b (Å)
c (Å)
b (°)3
V (Å )
Z
2
consistent spectral bands with those reported previously [28,29].
1
UV–Vis: kmax at 270, 285, 346, 414 nm [28,30]. H NMR (DMSO-d
d 3.19–3.24 (m) (b-CH ), 7.11 (m) (imidazol-5-H), 7.56–7.69 (m)
Ar–H), 8.37 (d) (imidazol-2-H), 9.00 (s) (CH@N), 12.82 (s) broad
6
)
2
Absorption coefficient (mmꢀ1)
(
(
(
(
1
3
ꢀ3
imidazol N–H). C NMR (DMSO-d
6
) d 116.0 (imidazole-C
Ar), 122.7 (Ar), 123.0 (Ar), 133.1 (A), 135.1 (imidazole-C
Ar), 162.5 (CH@N), 165.4 (Ph), 174.6 (C@O) [28].
2
), 118.8
Dcalc (Mg m
)
R
int
5
), 136.8
3
Crystal size (mm )
h Range for collection (°)
Index ranges
0.2 ꢁ 0.2 ꢁ 0.2
3.3680–29.3850
ꢀ10 6 h 6 10
ꢀ
ꢀ
13 6 k 6 14
23 6 l 6 23
Reflections collected
Independent reflections
Independent reflections [I > 2
Maximum and minimum transmission
Data/restraints/parameters
15698
2.3. Preparation of catalyst
3465
3132
1.00–0.87500
3465/0/202
r
(I)]
Al-MCM-41 (1 g), prepared according to the previously reported
method [31], was added to the complex (0.26 mmol in 20 mL eth-
anol). The mixture was heated at reflux for 8 h while stirring. The
resultant MoO (Sal-His)/Al-MCM-41 solid was then filtered,
2
washed with hot ethanol and dried in air at room temperature.
The molybdenum percentage was determined by AAS to be 3.36%.
Final R indices [I > 2
r
(I)]
R
1
= 0.0285
wR = 0.0572
= 0.0242
wR = 0.0545
2
R indices (all data)
R
1
2

E. Zamanifar et al. / Inorganica Chimica Acta 414 (2014) 63–70
65
Table 2
distances of 1.7012 (13) and 1.7090 (14) Å respectively, typical of
double bond. Bond angles in the octahedron vary from
Selected bond lengths (Å) and angles (°) for [MoO2(Sal-His].
a
Bond lengths (Å)
Mo(1)–O(1)
Mo(1)–O(2)
Mo(1)–O(12)
Mo(1)–O(82)
Mo(1)–N(4)
Mo(1)–N(3)
73.28(5)° to 107.26(7)° for cis-substitued and 167.49(6)° to
153.48(5)° for trans substituted, indicating the significance of dis-
tortion degree in the octahedron.
1.7012(13)
1.7090(14)
1.9376(13)
2.0450(13)
2.2287(15)
2.3694(15)
The aminoacid–salicylaldehyde ligand coordinates to the
2
+
MoO
2
moiety as a tetradentate [N3,O12,N4,O82] chelating ligand
via the deprotonated carboxylic acid and phenoxy oxygen atoms,
together with the two imine N atoms. In the process, two five
membered and one six-membered chelate rings are formed. Chela-
tion of this type to a Mo atom has no precedent in the Cambridge
Structural Database [37] although there are several instances of an
Bond angles (°)
O(1)–Mo(1)–O(2)
O(1)–Mo(1)–O(12)
O(2)–Mo(1)–O(12)
O(1)–Mo(1)–O(82)
O(2)–Mo(1)–O(82)
O(12)–Mo(1)–O(82)
O(1)–Mo(1)–N(4)
O(2)–Mo(1)–N(4)
O(12)–Mo(1)–N(4)
O(82)–Mo(1)–N(4)
O(1)–Mo(1)–N(3)
O(2)–Mo(1)–N(3)
O(12)–Mo(1)–N(3)
O(82)–Mo(1)–N(3)
107.26(7)
96.60(6)
99.90(6)
93.71(6)
100.25(6)
153.48(5)
95.02(6)
157.35(6)
81.11(6)
73.68(5)
167.49(6)
84.18(6)
86.12(6)
78.96(5)
2+
imidazole molecule binding through its imine N atom to an MoO
2
unit which in term is coordinated to a tridentate O,N,O ligand [38].
There is a distinct trend in Mo–O bond lengths in the complex with
Mo@O1/2 (terminal) ꢂ Mo–O12 (phenoxyl) < Mo–O82 (carboxyl).
Also the trans influence ensures that both Mo–N distances trans to
the M@O bonds are significantly extended (Table 2).
i
In the crystal structure, N1–H1Nꢃ ꢃ ꢃO82 hydrogen bonds bol-
stered by C2–H2ꢃ ꢃ ꢃO81 contact link molecules into zigzag chains
along b (Fig. 2).
Several other C–Hꢃ ꢃ ꢃO contacts (Table 3), and a
p p stacking
ꢃ ꢃ ꢃ
interaction between the benzene rings of adjacent molecules with
a centroidꢃ ꢃ ꢃcentroid distance of 3.5791(12) Å further stabilize the
structure stacking molecules along the crystallographic b axis
Fig. S2 (see Supplementary).
MoO
2 2 2
(acac) , affording [MoO (Sal-His)]. The ligand acts as a tetra-
2+
dental ONON donor towards the MoO
2
core.
3.1. Crystal structure determination
3.2. NMR studies
An ORTEP view (50% thermal ellipsoids) of [MoO
shown in Fig. 1. The Mo atom in the complex [MoO
2
(Sal-His] is
(Sal-His]
2
The coordinating modes of the ligand were confirmed by com-
1
adopts a severely distorted octahedral 6-coordinated geometry
Fig. S1 (see Supplementary) with the Mo@O1 and Mo@O2
parison of H NMR (DMSO-d
6
) spectrum of the ligand with that of
d = 0.85 ppm) of the
complex. A significant downfield shift (
D
H N
N
2
NaOAc
+
MoO (acac)
2 2
O
N
N
[
MoO (Sal-His)]
NH
2
HO
O
OH
OH
NH
HO
O
salicylaldehyde
L-histidine
N-salicylidene-L-histidine
Scheme 1. Preparation of MoO2(Sal-His).
Fig. 1. The structure of 1 showing the atom numbering and with ellipsoids drawn at the 50% probability level.

6
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E. Zamanifar et al. / Inorganica Chimica Acta 414 (2014) 63–70
Fig. 2. Zig-zag chains of molecules of 1 with hydrogen bonds drawn as dashed lines. Symmetry operation i = ꢀ1 + x,y,z.
Table 3
Hydrogen bond distances (Å), angles (°) for 1.
D–Hꢃ ꢃ ꢃA
D–H
Hꢃ ꢃ ꢃA
Dꢃ ꢃ ꢃA
hD–Hꢃ ꢃ ꢃA
N1–H1Nꢃ ꢃ ꢃO2i
C2–H2ꢃ ꢃ ꢃO81i
C6–H6Bꢃ ꢃ ꢃO2ii
C7–H7ꢃ ꢃ ꢃO2ii
C9–H9ꢃ ꢃ ꢃO2ii
0.79(3)
0.95
0.99
1.00
0.95
2.23(3)
2.55
2.43
2.60
2.53
2.985(2)
3.140(2)
3.102(2)
3.193(2)
3.349(2)
160(2)
121
125
118
145
Symmetry operations: i = ꢀ1 + x,y,z; ii = ꢀ1/2 ꢀ x,1/2 + y,1/2 ꢀ z.
complex azomethine (ACH@NA) proton relative to the corre-
sponding ligand demonstrates the coordination of azomethine
nitrogen atom. Aromatic protons appear in the expected regions
in the spectra of the ligand as well as of the complex with minor
variation in their positions. The signal appears at low field
d = 12.82 ppm) is assigned to the imidazole NH. The 1 C NMR
3
(
spectrum shows thirteen distinct carbon signals consistent with
the single crystal structure Fig. S3 (see Supplementary).
3.3. FTIR studies
Fig. 3. FT-IR spectra of (A) Ligand schiff-base N-salicyliden-L-histidine (Sal-His), (B)
MoO2(Sal-His).
2
The FT-IR spectra of Sal-His and MoO (sal-his) are shown in
Fig. 3A and B, respectively. As seen in Fig. 3A, the bands appear
ꢀ
1
ꢀ
at 1603 and 1347 cm are due to the asymmetric COO and sym-
ꢀ
ꢀ1
metric COO vibrations. The C@N stretching of the complex
Strong band appearing at 1315 cm is perhaps due to C–O vibra-
tion. The carboxylic acid group of the Schiff base ligand was depro-
tonated because the solution was kept alkaline upon addition of
sodium acetate in the preparation procedure. For this and what
was reported previously for vanadium complexes [29,30], the
coordination of the carboxylate oxygen to the metal center is con-
ꢀ
1
appearing at 1631 cm shows a slightly lower frequency in com-
parison to the corresponding vibration in the spectrum of the free
ꢀ1
ligand (1635 cm ). Recall that the involvement of the imine lone
pairs in bonding to metal ion shifts the C@N to lower frequency.
The rather large difference between asymmetrical and symmetri-
ꢀ
ꢀ1
cal COO frequencies indicates a monodentate coordination of this
cluded. The weak band displaying at 2590 cm in the complex
group [39]. The O–H stretching frequency of the free ligand dis-
playing at 3425 cm disappeared after complexation due to the
deprotonation of COO–H group and formation of Mo–O bond.
spectrum due to imidazole NH group indicates that it has not
undergone deprotonation. Due to the rather harsh synthetic
conditions of temperature and alkalinity, the dianionic ligand has
ꢀ1

E. Zamanifar et al. / Inorganica Chimica Acta 414 (2014) 63–70
67
donation lone pairs of the Schiff base to the metal ion (N ? M)
42]. The absorption maximum appearing at 414 nm could be as-
signed to ligand to metal charge transfer (LMCT) Mo (d ) ? O
). Recall that this is in the range usually observed for MoO com-
plexes [43].
[
p
(p
3.5. TGA/DSC
A deeper insight into the structure of complex was undertaken
using thermogravimetry (TG) and differential scanning calorimetry
techniques (Fig. 5). The [Mo(O )(C13 )] complex with 385 M
mass showed a thermal stability from room temperature up to
00 °C. Observing two weight looses at 317 and 450 °C are due
to the decomposition of organic ligand (obs.: 62%, calcd.: 62.53%)
2
11 3 3
H N O
3
and ended with molybdenum oxide MoO
calcd.:37.47%).
3
(obs.: 38%,
Fig. 4. Electronic spectra of [MoO2(Sal-His)] in acetonitrile.
2
3.6. MoO (Sal-His)/Al-MCM-41 characterization
undergone deprotanation at carboxylic acid and phenol groups on
ꢀ1
complexation, Observation of two bands at 930 and 910 cm
regions are due to the Mo@O vibrations [40]. The two bands dis-
The X-ray powder diffraction pattern of calcined Al-MCM-41
and MoO (Sal-His)/Al-MCM-41 are shown in Fig. S4 (see Supple-
mentary). Whereas the former is consistent with those reported
2
ꢀ1
ꢀ1
playing at 525 cm and 478 cm of the low frequency region
are attributed to the Mo–O and Mo–N, generated by Mo complex-
ation to the phenol O and azomethine N atoms, respectively [41].
Notably, the consistency of the elemental analysis results of the
complex with molecular structure obtained from X-ray crystallog-
before [31] and can be indexed on a hexagonal lattice, the latter
displays a decrease in the peak intensity in comparison to that of
calcined Al-MCM-41. Such changes clearly indicate that Mo com-
plex has been immobilized on Al-MCM-41. Observation of the shift
in reflection to lower angle (or higher d-value) also indicates
expansion in unit cell parameter due to the immobilization of
the complex within Al-MCM-41 pores (Table 4).
The low temperature N2 adsorption–desorption analyses was
performed in order to investigate the textural properties such as
surface area, pore volume and pore diameter of the resultant
materials after immobilization of dioxo molybdenum Schiff base
2
raphy clearly indicates that no solvent (H O or EtOH) has been in-
volved in bonding to the metal center. On the other hand, the
involvement of imidazole group of the aminoacid in coordination
is consistent with X-ray single crystal structure.
3.4. UV studies
Investigation of the electronic spectra of Schiff base ligand
complex. Observation of type IV isotherms [44] in the MoO
(Sal-His)/Al-MCM-41 adsorption–desorption (Fig. S5 (see
Supplementary) indicates that the adsorption takes place as a thin
layer on the walls at low relative pressures, P/P0 (monolayer cover-
age). In addition, the inflection heights are smaller than those of
2
3
ꢀ1
ꢀ1
exhibits three main peaks at 250 (
e
= 5.45 ꢁ 10 M cm ), 260
N
2
3
ꢀ1
ꢀ1
3
ꢀ1
ꢀ1
(
(
p
e
= 6.00 ꢁ 10
M
cm ) and 321 nm (
e
= 2.03 ꢁ 10
M
cm
)
Fig. 4). The first and second peaks are attributed to the benzene
⁄
⁄
? p and imine p ? p transitions, respectively. These bands
were slightly affected by chelation and shifted to longer wave-
Al-MCM-41 Fig. S5 (see Supplementary). This phenomenon is attrib-
uted to the reduced pore volume from 1.074 to 0.8416 cm3
g
ꢀ1
,
length 270 and 285 nm with relatively decrease in their intensities.
⁄
The third band is assigned to the n ?
p
transition. This band is
which reflects the decrease in surface area (Table 4), perhaps due
to the inclusion of MoO (Sal-His) within the Al-MCM-41. Despite
the decrease in the specific surface area of the prepared catalyst,
shifted to longer wavelength at 347 nm together with increasing
in intensity. This shift may be attributed to the nitrogen atom
2
Fig. 5. TGA and DSC curves of the MoO2(Sal-His) complex.

6
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E. Zamanifar et al. / Inorganica Chimica Acta 414 (2014) 63–70
Table 4
Sample texture parameters obtained from XRD and nitrogen sorption studies.
BET (m2
a
g
ꢀ1
)
Pore volume (cm3 g-1)
Average pore diameter (Å)
Material
XRD d value (Å)
Lattice parameter (Å)
Al-MCM-41
MoO2(Sal-His)/Al-MCM-41
32.5
33.90
37.53
39.14
1455
1030
1.074
0.8416
20.72
19.12
a
Specific surface area.
the remaining space in pores is still so large that many of the organic
substrates can enter into the channels.
second weight loss observed at about 400–700 °C is due to decom-
position of the organic ligand, consistent with DTA result.
2
The FT-IR spectra of MoO (Sal-His)/Al-MCM-41, Al-MCM-41
and the drift of FTIR or the difference between the immobilized
complex on Al-MCM-41 and Al-MCM-41 are shown in Fig. 6A–C,
respectively. The broad bands appearing at 3424 to 3445 cm
3
3
.7. Catalytic studies
ꢀ1
2
.7.1. Catalytic activity of MoO (Sal-His)/Al-MCM-41 in the oxidation
may be attributed to the surface silanol and –OH groups of the ad-
sorbed water, respectively. After immobilization of the complex
within Al-MCM-41, some weak peaks appearing in the region of
of sulfides
To find optimum reaction conditions, we initially investigated
the influence of factors such as oxidant, amount of catalyst and
reaction time on the conversion and selectivity of sulfide oxida-
tions. The oxidation of phenyl methyl sulfide was carried out as
the model reaction in the presence of H
perature in acetonitrile as solvent. It was found that H
ter oxidizing agent due to higher sulfide conversion.To optimize
the amount of catalyst, sulfoxidation of phenyl methyl sulfide
was carried out within 60 min using 30, 40 and 50 mg of catalyst.
As indicated in Table 5 and 40 mg of catalyst proved to be sufficient
for 97% conversion of methyl phenyl sulfide with 94% percent
selectivity toward the formation of methyl phenyl sulfoxide as
the major product. Whereas increasing of the catalyst to 50 mg
partially increased the conversion, but resulted in the correspond-
ing sulfoxide in lower yield (Table 5).
To obtain the best reaction time, the oxidation reactions were
carried out within different times (Fig. 7). Whereas 97% of the
methyl phenyl sulfide was converted resulting in 94% sulfoxide
during 60 min, no considerable oxidation occurred during the next
ꢀ
1
2
900 to 3160 cm due to the C–H and N–H vibrations. Whereas
ꢀ1
the band displaying at 1200 to 1080 cm belong to the asymmet-
rical stretching vibrations of Si–O–Si bridges, those displaing at
61 to 803 cm arise from Si–O–Al stretching vibrations. How-
2
O
2
and TBHP at room tem-
is a bet-
ꢀ1
9
2 2
O
ꢀ1
2
ever, observing a band at 930 cm is due to the MoO stretching
vibration. The stretching frequency of the C@N group of the immo-
bilized molybdenum complex within the Al-MCM-41 appears at
ꢀ1
1
632 cm . The subtracted spectrum of the immobilized complex
within Al-MCM-41 from that of Al-MCM-41 reveals a band at
ꢀ1
1
632 and two medium intensity bands at 909 and 937 cm that
are assigned to C@N and Mo@O stretchings, respectively. The
asymmetrical and symmetrical COO– and C–O vibration bands
ꢀ
1
are also observed at 1334 and 1594 and 1312 cm , respectively.
These observations confirm the presence of the immobilized com-
plex within Al-MCM-41.
2
TG/DTA patterns of MoO (Sal-His)/Al-MCM-41 are shown in
Fig. S6 (see Supplementary). The weight loss of the complex immo-
bilized within nanoreactors of Al-MCM-41 take places at two steps.
The first weight loss of 5.5% is observed below 350 °C, correspond-
ing to the evaporation of water and organic solvent (ethanol). The
3
0 min.
Encouraged by this result, the new method was applied to a
variety of sulfides. We have included the effect of Al-MCM-41 void
of complex as catalyst and those oxidation results obtained using
2 2
MoO (acac) /Al-MCM-41 and blank reaction in Table 6 in order
to make the comparison more convenient. As seen in Table 6, all
substrates were converted into the corresponding sulfoxides with
moderate to high yields and excellent selectivities under mild con-
ditions. Notably, sulfoxidation of dialkyl sulfide proceeded to al-
most completion within 60 min (entry 1, Table 6). Whereas
replacement of an alkyl with a phenyl group resulted in lower yield
(
entries 2 and 5, Table 6), sulfoxidation of diphenyl sulfides had to
be carried out under reflux condition (entries 3 and 4, Table 6).
Compared to using MoO (Sal-His) as a homogeneous catalyst
entry 7d, Table 6), Al-MCM-41 void of complex (entry 7e, Table 6)
and MoO (acac) /Al-MCM-41 (entry 7f, Table 6), oxidation of
methyl phenyl sulfide using the heterogeneous MoO (Sal-His)/Al-
2
(
2
2
2
MCM-41 catalysis system afforded the methyl phenyl sulfoxide
in 97% yield (entry 5, Table 6). These results clearly indicate the
Table 5
The influence of amount Mo-catalyst on the oxidation of methyl phenyl sulfide.
Entry Catalyst amount
mg)
Conversion
(%)
Selectivity (%)
TONa
(
Sulfoxide Sulfone
1
2
3
30
40
50
86
97
98
97
94
90
3
6
10
1638
1386
1120
Reaction conditions: substrate (20 mmol), oxidant (H2O2, 30%, 20 mmol), solvent
(
acetonitrile, 5 mL), Time (60 min), room temperature.
TON is the mmol of product to the mmol molybdenum present in catalyst.
Fig. 6. FT-IR spectra of (A) MoO2(Sal-His)/Al-MCM-41, (B) Al-MCM-41 and (C) the
spectrum of the difference of MoO2(Sal-His)/Al-MCM-41 and Al-MCM-41.
a

E. Zamanifar et al. / Inorganica Chimica Acta 414 (2014) 63–70
69
Compared to the catalytic activity of the reported homogeneous
catalysis systems [13,22,45] the heterogeneity and stability of
MoO (Sal-His)/Al-MCM-4 catalyst is notable. In fact, utilization of
2
solid support offers the advantages of easy catalyst separation
and possible catalyst recycling. Particularly significant in this case
is the high activity and selectivity of our catalysis system. In addi-
tion, the provided method is an interesting approach to the immo-
bilizing biomimetic Mo complex in order to investigate the
oxidation catalytic type reactions.
4
. Conclusions
An expedient procedure was provided for the synthesis of dioxo-
molybdenum(VI) with tetradental N-salicylidene-L-histidin Schiff
base complex. In fact, the coordination of the aminoacid as histidine
is also interesting regarding some enzymatic reactions such as nitro-
genase. The molybdenumcomplex [MoO (Sal-His)] was then immo-
2
Fig. 7. Conversion of methyl phenyl sulfide at different times in the presence of
MoO2(Sal-His)/Al-MCM-41 catalyst.
bilized within Al-MCM-41 nanoreactors. The prepared
heterogeneous molybdenum was found to successfully catalyze
the oxidation of sulfides. Compared to using MoO (Sal-His) alone
2
Table 6
Oxidation of sulfides using MoO2(Sal-His)/Al-MCM-41 as catalyst.
as a homogeneous catalyst, immobilization within Al-MCM-41
proved to enhances the sulfoxidation catalytic activity. The reusabil-
ity of the recovered catalyst with rather similar catalytic activity
along with detecting no molybdenum desorption into the filtrate
indicates the heterogeneous character of the examined catalysis
system. Overall, utilization of an easily prepared heterogeneous cat-
alyst and using environmentally benign oxidants together with mild
reaction conditions and obtaining moderate to high yields of the de-
sired product with excellent selectivity, short reaction time, and
reusability of the catalyst with no marked drop in conversion make
the present protocol a green and efficient alternative for the synthe-
sis of commercially important sulfoxides.
Entry Substrate
Conversion (%) Selectivity (%)
Sulfoxide Sulfone
90 10
TONa
1
2
99
98
1414
1400
95
5
3
4
5
6
7
70
80
86
94
94
85
20
14
6
1000
1286
1386
1357
1143
90b
97
Acknowledgement
The financial support from the University of Alzahra is grate-
fully acknowledged.
95c
80d,e
6
,
f,g
15
Appendix A. Supplementary material
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.ica.2014.01.028.
Reaction conditions: catalyst (40 mg), substrate (20 mmol), oxidant (H
0 mmol), solvent (acetonitrile, 5 mL, Time (60 min), room temperature.
2 2
O , 30%,
2
a
TON is the mmol of product to mmol molybdenum present in catalysts.
Under reflux conditions.
Reaction was carried out in the presence of recovered catalyst.
b
c
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