Synthesis, crystal structure, and catalytic properties of novel dioxidomolybdenum(VI) complexes with tridentate schiff base ligands in the biomimetic and highly selective oxygenation of alkenes and sulfides

Article abstract of DOI:10.1002/ejic.200900814

Four novel dioxidomolybdenum(VI) complexes [MoO₂(L ^x)(CH₃OH)] have been synthesized, using 2[(E)-(2-hydroxy-2-phenylethylimino)methyl]phenol derivatives as tridentate ONO donor Schiff base ligands (H₂L^x) and MoO ₂(acac)₂. A monoclinic space group was determined by X-ray crystallography from single-crystal data of a sample of these new complexes. The epoxidation of alkenes by using tert-butyl hydroperoxide and oxidation of sulfides to sulfoxides by urea hydrogen peroxide were efficiently enhanced with excellent selectivity under the catalytic influence these new Mo^VI complexes. The high efficiency and relative stability of the catalysts have been observed, by turnover numbers and UV/Vis investigations. The electron-poor and bulky ligands promoted the effectiveness of the catalysts. 2010 Wiley-VCH Verlag GmbH & Co. KGaA.

Full text of DOI:10.1002/ejic.200900814

FULL PAPER
DOI: 10.1002/ejic.200900814
Synthesis, Crystal Structure, and Catalytic Properties of Novel
Dioxidomolybdenum(VI) Complexes with Tridentate Schiff Base Ligands in the
Biomimetic and Highly Selective Oxygenation of Alkenes and Sulfides
Abdolreza Rezaeifard,*[a] Iran Sheikhshoaie,[b] Niaz Monadi,[a,b] and Helen Stoeckli-Evans[c]
Keywords: Molybdenum / Oxido ligands / Peroxides / Oxygenation / Schiff bases / Epoxidation
Four novel dioxidomolybdenum(VI) complexes [MoO2(Lx)-
(CH3OH)] have been synthesized, using 2[(E)-(2-hydroxy-2-
phenylethylimino)methyl]phenol derivatives as tridentate
ONO donor Schiff base ligands (H2Lx) and MoO2(acac)2. A
monoclinic space group was determined by X-ray crystal-
lography from single-crystal data of a sample of these new
complexes. The epoxidation of alkenes by using tert-butyl
hydroperoxide and oxidation of sulfides to sulfoxides by urea
hydrogen peroxide were efficiently enhanced with excellent
selectivity under the catalytic influence these new MoVI com-
plexes. The high efficiency and relative stability of the cata-
lysts have been observed by turnover numbers and UV/Vis
investigations. The electron-poor and bulky ligands pro-
moted the effectiveness of the catalysts.
on 2[(E)-(2-hydroxy-2-phenylethylimino)methyl]phenol and
evaluate their catalytic activity in the oxygenation of alk-
enes to epoxides and sulfides to sulfoxides (Scheme 1) by
using tert-butyl hydroperoxide (TBHP) and urea hydrogen
peroxide (UHP) both of which are industrially and environ-
mentally important oxidants.[8] The selective epoxidation of
olefins and sulfoxidation of thioethers are particular chal-
lenging problems in organic synthesis, leading to a great
interest in new and more efficient catalytic versions for
these reactions.[9] We also report the crystal structure of a
sample of these molybdenum complexes (MoL3, Scheme 1)
determined by single-crystal X-ray diffraction.
Introduction
The desire to construct biomimetic oxidation catalysts
has prompted an extensive area of research into biologically
important molybdenum complexes with various synthetic
ligands.[1] In this context, dioxidomolybdenum(VI) com-
plexes have been extremely investigated,[2] particularly with
respect to the catalytic role of transferase enzymes like ni-
trate reductase, in which their active sites consist of a cis
molybdenum dioxido moiety.[3] The ability of molybdenum
to form stable complexes with oxygen-, nitrogen-, and sul-
fur-containing ligands led to the development of molybde-
num Schiff base complexes that are efficient catalysts in
both homogeneous and heterogeneous reactions.[4] The ac-
tivity of these complexes varied markedly with the types of
ligands and coordination sites.[4,5] In continuation of our
ongoing research on the development of biomimetic oxi-
dation reactions,[6] very recently we introduced a (dioxido)-
MoVI Schiff base complex as an efficient and selective cata-
lyst for the oxidation of sulfides to sulfoxides and sul-
fones.[7] Now, we wish to introduce four novel dioxidomo-
lybdenum(VI) Schiff base complexes containing simple and
easy-to-prepare asymmetric ONO tridentate ligands based
[a] Catalysis Research Laboratory, Department of Chemistry,
Faculty of Science, University of Birjand,
Birjand 97179-414, Iran
Fax: +98-0561-2502515
E-mail: rrezaeifard@birjand.ac.ir
rrezaeifard@gmail.com
[b] Department of Chemistry, Faculty of Science, Shahid Bahonar
University of Kerman,
Kerman 76175-133, Iran
[c] Institute of Physics, University of Neuchâtel,
Rue Emile-Argand 11, 2009 Neuchâtel, Switzerland
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejic.200900814.
Scheme 1. Oxygenation of alkenes and sulfides catalyzed by dif-
ferent MoVI–Schiff base complexes utilized in this study.
Eur. J. Inorg. Chem. 2010, 799–806
© 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
799

A. Rezaeifard, I. Sheikhshoaie, N. Monadi, H. Stoeckli-Evans
FULL PAPER
Results and Discussion
Structural Discussion of (Dioxido)MoVI Complexes
Dioxidomolybdenum complexes (MoL1–MoL4) were
prepared by the reaction of 2[(E)-(2-hydroxy-2-phenylethyl-
imino)methyl]phenol derivatives (Scheme 1) and dioxidomo-
lybdenum(VI) acetylacetonate,[10] and these complexes were
characterized by elemental analysis, IR, NMR, and UV/Vis
spectroscopy, and mass spectrometry (Supporting Infor-
mation, Scheme S1). The Schiff base ligand is bonded to
Figure 2. Crystal packing of MoL3 viewed down the b axis, show-
ing the formation of the O–H···O hydrogen bonded centrosymmet-
ric dimer (hydrogen bonds are shown as dashed lines).
+2
the MoO2 ion through two phenolic oxygen atoms and
an imine nitrogen atom trans to an oxido group in a meridi-
onal arrangement. For example, the IR spectra of MoL3 in
KBr matrix confirm the presence of an imine bond (C=N)
at 1638 cm–1, the C=C vibration band of the aromatic ring
at 1476 cm–1, and Mo–O vibration bands at 909 and
920 cm–1. The ORTEP plot of MoL3 with relevant bond
lengths and angles is given in Figure 1. The O=Mo=O an-
gle is 106.36° and the Mo=O distances are 1.707 and
1.699 Å. According to the spectroscopic data and also the
molecular structure, which was determined for MoL3 by
crystal data, a methanol molecule is coordinated to the
MoVI central metal. The Mo–O–C angle and Mo–O dis-
tance for Mo–OHCH3 in MoL3 are 125.94° and 2.3877 Å,
respectively. The crystal packing diagram (Figure 2) shows
the formation of the O–H···O hydrogen bonded centrosym-
metric dimer. The electronic spectra of the H2L3 ligand
(λmax = 345 nm) and the MoL3 complex (λmax = 285 nm)
are shown in Figure 3.
Figure 3. Electronic spectra of H2L3 (a) and MoL3 (b) in methanol.
Catalytic Epoxidation of Alkenes
The oxidation of cyclooctene by using TBHP, which did
not proceed in the absence of catalyst under mild and reflux
conditions, was used as a model reaction. To find optimum
reaction conditions, the influence of different factors that
may affect the conversion and selectivity of the reaction was
investigated. A systematic examination of cyclohexene oxi-
dation in various solvents was performed in the presence of
1 mol-% of the simple MoL1 (Scheme 1) catalyst at different
temperatures. Considering the yield and reaction time,
dichloroethane (DCE) was found to be the best reaction
solvent; however, completion of the reaction required a
temperature of 80 °C (Table 1). The poorer yields obtained
with CH2Cl2 and CHCl3 is probably caused by the lower
reaction temperature at their reflux conditions.
Different catalyst/alkene molar ratios have been used in
the oxidation of cyclooctene for different reaction times
(Figure 4). It was observed that oxidation of cyclooctene
Figure 1. A view of the molecular structure of MoL3, showing the
numbering scheme and displacement ellipsoids drawn at the 50% required 45 min for completion with the use of 1 mol-% of
probability level. Selected bond lengths (Å) and angles (°): Mo1–
O1 1.9520(16), Mo1–O2 1.9213(16), Mo1–O4 1.707(2), Mo1–O5
1.6992(16), Mo1–O6 2.3877(17), Mo1–N1 2.268(2), N1–C7
1.282(3), N1–C8 1.457(3), O1–C1 1.342(3), O2–C9 1.425(3), O3–
the catalyst and an increase in this ratio up to 5 mol-% did
not affect noticeably the reaction rate (Figure 4).
The examination of various molar ratios of TBHP/alkene
in the catalytic oxidation of cyclooctene in DCE showed
that full oxidation of the starting material was obtained by
two equivalents of TBHP (Figure 5).
The influence of axial nitrogenous bases having dramatic
effects on the rate and selectivity in the oxidation reaction
catalyzed by various transition-metal complexes such as
C4 1.383(3), O3–C16 1.432(3); O4–Mo1–O5 106.36(9), O4–Mo1–
O6 85.66(8), O1–Mo1–O2 150.19(6), O1–Mo1–O4 99.79(7), O1–
Mo1–O5 98.18(8), O2–Mo1–O4 97.47(7), O2–Mo1–O5 100.04(8),
O1–Mo1–N1 80.30(7), O2–Mo1–N1 75.21(7), Mo1–O6–C17
125.94(16), O2–Mo1–O1–C1 –61.9(2), O4–Mo1–O1–C1 173.6(2),
O5–Mo1–O1–C1 65.3(2), N1–Mo1–O2–C9 28.58(15), C16–O3–
C4–C5 6.5(3), C8–C9–C10–C15 –77.9(3).
800
www.eurjic.org
© 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Inorg. Chem. 2010, 799–806

Dioxidomolybdenum(VI) Complexes with Tridentate Schiff Base Ligands
Table 1. The influence of the nature of the solvent on the epoxid- excellent substrates for this catalyst (Table 2, entries 1–11).
ation of cyclooctene by using TBHP catalyzed by MoL1.[a]
It led to complete conversion of cyclohexenes, cyclooctene,
% yield of epoxide[b]
and norbornene with the formation of the corresponding
epoxides as the sole products. Inspection of the results in
Table 2 indicates several useful features of this catalytic
method. The least reactive aliphatic terminal and nonter-
minal alkenes were oxidized in desired times in high/excel-
lent yields and excellent selectivities (Table 2, Entries 5 and
6). The chemoselectivity of the procedure was notable. The
oxidative hydroxy group was tolerated under the influence
of this catalytic system and the corresponding epoxide was
obtained in 100% selectivity (Table 2, Entries 7–9). A novel
feature of this simple catalytic method is its excellent stereo-
selectivity. Complete retention of configuration in the epox-
Solvent
(T, °C)[c]
CH3CN
(CH3)2CO
C2H5OH
CH2Cl2
CHCl3
0
0
0
35
76
DCE
5 (30), 60 (60), 100 (80)
[a] The molar ratio of cyclooctene/TBHP/catalyst was 100:200:1.
[b] GC yield based on the starting alkene after 45 min. [c] The reac-
tions were run in air under reflux except for DCE, which was per-
formed at different temperatures.
Table 2. Epoxidation of alkenes by using TBHP catalyzed by MoL1
in DCE.[a]
Figure 4. The influence of catalyst concentration on the reaction
rate and yield of cyclooctene epoxide.
Figure 5. The influence of TBHP/alkene molar ratio on the reac-
tion rate and yield of cyclooctene epoxide.
porphyrin[11] and salen catalysts[12] has also been studied.
However, the addition of imidazole to this catalytic system
retarded significantly the oxidation of cyclooctene (12%
yield after 45 min).[13] Under the optimized conditions
(100:200:1 molar ratio for alkene/TBHP/catalyst in DCE at
80 °C), cyclooctene converted completely within 45 min and
95% of epoxide product was secured as the sole product.
In order to establish the general applicability of the
method, various alkenes were subjected to the oxidation
protocol under the influence of the MoL1 catalyst (Table 2).
[a] The reactions were run for 45 min in air in DCE at 80 °C with
a molar ratio of alkene/TBHP/catalyst, 100:200:1. [b] Conversions
and selectivities were determined by GC based on the starting al-
1
kenes. [c] All products were identified by their IR, H NMR, and
GC–MS spectroscopic data in comparison with authentic samples.
1
As summarized in Table 3, different alkenes are generally [d] Determined by H NMR spectroscopy.
Eur. J. Inorg. Chem. 2010, 799–806
© 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
801

A. Rezaeifard, I. Sheikhshoaie, N. Monadi, H. Stoeckli-Evans
FULL PAPER
idation of trans-stilbene and excellent stereoselectivity in the dition of a strong π-donor imidazole ligand to the reaction
oxidation of cis-stilbene (Ͼ98%) were obtained (Table 2, containing the MoL1 complex, making it a sluggish catalyst
Entries 10 and 11). Although the mechanism of epoxidation for epoxidation of cyclooctene (12% conversion after
of alkenes is still controversially discussed, the ideas of Sob- 45 min at 80 °C). Presumably, imidazole forms an inactive
czak[14] may be accepted on the basis of structural analogy species (Scheme 2, IV) by coordinating to the molybdenum
of the catalyst (Scheme 2). According to this suggested ion,[13] and/or interference with the catalytic cycle by form-
mechanism the second stage of process is the interaction ing a hydrogen bond with TBHP and stabilizing the wrong
between the alkene and the TBHP molecule, activated in species.[18]
the coordination sphere of the molybdenum complex
(Scheme 2, I). The Lewis acidity of the Mo center increases
the oxidizing power of the peroxido group and the alkene
is subsequently oxygenated by nucleophilic attack on an
electrophilic oxygen atom of the coordinated peroxide.[15]
Spectroscopic and computational methods applied to mo-
lybdenum dioxido and oxidodiperoxido complexes support
a reaction mechanism involving an activation state formed
between the complex, oxidant, and alkene (Scheme 2,
II).[16,17] In this process, in higher conversions, tert-butyl
alcohol can compete with TBHP for coordination to the
molybdenum center, forming
a less reactive species
(Scheme 2, III) that leads to the decreasing reaction rate.[16]
Similarly, for complex MoL1, the initial reaction rates were
much higher than those observed later in the reactions
(Table 2, Entry 3). This suggestion may be supported by ad-
Table 3. Oxidation of various sulfides by UHP catalyzed by MoL1
in ethanol.[a]
Scheme 2. Suggested mechanism of cis-stilbene epoxidation with
TBHP catalyzed by MoLx complexes.
The excellent stereoselectivity obtained in the oxidation
of cis-stilbene indicates that the closure of the epoxide ring
occurs faster than the rotation around the C–C bond during
the oxygen transfer step from the active oxidizing species to
the alkene (Scheme 2, II).
Catalytic Oxidation of Sulfides to Sulfoxides
On the basis of the promising results obtained in the pre-
vious report on green oxidation of sulfides catalyzed by
[MoO2(L)(CH3OH)] using UHP in ethanol,[7] we evaluated
the sulfoxidation activity of these new MoVI catalysts in the
oxidation of thioanisole, which did not proceed in the ab-
sence of a catalyst. Performing the reaction in the presence
of 1 mol-% of MoL1 by using an equimolar ratio of UHP
and sulfide in ethanol under mild conditions afforded the
related sulfoxide as the sole product in excellent yield
(Ͼ96%) within 35 min. It should be mentioned that the use
[a] The reactions were run in air at room temperature with a molar
ratio of sulfide/UHP/catalyst, 100:100:1. [b] Conversions and yields
were measured by GC based on the starting sulfides after 10 min
for Entries 1 and 2 and 35 min for all others. [c] All products were
identified by their IR, H NMR, and GC–MS spectroscopic data
in comparison with authentic samples.
1
802
www.eurjic.org
© 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Inorg. Chem. 2010, 799–806

Dioxidomolybdenum(VI) Complexes with Tridentate Schiff Base Ligands
of dried ethanol as the reaction media decreases the selec- competing destruction of the catalyst, as confirmed by UV/
tivity of sulfoxide (86%) and the conversion rate was not Vis spectral changes in the complex during the oxidation
affected. Encouraged by the excellent catalytic performance reaction (Figure 6; Supporting Information, Figure S2). By
for the thioanisole oxidation to sulfoxide, different sulfides comparing the absorbance (λmax = 287 nm) at the end of
were subjected to the reaction system in the presence of the reaction (A = 0.46 after 45 min) with that observed be-
MoL1 and the results are listed in Table 3. All substrates fore the addition of oxidants (A = 0.86) we found that 53%
could be smoothly converted into sulfoxides with high to of the catalyst remained intact.
excellent conversion rates, and excellent selectivities were
obtained under mild conditions. The highest yields were ob-
tained for dialkyl and diallyl sulfide (98–100%) after
10 min. Replacement of one or both alkyl or aryl groups
with a phenyl or benzyl group enhanced the reaction times
(35 vs. 10 min); however, no regular trends in electronic and
steric requirements have been observed. A salient feature of
the present sulfoxidation system is its excellent selectivity.
Sulfoxides could be nearly stoichiometrically produced and
the generation of the corresponding sulfones was well con-
trolled, which makes this process a good alternative for sulf-
oxide production. Also, a sulfide having a carbon–carbon
double bond (Table 3, Entries 2 and 6) was cleanly trans-
formed into the corresponding sulfoxide in excellent yield.
In addition, benzyl phenyl sulfide (Table 3, Entry 8) was se-
lectively oxidized to its corresponding sulfoxide without for-
mation of any benzylic oxidation byproducts.
Figure 6. Plot of absorbance at 286 nm of MoL1 vs. time in the
oxidation of thioanisole by using UHP in ethanol at 25 °C and
It is noteworthy that the easy preparation method of the
catalyst offered ready scalability. Performing the oxidation epoxidation of cyclooctene by using TBHP in DCE at 80 °C.
reaction with the use of 100 mmol of thioanisole as sub-
strate gave 89% of the related sulfoxide after purification
These results encouraged us to investigate the influence
over silica chromatography, eluting with n-hexane/ethyl ace- of electronic and structural requirements in the Schiff base
tate (10:3).
ligand on the catalytic activity and stability of Mo com-
plexes in the epoxidation reaction.[6c–6e] As outlined in Fig-
ure 7, the order of catalytic activities of different electroni-
cally and structurally Mo complexes was found to be MoL4
Ͼ MoL2 Ͼ MoL1 Ͼ MoL3. As expected, an electron-with-
Activity and Stability of (Dioxido)MoVI Complexes
The high/excellent yields of epoxides (64–100%) and drawing group in the salicylidene ring (MoL2) increases the
sulfoxides (78–100%) obtained by using these new catalytic effectiveness of the catalyst, resulting from elevating the
methods in relatively short times (10–45 min) display the Lewis acidity of the molybdenum center and therefore acti-
superior catalytic activity of the present (dioxido)MoVI vation of TBHP (Scheme 2, I), promoting the catalytic ac-
complexes.[4] More evidence in this matter have been ob- tivity of epoxidation.[4g,17] However, the introduction of the
tained by determining the turnover numbers (TON) of the bulky and electron-rich tert-butyl substituents on the salicy-
MoL1 catalyst in the oxidation of sulfides by UHP and al- lidene ring near the coordination site (MoL4) enhanced re-
kenes by TBHP by using a 5000:10000:1 molar ratio for markably the catalytic performance of the catalyst. Presum-
substrate/oxidant/catalyst. The impressive turnover fre- ably, the partial ligand dissociation would open a coordina-
quency (TOF = 1640/h) and turnover number (TON ≈ tion site at the molybdenum center of hindered MoL4 for
5000/24 h) obtained in the oxidation of thioanisole by using coordination/activation of the TBHP and additionally its
UHP indicate the high efficiency and relative stability of the Lewis acidity.[19a,19b,20] Also, the absence of a methanol
catalyst, respectively. This was further supported by the molecule as a ligand in the coordination site of the MoL4
study of the electronic spectra of the complex in the oxi- complex, facilitating the coordination of TBHP to the five-
dation of thioanisole (Figure 6). The intensity of the char- coordinate molybdenum center, should be taken into ac-
acteristic absorption band at 287 nm remains approximately count. Moreover, the higher catalytic activity of the elec-
unchanged during a 1-h reaction time, indicating the high tron-deficient MoL2 complex, as well as the hindered MoL4
stability of the Mo catalyst under the oxidation reaction complex, is probably caused by their resistance to formation
conditions. The initial decrease in the intensity upon ad- of catalytically inactive species[21] during the oxidation pro-
dition of the oxidant may be attributed to the formation of cess, enhancing the stability of the catalysts towards oxidat-
active species in the oxidation reaction (Supporting Infor- ive degradation (Supporting Information, Figures S3 and
mation, Figure S1).[19] However, the lower TOF (1250/h) S4) as is well established for porphyrin- and salen-complex-
and TON (4700/24 h) observed in the oxidation of less reac- catalyzed oxidation reactions.[6c–6e,11,12,22,23] This suggestion
tive norbornene by using TBHP can be related to some was further supported by high TONs of the MoL2 and
Eur. J. Inorg. Chem. 2010, 799–806
© 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
803

A. Rezaeifard, I. Sheikhshoaie, N. Monadi, H. Stoeckli-Evans
FULL PAPER
MoL4 catalysts in the epoxidation of norbornene (TON ≈ stability in combination with TBHP and UHP as desired
5000/24 h). However, general statements such as this have oxygen sources highlight the novelty of these new MoVI cat-
to be regarded with caution, as the catalytic mechanisms alysts and make the present catalytic methods more attract-
may be different for oxidation reactions using different cata- ive for applied goals.
lytic systems.
Experimental Section
General: All reagents were used as received. Solvents were purified
by standard methods and dried if necessary. Methanol was distilled
from magnesium methoxide. Acetyl acetate and ammonium hepta-
molybdate was purchased from Fluka Company and these reagents
were used as received. All reactions and workups were carried out
in air.
Instrumentation: X-ray data collections were recorded with a Stoe
Image Plate Diffraction System. Purity determinations of the prod-
ucts were accomplished by GC with a Shimadzu GC-16A instru-
ment by using a 25-m CBP1–S25 (0.32 mm ID, 0.5 µm coating)
capillary column. IR spectra were recorded with a Perkin–Elmer
780 instrument. UV/Vis spectra were recorded with a 160 Shim-
adzu spectrophotometer. NMR spectra were recorded with a
Bruker Avance DPX 500 MHz instrument. Mass spectra were re-
corded with a Shimadzu GC–MS-QP5050A.
Figure 7. The influence of electronic and structural requirements
on the catalytic activity of the Mo catalysts in the epoxidation of
cyclooctene by using TBHP in DCE at 80 °C.
Synthesis of H2L3 Ligand: The asymmetric tridentate Schiff base
(H2L3) was obtained by addition of a solution of 1-amino-2-phen-
ylethanol (1.37 g, 0.01 mol) in methanol (10 mL) to a solution of
5-methoxysalicylaldehyde (1.52 g, 0.01 mol) in methanol (10 mL),
and the reaction mixture was stirred in air at reflux for 2 h, giving
an orange precipitate. The crude product was recrystallized from
CHCl3/hexane (1:4). Yield: 75% (2.03 g). M.p. 166–168 °C. IR
The promising results for activity and stability of these
easily prepared molybdenum Schiff base complexes during
the oxidation reactions leading to high/excellent yields of
products at reasonably low reaction times, in particularly
for full epoxidation of aliphatic alkenes, along with excel-
lent chemoselectivity in both epoxidation and sulfoxidation
reactions and absolute stereoselectivity in the epoxidation
of stilbenes, are strong points of the present cis-(dioxido)-
MoVI Schiff base complexes as oxidation cata-
lysts.[4c–4e,4g,4h,5,12] It is noteworthy that, as the ligand back-
bone is chiral, a complex bearing enantiomerically pure li-
gand may offer enantioselective reactions.
(KBr): ν = 3159 (υOH), 1640 (υC=N), 1488 (υC=C), 1263 (υCOphenolic
)
˜
cm–1. 1H NMR (500 MHz, CDCl3, 25 °C): δ = 3.8 (dd, J = 6.9,
7.8 Hz, 1 H, CH2), 3.8 (S, 3 H, OMe), 4.0 (dd, J = 14.8, 2.8 Hz, 1
H, CH2) 5.1 (dd, J = 8.9, 6.0 Hz, 1 H, CH), 6.8 (d, J = 2.8 Hz, 1
H, aromatic proton), 6.9 (d, J = 8.9 Hz, 1 H, aromatic proton), 6.9
(dd, J = 7.5, 2.8 Hz, 1 H, aromatic proton), 7.3 (t, J = 7.2 Hz, 1
H, aromatic proton), 7.42 (t, J = 7.2 Hz, 2 H, aromatic proton),
7.5 (d, J = 7.09 Hz, 2 H, aromatic proton), 8.4 (s, 1 H,C=N),12.6
(s, 1 H, OH) ppm. 13C NMR (125 MHz, CDCl3, 25 °C): δ = 56.37,
67.72, 74.0, 115.44, 118.22, 118.72, 120.08, 126.41, 128.44, 142.11,
152.48, 155.63, 167.32 ppm. C16H17NO3 (271.13): calcd. C 70.85,
H 6.28, N 5.17; found C 70.80, H 6.35, N 5.20. MS: m/z = 271
[M+]. UV/Vis: λmax = 345 nm.
Conclusions
In summary, the synthesis of four novel MoVI complexes
[MoO2(Lx)(CH3OH)] based on 2[(E)-(2-hydroxy-2-phenyl-
ethylimino)methyl]phenol as a ONO tridentate Schiff base
ligand was described. The crystal structure of a sample of
these complexes showed a monoclinic space group P21/n. A
highly efficient epoxidation method with the use of TBHP
activated by these simple MoVI complexes in desired times
with excellent chemo- and stereoselectivity has been devel-
oped. Also, green oxidation of sulfides by using UHP was
Synthesis of MoL3 Complex: To the H2L3 ligand (2.71 g) was added
dioxidomolybdenum(VI) acetylacetonate[10] (3.28 g, 0.01 mol) in
methanol (10 mL). The reaction mixture was stirred under reflux
for 1 h. The yellow precipitate was removed by filtration after 1 d.
By washing with cold methanol, recrystallizing from methanol, and
drying in vacuo, yellow rod crystals were secured in 70% (3.01 g)
yield. M.p. Ͼ250 °C. IR (KBr): ν = 1638 (υC=N), 1476 (υC=C), 909
˜
and 920 (υMo–O) cm–1. 1H NMR (500 MHz, DMSO, 25 °C): δ =
efficiently enhanced with excellent selectivity in ethanol un- 3.17 (d, J = 5.2 Hz, 3 H, methanol), 3.75 (s, 3 H, OMe), 4.08 (dd,
J = 10.5, 5.2 Hz, 1 H, CH2), 4.39 (dd, J = 13.5, 7.1 Hz, 1 H, CH2),
5.28 (dd, J = 10.7, 4.0 Hz, 1 H, CH), 6.87 (d, J = 8.9 Hz, 1 H,
aromatic proton), 7.13 (dd, J = 8.9, 3.2 Hz, 1 H, aromatic), 7.17
(d, J = 3.2 Hz, 1 H, aromatic proton), 7.32 (t, J = 7.2 Hz, 1 H,
aromatic proton), 7.39 (t, J = 7.7 Hz, 2 H, aromatic proton), 7.46
(d, J = 7.4 Hz, 2 H, aromatic proton) 8.74 (s, 1 H,C=N) ppm. 13C
NMR (125 MHz, DMSO, 25 °C): δ = 48.55, 55.62, 66.91, 82.36,
116.14, 120.04, 120.56, 122.11, 126.06, 127.58, 128.25, 141.45,
151.98, 156.00, 163.49 ppm. C17H19MoNO6 (429.50): calcd. C
47.54, H 4.42, N 3.26; found C 47.61, H 4.39, N 3.31. UV/Vis: λmax
der the influence of these Mo catalysts. Electron-poor and
bulky groups on the salicylidene ring of the ligand pro-
moted the effectiveness of the catalyst. The relatively high
TOFs and TONs obtained in the oxidation of sulfides and
alkenes, along with good/high percentages of the remaining
catalysts at the end of the reactions, confirmed clearly the
high activity and also relative stability of the Mo catalysts
towards oxidative degradation. The applicability of these
easily prepared Mo–Schiff base complexes in both epoxid-
ation and sulfoxidation reactions with high efficiency and = 285 nm.
804
www.eurjic.org
© 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Inorg. Chem. 2010, 799–806

Dioxidomolybdenum(VI) Complexes with Tridentate Schiff Base Ligands
Epoxidation Procedure: To a solution of the alkene (0.5 mmol) and
MoLx (Scheme 1; 0.05 mmol) in DCE (5 mL) was added TBHP
(1 mmol), and the reaction mixture was stirred in air at 80 °C for
the required time. The reaction progress was monitored by GC,
and the yields of the products were determined by GC and NMR
spectroscopic analysis. Further purification was achieved by silica
1 min, 2θ range 4.3–51.8°, dmin–dmax = 12.45–0.81 Å. The structure
was solved by direct methods by using SHELXS-97.[25] The refine-
ment and all further calculations were carried out by using
SHELXL-97.[25] The OH H atom was located in a difference elec-
tron-density map and freely refined. The remainder of the H atoms
could also be located from Fourier difference maps but were in-
chromatography (n-hexane/ethyl acetate, 10:1). Assignments of cluded in calculated positions and treated as riding atoms by using
products were made by comparison with authentic samples or were
identified by their IR, H NMR and MS spectroscopic data.
SHELXL default parameters. The non-H atoms were refined an-
isotropically by using weighted full-matrix least-squares on F2. The
ORTEP plot of the MoL3 complex was drawn by using OR-
TEP3,[26] at 50% probability level with atomic numbering along
with selected bond lengths and angles. The crystal packing diagram
(Figure 2) was drawn by using PLATON.[27] Data collection and
refinement processes are summarized in Table 4. CCDC-736287
contains the supplementary crystallographic data for this article.
These data can be obtained free of charge from The Cambridge
Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_
request/cif.
1
Sulfoxidation Procedure: To a solution of the sulfide (0.5 mmol)
and MoLx (Scheme 1) (0.05 mmol) in ethanol (5 mL) was added
urea hydrogen peroxide (0.5 mmol), and the reaction mixture was
stirred in air at room temperature for the required time. The reac-
tion progress was monitored by TLC, and the yields of the products
were determined by GC analysis. Higher purification of products
was achieved by chromatography over silica gel (n-hexane/ethyl ace-
tate, 10:3). Assignments of products were made by comparison with
authentic samples or were identified by their IR, 1H NMR, and
MS spectroscopic data.
Supporting Information (see footnote on the first page of this arti-
cle): Synthesis and characterization of tridentate Schiff base ligands
H2L1, H2L2, and H2L4 and their dioxidomolybdenum(VI) com-
plexes MoL1, MoL2, and MoL4; UV/Vis spectral changes of the
MoL1, MoL2, and MoL4 complexes during the oxidation reactions.
Crystallographic Data Collection and Structure Refinement: Suitable
X-ray quality crystals of MoL3 were obtained as yellow rods by
slow evaporation of solvent from a methanol solution of the com-
plex at room temperature. The intensity data were collected at
173°K with a Stoe Image Plate Diffraction System[24] using Mo-
Kα graphite monochromated radiation. Image plate distance was
70 mm, ω oscillation scans 0–200°, step ∆ω = 0.8°, exposures of
Acknowledgments
Support for this work by the Research Council of University of
Birjand and Shahid Bahonar University of Kerman are highly ap-
preciated.
Table 4. Crystal data and structure refinement for the MoL3 com-
plex.
[MoO2(L3)(CH3OH)]
[1] a) B. I. Ceylan, Y. D. Kurt, B. Ulkuseven, Rev. Inorg. Chem.
2009, 29, 49–67; b) F. E. Kuhn, A. M. Santos, M. Abrantes,
Chem. Rev. 2006, 106, 2455–2475; c) H. A. McManus, P. J.
Guiry, Chem. Rev. 2004, 104, 4151–4202; d) R. H. Holm, Co-
ord. Chem. Rev. 1990, 100, 183–221; e) J. T. Spence, Coord.
Chem. Rev. 1983, 48, 59–82.
Empirical formula
Formula weight
Temperature (K)
Wavelength (Å)
Crystal system
Space group
a (Å)
b (Å)
c (Å)
α (°)
β (°)
C17H19MoNO6
429.27
173
0.71073
monoclinic
P21/n
7.1829(6)
31.2340(19)
7.7220(7)
90
96.918(10)
90
1719.8(2)
4
[2] a) S. Gago, P. Neves, B. Monteiro, M. Pessêgo, A. D. Lopes,
A. A. Valente, F. A. Almeida Paz, M. Pillinger, J. Moreira,
C. M. Silva, I. S. Gonçalves, Eur. J. Inorg. Chem. 2009, 4528–
4537; b) P. Neves, S. Gago, C. C. L. Pereira, S. Figueiredo, A.
Lemos, A. D. Lopes, I. S. Gonçalves, M. Pillinger, C. M. Silva,
A. A. Valente, Catal. Lett. 2009, 132, 94–103; c) F.-L. Chai,
H.-L. Su, X.-Y. Wang, J.-C. Tao, Inorg. Chim. Acta 2009, 362,
3840–3844; d) M. Bagherzadeh, R. Latifi, L. Tahsini, L. Ke-
ith Woo, Inorg. Chim. Acta 2009, 362, 3698–3702; e) C. D.
Nunes, M. Pillinger, A. A. Valente, J. Rocha, A. D. Lopes, I. S.
Gonçalves, Eur. J. Inorg. Chem. 2003, 3870–3877; f) J. C.
Alonso, P. Neves, M. J. P. da Silva, S. Quintal, P. D. Vaz, C.
Silva, A. A. Valente, P. Ferreira, M. J. Calhorda, V. Felix,
M. G. B. Drew, Organometallics 2007, 26, 5548–5556; g) M. G.
Topuzova, S. V. Kotov, T. M. Kolev, Appl. Catal. A 2005, 281,
157–166; h) M. Gomez, S. Jansat, G. Muller, G. Noguera, H.
Teruel, V. Moliner, E. Cerrada, M. Hursthouse, Eur. J. Inorg.
Chem. 2001, 1071–1076; i) H. Arzoumanian, L. Maurino, G.
Agrifoglio, J. Mol. Catal. A 1997, 117, 471–478.
[3] a) A. Sigel, H. Sigel (Eds.), Metal Ions in Biological Systems 39,
Molybdenum and Tungsten: Their Roles in Biological Processes,
Marcel Dekker, New York, 2002; b) M. M. Abu-Omar, A.
Loaiza, N. Hontzeas, Chem. Rev. 2005, 105, 2227–2252; c) R.
Hille, Chem. Rev. 1996, 96, 2757–2816.
[4] a) G. Lyashenko, G. Saischek, M. E. Judmaier, M. Volpe, J.
Baumgartner, F. Belaj, V. Jancik, R. Herbst-Irmer, N. C.
Mosch-Zanetti, Dalton Trans. 2009, 5655–5665; b) M. Bagher-
zadeh, R. Latifi, L. Tahsini, V. Amani, A. Ellern, L. Ke-
ith Woo, Polyhedron 2009, 28, 2517–2521; c) S. M. Bruno, S. S.
Balula, A. A. Valente, F. A. Almeida Paz, M. Pillinger, C.
γ (°)
V (Å3)
Z
Dcalcd. (g/cm3)
1.658
Absorption coefficient (1/mm) 0.796
F(000)
Crystal size (mm)
872
0.19ϫ0.23ϫ0.45
θ range for data collection (°) 2.55 to 26.10
Index ranges
–8ՅhՅ7
–35ՅkՅ38
–9ՅlՅ9
11617
3320 (Rint = 0.053)
98
multiscan
0.8610 and 0.7108
Full-matrix least-squares on F2
3320/1/232
Reflections collected
Independent reflections
Completeness to θ = 26° (%)
Absorption correction
Max. and min. transmission
Refinement method
Data/restraints/parameters
Goodness-of-fit on F2
Final R indices [I Ͼ 2σ(I)]
R indices (all data)
1.04
[a]
[b]
R1[a] = 0.0299, wR2[b] = 0.0757
R1 = 0.0348, wR2 = 0.0781
0.419, –0.413
Largest diff. peak and hole
[a] R1 = Σ||Fo| – |Fc||/Σ|Fο|. [b] wR2 = Σw{(Fο – Fc ) /Σw[(Fο2)2]1/2}.
2
2 2
Eur. J. Inorg. Chem. 2010, 799–806
© 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
805

A. Rezaeifard, I. Sheikhshoaie, N. Monadi, H. Stoeckli-Evans
FULL PAPER
Sousa, J. Klinowski, C. Freire, J. Mol. Catal. A 2007, 270, 185–
194; d) M. Salavati-Niasari, M. Bazarganipour, J. Mol. Catal.
A 2007, 278, 173–180; e) M. Masteri-Farahani, F. Farzaneh,
M. Ghandi, Catal. Commun. 2007, 8, 6–10; f) S. N. Rao, N.
Kathale, N. N. Rao, K. N. Munshi, Inorg. Chim. Acta 2007,
360, 4010–4016; g) Y. Sui, X. Zeng, X. Fang, X. Fu, Y. Xiao,
L. Chen, M. Li, S. Cheng, J. Mol. Catal. A 2007, 270, 61–67;
h) K. Ambroziak, R. Pelech, E. Milchert, T. Dziembowska, Z.
Rozwadowski, J. Mol. Catal. A 2004, 211, 9–16; i) M. Cindric,
N. Strukan, V. Vrdoljak, B. Kamenar, Z. Anorg. Allg. Chem.
2002, 628, 2113–2117; j) J. Liimatainen, A. Lehtonen, R. Sil-
lanpaa, Polyhedron 2000, 19, 1133–1138; k) C. P. Rao, A.
Sreedhara, S. Venkateswara, P. V. Rao, N. K. Lokanath, M. A.
Sridhar, J. S. Prasad, K. Rissanen, Polyhedron 1999, 18, 289–
297.
590; c) C. Zhang, G. Rheinwald, V. Lozan, B. Wua, P.-G. Las-
sahn, H. Lang, C. Janiak, Z. Anorg. Allg. Chem. 2002, 628,
1259–1268.
a) J. M. Sobczak, J. J. Ziolkowski, J. Mol. Catal. 1981, 13, 11–
42; b) J. M. Sobczak, J. J. Ziolkowski, Appl. Catal. A 2003, 248,
261–268.
[14]
[15]
[16]
C. D. Nunes, A. A. Valente, M. Pillinger, J. Rocha, I. S. Gonç-
alves, Chem. Eur. J. 2003, 9, 4380–4390.
F. E. Kuhn, M. Groarke, E. Bencze, E. Herdtweck, A. Prazeres,
A. M. Santos, M. J. Calhorda, C. C. Romao, I. S. Gonçalves,
A. D. Lopes, M. Pillinger, Chem. Eur. J. 2002, 8, 2370–2383.
W. R. Thiel, J. Eppinger, Chem. Eur. J. 1997, 3, 696–705.
M. S. Saraiva, S. Quintal, F. C. M. Portugal, T. A. Lopes, V.
Félix, J. M. F. Nogueira, M. Meireles, M. G. B. Drew, M. J.
Calhorda, J. Organomet. Chem. 2008, 693, 3411–3418.
a) J. A. Brito, H. Teruel, S. Massou, M. Gomeza, Magn. Reson.
Chem. 2009, 47, 573–577; b) A. Hroch, G. Gemmecker, W. R.
Thiel, Eur. J. Inorg. Chem. 2000, 1107–1114; c) W. R. Thiel, M.
Angstl, T. Priermeier, Chem. Ber. 1994, 127, 2373–2379; d)
W. R. Thiel, Chem. Ber. 1996, 129, 575–580; e) W. R. Thiel, M.
Angstl, N. Hansen, J. Mol. Catal. A 1995, 103, 5–10.
J. A. Brito, M. Gomez, G. Muller, H. Teruel, J.-C. Clinet, E.
Dunach, M. A. Maestro, Eur. J. Inorg. Chem. 2004, 4278–4285.
a) C. J. Whiteoak, G. J. Britovsek, V. C. Gibson, A. J. P. White,
Dalton Trans. 2009, 2337–2344; b) L. Stelzig, S. Kötte, B.
Krebs, J. Chem. Soc., Dalton Trans. 1998, 2921–2926; c) C. J.
Doonan, D. A. Slizys, C. G. Young, J. Am. Chem. Soc. 1999,
121, 6430–6436.
D. Dolphin, T. G. Traylor, L. X. Xie, Acc. Chem. Res. 1997, 30,
251–259.
P. G. Cozz, Chem. Soc. Rev. 2004, 33, 410–421.
Stoe & Cie, IPDS-I Bedienungshandbuch, Stoe & Cie GmbH,
Darmstadt, Germany, 2000.
G. M. Sheldrick, Acta Crystallogr., Sect. A 2008, 64, 112–122.
L. J. Farrugia, J. Appl. Crystallogr. 1997, 30, 565.
A. L. Spek, Acta Crystallogr., Sect. D 2009, 65, 148–155.
Received: November 12, 2009
[17]
[18]
[5] a) K. C. Gupta, A. K. Sutar, C.-C. Lin, Coord. Chem. Rev.
2009, 253, 1926–1946; b) K. C. Gupta, A. K. Sutar, Coord.
Chem. Rev. 2008, 252, 1420–1450.
[19]
[6] a) D. Mohajer, A. Rezaeifard, Tetrahedron Lett. 2002, 43,
1881–1884; b) N. Iranpoor, D. Mohajer, A.-R. Rezaeifard, Tet-
rahedron Lett. 2004, 45, 3811–3815; c) A. Rezaeifard, M.
Jafarpour, G. Kardan Moghaddam, F. Amini, Bioorg. Med.
Chem. 2007, 15, 3097–3101; d) A. Rezaeifard, M. Jafarpour, S.
Rayati, R. Shariati, Dyes Pigm. 2009, 80, 80–85; e) A. Rezaei-
fard, M. Jafarpour, M. A. Nasseri, R. Haddad, Helv. Chim.
Acta, DOI: 10.1002/hlca.200900262.
[7] I. Sheikhshoaie, A. Rezaeifard, N. Monadi, S. Kaa, Polyhedron
2009, 28, 733–738.
[8] J.-M. Brégeault, Dalton Trans. 2003, 3289–3302.
[9] J. E. Bäckvall, Modern Oxidation Methods, Wiley-VCH,
Weinheim, 2004.
[20]
[21]
[22]
[23]
[24]
[10] G. J. J. Chen, J. W. McDonald, W. E. Newton, Inorg. Chem.
1976, 15, 2612–2615.
[11] B. Meunier, Chem. Rev. 1992, 92, 1411–1456.
[25]
[26]
[27]
[12] E. M. McGarrigle, D. G. Gilheany, Chem. Rev. 2005, 105,
1563–1602.
[13] a) Y.-W. Lin, Y.-P. Tong, C. Yang, Y.-R. Lin, Inorg. Chem.
Commun. 2009, 12, 252–254; b) M. Cindric, N. Strukan, V.
Vrdoljak, B. Kamenar, Z. Anorg. Allg. Chem. 2004, 630, 585–
Published Online: January 4, 2010
806
www.eurjic.org
© 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Inorg. Chem. 2010, 799–806