Chiral molybdenum(VI) complexes with tridentate Schiff bases derived from S(+)-1-amino-2-propanol: Synthesis, characterization and catalytic activity in the oxidation of prochiral sulfides and olefins

Article abstract of DOI:10.1016/j.poly.2016.06.015

Seven chiral dioxidomolybdenum(VI) complexes with tridentate Schiff bases were synthesized by monocondensation of S(+)-1-amino-2-propanol with salicylaldehyde and its derivatives. One- (¹H,¹³C) and two-dimensional (COSY, gHSQC and NOESY) NMR, IR, CD and UV-Vis spectroscopy were used for detailed characterization of the new molybdenum(VI) compounds. After optimization of the reaction conditions, the catalytic activities of these complexes were tested for the oxidation of olefins, i.e. styrene and cyclohexene, with aqueous 30% H₂O₂or tert-butyl hydroperoxide (TBHP) as an oxidant. Moreover, the molybdenum(VI) Schiff base complexes were also able to catalyze the oxidation of prochiral sulfides [PhSR (R = Me, Bz)] to optically active sulfoxides in the presence of aqueous 30% H₂O₂.

Full text of DOI:10.1016/j.poly.2016.06.015

Polyhedron 117 (2016) 352–358
Contents lists available at ScienceDirect
Polyhedron
journal homepage: www.elsevier.com/locate/poly
Chiral molybdenum(VI) complexes with tridentate Schiff bases derived
from S(+)-1-amino-2-propanol: Synthesis, characterization and catalytic
activity in the oxidation of prochiral sulfides and olefins
⇑
Grzegorz Romanowski , Jaromir Kira
Faculty of Chemistry, University of Gdan´sk, Wita Stwosza 63, PL-80308 Gdan´sk, Poland
a r t i c l e i n f o
a b s t r a c t
Article history:
Seven chiral dioxidomolybdenum(VI) complexes with tridentate Schiff bases were synthesized by mono-
condensation of S(+)-1-amino-2-propanol with salicylaldehyde and its derivatives. One- (1H, 13C) and
two-dimensional (COSY, gHSQC and NOESY) NMR, IR, CD and UV–Vis spectroscopy were used for detailed
characterization of the new molybdenum(VI) compounds. After optimization of the reaction conditions,
the catalytic activities of these complexes were tested for the oxidation of olefins, i.e. styrene and cyclo-
hexene, with aqueous 30% H2O2 or tert-butyl hydroperoxide (TBHP) as an oxidant. Moreover, the molyb-
denum(VI) Schiff base complexes were also able to catalyze the oxidation of prochiral sulfides [PhSR
(R = Me, Bz)] to optically active sulfoxides in the presence of aqueous 30% H2O2.
Received 20 April 2016
Accepted 9 June 2016
Available online 17 June 2016
Keywords:
Molybdenum(VI)
Schiff base
Sulfoxidation
Oxidation
Ó 2016 Elsevier Ltd. All rights reserved.
Olefins
1. Introduction
[8,9]. Moreover, their transition metal complexes, including diox-
idomolybdenum(VI) compounds, have been successfully employed
Molybdenum is essential transition metal for life, occurring as
an important constituent of certain enzymes in plants and animals.
In plants, nitrate reductase catalyzes nitrogen assimilation. In ani-
mals, xanthine dehydrogenase plays an important role in xanthine
oxidation to uric acid, in purine catabolism and aldehyde oxidase
in the final oxidation step of abscisic acid biosynthesis [1]. More-
over, as a trace element it is most abundant in seawater, with a
concentration of ca. 100 nM, including marine sediments. All
molybdenum enzymes found in living organisms are formed by
different variants of the pterin-based cofactor (Moco) [2]. Such
enzymes can be modelled by molybdenum complexes with an
appropriate ligand. In this context, Schiff-base ligands have been
particularly addressed due to their common availability via the
condensation of amino compounds with aldehyde moieties [3].
The application of molybdenum(VI) complexes is mainly focused
on the epoxidation of olefins and sulfoxidation reactions, but in
the case of enantioselective sulfoxidation, molybdenum complexes
are much less successful and explored [4,5].
as catalysts for the very efficient epoxidation of olefins [10] and
oxidation of sulfides to sulfoxides [11–13], the asymmetric alkyny-
lation of aldehydes [13], the stereoselective synthesis of cyclic
ethers [14,15] and trimethylsilylcyanations [16,17].
Transition metal complexes incorporating chiral Schiff base
ligands have kept our continuing interest [18–20]. In this paper
we describe new seven dioxidomolybdenum(VI) complexes with
ONO donor Schiff bases, products of a single condensation of S
(+)-1-amino-2-propanol with aromatic salicylaldehyde derivatives,
presented in Fig. 1. Examination of the spectroscopic properties, i.e.
UV–Vis, IR, circular dichroism, one- and two-dimensional NMR
have been also performed. Moreover, the catalytic ability for the
enantioselective sulfoxidation of methyl phenyl sulfide (PhSMe)
and benzyl phenyl sulfide (PhSBz), utilizing aqueous 30% H2O2 as
an oxidant, has been studied. Finally, the dioxidomolybdenum
(VI) complexes have also shown their catalytic potential for the
oxidation of styrene and cyclohexene in the presence of aqueous
30% H2O2 or tert-butyl hydroperoxide (TBHP) as an oxidant.
N-salicyl-b-amino alcohol Schiff base ligands, especially chiral
ones, are a member of the well-known ‘‘privileged ligands” group
[6,7]. These ‘‘tridentate salen ligands” are very attractive due to
their structural and electronic fine-tunability and extremely
simple synthesis from naturally available chiral amino acids
2. Experimental
2.1. Measurements
All chemicals and reagents were obtained from commercial
sources and used without further purification unless stated other-
wise. Carbon, hydrogen and nitrogen contents were determined on
⇑
Corresponding author. Fax: +48 585235012.
E-mail address: grzegorz.romanowski@ug.edu.pl (G. Romanowski).
http://dx.doi.org/10.1016/j.poly.2016.06.015
0277-5387/Ó 2016 Elsevier Ltd. All rights reserved.

G. Romanowski, J. Kira / Polyhedron 117 (2016) 352–358
353
2.3.1. {S(+)-2-[(2-oxidopropyl)iminomethyl]phenolato-j3N,O,O0}diox-
idomolybdenum(VI) (1)
1
2
3
4
R
H
R
R
R
1
2
R
R
1
2
3
4
5
6
7
H
H
H
H
H
H
H
H
H
H
H
H
H
H
O
Yield 87%. Anal. Calc. for C10H11NO4MoꢂCH3OH: C, 39.2; H, 4.5;
3
R
O
N
OCH3
H
N, 4.2. Found: C, 39.0; H, 4.5; N, 4.2%. IR (KBr, cmꢁ1): 1640 (
mC@N);
O
Mo
S
OCH3
H
4
R
O
931, 901
e (Mꢁ1 cmꢁ1)]: 271 (8300), 347 (2360). CD spectrum in DMSO [kmax
(nm),
e (Mꢁ1 cmꢁ1)]: 278 (13.07), 349 (5.52). 1H NMR (DMSO-d6,
(mMo@O). UV–Vis spectrum in DMSO [kmax (nm),
C(CH3)3
C(CH3)3
Br
C(CH3)3
CH3
D
Br
_
H
_
ppm) d: 8.70 (1H, s) (azomethine); 7.55 (1H, d, 3J = 7.7 Hz), 7.47
(1H, t, 3J = 7.7 Hz), 6.95 (1H, t, 3J = 7.7 Hz), 6.87 (1H, d, 3J = 8.3 Hz)
(aromatic); 4.46–4.48 (1H, m) (methine); 4.17 (1H, dd,
3J = 13.3 Hz, 4J = 4.1 Hz), 3.57 (1H, dd, 3J = 13.3 Hz, 4J = 9.6 Hz)
(methylene); 1.28 (1H, d, 3J = 6.1 Hz) (methyl); 4.11 (1H, br s),
3.18 (3H, s) (MeOH). 13C NMR (DMSO-d6, ppm) d: 164.1
(azomethine); 162.2, 135.2, 134.4, 121.4, 119.8, 119.7 (aromatic);
77.9 (methine); 67.1 (methylene); 49.1 (MeOH); 20.3 (methyl).
_
S
methanol
H
(CH)4
Fig. 1. Structural formulae of the molybdenum(VI) complexes.
a Carlo Erba MOD 1106 elemental analyzer. Electronic spectra were
recorded on a Perkin-Elmer LAMBDA 18 spectrophotometer. Circu-
lar dichroism spectra were measured with a Jasco J-815 spectropo-
larimeter. IR spectra of solid samples (KBr pellets) were run on a
Bruker IFS 66. NMR spectra were obtained in DMSO-d6 solutions
with a Bruker AVANCE III 700 MHz spectrometer using TMS as a
reference. A Shimadzu GC-2025 gas chromatograph with a Zebron
ZB-5 capillary column (30 m ꢀ 0.25 mm ꢀ 0.25 mm) and FID
detector were used to analyze the reaction products from the oxi-
dation of olefins. Confirmation of the identity of the reaction prod-
ucts has been made with a GC–MS model Shimadzu GCMS-QP2010
SE.
2.3.2. {S(+)-2-[(2-oxidopropyl)iminomethyl]-6-methoxyphenolato-
j3N,O,O0}dioxidomolybdenum(VI) (2)
Yield 91%. Anal. Calc. for C11H13NO5MoꢂCH3OH: C, 39.3; H, 4.7;
N, 3.8. Found: C, 39.1; H, 4.6; N, 3.9%. IR (KBr, cmꢁ1): 1642 (
mC@N);
1256 (
trum in DMSO [kmax (nm), e (Mꢁ1 cmꢁ1)]: 273 (8480), 377 (2510).
CD spectrum in DMSO [kmax (nm),
e (Mꢁ1 cmꢁ1)]: 284 (9.46),
masym(CAO)); 1040 (msym(CAO)); 924, 901 (mMo@O). UV–Vis spec-
D
374 (3.39). 1H NMR (DMSO-d6, ppm) d: 8.68 (1H, s) (azomethine);
7.14 (2H, t, 3J = 7.9 Hz), 6.89 (1H, t, 3J = 7.9 Hz) (aromatic); 4.43–
4.45 (1H, m) (methine); 4.16 (1H, dd, 3J = 13.3 Hz, 4J = 4.1 Hz),
3.56 (1H, dd, 3J = 13.3 Hz, 4J = 9.6 Hz) (methylene); 3.78 (3H, s)
(methoxy); 1.27 (1H, d, 3J = 6.1 Hz) (methyl); 4.12 (1H, q), 3.18
(3H, d) (MeOH). 13C NMR (DMSO-d6, ppm) d: 164.1 (azomethine);
152.3, 149.7, 125.6, 121.5, 119.5, 116.7 (aromatic); 77.9 (methine);
66.9 (methylene); 56.1 (methoxy); 49.2 (MeOH); 20.2 (methyl).
2.2. Catalytic activity
2.2.1. Sulfoxidation
To a solution of catalyst (0.010 mmol) in 3 ml of CH2Cl2/MeOH
mixture (7:3), thioanisole or benzyl phenyl sulfide (1.00 mmol)
was added at ꢁ20 °C or room temperature, with 1,3,5-trimethoxy-
benzene as an internal standard. Aqueous 30% H2O2 was added
(1.10 mmol) in small portions and the resulting mixture was stir-
red. After the appropriate reaction time, the solution was quenched
with 3 ml of sodium sulfite solution (0.1 M) and extracted with
ethyl acetate (3 ꢀ 3 ml). The combined organic layers were evapo-
rated to dryness. The solid product, dissolved in CDCl3, was ana-
lyzed (yield and ee value) by 1H NMR spectra in the presence of
the chiral shift reagent Eu(hfc)3 [21].
2.3.3. {S(+)-2-[(2-Oxidopropyl)iminomethyl]-4-methoxyphenolato-
j3N,O,O0}dioxidomolybdenum(VI) (3)
Yield 94%. Anal. Calc. for C11H13NO5MoꢂCH3OH: C, 39.3; H, 4.7;
N, 3.8. Found: C, 39.1; H, 4.7; N, 3.8%. IR (KBr, cmꢁ1): 1643 (
mC@N);
1259 (
trum in DMSO [kmax (nm), e (Mꢁ1 cmꢁ1)]: 276 (7780), 375 (3120).
CD spectrum in DMSO [kmax (nm),
e (Mꢁ1 cmꢁ1)]: 279 (14.31),
masym(CAO)); 1032 (msym(CAO)); 926, 898 (mMo@O). UV–Vis spec-
D
385 (4.76). 1H NMR (DMSO-d6, ppm) d: 8.67 (1H, s) (azomethine);
7.13 (1H, d, 3J = 3.2 Hz), 7.10 (1H, dd, 3J = 8.9 Hz, 4J = 3.2 Hz), 6.82
(1H, d, 3J = 8.9 Hz) (aromatic); 4.42–4.44 (1H, m) (methine); 4.14
(1H, dd, 3J = 13.3 Hz, 4J = 4.1 Hz), 3.55 (1H, dd, 3J = 13.3 Hz,
4J = 9.6 Hz) (methylene); 3.75 (3H, s) (methoxy); 1.27 (1H, d,
3J = 6.1 Hz) (methyl); 4.11 (1H, q), 3.18 (3H, d) (MeOH). 13C NMR
(DMSO-d6, ppm) d: 163.8 (azomethine); 156.5, 152.3, 122.5,
121.0, 120.5, 116.5 (aromatic); 77.6 (methine); 67.1 (methylene);
56.0 (methoxy); 49.0 (MeOH); 20.3 (methyl).
2.2.2. Oxidation of olefins
In typical procedure, styrene or cyclohexene (1.00 mmol), an
oxidant (2.00 mmol), i.e. aqueous 30% H2O2 or 5.5 M tert-butyl
hydroperoxide (TBHP) in decane, and the catalyst (0.010 mmol)
were heated at 80 °C for 1 h in 10 ml of 1,2-dichloroethane (DCE)
or for 5 h in 10 ml of acetonitrile (MeCN). The reactions were mon-
itored by GC and the yields were recorded as the GC yield based on
the starting styrene or cyclohexene. The identities of the oxidation
products were confirmed by GC–MS. Different amounts of catalysts
and oxidants were also added to study their effect on the conver-
sion and selectivity of the reaction products.
2.3.4. {S(+)-2-[(2-Oxidopropyl)iminomethyl]-6-tert-butylphenolato-
j3N,O,O0}dioxidomolybdenum(VI) (4)
Yield 89%. Anal. Calc. for C14H19NO4MoꢂCH3OH: C, 45.8; H, 5.9;
2.3. Synthesis of the dioxidomolybdenum(VI) complexes
N, 3.6. Found: C, 46.0; H, 5.7; N, 3.7%. IR (KBr, cmꢁ1): 1646 (
mC@N);
925, 889
(Mꢁ1 cmꢁ1)]: 288 (6280), 366 (1560). CD spectrum in DMSO [kmax
(nm),
e (Mꢁ1 cmꢁ1)]: 282 (11.57), 358 (4.79). 1H NMR (DMSO-d6,
(mMo@O). UV–Vis spectrum in DMSO [kmax (nm), e
The complexes were obtained in the following example proce-
dure. A solution of 1 mmol of S(+)-1-amino-2-propanol in methanol
(10 ml)was added to 1 mmolof anaromatico–hydroxyaldehyde (sal-
icylaldehyde, 3-methoxysalicylaldehyde, 5-methoxysalicylaldehyde,
3-tert-butylsalicylaldehyde, 3,5-di-tert-butylsalicylaldehyde, 3,5-
dibromosalicylaldehyde or 2-hydroxy-1-naphthaldehyde) in MeOH
(10 ml) and heated with stirring under reflux for 1 h. Bis(acetylaceto-
nato)dioxidomolybdenum(VI) (1 mmol) in MeOH (10 ml) was then
added and stirred atroom temperature for 2 h. After cooling, a precip-
itate separated and was filtered off, washed and recrystallized from
MeOH.
D
ppm) d: 8.68 (1H, s) (azomethine); 7.44 (1H, d, 3J = 7.6 Hz), 7.41
(1H, d, 3J = 7.6 Hz), 6.89 (1H, t, 3J = 7.6 Hz) (aromatic); 4.46–4.48
(1H, m) (methine); 4.15 (1H, dd, 3J = 13.3 Hz, 4J = 4.1 Hz), 3.57
(1H, dd, 3J = 13.3 Hz, 4J = 9.6 Hz) (methylene); 1.36 (9H, s)
(tert-butyl); 1.27 (1H, d, 3J = 6.1 Hz) (methyl); 4.10 (1H, q), 3.18
(3H, d) (MeOH). 13C NMR (DMSO-d6, ppm) d: 165.7 (azomethine);
159.6, 142.3, 139.7, 132.6, 132.0, 119.3 (aromatic); 77.6 (methine);
67.0 (methylene); 49.0 (MeOH); 34.2, 30.1 (tert-butyl); 20.6
(methyl).

354
G. Romanowski, J. Kira / Polyhedron 117 (2016) 352–358
2.3.5. {S(+)-2-[(2-Oxidopropyl)iminomethyl]-4,6-di-tert-butylpheno-
lato-j3N,O,O0}dioxidomolybdenum(VI) (5)
plexes 1–7 display two sharp bands at 924–931 and 889–
901 cmꢁ1 due to the
masym(O@Mo@O) and msym(O@Mo@O) modes,
Yield 85%. Anal. Calc. for C18H27NO4MoꢂCH3OH: C, 50.8; H, 7.0;
respectively, which indicate the presence of a cis-[MoVIO2] struc-
ture [23].
N, 3.1. Found: C, 50.5; H, 6.9; N, 3.2%. IR (KBr, cmꢁ1): 1637 (
mC@N);
929, 897 (
(Mꢁ1 cmꢁ1)]: 284 (6880), 362 (1980). CD spectrum in DMSO [kmax
(nm),
e (Mꢁ1 cmꢁ1)]: 283 (11.67), 368 (4.24). 1H NMR (DMSO-d6,
m
Mo@O). UV–Vis spectrum in DMSO [kmax (nm),
e
Circular dichroism and UV–Visible absorption spectra of the
molybdenum(VI) complexes were recorded in DMSO. The intrali-
D
gand p–
p⁄ transitions are strong and appear as very intense bands
ppm) d: 8.70 (1H, s) (azomethine); 7.47 (1H, d, 3J = 2.5 Hz), 7.40
(1H, d, 3J = 2.5 Hz) (aromatic); 4.44–4.46 (1H, m) (methine); 4.13
(1H, dd, 3J = 13.3 Hz, 4J = 4.1 Hz), 3.55 (1H, dd, 3J = 13.3 Hz,
4J = 9.6 Hz) (methylene); 1.37 (9H, s), 1.29 (9H, s) (tert-butyl);
1.26 (1H, d, 3J = 6.1 Hz) (methyl); 4.10 (1H, q), 3.18 (3H, d) (MeOH).
13C NMR (DMSO-d6, ppm) d: 165.1 (azomethine); 158.8, 140.6,
138.0, 129.1, 128.7, 120.5 (aromatic); 77.4 (methine); 67.0 (methy-
lene); 49.0 (MeOH); 35.2, 34.3, 31.6, 30.1 (tert-butyl); 20.4
(methyl).
in the 270–310 nm region with emax = 6280–12950 dm3 molꢁ1
cmꢁ1. On the other hand, the low-energy absorptions recorded for
all compounds between 347 and 385 nm (emax = 1560–4940 dm3
molꢁ1 cmꢁ1) are assigned to a ligand-to-metal charge transfer
(LMCT) transition arising from the phenolate oxygen p orbital to
p
an empty d orbital of the molybdenum atom [24]. The circular
dichroism spectra revealed the same bands in the 278–284 and
349–385 nm ranges, having the same origin as the electronic spectra
with a very strong positive sign of the Cotton effects, i.e. with De val-
ues in the 7.11–14.31 and 2.86–5.60 Mꢁ1 cmꢁ1 ranges, respectively.
The NMR spectra of all the molybdenum(VI) complexes were
recorded in DMSO-d6. The assignment of all the proton and carbon
signals were made on the basis of their intensity, coupling patterns
and chemical shifts using one- (1H and 13C) and two-dimensional
(COSY, gHSQC and NOESY) techniques. The 1H spectra of all the
complexes 1–7 show the presence of azomethine proton signals,
proving the monocondensation of all the salicylaldehyde deriva-
tives with S(+)-1-amino-2-propanol, as reported earlier for the
vanadium(V) complexes derived from the same Schiff base ligand
[25]. Two-dimensional NMR experiments were used for the com-
plete assignment and identification of all the 1H and 13C signals
and for establishing connections and proximity between all pro-
tons and carbon atoms. Taking compound 3 as an example, we
found the appearance of cross peaks in its COSY spectrum between
2.3.6. {S(+)-2-[(2-Oxidopropyl)iminomethyl]-4,6-dibromophenolato-
j3N,O,O0}dioxidomolybdenum(VI) (6)
Yield 87%. Anal. Calc. for Br2C10H9NO4MoꢂCH3OH: C, 26.7; H,
2.7; N, 2.8. Found: C, 26.6; H, 2.7; N, 2.9%. IR (KBr, cmꢁ1): 1644
(
m
C@N); 927, 900 (
e (Mꢁ1 cmꢁ1)]: 272 (12950), 361 (4940). CD spectrum in DMSO
[kmax (nm),
e (Mꢁ1 cmꢁ1)]: 281 (10.86), 361 (2.86). 1H NMR
mMo@O). UV–Vis spectrum in DMSO [kmax (nm),
D
(DMSO-d6, ppm) d: 8.69 (1H, s) (azomethine); 7.99 (1H, d,
3J = 2.5 Hz), 7.82 (1H, d, 3J = 2.5 Hz) (aromatic); 4.52–4.54 (1H, m)
(methine); 4.19 (1H, dd, 3J = 13.3 Hz, 4J = 4.1 Hz), 3.63 (1H, dd,
3J = 13.3 Hz, 4J = 9.6 Hz) (methylene); 1.28 (1H, d, 3J = 6.1 Hz)
(methyl); 4.10 (1H, br s), 3.17 (3H, s) (MeOH). 13C NMR
(DMSO-d6, ppm) d: 163.0 (azomethine); 157.7, 139.1, 135.9,
123.6, 115.6, 109.7 (aromatic); 78.6 (methine); 67.2 (methylene);
49.0 (MeOH); 20.3 (methyl).
two methylene protons doublet of doublets (at
d 4.14 and
3.55 ppm) and the methine proton signal at d 4.43 ppm. Further-
more, a cross-peak between the methyl proton doublet at d 1.27
and 4.43 ppm signal has been also observed. On the basis of struc-
tural information from through space dipole–dipole couplings in
the NOESY spectra, cross-peaks between the signals of two of the
methylene protons (d 4.14 and 3.55 ppm) and the azomethine pro-
ton at d 8.67 ppm have been found. Finally, just as expected, no
corresponding cross-peaks between the signals of the azomethine
proton and the methyl proton (d 1.27 ppm) or the methine proton
(d 4.43 ppm) could be observed. Moreover, the coordination of one
methanol molecule to the molybdenum atom in the case of all
complexes was confirmed by the appearance of signals at
d 3.17–3.18 and 4.10–4.12 ppm.
2.3.7. {S(+)-2-[(2-Oxidopropyl)iminomethyl]naphtholato-j
3N,O,
O0}dioxidomolybdenum(VI) (7)
Yield 87%. Anal. Calc. for C14H13NO4MoꢂCH3OH: C, 46.5; H, 4.4;
N, 3.6. Found: C, 46.5; H, 4.4; N, 3.6%. IR (KBr, cmꢁ1): 1625 (
C@N);
931, 897 Mo@O). UV–Vis spectrum in DMSO [kmax (nm),
e (Mꢁ1 cmꢁ1)]: 273 (8810), 310 (10710), 378 (3980). CD spectrum
in DMSO [kmax (nm),
e (Mꢁ1 cmꢁ1)]: 283 (7.11), 298 (7.11), 379
m
(m
D
(5.60). 1H NMR (DMSO-d6, ppm) d: 9.61 (1H, s) (azomethine);
8.40 (1H, d, 3J = 8.6 Hz), 8.03 (1H, d, 3J = 9.0 Hz), 7.90 (1H, d,
3J = 8.0 Hz), 7.62 (1H, t, 3J = 7.5 Hz), 7.42 (1H, t, 3J = 7.5 Hz), 7.13
(1H, d, 3J = 9.0 Hz) (aromatic); 4.54–4.56 (1H, m) (methine); 4.38
(1H, dd, 3J = 13.3 Hz, 4J = 4.1 Hz), 3.70 (1H, dd, 3J = 13.3 Hz,
4J = 9.6 Hz) (methylene); 1.31 (1H, d, 3J = 6.1 Hz) (methyl); 4.11
(1H, q), 3.18 (3H, d) (MeOH). 13C NMR (DMSO-d6, ppm) d: 159.7
(azomethine); 163.0, 136.1, 133.5, 129.4, 128.6, 128.2, 124.4,
122.3, 121.4, 112.4 (aromatic); 77.5 (methine); 67.6 (methylene);
49.1 (MeOH); 20.3 (methyl).
3.2. Catalytic activity studies
3.2.1. Sulfoxidation
The catalytic activity of the dioxidomolybdenum(VI) complexes
1–7 under optimized reaction conditions for the enantioselective
oxidation of model prochiral sulfides, i.e. methyl phenyl sulfide
(thioanisole) and benzyl phenyl sulfide into their corresponding
sulfoxides (Fig. 2) have been tested. A slight excess of 1.10 equiva-
lents of aqueous 30% H2O2 was used as an oxidant basing on the
sulfide substrate. Furthermore, optimized amounts of 1 mol% of
catalyst were used in the sulfoxidation reactions, based on a sub-
strate in a mixture of CH2Cl2 and CH3OH (7:3). The best enantiose-
lectivities were achieved with dichloromethane, but methanol was
necessary for a better mixing of the aqueous oxidant with the halo-
genated solvent [26]. Moreover, protic solvents can significantly
enhance yield and selectivity of sulfoxide [27]. The catalytic stud-
ies results are listed in Table 1.
3. Results and discussion
3.1. Spectroscopic properties
The spectroscopic properties of the complexes, i.e. infrared,
UV–Vis, circular dichroism, 1H and 13C NMR data, are listed in
Section 2.
Strong C@N vibrations (at 1625–1646 cmꢁ1) are displayed in
the IR spectra of the complexes, which may be assigned the azome-
thine group of the Schiff base ligands coordinated to the dioxido-
molybdenum(VI) moiety [22]. In the case of compounds 2 and 3,
with methoxy substituents attached to the aromatic ring of the sal-
icyl moiety, asymmetric and symmetric CAO stretches have been
found at ca. 1260 and 1040 cmꢁ1, respectively. In addition, com-
Complexes 1, 2, 3 and 7 showed the best results as catalysts in
the oxidation of thioanisole (Table 1, entries 1, 2, 3 and 7). All the
molybdenum(VI) catalysts yielded 79–93% overall conversion

G. Romanowski, J. Kira / Polyhedron 117 (2016) 352–358
355
HO
OH
U
U
O
S
S
1 mol% catalyst
30% H O
R
R
2
2
PhED
O
Fig. 2. Sulfoxidation of thioethers catalyzed by the molybdenum(VI) complexes.
O
HO
O
1 mol% catalyst
H O or TBHP
Table 1
2
2
Catalytic oxidation of PhSMe and PhSBz by aqueous 30% H2O2 in the presence of 1 mol
% of the molybdenum(VI) Schiff base complexes as catalysts.
styrene
StO
BzA
BzAC
Entry Catalyst Substrate Yield (%) T (°C) t (min) TON eea (%)
O
1
2
3
4
5
6
7
8
1
2
3
4
5
6
7
1
2
3
7
1
2
3
7
PhSMe
PhSMe
PhSMe
PhSMe
PhSMe
PhSMe
PhSMe
PhSBz
PhSBz
PhSBz
PhSBz
PhSMe
PhSMe
PhSMe
PhSMe
88
93
90
79
78
74
82
73
84
79
74
93
97
95
91
Rt
Rt
Rt
Rt
Rt
Rt
Rt
Rt
Rt
Rt
Rt
ꢁ20
ꢁ20
ꢁ20
ꢁ20
45
45
45
45
45
45
45
45
45
45
45
210
210
210
210
88
93
90
79
78
74
82
73
84
79
74
93
97
95
91
6
11
9
3
PhAA
rac
rac
7
3
6
5
3
11
17
15
12
Fig. 3. Various oxidation products of styrene catalyzed by the molybdenum(VI)
complexes.
9
10
11
12
13
14
15
the reaction mixture with the representative catalyst, 2, were
tested.
Considering the highest conversion and reaction rate, we found
1,2-dichloroethane (DCE) to be the best solvent during our exami-
nation of olefin oxidation in different solvents, such as methanol,
ethanol, acetonitrile, chloroform, methylene chloride and 1,2-
dichloroethane [24,29]. Moreover, as reported earlier [30], it was
concluded that a higher reactions temperature can also be respon-
sible for obtaining better yields and reaction rates.
a
All sulfoxides are in the S configuration.
within 45 min reaction time and enantiomeric excesses of up to
11% for the S-configured sulfoxide were obtained. We also
employed a substrate with a more bulky substituent, i.e. benzyl
phenyl sulfide. In this case (Table 1, entries 8–11) the overall yield
of benzyl phenyl sulfoxide was slightly lower (73–84%) and a con-
siderable decrease in the enantioselectivities (66%) was noticed.
When the sulfoxidation was carried out at ꢁ20 °C with 1, 2, 3
and 7 as catalysts, the enantioselectivities significantly increased
to value of 11–17% for methyl phenyl sulfide and also the conver-
sions improved up to 97% after 210 min reaction time (Table 1,
entries 12–15). It was reported earlier by Mimoun et al. that a suf-
ficient nucleophilic center is very important for the catalytic oxida-
tion processes of a number of types of organic substrates [28].
Compounds 2 and 3 have achieved the best enantioselectivity in
comparison to the remaining catalysts. In our opinion, the reason
for such behavior is the highest electron-donating resonance effect
of the ortho- and para-substituted methoxy groups, where a higher
electron density on the phenolate oxygen atom is observed, help-
ing to improve attainment of sufficient nucleophilicity of the metal
centre.
To study the effect of the amount of oxidant, i.e. tert-butyl
hydroperoxide (TBHP) in decane or aqueous 30% H2O2, to styrene,
different molar ratios (1:1, 2:1 and 3:1) styrene (1.00 mmol) and
catalyst 2 (0.010 mmol) were employed in DCE (10 ml), and the
oxidation was carried out with a contact time of 45 min at 80 °C.
Taking the 1:1 M ratio of H2O2 to styrene, no more than 18% con-
version was observed. The conversion improved to 30% with a
2:1 ratio, and finally with 3:1 M ratio a maximum of 32% conver-
sion was achieved. Further addition of the oxidant did not improve
the overall conversion yield, therefore a 2:1 M ratio has been
shown to be adequate. When TBHP was taken into consideration,
we tested the same molar ratios of TBHP:styrene (1:1 to 2:1 and
3:1) and the conversion changed from 32 to 53 and 55%, respec-
tively. Addition of this oxidant in higher molar ratios improved
the conversion only marginally, so again the 2:1 M ratio was cho-
sen for further studies.
Continuing the optimization of the oxidation conditions, differ-
ent amounts of catalyst 2 (0.5, 1 and 2 mol%) with the 2:1 M ratio
of oxidant to styrene have been studied. Adding 0.5 mol% catalyst,
only 12% (with 30% H2O2) and 27% (with TBHP) conversion was
achieved. Furthermore, when 1 and 2 mol% of the catalyst were
added to the reaction mixture, we found very similar results, with
30 and 53% conversion for 30% H2O2 and TBHP, respectively, for
45 min of contact time. With the above results we concluded to
perform all oxidation reaction with 1 mol% of catalysts. Under
the same conditions, reactions without the presence of any cata-
lysts were tested, giving only ca. 3–5% conversion for both
oxidants.
In general, when a smaller steric demand of the substrate was
involved, i.e. using thioanisole as compared to benzyl phenyl sul-
fide, much better results in both yield and enantioselectivity of
the corresponding sulfoxide were obtained in the sulfoxidation
reactions. In addition it is noteworthy that under the given condi-
tions no over oxidation to the sulfone could be observed.
3.2.2. Oxidation of styrene
The oxidation of styrene with complexes 1–7 as catalysts in 1,2-
dichloroethane was performed in the presence of aqueous 30%
H2O2 or tert-butyl hydroperoxide (TBHP) as an oxidant. Under
these reaction conditions styrene oxide, 1-phenylethane-1,2-diol,
phenylacetaldehyde, benzaldehyde and benzoic acid were
obtained as the oxidation products (Fig. 3). To achieve optimized
reaction conditions for the maximum conversion of styrene oxida-
tion, different parameters were taken under consideration, i.e.
amount of catalyst (0.5, 1 and 2 mol%) and oxidant (1:1, 2:1 and
3:1 M ratios to substrate), different solvents and temperature of
The selectivity for all the reaction products and the percentage
conversion of styrene are summarized in Table 2. Using tert-butyl
hydroperoxide (TBHP) in decane as an oxidant, under the opti-
mized reaction conditions, all the complexes gave significantly
higher conversions (68–76%) in comparison to the other earlier
reported dioxidomolybdenum(VI) complex with a Schiff base
derived from 2-[(1-hydroxy-2-methylpropane-2-ylimino)methyl]-
naphthol, which gave 50% conversion, but with less excess of the
oxidant and only 0.1 mol% of the catalyst [30]. The selectivities in

356
G. Romanowski, J. Kira / Polyhedron 117 (2016) 352–358
Table 2
Catalytic oxidation of styrene by aqueous 30% H2O2 or tert-butyl hydroperoxide (TBHP) in the presence of 1 mol% molybdenum(VI) Schiff base complexes.
Entry
Catalyst
Oxidant
Conv. (%)
Solvent
TON
Product selectivitya (%)
StO
BzA
BzAC
PhAA
PhED
1
2
3
4
5
6
7
8
1
2
3
4
5
6
7
1
2
3
4
5
6
7
1
2
3
4
5
6
7
1
2
3
4
5
6
7
H2O2
H2O2
H2O2
H2O2
H2O2
H2O2
H2O2
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
H2O2
H2O2
H2O2
H2O2
H2O2
H2O2
H2O2
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
17
30
24
27
26
20
23
68
73
76
70
71
70
74
26
29
28
21
30
27
18
88
91
93
79
81
85
82
DCE
DCE
DCE
DCE
DCE
DCE
DCE
DCE
DCE
DCE
DCE
DCE
DCE
DCE
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
17
30
24
27
26
20
23
68
73
76
70
71
70
74
26
29
28
21
30
27
18
88
91
93
79
81
85
82
68
59
56
58
55
51
55
85
77
81
84
78
85
73
7
8
5
7
6
4
5
25
29
34
31
33
25
23
28
32
33
38
39
42
40
14
19
17
14
19
13
24
79
83
85
80
78
84
88
65
59
56
61
55
64
67
–
–
–
–
–
–
–
–
–
–
–
–
–
–
1
1
1
1
1
1
1
5
7
7
4
5
6
4
–
–
–
–
–
–
–
–
–
–
–
–
–
–
6
3
4
6
7
4
4
2
2
3
2
3
2
2
4
9
11
4
6
7
5
1
4
2
2
3
2
3
7
5
5
5
8
7
2
3
3
4
2
4
3
4
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
a
StO – styrene oxide, BzA – benzaldehyde, BzAC – benzoic acid, PhAA – phenylacetaldehyde, PhED – 1-phenylethane-1,2-diol.
the case of all the molybdenum(VI) complexes are rather similar
and they are generally distinctly more selective toward styrene
oxide (73–85%) than benzaldehyde (13–24%). Furthermore, their
selectivity against 1-phenylethane-1,2-diol is relatively low (1–
4%). Judmaier et al. [23] described some new dioxidomolybde-
num(VI) Schiff base complexes with pendant OMe donor arms
where two ligands coordinate in a bidentate manner to the metal
center, and these were used as catalysts in similar reactions but
in chloroform at 50 °C. In this case, 71–75% conversions were
obtained in 5 h of reaction time with 97–98% styrene oxide selec-
tivity after 24 h. When a one-pot synthesis by reductive amination
was employed, dimeric dioxidomolybdenum(VI) complexes with
one Schiff base in a tridentate manner were obtained. Catalytic
reactions with these complexes under the same reaction condi-
tions gave only 35% conversion of styrene in 5 h and 44% in 24 h
of reaction time [31]. On the other hand, the catalytic properties
of dioxidomolybdenum(VI) complexes with naphtholate-oxazoline
ligands for the epoxidation of styrene in DCE at 80 °C have been
also reported [32]. At low catalyst loadings of 0.05 mol% with TBHP
after 6 h of reaction time, 76–83% conversion with 90–92% styrene
oxide selectivity resulted.
is slightly better in MeCN, using both H2O2 and TBHP as oxidizing
agents, and comparable with vanadium(V) complexes. On the
other hand, the selectivity, in the case of molybdenum(VI) and
vanadium(V) complexes, is very similar and both complexes with
both metals are generally distinctly more selective toward ben-
zaldehyde (ca. 60%) than styrene oxide (20–30%) using TBHP. The
selectivity of the other oxidation products is also comparable and
varies in the order: benzoic acid > 1-phenylethane-1,2-
diol > phenylacetaldehyde. When aqueous 30% H2O2 was used as
an oxidant under the same styrene oxidation conditions, distinctly
lower conversions (below 30%) for all of the catalysts have been
noticed. As can be also seen, the conversion of styrene is distinctly
lower than with TBHP, but the selectivity against benzaldehyde is
very impressive (over 78%) in all cases. The selectivity for the other
oxidation product is below 10%. In the presence of H2O2, a strong
oxidizing agent, oxidation of styrene results in styrene oxide in
the first step, but further reaction, via nucleophilic attack of the
oxidant to styrene oxide followed by the cleavage of the interme-
diate hydroperoxystyrene, is very fast, converting the product into
benzaldehyde [33]. Moreover, the formation of benzaldehyde is
probably facilitated via a radical mechanism by direct oxidative
cleavage of the styrene side-chain double bond. A significant
amount of water in aqueous 30% H2O2 can be blamed for the
decomposition of the catalyst and thus the very low conversion
of styrene. Formation of 1-phenylethane-1,2-diol by the hydrolysis
of styrene oxide is also caused by the presence of water. The other
processes, although surely much slower, can be responsible for the
formation of benzoic acid in the further oxidation of benzaldehyde
or isomerisation of styrene oxide to phenylacetaldehyde.
The catalytic oxidation of styrene has also been performed by us
under the same reaction conditions, but with aqueous 30% H2O2
acting as an oxidizing agent. Distinctly lower conversion in the
same contact time was found (17–30%). Moreover, much lower
selectivity against styrene oxide has been observed (51–68%) than
in the case of TBHP. Also, with H2O2 the catalysts are much more
selective toward benzaldehyde (28–42%) and 1-phenylethane-
1,2-diol (4–11%).
For a comparison of catalysts 1–7 to chiral tridentate oxi-
dovanadium(V) complexes with the same Schiff base ligands,
reported by us earlier [25], we have also tested the catalytic activ-
ity for the oxidation of styrene using 1 mol% of the catalysts in an
optimized 5 h of reaction time, 3:1 oxidant:styrene molar ratio at
80 °C, but in acetonitrile as the solvent. Generally, the conversion
3.2.3. Oxidation of cyclohexene
The catalytic activity of complexes 1–7 has been tested for the
oxidation of cyclohexene in the presence of tert-butyl hydroperox-
ide (TBHP) in decane or aqueous 30% H2O2 as an oxidant, giving
four products, i.e. cyclohexene oxide, cyclohexane-1,2-diol, 2-

G. Romanowski, J. Kira / Polyhedron 117 (2016) 352–358
357
OH
OH
reaction under the above conditions. A blank reaction under the
above reaction conditions gave ca. 4–5% conversion with both
oxidants.
O
R
ChO
ChDL
O
For the oxidation reactions with 1–7 as catalysts and aqueous
30% H2O2 as an oxidant, 51–63% conversion in 5 h of contact time
was found. On the other hand, the conversion of styrene under the
same reaction conditions is distinctly lower. The selectivity against
cyclohexene oxide (38–56%) is slightly more noticeable than
against 2-cyclohexen-1-one (23–40%), 2-cyclohexen-1-ol (10–
26%) and cyclohexane-1,2-diol (2–10%). The preferential attack of
the activated CAH bond over the C@C bond may be responsible
for the formation of the allylic oxidation products (2-cyclohexen-
1-ol and 2-cyclohexen-1-one) with higher selectivity [34].
Using the same optimized reaction conditions for the catalytic
oxidation of cyclohexene, but with tert-butyl hydroperoxide
(TBHP) in decane as the oxidant, catalysts 1–7 gave 85% conver-
sion. In contrast to the catalytic reactions with H2O2, complexes
1–7 are distinctly more selective toward cyclohexene oxide (69–
85%). The remaining amounts of 2-cyclohexene-1-ol found in the
reaction mixture are by-products. Very similar results were
reported by Rayati et al. [24] with MoO2{hnaphnptn} – a dioxido-
molybdenum(VI) complex with a symmetrical tetradentate Schiff
base.
1 mol% catalyst
H O or TBHP
2
2
OH
cyclohexene
R
ChOL
ChON
Fig. 4. Various oxidation products of cyclohexene catalyzed by the molybdenum
(VI) complexes.
Table 3
Catalytic oxidation of cyclohexene by aqueous 30% H2O2 or tert-butyl hydroperoxide
(TBHP) in the presence of 1 mol% molybdenum(VI) complexes in DCE.
Entry Catalyst TON Oxidant Conv. (%) Product selectivitya (%)
ChO ChOL ChON ChDL
1
2
3
4
5
6
7
8
1
2
3
4
5
6
7
1
2
3
4
5
6
7
60
56
51
61
57
60
63
72
83
81
85
80
77
79
H2O2
H2O2
H2O2
H2O2
H2O2
H2O2
H2O2
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
60
56
51
61
57
60
63
72
83
81
85
80
77
79
55
42
50
46
38
46
56
74
82
85
71
69
72
76
18
16
12
10
26
12
10
26
18
15
29
31
28
24
23
33
31
37
33
40
29
–
–
–
–
–
4
10
7
6
3
2
5
–
–
–
–
–
–
–
3.2.4. Reactivity of the catalysts with H2O2
A variety of molybdenum(VI) complexes have been found to
react with H2O2 to form the corresponding oxidoperoxido com-
plexes. Although the isolation of [MoO(O2)]2+ Schiff base com-
pounds was unsuccessful, the stepwise addition of a DMSO
solution of H2O2 to the dioxidomolybdenum(VI) complex solutions
and monitoring any UV–Vis spectral changes allowed us to estab-
lish the generation of such oxidoperoxido species and shed some
light on the mechanism of these catalytic reactions. In an example
procedure, the spectra were recorded after successive addition of
one drop portions of aqueous 30% H2O2 (1.70 g, 13 mmol) dis-
solved in 5 ml of DMSO to 15 ml of an 8.1 ꢀ 10ꢁ5 M solution of cat-
alyst 3 in DMSO and the resultant spectroscopic changes are
presented in Fig. 5. Such a titration with a dilute solution of com-
plex 3 caused a decrease with only a marginal change in intensity
of the 375 nm band, which belongs to a weak ligand-to-metal
charge transfer (LMCT) transition. On the other hand, the strong
9
10
11
12
13
14
–
–
a
ChO – cyclohexene oxide, ChOL – 2-cyclohexen-1-ol, ChON – 2-cyclohexen-1-
one, ChDL – cyclohexane-1,2-diol.
cyclohexene-1-ol and 2-cyclohexene-1-one (Fig. 4). The formation
of all these products, conversion and selectivity are presented in
Table 3.
Optimization of the reaction conditions has been performed
with complex 2 as a representative catalyst, as in the case for the
oxidation of styrene. Different amounts of the catalyst (0.5, 1 and
2 mol%) and both oxidants (1:1, 2:1 and 3:1 M ratios to cyclohex-
ene), different solvents and temperature of the reaction mixture
have been also employed.
intraligand
p–
p⁄ transition, with a 276 nm band, increases its
intensity considerably with a small shift to 265 nm, and then it
finally disappears. In our opinion these changes indicate the inter-
The best results were found when 1,2-dichloroethane (DCE) and
80 °C were chosen to run the catalytic reactions. Three different
molar ratios of aqueous 30% H2O2 or tert-butyl hydroperoxide
(TBHP) to cyclohexene, i.e. 1:1, 2:1 and 3:1, have been studied.
Cyclohexene (1.00 mmol) and catalyst (0.010 mmol) were taken
in DCE (10 ml), and the reaction was carried out for 1 h of contact
time at 80 °C. At a 1:1 H2O2 to styrene molar ratio, a maximum of
27% conversion was achieved. Increasing the ratio to 2:1 improved
the conversion to 56%, while a 3:1 ratio showed a maximum of 57%
conversion. Further increment of H2O2 improved the conversion
only marginally, therefore a 2:1 ratio was considered as adequate.
In the case using TBHP as an oxidizing agent, increasing the TBHP:
cyclohexene molar ratio from 1:1 to 2:1 and 3:1 improved the con-
version from 46 to 83 and 85%, respectively. As in the previous
case, there was no significant conversion improvement upon fur-
ther addition of the oxidant.
Similarly, for different amounts of catalyst (0.5, 1 and 2 mol%)
with a 2:1 M ratio of oxidant to cyclohexene and under the above
reaction conditions, 0.5 mol% gave only 18% (H2O2) and 34% (TBHP)
oxidative conversion, while 1 mol% and 2 mol% of the catalyst
showed a maximum conversion of 56% for H2O2 and 83% for TBHP.
Thus, 1 mol% of catalyst may be considered sufficient to run the
Fig. 5. Spectral changes observed during titration of catalyst 3. The spectra
recorded after successive addition of one drop portions of aqueous 30% H2O2
(1.70 g, 15 mmol) dissolved in 5 ml of DMSO to 15 ml of an 8.1 ꢀ 10ꢁ5 M solution of
3 in DMSO.

358
G. Romanowski, J. Kira / Polyhedron 117 (2016) 352–358
action of complex 3 with hydrogen peroxide and the plausible for-
mation of the oxidoperoxidomolybdenum(VI) complex in DMSO,
which in the catalytic reaction finally transfers oxygen to an appro-
priate organic substrate to give the various oxidation products.
Acknowledgement
This scientific work was supported by the Polish Ministry of
Science and Higher Education (DS/530-8216-D598-16).
References
4. Conclusion
[1] R. Hille, Coord. Chem. Rev. 255 (2011) 1179.
[2] M. Ribbe, Coord. Chem. Rev. 255 (2011) 1218.
New chiral dioxidomolybdenum(VI) complexes derived from
Schiff bases, products of a single condensation of salicylaldehyde
and its derivatives with S(+)-1-amino-2-propanol, were synthe-
sized and characterized by UV–Vis, CD, IR, one- (1H, 13C) and
two-dimensional (COSY, NOESY, gHSQC) NMR spectroscopy. More-
over, the catalytic properties of the chiral catalysts for the oxida-
tion of organic sulfides (thioanisole and benzyl phenyl sulfide)
and olefins (styrene and cyclohexene) have been studied.
The results of sulfoxidation studies showed that the overall
yield and enantiomeric excess distinctly depend on the nature of
the catalyst, i.e. the electron density at the metal center. Moreover,
when a bulky substituent was involved in the structure of the sul-
fide, with large steric demand, both the activity and enantioselec-
tivity dropped sharply. Finally, the sulfoxidation reactions carried
out at much lower temperatures result in both better yields and
enantioselectivities.
[3] K.C. Gupta, A.K. Sutar, Coord. Chem. Rev. 252 (2008) 1420.
[4] A. Basak, A.U. Barlan, H. Yamamoto, Tetrahedron Asymmetry 17 (2006) 508.
[5] A.P. da Costa, P.M. Reis, C. Gamelas, C.C. Romão, B. Royo, Inorg. Chim. Acta 361
(2008) 1915.
[6] T.D. Owens, A.J. Souers, J.A. Ellman, J. Org. Chem. 68 (2003) 3.
[7] E.N. Jacobsen, Acc. Chem. Res. 33 (2000) 421.
[8] A. Abiko, S. Masamune, Tetrahedron Lett. 33 (1992) 5517.
[9] M.J. McKennon, A.I. Meyers, J. Org. Chem. 58 (1993) 3568.
[10] M. Amini, M.M. Haghdoost, M. Bagherzadeh, Coord. Chem. Rev. 257 (2013)
1093.
[11] Q. Zeng, H. Wang, W. Weng, W. Lin, Y. Gao, X. Huang, Y. Zhao, New J. Chem. 29
(2005) 1125.
[12] A. Rezaeifard, I. Sheikhshoaie, N. Monadi, H. Stoeckli-Evans, Eur. J. Inorg. Chem.
(2010) 799.
[13] S.-H. Hsieh, Y.-P. Kuo, H.-M. Gau, Dalton Trans. (2007) 97.
[14] J. Hartung, Pure Appl. Chem. 77 (2005) 1559.
[15] J. Hartung, S. Drees, M. Grab, P. Schmidt, I. Svoboda, H. Fuess, A. Murso, D.
Stalke, Eur. J. Org. Chem. (2003) 2388.
[16] Á. Gama, L.Z. Flores-López, G. Aguirre, M. Parra-Hake, R. Somanathan, T. Cole,
Tetrahedron Asymmetry 16 (2005) 1167.
The catalytic potential of the dioxidomolybdenum(VI) com-
plexes for the oxidation of olefins was investigated, and a compar-
ison to similar oxidovanadium(V) catalysts was also studied,
choosing the oxidation of styrene and cyclohexene as model reac-
tions. These complexes are able to catalyze the oxidative conver-
sion of styrene to styrene oxide and benzaldehyde as the main
products, and cyclohexene to cyclohexene oxide and its successive
by-products in the presence of aqueous 30% H2O2 or tert-butyl
hydroperoxide in decane. The oxidation of styrene after 5 h of reac-
tion time in DCE at 80 °C with H2O2 can give five different products,
usually with a bigger excess of styrene oxide and benzaldehyde,
but low conversion. On the contrary, tert-butyl hydroperoxide
proved to be an excellent oxidant giving even 85% conversion of
styrene, with styrene oxide as the main product. Moreover, oxida-
tion of styrene in acetonitrile gave similar results as for oxidovana-
dium(V) complexes with the same Schiff base ligand. In the
oxidation of cyclohexene, even significantly higher conversions
were found than for styrene, especially when H2O2 was employed
as the oxidant. However, when TBHP was employed, the selectivity
against cyclohexene oxide achieved 85% with small amounts of
2-cyclohexene-1-ol as the by-product.
[17] L.Z. Flores-Lopéz, M. Parra-Hake, R. Somanathan, P.J. Walsh, Organometallics
19 (2000) 2153.
[18] G. Romanowski, T. Lis, Inorg. Chim. Acta 394 (2013) 627.
[19] G. Romanowski, M. Wera, Polyhedron 50 (2013) 179.
[20] G. Romanowski, J. Kira, M. Wera, J. Mol. Catal. A: Chem. 381 (2014) 148.
[21] M.E. Cucciolito, R. Del Litto, G. Roviello, F. Ruffo, J. Mol. Catal. A: Chem. 236
(2005) 176.
[22] V. Saheb, I. Sheikhshoaie, H. Stoeckli-Evans, Spectrochim. Acta A 95 (2012) 29.
[23] M.E. Judmaier, C. Holzer, M. Volpe, N.C. Mösch-Zanetti, Inorg. Chem. 51 (2012)
9956.
[24] S. Rayati, N. Rafiee, A. Wojtczak, Inorg. Chim. Acta 386 (2012) 27.
[25] G. Romanowski, J. Mol. Catal. A: Chem. 368–369 (2013) 137.
[26] M. Mancka, W. Plass, Inorg. Chem. Commun. 10 (2007) 677.
[27] I. Sheikhshoaie, A. Rezaeifard, N. Monadi, S. Kaafi, Polyhedron 28 (2009) 733.
[28] H. Mimoun, P. Chaumette, M. Mignard, L. Sausinne, Nouv. J. Chim. 7 (1983)
467.
[29] A. Rezaeifard, I. Sheikhshoaie, N. Monadi, M. Alipour, Polyhedron 29 (2010)
2703.
[30] A. Rezaeifard, M. Jafarpour, H. Raissi, M. Alipour, H. Stoeckli-Evans, Z. Anorg.
Allg. Chem. 638 (2012) 1023.
[31] M.E. Judmaier, C.H. Sala, F. Belaj, M. Volpe, N.C. Mösch-Zanetti, New J. Chem.
37 (2013) 2139.
[32] P. Traar, J.A. Schachner, B. Stanje, F. Belaj, N.C. Mösch-Zanetti, J. Mol. Catal. A:
Chem. 385 (2014) 54.
[33] V. Hulea, E. Dumitriu, Appl. Catal. A: Gen. 277 (2004) 99.
[34] J.D. Koola, J.K. Kochi, J. Org. Chem. 52 (1987) 4545.