Synthesis, characterization and catalytic activity of dioxidomolybdenum(VI) complexes with tridentate Schiff bases derived from 1R,2S(?)-norephedrine

Article abstract of DOI:10.1016/j.poly.2017.06.005

New chiral dioxidomolybdenum(VI) complexes with tridentate Schiff base ligands obtained by monocondensation of 1R,2S(?)-norephedrine with salicylaldehyde and its derivatives were synthesized. The complexes were characterized by elemental analysis and by their IR, CD, UV–vis, one- (¹H) and two-dimensional (COSY, gHSQC and NOESY) NMR spectra. After optimization of reaction conditions, catalytic activity of these complexes were tested in the oxidation of olefins, i.e. styrene and cyclohexene, using aqueous 30% H₂O₂ or tert-butyl hydroperoxide (TBHP) as an oxidant. Moreover, the dioxidomolybdenum(VI) Schiff base complexes have also ability to catalyze the oxidation of thioanisole to methyl phenyl sulfoxide in presence of aqueous 30% H₂O₂.

Full text of DOI:10.1016/j.poly.2017.06.005

Accepted Manuscript
Synthesis, characterization and catalytic activity of dioxidomolybdenum(VI)
complexes with tridentate Schiff bases derived from 1R,2S(-)-norephedrine
Grzegorz Romanowski, Jaromir Kira
PII:
S0277-5387(17)30421-7
DOI:
http://dx.doi.org/10.1016/j.poly.2017.06.005
POLY 12684
Reference:
To appear in:
Polyhedron
Received Date:
Accepted Date:
19 April 2017
6 June 2017
Please cite this article as: G. Romanowski, J. Kira, Synthesis, characterization and catalytic activity of
dioxidomolybdenum(VI) complexes with tridentate Schiff bases derived from 1R,2S(-)-norephedrine, Polyhedron
(
2017), doi: http://dx.doi.org/10.1016/j.poly.2017.06.005
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers
we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and
review of the resulting proof before it is published in its final form. Please note that during the production process
errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Synthesis, characterization and catalytic activity of dioxidomolybdenum(VI)
complexes with tridentate Schiff bases derived from 1R,2S(‒)-norephedrine
Grzegorz Romanowski* and Jaromir Kira
Faculty of Chemistry, University of Gdańsk, Wita Stwosza 63, PL-80308 Gdańsk, Poland
Abstract
New chiral dioxidomolybdenum(VI) complexes with tridentate Schiff base ligands
obtained by monocondensation of 1R,2S(‒)-norephedrine with salicylaldehyde and its
derivatives were synthesized. The complexes were characterized by elemental analysis and by
1
their IR, CD, UV-Vis, one- ( H) and two-dimensional (COSY, gHSQC and NOESY) NMR
spectra. After optimization of reaction conditions, catalytic activity of these complexes were
tested in the oxidation of olefins, i.e. styrene and cyclohexene, using aqueous 30% H
2
2
O or
tert-butyl hydroperoxide (TBHP) as an oxidant. Moreover, the dioxidomolybdenum(VI)
Schiff base complexes have also ability to catalyze the oxidation of thioanisole to methyl
phenyl sulfoxide in presence of aqueous 30% H
. Introduction
Molybdenum is a trace element, which is capable of forming complexes with many
2
O
2
.
1
compounds of biological significance. It is also an important cofactor in the active site of
many enzymes in plants and animals that catalyzes a number of diverse reactions. For plant
growth, molybdenum is essential in the processes of atmospheric nitrogen fixation by
convertion to ammonia (nitrogenases) and nitrate reduction to nitrite via nitrate reductase. In
mammalian enzymes, it is part of a complex called molybdenum cofactor (Moco) required for
xanthine oxidoreductase, xanthine dehydrogenase or xanthine oxidase that catalyzes oxidation
of hypoxanthine to xanthine, xanthine to uric acid or participates in the metabolism of
purines. Furthermore, sulfite oxidase is involved in the metabolism of sulfur-containing amino
acids and finally, the last step of abscisic acid (ABA) biosynthesis involves oxidation of its
aldehyde catalyzed by aldehyde oxidase [1,2].
Such enzymes can be modeled by molybdenum complexes with chiral N-salicyl-β-
amino alcohol Schiff base ligands [3,4]. These “tridentate salen ligands” are very attractive
due to their simple synthesis from naturally available chiral amino acids [5,6] and structural
and electronic fine-tunability [7]. Many transition metal complexes, including these with
Schiff bases and molybdenum(VI) or vanadium(V) cation have been successfully employed
as catalysts in epoxidation of olefins [8], the asymmetric alkynylation of aldehydes [9], the
stereoselective synthesis of cyclic ethers [10,11] and trimethylsilylcyanations [12] and
oxidation of sulfides to sulfoxides [9,13,14]. Especially in case of enantioselective
sulfoxidation, molybdenum complexes are much less successful and explored [15]. Until now,
various systems have been developed successfully for the asymmetric sulfoxidation [16] and
epoxidation reactions of alkenols [17] using in situ generated molybdenum and vanadium-
based catalysts.
In continuation of our studies on synthesis, structure, spectroscopic and catalytic
properties of transition metal complexes incorporating chiral tridentate Schiff base ligands
[
_
18-20], we prepared a series of new dioxidomolybdenum(VI) complexes with ONO donor
____________
1

*
5
Corresponding author. E-mail address: grzegorz.romanowski@ug.edu.pl, tel.: +48
85235102, fax: +48 585235012
Schiff base ligands, products of monocondensation of 1R,2S(‒)-norephedrine with aromatic o-
hydroxyaldehydes, presented in Fig. 1. Their spectroscopic properties by 1D and 2D NMR,
UV-Vis, CD and IR have been examined. The catalytic potential of these complexes in the
enantioselective sulfoxidation of thioanisole utilizing aqueous 30% H
been studied. Moreover, they were also used as catalysts in the oxidation of olefins, i.e.
styrene and cyclohexene, in presence of aqueous 30% H or tert-butyl hydroperoxide
TBHP) in decane.
2
2
O as an oxidant has
2
O
2
(
Figure 1. Structural formulae of dioxidomolybdenum(VI) complexes.
R1
R2
R3
R4
R2
R1
1
H
H
H
H
H
H
H
H
H
O
2 OCH
R3
O
N
3
O
3
4
5
6
7
8
9
H
OCH
H
3
Mo
S
R4
H
H
H
H
H
OCH3
OCH3
O
H
CH
H
3
H C
3
H
H
Br
H
H
H
NO
H
2
OH
H
_
S
methanol
C(CH3)3
H
C(CH3)3
_
H
_
1
0
H
(CH)4
2
. Experimental
.1. Measurements
All chemicals and reagents were obtained from commercial sources and used without
2
further purification unless stated otherwise. Carbon, hydrogen and nitrogen contents were
determined on a Carlo Erba MOD 1106 elemental analyzer. IR spectra of solid samples (KBr
pellets) were run on a Bruker IFS 66 and electronic spectra on the Perkin-Elmer LAMBDA 18
spectrophotometer. Circular dichroism spectra were measured with a Jasco J-815
spectropolarimeter. NMR spectra were obtained in DMSO-d solutions with a Bruker
6
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 of the oxidation of olefins. The identity of
the products was confirmed using a GC-MS model Shimadzu GCMS-QP2010 SE.
2
.2. Catalytic activity
2

2
.2.1. Sulfoxidation
In typical procedure, to a solution of catalyst (0.010 mmol) in 3 ml of CH
2
Cl
2
/MeOH
mixture (7:3), thioanisole (1.00 mmol) was added at room temperature or ‒20 C, together
with 1,3,5-trimethoxybenzene as an internal standard. Aqueous 30% H was added (1.10
o
2
O
2
mmol) by small portions and the resulting mixture was stirred. After the appropriate reaction
time, the solution was quenched with 3 ml of sodium sulphite solution (0.1 M) and extracted
with ethyl acetate (3 × 3 ml). The combined organic layers were evaporated to dryness. The
1
solid product dissolved in CDCl
presence of chiral shift
heptafluoropropylhydroxymethylene)-(+)-camphoric acid) [21].
3
was analyzed (yield and ee value) by H NMR spectra in the
reagent
Eu(hfc)
3
(where
Hhfc
is
3-
(
2
.2.2. Oxidation of olefins
In typical procedure, styrene or cyclohexene (1.00 mmol), an oxidant (2.00 mmol), i.e.
aqueous 30% H or 5.5 M tert-butyl hydroperoxide (TBHP) in decane, and catalyst (0.010
2
O
2
o
mmol) were heated at 80 C for 1 h of reaction time in 10 ml of 1,2-dichloroethane (DCE).
The reactions were monitored by GC and the yields were recorded as GC yield based on the
starting styrene or cyclohexene. The identity of oxidation products were confirmed by GC-
MS. The influence of amounts of catalyst and oxidant were also studied to check their effect
on the conversion and selectivity of the reaction products.
2
.3. Synthesis of dioxidomolybdenum(VI) complexes
The complexes were obtained in a following example procedure. A solution of 1 mmol
of 1R,2S(‒)-norephedrine in absolute ethanol (10 ml) was added with stirring to 1 mmol of an
aromatic
methoxysalicylaldehyde,
bromosalicylaldehyde,
o-hydroxyaldehyde
(salicylaldehyde,
3-methoxysalicylaldehyde,
5-methylsalicylaldehyde,
5-
5-
4,6-dimethoxysalicylaldehyde,
5-nitrosalicylaldehyde,
4-hydroxysalicylaldehyde,
3,5-di-tert-
butylsalicylaldehyde or 2-hydroxy-1-naphthaldehyde) in MeOH (10 ml) and heated under
reflux for 1 h. Then, bis(acetylacetonato)dioxidomolybdenum(VI) (1 mmol) in MeOH (10 ml)
was added and stirred at room temperature for 2 h. After cooling in a fridge a solid was
separated and filtered off and washed several times with MeOH.
2
.3.1. {1R,2S(‒)-2-[(1-oxido-1-phenylpropyl)iminomethyl]phenolato-
3
κ N,O,O’}dioxidomolybdenum(VI) methanol solvate (1)
Yield 85%. Anal. Calc. for C16
H
15NO
4
Mo·CH
3
OH: C, 49.4; H, 4.6; N, 3.4. Found: C,
-1
4
9.2; H, 4.6; N, 3.5%. IR (KBr, cm ): 1639 νC=N); 929, 900 (νMo=O). UV-Vis spectrum in
-1
-1
DMSO [λmax (nm), ɛ (M cm )] 275 (8240), 368 (2630). CD spectrum in DMSO [λmax (nm),
-
1
-1
1
Δɛ (M cm )] 279 (-6.27), 301 (0.42), 364 (-5.68). H NMR DMSO-d
6
, ppm): 8.69 (1H, s)
3
3
(
azomethine); 7.56 (1H, d, J=7.7 Hz), 7.48 (1H, t, J=7.7 Hz), 7.42-7.35 (4H, m), 7.30 (1H, t,
3
3
3
3
J=7.5 Hz), 6.96 (1H, t, J=7.7 Hz), 6.85 (1H, d, J=8.9 Hz) (aromatic); 5.56 (1H, d, J=4.3
3
Hz), 4.53 (1H, m) (methine); 0.94 (1H, d, J=6.9 Hz) (methyl); 4.10 (1H, q), 3.18 (3H, d,
J=5.2 Hz) (MeOH).
3
2
.3.2. {1R,2S(‒)-2-[(1-oxido-1-phenylpropyl)iminomethyl]-6-methoxyphenolato-
3
κ N,O,O’}dioxidomolybdenum(VI) methanol solvate (2)
3

Yield 88%. Anal. Calc. for C17
8.7; H, 4.6; N, 3.1%. IR (KBr, cm ): 1631 νC=N); 1257 (νasym(C-O)); 1034 (νsym(C-O)); 927, 883
(νMo=O). UV-Vis spectrum in DMSO [λmax (nm), ɛ (M cm )] 276 (7600), 376 (2590). CD
spectrum in DMSO [λmax (nm), Δɛ (M cm )] 282 (-5.46), 310 (0.72), 375 (-4.77). H NMR
H
17NO
5
Mo·CH OH: C, 48.8; H, 4.8; N, 3.2. Found: C,
3
-1
4
-1
-1
-1
-1
1
3

DMSO-d , ppm): 8.85 (1H, s) (azomethine); 7.46-7.41 (4H, m), 7.34 (1H, t, J=7.5 Hz), 7.26
6
3
3
3
(
1H, d, J=7.9 Hz), 7.23 (1H, d, J=7.9 Hz), 6.98 (1H, t, J=7.9 Hz) (aromatic); 5.59 (1H, d,
3
3
J=4.3 Hz), 4.58 (1H, m) (methine); 3.72 (3H, s) (methoxy); 0.96 (1H, d, J=6.8 Hz) (methyl);
3
4
.10 (1H, q), 3.18 (3H, d, J=5.2 Hz) (MeOH).
2
.3.3. {1R,2S(‒)-2-[(1-oxido-1-phenylpropyl)iminomethyl]-4-methoxyphenolato-
3
κ N,O,O’}dioxidomolybdenum(VI) methanol solvate (3)
Yield 82%. Anal. Calc. for C17
8.6; H, 4.7; N, 3.2%. IR (KBr, cm ): 1636 νC=N); 1258 (νasym(C-O)); 1029 (νsym(C-O)); 928, 897
(νMo=O). UV-Vis spectrum in DMSO [λmax (nm), ɛ (M cm )] 276 (7490), 373 (2540). CD
H
17NO
5
Mo·CH OH: C, 48.8; H, 4.8; N, 3.2. Found: C,
3
-1
4
-1
-1
-1
-1
spectrum in DMSO [λmax (nm), Δɛ (M cm )] 281 (-5.11), 298 (0.20), 319 (-1.60), 384 (-
1
4
.57). H NMR DMSO-d , ppm): 8.77 (1H, s) (azomethine); 7.40-7.36 (4H, m), 7.28 (1H, t,
6
3
3
3
4
3
J=7.5 Hz), 7.18 (1H, d, J=3.2 Hz), 7.15 (1H, dd, J=9.0 Hz, J=3.2 Hz), 6.88 (1H, d, J=9.0
Hz) (aromatic); 5.53 (1H, d, J=4.3 Hz), 4.50 (1H, m) (methine); 3.73 (3H, s) (methoxy); 0.91
1H, d, J=6.8 Hz) (methyl); 4.10 (1H, q), 3.18 (3H, d, J=5.2 Hz) (MeOH).
3
3
3
(
2
.3.4. {1R,2S(‒)-2-[(1-oxido-1-phenylpropyl)iminomethyl]-3,5-dimethoxyphenolato-
3
κ N,O,O’}dioxidomolybdenum(VI) methanol solvate (4)
Yield 78%. Anal. Calc. for C18
8.2; H, 5.0; N, 3.0%. IR (KBr, cm ): 1623 νC=N); 1256 (νasym(C-O)); 1027 (νsym(C-O)); 924, 894
H
19NO
6
Mo·CH OH: C, 48.2; H, 4.9; N, 3.0. Found: C,
3
-1
4
-1
-1
(νMo=O). UV-Vis spectrum in DMSO [λmax (nm), ɛ (M cm )] 306 (12700). CD spectrum in
-1
-1
1
DMSO [λmax (nm), Δɛ (M cm )] 264 (4.04), 299 (-5.84), 340 (-7.47). H NMR DMSO-d ,
6
3
ppm): 8.87 (1H, s) (azomethine); 7.39-7.34 (4H, m), 7.27 (1H, t, J=7.5 Hz), 6.16 (1H, d,
3
3
3
J=2.2 Hz), 6.12 (1H, d, J=2.2 Hz) (aromatic); 5.52 (1H, d, J=4.3 Hz), 4.52 (1H, m)
3
(
methine); 3.86 (3H, s), 3.81 (3H, s) (methoxy); 0.87 (1H, d, J=6.8 Hz) (methyl); 4.10 (1H,
3
q), 3.18 (3H, d, J=5.2 Hz) (MeOH).
2
.3.5. {1R,2S(‒)-2-[(1-oxido-1-phenylpropyl)iminomethyl]-4-methylphenolato-
3
κ N,O,O’}dioxidomolybdenum(VI) methanol solvate (5)
Yield 77%. Anal. Calc. for C17
H
17NO
4
Mo·CH
3
OH: C, 50.6; H, 5.0; N, 3.3. Found: C,
-1
5
0.5; H, 5.1; N, 3.2%. IR (KBr, cm ): 1632 νC=N); 919, 890 (νMo=O). UV-Vis spectrum in
-1
-1
DMSO [λmax (nm), ɛ (M cm )] 275 (9310), 364 (2120). CD spectrum in DMSO [λmax (nm),
-
1
-1
1
Δɛ (M cm )] 281 (-7.88), 300 (0.18), 363 (-6.22). H NMR DMSO-d
6
, ppm): 8.73 (1H, s)
3
4
3
(
6
azomethine); 7.40-7.36 (5H, m), 7.33 (1H, dd, J=8.4 Hz, J=2.2 Hz), 7.28 (1H, t, J=9.7 Hz),
3
3
.84 (1H, d, J=8.4 Hz) (aromatic); 5.54 (1H, d, J=4.3 Hz), 4.51 (1H, m) (methine); 2.29 (3H,
3
3
s), 0.90 (1H, d, J=6.8 Hz) (methyl); 4.10 (1H, q), 3.18 (3H, d, J=5.2 Hz) (MeOH).
2
.3.6. {1R,2S(‒)-2-[(1-oxido-1-phenylpropyl)iminomethyl]-4-bromophenolato-
3
κ N,O,O’}dioxidomolybdenum(VI) methanol solvate (6)
Yield 82%. Anal. Calc. for BrC16
C, 41.4; H, 3.8; N, 3.0%. IR (KBr, cm ): 1634 νC=N); 924, 895 (νMo=O). UV-Vis spectrum in
H
14NO
4
Mo·CH OH: C, 41.5; H, 3.7; N, 2.9. Found:
3
-1
4

-1
-1
DMSO [λmax (nm), ɛ (M cm )] 277 (9380), 366 (2260). CD spectrum in DMSO [λmax (nm),
-
1
-1
1
Δɛ (M cm )] 285 (-7.25), 304 (0.11), 366 (-5.79). H NMR DMSO-d
6
, ppm): 8.74 (1H, s)
3
4
(
azomethine); 7.75 (1H, s), 7.61 (1H, dd, J=8.4 Hz, J=2.2 Hz), 7.44-7.37 (5H, m), 6.93 (1H,
d, J=8.4 Hz) (aromatic); 5.54 (1H, d, J=4.3 Hz), 4.51 (1H, m) (methine); 0.92 (1H, d, J=6.8
Hz) (methyl); 4.10 (1H, q), 3.18 (3H, d, J=5.2 Hz) (MeOH).
3
3
3
3
2
.3.7. {1R,2S(‒)-2-[(1-oxido-1-phenylpropyl)iminomethyl]-4-nitrophenolato-
3
κ N,O,O’}dioxidomolybdenum(VI) methanol solvate (7)
Yield 79%. Anal. Calc. for C16
H
14
N
2
O
6
Mo·CH OH: C, 44.6; H, 4.0; N, 6.1. Found: C,
3
-
1
4
4.5; H, 3.9; N, 6.2%. IR (KBr, cm ): 1648 νC=N); 929, 907 (νMo=O). UV-Vis spectrum in
-1
-1
-1
DMSO [λmax (nm), ɛ (M cm )] 344 (14800). CD spectrum in DMSO [λmax (nm), Δɛ (M
-1
1
cm )] 258 (-7.11), 298 (0.84), 328 (-4.48). H NMR DMSO-d
6
, ppm): 9.04 (1H, s)
3
3
4
(
azomethine); 8.68 (1H, d, J=3.0 Hz), 8.38 (1H, dd, J=9.2 Hz, J=3.0 Hz), 7.47-7.31 (4H,
3
3
3
m), 7.36 (1H, t, J=8.8 Hz), 7.15 (1H, d, J=9.2 Hz) (aromatic); 5.73 (1H, d, J=4.3 Hz), 4.65
3
(
1H, m) (methine); 1.01 (1H, d, J=7.0 Hz) (methyl); 4.16 (1H, q), 3.23 (3H, s) (MeOH).
2
.3.8. {1R,2S(‒)-2-[(1-oxido-1-phenylpropyl)iminomethyl]-5-hydroxyphenolato-
3
κ N,O,O’}dioxidomolybdenum(VI) methanol solvate (8)
Yield 82%. Anal. Calc. for C 1- 6
H
15NO
5
Mo·CH
3
OH: C, 47.6; H, 4.5; N, 3.3. Found: C,
1
4
7.4; H, 4.4; N, 3.3%. IR (KBr, cm ): 1632 νC=N); 931, 893 (νMo=O). UV-Vis spectrum in
-1
-1
DMSO [λmax (nm), ɛ (M cm )] 288 (7410), 343 (2210). CD spectrum in DMSO [λmax (nm),
-
1
-1
1
Δɛ (M cm )] 263 (5.16), 294 (-8.47), 346 (-7.18). H NMR DMSO-d , ppm): 10.32 (1H, s)
6
3
(
hydroxyl); 8.59 (1H, s) (azomethine); 7.40-7.34 (5H, m), 7.27 (1H, t, J=7.5 Hz), 8.43 (1H,
3
3
d, J=8.4 Hz), 6.27 (1H, s) (aromatic); 5.53 (1H, d, J=4.3 Hz), 4.44 (1H, m) (methine); 0.88
3
3
(
1H, d, J=6.9 Hz) (methyl); 4.10 (1H, q), 3.17 (3H, d, J=5.2 Hz) (MeOH).
2
.3.9. {1R,2S(‒)-2-[(1-oxido-1-phenylpropyl)iminomethyl]-4,6-di-tert-butylphenolato-
3
κ N,O,O’}dioxidomolybdenum(VI) methanol solvate (9)
Yield 75%. Anal. Calc. for C 2- 4
H
31NO
4
Mo·CH
3
OH: C, 57.1; H, 6.7; N, 2.7. Found: C,
1
5
7.1; H, 6.6; N, 2.8%. IR (KBr, cm ): 1632 νC=N); 920, 891 (νMo=O). UV-Vis spectrum in
-1
-1
DMSO [λmax (nm), ɛ (M cm )] 280 (7780), 363 (1600). CD spectrum in DMSO [λmax (nm),
-
1
-1
1
Δɛ (M cm )] 284 (-7.95), 304 (0.13), 369 (-6.19). H NMR DMSO-d
6
, ppm): 8.79 (1H, s)
3
3
(
azomethine); 7.50 (1H, d, J=2.3 Hz), 7.34 (1H, d, J=2.3 Hz) 7.40-7.36 (4H, m), 7.28 (1H, t,
3
3
J=9.3 Hz) (aromatic); 5.54 (1H, d, J=4.3 Hz), 4.44 (1H, m) (methine); 1.40 (9H, s), 1.30
3
3
(
(
9H, s) (tert-butyl); 0.92 (1H, d, J=6.9 Hz) (methyl); 4.11 (1H, q), 3.18 (3H, d, J=5.2 Hz)
MeOH).
2
.3.10. {1R,2S(‒)-2-[(1-oxido-1-phenylpropyl)iminomethyl]naphtholato-
3
κ N,O,O’}dioxidomolybdenum(VI) methanol solvate (10)
Yield 77%. Anal. Calc. for C 2- 0
H
17NO
4
Mo·CH
3
OH: C, 54.4; H, 4.6; N, 3.0. Found: C,
1
5
4.2; H, 4.5; N, 3.1%. IR (KBr, cm ): 1623 νC=N); 932, 902 (νMo=O). UV-Vis spectrum in
-1
-1
DMSO [λmax (nm), ɛ (M cm )] 273 (8860), 308 (11230), 382 (4060). CD spectrum in
-1
-1
1
DMSO [λmax (nm), Δɛ (M cm )] 283 (-2.04), 306 (-5.05), 379 (-6.45). H NMR DMSO-d ,
6
3
3
ppm): 8.68 (1H, s) (azomethine); 8.29 (1H, d, J=9.1 Hz), 8.01 (1H, d, J=9.1 Hz), 7.80 (1H,
3
3
3
d, J=9.1 Hz), 7.60-7.55 (4H, m), 7.53 (1H, t, J=9.1 Hz), 7.45 (1H, t, J=9.3 Hz), 7.40 (1H, t,
5

3
3
3
J=9.1 Hz), 7.18 (1H, d, J=9.1 Hz) (aromatic); 5.56 (1H, d, J=4.3 Hz), 4.47 (1H, m)
3
3
(
methine); 0.95 (1H, d, J=6.9 Hz) (methyl); 4.10 (1H, q), 3.17 (3H, d, J=5.2 Hz) (MeOH).
. Results and discussion
.1. Spectroscopic properties
3
3
1
The electronic, circular dichroism, H NMR and selected solid-state IR spectral data
are listed in Section 2.
−1
The IR spectra of solid complexes display strong C=N stretch (at 1623-1648 cm )
which may be assigned the azomethine group of Schiff base ligands coordinated to
dioxidomolybdenum(VI) moiety [22]. In case of compounds 2, 3 and 4, with methoxy
substituents attached to aromatic ring of salicyl moiety, asymmetric and symmetric C-O
−1
stretches have been found at ca. 1260 and 1030 cm , respectively. In addition, the
-1
molybdenum(VI) complexes display two sharp bands at 919-932 and 883-907 cm due to the
asym(O=Mo=O) and νsym(O=Mo=O) modes, respectively, which indicate the presence of a
ν
VI
cis-[Mo O
2
] structure [23].
The electronic and circular dichroism spectra of all complexes were recorded in
3
-1
-1
DMSO. Strong intense bands, ɛmax = 7410-9380 dm mol cm , with λmax in region 273-288
*
nm are considered to arise from intraligand π-π transitions The low energy bands, recorded
3
-1
-1
for all complexes, between 343-382 nm (ɛmax = 1600-4060 dm mol cm ) may be assigned as
a ligand-to-metal charge transfer (LMCT) transition originating from the p orbital on the
π
phenolate oxygen to the empty d orbital of molybdenum atom [24]. The spectrum of 10,
examined in the same region, display additional band at 308 nm (ɛmax = 11200). In the case of
compounds 4 and 7, there are only single intense bands at around 306 (ɛmax = 12700) and 344
nm (ɛmax = 14 800), similar to vanadium(V) complex with the same chiral Schiff base ligand,
with bands at 316 and 348 nm, respectively [19].
The circular dichroism spectra of 1-3, 5-7 and 9 revealed the same bands in the 258-
2
85 nm and the 328-384 nm range of the same origin as electronic spectra with a very strong
negative sign of the Cotton effects and additional one with a positive sign of the Cotton
effects at the 294-310 nm range. Moreover, there are two exceptions, 4 and 8, which show
bands with a strong positive sign of the Cotton effects at 264 and 263 nm and with a very
strong negative sign of the Cotton effects at 299 and 294 nm, respectively. Furthermore,
complex 10 derived from 2-hydroxy-1-naphthaldehyde, displays three strong bands with a
negative sign of the Cotton effects at 283, 306 and 379 nm.
1
The one- ( H) and two-dimensional (COSY, gHSQC and NOESY) NMR spectra of all
the molybdenum(VI) complexes were recorded in DMSO-d . The signals were assigned on
6
the basis of intensity, spin-spin coupling pattern and chemical shifts. Single condensation of
salicylaldehyde and its derivatives with 1R,2S(‒)-norephedrine is confirmed by the presence
1
of azomethine proton signals in the H NMR spectra of all 1-10 complexes, similar as
reported earlier for the vanadium(V) complexes derived from the same chiral Schiff base
ligand [19]. Two-dimensional NMR experiments allow to unambiguously identify all protons
and carbon atoms and establish connection and proximity between all protons and their
attachment to carbon atoms, what we further discuss on the example of complex 3. For
example, its COSY spectrum shows cross-peaks between methine proton signal at 4.50 ppm
and second methine proton doublet at 5.53 ppm, and also with methyl proton doublet at 0.91
ppm. In aromatic parts of the ligand, there are cross-peaks between 7.40-7.36 ppm multiplet
and triplet at 7.28 ppm (protons of phenyl group in norephedrine fragment) and between
doublet at 6.88 ppm and doublet of doublets at 7.15 ppm (salicylidene fragment). Moreover,
NOESY spectra exhibit cross-peaks between the signal of the azomethine proton at 8.77 ppm
6

and methine proton signal at 4.50 ppm, also with doublet of methyl protons at 0.91 ppm and
with salicylidene aromatic proton doublet at 7.18 ppm. The latter aromatic proton (7.18 ppm)
also show cross-peak with signal of methoxy substituent protons at 3.73 ppm. As expected,
there is no corresponding cross-peak between the signals of the azomethine proton at 8.77
ppm and the methine proton at 5.53 ppm. The coordination of a methanol molecule to the all
molybdenum atoms of these complexes was confirmed by appearance of signals at around
3
.18 and 4.10 ppm.
3
3
.2. Catalytic activity studies
.2.1. Sulfoxidation
The dioxidomolybdenum(VI) complexes 1-10 have been tested for their ability to
catalyze the oxidation of prochiral sulfides using methyl phenyl sulfide (thioanisole) as model
substrate with the optimized reaction conditions (Fig. 2). In this purpose, aqueous 30% H
2
O
2
was used as an oxidant in a slight excess of 1.10 equivalents based on the sulfide substrate
and reactions were run with 1 mol% of catalyst based on the model substrate in a mixture of
CH
2
Cl
2
and CH OH (7:3). With dichloromethane the best enantioselectivities were achieved,
3
but methanol was necessary for a better mixing of the aqueous oxidant with the halogenated
solvent [25,26]. Moreover, protic solvents can significantly enhance yield and selectivity of
sulfoxide [27]. The results of catalytic studies are listed in Table 1.
Figure 2. Sulfoxidation of thioanisole catalyzed by dioxidomolybdenum(VI) complexes.
.
.
O
S
S
1
mol% catalyst
0% H O
R
R
3
2
2
The best results have been obtained at room temperature for complexes 2-5 and 8 as
catalysts (Table 1, entries 2-5 and 8). An overall yields for all catalysts were in the range of
7
7-89% within 45 min reaction time and enantiomeric excesses (ee’s) with up to 28% for the
o
S-configured sulfoxide were obtained. With the reaction carried out at ‒20 C for 2-5 and 8 as
catalysts in the oxidation of thioanisole, enantioselectivities improve significantly to 32-37%
and a conversion up to 91% is observed after 210 min of the reaction time (Table 1, entries
1
1-15). In addition it is noteworthy that under the given conditions no over oxidation to the
sulfone could be observed.
Generally, the best results have been obtained in the oxidation of thioanisole with
catalysts possessing electron donating groups. The best enantioselectivities for 2-5 and 8 as
compared to other catalysts may be result of a higher electron density on the phenolate
oxygen, e.g. due to the highest electron-donating resonance effect contributing to an
attainment of sufficient nucleophilicity by the metal centre. Mimoun et al. [28] pointed out
7

the importance of sufficiently nucleophilic centre for the oxidative catalysis of organic
substrates.
Table 1. Catalytic oxidation of thioanisole by aqueous 30% H
2
O
2
in presence of 1 mol%
molybdenum(VI) Schiff base complexes as catalysts.
o
a
Entry
Catalyst
Yield (%)
T ( C)
t (min)
ee (%)
1
2
3
4
5
6
7
8
9
1
2
3
4
5
6
7
8
9
10
2
3
4
5
89
87
84
81
83
79
77
86
82
87
84
89
91
88
91
rt
rt
rt
rt
rt
rt
rt
rt
rt
45
45
45
45
45
45
45
45
45
20
27
24
25
23
21
18
28
19
21
33
37
35
30
32
1
1
1
1
1
1
0
1
2
3
4
5
rt
45
-20
-20
-20
-20
-20
210
210
210
210
210
8
a
All sulfoxides are in S configuration.
.2.2. Oxidation of styrene
3
The oxidation of styrene with the 1-10 complexes as catalysts in 1,2-dichloroethane
was performed in presence of aqueous 30% H or tert-butyl hydroperoxide (TBHP) as an
2
O
2
oxidant. In these reaction conditions styrene oxide, benzaldehyde, 1-phenylethane-1,2-diol,
benzoic acid and phenylacetaldehyde were obtained as the oxidation products (Fig. 3). The
formation of all these products, conversion and selectivity are presented in Table 2.
In order to achieve suitable reaction conditions for a maximum oxidative conversion
of styrene, complex 3 was taken as a representative catalyst and different parameters, i.e.
amount of catalyst (0.5, 1 and 2 mol%) and an oxidant (in 1:1, 2:1 and 3:1 molar ratios to
styrene), different solvents and temperatures of the reaction mixture were tested. Considering
the highest conversion and reaction rate we found 1,2-dichloroethane (DCE) as the best
solvent during our examination of olefins 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 the higher reactions temperature can
be also responsible for obtaining better yields and reaction rates.
To study the effect of amount of an oxidant, three different molar ratios of aqueous
3
0% H
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 1:1 molar ratio of H to styrene, a maximum of 29%
conversion was achieved. Increasing the molar ratio to 2:1 improved the conversion to 51%,
while 3:1 ratio has shown a maximum of 54% conversion. Further increment of 30% H
2
O
2
or tert-butyl hydroperoxide (TBHP) to styrene, i.e. 1:1, 2:1 and 3:1, styrene (1.00
2
O
2
2
O
2
shows no improvement in conversion In case of using TBHP as an oxidant, increasing the
TBHP:styrene ratio from 1:1, 2:1 to 3:1 improved the conversion from 43 to 72 and 74%,
8

respectively. As in the previous case the oxidation improved only marginally upon further
addition of TBHP, therefore in both cases a 2:1 molar ratio being considered adequate.
Figure 3. Various products of catalytic oxidation of styrene.
HO
OH
PhED
O
O
HO
O
1
mol% catalyst
H O or TBHP
2
2
styrene
StO
BzA
BzAC
O
PhAA
Furthermore, three different amounts of 3 catalyst (0.5, 1 and 2 mol% in respect to
substrate) with oxidant to styrene molar ratio of 2:1 under above reaction conditions were
optimized. In this study, addition of 0.5 mol% gave only 19% (for 30% H
2
2
O ) and 34% (for
TBHP) conversion, while 1 and 2 mol% have shown the conversion results as 51 and 72% for
H
2
2
O
2
and TBHP, respectively, for 1 h of reaction time. Upon further increasing of catalyst to
mol% reactions improved only marginally, which suggested that a higher amount of catalyst
does not improve the oxidative conversion of styrene. Thus, 1 mol% of catalyst may be
considered sufficient enough to run the reaction under above conditions. A blank reaction
under the above reaction conditions gave with both oxidants ca. 2-5% conversion. Table 2
summarizes the percentage conversion of styrene and the selectivities for the various reaction
products.
Using tert-butyl hydroperoxide (TBHP) in decane as an oxidant, under the optimized
reaction conditions, i.e. 1.00 mmol of styrene, 2.00 mmol of oxidant, 1 mol% of catalyst and
DCE as a solvent, all the complexes gave significantly higher 62-73% conversion (Table 2,
entries 11-20), in comparison to the other reported earlier dioxidomolybdenum(VI) complex
with 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 conversion results were very similar to, reported by us earlier [18],
dioxidomolybdenum(VI) Schiff base complexes derived from S(+)-1-amino-2-propanol.
Selectivity, in case of 1-10 catalysts, is rather similar and they are generally distinctly more
9

selective toward styrene oxide (59-71%) than benzaldehyde (20-34%). Furthermore, their
selectivity against benzoic acid (2-4%), phenylacetaldehyde (2-4%) and 1-phenylethane-1,2-
diol (1-2%) is relatively low. Judmaier et al. [23] described new dioxidomolybdenum(VI)
Schiff base complexes with pendant OMe donor arms where two ligands coordinate in a
bidentate manner to the metal center, which were used as catalysts in similar reactions but in
o
chloroform at 50 C. After 24 h, 71-75% conversion has been obtained in 5 h of reaction time
with 97-98% of styrene oxide selectivity. When the dimeric dioxidomolybdenum(VI)
complexes with one Schiff base in a tridentate manner were used as catalysts in the same
reaction conditions only 35% conversion of styrene after 5 h and 44% conversion after 24 h
have been achieved [31]. Moreover, in styrene oxidation with presence of
dioxidomolybdenum(VI) catalysts derived from naphtholate-oxazoline ligands used at 0.05
o
mol% loadings in DCE at 80 C and TBHP as an oxidant after 6 h of reaction time, the 76-
8
3% conversion with very high selectivity to styrene oxide (90-92%) has been reported [32].
Table 2. Catalytic oxidation of styrene in presence of 1 mol% molybdenum(VI) Schiff base
complexes in DCE.
a
Product selectivity (%)
Entry Catalyst
Oxidant
Conv. (%)
StO
BzA
BzAC
PhAA
PhED
1
2
3
4
5
6
7
8
9
1
2
3
4
5
6
7
8
9
10
1
2
3
4
5
6
7
8
H
H
H
H
H
H
H
H
H
H
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
2
2
2
2
2
2
2
2
2
2
O
O
O
O
O
O
O
O
O
O
2
2
2
2
2
2
2
2
2
2
44
48
51
50
43
47
41
52
48
49
63
67
72
70
73
67
62
73
68
60
38
44
46
44
42
35
32
39
45
44
66
61
65
67
71
68
59
67
64
62
57
53
50
51
52
59
64
55
50
51
27
31
27
26
20
25
34
27
29
32
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
5
3
4
5
6
6
4
6
5
5
2
2
2
1
1
2
1
2
1
1
1
0
1
2
3
4
5
6
7
8
9
0
-
-
1
1
1
1
1
1
1
1
1
2
2
4
3
3
2
3
3
2
2
3
3
2
3
3
4
2
3
2
4
2
9
10
a
StO – styrene oxide, BzA – benzaldehyde, BzAC – benzoic acid, PhAA –
phenylacetaldehyde, PhED – 1-phenylethane-1,2-diol.
When catalytic oxidation of styrene in the same reaction conditions were performed,
but with aqueous 30% H
2
2
O as an oxidizing agent, distinctly lower 41-52% conversion was
found (Table 2, entries 1-10). Moreover, much lower selectivity against styrene oxide has
been observed (32-46%) than in the case of TBHP. Moreover, all catalysts are much more
selective toward benzaldehyde (50-64%) and slightly more selective toward 1-phenylethane-
1
,2-diol (3-6%).
1
0

Styrene oxide formed by epoxidation in the first step is very fast converted into
benzaldehyde via nucleophilic attack of H to styrene oxide followed by the cleavage of the
2
O
2
intermediate hydroperoxystyrene [33]. Benzaldehyde formation may also be facilitated by
direct oxidative cleavage of the styrene side-chain double bond via a radical mechanism. Low
conversion of styrene is probably caused by presence of significant amount of water in 30%
H
2
2
O , which can be responsible for the decomposition of catalyst and also the hydrolysis of
styrene oxide to form 1-phenylethane-1,2-diol. Formation of other products, e.g.
phenylacetaldehyde through isomerization of styrene oxide and benzoic acid through
oxidation of benzaldehyde, are distinctly much slower processes.
Figure 4. Vanadium(V) Schiff base complexes derived from 1R,2S(‒)-norephedrine.
R2
R1
R3
O
V
O
N
O-V=O(L)
O
R4
H C
3
1
2
3
4
1
2
4
3
1
1
1
1
1
1: R =R =R =R =H
16: R =R =R =H, R =Br
2
3
4
1
1
2
3
2: R =R =R =H, R =OCH
17: R =R =H, R =NO
2
3
1
2
4
3
1
3
4
2
3: R =R =R =H, R =OCH
18: R =R =R =H, R =OH
3
1
3
2
4
2
3
4
1
4: R =R =H, R =R =OCH
19: R =R =R =H, R =C(CH )
3
3 3
1
2
4
3
1
2
3
4
5: R =R =R =H, R =CH
20: R =R =H, R -C H -R
3
4
4
For comparison to so far mentioned molybdenum(VI) catalysts (1-10),
oxidovanadium(V) complexes (Fig. 4) with the same chiral tridentate Schiff base ligands, 11-
0, have been also tested as catalysts in the oxidation of styrene, employing the same oxidants
2
and using optimized amounts of catalysts (1 mol%), reaction time (6 h), oxidant to styrene
molar ratio (3:1), temperature (80 °C) and solvent (acetonitrile). Synthesis, structure,
spectroscopic characterization and catalytic properties in asymmetric sulfoxidation of these
compounds have been reported by us earlier [19]. Using TBHP as an oxidant (Table 3, entries
1
1-20), conversion is quite similar (65-76%), as in the cases of molybdenum(VI) complexes,
but much better than in case of the other dioxidovanadium(V) complexes (20-35%
conversion) reported earlier [34]. On the other hand, the selectivity toward styrene oxide is
considerably lower (52-63%). When aqueous 30% H
2
2
O is employed as an oxidant, distinctly
much lower conversion of styrene has been observed (27-36%). Moreover, in comparison to
1
1

1
-10, the selectivity much more differ (Table 3, entries 1-10) and, surprisingly, benzaldehyde
is the main product (78–83%). Moreover, as well as for reported earlier dioxidovanadium(V)
complexes [34], styrene oxide is the most expected product but its selectivity goes down
considerably (>16%).
Table 3. Catalytic oxidation of styrene by in presence of 1 mol% vanadium(V) Schiff base
complexes in acetonitrile.
Product selectivity (%)
Entry Catalyst
Oxidant
Conv. (%)
StO
BzA
BzAC
PhAA
PhED
1
2
3
4
5
6
7
8
9
11
12
13
14
15
16
17
18
19
20
11
12
13
14
15
16
17
18
19
20
H
H
H
H
H
H
H
H
H
H
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
2
2
2
2
2
2
2
2
2
2
O
O
O
O
O
O
O
O
O
O
2
2
2
2
2
2
2
2
2
2
31
36
34
32
27
28
31
34
35
30
74
76
70
68
73
67
65
72
70
73
13
14
16
13
14
15
12
13
14
13
55
63
61
58
57
53
52
62
57
55
81
79
80
82
81
79
79
82
78
83
39
27
32
35
34
40
39
29
37
38
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
6
7
4
5
5
6
9
5
8
4
-
-
-
-
-
-
-
-
-
1
0
1
2
3
4
5
6
7
8
9
0
-
-
1
1
1
1
1
1
1
1
1
2
4
6
4
4
5
5
6
5
4
4
2
4
3
3
4
2
3
4
2
3
-
3
.2.3. Oxidation of cyclohexene
The catalytic potential of the 1-10 complexes has been also found for the oxidation of
cyclohexene in presence of aqueous 30% H or tert-butyl hydroperoxide (TBHP) in decane
2
O
2
as an oxidant to give cyclohexene oxide, cyclohexane-1,2-diol, 2-cyclohexene-1-ol and 2-
cyclohexene-1-one (Fig. 5). The formation of all these products, conversion and selectivity
are presented in Table 4.
As in the case of the oxidation of styrene, complex 3 was taken as a representative
catalyst for optimizing reaction conditions. In this purpose, the same amounts of catalyst (0.5,
1
and 2 mol%) and the oxidants (1:1, 2:1 and 3:1 molar ratios to cyclohexene) were tested.
Different solvents and temperatures of the reaction mixture were also tested and, as earlier,
the best results were found with 1,2-dichloroethane (DCE) as a solvent and at 80 °C.
Three different molar ratios of aqueous 30% H
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 H to cyclohexene 1:1 molar ratio, a maximum of 32% conversion was
achieved. Increasing the ratio to 2:1 improved the conversion to 68%, while 3:1 ratio has
shown a maximum of 69% conversion. Further increment of H shows no improved
2
O
2
or tert-butyl hydroperoxide (TBHP)
(
2
O
2
2
O
2
1
2

conversion only marginally, therefore a 2:1 ratio being considered adequate In case of using
TBHP as an oxidizing agent, increasing the TBHP:cyclohexene molar ratio from 1:1 to 2:1
and 3:1 improved the conversion from 38 to 89 and 90%, respectively. As in the previous
case, there was no significant conversion improvement upon further addition of any oxidant.
Figure 5. Various products of catalytic oxidation of cyclohexene.
OH
O
+
OH
ChO
ChDL
1
mol% catalyst
H O or TBHP
2
2
OH
O
cyclohexene
+
ChOL
ChON
Table 4. Catalytic oxidation of cyclohexene in presence of 1 mol% molybdenum(VI) Schiff
base complexes in DCE.
a
Product selectivity (%)
Entry Catalyst
Oxidant
Conv. (%)
ChO
ChOL
ChON
ChDL
1
2
3
4
5
6
7
8
9
1
2
3
4
5
6
7
8
9
10
1
2
3
4
5
6
7
8
H
H
H
H
H
H
H
H
H
H
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
2
2
2
2
2
2
2
2
2
2
O
O
O
O
O
O
O
O
O
O
2
2
2
2
2
2
2
2
2
2
62
70
68
67
61
63
61
69
63
68
80
81
89
87
89
82
85
90
11
13
9
10
11
10
9
10
11
10
85
89
87
84
79
81
82
84
54
61
66
62
58
64
59
62
65
63
15
11
13
16
21
19
18
16
29
22
20
21
27
21
26
25
20
23
-
-
-
-
-
-
-
-
6
4
5
7
4
5
6
3
4
4
-
-
-
-
-
-
-
-
1
0
1
2
3
4
5
6
7
8
1
1
1
1
1
1
1
1
1
3

1
2
9
0
9
10
TBHP
TBHP
87
83
80
84
20
16
-
-
-
-
a
ChO – cyclohexene oxide, ChOL – 2-cyclohexene-1-ol, ChON – 2-cyclohexene-1-one,
ChDL – cyclohexane-1,2-diol.
Similarly, for three different amounts (i.e. 0.5, 1 and 2 mol%) of catalyst and oxidant
to cyclohexene molar ratio of 2:1 under above reaction conditions, 0.5 mol% gave only 22%
(
H
2
O
2
) and 31% (TBHP) oxidative conversion, while 1 mol% and 2 mol% of catalyst have
shown a maximum conversion with 68% for H and 89% for TBHP. Thus, 1 mol% of
O
2 2
catalyst may be considered sufficient enough to run the reaction under above conditions. A
blank reaction under the above reaction conditions gave with both oxidants ca. 4-5%
conversion.
Table 5. Catalytic oxidation of cyclohexene in presence of 1 mol% vanadium(V) Schiff base
complexes in acetonitrile.
Product selectivity (%)
Entry Catalyst
Oxidant
Conv. (%)
ChO
ChOL
ChON
ChDL
1
2
3
4
5
6
7
8
9
11
12
13
14
15
16
17
18
19
20
11
12
13
14
15
16
17
18
19
20
H
H
H
H
H
H
H
H
H
H
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
2
2
2
2
2
2
2
2
2
2
O
O
O
O
O
O
O
O
O
O
2
2
2
2
2
2
2
2
2
2
42
54
57
53
55
56
51
58
52
56
85
96
86
92
96
90
72
71
89
92
30
27
35
27
24
33
30
27
25
28
1
2
3
1
-
3
4
2
5
31
33
34
28
30
31
32
26
33
30
90
87
84
89
86
89
87
89
86
88
3
11
4
7
5
12
5
14
6
8
1
2
2
1
1
-
1
1
1
36
29
27
38
41
24
33
33
36
34
8
9
11
9
13
8
8
1
0
1
2
3
4
5
6
7
8
9
0
1
1
1
1
1
1
1
1
1
2
8
8
9
2
1
In these conditions, conversion of cyclohexene with 30% H
2
2
O as an oxidant (Table 4,
entries 1-10) is distinctly higher than for styrene (61-70%). Surprisingly, 1-10 catalysts are the
most selective toward 2-cyclohexene-1-ol (54-66%). Furthermore, the selectivity against 2-
cyclohexene-1-one (20-29%) is more noticeable than against cyclohexene oxide (9-13%) and
cyclohexane-1,2-diol (3-7%). The reason for the formation of the allylic oxidation products,
i.e. 2-cyclohexen-1-ol and 2-cyclohexen-1-one in higher selectivity may be preferential attack
of the activated C–H bond over the C=C bond [35].
When catalytic reactions have been performed under optimized conditions, but with
tert-butyl hydroperoxide (TBHP) in decane as an oxidant (Table 4, entries 11-20), 1-10 gave
1
4

much higher conversion of cyclohexene (80-90%). In contrast to catalytic reactions with
, molybdenum(VI) catalysts are distinctly more selective toward cyclohexene oxide (79-
9%). The rest amounts of 2-cyclohexene-1-ol found in the reaction mixture are by-products.
H
2
O
2
8
Very similar results were reported by Rayati et al. [24] with MoO {hnaphnptn} –
2
dioxidomolybdenum(VI) complex with symmetrical tetradentate Schiff base.
Vanadium(V) complexes have been also tested in the oxidation of cyclohexene and
results were compared with these for molybdenum(VI) complexes. When oxidation reactions
were performed with aqueous 30% H
2
O
2
as an oxidant, 42-58% conversion in acetonitrile and
6
h of contact time was found (Table 5, entries 1-10) and is distinctly lower than for 1-10. The
selectivity against cyclohexene oxide (24-35%), 2-cyclohexene-1-ol (26-34%) and
cyclohexane-1,2-diol (29-41%) is almost the same, only 2-cyclohexene-1-one (3-14%) is
present in the smallest amounts. When this catalytic reaction has been performed in the same
reaction conditions, but with tert-butyl hydroperoxide (TBHP) as an oxidant, 11-20 catalysts
gave very high up to 96% conversion (Table 5, entries 11-20). Surprisingly, in contrast to
reactions with 30% H
2
2
O and also molybdenum(VI) catalysts with TBHP as an oxidant, only
2
-cyclohexene-1-ol (84-90%) was the main product.
3
.2.4. Reactivity of catalysts with H
2
O
2
A variety of molybdenum(VI) complexes have been found to react with H
the corresponding oxidoperoxido complexes, but the isolation of such [MoO(O
2
O to form
2
2
+
2
)] Schiff
base compounds was unsuccessful in our attempts. Nevertheless, to establish generation of
such oxidoperoxido species and shed some light on the mechanism of these catalytic reactions
UV-Vis spectroscopy was involved. Spectral changes were monitored by recording a series of
spectra after the dropwise additions of DMSO solution of H
dioxidomolybdenum(VI) complex (Fig. 6).
2
O
2
to an example
Figure 6. Spectral changes observed during titration of 2 catalyst. The spectra recorded after
successive addition of one drop portions of aqueous 30% H (1.70 g, 15 mmol) dissolved in
2
O
2
.
-5
5
ml of DMSO to 2 ml of a 7.9 10 M solution of 2 in DMSO.
1
5

In an example procedure, the spectra have been recorded after successive addition of
one drop portions of aqueous 30% H (1.71 g, 15 mmol) dissolved in 5 ml of DMSO to 2
2
O
2
.
-5
ml of ca. 7.9 10 M solution of catalyst 2 in DMSO and the resultant spectroscopic changes
are presented in Fig. 5. Such titration with a dilute solution of the complex 2 causes a decrease
with only a marginal change in intensity of the 376 nm band, which belongs to a weak ligand-
*
to-metal charge transfer (LMCT) transition. On the other hand, the strong intraligand π-π
transition with 276 nm band increases its intensity considerably with a small shift to 267 nm
and finally disappears. In our opinion these changes indicate the interaction of complex 2 with
hydrogen peroxide and the plausible formation of the oxidoperoxidomolybdenum(VI)
complex in DMSO, which in the catalytic reaction finally transfers oxygen to an appropriate
organic substrate to give various oxidation products.
4
. Conclusion
New chiral dioxidomolybdenum(VI) complexes derived from Schiff base ligands,
monocondensation products of o-hydroxycarbonyl compounds with 1R,2S(‒)-norephedrine
were synthesized and characterized by IR, CD, UV-Vis techniques and also 1D and 2D NMR
spectroscopy. Moreover, the catalytic properties of the chiral catalysts in oxidation of
thioanisole and olefins (styrene and cyclohexene) have been studied.
The results of sulfoxidation reactions show that the observed yield and enantiomeric
excess significantly depend on the nature of the catalyst, especially in the aspect of catalysts
possessing electron donating groups to attain sufficient nucleophilicity by the molybdenum
atom. Finally, higher enantioselectivities can be obtain when the reactions are carried out in
lower temperatures.
The catalytic potentials of the dioxidomolybdenum(VI) complexes and
oxidovanadium(V) catalysts with the same chiral Schiff base ligands in oxidation of olefins
have been also studied and compared, choosing the oxidation of styrene and cyclohexene as
1
6

the model reactions. The molybdenum(VI) complexes are able to catalyze the oxidative
conversion of styrene to styrene oxide and benzaldehyde as main products. In comparison to
vanadium(V) complexes, the selectivities against styrene oxide are much more higher using
3
0% H
2
O
2
and slightly higher with TBHP as an oxidant. On the other hand, in the oxidation
was
of cyclohexene much better conversions were found, especially when 30% H
2
O
2
employed as an oxidant. Moreover, the excellent conversion (up to 96%), using TBHP as an
oxidant, for both molybdenum(VI) and vanadium(V) has been noted, with different
compounds as the main products, i.e. cyclohexene oxide and 2-cyclohexene-1-ol,
respectively.
Acknowledgements
This scientific work was supported by the Polish Ministry of Science and Higher
Education (DS/530-8216-D598-17).
References
[
[
[
[
[
[
[
[
[
[
[
1] R. Hille, Coord. Chem. Rev. 255 (2011) 1179.
2] M. Ribbe, Coord. Chem. Rev. 255 (2011) 1218.
3] T.D. Owens, A.J. Souers, J.A. Ellman, J. Org. Chem. 68 (2003) 3.
4] E.N. Jacobsen, Acc. Chem. Res. 33 (2000) 421.
5] A. Abiko, S. Masamune, Tetrahedron Lett. 33 (1992) 5517.
6] M.J. McKennon, A.I. Meyers, J. Org. Chem. 58 (1993) 3568.
7] K.C. Gupta, A.K. Sutar, Coord. Chem. Rev. 252 (2008) 1420.
8] M. Amini, M.M. Haghdoost, M. Bagherzadeh, Coord. Chem. Rev. 257 (2013) 1093.
9] S.-H. Hsieh, Y.-P. Kuo, H.-M. Gau, Dalton Trans. (2007) 97.
10] J. Hartung, Pure Appl. Chem. 77 (2005) 1559.
11] J. Hartung, S. Drees, M. Grab, P. Schmidt, I. Svoboda, H. Fuess, A. Murso, D. Stalke,
Eur. J. Org. Chem. (2003) 2388.
12] (a) Á. Gama, L.Z. Flores-López, G. Aguirre, M. Parra-Hake, R. Somanathan, T. Cole,
[
Tetrahedron: Asymmetry 16 (2005) 1167; (b) L.Z. Flores-Lopéz, M. Parra-Hake, R.
Somanathan, P.J. Walsh, Organometallics 19 (2000) 2153.
[
(
[
(
[
13] Q. Zeng, H. Wang, W. Weng, W. Lin, Y. Gao, X. Huang, Y. Zhao, New J. Chem. 29
2005) 1125.
14] A. Rezaeifard, I. Sheikhshoaie, N. Monadi, H. Stoeckli-Evans, Eur. J. Inorg. Chem.
2010) 799.
15] (a) A. Basak, A.U. Barlan, H. Yamamoto, Tetrahedron: Asymmetry 17 (2006) 508; (b)
A.P. da Costa, P.M. Reis, C. Gamelas, C.C. Romão, B. Royo, Inorg. Chim. Acta 361 (2008)
915.
1
[
[
[
[
[
[
16] C. Bolm, F. Bienewald, Angew. Chem. Int. Ed. Engl. 34 (1995) 2640.
17] Z. Li, H. Yamamoto, Acc. Chem. Res. 46 (2013) 506.
18] G. Romanowski, J. Kira, Polyhedron 117 (2016) 352.
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)
1
76.
[
[
[
[
22] V. Saheb, I. Sheikhshoaie, H. Stoeckli-Evans, Spectrochim. Acta A 95 (2012) 29.
23] M.ꢀ. ꢁudmaier, C. ꢂolꢃer, M. Volpe, ꢄ.C. Mꢅsch-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.
1
7

[
[
[
[
[
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, ꢆ. ꢇelaꢈ, M. Volpe, ꢄ.C. Mꢅsch-Zanetti, New J. Chem. 37
2013) 2139.
32] ꢉ. Traar, ꢁ.ꢊ. Schachner, ꢇ. Stanꢈe, ꢆ. ꢇelaꢈ, ꢄ.C. Mꢅsch-Zanetti, J. Mol. Catal. A:
Chem. 385 (2014) 54.
[
[
[
33] V. Hulea, E. Dumitriu, Appl. Catal. A: Gen. 277 (2004) 99.
34] M.R. Maurya, A. Kumar, M. Ebel, D. Rehder, Inorg. Chem. 45 (2006) 5924.
35] J.D. Koola, J.K. Kochi, J. Org. Chem. 52 (1987) 4545.
1
8

R2
R1
Grzegorz Romanowski*, Jaromir Kira
Polyhedron, (2017)
w
chiral dioxidomolybdenum(VI) complexes with
tridentate Schiff bases obtained by monocondensation
of 1R,2S(‒)-norephedrine with salicylaldehyde and its
derivatives were synthesized. The complexes were
O
3
R
O
N
O
Mo
S
Synthesis, characterization and catalytic characterized by IR, CD, UV-Vis and NMR
activity of dioxidomolybdenum(VI) complexes spectroscopy. Catalytic activity of these complexes
with tridentate Schiff bases derived from were tested in the oxidation of olefins using aqueous
R
4
O
30% H
2
O
2
or TBHP as an oxidant. Moreover, the
H C
3
1
R,2S(‒)-norephedrine
dioxidomolybdenum(VI) complexes have also ability
to catalyze the oxidation of thioanisole to methyl
2 2
phenyl sulfoxide in presence of aqueous 30% H O .
_
S
methanol
1
9