Synthesis, structure, spectroscopic characterization and catalytic activity of chiral dioxidomolybdenum(VI) Schiff base complexes derived from R(?)-2-amino-1-propanol

Article abstract of DOI:10.1016/j.ica.2018.08.022

Ten new chiral cis-dioxidomolybdenum(VI) complexes with tridentate Schiff bases were synthesized by monocondensation of R(?)-2-amino-1-propanol with salicylaldehyde and its derivatives. Their spectroscopic properties have been investigated UV-Vis, CD, IR, one- (¹H, ¹³C) and two-dimensional (COSY, gHSQC and NOESY) NMR techniques. The single-crystal X-ray analysis of the {R(?)-2-[(1-oxidopropyl)iminomethyl]-4-methylphenolato-κ³N,O,O’}(methanol)dioxidomolybdenum(VI) reveals a distorted octahedral geometry. After optimization of reaction conditions, catalytic activity of these complexes in oxidation of olefins, i.e. styrene and cyclohexene, with aqueous 30% H₂O₂ or tert-butyl hydroperoxide (TBHP) as an oxidant were tested. Moreover, the molybdenum(VI) Schiff base complexes are also able to catalyze the oxidation of thioanisole to methyl phenyl sulfoxide in presence of aqueous 30% H₂O₂.

Full text of DOI:10.1016/j.ica.2018.08.022

Accepted Manuscript
Synthesis, structure, spectroscopic characterization and catalytic activity of chi-
ral dioxidomolybdenum(VI) Schiff bases complexes derived from R(-)-2-ami-
no-1-propanol
Grzegorz Romanowski, Jaromir Kira, Michał Wera
PII:
S0020-1693(18)30925-3
https://doi.org/10.1016/j.ica.2018.08.022
ICA 18422
DOI:
Reference:
Inorganica Chimica Acta
To appear in:
Received Date:
Revised Date:
Accepted Date:
16 June 2018
16 August 2018
17 August 2018
Please cite this article as: G. Romanowski, J. Kira, M. Wera, Synthesis, structure, spectroscopic characterization
and catalytic activity of chiral dioxidomolybdenum(VI) Schiff bases complexes derived from R(-)-2-amino-1-
propanol, Inorganica Chimica Acta (2018), doi: https://doi.org/10.1016/j.ica.2018.08.022
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Synthesis, structure, spectroscopic characterization and catalytic activity
of chiral dioxidomolybdenum(VI) Schiff bases complexes
derived from R()-2-amino-1-propanol.
Grzegorz Romanowski*, Jaromir Kira, Michał Wera
Faculty of Chemistry, University of Gdańsk, Wita Stwosza 63, PL-80308 Gdańsk, Poland
Abstract
Ten new chiral cis-dioxidomolybdenum(VI) complexes with tridentate Schiff bases
were synthesized by monocondensation of R()-2-amino-1-propanol with salicylaldehyde and
its derivatives. Their spectroscopic properties have been investigated UV-Vis, CD, IR, one-
13
(1H, C) and two-dimensional (COSY, gHSQC and NOESY) NMR techniques. The single-
crystal X-ray analysis of the {R()-2-[(1-oxidopropyl)iminomethyl]-4-methylphenolato-
κ3N,O,O’}(methanol)dioxidomolybdenum(VI) reveals a distorted octahedral geometry. After
optimization of reaction conditions, catalytic activity of these complexes in oxidation of
olefins, i.e. styrene and cyclohexene, with aqueous 30% H2O2 or tert-butyl hydroperoxide
(TBHP) as an oxidant were tested. Moreover, the molybdenum(VI) Schiff base complexes are
also able to catalyze the oxidation of thioanisole to methyl phenyl sulfoxide in presence of
aqueous 30% H2O2.
1. Introduction
Molybdenum is used as an essential element in more than 40 naturally occurring
enzymes catalyzing various redox reactions. For plant growth, molybdenum is necessary in
the processes, which tie the element to the nitrogen cycle, e.g. atmospheric nitrogen fixation
by bacterial nitrogenases and key step of nitrogen assimilation by nitrate reductase [1]. The
FeMo-cofactor in bacterial nitrogenase is the only exception where molybdenum is directly
coordinated to amino acid side chains of the protein [2]. In case of all other enzymes this
second row transition element is bound to a unique tricyclic pterin complex called
molybdenum cofactor (Moco) in order to gain catalytic activity [3]. After its synthesis and
distribution to apoproteins, Moco was found to be 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, in
sulfite oxidase is involved in the oxidation of harmful sulfite to sulfate and finally, the last
step of abscisic acid (ABA) biosynthesis involves oxidation of its aldehyde catalyzed by
aldehyde oxidase [4].
Such enzymes can be modeled by molybdenum complexes with an appropriate ligand
and Schiff bases being a good example, have been successfully addressed due to their
common availability via condensation of amino compounds with aldehyde moieties [5]. The
application of molybdenum(VI) complexes as a stoichiometric reagent or as a catalyst are
mainly focused on the oxidation of olefins, alcohols, alkanes, sulfides and amines [6], but in
the context of using chiral molybdenum complexes as catalysts performing the
enantioselective oxidation still remains very limited [7,8]. Nevertheless, cis-
dioxidomolybdenum(VI) complexes with tridentate and tetradentate Schiff bases have been
successfully employed as catalysts in very efficient epoxidation of olefins (including styrene
_____________
* Corresponding author. E-mail address: grzegorz.romanowski@ug.edu.pl, tel.: +48 58
5235102, fax: +48 585235012
1

and cyclohexene) [9-12], oxidation of sulfides to sulfoxides [13-15] and synthesis of
bis(indolyl)methane derivatives [16].
Molybdenum(VI) complexes incorporating chiral Schiff base ligands have kept lately
our continuing interest [17,18]. In this paper we describe new ten cis-
dioxidomolybdenum(VI) complexes with ONO donor Schiff bases, products of a single
condensation of R()-2-amino-1-propanol with aromatic salicylaldehyde derivatives,
presented in Fig. 1. Very detailed investigation of their spectroscopic properties, i.e. UV-Vis,
IR, circular dichroism, one- and two-dimensional NMR have been also performed. Moreover,
the catalytic ability in enantioselective sulfoxidation of thioanisole utilizing aqueous 30%
H2O2 as an oxidant, has been studied. Finally, the dioxidomolybdenum(VI) complexes have
also shown their catalytic potential in oxidation of styrene and cyclohexene, in presence of
aqueous 30% H2O2 or tert-butyl hydroperoxide (TBHP) as an oxidant. Crystal and molecular
structures of dioxidomolybdenum(VI) complex, 4, {R()-2-[(1-oxidopropyl)iminomethyl]-4-
methylphenolato-κ3N,O,O’}(methanol)dioxidomolybdenum(VI), is also reported.
Figure 1. Structural formulae of dioxidomolybdenum(VI) complexes.
R1
R
R
R
2
3
4
2
R
R1
H
OCH3
H
H
H
H
H
H
H
H
H
1
2
O
R3
O
N
OCH3
3
O
Mo
S
4
H
H
H
CH3
Br
H
H
H
H
H
4
R
O
H
5
H
6
H
NO2
H
H3C
H
7
OH
H
C(CH3)3
C(CH3)3
H
H
8
_
S aqua or methanol
H
C(CH3)3
_
H
_
9
H
(CH)4
10
2. Experimental
2.1. Measurements
All chemicals and reagents were obtained from commercial sources and used without
further purification unless stated otherwise. Carbon, hydrogen and nitrogen contents were
determined on a Carlo Erba MOD 1106 elemental analyzer. Electronic spectra were measured
on the Perkin-Elmer LAMBDA 18 spectrophotometer and circular dichroism spectra with a
Jasco J-815 spectropolarimeter. 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
2

detector were used to analyze the reaction products in oxidation of olefins. Confirmation of
the identity of the reaction products have been made by GC-MS model Shimadzu GCMS-
QP2010 SE.
2.2. X-ray investigations
Diffraction measurements were made on a Oxford Diffraction Gemini R Ultra Ruby
CCD, using graphite-monochromatized Mo Kα radiation (λ = 0.71073 Å) at 295(2) K and
determination of the crystal class, orientation matrix, and accurate unit cell parameters was
performed according to established procedures. Data collection and processing parameters are
summarized in Table 1. The structure was solved by direct methods and refined with full-
matrix least squares on F2 with SHELXL-2014/7 [19]. All non-hydrogen atoms were refined
anisotropically. The positions of hydrogen atoms were calculated and treated as riding atoms
with fixed thermal parameters.
Table 1. Crystal data and structure refinement.
Compound
4
Empirical formula
Formula weight
Temperature (K)
Wavelength (Å)
Crystal system
Space group
C12H17NO5Mo
351.20
295(2)
0.71073
monoclinic
P21
Unit cell dimensions
a (Å)
b (Å)
11.1573(6)
10.9549(4)
12.3044(6)
110.141(6)
1411.97(13)
4
1.652
0.944
712
c (Å)
β (°)
Volume (Å3)
Z
Dcalc (g cm-3)
Absorption coefficient (mm-1)
F(000)
Crystal size (mm)
θ Range for data collection (°)
Limiting indices
0.990.720.36
3.39 to 26.00
-13  h  13,
-10  k  13,
-14  l  15
9728 / 4520 [Rint = 3.39%]
99.8
Reflections collected / unique
Completeness to 2θ (%)
Data / restraints / parameters
Goodness-of-fit on F2
4520 / 1 / 349
1.042
Final R indices [I>2σ(I)]
R indices (all data)
R1 = 3.09%, wR2 = 7.78%
R1 = 3.36%, wR2 = 8.00%
-0.01(5)
Absolute structure parameter
Largest diff. peak and hole (e Å-3)
0.414 and -0.657
3

2.3. Catalytic activity
2.3.1. Sulfoxidation
To a solution of catalyst (0.010 mmol) in 3 ml of CH2Cl2/MeOH mixture (7:3),
thioanisole (1.00 mmol) was added at 20 °C or room temperature, with 1,3,5-
trimethoxybenzene as internal standard. Aqueous 30% H2O2 was added (1.10 mmol) by small
portions and the resulting mixture was stirred. 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 evaporated to dryness. The solid product
dissolved in CDCl3 was analyzed (yield and ee value) by 1H NMR spectra in the presence of
chiral shift reagent - Eu(hfc)3 [20].
2.3.2. 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 catalyst (0.010
mmol) were heated at 80 °C for 1 h in 5 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. Different
amounts of catalysts and oxidants were also added to study their effect on the conversion and
selectivity of the reaction products.
2.4. Synthesis of dioxidomolybdenum(VI) complexes
The complexes were obtained in a following example procedure. A solution of 1 mmol
of R()-2-amino-1-propanol in MeOH (10 ml) was added to 1 mmol of an aromatic o-
hydroxyaldehyde (salicylaldehyde, 3-methoxysalicylaldehyde, 5-methoxysalicylaldehyde, 5-
methylsalicylaldehyde,
5-bromosalicylaldehyde,
5-nitrosalicylaldehyde,
4-
hydroxysalicylaldehyde, 3-tert-butylsalicylaldehyde, 3,5-di-tert-butylsalicylaldehyde, 2-
hydroxy-1-naphthaldehyde) in MeOH (10 ml) and heated with stirring 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 a precipitate was separated, filtered off
and washed with MeOH.
2.4.1. {R()-2-[(1-oxidopropyl)iminomethyl]phenolato-
κ3N,O,O’}(aqua)dioxidomolybdenum(VI) (1)
Yield 84%. Anal. Calc. for C10H11NO4Mo . H2O: C, 37.2; H, 4.1; N, 4.3. Found: C,
37.0; H, 4.2; N, 4.2%. IR (KBr, cm-1): 1640, 1598 νC=N, C=C); 895, 849 (νMo=O). UV-Vis
spectrum in DMSO [λmax (nm), ɛ (M-1 cm-1)] 269 (6700), 320 (1650). CD spectrum in DMSO
[λmax (nm), Δɛ (M-1 cm-1)] 276 (-3.06), 342 (-0.53). 1H NMR DMSO-d6, ppm): 8.66 (1H, s)
3
3
3
(azomethine); 7.62 (1H, d, J=7.5 Hz), 7.48 (1H, t, J=7.5 Hz), 6.96 (1H, t, J=7.5 Hz), 6.88
3
3
4
(1H, d, J=8.1 Hz) (aromatic); 4.53 (1H, dd, J=9.6 Hz, J=5.0 Hz), 4.06 (1H, ov)
(methylene); 4.05 (1H, ov) (methine); 1.36 (1H, d, 3J=5.8 Hz) (methyl). 13C NMR DMSO-d6,
ppm): 166.5 (azomethine); 162.7, 139.3, 131.4, 129.8, 121.4, 120.9 (aromatic); 79.0
(methylene); 65.8 (methine); 17.1 (methyl).
2.4.2. {R()-2-[(1-oxidopropyl)iminomethyl]-6-methoxyphenolato-
κ3N,O,O’}(aqua)dioxidomolybdenum(VI) (2)
4

Yield 91%. Anal. Calc. for C11H13NO5Mo . H2O: C, 37.4; H, 4.3; N, 4.0. Found: C,
37.3; H, 4.2; N, 3.9%. IR (KBr, cm-1): 1634, 1598 νC=N, C=C); 1259 (νasym(C-O)); 1042 (νsym(C-
O)); 928, 889 (νMo=O). UV-Vis spectrum in DMSO [λmax (nm), ɛ (M-1 cm-1)] 285 (6820), 359
(2570). CD spectrum in DMSO [λmax (nm), Δɛ (M-1 cm-1)] 284 (-4.17), 374 (-0.64). 1H NMR
DMSO-d6, ppm): 8.65 (1H, s) (azomethine); 7.15 (2H, t, 3J=7.9 Hz), 6.91 (1H, t, 3J=7.9 Hz)
3
4
(aromatic); 4.54 (1H, dd, J=9.6 Hz, J=5.0 Hz), 4.04 (1H, ov) (methylene); 4.03 (1H, ov)
3
13
(methine); 3.77 (3H, s) (methoxy); 1.36 (1H, d, J=5.8 Hz) (methyl). C NMR DMSO-d6,
ppm): 163.5 (azomethine); 151.6, 149.9, 125.7, 121.6, 119.9, 116.9 (aromatic); 78.1
(methylene); 64.7 (methine); 56.0 (methoxy); 17.2 (methyl).
2.4.3. {R()-2-[(1-oxidopropyl)iminomethyl]-4-methoxyphenolato-
κ3N,O,O’}(aqua)dioxidomolybdenum(VI) (3)
Yield 93%. Anal. Calc. for C11H13NO5Mo . H2O: C, 37.4; H, 4.3; N, 4.0. Found: C,
37.5; H, 4.3; N, 4.0%. IR (KBr, cm-1): 1638, 1560 νC=N, C=C); 1261 (νasym(C-O)); 1036 (νsym(C-
O)); 906, 861 (νMo=O). UV-Vis spectrum in DMSO [λmax (nm), ɛ (M-1 cm-1)] 273 (6900), 367
(2640). CD spectrum in DMSO [λmax (nm), Δɛ (M-1 cm-1)] 278 (-7.32), 384 (-1.04). 1H NMR
3
3
DMSO-d6, ppm): 8.64 (1H, s) (azomethine); 7.22 (1H, d, J=3.2 Hz), 7.11 (1H, dd, J=9.0
Hz, 4J=3.2 Hz), 6.84 (1H, d, 3J=9.0 Hz) (aromatic); 4.52 (1H, dd, 3J=9.6 Hz, 4J=5.0 Hz), 4.03
3
(1H, ov) (methylene); 4.03 (1H, ov) (methine); 3.75 (3H, s) (methoxy); 1.36 (1H, d, J=5.8
Hz) (methyl). 13C NMR DMSO-d6, ppm): 163.0 (azomethine); 155.6, 152.4, 122.6, 121.1,
120.5, 116.8 (aromatic); 78.0 (methylene); 65.0 (methine); 56.1 (methoxy); 17.3 (methyl).
2.4.4. {R()-2-[(1-oxidopropyl)iminomethyl]-4-methylphenolato-
κ3N,O,O’}(methanol)dioxidomolybdenum(VI) (4)
Yield 88%. Anal. Calc. for C11H13NO4Mo . CH3OH: C, 41.0; H, 4.9; N, 4.0. Found: C,
41.0; H, 4.9; N, 4.0%. IR (KBr, cm-1): 1632, 1561 νC=N, C=C); 933, 896 (νMo=O). UV-Vis
spectrum in DMSO [λmax (nm), ɛ (M-1 cm-1)] 278 (8140), 353 (2590). CD spectrum in DMSO
[λmax (nm), Δɛ (M-1 cm-1)] 278 (-7.21), 360 (-1.03). 1H NMR DMSO-d6, ppm): 8.61 (1H, s)
3
4
3
(azomethine); 7.35 (1H, dd, J=8.5 Hz, J=2.3 Hz), 7.26 (1H, t, J=9.5 Hz), 6.81 (1H, d,
3J=8.5 Hz) (aromatic); 4.54 (1H, dd, J=9.6 Hz, J=5.0 Hz), 4.05 (1H, ov) (methylene); 4.04
(1H, ov) (methine); 2.31 (3H, s) (methyl); 1.36 (1H, d, J=5.8 Hz) (methyl); 4.11 (1H, q),
3.18 (3H, d) (MeOH). C NMR DMSO-d6, ppm): 162.6 (azomethine); 156.3, 142.4, 123.6,
3
4
3
13
121.5, 120.7, 117.2 (aromatic); 78.3 (methylene); 65.2 (methine); 56.1 (methyl); 49.0
(MeOH); 17.3 (methyl).
2.4.5. {R()-2-[(1-oxidopropyl)iminomethyl]-4-bromophenolato-
κ3N,O,O’}(methanol)dioxidomolybdenum(VI) (5)
Yield 85%. Anal. Calc. for BrC10H10NO4Mo . CH3OH: C, 31.8; H, 3.4; N, 3.4. Found:
C, 31.7; H, 3.4; N, 3.3%. IR (KBr, cm-1): 1637, 1591 νC=N, C=C); 928, 896 (νMo=O). UV-Vis
spectrum in DMSO [λmax (nm), ɛ (M-1 cm-1)] 272 (9290), 347 (2990). CD spectrum in DMSO
[λmax (nm), Δɛ (M-1 cm-1)] 277 (-5.48), 355 (-0.76).1H NMR DMSO-d6, ppm ): 8.63 (1H, s)
3
4
3
(azomethine); 7.73 (1H, s), 7.58 (1H, dd, J=8.5 Hz, J=2.3 Hz), 6.91 (1H, d, J=8.5 Hz)
3
4
(aromatic); 4.53 (1H, dd, J=9.6 Hz, J=5.0 Hz), 4.04 (1H, ov) (methylene); 4.04 (1H, ov)
(methine); 1.37 (1H, d, J=5.8 Hz) (methyl); 4.11 (1H, q), 3.18 (3H, d) (MeOH). C NMR
3
13
5

DMSO-d6, ppm): 163.4 (azomethine); 155.8, 152.6, 122.4, 121.0, 120.4, 116.9 (aromatic);
78.2 (methylene); 65.4 (methine); 49.1 (MeOH); 17.4 (methyl).
2.4.6. {R()-2-[(1-oxidopropyl)iminomethyl]-4-nitrophenolato-
κ3N,O,O’}(methanol)dioxidomolybdenum(VI) (6)
Yield 87%. Anal. Calc. for C10H10N2O6Mo . CH3OH: C, 34.6; H, 3.7; N, 7.3. Found: C,
34.6; H, 3.8; N, 7.4%. IR (KBr, cm-1): 1646, 1612 νC=N, C=C); 928, 901 (νMo=O). UV-Vis
spectrum in DMSO [λmax (nm), ɛ (M-1 cm-1)] 345 (16610). CD spectrum in DMSO [λmax
1
(nm), Δɛ (M-1 cm-1)] 278 (-4.95), 356 (0.45). H NMR DMSO-d6, ppm): 8.85 (1H, s)
3
3
4
(azomethine); 8.68 (1H, d, J=2.7 Hz), 8.28 (1H, dd, J=9.2 Hz, J=2.8 Hz), 7.05 (1H, d,
3J=9.2 Hz) (aromatic); 4.64 (1H, dd, J=9.6 Hz, J=5.0 Hz), 4.14 (1H, ov) (methylene); 4.14
(1H, m) (methine); 1.42 (1H, d, 3J=5.8 Hz) (methyl); 4.14 (1H, ov), 3.16 (3H, s) (MeOH). 13C
NMR DMSO-d6, ppm): 166.5 (azomethine); 162.7, 139.3, 131.4, 129.8, 121.4, 120.9
(aromatic); 79.0 (methylene); 65.8 (methine); 48.9 (MeOH); 17.1 (methyl).
3
4
2.4.7. {R()-2-[(1-oxidopropyl)iminomethyl]-5-hydroxyphenolato-
κ3N,O,O’}(aqua)dioxidomolybdenum(VI) (7)
Yield 90%. Anal. Calc. for C10H11NO5Mo . H2O: C, 37.4; H, 4.3; N, 4.0. Found: C,
37.2; H, 4.2; N, 4.1%. IR (KBr, cm-1): 1630, 1608 νC=N, C=C); 926, 887 (νMo=O). UV-Vis
spectrum in DMSO [λmax (nm), ɛ (M-1 cm-1)] 288 (9680), 339 (3220). CD spectrum in DMSO
[λmax (nm), Δɛ (M-1 cm-1)] 287 (-3.06), 342 (-1.67). 1H NMR DMSO-d6, ppm): 10.34 (1H, s)
(hydroxyl); 8.47 (1H, s) (azomethine); 7.41 (1H, d, 3J=8.5 Hz), 6.41 (1H, d, 3J=8.5 Hz), 6.24
3
4
3
4
(1H, s) (aromatic); 4.49 (1H, dd, J=9.6 Hz, J=5.0 Hz), 4.04 (1H, dd, J=9.4 Hz, J=6.2 Hz)
(methylene); 3.99 (1H, m) (methine); 1.33 (1H, d, 3J=6.3 Hz) (methyl). 13C NMR DMSO-d6,
ppm): 164.0 (azomethine); 162.9, 162.0, 136.2, 114.4, 108.8, 105.0 (aromatic); 78.4
(methylene); 65.0 (methine); 17.8 (methyl).
2.4.8. {R()-2-[(1-oxidopropyl)iminomethyl]-6-tert-butylphenolato-
κ3N,O,O’}(methanol)dioxidomolybdenum(VI) (8)
Yield 87%. Anal. Calc. for C14H19NO4Mo . CH3OH: C, 45.8; H, 5.9; N, 3.6. Found: C,
46.0; H, 5.8; N, 3.7%. IR (KBr, cm-1): 1640, 1588 νC=N, C=C); 919, 880 (νMo=O). UV-Vis
spectrum in DMSO [λmax (nm), ɛ (M-1 cm-1)] 278 (7490), 352 (1810). CD spectrum in DMSO
[λmax (nm), Δɛ (M-1 cm-1)] 280 (-5.39), 357 (-0.92). 1H NMR DMSO-d6, ppm): 8.63 (1H, s)
3
3
3
(azomethine); 7.11 (1H, d, J=7.8 Hz), 7.07 (1H, d, J=7.8 Hz), 6.80 (1H, t, J=7.8 Hz)
3
4
(aromatic); 4.51 (1H, dd, J=9.6 Hz, J=5.0 Hz), 4.03 (1H, ov) (methylene); 4.02 (1H, ov)
(methine); 1.39 (9H, s) (tert-butyl); 1.31 (1H, d, 3J=5.8 Hz) (methyl). 4.12 (1H, q), 3.17 (3H,
d) (MeOH). 13C NMR DMSO-d6, ppm): 162.8 (azomethine); 155.4, 152.0, 121.9, 121.0,
120.1, 116.2 (aromatic); 77.8 (methylene); 65.2 (methine); 49.3 (MeOH); 34.0, 29.8 (tert-
butyl); 17.2 (methyl).
2.4.9. {R()-2-[(1-oxidopropyl)iminomethyl]-4,6-di-tert-butylphenolato-
κ3N,O,O’}(methanol)dioxidomolybdenum(VI) (9)
Yield 84%. Anal. Calc. for C18H27NO4Mo . CH3OH: C, 50.8; H, 7.0; N, 3.1. Found: C,
50.6; H, 6.8; N, 3.2%. IR (KBr, cm-1): 1633, 1561 νC=N, C=C); 925, 898 (νMo=O). UV-Vis
spectrum in DMSO [λmax (nm), ɛ (M-1 cm-1)] 278 (7620), 356 (1780). CD spectrum in DMSO
6

[λmax (nm), Δɛ (M-1 cm-1)] 281 (-5.95), 366 (-0.95). 1H NMR DMSO-d6, ppm): 8.67 (1H, s)
3
3
(azomethine); 7.43 (1H, d, J=2.5 Hz), 7.38 (1H, d, J=2.5 Hz) (aromatic); 4.53 (1H, dd,
3J=9.6 Hz, 4J=5.0 Hz), 4.03 (1H, ov) (methylene); 4.03 (1H, ov) (methine); 1.39 (9H, s), 1.31
(9H, s) (tert-butyl); 1.37 (1H, d, 3J=5.8 Hz) (methyl); 4.11 (1H, q), 3.18 (3H, d) (MeOH). 13C
NMR DMSO-d6, ppm): 163.1 (azomethine); 155.9, 152.6, 122.7, 121.3, 120.5, 116.9
(aromatic); 77.6 (methylene); 65.1 (methine); 49.1 (MeOH); 35.1, 34.2, 31.4, 30.0 (tert-
butyl); 17.3 (methyl).
2.4.10. {R()-2-[(1-oxidopropyl)iminomethyl]naphtholato-
κ3N,O,O’}(methanol)dioxidomolybdenum(VI) (10)
Yield 89%. Anal. Calc. for C14H13NO4Mo . CH3OH: C, 46.5; H, 4.4; N, 3.6. Found: C,
46.6; H, 4.5; N, 3.7%. IR (KBr, cm-1): 1624, 1551 νC=N, C=C); 912, 881 (νMo=O). UV-Vis
spectrum in DMSO [λmax (nm), ɛ (M-1 cm-1)] 274 (6400), 308 (8980), 380 (3150). CD
spectrum in DMSO [λmax (nm), Δɛ (M-1 cm-1)] 274 (-2.69), 322 (-0.43), 395 (0.41). 1H NMR
3
3
DMSO-d6, ppm): 9.53 (1H, s) (azomethine); 8.44 (1H, d, J=8.6 Hz), 8.05 (1H, d, J=9.0
3
3
3
Hz), 7.90 (1H, d, J=8.0 Hz), 7.62 (1H, t, J=7.4 Hz), 7.43 (1H, t, J=7.4 Hz), 7.15 (1H, d,
3J=9.0 Hz) (aromatic); 4.54 (1H, dd, J=9.6 Hz, J=5.0 Hz), 4.19 (1H, dd, J=9.4 Hz, J=5.2
3
4
3
4
3
Hz) (methylene); 4.29 (1H, m) (methine); 1.47 (1H, d, J=6.5 Hz) (methyl); 4.18 (1H, ov),
3.16 (3H, s) (MeOH). C NMR DMSO-d6, ppm): 162.0 (azomethine); 158.8, 136.1, 133.4,
13
129.3, 128.5, 128.3, 124.3, 122.2, 121.2, 112.0 (aromatic); 78.1 (methylene); 66.2 (methine);
48.9 (MeOH); 18.8 (methyl).
3. Results and discussion
3.1. Crystal and molecular structure
The crystallographic data of 4 are summarized in Table 1 and the selected bond
distances and angles are given in Table 2.
To the best of our knowledge, there are only one report on crystal and molecular
structures of cis-dioxidomolybdenum(VI) complex incorporating tridentate Schiff base ligand
derived from chiral amino alcohols [21]. Moreover, despite extensive studies, so far only few
reports describe such kind of compounds, but achiral ones [10, 22-26]. Similarly to compound
4, in most structures ethanol (solvent) and water molecules are coordinated, but after
crystallization from DMSO this solvent can also complete the coordination sphere. Moreover,
isolated case about formation of such five-coordinate complex without any solvent
coordinated has been also reported [21].
The asymmetric unit of 4 contains two symmetry independent complex molecules.
Our analysis is restrained to the molecule with Mo1 molybdenum atom, shown in Fig. 2. The
complex adopts a six-coordinate, distorted octahedral geometry. The tridentate Schiff base
ligand coordinate to the molybdenum atom by azomethine nitrogen (Mo1–N1 = 2.276(4) Å),
deprotonated phenolate (Mo1–O1 = 1.944(3) Å) and alcoholate (Mo1–O2 = 1.944(4) Å)
oxygen atoms. Finally, two oxido groups (O3 and O4) with similar distances to molybdenum
atom (1.708(4) and 1.686(4) Å, respectively) and one ethanol molecule (Mo1–O5 = 2.380(4)
Å) complete the coordination sphere. The equatorial plane is defined by the tridentate Schiff
base ligand and an oxygen atom of oxido group (O3), while the oxygen atom of second oxido
group (O4) and the oxygen atom from the solvent molecule (O5) occupy axial positions with
the trans angle of 170.28(19)°. The molybdenum atom is displaced from the least squares
equatorial plane toward the terminal oxido group by 0.328 Å. Generally, the values of all
these distances and the trans angle correspond well with those reported for similar cis-
7

dioxidomolybdenum(VI) complexes with tridentate Schiff bases derived from amino alcohols
[21-26]. In addition, it is worth to mentioning that distances between molybdenum atoms and
oxygen atom of ethanol or water molecules are in the range 2.324 to 2.388 Å, but can
distinctly decrease to the value of 2.303 or 2.264 Å with DMSO solvent molecule coordinated
to similar complex [10,25,26].
Table 2. Selected bond lengths (Å), angles (°) and hydrogen bonding parameters (Å,°) for 4.
Mo1-O1
Mo1-O2
Mo1-O3
Mo1-O4
Mo1-O5
Mo1-N1
O1-Mo1-O2
O1-Mo1-O3
O1-Mo1-O4
O2-Mo1-O3
O2-Mo1-O4
1.944(3)
1.944(4)
1.708(4)
1.686(4)
2.380(4)
2.276(4)
151.76(16)
101.20(19)
97.6(2)
O3-Mo1-O4
O1-Mo1-N1
O2-Mo1-N1
O3-Mo1-N1
O4-Mo1-N1
O1-Mo1-O5
O2-Mo1-O5
O3-Mo1-O5
O4-Mo1-O5
N1-Mo1-O5
106.2(2)
81.04(14)
75.24(16)
158.92(19)
94.11(18)
78.90(17)
80.78(15)
83.41(18)
170.28(19)
76.42(15)
95.72(19)
99.14(19)
DH···A
D-H
H···A
D···A
DH···A
O25-H25B···O2
O5-H5B···O22
C27-H27A···O4i
C7-H7A···O23ii
0.93
0.93
0.93
0.93
1.80
1.72
2.60
2.53
2.664(6)
2.637(6)
3.165(7)
3.253(7)
154
169
120
135
Symmetry code: i) x, y+1, z; ii) –x+2, y–½, –z+1.
Figure 2. An ORTEP drawing of 4 molecule with atom-numbering scheme. Thermal
ellipsoids drawn at the 50% probability level. H atoms are shown as small spheres of arbitrary
radius.
8

The tridentate Schiff base ligand combines with the molybdenum atom to give one
six-membered and one five-membered chelate rings. The six-membered chelate ring,
involving Mo1, O1, C1, C6, C7 and N1 atoms, is basically planar, whereas the five-
membered chelate ring containing Mo1, O2, C9, C8 and N1 atoms adopts an envelope
conformation on C9 with pseudorotation parameters P = 232.7(4)° and Tau(M) = 47.2(3)° for
reference bond Mo1–O2 [27].
The crystal structure of the complex 4 is stabilized by the intermolecular hydrogen
bonds linking two adjacent complex molecules, i.e. hydrogen bonds between alcoholate
oxygen atom of one molecule and hydroxyl group of coordinated ethanol molecule of
neighbouring complex molecule, result in formation of dimers (Fig. 3). Apart from the
intermolecular hydrogen bonds there are also C–H···O close contacts additionally stabilizing
interactions between complex molecules (Table 2).
Figure 3. The arrangement of the molecules of 4 in the crystal structure viewed
approximately along c-axis. The intermolecular hydrogen bonds and C–H···O close contacts
are represented by dashed lines.
3.2. Spectroscopic properties
Spectroscopic properties of the complexes, i.e. IR, UV-Vis, circular dichroism, 1H and
13C NMR data are listed in Section 2.
9

All compounds exhibit in their IR spectra strong C=C vibrations of the salicylaldehyde
moiety at 1551-1612 cm−1, but also sharp bands due to C=N stretches at 1624-1646 cm−1,
which may be assigned to the azomethine groups of Schiff base ligands coordinated to
dioxidomolybdenum(VI) cation [24,25]. In case of compounds 2 and 3, with methoxy
substituents attached to aromatic ring of salicylaldehyde moiety, antisymmetric and
symmetric C-O stretches have been found at ca. 1260 and 1040 cm−1, respectively.
Furthermore, all 1-10 complexes display two sharp bands at 895-933 and 849-901 cm-1 due to
the νasym(O=Mo=O) and νsym(O=Mo=O) modes, respectively, which indicate the presence of a
cis-[MoVIO2] structure [28].
The cis-dioxidomolybdenum(VI) complexes were also characterized by their UV-Vis
and circular dichroism spectra in DMSO. Intraligand π-π* transitions are strong and appear
very intensely in 269-288 nm region with ɛmax = 6400-9680 dm3mol-1cm-1 Furthermore, the
low-energy absorptions recorded for all compounds between 347-385 nm (ɛmax = 1650-3220
dm3mol-1cm-1) are assigned to a ligand-to-metal charge transfer (LMCT) transition arising
from the phenolate oxygen pπ orbital to an empty d orbital of molybdenum atom [29].
Additionally, the spectrum of 10 displays additional band at 308 nm (ɛmax = 8980) and, in the
case of complex 6 derived from 5-nitrosalicylaldehyde, there are only single intense bands at
345 nm (ɛmax = 16610), similar to oxidovanadium(V) complex with the same Schiff base
ligand, but opposite chirality, with band at 343 nm [30]. The circular dichroism spectra
revealed the same bands in the 274-287 nm and the 342-374 nm range of the same origin as
electronic spectra with very strong positive sign of the Cotton effects, i.e. with Δɛ values from
2.69 to 7.32 and from 0.43 to 1.67 M-1 cm-1, respectively. Similarly, as in the case of
UV-Vis spectrum, complex 10 derived from 2-hydroxy-1-naphthaldehyde displays two strong
bands with a negative sign of the Cotton effects at 274 and 322, but with a positive sign of the
Cotton effect at 395 nm. Moreover, compund 6 also exhibit band with a positive sign of the
Cotton effect at 356 nm.
The NMR spectra of all the molybdenum(VI) complexes were recorded in DMSO-d6.
The assignment of all proton and carbon signals were made on the basis of their intensity,
13
coupling patterns and chemical shifts using one- (1H and C) and two-dimensional (COSY,
1
gHSQC and NOESY) techniques. The H spectra of all 1-10 complexes have shown the
presence of azomethine proton signals proving monocondensation of all salicylaldehyde
derivatives with R()-2-amino-1-propanol, similar to reported earlier oxidovanadium(V)
complexes derived from the same Schiff base ligand [30]. Two-dimensional NMR
1
experiments were used for complete assignments and identification of all of H and 13C
signals and for establishing a connection and proximity between all protons and their
attachment to carbon atoms. For example, COSY spectrum of compound 3 exhibit cross-
peaks between two methylene protons (at 4.03 and 4.52 ppm) and the methine proton signal at
4.03 ppm. Chemical shifts of the methine proton and one of the methylene protons are
identical (4.03 ppm), but their origin have been confirmed by gHSQC spectrum, which shows
cross-peaks with 65.0 ppm and 78.0 ppm carbon signals, where the latter exhibits a second
cross-peak with 4.52 ppm proton signal. Moreover, a cross-peak between the methyl proton
doublet at 1.36 ppm and the methine proton signal at 4.03 ppm has been also observed. The
NOESY spectrum of 3 revealed additional information about close spatial proximity between
protons. The cross-peaks between its azomethine proton with signal at 8.64 ppm and the
signals at 1.36 (methyl protons), 4.03 (methine proton) and 7.22 ppm of one of the aromatic
protons doublet have been found. Finally, just as expected, no corresponding cross-peaks
between the signals of the azomethine proton and two methylene protons at 4.03 and 4.52
ppm could be observed.
10

3.3. Catalytic activity studies
3.3.1. Sulfoxidation
Under the optimized reaction conditions, the catalytic activity of the cis-
dioxidomolybdenum(VI) complexes 1-10 in enantioselective oxidation of model prochiral
sulfide, i.e. methyl phenyl sulfide (thioanisole) into its corresponding sulfoxide in the
presence of aqueous 30% H2O2 have been tested (Fig. 4). A slight excess of 1.10 equivalents
of the oxidant in these sulfoxidation reactions was used basing on a sulfide substrate.
Furthermore, optimized amounts of 1 mol% of catalyst were used in a mixture of two
solvents, i.e. CH2Cl2 and CH3OH (7:3). Asymmetric sulfoxidation reactions tend to give the
best enantioselectivities with dichloromethane as an aprotic solvent, but methanol was
necessary for a better mixing of the aqueous oxidant with the halogenated solvent [15,31].
Moreover, protic solvents can significantly enhance yield and selectivity of sulfoxide [22].
The sulfoxidation reaction performed without presence of any catalysts has given only ca. 2-
3% conversion. In addition it is noteworthy that under the given conditions no over oxidation
to the sulfone could be observed. The results of sulfoxidation studies are listed in Table 3.
Figure 4. Sulfoxidation of thioanisole catalyzed by dioxidomolybdenum(VI) complexes.
.
.
O
S
S
1 mol% catalyst
30% H O
CH3
CH3
2 2
Table 3. Catalytic oxidation of thioanisole by aqueous 30% H2O2 in presence of 1 mol%
molybdenum(VI) Schiff base complexes as catalysts.
Entry
Catalyst
Yield (%)
T (°C)
t (min)
ee (%) a
1
2
3
4
5
6
7
8
9
10
11
12
13
14
82
84
85
84
81
86
80
84
85
83
88
90
92
88
rt
rt
rt
rt
rt
rt
rt
rt
rt
45
45
45
45
45
45
45
45
45
15
18
22
19
13
16
13
15
16
21
22
26
20
24
1
2
3
4
5
6
7
8
9
10
2
3
4
10
rt
45
210
210
210
210
20
20
20
20
a All sulfoxides are in R configuration.
11

For complexes 2, 3, 4 and 10 as catalysts in oxidation of thioanisole (Table 2, entries
2, 3, 4 and 10) the best enatioselectivities have been observed. All molybdenum(VI) catalysts
yielded 80-86% overall conversion within 45 min reaction time and enantiomeric excesses
with up to 22% for the R-configured sulfoxide were obtained. When sulfoxidation was carried
out at 20 °C with 2, 3, 4 and 10 catalysts, enantioselectivities slightly increased to value of
20-26% and also conversions improved up to 92% after 210 min of the reaction time (Table 2,
entries 11-14). It was reported earlier by Mimoun et al. that for catalytic oxidation processes
for a number of types of organic substrates very important is sufficient nucleophilic center
[32]. In our opinion, the reason of the best enantioselectivity achieved by 3 is the highest
electron-donating resonance effect of para-substituted methoxy group where a higher electron
density on the phenolate oxygen is observed, helping to improve attainment of sufficient
nucleophilicity by the metal centre.
3.3.2. Oxidation of styrene
To achieve optimized reaction conditions for a maximum conversion of styrene
oxidation different parameters were taken under consideration, i.e. amount of a catalyst (0.5, 1
and 2 mol%) and oxidant (1:1, 2:1 and 3:1 molar ratios to substrate), different solvents and
temperature of the reaction mixture with the representative catalyst, 3, were tested.
Considering the highest conversion and reaction rate we found 1,2-dichloroethane 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 [33]. Moreover,
as reported earlier [34], it was concluded that the higher reactions temperature can be also
responsible for obtaining better yields and reaction rates.
After optimization of reaction conditions the oxidation of styrene with the 1-10
complexes as catalysts in 1,2-dichloroethane (DCE) was performed at 80 °C in the presence
of aqueous 30% H2O2 or tert-butyl hydroperoxide (TBHP) in decane as an oxidant. In these
reaction conditions styrene oxide, 1-phenylethane-1,2-diol, phenylacetaldehyde,
benzaldehyde and benzoic acid were obtained as the oxidation products (Fig. 5).
To study the effect of amount of oxidant to substrate, its different molar ratios (1:1,
2:1 and 3:1) to styrene (1.00 mmol) and catalyst 3 (0.010 mmol) were employed in DCE (5
ml), and the oxidation was carried out for 1 h of contact time at 80 °C Taking 1:1 molar ratio
of H2O2 to styrene, no more than 12% conversion was observed. Conversion improvement to
28% with to 2:1 ratio and finally with 3:1 molar ratio a maximum of 29% conversion was
achieved. Further addition of the oxidant did not improve the overall conversion yield,
therefore a 2:1 molar ratio has been shown to be adequate When TBHP was taken into
consideration, we have tested the same molar ratio of TBHP:styrene (1:1 to 2:1 and 3:1) and
the conversion has changed from 42 to 71 and 74%, respectively. Addition of this oxidant in
higher molar ratios has improved conversion marginally, so again 2:1 molar ratio was chosen
for further studies.
In the next step of reaction conditions optimization, three different amounts of catalyst
3 (0.5, 1 and 2 mol%) and 2:1 molar ratio of oxidant to styrene have been studied. After
addition of 0.5 mol%, only 9% (with 30% H2O2) and 47% (with TBHP) conversion has been
achieved. When 1 and 2 mol% of the catalyst was added to the reaction mixture very similar
results were found with 28 and 71% conversion for 30% H2O2 and TBHP, respectively.
Taking these results under consideration, all oxidation reactions with 1 mol% of catalysts
have been performed. Under the same reaction conditions the oxidation of styrene without
presence of any catalysts were tested, giving for both oxidants only ca. 2-4% conversion.
12

Figure 5. Various oxidation products of styrene catalyzed by molybdenum(VI) complexes.
HO
OH
PhED
O
O
HO
O
1 mol% catalyst
H O or TBHP
2
2
styrene
StO
BzA
BzAC
O
PhAA
The selectivity for all reaction products and the percentage conversion of styrene are
summarized in Table 4. Using tert-butyl hydroperoxide (TBHP) in decane as an oxidant,
under the optimized reaction conditions, all catalysts gave similar conversion (65-72%) in
comparison to our previous reports with such chiral catalyst derived from S(+)-1-amino-2-
propanol [17] and 1R,2S()-norephedrine [18]. Dioxidomolybdenum(VI) complex with Schiff
base derived from 2-[(1-hydroxy-2-methylpropane-2-ylimino)methyl]naphthol gave much
smaller conversion (50%), but with less excess of the oxidant (1.5:1 molar ratio to styrene)
and only 0.1 mol% of the catalyst in 25 min of reaction time [34]. Selectivity for all
molybdenum(VI) complexes is generally similar and the main oxidation product is
benzaldehyde (55-75%). They are much less selective toward styrene oxide (24-37%) and
their selectivity against 1-phenylethane-1,2-diol is relatively low (1-4%). Judmaier et al. [28]
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 chloroform at 50 °C. In this case, 71-75% conversion
has been obtained in 5 h of reaction time with 97-98% of styrene oxide selectivity after 24 h.
When one-pot synthesis by reductive amination was employed, the dimeric
dioxidomolybdenum(VI) complexes with one Schiff base in a tridentate manner, but in the
same reaction conditions only 35% conversion of styrene in 5 h and 44% in 24 h of reaction
time have been achieved [35]. On the other hand, catalytic properties of
dioxidomolybdenum(VI) complexes with naphtholate-oxazoline ligands in epoxidation of
styrene in DCE at 80 °C have been also reported [36]. 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 has been resulted.
In the same reaction conditions during the catalytic oxidation of styrene by aqueous
30% H2O2 distinctly lower conversion in the same contact time was found (21-29%), but
much higher selectivities against the most desired product, i.e. styrene oxide, were observed
13

(50-66%) than in the case of using TBHP as an oxidant. Furthermore, the selectivity against
by-products, i.e. benzaldehyde (28-39%) is distinctly lower and higher amounts of 1-
phenylethane-1,2-diol (2-10%) and also, in few cases, benzoic acid and phenylacetaldehyde
were found in comparison to the catalytic oxidations with TBHP. In the presence of H2O2, a
strong oxidizing agent, oxidation of styrene resulting in styrene oxide in the first step, but
further, via nucleophilic attack of the oxidant to styrene oxide followed by the cleavage of the
intermediate hydroperoxystyrene, can be converted into benzaldehyde [37]. Moreover, the
formation of benzaldehyde is probably facilitated via a radical mechanism by direct oxidative
cleavage of the styrene side-chain double bond. Significant amount of water in aqueous 30%
H2O2 can be blamed for the decomposition of a catalyst and thus very low conversion of
styrene. Formation of 1-phenylethane-1,2-diol by the hydrolysis of styrene oxide are also
caused by the presence of water. The other processes but surely much slower can be
responsible for the formation of benzoic acid in further oxidation of benzaldehyde or
isomerisation of styrene oxide to phenylacetaldehyde.
Table 4. Catalytic oxidation of styrene by aqueous 30% H2O2 or tert-butyl hydroperoxide
(TBHP) in presence of 1 mol% molybdenum(VI) Schiff base complexes.
Product selectivity (%) a
Conv.
Entry
Catalyst
Oxidant
(%)
StO
BzA
BzAC PhAA PhED
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
H2O2
H2O2
H2O2
H2O2
H2O2
H2O2
H2O2
H2O2
H2O2
26
21
28
22
26
25
26
22
26
29
72
66
71
65
65
73
69
68
72
68
59
51
63
53
66
57
52
44
63
50
37
31
24
27
30
41
29
34
31
25
39
28
34
35
31
37
38
34
35
32
59
67
75
71
66
55
69
63
67
73
-
11
-
4
-
-
3
-
-
-
-
-
3
-
2
-
-
-
-
-
-
-
-
-
-
2
7
3
8
3
6
6
10
2
6
4
2
1
2
4
4
2
3
2
2
1
2
3
4
5
6
7
8
9
10
1
2
3
4
5
6
7
8
-
4
9
-
10
-
-
-
-
-
-
-
-
-
H2O2
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
9
10
-
a
StO – styrene oxide, BzA – benzaldehyde, BzAC – benzoic acid, PhAA –
phenylacetaldehyde, PhED – 1-phenylethane-1,2-diol.
3.3.3. Oxidation of cyclohexene
The complexes 1-10 has been also tested as catalysts in the oxidation of cyclohexene
in the presence of aqueous 30% H2O2 or tert-butyl hydroperoxide (TBHP) in decane as an
oxidant. These reactions result in four products, i.e. cyclohexene oxide, cyclohexane-1,2-diol,
14

2-cyclohexene-1-ol and 2-cyclohexene-1-one (Fig. 6). The conversion and selectivity of these
products are presented in Table 5.
Figure 6. Various oxidation products of cyclohexene catalyzed by molybdenum(VI)
complexes.
OH
O
+
OH
ChO
ChDL
1 mol% catalyst
H O or TBHP
2
2
OH
O
cyclohexene
+
ChOL
ChON
Table 5. Catalytic oxidation of cyclohexene by aqueous 30% H2O2 or tert-butyl
hydroperoxide (TBHP) in presence of 1 mol% molybdenum(VI) complexes in DCE.
Product selectivity (%) a
Entry Catalyst
Oxidant
Conv. (%)
ChO
ChOL
ChON
ChDL
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
H2O2
H2O2
H2O2
H2O2
H2O2
H2O2
H2O2
H2O2
H2O2
95
100
94
95
90
100
100
99
98
100
50
50
43
45
50
57
46
46
47
74
78
80
81
79
80
74
87
91
88
96
97
94
94
96
96
93
95
95
94
9
5
7
6
6
5
9
4
2
3
-
-
-
-
-
-
-
-
-
2
1
1
1
2
2
2
1
1
1
-
-
-
-
-
-
-
-
-
15
16
12
12
13
13
15
8
6
8
4
3
6
6
4
4
7
5
5
6
1
2
3
4
5
6
7
8
9
10
1
2
3
4
5
6
7
8
H2O2
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
9
10
42
-
-
a ChO – cyclohexene oxide, ChOL – 2-cyclohexen-1-ol, ChON – 2-cyclohexen-1-one, ChDL
– cyclohexane-1,2-diol.
15

Optimization of the reaction conditions has been also performed with complex 3 as the
representative catalyst. As in the case of styrene oxidation, different amounts of the catalyst
(0.5, 1 and 2 mol%) and both oxidants (1:1, 2:1 and 3:1 molar ratios to substrate) have been
tested, as well as various solvents and temperatures of the reaction mixture.
The best results were found when 1,2-dichloroethane (DCE) and 80 °C were chosen to
run the catalytic reactions, as in the case for the oxidation of styrene. 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 (5 ml), and the reaction was carried out for 1 h of contact time at 80 °C At a 1:1
molar ratio, a maximum of 62% conversion was achieved. Increasing the ratio to 2:1
improved the conversion to 94%, while 3:1 ratio has shown a maximum of 95% conversion.
Further increment of H2O2 shows no improved conversion only marginally, therefore a 2:1
ratio being considered adequate In the case of using TBHP, increasing the
TBHP:cyclohexene molar ratio from 1:1 to 2:1 and 3:1 improved the conversion from 27 to
43 and 45%, respectively. As in the previous case, there was no significant conversion
improvement upon further addition of the oxidant.
Similarly, for different amounts of catalyst (0.5, 1 and 2 mol%) with a 2:1 molar ratio
of oxidant to cyclohexene and under above reaction conditions, 0.5 mol% gave only 47%
(H2O2) and 22% (TBHP) oxidative conversion, while 1 mol% and 2 mol% of catalyst have
shown a maximum conversion with 94% for H2O2 and 43% for TBHP. 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. 1-2%
conversion.
The oxidation of cyclohexene with 1-10 as catalysts and aqueous 30% H2O2 as an
oxidant gave overall excellent conversion (94-100%). As previously mentioned, the
conversion of styrene in the same reaction conditions is distinctly lower (21-29%).
Cyclohexene oxide is the main product with 74-91% conversion, and the selectivity against
cyclohexane-1,2-diol (6-16%) is slightly more noticeable than against 2-cyclohexen-1-ol (2-
9%) and 2-cyclohexen-1-one (1-2%). When tert-butyl hydroperoxide (TBHP) was used as the
oxidant, reactions with 1-10 catalysts gave much lower 42-57% conversion, but, in contrast to
catalytic oxidations with H2O2, are distinctly more selective toward cyclohexene oxide giving
excellent selectivity (93-97%). The rest amounts of cyclohexane-1,2-diol found in the reaction
mixture are by-products.
4. Conclusion
New chiral dioxidomolybdenum(VI) complexes derived from Schiff bases, products of
a single condensation of salicylaldehyde and its derivatives with R()-2-amino-1-propanol
13
were synthesized and characterized by UV-Vis, CD, IR, one- (1H, C) and two-dimensional
(COSY, NOESY, gHSQC) NMR spectroscopy. Moreover, crystal and molecular structures of
R()-2-[(1-oxidopropyl)iminomethyl]-4-methylphenolato-
κ3N,O,O’}(methanol)dioxidomolybdenum(VI), is also reported.
Their catalytic activity in room temperature was proved during the sulfoxidation of
thioanisole by aqueous 30% H2O2 revealing that the overall yield and enantiomeric excess
only slightly depend on the nature of a catalyst, i.e. the electron density at the metal center.
Moreover, the sulfoxidation reactions carried out in much lower temperatures results as
expected in both better yields and enantioselectivities. The oxidation of olefins by aqueous
30% H2O2 or tert-butyl hydroperoxide (TBHP) in decane was also studied, choosing styrene
and cyclohexene as the model substrates. These complexes are able to catalyze the oxidative
16

conversion of styrene to styrene oxide and benzaldehyde, and cyclohexene to cyclohexene
oxide as the main products. The oxidation of styrene after 1 h of reaction time in DCE at 80
°C by 30% H2O2 has resulted in five different products usually with the significant excess of
styrene oxide and benzaldehyde, but much lower conversion was observed than with TBHP as
oxidant which gives even 73% conversion of styrene. On the contrary, in the oxidation of
cyclohexene excellent conversions were found especially when 30% H2O2 was employed.
Using TBHP conversion decreased to ca. 50%, but selectivity against cyclohexene oxide
achieved 93-97% with small amounts of cyclohexane-1,2-diol as the by-product. The
promising results of the catalytic studies with presented chiral cis-dioxidomolybdenum(VI)
complexes and also those reported earlier show that designing of such chiral catalysts and
performing similar oxidations reactions also with different substrates is worth of further
investigations in this regard.
Acknowledgements
This scientific work was supported by the Polish Ministry of Science and Higher
Education (DS/530-8216-D598-18).
Appendix A. Supplementary data
CCDC 1847391 contains the supplementary crystallographic data for compound 4.
These
data
can
be
obtained
free
of
charge
via
http://www.ccdc.cam.ac.uk/conts/retrieving.html, or from the Cambridge Crystallographic
Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: (+44) 1223-336-033; or e-mail:
deposit@ccdc.cam.ac.uk.
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18

R1
2
R
O
R3
O
N
O
Mo
S
4
R
O
H3C
S aqua or methanol
_
19

 Chiral dioxidomolybdenum(VI) Schiff base complexes derived from
R(‒)-2-amino-1-propanol and salicylaldehyde derivatives have been
synthesized.
 The complexes were characterized by IR, CD, UV-Vis, one- and two-
dimensional NMR spectroscopy. The single-crystal X-ray analysis was
also performed.
 The dioxidomolybdenum(VI) complexes have ability to catalyze the
oxidation of thioanisole to methyl phenyl sulfoxide with good yields and
enantiomeric excesses.
 The molybdenum(VI) complexes are also catalytically active in the
oxidation of styrene and cyclohexene with 30% H2O2 or TBHP as an
oxidant.
20