Cis-dioxidomolybdenum(VI) complexes with chiral tetradentate Schiff bases: Synthesis, spectroscopic characterization and catalytic activity in sulfoxidation and epoxidation

Article abstract of DOI:10.1016/j.ica.2020.119832

New chiral mononuclear cis-dioxidomolybdenum(VI) complexes, MoO₂L¹-MoO₂L⁷, have been synthesized by the reaction of MoO₂(acac)₂ with tetradentate Schiff bases derived from various substitut

Full text of DOI:10.1016/j.ica.2020.119832

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Research paper
Cis-dioxidomolybdenum(VI) complexes with chiral tetradentate schiff bases:
Synthesis, spectroscopic characterization and catalytic activity in sulfoxida‐
tion and epoxidation
Marta Karman, Grzegorz Romanowski
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DOI:
Reference:
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https://doi.org/10.1016/j.ica.2020.119832
ICA 119832
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Inorganica Chimica Acta
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18 May 2020
11 June 2020
11 June 2020
Please cite this article as: M. Karman, G. Romanowski, Cis-dioxidomolybdenum(VI) complexes with chiral
tetradentate schiff bases: Synthesis, spectroscopic characterization and catalytic activity in sulfoxidation and
epoxidation, Inorganica Chimica Acta (2020), doi: https://doi.org/10.1016/j.ica.2020.119832
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Cis-dioxidomolybdenum(VI) complexes with chiral tetradentate Schiff
bases: Synthesis, spectroscopic characterization and catalytic activity in
sulfoxidation and epoxidation
Marta Karman and Grzegorz Romanowski*
Faculty of Chemistry, University of Gdańsk, Wita Stwosza 63, PL-80308 Gdańsk, Poland
Abstract
New chiral mononuclear cis-dioxidomolybdenum(VI) complexes, MoO2L1-MoO2L7,
have been synthesized by the reaction of MoO2(acac)2 with tetradentate Schiff bases derived
from various substituted salicylaldehydes and 1S,2S-(+)-2-amino-1-phenyl-1,3-propanediol.
All complexes have been characterized by elemental analysis, circular dichroism, electronic
1
and IR spectral studies. H NMR and also two-dimensional (COSY, NOESY and gHSQC)
NMR measurements made for MoO2L1-MoO2L7 complexes show that Schiff bases are
2+
coordinated to MoO2 cation creating facial (fac) and meridional (mer) types of geometrical
isomers. Moreover, the catalytic activity studies have been also performed for all complexes
in asymmetric sulfoxidation of thioanisole and epoxidation of styrene, cyclohexene and two
monoterpenes, i.e. S(‒)-limonene and (‒)-α-pinene, using aqueous 30% H2O2 or tert-butyl
hydroperoxide (TBHP) as the oxygen source.
1. Introduction
Chiral Schiff base ligands are considered “privileged ligands” [1] in modern
asymmetric catalysis and forming complexes with transition metals as synthetic catalysts are
active and moreover enantioselective in many various chemical reactions. Schiff base ligands
are able to stabilize many different metals in various oxidation states, controlling their
performance in a large variety of useful catalytic transformations. Lately, especially useful are
chiral N-salicyl-β-amino alcohol Schiff base ligands [2] which are very attractive due to their
simple synthesis from naturally available chiral amino acids [3] and which structural and
electronic properties can be fine-tuned. They are also a group of ligands which form stable
complexes with transition metals with proved catalytic activity, i.e. titanium, vanadium and
molybdenum, and therefore widely employed in asymmetric transformations such as oxygen
atom transfer (OAT) reactions (epoxidation and sulfoxidation) [4], alkynylation of aldehydes
[4a], enantioselective trimethylsilylcyanations [5], oxidation of bromide [6], stereoselective
synthesis of cyclic ethers [6,7] oxidative kinetic resolution of α-hydroxy esters [8] or found
applications in synthesis of biologically active compounds by multicomponent reactions
(MCRs) [9]. Recently, much better yields and selectivities have been achieved towards
cyclohexane oxidation using ionic liquid or supercritical carbon dioxide medium in contrast to
common solvent [10].
Although different molybdenum complexes have widely studied as catalysts, the chiral
ones especially useful in catalytic enantioselective oxidation reactions still remains very
limited [11]. 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 and cyclohexene) [12-15], oxidation of sulfides to
sulfoxides [16-18]. Lately, particular attention has been drawn on monocyclic and bicyclic
_____________
* Corresponding author. E-mail address: grzegorz.romanowski@ug.edu.pl, tel.: +48 58
5235102, fax: +48 585235012
1

monoterpenes, such as limonene and pinene, which are abundant natural products, but also
inexpensive by-product from the citrus fruit juice industry [19] and technical forestry resin or
wood pulp by-produced in the manufacture of cellulose [20], respectively.
We have an ongoing interest in the chemistry of dioxidomolybdenum(VI) complexes
using different ONO tridentate donor ligands [21, 22]. In this paper we describe new ten cis-
dioxidomolybdenum(VI) complexes with tetradentate ONOO Schiff base ligands, products of
a single condensation of 1S,2S-(+)-2-amino-1-phenyl-1,3-propanediol with salicylaldehyde
and its derivatives, presented in Fig. 1. Very detailed investigation of their spectroscopic
properties using IR, UV-Vis, circular dichroism, one- and two-dimensional NMR techniques
has been also performed. Moreover, their catalytic abilities in enantioselective sulfoxidation
of thioanisole and epoxidation of alkenes, i.e. styrene and cyclohexene, and monoterpenes, i.e.
S(-limonene and (-α-pinene, in the presence of aqueous 30% H2O2 or tert-butyl
hydroperoxide as the terminal oxidant, have been studied.
Figure 1. Structural formulae of dioxidomolybdenum(VI) complexes.
2. Experimental
2.1. Measurements
All chemicals and reagents were obtained from commercial sources and used without
further purification. Elemental analyses were performed with a Carlo Erba MOD 1106
instrument. Electronic spectra were measured on a Perkin-Elmer LAMBDA 18
spectrophotometer. CD spectra were recorded on 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 detector were used to during catalytic studies. The
identities of the oxidation products have been confirmed by GC-MS model Shimadzu GCMS-
QP2010 SE.
2.2. Synthesis of dioxidomolybdenum(VI) complexes
2

A similar procedure was employed for synthesis of all complexes. To a solution of 1
mmol of 1S,2S-(+)-2-amino-1-phenyl-1,3-propanediol in MeOH (10 ml) 1 mmol of one of
following aromatic o-hydroxyaldehyde was added, i.e. salicylaldehyde, 5-
methoxysalicylaldehyde,
5-bromosalicylaldehyde,
5-nitrosalicylaldehyde,
4-
hydroxysalicylaldehyde, 3-tert-butylsalicylaldehyde or 2-hydroxy-1-naphthaldehyde in 10 ml
of MeOH and reaction mixture was heated with stirring under reflux for 1 h. Then,
bis(acetylacetonato)dioxidomolybdenum(VI) (1 mmol) in MeOH (10 ml) was added and
stirred at under reflux for next 2 h. After cooling precipitates were separated as yellow solids,
filtered off and washed with MeOH.
MoO2L1: Yield 82%. Anal. Calc. for C16H15NO5Mo: C, 48.4; H, 3.8; N, 3.5. Found: C,
48.3; H, 3.7; N, 3.5%. IR (KBr, cm-1): 3355 νO-H); 1633 νC=N); 1589, 1480 (νC=C); 1265 (νC-
O); 926, 883 (νMo=O). UV-Vis spectrum in DMSO [λmax (nm), ɛ (M-1 cm-1)] 267 (7930), 346
1
(1630). CD spectrum in DMSO [λmax (nm), Δɛ (M-1 cm-1)] 279 (9.13), 349 (5.65). H NMR
3
DMSO-d6, ppm) mer-isomer (65%): 8.75 (1H, s) (azomethine); 7.61 (1H, dd, J=7.8 Hz,
3
3
3
4J=1.6 Hz), 7.51 (1H, t, J=7.8 Hz), 7.45 (2H, d, J=7.3 Hz), 7.39 (2H, t, J=7.3 Hz), 7.33
3
3
(1H, t, J=7.3 Hz), 6.98 (1H, ov), 6.92 (1H, d, J=9.0 Hz) (aromatic); 5.22 (1H, ov)
(hydroxyl); 5.24 (1H, d, 3J=8.0 Hz), 3.85 (1H, m) (methine); 4.10 (1H, dt, 3J=12.3 Hz, 4J=4.0
3
4
Hz), 3.60 (1H, dt, J=12.3 Hz, J=5.6 Hz) (methylene); fac-isomer (35%): 8.77 (1H, s)
(azomethine); 7.65 (1H, dd, J=7.8 Hz, J=1.6 Hz), 7.51 (1H, t, J=7.8 Hz), 7.45 (2H, ov),
3
4
3
3
7.39 (2H, ov), 7.33 (1H, ov), 7.00 (1H, ov), 6.94 (1H, d, J=9.0 Hz) (aromatic); 5.65 (1H, d,
3J=4.3 Hz) (hydroxyl); 4.07 (1H, ov), 4.05 (1H, ov) (methine); 4.73 (1H, dd, J=9.0 Hz,
3
4J=4.3 Hz), 4.07 (1H, ov) (methylene).
MoO2L2: Yield 86%. Anal. Calc. for C17H17NO6Mo: C, 47.8; H, 4.0; N, 3.3. Found: C,
47.7; H, 3.9; N, 3.3%. IR (KBr, cm-1): 3416 νO-H); 1638 νC=N); 1612, 1483 (νC=C); 1279 (νC-
O); 925, 898 (νMo=O). UV-Vis spectrum in DMSO [λmax (nm), ɛ (M-1 cm-1)] 274 (6650), 375
(1750). CD spectrum in DMSO [λmax (nm), Δɛ (M-1 cm-1)] 279 (10.46), 383 (4.84). 1H NMR
DMSO-d6, ppm) mer-isomer (65%): 8.75 (1H, s) (azomethine); 7.45 (2H, d, 3J=7.3 Hz), 7.40
3
3
3
3
(2H, t, J=7.3 Hz), 7.33 (1H, t, J=7.3 Hz), 7.20 (1H, d, J=3.2 Hz), 7.13 (1H, t, J=3.2 Hz),
3
3
3
6.87 (1H, d, J=9.0 Hz) (aromatic); 5.14 (1H, t, J=5.1 Hz) (hydroxyl); 5.21 (1H, d, J=8.0
Hz), 3.81 (1H, m) (methine); 3.96 (1H, dt, 3J=12.2 Hz, 4J=3.8 Hz), 3.61 (1H, dt, 3J=12.2 Hz,
4J=5.5 Hz) (methylene); 3.77 (3H, s) (methoxy); fac-isomer (35%): 8.77 (1H, s)
(azomethine); 7.43 (2H, ov), 7.39 (2H, ov), 7.34 (1H, ov), 7.25 (1H, d, 3J=3.2 Hz), 7.15 (1H,
t, J=3.2 Hz), 6.89 (1H, d, J=9.0 Hz) (aromatic); 5.63 (1H, d, J=4.4 Hz) (hydroxyl); 4.07
(1H, ov), 4.05 (1H, ov) (methine); 4.73 (1H, dd, 3J=9.1 Hz, 4J=4.4 Hz), 4.02 (1H, dd, 3J=10.2
Hz, 4J=4.9 Hz) (methylene); 3.78 (3H, s) (methoxy).
3
3
3
MoO2L3: Yield 77%. Anal. Calc. for BrC16H14NO5Mo: C, 40.4; H, 3.0; N, 2.9. Found:
C, 40.2; H, 3.1; N, 3.0%. IR (KBr, cm-1): 3405 νO-H); 1634 νC=N); 1613, 1494 (νC=C); 1235
(νC-O); 931, 906 (νMo=O). UV-Vis spectrum in DMSO [λmax (nm), ɛ (M-1 cm-1)] 268 (8310),
1
355 (1640). CD spectrum in DMSO [λmax (nm), Δɛ (M-1 cm-1)] 278 (9.54), 359 (5.26). H
3
NMR DMSO-d6, ppm) mer-isomer (65%): 8.74 (1H, s) (azomethine); 7.85 (1H, d, J=2.6
3
3
3
Hz), 7.62 (1H, t, J=2.6 Hz), 7.45 (2H, d, J=7.3 Hz), 7.40 (2H, t, J=7.3 Hz), 7.34 (1H, t,
3J=7.3 Hz), 6.90 (1H, d, 3J=8.8 Hz) (aromatic); 5.14 (1H, t, 3J=5.3 Hz) (hydroxyl); 5.28 (1H,
3
3
4
d, J=8.0 Hz), 3.84 (1H, m) (methine); 3.98 (1H, dt, J=12.3 Hz, J=4.0 Hz), 3.59 (1H, dt,
3J=12.3 Hz, J=5.6 Hz) (methylene); fac-isomer (35%): 8.77 (1H, s) (azomethine); 7.90 (1H,
4
3
3
d, J=2.6 Hz), 7.64 (1H, t, J=2.6 Hz), 7.46 (2H, ov), 7.38 (2H, ov), 7.34 (1H, ov), 6.92 (1H,
3

3
3
d, J=8.8 Hz) (aromatic); 5.68 (1H, d, J=4.4 Hz) (hydroxyl); 4.09 (1H, ov), 4.08 (1H, ov)
(methine); 4.73 (1H, dd, 3J=9.0 Hz, 4J=4.4 Hz), 4.07 (1H, ov) (methylene).
MoO2L4: Yield 83%. Anal. Calc. for C16H14N2O7Mo: C, 43.5; H, 3.2; N, 6.3. Found:
C, 43.6; H, 3.3; N, 6.3%. IR (KBr, cm-1): 3424 νO-H); 1623 νC=N); 1606, 1509 (νC=C); 1551,
1336 νNO2); 1509 (νC=C); 1249 (νC-O); 915, 897 (νMo=O). UV-Vis spectrum in DMSO [λmax
(nm), ɛ (M-1 cm-1)] 330 (14440). CD spectrum in DMSO [λmax (nm), Δɛ (M-1 cm-1)] 277
(8.75), 328 (7.64). 1H NMR DMSO-d6, ppm) mer-isomer (65%): 9.05 (1H, s) (azomethine);
3
3
3
3
8.72 (1H, d, J=2.6 Hz), 7.64 (1H, t, J=2.6 Hz), 7.47 (2H, d, J=7.3 Hz), 7.43 (2H, t, J=7.3
3
3
3
Hz), 7.36 (1H, t, J=7.2 Hz), 7.17 (1H, d, J=8.7 Hz) (aromatic); 5.17 (1H, t, J=5.2 Hz)
(hydroxyl); 5.30 (1H, d, 3J=8.2 Hz), 3.86 (1H, m) (methine); 3.99 (1H, dt, 3J=12.2 Hz, 4J=4.0
3
4
Hz), 3.61 (1H, dt, J=12.3 Hz, J=5.6 Hz) (methylene); fac-isomer (35%): 9.08 (1H, s)
(azomethine); 8.76 (1H, d, J=2.6 Hz), 7.67 (1H, t, J=2.6 Hz), 7.48 (2H, ov), 7.37 (2H, ov),
7.33 (1H, ov), 7.19 (1H, d, J=8.7 Hz) (aromatic); 5.71 (1H, d, J=4.3 Hz) (hydroxyl); 4.10
(1H, ov), 4.09 (1H, ov) (methine); 4.76 (1H, dd, J=9.0 Hz, J=4.3 Hz), 4.10 (1H, ov)
3
3
3
3
3
4
(methylene).
MoO2L5: Yield 77%. Anal. Calc. for C16H15NO6Mo: C, 46.5; H, 3.7; N, 3.4. Found: C,
46.5; H, 3.8; N, 3.5%. IR (KBr, cm-1): 3308 νO-H); 1638 νC=N); 1604, 1477 (νC=C); 1219 (νC-
O); 932, 902 (νMo=O). UV-Vis spectrum in DMSO [λmax (nm), ɛ (M-1 cm-1)] 287 (8280), 338
1
(2970). CD spectrum in DMSO [λmax (nm), Δɛ (M-1 cm-1)] 294 (8.35), 344 (7.76). H NMR
DMSO-d6, ppm) mer-isomer (65%): 10.28 (1H, s) (hydroxyl); 8.61 (1H, s) (azomethine);
3
3
3
3
7.43 (2H, d, J=7.3 Hz), 7.38 (2H, t, J=7.3 Hz), 7.31 (1H, t, J=7.3 Hz), 7.25 (1H, t, J=7.5
Hz), 6.41 (1H, d, 3J=8.3 Hz), 6.24 (1H, s) (aromatic); 5.15 (1H, t, 3J=5.2 Hz) (hydroxyl); 5.28
(1H, d, J=8.2 Hz), 3.86 (1H, m) (methine); 3.97 (1H, dt, J=12.1 Hz, J=4.1 Hz), 3.59 (1H,
dt, J=12.3 Hz, J=5.6 Hz) (methylene); fac-isomer (35%): 10.31 (1H, s) (hydroxyl); 8.63
3
3
4
3
4
3
(1H, s) (azomethine); 7.42 (2H, ov), 7.37 (2H, ov), 7.33 (1H, ov), 7.27 (1H, t, J=7.5 Hz),
3
3
6.43 (1H, d, J=8.3 Hz), 6.27 (1H, s) (aromatic); 5.70 (1H, d, J=4.3 Hz) (hydroxyl); 4.11
3
4
(1H, ov), 4.10 (1H, ov) (methine); 4.74 (1H, dd, J=9.0 Hz, J=4.3 Hz), 4.10 (1H, ov)
(methylene).
MoO2L6: Yield 81%. Anal. Calc. for C20H23NO5Mo: C, 53.0; H, 5.1; N, 3.1. Found: C,
53.2; H, 5.0; N, 3.0%. IR (KBr, cm-1): 3416 νO-H); 1630 νC=N); 1592, 1494 (νC=C); 1280 (νC-
O); 914, 878 (νMo=O). UV-Vis spectrum in DMSO [λmax (nm), ɛ (M-1 cm-1)] 271 (8110), 353
(1670). CD spectrum in DMSO [λmax (nm), Δɛ (M-1 cm-1)] 282 (10.60), 358 (5.35). 1H NMR
DMSO-d6, ppm) mer-isomer (65%): 8.73 (1H, s) (azomethine); 7.42 (2H, d, 3J=7.3 Hz), 7.38
3
3
3
3
(2H, t, J=7.3 Hz), 7.31 (1H, t, J=7.3 Hz), 7.15 (1H, d, J=7.6 Hz), 7.09 (1H, d, J=7.6 Hz),
3
3
3
6.82 (1H, t, J=7.6 Hz) (aromatic); 5.12 (1H, t, J=5.1 Hz) (hydroxyl); 5.20 (1H, d, J=7.9
Hz), 3.79 (1H, m) (methine); 3.94 (1H, dt, 3J=12.2 Hz, 4J=3.9 Hz), 3.60 (1H, dt, 3J=12.2 Hz,
4J=5.4 Hz) (methylene); 1.41 (9H, s) (tert-butyl); fac-isomer (35%): 8.75 (1H, s)
(azomethine); 7.41 (2H, ov), 7.37 (2H, ov), 7.32 (1H, ov), 7.17 (1H, d, 3J=7.6 Hz), 7.11 (1H,
d, J=7.6 Hz), 6.84 (1H, t, J=7.6 Hz) (aromatic); 5.62 (1H, d, J=4.4 Hz) (hydroxyl); 4.08
(1H, ov), 4.07 (1H, ov) (methine); 4.71 (1H, dd, 3J=9.0 Hz, 4J=4.2 Hz), 4.01 (1H, dd, 3J=10.1
Hz, 4J=4.9 Hz) (methylene); 1.43 (9H, s) (tert-butyl).
3
3
3
MoO2L7: Yield 80%. Anal. Calc. for C20H17NO5Mo: C, 53.7; H, 3.8; N, 3.1. Found: C,
53.5; H, 3.7; N, 3.2%. IR (KBr, cm-1): 3388 νO-H); 1630 νC=N); 1609, 1497 (νC=C); 1250 (νC-
O); 914, 889 (νMo=O). UV-Vis spectrum in DMSO [λmax (nm), ɛ (M-1 cm-1)] 272 (6820), 308
(6300), 381 (2460). CD spectrum in DMSO [λmax (nm), Δɛ (M-1 cm-1)] 279 (3.29), 303 (3.02),
4

1
380 (6.21). H NMR DMSO-d6, ppm) mer-isomer (65%): 9.57 (1H, s) (azomethine); 8.48
(1H, d, 3J=8.5 Hz), 8.09 (1H, d, 3J=9.0 Hz), 7.93 (1H, d, 3J=8.1 Hz), 7.65 (1H, t, 3J=7.3 Hz),
3
3
3
3
7.46 (1H, t, J=7.3 Hz), 7.40 (2H, d, J=7.3 Hz), 7.36 (2H, t, J=7.3 Hz), 7.30 (1H, t, J=7.3
3
3
Hz), 7.18 (1H, d, J=9.0 Hz) (aromatic); 5.15 (1H, t, J=5.1 Hz) (hydroxyl); 5.23 (1H, d,
3J=7.9 Hz), 3.81 (1H, m) (methine); 3.97 (1H, dt, J=12.1 Hz, J=3.9 Hz), 3.63 (1H, dt,
3
4
4
3J=12.1 Hz, J=5.3 Hz) (methylene); fac-isomer (35%): 9.59 (1H, s) (azomethine); 8.50 (1H,
d, 3J=8.5 Hz), 8.11 (1H, d, 3J=9.0 Hz), 7.96 (1H, d, 3J=8.1 Hz), 7.67 (1H, t, 3J=7.3 Hz), 7.48
3
3
(1H, t, J=7.3 Hz), 7.39 (2H, ov), 7.35 (2H, ov), 7.32 (1H, ov), 7.20 (1H, d, J=9.0 Hz)
3
(aromatic); 5.64 (1H, d, J=4.3 Hz) (hydroxyl); 4.11 (1H, ov), 4.09 (1H, ov) (methine); 4.74
(1H, dd, 3J=9.0 Hz, 4J=4.2 Hz), 4.04 (1H, dd, 3J=10.2 Hz, 4J=4.9 Hz) (methylene).
2.3. Catalytic activity
2.3.1. Sulfoxidation reactions
All dioxidomolybdenum(VI) complexes have been tested as catalysts for sulfoxidation
of thioanisole in the presence of aqueous 30% H2O2 or 5.5 M decane solution of tert-butyl
hydroperoxide (TBHP) as the terminal oxidant. The catalyst, thioanisole and oxidant amounts
were in 0.01, 1 and 1.1 mmol, respectively. The reactions were run in CH2Cl2 and MeOH
(7:3) solution for a better mixing of the aqueous oxidant with the halogenated solvent [23] and
enhancing the yield and selectivity of sulfoxide by protic solvent [24]. After the appropriate
reaction time, the solution was quenched with 3 ml of sodium sulfite solution (0.1 M),
extracted with ethyl acetate and organic layers were evaporated to dryness. The yield and
reaction rates were estimated on the basis of the integrated intensities of substrate and product
1
signals in CDCl3 using H NMR spectroscopy with 1,3,5-trimethoxybenzene as an internal
standard. After addition of chiral shift reagent - Eu(hfc)3 [25] the enantiomeric excesses of
methyl phenyl sulfoxide were calculated.
2.3.2. Epoxidation reactions
The catalytic abilities of all complexes were studied for epoxidation of alkenes, i.e.
styrene and cyclohexene, and monoterpenes, i.e. S(‒)-limonene and (‒)-α-pinene, using
aqueous 30% H2O2 or 5.5 M decane solution of tert-butyl hydroperoxide. Different amounts
of catalysts and oxidants were also tested to optimize reaction conditions. All reactions were
run in 1,2-dichloroethane (DCE) at 80 °C and monitored by GC using 1:100:200 molar ratio
of catalyst, substrate and oxidant, respectively. The yields were recorded as GC yield based
on the starting substrate. The identity of oxidation products were confirmed by GC-MS.
3. Results and discussion
3.1. IR spectra
The IR spectra for MoO2L1-MoO2L7 complexes exhibit medium bands centered at
1
3308-3424 cm− and are assigned to ν(O-H) vibrations of coordinated hydroxyl group. The
1
characteristic imine C=N band, which exist at 1623-1638 cm− indicating the presence of
azomethine nitrogen atom of all Schiff base ligands coordinated to the molybdenum ion [26,
1
27]. Moreover, the appearance of ν(C-O) bands at 1219-1280 cm− also suggest the
coordination alkoxide ions and OH groups. In case of MoO2L4, with nitro substituent attached
to aromatic ring of salicylaldehyde moiety, asymmetric and symmetric N-O stretches have
1
been found at 1551 and 1336 cm− , respectively. Finally, a pairs of sharp and strong bands at
5

914-932 and 878-906 cm-1 due to the streching νasym(O=Mo=O) and νsym(O=Mo=O) modes,
respectively, clearly confirm the presence of a cis-[MoVIO2] structure [28].
3.2. Electronic and circular dichroism spectra
Electronic absorption and circular dichroism spectra of cis-dioxidomolybdenum(VI)
complexes were recorded in spectroscopic grade DMSO. The UV-Vis spectra display
intraligand π-π* transitions in 267-287 nm region. The low-energy transitions appear between
338-381 nm 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]. The MoO2L4
compound is exception to this rule and exhibit only one strong broad band at 330 nm (ɛmax
=
14440) and the spectrum of MoO2L7 with naphthyl ring displays additional band at 308 nm
(ɛmax = 6300). The circular dichroism spectra revealed the same bands in 277-294 nm and
328-383 nm region of the same origin as electronic spectra with very strong positive sign of
the Cotton effects.
3.3. NMR measurements
The one- (1H) and two-dimensional (COSY, NOESY, gHSQC) NMR spectra of
1
dioxidomolybdenum(VI) Schiff base complexes were recorded in DMSO-d6. The H spectra
of all complexes have shown the presence of azomethine proton signals proving condensation
reaction between all salicylaldehyde derivatives with 1S,2S-(+)-2-amino-1-phenyl-1,3-
propanediol. Complete assignments and identification of all proton and carbon signals and
establishing a connection and proximity between all protons and their attachment to carbon
atoms have been achieved using two-dimensional NMR experiments and are listed in
Experimental section.
We reported earlier that cis-dioxidomolybdenum(VI), but also oxidovanadium(V) and
cis-dioxidovanadium(V) complexes with unsymmetrical tridentate Schiff base ligands,
products of monocondensation reaction of salicylaldehyde and its derivatives with amino
alcohols and diamines, have been proven to possess a rigid and nearly planar backbone
composed of three donor centers established by the Schiff base linkage, which prefer only a
meridional coordination mode [22, 30]. Moreover, such complexes with similar high-denticity
ligands, i.e. pentadentate Schiff bases also revealed only meridional arrangement of
coordination sites [31]. It was possible to observe in solution a second isomer in facial
coordination arrangement but after reduction of imine functionality obtaining flexible amine
ligand system [32]. In case of MoO2L1-MoO2L7 complexes their Schiff bases are coordinated
2+
to MoO2 cation creating meridional (mer) and facial (fac) types of geometrical isomers,
respectively (Fig. 2). Liimatainen et al. [33] reported crystallization of racemic MoO2L1 and
X-ray analysis revealed its molecular structure, which, in contrast to our chiral MoO2L1
complex, possess one hydroxyl group uncoordinated and a methanol molecule completes
coordination sphere. Moreover, as expected only meridional isomer is present and chemical
shifts in 1H NMR spectrum are in good agreement with our results.
1
The H NMR spectra of MoO2L1-MoO2L7 show that all protons of both isomers are
chemically different giving rise to two sets of signals in 65:35 ratio and the resonances of the
mer-isomers are generally observed at lower frequencies. The chemical shift differences
between the two methine protons of the amino alcohol chelate rings of the fac-isomers are
rather very small. Whereas for the mer-isomers a distinct separation between the two
resonances is observed (almost 1.5 ppm). On the other hand, the separations between the
methylene protons are for the fac-isomers ca. 0.3 ppm bigger and moreover, signal of the
proton of coordinated hydroxyl group is a doublet. Furthermore, taking MoO2L3 as an
6

example, COSY spectrum show cross-peak between coordinated hydroxyl proton doublet at
5.68 pm only with one of the methylene protons at 4.73 ppm, but the hydroxyl proton triplet
of mer-isomer (5.14 ppm) reveals cross-peaks with both methylene protons at 3.98 and 3.59
ppm. In case of both isomers the methylene protons show unambiguously connection with
methine proton (at 3.84 for mer-isomer and 4.09 for fac-isomer) neighboring with azomethine
nitrogen. Additionally, NOESY spectrum reveals spacial proximity between azomethine
proton with signal at 8.74 ppm in case of mer-isomer and one of the aromatic proton (doublet
signal at 7.85 ppm), the methine proton (at 3.84 ppm) and both methylene protons (at 3.98
and 3.59 ppm), whereas the only one cross-peak with methylene proton (signal at 4.07 ppm)
is present for the fac-isomer.
Figure 2. Facial (fac) and meridional (mer) geometrical isomers of MoO2L3 complex.
3.4. Catalytic activity studies
3.4.1. Enantioselective sulfoxidation of thioanisole
Catalytic activities of all cis-dioxidomolybdenum(VI) complexes MoO2L1-MoO2L7
were tested for sulfoxidation of thioanisole with a slight excess (1.1 equivalents) of tert-butyl
hydroperoxide (TBHP) or aqueous 30% H2O2 as the terminal oxidants (Fig. 3). The reactions
were run in CDCl3 at different temperatures (25 and 20 °C) with optimized amounts of the
catalysts (1 mol%) and under these conditions no overoxidation to the corresponding sulfone
was detected. In control experiments carried out without any molybdenum(VI) Schiff base
catalysts present or in the presence of MoO2(acac)2 no significant amounts of reactions
products have been detected. All studied complexes presented practically similar overall
catalytic ability, suggesting that they may involve the same catalytic species.
Overall conversion of thioanisole to methyl phenyl sulfoxide in the presence of all
molybdenum(VI) catalysts was slightly higher using 30% H2O2 as the oxidant, in comparison
to TBHP (Table 1). In all cases the R-configured sulfoxides were obtained with enantiomeric
excesses from 20 to 29% using 30% H2O2 (entries 1-10) and 13-26% when TBHP was
employed as the terminal oxidant (entries 11-20). These results clearly show that catalytic
activities of all complexes in sulfoxidation of thioanisole are lower with TBHP than in the
analogous reactions involving aqueous 30% H2O2. The reaction temperature seemed to have
some effect on the observed enantioselectivity and a slight increase in the enantiomeric excess
with lower reaction temperature was noticed. For example in the case of MoO2L2, MoO2L4
and MoO2L6 the decrease in the reaction temperature was accompanied with longer reaction
time but also with a slight increase in enantioselectivity and without additional sulfone
production (entries 3, 6, 9, 13, 16 and 19).
7

It is noteworthy to mention that in case of studied molybdenum(VI) complexes the
best enantioselectivities were achieved with catalysts possessing high electron-donating
substituents where a higher electron density on the phenolate oxygen in salicylaldimine
moiety helps to improve attainment of sufficient nucleophilicity by the metal centre. Mimoun
et al. reported [34] that sufficient nucleophilic center in d0 metal catalysts has significant
importance for a number of types of organic substrates used in catalytic oxidation processes.
Figure 3. Sulfoxidation of thioanisole catalyzed by cis-dioxidomolybdenum(VI) complexes.
Table 1. Asymmetric sulfoxidation of thioanisole in the presence of molybdenum(VI) Schiff
base complexes as catalysts.
entry
catalyst
oxidant
T (°C) a
yield (%)
ee (%) b
MoO2L1
MoO2L2
MoO2L2
MoO2L3
MoO2L4
MoO2L4
MoO2L5
MoO2L6
MoO2L6
MoO2L7
MoO2L1
MoO2L2
MoO2L2
MoO2L3
MoO2L4
MoO2L4
MoO2L5
MoO2L6
MoO2L6
MoO2L7
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
25
25
20
25
25
20
25
25
20
25
25
25
20
25
25
20
25
25
78
80
84
85
81
87
77
79
84
80
77
82
85
79
82
88
84
80
86
79
21
25
29
23
22
27
20
21
25
20
17
22
26
13
14
19
15
20
23
19
H2O2
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
20
25
a Optimized reaction times were 1.5 h at 25 °C and 5 h at 20 °C. b In all cases enantiomeric
excess of methyl phenyl sulfoxide was found to be in R configuration.
3.5.2. Epoxidation of alkenes and monoterpenes
Catalytic abilities of chiral cis-dioxidomolybdenum(VI) complexes with tetradentate
Schiff bases, MoO2L1-MoO2L7 have been studied in epoxidation of alkenes, such as styrene
and cyclohexene, but also monoterpenes, which are cyclohexene naturally occurring
derivatives, i.e. S(‒)-limonene and (‒)-α-pinene (Fig. 4). As the terminal oxidants 30%
8

aqueous H2O2 or tert-butyl hydroperoxide (TBHP) were used and 1,2-dichloroethane (DCE)
was found to be the most efficient solvent with regards to other solvents like toluene,
acetonitrile, ethanol, methanol, CHCl3 and CH2Cl2. The poorer yields obtained especially with
latter ones may be probably caused by the lower reaction temperature for their reflux
conditions. Considering our observations in conversion and selectivity, higher reaction
temperature, i.e. 80 °C, has an overall benefit to achieve the best yields for all epoxidation
reactions which required 1 h to reach completion. Similar conclusions that higher reactions
temperature can be responsible for obtaining better yields and reaction rates have been also
drawn previously [35]. In order to achieve suitable reaction conditions for a maximum
oxidative conversion the influence of different reaction parameters were taken into account,
i.e. amount of catalyst (0.5, 1, 2 and 3 mol% loadings) and oxidant molar ratios to substrate
(1:1, 2:1, 3:1 and 4:1). It was observed that using 1 mol% of each catalyst with 2:1 molar ratio
of both oxidants to all substrates were sufficient to run the epoxidations and an increase in
these ratios did not noticeably affect the reaction rates.
Figure 4. Substrates used for catalytic oxidation studies.
As we have previously reported [21, 22], the oxidation of styrene with catalytic
amounts of dioxidomolybdenum(VI) Schiff base complexes using aqueous 30% H2O2 or
TBHP as the terminal oxidants generally can result in five oxidation products, i.e. styrene
oxide, benzaldehyde, benzoic acid, phenylacetaldehyde and 1-phenylethane-1,2-diol.
Formation of as many as five products with similar molybdenum catalysts has been also
observed earlier [36]. Styrene oxide can be formed in the first step, but further reaction, via
nucleophilic attack of the oxidant to styrene oxide followed by the cleavage of the
intermediate hydroperoxystyrene, is very fast, converting the product into benzaldehyde [37],
which can be also further oxidized to benzoic acid. Moreover, the direct formation of
benzaldehyde can be also facilitated via a radical mechanism by direct oxidative cleavage of
the styrene side-chain double bond. The presence of water, in case of aqueous 30% H2O2, can
be blamed for the decomposition of the catalyst and thus the very low conversion of styrene.
Moreover, it can be also responsible for the formation of 1-phenylethane-1,2-diol by the
hydrolysis of styrene oxide and finally, styrene oxide isomerisation can lead to the formation
of phenylacetaldehyde.
During our studies it was observed that the epoxidation of styrene by aqueous 30%
H2O2 give as expected low conversions (16-21%), but when TBHP is added to reaction
mixtures in non-aqueous environment, the conversions of styrene increase significantly to 66-
82%. These reactions carried out with both oxidants lead to the formation of styrene oxide as
a major product along with only small amounts of benzaldehyde and without any additional
by-products (Table 2, entries 1-10). Similar conversion (71-75%) and excellent epoxide
selectivity were obtained by Judmaier et al. [38] with 0.5 mol% of molybdenum(VI) Schiff
base catalysts loading in 5 h of reaction time, but in chloroform at 50 °C.
The epoxidation of cyclohexene with catalytic amounts of cis-
dioxidomolybdenum(VI) Schiff base complexes generally results in epoxidation products, i.e.
9

cyclohexene oxide and, after its eventual hydrolysis, cyclohexene-1,2-diol, but also the
formation of allylic oxidation products is possible, i.e. 2-cyclohexen-1-ol and 2-cyclohexen-1-
one (Fig. 5). Mono- and bicyclic monoterpenes used in this study and possessing cyclohexene
ring, i.e. S(‒)-limonene and (‒)-α-pinene, gave analogous oxidation reactions products.
Table 2. Epoxidation of styrene and cyclohexene in the presence of molybdenum(VI) Schiff
base complexes as catalysts.
entry
catalyst
substrate
yield (%)
oxidant
epoxide (%)
MoO2L2
MoO2L4
MoO2L7
MoO2L1
MoO2L2
MoO2L3
MoO2L4
MoO2L5
MoO2L6
MoO2L7
MoO2L2
MoO2L4
MoO2L7
MoO2L1
MoO2L2
MoO2L3
MoO2L4
MoO2L5
MoO2L6
MoO2L7
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
styrene
styrene
styrene
styrene
styrene
styrene
styrene
styrene
styrene
18
21
16
74
82
78
74
72
73
66
19
25
22
67
77
74
71
73
71
69
H2O2
H2O2
H2O2
74
81
79
81
88
83
80
85
83
80
98
98
99
99
99
98
99
99
98
99
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
H2O2
styrene
cyclohexene
cyclohexene
cyclohexene
cyclohexene
cyclohexene
cyclohexene
cyclohexene
cyclohexene
cyclohexene
cyclohexene
H2O2
H2O2
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
Generally, cyclohexene and (‒)-α-pinene were converted to their corresponding
epoxides with roughly the same yields as in the case of styrene, but selectivities to their
epoxides are clearly much higher, especially in the case of cyclohexene (Table 2, entries 11-
20) with excellent up to 99% formation of cyclohexene oxide. In case of (‒)-α-pinene and
when TBHP was used the main product was (‒)-α-pinene oxide (up to 89%), but with aqueous
30% H2O2, lower conversions were achieved and even over 30% of verbenol, allylic oxidation
product, was formed (Table 3, entries 11-20).
Suprisingly, S(‒)-limonene was oxidized selectively to its epoxide with excellent
conversion, especially with TBHP (Table 3, entries 4-10), than in case of all the other
substrates, i.e. styrene, cyclohexene and (‒)-α-pinene. When TBHP was used as the terminal
oxidant cis- and trans-1,2-limonene oxide are formed almost in equal proportions, provided
epoxide practically quantitatively with and only small amounts of diepoxide, due to the
presence of additional exocyclic isopropenyl moiety, as by-product were obtained. On the
other hand, the epoxidation using aqueous 30% H2O2 resulted only in epoxide formation
(Table 3, entries 1-3) but with an high excess of trans-1,2-limonene oxide.
Under the same reaction conditions, but at very low 0.05% loadings of two
dioxidomolybdenum(VI) complexes equipped with naphtholate-oxazoline ligands as catalysts
the oxidation of R(+)-limonene resulted in ca. 60% conversion and up to 64% selectivity
towards epoxide [39]. On the other hand, Judmaier et al. [40] reported catalytic activity of
dimeric μ-oxido bridged dioxidomolybdenum(VI) complex with reduced Schiff base, which
10

in chloroform at 50 °C show excellent epoxide yield and selectivity in the epoxidation of
cyclohexene, but when R(+)-limonene was used as a substrate the reaction yielded in almost
equal amounts of epoxide and diepoxide.
Figure 5. Possible epoxidation and allylic oxidation products of cyclohexene.
Table 3. Epoxidation of monoterpenes with catalytic amounts of molybdenum(VI) Schiff
base complexes as catalysts.
entry
catalyst
substrate
yield (%)
oxidant
epoxide (%)
MoO2L2
MoO2L4
MoO2L7
MoO2L1
MoO2L2
MoO2L3
MoO2L4
MoO2L5
MoO2L6
MoO2L7
MoO2L2
MoO2L4
MoO2L7
MoO2L1
MoO2L2
MoO2L3
MoO2L4
MoO2L5
MoO2L6
MoO2L7
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
S(‒)-limonene
S(‒)-limonene
S(‒)-limonene
S(‒)-limonene
S(‒)-limonene
S(‒)-limonene
S(‒)-limonene
S(‒)-limonene
S(‒)-limonene
S(‒)-limonene
(‒)-α-pinene
(‒)-α-pinene
(‒)-α-pinene
(‒)-α-pinene
(‒)-α-pinene
(‒)-α-pinene
(‒)-α-pinene
(‒)-α-pinene
(‒)-α-pinene
(‒)-α-pinene
33
37
32
96
98
99
97
99
95
97
14
18
17
67
74
72
68
65
62
75
H2O2
H2O2
H2O2
83
89
85
77
82
80
74
73
76
74
64
69
67
83
85
74
81
87
79
89
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
H2O2
H2O2
H2O2
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
11

4. Conclusion
Within this paper we present the synthesis of ten new chiral cis-
dioxidomolybdenum(VI) complexes derived from tetradentate Schiff bases, products of a
single condensation of salicylaldehyde and its derivatives with 1S,2S-(+)-2-amino-1-phenyl-
1,3-propanediol, which have been characterized spectroscopically by UV-Vis, CD, IR, and
NMR techniques.
All these complexes have proved catalytic activity in the asymmetric sulfoxidation of
thioanisole by aqueous 30% H2O2 and tert-butyl hydroperoxide (TBHP) resulting in better
yields and enantioselectivities when reactions were carried out in much lower temperatures.
Furthermore, catalytic abilities of MoO2L1-MoO2L7 complexes have been tested in the
epoxidation of model olefinic substrates, i.e. styrene, cyclohexene and two monoterpenes, i.e.
S(‒)-limonene and (‒)-α-pinene using the same terminal oxidants. These complexes are able
to catalyze their oxidative conversion to corresponding epoxides with excellent yields and
selectivities. Under optimized reaction conditions, the best results have been achieved for
S(‒)-limonene, which was oxidized selectively to its epoxide with excellent conversion using
TBHP as the terminal oxidant. Formation of chiral limonene epoxide is especially important
since its application as a key raw material in the synthesis of pharmaceuticals, fragrances,
perfumes and food additives was proved. In the future we would like to concentrate our
efforts on epoxidation of a number of chiral monoterpenes.
Acknowledgements
This scientific work was supported by the Polish Ministry of Science and Higher
Education (Grant No. 531-T050-D686-20).
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13


14

 Chiral cis-dioxidomolybdenum(VI) Schiff base complexes have been
synthesized.
 The complexes were characterized by IR, CD, UV-Vis and NMR
spectroscopy.
 The molybdenum(VI) complexes have ability to catalyze sulfoxidation of
thioanisole.
 The complexes are also catalysts in the oxidation of alkenes and
monoterpenes.
15

Marta Karman: Methodology, Investigation;
Grzegorz Romanowski: Conceptualization, Visualization, Writing - Original
Draft, Writing - Reviewing and Editing.
16