Oxidovanadium(V) complexes with chiral tridentate Schiff bases derived from R(-)-phenylglycinol: Synthesis, spectroscopic characterization and catalytic activity in the oxidation of sulfides and styrene

Article abstract of DOI:10.1016/j.poly.2012.12.038

A series of vanadium(V) complexes with chiral tridentate Schiff base ligands, obtained by the single condensation of R(-)-phenylglycinol with salicylaldehyde and its derivatives, were prepared. The complexes were characterized by elemental analysis and by their ESI-MS, IR, CD, UV-Vis, 1D (¹H, ⁵¹V) and 2D (COSY, gHSQC) NMR spectra. The vanadium(V) complexes have the ability to catalyze the oxidation of sulfides [PhSR (R = Me, Bz)] in excellent yields and enantiomeric excesses (over 90%), utilizing aqueous 30% H₂O₂ or cumene hydroperoxide (CHP) as the oxidant. These complexes are also catalytically active in the oxidation of styrene, using aqueous 30% H₂O₂ or tert-butyl hydroperoxide (TBHP) as the oxidant, to styrene oxide, benzaldehyde, benzoic acid, phenylacetaldehyde and 1-phenylethane-1,2-diol.

Full text of DOI:10.1016/j.poly.2012.12.038

Polyhedron 53 (2013) 172–178
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Polyhedron
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Oxidovanadium(V) complexes with chiral tridentate Schiff bases derived
from R(ꢀ)-phenylglycinol: Synthesis, spectroscopic characterization
and catalytic activity in the oxidation of sulfides and styrene
⇑
Grzegorz Romanowski , Jaromir Kira
Faculty of Chemistry, University of Gdan´sk, Sobieskiego 18/19, PL-80952 Gdan´sk, Poland
a r t i c l e i n f o
a b s t r a c t
Article history:
A series of vanadium(V) complexes with chiral tridentate Schiff base ligands, obtained by the single con-
densation of R(ꢀ)-phenylglycinol with salicylaldehyde and its derivatives, were prepared. The complexes
were characterized by elemental analysis and by their ESI-MS, IR, CD, UV–Vis, 1D (1H, 51V) and 2D (COSY,
gHSQC) NMR spectra. The vanadium(V) complexes have the ability to catalyze the oxidation of sulfides
[PhSR (R = Me, Bz)] in excellent yields and enantiomeric excesses (over 90%), utilizing aqueous 30%
H2O2 or cumene hydroperoxide (CHP) as the oxidant. These complexes are also catalytically active in
the oxidation of styrene, using aqueous 30% H2O2 or tert-butyl hydroperoxide (TBHP) as the oxidant,
to styrene oxide, benzaldehyde, benzoic acid, phenylacetaldehyde and 1-phenylethane-1,2-diol.
Ó 2013 Elsevier Ltd. All rights reserved.
Received 16 October 2012
Accepted 31 December 2012
Available online 26 January 2013
Keywords:
Vanadium(V) complex
Schiff base
Sulfoxidation
Oxidation of styrene
1. Introduction
epoxides are very important intermediates in laboratory organic
synthesis, found as intermediate products in some biosynthetic
Vanadium is an abundant bioelement in diverse biological sys-
tems [1], being found in some classes of enzymes, e.g. nitrogenases
[2] and haloperoxidases [3]. The discovery of vanadate-dependent
haloperoxidases and their importance in various biological cata-
lytic and inhibitory processes has stimulated research on the cata-
lytic aspects of vanadium complexes as model compounds [4,5].
Many such model vanadium(V) Schiff base complexes show cata-
lytic activity towards various organic transformations [6,7]. The
salicylaldimine moiety was found to be suitable and found wide-
spread application among the ligands used for complexation to
vanadium [8–10]. The main reason for this is that Schiff base li-
gands and their metal complexes can be easily synthesized and
they are efficient catalysts under homogeneous and heterogeneous
conditions.
Chiral sulfoxides possess a wide range of biological activities,
e.g. antimicrobial properties [11], inhibition of the biosynthesis of
uric acid [12] and gastric acid secretion [13] or regulation of cho-
lesterol catabolism [14]. Numerous chemical and biological meth-
ods for synthesizing chiral sulfoxides have been reported, but
metal-catalyzed (including vanadium) enantioselective oxidation
of prochiral sulfides is undoubtedly the most direct and economi-
cal approach for the synthesis of optically pure sulfoxides. Also
pathways. Their importance arises mainly from the ring opening
of epoxides, which allows straightforward elaboration to useful
generation of new carbon–carbon bonds.
Until now, vanadium(V) complexes with tridentate Schiff base
ligands derived from chiral and achiral amino alcohols have been
used successfully as catalysts in the enantioselective oxidation of
organic sulfides [15,16], the asymmetric alkynylation of aldehydes
[16], the epoxidation of cyclooctene [17], the oxidation of bromide
[18], the stereoselective synthesis of functionalized tetrahydrofu-
rans [18,19] and oxidative kinetic resolution of
a-hydroxy esters
[20]. In continuation of our studies on the synthesis, structure,
spectroscopic and catalytic properties of vanadium(V) complexes
incorporating chiral tridentate Schiff base ligands [21–23], a series
of new oxidovanadium(V) complexes comprising coordinated sin-
gle condensation products of R(ꢀ)-phenylglycinol with aromatic
o-hydroxyaldehydes, presented in Fig. 1, have been described.
Their spectroscopic properties by 1D and 2D NMR, UV–Vis, CD, IR
and ESI-MS have been examined. The catalytic potential of the oxi-
dovanadium(V) complexes in asymmetric oxidation, i.e. enantiose-
lective sulfoxidation of methyl phenyl sulfide (PhSMe) and benzyl
phenyl sulfide (PhSBz), utilizing aqueous 30% H2O2 or cumene
hydroperoxide (CHP) as the oxidant has been studied. Moreover,
these complexes were also used as catalysts in the oxidation of sty-
rene, using aqueous 30% H2O2 or tert-butyl hydroperoxide (TBHP)
as the oxidant.
⇑
Tel.: +48 585235341.
E-mail address: greg@chem.univ.gda.pl (G. Romanowski).
0277-5387/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.poly.2012.12.038

G. Romanowski, J. Kira / Polyhedron 53 (2013) 172–178
173
R1
(TBHP) in decane, and catalyst (0.010 mmol) were taken in 10 ml
of CH3CN and the reaction was carried out for 6 h at 80 °C. The
reaction was monitored by GC and the yields were recorded as
the GC yield based on the starting styrene. The identity of the oxi-
dation products were confirmed by GC–MS. The influence of the
amounts of catalyst and oxidant were also studied to check their
effect on the conversion and selectivity of the reaction products.
2
R
R3
O
N
O
V
O-V=O(L)
O
R4
2.3. Complexes
1
2
3
4
R
H
R
R
R
The complexes were obtained by the following example proce-
dure. A solution of 5 mmol of R(ꢀ)-phenylglycinol in 10 ml abso-
lute ethanol was added with stirring to 5 mmol of an aromatic
o-hydroxyaldehyde (salicylaldehyde, 3-methoxysalicylaldehyde,
5-methoxysalicylaldehyde, 4,6-dimethoxysalicylaldehyde, 5-methyl-
salicylaldehyde, 5-bromosalicylaldehyde, 5-nitrosalicylaldehyde,
2,4-dihydroxybenzaldehyde, 3-tert-butylsalicylaldehyde, 2-hydro-
xy-1-naphthaldehyde) in 20 ml absolute EtOH and heated under
reflux for 1 h. Then vanadium(V) oxytriethoxide (5 mmol) in
10 ml of absolute EtOH was added and stirred at room temperature
for 2 h. After cooling in a fridge, a solid was separated and filtered
off, washed several times and recrystallized from absolute EtOH.
H
H
H
H
H
H
H
1
2
3
4
5
6
7
8
9
OCH3
H
OCH3
H
H
H
H
H
H
H
OCH3
OCH3
H
H
H
CH3
Br
H
H
OH
H
NO
2
H
H
H
H
C(CH )
H
3 3
10
H
H
(CH)4
Fig. 1. Structural formulae of the vanadium(V) complexes.
2.3.1.
l
-Oxido-bis({R(ꢀ)-2-[(1-oxido-2-phenylethyl)iminomethyl]
phenolato-j3N,O,O0}oxidovanadium(V)) (1)
2. Experimental
Yield 83%. Anal. Calc. for C30H26N2O7V2: C, 57.3; H, 4.2; N, 4.5.
Found: C, 57.4; H, 4.3; N, 4.5%. IR (KBr, cmꢀ1): 1621 (
m
C@N); 978
V@O). UV–Vis spectrum in MeOH [kmax (nm), e (Mꢀ1 cmꢀ1)]: 280
(8860), 343 (3710). CD spectrum in MeOH [kmax (nm),
e (Mꢀ1
2.1. Measurements
(m
D
All chemicals and reagents were obtained from commercial
sources and used without further purification unless stated other-
wise. Carbon, hydrogen and nitrogen contents were determined on
a Carlo Erba MOD 1106 elemental analyzer. IR spectra of solid sam-
ples (KBr pellets) were run on a Bruker IFS 66, and electronic spec-
tra on a Perkin-Elmer LAMBDA 18 spectrophotometer. Circular
dichroism spectra were measured with a Jasco J-815 spectropolar-
imeter. ESI–MS measurements were performed in MeOH solutions
with a Bruker Daltonics HCTultra mass spectrometer (ion trap).
NMR spectra were obtained in CD3OD solutions with a Varian
Mercury-400BB (400 MHz) spectrometer using TMS (1H) and
cmꢀ1)]: 279 (ꢀ1.84), 312 (4.02), 363 (ꢀ5.32). 1H NMR (CD3OD,
ppm) major (60%): 8.27 (1H, s) (azomethine); 7.51 (3H, s), 7.43
(3H, m), 7.36 (2H, m), 6.84 (1H, d, 4J = 3 Hz) (aromatic); 5.13 (1H,
t, 3J = 8 Hz), 4.81 (1H, ov) (methylene); 5.00 (1H, dd, 3J = 8 Hz,
4J = 3 Hz) (methine); minor (40%): 8.39 (1H, s) (azomethine); 7.49
(3H, s), 7.45 (3H, ov), 7.39 (2H, ov), 6.94 (1H, d, 4J = 3 Hz)
(aromatic); 5.65 (1H, t, 3J = 8 Hz), 5.15 (1H, ov), (methylene);
4.98 (1H, ov) (methine). 51V NMR (CD3OD, ppm) major (60%):
ꢀ532.1, minor (40%): ꢀ535.9. ESI–MS (m/z): 696.7 [V2O3L2–
H+ + CH3OH + 2H2O].
VOCl3
gas chromatograph with
0.25 mm ꢁ 0.25 mm) and FID detector was used to analyze the
reaction products. The identity of the products was confirmed
using a GC–MS model Shimadzu GCMS-QP2010 SE.
(
51V) as reference compounds. A Perkin-Elmer Clarus 500
2.3.2.
l
-Oxido-bis({R(ꢀ)-2-[(1-oxido-2-phenylethyl)iminomethyl]-6-
a
DB-5 capillary column (30 m ꢁ
methoxyphenolato-j3N,O,O0}oxidovanadium(V)) (2)
Yield 85%. Anal. Calc. for C32H30N2O9V2: C, 55.8; H, 4.4; N, 4.1.
Found: C, 55.7; H, 4.5; N, 4.0%. IR (KBr, cmꢀ1): 1628 (
m
C@N); 984
V@O). UV–Vis spectrum in MeOH [kmax (nm), e (Mꢀ1 cmꢀ1)]: 281
(8740), 346 (3680). CD spectrum in MeOH [kmax (nm),
e (Mꢀ1
(m
D
cmꢀ1)]: 283 (ꢀ2.18), 314 (2.84), 372 (ꢀ4.15). 1H NMR (CD3OD,
ppm) major (60%): 8.24 (1H, s) (azomethine); 7.38–7.50 (5H, m),
7.14 (1H, d, 3J = 8 Hz), 6.89 (1H, d, 3J = 8 Hz), 6.78 (1H, t, 3J = 8 Hz)
(aromatic); 5.16 (1H, t, 3J = 8 Hz), 4.82 (1H, ov) (methylene); 5.02
(1H, dd, 3J = 8 Hz, 4J = 3 Hz) (methine); 3.87 (3H, s) (methoxy);
minor (40%): 8.39 (1H, s) (azomethine); 7.32–7.42 (5H, m), 7.18
(1H, d, 3J = 8 Hz), 6.99 (1H, d, 3J = 8 Hz), 6.84 (1H, t, 3J = 8 Hz) (aro-
matic); 5.67 (1H, t, 3J = 8 Hz), 5.18 (1H, ov) (methylene); 5.00 (1H,
ov) (methine); 3.89 (3H, s) (methoxy). 51V NMR (CD3OD, ppm) ma-
jor (60%): ꢀ528.8; minor (40%): ꢀ531.7. ESI–MS (m/z): 756.8
[V2O3L2–H+ + CH3OH + 2H2O].
2.2. Catalytic activity
2.2.1. Sulfoxidation
In typical procedure, to a solution of a catalyst (0.010 mmol) in
3 ml of CH2Cl2/MeOH solution (7:3, v/v), a sulfide (1.00 mmol) was
added at room temperature or ꢀ20 °C, together with 1,3,5-
trimethoxybenzene as an internal standard. Aqueous 30% H2O2
or cumene hydroperoxide (CHP) as the oxidant was added
(1.10 mmol) in small portions and the resulting mixture was stir-
red. After the appropriate reaction time, the solution was quenched
with 2 ml of sodium sulfite solution (0.1 M) and extracted with
CH2Cl2 (3 ꢁ 5 ml). The combined organic layers were evaporated
to dryness. The solid product dissolved in CDCl3 was analysed
(yield and ee value) by 1H NMR spectra in the presence of the chiral
shift reagent Eu(hfc)3 (where Hhfc is 3-(heptafluoropropylhydr-
oxymethylene)-(+)-camphoric acid) [24].
2.3.3.
l
-Oxido-bis({R(ꢀ)-2-[(1-oxido-2-phenylethyl)iminomethyl]-4-
methoxyphenolato-j3N,O,O0}oxidovanadium(V)) (3)
Yield 87%. Anal. Calc. for C32H30N2O9V2: C, 55.8; H, 4.4; N, 4.1.
Found: C, 55.8; H, 4.5; N, 4.1%. IR (KBr, cmꢀ1): 1625 (
m
C@N); 982
(
m
V@O). UV–Vis spectrum in MeOH [kmax (nm), e (Mꢀ1 cmꢀ1)]: 284
2.2.2. Oxidation of styrene
In typical procedure, styrene (1.00 mmol), an oxidant
(3.00 mmol), i.e. aqueous 30% H2O2 or tert-butyl hydroperoxide
(9030), 353 (3860). CD spectrum in MeOH [kmax (nm), D
e (Mꢀ1
cmꢀ1)]: 281 (ꢀ2.07), 313 (2.56), 369 (ꢀ5.13). 1H NMR (CD3OD,
ppm) major (60%): 8.27 (1H, s) (azomethine); 7.38–7.50 (5H, m),

174
G. Romanowski, J. Kira / Polyhedron 53 (2013) 172–178
7.15 (1H, dd, 3J = 8 Hz, 4J = 3 Hz), 6.91 (1H, d, 3J = 8 Hz), 6.86 (1H, d,
4J = 3 Hz) (aromatic); 5.14 (1H, t, 3J = 8 Hz), 4.82 (1H, ov) (methy-
lene); 5.01 (1H, dd, 3J = 8 Hz, 4J = 3 Hz) (methine); 3.72 (3H, s)
(methoxy); minor (40%): 8.39 (1H, s) (azomethine); 7.34–7.43
(5H, m), 7.18 (1H, dd, 3J = 8 Hz, 4J = 3 Hz), 6.95 (1H, d, 3J = 8 Hz),
6.89 (1H, d, 4J = 3 Hz) (aromatic); 5.66 (1H, t, 3J = 8 Hz), 5.17 (1H,
ov) (methylene); 4.99 (1H, ov) (methine); 3.75 (3H, s) (methoxy).
51V NMR (CD3OD, ppm) major (60%): ꢀ527.2; minor (40%):
ꢀ531.1. ESI–MS (m/z): 756.8 [V2O3L2–H+ + CH3OH + 2H2O].
(
m
V@O). UV–Vis spectrum in MeOH [kmax (nm), e (Mꢀ1 cmꢀ1)]: 346
(11350). CD spectrum in MeOH [kmax (nm),
e (Mꢀ1 cmꢀ1)]: 283
D
(ꢀ2.76), 318 (4.07), 361 (ꢀ5.26). 1H NMR (CD3OD, ppm) major
(60%): 8.41 (1H, s) (azomethine); 8.22 (1H, d, 4J = 3 Hz), 7.92 (1H,
dd, 3J = 8 Hz, 4J = 3 Hz), 7.43–7.52 (5H, m), 6.95 (1H, d, 3J = 8 Hz)
(aromatic); 5.17 (1H, t, 3J = 8 Hz), 4.83 (1H, ov) (methylene); 5.02
(1H, dd, 3J = 8 Hz, 4J = 3 Hz) (methine); minor (40%): 8.43 (1H, s)
(azomethine); 8.24 (1H, ov), 7.94 (1H, ov), 7.38–7.44 (5H, ov),
7.04 (1H, d, 3J = 8 Hz) (aromatic); 5.68 (1H, t, 3J = 8 Hz), 5.19 (1H,
ov) (methylene); 4.98 (1H, ov) (methine). 51V NMR (CD3OD, ppm)
major (60%): ꢀ531.8; minor (40%): ꢀ534.5. ESI–MS (m/z): 786.7
[V2O3L2–H+ + CH3OH + 2H2O].
2.3.4.
l
-Oxido-bis({R(ꢀ)-2-[(1-oxido-2-phenylethyl)iminomethyl]-
3,5-dimethoxyphenolato-j3N,O,O0}oxidovanadium(V)) (4)
Yield 78%. Anal. Calc. for C34H34N2O11V2: C, 54.6; H, 4.6; N, 3.7.
Found: C, 54.7; H, 4.7; N, 3.6%. IR (KBr, cmꢀ1): 1627 (
m
C@N); 980
V@O). UV–Vis spectrum in MeOH [kmax (nm), e (Mꢀ1 cmꢀ1)]: 318
(14580). CD spectrum in MeOH [kmax (nm),
e (Mꢀ1 cmꢀ1)]: 278
2.3.8.
l
-Oxido-bis({R(ꢀ)-2-[(1-oxido-2-phenylethyl)iminomethyl]-5-
(m
hydroxyphenolato-j3N,O,O0}oxidovanadium(V)) (8)
D
Yield 80%. Anal. Calc. for C30H26N2O9V2: C, 54.6; H, 4.2; N, 4.0.
(ꢀ1.92), 293 (2.71), 354 (ꢀ4.92). 1H NMR (CD3OD, ppm) major
(60%): 8.57 (1H, s) (azomethine); 7.38–7.47 (5H, m), 6.12 (1H, d,
3J = 8 Hz), 6.00 (1H, d, 3J = 8 Hz) (aromatic); 5.14 (1H, t, 3J = 8 Hz),
4.82 (1H, ov) (methylene); 5.02 (1H, dd, 3J = 8 Hz, 4J = 3 Hz)
(methine); 3.83 (3H, s), 3.69 (3H, s) (methoxy); minor (40%):
8.68 (1H, s) (azomethine); 7.26–7.43 (5H, m), 6.16 (1H, d,
3J = 8 Hz), 6.06 (1H, d, 3J = 8 Hz) (aromatic); 5.68 (1H, t, 3J = 8 Hz),
5.17 (1H, t, 3J = 8 Hz), (methylene); 5.00 (1H, dd, 3J = 8 Hz,
4J = 3 Hz) (methine); 3.85 (3H, s), 3.78 (3H, s) (methoxy). 51V
NMR (CD3OD, ppm) major (60%): ꢀ528.8; minor (40%): ꢀ532.2.
ESI–MS (m/z): 816.8 [V2O3L2-H+ + CH3OH + 2H2O].
Found: C, 54.7; H, 4.1; N, 4.1%. IR (KBr, cmꢀ1): 1631 (
m
C@N); 973
V@O). UV–Vis spectrum in MeOH [kmax (nm), e (Mꢀ1 cmꢀ1)]: 292
(10270), 356 (3910). CD spectrum in MeOH [kmax (nm),
e (Mꢀ1
(m
D
cmꢀ1)]: 279 (ꢀ2.05), 314 (2.56), 361 (ꢀ4.87). 1H NMR (CD3OD,
ppm) major (60%): 8.11 (1H, s) (azomethine); 7.41–7.53 (5H, m),
7.13 (1H, d, 3J = 8 Hz), 6.34 (2H, s) (aromatic); 5.13 (1H, t,
3J = 8 Hz), 4.81 (1H, ov) (methylene); 5.01 (1H, dd, 3J = 8 Hz,
4J = 3 Hz) (methine); 3.93 (1H, s) (hydroxy); minor (40%): 8.22
(1H, s) (azomethine); 7.29–7.41 (5H, m), 7.21 (1H, d, 3J = 8 Hz),
6.36 (2H, s) (aromatic); 5.66 (1H, t, 3J = 8 Hz), 5.15 (1H, ov) (meth-
ylene); 4.99 (1H, ov) (methine); 4.43 (1H, s) (hydroxy). 51V NMR
(CD3OD, ppm) major (60%): ꢀ529.4; minor (40%): ꢀ531.7. ESI–
MS (m/z): 728.7 [V2O3L2–H+ + CH3OH + 2H2O].
2.3.5.
l
-Oxido-bis({R(ꢀ)-2-[(1-oxido-2-phenylethyl)iminomethyl]-4-
methylphenolato-j3N,O,O0}oxidovanadium(V)) (5)
Yield 81%. Anal. Calc. for C32H30N2O7V2: C, 58.5; H, 4.6; N, 4.3.
2.3.9.
l
-Oxido-bis({R(ꢀ)-2-[(1-oxido-2-phenylethyl)iminomethyl]-6-
Found: C, 58.6; H, 4.6; N, 4.3%. IR (KBr, cmꢀ1): 1628 (
m
C@N); 982
V@O). UV–Vis spectrum in MeOH [kmax (nm), e (Mꢀ1 cmꢀ1)]: 278
(8720), 329 (3660). CD spectrum in MeOH [kmax (nm),
e (Mꢀ1
tert-butylphenolato-j3N,O,O0}oxidovanadium(V)) (9)
(m
Yield 72%. Anal. Calc. for C38H42N2O7V2: C, 61.6; H, 5.7; N, 3.8.
D
Found: C, 61.5; H, 5.7; N, 3.7%. IR (KBr, cmꢀ1): 1629 (
m
C@N); 979
V@O). UV–Vis spectrum in MeOH [kmax (nm), e (Mꢀ1 cmꢀ1)]: 288
(12680), 341 (4520). CD spectrum in MeOH [kmax (nm),
e (Mꢀ1
cmꢀ1)]: 280 (ꢀ2.54), 313 (3.46), 365 (ꢀ5.33). 1H NMR (CD3OD,
ppm) major (60%): 8.21 (1H, s) (azomethine); 7.38–7.50 (5H, m),
7.42 (1H, ov), 7.10 (1H, s), 6.87 (1H, d, 3J = 8 Hz) (aromatic); 5.14
(1H, t, 3J = 8 Hz), 4.82 (1H, ov) (methylene); 5.01 (1H, dd,
3J = 8 Hz, 4J = 3 Hz) (methine); 2.24 (3H, s) (methyl); minor (40%):
8.34 (1H, s) (azomethine); 7.34–7.43 (5H, m), 7.44 (1H, ov), 7.19
(1H, s), 6.89 (1H, d, 3J = 8 Hz) (aromatic); 5.69 (1H, t, 3J = 8 Hz),
5.18 (1H, ov) (methylene); 4.99 (1H, ov) (methine); 2.28 (3H, s)
(methyl). 51V NMR (CD3OD, ppm) major (60%): ꢀ529.3; minor
(40%): ꢀ533.0. ESI–MS (m/z): 724.8 [V2O3L2–H+ + CH3OH + 2H2O].
(m
D
cmꢀ1)]: 282 (ꢀ2.24), 316 (3.12), 365 (ꢀ4.95). 1H NMR (CD3OD,
ppm) major (60%): 8.28 (1H, s) (azomethine); 7.53 (1H, dd,
3J = 8 Hz, 4J = 3 Hz), 7.41–7.48 (5H, m), 7.15 (1H, dd, 3J = 8 Hz,
4J = 3 Hz), 6.81 (1H, t, 3J = 8 Hz) (aromatic); 5.10 (1H, t, 3J = 8 Hz),
4.81 (1H, ov) (methylene); 5.02 (1H, dd, 3J = 8 Hz, 4J = 3 Hz)
(methine); 1.49 (9H, s) (tert-butyl); minor (40%): 8.40 (1H, s) (azo-
methine); 7.56 (1H, dd, 3J = 8 Hz, 4J = 3 Hz), 7.32–7.41 (5H, m), 7.25
(1H, dd, 3J = 8 Hz, 4J = 3 Hz), 6.87 (1H, t, 3J = 8 Hz) (aromatic); 5.71
(1H, t, 3J = 8 Hz), 5.13 (1H, ov) (methylene); 5.00 (1H, ov)
(methine); 1.50 (9H, ov) (tert-butyl). 51V NMR (CD3OD, ppm) major
(60%): ꢀ537.6; minor (40%): ꢀ539.8. ESI–MS (m/z): 808.9 [V2O3L2–
H+ + CH3OH + 2H2O].
2.3.6.
l
-Oxido-bis({R(ꢀ)-2-[(1-oxido-2-phenylethyl)iminomethyl]-4-
bromophenolato-j3N,O,O0}oxidovanadium(V)) (6)
Yield 77%. Anal. Calc. for Br2C30H24N2O7V2: C, 45.8; H, 3.1; N, 3.6.
Found: C, 45.7; H, 3.1; N, 3.5%. IR (KBr, cmꢀ1): 1626 (
m
C@N); 974
V@O). UV–Vis spectrum in MeOH [kmax (nm), e (Mꢀ1 cmꢀ1)]: 286
(9580), 341 (4140). CD spectrum in MeOH [kmax (nm),
e (Mꢀ1
(m
2.3.10.
l
-Oxido-bis({R(ꢀ)-2-[(1-oxido-2-
D
phenylethyl)iminomethyl]naphtholato-j3N,O,O0}oxidovanadium(V)
(10)
cmꢀ1)]: 281 (ꢀ2.13), 312 (2.37), 364 (ꢀ3.62). 1H NMR (CD3OD,
ppm) major (60%): 8.23 (1H, s) (azomethine); 7.58 (1H, dd,
3J = 8 Hz, 4J = 3 Hz), 7.47 (1H, ov), 7.42–7.51 (5H, m), 6.88 (1H, d,
3J = 8 Hz) (aromatic); 5.21 (1H, t, 3J = 8 Hz), 4.84 (1H, ov) (methy-
lene); 5.03 (1H, dd, 3J = 8 Hz, 4J = 3 Hz) (methine); minor (40%):
8.35 (1H, s) (azomethine); 7.52 (1H, ov), 7.42 (1H, ov), 7.37–7.44
(5H, ov), 6.91 (1H, d, 3J = 8 Hz) (aromatic); 5.66 (1H, t, 3J = 8 Hz),
5.21 (1H, ov) (methylene); 5.01 (1H, ov) (methine). 51V NMR
(CD3OD, ppm) major (60%): ꢀ533.4; minor (40%): ꢀ536.1. ESI–
MS (m/z): 854.5 [V2O3L2–H+ + CH3OH + 2H2O].
Yield 79%. Anal. Calc. for C38H34N2O7V2: C, 62.3; H, 4.7; N, 3.8.
Found: C, 62.2; H, 4.8; N, 3.8%. IR (KBr, cmꢀ1): 1633 (
m
C@N); 984
V@O). UV–Vis spectrum in MeOH [kmax (nm), e (Mꢀ1 cmꢀ1)] 308
(10470), 394 (4280). CD spectrum in MeOH [kmax (nm),
e (Mꢀ1
(m
D
cmꢀ1)]: 284 (ꢀ2.16), 319 (2.78), 363 (ꢀ4.06). 1H NMR (CD3OD,
ppm) major (60%): 9.14 (1H, s) (azomethine); 8.02 (1H, d,
3J = 8 Hz), 7.81 (1H, d, 3J = 8 Hz), 7.48–7.62 (5H, m), 7.54–7.58
(3H, ov), 7.21 (1H, d, 3J = 8 Hz) (aromatic); 5.23 (1H, t, 3J = 8 Hz),
4.86 (1H, ov) (methylene); 5.18 (1H, dd, 3J = 8 Hz, 4J = 3 Hz)
(methine); minor (40%): 9.40 (1H, s) (azomethine); 8.08 (1H, d,
3J = 8 Hz), 7.85 (1H, d, 3J = 8 Hz), 7.31–7.46 (5H, m), 7.42–7.54
(3H, ov), 7.24 (1H, d, 3J = 8 Hz), (aromatic); 5.96 (1H, t, 3J = 8 Hz),
5.33 (1H, t, 3J = 8 Hz), (methylene); 5.15 (1H, dd, 3J = 8 Hz,
4J = 3 Hz) (methine). 51V NMR (CD3OD, ppm) major (60%):
2.3.7.
l
-Oxido-bis({R(ꢀ)-2-[(1-oxido-2-phenylethyl)iminomethyl]-4-
nitrophenolato-j3N,O,O0}oxidovanadium(V)) (7)
Yield 75%. Anal. Calc. for C30H24N4O11V2: C, 50.2; H, 3.4; N, 7.8.
Found: C, 50.1; H, 3.5; N, 7.7%. IR (KBr, cmꢀ1): 1630 (
mC@N); 985

G. Romanowski, J. Kira / Polyhedron 53 (2013) 172–178
175
ꢀ531.1; minor (40%): ꢀ533.4. ESI–MS (m/z): 800.9 [V2O3L2–
tion of major species varies from 53% to 95% [16]. The 51V NMR
spectra exhibit two signals with the same major:minor ratios as
the signals in the 1H NMR spectra. According to Rehder and co-
workers [30], possible reasons for presence of two species in solu-
tion could be diastereomers, equilibria between monomeric and
dimeric (or oligomeric) complexes or flexibility with respect to
the coordination geometry and the arrangement of the ligands.
When dimers or oligomers are formed, the temperature depen-
dence of the chemical shift and the increasing intensity of signals
with an increase in the concentration of the complexes should be
observed in the 51V NMR spectra. These rather high-field reso-
nances are present in chloroform solution [31] and disappear when
methanol is used as the solvent, due to an additional coordination
of methanol leading to six-coordinated vanadium species. We as-
sume that in our case the existence of two diastereomers (AR
and CR), caused by the chiral R methine carbon atom in the 2-ami-
no-2-phenyl-1-ethanolate moiety and two possible (clockwise – C
and anti-clockwise – A) absolute configurations at the vanadium
atom (Fig. 2), are the most likely explanation for the presence of
the two 51V NMR signals [32,33].
H+ + CH3OH + 2H2O].
3. Results and discussion
3.1. Spectroscopic properties
The electronic, circular dichroism, NMR and selected solid-state
IR spectral data are listed in Section 2.
The IR spectra of the solid complexes display a strong C@N
stretch (around 1620 cmꢀ1) for the C@N group of the coordinated
Schiff base ligands [25,26]. These vanadium(V) species also show
absorption bands around 980 cmꢀ1, attributable to single V@O
stretching vibrations, which are close to the values reported for re-
lated oxidovanadium(V) Schiff base complexes [17,19]. The
appearance of a weak/medium band near 2800 cmꢀ1, separate
from other C–H stretching vibrations, in complexes 2, 3 and 4 indi-
cates the presence of the methoxy substituents [27].
The electronic and circular dichroism spectra of all the com-
plexes were recorded in methanol. Strong intense bands,
emax = 8720–12680 dm3 molꢀ1 cmꢀ1, with kmax in the region 278–
3.2. Catalytic activity studies
308 nm are considered to arise from intraligand
p–
p⁄ transitions.
The low energy bands, recorded for complexes 1–3, 5, 6, 8–10 be-
tween 329 and 394 nm (emax = 3660–4520 dm3 molꢀ1 cmꢀ1) may
be assigned as a ligand-to-metal charge transfer (LMCT) transition,
3.2.1. Sulfoxidation
In this study, the oxidovanadium(V) complexes 1–10 have been
tested for their ability to catalyze the oxidation of prochiral sul-
fides using methyl phenyl sulfide (thioanisol) and phenyl benzyl
sulfide as model substrates (Fig. 3). Aqueous 30% H2O2 or cumene
hydroperoxide (CHP) were used as the oxidants in the slight excess
of 1.10 equivalents based on the sulfide substrate. Reactions were
run with 1 mol% of catalyst based on the model substrate. The re-
sults of the catalytic studies are listed in Table 1.
The best results have been obtained for complexes 2, 3, 5 and 8
as catalysts in the oxidation of methyl phenyl sulfide with 30%
H2O2 as the oxidant (Table 1, entries 2, 3, 5 and 8). The overall
yields for all the catalysts were in the range 84–92% within a
30 min reaction time and enantiomeric excesses (ee’s) of 71–87%
of the S-configured sulfoxide were obtained. For the same time of
reaction, with catalysts 2, 3, 5 and 8, using methyl phenyl sulfide,
but employing CHP as the oxidant, similar yields (81–88%) but
much lower ee’s (69–73%) of the S-configured sulfoxide were ob-
tained (Table 1, entries 15–18). When benzyl phenyl sulfide was
used as the substrate, possessing a more bulky substituent, with
hydrogen peroxide as the oxidant, the overall yield of the sulfoxide
was distinctly lower (70–75%) and the enantioselectivity decreases
originating from the p orbital on the phenolate oxygen to the
p
empty d orbital of the vanadium(V) center [28,29]. The spectra of
complexes 4 and 7, examined in the same spectral region, display
only one very strong band at 318 nm (emax = 14580 dm3 molꢀ1
cmꢀ1) and 346 nm ( max = 11350 dm3 molꢀ1 cmꢀ1), respectively,
e
similar to the same strong bands at around 320 and 350 nm
for complexes derived from 4,6-dimethoxysalicylaldehyde and
5-nitrosalicylaldehyde [18,21,22]. The circular dichroism spectra
revealed an additional band in the 293–319 nm range with an
opposite sign of the Cotton effect to the bands at around 280 and
360 nm.
The vanadium(V) complexes were also characterized in MeOH
solution by ESI–MS analysis, which showed the presence of mass
peaks corresponding to the methanol and water adducts of oxi-
do-bridged dinuclear vanadium(V) complexes, 1–10.
The 1H NMR signals were assigned on the basis of intensity,
spin–spin coupling pattern, chemical shifts, COSY and gHSQC tech-
niques. Single condensation of salicylaldehyde and its derivatives
with 2-amino-2-phenyl-1-ethanol is confirmed by the presence
of azomethine proton signals in the spectra of all the complexes.
Signals of the methyl and methine protons in all the compounds
have been readily recognized. For this purpose 2D (COSY, gHSQC)
NMR spectroscopy was performed for all complexes which unam-
biguously established the attachment of the carbon atoms in the 2-
amino-2-phenyl-1-ethanolate fragment. For example, in case of
the COSY spectrum of 6, cross-peaks between the methine proton
signal (major species signal at 5.03 ppm) and two methylene pro-
tons signals, triplet at 5.21 ppm and overlapped resonance at
4.84 ppm, are observed. Moreover, the NOESY spectra exhibit
cross-peaks between the signals of the azomethine proton and
the aromatic protons, and also between the signals of the azome-
thine proton and the methine proton at 5.03 ppm. As expected,
there is lack of any cross-peak between the signals of the azome-
thine proton and two methylene protons at 4.84 and 5.21 ppm.
In the 1H NMR spectra in CD3OD, two sets of signals are ob-
served, suggesting the presence of two species in solution. The
chemical shifts for the two species are listed separately and desig-
nated as major and minor species. In all cases, the major:minor ra-
tios are about 60:40, in contrast to similar vanadium(V) Schiff base
complexes derived from different amino alcohols, where contribu-
R
R
O
V
O
N
O
N
O-V=O(L)
O
O-V=O(L)
O
V
O
AR
CR
Fig. 2. Visualization of AR and CR diastereomers caused by dissymmetric vana-
dium(V) ions.
O
S
S
1 mol% catalyst
H O or CHP
R
R
2
2
Fig. 3. Sulfoxidation of thioethers catalyzed by the oxidovanadium(V) complexes.

176
G. Romanowski, J. Kira / Polyhedron 53 (2013) 172–178
Table 1
HO
OH
Catalytic oxidation of PhSMe and PhSBz by 30% H2O2 or cumene hydroperoxide (CHP)
as the oxidant in presence of 1 mol% vanadium(V) Schiff base complexes as the
catalyst.
Entry Catalyst Substrate Oxidant Yield
(%)
T
(°C)
t
ee (%) a
PhED
(min)
O
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
1
2
3
4
5
6
7
8
9
10
2
3
5
8
2
3
5
8
2
3
5
8
PhSMe
PhSMe
PhSMe
PhSMe
PhSMe
PhSMe
PhSMe
PhSMe
PhSMe
PhSMe
PhSBz
H2O2
H2O2
H2O2
H2O2
H2O2
H2O2
H2O2
H2O2
H2O2
H2O2
H2O2
H2O2
H2O2
H2O2
CHP
CHP
CHP
CHP
H2O2
H2O2
H2O2
H2O2
88
85
84
87
90
92
86
85
89
87
72
75
70
71
88
83
87
81
89
90
87
85
RT
RT
RT
RT
RT
RT
RT
RT
RT
RT
RT
RT
RT
RT
RT
RT
30
30
30
30
30
30
30
30
30
30
30
30
30
30
30
30
30
71
84
87
77
86
77
71
82
66
69
63
61
59
63
71
69
70
73
91
94
90
92
O
HO
O
1 mol% catalyst
H O or TBHP
2
2
styrene
StO
BzA
BzAC
O
PhSBz
PhSBz
PhSBz
PhAA
Fig. 4. Various oxidation products of styrene catalyzed by the oxidovanadium(V)
PhSMe
PhSMe
PhSMe
PhSMe
PhSMe
PhSMe
PhSMe
PhSMe
complexes.
RT
RT
ꢀ20
ꢀ20
ꢀ20
ꢀ20
30
180
180
180
180
For three different molar ratios of styrene to aqueous 30% H2O2
or tert-butyl hydroperoxide (TBHP) in decane (i.e. 1:1, 1:2 and 1:3),
styrene (1.00 mmol) and a catalyst (0.010 mmol) were taken in
acetonitrile (10 ml), and the reaction was carried out with 6 h of
contact time at 80 °C. At a 30% H2O2 to styrene molar ratio of
1:1, a maximum of 8.3% conversion was achieved. Increasing the
ratio to 2:1 improved the conversion to 25.8%, whilst a 3:1 ratio
showed a maximum of 33.2% conversion. Further increments of
H2O2 showed no improvement in he conversion, therefore a 3:1 ra-
tio was considered as being adequate. In the case of using TBHP as
the oxidant, increasing the TBHP/styrene ratio from 1:1 to 2:1 and
3:1 improved the conversion from 36.4% to 62.1% and 84.8%,
respectively. As in the previous case, the oxidation improved only
marginally upon further addition of the oxidant.
Similarly for three different amounts (i.e. 0.5, 1 and 2 mol%) of
catalyst and oxidant to a styrene molar ratio of 3:1 under above
reaction conditions, 0.5 mol% gave only 6.3%, in the case of using
H2O2, and 36.8% (TBHP) oxidative conversion, while 1 and 2 mol%
showed nearly identical results, with 33.2 and 84.8% conversion
for H2O2 and TBHP respectively, with 6 h of contact time. Thus,
1 mol% of catalyst may be considered sufficient to run the reaction
under above conditions. A blank reaction under the above reaction
conditions gave ca. 2–3% conversion with both oxidants.
Using tert-butyl hydroperoxide (TBHP) as the oxidant, under the
optimized reaction conditions (i.e. 1.00 mmol of styrene,
3.00 mmol of oxidant, 1 mol% of catalyst), all the complexes gave
very high 80% conversion, especially in comparison to the other re-
ported earlier dioxidovanadium(V) complexes with Schiff bases, i.e.
[VO2(acac-ambmz)], [VO2(sal-ambmz)] and [VO2(sal-aebmz)]
(where Hacac is acetyloacetone, Hsal is salicylaldehyde, ambmz
is 2-aminomethylbenzimidazole and aebmz is 2-aminoethylbenz-
imidazole), which gave 20–35% conversion [35]. Table 2 summa-
rizes the percentage conversion of styrene and the selectivities
for the various reaction products. The selectivity for all the vana-
dium(V) complexes is rather similar and they are generally dis-
tinctly more selective toward styrene oxide (77.1–90.3%) than
benzaldehyde (5.8–13.2%). The conversion of the other oxidation
products is low (>6%) and the selectivity varies in the order: ben-
zoic acid > phenylacetaldehyde > 1-phenylethane-1,2-diol. Sur-
prisingly, in case of the [VO2(acac-ambmz)], [VO2(sal-ambmz)]
and [VO2(sal-aebmz)] complexes, styrene oxide and benzaldehyde
were two main products with 50–53% and 37–47% selectivity,
respectively. Moreover, the catalytic activity of the oxidovanadi-
um(IV) Schiff base complex derived from o-aminobenzyl alcohol,
[VO(sal-oaba)], using TBHP as the oxidant has been studied [36],
a
All sulfoxides are in the S configuration.
to value of 59–63% (Table 1, entries 11–14). When the reaction was
carried out at ꢀ20 °C with 30% H2O2 as the oxidant, for 2, 3, 5 and 8
as catalysts, the enantioselectivities improve significantly to 90–
94% ee and a conversion of over 85% of the substrate is observed
after 3 h (Table 1, entries 19–22). Mimoun et al. [34] pointed out
the importance of a sufficiently nucleophilic centre for the oxida-
tive catalysis of organic substances by peroxidovanadium(V) com-
pounds. Better enantioselectivities for 2, 3, 5 and 8 as compared to
other catalysts may be the result of a higher electron density on the
phenolato oxygen, e.g. due to the electronic effects of the ortho-,
meta- and para-substituted groups, contributing to an attainment
of sufficient nucleophilicity by the vanadium centre.
In general, the best results have been obtained in the oxidation
reactions of methyl phenyl sulfide with 30% H2O2 as the oxidant. A
significant decrease in reactivity is observed, i.e. enantioselectivity
as well as the overall yield of the oxidation of PhSMe, when the
bulkier oxidant cumene hydroperoxide was employed. Morever,
increasing the steric demand of the substrate on going from methyl
phenyl sulfide to benzyl phenyl sulfide leads to a decrease in both
the overall yield and enantioselectivity of corresponding sulfoxide.
In contrast to chiral oxidovanadium(V) Schiff base complexes de-
rived from optically active diamines and used as catalysts in the
sulfoxidation reactions, significantly shorter reaction times and
much higher values of the enantiomeric excesses of corresponding
sulfoxides were observed on employing 1–10 as catalysts [21–23].
3.2.2. Oxidation of styrene
The catalytic potential of the complexes 1–10 have been found
for the oxidation of styrene in presence of aqueous 30% H2O2 or
tert-butyl hydroperoxide (TBHP) as the oxidant to give styrene
oxide, benzaldehyde, 1-phenylethane-1,2-diol, benzoic acid and
phenylacetaldehyde (Fig. 4). The formation of all these products
is presented in Table 2.
In order to achieve suitable reaction conditions for a maximum
oxidative conversion of styrene, complex 6 was taken as a repre-
sentative catalyst and different parameters, i.e. amount of catalyst
(0.5, 1 and 2 mol%) and oxidant (1:1, 2:1 and 3:1 molar ratios to
styrene) were tested.

G. Romanowski, J. Kira / Polyhedron 53 (2013) 172–178
177
Table 2
Catalytic oxidation of styrene by 30% H2O2 or tert-butyl hydroperoxide (TBHP) as the oxidant in presence of 1 mol% vanadium(V) Schiff base complexes as the catalyst.
Entry
Catalyst
Oxidant
Conv. (%)
Product selectivity (%)
a
b
c
d
e
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
1
2
3
4
5
6
7
8
9
10
1
2
3
4
5
6
7
8
H2O2
H2O2
H2O2
H2O2
H2O2
H2O2
H2O2
H2O2
H2O2
H2O2
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
26.0
29.8
30.3
28.6
29.3
33.2
27.4
32.4
30.5
28.8
77.8
80.5
83.2
82.7
81.6
84.8
78.5
80.3
80.9
79.4
1.7
1.9
1.8
1.8
1.8
1.6
1.9
1.7
1.9
84.4
86.7
88.1
86.8
87.1
89.4
86.9
85.8
85.2
84.8
12.2
9.9
9.2
7.9
9.0
5.8
9.8
10.5
9.0
–
–
–
–
–
–
–
–
1.0
1.0
0.9
0.8
1.0
0.7
0.8
1.0
1.0
1.1
2.8
2.1
2.2
1.1
2.0
1.4
1.9
2.4
2.0
3.4
12.9
10.4
9.2
10.6
10.1
8.3
10.4
11.5
11.9
12.3
0.6
0.4
0.5
0.3
0.4
0.3
0.4
0.6
0.4
–
–
1.8
79.2
83.7
84.5
88.3
84.8
90.3
84.2
82.4
84.8
77.1
5.2
3.9
3.6
2.4
3.8
2.2
3.7
4.1
3.8
5.5
9
10
13.2
0.8
a
b
c
Styrene oxide.
Benzaldehyde.
Benzoic acid.
Phenylacetaldehyde.
1-Phenylethane-1,2-diol.
d
e
but it shows very low conversion (16%) and similar selectivity of
styrene oxide and benzaldehyde.
yl compounds with R(ꢀ)-phenylglycinol, were synthesized and
characterized by ESI–MS, IR, CD, UV–Vis, 1D (1H, 51V) and 2D
(COSY, gHSQC) NMR. Moreover, their catalytic properties towards
the oxidation of sulfides and styrene, using different oxidants were
tested.
In presented sulfoxidation reactions the enantioselectivity of
the vanadium(V) complexes is better and the reaction times are
much shorter than for other vanadium(V) complexes with Schiff
bases derived from optically active diamines. The results show that
the excellent yield and enantiomeric excess (over 90%) signifi-
cantly depend on the nature of the catalyst, substrate and peroxide
used, especially in the aspect of the bulky substituent, leading to
large steric demands, and finally depend on the temperature at
which the reaction is carried out.
In addition, to test the catalytic potential of the prepared vana-
dium(V) complexes in the oxidation of olefines, the oxidation of
styrene was chosen as a model reaction. These complexes are able
to catalyze the oxidative conversion of styrene, with 30% H2O2 or
TBHP as the oxidant, to styrene oxide and successive products de-
rived from styrene oxide, but according to our studies, no clear
trend of different catalytic reactivity was found upon changing
the substituent in the salicydene part of the Schiff base ligand.
The oxidation of styrene after 6 h of reaction time gives at least five
different products. Using 30% H2O2 as the oxidant, the conversion
of styrene was distinctly low, but due to the strong oxidizing nat-
ure of H2O2 the formation of benzaldehyde was preferred, whereas
the expected product styrene oxide was found only in a small yield.
The presence of a large amount of water in the reaction mixture
may be responsible for the lowering of the% conversion. On the
other hand, tert-butyl hydroperoxide (TBHP) proved to be an excel-
lent oxidant, giving ca. 80% conversion of styrene with styrene
oxide as the main product.
When this catalytic reaction was performed under the same
reaction conditions, but with 30% H2O2 as the oxidant, a very low
conversion with 6 h of contact time was found (ca. 30%). Although
the conversion of styrene is distinctly low, the selectivity for benz-
aldehyde is much better (84.4–89.4%) than in the case of TBHP. On
the other hand, as well as for the earlier reported [VO2(acac-
ambmz)], [VO2(sal-ambmz)] and [VO2(sal-aebmz)] complexes
[35], styrene oxide is the most expected product but its selectivity
goes down considerably (1.6–1.9%). Moreover, the catalytic activity
of dioxidovanadium(V) complexes encapsulated in the super cages
of zeolite–Y, i.e. [VO2(sal-ambmz)]–Y, [VO2(pydx-ambmz)]–Y and
[VO2(pydx-aebmz)]–Y (where Hpydx is pyridoxal, one of the forms
of vitamin B6), has recently been discovered [37,38]. Analysis for
[VO2(pydx-ambmz)]–Y and [VO2(pydx-aebmz)]–Y showed 65–
68% conversion, which is less than that observed for VO2(sal-
ambmz)]–Y (97%), but all these catalyst are less selective toward
benzaldehyde (ca. 55%) and more selective toward 1-phenyle-
thane-1,2-diol (ca. 25%) at the same time. In the presence of
H2O2, a strong oxidant, styrene oxide formed by epoxidation in
the first step is very quickly converted into benzaldehyde via
nucleophilic attack of H2O2 on the styrene oxide followed by cleav-
age of the intermediate hydroperoxystyrene [39]. Benzaldehyde
formation may also be facilitated by direct oxidative cleavage of
the styrene side-chain double bond via a radical mechanism. Very
low conversion of styrene is probably caused by presence of signif-
icant amounts of water in 30% H2O2, which can be responsible for
the decomposition of the catalyst and also the hydrolysis of styrene
oxide to form 1-phenylethane-1,2-diol (ca. 10%). Formation of
other products, e.g. phenylacetaldehyde through isomerisation of
styrene oxide and benzoic acid through oxidation of benzaldehyde,
are distinctly much slower processes.
4. Conclusion
Acknowledgement
Ten new chiral vanadium(V) complexes derived from Schiff
base ligands, the monocondensation products of o-hydroxycarbon-
This scientific work was supported by the Polish Ministry of Sci-
ence and Higher Education (DS/8210-4-0177-11).

178
G. Romanowski, J. Kira / Polyhedron 53 (2013) 172–178
[20] R.A. Shiels, K. Venkatasubbaiah, C.W. Jones, Adv. Synth. Catal. 350 (2008) 2823.
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