Vanadium(V) complexes with chiral tridentate Schiff base ligands derived from 1S,2R(+)-2-amino-1,2-diphenylethanol and with acetohydroxamate co-ligand: Synthesis, characterization and catalytic activity in the oxidation of prochiral sulfides and olefins

Article abstract of DOI:10.1016/j.molcata.2013.10.011

A series of vanadium(V) complexes with chiral tridentate Schiff base ligands were prepared. Five-coordinate complexes were obtained by a single condensation of 1S,2R(+)-2-amino-1,2-diphenylethanol with salicylaldehyde and its derivatives. Furthermore, six-coordinate complexes were synthesized with the same Schiff base ligands and also with monoanionic bidentate acetohydroxamate co-ligand. The complexes were characterized by elemental analysis and by their IR, CD, UV-vis, one- (¹H, ⁵¹V) and two-dimensional (COSY, gHSQC and NOESY) NMR spectra. The X-ray analyses of the ligand, 1S,2R(+)-2-[(1-hydroxy-1,2-diphenylethyl)iminomethyl]-4-nitrophenol, 7, and its complex, (acetohydroxamato-κ²O,O′){1S,2R(+)-2-[(1-oxido- 1,2-diphenylethyl)iminomethyl]-4-nitrophenolato-κ³N,O,O′} oxidovanadium(V), 7b, have been also performed. The five-coordinate vanadium(V) complexes have ability to catalyze the oxidation of prochiral sulfides [PhSR (R = Me, Bz)] in good yields and enantiomeric excesses. Moreover, catalytic activity of these complexes and also those derived from 1S,2S(-)-1,2-diphenyl-1, 2-diaminoethane were tested in the oxidation of styrene and cyclohexene.

Full text of DOI:10.1016/j.molcata.2013.10.011

Journal of Molecular Catalysis A: Chemical 381 (2014) 148–160
Contents lists available at ScienceDirect
Journal of Molecular Catalysis A: Chemical
journal homepage: www.elsevier.com/locate/molcata
Vanadium(V) complexes with chiral tridentate Schiff base ligands
derived from 1S,2R(+)-2-amino-1,2-diphenylethanol and with
acetohydroxamate co-ligand: Synthesis, characterization and
catalytic activity in the oxidation of prochiral sulfides and olefins
Grzegorz Romanowski∗, Jaromir Kira, Michał Wera
Faculty of Chemistry, University of Gdan´sk, Wita Stwosza 63, 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 were prepared.
Received 3 September 2013
Received in revised form 14 October 2013
Accepted 16 October 2013
Available online 26 October 2013
Five-coordinate complexes were obtained by single condensation of 1S,2R(+)-2-amino-1,2-
a
diphenylethanol with salicylaldehyde and its derivatives. Furthermore, six-coordinate complexes
were synthesized with the same Schiff base ligands and also with monoanionic bidentate ace-
tohydroxamate co-ligand. The complexes were characterized by elemental analysis and by their
IR, CD, UV–vis, one- (1H, 51V) and two-dimensional (COSY, gHSQC and NOESY) NMR spectra. The
X-ray analyses of the ligand, 1S,2R(+)-2-[(1-hydroxy-1,2-diphenylethyl)iminomethyl]-4-nitrophenol,
7, and its complex, (acetohydroxamato-ꢀ2O,Oꢀ){1S,2R(+)-2-[(1-oxido-1,2-diphenylethyl)iminomethyl]-
4-nitrophenolato-ꢀ3N,O,Oꢀ}oxidovanadium(V), 7b, have been also performed. The five-coordinate
vanadium(V) complexes have ability to catalyze the oxidation of prochiral sulfides [PhSR (R = Me, Bz)]
in good yields and enantiomeric excesses. Moreover, catalytic activity of these complexes and also
those derived from 1S,2S(−)-1,2-diphenyl-1,2-diaminoethane were tested in the oxidation of styrene
and cyclohexene.
Keywords:
Vanadium(V)
Schiff base
Sulfoxidation
Styrene
Cyclohexene
© 2013 Elsevier B.V. All rights reserved.
1. Introduction
Chiral sulfoxides possess a wide range of biological activities,
e.g. antimicrobial properties [19], inhibition of biosynthesis of uric
The coordination chemistry of vanadium is of interest for num-
ber of biological and therapeutic reasons. Vanadium complexes
have many attractive features that constitute the basis for their
utility as insulin-mimetic drugs stimulating glucose uptake [1],
potential anti-tumor agents exerting preventive effects against
carcinogenesis [2] or promising agents against some of parasitic
diseases [3]. The presence of vanadium [4–8] and hydroxamic acids
[9] in the active sites of some biological enzymes has also resulted
in increased interest in the study of their structural properties.
Optically active N-salicyl-␤-amino alcohol Schiff bases are con-
sidered as a member of the well-known “privileged ligands” group
[10]. These ligands can be readily synthesized from naturally avail-
able chiral amino acids [11] and have been widely employed as
catalysts in the enantioselective trimethylsilylcyanations [12] and
oxidation of organic sulfides [13,14], the asymmetric alkynylation
of aldehydes [13], the epoxidation of cyclooctene [15], the oxida-
tion of bromide [16], the stereoselective synthesis of cyclic ethers
[16,17] and oxidative kinetic resolution of ␣-hydroxy esters [18].
acid [20] and gastric acid secretion [21] or regulation of cholesterol
catabolism [22]. As efficient chiral auxiliaries they lead to many
important asymmetric transformations [23] and are valuable start-
ing materials in asymmetric synthesis as well as important chiral
ligands in enantioselective catalysis. Since epoxides are key starting
materials for a wide variety of products the epoxidation of alkenes
is one of the most widely studied reactions in organic chemistry.
Their importance arises mainly from the ring opening of epoxides,
which allows straightforward elaboration to useful generation of
new carbon–carbon bonds [24]. Some review articles that focus on
the vanadium catalysts (also complexes with Schiff base ligands)
in epoxidation of many olefinic compounds have been published
recently [25].
In continuation of our studies on synthesis, structure, spec-
troscopic and catalytic properties of vanadium(V) complexes
incorporating chiral tridentate Schiff base ligands [26–28], we
prepared
a series of new five-coordinate oxidovanadium(V)
complexes with ONO donor Schiff base ligands, products of
monocondensation of 1S,2R(+)-2-amino-1,2-diphenylethanol with
aromatic o-hydroxyaldehydes. Moreover, the same Schiff base
ligands were used to synthesize six-coordinate complexes with
monoanionic bidentate acetohydroxamate co-ligand (Fig. 1). Their
∗
Corresponding author. Tel.: +48 585235102; fax: +48 585235012.
E-mail address: grzegorz.romanowski@ug.edu.pl (G. Romanowski).
1381-1169/$ – see front matter © 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.molcata.2013.10.011

G. Romanowski et al. / Journal of Molecular Catalysis A: Chemical 381 (2014) 148–160
149
H
N
1
1
2
2
R
R
R
R
O
CH3
3
3
O
V
R
O
N
O-V=O(L)
O
R
O
N
O
O
V
4
R
4
R
O
R
S
R S
1a - 10a
1b - 10b
Fig. 1. Structural formulae of vanadium(V) complexes.
spectroscopic properties by 1D and 2D NMR, UV–vis, CD and IR
have been examined. The catalytic potential of five-coordinate
complexes in the asymmetric oxidation, i.e. enantioselective
sulfoxidation of methyl phenyl sulfide (PhSMe) and benzyl phenyl
sulfide (PhSBz), utilizing aqueous 30% H2O2 as oxidant, has been
studied. Moreover, they were also used as catalysts in the oxidation
of olefins, i.e. styrene and cyclohexene using aqueous 30% H2O2
or tert-butyl hydroperoxide (TBHP) as oxidant. For comparison,
the catalytic activity in styrene and cyclohexene oxidation of very
similar dioxidovanadium(V) Schiff base complexes derived from
1S,2S(−)-1,2-diphenyl-1,2-diaminoethane, synthesized and char-
acterized earlier [26], under the same reaction conditions has been
also tested. Crystal and molecular structure of the ligand, 1S,2R(+)-
2-[(1-hydroxy-1,2-diphenylethyl)iminomethyl]-4-nitrophenol,
7, and its complex, (acetohydroxamato-ꢀ2O,Oꢀ){1S,2R(+)-2-
[(1-oxido-1,2-diphenylethyl)iminomethyl]-4-nitrophenolato-
ꢀ3N,O,Oꢀ}oxidovanadium(V), 7b, are also reported.
anisotropically. The positions of hydrogen atoms were calculated
and treated as riding atoms with fixed thermal parameters.
2.3. Catalytic activity
2.3.1. Sulfoxidation
In typical procedure, to a solution of catalyst (0.010 mmol)
in 3 ml of CH2Cl2/MeOH solution (7:3, v/v), sulfide (1.00 mmol)
was added at room temperature or −20 ◦C, together 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 2 ml of sodium sulphite solution (0.1 M)
and extracted with CH2Cl2 (3 × 5 ml). The combined organic lay-
ers 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 (where Hhfc is 3-
(heptafluoropropylhydroxymethylene)-(+)-camphoric acid) [30].
2. Experimental
2.3.2. Oxidation of olefins
2.1. Measurements
In typical procedure, styrene or cyclohexene (1.00 mmol), an
oxidant (3.00 mmol), i.e. aqueous 30% H2O2 or tert-butyl hydroper-
oxide (TBHP) – 5.5 M 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 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 was confirmed by GC–MS. The
influence of amounts of catalyst and oxidant were also studied to
check their effect on the conversion and selectivity of the reaction
products.
All chemicals and reagents were obtained from commercial
sources and used without further purification unless stated oth-
erwise. Carbon, hydrogen and nitrogen contents were determined
on a Carlo Erba MOD 1106 elemental analyzer. IR spectra of
solid samples (KBr pellets) were run on a Bruker IFS 66, and
electronic spectra on the Perkin-Elmer LAMBDA 18 spectropho-
tometer. Circular dichroism spectra were measured with a Jasco
J-815 spectropolarimeter. NMR spectra were obtained in CD3OD
solutions with a Varian Mercury-400BB (400 MHz) spectrometer
using TMS (1H) and VOCl3 51V) as reference compounds. A Perkin-
(
Elmer Clarus 500 gas chromatograph with a DB-5 capillary column
(30 m × 0.25 mm × 0.25 mm) and FID detector were used to analyze
the reaction products. The identity of the products was confirmed
using a GC–MS model Shimadzu GCMS-QP2010 SE.
2.4. Synthesis of vanadium(V) complexes
The complexes were obtained in the following example
procedure.
A solution of 5 mmol of 1S,2R(+)-2-amino-1,2-
diphenylethanol in absolute ethanol (10 ml) was added with
stirring to 5 mmol of an aromatic o-hydroxyaldehyde (salicy-
laldehyde, 3-methoxysalicylaldehyde, 5-methoxysalicylaldehyde,
2.2. X-ray investigations
Diffraction measurements were made on a Oxford Diffraction
Gemini R Ultra Ruby CCD, using graphite-monochromatized Mo
K␣ radiation (ꢁ = 0.71073 A) at 295(2) K and determination of the
crystal class, orientation matrix, and accurate unit cell parameters
was performed according to established procedures. Data collec-
tion and processing parameters are summarized in Table S1. The
structure was solved by direct methods and refined with all data
on F2 with SHELXTL [29]. All non-hydrogen atoms were refined
4,6-dimethoxysalicylaldehyde,
5-methylsalicylaldehyde,
5-
bromosalicylaldehyde, 5-nitrosalicylaldehyde, 2,4-dihydroxyben-
zaldehyde, 3-tert-butylsalicylaldehyde, 2-hydroxy-1-naphthal-
dehyde) in absolute EtOH (20 ml) and heated under reflux for 1 h. In
case of 1b–10b, acetohydroxamic acid (5 mmol) in absolute EtOH
(10 ml) was also added with an aldehyde. Then a vanadium(V)
oxytriethoxide (5 mmol) in absolute EtOH (10 ml) was added and
stirred at room temperature for 2 h. After cooling in a fridge a
˚

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G. Romanowski et al. / Journal of Molecular Catalysis A: Chemical 381 (2014) 148–160
4J = 3 Hz), 7.12 (1H, ov), 7.02–7.16 (8H, m), 7.01 (1H, ov), 6.79 (2H,
dd, 3J = 8 Hz, 4J = 3 Hz) (aromatic); 6.76 (1H, d, 3J = 6 Hz), 5.91 (1H,
d, 3J = 6 Hz) (methine); 3.78 (3H, s) (methoxy); minor (40%): 8.74
(1H, s) (azomethine); 7.49 (1H, dd, 3J = 8 Hz, 4J = 3 Hz), 7.11 (1H, ov),
7.00–7.16 (8H, ov), 6.96 (1H, d, 3J = 8 Hz), 6.79 (2H, ov) (aromatic);
6.57 (1H, d, 3J = 6 Hz), 5.53 (1H, d, 3J = 6 Hz) (methine); 3.76 (3H, s)
(methoxy). 51V NMR (CD3OD, ppm) major (60%): −531.3; minor
(40%): −516.2.
solid was separated and filtered off, washed several times and
recrystallized from absolute EtOH.
During the monocondensation reaction of 1S,2R(+)-2-amino-
1,2-diphenylethanol with 5-nitrosalicylaldehyde, the ligand,
1S,2R(+)-2-[(1-hydroxy-1,2-diphenylethyl)iminomethyl]-4-
nitrophenol, 7, has crystallized and was isolated as yellow
needles, which were separated and washed several times with
EtOH. These crystals were suitable for X-ray diffraction measure-
ments. Yield 92%. Anal. Calc. for C21H18N2O4: C, 69.6; H, 5.0; N, 7.7.
Found: C, 69.7; H, 5.0; N, 7.6%. IR (KBr, cm−1): 3483 (␯ H); 1650
O
2.4.4. ꢃ-Oxido-bis({1S,2R(+)-2-[(1-oxido-1,2-
diphenylethyl)iminomethyl]-3,5-dimethoxyphenolato-
ꢀ3N,O,Oꢀ}oxidovanadium(V))
(␯ N); 1538, 1324 (␯NO2). UV–vis spectrum in MeOH [ꢁmax (nm), ε
C
(M−1 cm−1)]: 261 (19 810), 327 (10 030), 394 (7080). CD spectrum
in MeOH [ꢁmax (nm), ꢂε (M−1 cm−1)]: 257 (3.82), 280 (−4.15),
329 (−2.55), 398 (3.56). 1H NMR (DMSO-d6, ppm): 15.00 (1H, s)
(hydroxy); 8.73 (1H, s) (azomethine); 8.41 (1H, d, 4J = 3 Hz), 8.11
(1H, dd, 3J = 8 Hz, 4J = 3 Hz), 7.28 (3H, m), 7.21 (5H, m), 7.09 (2H, m),
6.82 (1H, d, 3J = 8 Hz) (aromatic); 5.97 (1H, d, 3J = 4 Hz) (hydroxy);
5.12 (1H, dd, 3J = 6 Hz, 4J = 4 Hz), 4.93 (1H, d, 3J = 6 Hz) (methine).
(4a)
Yield 74%. Anal. Calc. for C46H42N2O11V2: C, 61.3; H, 4.7; N, 3.1.
Found: C, 61.2; H, 4.8; N, 3.2%. IR (KBr, cm−1): 1599 (␯ N); 986
C
(␯ O). UV–vis spectrum in MeOH [ꢁmax (nm), ε (M−1 cm−1)]: 319
V
(14 380). CD spectrum in MeOH [ꢁmax (nm), ꢂε (M−1 cm−1)]: 257
(3.67), 297 (−5.64), 364 (9.79). 1H NMR (CD3OD, ppm) major (60%):
8.86 (1H, s) (azomethine); 6.98–7.13 (8H, m), 6.74 (2H, dd, 3J = 8 Hz,
4J = 3 Hz), 6.23 (1H, d, 3J = 8 Hz), 6.09 (1H, d, 3J = 8 Hz) (aromatic);
6.71 (1H, ov), 5.84 (1H, d, 3J = 6 Hz) (methine); 3.88 (3H, s), 3.82 (3H,
s) (methoxy); minor (40%): 8.87 (1H, ov) (azomethine); 6.98–7.13
(8H, ov), 6.73 (2H, ov), 6.18 (1H, d, 3J = 8 Hz), 6.04 (1H, d, 3J = 8 Hz)
(aromatic); 6.59 (1H, d, 3J = 6 Hz), 5.50 (1H, d, 3J = 6 Hz) (methine);
3.86 (3H, s), 3.80 (3H, s) (methoxy). 51V NMR (CD3OD, ppm) major
(60%): −531.8; minor (40%): −516.6.
2.4.1. ꢃ-Oxido-bis({1S,2R(+)-2-[(1-oxido-1,2-
diphenylethyl)iminomethyl]phenolato-
ꢀ3N,O,Oꢀ}oxidovanadium(V))
(1a)
Yield 76%. Anal. Calc. for C42H34N2O7V2: C, 64.6; H, 4.4; N,
3.6. Found: C, 64.5; H, 4.5; N, 3.7%. IR (KBr, cm−1): 1618 (␯ N);
C
984 (␯ O). UV–vis spectrum in MeOH [ꢁmax (nm), ε (M−1 cm−1)]:
V
282 (9620), 341 (4080). CD spectrum in MeOH [ꢁmax (nm), ꢂε
(M−1 cm−1)]: 261 (3.94), 293 (−5.87), 361 (10.53). 1H NMR (CD3OD,
ppm) major (60%): 8.73 (1H, s) (azomethine); 7.86 (1H, d, 3J = 8 Hz),
7.58 (1H, t, 3J = 8 Hz), 7.21 (1H, t, 3J = 8 Hz), 7.02–7.16 (8H, m), 6.97
(1H, d, 3J = 8 Hz), 6.83 (2H, dd, 3J = 8 Hz, 4J = 3 Hz) (aromatic); 6.82
(1H, ov), 5.93 (1H, d, 3J = 6 Hz) (methine); minor (40%): 8.75 (1H, s)
(azomethine); 8.03 (1H, d, 3J = 8 Hz), 7.71 (1H, t, 3J = 8 Hz), 7.38 (1H,
t, 3J = 8 Hz), 7.02–7.16 (8H, ov), 6.90 (1H, d, 3J = 8 Hz), 6.81 (2H, ov)
(aromatic); 6.58 (1H, d, 3J = 6 Hz), 5.53 (1H, d, 3J = 6 Hz) (methine).
51V NMR (CD3OD, ppm) major (60%): −534.5; minor (40%): −522.6.
2.4.5. ꢃ-Oxido-bis({1S,2R(+)-2-[(1-oxido-1,2-
diphenylethyl)iminomethyl]-4-methylphenolato-
ꢀ3N,O,Oꢀ}oxidovanadium(V))
(5a)
Yield 77%. Anal. Calc. for C44H38N2O7V2: C, 65.4; H, 4.7; N,
3.5. Found: C, 65.5; H, 4.8; N, 3.5%. IR (KBr, cm−1): 1619 (␯ N);
C
984 (␯ O). UV–vis spectrum in MeOH [ꢁmax (nm), ε (M−1 cm−1)]:
V
280 (9670), 343 (4210). CD spectrum in MeOH [ꢁmax (nm), ꢂε
(M−1 cm−1)]: 262 (2.63), 294 (−4.15), 367 (8.32). 1H NMR (CD3OD,
ppm) major (60%): 8.66 (1H, s) (azomethine); 7.41 (1H, dd, 3J = 8 Hz,
4J = 3 Hz), 7.28 (1H, d, 4J = 3 Hz), 7.00–7.15 (8H, m), 6.98 (1H, d,
3J = 8 Hz), 6.79 (2H, dd, 3J = 8 Hz, 4J = 3 Hz) (aromatic); 6.77 (1H, d,
3J = 6 Hz), 5.90 (1H, d, 3J = 6 Hz) (methine); 2.31 (3H, s) (methyl);
minor (40%): 8.68 (1H, s) (azomethine); 7.37 (2H, dd, 3J = 8 Hz,
4J = 3 Hz), 7.21 (1H, d, 4J = 3 Hz), 7.01–7.16 (8H, ov), 6.92 (1H, d,
3J = 8 Hz), 6.79 (2H, ov) (aromatic); 6.57 (1H, d, 3J = 6 Hz), 5.52 (1H, d,
3J = 6 Hz) (methine); 2.29 (3H, s) (methyl). 51V NMR (CD3OD, ppm)
major (60%): −532.4; minor (40%): −518.8.
2.4.2. ꢃ-Oxido-bis({1S,2R(+)-2-[(1-oxido-1,2-
diphenylethyl)iminomethyl]-6-methoxyphenolato-
ꢀ3N,O,Oꢀ}oxidovanadium(V))
(2a)
Yield 79%. Anal. Calc. for C44H38N2O9V2: C, 62.9; H, 4.6; N,
3.3. Found: C, 62.8; H, 4.5; N, 3.2%. IR (KBr, cm−1): 1615 (␯ N);
C
986 (␯ O). UV–vis spectrum in MeOH [ꢁmax (nm), ε (M−1 cm−1)]:
V
286 (9550), 349 (4130). CD spectrum in MeOH [ꢁmax (nm), ꢂε
(M−1 cm−1)]: 260 (3.86), 295 (−3.15) 365 (7.14). 1H NMR (CD3OD,
ppm) major (60%): 8.73 (1H, s) (azomethine); 7.22 (1H, dd, 3J = 8 Hz,
4J = 3 Hz), 7.01–7.15 (8H, m), 7.11 (1H, ov), 7.01 (1H, ov), 6.81 (2H,
dd, 3J = 8 Hz, 4J = 3 Hz) (aromatic); 6.71 (1H, d, 3J = 6 Hz), 5.95 (1H,
d, 3J = 6 Hz) (methine); 3.97 (3H, s) (methoxy); minor (40%): 8.75
(1H, s) (azomethine); 7.37 (1H, dd, 3J = 8 Hz, 4J = 3 Hz), 7.13 (1H, ov),
7.01–7.15 (8H, ov), 6.95 (1H, d, 3J = 8 Hz), 6.81 (2H, ov) (aromatic);
6.58 (1H, d, 3J = 6 Hz), 5.53 (1H, d, 3J = 6 Hz) (methine); 3.94 (3H, s)
(methoxy). 51V NMR (CD3OD, ppm) major (60%): −531.5; minor
(40%): −517.1.
2.4.6. ꢃ-Oxido-bis({1S,2R(+)-2-[(1-oxido-1,2-
diphenylethyl)iminomethyl]-4-bromophenolato-
ꢀ3N,O,Oꢀ}oxidovanadium(V))
(6a)
Yield 79%. Anal. Calc. for Br2C42H32N2O7V2: C, 53.8; H, 3.4; N,
3.0. Found: C, 53.7; H, 3.3; N, 3.1%. IR (KBr, cm−1): 1619 (␯ N);
C
985 (␯ O). UV–vis spectrum in MeOH [ꢁmax (nm), ε (M−1 cm−1)]:
V
280 (9710), 344 (4620). CD spectrum in MeOH [ꢁmax (nm), ꢂε
(M−1 cm−1)]: 258 (2.55), 291 (−4.49), 365 (8.73). 1H NMR (CD3OD,
ppm) major (60%): 8.68 (1H, s) (azomethine); 7.62 (1H, d, 4J = 3 Hz),
7.21 (1H, dd, 3J = 8 Hz, 4J = 3 Hz), 7.02–7.15 (10H, m), 6.98 (1H, d,
3J = 8 Hz), 6.87 (2H, dd, 3J = 8 Hz, 4J = 3 Hz) (aromatic); 6.90 (1H,
ov), 5.89 (1H, d, 3J = 6 Hz) (methine); minor (40%): 8.70 (1H, s)
(azomethine); 7.86 (1H, d, 4J = 3 Hz), 7.29 (1H, dd, 3J = 8 Hz, 4J = 3 Hz),
7.01–7.15 (10H, m), 6.94 (1H, d, 3J = 8 Hz), 6.87 (2H, ov) (aromatic);
6.54 (1H, d, 3J = 6 Hz), 5.51 (1H, d, 3J = 6 Hz) (methine). 51V NMR
(CD3OD, ppm) major (60%): −533.6; minor (40%): −524.3.
2.4.3. ꢃ-Oxido-bis({1S,2R(+)-2-[(1-oxido-1,2-
diphenylethyl)iminomethyl]-4-methoxyphenolato-
ꢀ3N,O,Oꢀ}oxidovanadium(V))
(3a)
Yield 75%. Anal. Calc. for C44H38N2O9V2: C, 62.9; H, 4.6; N,
3.3. Found: C, 62.7; H, 4.6; N, 3.4%. IR (KBr, cm−1): 1613 (␯ N);
C
984 (␯ O). UV–vis spectrum in MeOH [ꢁmax (nm), ε (M−1 cm−1)]:
V
284 (9420), 347 (4040). CD spectrum in MeOH [ꢁmax (nm), ꢂε
(M−1 cm−1)]: 259 (3.36), 296 (−2.61), 363 (6.23). 1H NMR (CD3OD,
ppm) major (60%): 8.72 (1H, s) (azomethine); 7.21 (1H, dd, 3J = 8 Hz,

G. Romanowski et al. / Journal of Molecular Catalysis A: Chemical 381 (2014) 148–160
151
8.08 (1H, d, 3J = 8 Hz), 7.82 (1H, d, 3J = 8 Hz), 7.55 (1H, t, 3J = 8 Hz),
7.38 (1H, t, 3J = 8 Hz), 7.27 (1H, d, 3J = 8 Hz), 6.99–7.18 (8H, m), 6.79
(2H, dd, 3J = 8 Hz, 4J = 3 Hz) (aromatic); 6.83 (1H, d, 3J = 6 Hz), 6.11
(1H, d, 3J = 6 Hz) (methine); minor (40%): 9.58 (1H, s) (azomethine);
8.07 (1H, ov), 8.01 (1H, d, 3J = 8 Hz), 7.78 (1H, d, 3J = 8 Hz), 7.47 (1H,
t, 3J = 8 Hz), 7.34 (1H, t, 3J = 8 Hz), 7.23 (1H, d, 3J = 8 Hz), 6.99–7.18
(8H, ov), 6.79 (2H, ov) (aromatic); 6.65 (1H, d, 3J = 6 Hz), 5.72 (1H, d,
3J = 6 Hz) (methine). 51V NMR (CD3OD, ppm) major (60%): −535.2;
minor (40%): −520.3.
2.4.7. ꢃ-Oxido-bis({1S,2R(+)-2-[(1-oxido-1,2-
diphenylethyl)iminomethyl]-4-nitrophenolato-
ꢀ3N,O,Oꢀ}oxidovanadium(V))
(7a)
Yield 71%. Anal. Calc. for C42H32N4O11V2: C, 57.9; H, 3.7; N, 6.4.
Found: C, 57.8; H, 3.8; N, 6.4%. IR (KBr, cm−1): 1609 (␯ N); 1560,
C
1338 (␯NO2); 988 (␯ O). UV–vis spectrum in MeOH [ꢁmax (nm), ε
V
(M−1 cm−1)]: 348 (15 360). CD spectrum in MeOH [ꢁmax (nm), ꢂε
(M−1 cm−1)]: 253 (3.21), 289 (−7.57), 360 (7.68). 1H NMR (CD3OD,
ppm) major (60%): 8.75 (1H, s) (azomethine); 8.45 (1H, d, 4J = 3 Hz),
8.31 (1H, dd, 3J = 8 Hz, 4J = 3 Hz), 7.56 (1H, d, 3J = 8 Hz), 6.98–7.11
(10H, m) (aromatic); 7.20 (1H, d, 3J = 6 Hz), 5.91 (1H, d, 3J = 6 Hz)
(methine); minor (40%): 8.79 (1H, s) (azomethine); 8.44 (1H, ov),
8.02 (1H, dd, 3J = 8 Hz, 4J = 3 Hz), 7.46 (1H, d, 3J = 8 Hz), 6.98–7.11
(10H, ov) (aromatic); 6.55 (1H, d, 3J = 6 Hz), 5.52 (1H, d, 3J = 6 Hz)
(methine). 51V NMR (CD3OD, ppm) major (60%): −528.0; minor
(40%): −526.1.
2.4.11. (Acetohydroxamato-ꢀ2O,Oꢀ){1S,2R(+)-2-[(1-oxido-1,2-
diphenylethyl)iminomethyl]phenolato-
ꢀ3N,O,Oꢀ}oxidovanadium(V)
(1b)
Yield 82%. Anal. Calc. for C23H21N2O5V: C, 60.5; H, 4.6; N, 6.1.
Found: C, 60.4; H, 4.5; N, 6.2%. IR (KBr, cm−1): 3436 (␯ H); 1623,
N
1601 (␯ O, ␯ N); 960 (␯ O). UV–vis spectrum in MeOH [ꢁmax
C
C
V
(nm), ε (M−1 cm−1)]: 259 (14 210), 483 (3460). CD spectrum in
MeOH [ꢁmax (nm), ꢂε (M−1 cm−1)]: 276 (0.38), 306 (−2.75), 375
(2.34), 463 (0.41). 1H NMR (CD3OD, ppm) major (60%): 8.57 (1H,
s) (azomethine); 7.52 (2H, m), 7.39 (1H, t, 3J = 8 Hz), 7.22 (1H, d,
3J = 8 Hz), 6.97–7.17 (8H, m), 6.82 (1H, t, 3J = 8 Hz), 6.73 (1H, d,
3J = 8 Hz) (aromatic); 6.24 (1H, d, 3J = 6 Hz), 5.38 (1H, d, 3J = 6 Hz)
(methine); 2.06 (3H, s) (acetyl); minor (40%): 8.71 (1H, s) (azome-
thine); 7.41 (2H, ov), 7.37 (1H, ov), 7.20 (1H, ov), 6.95–7.14 (8H,
m), 6.85 (1H, t, 3J = 8 Hz), 6.79 (1H, ov) (aromatic); 6.69 (1H, d,
2.4.8. ꢃ-Oxido-bis({1S,2R(+)-2-[(1-oxido-1,2-
diphenylethyl)iminomethyl]-5-hydroxyphenolato-
ꢀ3N,O,Oꢀ}oxidovanadium(V))
(8a)
Yield 76%. Anal. Calc. for C42H34N2O9V2: C, 62.1; H, 4.2; N,
3.4. Found: C, 62.2; H, 4.2; N, 3.3%. IR (KBr, cm−1): 1603 (␯ N);
C
983 (␯ O). UV–vis spectrum in MeOH [ꢁmax (nm), ε (M−1 cm−1)]:
V
285 (9830), 361 (4540). CD spectrum in MeOH [ꢁmax (nm), ꢂε
(M−1 cm−1)]: 259 (2.74), 298 (−4.66), 362 (8.31). 1H NMR (CD3OD,
ppm) major (60%): 8.62 (1H, s) (azomethine); 7.91 (1H, d, 3J = 8 Hz),
7.46 (1H, dd, 3J = 8 Hz, 4J = 3 Hz), 7.02–7.16 (8H, m), 6.77 (2H, dd,
3J = 8 Hz, 4J = 3 Hz), 6.46 (1H, d, 4J = 3 Hz) (aromatic); 6.71 (1H, d,
3J = 6 Hz), 5.84 (1H, d, 3J = 6 Hz) (methine); 4.89 (1H, s) (hydroxy);
minor (40%): 8.65 (1H, s) (azomethine); 7.87 (1H, d, 3J = 8 Hz), 7.39
(1H, dd, 3J = 8 Hz, 4J = 3 Hz), 7.02–7.16 (8H, ov), 6.77 (2H, ov), 6.38
(1H, d, 4J = 3 Hz) (aromatic); 6.29 (1H, d, 3J = 6 Hz), 5.49 (1H, d,
3J = 6 Hz) (methine); 4.62 (1H, s) (hydroxy). 51V NMR (CD3OD, ppm)
major (60%): −530.6; minor (40%): −516.0.
3J = 6 Hz), 5.57 (1H, d, 3J = 6 Hz) (methine); 2.29 (3H, s) (acetyl). 51
NMR (CD3OD, ppm) major (60%): −269.8; minor (40%): −239.7.
V
2.4.12. (Acetohydroxamato-ꢀ2O,Oꢀ){1S,2R(+)-2-[(1-oxido-1,2-
diphenylethyl)iminomethyl]-6-methoxyphenolato-
ꢀ3N,O,Oꢀ}oxidovanadium(V)
(2b)
Yield 84%. Anal. Calc. for C24H23N2O6V: C, 59.3; H, 4.8; N, 5.8.
Found: C, 59.3; H, 4.7; N, 5.8%. IR (KBr, cm−1): 3430 (␯ H); 1624,
N
1601 (␯ O, ␯ N); 962 (␯ O). UV–vis spectrum in DMSO [ꢁmax
C
C
V
(nm), ε (M−1 cm−1)]: 261 (14 570), 491 (3380). CD spectrum in
MeOH [ꢁmax (nm), ꢂε (M−1 cm−1)]: 280 (0.75), 323 (−3.17), 383
(4.64), 472 (0.63). 1H NMR (CD3OD, ppm) major (60%): 8.54 (1H,
s) (azomethine); 7.55 (2H, dd, 3J = 8 Hz, 4J = 3 Hz), 7.24 (1H, dd,
3J = 8 Hz, 4J = 3 Hz), 7.13 (1H, d, 4J = 3 Hz), 7.01–7.10 (8H, m), 7.02
(1H, ov) (aromatic); 6.44 (1H, d, 3J = 6 Hz), 5.56 (1H, d, 3J = 6 Hz)
(methine); 3.85 (3H, s) (methoxy); 2.06 (3H, s) (acetyl); minor
(40%): 8.69 (1H, s) (azomethine); 7.43 (2H, m), 7.26 (1H, ov), 7.15
(1H, ov), 6.99–7.10 (8H, ov), 6.93 (1H, d, 3J = 8 Hz) (aromatic); 6.67
(1H, d, 3J = 6 Hz), 5.77 (1H, d, 3J = 6 Hz) (methine); 3.88 (3H, s)
(methoxy); 2.26 (3H, s) (acetyl). 51V NMR (CD3OD, ppm) major
(60%): −264.2; minor (40%): −237.8.
2.4.9. ꢃ-Oxido-bis({1S,2R(+)-2-[(1-oxido-1,2-
diphenylethyl)iminomethyl]-6-tert-butylphenolato-
ꢀ3N,O,Oꢀ}oxidovanadium(V))
(9a)
Yield 78%. Anal. Calc. for C50H50N2O7V2: C, 67.3; H, 5.6; N,
3.1. Found: C, 67.4; H, 5.7; N, 3.0%. IR (KBr, cm−1): 1621 (␯ N);
C
981 (␯ O). UV–vis spectrum in MeOH [ꢁmax (nm), ε (M−1 cm−1)]:
V
278 (9520), 345 (4100). CD spectrum in MeOH [ꢁmax (nm), ꢂε
(M−1 cm−1)]: 258 (3.15), 299 (−2.89), 361 (6.68). 1H NMR (CD3OD,
ppm) major (60%): 8.75 (1H, s) (azomethine); 7.63 (1H, dd, 3J = 8 Hz,
4J = 3 Hz), 7.38 (1H, dd, 3J = 8 Hz, 4J = 3 Hz), 7.01–7.16 (8H, m), 6.93
(1H, t, 3J = 8 Hz), 6.82 (2H, dd, 3J = 8 Hz, 4J = 3 Hz) (aromatic); 6.73
(1H, d, 3J = 6 Hz), 5.90 (1H, d, 3J = 6 Hz) (methine); 1.56 (3H, s)
(tert-butyl); minor (40%): 8.77 (1H, s) (azomethine); 7.59 (1H, dd,
3J = 8 Hz, 4J = 3 Hz), 7.33 (1H, dd, 3J = 8 Hz, 4J = 3 Hz), 7.01–7.16 (8H,
ov), 6.88 (1H, t, 3J = 8 Hz), 6.81 (2H, ov) (aromatic); 6.58 (1H, d,
3J = 6 Hz), 5.52 (1H, d, 3J = 6 Hz) (methine); 1.53 (3H, s) (tert-butyl).
51V NMR (CD3OD, ppm) major (60%): −545.2; minor (40%): −532.7.
2.4.13. (Acetohydroxamato-ꢀ2O,Oꢀ){1S,2R(+)-2-[(1-oxido-1,2-
diphenylethyl)iminomethyl]-4-methoxyphenolato-
ꢀ3N,O,Oꢀ}oxidovanadium(V)
(3b)
Yield 83%. Anal. Calc. for C24H23N2O6V: C, 59.3; H, 4.8; N, 5.8.
Found: C, 59.4; H, 4.9; N, 5.8%. IR (KBr, cm−1): 3433 (␯ H); 1630,
N
1611 (␯ O, ␯ N); 966 (␯ O). UV–vis spectrum in DMSO [ꢁmax
C
C
V
2.4.10. ꢃ-Oxido-bis({1S,2R(+)-2-[(1-oxido-1,2-
diphenylethyl)iminomethyl]naphtholato-
ꢀ3N,O,Oꢀ}oxidovanadium(V))
(nm), ε (M−1 cm−1)]: 263 (15 320), 494 (3540). CD spectrum in
MeOH [ꢁmax (nm), ꢂε (M−1 cm−1)]: 275 (0.72), 318 (−3.66), 381
(3.23), 469 (0.58). 1H NMR (CD3OD, ppm) major (60%): 8.52 (1H,
s) (azomethine); 7.54 (2H, dd, 3J = 8 Hz, 4J = 3 Hz), 7.22 (1H, dd,
3J = 8 Hz, 4J = 3 Hz), 7.12 (1H, d, 4J = 3 Hz), 7.01–7.11 (8H, m), 7.02
(1H, ov) (aromatic); 6.43 (1H, d, 3J = 6 Hz), 5.54 (1H, d, 3J = 6 Hz)
(methine); 3.82 (3H, s) (methoxy); 2.05 (3H, s) (acetyl); minor
(40%): 8.68 (1H, s) (azomethine); 7.42 (2H, m), 7.25 (1H, ov), 7.14
(1H, ov), 6.99–7.10 (8H, ov), 6.94 (1H, d, 3J = 8 Hz) (aromatic); 6.69
(1H, d, 3J = 6 Hz), 5.78 (1H, d, 3J = 6 Hz) (methine); 3.87 (3H, s)
(10a)
Yield 72%. Anal. Calc. for C50H38N2O7V2: C, 68.2; H, 4.3; N,
3.2. Found: C, 68.1; H, 4.4; N, 3.2%. IR (KBr, cm−1): 1620 (␯ N);
C
985 (␯ O). UV–vis spectrum in MeOH [ꢁmax (nm), ε (M−1 cm−1)]:
V
311 (9340), 397 (3930). CD spectrum in MeOH [ꢁmax (nm), ꢂε
(M−1 cm−1)]: 259 (3.74), 310 (−7.03), 360 (10.02). 1H NMR (CD3OD,
ppm) major (60%): 9.56 (1H, s) (azomethine); 8.18 (1H, d, 3J = 8 Hz),

152
G. Romanowski et al. / Journal of Molecular Catalysis A: Chemical 381 (2014) 148–160
(methoxy); 2.28 (3H, s) (acetyl). 51V NMR (CD3OD, ppm) major
(60%): −263.5; minor (40%): −236.3.
2.4.17. (Acetohydroxamato-ꢀ2O,Oꢀ){1S,2R(+)-2-[(1-oxido-1,2-
diphenylethyl)iminomethyl]-4-nitrophenolato-
ꢀ3N,O,Oꢀ}oxidovanadium(V)
(7b)
Yield 77%. Anal. Calc. for C23H20N3O7V: C, 55.1; H, 4.0; N, 8.4.
2.4.14. (Acetohydroxamato-ꢀ2O,Oꢀ){1S,2R(+)-2-[(1-oxido-1,2-
diphenylethyl)iminomethyl]-3,5-dimethoxyphenolato-
ꢀ3N,O,Oꢀ}oxidovanadium(V)
Found: C, 55.0; H, 4.2; N, 8.2%. IR (KBr, cm−1): 3431 (␯ H); 1635,
N
1606 (␯ O, ␯ N); 1555, 1320 (␯NO2); 990 (␯ O). UV–vis spectrum
C
C
V
in DMSO [ꢁmax (nm), ε (M−1 cm−1)]: 343 (18 560), 482 (3240). CD
spectrum in MeOH [ꢁmax (nm), ꢂε (M−1 cm−1)]: 265 (0.78), 302
(−2.63), 377 (2.30), 468 (0.45). 1H NMR (CD3OD, ppm) major (60%):
8.59 (1H, s) (azomethine); 8.32 (1H, dd, 3J = 8 Hz, 4J = 3 Hz), 7.53
(2H, dd, 3J = 8 Hz, 4J = 3 Hz), 7.28 (1H, d, 4J = 3 Hz), 7.06–7.20 (8H, m),
7.04 (1H, d, 3J = 8 Hz) (aromatic); 6.57 (1H, d, 3J = 6 Hz), 5.55 (1H, d,
3J = 6 Hz) (methine); 2.11 (3H, s) (acetyl); minor (40%): 8.56 (1H, s)
(azomethine); 8.34 (1H, ov), 7.46 (2H, m), 7.26 (1H, ov), 7.06–7.20
(8H, ov), 6.99 (1H, d, 3J = 8 Hz) (aromatic); 6.93 (1H, d, 3J = 6 Hz), 5.70
(1H, d, 3J = 6 Hz) (methine); 2.65 (3H, s) (acetyl). 51V NMR (CD3OD,
ppm) major (60%): −256.2; minor (40%): −221.3.
(4b)
Yield 80%. Anal. Calc. for C25H25N2O7V: C, 58.1; H, 4.9; N, 5.4.
Found: C, 58.2; H, 5.0; N, 5.3%. IR (KBr, cm−1): 3428 (␯ H); 1601,
N
1573 (␯ O, ␯ N); 959 (␯ O). UV–vis spectrum in DMSO [ꢁmax
C
C
V
(nm), ε (M−1 cm−1)]: 320 (17 850), 489 (3670). CD spectrum in
MeOH [ꢁmax (nm), ꢂε (M−1 cm−1)]: 273 (0.24), 322 (−3.95), 371
(3.74), 465 (0.57). 1H NMR (CD3OD, ppm) major (60%): 8.88 (1H, s)
(azomethine); 7.51 (2H, dd, 3J = 8 Hz, 4J = 3 Hz), 7.01–7.12 (8H, m),
6.05 (1H, d, 3J = 8 Hz), 5.98 (1H, d, 3J = 8 Hz) (aromatic); 6.47 (1H, d,
3J = 6 Hz), 5.58 (1H, d, 3J = 6 Hz) (methine); 3.84 (3H, s), 3.78 (3H, s)
(methoxy); 2.01 (3H, s) (acetyl); minor (40%): 8.83 (1H, s) (azome-
thine); 7.43 (2H, m), 6.99–7.10 (8H, ov), 6.02 (1H, d, 3J = 8 Hz), 5.97
(1H, ov) (aromatic); 6.71 (1H, d, 3J = 6 Hz), 5.80 (1H, d, 3J = 6 Hz)
(methine); 3.87 (3H, s), 3.81 (3H, s) (methoxy); 2.21 (3H, s) (acetyl).
51V NMR (CD3OD, ppm) major (60%): −266.1; minor (40%): −238.4.
2.4.18. (Acetohydroxamato-ꢀ2O,Oꢀ){1S,2R(+)-2-[(1-oxido-1,2-
diphenylethyl)iminomethyl]-5-hydroxyphenolato-
ꢀ3N,O,Oꢀ}oxidovanadium(V)
(8b)
Yield 82%. Anal. Calc. for C23H21N2O6V: C, 58.5; H, 4.5; N, 5.9.
Found: C, 58.4; H, 4.6; N, 6.0%. IR (KBr, cm−1): 3439 (␯ H); 1621,
N
1592 (␯ O, ␯ N); 964 (␯ O). UV–vis spectrum in DMSO [ꢁmax
C
C
V
2.4.15. (Acetohydroxamato-ꢀ2O,Oꢀ){1S,2R(+)-2-[(1-oxido-1,2-
diphenylethyl)iminomethyl]-4-methylphenolato-
ꢀ3N,O,Oꢀ}oxidovanadium(V)
(nm), ε (M−1 cm−1)]: 262 (14 620), 491 (3380). CD spectrum in
MeOH [ꢁmax (nm), ꢂε (M−1 cm−1)]: 267 (1.25), 321 (−6.04), 380
(5.38), 474 (0.96). 1H NMR (CD3OD, ppm) major (60%): 8.52 (1H,
s) (azomethine); 7.88 (1H, d, 3J = 8 Hz), 7.54 (2H, m), 7.26 (1H, dd,
3J = 8 Hz, 4J = 3 Hz), 7.04–7.17 (8H, m), 7.01 (1H, d, 4J = 3 Hz) (aro-
matic); 6.42 (1H, d, 3J = 6 Hz), 5.45 (1H, d, 3J = 6 Hz) (methine); 4.87
(1H, s) (hydroxy); 2.05 (3H, s) (acetyl); minor (40%): 8.63 (1H, s)
(azomethine); 7.90 (1H, ov), 7.46 (2H, m), 7.04–7.17 (8H, ov), 7.28
(1H, m), 6.98 (1H, d, 4J = 3 Hz) (aromatic); 6.58 (1H, d, 3J = 6 Hz),
5.51 (1H, d, 3J = 6 Hz) (methine); 4.54 (1H, s) (hydroxy); 2.21 (3H, s)
(acetyl). 51V NMR (CD3OD, ppm) major (60%): −258.4; minor (40%):
−219.7.
(5b)
Yield 78%. Anal. Calc. for C24H23N2O5V: C, 61.2; H, 4.9; N, 6.0.
Found: C, 61.3; H, 4.9; N, 6.0%. IR (KBr, cm−1): 3415 (␯ H); 1635,
N
1602 (␯ O, ␯ N); 972 (␯ O). UV–vis spectrum in DMSO [ꢁmax
C
C
V
(nm), ε (M−1 cm−1)]: 266 (13 940), 487 (3360). CD spectrum in
MeOH [ꢁmax (nm), ꢂε (M−1 cm−1)]: 276 (0.89), 307 (−4.02), 368
(3.21), 463 (0.64). 1H NMR (CD3OD, ppm) major (60%): 8.49 (1H,
s) (azomethine); 7.49 (2H, dd, 3J = 8 Hz, 4J = 3 Hz), 7.30 (1H, dd,
3J = 8 Hz, 4J = 3 Hz), 7.16 (1H, d, 4J = 3 Hz), 6.97–7.10 (8H, m), 6.99
(1H, ov) (aromatic); 6.22 (1H, d, 3J = 6 Hz), 5.33 (1H, d, 3J = 6 Hz)
(methine); 2.31 (3H, s) (methyl); 2.06 (3H, s) (acetyl); minor (40%):
8.66 (1H, s) (azomethine); 7.33 (2H, m), 7.32 (1H, ov), 7.17 (1H,
ov), 6.97–7.10 (8H, ov), 6.88 (1H, d, 3J = 8 Hz) (aromatic); 6.61 (1H,
d, 3J = 6 Hz), 5.52 (1H, d, 3J = 6 Hz) (methine); 2.29 (3H, s) (methyl);
2.23 (3H, s) (acetyl). 51V NMR (CD3OD, ppm) major (60%): −259.0;
minor (40%): −224.5.
2.4.19. (Acetohydroxamato-ꢀ2O,Oꢀ){1S,2R(+)-2-[(1-oxido-1,2-
diphenylethyl)iminomethyl]-6-tert-butylphenolato-
ꢀ3N,O,Oꢀ}oxidovanadium(V)
(9b)
Yield 76%. Anal. Calc. for C27H29N2O5V: C, 63.3; H, 5.7; N, 5.5.
Found: C, 63.2; H, 5.6; N, 5.6%. IR (KBr, cm−1): 3432 (␯ H); 1620,
N
1592 (␯ O, ␯ N); 961 (␯ O). UV–vis spectrum in DMSO [ꢁmax
C
C
V
(nm), ε (M−1 cm−1)]: 265 (14 860), 493 (3410). CD spectrum in
MeOH [ꢁmax (nm), ꢂε (M−1 cm−1)]: 263 (2.11), 319 (−7.38), 375
(7.40), 466 (1.24). 1H NMR (CD3OD, ppm) major (60%): 8.75 (1H,
s) (azomethine); 7.56 (2H, m), 7.52 (1H, dd, 3J = 8 Hz, 4J = 3 Hz),
7.18 (1H, d, 3J = 8 Hz), 6.99–7.11 (8H, m), 7.02 (1H, ov) (aromatic);
6.42 (1H, d, 3J = 6 Hz), 5.63 (1H, d, 3J = 6 Hz) (methine); 1.95 (3H, s)
(acetyl); 1.44 (3H, s) (tert-butyl); minor (40%): 8.58 (1H, s) (azome-
thine); 7.50 (2H, ov), 7.21 (1H, d, 3J = 8 Hz), 7.17 (1H, ov), 6.99–7.11
(8H, ov), 6.98 (1H, ov) (aromatic); 6.76 (1H, ov), 5.83 (1H, d, 3J = 6 Hz)
(methine); 2.17 (3H, s) (acetyl); 1.41 (3H, s) (tert-butyl). 51V NMR
(CD3OD, ppm) major (60%): −265.4; minor (40%): −229.3.
2.4.16. (Acetohydroxamato-ꢀ2O,Oꢀ){1S,2R(+)-2-[(1-oxido-1,2-
diphenylethyl)iminomethyl]-4-bromophenolato-
ꢀ3N,O,Oꢀ}oxidovanadium(V)
(6b)
Yield 81%. Anal. Calc. for BrC23H20N2O5V: C, 51.6; H, 3.8; N, 5.2.
Found: C, 51.7; H, 3.8; N, 5.1%. IR (KBr, cm−1): 3421 (␯ H); 1626,
N
1598 (␯ O, ␯ N); 970 (␯ O). UV–vis spectrum in DMSO [ꢁmax
C
C
V
(nm), ε (M−1 cm−1)]: 262 (15 340), 497 (3410). CD spectrum in
MeOH [ꢁmax (nm), ꢂε (M−1 cm−1)]: 264 (1.05), 317 (−4.86), 378
(4.17), 471 (0.54). 1H NMR (CD3OD, ppm) major (60%): 8.53 (1H, s)
(azomethine); 7.65 (1H, d, 4J = 3 Hz), 7.53 (2H, dd, 3J = 8 Hz, 4J = 3 Hz),
7.47 (1H, dd, 3J = 8 Hz, 4J = 3 Hz), 6.99–7.12 (8H, m), 6.90 (1H, ov)
(aromatic); 6.19 (1H, d, 3J = 6 Hz), 5.31 (1H, d, 3J = 6 Hz) (methine);
2.07 (3H, s) (acetyl); minor (40%): 8.69 (1H, s) (azomethine); 7.67
(1H, d, 4J = 3 Hz), 7.51 (2H, m), 7.49 (1H, ov), 7.00–7.12 (8H, ov),
6.93 (1H, d, 3J = 8 Hz) (aromatic); 6.65 (1H, d, 3J = 6 Hz), 5.50 (1H, d,
3J = 6 Hz) (methine); 2.24 (3H, s) (acetyl). 51V NMR (CD3OD, ppm)
major (60%): −262.3; minor (40%): −227.8.
2.4.20. (Acetohydroxamato-ꢀ2O,Oꢀ){1S,2R(+)-2-[(1-oxido-1,2-
diphenylethyl)iminomethyl]-naphtholato-
ꢀ3N,O,Oꢀ}oxidovanadium(V)
(10b)
Yield 79%. Anal. Calc. for C27H23N2O5V: C, 64.0; H, 4.6; N, 5.5.
Found: C, 63.9; H, 4.6; N, 5.6%. IR (KBr, cm−1): 3437 (␯ H); 1618,
N
1604 (␯ O, ␯ N); 977 (␯ O). UV–vis spectrum in DMSO [ꢁmax
C
C
V
(nm), ε (M−1 cm−1)]: 312 (14 230), 490 (3550). CD spectrum in

G. Romanowski et al. / Journal of Molecular Catalysis A: Chemical 381 (2014) 148–160
153
Fig. 2. The molecular structure of 7, with displacement ellipsoids drawn at the 25%
probability level and H atoms are shown as small spheres of arbitrary radius.
MeOH [ꢁmax (nm), ꢂε (M−1 cm−1)]: 290 (1.13), 322 (−5.20), 394
(4.39), 483 (0.78). 1H NMR (CD3OD, ppm) major (60%): 9.41 (1H,
s) (azomethine); 7.87 (1H, d, 3J = 8 Hz), 7.71 (1H, d, 3J = 8 Hz), 7.55
(2H, m), 7.39 (1H, d, 3J = 8 Hz), 7.30 (1H, t, 3J = 8 Hz), 7.22 (1H, t,
3J = 8 Hz), 6.99–7.18 (8H, m), 7.03 (1H, ov) (aromatic); 6.27 (1H,
d, 3J = 6 Hz), 5.51 (1H, d, 3J = 6 Hz) (methine); 2.07 (3H, s) (acetyl);
minor (40%): 9.47 (1H, s) (azomethine); 7.91 (1H, d, 3J = 8 Hz), 7.74
(1H, d, 3J = 8 Hz), 7.46 (2H, m), 7.44 (1H, ov), 7.32 (1H, ov), 7.24
(1H, ov), 6.99–7.18 (8H, m), 6.99 (1H, ov) (aromatic); 6.66 (1H, d,
3J = 6 Hz), 5.70 (1H, d, 3J = 6 Hz) (methine); 1.94 (3H, s) (acetyl). 51
NMR (CD3OD, ppm) major (60%): −253.6; minor (40%): −216.5.
V
Fig. 3. View of the crystal structure of 7 approximately along the b axis. The O H· · ·O
and C H· · ·O contacts are represented by dashed lines and the C H· · ·␲ interactions
are represented by dotted lines. H atoms not involved in interactions were omitted.
3. Results and discussion
3.1. Crystal and molecular structure of 7 and 7b
ligand 7 and 1.436(4) and 1.432(4) Å for the Schiff base derived
from l-phenylalaninol. On the other hand, the rather shorter dis-
tances are observed for the C3 C4 and C5 C6 bonds, i.e. 1.374(3)
The crystallographic data of the ligand 7 and the complex 7b
are summarized in Table S1, and the selected bond distances and
angles are given in Tables S2 and S3, respectively.
˚
˚
and 1.363(4) A for the ligand 7 and 1.358(4) and 1.359(4) A for
the l-phenylalaninol Schiff base. Significant differences have been
also found in the aromatic valence angles between benzenoid and
quinoid form. For example, the ipso-C1 angle has smaller values
for keto-amino tautomers, i.e. 116.5(2)◦ for the ligand 7, 115.5(3)◦
or the l-phenylalaninol Schiff base [31], than for enol-imino tau-
tomers, ca. 120.0◦ [32]. Moreover, the donor acceptor (D· · ·A)
distance in the intramolecular N H· · ·O hydrogen bond in the
Salicylaldimine compounds are of interest mainly due to the
existence of either N· · ·H O or N H· · ·O type of intramolecular
hydrogen bond and the related tautomerism between such ben-
zenoid (enol-imino) and quinoid (keto-amino) forms. In most
crystal structures of salicylaldimines listed in the Cambridge Struc-
tural Database (CSD) the existence of enol-imino tautomer has
been established and keto-amino tautomer appears only in sev-
eral examples of such compounds. The ligand 7 crystallizes in
the polar space group P21 with two molecules in the asym-
metric unit. This Schiff base is an example of salicylaldimine
compound containing N H· · ·O hydrogen bond, which seems
to be imposed by through-molecule interactions between the
nitro group present in the para position, which is expected to
play a role in stabilizing the present keto-amino tautomeric
form of the molecule, and resonance-assisted hydrogen bond-
ing (Fig. 2). Such conclusion is taken into account, inter alia, due
˚
˚
ligand 7 is 2.563(3) A and the H· · ·A distance is 1.81(3) A, which
are even shorter than for l-phenylalaninol Schiff base with lengths
˚
˚
of 2.638 A and 1.926 A, respectively. Surprisingly, the N H· · ·O
hydrogen-bond angle with value of 143(2)◦ is much bigger than
134.00◦ for the l-phenylalaninol Schiff base, and very similar to
N· · ·H O hydrogen-bond angles, ca. 141.9◦ [32].
In the crystal network molecules of 7 are assembled via weak
intermolecular O H· · ·O hydrogen bonds (Table 2) forming chains,
additionally stabilized by C H· · ·␲ interactions between neighbor-
ing molecules, which are arranged into columns parallel to the b
axis (Fig. 3).
˚
to length of C1 O7 bond (1.274(3) A) which is characteristic of
˚
a double bond, and similar with 1.267(3) A distance for (S)-2-
[(1-benzyl-2-hydroxyethylimino)methyl]-4-nitrophenol, i.e. Schiff
base derived from 5-nitrosalicylaldehyde and l-phenylalaninol,
with quinoid structure [31]. On the other hand, the value of
The complex 7b crystallizes in the same polar space group P21
as its ligand precursor, with two molecules in the asymmetric unit.
The vanadium atom is six-coordinated by one oxido group, two
oxygen atoms and one nitrogen atom of the tridentate Schiff base
ligand, and by two oxygen atoms of the O,Oꢀ-bidentate hydrox-
amate co-ligand, defining a distorted octahedral VO(ONO)(OO)
geometry (Fig. 4). The equatorial plane is defined by one nitro-
gen atom (N12) and two oxygen atoms (O7 and O21) of the
tridentate Schiff base ligand and the oxime oxygen atom (O30)
of the acetohydroxamate co-ligand, while the oxido oxygen atom
˚
1.284(3) A for C11 N12 bond (azomethine) gives no significant
difference between such values reported for Schiff bases exist-
ing as enol-imino or keto-amino tautomer [31,32]. There is also
a certain regularity in the aromatic bond lengths. For the cases of
Schiff bases with benzenoid form, all the bond lengths in the aro-
matic ring are equal. However, keto-amino tautomers have much
˚
longer C1 C2 and C1 C6 bonds, i.e. 1.442(3) and 1.425(3) A for the

154
G. Romanowski et al. / Journal of Molecular Catalysis A: Chemical 381 (2014) 148–160
Fig. 4. The molecular structure of 7b, with displacement ellipsoids drawn at the 25%
probability level and H atoms are shown as small spheres of arbitrary radius.
Fig. 5. View of the crystal structure of 7b approximately along the a axis.
˚
oxygen (O21) atom is by 0.337 A shorter than in [VO(HSHED)SHI]
(O29) and oxygen carbonyl atom (O33) of the hydroxamate group
occupy axial positions. The vanadium atom is displaced from the
least squares equatorial plane toward the terminal oxido group
with coordination bond between vanadium and secondary amine
nitrogen atoms.
The crystal structure of the 7b complex is stabilized by the
intermolecular hydrogen bonds linking the neighboring complex
molecules and water molecules into two-dimensional infinite net-
work, i.e. hydrogen bonds between nitrogen oxime atom of a one
molecule, water molecule and carbonyl oxygen atom of neighbor-
ing molecule and also between water molecule and one of the
oxygen atom of nitro group, result in formation of chains extending
in the a axis direction (Fig. 5). Apart from the intermolecular hydro-
gen bonds there are also intra- and intermolecular C H· · ·O close
contacts additionally stabilizing interactions inside and between
those chains (Fig. 6).
˚
by 0.257 A. In trans position to the terminal oxido group is the
O33 atom of the acetohydroxamate co-ligand with the bond dis-
˚
tance from vanadium atom of 2.2341(19) A that is significantly
˚
longer than V28 O30(oxime) bond distance (1.8737(18) A), due to
this strong trans influence [33]. Also, the trans angle is 168.88(9)◦
˚
and these values correspond well with 2.164(2), 1.856(2) A dis-
tances and 169.3(1)◦ angle for [VVO(sal-gly)(bz)] reported earlier
˚
[34]. Moreover, the V28 O29 distance of 1.5960(18) A and also, the
values for V28 O7(phenolate) and V28 N12(imine) bonds being
˚
˚
1.913(2) A and 2.1119(18) A, respectively, are in the range found
in other similar vanadium(V) complexes with hydroxamate co-
ligands [34–38].
The tridentate Schiff base ligand and the acetohydroxamate co-
ligand combine with the vanadium atom to give one six-membered
and two five-membered chelate rings. The five-membered chelate
ring, involving V28, O30, N31, C32 and O33 atoms from acetohy-
droxamate co-ligand, is basically planar, whereas the Schiff base
five-membered chelate ring containing V28, N12, C13, C20 and
O21 atoms adopts an envelope conformation on C20 atom with
P = 54.5(1)◦ and Tau(M) = 46.8(1)◦ for reference bond V28 O21 [39].
Finally, the six-membered chelate ring, involving V28, O7, C1, C2,
C11 and N12 atoms, is non-planar, and concluded to be an enve-
lope on V28 atom. Moreover, the values of bond lengths and angles
in this chelate ring are comparable with those for corresponding
entries in the CSD, as expected for free Schiff base keto-amino
tautomers and Schiff base coordinated to a metal ion [40].
The loss of only one proton, from acetohydroxamic acid, yielding
the hydroxamate anion, not the hydroximate dianion, is revealed
by comparison of bond distances and angles between the hydrox-
amate ring of 7b, and the hydroximate ring in the vanadium(V)
complex, [VO(HSHED)SHI], with hydroximate co-ligand reported
earlier [35]. The distance of the N31 C32 bond is comparable,
showing its partial double-bond character, but there are significant
differences in some of the bond distances in this five-membered
chelate ring. The obvious decrease in C32 O33 bond length by
3.2. Spectroscopic properties
The electronic, circular dichroism, NMR and selected solid-state
IR spectral data are listed in Section 2.
To compare the changes that might have taken during the com-
plexation we have compared the IR spectra of the complexes with
the ligand. The ligand, 7, exhibit broad strong band at 3483 cm−1
due to the ␯(O H) vibration. Also, only in this case the strong
bands appear at 1324 and 1538 cm−1 which are typical for nitro
group and undergo minor changes in the IR spectra of 7a (1338 and
1560 cm−1) and 7b (1320 and 1555 cm−1) complexes. The strong
band at 1650 cm−1 may be assigned to the ␯(C N) (azomethine)
stretch. In this region, azomethine band may be associated with
the aromatic C C stretching band. Shifting of this band to a lower
wave number by 41–44 cm−1 in the corresponding vanadium(V)
complexes indicates the coordination of azomethine nitrogen to
the vanadium atom. A strong band appearing in the 959–990 cm−1
region in all the complexes may be attributed to single V
O
stretching vibrations, which are close to values reported for related
oxidovanadium(V) Schiff base complexes [15,17,28]. Additionally,
the spectra of 1b–10b complexes exhibit broad strong band in
3415–3439 cm−1 ranges, assigned to ␯(N H) stretch of acetohy-
droxamate co-ligand. The appearance of weak/medium band near
2800 cm−1 separate from other C H stretching vibrations in 2a–4a
and 2b–4b complexes indicated the presence of the methoxy sub-
stituents [41].
˚
0.019 A, owing to the stronger partial double-bond character, and
˚
increase in V28 O33 bond length by 0.112 A, owing to the signif-
icantly much weaker bond, where oxygen atom has neutral (not
negative) charge, are observed. It is also noteworthy to mention
that bond between vanadium atom and deprotonated alcoholate
The electronic and circular dichroism spectra of the ligand
7
and all the complexes were recorded in methanol. The

G. Romanowski et al. / Journal of Molecular Catalysis A: Chemical 381 (2014) 148–160
155
separately and designated as major and minor. In all the com-
plexes, the major:minor ratios are about 60:40, in contrast to
similar vanadium(V) Schiff base complexes derived from different
amino alcohols, where contribution of major species varies from
53% to 95% [14]. The 51V NMR spectra exhibit two signals with
the same major:minor ratios as the signals in the 1H NMR spectra.
According to Rehder et al. [46], possible reasons for the presence of
two species in solution 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 temperature
dependence of chemical shift and the increasing intensity of sig-
nals with an increase of the concentration of complexes should be
observed in 51V NMR spectra. These rather high-field resonances
are present in chloroform solution [47] and disappear when
methanol is used as solvent, due to an additional coordination of
methanol leading to six-coordinated vanadium species.
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-1,2-diphenylethanol is confirmed by the presence of
azomethine proton signals in the spectra of the ligand 7 and all
the complexes. Two-dimensional (COSY, gHSQC and NOESY) NMR
experiments were performed for all the compounds which allow to
unambiguously establish the attachment of all carbon atoms. Sur-
prisingly, the presence of the signal at 15.00 ppm in DMSO solution
originated from phenolic proton suggests the predominant con-
tribution of benzenoid form (enol-imino tautomer) in contrast to
the solid state where only quinoid form of the ligand (keto-amino
tautomer) was found. The COSY spectrum of the ligand 7 (Fig. S1)
exhibits cross-peaks between the methine proton (H20) signal at
5.12 ppm and hydroxyl proton (H21) signal at 5.97 ppm, provid-
ing evidence of a bond between C20 and O21 atoms. Moreover,
the connection between C13 and C20 atoms is proved by cross-
peaks between the two methine protons signals at 4.93 (H13) and
5.12 (H20) ppm. In salicylidene moiety there are also two cross-
peaks present, i.e. between aromatic proton signal at 6.82 (doublet,
3J = 8 Hz) and 8.11 ppm (doublet of doublets, 3J = 8 Hz and 4J = 3 Hz)
and second cross-peak between aromatic proton signal at 8.41
(doublet, 4J = 3 Hz) and 8.11 ppm (doublet of doublets, 3J = 8 Hz and
4J = 3 Hz).
On the other hand, in the NOESY spectra of the complexes 7a
and 7b, cross-peak between azomethine proton (C12) signal at 8.75
and methine proton (H13) signal at 5.91 ppm (major species sig-
nal), and between azomethine proton signal at 8.59 and methine
proton signal at 5.55 ppm, respectively, are observed proving the
connection between N12 and C13 atoms. As expected, there is lack
of any cross-peak between the signals of azomethine protons and
the other methine protons in both complexes. Downfield shifting
by 0.98 (H13) and 2.08 (H20) ppm in 7a, and by 0.62 and 1.45 ppm
in 7b of the major species signals of the two methine protons of the
aliphatic bridge, in comparison to the spectrum of the ligand, may
confirm coordination of the vanadium atom to azomethine nitro-
gen and alcoholate oxygen. Moreover, the absence of a peaks at
15.0 and 5.97 ppm, which are present in the spectrum of the ligand,
noted in all the complexes indicates the loss of the two hydroxyl
protons from phenolate and alcoholate moieties, respectively, due
to complexation reaction.
Fig. 6. The arrangement of the molecules of 7b in the crystal structure. The N H· · ·O,
O
H· · ·O and C H· · ·O contacts are represented by dashed lines. H atoms not
involved in interactions were omitted.
ligand 7 shows three bands at 261 (εmax = 19 810 M−1 cm−1
)
and 327 nm (εmax = 10 030 M−1cm−1), which may be assigned to
*
*
␲–␲ transitions, and at 394 nm (εmax = 7080 M−1 cm−1) to n–␲
transitions. For the nitro-substituted ligands, an overlap between
*
␲–␲ transitions involving the azomethine and ␲ molecular
orbitals of the aromatic ring of the salicylidene chromophores
*
may be observed [42]. In the complexes, the n–␲ transitions
of the azomethine group are shifted to lower energy and it
may be concluded that the azomethine nitrogen appears to
be coordinated to the vanadium atom. The spectra of 7a and
7b complexes examined in the same spectral region display
very strong bands at 348 (εmax = 14 580 M−1 cm−1) and 343 nm
(εmax = 11 350 M−1 cm−1), respectively, similar to the same strong
bands at around 350 nm for the other vanadium(V) complexes
derived from 5-nitrosalicylaldehyde [26,43]. On the other hand,
strong intense bands, εmax = 9340–14380 M−1 cm−1, with ␭
max
in region 278–319 nm for 1a–6a, 8a–10a and in the 259–320 nm
range (εmax = 13940–17850 M−1 cm−1
)
for 1b–6b, 8b–10b are
*
considered to arise from intraligand ␲–␲ transitions· The low
energy bands, recorded for 1a–3a, 5a, 6a, 8a–10a and 1b–10b
complexes, between 341–397 nm (εmax = 3930–4620 M−1 cm−1
)
and 482–497 nm (εmax = 3240–3670 M−1 cm−1), respectively, may
be assigned as a ligand-to-metal charge transfer (LMCT) transition
originating from the p orbital on the phenolate oxygen to the
␲
empty d orbital at the vanadium(V) center [44,45]. The circular
dichroism spectra of the ligand 7 and the complexes 1a–10a
revealed an additional band at 280 nm and in the 289–310 nm
*
range, respectively, of ␲–␲ transitions origin. On the other hand,
1b–10b complexes possessing acetohydroxamate co-ligand, in
comparison to their electronic spectra, revealed two additional
bands in the 302–323 nm and the 368–394 nm range with the
negative and positive Cotton effect, respectively.
3.3. Catalytic activity studies
3.3.1. Sulfoxidation
In this study, the oxidovanadium(V) complexes 1a–10a for their
ability to catalyze the oxidation of prochiral sulfides using methyl
phenyl sulfide (thioanisol) and phenyl benzyl sulfide as model sub-
strates have been tested (Fig. 7). Aqueous 30% H2O2 was used as
In the 1H NMR spectra of all the complexes in CD3OD, two
sets of signals are observed to suggest the presence of two
species in solution. Chemical shifts for the two species are listed

156
G. Romanowski et al. / Journal of Molecular Catalysis A: Chemical 381 (2014) 148–160
optically active diamines used as catalysts in the sulfoxidation reac-
tions, significantly shorter reaction times and much higher values
of enantiomeric excesses of corresponding sulfoxides employing
1a–10a as catalysts were observed [26].
3.3.2. Oxidation of styrene
Fig. 7. Sulfoxidation of thioethers catalyzed by vanadium(V) complexes.
The oxidation of styrene was achieved with the 1a–10a
complexes. In the presence of aqueous 30% H2O2 or tert-butyl
hydroperoxide (TBHP) as oxidant, the oxidation products obtained
are styrene oxide, benzaldehyde, 1-phenylethane-1,2-diol, benzoic
acid and phenylacetaldehyde (Fig. 8). 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 2a 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), different solvents and temperature of the reaction mix-
ture were tested.
To study the effect of amount of oxidant, three different molar
ratios of aqueous 30% H2O2 or tert-butyl hydroperoxide (TBHP)
to styrene, i.e. 1:1, 2:1 and 3:1, styrene (1.00 mmol) and catalyst
(0.010 mmol) were taken in CH3CN (10 ml), and the reaction was
carried out for 6 h of contact time at 80 ◦C. At a H2O2 to styrene
molar ratio of 1:1, a maximum of 6.8% conversion was achieved.
Increasing the ratio to 2:1 improved the conversion to 17.4%, while
3:1 ratio has shown a maximum of 21.3% conversion. Further incre-
ment of H2O2 shows no improvement in conversion, therefore a 3:1
ratio being considered adequate. In case of using TBHP as oxidant,
increasing the TBHP:styrene ratio from 1:1 to 2:1 and 3:1 improved
the conversion from 43.1% to 66.7% and 87.3%, respectively. As in
the previous case the oxidation improved only marginally upon
further addition of oxidant.
Similarly for three different amounts (i.e. 0.5, 1 and 2 mol%) of
catalyst and oxidant to styrene molar ratio of 3:1 under above reac-
tion conditions, 0.5 mol% gave only 6.3% (H2O2) and 36.8% (TBHP)
oxidative conversion, while 1 and 2 mol% have shown nearly iden-
tical results with 21.3% and 87.3% conversion, for H2O2 and TBHP,
respectively, for 6 h of contact time. Thus, 1 mol% of catalyst may
be considered sufficient enough to run the reaction under above
conditions. A blank reaction under the above reaction conditions
gave with both oxidants ca. 2–4% conversion.
Using tert-butyl hydroperoxide (TBHP) as 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 80–87% conversion, especially in comparison to the other
reported earlier dioxidovanadium(V) complexes with Schiff bases,
i.e. [VO2(acac-ambmz)], [VO2(sal-ambmz)] and [VO2(sal-aebmz)],
which gave 20–35% conversion [49]. Table 2 summarizes the per-
centage conversion of styrene and the selectivities for the various
reaction products. Selectivity, in case of all vanadium(V) com-
plexes, is rather similar and they are generally distinctly more
selective toward styrene oxide (72.3–81.4%) than benzaldehyde
(5.5–8.4%) and surprisingly high selectivity of 1-phenylethane-1,2-
diol (7.7–11.7%). The conversion of the other oxidation products
is low (>7%). In case of [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, catalytic activity of oxidovanadium(IV)
Schiff base complex derived from o-aminobenzyl alcohol, [VO(sal-
oaba)], using TBHP as oxidant, has been studied [50], but resulting
in very low conversion (16%) and similar selectivity of styrene oxide
and benzaldehyde.
oxidant in a 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 results of catalytic studies are listed in Table 1.
The best results have been obtained for complexes 2a, 3a, 5a,
6a and 8a as catalysts in the oxidation of methyl phenyl sulfide
(Table 1, entries 2, 3, 5, 6 and 8). An overall yields for all catalysts
were in the range of 79–86% within 30 min reaction time and enan-
tiomeric excesses (ee’s) of 48–69% for the S-configured sulfoxide
was obtained. When benzyl phenyl sulfide was used as substrate,
possessing more bulky substituent the overall yield of correspond-
ing sulfoxide was distinctly lower (69–73%) and enantioselectivity
decreases to value of 26–31% (Table 1, entry 11–15). With the reac-
tion carried out at −20 ◦C for 2a, 3a, 5a, 6a and 8a as catalysts in the
oxidation of methyl phenyl sulfide, enantioselectivities improve
significantly to 75–80% ee’s and a conversion of over 85% of the sub-
strate is observed after 3 h (Table 1, entries 16–20). Mimoun et al.
[48] pointed out the importance of sufficiently nucleophilic center
for the oxidative catalysis of organic substances by peroxidovana-
dium(V) compounds. Better enantioselectivities for 2a, 3a, 5a, 6a
and 8a as compared to other catalysts may be result of a higher
electron density on the phenolato oxygen, e.g. due to the electronic
effects of orto-, meta- and para-substituted groups, contributing to
an attainment of sufficient nucleophilicity by the vanadium center.
In general, the best results have been obtained in the oxidation
reactions of methyl phenyl sulfide. Increasing the steric demand
of the substrate on going from methyl phenyl sulfide to ben-
zyl phenyl sulfide leads to a decrease in both the overall yield
and enantioselectivity of corresponding sulfoxide. The yields for
methyl phenyl sulfoxide obtained in this paper are comparable
with the other vanadium(V) Schiff base catalysts derived from chi-
ral amino alcohols, but generally enantioselectivities in many cases
are better and only when 3,5-diiodosalicylaldehyde derivative was
employed values of ee’s in −20 ◦C were over 90% [13]. In contrast
to chiral oxidovanadium(V) Schiff base complexes derived from
Table 1
Catalytic oxidation of PhSMe and PhSBz by aqueous 30% H2O2 in the presence of
1 mol% vanadium(V) Schiff base complexes as catalysts.
Entry
Catalyst
Substrate
Yield (%)
T (◦C)
t (min)
Ee (%)a
1
2
3
4
5
6
7
8
1a
2a
3a
4a
5a
6a
7a
8a
9a
10a
2a
3a
5a
6a
8a
2a
3a
5a
6a
8a
PhSMe
PhSMe
PhSMe
PhSMe
PhSMe
PhSMe
PhSMe
PhSMe
PhSMe
PhSMe
PhSBz
PhSBz
PhSBz
PhSBz
PhSBz
PhSMe
PhSMe
PhSMe
PhSMe
PhSMe
80
83
82
83
79
86
85
81
82
84
72
69
73
70
71
88
86
85
87
90
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
52
66
64
56
67
65
53
69
48
51
28
31
26
31
30
75
80
76
75
78
9
10
11
12
13
14
15
16
17
18
19
20
−20
−20
−20
−20
−20
180
180
180
180
180
When this catalytic reaction have been performed in the same
reaction conditions, but with 30% H2O2 as oxidant, very low con-
version in 6 h of contact time was found (19.8–24.2%). Although
the conversion of styrene is distinctly low, the selectivity for
a
All sulfoxides are in S configuration.

G. Romanowski et al. / Journal of Molecular Catalysis A: Chemical 381 (2014) 148–160
157
Fig. 8. Various oxidation products of styrene catalyzed by vanadium(V) complexes.
benzaldehyde is much better (77.8–85.2%) than in the case of
TBHP. On the other hand, as well as for reported earlier [VO2(acac-
ambmz)], [VO2(sal-ambmz)], [VO2(sal-aebmz)] [49] and [VVO(hap-
dahp)] complexes [51], styrene oxide is the most expected product
but its selectivity goes down considerably (3.1–5.7%). Moreover,
the catalytic activity of vanadium(V) complexes encapsulated in
the super cages of zeolite-Y, i.e. [VVO(hap-dahp)]-Y [51], [VO2(sal-
ambmz)]-Y, [VO2(pydx-ambmz)]-Y and [VO2(pydx-aebmz)]-Y,
has recently been discovered [52,53]. The percentage conversion
of styrene for [VO2(pydx-ambmz)]-Y and [VO2(pydx-aebmz)]-Y
Table 2
Catalytic oxidation of styrene by aqueous 30% H2O2 or tert-butyl hydroperoxide (TBHP) in the presence of 1 mol% vanadium(V) Schiff base complexes as catalysts.
Entry
Catalyst
Oxidant
Conv. (%)
Product selectivity (%)
StOa
BzAb
BzACc
PhAAd
PhEDe
1
2
3
4
5
6
7
8
1a
2a
3a
4a
5a
6a
7a
8a
9a
10a
1a
2a
3a
4a
5a
6a
7a
8a
9a
10a
11
12
13
14
15
16
17
18
11
12
13
14
15
16
17
18
H2O2
H2O2
H2O2
H2O2
H2O2
H2O2
H2O2
H2O2
H2O2
H2O2
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
H2O2
H2O2
H2O2
H2O2
H2O2
H2O2
H2O2
H2O2
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
20.7
21.3
23.6
21.6
24.2
20.7
19.8
22.9
21.3
20.6
85.2
82.6
85.8
87.3
84.4
82.8
80.5
85.3
83.9
81.4
17.3
21.7
18.7
19.3
17.8
18.4
16.9
20.1
78.4
83.0
81.7
84.6
82.1
80.5
77.4
81.9
4.4
4.1
4.5
3.9
4.7
3.6
5.2
3.1
4.9
81.2
82.4
79.6
81.5
80.9
83.1
81.3
85.2
79.3
77.8
8.4
7.2
7.4
5.6
6.3
6.8
8.3
7.0
7.9
1.7
1.4
1.7
1.4
1.4
1.5
1.2
1.0
1.4
1.8
6.2
4.6
4.9
3.8
4.2
4.6
6.3
5.4
5.8
4.1
–
–
–
–
–
–
–
–
2.3
2.0
1.9
2.1
2.5
2.1
1.7
1.9
2.0
1.8
3.0
2.9
2.5
2.8
2.2
1.7
2.5
2.1
1.7
2.4
2.1
1.5
1.7
2.0
2.3
2.5
2.3
1.4
1.3
1.5
1.3
1.2
1.4
1.2
1.1
1.2
1.3
1.4
1.7
1.3
1.2
1.3
1.5
1.2
10.7
10.3
11.2
10.3
10.5
9.0
10.1
8.4
11.9
12.6
10.5
9.4
9.9
7.7
8.7
9.8
10.6
9.4
11.7
7.9
9.4
12.1
7.2
9.0
6.4
5.1
8.4
11.4
2.0
2.1
1.8
1.6
1.4
1.7
1.9
2.0
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
5.7
73.2
76.4
75.7
81.4
79.1
76.8
72.5
75.7
72.3
81.1
4.8
5.5
5.1
5.7
6.6
5.5
4.1
3.8
49.8
52.7
56.5
53.1
52.6
57.2
59.8
58.4
5.5
84.5
80.9
86.4
84.1
85.6
88.2
86.4
83.6
44.6
41.8
38.1
41.9
42.3
37.7
35.1
36.5
a
Styrene oxide.
Benzaldehyde.
Benzoic acid.
Phenylacetaldehyde.
1-Phenylethane-1,2-diol.
b
c
d
e

158
G. Romanowski et al. / Journal of Molecular Catalysis A: Chemical 381 (2014) 148–160
2
1
R
R
1
2
3
4
11: R =R =R =R =H
2
3
4
1
12: R =R =R =H, R =OCH3
O
V
3
R
O
N
O
1
2
4
3
13: R =R =R =H, R =OCH3
1
3
2
4
14: R =R =H, R =R =OCH3
4
R
NH2
1
2
4
3
15: R =R =R =H, R =CH3
1
2
4
3
S
S
16: R =R =R =H, R =Br
1
2
4
3
17: R =R =R =H, R =NO2
1
2
3
4
18: R =R =H, R -C H -R
4
4
Fig. 9. Structural formulae of vanadium(V) complexes containing ONN-tridentate
donor ligands.
Fig. 10. Various oxidation products of cyclohexene catalyzed by vanadium(V) com-
plexes.
is 65–68% and 78% for [VVO(hap-dahp)]-Y, which is less than that
observed for VO2(sal-ambmz)]-Y (97%), but all these catalysts are
less selective toward benzaldehyde (ca. 55%) and more selective
toward 1-phenylethane-1,2-diol (ca. 25%).
As acetonitrile and 80 ◦C were found again an ideal solvent
and temperature to run the catalytic reaction, we have studied
three different molar ratios of aqueous 30% H2O2 or tert-butyl
hydroperoxide (TBHP) to cyclohexene, i.e. 1:1, 2:1 and 3:1. Cyclo-
hexene (1.00 mmol) and catalyst (0.010 mmol) were taken in
CH3CN (10 ml), and the reaction was carried out for 6 h of contact
time at 80 ◦C. At a H2O2 to styrene molar ratio of 1:1, a maxi-
mum of 18.3% conversion was achieved. Increasing the ratio to 2:1
improved the conversion to 34.7%, while 3:1 ratio has shown a max-
imum of 61.4% conversion. Further increment of H2O2 shows no
improved conversion only marginally, therefore a 3:1 ratio being
considered adequate. In case of using TBHP as oxidant, increasing
the TBHP:cyclohexene ratio from 1:1 to 2:1 and 3:1 improved the
conversion from 47.2% to 72.4% and 93.1%, respectively. As in the
previous case, there was no significant conversion improvement
upon further addition of oxidant.
Similarly, for three different amounts (i.e. 0.5, 1 and 2 mol%)
of catalyst and oxidant to cyclohexene molar ratio of 3:1 under
above reaction conditions, 0.5 mol% gave only 6.3% (H2O2) and
36.8% (TBHP) oxidative conversion, while 1 mol% and 2 mol% of cat-
alyst have shown a maximum conversion with 61.4% for H2O2 and
93.1% for TBHP (6 h of contact time). 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. 3% conversion.
For 1a–10a as catalysts and with 30% H2O2 as oxidant,
46.5–64.8% conversion in 6 h of contact time was found. On the
other hand, the conversion of styrene in the same reaction con-
ditions is distinctly lower. The selectivity for cyclohexene oxide
(21.3–30.3%) and 2-cyclohexene-1-ol (31.0–40.8%) is much better
than for cyclohexane-1,2-diol (19.1–25.3%) and 2-cyclohexene-1-
one (12.2–20.7%).
When this catalytic reaction has been performed in the same
reaction conditions, but with tert-butyl hydroperoxide (TBHP) as
oxidant, under the optimized reaction conditions, 1a–10a catalysts
gave over 78% conversion. The percentage conversion of cyclo-
hexene and the selectivities for the various reaction products is
shown in Table 3. In contrast to H2O2 as oxidant, 1a–10a are
distinctly more selective toward 2-cyclohexene-1-ol (46.2–55.6%)
than cyclohexene oxide (35.4–41.8%) and expected low selectiv-
ity of cyclohexane-1,2-diol (1.4–2.3%) and 2-cyclohexene-1-one
(6.6–11.5%) is noticed. The reason for the formation of the allylic
oxidation products 2-cyclohexen-1-ol and 2-cyclohexen-1-one in
For comparison, very similar dioxidovanadium(V) complexes
but with ONN-type tridentate Schiff bases derived from 1S,2S(−)-
1,2-diphenyl-1,2-diaminoethane, 11–18, have been tested as
catalysts (Fig. 9), employing the same oxidants and under the
same optimized reaction conditions. Synthesis, structure, spec-
troscopic characterization and catalytic properties in asymmetric
sulfoxidation of these compounds have been reported earlier
[26]. Using TBHP as oxidant, conversion goes slightly down to
77.4–84.6%, but is still much better than in case of the other
dioxidovanadium(V) complexes, i.e. [VO2(acac-ambmz)], [VO2(sal-
ambmz)] and [VO2(sal-aebmz)] with 20–35% conversion [49] or
vanadium(IV) complexes with symmetrical tetradentate Schiff
bases derived from diamines, i.e. [VIVO(pydx-en)], [VIVO(pydx-
1,2-pn)] and [VIVO(pydx-1,3-pn)] with 63–65% conversion [54].
Moreover, the selectivity toward styrene oxide is much higher
(50–60%) and much lower toward benzaldehyde (35–45%). When
aqueous 30% H2O2 is employed as oxidant, distinctly the lowest
conversion of styrene has been observed (≥22%), but as in the case
of 1a–10a, benzaldehyde is the main product (80.9–88.2%). More-
over, as reported earlier for other dioxidovanadium(V) complexes
[49], styrene oxide is the most expected product but its selectivity
goes down considerably (>7%).
In the presence of H2O2, a strong oxidant, styrene oxide formed
by epoxidation in the first step is very fast converted into benzalde-
hyde via nucleophilic attack of H2O2 to styrene oxide followed by
the cleavage of the intermediate hydroperoxystyrene [55]. Benz-
aldehyde formation may also be facilitated by direct oxidative
cleavage of the styrene side-chain double bond via a radical mech-
anism. Very low conversion of styrene is probably caused by the
presence of significant amount of water in 30% H2O2, which can
be responsible for the decomposition of 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
isomerization of styrene oxide and benzoic acid through oxidation
of benzaldehyde, are distinctly much slower processes.
3.3.3. Oxidation of cyclohexene
The catalytic potential of the 1a–10a complexes has been
found for the oxidation of cyclohexene in the presence of aqueous
30% H2O2 or tert-butyl hydroperoxide (TBHP) as oxidant to give
cyclohexene oxide, cyclohexane-1,2-diol, 2-cyclohexene-1-ol and
2-cyclohexene-1-one (Fig. 10). The formation of all these products
is presented in Table 3.
higher selectivity may be preferential attack of the activated C
bond over the C C bond [56].
H
As in the case of the oxidation of styrene, complex 2a was taken
as a representative catalyst for optimizing reaction conditions. In
this purpose, amount of catalyst (0.5, 1 and 2 mol%) and oxidant
(1:1, 2:1 and 3:1 molar ratios to cyclohexene), different solvents
and temperature of the reaction mixture were also tested.
As in the case of the oxidation of styrene, 11–18 have been tested
as catalysts, employing the same oxidants and under the same
optimized reaction conditions. Using both, TBHP or aqueous 30%
H2O2 as oxidant, conversion goes distinctly down to 68.7–86.2%
and 36.8–48.6%, respectively. The difference in conversion is much

G. Romanowski et al. / Journal of Molecular Catalysis A: Chemical 381 (2014) 148–160
159
Table 3
Catalytic oxidation of cyclohexene by aqueous 30% H2O2 or tert-butyl hydroperoxide (TBHP) in the presence of 1 mol% vanadium(V) Schiff base complexes as catalysts.
Entry
Catalyst
Oxidant
Conv. (%)
Product selectivity (%)
ChOa
ChOLb
ChONc
ChDLd
1
2
3
4
5
6
7
8
1a
2a
3a
4a
5a
6a
7a
8a
9a
10a
1a
2a
3a
4a
5a
6a
7a
8a
9a
10a
11
12
13
14
15
16
17
18
11
12
13
14
15
16
17
18
H2O2
H2O2
H2O2
H2O2
H2O2
H2O2
H2O2
H2O2
H2O2
H2O2
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
H2O2
H2O2
H2O2
H2O2
H2O2
H2O2
H2O2
H2O2
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
52.7
61.4
63.1
58.3
64.8
56.3
49.3
55.2
50.9
46.5
84.2
93.1
92.6
89.4
85.6
86.7
78.3
84.2
82.6
81.3
36.8
48.6
44.2
42.5
41.4
39.3
40.6
45.7
75.6
85.4
86.2
84.8
77.5
76.2
71.4
68.7
30.3
21.3
24.5
23.7
27.2
28.5
27.8
25.3
29.9
23.0
37.0
35.8
35.4
36.1
38.2
36.4
41.8
39.2
40.5
41.5
38.2
42.1
43.5
42.3
37.5
40.0
37.7
36.4
24.6
33.6
31.2
32.7
33.5
30.8
26.3
22.8
33.7
37.8
40.8
37.7
39.4
34.2
31.3
35.8
32.4
31.0
55.0
54.3
55.6
53.0
53.6
54.8
48.8
47.6
46.2
47.5
35.1
29.6
28.3
30.3
28.4
36.7
29.9
33.1
65.1
57.2
60.6
58.0
60.4
64.5
68.7
70.9
13.5
16.2
12.2
15.7
13.7
18.2
18.8
19.3
16.3
20.7
6.6
7.9
6.7
8.8
6.6
6.9
7.4
10.9
11.5
8.9
12.6
16.4
15.2
17.4
18.5
10.0
17.7
13.8
9.2
8.0
7.3
8.3
5.4
22.5
24.7
22.5
22.9
19.7
19.1
22.1
19.6
21.4
25.3
1.4
2.0
2.3
2.1
1.6
1.9
2.0
2.3
1.8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
2.1
14.1
11.9
13.0
10.0
15.6
13.3
14.7
16.7
1.1
1.2
0.9
1.0
0.7
3.7
4.4
5.4
1.0
0.6
0.9
a
Cyclohexene oxide.
2-Cyclohexene-1-ol.
2-Cyclohexene-1-one.
Cyclohexane-1,2-diol.
b
c
d
bigger especially in contrast to vanadium(IV) complexes with
symmetrical tetradentate Schiff bases derived from diamines,
i.e. [VIVO(pydx-en)], [VIVO(pydx-1,2-pn)] and [VIVO(pydx-1,3-pn)],
which gave 23.1–32.2% conversion with 30% H2O2 as oxidant
[54]. Moreover, in case of H2O2, the selectivity toward cyclo-
hexene oxide is much higher (36.4–43.5%), but goes distinctly
down to 28.3–35.1% for 2-cyclohexen-1-ol and 10.0–16.7% for
cyclohexane-1,2-diol. When TBHP is employed as oxidant, dis-
tinctly the highest selectivity toward 2-cyclohexen-1-ol has been
noticed (57.2–70.9%).
In presented sulfoxidation reactions enantioselectivity of vana-
dium(V) complexes derived from chiral ␤-amino alcohols is better
and the reaction times are much shorter than for vanadium(V) com-
plexes with Schiff bases derived from optically active diamines.
The results show that the observed yield and enantiomeric excess
significantly depend on the nature of the catalyst and substrate
used, especially in the aspect of bulky substituent leading to large
steric demand, and finally depend on the temperature in which the
reaction is carried out.
The catalytic potential of the above-mentioned vanadium(V)
complexes and similar compounds derived from chiral diamine in
the oxidation of olefins was also studied, choosing the oxidation
of styrene and cyclohexene as the model reactions. These com-
plexes are able to catalyze the oxidative conversion of styrene
to styrene oxide and its successive products, and cyclohexene to
cyclohexene oxide, cyclohexane-1,2-diol, 2-cyclohexene-1-ol and
2-cyclohexene-1-one by 30% H2O2 or tert-butyl hydroperoxide as
oxidant. It is noticed that catalysts with the electron donating sub-
stituents in salicylaldimine moiety gave slightly better conversion.
The oxidation of styrene after 6 h of reaction time gives at least five
different products. Using 30% H2O2 as oxidant conversion of styrene
was distinctly low, but due to the strong oxidizing nature of H2O2
the formation of benzaldehyde was preferred, whereas expected
product styrene oxide was found only in small yield. On the other
hand, tert-butyl hydroperoxide (TBHP) proved to be an excel-
lent oxidant giving even 87.3% conversion of styrene with styrene
oxide as the main product. On the other hand, in the oxidation of
4. Conclusion
New chiral five-coordinate vanadium(V) complexes derived
from Schiff base ligands, monocondensation products of
o-hydroxycarbonyl compounds with 1S,2R(+)-2-amino-1,2-
diphenylethanol were prepared. Moreover, six-coordinate
complexes were synthesized with the same Schiff base lig-
ands and also with monoanionic bidentate acetohydroxamate
co-ligand. The catalytic properties of five-coordinate compounds
toward the oxidation of organic sulfides (thioanisole and ben-
zyl phenyl sulfide) and olefins (styrene and cyclohexene) have
been studied. For comparison, very similar dioxidovanadium(V)
complexes but with ONN-type tridentate Schiff bases derived
from 1S,2S(−)-1,2-diphenyl-1,2-diaminoethane in the oxidation of
styrene and cyclohexene have been tested.

160
G. Romanowski et al. / Journal of Molecular Catalysis A: Chemical 381 (2014) 148–160
cyclohexene much better conversions were found, especially when
H2O2 was employed as oxidant. But using TBHP as the terminal oxi-
dant the excellent conversion (up to 93%) and cyclohexene oxide
and 2-cyclohexene-1-ol as the main reaction products have been
noted.
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