Five- and six-coordinate vanadium(V) complexes with tridentate Schiff base ligands derived from S(+)-isoleucinol: Synthesis, characterization and catalytic activity in the oxidation of sulfides and olefins

Article abstract of DOI:10.1016/j.poly.2013.10.008

A series of vanadium(V) complexes with chiral tridentate Schiff base ligands, obtained by a single condensation of S(+)-isoleucinol with salicylaldehyde and its derivatives, and also six-coordinate complexes with additional monoanionic bidentate benzohydroxamate co-ligand, were prepared. 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 analysis of the complex, (benzohydroxamato- κ²O,O′){S(+)-2-[(1-oxido-3-methylpentyl)iminomethyl] phenolato-κ³N,O,O′}oxidovanadium(V), 1b, revealed a distorted octahedral VO(ONO)(OO) geometry. The five-coordinate vanadium(V) complexes have ability to catalyze the oxidation of sulfides [PhSR (R = Me, Bz)] using aqueous 30% H₂O₂ in good yields and enantiomeric excesses. Catalytic activity of these complexes were also tested in the oxidation of styrene and cyclohexene, using aqueous 30% H₂O ₂ or tert-butyl hydroperoxide (TBHP) as oxidant.

Full text of DOI:10.1016/j.poly.2013.10.008

Polyhedron 67 (2014) 529–539
Contents lists available at ScienceDirect
Polyhedron
journal homepage: www.elsevier.com/locate/poly
Five- and six-coordinate vanadium(V) complexes with tridentate Schiff
base ligands derived from S(+)-isoleucinol: Synthesis, characterization
and catalytic activity in the oxidation of 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, obtained by a single
condensation of S(+)-isoleucinol with salicylaldehyde and its derivatives, and also six-coordinate
complexes with additional monoanionic bidentate benzohydroxamate co-ligand, were prepared. 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 analysis of the complex, (benzohy-
droxamato-j2O,O0){S(+)-2-[(1-oxido-3-methylpentyl)iminomethyl]phenolato-j3N,O,O0}oxidovanadium(V),
1b, revealed a distorted octahedral VO(ONO)(OO) geometry. The five-coordinate vanadium(V) complexes
have ability to catalyze the oxidation of sulfides [PhSR (R = Me, Bz)] using aqueous 30% H2O2 in good
yields and enantiomeric excesses. Catalytic activity of these complexes were also tested in the oxidation
of styrene and cyclohexene, using aqueous 30% H2O2 or tert-butyl hydroperoxide (TBHP) as oxidant.
Ó 2013 Published by Elsevier Ltd.
Received 2 September 2013
Accepted 2 October 2013
Available online 16 October 2013
Keywords:
Vanadium(V)
Schiff base
Sulfoxidation
Oxidation
Styrene
Cyclohexene
1. Introduction
in organic chemistry. Their importance arises mainly from the ring
opening of epoxides, which allows straightforward elaboration to
The growing interest in recent years in the coordination chem-
istry of vanadium is generated by the wide range of biological and
catalytic properties of particular importance in the biosphere. Five-
and six-coordinated vanadium complexes, also with hydroxamic
acids as ligands, possess potential medicinal applications, such as
the treatment of diabetes type I and II [1], preventive activity
against carcinogenesis [2] or exert anti-parasitic effects against
American trypanosomiasis, human African trypanosomiasis and
leishmaniasis [3]. On the other hand, vanadium nitrogenase in
Azotobacter genus [4], vanadium-dependent haloperoxidases in
marine algae or terrestrial fungi and lichen [5–7], or amavadin in
the mushroom Amanita muscaria [8], are only few examples of
vanadium complexes which can be found in nature.
useful generation of new carbon–carbon bonds [14].
Chiral Schiff base ligands are considered ‘‘privileged ligands’’
[15], which can be readily synthesized from chiral aldehydes or
chiral amines, and are able to transmit chiral information to pro-
duce enantioselectively a number of different products through a
catalytic process. Moreover, chiral N-salicyl-b-amino alcohol Schiff
bases can be readily synthesized from naturally available chiral
amino acids [16] and have been widely employed as catalysts, i.e.
in the stereoselective synthesis of cyclic ethers [17], the asymmet-
ric alkynylation of aldehydes [18], the epoxidation of cyclooctene
[19], the oxidation of bromide [17], oxidative kinetic resolution
of a-hydroxy esters [20] or the enantioselective oxidation of organ-
ic sulfides [18,21] and trimethylsilylcyanations [22].
Optically active sulfoxides possess a wide range of biological
activities, e.g. antimicrobial properties [9], inhibition of biosynthe-
sis of uric acid [10] and gastric acid secretion [11] or regulation of
cholesterol catabolism [12]. As efficient chiral auxiliaries they lead
to many important asymmetric transformations [13] and are valu-
able starting materials in asymmetric synthesis as well as impor-
tant 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 continuation of our studies on synthesis, structure, spectro-
scopic and catalytic properties of vanadium(V) complexes incorpo-
rating chiral tridentate Schiff base ligands [23–26], we prepared a
series of new five-coordinate oxidovanadium(V) complexes with
ONO donor Schiff base ligands, products of monocondensation of
S(+)-isoleucinol with aromatic o-hydroxyaldehydes, and also six-
coordinate with additional monoanionic bidentate benzohydroxa-
mate co-ligand, presented in Fig. 1. Their spectroscopic properties
by 1D and 2D NMR, UV–Vis, CD and IR have been examined. The
catalytic potential of these complexes in the asymmetric oxidation,
i.e. enantioselective sulfoxidation of methyl phenyl sulfide (PhSMe)
and benzyl phenyl sulfide (PhSBz) by aqueous 30% H2O2 has been
⇑
Corresponding author. Tel.: +48 585235102.
E-mail address: greg@chem.univ.gda.pl (G. Romanowski).
0277-5387/$ - see front matter Ó 2013 Published by Elsevier Ltd.
http://dx.doi.org/10.1016/j.poly.2013.10.008

530
G. Romanowski et al. / Polyhedron 67 (2014) 529–539
Table 1
H
1
1
2
2
4
R
R
R
R
Crystal data and structure refinement.
N
O
Compound
1b
3
3
O
V
R
O
N
O-V=O(L)
O
R
O
N
O
O
Empirical formula
Formula weight
T (K)
C
20H23N2O5V
V
422.34
295(2)
4
R
R
O
k (Å)
Crystal system
Space group
0.71073
orthorhombic
P212121
H3C
H3C
H3C
H3C
Unit cell dimensions
a (Å)
b (Å)
c (Å)
V (Å3)
Z
9.0724(4)
10.2624(7)
21.3738(11)
1990.00(19)
4
1.410
0.532
880
0.3 ꢀ 0.2 ꢀ 0.05
2.95 to 25.05
ꢁ10 6 h 6 10,
ꢁ12 6 k 6 11,
ꢁ25 6 l 6 23
16574/3520 [Rint = 0.0808]
99.8
3520/0/269
0.744
R1 = 0.0345, wR2 = 0.0405
R1 = 0.1039, wR2 = 0.0470
0.01(2)
1a - 8a
1b - 8b
1
1
2
3
4
2
3
4
R
H
R
R
R
R
R
R
R
Dcalc (g cmꢁ3
)
H
H
H
H
H
H
H
H
H
H
H
CH3
Br
H
H
H
H
1
5
6
7
8
Absorption coefficient (mmꢁ1
F(000)
Crystal size (mm)
h Range for data collection (°)
Limiting indices
)
OCH3
H
H
OCH3
H
2
3
4
H
H
H
NO2
H
H
OCH3
OCH3
OH
Fig. 1. Structural formulae of vanadium(V) complexes.
Reflections collected/unique
Completeness to 2h (%)
studied. Moreover, they were also used as catalysts in the oxida-
tion of olefins, i.e. styrene and cyclohexene, using aqueous 30%
H2O2 or tert-butyl hydroperoxide (TBHP) as oxidant. Crystal and
molecular structure of the complex, (benzohydroxamato-j2
O,O0){S(+)-2-[(1-oxido-3-methylpentyl)iminomethyl]phenolato-j3
N,O,O0}oxidovanadium(V), 1b, is also reported.
Data/restraints/parameters
Goodness-of-fit (GOF) on F2
Final R indices [I > 2
R indices (all data)
r
(I)]
Absolute structure parameter
Largest difference in peak and hole (e Åꢁ3
)
0.178 and ꢁ0.181
2. Experimental
added at room temperature or ꢁ20 °C, together with 1,3,5-trime-
thoxybenzene as internal standard. Aqueous 30% H2O2 was added
(1.10 mmol) by 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 chiral
shift reagent Eu(hfc)3 (where Hhfc is 3-(heptafluoropropylhydr-
oxymethylene)-(+)-camphoric acid) [28].
2.1. Measurements
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 the Perkin-Elmer LAMBDA 18 spectrophotometer. Circular
dichroism spectra were measured with a Jasco J-815 spectropolar-
imeter. NMR spectra were obtained in CD3OD solutions with a
Varian Mercury-400BB (400 MHz) spectrometer using TMS (1H)
2.3.2. Oxidation of olefins
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) in decane, and catalyst (0.010 mmol) were taken in
CH3CN (10 ml) 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 iden-
tity of oxidation products were confirmed by GC–MS. The influence
of amounts of catalyst and oxidant were also studied to check their
effect on the conversion and selectivity of the reaction products.
and VOCl3
500 gas chromatograph with
(
51V) as reference compounds. A Perkin-Elmer Clarus
DB-5 capillary column
a
(30 m ꢀ 0.25 mm ꢀ 0.25 mm) and FID detector was used to ana-
lyze the reaction products. The identity of the products was con-
firmed using a GC–MS model Shimadzu GCMS-QP2010 SE.
2.2. X-ray investigations
Diffraction measurements were made on a Oxford Diffraction
Gemini R Ultra Ruby CCD, using graphite-monochromatized Mo
2.4. Complexes
Ka radiation (k = 0.71073 Å) at 295(2) K and determination of
the crystal class, orientation matrix, and accurate unit cell param-
eters was performed according to established procedures. Data col-
lection and processing parameters are summarized in Table 1. The
structure was solved by direct methods and refined with all data
on F2 with SHELXTL [27]. All non-hydrogen atoms were refined aniso-
tropically. The positions of hydrogen atoms were calculated and
treated as riding atoms with fixed thermal parameters.
The complexes were obtained in a following example proce-
dure. A solution of 5 mmol of S(+)-isoleucinol in absolute ethanol
(10 ml) was added with stirring to 5 mmol of an aromatic
o-hydroxyaldehyde (salicylaldehyde, 3-methoxysalicylaldehyde,
5-methoxysalicylaldehyde,
4,6-dimethoxysalicylaldehyde,
5-
methylsalicylaldehyde, 5-bromosalicylaldehyde, 5-nitrosalicylal-
dehyde, 2,4-dihydroxybenzaldehyde) in absolute EtOH (20 ml)
and heated under reflux for 1 h. In case of 1b–8b, benzohydroxa-
mic acid (5 mmol) in absolute EtOH (10 ml) was also added with
an aldehyde. Then a vanadium(V) oxytriethoxide (5 mmol) in abso-
lute EtOH (10 ml) 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.
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

G. Romanowski et al. / Polyhedron 67 (2014) 529–539
531
2.4.1.
l
-Oxido-bis({S(+)-2-[(1-oxido-3-methylpentyl)iminomethyl]
3J = 8 Hz), 0.95 (3H, t, 3J = 8 Hz) (methyl); 3.89 (3H, s), 3.85 (3H,
s) (methoxy); minor (40%): 8.85 (1H, s) (azomethine); 6.31 (1H,
ov), 6.24 (1H, ov) (aromatic); 4.97 (1H, dd, 3J = 8 Hz, 4J = 6 Hz),
4.66 (1H, br s), 1.40 (1H, m), 1.16 (1H, ov) (methylene); 4.25 (1H,
dd, 3J = 8 Hz, 4J = 6 Hz), 1.74 (1H, m) (methine); 1.09 (3H, ov),
0.90 (3H, t, 3J = 8 Hz) (methyl); 3.90 (3H, ov), 3.86 (3H, ov) (meth-
oxy). 51V NMR (CD3OD, ppm) major (60%): ꢁ526.5, minor (40%):
ꢁ530.2.
phenolato-j3N,O,O0}oxidovanadium(V)) (1a)
Yield 73%. Anal. Calc. for C26H34N2O7V2: C, 53.1; H, 5.8; N, 4.8.
Found: C, 53.0; H, 5.7; N, 4.7%. IR (KBr, cmꢁ1): 1623 (
m
C@N); 974
(Mꢁ1 cmꢁ1)]:
277 (10320), 328 (4370). CD spectrum in MeOH [kmax (nm),
(mV@O). UV–Vis spectrum in MeOH [kmax (nm), e
D
e
(Mꢁ1 cmꢁ1)]: 251 (ꢁ3.18), 280 (2.37), 347 (ꢁ6.15). 1H NMR (CD3-
OD, ppm) major (60%): 8.64 (1H, s) (azomethine); 7.62–7.52 (2H,
m), 7.01–6.92 (2H, m) (aromatic); 4.80 (2H, br s), 1.49 (1H, m),
1.13 (1H, m) (methylene); 3.85 (1H, dd, 3J = 8 Hz, 4J = 6 Hz), 2.51
(1H, m) (methine); 1.11 (3H, d, 3J = 8 Hz), 0.95 (3H, t, 3J = 8 Hz)
(methyl); minor (40%): 8.76 (1H, s) (azomethine); 7.62–7.52 (2H,
ov), 7.01–6.92 (2H, ov) (aromatic); 5.02 (1H, dd, 3J = 8 Hz,
4J = 6 Hz), 4.68 (1H, br s), 1.41 (1H, m), 1.18 (1H, ov) (methylene);
4.25 (1H, dd, 3J = 8 Hz, 4J = 6 Hz), 1.76 (1H, m) (methine); 1.10 (3H,
ov), 0.92 (3H, t, 3J = 8 Hz) (methyl). 51V NMR (CD3OD, ppm): major
(60%): ꢁ530.9; minor (40%): ꢁ533.8.
2.4.5. l-Oxido-bis({S(+)-2-[(1-oxido-3-methylpentyl)iminomethyl]-4-
methylphenolato-j3N,O,O0}oxidovanadium(V)) (5a)
Yield 72%. Anal. Calc. for C28H38N2O7V2: C, 54.6; H, 6.2; N, 4.5.
Found: C, 54.5; H, 6.1; N, 4.5%. IR (KBr, cmꢁ1): 1626 (
(mV@O). UV–Vis spectrum in MeOH [kmax (nm), e
m
C@N); 978
(Mꢁ1 cmꢁ1)]:
282 (9910), 349 (4050). CD spectrum in MeOH [kmax (nm),
D
e
(Mꢁ1 cmꢁ1)]: 252 (ꢁ3.89), 293 (3.18), 356 (ꢁ6.47). 1H NMR (CD3-
OD, ppm) major (60%): 8.60 (1H, s) (azomethine); 7.40–7.36 (2H,
m), 6.84 (1H, d, 3J = 8 Hz) (aromatic); 4.79 (2H, br s), 1.48 (1H,
m), 1.14 (1H, m) (methylene); 3.83 (1H, dd, 3J = 8 Hz, 4J = 6 Hz),
2.51 (1H, m) (methine); 2.34 (3H, s), 1.10 (3H, d, 3J = 8 Hz), 0.94
(3H, t, 3J = 8 Hz) (methyl); minor (40%): 8.72 (1H, s) (azomethine);
7.40–7.36 (2H, ov), 6.90 (1H, d, 3J = 8 Hz) (aromatic); 5.01 (1H, dd,
3J = 8 Hz, 4J = 6 Hz), 4.66 (1H, br s), 1.40 (1H, m), 1.16 (1H, ov)
(methylene); 4.23 (1H, dd, 3J = 8 Hz, 4J = 6 Hz), 1.73 (1H, m)
(methine); 2.36 (3H, s), 1.09 (3H, ov), 0.91 (3H, ov) (methyl). 51V
NMR (CD3OD, ppm): major (60%): ꢁ528.0; minor (40%): ꢁ531.8.
2.4.2. l-Oxido-bis({S(+)-2-[(1-oxido-3-methylpentyl)iminomethyl]-6-
methoxyphenolato-j3N,O,O0}oxidovanadium(V)) (2a)
Yield 77%. Anal. Calc. for C28H38N2O9V2: C, 51.9; H, 5.9; N, 4.3.
Found: C, 51.7; H, 5.8; N, 4.4%. IR (KBr, cmꢁ1): 1625 (
m
C@N); 981
(Mꢁ1 cmꢁ1)]:
285 (10270), 351 (4420). CD spectrum in MeOH [kmax (nm),
(mV@O). UV–Vis spectrum in MeOH [kmax (nm), e
D
e
(Mꢁ1 cmꢁ1)]: 252 (ꢁ3.89), 278 (2.04) 358 (ꢁ4.32). 1H NMR (CD3-
OD, ppm) major (60%): 8.65 (1H, s) (azomethine); 7.23–7.16 (2H,
m), 6.96 (1H, d, 3J = 8 Hz) (aromatic); 4.79 (2H, br s), 1.48 (1H,
m), 1.16 (1H, m) (methylene); 3.86 (1H, ov), 2.50 (1H, m)
(methine); 1.10 (3H, d, 3J = 8 Hz), 0.94 (3H, t, 3J = 8 Hz) (methyl);
3.86 (3H, s) (methoxy); minor (40%): 8.77 (1H, s) (azomethine);
7.23–7.16 (2H, ov), 6.95 (1H, ov) (aromatic); 5.00 (1H, dd,
3J = 8 Hz, 4J = 6 Hz), 4.68 (1H, br s), 1.40 (1H, m), 1.17 (1H, ov)
(methylene); 4.25 (1H, dd, 3J = 8 Hz, 4J = 6 Hz), 1.75 (1H, m)
(methine); 1.09 (3H, ov), 0.90 (3H, t, 3J = 8 Hz) (methyl); 3.88
(3H, s) (methoxy). 51V NMR (CD3OD, ppm): major (60%): ꢁ527.3;
minor (40%): ꢁ530.7.
2.4.6. l-Oxido-bis({S(+)-2-[(1-oxido-3-methylpentyl)iminomethyl]-4-
bromophenolato-j3N,O,O0}oxidovanadium(V)) (6a)
Yield 77%. Anal. Calc. for Br2C26H32N2O7V2: C, 41.8; H, 4.3; N,
3.8. Found: C, 41.7; H, 4.2; N, 3.9%. IR (KBr, cmꢁ1): 1627 (
m
C@N);
978
(mV@O). UV–Vis spectrum in MeOH [kmax (nm), e
(Mꢁ1
cmꢁ1)]: 280 (10620), 347 (4520). CD spectrum in MeOH [kmax
(nm),
D
e
(Mꢁ1 cmꢁ1)]: 255 (ꢁ2.76), 283 (1.84), 352 (ꢁ4.23). 1H
NMR (CD3OD, ppm) major (60%): 8.58 (1H, s) (azomethine); 7.71
(1H, d, 3J = 3 Hz), 7.60 (1H, dd, 3J = 8 Hz, 4J = 3 Hz), 6.85 (1H, d,
3J = 8 Hz) (aromatic); 4.75 (2H, br s), 1.49 (1H, m), 1.15 (1H, m)
(methylene); 3.86 (1H, ov), 2.48 (1H, m) (methine); 1.09 (3H, d,
3J = 8 Hz), 0.94 (3H, t, 3J = 8 Hz) (methyl); minor (40%): 8.69 (1H,
s) (azomethine); 7.74 (1H, d, 3J = 3 Hz), 7.61 (1H, ov), 6.90 (1H, d,
3J = 8 Hz) (aromatic); 5.00 (1H, dd, 3J = 8 Hz, 4J = 6 Hz), 4.65 (1H,
br s), 1.38 (1H, m), 1.16 (1H, ov) (methylene); 4.18 (1H, dd,
3J = 8 Hz, 4J = 6 Hz), 1.79 (1H, m) (methine); 1.10 (3H, ov), 0.92
(3H, ov) (methyl). 51V NMR (CD3OD, ppm) major (60%): ꢁ531.1,
minor (40%): ꢁ532.8.
2.4.3. l-Oxido-bis({S(+)-2-[(1-oxido-3-methylpentyl)iminomethyl]-4-
methoxyphenolato-j3N,O,O0}oxidovanadium(V)) (3a)
Yield 71%. Anal. Calc. for C28H38N2O9V2: C, 51.9; H, 5.9; N, 4.3.
Found: C, 51.8; H, 5.8; N, 4.3%. IR (KBr, cmꢁ1): 1631 (
m
C@N); 977
(Mꢁ1 cmꢁ1)]:
284 (9930), 349 (4120). CD spectrum in MeOH [kmax (nm),
(mV@O). UV–Vis spectrum in MeOH [kmax (nm), e
D
e
(Mꢁ1 cmꢁ1)]: 253 (ꢁ3.05), 280 (2.13), 362 (ꢁ4.55). 1H NMR (CD3-
OD, ppm) major (60%): 8.65 (1H, s) (azomethine); 7.22–7.12 (2H,
m), 6.89 (1H, d, 3J = 8 Hz) (aromatic); 4.79 (2H, br s), 1.47 (1H,
m), 1.16 (1H, m) (methylene); 3.84 (1H, ov), 2.52 (1H, m)
(methine); 1.12 (3H, d, 3J = 8 Hz), 0.96 (3H, t, 3J = 8 Hz) (methyl);
3.83 (3H, s) (methoxy); minor (40%): 8.76 (1H, s) (azomethine);
7.22–7.12 (2H, ov), 6.95 (1H, d, 3J = 8 Hz) (aromatic); 4.99 (1H,
dd, 3J = 8 Hz, 4J = 6 Hz), 4.67 (1H, br s), 1.42 (1H, m), 1.18 (1H, ov)
(methylene); 4.27 (1H, dd, 3J = 8 Hz, 4J = 6 Hz), 1.75 (1H, m)
(methine); 1.10 (3H, ov), 0.91 (3H, t, 3J = 8 Hz) (methyl); 3.85
(3H, s) (methoxy). 51V NMR (CD3OD, ppm): major (60%): ꢁ525.9;
minor (40%): ꢁ530.6.
2.4.7. l-Oxido-bis({S(+)-2-[(1-oxido-3-methylpentyl)iminomethyl]-4-
nitrophenolato-j3N,O,O0}oxidovanadium(V)) (7a)
Yield 72%. Anal. Calc. for C26H32N4O11V2: C, 46.0; H, 4.8; N, 8.3.
Found: C, 46.1; H, 4.9; N, 8.3. IR (KBr, cmꢁ1): 1636 (
mC@N); 1557,
1332 (mNO ); 986 (
m
V@O). UV–Vis spectrum in MeOH [kmax (nm),
e
e
2
(Mꢁ1 cmꢁ1)]: 345 (15910). CD spectrum in MeOH [kmax (nm),
D
(Mꢁ1 cmꢁ1)]: 254 (ꢁ3.18), 309 (2.48), 348 (ꢁ4.95). 1H NMR (CD3-
OD, ppm) major (60%): 8.70 (1H, s) (azomethine); 8.56 (1H, d,
4J = 3 Hz), 8.34 (1H, dd, 3J = 8 Hz, 4J = 3 Hz), 6.96 (1H, d, 3J = 8 Hz)
(aromatic); 5.00 (1H, dd, 3J = 8 Hz, 4J = 6 Hz), 4.77 (1H, br s), 1.49
(1H, m), 1.22 (1H, m) (methylene); 3.98 (1H, m), 2.48 (1H, m)
(methine); 1.10 (3H, d, 3J = 8 Hz), 0.99 (3H, t, 3J = 8 Hz) (methyl);
minor (40%): 8.77 (1H, s) (azomethine); 8.71 (1H, ov), 8.57 (1H,
ov), 7.04 (1H, d, 3J = 8 Hz) (aromatic); 5.10 (1H, dd, 3J = 8 Hz,
4J = 6 Hz), 4.67 (1H, ov), 1.36 (1H, m), 1.06 (1H, ov) (methylene);
4.11 (1H, dd, 3J = 8 Hz, 4J = 6 Hz), 1.85 (1H, m) (methine); 1.09
(3H, ov), 0.97 (3H, ov) (methyl). 51V NMR (CD3OD, ppm) major
(60%): ꢁ529.6, minor (40%): ꢁ528.9.
2.4.4. l-Oxido-bis({S(+)-2-[(1-oxido-3-methylpentyl)iminomethyl]-
3,5-dimethoxyphenolato-j3N,O,O0}oxidovanadium(V)) (4a)
Yield 74%. Anal. Calc. for C30H42N2O11V2: C, 50.9; H, 6.0; N, 4.0.
Found: C, 51.0; H, 6.1; N, 4.0%. IR (KBr, cmꢁ1): 1628 (
mC@N); 986
(m
V@O). UV–Vis spectrum in MeOH [kmax (nm),
e
D
(Mꢁ1 cmꢁ1)]:
e
(Mꢁ1 cmꢁ1)]:
317 (20180). CD spectrum in MeOH [kmax (nm),
251 (ꢁ2.91), 283 (2.03), 348 (ꢁ3.62). 1H NMR (CD3OD, ppm) major
(60%): 8.67 (1H, s) (azomethine); 6.33 (1H, d, 4J = 3 Hz), 6.26 (1H, d,
4J = 3 Hz) (aromatic); 4.77 (2H, br s), 1.46 (1H, m), 1.15 (1H, m)
(methylene); 3.82 (1H, ov), 2.50 (1H, m) (methine); 1.11 (3H, d,

532
G. Romanowski et al. / Polyhedron 67 (2014) 529–539
2.4.8.
l
-Oxido-bis({S(+)-2-[(1-oxido-3-methylpentyl)iminomethyl]-5-
2.4.11. (Benzohydroxamato-j2O,O0){S(+)-2-[(1-oxido-3-methylpentyl)
iminomethyl]-4-methoxyphenolato-j3N,O,O0}oxidovanadium(V) (3b)
Yield 78%. Anal. Calc. for C21H25N2O6V: C, 55.8; H, 5.6; N, 6.2.
hydroxyphenolato-j3N,O,O0}oxidovanadium(V)) (8a)
Yield 76%. Anal. Calc. for C26H34N2O9V2: C, 50.3; H, 5.5; N, 4.5.
Found: C, 50.5; H, 5.6; N, 4.4%. IR (KBr, cmꢁ1): 1618 (
m
C@N); 956
(Mꢁ1 cmꢁ1)]:
288 (11170), 355 (4730). CD spectrum in MeOH [kmax (nm),
Found: C, 55.6; H, 5.5; N, 6.2%. IR (KBr, cmꢁ1): 3433 (
m
N–H); 1628,
(mV@O). UV–Vis spectrum in MeOH [kmax (nm),
e
1605 (mC@O,
(nm),
MeOH [kmax (nm),
mC@N); 966 (mV@O). UV–Vis spectrum in MeOH [kmax
D
e
e
(Mꢁ1 cmꢁ1)]: 259 (13040), 484 (4080). CD spectrum in
(Mꢁ1 cmꢁ1)]: 251 (ꢁ3.85), 278 (3.64), 354 (ꢁ6.96). 1H NMR (CD3-
OD, ppm) major (60%): 8.46 (1H, s) (azomethine); 7.38 (1H, d,
3J = 8 Hz), 6.44 (1H, dd, 3J = 8 Hz, 4J = 3 Hz), 6.31 (1H, d, 4J = 3 Hz)
(aromatic); 4.78 (2H, br s), 1.48 (1H, m), 1.15 (1H, m) (methylene);
3.79 (1H, m), 2.47 (1H, m) (methine); 1.09 (3H, d, 3J = 8 Hz), 0.92
(3H, t, 3J = 8 Hz) (methyl); minor (40%): 8.60 (1H, s) (azomethine);
7.43 (1H, d, 3J = 8 Hz), 6.48 (1H, dd, 3J = 8 Hz, 4J = 3 Hz), 6.38 (1H, d,
4J = 3 Hz) (aromatic); 5.00 (1H, dd, 3J = 8 Hz, 4J = 6 Hz), 4.63 (1H, br
s), 1.39 (1H, m), 1.11 (1H, ov) (methylene); 4.22 (1H, dd, 3J = 8 Hz,
4J = 6 Hz), 1.71 (1H, m) (methine); 1.08 (3H, ov), 0.88 (3H, ov)
(methyl). 51V NMR (CD3OD, ppm) major (60%): ꢁ526.3, minor
(40%): ꢁ530.0.
D
e
(Mꢁ1 cmꢁ1)]: 273 (ꢁ4.42), 311 (2.57), 381
(ꢁ4.14), 519 (2.42). 1H NMR (CD3OD, ppm) major (60%): 8.78 (1H,
s) (azomethine); 7.83 (1H, d, 3J = 8 Hz), 7.57–7.37 (3H, m), 7.23–
7.10 (3H, m), 6.70 (1H, d, 3J = 8 Hz) (aromatic); 4.71 (1H, d,
3J = 8 Hz), 4.58 (1H, dd, 3J = 8 Hz, 4J = 3 Hz), 1.51 (1H, m), 1.16 (1H,
m) (methylene); 4.01 (1H, dd, 3J = 8 Hz, 4J = 6 Hz), 2.49 (1H, m)
(methine); 1.12 (3H, d, 3J = 8 Hz), 0.96 (3H, t, 3J = 8 Hz) (methyl);
3.83 (3H, s) (methoxy); minor (40%): 8.81 (1H, s) (azomethine);
7.79 (1H, d, 3J = 8 Hz), 7.57–7.37 (3H, ov), 7.23–7.10 (3H, ov), 6.88
(1H, d, 3J = 8 Hz) (aromatic); 4.98 (1H, dd, 3J = 8 Hz, 4J = 3 Hz), 4.79
(1H, d, 3J = 8 Hz), 1.42 (1H, m), 1.18 (1H, ov) (methylene); 4.18
(1H, dd, 3J = 8 Hz, 4J = 6 Hz), 2.04 (1H, m) (methine); 1.10 (3H, ov),
0.91 (3H, t, 3J = 8 Hz) (methyl); 3.85 (3H, s) (methoxy). 51V NMR
(CD3OD, ppm): major (60%): ꢁ453.8; minor (40%): ꢁ450.0.
2.4.9. (Benzohydroxamato-j2O,O0){S(+)-2-[(1-oxido-3-methylpentyl)
iminomethyl]phenolato-j3N,O,O0}oxidovanadium(V) (1b)
2.4.12. (Benzohydroxamato-j2O,O0){S(+)-2-[(1-oxido-3-methylpentyl)
iminomethyl]-3,5-dimethoxyphenolato-j3N,O,O0}oxidovanadium(V) (4b)
Yield 81%. Anal. Calc. for C22H27N2O7V: C, 54.8; H, 5.6; N, 5.8.
Yield 81%. Anal. Calc. for C20H23N2O5V: C, 56.9; H, 5.5; N, 6.6.
Found: C, 57.0; H, 5.6; N, 6.7%. IR (KBr, cmꢁ1): 3448 (
m
N–H); 1626,
1602 (mC@O,
(nm),
MeOH [kmax (nm),
mC@N); 965 (mV@O). UV–Vis spectrum in MeOH [kmax
Found: C, 54.9; H, 5.7; N, 5.7%. IR (KBr, cmꢁ1): 3435 (
mN–H); 1603,
e
(Mꢁ1 cmꢁ1)]: 258 (12530), 463 (3640). CD spectrum in
1547 (mC@O,
(nm),
MeOH [kmax (nm),
mC@N); 968 (mV@O). UV–Vis spectrum in MeOH [kmax
D
e
(Mꢁ1 cmꢁ1)]: 269 (ꢁ3.99), 296 (ꢁ2.84),
e
(Mꢁ1 cmꢁ1)]: 314 (16350), 487 (4570). CD spectrum in
356 (ꢁ4.81), 510 (2.21). 1H NMR (CD3OD, ppm) major (60%):
8.78 (1H, s) (azomethine); 7.84 (1H, d, 3J = 8 Hz), 7.67–7.32 (5H,
m), 7.01–6.90 (2H, m), 6.77 (1H, d, 3J = 8 Hz) (aromatic); 4.70
(1H, d, 3J = 8 Hz), 4.61 (1H, dd, 3J = 8 Hz, 4J = 4 Hz), 1.49 (1H, m),
1.13 (1H, m) (methylene); 4.02 (1H, dd, 3J = 8 Hz, 4J = 6 Hz), 2.48
(1H, m) (methine); 1.11 (3H, d, 3J = 8 Hz), 0.95 (3H, t, 3J = 8 Hz)
(methyl); minor (40%): 8.82 (1H, s) (azomethine); 7.79 (1H, d,
3J = 8 Hz), 7.67–7.32 (5H, ov), 7.01–6.90 (2H, ov), 6.86 (1H, d,
3J = 8 Hz) (aromatic); 4.92 (1H, ov), 4.79 (1H, d, 3J = 8 Hz), 1.41
(1H, m), 1.18 (1H, ov) (methylene); 4.22 (1H, dd, 3J = 8 Hz,
4J = 6 Hz), 2.05 (1H, m) (methine); 1.10 (3H, ov), 0.92 (3H, t,
3J = 8 Hz) (methyl). 51V NMR (CD3OD, ppm): major (60%):
ꢁ457.7; minor (40%): ꢁ451.4.
D
e
(Mꢁ1 cmꢁ1)]: 284 (ꢁ5.01), 306 (2.78), 348
(ꢁ5.74), 512 (2.31). 1H NMR (CD3OD, ppm) major (60%): 8.92
(1H, s) (azomethine); 7.82 (1H, d, 3J = 8 Hz), 7.60–7.37 (5H, m),
6.04 (1H, d, 4J = 3 Hz), 5.93 (1H, d, 4J = 3 Hz) (aromatic); 4.66 (1H,
d, 3J = 8 Hz), 4.60 (1H, dd, 3J = 8 Hz, 4J = 3 Hz), 1.54 (1H, m), 1.14
(1H, m) (methylene); 4.11 (1H, dd, 3J = 8 Hz, 4J = 6 Hz), 2.44 (1H,
m) (methine); 1.12 (3H, d, 3J = 8 Hz), 0.96 (3H, t, 3J = 8 Hz)
(methyl); 3.92 (3H, s), 3.78 (3H, s) (methoxy); minor (40%): 8.96
(1H, s) (azomethine); 7.78 (1H, d, 3J = 8 Hz), 7.60–7.37 (5H, ov),
6.11 (1H, d, 4J = 3 Hz), 6.06 (1H, d, 4J = 3 Hz) (aromatic); 4.99 (1H,
dd, 3J = 8 Hz, 4J = 3 Hz), 4.78 (1H, d, 3J = 8 Hz), 1.42 (1H, m), 1.19
(1H, ov) (methylene); 4.27 (1H, dd, 3J = 8 Hz, 4J = 6 Hz), 2.03 (1H,
m) (methine); 1.11 (3H, ov), 0.92 (3H, t, 3J = 8 Hz) (methyl); 3.93
(3H, s), 3.79 (3H, s) (methoxy). 51V NMR (CD3OD, ppm): major
(60%): ꢁ456.2; minor (40%): ꢁ450.8.
2.4.10. (Benzohydroxamato-j2O,O0){S(+)-2-[(1-oxido-3-methylpentyl)
iminomethyl]-6-methoxyphenolato-j3N,O,O0}oxidovanadium(V) (2b)
Yield 83%. Anal. Calc. for C21H25N2O6V: C, 55.8; H, 5.6; N, 6.2.
2.4.13. (Benzohydroxamato-j2O,O0){S(+)-2-[(1-oxido-3-methylpentyl)
iminomethyl]-4-methylphenolato-j3N,O,O0}oxidovanadium(V) (5b)
Yield 79%. Anal. Calc. for C21H25N2O5V: C, 57.8; H, 5.8; N, 6.4.
Found: C, 55.7; H, 5.5; N, 6.1%. IR (KBr, cmꢁ1): 3436 (
m
N–H); 1621,
1598 (mC@O,
(nm),
MeOH [kmax (nm),
mC@N); 961 (mV@O). UV–Vis spectrum in MeOH [kmax
e
(Mꢁ1 cmꢁ1)]: 261 (12870), 486 (3920). CD spectrum in
Found: C, 57.9; H, 5.9; N, 6.3%. IR (KBr, cmꢁ1): 3435 (
m
N–H); 1625,
D
e
(Mꢁ1 cmꢁ1)]: 275 (ꢁ4.15), 313 (3.46), 368
1590 (mC@O,
(nm),
MeOH [kmax (nm),
mC@N); 957 (mV@O). UV–Vis spectrum in MeOH [kmax
(ꢁ3.39), 517 (3.18). 1H NMR (CD3OD, ppm) major (60%): 8.78
(1H, s) (azomethine); 7.84 (1H, d, 3J = 8 Hz), 7.57–7.46 (3H, m),
7.37 (1H, d, 3J = 8 Hz), 7.21 (1H, d, 3J = 8 Hz), 7.12 (1H, d,
3J = 8 Hz), 6.76 (1H, d, 3J = 8 Hz) (aromatic); 4.72 (1H, d, 3J = 8 Hz),
4.59 (1H, dd, 3J = 8 Hz, 4J = 4 Hz), 1.51 (1H, m), 1.17 (1H, m) (meth-
ylene); 4.06 (1H, dd, 3J = 8 Hz, 4J = 6 Hz), 2.47 (1H, m) (methine);
1.13 (3H, d, 3J = 8 Hz), 0.95 (3H, t, 3J = 8 Hz) (methyl); 3.86 (3H, s)
(methoxy); minor (40%): 8.82 (1H, s) (azomethine); 7.78 (1H, d,
3J = 8 Hz), 7.59–7.50 (3H, ov), 7.35 (1H, ov), 7.19 (1H, ov), 7.10
(3H, ov), 6.89 (1H, d, 3J = 8 Hz) (aromatic); 4.97 (1H, dd, 3J = 8 Hz,
4J = 3 Hz), 4.81 (1H, d, 3J = 8 Hz), 1.47 (1H, ov), 1.14 (1H, ov) (meth-
ylene); 4.20 (1H, dd, 3J = 8 Hz, 4J = 6 Hz), 2.04 (1H, m) (methine);
1.10 (3H, ov), 0.97 (3H, t, 3J = 8 Hz) (methyl); 3.85 (3H, s) (meth-
oxy). 51V NMR (CD3OD, ppm): major (60%): ꢁ451.7; minor (40%):
ꢁ446.0.
e
(Mꢁ1 cmꢁ1)]: 266 (12430), 479 (3710). CD spectrum in
D
e
(Mꢁ1 cmꢁ1)]: 271 (ꢁ4.92), 299 (2.54), 369
(ꢁ4.66), 516 (2.27). 1H NMR (CD3OD, ppm) major (60%): 8.73 (1H,
s) (azomethine); 7.83 (1H, d, 3J = 8 Hz), 7.60–7.43 (3H, m), 7.40–
7.33 (3H, m), 6.67 (1H, d, 3J = 8 Hz) (aromatic); 4.70 (1H, d,
3J = 8 Hz), 4.58 (1H, dd, 3J = 8 Hz, 4J = 3 Hz), 1.50 (1H, m), 1.14 (1H,
m) (methylene); 4.02 (1H, dd, 3J = 8 Hz, 4J = 3 Hz), 2.49 (1H, m)
(methine); 2.34 (3H, s), 1.10 (3H, d, 3J = 8 Hz), 0.94 (3H, t,
3J = 8 Hz) (methyl); minor (40%): 8.77 (1H, s) (azomethine); 7.79
(1H, d, 3J = 8 Hz), 7.60–7.43 (3H, ov), 7.40–7.33 (2H, ov), 7.30 (1H,
dd, 3J = 8 Hz, 4J = 3 Hz), 6.84 (1H, d, 3J = 8 Hz) (aromatic); 4.90 (1H,
ov), 4.79 (1H, d, 3J = 8 Hz), 1.40 (1H, m), 1.16 (1H, ov) (methylene);
4.18 (1H, dd, 3J = 8 Hz, 4J = 3 Hz), 2.03 (1H, m) (methine); 2.36 (3H,
s), 1.09 (3H, ov), 0.91 (3H, t, 3J = 8 Hz) (methyl). 51V NMR (CD3OD,
ppm): major (60%): ꢁ455.0; minor (40%): ꢁ447.8.

G. Romanowski et al. / Polyhedron 67 (2014) 529–539
533
2.4.14. (Benzohydroxamato-j2O,O0){S(+)-2-[(1-oxido-3-methylpentyl)
3. Results and discussion
iminomethyl]-4-bromophenolato-j3N,O,O0}oxidovanadium(V) (6b)
Yield 82%. Anal. Calc. for BrC20H22N2O5V: C, 47.9; H, 4.4; N, 5.6.
3.1. Crystal and molecular structure of 1b
Found: C, 47.8; H, 4.5; N, 5.7%. IR (KBr, cmꢁ1): 3443 (
m
N–H); 1623,
The crystallographic data of the complex 1b are summarized in
Table 1, and the selected bond distances and angles are given in
Table 2.
1598 (mC@O,
(nm),
MeOH [kmax (nm),
mC@N); 959 (mV@O). UV–Vis spectrum in MeOH [kmax
e
(Mꢁ1 cmꢁ1)]: 264 (12180), 488 (3620). CD spectrum in
D
e
(Mꢁ1 cmꢁ1)]: 271 (ꢁ3.58), 309 (2.10), 367
(ꢁ4.12), 514 (1.84). 1H NMR (CD3OD, ppm) major (60%): 8.76
(1H, s) (azomethine); 7.76 (1H, d, 3J = 3 Hz), 7.57–7.48 (5H, m),
7.41 (1H, dd, 3J = 8 Hz, 4J = 3 Hz), 6.69 (1H, d, 3J = 8 Hz) (aromatic);
4.75 (1H, d, 3J = 8 Hz), 4.63 (1H, dd, 3J = 8 Hz, 4J = 3 Hz), 1.52 (1H,
m), 1.15 (1H, m) (methylene); 4.07 (1H, dd, 3J = 8 Hz, 4J = 3 Hz),
2.46 (1H, m) (methine); 1.14 (3H, d, 3J = 8 Hz), 0.97 (3H, t,
3J = 8 Hz) (methyl); minor (40%): 8.79 (1H, s) (azomethine); 7.81
(1H, d, 3J = 8 Hz), 7.56–7.47 (5H, ov), 7.62 (1H, dd, 3J = 8 Hz,
4J = 3 Hz), 6.83 (1H, d, 3J = 8 Hz) (aromatic); 4.94 (1H, dd,
3J = 8 Hz, 4J = 3 Hz), 4.85 (1H, ov), 1.45 (1H, m), 1.17 (1H, ov) (meth-
ylene); 4.22 (1H, dd, 3J = 8 Hz, 4J = 3 Hz), 2.04 (1H, m) (methine);
1.11 (3H, ov), 0.95 (3H, t, 3J = 8 Hz) (methyl). 51V NMR (CD3OD,
ppm): major (60%): ꢁ452.5; minor (40%): ꢁ443.7.
The complex 1b crystallizes in the orthorhombic system, space
group P212121 with four 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,O0-bidentate hydroxamate co-li-
gand, defining a distorted octahedral VO(ONO)(OO) geometry
(Fig. 2). The equatorial plane is defined by one nitrogen atom
(N8) and two oxygen atoms (O15 and O17) of the tridentate Schiff
base ligand and the oxime oxygen atom (O19) of the acetohydrox-
amate co-ligand, while the oxido oxygen atom (O18) and oxygen
carbonyl atom (O22) of the hydroxamate group occupy axial posi-
tions. The vanadium atom is displaced from the least squares equa-
torial plane toward the terminal oxido group by 0.265 Å. In trans
position to the terminal oxido group is the O22 atom of the acet-
ohydroxamate co-ligand with the bond distance from vanadium
2.4.15. (Benzohydroxamato-j2O,O0){S(+)-2-[(1-oxido-3-methylpentyl)
iminomethyl]-4-nitrophenolato-j3N,O,O0}oxidovanadium(V) (7b)
Yield 76%. Anal. Calc. for C20H22N3O7V: C, 51.4; H, 4.7; N, 9.0.
atom of 2.197(2) Å that is significantly longer than V16–
O19(oxime) bond distance (1.894(2) Å), due to this strong trans
influence [29]. Also, the trans angle is 169.66(11)° and these values
correspond well with 2.164(2), 1.856(2) Å distances and 169.3(1)°
angle for [VVO(sal-gly)(bz)] reported earlier [30]. Moreover, the
V16 = O18 distance of 1.5977(18) Å and also, the values for V16–
O17(phenolate) and V16–N8(imine) bonds being 1.874(2) and
2.104(3) Å, respectively, are in the range found in other similar
vanadium(V) complexes with hydroxamate co-ligands [30–34].
The tridentate Schiff base ligand and the benzohydroxamate
co-ligand combine with the vanadium atom to give one six-mem-
bered and two five-membered chelate rings. The Schiff base
five-membered chelate ring containing V16, O15, C14, C9 and N8
atoms adopts an envelope conformation on C14 atom with
P = 240.6(2)° and Tau(M) = 51.6(2)° for reference bond V16–O15
[35], whereas the five-membered chelate ring, involving V16,
Found: C, 51.3; H, 4.6; N, 9.1%. IR (KBr, cmꢁ1): 3440 (
m
N–H); 1636,
1606 (mC@O,
in MeOH [kmax (nm),
spectrum in MeOH [kmax (nm),
mC@N); 1554, 1325 (mNO ); 953 (mV@O). UV–Vis spectrum
2
e
(Mꢁ1 cmꢁ1)]: 343 (15270), 480 (4390). CD
D
e
(Mꢁ1 cmꢁ1)]: 274 (ꢁ4.03), 310
(2.34), 361 (ꢁ4.74), 513 (2.06). 1H NMR (CD3OD, ppm) major
(60%): 8.93 (1H, s) (azomethine); 8.63 (1H, d, 4J = 3 Hz), 8.28 (1H,
dd, 3J = 8 Hz, 4J = 3 Hz), 7.62–7.50 (5H, m), 6.88 (1H, d, 3J = 8 Hz)
(aromatic); 4.81 (1H, d, 3J = 8 Hz), 4.74 (1H, dd, 3J = 8 Hz,
4J = 3 Hz), 1.54 (1H, m), 1.23 (1H, m) (methylene); 4.14 (1H, dd,
3J = 8 Hz, 4J = 3 Hz), 2.52 (1H, m) (methine); 1.17 (3H, d,
3J = 8 Hz), 0.99 (3H, t, 3J = 8 Hz) (methyl); minor (40%): 8.95 (1H,
s) (azomethine); 8.65 (1H, d, 3J = 8 Hz), 8.33 (1H, dd, 3J = 8 Hz,
4J = 3 Hz), 7.38–7.47 (5H, m), 7.01 (1H, d, 3J = 8 Hz) (aromatic);
5.13 (1H, dd, 3J = 8 Hz, 4J = 3 Hz), 4.97 (1H, d, 3J = 8 Hz), 1.46 (1H,
m), 1.29 (1H, ov) (methylene); 4.31 (1H, dd, 3J = 8 Hz, 4J = 3 Hz),
2.08 (1H, m) (methine); 1.15 (3H, ov), 0.97 (3H, t, 3J = 8 Hz)
(methyl). 51V NMR (CD3OD, ppm): major (60%): ꢁ444.7; minor
(40%): ꢁ431.5.
Table 2
Selected bond lengths (Å), angles (°) and hydrogen bonding parameters (Å, °) for 1b.
V16–O18
V16–O17
V16–N8
V16–O15
V16–O19
V16–O22
O19–N20
O22–C21
N20–C21
C1–O17
C1–C2
1.5977(18)
1.874(2)
2.104(3)
1.873(2)
1.894(2)
2.197(2)
1.353(3)
1.245(4)
1.319(4)
1.314(4)
1.403(5)
1.433(5)
1.279(3)
O18–V16–O17
O18–V16–O19
O18–V16–O22
O18–V16–N8
O18–V16–O15
O17–V16–O19
O17–V16–O22
O17–V16–N8
O17–V16–O15
N8–V16–O19
N8–V16–O22
N8–V16–O15
O15–V16–O19
O15–V16–O22
O19–V16–O22
95.68(12)
93.71(10)
169.66(11)
106.15(10)
97.51(11)
96.32(10)
84.46(11)
83.86(10)
160.16(10)
160.04(10)
84.16(10)
78.28(10)
97.54(9)
2.4.16. (Benzohydroxamato-j2O,O0){S(+)-2-[(1-oxido-3-methylpentyl)
iminomethyl]-5-hydroxyphenolato-j3N,O,O0}oxidovanadium(V) (8b)
Yield 81%. Anal. Calc. for C20H23N2O6V: C, 54.8; H, 5.3; N, 6.4.
Found: C, 54.9; H, 5.4; N, 6.3%. IR (KBr, cmꢁ1): 3438 (
m
N–H); 1617,
1587 (mC@O,
(nm),
MeOH [kmax (nm),
mC@N); 956 (mV@O). UV–Vis spectrum in MeOH [kmax
e
(Mꢁ1 cmꢁ1)]: 265 (12240), 483 (3920). CD spectrum in
C2–C7
C7–N8
D
e
(Mꢁ1 cmꢁ1)]: 270 (ꢁ3.76), 306 (2.19), 357
(ꢁ4.14), 511 (1.97). 1H NMR (CD3OD, ppm) major (60%): 8.68
(1H, s) (azomethine); 7.60 (1H, d, 4J = 3 Hz), 7.56–7.45 (5H, m),
6.65 (1H, dd, 3J = 8 Hz, 4J = 3 Hz), 6.53 (1H, d, 3J = 8 Hz) (aromatic);
4.73 (1H, d, 3J = 8 Hz), 4.61 (1H, dd, 3J = 8 Hz, 4J = 3 Hz), 1.53 (1H,
m), 1.19 (1H, m) (methylene); 4.10 (1H, dd, 3J = 8 Hz, 4J = 3 Hz),
2.49 (1H, m) (methine); 1.15 (3H, d, 3J = 8 Hz), 0.98 (3H, t,
3J = 8 Hz) (methyl); minor (40%): 8.70 (1H, s) (azomethine); 7.63
(1H, d, 3J = 8 Hz), 7.50–7.39 (5H, ov), 6.69 (1H, dd, 3J = 8 Hz,
4J = 3 Hz), 6.57 (1H, d, 3J = 8 Hz) (aromatic); 4.98 (1H, dd,
3J = 8 Hz, 4J = 3 Hz), 4.87 (1H, d, 3J = 8 Hz), 1.46 (1H, m), 1.17 (1H,
ov) (methylene); 4.18 (1H, dd, 3J = 8 Hz, 4J = 3 Hz), 2.06 (1H, m)
(methine); 1.12 (3H, ov), 0.96 (3H, t, 3J = 8 Hz) (methyl). 51V NMR
(CD3OD, ppm): major (60%): ꢁ449.3; minor (40%): ꢁ440.6.
85.20(9)
76.02(10)
DHꢂ ꢂ ꢂA
D–H
Hꢂ ꢂ ꢂA
Dꢂ ꢂ ꢂA
DHꢂ ꢂ ꢂA
N20–H20ꢂ ꢂ ꢂO15i
1.07(3)
0.93
0.97
0.93
0.93
0.93
0.93
0.98
0.96
1.82(3)
2.42
2.53
2.59
2.39
2.49
2.71
2.83
2.34
2.845(3)
3.293(4)
3.022(5)
2.891(5)
3.110(4)
2.800(5)
3.552(5)
3.728(4)
3.238(7)
161
156
111
100
135
100
152
152
156
C3–H3ꢂ ꢂ ꢂO18ii
C14–H14Aꢂ ꢂ ꢂO22
C24–H24Aꢂ ꢂ ꢂN20
C24–H24Aꢂ ꢂ ꢂO18i
C28–H28Aꢂ ꢂ ꢂO22
C5–H5ꢂ ꢂ ꢂCg2iii
C9–H9ꢂ ꢂ ꢂCg1ii
C13–H13ꢂ ꢂ ꢂCg1iv
Symmetry code: (i) ꢁx + 1, y ꢁ ½, ꢁz + 3/2; (ii) x ꢁ ½, ꢁy + 3/2, ꢁz + 2; (iii) x + ½,
ꢁy + ½, ꢁz + 2; (i ) x + ½, ꢁy + 3/2, ꢁz + 2.
Notes: Cg1 and Cg2 denotes the centroids of the C1–C6 and C23–C28, respectively.
v

534
G. Romanowski et al. / Polyhedron 67 (2014) 529–539
Fig. 2. The molecular structure of 1b, with displacement ellipsoids drawn at the 25% probability level and H atoms are shown as small spheres of arbitrary radius.
O19, N20, C21 and O22 atoms from benzohydroxamate co-ligand,
is basically planar. Suprisingly, the six-membered chelate ring,
involving V16, O17, C1, C2, C7 and N8 atoms, is also planar.
The loss of only one proton, from acetohydroxamic acid, yield-
ing the hydroxamate anion, not the hydroximate dianion, is re-
vealed by comparison of bond distances and angles between the
hydroxamate ring of 1b, and the hydroximate ring in the vana-
dium(V) complex, [VO(HSHED)SHI], with hydroximate co-ligand
reported earlier [31]. The distance of the N20–C21 bond is compa-
rable, showing its partial double-bond character, but there are sig-
nificant differences in some of the bond distances in this five-
membered chelate ring. The obvious decrease in C21–O22 bond
length by 0.029 Å, owing to the stronger partial double-bond char-
acter, and increase in V16–O22 bond length by 0.075 Å, owing to
the significantly much weaker bond, where oxygen atom has neu-
tral (not negative) charge, are observed. It is also noteworthy to
mention that bond between vanadium atom and deprotonated
alcoholate oxygen (O15) atom is by 0.292 Å shorter than in
[VO(HSHED)SHI] with coordination bond between vanadium and
secondary amine nitrogen atoms. In the final stage of anisotropic
refinement conformation disorder of sec-butyl side chain of b-ami-
noalcohol moiety was recognized. The disorder of C13 atom with
its three hydrogens and also two hydrogens of C11 atom is dis-
played in Fig. 2. The refined site occupancy values are 0.483(6)
for C13 and 0.517(6) for C13A.
The crystal structure of the 1b complex is stabilized by the
intermolecular hydrogen bonds and C–Hꢂ ꢂ ꢂO contacts linking the
neighboring complex molecules into two-dimensional infinite net-
work, i.e. hydrogen bonds between nitrogen oxime atom (N20) of a
one molecule and alcoholate oxygen atom (O15) of neighboring
molecule and other close contact between C24 carbon atom and
oxide group (O18) result in formation of chains extending in the
b axis direction (Fig. 3). Apart from the intermolecular hydrogen
bonds and close contacts these chains are additionally stabilized
by C–Hꢂ ꢂ ꢂp interactions (Fig. 4).
3.2. Spectroscopic properties
The electronic, circular dichroism, NMR and selected solid-state
IR spectral data are listed in Section 2.
Fig. 3. The arrangement of the molecules of 1b in the crystal structure. The N–Hꢂ ꢂ ꢂO and C–Hꢂ ꢂ ꢂO contacts are represented by dashed lines. H atoms not involved in
interactions were omitted.

G. Romanowski et al. / Polyhedron 67 (2014) 529–539
535
Fig. 4. View of the crystal structure of 1b approximately along the a axis. The C–Hꢂ ꢂ ꢂ
p interactions are represented by dotted lines.
The IR spectra of solid complexes display strong C@N stretch
(around 1603–1636 cmꢁ1), additionally associated with the aro-
matic C@C stretching band, which indicates the C@N group of
the coordinated Schiff base ligands [36,37]. A strong band appear-
ing in the 944–986 cmꢁ1 region in all the complexes may be attrib-
uted to single V@O stretching vibrations, which are close to values
reported for related oxidovanadium(V) Schiff base complexes
[17,19,25,26]. Additionally, the spectra of 1b–8b complexes exhibit
for 1a–3a, 5a, 6a, 8a and in the 258–266 nm range (
e
max = 12180–
13040 Mꢁ1 cmꢁ1) for 1b–3b, 5b, 6b, 8b are considered to arise
from intraligand
for 1a–3a, 5a, 6a, 8a and 1b–8b complexes, between 328–355 nm
max = 4050–4730 Mꢁ1 cmꢁ1
and 463–488 nm max = 3620–
4570 Mꢁ1 cmꢁ1), respectively, may be assigned as a ligand-to-me-
p–
p⁄ transitions. The low energy bands, recorded
(e
)
(e
tal charge transfer (LMCT) transition originating from the p orbital
p
on the phenolate oxygen to the empty d orbital at the vanadium(V)
center [39]. The spectra of 4a, 4b, 7a and 7b complexes examined
in the same spectralꢁ1region display very strong bands at 317
broad strong band in 3433–3448 cmꢁ1 ranges, assigned to
m(N–H)
stretch of benzohydroxamate co-ligand. The appearance of weak/
medium band near 2800 cmꢁ1 separate from other C–H stretching
vibrations in 2a–4a and 2b–4b complexes indicated the presence
of the methoxy substituents [38]. Also, only in the cases of 7a
and 7b the strong bands appear at 1332, 1557 and 1325,
1554 cmꢁ1, respectively, which are typical for nitro group.
(
(
e
e
max = 20180 Mꢁ1 cm ), 314
(e
max = 16350 Mꢁ1 cmꢁ1), 345
max = 15910 Mꢁ1 cmꢁ1
) and 343 nm (e
max = 15270 Mꢁ1 cmꢁ1),
respectively, similar to the same strong bands at around 320 and
350 nm for the other vanadium(V) complexes derived from 4,6-
dimethoxysalicylaldehyde and 5-nitrosalicylaldehyde [24–26].
The circular dichroism spectra of the complexes 1a–8a revealed
The electronic and circular dichroism spectra of all the
complexes were recorded in methanol. Strong intense bands,
an additional band in the 251–255 nm range of
origin. On the other hand, 1b–8b complexes possessing
p–
p⁄ transitions
e
max = 9910–11170 Mꢁ1 cmꢁ1, with kmax in region 277–288 nm

536
G. Romanowski et al. / Polyhedron 67 (2014) 529–539
.
.
O
benzohydroxamate co-ligand, in comparison to their electronic
spectra, revealed two additional bands in the 296–313 nm and
the 348–381 nm range with the positive and negative Cotton ef-
fect, respectively.
S
S
1 mol% catalyst
30% H O
R
R
2
2
Fig. 5. Sulfoxidation of thioethers catalyzed by vanadium(V) complexes.
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 separately
and designated as major and minor. In all the complexes, the ma-
jor:minor ratios are about 60:40, in contrast to similar vana-
dium(V) Schiff base complexes derived from different amino
alcohols, where contribution of major species varies from 53% to
95% [18]. 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. [40], possible reasons for 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 signals with an in-
crease of the concentration of complexes should be observed in
51V NMR spectra. These rather high-field resonances are present
in chloroform solution [41] and disappear when methanol is used
as solvent, due to an additional coordination of methanol leading
to six-coordinated vanadium species.
Table 3
Catalytic oxidation of PhSMe and PhSBz by aqueous 30% H2O2 in 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
9
10
11
12
13
14
15
16
17
18
1a
2a
3a
4a
5a
6a
7a
8a
2a
3a
5a
6a
8a
2a
3a
5a
6a
8a
PhSMe
PhSMe
PhSMe
PhSMe
PhSMe
PhSMe
PhSMe
PhSMe
PhSBz
PhSBz
PhSBz
PhSBz
PhSBz
PhSMe
PhSMe
PhSMe
PhSMe
PhSMe
85
82
81
83
84
87
84
89
76
74
72
77
73
89
90
85
84
87
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
180
180
180
180
180
44
54
56
48
56
52
42
58
29
28
24
26
24
64
68
62
65
67
rt
ꢁ20
ꢁ20
ꢁ20
ꢁ20
ꢁ20
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-3-methyl-1-pentanol is confirmed by the presence
of azomethine proton signals in the spectra of all the complexes.
Two-dimensional (COSY, gHSQC and NOESY) NMR experiments
were performed for all the compounds which allow to unambigu-
ously establish the attachment of all carbon atoms. For example,
the COSY spectrum of the complex 1a exhibits cross-peaks be-
tween the methine proton (major species signal at 3.85 ppm) and
one of methylene proton (not separated signal for two protons at
4.80 ppm) and also another methine proton signal (sec-butyl sub-
stituent) at 2.51 ppm. The bonds between corresponding carbon
atoms in the sec-butyl side chain are also visible as cross-peaks be-
tween the signals of the methyl group protons (1.11 ppm) and the
methine proton (2.51 ppm), the methine proton and and both
methylene protons (1.13 and 1.49 ppm) and finally cross-peak
between methyl group signal at 0.95 ppm and both methylene pro-
tons. In case of 1b, the complex with additional benzohydroxamate
co-ligand, the COSY spectrum exhibits separated signals of methy-
lene protons in b-amino alcohol moiety, doublet at 4.70 ppm and
doublet of doublets at 4.61 ppm, with cross-peak between latter
signal and methine proton signal (doublet of doublets at
4.02 ppm). Moreover, in the NOESY spectra of 1a and 1b, cross-
peaks between azomethine proton signals and methine proton sig-
nals from the ethylene bridge of b-amino alcohol moiety, all of the
protons signals of sec-butyl moiety and one of the aromatic proton
signals are observed. As expected, there is lack of any cross-peaks
between the signals of azomethine protons and the methylene pro-
tons signals from the above-mentioned ethylene bridge.
a
All sulfoxides are in R configuration.
The best results have been obtained for complexes 2a, 3a, 5a, 6a
and 8a as catalysts in the oxidation of methyl phenyl sulfide (Ta-
ble 3, entries 2, 3, 5, 6 and 8). An overall yields for all catalysts were
in the range of 81–89% within 30 min reaction time and enantio-
meric excesses (ee’s) of 42–58% for the R-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 (72–77%) and enantioselectivity
decreases to value of 24–29% (Table 3, 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 im-
prove significantly to 62–68% ee’s and a conversion of over 84%
of the substrate is observed after 3 h (Table 3, entries 16–20). Mi-
moun et al. [42] pointed out the importance of sufficiently nucleo-
philic centre for the oxidative catalysis of organic substances by
peroxidovanadium(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 centre.
In general, the best results have been obtained in the oxidation
reactions of methyl phenyl sulfide with 30% H2O2 as oxidant.
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.
3.3. Catalytic activity studies
3.3.1. Sulfoxidation
3.3.2. Oxidation of styrene
In this study, the oxidovanadium(V) complexes 1a–8a 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. 5). Aqueous 30% H2O2 was used as
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 3.
The catalytic potential of the 1a–8a complexes have been found
for the oxidation of styrene in presence of aqueous 30% H2O2 or
tert-butyl hydroperoxide (TBHP) as oxidant to give styrene oxide,
benzaldehyde, 1-phenylethane-1,2-diol, benzoic acid and phenyl-
acetaldehyde (Fig. 6). The formation of all these products are pre-
sented in Table 4.
In order to achieve suitable reaction conditions for a maximum
oxidative conversion of styrene, complex 5a was taken as a

G. Romanowski et al. / Polyhedron 67 (2014) 529–539
537
HO
nearly identical results with 28.6 and 91.5% conversion, for H2O2
and TBHP respectively, for 6 h of contact time. Thus, 1 mol% of cat-
alyst may be considered sufficient enough to run the reaction un-
der above conditions. A blank reaction under the above reaction
conditions gave with both oxidants ca. 2–5% conversion.
OH
PhED
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
excellent over 85% 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 [43]. Table 4 summarizes
the percentage conversion of styrene and the selectivities for the
various reaction products. Selectivity, in case of all vanadium(V)
complexes, is rather similar and they are generally more selective
toward benzaldehyde (61.8–67.4%) than styrene oxide (26.7–
31.8%). The conversion of the other oxidation products, i.e. benzoic
acid, phenylacetaldehyde and 1-phenylethane-1,2-diol is very low
(>3%). In case of [VO2(acac-ambmz)], [VO2(sal-ambmz)] and [VO2(-
sal-aebmz)] complexes, styrene oxide and benzaldehyde were also
two main products but with inverted selectivity in comparison to
1a–8a, i.e. 50–53% and 37–47%, 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 [44], but resulting in very low conversion (16%)
and similar selectivity of styrene oxide and benzaldehyde.
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 (24.2–30.3%). Although
the conversion of styrene is distinctly low, the selectivity for benz-
aldehyde is much better (86.4–88.1%) 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)] [43] and [VVO(hap-dahp)]
complexes [45], styrene oxide is the most expected product but
exhibits negligible selectivity (1.2–1.9%). Moreover, the catalytic
activity of vanadium(V) complexes encapsulated in the super cages
of zeolite-Y, i.e. [VVO(hap-dahp)]-Y [45], [VO2(sal-ambmz)]-Y,
[VO2(pydx-ambmz)]-Y and [VO2(pydx-aebmz)]-Y, has recently
been discovered [46,47]. The percentage conversion of styrene
for [VO2(pydx-ambmz)]-Y and [VO2(pydx-aebmz)]-Y is 65–68%
and 78% for [VVO(hap-dahp)]-Y, which is less than that observed
O
O
HO
O
1 mol% catalyst
H O or TBHP
2
2
styrene
StO
BzA
BzAC
O
PhAA
Fig. 6. Various oxidation products of styrene catalyzed by vanadium(V) complexes.
representative catalyst and different parameters, i.e. amount of cat-
alyst (0.5, 1 and 2 mol%) and oxidant (1:1, 2:1 and 3:1 M 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 7.3% conversion was achieved.
Increasing the ratio to 2:1 improved the conversion to 18.5%, while
3:1 ratio has shown a maximum of 28.6% conversion. Further
increment of H2O2 shows no improvement in conversion, therefore
a 3:1 ratio being considered adequate. In case of using TBHP as oxi-
dant, increasing the TBHP:styrene ratio from 1:1 to 2:1 and 3:1 im-
proved the conversion from 46.4 to 70.3 and 91.5%, 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
reaction conditions, 0.5 mol% gave only 8.7% (H2O2) and 45.2%
(TBHP) oxidative conversion, while 1 and 2 mol% have shown
Table 4
Catalytic oxidation of styrene by 30% H2O2 or tert-butyl hydroperoxide (TBHP) as oxidant in 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
9
10
11
12
13
14
15
16
1a
2a
3a
4a
5a
6a
7a
8a
1a
2a
3a
4a
5a
6a
7a
8a
H2O2
H2O2
H2O2
H2O2
H2O2
H2O2
H2O2
H2O2
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
25.6
26.1
27.4
25.6
28.6
28.2
30.3
24.2
88.9
87.2
90.4
89.0
91.5
88.4
86.7
87.2
1.5
1.6
1.2
1.8
1.4
1.7
1.9
1.5
31.7
29.2
31.8
31.3
26.7
28.4
31.2
28.0
87.6
86.8
87.0
86.1
86.7
88.1
86.4
87.0
63.0
65.4
61.8
62.5
67.4
65.8
63.3
66.1
–
–
–
–
–
–
–
–
1.5
1.3
1.6
1.5
1.8
1.7
1.4
1.6
1.2
1.4
1.3
1.2
1.7
1.1
1.1
1.3
2.4
2.6
2.9
3.0
2.5
2.8
2.3
2.8
9.7
10.2
10.5
10.9
10.2
9.1
10.6
10.2
1.4
1.2
1.9
1.7
1.6
1.3
1.8
1.5
a
b
c
Styrene oxide.
Benzaldehyde.
Benzoic acid.
Phenylacetaldehyde.
1-Phenylethane-1,2-diol.
d
e

538
G. Romanowski et al. / Polyhedron 67 (2014) 529–539
Table 5
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%). In the presence of
H2O2, a strong oxidant, styrene oxide formed by epoxidation in
the first step is very fast converted into benzaldehyde via nucleo-
philic attack of H2O2 to styrene oxide followed by the cleavage of
the intermediate hydroperoxystyrene [48]. Benzaldehyde forma-
tion may also be facilitated by direct oxidative cleavage of the sty-
rene side-chain double bond via a radical mechanism. Very low
conversion of styrene is probably caused by 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 isomerisation of styrene
oxide and benzoic acid through oxidation of benzaldehyde, are dis-
tinctly much slower processes.
Catalytic oxidation of cyclohexene by 30% H2O2 or tert-butyl hydroperoxide (TBHP) as
oxidant in 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
9
10
11
12
13
14
15
16
1a
2a
3a
4a
5a
6a
7a
8a
1a
2a
3a
4a
5a
6a
7a
8a
H2O2
H2O2
H2O2
H2O2
H2O2
H2O2
H2O2
H2O2
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
57.7
62.6
59.3
56.4
54.7
63.1
53.8
68.4
75.7
89.6
86.2
81.3
88.0
87.2
80.5
88.6
34.8
40.6
37.4
39.6
40.2
43.4
37.2
43.7
19.0
16.5
18.4
22.4
16.3
17.7
20.6
18.9
48.2
42.3
46.6
42.5
44.8
41.7
44.9
37.1
75.4
79.0
76.6
72.3
78.2
76.4
74.9
76.0
3.8
3.0
3.2
3.4
3.7
3.3
2.7
2.9
4.4
3.5
3.9
4.1
4.4
4.9
3.6
3.9
13.2
14.1
12.8
14.5
11.3
11.6
15.2
16.3
1.2
1.0
1.1
1.2
1.1
1.0
0.9
1.2
3.3.3. Oxidation of cyclohexene
The oxidation of cyclohexene was achieved with the 1a–8a
complexes. In presence of aqueous 30% H2O2 or tert-butyl hydro-
peroxide (TBHP) as oxidant, the oxidation products obtained are
cyclohexene oxide, cyclohexane-1,2-diol, 2-cyclohexene-1-ol and
2-cyclohexene-1-one (Fig. 7). The formation of all these products
is presented in Table 5.
a
b
c
Cyclohexene oxide.
2-Cyclohexene-1-ol.
2-Cyclohexene-1-one.
Cyclohexane-1,2-diol.
d
As in the case of the oxidation of styrene, complex 5a 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 M ratios to cyclohexene), different solvents and
temperature of the reaction mixture were also tested.
shown a maximum conversion with 54.7 for H2O2 and 88.0% for
TBHP (6 h of contact time). Thus, 1 mol% of catalyst may be consid-
ered sufficient enough to run the reaction under above conditions.
A blank reaction under the above reaction conditions gave with
both oxidants ca. 2–3% conversion.
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 hydroper-
oxide (TBHP) to cyclohexene, i.e. 1:1, 2:1 and 3:1. Cyclohexene
(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
14.6% conversion was achieved. Increasing the ratio to 2:1 im-
proved the conversion to 29.3%, while 3:1 ratio has shown a max-
imum of 54.7% 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 41.4 to 67.7 and 88.0%, respectively. As in
the previous case, there was no significant conversion improve-
ment upon further addition of oxidant.
For 1a–8a as catalysts and with 30% H2O2 as oxidant, 53.8–
68.4% conversion in 6 h of contact time was found. On the other
hand, the conversion of styrene in the same reaction conditions
is distinctly lower. The selectivity for cyclohexene oxide (34.8–
43.7%) and 2-cyclohexene-1-ol (37.1–48.2%) is much better than
for cyclohexane-1,2-diol (11.3–16.3%) and 2-cyclohexene-1-one
(2.7–3.8%).
When this catalytic reaction have been performed in the same
reaction conditions, but with tert-butyl hydroperoxide (TBHP) as
oxidant, under the optimized reaction conditions, 1a–8a catalysts
gave over 75% conversion. The percentage conversion of styrene
and the selectivities for the various reaction products is shown in
Table 5. In contrast to H2O2 as oxidant, 1a–8a are distinctly more
selective toward 2-cyclohexene-1-ol (72.3–79.0%) than cyclohex-
ene oxide (16.3–22.4%) and surprisingly low selectivity of cyclo-
hexane-1,2-diol (ca. 1%) and 2-cyclohexene-1-one (3.5–4.9%) is
noticed. The reason for the formation of the allylic oxidation prod-
ucts 2-cyclohexen-1-ol and 2-cyclohexen-1-one in higher selectiv-
ity may be preferential attack of the activated C–H bond over the
C@C bond [49].
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 5.4% (H2O2) and 33.6%
(TBHP) oxidative conversion, while 1 and 2 mol% of catalyst have
4. Conclusion
OH
New chiral five-coordinate vanadium(V) complexes derived
from Schiff base ligands, monocondensation products of
o-hydroxycarbonyl compounds with S(+)-isoleucinol, and also
six-coordinate with the same Schiff bases and bidentate monoan-
ionic benzohydroxamate co-ligand, have been successfully
prepared. Those complexes have been characterized by IR, CD,
UV–Vis, 1D (1H, 51V) and 2D (COSY, NOESY, gHSQC) NMR and also
by X-ray diffraction technique. Moreover, their catalytic properties
towards the oxidation of organic sulfides (thioanisole and benzyl
phenyl sulfide) and olefins (styrene and cyclohexene) have been
studied.
O
OH
ChO
OH
ChDL
O
1 mol% catalyst
H O or TBHP
2
2
cyclohexene
ChOL
ChON
Sulfoxidation reactions have shown that enantioselectivity of
vanadium(V) complexes derived from chiral b-amino alcohols is
Fig. 7. Various oxidation products of cyclohexene catalyzed by vanadium(V)
complexes.

G. Romanowski et al. / Polyhedron 67 (2014) 529–539
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excess significantly depend on the nature of the catalyst and sub-
strate used, especially in the aspect of bulky substituent leading
to large steric demand, and finally depend on the temperature in
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