Oxovanadium(V) complexes with schiff base Ligands: Synthesis, Structure, and catalytic property

Article abstract of DOI:10.1080/15533174.2013.768655

The reaction of [VO(acac)2] with N,N-ethylenebis(3-ethoxysalicylideneimine) (H2L1) and N,N-ethylenebis(3-methylsalicylideneimine) (H2L2), respectively, in methanol, affords two new oxovanadium(V) complexes, [VOL1]H2O (1) and [VOL2] (2). Both complexes hav

Full text of DOI:10.1080/15533174.2013.768655

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Synthesis and Reactivity in Inorganic, Metal-Organic,
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Oxovanadium(V) Complexes with Schiff Base Ligands:
Synthesis, Structure, and Catalytic Property
Ning Wang a , Jing-Yi Guo a & Ji-Yong Hu a
a Department of Chemical Engineering , Henan University of Urban Construction ,
Pingdingshan , 467044 , People's Republic of China
Accepted author version posted online: 24 Aug 2013.
To cite this article: Synthesis and Reactivity in Inorganic, Metal-Organic, and Nano-Metal Chemistry (2013): Oxovanadium(V)
Complexes with Schiff Base Ligands: Synthesis, Structure, and Catalytic Property, Synthesis and Reactivity in Inorganic, Metal-
Organic, and Nano-Metal Chemistry, DOI: 10.1080/15533174.2013.768655
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ACCEPTED MANUSCRIPT
Oxovanadium(V) Complexes with Schiff Base Ligands: Synthesis, Structure, and Catalytic
Property
Ning Wang, Jing-Yi Guo, Ji-Yong Hu
Department of Chemical Engineering, Henan University of Urban Construction, Pingdingshan
467044, People's Republic of China
Abstract
The reaction of [VO(acac)2] with N,N’-ethylenebis(3-ethoxysalicylideneimine) (H2L1) and N,N’-
ethylenebis(3-methylsalicylideneimine) (H2L2), respectively, in methanol, affords two new
oxovanadium(V) complexes [VOL1]·H2O (1) and [VOL2] (2). Both complexes have been
characterized by elemental analysis, i.r., and single crystal X-ray diffraction methods. The V
atoms in the complexes are in distorted square pyramidal coordination. The dinegative Schiff
base ligands coordinate to the metal atoms through phenolate and imine donor atoms. Both
complexes have effective catalytic property for the sulfoxidation reaction.
Keywords: vanadium, Schiff base, crystal structure, hydrogen bonding, sulfoxidation
INTRODUCTION
 Author to whom correspondence should be addressed. E-mail: wangning7903@yahoo.com.cn
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Oxovanadium(V) complexes with N- and O-containing ligands have been extensively
investigated in recent years with respect to their remarkable efficiency as insulin mimetic
compounds [1-3]. The interaction of simple vanadium species with ligands having
pharmacological activity is of particular interest. Schiff base compounds have been widely used
as versatile ligands in coordination chemistry [4-6], and they have shown interesting biological
properties, such as antibacterial, antitumor, and antifungi activities [7-9], as well as catalytic
properties [10-12]. Recently, we have reported a few vanadium complexes [13]. As an extension
of the work on such complexes, in the present paper, we describe the synthesis, characterization,
crystal structure determination, and catalytic property of two new oxovanadium(V) complexes
with similar tetradentate Schiff base ligands N,N’-ethylenebis(3-ethoxysalicylideneimine) (H2L1)
and N,N’-ethylenebis(3-methylsalicylideneimine) (H2L2) (Scheme 1).
EXPERIMENTAL
Materials and Methods
3-Ethoxysalicylaldehyde, 3-methylsalicylaldehyde, and ethane-1,2-diamine were purchased from
Aldrich Chemical Co. All other reagent-grade chemicals and reagents were purchased
commercially and used without further purification. Elemental analyses (C, H, N) were obtained
with a Perkin-Elmer model 240C instrument. I.r. spectra were recorded in KBr discs on a Nicolet
170SX spectrophotometer. The 1H NMR spectra with tetramethylsilane (TMS) as an internal
standard were obtained using a Bruker Avance III 400MHz spectrophotometer.
Synthesis of the Schiff Base Compounds
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H2L1: A mixture of 3-ethoxysalicylaldehyde (3.32 g, 20 mmol) and ethane-1,2-diamine (0.60 g,
10 mmol) in 50 mL of methanol was refluxed for 1 h. After reducing the solvent to 10 mL by
distillation, and cooling to room temperature, the precipitated yellow solid was filtered off,
washed with methanol and dried in air. Recrystallization from methanol yielded pure product of
H2L1. Yield 2.60 g (73%). Found: C, 67.21, H, 6.87; N, 7.73. Calc. for C20H24N2O4: C, 67.40; H,
6.79; N, 7.86. IR (KBr, cm-1): 3427 (br, w), 2931 (w), 2092 (w), 2873 (w), 1635 (s), 1611 (m),
1575 (s), 1450 (s), 1465 (m), 1457 (m), 1423 (m), 1378 (w), 1322 (w), 1291 (w), 1281 (s), 1251
(w), 1237 (m), 1222 (m), 1154 (m), 1122 (w), 1037 (m), 1027 (m), 985 (w), 975 (w), 928 (w),
887 (w), 677 (w), 841 (s), 775 (m), 765 (m), 751 (m), 749 (m), 632 (w).
H2L2: This compound was prepared by following the same procedure outlined for H2L1, but with
3-ethoxysalicylaldehyde replaced by 3-methylsalicylaldehyde (2.72 g, 20 mmol). The pure
product was obtained as a yellow solid. Yield 2.03 g (68 %). Found: C, 72.83, H, 6.92; N, 9.35.
Calc. for C18H20N2O2: C, 72.95; H, 6.80; N, 9.45. IR (KBr, cm-1): 3421 (br, w), 2932 (w), 2093
(w), 2869 (w), 1636 (s), 1611 (m), 1578 (s), 1450 (s), 1463 (m), 1462 (m), 1421 (m), 1370 (w),
1319 (w), 1292 (w), 1283 (s), 1251 (w), 1220 (m), 1153 (m), 1118 (w), 1109 (w), 1037 (m),
1023 (m), 987 (w), 873 (w), 833 (w), 898 (w), 672 (w), 853 (s), 772 (m), 765 (m), 751 (m), 745
(m), 640 (w).
Synthesis of the Vanadium(V) Complexes
[VOL1]·H2O (1): [VO(acac)2] (0.263 g, 1.0 mmol) dissolved in 20 mL methanol was added
dropwise to a methanol solution (10 mL) of H2L1 (0.356 g, 1.0 mmol) with stirring, and then the
mixture was heated at reflux with stirring for 1 h and cooled to room temperature. When the
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solution was allowed to stand at room temperature for a few days to slow evaporation of the
solvent, X-ray quality brown block-shaped single crystals were collected by filtration. The
crystals were washed with methanol and dried in air. Yield 0.273 g (62 %). Found: C, 54.39; H,
5.63; N, 6.50. Calc. for C20H24N2O6V: C, 54.67; H, 5.51; N, 6.38. IR (KBr, cm-1): 3412 (w),
1617 (s), 1551 (m), 1427 (w), 1308 (m), 1220 (w), 1085 (w), 978 (m), 865 (w), 753 (m), 654
(w), 621 (w), 515 (w), 492 (w), 440 (w).
[VOL2] (2): This complex was prepared by following the same procedure outlined for (1), but
with H2L1 replaced by H2L2 (0.296 g, 1.0 mmol). The X-ray quality brown block-shaped single
crystals were obtained. Yield 0.232 g (64 %). Found: C, 59.71, H, 4.95; N, 7.83. Calc. for
C18H18N2O3V: C, 59.84; H, 5.02; N, 7.75. IR (KBr, cm-1): 1616 (s), 1550 (m), 1430 (w), 1302
(m), 1223 (w), 1082 (w), 978 (m), 868 (w), 750 (m), 643 (w), 612 (w), 509 (w), 460 (w).
X-Ray Structure Determination
The crystal and instrumental parameters used in the unit cell determination and data collection
are summarized in Table 1. Diffraction measurements were made at room temperature on a
Bruker SMART APEX II X-ray diffractometer using graphite-monochromated Mo Kα radiation
using ω scan mode. Unit-cell dimensions were determined and refined in the range 2.40-25.89°
for (1) and 2.33-25.73° for (2). Both structures were solved by the direct methods using
SHELXS-97 and refined by full-matrix least-squares techniques on F2 with SHELXL-97 [14].
The empirical absorption corrections were applied by the multi-scan method [15]. All non-
hydrogen atoms were refined with anisotropic displacement parameters. The water H atoms in
(1) were located in a difference Fourier map, and refined isotropically, with O–H and H···H
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distances restrained to 0.85(1) and 1.37 (2) Å, respectively, and with Uiso restrained to 0.08 Å2.
The remaining hydrogen atoms were included in their idealized positions and refined
isotropically. Selected bond distances (Å) and angles (°) for the complexes are listed in Table 2.
RESULTS AND DISCUSSION
The Schiff base compounds were prepared by the condensation of ethane-1,2-diamine with 3-
ethoxysalicylaldehyde and 3-methylsalicylaldehyde, respectively, in methanol. Each of them has
two dissociable protons at the phenolic-OH functionality. The reaction of
bis(acetylacetonato)oxovanadium(IV) with stoichiometric amounts of the Schiff base ligands in
methanol in air afforded the two vanadium(V) complexes as brown crystalline solids. It should
be pointed out that the vanadium in the starting materials is in V(IV) oxidation state, but it
appears to be V(V) in both complexes, indicating that it was oxidized by air during the reaction
procedures. Elemental analyses of the complexes are satisfactory with the empirical formulae.
Spectra Characterization of the Complexes
The i.r. spectra of the ligands exhibit broad bands in the region 3400–3450 due to the ν(O–H)
vibrations. The absence of these bands in the spectra of the complexes is consistent with the
subsequent proton replacement by the vanadium atom. A broad and weak band centered at 3412
cm-1 in the infrared spectrum of (1) is attributed to the O–H stretches of the water molecule. New
bands appearing at about 1220 cm-1 are assigned to ν(C–O). The strong bands at 1617 and 1616
cm-1 in complexes (1) and (2), respectively, are assigned to the azomethine groups, ν(C=N) [16],
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which are located at lower wave numbers when compared with those of the free Schiff bases.
The bands observed at 978 cm-1 for (1) and (2) are assigned to the V=O stretch [16,17].
Structure Description of the Complexes
The molecular structures of complexes (1) and (2) are presented as Figures 1 and 2, respectively.
Both complexes are mononuclear, with the V atoms coordinated by two phenolate oxygen atoms
and two imine nitrogen atoms of the Schiff base ligands in the basal plane, and by one oxo group
at the apical position. The The τ values for the complexes (1) and (2) are 0.19 and 0.08,
respectively, indicating the geometries of both complexes are square pyramid, instead of trigonal
bipyramid [18]. The bond lengths and angles in both complexes are comparable to each other,
and also similar to those observed in other vanadium(V) complexes with Schiff bases [19-21].
The dianionic tetradentate Schiff base ligands bind the metal atoms through the phenolate-O and
imine-N atoms, forming six- and five-membered chelate rings. The dihedral angles between the
two benzene rings in the complexes are 16.4(3)° for (1) and 5.1(3)° for (2). The mean deviation
of the four basal donor atoms from the least-squares planes are 0.098(3) Å for (1) and 0.038(3) Å
for (2). The displacement of the V atoms toward the oxo groups from the planes are 0.608(2) Å
for (1) and 0.606(3) Å for (2). The water molecule in complex (1) is bind to the [VOL1] molecule
via intramolecular hydrogen bonds (Table 3).
Sulfoxidation
The complexes were tested for their ability to catalyze the oxidation of prochiral sulfides, using
methyl phenyl sulfide (thioanisol) and benzyl phenyl sulfide as model substrates (Scheme 2)
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[22]. The results are summarized in Table 4. A 1.25-fold excess, based on the sulfide substrate,
of H2O2 or cumene hydroperoxide (CHPO) as oxidants was used. For all the reactions
performed, no sulfone formation as a side product was observed. The effective catalytic activity
of the complexes may be the result of a higher electron density on the phenolato oxygen due to
the electronic effects of the substituted ethoxy and methyl groups, contributing to an attainment
of sufficient nucleophilicity by the vanadium centre [23]. The best results have been obtained for
complex (1) as a catalyst in the oxidation of methyl phenyl sulfide with H2O2 as the oxidant. In
this case an overall yield of 98% within 24 h reaction time and a 20 % ee of the S-configured
sulfoxide were obtained. For the same time of reaction, catalyst and sulfide, but with CHPO as
the oxidant, a lower yield (93 %) and 15 % ee of the S-configured sulfoxide were observed.
When benzyl phenyl sulfide was used as the substrate, with a more bulky substituent, an increase
of reaction time to 48 h is noticed. Moreover, the yields of conversion are much smaller and the
enantioselectivity decreases to value of 5 % and 3 % with H2O2 and CHPO as oxidants,
respectively. As for complex (2), reduced catalytic activity on comparison to complex (1) is
noticed. With H2O2 as the oxidant in the sulfoxidation reaction, for methyl phenyl sulfide 95 %
was converted within 24 h and with 17 % ee of the S-configured sulfoxide, whereas with CHPO
as the oxidant, 91 % conversion was observed with 11 % ee of the S-configured sulfoxide. In the
case of benzyl phenyl sulfide, after a 48 h reaction time, with H2O2 and CHPO as oxidants,
overall yields of 77 % and 72 % were obtained, respectively.
Overall, despite having different substituent groups of the Schiff base ligands, both complexes
show effective catalytic properties. Nevertheless, the complexes in the present work have higher
transformation yields but lower ee values when compared to vanadium complexes with chiral
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Schiff base ligands [24,25], and have higher transformation yields when compared to vanadium
complexes with hydrazone ligands [26].
CONCLUSIONS
The results presented here clearly show that the Schiff base compounds N,N’-ethylenebis(3-
ethoxysalicylideneimine) and N,N’-ethylenebis(3-methylsalicylideneimine) can be used as
tetradentate ligands for the preparation of vanadium(V) complexes. The slight difference
between the Schiff base compounds can result in the cohesive or not of the water molecule. Both
complexes possess effective catalytic property for the oxidation of thioethers PhSMe and PhSBz
using H2O2 or CHPO as the oxidant.
Supplementary Material
Crystallographic data for the structures reported in this paper have been deposited with the
Cambridge Crystallographic Data Centre as supplementary publication no. CCDC – 917366 (1)
and no. CCDC – 917367 (2). Copies of the data can be obtained free of charge on application to
CCDC, 12 Union Road, Cambridge CB2 1EZ, UK (fax: +44-1223-336-033; e-mail:
deposit@ccdc.cam.ac.uk).
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N
N
HO
OH
R
R
R = OEt for H2L1 and R = Me for H2L2
Scheme 1. The Schiff base ligands
O
S
S
Complex
R
R
H2O2 or CHPO
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Scheme 2. Catalytic oxidation of thioethers PhSMe and PhSBz using H2O2 or CHPO as the
oxidant.
Figure 1. Molecular structure of (1) with 30% thermal ellipsoids. Dotted lines indicate hydrogen
bonds.
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Figure 2. Molecular structure of (2) with 30% thermal ellipsoids.
TABLE 1 Crystallographic data for the complexes
(1)
(2)
Formula
C20H24N2O6V
439.4
C18H18N2O3V
361.3
Mr
Crystal size (mm)
Crystal system
Space group
a (Å)
0.20 × 0.17 × 0.17
Monoclinic
P21/c
0.32 × 0.30 × 0.27
Monoclinic
P21/c
9.444(1)
9.664(1)
b (Å)
16.991(1)
17.472(1)
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c (Å)
 (°)
12.777(2)
10.348(1)
98.136(2)
106.464(2)
V (Å3)
2029.6(4)
4
1675.7(2)
4
Z
Dc (g/cm3)
1.438
0.528
916
1.432
µ (mm-1)
0.610
F(000)
748
Temperature (K)
Tmin
298(2)
0.9017
0.9156
20180
298(2)
0.8288
0.8526
15917
Tmax
Reflections measured
Independent reflections (Rint) 3937
3200
Observed reflections
3147
2390
Goodness of fit on F2
Final R indices [I  2(I)]a
1.036
1.029
R1 = 0.0341
wR2 = 0.0844
R1 = 0.0479
wR2 = 0.0918
R1 = 0.0374
wR2 = 0.0903
R1 = 0.0611
wR2 = 0.1021
R indices (all data)a
aR1 = Fo – Fc/Fo, wR2 = [w(Fo2 – Fc2)2/w(Fo2)2]1/2
TABLE 2 Selected bond lengths (Å) and bond angles (o) for the complexes
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(1)
(2)
V1-O1
1.9252(13)
1.9343(13)
2.0572(16)
2.0641(16)
1.5926(13)
112.92(7)
107.16(7)
87.39(5)
V1-O1
1.9093(16)
1.9244(15)
2.0501(18)
2.0509(18)
1.5921(17)
112.28(8)
109.02(8)
86.57(7)
V1-O2
V1-O2
V1-N1
V1-N1
V1-N2
V1-N2
V1-O5
V1-O3
O5-V1-O1
O5-V1-O2
O1-V1-O2
O5-V1-N1
O1-V1-N1
O2-V1-N1
O5-V1-N2
O1-V1-N2
O2-V1-N2
N1-V1-N2
O3-V1-O1
O3-V1-O2
O1-V1-O2
O3-V1-N1
O1-V1-N1
O2-V1-N1
O3-V1-N2
O1-V1-N2
O2-V1-N2
N1-V1-N2
102.35(7)
86.77(6)
103.99(8)
86.75(7)
149.85(6)
108.44(7)
138.15(6)
86.48(6)
146.39(7)
105.56(8)
141.72(8)
86.51(7)
78.42(6)
78.68(7)
TABLE 3 Distances (Å) and angles (º) involving hydrogen bonding of the complex (1)
D–H∙∙∙A
d(D–H)
d(H∙∙A)
d(D∙∙∙A)
Angle(D–H∙∙∙A)
O6–H6B∙∙∙O2
0.85(1)
2.35(2)
3.018(2)
137(2)
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O6–H6B∙∙∙O4
O6–H6A∙∙∙O1
O6–H6A∙∙∙O3
0.85(1)
0.85(1)
0.85(1)
2.21(2)
2.29(2)
2.29(2)
2.982(2)
2.991(2)
3.041(2)
153(2)
141(2)
149(2)
TABLE 4 Catalytic results of the complexes
Complex
(1)
Substrate
PhSMe
PhSMe
PhSBz
PhSBz
PhSMe
PhSMe
PhSBz
PhSBz
Oxidant
H2O2
Yield (%)
Time (h)
24
ee (%)
20 (S)
15 (S)
5 (S)
3 (S)
17 (S)
11 (S)
0
98
93
85
82
95
91
77
72
CHPO
H2O2
24
48
CHPO
H2O2
48
(2)
24
CHPO
H2O2
24
48
CHPO
48
0
17
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