Synthesis, crystal structures, and catalytic oxidation properties of oxidovanadium(V) complexes with Schiff base ligands

Article abstract of DOI:10.1007/s11243-014-9821-1

Two new Schiff base ligands 2-chloro-N′-(5-fluoro-2- hydroxybenzylidene)benzohydrazide (H₂L^a) and 4-fluoro-2-{[2-(2-hydroxyethylamino)ethylimino]methyl}phenol (HL^b) were synthesized and characterized. Their respective oxid

Full text of DOI:10.1007/s11243-014-9821-1

Transition Met Chem (2014) 39:469–475
DOI 10.1007/s11243-014-9821-1
Synthesis, crystal structures, and catalytic oxidation properties
of oxidovanadium(V) complexes with Schiff base ligands
Kui-Hua Yang
Received: 16 January 2014 / Accepted: 19 March 2014 / Published online: 2 April 2014
Ó Springer International Publishing Switzerland 2014
Abstract Two new Schiff base ligands 2-chloro-N0-(5-
fluoro-2-hydroxybenzylidene)benzohydrazide (H2La) and
4-fluoro-2-{[2-(2-hydroxyethylamino)ethylimino]methyl}
phenol (HLb) were synthesized and characterized. Their
respective oxidovanadium complexes, [VOLa(OMe)
(MeOH)]ꢀMeOH (1) and [VO(l-O)Lb]2 (2), were syn-
thesized and characterized by spectroscopy and single-
crystal X-ray diffraction. The coordination sphere of each
V atom is octahedral. Both complexes showed selective
heterogeneous catalytic properties with 74–98 % con-
version, for the oxidation of cyclohexene, cyclopentene,
and benzyl alcohol using H2O2 as primary oxidant.
characterization, and catalytic properties of two new oxi-
dovanadium(V) complexes, [VOLa(OMe)(MeOH)]ꢀMeOH
(1) and [VO(l-O)Lb]2 (2), with Schiff base ligands
2-chloro-N0-(5-fluoro-2-hydroxybenzylidene)benzohydraz-
ide (H2La) and 4-fluoro-2-{[2-(2-hydroxyethylamino)eth-
ylimino]methyl}phenol (HLb).
Experimental
Reagents and measurements
All chemicals of AR grade were obtained from Sigma-
Aldrich and used as received. 1H NMR spectra were
recorded on a 500 MHz Bruker NMR spectrometer using
DMSO-d6 solvent. IR spectra were obtained as KBr plates
using a Bruker FT IR spectrophotometer. Elemental anal-
yses (C, H, N) were obtained with a PerkinElmer CHN
analyzer 2400. GC analyses were carried out with a GC-
17A Shimadzu instrument.
Introduction
In recent years, the development of efficient new catalysts
for various organic reactions has received considerable
attention [1–6]. Among the transition metals, vanadium has
proved to be particularly useful for catalytic applications.
Oxidovanadium(IV) complexes with various types of
ligands have been shown to possess effective catalytic
ability [7–11]. Several reports have indicated that oxido-
vanadium complexes with Schiff base ligands can catalyze
the epoxidation of alkenes [12–17]. These epoxidation
products are important industrial materials. Hydrogen
peroxide is particularly attractive as an oxidant, as it is
cheap, reasonably stable, readily available, and gives only
water as a by-product. Herein, we report the synthesis,
Synthesis of H2La
5-Fluorosalicylaldehyde (10 mmol, 1.40 g) and 2-chloro-
benzohydrazide (10 mmol, 1.70 g) were dissolved in
methanol (30 mL), and the mixture was stirred under reflux
for 30 min. Most of the solvent was then removed by
distillation to give a colorless precipitate. This was col-
lected by filtration and recrystallized from methanol,
washed successively with cold methanol and diethyl ether,
and then air-dried. Yield: 83 %. Anal. Calc. for C14H10-
ClFN2O2: C, 57.4; H, 3.4; N, 9.6. Found: C, 57.6; H, 3.6;
N, 9.5 %. Selected IR data (m cm-1, KBr): 3,371 (mO–H),
3,218 (mN–H); 1,632 (mC=N). 1H NMR (ppm): 12.13 (1H, br,
K.-H. Yang (&)
School of Chemistry and Chemical Engineering, Mianyang
Normal University, Mianyang 621000, People’s Republic of
China
e-mail: kuihua_yang@163.com
123

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Transition Met Chem (2014) 39:469–475
Synthesis of complex 2
OH), 10.54 (1H, br, NH); 8.75 (1H, s, CH = N); 7.4–7.7
(5H, m, HAr), 7.21 (1H, d, HAr), 7.00 (1H, d, HAr).
HLb (0.2 mmol, 45.2 mg) and VO(acac)2 (0.2 mmol,
53.0 mg) were added to ethanol (20 mL), and the mixture
was boiled under reflux until the color of the solution
turned to yellow–green. Yellow single crystals of the
complex were obtained after slow evaporation of the
solution in air. These were isolated by filtration, washed
successively with cold methanol and diethyl ether, and then
air-dried. Yield: 45 %. Anal. Calc. for C22H28F2N4O8V2:
C, 42.9; H, 4.6; N, 9.1. Found: C, 43.1; H, 4.7; N, 8.9 %.
Selected IR data (m cm-1, KBr): 3,430 (mO–H); 3,246
(mN–H); 1,605 (mC=N); 972 (mV=O).
Synthesis of HLb
5-Fluorosalicylaldehyde (10 mmol, 1.40 g) and b-hydrox-
yethylenediamine (10 mmol, 1.04 g) were dissolved in
methanol (30 mL), and the mixture was stirred under reflux
for 30 min. Most of the solvent was then removed by
distillation to give a colorless precipitate, which was col-
lected by filtration, washed successively with cold metha-
nol and diethyl ether, and then air-dried. Yield: 57 %.
Anal. Calc. for C11H15FN2O2: C, 58.4; H, 6.7; N, 12.4.
Found: C, 58.3; H, 6.6; N, 12.3 %. Selected IR data
(m cm-1, KBr): 3,360 (mO–H), 3,232 (mN–H); 1,618 (mC=N).
1H NMR (ppm): 11.87 (1H, br, OH), 10.72 (1H, br, NH);
8.53 (1H, s, CH = N); 7.60 (1H, s, HAr), 7.20 (1H, d,
HAr), 6.98 (1H, d, HAr), 3.69 (2H, t, NCH2), 3.50 (2H, t,
CH2OH), 2.89 (2H, t, NCH2CH2NH), 2.73 (2H, t, NHCH2).
X-ray crystallography
Single-crystal X-ray diffraction measurements were carried
out on a Bruker Smart 1000 CCD diffractometer equipped
with a graphite crystal monochromator situated in the
incident beam for data collection at room temperature. The
determination of unit cell parameters and data collection
Synthesis of complex 1
˚
was performed with Mo-Ka radiation (k = 0.71073 A).
H2La (0.2 mmol, 58.6 mg) and VO(acac)2 (0.2 mmol,
53.0 mg) were mixed and added to methanol (20 mL), and
the mixture was boiled under reflux until the color of the
solution turned to deep brown. Block-shaped single crys-
tals of the complex were obtained by slow evaporation of
the solution in air. The crystals were isolated by filtration,
washed successively with cold methanol and diethyl ether,
and then air-dried. Yield: 69 %. Anal. Calc. for C17H19-
ClFN2O6V: C, 45.1; H, 4.2; N, 6.2. Found: C, 45.3; H, 4.3;
N, 6.1 %. Selected IR data (m cm-1, KBr): 3,422 (mO–H);
3,211 (mN–H); 1,621 (mC=N); 980 (mV=O).
The collected data were reduced with SAINT [18], and
multi-scan absorption corrections were made using SAD-
ABS [19]. Unit cell dimensions were obtained with least-
squares refinements, and both structures were solved by
direct methods. Both complexes were refined against F2 by
full-matrix least-squares methods using SHELXTL [20]. V
atoms in the complexes were located from electron density
maps. The non-hydrogen atoms were located in successive
difference Fourier syntheses. The final refinement was
performed by full-matrix least-squares methods with
anisotropic thermal parameters for non-hydrogen atoms on
Scheme 1 Synthesis of the Schiff bases and their complexes
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Transition Met Chem (2014) 39:469–475
471
F2. The methanol hydrogen atom of 1 and the amino
hydrogen atom of 2 were located from difference Fourier
maps and refined isotropically, with O–H and N–H dis-
The remaining hydrogen atoms were added theoretically as
riding on the concerned atoms. Crystallographic data
(excluding structure factors) for the structures reported in
this paper have been deposited with the Cambridge Crys-
tallographic Data Centre as supplementary publication no.
CCDC-980582 for 1 and 980583 for 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.com.ac.uk].
˚
tances restrained to 0.85(1) and 0.90(1) A, respectively.
Catalytic oxidation experiments
The liquid phase catalytic oxidations were carried out in a
25-mL round-bottom flask equipped with a magnetic stirrer
and immersed in a thermostated oil bath at 60 °C. H2O2
(3 mmol) was added to the flask containing the catalyst
(2.0 lmol V), NaHCO3 (1 mmol), and a representative
alkene, namely cyclohexene (1 mmol) in the required
Fig. 1 Perspective view of the complex molecule of 1 with atom
labeling scheme. Displacement ellipsoids are drawn at 30 % prob-
ability level; hydrogen atoms are depicted as spheres of arbitrary radii
˚
Table 2 Coordinate bond lengths (A) and bond angles (°)
Table 1 Crystal data, data collection, and structure refinement
1
Complex
1
2
Bond lengths
V1–O1
1.852(3)
1.580(4)
2.126(4)
1.291(6)
1.290(6)
V1–O2
V1–O4
V1–O5
N1–N2
C8–O2
1.976(3)
1.761(3)
2.293(4)
1.390(5)
1.296(6)
Molecular formula
Formula weight
Crystal system
Space group
C17H19ClFN2O6V
452.7
C22H28F2N4O8V2
616.4
V1–O3
V1–N1
Monoclinic
Cc
Monoclinic
P21/c
C7–N1
˚
a (A)
N2–C8
17.813(3)
12.017(3)
10.135(2)
98.211(2)
2,147.1(6)
4
10.3484(5)
11.0233(5)
10.6659(5)
92.000(2)
1216.0(1)
2
˚
b (A)
Bond angles
O3–V1–O4
O4–V1–O1
O4–V1–O2
O3–V1–N1
O1–V1–N1
O3–V1–O5
O1–V1–O5
N1–V1–O5
2
˚
c (A)
102.16(19)
102.76(17)
93.81(16)
95.51(18)
83.38(16)
176.15(18)
80.43(16)
80.75(15)
O3–V1–O1
O3–V1–O2
O1–V1–O2
O4–V1–N1
O2–V1–N1
O4–V1–O5
O2–V1–O5
100.2(2)
96.53(19)
153.33(16)
159.82(16)
74.39(15)
81.36(17)
81.60(16)
b (°)
3
˚
V (A )
Z
Dc (g cm-3
)
1.401
1.683
F(000)
l (mm-1
928
632
)
0.628
0.841
h range (°)
hkl range
3.00–25.50
-21 B h B 21
-14 B k B 13
-12 B l B 12
2.66–25.50
-12 B h B 12
-13 B k B 12
-12 B l B 12
Bond lengths
V1–O1
1.902(1)
1.612(1)
2.149(2)
V1–O3
V1–O3A
V1–N2
1.684(1)
2.299(1)
2.198(2)
V1–O4
Reflections
Collected
Unique
V1–N1
9,702
11,490
Bond angles
O4–V1–O3
O3–V1–O1
O3–V1–N1
O4–V1–N2
O1–V1–N2
O4–V1–O3A
O1–V1–O3A
N2–V1–O3A
3,919
2,265
107.61(7)
99.29(6)
155.32(6)
89.58(7)
159.12(6)
170.35(6)
85.13(5)
82.55(5)
O4–V1–O1
O4–V1–N1
O1–V1–N1
O3–V1–N2
N1–V1–N2
O3–V1–O3A
N1–V1–O3A
100.69(7)
95.87(6)
83.29(6)
94.76(6)
77.56(6)
78.72(6)
77.05(5)
Observed [I [ 2r(I)]
Rint
3,137
1,990
0.0399
0.0292
Parameters
260
176
Restraints
3
1
Final R index [I [ 2r(I)]
R index (all data)
Goodness-of-fit on F2
Maximum/minimum Dq
0.0576, 0.1452
0.0754, 0.1614
1.044
0.0281, 0.0722
0.0347, 0.0766
1.081
0.909, -0.383
0.274, -0.216
-3
˚
(e A
)
Symmetry code for A: 1-x, 2-y, -z
123

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Transition Met Chem (2014) 39:469–475
Table 3 Hydrogen bonding information
D–HꢀꢀꢀA
d (D–H)
d (HꢀꢀꢀA)
d (DꢀꢀꢀA)
\(D–HꢀꢀꢀA)
1
O5–H5ꢀꢀꢀO6i
O6–H6ꢀꢀꢀN2
2
0.85(1)
0.82
1.80(2)
2.23
2.634(6)
2.819(6)
169(7)
129
N2–H2ꢀꢀꢀO4ii
N2–H2ꢀꢀꢀO1iii
O2–H2AꢀꢀꢀO3
0.90(1)
0.90(1)
0.82
2.48(3)
2.46(2)
1.95
3.072(2)
3.142(2)
2.737(2)
124(2)
134(3)
161
Symmetry codes: (1) -1 ? x, -1 ? y, -1 ? z, (2) 1 - x, 1/2 ? y,
1/2 - z; (3) 1 - x, 2 - y, -z
Fig. 3 Perspective view of the dinuclear structure of 2 with atom
labeling scheme. Displacement ellipsoids are drawn at 30 % prob-
ability level; hydrogen atoms are depicted as spheres of arbitrary
radii. Intramolecular hydrogen bonds are shown as dashed lines
solvent (3 mL). The course of the reaction was monitored
using a gas chromatograph equipped with a capillary col-
umn and a flame ionization detector. The oxidation pro-
ducts were identified by comparing their retention times
with those of authentic samples or alternatively by 1H
NMR and GC-mass analyses. Yields based on the added
substrate were determined by a calibration curve. Control
reactions were carried out in the absence of catalyst, under
the same conditions as the catalytic runs.
resulted in the Schiff base ligands H2La and HLb, respec-
tively (Scheme 1). The complexes were readily prepared
by the reaction of the Schiff base ligands and VO(acac)2 in
methanol (Scheme 1). All the compounds are soluble in
polar solvents such as methanol, ethanol, and acetonitrile.
The IR spectra of the free Schiff bases contain bands at
1,632 cm-1 for H2La and 1,618 cm-1 for HLb, which are
due to the presence of C = N groups. In the IR spectra of
the vanadium complexes, these bands were observed at
lower wave numbers, 1,621 cm-1 for 1 and 1,605 cm-1 for
2. This suggests coordination through the nitrogen atom of
the C = N groups. Furthermore, broad and medium bands
observed at 3,300–3,500 cm-1 are indicative of the pre-
sence of O–H containing solvent molecules. Weak and
Results and discussion
Synthesis and spectra
The condensation reactions of 5-fluorosalicylaldehyde with
2-chlorobenzohydrazide and b-hydroxyethylenediamine
Fig. 2 Crystal packing of complex 1. The dashed lines denote hydrogen bonds
123

Transition Met Chem (2014) 39:469–475
473
Fig. 4 Crystal packing of complex 2 as seen along axis-b direction. The dashed lines denote hydrogen bonds
sharp bands located at 3,200–3,250 cm-1 are indicative of
the presence of amino groups. The typical bands at
980 cm-1 for 1 and 972 cm-1 for 2 are assigned to the
V = O coordinate bonds.
Table 4 Catalytic results for cyclohexene
Catalyst Oxidant Temperature (°C) Time (h) Conversion (%)a
1
1
2
2
H2O2
H2O2
H2O2
H2O2
25
60
25
60
3
1
3
1
0
75
0
Crystal structure descriptions
62
The molecular structure of complex 1 is shown in Fig. 1.
Crystal data for the structure are listed in Table 1, and
selected bond lengths are given in Table 2. The crystal
The complex (2.0 lmol V), cyclohexene (1 mmol), CH3CN (3 mL),
NaHCO3 (1 mmol), H2O2 (3 mmol)
a
Based on cyclohexene
123

474
Transition Met Chem (2014) 39:469–475
100
80
60
40
20
0
oxygen ligand. The three donor atoms of the Schiff base
and the deprotonated methanol ligand constitute the
equatorial plane of the octahedral coordination sphere,
such that the V atom deviates from the plane by 0.296(1)
(1)
(2)
CH3 CN
˚
A. The axial bond V1-O5 is much longer than the other
bonds in the complex, distorting the octahedron. This
distortion can also be observed from the bond angles,
which range from 74.4(2)° to 102.8(2)° for the cis angles
and from 153.3(2)° to 176.2(2)° for the trans angles. The
dihedral angle between the two aromatic rings of the
Schiff base ligand is 85.2(3)°. The bond length pattern
within the bridging C–C=N–C moiety reflects the delo-
calization of the electron density throughout this frag-
ment: The formally single bonds are shorter, and formally
double bonds are longer than the typical values (Table 2)
[20].
CH OH
3
CHCl3
0
1
2
3
4
5
Time (h)
Fig. 5 Solvent effect on the epoxidation of cyclohexene. The
complexes as catalyst (2.0 lmol V), cyclohexene (1 mmol), solvent
(3 mL), NaHCO3 (1 mmol), H2O2 (3 mmol); 60 °C
In the crystal structure of complex 1, the vanadium
complex molecules are linked by methanol molecules
through intermolecular O–HꢀꢀꢀO and O–HꢀꢀꢀN hydrogen
bonds (Table 3), to form 1D chains running along the c-
axis direction (Fig. 2).
compound contains a mononuclear vanadium complex
molecule and a methanol molecule of crystallization. The
Schiff base ligand coordinates to the V atom through the
phenolate oxygen, imino nitrogen, and enolate oxygen.
The other three coordinate sites are occupied by one
neutral methanol, one deprotonated methanol, and an oxo
The molecular structure of complex 2 is shown in Fig. 3.
Crystal data for the structure are listed in Table 1, and
Table 5 Catalytic oxidation results of various substrates
Substrate
Products
Complex
Yield (%)a
Conversion (%)
1
2
81
66
95
87
O
1
2
85
70
98
91
O
1
2
53
45
74
78
OH
1
2
90
81
96
92
CHO
CH2OH
The complex (2.0 lmol V), substrate (1 mmol), CH3CN (3 mL), NaHCO3 (1 mmol), H2O2 (3 mmol). Temperature: 60 °C
a
Based on the starting substrates
123

Transition Met Chem (2014) 39:469–475
475
selected bond lengths are given in Table 2. The compound
possesses a crystallographic inversion center of symmetry.
53 % for 1 and 45 % for 2, and with cyclohexanone as the
by-product (1.5 %). Oxidation of benzyl alcohol by the
complexes produced benzaldehyde with high yields and
quantitative selectivities.
˚
The VꢀꢀꢀV distance is 3.104(1) A. The Schiff base ligand
coordinates to the V atom through the phenolate oxygen,
imino nitrogen, and amino nitrogen. The other three
coordinate sites are occupied by one oxo oxygen and two
bridging oxo oxygen ligands. The three donor atoms of the
Schiff base and one bridging oxo oxygen (O3) constitute
the equatorial plane of the octahedral coordination sphere,
such that the V atom deviates from the plane by 0.301(1)
Conclusion
In summary, two new Schiff base ligands and their oxi-
dovanadium(V) complexes have been synthesized and
characterized. The complexes are effective catalysts for the
oxidation of cyclohexene, cyclopentene, and benzyl alco-
hol by H2O2 as oxidant. Complex 1 with a hydrazone-type
Schiff base ligand was a more effective catalyst than the
non-hydrazone Schiff base complex 2.
˚
A. As with complex 1, the axial bond V1-O3A is much
longer than the other bonds, resulting in distorted octahe-
dral coordination. The bond angles around vanadium range
from 77.05(5)° to 107.61(7)° for the cis angles and from
155.32(6)° to 170.35(6)° for the trans angles. The hydroxyl
groups of the Schiff base ligands and the bridging oxo
oxygen are involved in intramolecular hydrogen bonds.
In the crystal structure of complex 2, the vanadium
complex molecules are linked through intermolecular N–
HꢀꢀꢀO hydrogen bonds (Table 3), to form 1D chains run-
ning along the direction of the c-axis (Fig. 4).
Acknowledgments We would like to acknowledge the financial
supports from Mianyang Normal University.
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corresponding epoxides with quantitative selectivity.
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¨
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