Vanadium(V) complexes with hydrazone and benzohydroxamate ligands: Synthesis, structures and catalytic epoxidation

Article abstract of DOI:10.1080/00958972.2014.931577

Two structurally similar vanadium(V) complexes, [VOL¹L] (1) and [VOL²L] (2), where L¹ is the dianionic form of N'-(3,5-dibromo-2-hydroxybenzylidene)-4-methoxybenzohydrazide, L² is the dianionic form of N'-(3,5-dibromo-2-hydroxybenzylidene)nicotinohydrazide, and L is benzohydroxamate, were prepared and characterized by elemental analysis, infrared spectra, and single crystal X-ray diffraction. 1 crystallizes in the monoclinic space group P2_1/c, with unit cell dimensions a = 12.0262(7), b = 16.5629(9), c = 12.3449(8) A, β = 105.160(2)°, V = 2373.4(2) A³, Z = 4, R₁ = 0.0376, wR₂ = 0.0844, GOOF = 1.058; 2 crystallizes in the monoclinic space group P2 _1/c, with unit cell dimensions a = 10.6439(7), b = 16.1968(9), c = 13.1108(8) A, β = 104.180(2)°, V = 2191.4(2) A³, Z = 4, R₁ = 0.0521, wR₂ = 0.1142, GOOF = 1.018. X-ray analysis indicates that vanadium in the complexes are coordinated by phenolate oxygen, imino nitrogen, and enolate oxygen of the hydrazone, two oxygens of the benzohydroxamate, and one oxo, generating octahedral coordination. Oxidations of olefins with the complexes as catalysts were evaluated. The complexes showed effective catalytic efficiency in oxidation of several aliphatic and aromatic substrates under mild conditions, using tert-butyl hydrogen peroxide as oxidant. 2014

Full text of DOI:10.1080/00958972.2014.931577

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Vanadium(V) complexes with
hydrazone and benzohydroxamate
ligands: synthesis, structures and
catalytic epoxidation
An-Mei Lia
a College of Chemical Engineering and Pharmacy, Jingchu
University of Technology, Jingmen, PR China
Accepted author version posted online: 06 Jun 2014.Published
online: 30 Jun 2014.
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To cite this article: An-Mei Li (2014) Vanadium(V) complexes with hydrazone and
benzohydroxamate ligands: synthesis, structures and catalytic epoxidation, Journal of Coordination
Chemistry, 67:12, 2076-2085, DOI: 10.1080/00958972.2014.931577
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Journal of Coordination Chemistry, 2014
Vol. 67, No. 12, 2076–2085, http://dx.doi.org/10.1080/00958972.2014.931577
Vanadium(V) complexes with hydrazone and
benzohydroxamate ligands: synthesis, structures and catalytic
epoxidation
AN-MEI LI*
College of Chemical Engineering and Pharmacy, Jingchu University of Technology, Jingmen, PR
China
(Received 4 March 2014; accepted 23 April 2014)
Two structurally similar vanadium(V) complexes, [VOL1L] (1) and [VOL2L] (2), where L1 is the
dianionic form of N′-(3,5-dibromo-2-hydroxybenzylidene)-4-methoxybenzohydrazide, L2 is the
dianionic form of N′-(3,5-dibromo-2-hydroxybenzylidene)nicotinohydrazide, and L is benzohydroxa-
mate, were prepared and characterized by elemental analysis, infrared spectra, and single crystal
X-ray diffraction. 1 crystallizes in the monoclinic space group P21/c, with unit cell dimensions
a = 12.0262(7), b = 16.5629(9), c = 12.3449(8) Å, β = 105.160(2)°, V = 2373.4(2) Å3, Z = 4,
R1 = 0.0376, wR2 = 0.0844, GOOF = 1.058; 2 crystallizes in the monoclinic space group P21/c, with unit
cell dimensions a = 10.6439(7), b = 16.1968(9), c = 13.1108(8) Å, β = 104.180(2)°, V = 2191.4(2) Å3,
Z = 4, R1 = 0.0521, wR2 = 0.1142, GOOF = 1.018. X-ray analysis indicates that vanadium in the
complexes are coordinated by phenolate oxygen, imino nitrogen, and enolate oxygen of the
hydrazone, two oxygens of the benzohydroxamate, and one oxo, generating octahedral coordination.
Oxidations of olefins with the complexes as catalysts were evaluated. The complexes showed
effective catalytic efficiency in oxidation of several aliphatic and aromatic substrates under mild
conditions, using tert-butyl hydrogen peroxide as oxidant.
Keywords: Hydrazone ligand; Vanadium complex; Crystal structure; Catalytic epoxidation
*Email: amli2010@163.com
© 2014 Taylor & Francis

Vanadium(V) hydrazone complexes
2077
1. Introduction
The development of processes for oxidation of olefins is a goal that has long been pursued.
The challenge is to make such processes environmental friendly using nontoxic reagents
and energy-efficient catalytic methods [1]. Vanadium complexes are well known for use in
organic synthesis, in particular for oxidations of various organic compounds [2–7].
Vanadium complexes have attracted attention for sulfoxidation and epoxidation of olefins
[8–13]. The synthesis and catalytic properties of a number of vanadium complexes with
Schiff bases have been reported [4, 7, 14–16]. However, the number of documented
vanadium complexes catalyzing the peroxidic oxidation of sulfides is still limited and the
structure-activity relationship is not clear. In this work, two new structurally similar
vanadium(V) complexes, [VOL1L] (1) and [VOL2L] (2), where L1 is the dianionic form
of N′-(3,5-dibromo-2-hydroxybenzylidene)-4methoxybenzohydrazide (H2L1), L2 is the
dianionic form of N′-(3,5-dibromo-2-hydroxybenzylidene)nicotinohydrazide (H2L2), and L
is benzohydroxamate, were prepared and studied for their structures and catalytic oxidation
of olefins (scheme 1).
2. Experimental
2.1. Materials and methods
All chemicals and solvents were analytical reagent grade and purchased from Beijing
Chemical Reagent Company. H2L1 and H2L2 were prepared according to the literature
method [17]. Microanalyses (C, H, N) were performed using a Perkin-Elmer 2400
elemental analyzer. Infrared spectra were carried out using the JASCO FT-IR model 420
spectrophotometer with KBr disk from 4000 to 400 cm−1. GC analyses were performed on
a Shimadzu GC-2014 gas chromatograph. UV–vis spectra were measured with a
Perkin-Elmer Lambda 18 spectrophotometer.
2.2. Syntheses of the complexes
[VOL1L] (1). A methanolic solution (15 mL) of VO(acac)2 (0.26 g, 1.0 mM) was added to a
methanolic solution (15 mL) of H2L1 (0.43 g, 1.0 mM) and benzohydroxamic acid (0.14 g,
1.0 mM). The mixture was stirred at room temperature for 1 h to give a brown solution.
Single crystals of the complex, suitable for X-ray diffraction, were formed at the bottom of
the vessel on slow evaporation of the solution in air for a few days. Yield: 54%. Anal.
Calcd for C22H16Br2N3O6 V: C, 42.00; H, 2.56; N, 6.68. Found: C, 41.86; H, 2.67; N,
6.81%.
Scheme 1. The hydrazone ligands H2L1 (left) and H2L2 (right).

2078
A.-M. Li
[VOL2L] (2). This complex was prepared according to the same method as 1, but with
H2L1 replaced by H2L2 (0.40 g, 1.0 mM). Yield: 67%. Anal. Calcd for C20H13Br2N4O5 V:
C, 40.03; H, 2.18; N, 9.34. Found: C, 40.22; H, 2.30; N, 9.17%.
2.3. X-ray structure determination
X-ray measurement was performed using a Bruker Smart 1000 CCD diffractometer with
graphite monochromated Mo Kα radiation (λ = 0.71073 Å) using the ω-scan technique.
Determination of the Laue class, orientation matrix, and cell dimensions were performed
according to established procedures where Lorentz polarization and absorption corrections
were applied. Absorption corrections were applied by fitting a pseudoellipsoid to the ψ-scan
data of selected strong reflections over a range of 2θ angles. The positions of non-hydrogen
atoms were located with direct methods. Subsequent Fourier syntheses were used to locate
the remaining non-hydrogen atoms. All non-hydrogen atoms were refined anisotropically.
The amino hydrogens were located from difference Fourier maps and refined isotropically,
with N–H distances restrained to 0.90(1) Å. The remaining hydrogens were placed in calcu-
lated positions and constrained to ride on their parent atoms. The analysis was performed
with the aid of the SHELXS-97 and SHELXL-97 suite of codes [18, 19]. The crystallo-
graphic data for the complexes are summarized in table 1. Selected bond lengths and angles
are given in table 2.
Table 1. Crystal data for 1 and 2.
1
2
Chemical formula
Fw
C22H16Br2N3O6 V
629.1
C20H13Br2N4O5 V
600.1
Crystal shape/color
Crystal size (mm)
T (K)
Block/deep brown
0.20 × 0.18 × 0.16
298(2)
Block/deep brown
0.18 × 0.17 × 0.17
298(2)
λ (Mo Kα) (Å)
Crystal system
Space group
a (Å)
b (Å)
c (Å)
0.71073
Monoclinic
P21/c
12.0262(7)
16.5629(9)
12.3449(8)
105.160(2)
2373.4(2)
4
0.71073
Monoclinic
P21/c
10.6439(7)
16.1968(9)
13.1108(8)
104.180(2)
2191.4(2)
4
β (°)
V (Å3)
Z
μ (Mo Kα) (cm−1
T (min)
)
3.829
0.5148
4.140
0.5228
T (max)
0.5794
1.761
1240
0.5396
1.819
1176
DCalcd (g cm−3
F (000)
)
Collected reflections
Unique reflections
Observed reflections [I ≥ 2σ(I)]
Parameters
25,139
4402
3523
311
21,696
4492
3289
292
Restraints
1
1
Goodness of fit on F2
Rint
1.058
0.0431
1.018
0.0415
R1 [I ≥ 2σ(I)]
0.0376
0.0521
wR2 [I ≥ 2σ(I)]
R1 (all data)
0.0844
0.0551
0.1142
0.0807
wR2 (all data)
0.0923
0.1295

Vanadium(V) hydrazone complexes
2079
Table 2. Selected bond lengths (Å) and angles (°) for 1 and 2.
1
Bond lengths
V1–O1
V1–O5
V1–O4
1.873(2)
1.855(2)
2.168(3)
V1–O2
V1–O6
V1–N1
1.948(2)
1.581(3)
2.094(3)
Bond angles
O6–V1–O5
O5–V1–O1
O5–V1–O2
O6–V1–N1
O1–V1–N1
O6–V1–O4
O1–V1–O4
N1–V1–O4
97.24(13)
102.76(11)
93.51(11)
100.11(13)
83.84(11)
173.67(12)
84.57(11)
85.61(10)
O6–V1–O1
O6–V1–O2
O1–V1–O2
O5–V1–N1
O2–V1–N1
O5–V1–O4
O2–V1–O4
98.68(14)
97.85(14)
155.06(11)
160.29(11)
74.95(10)
76.69(10)
80.91(11)
2
Bond lengths
V1–O1
V1–O4
V1–O3
1.889(3)
1.848(3)
2.196(3)
V1–O2
V1–O5
V1–N1
1.944(3)
1.583(3)
2.073(3)
Bond angles
O5–V1–O4
O4–V1–O1
O4–V1–O2
O5–V1–N1
O1–V1–N1
O5–V1–O3
O1–V1–O3
N1–V1–O3
95.89(15)
105.29(13)
90.55(12)
98.92(15)
84.17(13)
171.30(15)
81.18(12)
89.73(12)
O5–V1–O1
O5–V1–O2
O1–V1–O2
O4–V1–N1
O2–V1–N1
O4–V1–O3
O2–V1–O3
98.69(15)
101.32(15)
152.99(13)
161.03(13)
74.99(12)
75.85(12)
81.72(12)
2.4. Oxidation
Catalytic experiments were carried out in a 50 mL glass round-bottom flask fitted with a
reflux condenser and placed in an oil bath at prearranged temperature under continuous stir-
ring. The oxidation was carried out as follows: the complexes (0.032 mM) were dissolved
in 10 mL 1,2-dichloroethane. Then 10 mM alkene was added to the reaction mixture and
30 mM TBHP was added. The reaction mixture was refluxed for 1 h. The reaction products
were monitored at periodic time intervals using gas chromatography. The oxidation prod-
ucts were identified by comparison with authentic samples (retention times in GC).
3. Results and discussion
3.1. Synthesis
The hydrazones were readily prepared by condensation of 3,5-dibromo-2-hydrox-
ybenzohydrazide with 4-methylbenzohydrazide and nicotinohydrazide, respectively, in
methanol. The vanadium complexes were synthesized by stirring equimolar quantities of
the hydrazone ligands, benzohydroxamic acid and VO(acac)2 in methanol at room tempera-
ture (scheme 2). The chemical formulas of the complexes have been confirmed by elemen-
tal analyses, IR spectra, and single crystal X-ray diffraction.

2080
A.-M. Li
Scheme 2. Preparation of the complexes. 1: R = p-OMePh; 2: R = m-pyridine.
Figure 1. Molecular structure of 1 with 30% probability thermal ellipsoids.
Figure 2. Molecular structure of 2 with 30% probability thermal ellipsoids.

Vanadium(V) hydrazone complexes
2081
Figure 3. Molecular packing of 1 viewed along the b axis. Hydrogen bonds are drawn as thin dotted lines.
Figure 4. Molecular packing of 2. Hydrogen bonds are drawn as thin dotted lines.

2082
A.-M. Li
1.0
0.5
0.0
1
2
300
400
500
600
700
800
Wavelength (nm)
Figure 5. UV–vis spectra of the complexes.
3.2. Structure description of the complexes
The molecular structures of 1 and 2 are shown in figures 1 and 2, respectively. The coordi-
nation geometry around V can be described as distorted octahedral, with the phenolate oxy-
gen, imino nitrogen, and enolate oxygen of the hydrazone, and one hydroxy oxygen of the
benzohydroxamate defining the equatorial plane, and with the carbonyl oxygen of the ben-
zohydroxamate and the oxo occupying axial positions. The hydrazone ligands coordinate to
V in a meridional fashion forming five- and six-membered chelate rings with bite angles of
74.95(10)° and 83.84(11)° for 1, and 74.99(12)° and 84.17(13)° for 2. The dihedral angles
between the aromatic rings of the hydrazone ligands are 1.9(2)° for 1 and 39.4(3)° for 2.
The displacements of V in 1 and 2 from the equatorial mean planes toward the apical oxo
are 0.281(1) and 0.284(1) Å, respectively. The hydrazone ligands are coordinated in their
dianionic form, which is evident from the N2–C8 and O2–C8 bond lengths. The abnormal
bond values indicate the presence of the enolate form of the ligand amide groups. The
V–O, V–N and V=O bonds are within normal ranges and are similar to those observed in
similar vanadium complexes [20, 21]. The distances between V and the hydroxy oxygen are
much longer than the other bonds, caused by the trans effect of the oxo.
In the crystal structure of 1, the molecules are linked through intermolecular N3–H3⋯N2
hydrogen bonds [N3–H3 = 0.90(1) Å, H3⋯N2i = 2.09(1) Å, N3⋯N2i = 2.990(4) Å,
N3–H3⋯N2i = 173(5)°; symmetry code for i: x, 1/2−y, −1/2 + z] to form chains along the
c-axis (figure 3). In the crystal structure of 2, adjacent molecules are linked through two
intermolecular N–H4⋯N3 hydrogen bonds [N4–H4 = 0.90(1) Å, H4⋯N3ii = 1.86(2) Å,
N4⋯N3ii = 2.749(5) Å, N4–H4⋯N3ii = 168(6)°; symmetry code for ii: 2 − x, −y, 1 − z]
(figure 4).
3.3. IR and UV–vis spectra
The hydrazones show stretching bands attributed to C=O, C=N, C–OH and NH at
1645–1672, 1619–1625, 1175–1183, and 3210–3230 cm−1, respectively [22]. For the

Vanadium(V) hydrazone complexes
2083
complexes, absence of the bands characteristic of N–H and C=O groups indicates enoliza-
tion of the hydrazone ligands and coordination through deprotonated enolic oxygens. The
V=O stretches occur as single sharp bands at 978 cm−1 for 1 and 973 cm−1 for 2 [23]. The
strong bands indicative of C=N groups of the complexes are located at 1605 cm−1 for 1 and
1610 cm−1 for 2 [24]. The weak peaks observed at low wavenumbers (420–600 cm−1) may
be attributed to V–O and V–N vibrations.
The UV–vis spectra of the complexes were recorded in acetonitrile. In the UV–vis
region, 1 shows bands at 230, 274, 346, and 433 nm, and 2 shows bands at 223, 262, 341,
and 445 nm (figure 5). The weak bands at 390–500 nm were attributed to ligand-to-metal
charge transfer (LMCT) from the pπ orbital on the phenolate oxygen to the empty d orbitals
of the vanadium ion [25]. The bands at 310–350 nm were assigned to n–π* transitions [26].
The intense bands at 250–280 nm were assigned to intra-ligand π–π* transitions [26].
Table 3. Catalytic oxidation of olefins catalyzed by 1 and 2.a
Substrate
Product
Conversion (%)b
1
2
72.0
75.4
1
2
68.2
65.3
1
2
77.8
82.0
1
2
87.2
90.3
1
2
85.5
88.0
1
2
92.7
95.6
1
2
98.3
99.1
1
2
95.5
93.0
aThe molar ratio of catalyst : substrate : TBHP is 1 : 300 : 1000. The reactions were performed in mixture of CH3OH/CH2Cl2 (V:
V = 6 : 4; 1.5 mL).
bThe GC conversion (%) was measured relative to the starting substrate after 1 h.

2084
A.-M. Li
3.4. Catalytic oxidation results
Vanadium complexes often show interesting catalytic properties [27–30]. When compared
to reported species, the present two complexes have excellent performance. The catalytic
results are listed in table 3. Effective epoxide yields and 100% selectivity were observed for
all aliphatic and aromatic substrates. In general, oxidation of aromatic substrates gave the
corresponding epoxides in over 80% yields, while in oxidation of aliphatic substrates the
conversion is lower than 80%. Based on this consideration, isolated double bonds are less
reactive than the conjugated ones, in agreement with those reported previously [31–33].
When comparing the results of 1 and 2, they are similar. This indicates that the slight differ-
ence of the hydrazone ligands is not essential for influence of the catalytic properties.
4. Conclusion
Two structurally similar vanadium complexes with hydrazone and benzohydroxamate
ligands have been prepared and structurally characterized by single crystal X-ray structure
determination and infrared spectra. The hydrazones coordinate to V through the phenolate
oxygen, imino nitrogen, and enolate oxygen with vanadium in octahedral coordination
geometry. The complexes are effective catalysts for oxidation of aliphatic and aromatic
olefins.
Supplementary material
CCDC 988425 (1) and 988426 (2) contain the supplementary crystallographic data for the
complex. These data can be obtained free of charge via http://www.ccdc.ac.uk/conts/retriev
ing.html, or from the Cambridge Crystallographic Data Center, 12 Union Road, Cambridge
CB2 1EZ, UK; Fax: (+44)1223-336-033; or Email: deposit@ccdc.cam.ac.uk.
Acknowledgement
The author greatly acknowledges Jingchu University of Technology for financial support.
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