Synthesis, X-Ray Crystal Structure, and Catalytic Epoxidation Property of an Oxovanadium(V) Complex with Hydrazone and Ethyl Maltol Ligands

Article abstract of DOI:10.1134/S0022476619080092

A new oxovanadium(V) complex [VO(BH)(EM)], where BH = N′-(2-hydroxybenzylidene)-4-trifluoromethylbenzohydrazide, EM = ethyl maltol, is synthesized and fully characterized based on the elemental analysis, FT-IR, UV-Vis, and ¹H NMR spectra. The c

Full text of DOI:10.1134/S0022476619080092

ISSN 0022-4766, Journal of Structural Chemistry, 2019, Vol. 60, No. 8, pp. 1299-1305. © Pleiades Publishing, Inc., 2019.
Text © The Author(s), 2019, published in Zhurnal Strukturnoi Khimii, 2019, Vol. 60, No. 8, pp. 1351-1357.
SYNTHESIS, X-RAY CRYSTAL STRUCTURE,
AND CATALYTIC EPOXIDATION PROPERTY
OF AN OXOVANADIUM(V) COMPLEX
WITH HYDRAZONE AND ETHYL MALTOL LIGANDS
1,2
Y. M. Cui , Y. Q. Wang , X. X. Su , H. Huang ,
2
2
2
1
and P. Zhang *
A
new oxovanadium(V) complex [VO(BH)(EM)], where BH = N′-(2-hydroxybenzylidene)-4-
trifluoromethylbenzohydrazide, EM = ethyl maltol, is synthesized and fully characterized based on the
1
elemental analysis, FT-IR, UV-Vis, and H NMR spectra. The complex is also characterized by single
crystal X-ray diffraction, which indicates that the V atom adopts the octahedral coordination. The
hydrazone ligand behaves as a tridentate ligand, and the ethyl maltol ligand behaves as a bidentate ligand.
The catalytic epoxidation property of the complex is investigated.
DOI: 10.1134/S0022476619080092
Keywords: oxovanadium complex, hydrazone, ethyl maltol, X-ray crystal structure, catalytic epoxidation.
INTRODUCTION
Schiff bases are a kind of interesting ligands in coordination chemistry. Metal complexes with Schiff bases have
attracted remarkable attention due to their facile synthesis and special biological, catalytic, and industrial applications [1-5].
Catalytic epoxidation of olefins is an important reaction in chemistry. Many transition metal complexes are active catalysts
for this process [6-10]. However, among the complexes, vanadium ones seem to be more interesting because of their
excellent catalytic ability in the oxidation of olefins and sulfides [11-13]. In this paper, a new vanadium(V) complex derived
from N′-(2-hydroxybenzylidene)-4-trifluoromethylbenzohydrazide (BH, Scheme 1) and ethyl maltol (EM) is prepared and
studied for its catalytic epoxidation property on cyclooctene.
Scheme 1. Hydrazone BH.
1
Engineering Research Center for Clean Production of Textile Printing and Dyeing, Ministry of Education, Wuhan
2
Textile University, Wuhan, P. R. China; *zhping654321@163.com. School of Environmental Engineering, Wuhan Textile
University, Wuhan, P. R. China. Original article submitted February 2, 2019; revised February 21, 2019; accepted March 15,
2019.
0022-4766/19/6008-1299 © 2019 by Pleiades Publishing, Ltd.
1299

EXPERIMENTAL
Materials and methods. 4-Trifluoromethylbenzohydrazide was prepared as described in the literature [14].
Salicylaldehyde and VO(acac)2 were purchased from Alfa Aesar and used as received. Reagent grade solvents were used as
received. Microanalyses of the complexes were performed with a Vario EL III CHNOS elemental analyzer. Infrared spectra
were recorded as KBr pellets with an FTS-40 spectrophotometer. Electronic spectra were recorded on a Lambda 900
1
spectrometer. H NMR spectra were recorded on a Bruker spectrometer at 300 MHz. The catalytic reactions were followed
by gas chromatography on an Agilent 6890A chromatograph equipped with an FID detector and a DB5-MS capillary column
(30 m × 0.32 mm, 0.25 μm). Molar conductance measurements were made by means of a Metrohm 712 conductometer in
acetonitrile.
Synthesis of the hydrazone compound BH. 4-Trifluoromethylbenzohydrazide (10 mmol, 2.04 g) and
salicylaldehyde (10 mmol, 1.22 g) were refluxed in methanol (50 mL). The reaction was continued for 1 h in an oil bath
during which a solid compound was separated. It was filtered and washed with cold methanol. The crude product was
–1
recrystallized from methanol and dried over anhydrous CaCl2. Yield: 2.75 g (89%). IR data (KBr pellet, cm ): 3337 ν(O–H),
3223 ν(N–H), 1655 ν(C=O), 1613 ν(C=N). UV-Vis data in methanol (nm): 275, 350, 430. Analysis: found (%): C 58.31,
1
6
H 3.72, N 9.23; calculated for C15H11F3N2O2 (%): C 58.45, H 3.60, N 9.09. H NMR (300 MHz, d -DMSO): δ 12.26 (s, 1H,
OH), 11.53 (s, 1H, NH), 8.72 (s, 1H, CH=N), 7.92 (d, 2H, ArH), 7.81 (d, 2H, ArH), 7.63 (d, 1H, ArH), 7.54 (t, 1H, ArH),
7.10-7.01 (m, 2H, ArH).
Synthesis of the [VO(BH)(EM)] complex. The hydrazone ligand (1.0 mmol, 0.31 g), ethyl maltol (1.0 mmol,
0.14 g), and VO(acac)2 (1.0 mmol, 0.26 g) were refluxed in methanol (30 mL). The reaction was continued for 1 h in an oil
bath to give a deep brown solution. Single crystals of the complex formed during slow evaporation of the reaction mixture in
air. The crystals were isolated by filtration, washed with cold methanol, and dried over anhydrous CaCl2. Yield: 0.29 g
–1
(57%). IR data (KBr pellet, cm ): 1607 ν(C=N), 1371 ν(C–Ophenolate), 1162 ν(N–N), 955 ν(V=O). UV-Vis data in acetonitrile
–3
–1
2
–1
(nm): 275, 320, 400. Molar conductance (10 M, acetonitrile): 20 Ω ·cm ·mol . Analysis: found (%): C 51.30, H 3.27,
1
6
N 5.61; calculated for C22H16F3N2O6V (%): C 51.58, H 3.15, N 5.47. H NMR (300 MHz, d -DMSO): δ 9.26 (s, 1H, CH),
8.46 (s, 1H, CH=N), 8.06 (d, 2H, ArH), 7.85 (m, 3H, ArH), 7.60 (d, 1H, ArH), 7.10 (t, 1H, ArH), 6.90 (t, 1H, ArH), 6.70 (t,
1H, ArH), 3.02 (t, 3H, CH2), 1.33 (s, 3H, CH3).
Crystal structure determination. Data were collected on a Bruker SMART 1000 CCD area diffractometer using
a graphite monochromator and MoK radiation (λ = 0.71073 Å) at 298(2) K. The data were corrected with SADABS
α
2
programs and refined on F with the Siemens SHELXL software [15, 16]. The structure was solved by direct methods and
difference Fourier syntheses. All non-hydrogen atoms were refined anisotropically. The hydrogen atoms were placed in
calculated positions and included in the last cycles of the refinement. Crystal data and details of the data collection and
refinement are listed in Table 1. Selected coordinate bond lengths and angles are listed in Table 2.
Catalytic epoxidation process. A mixture of cyclooctene (2.76 mL, 20 mmol), acetophenone (internal reference),
and the complex as the catalyst (0.05 mmol) was stirred and heated up to 80 °C before the addition of aqueous tert-butyl
hydroperoxide (TBHP; 70% w/w, 5.48 mL, 40 mmol). The mixture was initially an emulsion, but two phases became clearly
visible as the reaction progressed (a colorless aqueous one and a yellowish organic one). The reaction was monitored for 5 h
with the withdrawal and analysis of organic phase aliquots (0.1 mL) at required times. Each withdrawn sample was mixed
with 2 mL of diethylether, treated with a small quantity of MnO2, and then filtered through silica and analyzed by GC.
RESULTS AND DISCUSSION
Synthesis. The hydrazone compound and the complex were synthesized in a facile and analogous way (Scheme 2).
3+
Hydrazone acts as a tridentate dianionic ONO donor ligand toward the VO core. The oxovanadium complex was
obtained from a refluxing mixture of hydrazone, ethyl maltol, and VO(acac)2 in a 1:1 molar proportion in methanol. The
1300

Scheme 2. Synthesis of hydrazone and the complex.
TABLE 1. Crystallographic Data for the Complex
Parameters
Values
Empirical formula
Formula weight
Crystal size, mm
Crystal system
Space group
C22H16F3N2O6V
512.31
0.30×0.30×0.28
Triclinic
P1
a, b, c, Å
10.153(2), 10.790(2), 11.320(2)
114.884(2), 92.435(2), 94.069(2)
α, β, γ, deg
3
V, Å
1118.5(3)
2
Z
3
1.521
ρcalc, g/cm
–1
0.510
μ, mm
F(000)
θ range for data collection, deg
Index ranges
520
2.72-25.50
–12 ≤ h ≤ 12, –13 ≤ k ≤ 11, –13 ≤ l ≤ 13
Measured / Independent reflections
Observed reflections (I > 2σ(I ))
Parameters
10072 / 4082
3465
308
Restraints
0
Data completeness
0.981
Maximum and minimum transmission
Final R indices [I > 2σ(I )]
R indices (all data)
0.8621 and 0.8704
0.0567, 0.1552
0.0667, 0.1668
1.067
2
GOOF on F
3
Largest difference in peak and hole, e/Å
0.771 and –0.619
complex was isolated as brown single crystals from the reaction mixture by slow evaporation at room temperature. Crystals
of the complex are stable at room temperature and are found to be fairly soluble in most common organic solvents such as
methanol, ethanol, acetonitrile, DMF, and DMSO. The low molar solution conductance of the complex in acetonitrile
indicates its non-electrolyte behavior.
1301

–1
IR and electronic spectra. The IR spectrum of hydrazone shows bands centered at 3223 cm for ν(N–H), 3337 cm
–1
–1
for ν(O–H), and 1655 cm for ν(C=O) [17, 18]. The peaks attributed to ν(N–H) and ν(C=O) are absent in the spectrum of the
complex as the ligand binds in the dianionic form, resulting in losing a proton from the carbohydrazide group. A strong band
–1
observed at 1607 cm for the complex is attributed to ν(C=N), which is located at lower frequencies as compared to the free
–1
–1
hydrazone ligand, viz. 1613 cm [19, 20]. The complex exhibits a characteristic band at 955 cm for the stretching of the
V=O bond [21]. Based on the IR absorption, it is obvious that the hydrazone ligand exists in the uncoordinated form in the
keto-amino tautomer and in the complex inthe imino-enol tautomeric form. This is not uncommon in the coordination of
hydrazone compounds [22-24].
The electronic spectrum of the complex recorded in acetonitrile displays a strong absorption band centered at
400 nm, which is assigned as charge transfer transitions of N(pπ)–Mo(dπ) LMCT. The medium absorption band centered at
320 nm is assigned as charge transfer transitions of O(pπ)–Mo(dπ) LMCT [25, 26].
Description of the structure of the complex. The perspective view of the complex together with the atom
numbering scheme is shown in Fig. 1. The coordination geometry around the V atom reveals a distorted octahedral
environment with the NO5 chromophore. The hydrazone ligand behaves as a dianionic tridentate ligand binding through
TABLE 2. Selected Bond Lengths (Å) and Angles (deg) for the Complex
V1–O1
V1–O3
V1–O6
1.839(2)
2.294(2)
1.578(2)
V1–O2
V1–O4
V1–N1
1.926(2)
1.864(2)
2.087(2)
O6–V1–O1
O1–V1–O4
O1–V1–O2
O6–V1–N1
O4–V1–N1
O6–V1–O3
O4–V1–O3
N1–V1–O3
100.22(13)
105.22(10)
153.46(11)
98.27(11)
158.71(10)
175.64(11)
77.12(8)
O6–V1–O4
O6–V1–O2
O4–V1–O2
O1–V1–N1
O2–V1–N1
O1–V1–O3
O2–V1–O3
98.70(11)
98.85(12)
89.89(9)
84.26(10)
74.85(9)
82.23(10)
80.02(9)
85.53(9)
Fig. 1. ORTEP plots (30% probability level) and numbering scheme for the complex.
1302

phenolate oxygen, enolate oxygen, and imine nitrogen atoms, and occupies three positions in the equatorial plane. The fourth
position of the equatorial plane is occupied by the hydroxyl group of the ethyl maltol ligand. The carbonyl oxygen atom of
the ethyl maltol ligand occupies one axial position of the octahedral coordination, and another axial position is occupied by
the oxo group. The vanadium atom is found to deviate from the mean equatorial planes defined by the four donor atoms by
0.301(1) Å. The V1–O3 bond length is longer than the normal single bond lengths (2.29 Å against 1.9-2.0 Å). This shows
that the carbonyl oxygen atom is loosely attached to the V center. This is due to the trans effect generated by the oxo group
trans to the V1–O3 bond. The remaining V–O and V–N bond lengths are similar to other vanadium(V) complexes [27, 28].
The C8–O2 bond length in the complex is 1.31 Å, which is closer to single bond length rather than C=O double bond length.
However, the shorter length compared to the C–O single bond may be attributed to the extended electron delocalization in the
ligand [26]. A similar shortening of the C8–N2 bond length (1.29 Å instead of normal 1.38 Å) together with the elongation of
the N1–N2 bond length (1.40 Å) also supports the electron cloud delocalization in the ligand system. The hydrazone ligand
forms five- and six-membered chelate rings with the V center. The five-membered metallacycle is quite planar, but the six-
membered metallacycle is clearly distorted. The dihedral angle between the two benzene rings is 4.0(3)°. The trans angles are
in the range 153.46(11)–175.64(11)°, indicating a considerable distortion of the coordination octahedron around the V center.
The crystal packing diagram of the complex is shown in Fig. 2. The adjacent complex molecules are linked by π⋯π
stacking interactions (Table 3).
Catalytic epoxidation results. Before the addition of aqueous TBHP at 80 °C, the complex dissolved completely in
the organic phase. The aqueous phase of the solution was colorless and the organic phase was brown, indicating that the
catalyst is mainly confined in the organic phase. TBHP is mainly transferred into the organic phase under those conditions,
Fig. 2. Molecular packing diagram of the complex, viewed down the c
axis.
TABLE 3. π⋯π Stacking Interactions of the Complex
Cg(I)⋯Cg(J)
Cg(1)⋯Cg(2)
d (Å)
Cg(I)⋯Cg(J)
d (Å)
2.767(3)
4.798(3)
4.944(3)
3.572(3)
3.973(3)
Cg(1)⋯Cg(3)
4.682(3)
4.361(3)
3.709(3)
4.963(3)
#1
#2
Cg(1)⋯Cg(4)
Cg(1)⋯Cg(2)
Cg(3)⋯Cg(3)
Cg(4)⋯Cg(5)
Cg(1)⋯Cg(5)
Cg(2)⋯Cg(3)
Cg(4)⋯Cg(3)
#3
#3
#1
#3
#4
#1
Symmetry codes: 2–x, 1–y, 1–z; 1–x, 1–y, 1–z; 2–x, –y, –z; x, 1+y, z.
#2
#3
#4
1303

Fig. 3. Kinetic monitoring of cis-cyclooctene
epoxidation with TBHP–H2O in the presence of the
complex.
and for that reason the reactant and products in the organic layer were analyzed. Cyclooctene and cyclooctene oxide are not
significantly soluble in water, therefore the determination of the epoxide selectivity (epoxide formation/cyclooctene
conversion) is expected to be accurate. As for the cyclooctene epoxidation with aqueous TBHP, with no extra addition of
organic solvents, the present study shows the effective activity. Kinetic profiles of the complex as the catalyst are presented
in Fig. 3. No induction time was observed. The cyclooctene conversion for the complex is 91% after 5 h, and the selectivity
towards cyclooctene oxide is 61%. A possible mechanistic consideration involves the coordination of TBHP as a neutral
molecule, with the O–H⋯O hydrogen bond (Scheme 3).
Scheme 3. Proposed mechanism for the catalytic process of the complex.
CONCLUSIONS
In summary, a new oxovanadium(V) complex with tridentate hydrazone and ethyl maltol ligands has been
synthesized and characterized. The V atom of the complex is in the octahedral coordination. The complex can catalyze the
epoxidation of cyclooctene with a high conversion.
FUNDING
This work was supported by the Collaborative Innovation Plan of Hubei Province for Key Technology of Eco-
Ramie Industry.
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ADDITIONAL INFORMATION
CCDC number 1824512 contains the supplementary crystallographic data for the complex. These data can be
obtained free of charge via http://www.ccdc.cam.ac.uk/conts/retrieving.html, or from the Cambridge Crystallographic Data
Center, 12, Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336 033; or e-mail: deposit@ccdc.cam.ac.uk.
CONFLICT OF INTERESTS
The authors declare that they have no conflict of interests.
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