Synthesis, structural characterization and catalytic potential of oxidovanadium(IV) and dioxidovanadium(V) complexes with thiazole-derived NNN-donor ligand

Article abstract of DOI:10.1080/00958972.2018.1465941

Reaction between the tridentate NNN donor ligand, (E)-2-(2-(1-(pyridin-2-yl)ethylidene)hydrazinyl)benzo[d]thiazole (HL), and V₂O₅ in ethanol gave a new vanadium(V) complex, [VO₂L] (1), while the similar reaction by using [V^IVO(acac)₂] as the metal source gave two different types of crystals related to compounds [VO₂L] (1) and [V^IVO(acac)L] (2). The molecular structures of the complexes were determined by single-crystal X-ray diffraction and spectroscopic characterization was carried out by means of FT-IR, UV–vis and NMR experiments as well as elemental analysis. The oxidovanadium(IV) and dioxidovanadium(V) species were used as catalyst precursors for olefin oxidation in the presence of hydrogen peroxide (H₂O₂) as an oxidant. Under similar experimental conditions, the presence of 1 resulted in higher oxidation conversion than 2.

Full text of DOI:10.1080/00958972.2018.1465941

Journal of Coordination Chemistry
ISSN: 0095-8972 (Print) 1029-0389 (Online) Journal homepage: http://www.tandfonline.com/loi/gcoo20
Synthesis, structural characterization and
catalytic potential of oxidovanadium(IV) and
dioxidovanadium(V) complexes with thiazole-
derived NNN-donor ligand
Massomeh Ghorbanloo, Rahman Bikas, Solmaz Jafari, Marta S. Krawczyk &
Tadeusz Lis
To cite this article: Massomeh Ghorbanloo, Rahman Bikas, Solmaz Jafari, Marta S.
Krawczyk & Tadeusz Lis (2018): Synthesis, structural characterization and catalytic potential of
oxidovanadium(IV) and dioxidovanadium(V) complexes with thiazole-derived NNN-donor ligand,
Journal of Coordination Chemistry, DOI: 10.1080/00958972.2018.1465941
To link to this article: https://doi.org/10.1080/00958972.2018.1465941
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Journal of Coordination Chemistry, 2018
https://doi.org/10.1080/00958972.2018.1465941
Synthesis, structural characterization and catalytic potential
of oxidovanadium(IV) and dioxidovanadium(V) complexes
with thiazole-derived NNN-donor ligand
Massomeh Ghorbanlooa, Rahman Bikasb, Solmaz Jafaria, Marta S. Krawczykc and
Tadeusz Lisd
b
adepartment of Chemistry, university of Zanjan, Zanjan, iran; faculty of science, department of Chemistry,
c
imam Khomeini international university, Qazvin, iran; faculty of Pharmacy, department of analytical
d
Chemistry, Wroclaw medical university, Wroclaw, Poland; faculty of Chemistry, university of Wroclaw,
Wroclaw, Poland
ABSTRACT
ARTICLE HISTORY
received 31 october 2017
accepted 5 march 2018
Reaction between the tridentate NNN donor ligand, (E)-2-(2-(1-
(pyridin-2-yl)ethylidene)hydrazinyl)benzo[d]thiazole (HL), and V2O5 in
ethanol gave a new vanadium(V) complex, [VO2L] (1), while the similar
reaction by using [VIVO(acac)2] as the metal source gave two different
types of crystals related to compounds [VO2L] (1) and [VIVO(acac)L]
(2). The molecular structures of the complexes were determined by
single-crystal X-ray diffraction and spectroscopic characterization was
carried out by means of FT-IR, UV–vis and NMR experiments as well as
elemental analysis. The oxidovanadium(IV) and dioxidovanadium(V)
species were used as catalyst precursors for olefin oxidation in the
presence of hydrogen peroxide (H2O2) as an oxidant. Under similar
experimental conditions, the presence of 1 resulted in higher
oxidation conversion than 2.
KEYWORDS
oxidovanadium(iV);
dioxidovanadium(V);
thiazole-hydrazone; h2o2;
oxidation
CONTACT massomeh Ghorbanloo
m_ghorbanloo@yahoo.com
supplemental data for this article can be accessed at https://doi.org/10.1080/00958972.2018.1465941.
© 2018 informa uK limited, trading as taylor & francis Group

2
M. GHORBANLOO ET AL.
1. Introduction
Vanadium is a versatile metal ion which has been widely used in coordination chemistry in
a variety of oxidation states, from −3 to +5. The +4 and +5 oxidation states are the most
stable ones under aerobic conditions [1]. The +5 oxidation state of vanadium has received
considerable attention, not only due to its interesting chemistry associated with its ability
to exist in three different motifs viz., VO3+, V2O34+ and VO2+ but also due to its involvement
in many catalytic processes [2]. The chemistry of vanadium compounds in the +4 oxidation
state, also, has attracted particular attention for their interesting spectroscopic [3], biological
[4], mechanochemical [5] and many other activities and characteristics [6]. High-valent vana-
dium complexes have been considered as new versatile catalytic reagents for a wide range
of oxidation reactions, such as oxidation of olefins and alcohols [7], benzene alkylaromatic
compounds [8] and sulfides [9].
Vanadium easily switches between +4 and +5 oxidation states and the relative stability
of its species depends upon the basicity of the ligands and the pH of the reaction medium
[10]. Our work in this area led us to design monobasic tridentate NNN donor ligands to
prepare the respective both vanadium(IV) and vanadium(V) at ambient condition.
In general, N, O donor ligands (such as hydrazones) promote the aerial oxidation of oxi-
dovanadium(IV) complexes to highly stable oxidovanadium(V) compounds. Hydrazones
have a tendency to stabilize the vanadium in its highest oxidation state [11, 12] and, for that
reason, they seem to be good candidates for catalytic oxidation studies because of their
ability to resist oxidation [13]. Thiazoles are the significant class among heterocyclic com-
pounds containing both S and N atoms. Metal complexes of these ligands have attracted
much attention due to their appealing physico-chemical properties [14–16]. The combina-
tion of the thiazole moiety with hydrazides provides intriguing chemical properties and
catalytic activity for the resulting condensation products [17]. Herein, we report the synthe-
ses, crystal structures, spectroscopic properties and catalytic activity of two new vanadium
complexes of the tridentate Schiff base ligand, (E)-2-(2-(1-(pyridin-2-yl)ethylidene)hydrazi-
nyl)-benzo[d]thiazole (HL).
2. Experimental
2.1. Materials and instruments
All chemicals and solvents were purchased in the highest possible purity from Merck or
Sigma and used as received without further purification. FT-IR spectra were recorded in KBr
disks with a Bruker FT–IR spectrophotometer. The elemental analyses (carbon, hydrogen
and nitrogen) were obtained from a Carlo ERBA Model EA 1108 analyzer. The atomic absorp-
tion analysis was carried out using Varian Spectra AA 220 equipment. 1H and 13C NMR spectra
of the ligand in DMSO-d6 solution were recorded on a Bruker 250 and 62.9 MHz spectrometer,
respectively, and chemical shifts are indicated in ppm relative to tetramethylsilane (TMS).
The reaction products of oxidation were determined and analyzed using an HP Agilent 6890
gas chromatograph equipped with a HP-5 capillary column (phenyl methyl siloxane
30 m × 320 μm × 0.25 μm).

JOURNAL OF COORDINATION CHEMISTRY
3
2.2. Synthesis of (E)-2-(2-(1-(pyridin-2-yl)ethylidene)hydrazinyl)benzo[d]thiazole
(HL)
(E)-2-(2-(1-(pyridin-2-yl)ethylidene)hydrazinyl)benzo[d]thiazole (HL) was synthesized by the
reaction of 2-acetylpyridine (0.121 g, 1.00 mmol) with 2-hydrazinolbenzothiazole (0.165 g,
1.00 mmol) in methanol (30 mL). The mixture of mentioned compounds was refluxed for
6 h. The solution was then evaporated on a steam bath and cooled to room temperature.
The resulting precipitate was separated and filtered off, washed with 3 mL of cooled meth-
anol and recrystallized from ethanol. Yield: 80% (0.21 g). Anal. Calcd for C14H12N4S
(MW = 268.34 g mol−1): C, 62.66; H, 4.51; N, 20.88%. Found: C, 62.63; H, 4.69; N, 20.86%.
Selected FT-IR (KBr, cm−1): 3443 (br, w), 3066 (w), 2939 (w), 2860 (w), 1610 (vs), 1565 (s), 1450
(s), 1375 (w), 1277 (m), 1127 (s), 1050 (m), 991 (m), 878 (s), 755 (s), 735 (m), 676 (m), 565 (m),
497 (w). 1H NMR (250.13 MHz, DMSO-d6, 25°C, TMS, ppm): δ = 2.3 (3H, s), 4.11 (1H, NH), 7.09
(2H, m), 7.28 (2H, m), 7.83 (2H, d, J = 7.25), 8.04 (1H, d, J = 7.5), 8.57 (1H, d, J = 5.50). 13C NMR
(62.9 MHz, DMSO-d6, 25 °C, TMS, ppm): δ = 13.50, 116.53, 120.12, 122.78, 125.26, 126.02,
127.32, 128.76, 138.17, 143.64, 147.86, 153.28, 154.91, 167.16 ppm. UV–vis in MeOH, λmax
,
nm (ε, M−1 cm−1): 226 (3290), 339 (1780), 431 (640).
2.3. Synthesis of dioxido-(2-((1-(pyridin-2-yl)ethylidene)hydrazono)-2,3-dihydro-
1,3-benzothiazolato)-vanadium(V); [VO2L] (1)
Complex 1 was synthesized by the reaction of thiazole-hydrazone ligand, HL (0.268 g,
1.00 mmol), and V2O5 (0.091 g, 0.50 mmol) in ethanol solvent. Mentioned amounts of mate-
rials were placed in the main arm of a branched tube. Ethanol was carefully added to fill the
arms, the tube was sealed and the reagents containing arm immersed in an oil bath at 60
°C while the other arm was kept at ambient temperature. After a week, block-red crystals of
1 were deposited in the cooler arm. Yield based on V2O5: 60% (0.21 g). Anal. Calcd for
C14H11N4O2SV (MW = 350.27 g mol−1): C, 48.01; H, 3.17; N, 16.00; V, 14.54%. Found: C, 48.03;
H, 3.21; N, 15.97; V, 14.47%. Selected FT-IR (KBr, cm−1): 3446 (br), 2927 (w), 2856 (w), 1600 (s),
1560 (w), 1506 (w), 1428 (s), 1328 (w), 1281 (m), 1237 (m), 1152 (m), 1078 (w), 945 (m), 872
(w), 747 (m), 695 (w), 660 (w), 554 (w), 443 (w). 1H NMR (250.13 MHz, DMSO-d6, 25 °C, TMS,
ppm): δ = 2.57 (3H, s), 7.07 (1H, d), 7.30 (2H, m), 7.67 (2H, m), 8.07 (1H, d), 8.28 (1H, d), 8.73
(1H, d). UV–vis in MeOH, λmax, nm (ε, M−1 cm−1): 227 (16,140), 284 (10,080), 471 (7550).
2.4. Synthesis of (acetylacetonato)-oxido-(2-((1-(pyridin-2-yl)ethylidene)-
hydrazono)-2,3-dihydro-1,3-benzothiazolato)-vanadium(IV); [VO(acac)L] (2)
Complex 2 was synthesized by a similar method to that described for 1, but, in this case
VO(acac)2 was used instead of V2O5 as the vanadium source. In fact, thiazole-hydrazone
ligand, HL (0.268 g, 1.00 mmol), and VO(acac)2 (0.133 g, 0.50 mmol) were used. The reaction
was done in ethanol and the red plate-shaped crystals of 2 together with some pieces of
red block-shaped crystals were collected in the cooler arm after about one week. In this
reaction, two kinds of crystals (block and plate red) were obtained which various analyses
(like FT-IR, NMR, UV–vis and single crystal X-ray analysis) indicated that the block-red crystals
are the same as the crystals of 1. The reaction was repeated several times in various purity
of ethanol solvent. The results of reaction indicated that the crystals of both 1 and 2

4
M. GHORBANLOO ET AL.
simultaneously form in the branched tube; however, their ratio is controlled by the purity
of solvent. Complex 2 is the sole product in absolute ethanol (Merck) while in ethanol with
higher amount of water (ethanol 96%), 1 is the main product of this reaction. Anal. Calcd for
C19H18N4O3SV (MW = 433.37 g mol−1): C, 52.66; H, 4.19; N, 12.93; V, 11.75%. Found: C, 52.36;
H, 4.31; N, 12.99; V, 11.74%. Selected FT-IR (KBr, cm−1): 3439 (br), 2927 (w), 2858 (w), 1750 (w),
1600 (s), 1564 (w), 1517 (m), 1428 (s), 1399 (w), 1331 (w), 1279 (w), 1238 (m), 1149 (s), 1081
(m), 1005 (w), 952 (m), 776 (m), 750 (m), 660 (w), 553 (w), 432 (w). UV–vis (MeOH): λmax, nm
(ε, M−1 cm−1): 227 (15,550), 288 (7800), 493 (6500).
2.5. General oxidation procedure
Oxidation reactions were performed under air atmosphere in 2 mL of solvent and aqueous
H2O2 (30%) was used as oxidant. In a typical experiment, a mixture of catalyst precursor (1
or 2), 2.0 mL solvent and 0.5 mmol of substrate were placed in a 25 mL round bottomed
glass flask. The mixture was heated to the desired temperature and after addition of H2O2,
the solution was placed to continue the reaction in the stable temperature. At appropriate
intervals, aliquots were removed and analyzed immediately by GC. The oxidation products
were identified by comparison of their retention times with those of authentic samples.
2.6. X-ray crystallography
X-ray diffraction data for 1 and 2 were collected at 120 and 150 K, respectively, by the ω-scan
technique on Kuma KM4 CCD diffractometers with a Sapphire 2 CCD detector using graph-
ite-monochromated MoKα radiation (λ = 0.71073 Å). Data collection, cell refinement, and
data reduction and analysis were carried out with CrysAlis CCD and CrysAlis RED, respectively
(Table 1) [18]. Analytical absorption corrections were applied to the data with the use of
CrysAlis RED. The structures were solved by direct methods using SHELXS-97 and refined by
a full-matrix least squares technique on F2 with SHELXL-2014 with anisotropic thermal param-
eters for all non-H atoms [19]. All H atoms were found in difference Fourier maps, and in the
final refinement cycles the C-bonded H atoms were repositioned in their calculated positions
and refined using a riding model, with C–H = 0.95 Å, and with Uiso(H) = 1.2Ueq(C) for aromatic
CH. The structure plots were prepared with DIAMOND [20].
Crystallographic data for the structural analyses have been deposited with the Cambridge
Crystallographic Data Center, No. 1573423 and 1573424 for 1 and 2, respectively. Copies of
this information may be obtained free of charge from: The Director, CCDC, 12 Union Road,
Cambridge, CB2 1EZ, UK. Fax: +44 1223 336 033, deposit@ccdc.cam.ac.uk, or www: www.
ccdc.cam.ac.uk.
3. Results and discussion
3.1. Synthesis and characterization
The tridentate Schiff base ligand, HL, was derived from the reaction of 2-hydrazinylben-
zo[d]-thiazole with 2-acetylpyridine in methanol under refluxing condition. 1H and 13C NMR
spectra of HL in DMSO-d6 confirmed the proposed structure for the ligand. In the 1H NMR
spectrum of HL, signals at δ 2.3 and 4.11 ppm are assigned to the –CH3 and NH– groups,

JOURNAL OF COORDINATION CHEMISTRY
5
Table 1. Crystal data and structure refinement parameters for 1 and 2.
Identification code
1
2
net formula
formula weight
radiation
T (K)
C
14h11n4o2sV
350.27
moKα
C19h18n4o3sV
433.37
moKα
150
120
Crystal size (mm)
Crystal shape (color)
Crystal system
space group
a (Å)
0.55 × 0.30 × 0.18
0.40 × 0.16 × 0.08
Plate, red
orthorhombic
Block, red
monoclinic
P21/c
15.230(4)
13.414(3)
13.611(3)
96.09(2)
2765.0(11)
Pbca
8.003(2)
15.350(6)
32.120(12)
90
3946(2)
8
1.459
0.64
analytical
1784
2.8–26.5
4079
25,068
2465
b (Å)
c (Å)
β (°)
Volume (Å3)
Z
4
density (mg m−3
)
1.683
0.88
analytical
1424
3.0–28.5
6998
36,407
5383
absorption coefficient (mm−1
absorption correction
F(0 0 0)
)
Θ range (°)
independent reflections
measured reflections
reflections with I > 2σ(I)
index ranges (hkl)
Rint
−20 < h < 20; −10 < k < 17; −18 < l < 18
−9 < h < 10; −18 < k < 18; −40 < l < 34
0.058
0; 399
1.032
0.043
0.117
0.67
0.153
0; 256
1.002
0.061
0.150
0.36
restraints; parameters
Goodness of fit on F2
R[F2 > 2σ(F2)]
wR(F2)
max. electron density (e·Å−3
)
)
min. electron density (e·Å−3
−0.52
−0.50
respectively. The reaction of V2O5 and VO(acac)2 with HL in ethanol afforded [VO2L] (1) and
[VO(acac)L] (2) as red block-shaped and red plate-shaped crystalline compounds, respec-
tively (see Equations 1 and 2). Complex 1 was obtained as the sole product by the reaction
of HL and V2O5. In the case of VO(acac)2, 2 was not the sole product because some crystals
of 1 were formed beside the crystals of 2; their amount was dependent to the nature of the
solvent. It is seen that the amount of 1 in this reaction is dependent upon the purity of
ethanol and the presence of water in the solvent. However, in absolute ethanol it is possible
to obtain 2 as the sole product. In this reaction the formation of the dioxidovanadium(V)
core conceivably takes place according to the overall Equation (3). In this reaction the V(IV)
is oxidized to V(V) by aerial oxygen and the presence of aerial oxygen is also an effective
parameter in controlling the formation of 2 as sole product.
[
]
V2O5 + 2HL → 2 VO2L + H2O
(1)
(2)
(3)
VO(acac) + HL → [VO(acac)L] + Hacac
2
[
]
1
2VO(acac) + 2HL + ∕ O + H O → 2 VO L + 4Hacac
2
2
2
2
2

6
M. GHORBANLOO ET AL.
Figure 1. uV–vis spectra of hl (black), 1 (0.57 × 10−5 m) (blue) and 2 (0.57 × 10−5 m) (red).
Spectroscopic studies confirmed that both red, block-shaped crystals obtained from two
different metal sources (V2O5 and VO(acac)2) are the same. Checking the unit cell parameters
of these crystals was another tool which confirmed this finding, since the unit cell parameters
of the crystals obtained from two methods were the same. In the 51V NMR spectra of 1 and
2 a single signal related to the vanadium ions is observed at 533.8 and 535.8 ppm, respec-
tively [21]. In the case of 1, due to the presence of two independent V(V) molecules in the
crystal structure, two peaks were expected. However, as mentioned in section 3.2., the coor-
dination spheres around the V(V) ions in these molecules are almost the same and there are
not considerable differences in the electronic properties of vanadium ions in these
molecules.
A comparison of the FT-IR spectra of the complexes (Figure S1) with the ligand provides
some evidences for the coordination of the ligand to the vanadium core. On complexation,
the absence of the νNH band of the ligand shows coordination of the ligand as a mono-anionic
ligand, (L)−1. The infrared spectra of the complexes display an absorption band at 1600 cm−1
which can be assigned to the C=N stretching frequency of the coordinated ligand [22]. The
red shift of the strong C=N stretch in the FT-IR spectra of complexes indicates the C=N group
of the Schiff base ligand is coordinated to the metal ion [23]. The FT-IR spectrum of 2 exhibits
one sharp band at 952 cm−1 due to the ν(V=O) mode [24]. Complex 1 exhibits one sharp
band at 945 cm−1 due to ν(V=O) and a band at ca. 872 cm−1 due to the υ[V–(μ-O)–V] vibrations
[25].
The electronic absorption bands of the ligand and complexes were recorded in methanol
solution (Figure 1). Bands at 431 and 339 nm are attributed to the n→π* and the band at
226 nm is due to π→π* transitions. These bands are shifted in the UV–vis spectrum of the
complexes which confirm the coordination of hydrazone ligand to the vanadium ion. For 1
and 2, high energy bands in the range 471 and 493 nm are assigned to the ligand to metal
charge transfer (LMCT) transitions (ligand to an empty d orbital of the vanadium ion).

JOURNAL OF COORDINATION CHEMISTRY
7
Figure 2. (a) molecular structure of components in the asymmetirc unit of the crystal 1; (b) overlay of two
molecules. thermal ellipsoids are drawn in 30% probability level.
3.2. Description of the crystal structure of [VO2L] (1)
Single crystal X-ray analysis indicates that 1 is a molecular complex of dioxidovanadium(V).
The molecular structure of 1 is shown in Figure 2(a) and selected bond lengths and angles
are given in Table 2. The asymmetric part of the unit cell of 1 consists of two independent
complex molecules (labeled as molecules A and B) in which one of them (A) may be consid-
ered as a dimeric species (similar to dioxidovanadium(V) complexes presented in our previous
report [26]). Comparison of geometries of these two components is presented in Figure 2(b)
indicating significant similarity between them, with the exception of some differences
observed in the position of oxido ligands toward the vanadium(V) center. In the presented

8
M. GHORBANLOO ET AL.
Table 2. selected bond lengths (Å) and angles (°) in 1.
Bond
Length
Bond
Angle
V1a–o2a
V1a–o1a
V1a–n2a
1.6116(17)
1.6574(16)
2.0384(18)
2.0920(18)
2.1405(19)
2.4218(17)
1.751(2)
o2a–V1a–o1a
o2a–V1a–n2a
o1a–V1a–n2a
o2a–V1a–n1a
o1a–V1a–n1a
n2a–V1a–n1a
o2a–V1a–n3a
o1a–V1a–n3a
n2a–V1a–n3a
n1a–V1a–n3a
o2a–V1a–n1ai
o1a–V1a–n1ai
n2a–V1a–o1ai
n1a–V1a–o1ai
n3a–V1a–o1ai
o1B–V1B–o2B
o1B–V1B–n2B
o1B–V1B–n2B
o1B–V1B–n1B
n2B–V1B–n3B
105.98(8)
98.26(7)
103.84(7)
94.26(8)
102.03(7)
146.78(7)
105.11(8)
148.86(7)
73.20(7)
73.86(7)
175.30(7)
77.44(7)
83.87(6)
81.77(6)
71.42(6)
109.09(10)
102.04(8)
99.66(8)
98.08(8)
73.06(7)
V1a–n1a
V1a–n3a
V1a–o1ai
s1a–C7a
s1a–C8a
1.753(2)
o1a–V1ai
V1B–o1B
2.4219(17)
1.6203(17)
1.6213(17)
2.0222(18)
2.1109(18)
2.1495(19)
angle
V1B–o2B
V1B–n2B
V1B–n1B
V1B–n3B
Bond
o2B–V1B–n1B
n2B–V1B–n1B
o1B–V1B–n3B
o2B–V1B–n3B
n1B–V1B–n3B
99.18(8)
146.08(7)
126.88(8)
123.98(9)
73.02(7)
symmetry codes: (i) 1 – x, 1 – y, 1 − z.
molecules, the vanadium(V) ions have a five coordinate {N3O2} environment and the coor-
dination geometry around them may be described as distorted trigonal-bipyramidal with
equatorial angles in the range of 105–148° in molecule A and 109–126° in molecule B. In
both species the coordination environments around the vanadium(V) ions are composed
by three nitrogen atoms provided by tridentate thiazole-hydrazone Schiff base ligand, (L)−
and two oxygen atoms of oxido groups. In both molecules, the hydrogen atom of the NH
moiety of thiazole-hydrazone ligand was eliminated during the complexation. Consequently,
the ligand acts as tridentate mononegative NNN-donor ligand, (L1)−. The crystal structure of
1 is stabilized by the network of C–H⋯O and C–H⋯N interactions (Table 3).
Despite a great resemblance between A and B components of 1, they exhibit quite dif-
ferent intermolecular contacts. As was noticed before, molecule A is involved in the π⋯π
interactions with the neighboring molecule of A type, resulting in a centrosymmetric archi-
tecture of “dimer” [27], with the nearest distance between adjacent components (viz. cen-
troid⋯centroid distance) of about 3.60 Å, between N1A/C1A/C2A/C3A/C4A/C5A and C8A/
C9A/C10A/C11A/C12A/C13A rings (see Figure S2). Moreover, the V5+ ions are combined by
the dioxido bridge in this unit, with the V1A–O1A bond length of 1.6574(16) Å and V1A–O1Ai
of 2.4219(17) Å (i = −x + 1, −y + 1, −z + 1), forming a {V2O2} cyclic unit. The V⋯V distance
through this bridge is 3.2185(10) Å and the V1A–O1A–V1Ai angle is 102.56(7)°. By considering
this V⋯O interaction a distorted octahedral geometry can also be described for the vana-
dium ion in this unit. In the case of molecule B in the crystal 1, π⋯π interactions between
the adjacent species are also created (see Figure S3). The distance between V1B/N2B/C7B/
N4B/N3B ring and C8B/C9B/C10B/C11B/C12B/C13B ring from neighboring unit is about 3.66
Å. The plane of the ligand is approximately planar to the crystallographic plane (001). The
molecules from adjacent layers are related to each other by inversion through a center of
inversion whereby 5-membered S1B/C7B/N2B/C9B/C8B rings from these neighboring spe-
cies are arranged one above the other (the centroid⋯centroid distance equals 3.70 Å). Unlike
molecule A, there are no V⋯O interactions in the case of molecule B.

JOURNAL OF COORDINATION CHEMISTRY
9
Table 3. hydrogen-bond geometry (Å, °) in 1 and 2.
D–H⋯A
D‒H
H⋯A
D⋯A
D–H⋯A
Complex 1
C4A‒h4A⋯o2Aiii
0.95
0.98
2.54
2.60
3.097(3)
3.455(3)
118
146
C61A‒h61C⋯n4Biv
Complex 2
C61‒h61C⋯n4i
0.98
0.98
2.55
2.38
3.522(5)
3.350(6)
171
169
C14‒h14B⋯o1ii
symmetry codes: (i) x + 1/2, y, −z + 3/2; (ii) x + 1, y, z; (iii) −x + 1, y + 1/2, −z + 3/2; (iv) −x + 1, y−1/2, −z + 3/2.
Figure 3. molecular structure of 2. thermal ellipsoids are drawn in 30% probability level.
3.3. Description of the crystal structure of [VO(acac)L] (2)
The molecular structure of 2 is shown in Figure 3 and selected bond lengths and angles are
given in Table 4. X-ray studies indicated that the obtained product is a molecular mononu-
clear oxidovanadium(IV) complex, [VO(acac)L] (2). The vanadium(IV) ion adopts a distorted
octahedral geometry and its coordination environment is filled by three nitrogen atoms
provided by the tridentate thiazole-hydrazone Schiff base ligand, (L)−, two oxygen atoms of
an acetylacetonate ligand, (acac)−, and the oxygen atom of the oxido group. Three donor
atoms of the Schiff base ligand (N1, N2, N3) and one oxygen atom of (acac)− create the
equatorial plane of the octahedral geometry. One axial position is occupied by the oxygen
atom of the terminal oxido ligand, (V=O1 = 1.610(3) Å), and the second axial position is
occupied by a second oxygen atom of (acac)−, (V–O3 = 2.168(3) Å). The metal center (V) is
deviated by 0.275 Å from the mean plane defined by the equatorial donor atoms towards
the V=O moiety. In 2, the hydrogen of the NH moiety of thiazole-hydrazone ligand was
eliminated during the complexation. Consequently, the ligand acts as a tridentate monon-
egative N3-donor ligand, (L1)−. The crystal packing of 2 is stabilized by C–H⋯O and C–H⋯N
interactions (see Figure 4 and Table 3).
According to our previous report on dioxidomolybdenum and dioxidovanadium complexes
of thiazole-hydrazone ligands [17, 26], two resonance structures contribute to the thiazole
part of ligand in the deprotonated form. These resonance contributors can be easily identified
by comparing the C7–N2 and C7–N4 bond lengths. The C7–N2 (1.338(4) Å) and C7–N4

10
M. GHORBANLOO ET AL.
Table 4. selected bond lengths (Å) and angles (°) in 2.
Bond
Length
Bond
Angle
V–o1
V–o4
V–n3
V–n2
V–n1
V–o3
s–C8
s–C7
n1–C5
n1–C1
1.610(3)
1.976(3)
2.061(3)
2.087(3)
2.114(3)
2.168(3)
1.753(3)
1.755(4)
1.341(5)
1.357(5)
o1–V–o4
o1–V–n3
o4–V–n3
o1–V–n2
o4–V–n2
n3–V–n2
o1–V–n1
o4–V–n1
n3–V–n1
n2–V–n1
o1–V–o3
o4–V–o3
n3–V–o3
n2–V–o3
n1–V–o3
C5–n1–V
C1–n1–V
97.50(13)
98.49(13)
162.73(13)
97.60(14)
108.19(13)
76.10(13)
97.40(13)
95.27(14)
76.22(14)
150.11(13)
178.49(13)
83.37(11)
80.80(12)
80.94(12)
83.74(12)
125.7(3)
115.2(3)
(1.329(4) Å) bond lengths in complex 2 are different in comparison to the lengths reported
for double C=N (≈ 1.30 Å) or single C–N (≈ 1.35 Å) bonds. These bond lengths are close to the
delocalized C=N bond lengths (≈ 1.33 Å) and show that the negative charge is delocalized
over N2, C7 and N4 atoms. A similar effect is also observed in the case of complex 1.
3.4. Catalytic activity of 1 and 2
The catalytic oxidation of cyclooctene was carried in the presence of H2O2 by using 1 or 2
as a catalyst precursor. Complex 1 was used as a representative catalyst precursor for the
optimization of different reaction parameters viz. amounts of catalyst, oxidant (30% aqueous
H2O2), solvent, and temperature. It was observed that only one product, namely cyclooctene
epoxide, was formed in these catalytic reactions. To optimize the amount of catalyst precur-
sor, three different amounts of 1 (0.0005, 0.0010 and 0.0020 g equal to 1.43, 2.85 and
5.70 × 10−3 mmol, respectively) were taken while the amount of cyclooctene (0.5 mmol) and
other reaction parameters were constant (aqueous 30% H2O2 (1 mmol), acetonitrile (2 mL)
and temperature (60 °C)). By increasing the amount of 1 from 1.43 × 10−3 mmol to
2.85 × 10−3 mmol, the conversion increased from 47% to 94%, but there was no further
improvement in conversion on increasing its amount up to 5.70 × 10−3 mmol. Therefore,
2.85 × 10−3 mmol of 1 was chosen as an appropriate amount for optimizing the remaining
conditions. Considering 2.85 × 10−3 mmol of catalyst precursor for 0.5 mmol of cyclooctene,
three different amounts of oxidant (30% H2O2) viz. 0.5, 1.0 and 1.5 mmol were tested. The
lowest i.e. 49% conversion of epoxide was obtained with 0.5 mmol of oxidant under above
reaction conditions, whereas maximum of 94% conversion was obtained using 1.0 mmol of
oxidant (see Figure 5). Further increasing the amount of oxidant to 1.5 mmol did not improve
the conversion (95%). Thus, 1.0 mmol of oxidant (cyclooctene : oxidant ratio is 1:2) was found
to be suitable for the maximum conversion of cyclooctene.
The solvent also affects the oxidation of substrate (Figure 6). It was found that CH3CN
gave the highest conversion (94%). The difference is explained by the high solubility of
catalyst in this solvent and also the high miscibility of CH3CN with water in comparison with
n-hexane (immiscible). The effect of temperature on oxidation of cyclooctene was also

JOURNAL OF COORDINATION CHEMISTRY
11
Figure 4. hydrogen bonding interactions in the crystal of 2.
Figure 5. effect of different amount of h2o2 in the oxidation of cyclooctene by 1. Reaction conditions:
1 (2.85 × 10−3 mmol); temperature, 60 °C; cyclooctene, 0.5 mmol; Ch3Cn (as solvent), 2 ml; time, 5 h.

12
M. GHORBANLOO ET AL.
Figure 6. influence of solvent in the catalytic epoxidation of cyclooctene by 1.a Reaction conditions: 1
(2.85 × 10−3 mmol); temperature, 60 °C; cyclooctene, 0.5 mmol; solvent, 2 ml; time, 5 h; h2o2, 1 mmol.
studied. The reaction was monitored at four different temperatures viz. 40, 60 and 70 and
80 °C for 5 h. The conversion was lowest (61%) at 40 °C but improved to 97% at 70 °C.
Increasing the temperature to 80 °C did not improve the conversion.
Thus, the optimized reaction conditions for the oxidation of 0.5 mmol of cyclooctene are:
catalyst (2.85 × 10−3 mmol), 30% aqueous H2O2 (1 mmol), acetonitrile (2 mL) and temperature
70 °C. Blank reactions gave only ca. 5% conversion after 24 h [27]. Thus, the presence of 1 as
a catalyst precursor in running the oxidation reactions is necessary.
To establish the scope for the activity of [VO2L], this study was further extended to the
catalytic epoxidation of several linear and cyclic olefins, namely styrene, α-methyl styrene,
Table 5. oxidation of alkenes with h2o2 catalyzed by 1.a
Conversion (%)b
Product(s)
Selectivity (%)
TONc
Entry
Substrate
1
95
82
166.7
18
85
2
83
145.6
15
>99
3
60
105.3
4
5
49
30
>99
>99
86.0
52.6
areaction conditions: 1 (2.85 × 10−3 mmol), substrate (0.5 mmol), Ch3Cn (2 ml), h2o2 (1 mmol) and temperature 70 °C;
time 5 h.
bConversions are based on the starting substrates.
cton = (mmol of product)/(mmol of vanadium).

JOURNAL OF COORDINATION CHEMISTRY
13
1-octene, 1-decene, cyclohexene and indene, using [VO2L] (Table 5). Generally, the present
catalytic system was completely selective for the epoxidation of cyclooctene, indene,
1-octene and 1-decene while for styrene and α-methyl styrene, benzaldehyde and acetophe-
none were the byproducts, respectively (Table 5, entries 1–6).
In continuation, we studied catalytic oxidation of olefins using [VO(acac)L] as catalyst
precursor. As same as catalyst 1, we determined the optimized reaction conditions for 2.
Thus, the optimized reaction conditions for the oxidation of 0.5 mmol of cyclooctene are: 2
(2.85 × 10-3 mmol), 30% aqueous H2O2 (1 mmol), acetonitrile (2 mL) and temperature 70 °C.
Details of experimental conditions are presented in Table 6 and Figures 7 and 8.
This study was further extended to the catalytic epoxidation of several linear and cyclic
olefins, namely styrene, α-methyl styrene, 1-octene, 1-decene, cyclohexene and indene, by
using 2 (Table 7).
The catalytic efficiency of 1 and 2 for hydrocarbon oxidation are not same (as shown in
Tables 5 and 7) which can be attributed to their different coordination geometry and also
different oxidation state of vanadium centers. Probably, the high oxidation state of vanadium
Table 6. Catalytic activity of 2 on cyclooctene oxidation.a
[H2O2]/[C8H12] Molar
ratio
Catalyst amount (×
10−3 mmol)
Conversion (%)b
Temperature (°C)
TONc
Entry
1
2
3
4
5
6
7
8
0.5
1
1.5
1
1
1
37
83
85
38
86
39
92
94
2.85
2.85
2.85
1.43
5.70
2.85
2.85
2.85
60
60
60
60
60
40
70
80
64.9
145.6
149.1
132.9
75.4
68.4
161.4
164.9
1
1
aReaction conditions: 2 (2.85 × 10−3 mmol); cyclooctene, 0.5 mmol; Ch3Cn, 2 ml; time, 5 h.
bConversions are based on the starting substrate.
cton = (mmol of product)/(mmol of vanadium).
Figure 7. effect of different amount of h2o2 in the oxidation of cyclooctene by 2. Reaction conditions:
2 (2.85 × 10−3 mmol); temperature, 70 °C; cyclooctene, 0.5 mmol; Ch3Cn (as solvent), 2 ml; time, 5 h.

14
M. GHORBANLOO ET AL.
Figure 8. influence of solvent in the catalytic epoxidation of cyclooctene by 2. Reaction conditions: 2
(2.85 × 10−3 mmol); temperature, 70 °C; cyclooctene, 0.5 mmol; solvent, 2 ml; time, 5 h; h2o2, 1 mmol.
Table 7. oxidation of alkenes with h2o2 catalyzed by 2.a
Conversion (%)b
Product(s)
Selectivity (%)
TON
Entry
Substrate
1
86
78
150.9
22
84
2
71
124.6
16
>99
3
4
47
93
82.5
>99
163.2
5
6
32
18
>99
>99
56.1
31.6
areaction conditions: 2 (2.85 × 10−3 mmol), substrate (0.5 mmol), Ch3Cn (2 ml), h2o2 (1 mmol) and temperature 70 °C.
bConversions are based on the starting substrates.
cton = (mmol of product)/(mmol of vanadium).
in 1 makes it more active in hydrocarbon oxidation. Similar trends have been reported for
[VIVO(sal–his)(acac)] and [VVO2(sal–his)] [28] and for VIVOL1 (L1 = 2,2’-((1E,1’E)-((2-
hydroxybutane-1,4-diyl)bis(azanylylidene)) bis(methanylylidene))diphenol) and VVOL2
(L2 = 6,6’-((1E,1’E)-((2-hydroxybutane-1,4-diyl)bis(azanylylidene))bis(methanylylidene))
bis(2-methoxyphenol)) [29]. In comparison with the previously reported catalysts, both 1

JOURNAL OF COORDINATION CHEMISTRY
15
and 2 exhibited better activity for the oxidation of hydrocarbons. The previously reported
N,Nʹ-bis(salicylaldiminato)oxidovanadium(IV) [30] complex showed lower catalytic activity
than 1 and 2 for the oxidation of cyclooctene. Additionally, catalysts 1 and 2 showed higher
catalytic activity than the VO(tmbmz)2 (tmbmz = 2-thiomethylbenzimidazole) [31] and
VIVO{N,Nʹ-salicyl-R,R-cyclohexanediaminium} [32] for the oxidation of cyclohexene and sty-
rene. However, for the oxidation of cyclohexene and styrene, these catalytic systems are less
effective than the [VO(acac)(R-1,2-bis{(Cl-phenyl)imino}acenaphthene)]Cl [33] system.
Although the details of the catalytic mechanism were not studied in the present work,
on the basis of the electronic absorption spectroscopy studies (Figures S7 and S8) and also
by considering the previous studies on catalytic oxidation by oxidovanadium complexes
[13b, 14, 27, 34], it is predicted that the key step in this catalytic process is the oxidation of
substrates by a peroxovanadium-hydrazone Schiff base species formed after addition of
H2O2 to the solution of complex.
4. Conclusion
New oxidovanadium(IV) and dioxidovanadium(V) complexes with thiazole-hydrazone ligand
have been synthesized and characterized by spectroscopic methods and single crystal X-ray
analyses. The catalytic activity of these compounds has been tested in olefin epoxidation
by H2O2. The epoxide yields show that the catalytic activity of these complexes is strongly
dependent on the temperature and nature of solvent. These compounds can be considered
as active catalyst precursors for epoxidation reactions. Nevertheless, the results obtained
with the V(V) complex provide a good basis for further optimization aiming at efficient
catalytic activity.
Acknowledgements
The authors are thankful to the University of Zanjan and Imam Khomeini International University for
support of this study.
Disclosure statement
No potential conflict of interest was reported by the authors.
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