Dioxidovanadium(V) complexes containing thiazol-hydrazone NNN-donor ligands and their catalytic activity in the oxidation of olefins

Article abstract of DOI:10.1016/j.ica.2016.10.005

The reaction between tridentate NNN donor ligands, (E)-2-(2-(pyridin-2-ylmethylene)hydrazinyl)benzo[d]thiazole (HL¹) and (E)-2-(2-(phenyl(pyridin-2-yl)methylene)hydrazinyl)benzo[d]thiazole (HL²), with VO(acac)₂in ethanol gave [VO₂(L¹)] (1) and [VO₂(L²)] (2) complexes. Complex [VO₂(L²)]·0.2(H₂O) (3) was produced from the reaction of HL²and V₂O₅in ethanol. Molecular structures of complexes were determined by single-crystal X-ray diffraction. All of the investigated compounds were further characterized by elemental analysis, FT-IR, UV–vis and NMR spectroscopy. The dioxidovanadium(V) species were used as catalysts for olefin oxidation in the presence of hydrogen peroxide (H₂O₂) as an oxidant. Under similar experimental conditions with equal vanadium loading, the presence of the [VO₂(L¹)] (1) resulted in higher conversion than in the case of using [VO₂(L²)] (2) and [VO₂(L²)]·0.2(H₂O) (3) as catalytic agents.

Full text of DOI:10.1016/j.ica.2016.10.005

Accepted Manuscript
Research paper
Dioxidovanadium(V) complexescontainingthiazol-hydrazoneNNN-donor
ligandsand their catalytic activity in theoxidation of olefins
Massomeh Ghorbanloo, Solmaz Jafari, Rahman Bikas, Marta S. Krawczyk,
Tadeusz Lis
PII:
S0020-1693(16)30649-1
http://dx.doi.org/10.1016/j.ica.2016.10.005
ICA 17298
DOI:
Reference:
Inorganica Chimica Acta
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Received Date:
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Accepted Date:
11 August 2016
4 October 2016
5 October 2016
Please cite this article as: M. Ghorbanloo, S. Jafari, R. Bikas, M.S. Krawczyk, T. Lis, Dioxidovanadium(V)
complexes containing thiazol-hydrazone NNN-donor ligands and their catalytic activity in the oxidation of
olefins, Inorganica Chimica Acta (2016), doi: http://dx.doi.org/10.1016/j.ica.2016.10.005
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Graphical abstract
Highlights
ꢀ Synthesis of three new dioxidovanadium(V) complexes with NNN-donor ligands
ꢀ Spectroscopic properties of vanadium(V) complexes with thiazole-hydrazone ligands
ꢀ Crystal structure of dioxidovanadium(V) complexes
ꢀ Catalytic behavior of V(V) complexes in oxidation of olefins.

Dioxidovanadium(V) complexes containing thiazol-hydrazone NNN-
donor ligands and their catalytic activity in the oxidation of olefins
Massomeh Ghorbanloo,a,* Solmaz Jafari,a Rahman Bikas,a Marta S.
Krawczykb and Tadeusz Lis,c
a Department of Chemistry, Faculty of Science, University of Zanjan 45195-313, Zanjan, Iran
bDepartment of Analytical Chemistry, Faculty of Pharmacy, Wroclaw Medical University,
Borowska 211A, 50-556 Wroclaw, Poland
c Faculty of Chemistry, University of Wroclaw, Joliot-Curie 14, Wroclaw 50-383, Poland
Abstract
The
reaction
between
tridentate
NNN
(HL1)
donor
and
ligands,
(E)-2-(2-(pyridin-2-
ylmethylene)hydrazinyl)benzo[d]thiazole
(E)-2-(2-(phenyl(pyridin-2-
yl)methylene)hydrazinyl)benzo[d]thiazole (HL2), with VO(acac) in ethanol gave [VO2(L1)] (1)
2
and [VO2(L2)] (2) complexes. Complex [VO2(L2)]·0.2(H2O) (3) was produced from the reaction
of HL2 and V2O5 in ethanol. Molecular structures of complexes were determined by single-
crystal X-ray diffraction. All of the investigated compounds were further characterized by
elemental analysis, FT-IR, UV-Vis and NMR spectroscopy. The dioxidovanadium(V) species
were used as catalysts for olefin oxidation in the presence of hydrogen peroxide (H2O2) as an
oxidant. Under similar experimental conditions with equal vanadium loading, the presence of the
[VO2(L1)] (1) resulted in higher conversion than in the case of using [VO2(L2)] (2) and
[VO2(L2)]·0.2(H2O) (3) as catalytic agents.
Keywords: Dioxidovanadium(V) complexes; Thiazole-hydrazone ligands; Crystal structure,
Epoxidation; N3-donor ligands
*
Corresponding author. Tel.: +98 2433054084; fax: +98 2433052477. E-mail:
m_ghorbanloo@yahoo.com (M. Ghorbanloo).

Introduction
Vanadium has a growing influence on coordination chemistry, due to its biological and
catalytic properties [1]. The oxidovanadium(V) has ability to approve different structural motifs
such as: (i) [VO]3+, (ii) [VO2]+ and (iii) [V2O3]4+ [2]. The importance of vanadium as oxidometal
species in biological and catalytic systems is fine documented [3]. Coordination complexes of
dioxidovanadium(V), [VO2]+, are found to be (pre)catalysts for oxidation reactions [4, 5, 6],
especially accompanied by H2O2 [7]. Moreover, the [VO2]+ species are engaged in the
bromination reaction of compounds in the presence of peroxides [8].
In recent years, vanadium complexes containing VO2-moiety have been widely used in oxidation
reactions due to their high activities. For example, [VVO2(acac-ambmz)] (acac-ambmz = (Z)-4-
(((1H-benzo[d]imidazol-2-yl)methyl)imino)pentan-2-one), [VVO2(salambmz)] (sal-ambmz =
(E)-2-((((1H-benzo[d]imidazol-2-yl)methyl)imino)methyl)phenolate) and [VVO2(sal-aebmz)]
(sal-aebmz = (E)-2-(((2-(1H-benzo[d]imidazol-2-yl)ethyl)imino)methyl)phenol), (Scheme 1),
have been used in oxidation of sulfides and styrene [9]. In addition, [VVO2(sal–his)] {sal-his =
(E)-2-(((2-(1H-imidazol-4-yl)ethyl)imino)methyl)phenolate}, (Scheme 1), has been used as a
catalyst for the oxidation of methyl phenyl sulfide, diphenyl sulfide and benzoin with H2O2 as
oxidant [10]
Scheme 1. Schematic representation of ligands used in references 9 and 10
Thiazole-hydrazones are among the most vital class of heterocyclic compounds
containing S and N atoms, that belong to Schiff base family revealing polydentate chelating

properties, with interesting modes of ligation for a variety of metal ions [11]. The =N–NH–
thiazole moiety in thiazole-hydrazone based ligands can lose the –NH– hydrogen to form a
mono-negative part while in the NH form it can coordinate to metal centers as a neutral moiety.
Although keto-enol tautomerism is a well-known phenomenon in R''R'C=N–NH–C(=O)–R based
hydrazones [12], tautomerism is relatively uncommon in the R''R'C=N-NH–R type of
hydrazones. In the case of R''R'C=N–NH–C(=O)–R species, the C=O group can be involved in
the tautomerism, whereas for R''R'C=N–NH–R based hydrazones the tautomeric effect depends
on the structure of a R group. Due to the structure of the thiazole ring, tautomerism is expected in
ligands with R''R'C=N–NH–thiazole entity that plays a crutial role in determination of the overall
charge of the ligands coordinated to the metal ions (see Scheme 2). According to our previous
report on dioxidomolybdenum complexes of thiazole-hydrazone ligands [13], two tautomeric
structures are expected for thiazole part of ligand in the deprotonated form (Scheme 2c). The
tautomerism in thiazole-hydrazone structures can be determined on the base of N(hydrazone)–C and
C–N(thiazole ring) bond length analysis [viz. single C–N bond length is longer than 1.40 Å, double
C=N bond length equals ~1.27 Å, while delocalized C–N bond length is expected at about 1.33
Å [14].
As a continuation of our studies on coordination chemistry and catalytic properties of
transition metal complexes with hydrazone ligands [15], we undertook synthesis,
physicochemical characterization, crystal structure determination and catalytic studies of three
new dioxidovanadium(V) compounds of thiazole-hydrazone ligands. The obtained complexes 1-
3 are considered as mononuclear species, however in the crystals the interactions between ligand
moieties belonging to the adjacent molecules of complexes result in the formation of
characteristic motifs that may be described as patterns of dimeric units.

Scheme 2. a) General structure of R''R'C=N–NH–C(=O)–R kind of hydrazone ligands and their
tautomeric forms; b) general structure of R''R'C=N–NH–R type hydrazones and two examples
which do not exhibit tautomeric forms; c) thiazole-hydrazone ligands and their tautomeric forms
with approximately bond lengths around N–C–N moiety; the average bond lengths within
N−C−N moiety are given; d) structure of HL1 and HL2.
2. Experimental
2.1. Materials and instruments
All chemicals except for solvents were purchased from Sigma Aldrich and used as received
without further purification. Ethanol (96%) for syntheses of complexes was purchased from
Bidestan Company (Iran), while other solvents were purchased (in the highest available purity)
from Merck and used as received. IR spectra were recorded in KBr disks with a Bruker FT–IR

spectrophotometer. The elemental analyses (carbon, hydrogen and nitrogen) were obtained from
Carlo ERBA Model EA 1108 analyzer. The atomic absorption analyses were carried out using
Varian Spectra AA 220 equipment. 1H NMR and 13C{1H} NMR spectra in DMSO-d6 solution of
the ligand and complexes were recorded on Bruker 250 MHz and 62.9 MHz spectrometer,
respectively and chemical shifts are indicated in ppm relative to tetramethylsilane (TMS). 51V
NMR spectra were collected on a Bruker 300 MHz. 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).
2.2. Synthesis of (E)-2-(2-(pyridin-2-ylmethylene)hydrazinyl)benzo[d]thiazole (HL1)
The ligand (E)-2-(2-(pyridin-2-ylmethylene)hydrazinyl)benzo[d]thiazole (HL1) was synthesized
by the reaction of 2-pyridinecarbaldehyde (0.11 g, 1.0 mmol) with 2-hydrazinylbenzo[d]thiazole
(0.16 g, 1.0 mmol) in 6 mL of methanol. The mixture of mentioned compounds was refluxed for
6 h in methanol under air atmosphere. 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 methanol and recrystallized from ethanol. Yield: 86% (0.22 g). Anal. Calc. for
C13H10N4S (MW = 254.31 g/mol): C, 61.40; H, 3.96; N, 22.03%. Found: C, 61.36; H, 3.99; N,
22.11%. Selected FT-IR (KBr, cm-1): 3322 (w), 3214 (w), 3136 (w), 3062 (w), 2939 (w), 2869
(w), 1618 (vs), 1568 (m), 1469 (s), 1449 (s), 1426 (s), 1290 (s), 1278 (vs), 1195 (m), 1127 (vs),
1106 (m), 1019 (m), 994 (s), 967 (m), 881 (s), 794 (s), 755 (s), 742 (m), 699 (s), 649 (m), 622
1
(w), 565 (m), 497 (w), 447 (m). H NMR (250.13 MHz, DMSO-d6, 25 °C, TMS, ppm): δ = NH
Overlapped by H2O at ~ 4 ppm, 7.11 (2H, m), 7.32 (2H, m), 7.45 (1H, d, J = 7.25 Hz), 7.78 (1H,
d, J = 7.50 Hz), 7.87 (1H, d, J = 7.50 Hz), 8.12 (1H, s), 8.56 (1H, d, J = 6.95 Hz). 13C{1H} NMR

(62.9 MHz, DMSO-d6, 25 °C, TMS, ppm): δ = 118.67, 119.84, 122.03, 122.38, 124.3, 126.46,
129.87, 137.24, 144.39, 149.97 153.54, 167.56 ppm. UV-Vis in MeOH, λmax, nm (ε, M-1cm-1):
224 (3.4 × 104), 254 (1.8 × 104), 346 (3.0 × 104), 436 (0.7 × 104).
2.3. Synthesis of (E)-2-(2-(phenyl(pyridin-2-yl)methylene)hydrazinyl)benzo[d]thiazole (HL2)
HL2 was synthesized in the course of the reaction of 2-benzoylpyridine (0.18 g, 1.0 mmol) with
2-hydrazinylbenzo[d]thiazole (0.16 g, 1.0 mmol) in 6 mL of methanol under air atmosphere with
the same procedure reported for HL1. Yield: 79% (0.26 g). Anal. Calc. for C19H14N4S (MW =
330.41 g/mol): C, 69.07; H, 4.27; N, 16.96%. Found: C, 69.11; H, 4.31; N, 16.99%. Selected FT-
IR (KBr, cm-1): 3198 (w), 3158 (w), 3085 (w), 2969 (w), 1629 (vs), 1562 (m), 1475 (s), 1444 (s),
1358 (W), 1268 (m), 1244 (w), 1160 (m), 1155 (m), 1134 (s), 1049 (w), 1021 (m), 990 (s), 963
1
(w), 918 (s), 893 (vs), 780 (vs), 760 (s), 711 (m), 636 (m), 611 (m), 518 (s), 425 (m). H NMR
(250.13 MHz, DMSO-d6, 25 °C, TMS, ppm): δ = 4.11 (1 H, NH), 7.05 (1 H, m), 7.26 (2 H, m),
7.39 (3 H, m), 7.41 (2 H, d, J = 5.75 Hz), 7.63 (1 H, m), 7.85-7.90 (2H, m), 8.12 (1H, d, J = 7.75
13
Hz), 8.44 (1H, d, J = 7.25 Hz). C{1H} NMR (62.9 MHz, DMSO-d6, 25 °C, TMS, ppm): δ =
116.76, 122.88, 123.56, 125.63, 126.10, 127.41, 129.08, 130.26, 130.71, 137.54, 143.37, 147.73,
153.67, 155.88, 169.6 ppm. UV-Vis in MeOH, λmax, nm (ε, M-1cm-1): 224 (3.2 × 104), 255 (1.7 ×
104), 348 (1.9 × 104), 447 nm (0.6 × 104).
2.4. Synthesis of [VO2(L1)] (1)
Complex 1 was synthesized by the reaction of thiazole-hydrazone ligand, HL1, (0.25 g, 1.0
mmol) with VO(acac)2 (0.26 g, 1.0 mmol) in ethanol solvent under air. Mentioned amount of
materials were placed in the main arm of a branched tube. Ethanol (15 mL, 96%) was carefully
added to fill the arms, the tube was sealed and the arm containing reagents was immersed in an

oil bath at 60 °C while the other arm was kept at ambient temperature. After a week, red crystals
of complex 1 appeared in the cooled arm. Isolated yield: 80% (0.27 g). Anal. Calc. for
C13H9N4SO2V (MW = 336.24 g/mol): C, 46.44; H, 2.70; N, 16.66; V, 15.15%. Found: C, 46.39;
H, 2.74; N, 16.59; V, 15.12%. Selected FT-IR (KBr, cm-1): 3452 (br, w), 3079 (w), 2961 (w),
2928 (m), 2855 (w), 1598 (s), 1562 (m), 1517 (s), 1483 (m), 1453 (s), 1428 (s), 1402 (m), 1350
(m), 1279 (w), 1242 (m), 1155 (w), 1106 (m), 1088 (s), 969 (s), 948 (s), 885 (m), 770 (m), 748
(m), 689 (w), 659 (w), 516 (w), 464 (w). 1H NMR (250.13 MHz, DMSO-d6, 25 °C, TMS, ppm):
δ = 7.12 (1H, t, J = 6.75 Hz), 7.33 (2H, m), 7.80 (1H, m), 7.85 (3H, m), 8.11 (1H, s), 8.57 (1H, d,
J = 4.75 Hz). 51V NMR (131.44 MHz, DMSO-d6, 25 °C, ppm): δ = 527.18 (Fig. S10). UV-Vis in
MeOH, λmax, nm (ε, M-1cm-1): 230 (2.8 × 104), 278 (1.3 × 104), 498 nm (0.90 × 104).
2.5. Synthesis of [VO2(L2)] (2)
Complex 2 was synthesized in the course of the reaction of HL2 (0.33 g, 1.0 mmol) with
VO(acac)2 (0.26 g, 1.0 mmol) in 15 mL of ethanol solvent according to the same procedure
reported for complex 1. Red crystals of complex 2 were formed during two weeks. Isolated
yield: 60% (0.25 g). Anal. Calc. for C19H13N4O2SV (MW = 412.33 g/mol): C, 55.34; H, 3.18; N,
13.59; V, 12.35%. Found: C, 55.36; H, 3.13; N, 13.63; V, 12.24%. Selected FT-IR (KBr, cm-1):
3469 (br), 3013 (w), 2956 (w), 2934 (w), 2860 (w), 1609 (s), 1548 (m), 1498 (m), 1431 (m),
1336 (s), 1281 (m), 1243 (m), 1202 (w), 1155 (m), 1148 (m), 1088 (s), 1080 (s), 1025 (w), 999
(w), 944 (s), 877 (m), 786 (w), 757 (m), 708 (m), 667 (w). 1H NMR (250.13 MHz, DMSO-d6, 25
°C, TMS, ppm): δ = 7.11 (1H, m), 7.36 (3H, m), 7.44 (2H, d, J = 8.00), 7.60-7.73 (5H, m), 8.17
(1H, d, J = 8.00 Hz), 8.80 (1H, d, J = 5.25 Hz). 51V NMR (131.44 MHz, DMSO-d6, 25 °C, ppm):
δ = 529.82 (Fig. S11). UV-Vis in MeOH, λmax, nm (ε, M-1cm-1): 226 (3.3 × 104), 281 (1.8 × 104),
502 nm (1.3 × 104).

2.6. Synthesis of [VO2(L2)]·0.2(H2O) (3)
Compound 3 was synthesized by the reaction of HL2, (0.33 g, 1.0 mmol) and V2O5 (0.091 g, 0.50
mmol) in 15 mL ethanol solvent. After a week, violet crystals of complex 3 were deposited in the
cooler arm. Isolated yield: 75% (0.31 g). Anal. Calc. for C19H13.4N4O2.2SV (MW = 415.94
g/mol): C, 54.86; H, 3.25; N, 13.47; V, 12.25%. Found: C, 54.81; H, 3.22; N, 13.43; V, 12.20%.
Selected FT-IR (KBr, cm-1): 3433 (br, w), 2960 (w), 2929 (w), 2855 (w), 2364 (w), 2346 (w),
1610 (s), 1551 (m), 1498 (m), 1432 (m), 1388 (w), 1336 (s), 1281 (m), 1243 (m), 1202 (w), 1154
(w), 1088 (s), 1023 (m), 944 (s), 889 (m), 837 (m), 757 (w), 708 (w), 659 (m), 620 (m), 512 (w),
482 (w), 428 (w). 1H NMR (250.13 MHz, DMSO-d6, 25 °C, TMS, ppm): δ = 7.09 (1H, m), 7.35
(3H, m), 7.43 (2H, d, J = 7.5), 7.61-7.71 (5H, m), 8.13 (1H, d, J = 7.5 Hz), 8.78 (1H, d, J = 5.25
Hz). 51V NMR (131.44 MHz, DMSO-d6, 25 °C, ppm): δ = 529.92 (Fig. S12). UV-Vis in MeOH,
λmax, nm (ε, M-1cm-1): 224 (3.2 × 104), 281 (1.7 × 104), 494 nm (1.1 × 104).
2.7. General oxidation procedure
Oxidation reactions were carried out under air in a 2 mL of solvent and hydrogen peroxide (30%
solution of H2O2) was used as an oxidant. In a typical experiment, a mixture of 3.0 × 10-3 mmol
of complex 1, 2 mL solvent and 1.0 mmol of substrate were placed in a 25 mL round bottomed
glass flask. The mixture was heated to reach the set temperature and after addition of H2O2, the
solution was introduced 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 comparing the retention times with the literature data.
2.8. X-ray Crystallography

X-ray diffraction data for 1, 2 and 3 were collected at 150 K (for 1 and 3) and at 120 K for 2 by
the ω-scan technique on Kuma KM4 CCD diffractometer with a Sapphire CCD detector using
graphite-monochromatized MoKα radiation (λ = 0.71073 Å). Data collection, cell refinement,
and data reduction were carried out with CrysAlis CCD, Oxford Diffraction Ltd. software [16].
Analytical absorption correction was applied to data with the use of CrysAlis RED, Oxford
Diffraction Ltd. The structures were solved by Patterson methods using SHELXS-97 [17], and
refined by a full-matrix least-squares technique on F2 with SHELXL-2014 with anisotropic
thermal parameters for all non-H atoms [18]. 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 for complexes 1-3 were prepared with
DIAMOND [19].
Crystallographic data for the structural analyses have been deposited with the Cambridge
Crystallographic Data Centre, No. 1498335, 1498333 and 1498334 for 1, 2 and 3, 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.

Table 1. Crystal data and structure refinement parameters for 1, 2 and 3.
Identification code
Net formula
Formula weight
Radiation
1
C13H9N4O2SV
336.24
MoKα
150
2
C19H13N4O2SV
412.33
MoKα
120
3
C19H13N4O2SV·0.2(H2O)
415.94
MoKα
150
T, K
Crystal size, mm
Crystal shape, color
Crystal system
Space group
a, Å
b, Å
c, Å
0.38 × 0.23 × 0.06
Plate, red
Monoclinic
P21/n
7.831 (2)
21.579 (6)
7.857 (2)
101.61 (3)
1300.6 (6)
4
0.43 × 0.41 × 0.33
Block, red
Monoclinic
P21/n
9.687 (2)
15.914 (4)
11.480 (3)
98.38 (3)
1750.8 (7)
4
0.38 × 0.28 × 0.22
Block, violet
Monoclinic
P21/c
11.119 (3)
15.260 (4)
11.715 (3)
113.59 (3)
1821.6 (9)
4
β, deg
Volume, Å3
Z
Density, Mg m−3
1.717
1.564
0.71
1.517
0.68
Absorption coefficient, mm−1 0.93
F(000)
680
840
848
Θ range, deg
2.8-28.7
3141
8814
2544
-10<h<10; -24<k<29;
-9<l<10
3.0-28.7
4072
9880
3.3–28.7
4393
10857
Independent reflections
Measured reflections
Reflections with I > 2σ(I)
Index ranges hkl
3311
3603
-11<h<12; -
21<k<19;
-14<l<15
0.031
0/244
1.07
0.044
0.128
0.46
−0.57
-14<h<8; -19<k<20;
-14<l<15
Rint
0.054
0/190
1.05
0.058
0.153
0.52
0.030
0/248
1.09
0.043
0.125
0.38
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
−0.64
−0.46
Results and Discussion
3.1. Synthesis and characterization
The reaction of VO(acac)2 and V2O5 with the tridentate Schiff base ligands, HL1–2, in ethanol
resulted in the molecular dioxidovanadium(V) complexes in the form of red and violet crystals,
respectively. The precursor, [VIVO(acac)2], reacts with an equimolar amounts of HL1 and HL2

yielding [VVO2L1-2]. These complexes were achieved upon oxidation of V(IV) to V(V), according
to Eqn. (1) [20].
2[VO(acac)2] + 2 HL1-2 + H2O +1/2O2 • 2[VO2(L1-2)] + 4Hacac
Eqn. 1
In addition, vanadium(V) complex with HL2 ligand, was prepared by treating an ethanolic
solution of HL2 with V2O5, according to formal Eqn. (2).
V2O5 + 2HL2
2[V2O4(L2)2] + H2O
Eqn. 2
The ligands were found to coordinate to dioxidovanadium cores as tridentate mono-anionic (L1-
2 -1
)
ligands. The elemental analyses of the complexes are in agreement with the formulae
[VO2(L1)] (1), [VO2(L2)] (2) and [VO2(L2)]·0.2(H2O) (3). A comparison of the IR spectra of the
complexes with the spectra of ligands provides some evidences for the coordination mode of the
ligands in the complexes. On complexation the absence of υ(NH) band of the ligands shows
coordination of HL1–2 as mono-anionic ligands. In addition, the infrared spectra of complexes
display an absorption band around ~1600 cm-1 which can be assigned to the C=N stretching
frequency of the coordinated ligand [21]. The strong υ(V=O) band around 944-947 cm-1 could be
clearly identified for all the complexes (Figs. S1-S2) [22].
1H NMR and 13C{1H} NMR spectra of HL1-2 in DMSO-d6 (Figs. S3-S6), confirmed the proposed
structure for the ligands. In the 1H NMR spectrum of HL1, the signal at δ 8.12 is assigned to the
azomethine -CH=N- group and the peak of NH- hydrogen has been overlapped by H2O peak of
1
solvent, at ~ 4 ppm. The signal at 4.11 ppm in the H NMR spectrum of HL2 is assigned to the
peak of NH hydrogen. 13C{1H} NMR spectra of ligands were also recorded to provide diagnostic
tools for the elucidation of the structures. The presence of peaks at 153.54 and 155.88 ppm

confirmed the Schiff base formation in the HL1 and HL2, respectively [13,23,24]. In the 1H NMR
spectrum of compounds 1, 2 and 3 (Figs. S7-S9), the NH peak disappeared after complexation
and indicates the ligands act as mono-negative ligands in 1-3.
The UV–Vis spectra of HL1-2 and their oxidovanadium complexes were obtained in methanol
solution, as shown in Fig. 1-a and Fig. 2-a. The UV-Vis spectrum of the ligand shows four sharp
absorption maxima. Based on the extinction coefficients the most probable assignment of the
first two bands (224, 254 nm) is π → π*, while the two last bands (346, 436 nm) are assignable
to the n → π* transitions. On complexation, the intra-ligand transitions shift toward higher
wavelengths (230, 287 nm in complex 1, 226, 281 nm in complex 2 and 224, 281 nm in complex
3), Fig. 1-b and Fig. 2-b,c. The most important feature in the near-UV region of the V(V) Schiff-
base complexes is a band at ~500 nm in complexes 1, 2 and 3. That band indicates the
coordination of metal ion with the ligand and can be assigned to a ligand to metal charge-transfer
(LMCT) band [25].
Solid-state spectra of 1, 2 and 3 (as shown in Fig. 3) are expected to show modest shifts in intra
ligand transition and LMCT bands relative to those recorded in the solution spectra, due to
absence of solvent interaction effects [26], the crystals of 2 and 3 afford similar solution intra
ligand and LMCT spectra but striking differences observe in their solid-state LMCT absorptions.
Equally striking solid-state effects on LMCT absorptions are also shown by [(hypoxanthine-
κN7)(NH3)5Ru]·3Cl H2O [27] and [(1,3,9-trimethylxanthine-κN7)(NH3)5Ru][ZnCl4]Cl [28], but
we could not find similar reports for vanadium compounds.
Common solid-state effects like hydrogen bonding interactions, which might lead to splitting of
electronic absorption bands, were probed in an attempt to identify the origins of this unusual
observation. Although in general complexes 2 and 3 have relatively the same molecular

structures, in crystalline state there are several considerable differences in their intermolecular
interactions. The V···V, N···O and V···O distances are different in the crystals of 2 and 3 which
should have an influence on the electronic spectra of these compounds in the solid state.
Moreover, intermolecular C−H···S, C−H···O, C−H···N, O···S and C···S interactions are evidently
different in the crystal packing of complexes 2 and 3 which are the other important parameters
that could affect the solid state electronic spectrum of these materials. Solid-state spectroscopic
results reported previously include the effects of conformational polymorphism [29], allotropes
showing different metal-ligand distances [30], stacking changes which enable intermolecular
charge-transfer absorptions [31], and varying metal-metal distances [32].
Fig. 1. UV-Vis spectra of a) HL1, b) complex 1

Fig. 2. UV-Vis spectra of a) HL2, b) complex 2 c) complex 3
Fig. 3. UV–Vis reflectance spectra for a) 1, b) 2, c) 3 crystals

3.2. X-ray structures of [VO2(L1)] (1)
In order to compare forms of the complexes [VO2(L1)] (1), [VO2(L2)] (2) and
51
1
[VO2(L2)]·0.2(H2O) (3) in solutions and in the solid state, the V NMR, H NMR experiments
(collected in DMSO) as well as mass spectra (performed in MeCN) were carried out, which
revealed that the mononuclear dioxidovanadium species of 1, 2 and 3 are present in the solvents.
51
The V NMR spectra showed single signals coming from vanadium(V) centers. In the case of
51
1
the presence of dimers two peaks on each V NMR spectra should be observed [33]. The H
NMR experiments showed no broadening of peaks that would also be characteristic for dimeric
forms. However, X-ray investigations showed that in the crystals the dimeric units of 1, 2 and 3
may be distinguished, in which the vanadium(V) centers are coordinated by two oxido ligands,
where one is terminal and second one is involved in the interaction with the adjacent species
(Figs. 4-7).
Molecular structure of complex 1 is shown in Fig. 4 and selected bond lengths and angles
are given in Table 2. X-ray studies revealed that the crystal of 1 comprises of molecules of
mononuclear dioxidovanadium complex considered as a basic unit. The V5+ ion is bonded by the
polydentate thiazole-hydrazone ligand (L1) via three nitrogen atoms from the Schiff base (N1,
N2, N3) as well that means acting of the ligand as a tridentate mononegative N3-donor ligand,
(L1)־. The V–O1 and V–O2 bond lengths as well as the V–O1–Vi and O1–V–O2 angles (Table
2) are in the typical range of values for similar dinuclear oxidovanadium complexes [34]. The
coordination sphere of each metal centre adopts the geometry of intermediate form between a
square pyramid and a trigonal bipyramid. The metal ion is located nearly in the plane defined by
atoms of the thiazole-hydrazone ligand, wherease the O1 and O2 atoms are deviated of 1.612(3)

and 0.834(3) Å from the plane, respectively. In complex 1, the hydrogen of NH group of
thiazole-hydrazone ligand is eliminated during the complexation.
As was mentioned before, in the crystal structure the molecules of the complex form the
characteristic for dioxidovanadium(V) compounds architecture where neighboring mononuclear
species contact each other via π···π interractions with the nearest distance between adjacent
aromatic rings (viz. centroid – centroid distance) of about 3.63 Å. Such a composition of
molecules of [VO2(L1)] (1) may be described as a pattern of dimers (Fig. 4). Within a dimeric
unit vanadium(V) ions are bridged by two oxido ligands, with the V–O2 bond length of 1.660(2)
Å and the much longer distance V–O2i of 2.538(3) Å (i = 1-x, 1-y, 1-z) (Table 2). The V···Vi
distance equals 3.3428(13) Å and is comparable to the V···V ones observed in the previously
reported oxidovanadium complexes [33].
The crystal packing of complex 1 is stabilized by C–H···O, C–H···N, C–H···S, π···π, and C–H···π
interactions (Fig. 5, Table 3). According to our previous report on dioxidomolybdenum
complexes of thizaole-hydrazone ligands [13], two tautomeric structures are expected for
thiazole part of ligand in the deprotonated form (see Scheme 2). These tautomeric structures can
be easily identified by analysis of the C7–N2 and C7–N4 bond lengths as follows. The C7–N2
(1.338(4) Å) and C7–N4 (1.329(4) Å) bond lengths in complex 1 are out of the range for the
bond lengths reported for double C=N (≈ 1.27 Å) or single C–N (≈ 1.40 Å) bond lengths [14].
These bonds can be considered as delocalized C=N bonds (≈ 1.33 Å) with the delocalization of
the negative charge over N2, C7 and N4 atoms (Scheme 2c).

Table 2 Selected bond lengths (Å) and angles (°) in 1, 2 and 3.
Bond
Length, Å
1
Length, Å
Length, Å
3
Complex
V—O1
V—O2
V—O2i
V···Vi
V—N2
V—N1
V—N3
S—C8
S—C7
N2—C7
2
1.6230(16)
1.6108(17)
3.263(2)
3.9128(10)
2.0173(19)
2.0871(19)
2.1431(18)
1.753(2)
1.741(2)
1.334(3)
1.610(2)
1.660(2)
2.538(3)
3.3428(13)
2.037(3)
2.112(3)
2.152(3)
1.755(3)
1.755(3)
1.338(4)
1.6552(14)
1.6130(16)
2.4712(16)
3.2734(9)
2.0257(16)
2.1014(18)
2.1640(16)
1.754(2)
1.7480(19)
1.344(2)
N4—C7
1.329(4)
1.364(3)
1.357(2)
Bond
Angle, °
Angle, °
Angle, °
V–O1–Vi
108.58(9)
102.83(8)
101.08(8)
97.85(8)
96.91(8)
146.45(7)
114.67(8)
136.54(8)
73.71(7)
73.73(7)
O1—V—O2
O1—V—N2
O2—V—N2
O1—V—N1
O2—V—N1
N2—V—N1
O1—V—N3
O2—V—N3
N2—V—N3
N1—V—N3
107.70(12)
102.52(11)
98.39(11)
102.80(11)
94.70(11)
146.31(11)
146.99(11)
105.30(11)
72.93(10)
73.70(10)
106.83(7)
97.21(7)
103.10(7)
94.24(7)
103.55(7)
146.44(7)
105.72(7)
147.45(7)
73.13(6)
73.42(6)
Symmetry code: i = 1-x, 1-y, 1-z

Fig. 4. The architecture of “dimers” built up from complex 1 interracting each other via V···Oi
and π···π of aromatic rings. In the picture the V–O2i distances of 2.538(3) Å (i = 1-x, 1-y, 1-z)
are marked by dashed line. Displacement ellipsoids are drawn at the 30% probability level.
Fig. 5. C–H···O, C–H···N, C–H···S and C–H···π interactions in the crystal of complex 1

Table 3. Hydrogen-bond geometry (Å, °) in 1, 2 and 3
D–H···A
D–H
H···A
D···A
D–H···A
Compound 1
C5–H5···O2i
Compound 2
C2–H2···Sii
0.95
2.54
3.224(4)
129
0.95
0.95
2.86
2.51
3.657(2)
3.200(3)
143
130
C13–H13···O1iii
Compound 3
C4–H4···Siv
0.95
0.84
2.92
2.30
3.775 (2)
3.087 (15)
150
156
O1W–H1W···O1
Symmetry codes : (i) −x, −y+1, −z+1.; (ii) −x+1/2, y−1/2, −z+3/2; (iii) x+1/2, −y+3/2, z+1/2.; (iv)
−x+1, y−1/2, −z+1/2
3.3. X-ray structures of [VO2(L2)] (2) and [VO2(L2)]·0.2(H2O) (3)
[VO2(L2)] was obtained in two crystalline forms 2 and 3, in reactions of HL2 with VO(acac)2 and
V2O5, respectively (Figs. 6, 7, Table 2). The studied crystals differ in water content. Similarly to
1, complexes 2 and 3 are mononuclear dioxidovanadium (V) compounds of the analogous
tridentate coordination mode, where the ligand, (L2)־ is linked to the vanadium centre via N
atoms. In both 2 and 3 compounds the similar overall geometry as in 1 is observed (Table 2, Fig.
8). The O1 and O2 oxido ligands are deviated of 1.571(2) and 0.994(2) Å respectively in 2 and of
1.572(2) and 0.874(2) Å in 3 from the plane including atoms of ligand L1 along with the
vanadium ion. In both 2 and 3 crystals the architecture of dimeric oxidovanadium species with
the same coordination environment around the vanadium ions can be distinguished (Fig. 8) that
may be stabilized by π···π interactions occuring within dimeric units. The distinctive differences
between 2 and 3 crystal structures are obsered in values of V···Vi and V–O2i distances (i = 1-x,
1-y, 1-z) (Table 2) which might result from the interactions between adjacent molecules in
crystals.

By comparison with 1, the studied crystals 2 and 3 reveal the similar kinds of network of
hydrogen bonds as well as π···π and C–H···π interactions (see Table 3). Moreover, by analogy to
1, in 2 and 3 the other analyzed bonds of thizaole-hydrazone ligand L2 (viz. C7–N2 and C7–N4)
are assigned to be delocalized ones.
Fig. 6. Molecular structure of crystal 2 in the form of dinuclear unit (i = 1-x, 1-y, 1-z); V···O and
N···O interations are shown as dashed lines. Displacement ellipsoids are drawn at the 30%
probability level.

Fig. 7. Picture showing dinuclear species formed by molecules of mononuclear
oxidovanadium(V) complex 3 (i = 1-x, 1-y, 1-z); V···O interactions are shown as dashed lines.
Displacement ellipsoids are drawn at the 30% probability level.

Fig. 8. Overlay of dimeric units of molecules 2 (dashed line) and 3 (solid line).
3.4. Catalytic activity
The oxidation of cyclooctene as a substrate with 30% aqueous H2O2 was carried out in the
presence of complex 1 as a homogeneous catalyst. Each experiment was repeated three times and
the conversion are based on the average conversion of three experiments. In order to find the
optimum conditions for the oxidation reactions, oxidation of cyclooctene in various conditions
was studied. As shown in Table 4, the conversion increased with increasing amounts of hydrogen
peroxide (Table 4, entries 1-3) is presented. It may be pointed out that the addition of the catalyst
led to an increase in yield to cyclooctene epoxide. With the use of 1.5 × 10-3 mmol and 3.0 × 10-3
mmol of catalyst the yield reached to 54% and 96%, respectively, and with the increasing
amount of catalyst, the further boost to yield was not observed. In the blank reaction under
similar conditions with cyclooctene, only 5.0% of cyclooctene was oxidized after 24 hours [35].
Thus, the presence of complex 1 as a catalyst is necessary in running the oxidation reaction.

Table 4. Catalytic activity of [VO2(L1)] on cyclooctene oxidation a
Entry
[H2O2]/[C8H12]
Molar ratio
Conversion b
(TON)d
(σ)c
1
2
3
0.5
1.0
2.0
33 (0.408)
96 (1.63)
94 (2.08)
(110)
(320)
(313)
a Reaction conditions: catalyst [VO2(L1)] (3.0 × 10-3 mmol); reaction temperature,
80 °C; cyclooctene, 1.0 mmol; CH3CN (as solvent), 2 mL; reaction time, 5 h.;
b Conversions are based on the starting substrate; c σ = standard deviation;
d TON = (mmol of product)/(mmol of catalyst)
Alternatively, the reaction was carried out at different temperatures. When the temperature was
increased (from 40 to 80 °C), the conversion increased correspondingly (from 36% to 96%). In
order to further optimize the process to achieve the maximum oxidation of cyclooctene, the
effect of different solvents was studied (Table 5). The conversion was highest for CH3CN, and
equaled 96% after 5 hours; CH3CN was a better solvent medium than CH3OH, EtOH, CH2Cl2,
and CH3Cl as well as n-hexane was associated with the lowest conversion. 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.
Table 5. An effect of various solvents on cyclooctene oxidation by [VO2(L1)]a
Entry
Solvent
Conversion b
(σ)c
(TON)d
1
2
3
4
5
6
MeOH
EtOH
68 (1.41)
80 (1.31)
96 (1.32)
20 (0.82)
10 (0.51)
6 (0.41)
(227)
(267)
(320)
(67)
CH3CN
CH2Cl2
CHCl3
(33.3)
(20)
n-hexane
a Reaction conditions: catalyst [VO2(L1)] (3.0 × 10-3 mmol); reaction temperature, 80 °C;
cyclooctene, 1.0 mmol; solvent, 2 mL; reaction time, 5 h; H2O2, 1.0 mmol; b Conversions

c
d
are based on the starting substrate; σ = standard deviation; TON = (mmol of product)/(mmol of
catalyst);
To establish the scope for the activity of [VO2(L1)2], this study was further extended to the
catalytic epoxidation of other linear and cyclic olefins, namely styrene, α-methyl styrene, 1-
octene, 1-decene, cyclohexene and indene, using [VO2(L1)] (Table 6). Cis-cyclooctene, indene,
1-octene and 1-decene were converted to the corresponding epoxide with >99% selectivity which
indicates that the epoxide ring remains stable in the reaction mixture. It may be due to the
reaction condition [36], like the absence of strong nucleophiles and strong acidic or basic
medium [37]. For cyclohexene in addition to cyclohexene oxide, allylic oxidation products (2-
cyclohexene-1-ol) were formed. Although 2-cyclohexenone could form as another product in
cyclohexene oxidation, it was not detected in this work. Allylic oxidation has been reported by
the metalloporphyrin systems in the oxidation of alkenes such as cyclohexene and is due to the
radical nature of the active oxidizing species [38]. In addition, H.H. Monfared et al. used a
[VVO(OCH3)(L2)] complex (where H2L2 = (E)-3-hydroxy-N'-(2-hydroxybenzylidene)-2-
naphthohydrazide) as catalyst, in the presence of H2O2, as an oxidant. This catalytic system leads
to the oxidation of cyclohexene, cyclooctene and α methylstyrene with conversions >90%.
Cyclooctene is converted to the corresponding epoxide with 100% selectivity. For cyclohexene,
in addition to cyclohexene oxide (29%), allylic oxidation products (2-cyclohexene-1-ol, 55%,
and 2-cyclohexene-1-one, 8%) are also formed [35]. According to those results, the selectivity of
related vanadium(V) complex is not comparable with 1 in this work. In addition, for styrene and
α-methyl styrene catalytic system was not completely selective for epoxidation, which
benzaldehyde and acetophenone were the byproducts, respectively. Romanowski et al. has been
reported oxidation of styrene in the presence of different vanadium(V) complexes [39]. In

comparison with their results, [VO2(L1)] (1) could act as better catalytic system with different
selectivity.
Table 6. Oxidation of alkenes with H2O2 catalyzed by the [VO2(L1)] (1)a
Conversion b
Product(s)
Yield
(%)
Selectivity
(%)
TON d
290
Entry
Substrate
(σ)c
72.2
83
O
1
87 (2.39)
O
14.8
59.3
17
76
2
O
78 (1.93)
70 (1.15)
260
233
O
18.7
70
24
O
3
4
>99
O
90 (2.14)
80
89
OH
300
10
50
11
5
6
50 (0.92)
30 (0.73)
>99
167
100
O
30
>99
O
a Reaction conditions: catalyst [VO2(L1)] ( 3.0 × 10-3 mmol), substrate (1.0 mmol), CH3CN (2 mL),
H2O2 (1.0 mmol) and temperature 80 ºC; Time 5 h, b Conversions are based on the starting substrates;
c σ = standard deviation; d TON = (mmol of product)/(mmol of catalyst);
In the following, for investigate about substituent effect in catalytic activity, the catalytic
activity of [VO2(L2)] (2) was examined under the optimized conditions of [VO2(L1)] (1). Also,
catalytic activity of complex 3 was studied under optimized conditions. The conversion and

selectivity of complex 3 were comparable with complex 2 which can be attributed to their similar
structures. According to the results, complex 1 shows higher catalytic reactivity than 2 and 3 in
the oxidation of olefins with H2O2.
Table 7. Oxidation of alkenes with H2O2 catalyzed by complex (2) and (3)a
Product(s)
Yield
Selectivity TON
(%)
Entry
Conversion b
(σ)c
(%)
Substrate
57.7
78
1
O
247
74 (1.36)
64 (1.28)
O
16.3
47.5
22
2
74
O
213
O
16.5
56
26
O
3
4
56 (1.22)
88 (1.63)
100
100
187
293
88
69
5
83
O
83 (1.84)
277
OH
14
17
6
7
36.5 (0.72)
19 (0.41)
36.5
100
122
63
O
19
100
O
a Reaction conditions: catalyst [VO2(L2)] (3.0 × 10-3 mmol), substrate (1.0 mmol), CH3CN (2 mL), H2O2
b
c
(1.0 mmol) and temperature 80 ºC, Conversions are based on the starting substrates; σ = standard
deviation; d TON = (mmol of product)/(mmol of catalyst)

In comparison with the previously reported catalysts, complexes 1, 2 and 3 exhibited superior
catalytic activity than [{VO(CH3OH)(CH3O)}2(L2)] [24] (where H4L2 = N'1,N'2-bis(2-
hydroxybenzylidene)oxalohydrazine) for the oxidation of cyclohexene, cyclooctene and styrene.
However, these homogeneous catalysts are also less efficient compared to [VVO(OCH3)(L2)]
(where H2L2 = (E)-3-hydroxy-N′-(2-hydroxybenzylidene)-2-naphthohydrazide) and µ-oxido-
bis({R(-)-2-[(2-oxidopropyl)iminomethyl]-4-nitrophenolato-κ3N,O,O/}oxidovanadium(V)) for
the oxidation of α-methyl styrene [35] and styrene [39], respectively.
Although the details of catalytic mechanism are not studied in the present work, on the basis of
the electronic absorption spectroscopy studies (Fig. S13) and also by considering the previous
studies on catalytic oxidation by oxidovanadium complexes [15, 40, 41], 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.
Conclusion
In summary, new vanadium(V) complexes were synthesized and characterized by various
analyses and their structure were determined by single crystal X-ray analysis. In the present work
we found electron delocalization on Nhydrazide–C–Nthiazole atoms as a new feature for this kind of
ligands. The new complexes were used as catalyst in the oxidation of olefins in the presence of
hydrogen peroxide as a green oxidant. The catalytic potential of the above-mentioned
vanadium(V) complexes in the oxidation of olefins were studied, choosing the oxidation of
cyclooctene, styrene, α-methyl styrene, indene, cyclohexene, 1-octene and 1-decne as the model
reactions. These complexes are able to catalyze the oxidative conversion of styrene to styrene
oxide and its successive products, and cyclohexene to cyclohexene oxide, 2-cyclohexene-1-ol, α-

methyl styrene to α-methyl styrene epoxide and acetophenone, while the present catalytic system
was completely selective for the epoxidation of cyclooctene, indene, 1-octene and 1-decene. All
the catalytic processes have clean, safety and operationally simple characteristics.
Acknowledgments
The authors are grateful to the University of Zanjan support of this study.
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