Six-coordinated vanadium(IV) complexes with tridentate task-specific ionic liquid Schiff base ligands: Synthesis, characterization and effect of ionic nature on catalytic activity

Article abstract of DOI:10.1002/aoc.4078

By reaction of 5-(chloromethyl)salicylaldehyde with triphenylphosphine and N-methylimidazole in two separate reactions, salicylaldehydetriphenylphosphonium chloride (S²) and salicylaldehydemethylimidazolinium chloride (S³) were prepared. Reaction of 2-(aminomethyl)pyridine with these aldehydes resulted in the task-specific ionic liquid Schiff base ligands L¹ and L², respectively. Then six-coordinated vanadium(IV) Schiff base complexes of VO(acac)L^1–4 were synthesized by reactions of these tridentate Schiff base ligands and VO(acac)₂ in 1:1 stoichiometry. The aldehydes, ligands and VO(acac)L^1–4 complexes were characterized using infrared, ¹H NMR, ¹³C NMR, ³¹P NMR, UV–visible and mass spectroscopies, as well as elemental analysis. Paramagnetic property of the complexes was also studied using magnetic susceptibility measurements. The complexes were used as catalysts in epoxidation of cyclooctene and oxidation of methylphenyl sulfide and the reaction parameters were optimized. The effect of the ionic nature of the complexes was investigated in these oxidation reactions. The catalytic activity of the complexes could be varied by changing the ionic (cationic or anionic) character of VO(acac)L^1–4 catalysts in which counter anion variation showed a greater effect than cationic moiety variation.

Full text of DOI:10.1002/aoc.4078

Received: 8 May 2017
DOI: 10.1002/aoc.4078
Revised: 12 August 2017
Accepted: 13 August 2017
F U L L P A P E R
Six‐coordinated vanadium(IV) complexes with tridentate
task‐specific ionic liquid Schiff base ligands: Synthesis,
characterization and effect of ionic nature on catalytic
activity
Somayeh Azizi Talouki1 | Gholamhossein Grivani1
| Aliakbar Dehno Khalaji2
1 School of Chemistry, Damghan
By reaction of 5‐(chloromethyl)salicylaldehyde with triphenylphosphine and N‐
methylimidazole in two separate reactions, salicylaldehydetriphenylphosphonium
chloride (S2) and salicylaldehydemethylimidazolinium chloride (S3) were pre-
pared. Reaction of 2‐(aminomethyl)pyridine with these aldehydes resulted in the
task‐specific ionic liquid Schiff base ligands L1 and L2, respectively. Then six‐
coordinated vanadium(IV) Schiff base complexes of VO(acac)L1–4 were synthe-
sized by reactions of these tridentate Schiff base ligands and VO(acac)2 in
1:1 stoichiometry. The aldehydes, ligands and VO(acac)L1–4 complexes were
University, Damghan 3671641167, Iran
2 Department of Chemistry, Faculty of
Science, Golestan University, Gorgan, Iran
Correspondence
Gholamhossein Grivani, School of
Chemistry, Damghan University,
Damghan 3671641167, Iran.
Email: grivani@du.ac.ir
Funding information
Damghan University (DU)
characterized using infrared, H NMR, 13C NMR, 31P NMR, UV–visible and
1
mass spectroscopies, as well as elemental analysis. Paramagnetic property of
the complexes was also studied using magnetic susceptibility measurements.
The complexes were used as catalysts in epoxidation of cyclooctene and oxida-
tion of methylphenyl sulfide and the reaction parameters were optimized. The
effect of the ionic nature of the complexes was investigated in these oxidation
reactions. The catalytic activity of the complexes could be varied by changing
the ionic (cationic or anionic) character of VO(acac)L1–4 catalysts in which
counter anion variation showed a greater effect than cationic moiety
variation.
KEYWORDS
ionic liquid, methylphenyl sulfide, oxidation reaction, Schiff base ligand, vanadium(IV) complex
1 | INTRODUCTION
and preventive activity against carcinogenesis,[2] are only
a few examples of vanadium complexes which can be
The growing interest in recent years in the coordination
chemistry of vanadium has been generated by the wide
range of biological and catalytic properties of particular
importance in the biosphere. Five‐ and six‐coordinated
vanadium complexes possessing potential medicinal appli-
cations, such as the treatment of diabetes type I and II[1]
found in nature. Moreover, Schiff base complexes can be
widely employed as catalysts, e.g. in the stereoselective
synthesis of cyclic ethers,[3] asymmetric alkynylation of
aldehydes,[4] epoxidation of alkenes,[5] hydroxylation of
phenols,[6] oxidation of bromides,[3] oxidative kinetic reso-
lution of α‐hydroxyesters[7] or the enantioselective
Appl Organometal Chem. 2017;e4078.
wileyonlinelibrary.com/journal/aoc
Copyright © 2017 John Wiley & Sons, Ltd.
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https://doi.org/10.1002/aoc.4078

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TALOUKI ET AL.
oxidation of organic sulfides.[4,8] Optically active sulfoxides
possess a wide range of biological activities, e.g. antimicro-
bial properties,[9] inhibition of biosynthesis of uric acid[10]
and gastric acid secretion[11] or regulation of cholesterol
catabolism.[12]
materials were obtained using a Varian CP‐3800 instru-
1
ment with a silicon‐DC 200 column. All the H NMR
spectra of S1, S2, S3, L1 and L2 and 31P NMR spectra
of S2 and L1 as well as 13C NMR spectra of L1 and L2
are given in the supporting information.
Ionic liquids have not only become increasingly popu-
lar as reaction and extraction media in research and devel-
opment, but also they have widely been promoted as ‘green
solvents’, which are regarded as powerful alternatives to
volatile organic compounds in the field of organic synthe-
sis. Furthermore, task‐specific ionic liquids, where a func-
tional group is covalently tethered to the cation or anion
(or both) of the ionic liquid, are the latest generation of
ionic liquids. The incorporation of this functionality
should imbue an ionic liquid with a capacity to behave
not only as a reaction medium but also as a reagent or cat-
alyst in some reactions or processes.[13] Moreover, water‐
soluble ligands and their metal complexes containing ionic
liquid fragments have emerged in the last decade as a syn-
thetic strategy for homogeneous catalysis.
The investigation of the effect of the ionic nature of
task‐specific ionic liquid vanadyl Schiff base complexes
is very rare. Therefore, in this paper, we describe the
synthesis of task‐specific ionic liquid Schiff base ligands
and their vanadyl complexes, namely VO(acac)L1,
VO(acac)L2, VO(acac)L3 and VO(acac)L4 (Scheme 1),
and report an investigation of the catalytic activity of
them in the epoxidation of cyclooctene and oxidation
of methylphenyl sulfide as well as the effect of ionic
nature in these reactions.
2.2 | Synthesis of 5‐(Chloromethyl)
salicylaldehyde [C8H7ClO2] (S1)
Salicylaldehyde (15.25 g, 0.12 mol) and paraformaldehyde
(2.25 g, 0.075 mol) were added to concentrated hydrochlo-
ric acid (100 ml) and the content stirred for 48 h at room
temperature. After filtration, the white solid was washed
five times with 1 M sodium hydrogen carbonate solution
(each time with 70 ml) and dried under vacuum.[14] Yield:
10.85 g (51%). M.p. 84 °C. Anal. Calcd for (C8H7ClO2) (%):
C, 56.34; H, 4.10; N, 0. Found (%): C, 56.56; H, 4.07; N, 0. IR
(KBr pellet, cm−1): ν(O―H), 3340; ν(C―H)aromatic, 2890–
3010; ν(C―;H)aliphatic, 2780–2870; ν(C―H)aldehyde, 2784;
ν(C═O), 1654; ν(C═C), 1483–1578; ν(C―O), 1192. 1H
NMR (δ, ppm; CDCl3): 11.09 (1H, Ha); 7.03(1H, Hb); 7.52
(1H, Hc); 7.61 (1H, Hd); 9.90 (1H, He); 4.60 (2H, Hf).
2.3 | Synthesis of task‐specific ionic
liquids [C26H22O2P+]Cl− (S2) and
+
[C12H13O2N2 ]Cl− (S3)
A solution of triphenylphosphine (1.53 g, 5.8 mmol) in
dry acetonitrile (35 ml) was added to S1 (1 g, 5.8 mmol).
The solution was refluxed for 17 h. After filtration, light
yellow solid of S2 was obtained. Yield: 2.05 g (81%). M.p.
97 °C. Anal. Calcd for (C26H22O2P+)Cl− (%): C, 72.15;
H, 5.08. Found (%): C, 71.69; H, 4.74. IR (KBr pellet,
2 | EXPERIMENTAL
2.1 | Materials and techniques
cm−1): ν(O―H), 3345; ν(C―H)aromatic
ν(C―H)aliphatic 2785–2870; ν(C―H)aldehyde,
,
2990–3000;
2690;
All reagents and solvents for synthesis and analysis
were purchased from Merck and used as received with-
out further purifications. tert‐Butyl hydroperoxide (80%)
in di‐tert‐butyl peroxide (TBHP) solution and 30%
H2O2 solution from Merck were used in catalytic activ-
ity investigations. Infrared (IR) spectra were recorded
in the range 400–4000 cm−1 with a PerkinElmer FT‐IR
spectrophotometer using KBr pellets. Elemental analyses
were performed with a Heraeus CHN–O‐Rapid analyser
and results agreed with calculated values. UV–visible
spectra were recorded with a PerkinElmer Lambda 25
spectrometer. The NMR spectra of aldehydes and
ligands were recorded with a Bruker 400 MHz spectro-
photometer. The mass spectra of complexes were
recorded with a Shimadzu spectrometer (GCMS‐QP
1000 EX). Paramagnetic property of complexes was
studied with a magnetic susceptibility balance (Sher-
wood Scientific). All GC yields based on the starting
,
ν(C═O), 1667; ν(C═C), 1434–1587; ν(C―O), 1151. 1H
NMR (δ, ppm; DMSO‐d6): 11.26 (1H, Ha); 7.03(1H,
Hb); 7.05 (1H, Hc); 7.20 (1H, Hd); 10.15 (1H, He);
5.13–5.18 (2H, Hf); 7.64–7.76 (9H, Hg(3,4,5)); 7.86–7.90
(6H, Hg(2,6)). 31P{1H} NMR (δ, ppm; DMSO‐d6): 22.7.
According to the above method, S3 was obtained using
1‐methylimidazole (0.47 g, 5.8 mmol) instead of
triphenylphosphine. Yield: 1.39 g (94%). M.p. 101 °C.
Anal. Calcd for (C12H13O2N2+)Cl− (%): C, 57.06; H,
5.14; N, 11.08. Found (%): C, 56.91; H, 4.87; N, 10.96.
IR (KBr pellet, cm−1): ν(O―;H), 3445; ν(C―H)aromatic
3048–3138; ν(C―;H)aliphatic, 2705–2890; ν(C―H)aldehyde
2569; ν(C═O), 1674; ν(C═C), 1445–1613; ν(C―O),
1153. 1H NMR (δ, ppm; CDCl3): 11.43 (1H, Ha); 7.05
(1H, Hb); 7.11 (1H, Hc); 7.14 (1H, Hd); 9.98 (1H, He);
6.43 (2H, Hf); 8.05–8.06 (1H, Hg); 5.64 (1H, Hh); 7.61–
7.66 (1H, Hi); 3.99–4.05 (3H, Hj).
,
,

TALOUKI ET AL.
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SCHEME 1 Synthetic procedures for task‐specific ionic liquid aldehydes S1–3, Schiff base ligands L1,2 and complexes VO(acac)L1–4, and the
labelling of protons and carbons in aldehydes and ligands
(2CH3OH) (%): C, 69.67; H, 6.14; N, 4.78. Found (%): C,
2.4 | Synthesis of task‐specific ionic liquid
69.92; H, 5.82; N, 5.15. IR (KBr pellet, cm−1): ν(O―H)phe-
Schiff Base ligands [C32H28ON2P+]Cl−.
+
(2CH3OH) (L1) and [C18H19ON4 ]Cl−.
nolic, 3344–3382; ν(C―H)aromatic, 3053; ν(C―H)aliphatic
,
(2CH3OH) (L2)
2858–2954; ν(C═N), 1635; ν(C═C) 1436–1587; ν(C―O),
1
1159. H NMR (δ, ppm; DMSO‐d6): 13.39 (1H, Ha); 7.18
Aldehyde S2 (0.64 g, 1.48 mmol) was added to a solution
of 2‐(aminomethyl)pyridine (0.16 g, 1.48 mmol) in dry
methanol (25 ml) and stirred for 4 h in reflux conditions.
After evaporation of the solvent, an orange oily product of
L1 was obtained that was dried under vacuum for 24 h
and a yellow film‐like solid was obtained. Yield: 0.84 g
(97%). M.p. 110 °C. Anal. Calcd for (C32H28ON2P+)Cl−.
(1H, Hb); 7.23 (1H, Hc); 7.27 (1H, Hd); 8.42–8.44 (1H,
He); 5.24–5.28 (2H, Hf); 7.52–7.67 (15H, Hg); 4.74 (2H,
Hh); 6.78–6.81 (1H, Hi); 7.08–7.12 (1H, Hj); 6.52–6.55
(1H, Hk); 8.24 (1H, Hl). 13C{1H} NMR (δ, ppm; DMSO‐
d6): 33.1 (Cf), 59.0 (Ch), 117.2 and 117.3 (Cg‐Ipso), 118.3
(Cy), 119.7 (Cs), 122.2 (Cj), 122.5 (Cw), 129.9 (Ci), 130.1
(Cb), 130.2 (Cl), 133.9 (Cc), 134.1 (Cd), 134.7 (Cg‐ortho or

4 of 12
TALOUKI ET AL.
meta), 134.8 (Cg‐ortho or meta), 135.0 (Cg‐para), 137.0 (Ck),
157.4 (Ca), 160.8 (Ce). 31P{1H} NMR (δ, ppm; DMSO‐d6):
22.6. UV–visible (nm): 269, 321. According to the above
procedure, ligand L2 was synthesized using S3 (0.86 g,
3.4 mmol) instead of S2 with 2‐(aminomethyl)pyridine
(0.37 g, 3.4 mmol). Yield: 1.32 g (96%). M.p. 122 °C. Anal.
Calcd for (C18H19ON4+)Cl−.(2CH3OH) (%): C, 59.04; H,
6.64; N, 13.77. Found (%): C, 58.98; H, 5.97; N, 14.12. IR
(KBr pellet, cm−1): ν(O―H)phenolic, 3844–3401; ν(C―H)
aromatic, 3068; ν(C―H)aliphatic, 2843–2964; ν(C═N), 1637;
ν(C═C) 1436–1590; ν(C―O), 1163. 1H NMR (δ, ppm;
DMSO‐d6): 13.50 (1H, Ha); 7.65 (1H, Hb); 7.72 (1H, Hc);
7.84 (1H, Hd); 9.35 (1H, He); 5.37 (2H, Hf); 8.72 (1H,
Hg); 5.29 (1H, Hh); 8.16 (1H, Hi); 3.83–3.85 (3H, Hj); 4.91
(2H, Hk); 7.23–7.30 (1H, Hl); 7.38–7.44 (1H, Hm); 6.86–
6.97 (1H, Hn); 8.52–8.54 (1H, Ho). 13C{1H} NMR (δ, ppm;
DMSO‐d6): 35.8 (Cj), 48.8 (Cf), 51.8 (Ck), 118.2 (Cy),
119.6 (Cs), 120.6 (Ci), 121.6 (Ch), 121.9 (Cg), 122.0 (Cm),
123.7 (Cw), 123.8 (Cl), 125.5 (Co), 132.1 (Cb), 135.6 (Cc),
136.2 (Cd), 136.4 (Cn), 157.5 (Ca), 167.8 (Ce). UV–visible
(nm): 271, 319.
2.6 | Synthesis of task‐specific ionic liquid
Vanadyl Schiff Base complexes
−
[C37H34VO4N2P+]PF6 (VO(acac)L3) and
+
[C23H25VO4N4 ]PF6− (VO(acac)L4)
Anion exchange was carried out at room temperature in
water solution, between anions of Cl− and PF6 for
−
these complexes. The reaction of 1:1 solutions of
VO(acac)L1 (0.51 g, 0.65 mmol) and potassium
hexafluorophosphate (0.12 g, 0.65 mmol) resulted in
synthesis of complex VO(acac)L3. Yield: 0.44
g
(86%). M.p. 190 °C. Anal. Calcd for (C37H34VO4N2P+)
−
PF6 (%): C, 55.07; H, 4.26; N, 3.51. Found (%): C,
54.44; H, 4.01; N, 3.86. IR (KBr pellet, cm−1): ν(C―H)
aromatic, 3064–3317; ν(C―H)aliphatic, 2921, 2983; ν(C═N),
1635; ν(C═C), 1471–1598; ν(C―O), 1163; ν(V═O), 951.
Mass (m/z): 796 (M+), 410, 262, 230, 183, 115, 105, 89,
77, 63, 49 (M+). UV–visible (nm): 271, 382, 549 (d–d).
According to the above method, the anion exchange
for complex VO(acac)L2 was carried out in 1:1 solutions
of VO(acac)L2 (0.52 g, 0.91 mmol) and potassium
hexafluorophosphate (0.17 g, 0.91 mmol) resulting in
the synthesis of complex VO(acac)L4. Yield: 0.47 g
+
2.5 | Synthesis of task‐specific ionic liquid
Vanadyl Schiff Base complexes
(84%). M.p. 218 °C. Anal. Calcd for (C23H25VO4N4
)
−
PF6 (%): C, 44.5; H, 4.03; N, 9.03. Found (%): C,
[C37H34VO4N2P+]Cl−.(2CH3OH.2H2O)
45.01; H, 3.84; N, 9.16. IR (KBr pellet, cm−1): ν(C―H)
+
(VO(acac)L1) and [C23H25VO4N4 ]Cl−.
,
3157; ν(C―H)aliphatic, 2917; ν(C═N), 1633;
aromatic
(2CH3OH) (VO(acac)L2)
ν(C═C),1473–1593; ν(C―O), 1163; ν(V═O), 948. Mass
(m/z): 616 (M+), 615, 328, 286, 191, 164, 100, 92, 85,
82, 67, 43 (M+). UV–visible (nm): 271, 381, 551 (d–d).
VO(acac)2 (0.4 g, 1.5 mmol) was dissolved in 35 ml of dry
methanol. This solution was added to a solution of L1
(0.79 g, 1.5 mmol) in 25 ml of methanol and the content
was stirred for 4 h in reflux condition. By evaporation of
solvent, a dark brown oily product of VO(acac)L1 was
obtained. After drying in a vacuum oven a greenish
film‐like solid was obtained. Yield: 0.98 g (93%). M.p.
207 °C. Anal. Calcd for (C37H34VO4N2P+)Cl−.
(2CH3OH.2H2O) (%): C, 60.82; H, 5.51; N, 3.83. Found
(%): C, 60.53; H, 5.16; N, 4.04. IR (KBr pellet, cm−1):
ν(C―H)aromatic, 3311–3404; ν(C¼H)aliphatic, 2810, 2958;
ν(C═N), 1625; ν(C═C), 1519–1587; ν(C―O), 1164;
ν(V═O), 946. Mass (m/z): 787 (M+), 786, 683, 262, 183,
166, 152, 107, 92, 77, 63, 49, 43 (M+). UV–visible (nm):
271, 380, 550 (d–d). Complex VO(acac)L2 was synthesized
according to the above procedure by the reaction between
L2 (1.17 g, 3.4 mmol) and VO(acac)2 (0.91 g, 3.4 mmol).
Yield: 1.49 g (91%). M.p. 157 °C. Anal. Calcd for
(C23H25VO4N4+)Cl−.(2CH3OH) (%): C, 52.50; H, 5.77; N,
9.80. Found (%): C, 51.83; H, 5.41; N, 9.16. IR (KBr pellet,
cm−1): ν(C―H)aromatic, 3078–3390; ν(C–H)aliphatic, 2832,
2950; ν(C═N), 1634; ν(C═C), 1314–1592; ν(C―O), 1163;
ν(V═O), 947. Mass (m/z): 571 (M+), 388, 345, 265, 249,
183, 166, 158, 100, 92, 85, 82, 67, 43 (M+). UV–visible
(nm): 271, 381, 552 (d–d).
3 | RESULTS AND DISCUSSION
3.1 | Synthesis
S1 was prepared as described previously.[14] Then by reac-
tion of S1 with triphenylphosphine and methylimidazole,
the task‐specific ionic liquids S2 and S3 were synthesized,
respectively. After synthesis of Schiff base ligands L1 and
L2, the reactions of them with VO(acac)2 resulted in the
task‐specific ionic liquid vanadyl Schiff base complexes
VO(acac)L1,2. Then the anion exchange of Cl− by PF6
−
carried out for VO(acac)L1,2 led to synthesis of complexes
VO(acac)L3,4. Also, the solubility of the task‐specific ionic
liquid Schiff base ligands and their complexes was investi-
gated in various organic solvents and water. They are sol-
uble in most of solvents, especially in water. The synthetic
procedures are shown in Scheme 1.
3.2 | IR spectra
The IR spectra of S1, S2 and S3 show absorptions at 3340,
3345 and 3445 cm−1, which are related to O―H bonds

TALOUKI ET AL.
5 of 12
and also show absorptions at 1654, 1667 and 1674 cm−1 for
the C═O bonds. Broad bands at 3344–3382 and 3384–
3401 cm−1 in the IR spectra of ligands L1 and L2 can be
related to O―H of phenol and CH3OH. A similar broad
band is seen in the IR spectra of VO(acac)L1,2 related to
O―H of CH3OH and/or H2O which is not seen in the IR
spectra of complexes VO(acac)L3,4. The fundamental
stretching mode of the azomethine moiety νC═N for the free
ligands L1 and L2 appears at 1635 and 1637 cm−1, respec-
tively, which shifts to lower frequencies and seen at 1625,
1634, 1635 and 1633 cm−1 in the IR spectra of VO(acac)
L1–4. These changes are attributed to the coordination of
nitrogen in azomethine of ligands to the V(IV) centre. As
is generally seen, complexes VO(acac)L1–4 showed νV═O
at around 946, 947, 951 and 948 cm−1 as monomeric
vanadyl Schiff base complexes.[15,16]
7.08–7.12, 6.52–6.55 and 8.42 ppm attributed to the pro-
tons of Hh, Hi, Hj, Hk and Hl for L1 and at about 5.29,
8.16, 3.83–3.85, 4.91 and 7.23–7.30 ppm for L2, respec-
tively. Other signals are seen at about 7.38–7.44, 6.86–
6.97 and 8.52–8.54 ppm related to the protons of Hm, Hn
and Ho, respectively, for L2.
The 13C NMR spectra of L1 showed chemical shifts at
33.1, 59.0, 118.3, 119.7, 122.2, 122.5, 129.9 and 130.2 ppm
related to Cf, Ch, Cy, Cs, Cj, Cw, Ci and Cl, as well as sig-
or
nals of (Cg‐Ipso), (Cg‐ortho
meta) and (Cg‐para) that
appeared at 117.2, 134.7, 134.8 and 135.0 ppm, respec-
tively. For salicylate moiety, resonances at 130.1, 133.9
and 134.1 ppm were seen for Cb, Cc and Cd, respectively,
and other resonances at 137.0, 157.4 and 160.8 ppm are
related to Ck, Ca and Ce. Also, for L2 resonances at 35.8,
48.8, 51.8, 118.2, 119.6, 122.0, 123.7, 123.8 and
125.5 ppm are related to Cj, Cf, Ck, Cy, Cs, Cm, Cw, Cl
and Co, respectively, and the carbon signals of Ci, Ch
and Cg appeared at 120.6, 121.6 and 121.9 ppm, as well
as resonances at 132.1, 135.6 and 136.2 ppm correspond-
ing to Cb, Cc and Cd. Other resonances at 136.4, 157.5
and 167.8 ppm are related to Cn, Ca and Ce, respectively.
In addition, the 31P NMR spectra were recorded for S2
and L1. These showed a singlet resonance at 22.7 and
22.6 ppm for S2 and L1, respectively.
3.3 | NMR spectra
1
The obtained H NMR spectra of VO(acac)L1–4 did not
give useful information because of their paramagnetic
1
properties. But the H NMR spectra of S1, S2, S3, L1 and
L2 were obtained. The proton labelling of S1, S2, S3, L1
and L2 is given in Scheme 1. The chemical shifts for S1
at 11.09, 7.03, 7.52 and 7.61 ppm were seen for Ha, Hb,
Hc and Hd protons of salicylate moiety. These resonances
for S2 appeared at 11.26, 7.03, 7.05, 7.20 ppm and for S3 at
11.43, 7.05, 7.11, 7.14 ppm. The resonances at 9.90, 10.15
and 9.98 ppm corresponded to He in S1, S2 and S3, respec-
tively, and proton signals of methylene group also
appeared at 4.60, 5.13–5.18 and 6.43 ppm related to Hf
in S1, S2 and S3. The proton signals of Hg(3,4,5) from phenyl
group of triphenylphosphonium moiety in S2 were seen at
7.64–7.76 ppm and other proton signals of Hg(2,6) were
observed at 7.86–7.90 ppm. In addition, the proton signals
of Hg for S3 were seen at 8.05–8.06 ppm and the signals at
5.64, 7.61–7.66 and 3.99–4.05 ppm are related to Hh, Hi
and Hj of S3, respectively. For ligands L1 and L2 in the
downfield region of each spectrum, one singlet resonance
appeared at 13.39 and 13.50 ppm, respectively, attributed
to the phenolic proton of Ha. These signals were shifted
downfield because of the formation of strong intramolec-
ular hydrogen bonding between the phenolic proton and
nitrogen of imine. Some other signals appeared at about
7.18, 7.23, 7.27 and 8.42–8.44 ppm corresponding to the
aromatic protons of Hb, Hc, Hd and iminic proton He for
L1 and at 7.65, 7.72, 7.84 and 9.35 ppm for L2, respectively.
In addition, the resonance of Hf was observed at 5.24–5.28
and 5.37 ppm for L1 and L2 and the protons of aromatic
rings of triphenylphosphonium showed signals in the
region 7.52–7.67 ppm for L1 and the resonance of Hg pro-
ton of imidazolinum moiety in L2 appeared at 5.41 ppm.
Some other resonances are seen at about 4.74, 6.78–6.81,
3.4 | UV–visible spectra
The UV–visible spectra of ligands and complexes are
shown in Figure 1. The spectra of VO(acac)L1–4 were
broader than those of free ligands of L1,2 because of the
coordination of anionic form of ligands (as phenolate)
and resonance of charge in a longer route.[17] The UV–vis-
ible spectra of ligands in DMSO showed two bands at 269
and 321 nm for L1 and two bands at 271 and 319 nm for L2
due to the π → π* and n → π* transitions, respectively.
For VO(acac)L1–4, these bands appeared at 271 and
380 nm for VO(acac)L1, 271 and 381 nm for VO(acac)L2,
271 and 382 nm for VO(acac)L3 and finally 271 and
381 nm for VO(acac)L4. In addition, very weak bands cor-
responding to d–d transitions appeared at 550, 552, 549
and 551 nm for VO(acac)L1–4, respectively.
3.5 | Mass spectra
Mass spectrometric data for VO(acac)L1–4 are consistent
with the structures proposed on the basis of other spectro-
scopic data. In some cases, the ion molecule shows very
low intensity and in rare cases is not seen.[18] The mass
spectrum of VO(acac)L1 shows expected signals at m/z of
787 (M+), 786, 683, 262, 183, 166, 152, 107, 92, 77, 63, 49
and 43. The molecular weight of complex [VO(acac)L1].
(2CH3OH.2H2O) is 787 (M+) that is not seen, but with loss

6 of 12
TALOUKI ET AL.
molecular weight of [VO(acac)L2.2CH3OH] and the base
peak at m/z 85 is related to the loss of methyl group from
acac fragment. The sharp peak at m/z 388 is related to the
residue of molecular weight by loss of 2CH3OH and
HCNCH2Py fragment. Also, the peak at m/z 345 is due
to the CIm+―CH2C6H3OVO(acac) fragment by loss of
four hydrogens, with formula of C16H15N2VO4+. The
peaks at m/z 265 and 249 are related to the CIm
+―CH2C6H3VO3 and CIm+―CH2C6H3VO2 fragments
with formulas of C11H7N2VO3 and C11H7N2VO2, respec-
tively. Other peaks at m/z 183, 166, 158 and 100 are attrib-
utable to CIm+―CH2C6H3O with formula of C11H8N2O,
VO(acac), CH2PyVO and acac that gained one hydrogen,
respectively, and the signal at m/z 92 is related to CH2Py.
Also the signals at m/z 82 and 67 are due to
methylimidazole and the loss of methyl group from it,
with formulas of C4H6N2 and C3H3N2, respectively.
Finally the peak at m/z 43 is related to the HN═CH―NH2
fragment that resulted from decomposition of the imidaz-
ole part, with loss of one hydrogen.
Similarly, the mass spectra of complexes VO(acac)L3,4
confirm their chemical composition. The mass spectrum
of VO(acac)L3 shows signals at m/z 796 (M+), 410, 262,
230, 183, 115, 105, 89, 77, 63 and 49. The molecular weight
of VO(acac)L3 is represented by m/z 796, but this signal is
not seen because of the instability of the ligand in the ion
source of the mass spectrometer, but the molecular weight
will be proven by the fragments in the mass spectrum. The
signal at m/z 410 is related to the residue of molecular
weight by loss of [HCNCH2Py] and +PPh3 fragments and
also loss of five hydrogens from the complex. The base peak
at m/z 77 is attributed to the phenyl group and the signal at
m/z 262 is related to +PPh3. Signals at m/z 230, 183 and 115
are attributed to the CH2C6H3(CHN)VO4, +PPh2 and
CH2C6H3HC═N fragments, respectively. Also the signals
at m/z 105 and 89 are related to the CH2C6H3O and CH2Py
fragments by loss of three hydrogens. Finally, the signals at
m/z 63 and 49 are attributed to the loss of first and second
methylene groups from phenyl group, respectively.
FIGURE 1 UV–visible spectra of (a) L1 and VO(acac)L1,3 and (b)
L2 and VO(acac)L2,4 (10−4 M in DMSO)
of one hydrogen atom, a signal at m/z 786 is seen. The peak
at m/z 683 is attributed to the residue of molecular weight
by loss of 2CH3OH and 2H2O and also four hydrogen
atoms. The base peak at m/z 43 is related to the +PC frag-
ment and other signals at m/z 262, 183, 166 and 152 are
The mass spectrum of VO(acac)L4 exhibits peaks at m/
z 616 (M+), 615, 328, 286, 191, 164, 100, 92, 85, 82, 67 and
43. The molecular weight of VO(acac)L4 is equal to 616
(M+), the peak of which is not seen, but with loss of one
hydrogen atom, the signal at m/z 615 is seen. Also the
base peak at m/z 43 is related to the HN═CH―NH2 frag-
ment that results from decomposition of the imidazole
part, with loss of one hydrogen. The signal at m/z 328 is
attributed to the residue of molecular weight by loss of
HCNCH2Py and VO(acac) fragments and also three
hydrogen atoms from the complex. Signals at m/z 286
and 191 are related to the HCNCH2PyVO(acac) and
CH2PyVO3 fragments, respectively. Other peaks at m/z
164 and 100 are attributable to loss of two hydrogens from
+
+
related to the PPh3, PPh2 and VO(acac) fragments and
the ion produced from VO(acac) by loss of methyl group
and then adding one hydrogen atom resulting in formula
of C4H6VO3. Signals at m/z 107 and 92 are attributed to
the +PPh and acac fragments with formula of [C5H7O2].
Finally, the signal at m/z 77 is attributed to the phenyl
group, and by loss of the first and second methylene
groups, signals at m/z 63 and 49 appeared, respectively.
The mass spectrum of complex VO(acac)L1 is shown in
Figure 2.
The mass spectrum of VO(acac)L2 exhibits signals at
m/z 571 (M+), 388, 345, 265, 249, 183, 166, 158, 100, 92,
85, 82, 67 and 43. The peak at m/z 571 is assigned to the

TALOUKI ET AL.
7 of 12
FIGURE 2 Mass spectrum of VO(acac)L1 complex
VO(acac) and acac fragment that gained one hydrogen,
respectively. Also the signal at m/z 92 is related to the
CH2Py fragment and that at m/z 85 is attributed to loss
of methyl group from acac fragment. Finally the signals
at m/z 82 and 67 are due to the methylimidazole fragment
with formula C4H6N2 and loss of the methyl group from it
with formula C3H3N2, respectively.[18]
states +4 or +5.[19–21] As one‐electron system of six‐coor-
dinated V(IV) complexes with 3d1 electron configuration
and S = 1/2 possesses an orbitally nondegenerate ground
state (2B2g), by the static Jahn–Teller distortion (z‐in),
partially quenched orbital contributions to the magnetic
moment and also g|| and g⊥ are not substantially different
from 2. Therefore, the g‐values are close to 2. It is expected
to obey Curie–Weiss behaviour with near or equal to zero
and moments close to the spin‐only value. In fact, the
Curie law is followed with magnetic moments close to
the spin‐only value. The magnetic susceptibilities of the
complexes VO(acac)L1–4 were measured at 292 K and
the results are presented in Table 1. The low symmetry
of complexes VO(acac)L1–4 removes the orbital degener-
acy so the experimental magnetic moment values (μeff)
3.6 | Magnetic properties
The total measured magnetic susceptibility (χmeas) was
determined using the equation χmeas = χgMw, in which
χg is mass susceptibility, where χg =[CBall(R −R0)]/[(m
−m0) ×109]. On the other hand, χmeas is defined as the
sum of contributions: χmeas = χP + χD, where χD is dia-
magnetic correction made with Pascal's constants for all
constituent atoms and χP is paramagnetic susceptibility.
The latter can be related to the number of unpaired elec-
trons in a complex and it is measured after determination
of the value of χD using the equation χP = χmeas − χD.
Then the magnetic moment was calculated using the
equation μeff = 2.828(χPT)1/2, where T is absolute temper-
ature. This is equivalent to μeff = [n(n + 2)]1/2
(n = unpaired electrons) when the spin‐only formula
can be used which is usual for first row transition metals
when the orbital contribution is largely quenched. Hence,
if μeff = 1.73, then n = 1. Matching of μeff to the nearest
value to find n and comparison of this value with the the-
oretical one predicted knowing the element and its oxida-
tion state such as the emphasis on vanadium in oxidation
TABLE 1 Measurements of magnetic susceptibilities (χ) and μeff
μeff
(calc.)
Complex
(V4+),
=
μspin‐
χg (× χmeas
10−7 (×10−4) (×10−6
9.12) 7.186
χD
χP
μeff
(3d1)
)
)
(×10−3) (exp.)
only
VO(acac)
L1
−518.62 1.237
−333.37 1.207
−1122.36 1.231
−996.83 1.217
1.70
1.67
1.69
1.68
1.73
VO(acac) 15.3
L2
8.742
1.73
1.73
1.73
VO(acac)
L3
1.36 1.090
3.58 2.208
VO(acac)
L4

8 of 12
TALOUKI ET AL.
for paramagnetic vanadium complexes VO(acac)L1–4 at
292 K are very close to the spin‐only value expected for
one unpaired electron with S = 1/2 for uncoupled spin
systems (1.73). Thus these measurements confirm the oxi-
(Table 2). The results show that the activity of the complex
VO(acac)L1 in the epoxidation of cyclooctene in the pres-
ence of TBHP in CHCl3 is higher than in the presence of
H2O2 and NaIO4. This may be related to the ability of
TBHP and the inability of H2O2 and NaIO4 to mix with
the organic substrate phase. In addition, we optimized
the effect of cyclooctene/oxidant ratio (by using 1:1.2, 1:2,
1:3 and 1:4 ratios) and the effect of the catalyst amount
(0.005, 0.01 and 0.02 mmol) in the epoxidation reaction of
cyclooctene catalysed by complex VO(acac)L1. According
to these experiments, a cyclooctene/oxidant ratio of 1:3
and 0.01 mmol of catalyst can be chosen as optimal provid-
ing high epoxide yields. The results for other catalysts,
VO(acac)L2–4, under various conditions in the presence of
different oxidants and substrate/oxidant ratio of 1:3, were
obtained and are presented in Table 3.
dation state of complexes VO(acac)L1–4 [21]
.
3.7 | Catalytic activity
3.7.1 | Epoxidation of cyclooctene
In order to assess the catalytic activity of complexes
VO(acac)L1–4 in epoxidation reactions, cyclooctene was
used as a model substrate and various reaction parameters,
such as solvent, oxidant, alkene/oxidant ratio and amount
of catalyst, were optimized with complex VO(acac)L1.
Figure
3 illustrates the results of epoxidation of
cyclooctene in the presence of TBHP as an oxidant with a
catalytic amount of complex VO(acac)L1 in various sol-
vents. The trend of the observed solvent effect was CHCl3
> CH3CN–H2O > C2H4Cl2 > CCl4 > CH3CN > THF >
CH3OH > H2O. The low conversion in THF, CH3OH and
H2O can be related to the high coordination ability of these
solvents to the metal centre. As the coordination ability of
the solvent is increased, the activity of the complex is
decreased.[22,23] Dielectric constant of solvent is another
factor that could affect the catalytic activity. This trend
can be given as:[24] H2O (80.3) > CH3CN (37.5) > CH3OH
(32.7) > C2H4Cl2 (10.3) > THF (7.58) > CHCl3 (4.9) >
CCl4 (2.2). It seems that in protic solvents with high polar-
ity and high dielectric constant (H2O and CH3OH) the low-
est catalytic activity is observed. High activity of the
catalyst is observed in aprotic solvents with moderate
dielectric constants. Among them, high conversion (89%)
was observed in CHCl3 with moderate dielectric constant
and without any coordination ability to the metal centre.
We also investigated different reaction media to obtain
a suitable oxidant for the epoxidation of cyclooctene
In order to characterize the reaction products, in a typ-
ical reaction, after reaction completion under optimal con-
ditions using VO(acac)L1, the cyclooctene epoxide was
isolated by column chromatography. After evaporation of
the solvent and isolation the cyclooctene epoxide, it was
characterized using 1H NMR spectroscopy (see supporting
information (F10) for the 1H NMR spectrum of cyclooctene
epoxide). Thus reactions catalysed by VO(acac)L2–4 were
carried out under these optimum conditions. For
VO(acac)L1 in optimum conditions the yield was 89% in
140 min, whereas these yields when using VO(acac)L2,
VO(acac)L3 and VO(acac)L4 catalysts are 81, 89 and 45%,
respectively, under optimum conditions (Figure 4). Thus
the observed trend in catalytic activity of these catalysts
in epoxidation of cyclooctene was: VO(acac)
L1 ~ VO(acac)L3 > VO(acac)L2 > VO(acac)L4.
As seen from Scheme 1, the coordination spheres of
these catalysts are the same and only the ionic pendant
groups are different. Thus it seems that a change of the
ionic part in these complexes would affect their catalytic
activity. By changing the cationic part of the catalyst from
triphenylphosphonium cation in VO(acac)L1 to
methylimidazolium cation in VO(acac)L2 with constant
anion of Cl−, the catalytic activity was decreased. When
the counter anion of Cl− in VO(acac)L1 was substituted
TABLE 2 Epoxidation of cyclooctene in presence of various oxi-
dants under different conditionsa catalysed by VO(acac)L1
Solvent
Oxidant Time (min) Conversion (%)
CHCl3
TBHP
H2O2
140
140
140
140
140
89
CHCl3
CHCl3
2
NaIO4
12
FIGURE 3 Effect of solvent on conversion of cyclooctene to
cyclooctene epoxide under reflux conditions in the presence of
TBHP as oxidant with catalytic amount of VO(acac)L1 complex.
(Reaction conditions: CHCl3 as solvent, 5 ml; cyclooctene, 0.5 mmol;
TBHP, 1.5 mmol; catalyst, 0.01 mmol)
CH3CN–H2O (3:2) H2O2
CH3CN–H2O (3:2) NaIO4
No reaction
No reaction
aReaction conditions: solvent: 5 ml; cyclooctene: 0.5 mmol; TBHP: 1.5 mmol;
catalyst VO(acac)L1: 0.01 mmol.

TALOUKI ET AL.
9 of 12
TABLE 3 Epoxidation of cyclooctene under various conditionsa catalysed by VO(acac)L1 − 4
Entry
Complex
Solvent
Substrate/oxidant
Oxidant
Time (min)
Conversion (%)
1
VO(acac)L1
CHCl3
1:3
TBHP
140
89
2
VO(acac)L1
VO(acac)L1
VO(acac)L2
VO(acac)L2
VO(acac)L2
VO(acac)L3
VO(acac)L3
VO(acac)L3
VO(acac)L4
VO(acac)L4
VO(acac)L4
CHCl3
CHCl3
CHCl3
CHCl3
CHCl3
CHCl3
CHCl3
CHCl3
CHCl3
CHCl3
CHCl3
1:3
1:3
1:3
1:3
1:3
1:3
1:3
1:3
1:3
1:3
1:3
H2O2
140
140
140
140
140
140
140
140
140
140
140
2
3
NaIO4
TBHP
H2O2
12
81
2
4
5
6
NaIO4
TBHP
H2O2
8
7
89
5
8
9
NaIO4
TBHP
H2O2
9
10
11
12
45
‐
NaIO4
3
aReaction conditions: CHCl3 as solvent: 5 ml; cyclooctene: 0.5 mmol; TBHP: 1.5 mmol; catalyst: 0.01 mmol.
O
V
O
V
O
HO
O
O
O
O
Y
Y
X
X
N
N
O
O
OOH
N
N
X= PPh3, MeIm
Y= Cl- , PF6
-
O
I
O
LVV
O
HO
II
OH
LVV
O
O
O
LVV
O
HO
OH
FIGURE 4 Yields of cyclooctene epoxide obtained under reflux
conditions in the presence of VO(acac)L1–4 complexes. (Reaction
conditions: CHCl3 as solvent, 5 ml; cyclooctene, 0.5 mmol; TBHP,
1.5 mmol; catalyst, 0.01 mmol)
V
LVV
III
O
O
IV
by PF6 to give VO(acac)L3, and keeping the cation (tri-
−
SCHEME
2
Proposed mechanism for the epoxidation of
cyclooctene catalysed by oxovanadium complexes VO(acac)L1–4 in
the presence of TBHP
phenylphosphonium), the catalytic activity of the complex
was not changed appreciably, whereas this trend was not
seen in the case of methylimidazolium cation in which
the catalytic activity of VO(acac)L2, containing Cl− counter
anion, is higher than that of VO(acac)L4 with PF6− counter
anion. Thus the effect of the ionic part of these task‐specific
ionic liquid vanadyl Schiff base complexes in epoxidation
of cyclooctene depends on the nature of cation and anion
of the ionic part.
The mechanistic aspects of the catalytic epoxidation of
alkenes by V(IV) complexes were reviewed by Conte
et al.[25] Polar and radical mechanisms were proposed.
Therefore, we propose the mechanism sketched in
Scheme 2. We suggest that the nitrogen of pyridine in pen-
dant group of Schiff base ligand is coordinated at the oppo-
site site of V═O bond in a weak linkage and firstly it can
be substituted by TBHP in catalytic reaction (Scheme 2).
Thus in similar to five‐coordinated V(IV) Schiff base com-
plexes,[26–28] after coordination of TBHP to the vanadyl cen-
tre, it oxidizes to LVV═O moiety. As we discussed above
concerning the solvent effect, the change in the dielectric
constant and/or polarity of the solvent can have an effect
on the catalytic activity of the catalyst. Based on our search
we did not find any reports of the ionic character of V(IV)
Schiff base complexes in epoxidation reaction being investi-
gated. Thus, here, we study this by choosing the groups on
the ligands to give them ionic character, in order to investi-
gate the effect of the ionic nature of the complexes (catalysts)
on their catalytic activity. It seems that the catalytic activity
of a complex varies by changing in its ionic character. These
observations are in agreement with polar mechanism

10 of 12
TALOUKI ET AL.
(Scheme 2). Changing the ionic character of the catalyst
changes the interaction of the catalyst with its surrounding.
As this ionic change can impose a large change in polarity of
the catalyst, the catalytic activity can be highly affected. By
changing the cationic moiety from triphenyphosphonium
in L1 to imidazolinium in L2, the catalytic activity is
decreased (Figure 4). It can be suggested that positive charge
on phosphonium can be distributed on the three phenyl
groups and thus would have a small effect on the interaction
of the catalyst with its surrounding (substrate and TBHP)
resulting in favourable interaction with substrate and oxi-
dant in the transition state (V in Scheme 2), whereas, in
the case of imidazolinium, the positive charge is concen-
trated on this moiety and has a greater effect on the interac-
tion of the catalyst with reaction media and retards the
formation of activated complex V. This is in agreement with
the observation that changing the counter anion of
imidazolinium moiety from Cl− in VO(acac)L2 to PF6 in
−
VO(acac)L4 leads to a marked decrease in catalytic activity.
It is clear that there is a need for very deep experimental
and theoretical study to confirm these observations, which
is beyond the scope of this research.
Furthermore, the epoxidation of various alkenes
catalysed by VO(acac)L1 was investigated under opti-
mized conditions. The obtained results are presented in
Table 4. As seen from this table, cyclic alkenes were
more efficiently converted to their corresponding epox-
ides than linear alkenes.
3.7.2 | Oxidation of methylphenyl sulfide
It
is
known
that
vanadium‐dependent
haloperoxidases[29,30] and model complexes catalyse the
oxidation of sulfides (thioethers) to sulfoxides and sul-
fones.[31,32] We also investigated the catalytic activity of
Schiff base complexes VO(acac)L1–4 in mimicking
sulfoxidation using methylphenyl sulfide as a model
substrate. The reaction conditions were optimized for
the maximum oxidation of methylphenyl sulfide by
studying three different parameters (oxidation media,
catalyst amount and reaction time) under solvent‐free
conditions. The results for VO(acac)L1 catalyst are sum-
marized in Table 5 and for VO(acac)L2–4 catalysts in
Table 6. The blank reactions were carried out under
the optimum conditions using 4 mmol of MeSPh and
4.4 mmol of TBHP without any catalyst with solvent‐
free conditions, but no appreciable conversions were
seen. In all other cases, i.e. increasing the reaction time,
TABLE 4 Epoxidation of alkenes catalysed by VO(acac)L1 in
presence of TBHP under reflux conditionsa
Alkene
Time (min)
Conversion (%)
140
89
300
57
240
240
18
13
300
2
aReaction conditions: CHCl3 (5 ml), alkene (0.5 mmol), TBHP (1.5 mmol),
catalyst (0.01 mmol).
TABLE 5 Oxidation of methylphenyl sulfide under various conditionsa catalysed by VO(acac)L1
Run Substrate/oxidant Oxidant Catalyst amount (mmol) Time (min) Conversion (%) Sulfoxide (%) Sulfone (%)
1
1:1.1
1:1.1
1:1.2
1:1.5
1:1.1
1:1.1
1:1.1
1:1.1
1:1.5
1:3
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
H2O2
H2O2
H2O2
H2O2
TBHP
H2O2
0.024
0.024
0.024
0.024
0.012
0.048
0.024
0.024
0.024
0.024
—
2
85
100
—
9
—
2
10
2
100
100
91
3
95
4
2
100
—
51
—
100
67
81
42
—
—
100
49
5
2
89
6
2
100
100
—
7
2
10
8
10
2
18
33
9
13
19
10
11
12
2
29
58
1:1.1
1:1.1
5
No reaction
No reaction
—
—
5
—
aReaction conditions: solvent free; substrate (methylphenyl sulfide): 4 mmol. Optimum conditions with high conversion: substrate/oxidant (TBHP) ratio of 1:1.1
led to 85% yield for conversion of MePhS (sulfide) to MePhSO (sulfoxide) in 2 min with 0.024 mmol of VO(acac)L1 catalyst (entry 1).

TALOUKI ET AL.
11 of 12
TABLE 6 Oxidation of methylphenyl sulfide under various conditionsa catalysed by VO(acac)L2 − 4
Substrate/oxidant
Catalyst amount
(mmol)
Time
(min)
Conversion
(%)
Sulfoxide
(%)
Sulfone
(%)
Run
Complex
Oxidant
1
VO(acac)L2
1:1.1
1:1.1
1:1.1
1:1.1
1:1.5
1:1.1
1:1.1
1:1.1
1:1.5
1:1.1
1:1.1
1:1.1
1:1.1
1:1.5
1:1.1
TBHP
0.024
0.024
0.012
0.048
0.024
0.024
0.024
0.012
0.048
0.024
0.024
0.012
0.048
0.024
—
2
85
100
100
91
—
2
VO(acac)L2
VO(acac)L2
VO(acac)L2
VO(acac)L2
VO(acac)L3
VO(acac)L3
VO(acac)L3
VO(acac)L3
VO(acac)L4
VO(acac)L4
VO(acac)L4
VO(acac)L4
VO(acac)L4
VO(acac)L2–4
H2O2
2
9
—
3
TBHP
TBHP
TBHP
TBHP
H2O2
2
50
9
4
2
100
—
100
100
—
5
2
100
—
6
4
82
100
100
67
7
4
5
—
8
TBHP
TBHP
TBHP
H2O2
4
18
33
9
4
92
—
100
—
10
11
12
13
14
15
10
10
10
10
10
10
76
100
43
5
57
TBHP
TBHP
TBHP
TBHP
42
59
41
100
—
100
100
—
100
—
No reaction
—
aReaction conditions: solvent free; substrate (methylphenyl sulfide): 4 mmol. Optimum conditions with high conversion: substrate/oxidant (TBHP) ratio of 1:1.1
with 0.024 mmol of VO(acac)L2–4 catalyst (entries 1, 6, 10).
increasing the substrate/oxidant ratio and increasing the
catalyst concentration, the selectivity to sulfoxide was
decreased and the reaction proceeded towards sulfone
production. Thus the catalytic reactions of the
VO(acac)L1–4 catalysts were carried out under optimized
conditions: substrate/oxidant ratio of 1:1.1, catalyst
amount of 0.024 mmol and time of 2 min for
VO(acac)L1,2, 4 min for VO(acac)L3 and 10 min for
VO(acac)L4. The results are presented in Figure 5.
According to these results, the order of activity is
VO(acac)L1 ~ VO(acac)L2 > VO(acac)L3 > VO(acac)L4
with yields 85, 82 and 76%, respectively. It is seen that
the catalytic activity of these complexes in the oxidation
of methylphenyl sulfide was also affected by changing
the ionic part of them. By changing the cation from
phosphonium (in VO(acac)L1) to imidazolinium (in
VO(acac)L2) with constant counter anion of Cl−, the
catalytic activity was not changed, but by changing the
counter anion from Cl− in VO(acac)L1,2 to PF6 in
−
VO(acac)L3,4 the catalytic activity of the latter
(VO(acac)L3,4) was decreased where in the case of
imidazolinium chloride (VO(acac)L4) it was dramatic.
Therefore, the good yield of products obtained when
using complexes VO(acac)L1–3 can be related to the
good mixing of them with other reactants in contrast
to the poorly soluble complex VO(acac)L4 in solvent‐free
condition and also to the high solubility of them in
water and most organic solvents. Thus for these task‐
specific ionic liquid vanadyl Schiff base complexes,
changing the counter anion has a greater effect than
changing the cationic part on the catalytic activity in
methylphenyl sulfide oxidation reaction, because of the
effects on polarity and solubility of the complexes.
4 | CONCLUSIONS
FIGURE 5 Yields of methylphenyl sulfoxide obtained under
optimized conditions in the presence of VO(acac)L1–4 complexes.
(Substrate/oxidant ratio, 1:1.1; catalyst amount, 0.024 mmol;
reaction time, 2 min for VO(acac)L1,2, 4 min for VO(acac)L3 and
10 min for VO(acac)L4)
In summary, task‐specific ionic liquid Schiff base com-
plexes of V(IV), VO(acac)L1–4, have been synthesized
and characterized. Spectroscopic studies showed a six‐
coordinated V(IV) structure in the solid state. Complexes

12 of 12
TALOUKI ET AL.
VO(acac)L1–3 are quite soluble in water and also in other
organic solvents. We investigated the effect of the ionic
nature of these task‐specific ionic liquid Schiff base
complexes in epoxidation of cyclooctene and oxidation
of methylphenyl sulfide under optimized conditions. It
was found that the catalytic activity of these complexes
was affected by their ionic nature in epoxidation of
cyclooctene and oxidation of methylphenyl sulfide, with
the nature of the counter anion having a greater effect
on the catalytic activity than the cationic moiety.
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Khalaji AD. Six‐coordinated vanadium(IV)
complexes with tridentate task‐specific ionic liquid
Schiff base ligands: Synthesis, characterization and
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