Synthesis, spectral, catalytic, and thermal studies of vanadium complexes with quadridentate schiff bases

Article abstract of DOI:10.1134/S1070328410040032

The paper reports the synthesis and characterization of vanadium complexes of N,N′-(±)-transbis(2,4-dihydroxyacetophenone)-1,2- cyclohexanediamine (H₂L¹) and N,N′-(±)-trans- bis(2,4-dihyroxy-5-nitroacetophenone)-1,2-chyclohexanediamine (H ₂L²). All the complexes were characterized by elemental analysis, magnetic susceptibility measurements, infrared and electronic spectra, and thermogravimetric analysis. The X-ray patterns of the [VO(L¹)] · H₂O (I) and [VO(L²)] · H₂O (II) complexes show the monoclinic system with the unit cell parameters a = 26.1352, b = 11.7149, c = 6.0401 A, β = 115.38° and a = 29.3787, b = 12.9398, c = 5.9175 A, β = 96.84°, respectively. The complexes I and II catalyze the oxidation of styrene in the presence of hydrogen peroxide. Pleiades Publishing, Ltd., 2010.

Full text of DOI:10.1134/S1070328410040032

ISSN 1070ꢀ3284, Russian Journal of Coordination Chemistry, 2010, Vol. 36, No. 4, pp. 254–258. © Pleiades Publishing, Ltd., 2010.
Synthesis, Spectral, Catalytic, and Thermal Studies of Vanadium
Complexes with Quadridentate Schiff Bases1
A. R. Yaul, G. B. Pethe, and A. S. Aswar*
Department of Chemistry, Sant Gadge Baba Amravati University, Amravati 444602, India
*Eꢀmail: anandaswar@yahoo.com
Received September 6, 2009
Abstract
—The paper reports the synthesis and characterization of vanadium complexes of N,N'ꢀ( )ꢀtransꢀ
bis(2,4ꢀdihydroxyacetophenone)ꢀ1,2ꢀcyclohexanediamine (H2L1) and N,N'ꢀ( )ꢀtransꢀbis(2,4ꢀdihyroxyꢀ5ꢀ
nitroacetophenone)ꢀ1,2ꢀchyclohexanediamine (H2L2). All the complexes were characterized by elemental
analysis, magnetic susceptibility measurements, infrared and electronic spectra, and thermogravimetric
analysis. The Xꢀray patterns of the [VO(L1)] · H2O ( ) and [VO(L2)] · H2O (II) complexes show the monoꢀ
I
clinic system with the unit cell parameters
a
29.3787, = 12.9398, = 5.9175 = 96.84
= 26.1352,
b
= 11.7149,
c
= 6.0401
Å
,
β
= 115.38
°
and a =
b
c
of styrene in the presence of hydrogen peroxide.
Å,
β
°
, respectively. The complexes I
and II catalyze the oxidation
DOI: 10.1134/S1070328410040032
1
INTRODUCTION
We also report oxidation of styrene by hydrogen
peroxide using the [VO(L1)] · H2O ( and [VO(L2)] ·
H2O (II complexes.
I)
Vanadium chelates containing tetradentate Schiff
)
base ligands derived from 1,2ꢀdiamines were subjects
of several recent reports [1–3]. The salen complexes of
vanadium metal were used in solution as biomimetic
catalysts for oxygen atom transfer and as catalysts for
aziridination, sulfoxidation, and other oxidative proꢀ
cesses [4–6]. In addition, selective epoxidation of oleꢀ
fins catalyzed by the vanadium Schiff base complexes
became the most important industrial processes. More
recently the vanadium coordination compounds were
shown to catalyze the selective oxidation of alkenes by
molecular oxygen [7]. The Schiff base vanadium comꢀ
plexes are also very interesting as model compounds
for the clarification of several biochemical processes
[8–10]. The present work describes the synthesis and
characterization of the vanadium(IV) and (V) comꢀ
plexes with the tetradentate Schiff base ligands:
EXPERIMENTAL
Materials and Solvents
The solvents were used after purification employꢀ
ing standard methods [11]. The reagents used in the
synthesis of the Schiff base ligands and their comꢀ
plexes were used as received. Vanadyl sulfate (VOSO4 ·
5H2O), ammonium metavanadate, and vanadium
pentoxide were received from Qualigens Chemicals.
transꢀ1,2ꢀDiaminocyclohexane was purchased from
Across Organics.
Physical Measurements
N,N'ꢀ( )ꢀtransꢀbis(2,4ꢀdihydroxyacetophenone)ꢀ1,2
ꢀ
ꢀ
Carbon, hydrogen, and nitrogen contents were
recorded on a Carlo Erba 1108 elemental analyzer. 1H
NMR spectra of the ligands were recorded with a
Bruker DRXꢀ300NMR spectrophotometer in
DMSOꢀd6 using tetramethylsilane as internal referꢀ
ence. FTꢀIR spectra were obtained as KBr pellets
using a Shimadzu 8201 spectrophotometer in the
range 400–4000 cm–1. The solid state reflectance
spectra of the complexes were recorded in the range
200–1000 nm (as MgO) on a Beckman DKꢀ2A specꢀ
trophotometer. Magnetic measurements were carried
out at room temperature by the Gouy method using
Hg[Co(SCN)4] as calibrant. Thermogravimetric analꢀ
ysis was performed on a TGAꢀ2 PerkinElmer thermal
cyclohexanediamine (H2L1) and N,N'ꢀ( )ꢀtrans
bis(2,4ꢀdihyroxyꢀ5ꢀnitroacetophenone)ꢀ1,2ꢀchycloꢀ
hexanediamine (H2L2).
H3C
CH3
N
N
HO
R
R
OH
OH
HO
,
where R = H (for H2L1), NO2 (for H2L2).
1
The article is published in the original.
analyzer in the temperature range 40–800°C with a
254

SYNTHESIS, SPECTRAL, CATALYTIC, AND THERMAL STUDIES
Table 1. Elemental analysis data for the ligands and their complexes
255
Contents (found/calcd.), %
Compound
Formula weight
C
H
N
V
H2L1
382.45
465.39
482.83
538.50
472.44
555.38
572.82
628.50
69.27/69.09
56.92/56.78
54.80/54.73
49.12/49.07
55.87/55.93
47.65/47.58
46.29/46.13
42.16/42.04
6.81/6.85
5.69/5.63
5.12/5.01
5.09/5.24
5.27/5.12
4.32/4.36
3.56/3.87
4.21/4.17
7.57/7.32
5.89/6.02
5.87/5.80
5.23/5.20
11.81/11.86
10.24/10.09
9.83/9.78
9.02/8.91
[VO(L1)] · H2O
[VO(Cl)(L1)]
K[VO2(L1)] · 2H2O
H2L2
[VO(L2)] · H2O
[VO(Cl)(L2)]
K[VO2(L2)] · 2H2O
10.87/10.95
10.53/10.50
9.54/9.46
9.14/9.17
8.95/8.89
8.23/8.11
heating rate of 10°
C min–1. Gas chromatography ture was refluxed in a water bath for 3–4 h. The colꢀ
experiments were performed with a Shimadzu 14B gas ored solid obtained was filtered off, washed with
chromatograph equipped with a SEꢀ30 column and a diethyl ether, and dried over calcium chloride. The
flameꢀionization detector. Xꢀray diffraction patterns yield was 55–60%.
Synthesis of K[VO2(L1)] · 2H2O (V) and
K[VO2(L2)] · 2H2O (VI) complexes. V2O5 (0.46 g,
5 mmol) dissolved in 10 ml of KOH (0.29 g, 5 mmol)
was added to a hot solution of the ligands (5 mmol) disꢀ
solved in dry methanol (20 ml). The resulting mixture
vigorously was stirred, and on stirring a colored precipꢀ
itate was formed, filtered off, washed with diethyl ether,
and dried in vacuum. The yield was 47–50%.
were obtained with a BRUKER AXS, D8 ADVANCE
instrument (GmbH, Karlsruhe, West Germany)
equipped with a
tor.
θ
/ goniometer and a Lynx Eye detecꢀ
θ
Synthesis of Schiff base ligands. Both the Schiff
bases H2L1 and H2L2 were prepared by a similar
method. A solution of transꢀ1,2ꢀdiaminocyclohexane
(10 mmol) in 10 ml of methanol was added dropwise
to a hot methanolic solution of respective ketones
(20 mmol in 40 ml), and the reaction mixture was
heated under reflux on a water bath for 2 h. After coolꢀ
ing to room temperature, the yellow precipitates sepaꢀ
rated were filtered off, wash with methanol, and finally
dried over calcium chloride. The purity of the syntheꢀ
sized compounds was monitored by TLC. The yield
Oxidation of styrene. The catalytic oxidation of styꢀ
rene was carried out using the complexes I and II in a
50ꢀml reaction flask fitted with a water condenser. A
general procedure was applied for all reactions. In typꢀ
ical reactions, styrene (1.04 g, 10 mmol) and 30%
aqueous hydrogen peroxide (2.27 g, 20 mmol) were
mixed in 10 ml of MeCN, and the reaction mixture
was heated in an oil bath with continuous stirring at
was 75–78%, m.p. H2L1: 214
°C °
and H2L2: 229 C. 1H
NMR
(
δ
, ppm) H2L1: 16.87 (s., 2H, OH C2), 9.74 (s.,
80°C for 8 h. The catalyst to be tested (0.025 g) was
2H, OH C4), 7.32–6.00 (m., 6H, Ar–H), 3.84 (m.,
2H, CH), 2.22 (s., 6H, CH3), 1.03–1.87 (br., 8H,
CH2CH2); H2L2: 17.28 (s., 2H, OH C2), 10.02 (s., 2H,
OH C4), 8.33–6.08 (m, 4H, Ar–H), 4.24 (m., 2H,
CH), 2.43 (s., 6H, CH3), 1.97–1.06 (br., 8H,
CH2CH2).
Synthesis of complexes I and II. Equimolar quantiꢀ
ties (10 mmol) of metal salt (VOSO4 · 5H2O) and the
ligand were dissolved separately in absolute methanol
(25 ml). Both solutions were filtered and mixed under
hot conditions. The reaction mixture was refluxed for
4–6 h in a water bath, and a colored solution was conꢀ
centrated and cooled to yield the complexes. The latꢀ
ter were washed with diethyl ether and dried over calꢀ
cium chloride. The yield was 62–70%.
Synthesis of [VO(Cl)(L1)] (III) and [VO(Cl)(L2)] (IV) were compared with those of the free ligands. Both
complexes. An aqueous (15 ml) solution of NH4VO3 ligands exhibit a broad band at 2912–2947 cm–1 due to
(1.17 g) was treated with 6 ml of concentrated HC1 hydrogen bonding between phenolic hydrogen and
and stirred until a yellow solution was obtained. This nitrogen of the azomethine group. The ligands are relꢀ
solution was added to a methanolic solution (30 ml) of atively planar with an adequate intramolecular disꢀ
the ligand (10 mmol) with stirring. The reaction mixꢀ tance that favors the formation of hydrogen bonds
added to the reaction mixture, and the reaction was
considered to begin. The progress of the reaction was
monitored by withdrawing a small amount of the reacꢀ
tion mixture at different time intervals and analyzing
them quantitatively by gas chromatography. The effect
of an amount of the catalyst was studied to obtain suitꢀ
able reaction conditions.
RESULTS AND DISCUSSION
All the complexes are nonhygroscopic and stable in
the solid state at room temperature. Analytical and
physical data of the ligands and their vanadium comꢀ
plexes are presented in Table 1.
The infrared spectra of the vanadium complexes
RUSSIAN JOURNAL OF COORDINATION CHEMISTRY Vol. 36
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2010

256
YAUL et al.
quency as compared to the oxovanadium(IV) comꢀ
(а)
plexes, which may be due to the weakening of the V=O
bond by the trans effect of the added ligand. Concluꢀ
sive evidence of the bonding is also shown by the new
bands in the spectra of the complexes appeared at
468–480 and 527–549 cm–1 assigned to the V–O and
V–N stretching vibrations that are not observed in the
spectra of the ligands. On the basis of these results, we
conclude that the Schiff bases in the vanadium comꢀ
plexes behave as dibasic tetradentate molecules.
Complexes I and II exhibit the μeff value 1.73 and
1.82 μВ, respectively, which are very close to the spinꢀ
only value for the d1 system [19, 20]. These complexes
show all the expected three bands in the regions
10
20
30
40
1
13570–13630, 16680–16698, and 21837–22102 cm⎯
,
2θ, deg
which result from the 2B2
→
2E
,
2B2
→
2B1 and 2B2
→
2A1
(b)
transitions, respectively [21]. The fourth band
observed in the region 28010–29472 cm–1 may be due
to the charge transfer transition. Thus, keeping in view
the tetradentate nature of the ligands, 5ꢀcoordinated
squareꢀpyramidal structures in which the ligand lies in
the basal plane and oxygen occupies the apical posiꢀ
tion may be suggested for both VO(IV) complexes.
Vanadium(V) is a d0 species, and no d–d traditions are
expected or observed. However, these complexes disꢀ
play lowꢀenergy bands, which are assigned to the
ligandꢀtoꢀmetal charge transfer (LMCT) transition
from the phenolic oxygen to the empty
the vanadium [22].
d orbitals on
10
20
30
40
The Xꢀray diffraction (XRD) studies of the comꢀ
plexes and II (Fig. 1) were carried out using Cu
radiation with
both complexes were recorded at
and 45 . The Xꢀray crystal system was worked out by
I
K
2
θ
, deg
α
λ
= 1.5418 Å. The XRD patterns of the
2θ
values between 3°
Fig. 1. Xꢀray diffraction pattern of for I (a) and II (b).
°
the trial and error methods for finding the best fit
between the observed and calculated values. The unit
[12]. The presence of electronꢀdonating groups on
nitrogen also favors intramolecular hydrogen bond
formation [12]. The absence of these bands in the
spectra of the complexes indicates the destruction of
the hydrogen bond followed by the coordination of
phenolic oxygen after deprotonation. This is also supꢀ
ported by the upfield shift (by 9–24 cm–1) of the pheꢀ
cell parameters for complex
11.7149, = 6.0401 = 115.38
complex II = 29.3787, = 12.9398,
96.84
= 2233.51 Å3. It was found that both comꢀ
I:
a
°, V
= 26.1352,
= 1670.00 Å3; for
= 5.9175 , =
b =
c
Å,
β
:
a
b
c
Å
β
°, V
plexes belong to the monoclinic system.
The catalytic properties of complexes
I
and II were
investigated in the oxidation of styrene by using H2O2
as an oxidant. Styrene oxide, phenyl acetaldehyde,
benzaldehyde, benzoic acid, and 1ꢀphenylethaneꢀ1,2ꢀ
diol were formed along with minor amounts of uniꢀ
dentified products. These are common products and
were identified as well [23, 24]. Catalyst concentraꢀ
tions were varied to study their effect on the oxidation
activity of the catalytic system. The catalytic study of
both the complexes toward oxidation of styrene using
H2O2 as an oxidant results in the 27–32% styrene conꢀ
nolic
ν
(C O) mode [13, 14]. Both ligands exhibit a
−
strong band around 1620 cm–1 attributable to the C=N
stretching vibration, undergoing a shift to lower freꢀ
quency (by 9–25 cm–1) after complexation, indicating
the coordination of azomethine nitrogen to the vanaꢀ
dium ion. This can be explained by the donation of
electrons from nitrogen to the empty
vanadium ion [15–17].
d orbital of the
The oxovanadium(IV) complexes exhibit an addiꢀ
tional strong absorption band near ~980 cm–1, which
version. Three different amounts of I, viz., 0.015,
0.025 and 0.035 g were considered while keeping fixed
is assigned to (V=O) [18]. The dioxovanadium comꢀ
ν
plexes show a significant pair of the V=O stretching
frequency around 980 and 920 cm–1 for the asymmetric
and symmetric modes, respectively. Complexes III and IV
display one sharp band at 950 and 948 cm–1 respecꢀ
amounts of styrene (10 mmol) and H2O2 (20 mmol) in
10 ml of MeCN. The reaction was carried out at 80°C.
Figure 2 shows that the conversion intensifies on
increasing the amount of the catalyst and a maximum
tively, and a decrease reduction in the stretching freꢀ of 32% conversion were achieved with 0.025 g of the
RUSSIAN JOURNAL OF COORDINATION CHEMISTRY Vol. 36
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SYNTHESIS, SPECTRAL, CATALYTIC, AND THERMAL STUDIES
257
35
30
25
20
15
10
5
100
0.015 g
0.025 g
0.035 g
TG
80
60
40
20
0
DTG
100
200
300
400
500
Temperature,
°
C
Fig. 3. TG and DTG curves of complex V.
0
1
2
3
4
Time, h
5
6
7
8
60–115
the lattice water molecules [25], The second stage of
decomposition started at about 115 and completed
at 260 corresponding to a weight loss of 11.59%,
°C and corresponds to the loss of two moles of
Fig. 2. Effect of amount of catalyst on the oxidation of styꢀ
rene.
°C
°
C
which could be attributed to the loss of one part of the
ligand moiety. The third stage of decomposition
occurred in the range 260–490°C with a weight loss of
55.68%, which can be due to the loss of the remaining
part of the ligand and leaving KVO3 as a final product.
The three stages were denoted by the DTG peaks at
catalyst. The conversion decreases with further
increasing the catalyst conversion. Hence, 0.025 g of
the catalyst can be considered as optimum conditions
for the maximum conversion.
The present study aims at evaluating the thermal
behavior of the representative vanadium complexes on
the basis of the TG/DTG method. The stages of
decomposition, temperature ranges, and the observed
and calculated weight loss percentages are listed in
about 82, 208, and 423°C, respectively.
Complex slowly started to decompose in the temꢀ
I
perature range 40–110 . The first weight loss 3.80%
°C
(calcd. 3.86%) is attributed to the removal of one latꢀ
tice water molecule [25]. In the temperature range
Table 2. The TG/DTG curves of complex
in Fig. 3. The dioxovanadium complex undergoes
V is shown
threeꢀstage decomposition. The first weight loss 6.42% 110–340
°C, the weight loss of 14.02% is due to the
(calcd. 6.68%) was observed in the temperature range pyrolysis of one part of the ligand. The weight loss
Table 2. Thermal decomposition data for the vanadium complexes
Complex TG range, oC DTG peak, oC Weight loss, % (obs./calcd.)
Assignment
I
40–110
110–340
340–800
92
334
743
3.80/3.86
14.02
Loss of one mole of lattice water molecule
Removal of one part of the ligand
Removal of remaining part of the ligand
Leaving of metal oxide residue
63.98
81.80*/82.08
12.96
III
V
170–255
255–740
212
629
Removal of one part of the ligand
Removal of remaining part of the ligand
Leaving of metal oxide residue
69.18
82.14*/82.73
6.42/6.68
11.59
60–115
115–260
260–490
82
208
423
Loss of two moles of lattice water molecules
Removal of one part of the ligand
Removal of remaining part of the ligand
Leaving of metal oxide residue
55.68
73.69*/74.27
* Total weight loss.
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258
YAUL et al.
7. Mohebbi, J., Nikpour, S., and Rayati, S., J. Mol. Catal.,
, 2006, vol. 256, p. 265.
63.98% is observed in the range 340–800°C, which
corresponds to the loss of the remaining part of the
ligand. The three stages were also denoted by the DTG
A
8. Butler, A. and Carrano, C.J., Coord. Chem. Rev., 1991,
vol. 109, p. 152.
peaks at about 92, 334, and 743°C, respectively. The
final residue vanadium oxide has the observed weight
17.92% against the calculated value 17.81%.
The thermal decomposition of complex III showed
two decomposition stages in the temperatures ranges
9. Masuda, Y., Thermochim. Acta, 1983, vol. 60, p. 203.
10. Jeyraja, G.L. and House (Jr.), J.E., Thermochim. Acta
1983, vol. 66, p. 289.
,
11. Furniss, B.S., Hannaford, A.J., Smith, P.W.G., and
Tatchell, A.R., Vogel’S Textbook of Practical Organic
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170–255 and 255–740
DTG peaks at 212 and 629
°
C
. They were donated by the
°C. The first stage of
decomposition agreed with a weight loss of 12.96%,
which may be due to the loss of 50% of the ligand. The
second stage indicates a weight loss of 69.18%, which
may correspond, to the loss of the remaining part of
the ligand and the oxidation of the metal to a residue
of VO2, which is stable above this temperature [26].
12. Freedman, H.H., J. Am. Chem. Soc., 1961, vol. 83,
p. 2900.
13. Natarajan, K., Karvembu, R., Hemalatha, S., and
Prabhakaran, R., Inorg. Chem. Commun., 2003, vol. 6,
p. 486.
14. Gupta, K.C. and Sutar, A.K., J. Mol. Catal., A, 2007,
vol. 272, p. 64.
ACKNOWLEDGMENTS
15. Yaul, A.R., Dhande, V.V., Suryawanshi, N.J., and
Aswar, A.S., Pol. J. Chem., 2009, vol. 83, p. 565.
The authors are grateful to the Director, C.D.R.I. Lucꢀ
know for recording the IR spectra and elemental analyses;
S.A.I.F., IIT Chennai for recording the electronic spectra.
We also thank Prof. R. Prasad, School of Chemical Sciꢀ
ences, D.A. University, Indore, for recording the gas chroꢀ
16. Aranha, E.P., Dos Santos, M.P., Romera, S., and
Dockel, E.R., Polyhedron, 2007, vol. 26, p. 1373.
17. Vafazadef, R. and Kashfi, M., Bull. Korean Chem. Soc.
2007, vol. 2817, p. 1228.
,
matograms. One of the authors (A.R. Yaul) is grateful to 18. Garg, R., Fahmi, N., and Singh, R.V., J. Indian Chem.
Soc., 2009, vol. 86, p. 670.
the University Grant Commission (New Delhi, India) for
providing financial support.
19. Raman, N., Pitchaikaniraja, Y., and Kulandaisamy, A.,
Proc. Indian Acad. Sci., 2001, vol. 113, p. 183.
20. Dutta, S.K. and Edward, R.T., Polyhedron, 1997,
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2010