Microwave assisted synthesis and characterization of unsymmetrical tetradentate Schiff base complexes of VO(IV) and MoO(V)

Article abstract of DOI:10.1016/j.saa.2011.08.055

Microwave synthesis, is green chemical method, simple, sensitive, reducing solvent amount and reaction time. The attempt was made to synthesize the unsymmetrical tetradentate N₂O₂ ligands and their VO(IV) and MoO(V) unsymmetrical tet

Full text of DOI:10.1016/j.saa.2011.08.055

Spectrochimica Acta Part A 84 (2011) 51–61
Contents lists available at SciVerse ScienceDirect
Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy
journal homepage: www.elsevier.com/locate/saa
Microwave assisted synthesis and characterization of unsymmetrical
tetradentate Schiff base complexes of VO(IV) and MoO(V)
B.T. Thaker∗, R.S. Barvalia
Department of Chemistry, Veer Narmad South Gujarat University, Surat 395007, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 12 June 2011
Received in revised form 13 August 2011
Accepted 24 August 2011
Microwave synthesis, is green chemical method, simple, sensitive, reducing solvent amount and reaction
time. The attempt was made to synthesize the unsymmetrical tetradentate N2O2 ligands and their VO(IV)
and MoO(V) unsymmetrical tetradentate Schiff base complexes by classical and microwave techniques
using domestic microwave oven. The resulting unsymmetrical Schiff base ligands L1–L3 characterized by
different spectral methods. Their complexes with oxocations of VO(IV) and MoO(V) have been synthe-
sized and characterized by elemental analyses, conductometric measurements, infrared and electronic
absorption, 1H NMR spectra, mass spectrometry, ESR spectra, magnetic susceptibility measurement and
thermal study. The study suggests that the oxo metal ion is bonded to the ligand through the oxygen and
imino nitrogen and the geometry around metal ion is distorted octahedral.
Keywords:
Oxovanadium(IV)
Oxomolybdenum(V)
Pyrazolone
Dehydroacetic acid
Microwave assisted synthesis
© 2011 Elsevier B.V. All rights reserved.
1. Introduction
systems, molybdenum is believed to be associated with flavins
and amino acid residues [17]. Microwave assisted reactions
Unsymmetrical Schiff base complexes have been applied as
redox catalyst in some organic reactions [1], their use as lig-
ands in metal complexation [2–5]. These metalloenzymes facilitate
a variety of reactions, including redox reaction (carried out by
oxidases and oxygenases), acid-catalyzed hydrolysis (hydrolases),
and rearrangement of carbon–carbon bonds (synthases and iso-
merases) [6,7]. Schiff-base complexes of transition metal ions
display biological activities such as treatment of cancer, fungicides,
viracides, bactericides, and function as catalysts in industrial pro-
cesses [8–12].
in solvent or solvent-free conditions have gained popular-
ity because of rapid reaction rate, cleaner reactions, easy of
manipulation, higher yields, often improved selectivity with
respect to conventional reaction conditions [18]. Microwave
assisted reactions used the formation of trinuclear cluster
[Ni3(mimp)5(CH3CN)]ClO4, Co(II) cluster, [M7(OH)6(mimmp)6]X2
−
(M = Zn, X = NO3
;
M = Mn, X = ClO4−), [Ni(immp)2]·(H2O)2]
(mimmp = anion of 2-methoxy-6-methyliminomethyl-phenolic;
imp = anion of 2-iminomethyl-6-methoxyphenolic) [18]. The
microwave heating is a new and extremely attractive method for
synthesizing polymetallic clusters of paramagnetic transition metal
ions [18].
The coordination chemistry and reactivity of vanadium
have continued to play a significant role not only because of
the physiological relevance of the metal but also for its activ-
ity in various industrial processes. Most oxo-vanadium(IV)
Induced organic reaction enhancement (MORE) chemistry tech-
niques are lowered energy consumption compared to conventional
reactions performed under reflux, which also require the latent
heat of vaporization. A major advantage is given by the usual
reaction time which is a few minutes even on a few hundred
grams scale. The reduction of solvents and formation of low-
ered amounts of by-products decrease pollution at the source and
ensure high levels of “atomic economy”. The present study investi-
gates reaction of unsymmetrical tetradentate Schiff base and their
metal complexes synthesized by classical and microwave tech-
nology. In our continuation of the work on pyrazolones ligands
[19], an attempt has been made to synthesize, characterize and
find their thermal behavior of novel unsymmetrical Schiff base
complexes of oxocations such as VO(IV) and Mo(V) by both meth-
ods and to indicate in which by microwave process the yield is
better.
complexes with
a tetradentate Schiff-base ligand like salen
have green monomeric structures with square-pyramidal
coordination geometry. However, orange polynuclear lin-
ear chain structures have been observed for the six-member
N–N chelating ring for vanadyl Schiff-base complexes [13,14],
especially for Schiff-base ligands with electron-withdrawing
substituents [15]. Interest in the chemistry of metallacyclic
oxomolybdenum(V) and (VI) complexes grows continuously,
particularly due to their potential relevance as model systems
for molybdenum enzymes [16]. In
a number of enzymatic
∗
Corresponding author. Tel.: +91 261 2267957; fax: +91 261 2227312.
E-mail address: btthaker1@yahoo.co.in (B.T. Thaker).
1386-1425/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.saa.2011.08.055

52
B.T. Thaker, R.S. Barvalia / Spectrochimica Acta Part A 84 (2011) 51–61
2.4. Synthesis of unsymmetrical tetradentate Schiff base ligand L1
2. Experimental
2.1. Materials
to L3
2.4.1. Classical method
For the synthesis of unsymmetrical tetradentate Schiff base
ligands and their metal complexes, solvents like methanol,
petroleum ether, N,N-dimethyl formamide (AR grade) were used.
Dehydroacetic acid (DHA) (Merk), primary aromatic diamines
viz: o-phenylene diamines (Atul Ltd.), 1-phenyl-3-methyl-5-
pyrazolone, 1(2-chloro)-1-phenyl-3-methyl-5-pyrazolone and
1(3-sulfoamido)-1-phenyl-3-methyl-5- pyrazolone (Nutan Dye
Chem., G.I.D.C., Sachin) were used. The metal salt like VOSO4·5H2O
(National chemicals) and MoCl5 (Aldrich, U.S.A.) were used as
received.
2-Aminomono Schiff base ligand (3-[(1-E)-N-(2-aminophenyl)
ethanimidoyl]-4-hydroxy-6-methyl-2H-pyran-2-one) is synthe-
sized by refluxing of an equimolar mixture of dehydroacetic acid
(DHA) and aromatic diamines in dry ethanol. The dehydroacetic
acid (0.1 mol, 16.8 g) and o-phenylene diamine (10.8 g) dissolved
in (100 ml) dry methanol in a round bottom flask and add to it one
to two drops of concentrated sulphuric acid. The contents of the
flask were refluxed for about 2 h. Then it was cooled, up to room
temperature. On cooling, the solid Schiff base that separated out
was filtered and washed with methanol. The Schiff base was further
purified by recrystallization from dry methanol and dried.
0.01 mol of 2-amino mono Schiff base ligand dissolved in N,N-
dimethyl formamide in round bottom flask. 0.01 mol of each ligands
(I), (II) and (III) also dissolved in N,N-dimethyl formamide, and then
added gradually into the round bottom flask in which previously
containing the mono Schiff base. After the addition of ligands (I),
(II) and (III), respectively, was completed, the reaction mixture was
refluxed on water bath for 3–4 h, and then poured into 1 l ice water
in beaker. Brown colored precipitate of unsymmetrical tetradentate
Schiff base ligands obtained. The resulting mixture was allowed to
stand overnight. It was collected by filtration, washed with distilled
water and petroleum ether, and dried.
2.2. Instruments
Microwave assisted condensation of Schiff base and their com-
plexes were carried out in a domestic oven, 2450 MHz frequency,
800 W. The elemental analyses (C,H,N) were performed at RSIC,
Lucknow. The vanadium metal was estimated by gravimetrically
as V2O5. A 100 mg sample of VO(IV) complexes were placed in
a silica crucible, decomposed by gentle heating and then treated
with 2–3 ml of nitric acid, 2–3 times and igniting at 600 ◦C. Orange
colored residues of V2O5 were obtained after decomposition and
complete drying and weighing [20]. Similarly, molybdenum was
also estimated by gravimetrically as MoO3, yellowish white colored
residues of MoO3 were obtained after decomposition and complete
drying and weighing [21]. The conductance of oxo cations com-
plexes were carried out on the Equip-Tronics conductivity meter,
model No. EQ-664 with range 20 ␮ꢀ to 20 mꢀ at 306 K tempera-
ture. The IR spectra of the unsymmetrical Schiff base ligands and
their complexes were recorded at Centre of Excellence, Quality
Testing Facility & R&D Centre, GIDC, Vapi with a Perkin Elmer
Spectrum-BX, IR spectrophotometer (4000–450 cm−1) using KBr
discs. The electronic spectra of complexes in the 200–800 nm were
obtained in DMSO as a solvent on a SHIMADZU UV 160 A using
quartz cell of 1 cm optical path. The magnetic measurement of
metal complexes at room temperature was carried out on Gouy
balance as per the method suggested by Prasad and coworkers [22]
at M.S. University of Baroda, Vadodara. The mass spectra of VO(IV)
and MoO(V) complexes were recorded by Electro impact mass spec-
trometer (GC–MS). The mass spectra were recorded at SAIF, Punjab
University, Chandigarh. The ESR spectra of all VO(IV) and MoO(V)
complexes were recorded by SAIF, IIT, Mumbai, at RT and LNT for
polycrystalline state. TGA and DTG were performed on an Al2O3
crucible and samples under investigation decomposed in the N2
atmosphere at a heating rate 10 ◦C/min at SAIF, Indian Institute of
Technology, Madras, Chennai.
2.4.2. Microwave method
2-Amino mono Schiff base ligand was synthesized by ethano-
lic mixture of DHA (0.1 mol) and aromatic diamines (0.1 mol) were
placed in a flask and irradiated in a microwave oven for 30 s at
800 W. Completion of the reaction was monitored by TLC. The reac-
tion mixture was allowed to attain RT. The solid mass obtained and
recrystallized from methanol.
0.01 mol of 2-amiono mono Schiff base ligand and each pyra-
zolone ligands (I), (II) and (III) dissolved in DMF in a flask. The
resulting mixture was irradiated in a microwave oven for 1 min
at 800 W. The solid mass separated was poured into 1 l ice water in
beaker. Brown colored precipitate of unsymmetrical tetradentate
Schiff base ligands obtained. The resulting mixture was allowed to
stand overnight. It was collected by filtration, washed with distilled
water and petroleum ether, and dried.
2.5. Synthesis of unsymmetrical tetradentate Schiff base metal
complexes of VO(IV) and Mo(V)
2.5.1. Classical method
(A) VO(IV) complexes: 0.01 mol of each ligand in slight excess was
taken in round bottom flask containing 30 ml of dimethyl for-
mamide and dissolved completely. A solution of 0.01 mol of
VOSO4·5H2O in 20 ml of DMF was then added drop wise to the
solution of the ligand. The contents were refluxed for 2–3 h.
Then the resulting solution was poured on ice cold distilled
water. The precipitates were digested for 1 h. The solid metal
complex separated out and filtered in hot condition. The com-
plexes washed with petroleum ether and dried.
(B) MoO(V) complexes: In a flat bottom flask 0.01 mol of each ligand
in slight excess was dissolved in a mixed solvents of chloroform
(15 ml) + methanol (15 ml). The solution was stirred on mag-
netic stirrer for few minutes for dissolution of ligand. A solution
of 0.01 mol MoCl5 in 20 ml of methanol was added drop wise to
the solution of ligand. The pH of resulting mixture was adjusted
to 6 with NaOAc buffer and stirred for 8–10 h and checked the
pH of the solution. The solid mass was separated. The precipi-
tate was filtered, washed with methanol and petroleum ether
2.3. Synthesis of ligands: (I), (II) and (III)
The ligands (I), (II) and (III) were prepared by condensation of 1-
(2-chloro)-phenyl-3-methyl-5-pyrazolone (10.4 g, 0.05 mol), 1-(3-
chloro)-phenyl-3-methyl-5-pyrazolone (10.4 g, 0.05 mol) and 1-(3-
sulfoamido)-1 phenyl-3-methyl-5- pyrazolone (12.6 g, 0.05 mol)
dissolved in DMF (10 ml, 0.05 mol), then cooled to 0 ◦C in an ice bath.
The phosphoryl chloride (5.5 ml, 0.06 mol) was added drop wise at
a rate to maintain the temperature between 10 and 20 ◦C. After the
addition was completed, the reaction mixture was heated for 2.5 h
for (I) ligand and 3.5–4 h for (II) and (III) ligands, then poured into
1 l ice water in beaker. The resulting mixture was allowed to stand
overnight; it was collected by filtration, washed with water, and
dried. Recrystallization was done from ethanol.

B.T. Thaker, R.S. Barvalia / Spectrochimica Acta Part A 84 (2011) 51–61
53
H
C
(A)
3
O
O
O
NH
H
2
N
2
O
OH
N
metanol
reflux
CH
3
+
O
2 h
(1:1)
NH
H
C
O
2
3
H
C
3
o - phenylene diamine
Dehydroacetic acid (DHA)
2-amino mono Schiff base
(B)
H3C
H
C
H
H3C
C
CH
C
2
C=O
C
o
N
10-20 C
N
C
N
O
N
DMF + POCl
3
O
reflux
3-3.5 hs
X
X
X= H; (I), Cl; (II), SO NH (III)
2;
X= H; Cl; SO NH
2
2
2
X
H3C
DMF
-H2O
HO
O
OH
(A)
(B)
N
+
reflux
3-4 hs
(1:1)
C
N
O
N
C
C
C
H
N
H3C
CH3
X=H; L1, X=Cl; L2 and X=SO2NH2; L3
Ligand L1-3
DMF
VOSO4.5H2O
DMF
reflux 2-3 hours
[VOL1-L3(H2O)]
[MoOL1-L3Cl]
+
poured on ice cold
dist. H2O
Ligand L1-3
PH-6 with NaOAc
Stirred 8-10 hours
MoCl5
+
Methanol
chloroform + methanol
Fig. 1. Synthetic pathway of unsymmetrical tetradentate Schiff base complexes of VO(IV) and MoO (V).
successively and dried. Synthetic rout to unsymmetrical TSB
3. Result and discussion
complexes of VO(IV) and MoO(V) is shown in Fig. 1.
Physical data and elemental analysis of unsymmetrical
tetradentate ligands L1–L3 and their metal complexes are
shown in Table 1. The elemental analyses are also in good
agreement with expected stoichiometry of complexes which
indicate that the metal to ligand ratio in all VO(IV) and
Mo(V) complexes is 1:1. It is clear from the yield compari-
son plot shown in Fig. 2 of classical and microwave assisted
(MWI) synthesis of the unsymmetrical tetradentate Schiff base
and their metal complexes that MWI is easier, convenient
2.5.2. Microwave method
An equi molar solution of unsymmetrical tetradentate Schiff
base ligands L1–L3 in DMF were added to a DMF solution of
VOSO4·5H2O or methanolic solution of MoCl5 in a flask. The result-
ing mixture was irradiated in microwave oven for 1 min at 800 W in
both cases. The reaction mixture was kept at room temperature for
few minutes. Thus, solid colored compound obtained, was filtered,
washed thoroughly with methanol and petroleum ether and dried.

54
B.T. Thaker, R.S. Barvalia / Spectrochimica Acta Part A 84 (2011) 51–61
Table 1
Elemental analysis, molar conductivity and ꢁeff physical data of unsymmetrical tetradentate Schiff base ligand and their metal complexes.
Compounds
Elemental analysis found % (calculated %)
Molar conductivity (ohm−1 cm2 mol−1
)
ꢁeff (B.M.)
C
H
N
M
L1
68.01(67.71)
63.16(62.96)
57.85(57.57)
57.26(57.15)
53.78(53.53)
49.91(49.67)
45.15(45.02)
48.46(48.25)
51.32(51.10)
5.20(5.01)
4.25(4.43)
4.60(4.44)
4.44(4.22)
3.88(3.78)
4.10(3.83)
3.38(3.17)
3.28(3.07)
3.63(3.42)
12.75(12.66)
11.86(11.74)
13.67(13.42)
10.89(10.66)
17.21(17.14)
11.68(11.58)
10.62(10.50)
9.14(9.00)
–
–
–
–
–
–
0.24
0.96
1.16
0.85
1.16
1.47
–
–
–
1.76
1.76
1.77
1.71
1.71
1.69
L2
L3
[VOL1(H2O)]
[VOL2(H2O)]
[VOL3(H2O)]
[MoOL1Cl]
[MoOL2Cl]
[MoOL3Cl]
9.89(9.69)
9.21(9.09)
8.69(8.42)
14.45(14.38)
15.49(15.41)
16.45(16.32)
9.64(9.53)
Fig. 2. Graphical representation of yield comparison between classical and microwave irradiations.
and yields of all products are better than classical method
(Table 2).
C–O stretching vibrational mode is expected to appear around
1200 cm−1 [30]. A weak to strong intensity bands observed at
1250–1206 cm−1 is assigned to enolic C–O stretching vibrational
mode (Table 3).
3.1. Molar conductance
3.3. 1H NMR spectra of unsymmetrical tetradentate Schiff base
ligands
The molar conductance of the VO(IV) and Mo(V) complexes
(Table 1) in 10−3 M in DMSO as solvent observed in the range
1.47–0.24 and 1.47–0.85 ohm−1 cm2 mol−1, respectively. The low
conductivity values in DMSO reveal that all complexes are non-
electrolytic in nature and there is no counter ion present outside
the coordination sphere of oxocations complexes [23].
The chemical shift observed at ı 2.16–2.17 and ı 2.28–2.40 for
the –CH3 proton of three position of pyrazolone ring and DHA moi-
ety, respectively. The base value for chemical shift of –CH3 protons
of azomethine carbon is ı 2. But the aromatic ring is bonded to nitro-
gen and ꢂ electron cloud of aromatic ring may dishield the methyl
proton and thus ı value is shifted towards downfield to 2.63–2.66
[26]. Quartets signals are observed at ı 5.22–5.25 (ligands-L1and
L2) and ı 6.72–6.76 (ligand-L3) due to the coupling of C5 proton
of DHA with C6 methyl protons of DHA moiety. A signal observed
at ı 5.62–5.75 is due to –OH proton of pyrazolone ring. The H of
C3 of DHA is expected to resonate at ı 2.1 [20,21]. However, dehy-
droacetic acid exists in solution as a completely enolized species as
the acidic hydrogen at C3 being alpha to carbonyl groups and res-
onate at ı 15.35–15.65, which is the chemical shift of enolic –OH
involved in hydrogen bonding with azomethine group. This enolic
hydrogen undergoes tautomerism to form –NH shown in Fig. 3 and
the signals are shifted at ı 7.98 and 8.50 [22] (Table 4).
3.2. FT-IR spectra of unsymmetrical tetradentate Schiff base
ligands
The broad band appeared at 3464–3220 cm−1 due to the intra
molecular hydrogen bonding (O–H· · ·N) present in TSB ligand L1
to L3 [24,25]. But in the IR spectra of L3 ligand we observed new
bands at 3356 cm−1 (N–H asymm.) and 3220 cm−1 (N–H symm.)
of –SO2NH2 group of pyrazolone [26]. The lactone C O band
observed at 1710–1697 cm−1 [25]. The medium strong intensity
band observed at 1632–1660 cm−1 in IR of ligands are assigned to
azomethine C N stretching vibrations [27–29]. The strong medium
intensity bands appeared at 1569–1571 cm−1, due to C C aro-
matic ring stretching vibrations. The IR spectral band due to enolic
Table 2
Physical properties of unsymmetrical tetradentate Schiff base ligand and their metal complexes.
Compounds
Color
Melting point (◦C)
Microwave method
Reaction time (s)
Classical method
Reaction time (h)
Yield (%)
Yield (%)
L1
Dark brown
Dark brown
Brown
Greenish brown
Greenish brown
Greenish brown
Brown
69
65
130
>250
>250
>250
>250
>250
>250
60
60
60
60
60
60
60
60
60
79.35
81.47
82.79
80.32
83.63
84.21
77.29
77.80
78.79
3
3
4
2
2
3
8
9
10
69.30
69.74
66.79
69.52
67.79
67.50
60.07
59.42
61.18
L2
L3
[VOL1(H2O)]
[VOL2(H2O)]
[VOL3(H2O)]
[MoOL1Cl]
[MoOL2Cl]
[MoOL3Cl]
Brown
Brown

Table 3
FT-IR spectral data of unsymmetrical tetradentate Schiff base ligand and their metal complexes.
Functional group and IR frequencies (cm−1
)
Compounds
v(OH) of (H2O) coord.
v(O–H–N)
v(C O) lactone
v(C N) free azomethine
v(C N) after coord.
v(C–O) coord.
v(M O)
v(M–O) str.
v(M–N) str.
L1
3220
3225
3464
1700
1710
1697
1715
1716
1726
1705
1704
1700
1654
1660
1632
L2
–
–
–
–
–
–
–
–
–
–
–
–
L3
[VOL1(H2O)]
[VOL2(H2O)]
[VOL3(H2O)]
[MoOL1Cl]
[MoOL2Cl]
[MoOL3Cl]
3418
3429
3422
–
–
–
–
–
–
–
–
–
–
–
–
–
1621
1629
1612
1630
1629
1611
1290
1267
1236
1248
1246
1243
969
972
975
916
918
919
502
512
514
504
501
502
478
482
485
476
472
474
–
–
Table 4
1H NMR spectral data of unsymmetrical tetradentate Schiff base ligand and their metal complexes.
Functional group and chemical shifts ı in ppm
Compounds
–CH3 pyrazolone
C6-CH3 DHA
2.28–2.39
–CH3 bonded to C
N
C5-DHA
–OH pyrazolone
Phenyl proton pyrazolone
Phenyl proton o-phenylene diamine
–CHO
–OH DHA (hump)
L1
2.16
2.16
2.17
2.14
2.15
2.10
2.16
2.53–2.63
2.55–2.65
2.53–2.66
2.52–2.96
2.55–2.86
2.48–2.57
2.51–2.59
5.22–5.25
5.22–5.25
6.72–6.76
5.22
5.19
5.75
5.26
5.29
5.75
–
–
–
6.70–7.16
6.71–7.19
6.72–6.76
6.60–7.20
6.68–7.22
6.67–7.60
6.68–7.61
7.29–7.54
7.30–7.58
7.33–8.18
7.31–7.89
7.35–7.92
7.70–8.19
7.73–8.20
7.89
7.91
8.47
7.97
8.01
7.98
8.46
15.65
15.70
15.35
–
–
–
–
L2
2.29–2.40
2.29–2.40
2.29–2.45
2.31–2.47
2.39–2.40
2.40–2.41
L3
[VOL1(H2O)]
[VOL3(H2O)]
[MoOL1Cl]
[MoOL3Cl]
5.70
–

56
B.T. Thaker, R.S. Barvalia / Spectrochimica Acta Part A 84 (2011) 51–61
H 3C
O
H O
O
N
N
C
N
H
O
C
C
C
H
N
CH 3
H3C
H 3C
O
HO
O
N
C
N
H
O
C
C
N
C
N
CH 3
H3C
H
Fig. 3. Tautomerism form in unsymmetrical tetradentate ligands.
3.4. Mass spectra of unsymmetrical tetradentate L1
The molecular mass spectra of ligand L1 is depicted in Fig. 4,
which shows molecular ion peak at m/z = 443.6 (M+1)+ which
is also in agreement with molecular mass of the ligand L1
(C25H22O4N4 = 442.45).
3.5. FT-IR spectra of unsymmetrical tetradentate Schiff base
complexes
Some of the important applications of IR spectroscopy are the
identification of major types of bonds, various functional groups
and hydrogen bonding in metal complexes. One of the best features
of IR spectroscopy in qualitative analysis is that, the absorption or
lack of absorption in the specific frequency region can be corrected
with specific stretching and bending modes and in some cases, with
the relationship of these groups to rest of the molecule.
The broad band centred at ∼3400 cm−1 due to v(O–H) of coordi-
nated water molecule present in vanadium complexes. The sharp
bands observed in the region 1612–1630 cm−1 were assigned to
Fig. 4. Mass spectra of ligand L1.

B.T. Thaker, R.S. Barvalia / Spectrochimica Acta Part A 84 (2011) 51–61
57
C
N stretching vibrational mode. In the corresponding free TSB lig-
ands the frequencies for C N group are observed between 1632 and
1660 cm−1. A downward shift of the band by 20–33 cm−1 in com-
plexes indicate that the C N group of the ligands is coordinated
to the metal ion via its azomethine nitrogen [31,32]. This lower-
ing may be due to reduction in electron density in the azomethine
link as the nitrogen coordinates to the metal ion. The medium to
strong bands observed at 1236–1290 cm−1 appeared in the spec-
tra of complexes were assigned to enolic C–O stretching vibrations.
The observed higher C–O frequency in metal complexes may be
ascribed to delocalization ꢄ-electron density from the oxygen to
metal ion, resulting in an increase in electro negativity of oxygen
atom. This will lead to greater ionic character of bond and conse-
quently increase in C–O vibration frequency on metal complexation
[32].
Fig. 5. The mass spectra of [VOL2(H2O)] complex.
3.6. 1H NMR spectra of unsymmetrical tetradentate Schiff base
complexes
electronic spectra of VO(IV) Schiff base compounds exhibit three
d–d bands in the region 23,529–12,903 cm−1. These bands may be
assigned to the following transitions. The electronic data of oxo
complexes presented in Table 5.
The very low intense singlet observed at ı 5.19–5.22 and
5.70–5.75 due to the C5 -proton of DHA moiety. The disappearance
Above transition have major contribution for distorted Oh
geometry of oxovanadium(IV) complexes, which is supported
by the reported literature [35]. In addition, it also shows a
high energy band above 30,000 cm−1, which may be due to the
first spin forbidden charge transfer transition [36]. The elec-
tronic absorption spectra of [MoOL1Cl]–[MoOL3Cl] complexes, we
observed the high energy bands (in UV region or near UV region)
observed at 33,444 cm−1, 32,467 cm−1 and 32,501 cm−1 due to
n → ꢂ* or ꢂ → ꢂ* transitions. In the visible region two bands
observed with low intensity in the range 14,388–13,888 cm−1 and
18,315–16,750 cm−1 due to d–d band and this may be assigned to
of peaks –OH (hump) in the spectra of the complexes, which indi-
cates that coordination occur through the ketonic oxygen atom of
the –OH group (after enolization). One more signal observed near
downfield region at ı 5.57 and ı 5.90 due to water molecule pro-
ton, indicating the presence of H2O molecule in VO(IV) complexes.
The multiplet signals observed at ı 6.60–7.61 and at ı 7.31–8.20
due to the aromatic protons of phenyl ring of pyrazolone moiety
and o-phenylene diamines, respectively. In the spectra of the [VOL3
H2O] and [MOL3Cl] complexes we observed a signal at ı 8.47 and
ı 8.55 due to the proton of –SO2NH2 group substituted of phenyl
pyrazolone.
2
2
2B2g → Eg (ꢃ1) and 2B2g → B1g (ꢃ2), respectively. Thus, electronic
spectra of these compounds indicate octahedral environment for all
the complexes and are in conformity with an distorted octahedral
geometry [37].
3.7. UV/visible spectra of unsymmetrical tetradentate Schiff base
complexes
3.8. Mass spectra of unsymmetrical tetradentate Schiff base
complexes
The electronic spectra of oxovanadium (IV) complexes
show intense band at 26,455–23,923 cm−1, 28,945–28,409 cm−1
and 33,783–35,971 cm−1
,
respectively, assigned to either
a
The mass spectrum of [VOL2 (H2O)] complex is shown in
Fig. 5, which also gives molecular ion peak at m/z = 561.3
(M+2)+, which are in agreement with molecular mass of
metal–ligand charge transfer band or to an electronic transi-
tion within the ligand [33,34]. In the visible region, the solution

58
B.T. Thaker, R.S. Barvalia / Spectrochimica Acta Part A 84 (2011) 51–61
the [VOL2(H2O)] (VC25H21O6N4Cl = 559.82) complex. The mass
spectrum of [MoOL3Cl] complex is shown in Fig. 6, which
also gives molecular ion peak at m/z = 667.1 (M+1)+, which
is also in agreement with molecular mass of the [MoOL3Cl]
(MoC25H21O7N5Cl2S = 666.87) complex.
3.9. Magnetic susceptibility
The magnetic susceptibility measurement of unsymmetrical
tetradentate complexes of VO(IV) and MoO(V) complexes were
carried out by Gouy method at room temperature. The observed
magnetic moment (ꢁeff) values is in the range 1.77–1.76 B.M.,
indicating the VO(IV) compounds are normal paramagnetic hav-
ing one unpaired electron s = 1/2. There is no reduced magnetic
moment, ruling out any anti ferromagnetic interaction present in
the molecules [35]. Therefore, VO(IV) complexes are mono nuclear.
MoO(V) are paramagnetic with effective magnetic moments in
the range 1.69–1.71 B.M., with no significant magnetic interac-
tions between the neighboring oxomolybdenum(V) ions, indicating
mononuclear nature of unsymmetrical tetra dentate Schiff base
Fig. 6. The mass spectra of [MoOL3Cl] complex.
Fig. 7. The power ESR spectra of [VOL1(H2O)] complex at (a) RT and (b) LNT.
Fig. 8. The power ESR spectra of [MoOL3Cl] complex at (a) RT and (b) LNT.

B.T. Thaker, R.S. Barvalia / Spectrochimica Acta Part A 84 (2011) 51–61
59
Fig. 11. TGA/DTG curve of [MoOL3Cl] complex.
Fig. 9. TGA/DTG curve of [VOL1(H2O)] complex.
Table 6
complexes [35]. The ESR spectral studies indicate that the all com-
plexes are magnetically dilute in nature. The relevant data of
magnetic moments are reported in Table 1.
g and A value of the powder ESR spectra [VOL1(H2O)] and [MoOL3Cl] complexes at
RT and LNT.
Complex
g
||
g
⊥
|g|
Hyperfine contents × 10−4 cm−1
A
||
A
⊥
|A|
3.10. ESR spectra
[VOL1(H2O)]
The RT and LNT ESR spectrum of [VOL1(H2O)] complex shows
the typical eight line pattern indicating single vanadium is present
RT
LNT
1.93
1.92
1.97
1.96
1.94
1.95
153.75
155.15
66.60
63.65
95.65
94.15
[MoOL3Cl]
RT
in the molecules, i.e., it is a mononuclear. The g and g value were
||
⊥
1.92
1.94
1.96
2.0
1.95
1.98
6.35
10.81
5.54
11.71
5.14
11.41
computed from the spectra using DPPH free radical as g marker. The
LNT
observed g , g , |g| A , A , and |A| values are well in accordance with
||
⊥
||
⊥
|g| = 1/3(g + 2g ), |A| = 1/3(A + 2A ).
||
⊥
||
⊥
the complexes indicate that the unpaired electron localized in dxy
orbital [34–36] of a molecule which exists in distorted octahedral
ters are low because of the dipolar interactions of the unpaired
electron in the 3pz chlorine orbital should be near to zero [42]. The
geometry [37–40]. The g < g and A > A relations are consistent
with octahedral geometry in complexes with C4v symmetry. The
principal component of the g and A tensor were calculated from
||
⊥
||
⊥
g , g and |g| value are well in accordance with the complexes indi-
||
⊥
cates that the unpaired electron localized in the dxy orbital [43,44]
of a molecule which exists in distorted octahedral geometry. The
ESR spectral data of [VOL1(H2O)] and [MoOL3Cl] complexes are pre-
sented in Table 6 and their ESR spectra are given in Figs. 7 and 8,
respectively.
the observed spectra. A
A
and |A| are the hyperfine coupling
||
⊥
⊥
constants. The g , g , A and A were evaluated by using the expres-
||
⊥
||
sions given in literature [41].
The ESR spectra of [MoOL3Cl] complex which was recorded
in powder form at RT and LNT. The ESR spectra of [MoOL3Cl]
complex exhibited a six weak line in the powder form at room
temperature. But [MoOL3Cl] complex gave six intense and broad
ESR lines at LNT, the intensity of these lines are very high. A
typical six line pattern which shows that single molybdenum
is present in the molecules, i.e., it is a mononuclear. The ESR
parameters were found to be g = 1.92–1.94, g = 1.96–2.0 and |g|
3.11. Thermal study
From the TGA and DTG curves, it has been observed that there
are main three steps in the decomposition of [VOL1(H2O)] com-
plex. In complex, decomposition starts from 130 to 180 ◦C. The first
mass loss 3.50% (3.42% cala.) up to 180 ◦C is in good agreement with
loss of one coordinated water molecule. The second and third steps
start from 340 to 480 ◦C and 720 to 900 ◦C, respectively, which is
due to the pyrolysis of organic molecule into two steps. After 900 ◦C,
||
⊥
or gav = 1.95–1.98. An intensity point is that g > g in most of
⊥
||
MoO(V) unsymmetrical tetradentate Schiff base complexes [41].
The A and A value are 6.35 × 10−4 cm−1–10.81 × 10−4 cm−1 and
||
⊥
5.54 × 10−4 cm−1–11.71 × 10−4 cm−1, respectively; these parame-
Fig. 10. Broido plots of [VOL1(H2O)] complex.
Fig. 12. Broido plots of [MoOL3Cl] complex.

60
B.T. Thaker, R.S. Barvalia / Spectrochimica Acta Part A 84 (2011) 51–61
Table 7
Thermal behavior and thermodynamic parameters of [VOL1(H2O)] and [MoOL3Cl] complexes.
Complexes
Step
Step analysis
ꢅE* kcal/mol
Cs = Ws − Wf/W0 − Wf
Order of reaction
Temperature ◦C
Ti
Tmax
Tf
I
130
340
720
175
305
710
140
405
760
185
375
755
180
480
900
230
520
815
7.64
5.60
3.16
7.51
7.08
3.94
0.321
0.430
0.534
0.337
0.398
0.389
1
3/2
2
1
1
[VOL1(H2O)]
[MoOL3Cl]
II
III
I
II
III
1
Ti, initial decomposition temperature.
Tmax, temperature for maximum rate of decompositions.
Tf, final decomposition temperature.
X
H C
3
H C
3
O
O
O
O
O
O
O
N
O
O
N
N
V
V
C
N
C
N
O
C
C
N
CH
C
C
C
H
N
C
H
OH2
OH2
H C
3
H C
3
CH3
CH
3
Complex; [VOL1(H2O)]
X=Cl; Complex; [VOL2(H2O)] and
X =SO2NH2; Complex; [VOL3(H2O)]
Fig. 13. Probable structures of unsymmetrical tetradentate Schiff base complexes of oxovanadium.
the stable metal oxide V2O5 was formed. The energy of activation
Ea of thermal degradation of [VOL1(H2O)] is 7.64 kcal/mol in the
N2 for the first step and the corresponding values for the second
and third steps are 5.60 and 3.16 kcal/mol, respectively. The ther-
mal behavior and thermodynamic parameters presented in Table 7.
The TGA/DTG curves of [VOL1(H2O)] complex of unsymmetrical
tetradentate also shown in Fig. 9. The plot of lnln (1/Y) vs 1/T,
results in straight line are shown in Fig. 10. From these plots we
calculated the activation energy ꢅE*and also calculated order of
reaction.
mass loss 5.40% (5.31% cala.) is due to the loss of one coordinated
chlorine molecule, i.e., one chlorine atom coordinated with Mo(V)
atom. The second step and third step start from 305 to 520 ◦C and
710 to 815 ◦C, respectively, which is due to the pyrolysis of organic
molecule into two steps. After 850 ◦C, the stable metal oxide MoO3
was formed. The thermal behavior and thermodynamic parameters
presented in Table 7. The TGA/DTG curves of [MoOL3Cl] complex of
unsymmetrical tetradentate also shown in Fig. 11. The plot of lnln
(1/Y) vs 1/T, results in straight line are shown in Fig. 12. From these
plots we calculated the activation energy ꢅE*and also calculated
order of reaction.
From the TGA and DTG curves, it has been observed that there
are also main three steps in the decomposition of [MoOL3Cl] com-
plex. In complex, decomposition starts from 175 to 230 ◦C. The first
Application of Broido’s [45] method used to determine the
kinetic parameter for complexes.
X
H C
3
H C
3
O
O
O
O
O
O
O
N
O
O
O
Mo
N
C
N
Mo
C
N
N
C
C
Cl
N
C
H
N
C
C
N
C
H
Cl
H C
3
CH3
H3C
CH
3
Complex; [MoOL1Cl]
X=Cl; Complex; [MoOL2Cl] and
X =SO2NH2; Complex; [MoOL3Cl]
Fig. 14. Probable structures of unsymmetrical tetradentate Schiff base complexes of oxomolybdenum.

B.T. Thaker, R.S. Barvalia / Spectrochimica Acta Part A 84 (2011) 51–61
61
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