Synthesis, structural, and insulin-enhancing studies of oxovanadium(iv) complexes

Article abstract of DOI:10.1071/CH11291

Oxovanadium(iv) complexes with Schiff bases derived from substituted salicylaldehyde and diamine have been prepared and characterized. 1H NMR and IR spectral data revealed that the symmetrical Schiff base was isolated, and elemental analysis confirmed the

Full text of DOI:10.1071/CH11291

CSIRO PUBLISHING
Aust. J. Chem. 2011, 64, 1574–1579
Full Paper
http://dx.doi.org/10.1071/CH11291
Synthesis, Structural, and Insulin-Enhancing Studies
of Oxovanadium(IV) Complexes
A C D
, ,
A
A
Adeola A. Nejo,
Gabriel A. Kolawole, Ayorinde O. Nejo,
A
B
Tebogo V. Segapelo, and Christo J. Muller
A
Department of Chemistry, University of Zululand, Private Bag X1001,
Kwadlangezwa 3886, South Africa.
B
Diabetic Discovery Platform, South African Medical Research Council,
PO Box 19070, Tygerberg 7505, South Africa.
C
Department of Chemical and Environmental Sciences, Babcock University,
Ilisan-Remo, Ogun State, P.M.B. 21244, Ikeja, Lagos, Nigeria.
D
Corresponding author. Email: anejo@pan.uzulu.ac.za
Oxovanadium(IV) complexes with Schiff bases derived from substituted salicylaldehyde and diamine have been prepared
and characterized. 1H NMR and IR spectral data revealed that the symmetrical Schiff base was isolated, and elemental
analysis confirmed the purity of all the compounds as formulated. The room-temperature magnetic moments of
1.6–1.8 Bohr magneton for the complexes confirmed that the complexes are VIV complexes, with d1 configuration.
Cyclic voltammetry revealed only one quasi-reversible wave for each complex and they all showed redox couples with
peak-to-peak separation values (DEp) ranging from 76 to 84 mV, indicating a single-step one-electron transfer process. An
oral administration of these complexes supplied at a dose of 0.2 mmol kgꢀ1 to streptozotocin-induced diabetic rats elicited
a progressive reduction in plasma glucose over 6-h periods. Two of the complexes produced significant and consistent
reduction in fasting blood glucose levels over a 6-h monitoring period.
Manuscript received: 14 July 2011.
Manuscript accepted: 19 September 2011.
Published online: 17 October 2011.
Introduction
The insulin-like properties of vanadium compounds have
attracted a great deal of attention in recent years. Various studies
on vanadium compounds, such as those performed on
streptozocin-induced diabetes in rats, resulted in normalization
in blood glucose levels. Stimulation of glucose oxidation and
adipocyte transport has been demonstrated via vanadium by
several experiments, as well as enhancement of glycogen
synthesis and inhibition of hepatic gluconeogenesis. Investiga-
tors also believe vanadium to be responsible for the phosphory-
lation of the insulin receptor, resulting in activation and glucose
sensitivity.[8,9] Although several inorganic vanadium salts have
beneficial biological properties, the development of new
vanadium derivatives with organic ligands to improve their
bioavailability and to decrease the toxic side effects is of great
interest.[8–11] Organovanadium compounds, as opposed to the
inorganic salts, are recognized as safer, more absorbable, and
able to deliver a therapeutic effect up to 50 % greater than the
inorganic forms.[12] Some organovanadium compounds with a
VO(N2O2) coordination mode have been shown to normalize
blood glucose levels with high efficiency, even at low concen-
trations, with low toxicity.[13–15]
Schiff bases have been widely used as chelating ligands in
inorganic chemistry because of high stability of the coordination
compounds and their good solubility in common solvents.
Tetradentate Schiff bases are well known for their coordination
with various metal ions, forming stable compounds.[1–3] The
development of the field of bioinorganic chemistry has also
increased the interest in Schiff base complexes, because it has
been recognized that many of these complexes may serve as
models for biologically important species.[2,4–6]
Diabetes mellitus is
a chronic metabolic disorder
characterized by hyperglycaemia caused by a relative or abso-
lute deficiency of insulin, or inefficient use of insulin. Two types
of diabetes mellitus have thus far been identified. Type 1 or
insulin-dependent diabetes mellitus (IDDM) is caused by
destruction of pancreatic B-cells and typically affects the
younger population. Type 2 or non-insulin-dependent diabetes
mellitus (NIDDM) is caused by aging, lifestyle-related obesity
or other environmental factors and is the most common form of
diabetes in adults. Untreated or prolonged diabetes mellitus may
lead to many severe secondary complications, such as micro-
vascular disease that leads to kidney failure, blindness and nerve
damage, as well as macrovascular disease that leads to ampu-
tations, cardiovascular disease and stroke. It is estimated that
nearly 366 million people will suffer from diabetes mellitus by
the year 2030.[7]
Recently, some symmetrical and unsymmetrical Schiff base
complexes of oxovanadium(IV) and their insulin-enhancing
activities have been reported from our laboratory.[4,16] As a part
of our continuing work on symmetrical tetradentate Schiff-base
complexes containing N and O donor atoms and in the light of
Journal compilation Ó CSIRO 2011
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Oxovanadium(IV) Complexes with Schiff Bases
1575
the importance of the oxovanadium(IV) ion, we now report the
synthesis and characterization and insulin-enhancing studies of
oxovanadium(IV) complexes of tetradentate Schiff bases derived
from substituted salicylaldehyde and aliphatic or aromatic
diamine.
protons were observed as a doublet at 3.9 ppm. The C–H protons
present only in methylethylenediamine-bridged ligands were
observed at 3.5–3.8 ppm. The chemical shifts obtained were
similar to those of Schiff bases reported in the literature.[17,18]
The IR spectra of the complexes were compared with those of
the Schiff bases in order to determine the coordination sites that
may be involved in chelation. The band appearing at
1614–1640 cmꢀ1 due to the azomethine group in the ligands
was shifted to lower frequency at 1599–1634 cmꢀ1 in the
complexes, indicating the participation of the azomethine nitro-
gen atom in coordination. The nC–O band of the ligands, which
occurs at 1272–1285 cmꢀ1, was slightly displaced to higher
frequencies (1277–1315 cmꢀ1) on coordination to the metal
complexes. The C–O stretching frequency was generally shifted
to a higher frequency, indicating the participation of phenolic
oxygen in C–O–V bond formation.[19] Thus, it can be concluded
that the Schiff bases acted as tetradentate ligands coordinating
via the azomethine N and the phenolic O.
Results and Discussion
Synthesis and Characterization
A series of symmetrical tetradentate Schiff-base complexes of
oxovanadium(IV) were prepared by refluxing the relevant
symmetrical Schiff bases with oxovanadium(IV) sulfate in
methanolic medium. All the metal complexes are stable and
non-hygroscopic in nature. The purity of the Schiff bases and
their metal complexes as formulated were established by
microanalysis. Generalized structures for the Schiff bases and
their complexes are given in Fig. 1.
The 1H NMR spectral data of the free ligands were recorded
in CDCl3 against tetramethylsilane (TMS) as internal reference.
The 1H NMR spectra of the Schiff bases (Fig. 1) displayed the
O–H protons of the phenolic groups and azomethine protons
(H–C¼N) at 12.7–13.8 and 8.3–8.6 ppm, as singlets, respec-
tively. The aromatic protons, which appeared as a multiplet,
were observed at 6.7–7.7 ppm. The signal due to the methoxy
protons was observed as a singlet at 3.8 ppm. Three peaks were
observed for the CH2 protons of the trimethylenediamine-
bridged ligand at 2.2, 3.7 and 4.0 ppm, corresponding to a
multiplet, a triplet and a singlet respectively. The methyl protons
of the ethoxy substituent were observed as a triplet at 1.5 ppm
while its CH2 protons appeared as a quadruplet at 4.1–4.2 ppm.
The methyl protons of the methylethylenediamine-bridged
ligand were observed as a doublet at 1.4 ppm while its CH2
Further conclusive evidence of the coordination of the
Schiff bases with the metal ions was shown by the appearance
of new bands at 448–491 and 541–614 cmꢀ1 assigned to the
metal–nitrogen (V–N) and metal–oxygen (V–O) vibrations
respectively. These bands were absent in the spectra of the
uncomplexed Schiff bases, thus confirming participation of the
O and N atoms in coordination.
A significant change observed in the infrared spectra of the
complexes compared with their respective ligands was the
appearance of a strong absorption band due to the V¼O
stretching frequency in the complexes. All the complexes
exhibited a strong V¼O stretching band at 976–982 cmꢀ1
,
which suggests monomeric square pyramidal structures, except
for the trimethylene derivative in which the nV¼O stretching
frequency occurred at 848 cmꢀ1 [20,21]
.
Electronic spectral measurements were used for assigning
the stereochemistries of the oxovanadium(IV) complexes based
on the positions and number of d–d transition peaks. These
transitions were assigned using the molecular orbital treatment
of [VO(H2O)5]2þ by Ballhausen and Gray.[22] In this energy
OH
HO
Ã
treatment, band I is assigned as b2 - ep (11000–16000),
C
H
N
N
C
H
Ã
Ã
band II as b2 - b1 (14500–19000) and band III as b2 - a1
(21000–30000).
R
R
The electronic absorption spectra of the oxovanadium(IV)
complexes, VOL1–VOL6, were recorded in 10ꢀ3 and 10ꢀ5 M
solutions of each complex in DMSO and chloroform in the range
200–1100 nm at room temperature. The spectra of VOL1 in both
solvents show only two d–d transitions out of the three predicted
for oxovanadium(IV) complexes. The band I transition was
absent in the solution spectra of VOL1 in both solvents, and
band II experienced a shift of ,197 cmꢀ1 with a lowering of
intensity in the molar absorptivity from CHCl3 to DMSO that is
indicative of six-coordination geometry in DMSO solution but
five coordinate geometry in CHCl3. Complex VOL2 formed
with an associated methanol molecule, which reduced
nV¼O to 848 cmꢀ1, and appeared to be polymeric with
ꢁ ꢁ ꢁ V¼O ꢁ ꢁ ꢁ V¼O ꢁ ꢁ ꢁ linkages and in a distorted octahedral
coordination geometry in the solid state. This complex dis-
played all three bands predicted for VO2þ complexes in the
visible region in both solvents. This is due to dissociation of the
complex in solution to form five-coordinate species. A facile
interaction of DMSO with vanadium was indicated by the
reduction in the molar absorptivity of band II in this solvent.
The above spectral characteristics indicate that VOL2 is six-
coordinate in the solid and in DMSO. It may be concluded that
X
O
V
O
O
C
N
N
C
R
R
R
H
H
X
X
Ligand
H2(Omesal2-en)
CH2CH2CH2 H2(Omesal2-tn)
Complex
OCH3
OCH3
CH2CH2
[VO(Omesal2-en)]
[VO(Omesal2-tn)]
OCH3
C6H4
H2(Omesal2-opd) [VO(Omesal2-opd)]
OCH2CH3
CH2CH2
H2(Oetsal2-en)
H2(Oetsal2-pn)
H2(Oetsal2-opd)
[VO(Oetsal2-en)]
[VO(Oetsal2-pn)]
[VO(Oetsal2-opd)]
OCH2CH3 CH2CH(CH3)
OCH2CH3 C6H4
Fig. 1. The proposed structures of the Schiff bases and the complexes.

1576
A. A. Nejo et al.
this complex also exhibits six-coordinate geometry in CHCl3
owing to the observation of a band I transition, and it appeared
that the ꢁ ꢁ ꢁ V¼O ꢁ ꢁ ꢁ V¼O ꢁ ꢁ ꢁ linkages were not broken in
this solvent. Complex VOL3 exhibited only band III at
22422 cmꢀ1 in CHCl3 and two bands at 15649–22624 cmꢀ1 in
DMSO. It is difficult, on the strength of this result, to deduce the
absolute stereochemistry of this complex but it appeared to
exhibit six-coordinate geometry in DMSO and five-coordinate
in CHCl3.
The solution spectra of the ethoxysalicylaldiimino
complexes are similar to those of methoxysalicylaldiimino
complexes discussed above. Two d–d transitions were observed
in the solution spectra of this series in the visible region, except
for VOL5, which was characterized by a single band in DMSO.
A transition corresponding to band I was absent in the solution
spectra in both solvents. A shift in band II with a slight lowering
in the molar absorptivities of complexes VOL4 and VOL5 is
indicative of six-coordinate geometry in DMSO but five-
coordinate geometry in CHCl3. A slight shift was observed in
the spectrum of VOL6 moving from CHCl3 to DMSO. This
indicates a different stereochemistry in both solvents. The
electronic spectra of the complexes indicated a square-
pyramidal or trigonal-bipyramidal geometry with probably
C2v symmetry for the five-coordinate species and distorted
octahedral geometry for trimethylene-bridged complexes.[21]
One of the complexes was characterized by electron para-
magnetic resonance (EPR) spectroscopy after dissolution in
toluene/dichloromethane (90:10 v/v) at 290 and 120 K. The
room-temperature (290 K) spectrum is a typical eight-line
pattern, which shows that a single vanadium is present in
the molecule (Fig. 2). The frozen-solution EPR spectrum of
the complex VOL5 at 120 K displays rhombic vanadium(IV)
spectra with three g (gz , gx , gy) and three A values (Az .
Ax . Ay) characteristic for C2v symmetry, where g is the Lande´
splitting factor and A is the hyperfine coupling constant.[23]
Usually, this is due to a distortion of the square-pyramidal
geometry towards trigonal-bipyramidal, with the two longer
bonds in the axial direction and the three shorter ones in the
equatorial plane. Simulation gave the following parameters:
gx ¼ 1.976, gy ¼ 1.978, gz ¼ 1.948 and Ax ¼ 64.0, Ay ¼ 60.0,
Az ¼ 179.0.
The room-temperature magnetic moments of 1.6–1.8 Bohr
magneton (BM) are normal for VIV d1 configuration. Cyclic
voltammetry revealed that the complexes display one well-
defined oxidation–reduction wave at positive potentials. In an
ideal reversible process, the peak-to-peak separation, DEp E 59
and 30 mV for one-electron and two-electron processes respec-
tively and ia/ic E 1 for a reversible process, where ia/ic is the
ratio of the currents passed at oxidation (ia) and reduction (ic).
All the complexes showed redox couples with peak-to-peak
separation values (DEp) ranging from 76 to 84 mV at a scan rate
of 100 mV sꢀ1, indicating a single-step pseudoreversible one-
electron redox process.[24] The positive E1/2 values of 338.5–
470 mV may be assigned to metal-centred oxidation of VIV to
VV in which there appears not to be any change in the structure
of the parent complex when oxidized, and fall within the range
reported in the literature for similar complexes.[24,25] The cyclic
voltammetry parameters of all the complexes are listed in
Table 1 and a representative cyclic voltammograph of VOL5
is shown in Fig. 3.
- Experimental
- Simulation
In Vivo Insulin-Mimetic Activity
Both inorganic salts and coordination complexes of
oxovanadium(IV) have been studied extensively in order to
understand their insulin-mimetic effects. It has been demon-
strated that many vanadium compounds have therapeutic
effects as insulin-enhancing agents.[10,26–28] But it is very diffi-
cult to compare the efficacy of insulin-mimetic activities of
vanadium complexes in published studies unless the studies
are carried out for the specific purpose of comparison. This is due
to variables such as method of administration, dose, diet, liquid
consumption, gravity of the disease and the genetics, metabolism
and environment of the animal model. Comparison and evalua-
tion can therefore only be done if a reference compound has been
included in the study for the purpose of comparison.
2800
3000
3200
3400
3600
3800
Magnetic field [G]
Fig. 2. Electron paramagnetic resonance spectrum of VOL5 at 290 K in
The in-vivo study was carried out to determine the insulin-
enhancing activities of these oxovanadium(IV) complexes
toluene/CH2Cl2.
Table 1. Cyclic voltammetric data for oxovanadium(IV) complexes
(Ep.c: cathodic peak, Ep.a: anodic peak, ic: current passed at reduction, ia: current passed at oxidation, ia/ic: peak-current ratio, DEp: potential for cathodic
and anodic peak separation, E1/2: half-sum of the cathodic and anodic peaks)
Complexes
Redox couple
Ep.c [mV]
Ep.a [mV]
ic ꢂ 10ꢀ6
ia ꢂ 10ꢀ6
ia/ic
E1/2 [mV]
DEp [mV]
VOL1
VOL2
VOL3
VOL4
VOL5
VOL6
VIV/VV
VIV/VV
VIV/VV
VIV/VV
VIV/VV
VIV/VV
380
490
512
395
399
528
297
414
428
317
319
448
1.07
8.92
8.96
9.21
8.21
8.22
1.02
8.45
7.27
8.81
7.93
6.45
0.95
0.95
0.81
0.96
0.97
0.78
338.5
452
470
356
359
488
83
76
84
78
80
80

Oxovanadium(IV) Complexes with Schiff Bases
1577
(VOL1–VOL6). Rats were injected with streptozotocin (STZ) to
reduce or deplete the number of B-cells to induce an insulin-
deficient non-ketoacidotic hyperglycaemia at levels typical of a
stable type 1 (T1D), or late stage type 2 diabetes (T2D). This is to
test the effect of oxovanadium(IV) complexes in reducing fasting
glucose levels in the absence (T1D) or presence of compromised
insulin production (late stage T2D). The percentage changes in
the plasma glucose values of the treatment groups in the STZ rat
model were calculated for each hour and then subtracted from
the percentage changes in the control values (normalized against
the control) at each time-point. If the complexes do show a
lowering effect of blood glucose, it might be possible that
the complexes have an insulin-like effect or extra-pancreatic
action. The extra-pancreatic actions could be stimulation of
peripheral glucose utilization or enhancing glycolytic and gly-
cogenic processes with a decrease in glycogenolysis and
gluconeogenesis.
significantly reduced plasma glucose by 38.7 % (P , 0.01) and
by 29.9 % (P , 0.05) when compared with the control after
2 and 3 h respectively. Complex VOL4 was found to signifi-
cantly reduce plasma glucose by 30.2 % (P , 0.01) and 37.6 %
(P , 0.01) when compared with the control after 5 and 6 h
respectively. Therefore, the hypoglycaemic action of VOL3 and
VOL4 could be an insulin-like response with an additive effect
on glucose utilization. The percentage reduction in plasma
glucose after treatment of the STZ rats with the complexes is
shown in Fig. 4.
The optimal time-course procedure used in this study to
investigate the short-term insulin-enhancing effect of these
complexes showed a significant hypoglycaemic effect from 0
to 6 h when compared with the control. From the above results, it
may be inferred that the complexes under observation will also
enhance glucose utilization or lowering of glucose serum levels
if they are administered over a long-term period.
An oral administration of these symmetrical Schiff-base
complexes supplied at a dose of 0.2 mmol kgꢀ1 to STZ-induced
diabetic rats elicited a progressive reduction in plasma glucose
over 6-h periods. Two of the complexes, VOL3 and VOL4,
showed significant and consistent reduction in fasting blood
glucose levels over a 6-h monitoring period. Complex VOL3
Bis(maltolato)oxovanadium (BMOV) is a potent example of
a series of compounds designed specifically to be orally
absorbed by passive diffusion as a result of their properties of
water solubility, electrical neutrality and low molecular weight.
It has been extensively investigated in rats treated with STZ that
subsequently developed diabetes.[8,26] BMOV was proved ef-
fective in lowering plasma glucose in STZ-diabetic rats, when
administered in drinking water over a 25-week period. Over
time, BMOV has become the ‘benchmark’ compound against
which all other vanadium-based hypoglycaemic agents have
been compared. Both oral and intraperitoneal dose–response
curves have been conducted to compare the effectiveness of
BMOV and vanadyl sulfate following administration of a single
dose. The effective therapeutic dose of BMOV was calculated to
be 0.45 mmol kgꢀ1 compared with 0.55 to 0.64 mmol kgꢀ1 for
vanadyl sulfate.[26] The maintenance dose of BMOV of
0.18 mmol kgꢀ1 per day was ,50 % of that required for vanadyl
sulfate. The maintenance dose of BMOV was found to compare
favourably with the corresponding doses obtained in our experi-
ments with Schiff-base complexes of oxovanadium(IV)
(0.2 mmol kgꢀ1 per day).
0.000008
0.000006
0.000004
0.000002
0.000000
Ϫ0.000002
Ϫ0.000004
Ϫ0.000006
Ϫ0.000008
Ϫ0.000010
Ϫ0.000012
Ϫ0.2
0.0
0.2
0.4
0.6
0.8
1.0
Conclusions
Potential [V]
Schiff-base complexes of oxovanadium(IV) with N2O2 chro-
mophores have been synthesized and characterized. The
Fig. 3. Cyclic voltammograph of VOL5 complex.
0.00
Ϫ20.00
Ϫ40.00
Ϫ60.00
**
*
**
**
Ϫ80.00
1 h
2 h
VOL1
3 h
4 h
5 h
VOL5
6 h
Control
VOL2
VOL3
VOL4
VOL6
Fig. 4. The effect of the oxovanadium(IV) complexes on hyperglycemia in Wistar outbred rats with streptozotocin-
induced diabetes is shown. A bar-graph with s.d. error bars represents the hourly percentage changes in the whole-blood
glucose values over the 6-h monitoring period. A statistical significance of P # 0.05 is indicated by * and statistical
significance 0.01–0.0001 is indicated by **.

1578
A. A. Nejo et al.
comparison of the IR spectra of the Schiff bases and their metal
complexes indicated that the Schiff bases are tetradentate and
coordinated via the two azomethine N and the two phenolic O
atoms. The solution EPR spectrum of one of the complexes
shows a typical eight-line pattern for the oxovanadium ion. The
in-vivo study revealed that the oxovanadium(IV) complexes
elicited a progressive reduction in plasma glucose over 6 h in
STZ rats.
NCH2CH(CH3)N), 3.9 (d, 2H, NCH2CH(CH3)N), 3.8 (m,
1H, NCH2CH(CH3)N). nmax (ATR)/cmꢀ1 1631 n(C¼N),
1272 n(C–O). Anal. Calc. for C21H26N2O4: C 68.09, H 7.07,
N 7.56. Found: C 67.73, H 7.15, N 7.51 %.
N,N0-Bis(3-ethoxysalicylidene) Orthophenylenediamine
(H2L6): (12.9 g, 79.7 %) as orange-yellow crystals. mp 72–
738C. dH (400 MHz, CDCl3) 6.8–7.3 (m, 10H, Ar–H), 13.1
(s, 2H, OH), 8.6 (s, 2H, N¼CH), 1.5 (t, 6H, OCH2CH3), 4.2
(q, 4H, OCH2CH3). nmax (ATR)/cmꢀ1 1614 n(C¼N), 1282
n(C–O). Anal. Calc. for C21H26N2O4: C 71.27, H 5.98, N 6.93.
Found: C 71.43, H 5.98, N 7.01 %.
Experimental
Materials
All reagents and solvents were of Analar or spectroscopic grades
and used without further purification. Ethanol, methanol,
3-ethoxysalicylaldehyde, 5-methoxysalicylaldehyde, 1,3-
diaminopropane (trimethylenediamine), 1-methylethylenediamine
(propylenediamine) triethylamine, ethylenediamine, vanadyl
sulfate and o-phenylenediamine were purchased from Aldrich–
Sigma and were used as supplied.
Preparation of Oxovanadium(IV) Complexes
The complexes were prepared following the procedure
described in the literature.[29] Oxovanadium(IV) sulfate
(6 mmol, 0.978 g) was dissolved in hot absolute methanol
(300 mL) and a mixture of triethylamine (12 mmol, 1.214 g) and
the corresponding Schiff base (6 mmol) dissolved in methanol
(20 mL) was added with stirring, which resulted in an instant
colour change to green (or orange or brown). The mixture was
stirred for 3 h at 508C and then concentrated to half of its volume
using a rotary evaporator. The product was filtered and washed
twice with cold absolute ethanol and allowed to dry in a desic-
cator over silica gel.
N,N0-Bis(5-methoxysalicylidene)ethylenediiminato Oxova-
nadium(IV) (VOL1). (2.0 g, 86.7 %) as green crystals. mp
.2508C. nmax (ATR)/cmꢀ1 1627 n(C¼N), 1277 n(C–O), 979
n(V¼O), 491 n(V–N), 573 n(V–O). lmax/nm (e/Mꢀ1 cmꢀ1) in
CHCl3 16694 (19), 24631 (10). lmax/nm (e/Mꢀ1 cmꢀ1) in
DMSO 16891 (17), 25380 (1090). meff 1.72 BM. Anal. Calc.
for C18H18N2O5V: C 57.56, H 4.35, N 6.71. Found: C 57.14,
H 4.61, N 7.12 %.
Preparation of Schiff Bases
The ligands were synthesized as previously reported.[4] To a
stirred solution of the appropriate substituted salicylaldehyde
(0.08 mol) in absolute ethanol (60 mL) was added, dropwise, an
ethanolic solution of aliphatic or aromatic diamines (0.04 mol).
This mixture was then stirred for 2 h at 508C. Afterwards, the
mixture was cooled to room temperature or in ice, and the
products formed were collected by filtration. The crystals were
washed with cold absolute ethanol and recrystallized from
ethanol/chloroform (1:3 v/v) mixture. The yellow crystals were
dried in a desiccator over silica gel.
N,N0-Bis(5-methoxysalicylidene) Ethylenediamine (H2L1):
(10.95 g, 83.4 %) as yellow crystals. mp 165–1668C. dH
(400 MHz, CDCl3) 6.7–7.3 (m, 6H, Ar–H), 12.7 (s, 2H, OH),
8.3 (s, 2H, N¼CH), 3.8 (s, 6H, OCH3), 4.0 (s, 4H, NCH2CH2N).
nmax [attenuated total reflectance (ATR)]/cmꢀ1 1638 n(C¼N),
1285 n(C–O). Anal. Calc. for C18H20N2O4: C 65.84, H 6.14, N
8.53. Found: C 65.82, H 6.14, N 8.51 %.
N,N0-Bis(5-methoxysalicylidene) Trimethylenediamine
(H2L2): (10.9 g, 79.6 %) as yellow crystals. mp 82–838C. dH
(400 MHz, CDCl3) 6.7–7.3 (m, 6H, Ar–H), 12.9 (s, 2H, OH),
8.3 (s, 2H, N¼CH), 3.8 (s, 6H, OCH3), 3.7 (t, 4H,
NCH2CH2CH2N), 2.2 (m, 2H, NCH2CH2CH2N). nmax
(ATR)/cmꢀ1 1640 n(C¼N), 1275 n(C–O). Anal. Calc. for
C19H22N2O4: C 66.65, H 6.48, N 8.18. Found: C 66.58,
H 6.50, N 8.15 %.
N,N0-Bis(5-methoxysalicylidene) Orthophenylenediamine
(H2L3): (9.8 g, 65.1 %) as orange crystals. mp 103–1048C. dH
(400 MHz, CDCl3) 6.9–7.7 (m, 10H, Ar–H), 12.7 (s, 2H, OH),
8.6 (s, 2H, N¼CH), 3.8 (s, 6H, OCH3). nmax (ATR)/cmꢀ1 1622
n(C¼N), 1272 n(C–O). Anal. Calc. for C22H20N2O4: C 70.20,
H 5.36, N 7.44. Found: C 70.23, H 5.29, N 7.45 %.
N,N0-Bis(5-methoxysalicylidene)trimethylenediiminato
Oxovanadium(IV) (VOL2). (2.1 g, 81.5 %) as orange-yellow
crystals. mp .2508C. nmax (ATR)/cmꢀ1 1634 n(C¼N),
1307 n(C–O), 848 n(V¼O), 476 n(V–N), 549 n(V–O).
l
max/nm (e/Mꢀ1 cmꢀ1) in CHCl3 14728 (76), 19685 (90),
24876 (11). lmax/nm (e/Mꢀ1 cmꢀ1) in DMSO 13850 (19),
18762 (30), 25445 (1010). meff 1.82 BM. Anal. Calc. for
C20H24N2O6V: C 54.67, H 5.51, N 6.38. Found: C 54.80,
H 5.32, N 6.82 %.
N,N0-Bis(5-methoxysalicylidene)orthophenylenediiminato
Oxovanadium(IV) (VOL3). (2.0 g, 74.3 %) as brown crystals.
mp .2508C. nmax (ATR)/cmꢀ1 1599 n(C¼N), 1289 n(C–O),
981 n(V¼O), 497 n(V–N), 544 n(V–O). lmax/nm (e/Mꢀ1 cmꢀ1
)
in CHCl3 22422 (1790). lmax/nm (e/Mꢀ1 cmꢀ1) in DMSO
15649 (9), 22624 (1790). meff 1.83 BM. Anal. Calc. for
C22H18N2O5V: C 59.87, H 4.11, N 6.35. Found: C 59.55,
H 4.60, N 5.94 %.
N,N0-Bis(3-ethoxysalicylidene)ethylenediiminato Oxovana-
dium(IV) (VOL4). (2.3 g, 91.6 %) as green crystals. mp .2508C.
N,N0-Bis(3-ethoxysalicylidene) Ethylenediamine (H2L4):
(11.5 g, 80.2 %;) as yellow crystals. mp 138–1398C. dH
(400 MHz, CDCl3) 6.7–7.3 (m, 6H, Ar–H), 13.6 (s, 2H, OH),
8.3 (s, 2H, N¼CH), 1.5 (s, 6H,OCH2CH3), 4.0 (t, 4H,
NCH2CH2N), 4.1 (q, 4H, OCH2CH3). nmax (ATR)/cmꢀ1 1637
n(C¼N), 1275 n(C–O). Anal. Calc. for C20H24N2O4: C 67.40,
H 6.79, N 7.86. Found: C 67.52, H 6.98, N 7.96 %.
N,N0-Bis(3-ethoxysalicylidene)-1-methylethylenediamine
(H2L5): (13.2 g, 89.3 %) as yellow crystals. mp 70–718C. dH
(400 MHz, CDCl3) 6.7–7.3 (m, 6H, Ar–H), 13.8 (s, 2H, OH),
8.3 (s, 2H, N¼CH), 1.4 (d, 6H, OCH2CH3), 1.5 (t, 3H,
n
max (ATR)/cmꢀ1 1631 n(C¼N), 1313 n(C–O), 976 n(V¼O), 48,
n(V–N), 611 n(V–O). lmax/nm (e/Mꢀ1 cmꢀ1) in CHCl3
16694 (15), 25510 (620). lmax/nm (e/Mꢀ1 cmꢀ1) in DMSO
17065 (14). meff 1.77 BM. Anal. Calc. for C20H22N2O5V:
C 57.01, H 5.26, N 6.65. Found: C 56.93, H 5.32, N 6.79 %.
N,N0-Bis(3-ethoxysalicylidene))-1,2-propylenediiminato
Oxovanadium(IV) (VOL5). (2.3 g, 88.5 %) as green crystals.
mp .2508C. nmax (ATR)/cmꢀ1 1611 n(C¼N), 1304 n(C–O),
982 n(V¼O), 471 n(V–N), 614 n(V–O). lmax/nm (e/Mꢀ1 cmꢀ1
)
)
in CHCl3 16667 (16), 25316 (6). lmax/nm (e/Mꢀ1 cmꢀ1
in DMSO 16694 (15). meff 1.64 BM. Anal. Calc. for

Oxovanadium(IV) Complexes with Schiff Bases
1579
C21H24N2O5V: C 57.93, H 5.56, N 6.43. Found: C 57.45,
H 5.49, N 6.40 %.
Acknowledgements
G.A.K. and A.A.N acknowledge support from the National Research
Foundation (NRF), South Africa, under the Institutional Research Devel-
opment Programme funding.
N,N0-Bis(3-ethoxysalicylidene)orthophenylenediiminato
Oxovanadium(IV) (VOL6). (2.1 g, 75.5 %) as green crystals.
mp .2508C. nmax (ATR)/cmꢀ1 1602 n(C¼N), 1315 n(C–O),
982 n(V¼O), 448 n(V–N), 541 n(V–O).
lmax/nm
References
(e/Mꢀ1 cmꢀ1) in CHCl3 16051 (7), 22831 (1180). lmax/nm
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1.78 BM. Anal. Calc. for C24H22N2O5V: C 61.41, H 4.72,
N 5.97. Found: C 59.98, H 4.93, N 5.88 %.
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