Synthesis, characterization, and thermodynamics of some new unsymmetrical Co(III) Schiff base complexes derived from 2,3-diaminopyridine

Article abstract of DOI:10.1007/s00706-013-1045-0

Some new Schiff bases derived from 2,3-diaminopyridine and their new Co(III) complexes were synthesized and characterized. The formation constants of the Co(III) complexes vary according to the following trend: 5-MeO > 5-H > 5-Br and 5-MeO > 3-MeO > 4-MeO. For a given donor, the order of formation constants according to the axial ligand is PBu₃ > PPh₂Me. The binding trend of the donors toward a given cobalt(III) Schiff base complex is as follows: benzylamine > nbutylamine > sec-butylamine > tert-butylamine; Im > 2-MeIm > 2-EtIm > BzIm; and PBu₃ > PPh₂Me. Springer-Verlag Wien 2013.

Full text of DOI:10.1007/s00706-013-1045-0

Monatsh Chem (2013) 144:1635–1643
DOI 10.1007/s00706-013-1045-0
ORIGINAL PAPER
Synthesis, characterization, and thermodynamics of some new
unsymmetrical Co(III) Schiff base complexes derived
from 2,3-diaminopyridine
•
Mozaffar Asadi Susan Torabi Khosro Mohammadi
•
Received: 10 November 2012 / Accepted: 27 June 2013 / Published online: 16 August 2013
Ó Springer-Verlag Wien 2013
Abstract Some new Schiff bases derived from 2,3-
diaminopyridine and their new Co(III) complexes were
synthesized and characterized. The formation constants of
the Co(III) complexes vary according to the following
trend: 5-MeO [ 5-H [ 5-Br and 5-MeO [ 3-MeO [
4-MeO. For a given donor, the order of formation constants
according to the axial ligand is PBu3 [ PPh2Me. The
binding trend of the donors toward a given cobalt(III)
Schiff base complex is as follows: benzylamine [ n-
butylamine [ sec-butylamine [ tert-butylamine; Im [ 2-
MeIm [ 2-EtIm [ BzIm; and PBu3 [ PPh2Me.
condensation of o-hydroxyaromatic aldehydes and amino-
pyridines are interesting because of their thermo- and
photochromic properties [5, 6], tautomeric equilibria [7, 8],
potential applications in chemical analysis [2, 9], and use
as therapeutic agents [3]. However, much less information
is available regarding the thermodynamics of these kinds of
Schiff bases.
In our previous reports, the thermodynamics of a new
Co(III) Schiff base complex derivative from 2,3-diamino-
pyridine (2,3-DAP) with a salicylaldehyde [10] and its
application as a sensor [11] were investigated. In this
context, the present work focuses on the synthesis and
thermodynamic properties of some other unsymmetrical
cobalt(III) Schiff base complexes of 2,3-DAP with differ-
ent ligand donors. The spectral and thermodynamic
properties of the complexes were investigated and com-
pared in order to determine the effects of different
electronic and steric factors.
Keywords Cobalt(III) ꢀ Unsymmetrical Schiff base ꢀ
Thermodynamic free energy change ꢀ Formation constants
Introduction
Schiff bases involving a pyridine ring have received con-
siderable attention in the literature because of their very
important role in biological systems [1]. They may also be
used as analytical reagents for metals [2, 3] and as anti-
inflammatory agents [4]. Schiff bases resulting from the
Results and discussion
Because 2,3-diaminopyridine has two different condensa-
tion sites, one might expect two mono Schiff bases to be
formed by 1:1 condensation; however, only compound 1 was
obtained (Scheme 1). The lower reactivity of the 2-amino
group with respect to the 3-amino group may be attributed to
the contribution of the resonance structure II [12]
(Scheme 2). By mixing the mono Schiff base 1 with an
appropriate aldehyde in a 1:1 ratio, the bis Schiff bases 2
were obtained. The complexes 3 were prepared according to
Scheme 3. The resulting compounds are non-hygroscopic
and air-stable solids. The compounds were characterized by
various physicochemical techniques, viz. 1H NMR, 13C
NMR, IR, UV–Vis spectroscopy, and elemental analysis.
M. Asadi (&) ꢀ S. Torabi
Chemistry Department, College of Sciences, Shiraz University,
71454 Shiraz, Islamic Republic of Iran
e-mail: mozaffarasadi@yahoo.com; asadi@susc.ac.ir
S. Torabi
International Branch, Shiraz University of Medical Sciences,
Shiraz, Islamic Republic of Iran
K. Mohammadi
Chemistry Department, Faculty of Sciences, Persian Gulf
University, 75169 Bushehr, Islamic Republic of Iran
123

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M. Asadi et al.
Scheme 1
18
17
16
22
H
21H
14
13
CH
N
N
N
9
8
15
10
11
10
O
N
2
1H
11
12
H
H
O
12
1
CH
1O
N
CH 7
N
6
5
7
2
3
2
3
1
6
4
4
R
5
1
R = 5-Br (2a), 5-OCH3 (2b),
4-OCH3 (2c), 3-OCH3 (2d)
range of the complexes is assignable to mM–O [20]. The other
series of weak bands between 3,100 and 2,800 cm-1 are
related to (C–H) modes of vibrations [21]. All the IR data
suggest that the metal is bonded to the Schiff bases through
the phenolic oxygen and imino nitrogen.
Scheme 2
NH2
NH2
NH2
N
N
NH2
I
II
Electronic spectra
The spectra of the ligands exhibit bands at about
305–380 nm and a broad shoulder at approximately
450 nm. The first peak is attributed to p ? p* transitions.
This band was not significantly affected by chelation. The
second band in the spectra of the ligand is assigned to
n ? p* transition that involves the nonbonding electrons
of the azomethine nitrogen atom. This band is shifted to a
longer wavelength and is increased in intensity. This shift
may be attributed to the donation of the lone pairs of the
nitrogen atoms of the Schiff base to the metal ion (N ? M)
[22].
Infrared spectral studies
The IR spectra of the ligands exhibit broad medium
intensity bands in the 2,800–3,400 cm-1 range which are
assigned to the intramolecular hydrogen bonding vibration
(O–HꢀꢀꢀN). These bands disappeared on complexation to
cobalt(III) ions [13]. The IR spectra of the complexes
exhibit absorption around 3,450 cm-1 that is attributed to
the presence of lattice and coordinated water [14].
The band appearing at around 1,604–1,624 cm-1 due to
azomethine was shifted to a lower frequency by approxi-
mately 1–15 cm-1 in the complexes, indicating participation
of the azomethine nitrogen in the interaction with the metal
ion [15]. Coordination of azomethine the nitrogen is con-
firmed by the presence of new bands at 540–570 cm-1
assignable to mM–N for these complexes [16]. The mC–O
stretching frequency shifts in the complexes towards lower
or higher values as a result of coordination of the oxygen to
the metal ion [17–19]. A new band in the 460–470 cm-1
All the complexes show a band in the range of
440–500 nm, which is attributed to a d ? p* transition
that is mixed with d ? d transitions. The study of the
electronic spectra to identify the d ? d transitions in the
presence of a ligand field has encountered difficulties
because several bands fall in the near-infrared region with a
low intensity while a large part of the visible region is
obscured by intense charge transfer and intraligand
(p ? p*) transitions [14].
123

Synthesis, characterization, and thermodynamics
1637
Scheme 3
Scheme 4
1H and 13C NMR spectra
The number of signals of sharp peaks represents the
number of carbons of the compounds which are chemi-
cally non-equivalent. These spectra gave the correct
number of carbon signals required by the suggested
structures. The spectra of the complexes showed the
presence of two imine and two C(O) signals for carbon
atoms.
The 1H NMR spectra of all the Schiff bases provide
compelling evidence of the presence of either one or two
1
azomethine groups. By comparing the H NMR spectra of
the Schiff bases and their complexes, it is noted that there
is a down- or upfield shift in the frequency of the azome-
thine protons confirming coordination of the metal ion to
these groups. In complexes, the absence of hydroxyl pro-
tons at approximately 12 ppm provides evidence for
coordination through two oxygen atoms (after deprotona-
tion of the ortho hydroxyl groups). The 1H NMR spectra of
the complexes showed a differentiation of almost all
hydrogen atoms, because they are in different chemical
environments owing to the rigid structure in which the
ligand lies when it is coordinated to the metal center [23].
Three sets of signals were observed for the pyridyl groups
of the Schiff bases which coordinated to metal ion
(9.0–6.3 ppm). The formation of rigid structure also helps
the clearer visualization of aromatic protons. These protons
were observed in the range of approximately 6.9–8.5 ppm.
In the spectra of complexes, two sets of resonances were
observed for methine protons, which shows that they are in
different chemical environments. The signals for these
protons were observed at around 8.4–9.5 ppm [23] and
those for the hydrogens of the methoxy groups were are at
3.7 ppm. The proton chemical shifts for the coordinated
PBu3 in the Schiff base complexes appear at 0.7 up to
1.3 ppm. These results are in agreement with the previous
results observed for metal complexes of phosphine as axial
ligand [10, 24, 25].
Thermodynamic studies
The formation constants were determined by UV–Vis
absorption spectroscopy from the reaction of the acceptors
with the donors in 96 % methanol solvent (Scheme 4).
A solution of each complex at a concentration of about
3 9 10-5 M was prepared and constant ionic strength
(I = 0.1 M) was maintained by addition of sodium per-
chlorate. In a typical titration, 2.5 cm3 of this solution was
transferred into the thermostated cell compartment of the
UV–Vis instrument, which was kept at constant tempera-
ture ( 0.1 K) by circulating water and was titrated against
the given amine. The titration was done by adding aliquots
of the amine with a Hamilton microliter syringe. The
donor’s concentration was varied between 1- and 3-fold in
excess.
The absorption measurements were carried out at vari-
ous wavelengths where the difference in absorption was the
maximum after the equilibrium was assessed. The formed
adduct shows an absorption different from the acceptor,
whereas the donors show no absorption at those wave-
lengths. As an example, the variation of the electronic
spectra for [Co(2,3-Salpyr)(PBu3)]ClO4ꢀH2O titrated with
PBu3 at T = 293 K in 96 % methanol is shown in Fig. 1.
The isosbestic points of this system show that there is only
one reaction in equilibrium. The same is valid for other
systems.
The 13C NMR spectra provide further support for the
structural characterization of the Schiff bases. They were
recorded by employing TMS as internal reference and
CDCl3 and DMSO-d6 as solvents at ambient temperature.
123

1638
M. Asadi et al.
Table 1 Formation constants, 10-3/K/dm3 mol-1, for the interaction
0.6
[Co(5-Br-2,3-Salpyr)(PBu3)]? with various amines in 96 % methanol
T/K
Benzyl
n-Butyl
sec-Butyl
tert-Butyl
0.5
0.4
0.3
0.2
0.1
0
283
293
303
313
5.2 1.1
4.5 0.7
3.0 0.1
2.2 0.4
4.1 0.5
3.6 0.4
2.9 0.3
1.9 0.1
3.8 0.2
3.5 0.3
2.7 0.4
1.6 0.3
1.4 0.1
1.0 0.1
0.7 0.0
0.6 0.0
Table 2 Formation constants, 10-1/K/dm3 mol-1, for the interaction
[Co(2,3-Salpyr)(PPh2Me)]? with Im and its derivatives in 96 %
methanol
300
350
400
450
500
550
600
650
700
Wavelength/nm
Fig. 1 Variation of the electronic spectra of [Co(2,3-Salpyr)(PBu3)]
ClO4ꢀH2O titrated with PBu3 at 293 K in 96 % methanol
T/K
Im
2-MeIm
2-EtIm
BzIm
283
293
303
313
257.0 19.0
226.7 10.0
182.8 7.0
123.8 4.0
16.7 1.0
13.2 1.6
12.7 0.9
5.4 0.4
13.2 1.0
12.3 1.0
7.2 1.7
1.8 0.6
10.4 0.6
8.6 0.1
6.8 0.2
4.3 0.1
0
-0.2
283
-0.4
-0.6
-0.8
-1
293
303
313
Table 3 Formation constants, 10-2/K/dm3 mol-1, for the interaction
[Co(2,3-Salpyr)(PBu3)]? with phosphines in 96 % methanol at 293 K
PBu3
7.2 0.3
2.3 0.2
PPh2Me
-1.2
-1.4
0
0.4
0.8
1.2
1.6
2
103 C
trends [27, 28]. Four amines, n-butyl-, sec-butyl-, tert-
butyl-, and benzylamine, were chosen to measure the rel-
ative binding constants for a range of sterically and
electronically varied amines. Here, we attempt to separate
these two parameters and use cone angles to place amine
and phosphine ligands on a common steric scale. The
amines have an important steric factor that is shown by
Trogler’s cone angle [28]. The cone angles of sec-butyl-
amine, tert-butylamine, and benzylamine are 113°, 123°,
and 106°, respectively. The cone angle of n-butylamine is
unpublished. The formation constants were increased by
decreasing the cone angles.
Fig. 2 Typical plots of
P
against
C for [Co(5-Br-2,3-Sal-
pyr)(PBu3)]ClO4ꢀH2O titrated with n-butylamine at various
temperatures (283–313 K) in 96 % methanol, I = 0.1 M. P =
(C°AC°D)/(A - A°A - A°D) and C = (C°A ? C°D)
Effects of donor, axial and equatorial ligands,
and temperature on the formation constants
of the tributylphosphinecobalt(III) Schiff base
The formation constants of the various cobalt(III) Schiff base
complexes were calculated by using Ketelaar’s equation [10,
24, 26]. The linear plots of P against C, where P = (C°AC°D)/
(A - A°A - A°D) and C = (C°A ? C°D), for [Co(2,3-Sal-
pyr)(PBu3)]ClO4ꢀH2O titrated with PBu3 at various
temperatures in 96 % methanol are shown in Fig. 2, which
signify that only a 1:1 complex is formed. Similar plots were
obtained for other systems. The adduct formation constants
are collected in Tables 1, 2, 3, 4, and 5.
The electronic effect of amines is shown by Taft r*
values, which are ?0.85, ?0.77, ?0.68, and ?1.2 for
n-butylamine, sec-butylamine, tert-butylamine, and
benzylamine, respectively [29]. On the base of the results
in Table 1, the formation constants follow the sequence
below:
benzylamine [ n - butylamine [ sec - butylamine
[ tert - butylamine
Acceptor properties of Co(III) Schiff base complexes
Aromatic amines as donors
Aliphatic amines as donors
We previously studied the effect of the base strength of
the cyclic amines on the adduct formation constants of
some symmetrical pentacoordinated Co(III) Schiff base
The comparison of the reactivity of amine compounds
toward metal complexes has primarily focused on pKa
123

Synthesis, characterization, and thermodynamics
1639
Table 4 Formation constants,
T/K
[Co(5-Br-2,3-
Salpyr)(PBu3)]?
[Co(5-MeO-2,3-
Salpyr)(PBu3)]?
[Co(4-MeO-2,3-
Salpyr)(PBu3)]?
[Co(3-MeO-2,3-
Salpyr)(PBu3)]?
10-3/K/dm3 mol-1, for the
interaction Co(III) complexes
with n-butylamine in 96 %
methanol
283
4.1 0.5
3.6 0.4
2.9 0.3
1.9 0.1
6.3 1.0
6.0 0.4
3.5 0.3
2.4 0.3
4.7 1.1
4.4 0.3
3.1 0.6
2.1 0.8
5.9 1.2
5.5 1.2
3.4 0.3
2.3 0.0
293
303
313
Table 5 Formation constants, 10-2/K/dm3 mol-1, for the interaction
Co(III) complexes with Im and its derivatives in 96 % methanol at
293 K
Scheme 5
[Co(Chel)(PBu3)]+ + S
[Co(Chel)(PBu3)S]+
Compounds
Im
2-MeIm 2-EtIm
BzIm
S = H2O or solvent molecule
[Co(2,3-
45.2 2.0 4.0 0.6 1.6 0.2 1.4 0.2
22.6 1.0 1.3 0.1 1.2 0.1 0.8 0.0
Salpyr)(PBu3)]?
[Co(2,3-
Salpyr)(PPh2Me)]?
PBu3 are 136° and 132°, respectively [33]. The steric effect
increased with increase in the cone angle. So by increasing
the cone angle in PPh2Me, the formation constant is
decreased. So, the steric and the electronic properties affect
the formation constant of the complexes.
complexes [10]. In the present manuscript, the effects of
these amines were examined on the formation constants of
the unsymmetrical Co(III) Schiff base complexes derived
from 2,3-DAP according to equation (d) in Scheme 4
(Table 2). The results show the following trend in the
adduct formation between Co(III) Schiff base complexes as
acceptors and the aromatic amines as donors:
Electronic effect of the equatorial Schiff base ligands
The equatorial ligands play an important role in the sta-
bility and the reactivity of the complexes. We determined
the role of four equatorial ligands in the stability of the
five-coordinated complexes. The formation constants for
[Co(5-MeO-2,3-Salpyr)(PBu3)]? in methanol are larger
than those of all the other complexes with all donors
(Table 4).
Im [ 2-MeIm [ 2-EtIm [ BzIm
As one can see, the formation constants for these bases
with [Co(2,3-Salpyr)(PPh2Me)]? are decreased with
increasing the steric hindrance of the donor bases and it
is in contrast with trend of the basic constants, Kb, of these
amines [10, 30] (Table 2).
The addition of a methoxy group to 2,3-Salpyr
decreased the stability of the solvated five-coordinated
complex and increased the formation constants of the six-
coordinated adduct. It seems that in the five-coordinated
complexes, a water or a solvent molecule occupies the
sixth position (Scheme 5) and this would decrease the
tendency of the complexes toward the interaction with
donors.
Phosphines as donors
The adduct formations of some pentacoordinated cobalt(III)
Schiff base complexes were studied according to equation
(c) in Scheme 4 with two phosphines as donors. The steric
and the electronic effects of the entering ligands of the
Schiff base complexes were studied. Theses data are col-
lected in Table 3.
Concerning the stabilization of the five-coordinate
complex, the donation power of the Schiff base is impor-
tant. The cobalt atom in [Co(5-Br-2,3-Salpyr)(PBu3)]?
complex has a more powerful acceptor property than that in
the other types of complexes and forms more stable com-
plexes with H2O or a solvent molecule (Scheme 5).
Therefore its tendency for the reaction with donors
decreases and hence its formation constant, K, with donors
is lower [10].
The electronic effects of phosphines are expressed by
Taft r* values. The r* value for PBu3 is -0.390 and that
for PPh2Me was not reported in the literature (but for
PPh2Et is ?1.10 and for PPhMe2 is ?0.60) [10, 31, 32].
Also, the phenyl electron-withdrawing groups decrease the
basic properties of PPh2Me with respect to PBu3. The
formation constants increased according to the order
PBu3 [ PPh2Me.
The phosphine ligands have an important steric effect
that can decrease the tendency to form five-coordinated
complexes with the donor bases. This factor is shown by
Tolman’s cone angle. The cone angles for PPh2Me and
To study the effect of the position of substituents on the
interaction of the Co(III) complexes, 3-MeO, 4-MeO, and
5-MeO were selected. The results show the following trend
in the adduct formation between cobalt(III) Schiff bases
and n-butyl amine as donor: 5-MeO [ 3-MeO [ 4-MeO.
123

1640
M. Asadi et al.
Table 6 DG°/kJ mol-1 values for the interaction between [Co(5-Br-
thermodynamic formation constants, K, and the thermo-
dynamic free energy change values for the above adducts
of the mentioned complexes with various donors were
determined spectrophotometrically and the following con-
clusions have been drawn:
2,3-Salpyr)(PBu3)]? and various amines in 96 % methanol
Donors
283 K
293 K
303 K
313 K
Benzyl
-20.1 1.1 -20.5 0.7 -20.1 0.8 -20.0 0.2
-19.5 0.3 -19.9 0.9 -20.1 0.3 -19.6 0.1
n-Butyl
1. The reactions are exothermic, because by increasing
temperature, the formation constant is decreased:
DH° \ 0.
sec-Butyl -19.3 0.2 -19.8 0.7 -19.9 0.5 -19.2 0.6
tert-Butyl -17.06 0.4 -16.8 0.9 -16.5 0.4 -16.7 0.5
2. The formation constants change according to the
following trend owing to the steric and the electronic
factors of the Co(III) complexes:
Table 7 DG°/kJ mol-1 values for the interaction between [Co(2,3-
Salpyr)(PPh2Me)]? and Im and its derivatives in 96 % methanol
5 - MeO [ 5 - H [ 5 - Br
Donors 283 K
293 K
303 K
313 K
5 - MeO [ 3 - MeO [ 4 - MeO
Im -18.4 1.0 -18.7 0.7 -18.9 0.2 -18.5 0.3
2-MeIm -12.0 0.3 -11.9 0.5 -12.2 0.1 -10.4 0.0
2-EtIm -11.5 0.7 -11.7 0.4 -10.7 0.1 -7.5 0.2
3. The order of the formation constants of Co(III) Schiff
base complexes toward a given donor according to the
phosphine axial ligand is as follows:
BzIm
-10.9 0.6 -10.8 0.2 -10.6 0.8 -9.7 0.4
PBu3 [ PPh2Me
Effect of the axial ligands
4. The following binding trend of the donors toward a
given cobalt(III) Schiff base complex is as follows:
Concerning the stabilization of the five-coordinate com-
plex, the donation power of the Schiff base is important. By
decreasing the r* in PPh2Me, the cobalt atom in [Co(2,3-
Salpyr)(PPh2Me)]? complex has a more powerful acceptor
property than that in the other types of complexes and
forms more stable complexes with H2O or a solvent mol-
ecule (Scheme 5). Therefore, its tendency for the reaction
with donors decreases and its formation constant, K, with
donors is lower [10]. So, the formation constants increased
according to the order PBu3 [ PPh2Me.
benzylamine [ n - butylamine [ sec - butylamine
[ tert - butylamine
Im [ 2 - MeIm [ 2 - EtIm [ BzIm
PBu3 [ PPh2Me
Experimental
All chemicals were used as obtained from Merck, Fluka, or
Aldrich. Anal. grade solvent from Merck was used without
further purification. The diamine and salicylaldehyde were
distilled before use.
Effect of temperature on the formation constants
The infrared spectra of all ligands and their complexes
were recorded in the range 4,000–400 cm-1 using a Shi-
madzu FTIR-8300 spectrophotometer applying the KBr
disc technique. The UV–Vis absorption spectra were
recorded using a Perkin-Elmer Lambda 2 spectrophotom-
eter at room temperature. The elemental analysis was
The formation constants were determined at various tem-
peratures. Because of bond formation, by increasing the
temperature, the formation constants were decreased.
Thermodynamic free energy change
1
carried out using a Thermo Finnigan-Flash-1200. The H
The free energy change values, DG°, of the studied
cobalt(III) Schiff base complexes were calculated from
DGꢁ ¼ ꢂRT ln Kf at various temperatures (Tables 6, 7, 8).
The linear plot of [Co(5-Br-2,3-Salpyr)(PBu3)]ClO4ꢀH2O
titrated with n-butylamine at various temperatures in 96 %
methanol is shown in Fig. 3.
NMR spectra were recorded by a Bruker Avance DPX
250 MHz spectrometer.
Preparation of Schiff bases
The mono Schiff base 1 was prepared by mixing stoichi-
ometric amounts of analytically pure salicylaldehyde with
2,3-DAP dissolved in ethanol and refluxing for 3 h on a
water bath. The separated brown-yellow crystals were re-
crystallized from hot ethanol [7]. The mono Schiff base 1
were mixed with the appropriate aldehyde in a 1:1 ratio
using the same condensation procedure to afford the bis
Conclusions
The synthesis and characterization of some new Schiff
bases derived from 2,3-DAP and their new five- and six-
coordinated Co(III) complexes were performed. The
123

Synthesis, characterization, and thermodynamics
1641
Table 8 DG°/kJ mol-1 values
Compounds
283 K
293 K
303 K
313 K
for the Co(III) complexes with
n-butylamine in 96 % methanol
[Co(5-Br-2,3-Salpyr)(PBu3)]?
-19.5 0.3
-20.5 1.1
-19.9 0.7
-20.4 0.5
-19.9 0.2
-21.1 1.4
-20.4 0.9
-21.0 0.6
-20.1 0.9
-20.5 0.7
-20.2 0.8
-20.5 0.7
-19.6 1.0
-20.2 0.9
-19.8 0.6
-20.1 0.4
[Co(5-MeO-2,3-Salpyr)(PBu3)]?
[Co(4-MeO-2,3-Salpyr)(PBu3)]?
[Co(3-MeO-2,3-Salpyr)(PBu3)]?
(N-Salicyliden-N0-4-methoxysalycyliden)-2,3-diaminopyri-
dine (4-MeO-2,3-Salpyr) (2c, C20H17N3O3)
8.4
8.2
8
Yellow crystals; yield 55 %; m.p.: 200 °C; 1H NMR
(DMSO-d6): d = 6.6–7.2 (1H, H11), 8.0 (1H, H10), 8.6 (1H,
H12), 6.6–7.2 (6H, Ar–H), 8.6 (1H, H6), 8.8 (1H, H13), 9.0
(1H, H7), 12.3 (1H, H2), 12.6 (1H, H1), 3.8 (OCH3) ppm;
13C NMR (DMSO-d6): d = 160.5 (C7=N), 160.2 (C13=N),
148.4 (C12), 122.6 (C11), 135.9 (C10), 139.8 (C9), 150.3
(C8), 162.9, 159.9, 153.1, 132.3, 130.0, 120.8, 118.5, 118.2,
117.8, 116.3, 116.1, 110.3 (12 signals for Ar–C), 55.6
7.8
7.6
7.4
315
325
335
105(T /K)-1
345
355
Fig. 3 Plot of
ClO4ꢀH2O with n-butylamine in 96 % methanol
K against 1/T for [Co(5-Br-2,3-Salpyr)(PBu3)]
ꢀ
(OCH3) ppm; FT-IR (KBr): m = 3,444 (mO–H), 1,612
(mC=N), 1,558 (mC=C), 1,107 (mC–O
(MeOH): kmax = 388, 445 (sh) nm.
) ; UV–Vis
cm-1
(N-Salicyliden-N0-3-methoxysalycyliden)-2,3-diaminopyri-
dine (3-MeO-2,3-Salpyr) (2d, C20H17N3O3)
Schiff bases 2. The resulting solution was concentrated to
half its initial volume and the separated crystals were fil-
tered and recrystallized from ethanol.
Orange crystals; yield 59 %; m.p.: 205 °C; 1H NMR
(CDCl3): d = 6.8–7.3 (1H,H11), 7.4 (1H, H10), 8.4 (1H,
H12), 6.8–7.3 (6H, Ar–H), 7.8 (1H, H6), 8.8 (1H, H13), 9.5
(1H, H7), 12.5 (1H, H2), 13.2(1H, H1), 3.8 (OCH3) ppm;
13C NMR (CDCl3): d = 160.6 (C7=N), 160.1 (C13=N),
147.8 (C12), 121.0 (C11), 137.6 (C10), 143.5 (C9), 151.1
(C8), 163.4, 161.7, 161.1, 132.7, 132.3, 130.1, 121.4, 118.0,
116.3, 112.2, 108.1, 104.9 (12 signals for Ar–C), 55.9
(N-Salicyliden-N0-5-bromosalycyliden-2,3-diaminopyri-
dine (5-Br-2,3-Salpyr) (2a, C19H14BrN3O2)
Yellow crystals; yield 50 %; m.p.: 120 °C; 1H NMR
(DMSO-d6): d = 6.9 (1H, H11), 7.0 (1H, H10), 8.9 (1H,
H12), 7.4–7.9 (6H, Ar–H), 7.9 (1H, H6), 8.4 (1H, H13), 9.5
(1H, H7), 12.6 (1H, H2), 13.1 (1H, H1) ppm; 13C NMR
(DMSO-d6): d = 160.5 (C7=N), 160.1 (C13=N), 149.1
(C12), 123.6 (C11), 136.4 (C10), 141.3 (C9), 151.9 (C8),
163.6, 161.8, 133.1, 132.6, 130.3, 126.3, 122.0, 120.8,
118.1, 117.1, 114.7 (12 signals for Ar–C) ppm; FT-IR
ꢀ
(OCH3) ppm; FT-IR (KBr): m = 3,456 (mO–H), 1,605
(mC=N), 1,556 (mC=C), 1,107 (mC–O
(MeOH): kmax = 330, 465 (sh) nm.
) ; UV–Vis
cm-1
ꢀ
(KBr): m = 3,427 (mO–H), 1,612 (mC=N), 1,558 (mC=C),
Synthesis of the five-coordinate Co(III) complexes
1,184 (mC–O) cm-1; UV–Vis (MeOH): kmax = 330, 460
(sh) nm.
The cobalt(III) Schiff base complexes were prepared by the
methods described in the literature [34]. To a refluxing
solution of the tetradentate ligands (1 mmol) in 10 cm3
methanol was added 0.25 g Co(OAc)2ꢀ4H2O (1 mmol).
After 15 min, the appropriate phosphine (0.8 mmol) was
added to the solution. The reaction mixture was refluxed
for 1 h. The formed Co(II) complex was oxidized by
blowing air into the solution for 2 h, and the solution was
then filtered. An appropriate amount of sodium perchlorate
was added to the filtrate. The resulting brown crystals were
formed after 48 h. The crystals were washed with some
methanol and purified by recrystallization in dichloro-
methane/ethanol. The complex was dried in vacuo at 323 K
for 48 h (Fig. 4).
(N-Salicyliden-N0-5-methoxysalycyliden)-2,3-diaminopyri-
dine (5-MeO-2,3-Salpyr) (2b, C20H17N3O3)
Yellow crystals; yield 51 %; m.p.: 115 °C; 1H NMR
(DMSO-d6): d = 7.1 (1H, H11), 7.6 (1H, H10), 7.9 (1H,
H12), 6.6–7.1 (7H, Ar–H), 8.7 (1H, H13), 8.9 (1H, H7), 12.9
(1H, H2), 13.3 (1H, H1), 3.7 (OCH3) ppm; 13C NMR
(DMSO-d6): d = 160.6 (C7=N), 160.1 (C13=N), 148.2
(C12), 123.1 (C11), 137.1 (C10), 140.6 (C9), 152.4 (C8),
162.4, 161.8, 161.1, 133.2, 132.3, 130.1, 121.3, 117.9,
116.2, 111.8, 108.1, 107.3 (12 signals for Ar–C), 55.8
ꢀ
(OCH3) ppm; FT-IR (KBr): m = 3,417 (mO–H), 1,624
(mC=N), 1,556 (mC=C), 1,151 (mC–O
(MeOH): kmax = 305, 334, 504 nm.
) ; UV–Vis
cm-1
123

1642
M. Asadi et al.
6.3–7.7 (7H, Ar–H), 8.8 (1H, H13), 9.1 (1H, H7), 0.7–0.8
(9H, CH3), 1.2–1.3 (18H, CH2) ppm; 13C NMR (DMSO-
d6): d = 163.6 (C7=N), 163.2 (C13=N), 148.2 (C12), 123.1
(C11), 130.3 (C10), 142.1 (C9), 168.4 (C8), 158.4, 155.4,
152.3, 133.5, 131.2, 120.8, 118.7, 117.9, 117.5, 116.3,
114.1, 110.3 (12 signals for Ar–C), 55.7 (OCH3), 29.1,
23.8, 23.3, 13.8 (PBu3) ppm; FT-IR (KBr): mꢀ = 3,421
(mO–H), 1,608 (mC=N), 1,562 (mC=C), 1,122 (mC–O), 1,095
(mClO4), 524 (mCo–N), 462 (mCo–O) cm-1; UV–Vis (MeOH):
kmax = 307, 381, 464 nm.
[(N-Salicyliden-N0-3-methoxysalycyliden)-2,3-diamino-
pyridine](tributylphosphine)cobalt(III) perchlorate mono-
hydrate ([3-MeO-Co(Salpyr)(PBu3)]ClO4ꢀH2O)
Fig. 4 Proposed structure of the unsymmetrical Co(III) Schiff base
complexes
(3d, C32H44ClCoN3O8P)
Light brown crystals; yield 60 %; m.p.:[250 °C; 1H NMR
(DMSO-d6): d = 6.6–8.7 (3H, pyridine), 6.6–8.7 (7H, Ar–
H), 9.0 (1H, H13), 9.3 (1H, H7), 0.7–0.8 (9H, CH3), 1.2–1.3
(18H, CH2) ppm; 13C NMR (DMSO-d6): d = 163.8
(C7=N), 163.5 (C13=N), 147.6 (C12), 122.8 (C11), 130.4
(C10), 142.8 (C9), 167.8 (C8), 159.2, 158.8, 157.7, 133.6,
131.5, 130.8, 121.9, 118.1, 115.9, 112.1, 108.8, 106.8 (12
signals for Ar–C), 55.7 (OCH3), 29.1, 23.8, 23.3, 13.8
[(N-Salicyliden-N0-5-bromosalycyliden)-2,3-diaminopyri
dine](tributylphosphine)cobalt(III) perchlorate monohy
drate ([5-Br-Co(Salpyr)(PBu3)]ClO4ꢀH2O)
(3a, C31H41BrClCoN3O7P)
Brown crystals; yield 65 %; m.p.: [250 °C; 1H NMR
(DMSO-d6): d = 6.7–7.8 (2H, pyridine), 8.2 (1H, H12), 6.7–
7.8 (7H, Ar–H), 9.1 (1H, H13), 9.5 (1H, H7), 0.7–0.8 (9H,
CH3), 1.2–1.3 (18H, CH2) ppm; 13C NMR (DMSO-d6):
d = 163.9 (C7=N), 163.4 (C13=N), 148.8 (C12), 124.1 (C11),
131.4 (C10), 142.5 (C9), 168.5(C8), 160.2, 158.3, 133.1,
132.6, 130.3, 126.3, 122.0, 120.8, 118.9, 118.1, 117.1, 114.7
(12 signals for Ar–C), 29.1, 23.8, 23.3, 13.8 (PBu3) ppm; FT-
ꢀ
(PBu3) ppm; FT-IR (KBr): m = 3,404 (mO–H), 1,606 (mC=N),
1,564 (mC=C), 1,193 (mC–O), 1,093 (mClO4), 549 (mCo–N), 462
(mCo–O) cm-1; UV–Vis (MeOH): kmax = 342, 378,
491 nm.
ꢀ
IR (KBr): m = 3,444 (mO–H), 1,604 (mC=N), 1,510 (mC=C),
[N,N0-bis(Salicyliden)-2,3-diaminopyridine]-(diphenyl-
methylphosphine)cobalt(III) perchlorate monohydrate
([Co(2,3-Salpyr)(PPh2Me)]ClO4ꢀH2O)
1,170 (mC–O), 1,097 (mClO4), 563 (mCo–N), 462 (mCo–O) cm-1
UV–Vis (MeOH): kmax = 324, 400, 489 nm.
;
[(N-Salicyliden-N0-5-methoxysalycyliden)-2,3-diaminopyr-
idine](tributylphosphine)cobalt(III) perchlorate mono-
hydrate ([5-MeO-Co(Salpyr)(PBu3)]ClO4ꢀH2O)
(3b, C32H44ClCoN3O8P)
(3e, C32H28ClCoN3O7P)
Brown crystals; yield 60 %; m.p.: [250 °C; 1H NMR
(DMSO-d6): d = 6.6–7.4 (2H, pyridine), 8.6 (1H, H12),
6.6–7.4 (6H, Ar–H), 8.5 (1H, H6), 8.7 (1H, H13), 8.9 (1H,
H7), 1.4 (CH3) ppm; 13C NMR (DMSO-d6): d = 163.6
(C7=N), 163.3 (C13=N), 148.1 (C12), 123.6 (C11), 131.4
(C10), 142.8 (C9), 167.8 (C8), 158.2, 157.6, 133.0, 132.9,
131.2, 130.9, 122.5, 122.2, 122.1, 121.8, 116.9, 116.2 (12
signals for Ar–C), 142.8, 137.6, 130.1, 127.1, 12.1
Brown crystals; yield 62 %; m.p.: [ 250 °C; 1H NMR
(DMSO-d6): d = 6.7–7.6 (2H, pyridine), 8.7 (1H, H12),
6.7–7.6 (6H, Ar–H), 8.5 (1H, H ), 9.0 (1H, H13), 9.3 (1H,
6
H7), 0.7–0.8 (9H, CH3), 1.2–1.3 (18H, CH2) ppm; 13C
NMR (DMSO-d6): d = 163.5 (C7=N), 163.1 (C13=N),
148.4 (C12), 123.9 (C11), 131.3 (C10), 142.3 (C9), 168.7
(C8), 159.6, 158.2, 157.3, 133.1, 132.9, 130.5, 121.2, 118.0,
115.4, 107.8, 107.0, 100.5 (12 signals for Ar–C), 55.8
(OCH3), 29.1, 23.8, 23.3, 13.8 (PBu3) ppm; FT-IR (KBr):
ꢀ
(PPh2Me) ppm; FT-IR (KBr): m = 3,398 (mO–H), 1,608
(mC=N), 1,562 (mC=C), 1,149 (mC–O), 1,093 (mClO4), 543
(mCo–N), 469 (mCo–O
)
cm-1
;
UV–Vis (MeOH):
kmax = 330(sh), 375, 479 nm.
ꢀ
m = 3,402 (mO–H), 1,606 (mC=N), 1,525 (mC=C), 1,153
[N,N0-bis(Salicyliden)-2,3-diaminopyridine]bis(tributyl-
phosphine)cobalt(III) perchlorate monohydrate ([Co(2,3-
Salpyr)(PBu3)2]ClO4ꢀH2O) (4, C44H73ClCoN3O7P2)
(mC–O), 1,093 (mClO4), 540 (mCo–N), 466 (mCo–O) cm-1
UV–Vis (MeOH): kmax = 341, 381, 503 nm.
;
[(N-Salicyliden-N0-4-methoxysalycyliden)-2,3-diamino-
pyridine](tributylphosphine)cobalt(III) perchlorate mono-
hydrate ([4-MeO-Co(Salpyr)(PBu3)]ClO4ꢀH2O)
(3c, C32H44ClCoN3O8P)
To a solution 0.16 g of the ligand 2,3-Salpyr (0.5 mmol)
and 0.4 cm3 tributylphosphine (1.6 mmol) in 6 cm3 meth-
anol was added a solution of 0.136 g of Co(OAc)2ꢀ4H2O
(0.6 mmol) in 1.5 cm3 methanol. The reaction mixture was
refluxed for 2 h to give a brown-red solution. A solution of
0.42 g NaClO4ꢀH2O (3 mmol) in 7 cm3 methanol was then
Brown crystals; yield 64 %; m.p.: [250 °C; 1H NMR
(DMSO-d6): d = 6.3–7.7 (2H, pyridine), 8.4 (1H, H12),
123

Synthesis, characterization, and thermodynamics
1643
12. Albert A (1963) Ionization Constants. In: Katritsky AR (ed)
Physical methods in heterocyclic chemistry, vol 1. Academic,
New York
13. Tumer M, Koksal MH, Sener MK, Serin S (1999) Trans Metal
Chem 24:414
14. Lever ABP (1984) Inorganic electronic spectroscopy, 2nd edn.
Elsevier, New York
15. Tumer M, Koksal H, Senerk MK, Serin S (1996) Synth React
Inorg Metal Org Chem 26:1589
added and the mixture was stirred for 2 h. The resulting
brown precipitate was collected by filtration and recrystal-
lized from dichloromethane/methanol. The product was
dried in vacuo. Brown crystals; yield 61 %; m.p.:[250 °C;
1H NMR (DMSO-d6): d = 6.4–7.4 (2H, pyridine), 7.5 (1H,
H12), 6.4–7.4 (7H, Ar–H), 8.7 (1H, H13), 9.0 (1H, H7), 0.7–
0.8 (18H, CH3), 1.2–1.3 (36H, CH2) ppm; FT-IR (KBr):
ꢀ
m = 3,342 (mO–H), 1,602 (mC=N), 1,548 (mC=C), 1,149
16. West DX, Nassar AA (1999) Trans Metal Chem 24:617
17. Al-Shihri AS (2004) Spectrochim Acta A 60:1189
18. Garg BS, Kumar DN (2003) Spectrochim Acta A 59:229
19. Nakamoto K (1978) Infrared and Raman spectra of inorganic and
coordination compounds. Wiley, New York
(mC–O), 1,093 (mClO4), 536 (mCo–N), 472 (mCo–O) cm-1
UV–Vis (MeOH): kmax = 306, 437 nm.
;
Acknowledgments We are grateful to Shiraz University Research
Council for their financial support.
20. Djebbar-Sid S, Baitich OB, Deloume JP (2001) J Mol Struct
569:121
21. Gehad G, El-Wahab MZ (2005) Spectrochim Acta A 61:1059
22. Ghose BN, Lasisi KM (1986) Synth React Inorg Metal-Org Chem
16:1121
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