Synthesis, characterization, and thermodynamics of some new unsymmetrical Schiff bases of salicylaldehyde with 3,4-diaminopyridine and their cobalt(III) complexes

Article abstract of DOI:10.1016/j.saa.2013.09.098

Some new Schiff bases derived from 3,4-diaminopyridine (3,4-DAP) and their new unsymmetrical Co(III) five coordinate complexes described as [Co(Chel)(L)]ClO₄×H₂O where (Chel) is the deprotonated form of a series of unsymmetric ligands containing 3,4-diaminopyridine (3,4-DAP) and substituted salicylaldehyde moieties and a new Co(III) six coordinate Co(III) complex, were synthesized and characterized by ¹H NMR, IR, UV-Vis, and elemental analysis. For the new synthesized five coordinate complexes, the formation constants of the interaction of the Co(III) Schiff bases with various donors were measured spectrophotometrically. The trend of the formation constants of the five coordinate Co(III) Schiff base complexes toward a given phosphine is as follow: 5-H > 5-Br and the formation constants trend of these donors are as follow: PBu₃ > PPh ₂Me. Furthermore the adduct formation of the five coordinate [Co(3,4-Salpyr)(PBu₃)] ClO₄×H₂O, with aromatic amines shows the following binding trend: Im > 2-MeIm > 2-EtIm > BzIm. The trend of the formation constants of Co(III) Schiff base complexes toward a given donor according to the phosphine axial ligand is as follow: PBu₃ > PPh₂Me.

Full text of DOI:10.1016/j.saa.2013.09.098

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 122 (2014) 676–681
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Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy
journal homepage: www.elsevier.com/locate/saa
Synthesis, characterization, and thermodynamics of some new
unsymmetrical Schiff bases of salicylaldehyde with 3,4-diaminopyridine
and their cobalt(III) complexes
Mozaffar Asadi a, , Susan Torabi a,b, Khosro Mohammadi c
⇑
a Chemistry Department, College of Sciences, Shiraz University, Shiraz 71454, Islamic Republic of Iran
b International Branch, Shiraz University of Medical Sciences, Shiraz, Islamic Republic of Iran
c Chemistry Department, Faculty of Sciences, Persian Gulf University, Bushehr 75169, Islamic Republic of Iran
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
ꢀ
Synthesis and characterization of
unsymmetrical cobalt(III) Schiff base
complexes.
ꢀ
ꢀ
Thermodynamic studies of their
interactions with various donors.
Investigation the effects of different
electronic and steric characters of the
ligand’s substituent.
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 24 July 2013
Received in revised form 8 September 2013
Accepted 26 September 2013
Available online 9 October 2013
Some new Schiff bases derived from 3,4-diaminopyridine (3,4-DAP) and their new unsymmetrical Co(III)
five coordinate complexes described as [Co(Chel)(L)]ClO4ꢁH2O where (Chel) is the deprotonated form of a
series of unsymmetric ligands containing 3,4-diaminopyridine (3,4-DAP) and substituted salicylaldehyde
moieties and a new Co(III) six coordinate Co(III) complex, were synthesized and characterized by 1H NMR,
IR, UV–Vis, and elemental analysis. For the new synthesized five coordinate complexes, the formation
constants of the interaction of the Co(III) Schiff bases with various donors were measured spectrophoto-
metrically. The trend of the formation constants of the five coordinate Co(III) Schiff base complexes
toward a given phosphine is as follow: 5-H > 5-Br and the formation constants trend of these donors
are as follow: PBu3 > PPh2Me. Furthermore the adduct formation of the five coordinate [Co(3,4-Sal-
pyr)(PBu3)] ClO4ꢁH2O, with aromatic amines shows the following binding trend: Im > 2-MeIm > 2-
EtIm > BzIm. The trend of the formation constants of Co(III) Schiff base complexes toward a given donor
according to the phosphine axial ligand is as follow: PBu3 > PPh2Me.
Keywords:
Cobalt(III)
Unsymmetrical Schiff base
Thermodynamic free energy change
Formation constants
Ó 2013 Elsevier B.V. All rights reserved.
Introduction
organic ligands have been studied for a long time. The Schiff bases
involving a pyridine ring have received considerable attention in
Metal Schiff base complexes have contributed widely to the
inorganic chemistry of chelate systems. Metal complexes of these
literature [1,2]. The anti-inflammatory activity of 2-salicylidenea-
minopyridine and the corresponding complex with Co(II) has been
reported [3].
Schiff bases derived from 3,4-diaminopyridine (3,4-DAP) have
received much less attention [4,5], although many clinical studies
⇑
Corresponding author. Tel.: +98 2284822; fax: +98 (711)2286008.
E-mail addresses: mozaffarasadi@yahoo.com, asadi@susc.ac.ir (M. Asadi).
1386-1425/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.saa.2013.09.098

M. Asadi et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 122 (2014) 676–681
677
of DAP suggest that this amine, known as a K+ channel blocker
[6–8], effectively relieves the symptoms of various neurological
disorders such as the Lambert–Eaton myasthenia syndrome
(LEMS) [9–11]. In order to better understand the properties of iso-
mers of diaminopyridines, we studied the thermodynamics prop-
erty of their cobalt(III) complexes. In particular, we are interested
in comparing some thermodynamic features of new synthesized
Schiff bases derived from 3,4-DAP with our previously investigated
Schiff bases obtained from 2,3-DAP [12].
Recently we reported the thermodynamic properties of cobalt
Schiff base complexes with (N,N0-bis(Salicyliden)-2,3-diaminopyr-
idine) as ligand [12]. Sustained interest in the characteristics of the
Schiff bases derived from diaminopyridines has promoted us to
prepare condensation products of 3,4-DAP and various salicylalde-
hyde (1, 2, Fig. 1S). This report deals with their detailed character-
istic, including characterization and thermodynamic properties of
some new unsymmetrical cobalt(III) complexes derived from
3,4-DAP. Comparison of their properties, spectrally, and thermody-
namically aimed to investigate the effects of different electronic
and steric situations.
was 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
were purified by re-crystallization in dichloromethane/ethanol.
The complex was dried in vacuum at T = 323 K for 48 h (Fig. 2S).
Synthesis of the six coordinate Co(III) complex, [Co(3,4-
Salpyr)(PBu3)2]ClO4ꢁH2O (7)
To a methanolic solution (6 ml) of the ligand 3,4-Salpyr (0.16 g,
0.5 mmol) and tributylphosphine (0.4 ml, 1.6 mmol) was added a
methanolic solution (1.5 ml) of Co(OAc)2ꢁ4H2O (0.136 g, 0.6 mmol).
The reaction mixture was refluxed for 2 h to give a brownish red
solution. A methanolic solution (7 ml) of NaClO4ꢁH2O (0.42 g,
3 mmol) was then added and the mixture was stirred for 2 h. The
resulting brown precipitate was collected by filtration and recrys-
tallized by dichloromethane/methanol. The product (7) was dried
under vacuum.
Thermodynamic studies
The formation constants have been determined by UV–Vis
absorption spectroscopy from the reaction of the acceptors with
the donors in 96% methanol solvent, according to the following
equations:
Experimental
Materials
½Coð3; 4-SalpyrÞðLÞꢃþ þ Y ¢ ½Coð3; 4-SalpyrÞðLÞYꢃþ
ð1Þ
All chemicals were used as obtained from Merck, Fluka or Al-
drich. Anal. Grade solvent from Merck was used without further
purification. The diamine and salicylaldehyde were distilled before
use.
where L = PBu3, PPh2Me and Y = n-butylamine, PPh2Me and PBu3
½Coð5-Br-3; 4-SalpyrÞðPBu3Þꢃþ þ Y ¢ ½Coð5-Br-3; 4-SalpyrÞðPBu3YÞꢃþ
ð2Þ
Apparatus and techniques
where Y = PPh2Me and PBu3
½Coð3; 4-SalpyrÞðPBu3Þꢃþ þ Y ¢ ½Coð3; 4-SalpyrÞðLÞYꢃþ
ð3Þ
The infrared spectra of all ligands and their complexes were re-
corded in the range 4000–400 cmꢂ1 using a Shimadzu FTIR-8300
spectrophotometer applying the KBr disc technique. The UV–Vis
absorption spectra were recorded using Perkin–Elmer Lambda 2
spectrophotometer at room temperature. The Elemental analysis
was carried out by Thermo Finnigan-Flash-1200. The 1H NMR spec-
tra were recorded by a Bruker Avance DPX 250 MHz spectrometer.
where Y = Im, 2-MeIm, 2-EtIm and BzIm.
A solution from each complex with concentration at about
3 ꢄ 10ꢂ5 M and constant ionic strength (I = 0.1 M) by sodium per-
chlorate was prepared. In a typical titration 2.5 ml of this solution
was transferred into the thermostated cell compartment of the
UV–Vis instrument, which was kept at constant temperature
( 0.1 K) by circulating water and was titrated by the given donor.
The titration was done by adding aliquots of the donor with a
Hamilton microlitre syringe. The donor’s concentration was varied
in 1–3 folds in excess.
Preparation of the Schiff bases
A mono Schiff base (1) was prepared by mixing stoichiometric
amounts of analytically pure salicylaldehyde with 3,4-diaminopyr-
idine dissolved in ethanol and refluxing for three hours on a water
bath. The separated brownish yellow crystals were recrystallized
from hot ethanol [13]. Such compound as (1), commonly referred
to as half-unit, is a potential precursor for the synthesis of non-
symmetrical Schiff bases comprising 3,4-DAP moieties. By mixing
the half unit with appropriate aldehyde in the 1:1 ratio, using
the same condensation procedure the bis-Schiff bases (2), (3) were
obtained. After evaporation of the resulting solution to the half vol-
ume the separated crystals were filtered off and recrystallized from
ethanol.
The absorption measurements were carried out at various
wavelengths where the difference in absorption was the maximum
0.8
0.6
0.4
0.2
0
Synthesis of cobalt complexes
Synthesis of the five coordinate Co(III) complexes
The cobalt(III) Schiff base complexes were prepared by the
methods described in the literature [14], to a refluxing solution
of tetradentate ligands (1 mmol), in methanol (10 cm3) were added
Co(OAc)2ꢁ4H2O (0.25 g, 1 mmol). After 15 min, tri-n-butylphos-
phine (0.2 cm3, 0.8 mmol) was added to the solution. The reaction
mixture was refluxed for an hour. The Co(II) formed complex was
oxidized by blowing air into the solution for 2 h, and the solution
320
420
520
620
Wavelength / nm
Fig. 1. The variation of the electronic spectra of [Co(3,4-Salpyr)(PBu3)]ClO4ꢁH2O
titrated by PBu3 at T = 293 K in 96% methanol.

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M. Asadi et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 122 (2014) 676–681
after the equilibrium was assessed. The formed adduct shows an
absorption different from the acceptor, while the donors show no
absorption at those wavelengths. As an example the variation of
the electronic spectra for [Co(3,4-Salpyr)(PBu3)]ClO4ꢁH2O, titrated
with PBu3 at T = 293 K in 96% methanol is shown in Fig. 1. The isos-
bestic points for this system show that there is only one reaction in
equilibrium. The same is valid for other systems.
of the oxygen to the metal ion [20,21]. A new band in the
400–460 cmꢂ1 range of the complexes is assignable to mMAO [22].
The other series of weak bands between 3100 and 2800 cmꢂ1 are
related to (CAH) modes of vibrations [23]. At ꢅ1093 cmꢂ1 very
strong bands are ascribed to the stretching vibration of ClOꢂ4 .
All the IR data suggest that the metal is bonded to the Schiff
base through the phenolic oxygen and the imino nitrogen.
Results and discussion
Electronic spectra
It is interesting to note that 3,4-DAP is converted to Schiff bases
with difficulty in contrast to 2,3-DAP. This is supported by the fact
that 3,4-DAP, unlike 2,3-DAP, is unreactive to 3-(40-nitrophenyl)-3-
oxo-propanal [15]; several attempts to obtain the Schiff bases,
using different experimental procedures, failed.
In the asymmetric unit of compound 1, there are two identical
molecules (labeled 1A and 1B). The single X-ray diffraction analysis
shows that the condensation reaction takes place in the amino
group at 3-position of the pyridine ring (isomer 1A) [16].
By mixing the half unite (1) with appropriate aldehyde in the
1:1 ratio, the bis Schiff bases were obtained. The complexes were
prepared according to the following equations:
The electronic spectra of the Schiff bases and their complexes
are summarized in Table 3. The spectra of the ligands exhibit bands
at about 330 nm and a broad shoulder at 460 nm. The first peak is
attributed to
p ?
pꢆ transitions. This band was not significantly af-
fected by chelation. The second band in the spectra of the ligands is
assigned to n ? pꢆ transition. This band is disappeared via com-
plexation and a new band attribured to the donation of the lone
pairs of the nitrogen atoms of the Schiff base to the metal ion
(N ? M) appear [24]. Camparing this with n ? pꢆ transition, this
band was shifted to a longer wavelength along with increasing in
its intensity.
All the complexes show a band in the range of 440–500 nm,
which is attributed to d ? pꢆ transition that is mixed with d ? d
transition. 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 ob-
O
Methanol
H LþCoðOAcÞ ꢁ4H O
½CoðLÞðPX Þꢃþ NaClO4ꢁH2 ½CoðLÞðPXÞꢃClO ꢁH O
ꢀꢀꢀꢀꢀ!
ꢀꢀꢀꢀꢀ!
2
2
3
4
2
2
PX
Air blowing
where H2L = 3,4-SalpyrH2, 5-Br-3,4-SalpyrH2 and X = Bu3, PPh2Me.
The resulting compounds are non-hygroscopic and air stable
solids. Some physical properties, such as, decomposition tempera-
tures, colors and percentage of yields as well as the analytical data
of these compounds are given in Table 1. The compounds so ob-
tained were characterized by various physico-chemical techniques,
viz. 1H NMR, IR, UV–Vis spectroscopy and elemental analysis.
scured by intense charge transfer and intraligand (
tions [25].
p ?
pꢆ) transi-
1H NMR spectra
Infrared spectral studies
The 1H NMR spectra of all the Schiff bases (data shown in Table 4)
provide compelling evidence of the presence of either one or two
azomethine groups. By comparing the 1H NMR spectra of the Schiff
bases and their complexes, it is noted that there is a down- or up-
field shift in the frequency of azomethine protons confirming coor-
dination of the metal ion to these groups. In complexes, the absence
of hydroxyl protons in ꢅ12 ppm provides evidence for coordination
through two oxygen atoms (after deprotonation of the ortho hydro-
xyl groups). The 1H NMR spectra of the complexes showed a differ-
entiation of almost all hydrogen atoms, since they are in different
chemical environments due to the rigid structure in which the li-
gand lies when it is coordinated to the metal center [26]. Three sets
of signals were observed for the pyridyl groups of the Schiff bases
which coordinated to the metal ion (9.1–6.5 ppm). The formation
of rigid structure also supports the clearer visualization of aromatic
protons. These protons were observed in the range of ꢅ6.4–8.2 ppm
and CH3 proton was seen at 1.5 ppm. In the spectra of complexes,
two sets of resonances were observed for methine proton, which
The IR spectra provide valuable information regarding the nat-
ure of functional groups attached to the metal atom. The ligands
and the metal complexes were characterized mainly using the azo-
methine band. The main infrared bands and their assignments are
listed in Table 2. The IR spectra of the ligands exhibit broad med-
ium intensity bands in the range 3402–3456 cmꢂ1 which are as-
signed to the intra molecular hydrogen bonding vibration
(OAHꢁ ꢁ ꢁN) (see Table 2). These bands were disappeared via com-
plexation to the metal ions [17]. The band appearing at
ꢅ1616 cmꢂ1 due to azomethine was shifted to a lower frequency
by ꢅ1–15 cmꢂ1 in the complexes, indicating participation of azo-
methine nitrogen in the interaction with the metal ion [18]. Coor-
dination of azomethine nitrogen is affirmed with the presence of
new bands at 450–480 cmꢂ1 region assignable to mMAN for these
complexes [19]. The mCAO stretching frequency shifts in the com-
plexes towards lower or higher values as a result of coordination
Table 1
Analytical and physical data of the ligands and their Co(III) complexes.
Compounds
Color
Yields (%)
m.p. (°C)
Anal. found (Calcd)%
C
H
N
Half unit 0.25 (H2O)
3,4-SalpyrH2
Yellow
Oranges
yellow
Yellow
Brown
Brown
Brown
Brown
62
52
140
145
65.82(66.19)
69.70(69.93)
4.99(5.32)
4.99(4.94)
19.02(19.30)
12.50(12.88)
(3,4-5-Br-SalpyrH2)ꢁ0.5(H2O)
50
51
64
67
61
120
>250
>250
>250
>250
56.03(56.31)
53.79(53.65)
47.79(48.17)
55.23(55.55)
60.45(60.63)
4.08(3.73)
6.30(6.10)
5.30(5.35)
3.8(4.08)
9.95(10.37)
6.30(6.05)
5.33(5.44)
6.10(6.07)
5.00(4.93)
[Co(3,4-Salpyr) (PBu3)]ClO4ꢁ(H2O)
[Co(5-Br-3,4-Salpyr)(PBu3)]ClO4ꢁ(H2O)
[Co(3,4-Salpyr) (PPh2Me)]ClO4ꢁ(H2O)
[Co(3,4-Salpyr)(PBu3)2]ClO4ꢁ(H2O)
8.15(7.99)

M. Asadi et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 122 (2014) 676–681
679
Table 2
IR spectral data of the Schiff bases and their Co(III) complexes (cmꢂ1).
Compounds
1
t(NH2)
t(OH)
t(C@N)
t(C@C)
t(CAO)
t(MAN)
t(MAO)
tClO4
3463
3290
2750
1616
1589
1276
2
3
4
5
6
7
3456
3427
3427
3444
3436
3342
1610
1612
1608
1604
1606
1602
1554
1558
1573
1573
1573
1548
1274
1272
1188
1176
1190
1149
623
536
501
536
405
464
457
472
1093
1093
1095
1093
Thermodynamic properties of the cobalt(III) Schiff base complexes
with donors
Table 3
UV–Vis data of the compounds in methanol.
Aromatic amines as donors. In our earlier work of this series, the
base strength of the cyclic amines on the adduct formation con-
stants of some symmetrical pentacoordinated Co(III) Schiff base
complexes were studied [12]. In the present article, the effect of
these amines have been examined on the formation constants of
the unsymmetrical Co(III) Schiff base complexes derived from
3,4-DAP according to Eq. (3) (Table 5).
As one can see, the formation constants for these bases with
[Co(3,4-Salpyr)(PBu3)]+ 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 [12,30] (see Table 5).
Compounds
k max (nm)
2
3
4
5
6
7
334, 465(sh)a
330, 460(sh)a
308, 379, 490
319, 381, 503
307, 380, 487
306, 385, 500
a
sh = shoulder.
shows they are in different chemical environments. The signals for
these protons were observed in the ꢅ8.2–9.6 ppm range [26]. The
protons chemical shifts for the coordinated PBu3 in the Schiff base
complexes appear at d = 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 [27,28].
Phosphines as donors. The adduct formation of some pentacoordi-
nated cobalt(III) Schiff base complexes were studied according to
Eqs. (1) and (2) with two phosphines as donors. The steric and
the electronic effects of the entering ligands toward the Schiff base
complexes and the entering ligands were studied. These data are
collected in Table 6.
The study of the donor ligand, axial and equatorial ligand on the
formation constants of tributyl phosphinecobalt(III) Schiff base
complexes derived from 3,4-DAP
The electronic effects of phosphines have been expressed by rꢆ
values of Taft (Taft constants). rꢆ value for PBu3 is ꢂ0.390 and for
PPh2Me was not reported in literature (but for PPh2Et is +1.10 and
for PPhMe2 is +0.60) [12,31,32]. Also, the phenyl withdrawing
groups decrease the basic properties of PPh2Me with respect to
PBu3. The formation constants increased according to the following
trend:PBu3 > PPh2Me.
The phosphine ligands have an important steric factor that it
can decrease the trend of five-coordinated complex toward the do-
nor bases. This factor is shown by Tolman’s cone angle. The cone
angle for PPh2Me and PBu3 are 136° and 132°, respectively [33].
The steric effect increased with increase in the cone angle. So by
The formation constants of the various cobalt(III) Schiff-base
complexes titrated with various donors were calculated by using,
Ketelaar’s equation [12,27,29]. The linear plots of P against C,
ꢁ
ꢂ.ꢁ
ꢂ
where
P
is defined as P ¼ CꢇACꢇD
A ꢂ AꢇA ꢂ AꢇD and
C as
ꢁ
ꢂ
C ¼ CꢇA þ CDꢇ for [Co(3,4-Salpyr)(PBu3)]ClO4ꢁH2O 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 are obtained for other systems. The adduct forma-
tion constants are collected in Tables 5–8.
Table 4
1H NMR spectral data of the Schiff bases and the complexes in DMSO d6 (d, ppm).
Compounds
1
HAC@N
Pyridine-H
ArAH
OH
PBu3
NH2
ACH3
8.8(1H, H7)
7.0(1H, H5)
7.6(1H, H6)
7.8(1H, H2)
6.6–7.4(4H)
12.1(H1)
5.8(2H)
2
3
8.1(1H, H14
8.6(1H, H7)
8.9(1H, H14
9.0(1H, H7)
)
)
6.6–7.6(2H)
6.6–7.6(8H)
6.9–7.9(7H)
12.5(1H, H2)
12.6(1H, H1)
12.1(1H, H2)
12.5(1H, H1)
2
8.1(1H, H
)
5
6.9–7.9(1H, H
)
6
8.5(1H, H
)
8.6(1H, H2)
4
9.1(1H, H14
9.6(1H, H7)
)
8.3(1H, H5)
8.6(1H, H6)
9.1(1H, H2)
6.7–7.6(8H)
0.7–0.8(9H, CH3)
1.2–1.3(18H, CH2)
5
6
9.1(1H, H14
9.5(1H, H7)
)
)
7.6–8.6(2H)
9.1(1H, H2)
6.7–8.2(7H)
0.7–0.8(9H, CH3)
1.2–1.3(18H, CH2)
8.5(1H, H14
8.8(1H, H7)
6.6–7.4(1H, H5)
8.0(1H, H6)
6.6–7.4(18H)
1.4
8.5(1H, H2)
7
8.7(1H, H14
9.0(1H, H7)
)
6.4–7.4 (2H)
7.5(1H, H2)
6.4–7.4(8H)
0.6–0.8(18H, CH3)
1.2–1.3(36H, CH2)

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M. Asadi et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 122 (2014) 676–681
Table 7
increasing the cone angle in PPh2Me, the formation constant is de-
creased. The formation constants increased according to the fol-
lowing trend: PBu3 > PPh2Me So, the steric and the electronic
properties affect the formation constant of the complexes.
The formation constants (10ꢂ3)/K/(dm3 mol)ꢂ1, for the interaction between Co(III)
Schiff base complexes with n-butyl amine in 96% methanol.
Compounds
(10ꢂ3)/K/(dm3 mol)ꢂ1
[Co(3,4-Salpyr)(PBu3)]+
2.9 0.2
2.0 0.2
[Co(3,4-Salpyr)(PPh2Me)]+
aT = 293 K.
-0.1
283
293
-0.2
303
The electronic effect of the equatorial Schiff base ligands
313
The equatorial ligands play important role in the stability and
the reactivity of their complexes. Herein, the role of 2 equatorial li-
gands was shown on the stability of the five-coordinated com-
plexes. In the cases of the complexes studied here, the formation
constants for [Co(3,4-Salpyr)(PBu3)]+ in methanol are larger than
the other complexes with a given donor (Table 6).
-0.3
-0.4
-0.5
It is observed that in 3,4-Salpyr the stability of the solvated five-
coordinated complex has decreased and the formation constants of
the six-coordinated adduct has been increased. It seems that in the
five-coordinated complexes, a water or a solvent molecule occu-
pies the sixth position (Eq. (4)) which would decrease the tendency
of the complexes toward the interaction with the donors.
0
0.4
0.8
1.2
1.6
2
103C
Fig. 2. Typical plots of P against C for [Co(3,4-Salpyr)(PBu3)]ClO4ꢁH2O titrated with
PBu3 at various temperatures (T = 283 to 313 K) in 96% methanol, I = 0.1 M.
ꢁ
ꢂ.ꢁ
ꢂ
ꢁ
ꢂ
P ¼ CꢇACꢇD
A ꢂ AꢇA ꢂ AꢇD and C as C ¼ CAꢇ þ CꢇD
.
½CoðChelÞðPBu3Þꢃþ þ S ¢ ½CoðChelÞðPBu3ÞSꢃþ
ð4Þ
where S = H2O or solvent molecule.
Table 5
Concerning the stabilization of the five-coordinate complex, the
donation power of the Schiff base is important. Therefore the co-
balt atom in [Co(5-Br-3,4-Salpyr)(PBu3)]+ complex has more accep-
tor property than the other type of the complexes and forms more
stable complex with H2O or solvent molecule (Eq. (4)). Therefore
its tendency for the reaction with donors decreases, hence its for-
mation constant, K, with donors is lower and it is in agreement
with our previous work [12].
The formation constants, (10ꢂ1)/K/(dm3 mol)ꢂ1, for the interaction between [Co(3,4-
Salpyr)(PBu3)]+ with Im and its derivatives in 96% methanol.
Donors
(10ꢂ1)/K/(dm3 mol)ꢂ1
Im
118.7 3.0
11.2 1.1
8.0 1.0
2-MeIm
2-EtIm
BzIm
4.9 0.8
a
T = 293 K.
The effect of the axial ligands
Concerning the stabilization of the five-coordinate complex, the
donation power of the Schiff base is important. By decreasing the
rꢆ in PPh2Me, the cobalt atom in [Co(3,4-Salpyr)(PPh2Me)]+ com-
plex has more acceptor property than the other type of the com-
plexes and forms more stable complex with H2O or solvent
molecule (Eq. (4)). Therefore its tendency for the reaction with do-
nors decreases and its formation constant, K, with donors is lower
[12]. So, the formation constants increased according to the follow-
ing trend: PBu3 > PPh2Me.
Table 6
The formation constants, 10ꢂ1/K/(dm3 mol)ꢂ1, for the interaction between Co(III)
complexes with PBu3 and PPh2Me in 96% methanol.
283 K
293 K
303 K
313 K
PPh2Me
[Co(5-Br-3,4-Salpyr)(PBu3)]+ 58.3 7.7 42.7 5.8 33.0 1.3 26.8 1.0
[Co(3,4-Salpyr)(PBu3)]+
65.0 6.5 58.1 7.0 35.3 5.3 30.1 4.7
60.7 8.7 44.0 1.0 31.1 2.3 28.4 1.2
[Co(3,4-Salpyr)(PPh2Me)]+
PBu3
Effect of the temperature on the formation constants
The formation constants were carried out at various tempera-
tures. Because of bond formation, by increasing the temperature,
the formation constants were decreased.
[Co(5-Br-3,4-Salpyr)(PBu3)]+ 64.4 1.4 52.0 8.6 33.9 1.4 30.0 5.2
[Co(3,4-Salpyr)(PBu3)]+
73.9 9.3 67.6 1.0 58.9 4.8 49.9 7.1
68.0 5.2 55.6 1.5 34.7 1.0 31.2 1.1
[Co(3,4-Salpyr)(PPh2Me)]+
Table 8
The thermodynamic parameter values
D
G°,
D
H°, and
D
S° values for the interaction between Co(III) Schiff base complexes with various phosphines in 96% methanol.a
G°a/(kJ molꢂ1 H°/(kJ molꢂ1 S°/(kJ molꢂ1
)
Compounds
D
)
D
)
D
PPh2Me
[Co(5-Br-3,4-Salpyr)(PBu3)]+
[Co(3,4-Salpyr)(PBu3)]+
[Co(3,4-Salpyr)(PPh2Me)]+
ꢂ14.6 1.1
ꢂ15.0 5.6
ꢂ14.7 3.8
ꢂ19.1 0.7
ꢂ20.6 3.9
ꢂ19.4 2.7
ꢂ14.7 2.7
ꢂ18.6 13.1
ꢂ15.7 9.1
PBu3
[Co(5-Br-3,4-Salpyr)(PBu3)]+
[Co(3,4-Salpyr)(PBu3)]+
[Co(3,4-Salpyr)(PPh2Me)]+
ꢂ15.0 3.9
ꢂ16.0 1.5
ꢂ15.0 4.8
ꢂ20.0 2.7
ꢂ9.6 1.1
ꢂ20.7 1.1
ꢂ17.0 9.4
21.0 3.7
ꢂ18.8 11.3
a
D
G° values are calculated from
D
G° =
D
H° ꢂ TDS° at T = 293 K.

M. Asadi et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 122 (2014) 676–681
681
Table 9
G° (kJ molꢂ1) values for the interaction between Co(III) Schiff base complexes with various phosphines in 96% methanol.
D
Compounds
283 K
293 K
303 K
313 K
PBu3
[Co(3,4-Salpyr)(PBu3)]+
[Co(3,4-Salpyr)(PPh2Me)]+
[Co(5-Br-3,4-Salpyr)(PBu3)]+
ꢂ15.5 0.2
ꢂ15.3 0.5
ꢂ15.2 0.6
ꢂ15.8 0.9
ꢂ15.4 0.3
ꢂ15.2 0.6
ꢂ16.0 1.0
ꢂ14.7 0.8
ꢂ14.7 0.7
ꢂ16.2 0.2
ꢂ14.9 0.5
ꢂ14.8 0.1
PPh2Me
[Co(3,4-Salpyr)(PBu3)]+
[Co(3,4-Salpyr)(PPh2Me)]+
[Co(5-Br-3,4-Salpyr)(PBu3)]+
ꢂ15.2 0.7
ꢂ15.1 .08
ꢂ15.0 0.5
ꢂ15.5 0.8
ꢂ14.8 0.4
ꢂ14.7 0.6
ꢂ14.8 0.4
ꢂ14.4 0.8
ꢂ14.6 0.6
ꢂ14.8 0.6
ꢂ14.7 0.7
ꢂ14.5 0.5
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D
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We are grateful to Shiraz University Research Council for their
financial support.
Appendix A. Supplementary material
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.saa.2013.09.098.
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