PH-metric and spectroscopic properties of new 4-hydroxysalicylidene-2- aminopyrimidine Schiff-base transition metal complexes

Article abstract of DOI:10.1016/j.molstruc.2010.03.037

The new complexes, cis-[WO₂(Hsap)₂], [Ru(PPh ₃)₂(Hsap)₂], [Pd(Hsap)Cl(H₂O)], [Pd(PPh₃)₂(Hsap)]Cl, [Ag(Hsap)(H₂O) ₂], [Ni(Hsap)(AcO)(H₂O)₂], [Ni(Hsap) ₂] and [Cu(Hsap)Cl(H₂O)] are reported, where H ₂sap is 4-hydroxysalicylidene-2-amino pyrimidine Schiff-base. The complexes were characterized by elemental analyses, spectroscopic (IR, NMR, UV-vis, ESR and mass) and physical techniques (conductivity, magnetic and thermal measurements). The Schiff-base H₂sap behaves as a bidentate chelate with the deprotonated 2-hydroxy and azomethine nitrogen centers with the pendant pyrimidine cyclic nitrogen functionality playing no role in coordination. The dissociation constants of H₂sap and the stability constants of the metal complexes have been determined pH-metrically at various temperatures, and the thermodynamic activation parameters (ΔS, ΔG, ΔH) calculated.

Full text of DOI:10.1016/j.molstruc.2010.03.037

Journal of Molecular Structure 973 (2010) 69–75
Contents lists available at ScienceDirect
Journal of Molecular Structure
journal homepage: www.elsevier.com/locate/molstruc
pH-metric and spectroscopic properties of new 4-hydroxysalicylidene-2
-aminopyrimidine Schiff-base transition metal complexes
a
a
b
a,*
Abd El-Fatah Ouf , Mayada S. Ali , Eman M. Saad , Sahar I. Mostafa
a
Chemistry Department, Faculty of Science, Mansoura University, Mansoura, Egypt
Chemistry Department, Faculty of Science (Suez), Suez Canal University, Egypt
b
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 9 August 2009
Received in revised form 11 March 2010
Accepted 11 March 2010
Available online 16 March 2010
The new complexes, cis-[WO (Hsap) ], [Ru(PPh ) (Hsap) ], [Pd(Hsap)Cl(H O)], [Pd(PPh ) (Hsap)]Cl,
2
2
3
2
2
2
3 2
[Ag(Hsap)(H
2
O)
2
], [Ni(Hsap)(AcO)(H
2
O)
2
], [Ni(Hsap)
2
]
2
and [Cu(Hsap)Cl(H O)] are reported, where
2
H sap is 4-hydroxysalicylidene-2-amino pyrimidine Schiff-base. The complexes were characterized by
elemental analyses, spectroscopic (IR, NMR, UV–vis, ESR and mass) and physical techniques (conductiv-
ity, magnetic and thermal measurements). The Schiff-base H sap behaves as a bidentate chelate with the
deprotonated 2-hydroxy and azomethine nitrogen centers with the pendant pyrimidine cyclic nitrogen
functionality playing no role in coordination. The dissociation constants of H sap and the stability con-
stants of the metal complexes have been determined pH-metrically at various temperatures, and the
2
Keywords:
Hsap
Spectra
2
ꢀ
ꢀ
ꢀ
thermodynamic activation parameters (
D
S ,
D
G ,
D
H ) calculated.
Hydroxy
Ó 2010 Elsevier B.V. All rights reserved.
Pyrimidine
Azomethine
Stability constant
1
. Introduction
metal complexes and the thermodynamic activation parameters
(DS , DG , DH ) which are calculated pH-metrically at various tem-
ꢀ
ꢀ
ꢀ
Schiff-bases are important class of chelating agents in coordina-
peratures, as these studies can give some knowledge in their exam-
ination as anticancer agents.
tion chemistry as they easily form stable complexes with most
transition metal ions [1]. The bioinorganic chemistry field has in-
creased the interest in Schiff-bases complexes, as they serve as
models for important biological species [2]. Such complexes are
also of interest due to their catalytic activity [3–5], removal of toxic
metal ions [6], corrosion inhibitors [7], ability to reversibly bind
oxygen [8] and their photo-chromic properties [9].
The interaction of Schiff-bases derived from salicylaldehyde
moiety and primary amines, especially amino acids with variety
of transition metals have been reported [4,10–13]. Moreover,
pyrimidine rings are present in nucleic acids, several vitamins,
coenzymes and antibiotics [14] and act as valuable substrates in
the synthesis of antitumor agents [15]. We have previously re-
ported the chemotherapeutic potential of Pd(II) and Ag(I) com-
plexes with 4,6-diamino-5-hydroxy-2-mercaptopyrimidine. These
complexes were found to display significant anticancer activity
against Ehrlich ascites tumor cell (EAC) in albino mice [15]. As a con-
tinuation of our interest in anticancer activity of complexes con-
taining pyrimidine rings, here we describe the preparation and
characterization of new 4-hydroxysalicylidene-2-aminopyrimidine
2
. Experimental
All reagents were purchased from Aldrich and all manipulations
were performed under aerobic conditions using materials and sol-
vents as received. [Pd(PPh Cl ] was prepared from K [PdCl ] and
PPh in aqueous-ethanol solution [16] and [Ru(PPh Cl ] was syn-
3
)
2
2
2
4
3
3
)
3
2
thesised as previously reported [17]. DMF and DMSO used in con-
ductivity and electronic spectral measurements were dried over
molecular sieves. DMSO-d
.1. Preparation of H sap
The Schiff-base, H sap, was synthesised by the condensation of
6
(NMR) was referenced using TMS.
2
2
2
ethanolic solutions of 2-aminopyrimidine (0.095 g, 1 mmol) and 4-
hydroxysalicylaldehyde (0.138 g, 1 mmol) in the presence of drops
of glacial acetic acid. The orange product was filtered off during
hot, washed with hot ethanol, diethyl ether and dried in vacuo.
Schiff-base (H
the dissociation constants of H
2
sap) transition elements complexes. Also, we report
sap, the stability constants of the
2
2
.2. Preparation of complexes
2
.2.1. cis-[WO
A solution of Na
added to H sap (0.107 g, 0.5 mmol) in ethanol (15 cm ), The mix-
2
(Hsap)
2
]ꢁH
2
O
3
2
[WO ] (0.15 g, 0.5 mmol) in water (5 cm ) was
4
*
Corresponding author. Tel.: +20 108502625; fax: +20 50 2246781.
E-mail address: sihmostafa@yahoo.com (S.I. Mostafa).
3
2
0
022-2860/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.molstruc.2010.03.037

7
0
A.-E.F. Ouf et al. / Journal of Molecular Structure 973 (2010) 69–75
ꢂ4
ꢂ1
ture was heated and stirred for 3 h. The resulting pale orange pre-
Solution (B): 2.5 ml of Hsap (1 ꢃ 10 mol L ) + 2.5 ml etha-
ꢂ1
cipitate was filtered off, washed with ethanol, diethyl ether and
nol + 1 ml KNO
3
(1 mol L ) + 0.5 ml of metal ion, Co(II), Ni(II),
ꢂ3
ꢂ5
ꢂ1
dried in vacuo. Conductivity data (10
M
in DMSO):
Cu(II), Zn(II), Pd(II), Ag(I), (5 ꢃ 10 mol L ).
ꢂ1
2
ꢂ1
K
M
= 6.0
X
cm mol
.
These solutions were completed to 25 ml with bidistilled water.
The titrations were carried out for a ratio of metal to ligand 1:10.
Microanalyses were determined by the Micro Analytical Unit of
Cairo University. Magnetic moments at 25 °C were recorded using
2
.2.2. [Pd(Hsap)Cl(H
2
O)]ꢁH
2
O
3
2 4
K [PdCl ] (0.163 g, 0.5 mmol) in water (5 cm ) was added to
3
H
2
sap (0.107 g, 0.5 mmol) in ethanol (15 cm ). The mixture was
a
Johnson Matthey magnetic susceptibility balance with
stirred for 12 h until a pale brown precipitate was isolated; this
Hg[Co(SCN) ] as calibrant. IR spectra were measured as KBr discs
4
was washed with water, ethanol and dried in vacuo. Conductivity
on a Matson 5000 FT-IR spectrometer. Electronic spectra were re-
corded using a Unicam UV2–100 UV–vis spectrometer. NMR spectra
were measured on a Varian Gemini WM-200 and Varian Mercury
ꢂ3
ꢂ1
2
ꢂ1
data (10 M in DMSO):
K
M
= 8.0
X
cm mol .
2
.2.3. [Pd(PPh
3
)
2
(Hsap)]Clꢁ3H
2
O
400 MHz spectrometers. Thermal analysis measurements were
ꢂ1
2
H sap (0.107 g, 0.5 mmol) was dissolved in methanolic solution
made in the 20–800 °C range at the heating rate of 10 °C min
using -Al
,
3
of KOH (0.028 g, 0.5 mmol; 15 cm ). [Pd(PPh
3 2
)Cl ]
(0.219 g,
a
2
O
3
as a reference, on a Shimadzu Thermogravimetric
0
.5 mmol) was added and the mixture was warmed with stirring
Analyzer TGA-50. Conductometric measurements were carried
out at room temperature on a YSI Model 32 conductivity bridge.
Mass spectra were recorded on a Matson MS 5988 spectrometer.
The theoretically expected molecular structures were deduced
from the results of a geometry optimization process carried out
using the ab initio/STO-3G quantum mechanical method of compu-
tation. The hyperchem/6.03, accommodated on PIV-2.8 MHz per-
sonal computer was employed. The pH-metric titrations were
recorded using HANA instrument 8519 digital pH-meter. ESR spec-
trum was recorded on a Bruker EMX spectrometer working in the
X-band (9.78 GHz) with 100 kHz modulation frequency. The
microwave powder and modulation amplitudes were set at
1 mW and 4 Gauss, respectively. The low field signal was obtained
after 4 scans with 10-fold increase in the receiver gain. A powder
spectrum was obtained in a 2 mm quartz capillary at room temper-
ature (300 K).
for 10 h until a brown precipitate was formed. It was filtered off,
washed with methanol, diethyl ether and dried in vacuo. Conduc-
tivity data (10 M in H
ꢂ3
ꢂ1
2
ꢂ1
2
O):
K
M
= 89.0
X
cm mol
.
2 2
2.2.4. [Ag(Hsap)(H O) ]
3
Silver nitrate (0.085 g, 0.5 mmol) in water (2 cm ) was added to
sap (0.107 g, 0.5 mmol) in ethanol. The reaction mixture was
H
2
stirred in the dark for 10 h to produce a brown solid. It was filtered
off, washed with little water, ethanol, diethyl ether and dried in va-
cuo. Conductivity data (10 M in DMSO):
ꢂ3
ꢂ1
2
ꢂ1
K
M
= 2.0
X
cm mol .
2
.2.5. [Ru(Hsap)
The complex [Ru(PPh
an ethanolic solution of H
2 3 2
(PPh ) ]
3
)
3
Cl
2
] (0.25 g, 0.25 mmol) was added to
2
sap (0.16 g, 0.75 mmol). The reaction
mixture was refluxed for 2.5 h during which shiny brown micro-
crystals were isolated, washed with ethanol, diethyl ether and
ꢂ3
dried in vacuo. Conductivity data (10
M
in DMSO):
ꢂ1
2
ꢂ1
K
M
= 8.0
X
cm mol
.
3. Results and discussion
2
.2.6. [Ni(Hsap)
2
]ꢁH
2
O
The experimental section lists some new complexes of 4-
3
NiCl
2
ꢁ6H
2
O (0.119 g, 0.5 mmol) in water (5 cm ) was added to
hydroxysalicylidene-2-aminopyrimidine Schiff-base (H sap). The
2
3
H
2
sap (0.107 g, 0.5 mmol) in ethanol (15 cm ). The reaction mix-
elemental analyses (Table 1) of the complexes are in agreement
ture was heated under reflux for 4 h and a green-brown precipitate
with the assigned formulae. The molar conductivities (KM) in
DMSO at room temperature, suggest that all complexes are non-
electrolytes except [Pd(Hsap)(PPh ) ]Cl which behaves as 1:1 elec-
was isolated. It was filtered off, washed with water, ethanol and
ꢂ3
dried in vacuo. Conductivity data (10
M
in DMSO):
3
2
ꢂ1
2
ꢂ1
K
M
= 6.0
X
cm mol
.
2 2
trolyte, supporting its ionic formulation [20]. [WO (Hsap) ] and
[
Pd(Hsap)Cl(H
2
O)] complexes were prepared from the reaction of
ꢂ2
2
.2.7. [Ni(Hsap)(AcO)(H
Ni(AcO)
ꢁ4H
added to H
2
O)
2
]ꢁH
2
O
H sap with [WO ] or K [PdCl ] in aqueous ethanolic solutions.
[Pd(PPh ) (Hsap)]Cl was made from [Pd(PPh ) Cl ] and H sap in
3 2 3 2 2 2
potassium methoxide. [Ag(Hsap)(H O) ] was isolated from the
2 2
2
4
2
4
3
2
2
O (0.124 g, 0.5 mmol) in ethanol (10 cm ) was
sap (0.107 g, 0.5 mmol) in ethanol (20 cm ). The reac-
3
2
tion mixture was heated under reflux for 6 h to produce a reddish
brown precipitate, which was filtered off and washed with water,
2 3 2
reaction of H sap and AgNO in H O–EtOH in the dark. The reaction
of [Ru(PPh ) Cl ] and H sap in ethanol produce the complex
3
3
2
2
ꢂ3
ethanol and dried in vacuo. Conductivity data (10 M in DMSO):
3 2 2 2
[Ru(PPh ) (Hsap) ]. The complexes [Ni(Hsap) ] and [Ni(Hsap)(A-
ꢂ1
2
ꢂ1
K
M
= 5.0
X
cm mol
.
cO)(H O) ] were prepared by the reaction of hydrated NiCl or
2
2
2
Ni(AcO)
2
with H
2
sap in water–ethanol and ethanol, respectively.
2
.2.8. [Cu(Hsap)Cl(H
A similar procedure as for the nickel analogue was applied;
CuCl was used to produce a brown precipitate. Conductivity data
2
O)]ꢁH
2
O
The complex [Cu(Hsap)Cl(H O)] was made from CuCl and H sap
in aqueous ethanolic solution.
2
2
2
2
ꢂ3
The complexes are microcrystalline or powder-like, stable in
the normal laboratory atmosphere and partially soluble in DMF
and DMSO. We had hoped to structurally characterize one of the
complexes by single X-ray crystallography, but were thwarted on
numerous occasions by very small crystal dimensions. Thus, the
characterization of the complexes is based on physical and spectro-
scopic methods.
ꢂ1
2
ꢂ1
(
10 M in DMSO):
K
M
= 8.0
X
cm mol .
2
.3. pH-metric measurements
ꢀ
The pka value of Hsap at different temperatures (20, 30,
0 ± 0.05 °C) was determined pH-metrically by the Irving–Rossotti
4
method [18,19]. The following solutions were prepared and ti-
trated pH-metrically with standardized solution of sodium hydrox-
ꢂ3
ꢂ1
ꢂ1
ide (1.6 ꢃ 10 mol L ) at constant ionic strength (1 mol L
KNO ).
3.1. Electronic spectra and magnetic measurements
3
ꢂ4
ꢂ1
Solution (A): 2.5 ml of Hsap (1 ꢃ 10 mol L ) + 2.5 ml etha-
The electronic spectrum of the diamagnetic [Ru(PPh
complex is indicative of its low-spin octahedral structure. The
3 2 2
) (Hsap) ]
ꢂ1
nol + 1 ml KNO
3
(1 mol L ).

A.-E.F. Ouf et al. / Journal of Molecular Structure 973 (2010) 69–75
71
Table 1
Elemental analysis and 1H NMR spectral data of H
2
sap and its complexes.
Compounds
Elemental analysisa
1H NMR spectra
(d) (d)
C
H
N
Cl
M
HC (s)
9.94
H
3
(s)
H
5
H
6
H
4
0
,
6
0
(d)
5
H (t)
0
H
2
sap
61.3
4.2
19.0
(19.5)
7.32
7.44
7.47
–b
6.57
6.61
6.63
6.60
7.55
7.66
7.73
–b
8.22
8.24
8.23
8.24
8.80
8.82
8.84
8.81
(
61.4)
39.7
39.8)
33.7
33.7)
60.8
60.4)
37.0
36.9)
66.2
66.1)
52.1
52.3)
40.8
40.6)
40.4
40.4)
37.5
37.8)
(4.2)
Cis-[WO
2
(Hsap)
Pd(Hsap)Cl(H
O)]ꢁH
Pd(PPh
(Hsap)]Clꢁ3H
Ag(Hsap)(H O)
2
]ꢁH
2
O
2.5
(2.4)
12.8
(12.7)
10.05
10.08
10.04
(
[
[
[
[
[
[
[
[
2
2
O
3.0
(3.1)
10.4
(10.7)
9.0
(9.1)
27.0
(27.2)
(
3
)
2
2
O
4.7
(4.7)
4.3
(4.5)
3.6
(3.8)
11.8
(11.4)
(
2
2
]
3.3
(3.4)
11.5
(11.7)
30.0
(30.2)
(
Ru(PPh
3
)
2
(Hsap)
]ꢁH
2
]
4.7
(4.4)
8.0
(8.0)
(
10.02
10.02
–b
6.61
6.60
–b
8.23
8.25
8.81
8.82
Ni(Hsap)
2
2
O
3.7
(3.6)
16.4
(16.6)
7.43
7.70
(
Ni(Hsap)(AcO)(H
2
O)
O)]ꢁ2H
O)]ꢁH
2
]ꢁH
2
O
4.0
(4.2)
10.7
(10.9)
(
Co(Hsap)(AcO)(H
2
2
O
4.7
(4.4)
10.9
(10.8)
(
Cu(Hsap)Cl (H
2
2
O
3.3
(3.4)
12.1
(12.0)
10.4
(10.2)
(
a
Calculated values in parentheses.
Interference of H signals with PPh
b
3
protons signals.
ground term is 1A1g and the two spin-allowed transitions to 1
and T2g are observed at 520 and 400 nm, respectively [21,22].
T
1g
ing the metal ion environment in the complexes, i.e., the
geometry, nature of the ligation sites from the ligand to the metal
and the degree of covalency of the metal ligand bonds. To obtain
further information about the stereochemistry and the site of the
metal ligand bonding and to determine the magnetic interaction
in the metal complexes, ESR spectra of the complexes were re-
corded in the solid state.
1
The ligand field spectra of the diamagnetic [Pd(Hsap)Cl(H
and [Pd(PPh (Hsap)]Cl complexes are typical of a square-planar
environment around Pd(II). The bands near 475, 370 and 320 nm
2
O)]
3 2
)
1
1
1
1
1
1
g
are assigned to A1g ? A2g, A1g ? E and A1g ? B1g transitions,
respectively [23]. The absorption band near 370 nm is assigned to
combination of charge transfer transition from palladium d-orbital
The room temperature (300 K) powder ESR spectrum of [Cu(H-
ꢀ
to
p
orbital of PPh
3
and d–d bands [24].
2
sap)Cl(H O)] exhibits an axial signal with two g values (g// = 2.222,
The electronic spectrum of [Ag(Hsap)(H
2
2
O) ] shows bands at
g
\
= 2.049). The lowest g value is >2.04, indicating that Cu(II) is
4
40 and 332 nm; the latter one may arise from charge transfer of
present in an axial symmetry with all the principle axes aligned
parallel in a distorted octahedral geometry. The G factor ((g//
+
+
the type ligand (
p
) ? b1g (Ag ) and ligand (
r
) ? b1g (Ag ), respec-
tively, in distorted square-planar environment around Ag(I) [15].
The electronic spectrum of [Ni(Hsap)(AcO)(H O) ] can be inter-
preted in a terms of a distorted octahedral stereochemistry around
ꢂ2)/(g
only slightly misaligned and the exchange interactions between
Cu(II) ions are negligible [28]. As g// > g > 2, is consisting with a
ground state in which the unpaired electron is roomed in dx2–y2
orbital [29]. It have been reported that g// in Cu(II) complexes
can be used as a measure of the covalent character of metal–
ligand interaction. The environment is considered covalent as
\
ꢂ2)) is 4.53, suggesting that the local tetragonal axes are
2
2
\
3
3
the nickel center. The A2g ? T1g (F) transition appears at 870 and
05 nm, probably caused by a distortion from regular octahedral
while the strong band at 415 nm can be assigned to a combination
8
3
3
of
A2g ? T1g (P) and charge transfer transitions [25]. The mag-
netic moment of 3.08 B. M. lies within the range reported for octa-
hedral Ni(II) complexes [26]. On the other hand, the diamagnetic
g// < 2.3 [30].
[
Ni(Hsap)
2
] complex, may have a square-planar geometry. Its elec-
3
.3. NMR spectra
tronic spectrum exhibits a characteristic band at 410 nm may as-
sign to B1g ? E
2
2
g
transition [27].
1
Diagnostic H NMR assignments of H
2
sap and some of the rep-
In the electronic spectrum of [Cu(Hsap)Cl(H
2
O)], a broad band
6
resentative complexes (in DMSO-d ) are listed in Table 1. The
was observed at 640 nm, suggesting a tetragonal configuration
2
2
2
2
study was based on comparison with the data obtained for diamag-
netic complexes with such similar ligands. In all the studied spec-
tra, the integration ratio of the signals is consistent with the
around copper(II), which show
B
1g ? A1g
,
B
1g ? B2g and
2
2
B
1g ? E
g
transitions. These transitions usually overlap to give
one broad band [26]. Furthermore, the complex show band at
00 nm may attributes to charge transfer [26]. This complex is
assignments.
4
1
The H NMR spectrum of the free H
2
sap exhibits a triplet at d
paramagnetic with magnetic moment of 1.8 B. M., which is normal
for Cu(II).
0
8
.80 ppm assigned to H(5 ) (see Fig. 1 for numbering scheme)
and two singlets at d 9.94 and 7.30 ppm due to CH@N and H(3),
respectively. The four doublets at d 6.60, 7.50, 8.22 and 8.22 ppm
assigned to H(5), H(6), H(4 ) and H(6 ), respectively. The relatively
0
0
3
.2. ESR spectrum of [Cu(Hsap)Cl(H
2
O)]
broad singlets at d 12.20 and 11.01 ppm are due to protons of the
1
The ESR spectra of complexes provide information about hyper-
fine and superhyperfine structures which are important in study-
hydroxy O(2)H and O(4)H groups, respectively [31]. In the H NMR
spectra of the complexes, the resonance arising from the hydroxyl

7
2
A.-E.F. Ouf et al. / Journal of Molecular Structure 973 (2010) 69–75
1
5.10 ppm, suggesting the presence of two coordinated triphenyl
4
’
3
2
’ N
’
5’
phosphine ligands in these complexes [21,27].
*
6
*
6’
3.4. Vibrational spectra
5
N
N
1’
1
The quantum mechanical ab initio/STO-3G MO method was ap-
2
plied to study the geometric structure of the Schiff-base H sap.
4
2
HO
OH
Fig. 2 shows that the aromatic and pyrimidine rings are twisted
relative to each other by 70°. Conformation II (cis, O(2)H and
HC@N) was found to be more stable than conformation I (trans,
3
2
Fig. 1. Numbering scheme of H sap.
O(2)H and HC@N) and
DE = 3.2 kcal/mol. The calculated charge
densities on the active centers and the 3d mapped electrostatic po-
tential showed that the oxygen O(2)H and nitrogen HC@N centers
are then accessible for interaction with the metal (without interac-
tion from the cyclic nitrogen and O(4)H centers). The calculated
charge densities contour values (at 0.33 levels) indicates that these
two centers have the highest charge densities in this part of the
molecule; azomethine nitrogen atom has high charge density com-
pared to the pyrimidine ring cyclic nitrogen atoms. Moreover, the
spacing distance between the hydrogen of O(2)H and nitrogen of
HC@N are 2.100 Å, i.e., long enough to leave the O(2)H group free.
The solid-state properties of 4-hydroxysalicylidene-2-aminopy-
O(2)H proton is not observed while that arising from O(4)H is
shifted slightly downfield. These data confirm the de-protonation
of O(2)H and its coordination to the metal ion [31]. The signal
due to the azomethine proton (CH@N) is found to be considerably
deshielded (d > 10 ppm) relatively to that of the free H
2
sap
(
d = 9.94 ppm) as a consequence of electron donation to the metal
0
0
0
centre [32]. The H(4 ), H(5 ) and H(6 ) signals undergo marginal
shift to indicate the non-evolvement of the pyrimidine cyclic nitro-
gen centers in coordination; relatively large downfield shift would
be expected if coordination had occurred [33]. The resonance aris-
ing from H(3) and H(6) shift to higher field to a great extent than
the others, probably owing to a decrease in the electron density
in the aromatic ring more than the pyrimidine protons upon coor-
dination [31].
rimidine Schiff-base (H
copy. The spectrum of H
complexes. Tentative assignments of selected IR bands are re-
2
sap, Fig. 1) were examined by IR spectros-
2
sap was compared with those of the
ported in Table 2. The spectrum of H
at 1622 cm which is characteristic of
2
sap exhibits a strong band
(HC@N) group. It is ex-
In the 1H NMR spectra of [Ru(PPh
Hsap)], the phenyl protons experience downfield shifts as com-
pared with the free phenyl protons [24]. The PPh protons signals
show upfield shifts as compared with [Ru(PPh Cl and
]. This is interpreted in terms of stronger binding of
ꢂ1
3 2 2 3 2
) (Hsap) ] and [Pd(PPh )
m
(
pected that coordination of the nitrogen to the metal ion would re-
duce the electron density in the azomethine link and thus shifted
3
3
)
2
2
]
m
(HC@N) to lower wave numbers [3]. In the IR spectra of the com-
ꢂ1
[
Pd(PPh
3
2
) Cl
2
plexes, this band is shifted to the region at 1604–1615 cm [3,35].
The intense band at 1229 cm in the free H
ꢂ
ꢂ1
Hsap to Ru(II) and Pd(II) compared to binding of chloride ion
2
sap has been assigned
[
(
34]. The 1H NMR spectra of [Ru(PPh
Hsap)] show complicated multiplet in d 7.0–7.8 ppm region are as-
protons that interfere with the aromatic (H sap)
3 2 2 3 2
) (Hsap) ] and [Pd(PPh )
to the phenolic m(C–O) stretch. Upon complexation, this band is
shifted to higher frequency, indicating the coordination of H
through the deprotonated phenolic (C(2)–O ) [36]. These data
2
sap
ꢂ
signed to the PPh
3
2
protons resonances.
are further supported by the disappearance of the broad band at
The 31P NMR spectra of [Ru(PPh
) (Hsap)
2
] and [Pd(PPh
ꢂ1
3
2
3
)
2
3
387 cm attributed to
m
(O(2)H); the de-protonation occurs prior
ꢂ1
(
6
Hsap)] in DMSO-d show two sharp singlets near d 13.95 and
to coordination. The band at 3217 cm
2
in the H sap due to
2
Fig. 2. Geometric structure of the Schiff-base, H sap.

A.-E.F. Ouf et al. / Journal of Molecular Structure 973 (2010) 69–75
73
Table 2
2
IR spectral data of H sap and its complexes.
ꢂ1
Compounds
IR spectra (cm
(HC@N)
)
m
m(C@N)
m(C@C)
m(C–O)
m(M–O)
m
(M–N)
m(M–Cl)
H
2
sap
1622
1605
1560
1559
1575
1578
1229
1241
–
935
9
–
–
a
Cis-[WO
2
2 2
(Hsap) ]ꢁH O
b
10
[
[
[
[
[
[
[
[
Pd(Hsap)Cl(H
Pd(PPh
(Hsap)]Clꢁ3H
Ag(Hsap)(H O)
(Hsap)
2
O)]ꢁH
2
O
1604
1611
1610
1609
1615
1613
1615
1612
1558
1559
1557
1561
1558
1560
1545
1560
1560
1574
1577
1580
1572
1573
1557
1571
1238
1242
1245
1245
1246
1245
1249
1246
544
543
537
530
510
517
507
531
430
435
411
400
425
445
443
439
329
3
)
2
2
O
2
2
]
2
]
Ru(PPh
3
)
2
Ni(Hsap)
Ni(Hsap)(AcO)(H
Co(Hsap)(AcO)(H
2
]ꢁH
2
O
2
O)
2
]ꢁH
2
O
2
O
2
O)]ꢁ2H
Cu(Hsap)Cl (H
2 2
O)]ꢁH O
320
a
m
s
(WO
2
).
).
b
mas(WO
2
m
H
(O(4)H) stretch is unaffected by coordination [12]. In the free
m(P–C) and d(CCH) vibrations, respectively [21]; these bands are
ꢂ1
2
sap, strong bands at 1575 and 1560 cm , attributed to the
shifted to higher compared with the free PPh
3
indicating their par-
non-aromatic pyrimidine
tively [37], are not affected upon complexation. This means that
sap acts as a N,O-mononegative bidentate ligand. In the complex
Ni(Hsap)(AcO)(H O) ] (Fig. 3), two extra bands are observed at
m(C@C) and m(C@N) stretches, respec-
ticipation in complexation.
The spectrum of [WO
2
(Hsap)
2
] shows bands characteristic of
2
+
ꢂ1
H
[
2
the cis-WO
signed to the
The region of the complex spectra between 550 and 200 cm
contains several weak bands; these may assign to (M–O), (M–
N), (M–P) and (M–Cl) stretches, respectively [25,34].
2
core [39]. The IR bands at 935 and 910 cm are as-
2
2
ꢂ1
s 2 2
m (WO ) and mas(WO ) stretches, respectively.
ꢂ
ꢂ
ꢂ1
1
s
530 and 1410 cm assigned to mas(COO ) and m (COO ) vibra-
tions of the acetato group, respectively [38]. The separation be-
m
m
ꢂ
ꢂ
ꢂ1
tween these two bands {
D
=
m
as(COO ) ꢂ
m
s
(COO ) ꢄ 120 cm },
m
m
indicating asymmetric bidentate coordination of the carboxylic
group.
3.5. Mass spectra
3 2 3 2 2
In the IR spectra of [Pd(PPh ) (Hsap)]Cl and [Ru(PPh ) (Hsap) ]
(
Fig. 4), the presence of coordinated PPh
3
groups is manifested by
The mass spectra of the complexes [Pd(Hsap)Cl(H O)]ꢁH O,
2
2
ꢂ1
the strong IR bands at ꢄ1100 and ꢄ750 cm , attributed to the
[Pd(PPh ) (Hsap)]Clꢁ3H O, [Ni(Hsap) ]ꢁH O and [Cu(Hsap)Cl(-
3
2
2
2
2
H
2
O)]ꢁH
2
O are reported and their molecular ion peaks are in agree-
ment with their assigned formulae.
The mass spectrum of [Pd(Hsap)Cl(H
2 2
O)]ꢁH O shows fragmen-
tation patterns corresponding to the successive degradation of
the complex. The first peak at m/e 393 with 13.2% abundance rep-
resents the molecular ion (Calcd. 391.9). The peaks at 337 and 318
correspond to [Pd(Hsap)(H
respectively [23]. The mass spectrum of [Pd(PPh
shows the first signal at m/e 936 (Calcd. 933.9), in agreement with
+
+
2
O)] and [Pd(Hsap)] fragments,
3
)
2
(Hsap)]Clꢁ3H
2
O
+
3 2
the molecular ion of the complex, [Pd(PPh ) (Hsap)] , with 10.13%
abundance. There are signals which represent the loss of Hsap and
+
PPh
m/e 632 (Calcd. 630.4) and [Pd(PPh
respectively [24,40]. The mass spectrum of [Ni(Hsap)
3
fragments indicating stepwise ligand loss to [Pd(PPh
3
)
2
] with
)] with m/e 368 (Calcd. 368.4),
O shows
+
3
2
]ꢁH
2
a signal at m/e 507 (Calcd. 504.7) with 11.25% abundance. The frag-
mentation patterns indicates the stepwise ligand loss to [Ni(H-
+
+
Fig. 3. Structure of [Ni(Hsap)(AcO)(H
2
O)
2
].
sap)
Cu(Hsap)Cl(H
9.35% abundance. The fragmentation patterns indicate the step-
]
(488) and [Ni(Hsap)] (274) [24]. The mass spectrum of
2
[
1
2
O)]ꢁH O, shows signal at m/e 351 (Calcd. 349) with
2
+
+
OH
wise ligand loss to [Cu(Hsap)(H
2
O)] (297), [Cu(Hsap)] (277).
N
N
3
.6. Thermal analysis
PPh3
The thermal decomposition of the complexes, [Pd(Hsap)Cl
O)]ꢁH O, [Ru(PPh (Hsap) ], and [Ni(Hsap)(AcO)(H O) ]ꢁH
was studied using thermo-gravimetry (TG) technique.
The thermogram of [Pd(Hsap)Cl(H O, is characterized by
O
N
(
H
2
2
3
)
2
2
2
2
2
O
CH
Ru
PPh3
HC
O)]ꢁH
2
2
N
O
steps at 25–125, 126–306, 307–405 and 406–560 °C regions. The
elimination of crystal lattice water (Calcd. 4.6, Found 4.8%)
N
[
12,25], coordinated water (Calcd. 4.6, Found 4.6%), C
/2Cl (Calcd. 29.2, Found 30.1%), C O and 1/2N (Calcd. 30.4,
Found 30.9%) fragments, respectively, leaves PdO residue at
00 °C (31.3%) [15,23]. The data for [Ru(PPh (Hsap) ], show two
TG inflections in the ranges 250–336 and 337–467 °C. The first
4 3 2
H N and
N
1
2
7
H
5
2
HO
7
3
)
2
2
2 3 2
Fig. 4. Structure of [Ru(Hsap) (PPh ) ].

7
4
A.-E.F. Ouf et al. / Journal of Molecular Structure 973 (2010) 69–75
ꢂ1
weight loss may arise from the release of four Ph groups (C24
H
20
)
found to be 48.37 and 57.35 kJ mol , respectively at 20 ± 0.05 °C.
and two C
PPh and two C
4
H
3
N
2
fragments (Calcd. 44.3, Found 43.9%), and two
O and N fragments (Calcd. 43.1, Found 43.7%),
The positive values indicate the non-spontaneous dissociation
character of H sap [44].
2
7
H
5
2
respectively [24]. For most of the reported ruthenium complexes,
the residue is mainly metal or metal oxide, but in the presence of
non-stoichiometric oxide, unburned carbon or nitrogen is observed
3.7.2. Determination of the stability constants of the complexes
The stability constants of some metal ions {Co(II), Ni(II), Cu(II),
(
mixed ruthenium oxide and nitride residue (18.3%)) [34]. The TG
Pd(II) and Ag(I)} with H
20 ± 0.05 °C and 0.04 mol L
aqueous solution (1:4 V/V) solutions. The titration curves were
2
sap were determined pH-metrically at
ꢂ1
thermogram of [Ni(Hsap)(AcO)(H O) O shows the first endo-
thermic weight loss step between 32 and 125 °C, which corre-
2
2
]ꢁH
2
ionic strength in 20% ethanol-
sponding to the release of crystal lattice water (Cacld. 4.7, Found
found below and well separated from that of the free H sap con-
2
4
2
.6%). The second decomposition step occurs between 161 and
45 °C, this weight loss is attributed the loss of coordinated water
firming that the complex formation with liberation of protons
[43]. The formation curves of the complexes were obtained from
the nꢀ – pL plots, where nꢀ is the average number of ligand attached
per metal ion and pL the free ligand exponent. The nꢀ values were
(
Calcd. 9.4, Found 9.6%). The third TG inflection between 246 and
3
4 3 2
55 °C is due to the loss of C H N fragments (Calcd. 20.5, Found
1
9.2%). The TG inflection between 356 and 401 °C, may arise from
calculated from,
the elimination of the acetate species (Calcd. 15.3, Found 15.4%)
38]. The last TG inflection lies in the 402–500 °C range and may
arise from the release of C O and 1/2N fragments (Calcd. 30.9,
Found 31.0%), leaving NiO representing (Calcd. 19.4, Found 19.2%).
o
V
2
ꢂ V
1
ÞN
[
nꢀ ¼
ð2Þ
ðV
o
þ V
1
Þn
A
T
M
H
7 5
2
where V is the volume of NaOH required to reach the desired pH in
2
M
the complex solution and T is the initial concentration of the metal
n+
ion [M ] in the solution. The pL values were calculated from
3
3
.7. pH-metric measurements
þ
ð1 þ K
1
½H ꢅ ðV
2
þ V ÞÞ
o
pL ¼ log
ꢁ
ð3Þ
2
.7.1. Determination of the protonation constants of H sap
The protonation constants of H sap were determined pH-metri-
2
T
L
ꢂ nꢀ
T
V
o
M
cally. The titrations were carried out in 20% ethanol-aqueous solu-
The nꢀ – pL plots were obtained for solution containing 1:10 me-
tal to H sap ratio to ensure the formation of the most stable species
ꢂ1
tion (1:4 V/V) at constant ionic strength (0.04 mol L KNO
3
) at
2
different temperatures (20, 30, 40 ± 0.05 °C). The suitable titration
[44]. The metal–ligand stoichiometries were confirmed by the
analysis of the titration curves, indicating M:L and M:2L ratios
complex formation (Table 3); i.e., H sap behaves as monoprotic
2
species via O(2)H. The stability constants of the investigated sys-
tems have been evaluated by using both half and least square
ꢂ5
ꢂ1
curve was obtained by titrating H
2
sap (1 ꢃ 10 mol L ) against
ꢂ3
ꢂ1
NaOH (1.6 ꢃ 10 mol L ). The titration curve shows two sharp
jumps indicating two neutralization equilibrium corresponding
to the librated protons of O(2)H and O(4)H of the 4-hydroxysalicy-
lidene moiety. The average number of protons associated with
methods [19]. It is clear that log k
same complex. This can be explained by assuming the steric hin-
drance for the linking of the second species of H sap to the central
1 2
is higher than log K for the
H
2
sap; nꢀ
, was calculated at different pH values using Irving–Ross-
A
otti [19]:
2
metal ion [38]. The order of stability constants decreased in the or-
der Cu(II) > Ni(II) > Co(II) for the first row transition elements in
accordance with Irving and Williams order [45,46].
o
1
V N
þ V
nꢀ
¼ Y ꢂ
ð1Þ
A
ðV
o
1
ÞT
L
The values of the free energy of formation (
D
G) were calculated
where Y = 2, is the number of dissociable protons present in H
the volume of NaOH added, V
concentration of H
2
sap,
using
V
1
o
the initial volume (25 ml), T
l
the
ꢀ
ꢂ5
ꢂ1
D
G ¼ 2:303RT log b ¼ 2:303RTðlog K
1
þ log K Þ
2
ð4Þ
2
sap in the initial volume (1 ꢃ 10
mol L
)
o
ꢂ3
ꢂ1
and N the concentration of NaOH (1.6 ꢃ 10 mol L ).
The negative values of
tion [43,44]. Also, it has been reported that the higher the negative
value of G, the more stable is the complex produced (Table 4)
[41,43,44].
The stability constants of the complexes {Co(II), Ni(II), Cu(II),
Pd(II) and Ag(I) with sap} were calculated at 30 and
0 ± 0.05 °C. The stability constants and the thermodynamic func-
tions data are reported in Tables 4 and 5, respectively. The decrease
in the stability constants (log k and log K ) values {Ag(I) and
DG indicate the spontaneous complex forma-
The plot of nꢀ
indicating that H
against pH shows the maximum nꢀ
sap has two dissociable hydroxy protons; O(2)H
A
value to be 2,
A
2
D
ꢀ
ꢀ
and O(4)H. The de-protonation constants, (pK and pK ) are
determined by interpolation at nꢀ
tion, and were calculated from both the half and least square meth-
ods. They were found to be 8.5 and 10.2, respectively. These results
reveal to the basicity of 2-aminopyrimidine [41].
Table 3 shows the effect of temperature on the protonation con-
2
a1 a2
sap. The pK and pK values decrease with increasing
temperature, i.e., the acidity of H
a1
a2
¼ 1:5 and 0.5 from nꢀ
–pH rela-
A
A
H
2
4
1
2
ꢀ
ꢀ
stants of H
Pd(II)} with increasing the temperature suggests that these com-
plex formation are exothermic [42,43,47]. In case of Co(II), Ni(II)
2
sap increases as the temperature
G values for the dissociation of H sap were
increase [42,43].
D
2
and Cu(II), the complex formation is endothermic [47]. The nega-
ꢀ
tive values of
D
H
indicate that the complexation processes are
Table 3
2
The protonation constant of H sap and the formation constants of its complexes.
Table 4
Ligand
293 °C
pKa1
303 °C
pK
313 °C
pK
2
Metal–H sap complexes stability constant at 20 ± 0.05 °C.
pKa2
1
pK
2
1
pK
2
Metal ions
log k1
log k2
log bꢀa
ꢂ
D
G
b
H
2
sap
10.2
log K
8.5
log K
9.94
log K
8.32
log K
9.71
log K
8.12
log K
Co(II)
Ni(II)
Cu(II)
Pd(II)
Ag(I)
4.0
3.81
3.87
4.12
5.18
6.65
7.81
8.31
9.65
11.55
13.81
42.87
45.61
52.97
63.39
75.80
Complexes
1
2
1
2
1
2
4.44
5.53
6.37
7.16
Co(II)
Ni(II)
Cu(II)
Pd(II)
Ag(I)
4.0
3.81
3.87
4.12
5.18
6.65
4.90
5.36
6.11
5.65
6.15
4.30
4.55
4.55
4.45
5.55
5.55
6.08
6.55
5.10
5.30
5.16
5.29
5.35
4.20
4.90
4.44
5.53
6.37
7.16
a
Overall formation constant.
KJ mol
b
ꢂ1
.

A.-E.F. Ouf et al. / Journal of Molecular Structure 973 (2010) 69–75
75
Table 5
2
Thermodynamic parameters of metal–H sap complexes.
Metal ions
Thermodynamic parameters
ꢀ
ꢀ
ꢀ
ꢀ
DH
DH
D
S
DS
293 °C
303 °C
313 °C
1
2
1
2
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢂ
DG
ꢂ
DG
ꢂ
D
G
ꢂ
DG
ꢂ
DG
ꢂ
DG
1
2
1
2
1
2
Co(II)
Ni(II)
Cu(II)
Pd(II)
Ag(I)
114.9
175.4
74.7
102.1
104.0
89.2
110.9
132.3
128.3
91.9
101.5
101.5
103.8
74.7
22.1
24.4
30.6
35.3
39.7
20.9
21.4
22.8
28.7
37.0
38.4
31.1
35.5
32.6
35.8
25.0
25.8
26.3
25.7
38.1
33.3
36.4
39.2
30.5
32.3
31.0
30.7
32.1
25.1
29.1
ꢂ75.7
ꢂ76.6
ꢂ132.5
ꢂ134.0
95.7
86.2
exothermic in nature and favorable at low temperature for both
Ag(I) and Pd(II) complexes while the positive values indicate endo-
thermic nature and favorable at high temperature for Co(II), Ni(II)
and Cu(II) complexes. These features support the production of so-
lid Ag(I) and Pd(II) complexes at room temperatures while that of
Co(II), Ni(II) and Cu(II) were isolated after reflux at high tempera-
[14] S. Hadjikakou, M. Demertzis, M. Kubicki, D. Kovala-Demertzis, Appl.
Organomet. Chem. 14 (2000) 727.
[
[
15] S. Mostafa, F. Badria, Met. Based Drug, 2008, doi: 1155/2008/723634.
16] R. Cookson, D. Jones, J. Chem. Soc. (1965) 181.
[17] T. Stephenson, G. Wilkinson, J. Inorg. Nucl. Chem. 28 (1966) 945.
[
18] J. Bejerrum, Metal Amine Formation Aqueous Solutions, P. Hasse and Son,
Copenhagen, 1941.
[
19] H. Irving, H. Rossotti, J. Chem. Soc. (1954) 2904.
tures, as reported in the experimental section. The positive values
[20] W. Geary, Coord. Chem. Rev. 7 (1981) 81.
[21] S. Mostafa, C. Papatriantafyllopoulou, S. Perlepes, N. Hadjiliadis, Bioinorg.
Chem. Appl., 2008, doi: 1155/2008/647873.
ꢀ
of
D
S indicate that the disorder were increased more rapidly than
the increase in the order taking place in chelation; i.e., the order of
[
22] A.B.P. Lever, Inorganic Electronic Spectroscopy, second ed., Elsevier,
Amsterdam, The Netherlands, 1984.
arrangement of the solvent around H
lost upon complexation [47]. In all cases
2
sap and the metal ions are
ꢀ
[23] S. Mostafa, Transition Metal Chem. 32 (2007) 769.
D
G values are negative,
[
[
24] S. Mostafa, J. Coord. Chem. 61 (2008) 1553.
25] S. Mostafa, S. Perlepes, N. Hadjiliadis, Z. Naturforsch. 56b (2001) 394.
which means that the complexation processes are spontaneous.
ꢀ
In comparing
D
G values for Co(II), Ni(II) and Cu(II), i.e., the first
[26] F.A. Cotton, G. Wilkinson, C.A. Murillo, M. Bochmann, Advanced Inorganic
Chemistry, sixth ed., Wiley, New York, 1999.
row, it is found that the value is more negative for Cu(II), suggest-
ing high tendency of interaction between Cu(II) and H sap [48].
2
[
27] M. Tamizh, K. Mereiter, K. Kirchner, B. Ramachandra Bhat, R. Karvembu,
Polyhedron 28 (2009) 2157.
[
[
[
[
28] V. Singh, Spectochim. Acta 71 (2008) 17.
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Gopinathan, Polyhedron 16 (1997) 3977.
Acknowledgement
We wish to thank Prof. W.P. Griffith (Chemistry Department,
Imperial College of London) for the language corrections.
[
[
[
32] D. Bhattacharyya, S. Chakraborty, P. Munshi, G. Lahiri, Polyhedron 18 (1999)
2951.
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