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

Journal of Molecular Structure 973 (2010) 69–75

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pH-metric and spectroscopic properties of new 4-hydroxysalicylidene-2 -aminopyrimidine Schiff-base transition metal complexes Abd El-Fatah Ouf a, Mayada S. Ali a, Eman M. Saad b, Sahar I. Mostafa a,* a b

Chemistry Department, Faculty of Science, Mansoura University, Mansoura, Egypt Chemistry Department, Faculty of Science (Suez), Suez Canal University, Egypt

a r t i c l e

i n f o

Article history: Received 9 August 2009 Received in revised form 11 March 2010 Accepted 11 March 2010 Available online 16 March 2010 Keywords: Hsap Spectra Hydroxy Pyrimidine Azomethine Stability constant

a b s t r a c t The new complexes, cis-[WO2(Hsap)2], [Ru(PPh3)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 H2sap 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 (DS, DG, DH) calculated. Ó 2010 Elsevier B.V. All rights reserved.

1. Introduction Schiff-bases are important class of chelating agents in coordination chemistry as they easily form stable complexes with most transition metal ions [1]. The bioinorganic chemistry field has increased 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 reported the chemotherapeutic potential of Pd(II) and Ag(I) complexes 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 continuation of our interest in anticancer activity of complexes containing pyrimidine rings, here we describe the preparation and characterization of new 4-hydroxysalicylidene-2-aminopyrimidine Schiff-base (H2sap) transition elements complexes. Also, we report the dissociation constants of H2sap, the stability constants of the * Corresponding author. Tel.: +20 108502625; fax: +20 50 2246781. E-mail address: [email protected] (S.I. Mostafa). 0022-2860/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.molstruc.2010.03.037

metal complexes and the thermodynamic activation parameters (DS, DG, DH) which are calculated pH-metrically at various temperatures, as these studies can give some knowledge in their examination as anticancer agents. 2. Experimental All reagents were purchased from Aldrich and all manipulations were performed under aerobic conditions using materials and solvents as received. [Pd(PPh3)2Cl2] was prepared from K2[PdCl4] and PPh3 in aqueous-ethanol solution [16] and [Ru(PPh3)3Cl2] was synthesised as previously reported [17]. DMF and DMSO used in conductivity and electronic spectral measurements were dried over molecular sieves. DMSO-d6 (NMR) was referenced using TMS. 2.1. Preparation of H2sap The Schiff-base, H2sap, was synthesised by the condensation of ethanolic solutions of 2-aminopyrimidine (0.095 g, 1 mmol) and 4hydroxysalicylaldehyde (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. 2.2. Preparation of complexes 2.2.1. cis-[WO2(Hsap)2]H2O A solution of Na2[WO4] (0.15 g, 0.5 mmol) in water (5 cm3) was added to H2sap (0.107 g, 0.5 mmol) in ethanol (15 cm3), The mix-

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ture was heated and stirred for 3 h. The resulting pale orange precipitate was filtered off, washed with ethanol, diethyl ether and dried in vacuo. Conductivity data (103 M in DMSO): KM = 6.0 X1 cm2 mol1. 2.2.2. [Pd(Hsap)Cl(H2O)]H2O K2[PdCl4] (0.163 g, 0.5 mmol) in water (5 cm3) was added to H2sap (0.107 g, 0.5 mmol) in ethanol (15 cm3). The mixture was stirred for 12 h until a pale brown precipitate was isolated; this was washed with water, ethanol and dried in vacuo. Conductivity data (103 M in DMSO): KM = 8.0 X1 cm2 mol1. 2.2.3. [Pd(PPh3)2(Hsap)]Cl3H2O H2sap (0.107 g, 0.5 mmol) was dissolved in methanolic solution of KOH (0.028 g, 0.5 mmol; 15 cm3). [Pd(PPh3)Cl2] (0.219 g, 0.5 mmol) was added and the mixture was warmed with stirring for 10 h until a brown precipitate was formed. It was filtered off, washed with methanol, diethyl ether and dried in vacuo. Conductivity data (103 M in H2O): KM = 89.0 X1 cm2 mol1. 2.2.4. [Ag(Hsap)(H2O)2] Silver nitrate (0.085 g, 0.5 mmol) in water (2 cm3) was added to H2sap (0.107 g, 0.5 mmol) in ethanol. The reaction mixture was 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 vacuo. Conductivity data (103 M in DMSO): KM = 2.0 X1 cm2 mol1. 2.2.5. [Ru(Hsap)2(PPh3)2] The complex [Ru(PPh3)3Cl2] (0.25 g, 0.25 mmol) was added to an ethanolic solution of H2sap (0.16 g, 0.75 mmol). The reaction mixture was refluxed for 2.5 h during which shiny brown microcrystals were isolated, washed with ethanol, diethyl ether and dried in vacuo. Conductivity data (103 M in DMSO): KM = 8.0 X1 cm2 mol1. 2.2.6. [Ni(Hsap)2]H2O NiCl26H2O (0.119 g, 0.5 mmol) in water (5 cm3) was added to H2sap (0.107 g, 0.5 mmol) in ethanol (15 cm3). The reaction mixture was heated under reflux for 4 h and a green-brown precipitate was isolated. It was filtered off, washed with water, ethanol and dried in vacuo. Conductivity data (103 M in DMSO): KM = 6.0 X1 cm2 mol1. 2.2.7. [Ni(Hsap)(AcO)(H2O)2]H2O Ni(AcO)24H2O (0.124 g, 0.5 mmol) in ethanol (10 cm3) was added to H2sap (0.107 g, 0.5 mmol) in ethanol (20 cm3). The reaction mixture was heated under reflux for 6 h to produce a reddish brown precipitate, which was filtered off and washed with water, ethanol and dried in vacuo. Conductivity data (103 M in DMSO): KM = 5.0 X1 cm2 mol1. 2.2.8. [Cu(Hsap)Cl(H2O)]H2O A similar procedure as for the nickel analogue was applied; CuCl2 was used to produce a brown precipitate. Conductivity data (103 M in DMSO): KM = 8.0 X1 cm2 mol1. 2.3. pH-metric measurements 

The pka value of Hsap at different temperatures (20, 30, 40 ± 0.05 °C) was determined pH-metrically by the Irving–Rossotti method [18,19]. The following solutions were prepared and titrated pH-metrically with standardized solution of sodium hydroxide (1.6  103 mol L1) at constant ionic strength (1 mol L1 KNO3). Solution (A): 2.5 ml of Hsap (1  104 mol L1) + 2.5 ml ethanol + 1 ml KNO3 (1 mol L1).

Solution (B): 2.5 ml of Hsap (1  104 mol L1) + 2.5 ml ethanol + 1 ml KNO3 (1 mol L1) + 0.5 ml of metal ion, Co(II), Ni(II), Cu(II), Zn(II), Pd(II), Ag(I), (5  105 mol L1). 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 a Johnson Matthey magnetic susceptibility balance with Hg[Co(SCN)4] as calibrant. IR spectra were measured as KBr discs on a Matson 5000 FT-IR spectrometer. Electronic spectra were recorded using a Unicam UV2–100 UV–vis spectrometer. NMR spectra were measured on a Varian Gemini WM-200 and Varian Mercury 400 MHz spectrometers. Thermal analysis measurements were made in the 20–800 °C range at the heating rate of 10 °C min1, using a-Al2O3 as a reference, on a Shimadzu Thermogravimetric 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 computation. The hyperchem/6.03, accommodated on PIV-2.8 MHz personal computer was employed. The pH-metric titrations were recorded using HANA instrument 8519 digital pH-meter. ESR spectrum 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 temperature (300 K).

3. Results and discussion The experimental section lists some new complexes of 4hydroxysalicylidene-2-aminopyrimidine Schiff-base (H2sap). The elemental analyses (Table 1) of the complexes are in agreement with the assigned formulae. The molar conductivities (KM) in DMSO at room temperature, suggest that all complexes are nonelectrolytes except [Pd(Hsap)(PPh3)2]Cl which behaves as 1:1 electrolyte, supporting its ionic formulation [20]. [WO2(Hsap)2] and [Pd(Hsap)Cl(H2O)] complexes were prepared from the reaction of H2sap with [WO4]2 or K2[PdCl4] in aqueous ethanolic solutions. [Pd(PPh3)2(Hsap)]Cl was made from [Pd(PPh3)2Cl2] and H2sap in potassium methoxide. [Ag(Hsap)(H2O)2] was isolated from the reaction of H2sap and AgNO3 in H2O–EtOH in the dark. The reaction of [Ru(PPh3)3Cl2] and H2sap in ethanol produce the complex [Ru(PPh3)2(Hsap)2]. The complexes [Ni(Hsap)2] and [Ni(Hsap)(AcO)(H2O)2] were prepared by the reaction of hydrated NiCl2 or Ni(AcO)2 with H2sap in water–ethanol and ethanol, respectively. The complex [Cu(Hsap)Cl(H2O)] was made from CuCl2 and H2sap in aqueous ethanolic solution. 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 spectroscopic methods.

3.1. Electronic spectra and magnetic measurements The electronic spectrum of the diamagnetic [Ru(PPh3)2(Hsap)2] complex is indicative of its low-spin octahedral structure. The

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A.-E.F. Ouf et al. / Journal of Molecular Structure 973 (2010) 69–75 Table 1 Elemental analysis and 1H NMR spectral data of H2sap and its complexes. Compounds

a b

Elemental analysisa

1

H NMR spectra

HC (s)

H3 (s)

H5 (d)

H6 (d)

H40 ,

19.0 (19.5)

9.94

7.32

6.57

7.55

8.22

8.80

2.5 (2.4)

12.8 (12.7)

10.05

7.44

6.61

7.66

8.24

8.82

33.7 (33.7)

3.0 (3.1)

10.4 (10.7)

9.0 (9.1)

27.0 (27.2)

10.08

7.47

6.63

7.73

8.23

8.84

[Pd(PPh3)2(Hsap)]Cl3H2O

60.8 (60.4)

4.7 (4.7)

4.3 (4.5)

3.6 (3.8)

11.8 (11.4)

10.04

–b

6.60

–b

8.24

8.81

[Ag(Hsap)(H2O)2]

37.0 (36.9)

3.3 (3.4)

11.5 (11.7)

[Ru(PPh3)2(Hsap)2]

66.2 (66.1)

4.7 (4.4)

8.0 (8.0)

10.02

–b

6.61

–b

8.23

8.81

[Ni(Hsap)2]H2O

52.1 (52.3)

3.7 (3.6)

16.4 (16.6)

10.02

7.43

6.60

7.70

8.25

8.82

[Ni(Hsap)(AcO)(H2O)2]H2O

40.8 (40.6)

4.0 (4.2)

10.7 (10.9)

[Co(Hsap)(AcO)(H2O)]2H2O

40.4 (40.4)

4.7 (4.4)

10.9 (10.8)

[Cu(Hsap)Cl (H2O)]H2O

37.5 (37.8)

3.3 (3.4)

12.1 (12.0)

C

H

N

H2sap

61.3 (61.4)

4.2 (4.2)

Cis-[WO2(Hsap)2]H2O

39.7 (39.8)

[Pd(Hsap)Cl(H2O)]H2O

Cl

M

60

(d)

H50 (t)

30.0 (30.2)

10.4 (10.2)

Calculated values in parentheses. Interference of H signals with PPh3 protons signals.

ground term is 1A1g and the two spin-allowed transitions to 1T1g and 1T2g are observed at 520 and 400 nm, respectively [21,22]. The ligand field spectra of the diamagnetic [Pd(Hsap)Cl(H2O)] and [Pd(PPh3)2(Hsap)]Cl complexes are typical of a square-planar environment around Pd(II). The bands near 475, 370 and 320 nm are assigned to 1A1g ? 1A2g, 1A1g ? 1Eg and 1A1g ? 1B1g transitions, respectively [23]. The absorption band near 370 nm is assigned to combination of charge transfer transition from palladium d-orbital to p orbital of PPh3 and d–d bands [24]. The electronic spectrum of [Ag(Hsap)(H2O)2] shows bands at 440 and 332 nm; the latter one may arise from charge transfer of the type ligand (p) ? b1g (Ag+) and ligand (r) ? b1g (Ag+), respectively, in distorted square-planar environment around Ag(I) [15]. The electronic spectrum of [Ni(Hsap)(AcO)(H2O)2] can be interpreted in a terms of a distorted octahedral stereochemistry around the nickel center. The 3A2g ? 3T1g (F) transition appears at 870 and 805 nm, probably caused by a distortion from regular octahedral while the strong band at 415 nm can be assigned to a combination of 3A2g ? 3T1g (P) and charge transfer transitions [25]. The magnetic moment of 3.08 B. M. lies within the range reported for octahedral Ni(II) complexes [26]. On the other hand, the diamagnetic [Ni(Hsap)2] complex, may have a square-planar geometry. Its electronic spectrum exhibits a characteristic band at 410 nm may assign to 2B1g ? 2Eg transition [27]. In the electronic spectrum of [Cu(Hsap)Cl(H2O)], a broad band was observed at 640 nm, suggesting a tetragonal configuration around copper(II), which show 2B1g ? 2A1g, 2B1g ? 2B2g and 2 B1g ? 2Eg transitions. These transitions usually overlap to give one broad band [26]. Furthermore, the complex show band at 400 nm may attributes to charge transfer [26]. This complex is paramagnetic with magnetic moment of 1.8 B. M., which is normal for Cu(II). 3.2. ESR spectrum of [Cu(Hsap)Cl(H2O)] The ESR spectra of complexes provide information about hyperfine and superhyperfine structures which are important in study-

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 recorded in the solid state. The room temperature (300 K) powder ESR spectrum of [Cu(Hsap)Cl(H2O)] exhibits an axial signal with two g values (g// = 2.222, g\ = 2.049). The lowest g value is >2.04, indicating that Cu(II) is present in an axial symmetry with all the principle axes aligned parallel in a distorted octahedral geometry. The G factor ((g// 2)/(g\2)) is 4.53, suggesting that the local tetragonal axes are 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 g// < 2.3 [30]. 3.3. NMR spectra Diagnostic 1H NMR assignments of H2sap and some of the representative complexes (in DMSO-d6) are listed in Table 1. The study was based on comparison with the data obtained for diamagnetic complexes with such similar ligands. In all the studied spectra, the integration ratio of the signals is consistent with the assignments. The 1H NMR spectrum of the free H2sap exhibits a triplet at d 8.80 ppm assigned to H(50 ) (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(40 ) and H(60 ), respectively. The relatively broad singlets at d 12.20 and 11.01 ppm are due to protons of the hydroxy O(2)H and O(4)H groups, respectively [31]. In the 1H NMR spectra of the complexes, the resonance arising from the hydroxyl

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3’ N

*

6 5

HO

*

4

2 3

2’

N

1

4’

1’ N

5’

6’

OH

Fig. 1. Numbering scheme of H2sap.

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 H2sap (d = 9.94 ppm) as a consequence of electron donation to the metal centre [32]. The H(40 ), H(50 ) and H(60 ) signals undergo marginal shift to indicate the non-evolvement of the pyrimidine cyclic nitrogen centers in coordination; relatively large downfield shift would be expected if coordination had occurred [33]. The resonance arising 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 coordination [31]. In the 1H NMR spectra of [Ru(PPh3)2(Hsap)2] and [Pd(PPh3)2 (Hsap)], the phenyl protons experience downfield shifts as compared with the free phenyl protons [24]. The PPh3 protons signals show upfield shifts as compared with [Ru(PPh3)2Cl2] and [Pd(PPh3)2Cl2]. This is interpreted in terms of stronger binding of Hsap to Ru(II) and Pd(II) compared to binding of chloride ion [34]. The 1H NMR spectra of [Ru(PPh3)2(Hsap)2] and [Pd(PPh3)2 (Hsap)] show complicated multiplet in d 7.0–7.8 ppm region are assigned to the PPh3 protons that interfere with the aromatic (H2sap) protons resonances. The 31P NMR spectra of [Ru(PPh3)2(Hsap)2] and [Pd(PPh3)2 (Hsap)] in DMSO-d6 show two sharp singlets near d 13.95 and

15.10 ppm, suggesting the presence of two coordinated triphenyl phosphine ligands in these complexes [21,27].

3.4. Vibrational spectra The quantum mechanical ab initio/STO-3G MO method was applied to study the geometric structure of the Schiff-base H2sap. 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, 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 potential showed that the oxygen O(2)H and nitrogen HC@N centers are then accessible for interaction with the metal (without interaction 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 compared 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-aminopyrimidine Schiff-base (H2sap, Fig. 1) were examined by IR spectroscopy. The spectrum of H2sap was compared with those of the complexes. Tentative assignments of selected IR bands are reported in Table 2. The spectrum of H2sap exhibits a strong band at 1622 cm1 which is characteristic of m(HC@N) group. It is expected that coordination of the nitrogen to the metal ion would reduce the electron density in the azomethine link and thus shifted m(HC@N) to lower wave numbers [3]. In the IR spectra of the complexes, this band is shifted to the region at 1604–1615 cm1 [3,35]. The intense band at 1229 cm1 in the free H2sap has been assigned to the phenolic m(C–O) stretch. Upon complexation, this band is shifted to higher frequency, indicating the coordination of H2sap through the deprotonated phenolic (C(2)–O) [36]. These data are further supported by the disappearance of the broad band at 3387 cm1 attributed to m(O(2)H); the de-protonation occurs prior to coordination. The band at 3217 cm1 in the H2sap due to

Fig. 2. Geometric structure of the Schiff-base, H2sap.

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A.-E.F. Ouf et al. / Journal of Molecular Structure 973 (2010) 69–75 Table 2 IR spectral data of H2sap and its complexes.

a b

Compounds

IR spectra (cm1)

m(HC@N)

m(C@N)

m(C@C)

m(C–O)

m(M–O)

m(M–N)

m(M–Cl)

H2sap Cis-[WO2(Hsap)2]H2O

1622 1605

1560 1559

1575 1578

1229 1241

–

–

[Pd(Hsap)Cl(H2O)]H2O [Pd(PPh3)2(Hsap)]Cl3H2O [Ag(Hsap)(H2O)2] [Ru(PPh3)2(Hsap)2] [Ni(Hsap)2]H2O [Ni(Hsap)(AcO)(H2O)2]H2O [Co(Hsap)(AcO)(H2O)]2H2O [Cu(Hsap)Cl (H2O)]H2O

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

– 935a 910b 544 543 537 530 510 517 507 531

430 435 411 400 425 445 443 439

329

320

ms(WO2). mas(WO2).

m(O(4)H) stretch is unaffected by coordination [12]. In the free

m(P–C) and d(CCH) vibrations, respectively [21]; these bands are

H2sap, strong bands at 1575 and 1560 cm1, attributed to the non-aromatic pyrimidine m(C@C) and m(C@N) stretches, respectively [37], are not affected upon complexation. This means that H2sap acts as a N,O-mononegative bidentate ligand. In the complex [Ni(Hsap)(AcO)(H2O)2] (Fig. 3), two extra bands are observed at 1530 and 1410 cm1 assigned to mas(COO) and ms(COO) vibrations of the acetato group, respectively [38]. The separation between these two bands {D = mas(COO)  ms(COO)  120 cm1}, indicating asymmetric bidentate coordination of the carboxylic group. In the IR spectra of [Pd(PPh3)2(Hsap)]Cl and [Ru(PPh3)2(Hsap)2] (Fig. 4), the presence of coordinated PPh3 groups is manifested by the strong IR bands at 1100 and 750 cm1, attributed to the

shifted to higher compared with the free PPh3 indicating their participation in complexation. The spectrum of [WO2(Hsap)2] shows bands characteristic of the cis-WO22+ core [39]. The IR bands at 935 and 910 cm1 are assigned to the ms(WO2) and mas(WO2) stretches, respectively. The region of the complex spectra between 550 and 200 cm1 contains several weak bands; these may assign to m(M–O), m(M– N), m(M–P) and m(M–Cl) stretches, respectively [25,34].

Fig. 3. Structure of [Ni(Hsap)(AcO)(H2O)2].

OH N PPh3 O CH Ru HC

N O

The mass spectra of the complexes [Pd(Hsap)Cl(H2O)]H2O, [Pd(PPh3)2(Hsap)]Cl3H2O, [Ni(Hsap)2]H2O and [Cu(Hsap)Cl(H2O)]H2O are reported and their molecular ion peaks are in agreement with their assigned formulae. The mass spectrum of [Pd(Hsap)Cl(H2O)]H2O shows fragmentation patterns corresponding to the successive degradation of the complex. The first peak at m/e 393 with 13.2% abundance represents the molecular ion (Calcd. 391.9). The peaks at 337 and 318 correspond to [Pd(Hsap)(H2O)]+ and [Pd(Hsap)]+ fragments, respectively [23]. The mass spectrum of [Pd(PPh3)2(Hsap)]Cl3H2O shows the first signal at m/e 936 (Calcd. 933.9), in agreement with the molecular ion of the complex, [Pd(PPh3)2(Hsap)]+, with 10.13% abundance. There are signals which represent the loss of Hsap and PPh3 fragments indicating stepwise ligand loss to [Pd(PPh3)2]+ with m/e 632 (Calcd. 630.4) and [Pd(PPh3)]+ with m/e 368 (Calcd. 368.4), respectively [24,40]. The mass spectrum of [Ni(Hsap)2]H2O shows a signal at m/e 507 (Calcd. 504.7) with 11.25% abundance. The fragmentation patterns indicates the stepwise ligand loss to [Ni(Hsap)2]+ (488) and [Ni(Hsap)]+ (274) [24]. The mass spectrum of [Cu(Hsap)Cl(H2O)]H2O, shows signal at m/e 351 (Calcd. 349) with 19.35% abundance. The fragmentation patterns indicate the stepwise ligand loss to [Cu(Hsap)(H2O)]+ (297), [Cu(Hsap)]+ (277). 3.6. Thermal analysis

N N

3.5. Mass spectra

PPh3 N

N

HO Fig. 4. Structure of [Ru(Hsap)2(PPh3)2].

The thermal decomposition of the complexes, [Pd(Hsap)Cl (H2O)]H2O, [Ru(PPh3)2(Hsap)2], and [Ni(Hsap)(AcO)(H2O)2]H2O was studied using thermo-gravimetry (TG) technique. The thermogram of [Pd(Hsap)Cl(H2O)]H2O, is characterized by 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%) [12,25], coordinated water (Calcd. 4.6, Found 4.6%), C4H3N2 and 1/2Cl2 (Calcd. 29.2, Found 30.1%), C7H5O and 1/2N2 (Calcd. 30.4, Found 30.9%) fragments, respectively, leaves PdO residue at 700 °C (31.3%) [15,23]. The data for [Ru(PPh3)2(Hsap)2], show two TG inflections in the ranges 250–336 and 337–467 °C. The first

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weight loss may arise from the release of four Ph groups (C24H20) and two C4H3N2 fragments (Calcd. 44.3, Found 43.9%), and two PPh and two C7H5O and N2 fragments (Calcd. 43.1, Found 43.7%), 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 (mixed ruthenium oxide and nitride residue (18.3%)) [34]. The TG thermogram of [Ni(Hsap)(AcO)(H2O)2]H2O shows the first endothermic weight loss step between 32 and 125 °C, which corresponding to the release of crystal lattice water (Cacld. 4.7, Found 4.6%). The second decomposition step occurs between 161 and 245 °C, this weight loss is attributed the loss of coordinated water (Calcd. 9.4, Found 9.6%). The third TG inflection between 246 and 355 °C is due to the loss of C4H3N2 fragments (Calcd. 20.5, Found 19.2%). The TG inflection between 356 and 401 °C, may arise 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 C7H5O and 1/2N2 fragments (Calcd. 30.9, Found 31.0%), leaving NiO representing (Calcd. 19.4, Found 19.2%). 3.7. pH-metric measurements 3.7.1. Determination of the protonation constants of H2sap The protonation constants of H2sap were determined pH-metrically. The titrations were carried out in 20% ethanol-aqueous solution (1:4 V/V) at constant ionic strength (0.04 mol L1 KNO3) at different temperatures (20, 30, 40 ± 0.05 °C). The suitable titration curve was obtained by titrating H2sap (1  105 mol L1) against NaOH (1.6  103 mol L1). 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-hydroxysalicylidene moiety. The average number of protons associated with  A , was calculated at different pH values using Irving–RossH2sap; n otti [19]:

A ¼ Y  n

V 1 No ðV o þ V 1 ÞT L

ð1Þ

where Y = 2, is the number of dissociable protons present in H2sap, V1 the volume of NaOH added, Vo the initial volume (25 ml), Tl the concentration of H2sap in the initial volume (1  105 mol L1) and No the concentration of NaOH (1.6  103 mol L1).  A against pH shows the maximum n  A value to be 2, The plot of n indicating that H2sap has two dissociable hydroxy protons; O(2)H and O(4)H. The de-protonation constants, (pK a1 and pK a2 ) are  A –pH rela A ¼ 1:5 and 0.5 from n determined by interpolation at n tion, and were calculated from both the half and least square methods. 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 constants of H2sap. The pK a1 and pK a2 values decrease with increasing temperature, i.e., the acidity of H2sap increases as the temperature increase [42,43]. DG values for the dissociation of H2sap were Table 3 The protonation constant of H2sap and the formation constants of its complexes. Ligand

293 °C

303 °C

313 °C

found to be 48.37 and 57.35 kJ mol1, respectively at 20 ± 0.05 °C. The positive values indicate the non-spontaneous dissociation character of H2sap [44]. 3.7.2. Determination of the stability constants of the complexes The stability constants of some metal ions {Co(II), Ni(II), Cu(II), Pd(II) and Ag(I)} with H2sap were determined pH-metrically at 20 ± 0.05 °C and 0.04 mol L1 ionic strength in 20% ethanolaqueous solution (1:4 V/V) solutions. The titration curves were found below and well separated from that of the free H2sap confirming that the complex formation with liberation of protons [43]. The formation curves of the complexes were obtained from  – pL plots, where n  is the average number of ligand attached the n  values were per metal ion and pL the free ligand exponent. The n calculated from,

¼ n

V 2  V 1 ÞNo ðV o þ V 1 ÞnA T M

ð2Þ

where V2 is the volume of NaOH required to reach the desired pH in the complex solution and TM is the initial concentration of the metal ion [Mn+] in the solution. The pL values were calculated from

pL ¼ log

ð1 þ K1 ½Hþ  ðV 2 þ V o ÞÞ  T M TL  n Vo

ð3Þ

 – pL plots were obtained for solution containing 1:10 meThe n tal to H2sap ratio to ensure the formation of the most stable species [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., H2sap behaves as monoprotic species via O(2)H. The stability constants of the investigated systems have been evaluated by using both half and least square methods [19]. It is clear that log k1 is higher than log K2 for the same complex. This can be explained by assuming the steric hindrance for the linking of the second species of H2sap to the central metal ion [38]. The order of stability constants decreased in the order Cu(II) > Ni(II) > Co(II) for the first row transition elements in accordance with Irving and Williams order [45,46]. The values of the free energy of formation (DG) were calculated using

DG ¼ 2:303RT log b ¼ 2:303RTðlog K1 þ log K2 Þ

ð4Þ

The negative values of DG indicate the spontaneous complex formation [43,44]. Also, it has been reported that the higher the negative value of DG, 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 H2sap} were calculated at 30 and 40 ± 0.05 °C. The stability constants and the thermodynamic functions data are reported in Tables 4 and 5, respectively. The decrease in the stability constants (log k1 and log K2) values {Ag(I) and Pd(II)} with increasing the temperature suggests that these complex formation are exothermic [42,43,47]. In case of Co(II), Ni(II) and Cu(II), the complex formation is endothermic [47]. The negative values of DH indicate that the complexation processes are

Table 4 Metal–H2sap complexes stability constant at 20 ± 0.05 °C.

pKa1

pKa2

pK1

pK2

pK1

pK2

Metal ions

log k1

log k2

log ba

D G b

H2sap Complexes

10.2 log K1

8.5 log K2

9.94 log K1

8.32 log K2

9.71 log K1

8.12 log K2

Co(II) Ni(II) Cu(II) Pd(II) Ag(I)

4.0 4.44 5.53 6.37 7.16

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

Co(II) Ni(II) Cu(II) Pd(II) Ag(I)

4.0 4.44 5.53 6.37 7.16

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

a b

Overall formation constant. KJ mol1.

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A.-E.F. Ouf et al. / Journal of Molecular Structure 973 (2010) 69–75 Table 5 Thermodynamic parameters of metal–H2sap complexes. Metal ions

Thermodynamic parameters

DH1

Co(II) Ni(II) Cu(II) Pd(II) Ag(I)

114.9 175.4 74.7 75.7 132.5

DH2

102.1 104.0 89.2 76.6 134.0

DS1

110.9 132.3 128.3 91.9 95.7

DS2

101.5 101.5 103.8 74.7 86.2

exothermic in nature and favorable at low temperature for both Ag(I) and Pd(II) complexes while the positive values indicate endothermic nature and favorable at high temperature for Co(II), Ni(II) and Cu(II) complexes. These features support the production of solid 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 temperatures, as reported in the experimental section. The positive values of DS indicate that the disorder were increased more rapidly than the increase in the order taking place in chelation; i.e., the order of arrangement of the solvent around H2sap and the metal ions are lost upon complexation [47]. In all cases DG values are negative, which means that the complexation processes are spontaneous. In comparing DG values for Co(II), Ni(II) and Cu(II), i.e., the first row, it is found that the value is more negative for Cu(II), suggesting high tendency of interaction between Cu(II) and H2sap [48]. Acknowledgement We wish to thank Prof. W.P. Griffith (Chemistry Department, Imperial College of London) for the language corrections. References [1] A. El-Hendawy, E.G. El-Kourashy, M. Shanab, Polyhedron 11 (1992) 523. [2] N. Dharmaraj, P. Viswanalhamurthi, K. Natarajan, Transition Metal Chem. 26 (2001) 105. [3] T. Thangadurai, S. Ihm, J. Ind. Eng. Chem. 9 (2003) 563. [4] T. Thangadurai, S. Ihm, J. Ind. Eng. Chem. 9 (2003) 569. [5] S. Mostafa, S. Ikeda, B. Ohtani, J. Mol. Cat. A 225 (2005) 181. [6] M. Khalifa, A. Hassaan, J. Chem. Soc. Pak. 18 (1996) 115. [7] H. Ashassi-Sorkhabi, B. Shaabani, D. Seifzaden, Electrochim. Acta 50 (2005) 3446. [8] R. Jones, D. Summerville, F. Basolo, Chem. Rev. 79 (1979) 139. [9] J. Margerum, L. Miller, Photochromism, Wiley Interscience, New York, 1971. p. 569. [10] O. Sattari, E. Alipour, S. Shirani, J. Amighian, J. Inorg. Biochem. 45 (1992) 115. [11] N. Lee, J. Byun, T. Oh, Bull. Korean Chem. Soc. 26 (2005) 454. [12] Sh. Sallam, M. Ayad, J. Korean Chem. Soc. 47 (2003) 199. [13] V. Leovac, A. Petrovic, Transition Metal Chem. 8 (1983) 337.

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DG2

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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

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