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Synthesis, Characterization and Biological Activities of a Bidentate Schiff Base Ligand: N,N'-Bis(1-phenylethylidene)ethane-1,2-diamine and its Transition Metals (II) Complexes.

Byline: SHOUKAT PARVEZ, ITRAT ANIS, NIGHAT AFZA, MUHAMMAD ASLAM, MUHAMMAD TAHIR HUSSAIN, AJAZ HUSSAIN, TAUQIR ALI SHERAZI, MUHAMMAD RAZA SHAH, MUHAMMAD KHALID AND TANVEER HUSSAIN BOKHARI

Summary: Schiff base ligand: N,N'-bis(1-phenylethylidene)ethane-1,2-diamine (L), was derived from acetophenone and ethylenediamine by condensation and its complexes (1-5) were prepared with Pb2+, Ni2+, Co2+, Cu2+ and Cd2+ metal ions. Their structures were characterized by FAB-MS, IR spectra, elemental analyses and molar conductance. The octahedral geometry of the complexes was proposed by electronic spectra and magnetic moment data. The conductivity data showed that the complexes have non-electrolytic nature. The complexes (1-5) have higher in vitro antimicrobial activity than the Schiff base ligand (L). In the nuclease activity, the complexes cleave DNA as compared to control DNA in the presence of H2O2.

Keywords: Schiff base; Transition metal complexes; Biological activities; Nuclease activity.

Introduction

Schiff bases are imine ((greater then)C=N-) group containing compounds which are derived from primary amines and carbonyl compounds (aldehydes and ketones) through condensation catalyzed by acid or base and can coordinate to metal ions via nitrogen, have been extensively studied [1-3]. Schiff bases having other donor atoms (O, N, S) act as chelating agents and are important ligands for designing metal complexes [4]. These complexes make the ligands more effective catalysts in various reactions like oxidation, reduction and other transformations and they also show biological activity. The literature reveals that complexation of the ligands with various metals ions results in an increase biological activity [5-7]. Such complexes have shown diversified activities such as antimicrobial, antituberculosis, anti- inflammatory, anticonvulsant, antitumor and anti- HIV activities [8-13].

The transition metal ions used for complexation also have remarkable roles in biological systems such as functions of many enzymes are metal ions dependent like zinc in dehydrootrotase [9], copper in the synthesis of hemoglobin [10] and cobalt in vitamin B12 and its trace amount is necessary for hemoglobin [11] and has synergic effect for penicillin. The interface between metal ions and active ligands provide key route for designing metal-based antifungal, antibacterial, anticancer, etc. therapies. Therefore, this ethnopharmacological thought and attractive biological activities of Schiff bases and their metals complexes prompted us to synthesize Schiff base and study their biological activities via complexation with metal ions.

We herein report the synthesis and characterization of a Schiff base ligand, N,N'-bis(1- phenylethylidene)ethane-1,2-diamine and its complexes with Pb2+, Ni2+, Co2+, Cu2+ and Cd2+ ions along with their antimicrobial and DNA cleavage activities.

Results and Discussion

The Schiff base ligand, N,N'-bis(1- phenylethylidene)ethane-1,2-diamine (L), was obtained as a white transparent crystalline solid, soluble in water and common organic solvents except ether (Scheme-1). The mechanism of the formation of this compound may be a variation of the theme of neuclophilic addition to the carbonyl group. In this case, the amine reacts with the carbonyl compound to give an unstable addition compound called carbinolamine which loses a water molecule by acid catalyzed pathwayThe dehydration of the carbinolamine is the rate-determining step of Schiff base formation (Scheme-2). FAB (+ve) mass spectrum showed the [M+H]+ peak at m/z 265.3 confirming the molecular formula C18H20N2. The complexes (1-5) of the ligand were obtained as crystalline solids and their decomposition temperatures indicate the fair stability of these compounds. All the complexes (1-5) are soluble in dimethyl formamide (DMF) and dimethyl sulfoxide (DMSO) but insoluble in water and other common organic solvents.

FAB Mass Spectra

The FAB (+ve) mass spectra of the Schiff base ligand and its complexes were recorded and used to compare their stoichiometric composition. The Schiff base ligand shows a molecular ion peak [M+H]+ at m/z 265.3, confirms the theoretical molecular weight i.e., 264.1. The molecular ion peaks [M+H]+ for the complexes, supported the stoichiometry of metal to ligand ratio to be 1:2. It is also in good agreement with the elemental analysis values.

Infrared Spectra

Binding mode of the Schiff base ligand to metal ions was studied by comparing the IR spectrum of the free ligand with the spectra of the metal complexes. The absence of characteristic bands of the amino group, n(-NH2) of free ethylenediamine and carbonyl group, n(C=O) of acetophenone in the IR spectrum of the complexes confirms the complete condensation [12]. The band at 1621 cm-1 due to the azomethine group of the Schiff base ligand undergoes a shift to lower frequency (1551-1594 cm-1) after complexation, indicates that coordination occurred through the nitrogen of the -C=N- groups to metal atom and this can be explained by the donation of electrons from nitrogen to the empty d-orbitals of the metal atom [13-17]. This is further confirmed by the appearance of a medium intensity band at 419-498 cm-1 assignable to n(M-N) [18, 19] in all complexes. The bands in the region 734-763 cm-1 may be assigned to n(C-H) out of plane bending of the aromatic ring.

The bands in the region of 1401-1490 cm-1 and 1015-1093 cm-1 are assigned to n(C=C) aromatic stretching vibrations of the ring [20]. The absorption bands at 1636-1642 cm-1 in acetate complexes (i.e., Pb (II) and Ni (II) complexes) are assigned to the n(COO-)as asymmetric stretching vibrations of the acetate ion and also at 1263-1278 cm-1 in acetate complexes can be assigned to the n(COO-)s symmetric stretching vibrations of the acetate ion. This indicates that the acetate group may be coordinated with the central metal ion in a unidentate fashion (Scheme-3). The band present at 257 cm-1 in Co (II) complex and at 284 cm-1 in Cu (II) complex may be assigned to n(M-Cl) vibration [13, 21-23]. There is no precipitation of the chloride ions upon the addition of Ag+ and Pb2+ solutions, confirmed the presence of the chloride ions inside the coordination sphere [24].

The weak bands present in the region 2884-2949 cm-1 may be assigned to n(C- H) stretching vibrations of the methyl group of the ligand and complexes [25]. In Cd (II) complex, a broad band at 3600 cm-1 is assigned to n(O-H) stretching vibration, a feature indicating the presence of coordinated water [26, 27].

Table-1: Electronic and magnetic moment data of Schiff base ligand and its complexes.

Compound###Electronic spectra wavelength###Magnetic moment###Assignment###Geometry

###(cm-1)###(B.M.)

1###9300, 13400, 19800###5.02###4T1g(F) - 4T2g(F), 4T1g(F) - 4A2g(F), 4T1g(F), 4T1g(P)###Octahedral

2###8450,10100, 19500###2.02###3A2g(F) - 3T2g(F), 3A2g(F) - 3T1g(F), 3A2g(F) - 3T1g(P)###Octahedral

3###14500, 22000###3.98###4T1g(F) - 4T1g(P), 4T1g(F) - 4A2g(F)###Octahedral

4###16350, 19200###1.95###2B1g - 2B2g, 2B1g - 2E1g###Octahedral

5###9450, 14000, 20000###4.64###4T1g(F) - 4T2g(F), 4T1g(F) - 4A2g(F), 4T1g(F) - 4T1g(P)###Octahedral###

Electronic Spectra and Magnetic Moments

The electronic spectra were used for assigning the stereochemistry of the metal ions in the complexes based on position and number of d-d transitions (Table-1). There are three bands in the electronic spectrum of Pb (II) complex at 9300, 13400, and 19800 cm-1 assigned to 4T1g(F) - 4T2g(F), 4T1g(F) - 4A2g(F) and 4T1g(F) - 4T1g(P) transitions, confirming an octahedral geometry. The magnetic moment of the complex was 5.02 B.M. which is in agreement with the reported octahedral geometry. The electronic spectrum of Ni (II) complex showed three bands at 8450, 10100 and 19500 cm-1 attributed to 3A2g(F) - 3T2g(F), 3A2g(F) - 3T1g(F) and 3A2g(F) - 3T1g(P) transitions for octahedral geometry which was further confirmed by its magnetic moment value.

The electronic spectrum of Co (II) exhibits two bands at 14500 and 22000 cm-1 which may be assigned to the transitions 4T1g(F) - 4T1g(P) and 4T1g(F) - 4A2g(F), corresponding to an octahedral geometry, also supported by its magnetic moment value (3.98 B.M) [28, 29]. A broad band at 19200 cm-1 with a shoulder band at 16350 cm-1 in the electronic spectrum of Cu (II) complex, assigned to 2B1g - 2E1g and 2B1g - 2B2g transitions, considering a distorted octahedral geometry which was further confirmed by its magnetic moment value (1.95 B.M) [30-34]. The Cd (II) complex displays three bands at 9450, 14000 and 20000 cm-1, corresponding to transitions 4T1g(F) - 4T2g(F), 4T1g(F) - 4A2g(F) and 4T1g(F) - 4T1g(P) and suggesting octahedral geometry. The magnetic moment value of Cd (II) complex is 4.64 which further confirmed octahedral geometry [32, 33].

Molar Conductance

The molar conductance values of complexes are found in the range of 6.31-12.53 ohm-1 cm2 mol-1 indicating the non-electrolytic nature of all these complexes [35, 36].

Proposed Structures of the Complexes (1-5)

From the physical and spectral data of the Schiff base ligand and its complexes discussed above, one can assume that the metal ions are bonded to the Schiff base ligand via the nitrogen. The proposed structures of the complexes are illustrated in (Scheme-3).

Biological Activities

Antimicrobial Activity

The in vitro antimicrobial (antibacterial and antifungal) activities of the ligand (L) and its complexes (1-5) were tested against the bacterial species: Staphylococcus aureus, Escherichia coli and Bacillus subtilis, and the fungal species: Aspergillus flavus, Fusarium solani and Trichophyton longifusus. The results indicated that the ligand and all the complexes are more active than the standards (Table-2), which can be explained on the basis of Overtone's concept and chelation theory [37].

Nuclease Activity

In the present work, the CT DNA gel electrophoresis experiment was conducted in the presence of H2O2 as an oxidant at 35 degC using synthesized complexes. At very low concentration, the results show that complexes exhibit nuclease activity in the presence of H2O2. Even on longer exposure time, there was no significant cleavage of CT DNA in the control experiment using DNA alone. From the experimental results, the complexes cleave DNA in contrast to control DNA in the presence of H2O2 (Fig. 1). Probably this may be due to the formation of redox couple of the metal ions and its behavior [38].

Experimental

General Experimental Procedures

All reagents and solvents were used as purchased from Merck. The metal salts were used as metal (II) acetate or chloride in hydrated form [Pb(CH3COO)2.3H2O, Ni(CH3COO)2.4H2O, CuCl2.2H2O, Cd(CH3COO)2.2H2O]. The calf-thymus DNA (CT DNA) was purchased from Sigma. TLC was performed on pre-coated silica gel G-25-UV254 plates and detection at 254 and 366 nm or by spraying with ceric sulfate in 10 % H2SO4 (heating). The weighing was carried out on an electric Mettler Toledo balance. The IR spectra were recorded on Thermo Nicolet Avatar 320 FTIR spectrometer by using KBr pellet. Melting points were recorded on a Gallenkamp apparatus and are uncorrected. Elemental analyses were performed on Perkin Elmer 2400 Series II elemental analyzer while the molar conductance was recorded on Jenway 4010 in DMSO solution (1 x 10-3 M) at room temperature. The electronic spectra were recorded on Specord 200 UV- Vis spectrophotometer whereas magnetic moments were measured on Guy-type magnetic balance (Hertz SG8SHJ).

The FAB (Fast atomic bombardment) mass spectra were recorded on JEOL SX102/DA-6000 mass spectrometer using glycerol as matrix and ions are given in m/z (%).

Table-2: In vitro antimicrobial activities of the compounds and standard reagents.

Compound###Antibacterial activity (MIC,(mu)g/mL)###Antifungal activity (MIC, (mu)g/mL)

###S. aureus###E. coli###B. subtilis###A. flavus###F. solani###T. longifusus

L###19###20###22###21###19###20

1###20###24###26###23###21###24

2###24###21###20###25###24###25

3###26###25###19###23###21###24

4###24###25###23###22###25###25

5###25###19###25###25###22###21

Imipenem*###30###28###30###-###-###-

Amphotericin B* -###-###-###27###24###26

*Standards

Synthesis

Procedure for the Synthesis of Schiff Base Ligand (L)

The solution of acetophenone (2 moles in 30 mL MeOH) was refluxed with the solution of ethylenediamine (1 mole in 30 mL MeOH) with stirring in the presence of 3-4 drops of conc. H2SO4 for 4 hours at 100 degC on water bath. The reaction mixture was concentrated to its one-third volume by rotary evaporator, then added n-hexane and cooled on ice-water. The resulting mixture was kept for overnight at room temperature and white transparent crystals were obtained. The crystals were filtered off, washed with cold methanol, dried, and recrystallized with methanol and then dried over anhydrous CaCl2 under reduced pressure to obtain the pure product. Purity was checked by TLC (Scheme-1).

N,N'-Bis(1-phenylethylidene)ethane-1,2-diamine (L)

White transparent crystals; yield: 79 %; m.p.: 109 degC; IR (KBr) umax cm-1: 2899 (C-H), 1621 (C=N), 1490, 1081, 747 (benzene ring); FAB-MS (+ve) m/z: 265.3 [M+H]+ (calcd. 264.1 for C18H20N2); Elemental analysis (%): C 81.87, H 7.58, N 10.62 (clacd. C 81.78, H 7.63, N 10.60)

General Procedure for the Synthesis of Metal

Complexes (1-5)

To a hot stirring methanolic solution of ligand (L) was slowly added the methanolic solution of the respective metal salts [(Pb(CH3COO)2.3H2O, Ni(CH3COO)2.4H2O, CoCl2.6H2O, CuCl2.2H2O, Cd(CH3COO)2.2H2O] in 2:1 (L:M) molar ratio. The mixture was further refluxed for 45 min. The pH of the reaction mixture was maintained by drop wise addition of 1M NaOH in MeOH for the complex formation and the precipitation of complexes except Ni (II) complex. In case of Ni (II), the reaction mixture was further refluxed for 30 min and the mixture was concentrated half of its volume. The precipitates of the complexes were filtered off, washed with cold methanol but the Ni complex was washed with ether and all complexes dried over anhydrous CaCl2 under vacuum.

[PbL2(OAc)2] (1)

Pale golden solid; yield: 71 %; m.p.: 197 degC (decomp.); IR (KBr) umax cm-1: 2888 (C-H), 1637 (COO)as, 1551 (C=N), 1438, 1036, 739 (benzene ring), 1278 (COO)s, 498 (Pb-N);), Molar conductance (DMSO) lm (Ohm-1cm2mol-1): 8.12; FAB-MS (+ve) m/z: 855.1 [M+H]+ (calcd. 854.3 for C40H46N4O4Pb); Elemental analysis (%): C 56.21, H 5.47, N 6.51, Pb 24.29 (clacd. C 56.26, H 5.43, N 6.56, Pb 24.26)

[NiL2(OAc)2] (2)

Misty rose solid; yield: 78 %; m.p.; 214 degC (decomp.); IR (KBr) umax cm-1: 2934 (C-H), 1636 (COO)as, 1559 (C=N), 1401, 1093, 734 (benzene ring), 1275 (COO)s, 462 ((Ni-N);), Molar conductance (DMSO) lm (Ohm-1cm2mol-1): 10.33; FAB-MS (+ve) m/z: 705.0 [M+H]+ (calcd. 704.2 for C40H46N4O4Ni); Elemental analysis (%): C 68.13, H 6.51, N 7.99, Ni 8.38 (clacd. C 68.10, H 6.57, N 7.94, Ni 8.32)

[CoL2Cl2] (3)

Chocolate solid; yield: 59 %; m.p.: 216 degC (decomp.); IR (KBr) umax cm-1: 2884 (C-H), 1594 (C=N), 1447, 1015, 763 (benzene ring), 257 (Co-Cl), 459 (Co-N), Molar conductance (DMSO) lm (Ohm-1cm2mol-1): 6.31; FAB-MS (+ve) m/z: 658.8 [M+H]+ (calcd. 657.2 for C36H40Cl2N4Co); Elemental analysis (%): C 65.61, H 6.10, N 8.61, Co 8.87 (clacd. C 65.66, H 6.12, N 8.51, Co 8.95)

[CuL2Cl2] (4)

Blue stone solid; yield: 68 %; m.p.: 202 degC (decomp.); IR (KBr) umax cm-1: 2949 (C-H), 1572 (C=N), 1459, 1043, 740 (benzene ring), 284 (Cu-Cl), 473 (Cu-N), Molar conductance (DMSO) lm (Ohm-1cm2mol-1): 12.07; FAB-MS (+ve) m/z: 662.3 [M+H]+ (calcd. 661.1 for C36H40Cl2N4Cu); Elemental analysis (%): C 65.26, H 6.12, N 8.47, Cu 9.61 (clacd. C 65.20, H 6.08, N 8.45, Cu 9.58)

[CdL2(H2O)2] (5)

Antique white solid; yield: 64 %; m.p.: 179 degC (decomp.); IR (KBr) umax cm-1: 3600 (O-H), 2943 (C-H), 1589 (C=N), 1483, 1056, 753 (benzene ring), 419 (Cd-N), Molar conductance (DMSO) lm (Ohm-1cm2mol-1): 12.53; FAB-MS (+ve) m/z: 679.1 [M+H]+ (calcd. 678.2 for C36H44O2N4Cd); Elemental analysis (%): C 63.90, H 6.51, N 8.25, Cd 16.52 (clacd. C 63.85, H 6.55, N 8.27, Cd 16.60)

Biological Activities

Antimicrobial Activity

The antibacterial and antifungal activities of the ligand (L) and its complexes (1-5) were tested in vitro against the bacterial species: Staphylococcus aureus, Escherichia coli and Bacillus subtilis using imipenem as a standard drug and the fungal species: Aspergillus flavus, Fusarium solani and Trichophyton longifusus using amphotericin B as a standard drug by the disc diffusion method [39]. The pathogens were grown on nutrient agar medium in petri plates. The ligand and all complexes were prepared in DMSO and soaked in a filter paper disc of 1 mm thickness and 5 mm diameter. The discs were placed on the previously seeded plates and incubated at 37 degC and the inhibition zone developed around each disc was measured after 24 hours for antibacterial and 72 hours for antifungal activities. The minimum inhibitory concentration (MIC) was determined by serial dilution method.

Nuclease Activity

Gel electrophoresis was used for the cleavage of DNA. The cleavage was conducted using calf-thymus DNA (CT DNA) of the ligand (L) and its complexes (1-5) in the presence of H2O2 (acting as an oxidant). First the reaction mixture was incubated at 35 degC for 2 hours as follows: CT DNA 30 (mu)M, 50 (mu)M complex, 50 (mu)M H2O2 in 50 mM tris-HCl buffer (pH = 7.2) and then electrophoresed the samples for 2 hours at 50 V on 1% agarose gel using tris-acetic acid EDTA buffer (pH = 8.3). After electrophoresis, the gel was stained using 1 (mu)g/ml ethidiumbromide and documented under UV light [40].

Conclusion

A bidentate Schiff base ligand was derived from acetophenone and ethylenediamine by condensation and it was characterized by various spectroscopic and analytical techniques. The complexes of the ligand (1-5) were prepared with Pb2+, Ni2+, Co2+, Cu2+ and Cd2+ metal ions. Elemental analyses and FAB (+ve) mass spectra showed the 2:1 (L:M) ratios and conductance data exhibited the non- electrolytic nature of the complexes. Electronic and magnetic moments revealed the octahedral geometry of all the complexes. The complexes complexes which showed higher antimicrobial activity than the free ligand. The nuclease activity revealed that the complexes cleave DNA as compared to control DNA in the presence of H2O2.

Acknowledgments

The authors express their gratitude to Pakistan Council of Scientific and Industrial Research Laboratories Complex, Karachi and Department of Chemistry, University of Karachi, Karachi for providing research facilities.

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1Pakistan Council of Scientific and Industrial Research Head Office, 1-Constitution Avenue, Sector G-5/2, Islamabad, Pakistan., 2Department of Chemistry, University of Karachi, Karachi-75270, Pakistan., 3Pharmaceutical Research Centre, Pakistan Council of Scientific and Industrial Research Laboratories Complex, Karachi-75280, Pakistan., 4Department of Applied Sciences, National Textile University, Faisalabad-37610, Pakistan., 5Department of Chemistry, Government College University, Faisalabad-38040, Pakistan., 6Department of Chemistry, COMSATS Institute of Information Technology, Abbottabad-22060, Pakistan., 7International Center for Chemical and Biological Sciences, H.E.J. Research Institute of Chemistry University of Karachi, Karachi-75270 Pakistan., 8Institute of Chemistry, University of Sao Paulo, Sao Paulo-05513-970, Brazil., hocprc@yahoo.com*
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Author:Parvez, Shoukat; Anis, Itrat; Afza, Nighat; Aslam, Muhammad; Hussain, Muhammad Tahir; Hussain, Ajaz;
Publication:Journal of the Chemical Society of Pakistan
Article Type:Report
Geographic Code:9PAKI
Date:Dec 31, 2012
Words:4636
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