Characterization of Ni(II) and Cd(II) Metal Complexes using Schiff Base Ligand derived from 2-Thiophenecarboxyldehyde and 2-Aminothiophenol

Schiff base ligand derived from condensation of 2-aminothiophenol and 2-thiophenecarboxyldehyde was synthesized and used for the preparation of Ni(II) and Cd(II) complexes. The synthesized ligand and complexes were analyzed by decomposition temperature, solubility, magnetic susceptibility, molar conductance and infrared spectra. The decomposition temperatures of the complexes are 212 and 221°C. Molar conductance values are 16.12 and 12.60 ohm -1 cm 2 mol -1 respectively. New bands appeared in the IR spectra of the complexes in the range of 519 475 cm -1 and 462 448 cm -1 which indicate υ (M N) and υ (M S) vibrations respectively. Magnetic susceptibility measurement indicated that Ni (II) complex is paramagnetic whileCd(II) complex is diamagnetic. The solubility test revealed that all complexes and ligand are soluble in DMSO. The analytical data show the formation of 2:1 metal to ligand ratio for all complexes and suggested the formula [ML2].nH2O. The ligand and metal chelates have been studied for microbial activity using well diffusion method against selected bacteria and fungi. The results signify that Ni(II) and Cd(II) metal complexes inhibit more compared with Schiff base ligand against the same test organisms.

Where R and R′ are aryl, alkyl, cycloalkyl or heterocyclic groups, which may be variously substituted. The bonding ability of ligands depends on the nature of atoms (which act as co-ordination sites), steric factor and electro negativity. If the ligand have functional group such as -OH or -COOH near the site of condensation, a very stable five or six membered chelate ring can be formed. Tridentate Schiff base ligands form comparatively stable complexes. Coordination behaviour of Schiff base ligand with transition metals play a significant role in biological and pharmacological activities, especially, transition metal complexes derived from amino acid Schiff bases. The formation of a Schiff base from an aldehydes or ketonesis a reversible reaction and generally takes place under acid or base catalysis, or upon heating.
The Schiff bases form metal complexes with d-block metals have been known to act as highly efficient catalysts in various syntheses and other useful reactions. Many Schiff base complexes of ruthenium and palladium are used as catalyst in the syntheses of quality polymers. Imine complexes are used in the treatment of diabetes and AIDS. They aid in understanding the structure of biomolecules and biological processes taking place in the living organisms. They are used in the treatment of cancer and for immobilization of enzymes.
Prepared metal (II) Schiff base complexes of Cu, Ni, Fe and Zn from salicylaldehyde and o-amino benzoic acid, the complexes were characterized by IR and NMR spectral analysis. The Schiff base and its complexes were tested for their antibacterial activity against Pseudomonas aeruginosa, Proteus vulgarus, Proteus mirabilis, Klebsiella pneumonia and Staphylococcus aureus, the results indicated that metal (II) Schiff base complexes are biologically more active than the Schiff base ligand against the same test organisms. The synthesis of Ni(II), Co(II) Cu(II) and Zn(II) complexes with Schiff base (3ethoxy salicylidene amino benzoic acid) in alcoholic medium, prepared from 3-ethoxy salicylaldehyde and 2-amino benzoic acid. The complexes were non-electrolytes in dimethyl sulfoxide solvent (DMSO). The structure of the Schiff-base ligand and its metal complexes were confirmed by various spectroscopic studies like IR, UV-VIS, 1 H NMR, 13 C NMR, ESI-mass spectra, elemental analysis, molar conductance, thermogravimetric studies and magnetic susceptibility measurements and six-coordinated geometry was assigned to these complexes.

Materials and Methods
Reagents are of analytical grade purity and were obtained from Sigma Aldrich chemical limited. The glassware used were washed thoroughly with detergent, rinsed with distilled water and dried in an electrical oven. Melting point and decomposition temperature were determined using Gallenkamp melting point apparatus. Molar conductivity measurement was carried out using Jenway conductivity meter model 4010, while magnetic susceptibility measurement was done on MBS MKI magnetic susceptibility balance at 25°C. IR spectral analysis was carried out using FTIR Cary 630 (Agilent Technology) model in the range of 4000 -400cm -1 . Bacterial and fungal isolates were obtained and identified at the Department of Microbiology, Kano University of Science and Technology, Wudil.

Preparation of Schiff base
The Schiff base were prepared by mixing a solution of 2-aminothiophenol (2.50g, 0.1 mole) in 25ml of ethanol with 2-thiophenecarboxyldehyde (2.24g, 0.1mole) in the same solvent. The reaction mixture was left under refluxed for 2 hours. The product formed was separated by filtration, purified by crystallization from ethanol, and then dried in a desiccator over anhydrous calcium chloride.

Preparation of the metal complexes
The Ni (II) and Cd (II) complex of Schiff base was prepared by the addition of a solution of appropriate metal chloride (1mmol) in an ethanol water-mixture (1:1, 25ml) to the solution of Schiff base (0.219g, 2mmol) in the same solvent (25ml). The resulting mixture was stirred under reflux for 1hour upon the complex precipitated and collected by filtration and washed with a 1:1 ethanol-water mixture.

Solubility test
The solubility test of the Schiff base, Ni (II) and Cd(II) metal complexes was carried out in some solvents, ethanol, methanol, dimethylsulphoxide (DMSO), dimethylformamide (DMF), n-hexane, diethyl ether, chloroform and distilled water. Small amount of Schiff base and metal(II) complexes were each added into the test tube followed by addition of the solvent. Their solubility was observed after shaking the test tube.

Melting point/decomposition temperature
The melting point of the Schiff base and the decomposition temperature of the Ni(II) and Cd(II) metal complexes were carried by taking small amount of each into a capillary tube. The tube was inserted into the Gallenkamp apparatus, the temperature at which the ligand melt and that which the complexes decompose were taken and recorded.

Conductivity measurement
0.003M solutions of the Ni(II) and Cd(II) complexes were prepared in DMSO and the molar conductance was determined by Janway, 4010 conductivity meter. All measurements were carried out at room temperature; the molar conductance value was obtained from the relation where C = Molar concentrations K = specific conductance.

Magnetic susceptibility measurement
The magnetic susceptibility of the Ni(II) and Cd(II) metal complexes was obtained from magnetic susceptibility balance. Each separate sample of the metal complex was placed into a capillary tube and then inserted into the magnetic susceptibility balance, the readings was then recorded. The gram magnetic moment is calculated using the relation The molar magnetic moment is calculated as The effective magnetic moment (B.M) is given by µ eff = 2.828 (Xm T) ½ T = Absolute temperature (25°C).

Determination of percentage of water of crystallization in the complexes
About 0.2g of each of the Ni(II) and Cd(II) metal complexes was measured into a watch glass of known weight and placed in an oven at 110°C until a constant weight was obtained. The percentage composition of water in the complex was calculated using the below formula: . 100 × taken complex the of Weight lost Weight

Digestion of metal complex
About 0.2g of each of the Ni(II) and Cd(II) metal(II) complex was placed in a 100cm 3 beaker containing 25cm 3 of distilled water to which 5cm 3 of concentrated acid was added and then heated to about dryness. The contents in the beaker were allowed to cool to room temperature and 25cm 3 of distilled water was added and the mixture was stirred before the filtrate was collected which contains the metal ions.

Estimation of nickel in nickel(II) complex
Distilled water was added to the filtrate of digested nickel(II) complex to 97cm 3 . The filtrate was heated to 70-80°C and alcoholic solution of 1% dimethylglyoxime (DMG) was added in slight excess, followed by immediate drop wise addition of dilute ammonia solution. The content was allowed to stand on steam bath for 30 minutes to ensure complete precipitation, before allow to cool and the precipitate separated, washed, dried at 110°C and weight as [Ni(C 4 H 7 O 2 N 2 ) 2 ].

Estimation of cadmium in cadmium(II) complex
Distilled water was added to the filtrate obtained from the digested cadmium(II) complex to 75cm 3 , 1g of ammonium thiocyanate and 2cm 3 of pyridine were added to the filtrate. The mixture was vigorously stirred until crystal separated out, which were allowed to stand for 15 minutes before washing with a solution made by dissolving 3.9g of potassium thiocyanate and 2.7g mercuric chloride in 100cm 3 of distilled water. The precipitate was dried at 110 0 C and weight as [Cd(C 5 H 5 N) 2 ] (SCN) 2 .

Determination of the metal to ligand ratio in the complex compounds using Job's method of continuous variation
The number of coordinated Schiff base ligand in the metal ion were determined by Job's method in which 3 millimolar solution of the ligand and the metal(II) chloride were separately prepared. The following ligand to metal salt (mL); 1:15, 3:13, 5:11, 7:9, 9:7, 11:5, 13:3, 15:1 were taken from the ligand solution and each of the metal complexes. A total volume of 16ml was maintained (in the above order) throughout the process and mole fraction of the ligand was calculated in each mixture. The solutions of the metal chloride (blank) were scanned to obtained wave length of maximum absorption (λ max ) for each metal ion. The spectrophotometer was set at λ max before taking the absorbance value. A plot of absorbance against mole fraction of the ligand, the number of coordinated ligand was determined using the following relation: where n = number of coordinated ligand at maximum absorbance i X = mole fraction at maximum absorbance.

Determination of empirical formular
The composition of each complex was determined from the known percentage of the metal ion and water content in the complex. The percentage composition of the ligand was obtained by adding percentage composition of the metal and water in the complex and subtracted from 100 to get that of ligand. The empirical formula of each of the complex was calculated using the percentage composition of the species involved.

Anti-bacterial studies
The antibacterial activity of the Schiff base ligand and its metal complexes was carried out by using bacterial isolates of Staphylococcus aureus, streptococcus pneumoniae, and Escherichia coli. The suspension of each microorganism was smeared on the surface of the solidified Muller-Hinton Agar (MHA) already poured into petri dishes. The Schiff base and the metal complexes were separately dissolved in DMSO so as to have three distinct concentrations (60µg/disc, 30µg/disc and 15µg/disc) through serial dilution and placed on the surface of the culture media, incubated at 37°C for 24 hours. Activities were determined by measuring (mm) the diameter of the zone of inhibition and compared with a standard drug (Ciprofloxacin) (Khan et al. [13]).

Anti-fungal Studies
The antifungal activity of the Schiff base ligand and that of its metal complexes were tested against three pathogenic fungi; Candida albicans, Aspergillusflavus, and Aspergillusfumigatus, using disc diffusion method. Ketoconazole was used as standard fungicide and DMSO was used as a negative control (Khan et al. [13]). The fungal suspension was smeared on the solidified Potato Dextrose Agar (PDA) already poured into petri dishes. The Schiff base and the metal Complexes were separately dissolved in DMSO to have three different concentrations (60µg/disc, 30µg/disc and 15µg/disc) per well. They were placed on the surface of the culture media and allowed to stand at room temperature for good 48 hours. Activities were determined by measuring (mm) the diameter of the zone of inhibition and compared with the standard.

Results
Results of the physical properties, characterization and microbial activities of the prepared Schiff base ligand, Ni(II) and Cd(II) metal (II) complexes are presented in the following tables.

Discussion
The Schiff base ligand was prepared by condensation of 2-aminothiophenol and 2thiophenecarboxyldehyde to obtain yellow crystal with high yield (75%) and melting point of 128°C (Table 3.1). The metal complexes, Ni(II) and Cd(II) complexes were synthesized and found to be of various colors with percentage composition of 57% and 56%. The decomposition temperature of the metal complexes range from 213°C and 221°C showing that the decomposition temperature of the metal (II) complexes is higher than that of melting point of the ligand indicating that complexation has taken place.
The solubility test carried out on the Schiff base showed that the Schiff base was soluble in methanol, ethanol, DMSO, DMF, diethylether, and chloroform but insoluble only in water and n-hexane. However, the Ni(II) and Cd(II) metal complexes were soluble in ethanol, methanol, DMSO and DMF, but insoluble in water and n-hexane while slightly soluble in diethylether and chloroform (Table 3.2).
The infrared spectral results of the ligand shows a band at 1689 cm -1 assigned to azomethine υ (C=N) vibration, this confirms condensation between amino group of 2-288 aminothiophenol and aldehyde group of 2-thiophenecarboxyldehyde in the formation of Schiff base. The infrared spectral data of the Schiff base ligand, Ni(II) and Cd(II) metal complexes were listed in Table 3.3.  The molar conductance of Ni(II) and Cd(II) metal complex was measured in dimethylsulfoxide (DMSO). The values obtained were 16.12 and 12.60 ohm -1 cm 2 mol -1 respectively in (Table 3.4) which are relatively low, indicating the non-electrolytic nature of the metal complexes. The molar conductance ranges for non-electrolyte metal complexes in DMSO is 1-50 ohms -1 cm 2 mol -1 .
Magnetic susceptibility measurement values for Ni(II) and Cd(II) complexes at room temperature are arranged in Table 3.5. The values Ni(II) complexes indicated it is paramagnetic because it shows the present of unpaired electrons, while Cd(II) complex is diamagnetic, showing the absence of unpaired electrons.

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The metal-ligand ratio was determined by using Job's method of continuous variation (UV-Visible). The results were presented in Tables 3.9.1 -3.9.2. They revealed that the metal-ligand ratio was 1:2. Gravimetric analysis was used for the determination of the percentage of metal(II) ions in the complexes (Table 3.7). The metal (II) Schiff base complexes show variation in the co-ordination number of water molecules. Cd(II) has three molecules of water, while Ni(II) has two water molecules. The empirical formulae of the complexes were determined from known values of percentage composition of metals, Schiff base and water of crystallization. The results revealed that the metal-ligand was 1:2 for all the complexes and suggested the formula [ML 2 ].nH 2 O.  The antibacterial activity of the Schiff base and metal complexes was carried out on three bacteria isolates, (Staphylococcus aureus, Streptococcus pnemoniae and Eschericia coli) using well diffusion method by taking DMSO as solvent (Table 3.10). The result shows that both the ligand and metal complexes are found to be moderately effective against all tested bacteria, but the metal complexes exhibit higher antibacterial activity than the Schiff base and the activity increase with increase in concentration, this is probably due to chelation in the metal complexes. The Schiff base show activity against Staphylococcus aureus and Eschericia coli at all concentrations, but found to be active only at (60µg and 30µg) concentrations against Staphylococcus aureu, while the complexes of Ni (II) and Cd(II) is active at all concentrations. In Eschericia coli, Cd(II) complex is effective at (60µg and 30µg) only, while Ni(II) complex is active at (60µg) concentration only. However, in comparison to the standard drugs antibacterial activities of the Schiff base as well as the complexes is relatively low.
Antifungal studies were carried out by well diffusion technique on potato dextrose agar against three fungal isolate Aspergillusfumigatus, Aspergillusflavus and Candida albicans (Table 3.11). The result of anti-fungal screening for the Schiff base and metal complexes revealed that both the Schiff base and Cd(II) complex show an activity against Aspergillusfumigatus isolate and the activity increase with increase in concentration, while the Ni(II) complex is active only at (60µg) concentration. Also, the Schiff base and the Ni(II) complex show high activity against Aspergillusflavus at all concentrations, and the Cd(II) complex active at (60µg and 30µg) concentrations respectively. In Candida albicans, the Schiff base, Ni(II) and Cd(II) complexes shows an appreciable activity at all concentrations.

Conclusion
The Schiff base and its metal complexes of Ni(II) and Cd(II) have been synthesized and studied by various analytical techniques. Job's method of continuous variation shows that the metal-ligand ratio in all the complexes is 1:2. All the complexes are nonelectrolytes in DMSO solvent. The decomposition temperature of Ni(II) and Cd(II) complexes indicated that complexation has taken places. Based on electrical conductivity data, Ni(II) Schiff base complexes is paramagnetic while Cd(II) complex is diamagnetic. The antimicrobial studies of the Schiff base and its metal(II) complexes reveals that the metal(II) complexes show better activity when compared to that of the ligand.