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Spectrophotometric Studies of N-Benzylideneaniline Ligand and its Nickel Complex

Spectrophotometry is based on the measurement of the absorbance for monochromatic light passing through the solution containing the substance to be determined. Spectrophotometric methods can particularly be used for studying complexation equilibria and determination of the number of complexforming constituents as it is suited to the selective determination of very small concentrations of species without changing the composition of the solution. Schiff base ligand was synthesized using benzaldehyde and aniline to form a metal complex with the target analyte (Ni). The melting point and electrical conductivity of the synthesized nickel complex were 175.6C and 84x10Ώcmrespectively and the maximum absorbance at 360nm was 0.60. The complex was characterized using different spectroscopic techniques. The peak in FT-IR spectra at 1625.2 cm confirmed the (C=N) stretching vibration. The C and H NMR spectra showed 193.8 and 10.0 ppm respectively of (CH=N) which was assigned to the azomethine carbon atom and reveals a plausible structure of the complex.


INTRODUCTION
The degree of selectivity of a method is dependent on the complexity of the material or the sample being analyzed. In many cases the simultaneous determination of two or more metal ions enhances the selectivity of the method to a greater extent. Simultaneous spectrophotometric analysis eliminates wavelength independent matrix system effect as well as the analysis of two or more component systems. It also saves the sample, time and thereby enhances the speed of analysis [1,2,3]. In absorption spectroscopy, absorption measurements based upon ultraviolet light and visible radiation find application for the detection and quantitative determination of an absorbing species. Spectrophotometry is one such technique noted for its remarkable sensitivity and precision. A change in the intensity of the colour of the system with change in concentration of the system is termed as colorimetry [4]. A substance appears colored whenever it transmits or absorbs a part of a visible radiation. Absorption spectrum constitutes the optical activity of the substance. When a normal electronic structure of the substance is deformed there is either the production of the color (or) change of the color, thus when molecules containing one or more chromophores and auxochromes are subjected to irradiation, they undergo variation in electronic energy. The presence of chromophores and auxochromes in organic molecules causes deepening of color by displacing the absorption maximum towards lower wavelengths [5].
In spectrophotometric technique, when an absorbing medium is placed between the sources of a radiation, the spectroscope and the light absorbed are measured [6,7]. The plot of light absorbed on ordinates versus wavelength is a characteristic for an absorbing component, and forms the basis for qualitative analysis. The height of the ordinate in the plot due to the component under investigation at any particular wavelength is a measure of the concentration of the component and is thus useful for quantitative work [5,8]. The composition and structure of the metal chelates can be determined by various physical, chemical and spectral characteristics [9,10]. The change in colour of solution is an indication of complex formation. Coordinate complex contains coordinate linkages and they are less soluble in aqueous solutions and are more soluble in non-polar solvents. This decrease in the solubility of the compound in polar solvents indicates complex formation. Generally, ligands are weak acids or bases and the change in pH value is an indication of complex ion formation [11,12]. The absorption of light in the visible region by a metal ion depends on electron transitions within the ion and also upon ligand to metal bonding. Thus, the metal ion containing d electrons give variable colours depending on the nature of the ligand attached with the metal ion. Polarography is a powerful tool in the study of complex compounds [13,14,15]. This study focuses on synthesis of Schiff base ligand from benzaldehyde and aniline followed by synthesis of nickel (Ni) complex using the ligand as a complexing agent.
Characterization of the synthesized ligand and its Ni complex using melting point determination, electrical conductivity, nuclear magnetic resonance (NMR) and Infrared (IR) spectroscopic techniques.

Preparation of N-benzilideneaniline Ligand
Equimolar quantity of benzaldehyde 10.143 ml (1.025 mmol) and aniline 9.099 ml (1.010 mmol) were mixed with rapid stirring in a 250 ml round bottom flask provided with a stirring bar, thermometer and reflux condenser for 2 minutes. The reaction began after five seconds followed by evolution of heat and separation of water. The mixture was allowed to reflux at 800C for four hours and then poured with fast stirring into a 600 ml beaker containing 16.5 ml absolute ethanol. Crystallization of the product was achieved by allowing the mixture to further stand in an ice bath for 30 minutes. An off-white colored solid mass of the product, N-benzilideneaniline was quantitatively transferred to a filter paper and allowed to dry in a desiccator using activated silica gel for 48 hours. 83.986% product yield was obtained and the melting point was found to be 56.20C.

Synthesis of Metal Complex
The nickel metal complex of N-benzilideneaniline ligand was prepared by dissolving 2.377 g of 0.01 mole of NiCl2.6H2O in 5 ml dry methanol in a 100 ml round bottom flask. 3.625 g of 0.02 mole of the synthesized N-benzilideneaniline ligand was dissolved in 10 ml of methanol in a separate 100 ml round bottom flask. The methanolic solution of the metal chloride and the ligand were mixed and refluxed in a 250 ml round bottom flask provided with a magnetic stirrer, thermometer and reflux condenser. Refluxing was continued for six hours at 800C under constant stirring. Finally, the solution was allowed to cool for 24 hours and 10 ml of ethanol was added. An olive green colored complex was separated out, filtered, washed with petroleum ether and dried under desiccator using activated silica gel. The purity of the complex was checked by thin layer chromatography (TLC).

Physico-Chemical Characterization of N-Benzylideneaniline Metal Complex
The complex was air-stable with high melting point and was soluble in methanol, chloroform, diethylether, ethanol and dimethylsulphoxide (DMSO).

Measurement of Electrical Conductivity
The electrical conductance measurement was done to ascertain whether the anions of the metal complex remain inside or outside the coordination sphere of the central metal atom.The specific conductance of the synthesized Ni 2+ complex (Ni 2+ = 26.87 Ώ -1 ) was 84x10 -6 Ώ -1 cm -1 . The conductance of 0.2 M KCl used was 849 Ώ -1 . This was used to calculate the cell constant which gave 2.9x10 -6 cm -1 . The high value of the molar conductance suggests that the complex behaves like an electrolyte.

FT-IR Spectra
The infra-red (IR) spectra of the N-benzylidene aniline (Schiff base) ligand shown in Figure 3.

Nuclear Magnetic Resonance (NMR) Spectroscopy of Schiff Base-Nickel Complex
Nuclear magnetic resonance spectroscopy was conducted to determine the plausible structure of Nbenzylideneaniline nickel complex.

Proton ( 1 H) NMR
The Schiff base (SB) proton NMR in Figure 3.4a shows highly de-shielded singlet absorption at ð (10.0183 ppm 1H) which can be attributed to the azomethine proton (HC=N-). The region of ð (6.4941 -8.6123 ppm, 10 H) with multiple signal absorption were assigned to chemically nonequivalent aromatic protons (Ar-H). The strong signal which appeared at ð 3.3389 ppm can be attributed to unreacted or excess droplets of phenyl amine protons during ligand synthesis. Dimethyl sulphoxide (DMSO) used as solvent was not 100 % isotopically pure and contains some trace of DMSO-(CH3)6 which gave a peak at 2.4983 ppm.

Carbon 13 ( 13 C) NMR
The 13 C spectra recorded on dimethyl sulphoxide (DMSO) in Figure 3.4b showed a signal which appeared at ð 193.670 ppm and was assigned to the azomethine carbon atom (CH=N

Interference Studies
The effect of foreign ions on the absorbance of 4.0 ppm nickel ion concentration is shown in That is, concentrations of foreign ions which cause relative deviations equal to, or lower than 5.0 % were tolerated. Most of the ions tested were tolerated in a wide range of concentrations.

CONCLUSION
When a normal electronic structure of a substance is deformed there is either the production of the color (or) change of the color, thus when molecules containing one or more chromophores and auxochromes are subjected to irradiation, they undergo variation in electronic energy. Schiff bases play important roles in coordination chemistry as they easily form stable complexes with most transition metal ions and they are very efficient as ligands. The presence of the lone pair of electrons on the nitrogen atom of the imine bond means that they can be donated to the appropriated metal ion. The synthesized ligand and its Ni complex were characterized using nuclear magnetic resonance (NMR), melting point determination, electrical conductivity and infrared (IR) spectroscopic techniques.