Physico – Chemical Study of Transition Metal Complexes with Schiff ’ s Base derived from Naphthaldehyde and Substituted Aromatic Amines

In continuation of the previous work here we are going to report stability constant values of transition metals like Cu(II) , Ni(II), Co(II) and Zn (II) with Schiff;s base ligand. Schiff’s base ligands were synthesized by the condensation reaction of βNaphthaldehyde with 7Hydroxy naphthalene – 2 – amine .Nitrate salts of divalent cobalt, nickel, copper and zinc were estimated by usual methods. Ligand was analysed for elements by standard method. pH metric titrations were carried out with the help of digital pH meter and stability constant of complexes of these metals with the ligands synthesized were computed by IrvingRossotti technique modified by Calvin-Bjerrum. The stability constant values of metals for the given ligand were found to be in the order Cu(II) > Ni(II), Co(II) > Zn (II). This result is in agreement with the natural order proposed by Irving-William.


INTRODUCTION
2][3][4] Schiff's base offer a versatile and flexible series of ligands capable to bind with variety of metal ions to give complexes with varying properties.These complexes are biologically active 5 and have wide potential applications in many fields such as catalysis 6 , electrochemistry 7 and medicines Studies 8 have shown that metal complexes act as antitumour, antiviral, anti cancer 10 and other many anti bacterial agents.
A large number of polydentate Schiff's base compounds have been synthesized and their complexes have been structurally characterized and extensively investigated.But little is known for their stability in aqueous solution in which it is used.Hence, the title project have been under taken.Here in the stability constant of complexes of divalent transition metals i.e.Co(II), Ni(II), Cu(II) and Zn(II) with Schiff's base ligands have been determined.

Experimental
Nitrate salts of divalent Co, Ni, Cu and Zn all were E. Merck.All other chemicals used were AnalR grade and used without further purification.Elemental analysis of metal salts were done by volumetric and gravimetric methods.Double distilled and deionised water was used throughout the experiment.All titrations were done in aqueous-dioxane medium in the ratio 3:2 (v/v).Dioxane was purified by standard method.
Schiff's base ligands were synthesized by the condensation of b-Naphthaldehyde with 7-Hydroxy naphthalene -2 -amine.3.5g of aldehyde in solution was mixed with nearly 3.0 g amine.The mixture was boiled under reflux in the presence of glacial acetic acid for about 2 hours.The solution was concentrated and cooled to 0 0 C. The product obtained was filtered, washed several times and recrystallized from ethanol.The yield of product was nearly 2.6 g.Calvin -Bjerrum pH metric titration of acid, acid + ligand and acid + ligand + metal ions solutions were done at constant ionic strength of 0.1 M KNO 3 at 298 K temperature in an inert atmosphere of nitrogen.

Metal
The same process of titration were repeated for all the four Co, Ni, Cu and Zn metal ions.The change in colour and appearance of turbidity at particular pH value were recorded simultaneously.
The change in pH of the solutions with each addition of alkali was recorded in Table no.2.

RESULTS
A graph was plotted between pH meter reading [B] and volume of alkali added in each case, (Figure -1. ) Three titration curves obtained for each metal ions are acid titration curve (a), ligand titration curve (b) and complex titration curve (c) respectively.

The values of volumes
corresponding to the same pH values were read from acid, ligand and complex titration curves (a), (b) and (c) respectively obtained from the experiment at temperature 298 K given in Figure -1.

Proton ligand stability constant
The ligand titration curve is above the acid titration curve showing the basic nature of ligand and it is well separated from the acid titration curve at pH=6.The complex titration curve of the system crossed the ligand mixture curve at pH 5.35 for Co(II), pH 4.88 for Ni(II), at pH = 5.90 for Cu(II) and pH 5.68 for Zn(II) -ligand system indicating the start of complexation.
Metal titration curve run parallel to the ligand titration curve indicating the liberation of extra proton due to hydrolysis of metal ions.

In, Co (II) System
Precipitation was observed at pH 8.9.Hence in the calculation of n only the lower pH region of titration curve were used.

In Ni(II) System
The curve increased regularly up to pH 7.56 indicating constant rate of release of proton .No turbidity appears, which indicates that hydrolysis does not take place.

In Cu(II) System
Similarly in Cu(II) system the curve increased regularly up to pH 9.356 indicating constant rate of release of proton .No turbidity appears, indicating that hydrolysis does not take place As n value did not go beyond 2 for any of the metal indicating the formation of ML and ML 2 type of complexes.
From the formation curve of n vs P L (Figure no.-4a,4b,4c and 4d) the values of log K 1 and log K 2 were calculated in each case.It was further corroborated

Fig. 1 :
Fig. 1: Experimental curve with ligand HNNCI Temp.298 ±1 K µ°= 0.10(M) KNO 3 Water : dioxane = 3:2(v/v) 3 at temp 298 K.The ligand curves run parallel to the acid titration curve indicating the smooth dissociation of the ligand.The values of A n at various pH reading [B] was calculated from the acid and ligand titration curves and recorded in table 3. The formation curve obtained from the plot of A n vs [B] extends from 0.43 to 0.88 (Figure 2) at temp 298 K.The dissociation of ligand may be represented as HL H + + L - The value of proton ligand stability constant was calculated by half integral method and it was further corroborated by linear plot method.(log A n / (1 -A n ) vs [B] Figure.-3.

Table 5 : Values of P L at various values of log
n /(1-n ) and log (2n /( n -1)

Table 9 : Values of P L at various values of log
n /(1-n ) and log (2 -n /( n -1)