Conductometric Study of Complex Formation Between Cu (II) Ion and 2-hydroxyimino-3-(2'-hydazonopyridyl)-butane (HL)

The association constant ,formation constants and Gibbs free energies are calculated from the conductometric titration curves of CuCl2 with 2-hydroxyimino-3-(2’-hydazonopyridyl)-butane (HL) in absolute ethanol at different temperatures( 293.15 K , 298.15 K , 303.15 K and 308.15 k). On drawing the relation between molar conductance and the ratio of metal to ligand concentrations, different lines are obtained indicating the formation of 1:2 , 1:1 and 2:1 (M:L) stoichiometric complexes. The formation constants of different complexes in absolute ethanol follow the order: Kf (2:1) › Kf (1:1) > Kf (1:2) for (M: L). As the temperature increases, the formation constants and association constants of different complexes increase. The enthalpy and entropy of formation and association of CuCl2 with HL were also estimated and their values were also discussed. The solvation free energies (∆Gs) ,Enthalpy changes of solvation (∆Hs)and the entropy of salvation (∆Ss) were also calculated from solubility measurements for 2-hydroxyimino-3-(2hydrazonopyridyl)-butane (HL) at different temperatures (293.15 K,298.15 K,303.15 K and 308.15 K).


Introduction
Schiff bases hydrazone derivatives and their metal compl exes have been studied for their interesting and important properties, e.g., antibacterial [1,2], antifungal [3], antioxidant [4], anticancer [5] and catalytic activity in oxidation of cyclohexene [6]. Moreover, Schiff bases hydrazone derivati ves are versatile ligands and they offer the possibility of different modes of coordination towards transition metal ions. Also, some of these derivatives have been applied as iron chelate or drugs in therapy of anaemia [7] and treatment of neuropathic pain [8].
Therefore it prompted us to study Schiff base transition metal complexes. Transition metal ions have a strong role in bio-inorganic chemistry and redox enzyme systems and may provide the basis of models for active sites of biological systems [9]. Copper (II) ion is a biologically active, essentialon, cleating ability and positive redox potential allow participation in biological transport reactions. Cu(II) complexes possess a wide range of biological activity and are among the most potent antiviral, antitumor and anti inflammatory agents [10]. Schiff base transition metal complexes have been extensively studied because of their potential use as catalysts in a wide range of oxidation reactions [11][12][13][14]. In recent years many copper, nickel and manganese complexes of Schiff bases were prepared and characterized by several techniques [15,16].

Objectives
This work deals with the determination of solvation free energies (∆Gs) ,enthalpy changes of solvation(∆Hs) and the entropy of solvation (∆Ss) from solubility measurement and identification of coordination behaviour of Schiff base ligand HLtowards CuCl 2 .and the determination of the thermodyna mic stability constants and thermodynamic functions using the conductometric technique. Thus, thermodynamic studies of complexation reactions of this Schiff base with transition metal ions not only result in important information on the thermodynamics of complexation reaction, but also lead to a better understanding of the high selectivity of this ligand towards different metal cations.
The aim of this work the evaluation the non-covalent behavior of CuCl 2 with 2-hydroxyimino-3-(2'-hydazonopyr idyl)-butane (HL) in absolute ethanol solutions at 294.15 K. These non-covalent interactions can help us for analysis of salts in bodies and environnement [17].

Materials
All manipulations were performed under aerobic conditions. The cupper chloride and the used reagents were Merck pure.

Conductometric Measurement
The conductometric titration of the CuCl 2 (1x10 -4 ) mole/L against the ligand (1x10 -3 ) mole/L in absolute ethanol was performed with 0.2 ml interval additions from HL solution. The specific conductance values were recorded using conductivity bridge ADWA, AD 3000 with a cell constant equal to 1 cm -1 . The temperature was adjusted at 293.15 K, 298.15 K, 303.15 K and 308.15 K

Solubility Measurment
Saturated solutions of HL were prepared by dissolving an excess amount of the solid substances in 10 ml. of the corresponding solvent mixtures, using closed test tubes. The solutions were vigorously shaken in a thermostatic water-ba th at the desired temperature. The molal solubilities of the HL were analysed by drying 1ml. of the saturated solutions in small aluminium dishes. Evaporation of the solvent was performed carefully and slowly under a tungsten lamp to prevent any loss in salt weight. Solubility value was taken as an average of three consecutive independent measurements.

Association Constants
The specific conductance values (K s ) of different concentrations of CuCl 2 in absolute ethanol were measured experimentally in absence and in the presence of ligand at different temperatures (293.15 K , 298.15 K , 303.15 K and 308.15 K).
The molar conductance (/\ m ) values were calculated [19]     The experimental data of (/\ M ) and (/\ o ) were analyzed for the determination of association and formation constants for each type of the stoichiometric complexes.
The association constants of CuCl 2 in the presence of ligand (HL) in absolute ethanol at different temperatures ( 293.15 K , 298.15 K , 303.15K and 308.15 K) for 2:1 ,1:1 and 1:2 (M:L) were calculated by using equation [20,21]: where (/\ m , /\ 0 ) are the molar and limiting molar conductance of CuCl 2 in presence of Hl respectively; C m is molar concentration of CuCl 2 , S(Z) is Fuoss-Shedlovsky factor, equal with unity for strong electrolytes [22]. The calculated association constants are shown in Table (1).

Gibbs Free Energies of Association
The Gibbs free energies of association (ΔG A ) were calculated from the association constant [23,24] by applying equation: , where R is the gas constant (8.341 J) and T is the absolute temperature .The calculated Gibbs free energies were presented in Table (2).

The Formation Constants for Complexes
The formation constants (K f ) for CuCl 2 complexes were calculated for each type of complexes (1:2), (1:1) and (2:1) (M: L) [25,26] by using equation: (4) where /\ M is the limiting molar conductance of the CuCl 2 alone, /\ obs is the molar conductance of solution during titration and /\ ML is the molar conductance of the complex.
The obtained values (K f ) for CuCl 2 -ligand stoichiometric complexes are presented in Table (3)  ( )

Gibbs Free Energies of Complex Formation
The Gibbs free energies of formation for each stoichiome tric complexes were calculated by using the equation: ΔG f = -R T ln K f (5) . The calculated ΔG f values are presented in Table (4).
The calculated values of (ΔH f ) and (ΔS f ) for CuCl 2 -ligand stoichiometric complexes are presented in Table (6):

Acivation Energies
Since the conductance of an ion depends mainly on its mobility, it is quite reasonable to treat the rate process taking place with the change of temperature on the basis of equation (8) : /\ 0 =A e -Ea/RT (8) , where A is the frequency factor, R is the gas constant and Ea is the Arrhenius activation energy of the transfer process. Consequently, from the plot of log /\ 0 vs. 1/T, the E a values can be evaluated [ 27] as shown in Fig (9) . E a value is 14.6996 KJ/mol.

Soubility Measurement
The solubility (S) of 2-hydroxyimino-3-(2'-hydazonopyrr idyl)-butane (HL) in (EtOH-H2O) mixtures at different temperatures (293.15, 298.15, 303.15 and 308.15 K) was determined by gravimetric technique. The results are illustrated in Table 1. Solubility was calculated as an average of the two experimental data. The molal solubility is calculated by using equation (9): Molal solubility (S) = W.1000/do.M g.mole /1000 g . solvent (9) , where (W) is the weight of one ml. of saturated solution, after its complete evaporation in the aluminum dish under the effect of tungsten lamp,(M) is the molecular weight of HL and (d o ) is the density of pure solvent used as it shown in Table (7) ; Fig.(10) the molal solubility was increased with the increase of the content of the organic solvent used (EtOH).This can be explained on the basis of the fact that like dissolve like as well as the lower and higher ion-solvent interactions. The molal solubility of HL was increased with the increase of temperatures

Thermodynamics of Solvation
The solvation free energies ∆G S of HL in EtOH-H 2 O mixture at different temperatures(293.15 K,298.15 K, 303.15 K and 308.15 K) were calculated from the solubility measurements by using the following equation (10): (∆G) s =-2.303 RT log K sp (10) . The value of (log K sp ) depends mainly on the solvation of the solute in the solvent under investigation .In case of neutral compound (the activity coefficient is close to one), the values of (log K sp ) can be equal to log (S).
The enthalpy changes of solvation(∆H s ) of HL in EtOH-H 2 O mixtures were calculated from the plots of (log K sp ) versus (1/T) ,where the slope equals (-∆H s /2.303 R) using the following equation (11): log K sp = -(∆H s ) / 2.303 RT + constant (11)

Conclusions
The stability constants for the complexation of copper (II) ion with 2-hydroxyimino-3-(2'-hydazonopyridyl)-butane (HL) were determined conductometrically at different temperatures. Thermodynamic parameters of complexation were determined from the temperature dependence of the formation constant. The negative values of ∆G show the ability of the studied ligand to form stable complexes and the process trend to proceed spontaneously. However, the obtained positive values of ∆H means that enthalpy is not the driving force for the formation of the complexes. Furthermore, the positive values of ∆S indicate that entropy is responsible for the complexing process. The formation constants and Gibbs free energies of different complexes follow that order: K f (2:1) › K f (1:1) › K f (1:2) for (M:L), and ∆G f (2:1) › ∆G f (1:1) › ∆G f (1:2) for (M:L)