1. Introduction

Recently, there has been growing interest in the development of ligands that are able to effectively and selectively bind metal ions through multiple non-covalent interactions[1]. Iron, copper and zinc from ions which involved in metalloproteins. For example, metallo-enzymes belong to a subclass of metalloproteinase which perform a specific catalytic function. There are three known classes of dioxygen transport proteins involved in respiration: the haemoglobin- myoglobin family, haemocyanines and hemerythrins. These metal ions may differ from one organism to another[1], The concentrations of metal ions are very important for the normal function of biological systems. An excess of Cu(II) and Fe(II) may include thalaseamia disease[1].

The aim of this work the evaluation the non-covalent behaviour of CuCl₂ with (E)-3-(2-benzylidene hydrazinyl)-3- oxo-N-(thiazol-2-yl)propanamide (H₂BH) in absolute ethanol solutions at 294.15 K. These non-covalent interactions can help us for analysis of salts in bodies and environnement[1]. Evaluation of individual ionic molar volumes[2], preferential salvation of drugs[3] partial molar volumes[4], ligands used for selective metal extraction ⁵, ionic solvation⁶,potntiometric study ⁷ and specific solvation ⁸ are some trends of this area of physical inorganic chemistry.

2. Experimental

2.1. Material and Methods

All manipulations were performed under aerobic conditions. The copper chloride salt and absolute ethanol used were pure (Merck).

2.2. Preparation of the Ligand

(E)-3-(2-benzylidene hydrazinyl)-3-oxo-N-(thiazol-2-yl) propanamide (H₂BH) (Structure 1) was prepared by heating a mixture of 3-hydrazinyl-oxo-N-(thiazole-2-) propanamide (0.01 mol; 2.00 g) and benzaldehyde (0.01 mol; 1.06 g) under reflux in absolute ethanol for 3 h. On heating, white crystals were formed, filtered off, washed and recrystallized from absolute ethanol (M.P.: 230 ^oC; yield 80%). The purity of the compound was checked by TLC.

Structure 1. (E)-3-(2-benzylidenehydrazinyl) - 3-oxo – N - (thiazol-2-yl) propanamide

The conductometric titration of the ligand (1x10^-4) mole/L against CuCl₂ (1x10^-4) mole/L in absolute ethanol was performed with 0.5 ml interval additions from CuCl₂ solution, as explained in previous papers[9-11]. The specific conductance values were recorded using conductivity bridge HANNA, H1 8819N with a cell constant equal to one. The conductometer was connected to ultra thermostat of the type Kottermann 4130. The temperature was adjusted at 294.15 K.

3. Results and Discussion

The specific conductance values (K_s) of different concentrations of CuCl₂ in absolute ethanol were measured experimentally in absence and presence of ligand at 294.15 K. From the specific conductance, the molar conductance (/\_m) were calculated by using equation (1)[12]:

(1)

Figure 1. The relation between molar conductance (/\M) and () of (A) CuCl2 alone and (B) in presence of ligand in absolute ethanol at 294.15 K

Where (K_s) and (K_solv) are the specific conductance of solution and solvent, respectively; (K_cell) is the cell constant and (C) is the molar concentration of the CuCl₂ solutions. The limiting molar conductance (/\_o) at infinite dilutions were estimated for CuCl₂ in absolute ethanol in absence and presence of the ligand (H₂BH), by extrapolating the relation between (/\_m) and (C_m^½) to zero concentration (Fig. 1). By drawing the relation between molar conductance (/\_m) and the molar ratio of metal to ligand concentrations, different lines are obtained with sharp breaks indicating the formation of 1:2, 1:1 and 2:1 (M:L) stoichiometric complexes (Fig. 2).

Figure 2. The relation between molar conductance (/\M) and the molar ratio (M/L) of CuCl2 in the presence of ligand in absolute ethanol at 294.15 K

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 CuCl₂ in the presence of ligand (H₂BH) in absolute ethanol at 294.15 K for 1:2 asymmetric electrolytes were calculated by using equation (2)[13, 14].

(2)

Where (/\_m, /\₀) are the molar and limiting molar conductance of CuCl₂; (C_m) is molar Concentration of CuCl₂ and S (Z) is Fuoss-Shedlovsky factor and equal approximately one for strong electrolytes[15]. The calculated association constants are shown in Table 1. The Gibbs free energies of association (ΔG_A) were calculated from the association constant[15,16] by applying equation (3):

(3)

Where (R) is the gas constant (8.341 J) and (T) is the absolute temperature (294.15 K). The calculated Gibbs free energies were presented in Table 1.

Table 1. Association constants and Gibbs free energies of association for CuCl2 with H2BH in absolute ethanol at 294.15 K

Table 2. Formation constants and Gibbs free energies of formation for 1:2 (M/L) CuCl2-H2BH complexes in ethanol at 294.15 K

Table 3. Formation constants and Gibbs free energies of formation for 1:1 (M/L) CuCl2- H2BH complexes in ethanol at 294.15 K

Table 4. Formation constants and Gibbs free energies of formation for 2:1 (M/L) CuCl2- H2BH complexes in ethanol at 294.15 K

The formation constants (K_f) for CuCl₂ complexes were calculated for each type of complexes (1:2), (1:1) and (2:1) (M:L) by using equation (4)[17, 18]:

(4)

Where (/\_m) is the molar conductance of the CuCl₂ before addition of the ligand, (/\_obs) is the molar conductance of solution during titration and (/\_ML) is the molar conductance of the complex.

The obtained values (K_f) for CuCl₂-ligand stoichiometric complexes are represented in Table 2-4. Also the Gibbs free energies of formation for each stoichiometric complexes were calculated by using the equation (5):

(5)

The calculated ΔG_f values are represented in Tables 2-4. The association free energies evaluated for CuCl₂-ligand complexes are small and spontaneous indicating electrostatic attraction force. The formation constants and Gibbs free energies of different complexes in absolute ethanol at 294.15 K follow that order: K_f (2:1) › K_f (1:1) › K_f (1:2) for (M:L), and ΔG_f (2:1) › ΔG_f (1:1) › ΔG_f (1:2) for (M:L).

The formation of 2:1, 1:1 and 1:2 complexes indicate that H₂BH acts as flexi dentate ligand (Structures 2-4).

4. Conclusions

In this paper, the association constants, the formation constants and Gibbs free energies for CuCl₂ with (H₂BH) had been calculated. The association free energies evaluated for CuCl₂-ligand complexes are small and spontaneous indicating electrostatic attraction force. While, 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 ΔG_f (2:1) › ΔG_f (1:1) › ΔG_f (1:2) for (M:L)