Spectrophotometric study of Stability Constants of

2z-2-Benzylidenehydrazinecarbothioamide-Mn Schiff Base Complex at Different Temperatures

 

K.O Eberendu

Department of Chemistry, Michael Okpara University of Agriculture, Umudike, Abia State Nigeria

 

Abstract:

Schiff base ligands have structures which enable them to act as chelating agents. The formation of Mn(III) complex with 2z-2-benzylidenehydrazinecarbothioamide has been studied spectrophotometrically at an absorption maximum of 635 nm at different temperatures. The data show that Mn(III) and 2z-2-benzylidenehydrazinecarbothioamide combine in the molar ratio of 1:3. The stability constants of the complex were calculated to be 6.70 x 10¹⁴_, 5.60 x10¹⁴ and 1.16 x 10¹⁴, 4.30 x 10¹⁴ by continuous variation method and mole ratio method at 25°C and 40°C respectively. The immediately formed complex shows stability with respect to temperature.

 

 

 

INTRODUCTION:

Schiff base ligands have been playing an important part in the development of coordination chemistry. Schiff base metal complexes have been studied extensively because of their attractive chemical, physical properties and their wide range of applications in numerous scientific areas¹. Paramount to the goals of the present day inorganic chemists is the discovery and development of chelating agents as well as their applications. Chelating agents have broadly found applications in different fields of industrial, analytical, metal cleaning, scale removal as well as clinical chemistry².

 

In discovery and development of better drugs to fight diseases, this has led to numerous studies on drug metal complexes, various studies have been carried out on complexation of some antibacterial, antifungal, antimalarial, antioxidant and carcinostatic drugs with metals. In industrial area, for example, EDTA is used for estimation of Ca²⁺ and Mg³⁺ ions in hard water. Metals play a vital role in all living systems. Any malfunctioning of these metals can initiate a number of physiological abnormalities and symptoms of clinical disorders. Transition metal ions are responsible for proper functioning of different enzymes. In biology, manganese (III) ions function as cofactors for a large variety of enzymes with many functions ². Complexes are also used as catalysts for different reactions. In this study, Spectrophotometric methods were used for the determination of interaction of manganese (III) with the Schiff base ligand 2z-2-benzylidenehydrazine carbothioamide and stability constants were determined at 25, and 40°C. Chelating ability of this complex could result due to the presence of two electron-donating nitrogen and sulfur atoms separated by two carbon atoms³. The nitrogen and sulfur atoms of this Schiff base can thus form thermodynamically stable saturated five membered rings by interacting with transition metals⁴.

 

EXPERIMENTAL:

Apparatus:

Spectrophotometric measurements were performed on a UV- 1700 Labomed double beam spectrophotometer (Japan) using matched 10 mm quartz cells. A corning 425 F.8 pH meter (Spain), calibrated with standard buffer solutions of pH 4 and 10, was used for pH measurements.

 

Reagents:

Synthesis of

2z-2-benzylidenehydrazinecarbothioamide:

The ligand, 2z-2-benzylidenehydrazinecarbothioamide was synthesized via the following procedures. Methanol solution (10 ml) of Thiosemicarbazide (2.03g, 0.01 mol) was prepared. Benzaldehyde (0.01mol) was added to the solution and stirred gently. The precipitate formed was air-dried. Manganous carbonate and all other chemicals used were of analytical grade purity purchased from E-Merck Germany. MnCO₃ was taken in an accurate amount and was not further standardized. Double-distilled water was used throughout this study.

 

Preparation of 2x 10^-1 M MnCO_3:

MnCO₃ (2.23 g, 0.01 m mol, M. Wt. = 114.95 gmol^_1) was dissolved in freshly distilled and dry ethanol in a beaker and was made up to the mark in a 100 mL volumetric flask.

 

Preparation of 2 x 10^-1 M Schiff base:

Schiff base compound (5.83 g, 0.02 m mol, M. Wt. = 291.34 g mo1^-1) was dissolved in freshly distilled and dry ethanol in a beaker and was made up to the mark in a 100 mL volumetric flask.

 

Procedure for continuous variation method:

Manganese (II) (2 x 10^-1 M) chloride tetrahydrate solution (0, 1,2, . . ., 6mL) was pipetted out and transferred into seven 50mL volumetric flasks and an aliquot (6, 5, . . ., 0mL) of 2 x 10^-1  M Schiff base was added, respectively in such a way that the mole fraction of solution remained constant. Colour of the solution was changed from light brown to Champagne pink. Wavelength of maximum absorbance was noted against a blank, which appeared at 635 nm. All the measurements were made at 635 nm at 25 and 40°C, respectively.

 

Procedure for Mole Ratio Method:

From 2 x 10^-1 M manganous carbonate solution, 2 mL was pipetted out and transferred into each of the seven 50 mL volumetric flasks and an aliquot (1, 2, . . ., 7 mL) of 2 x 10^-1  of the Schiff base was added to each, respectively. Wavelength of maximum absorbance was noted against blank reagent manganous carbonate, which appeared at 635 nm. All the measurements were made at 635 nm and 25 and 40°C, respectively.

 

RESULTS AND DISCUSSION:

The Properties of Complex:

The reaction of 2-Z(2)-BHC Schiff base ligand with manganous carbonate was investigated at two different temperatures i.e. 25 and 40°C. The absorption spectra were recorded over the wavelength range of 400–800 nm. It was found that the Schiff base ligand with manganous carbonate formed a Champagne pink, water soluble complex. The complex gave an absorption maximum at 635 nm (Fig. 2), and was used as λ_max for the analytical measurements. Under the same conditions, pure 2-Z(2)-BHC Schiff base ligand does not absorb significantly over the investigated wavelength range. However manganous carbonate itself absorbs at 700 nm, which is maximum absorbance k_max (series 2, Fig. 1). In solution, magnesium was present as [Mn(H₂O)₆]³⁺ and showed  λ_max at 700 nm. Water behaves as a weak field ligand so manganese aquo complex acts as a labile complex, which can be easily replaced by a Schiff base ligand, to form a stable complex of stoichiometry ML₂ (max = 635 nm). Full colour development was observed immediately and the absorbance remained unchanged. Metal ion binding is not able to change some conformational features of cimetidine, which could be a biologically important factor ^6,5

 

Table 1: Experimental data of manganese (III)–cimetidine complex by continuous variation method

 

 

Fig. 1: Absorption spectra of manganous carbonate; and complex with cimetidine. [C(2Z(2)-BHC) = 2 x 10^-1 M, C[Mn(H₂O)₆] = 2 x 10^-1 M]

 

The Composition of Complex and Stability Constant:

The stochiometric ratio of 2-Z(2)-BHC Schiff base ligand to Mn(III) in the complex was determined by Job’s method of equimolar solutions [5]. Manganous carbonate standard solutions 2 x 10^-1 M were pipetted into seven volumetric flasks (0, 1, 2, 3, . . ., 6 mL) and an aliquot of 2 x 10^-1 M cimetidine (6, 5, 4, . . ., 0 mL) was added, respectively keeping the mole ratio constant. All the measurements were made at 635 nm and two different temperatures i.e. 25 and 40°C.

 

The Job’s method of Continuous Variation:

The stoichiometric ratio of Zinc (II) to 2Z(2)-BHC in the complex was determined by Job’s method of equimolar solutions. The curve displayed maximum absorbance at mole fraction of zinc ion, XMn³⁺ = 0.2, which indicates the formation of complex with zinc ion to ligand ratio of 1:4. (Fig. 2). Experimental data of Manganese (III)-2Z(2)-BHC complex obtained based on the continuous variation method is given in Table 2. to the total absorbance of the complex, indicating that the curve in (Figure 2) gave maximum absorbance at 1:4 metal to ligand ratio (that is one ion of the metal is coordinated with four molecule of the ligand). The extrapolated value at the point of cross section on the continuous variation plot was corresponds complex formation process has been completed. The curve in Fig. 3 displayed a maximum at a mole fraction X_Mn = 0.50, which indicates the formation of complex having 1:3 metal to ligand ratio. By applying continuous variation method, also called Job’s method, the metal to ligand ratio and the stability constant of the complex have been determined. It requires the series of solutions of varying concentration of two constituents where their sum is kept constant.

 

Figure 2: Job’s Job’s curves for stability constants of equimolar solutions of Mn (III)-2Z(2)-BHC complex at (A) 25°C and (B) 40°C

 

Mole Ratio Method:

Mole ratio method is taken as alternative to the Job’s method for determining stoichiometry of the complex. Accordingly, with the constant of concentration Mn(III) (2x10^-4 moles) and varying the concentration 2-Z(2)-BHC Schiff base ligand from (2 x 10^-4 to 14 x10-4 moles), at 25°C and 40°C (Table 2), a maximum absorbance was observed at 1:3 mole ratios of Mn(III) ion and 2-Z(2)-BHC Schiff base ligand. This was because of the concentration of the complex which maximum at 1:3 mole ratio of Zn(II) ion and the ligand.

 

Table 2: Experimental Data of Manganese(Iii)–2z(2)-Bhc Complex by Mole Ratio Method

S/N	Metal concentration	Ligand concentration	Absorbance at 635 nm
			25°C	40°C
1	0.08 M = 2 x 10-4 mole	0.04 = 2 x 10-4 mole	0.25	0.27
2	0.08 M = 2 x 10-4 mole	0.08 = 4 x 10-4 mole	0.45	0.46
3	0.08 M = 2 x 10-4 mole	0.12 = 6 x 10-4 mole	0.63	0.65
4	0.08 M = 2 x 10-4 mole	0.16 = 8 x 10-4 mole	0.84	0.86
5	0.08 M = 2 x 10-4 mole	0.20 = 10 x 10-4 mole	0.88	0.89
6	0.08 M = 2 x 10-4 mole	0.24 = 12 x 10-4 mole	0.92	0.90
7	0.08 M = 2 x 10-4 mole	0.28 = 14 x 10-4 mole	0.91	0.92

 

 

Table 3: Stability constants and Gibb's free energy for the complex

 

 

Fig.3. Curves for mole ratio method (a) 25°C (b) 40°C

 

Determination of Stability Constant:

Spectrophotometric method used for the determination of stability constants of a complex. Spectrophotometric methods such as the mole ratio and Job’s method were used to determine the stability constant of the Manganese (III) complex with 2z-2-benzylidenehydrazine carbothioamide and on the basis of data obtained, the stability constants have been determined, and the value of K_s obtained. The mole ratio and Job’s method were used to determine stability constant and free energy change of Mn (III) complex formation with 2z-2-benzylidenehydrazi6necarbothioamide ligand as follow.

The stability constant Ks can be written as:

 

Where, C is the total concentration of the complex assuming no dissociation, α degree of dissociation and αC and nαC are concentrations of the metal and ligand respectively, all concentrations are being taken in moles per liter. The value of α may be obtained by measuring the E_m and E_s from the mole ratio curve to be drawn using the following relation.

 

Where, E_m is the maximum excitation obtained from the horizontal portion of the curve indicating that all reactants are present in the form of complex and E_s is the excitation of stoichiometric mole ratio of metal to ligand in the complex. When the total concentration of the complex being equal to the concentration of the reactants. On putting the value of α from equation 2 and substituting it in equation 1, K_s can be calculated.

 

Calculation of Free energy change of the complex:

If the value of the stability constant of the Mn(III) complex is known, it would so easy to find the value of the free energy formation of the complex. The free energy formation of a complex is related to its stability constant by the relation:

 

ΔG^o = −2.303RTlogk_s.                                                      (3)

 

The value of the free energy change of the Mn(III) complex have been calculated using both the mole ratio and Job’s methods.

 

CONFLICT OF INTEREST:

The authors declare no conflict of interest.

 

REFERENCES:

1.     Schiff HA available at http//www.wikkipedia.com/Schiffbaseligands, retrieved 14/08/16

2.     Roth, Jerome; Ponzoni, Silvia; Aschner, Michael (2013). "Chapter 6 Manganese

3.     Homeostasis and Transport". In Banci, Lucia. Metallomics and the Cell. Metal Ions in Life Sciences. 12. Springer. doi:10.1007/978-94-007-5561-1_6. ISBN 978-94-007-5560-4. Electronic  c-book ISBN 978-94-007-5561-1 ISSN 1559-0836 electronic-ISSN 1868-0402 Penston, J., Wormsley, G., 1986. Adverse reactions and interactions with H₂-receptor antagonists. Med. Toxicol. Adverse Drug Exp. 1 (3), 192–216.

4.     Amigo, J.M., Reventos, M.M., Sancho, A., Soto-Tuero, L., Cantarero, A., 1987. Metal coordination in nickel cimetidine tetrafluoroborate by powder X-ray diffraction. Zeitschrift fu r Kristallographie 180 (1–4), 123–129.

5.     Sarwar, S., 2006. A Spectrophotometric Study of Interaction of 3-d Transition Metals with H2-receptor Drugs. M. Phil Thesis, Quaid-i-Azam University, Islamabad, Pakistan.

6.     Silverman, R.B., 2004. The Organic Chemistry of Drug Design and Drug Action. Amsterdam, Elsevier Academic Press, p. 159.

 

 

 

 

Received on 07.02.2017       Modified on 20.03.2017

Accepted on 11.04.2017      ©A&V Publications All right reserved