INTRODUCTION:

The metal complex formation in solution can be studied by a number of physico–chemical methods viz. reaction kinetics, electrical conductance and electrode potential measurements, polarography, spectrophotometry, solvent extraction. Ion exchange, freezing point, boiling point and vapour pressure, magnetic susceptibility, calorimetry, light scattering etc. In recent years paper electrophoresis has been employed as a handy tool for the investigation of complexation. This technique till now had very striking limitation in as much as it was applicable for study of only mononuclear complexes.

Here in these laboratories this technique has been modified to universalise it. In the present thesis, this technique has been followed for the investigation of several binary and ternary complexing reactions in solution. The details of theoretical and experimental aspects of the technique are described in subsequent chapters.

The formation of mixed ligand complexes of metal ions was recognised even during the time of Werner. A systematic interest in mixed ligand complex formation has been taken during the past few decades ^1-13. Mixed ligand complexes include those in which more than one kind of ligand, other than the solvent molecule, are present in the innermost coordination sphere of the central metal ion and it can be represented by the general formula MA_iL_jY_k¹⁴

The formation of such complexes may take place either by the combination of two different binary complexes or by the partial stepwise replacement of the lignad (or water molecules) from the coordination sphere of the simple complex MA_nor MA_n-x (H₂O)_x. the mixed complexes are often found to be more stable than the binary complexes ¹⁵. Considerable work has been reported to explain the increased stability of mixed ligand complexes and the factors such as statistical effect ^16-19, increased polarisation of the metal ion in the field of ligands of more than one type ^20,21, formation of bonds between the lignands of different type, charge neutralization with decreased solvation, asymmetry of the ligand field and others have been found to be responsible for the enhanced stability of the ternary complexes ^22,23.

The work on mixed ligand complexes include the conditions necessary for mixed complex formation ²⁴, correlation of ternary constants with binary constants ²⁵, and with redox potentials ^26,27 in some of these works the changes in free energy of coordination bonds as a result of mixed complex formation have been discussed ^28-31. The effect of charge, the effect of the number and types of grounds bound to the metal before and after mixed complex formation and the effect of chelation also been determined ³², these parameters are evaluated in terms of their contribution to the free energy of formation of the mixed complex.

The formation of mixed ligand complexes is common among the inert complexes. Work on their synthesis, kinetic behavior and isomers have been reported ³³.

Mixed complexes, like simpler species, are best studied by those methods which introduce fewest unknown parameters and which readily give the concentration variables. For the study of complexes in solution, various methods including electrode potential studies, polarography,^34,35, optical and spectroscopic methods^36-38 such as Raman ^40-43 infra-red ^44,45 nuclear magnetic resonance^46,47 and electronic absorption spectral^48,49 liquid – liquid partition and ion exchange⁵⁰ have been used.

Nitrilotriacetic acid is a powerful ligand. It forms very stable complexes with metal ions even at low pH values. Hydroxo complexes are sometimes found at higher pH value⁵¹. Carey and Martell⁵² studied the ternary complex system of uranyl-NTA-hydroxyquinoline sulphonic acid. Fe(lll)-NTA-Catechol-3,5-disulphonic acid system was investigated by Schwarzenbach and Willi ⁵³. Metal-NTA-amino acid systems have been extensively invesigated by Israeli and his co-workers ^54-56.

In the present work mixed ligand complexation involving 2-aminobenzoic acid, 3- aminobenzoic acid and 4-aminobenzoic acid as primary ligands and nitrilotriacetic acid (NTA) as secondary ligand with Zn(II) metal ion has been studied.

The various ternary complexes studied are –

1- Zn(II) – 2ABA-NTA

2- Zn(II) - 3ABA-NTA

3- Zn(II) - 4ABA-NTA

Experimental results relating to above ternary complexing systems are given in chapter III and IV and attampts have been made to interpret them.

MATERIALS AND METHODS:

Instrument

Systronics paper electrophoresis equipment horizontal cum vertical type no. 604 (India) has been used. The apparatus consisted of PVC moulded double tank vessel. Two hollow metal plates covered with thin polythene plates have been used for controlling the temperature. Thermostated water supply has been made through hollow plates. The tank is closed with a transparent PVC moulded lid. The whole assembly is tight which prevents moisture change that may upset the equilibrium in the paper. This assembly design thus keeps to minimum the disturbing effects of evaporation from the unwanted liquid flow in the paper.

Each electrolyte vessel contains a separate electrode chamber in which the cathode and anode are placed, respectively. The auxiliary unit is a specially designed one can be operated upon either voltage mode or on current mode. The voltage can be changed through three ranges viz. 0-100, 100-200, and 200-300 volts. The voltage of each set of experiment was kept at 200 volts and electrophoresis was carried out for 30 minutes.

Filter Paper

Strips of Whatman no. 1 filter paper for chromatography (30 X 1) cm² were used.

pH Indicator and Accessories

Elico model L_1-10having glass and calomel electrode assembly and working on 220 volts/50 cycles stabilized A.C. main, were employed for pH measurements.

Water

Double distilled water was boiled to expel free carbon dioxide and cooled in well stopped pyrex flask. This was used for preparing solutions and for dilutions throughout these studies.

Metal Solutions

Nickel perchlorate was prepared by precipitation of Zinc carbonate from 0.1 M solution of sulphates of Zn(II) with solution of sodium carbonate (chemically pure grade). The precipitate was thoroughly washed with boiling water and treated with calculated amount of 10% perchloric acid. This was boiled on a water bath and filtered. The final concentration solution was kept at 5 X 10^-3 M.

Sodium Hydroxide Solution

Carbon dioxide free sodium hydroxide was prepared by dissolving 500 gms of sodium hydroxide in 500 ml of water and was left overnight. The clear supernatant liquid was filtered rapidly through a scintered bed jena glass crucible using a high vacuum pump. A suitable volume of the filterate was diluted and the concentration determined by titrating against a standard oxalic acid solution. A solution of 2.0 M concentration was, then, obtained by suitable dilution. The concentration of stock solution was checked from time to time.

Solution of Complexing Reagent

Stock solution of the complexing reagents viz.2-aminobenzoic acid, 3- aminobenzoic acid and 4-aminobenzioc acid (all CDH Anala R grade) were prepared by dissolving accurately weighed amounts in water and two to three drops of concentrated sodium hydroxide solution. Solutions of required strengths were then prepared by suitable dilution.

Electro- Osmotic Indicator

5 X 10^-3 M glucose (BDH AnalaR) was prepared in water and used as an electro–osmotic indicator for the correction due to electro-osmosis.

Locating Reagent For Metal Ion

1–(2-pyridylazo)-2-napthopl (PAN) solution in ethanol was used for locating Zn(II).

Indicator For Glucose

A saturated aqueous solutions (0.9 ml) of silver nitrate (BDH AnalaR) was diluted with acetone (BDH AnalaR) to 20 ml. Glucose was deducted by spraying with this solution, and then with 2% ethanolic hydroxide, when a block spot was formed.

Background Electrolyte

The background electrolytes were (i) 0.1 M perchloric acid, and (ii) 1.0 x 10^-2M primarily ligands and varying amounts of secondary ligand (NTA). Electrophoretic observations on metal ion spots were recorded at various pH values of the background electrolyte obtained by adding sodium hydroxide solution to it. The observed mobility of the migrant was calculated by using the formula.

U = d/ x.t

After applying the correction factor , the observed mobility is given as

U = d ± dG/x.t

Where

U = mobility of metal ion/complex ion,

d = mean of duplicate distances travelled by metal ion/complex ion.

dG = mean of duplicate distances travelled by glucose spots,

x = field strength,

and t = time for electrophoresis .

The speed of the metal ion spots on the strips were thus calculated and reported along with pH values. The individual speeds of the duplicate spots are fairly equal and the variation was always less than 0.5 cm.

RESULTS AND DISCUSSION:

For the study of mixed complexes of metal ions with 2-aminobenzoic acid, 3-aminobenzoic acid and 4-aminobenzoic acid as primary ligand and NTA as secondary ligand, the usual experimental procedure adopted for simple complexation studies is modified. The background electrolyte contains primary ligand in addition to perchloric acid and the pH of the background electrolyte is maintained at 8.5. This is important because much ahead of this pH, the simple complexes of metals with primary ligands are formed and also the metal ions form 1:1 complex with NTA. Hence, if the pH of the background electrolyte is fixed at 8.5, and if the transformation of one simple complex into another ones takes place, the study will be easy. In the background electrolyte the concentration of secondary ligand NTA is progressively increased and the electrophoretic observation of the metal ion spot on paper strip is taken at every addition of the secondary ligand, pH being maintained at 8.5. All the observations have been recorded under the following conditions –

Potential - 200 volts

Time - 30 min.

Temp. - 25⁰C

Ionic strength - 0.1

Potential gradient - 6.66 volt/cm.

The present work deals with complexation of 2-aminobenzoic acid, 3-aminobenzoic acid and 4-aminobenzoic acid – NTA with the metal ion Zn(II). The primary ligands exist in protonated and deprotonated forms in solution depending upon its hydrogen ion concentration. The dissociation of ligands are given as follows :

The dissociation constant of these ligands are cited below :

2 –aminobenzoic acid, pK_a = 4.98 (1)

3 –aminobenzoic acid, pK_a = 4.79 (1)

4 –aminobenzoic acid, pK_a = 4.92 (1)

CALCULATION OF STABILITY CONSTATS

The equation for overall mobility modified for the system under investigation as follows :

U = u₀+ u₁k₁L/1 + k₁L

Where U₀ and U₁are mobility of uncomplexed metal ion and 1:1 cationic complex, respectively. L is unprotonated ligand and K₁ is stability constant of 1:1complex which can be expressed as :

M.L. = ML

K₁ = [ML] / [M] [L]

Using the principle of average mobility K₁ can be calculated with the help of mobilities of first plateau and second plateau at the point where k = I/L. the concentration of unprotonated ligands at different pH values, for calculation of stability constants, can be calculated with the help of equilibria of protonated and deprotonated species of the ligands. With the help of dissociation constants of ligands, concentration of unprotonated ligand can be calculated by the following equation.-

[L] = Lt/ 1 + K/H⁺

Where

[L] = concentration of unprotonated species.

Lt = Total ligand concentration.

K = Dissociation constant of the ligand.

COMPLEXATION WITH NTA

Nitrilotriacetic acid on ionization gives ultimately triply charged anion which acts as a powerful tridentate or tetradentate ligand. Coordination of metal ion with this ligand has been widely studied by several workers. The ionization of the acid is well known and can be represented as follows.

Our interest with it is to find out its role in acting as ligand in mixed complexes.

The study of mixed complexes involving 2-aminobenzoic acid, 3-aminobenzoic acid and 4-aminobenzoic acid as primary ligand and NTA as secondary ligand has been carried out at fixed concentration of primary ligand and progressive addition of secondary ligand, NTA, to the background electrolyte maintaining the pH at 8.5.

CONCLUSION:

The relevant observation on mobilities are recorded in tables 1, 2, 3, and are graphically represented in figure 1, 2, 3. These figures indicate the transformation of ML+NTA =ML-NTA complexes. There are two plateaus in each cases. In the first plateau constant values of mobility obviously corresponds to the mobility of Zn(II)-2-amino, Zn(II)-3-amino- and Zn(II)-4-aminobenzoic acid complexes.

Whereas the second plateau corresponds to the mobilities of new complex.

This new complex may be a binary complex of Zn(II) - NTA type (ML+NTA= M-NTA + L) where the primary ligand has been completely replaced by NTA, the secondary ligand, or it may also be a mixed complex of the type M-L+NTA= M-L-NTA, where M is Zn(II) ion.

Obviously the final plateau corresponds to the mobility of M-NTA or M-L-NTA whichever is formed in the interaction. It is found that the mobility of final plateau is greater than the mobility of M-NTA (Table 4).

Which indicates the formation of a new complex with higher charge. This new complex is the mixed complex formed in the interaction. The stability constants of these complexes are reported in table (5). The higher values or stability constant of ternary complexes of 3-aminobenzoic acid than those of 4-aminobenzoic acid and 2-aminobenzoic acid and that of 4-aminobenzoic acid than 2-aminobenzoic acid are attributed to the stronger acidic character of 3-aminobenzoic acid than 4-amino, and 2-aminobenzoic acid and that of 4-aminobenzoic acid than 2-aminobenzoic acid. It must be noted that higher values of stability constants in case of ternary complexes than binary complexes indicate their increased stability, which is further supported by the work of sillen and Martell⁵⁷.

TABLE 1: Zn (II) 2-Aminobenzoic acid –NTA system (Concentration of 2-aminobenzoic acid in the background electrolyte -1.0x10^-2M, Concentration of metal ion in the spot -5.0 x 10^-3M, Potential – 200 volts , Temp. 25⁰C, Time – 30 Min.)

S. No.	Conc. of NTA	-log NTA	Movement (cm)	Mobility (cm²volt^-1min^-1)x 10³
1	1x10^-7	7.00	1.7	8.5
2	2x10^-7	6.69	1.7	8.5
3	4x10^-7	6.39	1.7	8.5
4	6x10^-7	6.22	1.7	8.5
5	8x10^-7	6.09	1.7	8.5
6	1x10^-6	6.00	1.7	8.5
7	2x10^-6	5.69	1.7	8.5
8	4x10^-6	5.39	1.7	8.5
9	6x10^-6	5.22	1.6	8.0
10	8x10^-6	5.09	1.7	8.5
11	1x10^-5	5.00	1.7	8.5
12	2X10^-5	4.69	1.8	9.0
13	4 X 10^-5	4.39	1.7	8.5
14	6X10^-5	4.22	1.7	8.5
15	8x10^-5	4.09	1.7	8.5
16	1x10^-4	4.00	1.7	8.5
17	2x10^-4	3.69	1.7	8.5
18	4x10^-4	3.39	1.7	8.5
19	6x10^-4	3.22	1.7	8.5
20	8x10^-4	3.09	1.7	8.5
21	1x10^-3	3.00	1.7	8.5
22	2x10^-3	2.69	1.6	8.0
23	4x10^-3	2.39	1.0	5.0
24	6x10^-3	2.22	0.4	2.0
25	8x10^-3	2.09	0.0	0.0
26	1x10^-2	2.00	-0.7	-3.5

TABLE 2: Zn (II) 3-Aminobenzoic acid –NTA system ( Concentration of 3-aminobenzoic acid in the background electrolyte -1.0x10^-2M, Concentration of metal ion in the spot -5.0 x 10^-3M, Potential – 200 volts, Temp. 25⁰C, Time – 30 Min.)

S.No.	Conc. of NTA	-log NTA	Movement (cm)	Mobility (cm²volt^-1min^-1)x 10³
1	1x10^-7	7.00	1.7	8.5
2	2x10^-7	6.69	1.6	8.0
3	4x10^-7	6.39	1.7	8.5
4	6x10^-7	6.22	1.7	8.5
5	8x10^-7	6.09	1.7	8.5
6	1x10^-6	6.00	1.7	8.5
7	2x10^-6	5.69	1.8	9.0
8	4x10^-6	5.39	1.7	8.5
9	6x10^-6	5.22	1.7	8.5
10	8x10^-6	5.09	1.7	8.5
11	1x10^-5	5.00	1.7	8.5
12	2x10^-5	4.69	1.7	8.5
13	4x10^-5	4.39	1.7	8.5
14	6x10^-5	4.22	1.7	8.5
15	8x10^-5	4.09	1.7	8.5
16	1x10^-4	4.00	1.6	8.0
17	2x10^-4	3.69	1.0	5.0
18	4x10^-4	3.39	0.2	1.0
19	6x10^-4	3.22	0.4	-2.0
20	8x10^-4	3.09	-0.6	-3.0
21	1x10^-3	3.00	-0.8	-4.0
22	2x10^-3	2.69	-0.8	-4.0
23	4x10^-3	2.39	-0.8	-4.0
24	6x10^-3	2.22	-0.8	-4.0
25	8x10^-3	2.09	-0.8	-0.4
26	1x10^-2	2.00	-0.8	-4.0

TABLE 3: Zn (II) 4-Aminobenzoic acid –NTA system ( Concentration of 4-aminobenzoic acid in the background electrolyte -1.0x10^-2M, Concentration of metal ion in the spot -5.0 x 10^-3M, Potential – 200 volts, Temp. 25⁰C, Time – 30 Min.)

S. No.	Conc. of NTA	-log NTA	Movement (cm)	Mobility (cm²volt¹1min^-1) x 10³
1	1x10^-7	7.00	1.7	8.5
2	2x10^-7	6.69	1.7	8.5
3	4x10^-7	6.39	1.6	8.0
4	6x10^-7	6.22	1.7	8.5
5	8x10^-7	6.09	1.7	8.5
6	1x10^-6	6.00	1.7	8.5
7	2x10^-6	5.69	1.7	8.5
8	4x10^-6	5.39	1.8	9.0
9	6x10^-6	5.22	1.7	8.5
10	8x10^-6	5.09	1.7	8.5
11	1x10^-5	5.00	1.7	8.5
12	2x10^-5	4.69	1.7	8.5
13	4x10^-5	4.39	1.7	8.5
14	6x10^-5	4.22	1.7	8.5
15	8x10^-5	4.09	1.7	8.5
16	1x10^-4	4.00	1.7	8.5
17	2x10^-4	3.69	1.6	8.0
18	4x10^-4	3.39	1.4	7.0
19	6x10^-4	3.22	1.0	5.0
20	8x10^-4	3.09	0.8	4.0
21	1x10^-3	3.00	0.5	2.5
22	2x10^-3	2.69	-0.7	-3.5
23	4x10^-3	2.39	-0.7	-3.5
24	6x10^-3	2.22	-0.7	-3.5
25	8x10^-3	2.09	-0.7	-3.5
26	1x10^-2	2.00	-0.7	-3.5

TABLE 4: M-NTA system, Concentration of metal ion in the spot – 5.0 X 10^-3M, Potential – 200 volts, temp. -25⁰c, Time – 30 Min.

S. No.	Conc. Of NTA	-log NTA	Mobility of Zn(ll)
1	10^-5	5.00	7.5
2	10^-4	4.00	3.0
3	10^-3	3.00	1.0
4	2x10^-3	2.69	-1.0
5	4x10^-3	2.39	-2.0
6	6x10^-3	2.22	-3.0
7	8x10^-3	2.09	-3.0
8	10^-2	2.00	-3.0

TABLE 5: Stability constants of Zn (II) Aminobenzoic acid – NTA Complexes

Temp . – 25⁰C Ionic strength – 0.1

S.No.	Primary ligand	Secondary ligand	Calculated Value of log
1	2-aminobenzoic acid	NTA	3.08
2	3-aminobenzoic acid	NTA	4.38
3	4-aminobenzoic acid	NTA	3.86

Figure-1 Representing mobility curve [Zn(II)-2-Aminobenzoic acid-NTA] system.

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Received on 03.01.2016 Modified on 20.01.2016

Asian J. Research Chem. 9(1): Jan., 2016; Page 33-39

DOI: 10.5958/0974-4150.2016.00007.9