INTRODUCTION:
Schiff bases play important roles in coordination chemistry as they easily form stable complexes with most transition metal ions 1, 2. Many attempts have been made to evaluate different factors affecting the stability of the metal chelates along with their stability constants 3-5. In the present study the stability constants of the mixed ligand complexes of Co(II), Ni(II), Cu(II) and Zn(II) with 4-bromo-2-{(E)-[(2,3-dimethylphenyl) imino]methyl}phenol (X) as primary ligand (Figure 1) and 1-[(1E)-N-(2,6-dichlorophenyl)ethanimidoyl] naphthalen-2-ol (Y) as secondary ligand (Figure 2) in 75: 25 (v/v) 1,4-dioxane-water medium at 27 ± 0.5oC have been reported by employing pH-metric titration technique 6-10. Under identical conditions the stability constants of binary metal complexes of 1-[(1E)-N-(2,6-dichlorophenyl)ethanimidoyl]naphthalen-2-ol (Y) have also been investigated.
Figure1. Primary ligand 4-bromo-2-{(E)-[(2,3-dimethylphenyl)imino]methyl}phenol
Figure2. Secondary ligand 1-[(1E)-N-(2,6-dichlorophenyl)ethanimidoyl]naphthalen-2-ol
EXPERIMENTAL:
The pH-meter model no. EQ-614 supplied by Equiptronics, a precision research pH-meter with wide range of glass electrode and calomel reference electrode was used for pH measurements. The pH-meter was standardized with potassium hydrogen phthalate and phosphate buffers before performing the titrations.
The solutions of ligands were prepared in 1,4-dioxane. All the metal ion solutions were prepared in double distilled water and standardized by using conventional procedures 11. A solution of KOH (0.2 M) was prepared in double distilled water and standardized with standard solution of succinic acid. The titrations were carried out in an inert atmosphere of nitrogen. All the measurements were carried out at temperature 27 ± 0.5oC. The method of Bjerrum and Calvin as modified by Irving and Rossotti 6,7 was used to determine ñA (average number of protons associated with secondary ligand ); ñ (average number of secondary ligand molecules attached per metal ion; ñmix (average number of secondary ligand molecules attached per (M.X)- ion); pL (free ligand exponent for binary (M-Y) system) and pLmix (free ligand exponent for ternary (M-X-Y) system) values. All the solvents and chemicals used were of A R grade.
For the determination of proton-ligand stability constant of secondary ligand (Y) and metal-ligand stability constants of binary (M-Y) and ternary (M-X-Y) complexes, the following sets of solutions were prepared keeping the total volume Vo = 40 mL. All titrations were carried out at the ionic strength of 0.1 M using KCl as an electrolyte in 75:25 (v/v) 1,4-dioxane-water medium against standard carbonate free KOH (0.2 M) solution.
i. 4.0 mL HCl (0.16 M) + 3.36 mL KCl (1.0 M) + 2.64 mL distilled water + 30.0 mL 1,4-dioxane.
ii. 4.0 mL HCl (0.16 M) + 3.36 mL KCl (1.0 M) + 2.64 mL distilled water + 4.0 mL secondary ligand (0.04 M) + 26.0 mL 1,4-dioxane.
iii. 4.0 mL HCl (0.16 M) + 3.24 mL KCl (1.0 M) + requisite volume of metal(II) chloride solution to give 0.001 M metal(II) chloride concentration in the final solution + requisite amount of distilled water + 4.0 mL secondary ligand (0.04 M) + 26.0 mL 1,4-dioxane.
iv. 4.0 mL HCl (0.16 M) + 3.36 mL KCl (1.0 M) + 2.64 mL distilled water + 4.0 mL secondary ligand (0.01 M) + 26.0 mL 1,4-dioxane.
v. 4.0 mL HCl (0.16 M) + 3.24 mL KCl (1.0 M) + requisite volume of metal(II) chloride solution to give 0.001 M metal(II) chloride concentration in the final solution + requisite amount of distilled water + 4.0 mL primary ligand (0.01 M) + 4.0 mL secondary ligand (0.01 M) + 22.0 mL 1,4-dioxane.
The ratio of metal (M): secondary ligand (Y) was maintained at 1:4 in the binary system. In the ternary system the ratio of metal (M): primary ligand (X): secondary ligand (Y) was maintained at 1:1:1. The pH-meter readings were plotted against the volume of alkali (KOH) used for each titrations.
RESULT AND DISCUSSION:
Proton-ligand stability constants of secondary ligand (Y)
From
the titration curves of solutions (i) and (ii), ñA values at
various pH were calculated. The proton ligand formation curve was obtained by
plotting the values of ñA vs. pH-meter readings. From the graph the
values of log
and log
were
evaluated by half integral method (A). The values of log and
log were
also evaluated using graphical method (B) by plotting the graph of log [ñA
/ (1-ñA)] against pH and log [(2- ñA) / (ñA-1)]
against pH, respectively. The values obtained by method A and B are in
agreement with each other, the average values of log and
log has
been found to be 8.63 and 2.15 respectively.
Metal-ligand stability constants of the Binary (M-Y) complexes
The metal ligand stability constants of binary complexes were evaluated assuming the polynuclear complexes and hydrolyzed products were not formed. The metal-ligand binary binding proposed structure is shown in figure 3. An examination of titration curves indicate that complex formation takes place in the solution on the following grounds: (I) The metal titration curve of solution (iii) shows displacement with respect ligand (Y) titration curve of solution (ii) along the volume axis. This indicates the affinity of the ligand to metal ions which release proton and produce volume difference. (II) The color change of ligand appeared in the presence of metal ion shows the formation of new species due to coordination. (III) The hydrolysis of the metal ion was suppressed due to the complex formation and precipitation did not appear during the titrations.
From
the titration curves of solutions (ii) and (iii), ñ and pL values were
calculated. The formation curves were obtained by plotting the values of ñ
vs. pL. From the graph the values of log
and
log
were
evaluated by half integral method (A) and in the similar way the values of logand
logwere
evaluated using graphical method (B) by plotting the graph of log [ñ / (1-ñ)]
against pL and log [(2-ñ) / (ñ-1)] against pL respectively. The
values obtained by method A and B are in agreement with each other, the average
values of logand
log,
along with metal-ligand stability constants the log β values of the binary
complexes are given in Table 1.
The
variations of ñ was found to be between 0.0-2.0 for the binary (M-Y) complexes
of Co(II), Ni(II), Cu(II) and Zn(II) metal ions, which indicate that the composition of the
complexes were 1:2 in solution. The logvalues
for the binary complexes of the metal ions are in the following order: Cu(II)
> Ni(II) > Co(II) > Zn(II).
M = Co(II), Ni(II), Cu(II) and Zn(II).
Figure 3. The metal-ligand binary binding proposed structure.
Metal-ligand stability constants of the ternary (M-X-Y) complexes
The metal-ligand stability constants of ternary complexes were evaluated assuming that the formation of polynuclear complexes and hydrolyzed products were not formed. The metal-ligand ternary binding proposed structure is shown in figure 4. An examination of the titration curves indicate that the ternary complexes formation has taken place in the solution on the following grounds: (I) the horizontal distance was measured between ternary titration curves of solution (v) and secondary ligand titration curve of solution (iv), the positive difference shows the earlier release of protons in the formation of ternary complexes. (II) The hydrolysis of the metal ion was suppressed and precipitation did not appear during the titrations.
From
the titration curves of solutions (iv) and (v), ñmix and pLmix
values were calculated. The values of log
have
been evaluated from the formation curves (ñmix vs. pLmix).
At ñmix = 0.5, in the formation curve, pLmix = log (method
A). The values of log were
also evaluated by the graphical method (B), by plotting the graph of log [ñmix
/ (1-ñmix)] against pLmix. The values of ñmix
vary from 0.0-1.0, thus confirm the formation of 1:1:1 mixed ligand complexes.
The values obtained by method A and B are in agreement with each other, the
average values of log are
included in Table 1.
The
values of logare
slightly lower than logand
higher than log,
which is due to the fact that the tendency of the secondary ligand (Y) to get
bound with aquated metal ion [M(aq)]2+ is more than to combine with
the metal ion already bound with primary ligand (X) 12.
The
relative stability of the ternary complexes compared with corresponding binary
complexes can be qualitatively expressed in many different ways. We have
expressed the relative stabilities in terms of Δ log KT (Δ
log KT = log–
log).
The Δ log KT values for all the metal(II) ions in the present
study (Table 1) is negative. This indicates that ternary 1:1:1 (M-X-Y)
complexes are less stable than binary 1:1 (M-Y) complexes 13,14.
In the ternary system studied, the order of stability constants of mixed ligand complexes with respect to the metal ions was found to be Cu(II) > NI(II) > Co(II) > Zn(II); which is same as in the corresponding binary (M-Y) systems. This is in accordance with the Irving-Williams series of stability constant 15,16.
M = Co(II), Ni(II), Cu(II) and Zn(II).
Figure 4. The metal-ligand ternary binding proposed structure.