Potentiometric and conductometric studies on the binary and mixed ligand complexes in solution: MII-dipicolinic acid–glycine systems

Talanta 44 (1997) 1365 – 1369

Potentiometric and conductometric studies on the binary and mixed ligand complexes in solution: M II-dipicolinic acid–glycine systems M.M. Khalil *, S.A. Mohamed, A.M. Radalla Department of Chemistry, Faculty of Science, Cairo Uni6ersity, Beni-Suef Branch, Beni-Suef, Egypt Received 13 June 1996; received in revised form 6 November 1996; accepted 13 November 1996

Abstract The binary and mixed ligand complexes of some alkaline earth and transition metal (II) ions with dipicolinic acid (DPA) as a primary ligand and the biologically important secondary ligand (glycine), were studied using potentiometric technique. The acidity constants of the ligands were determined and used for determining the stability constants of the complexes formed in aqueous solutions under the experimental conditions (t =25°C, m= 0.1 M NaNO3 ). The dissociation constants of DPA were also determined in various water + dioxane mixtures under the same experimental conditions. It is concluded that a pronounced change in the pK values is observed as the solvent is enriched in dioxane. The values of Dlog K have been evaluated and discussed. In addition, the chelation mode of ternary complexes was ascertained by conductivity measurements. © 1997 Elsevier Science B.V. Keywords: Binary and ternary complexes; Dipicolinic acid; Potentiometric and conductometric studies

1. Introduction Over the years there has been a steadily increasing interest in the complexes of pyridine derivatives [1–4] arises due in part to their physiological properties. The importance of pyridinecarboxylic acids stems from their presence in many natural products (alkaloids, vitamins, coenzymes, etc.). They are also of great interest to medicinal chemists because of the wide variety of their physiological properties displayed by the natural

* Corresponding author.

and synthetic acids. It is well known that complexes of metal ions are among the prominent interactions in nature [5,6], and the glycine residue is an important and versatile binding site of protein. Among pyridine dicarboxylic acids, dipicolinic acid seems to have the best chelating properties because it is terdentate. As a continuation of our research program oriented to study binary and ternary complexes of biological importance, [7–11] the present work reports the formation and characterization of binary and mixed ligand complexes of the type M II-DPA–glycine, where M= Mg, Ca, Sr, Ba, Cu, Co, Ni, Zn, Mn or Cd ions.

0039-9140/97/$17.00 © 1997 Elsevier Science B.V. All rights reserved. PII S 0 0 3 9 - 9 1 4 0 ( 9 6 ) 0 2 1 7 3 - X

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2. Experimental

2.1. Materials and solutions Dipicolinic acid was obtained from Fluka. The reagent was repeatedly recrystallized from water, dried at 115°C, and checked by its melting point (250°C). Stock solutions were prepared by dissolving precisely weighed amounts of the anhydrous acid in suitably bidistilled water. Glycine was also provided by Fluka. The metal salts were provided by BDH as nitrates or chlorides. All solutions of metal (II) ions were prepared and standardized complexometrically by EDTA using suitable indicators [12]. Dioxane was of high purity (spectro grade product). Carbonate-free sodium hydroxide (titrant, prepared in 0.1 M NaNO3 solution) was standardized potentiometrically with KH-phthalate solution (Merck AG). Nitric acid and NaOH were from Merck p.a.

cm3)(e) (d)+ 0.01 M M II (2 cm3)(f) (a)+0.01 M M II (5 cm3)+ 0.01 M DPA (5 cm3)+0.01 M glycine (5 cm3)The total volume was adjusted to 50 cm3 by adding double-distilled water in each case. All pH-metric titrations were performed at 25°C and m=0.1 M (NaNO3). An Irving and Rossotti pH titration technique [13] with modifications [14,15] was used to determine the protonation constants of the ligands and formation constants of the metal complexes. The equilibrium constants were calculated from six independent titration curves. The errors given in Table 1 and Table 2 are three times the standard error of the mean or the sum of the probable systematic errors, whichever is the larger. Mixture (g) was titrated conductometrically against 0.1 M NaOH solution: g, 0.01 M CuII (10 cm3)+ 0.01 M DPA (10 cm3)+ 0.01 M glycine (10 cm3).

2.2. Apparatus

3. Results and discussion

Potentiometric pH measurements were carried out on solutions in a double-walled glass vessel at 25°C9 0.1°C using a Griffin pH J-300-010 G Digital pH meter. The temperature was controlled by circulating water through the jacket, from a constant temperature bath. The cell was equipped with magnetic stirrer and a tightly fitting rubber stopper, through which an Amel 882 delivery dispenser, readable to 1 ml, and electrode system were inserted. The electrode system was calibrated in terms of hydrogen-ion concentrations instead of activities. Thus, all constants determined in this work are concentration constants. Conductance of solutions was measured with PTI-10 Mini Digital Conductivity and temperature meter.

Dipicolinic acid, H2L, can be further protonated as the acidity of the medium increases forming a monopositive species, H3L + . However, in the pH range utilized, this cationic form always proved negligible as checked previously using spectrophotometric technique [16]. The first and second proton association constants of neutral DPA were determined potentiometrically in aqueous solutions, under the experimental conditions (t= 25°C, m =0.1 M

2.3. Procedure and measuring techniques The following solutions were prepared and titrated potentiometrically against standard carbonate- free NaOH (0.1053 M) solution:(a) 0.04 M HNO3 (5 cm3) +0.50 M NaNO3 (10 cm3)(b) (a) +0.01 M DPA (5 cm3)(c) (b)+ 0.01 M M II (2 cm3)(d) (a) + 0.01 M glycine (5

Table 1 pKa values of dipicolinic acid in aqueous dioxane media (t= 25°C, m=0.1 M NaNO3) Solvent composition %v/v

pK1

pK2

0.00 10 20 30 40 50 60

2.32 90.05 2.68 90.04 2.81 90.07 3.02 90.04 3.28 9 0.06 3.65 90.08 3.96 9 0.08

4.53 9 0.06 5.15 9 0.07 5.45 9 0.04 5.67 9 0.08 5.98 9 0.05 6.32 9 0.06 6.75 9 0.05

The pKa values were calculated from pH titration data without correction.

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Table 2 Acidity constants of dipicolinic acid and stability constants of binary and mixed ligand complexes at 25°C, m=0.1 M (NaNO3) Cation

log k H 1

log k H 2

H Mg Ca Sr Ba Cu Co Ni Zn Mn Cd

4.5390.06

2.329 0.05

a

log k M MA

log k M ML

log k MA MAL

log b M MAL

Dlog k

2.50 9 0.05 4.50 90.08 3.98 9 0.06 3.60 9 0.06 9.14a 6.65a 6.95a 6.35a 5.01a 6.75a

3.45 90.08 5.12 90.06 3.90 90.08 3.45 90.04 7.85 90.06 4.90 9 0.05 5.90 90.06 5.46 9 0.06 5.12 9 0.04 4.30 9 0.05

3.92 9 0.05 3.80 90.04 3.99 9 0.06 3.73 9 0.06 5.80 9 0.07 3.10 9 0.06 4.77 90.08 3.58 90.05 3.74 90.07 1.65 90.08

6.42 8.30 7.97 7.33 14.94 9.75 11.72 9.93 8.75 8.40

0.47 −1.32 0.09 0.28 −2.05 −1.80 −1.13 −1.88 −1.38 −2.65

The values were previously determined by Anderegg using pCu-method [21], at 20°C, m= 0.1 M (NaNO3).

NaNO3). The values obtained (Table 1) are in a good agreement with the literature values [17]. In order to shed more light on the dissociation of DPA in various water+ organic solvent mixtures, dioxane (an aprotic nonionizing coorganic solvent) was chosen. The observed increase in the pKa values of DPA upon enrichment of the solvent with dioxane may be attributed to a lowering of the dielectric constant (o = 76.55, 60.03 for 10 and 60% v/v solvent composition, respectively at 25°C) which increases in turn the fraction of associated ions to form Bjerrum ion pairs [18] and higher aggregates such as triple ions and dipole aggregates [19]. In this medium, free ions have very low concentration and acidity phenomena are governed largely by ionic association reactions, as previously reported by Kolthoff and Bruckenstein [20]. The pKa values of DPA in aqueous dioxane media are listed in Table 1. The stability constants of normal 1:1 binary complexes of DPA with alkaline earth metal ions have been determined. The values obtained (Table 2) agree well with those previously reported [21]. The stabilities decrease in the order Ca(II) \ Sr(II) \Ba(II) \Mg(II). In the cases of the 1:1 transition metal ions complexes with dipicolinic acid the acid strength of the complexing agent combined with the great stabilities of the complexes made the determination of stability constants by the pH method very inaccurate. The values of stability constants are

taken from the work of Anderegg [21] using a copper amalgam electrode (p Cu-method) as shown in Table 2. The second association constant of glycine was determined, the obtained value (9.76) agree quite well with that previously reported [22]. The stability constants of glycine complexes with Mg, Cu, Co, Ni, Zn or Cd ions, agree well with literature values [22–25]. The disagreement found for the values of Ca, Sr, Ba or Mn complexes my be attributed to the different methods and ionic strength used for determination [23,26,27]. For the formation of the ternary complexes of the selected bivalent metal ions in presence of DPA= A and glycine=L, the following equilibria may be considered: M+ A ? MA MA+ L ? MAL k MA MAL =

[MAL] [MA][L]

(1)

Here complex formation is considered to take place in a stepwise manner, i.e., the secondary ligand, glycine, starts complexation after the complete formation of the binary 1:1 complex of DPA. Representative titration curves for ternary systems investigated are shown in Fig. 1 and Fig. 2. The formation constants obtained are reported in Table 2.

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The relative stability of the ternary complexes as compared with that of the corresponding binary complexes can be quantitatively expressed in different ways. The most suitable comparison is in terms of Dlog K, which represents the difference in stabilities for the addition of ligand L to the 1:1 MA complex and to the aquated metal ion as shown by Eq. (2) [28].

M Dlog K=log K MA MAL −log K ML

M =log K ML MLA −log K MA

(2)

The overall stability constant b M MALwhich must be determined experimentally, is connected to K MA MALby Eq. (3): M M log K MA MAL =logb MAL −logK MA

(3)

Fig. 1. Titration curves for the Ca + + –DPA – gly. system

Fig. 2. Titration curves for the Ni + + – DPA – gly. system.

It is observed, that in general, Dlog K for the investigated ternary complexes is negative as expected from the statistical considerations (Table 2). There is p acidic character in the primary ligand (DPA), due to the possibility of M “Np bond formation. This behaviour is similar to that observed previously in [M-dipyridyl-L] complexes [14,29]. The conductometric titration curve of the ternary complex containing copper (II) with DPA and glycine (Fig. 3) shows an inflection at a =2, probably corresponding to the neutralization of protons resulting from the formation of Cu-DPA binary complex. Between 2B aB3, the slight increase of conductance is due to the formation of the ternary complex and is associated with the release of a proton from the secondary ligand (glycine). Beyond a= 3, the conductance increases more uniformly due to the presence of excess sodium hydroxide.

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Fig. 3. Conductometric titration curve of Cu + + – DPA – gly. system.

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