Spectrophotometric study of complex formation between iodoquinol (IQ) and Co²⁺, Mn²⁺, Cd²⁺, Pb²⁺, and Zn²⁺ in DMF/MeOH binary mixed solvents

1 Introduction

Because of their very low toxicity, the derivatives of halogenated 8-hydroxyquinoline have been widely used as anti-amebic compounds. 5,7-diiodo-8-hydroxyquinoline or iodoquinol (IQ) belongs to the group of medicines called antiprotozoals. The IQ is one of the iodinated derivations of 8-hydroxyquinoline used most often in the treatment of an intestinal and vaginal infection called amebiasis (Lopez-De-Alba et al., 1997; Padmanabhan et al., 1989). The chemical structure of IQ is shown in Scheme 1. The literature revealed different techniques for the analysis of IQ such as spectrophotometry (Ashour et al., 1990; Belal, 1984), polarography (Nezhadali et al., 2008; Hasebe et al., 1989), TLC (Parimoo et al., 1992, 1993), and HPLC in pharmaceutical preparations (Ray, 1990; Moore and Carter, 1988) and in biological fluids (Ezzedeen et al., 1983, 1981; Hayakawa et al., 1982).

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Scheme 1

The chemical structure of IQ.

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Scientists are interested to find the solvent effects on solution properties that depend on all possible intermolecular interactions between solvent molecules and solute (Matubayasi et al., 2007; Quiroga et al., 2007; Gharib et al., 2006; Kailasam et al., 2006; Ray and Leszczynski, 2006; Solomonov et al., 2006; Tekin et al., 2005; Wu et al., 2005; Ahmed and Khan, 2000; Hush and Reimers, 1998). The aim of many studies in complex formation (in mixed solvent systems) is investigation of the effect of solvent composition on the stability and nature of the formed complexes. It is clear that the selectivity and stability of complexation are strongly depending to the solvating ability of the solvent. This is important to inquire into the relation between the stability constant and the composition of binary mixed solvents. As in the literature, the nature of solvent types can produce significant changes in the binding properties of the ligands (Shamsipur and Ghasemi, 1995). The various spectrophotometry methods have proven to be a useful means for investigation of stoichiometry and stability of complexes. In this work, the effect of the solvent types on the selectivity and stability of complexes of M²⁺/IQ in a binary mixed solvents system (DMF/MeOH) was investigated by a spectrophotometry method.

2 Experimental details

3 Results and discussion

The electronic absorption spectra of metal ions, ligand, and formed complexes were obtained in DMF at 25 °C. The concentration of metal ions and ligand was 1 × 10⁻⁴ M (0.1 mM). As shown in Fig. 1, the maximum absorption of Cd⁺²/IQ is 409.5 nm. The stoichiometry of the complexes in DMF solvent was obtained by Job (Job, 1928) and molar ratio methods (Likussar and Boltz, 1971). A sample of resulting plots is shown in Fig. 2 (Job's method) and Fig. 3 (mole ratio method). In the molar ratio method, the M²⁺ solutions (5 mM) were separately added to the ligand solution (10 mL, 0.5 mM). The stoichiometry of all complexes was obtained 1:1.

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Figure 1

The UV–vis spectra of Cd²⁺ ( ); IQ ( ) and Cd²⁺/IQ ( ) solutions.

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Figure 2

The continuous variation plot of Cd⁺²/IQ in DMF solutions.

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Figure 3

The molar-ratio of [IQ] = 0.5 and [Cd²⁺] = 5 mM.

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The stability constants of the 1:1 M²⁺/IQ complexes were obtained in different ratios (v/v) of DMF/MeOH as binary mixed solvents by measuring the complex's absorbance at maximum wavelength (λ_max). During the experiments, the M²⁺ solutions at different concentrations (1–3 mM) were separately added to the ligand solution. The data given in Table 1 indicated the effect of solvent type on the stability of M²⁺/IQ complexes. In all the cases, there is a reverse relationship between the stabilities of complexes and the amount of DMF in the binary mixed solvents. The stability of metal cations complexes with IQ in the most cases was decreased in the order of Zn²⁺ > Co²⁺ > Pb²⁺ > Mn²⁺ > Cd²⁺. The stability of a complex formed in a solution strongly depends on the nature of the solvent medium (Rounaghi et al., 2000). The solvation of the ligands and metal cations was influenced by the donor ability, dielectric constant of the solvent, shape, and size of the solvent molecules (Strasser and Popov, 1985; Popov, 1979). Generally, it is expectable that the stability constant of the complex should decrease owing to the competition between the ligand and the solvent molecules for the M²⁺ in solvents with high donor ability and dielectric constant. However, it has been shown that the donor ability of the solvent plays the most important role in the behavior of complexes in non-aqueous solvents (Popov, 1979).

In during the complexation, the ligand can be able to replace the molecules of solvent as totally as possible in the first solvation shell of the M²⁺, or the M²⁺ should be able to replace the solvent molecules with the ligands. Therefore, the variation of the solvent composition produces a significant change in the binding properties and selectivity of the ligand for a certain M²⁺. There is no enough scientific data about the M²⁺/IQ complexes in binary systems as well as mixed solvents are available in the literature. The variation of the formation constants of IQ complexes with Co²⁺, Mn²⁺, Cd²⁺, Pb²⁺, and Zn²⁺ ions with solvent compositions in DMF/DMF binary mixture is illustrated in Fig. 4. As shown in Fig. 4, the stability constants of all complexes were increased with increasing amount of MeOH in the DMF/MeOH binary mixtures. In a weak solvating solvent system such as MeOH, which has a low donor number (DN = 20) (Burger, 1983), the solvating of the metal ions and the ligand should be less than DMF with a bigger donor number (DN = 26.6) (Rounaghi et al., 1997). Therefore, less energy is needed at the desolvation step of Co²⁺, Mn²⁺, Cd²⁺, Pb²⁺, and Zn²⁺ ions and IQ during the complex formation in MeOH.

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Figure 4

The variation of the stability constants of M²⁺/IQ in DMF/MeOH binary systems: Pb²⁺/IQ (×); Mn²⁺/IQ (▴); Zn²⁺/IQ (∗); Co²⁺/IQ (■), and Cd²⁺/IQ (♦).

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