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Journal of the Chilean Chemical Society

versión On-line ISSN 0717-9707

J. Chil. Chem. Soc. v.51 n.2 Concepción jun. 2006

http://dx.doi.org/10.4067/S0717-97072006000200006 

 

J. Chil. Chem. Soc., 51, Nº 2 (2006) , pags: 865-867

 

COPPER(II) COMPLEX WITH THE TETRADENTATE LIGAND 1,5-BIS

(4-DITHIOCARBOXYLATE-1-DODECYL-5-HYDROXY-3-METHYLPYRAZOLYL)PENTANE. LIQUID-LIQUID EXTRACTION STUDY

 

ALFONSO OLIVA*, AURORA MOLINARI, CAROLINA AVILA AND MARIA FERNANDA FLORES

Instituto de Química, Pontificia Universidad Católica de Valparaíso, Chile


ABSTRACT

The synthesis of the complex and the solvent extraction behaviour of copper(II) from acid solution (pH 0-5) was studied with the new reagent 1,5-bis(4-dithiocarboxylate-1-dodecyl-5-hydroxy-3-methylpyrazolyl)pentane (H2DTC) as extractant. The reagent acts as tetradentate ligand and the extracted specie was found to be CuDTC.

Keywords: 5- Pyrazolone, dithiocarboxylate, tetradentate ligand; copper complex, solvent extraction


INTRODUCTION

The pyrazolyl-4-alkyldithiocarboxylates 1 (Fig.1) are heterocyclic bidentate chelating ligands with interesting liquid-liquid extraction properties, where the carboxylate and the hydroxyl substituents are responsible of these properties. These ligands are prepared by the reaction of 5-pyrazolones with carbon disulfide and alkyl halides in n-butyllithium-THF, sodium acetate-DMF or under phase-transfer catalysis conditions [1-7]. Also, chelating polymers containing the pyrazolyl-4-dithiocarboxylate core have been prepared by vinylic homopolymerization of the pyrazolyl-4-allyldithiocarboxylate derivative [8]. Compounds 1 allow the extraction of several divalent transition metal ions [9,10], including the separation of Cu(II) from Fe(III) in acidic lixiviation solutions of a copper mineral with a high content of iron [11]. Furthermore, they extract toxic mercury, lead and cadmium and some of them can be used to separate Hg(II) from Pb(II) and/or Cd(II) at pH 1 in a single step [12].


1
R = Alkyl, Phenyl ; R1 = Methyl, Hydroxyl ; R2 = Alkyl, Allyl
Fig. 1. General structure of the pyrazolyl-4-alkyldithiocarboxylates

In order to get bis-pyrazolyl derivatives with tetradentate complexing properties we have studied the reaction of 5-pyrazolones with carbon disulfide and 1,2-dibromoethane or 1,3-dibromopropane, but the only isolated compounds were the cyclic ketene dithioacetals 2 (Fig. 2) (n = 2, 3), probably due to the stability of the five member dithiolane and six member dithiane cycle [1,5,6, 13-15]. With longer alkyl dibromides, molecular model analysis shows that a linear alkyl chain of five carbon atoms allows the better conformation to both pyrazolyl groups in adopting a planar tetradentate structure to chelate metal ions and recently we have reported the succesfully preparation of 1,5-bis(4-dithiocarboxylate-5-hydroxypyrazolyl)pentane derivatives 3 (Fig.2) (R = n-Dodecyl, Phenyl) with potential tetradentate characteristics [16].


 
Fig. 2. General structure of ketenedithioacetals 2 and 1,5-bis(4-dithiocarboxylate-3-methyl-5-hydroxypyrazolyl)pentane derivatives 3

The enolic 4-dithiocarboxylate-5-hydroxypyrazolyl structure of compounds 3 was assigned on the basis of the 1H and 13C NMR data. The 1H NMR spectra shows a downfield singlet at 12.30-13.74 ppm due to the enolic proton of the 5-hydroxy group , meanwhile in the 13C spectra the thiocarbonyl group is observed at 213 ppm, in agreement with previously reported values [1-7,17]. Following with these studies, in this paper we wish to report the synthesis of the copper (II) complex with the new tetradentate ligand 1,5-bis(4-dithiocarboxylate-1-dodecyl-5-hydroxy-3-methylpyrazolyl)pentane (H2DTC) and the solvent extraction behavior of copper (II) with this ligand.

EXPERIMENTAL

Reagents

All reagents used were of analytical grade and the ligand H2DTC 3 (R = n-C12H25) was synthesized by the reaction of 1-dodecyl-3-methyl-2-pyrazolin-5-one with carbon disulfide and 1,5-dibromopentane using sodium acetate as base and N,N-dimethylformamide as solvent [16]. Stock solutions 0.01 M in Cu(II) and 0.1 M in potassium nitrate at fixed pH, were prepared and standardized by iodometric titration with Na2S2O3 0.01 M. The pH was adjusted in the range 1-5 by the addition of hydrochloric acid solutions of variable concentration. Acetic acid-sodium acetate buffer was used in the extraction studies as a function of the ligand concentration at constant pH. Freshly distilled chloroform was used as solvent and all extraction experiments were achieved by the batch method with a variable-speed vortex mixer (Orbital Shaker). At 20ºC the solubility of H2DTC in water was close to 1x10-3 g/100 ml and 20.0 g/100 ml in chloroform. Elemental analysis of carbon, hydrogen, nitrogen and sulfur were obtained with a Perkin Elmer 2400 Serie II Analyser. IR spectra were recorded on a Perkin Elmer Spectrum One FT-IR spectrophotometer, using KBr discs. UV-VIS spectra were recorded on a Perkin Elmer UV/VIS Lambda EZ 201 spectrophotometer, with chloroform as solvent. Melting points were measured in a Kofler Bristol apparatus and are uncorrected.

Synthesis of the copper complex

0.3766 g (0.5 mmol) of the ligand H2DTC dissolved in 10.0 ml of CHCl3 was stirred overnight at room temperature with a solution of 0.1297 g (0.65 mmol) of Cu(OAc)2.H2O in 15 ml of methanol. The brown amorphous solid formed was filtered, dissolved in 30.0 ml of CHCl3, washed with water (3x20 ml) and dried with anhydrous Na2SO4. After filtration, the solvent was vacuum evaporated leaving 0.3910 g (0.48 mmol, 96% yield) of a bright brown solid.

mp 185ºC; IR (cm-1, potassium bromide): 2923, 2852 aliphatic CH, 1561 pyrazole ring, 413 Cu-O, 362 Cu-S ; UV-VIS (l, nm, chloroform): 324 (log e 4,14), 374 (log e 3,87), 490 (log e 2,72) ; Anal. Calcd. for CuC39H66N4O2 S4 : C, 57.49; H, 8.17; N, 6.88; S, 15.74. Found : C, 57.91; H, 7.79; N, 7.18; S, 16.04.

Extraction and Analytical Procedures

Extraction of Copper(II) from 0.01 M Stock Solution as a Function of pH

15.0 ml of the copper (II) stock solution adjusted to the desire pH, were shaken for 2 ° h in a bottle with an equal volume of a 0.1 M chloroform solution of H2DTC. After phase separation, the concentration of the copper(II) ion left in the aqueous phase was evaluated by titration with Na2S2O3 0.01 M . The concentration of the complexed metal ion in the organic phase was obtained from a material balance.

Stoichiometry of the Extracted Complex

15.0 ml of the copper(II) stock solution at a constant pH of 4.65, fixed with acetic acid-sodium acetate buffer, were treated as before with chloroform solutions of the ligand at variable concentrations. The distribution ratio, Kd, was taken as the ratio of the concentration of the copper(II) as complex in the organic phase to that present in the aqueous phase.

Reextraction of Copper(II) from the Organic Solution

The resulting organic phase containing the extracted CuDTC complex was shaken with 15.0 ml of 2 M H2SO4 aqueous solution until disappearance of the brown colour of the complex. The concentration of the reextracted copper(II) ion in the aqueous phase was evaluated as before

RESULTS AND DISCUSSION

Synthesis of the Copper Complex.

The synthesis of the complex was achieved by stirring overnight at room temperature a methanol solution of copper (II) acetate with a chloroform solution of the ligand and the complex was isolated as a bright brown solid of enough purity to be analyzed without further purification. Elemental analysis of carbon, hydrogen, nitrogen and sulfur is in agreement with a formula CuDTC and the structure 4 (Fig. 3) is proposed for this copper(II) complex obtained.


Fig. 3. Proposed structure of the CuDTC complex 4.

IR spectra of the ligand exhibit a wide and weak absorption at 3300-2500 cm-1 due to the strongly enolic associated hydroxyl-tiocarbonyl group [1,2], meanwhile this absorption is absent in the complex, due to the replacement of the hydrogen by the metal ion. Furthermore, in the 600-300 cm-1 region, two additional bands at 362 and 413 cm-1 are observed and these bands correspond to the í(Cu-S) and í(CuO) vibrations, respectively [18].

The UV-VIS spectra of the ligand shows two bands at 312 and 349 nm and this absorptions can be assigned to pyrazole and chelate ring conjugations. Cu(II) complex exhibit three bands at 324, 374 and 490 nm, the first two related to the ligand [19,20]. However, the 324 nm transition can also be assigned as a charged-transfer band, b1a ® b3g , observed in Cu(II) complexes with acetylacetone, b-acetodithioesters and thiohydroxamic acid [21-23]. The shoulder at 490 nm is typical of S ® Cu II ligandto-metal charge-transfer transition observed in square-planar copper(II) complexes with bidentate pyrazolyldithiocarboxylates [18].

Solvent Extraction of Cu(II) as a function of pH

The liquid-liquid extraction of Cu(II) with 0.01 M solutions of H2DTC in chloroform was studied as a function of pH (0-5) and the results are shown in Fig. 4. According to these results the % extraction vary from 20 to 92 % and is closely quantitative at pH 2-3 (92 and 90% respectively) in a single step.


Fig. 4. Percentage extraction of Cu(II) as a function of pH. Aqueous phase : 15.00 ml 0.01 M in Cu(II) and 0.1 M in KNO3 at fixed pH. Organic phase : 15.00 ml 0.01 M ligand H2DTC in chloroform

Effect of Ligand Concentration

Stoichiometry of the extracted complex and equilibrium constant of the extraction process were obtained by studying the effect of the ligand concentration at constant pH and metal concentration. Plot of log Kd vs log[H2DTC] at constant pH (Fig. 5) gave a straight line with a slope of 1.1 in agreement with a neutral CuDTC composition for the extracted complex and the release of two hydrogen ions in the complex formation. A value of log Kex = - 6.5 was also deduced from the slope analysis.


Fig. 5. Plot of log Kd as a function of the ligand H2DTC concentration. Aqueous phase : 15.00 ml 0.01 M in Cu(II) and 0.1 M in KNO3 and acetic acid-sodium acetate buffered pH 4.65. Organic phase : 15.00 ml 1.4x10-3-6.5x10-3 M ligand H2DTC in chloroform

Reextraction of Cu(II) from the Organic Phase

Cu(II) ion was reextracted from the organic phase with an aqueous 2M solution of sulfuric acid and at pH 2 the extraction/reextraction process was performed with an overall recovery of Cu(II) around 90%.

Furthermore, the organic phase from the reextraction was available to be used again and the recovery of Cu(II) was quantitative after two extraction/reextraction process with the same organic phase of the ligand.

CONCLUSIONS

From the results of this investigation, it is possible to establish the following conclusions :

1.- Both pyrazolyl groups of the ligand 1,5-bis(4-dithiocarboxylate-1- dodecyl-5-hydroxy-3-methyl-pyrazolyl)pentane can adopt a square- planar tetradentate conformation to chelate Cu(II).
2.- The ligand behaves as a good extractant agent of Cu(II) at pH 2-3.
3.- The Cu(II) ion can be reextracted from the organic phase with aqueous H2SO4 solutions.

ACKNOWLEDGEMENTS

The authors gratefully acknowledge financial support from the Dirección de Investigación de la Pontificia Universidad Católica de Valparaíso, Chile (Project DI 125.773/2004 and DI 125.791/2005).

 

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e-mail: aoliva@ucu.cl