Extraction and Complexation of Metal Cations with Methoxy Derivative of Octahomotetraoxacalix[4]arene

Extraction and Complexation of Metal Cations with Methoxy Derivative of Octahomotetraoxacalix[4]arene

B. Mellah Laboratory of Composite

Materials and Clay Minerals, National Center of Researches in Material Sciences,

Technopole Borj Cédria, Tunisia

N. Komatsu Department of Chemistry, Graduate School of Science,

Kyoto University, Sakyo-Ku, Japan

F. Arnaud-Neu

ECPM, University of Louis Pasteur, Strasbourg, France

R. Abidi

Faculty of Sciences of Bizerte, Bizerte, Tunisia

Abstract: The ionophoric properties of a methoxy derivative of p-methyl octahomotetraoxacalix[4]arene have been investigated and evaluated by UV/vis spectrophotometer and ESI-MS. The studies have been followed towards metal cations in acetonitrile and have given the formation of mononuclear complexes. Some

considerable selectivity has been detected mainly towards Cs+,

Me Me

OMe

O O

OMe

O

O

Me Me Me Me

O O O O

Ba2+, Cu2+ and Hg2+.

Keywords: Octahomotetraoxacalixarene, 1,2-alternate, selectivity, metal cations, extraction, complexation.

1. INTRODUCTION

   Calixarenes are macrocyclic receptors known by their molecular recognition properties [1-4]. They have been widely used in many fields, including selective extraction of metal ions [4-12].

   The oxacalixarenes, a new family among calixarenes, are distinguished by the presence of one or more -CH2OCH2- units in the ring instead of the methylene bridge. Therefore, the molecular size and flexibility are increased in comparison with their calixarenes homologous [13-15]. Although, few studies have been reported, possibly due to the low yield in their synthesis, our interests on oxacaliarenes have been focused on the preparation and the study of their binding properties towards metal cations [16-22]. In the present work, the complexation and extraction of metal cations by tetramethoxy p-methyl octahomotetraoxacalix[4]arene 1 have been studied. This ligand has been prepared by Komatsu and al. [23] The behavior of 1 towards metal cations have been discussed and compared to its homologous tetramethoxy p- tert-butylcalix[4]arene 2 (figure 1).

   Each aromatic units in the oxacalix[4]arene derivative 1 is carrying a methoxy function in the norrow rim and methyl substituent in the widder rim [23]. Its conformation is 1,2- alternate in its solid state while in solution the conformation is varying due to the small size and weak hydrogen interactions between methyl substituent.

   OMe OMe

   Me

   Me

   1 2

   Figure 1: Structures of methoxy derivative of oxacalix[4]arene (1) and its homologous calix[4]arene (2).

   This mobility has been shown by a conformational study followed by 1H-NMR demonstrating a coalescence of signals [23]. However, in the case of the calixarene homologous 2, its cone conformation is maintained. In the presence of Li+ and Na+, the cone conformation is kept and has been demonstrated by an NMR study in CDCl3/CD3OD (4:1) [24]. While the addition of KClO4 and AgClO4 vary the cone conformation to partial and 1,3-alternat conformations, respectively. Both cations Ag+ and K+ were encapsulated within the cavity due to the interactions with one/two oxygen atoms and two aromatics rings units in front [24-25].

   In the present work, two approaches have been used in order to quantify the interactions between different metal cations: the liquid-liquid extraction of picrates salts from water into dichloromethane at 20C, and the determination of the stchiomety and stability constants of complexes. Both studies were spectrophotometrically followed. The complexation study was performed in acetonitrile and the spectra were recorded in the range of wavelength 250-300 nm. The acetonitrile was chosen for the solubility of 1 in it. For the extraction study, the molar absorptivities used as reported by Pedersen (max=354 nm; =1.5 x l0-4 in water; and max =378 nm; =1.8 x l0-4 in dichloromethane).

2. RESULTS AND DISCUSSION

   1. Extraction study

      The extraction of picrate salts by 1 was established according to the standard method of extraction described in literature [26]. The use of picrate salt is due to its high solubility in aqueous phase and insolubility in organic solvent, for that the transfer of picrate anions (A-) ions from aqueous to organic phase is associated to the cation (M+) coordination with the neutral extractant (L), initially dissolved in dichloromethane. The equilibrium of extraction processes of metal salts by extractant at the liquid/liquid interfaces has been clarified by Kudo [27] and Agnihotri [28]. The extraction equilibrium constant defines quantitatively the extraction efficiency of metal from aqueous to organic phase. It describes also the strength of the complex formed and the ability of making ion pair between complex-cation and picrate anion.

      The two-phase equilibrium containing metal ion (M+), picrate anion (A-), and ligand (L) can be defined as:

      MLA org defines complex. To calculate Ke, the distribution of all species present in both phases has to be considered and determined from the following equilibriums:

      From these equations, the final equation of Ke is defined as:

      Where Pe: partition coefficient of the ligand in water- dichloromethane

      Kd: dissociation constant of MLA in the organic phase

      Ks: stability constant of ML+ in aqueous phase

      f: single ion activity coefficient of the aqueous phase

      A: concentration of picrate anion in organic phase

      M0, A0 and L0: initial concentrations of the metal ion, picrate and ligand, respectively.

      The results are expressed as percent extraction (%E) of cations from water into dichloromethane. The percent extractions of cations are the average of at least three experiments and reported on table 1. As shown on table 1, the percentage of extraction range between 0 and 20.5% for alkali and alkaline-earth cations and from 0 to 14.1 for transition and heavy metal cations. These results are slightly higher for hard acids (alkali and alkaline-earth cations), according to the Pearson acid base concept [29]. Moreover, the highest values of (%E) are in favor of largest cations such as rubidium and cesium and it is owing to the large size of the aromatic ring of 1 (the distances between faced aromatic rings are 8.36 and

      8.14 Ã… [23]). Furthermore, the flexibility of 1 is a considerable factor in the selectivity and efficiency in extraction as

      established by Talanova et al. [30]. They also reported that the picrate anions participate also in the extraction process due to both strong interactions between – stacking with aromatic ring and OH interaction between picrate and oxacalixarene 1 [30].

      Table 1: Percent extraction (%E) of metal cations from water into dichloromethane, at 20°C.

      a Metal picrates values with uncertainties less than 1%.

      The profiles of extraction of 1 and 2 are different. In fact, the extraction efficiency of 1 is proportional to the size of alkali metals. However, with ligand 2, the extraction affinity is less important and insignificant for Li+, Rb+ and Cs+ (%E1). The maximum value of (%E) found for ligand 2 is towards Na+ (%E= 6.2). Thus, the presence of four bridges CH2OCH2 in ligand 1 confers it a larger hole able to encapsulate largest cations, unlike 2. Similar behavior for 1 was found with alkaline-earth cations where the percentages of extraction increase with the size of cations. In the case of soft acids, the extraction affinity of transition metal cations is generally weak (%E 1) while in the case of heavy metal cations, cadmiumand lead are more extracted by 1 (%E = 13.5 and 14.1 respectively). The soft base character of benzene units could explain the slightly increase of affinity towards cadmium, lead and silver.

   2. Complexation study

   Absorbance

   The complexation of metal cations has been followed by spectrophotometry absorption UV in acetonitrile in the range of 250-300 nm, and has shown different spectral changes. The spectrum of free ligand exhibits a maximum of absorbance in 282 nm and a shoulder in 275 nm. Generally, the addition of metal cations leads to a decrease in absorbance. In some cases, an isobestic point appears (around 253 nm for Mn2+ and 260 for Hg2+). Figure 2 illustrates the spectral changes corresponding to the complexation of lithium by 1.

   Ligand 1

   0.50

   0.40

   0.30

   0.20

   0.10

   0.00

   250

   260

   270

   280 290

   300

   Wavelength (nm)

   Figure 2: Spectrum UV of complexation of Li+ by 1 in acetonitrile,

   = 25°C, I = 10-2 mol.L-1 (Et4NClO4), (CL = 1.0710-4 mol.L-1, 0 R

   16).

   The treatment of UV spectra by the digital program Letagrop- Spefo [32] has allowed the detection of stoichiometies M:L=1:1 of complexes formed with all cations. Their stability constants have been calculated and collected on table 2.

   Table 2: Stability constants of complexes ML (log 11) of metal complexes in acetonitrile, I=10-2 mol.L-1 (Et4NClO4 or Et4NNO3), at 25°C.

   a Standard deviation n-1on the mean of several experiments < 0.05.

   The stability constants of mononuclear complexes formed with hard cations (alkali and alkaline-earth cations) shift between 2.7 and 4.4 logarithmic units. They are almost similar with cations having close sizes. In fact, the stability constants of small sized cations Na+ and Ca2+ are around 3.9 logarithmic units while those with large size Cs+ and Ba2+ are around 4.2 logarithmic units. Moreover, the stability constants increase twice, firstly for Na+ and secondly for Cs+. Same behavior was observed for alkaline-earth cations where the affinity increases for Ca2+ and Ba2+. As comparison with its homologous 2, only titrations of lithium and sodium cations were followed in acetonitrile and the complexes formed were also mononuclears. Unlike ligand 1, the affinity of 2 decreases from Li+ (log 11= 5.0) to Na+ (log 11=3.9) [31].

   As regards to stability, the complex [Li.2]+ is 17 times more stable than [Li.1]+, while complexes [Na.2]+ and [Na.1]+ are similar. Therefore, the affinity of 1 could be explained by both size and conformation effects. This ligand is larger in cavity, than ligand 2, due to the presence of four CH2-O-CH2 oxa bridges. Its large size allows the easy inclusion of and encapsulation of larger cations. Moreover, its 1,2 alternate conformation increases the stability of the complex by O- donor interaction of a couple of methoxy substituents in both side: upper and lower rim.

   Cs Rb

   Furthermore, this ligand 1 has shown considerable selectivities: towards Ba2+ (SBa2+/Sr2+ = 50) and towards Cs+ (S +/ + = 16). In the case of ligand 2, complexes ML were formed with alkaline-earth cations and no significant selectivity was detected due to the similarity in their stability constants (log 11=3.1) [31].

   The complexation of transition (Mn2+, Co2+, Ni2+, Cu2+ and Zn2+) and heavy (Cd2+, Hg2+, Pb2+ and Ag+) metal cations has shown the formation of ML species. Their stability constants shift between 2.5 and 5.5 logarithmic units for transition metals and from 3.4 and 5.7 logarithmic units for heavy metals. The stability sequence in the transition metal series follows the Irving-Williams rule[33], increasing from Mn2+ to

   Figure 3: Profile of stability of complexes ML of transition metal cations with ligand 1.

   A further study of complexation has been followed by mass spectrometry ESI-MS in acetonitrile. The titration of 1 by Rb+ within different ratio R=CM/CL equal to 1, 2,..,10 has confirmed the stoichiometry ML of complex. In addition, some signals appearing on the free ligand spectrum were assigned to complexes [NH4.1]+, [Na.1]+ and [K.1]+, already present as impurities. The first addition of Rb+ make out the appearance of a signal corresponding to the complex [Rb.1]+ at 741 m/z. This signal becomes major at the end of titration. A second signal, very weak, has appeared corresponding to the complex [Rb2.1.(ClO4)]+ with a proportion less than 2% compared to ML (figure 4).

   Figure 4: Spectrum ESI-MS relative to the complexation of Rb+ by 1, in acetonitrile, R =0, 1, 2, 10.

3. EXPERIMENTAL

   1. Synthesis of tetramethoxy octahomotetraoxacalix[4]arene 1 [23]

      The reductive homocoupling reaction of the

      arenedialdehyde (1 mmol) was carried out in the presence of Et SiH (2.2 equiv), CH Cl (30 ml) and a catalytic amount of

      Cu2+ and then decreasing for Zn2+. This sequence highlights 3 2 2

      the selectivity of 1 towards Cu2+ (S 2+

      2+= 9.1 102)

      a Lewis acid Me3SiOTf (5 mol%). The crude residue was

      (figure 3).

      Cu / Zn

      separated by stepwise isolation by column chromatography on silica gel (1:3 or 1:2 ethyl acetate-hexane) followed by

      The complex [Hg.1]2+ is 200 times more stable than [Ag.1]+. While, complexes ML of Ag+ and of Cd2+ have similar stabilities.

      GPC. The mass of the macrocycle was determined by MALDI-TOF-MS spectra. The octahomotetraoxa- calix[4]arene 1 was obtained with a yield of 16%. The X-ray crystallographic analysis has revealed the 1,2-alternate

      conformations of the compound. The dihedral angles between each aromatic ring are of 23.3º and 83.2º, respectively, and the distances between faced aromatic rings are 8.36 and 8.14 Å [23].

   2. Extraction biphasic

      The determination of the percentages of extraction (%E) of picrate salts from water into dichloromethane has been followed as Pedersen method [26]. Two solutions were prepared: one aqueous (C= 2.5 10-4 mol.L-1) of picrate salts

      and one of ligand (C= 2.5 10-4 mol.L-1) in CH Cl . 5 ml of

      cases, the distance between successive peaks isotope was a unit m/z. If this species is multi-charged, the charge will be given by the ratio r=1/d, where d is the distance between two peaks.

4. CONCLUSION

The study of the ionophoric properties of a novel methoxy derivative of octahomotetraoxacalix[4]arene 1 towards metal cations has been followed via two approaches: complexation in homogenous solvent and biphasic extraction from water into dichloromethane. Both techniques have shown the high

2 2 affinity of 1 towards large sized cations as Cs+ and Ba2+. This

each solution are mixed and stirred during 30 min at 20.0°C (± 0.1). The mixture was then centrifuged to complete the phase separation before measurement. The percentage extraction (%E) was calculated from the following expression in which A0 is the absorbance of the aqueous solution of a blank experiment without ligand 1 and A is the absorbance of the aqueous solution after separation: %E = ( A0 A) 100/ A0.

1. Stability constants

   The stability constant xy are the concentration ratio: [MxLyxn+]/([Mn+]x[L]y) (where Mn+ = metal and L = ligand). They were calculated in acetonitrile (Riedel-deHaën, analytical reagent) by spectrophotometry of absorption UV at 25 °C. The ionic strength was maintained by 0.01 mol.L-1 of Et4NClO4 (Fluka, purum) or of Et4NNO3 (Acros) according to the literature. 21 The spectra have been recorded from 250 and 300 nm by a spectrophotometer Perkin Elmer Lambda

   11. The results were treated by a digital program Letagrop- Spefo.31 The metal salts were either perchlorates or nitrate: LiClO4, Ca(ClO4)24H2O, Co(ClO4)26H2O, Ni(ClO4)26H2O, Cu(ClO4)26H2O and Hg(NO3)2 (Fluka, purum), KClO4 and Ba(ClO4)2 (Prolabo), NaClO4 and RbClO4 (Sigma), CsClO4 and Sr(ClO4)26H2O (Alfa Aesar), Zn(ClO4)26H2O and Pb(ClO4)2 (Johnson Matthey Alfa Products), AgNO3 (Strem Chemicals), Mg(NO3)26H2O, Cd(NO3)24H2O and Mn(NO3)24HO (Merck). All these salts were dried under vacuum during at least 24 h before use. The concentrations of each metal solution (in the exception of alkali cations and Ag+) have been found by complexometry in the presence of color indicator [34].

2. ESI-MS

Solutions of ligand and cations have been prepared in acetonitrile (CL10-4 mol.L-1 and CM10-3 mol.L-1, respectively). They were then mixed according to different

ration R=C /C (0; 1; 2 and 10) and analyzed immediately by

predisposition has been explained by both flexibility and large size of 1 owing to the presence of four CH2-O-CH2 oxa bridges. Moreover, the complexation study has shown the formation of ML species. All the complexes formed by 1 are generally more stables than those formed by its homologous calixarènes 2. Some interesting selectivities have been found out mainly towards Cs+, Ba2+, Cu2+ and Hg2+.

The study of the biphasic transfer of metal cations picrates shows that 1 exhibits weak extraction efficiency mainly towards transition metal cations. However, it is more effective with alkali and heavy metals.

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