Synergistic extraction of some divalent cations into nitrobenzene by using dicarbollylcobaltate and substituted calix[5]arenes

1 Introduction

Calix[n]arenes are a well - known family of macrocyclic molecules with many potential applications in various branches of chemistry, which offer nearly boundless possibilities for chemical modification. A three-dimensional basket, cup or bucket shape characterizes them. Calixarenes have hydrophobic cavities that can hold smaller molecules or ions and belong to the class of cavitands known in host–guest chemistry. These compounds find applications as selective binders and carriers, as analytical sensors, as catalysts, model structures for biomimetic studies, ^{1 , 2} and finally, as efficient and selective extractants for nuclear waste treatment. ^{3 , 4}

The dicarbollylcobaltate anion ^{5 , 6} and some of its halogen derivatives are efficient reagents for the extraction of various metal cations (especially Cs⁺, Sr²⁺, Ba²⁺, Eu³⁺ and Am³⁺) from aqueous solutions into a polar organic phase, both, in laboratory or technological scale. We must point out that the DCC⁻ anion itself extracts significantly only heavy univalent cations, especially Cs⁺. Neutral lipophilic ligands such as crown ethers, calixarenes, different “ionophores” and phosphorous or nitrogen containing compounds present in the system form strong hydrophobic cationic complexes with these metals, which further increases their extraction, even by a few orders of magnitude. The additions of such ligand also substantially change the selectivity of the extraction.

In our previous works ^{7 , 8 , 9 , 10 , 11 , 12 , 13} we dealt with the extraction of some divalent cations by substituted calix[4]crowns and calix[6]crowns. High stability constants in nitrobenzene were found for the extraction of Ba²⁺ cation by hexaethyl calix[6]arene hexaacetate (Cesium ionophore I) and especially by hexaethyl p-tert-butylcalix[6]arene hexaacetate (Cesium ionophore II).

The aim of present work is investigation of extraction of Ca²⁺, Sr²⁺, Ba²⁺, Pb²⁺, Cd²⁺, Cu²⁺, Zn²⁺, Mn²⁺, Co²⁺, Ni²⁺, and UO₂ ²⁺ into nitrobenzene in the presence of pentaphenyl p-tert-butylcalix[5]arene pentaketone (1, see Scheme 1), p-tert-butylcalix[5]arene pentakis(N,N′-diethylacetamide (2, see Scheme 2) and pentaethyl p-tert-butylcalix[5]arene pentaacetate (3, see Scheme 3). Furthermore, the stability constants of cationic complexes of these metal cations with these substituted calix[5]crowns in the organic phase of the water–nitrobenzene extraction system have been determined.

Scheme 1:

The structural formula of pentaphenyl p-tert-butylcalix[5]arene pentaketone.

Scheme 2:

The structural formula of p-tert-butylcalix[5]arene pentakis(N,N′-diethylacetamide.

Scheme 3:

The structural formula of pentaethyl p-tert-butylcalix[5]arene pentaacetate.

2 Experimental

Pentaphenyl p-tert-butylcalix[5]arene pentaketone (1), p-tert-butylcalix[5]arene pentakis(N,N′-diethylacetamide) (2) and pentaethyl p-tert-butylcalix[5]arene pentaacetate (3) was supplied by Aldrich. The nitrobenzene solution of strontium dicarbollylcobaltate (Sr(DCC)₂) was prepared by the method, described in Ref. 14. The other chemicals used (Lachema, Brno, Czech Republic) were of reagent grade purity. The radionuclide ⁸⁵Sr²⁺ (DuPont, Belgium) was of standard radiochemical purity.

The extraction experiments were carried out in 10 mL glass test tubes with polyethylene stoppers.

The extraction of Sr²⁺ cation was investigated in the system aqueous solution of strontium picrate – nitrobenzene solution of the investigated ligand. 2 mL of an aqueous solution of SrA₂ (1 × 10⁻⁴–3 × 10⁻⁴ mol L⁻¹), A⁻ = picrate, and micro amounts of ⁸⁵Sr²⁺ were added to 2 mL of a nitrobenzene solution of ligand, the initial concentration of which varied from 1 × 10⁻³ to 3 × 10⁻³ mol L⁻¹. In all experiments, the initial concentration of ligand in nitrobenzene, C 1 in , nb , was always higher than the initial concentration of SrA₂ in water, C SrA 2 in , aq .

The extraction of other cations was investigated in the system aqueous solution of metal nitrate – the nitrobenzene solution of equimolar mixture of strontium dicarbollylcobaltate and the calixarene ligand. 2 mL of an aqueous solution of M(NO₃)₂ (M²⁺ = Ca²⁺, Ba²⁺, Pb²⁺, Cu²⁺, Zn²⁺, Cd²⁺, Mn²⁺, Co²⁺, Ni²⁺, UO 2 2 + ) of the concentration in the range from 1 × 10⁻³ to 3 × 10⁻³ mol/L and microamounts of ⁸⁵Sr²⁺ were added to 2 mL of a nitrobenzene solution of ligand and Sr(DCC)₂, whose initial concentrations varied also from 1 × 10⁻³ to 3 × 10⁻³ mol/L. In all experiments, the initial concentration of ligand in nitrobenzene, C L in , nb , was always equal to the initial concentration of Sr(DCC)₂ in this medium, C Sr ( DCC ) 2 in , nb and the initial concentration of metal nitrate in water, C M ( NO 3 ) 2 in , aq . The test tubes filled with the solutions were shaken for 12 h at 25 ± 1 °C, using a laboratory shaker. Then the phases were separated by centrifugation (5 min, 3,000 rpm). Afterwards, 1 mL samples were taken from each phase and their γ-activities were measured using a well-type NaI(Tl) scintillation detector connected to a γ-analyzer Triathler (Hidex, Turku, Finland).

The equilibrium distribution ratios of strontium, D _Sr, were determined as the ratios of the measured radioactivities of ⁸⁵Sr²⁺ in the nitrobenzene and aqueous samples.

3 Results and discussion

The two–phase water–SrA₂ (A⁻ = picrate) – nitrobenzene extraction system can be described by the following equilibrium: ¹⁵

with the corresponding extraction constant K _ex (Sr²⁺, 2A⁻); aq and nb denote the presence of the species in the aqueous and nitrobenzene phases, respectively. The extraction constant K _ex (Sr²⁺, 2A⁻) can be calculated from equation:

where K Sr 2 + i and K A − i are the individual extraction constants for Sr²⁺ and A⁻, respectively, in the water–nitrobenzene system. Knowing the values log   K Sr 2 + i  = −10.7 ¹⁶ and log   K A − i  = 0.8 (A⁻ = picrate) ¹⁷ , the extraction constant K _ex (Sr²⁺, 2A⁻) was simply calculated from Eq. (2) as log K _ex (Sr²⁺, 2A⁻) = −9.1.

The stability constants of the strontium complexes of L has been calculated by the method described in Ref. 18. Regarding the results of previous papers ^{7 , 18} , the two–phase water – M(NO₃)₂ (M²⁺ = Ca²⁺, Ba²⁺, Pb²⁺, Cu²⁺, Zn²⁺, Cd²⁺, Mn²⁺, Co²⁺, Ni²⁺, UO 2 2 + ) – nitrobenzene – Sr(DCC)₂ extraction system can be described by the following equilibrium:

with the corresponding exchange extraction constant K _ex (M²⁺, Sr²⁺). For the constant K _ex (M²⁺, Sr²⁺) one can write:

where K M 2 + i and K Sr 2 + i are the individual extraction constants for M²⁺ and Sr²⁺ in the water-nitrobenzene system. ¹⁶ The corresponding data are given in Table 1. ¹⁶

In terms of previous results, ^{7 , 8 , 9 , 10 , 11 , 12 , 13 , 18} the two–phase water– M(NO₃)₂ (M²⁺ = Ca²⁺, Ba²⁺, Pb²⁺, Cu²⁺, Zn²⁺, Cd²⁺, Mn²⁺, Co²⁺, Ni²⁺, UO 2 2 + ) – nitrobenzene–L (L = substituted calix[5]arenes) – Sr(DCC)₂ extraction system (see Experimental), chosen for determination of stability of the complex ML²⁺ in water-saturated nitrobenzene, can be characterized by the chemical equilibrium.

with the equilibrium extraction constant K _ex (M²⁺, SrL²⁺):

It is necessary to emphasize that the substituted calix[5]arenes are considerably hydrophobic ligands, practically present in the organic phase only, where they form the relatively stable complexes ML²⁺ with the mentioned divalent cations. Taking into account the conditions of electroneutrality in both phases, the mass balances of the divalent cations studied at equal volumes of the nitrobenzene and aqueous phases, as well as the measured equilibrium distribution ratio of strontium, D _Sr = [SrL²⁺]_nb/[Sr²⁺]_aq, combined with Eq. (4), we obtain the final expression for K _ex (M²⁺, SrL²⁺) in the form.

where C M ( NO 3 ) 2 in , aq is the initial concentration of M(NO₃)₂ (M²⁺ = Ca²⁺, Ba²⁺, Pb²⁺, Cu²⁺, Zn²⁺, Cd²⁺, Mn²⁺, Co²⁺, Ni²⁺, UO 2 2 + ) in the aqueous phase of the system under consideration.

If C L in , nb  =  C Sr ( DCC ) 2 in , nb  =  C M ( NO 3 ) 2 in , aq the Eq. (5) transforms to

The Equations (7) and (8) are valid if [SrL²⁺]_nb >> [Sr²⁺]_nb, and [ML²⁺]_nb >> [M²⁺]_nb i.e. the stability constants of these complexes in nitrobenzene saturated with water are higher, than 10⁵. This condition is fulfilled (see below).

In this study, from the extraction experiments (see Experimental) by means of Eq. (6), the logarithms of the constants K _ex (M²⁺, SrL²⁺) (M²⁺ = Ca²⁺, Pb²⁺, Cu²⁺, Zn²⁺, Cd²⁺, UO 2 2 + , Mn²⁺, Co²⁺, Ni²⁺) were determined.

Moreover, with respect to Refs. 7, 18, for the extraction constants K _ex (M²⁺, Sr²⁺) and K _ex (M²⁺, SrL²⁺) defined above, as well as for the stability constants of the complexes ML²⁺ and SrL²⁺ in nitrobenzene saturated with water, denoted by K _nb (ML²⁺) and K _nb (SrL²⁺), respectively, one gets.

These data are also summarized in Table 1 and in Figure 1. It is clear, that the stability constants generally increase in the sequence pentaphenyl p-tert-butylcalix[5]arene pentaketone < p-tert-butylcalix[5]arene pentakis(N,N′-diethylacetamide ≈ pentaethyl p-tert-butylcalix[5]arene pentaacetate. p-Tert-butylcalix[5]arene pentakis(N,N′-diethylacetamide has high selectivity for Ca²⁺, Pb²⁺ and UO₂ ²⁺. This is probably caused by the presence of nitrogen atoms in calixarene molecule. Pentaethyl p-tert-butylcalix[5]arene pentaacetate shows very high selectivity especially for Ba²⁺.

Figure 1:

Stability constants of substituted calix[5]arenes with some divalent metal cations.

It should be pointed out that the protonation constants of all three investigated ligands in nitrobenzene are about log K _nb(HL⁺) = 8.5. ¹⁹ It means that the extraction can be carried out from the diluted mineral acid and the concentrated mineral acids can make the reextraction. Only the reextration of the extracted Ba²⁺ cations from the pentaethyl p-tert-butylcalix[5]arene pentaacetate solution is rather complicated. It can be carried out by the very concentrated mineral acid solutions or by the solutions of bulky organic cations.

Table 2 summarizes our previous data for the extraction by substituted calix[4]arenes and calix[6]arenes. It is clear from Table 2 that hexaethyl p-tert-butylcalix[6]arene hexaacetate (Scheme 4) has even higher affinity for barium cation than pentaethyl p-tert-butylcalix[5]arene pentaacetate. Affinity for Ba²⁺ cation increases in the order tetraethyl p-tert-butylcalix[4]arene tetraacetate (Scheme 5) < pentaethyl p-tert-butylcalix[5]arene pentaacetate < hexaethyl p-tert-butylcalix[6]arene hexaacetate. The high affinity of pentaethyl p-tert-butylcalix[5]arene pentaacetate and p-tert-butylcalix[6]arene hexaacetate to barium cation is surprising and we have no explanation for it.

Scheme 4:

The structural formula of hexaethyl p-tert-butylcalix[6]arene hexaacetate.

Scheme 5:

The structural formula of tetraethyl p-tert-butylcalix[4]arene tetraacetate.

4 Conclusion

The extraction of several divalent metal cations by the synergistic mixture of dicarbollylcobaltate and three substituted calix[5]arenes has been investigated. These calix[5]arenes forms with these cations in nitrobenzene saturated with water very stable complexes. It was found, that the Ba²⁺ complex of pentaethyl p-tert-butylcalix[5]arene pentaacetate is about 3.5 orders of magnitude more stable than the analogous complex of strontium cation. The analogous behavior has been observed previously with hexaethyl p-tert-butylcalix[6]arene hexaacetate ligand.