Interaction of the thallium cation with 1,3-alternate-25,27-bis(1-octyloxy)calix[4]arene-crown-6: Experimental and theoretical study

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Highlights

  • Stability of the Tl+·1,3-alternate-25,27-bis(1-octyloxy)calix[4]arene-crown-6 complex was determined.

  • The quantum mechanical DFT calculations were carried out.

  • Structures A and B of the resulting cationic complex species were predicted.

Abstract

From extraction experiments and γ-activity measurements, the extraction constant corresponding to the equilibrium Tl+ (aq) + 1·Cs+ (nb)  1·Tl+ (nb) + Cs+ (aq) taking place in the two–phase water–nitrobenzene system (1 = 1,3-alternate-25,27-bis(1-octyloxy)calix[4]arene-crown-6; aq = aqueous phase, nb = nitrobenzene phase) was evaluated as log Kex (Tl+, 1·Cs+) = −2.1 ± 0.1. Further, the stability constant of the 1·Tl+ complex in nitrobenzene saturated with water was calculated for a temperature of 25 °C: log βnb(1·Tl+) = 11.6 ± 0.2. Finally, by using quantum mechanical DFT calculations, the most probable structures A and B of the cationic complex species 1·Tl+, which are obviously in a dynamic equilibrium, were indicated. In both of these structures of the resulting 1·Tl+ complex, the “central” cation Tl+ is bound by eight strong bond interactions to six oxygen atoms from the 18-crown-6 moiety and to two carbons of the respective two benzene rings of the parent ligand 1 via cation-π interaction.

Introduction

The dicarbollylcobaltate anion (DCC) [1] and some of its halogen derivatives are very useful reagents for the extraction of various metal cations (especially Cs+, Sr2+, Ba2+, Eu3+, and Am3+) from aqueous solutions into a polar organic phase, both under laboratory conditions for purely theoretical or analytical purposes [2], [3], [4], [5], [6], [7], [8], [9], and on the technological scale for the separation of some high-activity isotopes in the reprocessing of spent nuclear fuel and acidic radioactive waste [10], [11], [12].

Calixarenes are macrocyclic compounds which are not only easily available on a large scale, but also offer nearly boundless possibilities for chemical modification [13]. This makes them highly attractive as the building blocks for more sophisticated and elaborate host molecules. Among the numerous “tailor made” ligands for a large variety of metal cations, crown ether derivatives of calixarenes (calixcrowns) represent not only some of the earliest complexes [14], but also elegantly demonstrate the potential of these compounds [15]. Calixarenes find applications as selective binders and carriers, as analytical sensors, as catalysts and model structures for biomimetic studies [16].

New cesium selective extractants especially from the calix[4]arene-crown-6 and calix[4]arene-bis(crown-6) families have been introduced [17], [18], [19], [20], [21], [22], [23]. The corresponding crystal structures of model complexes with cesium salts have demonstrated a significant π-interaction between the facing aromatic rings and the Cs+ guest cation [17], [18], [19]. Besides, the calix[4]arene-crown-6 family of the mentioned compounds gives Cs+/Na+ separation factors exceeding 104 [18].

In the current work, the solvent extraction of the Tl+ cation into nitrobenzene by using a synergistic mixture of cesium dicarbollylcobaltate (CsDCC) and 1,3-alternate-25,27-bis(1-octyloxy)calix[4]arene-crown-6 (abbrev. 1; see Scheme 1) was studied. Moreover, the stability constant of the cationic complex species 1·Tl+ in the organic phase of the water–nitrobenzene extraction system was determined. Finally, applying quantum mechanical DFT calculations, the most probable structures of the considered 1·Tl+ complex were predicted on the basis of the thorough conformational analysis (i. e., different initial mutual positions of the ligand 1 and the Tl+ cation were considered during the geometry optimization) and the respective vibrational frequency calculations. It is apparent that these structures may be an important contribution to the theoretical study of calixarenes.

Section snippets

Experimental

Compound 1 (see Scheme 1) was kindly supplied by Prof. V.I. Kalchenko, Institute of Organic Chemistry, NASU, Kiev, Ukraine. Cesium dicarbollylcobaltate (CsDCC) was synthesized by means of the method published by Hawthorne et al. [24]. The other chemicals used (Lachema, Brno, Czech Republic) were of reagent grade purity. The radionuclide 137Cs+ was purchased from Techsnaveksport, Russia; its radionuclidic purity was 99.9%.

The extraction experiments were carried out in 10 mL glass test-tubes with

Extraction experiments

Regarding the results of previous papers [1], [25], [26], the two-phase water–TlNO3–nitrobenzene–cesium dicarbollylcobaltate (CsDCC) extraction system can be described by the following equilibriumTl+(aq)+Cs+(nb)Tl+(nb)+Cs+(aq);Kex(Tl+,Cs+)with the corresponding exchange extraction constant Kex (Tl+, Cs+); aq and nb denote the presence of the species in the aqueous and nitrobenzene phases, respectively. For the constant Kex (Tl+, Cs+) one can write [1], [25], [26]logKex(Tl+,Cs+)=logKTl+i-logKCs+

Conclusions

In the present work, we have shown that a complementary experimental and theoretical approach can provide important information on the substituted calix[4]arene-crown-6 ligand (1) complexation with the univalent thallium cation. From the experimental investigation of the resulting 1·Tl+ complex in the two-phase water–nitrobenzene extraction system, the strength of the considered 1·Tl+ cationic complex species in nitrobenzene saturated with water was characterized quantitatively by the stability

Acknowledgements

This work was supported by the Grant Agency of Faculty of Environmental Sciences, Czech University of Life Sciences, Prague, Project No.: 42900/1312/3114 “Environmental Aspects of Sustainable Development of Society,” by the Czech Ministry of Education, Youth, and Sports (Project MSM 6046137307), as well as by the Academy of Sciences of the Czech Republic (Project 203/09/1478).

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