Elsevier

Inorganica Chimica Acta

Volume 394, 1 January 2013, Pages 45-57
Inorganica Chimica Acta

A new series of Cs+, K+ and Na+ chelators: Synthesis, kinetics, thermodynamics and modeling

https://doi.org/10.1016/j.ica.2012.08.009Get rights and content

Abstract

The synthesis of two molecules, B1 and B2, based on elements of norbadione A, the natural Cs+ chelator in mushrooms, associated, in the case of B2, with an 18-crown-6 ether is reported. Thermodynamic and kinetic analyses performed in water, ethanol and ethanol/water 9/1 v/v (M1) show in M1 and ethanol that B1 and B2 form stable complexes with Na+, K+ and Cs+. Affinity constants, measured spectrophotometrically in ethanol and M1, by the use of the Specfit program, are in the 105 and 106 range for B1 and B2, respectively. The second-order rate constants are in the 106–107 M−1 s−1 range and the first-order rate constants about unity. The ratios of the second-order/first-order rate constants confirm the thermodynamic results in EtOH. The kinetic processes become much too fast to allow runs in M1. Molecular simulations in EtOH imply the existence of two isomers for each of the Cs+/B1 and Cs+/B2 complexes. With B1, the more stable one is that in which the two enolates are parallel and mimic the alkali-metal inclusion cavity already envisaged for norbadione A. With B2, two similar structures are extracted, in both of which Cs+ is included in the crown ether and capped by the enolate. The affinity of B1 for Cs+ is comparable to that of norbadione A, whereas that of B2 is higher. These results are encouraging as they introduce a new series of alkali chelators which can lead to molecules capable of complexing 137Cs+ for radioactive decontamination.

Graphical abstract

Eventual structure of the cisoid B1–Cs+ complex.

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Highlights

► A new series of alkali metal sequestering agents which may be useful for decontamination after a nuclear accident. ► Chelator based on tetronic acids construction elements of norbadione A. ► Mimicking the inclusion cavity of crown ethers or calixarenes with high affinities for Na+, K+ and Cs+.

Introduction

Alkali metals, such as sodium and potassium, are absolutely essential for life, whereas others, such as cesium or lithium, are not required for biological processes. However, despite this fact, up to 1.5 mg of cesium is present in the human body [1]. Its stable isotope, 133Cs is considered innocuous or slightly toxic because of it interference with sodium potassium pump [2], [3]. Nonetheless, 38 other radioactive isotopes, such as 135Cs and 137Cs, are generated during nuclear reactions. The most dangerous to health is 137Cs with a half-life of 30 years. 137Cs is produced in nuclear power plants by the chain decay of 235U to 137Te and then 137Cs [4]. During the Chernobyl disaster in 1986 and very recently during that of Fukushima, large amounts of 137Cs were released [5], [6], [7], [8]. It should also be noted that more than 20 million people live within a range of 30 km of a nuclear power plant. This renders 137Cs accumulation a potential major health problem [7], [8], [9]. In Europe, after Chernobyl, 137Cs was partly accumulated in norbadione A (NbA), the essential pigment of the bay boletus mushroom [10], [11], [12], [13]. NbA is a naphtholactone related to a family of mushroom pigments, the pulvinic acids (Fig. 1) [10], [14].

Complex formations of alkali metals are considered to be extremely fast processes, which are practically diffusion-controlled [15], [16], [17]. However, with some sterically hindered and capped calix[4-8]arenes, or with NbA, they can be slow to very slow [18], [19], [20]. Most of the other investigations concerning these complexes were performed by 1H, 13C and/or 133Cs NMR. They always indicated fast kinetic processes occurring in the sub-second time range [21]. These are also assumed to be host–guest processes that occur with crown ethers and with calixarenes [22], [23], [24], [25], [26]. On the other hand, apart from NbA, alkali metal complexes with classical chelators, that do not have an inclusion cavity, are quite rare [10], [11], [16], [18], [27]. Furthermore, the elementary tetronic and pulvinic acid building blocks that constitute NbA do not complex alkali metals (Fig. 1) [18]. Complex formation occurs because of the particular structure of NbA [18], [28]. The aim of this work is to mimic NbA in complex formation with alkali metals by synthesizing a structure based on tetronic acid, to associate a tetronic acid with crown ether, and to analyze by means of chemical relaxation, fast kinetics and molecular simulation the mechanisms involved in complex formation with Na+, K+ and Cs+.

Section snippets

Synthesis

Commercial reagents were used without further purification. Anhydrous tetrahydrofuran (THF) was obtained by distillation over sodium and benzophenone. Analytical thin layer chromatography (TLC) was performed using plates cut from glass sheets (silica gel 60F-254 from Merck). Detection was performed under a 254 or 365 nm UV light and by immersion in an ethanol solution of cerium sulfate, followed by heating. Compounds were purified on a Silica gel 60 chromatography column. IR spectra were

Synthesis

For the synthesis of B1 and B2, we expected to make use of the particular reactivity of bis-lactone type compounds. These compounds are obtained by dehydration of pulvinic acids (Scheme 1) [32]. In the presence of a nucleophile, and especially amines, one of the lactone functions will react by ring-opening, giving thus the two amides without any control of the regioselectivity (Scheme 2). In the case of bis-lactone 1, we recently reported that the addition of 2 equivalents of TBAF allows

Complex formation

In Table 2, we summarize the kinetic and thermodynamic data determined here. The kinetic runs were performed by the stopped-flow mixing technique, which allows to investigate kinetic processes occurring in the 10−3 – 1 s range. Complex formation with alkali metals usually occurs by inclusion of the cation in the cavity of a crown ether or a calixarene [22], [25], [50], [51]. Very few examples concerning complex formation between more classical ligands not bearing an inclusion cavity, such as

Conclusion

The decontamination of 137Cs+ following nuclear incidents, such as Chernobyl and Fukushima, remains a major public health problem. To the best of our knowledge, the elimination of this radioisotope is not yet possible. In nature, NbA is among the few molecules which form stable complexes with Cs+. To mimic its structure in order to achieve such a purpose is of interest. This is shown here by the capabilities of B1 and B2 to form stable complexes with alkali metals. Even if with B2 a slight

Acknowledgment

This work was supported by DGA (Délégation Générale pour l’Armement) Grant PEA PROPERGAL N°. 06.70.110. The authors are grateful to Dr. John S. LOMAS for helpful discussions.

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