Cation-Binding Properties of Sodium-Selective 16-Crown-5 Derivatives

Chapter 22

Cation-Binding Properties of Sodium-Selective 16-Crown-5 Derivatives 1

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Mikio Ouchi , Kenji Mishima , Reizo Dohno , and Tadao Hakushi 1

Department of Applied Chemistry, Faculty of Engineering, Himeji Institute of Technology, 2167 Shosha, Himeji, Hyogo 671-22, Japan Department of Chemical Engineering, Fukuoka University, Nanakuma, Fukuoka 814-80, Japan

Downloaded by MONASH UNIV on August 28, 2013 | http://pubs.acs.org Publication Date: February 1, 1997 | doi: 10.1021/bk-1997-0659.ch022

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New 16-crown-5 ether derivatives possessing a variety of side arm(s) are synthesized, and their cation-binding abilities are evaluated by solvent extraction technique. They show high sodium selectivity over potassium compared with typical crown ethers. The effect of sidearm, complex formation, and the role of donor atom on cation-binding ability/selectivity especially for sodium ion are discussed.

Crown Ethers have been considered as the phase transfer catalyst (PTC) for industrial applications, however, money wise, quartenary ammonium salts are used in most phase-transfer reactions. The advantage of using a crown ether as a PTC compared with ammonium salts is their thermal stability, and the following factors are suggested when considering a crown ether as PTC (i); catalytic ability, stability, separability, toxicity, and cost. Of the crown ethers reported, dibenzo-18-crown-6 has been typically used as the PTC, and further investigation of crown ethers has not been done yet. When dibenzo-18-crown-6is used in the reaction, potassium salt is needed in view of the hole-size concept (2), while sodium salts are generally less expensive than the analogous potassium salts. It has also been suggested that PTC possessing high sodium ion binding ability/selectivity have the advantage of being polymer-bound and readily recoverable (3). In this sense, when screening new PTC applications, a crown ether possessing high sodium ion binding ability should be considered. This chapter deals with the cation-binding ability/selectivity for sodium ion against alkali metal ions evaluated from the solvent extraction of metal picrates. Crown Ethers of Low Symmetry So called "Host Guest Chemistry" has been developed since the discovery of dibenzo-18-crown-6 ether (2), and now a number of papers in this field have been published to investigate the attractive filed of "Supramolecular Chemistry". Despite the extensive work in this field, the effect of methylene chain length between two adjacent oxygen atoms of a crown ether (a crown ether of low symmetry) on its complexation ability has not been investigated systematically until very recently. We have reviewed complexations by crown ethers of low symmetry, and demonstrated the specific alkali metal ion selectivity of unsymmetrical crown ethers (4). Compared with symmetrical crown ethers, crown ethers of low symmetry, possessing (3m+n)-crownm skeletons showed lower cation-binding ability in general, however, exhibit © 1997 American Chemical Society

In Phase-Transfer Catalysis; Halpern, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1997.

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PHASE-TRANSFER CATALYSIS

drastically different and , in some cases, higher selectivities for specific cations. In particular, 16-crown-5 (1,4,7,10,13-pentaoxacyclohexadecane) shows much higher cation selectivity for sodium than symmetrical 15-crown-5 (5). Cation Binding by 16-crown-5 The specific interaction of 16-crown-5 with sodium may be explained by cavity size and orientation of the donor oxygen atoms. The examination of a CPK molecular model shows that N a (cation diameter, 2.04 Â (6)) is better accommodated in the cavity (1.8 Â) of 16-crown-5 than in the cavity (1.7 Â) of 15-crown-5. The five oxygen atoms are directed inside because of the larger ring structure compared with that of 15-crown-5, which makes 1:1 complex formation with N a favorable but with larger cations (K , Rb , and Cs+) unfavorable. This specific behavior prompted our group to use the 16-crown-5 framework for the design of sodium selective crown ethers. Their cation-binding abilities are evaluated from solvent extraction technique. When we discuss the cation-binding ability of crown ethers, at the beginning, the complexation phenomena are often considered by "Hole-size " concept (2). It was reported that this relationship in homogeneous solution fails to explain the cation selectivity in flexible macrocycles and lariat ethers (7). On the other hand, the cationselectivity in the solvent extraction of metal picrates can be rationalized in terms of this concept. This indicates that the concept is still valid at least in the solvent extraction technique (8). +

+


+

+

15-Crown-5

16-Crown-5

Solvent Extraction of Metal Picrates A variety of measurement techniques for recording the cation-binding ability of crown ethers have been reported (9). Notable among them are ion-selective electrode techniques, conductance methods, NMR methods, solvent extraction tequniques, and calorimetry. In this paper, data obtained by solvent extraction of metal picrates are presented for general discussion. The solvent extraction of metal picrates has been employed as a convenient method for evaluating cation-binding ability of crown ethers, and , as far as alkali metal ions are concerned, it affords quantitative binding constants compatible with those obtained in homogeneous phase complexation (10). The solvent extraction of aqueous metal picrates were performed with crown ethers in dichloromethane. The percentage extractabilities (% Ex), defined as percentage picrate extracted into the organic phase, are measured. In order to discuss the interaction of crown ethers with cations from a quantitative point of view, extraction studies are carried out at different ligand concentrations to determine the extraction equilibrium constant (Kex) and the complex stoichiometry (8). This extraction experiment must be used with caution since the data obtained depend on several variables (9): solvent, temperature, salt, and ionic strength. When the conditions of extraction are fixed, the data obtained give invaluable information (11).


22. OUCHI ET AL.

Sodium-Selective 16-Crown-5 Derivatives

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The data presented in this paper are obtained under fixed and controlled conditions. Metal picrates are used as salts, and the dichloromethane-water system at 25 °C is adopted.


16-Crown-5 Lariats Lariat ethers have been designed to enhance the cation-binding ability of crown ethers by introducing a side arm carrying extra donor group (72). It has been demonstrated that some carbon- and nitrogen-pivot lariat ethers exhibit higher cationbinding abilities than the parent crown ethers. From the point of synthetic feasibility, lariat ethers reported are based on symmetrical crown ethers, e.g. 15-crown-5 lariats, or their aza crown ethers. Based on the sodium selectivity, 16-crown-5 with a variety of side arm(s) should be of interest. We have reported the syntheses and the cationbinding ability of double-armed 16-crown-5 ethers (8). The discussion on the number of donor atoms in a side arm, the difference between the single- and double arms, and the lipophilicity of crown ethers was made. The results suggest that the single arm with two oxygens of 16-crown-5 should be the factor of designing sodium selective 16-crown-5 ethers.

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PHASE-TRANSFER CATALYSIS

In this paper, we discuss the cation-binding ability of 16-crown-5 derivatives (1-9) from the data of solvent extraction experiments (13). All crown ethers in this study are synthesized by reaction of the corresponding diols and the tosylates in the presence of NaOH in THF as shown in scheme. The solvent extraction of aqueous metal picrates are performed with crown ethers in dichloromethane. The extraction equilibrium constants (Kex) are determined. (Table I, and II) As seen from Table I, lariat 16-crown-5 ethers, 4-5 with two oxygen atoms in a side arm show higher sodium ion binding ability/selectivity compared with 1-3. Especially, the crown ether, 5, with tetrahydrofurfuryl group in a side arm, showed the highest selectivity among the crown ethers used. On the other hand, as seen from Table II, aza-16-crown-5 derivatives, 6-9, do not show the drastic change of Na+/K+ selectivity with the change of the substituent on the side arm. The reason why aza16-crown-5 derivatives do not show the enhancement of the sodium selectivity from the results of extraction experiment is not clear from the data. It is natural that complexation in homogeneous solution should be compared with those in solvent extraction system. The effect of a side arm of 16-crown-5 derivatives (1,2, and 4) in homogeneous phase (methanol) is examined by conductance method (74).(Table III) Contrary to the pronounced lariat effect reported for several lariat

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Cation-Binding Properties of Sodium-Selective 16-Crown-5 Derivatives

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