Receptors anion

Design and application of macroheterocycles as neutral anion receptors 98CC443. 

Seel, C., Galan, A., de Mendoza, J. Molecular Recognition of Organic Acids and Anions -Receptor Models for Carboxylates, Amino Acids, and Nucleotides. 175, 101-132 (1995). 

A variety of organoboron polymer electrolytes were successfully prepared by hydroboration polymerization or dehydrocoupling polymerization. Investigations of the ion conductive properties of these polymers are summarized in Table 7. From this systematic study using defined organoboron polymers, it was clearly demonstrated that incorporation of organoboron anion receptors or lithium borate structures are fruitful approaches to improve the lithium transference number of an ion conductive matrix. 

Gale, P. A. Anion receptor chemistry highlights from 1999. Coord. Chem. Rev. 2001, 213, 79-128. 

Wu FY, Li Z, Wen ZC et al (2002) A novel thiourea-based dual fluorescent anion receptor with a rigid hydrazine spacer. Org Lett 4 3203-3205... 

The methods of anion detection based on fluorescence involve quenching, complex formation, redox reactions and substitution reactions (Fernandez-Gutierrez and Munoz de la Pena, 1985). This chapter will be restricted to anion molecular sensors based on collisional quenching (in general, they exhibit a poor selectivity) and on recognition by an anion receptor linked to a fluorophore (fluoroionophore). 

An interesting practical application is the detection of the citrate anion in soft drinks, as shown in Box 10.3. The strategy is quite different from that of the preceding examples because the anion receptor is not linked to a fluorophore. The latter simply acts in competition with the citrate anion in a fashion that resembles fluorescence-based immunoassays. 

There a great need for selective anion sensors, but the number of available sensors is rather limited because of difficulties in their design. However, new selective sensors are expected because of the considerable progress made in the synthesis of anion receptors. 

In an effort to impart selectivity and enhance complex stability for this class of anion receptor novel ditopic biscobaltocenium receptor molecules (Fig. 42) have been synthesized and their coordination and electrochemical properties have been studied (Beer et al., 1993f, 1995h). The two positively charged metallocene centres linked by various alkyl, aryl and calix[4]arene spacers... 

Proton nmr halide anion titrations reveal that the ethyl- [79], propyl-[80] and butyl- [81] linked derivatives (Fig. 43) form complexes of 1 1 stoichiometry in acetonitrile solution. Stability constant determinations suggest that the ethyl derivative [79] exhibits selectivity for the chloride anion in preference to bromide or iodide. As the chain length increases, so the selectivity for chloride decreases and also the magnitude of the stability constant which is evidence for an anionic chelate effect with the chloride anion. Receptors containing larger aryl [81], [83], [84] and alkylamino spacers [85] (Fig. 43) form complexes of 2 1 halide anion receptor stoichiometry. 

Recently a porphyrin unit has been incorporated into an anion receptor (Fig. 46) (Beer et al., 1995g). H nmr titration experiments with this compound demonstrated the formation of 1 1 stoichiometric complexes with tetrabutylammonium halides, nitrate, hydrogensulfate and dihydrogenphosphate. 

Fig. 53 Lower-rim-bridged ferrocenylcalix[4]arene anion receptor [98].

At the present time, there is a striking contrast between the extensive development of fluorescent probes for cation recognition and the limited number of available probes for anions notwithstanding the great need for the latter. This is due to the difficulty of the design of selective anion receptors progress made in the relevant field of supra-molecular chemistry will certainly lead in the future to new selective fluorescent signaling receptors of anions. 

The first family of the so-called anion receptors was aza-ethers that were based on cyclic or linear amides, where the nitrogen core was made electron-deficient by the perfluoroalkylsulfonyl substituents so that these amides would preferentially interact with the electron-rich anions through Coulombic attraction, contrary to how their unsubstituted counterparts would act. Two selected representatives from the aza-ether family are shown in Table 8. When used as additives in solutions of various lithium halides LiX in THF, these novel compounds were found to increase both the solubility and the ion conductivity of these solutions. For example, the ion conductivity of the LiCl/THF solution was 0.0016 mS cm while the LiCl/THF solution with one of the linear aza-ethers containing eight perfluoroalkylsulfonyl substituents n = 5 for the linear aza-ether shown in... 

Unfortunately, these aza-ethers showed limited solubility in the polar solvents that are typically preferred in nonaqueous electrolytes, and the electrochemical stability window of the LiCl-based electrolytes is not sufficient at the 4.0 V operation range required by the current state-of-the-art cathode materials. They were also found to be unstable with LiPFe. Hence, the significance of these aza-ether compounds in practical applications is rather limited, although their synthesis successfully proved that the concept of the anion receptor is achievable by means of substituting an appropriate core atom with strong electron-withdrawing moieties. 

In their continued efforts, McBreen and co-workers selected boron, an electron-deficient atom, as the core to build a series of new anion receptors using the same tactics with electron-withdrawing substituents. These new additives can be classified roughly into thesethreesubcategories borate, borane, and boronate. Selected representatives from each category are also listed in Table 8. 

Among the three subcategories, boronate compounds seemed to be the most efficient in coordinating with anions and enhancing lithium ion stability, although the number of electron-withdrawing substituents in boronate is only two. The authors thus inferred that the ability of these anion receptors to capture an anion depends not only on the electron-deficiency of the core atom but perhaps also on the steric hindrance presented by these substituents on the core. With only two substituents, the core of the boronates is obviously more exposed and therefore more easily accessible for an anion. The higher ion conductivity achieved by boronate additive therefore comes from the better balance between the electron-deficiency and steric openness of this compound as... 

Since these anion receptors based on boron offer a wide variety of merits, their application seems promising if the cost and toxicology do not constitute any serious barrier for industry-scale productions. 

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