Abstract
Two-phenoxy walled calix[4]pyrroles 1 and 2 strapped with small rigid linkers containing pyridine and benzene, respectively, have been synthesized. 1H NMR spectroscopic analyses carried out in CDCl3 revealed that both of receptors 1 and 2 recognize only F− and HCO3− among various test anions with high preference for HCO3− (as the tetraethylammonium, TEA+ salt) relative to F− (as the TBA+ salt). The bound HCO3− anion was completely released out of the receptors upon the addition of F− (as the tetrabutylammonium, TBA+ salt) as a result of significantly enhanced affinities and selectivities of the receptors for F− once converted to the TEAHCO3 complexes. Consequently, relatively stable TEAF complexes of receptors 1 and 2 were formed via anion metathesis occurring within the receptor cavities. By contrast, the direct addition of TEAF to receptors 1 and 2 produces different complexation products initially, although eventually the same TEAF complexes are produced as via sequential TEAHCO3 and TBAF addition. These findings are rationalized in terms of the formation of different ion pair complexes involving interactions both inside and outside of the core receptor framework.
Highlights
An increasing appreciation of the role played by anions in a variety of biological and environmental systems, as well as a number of industrial processes, has made a variety of anions speci c targets for selective recognition.1–3 Among such anions, small, basic anions such as the bicarbonate anion (HCO3À) and the uoride anion (FÀ) have recently drawn particular attention
In order to support the notion that tight ion pairing between TEA+ and FÀ could reduce the effective uoride anion binding affinity in the case of receptors 1 and 2, we performed 1H NMR spectral titrations involving the treatment of receptor 2 with TEAF in 10% aqueous DMSO-d6
The strapped calix[4]pyrroles 1 and 2 wherein two diametrical meso-phenol walls are linked via a rigid benzene and pyridine spacer, respectively, were found to bind the HCO3À anion and the FÀ anion with high selectivity over various other test anions in CDCl3
Summary
An increasing appreciation of the role played by anions in a variety of biological and environmental systems, as well as a number of industrial processes, has made a variety of anions speci c targets for selective recognition.1–3 Among such anions, small, basic anions such as the bicarbonate anion (HCO3À) and the uoride anion (FÀ) have recently drawn particular attention. As compared with the ion-free forms of receptors 1 and 2, their HCO3À complexes were found to bind the uoride anion (as the corresponding TBA+ salt) with remarkably enhanced affinity.
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