Binding anion selectivity

Modified porphyrins have been used to bind anions selectively. In 2001, Swager and coworkers reported the synthesis of the doubly strapped porphyrin 22 this compound proved to be able to coordinate two fluoride... 

Anions play key roles in chemical and biological processes. Many anions act as nucleophiles, bases, redox agents or phase transfer catalysts. Most enzymes bind anions as either substrates or cofactors. The chloride ion is of special interest because it is crucial in several phases of human biology and in disease regulation. Moreover, it is of great interest to detect anionic pollutants such as nitrates and phosphates in ground water. Design of selective anion molecular sensors with optical or electrochemical detection is thus of major interest, however it has received much less attention than molecular sensors for cations. 

Anion sensing by complexes involving macrocyclic and calix[4]arene domains has been reviewed. The receptors [Ru(bpy)2(252)] + (in (252), R = Ph, 2-HOCeH4, 3-HOCeH4, 4-HOCeH4, Bu, or 4- BuC6H4) bind Cl, Br, and I hydrogen-bonded interactions that contribute to this process are discussed. All the complexes show the same anion selectivity CC > Br > The crystal... 

In the case of cationic host molecules, the binding of a target anion usually must occur in the presence of other counterions and hence association constants often represent more a measure of the effectiveness with which the target anion is bound relative to the others in the system. Neutral receptor molecules do not suffer from this drawback and also have the potential for greater anion selectivity since they do not rely upon nondirectional electrostatic forces to achieve anion coordination. ... 

Much stronger extraction may be obtained through the dual-host concept, in which the binding of the cation and anion by separate receptors compensates their unfavorable partitioning. In addition to stronger extraction, mixing two receptors offers great flexibility in the choice of receptors and in the separate control of cation and anion selectivity. 

E ) to 32 M 1 (I ). However, when the counter cations were switched to l-phenylalanine methyl ester (H3N+CH(Bn)C02Me), these and other anion affinities were found to increase by an order of 103 M 1. It should be noted, however, that the inherent anion selectivities remained the same with both counter cations, namely N03 >CI >CE3C02 > Br sTsO->l. Ah initio calculations supported the assumption that the binding of both the anion and the cation was cooperative. 

Heteroditopic ruthenium(n) and rhenium(l) bipyridyl bis(benzo-15-crown-5) ion-pair receptors 19 and 20 display, upon K+ complexation, a remarkable switch in anion selectivity <1998CC825>. In the absence of K+, receptors 19 and 20 exhibit a selectivity preference for II2PO4 over CP. However, addition of KPF6 caused in both cases, the formation of a sandwich K+ complex, which shows a reversed CP over H2POP binding selectivity because of a more favorable electrostatic attraction between K+- and CP-bound ions (positive binding cooperativity). 

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