Anion hosts neutral receptors

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. ... 

Early in the development of the field, compounds were often prepared to define the limits of both the synthetic methods and the stable products that could be formed. To date, many thousands of heteromacrocycles have been prepared. The dominant application of the vast family of host or receptor molecules has been to bind or complex a guest structure. The guests can be metal cations, organic cations, neutral substrates, anions, or complementary molecules. The complexation process can be understood from the simple example of 18-crown-6 complexing K+Cl- in solution. Ignoring structural and solvation/desolvation issues, the process can be described simply as... 

The anticrown chemistry term has been coined to describe the way in which many electroneutral hosts were built to recognize anions. That is the incorporation of multiple and convergent Lewis-acid centers into preorganized cyclic molecular scaffolds. The Lewis-acid centers, usually transition metals or elements like B, Si, Sn or Hg, should expose their electron-deficient sites for interaction with the lone electron pairs of the anions. The hydrogen-bonding interaction of an anion with an amide N - H mentioned above has also been widely used in the preparation of neutral receptors for anionic guests [8]. 

Ferrocene has been by far the most used redox-active group in a supramolecular sensor due its stability, its ease of functionalization and its well-understood and reversible redox chemistry. A host of feiTOcene-contain-ing compounds are now known that bind cations anions, and neutral molecules. It can be argued that the first examples of redox-active receptors, for which the complexation of other species was demonstrated, were ferrocenyl crown ether compounds, as reported in the late 1970s and in 1980. " However, it was not until... 

The unique combination of binding sites for both cations and anions in a neutral receptor molecule leads to hosts with multifunctional complexation properties, eg. for the complexation of both constituents of a salts. 

While there is less intrinsic electrostatic affinity between an anion and a neutral host compared to that between an anion and a cationic receptor, neutral receptors do not automatically come accompanied by a counter-anion and so it has been suggested that they could potentially exhibit greater selectivity as a class of receptors. Of course, we have to remember that if the receptor is neutral then it will not only bind an anion but will also bind its counter-cation. If this aspect is neglected, then the identity and solvation characteristics of the counter-cation can play an important and unpredictable role in the overall host selectivity (Section 2.4.2)... 

Toward the synthesis of pincer-type anion receptors, Dehaen and Maes appended ureido and thioureido groups to oxacalix[2]arene[2]pyrimidines 42 (Fig. 15.7). A binding preference for oxyanionic guests H2PO4 , AcO and BzO was observed over the alternative anions Cl, Br , N03 and HSO4 . Association constants of 200-4(X) M were measured in wet DMSO, indicating that the oxacalixarene hosts have the potential to act as neutral receptors for anions even in highly competitive media [64]. 

In supramolecular chemistry, molecular recognition has evolved over the last 35 years and now much effort is directed towards the complexation of anionic [28], zwitterionic [29], ion-pairs [30] and neutral guests for various purposes, including catalysis [31[. Host molecules can be constructed covalently, or they can themselves also be assembled in a supramolecular fashion. This strategy, called receptor site self-assembly, has been exploited in recent years. Especially, dynamic host formation in the presence of a substrate is highly interesting [32]. 

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