Ferrocene-derived anion receptors

As already mentioned, in contrast to the cobaltocenium moiety, the ferrocene derivatives represent neutral redox-active receptors for anions. As we are losing coulombic interactions, the complexation is mediated solely via hydrogen bonding. Consequently, the corresponding association constants can be evaluated only in HB non-competitive solvents and they are usually much lower when compared with those of cobaltocenium derivatives. This apparent disadvantage is compensated by the much higher synthetic potential of ferrocene (commercial availability of many derivatives, much easier synthetic handling) and the excellent electrochemical properties of this compound. 

The accessible ferrocene/ferrocenium redox couple of ferrocene has led to its frequent use in electrochemical anion sensors. The chemical and structural similarity between ferrocene and cobaltocenium has meant that receptors based on these complexes often share the same design. The most relevant difference is that the ferrocene derivatives are neutral (until oxidised to ferrocenium),have no inherent electrostatic interaction with anions and therefore their complexes with anions exhibit lower stability constants. 

A series of ferrocene derivatives with amine substituents behave as anion or neutral molecule receptors and can recognize various anions, a property facilitated by their... 

Anion receptors based on metal centres can be classified into three broad categories those in which the metal plays a structural role, those in which it is a key component of the anion-binding site and those in which it acts as part of a redox, fluorescent or colourimetric reporter group. The latter types of compound will be covered in the next section, although, of course, there are examples of compounds that fall into more than one group, such as 2.77 in which the ferrocene-derived unit acts as both a colorimetric reporter and a structural element. 

Electrochemical-based anion receptors have been a versatile means of sensing since the pioneering work of Beer and coworkers in the late 1980s, principally using cyclic voltammetry. A range of redox active moieties, including cobaltocene, ferrocene, and Ru(bpy)3 derivatives, have been incorporated into these receptors. 

Two of the phenolic groups in rerr-butyl-calix[4]arene were condensed with a ferrocene carboxylic diamide bridge, to give a receptor, 91, which electrochemically recognizes the H2P04 anion in the presence of excess HS04 and Cl [180]. Many similar derivatives have also been reported [181, 182]. 

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