Ferrocene based anion receptors binding

Studies of synthetic porphyrin-based anion receptors should form the basis for more effective sensors. Metallocene-substituted porphyrins examined by Beer and coworkers have proven successful in the solution-phase binding of ions such as chloride, bromide and nitrate." The cobaltocenium-substituted and ferrocene-substituted porphyrins (Figure 134) bind ions in solution, as shown by H NMR and electrochemical studies. The latter measurements reveal that the porphyrin and ferrocene redox... 

Many of the receptors synthesised interact with a wide variety of anions however, selectivity has been introduced to these systems. Ferrocene-based systems have been shown to detect aqueous phosphate [99] and sulphate [100] ions. Ferrocene boronic acid has been shown to associate with fluoride ion with much stronger binding than to chloride,bromide and other anions [101]. 

Roy and co-workers have recently described a ferrocene-based receptor for amino acids. Strong 1 1 binding of 127 for amino acids in MeCN/H20 (55/45) mixtures at pH = 7.2 was observed by UV/VIS, fluorescence, electrochemistry, isothermal calorimetry (ITC), and NMR. Glu and Asp, which exist in their anionic form at this pH, were found to have the greatest affinity for the receptor (iTa(127 Glu) = 98 x and... 

Another class of mixed-metal anion receptors has been investigated which possess redox reporter groups based on two different metal complexes. This enables the quahtative comparison of their comparative anion-sensing abih-ties. Macrocycles 35 and 36 combine the Ru (bpy)3 moiety with a bridging ferrocene or cobaltocenium imit [29]. Electrochemical experiments in acetonitrile solution revealed that the Ru VRu redox potential was insensitive to anion binding, whereas the ferrocene/ferrocenium (in 35) and cobal-tocene/cobaltocenium (in 36) redox couples were shifted cathodically (by 60 mV and 110 mV respectively with chloride). However, the first reduction of Ru°(bpy)3, a Hgand-centred process based on the amide substituted bipyridyl, was also found to imdergo an anion induced cathodic shift (40 mV and 90 mV with chloride for 35 and 36, respectively). 

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. 

Receptor 93 incorporates a zinc porphyrin backbone with four ferrocene amides [65]. This shares the design of the cobaltocenium receptor 4, except that now a zinc atom occupies the centre of the porphyrin. The Lewis acid metal centre provides an additional binding site for anion recognition. In dichloro-methane solution no significant anion-induced shifts in the lH NMR signals of the amide protons were seen in the free-base precursor of 93, whereas the... 

The common reporter groups cobaltocenium and ferrocene have not frequently been used in optical anion sensing, since these chromophores are generally insensitive to anion binding. However, metaUocene-based receptors... 

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