Ferrocene-based anion receptors

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... 

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. 

Ferrocene-based bisphosphanes, in hydrogenations, 10, 10 Ferrocene-based receptors anion receptors, 12, 476 cation receptors, 12, 466 for neutral guest species, 12, 483 Ferrocene-based sensors, screen-printed, in biosensing,... 

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]. 

Molecules 9-13 are a selection of ferrocene-based receptors which include secondary amide groups for the hydrogen bonding of anions [ 12,13]. Measured... 

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). 

A new class of ferrocene (also cobaltocenium) receptors which sense anions both spectrally and electrochemically, is based upon attachment of metallocene moieties to a porphyrin skeleton. The porphyrin fragment can be simultaneously complexed by zinc [120]. 

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. 

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. 

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... 

Ferrocene units appended with secondary amides have also been used for anion recognition (113-115) (231). Being neutral, unlike cobaltocenium based systems, these receptors have no inherent electrostatic attraction making the NMR stability constants much lower in magnitude than for the analogous cobaltocenium systems. Electrostatic interactions can, however, be switched on by oxidation of ferrocene to ferrocenium and consequently these molecules show interesting electrochemical effects and have a potential as amperometric anion sensors. Of interest to this development of sensor technology were the novel results of electrochemical competition experiments. These results demonstrated... 

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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