Anion-selective receptors

These problems have been addressed in a wide variety of imaginative and novel ways and progress in anion complexation has been rapid in recent years " to the extent that it has now been described by Lehn as a full member of the field of supramolecular chemistry. It is important to note that the search for anion-selective receptors has not been limited to the mimicry of Mother Nature s approach. Indeed, a great number of the hosts developed are far from being biocompatible as the tools of the chemist are not limited to the building blocks of natural systems. This aspect of anion recognition chemistry lies at the heart of supramolecular chemistry, the interface between chemistry and biology. ... 

Anion-selective receptor design is still less developed than that reported for its cation counterpart. This lack of development can be related to the various differences that exist between anions/cations and include ... 

The regions involved in the cation vs. anion selectivity are not well defined, as they are for the nAChR. However, in contrast to the nAChR, the residues within the pore contribute to the selectivity, as receptors formed from fully edited AMPA or kainate receptors (e.g., having an R at the Q/R site) are also permeable to chloride. 

Beer P. D. (1996) Anion Selective Recognition and Optical/Electrochemical Sensing by Novel Transition-Metal Receptor Systems, Chem. Commun. 689—96. 

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. ... 

In a related study (78CC934,79HCA2763) Lehn and coworkers incorporated guanidinium groups into macrocycles (e.g. 29). The resulting polyguanidium salts form stable complexes in water with phosphate and carboxylate anions. Unlike polyammonium anion receptors, these species remain protonated over a wide pH range and hold considerable potential in the development of anion-selective electrodes. 

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. 

Beer, P. D., Szemes, F., Remarkable chloride over dihydrogen phosphate anion selectivity exhibited by novel macrocyclic bis [ruthenium(II) bipyridyl] and ruthenium(II) bipyridyl-metallocene receptors. J. Chem. Soc., Chem. Commun. 1995, 2245-2247. 

However, despite these obvious differences, various types of anion-selective sensor are being increasingly reported within the literature, with new aspects of development covering the transducer, receptor and membrane elements being described. Frant and Ross reported the mechanism of the first anion-... 

The majority of the devices mentioned thus far rely on the Hofmeister series for anion selectivity. However, for anions that deviate from this series, organometallic receptors can be utilised. The type of ligand or metal centre will influence the sensor selectivity due to the characteristics of the electron acceptance of the complex. An interesting development that is being explored here is the use of calixarenes. These have previously found use as cation-selective species, but with suitable substitution are now being incorporated within anion-selective devices. Compounds suitable as receptors for halides [61],benzoate [61] and acetate [62] have been developed. Reinhoudt and his co-workers have reported the production of a POj-selective CHEMFET based on a uranyl cation immobilised within a salophene ligand (Fig. 5), which shows selectivity over more lipophilic anions such as Br" and NOj [63]. 

As with the majority of ISEs, all of the aforementioned receptors are immobilised within close proximity to the transducer element. However, conducting polymers (electroactive conjugated polymers) are now emerging rapidly as one of the most promising classes of transducer for use within chemical sensors. Here, the receptor can be doped within the polymer matrix, i.e. within the transducer element itself. This will facilitate the production of reliable, cost-effective, miniaturised anion-selective sensors, as it will be possible to move away from plasticiser-based membranes, but allow for ion recognition sites in conjunction with all-solid-state ion-to-electron transducers. 

Cryptate 72, in which the aryl spacer of 71 is replaced with a furanyl unit, acts a colorimetric sensor for anions. UV-vis titrations in aqueous solution gave log K values for the 1 1 halide/receptor adducts of 3.98 for chloride, 3.01 for bromide and 2.39 for iodide. X-ray diffraction studies confirm that bromide is held between the two copper atoms. Under the same conditions 72 also interacts strongly with azide (log K=4.7) and thiocyanate (log X=4.28) anions. This receptor is interesting because of its lack of selectivity compared to 71. The complex appears to be able to expand and contract its bite length in order to accommodate anions of various sizes. 

When comparing transmitter-gated ion channels on a functional level, one can discern between anion and cation channels. The former ones comprise the GABAa and glycine receptors, which display a rank order of anion selectivity of I- > Hr > Cl- and which are also permeable to HC()3. All other ionotropic receptors mentioned here are cation channels, which discriminate rather poorly between various monovalent cations, at least when compared with voltage-gated ion channels. Some... 

Among the anion carriers, calixpyrrole receptors have found applications as components in anion-selective membrane electrodes. The potentiometric selectivity for membranes ISEs based on calix[4]pyrrole, 1 towards a range of anions, namely fluoride, chloride, bromide and dihydrogen phosphate was found to be pH-dependent. 

---