Ionic species are ubiquitous in nature, both in biological and mineralogical systems, where they play multifarious roles. These roles may either be beneficial, as in the case of electrolyte solutions in the body, or detrimental, as in ionic contaminants in waste water streams. There has thus been a considerable amount of historical interest displayed in preparing artificial systems to detect and extract ionic contaminants from solutions, and or to influence the transport of ionic species across membranes in vivo. Such artificial systems are normally reliant on binding one of the two ionic components, either positive (cationic) or negative (anionic). While a large degree of success has been achieved by individual ion recognition, this approach suffers from the inherently energetically unfavourable necessity of separating the two ionic components involved. To avoid this undesirable partition, Paul Beer from the University of Oxford, UK, has recently propounded the binding of the two ions as a contact pair species within a single 'ditopic' receptor, thus greatly improving the binding properties. 

In this work, Beer and co-workers present two such ditopic receptors, which bind an ion pair through the proximal arrangement of anion and cation binding sites. The binding properties of these new receptors are studied by 1H NMR spectroscopic methods. 

While the design and operation of such systems are significant in themselves, it is the ability to 'tune' the selectivity of these receptors for particular ion-pair species that is of particular importance. This tuning, achieved by careful choice of the cation and binding sites of the receptors, opens the door to the discrimination of different ion-pair species, which, given the numerous potential uses of such systems, is of great interest in future detection, extraction, and transport applications. These potential applications have been the main motivation of the Beer team to pursue investigations in this field. 

'Our exploration of these ditopic receptors currently has two main future areas of investigation', Beer says. 'First, we intend to thoroughly investigate the mechanism and thermodynamics of this recognition phenomenon, in order to assist the design of future systems capable of even stronger recognition of and greater discrimination between different ion-pairs. Second, we aim to use the design principles gleaned thus far, and from the additional investigations outlined above, to provide ion-pair receptor systems for a variety of interesting applications.' 

The team is now examining two particularly fascinating directions of research: the use of these receptors for the solubilisation of inorganic salts such as sodium chloride into organic solvent media and the transport of ion-paired species such as potassium chloride across cell membranes. Furthermore, through the harnessing of suitable signalling units within the receptors, Beer and colleagues aim to develop new receptors that are capable of the selective detection of certain ion-pair species, with a macroscopic readout being provided by electrochemical or optical means. 

'This field of study is still very much in its infancy, and it is difficult to accurately predict how it will evolve over the course of the next few years as our understanding develops, and new discoveries are made,' says Beer. According to him, due to the high level of sophistication required in the design and synthesis of these systems, there remain significant challenges involved in their study, particularly in the extension of their application towards more ambitious sensing or extraction purposes. In addition to these synthetic considerations, it will also be interesting to incorporate new receptor design features dependent on our greater understanding of the ion-pair recognition process. 

Comment about this work:

'There is increasing recognition that selectivity in a cation or anion recognition processes can be controlled to a finer degree by complexing both components of a salt. However competing equilibria, including the formation of ion-pairs in solution, can diminish the effective affinity of receptors for ion-pairs unless the receptor has been designed to synergistically couple the cation and anion binding environments. The series of receptors from Beer and co-workers elegantly illustrate how selectivity for a particular metal cation (lithium) can be enhanced by the presence of a co-bound anion (bromide) in a macrocyclic calixarene based host.' Philip Gale, University of Southampton