Simultaneous anion and cation binding

Henry Taube, Canadian-born US chemist (1915—2005) winner of the 1983 Nobel Prize for chemistry for his work on the kinetics and mechanism of inorganic electron transfer reactions in solution. 

Supramolecular Chemistry, 2nd edition J. W. Steed and J. L. Atwood 2009 John Wiley Sons, Ltd ISBN 978-0-470-51233-3 

The secondary amide group, -C(0)NHR, is ubiquitous in protein structures where hydrogen bonding from the amide NH group to the carbonyl oxygen atom is responsible for much of the secondary structure of proteins such as a-helices and (I-sheets. The electron withdrawing effect of the carbonyl oxygen 

The range and versatility of amide anion binding hosts is very extensive. Macrocyclic amides have also been reported, such as 4.42 which forms a remarkable sandwich compound with sulfate in the solid state. Sulfate binding is enhanced by protonation of the tertiary amine groups and the host is thus a hybrid between amide and ammonium binding sites. The triethylbenzene scaffold has been used to prepare 

While interesting neutral alternatives to the unsaturated expanded porphyrin derivatives, calix[n] pyrroles (n = 4, 5, 6, 8) are relatively limited in their scope for anion binding because of the constraints of the cavity size. Nevertheless pyrroles linked by an sp3 hybridised carbon atom are of considerable interest because of their resemblance to the anti-cancer agents the prodigiosins (Section 4.2.4), moreover pyrrole anion-receptor chemistry is synthetically versatile and hence a range of acyclic pyrroles and hybrid amidopyrroles such as 4.50-4.52 have been developed, particularly by the groups of 

In recent years anion receptors based on peptides (oligomers of amino acids) have become of increasing interest, not least because of their resemblance to biological receptors. The archetypal example is the antibiotic vancomycin illustrated in Section 1.5. Vancomycin binds to the L-Lys-D-Ala-D-Ala 

8-tt Beer, P. D., Hayes, E. J., Transition metal and organometallic anion complexation agents , Coord. Chem. Rev. 2003, 240, 167-189. 

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We have already seen in Section 2.2 how the transport of both anions and cations is a vital part of biochemistry. We will examine supramolecular models of biological ion channels in detail in Chapter 12 but here we focus on some simple ion transport systems (ionophores) relevant to simultaneous anion and cation binding. Because of the need to maintain overall and local charge neutrality during any transport process the transport of individual ions across a biological or artificial membrane never occurs in isolation. There are two kinds of primary transport processes. Ion exchange or antiport, occurs when chemically different ions of like charge such as Na+ and K+ are simultaneously transported in... 

The next phase of our research is the design of bifunctional neutral receptors which contain both anionic and cationic binding sites and therefore are capable to complex simultaneously anionic and cationic species in apolar solvents. There are only a few examples known in the literature related to this subject. 

Appropriately strapped calixarenes have also been shown to complex anions and cations simultaneously. Such a system is the ditopic, calixarene-based receptor 11 (Scheme 9.4) reported by Tumcharern and coworkers [51[. Receptor 11, in which the strap incorporates two different binding motifs (urea and amide), showed selectivity for the tetrahedral phenylphosphinate anion (PPhH02) over simple Y-shaped anions, such as acetate. After initial screening studies, quantitative analyses were carried out using NMR spectroscopic methods. Specifically, titrations were carried out in CD3CN using the TBA salts of three promising anions, namely acetate, phenylphosphinate, and diphenylphosphate. Standard analyses of the... 

A bifunctional receptor constructed by coupling two crown ether fragments to a UO2-salen complex has been reported for the simultaneous complexation of anions and cations [69]. This compound, shown in Figure 49, showed high selectivity in binding potassium dihydrogen phosphate. Selectivity of the membrane transport of H2PO4" (over Cl ) has also been shown [70]. 

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