Receptors ion-pair

Davis, J. T. Lipophilic G-quadruplexes are self-assembled ion pair receptors, and the bound anion modulates the kinetic stability of these complexes. J Am Chem Soc 2003, 125, 10830-10841. 

Faced with the challenge of ion pair binding, there are three basic ion pair receptor designs contact ion pair receptors (including cascade complexes) in which the anion and cation are bound as a contact ion pair, ditopic receptors with individual, well-separated anion and cation binding sites, and zwitterion receptors, Figure 5.2. We will discuss some examples of each type of complex in the following sections. 

Figure 5.2 Schematic diagram of the three different types of simultaneous ion pair receptors, (a) cascade receptor, (b) ditopic receptor and (c) zwitterion receptor.

One of the first ion pair receptors was the 21-membered crown ether 5.12, prepared by M. T. Reetz of the University of Marburg, Germany in 1991, which possesses a Lewis acidic boronate centre. Ligand 5.12 is capable of dissolving stoichiometric amounts of KF in dichloromethane at room temperature to give a KF adduct in which the K+ ion is bound by the crown ether ring while the I1 anion forms a dative bond with the boron centre out of the plane of the crown ether. Ligand 5.12 fails to dissolve either KC1 or KBr as a consequence of the weaker nature of the B-Cl and B-Br bonds. 

Ion pair receptors contain a cation and an anion binding region, often a combination of common binding motifs, and can bind both contact or separated ion pairs to give an overall neutral complex. 

Ion pair receptors are useful in mimicking biological symporl processes — the simultaneous transport of oppositely charged ions in the same direction. 

Ion pair receptors are also common used in the extraction of metal salts in refining or purification... 

Ion pair receptors can bind zwitterions such as amino acids. 

Tucker and co-workers have recently reported a ferrocene ion-pair receptor which acts as a chromogenic molecular switch [24]. Appended to one cyclo-pentadienyl ring of the ferrocene of molecule 28 is a phenyl urea unit for anion binding and to the other a crown ether for cation binding. On addition... 

Heteroditopic ruthenium(n) and rhenium(l) bipyridyl bis(benzo-15-crown-5) ion-pair receptors 19 and 20 display, upon K+ complexation, a remarkable switch in anion selectivity <1998CC825>. In the absence of K+, receptors 19 and 20 exhibit a selectivity preference for II2PO4 over CP. However, addition of KPF6 caused in both cases, the formation of a sandwich K+ complex, which shows a reversed CP over H2POP binding selectivity because of a more favorable electrostatic attraction between K+- and CP-bound ions (positive binding cooperativity). 

Similarly, nickel(ll) and copper(ll) transition metal dithiocarbamate ion-pair receptors 21, containing amide-and crown ether-recognition sites, bind alkali metal cations and various anions. The sandwich K+ complex of the nickel(ll) receptor cooperatively enhanced the binding of acetate anion, while the copper(ll) receptor electrochemi-cally can sense anions and cations via perturbation of the copper(n)/copper(m) dithiocarbamate redox couple <2002JSU89>. 

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