Heteroditopic receptors

Arenoxyacetic acid Methyl glycinate DCC Heteroditopic receptors 71 2006CJC58... 

The calix[4]arene platform has been widely used by Beer and co-workers to produce the heteroditopic receptors 14-18 by attaching benzo-15-crown-5 ether moieties at either the upper or lower rim through amide linkers. Compound 17 contains additional ferrocene subunits at the upper rim <1995JCD3117, 2003JCD4451>. Noteworthy, K+ ion forms 1 1 intramolecular sandwich complexes with the benzo-15-crown-5 moieties of 14, 15, and 17 enhancing the strength of... 

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). 

The tren-based heteroditopic receptor 22, featuring a tripodal tetrahedral amide hydrogen-bond anion-recognition site in combination with benzo-15-crown-5 ether cation-binding moieties, has been found to cooperatively bind chloride, iodide, and perrhenate anions via co-bound crown ether-complexed sodium cations. It also can efficiently extract the radioactive sodium pertechnetate from simulated aqueous nuclear waste streams. The anion-binding affinity of 22 is considerably reduced in the absence of a co-bound cation <1999CC1253>... 

The covalent anchoring of a benzo-15-crown-5 moiety to the meta or para phenolic oxygen of a zinc tetraphenyl porphyrin via ether formation has produced m-29 and p-29, respectively. These heteroditopic receptors can bind NaCN in a ditopic fashion with a visible color change, in contrast with other sodium salts (including F, CF, BF, I-, and SCN-), which are bound in a monotopic fashion without a color change <2002CC512>. 

Dual-Host Approach Heteroditopic Receptor Approach Assembly Approach... 

Another interesting development is the use of bicyclic guanidinium receptors as heteroditopic systems for the enantioselective recognition of amino acids. In this case the guanidinium fragment was combined via a flexible linker with a crown ether moiety in order to promote the simultaneous binding of the carboxylate anion and the ammonium cation belonging to a zwitterionic amino acid [45]. 

Beer. P.D. Dent. S.W. Potassium cation induced switch in anion selectivity exhibited by heteroditopic ruthenium(ll) and rhenium(I) bipyridyl h/.v(benzo-15-crown-5) ion pair receptors. Chern. Commun. 1998. (7). 825-826. 

Heteroditopic receptors for simultaneous cation-anion complexation... 

For instance, receptor 76b shows selectivity toward NaH2P04, as a consequence of the sodium-selective complexation at the lower rim by the cone calix[4]arene tetraester and the selective inclusion of H2P04 at the upper rim, as previously found for the analogous receptor l(i2kP Compound 77, acting as heteroditopic carrier, is able to transport the hydrophilic CsCl better than the more lipophilic CsNOs across a SLM, owing to the higher... 

Figure 20 Complexation of copper salts by the heteroditopic receptor 78.

Figure 21 Switching-on anion binding by sodium complexation at the lower rim of heteroditopic receptor 79.

A contact ion pair mechanism was also observed by Jabin et al. in the complexation of alkylammonium chlorides by the heteroditopic receptor 81, in which the calix[6]arene... 

We reported that complexation of sodium ion by a crown[4]- moiety at the lower rim of an heteroditopic receptor 83 (Figure 22), bearing two 2,2,2-trifluoroethanol anion-binding groups at the upper rim, causes an increase... 

Figure 22 Allosteric effect in the binding of CHbCOO" Na+ by heteroditopic receptor 83.

Heteroditopic receptors are those in which the distance between the two binding sites allows the accomodation of solvent-bridged ion pairs, in which solvent molecules help link the anion to the cobound cation. 

Another interesting example of a heteroditopic receptor capable of cooperative recognition of ion pairs throngh a contact binding mechanism was recently reported by Beer and coworkers.Receptors 5 and 6 were designed to provide a good complementarity to contact ion pairs. 

An interesting example reported by Barboiu is based on the neutral heteroditopic (ureido)crown ether, 4-phenylurea-benzo-15-crown-5 receptor 22, complexing both anions and cations and being able to give self-organized structures in solution and in solid state. 

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