Ditopic hosts

The respective association constants of the latter anion to free 376 is only 10 M 1 while for the self-assembled complex it is equal to 2.5 104 M 1 [39]. Kubik and Goddard obtained a cyclic pseudopeptide ditopic receptor 378 for the simultaneous complexation of cations and anions [40]. Simultaneous encapsulation of N03 and PF6 ions by a highly charged (+12) anion receptor 379 was achieved by Schnebeck and coworkers [41]. By coordinating of 2,4,6-tris[(4-pyridyl)methylsulfa nyl]-l,3,5-triazine 380 with Ag, Hong and coworkers obtained nanosized tubes (Fig.7.8.1) that could host solvent and anionic molecules [42]. 

To come to a water-soluble, ditopic receptor model, Lehn etal. attached JV6-[24] crown-8 with an acridine-derived DNA-intercalator, so combining the anion binding strength of the first (in protonated state) [67] with the n-stacking capacity of the latter. Indeed, increased binding of ATP by 46 was observed as compared with the unsubstituted azacrown [68]. Additionally, the multifunctional host 46 catalyzes the hydrolysis of ATP [69] with a higher selectivity over ADP as compared with the parent crown but with a somewhat reduced eflectiv-ity. The reaction intermediate was shown to be the Af-phosphorylated crown. 

In ditopic hosts 65 the two cationic substructures are linked by a naphthy-lene diether. Schmidtchen observed the formation of 1 1 phosphoester complexes in organic solution with disilylether 65a and in water with diol 65b. The flexible skeleton enables the hosts to wrap around phosphate forming two pairs of hydrogen bonds with perpendicular main planes as schemed in formula 66 for the 5 -TMP complex [98]. Similarly, the complexation of AMP derivatives, NAD, and other phosphates by the related host 16 (Sect. 2) and its desilylated modification have been reported recently [99]. 

The recognition of different re-donating arene guests with a common re-accepting host has been subjected to control by metal ion crown ether recognition in the next case history to be described. The 18-crown-6 derivative 18, which bears a DNP ring system [12], is a ditopic compound that can act (Figure 11) as a host for alkali metal (e.g., K+) cations as well as as a... 

Di- and polytopic host-guest systems have provided a convenient starting point for the construction of larger assemblies and many systems of this type are now known. For example, in an early study Kimura et synthesised the catecholamine complex 17. The crown ether unit of this ditopic host was known to be an effective receptor for primary alkyl ammonium salts, whereas the partially protonated form of the hexamine ring had been documented to bind anionic substrates (such as car-boxy lates) or electron-donor substrates (such as catechols). Accordingly, this host forms stable 1 1 complexes with zwitterionic guests such as amino acids, pep-... 

We have also synthesized a ditopic cobaltocenium host based on a ca-lix[4]arene framework (103) and reported its crystal structure (Fig. 13) (219, 222). This receptor was shown to form extremely stable 1 1 anion complexes in polar DMSO solutions as well as with the adipate anion in acetone. 

The performance of ditopic receptors will suffer if the spacer is not long and/or flexible enough to allow simultaneous full contact at all binding centers. In some cases one observes only weakening of affinities [41], while in other cases one of the possible intermolecular forces cannot materiahze at all. Thus, additive ion pairing as well as dispersive interactions with positively charged polyaromatic host compounds are present, e.g., in complexes of AMP with tetrapyridinium porphy-... 

Probably the first attempts to bind amino acids in a ditopic fashion were described by Scmidtchen [151 and Kimura and co-workers [152 with very similar crown—aza crown bis-macrocycles, the easily protonated aza crown being introduced for carboxylate binding. The host [152 is as follows. 

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