Self-assembly, anion hosts

Pseudorotaxanes are precursors of both rotaxanes and catenanes they consist of a guest molecule threaded through a macrocyclic host. Stoppering both ends of the threaded molecule gives a rotaxane, cycliza-tion of the thread gives a catenane. Pseudorotaxane formation may occur by spontaneous self-assembly, or may be template-controlled. Anion size can be of paramount importance for such templates - Cl- is effective, Br, I- less good, and PFe ineffective when the recognition motif demands a small template (454). 

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]. 

In supramolecular chemistry, molecular recognition has evolved over the last 35 years and now much effort is directed towards the complexation of anionic [28], zwitterionic [29], ion-pairs [30] and neutral guests for various purposes, including catalysis [31[. Host molecules can be constructed covalently, or they can themselves also be assembled in a supramolecular fashion. This strategy, called receptor site self-assembly, has been exploited in recent years. Especially, dynamic host formation in the presence of a substrate is highly interesting [32]. 

Such complexes form a precursor to a full discussion of the vast and highly topical field of self-assembly (Chapter 10). We consider them here since they resemble structurally the types of compounds discussed in Section 4.7, but unlike metal-based anion receptors the simple thermodynamic equilibrium between host, anion and complex is not the only process occurring in solution. In fact multiple equilibria are occurring covering all possible combinations of interaction between anions, cations and ligands. These systems have the appeal that the formation of particular metal coordination complexes are thus subject to thermodynamic anion templating (cf. the thermodynamic template effect in macrocycle synthesis, Section 3.9.1) and vice versa. 

In addition to calixarenes, Sherman has reported the ability of a resorcin[4]arene, a bowl-shaped molecule with eight hydroxyl groups at the upper rim, to form a homodimer in the solid state.39 Deprotonation of two hydroxyl groups of a re-sorcin[4]arene using l,8-diazabicyclo[5.4.0]undec-7-ene (DBU) as a base produced a dianion that self-assembled to form an anionic capsule. The dimer hosted a molecule of pyrazine. Four FI+ DBU ions also interacted with the periphery of the dimer via four N+ H O hydrogen bonds (Fig. 19). 

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