Cation-binding hosts podands

However, in one important respeet, the podands are undoubtedly superior to the eyelie analogues, which is the kinetics of complexation and deeomplexation of cations.In fact, it is a general phenomenon that the more rigidly preorganized a host is for cation binding, the slower the kinetics of the proeess are. The same is true for deeomposition of the complex. Thus, both the forward (ki) and reverse complexation rates ( k 1) of the podands are high, while the rates for the forward and back reactions of the macrocycles are slower, in practice, 10 -10 times slower than with open-chain ligands. The complex... 

As early as 1972, podand Ca2+ carriers with transmembrane ionophore-type behaviour were reported by the Eidgenossische Technische Hochschule (ETH) group of Simon in Zurich.5 These hosts (1e.g. 3.13) rely on the polar amide groups for their binding ability, and the fact that the long ester arms are flexible and able to bend back on themselves to simultaneously coordinate the metal cation. 

By way of comparison, acyclic podand hosts analogous to compound 3.33 have been produced in order to assess the importance of the rigid preorganisation afforded by a cyclic host. Comparison of the closely related podand 3.36 (which is estimated to possess over 10 000 possible conformations, only two of which can bind cations in a convergent manner) and 3.33 (which is locked in only one conformation) shows that the spherand binds Li+ more than 1012 times more effectively. This highlights the importance of preorganisation effects in host design. 

Other molecules include cryptophanes, for example, 24, hemispherands (an example is 25 " ), and podands The last-named are host compounds in which two or more arms come out of a central structure. Examples are 26 and 27 " and the latter molecule binds simple cations, such as Na, K, and Ca. Lariat ethers are compounds containing a crown ether ring with one or more side chains... 

Lariat ethers are a class of compounds that contain a single side arm attached to a macrocycle, often by the nitrogen atom of an azacrown. This side arm is flexible (as in a podand) but contributes to the envelopment of the guest cation in a three-dimensional array, as in the cryptands, while the macrocycle exerts stronger complexation than in podand systems. The pseudo-podand arm comes over the face of the macrocycle and binds to the cation that has been encapsulated within the host, as demonstrated in Figure 2.4. 

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