Host-guest Complexes with Organic Cations

Host-guest Complexes with Organic Cations 12.2.1 Cyclophanes... 

Not only metal cations but also organic cations such as arenediazonium ions can be complexed by polyethers. Benzenediazonium tetrafluoroborate forms 1 1 host/guest complexes with acyclic e.g. pentaglyme) and cyclic polyethers e.g. crown ethers) [302]. This solvent-dependent complexation increases the stability of the benzenediazonium... 

Inclusion of cationic molecules by hosts in organic solvents is also observed with other host molecules, such as crown ethers and calixarenes. Cat-ion/n and coordination interactions are the main driving forces in these complexes. However, the formation of host-guest complexes with neutral... 

The introduction of appropriate functional groups to the rims of pillar[5]arenes also can also allow for the host-guest complexation media to be changed from an organic solvent to aqueous media. We first reported the preparation of a water-soluble pillar[5] arene via the installation of ten carboxylate moieties on both rims of pillar[5]arene (Fig. 19.8a) [19]. This pillar[5]arene formed very stable host-guest complexes with molecules bearing cationic moieties and amino acids with two or more amino groups in aqueous media (Fig. 19.8b) [39]. 

Crown-ethers are macrocyclic polyethers capable of forming host-guest complexes, especially with inorganic and organic cations. Modification of the crown-ether by the introduction of four carboxylic groups makes it possible to use this class of compounds as chiral selectors in CE. ... 

Practically motivated, the aim was to develop methods for recovery and determination of amino acids in the context of analytical chemistry and biotechnology. Amino acids are hydrophilic compounds, which therefore are difficult targets for conventional solvent extraction. Extraction to an organic solvent may be enhanced by the addition of lipophilic cationic or anionic extractants, forming extractable complexes with amino acids, or by the use of macrocyclic compounds, which form stable hydrophobic host-guest complexes. The most popular reagents from the latter group are crown... 

The whole set of compounds methylated at the narrower rim III-1-III-7 (Scheme 12.14) has been synthesized and the binding of picrates of organic cations B-2, B-10 B-lland C-3 (Scheme 12.6, Scheme 12.14) was studied. Despite having the same macrocyclic structure of the hosts, there are remailcable differences in both the measured association constants with different guests (see Table 12.3) and the CIS values extrapolated to indicate complete complexation of the guest. These results indicate that the cavity must be somewhat differently arranged in the complexes with different cations due to the balance between cation- r interaction and the strain energy of the host. 

In view of the compensatory enthalpy-entropy relationship observed for a wide variety of ionophore types, we may conclude that the cation-binding behavior, where the weak ion-dipole and dipole-dipole interaction is the major driving force for complexation, can be quantitatively analyzed and characterized by the slope and intercept of the AH-TAS plot without any exception. In this context, it is stimulating to extend the scope of this theory to the inclusion complexation of organic guests with molecular hosts. 

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