Partial-cone

Calixarenes are cyclophane-type molecules, in which at least four aromatic units are joined via methylene or related groups [9]. It is known that calix[4]arenes with hydroxy groups in the 2-position normally prefer a conelike conformation, however, depending on the substituents a more or less rapid dynamic interconversion with three possible partial cone conformations is possible (Fig. 1) [9, 10]. 

Interestingly, the macrocycles can adopt two different conformations A and B (Fig. 34), which are reminiscent of the partial cone and cone conformations of the calixarenes. A detailed... 

The calix[4]arenes are known to assume one of four conformations cone (from which its name was coined), partial cone, 1,2-altemate and 1,3-altemate shown in Fig. 7.2.5. The parent calix[4]- and [5]-arenes exhibit only cone conformation. However, at elevated temperatures they undergo ring inversion between two cones shown in Fig. 7.2.6. Other conformations are realized only for O- substituted calix[4]arenes. 

Figure 7.2.7. Cone-partial cone-cone isomerization driven by complexation with cations.

Calixarene derivatives involving crown ethers like 233 exhibit complexation ofalkali or ammonium cations typical of crown ethers that is especially effective in the partial cone conformation [26]. By a proper functionalization selective synthetic receptors for specific hosts have been developed. For instance, two 2,4-diaminotriazine groups at diametrical positions of the upper rim of a... 

Calix[4]arenes are characterized by a three-dimensional basket shape, the internal volume being around 10 nm3. Calixarenes exist in different chemical conformations because rotation around the methylene bridge is possible. In calix[4]arene, four conformations are possible cone, partial cone, 1,2-altemate, and 1,3-altemate. These four conformations are in dynamic equilibrium. Conformations can be locked, for instance, by placing a bulkier substituent than the ethyl group on the lower (or narrow) rim, this chain prevents the benzene units from rotating inside the calixarene cavity. 

The ligands initially considered are the conformationally mobile dimethoxy-calix[4] arene-crown-6 and its counterpart bearing a tert-butyl moiety wide-rim. Each has be considered in three conformations cone, partial cone, and 1,3 alternate in the free state and then complexed by Na, K, Rb, and Cs. The conclusions are summarized below. 

The Na+, K+, Rb+, and Cs+ complexes were simulated in the cone, partial-cone, and 1,3-alternate conformations, first without counterion. It was found that the smallest Na+ lies in a very deep position, surrounded by phenoxy-oxygens, not involving crown ether oxygens. As M+ gets bigger, it moves up to the crown. Picrate as the counterion remains an intimate pair with the complexed M+. The structure of the Na+ complex is different, compared with the host-M+ complex, because the picrate counterion pulls Na+ more exo to the crown region. On the contrary, for Cs+, the structures with and without picrate are very close, which demonstrates the good fit between Cs+ and the 1,3-alternate host. 

Zhou et al. studied the extraction of alkaline earth metal cations with p-tert-calix[4]arene-l,2-crown-5 under different conformations. The pH for half loading, pH0 5, is a qualitative measure of ligand acidity. The pH0 5 values for a given conformation decrease as the electron-withdrawing ability of X increases. Moreover, the ligand acidity increases uniformly as the conformation is varied in the order 1,3-alternate < partial-cone < cone. This order is related to the proximity of the ionized groups to the complexed cations.140... 

When these crowned, ionizable calixarenes contain no t-Bu groups in the 4-position (MCI1, MCI2, MCI3, MCI4, and MCI5), the increased molecular flexibility causes the extraction to shift to higher pH (cone conformer), the selectivity for Ba2+ to disappear (partial cone conformer) or to be less pronounced (1,3-alternate conformer).141... 

Figure 12 Molecular structure and stable conformers of p /ert-butycalix14 arcnc (a) cone (b) partial-cone (c) 1,3-alternate (d) 1,2-altemate.

Shinkai and coworkers have capitalized on this conformation flexibility in their designs of several pyrene-derivatized calixarene chemosensors. The rotated phenyl ring of the partial cone conformer of 37 allows two pyrene units to more easily interact by decreasing steric hindrance at the lower rim [376], Addition of Li+, Na+, and K+ ions enforces cone formation and the disruption of the initially formed excimer. Accordingly, the metal ions are detected by a decrease in pyrene excimer emission and concomitant increase in the pyrene fluorescence. 

For Ag+, complexation via the calixarene aromatic rings has been firmly established for both the partial cone and cone conformations of the immobile tetra/j-propyl ethers by X-ray crystallography.60 In both cases, the Ag+ cation is sandwiched between two distal (opposite sides of the molecule, as opposed to proximal, meaning adjacent to one another) calixarene rings, which are nearly mutually perpendicular giving a pinched cone conformation. The exposed face of the cation is coordinated to a triflate anion in both cases. For the partial cone case, the Ag+ ion also interacts with one of the anisole oxygen atoms, which is inverted with respect to the other three at the lower rim (Figure 3.85). 

Fig. 1 Four basic conformations of calix[4]arene a cone,b partial cone, c 1,3-alternate and d 1,2-alternate...

---