Calixarene core ligand

Dodecaferrocene 64 (Figure 8.3) was prepared for the study of multiple redox centers. The calixarene core was constructed by the acid-catalyzed condensation of ferroce-necarboxaldehyde with resorcinol. Beer et alJ58 have also constructed poly Na+ salt 65 by the connection of crown ethers to bipyridine ligands, which were subsequently organized by the complexation to a Ru(n) metal core. 

The lower rim O-alkylation of a calixarene core with pyridine-based appendages would provide universal ligands for both hard and soft metal ions. Since minor changes in the regioselectivity of derivatization and conformation of modified calixarenes would influence their complexation, a variety of such pyridine conjugates varying in nature of linkers, site of attachment in pyridine, have been investigated. 

Fig. 37.17 Chemical structure of the calixarene-phosphine ligand (a) and graphical representation of the macrocycle capped Au nanoparticle with the Au core diameter of 0.9 nm and five calixarenes bound to metal surface (b). Two calixarenes bind the metal core with two phosphines while the other three with only one phosphine. The monodentate ligand possesses one phosphine bound to the metal surface and one unbounded phosphine oxide (Reproduced with permission from Ref. [106], Copyright 2010, Nature Publishing Group)...

FIGURE 25 The wavy moieties represent the contour of the calixarene ligands in [(H0)2(0C02)Eu4(calix[7])2(DMS0)6] (upper left) and [(HO)9(H20)2Eu7(calix[9])2(DMSO)6] (bottom left). Their respective core structures are shown on the right (redrawn after Fleming et al., 2003). 

Calixarenes can play a relevant role in molecular recognition not only as receptors but also as ligands. In this latter case, they mainly work as scaffolds, which suitably organize several identical substituents in space linked to the lower or the upper rim and simultaneously orient these moieties toward the surface areas of large macromolecules or toward the recognition points of multiple binding site receptors. In other words, they act as core for the construction of multivalent ligands (see Multivalency, Concepts). 

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