2- pyridine calixarenes

The specific structure of [(H20)5Ni(py)]2+ was observed in the complexes with the second-sphere coordination of calix[4]arene sulfonate.715 There are two different [(H20)5Ni(py)]2+ cations in the complex assembly. In one the hydrophobic pyridine ring is buried in the hydrophobic cavity of the calixarene with the depth of penetration into the calixarene cavity being 4.3 A (Figure 9). The second independent [(H20)5Ni(py)]2+ cation is intercalated into the calixarene bilayer. 

Figure 7.40 Structure of the spherical lanthanide p-sulfonatocalix[4]arene assembly (a) partial space filling view along the pseudo-fivefold axis. Pyridine N oxide and one calixarene are shown in stick mode. S03 groups line the surface of the sphere, aryl rings define the hydrophobic shell and the polar core comprises 30 water molecules and two Na+ ions, (b) cut away view showing an [Na(H20)6] 2 cluster within the core. (Reprinted with permission from AAAS from [52]).

Figure 7.41 Structure of the tubular lanthanide p-sulfonatocalix[4] arene assembly (a) partial spacefilling view perpendicular to the tube showing six turns of the helix. p-Sulfonatocalix[4] arene anions shown in stick mode mark two helix turns. Helix turns involve TT-stacking between calixarenes and pyridine-TV-oxide guests (also shown as sticks), (b) cutaway view showing hydrated Na+ and La3+ ions within the cylindrical channel. (Reprinted with permission from AAAS from [52]).

Besides these classical aromatics and polyaromatic hydrocarbons, other very important classes or arene molecules are porphyrins [60, 61], phthalocyanins [61, 62], porphycens [63], calixarenes [64], resorcarenes [64], cydophanes [47], dendrimers [65], elementa-arenes [66], organometallic arene (hexahapto) [67], benzyne (dihapto), and aryl- and benzyl (monohapto) complexes [68], inorganic pyridine and polypyridine complexes [69], fullerenes [70, 71], and... 

Very appealing supramolecular structures have been obtained289 with the three-component system consisting of the p-sulfonatocalix[4]arene 143290 (used as its pentasodium salt), pyridine N-oxide and La(N03)3-6H20. If these components are mixed in a 2 2 1 molar ratio 12 calixarene pentaanions, shaped like a truncated pyramid, form spherical supramolecules with the phenolic OH functions inside and the sulfonato groups outside. Coordination of the sulfonato groups and of pyridine N-oxide included in the calixarene cavity to the La3+ stabilizes this assembly. Each sphere is in contact with six other spheres via C-shaped dimers, as shown in Figure 41. 

During nearly four decades of study, it has become clear that extremely large heteromacrocyclic rings can be prepared and that nearly any combination of O, N, and/or S can be included within the cycle. Numerous subcyclic units including hydrocarbons (cyclohexane, naphthalene, etc.) and heterocycles (furan, pyridine, imidazole, quinoline, etc.) have also been incorporated. Heteromacrocycles have also been fused with other receptors such as calixarenes to make calixcrowns. In recent years, the fusion of receptors and variations in the attached sidearms has been a dominant theme. 

Subvalent germanium complexes combined with a dianionic calixarene ligand have been found. The reaction of / -tert-butylcalix[4] arene with MesSil in the presence of pyridine provides a useful synthetic route for [Butcalix ]H2. 

Calix[8]arenes are also found to act as a ditopic receptor in most cases. The calixarenes assume various pinched conformations in these complexes, resembhng two calix[4]arene cone units hnked together. Two 1 1 complexes, of Ca° and Eu, have been characterized where a roughly planar circular conformation is observed, with only two of the phenolic O atoms interacting with the metal atom. It is interesting to note that the free jo-t-butylcalix[8]arene has been found to crystallize as a pyridine solvate with a planar, circular conformation in one case, and a pinched conformation in a subsequent report. 

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