Second-spheres 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. 

Calixarenes acting simultaneously as first- and second-sphere ligands... 

In this category, we deal with calixarenes that are directly connected to a transition metal atom (Zn, Pd, etc.) and whose cavity can wrap around other first-sphere ligands. In that case, the calixarenes behave as second-sphere ligands that partly protect the metal center. 

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