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The macrocycles known as calix[n]arenes, where n represents the number of phenolic units bridged by methylene groups, represent ideal building blocks in supramolecular chemistry for the development of scaffolds with a preorganized structure, a well-defined cavity size, and modifiable positions for the introduction of a variety of functional groups, as shown in Fig. 1 (Bohmer, 1995; Asfari et al., 2001). The development of novel calixarene derivatives with the capability to act as receptors, sensors, catalysts, or ion transporters designed for specific purposes has been exploited to a great extent with the smaller member of the family calix[4]arene, and to a lesser degree with calix[6]arene. In the particular case of calix[4]arenes, the ease of modification by introduction of several types of functional groups at the phenolic rim has led to the development of numerous examples of versatile compounds (Baklouti et al., 2006; Baldini et al., 2007). The variety of derivatives reported to date is related to the well established synthetic protocols, which allow the preparation of calix[4]arenes with regioand atropisomeric control by deprotonation of the phenolic OH groups with specific alkali-metal bases. These synthetic methods have been extended to the more recently developed thiacalix[4]arenes, which feature sulfur atoms as bridging groups between the phenolic components. The development of systems based on the larger members of the calixarene and thiacalixarene families, namely calix[8]arene and thiacalix[8]arene (from now on referred to indiscriminately as calix[8]arenes), has been slow relative to its smaller analogues. This is likely due to the number of phenolic OH and aromatic positions available for functionalization, for which the regioselective introduction of substituents remains a challenging synthetic task. As a consequence, reports on crystallographically characterized calix[8]arene derivatives are relatively sparse. While the solution structures can be determined by a variety of methods, notably NMR spectroscopy, crystallographic characterization still represents the most reliable proof of the spatial arrangement of the macrocycles, particularly when the mobility of the large calix[8]arene is concerned. The limited availability of structural data is likely related to the large number of degrees of freedom present in the larger macrocycles, which does not allow the long-range ordering required for single-crystal formation. A search of the Cambridge Structural Database affords 89 structures of methylene-bridged calix[8]arenes, compared to the numbers of the fourand six-member macrocycles (Table 1).

Calix[8]arenes Solid-State Structures: Derivatization and Crystallization Strategies