Induced-Fit Molecular Recognition of Alkyl Chains in p-tert-Butylcalix[5]arene in the Solid State

p-tert-Butylcalix[5]arene 1 recognized conformationally flexible aliphatic hydrocarbons (hexane 2, heptane 3, octane 4, nonane 5, decane 6, undecane 7, and dodecane 8) and monoalkyl benzenes (toluene 10, ethylbenzene 11, propylbenzene 12, butylbenzene 13, hexylbenzene 14, and octylbenzne 15) to form host–guest complexes. X-ray diffraction study has revealed that the alkyl chains of 11–15 were selectively recognized by 1, whereas host 1 recognized the aromatic ring of 10. The alkyl chain termini of 3, 4, 6–8, 11–13, and 15 adopted unusual folded conformations in the cavity. M06-2X/6-31G(d,p) level of calculations revealed that the mutual induced-fit shape adjustments between the calix[5]arene cavity and the flexible guests play a key role in maximizing the host–guest interactions directed by the C(sp3)–H/π interactions, leading to the unusual conformations of the alkyl chain termini of the guests and the selective binding of the alkyl chains in the solid state.