Influence of exo-Adamantyl Groups and endo-OH Functions on the Threading of Calix[6]arene Macrocycle

Calix[6]arenes bearing adamantyl groups at the exo -rim form pseudorotaxanes with dialkylammonium axles paired to the weakly coordinating [B(Ar F ) 4 ] - anion. The exo -adamantyl groups give rise to a more efficient threading with respect to the exo - tert -butyl ones, leading to apparent association constants more than one order of magnitude higher. This improved stability has been ascribed to the more favorable van der Waals interactions of exo -adamantyls versus exo - tert -butyls with the cationic axle. Calix[6]arenes bearing endo -OH functions give rise to a less efficient threading with respect to the endo -OR ones, in line with what was known from the complexation of alkali metal cations.