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The conformationally mobile tetramethoxy-dihomooxacalix[4]arene 1c is able to interact with linear and branched alkylammonium guests through an induced-fit process mainly driven by H-bonds and cation–π interactions. Although free 1c adopts a 1,4-alternate conformation in solution, in the presence of linear nBuNH3+ and branched sBuNH3+ and iPrNH3+ guests, it preferentially forms endo-complexes in a cone conformation through adaptive structural change in response to the presence of the ammonium guests. In accordance with previous results, the DFT-optimized structures of the complexes indicate that they are mainly stabilized by H-bonding interactions between the guest ammonium group and the host CH2OCH2 ethereal bridge.

The binding of representative alkali, alkaline earth, transition and heavy metal cations by 2-pyridylmethoxy derivatives (1b, in cone and partial cone conformations) of p-tert-butylhexahomotrioxacalix[3]arene (1a) was assessed by spectrophotometry, microcalorimetry, proton NMR spectrometry and density functional theory methods.

The tetraphenyl ketone of p-tert-butyldihomooxacalix[4]arene (2) was synthesized for the first time. This derivative possesses the cone conformation in solution at room temperature, which was confirmed by NMR measurements (1H, 13C and NOE 1D). Extraction studies with metal picrates from neutral aqueous solution into dichloromethane, transport experiments with metal picrates through a dichloromethane membrane and stability constant measurements by UV absorption spectroscopy in methanol and acetonitrile were performed to evaluate the binding ability of ketone 2 towards alkali and alkaline earth metal cations. The tetraphenylketone of p-tert-butylcalix[4]arene (3) was also studied, and the results for the two derivatives were compared. Compound 2 shows a preference for K+ and Ba2+ cations, being selective only in the alkaline earth metal cation series. Compound 3 is a stronger binder than 2, but is a weaker extractant. Both show transport rates that are not proportional to complex stability or to extraction efficiency. Ketone 2 can be characterized as a selective receptor. Copyright © 1999 John Wiley & Sons, Ltd.