Synthesis, structural characterisation and metal ion-binding properties of a new p-tert-butyldihomooxacalix[4]crown-6

Abstract

1,3-Diethoxy-p-tert-butyldihomooxacalix[4]arene-crown-6 (3) was synthesised for the first time. Compound 3 was isolated in a partial cone A conformation in solution at room temperature, as established by NMR measurements (¹H, ¹³C and NOESY). The assignment of both proton and carbon NMR spectra was achieved by a combination of COSY and HSQCTOCSY experiments. The binding affinities of ligand 3 for alkali, alkaline earth, transition and heavy metal cations have been assessed by phase transfer and proton NMR titration experiments. Compound 3 is a reasonable extractant for the alkali cations and also for Ag⁺, showing preference for Cs⁺. Ligand 3 forms 1:1 complexes with K⁺, Ca^2 + , Ag⁺ and Pb^2 + , as determined by proton NMR titrations, which also indicated that these cations should be encapsulated into the cavity defined by the crown ether unit, the ethoxy group and the inverted aryl ring.

A new dihomooxacalix[4]crown-6 (3) was synthesised and isolated in a partial cone A conformation in solution at room temperature, as established by NMR (¹H, ¹³C and NOESY). The binding properties of 3 towards alkali, alkaline earth, transition and heavy metal cations have been assessed by phase transfer and proton NMR titration experiments. The results are compared to those obtained with calixcrowns 1 and 2.

Keywords:

• calixcrowns
• dihomooxacalix[4]crown-6
• conformational analysis
• metal cation extraction
• ¹H NMR titration

Acknowledgements

Authors thank the Portuguese NMR Network for providing NMR facilities.