Synthesis, NMR characterization and ion binding properties of 1,3-bridged p- tert-butyldihomooxacalix[4]crown-6 bearing pyridyl pendant groups

Abstract

1,3-Di(2-pyridylmethoxy)- p- tert-butyldihomooxacalix[4]arene-crown-6 ( 2 ) was synthesized for the first time. 2 was isolated in a cone conformation in solution at room temperature, as established by NMR spectroscopy ( 1H, 13C and NOESY). Complete assignment of both proton and carbon NMR spectra was achieved by a combination of COSY, HSQC and HMBC experiments. The binding properties of ligand 2 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 other dihomooxacalix[4]arene-crowns-6 and closely-related calix[4]arene-crown derivatives. 2 shows a preference for the soft heavy metal cations (except for Cd 2+), with a very strong affinity for Ag +. Some transition metal cations are also well extracted. 2 forms 1:1 complexes with K +, Ca 2+ and Ag +, and 1H NMR titrations indicate that they should be encapsulated into the cavity defined by the crown ether unit and by the two pyridyl pendant arms. A 1:2 (ML 2) complex is formed with Zn 2+ and two species, probably 1:1 and 1:2 complexes, are obtained with Pb 2+.