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-pyridymethoxy)-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 (H-1, C-13 and NOESY). Complete assignment of both proton and carbon NMR spectra wits 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-crowns-6 derivatives. 2 shows a preference for the soft heavy metal cations (except for Cd2+), with it very strong affinity for Ag+. Some transition metal cations are also well extracted. 2 Forms 1: 1 complexes with K+, C2+ and Ag+, and H-1 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 (ML2) complex is formed with Zn2+ and two species, probably 1: 1 and 1:2 complexes, are obtained with Ph (c) 2006 Elsevier Ltd. All rights reserved.