Di-ionizable calix[4]arene-1,2-crown-5 and -crown-6 ethers in cone conformations: synthesis and divalent metal ion extraction
Graphical abstract
Di-ionizable calix[4]arene-1,2-crown-5 and -crown-6 ethers in the cone conformation exhibit selectivity for Ba2+ in competitive solvent extraction of alkaline earth metal cations and high extraction ability for Pb2+ and for Hg2+ in single species extraction.
Introduction
The development of the field of liquid–liquid separations has been paralleling the progress of coordination chemistry from the use of simple ligands to pre-organized metal ion receptors. The advent of macrocyclic receptors, i.e., crown ethers and calixarenes, produced a second revolution in liquid–liquid separation of metal ions following the monodentate and bidentate extractants, which revolutionized the chemistry for metal ion separations from aqueous solutions, from separatory funnels to the industrial scale.1 The macrocyclic ligands select cations primarily by their sizes, and the most stable complexes are formed with the closest match between host cavity and cation size.2 Another important factor is pre-organization of the host macrocyclic molecules. The binding of a specific metal ion by a pre-organized ligand requires only minimal adjustment of the ligand conformation. Since such adjustment is both enthalpically and entropically expensive, this improvement furnishes much more effective and selective metal ion separations.1, 2
Calix[4]crowns, macrobicyclic compounds that combine calix[4]arene and polyether units, have been widely used as hosts for selective metal ion recognition for more than two decades, since the earliest example of this family, p-tert-butylcalix[4]arene-1,3-crown-6-ether, was reported in 1983.3, 3(a), 3(b), 3(c), 3(d) The complexation studies of calix[4]arene-crowns have mainly been focused on dialkylated 1,3-bridged calix[4]arene-crown ethers, which were found to exhibit high binding affinity and selectivity in alkali and alkaline earth metal cation extractions.4, 4(a), 4(b), 4(c), 4(d) In contrast, very little is known about the ligating ability of the corresponding 1,2-bridged regioisomers, probably because they are more difficult to synthesize. Known members in this category are limited. In general, they exhibit poor binding ability and little selectivity toward metal cations.5, 5(a), 5(b), 5(c), 5(d), 5(e), 5(f)
Calix[4]arenes with pendant proton-ionizable groups are found to be much more efficient interfacial carriers for metal cations than related unfunctionalized calix[4]arenes with phenolic groups on the lower rim.6, 6(a), 6(b), 6(c), 6(d), 6(e) Recently, we have undertaken the synthesis and evaluation of di-ionizable calix[4]arene-1,2-crown ether ligands as divalent metal ion extractants.7, 7(a), 7(b), 7(c) Di-ionized calix[4]arene-1,2-crown-4 ethers in the cone conformation were found to be efficient alkaline earth metal cation extractants with extraordinarily high selectivity for Ba2+.7c Since the crown ether ring is too small to accommodate Ba2+, we proposed the extraction complex shown in Figure 1. In this complex, the large divalent metal ion is sandwiched between the four polyether ring oxygens on one side and the two ionized groups on the other. In an extension of that study, we have now prepared analogues with larger crown-5 and crown-6 ether rings and evaluated their alkaline earth metal cation extraction abilities. In addition, the efficacy of these new ligands as Hg2+ and Pb2+ extractants has been assessed.
Section snippets
Synthetic routes
Calix[4]arene (1), tetraethylene glycol dimesylate, and pentaethylene glycol ditosylate were prepared according to reported methods.8, 9, 10, 10(a), 10(b) Calix[4]arene-1,2-crown-5 (2) and calix[4]aene-1,2-crown-6 (3) (Scheme 1) were synthesized by modification of reported procedures.5f For the preparation of 2, the reaction time was extended from 10 to 72 h and tetraethylene glycol ditosylate was replaced with tetraethylene glycol dimesylate, giving a 56% yield. For 3, the reaction time was
General
Ligands 16–25 were prepared by the reported methods.7b Reagents were obtained from commercial suppliers and used directly, unless otherwise noted. THF was dried over sodium wire with benzophenone ketyl as an indicator. DMF was stored over 4 Å molecular sieves. Infrared spectral analyses were performed with a Perkin–Elmer 1600 FTIR spectrophotometer by deposit from CH2Cl2 solution onto a NaCl plate. The absorptions are expressed in wavenumbers (cm−1). NMR spectra were measured with a Varian Unity
Acknowledgements
We thank the Division of Chemical Sciences, Geosciences, and Biosciences of the Office of Basic Energy Sciences of the U.S. Department of Energy (Grant DE-FG02-90ER14416) for support of this research.
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Present address: Department of Chemistry, Czestochowa University of Technology, 42-200 Czestochowa, Poland.