Mono-ionizable calix[4]arene-benzocrown-6 ligands in 1,3-alternate conformations: synthesis, structure and silver(I) extraction

doi:10.1016/j.tet.2009.07.006

Tetrahedron

Volume 65, Issue 37, 12 September 2009, Pages 7777-7783

https://doi.org/10.1016/j.tet.2009.07.006 Get rights and content

Abstract

Two series of novel mono-ionizable calix[4]arene-benzocrown-6 ligands in 1,3-alternate conformations are synthesized. In one series, the proton-ionizable group (PIG) is attached to the para position of one aromatic ring in the calixarene framework, thereby positioning it over the polyether ring cavity. In the other series, the PIG is a substituent on the benzo group in the polyether ring. This orients the PIG away from the crown ether cavity. In addition to carboxylic acid functions, the PIGs include N-(X)sulfonyl carboxamide groups. With X group variation from methyl to phenyl to 4-nitrophenyl to trifluoromethyl, the acidity of the PIG is ‘tuned’. Solvent extraction of Ag⁺ from aqueous solutions into chloroform is used to probe the influence of structural variation within the mono-ionizable calixcrown ligand on metal ion extraction efficiency, including the identity and acidity of the PIG and its orientation with respect to the polyether ring.

Editor's abstract

Introduction

Calixarene-crown ether compounds, also called calixcrowns, combine a calixarene scaffold with a crown ether unit that bridges two phenolic oxygens of the former with a polyether chain.1, 2, 3, 4, 5, 6 Linking of distal phenolic oxygens in calix[4]arenes gives 1,3-bridged calix[4]crowns, while connection of the proximal oxygens produces less common, 1,2-bridged calix[4]crown compounds. It has been found that 1,3-bridged calix[4]crowns exhibit high binding affinities towards alkali and alkaline earth metal cations.7, 8, 9

By use of appropriate substituents, the calix[4]arene platform may be locked in different conformations (cone, partial cone, 1,3-alternate and 1,2-alternate). In calix[4]arene-crown compounds, this controls the spatial relationship between the polyether ring and constituents of the calixarene moiety, including the aromatic rings of the calixarene unit itself, as well as substituents with potential coordination sites attached via the remaining phenolic oxygens. Of particular interest in our metal ion separations research program are substituents that bear proton-ionizable groups (PIGs). When the number of acidic hydrogens in a calixcrown ligand matches the charge of the metal ion to be complexed, solvent extraction proceeds by an ion-exchange mechanism. Formation of an electroneutral, ionized ligand-metal ion complex in the organic diluent markedly enhances the efficiency of metal ion extraction by avoiding the need for concomitant transfer a hydrophilic aqueous phase anion, such as chloride, nitrate or sulfate, into the low polarity medium.

In continuation of our studies of metal ion separations by mono-ionizable calix[4]arene compounds,10, 11, 12, 13 we have now synthesized two series of novel mono-ionizable calix[4]arene-benzocrown-6 ligands in 1,3-alternate conformations (Fig. 1). These two series have identical crown ether units and calix[4]arene scaffolds including two lipophilic octyl groups, but differ in the spatial relationship between the PIG and the polyether unit. In Series 1, the PIG is a substituent on the benzo group in the polyether ring. This orients the PIG away from the crown ether cavity. In Series 2, the PIG is attached to an aromatic ring of the calixarene framework. This positions the PIG over the polyether ring cavity.

We now report synthetic routes to these two series of proton-ionizable calixcrown ligands with carboxylic acid and N-(X)sulfonyl carboxamide PIGs. A solid-state structure of one of the Series 2 ligands is presented. The influence of structural variation within the mono-ionizable calixcrown ligand, including the identity and acidity of the PIG and its orientation with respect to the polyether ring, on metal ion complexation efficiency, is probed by solvent extraction of Ag⁺ from aqueous solutions into chloroform. Ag⁺ was selected as the metal ion to study due to its reported ability to coordinate at either ethereal or π-basic sites in 1,3-alternate calix[4]crown ligands.¹⁴

Section snippets

Synthesis of series 1 ligands

The synthetic route for the preparation of 4′-carboxy-25,27-di(octyloxy)-26,28-calix[4]arene-benzocrown-6 (4) and 4′-[N-(X)sulfonyl carbamoyl]-25,27-di(octylox)y-26,28-calix[4]arene-benzocrown-6 compounds 5–8 in the 1,3-alternate conformation is presented in Scheme 1.

In retrosynthetic analysis, the target mono-ionizable 1,3-alternate calix[4]arene-benzocrown-6 ligands 4–8 can be assembled from a benzopolyether diol fragment with an appropriate substituent on the 4-position of the benzene ring

General

Infrared spectra were taken with a Perkin–Elmer 1600 FTIR spectrophotometer as deposits from CDCl₃ or CH₂Cl₂ solution on sodium chloride plates. NMR spectra were obtained with a Varian Unity INOVA 500 MHz FT-NMR (¹H 500 MHz and ¹³C 126 MHz) spectrometer in CDCl₃ with TMS as internal standard. Melting points were determined with a Mel-Temp melting point apparatus in capillary tubes. Elemental analysis was performed by Desert Analytics Laboratory of Tucson, Arizona.

Reagents were purchased from

Acknowledgements

This research was supported at TTU by the Office of Biological and Environmental Research of the U.S. Department of Energy (Grant Number FG02-03ER63676). This research was supported at ORNL by the Environmental Management Science Program of the Offices of Science and Environmental Management of the U.S. Department of Energy (Contract Number DE-AC05-00OR22725).

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Mono-ionizable calix[4]arene-benzocrown-6 ligands in 1,3-alternate conformations: synthesis, structure and silver(I) extraction

Abstract

Editor's abstract

Introduction

Section snippets

Synthesis of series 1 ligands

General

Acknowledgements

Tetrahedron Lett.

Tetrahedron Lett.

Tetrahedron

J. Inclusion Phenom. Macrocyclic Chem.

Calixarenes. An Introduction

J. Inclusion Phenom. Macrocyclic Chem.

Calixarenes and Resorcinarenes. Synthesis, Properties and Applications

J. Inclusion Phenom. Macrocyclic Chem.