Membrane transport systems. VI. Complexation of cations by lipophilic crown ether carboxylic acids

Abstract

. J. Chem. 62, 507 (1984).The complexation of alkali metal and alkaline earth cations by crown ether carboxylic acids has been studied by potentiometric titration and by infrared and nuclear magnetic resonance spectroscopy. Complexation in methanol-water solution is governed by the electrostatic interactions between the cation and the carboxylate, hence discrimination among cations on the basis of ionic radius is poor. The isomers syrz and anti (8NH)218-6A2 exhibit differing complexation behaviour towards cations with the anti isomer giving the stronger complexes. Interaction between the carboxylates across the face of the macrocycle of syn (8NH)?18-6Az can be inferred from the potentiometric data. Combined infrared and nuclear magnetic resonance studies of the complexes in chloroform solution show that the cation is held as an asymmetric tight ion pair with the carboxylate. The dimeric salt (4NH18-6)>Ca exhibits both a tight and a symmetric or loose ion pair between the cation and the two carboxylates. The substituents on the macrocycle are pseudoaxially disposed and in some cases gauche ethylenedioxy units can be recognized from the spectra. The complexes are monomeric in solution and in most cases the conformations are unaffected by small amounts of water in the chloroform solution. The discovery of the complexing properties of synthetic macrocyclic polyethers (crown ethers) has sparked a wide range of activity in fields ranging from synthesis of related compounds to analytical applications (1-3). An important property of crown ethers is their ability to dissolve ionic solids in nonpolar media leading to solutions of "naked anions" for synthetic applications (1, 2) to phase transfer catalysis (4) and to cation extraction and membrane transport across artificial membranes (5). The structures of the ligands and their complexes in the solid state and in solutions have been the subject of numerous studies (1,2). In a "typical" complex the metal ion occupies a site in the center of the macrocycle with ligating donor atoms from the ring providing a full or partial inner coordination sphere for the ion. The exterior of the complex is lipophilic to provide for dissolution in the nonpolar solvent.Our interest in crown ethers centers on a series of lipophilic crown ether carboxylic acids (6-10) which are capable of mediating cation-proton coupled countertransport (1 1). Representative structures of the carriers examined to date are shown below together with a line notation (10) indicating structural features. A simple schematic mechanism for the net countertransport of monocations and protons is given in Fig. 1. At the basic interface, the carrier gives up a proton in exchange for a cation from the aqueous phase. The resultant complex is neutral and readily traverses the membrane phase to the acidic interface, whereupon the reverse ion exchange process occurs. The net result is the exchange of a cation for a proton across the membrane barrier. As long as the proton gradient exceeds the cation gradient the system will "...

Membrane transport systems. V. Coupled countertransport of alkaline earth cations and protons

“…1, C). The accompanying paper (32) examines this point in more detail and we propose a structure based on nmr studies for the 2: 1 complex in which the metal ion occupies one macrocycle while the other macrocycle provides a coordinating carboxylate. Thus the second carrier is simply a liposoluble anion which presumabIy could be replaced by ion exchange with picrate or 1-naphthylacetate when present.…”

Synthesis and metal ion complexation behavior of polycarboxylate 18-crown-6 ethers derived from tartaric acid

“…The carboxylates could act as additional ligating sites in a fashion similar to the neutral donor arms of lariat ethers (15,16). Evidence from infrared and nuclear magnetic resonance spectroscopy (12), from electron spin resonance spectroscopy (1 I), and from analysis of stability constant data (12,17) suggests that direct and cooperative carboxylate-cation interactions occur, especially with small cations in solvents of moderate polarity. Such contacts probably distort the crown ether such as seen in the crystal structures of the syn and anti diacid diamide 18-crown-6 derivatives (9).…”

“…Firstly, the carboxyl groups provide an easy synthetic entry to a wide range of derivatives (3)(4)(5)(6)(7). Secondly, the tartaric acid units show a strong conformational preference for an anti disposition of the carboxylates and other carboxylate derived groups (8)(9)(10)(11)(12). This limits overall conformational flexibility (8,11,12) and leads to well-defined conformations in which the carboxylate derived groups lie axial to the plane of the macrocycle (8)(9)(10)(11)(12).…”

“…Secondly, the tartaric acid units show a strong conformational preference for an anti disposition of the carboxylates and other carboxylate derived groups (8)(9)(10)(11)(12). This limits overall conformational flexibility (8,11,12) and leads to well-defined conformations in which the carboxylate derived groups lie axial to the plane of the macrocycle (8)(9)(10)(11)(12). Lateral interactions between substituent groups and bound substrates are thus favored (3,4,13,14).…”

“…Lateral interactions between substituent groups and bound substrates are thus favored (3,4,13,14). Finally, the presence of charged groups on the macrocyclic periphery leads to markedly enhanced cation complexation relative to uncharged ligands of similar structure (3,8,12). This is primarily a result of a favorable electrostatic interaction between the anionic ligand and the complexed cation (5,8).…”