Metal ion separations by acyclic and cyclic polyethers and their polymers
The serendipitous discovery by Charles J. Pedersen'^ that cychc polyether compounds, such as dibenzo-18-crown-6 (Figure l.lf), possess specific cation complexation capabihties catalyzed an avalanche of synthetic efforts to estabhsh novel polyether compounds which could be utilized for complexation of a wide variety of metal ions. The field progressed to include not only cychc polyether compounds, but also bicyclic, tricychc and other related three-dimensional structures. In 1987 the Nobel prize was jointly awarded to D. J. Cram, J.-M. Lehn, and C. J. Pedersen^ for their achievements in this area of host-guest chemistry. Polyether metal ion chelates, both cychc (crown ethers) and pseudocychc (podands) have been apphed in organic synthesis, metal ion separation processes, and analytical chemistry. As phase transfer catalysts, crown ethers and related compounds have provided an additional tool to the organic chemist for dual phase synthesis. Thus, the facile transfer of reactive salts, such as KMn04, into the organic phase may be performed with polyether compounds. A variety of analytical methods, primarily for the determination of Groups 1 and 2 metal ions, but also for the detection of heavy metal ions and anions, have been developed which incorporate crown ether hgands. Selective chromogenic agents for the colorimetic determination of alkah and alkaline earth metal ions have potential for apphcation in environmental and clinical laboratories. The principle by which these ligands fimction is a color change that occurs upon complexation with the metal ion. Molecules of this type generally have a side arm that mteracts with the bound metal ion and perturbs molecular conjugation. The superior complex stabihty of certain aza-crown ethers for Ca has enabled these polyether hgands to be used effectively as masking agents for the determination of other less concentrated metal ions in water. ^ In research for which the primary objective is the development of ion-selective electrodes, polyether hgands are a primary source of lonophores due to their selectivity and resihence. A promising analytical apphcation of crown ether compounds is in chromatography. Cation and anion separations may be performed with water, methanol or aqueous methanol eluents on columns packed with polystyrene or sihca gel-bound neutral polyether hgands. Good resolution of alkah metal saks has been demonstrated by sihca gel-bound benzo-15-crown-5.'^ Tliese separations are unique since conductivity detection may be used without the requirement for background suppression.