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Macrocyclic Exchange:

Selectivity of crown ether ionophores for small metal cations

An investigation into the 14-crown-4 structure and capability

Bill Brower

169 1/2 South Schuyler Ave.
Kankakee, IL 60901-3807
U.S.A.

Doug Armstrong

Professor of Chemistry
Olivet Nazarene University
Kankakee, IL 60901-0592
U.S.A.

Abstract

Whether it be for commercial collection or ecological purification, the competent ionophoric extraction of small metal cations is a phenomenon worthy of investigation. Dependent upon which ion is targeted, ring radius and heteroatomic configuration are variable factors which can be manipulated to achieve optimum customization. The smaller crown ethers are not useless. However, it is the 14-crown-4 that has received the bulk of commendatory research. Four such structures which have displayed extractive proficiency will be investigated. They are found in the research of Richard Bartsch, Hiroshi Tsukube, and Shijiro Ogawa. The optimal ionophore may yet be engineered, for none have shown unrivalled efficiency.

Structures

Bartsch Structurea
Bartsch Structureb

The two Bartsch 14C4 rings displayed high selectivity for the Li+
in aqueous solutions, in a study of ten rings ranging in sizes from 12C4 to 15C4.
The maximum Li+/Na+ ratio for structure a was 20, for structure b, 19.

Tsukube Structure

In a test aimed primarily at aza-12C4 structures
the Tsukube ring was named, "one of the best ionophores for
Li+ ion." The study also introduced the intriguing
idea of an amine side arm (not pictured here).

Ogawa Structure

The Ogawa azacycle was thought to demonstrate powerful selectivity
because of its inflexibility, and because the ring radius was "too small
to bind to other alkali and alkaline-earth ions." The aza-configuration
shows favourable partiality to the Li+ cation.

Conclusion

The complete overall structure of the ring is important when theorizing the flexibility of the ionophore. The nitrogen heteroatoms may be more selective than an oxygen-dominated crown ether. The Tsukubian amine side arm may prove more useful in regards to this aza-attractiveness, as well. When engineered ideally, after all these factors are fine-tuned, the smaller rings, such as the 9C3, 12C3, or 12C4, may prove more viable. The numerous factors and variables allow the search for the ideal ring to continue.



"One ring to bring them all, and in the darkness bind them."
--J.R.R. Tolkien, The Lord of the Rings

References

Bartsch, Richard A. et al. "High lithium selectivity in competitive alkali-metalic solvent extraction by lipophilic crown carboxylic acids." Journ. Am. Chem. Soc. 1985. Vol. 107, pp. 4997-4998.

Ogawa, Shojiro. et al. "Aza macrocycle that selectively binds to lithium ion with color change." Journ. Am. Chem. Soc. 1984. Vol. 106, pp. 5760-62.

Tsukube, Hiroshi. et al. "Amine-armed aza-12-crown-4 as a new Li+ ion-specific ionophore." Tetrahedron Letters. 1993. Vol. 34, No. 42, pp. 6749-6752.