This presentation consists only of an on-line literature search done by undergraduate student, Bill Brower, and no actual laboratory component, because he graduated before any lab work could be done. All comments and questions should be directed to Dr. Armstrong, his mentor.
Selective ionophoric extraction of small metal cations is a phenomenon worthy of investigation, whether it be for reasons of commercial collection, ecological considerations, or other reasons. Relatively small macrocyclic oxygen-containing compounds ("crown ethers") and analogs containing oxygen and/or other heteroatoms (such as nitrogen and/or sulfur) may be used to sequester small metal cations selectively, such as lithium and beryllium cations. Depending on which ion is targeted, selectivity for certain metal cations over others may be achieved by manipulating certain factors, such as the size of the cavity within the macrocyclic compound, the choices among the heteroatoms, and the arrangement (sequence) of the heteroatoms relative to the carbon atoms within each cycle. Although smaller crown ethers have been investigated, it is the 14-crown-4 system (abbreviated as 14C4) which has received most attention. Four such structures which have displayed extractive proficiency are reported here.
In a study by Richard Bartsch, et al. (reference #1), out of ten rings ranging in size from 12C4 to 15C4, two 14C4 compounds displayed high selectivity for lithium cations in aqueous solution, with maximum lithium cation to sodium cation ratios of 20 and 19, respectively.
In a study by Hiroshi Tsukube, et al. (reference #2), directed primarily toward aza-12C4 structures, one of the compounds in this study was named as one of the best ionophores for lithium cations. This study also introduced the intriguing idea of an amine side arm.
In a study by Shojiro Ogawa, et al. (reference #3), one compound in which all four heteroatoms were nitrogen, in an aromatic system, was thought to demonstrate powerful selectivity because of its inflexibility, and because the ring cavity was considered "too small to bind to other alkali and alkaline earth ions." The aza-configuration showed favorable partiality toward lithium cations.
In conclusion and in looking ahead to possible future work, the complete overall structure of the ring is important when theorizing the flexibility of the ionophore. The nitrogen atoms may be more selective than an oxygen-dominated crown ether. The amine side arm (Tsukube) may prove to be very powerful in regards to this aza-attractiveness, as well. When engineered ideally, after all of the factors have been fine-tuned, the smaller rings, such as 9C3, 12C3 and 12C4, may prove to be more viable. The numerous factors and variables allow the search for the ideal ring to continue, since none of the rings reported so far have demonstrated extremely high efficiency and selectivity for small cations.
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