Specific Cooperative Effect of a Macrocyclic Receptor for Metal Ion Transfer into an Ionic Liquid

An intramolecular cooperative extraction system for the
removal
of strontium cations (Sr2+) from water by use of a novel
macrocyclic receptor (H2βDA18C6) composed of diaza-18-crown-6
and two β-diketone fragments in ionic liquid (IL) is reported,
together with X-ray spectroscopic characterization of the resulting
extracted complexes in the IL and chloroform phases. The covalent
attachment of two β-diketone fragments to a diazacrown ether
resulted in a cooperative interaction within the receptor for Sr2+ transfer, which remarkably enhanced the efficiency of Sr2+ transfer relative to a mixed β-diketone and diazacrown
system. The intramolecular cooperative effect was observed only in
the IL extraction system, providing a 500-fold increase in extraction
performance for Sr2+ over chloroform. Slope analysis and
potentiometric titration confirmed that identical extraction mechanisms
operated in both the IL and chloroform systems. Extended X-ray absorption
fine structure spectroscopy revealed that the average distance between
Sr2+ and O atoms in the Sr2+ complex was shorter
in IL than in chloroform. Consequently, Sr2+ was held by
H2βDA18C6 more rigidly in IL than in chloroform,
representing an important factor dominating the magnitude of the intramolecular
cooperative effect of H2βDA18C6 for Sr2+. Furthermore, competitive extraction studies with alkaline earth
metal ions revealed that the magnitude of the intramolecular cooperative
effect depended on the suitability between metal ion size and the
cavity size of H2βDA18C6. Sr2+ was successfully
recovered from IL by controlling the pH in the receiving phase, and
the extraction performance of H2βDA18C6 in IL was
maintained after five repeated uses.