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Specific cooperative effect of a macrocyclic receptor for metal ion transfer into an ionic liquid

Okamura, Hiroyuki  ; Ikeda, Atsushi*  ; Saito, Takumi*; Aoyagi, Noboru  ; Naganawa, Hirochika ; Hirayama, Naoki*; Umetani, Shigeo*; Imura, Hisanori*; Shimojo, Kojiro  

An intramolecular cooperative extraction system for the removal of strontium cations (Sr$$^{2+}$$) from water by use of a novel macrocyclic receptor (H$$_{2}$$$$beta$$DA18C6) composed of diaza-18-crown-6 and two $$beta$$-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 $$beta$$-diketone fragments to a diazacrown ether resulted in a cooperative interaction within the receptor for Sr$$^{2+}$$ transfer, which remarkably enhanced the efficiency of Sr$$^{2+}$$ transfer relative to a mixed $$beta$$-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 Sr$$^{2+}$$ 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 Sr$$^{2+}$$ and O atoms in the Sr$$^{2+}$$ complex was shorter in IL than in chloroform. Consequently, Sr$$^{2+}$$ was held by H$$_{2}$$$$beta$$DA18C6 more rigidly in IL than in chloroform, representing an important factor dominating the magnitude of the intramolecular cooperative effect of H$$_{2}$$$$beta$$DA18C6 for Sr$$^{2+}$$.

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Category:Chemistry, Analytical

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