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

イオン液体への金属イオン抽出における大環状レセプターの特異的協同効果

岡村 浩之   ; 池田 篤史*  ; 斉藤 拓巳*; 青柳 登   ; 長縄 弘親  ; 平山 直紀*; 梅谷 重夫*; 井村 久則*; 下条 晃司郎   

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) from water by use of a novel macrocyclic receptor (HDA18C6) composed of diaza-18-crown-6 and two -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 -diketone fragments to a diazacrown ether resulted in a cooperative interaction within the receptor for Sr transfer, which remarkably enhanced the efficiency of Sr transfer relative to a mixed -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 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 and O atoms in the Sr complex was shorter in IL than in chloroform. Consequently, Sr was held by HDA18C6 more rigidly in IL than in chloroform, representing an important factor dominating the magnitude of the intramolecular cooperative effect of HDA18C6 for Sr.