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Effective extraction of strontium(II) into an ionic liquid by cooperative intramolecular interaction

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

We recently reported that diaza-18-crown-6 bearing two $$beta$$-diketone fragments (H2$$beta$$DA18C6) provides a considerably high extractability of Sr$$^{2+}$$ due to a cooperative intramolecular interaction in the ionic liquid (IL) extraction system. In the present study, we have investigated the extraction capability of H2$$beta$$DA18C6 for Sr$$^{2+}$$ into 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide ([C$$_{2}$$mim][Tf$$_{2}$$N]) and chloroform in detail. From the p$${it K}$$$$_{a}$$ measurements and the slope analysis, the transfer of Sr$$^{2+}$$ with H2$$beta$$DA18C6 into [C$$_{2}$$mim][Tf$$_{2}$$N] proceeds via a proton exchange reaction, which is identical to the extraction mechanism in the chloroform system. The superior extraction performance in the IL system is not attributed to the difference in the extraction mechanism between IL and chloroform systems. Extended X-ray absorption fine structure measurements showed that the Sr coordination environment of the Sr$$^{2+}$$-$$beta$$DA18C6 complex in [C$$_{2}$$mim][Tf$$_{2}$$N] is similar to that in chloroform. However, the distance between Sr$$^{2+}$$ and O atom in the Sr$$^{2+}$$-$$beta$$DA18C6 complex in [C$$_{2}$$mim][Tf$$_{2}$$N] was shorter than that in chloroform. This result reveals that the shortening of the distance causes remarkably high extractability of Sr$$^{2+}$$, and is ascribable to the cooperative intramolecular interaction of this ligand in IL.

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