Selective Anion Extraction and Recovery Using a FeII4 L4 Cage.

Selective anion extraction is useful for the recovery and purification of valuable chemicals, and in the removal of pollutants from the environment. Here we report that FeII4 L4 cage 1 is able to extract an equimolar amount of ReO4- , a high-value anion and a nonradioactive surrogate of TcO4- , from water into nitromethane. Importantly, the extraction was efficiently performed even in the presence of 10 other common anions in water, highlighting the high selectivity of 1 for ReO4- . The extracted guest could be released into water as the cage disassembled in ethyl acetate, and then 1 could be recycled by switching the solvent to acetonitrile. The versatile solubility of the cage also enabled complete extraction of ReO4- (as the tetrabutylammonium salt) from an organic phase into water by using the sulfate salt of 1 as the extractant.