Selective Anion Extraction and Recovery Using a Fe II 4 L 4 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 Fe II 4 L 4 cage 1 is able to extract an equimolar amount of ReO 4 − , a high‐value anion and a nonradioactive surrogate of TcO 4 − , 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 ReO 4 − . 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 ReO 4 − (as the tetrabutylammonium salt) from an organic phase into water by using the sulfate salt of 1 as the extractant.