Molecular Insights into the Separation of Iron by 18-Crown‑6 Ethers from Hydrochloric Acid.

Anomalous iron extraction has been observed in 18-crown-6-based extraction chromatographic resins for radiochemical and nuclear forensic separations. The molecular basis for this behavior was examined using three crown ethers: 18-crown-6 (18C6), dicyclohexano-18-crown-6 (DCH18C6), and dibenzo-18-crown-6 (DB18C6). Batch solvent extraction, spectroscopy, and density functional theory (DFT) calculations show that Fe is maximally extracted in highly concentrated hydrochloric acid (HCl) (>6 M), where FeCl4 - predominates in solution. Data support hydrogen-bonding interactions of hydronium (H3O+) within the macrocyclic cavity, followed by ion-pair association with FeCl4 -. The DC18C6 and 18C6 form predominantly 1:1 complexes, whereas DB18C6 exhibits features of composite binding. Extraction is spontaneous, with negligible enthalpic contributions within experimental uncertainty. These results provide quantitative data for the unexpected iron extraction by crown ether-based systems, offering mechanistic insight into the expanded use and design of crown ether-based radiochemical separations.