Superiority of the Supramolecular Halogen Bond Receptor over Its H‑Bond Analogue toward the Efficient Extraction of Perrhenate from Water

A halogen bond-based water-soluble tetrapodal iodoimidazolium receptor, (L-I)(4Br), exhibited a high degree of efficiency (∼96%) in extracting ReO4– from 100% aqueous medium within a wide range of concentrations and of pH values along with excellent reusability. The solid-state X-ray diffraction study showed the trapping of ReO4– by (L-I)(4Br) via the Re–O····I halogen bonding interaction. XPS studies also suggested the interaction between I and ReO4– through polarization of the electron density of I atoms by ReO4–. (L-I)(4Br) is found to be capable of retaining its high extraction efficiency in the presence of competing anions such as F–, Cl–, I–, SO42–, H2PO4–, CO32–, NO3–, BF4–, ClO4–, Cr2O72–, and a mixture of these anions. Interestingly, (L-I)(4Br) was found to be superior in ReO4– extraction as compared to its hydrogen-bond donor analogue, (L-H)(4Br), as confirmed by a series of control experiments and theoretical calculations. Our synthesized dipodal and tripodal halogen bond donor receptors and their H-analogues validated the superiority of these classes of supramolecular halogen bond donor receptors over their hydrogen-bond analogues. (L-I)(4Br) also showed superior practical applicability in terms of the removal of ReO4– at anion concentrations as low as ∼100 ppm, which was a major shortcoming of (L-H)(4Br).