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).