Supramolecular chemistry of liquid-liquid extraction

Liquid-Liquid Extraction (LLE) is a venerable and widely used method for the separation of a targeted solute between two immiscible liquids. In recent years, this method has gained popularity in the supramolecular chemistry community due to the development of various types of synthetic receptors that effectively and selectively bind specific guests in an aqueous medium through different supramolecular interactions. This has eventually led to the development of state-of-the-art extraction technologies for the removal and purification of anions, cations, ion pairs, and small molecules from one liquid phase to another liquid phase, which is an industrially viable method. The focus of this perspective is to furnish a vivid picture of the current understanding of supramolecular interaction-based LLE chemistry. This will not only help to improve separation technology in the chemical, mining, nuclear waste treatment, and medicinal chemistry sectors but is also useful to address the purity issue of the extractable species, which is otherwise difficult. Thus, up-to-date knowledge on this subject will eventually provide opportunities to develop large-scale waste remediation processes and metallurgy applications that can address important real-life problems.

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^-, SO₄^2-, H₂PO₄^-, CO₃^2-, NO₃^-, BF₄^-, ClO₄^-, Cr₂O₇^2-, 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).

A Promising 1,3,5-Triazine-Based Anion Exchanger for Perrhenate Binding: Crystal Structures of Its Chloride, Nitrate and Perrhenate Salts

The reaction of pyridine with cyanuric chloride was studied under microwave activation as well as in the presence of silver nitrate. The product of hydrolysis containing two pyridinium rings and chloride anion was isolated. The structures of these anion exchanger salts with chloride, nitrate and perrhenate anions are discussed.

Immersion grinding and in-situ polymerization synthesis of poly(ionic liquid)s incorporation into MOF composites as radioactive TcO4- scavenger

Imidazolium-based ionic liquids (ILs) are a promising candidate for efficient separation of radioactive pertechnetate (TcO₄^-) from nuclear waste. However, their effective fixation, availability of active sites and slow adsorption kinetics remain challenges. Here, we incorporated the bisimidazolium-based ILs into porous metal-organic frameworks (MOFs) via a combination of immersion grinding and in-situ polymerization. 3,3'-divinyl-1,1'(1,4-butanediyl) diimidazolium dichloride is tightly bound inside and outside the porous MOFs matrix by uniform immersion grinding, which facilitates the exposure of more adsorption sites and provides channels for the anions to travel through quickly. Solvent-free polymerization reduces environmental pollution and energy consumption. Notably, the composite P[C₄(VIM)₂]Cl₂@MIL-101 possesses an admirable removal efficiency (673 mg g^-1) compared with the pristine poly(ionic liquid)s (215 mg g^-1). Meanwhile, it exhibits fast sorption kinetics (92% in 2 min), good β and γ radiation-resistance, excellent regeneration and eminent removal efficiency in high alkaline conditions (83%). These superior traits endow that P[C₄(VIM)₂]Cl₂@MIL-101 effectively separated TcO₄^- from simulated Hanford Low-activity Waste (LAW) Melter off-gas scrubber solution tested in this work. DFT density functional theory confirms that the strong electrostatic attraction and minimum Gibbs free energy (-6.2 kcal mol^-1) achieve high selective adsorption for TcO₄^-. P[C₄(VIM)₂]Cl₂@MIL-101 demonstrates the considerable potential to remove TcO₄^- from radioactive contaminants.

Superiority of a polymeric scavenger over its hexapodal monomer towards efficient ReO4- removal in water

A new imidazolium functionalized hexapodal polymeric receptor, [PHIm-Br], showed selective and efficient removal (>99%) of perrhenate (ReO4-), from 100% aqueous medium via solid-liquid extraction, which was 13% higher as compared to its monomeric analouge [HIm-Br]. Most importantly, [PHIm-Br] overcomes the drawback of [HIm-Br] in terms of removal of ReO4- at lower anion concentration of ∼100 ppm along with excellent radiation resistivity and reusability within a wide pH range, which implies its potential towards practical applications.

Selective encapsulation and extraction of hydrogenphosphate by a hydrogen bond donor tripodal receptor

Intramolecular N–H⋯OC hydrogen bonding between the inner amide groups dictates the receptor–anion complementarity in a tripodal receptor towards selective encapsulation of hydrogenphosphate in the outer urea cavity by multiple hydrogen bonds.