Anion Selectivity in Zwitterionic Amide‐Functionalised Metal Salt Extractants

Abstract

Amide-functionalised salen ligands capable of extracting metal salts have been synthesised and characterised. Single-crystal X-ray structure determinations of complexes of NiSO4, [Ni(L)(SO4)], confirm that the ionophores are in a zwitterionic form with Ni(II) bound in the deprotonated salen moiety and the SO4(2-) ion associated with protonated pendant N'-amidopiperazine groups. Treatment of [Ni(L)(SO4)] with base removes the protons from the pendant amido-amine group resulting in loss of the SO4(2-) ion and formation of metal-only complexes of type [Ni(L-2H)], which have been characterized by single-crystal X-ray diffraction. Three of the ligands with solubilities suitable for solvent extraction studies show loading and stripping pH-profiles that are suitable for the recovery of CuSO4 or CuCl2 from industrial leach solutions. The copper-only complexes, [Cu(L-2H)], are selective for Cl- over SO4(2-) in both solvent extraction and bulk liquid membrane transport experiments and were found to bind Cl- in two steps via the formation of a 1:1:1 [Cu(L-H)Cl] assembly, followed by a 1:1:2 [Cu(L)Cl2] assembly as the pH of the aqueous phase is lowered. The anion transport selectivity was evaluated for a number of other mono-charged anions and interestingly the ligands were found to display a preference for the Br- ion. To probe the influence of the Hofmeister bias on the selectivity of anion complexation, single-phase potentiometric titration experiments were employed to investigate the binding of SO4(2-) and Cl- by one of the copper only complexes, [Cu(L-2H)] in 95 %/5 % MeOH/water. Under these conditions selectivity was reversed (SO4(2-)>Cl-) confirming that the Hofmeister bias, which reflects the relative hydration energies of the anions, dominates the selectivity of anion extraction from aqueous media into CHCl3.

Outer‐Sphere Coordination Chemistry: Selective Extraction and Transport of the [PtCl6]2− Anion

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Selectivity in the transport of platinum group metal ions by solvent extraction in industrial processes depends critically upon control of the metal coordination chemistry through the formation of either inner-sphere complexes with dialkyl sulfides or hydroxyoximes, [1] or through formation of outersphere organic-soluble salts with hydrophobic trialkylamine and related reagents of the Alamine type. [1,2] The latter rely upon control of partition coefficients and solubilities such that anion exchange can be used to transfer the chlorometallate to a water-immiscible solvent in a pH-dependent equilibriumAlthough the solvent extraction of base metals such as Cu and Zn usually involves the formation of inner-sphere coordination complexes, [3] the very slow ligand exchange for the [PtCl 6 ] 2À ion [4] makes it necessary to address and recognise the outer coordination sphere of this species to form neutral anion-ligand packages. Selectivity continues to be a challenge in the development of supramolecular recognition of anions [5] and is a pervasive problem in extractive metallurgy because the generation of electrolytes of high purity is essential for efficient electrolytic reduction to produce metals.

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“…Although the solvent extraction of base metals such as Cu and Zn usually involves the formation of inner-sphere coordination complexes, [3] the very slow ligand exchange for the [PtCl 6 ] 2À ion [4] makes it necessary to address and recognise the outer coordination sphere of this species to form neutral anion-ligand packages. Selectivity continues to be a challenge in the development of supramolecular recognition of anions [5] and is a pervasive problem in extractive metallurgy because the generation of electrolytes of high purity is essential for efficient electrolytic reduction to produce metals.…”

Enhanced liquid–liquid anion exchange using macrocyclic anion receptors: effect of receptor structure on sulphate–nitrate exchange selectivity

“…This selectivity trend has its origins in the fact that the thermodynamics of partitioning anions from water into an organic medium exhibits a strong bias that favours chargediffuse ions (11)(12)(13)(14). This bias persists in most solvent systems (15) and can even still dominate in the presence of anion receptors (16,17). The challenge of gaining control of anion selectivity in liquid -liquid extraction can in principle (10)(11)(12) be met with anion receptors bearing a sufficient number of strong hydrogen-bond donors rigidly positioned in space so as to be complementary for a targeted anion.…”

Synthesis of 1,2,3‐Triazole‐4‐carboxamide‐Containing Foldamers for Sulfate Recognition

“…Anions are present everywhere in living systems and cause both beneficial and deleterious effects to human beings. With regard to the various important anions, anion receptors specific to sulfate currently constitute an area of intense interest, due to the important roles the sulfate anion plays in biological systems and disease,2 in hydrometallurgy,3 and as a pollutant 4. Typical anion recognition moieties containing NH groups, such as pyrrole,5 urea,6 amide,7 ammonium,8 and imidazolium systems,9 have been widely studied as hydrogen donors to bind the anion species (N–H ··· X – ).…”