Using the Outer Coordination Sphere to Tune the Strength of Metal Extractants

Abstract

A series of 3-substituted salicylaldoximes has been used to demonstrate the importance of outer-sphere interactions on the efficacy of solvent extractants that are used to produce approximately one-quarter of the world's copper. The distribution coefficient for extraction of copper by 5-tert-butyl-3-X-salicylaldoximes (X = H, Me, (t)Bu, NO(2), Cl, Br, OMe) varies by more than two orders of magnitude. X-ray structure determinations of preorganized free ligand dimers (10 new structures are reported) indicate that substituents with a hydrogen-bond acceptor atom attached to the 3-carbon atom, ortho to the phenolic oxygen, buttress the intermolecular hydrogen bond from the oximic proton. Density functional theory calculations demonstrate that this hydrogen-bond buttressing is maintained in copper(II) complexes and contributes significantly to their relative stabilities in energy-minimized gas-phase structures. A remarkable correlation between the order of the calculated enthalpies of formation of the copper complexes in the gas phase and the observed strength of the ligands as copper solvent extractants is ascribed to the low solvation energies of species in the water-immiscible phase and/or the similarities of the solvation enthalpies of the preorganized ligand dimers and their copper(II) complexes.

Two New Schiff Base Ni Ii and Cu Ii Complexes: Synthesis and Structures

“…The bond distances subtended at the Cu atom are comparable to those observed in the similar copper(II) complexes with Schiff bases. 17,18 The morpholine rings adopt well-shaped chair conformation.…”

New Insights into the Recovery of Strategic and Critical Metals by Solvent Extraction

“…It was reported previously that the X-substituent of the phenolic oxime buttressed the hydrogen-bonding interaction between the phenolic oxygen atom and the adjacent oxime hydrogen atom, and that the nature of the X-substituent was a dominant effect on the strength of solvent extraction of Cu ( Figure 4) [12,13]. This is perhaps, surprising, as the bond dissociation energies of metal-ligand bonds are at least one order of magnitude greater than those of hydrogen bonds.…”

New diazo-containing phenolic oximes: structural characterization, computationalstudies, and solvent extraction of Cu(II), Ni(II), and Zn(II) ions

“…At pH 0, 30% of Cu(II) content was transferred from the methoxy group of HL3ox enters into weak hydrogen bond interaction with the oxime group of the neighboring molecule. 17,18 These weak hydrogen bond interactions are thought to contribute to the overall stability of the resulting complex. 17,18 When the extraction results are examined, it is found that HL1ox, HL2ox, and HL4ox have about 30% extraction capacities at pH 0, while HL3ox has about 55% extraction capacity.…”