Contributions of inner and outer coordination sphere bonding in determining the strength of substituted phenolic pyrazoles as copper extractants

Abstract

Alkyl-substituted phenolic pyrazoles such as 4-methyl-2-[5-(n-octyl)-1H-pyrazol-3-yl]phenol (L2H) are shown to function as Cu-extractants, having similar strength and selectivity over Fe(iii) to 5-nonylsalicylaldoxime which is a component of the commercially used ACORGA® solvent extraction reagents. Substitution in the phenol ring of the new extractants has a major effect on their strength, e.g. 2-nitro-4-methyl-6-[5-(2,4,4-trimethylpentyl)-1H-pyrazol-3-yl]phenol (L4H) which has a nitro group ortho to the phenolic hydroxyl group unit and has an extraction distribution coefficient for Cu nearly three orders of magnitude higher than its unsubstituted analogue 4-methyl-6-[5-(2,4,4-trimethylpentyl)-1H-pyrazol-3-yl]phenol (L8H). X-ray structure determinations and density functional theory (DFT) calculations confirm that inter-ligand hydrogen bonding between the pyrazole NH group and the phenolate oxygen atom stabilise the Cu-complexes, giving pseudomacrocyclic structures. Electron-accepting groups ortho to the phenol oxygen atoms buttress the inter-ligand H-bonding, enhancing extractant strength but the effectiveness of this is very dependent on steric factors. The correlation between the calculated energies of formation of copper complexes in the gas phase and the observed strength of comparably substituted reagents in solvent extraction experiments is remarkable. Analysis of the energies of formation suggests that big differences in strength of extractants arise principally from a combination of the effects of the substituents on the ease of deprotonation of the proligands and, for the ortho-substituted ligands, their propensity to buttress inter-ligand hydrogen bonding.

Salicylaldehyde Hydrazones: Buttressing of Outer-Sphere Hydrogen-Bonding and Copper Extraction Properties

“…As discussed recently for other systems where this occurs, [62] this correlation will only 13 be observed when the process occurring in the aqueous phase is the same, in this case two protons replacing one copper dication, for the series of extractions. Consequently, as in this work, hydration energies of species do not contribute to differences in the formation energies of metal complexes.…”

The Design of Tetradentate Ligands for Beryllium Encapsulation

“…Tasker and co-workers have also shown that the metal binding strength of a chelating ligand can be enhanced through the use of a hydrogen-bond buttress. 18 An initial computational investigation on a series of tetradentate receptors designed to have a selective, compact beryllium binding pocket ( Figure 5) was performed. 19 Ligands L16L18 and L20 provided an N 3 O donor set while ligand L19 presented an N 4 donor set.…”

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

“…Phenolic pyrazoles (Figure 4, right) have also been evaluated as extractants for Cu and the structures of the resulting complexes studied by X-ray diffraction and DFT calculations [15].…”