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.
A series of 22 tridentate unsaturated mono-anionic ligands having the atom-sequence Y-C[double bond, length as m-dash]C-N=CH-C=C-Z(-1), with Y = N, O, or S and Z = O or S, has been studied to establish whether this backbone could be used to develop strong solvent extractants for nickel(II) which will preferably also show a high selectivity over iron(III) in the pH-dependent process: 2LH(org) + NiSO4 ⇌ [(L)2Ni]org + H2SO4. All are capable of forming octahedral [(L)2Ni] complexes with a mer-arrangement of the YNZ(-1) donor set. X-ray crystal structures of three salicylaldimine proligands derived from 3-bromo-5-t-butyl-2-hydroxybenzaldehyde show these to have pre-organised donor sets in which the three donors are held in an approximately orthogonal arrangement by intramolecular hydrogen bonds. The tautomers observed are dependent on the nature of the Y atom and the extent to which it is favourable for this to form a bonding interaction with the acidic hydrogen atom on the salicylaldimine unit. X-ray crystal structure determinations of seven of the [(L)2Ni] complexes show these to have significantly distorted octahedral coordination geometries which partly account for the proligands proving to be fairly weak Ni-extractants. DFT calculations show that extractant strength is dependent on a combination of the binding strength of the YNZ(-1) donor set to the nickel ion and on the ease of deprotonation of the extractant. On this basis 3-nitro-4-t-octyl-6-(quinolin-8-imino)phenol is predicted, and is found, to be the strongest Ni-extractant. The extractants have low hydrolytic stability, reverting to their aldehyde precursors when solutions in water-immiscible solvents are contacted with aqueous acid, making them poor candidates for development as reagents for nickel recovery based on pH-swing processes of the type shown above.
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