Modification of Phenolic Oximes for Copper Extraction
Forgan, Ross Stewart
The thesis deals with the modification of salicylaldoxime-based reagents used in hydrometallurgical extraction, addressing rational ligand design to tune copper(II) extractant strengths and also the development of reagents which are capable of transporting transition metal salts. Chapter 1 reviews current solvent extractant technology for metal recovery, including the limited knowledge of the effect of substituents on extractive efficacy. Advances in leaching technology have led to systems wherein increases in process efficiency could be obtained using reagents which can transport both a transition metal cation and its attendant anion(s), and the potential advantages of metal salt extractants are discussed. The problems encountered when trying to extract hydrophilic anions selectively into organic media are also considered. Chapter 2 discusses techniques used in industry to tune reagent properties, many of which depend on the importance of H-bonding in non-polar solvents. Synthesis of a series of 5-alkyl-3-X-2-hydroxybenzaldehyde oximes (X = a range of substituents) is described and copper extraction experiments are reported. 3-Substitution is found to alter reagent strength by two orders of magnitude, with 3-bromo-5-tert-butyl-2- hydroxybenzaldehyde oxime the strongest extractant. An analysis of X-ray structures of several ligands and copper(II) complexes is given in an attempt to establish whether trends in the solid state structures can account for variations in extractant strength. A more detailed analysis of the hydrogen bonding in salicylaldoximato copper(II) complexes and ligand dimers is carried out in Chapter 3, with the aim of defining how substituent effects could be used to design reagents with appropriate extractive behaviour. 3-X-2-Hydroxybenzaldehyde oximes with no 5-alkyl substituent are synthesised and subjected to a detailed study by X-ray crystallography and computational techniques, which, alongside evidence provided by CID-MS experiments, suggest that the dominant substituent effect in determining extractant strength is the ability to “buttress” the pseudomacrocyclic hydrogen bonding motif involving the oximic hydrogen and phenolic oxygen. Ligands with 3-substituents capable of accepting H-bonds were found to be stronger extractants than those which could not, and the steric hindrance afforded by bulky substituents made 3,5-di-tert-butyl-2-hydroxybenzaldehyde oxime the weakest extractant. Ligand acidity is also noted to have a significant effect on reagent strength, with electronwithdrawing substituents lowering the pKa of the phenolic proton and increasing extractive efficacy. Chapter 4 focuses on metal salt extraction, and the development of selective, robust and hydrolytically stable reagents. Six novel extractants, based on a salicylaldoxime scaffold with a pendant dialkylaminomethyl arm, are described. Only 5-tert-butyl-3- dihexylaminomethyl-2-hydroxybenzaldehyde oxime and 3-tert-butyl-5- dihexylaminomethyl-2-hydroxybenzaldehyde oxime have sufficient solubility to be effective reagents. The former extracts CuCl2 and ZnCl2 in a highly efficient manner, with one mole of metal salt extracted per mole of ligand, twice the expected capacity. X-ray structure determination of complexes of the related ligand 5-tertbutyl- 2-hydroxy-3-piperidin-1-ylmethylbenzaldehyde oxime defines the binding mode, with the chloride anions bound to the inner sphere of the metal cations. Loading and stripping experiments show it to be an extractant with potential commercial application. Cation and anion selectivity of the two extractants defined above is the focus of Chapter 5, which begins with an overview of techniques and attempts to attenuate the Hofmeister bias, the main factor in the selective extraction of hydrophilic anions into organic media. pH loading profiles show the 3- dihexylaminomethyl isomer to be an effective CuCl2 and CuSO4 extractant, but the cation extractive efficacy of the 5-isomer is hampered by the 3-tert-butyl group. Both ligands are found to be selective for Cl- > SO4 2-, following the Hofmeister bias. Further information on anion binding is provided by solid state structures of copper salt complexes, showing that in all cases the copper(II) cation interacts in some way with the anion. Cation extraction is affected significantly by the anion present, with FeIII selectively extracted against CuII in the presence of SO4 2- which is consistent with cation-anion interactions having great influence on the overall stability of the ligand-metal salt assembly.