The thesis deals with the modification of salicylaldoxime-based reagents used inudhydrometallurgical extraction, addressing rational ligand design to tune copper(II)udextractant strengths and also the development of reagents which are capable ofudtransporting transition metal salts.udChapter 1 reviews current solvent extractant technology for metal recovery,udincluding the limited knowledge of the effect of substituents on extractive efficacy.udAdvances in leaching technology have led to systems wherein increases in processudefficiency could be obtained using reagents which can transport both a transitionudmetal cation and its attendant anion(s), and the potential advantages of metal saltudextractants are discussed. The problems encountered when trying to extractudhydrophilic anions selectively into organic media are also considered.udChapter 2 discusses techniques used in industry to tune reagent properties, many ofudwhich depend on the importance of H-bonding in non-polar solvents. Synthesis of audseries of 5-alkyl-3-X-2-hydroxybenzaldehyde oximes (X = a range of substituents) isuddescribed and copper extraction experiments are reported. 3-Substitution is found toudalter reagent strength by two orders of magnitude, with 3-bromo-5-tert-butyl-2-udhydroxybenzaldehyde oxime the strongest extractant. An analysis of X-rayudstructures of several ligands and copper(II) complexes is given in an attempt toudestablish whether trends in the solid state structures can account for variations inudextractant strength. A more detailed analysis of the hydrogen bonding inudsalicylaldoximato copper(II) complexes and ligand dimers is carried out in Chapterud3, with the aim of defining how substituent effects could be used to design reagentsudwith appropriate extractive behaviour. 3-X-2-Hydroxybenzaldehyde oximes with noud5-alkyl substituent are synthesised and subjected to a detailed study by X-rayudcrystallography and computational techniques, which, alongside evidence providedudby CID-MS experiments, suggest that the dominant substituent effect in determiningudextractant strength is the ability to “buttress” the pseudomacrocyclic hydrogenudbonding motif involving the oximic hydrogen and phenolic oxygen. Ligands with 3-substituents capable of accepting H-bonds were found to be stronger extractants thanudthose which could not, and the steric hindrance afforded by bulky substituents madeud3,5-di-tert-butyl-2-hydroxybenzaldehyde oxime the weakest extractant. Ligandudacidity is also noted to have a significant effect on reagent strength, with electronwithdrawingudsubstituents lowering the pKa of the phenolic proton and increasingudextractive efficacy.udChapter 4 focuses on metal salt extraction, and the development of selective, robustudand hydrolytically stable reagents. Six novel extractants, based on a salicylaldoximeudscaffold with a pendant dialkylaminomethyl arm, are described. Only 5-tert-butyl-3-uddihexylaminomethyl-2-hydroxybenzaldehyde oxime and 3-tert-butyl-5-uddihexylaminomethyl-2-hydroxybenzaldehyde oxime have sufficient solubility to beudeffective reagents. The former extracts CuCl2 and ZnCl2 in a highly efficientudmanner, with one mole of metal salt extracted per mole of ligand, twice the expectedudcapacity. X-ray structure determination of complexes of the related ligand 5-tertbutyl-ud2-hydroxy-3-piperidin-1-ylmethylbenzaldehyde oxime defines the bindingudmode, with the chloride anions bound to the inner sphere of the metal cations.udLoading and stripping experiments show it to be an extractant with potentialudcommercial application. Cation and anion selectivity of the two extractants definedudabove is the focus of Chapter 5, which begins with an overview of techniques andudattempts to attenuate the Hofmeister bias, the main factor in the selective extractionudof hydrophilic anions into organic media. pH loading profiles show the 3-uddihexylaminomethyl isomer to be an effective CuCl2 and CuSO4 extractant, but theudcation extractive efficacy of the 5-isomer is hampered by the 3-tert-butyl group.udBoth ligands are found to be selective for Cl- > SO4ud2-, following the Hofmeister bias.udFurther information on anion binding is provided by solid state structures of copperudsalt complexes, showing that in all cases the copper(II) cation interacts in some wayudwith the anion. Cation extraction is affected significantly by the anion present, withudFeIII selectively extracted against CuII in the presence of SO4ud2- which is consistentudwith cation-anion interactions having great influence on the overall stability of theudligand-metal salt assembly.
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