Phenolic oxime copper complexes: a gas phase investigation

Abstract

This thesis explores the use of mass spectrometry to define the strengths, and understand solution phase speciation of phenolic oxime-based solvent extractants of the types used in the hydrometallurgical recovery of copper.

Chapter 1 reviews briefly the current extraction technology for copper and focuses on hydrometallurgy and the use of phenolic oximes such as 5-nonylsalicylaldoxime. The modification of the latter to improve extraction efficiency is discussed, focussing on the introduction of X-substituents in the 3 position of the benzene ring. Modern mass spectrometry techniques are also discussed with a focus on their application to inorganic systems and their use in achieving the aims of this thesis, as defined above. The work described in chapter 2 involves the development of collision induced dissociation (CID) techniques to determine the relative gas phase stabilities of copper complex anions of the type [Cu(L)(L-H)]-, where LH is a 5-alkyl-3-X-2- hydroxybenzaldehyde oximes and X a range of substituents. The importance of interligand interactions in the outer-coordination sphere and their influence on gas phase anion stability, as defined by CID, is reported.

The work described in Chapter 2 on CID is extended in chapter 3 and looks at the effect of charge, of ligand type, LH, and of the nature of the metal on the stability of ionic forms of [M(L)₂] complexes, where LH is extended to include 5-alkyl-2- hydroxyphenylethanone oximes. The effects of substitution at the azomethine carbon atom and at the 3-position of the benzene ring and of variation of the nature of the metal on the ion dissociation mechanisms are shown to have a major influence on ion stability under CID conditions.

In chapter 4 density functional theory calculations have been used to investigate the influences of substitution at the azomethine carbon atom and at the 3-position of the benzene ring and of variation of the nature of the metal on the gas phase structures of the neutral complexes, [M(L)₂]. Gas phase deprotonation and dimerisation enthalpies of the ligands, LH, and enthalpies of formation of [M(L)₂] complexes have been calculated and correlates with experimentally determined ligand extraction strength. The ligand type has been extended to include 3-X-2-hydroxybenzaldehyde hydrazones, which have previously been shown to have lower distribution coefficients for copper than the analogous 3-X-substituted oximes. The calculated gas phase formation enthalpies for [M(L)₂] show a strong correlation with the strengths as extractants LH, measured as their pH0.5 values for metal uptake.

Chapter 5 considers whether mass spectrometry can be used to define the solution equilibria when two different oxime-based ligands, LXH and LYH, compete for Cu(II) in a single phase solution. It has been established that shifts in the relative peak intensities of deprotonated ions derived from the Cu(II) complexes, [Cu(LX)₂], [Cu(LY)₂] and [Cu(LX)(LY)] reflect changes in the solution composition.

The work described in chapter 6 extends the study of solution phase speciation using mass spectrometry. When the Cu(II) and proton concentrations of solutions were varied distinct changes in the resulting electrospray mass spectra were observed and the resulting species were identified using CID and high resolution mass spectrometry. A novel, [Cu₃(L-H)₃-μ₃-O/OH]- species is determined to be a major component of solutions where Cu(II) concentrations are equal to/greater than the LH concentration. Various 3-X-2-hydroxybenzaldehyde oximes (X = CH₂NR₂) were synthesised. The incorporation of a protonatable arm in the 3-position enabled trinuclear complexes, [Cu₃(L-H)₃-μ₃-OH], to be isolated and fully characterized, including two X-ray determined crystal structures.