John Campbell

Using the outer coordination sphere to tune the strength of metal extractants.

A series of 3-substituted salicylaldoximes has been used to demonstrate the importance of outer-sphere interactions on the efficacy of solvent extractants that are used to produce approximately one-quarter of the worlds copper. The distribution coefficient for extraction of copper by 5-tert-butyl-3-X-salicylaldoximes (X = H, Me, (t)Bu, NO(2), Cl, Br, OMe) varies by more than two orders of magnitude. X-ray structure determinations of preorganized free ligand dimers (10 new structures are reported) indicate that substituents with a hydrogen-bond acceptor atom attached to the 3-carbon atom, ortho to the phenolic oxygen, buttress the intermolecular hydrogen bond from the oximic proton. Density functional theory calculations demonstrate that this hydrogen-bond buttressing is maintained in copper(II) complexes and contributes significantly to their relative stabilities in energy-minimized gas-phase structures. A remarkable correlation between the order of the calculated enthalpies of formation of the copper complexes in the gas phase and the observed strength of the ligands as copper solvent extractants is ascribed to the low solvation energies of species in the water-immiscible phase and/or the similarities of the solvation enthalpies of the preorganized ligand dimers and their copper(II) complexes.

Polyacidic multiloading metal extractants

Novel polynucleating, di- and tri-acidic ligands have been designed to increase the molar and mass transport efficiencies for the recovery of base metals by solvent extraction.

Exploiting supramolecular chemistry in metal recovery: novel zwitterionic extractants for nickel(ii) saltsBased on the presentation given at Dalton Discussion No. 5, 10?12th April 2003, Noordwijkerhout, The Netherlands.

Hexadentate tris-salicylaldimine ligands bearing ortho-N-dialkylaminomethyl substituents have been shown to function as ditopic ligands for NiSO4 or NiCl2. The incorporation of the Ni-ion into the N3O33− site templates the pendant alkylammonium groups to allow them to hydrogen bond to the attendant anion(s). Formulation as complexes of the trianionic/tricationic ligand is supported by X-ray structure determinations of solvated forms of the complexes [Ni(L)SO4] and [Ni(L)Cl]Cl, where L = tris-1,1,1-{2-hydroxy-3-(piperidin-1-ylmethyl)-5-tert-butylbenzaldiminomethyl}ethane. The kerosene-soluble ligand, tris-2,2′,2″-{2-hydroxy-3-(di-n-hexylaminomethyl)-5-nonylbenzaldiminomethyl}amine, functions as a good extractant for nickel salts, showing high selectivity for recovery of NiCl2 over NiSO4. This reagent can be stripped and recycled directly using aqueous ammonia or by first displacing the Ni2+ with acid and then neutralising the pendant alkylammonium groups. The acid-stripping is accompanied by hydrolytic degradation of the reagent, but in many other respects this novel class of reagent shows promise in opening up novel flowsheets for nickel recovery, based on transport of the metal salt as opposed to ion-exchange based processes.

Supramolecular chemistry in metal recovery; H-bond buttressing to tune extractant strength

3-Substitution of salicylaldoximes alters their copper(II) binding strengths by buttressing stabilising hydrogen bonding.

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

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.

Collision induced dissociation (CID) to probe the outer sphere coordination chemistry of bis-salicylaldoximate complexes.

Ligand-ligand interactions in the outer coordination sphere make an important contribution to the effects of 3-substituents on the stabilities of anionic Cu(II) salicylaldoximato complexes [CuL(L-H)](-). When substituents contain a different number of bonds the interpretation of CID tandem mass spectrometry must take into account the ability of ions to redistribute energy acquired in collisions within different numbers of vibrational modes.

COPPER EXTRACTANT STRENGTH: THE EFFECT OF SUBSTITUENTS IN THE 3-POSITION ON HYDROXYOXIME PERFORMANCE

AbstractThe synthesis and study of a systematic series of phenolic oximes have shown that substitution in the 3-position can greatly affect extractive efficacy. Solid state, solution and gas phase analytical techniques including X-ray crystallography, Electron Paramagnetic Resonance (EPR) and IR spectroscopy and collision induced dissociation mass spectrometry have been assessed for their applicability in interpreting the origins of differences in extractant strength. The dominant substituent effect on pH0.5 is the change in ligand pKa , with the more acidic ligands being stronger extractants. The influence of the 3-substituent on the stabilizing intracomplex H-bonding of their copper(II) complexes also affects extractant strength. The 3-NO2 substituted ligand shows the lowest pH0.5 value due to a combination of its low pKa and a stabilising bifurcated H-bond assembly around the copper(II) centre. Appending an aminomethyl group to the 3-position gives a ligand capable of binding a metal and its attendant ...