title: Preorganized tridentate analogues of mixed hydroxyoxime/carboxylate nickel extractants.
abstract: 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.
authors.affiliation: EaStCHEM School of Chemistry, University of Edinburgh, David Brewster Road, Edinburgh, Scotland EH9 3FJ, UK. p.a.tasker@ed.ac.uk.
authors.forename: James W
authors.initials: JW
authors.lastname: Roebuck
authors.affiliation: EaStCHEM School of Chemistry, University of Edinburgh, David Brewster Road, Edinburgh, Scotland EH9 3FJ, UK. p.a.tasker@ed.ac.uk.
authors.forename: Jennifer R
authors.initials: JR
authors.lastname: Turkington
authors.affiliation: EaStCHEM School of Chemistry, University of Edinburgh, David Brewster Road, Edinburgh, Scotland EH9 3FJ, UK. p.a.tasker@ed.ac.uk.
authors.forename: David M
authors.initials: DM
authors.lastname: Rogers
authors.affiliation: EaStCHEM School of Chemistry, University of Edinburgh, David Brewster Road, Edinburgh, Scotland EH9 3FJ, UK. p.a.tasker@ed.ac.uk.
authors.forename: Philip J
authors.initials: PJ
authors.lastname: Bailey
authors.affiliation: Cytec Industries, 1937 West Main Street, Stamford, Connecticut 06902, USA.
authors.forename: Violina
authors.initials: V
authors.lastname: Griffin
authors.affiliation: Cytec Industries, 1937 West Main Street, Stamford, Connecticut 06902, USA.
authors.forename: Adam J
authors.initials: AJ
authors.lastname: Fischmann
authors.affiliation: EaStCHEM School of Chemistry, University of Edinburgh, David Brewster Road, Edinburgh, Scotland EH9 3FJ, UK. p.a.tasker@ed.ac.uk.
authors.forename: Gary S
authors.initials: GS
authors.lastname: Nichol
authors.affiliation: Anglo American, 8 Schonland Street, Theta, Johannesburg, 2091, South Africa.
authors.forename: Max
authors.initials: M
authors.lastname: Pelser
authors.affiliation: EaStCHEM School of Chemistry, University of Edinburgh, David Brewster Road, Edinburgh, Scotland EH9 3FJ, UK. p.a.tasker@ed.ac.uk.
authors.forename: Simon
authors.initials: S
authors.lastname: Parsons
authors.affiliation: EaStCHEM School of Chemistry, University of Edinburgh, David Brewster Road, Edinburgh, Scotland EH9 3FJ, UK. p.a.tasker@ed.ac.uk.
authors.forename: Peter A
authors.initials: PA
authors.lastname: Tasker