Strong and Selective Ni(II) Extractants Based on Synergistic Mixtures of Sulfonic Acids and Bidentate N-Heterocycles

James W. Roebuck, Philip J. Bailey, Euan D. Doidge, Adam J. Fischmann, Mary R. Healy, Gary S. Nichol, Niall O’Toole, Max Pelser, Thomas Sassi, Kathryn C. Sole, Peter A. Tasker*

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

Abstract / Description of output

Bidentate 5,5ʹ-alkyl-3,3ʹ-bi-1H-pyrazole and 2-(5-alkyl-1H-pyrazol-3-yl)pyridine ligands, L5 and L6, have been shown to be stronger synergists for the solvent extraction of Ni(II) from sulfate solutions by dinonylnaphthalene sulfonic acid (DNNSAH) than the structurally related tridentate ligand 2,6-bis-[5-n-nonylpyrazol-3-yl]pyridine, L1, previously reported by Zhou and Pesic. The bidentate ligands are highly selective, providing the option of sequential recovery of Ni(II) and Co(II) and rejection of other metals commonly found in the liquors resulting from the acidic sulfate leaching of laterite ores. They were the strongest synergists identified in a screening carried out on 18 types of bidentate and tridentate N-heterocyclic ligands, including the recently reported 2-(2ʹ-pyridyl)imidazoles, L9−11. X-ray crystal structures of Ni(II) complexes of model ligands for L5 and L6, having t-butyl rather than long-chain alkyl groups and with 2-naphthalene sulfonate rather than DNNSA as counteranions, show that the [Ni(L)3]2+ complexes form strong H-bonds from the pyrazolyl NH groups to the oxygen atoms of the sulfonate groups, an arrangement that will stabilize [Ni(L)3·(DNNSA)2] assemblies and shield their polar functionalities from diluent molecules of the water-immiscible phase. UV–visible spectra and mass spectrometry provide evidence for the strong synergists displacing all water molecules from the inner coordination sphere of the Ni(II) ions.

Original languageEnglish
Pages (from-to)437-458
Number of pages22
JournalSolvent extraction and ion exchange
Issue number5
Early online date2 Nov 2018
Publication statusE-pub ahead of print - 2 Nov 2018

Keywords / Materials (for Non-textual outputs)

  • laterite processing
  • Ni recovery
  • outer-sphere bonding
  • supramolecular chemistry
  • synergistic solvent extraction


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