Design and Function of Pre-organised Outer-Sphere Amidopyridyl Extractants for Zinc(II) and Cobalt(II) Chlorometallates: The Role of CH Hydrogen Bonds

• Published in: Chemistry : a European journal 2012-06, Vol.18 (25), p.7715
• Main Authors: Ellis, Ross J, Chartres, Jy, Henderson, David K, Cabot, Rafel, Richardson, Patricia R, White, Fraser J, Schroder, Martin, Turkington, Jennifer R, Tasker, Peter A, Sole, Kathryn C
• Format: Article
• Language: English
• Subjects: Chemistry; Hydrogen bonds; Zinc
• ISSN: 0947-6539; 1521-3765
• DOI: 10.1002/chem.201103616

Four new sterically hindered pyridines, L1-L4-containing amido substituents at the 2-position act as efficient solvent extractants for [CoCl4]2- or [ZnCl4]2- from acidic chloride solutions through protonation of the pyridino N-centre to form the neutral outer-sphere complexes [(LH)2MCl4]. These ionophores show very high selectivity for chlorometallate anions over chloride ion and are readily stripped to liberate the free-metal chlorides without the formation of inner-sphere complexes [ML2Cl2]. Single-crystal X-ray structure determinations of [(L2H)2CoCl4] and [(L2H)2ZnCl4] (L2=2-(4,6-di-tert-butylpyridin-2-yl)-N,N'-dihexylmalonamide) coupled with 1HNMR spectroscopy and DFT calculations on L2H+ and other complexes of [ZnCl4]2- confirm that the pyridinium NH group does not address the outer co-ordination sphere of the metallanion, but rather forms a hydrogen bond to the pendant amide groups and thus pre-organizes the ligand to present both CH and amido NH hydrogen-bond donors to the [MCl4]2- ions. The selectivity for chlorometallates over chloride ions shown by this class of extractants arises from their ability to present several polarized CH units towards the charge-diffuse ions [MCl4]2-, whereas the smaller, "harder" chloride anion prefers to be associated with the amido NH hydrogen-bond donors.