title: Design and function of pre-organised outer-sphere amidopyridyl extractants for zinc(II) and cobalt(II) chlorometallates: the role of C-H hydrogen bonds.
abstract: Four new sterically hindered pyridines, L(1)-L(4)-containing amido substituents at the 2-position act as efficient solvent extractants for [CoCl(4)](2-) or [ZnCl(4)](2-) from acidic chloride solutions through protonation of the pyridino N-centre to form the neutral outer-sphere complexes [(LH)(2)MCl(4)]. 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 [ML(2)Cl(2)]. Single-crystal X-ray structure determinations of [(L(2)H)(2)CoCl(4)] and [(L(2)H)(2)ZnCl(4)] (L(2) = 2-(4,6-di-tert-butylpyridin-2-yl)-N,N'-dihexylmalonamide) coupled with (1)H NMR spectroscopy and DFT calculations on L(2)H(+) and other complexes of [ZnCl(4)](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 C-H and amido N-H hydrogen-bond donors to the [MCl(4)](2-) ions. The selectivity for chlorometallates over chloride ions shown by this class of extractants arises from their ability to present several polarized C-H units towards the charge-diffuse ions [MCl(4)](2-), whereas the smaller, "harder" chloride anion prefers to be associated with the amido N-H hydrogen-bond donors.
authors.affiliation: School of Chemistry, University of Edinburgh, West Main Road, Edinburgh EH9 3 JJ, UK.
authors.forename: Ross J
authors.initials: RJ
authors.lastname: Ellis
authors.forename: Jy
authors.initials: J
authors.lastname: Chartres
authors.forename: David K
authors.initials: DK
authors.lastname: Henderson
authors.forename: Rafel
authors.initials: R
authors.lastname: Cabot
authors.forename: Patricia R
authors.initials: PR
authors.lastname: Richardson
authors.forename: Fraser J
authors.initials: FJ
authors.lastname: White
authors.forename: Martin
authors.initials: M
authors.lastname: Schröder
authors.forename: Jennifer R
authors.initials: JR
authors.lastname: Turkington
authors.forename: Peter A
authors.initials: PA
authors.lastname: Tasker
authors.forename: Kathryn C
authors.initials: KC
authors.lastname: Sole