Design and function of pre-organised outer-sphere amidopyridyl extractants for zinc(II) and Cobalt(II) chlorometallates: The role of C-H hydrogen bonds

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 pyridine N-centre to form the neutral outersphere complexes [(LH)2MCl 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 [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 1H NMR spectroscopy and DFT calculations on L 2H+ 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 C-H and amido N-H hydrogenbond donors to the [MCl4]2- ions. The selectivity for chlorometallates over chloride ions shown by this class of xtractants arises from their ability to present several polarized C-H units towards the charge-diffuse ions [MCl4]2-, whereas the smaller, "harder" chloride anion prefers to be associated with the amido N-H hydrogen-bond donors © 2012 Wiley-VCH Verlag GmbH&Co. KGaA, Weinheim.