Selective Extraction and Transport of the [PtCl6](2-) Anion through Outer-Sphere Coordination Chemistry

A series of tripodal receptors designed to recognise the outer coordination sphere of the hexachlorometallate anion [PtCl6](2-), and thus show selectivity for ion-pair formation over chloride binding, has been synthesised and characterised. The tripodal ligands contain urea, amido or sulfonamido hydrogen-bond donors, which are aligned to bind to the regions of greatest electron density along the faces and edges of the octahedral anion. The ligand structure incorporates a protonatable bridgehead nitrogen centre that provides a positive charge to ensure the solubility of a neutral 2:1 [LH](+)/[PtCl6](2-) complex in water-immiscible solvents. The extraction Of [PtCl6](2-) from acidic chloride solutions was evaluated by using a pH-swing mechanism to control the loading and stripping of the metallate anion. The ligands L-1-L-3, L-5-L-9, L-11-L-13 and L-15 showed extremely high loading (up to 95% in some cases) and high selectivity for [PtCl6](2-) over chloride ions (present in a 60-fold excess) compared with trioctylamine, a model Alamine reagent, which, under identical conditions, only extracted 10% of the Pt-IV anions. Generally, extraction was observed to be greater for urea-containing ligands than their amido analogues, and a quantitative recovery of platinum from feed solutions was achieved. The formation of neutral ([LH](+))(2)[PtCl6](2-) packages in organic media is supported by single-crystal Xray structure determinations of [((LH)-H-2)(2)PtC6]center dot 2CH(3)CN, [((LH)-H-8)(2)PtCl6(MeOH)(2)], [((LH)-H-12)(2)PtCl6]center dot 2CH(3)CN and [((LH)-H-14)(2)PtCl6], which confirm the presence of significant hydrogen bonding between the anion and urea or amido moieties of the protonated ligand and the anion. The structure of [((LH)-H-1)(H3O)PtCl6]center dot C6H6 center dot CH3CN shows hydrogen bonding of a H3O+ cation to the receptor and confirms that other stoichiometries are also possible, indicating that speciation in solution may be more complex.