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 [PtCl(6)](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](+)/[PtCl(6)](2-) complex in water-immiscible solvents. The extraction of [PtCl(6)](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 [PtCl(6)](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)[PtCl(6)](2-) packages in organic media is supported by single-crystal X-ray structure determinations of [(L(2)H)(2)PtCl(6)] x 2 CH(3)CN, [(L(8)H)(2)PtCl(6)(MeOH)(2)], [(L(12)H)(2)PtCl(6)] x 2 CH(3)CN and [(L(14)H)(2)PtCl(6)], 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 [(L(1)H)(H(3)O)PtCl(6)] x C(6)H(6) x CH(3)CN shows hydrogen bonding of a H(3)O(+) cation to the receptor and confirms that other stoichiometries are also possible, indicating that speciation in solution may be more complex.