Selective Extraction and Transport of the [PtCl₆]²⁻ Anion through Outer‐Sphere Coordination Chemistry

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Platinum recovery : Tripodal receptors incorporating urea, amido or sulfonamido hydrogen‐bond donors show high loading and selectivity for extraction of [PtCl 6 ] 2− over chloride (present in 60‐fold excess) from a low pH aqueous phase to organic media (see figure). The formation of neutral 2:1 [LH] + /[PtCl 6 ] 2− packages in organic solvents is supported by single‐crystal X‐ray structure determinations. magnified image 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 ] ⋅ 2 CH 3 CN, [(L 8 H) 2 PtCl 6 (MeOH) 2 ], [(L 12 H) 2 PtCl 6 ] ⋅ 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 ] ⋅ C 6 H 6 ⋅ 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.