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.

What's on the outside? Selective extraction and transport of [PtCl6]2− from aqueous acidic solutions in the presence of excess chloride has been demonstrated through outer‐sphere coordination of the metalloanion by tripodal polyamide and polyurea receptors (see picture; Pt purple, N blue, O red, Cl green). Loading and stripping of the organic phase can be controlled by variation of the pH value of the aqueous solution.

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A C2 hexadentate, in which two pyridine-2-aldehyde 2'-pyridylhydrazone (PAPHY) groups are linked to a chiral auxiliary derived from (R,R)-tartaric acid, (R,R)-1, reacts with iron(II) benzenesulfonate to give the two-bladed propeller, octahedral complex (P(Fe))-[Fe{(R,R)-1}](PhSO3)2 with complete diastereoselectivity, as determined by 1H NMR spectroscopy and X-ray crystallography. Saponification of the ester linkages and deprotonation of the hydrazone-NH groups in the configurationally pure diastereomer affords the complex (P(Fe))-[Fe(5-HOCH2PAPY)2] with 85% retention of configuration at the iron stereocenter, as determined by reprotonation of the neutral complex with enantiomerically pure (aR)-binaphthyl phosphoric acid and analysis of the 1H NMR spectrum of the mixture of diastereomeric salts produced. This is the first asymmetric synthesis of a two-bladed propeller, octahedral metal complex by the classical organic methodology of chiral auxiliary-directed, asymmetric synthesis.

Extraction and binding studies of [PtCl6](2-) are reported for 24 mono-, bi-, and tripodal extractants containing tris(2-aminoethyl)amine (TREN) or tris(3-aminopropyl)amine (TRPN) scaffolds. These reagents are designed to recognize the outer coordination sphere of [PtCl6](2-) and to show selectivity over chloride anion under acidic conditions. Extraction from 0.6 M HCl involves protonation of the N-center in tertiary amines containing one, two, or three urea, amide, or sulfonamide hydrogen-bond donors to set up the following equilibrium: 2L(org) + 2H(+) + [PtCl6](2-) ⇌ [(LH)2PtCl6](org). All reagents show higher Pt loading than trioctylamine, which was used as a positive control to represent commercial trialkylamine reagents. The loading of [PtCl6](2-) depends on the number of pendant amides in the extractant and follows the order tripodal > bipodal > monopodal, with urea-containing extractants outperforming amide and sulfonamide analogues. A different series of reagents in which one, two, or three of the alkyl groups in tris-2-ethylhexylamine are replaced by 3-N'-hexylpropanamide groups all show a comparably high affinity for [PtCl6](2-) and high selectivity over chloride anion in extractions from aqueous acidic solutions. (1)H NMR titration of three extractants [LH·Cl] with [(Oct4N)2PtCl6] in CDCl3 provides evidence for high selectivity for [PtCl6](2-) over chloride for tri- and bipodal extractants, which show higher binding constants than a monopodal analogue.