A Comparison of the Selectivity of Extraction of [PtCl6](2-) by Mono-, Bi-, and Tripodal Receptors That Address Its Outer Coordination Sphere

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. 1H 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.