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

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posted on 02.06.2016, 19:37 by Rebecca J. Warr, Katherine J. Bell, Anastasia Gadzhieva, Rafel Cabot, Ross J. Ellis, Jy Chartres, David K. Henderson, Eleni Lykourina, A. Matthew Wilson, Jason B. Love, Peter A. Tasker, Martin Schröder
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.