Ditopic Extractants to Separate Palladium(II) and Platinum(IV) Chloridometalates via Inner or Outer Sphere Binding

New thioetheramide ligands (L, PhS(CH₂)_nCONRR’) co-extract Pd(II) and Pt(IV) from acidic chloride solutions. The Pd is transferred to a water-immiscible phase as a [Pd(L)₂Cl₂] complex with thioether groups in the inner sphere whilst Pt is extracted in an outer-sphere assembly, [(LH)₂·PtCl₆], containing protonated reagent molecules LH⁺ that charge-balance the chloridoplatinate dianion, [PtCl₆]^2-. The much higher kinetic and thermodynamic stability of the Pd(II) complex makes it possible to strip the Pt into a weakly acidic aqueous phase before recovering the Pd by back-extraction into aqueous ammonia to form [Pd(NH₃)₄]Cl₂, thereby separating the two elements. An alkyl spacer group with two methylene units between the thioether (S) and amide (C) atoms is a stronger extractant for both metals than those with one or three methylene units. The extractants reject trianionic chloridometalates with higher hydration energies such as [IrCl₆]^3-. X-ray structures of two [Pd(L)₂Cl₂] complexes (L, PhSCH₂CONH-n-C₄H₉ or PhS(CH₂)₂CONH-n-C₄H₉) have planar coordination with a transarrangement of the thioether groups and geometries very similar to those predicted by DFT calculations. These calculations show that addition of a proton to the proligands L generates a pseudochelate with the added H⁺ located between the S atom and the carbonyl O atom. In contrast to related ether- and amino-amide extractants, this pseudochelate ring is broken in the [(LH)₂.PtCl₆] assemblies formed by the thioetheramides and the OH⁺ and NH units make the close contacts to the PtCl₆^2- ion.