Anion receptor design; exploiting outer sphere coordination chemistry to obtain high selectivity for chloridometalates over chloride.

High anion selectivity for PtCl₆^2– over Cl^– is shown by a series of amidoamines, R¹R²NCOCH₂CH₂NR³R⁴ (L1 with R¹ = R⁴ = benzyl and R² = R³ = phenyl and L3 with R¹ = H, R² = 2-ethylhexyl, R³ = phenyl and R⁴ = methyl), and amidoethers, R¹R²NCOCH₂CH₂OR³ (L5 with R¹ = H, R² = 2-ethylhexyl and R³ = phenyl), which provide receptor sites which extract PtCl₆^2– preferentially over Cl^– in extractions from 6 M HCl solutions. The amidoether receptor L5 was found to be a much weaker extractant for PtCl₆^2– than its amidoamine analogues. Density functional theory calculations indicate that this is due to the difficulty in protonating the amidoether to generate a cationic receptor, LH⁺, rather than the latter showing weaker binding to PtCl₆^2–. The most stable forms of the receptors, LH⁺, contain a tautomer in which the added proton forms an intramolecular hydrogen bond to the amide oxygen atom to give a six-membered proton chelate. Dispersion-corrected DFT calculations appear to suggest a switch in ligand conformation for the amidoamine ligands to an open tautomer state in the complex, such that the cationic N–H or O–H groups are also readily available to form hydrogen bonds to the PtCl₆^2– ion, in addition to the array of polarized C–H bonds. The predicted difference in energies between the proton chelate and nonchelated tautomer states for L1 is small, however, and the former is found in the X-ray crystal structure of the assembly [(L1H)₂PtCl₆]. The DFT calculations and the X-ray structure indicate that all LH⁺ receptors present an array of polarized C–H groups to the large, charge diffuse PtCl₆^2– anion resulting in high selectivity of extraction of PtCl₆^2– over the large excess of chloride