Anion Receptor Design: Exploiting Outer-Sphere Coordination Chemistry To Obtain High Selectivity for...

High anion selectivity for PtCl6(2-) over Cl(-) is shown by a series of amidoamines, R(1)R(2)NCOCH2CH2NR(3)R(4) (L1 with R(1) = R(4) = benzyl and R(2) = R(3) = phenyl and L3 with R(1) = H, R(2) = 2-ethylhexyl, R(3) = phenyl and R(4) = methyl), and amidoethers, R(1)R(2)NCOCH2CH2OR(3) (L5 with R(1) = H, R(2) = 2-ethylhexyl and R(3) = 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.