Anion Receptor Design: Exploiting Outer-Sphere Coordination Chemistry To Obtain High Selectivity for Chloridometalates over Chloride

High anion selectivity for PtCl62- over Cl- is shown by a series of amidoamines, (RRNCOCH2CH2NRR4)-R-1-N-2-R-3 (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, (RRNCOCH2CH2OR3)-R-1-N-2 (L5 with R-1. = H, R-2 = 2-ethylhexyl and R3 = phenyl), which provide receptor sites which extract PtCl62- preferentially over Cl- in extractions from 6 M HCl solutions. The amidoether receptor LS was found to be a much weaker extractant for PtCl62- 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 PtCl62-. 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 PtCl62- 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)(2)PtCl6]. 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 PtCl62- anion resulting in high selectivity of extraction of PtCl62- over the large excess of chloride.