Anion Receptor Design: Exploiting Outer-Sphere Coordination Chemistry To Obtain High Selectivity for Chloridometalates over Chloride
I Carson and KJ MacRuary and ED Doidge and RJ Ellis and RA Grant and RJ Gordon and JB Love and CA Morrison and GS Nichol and PA Tasker and AM Wilson, INORGANIC CHEMISTRY, 54, 8685-8692 (2015).
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.
Return to Publications page