Anion effects in selective bifunctional metal salt extractants based on aza-thioether macrocycles:: co-operative cation-anion binding?

The binding of AgX salts (X=NO3-, BF4- and ClO4-) to [15] aneS(2)ON(2)-(CH2CONHCONH-Bu-t)(2) (L-1) has been assessed by H-1 NMR spectroscopy. H-1 NMR titration studies of AgX with L-1 and related ligands are interpreted in terms of a range of metal-salt binding interactions. Anion binding occurs via a switching mechanism whereby co-ordination of a metal cation to L-1 breaks internal hydrogen-bonding to allow anions to bind to the inner urea hydrogen site on the attached pendant arm. No anion binding occurs in the absence of a bound metal ion. Crystal structures of two complexes L.AgNO3 confirm this feature [for N-H...ONO2, N...O=2.866(5)-2.940(5) Angstrom in L-1.AgNO3], but also show chelate binding of the amide O-donor of the acylurea pendant arm to Ag-I in the solid state. Two-phase metal extraction studies with AgX salts and L-1 confirm that the anion plays an important role, although it is likely that solubility of the resulting metal complexes in the organic phase is the predominant factor. A range of related amide substituted macrocycles has also been prepared and anion binding interactions appear strongest with those ligands containing the highest number of amide/urea units. At high anion concentrations, and in the presence of Ag-I all of the ligands participate in anion-ligand interactions.