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

Abstract

The binding of AgX salts (X = NO₃⁻, BF₄⁻ and ClO₄⁻) to [15]aneS₂ON₂–(CH₂CONHCONH–^tBu)₂ (L¹) has been assessed by ¹H NMR spectroscopy. ¹H NMR titration studies of AgX with L¹ 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 L¹ 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) Å in L¹·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 L¹ 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.