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

Dalton Transactions Pub Date: 2003-04-23 DOI: 10.1039/B210391P

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) ? in L1·AgNO3], but also show chelate binding of the amide O-donor of the acylurea pendant arm to AgI 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 AgI, all of the ligands participate in anion–ligand interactions.

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