Examples of Supramolecular Extraction

It must be stressed that factors such as the hydration (or solvation) of the metal ion and anion effects on the extracted complex often make it difficult to predict the order of extractability for such systems. Such factors may even influence the stoichiometry of the extracted species. Thus, the simple match of the metal to the whole concept is only of limited utility. For example, potassium, rubidium and sodium nitrates are extracted in the presence of dibenzo-18-crown-6 (2) as 1 1 1 complexes. On the other hand, cesium forms a 1 2 1 sandwich complex with this crown (metal crown nitrate) in the organic phase and this affects the extraction order for the above metal ions, with the order being dependent on ligand concentration. In contrast, for picrate as the anion the composition of the extracted cesium complex is 1 1 1 (Fig. 4.8) [27]. 

The use of ligands that are conformationally reinforced and have the appropriate number, nature and arrangement of their donor atoms for a particular metal ion. 

The introduction of intermediate and soft donor atoms like nitrogen and sulfur into a ligand architecture leads to a stronger preference for softer metal ions, such as the transition and precious metals. For example, the macrobicyclic tris(pyridine) cage 6 was shown to transport Ag+ with high selectivity into an organic phase [31]  

Over many years there has been continuing strong interest in the chemistry of polyazacompounds of different structure (open-chain, macrocyclic or macrobicy-dic) because such ligands are able to bind and extract both metal cations [32, 33] and anionic species [34, 35], depending on the pH of the aqueous solution. 

In their protonated form polyamines are often attractive receptors for a variety of anions, as exemplified by many examples now known in nature, industry and model systems [34, 36]. The characteristic extraction properties of a range of such ligands toward halide, pertechnetate and perrhenate ions are strongly influenced by the ligand s protonation behavior and lipophilicity. Extraction therefore shows a pH-dependence, as well as being influenced by all manner of structural factors [35, 37, 38]. Some results for the perrhenate extraction by three different substituted tripodal tetraamines 7a-c and two structure-related octaamino cryptands 7d,e (Fig. 4.11) are shown in Fig. 4.12 [35, 37]. 

---