Labile Complexes as Anion Hosts

Dual-host approaches have also been used to good effect for CsNOs extraction. Nitrate is a common anion found upon nitric acid extraction of Cs in the nuclear industry. Cs is conveniently com-plexed by a large crown ether such as tetrabenzo[24]crown-8 while simple tripodal amide hosts of type 

31 (where R is a long alkyl group to impart lipophilicity) are effective at binding NO3. Extraction efficiency of Cs from water into 1,2-dichloroethane was found to be enhanced by a factor of up to 4.4 in the presence of the anion host.  

Metal-assembled anion receptors , Coord. Chem. Rev., 2006, 250, 3190-3199. 

Such complexes form a precursor to a full discussion of the vast and highly topical field of self-assembly (Chapter 10). We consider them here since they resemble structurally the types of compounds discussed in Section 4.7, but unlike metal-based anion receptors the simple thermodynamic equilibrium between host, anion and complex is not the only process occurring in solution. In fact multiple equilibria are occurring covering all possible combinations of interaction between anions, cations and ligands. These systems have the appeal that the formation of particular metal coordination complexes are thus subject to thermodynamic anion templating (c/ the thermodynamic template effect in macrocycle synthesis. Section 3.9.1) and vice versa. 

The related monodentate ligands 5.33 and 5.34 form ML4 type complexes with the relatively inert platinum(II). The complex [Pt(5.33)4] adopts a 1,2-alternate conformation (by analogy with the calixarenes. Section 3.14) giving two anion-binding pockets that can bind planar anions such as nitrate and acetate in a 1 2 ratio. Acetate displays a positive allosteric effect with A 12 being more than twice Kii. Tetrahedral anions are bound in a 1 1 ratio, however, and it is likely the complex can adopt a variety of conformations in solution. Related to [Pt(5.33)4] + is [Pt(5.34)4] +. Like the complex with 5.33, X-ray diffraction results show that the chloride complex adopts a 1,2-altemate conformation with a 

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Hosts with metals as structural elements may be further divided into inert and labile coordination complexes. In the former case, the metal-ligand bonds are long-lived on the human time scale and the host compound binds anions as a unit in exactly the same way as non-metal-containing analogues. For example, in compound 2.85, two thiourea-derived terpyridyl ligands are held together in a well-defined way by a relatively inert (low-spin d ) ruthenium(ii) centre (Scheme 2.12). The host binds long dicarboxylates, particularly pimelate. ... 

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