Macrocyclic ligands alkali metal complexes

Novel electron-transfer reactions mediated by alkali metals complexed with crown ethers as macrocyclic ligands 98ACR55. 

Polypyridine Torand 1. Complexes of alkali metal triflates and picrates have been prepared from the monotriflate salt of torand 1, which is isolated directly from the macrocyclization reaction mixture. As shown in Figure 3, neutralization of the triflate salt with alkali metal hydroxides or carbonates gives the alkali metal complexes. With triflate or picrate counterions, torand complexes and salts partition selectively into chlorofonn rather than water. Thus 1 can be shuttled back and forth between the triflate salt and various complexes simply by washing the chloroform solution with aqueous acid or base. The free ligand is prepared by reaction of the triflate salt with tetra-n-butylammonium hydroxide in butanol/acetonitrile. Combustion microanalysis showed that the Li, Na, K, Rb and Cs complexes all have 1 1 host/guest stiochiome-try, despite the cavity/ion size mismatch at both ends of the series. The X-ray crystal... 

Poly (macrocyclic) compounds. The analytical application of compounds such as crown polyethers and cryptands is based on their ability to function as ligands and form stable stoichiometric complexes with certain cations. Special importance is due to their preference for alkali metal ions which do not form complexes with many other ligands. A number of these compounds are commercially available and their properties and analytical applications have been described by Cheng et a/.11... 

Based on donor atom type, macrocyclic ligands can be considered to span two extreme types. First there are those systems which chiefly contain nitrogen, sulfur, phosphorus, and/or arsenic donors. These macrocycles tend to have considerable affinity for transition and other heavy metal ions they usually show much less tendency to form stable complexes with ions of the alkali and alkaline earth metals. The present discussion will be restricted to a consideration of a selection of such ligands and their complexes. 

Crown polyethers. Macrocyclic effects involving complexes of crown polyethers have been well-recognized. As for the all-sulfur donor systems, the study of the macrocyclic effect tends to be more straightforward for complexes of cyclic polyethers especially when simple alkali and alkaline earth cations are involved (Haymore, Lamb, Izatt Christensen, 1982). The advantages include (i) the cyclic polyethers are weak, uncharged bases and metal complexation is not pH dependent (ii) these ligands readily form complexes with the alkali and alkaline earth cations... 

In a similar manner the so-called Zintl salts composed of alkali metal cations and clusters of metals as anions (see Chapter 16) were known in liquid ammonia solution but proved to be impossible to isolate Upon removal of the solvent they reverted to alloys. Stabilization of the cations by complexation with macrocyclic ligands allowed the isolation and determination of the structures of these compounds. 

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