Macrocyclic ligands arsenic

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. 

These examples not only indicate an interesting chemistry of arsenic (III) rings but also illustrate their versatility as macrocyclic ligands. It is important to underscore that the coordinated ligand is formed in a metal induced ring... 

As well as sulfur, macrocycles containing other large donors such as tertiary phosphorus or arsenic atoms are also known, although the metal-ion chemistry of such ligands has been somewhat less explored. In part, this reflects the synthetic difficulties often encountered in the preparation of ligands containing these heteroatoms structures (49) (Horner, Walach Kunz, 1978), (50) (Kauffmann Ennen, 1981), and (51) (Mealli etal., 1985) provide three representative examples of such macrocycles. 

The phosphorus macrocycles are made via template condensation of coordinated polyphosphine ligands and a dibromoalkane (equation 8).60 A more recently reported method involves a template-assisted single-stage ring closure (equation 9).61 The arsenic donor macrocycles are synthesized by reacting lithiated polyarsanes with a dichloroalkane (equation 10).62... 

Macrocyclic polyarsanes (43) have been reported by Ennen and Kauffman.62 The complexation tendencies of these ligands are as yet unexplored, although they should, due to the soft arsenic donors, form stable transition metal complexes. 

Probably one of the commonest reactions encountered in the template synthesis of macrocycles is the formation of imine C=N bonds from amines and carbonyl compounds. We have seen in the preceding chapters that co-ordination to a metal ion may be used to control the reactivity of the amine, the carbonyl or the imine. If we now consider that the metal ion may also play a conformational role in arranging the reactants in the correct orientation for cyclisation, it is clear that a limitless range of ligands can be prepared by metal-directed reactions of dicarbonyls with diamines. The Tt-acceptor imine functionality is also attractive to the co-ordination chemist as it gives rise to strong-field ligands which may have novel properties. All of the above renders imine formation a particularly useful tool in the arsenal of preparative co-ordination chemists. Some typical examples of the templated formation of imine macrocycles are presented in Fig. 6-12. 

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