Macrocycle oxygen-donor

Fig. 18. Plot of change in complex stability, Alogf , that occurs for the pairs of ligands cryptand-2,2,2 and EN ( ), and 18-aneN204 and EN (O), as a function of metal ion radius (20). The diagram shows how neutral oxygen donors tend to stabilize the complexes of large relative to small metal ions in macrocyclic ligands. Data from Ref. (11).

Impetus was given to work in the field of selective cation complex-ation by the observation of Moore and Pressman (5) in 1964 that the macrocyclic antibiotic valinomycin is capable of actively transporting K+ across mitochondrial membranes. This observation has been confirmed and extended to numerous macrocyclic compounds. There is now an extensive literature on the selective complexation and transport of alkali metal ions by various macrocyclic compounds (e.g., valinomycin, mo-nactin, etc.) (2). From solution spectral (6) and crystal X-ray (7) studies we know that in these complexes the alkali metal cation is situated in the center of the inwardly oriented oxygen donor atoms. Similar results are found from X-ray studies of cyclic polyether complexes of alkali metal ions (8) and barium ion (9). These metal macrocyclic compound systems are especially noteworthy since they involve some of the few cases where alkali metal ions participate in complex ion formation in aqueous solution. 

Transition-metal systems with macrocyclic nitrogen and oxygen donors are more analogous to biological 02-oxidation centers than the more organometallic type systems considered thus far. Studies on the macrocyclic ligand systems follow earlier ones such as Udenfriend s systems (e.g., Fe(II)-EDTA), and the use of other transition-metal salts, sometimes with added ligands (43, 113). 

The template synthesis of mixed nitrogen and oxygen donor macrocycles are mainly based on substituted salicylaldehydes or salicyl-aldehyde-metal complexes (82), e.g.,... 

Schiff basses related compounds form the third group containing both nitrogen and oxygen donors. Complexes of Schiff base derived ligand will be discussed at length under the section Macrocyclic complexes . 

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