Lanthanide complexes nitrogen donor macrocycles

Figure 4.43 The structure of [La(24)(N03)3] [70]. (Reproduced from J.D.J Backers-Dirks et aL, Preparation and properties of complexes of lanthanides with a hexadentate nitrogen-donor macrocycle X-ray crystal structure of the complex [La(N03)31], Journal of the Chemical Society, Chemical Communications, 11 A, 1979, by permission of The Royal Society of Chemistry.)...

The size of the metal ion has an evident control on the synthesis of the monometallic macrocyclic complexes. For example, all lanthanide(III) ions promote the formation of the six-nitrogen-donor macrocycles with an 18-atom cavity and flexible aliphatic diimine side-chains, even though the yields become progressively lower as the metal atom size decreases. However, when the six-nitrogen, 18-atom macrocycles contain rigid o-phenylene side chains, the smaller lanthanides actually fail to promote... 

Both the synthetic trends and the reactivity of the metal macrocycle complexes considered in this review find some rationalization in the strudural data obtained from X-ray analysis. In the series of complexes containing the six-nitrogen, 18-atom macrocycles with aliphatic side-chains, the macrocydic moiety can flex to a different extent in each metal complex, thus allowing the most favorable metal-donor distance to be achieved for all donor atoms. These favorable metal-donor distances, together with appropriate geometries of the donor atoms and low steric strain, must account for the unique inertness displayed by these metal-macrocycle entities under conditions (dilute aqueous solution, presence of acids, bases and competing ligands) that would cause most other lanthanide complexes to decompose instantly. 

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