Coordination complexes of 15-membered pentadentate aza, oxoaza and thiaaza Schiff base macrocycles “Old Complexes Offer New Attractions”

• A family of 15-membered N3X2 Schiff base macrocycles are reviewed, where X = N, O or S.
• A historical review of the discovery of mononuclear N3X2 complexes is presented.
• The spin crossover properties of [Fe(N3X2)CN2] complexes are reviewed.
• The structural and magnetic properties of polynuclear N3X2 complexes are summarized.

Two classes of complexes of 15-membered Schiff base macrocycles with a pentadentate (N3X2) cavity (where X = O, N, S) prepared via a metal templated condensation reaction between suitably functionalized 2,6-diacetylpyridine and polyamines are surveyed. The structures and properties of transition metal and lanthanide complexes are presented. Particular emphasis is placed on the unique structural changes accompanying the thermal and light induced spin crossover transition of a [Fe(N3O2)(CN)2] complex as well as a detailed exploration of Mn(II) complexes that in addition to their magnetic properties have been investigated for potential applications in biomedical diagnostics, as enzyme mimics for superoxide dismutase as well as for their antimicrobial activity. The remarkable versatility of this family of complexes as building blocks for the self-assembly of discrete and polymeric systems which include clusters, chains and photoswitchable molecule-based magnets is also reviewed.

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