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New [3+3] Schiff-base macrocycles and their complexes Gallant, Amanda Jane

Abstract

A series of conjugated [3+3] Schiff-base macrocycles containing both a central crown ether-like pocket and three tetradentate N[sub 2]O[sub 2] binding sites were prepared and investigated. The formation mechanism was investigated through the synthesis and study of macrocycle fragments. Further understanding of the macrocycle conformations and dynamics was obtained through computational studies. A monoreduced macrocycle where one of the six imines has been reduced was obtained as a by-product of macrocycle formation. Reactivity studies and deuterium labeling investigations revealed that the selective reducing agent is likely a benzimidazoline. This intermediate is generated in situ during the formation of the nonreduced macrocycle and with macrocycle reduction is converted to a stable benzimidazole unit. Upon addition of small cations, the conjugated Schiff-base macrocycles assemble into tubular structures. Spectroscopic and mass spectrometric studies have shown that the cations bind to the crown ether-like centre of the macrocycle and induce aggregation to form structures composed of alternating cations and macrocycles. With the addition of seven equivalents of Zn[sup 2+] or Cd[sup 2+] to these fully conjugated macrocycles surprising heptametallic complexes were obtained. Here, the trimetallated macrocycle is first formed (with metal ions bound to the three N[sub 2]O[sub 2] pockets) and then this templates the formation of a [M[sub 4]O][sub 6+] cluster that caps the cone-shaped macrocycle. NMR studies indicated that these zinc complexes dimerize under certain solvent conditions forming capsule-like structures resembling cavitands used in host-guest chemistry. Variations of these [3+3] Schiff-base macrocycles were prepared by modifying the substituents of the diformyl diol unit. In this way naphthalene-based macrocycles were prepared. Studies on a series of related model compounds revealed that the ketoenamine isomer is stabilized in these macrocycles rather than the enolimine isomer as observed in the analogous phenyl-based macrocycles.

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