Formation of Macrocycles Containing Metal Ions as Ring Atoms through Chelation of a Noncyclic Di-Imidazolylazo Ligand

As a first step toward design of biomimetic functional molecules based on macrocycles containing metal ions as ring atoms, metal complexation by a noncyclic ligand containing distant chelating sites was undertaken. For this purpose, tetradentate ligand N,N′-di[3-(2-imidazolylazo)phenyl]-N,N′-dimethyl-isophthalimide (1) and bidentate ligand N-[3-(2-imidazolylazo)phenyl]-N-methylbenzamide (2) were prepared. The Kf values of the Cu(II) or Ni(II) complex of 1 in water, in 40% (v/v) dimethyl sulfoxide (DMS0), in 95% (v/v) DMSO, or in 95% (v/v) CH3CN were l04-l0l0, whereas those of the corresponding complex of 2 were l02-l03. Spectral titration showed the formation of 1:1-type metal complexes for 1 in the complexation process represented by the Kf values. The results indicate that highly efficient binding of metal ions by 1 is due to macrochelation by the two imidazolylazo moieties with the formation of 16-membered rings. The macrocylce formed by metal complexation of 1 contains a metal ion as a ring atom as well as three benzene moieties on the ring, and, therefore, may be considered as a new type of cyclophanes.