Reactions at the azomethine C=N bonds in the nickel(II) and copper(II) complexes of pyridine-containing Schiff-base macrocyclic ligands

Template condensation between 2,6-dicarbonylpyridines and a tripodal tetramine tris(3-aminopropyl)amine in the presence of copper(II) or nickel(II) yielded macrocyclic complexes that contained a coordinated primary amine as an appended functional group. The derivatives of 2,6diformylpyridine contained labile azomethine bonds susceptible to water or alcohol addition, and were isolated in the form of crystallographically characterized hemiaminals or O-ethyl hemiaminals. The nickel(II) hemiaminal complex produced a macrocyclic product upon reduction with sodium borohydride, and the structures of the two complexes had very similar ligand conformations. The derivatives of 2,6-diacetylpyridine formed stable Schiff-base macrocycles with rigid, nearly planar structures in which azomethine bonds were conjugated with the pyridine ring. The steric strain was relieved upon heating of the complexes in water–alcohol media. The products were determined by the coordination requirements of the metal ions: the copper(II) underwent a C=N double bond shift, forming a more flexible five-coordinate macrocyclic complex, whereas the nickel(II) compound underwent a partial ring-opening hydrolysis, giving rise to a pseudo-octahedral sixcoordinate species. The unusual isomerization of the copper(II) complex was accompanied by H/D exchange at CH-bonds, and was facilitated by alcohol addition. The mechanism of isomerization is similar to B6-catalyzed transamination reactions (D.E. Metzler, M. Ikawa and E.S. Snell. J. Am. Chem. Soc., 1954, 76, 648). The corresponding four-coordinate copper(II) complex lacking a pendant arm was more thermally stable than its five-coordinate counterpart, and underwent partial double-bond migration under harsh conditions (prolonged heating at reflux in water–ethanol mixtures at pH > 8.5).