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 2,6-diformylpyridine 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 six-coordinate 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).