Nickel(ii) and copper(ii) complexes with pyridine-containing macrocycles bearing an aminopropyl pendant arm: synthesis, characterization, and modifications of the pendant amino group

Abstract

The synthesis of three five-coordinate nickel(II) complexes with pendant arm-containing macrocycles has been achieved by the reduction of CN bonds in the Schiff base precursors derived from diacetyl- or diformyl-pyridine and a tripodal tetramine. Demetallation of the nickel(II) macrocycles yielded stable pentadentate ligands that were used for the preparation of the copper(II) complexes. The structures of three nickel(II) complexes and two copper(II) complexes were determined by X-ray crystallography. Protonation of the pendant arm (pK_a = 6.3–6.6 for the nickel complexes, and 6.5–7.3 for the copper complexes) produced four-coordinate macrocycles, one of which was structurally characterized. The primary amino group of the pendant arm coordinated to the nickel(II) reacted with acetic anhydride or benzoyl chloride. The resulting mono-functionalized nickel(II) complexes and their copper(II) counterparts obtained by transmetallation displayed square-planar geometry in the solid state, as determined by X-ray crystallography, and remained four-coordinate in solutions below pH 11.