Formation of Trinuclear Copper(II) Complexes with Three Pyridine Oxime Ligands in Aqueous Solution

The protonation and complex formation equilibria of pyridine-2-carboxaldehyde oxime (1), 1-(2-pyridinyl)elhanone oxime (2) and 6-methylpyridine-2-carboxaldehyde oxime (3), HL, with copper(II) ion were studied in aqueous 0.1 M NaCl solution at 25°C by potentiometric titrations with the use of a glass electrode. Application of the least-squares computer program SUPERQUAD to the experimental data indicated that the following complex species are formed: Cu(HL)2 +, CuL+ (ligand 1), Cu(HL)2 2 + (ligands 1 and 3), Cu(HL)L+ CuL2, Cu3L3OH2+ and Cu3L3O + (or Cu3L3(OH)2 +, ligands 1 and 2). The trinuclear complex Cu3L3OH2 + most probably has a triangular Cu3OH central core. In 1:1 molar ratio of ligand to Cu2+ the trimer exists as the predominating complex around pH 4–6 and gives way to the next deprotonated trimer as the pH is increased.