Synthesis, crystal structures and electrochemical properties of Cu^II and Ni^II complexes with hexadentate ligands containing thioether-amido-pyridyl donor set

Novel two acyclic hexadentate ligands, 1,4-bis[o-(pyridine-2-carboxamidophenyl)]-1,4-dithiobutane (H₂bpctb) and 1,4-bis[o-(pyridine-2-carboxamidophenyl)]-1,5-dithiopentane (H₂bpctp), containing thioether functions were synthesized and their copper(II) and nickel(II) complexes [(Cu(bpctb)], 1; [Cu(bpctp)], 2; [Ni(bpctb)], 3 and [Ni(bpctp)], 4) were prepared. The structures of 1-4 were determined by X-ray crystallography. The coordination geometries of 3 and 4 are octahedral with two amido and two pyridyl nitrogen atoms and two thioether sulfur atoms of hexadentate ligand. However, the coordination geometry of 2 is a flattened tetrahedron combining two amido nitrogen atoms and two pyridyl nitrogen atoms of hexadentate ligand without thioether coordination. X-ray crystallography and ESR spectroscopic studies revealed that the coordination geometry of 1 is an uncommon equatorially elongated octahedron in solid state. However, this coordination geometry changes to an axially elongated one in solution. The copper(II/I) redox processes for 1 and 2 are around - 1.2 V and the nickel(II/III) processes for 3 and 4 at around 0.3 V vs ferrocene/ferrocenium couple were observed in DMSO.