New ternary ligands consisting of an N4 bridging ligand and two terpyridines, and their Co(ii) and Ni(ii) dinuclear complexes. Structure, redox properties, and reaction with acid

Abstract

Two ternary ligands consisting of two 2,2′:6′,2′′-terpyridines and one N4-quadridentate μ₂,η²-bridging ligand were synthesized. The N4 bridge is 1,4-bis(2-pyridyl)phthalazine in ligand 1, and 3,6-bis(2-pyridyl)pyridazine in ligand 2. Two Co(II) dinuclear complexes [(1)Co₂(μ-OH)]³⁺ and [(2)Co₂(μ-OH)]³⁺, and one Ni(II) dinuclear complex [(1)Ni₂(μ-Cl)]³⁺ were obtained. In the crystal structures of [(1)Co₂(μ-OH)]³⁺ and [(1)Ni₂(μ-Cl)]³⁺, two pyridine rings are twisted around the pyridine–phthalazine bonds to avoid steric repulsion between the hydrogen atoms. The pyridine rings also showed a significant tilt from the octahedral coordination plane, which causes the large positive shift of the first reduction potentials. Upon the addition of a proton, the cobalt dinuclear complexes can release one cobalt ion selectively, and the dinuclear complexes can be easily restored by the addition of a tertiary amine.