Synthesis, characterization and electrochemical behaviour of cobalt(II) and cobalt(III):O2- complexes, respectively, with linear and tripodal tetradentate ligands derived from Schiff bases

New octahedral cobalt complexes with linear and tripodal tetradentate ligands derived from Schiff bases have been synthesized and characterized using elemental analysis, molar conductance, IR spectra, magnetic measurements, electronic and ESR spectra. The experimental results support the binding of linear ligands with two N and two O donor sites to cobalt ion. They show a square planar geometry and tripodal ligands coordinated to the metal ion by only one nitrogen atom, giving an arrangement of NO3 donor groups, the other axial sites being occupied by the molecular oxygen and/or the aquo molecules. From the results of cyclic voltammetry it is shown that chelate structure and ligand geometry and electron donating effect of the ligand substituents are among the factors influencing the redox potentials of the complexes. Linear ligands lead to high-spin cobalt(II) complexes. They do not interact with dioxygen and stabilize the Co(II) state counter to their related Schiff-base complexes. The low-spin complexes with tripodal ligands are O2 adducts and the configuration in these complexes is best formulated as [CoIIIO2-].