Co(II) complexes with tripodal N-donor ligands: Thermodynamics of formation in anaerobic conditions and oxygen binding

The complex formation of Co(II) with N-donor ligands in dimethylsulfoxide (DMSO) is investigated by means of calorimetric and spectroscopic methods. The ligands considered in this work are tripodal polyamines and polypyridines: 2,2′,2′′-triaminotriethylamine (TREN), tris(2-(methylamino)ethyl)amine (Me₃TREN), tris(2-(dimethylamino)ethyl)amine (Me₆TREN), tris(2-pyridyl)methyl]amine (TPA) and 6,6′-bis-bis-(2-pyridylmethyl)aminomethyl]-2,2′-bipyridine (BTPA).These ligands are characterized by a systematic modification of the donor groups in order to study how their structure is related to the stability of the complexes formed and to their ability to bind oxygen. A comparison with thermodynamic data for similar Cd(II) systems as well as with data referred to linear tetra-amines in DMSO is also made. The solvent effect on the nature and stability of the species formed is discussed. DFT calculations are carried out to explain the trend in thermodynamic parameters for Me₆TREN. Only Co(TREN)²⁺ is able to bind oxygen and two successive species (μ-superoxo and μ-peroxo) are formed. The kinetics of oxygen uptake by Co(TREN)²⁺ is found to be less solvent-dependent than other Co(II)-polyamine complexes when the formation of the mononuclear μ-superoxo complex is considered.