Thermodynamics of complex formation in dimethylsulfoxide: The case of Co(II) complexes with nitrogen donor ligands and their O2 adducts

[Display omitted] •The stability of the Co(II) species decreases with the number of N-methyl groups.•Solvation effects of metal ion and ligand solvation compensate each other in DMSO.•N-methylation modulates the CoLj3+/2+ redox potential and thus the O2 affinity.•Correlation between the ΔH°β1, electrochemical data and O2 affinity is shown. Data for complex formation of Co(II) with N-donors in dimethylsulfoxide are reviewed and rationalized on the basis of both solvational effects and ligand structural and electronic features. For the polyamine ligands, the number and stability of the species formed decreases with increasing number of N-methyl groups, due to the increase of the steric crowding and the relatively low basicity of the donor nitrogen. However also the stabilization through H-bond formation with the second-sphere solvent molecules is an important factor. The same factor causes a greater electron donation from the ligand to the metal ion stabilizing the complex as shown by the thermodynamic and structural data. When comparing the thermodynamic parameters with those available in water, it results that the stability constants are higher than those in water despite the stronger solvation of the metal ion in DMSO. This is assigned to the ligand solvation playing an important role, especially for primary and secondary amines. When ligands able to be only H-bond acceptors are considered (tertiary amines and pyridines) the stability of the complex expected on the basis of the metal ion solvation is observed. When considering the oxygen affinity of the Co(II) complexes in DMSO it results that the complexes with primary and secondary amines are able to bind O2. As O2 uptake is a redox process it strongly depends on the ligand-metal charge transfer and on the solvation of the resulting complex. Thus, the same factors affecting the stability of the Co(II) complexes with the N-donor ligands considered also influence their dioxygen affinity. A correlation between enthalpy and electrochemical data in DMSO supports this hypothesis.