Thermodynamic studies on the addition of molecular oxygen to cobalt(II)-1,4,8,12-tetraazacyclopentadecane in aqueous solution at 25°C

Aqueous solutions containing Co2+ and the macrocyclic ligand 1,4,8,12-tetraazacyclopentadecane (L) are able to bind reversibly molecular oxygen to form the species Co2L2O24+ between pH 5 and 7. This species at pH greater than 8 is replaced by the complex Co2L2O2 (OH)22+ whose concentration in the solution increases to about pH 10.5. At pH greater than 10.5, Co(II) exists totally in this μ-peroxobisydroxo complex. Both the above mentioned oxygenated forms suffer a relatively slow irreversible oxidation to Co(III). From oxygen binding curves, approximate values of the equilibrium constants were calculated for the addition of oxygen to CoL2+ to form the μ-peroxo and the μ-peroxo-bishydroxo complexes. The enthalpies of formation of this latter complex starting from both Co2+ or CoL2+ have been obtained by means of calorimetric measurements. The effects of the size of the macrocyclic ligand on the affinity and the enthalpy of oxygen binding have been considered. © 1986 Plenum Publishing Corporation.