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

Abstract

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 Co₂L₂O ⁴⁺₂ between pH 5 and 7. This species at pH greater than 8 is replaced by the complex Co₂L₂O₂ (OH) ²⁺₂ 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.