Abstract
Aqueous solutions containing Co/sup 2 +/ and the macrocyclic ligand 1,4,8,12-tetraazacyclopentadecane (L) are able to bind reversibly molecular oxygen to form the species Co/sub 2/L/sub 2/O/sub 2//sup 4 +/ between pH 5 and 7. This species at pH greater than 8 is replaced by the complex Co/sub 2/L/sub 2/O/sub 2/(OH)/sub 2//sup 2 +/ whose concentration in the solution increases to about pH 10.5. At pH greater than 10.5, Co(II) exists totally in this ..mu..-peroxobisydroxo complex. Both the above mentioned oxygenated forms suffer a relatively slow irreversible oxidation into Co(III). From oxygen binding curves, approximate values of the equilibrium constants were calculated for the addition of oxygen to CoL/sup 2 +/ to form the ..mu..-peroxo and the ..mu..-peroxo-bishydroxo complexes. The enthalpies of formation of this latter complex starting from both Co/sup 2 +/ or CoL/sup 2 +/ 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.
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