Calorimetric study on oxygen addition to cobalt(II) complexes with tris(2-aminoethyl)amine and 3,7-diazanonane-1,9-diamine

Abstract

Through spectrophotometric and potentiometric measurements it has been verified that in aerated aqueous solutions of Co^II containing tris(2-aminoethyl)amine (tren) as ligand, the doubly bridged µ-peroxo–µ-hydroxo dicobalt ion [Co₂L₂(O₂)(OH)]³⁺(L = tren) is totally formed at pH > 6.5. In aerated aqueous solutions of Co^II and 3,7-diazanonane-1,9-diamine (dadn) at pH 6.5 only the singly bridged µ-peroxo dicobalt ion [Co₂L₂(O₂)]⁴⁺(L = dadn) is present. This complex at higher pH adds OH^– ions in its axial positions; at pH > 10.5 the singly bridged µ-peroxo–dihydroxo dicobalt ion [Co₂L₂(O₂)(OH)₂]²⁺ is totally formed. The singly bridged complex [Co₂L₂(O₂)]⁴⁺, after several hours, converts irreversibly into the doubly bridged µ-peroxo–µ-hydroxo ion [Co₂L₂(O₂)(OH)]³⁺, while the single bridged [Co₂L₂(O₂)(OH)₂]²⁺ ion slowly autoxidizes to Co^III. By means of calorimetric measurements, usually carried out at constant pH, the enthalpies of formation of [Co₂L₂(O₂)]⁴⁺ and [Co₂L₂(O₂)(OH)₂]²⁺(L = dadn) and [Co₂L₂(O₂)(OH)]³⁺(L = tren) have been obtained. These results are compared with the enthalpies relative to the oxygen addition to [Co(trien)]²⁺ ion (trien = triethylenetetra-amine), and the enthalpy effects, connected with the change from a linear amine (trien) to its branched isomer (tren) and from the sequence 5,5,5 (trien) to the sequence 5,6,5 (dadn) of the chelate rings, are discussed.