Thermodynamic and kinetic studies on the addition of dioxygen to Co(II) complexes with open-chain saturated polyamines

The systems Co(II)/N,N'-bis(3-aminopropyl)ethylenediamine, NH2(CH2)3NH(CH2)2NH (CH2)3NH2 (3,2,3-TET) and Co(II)/N,N'-bis(3-aminopropyl)propylenediamine, NH2(CH2)3NH(CH2)3NH (CH2)3NH2 (3,3,3-TET), have been investigated in aqueous solutions at 25-degrees-C, both under anaerobic and aerobic conditions, by means of potentiometric, spectrophotometric, calorimetric and dioxygen measurements carried out over a wide range of pH, in some cases at variable temperature. Under anaerobic conditions, at ligand-to-cobalt molar ratio C(L)o/C(Co)o of 1 and cobalt concentrations C(Co)o > 5 x 10(-3) M, the formation of cobalt(II) complexes with 3,2,3-TET and with 3,3,3-TET is complete at pH > 8.5 and at pH > 10, respectively. Under aerobic conditions, in the pH range from 6.5 to 8, the monobridged dicobalt mu-peroxo monohydroxo 3,2,3-TET complex is suggested to be formed which, in about thirty minutes, converts extensively to the dibridged mu-peroxo mu-hydroxo (olate) species. When Co(II) is added to aerated solutions of 3,2,3-TET at pH > 10.5, only the monobridged dicobalt-mu-peroxo dihydroxo complex is formed. This complex may also be slowly produced by increasing the pH of solutions containing the dibridged olate species to a value higher than 10.5. The three dioxygen adducts are also obtained in aerated solutions of Co(II) and 3,3,3-TET at different pH's. The thermodynamic standard functions DELTAX(o) (X = G, H, S) for the formation of the Co(II) complexes in anaerobic conditions are reported; kinetic and thermodynamic data concerning the dioxygen addition reactions, the isomerization of the monobridged mu-peroxo monohydroxo complexes to the debridged olate form, the conversion of the dibridged olate complex into the monobridged dicobalt mu-peroxo dihydroxo complex, and the irreversible oxidation of the Co(II) dioxygen adducts to Co(III) complexes have been evaluated. Finally, a comprehensive analysis has been carried out of the thermodynamic and kinetic features exhibited by the reaction of addition of dioxygen to Co(II) complexes with open-chain saturated tetraamines with ethylenic and/or propylenic chains.