5,7,7,12,14,14-Hexamethyl-1,4,8,11-tetraazacyclotetradecane-nickel(II) as a catalyst for oxidations by superoxide in aqueous solutions. A pulse radiolysis study

The title complex, NiL²⁺ is oxidized in aqueous solutions in the pH range 3-7 in the presence of stabilizing anions X (XHCO₂^-, SO₄^2- and PO₄^3-), by HO₂^· free radicals to Ni(III)LX₂. The latter complexes are powerful oxidizing agents which oxidize HCO₂^-, S₂O₃^2-, NO₂^- and ascorbate. Thus the title complex acts as a catalyst for oxidations by the superoxide free radical which is in equilibrium with the HO₂^· free radical. The results point out that in the presence of appropriate transition metal complexes the superoxide free radical is a potent oxidizing agent and suggest that this might cause some of the aspects of its toxicity.