Oxidation of Ascorbate by Ni(III) Complexes with Tetraaza-macrocyclic Ligands in Neutral Aqueous Solutions. A Pulse-Radiolysis Study

The mechanisms and kinetics of oxidation of ascorbate, AH^-, by Ni(III)Lⁱ_aq and by LⁱNi(III) (HPO₄)₂^- complexes (L¹ = meso-(5,12)-7,7,14,14-hex-amethyl-1,4,8,11-tetraazacyclotetradecane; L² = 1,8-dimethyl-1,3,6,8,10,13-hexaazacyclotetradecane) in neutral aqueous solutions have been investigated. The oxidation of ascorbate by the LⁱNi(III) (HPO₄)₂^- and Ni(III)L¹_aq proceeds via two consecutive reactions well separated in time. The products of the first reaction are the A^.- radical anion and the corresponding Ni(II) complex. The oxidations by the LⁱNi(III)(HPO₄)₂^- complexes proceed via the outer sphere mechanism, whereas the detailed mechanism of reaction of Ni(III)L¹_aq cannot be determined. The rate of reaction decreases with the increase in the concentration of phosphate, thus indicating that LⁱNi(III)(HPO₄)(H₂O)⁺ and LⁱNi(III)OH²⁺ are stronger oxidizing agents than LⁱNi(III)(HPO₄)₂^-. The oxidation of ascorbate by Ni(III)L²_aq proceeds via three consecutive reactions which are well separated in time. Thus the results clearly point out that this process occurs via the inner sphere mechanism. The first transient observed is tentatively identified as L²(H₂O)Ni(II)(A^.-)²⁺, i.e., an unexpected complex of the ascorbate anion radical. Also in this process the last transient observed is the A^.- anion radical. The stabilization of the ascorbyl radical in a transient complex might be of biological significance.