Kinetic studies on the reduction of nickel(IV) and nickel(III) oxime–imine complexes by ascorbic acid

Abstract

The oxidation of ascorbic acid by [Ni^IIIL¹]²⁺ and [Ni^IVL³₂]²⁺ complexes (where HL¹= 15-amino-3-methyl-4,7,10,13-tetraazapentadec-3-en-2-one oxime and HL³= 6-amino-3-methyl-4-azahex-3-en-2-one oxime) has been investigated by stopped-flow spectrophotometry in the range pH 2.50–8.20, with /= 0.20 mol dm^–3 NaClO₄ and T= 30 °C, using variable concentrations of ascorbic acid. At a particular pH both the reactions are second order, first order with respect to ascorbic acid and the complexes, and follow the general rate law –1/m d[NiL_xⁿ⁺]/dt=k[NiL_xⁿ⁺][H₂A]_T where H₂A = ascorbic acid and m represents the stoichiometric factor (m= 2 for [Ni^IIIL¹]²⁺ and 1 for [Ni^IVL³₂]²⁺). In the reduction of [Ni^IVL³₂]²⁺, the monophasic reaction traces throughout the experimental pH range (2.50–8.20) imply the involvement of nickel(III) complexes in the rate-determining step. A detailed evaluation of the reduction was achieved by considering suitable pH regions and employing appropriate computer programs to fit the experimental data. Application of the Marcus theory in calculating the theoretical rate constants and a comparison of these constants with the respective experimental values reveals the occurrence of an outer-sphere mechanism for the oxidation of A^2– by both [Ni^IIIL¹]²⁺ and [Ni^IVL³₂]²⁺ as well as by [Ni^IIIL²]⁺ where H₂L²= 3,14-dimethyl-4,7,10,13-tetra-azahexadec-3,13-diene-2,15-dione dioxime. The oxidation of H₂A and HA^– seems to follow a concerted electron–proton transfer with initial association of the reactants.