A kinetic study and mechanisms of reduction of N, N′-phenylenebis(salicyalideneiminato)cobalt(III) by L-ascorbic acid in DMSO-water medium

The kinetics of reduction of N, N(1)-phenylenebis-(salicylideneiminato)cobalt (III), referred to as [Co(Salophen)](+) by L-ascorbic acid (H(2)A) was studied in mixed aqueous medium (DMSO:H(2)O; 1:4 v/v) under pseudo-first-order conditions at 33 ± 1 °C, μ = 0.1 mol dm(−3) (NaCl) and λ(max) = 470 nm. L-ascorbic acid was oxidized to dehydroascorbic acid with kinetics that was first order in both the [H(2)A] and [Co(Salophen)(+)] and second-order overall. The reaction involves two parallel reaction pathways; an acid-dependent and the inverse acid-dependent pathways. The inverse acid pathway shows that there is a pre-equilibrium step before the rate determining-step in which a proton is lost. The kinetics followed negative Brønsted–Debye salt effect. Evidence was obtained for the presence of free radicals but none to support the formation of an intermediate complex of significant stability during the reaction. Overall, the data obtained suggest an outer-sphere mechanism for the reaction. A plausible mechanism is proposed.