Kinetics and mechanism of the reaction of <em>trans-(diaqua)(N,N' - </em>ethylene bis-(salicylidineiminato)cobalt(III) with ascorbic acid

Abstract

The kinetics of the reactions of <em>trans-</em>[Co(Salen)(OH₂)₂]⁺ (Salen = N.N¢-ethylene bis (salicylidineiminate) ·jth ascorbic acid (H₂Asc) have been studied under varying conditions of <em>p</em>H<em>, </em>[ascorbic acid]_T, and temperature at .5 mol dm^-3 ionic strength. The initial fast reactions observed in the stopped flow time scale are due to the complex formation between the reactants. This occurs in two phases i.e., the formation of the <em>trans- </em>[(2 ua)(ascorbato)Co^III(Salen)] and its transformation to the corresponding ascorbate chelate. The rate constants are the activation parameters for the formation of the monobonded and chelate ascorbate complexes are reported. T low values of Δ<em>H⁺</em> and negative values of ΔS⁺ for the complexation reaction favour associative interchange mechanism(l_a). The hydroxide in <em>trans-</em>[Co(Salen)(OH)(OH₂)] marginally accelerates substitution of the aqua and by HAsc- and the <em>trans-</em>[Co(Salen)(OH)(AscH)]<em>^- </em>is considered to undergo fast internal proton transfer to .nerate <em>trans-</em>[Co(Salen)(OH₂)(Asc)]<em>^- </em>which undergoes chelation of the Co^lll centre by the bound ascorbate noiety; the latter reaction is, however, 15 times slower than the corresponding reaction of <em>trans- </em>[Co(Salen)(OH₂)(AscH)]. The faster complexation reactions are followed by the slow redox reactions. The rate constant for the internal reduction of Co^lll by the coordinated ascorbate in the chelate [Co(Salen)(AscH)] is 5 - 10 times (25°C<em> – </em>45°C) faster than the same for [Co(Salen)(Asc)]^-. This trend in reactivity is due to the low value of Δ<em>H</em>for the former although the high negative value of ΔS compensates at least partly the overriding effect of the activation enthalpy. The internal redox occurs via innersphere mechanism. We also have observed a redox path involving <em>trans-</em>[Co(Salen)(OH₂)(AscH₂)]⁺ and H₂Asc for which electron transfer most likely involves outersphere mechanism.