Kinetics and mechanism of reduction of 2,8-dimethyl-1,9-diphenyl-3,7-diaza-2,7-nonadiene-1,9-dione dioximatocopper(III) ([CuIII A]+) and 4,6,9-trimethyl-5,8-diaza-4,8-dodecadiene-2,3,10,11-tetraone 3,10-dioximatocopper(III) ([CuIIIB]+) by p-methoxyphenol, 1,2-dihydroxybenzene, 1,4-dihydroxybenzene and some of its derivatives

The kinetics of reduction of two copper(III)-imine-oxime complexes, Cu^IIIA]⁺ and Cu^IIIB]⁺, (H₂A and H₂B=2,8-dimethyl-1,9-diphenyl-3,7-nonadiene-1,9-dione dioxime and 4,6,9-trimethyl-5,8-diaza-4,8-dodecadiene-2,3,10,11-tetraone 3,10-dioxime respectively) by hydroquinone (H₂Q), 2-methylhydroquinone (MH₂Q), 2-chlorohydroquinone (ClH₂Q), catechol (H₂Cat) and p-methoxyphenol (pMHP) have been examined in aqueous acidic solution. Under fixed reaction conditions, the kinetics display first-order dependence on each oxidant and reductant. The pH-dependence is complex for the reduction of Cu^IIIA]⁺, since both the copper(III) complex and the reductants undergo protonation–deprotonation equilibria. In the lower pH range, the second-order rate constant, k ₂, decreases with increasing pH. In the higher pH range, k ₂ increases with increasing pH. In the lower pH range the most important oxidant is Cu^IIIHA]²⁺, whereas, in the higher pH range the most important reactants are deprotonated reductants. However for H₂Cat, as was observed before, two reaction pathways seem to operate in the high pH range. In one pathway, HCat^? seems to be involved; whereas, in the other pathway Cat^2? seems to be the reactive species. Doubly deprotonated catechol, Cat^2?, is very unlikely to be formed at pH ≤ 5. It was therefore necessary to invoke a strong interaction between Cu^IIIA]⁺ and HCat^? followed by loss of the second proton. The pH dependence for the reduction of Cu^IIIB]⁺ is less complex. Thus H₂Q and MH₂Q showed no pH dependence up to pH ～ 4.60, whereas ClH₂Q, pMHP and H₂Cat displayed an inverse first-order dependence on H⁺]. Observed rate constants showing first-order dependence and inverse first-order dependence on H⁺] correlate reasonably well with those calculated using the Marcus equation. The reaction path involving Cat^2? is believed to proceed by an inner-sphere mechanism. The agreement between the calculated and observed values for the Cu^IIIA]⁺ complex is lower than was found for the Cu^IIIA₁]⁺(A₁=3,9-diethyl-4,8-diaza-3,8-undeca-2,10-dionedioxime). It seems that the replacement of methyl groups in the latter complex by phenyl groups in the former complex causes both electronic and steric effects, and both effects seem to retard electron transfer. The electronic effect is readily seen in the decrease of the reduction potential of Cu^IIIA]⁺ (E ⁰=1.09 V) compared to the reduction potential of Cu^IIIA₁]⁺(E ⁰=1.16 V) and thus making the former a weaker oxidant. The self-exchange rate constant (5 × 10⁵ M ^?1 s^?1) estimated for complexes with type H₂A ligands seem to work well for complexes with type H₂B ligands. This situation is supported by the findings of a fairly constant value for the self-exchange rate constant for Cu ^III/II –peptide complexes with varying substituents.