Inner sphere oxidation of N,N′-ethylenebis(isonitrosoacetyleacetoneimine)copper(II) by N-bromosuccinimide in aqueous weakly acidic solutions

Abstract

The kinetics of oxidation of N,N′-ethylenebis(isonitrosoacetyleacetoneimine)copper(II) complex, CuIIL, by N-bromosuccinimide (SBr) in weakly aqueous acidic solutions was studied under pseudo-first-order conditions. Plots of ln(A∞ − At) versus time where At and A∞ are absorbance values of the Cu(III) product at time t and infinity, respectively, showed marked deviations from linearity. The curves showed an acceleration of reaction rate consistent with an autocatalytic behavior. In the presence of Hg(II) ions, plots of ln(A∞ − At) versus time are linear up to >85 % of reaction. The value of the observed rate constant, kobs, increases with decreasing pH. At constant reaction conditions, the dependence of the observed rate constants, kobs, is described by Eq. (1). $$ k_{\text{obs}} = k_{\text{o}} + k_{1} \left[ {{\text{H}}^{ + } } \right] $$ (1) The dependence of both ko and k1 on [SBr] is not linear. The mechanism of the title reaction is consistent with an inner sphere mechanism in which a pre-equilibrium step precedes the electron transfer step. The overall rate law is represented by Eq. (2) where [CuIIL]t and K1 represent the total copper(II) complex concentration and the pre-equilibrium formation constant, respectively. $$ d\left[ {{\text{Cu}}^{\text{III}} {\text{L}}^{ + } } \right]/dt = \left\{ {\left( {k_{\text{o}} + k_{1} \left[ {{\text{H}}^{ + } } \right]} \right)\left[ {\text{SBr}} \right]\left[ {{\text{Cu}}^{\text{II}} {\text{L}}} \right]_{t} } \right\}/\left( {1 + K_{1} \left[ {\text{SBr}} \right]} \right) $$ (2) .