Inner sphere oxidation of 3,9-dimethyl-4,8-diaza-3,8-undecadiene-2,10- dione dioximecopper(II) complex by N-bromosuccinimide to copper(III) in aqueous acidic solutions

The kinetics of the oxidation of the 3,9-dimethyl-4,8-diaza-3,8-undecadiene-2,10-dionedioximecopper(II) complex, [Cu^IIHL₂]⁺+, by N-bromosuccinimide (NBS) were studied in weakly acidic aqueous media (pH 3.36-5.25). The reaction exhibited autocatalysis, characterized by S-shaped absorbance-time curves. Under pseudo-first-order conditions in the presence of Hg(II), the reaction rate showed a first-order dependence on the total copper(II) complex concentration. The pseudo-first-order rate constant, k_obs ​, displayed Michaelis-Menten type saturation kinetics with respect to [NBS]. The dependence of k_obs ​ on acidity was complex, increasing only by a factor of ~2 across the pH range studied. An inner-sphere mechanism is proposed, consistent with the derived rate law and the known ability of the four-coordinate [CuIIHL]+ complex to form five-coordinate species. Considering [Cu^IIHL]⁺, [Cu^IIL], [NBS] and [HNBS]⁺ as the reactive species within four possible pathways, the following rate law was derived:
Rate =  {k1K2K3 + (k2K1K2K4+k4K6)[H+] + k3K2K5[H+]2}[NBS][CuIILH+]t  (i)
                     [H+] + K1 + {K1K3 + (K1K2K4 + K6)[H+] + K2K5[H+]2}[NBS]