Kinetic and mechanistic studies on the electron-transfer reactions between diaquabisethylenediaminecobalt(III) and ethylenediaminetetraacetatocobaltate(III) with promazine in acidic aqueous media

Abstract

The kinetics of the electron-transfer reactions between promazine (ptz) and [Co(en)2(H2O)2]3+ in CF3SO3H solution ([CoIII] = (2–6) × 10−3m, [ptz] = 2.5 × 10−4m, [H+] = 0.02 − 0.05 m, I = 0.1 m (H+, K+, CF3SO3−), T = 288–308 K) and [Co(edta)]− in aqueous HCl ([CoIII] = (1 − 4) × 10−3m, [ptz] = 1 × 10−4m, [H+] = 0.1 − 0.5 m, I = 1.0 m (H+, Na+, Cl−), T = 313 − 333 K) were studied under the condition of excess CoIII using u.v.–vis. spectroscopy. The reactions produce a CoII species and a stable cationic radical. A linear dependence of the pseudo-first-order rate constant (kobs) on [CoIII] with a non-zero intercept was established for both redox processes. The rate of reaction with the [Co(en)2(H2O)2]3+ ion was found to be independent of [H+]. In the case of the [Co(edta)]− ion, the kobs dependence on [H+] was linear and the increasing [H+] accelerates the rate of the outer-sphere electron-transfer reaction. The activation parameters were calculated as follows: ΔH≠ = 105 ± 4 kJ mol−1, ΔS≠ = 93 ± 11 J K−1mol−1 for [Co(en)2(H2O)2]3+; ΔH≠ = 67 ± 9 kJ mol−1, ΔS≠ = − 54 ± 28 J K−1mol−1 for [Co(edta)]−.