Kinetic study of the promazine oxidation to promazine 5-oxide by trisoxalatocobaltate(III) in basic aqueous media

Abstract

The kinetics of the oxidation of promazine by trisoxalatocobaltate(III) were studied in the presence of a large excess of the cobalt(III) in tris buffer solution using u.v.–vis spectroscopy ([CoIII] = (0.6 − 2) × 10−3M, [ptz] = 6 × 10−5M, pH = 6.6–7.8, I = 0.1 M (NaCl), T = 288−308 K, l = 1 cm). The reaction proceeds via two consecutive reversible steps. In the first step, the reaction leads to formation of cobalt(II) species and a stable cationic radical. In the second step, cobalt(III) is reduced to cobalt(II) ion and a promazine radical is oxidized to the promazine 5-oxide. Linear dependences of the pseudo-first-order rate constants (k1 and k2) on [CoIII] with a non-zero intercept were established for both redox processes. Rates of reactions decreased with increasing concentration of the H+ ion indicating that the promazine and its radical exist in equilibrium with their deprotonated forms, which are reactive reducing species. The activation parameters for reactions studied were as follows: ΔH≠ = 44 ± 1 kJ mol−1, ΔS≠ = −100 ± 4 JK−1 mol−1 for the first step and ΔH≠ = 25 ± 1 kJ mol−1, ΔS≠ = −169 ± 4 J K−1 mol−1 for the second step, respectively. Mechanistic consequences of all the results are discussed.