The Reduction of Cobalt(III) Complexes by Ascorbic Acid. II. The Kinetics and Mechanisms of the Reactions of Diaqua and Aquahydroxo Macrocyclic N4 Complexes

Abstract

Abstract The kinetics of the reductions of several macrocyclic N4 cobalt(III) complexes by l-ascorbic acid (H2A, have been studied at 25.0 °C, at an ionic strength of 0.10 mol dm−3 (LiClO4), and over the pH range from 1.64 to 3.47. The second-order rate constants in dm3 mol−1 s−1 are 3.4±0.6 (Me6[14]4,11-dieneN4), (4.2±0.2)×10 (Me4[14]tetraeneN4), and (4.2±0.1)×10 (ms-Me6[14]aneN4) for the reactions of [Co(N4)(H2O)2]3+ with HA− ions and (1.1±0.1)×102 (Me6[14]4,11-dieneN4), (1.3±0.1)×103 (Me4[14]tetraeneN4), (5.5±0.1)×10 (ms-Me6[14]aneN4), and (2.9±0.6)×10 ([14]aneN4) for the reactions of [Co(OH)(N4)(H2O)]2+ with HA− ions. The mechanisms of these reactions are discussed, and it is concluded that the lack of saturation of macrocyclic ligands facilitates the electron-transfer reaction between cobalt(III) and ascorbic acid.