A kinetic study of the anation reactions of some cobalt(III) complexes containing macrocyclic quadridentate amine ligands.

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The kinetics of the anation of trans-[Co(OH)(N4)(H2O)]2+ by acetate ions (OAc−) have been studied at 25 °C and at an ionic strength of 0.10 mol dm−3 (LiClO4), and over the pH range from 3.62 to 5.17; N4 represents a macrocyclic ligand. The reaction is considered to proceed through a dissociative mechanism. The rate constants of the loss of the coordinated water of [Co(OH)(N4)(H2O)]2+ are estimated to be (1.4±0.2)×10−2 s−1, (8.9±1.6)×10−2 s−1, (1.0±0.1)×10−1 s−1, and (4.0±0.1)×10−1 s−1 for [14]aneN4, Me6[14]4,11-dieneN4, ms-Me6[14]aneN4, and rac-Me6[14]aneN4 complexes respectively. These represent the steric acceleration by the methyl groups of the macrocyclic ligands. The anation of the [14]aneN4 complex by chloride and bromide ions in acetate-buffer solutions has also been investigated. The relative effectiveness of entering groups for the coordination to the pentacoordinate intermediate follows the order: H2O<OAc−<Cl−<Br−. The substitution reaction of [Co(OH)(N4)(H2O)]2+ is slower than the ascorbate reduction of this complex reported previously. It is concluded that the electron-transfer reaction does not proceed through an inner-sphere mechanism, but through an OH-bridged mechanism.
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