Kinetics of oxidation of nitrosodisulfonate anion radical with a metallo-superoxide

Abstract

The metal bound superoxide in μ-superoxo-bis[pentaamminecobalt(III)]⁵⁺ (1) oxidizes the nitrosodisulfonate anion radical (NDS²⁻) by two electrons. Oxidized NDS²⁻ quickly decomposes to SO₄²⁻ and NO. 1 is itself reduced to the corresponding hydroperoxo complex which also decomposes fast to Co(II), NH₄⁺ ions and oxygen. 1.5 moles of volatile products formed per mole of 1 mixed with excess NDS²⁻. In the absence of superoxide in a bridged complex, e.g. the μ-amido-bis[pentaamminecobalt(III)]⁵⁺ complex fails to oxidize the nitroxyl radicals, NDS²⁻, TEMPO and 4-oxo TEMPO. With excess NDS²⁻ over 1, the reaction is first-order with respect to [1], [NDS²⁻] and inverse first order in [H⁺]. The activation entropy, ΔS^≠, is largely negative, increased ionic strength decreased the rate and a Brønsted plot is fairly linear with a negative slope. Oxidant μ-superoxo-bis[(ethylenediamine)(diethylenetriamine)cobalt(III)]⁵⁺ has ligands sterically more crowded though more basic than ammonia in 1. It oxidizes NDS²⁻ much more slowly. No solvent kinetic isotope effect (kH_2O/D2O ≈ 1) could be seen; a spin-adduct formation by the conjugate base of 1 followed by electron transfer is postulated.