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 (NDS(2-)) by two electrons. Oxidized NDS(2-) quickly decomposes to SO(4)(2-) and NO. 1 is itself reduced to the corresponding hydroperoxo complex which also decomposes fast to Co(ii), NH(4)(+) ions and oxygen. 1.5 moles of volatile products formed per mole of 1 mixed with excess NDS(2-). 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, NDS(2-), TEMPO and 4-oxo TEMPO. With excess NDS(2-) over 1, the reaction is first-order with respect to [1], [NDS(2-)] 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 NDS(2-) much more slowly. No solvent kinetic isotope effect (k(H(2)O/D(2)O)≈ 1) could be seen; a spin-adduct formation by the conjugate base of 1 followed by electron transfer is postulated.