Radiation-induced reactions of (2-methyl-5-nitro-1H-imidazole-1-ethanol)nickel(II) in aqueous solution: a flash-photolysis and steady-state gamma-radiolysis study

Abstract

On gamma radiolysis of the nickel(II) complex of metronidazole (2-methyl-5-nitro-1H-imidazole-1-ethanol), hydroxyl radicals and the hydrated electron e^–(aq) react with the complex leading to the destruction of the nitroimidazole structure. The loss of the complex and the production of nitrite have been followed by steady-state experiments, while the transient kinetics of the hydroxyl reactions have been probed by flash photolysis. The OH radicals react with the complex with a rate constant of ≈ 4.0 × 10⁹ dm³ mol^–1 s^–1, resulting in elimination of the nitro group either by adding to the C⁵ position or through generation of a nickel(III) species. The latter undergoes intramolecular electron transfer from the ring with a rate constant of ≈(4.6 ± 0.4)× 10³ s^–1, while the imidazole radicals formed decay by second-order kinetics with a 2k/ε value of (9.1 ± 1.3)× 10⁵ cm s^–1. The hydrated electrons generate the nitroanion radicals, a fraction of which undergoes denitration while the rest is reduced by a successive four-electron scheme to a hydroxylamino derivative.