Oxidation of N‐(4‐Chlorophenyl)‐N′‐hydroxyguanidine to N‐(4‐Chlorophenyl)urea and Nitric Oxide by Photoexcited Iron Porphyrins

Abstract

AbstractPhotochemical excitation (λ > 350 nm) of chloro[meso‐tetrakis(2,6‐dichlorophenyl)porphyrin]iron(III) [FeIII(TDCPP)Cl] and chloro[meso‐tetra(α,α,α,α‐pivalamidophenyl)porphyrin]iron(III) [FeIII(TpivPP)Cl] induces the oxidation of coordinated N‐(4‐chlorophenyl)‐N′‐hydroxyguanidine (1) by molecular oxygen, to give iminoxyl radicals and the FeII(O2) [or FeIII(O2·)] adduct. This complex can be accumulated in significant amounts using [FeIII(TpivPP)Cl]. The primary photoproducts give rise to secondary reactions that lead to the formation of N‐(4‐chlorophenyl)urea (2) as the main end‐product of 1. The conversion of 1 into 2 is accompanied by the formation of NO, as revealed both by an ESR spin‐trapping technique and in the form of its stable end‐products NO2− and NO3−. The presence of 1‐methyl imidazole (1‐MeIm) coordinated in axial position has a significant positive effect on the photoinduced production of 2 and NO from the hydroxyguanidine 1. The observation that radical scavengers inhibit the photooxidation process strongly supports the possibility that the reaction pathway resembles a radical‐type autoxidation mechanism, where the very fast reaction of O2 with the ferrous porphyrin in the presence of the photogenerated iminoxyl radical should yield an iron‐peroxo intermediate as precursor of the urea 2 and NO. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004)