Oxidation Of N-(4-Chlorophenyl)-N′-hydroxyguanidine To N-(4-Chlorophenyl)urea And Nitric Oxide By Photoexcited Iron Porphyrins

Photochemical 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)