Catalytic oxidation of hydroxyurea to bound NO+/ mediated by pentacyano(L)ferrates. Characterization of the nitroxide radical, bound C-nitrosoformamide and NO as reaction intermediates

Hydroxyurea (HU, NH2CONHOH), reacts with several oxidizing agents (O2, H2O2, IO 4 - , Fe(III) complexes) in aqueous solutions containing either of the pentacyano(aqua)ferrate(II,III) ions, [FeII,III(CN)5H2O]3,2−. In the initial 1-electron step, HU reacts with [FeIII(CN)5H2O]2− through an outer-sphere process, leading to [FeII(CN)5H2O]3− and the EPR active nitroxide radical, NH2C(O)NHO. This is further oxidized to C-nitrosoformamide, NH2C(O)NO, in a similar 1-electron step. The latter compound binds to [FeII(CN)5H2O]3− giving the [FeII(CN)5(NOC(O)NH2)]3− ion (I3−). I3− has been isolated as a solid, Na3[FeII(CN)5(NOC(O)NH2)]·2H2O, (I), and was identified by chemical analysis, IR and UV–Vis spectroscopies (λmax = 465 nm, ε = 4.0 ± 0.3 × 103 M−1 cm−1). I and I3− are very stable species in anaerobic media, as solids and in aqueous solutions with a pH close to 7. I3− decomposes in alkaline media through hydrolysis, giving CO2, NH3 and, presumably, the nitroxyl complex [FeII(CN)5NO]4−. The latter ion appears as a source of N2O, detected by mass spectrometry, and affords, in oxygenated media, two subsequent 1-electron processes, without the rupture of the NO bond, giving the radical intermediate [FeII(CN)5NO]3− (which was identified by UV–Vis and EPR), and finally the nitroprusside ion, [Fe(CN)5NO]2−. At pHs > 10, the latter ion may convert to [FeII(CN)5NO2]4−, and leads to [FeII(CN)5H2O]3− through the release of nitrite. Thus, the aqua-ion might afford a new 4-electron oxidation process if HU and the external oxidant were in excess.