Catalytic oxidation of hydroxyurea to bound NO+/NO2  mediated by pentacyano(L)ferrates. Characterization of the nitroxide radical, bound C-nitrosoformamide and NO as reaction intermediates

ANDREA C. MONTENEGRO, SERGIO G. DABROWSKI, MARÍA M. GUTIÉRREZ, VALENTÍN T. AMOREBIETA, SARA E. BARI, JOSÉ A. OLABE

INORGANICA CHIMICA ACTA

ELSEVIER SCIENCE SA

Lugar: Amsterdam; Año: 2011 vol. 374 p. 447 - 452

0020-1693

Hydroxyurea (HU, NH2CONHOH), reacts with several oxidizing agents (O2, H2O2, IO4, 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 (kmax = 465 nm, e = 4.0 ± 0.3 103 M1 cm1). 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 subsequent1-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.