title: Reactivity of iron(II)-bound nitrosyl hydride (HNO, nitroxyl) in aqueous solution.
abstract: The reactivity of coordinated nitroxyl (HNO) has been explored with the [Fe(II)(CN)(5)HNO](3-) complex in aqueous medium, pH 6. We discuss essential biorelevant issues as the thermal and photochemical decompositions, the reactivity toward HNO dissociation, the electrochemical behavior, and the reactions with oxidizing and reducing agents. The spontaneous decomposition in the absence of light yielded a two-electron oxidized species, the nitroprusside anion, [Fe(II)(CN)(5)NO](2-), and a negligible quantity of N(2)O, with k(obs)≈5×10(-7)s(-1), at 25.0°C. The value of k(obs) represents an upper limit for HNO release, comparable to values reported for other structurally related L ligands in the [Fe(II)(CN)(5)L](n-) series. These results reveal that the FeN bond is strong, suggesting a significant σ-π interaction, as already postulated for other HNO-complexes. The [Fe(II)(CN)(5)HNO](3-) ion showed a quasi-reversible oxidation wave at 0.32 V (vs normal hydrogen electrode), corresponding to the [Fe(II)(CN)(5)HNO](3-)/[Fe(II)(CN)(5)NO](3-),H(+) redox couple. Hexacyanoferrate(III), methylviologen and the nitroprusside ion have been selected as potential oxidants. Only the first reactant achieved a complete oxidation process, initiated by a proton-coupled electron transfer reaction at the HNO ligand, with nitroprusside as a final oxidation product. Dithionite acted as a reductant of [Fe(II)(CN)(5)HNO](3-), in a 4-electron process, giving NH(3). The high stability of bound HNO may resemble the properties in related Fe(II) centers of redox active enzymes. The very minor release of N(2)O shows that the redox conversions may evolve without disruption of the FeN bonds, under competitive conditions with the dissociation of HNO.
authors.affiliation: Departamento de Química Inorgánica, Analítica y Química Física, and INQUIMAE-CONICET, Facultad de Ciencias Exactas y Naturales, Universidad de Buenos Aires, Ciudad Universitaria, Pabellón 2, Buenos Aires, C1428EHA, Argentina.
authors.forename: Andrea C
authors.initials: AC
authors.lastname: Montenegro
authors.forename: Sara E
authors.initials: SE
authors.lastname: Bari
authors.forename: José A
authors.initials: JA
authors.lastname: Olabe
mesh_terms: D056831:Coordination Complexes / Q000138:chemical synthesis / Q000737:chemistry*; D004227:Dithionite / Q000737:chemistry; D004563:Electrochemistry; D005292:Ferricyanides / Q000737:chemistry; D005296:Ferrous Compounds / Q000138:chemical synthesis / Q000737:chemistry*; D007700:Kinetics; D009589:Nitrogen Oxides / Q000737:chemistry*; D009599:Nitroprusside / Q000737:chemistry; D016877:Oxidants / Q000737:chemistry; D010084:Oxidation-Reduction; D010269:Paraquat / Q000737:chemistry; D055668:Photochemical Processes; D019163:Reducing Agents / Q000737:chemistry; D012996:Solutions; D013056:Spectrophotometry, Ultraviolet
chemical_list: D056831:Coordination Complexes; D005292:Ferricyanides; D005296:Ferrous Compounds; D009589:Nitrogen Oxides; D016877:Oxidants; D019163:Reducing Agents; D012996:Solutions; C007931:hexacyanoferrate III; D004227:Dithionite; D009599:Nitroprusside; C039900:nitroxyl; D010269:Paraquat