title: Oxidation of hydroxyurea with oxovanadium(V) ions in acidic aqueous solution.
abstract: Hydroxyurea (HU) effectively reduces vanadium(V) into vanadium(IV) species (hereafter V(V) and V(IV) species, respectively) in acidic aqueous solution via the formation of a transient complex followed by an electron transfer process that includes the formation and subsequent fading out of a free radical, U* (U* identical with H(2)N-C(=O)N(H)O*). The electron paramagnetic resonance (EPR) spectra of U* in H(2)O/D(2)O solutions suggest that the unpaired electron is located predominantly on the hydroxamate hydroxyl-oxygen atom. Visible and V(IV)-EPR spectroscopic data reveal HU as a two-electron donor, whereas formation of U*, which reduces a second V(V), indicates that electron transfer occurs in two successive one-electron steps. At the molarity ratio [V(V)]/[HU]=2, the studied reaction can be formulated as: 2 V(V)+HU-->2 V(IV)+0.98 CO(2)+0.44 N(2)O+1.1 NH(3)+0.1 NH(2)OH. Lack of evidence for the formation of NO is suggested to be a consequence of the slow oxidation of HNO due to the too low reduction potential of the V(V)/V(IV) couple under the experimental conditions used. The nuclear magnetic resonance ((51)V-NMR) spectral data indicate protonation of (H(2)O)(4)V(V)O(2)(+), and the protonation equilibrium constant was determined to be K=0.7 M(-1). Spectrophotometric titration data for the V(V)-HU system reveal formation of (H(2)O)(2)V(V)O(OH)U(+) and (H(2)O)(3)V(V)OU(2+). Their stability constants were calculated as K(110)=5 M(-1) and K(111)=22 M(-2), where the subscript digits refer to (H(2)O)(4)V(V)O(2)(+), HU and H(+), respectively.
authors.affiliation: Faculty of Pharmacy and Biochemistry, University of Zagreb, A. Kovacića 1, 10000 Zagreb, Croatia.
authors.forename: Mario
authors.initials: M
authors.lastname: Gabricević
authors.forename: Erim
authors.initials: E
authors.lastname: Besić
authors.forename: Mladen
authors.initials: M
authors.lastname: Birus
authors.forename: Achim
authors.initials: A
authors.lastname: Zahl
authors.forename: Rudi van
authors.initials: Rv
authors.lastname: Eldik
mesh_terms: D004578:Electron Spin Resonance Spectroscopy; D005292:Ferricyanides / Q000737:chemistry; D005609:Free Radicals / Q000737:chemistry; D006918:Hydroxyurea / Q000737:chemistry*; D007477:Ions; D009682:Magnetic Resonance Spectroscopy; D010084:Oxidation-Reduction; D012996:Solutions; D014638:Vanadates / Q000737:chemistry*; D014867:Water
chemical_list: D005292:Ferricyanides; D005609:Free Radicals; D007477:Ions; D012996:Solutions; D014867:Water; C007931:hexacyanoferrate III; D014638:Vanadates; D006918:Hydroxyurea