engleski

Oxidation of Hydroxyurea with Oxovanadium(V) Ions in Acidic Aqueous Solution

Hydroxyurea (HU) effectively reduces vanadium(V) into vanadium(IV) in acid aqueous solution via the formation of a transient complex species followed by the electron transfer process including the formation and subsequent fading out of a free radical. The measured g-value of the radical (g = 2.0067) shows that the unpaired electron is located predominantly in the p-orbital of the hydroxamate hydroxyl-oxygen atom. The visible and VIV-EPR spectroscopic data indicate that HU acts as a two-electron donor. The studied redox reaction can be delineated as: 2VVO2+ + HU ? ; ; ; 2VIVO2+ + products. FTIR data reveal that one of the products is isocynate, but there is no evidence of NO as the product. The 51V-NMR spectral data indicates protonation of dioxovanadium(V) ions according to equation ; and value of K calculated as 0.7 M-1. Spectrophotometric titration data of V(V)-HU system reveal following equilibria ; characterized by K0 = 5 M-1 and K1 = 22 M-1. When HU is in molar excess, and the above three reactions featuring fast equilibria, the following inner-sphere electron-transfer mechanism satisfies the kinetic experiments: (H2O)3VVOU2+ ? ; ; ; (H2O)3VIVOU? ; ; ; 2+ (slow, k1) (H2O)3VIVOU? ; ; ; 2+ + 2H2O ? ; ; ; (H2O)5VIVO2+ + U? ; ; ; (fast) U? ; ; ; + (H2O)4VVO2+ ? ; ; ; (H2O)5VIVO2+ + products (fast) characterized by k1 = 1.4 s-1. When V(V) is in excess, two outher-sphere electron-transfer pathways must be added: (H2O)3VVOU2+ + (H2O)4VVO2+ ? ; ; ; (H2O)3VVOU? ; ; ; 3+ + (H2O)4VIVO2 k4 (H2O)3VVOU2+ + (H2O)4VVO(OH)2+ ? ; ; ; (H2O)3VVOU? ; ; ; 3+ + (H2O)4VIVO(OH)+ k5 characterized by k4 = 22 s-1 mol-1 dm3, and k5 = 3.8 × 103 s-1 mol-1 dm3.

Dioxovanaium(V); Hydroxyurea; Redox; Mechanism; EPR spectroscopy

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