Oxidations Of 2-furoic Hydrazide And DL-ethionine By Metal(Ⅳ) Complexes: A Kinetic Analysis

Oxidation of 2-furoic hydrazide（FH）by hexachloroiridate（Ⅳ）([IrCl₆]^2–)was studied kinetically in a wide pH range in aqueous solution of 1.0 M ionic strength.The oxidation reaction followed well-defined second-order kinetics:-d[IrCl₆^2–]/dt=k′[FH]_tot[IrCl₆^2–],where[FH]_totot denotes the total concentration of FH and k′stands for the observed second-order rate constants.The established k′-pH profile displays that k′increases drastically with pH and a plateau region exists between pH 4 and 6.A stoichiometric ratio of?[FH]_tot/?[IrCl₆^2–]=1/4 was revealed by spectrophotometric titrations.¹H NMR spectroscopic studies indicated that FH was cleanly oxidized to 2-furoic acid.The kinetic data suggest a reaction mechanism in which all the three protolysis species of FH react with[IrCl₆]^2–in parallel,forming the rate-determining steps.Two stabilized hydrazyl radicals are generated in the rate-determining steps,in which a single electron is transferred to[IrCl₆]^2–.The two hydrazyl radicals react rapidly in consecutive steps requiring 3 moles of Ir（Ⅳ）to form 2-furoic acid as the final product.Rate constants of the rate-determining steps were deduced through a simulation of the rate expression to the k′-pH dependency data.Values of these rate constants demonstrate that the three protolysis species of FH have a huge reactivity span,changing by about 10⁹ times towards reduction of[IrCl₆]^2–and that FH can be readily oxidized in neutral and basic media.Rapid scan spectra and the measured activation parameters suggest that an outer-sphere electron transfer is probably taking place in each of the rate-determining steps.This is the first kinetic study on the oxidation reactions of FH and provides concurrently the protolysis constants of FH(pK_a1=3.04±0.08 and pK_a2=11.6±0.1)at 25.0 ^oC and 1.0 M ionic strength.Ethionine is an S-ethyl analog of methionine（Met）having a small change in structure.But it is a chemical carcinogen and an antagonist of Met,thus displaying a disparate biological profile.The oxidations of ethionine by biologically important oxidants have not been exploited.Oxidations of DL-ethionine by Pt（IV）anticancer model complexes trans-[PtX₂（CN₄）]^2–（X=Cl or Br）were thus analyzed by time-resolved and stopped-flow spectral techniques.Overall second-order kinetics was established,being first-order in[Pt（IV）]and[Ethionine]_tot（the total concentration of ethionine）;the observed second-order rate constant k’versus pH profiles were obtained.A stoichiometry ofΔ[Pt（Ⅳ）]:Δ[Ethionine]_tot=1:1 was unraveled,indicating that ethionine was oxidized to ethionine-sulfoxide which was confirmed by NMR spectroscopic and high-resolution mass spectral analyses.In the proposed reaction mechanism which is similar to that for the oxidation of Met by the same Pt（Ⅳ）compounds,the rate-determining steps are rationalized in terms of a bridge formation between one of the coordinated halides in[PtX₂（CN₄）]^2–and the sulfur atom in ethionine,followed by an X⁺transfer.Moreover,a large rate enhancement for the reaction of ethionine with[PtBr₂（CN₄）]^2–compared with[PtCl₂（CN₄）]^2–strongly supports an X⁺transfer mechanism.Rate constants of the rate-determining steps have been derived.Ratios of k（ethionine）/k（Met）are between 2.2 and 2.6 obtained for the three protolytic species of ethionine and Met;the enhanced reactivity might be partially responsible for the disparate biological profiles.