Redox, kinetics, and complexation chemistry of the Cr~(VI)/Cr ~V/Cr~(IV) -D-glycero-D-gulo-heptono- 1,4-lactone System

摘要

When a 60-times or higher excess of D-glycero-D-gulo-heptono-1,4-lactone (GHL) over Cr~(VI) is used, reaction yields D-gluconic acid, formic acid and CrIII as final products. The redox reaction involves formation of intermediates, Cr~(IV) and CrV species, reacting with GHL at comparable rates. CrIV is a very reactive intermediate and does not accumulate during this reaction; its rate of disappearance is 2.0×10~4 and 4.0×10~3 times higher than Cr~(VI)or Cr~V reaction with GHL, respectively. Kinetic studies show that the redox reaction proceeds through a mechanism combining CrVI- Cr~(IV)Cr~(II) and Cr~(VI) Cr~(IV)Cr~(III) pathways. This mechanism is supported by the observation of free radicals, superoxoCrIII (CrO ~(2+)R_(2)) and oxo-Cr~Vas intermediates species. Complete rate laws for the GHL/chromium redox reaction are described in the present work. EPR spectra show that five-coordinate oxo-CrV bischelates (g _(iso1)=1.9802; g_(iso2)=1.9803) are formed at pH ≤4 where the OH and O-ring groups of GHL participate in the bonding to oxo-CrV. Penta-coordinated oxo-Cr~V monochelates are observed as minor species (g_(iso3)=1.9866; giso4=1.9879) in addition to the major penta-coordinated oxo-Cr~V bischelates.