Redox, Kinetics, And Complexation Chemistry Of The Cr-Vi/Cr-V/Cr-Iv -D-Glycero-D-Gulo-Heptono-1,4-Lactone System

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 Cr-III as final products. The redox reaction involves formation of intermediates, Cr-IV and Cr-V species, reacting with GHL at comparable rates. Cr-IV is a very reactive intermediate and does not accumulate during this reaction; its rate of disappearance is 2.0 x 10(4) and 4.0 x 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 Cr-VI -> -Cr-IV -> Cr-II and Cr-VI -> Cr-IV -> Cr-III pathways. This mechanism is supported by the observation of free radicals, superoxoCr(III) (CrO22+) and oxo-Cr-V as intermediates species. Complete rate laws for the GHL/chromium redox reaction are described in the present work. EPR spectra show that five-coordinate oxo-Cr-V 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-Cr-V. Penta-coordinated oxo-Cr-V monochelates are observed as minor species (g(iso3) = 1.9866; g(iso4) = 1.9879) in addition to the major penta-coordinated oxo-Cr-V bischelates. Copyright (C) 2010 John Wiley & Sons, Ltd.