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 × 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 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 (CrO   2 2+ ) 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 © 2010 John Wiley & Sons, Ltd.