The reduction of CrVI to CrIII by the α and β anomers of d-glucose in dimethyl sulfoxide. A comparative kinetic and mechanistic study Original Research Article

The reduction of CrVI by α-d-glucose and β-d-glucose was studied in dimethyl sulfoxide in the presence of pyridinium p-toluensulfonate, a medium where mutarotation is slower than the redox reaction. The two anomers reduce CrVI by formation of an intermediate CrVI ester precursor of the slow redox step. The equilibrium constant for the formation of the intermediate chromic ester and the rate of the redox steps are different for each anomer. α-d-Glucose forms the CrVI–Glc ester with a higher equilibrium constant than β-d-glucose, but the electron transfer within this complex is slower than for the β anomer. The difference is attributed to the better chelating ability of the 1,2-cis-diolate moiety of the α anomer. The CrV species, generated in the reaction mixture, reacts with the two anomers at a rate comparable with that of CrVI. The EPR spectra show that the α anomer forms several linkage isomers of the five-coordinate CrV bis-chelate, while β-d-glucose affords a mixture of six-coordinate CrV mono-chelate and five-coordinate CrV bis-chelate. The conversion of the CrV mono- to bis-chelate is discussed in terms of the ability of the 1,2-cis- versus 1,2-trans-diolate moieties of the glucose anomers to bind CrV.