The reduction of Cr(VI) to Cr(III) by the α and β anomers of D-glucose in dimethyl sulfoxide. A comparative kinetic and mechanistic study

Article abstract of DOI:10.1016/S0008-6215(99)00282-7

The reduction of Cr(VI) 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 Cr(VI) by formation of an intermediate Cr(VI) 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 Cr(VI)-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 Cr(V) species, generated in the reaction mixture, reacts with the two anomers at a rate comparable with that of Cr(VI). The EPR spectra show that the α anomer forms several linkage isomers of the five-coordinate Cr(V) bis-chelate, while β-D- glucose affords a mixture of six-coordinate Cr(V) mono-chelate and five- coordinate Cr(V) bis-chelate. The conversion of the Cr(V) 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 Cr(V). (C) 2000 Elsevier Science Ltd.