Redox and Complexation Chemistry of the Cr(VI)/Cr(V)-D-Galacturonic Acid System

JUAN CARLOS GONZÁLEZ; VERÓNICA DAIER; SILVIA GARCÍA; BERNARD GOODMAN; LUIS F. SALA; SANDRA SIGNORELLA

JOURNAL OF THE CHEMICAL SOCIETY, DALTON TRANSACTIONS

Royal Society of Chemistry

Año: 2004 vol. 2004 p. 2288 - 2296

0300-9246

The oxidation of D-galacturonic acid by CrVI yields the aldaric acid and CrIII as final products when a 30-times or higher excess of the uronic acid over CrVI is used. The redox reaction involves the formation of intermediate CrIV and CrV species, with CrVI and the two intermediate species reacting with galacturonic acid at comparable rates. The rate of disappearance of CrVI, CrIV and CrV depends on pH and [substrate], and the slow reaction step of the CrVI to CrIII conversion depends on the reaction conditions. The EPR spectra show that five-co-ordinate oxo-CrV bischelates are formed at pH £ 5 with the uronic acid bound to CrV through the carboxylate and the a-OH group of the furanose form or the ring oxygen of the pyranose form. Six-co-ordinated oxo-CrV monochelates are observed as minor species in addition to the major five-co-ordinated oxo-CrV bischelates only for galacturonic acid:CrVI ratio £ 10, in 0.25-0.50 M HClO4. At pH 7.5 the EPR spectra show the formation of a CrV complex where the vic-diol groups of Galur participate in the bonding to CrV. At pH 3-5 the Galur-CrV species grow and decay over short periods in a similar way to that observed for [Cr(O)(a-hydroxyacid)2]-. The lack of chelation at any vic-diolate group of Galur when pH ≤ 5 differentiates its ability to stabilise CrV from that of neutral saccharides that form very stable oxo-CrV-diolato2 species at pH > 1.