Redox and complexation chemistry of the hypervalent chromium-D.glucuronic acid system

GONZÁLEZ, JUAN CARLOS; BELLÚ, SEBASTIÁN; GARCÍA, SILVIA; SIGNORELLA, SANDRA; SALA, LUIS F.

Austria

Congreso; 9th FIGIPAS Meeting in Inorganic Chemistry; 2007

Viena University of Technology

Compounds of CrVI represent a potential environmental hazard because of their mammalian toxicity and carcinogenicity. The observation of CrV and CrIV intermediates in the selective oxidation of organic by CrVI and their implication in the mechanism of Cr-induced cancers has generated a considerable amount of interest in the chemistry and biochemistry of this element. Pectic substances are a group of polysaccharides present in all plant, and D-glucuronic acid (Glucur) is a low-molecular-weight metabolite of these biopolymers. The objective of this work was to study the redox chemistry of the CrVI- D-glucuronic acid system in acidic medium and the ability of Glucur to coordinate CrV, CrIV and CrIII. Kinetic measurements were performed on a reaction mixture of Glucur and CrVI in 1.0-0.2 M HClO4 following the time-evolution of the absorption spectra in the 350-500 nm range. Rate constants were calculated from the absorbance data obtained at 350 nm, for five different [Glucur] and [H+]. The decrease in absorbance with time was not monophasic. EPR spectroscopy revealed the presence of long-lived CrV-Glucur species. In the range of pH 1-4, the EPR spectra showed two triplets at giso = 1.9781 and 1.9783 and two doublets at giso = 1.9753 and 1.9763. The giso values for the triplets correspond to those calculated for five-coordinate oxo-CrV bischelates with two carboxilate and two alcoholate ligands. Doublets at giso = 1.9754 and 1.9763 correspond to those calculated for five-coordinated oxo-CrV monochelates with one carboxilate and one alcoholate ligands, and two alcoholate ligands, respectively. Differential UV/vis spectra of mixtures of CrVI and Glucur exhibited an absorption band with lmax = 377 nm consistent with that ascribed to CrVI oxy esters. A periodic scanning of the O2-saturated solution of a CrVI + Glucur reaction mixture in 2.0M HClO4 showed two absorption bands at 290 nm and 247 nm characteristic of CrO22+. These spectroscopic results reveal that CrII forms in the redox reaction, and can be taken as evidence that CrIV is implied in the redox mechanism of the reaction between CrVI and Glucur. The redox reaction is proposed to proceed through a mechanism combining CrVI ® CrIV ® CrII and CrVI ® CrIV ® CrIII pathways. Free radicals formed in the CrIV ® CrIII path were detected by the induced polymerization of acrylonitrile. Glucaric acid was found as chromic oxidation product of Glucur. CrIII-Glucur complexes were detected after redox reaction. Green solid [(glucuronate)2CrIII(OH)(OH2)] 2H2O was synthesized. UV/vis spectrum shows that CrIII is hexa-coordinate and IR spectrum demonstrates that carboxilate participate in coordination as mono-dentate ligand.