Formation of Superoxochromium(III) by a Novel Mechanism

Abstract

The reduction of chromate ion by L-cysteine in near-neutral aqueous solutions was studied by u.v.-vis spectroscopy. The rate constants for the formation, decomposition and redox degradation of a Cr VI thioester intermediate (both in the absence and in the presence of dissolved O 2 ) were determined by the use of a nonlinear least-squares regression to fit the experimental data to a double-exponential integrated rate law. Superoxochromium(III), CrOO 2+ , could be observed as a long-lived intermediate by the use of u.v. difference spectroscopy (peaks at 238 and 292 nm). The effects of several reaction variables on the yield of the intermediate CrOO 2+ were studied. A striking result was that the yield decreased as the concentration of dissolved O 2 increased. In the presence of phosphate buffer, the yield showed a maximum at pH 6.8. Formation of CrOO 2+ was suppressed by the additives Mn 2+ , H 2 O 2 , D-ribose, 2-deoxy-D-ribose and a high concentration of Ce 3+ , whereas a low concentration of the latter enhanced its appearance. A novel mechanism for the formation of CrOO 2+ from Cr VI and L-cysteine (involving the intramolecular formation of an O-O bond) is proposed, and the possible routes for the decay of this intermediate are discussed. This novel mechanism might offer new insights into the redox chemistry of chromate ions.

Reaction mechanisms in solution

“…7 In the reduction of cysteine by CrO 4 2À , CrOO 2+ was observed as a long-lived intermediate. 8 The proposed mechanism involves inner-sphere complexation between HCrO 4…”