141018-07-7Relevant academic research and scientific papers
Preparation and reactivity of a cationic dichromium-semiquinone complex
Scott, Susannah L.,Bakac, Andreja,Espenson, James H.
, p. 4605 - 4610 (2007/10/02)
The complex described in the literature as the μ-oxo complex (H2O)5CrOCr(H2O)54+ is shown to be a dichromium(III)-semiquinone complex, (H2O)5CrOC6H4OCr(H 2O)55+. It is prepared by the reaction of 1,4-benzoquinone with Cr2+ in acidic, aqueous solution. The reaction also yields Cr(H2O)63+ and a dichromium(III)-hydroquinone complex of 4+ charge. The semiquinone complex is reversibly reduced by outer-sphere electron transfer to the dichromium(III)-hydroquinone complex, (H2O)5CrOC6H4OCr(H 2O)54+. This reduction is accomplished by any of Cr2+, Ru(NH3)62+, or V2+. The hydroquinone complex is reoxidized by Fe3+, 1,4-benzoquinone, Br2, or Ce(IV) to the semiquinone complex. Cyclic voltammograms obtained with either the hydroquinone complex or the semiquinone complex are identical and correspond to a reversible, one-electron process. The standard reduction potential of the semiquinone complex is +0.61 V (NHE). The semiquinone complex decomposes, over a period of hours, by aquation of Cr(III) followed by disproportionation of the organic ligand, yielding hydroquinone, benzoquinone, and Cr(H2O)63+. A literature report of the quinone oxidation of ethanol catalyzed by Cr3+ is reinterpreted as the reversible formation of a dichromium(III)-semiquinone radical complex.
