Preparation and reactivity of a cationic dichromium-semiquinone complex

Article abstract of DOI:10.1021/ja00038a024

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.