Oxidation of hydroquinones by [(bpy){sub 2}(py)Ru{sup IV}(O)]{sup 2+} and [(bpy){sub 2}(py)Ru{sup III}(OH)]{sup 2+}. Proton-coupled electron transfer
- Univ. of North Carolina, Chapel Hill, NC (United States); and others
The oxidations of hydroquinone, 2-chlorohydroquinone, and 2,6-dichlorohydroquinone by cis-((bpy){sub 2}(py)Ru{sup IV}(O)){sup 2+})(bpy is 2,2{prime}-bipyridine; py is pyridine) to their corresponding quinones occur in distinct le{sup {minus}} steps and follow pH dependent rate laws. Within the pH range 1-8 the rate laws have the form -d(Ru{sup IV}=O{sup 2+})/dt=(Ru{sup IV}=O{sup 2+})(QH{sub 2})k{sub 1}{prime}(H{sup +}) + k{sub 2} + k{sup {prime}}{sub 3}(H{sup +}){sup {minus}1}. The k{sub 2} pathway displays a large H{sub 2}O/D{sub 2}O kinetic isotope effect of 30 {plus_minus} 1 at 20 {degrees}C, which arises largely from the temperature dependence of the reaction. In strongly acidic solution it is concluded that the oxidation of hydroquinone involves ((BPY){sub 2}(PY)rU{sup iii}(oh{sub 2})){sup 3+}. The k{sub 2} pathway displays an H{sub 2}O/D{sub 2}O kinetic isotope effect of 9.7 {plus_minus} 0.1 at 20 {degrees}C which also arises almost entirely from the temperature dependence of the reaction. Both oxidants display a linear H{sub 2}O/D{sub 2}O mole fraction dependence for k{sub 2} suggesting that the isotope effects for these pH independent pathways involve a single proton transfer. With regard to mechanism, it is concluded that (1) the k{sub 3} pathway for both oxidants occurs by oxidation of the hydroquinone monoanions, (2) the k{sub 1} pathway for Ru(III) involves the outer-sphere electron transfer with the aqua ion, ((bpy){sub 2} (py)Ru{sup III}(OH{sub 2}){sup 3+}, as oxidant, (3) the k{sub 2} pathway for both oxidants occurs by proton-coupled electron transfer for which, in contrast to H-atom transfer, the electron and proton donor functions of the reducing agent reside at different sites within the molecule. 31 refs., 16 figs., 6 tabs.
- Sponsoring Organization:
- USDOE
- OSTI ID:
- 171658
- Journal Information:
- Journal of the American Chemical Society, Vol. 114, Issue 1; Other Information: PBD: 1 Jan 1992
- Country of Publication:
- United States
- Language:
- English
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