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Oxidation of Benzyl Alcohol by a Dioxo Complex of Ruthenium(VI)

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journal contribution
posted on 1999-04-17, 00:00 authored by Estelle L. Lebeau, Thomas J. Meyer
The kinetics and mechanism of reduction of trans-[RuVI(tpy)(O)2(L)]2+ (L is H2O or CH3CN; tpy is 2,2‘:6‘,2‘ ‘-terpyridine) by benzyl alcohol have been studied in water and acetonitrile. The reactions are first order in alcohol and complex in both solvents and give benzaldehyde as the sole oxidation product. In acetonitrile, sequential RuVI → RuIV and RuIV → RuII‘ steps occur. As shown by FTIR and UV−visible measurements, RuII‘ solvolyzes to give [RuII(tpy)(CH3CN)3]2+ and benzaldehyde. With 18O-labeled RuVI, ∼50% of the label ends up in the aldehyde product for both the RuVI → RuIV and RuIV → RuII steps as shown by FTIR. In water, RuVI → RuIV reduction is followed by rapid dimerization by μ-oxo formation. Kinetic parameters for the individual redox steps in 0.1 M HClO4 at 25 °C are kVIIV = 13.3 ± 0.8 M-1 s-1H = 11.4 ± 0.2 kcal/mol, ΔS = −15.0 ± 1 eu, kH/kD = 10.4 for α,α-d2 benzyl alcohol). In CH3CN at 25 °C, kVIIV = 67 ± 3 M-1 s-1H = 7.5 ± 0.3 kcal/mol, ΔS = −33 ± 2 eu, kH/kD = 12.1) and kIVII = 2.4 ± 0.1 (ΔH = 5.1 ± 0.3 kcal/mol, ΔS = −47 ± 2 eu, kH/kD = 61.5). On the basis of the 18O labeling results in CH3CN, the O atom of the oxo group transfers to benzyl alcohol in both steps. Mechanisms are proposed involving prior coordination of the alcohol followed by O insertion into a benzylic C−H bond.

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