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

The kinetics and mechanism of reduction of trans-[Ru^VI(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 Ru^VI → Ru^IV and Ru^IV → Ru^II‘ steps occur. As shown by FTIR and UV−visible measurements, Ru^II‘ solvolyzes to give [Ru^II(tpy)(CH₃CN)₃]²⁺ and benzaldehyde. With ¹⁸O-labeled Ru^VI, ∼50% of the label ends up in the aldehyde product for both the Ru^VI → Ru^IV and Ru^IV → Ru^II steps as shown by FTIR. In water, Ru^VI → Ru^IV 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 k_VI→IV = 13.3 ± 0.8 M^-1 s^-1 (ΔH^⧧ = 11.4 ± 0.2 kcal/mol, ΔS^⧧ = −15.0 ± 1 eu, k_H/k_D = 10.4 for α,α-d₂ benzyl alcohol). In CH₃CN at 25 °C, k_VI→IV = 67 ± 3 M^-1 s^-1 (ΔH^⧧ = 7.5 ± 0.3 kcal/mol, ΔS^⧧ = −33 ± 2 eu, k_H/k_D = 12.1) and k_IV→II‘ = 2.4 ± 0.1 (ΔH^⧧ = 5.1 ± 0.3 kcal/mol, ΔS^⧧ = −47 ± 2 eu, k_H/k_D = 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.