Solvent effects on the oxidation of Ru^IV=O to O=Ru^VI=O by MnO₄⁻. hydrogen-atom versus oxygen-atom transfer

The kinetics of the oxidation of trans-[Ruˡⱽ(tmc)(O)(solv)]²⁺ to trans-[Ruⱽˡ(tmc)(O)₂]²⁺ (tmc is 1,4,8,-11-tetramethyl-1,4,8,11-tetraazacyclotetradecane, a tetradentate macrocyclic tertiary amine ligand; solv = H₂O or CH₃CN) by MnO₄⁻ have been studied in aqueous solutions and in acetonitrile. In aqueoussolutions the rate law is -d[MnO₄]/dt = ᵏH₂O[Ruˡⱽ][MnO₄⁻] = (kₓ + (kᵧ)/(Kₐ)[H⁺])[Ruˡⱽ][MnO₄⁻], Kₓ = (1.49 ± 0.09) x 10¹ M⁻¹s⁻¹ and kᵧ = (5.72 ± 0.29) × 10⁴ M⁻¹ s⁻¹ at 298.0 K and I = 0.1 M. The terms Kₓ and kᵧ are proposed to be the rate constants for the oxidation of Ruˡⱽ by MnO₄⁻ and HMnO₄, respectively, and Kₐ is the acid dissociation constant of HMnO₄. At [H⁺] = / = 0.1 M, ΔH^‡ and ΔS^‡ are (9.6 ± 0.6) kcal mol⁻¹ and -(18 ± 2) cal mol⁻¹ K⁻¹, respectively. The reaction is much slower in D₂O, and the deuterium isotope effects are Kₓ/kₓᴰ = 3.5 ± 0.1 and kᵧ/kᵧᴰ = 5.0 ± 0.3. The reaction is also noticeably slower in H₂¹⁸O, and the oxygen isotope effect is ᵏH₂ ¹⁶O/ᵏH₂ ¹⁸O= 1.30 ± 0.07. ¹⁸O-labeled studies indicate that the oxygen atom gained by Ruˡⱽ comes from water and not from KMnO₄. These results are consistent with a mechanism that involves initial rate-limiting hydrogen-atom abstraction by MnO₄⁻ from coordinated water on Ruˡⱽ. In acetonitrile the rate law is -d[MnO₄⁻]/dt = ᵏCH₃CN[Ruˡⱽ][MnO₄⁻], ᵏCH₃CN = 1.95 ± 0.08 M⁻¹ s⁻¹ at 298.0 K and / = 0.1 M. ΔH^‡ and ΔS^‡ are (12.0 ± 0.3) kcal mol⁻¹ and -(17 ± 1) cal mol⁻¹ K⁻¹, respectively. ¹⁸O-labeled studies show that in this case the oxygen atom gained by Ruˡⱽ comes from MnO₄⁻, consistent with an oxygen-atom transfer mechanism. Copyright © 2007 American Chemical Society.