Solvent Effects on the Oxidation of Ru~Ⅳ=O to O=Ru~Ⅳ=O by MnO_4~- · Hydrogen-Atom versus Oxygen-Atom Transfer

摘要

The kinetics of the oxidation of trans-[Ru~Ⅳ(tmc)(O)(solv)]~(2+) to trans-[Ru~Ⅵ(tmc)(O)_2]~(2+) (tmc is 1,4,8,-11-tetramethyl-1,4,8,11-tetraazacyclotetradecane, a tetradentate macrocyclic tertiary amine ligand; solv = H_2O or CH_3CN) by MnO_4~- have been studied in aqueous solutions and in acetonitrile. In aqueous solutions the rate law is -d[MnO_4]/dt =k_(H_2O)[Ru~Ⅳ][MnO_4~-] = (k_x + (ky)/(k_a)[H~+])[Ru~Ⅳ][MnO_4~-], k_x = (1.49 ± 0.09) × 10~1 M~(-1) s~(-1) and k_y = (5.72 ± 0.29) × 10~4 M~(-1) s~(-1) at 298.0 K and / = 0.1 M. The terms k_x and k_y are proposed to be the rate constants for the oxidation of Ru~Ⅳ by MnO_4~- and HMnO_4, respectively, and K_a is the acid dissociation constant of HMnO_4· At [H~+] = I= 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 D_2O, and the deuterium isotope effects are k_x/k_x~D = 3.5 ± 0.1 and k_v/k_y~D = 5.0 ± 0.3. The reaction is also noticeably slower in H_2~(18)O, and the oxygen isotope effect is k_(H_2)~(16)o/k_(H_2)~(18)_o = 1.30 ± 0.07.~(18)O-labeled studies indicate that the oxygen atom gained by Ru~Ⅳ comes from water and not from KMnO_4. These results are consistent with a mechanism that involves initial rate-limiting hydrogen-atom abstraction by MnO_4~- from coordinated water on RuⅣ. In acetonitrile the rate law is -d[MnO_4~-]/dt = k_(CH_3CN)[Ru~Ⅳ][MnO_4~-], k_(CH_3CN) = 1.95 ± 0.08 M~(-1) s~(-1) at 298.0 K and I= 0.1 M. ΔH and ΔS are (12.0 ± 0.3) kcal mol~(-1) and -(17 ± 1) cal mol~(-1) K~(-1), respectively, ~(18)Olabeled studies show that in this case the oxygen atom gained by Ru~Ⅳ comes from MnO_4~-, consistent with an oxygen-atom transfer mechanism.