Oxygen Transfer in the Oxidation of Triphenylphosphine by (bpy)2pyRuO2

The oxidation of PPh3 by (bpy)2pyRuO2+ (bpy is 2,2'-bipyridine) has been studied in acetonitrile solution. The observation of (bpy)2pyRuIIO=PPh32+ as an intermediate demonstrates that the reaction proceeds via an initial redox step (bpy)2pyRuO2+ + PPh3 -(bpy)2pyRuIIO=PPh32+ which is followed by relatively slow ((25 °C) = 1.15 (±0.10) X 10-4 s-1) solvolysis; CH₃CN + (bpy)2pyRunO=PPh32+ - (bpy)2pyRuCH₃CN2+ + O==PPh3. From the results of an 180-labeling experiment, oxygen transfer from Rurv=O to PPh3 is quantitative within experimental error. The kinetics of the redox step were studied by stopped-flow spectrometry. The reaction is first order in both reactants with k(26.6 °C) = 1.75 ± 0.10 X 10s M-1 s-1. Activation parameters were calculated from the temperature dependence of the rate constant, an analysis of which gave •H* = 4.7 ± 0.5 kcal/mol and •S* = -19 ± 3 eu. The probable microscopic details of the reaction are discussed in terms of a 2-equiv step with strong electronic-vibrational coupling between the electron-donor (P) and -acceptor (Ru) sites through Ru-O and O-P stretching vibrations. The possible role of the spin change which occurs in the net reaction is discussed.