Mechanisms of Oxidation of 2-Propanol by Polypyridyl Complexes of Ruthenium(III) and Ruthenium (IV)

Article abstract of DOI:10.1021/ja00379a011

Kinetic and mechanistic studies have been carried out on the oxidation of 2-propanol to acetone in water by Ru^IV(trpy)(bpy)O²⁺ (trpy is 2,2',2''-terpyridine; bpy is 2,2'-bipyridine) and in acetonitrile by Ru^IV(bpy)2(py)O²⁺ (py is pyridine).The reactions proceed by oxidation of 2-propanol by Ru(IV) followed by a slower oxidation by the Ru(III) complexes Ru(trpy)(bpy)OH²⁺ or Ru(bpy)2(py)OH²⁺.For the reactions: in water, k_IV(25 deg C) = 6.7 * 10^-2 M^-1 s^-1, ΔH^<*> = 9 +/- kcal/mol, ΔS^<*> = -34 +/- 4 eu, k_H/k_D = 18 +/- 3; in <(CH3)2CHOH/(CD3)2CDOH>, k_III(25 deg C) = 6 * 10^-5 M^-1 s^-1, ΔH^<*> = 19 +/- 2 kcal/mol, ΔS^<*> = -12 +/- 6 eu, k_H/k_D = 2.7 +/- 1.4.An 18O-labeling experiment in 2-propanol and a spectral experiment in CH3CN show that oxo transfer from the oxidant to the substrate does not occur.It is concluded that the most likely mechanism of oxidation for Ru(IV) is a concerted, two-electron hydride transfer from the α-C-H bond to Ru^IV=O with the oxo group acting as a lead-in atom to the Ru(IV) acceptor site.The Ru(III) reaction in water appears to occur by an initial one-electron, outer-sphere electron transfer.In acetonitrile there appears to be a change in mechanism for this reaction, apparently to a H-atom transfer, once again involving the α-C-H group.For this path: k(25 deg C) = (8+/- 2) * 10 ^-4 M^-1 s^-1, ΔH^<*> = 10 +/- 2 kcal/mol, ΔS^<*> = -38 +/- 7 eu, k_H/k_D <*> 8.