Mechanistic studies of the oxidations of hydrocarbons by manganese and ruthenium transition metal complexes

Abstract

The oxidation of hydrocarbon C-H and O-H bonds can occur by a variety of mechanisms. These include stepwise processes such as initial electron or proton transfer, or concerted mechanisms such as hydride transfer or hydrogen-atom abstraction. Hydrocarbons containing weak C-H and O-H bonds are oxidized by Mn(hfacac)3 (hfacac = hexafluoroacetylacetonate) and [(bpy) 2(py)Ru=O]2+ (bpy = bipyridine and py = pyridine). Mn(hfacac) 3 is an easily prepared and reactive oxidant, forming stable solutions in benzene and methylene chloride. Based on an equilibrium established with tris(2,4-dibromophenyl)amine, a redox potential of 0.9 +/- 0.1 V vs. Cp2Fe+/° is calculated. Mn(hfacac)3 oxidizes 9,10-dihydroanthracene (DHA) cleanly to anthracene with a bimolecular rate constant of 6.8 x 10-4 M-1 s-1. Mn(hfacac)3 is also capable of oxidizing xanthene, 1,4-cyclohexadiene, 2,4-di-tert-butylphenol, toluene, and p-methoxytoluene. Product analyses and relative rates indicate that the more electron-rich substrates react by initial electron transfer to manganese. For the less electron-rich substrates, such as 1,4-cyclohexadiene, a mechanism of hydrogen atom abstraction is suggested.The oxidations of DHA, xanthene, and fluorene by [(bpy)2(py)Ru IVO]2+ give mixtures of products including oxygenated and non-oxygenated compounds. The products include those formed by organic radical dimerization, such as 9,9'-bixanthene, as well as by oxygen-atom transfer. The kinetics of these reactions and those of indene, cyclohexene, cumene, ethylbenzene, and toluene have been measured and display a clear correlation with substrate C-H bond dissociation energy. The kinetic isotope effect for the reaction of Ru=O2+ with DHA vs. DHA-d4 gives kH /kD ≥35. A mechanism of initial hydrogen-atom abstraction followed by competitive pathways of radical dimerization and trapping by the oxidant is indicated. The hydrogen-atom selfexchange rate for the transfer of H· between [(bpy)2(py)RuIIIOH] 2+ and [(bpy)2(py)RuIVO]2+ has also been measured (k'HSE = (7.6 +/- 0.4) x 104 M-1 s -1). When combined with an estimate for the DHA/HA· self-exchange rate in the Marcus cross relation, this value agrees well with the observed hydrogen-atom abstraction rate. The deuterium-atom self-exchange rate is also reported for the D· exchange between [(bpy)2(py)RuIIIOD]2+ and [(bpy)2(py)RuIVO]2+. The kinetic isotope effect for this self-exchange reaction is surprisingly near unity.