Activation Parameters as Mechanistic Probes in the TAML Iron(V)–Oxo Oxidations of Hydrocarbons

The results of low-temperature investigations of the oxidations of 9,10-dihydroanthracene, cumene, ethylbenzene, [D10]ethylbenzene, cyclooctane, and cyclohexane by an iron(V)–oxo TAML complex (2; see Figure 1) are presented, including product identification and determination of the second-order rate constants k2 in the range 233–243 K and the activation parameters (ΔH≠ and ΔS≠). Statistically normalized k2 values (log k2′) correlate linearly with the CH bond dissociation energies DCH, but ΔH≠ does not. The point for 9,10-dihydroanthracene for the ΔH≠ vs. DCH correlation lies markedly off a common straight line of best fit for all other hydrocarbons, suggesting it proceeds via an alternate mechanism than the rate-limiting CH bond homolysis promoted by 2. Contribution from an electron-transfer pathway may be substantial for 9,10-dihydroanthracene. Low-temperature kinetic measurements with ethylbenzene and [D10]ethylbenzene reveal a kinetic isotope effect of 26, indicating tunneling. The tunnel effect is drastically reduced at 0 °C and above, although it is an important feature of the reactivity of TAML activators at lower temperatures. The diiron(IV) μ-oxo dimer that is often a common component of the reaction medium involving 2 also oxidizes 9,10- dihydroanthracene, although its reactivity is three orders of magnitude lower than that of 2.