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, [D 10 ]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 k 2 in the range 233–243 K and the activation parameters (Δ H ≠ and Δ S ≠ ). Statistically normalized k 2 values (log  k 2 ′) correlate linearly with the CH bond dissociation energies D CH , but Δ H ≠ does not. The point for 9,10‐dihydroanthracene for the Δ H ≠ vs. D CH 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 [D 10 ]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 .